Engineering Controls Database

Best Practices for Dust Control in Metal/Nonmetal Mining –
Mineral Processing Operations – Controlling Respirable Silica at Surface Mines – Controlling Dust at the Primary Hopper Dump

Problem:	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.
What this means to you:	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 and generate high levels of silica. Surface mining operations present dynamic and highly variable silica dust sources. Most of the dust generated at surface mines is produced by mobile earth-moving equipment such as drills, bulldozers, trucks, and front-end loaders excavating silica-bearing rock and minerals. Four practical areas of engineering controls designed to mitigate exposure of surface mine workers to all airborne dusts, including silica, are controlling dust at the primary hopper dump, dust on unpaved haulage roads, enclosed cab filtration systems, and drill dust collection systems. Ore is normally loaded into haul trucks from the pit or quarry and driven to the primary crusher location. This ore is either dumped directly from the haul truck into the primary ore hopper feeding a crusher or dumped into a stockpile. If it is stockpiled, a front-end loader then takes the ore and dumps it into the primary hopper. In either case during this dumping process, a dust cloud is billowed out of the hopper and rolled back under the truck bed or front-end loader bucket. Dust in the ore is released from the large volume of ore product being dumped in a short period of time, which quickly displaces the air in the hopper and transports the airborne dust released from dumping. If the equipment operators dumping the ore into the hopper have an effective enclosed cab filtration system their exposure to this dust would be reduced. However, if other mine personnel, such as crusher operators and/or maintenance workers, work near this primary dump, they can be exposed to this airborne dust.
Description of control:	Effective control methods include enclosing the hopper dump and using water sprays to suppress and contain the dust from rolling back out of the enclosure. Key Factors for Controlling Dust from the Primary Dump • Enclose the primary hopper dump. Walls can be constructed around the primary dump location to form an enclosure that must be custom designed to accommodate the dump vehicles being used. Walls can be either stationary (rigid) or movable (flexible material or curtains) based upon maintenance access within parts of the enclosure. Staging curtains (sometimes called stilling curtains) can be used in the enclosure to break up the natural tendency for dust to billow out of the primary dump hopper when a large volume of product is dumped in a very short time period (see Figure 1) [Weakly 2000]. Another option to restrict the dust from escaping the enclosure is using panels of flexible plastic stripping on the dump side of the enclosure. This plastic stripping employs an overlapping sequence which provides for a very effective seal and resists damage if contacted by the bucket of the front-end loader or the bed of the haul truck during dumping. Finally, a local exhaust ventilation (LEV) system can be used to filter the dust-laden air from the enclosed hopper area. This would be most appropriate when the primary dump is at a location where the dust could enter an adjoining structure or impact outside miners. Since hoppers are usually large, a significant amount of airflow would be required to create sufficient negative pressure to contain the dust cloud. This approach would be a more expensive alternative than using wet suppression [MSA Research Corp 1978]. Figure -1- Staging curtains used to prevent dust from billowing out of enclosure. • Use water sprays to suppress the dust in the enclosure. Water sprays directed at the ore dumped into the hopper will wet the material and suppress some of the generated airborne dust. A good starting point is to add 1% moisture by weight [MVS 1974]. This percentage can be adjusted based upon the improvement gained from additional moisture versus any consequences from adding too much. Since continuous use of water sprays during long periods of idle time between dumping can have adverse operational effects, the water sprays can be activated during the actual dump cycle through the use of a photo cell or a mechanical switching device. A delay timer can also be used in this application so that the sprays continue to operate and suppress dust for a short time period after the dump vehicle has moved away. • Prevent the dust from rolling back under the dump vehicle. A tire-stop water spray system is recommended to reduce the dust liberated due to rollback under the dumping mechanism. A tire stop or Jersey barrier should be positioned at the most forward point of dumping for the primary hopper. A water spray system should be attached to the back side of this tire stop to knock down and force the dust that would otherwise roll back under the dumping mechanism into the hopper. Additionally, a shield should be placed over this water spray manifold to protect it from damage from falling ore (Figure 2). Finally, a system should also be incorporated that allows the water sprays to only be activated during the actual dumping process, as previously discussed. Figure -2- Tire-stop water spray system reduces dust rollback under the dumping vehicle.
References:	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. 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 [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. MSA Research Corp [1978]. Improved dust control at chutes, dumps, transfer points, and crushers in noncoal mining operations. NTIS no. PB 297-422. By Rodger SJ, Rankin RL, Marshall MD. Evans City PA: MSA Research Corp. U.S. Bureau of Mines contract no. H0230027. MVS [1974]. Sources and methods of control of dust. By Quilliam JH. In: The ventilation of South African gold mines. Yeoville, Republic of South Africa: The Mine Ventilation Society of South Africa. NIOSH [1992]. Weakly A [2000]. Controlling dust without using bag houses. Coal Age November:24–26.
Keywords:	dust control metal/nonmetal mining mineral mining mineral processing miners surface mines