r 
 
 
 •i", -€ 
 ^^'O* 
 
 
 
 
 
 .'^^'^ 
 
 ^oV^ 
 
 
 •^-^ • 
 
 ,^o^ 
 
 
 
 
 
 5 
 
 
 
 .^''^ 
 
 
 
 
 .^'^^ 
 
 
 ^^-n^. ^-^ 
 
 • ■«' 
 
 .^ .,. 
 
 
 
 
 
 
 • a 
 
 ^Cr^9^ 
 
 0*^ r'^.^' 
 
 ^""^ 
 
 
 
 '^ A*' 
 
 
 ♦ aV "^^ 
 
 1^ 
 
 V-S' 
 
 
 0' 
 
 'bV- 
 
 .^S'' 
 
 "^ 'WI^W» A^-^ ^^6^= c'^^rv o^^^^» aV«^. - 
 
 ^^-n^^ 
 
 r^ 
 
 ^ 
 

 
 «^ xV ^^ - * * • 4 ' 
 
 
 
 
 
 
IIIIIIMMIIMMMIMnilllMMIMIIIIIIMMIMIIIII 
 
 IC 9248 
 
 BUREAU OF MINES 
 INFORMATION CIRCULAR/1990 
 
 y93 
 
 Dust Control In Coal Preparation and 
 Mineral Processing Plants 
 
 By Edward F. Divers and Andrew B. Cecala 
 
 U.S. BUREAU OF MINES 
 1910-1990 
 
 THE MINERALS SOURCE 
 
 ^AU OF >^' 
 
Mission: Asthe Nation's principal conservation 
 agency, the Department of the Interior has respon- 
 sibility for most of our nationally-owned public 
 lands and natural and cultural resources. This 
 includes fostering wise use of our land and water 
 resources, protecting our fish and wildlife, pre- 
 serving the environmental and cultural values of 
 our national parks and historical places, and pro- 
 viding for the enjoyment of life through outdoor 
 recreation. The Department assesses our energy 
 and mineral resources and works to assure that 
 their development is in the best interests of all 
 our people. The Department also promotes the 
 goals of the Take Pride in America campaign by 
 encouraging stewardship and citizen responsibil- 
 ity for the public lands and promoting citizen par- 
 ticipation in their care. The Department also has 
 a major responsibility for American Indian reser- 
 vation communities and for people who live in 
 Island Territories under U.S. Administration. 
 
Information Circular 9248 
 
 Dust Control in Coal Preparation and 
 IVIineral Processing Plants 
 
 By Edward F. Divers and Andrew B. Cecala 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Manuel Lujan, Jr., Secretary 
 
 BUREAU OF MINES 
 T S Ary, Director 
 
^^ 
 
 \ 
 
 ^ 
 
 
 <o 
 
 Library of Congress Cataloging in Publication Data: 
 
 Divers, Edward F. 
 
 Dust control in coal preparation and mineral processing plants / by Edward F. 
 Divers and Andrew B. Cecala. 
 
 p. cm. — (Information circular / Bureau of Mines; 9248) 
 
 Includes bibliographical references. 
 
 1. Coal preparation plants — Dust control. 2. Ore-dressing plants — Dust control. 
 I. Cecala, Andrew B. II. Title. III. Series: Information circular (United States. 
 Bureau of Mines); 9248 
 
 TN295.U4 
 
 [TN816] 622 s--dc20 [622'.83] 
 
 90-1528 
 CIP 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Background 2 
 
 Coal preparation 2 
 
 Mineral processing 2 
 
 Dust sampling instruments 2 
 
 Ventilation 3 
 
 Whole plant 3 
 
 Local exhaust 3 
 
 Air cleaners 4 
 
 Baghouse-type dust collectors 4 
 
 Scrubbers 5 
 
 Electrostatic precipitators 6 
 
 Control at source 7 
 
 Water sprays 7 
 
 Good housekeeping 7 
 
 Personal protection devices 8 
 
 Dust helmets 8 
 
 Respirators 9 
 
 Special problems 9 
 
 Pipe and duct clogging 9 
 
 Control room dust 10 
 
 Conclusions 11 
 
 References 11 
 
 ILLUSTRATIONS 
 
 1. Effects of bulk-loading outside on worker's exposure inside mill 3 
 
 2. Wall exhaustor — typical layout 4 
 
 3. Baghouse-type dust collector 4 
 
 4. Small-diameter cyclone type scrubber 5 
 
 5. Flooded fibrous bed-type scrubber 5 
 
 6. Venturi-type scrubber — horizontal arrangement 5 
 
 7. Venturi-type scrubber — vertical arrangement 6 
 
 8. Electrostatic precipitator 7 
 
 9. Increase in worker's dust exposure from broom sweeping 8 
 
 10. High capacity remote vacuum system 8 
 
 11. Dust helmet 8 
 
 12. Disposable-type respirator 9 
 
 13. Positive pressure-type respirator 9 
 
 14. Settling type drop-out chamber 10 
 
 15. Baffle type drop-out chamber 10 
 
 16. Centrifugal type drop-out chamber 10 
 
 TABLE 
 
 1. Respirable dust test results 6 
 

 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 
 
 Btu/h 
 
 British thermal unit per 
 
 hour 
 
 hp 
 
 horsepower 
 
 cfm 
 
 cubic foot per minute 
 
 
 in 
 
 inch 
 
 ft 
 
 foot 
 
 
 inHjO 
 
 in of water (pressure) 
 
 ft' 
 
 cubic foot 
 
 
 mg/m' 
 
 milligram per cubic meter 
 
 ft/min 
 
 foot per minute 
 
 
 min 
 
 minute 
 
 gpm 
 
 gcdlon per minute 
 
 
 pet 
 
 percent 
 
 h 
 
 hour 
 
 
 st/h 
 
 short ton per hour 
 
DUST CONTROL IN COAL PREPARATION 
 AND MINERAL PROCESSING PLANTS 
 
 By Edward F. Divers^ and Andrew B. Cecala^ 
 
 ABSTRACT 
 
 This U.S. Bureau of Mines report briefly evciluates the advjmtages and disadvantages of basic dust 
 control techniques presently used by U.S. coal preparation and mineral processing plants. These include 
 ventilation, baghouse-type collectors, wet scrubbers, electrostatic precipitators, source control, sprays, 
 good housekeeping, and personal protection devices. Two specific problems in these types of operations 
 are also considered: dust collector system duct clogging, and control room dust control. 
 
 Information provided in this report results from dust control research projects conducted by the 
 Bureau at various coal preparation and mineral processing plants over the past decade to reduce 
 workers' dust exposure. These studies indicate that plant ventilation systems normally provide the most 
 cost effective method for dust control. Baghouses and scrubbers were also effective in specific 
 appUcations, and examples of each are given. In extreme dust conditions, personal protection devices, 
 such as respirators or the dust helmet, can also be highly cost effective. 
 
 Mining engineers, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
 
INTRODUCTION 
 
 Coal preparation and mineral processing plants can 
 present a wide vciriety of dust problems. Fortunately, 
 the problems in both types of plants can be reduced or 
 resolved by the same basic techniques previously 
 mentioned. This U.S. Bureau of Mines report provides 
 
 information to operators considering various options for 
 dust control. Its conclusions are primarily based on the 
 field experience of Bureau personnel and plant operators 
 and can be applied to most, if not all, plant dust problems. 
 
 BACKGROUND 
 
 COAL PREPARATION 
 
 There are approximately 500 coal preparation plants in 
 the United States. The number of operators in each plant 
 ranges from about 3 to 10 per shift; raw capacity ranges 
 from roughly 250 to 1,250 st/h. 
 
 A recent Bureau survey of 21 coal preparation plants 
 showed that one-third had highly localized respirable dust 
 levels. These were dust levels in given specific areas of 
 the plant and not exposure levels of plant personnel. No 
 relationship was found between dust concentration and age 
 or capacity of the plant. Some old plants show 1 mg/m^ 
 or less in all areas. Some new plants had respirable dust 
 concentrations up to 11 mg/m^ in specific areas. In 
 general, these areas only had temporary occupancy, seldom 
 exceeding several hours. High dust concentrations were 
 closely associated with dry coal. With a few exceptions, 
 these high dust concentrations were found in poorly 
 ventilated areas on the ground floor. Excluding venti- 
 lation, few plants use operating dust control devices. Silica 
 (quartz) analysis made on gravimetric samples from coal 
 preparation plants show that the percent siHca was usually 
 below Federal limits. 
 
 MINERAL PROCESSING 
 
 There are well over 1,000 mineral processing plants in 
 the United States. While dust concentrations in these 
 plants are generally much lower than in coal preparation 
 plants, the associated respiratory problem can be more 
 severe due to siUca. In recent years. Bureau studies have 
 concentrated on those mineral processing plants having 
 silica problems; the percent silica on gravimetric samples 
 varies from to over 90 pet. The problem can be espe- 
 cially severe in the silica sand industry where respirable 
 silica usually is in the 80 to 90 pet range. When silica is 
 present, the FederaJ dust standcu^d is tightened below the 
 usual 2 mg/m^. 
 
 Unlike coal preparation, mineral processing plants 
 usually process the product completely dry because of 
 screen and duct clogging problems associated with wet 
 products. When products JU"e hygroscopic, the possibility 
 
 for any type of wet processing is excluded. Another major 
 difference is that many workers at mineral processing 
 plants spend the majority of the workday at a specific 
 location, which is not common in coal preparation plants. 
 This can extend ventilation to year-round use, and fixed 
 worker locations can be economically heated by infrared 
 devices. Although they are not generally recommended 
 due to costs, the use of steam, foams, and water additives 
 (surfactants) may be appropriate for some mineral 
 processing plants. Water sprays can also be an effective 
 seasonal or occasional dry product (such as strip coal) 
 control technique for both types of plcmts. 
 
 DUST SAMPLING INSTRUMENTS 
 
 Two types of dust sampling equipment were used 
 during these evaluations: gravimetric dust samplers and/or 
 instantaneous dust monitors called real-time aerosol 
 monitors (RAM) (1)} 
 
 Gravimetric dust samples have been the primary means 
 of determining respirable dust concentrations used by the 
 Mine Safety and Health Administration (MSHA) since 
 dust monitoring became a mandated practice as estab- 
 Ushed by the 1969 Federeil Mine Health and Safety Act. 
 Although gravimetric sampling is performed for all com- 
 pUance testing, it is not the most advantageous technique 
 for many studies because of the time required to obtain 
 dust data. 
 
 The instantaneous RAM-1 dust monitor built by 
 Monitoring Instruments for the Environment (MIE), Inc., 
 Bedford, MA overcomes the drawbacks of gravimetric 
 sampling. The RAM-1 unit instantaneously calculates 
 respirable dust concentrations. However, the RAM-1 
 cannot distinguish the quartz concentration of the dust. 
 
 Visucd observation can be an effective technique for 
 estimating dust concentrations, even though water mist can 
 cause problems. As a general guideUne, respirable dust 
 concentrations below 2 mg/m' will not create visibiUty 
 problems or hcdoing around lights; these are especially 
 
 ntalic numbers in parentheses refer to items in the list of references 
 at the end of this report. 
 
evident above 5 or 6 mg/m'. Visibility problems beyond 
 50 or 60 ft typically start at 8 to 10 mg/m^, and distinct 
 visibility problems beyond 20 to 30 ft occur above 15 or 20 
 
 mg/m^. At these high respirable concentrations, coal dust 
 odor can be detected and rapid accumulation of all types 
 of larger dust p2u-ticles on most surfaces is evident. 
 
 VENTILATION 
 
 WHOLE PLANT 
 
 Ventilation can be an excellent low-cost dust control 
 technique for all plants during mUd weather. Plants using 
 effective ventilation systems have low dust concentrations, 
 with peaks generally below 2 mg/m^ respirable. These 
 systems consists of powered exhaustors (fans) or gravity 
 ventilators in the roof, open access doors, and open-close- 
 type wall louvers at grade. Due to a sUght increase in 
 initial plant costs, ventilation systems are not built into 
 most plants. However, since ductwork is not required, 
 subsequent addition of a ventilation system can be 
 relatively inexpensive, about $1 per cubic foot per minute 
 for a powered roof exhaustor system with adjustable 
 louvers at grade. Powered roof exhaustors should be sized 
 to provide between 6 to 12 air changes per hoiu". Where 
 dust concentrations are known or safely assumed to be 
 low, 6 to 8 air changes per hour can be used. 
 
 Air changes per hour 
 
 Total volume exhausted per hour 
 Total volume of ventilated building 
 
 However, long-term, high dust concentrations, above 
 6 mg/m^, may not be adequately controlled by ventilation 
 alone. In these cases, primary efforts should be directed 
 to control at the dust source, or using dust collectors such 
 as baghouses or scrubbers. 
 
 Design guidelines for the installation of plant 
 ventilation systems am be found in the "ASHRAE 
 Handbook of Fimdamentals" or their systems handbook 
 published by the American Society of Heating, 
 Refrigeration, and Air-Conditioning Engineers, Inc., 
 Atlanta, GA, or "Industrial Ventilation" published by the 
 Committee on Industrial Ventilation, Cincinnati, OH. 
 
 For ventilation to be effective as a dust control 
 technique, it is critical that the design be capable of 
 supplying relatively clean makeup air to the system. If air 
 supplied to the ventilation system is contaminated, the 
 ventilation system can be ineffective and may increase a 
 
 worker's dust exposure. This occiu-red at a mineral 
 processing plant where trailer trucks were being bulk- 
 loaded outside a mill building (fig. 1). The dusty air 
 generated from the bulk-loading process was drawn into 
 the mill as makeup jiir for a ventilation system; worker's 
 dust exposure was increased 147 pet over previous 
 (normal) levels diuing the buUc-loading. Inlet louvers 
 should not be placed neai outside dust or other 
 contaminant sources. 
 
 LOCAL EXHAUST 
 
 Local exhaust through wzdl exhaustors, fans, and other 
 devices can be a highly cost-effective technique for 
 removing dust from specific, generally small, cueas. This 
 is frequently done with wall-mounted propeller fans 
 properly protected cmd mounted close to the dust source , 
 usually without ductwork (fig. 2). Fan volumetric capac- 
 ities generally range from 10,000 to 25,000 cfm or more, 
 with motors from 1 to 5 hp. The exhaustors aie controlled 
 by operation of the dust-generating soiu-ce, such as a 
 crusher, emd/or mjmual control. Because strong outside 
 winds can negatively affect the performance of exhaustors, 
 their placement should normally be on the nonwindward 
 (lee) side of buildings. 
 
 Bulk loading outside 
 
 8 12 
 
 TIME, min 
 
 Figure 1. -Effects of bulk-loading outside on worker's 
 exposure inside mill. 
 
Airflow 
 
 Guard 
 (outside) 
 
 and boot 
 
 Manual shutter 
 (aluminunn) 
 
 Figure 2.-Wall exhauster — typical layout 
 
 AIR CLEANERS 
 
 BAGHOUSE-TYPE DUST COLLECTORS 
 
 Baghouses collect airborne dust by filtration through 
 special cloths, somewhat similar to a home vacuum 
 sweeper. The collected dust is normally shaken auto- 
 matically from the filters into a hopper (fig. 3). Baghouses 
 have several advantages, especially for mineral processing 
 plants: they offer high dust collection efficiency that is 
 essentially independent of mineral type, cmd they do not 
 use water. When in-plant space is not available, outside 
 installations are generally successful. Disadvantages are 
 size (especially height), duct routing and cost, handling of 
 the collected dust, and frequent problems when excessive 
 moisture or water is in the air. Initial cost generailly 
 exceeds $4 per cubic foot per minute. Nevertheless, 
 baghouses are especially recommended for those plamts on 
 a reduced silica standard. Where very high dust collection 
 efficiency is required, use a baghouse. 
 
 Visible dust in the exhaust plume always indicates 
 problems in baghouse systems. For example, at one coal 
 preparation plant a 38,000 cfm outside unit discharged to 
 the atmosphere. A visible plume was emitted from its 
 exhaust and respirable dust sampling showed a concen- 
 tration of 5.0 mg/m^. This was caused by torn bags. 
 Properly designed and maintained systems will show less 
 than 1 mg/m in the exhaust plume, and no visible dust. 
 
 Another baghouse system was located on the second 
 floor of a 5-year old plant. Its capacity was approximately 
 15,000 cfm, and it eilso exhausted to the outside. A simple 
 
 manually operated spUtter was installed in the fan 
 discharge duct that also allowed inside discharge during 
 cold weather. Since the system was adequately main- 
 tained, exhaust concentrations averaged 0.70 mg/m^. 
 Inside discharge during severe cold weather saved over 1 
 milhon Btu/h by reducing makeup air heating cost. Inside 
 discharge should not be used when sihca dust is present. 
 
 Collector-fan 
 
 Clean air 
 out 
 
 Solid floor 
 perforated 
 at bag inlet 
 
 Figure 3.-Baghouse-type dust collector. 
 
Silica dust presents a serious dust hazard and requires the 
 plant to meet a lower Federal dust standard. 
 
 Baghouses can achieve very high respirable dust 
 collection efficiency, well above 99 pet with special bag 
 filter material. This high efficiency cannot be readily 
 achieved by scrubbers. However, scrubber efficiency can 
 be adequate for most coal preparation appUcations. 
 
 SCRUBBERS 
 
 Scrubbers frequently offer another practical control 
 method, especially for coal preparation plant dust control. 
 They collect airborne dust by highly turbulent collision 
 with water droplets or a wetted surface, the water then 
 being separated from the airstreemi. The water consumed 
 does not present problems to most coal preparation plant 
 operators, but can make them imsuitable for minereds 
 
 processing. Dust collector efficiency can be sufficient to 
 allow the scrubbed air to be discharged back into the 
 plant, and they £ire small enough to allow them to be 
 retrofitted into operating plants. Typical components 
 consist of a 25,000 to 50,000 cfm fan generally operating 
 between 6 to 10 in HjO, the scrubbing device, water sprays 
 at a flow rate between 0.5 and 3 gpm per 1,000 cfm of 
 airflow, a water droplet eliminator and duct work to 
 various dust sources. Initial costs are $2 to $3 per cubic 
 foot per minute, and mziintenance is uncomplicated. 
 
 Disadvantages are fan power cost and fan noise, 
 potential clogging, and cold weather freezing problems. 
 
 Three commonly used types of scrubbers were eval- 
 uated for this work: the small-diameter cyclone (fig. 4), 
 the flooded bed (fig. 5), and the venturi (fig. 6) (2). The 
 primary purpose was to determine if the cleaned air could 
 be safely dischairged back into the plant. 
 
 -Turning vanes 
 
 Airflow 
 U^ to fan 
 
 Figure 4.-Small-diameter cyclone type scrubber. 
 
 -Spray Blade type 
 
 nozzles rnist eliminator-, 
 
 ''^ [ pLj^ Airflow 
 _T^ to fan 
 
 / 
 
 ' — ^ Drain 
 Figure 5.-Fiooded fibrous bed-type scrubber. 
 
 Long 
 transition 
 
 Blade type mist 
 
 eliminator-i transition- 
 
 Centrifugal 
 fan 
 
 Dusty air 
 inlet duct 
 
 Figure 6.-Venturi-type scrubber — horizontal arrangement 
 
Dust sampling results showed that all three can do jui 
 adequate job of dust control at the pickup locations. 
 Results at the vicinity of the scrubber fiui discharge were 
 determined (table 1). 
 
 Table 1 .-Respirable dust test results' 
 (8-h gravimetric averages at scrubber) 
 
 Scrubber type 
 
 Discharge, 
 mg/m^ 
 
 Efficiency, 
 pet 
 
 Small-diameter cyclone . . . 
 
 Flooded fibrous bed 
 
 Venturl 
 
 2.47 
 1.12 
 3.09 
 
 88 
 95 
 85 
 
 Mnlet was 21 for all three types of scrubbers. 
 
 The flooded bed scrubber was more than twice as 
 efficient as the small-diameter cyclone, allowing unlimited 
 exposiu-e time. The results are consistent with previous 
 Biu-eau studies comparing both scrubber types (3-4). 
 However, both scrubbers clogged with coal particles, 
 creating imacceptable maintenance problems. Air pressure 
 drop across the flooded bed panel increased from 3.8 to 
 6.5 in of water during 9 h of typical operation due to 
 clogging. The cyclone panel increased about hadf of this 
 2.7 in rise, i.e., 1.37 in, under identical conditions. Proper 
 operation of these scrubbers is highly dependent upon a 
 steady and uniform application of spray water. Clogged 
 and partly clogged sprays decrease the dust collection 
 efficiency, and increase the pressure buildup across both 
 cyclone and flooded bed types. 
 
 Aucdysis of the used flooded bed panels showed that 
 clogging was primarily caused by large particulate (up to 
 1/16-in) embedded in the mesh. This indicates that the 
 dust inlet duct was located too close to the dust source. In 
 most underground applications, the flooded bed panels are 
 cleaned once per shift by drying and shaking, or by direct 
 flushing with a hose. 
 
 The venturi showed no indication of clogging. Its 
 efficiency was limited by the fan pressure capabihty in this 
 installation that only allowed a maximimi of 6 in HjO 
 gauge differential pressure across the ventiu-i throat. The 
 dust collection efficiency of a venturi scrubber is essen- 
 tially unlimited, and primarily dependent on fan pressme 
 capability and waterflow rate. Past Bureau work shows 
 that a throat differential above 10 in H2O gauge would 
 keep discharge concentrations below 2 mg/m in this 
 installation (5). 
 
 Owing to its simplicity and low cost, venturi-type 
 scrubbers can be readily recommended for coal prep- 
 aration plants. They are commercially available in a wide 
 
 range of configiu-ations, including vertical (fig. 7). As a 
 gmdeline, minimiun throat differential pressures should 
 be 6, 10, and 15 in HjO gauge for respective inlet con- 
 centrations of 10, 20, and 30 mg/m^ respirable dust. A 
 venturi should not be operated at less than 4 in HjO gauge 
 pressiu-e drop across the throat. 
 
 ELECTROSTATIC PRECIPITATORS 
 
 Electrostatic precipitators (5) pass airborne dust 
 between electrically charged plates (fig. 8). This causes 
 the dust to migrate toward and adhere to the plates; this 
 dust is then automatically shaken or washed from the 
 plates. Precipitators offer several advantages, including 
 very-high dust collection efficiency, and very-low air 
 pressure drop (1/2-in HjO gauge across the precipitator). 
 This results in low fan power requirements. Initial, $1 to 
 $1.50 per cubic foot per minute, and operating costs are 
 lower than scrubbers or baghouses. 
 
 Dusty 
 air in 
 
 -Drain 
 Figure 7.-Venturl-type scrubber — vertical arrangement 
 
Disadvantages include fairly large in size, and a 
 requirement for relatively experienced m2dnten2mce 
 personnel to work with the high voltages, and potential 
 sparking problems associated with high voltages. 
 Efficiency is reduced if subjected to vibration, if inlet air 
 velocity is not uniform, and if shutdown time is required 
 for cleaning. Detergent water is frequently required for 
 cleaning. 
 
 Results of efficiency tests of electrostatic precipitators 
 showed a wide efficiency difference between various plants. 
 One 5,000 cfm installation was only 30 pet efficient on 
 respirable coed dust. Minerals containing sodiimi can also 
 present problems when high dust collection efficiency is 
 required. For best results, electrostatic efficiency data on 
 the specific dust and installation should be obtained before 
 purchasing a precipitator. 
 
 Gas 
 passage 
 between 
 plates 
 
 Dischorge 
 electrode 
 
 Hopper- 
 Figure 8.-Electrostatic precipitator. 
 
 CONTROL AT SOURCE 
 
 For maximum cost effectiveness, dust should be 
 controlled at its source prior to dilution and dispersal. 
 Usually, dust sources are determined by visual observation, 
 especially diuing startup or on-off procedures. Precise 
 determination of the appropriate control technique de- 
 pends on many factors, some beyond the scope of this 
 report. However, these general guidelines should be 
 followed: 
 
 * A thorough dust survey should be conducted when 
 it appeeu-s that control costs will be significant. 
 
 * The survey should at least determine the dust type 
 (if unknown), its sources, extent, and concentration. 
 
 * The control technique should be primarily based on 
 survey data. For example, simple ventilation may be 
 suitable for a dust concentration of 4 mg/m^, but not 
 for 10 mg/m^. 
 
 Although not pointed out in most literature, high dust 
 concentrations or low threshold limit value (TLV) usually 
 require more severe and costly controls, such as baghouses 
 or scrubbers. Frequently, two control techniques such as 
 water sprays and ventilation can work together for maxi- 
 mum cost effectiveness. 
 
 WATER SPRAYS 
 
 Where the product aillows, water sprays can be very cost 
 effective for dust source control (6-7). Their primary 
 effectiveness is in keeping the dust from becoming 
 airborne, accordingly the effectiveness is primarily de- 
 pendent on flow rate and coverage, not water pressure. 
 Once airborne, respirable dust is very difficult to knock 
 down. This knockdown effectiveness is dependent on flow 
 rate emd pressure, and the best spray systems seldom 
 exceed 60 pet efficiency on respirable dust. Although not 
 
 generally recommended due to cost, water additives 
 (surfactants) (8), foams (9), and steam (10) can occasion- 
 cdly provide additional source control. 
 
 GOOD HOUSEKEEPING 
 
 Effective housekeeping is essential for maintaining 
 acceptable dust levels. Product material allowed to 
 buildup on the walls, beams, equipment, floors, and 
 walkways can be readily dispersed. Dust buildup on grated 
 floors and walkways is a significant dust contributor, 
 especially with the higher floor levels; a worker can create 
 a significant amount of dust just walking across this 
 grating, causing dust to drop three or four floors. An 
 effective housekeeping practice is to clean the mill or plant 
 at least once a day. Many operations will wash down all 
 floors and equipment to achieve this, although in some 
 plants that do not allow for the use of water, vacuum 
 cleaning systems are used. Sweeping or blowing clean air 
 with compressed air should not be used. Figure 9 shows 
 the increase in the respirable dust exposure to a worker 
 when a co-worker in a mill was sweeping the floor one 
 level below with a push broom. The operator's dust 
 exposure increased to nearly six times the previous 
 concentration because of this dust. 
 
 Plants or mills with low dust concentrations generally 
 had good housekeeping practices. Most of these opera- 
 tions would wash down or vacuum the plant or mill each 
 shift. This is not to suggest that effective housekeeping 
 was the sole reason for low dust concentrations, but to 
 emphasize that those operations who were trying to 
 maintain low dust levels realized the importance of this 
 practice. Various types of commercial vacuum cleaning 
 devices are available for this purpose. They range from 
 high capacity remote vacuum collectors (fig. 10) to easily 
 portable tank-type units. 
 
to 
 
 E 
 
 E 
 
 o 
 o 
 
 I- 
 
 -'^t . 
 
 TIME, min 
 
 Figure 9.-lncrease in worlter's dust exposure from broom 
 sweeping. 
 
 To dust 
 collector 
 
 ■^:>^mH^..:::: 
 
 Figure lO.-IHigh capacity remote vacuum system. 
 
 PERSONAL PROTECTION DEVICES 
 
 Occasionally, dust control with baghouses or scrubbers 
 may not be practical, as from various sources spread over 
 large areas. In these cases, control at the dust sources is 
 suggested. Where this is prohibitive due to costs, space 
 limitations, or other reasons, use of the dust helmet or 
 other personal protection devices such as respirators is 
 strongly recommended. 
 
 DUST HELMETS 
 
 Dust helmets have been designed to provide dust 
 protection for the wearer without some of the talking, 
 spitting, and fit problems associated with typical respi- 
 rators. They use a small fan in the hebnet to provide 
 filtered air to the breathing zone of the wearer (fig. 11). 
 The fan is powered by a rechargeable belt-mounted battery 
 suitable for 8-h continuous operation. A high efficiency 
 filter is fitted to the fan discharge, a coarse filter at its 
 inlet; both are throw-away types. The helmet is used with 
 a full-face lucite lens. Tests of the helmet's efficiency on 
 various dusts and conditions generally show excellent 
 results and reasonably good acceptance by personnel, 
 except when they must work in tight places. Various types 
 of side shields are available to increase the helmet's dust 
 protection efficiency as ambient ak velocity rises. Bureau 
 tests of the helmet with two types of side shields show dust 
 reductions greater than 90 pet between ambient and inside 
 helmet respirable concentrations (11). Typical in-plant 
 tests would show 92 pet without a full-face side shield, and 
 98 pet with. 
 
 The hebnet can be very useful in extreme dust 
 conditions, such as in coal preparation plcmts with high 
 respirable concentrations or in mineral processing plzmts 
 
 Main filter 
 
 Safety helmet 
 shell 
 
 Motor and 
 fan assembly 
 
 Battery 
 pack 
 
 Hinged 
 clear 
 visor 
 
 Figure 1 1 .-Dust helmet 
 
Figure 1 2.-Disposable-type respirator. 
 
 with high siHca dust levels. Primary disadvantages are 
 glare from the lens, especially for those who must wear 
 glasses, and the excessive bulk for tight work spaces. 
 
 RESPIRATORS 
 
 Many types of dust respirators are also commercially 
 available for this use. Dust protection efficiency is roughly 
 dependent on cost, type, face fit, and ranges from 
 approximately 60 pet for simple throw-away or negative- 
 pressure types (fig. 12), to the upper 90 pet for powered 
 positive-pressure types (fig. 13). When respirable silica 
 dust concentrations exceed 5 mg/m^, or long-term ex- 
 posure is anticipated, the latter types should be used. 
 However, dust helmets and respirators only help protect 
 the worker from usual dust and mist, they are not to be 
 used for harmful chemical dusts or gasses. 
 
 Figure 13.-Posltive pressure-type respirator. 
 
 SPECIAL PROBLEMS 
 
 PIPE AND DUCT CLOGGING 
 
 Serious duct clogging problems occur in some of the 
 duct collector systems in coal preparation plants; dust 
 peirticles graduidly accumulate in the horizontal duct rims 
 between the pickup points and fan, and zdmost completely 
 block the airflow. This not only cripples the performance 
 of the duct collector system, but also presents safety 
 problems. The added weight of the dust can occasionedly 
 break the duct hangers, allowing sections to drop. 
 Clogging could occur as often as every 6 months. In these 
 cases, the prepjiration plant operators would occasionally 
 drop these duct sections and flush with high-pressure 
 water. The common remedy for this problem is to 
 
 maintain an adequate duct ak velocity as indicated in the 
 American Conference of Government-Industrial Hygienist 
 (ACGIH) Handbook, however, the duct air velocity in 
 some plants is aheady above 4,000 ft/min. Conversations 
 with material handling experts show that these problems 
 are not rare, even in hi^-velocity systems. No simple 
 solution has been foimd to the duct clogging problem. 
 One possibility has been to paint the duct interior, first 
 with an epoxy nonlift primer followed by an epoxy topcoat, 
 both with suitable catalyst. Extra water sprays in the duct 
 sections that clog have also been successful. In some 
 cases, a dropout box or inertial separator can help. Use 
 of inertial separators can also help prevent scrubber 
 clogging. Inertifd separators separate large dust particulate 
 
10 
 
 from the airstream xising a centrifugal, gravitational, or 
 inertial force. The separate dust faiUs into a hopper, where 
 it is temporarily stored. The three primary types are 
 settling chambers, baiffle chambers, and centrifugal 
 collectors. Neither settling chambers nor baffle chambers 
 are commonly used in the minerals processing industry, 
 mostly due to space requirements, and difficulties with 
 cleaning and disposal of the collected dust. 
 
 Settling chambers (fig. 14) consist of a large box 
 installed in the ductwork. The sudden enlargement at the 
 chamber reduces the speed of the dust-filled airstream, 
 and heavier particles settle out. They are simple in design 
 and can be readily manufactured. 
 
 Baffle chambers (fig. 15) use a fixed baffle plate that 
 causes the airstreeun to make a sudden change of direction. 
 Large-diameter particles do not follow the gas stream, but 
 continue into a dead air space and settle. 
 
 Centrifugal collectors (fig. 16) use cyclone-type action 
 to separate dust particles from the air. In a typical 
 cyclone, the high velocity dusty air enters at a tangent. 
 The centrifugal force created by the circular flow forces 
 
 Dusty 
 air inlet 
 
 Outlet 
 
 Dust collection 
 hoppers 
 
 SETTLING CHAMBER 
 Figure 14.-Settling type drop-out chamber. 
 
 the dust particles toward the wall of the cyclone, where 
 they fall into a hopper. These collectors are more 
 expensive and more efficient in removing particulate than 
 settling and baffle chambers. 
 
 CONTROL ROOM DUST 
 
 Control room dust is generally not a serious problem, 
 except when the dust contains silica. Sihca is not a 
 problem in coal preparation plant control rooms, but it can 
 be a serious health problem in many mineral processing 
 plant rooms, especially because of the long-term, 8-h 
 occupjmcy. Dust can Jilso be detrimental to electrical 
 equipment. 
 
 Wall- or window-mounted air conditioners admitting 
 filtered outside air are adequate for control in most coal 
 preparation plants. For mineral processing plants, high 
 efficiency filters are strongly recommended. Minimum 
 makeup airflows should be above several hundred cubic 
 feet per minute even for small rooms (say 1,000 ft^); this 
 air should not be recirculated, but allowed to leak from the 
 room. This will help insure a positive pressure within the 
 room, and reduce dust infiltration. Conventional through- 
 the-wall or window-type ziir conditioners should not be 
 used for room pressurization; their makeup (fresh) airflow 
 is usually too low for mineral processing plants. 
 
 Inner vortex 
 
 To hopper 
 BAFFLE CHAMBER 
 
 Outer vortex 
 
 hopper 
 CENTRIFUGAL TYPE 
 
 Figure 1 5.-Baffle type drop-out chamber. 
 
 Figure 1 6.-Centrifugal type drop-out chamber. 
 
11 
 
 CONCLUSIONS 
 
 Currently, about one-third of the coal preparation in both cases are essentially indentical and depend on dust 
 
 plants in the U.S. have excessive respirable dust concen- 
 trations in specific areas. Results from extensive work at 
 mineral processing plants with dust problems indicate that 
 dust concentrations are lower, but frequently compoimded 
 by sihca and full-shift exposure. The primary solutions 
 
 soiu"ces, type, and concentration. In broad order of practi- 
 cahty and cost-effectiveness, these solutions are ventilation, 
 baghouses, scrubbers, good housekeeping, and personal 
 protection devices. 
 
 REFERENCES 
 
 1. Williams, K. L., and R. J. Timko. Performance Evaluation of a 
 Real-Time Aersol Monitor. BuMines IC 8968, 1984, 20 pp. 
 
 2. Grigal, D., G. Ufken, J. Sandstedt, M. Blom, and D. Johnson. 
 Development of Improved Scrubbers for Coal Mine Applications 
 (contract H0199055). BuMines OFR 91-83, July 1982, 124 pp.; NTIS PB 
 83-205385. 
 
 3. Divers, E. F., and J. T. Janosik. Scrubbers for Dust Control: A 
 Comparison of Six Medium-Energy Use Types. BuMines RI 8449, 1980, 
 29 pp. 
 
 4. . Comparison of Five Types of Low-Energy Scrubbers for 
 
 Dust Control. BuMines RI 8289, 1978, 38 pp. 
 
 5. McDonald, J. J., and A. H. Dean. Electrostatic Precipitator 
 Manual, Noyes Data Corp., 1982, 480 pp. 
 
 6. Mody, v., and R. Jakhete. Conveyor Belt Dust Control (contract 
 H0113007, Martin-Marietta Lab.). BuMines OFR 31-86, Feb. 1984, 
 410 pp. 
 
 7. Mody, v., and R. Jakhete. Dust Control Handbook for Minerals 
 Processing (contract J0235005). BuMines OFR 2-88, Feb. 1987, 220 pp.; 
 NTIS PB-88-159108. 
 
 8. Volkwein, J. C, A. B. Cecala, and E. D. Thimons. Moisture 
 Application for Dust Control. Appl. Ind. Hyg., v. 4, No. 8, Aug. 1989, 
 pp. 198-200. 
 
 9. . Use of Foam for Dust Control in Minerals Processing. 
 
 BuMines RI 8808, 1983, 11 pp. 
 
 10. Cecala, A. B., J. C. Volkwein, and E. D. Thimons. Adding Steam 
 To Control Dust in Mineral Processing. BuMines RI 8935, 1985, 9 pp. 
 
 11. Cecala, A. B., J. C. Volkwein, E. D. Thimons, and C. W. Urban. 
 Protection Factors of the Airstream Helmet. BuMines RI 8591, 1981, 
 17 pp. 
 
 INT.BU.OF MINES,PGH.,PA 29136 
 
■0 
 
 m 
 
 
 Si o 
 
 5 W 
 
 z 
 
 
 "> ZI 
 
 m ■ 
 
 > 
 
 5 
 
 o 
 
 ■n 
 ■n 
 
 3" 
 
 C <B 
 ©■p. 
 
 ■71 
 
 o 
 
 O 
 
 > 
 
 treet 
 ton, 
 
 
 "V 
 
 r- 
 
 0- 
 
 5 «> 
 
 a 
 
 1 
 
 00 
 
 c 
 
 V) 
 
 
 
 
 m 
 
 c 
 
 V) 
 
 z 
 m 
 
 S2 
 
 3" 
 
 (D 
 
 V) 
 
 
 3 
 
 
 
 (O 
 
 n 
 
 i 
 
 
 00 
 
 ^ 
 
 ® 
 
 
 
 
 5' 
 
 
 
 
 
 ^ 
 
 > 
 z 
 m 
 D 
 c 
 > 
 
 o 
 
 "D 
 TJ 
 O 
 3) 
 
 m 
 
 O 
 
 -< 
 m 
 3) 
 
 413-90 
 

 
 
 
 '^o^ 
 
 
 
 
 
 
 
 < * ^.^ * • • - -a 
 
 ^ "■• ^V^ '^ '•-» 
 
 * A <^ ♦'TV.- 4 
 
^•\ «c°^;^^% /^-^'X .^<*;^'>- X'-^i'X c^ *:^^'> .^^ 
 
 
 S°^ 
 
 ...i^m^. ^^^^^^^^ y^^^ t^^^^<i> ,»^^\ %^^^^' 
 
 
 4ECKMAN 
 
 "NDERY INC. 
 
 • ■ o ' 
 
 "^ '•-^K*' ^^^^'\ °''^^^*' •j.'^^^ ■"• 
 
 '%.n<i^ *^^^ ^r.'^ 
 
 <?5 °^ 
 
 0^ ^^/^TT,.* ^^^'^ -o^' 
 
 ^^ N. MANCHESTER, -* ♦><^* tV K'i 'f^V A. -A '^^S^K'^ .'V ^-. 'X^T* A* ^^ "^'^i^J^*