RI 9689
 

REPORT OF INVESTIGATIONS/2012 
Report of Investigations 9689 

Dust Control Handbook for Industrial Minerals 

Mining and Processing 

Andrew B. Cecala, Andrew D. O’Brien, Joseph Schall, Jay F. Colinet, William R. Fox, 

Robert J. Franta, Jerry Joy, Wm. Randolph Reed, Patrick W. Reeser, John R. Rounds, 

Mark J. Schultz 

DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

Office of Mine Safety and Health Research 

Pittsburgh, PA • Spokane, WA
	

 January 20122 
 

This document is in the public domain and may be freely 

copied or reprinted. 

Disclaimer 

Mention of any company or product does not constitute endorsement by the National Institute for 

Occupational Safety and Health (NIOSH). In addition, citations to Web sites external to NIOSH 

do not constitute NIOSH endorsement of the sponsoring organizations or their programs or 

products. Furthermore, NIOSH is not responsible for the content of these Web sites. All Web 

addresses referenced in this document were accessible as of the publication date. 

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DHHS (NIOSH) Publication No. 2012–112

January 2012

SAFER • HEALTHIER • PEOPLETM 

ii 
CHAPTER CONTENTS

FUNDAMENTALS OF DUST 

COLLECTION SYSTEMS 

WET SPRAY SYSTEMS 

DRILLING AND BLASTING 

CRUSHING, MILLING, 

AND SCREENING 

CONVEYING AND TRANSPORT 

BAGGING 

BULK LOADING 

CONTROLS FOR 

SECONDARY SOURCES 

OPERATOR BOOTHS, CONTROL 

ROOMS, AND ENCLOSED CABS 

HAUL ROADS, STOCKPILES, 

AND OPEN AREASiii

 

TABLE OF CONTENTS 

ACRONYMS AND ABBREVIATIONS USED IN THIS HANDBOOK....................................... xxiii 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS HANDBOOK .................................. xxiv 

ACKNOWLEDGMENTS............................................................................................................. xxv 

ABOUT THIS HANDBOOK ...................................................................................................... xxvi 

TASK FORCE COMMITTEE COCHAIRS............................................................................... xxvi 

TASK FORCE COMMITTEE MEMBERS ............................................................................... xxvi 

EDITOR................................................................................................................................. xxvii 

HANDBOOK LAYOUT COORDINATOR ............................................................................. xxvii 

IMA-NA COORDINATOR..................................................................................................... xxvii 

INTRODUCTION ........................................................................................................................... 1 

SILICA EXPOSURE AND POTENTIAL HEALTH IMPACTS ...................................................... 2 

REFERENCES ............................................................................................................................. 4 

CHAPTER 1: FUNDAMENTALS OF DUST COLLECTION SYSTEMS ...................................... 7 

BASICS OF DUST COLLECTOR SYSTEMS ............................................................................... 8 

AIRFLOW AND DUST CONTROL.............................................................................................. 9 

EXHAUST SYSTEMS DESIGN ................................................................................................. 11 

HOODS ..................................................................................................................................... 11 

Hoods and Blowing versus Exhausting Ventilation ........................................................................... 11 

Hood Types .............................................................................................................................. 12 

Hood Design............................................................................................................................. 14 

Air Induction ........................................................................................................................ 15 

Capture Velocity.................................................................................................................... 16 

Other Hood Considerations.......................................................................................................... 18 

Checklist for Hood Effectiveness .................................................................................................. 19 

DUCTWORK AND AIR VELOCITIES....................................................................................... 19 

High-Velocity Systems ............................................................................................................... 20 

Low-Velocity Systems ................................................................................................................ 21 

Airflow Target of 1,800 fpm .................................................................................................... 23 

Avoidance of Horizontal Ductwork ........................................................................................... 23 

The Use of Main Duct Trunk Lines ........................................................................................... 24 

v 
 

Minimizing Field Fits and Welds .............................................................................................. 24 

Avoiding Mitered Elbows Greater Than 90 Degrees ..................................................................... 25 

Sizing and Locating Orifice Plates............................................................................................. 25 

Minimizing the Use of Flexible Hoses ....................................................................................... 26 

Modified Low-Velocity (MLV) System ......................................................................................... 26 

AIR CLEANING DEVICES........................................................................................................ 28 

Gravity Separators (Drop-Out Boxes) ............................................................................................ 28 

Centrifugal Collectors or Cyclones ................................................................................................ 29 

Baghouse Collectors................................................................................................................... 31 

Mechanical Shaker Collectors .................................................................................................. 34 

Reverse Air Collectors ............................................................................................................ 36 

Reverse Jet (Pulse Jet) Collectors.............................................................................................. 37 

Cartridge Collectors ............................................................................................................... 38 

Wet Scrubbers........................................................................................................................... 40 

Venturi Scrubbers .................................................................................................................. 41 

Impingement Plate Scrubbers ................................................................................................... 41 

Spray Tower Scrubbers ........................................................................................................... 42 

Wet Cyclone Scrubbers........................................................................................................... 42 

Electrostatic Precipitators (ESPs) .................................................................................................. 42 

COLLECTOR DISCHARGE DEVICES ...................................................................................... 43 

Rotary Airlock Valves ................................................................................................................ 43 

Double Dump Valves ................................................................................................................. 43 

Tilt Valves................................................................................................................................ 44 

Vacuum or Dribble Valves........................................................................................................... 44 

FILTER FABRICS ..................................................................................................................... 44 

Particulate Collection of Fabrics ................................................................................................... 46 

DESIGNING MINE/PLANT DUST COLLECTION SYSTEMS ................................................... 48 

Clay Application........................................................................................................................ 49 

FANS......................................................................................................................................... 50 

Fan Operating Characteristics....................................................................................................... 50 

Fan Laws ................................................................................................................................. 52 

Fan Types................................................................................................................................. 53 

Axial-Flow Fans .................................................................................................................... 53 

vi 
 

Centrifugal Fans .................................................................................................................... 54 

Other Fan Types ........................................................................................................................ 56 

Roof Ventilators ........................................................................................................................ 56 

REFERENCES ........................................................................................................................... 57 

CHAPTER 2: WET SPRAY SYSTEMS......................................................................................... 61 

PRINCIPLES OF WET SPRAY SYSTEMS................................................................................. 61 

Water Application...................................................................................................................... 61 

Nozzle Location ........................................................................................................................ 62 

Controlling Droplet Size.............................................................................................................. 63 

Methods of Atomization.............................................................................................................. 64 

Chemical Additives to Control Droplets ......................................................................................... 64 

NOZZLE TYPES AND SPRAY PATTERNS............................................................................... 65 

Air Atomizing Nozzles ............................................................................................................... 66 

Hydraulic Full Cone Nozzles........................................................................................................ 67 

Hydraulic Hollow Cone Nozzles ................................................................................................... 67 

Hydraulic Flat Fan Nozzles.......................................................................................................... 68 

MAINTENANCE ISSUES WITH WET SPRAY SYSTEMS ........................................................ 70 

Water Quality............................................................................................................................ 70 

Nozzle Maintenance ................................................................................................................... 72 

Erosion and Wear ...................................................................................................................... 72 

Corrosion ................................................................................................................................. 72 

Clogging .................................................................................................................................. 73 

Caking..................................................................................................................................... 73 

Temperature Damage.................................................................................................................. 73 

Improper Reassembly ................................................................................................................. 73 

Accidental Damage .................................................................................................................... 74 

Checking Spray Nozzle Performance ............................................................................................. 74 

Performance Testing of Equipment................................................................................................ 75 

REFERENCES ........................................................................................................................... 75 

CHAPTER 3: DRILLING AND BLASTING ................................................................................ 79 

SURFACE DRILLING DUST CONTROL................................................................................... 79 

Wet Drilling.............................................................................................................................. 80 

Advantages to Wet Drilling...................................................................................................... 81 

vii 
 

Recommendations for Proper Wet Drilling ................................................................................. 82 

Disadvantages to Wet Drilling .................................................................................................. 82 

Water Separator Sub............................................................................................................... 83 

Dry Drilling .............................................................................................................................. 84 

Medium- to Large-Diameter Drill Dust Collection Systems............................................................ 84 

Sources of Dust Emissions from Collection Systems..................................................................... 84 

Deck Shroud Leakage Solutions ............................................................................................... 85 

Rectangular Shroud ............................................................................................................ 85 

Circular Shroud ................................................................................................................. 86 

Collector to Bailing Airflow Ratio......................................................................................... 86 

Airflow Maintenance for Dust Control ................................................................................... 87 

Use of Collector to Bailing Airflow Ratios for Dust Control ....................................................... 88 

Air-Blocking Shelf ................................................................................................................. 90 

Collector Dump Discharge....................................................................................................... 93 

Preventing Collector Dump Dust Entrainment ......................................................................... 93 

Drill Stem/Deck Leakage ........................................................................................................ 95 

Preventing Drill Stem/Deck Leakage ..................................................................................... 95 

Small Diameter Drill Dust Collection Systems............................................................................. 96 

UNDERGROUND DRILLING DUST CONTROL....................................................................... 97 

Wet Drilling.............................................................................................................................. 98 

Water Mists and Foams........................................................................................................... 99 

Wetting Agents...................................................................................................................... 99 

External Water Sprays .......................................................................................................... 100 

RESPIRABLE DUST CONTROL FOR BLASTING .................................................................. 100 

Blasting Dust Control Measures.................................................................................................. 101 

Wetting Down Blasting Area.................................................................................................. 101 

Water Cartridges.................................................................................................................. 102 

Fogger Sprays ..................................................................................................................... 102 

Air Filtration Systems ........................................................................................................... 103 

Ventilation System Dispersal ................................................................................................. 105 

REFERENCES ......................................................................................................................... 105 

CHAPTER 4: CRUSHING, MILLING, AND SCREENING ...................................................... 111 

PREVENTION AND SUPPRESSION APPLICATIONS ............................................................ 112 

viii 
 

Wet Control Methods ............................................................................................................... 112 

Dry Control Methods................................................................................................................ 112 

CRUSHING ............................................................................................................................. 113 

Work Practices to Minimize Dust Exposure from Crushers .............................................................. 122 

MILLING................................................................................................................................. 122 

Design and Work Practices to Minimize Dust Exposure from Milling ................................................ 124 

SCREENING ........................................................................................................................... 125 

Work Practices to Minimize Dust Exposure from Screens................................................................ 127 

MAINTENANCE ..................................................................................................................... 128 

REFERENCES ......................................................................................................................... 129 

CHAPTER 5: CONVEYING AND TRANSPORT....................................................................... 133 

BELT CONVEYORS................................................................................................................ 133 

Controlling Material Spillage ..................................................................................................... 134 

Carryback............................................................................................................................... 141 

Belt Scrapers....................................................................................................................... 141 

Belt Washing ...................................................................................................................... 142 

CONVEYOR DESIGN AND MAINTENANCE ISSUES ........................................................... 143 

Belt Splices............................................................................................................................. 143 

DESIGN CONSIDERATIONS FOR TRANSFER POINTS ........................................................ 144 

WATER SPRAYS FOR PREVENTION OF AIRBORNE DUST ................................................ 146 

SCREW CONVEYORS ............................................................................................................ 147 

BUCKET ELEVATORS ........................................................................................................... 148 

PNEUMATIC CONVEYANCE ................................................................................................ 150 

REFERENCES ......................................................................................................................... 153 

CHAPTER 6: BAGGING ........................................................................................................... 157 

BAGS AS DUST SOURCES..................................................................................................... 157 

BAG CONSIDERATIONS........................................................................................................ 159 

Bag Construction ..................................................................................................................... 159 

Bag Perforations ...................................................................................................................... 160 

Overall Perforations ............................................................................................................. 161 

Undervalve Perforations........................................................................................................ 161 

Bag Fill Type .......................................................................................................................... 162 

Effects of Bag Failures.............................................................................................................. 165 

ix 
 

CONTROL TECHNOLOGIES FOR FILLING 50- TO 100-POUND BAGS ................................ 166 

Exhaust Hood Placed around Bag Loading Nozzles ........................................................................ 166 

Exhaust Hoppers (Below Entire Fill Station) ................................................................................. 167 

Dual-Nozzle Bagging System..................................................................................................... 168 

Overhead Air Supply Island System ............................................................................................ 170 

Automated Bag Placer and Filling Systems ................................................................................... 172 

CONTROL TECHNOLOGIES FOR CONVEYING AND PALLETIZING 50- to 100-POUND 

BAGS.................................................................................................................................. 174 

Bag and Belt Cleaning Device .................................................................................................... 174 

Semi-Automated Bag Palletizing Systems..................................................................................... 175 

Automated Bag Palletizing Systems ............................................................................................ 177 

Plastic Pallet Wrap ................................................................................................................... 180 

FLEXIBLE INTERMEDIATE BULK CONTAINERS ............................................................... 181 

MAINTAINING CLEAN FLOORS IN BAG LOADING, PALLETIZING, AND WAREHOUSE 

LOCATIONS....................................................................................................................... 182 

ENCLOSING DUST-LADEN AREAS WITH PLASTIC STRIPPING AND USING AN LEV 

SYSTEM ............................................................................................................................. 183 

WORK PRACTICES TO MINIMIZE DUST EXPOSURES FROM BAGGING AND 

PALLETIZING.................................................................................................................... 184 

REFERENCES ......................................................................................................................... 185 

CHAPTER 7: BULK LOADING ................................................................................................ 189 

LOADING SPOUTS................................................................................................................. 189 

Dust Collection Systems and Loading Spouts ................................................................................ 192 

Cascading Loading Spout .......................................................................................................... 193 

DUST SUPPRESSION HOPPER .............................................................................................. 194 

ENCLOSURES ........................................................................................................................ 194 

Enclosed Loadout .................................................................................................................... 194 

Operator Booths/Control Rooms ................................................................................................. 196 

REFERENCES ......................................................................................................................... 197 

CHAPTER 8: CONTROLS FOR SECONDARY SOURCES...................................................... 201 

CLEANING DUST FROM SOILED WORK CLOTHES ............................................................ 201 

NEW TECHNOLOGY FOR CLEANING DUST FROM WORK CLOTHING ............................ 202 

Clothes Cleaning Booth ............................................................................................................ 203 

Air Reservoir .......................................................................................................................... 203 

x 
 

Air Spray Manifold .................................................................................................................. 203 

Exhaust Ventilation .................................................................................................................. 204 

Safety Issues ........................................................................................................................... 205 

EFFECTIVENESS OF CLOTHES CLEANING TECHNOLOGY ............................................... 205 

CURRENT STATUS OF CLOTHES CLEANING REGULATIONS........................................... 206 

HOUSEKEEPING PRACTICES ............................................................................................... 207 

TOTAL STRUCTURE VENTILATION DESIGN...................................................................... 210 

PRINCIPLES OF TOTAL STRUCTURE VENTILATION ......................................................... 210 

Clean Makeup Air Supply ......................................................................................................... 211 

Effective Upward Airflow Pattern ............................................................................................... 212 

Competent Shell of Structure...................................................................................................... 212 

VARIABLES AFFECTING TOTAL STRUCTURE VENTILATION.......................................... 212 

Costs ..................................................................................................................................... 213 

OPEN STRUCTURE DESIGN.................................................................................................. 213 

BACKGROUND DUST SOURCES .......................................................................................... 214 

EXAMPLES OF SECONDARY DUST SOURCES.................................................................... 215 

Dusts from Outside Sources Traveling Inside Structures .................................................................. 215 

Personal Dust Exposure from Differences in Job Function Work Practices .......................................... 215 

Broken Bags of Product ............................................................................................................ 217 

Cloth Seat Material/Worn Acoustical Material............................................................................... 218 

Dirty Work Boots .................................................................................................................... 218 

MAINTENANCE ..................................................................................................................... 220 

Work Practices to Reduce Dust Exposure during Maintenance Activities ............................................ 220 

REFERENCES ......................................................................................................................... 221 

CHAPTER 9: OPERATOR BOOTHS, CONTROL ROOMS, AND ENCLOSED CABS 

EFFECTIVENESS TERMINOLOGY ..................................................................................... 225 

FILTRATION AND PRESSURIZATION SYSTEMS ................................................................ 227 

Field Studies ........................................................................................................................... 227 

Laboratory Study ..................................................................................................................... 228 

Mathematical Model to Determine Enclosure Protection Factor .................................................... 229 

RECOMMENDATIONS FOR FILTRATION/PRESSURIZATION SYSTEMS ........................... 230 

Ensuring Enclosure Integrity by Achieving Positive Pressurization against Wind Penetration ................. 230 

Keeping Doors and Windows Closed ........................................................................................... 231 

xi 
 

Effective Filtration ................................................................................................................... 232 

Effective Outside Air Filtration and Enclosure Pressurization ....................................................... 232 

Outside Air Inlet Location ..................................................................................................... 233 

PreCleaner.......................................................................................................................... 233 

Effective Recirculation Filtration ............................................................................................ 233 

Floor Heaters .......................................................................................................................... 234 

Good Housekeeping (Enclosure Cleanliness)................................................................................. 235 

Mechanical Filter Media and Visual Indicator for Filter Changing ..................................................... 235 

Ease of Filter Change................................................................................................................ 236 

Uni-Directional Design ............................................................................................................. 236 

Generalized Parameters for an Effective System ............................................................................ 237 

OTHER METHODS TO LOWER RESPIRABLE DUST IN ENCLOSURES............................... 238 

Small In-Unit HEPA Filters ....................................................................................................... 238 

REFERENCES ......................................................................................................................... 239 

CHAPTER 10: HAUL ROADS, STOCKPILES, AND OPEN AREAS ....................................... 243 

HAUL ROADS ........................................................................................................................ 244 

Basics of Road Construction ...................................................................................................... 244 

Haul Road Dust Control Measures .............................................................................................. 249 

Road Preparation ................................................................................................................. 250 

Water ................................................................................................................................ 251 

Surfactants...................................................................................................................... 253 

Salts .............................................................................................................................. 254 

Petroleum Emulsions ........................................................................................................ 255 

Polymers ........................................................................................................................ 255 

Adhesives....................................................................................................................... 256 

Other Dust Control Methods .................................................................................................. 257 

Speed Control ................................................................................................................. 257 

Traffic Control ................................................................................................................ 257 

Load Covers ................................................................................................................... 257 

Cab Maintenance ............................................................................................................. 258 

STOCKPILES AND OPEN AREAS.......................................................................................... 258 

Surface Roughness ................................................................................................................... 259 

Water .................................................................................................................................... 262 

xii 
 

Coatings................................................................................................................................. 263 

Wind Barriers.......................................................................................................................... 265 

REFERENCES ......................................................................................................................... 267 

GLOSSARY ................................................................................................................................. 275 

xiii 
 

ILLUSTRATIONS 

Figure 1.1. Relationship between static, velocity, and total pressure .......................................... 10 

Figure 1.2. A basic depiction of a simple exhaust system with the major components being 

the hood, duct, air cleaning device, and fan ............................................................................. 11 

Figure 1.3. Comparison of ventilation characteristics for blowing versus exhausting system.... 12 

Figure 1.4. Simple design of basic push-pull ventilation system ................................................ 13 

Figure 1.5. Basic setup for a type of receiving hood ................................................................... 14 

Figure 1.6. Demonstration of air induction as material falls from a conveyor............................ 15 

Figure 1.7. Hood entry loss calculation ....................................................................................... 17 

Figure 1.8. Hood entry loss coefficients for different hood types ............................................... 18 

Figure 1.9. Schematics of high- and low-velocity systems ......................................................... 20 

Figure 1.10. Sawtooth design of a low-velocity system with a dropout duct back into the 

process at the base of each leg.................................................................................................. 22 

Figure 1.11. Duct design angles for low-velocity systems .......................................................... 24 

Figure 1.12. Demonstration of poor orifice plate placement ....................................................... 25 

Figure 1.13. Depiction of the horizontal and vertical velocity relationship in a modified lowvelocity system ......................................................................................................................... 27 

Figure 1.14. Typical design of gravity separator (drop-out box)................................................. 29 

Figure 1.15. Typical design of a cyclone dust collector .............................................................. 30 

Figure 1.16. Basic design of a baghouse dust collector............................................................... 31 

Figure 1.17. Typical design of a mechanical shaker dust collector ............................................. 35 

Figure 1.18. Typical design of a reverse air dust collector.......................................................... 36 

Figure 1.19. Typical design of reverse jet (pulse jet) dust collector............................................ 37 

Figure 1.20. Cartridge collector with dust filter canisters ........................................................... 39 

Figure 1.21. Typical design of a wet scrubber dust collector ...................................................... 40 

Figure 1.22. Typical designs of a venturi scrubber...................................................................... 41 

Figure 1.23. Typical design of an impingement plate scrubber................................................... 42 

Figure 1.24. Double dump weight-based valves.......................................................................... 44 

Figure 1.25. Demonstration of how a complex exhaust system is a combination of branches 

linking simple exhaust systems ................................................................................................ 49 

Figure 1.26. A typical fan performance curve. Each fan curve is associated with a certain 

fan model at a selected RPM.................................................................................................... 51 

Figure 1.27. Fan performance curve showing stalling and operating ranges .............................. 51 

xiv 
 

Figure 1.28. Wall-mounted propeller fan .................................................................................... 53 

Figure 1.29. Typical tubeaxial fan (left) and vaneaxial fan (right).............................................. 54 

Figure 1.30. Typical centrifugal fan ............................................................................................ 55 

Figure 1.31. Axial centrifugal fan showing airflow..................................................................... 56 

Figure 1.32. Typical roof ventilators using axial fans (left) and centrifugal fans (right) ............ 57 

Figure 2.1. Common dust control application illustrating nozzle positioning............................. 62 

Figure 2.2. Effect of droplet size on dust particle impingement.................................................. 63 

Figure 2.3. Contact angle resulting from a liquid meeting a solid surface .................................. 65 

Figure 2.4. Typical internal mix nozzle (top) and external mix nozzle (bottom)........................ 66 

Figure 2.5. Typical air atomizing nozzle round spray pattern (top) and fan spray pattern 

(bottom).................................................................................................................................... 66 

Figure 2.6. Typical full cone nozzle and spray pattern................................................................ 67 

Figure 2.7. Typical hollow cone whirl chamber nozzle and spray pattern. Right angle design 

shown........................................................................................................................................ 67 

Figure 2.8. Typical hollow cone spiral nozzle and spray pattern ................................................ 68 

Figure 2.9. Typical flat fan nozzle and spray patterns................................................................. 68 

Figure 2.10. Typical loader dump dust control application ......................................................... 69 

Figure 2.11. Typical conveyor dust control application .............................................................. 69 

Figure 2.12. Airborne suppression performance of four types of spray nozzles and flat fan 

spray nozzles are hydraulically atomizing ............................................................................... 70 

Figure 2.13. Typical self-contained water delivery system ......................................................... 71 

Figure 2.14. Typical duplex basket strainer................................................................................. 72 

Figure 2.15. Nozzle erosion—new versus used........................................................................... 72 

Figure 2.16. Nozzle corrosion—new versus used ....................................................................... 73 

Figure 2.17. Nozzle caking—new versus used............................................................................ 73 

Figure 2.18. Temperature damage—new versus used................................................................. 73 

Figure 2.19. Nozzle damage—new versus used .......................................................................... 74 

Figure 2.20. Good spray tip showing pattern and distribution graph. Height of bar indicates 

distribution of water over pattern width and indicates relatively uniform flow over the 

width of the pattern................................................................................................................... 74 

Figure 2.21. Worn spray tip showing pattern and distribution graph. Height of bar indicates 

distribution of water over pattern width and indicates an increase of and excessive flow in 

the center of the pattern due to orifice wear............................................................................. 74 

Figure 3.1. Illustrations depicting a small surface crawler drill rig, a truck-mounted drill rig, 

and a large track-mounted drill rig, respectively...................................................................... 80 

xv 
 

Figure 3.2. The air and water flow during drilling operation to demonstrate water flushing 

of the drill cuttings. The water flows through the center of the drill steel and out the end 

of the drill bit to remove the cuttings from the drill hole ......................................................... 81 

Figure 3.3. Internal workings of a water separator sub................................................................ 83 

Figure 3.4. A basic dry dust collection system on a drill............................................................. 84 

Figure 3.5. Reinforcing flaps added to each corner of a rectangular deck shroud to reduce 

leakage...................................................................................................................................... 85 

Figure 3.6. Circular shroud design............................................................................................... 86 

Figure 3.7. Results of changing dust collector to bailing air ratio at various shroud gap 

heights ...................................................................................................................................... 87 

Figure 3.8. Graphs of the model that represents the severity of dust emissions based upon 

collector to bailing airflow ratios, showing that the respirable dust concentrations become 

lower as the collector to bailing airflow ratio increases........................................................... 89 

Figure 3.9. Qualitative models of airflow patterns underneath the shroud without the airblocking shelf (left) and with the air-blocking shelf (right)..................................................... 90 

Figure 3.10. Air-blocking shelf installed on the inside perimeter of the drill shroud of a 

blasthole drill. Note the overlap in the rear corners to eliminate any gaps in the shelf 

perimeter................................................................................................................................... 91 

Figure 3.11. Buildup of material on the air-blocking shelf (left) and the resulting dust 

emissions that are created during lowering of the drill mast (right) ........................................ 92 

Figure 3.12. Air-blocking shelf with modifications incorporated into installation. This 

shelf was installed in short sections in an attempt to prevent potential damage that could 

occur during tramming with the drill mast up.......................................................................... 92 

Figure 3.13. Dust collector dump point prior to shroud installation (A). Two men installing 

the shroud onto the dust collector dump point (B). The dust collector dump point after 

installation of the shroud (C).................................................................................................... 93 

Figure 3.14. Comparison of dust concentrations of uncontrolled dust collector dump to 

shrouded dust collector dump................................................................................................... 94 

Figure 3.15. Air ring seal used to impede the movement of dust particles through an opening . 96 

Figure 3.16. Typical dust collection system used by small crawler or "buggy" drills ................ 97 

Figure 3.17. Illustrations showing a typical jackleg drill, stoper drill, and 3-boom jumbo, 

respectively............................................................................................................................... 98 

Figure 3.18. Water flow during a drilling operation to demonstrate water flushing of the drill 

cuttings. The water flows through the drill into the center of the drill steel and out the end 

of the drill bit to remove the cuttings from the drill hole ......................................................... 98 

Figure 3.19. Drill boom showing external water sprays directed onto the drill steel ................ 100 

Figure 3.20. A typical blasthole containing an explosive charge utilizing a water cartridge to 

suppress dust generated during blasting................................................................................. 102 

xvi 
 

Figure 3.21. A fogger spray used to create a mist for dust suppression in the heading where 

blasting will occur .................................................................................................................. 103 

Figure 3.22. A filtration unit, located adjacent to the blast heading, used for filtering the 

contaminated ventilation air from the blast heading after blasting occurs............................. 104 

Figure 3.23. A filtration unit, located in the blast heading, used for filtering the contaminated 

ventilation air from the blast heading after blasting occurs ................................................... 104 

Figure 4.1. Illustration of a jaw crusher showing material being crushed between fixed and 

reciprocating jaws................................................................................................................... 113 

Figure 4.2. Examples of compressive crushers. Left—illustration of a cone crusher showing 

material being crushed between the cone mantle and bowl liner. Right—illustration of a 

gyratory crusher with material being crushed between the gyratory head and the frame 

concave................................................................................................................................... 114 

Figure 4.3. Examples of impactive crushers. Left—a hammermill-type crusher showing 

material crushed between the rotating hammers and fixed grinding plate. Center—
	

illustration of material being crushed between rotating hammers and fixed anvil plates in 

an impact breaker. Right—illustration of the size reduction action of a roll crusher ........... 114 

Figure 4.4. Illustration of a wet dust control approach with partial enclosure at a crusher 

dump loading operation. Note the blue "fan patterns" signifying water sprays.................... 115 

Figure 4.5. Illustration of a dry (exhaust) dust control system with a partial enclosure at a 

crusher dump loading operation ............................................................................................. 116 

Figure 4.6. Illustration of a dry (exhaust) dust control system at the transfer point of a 

conveyor discharge to a crusher feed hopper ......................................................................... 118 

Figure 4.7. Illustration of a dry (exhaust) dust control system on a feed chute into a transfer 

chute feeding a crusher........................................................................................................... 118 

Figure 4.8. Illustration of a dry (exhaust) dust control system at the discharge of a jaw 

crusher onto a belt conveyor .................................................................................................. 119 

Figure 4.9. Illustration of a dry (exhaust) dust control system at the discharge of a 

hammermill crusher onto a belt conveyor.............................................................................. 119 

Figure 4.10. Illustration of a wet dust control approach with a transfer chute/impact bed 

enclosure at a crusher loading operation ................................................................................ 120 

Figure 4.11. Illustration of a wet dust control approach on a crusher discharge/belt loading 

operation................................................................................................................................. 121 

Figure 4.12. Cut-away illustration of a ball mill showing the charge consisting of the 

material being processed and balls......................................................................................... 123 

Figure 4.13. Illustration of a stirring mill .................................................................................. 123 

Figure 4.14. Sampling doors located on top of a collection/transfer point to prevent leakage . 124 

Figure 4.15. Illustration of a dry (exhaust) dust control system on the feed to a rotary screen 

with enclosed transfer chute ................................................................................................... 126 

Figure 4.16. Illustration of a dry (exhaust) dust control system on a vibrating screen.............. 127 

xvii 
 

Figure 5.1. Basic components of a conveyor belt...................................................................... 133 

Figure 5.2. Types of fugitive dust emissions from conveyor belts............................................ 134 

Figure 5.3. Basic depiction of material transfer from a top conveyor to a bottom conveyor .... 135 

Figure 5.4. Depiction of impact bed used to control material direction and speed ................... 135 

Figure 5.5. Depiction of curved loading plate to help steer loaded material in a particular 

direction.................................................................................................................................. 136 

Figure 5.6. Spillage as a result of material being improperly fed onto a belt............................ 136 

Figure 5.7. Calculations for determining material impact loading at a conveyor transfer 

point........................................................................................................................................ 137 

Figure 5.8. Depiction of an impact cradle with underlying rubber layers ................................. 138 

Figure 5.9. Side support cradle .................................................................................................. 138 

Figure 5.10. Skirtboard used within the transfer point to help manage loaded material ........... 139 

Figure 5.11. Typical dust curtain used at the entrance and exit of the chute enclosure ............ 140 

Figure 5.12. Typical method for scraping material from a belt ................................................. 141 

Figure 5.13. Belt conveyor discharge chute used to return scrapings to the primary material 

flow......................................................................................................................................... 142 

Figure 5.14. Illustration of a mechanical belt splice (left) and a vulcanized belt splice 

(right)...................................................................................................................................... 143 

Figure 5.15. Conveyor transfer enclosure used with an exhaust ventilation system ................. 145 

Figure 5.16. Typical impact bed designed for ore-on-ore contact at transfer locations or 

chutes...................................................................................................................................... 145 

Figure 5.17. Depiction of a typical screw conveyor .................................................................. 147 

Figure 5.18. Packing gland and pillow block support bearing .................................................. 148 

Figure 5.19. Depiction of a bucket elevator............................................................................... 149 

Figure 5.20. Depiction of a typical bucket elevator dust collection process ............................. 150 

Figure 5.21. Depiction of two types of pneumatic conveying systems ..................................... 151 

Figure 5.22. HammerTek wear-resistant elbow installed in conveying ductwork .................... 151 

Figure 5.23. Rotary airlock feeding pneumatic conveying line................................................. 152 

Figure 5.24. Conveyance system utilizing a venturi eductor device ......................................... 152 

Figure 5.25. Venturi eductor utilizing suction to draw the feed material into the conveying 

line .......................................................................................................................................... 153 

Figure 6.1. Product "rooster tail" shown on left, product blowback shown on top right, and 

dust-soiled bag shown on bottom right .................................................................................. 158 

Figure 6.2. Typical bag valve—interior view (left). Artist concept of fill nozzle and bag 

valve (right). Fill nozzle enters through the bag valve and delivers product into bag .......... 159 

xviii 
 

Figure 6.3. Undervalve bag perforations (left) and overall perforations (right)........................ 161 

Figure 6.4. Undervalve perforations relieve excess pressure from bag during filling process 

to minimize the possibility of bag failure. Product and dust are seen escaping along with 

excess air ................................................................................................................................ 162 

Figure 6.5. Open-top bag being loaded with product (normal whole-grain material, which is 

less dusty) ............................................................................................................................... 163 

Figure 6.6. Extended polyethylene bag valve, interior view ..................................................... 164 

Figure 6.7. Dust reductions with extended polyethylene and foam valves as compared with 

the use of the standard paper valve ........................................................................................ 164 

Figure 6.8. Exhaust hood to capture and exhaust respirable dust away from each fill nozzle .. 167 

Figure 6.9. Exhaust hopper below bag loading station with integrated LEV system to 

capture dust generated during bag loading process................................................................ 168 

Figure 6.10. Dual-nozzle bagging system design ...................................................................... 169 

Figure 6.11. Bag operator's dust exposure with and without the use of a dual-nozzle 

bagging system ....................................................................................................................... 170 

Figure 6.12. OASIS positioned over bag operator to deliver clean filtered air down over 

work area ................................................................................................................................ 171 

Figure 6.13. Bag storage shelves shown on left; a robotic arm with suction cups to place 

empty bags on fill nozzle shown on right............................................................................... 173 

Figure 6.14. Bag and belt cleaner device. Note the use of a closed system under negative 

pressure................................................................................................................................... 175 

Figure 6.15. Semi-automated bag palletizing system to ergonomically improve bag stacking 

process with a push-pull ventilation system used to capture the dust generated ................... 176 

Figure 6.16. Semi-automated bag palletizing systems in which a worker removes bags from 

a conveyor and lifts and positions each layer on an air slide before automated loading 

onto the pallet. Both an OASIS system and an exhaust ventilation system are used to 

lower the worker's dust exposure during this bag palletizing process................................... 177 

Figure 6.17. Figure of the automated high-level palletizing process......................................... 178 

Figure 6.18. New innovation of low-level pallet loading process using a robotic arm device.. 179 

Figure 6.19. Automated spiral stretch wrap machine ................................................................ 180 

Figure 6.20. Expandable neoprene rubber bladder used in the fill spout of the bagging unit 

for Flexible Intermediate Bulk Containers............................................................................. 181 

Figure 6.21. Exhaust ventilation system used during loading of Flexible Intermediate Bulk 

Containers............................................................................................................................... 182 

Figure 7.1. Loading spouts designed with telescoping cups or pipes and dust collection 

capabilities.............................................................................................................................. 190 

Figure 7.2. Loading spout discharge cones designed to seal against the loading port of 

closed vehicles to minimize dust liberation ........................................................................... 191 

xix 
 

Figure 7.3. Articulated loading spout positioner ....................................................................... 191 

Figure 7.4. Skirt (in yellow box) and tilt sensors (in red boxes) installed on the discharge 

end of loading spouts to help reduce dust emissions.............................................................. 192 

Figure 7.5. Cascade loading spout ............................................................................................. 193 

Figure 7.6. Dust Suppression Hopper creating a solid stream of material during loading to 

reduce dust liberation ............................................................................................................. 194 

Figure 7.7. Rail car loadout area enclosed with plastic strips to contain dust ........................... 195 

Figure 7.8. Environmentally controlled operator's booth near loadout area ............................. 196 

Figure 8.1. Design of clothes cleaning system .......................................................................... 202 

Figure 8.2. Air spray manifold design with 26 nozzles spaced 2 inches apart .......................... 204 

Figure 8.3. Test subject wearing polyester/cotton blend coveralls before and after using the 

clothes cleaning booth ............................................................................................................ 206 

Figure 8.4. Increase in coworker's dust exposure one floor up from floor sweeping activities. 207 

Figure 8.5. Various types of floor sweep units used to clean dust-laden floors ........................ 208 

Figure 8.6. Portable system used to perform vacuuming........................................................... 209 

Figure 8.7. Basic design of total structure ventilation system ................................................... 211 

Figure 8.8. Open-structure design compared to standard walled structure................................ 214 

Figure 8.9. Increase in a worker's respirable dust exposure inside a mineral processing 

facility while bulk loading was being performed outside ...................................................... 215 

Figure 8.10. Comparison of two workers' respirable dust exposure while performing their 

job function with different control techniques being implemented ....................................... 216 

Figure 8.11. Worker closing bag valve with his hand as he removes the bag from the fill 

nozzle, significantly reducing his dust exposure.................................................................... 217 

Figure 8.12. Increase in worker's respirable dust exposure from broken bag during the 

conveying process .................................................................................................................. 218 

Figure 8.13. Product tracked onto the floor of a piece of mobile equipment from dirty work 

boots ....................................................................................................................................... 219 

Figure 8.14. Respirable dust concentrations inside enclosed cab of two different pieces of 

equipment with and without the use of a gritless, nonpetroleum sweep compound .............. 220 

Figure 9.1. General design of an effective filtration and pressurization system........................ 226 

Figure 9.2. Laboratory test setup to evaluate various operational parameters on a filtration 

and pressurization system for an enclosed cab....................................................................... 228 

Figure 9.3. Positive cab pressure necessary to prevent dust-laden air from infiltrating the 

enclosed cab at various wind velocities ................................................................................. 231 

Figure 9.4. Respirable dust concentrations inside enclosed cab for three days of testing with 

cab door closed and open ....................................................................................................... 232 

xx 
 

Figure 9.5. Problem created by heater stirring up dust from the floor and blowing dust off 

of the worker's clothing ......................................................................................................... 234 

Figure 9.6. Filtration unit showing lack of maintenance and care (left side) and after 

cleaning and the addition of a new intake air filtration unit (right side) ................................ 236 

Figure 9.7. Airflow pattern for intake and return at roof of cab and uni-directional airflow 

design...................................................................................................................................... 237 

Figure 10.1. Example of haul road dust from a typical mine haul truck ................................... 243 

Figure 10.2. Cross section of haul road ..................................................................................... 246 

Figure 10.3. Use of a spreader box to lay gravel onto road surface .......................................... 247 

Figure 10.4. Use of end-dumping with grader to lay gravel onto road surface ......................... 248 

Figure 10.5. Subbase thickness from California Bearing Ratio tests on road material ............. 249 

Figure 10.6. Water truck equipped with a front water cannon and rear water sprays ............... 251 

Figure 10.7. Various types of manufactured fan spray nozzles for use on a water truck .......... 252 

Figure 10.8. Respirable dust concentrations measured from haul road after water application 

occurred at 10:00 a.m. ............................................................................................................ 253 

Figure 10.9. Self-tarping dump truck. The blue arrow indicates the direction of movement 

when tarping. Section A-A shows the recommended freeboard ........................................... 258 

Figure 10.10. Chisel plow used for creating large clods in open areas to reduce wind 

erosion .................................................................................................................................... 260 

Figure 10.11. Example of tillage (from chisel plowing, for example) exposing large clods..... 261 

Figure 10.12. The armoring process for tillage rows. Blue arrow denotes wind direction 

perpendicular to tillage rows. Magenta arrows depict fine material being eroded from the 

windward side of ridges leaving large-sized particles, with the fine material being carried 

over ridge tops and deposited in the troughs on the leeward side of the ridges..................... 262 

Figure 10.13. Dozer being used to compact stockpile for long-term storage ............................ 264 

Figure 10.14. Example of a vegetated topsoil stockpile ............................................................ 265 

Figure 10.15. Demonstration of how fences are used as wind barriers ..................................... 266 

Figure 10.16. Barrier built to enclose a stockpile ...................................................................... 266 

Figure 10.17. Layout demonstrating dispersed wind barriers.................................................... 267 

xxi 
 

 

TABLES 

Table 1.1. Typical quantities of airflow in dust collection system components.......................... 14 

Table 1.2. Emission factors for crushed stone processing operations (lb/ton)* .......................... 33 

Table 1.3. Minimum Efficiency Reporting Values (MERV) according to ASHRAE Standard 

52.............................................................................................................................................. 45 

Table 2.1. Particle/droplet size in comparison to common precipitation classification .............. 64 

Table 2.2. Typical applications by spray nozzle type.................................................................. 68 

Table 4.1. Relative emission rate ratios of crushing and screening equipment......................... 111 

Table 5.1. Impact cradle ratings to be used based on impact loading ....................................... 137 

Table 5.2. A comparison of CEMA and Army Corps of Engineers recommendations for 

skirting for a belt speed of 600 fpm ....................................................................................... 140 

Table 8.1. Amount of dust remaining on the coveralls after cleaning, and the cleaning time 

for 100 percent cotton and polyester/cotton blend coveralls.................................................. 205 

Table 9.1. Comparison of three different descriptors for the effectiveness at providing clean 

air to an enclosed cab, operator booth, or control room......................................................... 227 

Table 9.2. Summary of field studies evaluating upgraded cabs ................................................ 227 

Table 9.3. Calculated PF derived from mathematical model..................................................... 238 

xxii 
 

ACRONYMS AND ABBREVIATIONS 

USED IN THIS HANDBOOK 

AASHTO American Association of State Highway and Transportation Officials 

ACGIH American Conference of Governmental Industrial Hygienists 

AEK airflow extensible Kraft 

AIRRS air ring seal 

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers 

ASTM American Society for Testing and Materials 

B&BCD bag and belt cleaning device 

BHP brake horsepower 

CBR California Bearing Ratio 

CEMA Conveyor Equipment Manufacturers Association 

CFR Code of Federal Regulations 

DSH Dust Suppression Hopper 

EPA Environmental Protection Agency 

ESP electrostatic precipitator 

FIBC Flexible Intermediate Bulk Container 

FK flat Kraft 

HEPA high-efficiency particulate air 

HVAC heating, ventilation, and air-conditioning 

LEV local exhaust ventilation 

MERV minimum efficiency reporting value 

MLV modified low-velocity 

MSD musculoskeletal disorder 

MSDS Material Safety Data Sheet 

MSHA Mine Safety and Health Administration 

NIOSH National Institute for Occupational Safety and Health 

NISA National Industrial Sand Association 

NK natural Kraft 

OASIS overhead air supply island system 

OSHA Occupational Safety and Health Administration 

PF protection factor 

PM particulate matter 

PSE particle size efficiency 

PTFE polytetrafluoroethylene 

SOP standard operating procedure 

SP static pressure 

TP total pressure 

TSP total suspended particulates 

TWA time-weighted average 

USBM United States Bureau of Mines 

VP velocity pressure 

xxiii 
 

UNIT OF MEASURE ABBREVIATIONS 

USED IN THIS HANDBOOK 

acph air changes per hour 

cfm cubic feet per minute 

cfm/ft cubic feet per minute per feet 

2 cfm/ft cubic feet per minute per feet squared 

fpm feet per minute 

ft feet 

2 ft feet squared 

ft/min feet per minute 

2 gal/yd gallons per square yard 

gpm gallons per minute 

gr/cf grains per cubic foot 

gr/dscf grains per dry standard cubic foot 

3 kg/m kilograms per cubic meter 

lb pounds 

lbs/hour pounds per hour 

lbs/min pounds per minute 

3 μg/m micrograms per cubic meter 

μm micrometers 

3 mg/m milligrams per cubic meter 

mm millimeters 

NPT national pipe taper 

pH potential of hydrogen 

psi pounds per square inch 

psig pound-force per square inch gauge 

RPM revolutions per minute 

scfm standard cubic feet per minute 

TPH tons per hour 

wg water gauge 

xxiv 
 

ACKNOWLEDGMENTS 

The developers of this handbook are indebted to the following individuals, listed alphabetically, 

for their review and valuable contributions to the handbook's production: 

Roger Bresee, Vice President, Technical Services, Unimin Corporation, Peterborough, 

Ontario, Canada; 

Chris Bryan (retired), C.M.S.P., Occupational Health & Safety Manager, U.S. Silica 

Company, Berkeley Springs, WV; 

Robert M. Castellan, M.D., M.P.H., Expert—Division of Respiratory Disease Studies, 

National Institute for Occupational Safety and Health; 

Cynthia Farrier, Graphics Specialist, National Institute for Occupational Safety and 

Health; 

Christopher Findlay, C.I.H., C.M.S.P., U.S. Department of Labor, Mine Safety and 

Health Administration, Arlington, VA; 

Ian Firth, Principal Adviser—Occupational Health, Rio Tinto Limited, Bundoora, 

Victoria, Australia; 

Marion Molchen, Molchen Photography, Washington, PA; 

Jan Mutmansky, Ph.D., P.E., Professor Emeritus, Pennsylvania State University, 

University Park, PA; 

Kenneth Vorpahl (retired), C.I.H. (1973-2009), C.S.P. (1976-2009), General 

Manager/Safety & Health, Unimin Corporation, Winchester, VA; 

Richard Wood, International Union of Operating Engineers, National HAZMAT 

Program, Beaver, WV. 

xxv 
 

ABOUT THIS HANDBOOK 

This handbook represents a successful collaborative effort by government and industry in 

protecting the health of U.S. mine workers. The two principal partnerships active in creating this 

handbook were between the Office of Mine Safety and Health Research (OMSHR) of the 

National Institute for Occupational Safety and Health (NIOSH) and the Industrial Minerals 

Association–North America (IMA-NA). The mission of the NIOSH OMSHR is to eliminate 

mining fatalities, injuries, and illnesses through research and prevention, while the IMA-NA is 

the representative voice of companies that extract and process the raw materials known as 

industrial minerals. 

This handbook was written by a task force of safety and health specialists, industrial hygienists, 

and engineers (listed below) to provide information on proven and effective control technologies 

that lower workers' dust exposures during all stages of minerals processing. The handbook 

describes both the dust-generating processes and the control strategies necessary to enable mine 

operations to reduce workers' dust exposure. Implementation of the engineering controls 

discussed can assist mine operators, health specialists, and workers in reaching the ultimate goal 

of eliminating pneumoconiosis and other occupational diseases caused by dust exposure in the 

mining industry. 

Designed primarily for use by industrial minerals producers, this handbook contains detailed 

information on control technologies to address all stages of the minerals handling process, 

including drilling, crushing, screening, conveyance, bagging, loadout, and transport. The 

handbook's aim is to empower minerals industry personnel to apply state-of-the-art dust control 

technology to help reduce or eliminate mine and mill worker exposure to hazardous dust 

concentrations—a critical component in ensuring the health of our nation's mine workers. 

TASK FORCE COMMITTEE COCHAIRS 

Andrew B. Cecala, Senior Research Engineer 

Dust Control, Ventilation, and Toxic Substances Branch, NIOSH 

Andrew D. O'Brien, C.S.P., General Manager/Safety & Health 

Unimin Corporation 

TASK FORCE COMMITTEE MEMBERS 

Jay F. Colinet, Senior Scientist 

Dust Control, Ventilation, and Toxic Substances Branch, NIOSH 

William R. Fox, Manager/Safety & Health 

Unimin Corporation 

xxvi 
 

Robert J. Franta, Quotation Engineer 

Spraying Systems Co. 

Jerry Joy, C.I.H., C.S.P., Research Scientist—Industrial Hygienist 

Dust Control, Ventilation, and Toxic Substances Branch, NIOSH 

Wm. Randolph (Randy) Reed, Ph.D., P.E., Research Mining Engineer 

Dust Control, Ventilation, and Toxic Substances Branch, NIOSH 

Patrick W. Reeser, Engineering Manager 

U.S. Silica Company 

John R. Rounds, Director Project Engineering 

Unimin Corporation 

Mark J. Schultz, P.E., Senior Mining Engineer 

Pittsburgh Safety and Health Technology Center (PS&HTC), Dust Division, Mine Safety and 

Health Administration (MSHA) 

EDITOR 

Joseph Schall, Health Communications Specialist 

NIOSH 

HANDBOOK LAYOUT COORDINATOR 

Jeanne A. Zimmer, Physical Science Technician 

Dust Control, Ventilation, and Toxic Substances Branch, NIOSH 

IMA-NA COORDINATOR 

Mark G. Ellis, President 

IMA-NA 

xxvii 

 

 

INTRODUCTION 

Throughout the mining and processing of minerals, the mined ore undergoes a number of 

crushing, grinding, cleaning, drying, and product sizing operations as it is processed into a 

marketable commodity. These operations are highly mechanized, and both individually and 

collectively these processes can generate large amounts of dust. If control technologies are 

inadequate, hazardous levels of respirable dust may be liberated into the work environment, 

potentially exposing workers. Accordingly, federal regulations are in place to limit the 

respirable dust exposure of mine workers. Engineering controls are implemented in mining 

operations in an effort to reduce dust generation and limit worker exposure. 

For the purposes of this handbook, dust is broadly defined as small solid particles created by the 

breaking up of larger particles. Depending on their size, these particles can become hazardous to 

worker health, particularly when suspended in air. The largest size particle that can be 

suspended in air for long periods of time from wind velocity acting upon it is about 60 

micrometers (μm), which is about the thickness of a human hair. Particles ranging from about 60 

to 2,000 μm can also become suspended in air, but they only reach heights up to approximately 

three feet above the ground before they fall back to the surface. Particles larger than about 2,000 

μm generally creep or roll along the surface due to wind velocity acting upon them [EPA 1996]. 

These larger particles of dust can affect the nasal passages, causing an irritated and congested 

nose, and might also cause an irritant cough should they deposit in the throat. Smaller airborne 

particles of dust, which can remain suspended in air for hours, pose a greater risk to the 

respiratory system when inhaled. In general, the smaller the aerodynamic diameter of the 

inhaled dust particle, the more likely it will be deposited more deeply in the respiratory tract. 

Established sampling procedures measure various fractions of airborne dust according to 

aerodynamic diameter. For example, inhalable dust samplers collect dust that tends to deposit 

from the upper airways to the alveolar region of the lungs. Inhalable dust samplers collect 

approximately 97 percent of particles less than 1 μm aerodynamic diameter, but only about 50 

percent of particles of 100 μm aerodynamic diameter [ACGIH 2007]. Respirable dust samplers 

more selectively collect dust that tends to deposit in the gas exchange region of the lungs. 

Respirable dust samplers collect about 50 percent of particles of 4 μm aerodynamic diameter. 

Their collection efficiency for other particle sizes ranges from about 1 percent of 10-μm particles 

to approximately 97 percent of particles less than 1 μm in diameter [ACGIH 2007]. Individually, 

such particles cannot be seen by the unaided eye. 

The safety and health regulations for the U.S. mining industry are enforced by the U.S. Mine 

Safety and Health Administration (MSHA). The Federal Mine Safety and Health Act of 1977 

created MSHA, giving it the authority to develop and revise improved mandatory health and 

safety standards to prevent injuries and protect the lives of those working in mines. These 

Introduction 1 
 

 

 

1 regulations are found in Chapter 1 of Title 30, Code of Federal Register (CFR).

1 Subchapter K—Metal and Nonmetal Mine Safety and Health—details the safety and health standards for metal and 

nonmetal mines (Part 56 covers surface mines, while Part 57 covers underground mines). Section 56.5001 sets forth 

the exposure limits for airborne contaminants for surface metal and nonmetal mines, while 57.5001 sets forth the 

exposure limits for airborne contaminants for underground metal and nonmetal mines. Both sections state that "the 

exposure to airborne contaminants shall not exceed, on the basis of a time weighted average, the threshold limit 

values adopted by the American Conference of Governmental Industrial Hygienists, as set forth and explained in the 

1973 edition of the Conference's publication entitled 'TLV's Threshold Limit Values for Chemical Substances in 

Workroom Air Adopted by ACGIH for 1973,' pages 1 through 54, which are hereby incorporated by reference and 

made part hereof" Title 30 CFR is available at the MSHA website at http://www.msha.gov/30cfr/CFRINTRO.HTM. 

Among the 

listed threshold limit values, this publication establishes a total airborne dust standard of 10 

3 mg/m for those listed substances. However, if potential exposure to silica dust is suspected, a 

respirable dust sample is obtained, and x-ray diffraction following NIOSH Analytical Method 

7500 [NIOSH 2003] is used to quantify crystalline silica content. The three most common forms 

(polymorphs) of respirable crystalline silica are quartz, tridymite, and cristobalite. If the sample 

contains greater than 1 percent quartz, a respirable dust standard is calculated with the following 

equation: 

Respirable dust standard = 

respirable% 2quartz

10mg/m 

3

Enforcement of a respirable standard based upon this equation effectively limits respirable quartz 

3 exposure to less than 100 μg/m . NIOSH has a recommended exposure limit (REL) for 

3 respirable crystalline silica of 50 μg/m , as a time-weighted average for up to a 10-hour day 

during a 40-hour week [NIOSH 1974]. The exposure limits for tridymite and cristobalite are 

1 limited to 50 percent of the value calculated from the formula for quartz.

In consideration of the above standards, a critical aspect of the mining and processing of 

minerals is to determine when workers are exposed to unacceptable levels of respirable dust, and 

to implement and ensure the effectiveness of the controls described in this handbook as needed. 

To this end, the National Industrial Sand Association (NISA) recently published the second 

edition of the "Occupational Health Program for Exposure to Crystalline Silica in the Industrial 

Sand Industry" [NISA, 2010]. This document provides the necessary information for the proper 

surveillance and assessment of worker dust exposure. 

SILICA EXPOSURE AND POTENTIAL HEALTH IMPACTS 

2 Introduction 

As reported in "NIOSH Hazard Review: Health Effects of Occupational Exposure to Respirable 

Crystalline Silica," occupational exposure to respirable crystalline silica dust can have several 

adverse health consequences, including silicosis, tuberculosis, chronic bronchitis, emphysema, 

and chronic renal disease [NIOSH 2002]. In addition, NIOSH has classified crystalline silica as 

2 a potential occupational carcinogen [NIOSH 2002; 54 Fed. Reg. 2521 (1989) ]. 

2 Federal Register. See Fed. Reg. in references. 

Silicosis is the disease predominantly associated with crystalline silica exposure. Silicosis is an 

incurable and potentially fatal lung disease caused by the inhalation of respirable crystalline 
 

 

 

silica. Quartz is the most common single mineral in the earth's crust, and many mining 

operations involve direct contact with overburden and ore containing quartz. Thus, workers 

throughout much of the mining industry are potentially exposed to respirable crystalline silica 

through routine mining activities such as drilling, crushing, sizing, transporting, and loading. 

When workers inhale respirable dust, the particles can penetrate the body's defense mechanisms 

and reach the alveolar region of the lungs. Crystalline silica particles that deposit in the alveolar 

region can stimulate an inflammatory and toxic process that can ultimately develop into 

clinically recognizable silicosis. Depending on the concentration of respirable crystalline silica 

and duration to which they are exposed, workers may develop any of several forms of silicosis 

[NIOSH 2002]: 

chronic—resulting from long-term excessive exposures, and first clinically apparent 10– 

30 years after first exposure; 

accelerated—resulting from exposure to higher concentrations of crystalline silica, first 

clinically apparent 5–10 years after the initial exposure; 

acute—resulting from exposure to unusually high concentrations of crystalline silica, 

clinically apparent within weeks to 5 years after the initial exposure. 

Chronic silicosis, the most common form of the disease, results in characteristic nodular scarring 

in the lungs and occurs after many years of inhaling respirable crystalline silica dust. Over time, 

the initial small nodules can eventually coalesce into large fibrotic masses, a condition called 

progressive massive fibrosis. Accelerated silicosis is much less common than chronic silicosis 

but progresses more rapidly, and acute silicosis, a rarely occurring form of the disease, is the 

most serious and most rapidly fatal form. Unlike chronic and accelerated silicosis, where a chest 

x-ray examination typically reveals scattered discrete small (and possibly also large) opacities, 

the chest x-ray appearance of acute silicosis resembles that of a diffuse pneumonia. This 

appearance results from extensive damage to the lining of the air spaces of the lungs, causing the 

alveoli to become filled with an abnormal fluid containing protein, degenerating cells, and other 

materials [Davis 2002]. 

Not all workers diagnosed with silicosis will be symptomatic; some with chronic disease will 

have no notable symptoms despite characteristic abnormalities on their chest radiograph. 

However, many workers with chronic silicosis will develop symptoms over time and essentially 

all those with accelerated and acute forms of the disease will be symptomatic even before they 

are diagnosed. Though silicosis is incurable, various symptoms, including chest irritation with 

uncontrollable coughing and shortness of breath, can be debilitating and warrant therapeutic 

intervention. Additionally, those with silicosis are at substantial risk for developing tuberculosis 

3 or other mycobacterial diseases [NIOSH 2002; Davis 2002].

3 Additional information on silica and silica-related diseases can be found at the National Institute for Occupational 

Safety and Health (NIOSH) website at http://www.cdc.gov/niosh/topics/silica/. 

Introduction 3 

This introduction has highlighted the need to control exposures to respirable silica dust. The 

control technologies discussed in this handbook for lowering dust levels below permissible or 

recommended occupational exposure limits are designed to control exposures not only to silica 

dust, but also to other types of dust. Also, while this handbook is focused on protecting workers 
 

 

in the mining and mineral processing industry, the concepts and approaches presented can 

provide useful guidance for controlling airborne dust in other industries. 

REFERENCES 

ACGIH [2007]. 2007 Threshold limit values for chemical substances and physical agents and 

biological exposure indices. Cincinnati, OH: American Conference of Governmental Industrial 

Hygienists. 

Davis GS [2002]. Silicosis. In: Hendrick DJ, Burge PS, Beckett WS, Churg A, eds. 

Occupational disorders of the lung: recognition, management, and prevention. W.B. Saunders, 

pp. 105–127. 

EPA [1996]. Air quality criteria for particulate matter, Vol. 1. Research Triangle Park, NC: 

National Center for Environmental Assessment, Office of Research and Development, 

Environmental Protection Agency. 

54 Fed. Reg. 2521 [1989]. Occupational Safety and Health Administration: air contaminants; 

final rule; silica, crystalline-quartz. (Codified at 29 CFR 1910.) 

NIOSH [1974]. NIOSH criteria for a recommended standard: occupational exposure to 

crystalline silica. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public 

Health Service, Center for Disease Control, National Institute for Occupational Safety and 

Health, DHEW (NIOSH) Publication No. 75–120. 

NIOSH [2002]. NIOSH hazard review: health effects of occupational exposure to respirable 

crystalline silica. By Schulte PA, Rice FL, Key-Schwartz RJ, Bartley DL, Baron P, Schlecht PC, 

Gressel M, Echt AS. U.S. Department of Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) 

Publication No. 2002–129. 

th rd NIOSH [2003]. NIOSH manual of analytical methods, 4 ed., 3 supplement. Cincinnati, OH: 

U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003– 

154. 

NISA [2010]. Occupational health program for exposure to crystalline silica in the industrial 

sand industry, 2nd ed. Washington, DC: National Industrial Sand Association. 

4 Introduction 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Introduction 1 

CHAPTER 1: 

FUNDAMENTALS OF 

DUST COLLECTION 

SYSTEMS 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

2 Introduction 
 

 

CHAPTER 1: FUNDAMENTALS OF 

DUST COLLECTION SYSTEMS 

Dust collection systems are the most widely used engineering control technique employed by 

mineral processing plants to control dust and lower workers' respirable dust exposure. A wellintegrated dust collection system has multiple benefits, resulting in a dust-free environment that 

increases productivity and reclaims valuable product. 

The most common dust control techniques at mineral processing plants utilize local exhaust 

ventilation systems (LEVs). These systems capture dust generated by various processes such as 

crushing, milling, screening, drying, bagging, and loading, and then transport this dust via 

ductwork to a dust collection filtering device. By capturing the dust at the source, it is prevented 

from becoming liberated into the processing plant and contaminating the breathing atmosphere 

of the workers. 

LEV systems use a negative pressure exhaust ventilation technique to capture the dust before it 

escapes from the processing operation. Effective systems typically incorporate a capture device 

(enclosure, hood, chute, etc.) designed to maximize the collection potential. 

As part of a dust collection system, LEVs possess a number of advantages: 

the ability to capture and eliminate very fine particles that are difficult to control using 

wet suppression techniques; 

the option of reintroducing the material captured back into the production process or 

discarding the material so that it is not a detriment later in the process; and 

consistent performance in cold weather conditions because of not being greatly impacted 

by low temperatures, as are wet suppression systems. 

In addition, LEVs may be the only dust control option available for some operations whose 

product is hygroscopic or suffers serious consequences from even small percentages of moisture 

(e.g., clay or shale operations). 

In most cases, dust is generated in obvious ways. Anytime an operation is transporting, refining, 

or processing a dry material, there is a great likelihood that dust will be generated. It also 

follows that once the dust is liberated into the plant environment, it produces a dust cloud that 

may threaten worker health. In addition, high dust levels can impede visibility and thus directly 

affect the safety of workers. 

The five areas that typically produce dust that must be controlled are as follows: 

1.	 The transfer points of conveying systems, where material falls while being transferred to 

another piece of equipment. Examples include the discharge of one belt conveyor to 

another belt conveyor, storage bin, or bucket elevator. 

2.	 Specific processes such as crushing, drying, screening, mixing, blending, bag unloading, 

and truck or railcar loading. 

Fundamentals of Dust Collection Systems 7 
 

 

3.	 Operations involving the displacement of air such as bag filling, palletizing, or pneumatic 

filling of silos. 

4.	 Outdoor areas where potential dust sources are uncontrolled, such as core and blast hole 

drilling. 

5.	 Outdoor areas such as haul roads, stockpiles, and miscellaneous unpaved areas where 

potential dust-generating material is disturbed by various mining-related activities and 

high-wind events. 

While areas 4 and 5 can be significant sources of dust, they are generally not included in plant or 

mill ventilation systems design because of the vast area encompassed and the unpredictability of 

conditions. Therefore, dust control by methods alternative to LEVs is required, as discussed in 

later chapters of this handbook. 

Dust control systems involve multiple engineering decisions, including the efficient use of 

available space, the length of duct runs, the ease of returning collected dust to the process, the 

necessary electrical requirements, and the selection of optimal filter and control equipment. 

Further, key decisions must be made about whether a centralized system or multiple systems are 

best for the circumstances. Critical engineering decisions involve defining the problem, 

selecting the best equipment for each job, and designing the best dust collection system for the 

particular needs of an operation. 

This chapter will make a number of references to the industry handbook Industrial Ventilation: A 

Manual of Recommended Practice for Design, by the American Conference of Governmental 

Industrial Hygienists [ACGIH 2010], with several figures from that handbook adapted for use 

here. The ACGIH handbook should be considered as a primary resource for anyone interested in 

protecting workers from dust exposure in the mineral industry using dust collector systems, and 

especially for engineers who are involved in designing such systems. The material in this 

chapter will complement the information available in the ACGIH handbook. 

BASICS OF DUST COLLECTOR SYSTEMS 

Well-designed dust collection systems need to consider not only the dust as a potential 

contaminant, but also the attributes of the dust capturing system. In defining the nature of dust 

as a potential contaminant to workers, a number of issues must be examined. These include the 

particle size and distribution, shape, physical characteristics, and the amount of dust emitted. 

Particle size describes how coarse or fine particles are, and is normally defined by their upper 

th and lower size limits. Particle sizes are measured in micrometers (μm) (1/1,000 millimeter). 

The respirable dust range harmful to workers' health is defined by those particles at, or below, 

the 10 μm size range. To put this size in perspective, 325 mesh is approximately 44 μm and is 

the smallest micrometer size that one can see with the unaided human eye. In dust collector 

systems, the larger particle sizes are easy to collect, often aided significantly by gravity. 

8 Fundamentals of Dust Collection Systems 
 

 

The shape of particles affects how they are collected and how they are released from the 

collection media. Particle shape is a common terminology used in aerosol technology, while the 

term aerodynamic diameter is frequently used to describe particle diameters. The aerodynamic 

3 diameter of a particle is the diameter of a spherical particle that has a density of 1,000 kg/m (the 

standard density of a water droplet) and the same settling velocity as the particle [Hinds 1999]. 

Aerodynamic diameter is used in many designs of filtration systems and air cleaners. Additional 

properties of the material that are key design considerations for dust collection systems are 

moisture and temperature. Moisture and temperature play a significant part in equipment 

selection for dust collector systems. 

AIRFLOW AND DUST CONTROL 

To control how air flows in a ventilation system, one must manage air velocities, air quantities, 

and temperature, as well as apply basic principles of static pressure (SP) and velocity pressure 

(VP). 

Air velocity is measured in feet per minute and impacts the size of particle that can be carried by 

the airstream. Air quantity is measured in cubic feet of air per minute (cfm), which is the amount 

of air used in ventilating the process. Air temperature is measured in degrees Fahrenheit or 

degrees Celsius. It is used to determine the type of gaskets and filter media needed. Many 

applications where dust is being collected are thermal in nature, with examples including 

furnaces, kilns, and dryers. 

Pressure (or head) in ventilation design is generally measured in inches of H2O, also referred to 

as inches water gauge (wg). In a ventilation system, this pressure is known as the static pressure 

and is generally created by a fan. Static pressure is the difference between the pressure in the 

ductwork and the atmospheric pressure. Negative static pressure would want to collapse the 

walls of the duct, while positive static pressure would want to expand the walls of the duct. 

Static pressure values are used to overcome the head loss (Hl) of the system, which is made up of 

two components: frictional resistance to airflow in the ductwork and fittings (frictional losses 

(Hf)) and the resistance of obstacles such as cyclones and dust collectors (shock losses (Hx)) 

[Hartman et al. 1997]. Static pressure is measured by inserting a pitot tube into the ductwork, 

perpendicular to the side walls, to determine the difference between atmospheric and duct 

pressures (Figure 1.1). 

Fundamentals of Dust Collection Systems 9 
 

 

Figure 1.1. Relationship between static, velocity, and total pressure [adapted from ACGIH 2010]. 

Air traveling through a duct at a specific velocity will create a corresponding pressure known as 

the velocity pressure (VP). Velocity pressure is the pressure required to accelerate the air from 

rest to a particular velocity. It only exists when air is in motion, always acts in the direction of 

airflow, and always has a positive value. For ventilation purposes, VP is measured with a test 

probe facing directly into the airstream. The algebraic sum of static pressure and velocity 

pressure is total pressure (TP) [Hartman et al. 1997], as expressed by the following equation: 

TP SP VP

(1.1) 

where TP = total pressure, inches wg; 

SP = static pressure, inches wg; and 

VP = velocity pressure, inches wg. 

The ACGIH handbook, Industrial Ventilation: A Manual of Recommended Practice for Design, 

[ACGIH 2010], gives a number of definitions and equations that are useful for describing airflow 

in an operation's ventilation system. The handbook also details fundamental characteristics in 

relation to blowing and exhausting air through a plant ventilation system. A handbook from 

Martin Engineering, Foundations: The Practical Resource for Cleaner, Safer, More Productive 

Dust & Material Control [Swinderman et al. 2009], also devotes a chapter to the control of air 

movement, including a section on effective measurement of air quantities. Finally, a 

recommended journal article is "Dust Control System Design: Knowing your Exhaust Airflow 

Limitations and Keeping Dust out of the System" [Johnson 2005]. 

10 Fundamentals of Dust Collection Systems 
 

 

EXHAUST SYSTEMS DESIGN 

All exhaust systems, whether simple or complex, have in common the use of hoods, ductwork, 

and an air cleaning and collection device that leads to the exhaust fan (Figure 1.2). 

Figure 1.2. A basic depiction of a simple exhaust system with the major 

components being the hood, duct, air cleaning device, and fan. 

 The ACGIH 

handbook, Industrial Ventilation: A Manual of Recommended Practice for Design [ACGIH 

2010], discusses all aspects of these systems in great detail. To supplement the ACGIH 

handbook, the discussion below outlines some basic system design parameters and sets forth 

some important considerations about air velocity. 

HOODS 

Hoods are specifically designed to meet the characteristics of the type of ore or product being 

processed. An effective hood is a critical part to any system because if the hood does not capture 

the dust, the rest of the exhaust ventilation system becomes meaningless. A properly designed 

hood will create an effective flow rate and airflow pattern to capture the dust and carry it into the 

ventilation system. The effectiveness of the hood is determined by its ability to induce an inward 

airflow pattern for the dust-laden air in the work environment. 

Hoods and Blowing versus Exhausting Ventilation 

When considering the effectiveness of a hood at capturing dust, the limitations of exhausting 

systems need to be considered. This issue is most evident when comparing the characteristics of 

blowing versus exhausting air from a duct. With a blowing system, the air delivered from the 

fan maintains its directional effect for a substantial distance once exiting the duct. With a 

blowing system, at a distance of 30 diameters (dimension of the exiting duct), the air velocity is 

reduced to approximately 10 percent of the exiting velocity (Figure 1.3). This blowing air tends 

to maintain its conical shape and actually entrains additional air, a process commonly referred to 

as induction. When one compares a blowing system to an exhaust system, the air velocity is at 

this approximate 10 percent level at only one duct diameter from the exhaust inlet. 

Fundamentals of Dust Collection Systems 11 
 

 

Figure 1.3. Comparison of ventilation characteristics for blowing versus exhausting system 

[adapted from Hartman et al. 1997]. 

The airflow characteristic for an exhaust system is substantially different. The air exhausted, or 

pulled into the duct, is captured from all directions around the duct opening and thus forms a 

nearly spherical shape, as opposed to the conical shape of the blowing system. Another major 

difference is the air velocity. The air velocity for an exhaust system is approximately 10 percent 

of the intake velocity at the duct opening at only 1 diameter away, as compared to 30 diameters 

away at the 10 percent level for the blowing system. These ventilation principles underscore 

how critical it is for an effective hood design to be very close to the dust generation source. 

Hood Types 

Hoods have a vast range of different configurations, but usually fall into three different 

categories: enclosing, capturing, and receiving. 

Enclosing hoods are those in which the source is either partially or totally enclosed to provide the 

required airflow to capture the dust and prevent it from contaminating the work environment. 

The most effective way to capture dust generated is a hood that encompasses the entire dust 

generation process. Openings into the enclosure (hood) are minimized with doors and access 

points into the contaminated work process. This situation is normally used when worker access 

is not necessary and openings are only necessary for the product to enter and exit a piece of 

machinery or a work process. These types of enclosing hoods can have numerous applications 

throughout the mining and minerals processing sequence, and are most often used in crushing, 

grinding, milling, and screening applications. 

When access is necessary into the dust generation process or area, it is then common to use some 

type of booth or tunnel—a type of partial enclosure application. In these partial enclosure 

systems, the key is to provide sufficient intake airflow to eliminate, or at least minimize, any 

escape of dust from the enclosed area. This is best accomplished by enclosing the dust 

generation area or zone as much as possible. One common method to do this is with clear plastic 

12 Fundamentals of Dust Collection Systems 
 

 

stripping, which allows workers to have ingress and egress while maintaining an effective seal to 

the contaminated area. A partial booth or tunnel (hood) requires higher exhaust volumes to be 

effective than do totally enclosed systems. 

When it is not applicable to either totally or partially enclose the dust generation source or area, 

capturing hoods are normally used and are located as near as possible to the dust source. 

Because the dust generation source is exterior to the hood, the ability of the hood to capture the 

dust-laden air is paramount to the success of the system. These types of hoods must be able to 

overcome any exterior air current around this area. They can be very effective when the dust is 

emitted in a specific area and the exhaust hood is placed in relatively close vicinity to this area. 

The capture velocity of the hood decreases inversely with the square of the distance from the 

hood. In cases where this distance becomes too great, one should consider the use of a push-pull 

ventilation system (Figure 1.4). 

Figure 1.4. Simple design of basic push-pull ventilation system. 

In a push-pull ventilation system, a blowing jet of air provides a 

blast of air movement to provide the necessary quantity to overcome the distance from the hood. 

This air jet is normally directed across a contaminant source and towards the exhaust hood. As 

this jet travels towards the exhaust hood, this airflow entrains additional air with the intent to 

capture and move the dust-laden air. The goal is to move this total volume of air into the exhaust 

hood. This blowing jet coupled with an exhaust (capturing) hood provides a very effective 

ventilation design. 

The third and most infrequently used type of hood is a receiving hood (Figure 1.5). Receiving 

hoods are normally located close to the point of generation to capture the dust and not allow it to 

escape. In most cases, these hoods are relatively small in size. The hood uses the directional 

inertia of the contaminant to lower the necessary capture velocity. These types of hoods have 

only minor applications in mining and mineral processing and are most common in small 

machinery and tool applications in laboratory and shop areas. 

Fundamentals of Dust Collection Systems 13 
 

 

 

Figure 1.5. Basic setup for a type of receiving hood. 

Hood Design 

The most important parameters in the design of an exhaust hood are as follows: 

1. The rate of airflow through the hood. 

2. The location of the hood. 

3. The shape of the hood. 

Of these three parameters, the rate of airflow through the hood is the most important. As 

previously mentioned, if the hood is not able to capture the dust, the rest of the dust collector 

system becomes meaningless. Without an adequate air velocity, dust capture may not be 

sufficient. In order to maintain an acceptable negative internal pressure, new or "tightly"

enclosed equipment needs less airflow than older or "loosely" enclosed equipment. Because of 

this, the airflow volume (in cubic feet per minute, or cfm) for similar pieces of equipment can 

vary widely yet still maintain good dust control ability. 

As an approximation, the quantities of airflow in Table 1.1 will generally provide good dust 

control when applied to the listed pieces of equipment, properly hooded. 

Table 1.1. Typical quantities of airflow in dust collection system components 

Equipment Airflow, cfm 

Bucket elevator—sealed 400 at top and bottom 

Bucket elevator—sealed 800 if top-only 

Belt conveyor 1,000–1,500 per transfer point 

Low-speed oscillating screens 300–500 hood 

High-speed vibratory screens Length of the hood seal x 250 cfm/ft 

Screens 300–500 on each discharge chute 

Loading spouts 800–1,200 

Storage bins 300–400 

Hoods, hoppers, and canopies 250 cfm/ft2 of vertical curtain area around perimeter of unit 

14 Fundamentals of Dust Collection Systems 
 

 

When using the air quantities in Table 1.1, it should be noted that feed rate into the bin (in tons 

per hour, or TPH) is to be taken into account when sizing hood air volume to allow for air that is 

entrained by incoming material. 

Other equipment fitted with dust control hoods, such as baggers, packers, crushers, magnetic 

separators, palletizers, etc., will likely include a manufacturer's recommended airflow. In some 

instances, it may be difficult to install hoods on process equipment and maintain high collection 

efficiencies and easy access for operation and maintenance. In these rare cases, it may be 

necessary to build an enclosure for the entire piece of equipment, possibly by erecting a dust 

control hood above the equipment and surrounding it with flexible curtains. 

There are two issues that need to be considered when determining the rate of airflow to a hood: 

air induction and capture velocity. 

Air Induction 

Air induction is based on the concept that material falling through air imparts momentum to the 

surrounding air (Figure 1.6). 

Figure 1.6. Demonstration of air induction as material falls from a conveyor. 

Fundamentals of Dust Collection Systems 15 

 Due to this energy transfer, a stream of air always travels with the 

falling material. For example, a chute feeding sand to an elevator will drag air into the elevator. 

This air must be removed from the elevator through an exhaust hood or it will escape and carry 

dust with it into the plant through openings in the elevator casing, creating dust emissions. 
 

 

The following air induction equation can be used to estimate exhaust volumes for hoods based 

on material feed rate, height of free fall, size, and feed open area: 

10 3

2

D

SR

AQ U

(1.2)

where Q = air quantity, cubic feet per minute; 

AU = enclosure upstream open area, square feet; 

R = rate of material flow, tons per hour; 

S = height of material fall, feet; and 

D = average material size, feet. 

The most important parameter in the air induction equation is AU, the opening through which the 

air induction occurs (i.e., the cross-sectional area of the feed inlet). The tighter the feed 

enclosure, the smaller the value for AU and the smaller the exhaust quantity required. Also, the 

lower the value for S the smaller the exhaust quantity required. In designing a material handling 

circuit, it is important to keep the values for AU and S as low as possible to prevent excessive 

dust generation and to reduce the quantity of ventilation air. The values for R and D also affect 

exhaust quantity requirements, but are typically a constant in the mining or processing operation 

and cannot be altered. 

The air quantity (Q) for exhaust hoods can also be estimated using the air induction approach. 

This concept is important because it relates various factors that affect the required air volumes, 

which may not be accounted for in standard tables or charts. The airflow required using air 

induction calculations should be compared to the published standards, with corrections made if 

deemed necessary. 

Capture Velocity 

Capture velocity is a measure of the required airflow necessary to seize the dust released at the 

source and then pull this dust into the exhaust hood. The capture velocity must be powerful 

enough to overcome all the opposing factors and air currents in the surrounding area. There are 

various tables available that provide a range of recommended air velocities under a variety of 

conditions. For mining and minerals processing, this range is normally between 100 and 

200 fpm, but can increase up to 500 fpm in special cases. Most operations establish their own 

capture velocities for their exhaust hoods based on years of experience. 

After this capture velocity is determined, the exhaust volume for the hood can be calculated. The 

following "DallaValle" equation is used to determine the exhaust volume needed for a basic free

standing hood arrangement (Figure 1.7) [DallaValle 1932; Fletcher 1977]: 

-

Q Vx(10X

2 Ah

)

(1.3) 

where Q = the rate of air exhausted, cubic feet per minute; 

Vx = the required air velocity at the most remote point of contaminant dispersion, feet per minute; 

16 Fundamentals of Dust Collection Systems 
 

 

X = distance in feet from the face of the hood to the most remote point of contaminant 

dispersion; and 

Ah = the area of hood opening, square feet. 

Figure 1.7. Hood entry loss calculation [adapted from ACGIH 2010]. 

From this equation, it becomes obvious that the air velocity, the size of the hood, and the 

distance from the hood to the dust source are all critical factors for the required air volume. The 

distance (X) from the dust source to the hood is extremely important because it is a squared 

relationship. 

Another issue that should be noted is that for a free-standing hood, air is also being pulled from 

behind the hood. This lessens the hood's ability to capture and pull the dust-laden air in the 

source area. In order to minimize this effect, there are several approaches that need to be 

considered. First, if the hood is positioned on a tabletop, the airflow requirement is reduced to 

the following equation: 

Q Vx(5X

2 Ah

)

(1.4) 

Another simple technique to improve the airflow from around a hood is to place a flange around 

it. By doing so, the equation becomes: 

Q 0.75V (10 2

x X Ah

)

(1.5) 

A flange provides a barrier that prevents unwanted air from being drawn from behind the hood 

and is a very simple design modification to improve the effectiveness of the hood, as well as 

reducing operating costs. Numerous other factors and considerations for capture velocities can 

be found in the ACGIH handbook, Industrial Ventilation: A Manual of Recommended Practice 

for Design [ACGIH 2010]. 

Fundamentals of Dust Collection Systems 17 
 

 

Other Hood Considerations 

When dust is captured and pulled into a hood from a dust source, the hood converts static 

pressure to velocity pressure and hood entry losses. Hood entry loss is calculated according to 

the following equation: 

He(K)(VP) SPh VP

(1.6) 

where He = hood entry loss, inches wg; 

K = loss coefficient; 

VP = velocity pressure in the duct, inches wg; and 

SPh = absolute static pressure about 5 duct diameters down the duct from the hood, inches wg. 

Air flowing through an exhaust hood will cause pressure changes that need to be calculated when 

evaluating the impact on each individual hood in a multiple hood system. Figure 1.8 provides 

the hood entry loss coefficients for three different types of hoods commonly used in mining and 

mineral processing facilities. 

Figure 1.8. Hood entry loss coefficients for different hood types [adapted from ACGIH 2010]. 

18 Fundamentals of Dust Collection Systems 

The first case shows three different hood types: circular, square, 

and rectangular with plain openings. The second case shows loss coefficients with flanged 

openings, and the last case shows a bell mouth inlet for just a circular duct. This figure 

demonstrates the significant improvement in the design, and thus the lowering of the hood entry 

loss coefficient, with each improvement in the hood type. 
 

 

Checklist for Hood Effectiveness 

The following is a checklist of effective practices or considerations regarding the use of hoods in 

exhaust ventilation systems. 

The most effective hood design is one that encompasses the entire dust generation 

process. This virtually eliminates dust escaping and contaminating mine/plant air and the 

exposure of workers. 

Openings/doors/access points into the hood should be minimized as much as is 

reasonably possible. When access is necessary, a partial enclosure application, normally 

referred to as a booth or tunnel, is then recommended. When access points are not able to 

be closed, sealing these areas with clear plastic stripping is a common and effective 

technique. 

When neither total nor partial hood enclosures are possible, capture hoods should be 

used. The hoods need to be located as close to the dust source as feasibly possible. 

Remember that the distance component is a squared relationship that affects the required 

hood air volume. 

Hood capture velocities must be able to overcome any exterior air current between the 

dust source and the hood. 

The use of a push-pull ventilation system should be considered when the distance 

between the dust source and the hood becomes too great, or when there are significant 

exterior air currents in the area. 

It is critical that makeup air drawn past a worker before entering an exhaust hood be 

essentially dust-free. If the air is drawn from a contaminated area, the potential exists to 

increase a worker's respirable dust exposure. Dust-laden air from a dust source should 

never be pulled through a worker's breathing zone as it is being drawn into an exhaust 

hood. 

Flanges on hoods should be used because they significantly improve the airflow from the 

front of the hood area, which should be directed towards the dust source. In cases where 

bell shaped hoods and ducts can be used, they provide the optimum design. 

All hoods should be designed to meet the criteria established by the ACGIH handbook, 

Industrial Ventilation: A Manual of Recommended Practice for Design [ACGIH 2010]. 

DUCTWORK AND AIR VELOCITIES 

There are three basic types of systems used to transport dust to the collector: high-velocity, lowvelocity, and modified low-velocity. Most familiar to industry is the high-velocity system, 

where air-carrying velocities are in the range of 3,000–4,500 fpm. Fundamentally, this is a dust 

collection system, while a low-velocity system can best be described as a dust containment 

system, with duct velocities always less than 1,800 fpm. Low-velocity systems are designed so 

that they will not transport nonrespirable size dust particles (generally particles over 10 μm). It 

is important to keep in mind that low-velocity transport does not imply low airflow. Capture 

velocities (the air velocity at the hood opening required to capture the contaminant) and hood air 

volumes are the same in all designs, be they high-velocity, low-velocity, or modified lowvelocity systems. Figure 1.9 depicts the basic difference between high- and low-velocity 

systems. 

Fundamentals of Dust Collection Systems 19 
 

 

The implication of the figure is that, in a high-velocity system, the dust will be carried to the 

collector regardless of the slope or angle of the duct. However, in a low-velocity system, it is 

necessary to slope the duct to an inlet hood or discharge point, because some of the larger 

particles, inadvertently collected, will settle out in transport, and must be removed. 

Figure 1.9. Schematics of high- and low-velocity systems. 

High-Velocity Systems 

The high-velocity system is characterized by its ability to carry dust-laden air (particles larger 

than 10 μm) from the entry points of the system to the dust collector without having the particles 

settle out in the duct. To keep the particles from settling, high transport velocities are required. 

Because the ducting in a high-velocity system can be run both horizontally and vertically, there 

are few engineering restrictions. Effective ducting layouts can be easily designed, normally with 

a central horizontal duct and smaller collection ducts branching off to the dust-producing 

equipment/hood. 

20 Fundamentals of Dust Collection Systems 
 

 

A major disadvantage of the high-velocity system is that the ductwork is subjected to highly 

abrasive "blasting" by the dust particles moving at a high rate of speed, especially when the air 

changes direction. Elbows and branch entries are particular areas subjected to high wear. 

Abrasion first shows in these areas of air direction change. This wear or abrasion, if not 

addressed through additional engineering design or maintenance, will result in the long-term 

deterioration of the collection system due to formation of holes or openings in the ductwork. 

Because of this abrasive wear, most ductwork and fittings have to be fabricated from heavier 

materials and the fittings require longer radii. The high wear rate also results in high 

maintenance labor and material costs. Any system with holes in the ducting loses its 

effectiveness at the pickup point, and even a single hole in the duct can have a strong impact on 

the system. Because high-velocity systems are prone to required maintenance, they may not 

always be operating at optimum efficiency. 

High humidity, coupled with inadequate airflow, leads to further wear problems on systems with 

ducts that are run horizontally. Upon shutdown, the dust in the airstream settles in the horizontal 

runs of duct. As the air cools to the dew point, moisture can form on the surface of the duct, 

causing some attachment of dust particles to the wall. Over time, this buildup robs the system of 

airflow due to increased friction losses in the system and increases in velocity through the 

narrower opening. The loss of airflow in the system reduces collection efficiency, and the 

resulting increased velocity increases wear and can cause an even more rapid deterioration to the 

system than would normally be expected. 

Finally, high velocity means high pressure drops throughout the system. This equates to 

increased horsepower and higher power consumption, raising overall operating costs. Also, 

although the initial installation costs for high-velocity systems are comparatively low, higher 

maintenance and operating costs lead to high overall lifetime costs of the system. 

Low-Velocity Systems 

The basis of the low-velocity collection system is to create the same negative static pressure in 

the area surrounding the dust source and maintain the same airflow into the collection hood, as in 

a high-velocity system. The difference between the systems is that, after collection, the air 

transport velocity in the low-velocity system is much lower. Thus the pipe is always sloped to 

allow the oversized particles to slide to a discharge point for easy removal. Transport velocity is 

designed to move only the particles in the respirable size range, generally below 10 μm. 

Respirable dust is carried through the ductwork by the low-velocity airflow, while heavier 

particles fall out into the ductwork and slide back into the process. 

Ductwork in the low-velocity system cannot be run horizontally. The ductwork is designed so 

that the larger particles that fall from the airstream are reintegrated into the process. Therefore, a 

sloped sawtooth design (Figure 1.10) is used instead of long horizontal runs. In order to control 

the airflow, a fixed orifice plate or blast gate is positioned in the duct segment, preferably on a 

downward run, so as not to trap material above the high-velocity flow through the orifice. 

Fundamentals of Dust Collection Systems 21 
 

 

Figure 1.10. Sawtooth design of a low-velocity system with a dropout 

duct back into the process at the base of each leg. 

The particle-carrying ability of a low-velocity system is confined to particles smaller than 10 μm. 

Because of the absence of larger particles, as well as the lower velocities involved, abrasion is 

low even at points of air direction change. This allows for the use of short radius or mitered 

elbows without fear of extreme wear. 

From a design standpoint, dust control systems are engineered for expected pressure losses 

within the system. These losses are due to air friction and pressure losses across the dust 

collection unit. By reducing the air velocity, frictional losses in the ductwork and fittings are 

reduced, lowering overall power requirements. 

Because of the behavior of low-velocity air, moderate orifice size changes can be made without 

completely upsetting the system. Even opening a branch completely will not drastically change 

the airflow in other branches because the pressure drop change is minimal. Should one branch of 

the network fail or change airflows dramatically, the system tends to stay in balance and other 

branches do not lose effectiveness. Therefore, the airflow can vary substantially on two identical 

pieces of equipment while maintaining excellent dust containment. Thus, overall losses through 

the dust collection system are lower. 

The disadvantages of low-velocity systems are the higher initial cost and more complex design. 

In these systems, the ability to pass a certain volume of air (cfm) through the duct at a 

predetermined velocity (fpm) dictates the diameter of the duct, requiring larger diameter 

ductwork. Also, because ductwork cannot be run horizontally, a sawtooth design is used, which 

adds expense and complicates installation. 

22 Fundamentals of Dust Collection Systems 
 

 

A common misconception when designing a low-velocity system is that an exhaust hood is not 

required due to the low-velocity ducting design. To the contrary, a properly designed exhaust 

hood on a low-velocity system is still a requirement for a good dust containment system. A 

properly designed hood will help maintain lower system pressure losses by minimizing the shock 

loss that occurs from the airflow entering the system. Hood design is essentially the same for 

both high- and low-velocity systems. However, all exhaust hoods should be located away from 

the point of material impact to lower the likelihood of material being drawn into the hood. The 

principle of low-velocity dust containment is to exhaust air from a well-enclosed area. This will 

impart a negative inward pressure, trapping fugitive dust within the enclosure. The shape of the 

exhaust hood is also important for two reasons: 

1.	 A well-designed hood has a lower resistance to air movement. 

2.	 A well-designed hood prevents material from being vacuumed into the dust containment 

system. Even with low-velocity ducting, coarse material can still be drawn into the 

system. It is therefore important to have a hood with a face velocity of 200 to 300 fpm or 

less in order to avoid entraining saleable product. 

Finally, the design and layout for a low-velocity system requires more engineering. Because of 

the constraint that runs cannot be made horizontally, it is sometimes difficult to physically find 

room for the required sawtooth design of the ducting. However, a properly designed and 

balanced low-velocity system provides a virtually nonplugging and low-maintenance dust 

control system. More specific engineering principles for low-velocity systems are detailed 

below. 

Airflow Target of 1,800 fpm 

Because of the nature of a low-velocity system, it is not necessary to maintain a specific air 

velocity within the ductwork. However, the desired target of 1,800 fpm should move most of the 

respirable dust particles (minus 10 μm) through the system to the filter equipment while 

containing it within the confines of the individual pieces of dust-producing equipment/hood. The 

basic goal of low-velocity dust control is to minimize the amount of contaminants (respirable 

dust) that escape the system, thus becoming a hazard or nuisance. The 1,800 fpm target 

maintains a slightly negative pressure within the piece of equipment, while providing adequate 

airflow to discourage dust particles from escaping the unit. This ensures that any airflow at the 

equipment is moving inwards rather than outwards. 

Avoidance of Horizontal Ductwork 

As a result of the system's low-velocity airflow, it is imperative that ductwork is not run 

horizontally. This is because the heavier dust particles drawn into the airstream may drop out 

and attach themselves to the sides of these ducts. The net result is an eventual narrowing of the 

duct opening, leading to an increase in velocity and ductwork erosion. Therefore, ductwork 

should be designed for a minimum upflow angle of 45 degrees and a minimum downflow angle 

of 30 degrees (Figure 1.11). This allows any particles drawn into the airstream to slide back to 

their source. This design also requires that a dropout point be located at the low point of all duct 

runs to allow these particles to fall out, as shown in Figure 1.10. 

Fundamentals of Dust Collection Systems 23 
 

 

Figure 1.11. Duct design angles for low-velocity systems. 

The Use of Main Duct Trunk Lines 

From a design standpoint in low-velocity systems, consideration should be given to using one or 

more main duct trunk lines, running from the air filter to a centrally located proposed dust 

control point in the plant. This allows for smaller branch duct lines to emanate from the main 

duct trunk line and enables easy connection to individual pieces of equipment. These trunk lines 

can be either rectangular or round. In areas where space is a problem, rectangular duct cross 

sections that are shallow in depth and wide are easier to install. Many times these trunk lines can 

be run vertically up the side of an existing column or bucket elevator. As previously noted, a 

dropout point—possibly the bucket elevator—must be provided at the low point of all duct runs. 

Minimizing Field Fits and Welds 

New ductwork and fittings should be shop fabricated and be as complete as possible before being 

sent to the field for erection. Welding or cutting duct joints in the field are costly, and proper 

engineering design can minimize the welding required. Extra time spent at the design stage 

preplanning and prefabricating hoods, fittings, and duct segments, pays dividends later during 

installation. 

Frequently, field fits must be made, and these can be anticipated before the material is sent to the 

field. It is best to make field cuts and welds at the flange point of the ductwork. In these 

instances the ductwork can intentionally be shop fabricated to lengths longer than necessary. 

Also, a rolled angle ring can be shop installed at the end of the duct, then tack welded. This 

keeps the ductwork from becoming distorted during handling and transit, and the tack weld can 

be cut in the field, the duct shortened, and the flange moved, rotated, and welded as needed. 

Flanged connections are an efficient and easy means to install ductwork in the field. Rolled 

angle ring flanges are readily available, inexpensive, and easy to install. These flanges should 

normally be used on all shop fabricated fittings, hoods, and duct segments. These types of 

connections also help eliminate costly welding requirements and allow for ease of replacement 

sections should maintenance be required. 

24 Fundamentals of Dust Collection Systems 
 

 

Avoiding Mitered Elbows Greater Than 90 Degrees 

As with high-velocity systems, pressure losses increase rapidly in ductwork with abrupt 

directional changes greater than 90 degrees. Since duct pressure losses are a squared function of 

the velocity, the low-velocity system still results in lower pressure losses. However, a 

segmented elbow can be used in areas of unusually abrupt air directional change if maintaining 

low pressure losses in the system is critical. 

Sizing and Locating Orifice Plates 

In many complex ventilation systems, orifice plates are used to balance airflows by changing the 

pressure requirement in a selected section. It is recommended that an orifice plate, sized for a 

minimum pressure loss of 2 inches, be installed in each individual branch line. This allows for 

easy expansion of the system in the future. As additional branch lines are added to the main 

trunk line, the orifice plate can be resized for a larger, and therefore less restrictive, orifice 

opening. 

Figure 1.12 demonstrates inappropriate locations for orifice openings. 

Figure 1.12. Demonstration of poor orifice plate placement. 

Fundamentals of Dust Collection Systems 25 

 Orifice plates should be 

installed a minimum of 4 to 5 duct diameters upstream and 4 diameters downstream from air 

directional changes. This permits the airflow to become closer to laminar and to reestablish 

normal velocity. If an upstream orifice plate is placed too near a downstream air directional 

change, the higher velocities encountered through the orifice opening will produce a sandblasting 

effect on the far-side wall. Orifice plates should be installed in the upflow branch leg whenever 

possible. This helps avoid a buildup of heavier dust particles above the orifice plate, as could 

happen in the upflow branch line installation, where particles are unable to drop out through the 

higher velocities of the orifice plate. 
 

 

Orifice plates should also not be installed near hood inlet openings. The higher velocities near 

the orifice tend to capture unwanted material. Hood capture velocities below 500 fpm should be 

maintained. On fine-grind materials, a target capture velocity of 200 fpm is desirable. 

Minimizing the Use of Flexible Hoses 

Flexible hoses should not, as a rule, be used as part of the ductwork in low-velocity systems. A 

failure could compromise the dust control ability of the branch lines in the general area. Flexible 

hoses also increase the frictional losses in the system. If flexible hoses are used, they should be 

placed between the orifice plate and the inlet hood where their failure would have the least 

impact. 

Modified Low-Velocity (MLV) System 

The objective of the modified low-velocity (MLV) system is to combine the advantages of the 

high- and low-velocity systems, while avoiding the disadvantages of both. The key to the 

success of the MLV resides at the pickup hood. Here the first duct run must be vertical for at 

least 6 feet, and preferably 8 to 10 feet. The velocity in this first vertical run is extremely low, 

even compared to the standard low-velocity system (i.e. 1,000 to 1,200 fpm). As in other 

systems, dust is contained, but only the smaller micron particles are transported. The 6- to 10

foot vertical rise is necessary to reduce turbulence, to achieve a smooth laminar flow, and to 

optimize elutriation (separation of lighter and heavier particles) at the first bend. 

Because there are minimal particles in the airstream after this first vertical rise, the need to 

maintain high velocities in subsequent horizontal runs is not necessary. Similarly, there is no 

need for an overall sawtooth design or mid-run discharge points. Velocity in the first horizontal 

run following a 1,200-fpm vertical duct should be about 2,000 fpm. 

Another important design consideration with MLV systems is that as additional lateral lines are 

connected to the main trunk line, the exit velocity of the trunk line after the initial lateral 

connection should be increased by about 100 fpm (see Figure 1.13). Even in long runs with 

many lateral lines, it is not likely that the line velocity upon reaching the collector will exceed 

2,500 to 3,000 fpm. Given the low pressure drops in such a system, it is not necessary to 

construct expensive elbows, mitered bends, or laterals. Instead, box elbows and fittings can be 

used. 

-

26 Fundamentals of Dust Collection Systems 
 

 

Figure 1.13. Depiction of the horizontal and vertical velocity 

relationship in a modified low-velocity system. 

The MLV system contains all the advantages of the low-velocity system relative to low abrasion, 

reduced maintenance, reduced power, and stability of the system balance. At the same time, it 

does not require the space-consuming sawtooth installation. Duct size (diameter) is a 

compromise between the high- and low-velocity systems, since velocities in all trunk lines will 

be somewhere between the two. Another advantage of the MLV system is that it can readily be 

used in extensions to, or modifications of, existing high-velocity systems. Only under certain 

conditions can it be used in an extension of a low-velocity system. One would need to compare 

velocities at the connecting points, recognizing that it is not feasible to move from a higher to a 

lower velocity within a duct. A low-velocity extension can, however, be made to a MLV system. 

The basic design tips for low-velocity systems as detailed previously can be applied to MLV 

systems, with a few exceptions: 

1.	 The reference to avoiding horizontal ducts does not apply. 

2.	 Orifice plates or blast gates should be installed at the top of the low-velocity vertical leg, 

not the bottom. Plates/gates should be as near to the higher-velocity leg as possible. 

3.	 Vertical leg velocity should be 1,000 to 1,200 fpm. 

4.	 Horizontal leg velocity should begin at a minimum of at least 2,000 fpm. 

Fundamentals of Dust Collection Systems 27 
 

 

AIR CLEANING DEVICES 

Air cleaning devices used within the industrial minerals mining industry are used to clean 

ventilation airstreams of harmful particulate matter. The choice of air cleaner for any particular 

installation will depend on the following: 

dust concentrations and dust characteristics, 

particle size, 

efficiency of particulate removal required, 

airstream temperature, airstream moisture content, and 

methods of disposal. 

Distinguishing dust characteristics that affect the collection process include abrasive, explosive, 

sticky or tacky, and light or fluffy. The shape of the dust particle is also important because it 

factors into whether the particles are agglomerating (irregular) or nonagglomerating (spherical), 

which is important when using a filter cloth. For collection purposes, agglomerating particles are 

ideal as they allow dust cakes to build up easily on the filter cloth, allowing for more efficient 

collection at the dust collector. However, agglomerating particles may have a tendency to not 

release from the filter cloth very easily. 

The types of dust control equipment used for air cleaning range from very crude gravity 

separators to more sophisticated electrostatic precipitators. The following is a list of the types of 

collectors used for particulate removal. 

1. Gravity separators (drop-out boxes). 

2. Centrifugal collectors or cyclones. 

3. Baghouse collectors. 

4. Cartridge collectors. 

5. Wet scrubbers. 

6. Electrostatic precipitators (ESPs). 

A brief overview of each type of collector, along with its advantages and disadvantages, follows. 

Gravity Separators (Drop-Out Boxes) 

Gravity separators (also called drop-out boxes) are large chambers where the velocity of the 

airstream is drastically reduced in order to facilitate the vertical drop of particles. The separator 

works by not only slowing down the air but by changing its direction as well. Airflow enters 

horizontally and is immediately directed vertically downward by a target plate (Figure 1.14). As 

the air slows and moves downward, gravity takes over the large particles and drops them out of 

the airstream. Finer particles not affected by this will continue to flow in the airstream and exit 

the separator. 

28 Fundamentals of Dust Collection Systems 
 

 

Figure 1.14. Typical design of gravity separator (drop-out box). 

The benefits of using gravity separators are that they require little maintenance and they reduce 

the load on the primary dust collector. However, they also take up significant plant space and 

have a low collection efficiency. 

Centrifugal Collectors or Cyclones 

Cyclones are a dust collection device that separates particulate from the air by centrifugal force. 

The cyclone works by forcing the incoming airstream to spin in a vortex. As the airstream is 

forced to change direction, the inertia of the particulates causes them to continue in the original 

direction and to be separated from the airstream (Figure 1.15). Although, the cyclone is simple 

in appearance and operation, the interactions inside a cyclone are complex. A simple way to 

explain the action taking place inside a cyclone is that there are two vortices that are created 

during operation. The main vortex spirals downward and carries the coarser particles. An inner 

vortex, created near the bottom of the cyclone, spirals upward and carries finer dust particles. 

Fundamentals of Dust Collection Systems 29 
 

 

Figure 1.15. Typical design of a cyclone dust collector. 

Cyclones are cost-effective and low-maintenance devices, and they can handle high 

temperatures. They also reduce loading on the primary collector and allow for the dry recovery 

of product. However, it is difficult to predict the performance of cyclones and they pose 

particular design challenges. Accurate inlet data are necessary and they require significant plant 

space. 

Cyclones have low efficiencies in removing fine particulate. They are typically used as a 

precleaner to remove coarser particles that could otherwise damage the bags in fabric collectors 

or plug wet scrubbers. It should be noted that adding a cyclone to a ventilation system may not 

reduce the overall system resistance because the drop in resistance at the baghouse, due to lower 

dust loading, may be more than offset by the pressure drop of the inertial cyclone collector. 

Pressure drops range from 3 inches wg for low efficiency inertial cyclone collectors and up to 8 

inches wg for higher efficiency models. 

30 Fundamentals of Dust Collection Systems 
 

 

Baghouse Collectors 

Baghouse dust collectors capture the particulate in an airstream by forcing the airflow through 

filter bags. A baghouse works by taking the inlet dust-laden air and initially reducing the 

velocity to drop out larger particles, then filtering the remainder of the particles by passing the 

air through a fabric bag (Figure 1.16). 

Figure 1.16. Basic design of a baghouse dust collector. 

 Separation occurs by the particles colliding and attaching 

to the filter fabric and subsequently building upon themselves, creating a dust cake. Since the 

dust has been deposited on the outside of the bag, when the dust cake is removed from the bag or 

cleaned, it falls by gravity into the collection hopper located below the bag section. Collected 

dust is then removed from the collector through a hopper valve. 

Baghouse collectors are generally designed and sized to operate with a differential pressure 

between 4 and 6 inches wg. These collectors can achieve air cleaning efficiencies of more than 

99.97 percent (high-efficiency particulate air, or HEPA) for fine particles. The fabric bags can 

be made from cotton, synthetic materials, or glass fiber. The type of bag used depends on the 

type of fabric collector and application. For most applications involving ambient temperature, a 

cotton bag is the most economical. However, in a corrosive or high-temperature environment a 

bag material other than cotton should be employed. Since bags must be changed periodically, 

fabric collector designs that facilitate bag changes should be purchased. Designs where the bags 

can be changed from outside the collector are preferred. 

Fundamentals of Dust Collection Systems 31 
 

 

Baghouse systems can also be designed for economic optimization. For a given emission control 

problem, factors such as the overall pressure drop, filtration cleaning cycle, and total filtration 

surface area can be addressed simultaneously. The article "Baghouse System Design Based on 

Economic Optimization" [Caputo and Pacifico 2000] provides a useful model, in particular for 

operations in the preliminary design phase. 

Bulk density of the material requires special engineering attention. The effect that upward 

velocity (interstitial velocity) can have on the operation of a dust collector can be enormous. 

Materials with low bulk density (<30 pounds per cubic feet) must have specialized designs. In 

these cases, collector designs must be modified to accommodate lower interstitial velocities. 

Typical modifications include wider bag to bag spacing, shorter length bags, or high side inlets. 

Finally, particle size distribution plays a key role in determining the air to cloth ratio and filter 

bag selection. It is generally understood that the finer the dust the lower the air to cloth ratio 

needed. Proper bag or cartridge selection based on the material to be collected is fundamental to 

a successful system. The article "Fine Filtration Fabric Options Designed for Better Dust 

Control and to Meet PM2.5 Standards" [Martin 1999] provides a useful fabric characteristics and 

capabilities chart, matching fabric type to operating conditions. Another recommended resource 

is the article "Pick the Right Baghouse Material" [Mycock 1999], which includes a chart 

detailing properties of textile fabrics for filtration. 

Inlet loading refers to the amount of dust arriving at the inlet of the dust collector. It is typically 

expressed in pounds per minute (lbs/min) or pounds per hour (lbs/hour) and converted into grain 

loading expressed in grains/cubic foot (gr/cf) of airflow. The grain loading within an airstream is 

dependent on many factors, which include the number of dust sources serviced by the dust 

collection system, the types of dust sources (e.g., crushers, screens, etc.), the dust emissions from 

these individual sources, and the capture effectiveness of the dust collection system at each 

source. The amount of dust emitted by each source is impacted by a number of parameters, 

including the particle size distribution (dustiness) of the material being handled in the process, 

the moisture content, and the throughput rate. 

The Environmental Protection Agency has compiled data on dust emission factors for a number 

of processes that are involved in mineral processing [EPA 1995]. Table 1.2 lists some of these 

factors, which are averages of dust emissions from available data and should be viewed as a 

general starting point when attempting to determine inlet loading. These emission factors would 

have to be multiplied by the processing rate at the specific source to calculate inlet loading in 

lbs/min. Recommendations based on experience from dust collector manufacturers and from 

filter cloth manufacturers should also be utilized in efforts to effectively quantify inlet loading. 

This inlet loading or grain loading helps determine the air to cloth ratio, filter media, type of 

collector, type of inlet to be used, and how the filter cleaning system will be configured. 

32 Fundamentals of Dust Collection Systems 
 

 

Table 1.2. Emission factors for crushed stone processing operations (lb/ton)* 

Source Total PM-10** 

Tertiary crushing 0.0024 

Screening 0.0087 

Conveyor transfer point 0.0011 

Wet drilling—unfragmented stone 0.00008 

Truck unloading—fragmented stone 0.000016 

Truck loading—conveyor, crushed stone 0.0001 *

Uncontrolled emissions in lb/ton of material throughput. 

**PM-10 is particulate matter less than 10 micrometers (μm), in size. 

3 The following formula can be used for converting lbs/min into grains/ft : 

3

gr/ft

cfm

lbs/min)(grains 7,000

(1.7) 

where lb/min = loading rate of the material, and cfm = airflow of the dust collector. 

The following expressions define various inlet loading relationships: 

3 low concentration = <1 gr/ft ; 

3 typical concentration = 1–5 gr/ft ; 

3 high concentration = 5–10 gr/ft ; 

3 very high concentration = >10 gr/ft . 

The air to cloth ratio is a measure of the volume of gas or air per minute per unit area of bag 

3 2 (ft air per minute/ft of bag area), and can be expressed mathematically as follows: 

air to cloth ratio = Q/A 

where Q = quantity of gas (air) in cfm and, A = area of the filter cloth or total number of bags in 

2 ft . 

The porosity of the filter media (cloth) will determine the amount of air that can be drawn 

through before the static pressure becomes too high for the collector housing or the fan capacity. 

By estimating the inlet loading, one can also determine the grains of dust striking each square 

foot of filter media per unit time. 

Air to cloth ratio is also a measure of the average velocity of the air moving towards the bags. 

Inlet loading directly affects the air to cloth ratio. The heavier the inlet loading the lower the air 

to cloth ratio should be. High inlet loading results in more dust being retained on the filter media 

and causing higher pressure drops. Air to cloth ratios below 4:1 are considered low, from 4:1 to 

7:1 are moderate, and above 7:1 are considered high. By lowering the air to cloth ratio, the filter 

Fundamentals of Dust Collection Systems 33 
 

 

has more filter area to distribute the dust, thereby helping to keep the pressure drop lower on the 

residual dust cake. 

There are basically two methods to reduce the air to cloth ratio—lowering the cfm of air or 

increasing the filter cloth area. However, in overall dust collection system design, changing the 

airflow volume can be impractical. Therefore, increasing the filter cloth area is more commonly 

used. 

In addition to air to cloth ratio, inlet loading can also affect the method used for cleaning the 

bags. Since virtually all filter materials work better with a consistent filter cake to optimize 

collection efficiency, varying the duration of the cleaning cycle, in conjunction with the rest 

period between cycles, will allow the operator to maintain a consistent filter cake. This is 

manifested by a low variation in pressure drop across the filter media. This approach is called 

"on-demand" cleaning. Bag cleaning is initiated at a predetermined high pressure drop and stops 

when the pressure drop reaches a predetermined low set point. This method ensures that the 

bags always have a sufficient amount of dust cake. Experience and manufacturers'

recommendations are the best means of determining the optimum cleaning cycle for each system. 

There are three techniques used with baghouse collectors to clean the dust cake from the filter 

media. These techniques are accomplished by mechanical shaker collectors, reverse air 

collectors, and reverse jet collectors. 

Mechanical Shaker Collectors 

Mechanical shakers use a mechanical rapping device to remove the excess dust cake; however, 

the airflow through the collector must be temporarily stopped to clean the bags. These shakers 

involve low maintenance and low operating costs, but they require large amounts of space, have 

limited design flexibility, and have low air to cloth ratios (2:1). 

Shaker collectors employ tubular filter bags fastened on the bottom and suspended from a shaker 

mechanism at the top. Dust-laden air enters the collector and is deposited on the outside of the 

tubular bags. Continuous processes use compartmentalized collectors where airflow can be 

diverted to other compartments during the bag cleaning cycle (Figure 1.17). Bag materials must 

be made from woven fabrics such as cotton to withstand shaking. 

Some shaker collectors have been converted to pulse jet collectors. However, this is an 

expensive operation and should not be necessary if the collector is properly maintained. 

34 Fundamentals of Dust Collection Systems 
 

 

Figure 1.17. Typical design of a mechanical shaker dust collector. 

When using mechanical shakers, a number of recommendations should be followed: 

Magnehelic gauges should be installed on each compartment to monitor differential 

pressures during the cleaning cycles. 

Differential pressures should be as close as possible to 0.0 inches wg to ensure that the 

dust cake breaks and is released from the bags. Differential pressures above 1/4-inch wg 

during the cleaning cycle will dramatically interfere with bag reconditioning, which can 

lead to reduced airflow, high static pressures, and low bag life. 

Bags should only be shaken when differential pressures across a section of bags have 

increased by 1/2-inch wg. 

Experiments to determine the optimum time interval between the shaking of bags should 

be performed so that bags are not shaken excessively. This also lessens wear on the bags 

as well as the mechanical parts. 

Fundamentals of Dust Collection Systems 35 
 

 

Reverse Air Collectors 

Reverse air collectors use a traveling manifold to distribute low-pressure cleaning air (3–7 psig) 

to the filter bags for reconditioning. They have no compressed air requirements and involve no 

freezing of air lines. On the negative side, they have larger footprints than conventional 

collectors and maintaining their cleaning mechanisms is difficult. They also perform poorly in 

corrosive environments. 

Reverse air collectors employ tubular bags fastened onto a cell plate on the bottom and 

suspended from the top of the collector. The collectors must be compartmentalized for 

continuous service. Dust-laden air enters the collector and deposits dust on the outside of the 

bags. Before a cleaning cycle begins, filtration in the compartment to be cleaned is suspended. 

Bags are cleaned by blowing low-pressure air into the compartment in the reverse direction to 

normal airflow. This causes the bags to partially collapse and release the dust cake. The bags 

have rings located at various intervals to prevent total collapse so that the dust cake can escape 

and fall into the hopper (Figure 1.18). 

Figure 1.18. Typical design of a reverse air dust collector. 

36 Fundamentals of Dust Collection Systems 

Reverse air collectors were originally developed primarily with fragile glass cloth for use in 

high-temperature operations. These collectors have declined in popularity with the use of new 

materials that can withstand high temperatures and greater physical action. Air to cloth ratios for 

reverse air collectors are similar to those for shakers due to the low bag cleaning efficiency of the 

reverse air. 
 

 

Reverse Jet (Pulse Jet) Collectors 

Reverse jet (also called pulse jet) collectors use bags supported from a metal cage fastened onto a 

tube sheet at the top of the collector (Figure 1.19). 

Figure 1.19. Typical design of reverse jet (pulse jet) dust collector. 

Fundamentals of Dust Collection Systems 37 

Dust-laden air enters the collector and flows 

from outside to inside the bag. The dust cake deposits on the outside of the bag and is cleaned 

by short bursts of compressed air injected inside the bag. The burst of air causes the bags to flex, 

breaking and releasing the dust cake. The compressed air must be clean and dry or moisture can 

build up on the bags, hindering the bag cleaning efficiency. Pulse jet collectors are not 

compartmentalized, allowing bags to be reconditioned without removing a section from service. 
 

 

Reverse jet collectors use timed blasts of compressed air (60–90 psig) for filter bag 

reconditioning. This is the most common type of baghouse collector and has been in use since 

the late 1950s. Reconditioning or cleaning of the filter bags allows the dust collection system to 

maintain pressure drops and operate at designed airflows. The advantage of using reverse jet 

collectors is high product recovery and high collection efficiency. They also enjoy high 

flexibility of application with many inlet design options. Their limitation is that their 

performance varies with temperature and moisture. 

Due to more frequent cleaning intervals, these collectors provide more complete bag cleaning 

than previous styles of collector already discussed. Thus, the air to cloth ratios can be higher— 

typically, suppliers specify air to cloth ratios of 6:1 or higher. However, ratios of 4:1 should be 

used for applications involving abrasive minerals. High air to cloth ratios can cause high air 

velocity impingement on the bags, resulting in dust re-entrainment on neighboring bags after 

cleaning and low bag life. 

Bags are typically made from nonwoven felt material. Woven materials are not used in pulse jet 

collectors because they require a buildup of a permanent dust layer to provide efficient air 

cleaning. Since pulse jet collectors aggressively clean the bags, excessive penetration of dust 

particles through the woven fabric can occur. 

Pulse jet collectors are more cost-effective than earlier styles of collector such as mechanical 

shaker collectors. They can operate with higher air to cloth ratios, have no moving parts to 

maintain, and involve lower capital costs. 

Cartridge Collectors 

Cartridge collectors capture particulate from an airstream by forcing the air through filter 

canisters in which the filter media is fabricated in a pleated configuration. There are two basic 

configurations of cartridge collectors: those that suspend the filter canisters vertically and those 

that are mounted horizontally (Figure 1.20). 

38 Fundamentals of Dust Collection Systems 
 

 

Figure 1.20. Cartridge collector with dust filter canisters. 

Cartridge collectors are the latest generation of fabric collector. Unlike other fabric collectors in 

which the filtering media are woven or felt bags, this type of collector employs cartridges that 

contain a pleated filtering media. The pleated cartridges can be made from a variety of media 

including polyester or synthetic material. Due to the pleated design, the total filtering surface 

area is greater than with conventional bags of the same diameter. However, the pleated design 

cartridges produce very high approach velocities and thus greater re-entrainment onto the 

cartridge can occur. Filtration velocities are therefore limited to air to cloth ratios of less than 

2:1. 

The cartridge filter works like the pulse jet baghouse collector in that dust is filtered on the 

outside of the elements. Cleaning air (compressed) is blown into the center of the elements, 

discharging the excess collected material. Cartridge collectors involve lower capital costs than a 

baghouse because the cartridges have more area than bags, therefore fewer are required. 

Cartridge collectors have lower headroom requirements, because the cartridges are inserted from 

the side of the unit as opposed to the top. Cartridges are shorter than bags, and there are fewer of 

them. Media can also be changed rapidly, facilitating a higher level of maintenance. Limitations 

to cartridge collectors include a lack of flexibility based on extreme temperature and moisture 

applications, since there are not as many varieties of filter fabrics available as there are for 

baghouses. Because of the high filter area per unit and the pleated design, media replacement 

costs are high on a per-cartridge basis. 

The main advantages of cartridge collectors are the compact design and ease of cartridge 

changing, resulting in reduced dust exposure to workers. New cartridges are packaged in a 

cardboard box, and to change cartridges the worker simply removes the new cartridge from the 

box, extracts the used cartridge and places it in the box, then installs the new cartridge, with 

minimal dust exposure to the worker. Within the collector, the cartridges are cleaned by a 

conventional pulse jet cleaning system. In practice, the cartridges can have a two-year life in 

abrasive applications. However, purchasing this type of collector may require the user to 

purchase cartridges from the equipment supplier, reducing a competitive pricing advantage. 

Fundamentals of Dust Collection Systems 39 
 

 

It should be noted that cartridge collectors do not work well with moist or sticky materials, and 

are generally restricted to applications with temperatures less than 180oF. The inlet loading is typically lower than with bags, because with the pleated design the cartridges do not clean as 

well. In addition, the horizontal alignment of the cartridges allows dust from the cartridges 

above to fall on the cartridges below during cleaning. 

Wet Scrubbers 

Wet scrubbers accomplish particulate collection by employing water or another liquid as the 

collection media. There are many scrubber designs; most particulate scrubbers work by creating 

a wetted target for particle collection. This wetted target plate can be a bed of water or a zone 

where the particle and water droplet collide (Figure 1.21). 

Figure 1.21. Typical design of a wet scrubber dust collector. 

 Benefits to wet scrubbers are their 

ability to perform in various moisture and temperature conditions, their resistance to chemical 

corrosion, and their low maintenance needs. However, wet scrubbers require a significant 

amount of water, which must be disposed of along with the collected particulate, all of which 

lower the collection efficiency and increase the energy costs. Settling ponds are one common 

method of collected particulate disposal in the mineral processing industry, and the water from 

these ponds can often be reused within the operation. 

The air cleaning efficiency of this type of collector primarily depends on its pressure drop. 

Scrubbers with high differential pressures have higher air cleaning efficiencies than scrubbers 

with lower differential pressures. These pressures range from 1 to over 15 inches wg. The style 

of scrubber chosen for a particular application depends on the air cleaning required, the dust 

loading, and the particle sizes involved. 

40 Fundamentals of Dust Collection Systems 
 

 

Wet scrubbers are particularly advantageous when handling moist hot gases. Problems such as 

bag blinding and condensation can arise when fabric collectors are used to clean moist hot gases. 

These problems are eliminated when using scrubbers. However, wet scrubbers discharge 

contaminated water, which requires further treatment in a settling pond or sewage system. 

Venturi Scrubbers 

Venturi scrubbers are a type of wet scrubber and consist of a venturi-shaped inlet and a separator 

(Figure 1.22). 

Figure 1.22. Typical designs of a venturi scrubber. 

 Dust-laden air is accelerated to velocities between 12,000 and 36,000 feet per 

minute in the throat of the venturi. These high velocities atomize the coarse water spray and 

create pressure drops ranging from 5 to over 15 inches wg. The venturi can be ceramic-lined for 

abrasive applications. The extreme turbulence promotes collision between water droplets and 

dust particles in the throat of the venturi. An inertial separator then removes these agglomerates. 

Impingement Plate Scrubbers 

In another type of wet scrubber—an impingement plate scrubber (see Figure 1.23)—the exhaust 

passes upward through openings in perforated plates, which hold a layer of water. An 

impingement baffle is located above each hole, resulting in the formation of small droplets. 

Intimate gas/liquid contact results in efficient particle collection. A pressure drop of 4 inches wg 

is typical. 

Fundamentals of Dust Collection Systems 41 
 

 

Figure 1.23. Typical design of an impingement plate scrubber. 

Spray Tower Scrubbers 

Gravity spray tower scrubbers employ atomized water, which falls counter-current through a 

rising dust-laden airstream, to remove dust particles. These scrubbers are generally of lower 

efficiency and operate at pressures of 1 to 2 inches wg. They are approximately 70 percent 

efficient on 10-μm particles and above with poor efficiency on particles smaller than 10 μm. 

However, they are capable of treating high dust concentrations without becoming plugged. 

Wet Cyclone Scrubbers 

Similar to a dry cyclone, wet cyclone scrubbers use centrifugal forces to throw particles on the 

collector's wetted walls. Water is introduced from the top of the scrubber to wet the walls and 

wash the particles away. Pressure drops for these collectors range from 2 to 8 inches wg with a 

good efficiency for removing particles 5 μm and larger. 

Electrostatic Precipitators (ESPs) 

Electrostatic Precipitators (ESPs) are particulate control devices that use electrical forces to 

move particles from the airstream to collection plates. Particles passing through the precipitator 

are given a negative electrical charge by being forced to pass through a region, called a corona, 

in which gas ions flow. Once the particle has been negatively charged, it is forced to the 

positively charged plate. Particles are removed from the plate by knocking action. 

42 Fundamentals of Dust Collection Systems 
 

 

Electrostatic precipitators normally have a higher initial cost than local exhaust ventilation 

systems, but a number of advantages that make them worth considering. Once installed, ESPs 

require very little maintenance because there are no moving parts. The installation time and the 

operating costs are also lower than for a local exhaust ventilation system. One last advantage is 

that the product is easily recovered and recycled right back into the process. 

There are four basic types of ESPs: plate and wire (dry), flat plate (dry), wet, and two-stage. 

ESPs provide a large air volume, operate favorably in various temperatures, and require little 

maintenance. Their limitations include their physical size, operation expenses, and inconsistent 

collection efficiencies. A more thorough discussion of electrostatic precipitators, including 

distinctions between single-stage and two-stage types, is available in the ACGIH handbook, 

Industrial Ventilation: A Manual of Recommended Practice for Design [ACGIH 2010]. 

COLLECTOR DISCHARGE DEVICES 

The dust collected in a baghouse falls into a hopper below the bags. This hopper must have a 

discharge device that not only releases the accumulated dust, but ensures a seal between the 

baghouse and outside air to maintain negative pressure within the unit. Common hopper waste 

discharge devices available for baghouses are the rotary airlock valve, double dump valve (flap 

valve), tilt valve, and vacuum valve (dribble valve). Each of these is readily available in various 

designs from several manufacturers. 

Rotary Airlock Valves 

Rotary airlock valves are normally used in applications where both an active airlock and material 

metering are required. Although they are used in many baghouse installations, such common use 

may not be the best application for this valve. The initial cost is high, and maintenance costs 

may also be high in abrasive applications. 

Double Dump Valves 

Double dump valves are dual flapper valves mounted in tandem off the discharge of the 

baghouse and can be either automated or weight-based. For automated double dump valves, an 

automated rotating cam briefly opens the normally closed spring-loaded door to each valve. The 

dual valves are opened individually in a sequence that allows the top door to open, dump, and 

close before the lower door follows the same sequence. This allows a negative pressure to be 

maintained in the discharge hopper. The weight-based valve is very similar except that the valve 

opens based upon the weight of the product and not on an automated basis (Figure 1.24). Both 

types of double dump valves require sufficient vertical room below the baghouse hopper. Some 

wear can be expected at the actuator cam lever of each door. 

Fundamentals of Dust Collection Systems 43 
 

 

Figure 1.24. Double dump weight-based valves. 

Tilt Valves 

Tilt valves are similar to double dump valves and are normally used in pairs. These valves 

commonly have a near horizontal and counterweighted flap door that is adjustable for closed 

tension. The function of the valve is to allow material to build up on the near horizontal flap gate 

until it overcomes the pull of the negative pressure in the baghouse. The material then dumps to 

the next valve as the gate shuts behind it. Typically a divider chute 24–30 inches long is used 

between the two valves to allow one valve to close completely before the other opens. 

Tilt valves are less expensive than double dump and rotary valves, but can be hard to adjust. 

They have been known to stick open after being in service for a period of time, and they require 

4 feet 0 inches clearance below the baghouse for installation. 

Vacuum or Dribble Valves 

A vacuum valve basically consists of a soft gum rubber tube mounted in a steel housing, which 

attaches to the bottom of a baghouse hopper discharge. Manufacturers have various designs 

available for purchase, some using a "fishtail" dribble bladder mounted inside a flanged pipe. 

The valve works using the negative pressure of the hopper to collapse the rubber tube and seal 

the discharge until sufficient material builds up and "dribbles" through. This valve normally has 

low initial cost, high reliability, and low maintenance, and can fit in a short discharge space. 

FILTER FABRICS 

The majority of development work that has been done in the industry over the last ten years has 

not been in the area of the collector itself but in filter fabrics. Some of these fabrics have been 

designed to handle the more challenging applications. Problems that particular fabrics address 

44 Fundamentals of Dust Collection Systems 
 

 

 

may be related to specific applications or the fabrics may simply provide a more efficient way of 

4 optimizing the collector's potential. 

Fundamentals of Dust Collection Systems 45 

4 For instance, many applications that involve temperature and corrosive dust or gases had been previously handled 

by using wet scrubbers. However, as emission requirements become more stringent, the use of scrubbers must 

diminish. 

A wider variety of filter fabrics are now available, giving 

the design engineer more options to address the problem. 

One of the most critical aspects to any dust collector device is the filter fabric's ability to remove 

the dust particles from the airstream as the dust-laden air passes through the filter fabric material 

in the collector. The term "filter fabric" can be used interchangeably with the term "filter 

media." For use in dust collector devices, there are numerous types of filter fabrics with a wide 

variety of properties and characteristics produced by different manufacturers. Regardless of this 

variability, filter fabric effectiveness is fundamentally dependent on the ability to capture or 

remove dust particles from the airstream. A filter fabric's ability to let air pass through the 

media (i.e., its permeability) is usually defined as the volume of air that can pass through one 

square foot of filter fabric each minute, at a pressure drop of 0.5 inches wg. 

Along with permeability, another measure of a filter fabric's effectiveness is its efficiency at 

removing dust particles. For instance, a HEPA quality filter is one that is 99.97 percent efficient 

at removing 0.3-μm particles. A practice that is becoming more prevalent in the industry is to 

refer to a fabric's dust removal efficiency by its minimum efficiency reporting value (MERV) 

rating (see Table 1.3). As Table 1.3 illustrates, the higher the MERV rating, the more efficient 

the filter is at capturing the various size ranges of dust particles. 

Table 1.3. Minimum Efficiency Reporting Values (MERV) according to 

ASHRAE Standard 52 

Group MERV Rating 

Average Particle 

Size Efficiency 

(PSE) 0.3–1.0 

μm 

Average Particle 

Size Efficiency 

(PSE) 1.0–3.0 

μm 

Average Particle 

Size Efficiency 

(PSE) 3.0–10.0 

μm 

1 < 20% 

1 2 < 20% 

3 < 20% 

4 < 20% 

5 20–34.9% 

2 6 35–49.9% 

7 50–69.9% 

8 70–84.9% 

9 < 50% ≥ 85% 

3 10 50–64.9% ≥ 85% 

11 65–79.9% ≥ 85% 

12 80–89.9% ≥ 90% 

13 < 75% ≥ 90% ≥ 90% 

4 14 75–84.9% ≥ 90% ≥ 90% 

15 85–94.9% ≥ 90% ≥ 90% 

16 ≥ 95% ≥ 95% ≥ 95% 
 

 

A filter fabric becomes more efficient, or achieves a higher MERV rating, by its ability to 

capture particles by interception, impaction, diffusion, and electrostatic charge. Distinctions 

among these capture processes are as follows: 

Interception occurs with smaller-sized particles that follow the airstream flow lines and 

come within one particle radius of a fabric fiber, then adhere to the fabric. 

Impaction occurs with larger particles that are not able to stay on airstream flow contours, 

travelling through the filter media and then embedding into one of the fibers directly. 

The amount of impaction increases with higher airflow velocities and with decreasing 

distances between the fibers. 

Diffusion occurs when the smallest dust particles collide with gas molecules, especially 

those smaller than 0.1 μm, and then alter their flow path so that they are captured by 

either interception or impaction. 

With electrostatic collection, the filter fabric is made from a fabric material or media that 

is able to sustain an electrical charge. Because dust particles also have an electrical 

charge, as they pass through the filter fabric, they are attracted to and adhere to the fabric 

material. 

Filter fabrics are constructed of either natural or man-made fibrous material and are either woven 

or nonwoven. Woven fabrics are normally identified by a weight per unit area as well as a 

thread count. Nonwoven fabrics are also identified by the weight per unit area, but typically 

include a filter thickness classification. Nonwoven material is usually more efficient than woven 

fabric of the same thickness because the open areas or pores are smaller. It is important to note 

that, from a dust collection standpoint, any type of filter fabric can be made more efficient or be 

given a higher MERV rating, simply by the use of smaller fiber diameters, the inclusion of a 

greater weight of fibers per unit area, or by the fibers being packed more tightly. 

With any filter fabric, the efficiency or MERV rating improves as the filter forms a filter dust 

cake. After the initial collection by one of the methods described above, as particle collection 

continues, a layer of dust particles forms on the fabric material and this layer then becomes the 

principal collection medium. As more dust particles continue to impinge on the previously 

collected particles, this dust filter cake continues to grow. Over a period of time, this dust filter 

cake will grow to such an extent that it will become increasingly difficult for the air molecules to 

pass through the fabric material, and this is evidenced by an increasing pressure differential 

across the filter. Once this pressure reaches a certain level, the dust filter cake needs to be 

removed from the filter fabric by some type of mechanical process (such as by a shaker, reverse 

air, or reverse jet process, described earlier). 

Particulate Collection of Fabrics 

There are two methods for collecting particulate on fabrics. The first is depth loading. This 

means that the fabric depends on a filter cake to optimize collection efficiencies. The second is 

surface loading, meaning that the fabric does not depend on a filter cake to obtain maximum 

collection efficiencies. 

Felted fabrics are depth loaded. Polyesters, polypropylene, aramids, and similar fabrics rely on 

the ability of the dust collection system to develop and maintain a dust cake or filter cake at all 

46 Fundamentals of Dust Collection Systems 
 

 

times. When considering the depth of loading onto felted fabrics, the filter bag is actually acting 

as a support for the dust cake. Dust is allowed to penetrate into the fabric to form a layer of dust 

on the bag. Thus, the dust creates a "porous" cake with much smaller air passages, allowing the 

dust to filter itself. Without this cake, the filter would bleed through and never obtain acceptable 

outlet emissions. Therefore, in the design and operation of a baghouse using felted media, it is 

important to use appropriate air to cloth ratios, cleaning cycles, and cleaning pressures. The 

optimum collector operation maintains a constant pressure drop of 3–6 inches wg and minimizes 

cleaning cycles and compressed air usage. This does not mean that a system running higher than 

a 6-inch pressure drop is not operating satisfactorily. Some systems can operate effectively at 

higher pressure drops. The system is operating correctly as long as the pressure drop is constant 

and the required air volume is being achieved. 

Surface loaded media do not require a dust cake to achieve optimum performance. As the name 

implies, the dust is collected on the very surface of the fabric. Some surface loaded media use 

membranes composed of a very thin layer (1 mil) of PTFE (polytetrafluoroethylene) material 

that is laminated to a substrate. This substrate material is a conventional felt. The laminate is 

applied in a special process that controls the size of the pore openings; thereby the collection 

efficiency is predictable. These pore openings are so small that they will only allow extremely 

small submicron particles to pass, making these fabrics extremely efficient. 

Dust collectors using felted media typically run higher pressure drops than those operating with 

surface-loaded media because the filter or dust cake adds resistance to the system. As previously 

mentioned, the pressure drop tends to range from 3–6 inches wg, while surface-loaded media 

operating without a filter cake typically run in the 2–4 inches wg range. 

The standard fabrics used in shaker collectors are woven cotton and polyester sateens and 

lightweight felts. Pulse jet and reverse air collectors use felts ranging in weight from 14 to as 

heavy as 22 ounces. Woven fabrics are not used in the last two types of collectors because the 

cleaning system is much more aggressive than shaking. Pulsing woven fabrics opens up the 

weave and causes leakage and undesirable emission levels. Cartridge filters typically use 

cellulose/synthetic blends and spun-bonded polyester media. 

When matching fabric choice to dust collection needs, the basic criteria are temperature, inlet 

loading, particle size distribution, gas composition, abrasion, static charge, release, and 

efficiency requirements. The following are some basic guidelines for each criterion. 

1.	 Temperature. Inlet gas temperature should be matched with a fabric capable of 

continuous operation at or above the maximum temperature the system might experience. 

2.	 Inlet loading. Inlet grain loading (normally expressed in grains/dry standard cubic feet) 

is especially important when the loading is very light. Light loadings can make it 

difficult for the media to operate with sufficient dust cake. 

3.	 Particle size distribution. Finer particle size distributions often require using higher 

efficiency media. 

4.	 Gas composition. The designer must make sure that if corrosive compounds or 

hydrocarbons are present in the airstream, then the media selected will be able to 

withstand that environment. 

Fundamentals of Dust Collection Systems 47 
 

 

5.	 Abrasion. When the dust is abrasive, the designer should consider using heavier felt or 

adding wear cuffs to the bottom of the filter bags on hopper entry inlets. 

6.	 Static charge. Explosive dusts and gases or dust that can develop a static charge in the 

duct system require some type of static grounding. This can come in the form of a 

braided grounding wire sewn into the filter bag and attached to the tubesheet. Grounding 

can also be accomplished through the bag material by way of epitropic fibers (using 

carbon fibers, graphite fibers, or 3–4 percent stainless steel fibers throughout the bag). In 

these cases, it is important to make sure the bag collar (in top-loaded bags) is fabricated 

out of the same material so that there is conductivity to ground. 

7.	 Release. A properly designed dust collector has two functions: the first is to collect the 

dust; the second is to release the dust off of the filter bag after it is collected. In the case 

where the dust is oily or sticky, the fabric may require a treatment to the filtering surface 

in order to aid in the release. This can take the form of modifying the surface with 

treatments such as singeing or glazing. The filtering surface can also be treated with a 

coating such as a PTFE bath or even a membrane. There are also specialty fabrics that 

treat the fiber before the felting process, providing a longer lasting surface than typical 

coatings. 

8.	 Efficiency. Expressed as a mass loading (grains per standard cubic feet, or gr/dscf), the 

required collection efficiency is determined by the environmental engineer based on 

EPA, state, or regional regulations that correspond to the particular plant's allowable 

discharges to the atmosphere. 

DESIGNING MINE/PLANT DUST COLLECTION SYSTEMS 

Applying the above principles presented in this chapter for simple systems provides the 

foundation for designing more complex exhaust ventilation systems. In reality, a complex 

exhaust ventilation system is just a combination of a number of simple systems combined and 

pieced together (Figure 1.25). When designing a complex system, the following basic approach 

should be taken: 

1.	 Consider the layout of the building, equipment, supports, etc. 

2.	 Begin the design at the hood farthest away from the fan. 

3.	 Create a line sketch of the proposed duct system layout (including plan and elevation 

dimensions), fan location, collector location, and equipment locations, with each branch 

and section of main on the line sketch numbered or lettered for convenience. 

4.	 Select from an existing design or design an exhaust hood tailored to suit the operation 

and determine its airflow rate specifications. 

5.	 Create a rough sketch design of the desired hood for each piece of equipment, including 

orientation and elevation of the outlet. 

48	 Fundamentals of Dust Collection Systems 
 

 

Figure 1.25. Demonstration of how a complex exhaust system is a combination 

of branches linking simple exhaust systems [adapted from ACGIH 2010]. 

To begin designing a dust control system, the following are some basic preliminary 

considerations: 

The amount of dust emissions to be collected by a system may be the most important 

design consideration. Different collector systems possess different capabilities at 

removing particulate. 

The overall system pressure (total head) helps to determine the type of collector to use. 

Most fugitive dust applications will have inlet static pressures below 20 inches wg. 

Standard baghouses and cartridge filters are capable of handling this pressure. 

Some applications require higher system pressures (in some cases, pressures exceed 

40 inches wg), and therefore the equipment must be reinforced. 

In most LEVs, square or rectangular dust collector housing designs are adequate; 

however, in high-pressure systems (>40 inches wg), cylindrical housings, which are 

inherently stronger, would be used. 

Once the entire system is laid out including all the hoods, ductwork, and dust collector system(s), 

this information is then used to determine the required fan capacity for the system. Many times, 

fan manufacturers will provide information and assistance with determining the correct fan and 

settings. 

Clay Application 

Particular caution should be exercised in designing a system for any product that contains clay 

dust. Typically, clay products are dried to 10 percent moisture compared to silica products, 

which are dried to 0.5 percent moisture or less. The higher moisture content of the clay becomes 

a significant factor if the conveying air temperature approaches dew point temperature. This will 

usually occur at the wall of the ductwork or baghouse. The surface of clay particles will become 

Fundamentals of Dust Collection Systems 49 
 

 

sticky and adhere to any surface contacted. Over a period of time, plugging or blinding will 

occur. Therefore, additional measures may be required to prevent material buildup, such as 

insulation of additional equipment, duct heaters, location of equipment indoors, etc. 

FANS 

Fans are a critical feature in the design of ventilation systems for dust control. They are used to 

move the air through the ventilation system, whether to create an exhausting or blowing 

ventilation system. In an exhausting system the fan is located at the end or discharge of the 

ventilation system and is used to "pull" air through the entire system. In a blowing system, the 

fan is located at the inlet of the ventilation system and is used to "push" air through the entire 

system [Hartman et al. 1997]. 

There are different types of fans used in ventilation systems, with their selection being dependent 

upon their operating characteristics. Several basics of fan operation need to be understood in 

order to properly select a fan for a ventilation system. 

Fan Operating Characteristics 

The operating characteristics of the fan are provided by the manufacturer. This information can 

be in the format of fan performance tables and/or fan performance curves. Fan performance 

tables provide only the minimum information, i.e., static pressure, airflow, and brake horsepower 

(BHP) required for selecting a fan [Greenheck Fan Corporation 1999]. Static pressure is based 

upon the amount of pressure required to overcome the friction loss of the entire ventilation 

system. Airflow is the amount of air required for the ventilation system. BHP is the horsepower 

required of the motor to operate the fan at the desired static pressure and airflow. 

Fan curves or performance curves are graphs which also describe the performance characteristics 

of the fan. The fan performance curve is provided for a particular model of fan at a given 

revolutions per minute (RPM). There can be a series of fan performance curves to cover the 

performance of a selected fan with each graph representing a different RPM. A typical fan 

performance curve is in the format shown in Figure 1.26, with static pressure on the y-axis and 

airflow on the x-axis. Additionally, BHP curves for the fan can be included on the graph with 

the BHP scale on a separate scaled y-axis. Information about mechanical efficiency and noise 

can sometimes be included, if the manufacturer provides this information [Hartman et al. 1997]. 

50 Fundamentals of Dust Collection Systems 
 

 

Figure 1.26. A typical fan performance curve. Each fan curve 

is associated with a certain fan model at a selected RPM. 

Fans are selected based upon the required static pressure and airflow of the ventilation system. It 

is important that the static pressure and airflow be located in the operating range of the fan 

curve—not in the stalling range of the curve. Figure 1.27 shows a graph presenting the operating 

and stalling ranges for the fan curve. 

Figure 1.27. Fan performance curve showing stalling and operating ranges. 

When the fan operates in the operating range, the airflow passes through the blades smoothly and 

quietly without any eddies. Once the static pressure and airflow are located in the operating 

range of the fan curve, the BHP of the motor required to operate the fan at this condition can be 

determined from the graph. If the static pressure and airflow are located in the stalling range of 

the fan, it will not operate correctly, operating in a condition known as aerodynamic stall. Stall 

Fundamentals of Dust Collection Systems 51 
 

 

occurs when the airflow and resulting air velocity becomes so low that the airflow is not able to 

follow the designed contours of the fan blade. The blades throw the approaching inlet air 

outward to produce high static pressures with low airflows, which creates high turbulence and 

eddies. Thus, the fan will operate noisily and with low efficiency due to the air turbulence and 

eddies caused by this condition [Bleier 1998]. 

Fan Laws 

Several equations can be used to determine the effects to airflow, static pressure, and BHP from 

changes in RPM. These simplified equations are the general fan laws and are applicable as long 

as fan diameter and air density are constant [Bleier 1998]. The laws are represented in the 

following way. 

1

2

1

2

RPM

RPM

CFM

CFM

(1.8) 

2

1

2

1

2

RPM

RPM

SP

SP

(1.9) 

3

1

2

1

2

RPM

RPM

BHP

BHP

(1.10) 

1

2

12 10 50log 

BHP

BHP

NN

(1.11) 

where RPM = speed of fan, revolutions per minute; 

CFM = airflow, cfm; 

SP = static pressure (this can be for total, velocity, or static pressure as long as the pressures used 

are consistent—i.e., all total, all velocity, or all static pressure); 

BHP = horsepower (this can be for air or total horsepower or brake horsepower as long as the 

horsepower used is consistent); and 

N = noise level, decibels. 

The subscripts represent the different fan characteristics—i.e., 1 for the original fan 

characteristics and 2 for the new/modified characteristics. There are also equations that can 

account for changes in fan diameter and air density, but these are beyond the scope of this 

chapter [Hartman et al. 1997]. Further information on changes in fan diameter and air density 

can be found by reviewing the references Bleier [1998] and Hartman et al. [1997]. 

52 Fundamentals of Dust Collection Systems 
 

 

Fan Types 

There are two basic types of fans: axial-flow fans and centrifugal fans. There are also other fan 

designs that use or combine the concepts of axial or centrifugal flow; these are axial-centrifugal 

fans and roof ventilators. Again, the selection of the fan type is based upon the requirements of 

the ventilation system design. 

Axial-Flow Fans 

The axial-flow fan category includes propeller fans, tubeaxial fans, vaneaxial fans, and two-stage 

axial-flow fans. Axial flow fans move the air in a direction that is "axial" or parallel to the axis 

of rotation of the fan. Propeller fans are the most common type. They are generally mounted in 

the wall of the building near a heat source to exhaust the hot air into the outside atmosphere 

(Figure 1.28). 

Figure 1.28. Wall-mounted propeller fan. 

 They can have different drive configurations, being either direct drive or belt 

drive. The wall-mounted fans can also be constructed with shutters that close when the fan is 

turned off. Propeller fans are designed to move large volumes of air at low static pressures 

[Bleier 1998]. 

Tubeaxial fans are used for exhausting air from an inlet duct (Figure 1.29, left). They consist of 

a fan with many blades in a cylindrical housing. The blades are generally shaped as an airfoil to 

help with air movement. The hub diameter can be 30 to 50 percent of the blade outside diameter 

[Bleier 1998]. The housing is connected to the inlet duct and also contains the motor support. 

These fans are used in conditions that require moderate static pressures (higher than those 

required for propeller fans). 

Fundamentals of Dust Collection Systems 53 
 

 

Vaneaxial fans are similar to tubeaxial fans. The vaneaxial fan has a housing that contains guide 

vanes that are oriented parallel to the airflow (Figure 1.29, right). 

Figure 1.29. Typical tubeaxial fan (left) and vaneaxial fan (right). 

These vanes are used to 

recover the tangential airflow velocity from the fan blade to convert it into static pressure. This 

tangential airflow velocity is not recovered as static pressure in propeller or tubeaxial fans (this 

component is lost energy). The hub diameter of a vaneaxial fan is much larger being 50 to 80 

percent of the blade outside diameter [Bleier 1998]. The vaneaxial fan is used for high static 

pressure conditions. 

Two-stage axial-flow fans are basically two axial flow fans configured in series. This 

configuration allows for operation in high static pressure conditions as each fan's operating 

pressure is added together when in series. This fan can be designed to have each fan rotate in the 

same direction with guide vanes located between each of the fans, or the fans may counter-rotate. 

Centrifugal Fans 

The airflow for a centrifugal fan is different from that of axial flow fans. For a centrifugal fan 

the airflow is drawn into a rotating impeller and discharged radially from the fan blade into a 

housing. The resulting flow of air is perpendicular to the axial rotation or parallel to blade 

motion [Hartman 1997] and the housing is used to direct the airflow to the desired location 

(Figure 1.30). 

54 Fundamentals of Dust Collection Systems 
 

 

Figure 1.30. Typical centrifugal fan. 

There are numerous types of centrifugal fans. The flow through the fan is basically the same for 

all types, the difference being in the configuration of the blades. Each blade type has its 

advantages for different applications, as detailed below. 

Airfoil type blades have the best mechanical efficiency and lowest noise level. 

Backward curved blades have slightly lower efficiencies compared to airfoil blades. 

These blades are better suited to handle contaminated air because they are single 

thickness and can be made of heavier material that can resist the effects to fan blades by 

the contaminated air. 

Backward inclined blades have lower structural strength and efficiencies. They are easier 

to produce due to the elimination of the blade curvature. 

Radial tip blades are curved at the tips. These types are used mainly in large diameters 

(30 to 60 inches) under severe conditions of high temperatures with minimal air 

contamination [Bleier 1998]. 

Forward curved blades produce airflow rates higher than other centrifugal fans of the 

same size and speed. This allows for the fan to be more compact than other types of 

centrifugal fans. These fans are often used in furnaces, air conditioners, and electronic 

equipment cooling. 

Radial blades are rugged and self-cleaning, but have low efficiencies. They are suited for 

airflows containing corrosive fumes and abrasive material from grinding operations. 

Fundamentals of Dust Collection Systems 55 
 

 

Other Fan Types 

Axial-centrifugal fans, also called tubular centrifugal or in-line centrifugal fans, use a centrifugal 

fan to move air in an in-line configuration. To accomplish this, the air flows into the inlet and 

makes a 90-degree turn at the fan blade, travels radially along the blade, and at the tip makes 

another 90-degree turn in order to flow out the outlet (Figure 1.31). 

Figure 1.31. Axial centrifugal fan showing airflow [adapted from Bleier 1998]. 

These fans are easily 

installed in-line with the ductwork, and they produce more static pressure than vaneaxial fans of 

the same fan diameter and speed. However, their mechanical efficiency is lower than for 

vaneaxial fans due to the two 90-degree turns required of the airflow. 

Roof Ventilators 

Roof ventilators use either axial or centrifugal fans in their configuration (Figure 1.32). They are 

generally installed in the roof of the mill building and are an integral part of total structure 

ventilation design as described in Chapter 8—Controls for Secondary Sources. Most roof 

ventilators are used for exhausting air from the building, creating an updraft throughout the 

building. However, they can also be used for supplying clean air by blowing it down into the 

building by creating a downdraft if required by the ventilation design. The exhaust discharge can 

be radial or upblast, with upblast used mainly for air laden with oil/grease or dust. Roof 

ventilators using axial fans generally do not require ductwork. However, those using centrifugal 

fans in their units may require some ductwork. 

56 Fundamentals of Dust Collection Systems 
 

 

Figure 1.32. Typical roof ventilators using axial fans (left) and centrifugal fans (right). 

REFERENCES 

th ACGIH [2010]. Industrial ventilation: a manual of recommended practice for design. 27 ed. 

Cincinnati, OH: American Conference of Governmental Industrial Hygienists. 

Bleier FP [1998]. Fan handbook: selection, application, and design. New York: McGraw-Hill. 

Caputo AC, Pacifico MP [2000]. Baghouse system design based on economic optimization. 

Env Prog 19(4):238–245. 

DallaValle JM [1932]. Velocity characteristics of hoods under suction. ASHVE Transactions 

38:387. 

EPA [1995]. AP 42, Fifth Edition, Compilation of air pollutant emission factors, volume 1: 

stationary point and area sources. Environmental Protection Agency. 

[http://www.epa.gov/ttn/chief/ap42]. 

Fletcher B [1977]. Centreline velocity characteristics of rectangular unflanged hoods and slots 

under suction. Ann Occup Hyg 20:141–146. 

Greenheck Fan Corporation [1999]. The basics of fan performance tables, fan curves, system 

resistance curves, and fan laws. Schofield, WI: Greenheck Fan Corp. 

[http://www.greenheck.com/library/articles/10]. 

Hartman HL, Mutmansky JM, Ramani RV, Wang YJ [1997]. Mine ventilation and air 

rd conditioning. 3 ed. New York: John Wiley & Sons, Inc. 

Hinds WC [1999]. Aerosol technology, properties, behavior, and measurement of airborne 

nd particles. 2 ed. New York: John Wiley & Sons, Inc. 

Fundamentals of Dust Collection Systems 57 
 

 

Johnson GQ [2005]. Dust control system design: knowing your exhaust airflow limitations and 

keeping dust out of the system. Powder and Bulk Eng 19(4):51–59. 

Martin CT [1999]. Fine filtration fabric options designed for better dust control and to meet 

PM2.5 standards. Cement Industry Technical Conference IEEE IAS/PCA. pp. 385–393. 

Mycock JC [1999]. Pick the right baghouse material. Power Eng 103(7):43–46. 

Swinderman RT, Marti AD, Goldbeck LJ, Strebel MG [2009]. Foundations: the practical 

resource for cleaner, safer, more productive dust & material control. Neponset, Illinois: Martin 

Engineering Company. 

58 Fundamentals of Dust Collection Systems 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Fundamentals of Dust Collection Systems 7 

CHAPTER 2: 

WET SPRAY SYSTEMS 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

8 Fundamentals of Dust Collection Systems 
 

 

CHAPTER 2: WET SPRAY SYSTEMS 

Probably the oldest and most often used method of dust control at mineral processing operations 

is the use of wet spray systems. In essence, as the fines are wetted each dust particle's weight 

increases, thus decreasing its ability to become airborne. As groups of particles become heavier, 

it becomes more difficult for the surrounding air to carry them off. The keys to effective wet 

spray dust control are proper application of moisture, careful nozzle location, controlling droplet 

size, choosing the best spray pattern and spray nozzle type, and proper maintenance of 

equipment. 

In the vast majority of cases for mineral processing operations, the wet spray system used is a 

water spray system. Although the use of water sprays is a very simple technique, there are a 

number of factors that should be evaluated to determine the most effective design for a particular 

application. The following two methods are used to control dust using wet sprays at mineral 

processing operations. 

Airborne dust prevention, achieved by direct spraying of the ore to prevent dust from 

becoming airborne. 

Airborne dust suppression, which involves knocking down dust already airborne by 

spraying the dust cloud and causing the particles to collide, agglomerate, and fall out 

from the air. 

Most operations will use a combination of both of these methods in the overall dust control plan. 

PRINCIPLES OF WET SPRAY SYSTEMS 

To use wet sprays effectively, it must be remembered that each ore type and application point is 

a unique situation and needs to be evaluated separately to achieve the optimal design. For 

example, wet sprays cannot be used with all ores, especially those that have higher 

concentrations of clay or shale. These minerals tend to cause screens to blind and chutes to clog, 

even at low moisture percentages. Also, water cannot be used at all times throughout the year in 

various climates where low temperatures may cause freezing. 

Water Application 

When water is used to control dust, it has only a limited residual effect due to evaporation, and 

will need to be reapplied at various points throughout the process to remain effective. Overapplication in the amount/volume of moisture can be a problem in all operations and may impact 

the equipment as well as the total process and transportability of the final product if shipped in 

bulk. In most cases, a properly designed spray system using finely atomized water sprays will 

not exceed 0.1 percent moisture application [USBM 1987]; however, systems that address 

prevention over larger areas with larger droplet sprays may add up to 1.5 percent moisture to the 

process. 

Wet Spray Systems 61 
 

 

The vast majority of dust particles created during crushing are not released into the air, but stay 

attached to the surface of the broken material. Therefore, adequate wetting is extremely 

important because it ensures that the dust particles stay attached to the broken material. 

Uniformity of wetting is also an important issue for an effective system. By far the best dust 

reductions can be achieved by spraying the ore with water and then mechanically mixing the ore 

and water together to achieve a uniformity of wetting. 

Ideally, the spray system should be automated so that sprays are only activated when ore is 

actually being processed. For dust knockdown, or suppression, a delay timer may be 

incorporated into some applications to allow the spray system to operate for a short time period 

after a dust-producing event. 

Nozzle Location 

Due to the unique characteristics of each application, there are no hard and fast rules for 

specifically locating spray nozzles in dust control applications; however, the following 

guidelines will contribute to spray system efficiency. 

For wet dust prevention systems, nozzles should be located upstream of the transfer point 

where dust emissions, in most cases, are being created [Blazek 2003]. 

Care should be taken to locate nozzles for the best mixing of material and water 

[Blazek 2003]. 

For airborne dust prevention, the nozzles should be located at an optimum target distance 

from the material—far enough to provide the coverage required but close enough so that 

air currents do not carry the droplets away from their intended target [Blazek 2003]. 

Droplet size also needs to be considered when setting the correct target distance. 

For airborne dust suppression, nozzles should be located to provide maximum time for 

the water droplets to interact with the airborne dust. 

Figure 2.1 illustrates a common dust control application at a conveyor dump point into a bin. 

Figure 2.1. Common dust control application illustrating nozzle positioning [SSCO 2003]. 

62 Wet Spray Systems 

 In 

this dust suppression application, spray nozzles are positioned in a manner which allows the 

spray patterns of the individual nozzles to properly interact with the dust particles and at a 

distance where the droplets will not be carried off by air currents. 
 

 

Controlling Droplet Size 

When using sprays, one of the primary considerations is the droplet size. If the droplet diameter 

is much greater than the diameter of the dust particle, the dust particle simply follows the air 

stream lines around the droplet. If the water droplet is of a size comparable to that of the dust 

particle, contact occurs as the dust particle follows the stream lines and collides with the droplet 

(Figure 2.2). 

Figure 2.2. Effect of droplet size on dust particle impingement. 

For optimal agglomeration, the particle and water droplet sizes should be roughly 

equivalent. The probability of impaction also increases as the size of the water spray droplets 

decreases, because as the size of the droplets decreases, the number of droplets increases 

[Rocha 2005a]. 

When wetting the bulk ore to achieve airborne dust prevention, droplet sizes above 100 

micrometers (μm) (preferably 200 to 500 μm) should be used. In contrast, for airborne dust 

suppression, where the goal is to knock down existing dust in the air, the water droplets should 

be in similar size ranges to the dust particles. The intent is to have the droplets collide and attach 

themselves (agglomerate) to the dust particles, causing them to fall from the air. To achieve this 

goal, droplets in the range of 10 to 150 μm have been shown to be most effective [Rocha 2005b]. 

For context, Table 2.1 relates particle/droplet size to common precipitation classification. 

Wet Spray Systems 63 
 

 

Table 2.1. Particle/droplet size in comparison to common 

precipitation classification [Bartell and Jett 2005] 

Particle size in micrometers Reference Seconds for particle 

(μm) to fall 10 feet 

5,000 to 2,000 Heavy rain 0.85 to 0.90 

2,000 to 1,000 Intense rain 0.9 to 1.1 

1,000 to 500 Moderate rain 1.1 to 1.6 

500 to 100 Light rain 1.6 to 11 

100 to 50 Mist 11 to 40 

50 to 10 Thick fog 40 to 1,020 

10 to 2 Thin fog 1,020 to 25,400 

Methods of Atomization 

Atomization is the process of generating droplets by forcing liquid through a nozzle, which is 

accomplished by one of two methods. 

Hydraulic or airless atomization controls droplet size by forcing the liquid through a 

known orifice diameter at a specific pressure. This method utilizes high liquid pressures 

and produces relatively small- to medium-sized droplets in uniformly distributed fan, full 

cone, or hollow cone spray patterns. Hydraulic fine spray nozzles are preferred in most 

areas because operating costs are lower since compressed air is not required. 

Air atomizing controls droplet size by forcing the liquid through an orifice at lower 

pressures than the hydraulic atomizing method, using compressed air to break the liquid 

into small droplets. This method produces very small droplets and uniform distribution 

in a variety of spray patterns. However, it is more complex and expensive because it 

requires compressed air. In most cases, air atomizing nozzles are effective in locations 

where dust particles are extremely small and the nozzles can be located in close 

proximity to the dust source, although some applications will require large capacity air 

atomizing nozzles to throw their sprays long distances to reach the dust. 

Chemical Additives to Control Droplets 

Surfactants are sometimes used in wet spray applications because they lower the surface tension 

of the water solution, which has the following effects: 

reduced droplet diameter; 

an increase in the number of droplets for a given volume of water; and 

a decrease in the contact angle [Blazek 2003], defined as the angle at which a liquid 

meets a solid surface (θ as shown in Figure 2.3). 

64 Wet Spray Systems 
 

 

Figure 2.3. Contact angle resulting from a liquid meeting a solid surface [NDT 2009]. 

The use of surfactants increases the rate at which the droplets are able to wet or coat dust 

particles; thus less moisture is used to produce the same effects as a typical water application. 

Small amounts of surfactants can be injected into the spray water, typically in a dilution range of 

1:700 to 1:1500, to improve the wetting and subsequent control of dust particles [Swinderman et 

al. 2002]. Despite the effectiveness of chemical additives, it must be noted that they are not 

often used in the metal/nonmetal mining industry based upon several limitations. 

Surfactants are significantly more expensive than a typical water application. 

They can alter the properties of the mineral or material being processed. 

They can damage some equipment such as conveyor belts and seals. 

Surfactant systems require more upkeep and maintenance than typical water systems. 

Surfactants have limited usefulness in the metal/nonmetal mining industry, as opposed to 

in the coal industry, since ore or stone are much easier to wet than is coal due to its 

hydrophobic nature [NIOSH 2003]. 

The effectiveness of chemical additives depends on: 

the type of wetting agent; 

hydrophobic nature of the mineral particles; 

dust particle size; 

dust concentration; 

water pH; and 

minerals present in the water used [Rocha 2005c]. 

NOZZLE TYPES AND SPRAY PATTERNS 

Spray nozzles are categorized by the type of atomization method used and by the spray patterns 

they produce. The most commonly used spray nozzles produce full cone, hollow cone, round, or 

flat fan patterns. Air atomizing nozzles (which utilize compressed air) are typically used to 

produce round or flat fan spray patterns, while hydraulically atomizing nozzles are typically used 

to produce full or hollow cone spray patterns; however, some hydraulically atomizing nozzles 

can also produce flat fan spray patterns. 

Wet Spray Systems 65 
 

 

Air Atomizing Nozzles 

Air atomizing nozzles are sometimes called two-fluid nozzles because they inject compressed air 

into the liquid stream to achieve atomization. Figure 2.4 depicts two styles of air atomizing 

nozzles known as internal mix and external mix. 

Figure 2.4. Typical internal mix nozzle (top) and external mix nozzle (bottom). 

 Internal mix nozzles use an air cap that mixes 

the liquid and air streams internally to produce a completely atomized spray and external mix 

nozzles use an air cap that mixes the liquid and air streams outside of the nozzle. With an 

internal mix nozzle, the atomization air pressure acts against the liquid pressure to provide 

additional liquid flow rate control. With an external mix nozzle, the liquid pressure is unaffected 

by the atomization air pressure. 

Internal mix nozzles can produce either round or flat spray patterns and external mix nozzles 

produce flat spray patterns as illustrated in Figure 2.5. 

Figure 2.5. Typical air atomizing nozzle round spray pattern (top) 

and fan spray pattern (bottom). 

66 Wet Spray Systems 
 

 

Hydraulically atomizing nozzles force liquid through a fixed orifice at high pressures to achieve 

atomization into droplets. The orifice geometry produces various spray patterns and droplet 

sizes. 

Hydraulic Full Cone Nozzles 

Hydraulic full cone nozzles produce a solid cone-shaped spray pattern with a round impact area 

that provides high velocity over a distance (Figure 2.6). 

Figure 2.6. Typical full cone nozzle and spray pattern. 

 They produce medium to large droplet 

sizes over a wide range of pressures and flows. They are normally used when the sprays need to 

be located further away from the dust source [Bartell and Jett 2005]. 

Hydraulic Hollow Cone Nozzles 

Hydraulic hollow cone nozzles produce a circular ring spray pattern and typically produce 

smaller drops than other hydraulic nozzle types of the same flow rate (Figure 2.7). They also 

have larger orifices which results in reduced nozzle clogging. Hollow cone nozzles are normally 

useful for operations where airborne dust is widely dispersed and are available in two different 

designs: whirl chamber and spiral sprays. In most whirl chambers, the spray pattern is at a right 

angle to the liquid inlet; however, in-line designs are also available. Both produce a more 

uniform pattern with smaller droplets (Figure 2.7). 

Figure 2.7. Typical hollow cone whirl chamber nozzle and 

spray pattern. Right angle design shown. 

Wet Spray Systems 67 

Spiral sprays are used when greater water 

flow is needed, resulting in less clogging due to the large orifices. There is also less pattern 

uniformity and larger droplets are created (Figure 2.8) [Bartell and Jett 2005]. 
 

 

Figure 2.8. Typical hollow cone spiral nozzle and spray pattern. 

Hydraulic Flat Fan Nozzles 

Hydraulic flat fan nozzles produce relatively large droplets over a wide range of flows and spray 

angles and are normally located in narrow enclosed spaces (Figure 2.9). These nozzles are 

useful for wet dust prevention systems. Flat fan nozzles are available in three different designs: 

tapered, even, and deflected [Bartell and Jett 2005]. 

Figure 2.9. Typical flat fan nozzle and spray patterns. 

Table 2.2 lists some common dust control application areas and the type of spray nozzle typically 

used for that application. 

Table 2.2. Typical applications by spray nozzle type 

68 Wet Spray Systems 

Air 

Atomizing 

Hydraulic 

Fine Spray 

Hydraulic 

Full Cone 

Hydraulic 

Hollow Cone 

Hydraulic 

Flat Fan 

Typical Liquid Pressures 10–60 psi 30–1,000 psi 10–300 psi 5–100 psi 10–500 psi 

Typical Air Pressures 10–70 psi N/A N/A N/A N/A 

Airborne Dust Suppression 

Jaw crushers • • • 

Loading terminals 

Primary dump hoppers 

Transfer points 

• 

• 

• 

• 

• 

• 

Dust Prevention 

Stackers, reclaimers • • 

Stockpiles 

Transfer points 

Transport areas/roads 

• 

• • • 

• 

• 
 

 

Figure 2.10 illustrates a typical dump application with both dust suppression and prevention 

spray nozzles. 

Figure 2.10. Typical loader dump dust control application. 

 In this case, dumping action produces airborne dust in the upper level of the 

enclosure and the impact of the falling material with the material already in the enclosure causes 

additional dust to become airborne. The dust suppression spray nozzles in the headers at the top 

of the enclosure would be either air atomizing or hydraulic fine spray hollow cone nozzles 

because of the smaller droplet sizes they produce. The dust prevention spray nozzles at the 

lower level of the enclosure would be hydraulic atomizing full cone nozzles because of their 

larger droplet sizes and full cone spray pattern for large surface area coverage. 

Figure 2.11 illustrates a typical conveyor application with both dust suppression and prevention 

spray nozzles. 

Figure 2.11. Typical conveyor dust control application. 

 In this case, dust prevention spray nozzles are positioned above the moving 

conveyor to prevent dust on the material from becoming airborne. Dust suppression nozzles are 

positioned at the discharge end transfer point to control airborne dust. 

Figure 2.12 shows the airborne suppression performance of the different spray nozzles 

performing at different operating pressures. As shown, atomizing sprays are the most efficient 

Wet Spray Systems 69 
 

 

for dust knockdown or suppression, followed by the hollow cone sprays. Hollow cone sprays are 

a good choice for many applications in mineral processing operations because significant 

coverage or wetting of the ore occurs, even at low moisture percentages. Full cone sprays would 

be most applicable in the early stages of the process where the quantity of moisture added is not 

as critical. Flat fan sprays are most appropriate for spraying into a narrow rectangular space 

because less water is wasted by spraying against an adjacent rock or metal surface. 

Figure 2.12. Airborne suppression performance of four types of spray nozzles 

[NIOSH 2003]. Atomizing nozzle is air atomizing, and hollow cone, full cone, 

and flat fan spray nozzles are hydraulically atomizing. 

MAINTENANCE ISSUES WITH WET SPRAY SYSTEMS 

Water Quality 

A fundamental consideration with any wet spray control system is the quality of the water being 

sprayed, with water hardness and cleanliness being of greatest concern. If the available water 

supply contains a high level of minerals, nozzle wear can be accelerated. The user should 

consider nozzles fabricated in stainless steel if this condition is present. A high level of minerals 

in the supply water can also introduce caking, which is a buildup of carbonates on the nozzle, 

and can increase the amount of maintenance required. Although it may not be possible to 

modify the hardness of the water being sprayed, a properly designed and applied maintenance 

program can help to minimize the adverse effects of high levels of minerals in the available 

water supply. 

If spray nozzles become plugged with sediment or debris, they render the spray system 

ineffective. Since the water to be used for spray systems at most mineral processing operations 

is drawn from settling ponds, water purity is a concern. It is recommended to use a water 

70 Wet Spray Systems 
 

 

filtering system to eliminate the possibility of sediment and debris clogging the water sprays. 

Most spraying companies offer filtering systems for this purpose. 

An effective method of supplying water to the system is to use a self-contained water delivery 

system specifically designed for the application. Such a system includes a water pump selected 

to provide water at the specific flow and pressure required for the application. Also included are 

a manual inlet valve, an inlet strainer, a flow switch to prevent the pump from operating without 

any water supply, pressure indicators, a bypass circuit to allow manual adjustment of water flow, 

a manual outlet valve, and a manual or timer-based control panel. A typical water delivery 

system is illustrated in Figure 2.13.

Figure 2.13. Typical self-contained water delivery system [SSCO 2006]. 

Water supply systems can be as simple as one which delivers a fixed water flow rate or one 

which can adjust the water flow rate in reaction to the needs of an application. Today's 

electronic technology offers the ability to sense factors such as the dust loading and 

automatically adjust the liquid flow rate and pressure to optimize the dust control system 

performance. 

In applications where water quality is poor and where the water contains excessive amounts of 

particulate, such as water drawn from settling ponds, additional filtration should be provided. In 

these cases, it is recommended that a duplex basket strainer be used prior to the water delivery 

system inlet. These units provide two basket strainers and a manual valve, which allows the 

operator to switch from one strainer to the other to allow the inactive strainer to be removed and 

cleaned. The strainer mesh should be selected based on the particulate size present in the water 

supply. The function of the strainer is to stop particles that could ultimately clog the spray 

nozzle orifice. For this reason the strainer mesh should be such that it will stop solid particles 

that are larger diameter than the orifice of the spray nozzles in the system. A typical duplex 

basket strainer is shown in Figure 2.14. 

Wet Spray Systems 71 
 

 

Figure 2.14. Typical duplex basket strainer [SSCO Catalog 70]. 

Nozzle Maintenance 

Nozzles require maintenance, regular inspection, cleaning, and even replacement to preserve 

final production quality and to maintain production processes on a cost-efficient basis. The type 

and frequency of the maintenance schedule depends on the particular application. In some 

operations, nozzles can spray usefully after hundreds of hours of operation, while in others, 

nozzles require daily attention. Most nozzle applications fall between these extremes. At a 

minimum, nozzles should be visually surveyed for damage on a regular basis. Additional 

maintenance will depend on application specifications, the water quality used, and nozzle 

material. 

Erosion and Wear 

The gradual removal of material from the surfaces of the nozzle orifice and internal flow 

passages causes them to become larger and/or distorted (Figure 2.15), which can affect flow, 

pressure, and spray pattern. 

Figure 2.15. Nozzle erosion—new versus used [Schick 2006]. 

Corrosion 

The chemical action of sprayed material or the environment causes corrosion breakdown of the 

nozzle material (Figure 2.16). 

72 Wet Spray Systems 
 

 

Figure 2.16. Nozzle corrosion—new versus used [Schick 2006]. 

Clogging 

Unwanted dirt or other contaminants blocking the inside of the orifice can restrict the flow and 

disturb spray pattern uniformity. 

Caking 

Overspraying, misting, or chemical buildup of material on the inside or outer edges of the orifice 

from evaporation of liquid can leave a layer of dried solids and obstruct the orifice or internal 

flow passages (Figure 2.17). 

Figure 2.17. Nozzle caking—new versus used [Schick 2006]. 

Temperature Damage 

Heat may have an adverse effect on nozzle materials not intended for high-temperature 

applications (Figure 2.18). 

Figure 2.18. Temperature damage—new versus used [SSCO 2004]. 

Improper Reassembly 

Misaligned gaskets, over-tightening, or other repositioning problems can result in leakage as well 

as poor spray performance. 

Wet Spray Systems 73 
 

 

Accidental Damage 

Inadvertent harm to an orifice can be caused by scratching through the use of improper tools 

during installation or cleaning (Figure 2.19). 

Figure 2.19. Nozzle damage—new versus used [SSCO 2003]. 

Checking Spray Nozzle Performance 

Visual nozzle inspections alone will not always indicate whether the nozzle is performing to 

specifications. The spray tips pictured in Figures 2.18 and 2.19 illustrate that factors such as 

wear that affect performance are not always obvious. 

Figure 2.20. Good spray tip showing pattern and distribution graph [Schick 2006]. 

Height of bar indicates distribution of water over pattern width and indicates 

relatively uniform flow over the width of the pattern. 

Figure 2.21. Worn spray tip showing pattern and distribution graph [Schick 2006]. 

Height of bar indicates distribution of water over pattern width and indicates an 

increase of and excessive flow in the center of the pattern due to orifice wear. 

74 Wet Spray Systems 
 

 

Performance Testing of Equipment 

Due to the number of possible maintenance issues, performance testing should be done to 

monitor the condition of spray equipment on a regular basis, considering numerous factors. 

Application-specific analysis or measurements. The dust content of the air should be 

surveyed and checked either visually, to determine the effectiveness of the spray at 

removing dust, or by using dust monitoring equipment that measures dust content in air. 

Visual inspection consists of simply observing the dust-producing area to determine if the 

spray system is effectively controlling the dust condition. Because this method of 

monitoring does not quantify the dust loading and is subject to operator interpretation, it 

does not specifically measure the efficiency of the spray system. Dust particle 

monitoring equipment is available that will quantify the dust loading in the problem area 

and provide a more accurate assessment of the system efficiency. 

Flow rate. The eye cannot necessarily detect increased flow, so the flow rate of each 

nozzle should be checked periodically by reading the flow meter or collecting the spray 

in a container. The results should be compared to specifications or to the performance of 

new nozzles. 

Spray pressure. Pressure in the nozzle manifold should be checked using a properly 

calibrated pressure gauge. 

Spray pattern. In many instances visual inspection is adequate for monitoring pattern 

uniformity. Changes caused by orifice damage, clogging, or caking are usually 

noticeable. However, to detect gradual orifice wear, special measuring equipment will be 

required. A flow meter may be used to compare the flow of a new nozzle at a specific 

liquid pressure to the flow of a used nozzle at the same pressure. Nozzle wear causes the 

flow to increase so the flow meter reading is a quantifiable indicator of the degree of 

orifice wear. 

Nozzle alignment. To provide uniform coverage, nozzles should be oriented correctly in 

relation to one another so that all like patterns are parallel. 

REFERENCES 

Bartell W, Jett B [2005]. The technology of spraying for dust suppression. Cement Americas, 

May/June pp. 32–37. 

Blazek CF [2003]. The role of chemicals in controlling coal dust emissions. Presented at the 

American Coal Council PRB Coal Use: Risk Management Strategies and Tactics Course. 

Dearborn, Michigan, June 2003. 

NIOSH [2003]. Handbook for dust control in mining. By Kissell FN. Pittsburgh, PA: U.S. 

Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, NIOSH Information Circular 9465, 

DHHS, (NIOSH) Publication No. 2003–147. 

Wet Spray Systems 75 
 

 

NDT Educational Resource Center, Iowa State University. [http://www.ndt

ed.org/EducationResources/CommunityCollege/PenetrantTest/PTMaterials/surfaceenergy.html]. 

Date accessed: February 13, 2009. 

Rocha E [2005a]. PowerPoint presentation slide # 30 presented by E. Rocha, General Manager, 

Spraying Systems do Brasil Ltda. Spray Technology Workshop for Pollution Control at 

Spraying Systems do Brasil Ltda, São Bernardo do Campo, Brazil. 

Rocha E [2005b]. PowerPoint presentation slide # 43 presented by E. Rocha, General Manager, 

Spraying Systems do Brasil Ltda. Spray Technology Workshop for Pollution Control at 

Spraying Systems do Brasil Ltda, São Bernardo do Campo, Brazil. 

Rocha E [2005c]. PowerPoint presentation slide # 75 presented by E. Rocha, General Manager, 

Spraying Systems do Brasil Ltda. Spray Technology Workshop for Pollution Control at 

Spraying Systems do Brasil Ltda, São Bernardo do Campo, Brazil. 

Schick R [2006]. Spray technology reference guide—understanding drop size. 

[http://www.spray.com]. 

SSCO (Spraying Systems Co. Catalog 70). Industrial spray products. [http://www.spray.com]. 

SSCO (Spraying Systems Co.) 2006. A guide to safe and effective tank cleaning. 

[http://www.spray.com]. 

SSCO (Spraying Systems Co.) 2003. Spray optimization handbook TM–410. 

[http://www.spray.com]. 

Swinderman RT, Goldbeck LJ, Marti AD [2002]. Foundations 3: the practical resource for total 

dust & material control. Neponset, Illinois: Martin Engineering. 

USBM [1987]. Dust control handbook for mineral processing. U.S. Department of the Interior, 

Bureau of Mines. Contract No. J0235005. NTIS No. PB88–159108. 

[http://www.osha.gov/SLTC/silicacrystalline/dust/dust_control_handbook.html]. Date accessed: 

December 13, 2008. 

-

76 Wet Spray Systems 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Wet Spray Systems 77

CHAPTER 3: 

DRILLING AND 

BLASTING 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

78 Wet Spray Systems 
 

 

CHAPTER 3: DRILLING AND BLASTING 

The drilling process is used in both surface and underground mines for blasting operations which 

are conducted to fragment the rock. While the drilling process is similar for both types of mines, 

the different operating environments require specialized techniques to accomplish the same task. 

Drilling operations are notorious sources of respirable dust, which can lead to high exposure 

levels for the drill operator, drill helper, and other personnel in the local vicinity during 

operation. Therefore, dust controls on drills are necessary and involve both wet and dry 

methods. 

Operator cabs are increasingly becoming an acceptable method for protecting the drill operator 

from respirable dust generated by the drilling operation, and the use of cabs is fully discussed in 

Chapter 9—Operator Booths, Control Rooms, and Enclosed Cabs. Enclosing the operator in an 

environmentally enclosed cab is extremely effective. However, the protection provided, is only 

available to the personnel inside the cab. Other personnel working in the vicinity of the drilling 

operations, the drill helper, shotfirer, mechanics, etc., cannot be protected in this manner. They 

can attempt to maintain a work location upwind of the drill to avoid respirable dust, but this is 

not always practical. Therefore, methods for dust control on drilling operations are still required. 

There are two basic methods for controlling dust on drills: either a wet suppression system or a 

dry cyclone/filter type collector. Wet systems operate by spraying water into the bailing air as it 

enters the drill stem. Dust particles are conglomerated as the drill cuttings are bailed out of the 

hole. Dry collectors operate by withdrawing air from a shroud or enclosure surrounding the area 

where the drill stem enters the ground. The air is filtered and exhausted to the atmosphere. 

When dust controls are implemented effectively at drilling operations, both wet suppression and 

dry collection systems can achieve good dust control efficiency. 

This chapter reviews methods for efficient and effective dust control for both underground and 

surface drilling operations. Much of the research for dust control has been conducted throughout 

the past century. The timeline for underground dust control research was during the 1920–1950 

time period, when there was a significant program undertaken by the U.S. Bureau of Mines to 

prevent silicosis in underground miners. While this research could seem outdated, the principles 

still apply today and are confirmed to be still effective by practices in the field. Additionally, 

many of the results from underground research form the basis for dust controls developed for 

surface mine drilling. Surface mine dust control research began around 1980, when it was 

recognized that surface drillers were also susceptible to silicosis. Research continues today to 

discover new methods for dust control for both surface and underground drilling. 

SURFACE DRILLING DUST CONTROL 

Surface mine drilling is accomplished using both rotary and percussion drilling methods. Rotary 

drilling achieves penetration through rock by a combination of rotation and high down pressures 

on a column of drill pipe with a roller drill bit attached to its end. Percussion drilling also 

achieves rock penetration through rotation and down pressure, but with a pneumatic drill, which 

Drilling and Blasting 79 
 

 

contains a piston which delivers hammer blows to the drill column or the drill bit, depending 

upon the location of the drill (top hammer or down-the-hole hammer), eliminating the need for 

the high down-pressures required in rotary drilling. 

Typical holes can be any size up to 15 inches in diameter, with the larger hole diameters 

commonly produced using rotary drill bits. Generally, these holes are oriented vertically, 

although some operations do use angled holes in their blast design, and the holes are drilled in a 

pattern where they are aligned in rows. The type of drilling equipment can range from small 

surface crawler rigs to truck-mounted drills to large track-mounted drill rigs, as seen in 

Figure 3.1. 

Figure 3.1. Illustrations depicting a small surface crawler drill rig, 

a truck-mounted drill rig, and a large track-mounted drill rig, respectively. 

Dust control methods for surface drilling use wet drilling or dry drilling with dust collection 

systems. There are variations to these methods due to the operating environment of the surface 

drilling operation and the type of equipment used, but generally the principles of dust control 

presented in this chapter are applicable to all types of surface drilling; including small crawler, 

truck-mounted, and large track-mounted drills. 

Wet Drilling 

The best method for dust control in surface drilling is to use wet drilling techniques. Wet drilling 

injects water along with the air to flush the cuttings out of the hole as shown in Figure 3.2. 

Testing has shown that this technique can provide dust control efficiencies up to 96 percent 

[USBM 1987]. 

Water injection requires monitoring by the drill operator to efficiently control dust. The amount 

of water required for dust control is not large. Typical water flow rates in wet drilling systems 

generally range from 0.1 to 2.0 gpm, but this varies based on the drill type, geology, and 

moisture level of the material being drilled. For example, testing at a surface mine site showed 

80 Drilling and Blasting 
 

 

that dust control efficiencies greatly increased from 0.2 to 0.6 gallons per minute (gpm), then 

leveled off above this flow rate. However, once the flow rate approached 1.0 gpm at this site, 

operational problems were encountered such as the drill bit plugging and the drill steel rotation 

binding due to the water causing the drill cuttings to become too heavy to be removed by the 

bailing air [USBM 1987]. Therefore, too little water reduces dust control efficiency while too 

much water creates operational problems. The amount of water needed would be dependent 

upon the surface drill type and the material being drilled. 

Figure 3.2. The air and water flow during drilling operation to demonstrate 

water flushing of the drill cuttings. The water flows through the center of the 

drill steel and out the end of the drill bit to remove the cuttings from the drill hole. 

To provide for wet drilling, a water tank mounted on the drill is used to pump water into the 

bailing air, and the water droplets trap dust particles as they migrate up the annular space of the 

drilled hole (the annulus is the open area of the drill hole between the drill steel and the wall of 

the drill hole), thus controlling the dust as the air bails the cuttings. The drill operator controls 

the water flow from the cab, and some cabs are equipped with a meter to indicate water flow 

rate. 

Advantages to Wet Drilling 

A potential advantage to the use of water in percussion drilling operations, besides better dust 

control, is that the use of water may improve the penetration rate of percussion drilling for 

surface mining. There is very little information on wet drilling effects on penetration rate, but 

sources have shown that the use of water while percussion drilling in underground mining can 

Drilling and Blasting 81 
 

 

produce an increase in the penetration rate of the drilling operation [Hustrulid 1982; USBM 

1995]. Since this is applicable for underground mining, it may also correlate to increased 

penetration rates in surface mining situations. Further testing would be required to definitively 

assert that wet drilling would increase penetration rates of surface percussion drilling operations. 

Recommendations for Proper Wet Drilling 

Based on testing results and observational best practices, the following recommendations apply 

to surface drilling using wet drilling techniques. 

In order to operate at close to the optimum water flow rate, the operator should slowly 

increase the amount of water just to the point where visible dust emissions are abated. 

Addition of more water beyond this point will not provide any significant improvement 

in dust control, but will most likely create operational problems such as bit degradation 

(when using tri-cone bits) and possible seizing of the drill stem. Using less water will 

give poor dust control. 

It is important that the water be increased slowly to account for the lag time as the 

air/water/dust mixture travels from the bottom to the top of the hole. 

Continuous monitoring of the water flow during drilling is necessary to provide optimum 

dust control and prevent drill steel binding. 

The water used in the wet drilling system should be filtered to prevent debris from 

plugging the drill's wet drilling system. 

When using a wet suppression system in outside temperatures below freezing, the system 

must be heated when the drill is in operation, and during downtimes the system must be 

drained. For most drilling machines the proximity of the water tank and lines to the 

engine and hydraulic lines is sufficient to prevent freezing during operation (except in 

extreme cold weather situations). The water tank and lines must be drained when the 

drill is not in operation. 

Disadvantages to Wet Drilling 

While wet drilling may be an advantage when percussion drilling, there is a disadvantage to wet 

drilling when rotary drilling. The use of water degrades the tri-cone roller drill bits and shortens 

their lives by 50 percent or more [Page 1991]. This is due to rapid bearing material degradation 

through hydrogen embrittlement and accelerated bit wear. The bit wear is a result of operating in 

the abrasive rock dust-water slurry environment. 

There is a solution to the disadvantage of short bit lives when wet drilling. In order to obtain 

acceptable drill bit life, the water must not reach the drill bit. This can be achieved through 

effective water optimization and water separation. Data gathered by the Bureau of Mines 

(BOM) from a mine where drilling occurred in monzonite, sandstone, limestone, and iron ore 

over a 14-year period showed that drill bit life averaged 1,938 ft/bit when wet drilling without 

water separation. With water separation, the drill bit life increased over 450 percent to an 

average of approximately 9,000 ft/bit [USBM 1988]. 

82 Drilling and Blasting 
 

 

Water Separator Sub 

Water separation can be achieved through the use of a water separator sub. A water separator 

sub uses inertia to remove the injected water from the bailing air. A sub is a short length of a 

drill collar assembly which is placed between the drill bit and the drill steel. This assembly has 

male threads on one end to accommodate the drill steel and female threads on the other for the 

drill bit. It generally incorporates stabilizer wear bars affixed on its circumference which are 

used to center the drill bit in the drill hole. 

Water separation is accomplished by forcing the bailing air stream to make a sharp turn or turns 

within its course, just above the bit (see Figure 3.3). 

Figure 3.3. Internal workings of a water separator sub. 

Basically, the air/water mixture flows down 

through the center of the drill steel, down past the standpipe, and makes a u-turn at the water 

reservoir. The airflow then flows up the outside of the stand pipe, through the slots, at which 

point the flow turns down the inside center of the standpipe, exiting through the nozzles in the 

drill bit. Due to the much higher inertia of the water, it cannot negotiate the turns and thus 

separates from the air. Since the drill stem interior is under positive air pressure, the 

accumulated water ejects through weep holes. These holes are in the separator perimeter above 

the bit and the water ejects into the drill stem annulus. All drill cuttings cleared by the airflow 

travel through this annulus and are wetted. This prevents water from reaching the drill bit and 

inhibits the formation of slurry at the bit/rock interface. 

Testing has demonstrated that dust control efficiencies of up to 98 percent can be obtained using 

the water separator sub while dust control efficiencies of wet drilling without the water separator 

sub were 96 percent [Page 1991]. Most importantly, the use of the water separator sub increased 

bit life. 

Drilling and Blasting 83 
 

 

Dry Drilling 

Dry drilling is accomplished without the use of water for dust control. Dust control is 

accomplished using a dust collection system mounted on the drill. These systems have the 

ability to operate in various climates, i.e., they are not subject to freezing at lower temperatures 

as with the use of water, and they can be up to 99 percent efficient if properly maintained 

[USBM 1987]. There are different types of dust collector configurations used which are 

dependent upon the size of the drill. 

Medium- to Large-Diameter Drill Dust Collection Systems 

Figure 3.4 shows a typical dry dust collection system on a medium- to large-diameter drill. 

Figure 3.4. A basic dry dust collection system on a drill. 

 Drill 

dust is generated by the bailing air, which is compressed air that is forced through the drill steel 

out the end of the drill bit and used to flush the cuttings from the hole (Figure 3.2 depicts a 

similar scenario, but includes water). In a properly operating collection system, these cuttings 

are contained by the drill deck and shroud located over the drilling area. This dusty air from 

underneath the shrouded drill deck is removed by the dust collector system. The collector is 

composed of an exhaust fan and filters which filter the air from underneath the shroud area. It is 

generally self-cleaning, using compressed air to pulse through the filters at timed intervals to 

clean them and prevent clogging. The filtered fine-sized material then drops out the bottom of 

the collector to the collector dump. 

Sources of Dust Emissions from Collection Systems 

Damaged, nonfunctional, or missing dust collector filters are generally the root cause of dusty 

exhaust air from the collector. Other emission sources in the collection system can be caused by 

damaged or worn collector system enclosure components or operational conditions impeding the 

enclosure components from containing and capturing dust. 

84 Drilling and Blasting 
 

 

The integrity of the drill deck shroud and its sealing capability to the ground are critical factors 

in the effectiveness of a dry collection system for medium- to large-diameter drills. Over half of 

the dry dust collector emissions are from shroud and drill stem bushing leakage [USBM 1985]. 

Openings or gaps greater than six inches between the drill deck shroud and ground can 

significantly diminish the dust collector's inlet capture effectiveness [USBM 1986; Zimmer et al. 

1987]. Three areas of vulnerability have been identified related to dry dust collection: 

deck shroud leakage, 

collector dump discharge, and 

drill stem/deck leakage. 

Deck Shroud Leakage Solutions 

With the exception of air track drills, most rock drills have the same basic configuration and 

collector airflow volumes. The only parameter that varies is the shroud leakage area, and such 

variances obviously change the capture efficiency. Some of the most common shroud enclosure 

openings are between the bottom of the shroud and ground, caused by uneven or sloping drill 

bench surfaces and at the corners of the drill deck. 

Rectangular Shroud 

Some shrouds have gaps at the corners of the front side of the shroud, which can be hydraulically 

lifted up so that drill cuttings are not dragged over the hole when the drill moves with its boom 

up. Because most deck shrouds are rectangular and constructed from four separate pieces of 

rubber belting attached to the deck, leakage can also occur as the seams separate from one 

another. To combat this problem, corner flaps can be added, attached with brackets, to help 

preserve the integrity of the shroud (see Figure 3.5), or the shroud can be constructed with a 

single piece of material (rubber belting) encompassing the drill deck perimeter. 

Figure 3.5. Reinforcing flaps added to each corner of 

a rectangular deck shroud to reduce leakage. 

Drilling and Blasting 85 
 

 

Circular Shroud 

In contrast to the use of a typical rectangular shroud, a specialized circular deck shroud can be 

used to effectively accomplish dust capture during dry collection. A circular deck shroud is 

slightly conical in design with no open seams, and steel banding is used to attach the shroud to 

the bottom of the drill deck. The shroud can be hydraulically raised to the drill deck and lowered 

to make contact with the ground. A steel band is also attached to the bottom of the shroud to 

help maintain its shape and provide weight for lowering it to the ground, and guide wires are 

attached to the bottom of the steel band and a hydraulic cylinder (see Figure 3.6). 

Figure 3.6. Circular shroud design. 

 A thin sheet 

rubber material is used on the shroud to give it flexibility, and the shroud has a small trap door, 

operated manually so that the cuttings can be shoveled from inside the shroud without 

compromising capture efficiency. 

During drilling, a large amount of cuttings may be generated and it is frequently necessary to 

raise the circular drill shroud to prevent the cuttings from falling back into the hole. As a result, 

there are times when it is unavoidable to have a broken seal between the shroud and the ground 

or cuttings. Therefore, it is important that the driller make a conscientious effort to keep the 

leakage area to a minimum. This may involve making frequent inspections underneath the 

shroud by raising the drill shroud to visually assess the circumstances. 

Collector to Bailing Airflow Ratio 

Dry collection can be performed most efficiently by maintaining an appropriate dust collector to 

bailing airflow ratio. The dust collector airflow to bailing airflow ratio, which can be 

represented by QC/QB, has been found to be an important indicator of dust collection 

performance. Dust collector airflow (QC) in standard cubic feet per minute (scfm) is the quantity 

of airflow generated by the dust collector fan, which pulls the air from underneath the drill 

shroud. Bailing airflow (QB) in scfm is the quantity of compressed air blown down the drill stem 

through the bit in order to flush the cuttings out of the hole. 

86 Drilling and Blasting 
 

 

Bailing airflow (QB) can be obtained from the drill manufacturer as the quantity airflow rating of 

the drill's compressor, which has units in scfm, while measurements of the collector airflow (QC) 

can be reasonably made by using a hot wire anemometer, vane anemometer, or pitot tube at the 

collector exhaust. More accurate measurements can be obtained by attaching a short (4-foot) 

duct extension to the collector exhaust and inserting the hotwire or pitot tube, to measure the 

airflow in cubic feet per minute (cfm), into a hole made at the halfway point in the duct. This 

extension can be simply made from cardboard and fitted to the outside of the collector exhaust 

duct. The collector airflow can be converted to scfm, but this is not necessary as the corrections 

from the ambient conditions generally have a small effect on the measurement. Also, due to the 

changes in the pressure drop across the filters caused by dust buildup, the quantity of air from the 

dust collector can vary by an amount that is much more significant than changes from standard 

atmospheric conditions. 

Common collector to bailing airflow ratios on operating drilling rigs were found to be as high as 

3:1 in the field. However, 2:1 ratios were more typical of operating collectors with normal filter 

loading [Page and Organiscak 2004]. Poorly operating collectors were found to operate at ratios 

of 1:1 or lower. Testing has shown a notable reduction in dust concentrations escaping through 

the bottom of the deck shroud gap by increasing the dust collector to bailing airflow ratio 

[NIOSH 2006; Organiscak and Page 2005]. Figure 3.7 shows the results of changing the dust 

collector to bailing air ratio at various drill deck shroud gap heights from the ground. 

Figure 3.7. Results of changing dust collector to bailing air ratio at various shroud gap heights. 

As shown, 

the largest decrease in dust levels is observed when the dust collector to bailing airflow ratio is 

increased from 2:1 to 3:1, with further decreases in dust levels as the ratio is increased to 4:1. 

Airflow Maintenance for Dust Control 

The collector and bailing airflows need to be maintained to sustain the high collector to bailing 

airflow ratios that prevent high dust emissions. The bailing airflow from the drill's compressor 

is the maximum rated airflow and generally does not vary much over the life of the drill. 

Therefore, its impact on the collector to bailing airflow ratio is minor compared to the impact of 

the collector airflow. Performing the normal maintenance on the compressor as required by the 

manufacturer will keep it in proper operating condition. 

Drilling and Blasting 87 
 

 

The collector airflow has a more substantial impact on the ratio due to the many components that 

make up the collector. The dust collector airflow can decrease over time through damaged 

components and neglected maintenance. Therefore, proper maintenance of the collector to 

maintain maximum airflow is required. Collector maintenance involves five actions by the 

operator: 

ensuring that filter backflushing is operating properly and within specifications; 

verifying that the intake duct and collector housing are tightly sealed and free of holes; 

changing filters at recommended intervals or when damaged; 

ensuring that the tubing and inlet to the collector are free of obstructions; and 

ensuring that the collector fan is operating properly and within its specified speed. 

Use of Collector to Bailing Airflow Ratios for Dust Control 

The collector to bailing airflow ratios are useful in that they form the basis of a model to predict 

the relative severity of the drill dust emissions and to estimate how much of a reduction is 

possible by measuring several basic parameters of the drilling operation. The parameters needed 

are: dust collector airflow (QC), bailing airflow (QB), drill deck shroud cross-sectional area (AS), 

and shroud leakage area (AL) or an approximate estimate for the leakage area [Page et al. 

2008a,b]. 

The model depicted as a graph in Figure 3.8 (top) shows the relative reductions possible by 

reducing the leakage area [Page et al. 2008a,b]. To demonstrate how the graph can be used, the 

following example is given. 

An operator has a drill rig with drill deck dimensions of 4 feet by 5 feet. The rated 

compressor QB is 260 scfm and QC was measured at 530 scfm which represents a 

collector to bailing airflow ratio QC/QB of approximately 2. The area of the shroud is 

2 calculated by multiplying the width by the length resulting in AS = 20 ft . AL is 

calculated by multiplying the leakage height (LH) in feet by the perimeter of the 

2 shroud which results in AL = LH x 18 ft. Therefore, the ratio AS/AL = 20 ft /(LH x 

18 ft) and can be calculated by estimating LH. 

Since QC/QB = 2, the top graph in Figure 3.8 can be used to show how reducing the 

leakage gap between the shroud and the ground will reduce the severity of the dust 

concentrations. A gap of 14 inches corresponds to AS/AL = 0.95, showing a relative 

3 airborne respirable dust concentration of approximately 16 mg/m , while a gap of 

2 inches corresponds to AS/AL = 6.7, resulting in a relative airborne respirable dust 

3 concentration of approximately 5 mg/m . This example demonstrates that reducing 

the leakage height of the drilling operation will result in a substantial improvement in 

dust reductions. 

It should be noted that any leakage area due to vertical shroud seam gaps should also be included 

by estimating the area for vertical leakage and adding this value to the shroud leakage area (AL), 

but many times vertical leakage may not be significant. Graphs for QC/QB greater than 2 are 

similar with the difference being that airborne respirable dust values at any value of AS/AL will 

88 Drilling and Blasting 
 

 

become smaller as QC/QB increases [Page et al. 2008a], as demonstrated in the comparison of the 

two graphs in Figure 3.8. 

It is important to keep in mind that the calculated value of airborne respirable dust is a relative 

value only and is not as important as the estimated value of AS/AL. The key considerations are 

where on the curve the drill operates, as determined by AS/AL, and which curve is applicable (i.e., 

what value is QC). These determinations will indicate the long-term average improvement that 

can be expected from either increasing the collector airflow (installing clean filters or a larger 

collector) or reducing the amount of shroud leakage. 

To demonstrate, a drill currently operating on the left side of the curves in Figure 3.8 can readily 

make significant airborne respirable dust reductions. 

Figure 3.8. Graphs of the model that represents the severity of dust emissions 

based upon collector to bailing airflow ratios, showing that the respirable dust 

concentrations become lower as the collector to bailing airflow ratio increases. 

Drilling and Blasting 89 

A drill operating on the right side of the 

curves indicates that only minimal reductions are achievable. However, operation on the right 

side of the curves also usually indicates a drill that has good operating dust controls [Page et al. 

2008b]. As stated previously, typical values of QC/QB in actual operation with dirty filters are on 

the order of 2. Therefore, on surface blasthole drill rigs it is important to maintain collector to 

bailing airflow ratios greater than 2, with ratios of 3 or more being more desirable. 
 

 

Air-Blocking Shelf 

The air-blocking shelf is a dust control that has been found to be effective for medium- to largesized track-mounted blasthole drills. However, it would be effective for any drill with a largesized drill shroud with approximate minimum dimensions of 4 feet by 4 feet. A 6-inch-wide 

shelf is placed underneath the drill shroud along the inside perimeter of the shroud. Its purpose 

is to reduce dust emissions from the drill shroud while the drill is operating. It was developed by 

observing the airflow patterns underneath the drill shroud at a drill shroud test facility [Potts and 

Reed 2008; Reed and Potts 2009]. 

Normally during drilling, the airflow pattern occurs as shown in Figure 3.9 (left), and consists of 

the bailing air exiting the drill hole along with an influence from the dust collector. 

Figure 3.9. Qualitative models of airflow patterns underneath the shroud 

without the air-blocking shelf (left) and with the air-blocking shelf (right). 

 The bailing 

air travels from the drill hole through the middle of the shrouded area, maintaining its course 

along the drill steel to the underneath side of the drill table, where it exhibits a coanda effect (a 

moving fluid's tendency to be attracted to a nearby surface) as it fans out across the bottom of 

the drill table and continues down the sides of the shroud. All of this occurs at a high velocity. 

Dust emissions at the ground surface occur when the air strikes the ground and fans out from 

underneath the shroud enclosure. 

The 6-inch-wide shelf is placed along the inside perimeter of the shroud where it disrupts the 

airflow and redirects it toward the center of the enclosure to prevent the air from striking the 

ground (Figure 3.9, right). This redirection of air reduces dust leakage from underneath the 

shroud [Potts and Reed 2008; Reed and Potts 2009]. 

Figure 3.10 shows a shelf installed on the blasthole drill during testing. The shelf was 

constructed of 6-inch conveyor belting material that was bolted to 2-inch light-gauge angle iron. 

The angle iron was then bolted to the inside perimeter of the shroud. A section of shelf (not 

90 Drilling and Blasting 
 

 

pictured) was added to the door flap of the shroud to ensure complete coverage of the parametric 

cross-section. The shelf was installed at a location that is approximately halfway between the 

top of the shroud and the ground surface. Laboratory testing demonstrated that the individual 

shelf pieces, which were located on each side of the shroud, did not have to be installed on the 

same horizontal plane in order to be effective. Rather, coverage of the parametric cross section 

of the shroud seemed to be the important design criterion (i.e., no gaps should be allowed at the 

corners or along the length of the shelf). Installation was simple with two persons able to 

assemble the shelf in less than an hour [Potts and Reed 2008]. 

The air-blocking shelf requires no maintenance once installed, unless it is damaged during 

tramming from hole to hole. Field testing of the air-blocking shelf on blasthole drills has shown 

dust reductions from the shroud ranging from 66–81 percent [Potts and Reed, 2010] with the 

differences being attributable to differing wind directions and rock type drilled. However, it 

3 should be noted that when the dust levels from drilling operations are very low (<0.5 mg/m ), the 

effectiveness of the air-blocking shelf is reduced. This is due to the effect of the many other dust 

sources (dust collector, table bushing, other mining operations, etc.) surrounding the drill which 

can impact the dust levels [Potts and Reed 2010]. 

Figure 3.10. Air-blocking shelf installed on the inside perimeter of the drill shroud of a blasthole 

drill. Note the overlap in the rear corners to eliminate any gaps in the shelf perimeter. 

A disadvantage that was found with the shelf is that material buildup from the drill cuttings can 

occur on the air-blocking shelf (Figure 3.11, left). This can cause significantly higher dust levels 

when lowering the mast due to the additional built-up material falling off the air-blocking shelf 

(Figure 3.11, right). Dust emissions from material falling from the shroud during drill mast 

lowering normally occur without the shelf. Therefore, the shelf does not create a new problem of 

Drilling and Blasting 91 
 

 

dust emissions from mast lowering, but it does have the potential to exacerbate this existing 

problem. This disadvantage can be overcome by installing the shelf at a 45 degree angle from 

the horizontal and constructing it from different material (Figure 3.12). 

Figure 3.11. Buildup of material on the air-blocking shelf (left) and the resulting 

dust emissions that are created during lowering of the drill mast (right). 

Figure 3.12. Air-blocking shelf with modifications incorporated into 

installation. This shelf was installed in short sections in an attempt to prevent 

potential damage that could occur during tramming with the drill mast up. 

Note that each of the sections overlap, eliminating any gaps in the shelf perimeter. 

When installing the shelf at a 45-degree angle, it is important that the shelf width be increased 

from 6 inches to 8 inches to maintain an equivalent 6-inch horizontal width. The shelf should be 

constructed of 1/4-inch high-density polyethylene (HDPE). The HDPE has a slippery surface 

which minimizes adhesion of the dust to its surface. It can be purchased in 4 foot by 8 foot 

sheets and cut to the required dimensions. These modifications eliminate the buildup of material 

on the air-blocking shelf. Additional modifications include shortening the shelf sections to 

prevent possible damage during tramming, and the addition of chains to support the shelf to 

prevent it from sagging. It is important that when using shortened shelf sections that they 

92 Drilling and Blasting 
 

 

overlap to eliminate any gaps that may occur in the air-blocking shelf perimeter. These 

modifications should not diminish the effectiveness of the air-blocking shelf. 

Collector Dump Discharge 

The collector dump cycle can account for as much as 40 percent of the respirable dust emissions 

[USBM 1985]. The dust collector dump cycle operates in one of two modes: trickle mode or 

batch mode. In the first mode (trickle mode), the collector filter cleaning mechanism operates at 

preset time intervals, e.g., every 1 minute. This allows the fine material from the collector to 

trickle out onto the ground. In the second mode (batch mode), the collector cleaning mechanism 

operates when the bailing airflow stops. This condition occurs during intervals when adding or 

removing drill steels, or when removing the drill bit from the hole. The batch mode creates the 

highest dust levels because of the longer accumulation time of fine material in the collector 

before dumping. During the batch mode, the dust is usually emitted into the ambient air when 

the fine material is dropped, which creates significant dispersion. Also, this fine material strikes 

the ground, producing more dust dispersion. Finally, the drill itself and service vehicles 

frequently drive through the dust pile formed by dumping, stirring up clouds of dust. Avoiding 

disturbing the collector dump piles will prevent dust from being entrained into the air. 

Preventing Collector Dump Dust Entrainment 

The dust collector dump point is generally anywhere from 24 to 36 inches above the drill bench. 

Dumping the fine material from this height causes entrainment of the respirable sized fraction of 

this material into the air. To reduce this entrainment and lower the respirable dust concentrations 

at the collector dump point, a piece of brattice cloth can be attached to the dust collector dump 

using a large hose clamp. Figures 3.13 A, B, and C show the installation of the dust collector 

dust shroud. This dust shroud was installed over the existing rubber boot attached to the dust 

collector dump. 

Figure 3.13. Dust collector dump point prior to shroud installation (A). 

Two men installing the shroud onto the dust collector dump point (B). 

 The dust collector dump point after installation of the shroud (C). 

Drilling and Blasting 93 
 

 

This simple procedure of creating a dust collector dump shroud has been shown to be very 

effective in reducing the respirable dust. Respirable dust concentrations measured after 

3 installation of the dust collector shroud ranged from 0.16 to 0.24 mg/m . Figure 3.14 shows the 

reduction of the respirable dust concentrations near the collector dump point with and without 

the dust collector shroud. 

Figure 3.14. Comparison of dust concentrations of uncontrolled dust 

collector dump to shrouded dust collector dump. 

It can be seen that the respirable dust generated by the dust collector 

dump point can be reduced by between 63 and 88 percent using the shroud [Reed et al. 2004]. It 

should be noted that this reduction is highly dependent upon wind direction and wind speed. 

Advantages to this method of respirable dust reduction are that the material is inexpensive and 

requires almost no maintenance. If the shroud becomes damaged, it can easily be replaced in 

10–15 minutes requiring little, if any, downtime for the drill. 

To install the shroud, the length of brattice cloth (or similar material) should be sufficient to 

allow it to extend from the dust collector dump point to the ground. It should be cut so that it is 

only long enough to just touch the ground when the drill is lowered. When wrapping the cloth 

around the dust collector dump, the overlap should be placed so that it is on the outside of the 

dust collector dump (i.e., it should be visible as the installer looks directly at the dust collector 

dump, as shown in Figure 3.13 C). This overlap allows the cloth to expand as fine material is 

dumped to the ground, while containing the entrained respirable fraction within its confines. 

Placement of the overlap on the outside also keeps the fine material off the drill tracks, which 

otherwise could cause re-entrainment of the respirable size fraction of the material when the drill 

starts in motion. 

94 Drilling and Blasting 
 

 

Drill Stem/Deck Leakage 

Drill cuttings and dust leaking through the drill table bushing can be a large source of respirable 

dust exposure to the drill operator and surrounding personnel. The table bushing, when new, 

generally prevents drill cuttings from coming through the drill deck. However, the table bushing 

is susceptible to wear during drilling operations, creating gaps between the bushing and the drill 

steel and deck surfaces which can allow the drill cuttings and dust to escape from underneath the 

shroud. 

Preventing Drill Stem/Deck Leakage 

In an attempt to minimize this source of dust, many drill operations install a rubber shield 

(generally a piece of used conveyor belting) underneath the drill deck covering the area of the 

table bushing. A hole is placed in this shield to allow the drill bit to pass through. This shield is 

also susceptible to wear and damage, and over time dust emissions through the drill table 

bushing soon occur. Figure 3.10 shows this rubber shield, which is located at the drill bit 

underneath the drill deck. 

An innovation to help keep dust from escaping around the drill stem/deck is the Air Ring Seal 

(AIRRS). The AIRRS is a nonmechanical, virtually maintenance-free system consisting of a 

donut-shaped compressed air header with closely spaced holes (see Figure 3.15) along its inside 

perimeter [Page 1991]. When compressed air is supplied to the AIRRS, high-velocity air jets are 

produced through the holes. These jets are directed across the opening to be sealed in order to 

impede movement of dust particles through the opening. 

The AIRRS is installed underneath the drill deck at the location where the drill steel penetrates 

the drill deck, positioned so that the drill bit and steel penetrate the inside diameter of the 

AIRRS. The diameter of the AIRRS should be as small as possible, but maintaining clearance 

for the drill bit and table bushing is an important consideration to minimize damage when raising 

the drill steel to remove the table bushing and drill bit. It is also very important that the drill's 

dust collector be maintained to achieve a collector to bailing airflow ratio of at least 2 to 1; 

otherwise more dust will be generated from the shroud area when using the AIRRS. 

On surface mine drills equipped with the AIRRS, it successfully and significantly reduced 

respirable dust levels, as well as helped to keep the drill deck clean [Page 1991]. Solid-mounting 

the AIRRS flush with the deck virtually eliminates all large cuttings as well as the visible dust 

above the deck. The amount of air supplied to the AIRRS would be limited to what is available 

from the drill's compressor. An air pressure of 30 psi with 1/16-inch holes in the AIRRS was 

successful in reducing respirable dust levels [Page 1991]. However, this amount of air would be 

highly dependent upon the drill configuration. 

In addition to using the AIRRS, installing a low-clearance deck bushing can minimize drill 

cuttings through the drill deck by reducing the gap size through which cuttings can escape. 

Replacement of the deck bushing as necessary can also enhance the performance of the air ring 

seal. 

Drilling and Blasting 95 
 

 

Figure 3.15. Air ring seal used to impede the movement of dust particles through an opening. 

Small Diameter Drill Dust Collection Systems 

A dust collection system used for small- to medium-diameter drills, small surface crawler, or 

"buggy" drills is shown in Figure 3.16. A schematic shows the operation of this type of system. 

The difference from the large diameter drill dust collection system is that this system collects all 

the drill cutting material, sends it to a large separation cyclone on the drill boom to drop out the 

large diameter material, and then sends the remainder to the dust collector at the back of the drill 

where the fine-sized material is discharged. 

Maintenance of the dust collection system is important in sustaining efficient dust control with 

these systems. Two important maintenance issues are: 

filters should be replaced before they become clogged; and 

dust leakage from the bushing at the drill steel and the shroud device can be severe if not 

properly maintained (this has been shown to be even more important to dust control than 

the seal between the shroud device and the rock) [USBM 1956]. 

96 Drilling and Blasting 
 

 

These drills may also contain a water misting system for dust control. This system includes a 

tank containing a dust suppressant mixture which is injected into the primary and fine collectors 

to reduce dust from their discharge [Sandvik 2005]. Monitoring the dust suppressant is 

important to maintain the integrity of the dust collection system. 

Figure 3.16. Typical dust collection system used by small crawler or "buggy‖ drills. 

UNDERGROUND DRILLING DUST CONTROL 

Underground drilling is generally accomplished using percussion drilling with small diameter 

holes (up to 3 inches). Depending upon the type of underground mining method used, these 

holes can be oriented in almost any direction. Generally the holes for a blast are consistently 

oriented horizontally or vertically and are drilled in a symmetrical pattern. The type of drilling 

equipment used can range from jackleg drills to stopers to jumbos which operate two to three 

drill booms, as shown in Figure 3.17. The most common method of dust control for 

underground drilling is using wet drilling techniques. Dry collectors have been used in the past, 

but are not commonly used due to the bulkiness of the collectors and their associated 

maintenance issues [Westfield et al. 1951]. 

Drilling and Blasting 97 
 

 

Figure 3.17. Illustrations showing a typical jackleg drill, 

stoper drill, and 3-boom jumbo, respectively. 

Wet Drilling 

Wet drilling uses water to flush the drill cuttings from the hole. Figure 3.18 illustrates the water 

being forced through the center of the drill steel, out the end of the drill bit, and back through the 

drill hole, forcing the cuttings out of the hole. The water pressure required to accomplish 

flushing of the drill hole is equal to the air pressure required for drilling, or, at a minimum, at 

least 10 psi less than this amount of air pressure [Hustrulid 1982]. 

Figure 3.18. Water flow during a drilling operation to demonstrate water 

flushing of the drill cuttings. The water flows through the drill into the center of the 

drill steel and out the end of the drill bit to remove the cuttings from the drill hole. 

98 Drilling and Blasting 
 

 

Wet drilling has been found to be the best method of dust control, with dust reductions ranging 

from 86 to 97 percent depending upon the type of drilling involved [USBM 1921; USBM 1939]. 

The angle of the drill hole directly impacts wet drilling's effectiveness, with the amount of dust 

reduction being less for overhead vertical holes. Angle holes from the vertical were found to 

produce 50–60 percent less dust than overhead vertical holes [USBM 1938a]. The reason the 

overhead vertical holes produce more dust is due to the short amount of time that water contacts 

the drill hole face and the increase in sludge dropping through the air and running down the drill 

steel. The sludge running down the drill steel gets slung into the air from the rotation of the 

steel. Methods used to reduce dust from overhead vertical holes include [USBM 1938c]: 

increasing the amount of water used in wet drilling; 

creating a trap to capture the sludge from the drillhole and divert it away from the drill 

steel; 

designing drills that reduce the amount of air leaking from the front head along the drill 

steel; and 

preventing the exhaust air generated by the drill from dispersing sludge into the air. 

The relationship of water flow to dust concentrations was tested in both stoper and drifter drills. 

For stoper drills, as the water flow increased to 1.3 gpm, it was found that the dust concentrations 

decreased rapidly. Above 1.3 gpm the dust concentrations decreased at a slower rate. This 

phenomenon was also seen in drifter drills, but at a water flow rate of 1.0 gpm. Therefore, it was 

recommended that the optimal water flow should be 1.3 gpm for stoper drills and 1.0 gpm for 

drifter drills [USBM 1938c]. Additionally, there are advantages gained through wet percussion 

drilling such as less drill steel breakage and greater penetration rate with wet drilling than with 

dry drilling [USBM 1921]. 

To characterize the typical dust emissions during wet drilling, it was shown through testing that 

the first 1 or 2 feet of drill hole was the dustiest when wet drilling [USBM 1938b]. After the first 

1–2 feet was completed, the dust concentrations dropped rapidly and maintained a constant level. 

This study also demonstrated that collaring a hole was not responsible for the high dust 

concentrations, since the holes were precollared prior to testing [USBM 1938b]. 

Water Mists and Foams 

Water mists, which use a mixture of compressed air and water, and foams injected through the 

drill steel, have also been shown to reduce dust concentrations by 91–96 percent, respectively 

[USBM 1982a,b]. However, the limited improvement in dust reduction by comparison to wet 

drilling does not justify the cost involved with using these methods. 

Wetting Agents 

The use of "wetting" agents or additives to the water used in drilling has also been evaluated, 

without much success. Results showed that wetting agent solutions applied at varying flow rates 

provided better dust control [USBM 1943]. However, the dust control provided by wet drilling 

alone was so good that the additional dust reduction from the use of wetting agent solutions was 

insignificant. The claim that wetting agents can improve penetration rate was shown to be 

Drilling and Blasting 99 
 

 

unsubstantiated due to the inconsistent drilling rates (ranging from -24.9 to +57.5 percent) and 

the dependency upon drilling circumstances [USBM 1943]. 

External Water Sprays 

External water sprays, depicted in Figure 3.19, have been used in the past, but when dry drilling 

they only produced dust reductions up to 25 percent compared with dry drilling alone—i.e., 

drilling without the use of any water [USBM 1921]. 

Figure 3.19. Drill boom showing external water sprays directed onto the drill steel. 

In an attempt to improve dust reductions, an 

external water spray device, a ring which contained spray holes on the inside diameter for dust 

control during dry drilling, was developed and tested. This device was slipped over the drill steel 

and sprayed water on the drill steel outside of the hole. It produced dust reductions that were 

variable, ranging from 75 to 88 percent, over dry drilling [USBM 1939]. Shrouding of the 

drilling area was tested through the addition of a rubber shroud mounted at the area at the end of 

the boom to surround the external spray and drill bit. It was shown to improve dust reduction to 

53 percent [USBM 1982a]. Although these devices were an improvement over external water 

sprays, results showed that these devices along with external water sprays could not be used to 

replace wet drilling and maintain proper dust control during drilling operations. They could be 

used to enhance dust control in conjunction with wet drilling. 

RESPIRABLE DUST CONTROL FOR BLASTING 

Blasting is a controlled operation which generally occurs intermittently at mining operations and 

is used to fragment the rock being mined. This is an important step in industrial mineral 

production as it is required to break up the mineral product, and the effectiveness of blasting may 

100 Drilling and Blasting 
 

 

be a significant factor when optimizing the fragmentation and cost of the crushing and grinding 

circuit [Hustrulid 1999]. While blasting seemingly generates large amounts of dust, the 

operation occurs infrequently enough that it is not considered to be a significant contributor to 

particulate matter less than 10 micrometers (μm), or PM10 [EPA 1991; Richards and Brozell 

2001]. Testing has been conducted to create an estimator for the amount of dust generated from 

blasting. The estimator is shown as the following equation: 

e 0.00050 A

1.5

where e = total suspended particulate emission factor (lb/blast); and 

2 A = area blasted (m ) as long as hole depth does not exceed 70 feet. 

To estimate the PM10 emissions from blasting, the following equation is used: 

PM10 e x 0.5

where PM10 = pounds per blast of particulate matter less than 10μm; and 

e = total suspended particulate emission factor (lb/blast). 

It must be noted here that the U.S. EPA determined that this estimator should only be used for 

guidance. More reliable emission factors for particulate emissions should be based upon sitespecific field testing in order to determine the amount of particulate matter generated from 

blasting [EPA 1998]. 

Blasting Dust Control Measures 

As a result of blasting being considered an insignificant dust source, there is very little 

documentation for dust control of blasting operations. There are five methods of dust 

suppression that can be used for the control of dust during blasting, many of which are effective 

for underground mining only [Cummins and Given 1973]: 

wetting down the entire blasting area prior to initiating the blast; 

the use of water cartridges alongside explosives; 

the use of an air-water fogger spray prior to, during, and after initiating the blast; 

the use of a filtration system to remove pollutants from the air after the blast; and 

dispersal and removal of the dust and gases using a well-designed ventilation system. 

Wetting Down Blasting Area 

A common method of dust control for blasting operations is to wet down the entire blasting area 

prior to initiating the blast. This procedure minimizes dust being entrained into the air from the 

blasting activity by allowing it to adhere to the wet surfaces [Cummins and Given 1973]. This 

method has been shown to be effective for dust control during blasting in underground mines. It 

could also be effective for surface mining depending upon the time interval between watering 

and blast initiation, with the possibility of exposure to the surface atmospheric conditions 

causing the moisture to quickly evaporate and rendering the watering ineffective. However, 

Drilling and Blasting 101 
 

 

safety considerations may preclude wetting the blast area during the "tying in" phase of the blast 

where all the individual hole charges are connected together prior to blasting. 

Water Cartridges 

Water ampoules or cartridges which are inserted into the blasthole with the explosive have been 

used successfully for dust reduction in past underground coal mining blasting operations 

[ILO 1965]. The water cartridges consist of a properly sized plastic bag which is prefilled with 

water or can be filled in the hole. The cartridges can be placed in front of, alongside, or behind 

the explosive without causing any adverse effects to fragmentation. There is another type of 

cartridge which can be used in the place of stemming as shown in Figure 3.20. 

Figure 3.20. A typical blasthole containing an explosive charge utilizing 

a water cartridge to suppress dust generated during blasting. 

This cartridge 

uses a PVC bag which is inserted into the hole after the explosive and is then filled with water to 

maintain a tight seal with the blasthole. In coal mining operations, the use of these cartridges is 

claimed to have reduced dust by 40–60 percent. 

Fogger Sprays 

In the past, many underground operations used a fogger type spray in the heading where the 

blasting is conducted, shown in Figure 3.21. This setup uses both compressed air and water 

forced through a nozzle to create a fogged-in blast area. The nozzle is located at an approximate 

distance of 100 feet from the face, is turned on prior to blast initiation, and remains operational 

for 20 to 30 minutes after the blast [ILO 1965]. While a fogger type spray is effective for 

underground blasting, it is not known if such a system would be viable for surface blasting. 

102 Drilling and Blasting 
 

 

Figure 3.21. A fogger spray used to create a mist for dust 

suppression in the heading where blasting will occur. 

Air Filtration Systems 

Another dust control method that has been used in underground mining operations in the past is 

to filter the return air of the ventilating air in the blasting area as seen in Figure 3.22. One such 

filtration unit, used in South Africa, consists of a filter and a bed of vermiculite treated with 

sodium carbonate and potassium permanganate which removes the dust and nitrous fumes from 

blasting [Cummins and Given 1973]. Figure 3.23 shows another method, which is to place 

filters inside the exhausting ventilation duct with water sprays, which spray on the filters and are 

oriented in the same direction as the airflow. The filters are only used during blasting, and the 

duct containing the filters is approximately twice the diameter of the ventilation tubing 

[ILO 1965]. Also, dry filters have been used successfully in the past for the same purpose. 

Drilling and Blasting 103 
 

 

Figure 3.22. A filtration unit, located adjacent to the blast heading, used for filtering 

the contaminated ventilation air from the blast heading after blasting occurs. 

Figure 3.23. A filtration unit, located in the blast heading, used for filtering the 

contaminated ventilation air from the blast heading after blasting occurs. 

104 Drilling and Blasting 
 

 

Ventilation System Dispersal 

The most common method of dust control is to allow the dust and gases from blasting to be 

dispersed and removed through the ventilation system in the case of underground operations, or 

through atmospheric dispersion in the case of surface operations. Underground operations 

generally schedule the blasting during off-shift times to allow sufficient time for the area to 

ventilate, to disperse, and to remove the dust and gases from the blasting [Cummins and Given 

1973]. If off-shift blasting is not feasible, then areas affected by the blasting should be cleared 

and work should not commence until the dust and gases are removed. 

The amount of time for dust and gas dispersal is mine-site specific, depending upon the 

efficiency of the mine's ventilation system. Therefore, it is important that the mine's ventilation 

system be maintained in good operating condition to optimize dust and gas removal and to 

minimize the time for the removal. 

At surface operations, the area is cleared of personnel just prior to blasting. Scheduling the blast 

to take into consideration the meteorological conditions, i.e. low wind speed and low inversion 

potential, can be used to minimize the impacts of dust generation from blasting. Generally the 

dispersion of dust and gases occurs quickly after the blast, depending upon the wind speed and 

direction, and work is not allowed in the affected area until dispersion is completed. 

Additionally, it has been noted that the use of multi-delay detonators to initiate the individual 

explosive charges in millisecond time intervals may reduce dust generation from blasting, but 

this has not been verified [Miller et al. 1985]. 

REFERENCES 

Cummins AB, Given IA, eds. [1973]. SME Mining engineering handbook, Vol. 1. New York: 

Society of Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum 

Engineers, Inc. 

EPA [1991]. Review of Surface Coal Mining Emissions Factors. U.S. Environmental Protection 

Agency, Office of Air Quality Planning and Standards, Research Triangle Park, North Carolina, 

July. 

EPA [1998]. Review of Emission Factors for AP—42 Section 11.9 Western Surface Coal 

Mining. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, 

Research Triangle Park, North Carolina, September. 

Hustrulid WA, ed. [1982]. Underground mining methods handbook. New York: Society of 

Mining Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers, 

Inc. 

Hustrulid WA [1999]. Blasting principles for open pit mining, Vol. 1—General design concepts. 

Rotterdam, Netherlands: A.A. Balkema. 

ILO [1965]. Guide to the prevention and suppression of dust in mining, tunnelling, and 

quarrying. International Labour Office, Geneva, Switzerland; Atar, South Africa. 

Drilling and Blasting 105 
 

106 Drilling and Blasting 

Miller S, Emerick JC, Vogely WA [1985]. The secondary effects of mineral development. 

Economics of the mineral industries, chapter 5.15B. Littleton, Colorado: Society of Mining, 

Metallurgy, and Exploration. 

NIOSH [2006]. Technology news 512: Improve drill dust collector capture through better 

shroud and inlet configurations. Pittsburgh, PA: U.S. Department of Health and Human 

Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety 

and Health Publication No. 2006–108. 

Organiscak JA, Page SJ [2005]. Development of a dust collector inlet hood for enhanced surface 

mine drill dust capture. Inter J of Surface Min, Rec, and Env. 19(1):12–28. 

Page SJ [1991]. Respirable dust control on overburden drills at surface mines. Coal mining 

technology, economics, and policy. Session papers from the American Mining Congress Coal 

Convention, Pittsburgh, Pennsylvania, June 2–5, pp. 523–539. 

Page SJ, Organiscak JA [2004]. Semi-empirical model for predicting surface coal mine drill 

respirable dust emissions. Inter J of Surface Min, Rec, and Env 18(1):42–59. 

Page SJ, Listak JM, Reed WR [2008a]. Drill dust capture-,mprove dust capture on your 

surface drill. Coal Age. 

Page SJ, Reed WR, Listak JM [2008b]. An expanded model for predicting surface coal mine 

drill respirable dust emissions. Inter J of Surface Min, Rec, and Env 22(3):210–221. 

Potts JD, Reed WR [2008]. Horizontal air blocking shelf reduces dust leakage from surface drill
	

shroud. Transactions of the Society of Mining, Metallurgy, and Exploration. 

Littleton, Colorado: Society for Mining, Metallurgy, and Exploration, Inc. 324:55–60. 

Potts JD, Reed WR [2010]. Field evaluation of air-blocking shelf for dust control on blasthole 

drills. Submitted to Mining Engineering, 2010.
 

Reed WR, Organiscak JA, Page SJ [2004]. New approach controls dust at the collector dump 

point. Coal Age, June, pp. 20–22. 

Reed WR, Potts JD [2009]. Improved drill shroud capture of respirable dust utilizing air nozzles 

underneath the drill deck. Transactions of the Society of Mining, Metallurgy, and Exploration. 

Littleton, Colorado: Society for Mining, Metallurgy, and Exploration. 326:1–9. 

Richards J, Brozell T [2001]. Compilation of National Stone, Sand and Gravel Association 

Sponsored Emission Factor and Air Quality Studies 1991–2001. Arlington, Virginia: National 

Stone, Sand and Gravel Association. 

Sandvik [2005]. Tamrock ranger drills feature rock pilot, dust minimizor systems for enhanced 

and cleaner operation. Atlanta, Georgia: Sandvik Mining and Construction. 

[http://www2.sandvik.com/__C1256A1E004428BE.nsf/0/7C249CA00F258C2D85257062000D
 

57EC].
 

USBM [1921]. Dust reduction by wet stopers. By Harrington D. U.S. Department of the 

Interior, Bureau of Mines Report of Investigations 2291. 
 

 

USBM [1938a]. Effect of angle of drilling on dust dissemination. By Brown CE, Schrenk HH. 

U.S. Department of the Interior, Bureau of Mines Report of Investigations 3381. 

USBM [1938b]. Relation of dust concentration to depth of hole during wet drilling. By 

Littlefield JB, Schrenk HH. U.S. Department of the Interior, Bureau of Mines Report of 

Investigations 3369. 

USBM [1938c]. Relation of dust dissemination to water flow through rock drills. By Brown 

CE, Schrenk HH. U.S. Department of the Interior, Bureau of Mines Report of Investigations 

3393. 

USBM [1939]. Dust produced by drilling when water is sprayed on the outside of the drill steel. 

By Johnson JA, Agnew WG. U.S. Department of the Interior, Bureau of Mines Report of 

Investigations 3478. 

USBM [1943]. Use of wetting agents in reducing dust produced by wet drilling in basalt. By 

Johnson JA. U.S. Department of the Interior, Bureau of Mines Report of Investigations 3678. 

USBM [1951]. Roof bolting and dust control. By Westfield J, Anderson FG, Owings CW, 

Harmon JP, Johnson L. U.S. Department of the Interior, Bureau of Mines Information Circular 

7615. 

USBM [1956]. Dust control in mining, tunneling, and quarrying in the United States. By 

Owings CW. U.S. Department of the Interior, Bureau of Mines Information Circular 7760. 

USBM [1982a]. An evaluation of three wet dust control techniques for face drills. Page SJ. 

U.S. Department of the Interior, Bureau of Mines Report of Investigations 8596. 

USBM [1982b]. Evaluation of the use of foam for dust control on face drills and crushers. By 

Page SJ. U.S. Department of the Interior, Bureau of Mines Report of Investigations 8595. 

USBM [1985]. Quartz dust sources during overburden drilling at surface coal mines. By 

Maksimovic SD, Page SJ. U.S. Department of the Interior, Bureau of Mines Informational 

Circular 9056. 

USBM [1986]. Investigation of quartz dust sources and control mechanisms on surface mine 

operations, Vol. I—Results, analysis, and conclusions. By Zimmer RA, Lueck SR. U.S. 

Department of the Interior, Bureau of Mines Final Contract Report, NTIS: PB 86–215852, 72 

pp. 

USBM [1987]. Optimizing dust control on surface coal mine drills. By Page SJ. U.S. 

Department of the Interior, Bureau of Mines Technology News 286. 

USBM [1988]. Impact of drill stem water separation on dust control for surface coal mines. By 

Page SJ. U.S. Department of the Interior, Bureau of Mines Technology News 308. 

Drilling and Blasting 107 
 

 

USBM [1995]. The reduction of airborne dust generated by roof bolt drill bits through the use of 

water. By Sundae LS, Summers DA, Wright D, Cantrell BK. U.S. Department of the Interior, 

Bureau of Mines Report of Investigations 9594. 

Zimmer RA, Lueck SR, Page SJ [1987]. Optimization of overburden drill dust control systems 

on surface coal mines. Inter J of Surface Min 1(2):155–157. 

108 Drilling and Blasting 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Drilling and Blasting 109 

CHAPTER 4: 

CRUSHING, MILLING, 

AND SCREENING 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

110 Drilling and Blasting 
 

 

CHAPTER 4: CRUSHING, MILLING, AND SCREENING 

Mineral crushing, milling, and screening operations can be major sources of airborne dust due to 

the inherent nature of size reduction and segregation processes. Control of dust generated by 

these operations can be achieved with proper analysis of the sources, identification of appropriate 

control technologies, and consistent application and maintenance of selected controls. 

Worker exposure may be managed through engineering controls to suppress or enclose the dust 

sources—as described below—or by isolating the worker from the dust source, as discussed in 

Chapter 9—Operator Booths, Control Rooms, and Enclosed Cabs. Administrative controls such 

as operating procedures, work practices, and worker training are also commonly applied to 

supplement engineering controls. Until feasible engineering and administrative controls are 

installed, or when they do not achieve the desired level of exposure reduction, and during 

maintenance, repair, and other unusual operating conditions, personal protective equipment may 

also be necessary. 

As with any process component, installation of the selected dust controls, even if they are 

appropriate, will not guarantee continuing effective performance. The performance of installed 

dust control systems, which often represent large capital expenditures, should be periodically 

evaluated, maintained, and, when necessary, modified to maximize performance. For example, 

the effectiveness of dust control systems installed to protect worker health can only be 

demonstrated by collecting personal air samples for comparison to the occupational exposure 

limit for the substances in question. 

Different mineral processing equipment generates different amounts of dust emissions. Relative 

dust emission rate ratios (setting the primary crusher emission rate as the baseline) for common 

mineral processing equipment are presented in Table 4.1. 

Table 4.1. Relative emission rate ratios of crushing and screening equipment 

Crushing, Milling, and Screening 111 

Equipment Relative Emission Rate Ratio 

Primary crusher 1 

Secondary crusher (No data, tertiary crusher rate would be an upper limit) 

Tertiary crusher (dry) 51 

Tertiary crusher (wet) 2 

Screen (dry) 214 

Screen (wet) 12 

This ranking is based on EPAestimated particulate emissions for crushed stone operations, and is presented only to illustrate 

the relative magnitude of the various dust sources [EPA 2003]. It can be seen that emissions 

increase as the size of the material processed decreases, as one would expect. 
 

 

PREVENTION AND SUPPRESSION APPLICATIONS 

Wet Control Methods 

Wet dust control systems can be very effective and are less costly to install and operate, but they 

may not be feasible due to characteristics of the mineral, subsequent processing steps, or 

customer specifications. Also, when operations are in northern climates, freeze protection is 

necessary, and ice buildup can create additional safety hazards for workers. 

As discussed more fully in Chapter 2—Wet Spray Systems, the use of water to control dust may 

be classified into prevention applications and suppression applications. Prevention is the 

application of water to prevent dust from becoming airborne. Suppression is the use of water to 

wet dust particles which have already become airborne, increasing their mass and causing them 

to settle more rapidly. 

In general, prevention is more effective than suppression [NIOSH 2003; USBM 1978]. 

However, when the wetted material is subject to further size reduction as in crushing operations, 

effective prevention requires application of additional water to the dry—and larger—surface area 

of the material exposed by the size reduction process. This may create a complication because 

additional application of water to improve prevention may cause the material to become too wet, 

interfering with efficient handling, subsequent processing operations, or product specifications. 

As a result, some trade-off between wet dust control and process efficiency is often unavoidable. 

This trade-off can necessitate the use of other control approaches to achieve acceptable 

conditions. 

Suppression of respirable airborne dust using water, usually through sprays directed into the dust 

cloud, is not always highly efficient. It is difficult with hydraulically atomizing spray nozzles to 

produce water droplets small enough to suppress respirable particles effectively. Appropriately 

sized water droplets can be produced with air-atomizing nozzles, but this method requires a 

source of compressed air to atomize the water, and the very small nozzle orifices are subject to 

clogging, requiring increased maintenance attention. Additionally, air-atomized spraying, due to 

the volume of the air/water mixture released, typically requires an enclosed area to avoid 

spreading dust into surrounding areas. All of these factors increase operating costs. 

Dry Control Methods 

Control of dust generated by transport (see Chapter 5—Conveying and Transport) and 

processing of minerals can be achieved through containment, exhaust, and cleaning of dusty air. 

This approach, termed dry control, can be more costly to install and operate than wet control 

methods, but can be very effective. Additionally, dry dust control may be necessary when the 

product is adversely affected by the addition of water, such as trona or clayey materials. 

Dry dust control must create conditions that will prevent the escape of dusty air from the 

controlled space to areas occupied by workers. This control is achieved by using exhaust 

ventilation to create a negative air pressure inside the controlled space relative to outside of the 

controlled space (see Chapter 1—Fundamentals of Dust Collection Systems). The amount of 

ventilation necessary to achieve control is affected by: 

112 Crushing, Milling, and Screening 
 

 

the degree of enclosure of the controlled space, with required exhaust volume increasing 

as the area of unsealed openings into the enclosure increases; 

the airflow created by the movement and processing of the mineral, including air that is 

entrained within moving material, airflow induced by moving material and equipment, 

and air displaced by the material flowing into or out of an enclosure; 

the effect of ambient wind speed and direction that can overcome the pressure differential 

between the interior and exterior of the controlled space. 

CRUSHING 

Size reduction processes will always contain at least one crushing circuit, and many times will 

have multiple crushers, often of different types. Selection of crushers is based primarily on 

material size-reduction and throughput requirements. Secondary selection considerations include 

the composition, hardness, and abrasiveness of the feed material(s). The most common primary 

crusher type is the jaw crusher (Figure 4.1) which operates by compressing the feed material 

between a fixed and moving plate or jaw. 

Figure 4.1. Illustration of a jaw crusher showing material being crushed 

between fixed and reciprocating jaws. 

Crushing, Milling, and Screening 113 

Cone (Figure 4.2 left) or gyratory (Figure 4.2 right) 

crushers may be used as primary or secondary crushers and also operate through compression. 
 

 

Figure 4.2. Examples of compressive crushers. Left—illustration of a 

cone crusher showing material being crushed between the cone mantle 

and bowl liner. Right—illustration of a gyratory crusher with material 

being crushed between the gyratory head and the frame concave. 

Hammermills (Figure 4.3 left), impact breakers (Figure 4.3 center), and roll crushers (Figure 4.3 

right) operate primarily by impaction. 

Figure 4.3. Examples of impactive crushers. Left—a hammermill-type crusher showing 

material crushed between the rotating hammers and fixed grinding plate. Center—

illustration of material being crushed between rotating hammers and fixed anvil plates 

in an impact breaker. Right—illustration of the size reduction action of a roll crusher. 

The difference between compression and impaction is the 

speed at which the breaking force is transmitted from the crusher to the material. Compression 

involves a slower energy transfer than impaction, and compressive crushers produce somewhat 

less dust than impactive crushers. 

Dust control at crushers may be achieved through application of water, by enclosure of the dust 

source with or without exhaust ventilation, or a combination of wet and dry methods. Generally, 

the upper portion of the crusher (feed side) should be enclosed as completely as possible 

[USBM 1974]. Enclosures should be constructed of physically robust materials appropriate for 

the operating environment and climate conditions. 

114 Crushing, Milling, and Screening 
 

 

 

Figure 4.4 illustrates a wet dust control system at a crusher dump loading operation. 

Figure 4.4. Illustration of a wet dust control approach with partial enclosure at a 

crusher dump loading operation. Note the blue "fan patterns" signifying water sprays. 

The frontal 

open area of the enclosure may be reduced by installing hanging curtains and installing a high 

curb or berm at ground level. To conserve resources, operators should consider installing a 

presence-sensing control with a timed duration to activate the water spray system. 

As described in Chapter 2—Wet Spray Systems, water sprays used for dust prevention typically 

use solid spray patterns or full cone spray patterns. Sprays used for dust suppression typically 

use hollow cone spray patterns. Sprays should overlap slightly to provide good coverage. 

A local exhaust ventilation dust control system at a crusher dump loading operation is illustrated 

in Figure 4.5. 

Enclosure of the volume to be controlled should be maximized; in other words, the number and 

area of openings should be minimized as much as possible. Initial exhaust rates for the type of 

enclosure depicted in Figure 4.5 can be estimated by Equation 4.1, which accounts for air 

displaced by a dumping operation: 

G

T

QE

600 33 3.

(4.1) 

where QE = exhaust air volume, cubic feet per minute; 

T = weight of material dumped, tons per minute; and 

G = bulk density of material, pounds per cubic foot. 

Crushing, Milling, and Screening 115 
 

 

Figure 4.5. Illustration of a dry (exhaust) dust control system 

with a partial enclosure at a crusher dump loading operation. 

If the dumping operation is not continuous, operators should consider utilizing a presencesensing control with a timed duration to activate the exhaust system fan and water sprays as 

dumping occurs. 

Whenever local exhaust ventilation is selected as a means of dust control, maintenance and 

clean-out openings should be designed into the enclosure and equipped with tight-fitting 

closures. Properly located and sized maintenance access openings will allow workers to perform 

their assigned tasks without having to modify the enclosure. This will reduce the potential for 

dust to escape through unintended openings. 

A minimum air capture velocity of 200 feet per minute (fpm) is recommended to be maintained 

at all openings into enclosures [USBM 1974]. Where feed or discharge belt openings penetrate 

dust control enclosures, it is recommended to add the belt speed in fpm to the 200 fpm design 

velocity to account for the process material volumetric flow and belt-induced air movement 

[Yourt 1969]. 

As an example, for an enclosure with an initial design capture velocity (VINIT) of 200 feet per 

minute (fpm) penetrated by a belt moving at 250 fpm (VBELT), the recommended capture velocity 

(VCAP) at openings into the enclosure becomes: 

Capture velocity (VCAP) = 200 fpm (VINIT) + 250 fpm (VBELT) 

Capture velocity (VCAP) = 450 fpm 

116 Crushing, Milling, and Screening 
 

 

 

Enclosure exhaust air volume (QE) in cubic feet per minute (cfm) can be estimated using the total 

area (in square feet) of all openings into the enclosure, with the capture velocity as: 

QE (cfm) = VCAP (fpm) x open area (sq ft) 

Again, these initial velocity and volume estimates should be verified as (1) providing acceptable 

dust control, and (2) not excessively consuming energy. Exhaust ventilation concepts are 

discussed more fully in Chapter 1—Fundamentals of Dust Collection Systems. 

When possible, dust control enclosures should be large in volume to allow settling of coarse 

dust. Removing large airborne dust particles by this method will reduce the load on subsequent 

air handling and cleaning systems. 

Falling material at transfer points will generate airflow both by induction and displacement 

[Anderson 1964; Goldbeck and Marti 1996]. When materials have an average size greater than 

1⁄8 inch (0.01 foot), this airflow can be estimated by Equation 4.2: 

10 3

2

E

D

SR

AQ U

(4.2) 

where QE = exhaust volume, cubic feet per minute; 

AU = enclosure upstream open area, square feet; 

R = rate of material flow, tons per hour; 

S = height of material fall, feet; and 

D = average material size, feet. 

Determining exhaust volume through the use of Equation 4.2 is illustrated in Figures 4.6 through 

4.9. In each case the height of material fall (variable S in Equation 4.2) is depicted. The value 

for the upstream open area variable—AU —for a transfer point is the cross-sectional area of the 

chute transferring the material and any unsealed area between the chute and the enclosure 

structure. When chutes are not used, an estimate of the open area around penetrations (e.g., belt 

conveyor structure) into the enclosure is used for the AU variable. 

Figures 4.8 and 4.9 depict transfer from the discharge of a crusher to a conveyor. For jaw 

crushers, which have variable cross-sectional area, the upstream open area value (AU) is the area 

of the bottom of the jaw crusher opening into the enclosure structure (Figure 4.8). For crushers 

with constant cross section, such as a cone crusher, the upstream open area is the crusher feed 

throat area (Figure 4.9). 

More information on dust control at transfer points is found in Chapter 5—Conveying and 

Transport. 

Crushing, Milling, and Screening 117 
 

 

Figure 4.6. Illustration of a dry (exhaust) dust control system at the transfer 

point of a conveyor discharge to a crusher feed hopper. 

Figure 4.7. Illustration of a dry (exhaust) dust control system 

on a feed chute into a transfer chute feeding a crusher. 

118 Crushing, Milling, and Screening 
 

 

Figure 4.8. Illustration of a dry (exhaust) dust control system 

at the discharge of a jaw crusher onto a belt conveyor. 

Figure 4.9. Illustration of a dry (exhaust) dust control system 

at the discharge of a hammermill crusher onto a belt conveyor. 

Crushing, Milling, and Screening 119 
 

 

Wet dust control methods for crushing operations, illustrated in Figures 4.10 and 4.11, involve 

wetting the process material before crushing, after crushing, or both. 

Figure 4.10. Illustration of a wet dust control approach with a transfer 

chute/impact bed enclosure at a crusher loading operation. 

120 Crushing, Milling, and Screening 

 This is essentially treating 

the crushing operation as two transfer points, the feed side and discharge side. Crushing creates 

smaller sized material with an attendant increase in surface area, which will be dry. Thus, 

material wetted prior to crushing for dust control will likely have to be wetted again to address 

the additional dry surface area. 

Where dust control through water application is selected, recall that solid spray nozzles, or full 

cone spray nozzles, produce larger drop sizes than hollow cone nozzles. Hollow cone nozzles 

should be used when dust suppression (reduction of airborne dust) is desired, with spray patterns 

arranged to cover the entire area of the dust cloud. When prevention of airborne dust is desired 

in a static application such as a bin or hopper, full cone nozzles should be used with spray 

patterns overlapping the entire surface of the material to be wetted. In a moving application such 

as a conveyor, fan spray nozzles should be used with spray patterns oriented perpendicular to 

conveyor travel and overlapping approximately 30 percent of the fan width. Further information 

on wet methods is found in Chapter 2—Wet Spray Systems. 
 

 

Figure 4.11. Illustration of a wet dust control approach 

on a crusher discharge/belt loading operation. 

The mechanical action of crushers can generate air movement; i.e., jaw crushers can have a 

bellows-type effect, and cone or gyratory crushers can act as fans, although neither of these 

classes of crushers operate at high speeds. In contrast, hammermills operate with higher speed 

components and can act as centrifugal fans. A method to estimate maximum generated airflow 

from this type of crusher has been described [Burton 1999]. The maximum volume (cubic feet) 

of air generated per shaft revolution can be estimated by treating the hammer/shaft assembly as a 

fan using Equation 4.3: 

WD

AGEN REV 2

(4.3) 

where D = diameter of the hammer assembly from the tip to tip, feet; and 

W = the width of the hammers, feet. 

The volume of air generated per minute (cfm) is then the product of AGEN REV and shaft RPM. 

A more general method to evaluate this issue for any type of crusher is to measure inflow 

velocity at permanent openings into the dust control enclosure with the crusher on, then off. If 

the inflow velocity is not satisfactory when the crusher is operating, it will be necessary to 

increase exhaust volume or install baffles inside of the enclosure. 

Crushing, Milling, and Screening 121 
 

 

Work Practices to Minimize Dust Exposure from Crushers 

The following practices can serve to reduce operator exposure to airborne dust at crusher 

operations. 

Maintain closure/locking devices such as clamps and other fasteners. Fasteners are only 

effective when used. 

Where compatible with the process, wash down areas on a periodic basis. Periodic 

washing prevents the accumulation of material which can eventually become too large for 

wash down, necessitating a dry cleanup problem with the generation of dust. 

Keep makeup air and recirculating air dust filtration systems on operator booths, frontend loaders or other mobile equipment in place. Ensure that only correct filters are 

utilized, and maintain the systems to manufacturer specifications. 

Wear approved respiratory protection when working in dust collectors, mills, air 

classifiers, screens, and crushers. 

Where feasible, automate the crushing process using sensing devices and/or video 

cameras. This removes the operator from the crusher area and reduces the potential for 

dust exposure. 

Where crusher operation must be supervised continuously, provide an enclosed booth 

with a positive-pressure, filtered air supply for the operator. See Chapter 9—Operator 

Booths, Control Rooms, and Enclosed Cabs for more information. 

MILLING 

Milling, often called grinding, is a process by which granular minerals are reduced in size by 

compression, abrasion, and impaction. Mills may be classified into two broad types based on 

how they operate: tumbling mills (Figure 4.12) and stirring mills (Figure 4.13). Tumbling mills 

generally operate in a horizontal orientation, and the shell of the mill rotates to impart motion to 

the contents or charge. Stirring mills may be horizontal or vertical and motion is imparted to the 

charge by an internal stirring element. 

Tumbling mills employ some type of medium to perform the size reduction, often rods or balls, 

usually manufactured from iron or a steel alloy, or high density ceramic material when metal 

contamination of the product is a concern. Autogenous mills use the feed stock (incoming ore) 

itself as the medium, and semi-autogenous mills use a combination of the feed stock and balls. 

Stirring mills reduce the size of the feed stock between fixed and rotating mill components. 

Materials discharged from mills can range in size from less than 40 to 300 micrometers (μm) for 

tumbling mills and from 40 to less than 15 μm for stirring mills [Wills 2008]. 

Although mineral processing milling is often done wet, the industrial minerals industry primarily 

utilizes dry milling. At some operations, the grinding mill is fed with coarse rejects or one of the 

products from their dry screening operation. These milling operations are fed dried granular 

minerals, usually less than 12 mesh (1.7 millimeters), and produce products in the 50

micrometer (μm) range, but some fine-grind products are less than 10 μm in size, which means 

that all of the fine-grind product is in the respirable size range. 

-

122 Crushing, Milling, and Screening 
 

 

Figure 4.12. Cut-away illustration of a ball mill showing the charge 

consisting of the material being processed and balls. 

Figure 4.13. Illustration of a stirring mill. 

Crushing, Milling, and Screening 123 
 

 

Some specialty grinding operations purchase damp granular minerals and dry them before they 

are fed into the milling process. Regardless, all the process equipment involved in preparing and 

feeding the grinding mill have their respective dust collection systems. 

The most important consideration for dust control within a grinding/classification/product 

storage circuit is dust containment, and the second is dust collection. All of the material 

conveying equipment related to a milling or grinding circuit must be enclosed. In addition, all 

transfer points must be fully enclosed. 

Design and Work Practices to Minimize Dust Exposure from Milling 

The following design and work practices can serve to reduce operator exposure to airborne dust 

around milling operations. 

Grinding circuit equipment must be tightly closed and maintained under a slight negative 

pressure 100 percent of the operational time by a dust collection system. This means that 

any air leakage will flow into and not out from the equipment, keeping the dust 

contained. 

Sampling points must be designed so that when the access doors are opened, the milled 

material stays inside. All sample and inspection doors should be installed at least 45 

degrees off the vertical or greater, with horizontal doors being preferable for easy access 

and lower risk of spillage (see Figure 4.14). 

Figure 4.14. Sampling doors located on top of a collection/transfer point to prevent leakage. 

124 Crushing, Milling, and Screening 
 

 

Grinding circuits and buildings should be designed so that they can be washed down for 

cleaning. 

A vacuum system should be installed and used for cleanup. Equipment should not be dry 

swept or brushed off because of the possibility of dust liberation. Never use compressed 

air to clean equipment or work areas. 

Upper levels around process equipment should have solid floors. If a spill or leak were to 

occur over a solid floor, it immediately piles up instead of falling towards the next level 

where it will become airborne and contaminate the mill. 

Sections of the milling process should be isolated to improve containment efforts. 

Examples include uncovered storage, bagging areas, and bulk load out areas. 

Above freezing conditions, floors should be kept wet so that any falling dust will 

immediately hydrate and be trapped. 

Whole room ventilation with at least 10 air changes per hour (acph) is recommended. 

For classifier circuits, vendor guidelines should be followed for ducting and airflow to 

achieve optimum classification results. With other components of the grinding/loadout 

operation, the containment, pickup points, and ducting should be designed so that areas 

with high air movement do not occur at transfer points and other potential disturbance 

areas where the air stream will entrain product. 

When new or recently lined tumbling mills are started up for the first time, the air leaving 

the mill is fairly humid until the grinding action builds up enough heat to drive off the 

moisture out of the curing grout. This can lead to a buildup of moist material in ducting 

and may cause blinded bags and cartridges in the dust collectors. These problems will 

typically be resolved once the mill reaches operating temperature and the grout has cured. 

SCREENING 

Screening operations can produce high levels of dust because smaller sized material is handled. 

Airborne dust is generated from the vibrating screen decks that accomplish the size separation. 

Additionally, material must fall some distance as part of the separation process, and some dust 

will be suspended by this process. 

Dust control for screening systems is similar to that for crushers, although wet systems are 

generally not used due to blanking of the screen openings by the wet material. Also, screens are 

not normally subject to large surges of material flow as are crushers. This is due to the fact that 

screens operate most efficiently within a designed flow range. Overloading screens, whether 

through excessive feed rate or large surges of material, can cause accelerated wear of 

components, reducing the efficiency of the operation and potentially increasing dust emissions. 

Screens should be totally enclosed, and water suppression systems (when compatible with the 

process) or dust collection and exhaust systems should be incorporated. Necessary openings in 

the screen enclosure must be minimized, and inspection and maintenance openings must be 

provided with tight-fitting closures. 

Flexible materials (e.g., rubber or synthetic sheeting) must be used to seal openings between the 

moving screen components and stationary equipment and structures. Because the seal between 

moving and stationary components is under dynamic stresses whenever the screen is operating, it 

Crushing, Milling, and Screening 125 
 

 

should be frequently inspected for signs of failure, on a schedule that incorporates data from its 

operating history. Some screen original equipment manufacturers offer parts and materials such 

as covers and flexible sealing components designed specifically to fit their equipment well, 

enabling creation of an effective dust control enclosure. Equipment vendors should be consulted 

to explore available control options. 

Initial design exhaust air volume may be estimated using the induced air method via 

Equation 4.2. Figures 4.15 and 4.16 illustrate exhaust ventilation dust control on screens. 

Figure 4.15. Illustration of a dry (exhaust) dust control system on the feed 

to a rotary screen with enclosed transfer chute. 

126 Crushing, Milling, and Screening 

 The 

dimension for material fall (Equation 4.2, variable S) is indicated in the figures. The upstream 

open area dimension (Equation 4.2, variable AU) is the estimated area of the transfer chute plus 

any unsealed open area around the chute penetration into the dust control enclosure. 

Alternatively, an exhaust volume of 50 cfm per square foot of screen area may be used as an 

initial value for flat deck screens [ACGIH 2010]. This value is not increased for multiple level 

screens because the stacked construction of multiple level screens presents no more "source 

area" than does a single screen. For cylindrical rotary screens, an exhaust volume of 100 cfm per 

square foot of screen cylindrical cross section with an in-draft velocity minimum of 400 fpm is 

recommended [ACGIH 2010]. 
 

 

Figure 4.16. Illustration of a dry (exhaust) dust control system on a vibrating screen. 

Exhaust air take-off location must be carefully considered so that desirable fines (product) are 

not collected. Locating the take-off closer to the discharge (away from the feed end) of the 

screen will help avoid the loss of desirable fines. Also, collection of undersized material can be 

reduced by using take-off openings large in area to reduce the air entry velocity. 

Work Practices to Minimize Dust Exposure from Screens 

The following practices should be observed where compatible with installed equipment and 

existing operating procedures. 

Clean equipment and area before and during work, when needed, preferably by washing 

or vacuuming. 

Stop material flow before cleaning screens, if operationally feasible. The absence of 

flowing material minimizes the potential for dust emissions. 

When cleaning, only open the top or the bottom part of screen decks at a time. Do not 

open both at the same time. The larger the size of the opening, the less effective the 

ventilation system can be. 

Only open one screen at a time during cleaning. Do not open multiple screens 

simultaneously. Again, numerous openings decrease the capture effectiveness of a 

ventilation system. 

When cleaning screens, only use long-handled brushes that will provide more distance 

between the worker and the dust source. 

Consider variable speed vibrators that significantly reduce the frequency of manual 

cleaning of screens. The less time cleaning screens, the less the potential for dust 

exposure. 

Crushing, Milling, and Screening 127 
 

 

Open screen decks slowly to allow internal exhaust system to function at the deck 

periphery. Opening decks rapidly can cause a swirling effect with dust being released 

into the work area, potentially exposing personnel. 

Do not slam screen decks closed. Any material, no matter how slight, has the potential of 

becoming airborne due to the shock of deck closure. 

Decks must be kept clear of dust. A clean deck lessens the potential of dust emissions 

resulting from opening and closing decks, as well as from environmental factors such as 

wind. 

Maintain seals on screen decks where an airtight closure is desired. Decks that are 

properly sealed should remain sealed when all fasteners and sealing materials are in 

place. 

Closure and locking devices such as clamps and other fasteners must be well maintained. 

Fasteners are ineffective unless they are utilized. 

Where compatible with the process, wash down areas around screens on a periodic basis. 

Periodic washing removes material before it accumulates to a degree where dry cleanup 

methods become necessary. Dry cleanup activities present a much higher potential for 

worker exposure. 

MAINTENANCE 

Maintenance of dust control systems is critical to ensure continuing worker protection [USBM 

1974]. Crushers and screens are subject to constant vibration when in use and this condition can 

cause accelerated wear on all components. Even small openings from worn seals, missing 

fasteners, and damaged flexible connectors can cause unwanted air infiltration, degrading 

capture velocity at necessary penetrations [Goldbeck and Marti 1996]. This could potentially 

result in worker overexposure, particularly when exposure limits are low. 

Operators should document baseline parameters of an effectively functioning dust control 

system, including for example: 

inflow velocity at enclosure openings, 

static pressure in ducts, 

velocity pressure in ducts and at hood entry points, and 

water pressure and flow in wet systems. 

Results of subsequent periodic checks should be compared to the baseline values. Significant 

deviations from the baseline values should be explored, and corrective action implemented when 

necessary. 

A preventive maintenance schedule should be established based on manufacturer 

recommendations, observed component wear, system performance measures, and exposure 

sampling results. 

128 Crushing, Milling, and Screening 
 

 

REFERENCES 

th ACGIH [2010]. Industrial ventilation: a manual of recommended practice. 27 ed. Cincinnati, 

Ohio: American Conference of Governmental Industrial Hygienists. 

Anderson DM [1964]. Dust control design by the air induction method. Industrial Medicine and 

Surgery. 33:68–72. 

rd Burton DJ, ed. [1999]. Hemeon's plant & process ventilation. 3 ed. Boca Raton, Florida: 

Lewis Publishers. 

EPA [2003]. AP 42, Fifth Edition, Compilation of air pollutant emission factors, volume 1: 

stationary point and area sources. United States Environmental Protection Agency. Chapter 11: 

Mineral Products Industry; Background Information for Revised AP-42 Section 11.19.2, 

Crushed Stone Processing and Pulverized Mineral Processing [online] available at URL: 

[http://www.epa.gov/ttn/chief/ap42/ch11/]. Date accessed 09/04/2008. 

Goldbeck LJ, Marti AD [1996]. Dust control at conveyor transfer points: containment, 

suppression and collection. Bulk Solids Handling 16(3):367–372. 

NIOSH [2003]. Handbook for dust control in mining. By Kissell FN. Pittsburgh, PA: U.S. 

Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, NIOSH Information Circular 9465, 

DHHS, (NIOSH) Publication No. 2003–147. 

USBM [1974]. Survey of past and present methods used to control respirable dust in noncoal 

mines and ore processing mills—final report. U.S. Department of the Interior, Bureau of Mines 

Contract No. H0220030. NTIS No. PB 240 662. 

USBM [1978]. Improved dust control at chutes, dumps, transfer points and crushers in noncoal 

mining operations. U.S. Department of the Interior, Bureau of Mines Contract No. H0230027. 

NTIS No. PB 297–422. 

Wills BA [2008]. Wills th ' Mineral Processing Technology, 7 ed. Oxford, UK: Elsevier Ltd. 

Yourt GR [1969]. Design for dust control at mine crushing and screening operations. 

Canadian Mining Journal 90(10):65–70. 

Crushing, Milling, and Screening 129 

DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Crushing, Milling, and Screening 131

CHAPTER 5: 

CONVEYING AND 

TRANSPORT 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

132 Crushing, Milling, and Screening 
 

 

CHAPTER 5: CONVEYING AND TRANSPORT 

There are numerous types of equipment used at mineral processing operations to transfer 

material from one location to another. The material being transferred can range from raw 

unprocessed ore to fully processed, finished product. Each material transfer methodology has its 

own strengths and weaknesses. Proper selection of the correct type of equipment is a function of 

the specific application, taking into account the material to be transferred, transfer distance, and 

nature of the transfer (i.e. horizontal, vertical, incline or decline). This chapter will discuss some 

of the most commonly used transport systems and the methods available for controlling dust and 

spillage. 

BELT CONVEYORS 

Belt conveyors are among the most commonly used piece of equipment at mineral processing 

operations. A conveyor, and the associated transfer points, can generate significant quantities of 

respirable dust and be one of the greatest sources of fugitive dust emissions within an operation. 

Operations must control these emissions by containing, suppressing, or collecting dust 

mechanically, either before or after it becomes airborne, giving special attention to transfer 

points. A conveyor belt consists of many different parts as seen in Figure 5.1. 

Figure 5.1. Basic components of a conveyor belt. 

Conveying and Transport 133 
 

 

There are three primary root causes for fugitive dust emissions associated with conveyor belts: 

spillage, carryback, and airborne dust (Figure 5.2). Control of all three primary dust sources is 

necessary to eliminate fugitive dust emissions. 

Figure 5.2. Types of fugitive dust emissions from conveyor belts. 

Controlling Material Spillage 

Material spillage from a conveyor belt is caused by a lack of material control, either at a transfer 

point or along the transfer route. Spillage along the transfer route is generally associated with 

carryback, a condition discussed later in this chapter. 

The most prevalent location for material spillage is at transfer points. Material may be deposited 

virtually anywhere along a conveyor, but always leaves the conveyor at the head pulley. In order 

to effectively control material at transfer points, the following four steps are used. 

1.	 Feeding material onto the belt in the direction of travel and at the same speed as the 

receiving conveyor. 

2.	 Keeping the fed material centered on the conveyor belt. 

3.	 Minimizing the transfer distance which, in turn, minimizes material impact on the belt 

and air entrainment within the material. 

4.	 Using skirtboard ("skirting") to manage the loaded material as it travels on the belt. 

Each of these steps will be discussed in detail below. 

Step 1. Feeding material onto the belt in the direction of travel and at the same speed as the 

receiving conveyor. Figure 5.3 shows a conveyor head pulley (top conveyor) feeding material 

onto a conveyor tail pulley (bottom conveyor). This figure depicts the simplest of transfer 

points, where the feeding and receiving conveyors are in-line with each other. The material is 

being fed in the same direction and the speed of the bottom conveyor should match, as closely as 

is possible, the top conveyor's speed. 

134 Conveying and Transport 
 

 

Figure 5.3. Basic depiction of material transfer from a top conveyor to a bottom conveyor. 

Achieving effective transfer of material in the same direction and at the same speed when 

conveyors are, for example, perpendicular to one another, is more challenging. Impact beds are 

often used to allow ore to fall onto ore rather than directly onto the belt. This is done to control 

the direction and speed of the material being transferred and to minimize wear on the inner 

surfaces of the chute and skirting (Figure 5.4). 

Figure 5.4. Depiction of impact bed used to control material direction and speed. 

At times, curved plates are used near the surface of the receiving belt to help steer the material 

being fed. Plates and impact beds can help to encourage the material being fed to flow in the 

Conveying and Transport 135 
 

 

direction of the receiving conveyor as well as control the placement of the material on the 

receiving conveyor (Figure 5.5). 

Figure 5.5. Depiction of curved loading plate to help steer loaded material in a particular direction. 

Step 2. Keeping the fed material centered on the conveyor belt. Keeping the fed material 

centered on the conveyor belt is extremely important in controlling material spillage at transfer 

points. Material fed more to one side of a belt than another (Figure 5.6) can cause the belt to 

shift sideways and run off-center. A belt that runs off-center can spill material over the edge of 

the belt outside of the transfer point. 

Figure 5.6. Spillage as a result of material being improperly fed onto a belt. 

Step 3. Minimizing the transfer distance which, in turn, minimizes material impact on the belt 

and air entrainment within the material (see Figure 5.7). The transfer distance is the distance 

136 Conveying and Transport 
 

 

that the material falls before landing on the belt. Longer transfer distances permit fine particles 

to separate from the material being transferred, while shorter transfer distances reduce the time 

available for fine material to separate from the material being transferred, thus reducing dust 

emissions. Additionally, longer transfer distances also contribute to belt sagging. When belts 

sag, they allow material to become trapped between the idlers, which in turn allow the material 

to release fugitive dust, as well as directly damaging the belt. To address this issue, some 

operations use impact cradles (often called impact beds). An impact cradle system may include a 

slick top cover to allow smooth belt movement as well as underlying rubber layers to absorb 

impact (Figure 5.8). In areas of low impact or no-impact, even lighter duty side-support cradles 

can be used (Figure 5.9). For effective impact cradle design, refer to Conveyor Equipment 

Manufacturers Association (CEMA) Standard 575–2000 (Table 5.1 and Figure 5.7) [CEMA 

Standard 575–2000]. 

Table 5.1. Impact cradle ratings to be used based on impact loading [CEMA Standard 575–2000] 

Code Rating Impact Force W x h (ref.) 

Lbf lb-ft 

L Light duty <8,500 <200 

M Medium duty 8,500 to 12,000 200 to 1,000 

H Heavy duty 12,000 to 17,000 1,000 to 2,000 

Figure 5.7. Calculations for determining material impact loading at 

a conveyor transfer point [CEMA Standard 575–2000]. 

Conveying and Transport 137 
 

 

Figure 5.8. Depiction of an impact cradle with underlying rubber layers. 

Figure 5.9. Side support cradle. 

Step 4. Using skirtboard (skirting) to manage the loaded material as it travels on the belt. 

Skirting (Figure 5.10) is the horizontal extension of the loading chute used to contain fines (very 

fine pieces of ore) and dust within the transfer point and to shape the material until it has settled 

into the desired profile on the center of the belt. 

138 Conveying and Transport 
 

 

Figure 5.10. Skirtboard used within the transfer point to help manage loaded material. 

Skirtboards are employed to retain material on the belt after leaving the loading chute and until 

the material reaches belt speed. Skirtboards are generally made of steel with the lower edges 

positioned some distance above the belt. The gap between the boards and the belt is in turn 

sealed by a flexible elastomer sealing strip attached (clamped or bolted) to the outside wall of the 

boards (see Figure 5.10). 

Skirtboard length should, in theory, extend in the belt's direction beyond the point where the 

material has settled into the profile that it will maintain for the remainder of its journey to the 

head pulley. CEMA notes that the length of skirtboarding is "a function of the difference 

between the velocity of the material at the moment it reaches the belt and the belt speed" [CEMA 

Standard 575–2000]. In essence, CEMA recommends two feet of skirtboard for every 100 feet 

per minute (fpm) of belt speed, with a minimum length of not less than three feet. While this 

rule of thumb is generally sufficient for easily conveyed materials, it is insufficient for materials 

that typically require dust collection or materials that roll on the conveyor, making their 

management more complex. Other rules of thumb also exist from various organizations. For 

example, the U.S. Army Corps of Engineers technical manual Design of Steam Boiler Plants 

recommends that skirting extend 5 feet from material impact and extend an additional 1 foot for 

every 100 fpm of belt speed [Swinderman et al. 2002]. 

As one demonstration, using the rules of thumb from CEMA and the Army Corps of Engineers, 

Table 5.2 compares the two recommendations using a belt speed of 600 fpm. Based on this 

scenario, CEMA recommends 12 feet of skirting while the Army Corps of Engineers 

recommends 11 feet of skirting. While the 1-foot difference may not seem significant, in 

Conveying and Transport 139 
 

 

operational conditions this could mean the difference between no material spillage and 

significant material spillage. Therefore, each circumstance must be evaluated individually with 

the behavior of the material under conditions of conveyance as a primary consideration. 

Table 5.2. A comparison of CEMA and Army Corps of Engineers 

recommendations for skirting for a belt speed of 600 fpm 

CEMA Army Corps of Engineers 

2 ft of skirting for every 100 fpm 5 ft plus 1 ft of skirting for every 100 fpm 

 of belt speed of belt speed 

2 ft x (600 fpm/100 fpm) = 12 ft of skirting 5 ft + (600 fpm/100 fpm) = 11 ft of skirting 

An inclined skirting design, in which the skirting belt is angled at approximately 30 degrees from 

vertical, is more advantageous over a standard vertical design because of wear issues. This 

skirting design improves the loading of ore onto the conveyor and reduces the amount of dust 

generated. 

Dust curtains (Figure 5.11) are an effective and inexpensive containment device that should be 

installed at the entrance and exit of the chute enclosure. 

Figure 5.11. Typical dust curtain used at the entrance and exit of the chute enclosure. 

140 Conveying and Transport 

 Some design guidelines include: 

Exit air velocity from the enclosure or chute should be kept below 500 fpm to minimize 

the entrainment of large particles [Yourt 1990]. 

Dual rubber curtains should be hung roughly 18 inches (450 mm) apart from each other 

to form a "dead" area where dust can settle [Goldbeck and Marti 2010]. 

Entrance curtains should extend to the conveyor belt to maximize the containment area. 

Exit curtains should extend to approximately one inch (25 mm) below the top of the 

product pile [Goldbeck and Marti 2010]. 
 

 

Carryback 

Material that sticks or clings to a conveyor belt after passing over the head pulley is called 

carryback. Carryback tends to fall from the belt as it passes over return idlers. This creates piles 

of material that require clean-up, which can increase worker dust exposure. Also, respirable 

portions of carryback can become airborne and increase fugitive dust exposure levels. The goal 

is to remove carryback before it is released into the air and becomes a source of contamination to 

the workers or creates piles of material that require clean-up. 

The primary means of controlling carryback is to clean the belt as it passes over or past the head 

pulley (i.e. shortly after material is discharged from the belt). The two most common means of 

cleaning a conveyor belt of carryback are to mechanically "scrape" the belt via scrapers or 

brushes or to wash the belt. 

Belt Scrapers 

There are a myriad of different types and configurations of scrapers and addressing them all is 

beyond the scope of this handbook. Instead, the basic principles and types of scrapers will be 

addressed. Although belt cleaner systems come in many different styles, types, and trade names 

sold by numerous commercial manufacturers, their function remains the same: to reduce the 

amount of carryback on the belt once the ore is discharged. 

Material can be either scraped or peeled from a belt. When material is scraped from a belt, the 

scraper is angled with the direction of belt travel (Figure 5.12). 

Figure 5.12. Typical method for scraping material from a belt. 

Multiple scraper belt cleaner systems provide a way to address carryback, with the use of a 

precleaner on the face of the head pulley to remove most of the material, followed by secondary 

cleaners to perform final cleaning. The secondary scrapers can be installed either where the belt 

leaves the head pulley or further along the conveyor return. To be effective as well as to reduce 

belt wear, multiple cleaners should be incorporated with low blade-to-belt pressure [Goldbeck 

and Marti 2010]. When dust levels are high, it is not uncommon to use two or three belt scrapers 

Conveying and Transport 141 
 

 

at different locations in an effort to further reduce the amount of carryback material on the belt 

[Roberts et al. 1987]. 

It is most desirable to return the scrapings to the primary material flow through chutes 

(Figure 5.13). Chutes should therefore be designed large enough to capture the scrapings and 

their walls steep enough to prevent scrapings from building and stacking up within the chute. 

Figure 5.13. Belt conveyor discharge chute used to return scrapings to the primary material flow. 

Belt Washing 

Some studies have shown that oversized material is more easily removed through scraping, but 

that the smaller respirable size particles tend to remain adhered to the conveyor. Washing is an 

alternative when belt scraping does not remove particles adhering to the belt. A belt wash 

system sprays the conveyor belt with water while simultaneously scraping it to remove the 

product. A handbook from Martin Engineering, Foundations: The Practical Resource for 

Cleaner, Safer, More Productive Dust & Material Control [Swinderman et al. 2009], devotes a 

chapter to a thorough discussion of belt washing systems. 

142 Conveying and Transport 
 

 

CONVEYOR DESIGN AND MAINTENANCE ISSUES 

The great challenge involved with conveyors is the number of belts involved and the total 

distance traveled throughout a mineral processing plant. Some belts are located outside where 

dust liberation is not as critical as when they are located within a building. Another challenge 

particular to conveyors is their ability to generate or liberate dust whether they are loaded heavily 

with ore or nearly empty. Controlling dust from conveyors requires a constant vigilance by the 

maintenance staff to repair and replace worn and broken parts, including conveyor belting. Basic 

maintenance and inspection are required to ensure that all parts of the system are performing to 

their capacity. Material can escape through chutes worn from rust or abrasion, and even small 

holes created by missing bolts or larger holes created from open access doors can be a pathway 

for fugitive dust. In some cases, it may become necessary to replace an entire loading chute to 

ensure proper containment. 

Belt Splices 

All conveyor belts require at least one splice to connect the ends together and thus close the loop. 

Due to belt failures, conveyors can often have more than one splice. Mechanical and vulcanized 

splices, similar to the styles shown in Figure 5.14, are commonly used in the mining industry. 

Figure 5.14. Illustration of a mechanical belt splice (left) and a vulcanized belt splice (right). 

Mechanical belt splices can often be a source of significant belt spillage because product and 

dust can fall through small openings in the splice. Vulcanized belt splices can be used to 

eliminate this source of spillage. Vulcanized splices provide a method of joining the ends of 

conveyor belts without interrupting the continuity of the belts, and usually without altering the 

geometry or dimensions of the belts. It is recommended that vulcanized belt splices be used, 

where feasible, to reduce dust and spillage. 

Conveying and Transport 143 
 

 

DESIGN CONSIDERATIONS FOR TRANSFER POINTS 

Enclosures at both the head and tail ends of a conveyor are a very common practice because they 

are effective at controlling dust at these locations and provide guarding safety. While the 

elimination of dust generation at conveyor transfer points is not feasible, effective dust control is. 

Designing the proper size enclosure is a critical factor because as the ore is dumped onto the 

conveyor, it entrains a measurable amount of air (venturi effect), and this can pressurize the 

enclosure if it is undersized. Enclosures for both conveyor and transfer points can be either full 

or partial, depending on the various components of the system. 

Although this seems like a very simple process, significant dust generation and liberation can 

result from transfer points if they are not properly designed and installed. The following are 

some important design considerations for an effective transfer point or chute used with an 

exhaust ventilation system (also see Figure 5.15): 

It has been shown that the exhaust port of the transfer point enclosure should be a least 

6 feet from the dump point to minimize the entrainment and pickup of oversized particles 

[MAC 1980]. The air velocity at the base of the exhaust port cone should also be kept 

below 500 fpm to avoid the pickup of larger particles [Yourt 1990]. 

Transfer point enclosures should be designed to have a 200 fpm intake velocity at any 

unavoidable opening to eliminate dust leakage from the openings [USBM 1974]. The 

Mining Association of Canada (MAC) recommends adding 25 percent to the 200 fpm 

guideline [MAC 1980]. 

It is important to minimize openings and thus preserve intake velocity by using plastic 

stripping and other types of sealing systems. It is recommended that the transfer point 

enclosure be made large enough so that the air velocity within the enclosure is below 

200 fpm (1 m/sec) [Goldbeck and Marti 2010]. The larger enclosure can serve as a 

plenum where air velocity can dissipate. If an enclosure is undersized, air will be forced 

from the interior high pressure of the enclosure to the lower pressure area outside, 

carrying dust particles outside the system. 

Transfer chutes should be sized to allow ore to flow without clogging or jamming. A 

general rule of thumb is that the chute width should be at least three times the maximum 

lump size to avoid clogging [USBM 1987]. 

The skirtboard for the transfer point should be high and long enough to serve as a plenum 

so that the dust has a chance to settle. On belts with minor air movement, a good rule-of

thumb is two feet per 100 feet/min of belt speed [Goldbeck and Marti 2010]. When belts 

have a greater calculated air movement, the transfer point should approach three feet per 

100 feet/min of belt speed [Goldbeck and Marti 2010]. 

The dump point of the ore should be designed to impact on a sloping bottom or an impact 

bed. Impact beds are designed to allow ore product to build up so that ore contacts ore at 

transfer locations or chutes to reduce wear and abrasion (Figure 5.16). 

Fall height of ore should be minimized whenever possible. Some methods to perform 

this are through the use of rock ladders, telescopic chutes, spiral chutes, and bin-lowering 

chutes. 

Any abrupt changes in product direction or flow should be avoided. 

-

144 Conveying and Transport 
 

 

Figure 5.15. Conveyor transfer enclosure used with an exhaust ventilation system. 

Figure 5.16. Typical impact bed designed for ore-on-ore contact at transfer locations or chutes. 

Assessing the correct amount of the required air volume is essential to effective dust collection. 

Total airflow at a given transfer point can be calculated by Equation 5.1 [Goldbeck and 

Marti 2010]: 

Qtot Qdis Qind Qgen

(5.1) 

where Qtot = total air movement, cubic feet per minute; 

Conveying and Transport 145 
 

 

Qdis = calculated displaced air, cubic feet per minute; 

Qind = calculated induced air, cubic feet per minute; and 

Qgen = determined generated air, cubic feet per minute. 

The amount of displaced air increases as the volume of moving material increases. The amount 

of displaced air can be calculated by Equation 5.2 [Goldbeck and Marti 2010]: 

bulk )(lb/ftdensity 

conveyed product (lb/min) Qtot 3

60

2000t/h x 

lb/min

(5.2) 

Material conveyed along the belt will have a small amount of air entrapped in the product bed. 

As the product leaves the head pulley in the normal trajectory it increases in volume as each 

particle of material collects an amount of air. Once the product lands, this induced air is 

released, causing substantial positive pressure flowing away from the center of the load zone. 

The movement of this induced air can be calculated as follows in Equation 5.3 [Goldbeck and 

Marti 2010]: 

3 10

2

D

SR

ind AQ U

(5.3) 

where Qind = induced air, cubic feet per minute; 

AU = enclosure upstream open area, square feet (where air is induced into the system by the 

action of falling material); 

R = rate of material flow, tons per hour; 

S = height of material free fall, feet; and 

D = average material diameter, feet. 

Other sources of moving air (Qgen) may be devices feeding the load zone such as crushers, and 

these devices can create a fan-like effect. Equipment manufacturers can provide operators with 

additional information to help quantify the amount of air generated by these mechanical devices. 

WATER SPRAYS FOR PREVENTION OF AIRBORNE DUST 

As discussed more fully in Chapter 2—Wet Spray Systems, the use of water to control dust may 

be classified into prevention applications and suppression applications. When properly designed 

and installed, water sprays are a cost-effective method of controlling dust from conveyors. The 

most common and effective practice for conveyor sprays is to wet the entire width of product on 

the belt. The best practices for the application of water sprays include: 

146 Conveying and Transport 
 

 

High-volume, high-pressure sprays should be avoided. The energy of the spray transfers 

to the material particles, resulting in more dust production [Goldbeck and Marti 2010]. 

The amount of moisture applied should be varied and tested at each operation to 

determine the optimum quantity, but a one percent moisture to product ratio is a good 

starting point. Excess moisture can promote slippage and adversely affect conveyor belt 

performance in cold weather conditions. 

Some studies indicate that wetting the return side of the conveyor belt also helps to 

minimize dust liberation. This practice reduces dust generation from the idlers as well as 

at the belt drives and pulleys. 

Water sprays located on the top (wetting the product) and the bottom (reducing dust from 

the idlers) at the same application point can be a very effective strategy [Courtney 1983; 

Ford 1973]. These locations are also beneficial from an installation and cost standpoint. 

Fan spray nozzles are the most commonly used because they minimize the volume of 

water added for the amount of coverage. It is more advantageous to locate the water 

sprays at the beginning of the dust source (i.e., the dump or transfer location) because as 

the water and ore continually mix together, the wetted surface of the ore grows in 

volume, increasing the suppression potential and reduction in dust liberation. 

Using more spray nozzles at lower flow rates and positioning them at locations closer to 

the ore is more advantageous than using fewer sprays at higher flow rates [NIOSH 2003]. 

Suppression of airborne dust involves the application of water, either untreated or chemically 

treated, in the form of spray, fog, or foam to prevent the dust fines from escaping into the air. 

Fog and foam are less used than water application alone, in particular because of the potentially 

higher costs and chemical contamination issues. 

SCREW CONVEYORS 

Screw conveyors are one of the oldest and simplest methods for moving bulk materials 

(Figure 5.17). 

Figure 5.17. Depiction of a typical screw conveyor. 

Conveying and Transport 147 

 They consist of a conveyor screw rotating in a stationary trough. Material placed 

in the trough is moved along its length by rotation of the screw. Screw conveyors can be 

mounted horizontally, vertically, and in inclined configurations. 
 

 

Normally, screw conveyors are totally enclosed except at the ends, where dust emissions can be 

controlled by proper transfer chute design. The screw conveyor cover is usually fastened by nuts 

and bolts. However, to maintain a proper dust seal, a self-adhesive neoprene rubber gasket 

should be installed. 

Screw conveyors are essentially dust- and spillage-free as long as the casings, packing glands, 

and transfer chutes are properly designed and maintained. The only location where spillage 

typically may occur is where the screw shaft exits the screw conveyor housing. Pedestalmounted pillow block support bearings (Figure 5.18) should be used and mounted outside the 

casing. Packing gland seals can be installed where the shaft exits the casing and before the 

bearing. This will prevent spillage and protect the bearings. 

Figure 5.18. Packing gland and pillow block support bearing. 

BUCKET ELEVATORS 

A typical bucket elevator (Figure 5.19) consists of a series of buckets mounted on a chain or belt 

that operates overhead and tail pulleys. The buckets are loaded by scooping up material from the 

boot (bottom) or by feeding material into the buckets. Material is discharged as the bucket 

passes over the head pulley. 

148 Conveying and Transport 
 

 

Figure 5.19. Depiction of a bucket elevator. 

A steel casing usually encloses the entire assembly and effectively contains dust unless there are 

holes or openings in the casings. Dust emissions typically occur at the boot of the elevator where 

material is being fed into the elevator or at the head of the elevator where material is being 

discharged. 

Emissions at the boot of the elevator can be controlled by proper design of a transfer chute 

(similar to belt conveyors) between the feeding equipment and the elevator. Dust can be reduced 

significantly by keeping the height of material fall to a minimum and by gently loading material 

into the boot of the elevator. 

Controlling dust emission at the discharge end of the bucket elevator can be accomplished by 

proper venting to a dust collector (see Chapter 1—Fundamentals of Dust Collection Systems), as 

well as through the use of proper enclosures and chutes between the elevator discharge and the 

receiving equipment. It is recommended that at least 100 cfm of dust collection air be provided 

for every square foot of casing cross-sectional area [ACGIH 2010]. Dust collection pickup 

should be provided from the top of the casing, just above the head pulley (Figure 5.20). 

Additional ventilation may be needed for belts traveling over 200 fpm. If the elevator is over 30 

feet tall a second pickup should be installed on the side of the casing just above the tail pulley. 

Dust collection ventilation should also be provided where the material is being discharged into 

the elevator. 

Conveying and Transport 149 
 

 

Figure 5.20. Depiction of a typical bucket elevator dust collection process. 

PNEUMATIC CONVEYANCE 

Pneumatic conveyors are tubes or ducts through which material is moved by pressure or vacuum 

(suction) systems. Positive pressure systems can be either dilute phase or dense phase. Dilute 

phase uses a low (dilute) product to air ratio for transport, while dense phase uses a high (dense) 

product to air ratio. Dilute phase flow is when the air velocity in the conveyor line is high 

enough to keep the product being conveyed airborne. Dense phase does not require the product 

to be airborne. Material being conveyed lies for periods of time in the bottom of a horizontal 

line and sometimes flows through the line in slugs. Dilute phase systems typically operate at 

pressures obtainable from a fan and dense phase systems use a high-pressure compressed air 

source. 

When material is fed into a pressure system, the material is conveyed to a storage bin with dust 

collection, cyclone, or filter-type collector. The conveying air then escapes through the cyclone 

vent or a filter (Figure 5.21). 

150 Conveying and Transport 
 

 

Figure 5.21. Depiction of two types of pneumatic conveying systems. 

Since positive pressure pneumatic systems are totally enclosed, dust emissions do not usually 

occur unless the system has worn-out areas. Because maximum wear in the conveying ductwork 

occurs at elbows, long-radius elbows made of heavy gauge material should be used. Numerous 

styles of wear-resistant elbows are available (Figure 5.22). 

Figure 5.22. HammerTek wear-resistant elbow installed in conveying ductwork. 

The elbows can also be lined with 

refractory or ceramic material to further reduce wear and abrasion. In low-pressure pneumatic 

systems, dust may also leak through joints. Self-adhesive neoprene gaskets should be used at all 

joints to provide a dust-tight seal. 

Dilute phase pneumatic conveying systems operate under pressure and require some mechanical 

means, such as a rotary valve or double dump valve, to provide an airlock and prevent transport 

air from short circuiting into the feed material storage bin (Figure 5.23). These types of devices 

employ moving parts and seals that are often subject to high wear and significant maintenance. 

Conveying and Transport 151 
 

 

The dismantling of these airlock devices can also be a significant source of dust for mechanics 

and requires downtime for process equipment. 

Figure 5.23. Rotary airlock feeding pneumatic conveying line. 

Venturi eductors are often a good alternative to the mechanical airlock devices. Eductors 

convert the blower output into a suction that is used to draw the feed material into the conveying 

line (Figures 5.24 and 5.25). 

Figure 5.24. Conveyance system utilizing a venturi eductor device. 

152 Conveying and Transport 

These devices have no moving parts and typically require minimal 

repair and downtime for maintenance activities. The venturi eductor can be used on dust 

collectors, grinders/mills, belt and vibratory feeders, etc. Systems utilizing this type of device 

are generally designed by the manufacturer due to variables associated with the process. 
 

 

Figure 5.25. Venturi eductor utilizing suction to draw the feed material into the conveying line. 

Vacuum systems offer clean, efficient pickup of material from rail cars, trucks, or bins and 

hoppers for unloading into other types of equipment. Since the systems are under a negative 

pressure dust leakage is not normally a problem. Cyclone receivers or filters are used at the end 

of such systems to separate the material. The level of separation desired will determine the type 

of separation equipment required. 

REFERENCES 

th ACGIH [2010]. Industrial ventilation: a manual of recommended practice for design. 27 ed. 

Cincinnati, OH: American Conference of Governmental Industrial Hygienists. 

CEMA Standard 575–2000. Bulk material belt conveyor impact bed/cradle selection and 

dimensions. [http://www.cemanet.org/publications/previews/CEMA%20Standard%20575%20– 

2000pv.pdf]. 

Courtney WG [1983]. Single spray reduces dust 90%. Coal Mining & Processing. June, 

pp. 75–77. 

Ford VHW [1973]. Bottom belt sprays as a method of dust control on conveyors. Min 

Technology (United Kingdom) 55(635):387–391. 

Conveying and Transport 153 
 

 

Goldbeck LJ, Marti AD [2010]. Dust control at conveyor transfer points: containment, 

suppression and collection. 

[http://www.ckit.co.za/secure/conveyor/troughed/transfer_points/transfer_ponits_dust_control.ht 

ml]. Date accessed August 23, 2010. 

rd MAC [1980]. Design guidelines for dust control at mine shafts and surface operations. 3 ed. 

Ottawa, Ontario, Canada: Mining Association of Canada. 

NIOSH [2003]. Handbook for dust control in mining. By Kissell FN. Pittsburgh, PA: U.S. 

Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, NIOSH Information Circular 9465, 

DHHS, (NIOSH) Publication No. 2003–147. 

Roberts AW, Ooms M, Bennett D [1987]. Bulk solid conveyor belt interaction in relation to belt 

cleaning. Bulk Solids Handling 7(3):355–362. 

Swinderman RT, Goldbeck LJ, Marti AD [2002]. Foundations 3: the practical resource for total 

dust & material control. Neponset, Illinois: Martin Engineering. 

Swinderman RT, Marti AD, Goldbeck LJ, Strebel MG [2009]. Foundations: the practical 

resource for cleaner, safer, more productive dust & material control. Neponset, Illinois: Martin 

Engineering Company. 

USBM [1974]. Survey of past and present methods used to control respirable dust in noncoal 

mines and ore processing mills—final report. U.S. Department of the Interior, Bureau of Mines 

Contract No. H0220030. NTIS No. PB 240 662. 

USBM [1987]. Dust control handbook for minerals processing. U.S Department of the Interior, 

Bureau of Mines. U.S. Bureau of Mines Contract No. J0235005. Martin Marietta Corporation 

and Marcom Associates, Inc. 

Yourt GR. [1990]. Design principles for dust control at mine crushing and screening operations. 

Canadian Mining J 10:65–70. 

154 Conveying and Transport 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Conveying and Transport 155

CHAPTER 6: 

BAGGING 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING

156 Conveying and Transport 
 

 

CHAPTER 6: BAGGING 

This chapter discusses techniques used by mineral producers for controlling worker dust 

exposures while bagging and stacking product in various types of bags for shipment to 

customers. The loading of product into some type of container is normally called "bagging." 

The stacking of these bags of product onto pallets for shipment to customers is typically called 

"palletizing." There is a wide spectrum of different types of bags that are used to ship product to 

customers, ranging from 50-pound to over one-ton bulk bags. Only the smaller type bags 

(100 pounds or less) are typically palletized because the larger bulk bags are in most cases, 

individually shipped. Both the bagging and palletizing process can be performed manually or 

through some type of semi-automated or totally automated process. Dust generated by the 

manual bagging and palletizing of 50- to 100-pound bags directly affects a number of worker's 

dust exposure. 

Workers performing bagging and stacking tasks, or working in and around these areas, typically 

have some of the highest dust exposures of all workers at mineral processing operations. A few 

of the difficulties with controlling dust in work processes associated with bagging and palletizing 

are the wide range of different equipment being used, as well as the variety of bag types. The 

bagging process can range from single-station manual bagging units to fully automated multistation machines. 

In addition to the potential for high respirable dust exposures from the bagging and palletizing 

process, there is also a significant risk for musculoskeletal disorder (MSD) from all the repetitive 

motion and the significant weight lifted by workers while performing tasks associated with these 

processes. The dust exposure and MSD concerns have motivated many operations to pursue the 

implementation of equipment that either semi-automates or totally automates these work tasks. 

BAGS AS DUST SOURCES 

To address problems associated with the bagging process, a number of different dust sources 

need to be addressed and controlled, specifically product blowback, product "rooster tail," and 

contaminated bags. When manually bagging and stacking 50- to 100-pound bags, these dust 

sources directly affect the worker's exposure. Two different types of bags that are used to 

transport product within this weight range are open-top bags and closed bags with an internal 

valve. 

Open-top bags are slid up over the loading chute and filled with product in a mass loading 

technique. Open-top bags are typically used for larger particle sizes called whole-grain, which 

are normally greater than 120 mesh (0.125 mm) in size. The dust sources when loading open-top 

bags are leakage from the loading chute once the bag is removed and liberation from the open 

bag before it is sealed. Sometimes these open-top bags are moved into a cage device to properly 

position the bag before sealing; this task of moving the bag into the cage can also be a dust 

source. Since these bags are typically used with large particle sizes, dust generated or liberated 

is not normally as significant as with the valve-typed bags (closed-top). 

Bagging 157 
 

 

For valve-type bags, three major dust sources need to be addressed for effective dust control. 

The first dust source is from product blowback, which occurs during bag filling and results from 

product spewing out of the bag valve. Product blowback occurs as excess pressure builds inside 

the bag during bag filling and is then relieved by air and product flowing out of the bag valve 

around the fill nozzle. The second major dust source is product spewing from the fill nozzle and 

bag valve as the bag is ejected from the filling machine. This is typically called a "rooster tail"

because of the spray pattern of the escaping product. Both product blowback and product 

"rooster tail" occur because of the air pressure injected into the bag with product during filling. 

Both release dust into the air and contaminate the outside surface of the bag. The contaminated 

bags then become the third significant source of dust exposure for the bag stackers, or for any 

other individuals handling the bags, including the end user of the product. Figure 6.1 shows 

product blowback during bag filling, the product "rooster tail" as the bag is ejected from the fill 

nozzle, and dust contamination on the outside of the bag after loading is completed. If bags are 

undersized, they create a greater amount of product blowback and "rooster tail" during the 

bagging process, and this needs to be considered when evaluating these dust sources. 

Figure 6.1. Product "rooster tail" shown on left, product blowback 

shown on top right, and dust-soiled bag shown on bottom right. 

With 50- to 100-pound bags, another area that impacts the amount of dust generated is the bag 

valve. The valve is an internal sleeve in the bag through which the fill nozzle is inserted prior to 

the bag being filled with product (Figure 6.2). The type of bag valve used impacts the amount of 

product blowback and product "rooster tail." Once the bag falls from the fill station, the weight 

and volume of the product inside the bag is supposed to force the valve closed, sealing the area 

and keeping product from leaking from it during the conveying, stacking, and the transportation 

process. However, many times the bag valve is lined with product and does not seal properly, 

causing product and dust to leak from the valve during movement/transportation. This sealing 

ability also varies for different types of bag valves. 

One last dust source at bagging operations is from broken, torn, or ruptured bags. When a bag 

breaks, a significant amount of product and dust is released into the mill atmosphere. Defects 

158 Bagging 
 

 

caused during manufacturing, including improper gluing and improper storage of empty bags, 

are just a few of the factors that can cause bag breakage. The location of the break has a 

significant impact on the amount of dust liberated. Bags that break during filling tend to be more 

violently affected than those that break during ejection or conveying. In all cases, bag breakage 

liberates a substantial amount of dust and significantly impacts workers in these areas. 

Figure 6.2. Typical bag valve—interior view (left). Artist concept of fill nozzle and bag 

valve (right). Fill nozzle enters through the bag valve and delivers product into bag. 

BAG CONSIDERATIONS 

A number of different factors ultimately impact the amount of dust generated or liberated from 

bags of product material: bag construction, bag perforations, bag fill type, and the effects from 

bag failures. 

Bag Construction 

The multi-wall pasted valve bag is by far the most common bag used for the packaging of 

mineral products and is the only style discussed below. Many of the considerations discussed for 

pasted valve bags are also applicable to open-top bags. 

A very important element in controlling the amount of dust liberated during the bagging process 

is the design and construction of the paper bag itself. The composition of the paper, its porosity, 

the number of plies or layers used, the "basis weight/thickness" of the paper sheets which 

provides strength, the type of pasted valve, and the use of bag perforations, are all factors that 

affect and can minimize the amount of dust generated or liberated from the bag. 

One area of significant advances by bag manufacturers over the years has been the improvements 

and advances in "paper" technology [Kearns 2004]. Currently, conventional natural kraft (NK) 

paper, also known as flat kraft (FK) paper, is the industry standard. Normal constructions for 50

to 100-pound bags are to use either three or four plies of NK paper. Due to the low porosity of 

NK paper, meaning that air does not flow through the paper material, bag perforations are 

-

Bagging 159 
 

 

normally required for most industrial mineral processing applications to keep the bag from 

rupturing/exploding. 

Another development in paper technology is the use of extensible kraft (EK) paper, sometimes 

referred to as "high performance paper." The strength of this EK paper has been increased to 

such an extent that the number of plies can be reduced by approximately 30 percent in multilayer bags when compared to conventional kraft papers. A savings in paper can be achieved in 

one of two ways. The first, and the most common approach, is to reduce the number of 

plies/layers in the bag. For instance, rather than using NK or FK paper that would normally 

require three or four plies/layers to obtain the needed strength, by using EK paper, the number of 

plies/layers can normally be reduced down to just two. For every ply/layer that is reduced, there 

is a benefit from an increase in the bag's porosity, thus allowing the bag to breathe better. 

Despite its popularity, extensible kraft paper does not allow for the proper ventilation required 

for many packaging applications; therefore, bag perforations are still required to provide the 

necessary air escape during bagging to avoid ruptures. The second way to reduce paper is by 

reducing the weight/thickness of each ply/layer of paper. 

Airflow extensible kraft (AEK) paper was developed in the 1990s and offers the same crossdirectional and machine directional strength as EK paper, but with an air-permeable (breathable) 

characteristic. A great benefit from this air-permeability characteristic is that it eliminates the 

requirement for void spaces in the bag construction, allowing for a smaller bag size. The porous 

paper is superior to perforated paper as far as strength and air evacuation (ability to relieve air 

pressure). No bag perforations are required when using this AEK paper type, and by eliminating 

perforations, bag strength can be increased by as much as 20 percent. The faster the air is 

evacuated from bags, the faster they can be filled and palletized, thus increasing production and 

profitability. 

The AEK paper also allows the flow of air to depressurize the bag at a rate of three to four times 

faster than standard kraft (NK or FK) paper and at the same time retains bag strength. The paper 

acts as a filter during the bag filling process which allows the product to flow freely into the bag 

while containing dust that could be liberated into the work environment and expose workers. 

Because of this, the outer surface of the bag stays cleaner. When considering the various options 

available for bag construction, the use of AEK paper appears to be the best choice to minimize 

the amount of dust generated and liberated from the bag during the filling process. 

Bag Perforations 

As product enters the paper bag during the filling process, fluidizing air entrapped in the product 

must be evacuated in order to fill the bag with product. In order to remove the air as the product 

is introduced into the bag, perforations or vent holes are integrated in the paper used in the bag. 

For this purpose, manufacturers use multi-wall bag perforations, which are small vent holes 

punched through the walls of the paper bag to allow air to escape rapidly during the filling 

process. 

There are two common types of perforations used in bags: overall perforations and undervalve 

perforations. Both types of perforations reduce the strength of a paper bag by as much as 

20 percent. 

160 Bagging 
 

 

Overall Perforations 

The use of overall perforations is a critical factor to consider when evaluating the effectiveness 

of bag performance. Each ply of paper is individually perforated during the manufacturing 

process, creating staggered perforations throughout the bag. The perforations (pinholes) do not 

go directly through all layers (2, 3, or 4 plies/layers) and are not aligned with each other. The 

goal is to allow the fluidizing air to work its way out of the bag while still containing the 

product/material. Perforations allow the bag to depressurize, minimizing the possibility of it 

rupturing during bag filling. Since the perforations are staggered for each of the different 

plies/layers, it is uncommon for the product being bagged to escape through the perforations, 

however, this can sometimes occur for very fine product sizes—i.e., 325 mesh (0.045 mm) and 

finer. As previously mentioned, the bag strength can be lessened by up to 20 percent when using 

perforations. Standard perforation sizes are 1/8, 3/32, and 1/16 inches, and patterns range from 

3/4 x 3/4 inch centers up to 2 x 3 inch centers (Figure 6.3). 

Figure 6.3. Undervalve bag perforations (left) and overall perforations (right). 

Undervalve Perforations 

Unlike overall perforations, undervalve perforations are created after all plies are positioned into 

the finished bag stage. They are punched through all 2, 3, or 4 plies/layers at the same time and 

are aligned. Since the fluidizing air is most likely to escape out of the bag valve around the fill 

nozzle, (product blowback), having perforations as close as possible to the valve area is most 

advantageous/effective. In addition, this location also minimizes the amount of bag 

contamination as the excess air and product escape. The perforation pattern can range from 1/4 x 

1/4 inch centers up to 1 x 1 inch centers. Perforation sizes are the same as for the overall 

perforations, being: 1/8, 3/32, or 1/16 inches. Using an exhaust hood with a local exhaust 

ventilation (LEV) system is most beneficial in this application because of the likelihood of 

product escaping from the bag during the filling process. Figure 6.4 shows an example of 

undervalve perforations and how they depressurize the bag to keep it from rupturing or 

exploding during bag filling. 

Bagging 161 
 

 

Figure 6.4. Undervalve perforations relieve excess pressure from 

bag during filling process to minimize the possibility of bag failure. 

 Product and dust are seen escaping along with excess air. 

Because of the product and dust emitted from the perforations during bag filling, most filling 

operations require some type of ventilation system to capture this dust. Two common techniques 

to achieve this dust capture are to place exhaust hoods around the bag loading nozzles and/or to 

use an exhaust hopper located below the bag loading station with an LEV system incorporated. 

These techniques can be used individually or together, depending on the level of dust control 

needed. Both of these techniques are discussed in greater detail later in this chapter. 

Bag Fill Type 

Two types of bags will be discussed in this section: open-top and valve bags. Figure 6.5 shows 

a typical open-top bag loading process. The bag operator slides an open-top bag up over a 

loading spout and then activates a start button to begin the loading process. The bag is then 

filled relatively quickly in a bulk loading fashion. During filling, the top of the bag is wellsealed and typically tied into an LEV system to exhaust any dust generated during the loading 

process. When filling is completed, the bag automatically releases from the loading spout and 

drops a short distance down onto a conveyor. The top of the bag is then sealed using either heat 

sealing/gluing or is sewn (stitched) by a machine with nylon thread. Both of these sealing 

techniques are effective and create minimal amounts of dust. 

162 Bagging 
 

 

Figure 6.5. Open-top bag being loaded with product 

(normal whole-grain material, which is less dusty). 

The second and most common bag type used in mineral processing operations is the pasted valve 

bag. The choice of valve type from among different commercially available bag valves can be a 

significant factor in the ability to effectively seal 50- to 100-pound bags of product and to 

minimize the amount of dust liberated during the bag filling, conveying, and stacking process. 

From a study performed a number of years ago, five different valves were tested to compare their 

effectiveness at sealing the bag and minimizing dust liberation. The valves were: standard 

paper, polyethylene, extended polyethylene, double trap, and foam [USBM 1986b; Cecala and 

Muldoon 1986]. By far the most effective valve was the extended polyethylene (Figure 6.6). 

This is simply a plastic valve approximately two inches longer than the standard paper or 

polyethylene valve. 

In the study, two factors determined the effectiveness of the bag valve: valve length and valve 

material. The longer valves were more effective in reducing the amount of product blowback 

and bag-generated dust. However, it is speculated that when the valve length became too much 

longer than the fill nozzle, it began to negatively impact the bag filling performance. For the 

second factor, which is valve material, foam was the most effective material tested. The foam 

valve was the shortest valve tested for the evaluation at four inches in length, while the extended 

polyethylene valve was six inches long. The rankings of valve types from the most to least 

effective were as follows: extended polyethylene, foam, standard paper, polyethylene, and 

double trap. Figure 6.7 shows a comparison of the extended polyethylene and the foam valve 

compared to the standard paper valve. Respirable dust levels ranged from approximately 45 to 

65 percent lower with the extended polyethylene as compared to the standard paper valve 

[USBM 1986b; Cecala and Muldoon 1986]. 

Bagging 163 
 

 

Figure 6.6. Extended polyethylene bag valve, interior view. 

Figure 6.7. Dust reductions with extended polyethylene and foam valves 

as compared with the use of the standard paper valve. 

164 Bagging 
 

 

Bagging operators should be aware that changes in product leakage and dust liberation are 

directly based on the type and effectiveness of the bag valve used in their bags. The extended 

polyethylene valve was the most effective valve with only a minor cost increase over the 

standard paper valve. 

A recent review of bag valves revealed that the standard paper, polyethylene, and extended 

polyethylene valve types are still the main valves being used by industry. Based upon these 

choices, operators should definitely consider using the extended polyethylene valve type to 

minimize the amount of dust liberated during bag loading, conveying, and bag stacking process. 

Another commercially available bag valve type is the sonic energy sealed bag valve, in which the 

valve is glued after filling. This valve construction has adhesive on the inner surface of the 

valve, activated by sonic energy applied either at the packer fill nozzle or from ancillary 

equipment downstream. Theoretically, heating/melting of the adhesive should provide improved 

sealing of the inner surface of the valve and thus minimize leakage. 

A problem that sometimes occurs with this valve type is that the glue does not properly seal 

because of product in the valve area, which then leads to product spillage. Another drawback 

with this valve type is that the cost to perform this sonic energy sealing is typically much higher 

than for the other types of valves. 

Effects of Bag Failures 

In addition to the considerations mentioned above, bag failure from broken, torn, or ruptured 

bags can occur during the bag filling, transportation, or handling process and can have 

catastrophic effects on the amount of dust generated or liberated into the work environment. 

When a bag breaks during bag filling, it can be a violent occurrence which quickly releases a 

significant amount of dust and product into the mill atmosphere. Another concern from this type 

of bag failure is that most dust control equipment is not normally sized adequately to handle the 

large volume of dust released when this occurs. Gluing defects during manufacturing and the 

improper storage of empty bags (storing for long periods of time in high humidity and/or high 

temperatures, or damages to bags during storage) are just a few of the circumstances that can 

cause bag failure problems during filling. 

In addition to bags rupturing during loading, another problem can be bag failures during the 

conveying or pallet loading process, which can also cause a significant amount of dust to be 

liberated into the work environment. Bag failures most often occur from rips and tears from 

sharp edges during conveying, or damage from mishandling during pallet loading. 

Every operation needs to address how it will deal with bag failures because of the significant 

impact that they can have on overall mill respirable dust concentrations, and ultimately to 

workers in the plant. Some operations use a bag scale (weighing) system during conveying 

which identifies underweight (broken) bags and diverts them onto another conveyor line. This 

secondary conveyor then takes the bags and discards them into a disposal device, normally a 

dumpster unit so the bags do not need to be rehandled. It is recommended that this dumpster unit 

be located outside the plant to minimize the possibility that the dust liberated as the bags are 

discarded contaminates the plant air. Once the dumpster is full, it is taken away and the bags are 

Bagging 165 
 

 

disposed of in a safe manner. A new dumpster is then positioned at the dump point and the cycle 

is repeated. 

CONTROL TECHNOLOGIES FOR FILLING 50- TO 100-POUND BAGS 

The bag filling process can range from a single-station manual bagging unit to a fully automated 

multi-station machine. This section addresses various control techniques to reduce respirable 

dust during bag filling of 50- to 100-pound bags. 

Exhaust Hood Placed around Bag Loading Nozzles 

One of the earliest techniques developed to capture dust generated from bag filling machines was 

a very simple exhaust hood, which surrounds the bag to capture the dust generated and liberated 

during the bag filling process [USBM 1983]. If the fill machine is a multiple bag filling unit, an 

individual exhaust hood is placed around each fill nozzle. The exhaust hood is normally shopfabricated from sheet metal and can be adapted to fit most valve type bag fill machines 

(Figure 6.8). 

To minimize installation and maintenance time, the hood is normally constructed in quarterpiece sections to allow it to be more easily installed and bolted together. The bottom of the hood 

should be sloped to allow product to slide from the hood and fall down into a hopper, normally 

located below the bag filling station. The exhaust hood also needs to be designed to allow the 

bag clamp, fill nozzle, or pinch tube to enter through the back of the hood. 

To accommodate the installation of an exhaust hood on retrofit systems, one item that sometimes 

needs to be relocated is the start/stop button which can easily be moved to a location outside the 

hood. The intent is for the hood to capture the respirable dust generated during the bag filling 

process (blowback and "rooster tail"), while allowing the oversized and heavier product to fall 

into the hopper and be recycled/reused. 

Each exhaust hood should be connected to an LEV system to capture the dust generated and 

liberated during bag filling and bag ejection. The hood's exhaust volume requirement is a 

function of the open hood area, with tests showing that air velocities of 200 feet per minute (fpm) 

into the hood are adequate for dust capture/containment. A typical hood size is in the range of 

2 4 ft of open face area, which requires an exhaust volume of 800 cubic feet per minute (cfm). 

Exhaust ventilation hoods have been shown to be from 90 to 100 percent effective at reducing 

dust leakage into the plant [USBM 1981]. For information on tying these hoods into LEV 

systems, refer to Chapter 1—Fundamentals of Dust Collection Systems. 

166 Bagging 
 

 

Figure 6.8. Exhaust hood to capture and exhaust respirable dust away from each fill nozzle. 

Exhaust Hoppers (Below Entire Fill Station) 

Similar to the above technique, which places an exhaust hood around each individual bag 

loading unit, a hopper can also be placed below the entire fill station to capture and recycle all 

the dust and product lost from the entire bag loading area or station. This is especially beneficial 

with three- and four-fill-nozzle bag loading systems. In these cases, incorporating the LEV into 

a single-hopper design is easier and more financially advantageous than trying to connect 

ductwork to individual hoods around each fill nozzle. 

Figure 6.9 shows a typical design for a single-hopper LEV type system. One critical design 

component is to locate the exhaust system takeoff points high in the hopper unit and in close 

proximity to the actual bag loading area. The heavier dust and product that is not captured by the 

exhaust system simply falls into the base of the hopper and is recycled back into the product line 

using a screw conveyor. 

Bagging 167 
 

 

Figure 6.9. Exhaust hopper below bag loading station with integrated 

LEV system to capture dust generated during bag loading process. 

One critical component for both the exhaust hood being placed around each bag loading nozzle 

and the exhaust hopper being below the entire fill station is to provide clean (dust-free) makeup 

air to the bag filling area. Any time an LEV system is used to capture dust generated at a 

bagging station, air will be drawn from the surrounding area as makeup air. Since this makeup 

air flows over the bag operator before being captured by the LEV system, it is important that it 

be dust-free so as to not increase the exposure of the bag operator [USBM 1986a; Constance 

2004]. Outside air can be used as makeup air when there are no outside dust sources near the 

intake location and when outside air temperatures are acceptable. 

Dual-Nozzle Bagging System 

The primary dust sources for manual bagging have been identified as product blowback, product 

"rooster tail," and contaminated bags. The dual-nozzle bagging system is designed to reduce 

these major dust sources and lower the exposure of the bag operator. Figure 6.10 depicts the 

major components of the dual-nozzle bagging system. This system uses a two-nozzle 

arrangement with an improved bag clamp to reduce the amount of product blowback and product 

"rooster tail."

168 Bagging 
 

 

In the original design, a single nozzle is used to fill the bag with product. With the two-nozzle 

arrangement, the inner nozzle is used to fill the bag and the outer nozzle relieves excess pressure 

from the bag after it has been filled. Once filling is completed, depressurization of the bag is 

accomplished with the aid of an eductor, which uses the venturi principle (compressed air 

entraining additional exhaust air) to exhaust excess air from the bag at approximately 50 cfm. 

This exhaust is controlled by a pinch valve, which is opened when the exhaust is on. The bag is 

slightly overfilled and held in place by the bag clamps, then the pinch valve is opened and the 

exhaust is initiated to depressurize the bag. After a few seconds, the bag clamp opens and the 

bag falls from the fill station. The exhaust system also continues to operate as the bag falls 

away, cleaning the bag valve area. The exhausted material can then be recycled back into the 

system. 

Figure 6.10. Dual-nozzle bagging system design. 

Another key component of this dual-nozzle bagging system is an improved bag clamp. With the 

original bag clamp, there were only two small points of contact with the fill nozzle. The 

improved bag clamp makes direct contact with approximately 60 percent of the nozzle 

(concentrated at the top and sides), thus reducing the amount of product blowback during bag 

filling. A controlled amount of blowback is necessary so the bag does not rupture during filling, 

but this should occur at the bottom of the nozzle to minimize the amount of dust contamination 

to the outside of the bag. 

Bagging 169 
 

 

The dual-nozzle bagging system can be very effective at lowering respirable dust levels to 

workers performing the bagging and palletizing tasks [USBM 1984a,b]. Figure 6.11 indicates an 

83 percent reduction in the bag operator's respirable dust exposure before and after the 

installation of a dual-nozzle bagging system at one plant.

Figure 6.11. Bag operator’s dust exposure with and without 

the use of a dual-nozzle bagging system. 

 A 90 percent reduction was also 

measured in the hopper below the fill station. This represents a substantial reduction in product 

blowback and product "rooster tail" during the bagging process. The dual-nozzle bagging 

system also resulted in less product and dust on the outside of the bags and accounted for a 90 

percent reduction in the bag stacker's dust exposure while bags were being loaded into an 

enclosed vehicle. 

The dual-nozzle bagging system is mainly recommended for operations with three- and four-fill

nozzle bag machines because there is a slight decrease in production due to the time needed to 

depressurize the bags after filling is completed. The system can be used on one- and two-nozzle 

filling machines, but the decrease in the production rate is even more significant because the bag 

operator must wait for each individual bag being depressurized, rather than waiting on a cycle of 

bags. Other similar systems that depressurize the bag after filling is completed have also been 

designed and sold [Case History 2006a]. 

-

Overhead Air Supply Island System 

The overhead air supply island system (OASIS) can be used to provide an envelope of clean, 

filtered air to a worker at a stationary bagging or palletizing unit. One of the main advantages of 

the OASIS is that it is suspended over a worker and operates independently of any processing 

equipment. Figure 6.12 shows an OASIS located over a bag operator. 

170 Bagging 
 

 

Figure 6.12. OASIS positioned over bag operator to deliver clean filtered air down over work area. 

The OASIS is a relatively simple design and system. Mill air is drawn into the unit and passes 

through a primary filtering chamber, normally using HEPA quality filters. After the air exits the 

primary chamber, it passes through an optional heating or cooling area, which can be 

incorporated into the unit if temperature control is desired. The air then flows through a 

distribution manifold filter when exiting the unit. This provides an even distribution of clean 

filtered airflow down over the worker, while providing backup filtering in case there is a problem 

with the primary filtering chamber. If the bagging or palletizing process generates a significant 

amount of dust, it will contaminate the envelope of clean air from the OASIS after it flows down 

over the worker. In these cases, it is normally advisable to then capture this dust-laden air with 

an LEV system. 

With the OASIS, the target is to have an average velocity of roughly 375 fpm flowing down over 

the worker, which normally keeps any dust-laden mill air from entering the clean air core 

[Volkwein et al. 1986]. One important feature with an OASIS system is to incorporate a selfcleaning primary filter design. There are a number of different self-cleaning techniques used to 

remove the dust cake from the filters. For example, the reverse air jet pulse collector is a very 

common and effective method to perform this task (refer to Chapter 1—Fundamentals of Dust 

Collection Systems). 

The OASIS is very effective at lowering workers' respirable dust exposures. A bag operator's 

respirable dust exposure was reduced by 82 and 98 percent at two different operations using the 

OASIS, as compared to when the unit was not being operated [Robertson 1986]. At both of 

Bagging 171 
 

 

these operations, the dust concentration within the clean filtered air of the OASIS remained 

3 under 0.04 mg/m . 

When the OASIS is used with standing workers, it can be advantageous to place clear plastic 

stripping down the sides of the device. This allows the workers to recognize the boundary of the 

clean air zone and the inability to be protected once exiting the physical barrier of the plastic 

stripping. 

An additional benefit provided by the OASIS is a general improvement to the overall mill air 

quality. This occurs because the OASIS is drawing air from the plant environment, filtering it, 

and then blowing this clean air down over a worker. After flowing over the worker, it then 

becomes part of the general mill air again. At one site using the OASIS, there was a 12 percent 

reduction in respirable dust levels throughout the entire mill building [Volkwein et al. 1986]. 

The volume of clean air delivered by the OASIS is variable based upon the size of the unit, but it 

is normally in the range of 6,000 to 10,000 cfm. Other studies have also shown the benefit of 

recirculating air back into a plant after it has been cleaned [Godbey 2005]. The OASIS is 

generic in design and can be fabricated and installed in-house or through any local engineering 

company that handles ventilation and dust control systems. 

Automated Bag Placer and Filling Systems 

Many manufacturers provide automated bagging systems that eliminate the operator from 

performing the bagging task manually. These systems normally include both a mechanical bag 

placer and a product filling system. For most automated systems, a worker is only required to 

place empty 50- to 100-pound bags on a storage rack. From this point forward, the system 

becomes automated. Normally some type of robotic arm using suction cups takes the unfilled 

bags from a storage rack and places them on the individual fill nozzles of the bag filling unit. 

When the bag is in place, the filling process starts and continues until a sensor indicates that the 

bag has reached the correct weight. The bag saddle then actuates to eject the loaded bag from 

the fill nozzle/fill station. Figure 6.13 shows an example of a bag storage area and an automated 

device to place the bag on the fill nozzles. 

For open-top bags, typically a semi-automated bagging system places, fills, and weighs the bags 

of product, then a worker manually feeds the bags through a closer device. For fully automated 

systems, all tasks are performed automatically and the operator/worker only needs to monitor the 

system's performance. 

There are many different factors to evaluate when an operation is considering some level of 

automation (semi-automatic to fully automatic) [White 2006]. For facilities only bagging one 

shift per day, semi-automated systems are more commonly used. When operations bag many 

different product types, the cleanout or change over between batches is also easier with semi

automatic systems. Automated bagging systems require more care and maintenance than 

nonautomated systems. It is also important to stock some spare parts for these systems, 

especially if the vendor is not located nearby. There are fewer parts necessary for semiautomated systems; thus a smaller inventory of these critical parts is necessary. 

-

172 Bagging 
 

 

Figure 6.13. Bag storage shelves shown on left; a robotic arm with suction 

cups to place empty bags on fill nozzle shown on right. 

With a totally automated system, greater production can be achieved. In addition, savings can be 

achieved with automated systems because they eliminate the worker at the bagging process. 

Eliminating the need to have workers performing tasks manually reduces their respirable dust 

exposure, MSD, and lost-time injuries. 

One aspect that does not change from manual to automated bagging systems is to have an LEV 

system incorporated into the bagging station to capture and remove the dust liberated from 

product blowback and product "rooster tail." Although workers are not located directly within 

the bag filling area, if the dust is not captured during bag loading and ejection from the fill 

station, it will flow out into the work environment and contaminate workers in and around the 

automated bag filling system. One very effective technique often used is to enclose the bag 

filling area with clear plastic stripping. This provides a physical barrier that indicates the 

potential dust-laden air zone to workers and allows the LEV a better opportunity to contain and 

capture the dust liberated from bagging. 

The initial costs of automated systems are much greater than for manual systems. If a cost 

analysis is performed, costs must be evaluated on a long-term basis, considering the respirable 

dust exposure and MSD exposure to workers performing the tasks manually. 

Bagging 173 
 

 

CONTROL TECHNOLOGIES FOR CONVEYING AND PALLETIZING 

50- TO 100-POUND BAGS 

Bag and Belt Cleaning Device 

The bag and belt cleaning device (B&BCD) is designed to reduce the amount of dust escaping 

from bags as they travel from the bag loading station to the stacking/palletizing area. This 

device reduces the dust exposure of all workers involved in and around the conveying process, as 

well as anyone handling the bags once they are filled at the loading station. 

These systems are applicable to any mineral processing operation that loads product into 50- to 

100-pound paper bags. This system should be located in-line on the conveyor at the closest 

available position to the filling station. 

In one particular application, the B&BCD was 10 feet long and used a combination of brushes 

and air jets to clean all sides of the bags (Figure 6.14). Initially, the bags enter the B&BCD 

through a curtain made of clear, heavy-duty flexible plastic stripping to seal it from the plant air. 

The bags then travel past a stationary brush on a swing arm that cleans the front and top of each 

bag. Next, the bags go through a second set of plastic stripping and into the main cleaning 

chamber. The bags then travel under a rotating circular brush that further cleans the top of each 

bag. The sides are cleaned by a stationary brush located on each side of the bag, and an air jet 

located at the end of these brushes provides for additional cleaning. The side of the unit facing 

the valve uses a higher volume (velocity) air jet to provide the maximum amount of cleaning of 

the valve area. After passing through the air jets, the bag travels over a rotating circular brush 

beneath the bag which cleans the bottom of the bag. Finally, the bag exits the device by 

traveling through another air lock chamber with flexible plastic stripping. 

A chain conveyor is used for the entire length of the device to allow product removed from the 

bags during cleaning to fall into a hopper. Product collected in this hopper is then recycled back 

into the process normally using a screw conveyor. Once exiting the B&BCD, both the bags of 

product and the conveyor belt should be essentially dust-free. 

During the evaluation of this specific B&BCD at two mineral processing plants, the device 

showed very favorable results in lowering respirable dust levels. The most relevant data from 

this testing was the amount of dust removed from the surface of the bags, with a 78 to 90 percent 

range of reduction [Cecala et al. 1997; USBM 1995]. 

Another common source of dust generation and liberation from bags is during the bag flattening 

process. In operations that use bag flatteners, this task could be performed within the B&BCD to 

capture the dust released during this process. 

174 Bagging 
 

 

Figure 6.14. Bag and belt cleaner device. Note the use 

of a closed system under negative pressure. 

Although this section describes one particular B&BCD system, there are numerous types that can 

be purchased from manufacturers, fabricated in-house by mineral processing plants, or fabricated 

by a local engineering firm. One key factor for a system is to be self-supporting so it can be 

implemented anywhere along the conveyor belt line. It is more logical to place a B&BCD in the 

closest possible proximity to the filling station to eliminate the dust hazards as quickly as 

possible. These devices normally need electrical and compressed air to power the cleaning unit 

and the blow-off nozzles, respectively. 

One last critical aspect is that a B&BCD must be under sufficient negative pressure to ensure that 

any dust removed from the bags or conveyor belt does not leak from the cleaning unit into the 

mill. Refer to Chapter 1—Fundamentals of Dust Collection Systems for the specifics on the 

HVAC system. 

Semi-Automated Bag Palletizing Systems 

Semi-automated systems use workers in conjunction with an automated system to perform the 

bag stacking process. These can include a vast array of different setups and types of systems. In 

one case, a worker performs the bag stacking task manually, but is assisted by a hydraulic lift 

table. This lift table allows the height for stacking the bags to remain constant throughout the 

Bagging 175 
 

 

entire pallet loading cycle. The lift table is set to approximately knuckle-high for the worker, 

which is the most ergonomic loading height (Figure 6.15). 

Figure 6.15. Semi-automated bag palletizing system to ergonomically improve bag stacking 

process with a push-pull ventilation system used to capture the dust generated. 

 A push-pull ventilation system is 

used on either side of this pallet to capture the dust liberated during the bag stacking (palletizing) 

process [Cecala and Covelli 1990]. This system is composed of a low-volume, high-velocity 

blower operating at approximately 120 cfm to direct a stream of air over the top layer of bags on 

the pallet. The blower system is composed of two 3-inch air jets (approximately 1,200-fpm 

velocity) directed towards an exhaust system on the opposite side of the pallet. As these air jets 

travel approximately 10 to 12 inches above the top of the pallet, they entrain the dust generated 

during bag stacking. The exhaust ventilation system pulls approximately 2,500 cfm of air and 

dust through the exhaust hood. This exhaust air can then be dumped into an LEV system. 

In other cases involving palletizing systems, the worker slides the bag of product on an air table 

one layer at a time, but the actual stacking of the bags onto the pallet is performed automatically. 

Since back injuries represent such a major lost-time injury potential for bag stackers, this design 

significantly reduces back stress by eliminating the need to manually lift any bags of product. 

One problem with an air slide device is that it can cause dust to be blown from the bags of 

product into the worker's breathing zone. An OASIS system (see this chapter's earlier 

"Overhead Air Supply Island System" section) can work very effectively in conjunction with an 

air slide device. 

In addition to the OASIS system, an exhaust hood was also used next to the air slide area to 

capture the dust blown from the bag (Figure 6.16) [Cecala et al. 2000]. With this setup, or any 

176 Bagging 
 

 

other industrial application using an exhaust hood, one area of concern for overexposure is when 

a worker positions himself/herself between the dust source and the exhaust hood. In this 

application, if the worker leans out over the air slide, he or she will be exposed to any dust being 

blown off the bags by the air slide as it is pulled into the exhaust hood. 

Figure 6.16. Semi-automated bag palletizing systems in which a worker removes 

bags from a conveyor and lifts and positions each layer on an air slide before 

automated loading onto the pallet. Both an OASIS system and an exhaust ventilation 

system are used to lower the worker’s dust exposure during this bag palletizing process. 

Automated Bag Palletizing Systems 

Various manufacturing companies have developed advanced designs for automatic palletizing 

equipment that utilize either the traditional high-level pallet loading or low-level pallet loading 

technology. With high-level pallet loading systems, the pallets enter the automated palletizer 

system near the floor and are then raised to perform bag loading. When the pallet is completely 

loaded, the full pallet is then lowered back down to near floor level before being delivered to a 

plastic pallet wrapping system or loadout area. 

With these high-level pallet loading systems, the bags arrive at the unit from the filling station 

via a conveyor belt. Once at the unit, the bags are guided onto a stripper plate that is composed 

of a series of rollers. Bags are arranged onto the stripper plate in a predetermined pattern as 

programmed into the unit by the worker overseeing the automated unit. Once a full layer of bags 

is positioned, the system stripper plate is automatically opened and the entire layer of bags is 

Bagging 177 
 

 

loaded onto the previous layer. The stripper plate then closes, allowing another layer of bags to 

begin loading in a prearranged pattern. Each layer of bags is loaded in an opposite pattern than 

the previous layer, typically with any manually or automated pallet loading system. Each time a 

full layer of bags is completed, the stripper plate is opened and the new layer of bags is stacked 

onto the pallet. When the predetermined number of layers of bags is achieved, the full pallet is 

lowered down near the floor level which ends the automated pallet loading process. Figure 6.17 

shows a typical high-level automated bag palletizing system. There are many different 

manufacturers of these types of systems, with each system having its own unique design 

specifications. 

Figure 6.17. Figure of the automated high-level palletizing process. 

With low-level automated pallet loading systems, the pallets are located near the floor and 

remain stationary throughout the entire loading process. A mechanical device takes the loaded 

bags and places them directly onto the pallet. The use of a robotic mechanical arm type 

palletizer is the most common type of automated low-level system. 

The robotic arm automatic palletizer illustrated in Figure 6.18 has become very popular due to its 

productivity and lower cost of maintenance compared to the in-line/high-level palletizers. Filled 

bags travel from the bagging station to the palletizing area via a conveyor belt. At the palletizing 

area, a photocell detects that a bag has arrived and is ready to be loaded onto a pallet. Each bag 

is grasped by the fingers of the robotic arm, then taken and gently placed onto the pallet in a 

programmed pattern established by a worker overseeing the automated unit. The arm can be 

programmed to close tolerances to avoid the bag from being dropped onto the pallet, which 

would cause dust to be emitted from the bag valve. The robotic arm can also stack bags onto two 

separate pallets and at a high rate of speed to increase productivity. 

178 Bagging 
 

 

The robotic arm automatic palletizers are proving themselves to be the preferred method for 

palletizing bags because they are much simpler, and thus more cost-effective by comparison to 

the high-level pallet loading systems. With these systems, operating costs, breakdowns, and 

maintenance are greatly minimized [Case History 2006b]. They also require much less space 

and minimize all the various dust generation and liberation potential when compared to highlevel pallet loading systems. 

Figure 6.18. New innovation of low-level pallet loading process using a robotic arm device. 

One aspect which is very important in any automated system is to provide adequate guarding to 

protect workers from entering automated work areas while these units are operating, or even 

activated. Because of the operating speeds and the mass of the unit in robotic arm systems, the 

swing pattern can span significant distances and the mechanical arm can cause severe injury, or 

even death, if it strikes a worker. 

When combining these automated palletizing systems with automated bag placers, in-line bag 

cleaning devices, automatic bag weighing systems, printing systems, and plastic pallet wrap 

systems, the worker is removed from the work processes. This in turn substantially reduces the 

worker's dust exposure potential, and the worker has more of an oversight responsibility to 

ensure that the system is working properly. Another significant benefit over and above the dust 

exposure reduction is the elimination of MSD bag stress injuries common to performing these 

various tasks manually. 

It should be stressed that when using automated systems, there is still a concern for dust. The 

amount of respirable dust generated and liberated from automated systems still needs to be 

evaluated and controlled to ensure that it does not flow into the surrounding mill areas and 

contaminate the breathing atmosphere of workers. This is normally achieved by enclosing the 

dust generation areas with plastic stripping and using an LEV system to exhaust the respirable 

dust. 

Bagging 179 
 

 

Plastic Pallet Wrap 

The final consideration for dust control in the palletizing of paper bags is plastic pallet wrap 

systems. The purpose of plastic pallet wrap is to contain product and dust leakage from bags 

loaded onto full pallets from becoming airborne while also providing protection from torn or 

ruptured bags during the pallet transportation process. Currently, there are two types of plastic 

pallet wrap systems being used: (1) spiral stretch wrap, and (2) shrink wrap/stretch hoods. Both 

systems are commonly used in the minerals processing industry and they can be used in both 

manual and automated applications. The size of the packaging system will normally dictate 

which pallet wrapping system is most practical. 

A consideration when using plastic pallet wrap systems is that the bags should be completely 

void of entrapped air before the plastic wrap is applied. If a protective plastic coating is applied 

before the bags have been deflated, it will become "loose" and thus, ineffective. In operations 

where entrapped air is a problem, a pallet press should be used to "squeeze" the excess air from 

the bags immediately before the plastic wrap is applied. 

Spiral stretch wrap was one of the first types of plastic wrapping techniques developed and used 

in the industry. This can be performed either through a manual or automated process. For 

manual operations, the full pallet is placed in an open area and a worker manually walks around 

the pallet, unwinding spiral stretch wrap over the pallet. Once completing multiple revolutions 

to the point that the pallet is adequately covered, the worker cuts the stretch wrap from the roll 

and connects the cut end to the pallet. 

In the automated process, the full pallet of bags is placed on a rotating table. As this table spins 

the pallet round and round, a roll of stretch plastic is continually wrapped around the pallet while 

slightly moving up and down at a predetermined wrap tension (Figure 6.19). After enough 

revolutions occur that the pallet is completely sealed, a mechanical device cuts the plastic wrap 

from the roll and assists the unrolled end in being attached to the pallet. 

Figure 6.19. Automated spiral stretch wrap machine. 

180 Bagging 
 

 

For many years, this plastic wrapping technique was the best and least expensive method of 

protecting loads for shipment [Packaging system 1978]. One shortcoming with the spiral stretch 

wrap is that the top of the pallet is not normally sealed and dust can be liberated from this top 

area. In addition, when the pallets are outside, rain or moisture can get down in and saturate the 

bags of product. To correct this shortcoming, a top sheet can first be placed over the top of the 

pallet before the process is started, but this normally has to be performed manually by a worker, 

which increases the time and the cost of performing this process. 

Another method to perform this plastic wrapping technique is through the use of shrink wraps or 

stretch hoods. This technique uses a single sheet of plastic which is placed over the entire pallet 

and then heated to cause it to shrink tightly down completely around the pallet of bags. This 

technique can also be performed manually by a worker or through the use of an automated 

system. When using this technique, pallets can be stacked and stored outside without worrying 

about water damage during storms. Stretch-hooded pallets are safer than shrink wrap pallets 

because the pallets are more stable and easier to handle [New Installations 2006]. 

FLEXIBLE INTERMEDIATE BULK CONTAINERS 

Flexible Intermediate Bulk Containers (FIBCs), also called "bulk bags," "semi-bulk bags,"

"mini-bulk bags," and "big bags," have become more popular over the recent years for shipment 

of product material. They are often more cost-effective than the 50-, 80-, or 100-pound bags for 

both the mineral producer and the end user. The most effective method to control the dust 

liberated during filling of FIBCs is by using an expandable neoprene rubber bladder in the fill 

spout of the bagging unit. This bladder expands against the interior of the FIBC loading spout 

and completely seals it, eliminating any product and dust escaping from the spout during loading 

(Figure 6.20). 

Figure 6.20. Expandable neoprene rubber bladder used in the fill spout 

of the bagging unit for Flexible Intermediate Bulk Containers. 

Bagging 181 

 When using this expansion bladder, the feed spout must also incorporate an 

exhaust ventilation system to exhaust the excess pressure from the FIBC during loading 

(Figure 6.21). 
 

 

Figure 6.21. Exhaust ventilation system used during 

loading of Flexible Intermediate Bulk Containers. 

If dust is still being liberated, another effective technique is to isolate the bag loading area from 

the rest of the plant. A worker can enter this area to manually attach the empty bag onto the 

loading device. The worker then leaves the area and once outside, remotely activates a start 

button to begin the bag filling cycle. The area is under negative pressure by virtue of being tied 

into an LEV system, and any dust generated will be exhausted and not allowed to contaminate 

the operator's work space. Once the FIBC loading is completed, the worker re-enters the area 

and removes the bag for shipping and begins the process again. 

A cautionary note is made here regarding work habits of employees performing the sealing of the 

filled FIBC spout. Most FIBCs have a fabric fill-spout that requires cords to be tied or secured 

in some manner. In this case, it is very important that the operator "point" the position of the 

spout away from the body to avoid the dust-laden exhaust air exiting the bag from being directed 

toward the operator's breathing zone. 

MAINTAINING CLEAN FLOORS IN BAG LOADING, PALLETIZING, 

AND WAREHOUSE LOCATIONS 

Pallets loaded with 50- to 100-pound bags, or bulk bags which are also loaded onto pallets, are 

normally moved by a forklift or tow motor directly to trailer trucks or railcars, or are stored in a 

182 Bagging 
 

 

warehouse location until a later time when they are then loaded into a transportation vehicle. As 

these forklifts and/or tow motors move around plant and warehouse locations, they can stir up 

significant amounts of dust if floors are not kept in a clean condition. The two most common 

floor cleaning techniques in mineral processing plants are washing with water and/or through the 

use of floor cleaning units. 

To serve as an effective cleaning technique, hosing down floors with water needs to be built into 

the structure right from the beginning. Floors need to be sloped toward floor drains to be truly 

effective and to minimize the amount of standing water. Typically, hosing down floors, needs to 

occur on a shift-by-shift basis to be effective. 

For floor cleaner units, there are many different manufacturers and companies that produce and 

sell a vast array of different types of units. These units vary from one that an individual rides to 

smaller units that an individual walks behind. One area that should be closely investigated is the 

disposal of the contaminated material once it has been collected. It would be counter-productive 

if the units were very efficient during the floor cleaning process but contaminated employees as 

the floor cleaning units were being cleaned or during disposal of the accumulated material. For 

more information on floor cleaning units, refer to "Housekeeping Practices" in Chapter 8— 

Controls for Secondary Sources. 

ENCLOSING DUST-LADEN AREAS WITH PLASTIC STRIPPING 

AND USING AN LEV SYSTEM 

Many of the different dust control techniques discussed in the chapter for bagging and palletizing 

operations can be further improved by enclosing the dust-laden air zone with clear plastic 

stripping and using an LEV system to capture and remove the dust. This "enclose and capture"

technique has a number of benefits. The plastic stripping contains the dust within the job process 

and minimizes the possibility of it being liberated throughout the entire plant. It also provides a 

visual indication for the plant personnel of the boundary of the dust-laden air zone. Workers 

know not to enter the dust-laden air zone while the job processes or functions are operating. 

Another benefit of the boundary area created by the plastic stripping is that it allows the LEV 

system to perform more effectively. Since the capture efficiency of exhaust ventilation systems 

in an open environment is not very efficient, the plastic stripping barrier allows the exhaust 

system to operate much more effectively than without one in place. From an economical 

standpoint, plastic stripping is relatively inexpensive compared to other types of dust control 

techniques. 

One last area to consider when using clear plastic stripping is that it allows workers to see 

through the curtain for safety reasons. This technique has numerous applications not only in the 

bagging and palletizing area, but throughout the entire mineral processing operation. When 

evaluating the correct connection technique for the LEV system, as well as recommended 

exhaust airflow quantities, refer to Chapter 1—Fundamentals of Dust Collection Systems. 

Bagging 183 
 

 

WORK PRACTICES TO MINIMIZE DUST EXPOSURES 

FROM BAGGING AND PALLETIZING 

The following list of best practices should be considered by operations wishing to minimize 

respirable dust exposures to workers performing bagging and palletizing. 

Use slides or chutes whenever possible to minimize the distance that product free-falls in 

open areas. 

Keep floors as clean as possible to minimize dust liberation as forklifts/tow motors travel 

around the area transporting bag material and other items. 

Maintain the work area such that there are no sharp edges at bag drop-off locations, 

turning points, and/or conveyor lines that could tear bags of product material. 

Maintain material flow cut-off valves at bagging machines to minimize product dribble 

from fill nozzles/fill chutes. 

Determine the optimal bag construction and bag valve type for the material being 

processed and bagging machine being used. 

Minimize product blowback and product "rooster tail" during bag filling and ejection 

from the fill station. Both of these dust sources can also cause extensive dusting on the 

outside of bags, as well increase the overall demand on the LEV system. 

Consider using ventilated filling spouts as discussed in the "Dual-Nozzle Bagging 

System" section to remove product from the fill nozzle and bag valve areas after bag 

filling is completed. 

Slow bag filling rates when necessary to minimize the amount of product blowback. 

Carefully place loaded bags onto pallets. Throwing or dropping bags onto conveyors or 

pallets increases the potential for dust generation and/or bag breakage. 

Use a hydraulic lift table to minimize dust generation and MSDs to workers during bag 

loading onto pallets by keeping the loading height at knuckle height as described in the 

"Semi-Automated Bag Palletizing Systems" section. This allows for a push-pull 

ventilation system to more effectively capture any dust generated during bag palletizing. 

Use dust-free empty bags manufactured to Flexible Intermediate Bulk Container (FIBC) 

standards. Many reusable FIBC bags are returned with clean interiors, but dust is often 

trapped in the fabric on the bag exterior. This dust is then expelled during handling of the 

bag. 

Store all empty bags in a clean environment and keep them covered to prevent dust from 

settling on them. 

When tying or sealing the spout of a filled FIBC bag, the operator should always hold the 

flexible loading spout away from his or her breathing zone to minimize the escaping dust. 

Avoid positioning the bag operator or the bag palletizer between any dust source and the 

pickup point of an LEV system. When this is not done, dust will be drawn directly over 

the worker, significantly increasing the worker's dust exposure. 

Consider using a mechanically assisted bag handling system to prevent dropping filled 

bags onto pallets. These systems also greatly reduce the potential for workers developing 

MSDs. 

Consider the use of a bag cleaning device prior to bag palletizing, as discussed in the 

"Bag and Belt Cleaning Device" section. Mechanical brushes and air nozzles can be 

184 Bagging 
 

 

used successfully to perform this cleaning. Brushes need to be adjusted or replaced often 

due to high wear potential. It is also critical to capture the dust removed from the bags 

with an LEV system. 

Develop and implement a plan for dealing with broken bags that minimizes manual 

handling. This can include a vacuuming system or a ventilated waste hopper device. 

Every effort should be made to eliminate plant personnel from manually handling any 

broken bags. 

Enclose bagging and palletizing areas from the general plant environment through the use 

of clear plastic stripping. If entry is not necessary, a solid type curtain could be used. 

These enclosed areas should be under negative pressure through an LEV system to 

prevent dust from migrating to other areas of the plant. 

Collect dust at the source through an LEV system rather than trying to redirect or to 

dilute with ventilation fans. The use of free-standing fans only circulates the air and dust 

and does not remove it from the plant. 

When using an OASIS, periodically clean and replace filters and ensure that clean filtered 

air is evenly distributed down over the worker. Significant airflow reductions can occur 

from dust-laden filters. 

Check and lubricate bearings on LEV systems, including the OASIS, as well as check 

power transmission belts. In addition, consider the use of a pressure differential gauge or 

monitor to indicate system problems or when maintenance needs to be performed. 

Ductwork to the LEV system should be examined and cleaned periodically to ensure that 

the system is working properly. Capture velocities at the face of shrouds and hoods 

should be monitored periodically to ensure proper functioning. 

REFERENCES 

Case History [2006a]. Dustless bag filling system puts material in, takes dust out. Powder and 

Bulk Eng 20(5):82–87. 

Case History [2006b]. Robotic palletizer improves company's bottom line. Powder and Bulk 

Eng 20(5):88–93. 

Cecala AB, Covelli A [1990]. Automation to control silica dust during pallet loading process. 

In: Proceedings of SME Annual Meeting. Preprint 90–28. 

Cecala AB, Muldoon T [1986]. Closing the door on dust - dust exposure of bag operator and 

stackers compared for commercial bag valves. Pit & Quarry 78(11):36–37. 

Cecala AB, Timko RJ, Prokop AD [1997]. Bag and belt cleaner reduces employee dust 

exposure. Rock Products 100(3):41–43. 

Cecala AB, Zimmer JA, Smith B, Viles S [2000]. Improved dust control for bag handlers. Rock 

Products 103(4):46–49. 

Constance JA [2004]. Ways to control airborne contaminants in your plant. Powder and Bulk 

Eng 18(5):118–119. 

Bagging 185 
 

 

Godbey T [2005]. Recirculating your cleaned air: is it right for your plant? Powder and Bulk 

Eng 19(10):31–37. 

Kearns P [2004]. What's new in paper and plastic bags. Powder and Bulk Eng 18(5):75–79. 

New Installations [2006]. Stretch-hood fits like a glove. Powder and Bulk Eng 20(5):76–81. 

Packaging system eases mineral dust problems [1978]. Pit and Quarry 70(10):104–106. 

Robertson JL [1986]. Overhead filters reduce dust level in air supply. Rock Products 89(7):24. 

USBM [1981]. SF6 tracer gas tests of bagging-machine hood enclosures. By Vinson RP, 

Volkwein JC, Thimons ED. U.S. Department of the Interior, Bureau of Mines Report of 

Investigations 8527. 

USBM [1983]. Technology News 54: Dust control hood for bag-filling machines. By 

Volkwein JC, Pittsburgh, Pennsylvania: U.S. Department of the Interior, Bureau of Mines. 

USBM [1984a]. Technology News 207: New nozzle system reduces dust during bagging 

operation. By Cecala AC. Pittsburgh, Pennsylvania: U.S. Department of the Interior, Bureau of 

Mines. 

USBM [1984b]. New bag nozzle to reduce dust from fluidized air bag machine. By Cecala AB, 

Volkwein JC, Thimons ED. U.S. Department of the Interior, Bureau of Mines Report of 

Investigations 8886. 

USBM [1986a]. Impact of background sources on dust exposure of bag machine operator. By 

Cecala AB, Thimons ED. U.S. Department of the Interior, Bureau of Mines Information 

Circular 9089. 

USBM [1986b]. Dust reduction capabilities of five commercial bag valves. By Cecala AB, 

Covelli A, Thimons ED. U.S. Department of the Interior, Bureau of Mines Information Circular 

9068. 

USBM [1995]. Reducing respirable dust levels during bag conveying and stacking using bag 

and belt cleaner device. By Cecala AB, Timko RJ, Prokop AD. U.S. Department of the Interior, 

Bureau of Mines Report of Investigations 9596. 

Volkwein JC, Engel MR, Raether TD [1986]. Get away from dust with clean air from overhead 

air supply island (OASIS). International Symposium on Respirable Dust in the Minerals 

Industry, University Park, Pennsylvania. 

White D [2006]. Is it time to automate your bagging line? Powder and Bulk Eng 20(5):71–75. 

186 Bagging 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Bagging 187

CHAPTER 7: 

BULK LOADING 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

188 Bagging 
 

 

CHAPTER 7: BULK LOADING 

During the bulk loading of product, dust contained within the product can be liberated and 

emitted into the ambient air as the product falls from the loadout area to the transport container 

(truck, rail car, barge, or ship). This dust-laden air can expose workers to respirable dust, as well 

as create nuisance dust problems. 

A number of factors impact the severity of dust liberation during bulk loading, including: 

the type of product and its size distribution, 

moisture content of the product, 

volume of the product being loaded and the loading rate, 

the falling distance, 

environmental factors such as wind velocity and rain, and 

physical characteristics of the receiving vessel. 

The first three factors represent the characteristics of the product being loaded and may be 

defined by the process or by the customer. Past research has shown that larger size distributions 

and higher moisture contents typically result in less respirable dust liberation. As a general rule, 

a moisture content of 1 percent by weight is recommended as a starting point for adding moisture 

to a crushed product [NIOSH 2003]. Chapter 2—Wet Spray Systems provides a detailed 

discussion on using water to reduce dust liberation. 

The last three factors represent the characteristics of the loading process. Engineering controls 

can be implemented to minimize the impact of these factors. For example, the falling distance 

can be reduced and physical barriers can be installed to protect against high winds. 

The following dust controls, which have been implemented in the industry, will be discussed in 

this chapter: 

loading spouts, 

the Dust Suppression Hopper, and 

enclosures. 

LOADING SPOUTS 

The loading spout is designed to transfer product from a plant/storage container into the vehicle 

that will be used to transport the product. Typically, these spouts have the capability to extend 

down to the vehicle being loaded and then be retracted to allow the vehicle to move away from 

the loading station. A review of several loading spout manufacturer specifications shows that 

this spout travel can be as little as a few feet and can extend up to 100 feet. As shown in 

Figure 7.1, these loading spouts are equipped with a series of telescoping cups or pipes that 

extend and retract. This figure also shows that the spout typically has an outer shroud which 

Bulk Loading 189 
 

 

encases the product transfer section of the spout. This outer shroud shields the product from the 

elements (rain, wind, etc.) and helps to contain any dust that is liberated. 

Figure 7.1. Loading spouts designed with telescoping cups 

or pipes and dust collection capabilities. 

Depending upon the loading application, the discharge end of the spout may be specially 

designed to seal against the loading port of the receiving vehicle. Often times, the spout 

discharge is cone-shaped to partially enter and seal against the vehicle port. Figure 7.2 illustrates 

examples of these cone discharges designed to reduce dust emissions during loading into a 

closed vehicle. If the vehicle has multiple loading ports, only one port should be opened during 

loading to minimize dust liberation from the vehicle. 

In order to realize the dust control benefits of the discharge cone, the loading spout must be 

properly positioned to minimize the chance of dust escaping into the ambient air during loading. 

This can be accomplished through the use of articulated loading systems. These systems can 

move the loading spout side-to-side as well as forward and back to ensure that the spout 

discharge is located in the optimum position above the vehicle, which minimizes dust liberation 

and product spillage. The operator can control these articulated loading systems from a control 

room or from a pendant station near the loading site. Figure 7.3 shows a schematic and 

photograph [Midwest International Standard Products 2010] of an articulated loading system. 

190 Bulk Loading 
 

 

Figure 7.2. Loading spout discharge cones designed to seal against the 

loading port of closed vehicles to minimize dust liberation. 

Figure 7.3. Articulated loading spout positioner. 

When loading into open vehicles, the discharge end of the loading spout can be equipped with a 

skirt or apron which can seal against the product pile in an effort to minimize dust liberation. 

The skirts are typically fabricated from belting or fabric and flare out as the loaded material 

builds up. For the most effective dust control and to prevent clogging of the loading spout, it is 

important that the spout position be periodically raised as the height of the loaded material 

increases. To automate this procedure, the loading spouts can be equipped with tilt sensors that 

are suspended vertically from the spout. As the loaded material builds up, it moves these sensors 

from their vertical orientation (e.g. 15 degrees or more) which activates the automatic raising of 

the loading spout. Examples of these discharge skirts and tilt sensors are illustrated in 

Figure 7.4. 

Bulk Loading 191 
 

 

Figure 7.4. Skirt (in yellow box) and tilt sensors (in red boxes) installed on the 

discharge end of loading spouts to help reduce dust emissions. 

Dust Collection Systems and Loading Spouts 

As material falls through the loading spout, airflow is induced which can entrain dust liberated 

from the product. As the falling product hits the vehicle floor or previously loaded material, the 

material is compressed and the induced air is driven off, creating entrained dust. As the falling 

distance increases, the induced air is driven off with a higher velocity, which creates larger 

amounts of dust [Biere et al. 2010]. When a given volume of material is loaded into a receiving 

vehicle, an equal volume of air in the vehicle is displaced, which can carry liberated dust with it. 

Chapter 1—Fundamentals of Dust Collection Systems discusses and illustrates air induction and 

dust generation for falling material. 

To address the above dust emissions during bulk loading, a common control measure is to equip 

the loading spout with a dry dust collection system. These dust collection systems induce 

airflow in the annular region between the outer shroud and the inner loading portion of the 

spouts, as shown in Figure 7.1. These systems also typically utilize filter cartridges or bags to 

capture the airborne dust. Compressed air is periodically introduced to back-flush the 

filters/bags to remove captured dust from the cartridges/bags and return this dust to the product 

stream. The dust collector unit can be located away from the loading spout and connected to the 

loading spout through ductwork as shown on the left in Figure 7.1. Another option is to have the 

dust collector incorporated into the top of the loading spout. The loading spout shown on the 

right in Figure 7.1 illustrates an internal filter cartridge system and fan mounted at the top of the 

loading spout. 

A review of various loading spout manufacturers' specifications indicates that dust collection 

systems are offered with available exhaust air volumes ranging from 1.8 [SLY 2010] to 4.9 

[PEBCO 2010] times the rated loading volume of the spout. Equation 1.2 in Chapter 1— 

Fundamentals of Dust Collection Systems can be used to calculate a recommended exhaust air 

volume based upon material feed rate, height of free-fall, product size, and feed open area. It 

should also be noted that if the exhaust airflow is too high, product can be pulled into the dust 

collection system. 

192 Bulk Loading 
 

 

Cascading Loading Spout 

Figure 7.5 illustrates a schematic and photograph [Cleveland Cascade 2010] of a loading spout 

designed with cascading cones that prevent the product from dropping straight down through the 

spout. 

Figure 7.5. Cascade loading spout. 

Bulk Loading 193 

 As mentioned earlier, free-falling product can promote dust entrainment and liberation 

into the ambient air during the loading process. Cascading-type loading spouts are designed with 

a series of inclined cones that do not permit the product to free-fall through the loading spout. 

The goal is to minimize product velocity and induced airflow movement, by allowing the 

product to be transferred smoothly from one cone to the next. Originally designed to reduce 

degradation of the product, the cascading action was also thought to have the potential to lower 

dust emissions by reducing product velocity and promoting mass flow [Maxwell 1999]. 

Improved mass flow of the product allows the particles to maintain greater contact with one 

another and thus reduce the release of small dust particles into the air. Wind shrouds and 

discharge skirts are the primary dust control devices utilized in these designs. Typically, 

cascading loading spouts are not equipped with dry dust collection systems. Within the loading 

spout, cone size, spacing, and inclination are adjusted to accommodate different product flow 

rates. Also, since there is more contact between the product and the cones than found in freefalling spouts, abrasion-resistant liners are frequently utilized inside the cones. 
 

 

DUST SUPPRESSION HOPPER 

Another modification that can be made in an effort to reduce dust emissions during bulk loading 

is the utilization of the Dust Suppression Hopper (DSH) [DSH Systems 2010]. The DSH is 

located at the product discharge point and is designed to help the loaded product flow more as a 

solid column of material with less exposed surface area to minimize entrainment into the ambient 

air [Wypych 2009]. The hopper is equipped with a central plug at the bottom to prevent 

discharge of product until a predefined quantity has accumulated in the hopper. Either springs or 

a programmable logic controller are used to set and control the amount of clearance between the 

plug and the hopper. Material is loaded into the top of the hopper and contained until sufficient 

weight forces clearance between the hopper and plug. Material then flows from the hopper in 

more of a solid column, as shown in Figure 7.6, rather than in a free-falling unconsolidated 

stream. This overall process results in less dust liberation [Biere et al. 2010]. 

Figure 7.6. Dust Suppression Hopper creating a solid stream of 

material during loading to reduce dust liberation. 

ENCLOSURES 

During bulk loading, the preferred method of dust control is to prevent dust from becoming 

airborne. However, if dust is released into the ambient air, efforts must be made to keep the dust 

from reaching the breathing zone of surrounding workers. This can be accomplished by 

enclosing the dust-generating process or enclosing the worker. Both of these methods will be 

discussed and illustrated below. 

Enclosed Loadout 

The goal of enclosing the loadout area is to contain and capture the airborne dust so that it cannot 

reach the breathing zone of nearby workers or contaminate the surrounding environment. 

Typically, this type of control will be used for the loading of trucks or railcars. Essentially, a 

physical barrier is constructed that confines the dust that is generated as loading occurs. The 

barrier also minimizes the impact of environmental factors such as high wind velocities. These 

barriers can be permanent walls with flexible plastic stripping, belting, or curtain used to provide 

194 Bulk Loading 
 

 

a seal at the entry and exit points of the vehicles. Alternatively, flexible plastic stripping or 

curtains can be used to construct the entire enclosure as shown in Figure 7.7. 

Figure 7.7. Rail car loadout area enclosed with plastic strips to contain dust. 

In either case, the 

individual pieces of flexible material should overlap one another to provide a more effective 

barrier against the possibility of dust escaping. To further prevent dust escape, the enclosure 

should be supplemented with a dry dust collection system. Equation 4.1 in Chapter 4— 

Crushing, Milling, and Screening, can be used to calculate an initial exhaust volume for the dry 

dust collector. If dust is seen escaping the enclosure, it is an indication that an effective seal is 

not being maintained, that the exhaust air quantity is not sufficient, or both. 

The following is a list of some issues that should be addressed in designing the loading process 

and the loadout area so that personnel do not need to enter the enclosure. 

Vehicle hatches should be opened and closed outside of the enclosure. 

Cameras can be installed to assist the operator in properly positioning the loading spout 

to minimize dust generation and spillage. 

An automatic level sensing kit should be installed to signal the operator when the vehicle 

is fully loaded or to automatically shut off the feed. Several types of sensor technologies 

are available and include tilt, pressure, capacitance, or infrared. This automation allows 

the operator to maintain a more remote position from the area where dust might be 

generated. 

The loading spout can be raised automatically after the vehicle is loaded to a predefined 

height, which allows the vehicle to move away without the spout being fully retracted. 

This ensures that sufficient clearance is available and reduces the time required to begin 

the loading of the next vehicle. Level sensors and limit switches can be used to help 

accomplish this effort. Once again, this automation allows the operator to maintain a 

more remote position from the area where dust might be generated. 

During loading and until all liberated dust is cleared, personnel should not be allowed to 

enter the enclosed area. The loading process should be designed so that it is not 

necessary for personnel to enter the enclosure. If personnel must enter the enclosure, dust 

concentrations within the enclosure should be quantified in order to ensure that 

appropriate respiratory protection is utilized. 

Bulk Loading 195 
 

 

Operator Booths/Control Rooms 

One of the most effective means of protecting a worker associated with a dust-generating 

operation can be the utilization of a booth/control room that provides filtered air inside the 

enclosure. Previous research has shown that properly operating filtration systems installed on 

enclosed compartments can provide over 90 percent reductions in respirable dust exposures 

[NIOSH 2008]. Another advantage of these booths is that they can be equipped with airconditioning and heating systems to make the work environment more comfortable for the 

operator. Figure 7.8 shows an operator's booth overlooking the loadout area in a plant. 

Figure 7.8. Environmentally controlled operator’s booth near loadout area. 

196 Bulk Loading 

From a dust control engineering standpoint, the goal is to provide a competent operator's booth 

that is positively pressurized with filtered air. The positive pressure within the booth prevents 

dust that may be present outside of the booth from leaking in and exposing the worker. 

Chapter 9—Operator Booths, Control Rooms, and Enclosed Cabs provides detailed information 

regarding the design and operation of filtration/pressurization systems for operator 

compartments. 
 

 

REFERENCES 

Biere G, Swinderman RT, Marti AD [2010]. Conveyor upgrades increase plant availability, 

reduce airborne dust. Coal Power Magazine [online] Feb 16. 

[http://www.coalpowermag.com/plant_design/Conveyor-Upgrades-Increase-Plant-Availability

Reduce-Airborne-Dust_242.html]. Date accessed: October 18, 2010. 

Cleveland Cascade [2010]. [http://www.clevelandcascades.co.uk/brochures.php]. Date 

accessed: November 18, 2010. 

DSH Systems [2010]. Dust Suppression Hopper Systems. [http://www.dshsystems.co.nz/]. 

Date accessed: October 7, 2010. 

Maxwell AS [1999]. Degradation, segregation and dust control of bulk materials. Solids 

Handling 19(3):399 403. 

Midwest International Standard Products [2010]. 

[http://www.midwestmagic.com/products_positioners_articuloader.php]. Date accessed: 

October 6, 2010. 

NIOSH [2003]. Handbook for dust control in mining. By Kissell FN. Pittsburgh, PA: U.S. 

Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, NIOSH Information Circular 9465, 

DHHS, (NIOSH) Publication No. 2003–147. 

NIOSH [2008]. Key design factors of enclosed cab dust filtration systems. By Organiscak JA, 

Cecala AB. U.S. Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, Report of Investigations 9677, 

DHHS (NIOSH) Publication No. 2009–103. 

PEBCO [2010]. [http://catalog.pebco.com/category/dustless-loading-spout]. Date accessed: 

October 8, 2010. 

SLY, Inc. [2010]. [http://slyinc.com/loading-spouts/xp-bulk-material-loading-spouts/]. Date 

accessed: October 8, 2010. 

Wypych P [2009]. Award winners can solve grain's dust problems. Australian Bulk Handling 

Review. [http://www.bulkhandling.com.au/news/april-8th-09/award-winners-can-solve

grain2019s-dust-problems-says-wypych]. Date accessed: October 13, 2010. 

-

-

Bulk Loading 197 

DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Bulk Loading 199

CHAPTER 8: 

CONTROLS FOR 

SECONDARY SOURCES
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

200 Bulk Loading 
 

 

CHAPTER 8: CONTROLS FOR SECONDARY SOURCES 

This chapter discusses various types of secondary dust sources frequently found in mineral 

processing facilities and that may have a significant impact on workers' respirable dust exposure. 

In some cases, the worker's personal respirable dust exposure can be more significant from these 

secondary sources than from the exposure resulting from the worker's primary job function. 

Because of this, these dust sources need to be recognized, identified, and controlled. Not only 

does this chapter identify these sources, but it provides methods and techniques that are available 

to minimize dust exposure from these sources. 

CLEANING DUST FROM SOILED WORK CLOTHES 

A significant area of respirable dust exposure to workers at mineral processing operations is from 

contaminated work clothing. It has been documented that the level of contamination from dustladen work clothes was so significant in some cases that it caused a ten-fold increase in a 

worker's respirable dust exposure [USBM 1986]. It must be noted also that once clothing 

becomes contaminated, it is a continual source of dust exposure until being changed or cleaned. 

One option that operations can use to deal with contaminated work clothing is to use dustblocking coveralls that can be worn by workers. The coveralls can be removed before the 

worker enters a clean room/area, then discarded at the end of the shift to minimize dust being 

transferred to the worker's personal clothing. These types of coveralls are also very popular 

when dealing with different types of toxic dusts. One drawback with these coveralls is that they 

become uncomfortable for workers because they do not dissipate heat very well and a secondary 

consideration is the cost. 

Currently in the United States, the only Mine Safety and Health Administration-approved 

(MSHA) method for cleaning work clothes requires the use of a HEPA-filter vacuuming system, 

which makes the clothes cleaning effort both time-consuming and unlikely to be performed 

effectively. It is very difficult for a worker to effectively vacuum his/her clothing, particularly in 

hard-to-reach areas such as the legs and back. 

The use of compressed air to remove dust from work clothing is prohibited by MSHA; however, 

because the vacuuming method is very time-consuming and ineffective, workers may attempt to 

use this illegal method of a single compressed air hose to blow the dust from their clothing. 

Although this is a slightly more effective cleaning method than the vacuuming technique, it is 

also time consuming and equally as difficult to clean the same hard-to-reach areas. The primary 

concern with the blowing technique is that it creates a dust cloud, elevating respirable dust levels 

for both the worker and coworkers in the work environment [Pollock et al. 2005]. 

Once a worker's clothing becomes contaminated, dust is continually emitted from the material as 

the worker performs his/her normal work activities. Given this problem and the reality that 

workers can be exposed to multiple dust sources throughout the day, the most effective solution 

is to change the contaminated clothing. Although disposable coveralls have been in use for 

Controls for Secondary Sources 201 
 

 

many years, as well as some new and improved clothing material that is less susceptible to dust 

retention, the vast majority of miners continue to wear standard work clothes (typically demin) 

that they have worn for years [Langenhove and Hertleer 2004; Hartsky et al. 2000; 

Salusbury 2004]. To address the above issues, a quick, safe, and effective method was 

developed that allows workers to clean their dust-contaminated clothing throughout the workday. 

NEW TECHNOLOGY FOR CLEANING DUST FROM WORK CLOTHING 

A cooperative research effort between Unimin Corporation and the National Institute for 

Occupational Safety and Health (NIOSH) led to the development of a new clothes cleaning 

system. This technology was not intended to eliminate the need to launder work clothing, but to 

provide an interim solution to allow workers to safely remove dust from their work clothing 

periodically throughout the day until laundering could be performed. 

The clothes cleaning system developed under this cooperative research effort consists of four 

major components: (1) a cleaning booth, (2) an air reservoir, (3) an air spray manifold, and, 

(4) an exhaust ventilation system. Figure 8.1 represents the design of the clothes cleaning 

system. 

Figure 8.1. Design of clothes cleaning system. 

202 Controls for Secondary Sources 

In order to perform the clothes cleaning process with this new system, a worker wearing personal 

protective equipment (respirator, eye, and hearing protection) simply enters the cleaning booth, 

activates a start button and rotates in front of the air spray manifold while dust is blown from the 

clothing via forced air. After a short time period (18 seconds), the air spray manifold is 

electronically deactivated and the worker can exit the booth with significantly cleaner work 

clothing. 
 

 

Clothes Cleaning Booth 

The clothes cleaning booth is 48 by 42 inches which provides the worker with sufficient space to 

rotate in front of the air nozzles to perform the cleaning process. Intake air enters the booth 

through a 2-foot opening in the roof, then flows down through the enclosure before exiting 

through an air plenum on the bottom back wall of the booth. As this intake air flows through the 

booth, it entrains dust removed from the worker's clothing during the cleaning process and 

forces it down towards the air plenum and away from the worker's breathing zone. The 

exhausted dust-laden air then travels from this air plenum at the base of the booth to the exhaust 

ventilation system. 

Air Reservoir 

The air reservoir is necessary to supply the required air volume to the air nozzles used in the 

spray manifold. The size requirement for the reservoir was calculated based upon the design of 

the manifold. Either a 120- or 240-gallon reservoir should be used, depending on the number of 

workers needing to clean their clothes in sequence. The average cleaning time required during 

field testing was about 18 seconds, and the 120-gallon reservoir provides approximately 

22 seconds of air capacity. If multiple individuals will be using the booth one after another, then 

the 240-gallon reservoir should be used. This reservoir should be pressurized to at least 150 psi 

and it should be located close to the cleaning booth and hard-piped to the air spray manifold 

located in the booth. A pressure regulator must be installed immediately before the air spray 

manifold to regulate the nozzle pressure to a maximum of 30 psi. 

Air Spray Manifold 

Figure 8.2 represents the air spray manifold design. The air spray manifold is composed of 

26 spray nozzles spaced 2 inches apart. The bottom nozzle is located 6 inches from the floor and 

is a circular designed nozzle for cleaning the worker's boots. This nozzle is used in conjunction 

with an adjustable ball-type fitting so that it can be directed downward. The other 25 air spray 

nozzles are flat fan sprays, which lab testing proved to be the most effective for cleaning at close 

distances. The air spray nozzles deliver slightly less than 500 cfm of air. 

The original air spray manifold was designed for an individual around 5 feet 10 inches, the 

average height for a male worker in the United States. For shorter workers, a sliding mechanism 

is used to cover the top air nozzles to prevent discharged air from directly hitting the individual's 

face. 

An in-line filter should also be incorporated between the compressed air supply and the air spray 

manifold. This filter substantially reduces the potential that foreign material such as a metal burr 

or rust particle could be blown from an air spray nozzle during cleaning. 

Controls for Secondary Sources 203 
 

 

Figure 8.2. Air spray manifold design with 26 nozzles spaced 2 inches apart. 

Exhaust Ventilation 

For effective dust removal, it is critical that the cleaning booth be ventilated under negative 

pressure at all times so as to not allow any dust liberated from the clothing to escape from the 

booth and into the work environment. In the design stage of this technology, testing validated 

that an exhaust volume of 2,000 cfm was sufficient to maintain a negative pressure throughout 

the entire clothes cleaning cycle. A pressure control switch is also incorporated into the system 

which will not allow the air spray manifold to activate unless an acceptable negative pressure is 

maintained within the cleaning booth. 

To exhaust the dust-laden air from the cleaning booth, there are two recommended methods: 

(1) the dust can be vented to a dust collector system (e.g., baghouse [Cecala et al. 2007]), or 

(2) the dust can be vented to an area outside the plant where it will not contaminate other 

workers nor be entrained back into the plant [Cecala et al. 2008]. There are many dust sources 

204 Controls for Secondary Sources 
 

 

outside all mineral processing plants, and the amount of dust-laden air ducted outside for this 

application is minor in comparison to other sources. 

Safety Issues 

All personnel entering the cleaning booth must wear personal protective equipment, including an 

approved fit-tested respirator with filters that are acceptable for the material being processed, and 

hearing and eye protection. In addition, based upon the Occupational Safety and Health 

Administration (OSHA) mandated pressure regulation, the air pressure at the air spray manifold 

must be regulated to a maximum of 30 psi before being directed toward the individual 

performing the clothes cleaning process. 

EFFECTIVENESS OF CLOTHES CLEANING TECHNOLOGY 

During development, the new clothes cleaning technology was compared to both the MSHAapproved vacuuming approach and a single handheld compressed air hose. In this testing, 

several 100-percent cotton and cotton/polyester blend type coveralls were soiled with limestone 

dust before the worker entered the clothes cleaning booth. Table 8.1 shows the three cleaning 

methods, their effectiveness in cleaning dust off the two types of coveralls, and the time it took 

the worker to perform the cleaning task. Clearly, the use of the clothes cleaning system was 

much more effective at removing the dust than the other two approaches, and required only a 

fraction of the time, 17 to 18 seconds compared to an average of 372 seconds for vacuuming and 

an average of 178 seconds for the handheld compressed air hose. Table 8.1 also shows that the 

polyester/cotton blend coveralls were cleaned more effectively than the 100 percent cotton type. 

Figure 8.3 depicts a worker before and after entering the clothes cleaning booth. 

Table 8.1. Amount of dust remaining on the coveralls after cleaning, and the 

cleaning time for 100 percent cotton and polyester/cotton blend coveralls 

Controls for Secondary Sources 205 

100% Cotton Polyester/Cotton blend 

Dust 

remaining on 

coveralls, 

grams 

Dust 

remaining on 

coveralls, 

grams 

Cleaning method Cleaning time, 

seconds 

Cleaning time, 

seconds 

Vacuuming 63.1 398 45.5 346 

Air hose 68.8 183 48.4 173 

Clothes cleaning booth 42.3 17 21.9 18 
 

206 Controls for Secondary Sources 

Figure 8.3. Test subject wearing polyester/cotton blend coveralls 

before and after using the clothes cleaning booth. 

CURRENT STATUS OF CLOTHES CLEANING REGULATIONS 

In the U.S., two federal regulations currently impact the clothes cleaning process. The first 

regulation is an MSHA standard 30 CFR Part 56.13020, which states: "At no time shall 

compressed air be directed toward a person. When compressed air is used, all necessary 

precautions shall be taken to protect persons from injury." A deflector mechanism is 

incorporated into the air spray manifold which allows the top air nozzles to be covered over 

when shorter individuals are performing the cleaning process. This only allows the nozzles to 

impact the worker's clothing and not the worker's face. 
 

 

 

A second regulation is a general industry standard established by OSHA in 29 CFR 1910.242(b), 

which states: "Compressed air shall not be used for cleaning purposes except where reduced to 

less than 30 psi and then only with effective chip guard and personal protective equipment." 

This regulation established the 30 psi limit for the maximum air pressure for the air spray 

manifold. 

Because of these two federal standards, operations must file a petition of modification to their 

MSHA District Manager to obtain approval to use this clothes cleaning system at their facility. 

Since this system has already been approved for use at other operations, obtaining approval for 

similar systems should not be difficult. Manufacturers producing the clothes cleaning system 

5 can be contacted by those interested in taking advantage of this technology.

5 One manufacturer of the clothes cleaning booth technology is Clothes Cleaning Systems 

(http://www.cleanclothbooth.com/), which is selling the system that was developed under this cooperative work 

effort. 

Controls for Secondary Sources 207 

HOUSEKEEPING PRACTICES 

Although good housekeeping practices might seem to be a minor or common sense issue, they 

can be a significant factor in a worker's respirable dust exposure at mineral processing 

operations. When housekeeping is performed properly and on a scheduled time frame, it can be 

a key factor in minimizing respirable dust exposures to workers. On the other hand, when it is 

not performed, or performed improperly, it can have just the opposite effect. In one study, an 

example of this was documented when a worker was dry sweeping the floor with a push broom. 

In this instance, the average exposure of a coworker located one floor up from the person 

3 3 sweeping the floor increased from 0.03 mg/m before sweeping to 0.17 mg/m during, and 

immediately after, the occurrence (Figure 8.4). Dry sweeping is discouraged as a viable cleaning 

technique because of the dust generated and liberated into the work environment [USBM 1986]. 

Figure 8.4. Increase in coworker’s dust exposure one floor up from floor sweeping activities.
 

 

Spills of product material are a common occurrence at processing operations and when they 

occur, the initial removal of material is most often performed with a scoop, bobcat, or through 

having workers using shovels. When this task is performed manually, the appropriate respiratory 

protection must be worn by workers. 

Once the bulk of this material is removed, or for general housekeeping, it is very effective to 

wash down the facility with water on a regular basis. Ideally for this to work, the design should 

be incorporated in during construction so that drains are installed and the floor is sloped correctly 

to ensure that the water runs toward the drains. Although washing down is very effective, it 

must also be noted that this can unintentionally increase respirable dust levels during the hosing 

process. The problem occurs when water strikes piles of accumulated fine material, which can 

cause dust to be liberated into the air. When washing down is being used as a housekeeping 

technique, it should be done shift by shift, or at least daily, to keep product from accumulating. 

In addition to washing down, the next most common housekeeping techniques are the use of 

floor cleaning devices and/or vacuum systems. Although both of these techniques are much 

more costly than washing down, neither typically generate or liberate significant levels of dust 

during the cleaning process, as is the case with washing down. 

When considering floor cleaning options, there are many different manufacturers and companies 

that sell a vast array of different types of floor cleaning units. These units vary from ones that an 

individual rides to smaller units that an individual walks behind (Figure 8.5). 

Figure 8.5. Various types of floor sweep units used to clean dust-laden floors. 

One area that 

should be closely investigated is the disposal of the contaminated material once it has been 

collected. It would be counterproductive if the unit is very efficient during the floor cleaning 

process but contaminates employees when the unit is being cleaned or during disposal of the 

accumulated material. 

In addition, there are many different types of vacuuming units. These can range from highcapacity, in-plant vacuuming systems with multiple collector pickup locations to portable tank208 Controls for Secondary Sources 
 

 

type units. Some operations also contract this work to independent companies who bring in 

high-powered exhaust trucks to perform the vacuuming process. In almost all cases, a worker 

uses an exhaust system collector device to manually exhaust the product and dust into the 

collector system (Figure 8.6). 

Figure 8.6. Portable system used to perform vacuuming. 

 This is very time consuming and can be difficult to perform in 

areas that are not easily accessible in the plant. In most cases, this is not something that can be 

performed on a shift-by-shift or daily basis because it is so labor-intensive. One safety issue 

with these vacuuming devices is from static electricity charges that can be created while 

performing this cleaning technique. Using a nonconductive or grounded hosing material should 

eliminate the risk of this safety issue. 

In addition to hosing with water, sweeping, or using mechanical devices to clean the floors at 

mineral processing facilities, one last option that can be considered is to use a floor sweep 

compound. Although it is not a common practice to use a floor sweep compound throughout an 

entire facility, some operations are using it in high traffic or extremely dusty areas to assist in 

minimizing the dust generated. Since there are many different manufacturers selling floor sweep 

compounds, operations may want to consider evaluating some different types of material in some 

problem areas to determine the effectiveness obtained within their facilities. 

Housekeeping should be performed at the end of each shift so that workers coming on to the next 

shift start with a clean work environment. When housekeeping is performed at the beginning of 

a shift, it increases the potential for worker clothing to become contaminated, which will increase 

the worker's personal dust exposure throughout the work shift. 

One final aspect of effective housekeeping is proper upkeep and maintenance of plant equipment 

and processes. When product is observed building up on floors, it indicates that some function 

or process is causing the leakage. In some cases, visible dust can be seen leaking from holes or 

damaged equipment, and this must be quickly corrected to minimize dust leakage. In addition, 

Controls for Secondary Sources 209 
 

 

repetitive dust leaks should not simply be, "patched," but rather, the underlying cause should be 

determined and corrective action taken to both fix the leak and prevent future reoccurrences. 

Each process or function within mineral processing operations should have a Standard Operating 

Procedure (SOP) that includes routine inspections, maintenance, and cleaning schedules. When 

equipment is cleaned and maintained on a routine basis, it can have a significant impact on 

minimizing dust liberation and generation. 

TOTAL STRUCTURE VENTILATION DESIGN 

The first strategy to lower dust exposures in any structure is to have an effective primary dust 

control plan that captures major dust sources at their point of origin, before they are allowed to 

escape out into the plant and contaminate workers. Most mineral processing plants control the 

dust by using standard engineering controls such as local exhaust ventilation systems, water 

spray applications, scrubbers, and electrostatic precipitators (refer to Chapter 1—Fundamentals 

of Dust Collection Systems). These engineering controls are effective at reducing and capturing 

the dust generated and liberated from the primary sources at mineral processing plants; however, 

they do not address the continual buildup of dust from background sources, such as the 

following: 

product residue on walls, beams, and other equipment which becomes airborne from 

plant vibration or high-wind events; 

product that accumulates on walkways, steps, and access areas, and which may be 

released as workers move through the plant and by plant vibration; 

leakage or falling material from chutes, beltways, and dust collectors; 

lids and covers of screens that are damaged or when they are removed for inspection and 

cleaning; and 

product released because of imperfect housekeeping practices. 

Since most mineral processing structures can be considered closed systems, the background dust 

sources stated above, along with numerous other unnamed sources, can cause dust concentrations 

to continually increase as the day or shift progresses inside these closed buildings. In order to 

keep dust levels at safe and acceptable levels, a method needs to be used to control these 

background dust sources. The most effective way to achieve this is to use a total structure 

ventilation system. 

PRINCIPLES OF TOTAL STRUCTURE VENTILATION 

The basic principle behind the total structure ventilation design is to use clean outside air to 

sweep up through a building to clear and remove the dust-laden air. This upward airflow is 

achieved by placing exhaust fans at or near the top of the structure and away from plant 

personnel working both inside and outside the structure. The size and number of exhaust fans is 

determined based upon the initial respirable dust concentration and the total volume of the 

structure. It must be noted that all the processing equipment within a mill generates heat and this 

produces a thermodynamic "chimney effect" that works in conjunction with the total structure 

210 Controls for Secondary Sources 
 

 

ventilation design [Cecala and Mucha 1991; Cecala 1998; Cecala et al. 1995]. Figure 8.7 shows 

the concept of the total structure ventilation design. 

Figure 8.7. Basic design of total structure ventilation system. 

Controls for Secondary Sources 211 

To be effective, the total structure 

ventilation design must meet three criteria, which are: to have a clean makeup air supply, an 

effective upward airflow pattern, and a competent shell structure. 

Clean Makeup Air Supply 

Intake air needs to be brought in at the base of the structure by strategically located wall louvers 

or open plant doors. It is very important that this air be free from outside dust sources such as 

bulk loading, high traffic areas, etc., which could cause the dust-laden air to be drawn into the 

structure and could increase respirable dust levels. Through the use of wall louvers or closing 

doors, the intake air locations could be changed based upon the outside dust conditions. 
 

 

Effective Upward Airflow Pattern 

The system should provide an effective upward airflow pattern that ventilates the entire structure 

by sweeping the major dust sources and work areas. Strategic positioning of both exhaust fans 

(high in the walls or roof) and makeup air intakes (at the base) create the most effective airflow 

pattern for purging the entire building. It must be noted that the total structure ventilation is not 

applicable to buildings with multi-story solid floors since solid floors do not allow the air to flow 

up through the structure. 

Competent Shell of Structure 

A competent outer shell of the structure is necessary because the ventilation system draws the 

makeup air through the points of least resistance. Therefore, the structure's outer shell must be 

free of open or broken windows, holes, cracks, and openings, especially in the vicinity of the 

exhaust fans. The exhaust fans create a pressure, and if the structure is not competent, air will be 

brought in from unwanted dust sources and will not allow for an effective airflow pattern for 

purging the building. 

A normal range of airflow for a total structure ventilation design would be in the 10–35 air 

changes per hour (acph) level. During the development of the total structure ventilation design 

concept, two different field studies were performed in an effort to document its effectiveness. In 

the first study, with a 10-acph ventilation system, a 40 percent reduction in respirable dust 

concentrations was achieved throughout the entire structure. In the second study, the total 

structure ventilation system was capable of providing both 17 and 34 acph. Average respirable 

dust reductions throughout the entire structure ranged from 47 to 74 percent for the two exhaust 

volumes, respectively [Cecala et al. 1995]. 

VARIABLES AFFECTING TOTAL STRUCTURE VENTILATION 

The direction of the prevailing wind can affect the total structure ventilation design. When roof 

exhausters are used, the impact is minor; but when wall-type exhausters are used, fans should not 

be placed in locations where the prevailing winds will work against them. The ideal design is 

that the fan should exhaust with the direction of the prevailing wind. This also minimizes the 

possibility of dust being recirculated back into the plant through the makeup air. 

When extremely low winter temperatures occur, the total structure ventilation system may 

somewhat increase the vulnerability to plant freeze-up problems. The makeup air can be heated 

to avoid this problem, but this is an extremely expensive proposition. A more affordable and 

realistic approach is to lower the ventilation volume during freeze-up air temperatures. This can 

be achieved by using variable-speed control fans or selectively turning off fans in a multiple fan 

design. When lowering the ventilation volume is not an option, operations may want to consider 

providing localized heating to individual work areas/stations. 

212 Controls for Secondary Sources 
 

 

Costs 

The total structure ventilation design has proven to be one of the most cost-effective systems to 

lower respirable dust concentrations throughout an entire closed mineral processing structure. 

Not only is the initial cost of this technique inexpensive when compared to other engineering 

controls, but its operation and maintenance are also minimal. Operations can install all the 

components for this system in-house to reduce their costs. 

OPEN STRUCTURE DESIGN 

The outside ambient environment can be an effective source of ventilation to dilute and carry 

away dust generated and liberated within a structure. Therefore, in some cases, an openstructure design can be used as an optimal way to provide for whole structure ventilation. The 

open design is similar to the total structure ventilation design in that it has the potential to be a 

global approach to lower dust levels throughout an entire building or structure. In the openstructure design, the natural environment acts as an effective method of diluting and carrying 

away dust liberated during operations in mineral processing structures. No dust plume should 

ever be visible with an open-structure design. As with the total structure ventilation system, the 

first goal is to capture the major dust sources at their point of origin, before they are allowed to 

escape out into the plant and contaminate the work environment. The open-structure design is a 

secondary technique to control the residual dust not captured at the source. 

In a study that compared three different building types: masonry, an open-structure design, and a 

steel-sided design, respirable dust concentrations were significantly lower in the open-structure 

building [Cecala et al. 2007; USBM 2006]. Figure 8.8 shows a conceptual drawing of a typical 

open-structure design with a roof in comparison with an identical sized walled processing 

facility. Obviously, a roof provides a little more protection from the natural elements than a 

totally open design. If an open-structure design is considered for an operation, a number of 

issues need to be addressed: 

Product residue that becomes airborne from plant vibration or high-wind events can 

accumulate on walls, beams, and other equipment. 

An open-structure design must be considered a secondary design. Engineering controls 

are needed to eliminate the major dust sources at their point of origin before being 

liberated into the plant and contaminating the work environment. 

A great effort must be made to provide safety railings and guards to minimize the 

potential for any personnel falling from the structure. 

Equipment and personnel must be protected from environmental elements such as rain, 

snow, sleet, and hail. One possibility to minimize this concern would be to design a 

structure with a roof and sufficient overhang. In addition, equipment freeze-up problems 

can happen when low outside air temperatures occur. 

Controls for Secondary Sources 213 
 

 

Figure 8.8. Open-structure design compared to standard walled structure. 

From a federal standard basis, the only consideration would be the Environmental Protection 

Agency's opacity dust measurement, which is a qualitative measurement, taken by a federal 

regulator based on the presence of a visible dust plume. If any plumes are visible from an 

operation using an open-structure design, primary dust control methods need to be implemented 

to control the problem. 

In addition to lowering dust levels throughout an entire building or structure, the open-structure 

design has also been determined to lower worker's noise exposure because it eliminates the 

reverberation effects within a closed structure. When building new facilities, obviously the 

open-structure design is more cost-effective because it lowers material and construction costs. 

Some companies may also want to consider modifying their existing structure with a more open 

design to further reduce dust and noise levels. 

BACKGROUND DUST SOURCES 

When a worker is overexposed to respirable dust, most often the assumption is that the dust 

exposure came from the worker's primary job function. A study was performed that documented 

a number of instances where this was not the case. When a worker obtains a high respirable dust 

measurement, the correct course of action is to evaluate the worker's job function, to determine 

the dust sources that contributed to this exposure, and the magnitude of the exposure from each 

of these sources. Sometimes a secondary or background dust source can be the major contributor 

to the worker's overall exposure. Controlling these less obvious dust sources can have a major 

impact on lowering the worker's respirable dust exposures. 

214 Controls for Secondary Sources 
 

 

EXAMPLES OF SECONDARY DUST SOURCES 

The following are a few examples that were documented in actual field studies and will 

demonstrate the impact that secondary dust sources can have on a worker's respirable dust 

exposure [Cecala and Thimons 1987; USBM 1988; Cecala and Thimons 1992]. 

Dusts from Outside Sources Traveling Inside Structures 

When dusts from outside sources travel inside structures, every worker inside the structure is 

impacted. Most bagging operations at mineral processing plants use an exhaust ventilation 

system to draw the dust generated from the bagging process down into the fill hopper and away 

from the breathing zone of the bagger (refer to Chapter 6—Bagging). It is important that the air 

being drawn into this exhaust ventilation system, commonly called makeup air, be clean air. At 

one operation, the makeup air was drawn directly from the bulk loading area outside the mill. 

The dust generated from this bulk loading process traveled through an open door into the mill, 

substantially contaminating the workers inside the mill. During periods when bulk loading was 

3 not performed, the bag operator's average dust exposure was 0.17 mg/m . As trucks were loaded 

3 at the bulk loading area, the worker's average exposure increased to 0.42 mg/m due to this 

contaminated air (Figure 8.9). As this study demonstrates, if outside air is used as makeup air, it 

must be from a location where the air is not contaminated. 

Figure 8.9. Increase in a worker’s respirable dust exposure inside a 

mineral processing facility while bulk loading was being performed outside. 

Personal Dust Exposure from Differences in Job Function Work Practices 

During an evaluation of four different dust control systems at a mineral processing operation, it 

was determined that substantial variations occurred in the respirable dust exposure of two 

workers based upon their personal work practices in performing their same job function. In this 

case, these two workers were loading bags of product on a bag filling machine and then 

Controls for Secondary Sources 215 
 

 

manually removing the bags and placing them onto a conveyor belt. Figure 8.10 shows a 

comparison of the respirable dust exposure of these two workers while performing the same job 

function when different control techniques were implemented (system type 1–4). 

Figure 8.10. Comparison of two workers’ respirable dust exposure while performing 

their job function with different control techniques being implemented. 

 The point to be 

highlighted from this testing was that regardless of the system being evaluated, worker #2's 

respirable dust exposure was always significantly less than that of his coworker (worker #1). 

The main reason for these differences was worker #1 performed his job function in a rough 

manner relative to worker #2, who performed the exact same tasks in a very conscientious or 

careful manner. Ultimately, this resulted in worker #2's respirable dust exposure being 

approximately 70 percent less than that of his coworker. 

One key element in how worker #2 performed his tasks was the way he manually closed the bag 

valve with his hand as he removed the bag from the fill station and transferred it to the conveyor 

(Figure 8.11). This technique was believed to be instrumental in lowering the worker's 

respirable dust exposure. Comparing the difference between these two workers indicates the 

impact that individuals can have on their dust exposure based upon how they perform their job 

function. 

216 Controls for Secondary Sources 
 

 

Figure 8.11. Worker closing bag valve with his hand as he removes the bag 

from the fill nozzle, significantly reducing his dust exposure. 

Broken Bags of Product 

In most cases, bag breakage occurs because of flaws in the bags delivered from the bag 

manufacturer. At one particular operation, a bag operator's average dust exposure went from 

3 3 0.07 mg/m before the bag break to 0.48 mg/m afterwards. Since the bag broke during the 

conveying process and not directly in front of the worker, the dust substantially contaminated the 

surrounding mill air, which in turn, flowed over the bag operator (Figure 8.12). This occurred 

because the exhaust ventilation system in the bag loading area creates a negative pressure that 

draws background air from the contaminated mill air. 

Controls for Secondary Sources 217 
 

 

Figure 8.12. Increase in worker’s respirable dust exposure

from broken bag during the conveying process. 

Cloth Seat Material/Worn Acoustical Material 

At many locations throughout mineral processing operations, workers are required to sit in chairs 

to perform their job functions. This includes all mobile equipment operators, control room 

operators, crusher operators, some bag loading operators, bulk loading operators, fork lift 

operators, lab workers, as well as other site-specific applications. In addition to these job 

functions where workers are required to sit, there are many chairs located throughout facilities 

such as in break rooms, lunch rooms, and other locations, which allow workers an opportunity to 

get off their feet for a break. When cloth is used for the seat material, it can retain a substantial 

amount of dust which is liberated each time a worker sits or moves on the material. Because of 

this, cloth material should not be used and should be replaced with some type of vinyl material. 

It should also be noted that even with vinyl seats, if the vinyl material has deteriorated to a point 

where the foam padding is being exposed, this also need to be changed because the foam will 

also hold and liberate dust. 

Material selection can also be a problem when acoustical material is used in equipment, enclosed 

cabs, controls rooms, operator's compartments, etc. to minimize the effects of noise. As this 

material gets old and starts to deteriorate, if the foam material becomes exposed, it also becomes 

a source of dust accumulation and exposure to the worker, similar to cloth or exposed foam in 

seats or chairs. 

Dirty Work Boots 

As with dust-laden work clothing, dirty work boots can also be a source of background dust 

exposure to workers. This occurs when a worker walks through wet product, overburden, and/or 

mud and it adheres to his/her boots. Obviously, wet conditions are common at mineral 

processing operations from rain and/or any wet plant processes. Once a worker's boots are 

218 Controls for Secondary Sources 
 

 

soiled with product, it is then tracked everywhere the worker goes until it either all falls off or 

the boots are cleaned. Figure 8.13 shows pr oduct tracked onto the floor of a piece of mobile 

equipment. It is easy to understand the amount of dust that can be generated when this level of 

material is accumulated on the floor. 

Figure 8.13. Product tracked onto the floor of a piece of mobile equipment from dirty work boots. 

In cases where a substantial amount of dust and product gets tracked into a work area, 

housekeeping should be performed on a daily or shift basis. In other cases where the 

contamination is not as great, housekeeping could be done on a more infrequent basis, but in any 

case, it is critical that this contamination is dealt with to minimize the source of dust to workers. 

One strategy that was tried and shown to be effective in reducing the amount of dust generated 

from dirt and product on the floor was through the use of sweeping compounds. A study was 

conducted to determine the effectiveness of using a gritless, nonpetroleum based sweep 

compound in a number of enclosed cabs [NIOSH 2001]. Figure 8.14 shows the average 

respirable dust concentrations measured inside two enclosed cabs for three to five shifts for both 

test conditions (baseline and sweep compound). The respirable dust levels were reduced from 

3 3 0.18 and 0.68 mg/m to 0.07 and 0.12 mg/m , respectively, for these two drills. These results 

indicate that the sweeping compound had a positive effect on suppressing dust when it was able 

to make direct contact with and bind up the dirt on the steel and rubber floor mats. 

It should be noted that before any sweeping compound is used, companies should examine the 

Material Safety Data Sheet (MSDS) to ensure there are no ingredients that would cause problems 

in an enclosed area. The sweeping compound should also be changed and disposed of properly 

once it has reached a state where it is no longer containing the dust. 

Controls for Secondary Sources 219 
 

 

In other cases, an operation may want to consider installing a boot wash or boot cleaning device 

to minimize the negative effects of dirty work boots. These boot cleaning stations are relatively 

inexpensive and can be placed at various locations around an operation. 

Figure 8.14. Respirable dust concentrations inside enclosed cab of two different pieces 

of equipment with and without the use of a gritless, nonpetroleum sweep compound. 

MAINTENANCE 

Maintenance of dust control systems is critical to ensure continuing worker protection 

[USBM 1974]. Preventive maintenance schedules should be established based on observed 

component wear, system performance measures, and exposure sampling results. 

Work Practices to Reduce Dust Exposure during Maintenance Activities 

Workers performing maintenance can be exposed to higher than normal dust concentrations 

because the maintenance task may involve working in areas where dust controls are not 

functioning effectively, working on highly contaminated equipment, or the work may have to be 

performed in unusual nonroutine operating conditions. 

Clean equipment and work areas when needed by washing or vacuuming before and 

during maintenance work. Working in dusty work areas results in increased dust 

exposure to maintenance workers. 

220 Controls for Secondary Sources 
 

 

Maximize the use of remote access fittings such as extension grease lines. This practice 

lessens the potential for dust exposure to maintenance workers from having to remove 

guards, covers, and other items of equipment that may have surface dust or exposed dusty 

areas. 

Keep outside intake air and recirculating air dust filters clean and replace when necessary 

in mobile equipment, cabs, control rooms, and operator booths. Proper attention must be 

given to the filter efficiency when filter changes are necessary. Refer to Chapter 9— 

Operator Booths, Control Rooms, and Enclosed Cabs for more information. 

Provide travelways to routine maintenance sites. This eliminates dust exposure from 

climbing in and around equipment to access maintenance sites. 

Provide lifting and mechanical handling equipment to reduce handling of dust-laden 

equipment. 

Clean dust-laden work clothing immediately following the completion of maintenance 

work activity using a clothes cleaning booth system or another approved technique. 

(Refer to the "Cleaning Dust from Soiled Work Clothes" section, earlier in this chapter.) 

In addition, disposable coveralls made of dust-blocking material, such as Tyvek® or 

KleenguardTM, can be worn to greatly reduce dust contamination of work clothing and the 

transfer of dust to clean work areas or to the worker's home. 

When gloves are worn, a nonporous material that does not retain dust should be used. 

Using an open weave fabric material that holds the dust is discouraged because of its 

ability to increase the workers dust exposure. 

Where feasible, wash down mobile equipment prior to entering maintenance bays for 

conducting maintenance work. 

Avoid the use of compressed air to clean surfaces. 

Periodically clean the inside of mobile equipment, control rooms, and operator booths to 

remove accumulated product, dust, and debris. 

REFERENCES 

Cecala AB [1998]. Supplementing your dust control equipment with whole-plant ventilation. 

Powder & Bulk Engineering 12(1):19–32. 

Cecala AB, Mucha R [1991]. General ventilation reduces mill dust concentrations. Pit & 

Quarry 84(1):48–53. 

Cecala AB, Thimons ED [1987]. Significant dust exposures from background sources. Pit & 

Quarry 79(12):46–51. 

Cecala AB, Thimons ED [1992]. Some factors impacting bag operator's dust exposure. Pit & 

Quarry 85(5):38–40. 

Cecala AB, Klinowski GW, Thimons ED [1995]. Reducing respirable dust concentrations at 

mineral processing facilities using total mill ventilation system. Min Eng 47(6):575–576. 

Controls for Secondary Sources 221 
 

 

Cecala AB, O'Brien AD, Pollock DE, Zimmer JA, Howell JL, McWilliams LJ [2007]. Reducing 

respirable dust exposure of workers using an improved clothes cleaning process. Inter J of Min 

Res Eng 12(2):73–94. 

Cecala AB, Rider JP, Zimmer JA, Timko RJ [2007]. Dial down dust and noise exposure. 

Aggregates Manager 12(7):50–53. 

Cecala AB, Pollock DE, Zimmer JA, O'Brien AD, Fox WF [2008]. Reducing dust exposure 

from contaminated work clothing with a stand-alone cleaning system. In: Wallace, ed. 

Proceedings of 12th U.S./North American Mine Ventilation Symposium. Omnipress. ISMN 

978–0–615–20009–5, pp. 637–643. 

Hartsky MA, Reed KL, Warheit DB [2000]. Assessments of the barrier effectiveness of 

protective clothing fabrics to aerosols of chrysotile asbestos fibers. Performance of Protective 

Clothing: Issues and Priorities for the 21st Century: Seventh Volume, ASTM STP 1386, 

pp. 141–154. 

Langenhove LV, Hertleer C [2004]. Smart clothing: a new life. Inter J of Clothing Science and 

Tech. 16(1&2):63–72. 

NIOSH [2001]. Technology news 487: Sweeping compound application reduces dust from 

soiled floor within enclosed operator cabs. By Organiscak JA, Page SJ, Cecala AB. Pittsburgh, 

PA: U.S. Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health. 

Pollock DE, Cecala AB, O'Brien AD, Zimmer JA, Howell JL [2005]. Dusting off. Rock 

Products. March. pp. 30–34. 

Salusbury I [2004]. Tailor-made-self-cleaning clothing, chemical warfare suits that trap toxins. 

Materials World. August. pp. 18–20. 

USBM [1974]. Survey of past and present methods used to control respirable dust in noncoal 

mines and ore processing mills–final report. U.S. Department of the Interior, Bureau of Mines 

Contract No. H0220030. NTIS No. PB 240 662. 

USBM [1986]. Impact of background sources on dust exposure of bag machine operator. By 

Cecala AB, Thimons ED. U.S. Department of the Interior, Bureau of Mines Information 

Circular 9089. 

USBM [1988]. Technology news 299: Reduce bagging machine operator's dust exposure by 

controlling background dust sources. U.S. Department of the Interior, Bureau of Mines. 

USBM [2006]. Lower respirable dust and noise exposure with an open structure design. By 

Cecala AB, Rider JP, Zimmer JA, Timko RJ. U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute for Occupational Safety and 

Health, NIOSH Report of Investigations 9670. 

222 Controls for Secondary Sources 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Controls for Secondary Sources 223

CHAPTER 9: 

OPERATOR BOOTHS, 

CONTROL ROOMS, AND

ENCLOSED CABS 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

224 Controls for Secondary Sources 
 

 

CHAPTER 9: OPERATOR BOOTHS, CONTROL ROOMS, 

AND ENCLOSED CABS 

Operator booths, control rooms, and enclosed cabs have all been used for many years to isolate 

workers from dust sources in mineral processing operations. When they are properly designed, 

installed, and maintained, they can provide a safe work environment that supplies clean and 

acceptable air quality to the worker. There has been a substantial amount of research performed 

in this area over the past decade identifying and correcting problematic issues and determining 

the critical components that are necessary for an effective system. 

Workers in operator booths, control rooms, and enclosed cabs at mining operations are 

surrounded by dynamic working conditions that have highly variable dust sources. These 

enclosures create a microenvironment for the workers where they can be either more protected or 

more vulnerable to respirable dusts. Workers can be more vulnerable to in-cab dust sources 

(floor heaters, dirt on floors/walls, or on operator's clothing, etc.) that are trapped within the 

enclosure. Enclosed cabs on mobile equipment are actually harder to control and maintain than 

enclosed stationary areas (operator booths and control rooms) since the moving of the equipment 

constantly stresses and can compromise the integrity of the enclosure. 

Figure 9.1 shows a general design of a filtration and pressurization system. At mineral 

processing operations, filtration and pressurization systems are the primary engineering control 

to reduce worker exposure to airborne dust in operator booths, control rooms, and enclosed cabs. 

These enclosures should have an effective filtration system that cleans both outside and inside air 

that is brought into the enclosure and sufficient enclosure integrity in order to achieve 

pressurization. The most effective filtration and pressurization systems are integrated into the 

heating, ventilating, and air-conditioning (HVAC) units. A substantial amount of research has 

been performed to improve the air quality in enclosed cabs of surface mining equipment, and this 

research is directly applicable to operator booths and control room dust control systems. 

EFFECTIVENESS TERMINOLOGY 

When evaluating a pressurization and filtration system—comparing outside with inside 

respirable dust concentrations—a number of different descriptors can be used to provide a 

numerical value that ranks the system's effectiveness. The following three descriptors are 

commonly used for this purpose: 

Protection Factor (PF) = 

;

i

o

C

C

(ratio) 

Efficiency (η) = 

o

io

C

CC

; (fraction, or multiplied by 100 for percent value) 

Penetration (Pen) = 1 – η; (fraction) 

Operator Booths, Control Rooms, and Enclosed Cabs 225 
 

 

 

where Co = outside respirable dust concentration, and 

Ci = inside respirable dust concentration. 

A comparison of these descriptors can be provided by the following: 

Pen

1

1

1

PF 

i

o

C

C

(9.1) 

Figure 9.1. General design of an effective filtration and pressurization system. 

226 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

Average Average Cab Protection inside cab outside cab Cab evaluation pressure, inches factor, out/ dust level, dust level, wg 3 mg/m 3 mg/m in 

1. Rotary drill None detected 0.08 0.22 2.8 

2. Haul truck 0.01 0.32 1.01 3.2 

3. Front-end loader 0.015 0.03 0.30 10.0 

4. Rotary drill 0.20 to 0.40 0.05 2.80 56.0 

5. Rotary drill 0.07 to 0.12 0.07 6.25 89.3 

Obviously, the higher the value for both protection factor and efficiency, and the lower the value 

for penetration that can be achieved, the better the air quality inside the operator booth/control 

room/enclosed cab. Table 9.1 shows a comparison of the three different descriptors. 

Table 9.1. Comparison of three different descriptors for the effectiveness 

at providing clean air to an enclosed cab, operator booth, or control room 

Protection Factor Efficiency, % Penetration 

2 50 0.50 

5 80 0.20 

10 90 0.10 

100 99 0.01 

1,000 99.9 0.001 

For this chapter, the term protection factor (PF) will be used when discussing enclosed cab, 

operator booth, and control room effectiveness. 

FILTRATION AND PRESSURIZATION SYSTEMS 

Field Studies 

A health screening study performed during the mid-1990s in central Pennsylvania identified a 

significant number of silicosis cases attributed to operators of mobile mining equipment in 

enclosed cabs that were not providing an acceptable level of protection [CDC 2000]. Because of 

this, a number of organizations began investigating enclosed cabs to better understand the 

problem and to determine methods and solutions to improve the air quality and protect workers. 

This led to a number of studies in which new filtration and pressurizations systems were installed 

on older pieces of mining equipment in an attempt to improve the air quality inside these 

enclosed cabs. The results of a few of these studies can be seen in Table 9.2, listed in ascending 

order of effectiveness [Organiscak et al. 2004; Chekan and Colinet 2003; Cecala et al. 2005; 

Cecala et al. 2004]. 

Table 9.2. Summary of field studies evaluating upgraded cabs 

These studies highlighted some very important factors relevant to improving the air quality in 

enclosed cabs and ultimately protecting the workers. Cab integrity, and the related ability to 

achieve positive pressurization, was found to be a critical component. As seen in the first two 

studies listed in Table 9.2, when there was very little to no cab pressure detected, this resulted in 

Operator Booths, Control Rooms, and Enclosed Cabs 227 
 

 

minimal improvement in the cab's air quality. In fact, similar filtration and pressurization 

systems were installed on the rotary drill and front-end loader listed as items 1 and 3 in 

Table 9.2, with the PF varying from 2.8 to 10. One notable difference between these two 

systems was that a small amount of pressurization was achieved in the front-end loader, whereas 

it was not possible to achieve any pressurization in the rotary drill. It should also be noted that 

low outside dust concentrations may have a negative impact on the PF. 

Another critical factor was the quality and effectiveness of the filtration system. The various 

studies presented in Table 9.2 indicated substantial improvement in the interior air quality from 

effectively removing the dust particles from the outside air and delivering this clean filtered air 

into the enclosed cab. When sufficient pressurization was achieved along with an effective 

filtration system, very good air quality was obtained in these cabs as indicated by significant PFs. 

Laboratory Study 

From these various field studies, a number of different factors emerged that were relevant to the 

effectiveness of filtration and pressurization systems. In an effort to evaluate this area, a 

controlled laboratory experiment was performed to systematically examine multiple cab designs. 

Figure 9.2 shows the cab filtration system setup used for this controlled laboratory study and the 

various parameters evaluated [NIOSH 2008b; Organiscak and Cecala 2008]. 

Figure 9.2. Laboratory test setup to evaluate various operational parameters 

on a filtration and pressurization system for an enclosed cab. 

228 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

 

In Figure 9.2, the parameter values are as follows: 

PF = protection factor, Co/Ci; 

Co = outside cab concentration; 

Ci = inside cab concentration; 

ηI = intake filter efficiency, fractional; 

QI = intake air quantity; 

QL = leakage air quantity; 

l = intake air leakage, QL/QI; 

ηR = recirculation filter efficiency, fractional; 

QR = recirculation air quantity; 

QW = wind quantity infiltration; and 

VC = cab volume. 

The results of this laboratory study indicate that intake filter efficiency and the use of a 

recirculation filter had the greatest impact on improving the air quality. When considering the 

use of an intake air filter, the addition of the recirculation component significantly improved the 

air quality due to the repeated filtration of the cab's interior air. The addition of an intake
	

pressurizer fan to the filtration system increased both intake airflow and cab pressure 

significantly. The cab air quality was also affected by intake filter loading and air leakage. 

Mathematical Model to Determine Enclosure Protection Factor 

In the course of the laboratory study, the significance of the filtration system parameters was 

6 evaluated and the following mathematical model was developed. Equation 9.2

6 This equation is dimensionless; therefore, air quantities used must be in equivalent units. Also, filter efficiencies 

and intake air leakage must be fractional values (not percentage values). 

Operator Booths, Control Rooms, and Enclosed Cabs 229 

was formulated 

from a basic time-dependent mass balance model of airborne substances within a control volume 

with steady state conditions. The equation determines the PF in terms of intake air filter 

efficiency, intake air quantity, intake air leakage, recirculation filter efficiency, recirculation 

filter quantity, and outside wind quantity infiltration into the cab. 

WIII

RRI

i

o

QlQ

QQ

C

C

)1(

P F

(9.2) 

Equation 9.2 allows for a comparison of how changes in the various parameters and components 

in the system impact the PF. The wind quantity infiltration (Qw) can be assumed to be zero if the 

cab pressure exceeds the wind velocity (Figure 9.3). By using Equation 9.2, operations have the 

ability to determine the desired parameters necessary to systematically achieve a desired PF in an 

operator's booth, control room, or enclosed cab to improve the air quality to safe levels and to 

ultimately protect their workers. 
 

 

 

)p4000(

cab

 

RECOMMENDATIONS FOR FILTRATION/PRESSURIZATION SYSTEMS 

Based on the knowledge gained from both laboratory and numerous field studies, the practices 

recommended in this section need to be incorporated in order to achieve an effective filtration 

and pressurization system. 

Ensuring Enclosure Integrity by Achieving Positive Pressurization against Wind 

Penetration 

Enclosure integrity is necessary in order to achieve pressurization, which is critical for an 

effective system. Testing has shown that the installation of new door gaskets and plugging and 

sealing cracks and holes in the shell of the enclosure have a major impact on increasing the 

enclosure pressurization. To prevent dust-laden air from infiltrating into the enclosure, the 

enclosure's static pressure must be higher than the wind's velocity pressure [Heitbrink et al. 

2000]. Equation 9.3 is used to determine the wind velocity equivalent for an enclosure (the wind 

velocity at which the cab is protected from outside infiltration as determined by the static 

pressure): 

Wind velocity equivalent 

fpm x 0.011364 mph/fpm 

@ standard air temperature and pressure 

(9.3) 

where Δр = cab static pressure in inches wg. 

Figure 9.3 provides a graphical display of this wind velocity equivalent. This shows that an 

enclosure pressure of 0.05, 0.1, 0.25, 0.5, and 1.0 inches wg would be able to withstand wind 

velocities of 10.2, 14.4, 22.7, 32.1, and 45.5 mph from penetrating dust into the enclosure. 

Although minimum pressurization has been shown to have positive results from field studies, a 

good rule of thumb is to have at least 0.05 inches wg of positive pressure in enclosures. A 

reasonable range of enclosure pressure is between 0.05 to 0.25 inches wg. 

230 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

Figure 9.3. Positive cab pressure necessary to prevent dust-laden air 

from infiltrating the enclosed cab at various wind velocities. 

Keeping Doors and Windows Closed 

In order to achieve and maintain enclosure pressurization, doors and windows must be closed at 

all times except while the operator is entering or exiting the enclosure. This problem was noted 

during a field study on a surface drill when the operator repeatedly opened the cab door to 

manually guide the drill steel into place each time an additional section was needed [Cecala et al. 

2007; NIOSH 2008a]. The cab door was usually open somewhere between 20 and 45 seconds 

each time this process took place before being closed again. Because no drilling was occurring 

and no dust cloud was visible as the cab door was opened, the impact to the drill operator's 

respirable dust exposure was initially thought to be insignificant. However, when dust data from 

inside the enclosed cab were analyzed, a substantial increase in respirable dust concentrations 

was noted during the periods when the door was open. This significant increase was unexpected 

when one considers that drilling had ceased approximately 2 minutes before the door was 

opened. Figure 9.4 shows average concentrations for each of the 3 days of testing for the time 

3 period when the cab door was closed and open. The average concentration was 0.09 mg/m with 

3 the cab door closed and 0.81 mg/m with the door open. Despite no visible dust cloud during the 

time when the cab door was open, respirable dust concentrations inside the cab were nine times 

higher than when the door was closed and drilling was being performed. 

The results of this testing clearly stress the importance of keeping doors and windows closed at 

all times in an effort to keep the compartment pressurized and working properly. Again, the only 

exception to keeping the door closed should be when the equipment operator enters or exits the 

cab. It also needs to be stressed that even when dust clouds are not visible outside, respirable 

dust levels can be significantly higher than filtered levels inside cabs. 

Operator Booths, Control Rooms, and Enclosed Cabs 231 
 

 

Figure 9.4. Respirable dust concentrations inside enclosed cab 

for three days of testing with cab door closed and open. 

Effective Filtration 

In order to achieve effective filtering and provide clean air to an operator's enclosure, the system 

needs to be composed of both an effective outside intake air and a recirculation filtration 

component. 

Effective Outside Air Filtration and Enclosure Pressurization 

A minimum quantity of at least 25 cfm of intake/outside air per person is necessary to dilute CO2 

quantities exhaled by each worker [ASABE 2003]. A typical range for most enclosed cabs on 

surface equipment is 40 to 70 cfm. The outside air delivered into an enclosure is the only 

component that creates pressurization, which is critical for an effective system. The amount of 

intake air delivered to create this pressurization must be carefully controlled and optimized. 

Once the optimal point is determined, increasing the air quantity degrades the system by 

increasing particle penetration and decreasing filter efficiency by allowing more contaminants to 

flow through the filter media. 

With outside air, the optimal design is to provide enough air to achieve an acceptable level of 

pressurization, but further increasing airflow only decreases the air quality in the enclosure. The 

mathematical model derived from the laboratory study to determine the PF can be used to 

emphasize this point. The following is a hypothetical situation: a recirculation system that 

provides 300 cfm (QR), a recirculation filter efficiency of 0.7 (70 percent) (ηR), an intake filter 

efficiency of 0.95 (95 percent) (ηI), and assuming no air leakage (l=0)—increasing the intake air 

232 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

quantity from 50 to 70 cfm (QI) will cause the PF to decrease from 104 to 80, respectively, just 

by increasing the intake air quantity by 20 cfm. 

High efficiency filters are a necessity for the outside intake air. Laboratory experiments showed 

a 10-fold increase in protection factors when using a 99-percent efficient filter versus a 38

percent efficient filter on respirable sized particles. The balance is to have a sufficient quantity 

of intake air to create a positive enclosure pressurization to eliminate the wind from blowing dust 

into the enclosure, but not so much airflow as to burden the filtration system beyond what is 

necessary. In addition, all intake air must be conditioned for temperature, either via heating in 

the winter or air-conditioning in the summer, and thus, the greater the intake air quantities, the 

greater the cost for providing for this temperature component. 

Outside Air Inlet Location 

Another key consideration is the location of the outside inlet for the intake air. Locating the cab 

air inlet near major dust sources causes high dust loading on the air filtration system. This high 

dust loading burdens the filtration system and reduces its effectiveness by increasing the pressure 

drop across the loaded filter and decreasing the quantity of air and cab pressurization. In 

addition, the increased pressure drop across the loaded filter also increases the potential for dust 

leakage around the filter cartridge. This requires that the filter cartridge be cleaned or changed 

more frequently which also increases the filter cost. Finally, air filtration is based on relative 

dust capture efficiency, so filtering higher outside dust levels creates higher inside cab dust 

concentrations. 

In an effort to minimize these effects, it is recommended to place the enclosure's air inlet 

location strategically away from dust sources to reduce dust loading of the filter cartridge 

[NIOSH 2001a]. This can usually be accomplished by locating the outside air intake inlet at 

higher levels away from the ground and on the opposite side of the enclosed cab, operator booth, 

or control room away from dust sources. This location also enables the enclosure to shield some 

of the dust from the inlet. 

PreCleaner 

It is recommended that the outside air filtration system use a precleaning and final-filtering 

arrangement. The precleaning should be performed to remove oversized dust particles and to 

increase the life cycle and the filtering capacity of the final intake filter. This could be 

performed by using some type of centrifugal design to spin out oversized particles or a backflushing compressed air system, similar to those in a local exhaust ventilation (LEV) system. 

These back-flushing systems can be set to clean the filter cartridge on a predetermined time basis 

or when the pressure builds to a predetermined level. 

Effective Recirculation Filtration 

The use of the recirculation component with effective filtration is critical for an optimal design. 

Laboratory experiments showed a 10-fold increase in protection factors when using an 85 to 

94.9 percent efficient filter on respirable sized dusts as compared to no recirculation filter. 

Laboratory testing also showed that the time for the interior to stabilize after the door was closed 

(decay time) was reduced by more than 50 percent when using the recirculation filter. The 

-

Operator Booths, Control Rooms, and Enclosed Cabs 233 
 

 

average decay times were between 16 and 29 minutes without the recirculation filter and 

between 6 and 11 minutes with the recirculation filter. Thus, the use of a recirculation filter 

greatly improved the air quality and reduced the exposure time after the cab door was closed 

[NIOSH 2007]. One additional benefit of using a recirculation filter is that it allows cleaner air 

to be circulated through the HVAC system, thus providing better thermal efficiency. 

Floor Heaters 

Any type of floor heater or fan located low in the enclosure that will stir up dust should be 

eliminated. During a field study, it was found that a floor heater fan used during the winter 

months to provide heat to an operator of a surface drill greatly increased the respirable dust 

concentrations inside the enclosed cab (Figure 9.5) [Cecala et al. 2001; NIOSH 2001b]. 

Figure 9.5. Problem created by heater stirring up dust from 

the floor and blowing dust off of the worker’s clothing.

234 Operator Booths, Control Rooms, and Enclosed Cabs 

 The 

floor heater can be a serious problem because the floor is the dirtiest part of the cab from the 

operator bringing dirt in on his or her work boots. Then as the operator moves his or her feet 

around, dust is created, which is then blown throughout the cab by the fan on the floor heater. 

This fan also tends to stir up dust that may be on the drill operator's clothes. 
 

 

Because of the significant increase in dust levels with floor heaters, it is recommended that they 

not be used. If removal is not an option, they should be repositioned to a higher area in the 

enclosure where they are less prone to pick up dust from the floor and operator's clothing. Also, 

no type of fan should be used low in the cab because of the potential to stir up in-cab dust 

sources. Ideally, the heater unit should be tied into the filtration and pressurization unit to 

deliver the heated air at the roof of the cab. 

Good Housekeeping (Enclosure Cleanliness) 

To maintain pressurization and filtration systems, good housekeeping practices are essential in 

that systems need to be cleaned periodically and filters need to be changed when necessary. In 

addition, the integrity of the enclosure must also be inspected to ensure that pressurization is 

maintained by replacing gaskets and seals when wear appears, and by plugging and sealing holes 

and cracks in the walls. It must be understood that a system that is not properly maintained will 

deteriorate over time to a point where it is no longer providing an acceptable level of protection, 

thus causing workers to be exposed to respirable dusts. 

During the field studies, a number of filtration units were found in all forms of disarray and had 

deteriorated to a condition where they were no longer providing acceptable levels of protection 

to the worker (Figure 9.6, left). In some cases, it appeared that the air quality or the protection 

provided to the worker was not a priority as long as the operator remained comfortable in regard 

to temperature controls. Although many cabs used standard heaters and air conditioners to 

control temperatures, in some instances workers resorted to just opening windows in an effort to 

be comfortable, thus bypassing the protection provided by the enclosed cab. With a little time, 

effort, and finances, effective maintenance can be performed on filtration and pressurization 

systems to transition them from poor systems to ones that will again provide clean and 

acceptable air quality to workers (Figure 9.6, right). 

Enclosure floors are commonly soiled from workers tracking dirt and product inside the 

enclosure upon entering from the mine site. In cases when a substantial amount of dust and 

product gets tracked into the compartment, housekeeping should be performed on a daily or shift 

basis. In other cases where the contamination is not as great, housekeeping could be done on a 

more infrequent basis. It is critical that the inside of an enclosed cab, operator's compartment, or 

control room be maintained in a manner which minimizes the worker's respirable dust exposure. 

Mechanical Filter Media and Visual Indicator for Filter Changing 

It is highly recommended that both the outside air and recirculation filters be a mechanical type 

filter media, as compared to an electrostatic media. Mechanical filters become more effective as 

they load with product. This occurs because as the filter loads with dust, a dust cake forms on 

the filter media and captures additional dust particles which further improves the filter efficiency. 

As the size of this filter cake continues to increase, the efficiency continues to increase. This 

causes the pressure differential across the filter to increase, which in turn causes the airflow to 

decrease. As the pressure increases further, it will become so restrictive that the filter will need 

to be cleaned or replaced. The ideal filtration and pressurization system includes a pressure 

differential visual indicator that notifies the operator when the filter needs to be changed. 

Operator Booths, Control Rooms, and Enclosed Cabs 235 
 

 

Figure 9.6. Filtration unit showing lack of maintenance and care (left side) and 

after cleaning and the addition of a new intake air filtration unit (right side). 

Ease of Filter Change 

When designing filtration and pressurization systems, one key component is the ease with which 

filters can be replaced when necessary. It defeats the purpose of a good system if a filter to be 

changed is so difficult to access that the operator or maintenance workers do not want to take the 

time to perform the task. Another consideration is dust contamination during the filter change. 

The easier a filter is to change, the less contamination should occur to the worker performing the 

task and to the work area. When changing a canister filter, a common and effective technique is 

to remove the new filter from the cardboard box and then insert the old dust-laden filter into the 

box, tape it closed, and dispose of it. 

Uni-Directional Design 

The use of a uni-directional airflow pattern, as shown in Figure 9.7, should be considered 

whenever possible to maximize the air quality at the breathing zone of the operator inside the 

enclosure. In most systems, both the intake and discharge for the recirculation air are located in 

the roof. Unfortunately, this location causes the dust-laden air within the enclosure to be pulled 

directly over the worker as it is drawn into the ventilation system. Further, in many designs, the 

contaminated return air and clean filtered air are ducted within inches of each other at the ceiling. 

This poor design allows for recirculated air to be short-circuited and allows dust-laden return air 

236 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

to be pulled directly back into the ventilation system and over the operator's breathing zone. A 

more effective design is to draw the recirculated air from the bottom of the enclosure, away from 

the worker's breathing zone. 

Generalized Parameters for an Effective System 

The key for an effective retrofit enclosed cab filtration and pressurization system is to balance 

the various components and factors. Obviously, if one is dealing with hazardous dust, the design 

should be directed to the higher efficiency range. On the other hand, if one is dealing with more 

of a nuisance dust, a lower range value can be chosen. 

Figure 9.7. Airflow pattern for intake and return at roof of cab and uni-directional airflow design. 

Table 9.3 shows a summary of the impact of the various critical components to an effective 

filtration and pressurization system. This table presents the calculated PF using the mathematical 

model derived from the laboratory study (Equation 9.2). In this table, the same component 

parameters have been chosen from the previous examples, being: intake air quantity (QI) of 

50 cfm, recirculation system (QR) of 300 cfm, a recirculation filter efficiency (ηR) of 0.7 

(70 percent), an intake filter efficiency (ηI) of 0.95 (95 percent), and leakage (l) of 0, 5 percent, 

10 percent, and 15 percent (l = 0, 0.05, 0.10, 0.15). 

Operator Booths, Control Rooms, and Enclosed Cabs 237 
 

 

This table highlights a number of critical factors. First, it shows how the effectiveness of each 

system deteriorates as the leakage into the enclosure increases. Leakage occurs from either a 

lack of cab integrity or an insufficient intake component. The second critical factor is the need 

for a recirculation system. When one evaluates an enclosure with no leakage, the PF is increased 

from 20 to 104 by the addition of a 300 cfm recirculation system with a 70 percent efficient 

filter. This highlights the critical need of the recirculation component. A high efficiency 

recirculation filter is not critical as long as a filter within reasonable filter efficiency range (70– 

95 percent) is used and maintained. This is demonstrated by viewing the first column in Table 

9.3 where l = 0 (leakage equals zero) and the PF only increases by 24 points when going from a 

70 to a 90 percent efficiency recirculation filter. Since the air is constantly being recirculated, 

any respirable dust inside the cab will be removed within a few passes through the recirculation 

system. 

Table 9.3. Calculated PF derived from mathematical model 

(QI) = 50 cfm 

(QR) = 300 cfm 

(ηI) = 0.95 

(ηR) = 0 

l 0 l .0 05 l .0 10 l .0 15

20.0 10.3 6.9 5.2 

(QI) = 50 cfm 

(QR) = 300 cfm 

(ηI) = 0.95 

(ηR) = 0.70 

104.0 53.3 35.9 27.0 

(QI) = 50 cfm 

(QR) = 300 cfm 

(ηI) = 0.95 

(ηR) = 0.90 

128.0 65.6 44.1 33.2 

OTHER METHODS TO LOWER RESPIRABLE DUST IN ENCLOSURES 

Small In-Unit HEPA Filters 

In some operator booths and control rooms, stand-alone air purifier units containing a HEPA 

filter have been installed. These stand-alone systems are small and portable units that are 

available at a fraction of the cost of permanent systems. However, these systems can only be 

effective if they are sized to handle the volumetric capacity of the booth or control room, and if 

the filters are replaced when necessary [Logson 1998/1999]. An obvious and significant 

shortcoming with these types of systems is that they do not provide any pressurization to keep 

dust from leaking into the booth or room. A few studies have shown that in many instances, they 

do not provide sufficient protection and companies would be better served to invest their money 

and time into improving their existing roof-mounted unit [Cecala and Zimmer 2004a,b]. 

Therefore, stand-alone air purifier units containing a HEPA filter are not recommended in most 

cases and should be replaced with more comprehensive and properly sized recirculation systems. 

238 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

REFERENCES 

ASABE [2003]. Agricultural Cabs—Engineering Control of Environmental Air Quality, Part 1: 

Definitions, Test Methods, and Safety Practices [Standard 5525–1.1]. St. Joseph, Michigan: 

American Society of Agricultural and Biological Engineers. 

CDC [2000]. Notice to readers: NIOSH Alert; silicosis screening in surface coal miners— 

Pennsylvania, 1996–1997. Cincinnati, OH: U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute for Occupational Safety and 

Health.MMWR Weekly 49(27):612–615. 

Cecala AB, Zimmer JA [2004a]. Clearing the air. Aggregates Manager J 9(4):12–14. 

Cecala AB, Zimmer JA [2004b]. Filtered recirculation—a critical component to maintaining 

acceptable air quality in enclosed cabs for surface mining equipment. Proceedings of 10th 

U.S./N.A. Mine Ventilation Symposium. Anchorage, Alaska. May 16–19, pp. 377–387. 

Cecala AB, Organiscak JA, Heitbrink WA [2001]. Dust underfoot—enclosed cab floor heaters 

can significantly increase operator's respirable dust exposure. Rock Products 104(4):39–44. 

Cecala AB, Organiscak JA, Heitbrink WA, Zimmer JA, Fisher T, Gresh RE, Ashley JD II 

[2004]. Reducing enclosed cab drill operator's respirable dust exposure at surface coal 

operations with a retrofitted filtration and pressurization system. SME Transactions 2003, 

Littleton, Colorado: Society for Mining, Metallurgy and Exploration, Inc, 314:31–36. 

Cecala AB, Organiscak JA, Zimmer JA, Heitbrink WA, Moyer ES, Schmitz M, Ahrenholtz E, 

Coppock CC, Andrews EH [2005]. Reducing enclosed cab drill operator's respirable dust 

exposure with effective filtration and pressurization techniques. J of Occ and Env Hyg 2:54–63. 

Cecala AB, Organiscak JA, Zimmer JA, Moredock D, Hillis M [2007]. Closing the door to dust 

when adding drill steels. Rock Products, October, pp. 29–32. 

Chekan GJ, Colinet JF [2003]. Retrofit options for better dust control—cab filtration, 

pressurization systems prove effective in reducing silica dust exposures in older trucks. 

Aggregates Manager, December, 8(9):9–12. 

Heitbrink WA, Thimons ED, Organiscak JA, Cecala AB, Schmitz M, Ahrenhottz E [2000]. 

Static pressure requirements for ventilated enclosures. Proceedings of the Sixth International 

Symposium on Ventilation for Contaminant Control, Helsinki, Finland, June 4–7. 

Logson R [1998/1999]. Controlling respirable dust in plant control rooms. Stone Review, The 

Bimonthly Publication of the National Stone Association. December 1998/January 1999, 

14(6):43–44. 

NIOSH [2001a]. Technology news 485: Improved cab air inlet location reduces dust levels and 

air filter loading rates. By Organiscak JA, Page SJ. Pittsburgh, PA: U.S. Department of Health 

and Human Services, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health. 

Operator Booths, Control Rooms, and Enclosed Cabs 239 
 

 

NIOSH [2001b]. Technology news 486: Floor heaters can increase operator's dust exposure in 

enclosed cabs. By Cecala AB, Organiscak JA. Pittsburgh, PA: U.S. Department of Health and 

Human Services, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health. 

NIOSH [2007]. Technology news 528: Recirculation filter is key to improving dust control in 

enclosed cabs. By Organiscak JA, Cecala AB. Pittsburgh, PA: U.S. Department of Health and 

Human Services, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, DHHS (NIOSH) Publication No. 2008–100. 

NIOSH [2008a]. Technology news 533: Minimizing respirable dust exposure in enclosed cabs 

by maintaining cab integrity. By Cecala AB, Organiscak JA. Pittsburgh, PA: U.S. Department of 

Health and Human Services, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication No. 2008–147. 

NIOSH [2008b]. Key design factors of enclosed cab dust filtration systems. By Organiscak JA, 

Cecala AB. U.S. Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health: NIOSH Report of 

Investigations 9677. 

Organiscak JA, Cecala AB [2008]. Laboratory investigation of enclosed cab filtration system 

performance factors. Min Eng 60(12):74–80. 

Organiscak JA, Cecala AB, Thimons ED, Heitbrink WA, Schmitz M, Ahrenholtz E [2004]. 

NIOSH/Industry collaborative efforts show improved mining equipment cab dust protection. 

SME Transactions 2003 Littleton, Colorado: Society for Mining, Metallurgy and Exploration, 

Inc., 314:145–152. 

240 Operator Booths, Control Rooms, and Enclosed Cabs 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Operator Booths, Control Rooms, and Enclosed Cabs 241 

CHAPTER 10: 

HAUL ROADS, 

STOCKPILES, 

AND OPEN AREAS 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

242 Operator Booths, Control Rooms, and Enclosed Cabs 
 

 

CHAPTER 10: HAUL ROADS, STOCKPILES, 

AND OPEN AREAS 

This chapter examines dust control for emissions from haul roads and wind erosion of 

stockpiles and open areas. Generally classified as a fugitive dust, the source material for 

these emissions is typically soil and mineral particles disturbed during the mining or 

stockpiling processes. 

The main focus of this chapter is on haul road dust control as it is a significant issue at 

surface mine sites (Figure 10.1). 

Figure 10.1. Example of haul road dust from a typical mine haul truck. 

 Past research has shown that haul trucks generate the 

majority of dust emissions at these sites, with their contribution being 78–97 percent of 

total dust emissions for particulate matter less than 10 micrometers (μm) [Cole and Zapert, 

1995; Amponsah-Dacosta and Annegarn 1998; Reed et al. 2001]. The emissions are 

determined using the United States Environmental Protection Agency's (U.S. EPA) 

published emission factors, which is the accepted method for calculating emissions from 

the various operations at industrial sites (including mining). The high contribution of dust 

emissions from haul trucks has the potential to expose personnel working nearby (within 

100 feet or downwind) to significant amounts of respirable dust, creating potential 

3 overexposures. Further, high concentrations of respirable dust (up to 21.50 mg/m ) have 

been documented in areas near the vehicles [Reed and Organiscak 2005], and elevated 

percentages of silica can be associated with haul road dust [Organiscak and Reed 2004]. 

Fugitive dust from haul roads, stockpiles, and open areas can contain particle sizes ranging 

-5 -3 from 10 to 1x10 mm, with the majority of the material ranging from 10 to 1x10 mm 

[Bagnold 1960]. The amount of larger material residing within this dust is dependent 

Haul Roads, Stockpiles, and Open Areas 243 
 

 

upon the wind velocity (or disturbance) at the source of emissions. Generally, the 

respirable dust fraction is only a small part of the total mass of total dust emissions. For 

example, an evaluation of dust particle size from haul truck emissions demonstrated that 

85.5 percent of the fugitive dust is >10 μm in diameter, with 14.5 percent <10 μm 

[Organiscak and Reed 2004; Reed and Organiscak 2005]. Albeit a small amount, these 

smaller particle sizes are a concern from a health and safety standpoint, especially if silica 

dust is present. 

Reduced visibility can be a safety problem with the generated fugitive dust from haul 

roads, stockpiles and open areas. Although there is no known documented correlation 

between dusty conditions and vehicle accidents at mine sites, observations of extreme 

dusty conditions make it obvious that there could be a connection. Coarse materials 

contribute to obstructing visibility by obstructing light, whereas fine materials obstruct 

visibility by scattering the light. Light scattering generally occurs with fine particles <2.5 

μm [Cowherd and Grelinger 1997; Moosmüller et al. 2005]. Visibility reduction occurs 

3 with dust concentrations ranging from 100–400 mg/m . However, of greatest concern 

here is the ongoing health problem associated with high dust concentrations being inhaled 

by workers at mine operations, especially respirable silica dust. 

HAUL ROADS 

Haul roads and access roads are used extensively in mining operations by mobile 

equipment to move material in and out of the mining areas. The road network at a surface 

mine site can be quite extensive over the entire property and the potential for dust 

generation is dependent upon the traffic patterns at the site. In areas where the traffic 

pattern is heavy, the possibility for dust generation is much higher than areas where the 

traffic pattern is light, creating areas of potentially high dust exposures. Overexposures to 

respirable dust can occur to both equipment operators and workers in the vicinity of the 

road. The majority of the fugitive dust is generated through the forces of the wheels on 

the road surface and by the turbulence created by the vehicles [Moosmüller et al. 2005]. 

Therefore, the first step in haul road dust control is proper road construction. 

Access roads are similar in nature to haul roads, except that vehicle traffic is less frequent 

and is generally smaller in weight, although this may not always be the case. The 

following principles that are discussed for haul road dust control can also be applied to 

access roads. In particular, the road construction discussion pertains to haul roads for 

large haul vehicles, but it can easily be adapted to be applied to access roads. It is 

generally accepted to overdesign access roads; because of the transient nature of mining, 

there may be times when an access road may be upgraded to a haul road. Additional 

information for the proper design of the smaller access roads can be found in Gravel 

Roads, Maintenance and Design Manual [Skorseth and Selim 2000]. 

Basics of Road Construction 

The proper construction of haul roads is an important consideration for effective dust 

control. A properly constructed road will have higher initial costs, but will require less 

244 Haul Roads, Stockpiles, and Open Areas 
 

 

 

road maintenance, reduce equipment maintenance cost, and will aid in effective dust 

control and increased tire life. In particular, dust control will be augmented from a 

reduction of fine material generated and through an increase in the longevity of dust 

suppressants. 

When a vehicle travels over a haul road, the wheels exert forces on the road surface. Of 

all the forces, the normal shear stresses created by the vehicle are the most critical. At the 

location of the wheel, the road is put into compression. Once the wheel passes this 

location, the road rebounds to its initial position through tension. On a properly designed 

road, this compression and tension cycle will occur within the elastic limits of the road 

structure. 

Permanent deformation of the road can occur if it is not properly designed. Roads 

constructed of weak materials will readily degrade, which can produce fine material that 

can be entrained by mobile equipment traveling the road. Roads constructed of the proper 

materials will degrade less rapidly, which in turn lessens the dust emission potential of the 

road over the same period of use. Additionally, road failure will cause any dust 

suppressant to exceed its bonding strength, thereby reducing the effectiveness of the 

suppressant. Generally, dust suppressants work by forming a layer, or crust, over the top 

of the road. If the bonding strength of the dust suppressant is exceeded, the suppressant 

will break up, deteriorate, and wear away. 

Materials selected for road construction must possess certain physical properties. They 

are: resistance to wear, soundness, maximum size, particle shape, and gradation [Midwest 

Research Institute 1981]. 

A high resistance to wear is desired for the road material. It should be hard and 

tough such that it will not be easily disintegrated under traffic load. The resistance 

to wear can be measured by conducting abrasion tests such as the Los Angeles 

7 Abrasion test. 

7 The Los Angeles Abrasion test provides information on aggregate resistance to crushing, degradation, and 

disintegration from activities such as manufacturing, stockpiling, production, placing, and compaction. 

Testing is conducted by placing a coarse aggregate sample in a rotating drum with steel spheres. As the 

drum rotates the material degrades by abrasion and impaction, with itself and with the spheres. Once the test 

is complete, the calculated mass of aggregates that has broken apart to smaller sizes is expressed as a 

percentage of the total mass of aggregate. Lower Los Angeles Abrasion values indicate that the aggregate is 

tougher and more wear resistant to abrasion. Specifications for the test can be found in AASHTO T96 or 

ASTM C 131 "Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the 

Los Angeles Machine."

Haul Roads, Stockpiles, and Open Areas 245 

Generally, the most desirable materials include granite or 

limestone. Soft and unsound materials such as shale, coal, mica, or vermiculite 

should be avoided as the use of such material will lower the road's strength and 

durability. 

Soundness represents the ability of the material to withstand climatic conditions. 

A material is desirable if it can resist weathering. Materials that are weak, 

extremely absorptive, easily cleavable, swell when saturated, or are susceptible to 

breakdown through natural weathering processes are not recommended for road 

construction. 
 

 

Maximum size is the largest particle size allowable in the road material. Generally 

it is undesirable to use aggregate sizes larger than 1 inch for the surface course, in 

order to maintain ease of maintenance with the road grader. 

Particle shape of the road material affects the stability, density, and durability of 

the road surface. Shapes which contain angular and rough-surfaced material result 

in good interlocking and produce a desirable road surface. 

Gradation is the distribution of particle sizes throughout the road material. A 

well-graded aggregate material contains a good representation of particle size 

fractions from large to small sizes of material. An acceptable aggregate particle 

size distribution of a material for structural purposes is generally represented by 

Equation 10.1 [National Stone Association 1991]: 

n

D

d

P 100

(10.1) 

where P = percent by weight finer than the sieve; 

d = sieve opening dimensions (inches) where common sieve sizes are 2-inch, 

1-inch, 3/4-inch, 3/8-inch, No. 4 (0.187 inches), No. 40 (0.0165 inches), and No. 

200 (0.0029 inches); 

D = maximum size of aggregate, inches; and 

n = an empirical gradation exponent ranging from 0.33 to 0.5. Generally 0.5 

represents the maximum density of material. 

In addition to possessing the appropriate material properties, a properly constructed road 

will consist of three components: subgrade, subbase, and wearing surface. These features 

are displayed in Figure 10.2. The American Society for Testing and Materials (ATSM) 

and the American Association of State Highway and Transportation Officials (AASHTO) 

have standards which define the minimum properties required. 

Figure 10.2. Cross section of haul road. 

The subgrade is the underlying soil or rock that serves as the foundation of the road. It is 

important because it supports the entire load of the vehicles traveling the road. There are 

two techniques for maximizing subgrade strength: compaction at optimum moisture and 

adequate drainage. Most subgrade soils will compact to their maximum density at a given 

moisture. To achieve this, the subgrade area should be scarified prior to adding water to 

the location. Then compaction is generally accomplished using a vibratory compactor 

246 Haul Roads, Stockpiles, and Open Areas 
 

 

with a smooth drum roller [National Stone Association 1991]. Additionally, the subgrade 

should be designed with a crown in the center to promote water drainage away from the 

road. Proper drainage ditches should be designed and built to carry the drainage away 

from the road surface. Another consideration is that any areas of the subgrade consisting 

of poor quality material should be upgraded/repaired prior to adding additional bases; 

otherwise the location of these areas will be problematic in the future, requiring frequent 

maintenance [Midwest Research Institute 1981]. 

The subbase is the layer between the subgrade and the wearing surface. The material for 

this layer is a compacted angular aggregate. This layer has less stringent requirements for 

strength, aggregate type, and gradation. If the subgrade meets the requirements for the 

subbase, the subbase can generally be omitted from the road design [Merritt et al. 1996]. 

The road subbase is well-graded and consists of material that has a broad particle size 

range, which does not bias toward any particular size. Generally, the maximum size of the 

material is 3 inch, with a 1 1/2-inch maximum size material being more common for light 

duty traffic [Midwest Research Institute 1981]. It is best to spread the material for the 

base using a spreader box similar to those used in paving applications (Figure 10.3). 

Figure 10.3. Use of a spreader box to lay gravel onto road surface. 

Haul Roads, Stockpiles, and Open Areas 247 

 The 

material is placed in layers to a uniform loose depth of 8 to 10 inches, then compacted 

with a vibratory roller using a minimum amount of blading to avoid material particle size 

segregation. End-dumping (also called "tailgating"), which is commonly used, is 

undesirable in that segregation of the uniform layer into areas of all fines or all coarse can 

occur easily with this method [National Stone Association 1991]. However, end-dumping 

is commonly used. To minimize the segregation of the fine and coarse material from enddumping, it is important that a grader be used to blade the material to a uniform surface 

(Figure 10.4). A vibratory roller can then be used to compact the road material. 
 

 

 

Figure 10.4. Use of end-dumping with grader to lay gravel onto road surface. 

The wearing surface should be constructed of material that is resistant to grinding, 

weathering, and large particle displacement from the road. The material should contain 

sufficient fines to fill the voids between the larger particles. Sufficient fines will minimize 

large particle loss. The wearing surface should be compacted at optimum moisture to 

achieve maximum strength. Additionally, the crown should be maintained throughout the 

construction process to remove water away from the road surface. 

Thicknesses of these three layers are important considerations in road designs. Sufficient 

thickness of these layers enables the road to support the load for which it was designed. A 

road that is not able to support the vehicle loading will deteriorate rapidly, which can in 

turn result in higher dust generation. The thickness of the subbase is an important design 

consideration to ensure that the surface load will be properly distributed over the subgrade 

area. As an example, a doubling of the thickness of the subbase can result in a reduction 

of more than twice the pressure on the subgrade. This design consideration can improve 

the life of the road by preventing the rapid deterioration of the road that can occur due to 

heavy loading [Midwest Research Institute 1981]. 

Mine haul roads have unique construction requirements due to the higher wheel loads 

from off-road mine haul trucks that they are required to support. Ordinary design 

guidelines for dirt and gravel roads for normal highway traffic are not adequate for mine 

roads. To determine the proper thickness for the subbase of a mine haul road, Figure 10.5, 

which was developed by the U.S. Bureau of Mines, should be used. This graph uses the 

8

8 The California Bearing Ratio test provides results as a ratio from the comparison of the bearing capacity of 

a material to the bearing capacity of a well-graded crushed stone. It is used for evaluating the strength of 

cohesive materials having maximum particle sizes of 0.75 inch. The test involves applying a load to a small 

diameter penetration piston, which is applied to the sample material at a rate of 0.05 inches per minute, 

recording total load at penetration distances ranging from 0.025 inches up to 0.300 inches. Specifications 

for the test can be found in AASHTO T193 "The California Bearing Ratio," or ASTM D 1883 "Bearing 

Ratio of Laboratory Compacted Soils."

248 Haul Roads, Stockpiles, and Open Areas 

California Bearing Ratio (CBR) and the wheel load of the haul truck—which is 

calculated using haul truck specifications provided by the manufacturer—to determine the 
 

 

thickness of the subbase, wearing surface, and any intermediate layer. These same 

specifications can be used as design guidelines for mine haul roads. 

Figure 10.5. Subbase thickness from California Bearing Ratio tests 

on road material [after USBM 1977]. 

Haul Road Dust Control Measures 

The most common method of dust control on haul roads is using dust suppressants, with 

water being the most common. Other suppressants are surfactants, salts, petroleum 

emulsions, polymers, and adhesives. Although haul road construction is important to the 

reduction of dust emissions from trucks, there is still the need to implement dust control 

measures for haul roads, as even the best constructed haul roads will provide material for 

dust entrainment. Selection of the proper agent for use is dependent upon mine site 

Haul Roads, Stockpiles, and Open Areas 249 
 

 

 

conditions. Performance of each of the above dust control agents is evaluated through the 

use of control efficiency. Control efficiency compares the dust concentration of a treated 

road surface to that of an untreated or uncontrolled road surface and is defined in Equation 

10.2: 

1 100

U

T

CE

(10.2) 

where CE = control efficiency, percent; 3 U = untreated or uncontrolled road dust concentration, mg/m ; and 3 T = treated or controlled road dust concentration, mg/m . 

Generally, the higher the control efficiency for a dust control agent, the better its 

performance, which results in less dust emissions from the road surface. The control 

efficiencies presented are for total suspended solids (TSP). 

Road Preparation 

Application of many of the dust suppressants requires the haul road to be properly 

prepared. The equipment used for the application of the dust suppressants includes: a road 

grader equipped with scarifiers (refer to Figure 10.4), a compactor, and a water truck for 

distribution of the suppressant to the road surface. 

Proper planning is necessary when applying many dust suppressants, as many require a 

significant amount of time to cure before traffic can return to the road. Additionally, 

depending upon the amount of road that requires application, several days of dry weather 

will be needed to properly apply the suppressant. The road should be fully prepared 

before applying the suppressant, as follows: 

any blade work required to eliminate road corrugations and potholes should be 

completed before dust suppression is initiated; 

any large material that would not make for a good road surface should be bladed 

off the road; 

the road should have a good crown to eliminate the potential for any standing 

water because it can have a detrimental effect on the road surface, creating 

potholes; and 

the dust suppressant is applied as the final step. 

Accomplishing the proper road preparation during application will allow the dust 

suppressant to work at its maximum efficiency. For the application of the dust 

suppressant, the road surface should be scarified to loosen the top 1 to 2 inches of road 

surface. There are two variations to applying the dust suppressant at this point, as 

discussed below. 

The first variation, generally used with salt suppressants, is to ensure that a 1- to 2

inch loose layer is spread evenly across the road surface. Next, the dust 

-

250 Haul Roads, Stockpiles, and Open Areas 
 

 

suppressant is applied to the loose layer at the recommended rate. Finally, the 

surface is compacted, preferably with a pneumatic roller, although other types can 

be used. Traffic should be kept off the road until the dust suppressant cures 

[Skorseth and Selim 2000]. Curing time is dependent upon the dust suppressant 

applied (generally 24 hours is sufficient). 

The second variation is to blade most of the loose material into windrows on both 

sides of the road. These windrows are used to ensure that application of the dust 

suppressant does not run off the road surface. One-third of the dust suppressant is 

applied to the road surface at the recommended rate between the windrows. Next, 

the windrows are bladed to the center, spreading evenly across the road surface. 

Next, another 1/3 of the dust suppressant is applied to the road surface. The road 

is regraded to mix the dust suppressant and the aggregate. The final 1/3 of dust 

suppressant is applied to the road surface as a topcoat, after regrading is complete. 

Excess runoff of the dust suppressant should be avoided when applying this final 

topcoat. Finally, the road surface is compacted, preferably with a pneumatic roller, 

completing compaction before the dust suppressant dries. The road should be 

allowed to cure (generally 24 hours, but dependent upon the dust suppressant used) 

before traffic returns to it [Midwest Research Institute 1981]. 

Keep in mind that these are general guidelines for dust suppressant application. Specific 

guidelines for dust suppressant application should be obtained from the manufacturer of 

the dust suppressant chosen for use on the road. 

Water 

Watering roads is the most common method used for haul road dust control. Its 

application is the simplest and the easiest of all dust control measures, as it does not 

require any road preparation prior to application. Application is generally achieved 

through the use of a water truck which sprays water onto the road (Figure 10.6). 

Figure 10.6. Water truck equipped with a front water cannon and rear water sprays. 

Haul Roads, Stockpiles, and Open Areas 251 
 

 

A water truck, used to apply water to the haul roads, consists of a tank, a pump, and the 

plumbing associated to send water through nozzles which are located at the rear of the 

truck. Additional side sprays may be available with some spray configurations but are 

optional. The tank capacity can range up to 30,000 gallons depending upon the size of the 

truck, with the larger capacities used with large off-road truck chassis. Some water trucks 

may have a water cannon mounted (see Figure 10.6) that can be used for wetting down 

muck piles for dust control during loading operations. The water cannon is controlled by 

the operator and can be rotated and elevated to send water to the desired location. 

The spray nozzles for road watering are generally fan sprays and are mounted on the truck 

at a stationary position. These nozzles can be elaborate fan spray nozzles fabricated by a 

manufacturer (Figure 10.7) or they can be simple nozzles. 

Figure 10.7. Various types of manufactured fan spray nozzles for use on a water truck. 

 Simple nozzles are constructed 

by drilling holes along the bottom of a water pipe along the back of the water truck to 

allow water to wet the road or by cutting horizontal slots in water stand-pipes (or endcaps) 

at the corners of the water truck. It is best to orient the nozzles in a manner to minimize 

overlap of the water spray from other nozzles [James and Piechota, 2008] in order to 

achieve the best spray coverage. The operator has the ability to start and stop the water 

spraying by turning the pump on or off through a switch located in the cab. Some water 

trucks allow the operator to turn on or off each individual water spray on the truck, 

allowing flexibility in water application. 

The disadvantage of the use of water is that it must be reapplied on a consistent basis. The 

U.S. EPA has reported several test results of watering haul roads. An application of water 

2 at the rate of 0.13 gallons per square yard (gal/yd ) had a control efficiency of 95 percent 

for total suspended particles (TSP) for 0.5 hours after application. Another test showed 

2 that an application of water at a rate of 0.46 gal/yd had a control efficiency of 74 percent 

for TSP for the 3-4 hours following the application of water [EPA 1998]. However, the 

control efficiency for water can be highly variable as it is highly dependent upon road 

material type, traffic, and weather conditions. For example, another independent study 

showed that watering haul roads with a water truck once an hour had a control efficiency 

of 40 percent for total suspended particulates (TSP). When watering increased to once 

every half hour, the control efficiency for TSP increased to 55 percent [USBM 1983]. 

252 Haul Roads, Stockpiles, and Open Areas 
 

 

 

During a field study by the National Institute for Occupational Safety and Health 

(NIOSH), the application of water was shown to prevent high dust emissions from the 

haul road for several hours after application during a hot dry day when the skies were clear 

of any clouds [Organiscak and Reed 2004]. The application rate of the water was not 

measured, but was observed to be very liberal as observations of the water truck 

application noted water runoff from the haul road. During the day the temperature ranged 

from a low of 77°F increasing to 90°F and the relative humidity ranged from a high of 64 

percent decreasing to 37 percent throughout the day. Figure 10.8 shows the dust 

concentrations measured throughout the day with one water application at approximately 

10:00 a.m. 

Figure 10.8. Respirable dust concentrations measured from haul road 

after water application occurred at 10:00 a.m. [Organiscak and Reed 2004] 

There are no published guidelines for the amount of water to use for dust control on haul 

roads. In the past, an old U.S. EPA emissions factor had stated that days having as little as 

0.01 inches of precipitation resulted in no dust emissions from haul trucks [EPA 1998]. 

However, it was not until recently that the optimal haul road moisture content for best 

reduction of respirable dust was found to be approximately 2 percent in a study completed 

in Kansas [Mulesky and Cowherd 2001]. Additionally, except for the application 

intervals previously mentioned, there are no published guidelines for determining 

optimum haul road watering intervals. Determining optimum haul road watering intervals 

would be dependent upon the amount of water applied to the road, the time between water 

applications, the traffic volume of the road, and the prevailing meteorological conditions 

[Cowherd et al. 1988]. 

Surfactants 

Surfactants or wetting agents can be added to the water to extend its life as a dust control 

agent. Like water, surfactants generally do not require any special application procedures, 

except to remove any excess loose material from the road surface. 

Haul Roads, Stockpiles, and Open Areas 253 
 

 

Surfactants work by reducing the surface tension of water. This allows the particles from 

the road surface to better penetrate the water droplets, thereby increasing the potential 

surface area coverage of the droplet (i.e., the penetration of the water droplet lets the water 

wet a larger surface area). The reduced surface tension also permits easier penetration of 

the treated water into a few inches of the sub-surface. This action allows the wetted subgrade to act as a reservoir of water such that, once the surface moisture evaporates, the 

sub-grade can provide moisture for dust control through capillary action, thus extending 

the life of the water [Midwest Research Institute 1981]. 

There is very little information about the use of surfactants to extend the effective life of 

watering haul roads. However, observations have noted that the time between watering 

roads can be extended 33–50 percent when surfactants are used [Midwest Research 

Institute 1981]. 

Salts 

Salt solutions are commonly mixed with water and used for haul road dust control. 

Magnesium chloride is one of the most common salt-based dust control agents. Other 

agents are calcium chloride, hydrated lime, and sodium silicates, with calcium chloride 

being the most common and having properties similar to magnesium chloride. Calcium 

chloride is hygroscopic and deliquescent, meaning that it absorbs moisture from the 

atmosphere to keep the road in a moist condition. In very dry climates, periodic 

application of water may be required to maintain road performance at acceptable levels 

[Midwest Research Institute 1981]. Application procedures for magnesium 

chloride/calcium chloride are similar to those described for dust suppressants in the 

section titled "Road Preparation."

After application to control haul-truck-generated dust, magnesium chloride control 

efficiencies were shown to average 95 percent for up to 22 days after application [USBM 

1987]. For calcium chloride, two weeks after the initial application, the control efficiency 

was measured as 82 percent and decreased over time to 14 percent at seven weeks after 

application [USBM 1983]. This decline has been attributed to road surface wear due to 

traffic volume and precipitation, in that chlorides are water soluble and can be washed 

away. 

Advantages of using chlorides for dust control are that they absorb moisture from the 

atmosphere to maintain the moisture content of the road at a higher level than normal. 

They can also act as a de-icer to thaw ice and snow on roads in freezing conditions. Also, 

chlorides generally do not require time to cure after application. 

Disadvantages are that chlorides may cause corrosion on equipment using the roads 

sooner than normal. They can also be harmful to vegetation and to personnel if skin or 

eye contact occurs. Chlorides are also water soluble and may leach from the road surface 

during precipitation, thereby degrading in performance over time [Midwest Research 

Institute 1981]. 

254 Haul Roads, Stockpiles, and Open Areas 
 

 

Petroleum Emulsions 

Petroleum resins are generally engineered products or byproducts of lubrication oil 

manufacturing. They generally consist of stable emulsions of petroleum residuals, solvent 

extracts, and acid sludge. If the road surface is in good condition, then application is 

relatively simple. Some products require the steps of scarifying to a 1-inch depth, 

applying the resin, and compacting the road surface; others can be applied by direct 

application of the resin to the road surface without any conditioning. Most petroleum 

resins do require a 24-hour time period allowing the road surface to cure. Therefore, it is 

important to keep traffic off the road for the 24-hour time period after application. 

The practice of using petroleum resins for dust control on access roads has reduced dust 

potential considerably, with reports that the access roads have been kept dust free for 

periods of 6 months to 1 year. Haul roads have been reported to be relatively dust free for 

a period of 3 to 4 weeks. 

For petroleum emulsions, documented control efficiencies are highly variable. For haultruck-generated dust at a sand and gravel operation, control efficiencies can be up to 70 

percent for petroleum derivatives for up to 21 days after application [USBM 1987]. Other 

testing has shown control efficiencies ranging from 4 to 38 percent during a 4-week period 

after application at several surface coal mines [USBM 1983]. Other differences in the 

variations of control efficiency could be due to the type of petroleum emulsion used, 

method of application, type of vehicle traffic, and measurement method. 

A primary advantage to using petroleum emulsions is that they are not corrosive. They 

are also not water soluble; therefore, rain will not displace them (nor wash them away 

unless improperly applied) and they will not evaporate. They are relatively nontoxic and 

nonflammable, and do not have adverse effects on plant growth (for revegetation needs). 

Disadvantages are that they must cure for 24 hours after application. Therefore, traffic 

should not be allowed during this time period. Traffic should be limited to wheeled 

equipment only, because tracked equipment running on the road surface will break down 

the treated surface, resulting in poor dust control of the road surface. Finally, storage 

temperatures of the emulsion products prior to application must be controlled as they 

cannot endure freezing or boiling conditions [Midwest Research Institute 1981]. 

Polymers 

Polymers include acrylics and vinyls, which are chemical additives, mixed with water to 

form a diluted solution, then applied to the road surface topically. Application is 

dependent upon the brand of polymer used, but it is generally a simplified procedure of 

preparing the road surface prior to application, then spraying the solution topically onto 

the surface in a series of applications. 

Although many brands of polymers are used in haul road dust control, only a few studies 

have documented their control efficiency. One study conducted on mine haul roads 

reported that a specific polymer had control efficiencies of 74 to 81 percent within 4 

weeks of application, reducing to 3 to 14 percent after 5 weeks of application [USBM 

1983]. The variability in the control efficiency was stated to be caused by precipitation, 

Haul Roads, Stockpiles, and Open Areas 255 
 

 

which differed for the various sites. Another study, conducted on a public dirt road, 

demonstrated control efficiencies of 94 to 100 percent within a week of application of the 

same specific polymer, reducing to control efficiencies of 37 to 65 percent at 

approximately 11 months after application [Gillies et al. 1999]. 

The advantage to using polymers is that they are generally noncorrosive and nontoxic. As 

a dust control agent, they are also generally long-lived, although it has been shown that 

precipitation can affect their longevity. Application procedures can vary by product from 

the simple procedure of initial road preparation and spraying the solution onto the road 

surface with a water truck to complex procedures requiring initial road preparation and 

additional blading and regrading during polymer solution application. Additionally, 

polymers can be utilized for soil stabilization. 

Adhesives 

Adhesives are compounds, solutions, formulas, etc. that are mixed with the soil surface to 

form a new road surface. One of the most common adhesives is lignin sulfonate, which is 

basically a waste product from the paper/pulp industry and a well-established dust control 

product. It is created when the wood chips are placed in a sulfonate solution. The 

solution absorbs the lignin or binder from the wood chips, creating the lignin sulfonate 

solution used for dust control [Midwest Research Institute 1981]. 

Application of lignin sulfonate involves procedures mentioned previously in the section 

titled "Road Preparation." When properly applied, lignin sulfonate has been observed to 

keep access roads dust free for periods ranging from 6 months to two years with periodic 

application of the solution [Midwest Research Institute 1981]. Heavily traveled haul roads 

have been observed to be kept dust free for periods of 3 to 4 weeks, after which time 

another application of the solution should be applied. The reapplication can be a simple 

process by spraying the solution onto the haul road, unless the road is in poor condition, in 

which case the procedures previously mentioned in "Road Preparation" would need to be 

followed [Midwest Research Institute 1981]. 

Field testing of lignin sulfonate demonstrated control efficiencies ranging from 50 to 63 

percent for up to 4 weeks after application, with the highest control efficiencies occurring 

directly after application [USBM 1983]. Other test sites had lower control efficiencies of 

31 to 45 percent, but this could be due to different road material types, equipment types, 

and precipitation events. Precipitation events can have a significant influence on control 

efficiency, especially since lignin sulfonate is water soluble and can be washed away with 

large rainfall events [USBM 1983]. 

Other advantages to using lignin sulfonate as a haul road dust control agent are that it is 

noncorrosive and is easily obtainable due to the large size of the paper/pulp mill industry. 

Disadvantages are that it can interfere with some mineral processing processes, such as 

flotation, and since it is water soluble it can be washed away from the road surface, 

requiring reapplication of the lignin sulfonate to maintain proper dust control [Midwest 

Research Institute 1981]. 

256 Haul Roads, Stockpiles, and Open Areas 
 

 

Other Dust Control Methods 

Speed Control 

Reducing speed of the vehicles traveling on haul roads can be an effective method for dust 

control. However, this method conflicts with maximizing production. The haul trucks are 

designed to operate at an optimum speed; to lower the speed of the trucks would lower the 

production rate of the mine, which may not be desirable. Nevertheless, reducing the speed 

of vehicles has been shown to reduce the potential generation of dust particles <10 μm by 

approximately 58 percent when speeds were reduced from 25 to 10 miles per hour (mph), 

and 42 percent when speeds were reduced from 25 to 15 mph [Watson et al. 1996]. In 

another study, limiting speeds on unpaved roads to 25 mph demonstrated a control 

efficiency of 44 percent [Countess Environmental 2006]. 

Traffic Control 

In addition to the mine haul trucks on-site, many mine sites have on-road trucks that enter 

the site to ship material to their customers. These trucks can often times come in batches 

of 2-3 trucks or more at a time. If the trucks travel on unpaved roads, maintaining a 

following distance between trucks by a time interval of 20 seconds has been shown result 

in up to a 52 percent reduction of respirable dust exposure to the trailing truck driver 

[Reed and Organiscak 2005]. Additionally, this 20-second time interval allows for the 

dust cloud generated from the lead truck, which can impair the visibility of the trailing 

driver, to dissipate. This can also reduce the possibility of an accident due to impaired 

visibility from the dust generated by trucks. 

Load Covers 

Covering the load of a haul truck using tarps can prevent the loaded material from 

becoming airborne during transportation. This method is not commonly used for off-road 

mine trucks as the speed of loaded trucks rarely exceeds speeds where entrainment of 

material occurs (neglecting ambient wind effects). Entrainment of material can occur 

when airflows coming in contact with the material exceed 13 mph for small diameter 

material (0.10 mm); higher velocities are required for larger material [Chepil 1958]. 

However, loaded highway trucks can exceed speeds where airflows contacting the loaded 

material exceed this threshold, resulting in dust emissions as the material becomes 

airborne. Therefore, covering with tarps (Figure 10.9) or surface wetting of the loaded 

material should be done to prevent it from being windblown. Additionally, the truck 

should not be overloaded. Material should be centered and a freeboard—which is the 

distance from the top of the dump truck bed down to the top of the material loaded into the 

bed—of three inches or more should be maintained. In cases where the material in the 

dump bed is mounded, the crest of the mound should not exceed the height of the dump 

bed [Blue Skies Campaign, Arizona 2010]. This will minimize spillage and allow the tarp 

to effectively trap the material in the truck bed. 

Haul Roads, Stockpiles, and Open Areas 257 
 

 

Figure 10.9. Self-tarping dump truck. The blue arrow indicates the direction 

of movement when tarping. Section A-A shows the recommended freeboard. 

Cab Maintenance 

Maintaining equipment cabs in good operating condition also reduces operator exposure to 

respirable dust. It has been demonstrated that properly maintained cabs can attain dust 

reductions of 90 percent or more for drills [Chekan et al. 2003] and between 44 and 100 

percent for dozers, with the variations of the dust reductions for dozers attributable to reentrainment of internal cab dust [Organiscak and Page 1999]. For haul truck cabs that 

were retrofitted with a filtration/pressure air conditioning system to produce positive 

pressure in the cab, it was demonstrated that properly maintained cabs can also reduce 

respirable dust exposures by 59 to 84 percent [Chekan and Colinet 2003; Chekan et al. 

2003]. It should be noted that the retrofitted trucks tested were old model (20-30 years 

old) haul trucks, and that newer haul trucks would have better cab integrity. The proper 

use and maintenance of cabs is discussed in more detail in Chapter 9—Operator Booths, 

Control Rooms, and Enclosed Cabs. 

STOCKPILES AND OPEN AREAS 

Stockpiles and open areas are common at surface mine sites. There are generally two 

types of stockpiles: long-term and short-term. A long-term stockpile undergoes no 

disturbance to the pile over a long time period (several months or more). An example 

would be a topsoil stockpile where topsoil is stored until reclamation of the mine site is 

258 Haul Roads, Stockpiles, and Open Areas 
 

 

 

performed. Short-term stockpiles could consist of product or material that is being stored 

temporarily until it is used in a process. Surge piles are common short-term stockpiles. 

Open areas are self-explanatory as they are large barren areas. 

Dust can be generated from stockpiles and open areas by wind velocities that entrain 

material into the air. The United States Department of Agriculture states that erosion will 

take place when "the surface soil is finely divided, loose, and dry; the surface is smooth 

9 and bare; and the wind is strong."

9 Chepil, W.S., "Soil Conditions that Influence Wind Erosion." United States Department of Agriculture 

Technical Bulletin No. 1195. June 1958. pg. 4. 

Haul Roads, Stockpiles, and Open Areas 259 

As stated previously, wind speeds as low as 13 mph 

can result in entrainment of material for small diameter material (0.10 mm) [Chepil 1958]. 

Conversely, there are several factors that can reduce wind erosion: 

soil that contains stable aggregates or clods that are large and dense to resist wind 

erosion; 

soil that is compacted, roughened, or kept moist; 

vegetated soil or soil covered by vegetative residue (for example, straw); and 

when wind velocity near the ground surface is reduced or eliminated [Chepil 

1958]. 

These four factors translate to the four dust control measures that can be applied to both 

stockpiles and open areas: surface roughness, water, coatings, and wind barriers. Water, 

coatings, and to some extent wind barriers are generally applicable to stockpiles, while all 

four measures can be applied to open areas. 

Surface Roughness 

Surface roughness is important in wind erosion from open areas, but less important for 

stockpiles. Generally, the rougher the surface the less wind erosion affects it. This is true 

for both land plots containing vegetative stubble and bare surface plots. The roughness for 

each type of plot is created by the amount and orientation of the stubble for the stubble 

plots, and the amount of surface irregularities and large clods for the bare surface plot. 

The mechanisms for controlling wind erosion are to decrease the force of the wind on the 

surface or to trap the eroding material with larger particles. Increasing the magnitude of 

the roughness for each plot can result in a decrease in soil erosion [Zingg 1951]. The best 

dust control for open areas is to plant vegetation to cover the entire area. Typically the 

vegetation is a grass, clover, alfalfa, etc. 

One control measure for open areas is to roughen the surface by using chisels, such as that 

provided by a chisel-pointed cultivator (Figure 10.10) used in agriculture [Chepil and 

Woodruff 1955]. The depth of tilling should be 3 to 6 inches. If higher wind speeds are 

expected then the depth should be increased to 8 to 14 inches. The objective of this tilling 

is to expose the rough large cloddy soil particles to the surface (Figure 10.11). The large 

clods protect the smaller particle material from wind erosion [Woodruff et al. 1977]. In 

order for the clods to be truly effective, they must cover the majority of the surface. This 

tillage also produces ridges and furrows which have the potential to reduce erosion 50–90 
 

 

percent [Fryrear and Skidmore 1985]. However, this method is considered to be a 

temporary emergency measure to be used as a last resort in controlling wind erosion in 

agricultural settings [Chepil 1958]. Additionally, for this method to be most effective, the 

tilling must be accomplished at right angles to the wind direction and completed slowly to 

prevent the large clods from disintegrating [Woodruff 1966]. 

Figure 10.10. Chisel plow used for creating large clods 

in open areas to reduce wind erosion. 

260 Haul Roads, Stockpiles, and Open Areas 
 

 

Figure 10.11. Example of tillage (from chisel plowing, for example) exposing large clods. 

Tillage ridges prevent wind erosion by increasing the surface roughness so that the 

velocity required for material entrainment for a tilled surface is higher than that for a 

smooth surface. The ridge tops generally consist of loose erodible soils (unlike the un

ridged surfaces) that become armored through a process where the larger particles are left 

behind when the smaller loose particles erode away [Hagen and Armbrust 1992; Lyles et 

al. 1974]. After this armoring occurs, the wind velocities required for entrainment for the 

ridge tops become higher than those for smooth surfaces, thus helping to prevent wind 

erosion. For a series of tillage rows, this same armoring process also occurs at the upwind 

side of the ridges, which results in any loose particles leaving the armored upwind side to 

have to cross over many ridges, and eventually many of the loose particles become 

trapped in the troughs and on the armored faces (Figure 10.12). These processes can 

result in the tilled or ridged surfaces that reduce emissions up to 50 percent as compared to 

smooth untilled surfaces [Hagen and Armbrust 1992]. 

-

Haul Roads, Stockpiles, and Open Areas 261 
 

 

Figure 10.12. The armoring process for tillage rows. Blue arrow denotes wind direction 

perpendicular to tillage rows. Magenta arrows depict fine material being eroded from the 

windward side of ridges leaving large-sized particles, with the fine material being carried 

over ridge tops and deposited in the troughs on the leeward side of the ridges. 

Another method for reducing wind erosion is the use of gravel blankets. The use of fine 

(0.02- to 0.25-inch), medium (0.1- to 0.5-inch), and coarse gravel (0.25- to 1.5-inch) 

applied at 20, 50, and 100 tons per acre, respectively, results in insignificant amounts of 

wind erosion (<25 pounds per acre). The smaller the diameter of the gravel, the smaller 

the amount of gravel required to prevent wind erosion. However, this only applies to a 

diameter down to 2 mm. Once the gravel diameter is less than 2 mm, wind erosion will 

increase, as particles <2 mm in the gravel will make the surface more erodible [Chepil et 

al. 1963]. 

Water 

Water is an obvious method of dust control for stockpiles and open areas that has been 

recommended for use at mine sites. The water mixes with the material to create a crust 

which can be resistant to wind erosion. This crust generally requires a higher wind 

velocity to erode material. However, once any erodible material has scoured and the crust 

has worn through, then lower wind velocities may potentially cause erosion [Chepil 1957]. 

Cementation of the soil also affects soil erodibility and can be used for dust control of 

both stockpiles and open areas. Cementation occurs from alternate wetting and drying of 

the soil, forming a surface crust generally 1/16-inch in thickness. The cementation of the 

262 Haul Roads, Stockpiles, and Open Areas 
 

 

soil surface becomes compact and mechanically stable, thus becoming more resistant to 

wind erosion. The soil must have an appropriate amount of fine material in order to bind 

the larger materials to form the crust. However, the degree of cementation is hard to 

detect, making it difficult to measure the amount of cementation required for wind 

resistance, i.e., the number of wet/dry cycles required to prevent erosion [Chepil 1958]. 

The amount of moisture in the soil also affects the erodibility of soil. Basically, as the 

moisture is increased, the erodibility of the soil decreases [Chepil 1956]. This is important 

for dust control of open areas. However, it has been shown that even soils with high 

moisture contents can be susceptible to wind erosion. With loose soil, with a depth of 5 

cm, the erodibility of a moist soil can be similar to that of a dry soil if the wind speed 

applied to the moist soil is 15–20 mph more than that applied to the dry soil [Cowherd and 

Grelinger 1996]. Therefore, while wetting is a good dust control measure, the area must 

still be monitored to maintain the proper amount of moisture to prevent erosion. 

Coatings 

Coatings as a dust control method generally include water or a myriad of chemical 

stabilizers in commercial production. The best coatings that prevent wind erosion form a 

crust on the surface of the material or soil. Additionally, the best coatings are nontoxic to 

plants, water permeable, and allow the material to be handled easily. The drawbacks to 

coatings are that they are expensive, and once the crust created by the coating is 

penetrated, erosion readily resumes [Diouf et al. 1990; Armbrust and Dickerson 1971; 

Lyles et al. 1969]. 

Mixing bentonite clay with soil is a dust control for open areas. Water is applied, 

allowing the soil-bentonite mixture to form a crust on only the top of the mixture. 

Beneath the crust the material is unconsolidated, as water is not able to penetrate the crust. 

Bentonite concentrations that are too high leave easily erodible sand particles (<1.0 mm 

diameter), while lower bentonite concentrations are desirable as the surface becomes 

tightly packed. However, it should be noted that a sand-only soil mixture with water 

application can resist erosion better than the soil-bentonite mixture, at least until an 

abrader is introduced. An abrader is an implement used to scrape or scratch the soil 

surface. The soil-bentonite mixture is the better choice if the surface will be abraded, 

because it will be more resistant to wind erosion than the sand-only soil mixture [Diouf et 

al. 1990]. 

Recently, as a dust control for open areas, a mineral operation in Wyoming applied a Guar 

Gum polymer to a tailings pond surface at a trona mine (sodium carbonate and sodium 

bicarbonate mixture). Guar Gum is extracted from the Guar Bean which can be fed to 

cattle. Industrial uses are in the oil industry to facilitate drilling and prevent fluid loss. It 

also has use in the minerals industry as a flocculant. At the trona operation in Wyoming, 

when the Guar Gum solution contacted the sodium bicarbonate strata of the tailings pond 

surface, bonds were formed which, once dry, created a crust which made the material 

resistant to wind erosion. This crust, because of the polymer involved, was high-strength, 

long-lived, and was resistant to microbial activity [Fuller and Marsden 2004]. 

Haul Roads, Stockpiles, and Open Areas 263 
 

 

A current method of stockpile formation for long-term storage is to build the stockpile and 

compact it using bulldozers (Figure 10.13). 

Figure 10.13. Dozer being used to compact stockpile for long-term storage. 

 The compaction helps to prevent wind 

erosion. Additionally, coatings or binders can be used to aid in dust control. Wax 

mixtures (hot-melt formulations) and polyethylene and polypropylene latex emulsions 

have been found to be useful in past applications for coal stockpiles and could be adapted 

to be applied to other mineral stockpiles. The latex emulsion coatings are thinner and 

more water-permeable than the hot-melt formulations. The latex emulsion coatings also 

require a surface preparation of spreading fines over the entire stockpile prior to 

application. However, all coatings, both latex emulsion and hot-melt formulations, are 

significantly better at dust control over stockpiles with or without compaction (with the 

compacted stockpile better at dust control than stockpile without compaction). 

One disadvantage to latex emulsion coatings for dust control is that this type of coating 

must be applied at temperatures >50°F, whereas hot-melt formulations can be applied in 

any weather condition [Kromrey et al. 1978]. Acrylic resins or polymers can also be used, 

but require a three-day curing period to form a crust on the surface of the stockpile. These 

coatings are very wind resistant to erosion, except if the stockpile surface is disturbed 

through settling or cracking, in which case wind erosion can resume as occurs with an 

untreated stockpile [Smitham and Nicol 1991]. 

Micro-foam is another method for controlling dust from fugitive dust sources, particularly 

load-out stockpiles. Typical foams used in firefighting (large 5-mm bubbles) are not 

effective at dust control, unlike micro-foam (small <100-μm bubbles). Micro-foam is 

stated to be better than water for dust suppression, because the water droplets, to be 

effective for dust control, must be similar in size to the dust particles which micro-foam 

can replicate. In addition, the velocity of the water must be high to break its surface 

tension upon contact with the dust particle. These dust particles penetrate the foam 

bubble, causing the bubble to break and wet the particles. Large particles are not a 

problem, as the small micro-foam bubbles wet the larger particles without affecting the 

bubble. Micro-foam can be injected at material transfer points in order to obtain optimum 

dust control, requiring approximately 0.4 gallons of water for each ton of bulk material 

treated for dust control [Eslinger 1997]. 

264 Haul Roads, Stockpiles, and Open Areas 
 

 

Wind Barriers 

Wind barriers for wind erosion of open areas generally consist of vegetation (grasses, 

shrubs, trees, etc.). This vegetation can be grown on the edges of fields to attempt to stop 

the erosion, or as crop strips through the field [Woodruff et al. 1977]. Collection 

efficiencies of different types of plants and their differing optimal spacings, for <10 μm 

particles, can vary from 35 to 80 percent [Hagen and Skidmore 1977]. 

Vegetation can also be grown on topsoil stockpiles as they are most amenable to being 

vegetated (Figure 10.14). 

Figure 10.14. Example of a vegetated topsoil stockpile. 

 The topsoil is generally seeded after placement of the stockpile 

and all work activity has ceased. The vegetation creates a barrier preventing the high wind 

velocities from contacting the stockpile surface, thereby preventing entrainment of topsoil 

particles. The vegetation also prevents water erosion of the topsoil from the stockpile. 

Most materials other than topsoil that are stockpiled are not amenable to vegetation and 

must be watered or coated. 

Solid board fences, snow fences, burlap fences, etc. can be used to control wind erosion of 

open areas. The fences should be 3 to 5 feet in height to be effective [Woodruff et al. 

1977]. These fences are used to trap suspended dust. For use near roadways, optimal 

placement for maximum trapping efficiency is approximately 10–20 barrier heights 

upwind of the road (Figure 10.15). Additionally, the barrier material should have a 

porosity of approximately 30 to 40 percent for best performance [Hagen and Skidmore 

1977]. Barriers or wind fences that are oriented perpendicular to the prevailing wind 

direction reduce saltation at the soil surface, and reduce the amount of collected airborne 

particles downwind of the fences by 90 percent [Grantz et al. 1998]. Barriers can also be 

used for stockpiles, but the barriers must be built to enclose the entire stockpile, making 

the barrier quite large, but effective (Figure 10.16). The stockpile barriers are generally 

constructed of common building materials. 

Haul Roads, Stockpiles, and Open Areas 265 
 

 

Figure 10.15. Demonstration of how fences are used as wind barriers. 

Figure 10.16. Barrier built to enclose a stockpile. 

Other types of barriers that control wind erosion from open areas are dispersed barriers 

and earthen banks. Widely dispersed barriers can be effective in reducing the amount of 

windblown soil up to a height of 6 feet [Grantz et al. 1998]. Figure 10.17 illustrates the 

widely dispersed barriers using shrubs that were placed on a 30 x 7.5 foot rectangular grid 

and were protected from herbivores using cones having a base diameter of 8 inches, top 

diameter of 4 inches, and height of 2 feet. The cones were anchored to the ground using 

stucco wire netting. Earthen banks with heights of 2 feet are also effective for preventing 

wind erosion from open areas [Woodruff et al. 1977]. 

266 Haul Roads, Stockpiles, and Open Areas 
 

 

Figure 10.17. Layout demonstrating dispersed wind barriers. 

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Skorseth K, Selim AA [2000]. Gravel roads, maintenance and design manual. South 

Dakota Local Transportation Assistance Program, U.S. Department of Transportation, 

Federal Highway Administration. 

Smitham JB, Nicol SK [1991]. Physico-chemical principles controlling the emissions of 

dust from coal stockpiles. Powder Tech 64(3):259–270. 

USBM [1977]. Design of surface mine haulage roads—A manual. By Kaufman WW, 

Ault JC. U.S. Department of the Interior, Bureau of Mines Information Circular 8758. 

USBM [1983]. Cost effectiveness of dust control used on unpaved haul roads, Vol. 1 

results, analysis, and conclusions. By Rosbury KD, Zimmer RA. U.S. Department of the 

Interior, Bureau of Mines Open File Report 106–85. 

USBM [1987]. Fugitive dust control for haulage roads and tailings. By Olson KS, Veith 

DL. U.S. Department of the Interior, Bureau of Mines Report of Investigations 9069. 

270 Haul Roads, Stockpiles, and Open Areas 
 

 

Watson JG, Rogers CF, Chow JC, DuBois D, Gillies JA, Derby J, Moosmüller H [1996]. 

Effectiveness demonstration of fugitive dust control methods for public unpaved roads and 

unpaved shoulders on unpaved roads. Final Report, DRI Document No. 685–5200.1F1, 

Prepared for California Regional Particulate Air Quality Study, Sacramento, California: 

California Air Resources Board. 

Woodruff NP [1966]. Wind erosion mechanics and control. Proc First Pan Amer Cong of 

Soil Cons, Sao Paulo, Brazil, pp. 253–262. 

Woodruff NP, Lyles L, Siddoway FH, Fryrear DW [1977]. How to control wind erosion. 

U.S. Department of Agriculture, Agricultural Research Service, Agriculture Information 

Bulletin No. 354. 

Zingg AW [1951]. Evaluation of the erodibility of field surfaces with a portable wind 

tunnel. Soil Sci Soc of Amer Proc, pp. 11–17. 

Haul Roads, Stockpiles, and Open Areas 271 

DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

Haul Roads, Stockpiles, and Open Areas 273

GLOSSARY 
DUST CONTROL HANDBOOK FOR INDUSTRIAL MINERALS MINING AND PROCESSING 

274 Haul Roads, Stockpiles, and Open Areas 
 

 

GLOSSARY 

Abrader. An implement used to scrape or scratch a surface. 

Accelerated silicosis. A form of silicosis resulting from exposure to high concentrations of 

crystalline silica and developing 5–10 years after the initial exposure. While accelerated silicosis 

presents with the nodular characteristics that typify chronic silicosis, accelerated silicosis 

develops only 5–10 years after the initial exposure and generally progresses more rapidly than 

chronic silicosis. 

Access road. A road that is generally used for utility or maintenance purposes. This is usually a 

secondary road, which is smaller in design than a haul road. An access road is not normally used 

for moving ore to and from the pit and crusher locations. 

Acute silicosis. A form of silicosis resulting from exposure to unusually high concentrations of 

respirable crystalline silica dust. Symptoms develop within only a few weeks to 4–5 years after 

the initial exposure. In contrast to the more common nodular forms of silicosis, acute silicosis 

involves a flooding of the alveolar region of the lungs with abnormal fluid containing fats and 

proteins, which gives rise to an alternative term for the disease—silicoproteinosis. Acute 

silicosis is uniformly fatal, often within several months of diagnosis. 

3 Aerodynamic diameter. The diameter of a spherical particle that has a density of 1,000 kg/m

(the standard density of a water droplet) and the same settling velocity as that of the particle in 

question. Although particles have irregular shapes, this is an expression of a given particle's 

behavior as if it were a perfect sphere. 

Agglomeration. The act or process of gathering into a mass. Using wet spray systems, the intent 

is to have the droplets collide with the dust particles and agglomerate, causing them to fall from 

the air. 

Air atomizing. Mixing compressed air with a liquid to break up the liquid into a controlled 

droplet size. 

Airborne dust prevention. Dust control measures taken to prevent dust from becoming airborne. 

Airborne dust suppression. Dust control measures taken to knock down dust already airborne by 

spraying water into the dust cloud and causing the particles to collide, agglomerate, and fall from 

the air. 

Air quantity. The amount of air used in ventilating a process, which is a combination of the air 

velocity or speed and the area of the duct, measured in cubic feet of air per minute (cfm). 

Air to cloth ratio. In dust collection systems, a measure of the volume of gas per minute per unit 

area of the bag collecting dust. 

Glossary 275 
 

276 Glossary 

Air velocity. The speed of air travelling in an area or duct measured in feet per minute (ft/min). 

Annulus. The open area of the drill hole between the drill steel and the wall of the drill hole. 

Armoring. A process used in wind erosion where large particles are left behind as smaller 

particles erode. The large particles then act as an erosion barrier, preventing further wind 

erosion. 

Articulated loading spout positioner. A mechanical device that transfers product to a loading
 

spout, while also providing the capability of moving the loading spout from side-to-side as well
 

as forward-and-back to ensure proper positioning of the spout discharge.
 

Atomization. The process of breaking up a liquid stream into droplets.
 

Autogenous grinding. Size reduction of materials accomplished by impact and compressive
 

grinding of the material by large units of the material itself—i.e., no balls or rods are employed.
 

Axial-flow fan. A fan that moves air in a direction parallel to the axis of the rotation of the fan. 

Axial-flow fans types include propeller, tubeaxial, vaneaxial, and two-stage axial-flow. 

Back-flushing. The action of reversing the flow of air in a system to remove contaminants from 

a filter.
 

Bagging. Loading of product into some type of bag to be shipped/delivered to customers.
 

Baghouse collector. In dust collection systems, a type of collector that captures the particulate in 

a gas stream by forcing the gas through filter bags.
 

Bag perforations. Vent holes integrated into bags of product to allow air to escape rapidly and to 

reduce bag failures (e.g., rupturing or exploding).
 

Bag valve. Sleeve placed inside 50- to 100-lb product bags to allow for the insertion of the fill 

nozzle during bag loading and the sealing of the bag when loading is completed.
 

Bailing airflow. Compressed air blown down the drill stem through the bit in order to flush the 

cuttings out of the hole.
 

Blowback. Product spewing out from the bag valve during bag filling.
 

Bonding strength. The ability of the material to adhere to itself and to the material to which it is 

applied.
 

Capture velocity. A measure of the required airflow necessary to seize dust released at a source
 

and then pull this dust-laden air into a (capturing) hood. Capture velocity is measured in feet per
 

minute (ft/min).
 

Capturing hood. A hood positioned as close to a dust source as possible to capture the dust-laden 

air and pull it into an exhaust ventilation system.
 
 

 

Carryback. Material that sticks or clings to a conveyor belt after passing over the head pulley. 

Cartridge collector. In dust collection systems, a type of collector that captures particulate from 

a gas stream by forcing the gas through filter elements, which are arranged in a pleated 

configuration. 

Centrifugal collector. In dust collection systems, a device that separates particulate from the air 

by centrifugal force. Also called a cyclone. 

Centrifugal fan. A fan in which the airflow is drawn into the rotating impeller and discharged 

radially from the fan blade into the housing. Centrifugal fans include radial, backward, and 

forward blade type. 

Chimney effect. The behavior of heated air or gas rising in a vertical passage, as in a chimney, 

due to its lower density compared to the surrounding air or gas. 

Chronic silicosis. A slowly progressive, nodular form of silicosis that typically develops after 

10–30 years of exposures to respirable crystalline silica dust. 

Coanda effect. The tendency of a moving fluid to be attracted to a nearby surface. 

Collaring. The preliminary step in drilling, forming the beginning of a drill hole, or collar. 

Contact angle. The angle at which a liquid meets a solid surface. 

Control efficiency. A metric usually expressed as a percentage used to compare the results of 

two or more outcomes. 

Crown. The mid-point of a road raised such that water drains off either side of the road surface 

into a ditch. In cross-section, the shape of the road is depicted as an A-shape, with the highest 

point in the center of the road. 

Cure. The hardening of a chemical additive and base material. 

Cuttings. Material that is produced during the drilling process. Cuttings are usually a fine-sized 

material. 

Cyclone. In dust collection systems, a conical device that uses centrifugal force to separate 

large- and small-diameter particles. 

Deliquescent. The ability of a material to absorb moisture from the air and become liquid. 

Depth loading. In dust collection systems, a method of collecting particulate on a fabric by 

maintaining a filter cake on the fabric to optimize collection efficiency. When depth loading is 

used on a particular fabric, dust is intentionally allowed to penetrate into the fabric to form a 

layer of dust on the bag. 

Glossary 277 
 

 

Diffusion. The spread of dust particles from high concentration areas to lower concentration 

areas. Many times this is achieved in ventilation systems by dust-laden air being mixed with 

clean, dust-free air. 

Drifter drill. A drill used in underground mining to drill horizontal holes. 

Drill deck (drill table). A deck or table on the drill located 2–3 ft above the ground surface and 

adjacent to the drill operator's cab. This deck is used by the operator to gain access to the drill 

rods when needed to add more rods or to change drill bits. 

Drill rods. Metal rods, also called "steels," used to connect the rotary head to the drill bit as 

drilling proceeds through the ground. 

Dual-nozzle bagging system. A type of fill nozzle for filling 50- to 100-lb bags of product where 

the inner nozzle is used for filling and the outer nozzle is used to exhaust excess pressure from 

the bag. 

Dust. Small solid particles, often created by the breaking up of larger particles. 

Dust collector. An air cleaning device used to remove air-entrained particles. 

Dust collector airflow. The quantity of air that flows through a dust collector. 

Dust collector airflow to bailing airflow ratio. A ratio of dust collector airflow and bailing 

airflow, mathematically represented by the dust collector airflow divided by the bailing airflow. 

Dust suppressants. Chemicals or substances applied to a surface to prevent dust emissions from 

adhering to that surface. 

Elastic limit. The maximum stress that can be applied to a material without causing permanent 

deformation. 

Electrostatic charge. The physical electrical property of a dust particle that will cause it to 

experience a force, either attracting or opposing, from an oppositely charged electrical 

component or device. 

Electrostatic precipitator. A particulate control device that uses electrical forces to move 

particles from the gas stream to collection plates. 

Elutriation. The process of separating lighter from heavier particles by flow between horizontal 

plates. 

Enclosing hood. A hood that either partially or totally encloses an area to capture the dust source 

or dust-laden air and prevent it from flowing out into a mine or plant environment where it could 

contaminate workers. 

278 Glossary 
 

 

End-dumping. A process of spreading material (commonly gravel or sand) directly from the end 

of the dump truck. The material flow is controlled by limiting the tailgate opening to allow for 

the desired flow of material. Also called "tailgating." 

Entrainment. The process of particles being lifted into the air and carried with the airflow. 

Epitropic fiber. A fiber whose surface includes embedded particles designed to modify the fiber 

properties, typically electrical conductivity. Commonly used epitropic fiber materials include 

carbon, graphite, and stainless steel. 

Exhaust. In ventilation systems, to discharge air into the atmosphere using an energy source 

(typically a fan). 

Exhaust ventilation system. An engineered system designed to draw air through a network of 

openings via a fan. 

Flexible Intermediate Bulk Containers (FIBCs). Large product containers, normally 1,000 lbs 

and greater when filled, used to ship bulk material to customers. Also called "bulk bags," "mini

bulk bags," "semi-bulk bags," and "big bags." 

Fogger spray. Compressed air and water forced through a nozzle to create a fogged-in area, 

resulting in a mist that aids in dust suppression. 

Freeboard. The distance from the top of a dump truck bed down to the top of the material loaded 

into the bed. 

Fugitive dust. Solid airborne particulate matter that escapes from any dust-generating source. 

Gradation. The distribution of different particle sizes in aggregate material. 

Gravity separator. In ventilation systems, a large chamber where the velocity of the air stream is 

drastically reduced in order to facilitate the vertical drop of particles. Also called a "drop-out 

box." 

Haul road. A road used for hauling ore to and from pit and crusher locations at a surface mine. 

Hauling is generally conducted using large off-road haul trucks. 

Hydraulic (airless) atomization. Controlling droplet size by forcing liquid through a known 

orifice diameter at a specific pressure. 

Hydraulic flat fan nozzles. In wet spray systems, nozzles used to produce relatively large 

droplets over a wide range of flows and spray angles, normally used in narrow enclosed spaces. 

Hydraulic full cone nozzles. In wet spray systems, nozzles used to produce a solid cone-shaped 

spray pattern with a round impact area that provides high velocity over a distance. 

-

Glossary 279 
 

 

Hydraulic hollow cone nozzles. In wet spray systems, nozzles used to produce a circular ring 

spray pattern, typically producing smaller drops than other hydraulic nozzle types of the same 

flow rate. 

Hydrogen embrittlement. A process where metal (high-strength steel) can become brittle and 

fracture due to exposure to a hydrogen source. 

Hygroscopic. The ability to absorb moisture from the air. 

Impact plate. When material is being conveyed, a device used to control the direction and speed 

of the material. 

Impingement plate scrubber. A type of wet scrubber in which the dust-laden air passes upward 

through openings in perforated plates, which hold a layer of water. 

Induction. Airflow or product movement that creates sufficient momentum energy to pull 

additional surrounding air into the airflow or product movement. 

Inhalable dust. A fraction of airborne dust intended to represent those particles likely, when 

inhaled, to be deposited in the respiratory tract from the mouth and throat to the deep lung. The 

collection efficiency of an inhalable dust sampler varies by aerodynamic diameter of the 

particles—to demonstrate, about 50 percent of 100-micrometer particles (but nearly 100 percent 

of particles less than 1 micrometer) are collected. 

Inlet loading. In ventilation systems, the amount of dust traveling to the collector. 

Intake air. In ventilation systems, the clean air supplied into an area or space to replace existing 

(contaminated) air. 

Laminar. Sometimes referred to as streamline flow, the occurrence of air flowing in parallel 

paths with no disruptions between the paths. 

Local exhaust ventilation (LEV). A system used to capture dust and remove it from a mine or 

plant environment. The concept is to capture dust-laden air, pull this air into an exhaust hood, 

transport the dust-laden air through ductwork to some type of dust collection system, and duct 

the dust-free air to some type of exhaust fan, which creates negative pressure, resulting in the 

airflow for the entire system. 

Makeup air. In ventilation systems, the clean air necessary to supply a specific ventilation 

application (e.g., a local exhaust ventilation system). 

Mechanical shaker collector. In dust collection systems, a type of collector that uses mechanical 

shaking to remove the excess dust cake from the collection media. 

Musculoskeletal Disorder (MSD). Health problems in the body's muscles, joints, tendons, 

ligaments, and nerves. When work-related, these problems are caused by the work process or 

environment. 

280 Glossary 
 

 

Open structure building design. A processing structure or building designed without exterior 

solid walls in an effort to lower respirable dust and noise levels. 

Overhead Air Supply Island System (OASIS). A filtration system located over a worker at a 

stationary position to filter dust-laden air from the work environment and deliver a clean 

envelope of air down over the worker. 

Palletizing. In the mineral processing industry, when bags of product, normally ranging in 

weight between 50 and 100 lbs, are loaded onto a pallet for shipment to customers. 

Percussion drilling. Rock penetration through rotation and down pressure with a pneumatic drill, 

which contains a piston that delivers hammer blows to the drill column or the drill bit. 

Pitot tube. A pressure-measuring instrument normally made out of two concentric metal tubes 

that can be used in conjunction with a manometer to measure the static pressure (SP), total 

pressure (TP), and velocity pressure (VP) due to airflow in the duct. 

Protection factor. A comparison of outside versus inside dust concentrations. 

Push-pull ventilation system. The technique of using an air jet (blowing system) directed 

towards a dust source and receiving hood (exhaust system) to capture and move dust-laden air 

into the hood. 

Respirable dust. A fraction of airborne dust intended to represent those particles likely to be 

deposited in the alveolar region of the lungs when inhaled. The collection efficiency of a 

respirable dust sampler varies by aerodynamic diameter of the particles—to demonstrate, only 

about 1 percent of 10-micrometer particles (but nearly 100 percent of particles less than 1 

micrometer) are collected. 

Reverse air collector. In dust collection systems, a type of collector, that uses a traveling 

manifold to distribute low-pressure cleaning air to the filter bags for reconditioning. 

Reverse jet collector. In dust collection systems, a type of collector that uses timed blasts of 

compressed air for filter bag reconditioning. Also called a pulse jet collector. 

Roof ventilators. Either axial or centrifugal fans placed on the roof of a building and used to pull 

air out of the structure and discharge it into the atmosphere. 

"Rooster tail." Product spewing from the fill nozzle and bag valve as the bag is ejected from the 

bag filling machine. 

Rotary drilling. Penetration through rock by a combination of rotation and high down pressures 

on a column of drill pipe with a roller drill bit attached to its end. 

Saltation. The movement of particles (usually soil or sand) in a jumping or bouncing motion 

caused by the wind. 

Scarify. To break up or loosen the ground surface. 

Glossary 281 
 

 

Semi-autogenous. Size reduction of materials accomplished by impact and compressive grinding 

of the material by large units of the material itself, with the use of balls or rods to provide 

additional crushing force. 

Silicosis. A progressive and incurable lung disease that belongs to a group of lung disorders 

called the pneumoconioses. Silicosis is caused by repeated inhalation of respirable crystalline 

silica (e.g., quartz) dust. 

Skirting. On conveyors, a horizontal extension of the loading chute used to contain ore and dust 

within the transfer point. On loading spouts, strips of belting or fabric installed on the spout 

discharge end, creating a physical seal with the product pile to reduce dust liberated during bulk 

loading. 

Spoon. When material is being conveyed, a curved plate that helps steer the material in the 

desired direction. 

Spray tower scrubber. A type of scrubber that employs water, which falls counter-current 

through a rising dust-laden air stream, to remove dust particles. 

Spreader box. An implement used in road construction to spread road material (typically gravel 

or sand) in a uniform layer. The implement, commonly in the shape of a box, is situated behind 

an end-dump truck with material from the truck dumped into the box for spreading into the 

uniform layer. 

Static pressure (SP). In ventilation systems, a measure of the pressure in a ventilation duct 

relative to the atmospheric pressure. Static pressure can be either a positive or negative value. 

Static pressure summed together with velocity pressure (VP) provides the total pressure (TP) for 

a duct in a ventilation system. 

Stoper drill. A drill used in underground mining to drill vertical holes overhead. 

Sub. A short length (usually 2–3 ft) of drill collar assembly, made of the same drill steel 

material, which is placed between the drill bit and the drill steel. 

Subbase. In road construction, the layer between the subgrade and the wearing surface. 

Subgrade. In road construction, the underlying soil or rock that serves as the foundation for the 

road. 

Surface loading. In dust collection systems, a method of collecting particulate on the surface of a 

fabric. 

Surface roughness. In wind erosion, a qualitative measure of the roughness of a ground surface. 

The higher the surface roughness, the more wind erosion is inhibited. 

Surfactants. Substances that, when dissolved in water, lower the surface tension of the water, 

allowing the water to "wet" the dust more easily. 

282 Glossary 
 

 

Table bushing. A bushing used to seal the opening where the drill steel passes through the drill 

deck. 

Tilt sensor. On bulk loading spouts, a vertically mounted sensor designed to prevent blockages 

during loading. As the product pile grows and moves the sensor off of vertical by a 

predetermined amount (e.g., 15 degrees), an electronic signal is transmitted to initiate the raising 

of the spout. 

Total pressure (TP). In duct ventilation systems, the sum of static pressure (SP) and velocity 

pressure (VP). 

Total structure ventilation system. The use of exhaust fans high on the outside walls or roof of a 

structure, bringing in clean outside air at the base of a building, which sweeps up through the 

structure to clear dust-prone areas. 

Total suspended solids. An air quality term used to denote a particulate emission's entire particle 

size distribution. 

Tri-cone roller bit. A rotary drill bit with three roller cones. As the drill bit rotates, the roller 

cones, which have carbide components, roll against the bottom of the drill hole, causing the 

material to break into small particles. This type of drill bit requires high down-pressure on the 

drill bit. 

Velocity pressure (VP). In ventilation systems, the pressure required to accelerate air from being 

at rest to a particular velocity. Velocity pressure summed with static pressure (SP) provides the 

total pressure (TP) in a duct. 

Venturi eductor. In pneumatic conveying systems, a device that converts blower output into 

suction to draw feed material into the conveying line. 

Venturi effect. A reduction in static pressure resulting from a liquid or gas flowing through a 

constricted space. 

Venturi scrubber. A type of wet scrubber consisting of a venturi-shaped inlet and a separator. 

Water cartridges. A plastic or PVC bag filled with water and inserted into the blasthole along 

with the explosive, resulting in dust suppression during blasting. 

Water separator sub. A short length of a drill collar assembly placed between the drill bit and the 

drill steel. A water separator sub uses inertia to remove the injected water from the bailing air. 

Wearing surface. The top layer of the road surface which is exposed to traffic. 

Wet cyclone scrubber. A type of scrubber that uses centrifugal forces to throw particles on the 

collector's wetted walls. 

Glossary 283 
 

 

Wet drilling. In surface drilling, the injection of water along with air to flush the cuttings out of 

the hole. 

Wet scrubber. An air pollution control device used for particulate collection. Wet scrubbers 

employ water or another liquid as the collection media. 

Wet spray systems. A system of water sprays used to wet fines so that each dust particle's 

weight increases, thus decreasing the particle's ability to become airborne. 

Windrow. A long linear pile of material. 

284 Glossary 
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