IC 9521 

INFORMATION CIRCULAR/2010 

Best Practices for Dust Control 

in Metal/Nonmetal Mining 

Department of Health and Human Services 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
 

Information Circular 9521 

Best Practices for Dust Control in Metal/Nonmetal Mining 

By Jay F. Colinet, Andrew B. Cecala, Gregory J. Chekan, John A. Organiscak, 

 and Anita L. Wolfe 

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 

May 2010 
This document is in the public domain and may be freely copied 

or reprinted. 

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DHHS (NIOSH) Publication No. 2010–132 

May 2010 

SAFER • HEALTHIER • PEOPLETM 
 CONTENTS

Introduction......................................................................................................................................

Page

1 

Chapter 1.—Health effects of overexposure to respirable silica dust ..............................................3 

 Silicosis...................................................................................................................................3 

Diagnosis and treatment .........................................................................................................5 

References ..............................................................................................................................6 

Chapter 2.—Sampling to quantify respirable dust generation .........................................................8 

Respirable dust samplers for use in mining ............................................................................8 

 Sampling strategies ...............................................................................................................10 

References ............................................................................................................................13 

Chapter 3.—Controlling respirable silica dust in underground stone and metal/nonmetal mines 15 

 Crushing facilities .................................................................................................................15 

 Production shots ...................................................................................................................19 

 Mucking operations ..............................................................................................................21 

Drilling .................................................................................................................................23 

References ............................................................................................................................23 

Chapter 4.—Controlling respirable silica dust in mineral processing operations .........................26 

 Primary dumping ..................................................................................................................27 

Crushing and grinding ..........................................................................................................30 

 Transfer points ......................................................................................................................31 

Conveying.............................................................................................................................32 

 Wet suppression ....................................................................................................................34 

Local exhaust ventilation (LEV) systems.............................................................................38 

Low velocity transport systems ............................................................................................39 

Total mill ventilation systems ..............................................................................................41 

Operator booths, control rooms, enclosed cabs ....................................................................43 

Screening ..............................................................................................................................45 

Packaging/bagging product for shipment.............................................................................46 

Clothes cleaning system .......................................................................................................50 

 Background issues ................................................................................................................54 

References ............................................................................................................................58 

Chapter 5.—Controlling respirable silica dust at surface mines ....................................................64 

Drill dust collection systems ................................................................................................65 

Enclosed cab filtration systems ............................................................................................67
 

Controlling haulage road dust...............................................................................................69

 Controlling dust at the primary hopper dump ......................................................................70 

References ............................................................................................................................72 

ILLUSTRATIONS 

1-1. Section of freeze-dried human lung with silicosis .................................................................4 

1-2. Inspector samples for select occupations from 2004-2008 that exceeded the PEL ...............5 

2-1. Gravimetric sampling pump, cyclone and filter cassette .......................................................8 

2-2. Example of dust measurements obtained with pDR ..............................................................9 
CONTENTS—Continued 

 2-3. Personal dust monitor (PDM) with TEOM unit removed and shown on right ...................

Page

10 

2-4. Sampling locations used to isolate dust generated at an underground crusher ...................11 

2-5. Dust samplers mounted on haul truck in gold mine ............................................................12 

2-6. Average dust generated by each segment of sampled haul truck cycle ...............................12 

2-7. Sampling locations around surface drill ..............................................................................13 

3-1. Typical method used to isolate crushing facility from mine air ..........................................16 

3-2. Closed ventilation system using plenum .............................................................................17 

3-3. Filtration and pressurization system components and design .............................................18 

3-4. Canopy air curtain blows filtered air over worker ...............................................................19 

3-5. Movement of dust after production shot as recorded by real-time dust sampler ................20 

3-6. Axial vane fan (left) versus propeller fan (right) .................................................................21 

3-7. Fans positioned to ventilate dead-end entries......................................................................22 

4-1. Staging curtains reduce dust billowing during dumping .....................................................29 

4-2. Plastic stripping holds dust inside enclosure allowing water sprays to knock down dust ...29 

4-3. Tire-stop water spray system reduces rollback under dumping mechanism .......................30 

4-4. Techniques for reducing respirable dust liberation from conveyor belts and transfers .......34 

4-5. Spray nozzles commonly used in mineral processing operations .......................................36 

4-6. Airborne dust capture performance of four types of spray nozzles .....................................37 

4-7. Canister-type dust collector system .....................................................................................39 

4-8. Sawtooth design for low velocity transport system .............................................................41 

4-9. Design concept of TMVS showing clean-air intakes and dust-laden air exhausts ..............42 

4-10. Airflow pattern for one-directional filtration system for an enclosed cab ........................44 

4-11. Screening unit with LEV system.......................................................................................45 

4-12. Dual bag nozzle design......................................................................................................46 

4-13. Bag and belt cleaner device ...............................................................................................47 

4-14. Semiautomated pallet loading system using push-pull ventilation ...................................49 

4-15. Telescoping bulk loading spout with an exhaust system ...................................................50 

4-16. Clothes cleaning system design .........................................................................................52 

4-17. Test subject before and after using the clothes cleaning booth .........................................53 

4-18. Drawing of a conventional structure and an open structure with a protective overhang ..56 

4-19. Overhead air supply island system ....................................................................................57 

5-1. Typical dry dust collection system used on surface drills ...................................................65 

5-2. Water separator discharging water before it reaches the drill bit ........................................67 

5-3. Increase in dust when a haul road dries ...............................................................................70 

5-4. Staging curtains used to prevent dust from billowing out of enclosure ..............................71 

5-5. Tire-stop water spray system reduces dust rollback under the dumping vehicle ................72 

TABLES

 4-1. Percent of samples exceeding PEL for select occupations ..................................................26 

5-1. Respirable dust sampling results of enclosed cab field studies ...........................................68 
ACRONYMS AND ABBREVIATIONS USED IN THIS REPORT

ACPH air changes per hour 

ACGIH American Conference of Governmental Industrial Hygienists 

CT computed tomography 

HEPA high efficiency particulate air 

HVAC heating, ventilation, and air conditioning 

IARC International Agency for Research on Cancer 

LEV local exhaust ventilation 

LHD load-haul-dump 

MSHA Mine Safety and Health Administration 

NIOSH National Institute for Occupational Safety and Health 

OASIS overhead air supply island system 

PDM personal dust monitor 

pDR personal DataRAM 

PEL permissible exposure limit 

PMF progressive massive fibrosis 

PPE personal protective equipment 

PVC poly vinyl chloride 

TEOM tapered-element oscillating microbalance 

TMVS total mill ventilation system 

XRD X-ray diffraction 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cfm cubic foot per minute 

fpm foot per minute 

gpm gallon per minute 

in w.g. inches water gauge 

lpm liter per minute 

mg/m3 milligram per cubic meter 

mm millimeter 

mph miles per hour 

μg/m3 microgram per cubic meter 

psi pound-force per square inch 

 

BEST PRACTICES FOR DUST CONTROL IN METAL/NONMETAL 

MINING 

By Jay F. Colinet,1

 1

Senior scientist, Office of Mine Safety and Health Research, National Institute for Occupational Safety and 

Health, Pittsburgh, PA. 

Andrew B. Cecala,2

 Gregory J. Chekan,2 John A. Organiscak,2

 2

Mining engineer, Office of Mine Safety and Health Research, National Institute for Occupational Safety and 

Health, Pittsburgh, PA. 

 and Anita L. 

Wolfe3
 

 3

Public health advisor, Division of Respiratory Disease Studies, National Institute for Occupational Safety and 

Health, Morgantown, WV. 

INTRODUCTION 

Respirable silica dust exposure has long been known to be a serious health threat to workers in 

many industries. Overexposure to respirable silica dust can lead to the development of silicosis— 

a lung disease that can be disabling and fatal in its most severe form. Once contracted, there is no 

cure for silicosis so the goal must be to prevent development by limiting a worker’s exposure to 

respirable silica dust. In addition, the International Agency for Research on Cancer (IARC) has 

concluded that there is sufficient evidence to classify silica as a human carcinogen. 

For workers in the metal/nonmetal mining industry, the Mine Safety and Health Administration 

(MSHA) regulates and monitors exposure to respirable silica dust through personal dust 

sampling. Recent MSHA personal sampling results indicate that overexposures to respirable 

silica dust continue to occur for miners in metal/nonmetal mining operations. From 2004 to 

2008, the percentages of samples that exceeded the applicable respirable dust standard for the 

different mining commodities were: 

• 12% for sand and gravel 

• 13% for stone 

• 18% for nonmetal 

• 21% for metal 

Of the 2,407 deaths attributed to silicosis in the United States from 1990–1999, employment 

information was available for 881 deaths. Metal/nonmetal mining was the industry recorded for 

over 15% of these 881 deaths, with mining machine operator the most frequently recorded 

occupation. 

In light of ongoing silica overexposures and reported silicosis deaths in metal/nonmetal miners, 

an ongoing threat to miners’ health is evident. This handbook was developed to identify 

available engineering controls that can assist the industry in reducing worker exposure to 

respirable silica dust. The controls discussed in this handbook range from long-used controls 

which have developed into industry standards, to newer controls, which are still being optimized. 

The intent is to identify the “best practices” that are available for controlling respirable dust 
levels in underground and surface metal/nonmetal mining operations. This handbook provides 

general information on the control technologies along with extensive references. In some cases, 

the full reference(s) will need to be accessed to gain in-depth information on the testing or 

implementation of the control of interest. 

The handbook is divided into five chapters. Chapter 1 discusses the health effects of exposure to 

respirable silica dust, while Chapter 2 discusses dust sampling instruments and sampling 

methods. Chapters 3, 4 and 5 are focused upon dust control technologies for underground 

mining, mineral processing, and surface mining, respectively. 

Finally, it must be stressed that after control technologies are implemented, the ultimate success 

of ongoing protection for workers is dependent upon continued maintenance of these controls. 

On numerous occasions, National Institute for Occupational Safety and Health (NIOSH) 

researchers have seen appropriate controls installed, but worker overexposures continued to 

occur in the absence of proper maintenance of these controls. 

2 
CHAPTER 1. HEALTH EFFECTS OF OVEREXPOSURE TO 

RESPIRABLE SILICA DUST 

By Anita Wolfe and Jay Colinet 

Pneumoconioses are lung diseases caused by the inhalation and deposition of mineral dusts in the 

lungs. Known pneumoconioses include, but are not limited to, coal workers’ pneumoconiosis 

and silicosis. These diseases are usually associated with working in a high-risk, mineral-related 

industry such as mining. 

SILICOSIS 

Occupational exposures to respirable crystalline silica occur in a variety of industries and 

occupations because of its extremely common natural occurrence. Respirable crystalline silica is 

defined as particles with aerodynamic diameters less than 10 microns [NIOSH 2002]. Workers 

with high exposure to crystalline silica include miners, sandblasters, tunnel builders, silica 

millers, quarry workers, foundry workers, and ceramics or glass workers. Silica refers to the 

chemical compound silicon dioxide (SiO2), which occurs in a crystalline or noncrystalline 

(amorphous) form [NIOSH 2002]. Crystalline silica may be found in more than one form: alpha 

quartz, beta quartz, tridymite, and cristobalite [USBM 1992a; Heaney 1994]. In nature, the alpha 

form of quartz is the most common [Virta 1993]. This form is so abundant that the term quartz is 

often used in place of the general term crystalline silica [USBM 1992b; Virta 1993]. 

Quartz is a common component of rocks; consequently, mine workers are potentially exposed to 

quartz dust when rock is cut, drilled, crushed, and transported. Occupational exposures to 

respirable crystalline silica are associated with the development of silicosis, lung cancer, 

pulmonary tuberculosis, and airways diseases. These exposures may also be related to the 

development of autoimmune disorders, chronic renal disease (loss of kidney function), and other 

adverse health effects. In 1996 and 2009, the International Agency for Research on Cancer 

(IARC) reviewed the published experimental and epidemiologic studies of cancer in animals and 

workers exposed to respirable crystalline silica and concluded that there was sufficient evidence 

to classify silica as a human carcinogen [IARC 1997; Straif et al. 2009]. 

Silicosis is also a fibrosing disease of the lungs caused by the inhalation, retention, and 

pulmonary reaction to the crystalline silica. When silicosis becomes symptomatic, the primary 

symptom is usually dyspnea (difficult or labored breathing and/or shortness of breath), first noted 

with activity or exercise and later, as the functional reserve of the lung is also lost, at rest. 

However, in the absence of other respiratory diseases, there may be no shortness of breath and 

the disease may first be detected through an abnormal chest x-ray. The x-ray may at times show 

quite advanced disease with only minimal symptoms. The appearance or progression of dyspnea 

may indicate the development of complications including tuberculosis, airways obstruction, 

progressive massive fibrosis (PMF), or cor pulmonale (enlargement of the right side of the 

heart). Productive cough is often present. 

A worker may develop one of three types of silicosis, depending on the airborne concentrations 

of respirable crystalline silica: 

3 
 

(1) Chronic Silicosis: Usually occurs after 10 or more years of exposure at relatively 

low concentrations. Swellings caused by the silica dust form in the lungs and lymph 

nodes of the chest. This disease may cause people to have trouble breathing and 

may be similar to chronic obstructive pulmonary disease. 

(2) Accelerated Silicosis: Develops 5 to 10 years after the first exposure. Swelling in 

the lungs and symptoms occur faster than in chronic silicosis. 

(3) Acute Silicosis: Develops after exposure to high concentrations of respirable 

crystalline silica and results in symptoms within a period of a few weeks to 5 years 

after the initial exposure [NIOSH 1986; Parker and Wagner 1998]. The lungs 

become very inflamed and can fill with fluid, causing severe shortness of breath and 

low blood oxygen levels. 

PMF can occur in either simple or accelerated silicosis but is more common in the accelerated 

form. Figure 1-1 shows a lung that has been damaged by silicosis. 

Figure 1-1. Section of freeze-dried human lung with silicosis. 

In an effort to prevent the development of silicosis in miners working in metal/nonmetal mines, 

MSHA regulates their exposure to respirable silica. When quartz levels in respirable dust 

samples are greater than 1%, a respirable dust standard (permissible exposure limit) is calculated 

by dividing 10 mg/m3

 by the sum of the percent quartz plus 2. For example, if a sample contains 

8% quartz, the respirable standard would be equal to 1 mg/m3 (i.e., 10 ÷ (8 + 2)). This regulation 

places the upper limit of exposure to respirable quartz at 100 μg/m3

. 

MSHA compliance sampling data identify those occupations in metal/nonmetal mining that are 

high-risk occupations for overexposure to quartz. Figure 1-2 shows the percent of samples 

collected by MSHA inspectors that exceeded the permissible exposure limit (PEL) for a number 

of high-risk occupations in metal/nonmetal mining. 

4 
10 

20 

30 

40 

Percent > PEL 

0 

Bagging Crusher Laborer Stone Polisher 

operator operator 

Figure 1-2. Inspector samples for select occupations from 2004–2008 that exceeded the PEL. 

DIAGNOSIS AND TREATMENT 

A doctor may diagnose silicosis based on the combination of an appropriate history of exposure 

to silica dust, compatible changes in chest imaging or lung pathology, and absence of plausible 

alternative diagnoses. A chest radiograph is often sufficient for diagnosis, but in some cases a 

computed tomography (CT) scan of the chest can be helpful. Lung biopsy, a procedure where a 

sample of lung tissue is taken for lab examination, is not usually required if a compatible 

exposure history and findings on chest imaging are present. Pulmonary function tests and blood 

tests to measure the amounts of oxygen and carbon dioxide in the blood (arterial blood gases) 

can help in objectively assessing the level of impairment caused by silicosis. 

Epidemiologic studies of gold miners in South Africa, granite quarry workers in Hong Kong, 

metal miners in Colorado, and coal miners in Scotland have shown that chronic silicosis may 

develop or progress even after occupational exposure to silica has been discontinued [Hessel et 

al. 1988; Hnizdo and Sluis-Cremer 1993; Ng et al. 1987; Kreiss and Zhen 1996; Miller et al. 

1998]. Therefore, removing a worker from exposure after diagnosis does not guarantee that 

silicosis or silica-related disease will stop progressing or that an impaired worker’s condition will 

stabilize. 

Treatment of silicosis may include use of bronchodilators (medications to open the airways) or 

supplemental oxygen. Once disease is detected, it is important to protect the lungs against 

respiratory infections, therefore a doctor may recommend vaccinations to prevent influenza and 

pneumonia. In some cases of severe disease, a lung transplant may be recommended. Prognosis 

depends on the length and level of exposure to respirable quartz dust. There is no cure for this 

lung disease and it cannot be reversed. Consequently, control technologies must be implemented 

in an effort to prevent the development of the disease. As an added measure of protection, a 

respirator program can be implemented for workers exposed to silica dust. 

5 
REFERENCES
 

Heaney PJ [1994]. Structure and chemistry of the low-pressure silica polymorphs. In: Heaney PJ, 

Prewitt CT, Gibbs GV, eds. Silica: physical behavior, geochemistry, and materials applications. 

Reviews in mineralogy. Vol. 29. Washington, DC: Mineralogical Society of America, pp.140. 

Hessel PA, Sluis-Cremer GK, Hnizdo E, Faure MH, Thomas RG, Wiles FJ [1988]. Progression 

of silicosis in relation to silica dust exposure. Ann Occup Hyg 32(Suppl 1):689–696.
 

Hnizdo E, Sluis-Cremer GK [1993]. Risk of silicosis in a cohort of white South African gold 

miners. Am J Ind Med 24:447–457. 

IARC [1997]. IARC monographs on the evaluation of carcinogenic risks to humans: silica, some
 

silicates, coal dust and para-aramid fibrils. Vol 68. Lyon, France: World Health Organization, 

International Agency for Research on Cancer.
 

Kreiss K, Zhen B [1996]. Risk of silicosis in a Colorado mining community. Am J Ind Med 

30:529–539. 

Miller BG, Hagen S, Love RG, Soutar CA, Cowie HA, Kidd MW, Robertson A [1998]. Risks of 

silicosis in coal workers exposed to unusual concentrations of respirable quartz. Occup Environ 

Med 55:52–58. 

 

Ng TP, Chan SL, Lam KP [1987]. Radiological progression and lung function in silicosis: a ten 

year follow up study. Br Med J 295:164–168. 

NIOSH [1986]. Silicosis. By Peters JM. In: Merchant JA, Boehlecke BA, Taylor G, PickettHarner M, eds. Occupational respiratory diseases. Cincinnati, OH: U.S. Department of Health 

and Human Services, Centers for Disease Control, National Institute for Occupational Safety and 

Health, DHHS (NIOSH) Publication No. 86-102, pp. 219–237. 

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

to respirable crystalline silica. 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. 2002-129. 

Parker JE, Wagner GR [1998]. Silicosis. In: Stellman JM, ed. Encyclopaedia of occupational 

health and safety. 4th ed. Geneva, Switzerland: International Labour Office, pp. 10.43–10.46. 

Straif K, Benbrahim-Tallaa L, Baan R, Grosse Y, Secretan B, El Ghissassi F, Bouvard V, Guha 

N, Freeman C, Galichet L, Cogliano V [2009]. A review of human carcinogens—Part C: metals, 

arsenic, dusts, and fibres. Lancet Oncol 10(5):453–454. 

USBM [1992a]. Crystalline silica overview: occurrence and analysis. By Ampian SG, Virta RL. 

Washington, DC: U.S. Department of the Interior, U.S. Bureau of Mines, Information Circular 

IC 9317. 

6 
USBM [1992b]. Crystalline silica primer. Washington, DC: U.S. Department of the Interior, U.S. 

Bureau of Mines. 

Virta RL [1993]. Crystalline silica: what it is and isn’t. Minerals Today Oct:12–16. 

7 
 

CHAPTER 2. SAMPLING TO QUANTIFY RESPIRABLE DUST 

GENERATION 

By Jay F. Colinet 

The respirable fraction of airborne dust is the dust that reaches the lungs and leads to the 

development of silicosis. Respirable dust cannot be seen with the eye. Conversely, if a dust cloud 

is visible, it is likely that a portion of the airborne dust will be in the respirable size range. In 

order to quantify the amount of harmful respirable dust that is in the mine air, sampling 

instrumentation must be used. 

RESPIRABLE DUST SAMPLERS FOR USE IN MINING 

The most common type of sampler used in the mining industry is the gravimetric sampler, which 

is designated by the Federal Coal Mine Health and Safety Act of 1969 for use in compliance dust 

sampling (Figure 2-1). 

Figure 2-1. Gravimetric sampling pump, cyclone and filter cassette. 

This sampler consists of a constant-flow sampling pump, a size-selective 

cyclone, and a filter cartridge. In metal/nonmetal mining operations, the pump should be 

operated at 1.7 lpm. The 10-mm Dorr-Oliver cyclone separates the oversize dust from the 

respirable fraction (usually considered to have an aerodynamic diameter of 10 microns or less). 

The oversize dust is deposited into the grit pot at the bottom of the cyclone, while the respirable 

fraction is deposited onto a 37-mm-diameter polyvinyl chloride (PVC) filter. Care must be taken 

after a sample is collected to ensure that the cyclone assembly stays in an upright position. 

Otherwise, the oversize dust particles that are in the grit pot can be deposited onto the filter and 

invalidate the sample. The filter collects the respirable dust and is weighed to determine the mass 

of dust that has been collected during sampling. The mass of dust and the volume of sampled air 

are used to calculate the average concentration of respirable dust in mg/m3

. 

In order to determine the silica content of a gravimetric sample, the filter should be sent to an 

accredited laboratory for analysis. For samples collected in metal/nonmetal mines, x-ray 

8 
diffraction (XRD) using NIOSH Analytical Method 7500 [NIOSH 1994, 2003] is the analytical 

technique used by MSHA’s accredited laboratory [Parobeck and Tomb 2000] to quantify silica 

in the samples for compliance purposes. 

In addition to the gravimetric samplers, a real-time dust sampler is available for use in mining. 

The personal DataRam (pDR) has dust-laden air pass through a sensing chamber in the sampler 

and passes a light beam through this dust. A sensor in the sampler measures the amount of light 

scatter caused by the dust and relates this scatter to a relative dust concentration. This 

concentration is correlated to the time when the sample was measured and stores this information 

in the internal data logger. The sample data can then be downloaded to a computer for analysis. 

Figure 2-2 illustrates a typical graph obtained with the pDR, as well as, a photo of the pDR. This 

data can be analyzed for specific time intervals (e.g., loading a cut), with average dust 

concentrations calculated for these intervals. 

Figure 2-2. Example of dust measurements obtained with pDR. 

9 

Unfortunately, the accuracy of the light-scattering monitor can be compromised by measuring 

dust clouds with different size distributions, different dust compositions, and/or water mist/fog in 

the ambient air. Consequently, when NIOSH utilizes pDR samplers, gravimetric samplers are 

placed adjacent to the pDR and individual pDR dust measurements are adjusted based upon the 

ratio between the average gravimetric concentration and the average pDR concentration [Thermo 

Scientific 2008]. For example, if the gravimetric concentration was 1.3 mg/m3

 over a 6-hour 

measurement period and the pDR average concentration was 1.0 mg/m3

 for the same 6 hours, 

then all individual pDR measurements would be multiplied by 1.3. 
The personal dust monitor (PDM) [NIOSH 2006] is another real-time sampler, which has been 

developed and tested by NIOSH, has been approved for use in underground mines by MSHA, 

and has reached commercial production. The PDM uses tapered element oscillating 

microbalance (TEOM) technology to obtain a real-time, gravimetric-based measure of respirable 

dust concentrations. The TEOM is a hollow tube that vibrates at a known frequency and has a 

filter mounted on the end. As respirable dust is deposited onto this filter, the TEOM frequency 

changes and this change can be related to a dust concentration. The PDM provides the wearer 

with a readout of the cumulative dust concentration to that point in the shift and the percent of 

the permissible exposure limit (PEL) that has been reached. This information can be used by the 

wearer to reduce their dust exposure prior to becoming overexposed. The sampler is incorporated 

into a standard cap lamp housing and has the sampling inlet located at the cap lamp (see Figure 

2-3). 

Figure 2-3. Personal dust monitor (PDM) with TEOM unit removed and shown on right. 

SAMPLING STRATEGIES 

In order to effectively control the respirable silica dust exposure of mine workers, it is necessary 

to identify the sources of dust generation and quantify the amount of dust liberated by these 

sources. Once the dust sources are identified and dust liberation from each source has been 

quantified, appropriate dust control technologies can be applied that offer the greatest protection 

to the mine workers. 

In order to quantify the amount of dust liberated by a source, area dust sampling can be 

conducted in a manner that isolates the potential dust source. This is achieved by placing dust 

samplers upwind and downwind of the source in question and utilizing the difference between 

these sampling results to determine the quantity of dust liberated by the source. An example of 

area sampling from a NIOSH research project is provided next to illustrate this sampling method. 

In an underground limestone mine, samplers were placed in the immediate intake and return of 

an underground crusher to determine the amount of dust liberated during the dumping/crushing 

10 
of the limestone. Figure 2-4 illustrates these sampling locations. Samplers are placed on both 

sides of the entry at both sampling locations to obtain more representative measurements of the 

airborne dust concentrations. 

Figure 2-4. Sampling locations used to isolate dust generated at an underground crusher. 

The concentrations from both sides of the entry are then averaged. 

If gravimetric samplers are used for this evaluation, the samplers must operate long enough to 

ensure that sufficient mass is collected during sampling. 

In addition, a great number of variables that can impact dust liberation are encountered in mining 

operations. It is often desirable to place multiple gravimetric samplers at a single area sampling 

location. An average dust concentration from the multiple samplers can be calculated, increasing 

the confidence that the measured dust levels are representative of the true dust concentration. 

For quantifying exposures for an operator of a more mobile piece of equipment, such as a haul 

truck in an underground gold mine, the use of a real-time sampler such as the pDR would be 

beneficial to quantify dust exposure from multiple sources. Similar to Figure 2-2, the haul truck 

operator would be exposed to dust generated during loading of the truck, hauling to and from the 

dump site, and during the dumping of the load. Gravimetric and real-time samplers were 

mounted near the operator’s compartment on the underground haul truck (see Figure 2-5) to 

monitor dust exposures throughout the normal haul cycle. Time study information was collected 

and used to separate each of these operations during analysis of the real-time data [Chekan et al. 

2002]. In this manner, the average dust exposure during each segment of the haul cycle could be 

isolated to determine where the greatest dust exposure was realized. Figure 2-6 shows the dust 

11 
concentrations from each segment of the cycle. Even though the dumping operation took the 

shortest time, it resulted in the highest dust levels. 

Figure 2-5. Dust samplers mounted on haul truck in gold mine. 

Figure 2-6. Average dust generated by each segment of sampled haul truck cycle. 

If a consistent ventilation pattern is not present, it would be necessary to use multiple samplers in 

an effort to quantify dust from a particular source. For example, in order to quantify the amount 

of respirable dust that is generated by a drill at a surface mine, it would be necessary to place an 

array of samplers around the drill to account for dust liberated during changing wind directions. 

12 
The dust concentrations from these samplers would be averaged to quantify dust liberation 

around the drill. It would also be necessary to place a background dust sampler far enough away 

from the drill to monitor ambient dust levels. The dust levels from the ambient sample would be 

subtracted from the drill samples that have been averaged to determine dust liberated by the drill. 

Figure 2-7 shows sampling locations around a surface drill. 

Figure 2-7. Sampling locations around surface drill. 

After identifying the most significant dust sources, appropriate dust controls should be selected 

and implemented. In order to determine the impact of the additional controls, sampling would 

once again be conducted. Typically, an A-B comparison would be needed to quantify the impact 

of added control technologies. The A portion of the test would be completed under original 

operating conditions (e.g., mining equipment and methods, production, geologic conditions) and 

used to establish baseline dust levels. The control technology of interest would then be installed 

and the B portion of the testing completed. The most valid comparisons can be made if the 

operating conditions do not change between the A and B segments of testing. The difference in 

dust levels measured for each test condition would be calculated to quantify the effectiveness of 

the installed control. 

REFERENCES 

Chekan GJ, Colinet JF, Grau RH III [2002]. Silica dust sources in underground metal/nonmetal 

mines—two case studies. In: SME Transactions, Vol. 312. Littleton, CO: Society for Mining, 

Metalury, and Exploration, Inc., pp 187–193. 

NIOSH [1994]. NIOSH manual of analytical methods. 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. 94-113. 

13 
NIOSH [2003]. NIOSH manual of analytical methods, 4th ed., 3rd 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. 

NIOSH [2006]. Laboratory and field performance of a continuously measuring personal 

respirable dust monitor. By Volkwein JC, Vinson RP, Page SJ, McWilliams LJ, Joy GJ, Mischler 

SE, Tuchman DP. 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. 2006-145. 

Parobeck PS, Tomb TF [2000]. MSHA’s programs to quantify the crystalline silica content of 

respirable mine dust samples. 2000 SME Annual Meeting, Preprint 00-159, 5 p. 

Thermo Scientific [2008]. Model pDR-1000AN/1200 personal DATARAM instruction manual. 

Waltham, MA: Thermo Scientific, pp. 35-36. 

14 
CHAPTER 3. CONTROLLING RESPIRABLE SILICA DUST IN 

UNDERGROUND STONE AND METAL/NONMETAL MINES 

By Gregory J. Chekan 

The health hazards associated with overexposure to respirable crystalline silica dust in the 

mining industry have been well documented. Studies of molybdenum, lead, and gold miners in a 

Colorado mining community found that silica exposure was strongly associated with silicosis 

prevalence rates, with 13% silicotics among those with an average exposure of 0.025–0.05 

mg/m3, 34% among those with exposures > 0.05–0.1 mg/m3, and 75% among those with 

exposures > 0.1mg/m3 [Steenland and Brown 1995; Kreiss and Zhen 1996]. An analysis of the 

Mine Safety and Health Administration (MSHA) compliance dust sampling data has shown that 

a high percentage of samples with more than 1% silica from underground stone, metal, and 

nonmetal mines exceeded the applicable permissible exposure limit (PEL). For inspector samples 

collected from 2004 through 2008, over 17% of samples exceeded the PEL. High risk 

occupations that had samples over the PEL include crusher operator at 36%, front end loader 

operator at 16%, and truck driver at 11% [MSHA 2008]. 

The stone and metal/nonmetal mining industry encompasses many types of commodities. The 

potential for respirable silica dust exposure to workers in the stone and metal/nonmetal mining 

industry is related to the percentage of silica in the product being mined or processed. For 

crushed and broken stone or dimension stone, silica percentages are on the high end, with 

sandstones and granites averaging 70% to 90%. On the low end are limestones, averaging 20% 

to 30%. For all metal/nonmetal ores, silica percentages average from 5% to 20% [USBM 1992]. 

Therefore, airborne concentrations of silica dust are dependent upon the silica percentage in the 

rock and ore being mined. Each commodity has common dust sources related to the mining 

cycle, which includes drilling, blasting, loading, hauling, and crushing. 

The purpose of this chapter is to address best practices in controlling respirable silica dust in 

underground stone and metal/nonmetal mines. Dust control methods commonly used in 

underground operations can be divided into three distinct areas: (1) the application of local and 

mine-wide ventilation systems to dilute, transport, and remove dust from the ambient air and 

direct dust away from workers, (2) the isolation of workers from airborne dust using dust 

filtration systems on enclosed cabs and booths, and (3) the capture of dust after generation using 

water sprays and wetting agents to mitigate dust entrainment. 

This chapter addresses best practices for respirable silica dust control generated from four 

primary dust sources: (1) crushing facilities, (2) production shots, (3) mucking operations, and 

(4) drilling. 

CRUSHING FACILITIES 

Sampling surveys have shown that underground crushing facilities, which include the dump, the 

crushers, and the associated conveyor belts and transfer points, can be a significant source of 

silica dust generation. Airborne silica concentrations can be extremely high depending on the 

bulk content of silica in the rock and crusher production capacity. At one limestone mine, with 

15 
rock composed of 30% silica by weight and the crusher operating at 1,000 tons per hour, the 

silica concentration measured directly above the crusher jaws was 1.8 mg/m3

 [Cheken and 

Colinet 2002; Chekan et al. 2003]. Occupations typically exposed to silica dust from this source 

obviously include the crusher operators and truck drivers, and also the mechanics, cleanup men, 

and laborers whose tasks require them to work in this area. Several methods for reducing worker 

exposure to silica dust at crusher locations are recommended, as follows: 

•	 Isolate the facility from the general mine air circuit. Dust generated from this 

source can be adequately contained using brattice or permanent stoppings to isolate 

the entire facility (dump, crusher, and belt). Booster fans using blowing ventilation 

should be positioned in key locations to increase airflow around the facility and dilute 

and transport dust away from the location to a return entry. Booster fans may be 

either axial vane or propeller, but recent studies have shown that propeller fans dilute 

and transport dust more effectively, especially in large-opening mines [Chekan et al. 

2006]. Figure 3-1 shows the plan view of a limestone crusher facility isolated from 

the other mine developments using stoppings and a blowing propeller fan to move 

dust-laden air into the return. 

Figure 3-1. Typical method used to isolate crushing facility from mine air. 

•	 Ventilate with a closed ventilation system. A closed ventilation system, where a 

plenum is located under the crusher, may be required in cases where the facility 

16 
cannot be isolated and dust cannot be directed to the return entries. Air is exhausted 

from under the plenum, creating an indraft at the crusher jaws to capture the dust. The 

dust-laden air is then directed to a nearby return, a bag house, or a fan-powered dust 

collector where it is captured by filters and the clean air can be discharged into the 

mine air [NIOSH 2003a]. Figure 3-2 shows a conceptual approach to control crusher 

dust in a stone mine using a closed ventilation system. 

Figure 3-2. Closed ventilation system using plenum. 

•	 Use filtration/pressurization systems in mobile equipment cabs and operator 

booths. As mining equipment ages, many of the original components on the cab 

enclosure deteriorate through normal operation in harsh mine environments. As a 

result, the effectiveness of the air filtration system and cab seals is lessened and the 

protection initially afforded to operators is compromised, possibly exposing them to 

elevated levels of respirable silica dust. NIOSH has worked with a number of 

manufacturers to develop cost-effective methods to improve both filtration 

effectiveness and cab integrity on these older cabs with the goal of reducing silica 

dust levels inside the cabs. Research results show dust levels inside upgraded cabs 

were reduced from 65% to 95% when compared to levels outside the cabs [NIOSH 

2008]. Retrofit options from several manufactures are available for both enclosed 

cabs and booths. Figure 3-3 shows an effective design of an enclosed filtration and 

pressurization system. 

17 
Figure 3-3. Filtration and pressurization system components and design. 

•	 Consider five key factors for maintaining and operating enclosed cabs and 

booths. 

(1) Ensure good cab enclosure integrity to achieve positive pressurization against 

wind penetration into the enclosure. Studies show that significant improvements in 

cab protection factors were achieved when cab pressures exceeded 0.01 inches of 

water gauge [Cecala et al. 2005]. 

(2) Utilize high-efficiency respirable dust filters on the intake air supply into the cab. 

Filter efficiency performance specifications used in the field were 95% or greater on 

respirable-sized dusts. Laboratory experiments showed an order of magnitude 

increase in cab protection factors when using a 99%-efficient filter versus a 38%­

efficient filter on respirable-sized particles [NIOSH 2008]. 

(3) Use an efficient respirable dust recirculation filter. All the cab field 

demonstrations used recirculation filters that were 95% efficient, or greater, on 

respirable-sized dusts. Laboratory experiments showed an order of magnitude 

increase in cab protection factors when using an 85%- to 94.9%-efficient filter on 

respirable-sized dusts as compared to using no recirculation filter [NIOSH 2008]. 

Laboratory testing also showed that the time for interior cab concentration to decrease 

and reach stability after the cab door is closed was cut by more than half when using 

the recirculation filter. 

18 
(4) Minimize dust sources in the cab by using good housekeeping practices, such as 

periodically cleaning soiled cab floors, using a sweeping compound on the floor, or 

vacuuming dust from a cloth seat [NIOSH 2001b]. Also, relocate heaters that are 

mounted near the floor. These units have been shown to blow air across soiled cab 

floors and increase dust levels inside the cab [NIOSH 2001a]. 

(5) Keep doors closed during equipment operation. One study showed a ninefold 

increase in dust concentrations inside the cab when doors were frequently opened 

during the sampling period [Cecala et al. 2007]. 

•	 Use canopy air curtains. In many underground mines, operator enclosures cannot be 

used due to various mining or operational parameters. An alternative option for 

operators in open cabs and crusher compartments is a canopy air curtain, which filters 

and blows clean air over the operator’s breathing zone (Figure 3-4) [Goodman et al. 

2006; Goodman and Organiscak 2001]. In one case study, NIOSH research has 

shown that the primary dust source for load-haul-dump operators without enclosed 

cabs occurred while dumping at the crusher. Dumping accounted for 34% of the 

operator’s silica exposure, despite being the shortest segment of the haulage cycle 

[Chekan and Colinet 2002]. 

Figure 3-4. Canopy air curtain blows filtered air over worker. 

PRODUCTION SHOTS 

In underground stone and metal mines, production shots generate a considerable volume of dust 

and can be a distinct point source of respirable silica dust [Chekan et al. 2004]. Gravimetric filter 

samples collected 100 feet from the faces being shot showed that silica can account for over 10% 

of the respirable dust sample by weight, reaching concentrations as high as 0.1 mg/m3

 [Chekan 

and Colinet 2002]. In large mine openings, low air velocities (< 25 fpm) are common because of 

the large open-space volume and the extremely low airflow resistance [Krog and Grau 2006]. As 

a result, airflow in the entries can be stratified, or the direction of airflow can be readily affected 

by the movement of mine equipment. The respirable dust that becomes airborne after the 

production shot will remain entrained in the air and circulate with the general airflow patterns in 

the mine. Typically, several faces are shot at the same time, usually during an off-shift with no 

personnel in the mine. However, if adequate ventilation is not present or air recirculation is 

occurring, significant levels of silica dust can remain at the mine face and in the general mine 

atmosphere when workers return to begin the production cycle. 

19 
To effectively remove and reduce the retention time of silica dust after production shots, three 

key design parameters should be included in the mine ventilation plan: 

•	 A main mine fan used to establish air circuits on a mine-wide scale. These fans 

include axial vane fans, jet fans, and more recently low-pressure, high-volume 

propeller fans. Depending on mine size, minimum air volumes of 250,000 cfm are 

required to adequately ventilate and maintain air velocities necessary to remove dust. 

Main mine fans should be mounted at the bulkhead and operated in the exhaust mode 

[Krog and Grau 2006; Grau et al. 2002]. 

•	 Permanent or brattice stoppings installed in key locations throughout the mine 

to more efficiently direct and control the airflow. Incorporating a stopping line into 

the ventilation plan using a combination of long pillars, permanent stoppings 

(metal/block) or temporary stoppings (brattice/curtain) has been shown to 

significantly improve airflow in main entry developments by providing a directional 

flow of air which did not exist before the systems were installed [Grau et al. 2006; 

Timko and Thimons 1987]. Studies showed that stopping lines reduced the retention 

time of dust generated by production shots and decreased the length of time for the 

dust to travel from the shot location to the main mine fans [Chekan et al. 2004]. 

Figure 3-5 shows an example of shot dust exiting the mine as recorded by a real-time 

dust monitor. 

20 

 

 

 

 

 

 

 

9.0 

pDR CONCENTRATION, mg/m3 

8.0 

7.0 

6.0 

5.0 

4.0 

3.0 

2.0 

1.0 

0.0 

2 

Shot Time 

@ 3.0 hrs

 3 

Arrival Time 

@ 3.5 hrs 

30 Minutes 

5 

Peak Time 

@ 5.0 hrs 

120 Minutes

 7 

End Time 

@ 9.0 hrs 

360 Minutes 

1

 Baseline 

Concentration 

4 

Peak Concentration 

6 

Average Concentration 

9 

Return to Baseline 

Example: pDR located 

 610 m (2000 ft) from shot. 

Velocity, 

m/s 

 Arrival 0.340 

 Peak 0.085

 End 0.028 

Average 0.151 

 (29.8 fpm) 

8 

Duration of Dust Cloud: 

End Time - Arrival Time = 330 minutes 

0.5

1

1.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

10.010.511.011.512.012.513.013.514.014.515.0 

TIME, hrs 

Figure 3-5. Movement of dust after production shot as recorded by real-time dust sampler. 
•	 Booster fans to improve local ventilation. Booster fans, either axial vane or 

propeller, should be in operation near production shot locations to move dust to the 

primary ventilation circuit where the main mine fan can remove the dust from the 

mine [Chekan et al. 2004]. Permanent installation of electric axial vane fans should 

be located so that they blow through the fresh air stream and assist the main mine fan 

[Krog and Grau 2006]. Because of their mobility, diesel-powered propeller fans can 

be positioned closer to the shot location and oriented to produce turbulence to dilute 

and transport dust to the primary ventilation circuit [Chekan et al. 2006]. Figure 3-6 

shows an axial vane and a diesel-powered propeller fan used to assist ventilation and 

remove dust near production faces. 

Figure 3-6. Axial vane fan (left) versus propeller fan (right). 

MUCKING OPERATIONS 

Hard rock mining requires drilling and shooting of faces to produce a “muck” which is loaded 

and hauled using different types of production vehicles depending on commodity and mining 

type. Production equipment is usually diesel-powered, and vehicle cabs may be either enclosed 

or open depending on commodity. For instance, limestone and granite mines generally use the 

room-and-pillar mining method, with entry widths ranging from 30 to 60 feet and entry heights 

on development ranging from 20 to 45 feet. Production equipment includes large front-end 

loaders and 50- to 100-ton capacity trucks with enclosed cabs. 

Gold and other metal operations may use sublevel caving, long-hole open stoping, or cut-and-fill 

methods with entries ranging from 15 to 20 feet wide and 12 to15 feet high. Open cab load-hauldump vehicles and muckers are commonly used in the above operations. Local ventilation and 

water application to the muck pile are the primary means of dust control during the loading and 

hauling cycle. 

In mucking operations, several dust control methods should be considered to lower airborne 

levels of silica dust: 

• Establish an air circuit and keep fans as close to the loading area as possible. 

Dead-end entries and stopes are difficult to ventilate and they create conditions where 

exposure to silica dust is most prevalent. In stone mines, booster fans located in key 

locations are commonly used to improve local ventilation and provide a more direct 

and controlled volume of airflow. Using a combination of booster fans in both the 

blowing and exhaust mode will provide both turbulent air to dilute dust and an air 

21 
circuit to sweep the face and remove airborne particulate. In metal operations, 

blowing and exhaust systems using ventilation tubing directed into the dead-entry are 

more applicable. A blowing system delivering 10,000 cfm and kept within 100 feet of 

the face is required to provide adequate dilution. For exhaust systems to effectively 

transport dust-laden air into return entries, the tubing needs to be kept within 10 feet 

of the dust source for adequate dust capture [NIOSH 2003b]. Figure 3-7 shows a 

typical fan set-up for ventilating a dead-end entry in a limestone mine. 

Figure 3-7. Fans positioned to ventilate dead-end entries. 

•	 Keep muck wet when loading. The amount of water applied to the muck pile will 

differ between commodities, depending on the acceptable amount of moisture 

allowed during processing. Keeping the muck wet to reduce airborne dust levels 

while loading is a widely accepted practice, and studies have shown it also reduces 

silica dust levels in gold mining [Chekan 2002]. Silica generation in stopes that had a 

wet muck was 28% less than that produced by a dry muck. Silica exposures for loadhaul-dump (LHD) machine operators were impacted during the loading and dumping 

activities with wet muck, with 32% and 35% dust reductions, respectively. 

•	 Control haul road dust. The most common method of controlling haul road dust is 

surface wetting with water, but other dust control methods include adding 

hygroscopic salts, surfactants (commonly referred to as wetting agents), soil cements, 

bitumens, or films (polymers) to the road surface [Organiscak and Reed 2004]. Haul 

road wetting with water trucks in stone mines has been demonstrated to be very 

effective when continual wetting is practiced. Wetting is primarily conducted on the 

main tram roads; however, other trucks and mine equipment using secondary tram 

roads can raise the silica levels in the mine atmosphere and have the potential for 

exposing other mine workers conducting tasks unrelated to the production cycle. 

22 
DRILLING 

Wet drilling has been shown to be effective in controlling dust and is commonly found on face 

drills or jumbo drills. Regular maintenance, as recommended by the drill manufacturers, should 

be completed to ensure proper operation and maximized protection of these systems. 

Dry hole drills, where water is not used to suppress dust, are commonly used on downhole drills 

in preparation for shooting benches. Silica dust is generated by compressed air (bailing airflow) 

flushing the drill cuttings from the hole. Dry dust collection systems, incorporated into the 

drilling machine by the original equipment manufacturers, tend to be the most common type of 

dust control. Ninety percent of dust emissions with this type of system are attributed to drill deck 

shroud leakage, drill stem bushing leakage, and dust collector dump discharge. 

•	 Minimize silica dust generation and reduce levels in the ambient mine air by 

using the following recommended drill operating parameters: (a) maintain a tight 

drill deck shroud enclosure with the ground, (b) maintain a collector-to-bailing 

airflow ratio of at least 3:1, (c) install a shroud on the collector dump discharge that 

extends close to the ground, and (d) maintain the dust collector as specified by the 

manufacturer [NIOSH 1998, 2005; Reed et al. 2004]. 

•	 Use booster fans to improve local ventilation. Booster fans should be used to 

improve local ventilation and remove dust from the drill site when drilling benches in 

areas where ventilation provided by the main mine fan is not adequately diluting and 

transporting dust. Studies have shown that diesel-powered propeller fans, because of 

their mobility and entry coverage, have favorable ventilation characteristics for this 

application [Chekan et al. 2006; NIOSH 2003a]. 

To protect drill operators from dust that escapes the controls discussed above, enclosed cabs 

should be used on drills and should be properly equipped with an upgraded filtration and 

pressurization system. 

REFERENCES 

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 Occup Environ Hyg 2(1):51– 

63. 

Cecala AB, Organiscak JA, Zimmer JA, Moredock D, Hillis M [2007]. Opening door on drill 

cab during non-drilling can significantly increase operator’s dust exposure. Rock Prod J 

110(10):29–32. 

Chekan GJ, Colinet JF [2002]. Silica dust sources in underground limestone mines. In: 

Proceedings of the Thirty-Third Annual Institute on Mining Health, Safety and Research. 

Blacksburg, VA: Virginia Polytechnic Institute and State University, Department of Mining and 

Minerals Engineering, pp. 55–70. 

Chekan GJ, Colinet JF, Grau RH III [2003]. Silica dust sources in underground metal/nonmetal 

mines. Trans Soc Min Met Explor 1(312):187–193. 

23 
Chekan GJ, Colinet JF, Grau RH III [2004]. Evaluating ventilating air movement in underground 

limestone mines by monitoring respirable dust generated from production shots. In: Proceedings 

of the 10th U.S./North American Mine Ventilation Symposium, Anchorage, AK, May 16–19, pp. 

221–232. 

Chekan GJ, Colinet JF, Grau RH III [2006]. Impact of fan type for reducing respirable dust in an 

underground limestone crushing facility. In: Proceedings of the 11th North American/Ninth U.S. 

Ventilation Symposium, University Park, PA, June 5–7, pp. 203–210. 

Goodman GVR, Organiscak JA [2001]. Laboratory evaluation of a canopy air curtain for 

controlling occupational exposures of roof bolters. In: Proceedings of the 7th International Mine 

Ventilation Congress, Krakow, Poland. 

Goodman GVR, Beck TW, Pollock DE, Colinet JF [2006]. Emerging technologies control 

respirable dust exposures for continuous miner and roof bolter personnel. In: Proceedings of the 

11th North American/Ninth U.S. Ventilation Symposium, University Park, PA, June 5–7. 

Grau RH III, Mucho TP, Robertson SB, Smith AC, Garcia F [2002]. Practical techniques to 

improve the air quality in underground stone mines. In: Proceedings of the 9th North 

American/U.S. Ventilation Symposium, Kingston, Ontario, June 8–12, pp.123–129. 

Grau RH III, Krog RB, Robertson SB [2006]. Maximizing the ventilation of large-opening 

mines. In: Proceedings of the 11th North American/Ninth U.S. Ventilation Symposium, 

University Park, PA, June 5–7, pp.53–59. 

Kreiss K, Zhen B [1996]. Risk of silicosis in a Colorado mining community. Am J Ind Med 

30:529–539. 

Krog RB, Grau III RH [2006]. Correct fan selection for large opening mines: axial, vane or 

propeller fans—which to choose. In: Proceedings of the 11th North American/Ninth U.S. 

Ventilation Symposium, University Park, PA, June 5–7, pp.535–542. 

MSHA [2008]. MSHA Standardized Information System, Arlington VA: U.S. Department of 

Labor, Mine Safety and Health Administration. 

NIOSH [1998]. New shroud design controls silica dust from surface mine and construction blast 

hole drills. By Page SJ, Organiscak JA, Flesch JP, Hagedorn RT. Cincinnati, OH: U.S. 

Department of Health and Human Services, Centers for Disease Control, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication No. 98-150. 

NIOSH [2001a]. Technology News 486: Floor heaters can increase operator’s dust exposure in 

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

Prevention, National Institute for Occupational Safety and Health. 

NIOSH [2001b]. Technology News 487: Sweeping compound application reduces dust from 

soiled floors within enclosed operator cabs. U.S. Department of Health and Human Services, 

24 
Centers for Disease Control and Prevention, National Institute for Occupational Safety and 

Health. 

NIOSH [2003a]. Dust control in stone mines. By Kissell FN, Chekan GJ. In: Handbook for dust 

control in mining. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for 

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

Publication No. 2003-147, pp. 57–72. 

NIOSH [2003b]. Underground hard-rock dust control. By Kissell FN, Stachulak JS. In: 

Handbook for dust control in mining. Cincinnati, OH: U.S. Department of Health and Human 

Services, Centers for Disease Control, National Institute for Occupational Safety and Health, 

DHHS (NIOSH) Publication No. 2003-147, pp. 83–96. 

NIOSH [2005]. Technology News 512: Improve drill dust collector capture through better 

shroud and inlet configurations. By Organiscak JA, Page SJ. 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. 2006–108. 

NIOSH [2008]. Key design factors of enclosed cab dust filtration systems. By Organiscak JA, 

Cecala AB. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for 

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

Publication No. 2009-103. 

Organiscak JA, Reed WR [2004]. Characteristics of fugitive dust generated from unpaved mine 

haulage roads. Int J Surf Min Reclam Environ 18(4):236–252. 

Reed WR, Organiscak JA, Page SJ [2004]. New approach controls dust at the collector dump 

point. Eng Min J July:29–31. 

Steenland K, Brown D [1995]. Silicosis among gold miners: exposure-response analyses and risk 

assessment. Am J Pub Health 85(10):1372–1377. 

Timko RJ, Thimons ED [1987]. Damage resistant brattice stoppings in mine with large entries. 

Eng Min J 188(5):34–36. 

USBM [1992]. Crystalline silica primer. Washington, DC: U.S. Department of the Interior, U.S. 

Bureau of Mines. 

25 
 

CHAPTER 4. CONTROLLING RESPIRABLE SILICA DUST IN MINERAL 

PROCESSING OPERATIONS 

By Andrew B. Cecala 

The U.S. surface metal/nonmetal mining industry is composed of a wide range of different 

mineral types and processes. In 2007, there were 12,270 mines (256 underground, 12,014 

surface) in the metal, nonmetal, stone, and sand and gravel industries, employing 149,177 miners 

(17,862 underground, 131,315 surface) [MSHA 2009]. A significant portion of these miners 

were working in mineral processing operations as the ore removed from the pit or quarry was 

processed. Throughout the mineral processing cycle, mined ore goes through a number of 

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

marketable commodity. Because these operations are highly mechanized, they are able to process 

high tonnages of ore. This in turn can generate large quantities of dust, often containing elevated 

levels of respirable crystalline silica, which can be liberated into the work environment. 

The Mine Safety and Health Administration’s (MSHA) respirable quartz personal dust samples 

from 2003–2007 verify that there were a significant number of over-exposures in many job 

classifications within mineral processing operations. Table 4-1 shows the job classification and 

the percentage of samples taken that exceeded the permissible exposure limit (PEL) over this 

time period. 

Table 4-1. Percent of samples exceeding PEL for select occupations. 

Job Classification Samples exceeding PEL (percent) 

Underground Crusher Operator 36
 

Stone Polisher/Cutter 33
 

Bagging Operator 27
 

Belt Cleaner 25 

Hammer Mill Operator 23 

Kiln/Dryer Operator 19 

Cleanup Man 19
 

Underground Frontend Loader Operator 18
 

Laborer, Bullgang 16
 

Drill Operator 11 

The purpose of this chapter is to summarize current state-of-the-art control technology available 

for lowering respirable crystalline silica dust at mineral processing operations. This chapter 

addresses dust control techniques from the time the ore reaches the primary crusher until it is 

packaged into some type of shipping container to be delivered to the customer. By applying the 

current state-of-the-art engineering controls, methods, and techniques to lower respirable dust 

levels, managers, engineers, and health specialists can lower respirable dust exposures to miners 

and work toward the ultimate goal of eliminating silicosis and other chronic respiratory diseases 

from miners in this industry. 

The following is a listing of sections presented in this chapter: 

26 
•	 Primary Dumping 

•	 Crushing and Grinding 

•	 Transfer Points 

•	 Conveying 

•	 Wet Suppression (Water Sprays) 

•	 Local Exhaust Ventilation (LEV) Systems 

•	 Low Velocity Transport System 

•	 Total Mill Ventilation System 

•	 Operator Booths, Control Rooms, Enclosed Cabs 

•	 Screening 

•	 Packaging/Bagging Product for Shipment 

•	 Clothes Cleaning System 

•	 Background Issues 

PRIMARY DUMPING 

Ore is normally loaded into haul trucks from the pit or quarry and driven to the primary crusher 

location. This ore is then either dumped directly into the primary hopper, which feeds the 

primary crusher, or it is dumped in a stockpile. If it is stockpiled, a front-end loader then takes 

the ore product and dumps it into the primary hopper. In either case, a dust cloud is created 

during this dumping process. There are two dust sources that must be addressed during this 

primary dumping process—billowing and rollback. 

Billowing. The first dust source is from dust that billows out from the hopper as the large volume 

of product is dumped from the truck or front-end loader in a very short time period. During the 

dumping process, ore is grinding on ore and creating dust. In addition, there is already a 

substantial amount of dust contained within the ore from blasting and haulage to the primary 

dump. As the air in the hopper is quickly displaced from the incoming ore during dumping, it 

entrains (carries along in the air current) these dust particles and billows out from the hopper. 

Rollback. The second dust source is from rollback under the dumping mechanism. This rollback 

occurs either under the bed of the haul truck or the bucket of the front-end loader. 

For a dust control system to be effective at the primary dump location, the dust generated from 

both billowing and rollback must be controlled. There are three methods to control the billowing 

of dust from the hopper (suppress, enclose, and filter) and one method to control rollback (a tirestop water spray system). 

Controlling Dust Billowing From Enclosure 

•	 Suppress. Normally, the first dust control technique attempted for primary dump locations is 

water spray application. The general rule of thumb is to add enough moisture to the product 

where the weight of water added is equivalent to 1% of the processed ore [Quilliam 1974]. 

From this point, the percentage can be adjusted based upon the improvement gained from 

27 
additional moisture versus any consequences from adding too much water. The amount of 

moisture that can be added at primary dump locations is normally not as sensitive as in later 

stages of the mineral processing cycle, thus higher rates or percentages can usually be tried. 

One important feature with a primary dump application is to only activate the water sprays 

during the actual dump cycle through the use of a photo cell or a mechanical switching 

device. Since the actual dump cycle is a very small portion of the total time, it is not 

appropriate to continually use water sprays during the idle time because this can cause 

clogging problems, as well as, wasting water. A delay timer should also be used in this 

application so that the sprays continue to operate and suppress dust for a short time period 

after the dump vehicle has moved away. For specifics about water spray types, refer to the 

“Wet Suppression” section of this chapter. 

•	 Enclose. Enclosures for primary dump application normally require a custom design and are 

usually dependant on the type and size of dump vehicles being used. In some cases, walls can 

be constructed around the primary dump location to form an enclosure. The walls can be 

either stationary or removable, based in particular on whether maintenance work is 

necessary. In some cases for a removable enclosure, a breathable tarp fabric material, similar 

to the material used on over-the-road haul trucks, can be laid over to seal the top of the 

enclosure. 

Another technique gaining in popularity is the use of staging curtains (Figure 4-1). Staging 

curtains, also called stilling curtains, are curtains of varying lengths which physically prevent 

the natural tendency for dust to billow out of the primary dump as a large volume of product 

is dumped [Weakly 2000]. By minimizing the billowing airflow effect, the amount of dust 

released from the primary hopper is reduced. 

If staging curtains are not used, another option is to enclose the front of the enclosure using 

panels of flexible plastic stripping. This plastic stripping employs an overlapping sequence 

which provides for a very effective seal (Figure 4-2). One noteworthy benefit to the plastic 

stripping is that the panels are not damaged if contacted by the bucket of the front-end loader 

or the bed of the haul truck during dumping. 

•	 Filter. When using an enclosure, it is also possible to incorporate a local exhaust ventilation 

(LEV) system to filter the dust-laden air from the hopper area. This would be most applicable 

when the primary dump is at a location where the dust could enter an adjoining structure or 

impact outside miners. The enclosure helps to contain the dust cloud that billows up from the 

hopper during the dumping process, but the dust remains airborne unless it is suppressed or 

removed. An LEV can be an effective technique incorporated to remove and filter this dust if 

it is properly designed and sized to the hopper. Since hoppers are usually large, a significant 

amount of airflow is typically required to create a negative pressure necessary to contain the 

dust cloud [MSA 1978]. Because of the large volume that must be ventilated in this 

application, using an LEV system would be a much more expensive control technique than 

the wet suppression technique. 

28 
Figure 4-1. Staging curtains reduce dust billowing during dumping. 

Figure 4-2. Plastic stripping holds dust inside enclosure allowing water sprays to knock down dust. 

Controlling Rollback Dust 

A tire-stop water spray system is recommended for reducing the dust source that causes liberated 

dust to rollback under the dumping mechanism. A tire stop or Jersey barrier should be positioned 

29 
 

at the most forward point of dumping for the primary hopper. To the back side of this tire stop, a 

water spray manifold should be attached to knock down and force the dust, which would 

otherwise roll back under the dumping mechanism, to remain in the hopper. Additionally, a 

shield should be placed over this water spray manifold to protect it from damage from falling ore 

(Figure 4-3). Finally, a system should also be incorporated that allows the water sprays to only 

be activated during the actual dumping process. 

Figure 4-3. Tire-stop water spray system reduces rollback under dumping mechanism. 

CRUSHING AND GRINDING 

Crushing and grinding at mineral processing operations include a wide range of different types of 

equipment and processes. On the crushing side, primary crushers are typically jaw crushers, but 

may sometimes include gyratory and/or cone crushers. These crushers use compressive forces to 

break the ore and do not normally generate large volumes of dust. Secondary crushers may 

include the gyratory and cone, as well as hammermill and impact crushers. Hammermill and 

impact crushers use a rotating device (hammers) to thrust the ore against the outer walls of the 

crusher with the intent to break the ore by impaction against the outer surface. Because the ore is 

impacted at high velocities to induce breakage, high dust generation and liberation rates can 

occur from these types of crushers. After ore is fed into the crusher, it remains in the unit and 

continues to be crushed until it reaches a size small enough to be discharged from the unit. 

Grinding and pulverizing the ore is performed later in the mineral process to reduce the product 

down to the smaller size ranges, normally measured in mesh sizes. Grinding mills are used to 

perform this process and are cylindrical, horizontal drums that rotate and have rods, balls, or 

pebbles inside to grind the ore down to the desired size ranges. 

The two primary dust emission points of all crushing and grinding units are at the feed and 

discharge points. Controlling this dust by properly designing chutes or transfer points with 

rubber seals between stationary and moving components, as well as enclosing this area, is critical 

to an effective dust control plan. 

30 
Dust control for the crushing and grinding processes is normally achieved by either wet 

suppression or LEV systems, or a combination of both. Spraying the ore with water sprays to 

coat the outer surface helps to prevent dust from becoming liberated. Applying the water to the 

ore before it enters the crushing or grinding unit is most effective. In addition, it has been shown 

that the water pressure at early stages of crushing should be kept below 60 psi to avoid 

pressurizing and forcing dust from the feed chute enclosures [NIOSH 2003]. The amount of 

moisture is not as critical during the early stages of the process but should be closely evaluated 

as the ore enters the later stages when the finer product sizing is taking place. In these cases, full 

and hollow cone sprays would normally be used to wet the ore and minimize dust liberation. 

(See the “Wet Suppression” section for more information on water spray systems.) 

When using an LEV system to capture and remove the dust from the crushing and grinding 

processes, a critical component to maintaining an effective system is determining the amount of 

air volume required to keep the process under negative pressure. As ore is fed into the crusher or 

grinder, it entrains air along with the product, creating a significant volume of air which must be 

exhausted to overcome the induction effect [MSA 1978; Yourt 1990]. The volume of exhaust air 

is also dependant on the effectiveness of sealing the crusher’s or grinder’s intake opening. By 

minimizing the area of the opening using belting and plastic stripping, the volume of exhaust air 

can be lowered while still maintaining an acceptable negative pressure necessary to contain the 

dust liberated during this transfer process. 

One final component that must be considered in all crushing and grinding processes is 

maintaining a proper seal on the device. If product is observed on the floor below the device or if 

visible dust is seen liberating from a unit, this indicates that a hole has been created or a seal has 

worn out, and maintenance needs to be performed to repair the problem. 

TRANSFER POINTS 

Transfer points are used to move ore from one process or one piece of equipment to another. 

Although this seems like a 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: 

•	 Transfer chutes should be sized to allow ore to flow without clogging or jamming. A 

general rule of thumb is that the chute depth should be at least three times the 

maximum lump size to avoid clogging [Martin Marietta Corp 1987]. 

•	 The dump point of the ore should be designed to impact on a sloping bottom or a 

rockbox. Rockboxes are designed to allow ore product to build up so that ore contacts 

ore during transfer, which reduces wear and abrasion of the chute. 

•	 Any abrupt changes in product direction or flow should be avoided. 

•	 Fall height of ore should be minimized whenever possible through the use of rock 

ladders, telescopic chutes, spiral chutes, and bin-lowering chutes. 

•	 A head enclosure should be used when transferring ore onto a conveyor. The head 

enclosure should be designed with strip curtains to minimize air induction into the 

enclosure and skirt boards to position the ore on the center of the belt. 

31 
•	 An LEV system should be used at transfer enclosures and chutes to capture and filter 

the dust from the air. These enclosures should be designed to have approximately a 

250-fpm intake velocity at any opening to eliminate dust leakage from the area. To 

accomplish this, plastic stripping and other types of sealing systems should be used to 

minimize openings and maximize intake velocity. One study also recommended that 

the exhaust port to the LEV system be located at least 6 feet away from the transfer 

dump point to minimize the possibility of entraining large particles [MAC 1980]. 

•	 The exit velocity from the enclosure or chute should be kept below 500 fpm to 

minimize the entrainment of large particles of ore [Yourt 1990]. 

CONVEYING 

At mineral processing operations, conveyors are the major component used to transfer ore from 

one process to another. A conveyor can generate significant quantities of respirable dust and be 

one of the greatest sources of dust emissions within an operation. There are four main areas of 

dust generation and liberation from conveyors: 

•	 When ore is dumped onto the belt. 

•	 As ore travels on the belt. 

•	 From the underside return idlers due to carryback on the belt. 

•	 When ore is dumped or transferred to another belt or process. 

One of the challenges with conveyors is the number of belts used 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 within a facility. Another challenge particular to conveyors is their 

ability to generate or liberate dust while operating, whether they are loaded with ore or empty. 

Controlling dust from conveyors requires constant vigilance by the maintenance staff to repair 

and replace worn and broken parts. There are a number of techniques to reduce dust liberation 

from conveyors, as follows: 

•	 Suppress. 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 amount of 

moisture applied should be varied and tested at each operation to determine the 

optimum quantity, but 1% moisture added to product ratio is a good starting point. A 

number of studies have indicated that wetting the return side of the conveyor belt also 

helps reduce dust liberation. This is effective because it reduces dust generation from 

the idlers as well as at the belt drives and pulleys. In many cases, water sprays located 

on the top (wetting the product) and the bottom (reducing dust from the idlers) at the 

same application point can be an effective strategy [Courtney 1983; Ford 1973]. 

These locations are also beneficial from an installation and cost standpoint. 

When considering nozzle type for these suppression systems, fan spray nozzles are 

normally the most common design because they minimize the volume of water added 

for the amount of coverage. For these types of applications, it is more advantageous 

to locate the water sprays at the beginning of the dust source (i.e., the dump or 

32 
transfer location), because as the water and ore continually mix together, the amount 

of wetted surface area of the ore increases, thus increasing the suppression potential 

and reduction in dust liberation. Using sprays at higher flow rates can sometimes 

create air turbulence, which makes it more difficult to contain/suppress the dust. 

Because of this, it is normally recommended to use more spray nozzles at lower flow 

rates and position them at locations closer to the ore [NIOSH 2003]. 

•	 Enclose. Enclosures are an effective dust control technique for many applications 

within mineral processing plants if they are correctly designed and installed, and this 

principle also applies to conveyors. Enclosures for conveyor and transfer points can 

be either full or partial type [Zimmer 2003]. One of the most common partial types of 

enclosures is with the use of skirting, which keeps the material on the belt, especially 

immediately after it exits a loading chute. 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. 

Enclosures at head and tail ends of the conveyor are a common practice because they 

are effective at controlling dust at these locations. 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. Dust curtains are another form of enclosure used to contain dust within a 

conveyor and are very cost effective to install. These curtains are normally installed at 

the head and/or tail ends of the conveyor. 

In many instances, a LEV is tied into the enclosure at these conveyor dump or 

transfer locations to capture generated dust. It has been shown that the takeoff port to 

the LEV system should be a least 6 feet from the dump point to minimize the pickup 

of oversized particles [MAC 1980]. The air velocity at this exhaust port should also 

be kept below 500 fpm to avoid the pickup of larger particles [Yourt 1990]. 

•	 Belt scraper. An effective method to reduce dust being liberated from conveyors is 

with belt scrapers. Although belt scrapers come in many different styles, types, and 

trade names sold by numerous commercial manufacturers, their function remains the 

same, which is to reduce the amount of carryback on the belt once the ore is 

discharged. Carryback is the material that sticks or clings to the conveyor belt after 

the material is discharged at the head pulley. As this material dries and passes over 

the return idlers, it falls from the belt and the respirable portion of this dust becomes 

airborne. The goal is to remove this carryback product before it is released into the air 

and becomes a source of contamination to the workers. When dust levels are high, a 

common practice is to use two or three belt scrapers at different locations in an effort 

to further reduce the amount of carryback material on the belt [Roberts et al. 1987]. 

•	 Belt wash. Some studies have shown that the oversized material is more easily 

removed through scraping but the smaller respirable-sized particles tend to remain 

adhered to the conveyor. When this occurs, a belt wash should be installed. A belt 

wash sprays the conveyor belt with water while simultaneously scraping it to remove 

33 
the product. In a number of published studies in this area, this technique has been 

shown to increase the cleaning effectiveness by approximately 14% [Planner 1990]. 

•	 Effective belt loading. Providing an effective belt loading area helps to reduce the 

amount of dust generated during belt loading. The first design goal of an effective 

system is to reduce belt sag and vibration during ore loading. To do this, some 

operations are using slider-bar cradles which stabilize this area [Stahura and Marti 

1995]. These cradles can be made to provide a shock-absorbing action to cushion the 

impact during belt loading. This goal can also be achieved with low-friction bars that 

provide a flat support area for loading, which minimizes belt sag. Effective belt 

loading should also include side shields which contain the fine-sized particles and 

help to minimize the liberation of dust. 

Figure 4-4 shows a combination of the various techniques discussed that help enclose and 

minimize dust generated during conveying. 

Figure 4-4. Techniques for reducing respirable dust liberation from conveyor belts and transfers. 

This includes the use of plastic stripping to minimize 

the opening size, a rockbox, belt skirting, and the dust collection pick-up point. In addition, wet 

suppression is also shown, which will be discussed in the next section of this report. 

WET SUPPRESSION 

Wet suppression systems are probably the oldest and most often used method of dust control at 

mineral processing operations. In the vast majority of cases for mineral processing operations, 

the wet suppression system used is a water spray system. Although the use of water sprays is a 

simple technique, there are a number of factors that should be evaluated to determine the most 

effective design for a particular application. There are two methods to control dust using water 

sprays at mineral processing operations: 

•	 Preventing dust from becoming liberated and airborne by directly spraying the ore. 

•	 Knocking airborne dust down by spraying the dust cloud and causing the particles to 

collide with water droplets and fall out of the air. 

34 
Most operations use a combination of both methods in the overall dust control plan. When 

considering the use of a wet suppression system, some general considerations and guidelines 

apply: 

•	 The effectiveness of water spray application is dependent on nozzle type, droplet size, 

spray pressure, spray pattern, spray angle, spray volume, spray droplet velocity, and 

spray droplet distribution. 

•	 Each ore type and application point is a unique situation and needs to be evaluated 

separately to achieve the optimal design. 

•	 Water evaporates and needs to be reapplied at various points throughout the process 

to remain effective. 

•	 Water freezes and its use is limited during certain times of the year and in certain 

climates. 

•	 Wet suppression cannot be used with all ores, especially those that have higher 

concentrations of clay or shale. These minerals tend to cause screens to bind and 

chutes to clog, even at low moisture percentages. 

•	 Over application in the volume of moisture is a problem in all operations and can 

impact the equipment as well as the total process. In most cases, a well-designed 

suppression system will not exceed 0.5% moisture application, which is roughly 

equivalent to one gallon per ton of ore. 

•	 The suppression system should be automated so that sprays are only used during 

times of production when ore is actually being processed. For dust knockdown, a 

delay timer may be incorporated into some applications to allow the suppression 

system to operate for a short time period after a dust-producing event. 

When considering sprays, one of the primary aspects is the droplet size. When wetting the ore to 

keep dust from becoming airborne, droplet sizes above 100 microns should be used. In contrast, 

when the goal is to knock down existing dust in the air, the water droplets should be in size 

ranges similar to the dust particles. The intent is to have the droplets collide and attach 

themselves to the dust particles, causing them to fall from the air. In these cases, droplets in the 

range of 10 to 50 microns have been shown to be most effective [Bartell and Jett 2005]. 

Uniformity of wetting is also a very 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. 

The following is a list of the spray nozzles used in the mineral processing industries and some of 

their defining characteristics (Figure 4-5): 

•	 Full-cone. Full-cone nozzles employ a solid cone-shaped spray pattern with a round 

impact area that provides high velocity over a distance. They produce medium to 

large droplets 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. 

35 
Figure 4-5. Spray nozzles commonly used in mineral processing operations. 

•	 Hollow-cone. These nozzles use a circular outer ring spray pattern (hollow cone) in 

three different designs: whirlchamber, deflector, and spiral sprays. They produce 

small to medium droplet sizes. 

•	 Flat-fan. The flat-fan pattern is produced in three different designs: tapered, even, 

and deflected type sprays. Flat-fan nozzles produce small to medium droplet sizes 

over a wide range of flows and spray angles and are normally located in narrow, 

enclosed spaces. 

•	 Air atomizing. Different spray patterns are available in two different designs: 

hydraulic and air-assisted. Hydraulic nozzles produce fine-mist droplet sizes and have 

low-volume capacities. Air-assisted nozzles produce the smallest droplets of all 

sprays but are the most expensive because they require compressed air. To be 

effective, both types of air-atomizing sprays need to be located close to the dust 

source. 

36 
 

Figure 4-6 shows the airborne capture performance of the different spray nozzles performing at 

different operating pressures.

Figure 4-6. Airborne dust capture performance of four types of spray nozzles. 

 As shown, atomizing sprays are the most efficient for dust 

knockdown, 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. They are also very beneficial because they 

have large orifice sizes and are less likely to clog as compared to the other nozzles. 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. 

Surfactants, commonly called wetting agents, are sometimes used in wet suppression 

applications because they lower the surface tension of the water solution. This in turn increases 

the solution’s ability to produce finer and a greater number of water droplets, while increasing 

the rate at which the droplets are able to wet dust particles. They use less moisture to produce the 

same effects as a straight water application. However, they are not very often used in the 

metal/nonmetal mining industry based upon the following limitations: 

• Surfactants are more expensive than a typical water application. 

37 
•	 Surfactants may alter the properties of the mineral or material being processed. 

•	 Surfactants can damage some equipment such as conveyor belts and seals. 

• Surfactant systems require more upkeep and maintenance than typical water systems. 

A fundamental consideration with any suppression system is the cleanliness of the water. If spray 

nozzles become plugged with sediment or debris, they render the suppression system ineffective. 

Since the water to be used for suppression systems at most mineral processing operations would 

be drawn from a settling pond, the issue of water purity is a legitimate concern. It is 

recommended to use some type of water filtering system to eliminate the possibility of sediment 

and debris clogging the water sprays. Most spraying companies offer filtering systems for this 

purpose. A hydrocyclone with a built-in accumulator flush should be considered in these 

applications [USBM 1976, 1981]. 

LOCAL EXHAUST VENTILATION (LEV) SYSTEMS 

The most common dust control technique at mineral processing plants is local exhaust 

ventilation (LEV) systems. These systems capture dust at the various processes such as crushing, 

milling, screening, drying, bagging, and loading and then transport this dust via ductwork to a 

dust collection filtering device. LEV systems use a negative pressure exhaust ventilation 

technique in an attempt to capture the dust before it escapes from the process. By capturing the 

dust at the source, it is prevented from becoming liberated into the processing plant and exposing 

workers. This technique is most effective when a capture device (e.g., enclosure, hood, or chute) 

is incorporated at the dust source to maximize the collection potential. LEVs have a number of 

advantages: 

•	 Ability to capture and eliminate very fine particles, which are difficult to control 

using wet suppression techniques. 

•	 Providing the option of reintroducing the captured material back into the production 

process or discarding the material so it is not a detriment later in the process. 

•	 Effectiveness in cold weather conditions because of not being greatly impacted by 

low temperatures as with the wet suppression technique. 

•	 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). 

There are many different types of dust collection systems available for use at mineral processing 

operations, including electrostatic precipitators, fabric collectors, wet collectors, and dry 

centrifugal collections; however, an LEV system using a canister-type collector is often the best 

choice for the majority of the minerals processing industry. The American Conference of 

Governmental Industrial Hygienists (ACGIH) produces a manual entitled Industrial Ventilation 

Handbook—A Manual of Recommended Practice for Design, currently in the 26th edition. This 

manual provides extensive and authoritative information for designing an effective LEV system. 

It is not the intent of this chapter to duplicate this information, but to briefly describe a canistertype LEV system. For more information on various dust collector types and design 

considerations, please refer to the ACGIH Industrial Ventilation Handbook [ACGIH 2007]. 

38 
The canister-type collector is the most recent generation of fabric-type collectors. Currently, all 

other fabric collectors use filter media made of felt-type collector bags. Instead, the canister-type 

collector uses a rigid cartridge that houses a pleated filter media (Figure 4-7).

Figure 4-7. Canister-type dust collector system. 

 The following are 

some advantages of using this system: 

•	 Various types of filter cartridges are available to meet a wide range of ore types and 

filtering needs. 

•	 The filter canisters are self-cleaned in the collector unit when the internal pressure 

reaches a set value. This self-cleaning is performed using the conventional pulse-jet 

cleaning action. 

•	 The pleated-type canister filters increase the surface area of the filter media and 

provide for a greater cycle time before cleaning, when compared to the typical bagtype filters. 

•	 Workers are exposed to very low respirable dust concentrations when replacing filter 

canisters. The normal replacement procedure is to remove the new filter canister from 

the cardboard shipping package, then remove the used canister from the collector unit 

and place it in the cardboard shipping container to be discarded. The cardboard 

container lid is closed and taped shut to minimize the potential for any dust leakage. 

LOW-VELOCITY TRANSPORT SYSTEMS 

As stated in the previous section, LEV systems are the most common dust control technique used 

at mineral processing plants to capture and filter dust from the air. With these systems, once the 

39 
dust is captured at the source, it is carried through ductwork to the filtering unit. This section 

deals with a novel method of transporting dust from the collection point to the filtering unit. 

For many years, the only recommended practice was to transport dust particles in a high-velocity 

design in which the air velocities within the duct were kept in the 3,500 to 4,000 feet per minute 

range. The intent with a high-velocity system is to prevent the dust particles from settling out 

from the airstream and clogging the duct. However, since the air velocities are so high, duct wear 

is a significant problem, especially at any elbows or transitions where holes are constantly 

developing. As holes develop, they compromise the system’s performance and cause dust to leak 

from the ductwork and into the work environment. 

A low-velocity system has been developed [Bresee 2008 ]where the transport velocities within 

the ductwork are held below 1,800 feet per minute. This section will present the advantages to 

the low-velocity design, which may be advantageous over the high-velocity design for some 

applications. This is especially true when dealing with an ore that contains silica due to the 

abrasiveness of the mineral. It must be noted that low velocity does not imply low airflow; in 

fact, hood capture velocities and negative pressure at the capture point are identical in both lowand high-velocity transport systems. 

There are significant differences between high- and low-velocity system designs. In highvelocity systems, the ductwork is mainly oriented in either the horizontal or vertical format, and 

at the high-transport velocities, large-size dust particles are moved through the ductwork to the 

dust collector unit. This is not the case in a low-velocity system in which the basic principle is to 

only move respirable-sized particles. The low-velocity system is based upon a “sawtooth design” 

in which the up-slopes are positioned at a 45 degree angle and all down-slopes are oriented at 30 

degrees (Figure 4-8). This allows the larger particles to fall out from the airstream, slide down 

the slope, and be recycled back into the process at various points. Advantages to the low-velocity 

transport system include the following: 

•	 Wear and maintenance. Since the system is only moving particles in the respirable 

range at relatively low velocities, there is reduced abrasion to the duct, which allows 

for shorter radius and elbows to be used. 

•	 Energy costs. Friction and pressure losses in this system are significantly less than in 

the conventional high-velocity system, making overall power requirements 

significantly less. 

•	 Availability and reliability. The low-velocity system maintains a more consistent 

balance, since changes in airflow or pressure drops in ducts do not have significant 

impacts in the overall system. Since the system stays in balance, it does not lose its 

transport effectiveness as with the high-velocity transport system. 

•	 Product recovery. Product losses are significantly less since larger particles are 

captured by the LEV system and ultimately recycled back into the process at various 

points in the system. 

Although low-velocity transport systems have higher initial costs, the break-even point usually 

occurs sometime around the three-year time frame. The low-velocity transport system has been 

40 
tested and utilized in operating plants, with a projected life between 15 and 25 years. This 

performance time frame accounts for significant cost savings over the life of the system. 

Figure 4-8. Sawtooth design for low velocity transport system. 

TOTAL MILL VENTILATION SYSTEMS 

Although LEV is the most common dust control technique used at mineral processing operations 

to capture and filter dust from the major dust sources, it is not possible to capture and control all 

the minor dust sources within a mineral processing operation using this same technique. As these 

minor sources continually generate and liberate dust over a work shift, they can have a 

cumulative effect and cause respirable dust concentrations to gradually increase to unacceptable 

levels. 

The best way to address and deal with these minor dust sources is to install a total mill 

ventilation system (TMVS). A TMVS provides a general purging of the plant air to minimize 

dust throughout the entire mill building, thus lowering respirable dust levels for all workers 

within the structure [Cecala and Mucha 1991; Cecala et al. 1995]. The TMVS lowers respirable 

dust levels by using 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. The size and number of exhaust fans is based upon the initial respirable dust 

concentration and the total volume of the structure. All the processing equipment within a mill 

generates heat and produces a thermodynamic chimney effect that works in conjunction with the 

TMVS. To be effective, a TMVS must meet three design criteria: 

•	 Supply of clean make-up air. The TMVS must supply clean make-up air at the 

plant’s base. Any outside dust sources, such as bulk loading near an air inlet location, 

can cause outside dust-laden air to be drawn into the plant and make the problem 

worse. It is critical to ensure that the make-up air is clean by controlling the air’s 

entry location through inlets such as wall louvers, plants doors, or other openings. 

•	 Effective upward airflow pattern. The TMVS should provide an effective upward 

airflow pattern that ventilates the entire plant and also sweeps through dust sources, 

41 
work areas, and dust-laden areas. To create the most effective airflow pattern for 

purging the entire mill, the exhaust fans must be properly located on the roof or high 

outer walls, and the make-up air inlets must be located at the base of the structure. 

•	 Competent mill shell. Because a TMVS uses exhaust fans to draw make-up air 

through the points of least resistance, the plant’s outer shell needs to be intact and 

competent. Any unwanted openings, especially near the exhaust fans, can shortcircuit the ventilation system’s designed airflow pattern and reduce the effectiveness. 

Figure 4-9 shows the concept of the TMVS from an outside perspective.

Figure 4-9. Design concept of TMVS showing clean-air intakes and dust-laden air exhausts. 

42 

 A normal range of 

airflow for a TMVS would be 10–35 air changes per hour (ACPH). During the development of 

this technique, two different field studies were performed to document the effectiveness of a 

TMVS [USBM 1993]. In the first study, with a 10-ACPH ventilation system, a 40% reduction in 

respirable dust concentration was achieved throughout the entire mill building. In a second field 

evaluation, two ventilation volumes were tested—17 ACPH and 34 ACPH—and average 

respirable dust reductions were recorded at 47% and 74%, respectively. 

The TMVS has proven to be a cost-effective system to lower respirable dust concentrations 

throughout an entire mineral processing structure. Not only is the initial cost of this technique 

inexpensive when compared to the other engineering controls, the operation and maintenance are 

also minimal. To further reduce their costs, operations can potentially install all the components 

for this system with in-house personnel. Therefore, the TMVS can be a very cost-effective 

system to lower respirable dust levels at mineral processing operations [Cecala 1998; Cecala and 

Thimons 1997; Cecala, et al. 1996]. 
OPERATOR BOOTHS, CONTROL ROOMS, ENCLOSED CABS 

Frequently at mineral processing plants, workers will be located in an operator’s booth, control 

room, or enclosed cab to give them a safe work area and to isolate them from dust sources. If 

these areas are properly designed, they can provide good air quality to the worker. On the other 

hand, if these enclosed areas are not properly designed and maintained, the air quality can 

deteriorate to unacceptable and unsafe levels. 

The most effective technique for reducing operators’ exposure to airborne dust in booths/control 

rooms/enclosed cabs at mineral processing operations is with filtration and pressurization 

systems. The most effective filtration and pressurization systems have the heating and air 

conditioning (HVAC) components tied in as an integral part of the system. A substantial amount 

of research has been performed over the past few years evaluating the air quality in enclosed 

cabs of surface mining equipment. This research is directly applicable to operator booths and 

control room dust control systems. As the research demonstrates, enclosed cabs on mobile 

equipment are harder to control and maintain since the moving of the equipment constantly 

stresses and compromises the competency of the enclosure. NIOSH recently conducted a 

controlled laboratory study to evaluate the key factors necessary for achieving an effective 

enclosure filtration and pressurization system [NIOSH 2008]. Through this laboratory and 

numerous field studies, the following items were identified as key components to an effective 

system: 

•	 Ensure booth/control room/cab integrity. Effective protection factors were realized 

in various field studies when positive pressures between 0.01 and 0.40 inches of 

water gauge were achieved within the booth/cab because of good enclosure integrity. 

These pressures correspond to wind velocity equivalents of 4.5 to 29 miles per hour 

and prevent against wind forcing dust laden air into the enclosure. 

•	 Use high-efficiency filters on intake air. Only intake filters with an efficiency of 

95% or greater were used during field studies [Cecala et al. 2004, 2005a; Chekan and 

Colinet 2003; Organiscak et al. 2004]. Laboratory experiments showed an order of 

magnitude increase in protection factors when using a 99%-efficient filter versus a 

38%-efficient filter on respirable-sized particles [NIOSH 2007]. 

•	 Use an efficient recirculation filter. All the field evaluations used recirculation 

filters that were 95% or greater on respirable-sized dusts [Cecala et al. 2004, 2005a; 

Chekan and Colinet 2003; Organiscak et al. 2004]. Laboratory experiments showed a 

tenfold increase in protection factors when using an 85%- to 94.9%-efficient filter on 

respirable-sized dusts as compared to using no recirculation filter [NIOSH 2007]. 

Laboratory testing also showed that the time needed for the interior to stabilize after 

the door was closed was reduced by more than 50% when using the recirculation 

filter. 

•	 Minimize interior dust sources. Good housekeeping practices are needed to keep 

enclosure interiors clean, which eliminates inside dust sources. One field study 

showed a significant increase in dust levels (0.03 to 0.26 mg/m3

) when a floor heater 

was used. The fan from the floor heater stirred up dust lying on the cab floor [Cecala 

et al. 2001; NIOSH 2001b]. 

43 
 

•	 Keep doors closed. In a study on an enclosed cab of a surface drill, the operator’s 

dust exposure averaged 0.09 mg/m3

 inside the cab with the door closed and 0.81 

mg/m3 when the door was briefly opened to add drill steels [Cecala et al. 2007b]. 

Although this procedure was performed after drilling stopped and the visible dust 

dissipated, it nevertheless produced a ninefold increase in dust concentrations inside 

the cab each time a drill steel was added. 

The above research findings also suggest that the use of a one-directional airflow pattern could 

be beneficial. In most systems, both the intake and discharge for the recirculation air are located 

in the roof. This could cause a portion of the air to short-circuit without penetrating deeply into 

the cab. Also, as cab air is drawn into the ventilation system at the roof, dust generated in lower 

portions of the cab may be pulled through the breathing zone of the worker. In a one-directional 

design, recirculated air is drawn from the bottom of the enclosure and away from the worker’s 

breathing zone. Figure 4-10 shows this one-directional airflow pattern. 

Figure 4-10. Airflow pattern for one-directional filtration system for an enclosed cab. 

44 

For some cases in operator booths and control rooms, stand-alone table-top air purifier units 

containing a high efficiency particulate air (HEPA) filter have been installed and shown to 

improve the air quality. Obviously, these table-top systems are small, portable units available at a 

fraction of the costs of permanent systems. These systems can 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]. Obviously, one shortcoming with this type of system is that they do 

not provide any pressurization to keep dust from leaking into the booth or room. 
SCREENING 

Screens are the most common device used at mineral processing operations to separate dry ore 

material into different size ranges, normally measured in mesh sizes. By having the ore 

processed on different mesh-sized screens, an array of various-sized materials can be produced. 

Screen sizes for mineral processing operations range from large openings that can be inches in 

size all the way down to 400 mesh, which has a 35-micron cut point. The amount of dust 

generated during screening is dependent on the ore type, the particle size, the moisture content, 

and the type of equipment. Normally, screening finer-sized ore material produces more dust. 

Screening has been performed at mineral processing operations for many years and has been 

perfected by most screen manufacturers. New screening equipment units today are well-sealed 

units which liberate very little dust into the work environment when they are properly maintained 

and operated (Figure 4-11). 

Figure 4-11. Screening unit with LEV system. 

45 

When processing any ore that has significant silica content, screens 

should be tied into an LEV dust collector system to help keep the process under negative 

pressure and capture respirable-sized particles liberated within the unit. If visible dust is seen 

leaking from a screening unit, or if product is visible on the ground underneath a screen, this 

indicates that a problem exists and needs to be corrected. The first course of action is to 

determine if a seal or part has worn and is leaking product. In addition, the LEV exhaust port 

should be examined to ensure it is operating properly and is exhausting the screening unit at the 

required airflow and pressure. 

Another problem occurs when a screen cover is lifted during operation to clean the screen 

surface. This basically renders the LEV system ineffective because the increased area with the 

cover lifted is so great that the screen area is not able to be kept under negative pressure. This is 

a known problem throughout the industry. The only safety option currently available to workers 

when performing this task is to wear fit-tested personal protective equipment (PPE) that is rated 

for the levels and type of dust being processed. 
PACKAGING/BAGGING PRODUCT FOR SHIPMENT 

Ore is processed so that it can be packaged and sold to customers. Mineral processing operations 

package their product in a wide spectrum that includes 50- and 100-pound bags as well as bulk 

loading into railcars and trailer trucks. This section will provide some recommended methods to 

control respirable dust while loading ore into each of the different packaging containers to be 

delivered to the customer. 

Bag Filling Machines Packaging 50- and 100-Pound Bags 

•	 Dual bag nozzle system. The dual bag nozzle system was designed to reduce dust 

from three major sources during bagging [USBM 1984a, b, 1986c; Cecala and 

Muldoon 1985; Cecala and Thimons 1989]. The system is composed of an improved 

bag clamp designed to reduce the amount of product blowback during bag filling 

(Figure 4-12). 

Figure 4-12. Dual bag nozzle design. 

The clamp reduces blowback by making direct contact with 

approximately 80% of the fill nozzle. The system incorporates a dual nozzle system, 

which is a nozzle within a nozzle. The inner nozzle is the normal fill nozzle. The 

outer nozzle incorporates an air exhaust system, which exhausts excess pressure from 

the bag when it has finished filling. The exhaust system is powered by an eductor, 

which uses the venturi effect to exhaust the bag at approximately 50 cfm. 

Depressurizing the bag reduces the “rooster-tail” of product which spews from the 

bag when it is ejected from the fill station. By reducing the blowback and rooster tail, 

the amount of product and dust on the outside of the bag are minimized, which also 

reduces the dust liberated during this process. During a field analysis on this dual bag 

nozzle system, the bag operator’s dust exposure was reduced by 83% [USBM 1984a]. 

•	 Overhead air supply island system (OASIS). The OASIS is used to reduce bag 

operators’ dust exposures while performing the bag loading processes. Normally, the 

46 
bag operator sits while loading bags, and the OASIS would be installed over his/her 

position to provide a clean envelope of filtered air down over the worker. (For more 

information, see the “Background Issues” section of this chapter.) 

•	 LEV. A local exhaust ventilation system is very effective at minimizing the dust 

exposure to the bag operator while loading bags of product. When designing these 

systems, the dust generated during bag loading must be pulled down toward the floor 

to prevent the dust from passing over the worker’s breathing zone as it is captured by 

the exhaust ventilation system. 

•	 Automated systems. A number of manufacturers have been making advances in 

automated equipment that mechanically takes 50- or 100-pound bags, loads them onto 

fill spouts, then ejects the bags once they have reached the desired bagging weight. 

Normally, the bag loading area is sealed with plastic stripping to isolate it. This area 

is connected to an LEV system and kept under slight negative pressure to capture and 

remove respirable-sized dust particles liberated during the bag loading process. 

Bag Conveying 

•	 Bag and belt cleaning devices. These automated systems use a combination of 

mechanical devices such as brushes and air sprays to clean dust and product from the 

outside of the bags as they are being conveyed from the loading station to the stacking 

area [USBM 1995; Cecala et al. 1997]. It is recommended that bag cleaning be performed 

in an enclosed system so that all the dust removed is contained and exhausted to an LEV 

system. By having a hopper located under this cleaning unit, product removed from the 

bags can be recycled back into the process (Figure 4-13). 

Figure 4-13. Bag and belt cleaner device. 

47 
•	 Bag valve type. A study was performed that evaluated the dust liberation differences 

in relation to five commercial bag valves: (1) standard paper, (2) polyethylene, (3) 

extended polyethylene, (4) double trap, and (5) foam. This study showed that the bag 

operator’s and bag stacker’s respirable dust exposures were reduced by 62% and 

66%, respectively, when using the extended polyethylene valve, as compared to the 

standard paper valve [USBM 1986a; Cecala and Muldoon 1986]. The extended 

polyethylene valve was a plastic liner 41⁄2 inches inside a standard paper valve. The 

plastic was then extended for another 11⁄2 inches. It appears that this plastic section 

allowed the bag valve to seal very effectively to keep product and dust from escaping 

from this valve. This study was performed two decades ago, and during the time of 

the study the additional cost was less than one cent per bag, which was very costeffective when considering the significant reductions measured to both the bag 

operator and the bag stacker. 

Pallet Loading 

•	 LEV. When the pallet loading process is manually performed by bag stackers, an 

LEV system should be incorporated to capture and remove the dust generated during 

this process. When designing these systems, the dust generated during bag loading 

must be pulled down toward the floor to prevent the dust from passing over the 

worker’s breathing zone as it is captured by the exhaust ventilation system. 

•	 OASIS. The OASIS is also applicable to the pallet loading process to reduce the dust 

exposure to the bag stacker(s). The OASIS should be installed over the work station 

and provide a clean envelope of filtered air down over the bag stacker(s). 

•	 Semiautomated system. Semiautomated systems use workers in conjunction with an 

automated system to perform the bag stacking process. This 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 using a hydraulic lift table. This lift table 

allows the height for stacking the bags to remain constant throughout the entire pallet 

loading cycle. The bag loading height is set to approximately knuckle-high for the 

worker, which is the most ergonomic loading height. A push-pull ventilation system 

is used on either side of this pallet (Figure 4-14) to capture the dust liberated during 

bag stacking processing [USBM 1988a, 1989]. 

In other cases, the workers slide the bags 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 are such a major lost-time injury for bag stackers, this design 

significantly reduces stress by not requiring them 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. In this case, either an 

exhaust hood should be placed over the air slide area that is tied into an LEV system, 

or an OASIS-type system should be placed over the worker [Cecala et al. 2000; 

NIOSH 2001a]. 

48 
Figure 4-14. Semiautomated pallet loading system using push-pull ventilation. 

•	 Automated systems. Significant advances have been made by different 

manufacturing companies to design systems that either mechanically or robotically 

performs the bag stacking process, and these systems are being used at mineral 

processing plants. Automated systems reduce fatigue (back and other body parts) of 

the worker performing the task manually, as well as, the respirable dust exposure to 

the worker [Cecala and Covelli 1990]. When using these automated systems, the 

amount of respirable dust liberated still needs to be evaluated and controlled to ensure 

that it does not remain on the product bags and does not escape to the surrounding 

environment. This is normally performed by enclosing the area and using an LEV 

system to exhaust the respirable dust from the area. 

One-Ton Bulk Bags 

One-ton bulk bags of product material have become more popular over recent years because they 

are more cost effective than the 50- or 100-pound bags. The most effective method to control the 

dust liberated during the one-ton bulk bagging is to isolate this area from the rest of the plant. A 

worker enters this area and manually attaches an empty bag to the loading device. The worker 

then leaves the area and once outside, remotely activates a start button to begin bag filling. The 

area is exhausted and under negative pressure by virtue of being tied into a LEV system. Once 

bag loading is completed, the worker reenters the area and removes the bag for shipping and 

begins the process again. 

Bulk Loading 

•	 Enclosed cabs. Frequently, bulk loading is performed using a front-end loader to load 

open-container trailer trucks. This is normally performed outdoors with coarser 

product material, which does not generate a substantial amount of respirable dust. 

Nevertheless, the front-end loader should have a structurally competent enclosed cab 

49 
with an effective filtration and pressurization system. (See the “Operator 

Booths/Control Rooms/Enclosed Cabs” section.) 

•	 Telescoping loadout spout. Many different manufacturers produce telescoping 

loadout spouts, which minimize the amount of dust generated during the bulk loading 

process. Allowing ore to fall or drop significant distances creates more dust than 

when the drop distance is minimized. To minimize the drop distance, telescoping 

loadout spouts extend the loading location so that it is directly above the ore. As the 

shipping containers fill with ore, the spout continually repositions upward until the 

container is completely loaded. Another feature of this device that further minimizes 

dust liberation is a dual tube arrangement. The inner tube delivers the ore product to 

the loading container. The outer tube is tied into an LEV system to create a negative 

pressure at the loading area, therefore exhausting the respirable-sized particles to a 

dust collector system (Figure 4-15). These systems have been used for many years 

and are effective at minimizing dust liberation during bulk loading. 

Figure 4-15. Telescoping bulk loading spout with an exhaust system. 

CLOTHES CLEANING SYSTEMS 

One significant area of respirable dust exposure to workers at mineral processing operations is 

from contaminated work clothing. For the mineral processing industry, a U.S. Bureau of Mines 

report documented two cases where a tenfold increase in a worker’s respirable dust exposures 

50 
came from dusty work clothes [USBM 1986b]. This study indicated that respirable dust levels 

liberated from the soiled clothes were elevated to the extent that workers could be over their 

permissible exposure limit (PEL) in less than two hours. As the individuals performed their work 

duties, dust was continually emitted from their clothing, and the only way to eliminate this dust 

source was to clean or change their work clothing. Although disposable coveralls have been in 

use for many years, as well as some new and improved clothing material less susceptible to dust 

capture, the vast majority of workers continue to wear clothing similar to what they have worn 

for years. In addition, dirty work clothing, if it is not cleaned or changed at the end of the shift, 

can also contaminate personal vehicles and expose family members [Langenhove and Hertleer 

2004; Hartsky et al. 2000; Salusbury 2004]. 

The MSHA-approved method of cleaning work clothing involves a filtered vacuuming system, 

which is both difficult and time-consuming for the worker. Because of this, workers sometimes 

use a single compressed-air hose to blow dust from their clothing, even though this is not an 

approved practice. This technique creates a significant dust cloud which increases the worker’s 

respirable dust exposure and contaminates the work area. 

To address these problems, NIOSH and Unimin Corporation developed a clothes cleaning 

system that is able to quickly, effectively, and safely remove dust from a worker’s clothing 

without exposure to the worker, the work environment, or coworkers during the cleaning process 

[NIOSH 2005]. This system has been shown to be significantly more effective than the 

vacuuming or single air hose technique, while being performed in a fraction of the time. 

The clothes cleaning system consists of four major components. Figure 4-16 shows the various 

components of the clothes cleaning system. 

•	 Cleaning booth. The cleaning booth used for testing has a base dimension of 48 

inches by 42 inches and provides a safe and controlled area to perform the clothes 

cleaning process. All intake air enters the cleaning booth through a 24-inch cutout on 

the roof. The air flows directly down and over the worker in the booth before flowing 

through expanded metal grating on the floor and exiting through a return air plenum 

at the bottom and back of the booth. All dust and product cleaned from the worker’s 

clothing is contained within the booth and then exits via the exhaust ventilation 

system. 

•	 Air spray manifold. The air spray manifold is composed of 26 spray nozzles, spaced 

2 inches apart, to remove the dust and product from the worker’s clothing [Pollock et 

al. 2005]. These spray nozzles are regulated to limit the operating pressure to a 

maximum of 30 psi. The top 25 spray nozzles are flat-fan air nozzles and are used to 

clean the clothing. The bottom nozzle is a circular design and is used for cleaning the 

individual’s work boots. 

•	 Air reservoir. The air reservoir supplies the required air volume necessary for the air 

nozzles used in the air spray manifold. A 240-gallon unit reservoir is recommended 

for the system because it allows for multiple cleanings to be performed one after 

another. The air reservoir is located next to the cleaning booth and hard-piped to the 

air spray manifold located inside the booth. 

51 
•	 Exhaust ventilation system. Exhaust ventilation is used to keep the cleaning booth 

under negative pressure throughout the entire clothes cleaning process. This exhaust 

ventilation system can be tied into a local exhaust ventilation system or directed 

outside the plant and exhausted from an elevated stack. Testing on this system has 

verified that an exhaust volume of 2,000 cfm is required to maintain a negative 

pressure throughout the entire clothes cleaning cycle [Pollock et al. 2006]. This 

exhaust ventilation system should only be operated when a worker enters the booth to 

perform a clothes cleaning cycle and can be de-activated once the process is 

completed and the worker exits the booth. 

Figure 4-16. Clothes cleaning system design. 

During the development of the clothes cleaning system, a matrix of tests was performed to 

evaluate the effectiveness of this technique in comparison to that of the HEPA vacuuming and 

the single compressed-air hose approach. For this testing, both 100% cotton and cotton/polyester 

blend coveralls were soiled with dust before a worker entered the cleaning booth. The new 

clothes cleaning technique was proven to be 40.8% and 50.6% more effective than the 

vacuuming and single compressed-air hose technique, respectively [Cecala et al. 2007a]. The 

clothes cleaning system was also superior in its ability to uniformly remove dust from all areas of 

the worker’s clothing. Another major benefit was that the complete cleaning process was 

performed in a fraction of the time. The average cleaning times were 317 seconds for 

vacuuming, 178 seconds for the air hose, and 18 seconds for the clothes cleaning system. This 

52 
test also indicated that polyester/cotton blend coveralls were cleaned more effectively than the 

100% cotton coveralls. Figure 4-17 shows the effectiveness of the technique before and after a 

worker performed the clothes cleaning process while weaing polyester/cotton blend coveralls. 

Figure 4-17. Test subject before and after using the clothes cleaning booth. 

All workers performing the cleaning process are required to wear a half-mask, fit-tested 

respirator with N100 filters, hearing protection, and eye protection. To perform the clothes 

cleaning process, the worker enters the booth wearing his/her PPE, pushes the start button, 

slowly spins in front of the air spray manifold (18 seconds), and exits the booth with clean 

clothing. This clothes cleaning system provides a quick and effective method for workers to 

clean dusty clothes during the workday without risk to the worker, coworkers, or the work 

environment [Cecala et al. 2005b]. MSHA recognized the benefits of this system and has 

approved Petitions for Modification so that the system can be used in place of HEPA vacuuming. 

53 
BACKGROUND ISSUES 

This area of the report will focus on four different background issues: secondary dust sources, 

open structure design, overhead air supply island system, and housekeeping practices. 

Secondary dust sourcesWhen 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 cases where this was not the case [Cecala and Thimons 

1987]. 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 bringing levels back to 

acceptable concentrations. 

The following examples will demonstrate the impact that secondary dust sources can have on a 

worker’s respirable dust exposure: 

•	 Outside dust sources traveling inside structures. When outside dust 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. It is important that 

the air being drawn into this exhaust ventilation system, commonly called make-up 

air, be clean air. At one operation, the make-up 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 not performed, the bag 

operator’s dust exposure was 0.17 mg/m3

. As trucks were loaded at the bulk loading 

area, the bag operator’s exposure increased to 0.42 mg/m3 due to this contaminated air 

[USBM 1986b]. If outside air is used as make-up air, it must be from a location where 

the air is not contaminated. 

•	 Performance of job function. During an evaluation of a dust control system at one 

processing plant, substantial variations existed in the dust exposures of two different 

workers due to differences in their work practices. A number of factors were 

identified that impacted these differences. 

One factor was the amount of time the bag operator allowed the bag to remain on the 

fill spout before removing it. If the bag remained on the fill spout for a few seconds 

after it was filled, there was less dust generated from the rooster tail of product that 

spewed from the bag valve and fill nozzle as the bag was removed. A second factor 

was the extent to which the bag valve was sealed by the bag operator. One operator 

did not pay attention to where he grasped the bag as he lifted it from the fill spout to 

transfer it onto a conveyor belt. A second operator grasped the bag at the fill spout 

and crimped it closed as he placed the bag on the conveyor. This substantially 

lowered the amount of product that spewed from the bag as it was placed on the 

conveyor. A third factor impacting the operator’s dust exposure was the general 

manner in which the operator removed the bag from the bag spout and placed it on 

54 
the conveyor. More dust was generated when this was done in a forceful manner, as 

compared to a more continuous and gentle fashion [Cecala and Thimons 1993]. 

A number of modifications were tested to lower these workers’ respirable dust 

exposures. However, regardless of the effectiveness of the dust control system, the 

worker who performed his work duties in a rough and careless manner had 

approximately a 70% higher respirable dust exposure when compared to his coworker 

who performed the tasks in a conscientious and gentler manner. 

•	 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 dust exposure went from 0.07 mg/m3 before the bag break to 0.48 mg/m3 

afterwards. Although the bag broke during the conveying process and not directly in 

front of the worker, the dust substantially contaminated the surrounding mill air, 

which flowed over the bag operator. Once again, this occurred because the exhaust 

ventilation system in the bag loading area created a negative pressure that draws 

background air from the mill. 

For mineral processing operations to keep workers at acceptable dust levels, management must 

be aware of the various dust contamination sources and methods to reduce these sources. The 

substantial effects of the various secondary dust sources should be recognized, identified, and 

controlled in an effort to minimize workers’ dust exposures. 

Open-structure design 

Many different types of structures and materials have been used to build mineral processing 

facilities through the years. Although structure type and building material were not viewed as 

significant factors affecting the health of employees in these facilities when they were built, a 

recent study was performed that compared dust levels with three different building types: 

masonry, an open-structure design, and a steel-sided design [Cecala et al. 2007c]. Respirable 

dust measurements were taken within these structures to evaluate and compare levels. When 

considering the data, the most effective structural design of these three building types from a dust 

standpoint was an open-structure design. Respirable dust concentrations were significantly lower 

in the open structure because the natural environment acts as the best source of ventilation to 

dilute and carry away dust generated and liberated within the structure. In this study, when 

production levels were normalized, respirable dust levels at the open structure were more than 

four times lower than at the masonry structure and over 1,000 times lower than at the steel-sided 

structure [Cecala et al. 2006]. 

From a federal standard basis, the only consideration for an open-structure design would be the 

Environmental Protection Agency’s opacity dust measurement, which is a qualitative 

measurement taken by a federal regulator based on a visible dust plume. If any plumes are 

visible, source dust controls must be implemented to address the problem. 

Figure 4-18 shows a conceptual drawing of a typical walled processing facility, then an 

identically sized facility with an open-structure design and a roof. Obviously, a roof would 

provide a little more protection from the natural elements than a totally open design. When 

building new facilities, the open-structure design is more cost-effective because there are lower 

55 
material and construction costs involved. Some companies may also want to consider modifying 

their existing structures with a more open design to further reduce dust levels. If an openstructure design is considered for an operation, a number of issues need to be addressed: 

•	 Safety railings, guards, and/or chain-link wall containments must be installed to 

minimize the potential for any personnel or objects 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 sufficient overhang. 

•	 An open-structure design must be considered a secondary design. The first approach 

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 liberate into the plant and contaminate workers. 

Figure 4-18. Drawing of a conventional and an open structure with a protective overhang. 

Overhead air supply island system 

A successful control technique to reduce respirable dust exposures at mineral processing 

operations when workers are at stationary positions is with Overhead Air Supply Island System 

(OASIS). The OASIS air cleaning device is suspended over a worker and provides a flow of 

filtered air over the work station. Mill air is drawn into the system and passed through a primary 

cartridge filter. This primary filter is self cleaning, automatically using the reverse pulse 

technique when excessive filter pressure is sensed. The air can then pass through a heating or 

cooling chamber, which is optional depending on the mill air temperature, and from there into a 

distribution manifold, which also serves as a secondary filter (Figure 4-19). The resulting filtered 

56 
air flows down over the worker at an average velocity of approximately 400 fpm, which restricts 

mill air from entering the clean air zone. 

During development, OASIS systems were installed over bag operators at two different mineral 

processing operations. Testing at these two operations showed 98% and 82% reductions in the 

bag operators’ respirable dust exposures. An additional benefit with this system was a 12% 

reduction in general dust levels throughout the mill building as a result of the OASIS cleaning 

the mill air [Volkwein et al. 1988]. 

Figure 4-19 - Overhead air supply island system. 

Housekeeping practices 

Although good housekeeping practices seem to be a minor or common-sense issue, it 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 play a key factor in 

minimizing respirable dust exposure to workers at processing plants. When it is not performed, 

or performed improperly, it can have just the opposite effect. One example of this was 

documented when a worker was dry sweeping the floor with a push broom at a mineral 

processing operation. In this instance, the exposure of a coworker located one floor up from the 

person sweeping the floors increased from 0.03 mg/m3 before sweeping occurred to 0.17 mg/m3 

during and immediately after the occurrence. Dry sweeping is an unacceptable method of 

cleaning because of the dust it liberates into the work environment [USBM 1986b]. 

The most effective method of housekeeping is to wash down the plant with water on a regular 

basis. For the ideal system, floor drains and floors that are sloped correctly toward the drains 

should be incorporated into the structure’s design during construction. Housekeeping should be 

57 
performed at the end of each shift so that workers coming on to the next shift start with a clean 

work environment. 

A final component of effective housekeeping is proper upkeep and maintenance of plant 

equipment and processes. When product is observed building up on the floors of the plant, it 

indicates that some function or process is leaking ore. In some cases, visible dust can be seen 

leaking from holes or damaged equipment and this must be quickly corrected to minimize dust 

leakage. 

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63 
CHAPTER 5. CONTROLLING RESPIRABLE SILICA DUST AT 

SURFACE MINES 

By John A. Organiscak 

Overexposure to airborne respirable crystalline silica dust (referred to here as “silica dust”) can 

cause silicosis, a serious and potentially fatal respiratory lung disease. Mining continues to have 

some of the highest incidences of work-related silicosis, with mining machine operators being 

the occupation most commonly associated with the disease [NIOSH 2003b]. Some of the most 

severe cases of silicosis have been observed in surface mine rock drillers [NIOSH 1992]. A 

voluntary surface coal miner lung screening study initiated in Pennsylvania in 1996 indicated 

that silicosis was directly related to age and years of drilling experience [CDC 2000]. 

Mine workers continue to be at risk of exposure to excessive levels of silica dust in the United 

States. The percentage of the Mine Safety and Health Administration (MSHA) dust samples 

from 2004 to 2008 that exceeded the applicable or reduced respirable dust standard due to the 

presence of silica were 12% for sand and gravel mines, 13% for stone mines, 18% for nonmetal 

mines, and 21% for metal operations [MSHA 2009]. At surface mining operations, occupations 

most frequently exceeding the applicable respirable dust standard are usually operators of 

mechanized equipment such as drills, bulldozers, scrapers, front-end loaders, haul trucks, and 

crushers. 

This chapter summarizes the current state-of-the-art dust controls for surface mines. Surface 

mining operations present dynamic and highly variable silica dust sources. Most of the dust 

generated at surface mines is produced by mobile earth-moving equipment such as drills, 

bulldozers, trucks, and front-end loaders excavating silica-bearing rock and minerals. Four 

practical areas of engineering controls designed to mitigate exposure of surface mine workers to 

all airborne dusts, including silica, are drill dust collection systems, enclosed cab filtration 

systems, controlling dust on unpaved haulage roads, and controlling dust at the primary hopper 

dump. 

Many surface mine dust control problems can be visually observed and diagnosed. Visible 

airborne dust emissions generated from a particular surface mine process usually indicate that 

respirable silica dust can be present and potentially become a worker exposure problem. Visual 

dust emissions affecting nearby workers indicates that an engineering control is needed or an 

existing control needs maintenance. Investigating possible causes of visual dust emissions when 

using an engineering control can often uncover the reason for its poor dust control effectiveness. 

Frequent visual inspections of engineering control systems can identify needed maintenance to 

optimize its dust control effectiveness. Area dust sampling should be conducted in conjunction 

with personal sampling when workers are being overexposed to respirable silica dust that cannot 

be observed. Area dust sampling locations are usually selected near potential dust sources to 

examine their contribution to the worker dust exposure problem. 

64 
DRILL DUST COLLECTION SYSTEMS 

Drill dust is generated by compressed air (bailing airflow) flushing the drill cuttings from the 

hole being drilled. Because of their ability to be operated in freezing temperatures, dry dust 

collection systems tend to be the most common type of dust control method incorporated into 

drilling machines by original equipment manufacturers. The typical dry dust collection system as 

shown in Figure 5-1 is comprised of a self-cleaning (compressed air back-pulsing of filters) dry 

dust collector sucking the dusty air from underneath the shrouded drill deck located over the 

hole. 

Figure 5-1. Typical dry dust collection system used on surface drills. 

Ninety percent of dust emissions with this type of system are attributed to drill deck shroud 

leakage, drill stem bushing leakage, and dust collector dump discharge. Wet suppression is 

another drill dust collection method and involves injecting water into the bailing airflow 

traveling down the drill stem. The process of the bailing airflow, water droplets, and cuttings 

mixing together captures the airborne dust as it travels back up the hole. However, wet 

suppression is infrequently used because of operational problems in cold climates, lack of a 

readily accessible water supply, and shorter bit life. Studies by the U.S. Bureau of Mines and the 

National Institute for Occupational Safety and Health (NIOSH) have shown the practical aspects 

of optimizing these dust collection systems. These are discussed below for each dust collection 

method. 

Dry Dust Collector System 

•	 Maintain a tight drill deck shroud enclosure with the ground. Dust emissions are 

significantly reduced around the drill deck shroud by maintaining the ground-to-shroud gap 

height below 8 inches [NIOSH 2005; USBM 1987b]. This can be accomplished by better 

vertical positioning of the drill table shroud by the operator to minimize the ground-to-shroud 

gap. Dust levels were significantly reduced from 21.4 mg/m3

 to 2.5 mg/m3 next to the drill 

deck shroud when the drill operator changed his drill setup procedure to minimize this gap 

[Organiscak and Page 1999]. Also, the ground-to-shroud gap can be more tightly closed by 

using a flexible shroud design that can be mechanically raised and lowered to the ground via 

65 
cables and hydraulic actuators. An adjustable-height shroud design maintains a better seal 

with uneven ground and was found to keep dust emissions next to the shroud below 0.5 

mg/m3

 at several drill operations [NIOSH 1998, 2005]. Finally, a shroud constructed in 

sections with vertical gaps along sections or corners can also be a source of shroud leakage. 

Overlapping sections of shroud material reduces gaps and leakage. One conceptual shroud 

design for a rectangular drill table is construction with corner sections and overlapping side 

sections of shroud material [Page and Organiscak 1995]. 

•	 Maintain a collector-to-bailing airflow ratio of at least 3:1. Dust emissions are 

significantly decreased around the shroud at or above a 3:1 collector-to-bailing airflow ratio 

[NIOSH 2005]. Dust collector airflow reductions under the shroud are generally caused by 

restrictions and/or leakages in the system. Loaded filters and material in the ductwork are 

likely causes of restrictions, while damaged duct work and holes are likely causes of leakage 

in the system. Thus, inspection and maintenance of the dust collection system is vital to 

achieving and maintaining optimal collector operation and airflow. 

•	 Maintain a good drill stem seal with the drill table. A rubber drill stem bushing (see 

Figure 5-1) restricts bailing airflow from blowing dust and cuttings through the drill deck 

and, therefore, needs to be replaced after mechanical wear. An alternative sealing method 

involves using a nonmechanical compressed air ring seal manifold under the drill deck. This 

manifold consists of a donut-shaped pipe with closely spaced holes on the inside perimeter 

which discharges air jets in a radial pattern at the drill stem. The high-velocity air jets block 

the gap between the drill stem and deck, reducing respirable dust leakage through the drill 

deck by 41%–70% [Page 1991]. 

•	 Shroud the collector dump discharge close to the ground. Dumping dust from the 

collector discharge several feet above ground level can disperse significant amounts of 

airborne respirable dust. Dust emission reductions of greater than 63% were measured by the 

collector discharge dump after installing an extended shroud near ground level (Figure 5-1) 

[Reed et al. 2004; USBM 1995]. These shrouds can be quickly installed by wrapping brattice 

cloth around the perimeter of the collector discharge dump and securing it to the discharge 

dump with hose clamps. 

•	 Maintain dust collector as specified by manufacturer. Collector system components 

should be frequently inspected and damaged components repaired or replaced. A 51% 

reduction in dust emission was measured at one drill after a broken collector fan belt was 

replaced, while another drill showed a reduction of 83% after the torn deck shroud was 

replaced [Organiscak and Page 1999]. 

Wet Suppression 

•	 Add small amounts of water into the bailing air until the visible dust cloud has been 

significantly reduced. Drill dust emissions are significantly reduced by increasing the water 

flow rate from 0.2 gpm to 0.6 gpm [USBM 1987b]. A needle valve and water flow meter 

installed on the water supply line provide adjustable control for wet suppression systems. 

However, adding excessive water down the hole can cause operational problems with no 

appreciable improvement in dust control. 

•	 Minimizing water flow to a rolling cutter bit can increase bit life. Wet drilling with 

rolling cutter bits can cause premature bit wear. A drill stem water separator installed 

66 
upstream of a rolling cutter bit can increase bit life without adverse effects on dust control 

[Listak and Reed 2007; USBM 1988]. The water separator is a bit stabilizer with an internal 

cyclonic or impaction water droplet classifier, which removes most of the water from the 

bailing airflow before it is flushed through the drill bit. The water removed by the internal 

separator is released through external holes in the bit stabilizer (Figure 5-2). 

Figure 5-2. Water separator discharging water before it reaches the drill bit. 

ENCLOSED CAB FILTRATION SYSTEMS 

Enclosed cab filtration systems are one of the mainstay engineering controls for reducing mobile 

equipment operators’ exposure to airborne dust at surface mines. Enclosed cabs with heating, 

ventilation, and air conditioning (HVAC) systems are typically integrated into the drills and 

mobile equipment to protect the operator from the outside environment. Air filtration is often 

part of the HVAC system as an engineering control for airborne dusts. Surface mining dust 

surveys conducted by NIOSH on drills and bulldozers have shown that enclosed cabs can 

effectively control the operator’s dust exposure, but cab performance can vary [Organiscak and 

Page 1999]. The enclosed cab protection factors (outside ÷ inside dust concentration) measured 

on rotary drills ranged from 2.5 to 84, and those measured on bulldozers ranged from 0 to 45. 

NIOSH also conducted field studies of upgrading older equipment cabs to improve their dust 

control effectiveness. These studies involved retrofitting older enclosed cabs with airconditioning, heating, and air filtration systems to demonstrate the effectiveness of upgrading 

older mine equipment cabs. During these retrofits, cab enclosure cracks, gaps, or openings were 

sealed with silicone and closed cell foam tape. Varying degrees of enclosure integrity were 

achieved. Table 5-1 shows the results in ascending order of performance achieved with these 

retrofitted installations. Additionally, NIOSH conducted controlled laboratory experiments to 

examine the key design factors of enclosed cab dust filtration systems. The key performance 

factors for effective enclosed cab dust filtration systems are summarized below. 

67 
Table 5-1. Respirable dust sampling results of enclosed cab field studies. 

Cab Evaluation 

[Ref] 

Cab Pressure 

in w.g. 

#

Wind 

Velocity 

Equivalent 

mph 

Average 

Inside Cab 

Dust Level 

mg/m3 

Average 

Outside Cab 

Dust Level 

mg/m3 

Protection 

Factor 

Out/In 

Rotary Drill 

[Organiscak et al. 2004]
 

Haul Truck 

[Chekan and Colinet 2003]
 

Front-end Loader 

[Organiscak et al. 2004]
 

Rotary Drill 

[Cecala et al. 2004]
 

Rotary Drill 

[Cecala et al. 2005]
 

None Detected 

0.01

0.015 

0.20 to 0.40 

0.07 to 0.12 

0 

 4.5 

5.6 

20.3 to 28.7 

12.0 to 15.7 

0.08 

0.32 

0.03

0.05 

0.07 

0.22 

1.01

 0.30

2.80 

6.25 

2.8
 

 3.2
 

 10.0
 

56.0
 

89.3
 

 

# Wind Velocity Equivalent = (4000 √ Δpcab) fpm × 0.11364 mph/fpm @ Standard Air Temp & Pressure. 

Key Performance Factors for Enclosed Cab Filtration Systems 

•	 Ensure good cab enclosure integrity to achieve positive pressurization against wind 

penetration into the enclosure. As shown in Table 5-1, significant improvements in cab 

protection factors were achieved in the field studies when cab pressures exceeded 0.01 inches 

of water gauge. This corresponded to wind velocity equivalents (an indicator of cab wind 

velocity resistance) greater than 4.5 miles per hour. The cab enclosures with greater than 0.01 

inches of water gauge pressure were of close-fitted construction and their integrity could be 

readily improved by sealing cab enclosure cracks, gaps, or openings with silicone and closed 

cell foam tape. The loosely fitted cab construction on one of the drills and the truck were 

difficult to seal, which limited the amount of cab pressure that could be attained. 

•	 Use high-efficiency respirable dust filters on the intake air supply into the cab. Filter 

efficiency performance specifications used in the field were 95% or greater on respirablesized dusts [Checkan and Colinet 2003; Cecala et al. 2004, 2005; Organiscak et al. 2004]. 

Laboratory experiments showed an order of magnitude increase in cab protection factors 

when using a 99%-efficient filter versus a 38%-efficient filter on respirable-sized particles 

[NIOSH 2007]. 

•	 Use an efficient respirable dust recirculation filter. All the cab field demonstrations used 

recirculation filters that were 95% efficient or better in removing respirable-sized dusts 

[Checkan and Colinet 2003; Cecala et al. 2004, 2005; Organiscak et al. 2004]. Laboratory 

experiments showed an order of magnitude increase in cab protection factors when using an 

85%- to 94.9%-efficient filter as compared to no recirculation filter [NIOSH 2007]. 

Laboratory testing also showed that when using a recirculation filter the time for interior cab 

concentration to decrease and reach stability after the cab door is closed was cut by more 

than half. 

•	 Minimize dust sources in the cab. Use good housekeeping practices and move heater 

outlets that blow across soiled cab floors. Dust levels were shown to increase from 0.03 to 

0.26 mg/m3

 by turning on a floor heater inside the cab [Cecala et al. 2005]. The floor heater 

was removed and cab heating was discharged down from the ceiling HVAC system, reducing 

68 
dust entrainment in the cab during colder winter months [Cecala et al. 2005]. Another 

method of reducing entrainment of dust from a soiled cab floor is placing a gritless (without 

sand added) sweeping compound on the floor during the working shift. Most commercial 

sweeping compounds have petroleum-based oils or wax added to the cellulose material. 

However, people sensitized to petroleum distillates could have allergic reactions to these 

sweeping compounds if used in enclosed cabs. A few companies offer nonpetroleum-base 

sweeping compounds that use either a natural oil or chemical additive for dust adhesion 

[NIOSH 2001]. It is also recommended to cover the floor with rubber matting instead of 

carpeting for easy cleaning. More frequent cleaning of heavily soiled floors may be a more 

straightforward alternative than using sweeping compounds to minimize this type of dust 

entrainment. 

•	 Keep doors closed during equipment operation. On one drill operation, the respirable dust 

concentrations inside the cab averaged 0.09 mg/m3

 with the door closed and averaged 0.81 

mg/m3 when the door was briefly opened to add drill steels [Cecala et al. 2007]. Although 

this occurred after drilling stopped and the visible dust dissipated, opening the door, even 

briefly, produced a ninefold increase in respirable dust concentrations inside the cab during 

many drill steel changes made over a working shift. 

CONTROLLING HAULAGE ROAD DUST 

Off-road haul trucks used in the mining industry typically contribute most of the dust emissions 

at a mine site. Although most of the airborne dust generated from unpaved haulage roads is 

nonrespirable, up to 20% is in the respirable size range [Organiscak and Reed 2004]. The most 

common method of haul road dust control is surface wetting with plain water, but others include 

adding hygroscopic salts, surfactants, soil cements, bitumens, and films (polymers) to the road 

surface [NIOSH 2003a; USBM 1987a]. Figure 5-3 shows the effectiveness of road wetting with 

water on airborne respirable dust generation measured next to an unpaved haul road [Organiscak 

and Reed 2004]. The road was wetted in the morning and dried out in the afternoon. Although 

the road treatment methods have been shown to be very effective, their application generally 

involves continual maintenance due to road degradation from traffic, dry climatic conditions, and 

material spillage on the road. Road dust generation can be inevitable at times during the mining 

operation, until controls are applied. Given their mobility, trucks have the potential for exposing 

downwind mine workers to respirable dust, as well as other truck drivers traveling on the haul 

road. NIOSH has recently studied the size characteristics, concentrations, and spatial variation of 

airborne dust generated along unpaved mine haulage roads to examine the potential human 

health and safety impacts of this dust source and is examining other avenues of truck dust 

mitigation. Techniques for controlling haulage road dust are summarized below. 

69 
Figure 5-3. Increase in dust when a wet haul road dries. 

Methods for Controlling Haulage Road Dust Exposures 

•	 Treatment of unpaved road surfaces. Figure 5-3 shows the effectiveness of road wetting 

on respirable dust next to the road and its time frame of effectiveness [Organiscak and Reed 

2004]. Other haulage road treatments, such as hygroscopic salts, surfactants, soil cements, 

bitumens, and films (polymers), can extend the time of effectiveness between treatments to 

up to several weeks [NIOSH 2003a; USBM 1987a]. 

•	 Increase distance between vehicles traveling the haul road. Airborne dust concentrations 

generated from haulage roads rapidly decreased and approached ambient air dust levels 100 

feet from the road [Organiscak and Reed 2004]. This road dust dissipation and dilution 

occurrence provides administrative and mine planning controls to reduce worker dust 

exposure. The distance placed between trucks not following within 20 seconds of each other 

can result in a 41–52% reduction in airborne respirable dust exposure to the following truck 

[Reed and Organiscak 2006]. Finally, road layout and traffic patterns that can be 

economically incorporated into the mine plan could also isolate the haul road dust sources 

from other workers [Organiscak and Reed 2004]. 

CONTROLLING DUST AT THE PRIMARY HOPPER DUMP 

Ore is normally loaded into haul trucks from the pit or quarry and driven to the primary crusher 

location. This ore is either dumped directly from the haul truck into the primary ore hopper 

feeding a crusher or dumped into a stockpile. If it is stockpiled, a front-end loader then takes the 

ore and dumps it into the primary hopper. In either case during this dumping process, a dust 

cloud is billowed out of the hopper and rolled back under the truck bed or front-end loader 

bucket. Dust in the ore is released from the large volume of ore product being dumped in a short 

period of time, which quickly displaces the air in the hopper and transports the airborne dust 

released from dumping. If the equipment operators dumping the ore into the hopper have an 

effective enclosed cab filtration system (as described earlier) their exposure to this dust would be 

reduced. However, if other mine personnel, such as crusher operators and/or maintenance 

70 
workers, work near this primary dump, they can be exposed to this airborne dust. Several 

effective control methods include enclosing the hopper dump and using water sprays to suppress 

and contain the dust from rolling back out of the enclosure. 

Key Factors for Controlling Dust from the Primary Dump 

•	 Enclose the primary hopper dump. Walls can be constructed around the primary dump 

location to form an enclosure that must be custom designed to accommodate the dump 

vehicles being used. Walls can be either stationary (rigid) or movable (flexible material or 

curtains) based upon maintenance access within parts of the enclosure. Staging curtains 

(sometimes called stilling curtains) can be used in the enclosure to break up the natural 

tendency for dust to billow out of the primary dump hopper when a large volume of product 

is dumped in a very short time period (see Figure 5-4) [Weakly 2000]. Another option to 

restrict the dust from escaping the enclosure is using panels of flexible plastic stripping on 

the dump side of the enclosure. 

Figure 5-4. Staging curtains used to prevent dust from billowing out of enclosure. 

This plastic stripping employs an overlapping sequence 

which provides for a very effective seal and resists damage if contacted by the bucket of the 

front-end loader or the bed of the haul truck during dumping. Finally, a local exhaust 

ventilation (LEV) system can be used to filter the dust-laden air from the enclosed hopper 

area. This would be most appropriate when the primary dump is at a location where the dust 

could enter an adjoining structure or impact outside miners. Since hoppers are usually large, 

a significant amount of airflow would be required to create sufficient negative pressure to 

contain the dust cloud. This approach would be a more expensive alternative than using wet 

suppression [MSA Research Corp 1978]. 

•	 Use water sprays to suppress the dust in the enclosure. Water sprays directed at the ore 

dumped into the hopper will wet the material and suppress some of the generated airborne 

dust. A good starting point is to add 1% moisture by weight [MVS 1974]. This percentage 

71 
can be adjusted based upon the improvement gained from additional moisture versus any 

consequences from adding too much. Since continuous use of water sprays during long 

periods of idle time between dumping can have adverse operational effects, the water sprays 

can be activated during the actual dump cycle through the use of a photo cell or a mechanical 

switching device. A delay timer can also be used in this application so that the sprays 

continue to operate and suppress dust for a short time period after the dump vehicle has 

moved away. 

•	 Prevent the dust from rolling back under the dump vehicle. A tire-stop water spray 

system is recommended to reduce the dust liberated due to rollback under the dumping 

mechanism. A tire stop or Jersey barrier should be positioned at the most forward point of 

dumping for the primary hopper. A water spray system should be attached to the back side of 

this tire stop to knock down and force the dust that would otherwise roll back under the 

dumping mechanism into the hopper. Additionally, a shield should be placed over this water 

spray manifold to protect it from damage from falling ore (Figure 5-5). Finally, a system 

should also be incorporated that allows the water sprays to only be activated during the actual 

dumping process, as previously discussed. 

Figure 5-5. Tire-stop water spray system reduces dust rollback under the dumping vehicle. 

REFERENCES 

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75 
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