2013 TRANSACTIONS • Vol. 334 SOCIETY FOR MINING, METALLURGY, AND EXPLORATION 444

Paper number TP-12-066. Original manuscript submitted November 2012. Revised manuscript accepted for publication July 

2013. Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. 

prior to Sept. 30, 2014. Copyright 2014, Society for Mining, Metallurgy, and Exploration, Inc.

444 • 2013 TRANSACTIONS OF THE SOCIETY FOR MINING, METALLURGY, AND EXPLORATION

Case study of controlled recirculation

at a Wyoming trona mine

C. Pritchard, D. Scott and G. Frey

Mining engineer, research physical scientist and general engineer, respectively,

National Institute for Occupational Safety and Health (NIOSH),

 Office of Mine Safety and Health Research, Spokane, WA

Abstract

Controlled recirculation has been used in the metal/nonmetal mining industry for energy savings when 

heating and cooling air, in undersea mining and for increasing airflow to mining areas. For safe and 

effective use of controlled district recirculation, adequate airflow to dilute contaminants must exist prior 

to implementation, ventilation circuit parameters must be accurately quantified, ventilation network 

modeling must be up to date, emergency planning scenarios must be performed and effective monitoring 

and control systems must be installed and used. Safety and health issues that must be considered and 

may be improved through the use of controlled district recirculation include blasting fumes, dust, diesel 

emissions, radon and contaminants from mine fires. Controlled recirculation methods are expected to 

become more widely used as mines reach greater working depths, requiring that these health and safety 

issues be well understood. The U.S. National Institute for Occupational Safety and Health (NIOSH) 

conducted two controlled recirculation tests over three days at a Wyoming trona mine, utilizing an inline 

booster fan to improve airflow to a remote and difficult-to-ventilate development section. Test results 

were used to determine the effect that recirculation had on air qualities and quantities measured in that 

section and in other adjacent areas. Pre-test conditions, including ventilation quantities and pressures, 

were modeled using VnetPC. During each test, ventilation quantities and pressures were measured, as 

well as levels of total dust. Sulfur hexafluoride (SF6) tracer gas was used to simulate a mine contaminant 

to monitor recirculation wave cycles. Results showed good correlation between the model results and 

measured values for airflows, pressure differentials, tracer gas arrival times, mine gasses and dust levels.

Introduction

Controlled district recirculation has been effectively used

in U.S. metal and nonmetal mines. Marks and Shaffner (1989)

document its use at the Homestake Mine, SD, where return

air was recirculated from the exhaust shaft through an underground cooling spray chamber and returned to deep, hot mine

workings. Controlled district recirculation can be an important

consideration, as active workings progress deeper or extend a

considerable distance undersea (Hannon, 1987). Mine return

air is often of good quality, and controlled recirculation can

augment airflow to mining areas, improving miner safety and

health, working conditions and providing operational savings

(Hall et al., 1987; Meyer, 1993; Pritchard, 1995). Robinson

(1989) showed that additional air quantity at coal mining faces

and in development sections can improve mixing of face methane emissions, reducing the risk of explosion. Safety and health

issues associated with controlled recirculation include blasting

fumes (Wu, 2001), dust control (Hardcastle, 1985), face and

section methane dilution (Longson et al., 1987), diesel emissions (Hall et al., 1987; Cecala et al., 1991), mine monitoring

(Saindon, 1987) and mine fires (Cecala et al., 1991). The U.S.

National Institute for Occupational Safety and Health (NIOSH)

investigated several of these health and safety issues related to

controlled district recirculation systems through field testing

at a nonmetal trona mine in Wyoming. 

The Mine Safety and Health Administration (MSHA)

defines trona operations as Gassy Type III metal/ nonmetal

operations that liberate methane, often in significant amounts

(Pritchard et al., 2004). The mining process and equipment

are very similar to room-and-pillar coal mining, with permissible face equipment inby and diesel-powered, rubber-tired

support equipment outby. Ore is mined by a continuous miner

or longwall system, hauled from the face to feeder breakers

by shuttle cars, transported to the shaft by conveyor belts and

hoisted in skips to the surface. Typical mining depth is 450

m (1,500 ft). Recirculation of return air from mining areas is

not permitted due to MSHA 30 CFR Part 57 Subpart T gassy

mine regulations (CFR, 2011) and Wyoming State Statutes,

which incorporate MSHA laws (Wyoming Statutes, 2011). The

primary source of methane is oil shale above and below the

trona bed. Emissions during development are minor and decay

rapidly, but are significant where caving takes place, such as

during longwall retreat. Emissions arise from fragmented oil

shale from the immediate roof and from floor cracks due to

Key words: Ventilation, Dust control, Air quality, Health and safety, Trona

2013 Transactions of the Society for Mining, Metallurgy and Exploration, Vol. 332, pp. 444-448.
SOCIETY FOR MINING, METALLURGY, AND EXPLORATION Vol. 334 • 2013 TRANSACTIONS 445

2013 TRANSACTIONS OF THE SOCIETY FOR MINING, METALLURGY, AND EXPLORATION • 445 

Figure 1 — Booster fan/controlled recirculation test area.

Figure 2 — Booster fan/controlled recirculation test panel detail from Fig. 1.

heaving. Sealing of mined-out areas has been used in the past 

but is not presently practiced.

Previous testing used a booster fan at the Wyoming trona 

mine operation to increase airflow to a remote mining panel 

(Pritchard et al., 2012). The study examined the impacts of 

booster fan use on ventilation airflow quantity and quality in a 

mining district consisting of an idle test panel, a longwall retreat 

panel and three outby longwall development panels (Fig. 1). 

Those tests showed that booster fan operation dramatically 

increased airflow in the idle panel from 5 m3/s (10,000 cfm) to 

14.2 m3/s (30,000 cfm) and, finally, to 23.6 m3/s (50,000 cfm). 

Results showed a reduction in outby airflow by approximately 

the same amount that the booster fan increased airflow to the 

test panel. Also, the booster fan overpressurized the panel return 

airway, causing a high percentage of the additional test panel 

airflow to leak back into fresh air. Those test results showed that 

booster fans may provide a potential option to increase section 

airflow, but outby recirculation effects must be considered.

Controlled recirculation test design

NIOSH used the same trona mine and test location for two 

additional controlled district recirculation tests over three

working shifts. Baseline readings were taken during the first 

shift, with testing conducted during the last two. The booster 

fan, shown in Fig. 2, increased airflow by recirculating return 

air through an open stopping/regulator (“R”) at the panel return 

corner. It was hypothesized that local recirculated air would 

affect the outby longwall and development panel airflows less 

than the previous booster fan test, due to lower booster faninduced pressure in the ventilation circuit. Recirculated airflows 

of 50% and 100% greater than the present 4.7 m3/s were tested 

while monitoring for dust, ventilation airflows and differential 

pressures to assess the health and safety aspects of controlled 

recirculation. The recirculated airflows of 7.1 and 9.4 m3/s 
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446 • 2013 TRANSACTIONS OF THE SOCIETY FOR MINING, METALLURGY, AND EXPLORATION

(15,000 and 20,000 cfm) were at 

the high end of practical controlled 

recirculation levels in order to show 

“worst case” scenarios.

Prior to the previous booster fan 

test, a mine network model was 

developed using VnetPC software 

from the engineering consulting 

firm Mine Ventilation Services. 

The model was then updated and 

analyzed to examine the impact of 

recirculation. The analysis indicated that the recirculation would 

show negligible effects on outby 

active mining section airflows. 

Theoretically, allowing the inline 

fan to perform as a controlled recirculation fan instead of as a pure 

booster fan, which increased test 

panel circuit pressure and airflow, would not affect the 

remainder of the ventilation circuit nearly as much. 

The testing occurred in an isolated room-and-pillar 

panel development at the south end of the submain 

that was idle, had no active production and was not 

considered return air, which would have prohibited 

recirculation in a gassy mine. The test panel was a 

three-entry development approximately 2.5 m (8 ft) 

high by 4.3 m (14 ft) wide, 1,500 m (5,000 ft) long, 

with two intake entries and a single return separated 

by styrofoam block stoppings. Using standard areas 

and scaled lengths, approximate airflow transit times 

through the test panel from booster fan to the face 

and back to the recirculation point were calculated 

to be 1.9 hours, with the booster fan operating at 7.1 

m3/s (15,000 cfm) and 1.4 hours when operating 

at 9.4 m3/s (20,000 cfm). Controlled recirculation 

would be induced by an inline 1.5-m (60-in.) diameter, 93-

kW (125-hp), booster fan with airflow adjusted by a variable 

frequency drive (VFD) controller. Outby panels included a 

longwall retreat panel and three longwall development panels 

(Fig. 1). Active mining was on the opposite (west) side of the 

submain, sharing a common intake with the test area but having 

separate returns. The longwall return was on a separate bleeder 

circuit and development sections used the submain west returns 

flowing north. The recirculation area used the east submain 

return, joining the west return to the north. Submain airflows 

and pressure differentials were measured prior to and during 

tests to gauge the effects of district recirculation on the local 

ventilation circuit. Personal data rams (PDRs) (Volkwein et al., 

2004) were used to monitor total dust in four locations: outby 

the fan, six crosscuts inby the fan, 12 crosscuts inby the fan, 

and at the outby end of the test panel return just prior to the 

recirculation regulator opening. Dust levels were monitored to 

determine the amount of dust settling at different points in the 

ventilation circuit and to quantify the amount of dust entering 

the intake air. As a rule, recirculated air should not contain 

higher dust concentrations than intake air (Cecala et al., 1991). 

Other than total dust, no additional contaminant sources were 

expected due to panel inactivity. To simulate such sources, a 

steady-state release of SF6 tracer gas was performed (Timko 

et al., 1982). A continuous SF6 release apparatus placed just 

outby the booster fan was assembled consisting of a lecture 

canister of gas, regulator, an adjustable valve to act as a variable critical orifice and a magnehelic pressure gauge attached 

to both ends of a laminar flow element (LFE). SF6 flow rates 

Table 1 — Controlled recirculation base, test and modeled airflow (m3/s).

Location

Base

Measured

m3/s 

Test 2

Measured

m3/s 

Base

Model

m3/s 

Test 2

Model

m3/s 

Test area intake 4.3 7.1 4.5 7.1

Test area return 4.3 7.0 4.5 7.0

Test area return outby 4.4 4.0 4.7 4.7

Longwall intake 72.9 65.7 73.5 73.5

HG #6 return 15.7 15.8 18.4 18.4

HG #7 return 16.4 16.5 19.8 19.8

HG #8 return 9.2 10.1 10.4 10.4

West return outby 11.1 11.4 12.7 12.7

Submain intake outby 122.9 115.8 134.6 134.6

East return outby 6.0 5.5 6.6 6.6

Table 2 – Detailed differential pressures in controlled recirculation 

circuit for base, test and model. Note: Crosscut (XC) values increase 

from 139 at the north end (Fig. 1) to 207 at the south end.

Location

Base

Measured

Pa

Test 2

Measured

Pa

Base

Model

Pa

Test 2

Model

Pa

207 XC - West 195 199 223 223

182 XC - West 58 57 64 64

174 XC - West 72 69 71 71

174 XC - East 46 35 69 67

165 XC - West 235 213 211 211

139 XC - West 344 310 372 372

139 XC - East 205 173 298 286

were measured with the calibrated magnehelic gauge using a 

secondary standard relating flow to measured pressure loss 

across the small diameter LFE. Gas canister pressure was 

reduced using the regulator to a point at which the valve could 

regulate the tracer gas. SF6 release rates of 480 cc/min were 

calculated for test panel airflow to provide the necessary gas 

chromatography analysis concentration range and to allow 

adequate gas flow for the eight-hour test period. SF6 samples 

were taken at the test panel intake and exhaust locations.

Controlled recirculation tests

The airflow quantity in the test panel measured on Day 1 

during normal mine operation was 4.3 m3/s (9,100 cfm), close 

to the modeled 4.7 m3/s (10,000 cfm) value. Airflow rates for 

the following two days were a 50% and 100% increase above 

baseline airflow. Controlled recirculation Test 1 was plagued 

by multiple mine power interruptions, compromising its value. 

Thus, contaminant and SF6 data were not used. Test 2 airflow 

was 7.1 m3/s (14,900 cfm) and that test experienced only one 

brief local mine power outage. Airflow and pressure differential 

readings for Test 2 are shown in Tables 1 and 2, respectively. 

Controlled recirculation Test 2 results. The booster fan 

increased the baseline airflow of 4.3 m3/s by 63% (the increase 

was actually 75% during the test in that panel intake baseline 

airflow had dropped slightly to 4 m3/s), corresponding to a 

recirculation percentage of 44% (3.1 m3/s ÷ 7 m3/s). Airflow 

readings prior to and during the test showed a negligible effect 

of the controlled recirculation on the submain intake, longwall 
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2013 TRANSACTIONS OF THE SOCIETY FOR MINING, METALLURGY, AND EXPLORATION • 447 

6

Figure 3 — Total dust concentrations at 12 XC inby booster fan.

Figure 4 — Return dust concentration prior to recirculation point, with time lag 

and activity spikes.

intake, and longwall development section (HG #6, 7 and 8) 

airflows, thus validating modeling data (Table 1). Detailed 

pressure differential readings across various submain stoppings/

mandoors (Table 2) showed good correlation to relative pressure changes when comparing modeling to measured readings. 

The various differences are explained below.

Longwall airflow gradually dropped by 7.2 m3/s (15,000 

cfm) over the three days of testing as the longwall passed 

through an intersection, increasing submain circuit resistance, 

which reduced longwall and submain intake airflow. The outby 

development panels showed a slight airflow increase during 

this period due to the longwall resistance increase. Because 

longwall development panels and the test panel shared a common return further outby, this higher airflow may have contributed to lowering the outby recirculation panel airflow from 

4.4 to 4 m3/s. Deviations noted in test airflows are a reminder 

of daily ventilation system variations and must be taken into 

consideration during mine ventilation planning.

Measured submain differential pressures shown in Table 2 

also showed negligible change except on the East Return. The 

controlled recirculation entry was not opened to full entry width 

(see regulator in Fig. 2), causing some minor backpressure in 

the controlled recirculation system. This is evidenced by the 

lower pressure differentials across the

East Return in Table 2 and the reduced

airflow in Table 1. Higher longwall

system resistance from passing through

the intake crosscut and lower intake submain airflow correlates with other lower

pressures measured. Modeled pressure

differentials correlated well, especially

at the inby end of the ventilation circuit,

where differential pressures were low

and airflows were often variable.

Dust levels at 12 XC inby the fan at

the third monitoring location showed

spikes from outby mobile equipment

and foot traffic, which were elevated

at the beginning of the shift and tailed

off afterward, as shown in the activity

peak in Fig. 3. All three monitoring

locations from XC 1 to XC 12 had the

same total dust concentration of 0.50

mg/m3, with minimal dust settling in this

400-m (1,300-ft) zone. Figure 4 shows

the average total dust concentration at

the return location regulator (Fig. 2)

prior to recirculation as 0.16 mg/m3,

with many concentration spikes. During the field studies, results showed it

became important not to locate dust

monitoring equipment downwind from

activity, as local foot traffic disturbed

floor dust while checking monitors,

causing localized dust concentration

spikes. Even with the induced spikes,

approximately 70% of the total dust

settled in the remainder of the intake

and the 3,100-m-long (10,000-ft-long)

return, highlighting the valuable contribution of return airways as dust settling

zones and emphasizing the importance

of selecting the correct recirculation

point. Previous work (Cecala et al.,

1991; Hardcastle, 1985) indicates that

the recirculation fan should be located a minimum of 600 to 915

m (2,000 to 3,000 ft) from an active mining area for this reason.

Comparing the activity peak dust level in Fig. 3 with the

corresponding peak in Fig. 4 shows a transit time of approximately two hours through the panel, corresponding to the pretest calculated panel SF circulation time. 

Figure 5 shows contaminant surrogate SF6 concentrations

at the return sampling point prior to recirculation. Anticipated

concentrations would increase to 200 ppb at Wave 1 after the 

1.9-hour test panel transit time, asymptotically approaching

330 ppb after a few recirculation cycles. Results show that

SF6 was released at about twice the 480 cc/min design rate

due to instrumentation issues, as shown in the upper SF6 line. 

The gradual increase in measured concentration instead of

instantaneous arrival was due to SF6 mixing and diffusion as it 

traveled the 3,100-m (10,000-ft) distance from fan to recirculation point in the pre-existing test panel air volume. The delayed

mixing process was nearing expected values after wave three. 

SF6 testing results and recirculation calculations confirm

that contaminant concentrations rise to a defined level based

on the percentage of air recirculated with constant face emission rates (Robinson, 1989). Figure 6 shows that, even with

previously stated SF6 mixing issues, the return concentration
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Figure 5 — Measured return location of SF6 tracer gas 

concentration (ppb) vs. time. SF6 planned release of 480 

cc/min vs. higher estimated 960 cc/min rate concentrations. 

Wave arrival times are based on calculated system transient 

time of 1.9 hrs.

Figure 6 — Relative relationship between SF6 intake (solid) 

and return SF6 concentration (dashed) with time.

does not exceed the original nonrecirculated concentration

and corresponds well with calculations. Intake contaminant

concentrations increased after three recirculation cycles, to

approximately 40% of total return concentration, near the

ultimate test intake recirculation percentage value of 44%,

which would have been attained in a few additional cycles.

Summary and conclusions

A series of tests were performed in a room-and-pillar environment to examine the effectiveness of controlled recirculation 

on improving airflow quantity and quality in a remote mining 

panel, and to assess the impacts on airflows in adjacent active 

areas. These tests used an inline booster fan to increase the 

4.3-m3/s panel airflow by 50% and 100%. Recirculation was 

controlled by opening a regulator installed between panel intake and return at the panel mouth. Pretest modeling showed 

similarities to in-mine controlled recirculation test-measured 

airflows and pressure differentials, with negligible effects on 

airflows in adjacent areas. Controlled recirculation involved 

no leakage from return to intake outby the test panel, and no 

change in airflow quantities in the outby mining sections. 

Recirculated air contained less total dust by virtue of return 

airway settling. Negligible dust settled out in the first 400 m 

(1,300 ft) of the panel intake, but the concentration was reduced by 70% in the remainder of the intake and the 3,000-m 

(10,000-ft) return distance prior to recirculation. Recirculation 

in an active mining panel would have to be studied prior to 

implementation, but previous studies have indicated that return 

air often has lower dust levels than intake air. Steady-state SF6

testing was also conducted to simulate contaminants such as 

diesel DPM and showed that intake contaminant concentrations approached a level corresponding to the percentage of 

recirculated air.

Test results showed that test panel airflow quantity could 

be improved with predictable increases in contaminant levels 

based upon knowledge of panel contaminant emissions and 

recirculation percentage. Dust levels were dependent upon 

ambient concentrations and available settling distances. Inline recirculation had minimal effect on outby panel airflows.

Acknowledgments

The authors thank Andrew Mazzella, Ken Strunk, Roy 

Grau and Heather Dougherty of the NIOSH OMSHR Fires 

and Explosions Branch and the many mine employees for their 

valuable assistance in conducting this research.

Disclosure

The findings and conclusions in this report are those of 

the authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. Mention of any company or product does not imply endorsement 

by NIOSH.

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