Reducing Respirable Dust Concentrations at

Mineral Processing Facilities Using Total Mill

Ventilation Systems

By Andrew B. Cecala, George W. Klinowski, and Edward D. Thimon
Report of Investigations 9469

Reducing Respirable Dust Concentrations at

Mineral Processing Facilities Using Total Mill

Ventilation Systems

By Andrew B. Cecala, George W. Klinowski, and Edward D. Thimons

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES
Library of Congress Cataloging in Publication Data:

Cecala, Andrew B.

Reducing respirable dust concentrations at mineral processing facilities using to-tal mill

ventilation systems / by Andrew B. Cecala, George W. Klinowski, and Edward D.

Thimons.

 p. cm. — (Report of investigations; 9469)

Includes bibliographical references (p. 11).

1. Ore-dressing plants—Dust control. 2. Ore-dressing plants—Heating and

ventilation. I. Klinowski, George W. II. Thimons, Edward D. III. Title. IV. Series:

Report of investigations (U.S. Bureau of Mines); 9469.

TN23.U43 [TH7697.07] 622 s—dc20 [622'.7] 93-17142 CIP
CONTENTS

Page

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Testing equipment and procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Mill 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Mill 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Tracer gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

ILLUSTRATIONS

1. Increases in total mill dust levels as day or shift progresses and dust sources are not addressed using

control techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Five dust monitoring locations at mill 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Respirable dust levels at location 5 with and without total mill ventilation system . . . . . . . . . . . . . . . . . . . 5

4. Effectiveness of total mill ventilation system at removing smoke from mill building . . . . . . . . . . . . . . . . . . 5

5. Six dust monitoring locations at mill 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

6. Percent reduction of respirable dust at each monitoring location for 2 days of testing for both two and

four fans using RAM-1 devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

7. Temperature readings at three levels in mill building during day 1 of testing with and without total mill

ventilation system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

8. Worker taking gas sample for tracer gas analysis at exhaust fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

9. SF6

 analysis—comparing four fans versus two fans for total mill ventilation system . . . . . . . . . . . . . . . . . . 9

10. SF6

 analysis—effectiveness of total mill ventilation system with windows open versus closed . . . . . . . . . . 9

11. Ventilation profile curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

TABLES

1. Dust reduction for gravimetric and RAM-1 instruments at five monitoring locations for both field

evaluations at mill 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2. Dust concentration and percent reduction for RAM-1 instruments at five monitoring locations at mill 2 . . . . 7

3. Reduction of respirable dust levels with gravimetric samplers at three monitoring locations at mill 2 . . . . . . 7
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cc/min cubic centimeter per minute mm millimeter

cfm cubic foot per minute μm micrometer

°F degree Fahrenheit pct percent

ft foot ppb part per billion

ft3

cubic foot ppm part per million

h hour ppt part per trillion

mg/m3 milligram per cubic meter s second

min minute
REDUCING RESPIRABLE DUST CONCENTRATIONS AT MINERAL

PROCESSING FACILITIES USING TOTAL MILL

VENTILATION SYSTEMS

By Andrew B. Cecala,1

 George W. Klinowski,2 and Edward D. Thimons3

 

ABSTRACT

The U.S. Bureau of Mines has designed and evaluated total mill ventilation systems at two different

mineral processing operations. Both systems have proven very effective at reducing respirable dust levels

throughout the mill in a cost-effective manner. A 25,500-cfm system installed at a clay processing mill

provided approximately 10 air changes per hour. This system reduced respirable dust concentrations by

approximately 40 pct throughout the mill building. The second evaluation was performed at a silica sand

operation. Tests were performed with 50,000 and 100,000 cfm of ventilation to the mill building,

corresponding to 17 and 34 air changes per hour. Average mill-wide respirable dust reductions were 36

and 64 pct, respectively. Not only did these systems reduce respirable dust concentrations and increase

visibility throughout the mills, they were also easy to install and required minimal maintenance. A total

mill ventilation system provides a general purging of the mill air; the system should be viewed as a

supplemental technique to assist other dust control systems in operation.

 

1Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.

2

Project engineer, Canada Centre for Mineral and Energy Technology, Sydney, Nova Scotia, Canada.

3Research supervisor, Pittsburgh Research Center.
2

BACKGROUND

The U.S. Bureau of Mines has been performing research

on methods of lowering the dust exposure of workers at

mineral processing operations for a number of years. This

research has mainly been directed at source control

techniques in an effort to lower a worker's dust exposure at

a particular job function. In some cases, this control

technology provided a carryover effect in lowering the

exposure of workers at other locations, but for the most part

the technology had very little effect in lowering dust levels

throughout an entire mill building.

A recent Bureau study identified the various types of dust

control techniques that were being used by the U.S. mining

industry in coal preparation plants and mineral processing

operations (1).4

 This work briefly evaluated the advantages

 

4

Italic numbers in parentheses refer to items in the list of references at

the end of this report.

and disadvantages of the following tech-niques: ventilation,

baghouse-type collectors, wet scrub-bers, electrostatic

precipitators, source control, sprays, good housekeeping, and

personal protection devices. Looking specifically at mineral

processing operations, the authors' experience has indicated

that baghouse-type col-lectors are the most common

technique used by the in-dustry. At the other extreme is

ventilation, which is given little consideration by the

industry.

Recently, the Bureau became aware of several mineral processing operations having problems with high overall mill dust levels. These operations were all using

baghouse-type collectors to address their most serious dust

contamination areas. Although these major dust sources

were being controlled, there were numerous minor dust

sources that were not. These minor dust sources were

causing dust levels in these operations to gradually in-crease

as the day or shift progressed. In some cases, dust

concentrations continually rose for the entire shift (fig-ure 1,

case A). In other cases, dust levels would stabilize at a

certain level as natural ventilation provided a purging of the

structure (figure 1, case B).

These minor dust sources can be generated from numerous processes or events. Some sources are common

throughout the industry, whereas others are site specific.

Some common sources are product dropping off from

the bottom of conveyor belts or being knocked off by

the rollers on a conveyor line; product residue on walls,

beams, and equipment becoming airborne from the vibra-tion

of the processes and equipment operating within the mill;

product on walkways and access areas being gener-ated as

workers walk throughout the mill; product leakage from

chutes or transfer points; leakage from dust control

equipment; dust released or generated from improper

housekeeping practices; and product released when inspecting screens or other milling processes when covers or

lids are opened. Every effort should be made by plant

managers to ensure that these dust sources listed, many

of which are housekeeping practices, are continually addressed to minimize their effects.

A cost-effective supplemental dust control method

to control a gradual increase in dust levels over the shift is

to use a total mill ventilation system. During a Bureau

survey of 25 coal prep plants, it was observed that those

operations that effectively used some type of general ventilation system had the lowest overall dust concentrations (2).

One possible reason for the lack of general mill ventilation systems in the minerals processing industry is the

shortage of published information addressing this area. The

Industrial Ventilation Manual is the primary reference source

on general plant ventilation systems in the United States (3).

Although this manual is excellent for engineers doing plant

design work and is quoted by some authors as "the Bible" in

this area (4), there is no specific information provided on the

proper design of total mill ventilation systems. Eshelman (5)

took the information provided in the Industrial Ventilation

Manual on general and localized exhaust systems, expanded

it, and noted the importance of mill ventilation systems, but

did not provide plant en-gineers the needed information to

implement the tech-nique. Because of the lack of published

information and the limited use of general ventilation

systems in the industry, the Bureau decided to perform a

short-term study in this area. This research effort was

performed in an effort to improve the health and safety of

men and women working in mineral processing operations

through-out the Nation.

Figure 1.-Increases in total mill dust levels as day or shift

progresses and dust sources are not addressed using control

techniques.
3

INTRODUCTION

The authors could not find published guidelines as to

what ventilation volumes should be provided to mineral

processing plants or mills. The Mine Safety and Health

Administration (MSHA) uses 30 CFR 56.5005, which states

that "control of employee exposure to harmful airborne

contaminants shall be, insofar as feasible, by pre-vention of

contamination, removal by exhaust ventilation, or by dilution

with uncontaminated air." This general standard states that

the use of exhaust ventilation is a possible technique to

control employee exposure but does not give suggested

guidelines for air volumes. The intent of the present study

was to determine general guidelines and design criteria for

operations that may be interested in implementing such a

total mill ventilation system.

Most mill buildings can be considered closed systems,

and thus any dust that is not being controlled within the

structure will cause dust levels to gradually increase over the

shift. A total mill ventilation system should be ••••••••••

designed to draw clean makeup air in near the base of the

mill. This air provides general purging and may clear some

dust-laden areas throughout the mill structure. This air

should be discharged at or near the top of the struc-ture,

where it will not contaminate plant personnel work-ing

outside. In addition, thermodynamic effects from heat

generated by mill equipment will produce a chimney effect,

thus assisting the basic flow pattern of this ventilation

system. This technique should not present any environmental problems since the exit velocity and dust concentrations are relatively low. Without this exhaust system, dust

can exit the building through open doors, windows, etc., and

contaminate outside plant workers. This dust can also be

recirculated back into the mill building, causing ad-ditional

contamination problems. A total mill ventilation system is

not a stand-alone technique, but is a supple-mental technique

to assist other localized dust control systems.

TESTING EQUIPMENT AND PROCEDURES

Total mill ventilation systems were evaluated at two

different processing operations. For both of these field test

sites, the primary goal was to determine the reduction in

respirable dust concentrations throughout the structure. The

respirable dust monitoring equipment and analysis were

identical at both test sites, except for the actual location of

the equipment. The evaluation was conducted by monitoring

with and without the total mill ventilation system.

Dust sampling was performed using both gravimetric

samplers and real-time aerosol dust monitors (RAM-1's).

Both of these instruments were used with the 10-mm DorrOliver cyclone to classify the respirable portion of dust,

usually considered to have aerodynamic diameters of 10 μm

or less.

Gravimetric dust samples were taken at most evaluation

points. Gravimetric dust filters were weighed before and

after use at the Bureau. This sampling procedure allows for

a determination of the average respirable dust concen-tration

over the entire time that the device was operated. Since the

goal was to determine the change in dust levels ••••••••••

with and without the total mill ventilation system, two sets

of gravimetric samplers were used, one operating when the

ventilation system was on and the second operating with the

ventilation system off. In all cases, dust concentrations were

determined by operating two or three sampling pumps side

by side to determine an average concentration. Each

gravimetric sampler provided one dust concentration value

for each day of testing. Gravimetric sampling is the primary

means for compliance sampling performed by MSHA for the

mining industry.

RAM-1's located at all dust sampling locations were also

used for testing. The RAM-1 is an instantaneous de-vice

that determines respirable dust concentrations by the light

scatter of particles drawn through an internal sensing

chamber by a sampling pump. This instrument has been

used for many years in dust research and has proven to be

a very reliable and accurate device. The RAM-1 was ideal

for comparing the effectiveness of a total mill ventilation

system in that it permitted respirable dust concentrations to

be determined for short time segments throughout the

evaluation.

MILL 1

TESTING

The first evaluation was performed at a clay processing

facility in New York State. The general mill ventilation

system was designed and installed at the crushing and

screening mill. The base dimensions of this mill were 130

by 32 ft with a volumetric capacity of 150,000 ft3

. The total

mill ventilation system was designed to provide 25,500 cfm

of ventilation air to the mill, representing ap-proximately

10 air changes per hour. This ventilation was provided by

three 8,500-cfm roof powered exhaustors that were evenly

spaced across the roof of the mill building (fig. 2). Each
4

exhaustor was wired separately to permit repair of one unit

while the other units remained func-tional. This also

allowed plant personnel the flexibility to turn one or two

units off during the winter months when outside air

temperatures are low and could cause freeze-up problems.

Three wall louvers were installed to provide an inlet for

makeup air near the base of the mill. These louver locations

were chosen to provide for minimal make-up air dust levels

and a good distribution profile for the entire mill.

Five locations were monitored for dust concentrations in

the mill building to provide a good dust profile (fig. 2).

Sampling locations 1, 4, and 5 were near the screening

process because of fluctuations at all three levels of the mill

in this area. Sample location 2 was at a central main access

way. Location 3 was at the other end of the mill building to

establish the entire mill profile. Each moni-toring location

had both gravimetric sampling packages and a RAM-1. The

analysis was performed by monitoring dust levels for 1-h

periods with and without the total mill ventilation system.

Two evaluations were performed at this mill. The first

was in December, when outside air temperatures ranged

from 10° to 40° F. Only 2 days of testing were performed

because of extremely high winds and mill freeze-up problems. Temperatures with the windchill factor dropped as

low as -30° to -40° F during the first part of the shift. The

second evaluation was in April, when outside air temperatures ranged between 50° and 80° F.

Figure 2.-Five dust monitoring locations at mill 1.

All monitors were placed in the same locations for both

evaluations except for the monitor at location 5, which had

to be moved from its original position in December to a

nearby location in April because of an open door used to

clean the area around the screens. This open door should

not have affected the total mill ventilation system, but

••••••••••

it would have biased the dust levels recorded by this

monitor.

RESULTS

The 2 weeks of testing at this mill verified the effectiveness of the exhaust ventilation system in lowering respirable dust concentrations throughout the entire mill.

Comparing the gravimetric sampler results, respirable dust

concentrations ranged from 0.22 to 2.39 mg/m3

 with the

ventilation system off, compared with 0.13 to 1.55 mg/m3

with the ventilation system on for the December analysis.

In April, respirable dust concentrations ranged from 0.29 to

4.84 mg/m3

 with the ventilation system off, compared with

0.21 to 2.37 mg/m3

 with the system on. Visibility

throughout the mill building was greatly improved with the

ventilation system in use, also indicating lower mill dust

levels.

Table 1 lists the percent dust reductions for gravimetric

and RAM-1 instruments at the five monitoring locations for

both weeks of testing at mill 1. Each value was deter-mined

by comparing the average concentration with the ventilation

system off and on for the entire day of moni-toring.

Although the correlation between the gravimetric and RAM1 results on an individual location basis was less than

expected, this was not the case for the overall average

reduction for the results. The mean value and standard

deviation for gravimetric and RAM-1 results were x 42.5,

s.d. 17.6 and x 37.1, s.d. 16.8, respectively.

Generally, the system reduced total mill respirable dust

concentrations by approximately 40 pct. Figure 3 shows an

approximate 3-h period recorded by the RAM-1 located at

sample location 5 for day 2 of testing in December. The

graph shows approximately 1-h periods with the system off,

then on, and then off again. When the total mill ventila-tion

system was started, it took approximately 8 to 10 min to get

dust levels down to stabilized levels. Since the system was

designed to provide 10 air changes per hour, 1 air change

would occur every 6 min.

Figure 4 indicates the effectiveness of the total mill

ventilation system at clearing a contaminant from the mill

building. A smoke flare was released inside the mill at

8:50 a.m.; it took approximately 2 min to discharge. To

contaminate the entire mill, the flare was constantly moved

around on the ground level during this 2-min discharge

period. The graph indicates the contaminant level recorded by the RAM-1 at location 2 during this simulated

contaminant test. It took 8 to 10 min to get the contaminant cleared from the mill. This result corresponds with

the time period necessary for respirable dust levels to

stabilize after the ventilation system was turned on under

normal test conditions, as previously mentioned.
5

Table 1.—Dust reduction for gravimetric and RAM-1 instruments at five monitoring

 locations for both field evaluations at mill 1, percent

 Day 1 2 3 4 5

Gravimetric RAM-1 Gravimetric RAM-1 Gravimetric RAM-1 Gravimetric RAM-1 Gravimetric RAM-1

DECEMBER 1989

1 . . . . . 64.9 54.8 33.3 18.5 40.7 55.0 55.0 53.4 33.5 (1)

2 . . . . . 49.0 18.4 54.2 43.8 40.9 35.0 67.4 55.3 (

1

) 72.5

APRIL 1990

3 . . . . . 37.4 20.1 66.7 53.5 14.6 22.7 48.7 38.2 53.4 12.1

4 . . . . . 63.3 44.3 27.7 46.3 0 33.3 27.6 37.2 44.8 29.5

5 . . . . . 48.3 16.8 63.5 56.9 27.3 26.2 39.5 35.9 19.2 9.9 

1Equipment malfunctioned.

Figure 3.-Respirable dust levels at location 5 with and without total mill ventilation system.

Figure 4.-Effectiveness of total mill ventilation system at removing smoke from mill

building.
6

MILL 2

TESTING

The second evaluation was performed at a silica sand

operation in central Texas for two 14-h days of testing. The

mill was a six-story structure located on three-story storage

silos. The structure was 79 ft high, with base dimensions of

66 by 34 ft, having a volumetric capacity of 177,000 ft3

.

The system was composed of four 25,000-cfm belt-driven

propeller-type wall exhaustors providing 100,000 cfm of

ventilation to the mill building, corre-sponding to about 34

air changes per hour. One fan was located on the top

outside wall on each side of the building. These fans were

designated as north, south, east, and west fans. Each fan

was separately wired to the con-trol room. Some tests were

performed with only two fans operating (east and west sides

of the building), pro-viding approximately 17 air changes per

hour.

Since there were a number of large doors at the base of

this mill, there was no need to install additional inlets for

incoming makeup air. These doors remained open at all

times during testing. There was also a bank of win-dows

between levels A and B of the building that mill workers

liked to keep open. These windows were closed during all

testing except for one test sequence conducted to determine

the effects of opening them.

There were six dust monitoring locations inside the mill

building (fig. 5). RAM-1 instruments were placed at all

locations. Gravimetric samples were located on the south

side of the building at sample locations 2, 4, and 6, corresponding to levels A, B, and C, respectively.

Continuous monitoring digital thermometers were lo-cated

at levels A, B, and C inside the mill building to examine

changes in temperature due to the total mill ventilation

system. This operation is located in central Texas and

temperatures rise to high levels in the summer months.

Also, the equipment, motors, etc., located inside the mill

building generate a substantial amount of heat. Both of

these factors contribute to high temperatures inside the

structure in summer months. If the total mill ventilation

system cooled the structure, this would be an additional

benefit.

RESULTS

The results of testing at this operation showed the

effectiveness of the ventilation system at reducing res-pirable

dust levels while providing a slight general cooling. Testing

also confirmed that the effectiveness of the gen-eral

ventilation system was reduced in the bottom of the structure

when the windows were open.

Table 2 shows results with RAM-1's for both days

of testing at five monitoring locations. The ••••••••••

instrument at location 5 malfunctioned, and no valid information was obtained for this location. Table 3 lists the

results for the gravimetric samplers at monitoring locations

2, 4, and 6.

The reduction in respirable dust concentrations with

the ventilation system ranged from 47 to 74 pct as re-corded

by the RAM-1's. For the gravimetric samplers, this

reduction ranged from 60 to 86 pct. When only two exhaust

fans were used (east and west sides of building), the

respirable dust reduction recorded by the RAM-1's ranged

from 6 to 55 pct, as compared with 25 to 78 pct for the

gravimetric samplers.

Using the RAM-1 results, the average respirable mill dust

concentration with no fans was 2.66 mg/m3

. The average

concentrations with two and four fans were 1.7 and 0.95

mg/m3

, respectively. This corresponds to average reductions

for all five dust monitoring locations of 36.1 and 64.3 pct,

respectively. The effectiveness of the total mill ventilation

system can be seen in figure 6, which indi-cates the percent

reduction in respirable dust levels for both days of testing

with both two and four fans.

Figure 5.- Six dust monitoring locations at mill 2.
7

Table 2.—Dust concentration and percent reduction for RAM-1 instruments at five monitoring locations at mill 2

Fan off 2 fans 4 fans 4 fans, windows open

Location concentration, Concentration, Reduction, Concentration, Reduction, Concentration, Reduction,

mg/m3 mg/m3pct mg/m3pct mg/m3pct 

DAY 1

1 . . . . . . . 2.17 1.17 46.08 0.88 59.45 (

1

) (1) 

2 . . . . . . . 2.53 2.39 5.53 1.35 46.64 (

1

) (1) 

3 . . . . . . . 2.36 1.43 39.41 .85 63.98 (

1

) (1) 

4 . . . . . . . 2.04 .92 54.90 .71 65.20 (

1

) (1) 

6 . . . . . . . 1.92 1.16 39.58 .89 53.65 (

1

) (1) 

DAY 2

1 . . . . . . . 2.59 1.69 34.75 1.06 59.07 1.02 60.62 

2 . . . . . . . 3.67 2.10 42.78 1.18 67.85 1.18 67.85 

3 . . . . . . . 3.31 2.13 35.65 .97 70.70 1.35 59.22 

4 . . . . . . . 3.68 2.46 33.15 1.02 72.28 1.68 54.35 

6 . . . . . . . 2.32 1.58 31.90 .61 73.71 1.48 36.21 

1No testing performed.

Table 3.—Reduction of respirable dust levels

with gravimetric samplers at three monitoring

locations at mill 2, percent

Number Level A, Level B, Level C, 

of fans location 2 location 4 location 6

DAY 1

2 . . . . . . 24.8 56.4 45.6

4 . . . . . . 70.5 72.6 59.5

DAY 2

2 . . . . . . 54.9 52.4 77.5

4 . . . . . . 76.9 80.5 86.4

Figure 6.-Percent reduction of respirable dust at each

monitoring location for 2 days of testing for both two and four

fans using RAM-1 devices.

Figure 7 shows the temperature readings at three levels in

the structure for day 1 of testing. One can determine when

the exhaust fans were or were not in use by looking at the

direction of temperature values at the three moni-toring

locations. With no exhaust fans, the temperature levels rose;

when the ventilation system was used, the tem-perature

readings decreased.

The ventilation system was less effective in the bottom of

the structure when the windows were open. During day 2 of

testing, the windows were opened for 1 h between 4:10 and

5:10 p.m. with all four fans in operation. The lack of

Figure 7.-Temperature readings at three levels in mill building

during day 1 of testing with and without total mill ventilation

system.

ventilation on level C of the building was obvious ••••••••••
8

from both dust and temperature levels. At location 6, on

level C of the building, the RAM-1 recorded an average

respirable dust concentration of 0.61 mg/m3

 with the windows closed, compared with 1.48 mg/m3

 with the windows

opened. The only time the temperatures on level C exceeded temperatures on levels A and B was when the

••••••••••

windows were open. Makeup air was mainly being drawn

into the mill from the open windows, making the ven-tilation

system ineffective in the bottom portion of the structure.

The windows should remain closed if the ven-tilation system

is used.

TRACER GAS ANALYSIS

A tracer gas study was also performed at mill 2 to

analyze the effectiveness of the total mill ventilation system.

Sulfur hexafluoride (SF6

), the tracer gas used in this analysis,

is a colorless, odorless, nontoxic, inert gas not normally

found in the environment and detectable at concentrations in

the parts-per-trillion range. A similar study was recently

performed by the Canada Centre for Mineral and Energy

Technology in Elliot Lake, Ontario, Canada, to determine

ventilation patterns in a fluorspar milling plant (6).

For this analysis, the SF6

 tracer gas was released from a

small lecture cylinder at a rate of 5.0 cc/min. Six different

tests were performed using the tracer gas. For tests 1

through 4, the gas was released by a person moving the

tracer gas release cylinder around on the bottom floor of the

mill to provide good dispersion throughout the bottom of the

structure. For tests 5 and 6, the tracer gas was released from

a single point at the center of the building on the bottom

floor. For all six tests, the tracer gas was released for

several minutes before the first air sample was taken and

then was released for a predeter-mined time afterward.

Gas samples were taken at the various exhaust fans at the

top of the mill structure using disposable plastic syringes

(fig. 8). Each syringe was numbered and labeled with test

number, location, and time. Sampling rates varied from

every 30 s to every 2 min. After a test was completed, the

samples were analyzed on-site using electron-capture gas

chromatography. This portable chro-matograph was capable

of detecting SF6

 from 50 ppt to 5 ppm. This chromatograph

was a prototype unit built by the Canada Centre for Mineral

and Energy Technology in Sydney, Nova Scotia, Canada.

The chromatograph was calibrated on site to ensure an

accurate analysis.

Four tests were conducted to compare the changes in

tracer gas levels with two versus four fans operating. Tests

1 and 3 were performed with four fans operating and had

similar results. Tests 2 and 4 were performed with two fans

and were also similar. Figure 9 shows a comparison of

concentrations recorded at the various fan locations for tests

3 and 4. Two points should be noted. The first is that the

tracer gas was cleared from the structure much ••••••••••

faster with all four fans operating. In this case, the tracer

gas was significantly removed from the structure at the

5-min mark; with only two fans operating, the gas was

evident until approximately the 8-min mark.

The second point is that the tracer gas was being drawn

from the east and west fans much more effectively than from

the north fan. The maximum concentration recorded at the

north fan was 80 ppb, compared with concentrations of over

300 ppb at both the east and west fans for test 3. It appears

that the east and west fans were more effective in ventilating

the mill than the north fan was.

Tests 5 and 6 were run to compare the effectiveness of

the ventilation system with the windows in the structure

closed versus open. Figure 10 shows the tracer gas concentration measured at the east fan for these two tests. The

findings support the dust and temperature results. With the

windows closed, the contaminant was cleared from the mill

Figure 8.-Worker taking gas sample for tracer gas analysis

at exhaust fan.

faster than when the windows were opened.
9

Figure 9.-SF_6 analysis-comparing four fans (T3) versus two

fans (T4) for total mill ventilation system. W, concentration

recorded at west fan; E, concentration measured at east fan

Figure 10.-SF_6 analysis-effectiveness of total mill

ventilation system with windows open versus closed.

Concentrations recorded at east fan.

DISCUSSION

One goal of this research effort was to develop a

ventilation profile and approximate the optimal size for a

total mill ventilation system. This was not successful. Numerous engineering contractors who design coal prepara-tion

Figure 11.-Ventilation profile curves. A, Theorized ventilation

profile curve with total mill ventilation system; b, ventilation

profile curve for mill 2.

plants suggested that an exhaust ventilation system should

provide between 8 and 12 air changes per hour. This value

was used to size the ventilation system at mill 1 to provide

10 air changes per hour. After this evaluation, although the

dust reductions achieved were very accept-able, there was no

way to determine that this was the optimal size and that

additional reductions could not be achieved with increased

ventilation. The testing and eval-uation plan for mill 2 was

designed to determine this. The investigators believed that

having a system capable of pro-viding up to 34 air changes

per hour would allow estima-tion of an approximate optimal

operating point. By having a number of points on a

ventilation profile curve, the ap-proximate optimal size could

be determined, as shown theoretically in figure 11A. The

curve would be linear up to an area where it would start to

flatten out. This area of flattening out would be the optimal

ventilation volume. As the ventilation volume continues to

increase past this point, it is theorized that the curve may

start to show a decrease in efficiency as the total mill

ventilation system starts to overpower other dust control

techniques in the structure, such as baghouse-type collectors.

At mill 2, it was anticipated that the four fans would

allow for a calculation of a ventilation profile curve. Two

things prevented this. First, with all four fans operating, it

appeared that the ventilation profile was still in the linear

portion of the curve (fig. 11B). Second, there appeared to be

a substantial difference between the effi-ciency of the east

and west fans and the north fan as determined from the

tracer gas testing. The south fan was not even tested

because of manpower limitations during ••••••••••
10

the evaluation and because it faced the primary direction of

the prevailing wind at this operation. The ideal setup, in

hindsight, would have been to have two fans on both the east

and west sides of the building.

It would appear from all this that the optimal ven-tilation

volume may vary substantially from structure to structure

and may be higher than originally anticipated at some

operations.

For a mill exhaust ventilation system to be effective, there

are three design criteria that must be achieved. First, the

system should be capable of supplying clean makeup air to

the base of the mill. Contaminated makeup air will cause

the ventilation system to increase dust con-centrations and

worker exposures. This was seen in a pre-vious Bureau

study, in which an inside bag operator was exposed to dust

generated during bulk loading of trailer trucks outside the

mill building. By controlling where the intake air is brought

into a structure, using either wall louvers positioned at

strategic locations at the base of the mill or other mill

openings including doors, etc., dust-free makeup air can be

brought into the mill.

The second design criteria is that the system should

provide an effective flow pattern to ventilate the entire mill

while providing a sweeping action in the major dust generation areas. This is achieved by the proper positioning of

both the fans and makeup air intakes. The location of air

intakes are then twofold in purpose: (1) to provide clean

outside air and (2) to provide the most effective flow pattern

to purge the entire inside of the structure.

The third design criteria is that the outer shell of the

structure should be competent. An exhaust ventilation system draws makeup air into the structure from the point(s) of

least resistance. A fan creates a pressure differential,

causing a negative pressure inside the building. Air at a

higher pressure outside the structure flows into the mill

building to compensate for this pressure differential at the

point(s) of least resistance. If the point(s) of least resist-ance

are open or broken windows, holes or cracks in the wall or

roof, or any opening in the vicinity of the exhaust fan(s), the

designed ventilation flow pattern will be short circuited,

causing the system to be ineffective. Mill 2 clearly shows

this problem. When the windows halfway up the structure

were open, the ventilation system was basi-cally only

working in the top portion of the structure.

Another consideration when designing a total mill ventilation system is to take into account prevailing wind

direction. Wind direction would have a minor effect when

using roof exhaustors as in mill 1, but it should be

considered when using wall-type exhaustors, as used in

mill 2. With wall exhaustors, fans should not be placed

where the prevailing wind will work against them. Where

possible, the fan should exhaust with the direction of the

••••••••••

prevailing wind. This also minimizes the possibility of recirculation or reentrainment of dust back into the struc-ture.

Variability in the wind direction and speed should be taken

into account, and the system should be designed to

compensate for times when it will operate against the wind.

Total mill ventilation systems also pose possible freeze-up

problems during winter months when extremely low outside

air temperatures occur. There are a number of options that

may be considered if this is a concern. One option is to heat

the supply air coming into the mill building. If this is

necessary, it will significantly increase the cost of using this

technique. A second option is to back the system down by

turning off some of the fans in a multifan system. However,

in some cases, the ventilation system may not really impact

freeze-up problems. Mill freeze-up problems were

encountered during the first week of testing at the first

evaluation site, but it appeared that the total mill ventilation

system had little effect on them. As previously stated, this

operation was in central New York State, where extremely

low temperatures are encountered several months of the year.

There was no designed heating of the mill building, and the

only heating that occurred was from the motors, equipment,

and processes being performed within the mill. Air temperatures were similar inside and outside the building with or

without the total mill ventilation system in operation. Most

freeze-up problems occurred through the night, when both

the mill and the ventilation system were shut down. This

system has been in operation for over 2 years with no

evidence of increased mill freeze-up problems.

Additionally, a total mill ventilation system must be

looked at from an environmental standpoint. The Environmental Protection Agency has four categories of compliance for crushed stone industry with the passage of the

Clean Air Act. These four areas are process weight, fugitive, ambient, and permits concerns. At both of the evaluation sites in this study, each of the Federal standards were

complied with. Visual examination of the exhaustors from

outside showed no dust plume evident. The dust being

exhausted was quickly diluted with atmospheric air, reducing

its potential as a contaminant to the environment or to

outside personnel. Measurements taken periodically at the

base of the mill using handheld instruments at both

evaluation sites did not indicate any increase in dust levels.

One would need to consider any State regulations that may

be more stringent than the Federal standards.

The total mill ventilation system is probably the most

cost-effective method that an operation could consider to

lower total mill dust levels. At mill 1, the total cost of the

ventilation system, installation and materials, was approximately $10,000; this included having an outside contractor

••••••••••
11

perform the installation work. At mill 2, the total cost of the

system was approximately $6,000. The four fans each cost

$880. Since these fans were placed on the walls of the

structure, the work was easily done by the maintenance

crew. Considering the respirable dust reductions obtained,

••••••••••

no other engineering control technique available can yield

these reductions for the cost. Not only are initial costs of

this technique small, but operating and maintenance costs are

also minimal.

CONCLUSIONS

The results of the field evaluations of the total mill

ventilation system clearly indicate that this technique has

potential application throughout the minerals processing

industry. A 40-pct reduction in respirable dust was measured at the first evaluation site at a cost of only $10,000.

An average 64-pct reduction was recorded at the second site

at a cost of approximately $6,000.

The system uses dust-free outside air brought in at the

base of the mill to sweep and clear contaminated areas, then

discharges this air out of the top of the structure. Since the

volume of ventilating air is minor relative to the size of the

structure, the discharge air does not pose any contamination

hazards to outside plant workers or the environment.

If wall-type exhaustors are used near the top of the

structure, as with mill 2, the system should operate with the

prevailing wind direction. There is always variability

••••••••••

in the wind, which should be taken into account; the sys-tem

should be designed to compensate for this. It must also be

noted that in some cases, the total mill ventilation system

may accelerate mill freeze-up problems. Opera-tions could

heat the supply air or back down the ventila-tion volume

when air temperatures drop to levels that may cause freezeup problems, as previously discussed.

If dust control techniques presently available to the

industry are compared on a cost-benefit basis, total mill

ventilation ranks near, if not at, the top of the list. However, the authors' experience shows it is an infrequently used

technique. Considering the cost awareness of the industry

and the new pending respirable dust standards that may

establish the Threshold Limit Value at 5.0 mg/m3

 respirable

limit for all metal-nonmetal operations, total mill ventilation

is a technique that deserves industry attention.

REFERENCES

1. Divers, E. F., and A. B. Cecala. Dust Control in Coal Preparation

and Mineral Processing Plants. BuMines IC 9248, 1990, 11 pp.

2. Divers, E. F., and R. A. Jankowski. Respirable Dust Control in Coal

Preparation Plants. Paper in Transactions of 1988 SME-AIME Meeting.

Soc. Min. Eng. AIME, v. 40, 1988, pp. 223-228.

3. American Conference of Governmental Industrial Hygienists, Inc.

(Cincinnati, OH). Industrial Ventilation, A Manual of Recommended

Practice. 20th ed., 1988, 410 pp.

4. Goodfellow, H. D., and J. W. Smith. Industrial Ventilation—A

Review and Update. Am. Ind. Hyg. Assoc. J., v. 43, March 1982, pp. 175-

184.

5. Eshelman, P. S. Correcting OSHA's Citations With Ventilation.

Paper in Symposium on Silica (March 12-13, 1975). Ind. Health Found.,

Inc., 1975, pp. 105-120.

6. Grenier, M. G., S. G. Hardcastle, G. Kunchur, and K. Bulter. The

Use of Tracer Gases To Determine Dust Dispersion Patterns and Ventilation

Parameters in a Mineral Processing Plant. Am. Ind. Hyg. Assoc. J., v. 53,

June 1992, pp. 387-394.

INT.BU.OF MINES,PGH.,PA 29771
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