Lowering respirable dust at an iron ore concentrator plant through 

improved ventilation practices 

A.B. Cecala, J.A. Zimmer, J.F. Colinet, & R.J. Timko 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, USA 

INTRODUCTION 

In August 2000, the United Steelworkers of America 

held a health and safety workshop in Eveleth, Minnesota to discuss methods and techniques to improve 

working conditions for its membership in the iron

ore mines in Minnesota and Michigan. For this 

workshop, the National Institute for Occupational 

Safety and Health (NIOSH) was requested to provide a 90-minute presentation on dust control research applicable to the iron ore industry. Shortly 

after this workshop, NIOSH was contacted by the 

Tilden Mining Company LC who stated their interest in working with NIOSH and the United Steelworkers of America union membership at their operation to lower respirable dust concentrations in the 

grinding and concentrator mill. Shortly after receiving this request, a trip was scheduled (April 2001) 

for two individuals from NIOSH’s Respiratory Hazards Control Branch to visit the Tilden operation in

Ishpeming, Michigan. The first part of this visit was

a meeting with all three parties to discuss a number

of potential dust control areas of interest. After this 

meeting, NIOSH was given a tour of the entire facility with special emphasis given to the grinding and 

concentrator mill. Shortly after this visit, NIOSH 

submitted a proposal for a cooperative research effort to reduce respirable dust levels in the grinding 

and concentrator facility to the Tilden Mining Company and the United Steelworkers of America. A 

short time later, NIOSH received notification that 

both parties were in agreement and wanted to proceed on this cooperative research effort. 

This cooperative effort has been composed of 

three different studies at the Tilden grinding and 

concentrator facility: March 11-13, 2002, March 5-

7, 2003, and March 9-11, 2004. The first study was

mainly a baseline analysis to determine dust levels

throughout this huge facility and determine research 

areas that could have the most significant impact on 

lowering dust. The next two visits were directed 

towards specific areas of dust control technology. 

All three studies were performed in March for a 

number of reasons. First, the Tilden grinding and 

concentrator mill would switch from producing 

magnetite to hematite throughout the course of the 

ABSTRACT: A cooperative research effort was established between the Tilden Mining Company LC, the 

United Steelworkers of America, and the National Institute for Occupational Safety and Health to lower respirable dust levels in an iron ore grinding and concentrator plant. This cooperative effort involved three field 

studies to evaluate different techniques to lower respirable dust levels. The greatest impact on lowering respirable dust concentrations throughout this facility was achieved through ventilation changes. The most significant improvement was realized by changing the ventilation profile throughout the entire facility. The ventilation to this grinding and concentrator facility was provided by approximately 40 roof fans operating in

either the intake or exhaust mode. The operation of these fans was significantly changed in an attempt to provide more of a directional flow pattern throughout the facility, as well as to more closely balance the intake 

and exhaust air volume. This change accounted for a 31 pct reduction in respirable dust levels in the primary 

grinding area of the mill. Another successful modification was to improve the sealing of a reclaim tunnel to 

the outside and by barricading the access point from the reclaim tunnel to the concentrator plant. This minimized the dust liberated in the conveyor tunnels from traveling through the reclaim tunnel and into the plant. 

These two modifications illustrate the impact that improvements in ventilation can have on lowering respirable dust concentrations in iron ore processing facilities. 
year. Hematite is a dustier product and was typically produced from November through March. All 

parties agreed that this research should be performed 

during the production of hematite in an effort to 

have the greatest impact on lowering respirable dust 

concentrations. Another factor in the timing of this 

research involved traveling in the winter months. 

The Tilden operation is in the upper peninsula of 

Michigan, where winters can be, and usually are 

very severe. In March, winter conditions usually 

begin to ease somewhat, facilitating personnel going

onto the roof of the structure to check on fan operation. The last factor for performing this research 

during the winter months is because the facility is 

typically closed to limit the inflow of cold air into 

the structure. 

The grinding and concentrator mill at the Tilden

operation was an 87,300 m2

 (940,000 ft2) structure 

with an internal air volume of 1,274,000 m3 

(45,000,000 ft3). The ventilation provided to this 

building was through a network of roof fans that can 

be used, for the most part, in either an intake or exhaust ventilation mode. In addition to the numerous

roof-powered fans, each of two replacement air systems supplied 4250 m3

/min (150,000 cfm) of heated 

intake air in the basement of the structure, one on 

the eastern and the other on the western side of the

structure. With this setup, the building had an exhaust capacity of approximately 121,776 m3

/min 

(4,300,000 cfm) and an intake air capacity of 23,500 

m3

/min (830,000 cfm) based upon the rated capacity 

of the fans. Again, 8500 m3

/min (300,000 cfm) of 

this air was from the intake air heaters. The roofmounted fans are the only source of powered exhaust ventilation to the facility, although there may 

have been some natural ventilation as the warm air 

within the structure rises and escapes through cracks 

or openings in the exterior walls or the roof. The 

amount of natural ventilation was minimal in comparison to the amount of air exhausted through the 

roof fans. 

The ventilation setup at this facility changed 

somewhat over the course of the year based on outside air temperatures. Typically, the air exhausted 

from the structure was significantly less during the 

winter months, with the goal being to provide 

enough ventilation to remove contaminants while 

trying to maintain an adequate temperature within 

the structure. During the summer months, the facility is more open to the outside air and the exhaust air 

from the roof fans is maximized to remove the heat 

generated from all the electrical motors and other 

processes within the facility. 

One last area to consider for this research effort is 

the location of various ore processes within the 

structure. Because some of the processes are more 

inherently dusty, certain areas of the grinding and 

concentrator facility typically have higher dust levels. Figure 1 shows an overview of the major processes in this facility. The raw ore mined at the operation is taken to a primary crusher where it is sized 

to less than 25.4 cm (10 inches). The ore is then 

conveyed to a covered storage building that is located along the entire northern portion of the grinding and concentrator facility. When the ore is ready 

to be processed, it flows by gravity into the facility 

onto one of 12 conveyors lines which brings the ore 

up a slope that feeds a primary grinding mill. Each 

of these 12 primary grinding mills then feed 2 pebble mills that further reduce the size of the material. 

After the ore exits the pebble mills and has been 

properly sized, it enters thickener tanks, then flotation tanks, and then back into concentrator thickener 

tanks. From this point, it enters concentrate slurry 

tanks and is filtered. The ore exits the structure and 

is delivered to the palletizing building. 

 The 12 conveyor lines and the primary grinding 

mills are located in the northern portion of the structure, which encompasses approximately half of the 

total area of the building. This is also the dustiest

area of the structure and Tilden management requires that all personnel wear personal protective 

respiratory equipment when in this portion of the facility. The 24 pebble mills encompass approximately a quarter of the facility. The last quarter is 

made up of the concentrator, slurry, thickener tanks, 

and filtering area. The southern portion of the building is much less dusty and does not require work 

personnel to wear respiratory protection. 

Figure 1. Location of major processes in grinding and 

concentrator building. 

 

ACKNOWLEDGEMENT 

The authors would like to acknowledge Leonard R. 

Parker, Manager-Safety & Environmental, Dave

Wickstrom, (Previous) Union Safety Chair, Bill 

Vivian (Union Safety), Greg Magnuson (Ventila-
tion), Dan Sleeman (Ventilation), and Scott Rasmussen, Area Manager-Concentrator Operations, for all 

their help and assistance in this research effort. 

SAMPLING INSTRUMENTATION 

The following section describes some of the instrumentation used in this cooperative research effort. 

Although many types of monitors and instruments 

were used, this discussion will be limited to the dust 

monitoring and air flow instruments used in the two 

ventilation research efforts discussed in this report. 

Gravimetric dust sampling packages and PDR instantaneous dust sampling devices will be discussed, 

followed by the two types of instruments used for 

measuring air velocity. 

Gravimetric dust sampling packages consisted of 

two sampling units, each containing a sampling 

pump, flexible tygon tubing, a respirable dust classifier, and a filter. All dust samples were collected 

with the 10-mm Dorr-Oliver cyclone, which classifies the respirable portion of dust, (usually considered having an aerodynamic diameter of 10 microns 

or less). Each gravimetric pump was calibrated to a 

flow rate of 1.7 liters/min, (the Mine Safety and 

Health Administration’s (MSHA) required flow rate 

for the metal/non-metal industry). The respirable 

dust classified by the 10-mm cyclone was deposited 

on a 37-mm dust filter cassette. These filters were 

pre- and post-weighed to the nearest microgram on a 

microbalance at the Pittsburgh Research Laboratory 

(PRL) and the results of each sampling unit’s filters 

were averaged together to determine an average respirable dust mass. From this mass value, the average respirable dust concentration over the sampling 

period was mathematically derived. 

The instantaneous monitors used for this testing 

were the Personal Data RAM (PDR) from MIE, Inc. 

of Bedford MA. This is a real-time aerosol sampler 

that measures the respirable dust concentration 

based upon the light scattering of particles drawn 

through an internal sensing chamber. The respirable 

dust levels obtained by light-scattering were recorded on an internal data logger and then 

downloaded to a laptop computer at the end of each 

sampling shift. After the dust traveled through the 

PDR, it was deposited on a filter cassette, identical 

to that of a gravimetric sampler. A new filer cassette was used for each shift of testing and was analyzed with the other gravimetric samples upon returning to PRL. For each sampling location, the 

PDR dust value was divided into the average gravimetric concentration to determine a correction factor, which was then applied to all the measurements 

taken with the corresponding PDR for that shift. 

This allowed the instantaneous respirable dust concentrations to be corrected to equivalent measurements taken using the gravimetric method. All PDR 

instrument values shown in this report have been 

corrected in this manner. Using both of these types 

of respirable dust monitoring equipment provided a 

good profile of average dust concentrations throughout the sampling period, as well as variations and 

changes in respirable dust concentrations relative to 

time throughout the workday. 

In addition to the respirable dust measurements, 

airflow measurements were taken using a Solent 

Velometer from Gill Instruments Limited of Hampshire, England to evaluate the airflow at different locations throughout the grinding and concentrator 

mill building. This velometer is an ultrasonic anemometer that operates by transmitting an ultrasonic 

pulse along the wind path and measuring the time 

taken to reach the receiver. In essence, the faster the 

wind speed, the faster the pulse reaches the receiver. 

The velometer was held stationary and a 10-sec 

sample period was started. At the end of the 10 seconds, the instrument provided average, maximum 

and minimum air velocities for that specific time period. 

In addition, velocity measurements were also 

taken with a vane-anemometer from Davis Instruments of Baltimore, MD. The Davis anemometer 

was a 10 cm (4-in) diameter, eight-blade instrument 

that can measure air velocities from 0.15 – 25 m/s 

(30 to 5000 ft/min). It contains a balanced vane 

wheel that rotates in response to airflow. Rotations 

are mechanically transmitted to an indicator through 

a clock-like movement. 

 

TESTING AND RESULTS 

In this cooperative research effort, a number of different areas were investigated in an effort to lower 

respirable dust concentrations at the Tilden iron ore 

grinding and concentrator facility. It should be 

noted that, although the goal was to lower respirable 

dust concentrations throughout the entire facility, the 

main priority was the primary grinding area. In this 

report, we will describe the two most successful efforts, both of which were directly tied to ventilation 

changes. The first area to be presented is the impact 

that lowering the amount of ventilation flowing 

through the reclaim tunnel had on lowering respirable dust levels in the western portion of the structure. The second area is the efforts to improve the 

ventilating airflow pattern throughout the entire 

grinding and concentrator mill using the roofmounted fans in a more direct and balanced flow 

pattern and the impact that this had on respirable 
dust levels, specifically in the primary grinding mill

area. 

Improving ventilation and dust in reclaim tunnel 

When performing our baseline testing during the 

first evaluation in March 2002, it was determined 

that dust levels increased from the eastern to the 

western portion of the reclaim tunnel. The reclaim 

tunnel is a 2.1 m (7 ft) wide by 2.3 m (7 1⁄2 ft) high 

opening that runs the entire length of the facility 

along the northern portion of the structure. The 

function of the reclaim tunnel is to provide access 

for workers and equipment to the 12 conveyor lines. 

Workers are not normally in the reclaim tunnel 

unless they need to access one of the discharge 

chutes or conveyors, which moves iron ore from the 

storage area to one of the 12 primary grinding mills. 

During a walk-through, it appeared that there was a 

substantial amount of air moving through this tunnel 

and there appeared to be no practical reason for this

amount of ventilation. 

To focus on the reclaim tunnel, a test sequence 

was performed on the daylight shift of March 6, 

2002. Eleven different dust sampling locations provided a profile of respirable dust concentrations

along the tunnel, as well as in some of the conveyor

slopes. These monitors were started over a one-anda-half hour time period and sampled for between 

five and six hours. Six of these samples were located along the reclaim tunnel, with a seventh sample location at the midpoint of a lower north-south 

tunnel that connected the reclaim tunnel to the main

building basement. This connecting tunnel was located between primary mills 10 and 11 and connected the reclaim tunnel to the primary grinding 

and concentrator mill about 9 m (30 ft) below the

main working elevation. The remaining four sampling locations were at conveyor transfer locations 

in tunnels 6, 7, 9, and 10. These 11 dust sampling 

locations provided a good overview of respirable 

dust concentrations for the tunnel In addition, velocity measurements were also taken along the entire 

length of the reclaim tunnel, in the connecting tunnel 

to the basement of the grinding and concentrator 

building, and in each of the 12 conveyor slope tunnels. 

The dust concentrations and ventilation measurements taken are shown in Figure 2. The values 

listed in this figure show a correlation between respirable dust concentrations and airflow measurements. From a dust control standpoint, respirable 

dust levels increased as one moved from the eastern 

to the western side of the reclaim tunnel. Once the 

airflow traveled a short distance past tunnel 11, it 

would make a 90 degree turn and flow down the 

connecting tunnel to the main structure. As seen, 

the respirable dust concentration in this connecting 

tunnel was lower than the eastern dust values (tunnels 1-11) along the reclaim tunnel because the dust 

Figure 2. Respirable dust and airflow measurements during 

baseline testing in 2002. 

was being diluted with an additional 144 m3

/min 

(5,100 cfm) of ventilating airflow that was coming 

from conveyor tunnel 12 and was traveling in the reclaim tunnel from west to east. Although no dust 

measurements were taken, it is assumed that the respirable dust concentration of this air was very low. 

When this airflow was combined with the airflow 

traveling from east to west, it diluted the respirable 

dust concentration down to 0.65 mg/m3

. The velocity measurements in the reclaim tunnel show the

magnitude of airflow in this tunnel. The airflow direction in the conveyor slopes varied with some air 

moving up the slope towards the grinding and concentrator building and others with air moving down 

towards the reclaim tunnel. For those tunnels that 

were ventilating downwards, they were introducing 

respirable dust generated during conveying and at 

transfer points into the reclaim tunnel. As this air 

moved from east to west in the tunnel, each additional conveyor slope would add additional dust to 

the airstream. 

After analyzing the data from this test, researchers 

concluded that the 620 m3

/min (21,900 cfm) of air 

being discharged into the basement of the structure 

from the connecting tunnel needed to be reduced. 

This volume of air had a respirable dust concentration of 0.65 mg/m3

, which was introducing a significant quantity of dust into the grinding and concentrator mill building. This volume of dusty air caused 

respirable dust concentrations to be higher in the

basement and first floor of the structure in the western portion of the building, as compared to the center or right portions of the structure. 

A factor in the amount of air moving through the 

reclaim tunnel was that the seal at the eastern end of 
the reclaim tunnel was badly damaged and was allowing a significant quantity of air into the tunnel. 

Air measurements at this location indicated that approximately 144 m3

/min (5,100 cfm) of air was

coming into the reclaim tunnel from this damaged 

seal. 

The other area of concern was the amount of air 

flowing from the reclaim tunnel into the grinding 

and concentrator structure basement from the connecting tunnel. Through discussions with all the cooperating parties in this research effort, it was decided to replace the damaged seal at the eastern end

of the tunnel and to install a seal where the connecting tunnel accessed the main structure. 

When returning for the 2003 study, NIOSH was

pleased with the quality of the seals installed at both 

locations by Tilden personnel, shown in Figure 3a,b. 

The first test performed during this study was to repeat the evaluation of the reclaim tunnel area. Identical respirable dust sampling and airflow measurement locations were used as performed in the 

previous study. 

Figure 3a. Seal replaced at eastern end of reclaim tunnel to

access outside

Figure 3b. Seal installed where connecting tunnel accesses 

main structure in basement of facility. 

The results of this test can be seen in Figure 4. 

As shown, dust levels constantly increased when 

moving from the east to west in the reclaim tunnel. 

For a number of areas, one could visually see dust 

flowing down the conveyor slope and into the reclaim tunnel. Average respirable dust levels increased from 0.21 mg/m3

 between tunnels 2 and 3 to 

2.44 mg/m3

 between tunnels 10 and 11. Respirable 

dust levels in the lower connecting tunnel averaged a 

concentration of 1.27 mg/m3

 over the entire sampling period. 

Figure 4. Respirable dust and airflow measurements during 

2003 testing after installation of seals. 

When comparing the results from pre to post-seal 

conditions, there are a number of areas that standout. 

First, during pre-seal testing, there was 144 m3

/min 

(5100 cfm) of air flowing from the door at the eastern most portion of the reclaim tunnel; this was reduced by 76 pct to a level of 34 m3

/min (1200 cfm)

under post-seal conditions. In addition, there was a 

reversal in the airflow direction in a number of the 

sloped conveyor tunnels. For the lower numbered 

tunnels, the air quantity moving up or down the tunnel during post-seal was substantially lower than 

during pre-seal conditions. Conveyor tunnel 9 

showed a significant increase in airflow quantity

traveling up the tunnel with 215 m3

/min (7600 cfm)

for post-seal, as compared to 2.4 m3

/min (5100 cfm)

for pre-seal. The effectiveness of the barricade on 

the lower level connecting tunnel is also verified by 

these values. The airflow measured in the connecting tunnel during post-seal was 22.7 m3

/min (800 

cfm), which is a 96 pct reduction from the 620 

m3

/min (21,900 cfm) measured during pre-seal conditions. 

When considering the dust values, it may appear

that improving the seal in the reclaim tunnel was

counter productive because respirable dust concentrations increased at some locations. This increase

was first noticed at the midpoint for mills 9 and 10 

and remained evaluated at the next two sample loca-
tions. The dust liberated in the various conveyor 

slope areas and drawn into the reclaim tunnel was 

diluted by a much lower quality of air, and thus, the 

reason for the higher dust concentration at the western portions. To consider the true impact of the

seals on lowering respirable dust flowing into the 

grinding and concentrator building from the reclaim 

tunnel, both the airflow and the respirable dust concentrations must be considered. For pre-seal test 

conditions in 2002, there was 620 m3

/min (21,900 

cfm) of air traveling into the main structure at a respirable dust concentration of 0.65 mg/m3

. Through 

a simple calculation, it was determined that 24,188 

mg of respirable dust was being delivered into the 

main building every hour under pre-seal conditions. 

And although the respirable dust levels are higher

under post-seal conditions at a level of 1.27 mg/m3

, 

when one considers this value with the reduction in 

airflow, only 1729 mg of respirable dust is being delivered into the main building every hour, a 93 pct 

reduction over pre-seal conditions. 

The most significant benefit from the installation 

of seals and minimizing the airflow through the reclaim tunnel is the significant reduction in the 

amount of respirable dust being introduced into the 

western portion of the building. In the pre-seal testing, there was a 67 pct increase in respirable dust 

concentrations at primary mill 12 (0.67 mg/m3

) on 

the western side of the building as compared to respirable dust levels at mill 1 (0.44 mg/m3

) on the 

eastern side. When the exact study was performed 

again with post-seal conditions, both dust measurements were almost identical: 0.84 mg/m3

 at mill 12 

and 0.87 mg/m3

 at mill 1. Although the respirable 

dust concentrations at both locations went up, this is 

a relative value based on the amount of production 

and ore being processed at any given time period. 

Respirable dust levels being identical on the eastern 

and western side of the structure indicates the impact 

that the seals had on minimizing the amount of airflow traveling through the reclaim tunnel. 

Entire structure ventilation 

During the testing performed during 2003, an entirestructure ventilation change was performed which 

indicated promising results. This led to a more indepth study performed during 2004 which involved 

changing the quantity of air that entered and was exhausted from the structure. During the first day of 

testing, the operation’s normal ventilation setup was 

evaluated. Figure 5 shows a summary of this ventilation setup with the exhaust and intake fans differentiated by color-coding of the fan symbol. To 

evaluate this original ventilation design, the structure was broken down into three distinct areas: primary grinding (northern-most portion), secondary 

grinding with pebble mills, and filtering and flota

tion (southern-most portion). Respirable dust samplers were located in these three areas, with the 

greatest focus being on the primary grinding mills. 

This original ventilation setup included twelve exhaust fans in the primary grinding area, eight exhaust fans and three intake fans in the secondary 

grinding area, and nineteen exhaust fans in the filter 

and flotation area. The only other mechanical ventilation provided was from two intake heater fans located at the base of the facility, one located on the 

east and the other on the west side of the building. 

The volume of air being exhausted from the structure was 54,670 m3

/min (1,930,500 cfm), as compared to 17,000 m3

/min (600,000 cfm) being 

brought into the structure. These ventilation air volumes were calculated based upon the fan quantities 

provided by the facility and the assumption that all 

the fans were operating at their rated output. The 

two intake heater fans accounted for 8,500 m3

/min 

(300,000 cfm), or 50 pct of the total intake airflow. 

Dust sampling under this ventilation setup was performed for approximately 20 hours of testing. 

Figure 5. Baseline or normal ventilation setup.

 

The imbalance between the intake and exhaust air

volumes with this ventilation setup was substantial; 

therefore it was decided to modify the ventilation to

a more balanced design. It should to be noted that 

although the goal was to lower respirable dust concentrations throughout the entire facility, the main 

priority was to lower respirable dust concentrations 

in the primary grinding area. Dust levels in the primary grinding area were higher than in the other areas because of the amount of dust liberated from the 

numerous conveyor lines feeding the primary grinding mills, as well as the amount of dust liberated

from the mills themselves. 

To focus on this area, a ventilation change was 

initiated as shown in Figure 6. The two intake

heater fans continued to bring a significant portion 

of intake air into the facility. For this modified design, there were eleven exhaust fans operating in the 

primary grinding area, twelve intake fans in the sec-
ondary grinding area, and eight exhaust fans operating in the filtering and flotation area. The volume of 

air being exhausted from the structure was approximately 26,600 m3

/min (940,500 cfm), with approximately 29,600 m3

/min (1,045,000 cfm) brought 

into the structure through the roof intake fans and 

two heater fans. This ventilation design was again 

tested for approximately 20 hours. 

Figure 6. Ventilation setup # 2. 

During this phase of testing, a smoke flare was 

released into one of the intake fans over the pebble 

mill. Since properly operating intake fans should 

move an air parcel 10 fan diameters before being reduced to 10 pct of the fan’s air velocity, the roofmounted fans should introduce a parcel of air into 

the lower parts of the building (Industrial Ventilation, 2001). These roof-mounted fans are all 1.2m 

(4 ft) in diameter and should introduce a parcel of air 

at least 12.2 to 15.2 m (40 to 50 ft) into the structure. 

Since this testing was performed during the winter 

months, introducing cold outside air into the structure should also assist the air parcel’s ability to flow 

down into the lower levels of the structure. From 

visual observations during this smoke flare release,

it appeared that the intake fan chosen for this testing 

was not operating at the calculated airflow. The 

smoke remained within the top one-quarter of the 

structure and tended to drift towards the southern 

portion of the building (toward the flotation and filtration area). As test personnel walked through the 

secondary grinding area, it was very difficult to feel 

the intake air being delivered into the structure by 

the intake fans. Since the walkway was only 30 ft

below the intake fans, airflow should have been perceivable at this level. 

Because the intake air appeared to be drifting towards the flotation and filtration area, it was decided 

to modify the ventilation setup to provide a greater 

quantity of the intake air moving in the northern direction towards the primary grinding mills. Figure 7 

indicates the second modification to the ventilation

design which was evaluated for approximately 8 

hours during the last day of testing. This ventilation 

setup was composed of 24 exhaust fans and 12 intake fans, along with the two heater intake fans. The 

volume of air exhausted from the structure was ap 

proximately 32,000 m3

/min (1,128,000 cfm), with 

29,600 m3

/min (1,045,000 cfm) being brought into 

the structure through the roof intake fans and two 

heater fans. The amount of intake air was identical 

to the previous test, but the exhaust was increased in 

an attempt to decrease the flow toward the flotation

and filtration area while increasing the airflow to the 

primary grinding mill area. 

Figure 7. Ventilation setup # 3.

Table 1 shows the average respirable dust concentration as determined by the instantaneous respirable dust monitors used in this study and provides a

comparison of the three different ventilation designs 

tested. Obviously, the last ventilation design was

the most effective at lowering respirable dust levels, 

Table 1. Average respirable dust concentrations in mg/m3 for 

three ventilation setups. 

__________________________________________________ 

Location Setup # 1 Setup # 2 Setup # 3

Day Night Day Night Day __________________________________________ 

Primary # 2 1.06 1.21 0.66 0.38 0.44 

Primary # 4 0.72 0.70 0.59 0.59 0.45 

Primary # 6 1.15 1.34 1.37 1.49 0.98 

Primary # 8 1.23 1.34 1.18 1.23 0.86 

Primary # 10 - 0.97 1.05 0.91 0.72 

Primary # 12 0.67 0.72 0.79 0.88 0.75 

 Average 0.97 1.05 0.94 0.91 0.72 

Pebble # 1 0.32 0.38 0.38 0.31 0.29 

Pebble # 3 0.42 0.42 0.47 0.51 0.42 

Pebble # 6 0.34 0.35 0.48 0.50 0.35 

Pebble # 9 0.49 0.50 0.51 0.56 0.49 

Pebble # 12 0.38 0.41 0.36 0.43 0.38 

 Average 0.39 0.41 0.44 0.46 0.38 

Flotation # 3 0.25 0.32 0.35 0.34 0.24 

Flotation # 9 0.27 0.34 0.45 0.40 0.31 

 Average 0.26 0.33 0.40 0.37 0.27 

particularly in the primary grinding area, which was

the main area of concern. When compared to the 

first two ventilation designs, this design lowered 

respirable dust concentrations in the primary grinding area by 31 and 25 pct, respectively. This is a

very significant reduction when one considers it was

achieved by using the existing fans and simply varying the intake and exhaust flow quantities. Additionally, respirable dust levels were also slightly 
lower in the pebble mill and the flotation and filtration areas with the last ventilation setup. This study 

indicates the impact that ventilation can have on 

lowering respirable dust levels throughout a largevolume iron ore processing structure. 

CONCLUSIONS 

A cooperative research effort was established between the Tilden Mining Company LC, the United 

Steelworkers of America, and the National Institute 

for Occupational Safety and Health in an effort to 

lower respirable dust levels in Tilden’s grinding and 

concentrator facility located in Ishpeming, Michigan. The two most significant impacts in this cooperative effort dealt with ventilation changes. The 

first change was to minimize the amount of air traveling through a reclaim tunnel before flowing into 

the basement of the structure. As the air moved 

through this reclaim tunnel, it entrained a substantial 

quantity of respirable dust generated from the twelve 

conveyor lines. As the air moved through this reclaim tunnel, it entrained a substantial quantity of 

respirable dust generated from the twelve conveyor 

lines. As this dust laden air then entered the main 

structure through a connecting tunnel, it caused respirable dust levels to be 61 pct higher on the western 

side of the building as compared to the eastern side. 

In an effort to eliminate this occurrence, a seal on 

the very eastern side of the structure, where the reclaim tunnel accessed the outside environment, was 

rebuilt. In addition, a new seal was installed where 

the reclaim tunnel accessed the main structure. Both 

of these seals substantially lowered the amount of air 

flowing through the reclaim tunnel and entering the 

main structure. Although this lower air flow caused 

dust levels in the reclaim tunnel to be higher at some 

locations, this was not a significant factor since personnel are not normally in this tunnel. The benefit 

of this change was that respirable dust levels were 

lowered in the western side of the main grinding 

area to a point that they were the same as on the 

eastern side of the structure. 

The second and more significant improvement 

identified in this cooperative research effort was accomplished by modifying the way the entire structure was ventilated. The effort involved monitoring 

respirable dust levels throughout the entire structure 

for three ventilation schemes. The first setup tested 

was as the structure was normally ventilated (by Tilden). For this setup, the exhausted air was over 

three times greater than the intake air, which caused 

the structure to be under negative pressure. For the 

second setup, the ventilation was changed to a more 

balanced design and improved airflow pattern within 

the structure. Based upon the configuration of the 

fans, it was decided to bring the intake air into the 

building at the center of the structure and exhaust it 

northward toward the primary grinding mills and 

southward toward the flotation and filtering areas. 

A smoke flare released into one of the intake fans 

indicated that a substantial portion of the air was 

drifting south towards the flotation and filtering areas, than was flowing north. Moreover, although the 

intent was to lower respirable dust levels throughout 

the entire structure, the main priority was to lower 

levels in the primary grinding area, where workers 

were required to wear respirators. 

A third ventilation setup was tested which increased the number of exhaust fans in the northern 

portion and reduced them in the southern portion of 

the structure. This last ventilation design was the 

most effective of the three tested and provided a 31 

pct reduction in respirable dust levels in the primary 

grinding area when compared to levels measured for 

the first setup. Respirable levels were also slightly 

lower in the pebble mill and flotation and filtration 

areas. This is a substantial reduction when considering that it came about by creating a more balanced 

flow pattern, but still using the same amount of fans. 

In closing, both of these research efforts indicate 

the significant and cost-effective impacts that ventilation changes can have on lowering respirable dust 

levels in a very large iron ore facility. 

REFERENCES 

 Industrial Ventilation – A Manual of Recommended Practices, 

2001 – 24th edition (American Conference of Governmental 

Industrial Hygienist: Cincinnati)