Engineering Controls Database
Best Practices for Dust Control in Metal/Nonmetal Mining – Mines and Mineral Processing Operations – Enclosed Cabs, Operator Booths, Control Rooms
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Respirable crystalline silica dust exposure has long been known to be a serious health threat to workers in many industries and occupations. Workers with high exposure to crystalline silica include miners, sandblasters, tunnel workers, silica millers, quarry workers, foundry workers, and ceramics and glass workers Overexposure to respirable crystalline silica dust can has been associated with development of silicosis, lung cancer, pulmonary tuberculosis, and airways disease. The International Agency for Research on Cancer (IARC) reviewed the published experimental and epidemiologic studies of cancer in animals and workers exposed to respirable crystalline silica and concluded that there was sufficient evidence to classify silica as a human carcinogen [IARC 1997]. Silicosis is also a fibrosing disease of the lungs caused by the inhalation, retention, and pulmonary reaction to the crystalline silica. When silicosis becomes symptomatic, the primary symptom is usually dyspnea (difficult or labored breathing and/or shortness of breath), first noted with activity or exercise and later, as the functional reserve of the lung is also lost, at rest. Once contracted, there is no cure for silicosis. The goal, therefore, is to limit worker exposure to respirable dust to prevent development of these diseases. |
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Silica refers to the chemical compound silicon dioxide (SiO2), which occurs in a crystalline or noncrystalline (amorphous) form [NIOSH 2002]. Silica is a common component of rocks; and; throughout the mineral processing cycle, mined ore goes through a number of crushing, grinding, cleaning, drying, and product-sizing sequences as it is processed into a marketable commodity. Because these operations are highly mechanized, they are able to process high tonnages of ore. This in turn can generate large quantities of dust, often containing elevated levels of respirable crystalline silica, which can be liberated into the work environment. Frequently at mines and mineral processing plants, workers will be located in enclosed cabs, operator’s booths, or control rooms to provide a safe work area and to isolate them from dust sources. If these areas are properly designed, they can provide good air quality to the worker. On the other hand, if these enclosed areas are not properly designed and maintained, the air quality can deteriorate to unacceptable and unsafe levels. |
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The most effective technique for reducing operators’ exposure to airborne dust in booths/control rooms/enclosed cabs at mineral processing operations and mines is with filtration and pressurization systems. The most effective filtration and pressurization systems have the heating and air conditioning (HVAC) components tied in as an integral part of the system. A substantial amount of research has been performed over the past few years evaluating the air quality in enclosed cabs of surface mining equipment. This research is directly applicable to operator booths and control room dust control systems. As the research demonstrates, enclosed cabs on mobile equipment are harder to control and maintain since the moving of the equipment constantly stresses and compromises the competency of the enclosure. NIOSH recently conducted a controlled laboratory study to evaluate the key factors necessary for achieving an effective enclosure filtration and pressurization system [NIOSH 2008]. Through this laboratory and numerous field studies, the following items were identified as key components to an effective system: • Ensure booth/control room/cab integrity. Effective protection factors were realized in various field studies when positive pressures between 0.01 and 0.40 inches of water gauge were achieved within the booth/cab because of good enclosure integrity. These pressures correspond to wind velocity equivalents of 4.5 to 29 miles per hour and prevent against wind forcing dust laden air into the enclosure. • Use high-efficiency filters on intake air. Only intake filters with an efficiency of 95% or greater were used during field studies [Cecala et al. 2004, 2005a; Chekan and Colinet 2003; Organiscak et al. 2004]. Laboratory experiments showed an order of magnitude increase in protection factors when using a 99%-efficient filter versus a 38%-efficient filter on respirable-sized particles [NIOSH 2007]. • Use an efficient recirculation filter. All the field evaluations used recirculation filters that were 95% or greater on respirable-sized dusts [Cecala et al. 2004, 2005a; Chekan and Colinet 2003; Organiscak et al. 2004]. Laboratory experiments showed a tenfold increase in protection factors when using an 85%- to 94.9%-efficient filter on respirable-sized dusts as compared to using no recirculation filter [NIOSH 2007]. Laboratory testing also showed that the time needed for the interior to stabilize after the door was closed was reduced by more than 50% when using the recirculation filter. • Minimize interior dust sources. Good housekeeping practices are needed to keep enclosure interiors clean, which eliminates inside dust sources. One field study showed a significant increase in dust levels (0.03 to 0.26 mg/m3) when a floor heater was used. The fan from the floor heater stirred up dust lying on the cab floor [Cecala et al. 2001; NIOSH 2001b]. • Keep doors closed. In a study on an enclosed cab of a surface drill, the operator’s dust exposure averaged 0.09 mg/m3 inside the cab with the door closed and 0.81 mg/m3 when the door was briefly opened to add drill steels [Cecala et al. 2007b]. Although this procedure was performed after drilling stopped and the visible dust dissipated, it nevertheless produced a nine fold increase in dust concentrations inside the cab each time a drill steel was added. The above research findings also suggest that the use of a one-directional airflow pattern could be beneficial. In most systems, both the intake and discharge for the recirculation air are located in the roof. This could cause a portion of the air to short-circuit without penetrating deeply into the cab. Also, as cab air is drawn into the ventilation system at the roof, dust generated in lower portions of the cab may be pulled through the breathing zone of the worker. In a one-directional design, recirculated air is drawn from the bottom of the enclosure and away from the worker’s breathing zone. Figure 1 shows this one-directional airflow pattern. For some cases in operator booths and control rooms, stand-alone table-top air purifier units containing a high efficiency particulate air (HEPA) filter have been installed and shown to improve the air quality. Obviously, these table-top systems are small, portable units available at a fraction of the costs of permanent systems. These systems can be effective if they are sized to handle the volumetric capacity of the booth or control room and if the filters are replaced when necessary [Logson 1998]. Obviously, one shortcoming with this type of system is that they do not provide any pressurization to keep dust from leaking into the booth or room.  NOTE: The above information is taken directly from the following publication: NIOSH [2010]. Information circular 9517. Best practices for dust control in metal/nonmetal mining. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-132. |
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Cecala AB, Organiscak JA, Heitbrink WA [2001]. Dust underfoot—enclosed cab floor heaters can significantly increase operator’s respirable dust exposure. Rock Prod 104(4):39–44. Cecala AB, Organiscak JA, Heitbrink WA, Zimmer JZ, Fisher T, Gresh RE, Ashley JD II [2004]. Reducing enclosed cab drill operator’s respirable dust exposure at surface coal operations with a retrofitted filtration and pressurization system. In: SME Transactions 2003, Vol. 314. Littleton, CO: Society for Mining, Metallurgy and Exploration, Inc., pp. 31–36. Cecala AB, Organiscak JA, Zimmer JA, Heitbrink WA, Moyer ES, Schmitz M, Ahrenholtz E, Coppock CC, Andrews EH [2005a]. Reducing enclosed cab drill operator’s respirable dust exposure with effective filtration and pressurization techniques. J Occup Environ Hyg 2:54–63. Cecala AB, Organiscak JA, Zimmer JA, Moredock D, Hillis M [2007b]. Closing the door to dust when adding drill steels. Rock Prod Oct:29–32. Chekan GJ, Colinet JF [2003]. Retrofit options for better dust control—cab filtration, pressurization systems prove effective in reducing silica dust exposures in older trucks. Aggregates Manag 8(9):9–12. Logson R [1998]. Controlling respirable dust in plant control rooms. Stone Rev 14(6):43–44. IARC [1997]. IARC monographs on the evaluation of carcinogenic risks to humans: silica, some silicates, coal dust and para-aramid fibrils. Vol 68. Lyon, France: World Health Organization, International Agency for Research on Cancer. NIOSH [2001b]. Technology news 486: floor heaters can increase operator’s dust exposure in enclosed cabs. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. NIOSH [2002]. NIOSH hazard review: health effects of occupational exposure to respirable crystalline silica. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2002-129. NIOSH [2007]. Technology news 528, recirculation filter is key to improving dust control in enclosed cabs. By Organiscak JA, Cecala AB. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2008-100. NIOSH [2008]. Key design factors of enclosed cab dust filtration systems. By: Organiscak JA, Cecala AB. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2009-103. Organiscak JA, Cecala AB, Thimons ED, Heitbrink WA, Schmitz M, Ahrenholtz E [2004]. NIOSH/Industry collaborative efforts show improved mining equipment cab dust protection. In: SME Transactions 2003, Vol. 314. Littleton, CO: Society for Mining, Metallurgy and Exploration, Inc., pp. 145–152. |
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dust control metal/nonmetal mining mineral mining mineral processing miners |