Engineering Controls Database
Best Practices for Dust Control in Metal/Nonmetal Mining –
Mineral Processing Operations – Crushing & Grinding
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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. 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. |
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| Crushing and grinding at mineral processing operations include a wide range of different types of equipment and processes. On the crushing side, primary crushers are typically jaw crushers, but may sometimes include gyratory and/or cone crushers. These crushers use compressive forces to break the ore and do not normally generate large volumes of dust. Secondary crushers may include the gyratory and cone, as well as hammermill and impact crushers. Hammermill and impact crushers use a rotating device (hammers) to thrust the ore against the outer walls of the crusher with the intent to break the ore by impaction against the outer surface. Because the ore is impacted at high velocities to induce breakage, high dust generation and liberation rates can occur from these types of crushers. After ore is fed into the crusher, it remains in the unit and continues to be crushed until it reaches a size small enough to be discharged from the unit. Grinding and pulverizing the ore is performed later in the mineral process to reduce the product down to the smaller size ranges, normally measured in mesh sizes. Grinding mills are used to perform this process and are cylindrical, horizontal drums that rotate and have rods, balls, or pebbles inside to grind the ore down to the desired size ranges. The two primary dust emission points of all crushing and grinding units are at the feed and discharge points. Controlling this dust by properly designing chutes or transfer points with rubber seals between stationary and moving components, as well as enclosing this area, is critical to an effective dust control plan. | |
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Dust control for the crushing and grinding processes is normally achieved by either wet suppression or local exhaust ventilation (LEV) systems, or a combination of both. Spraying the ore with water sprays to coat the outer surface helps to prevent dust from becoming liberated. Applying the water to the ore before it enters the crushing or grinding unit is most effective. In addition, it has been shown that the water pressure at early stages of crushing should be kept below 60 pounds per square inch (psi) to avoid pressurizing and forcing dust from the feed chute enclosures [NIOSH 2003]. The amount of moisture is not as critical during the early stages of the process but should be closely evaluated as the ore enters the later stages when the finer product sizing is taking place. In these cases, full and hollow cone sprays would normally be used to wet the ore and minimize dust liberation. When using an LEV system to capture and remove the dust from the crushing and grinding processes, a critical component to maintaining an effective system is determining the amount of air volume required to keep the process under negative pressure. As ore is fed into the crusher or grinder, it entrains air along with the product, creating a significant volume of air which must be exhausted to overcome the induction effect [MSA 1978; Yourt 1990]. The volume of exhaust air is also dependant on the effectiveness of sealing the crusher’s or grinder’s intake opening. By minimizing the area of the opening using belting and plastic stripping, the volume of exhaust air can be lowered while still maintaining an acceptable negative pressure necessary to contain the dust liberated during this transfer process. One final component that must be considered in all crushing and grinding processes is maintaining a proper seal on the device. If product is observed on the floor below the device or if visible dust is seen liberating from a unit, this indicates that a hole has been created or a seal has worn out, and maintenance needs to be performed to repair the problem. |
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NIOSH [2010]. Information circular 9517. Best practices for dust control in metal/nonmetal mining. Pittsburgh, PA: 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 [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. MSA Research Corp., USBM contract no. H0230027. NIOSH [2003]. Handbook for dust control in mining. By Kissell FN. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health NIOSH IC 9465. Yourt GR [1990]. Design principles for dust control at mine crushing and screening operations. Canadian Min J 10:65–70. |
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crushing or grinding minerals metal/nonmetal mining mineral mining mineral processing miners |