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Engineering Controls Database

Best Practices for Dust Control in Metal/Nonmetal Mining – Mineral Processing Operations – Screening

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.

Screens are the most common device used at mineral processing operations to separate dry ore material into different size ranges, normally measured in mesh sizes. By having the ore processed on different mesh-sized screens, an array of various-sized materials can be produced. Screen sizes for mineral processing operations range from large openings that can be inches in size all the way down to 400 mesh, which has a 35-micron cut point. The amount of dust generated during screening is dependent on the ore type, the particle size, the moisture content, and the type of equipment. Normally, screening finer-sized ore material produces more dust.
Screening has been performed at mineral processing operations for many years and has been perfected by most screen manufacturers. New screening equipment units today are well-sealed units which liberate very little dust into the work environment when they are properly maintained and operated (Figure 1). When processing any ore that has significant silica content, screens should be tied into a local exhaust ventilation (LEV) dust collector system to help keep the process under negative pressure and capture respirable-sized particles liberated within the unit. If visible dust is seen leaking from a screening unit, or if product is visible on the ground underneath a screen, this indicates that a problem exists and needs to be corrected. The first course of action is to determine if a seal or part has worn and is leaking product. In addition, the LEV exhaust port should be examined to ensure it is operating properly and is exhausting the screening unit at the required airflow and pressure.

Another problem occurs when a screen cover is lifted during operation to clean the screen surface. This basically renders the LEV system ineffective because the increased area with the cover lifted is so great that the screen area is not able to be kept under negative pressure. This is a known problem throughout the industry. The only safety option currently available to workers when performing this task is to wear fit-tested personal protective equipment (PPE) that is rated for the levels and type of dust being processed.

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.
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.
dust control
metal/nonmetal mining
mineral mining
mineral processing