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Engineering Controls Database

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

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.
Transfer points are used to move ore from one process or one piece of equipment to another. Although this seems like a simple process, significant dust generation and liberation can result from transfer points if they are not properly designed and installed.
The following are some important design considerations for an effective transfer point or chute:

• Transfer chutes should be sized to allow ore to flow without clogging or jamming. A general rule of thumb is that the chute depth should be at least three times the maximum lump size to avoid clogging [Martin Marietta Corp 1987].

• The dump point of the ore should be designed to impact on a sloping bottom or a rockbox. Rockboxes are designed to allow ore product to build up so that ore contacts ore during transfer, which reduces wear and abrasion of the chute.

• Any abrupt changes in product direction or flow should be avoided.

• Fall height of ore should be minimized whenever possible through the use of rock ladders, telescopic chutes, spiral chutes, and bin-lowering chutes.

• A head enclosure should be used when transferring ore onto a conveyor. The head enclosure should be designed with strip curtains to minimize air induction into the enclosure and skirt boards to position the ore on the center of the belt.

• A Local Exhaust Ventilation (LEV) system should be used at transfer enclosures and chutes to capture and filter the dust from the air. These enclosures should be designed to have approximately a 250-feet/per/minute (fpm) intake velocity at any opening to eliminate dust leakage from the area. To accomplish this, plastic stripping and other types of sealing systems should be used to minimize openings and maximize intake velocity. One study also recommended that the exhaust port to the LEV system be located at least 6 feet away from the transfer dump point to minimize the possibility of entraining large particles [MAC 1980].

• The exit velocity from the enclosure or chute should be kept below 500 fpm to minimize the entrainment of large particles of ore [Yourt 1990].

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.

MAC [1980]. Design guidelines for dust control at mine shafts and surface operations, third edition. Ottawa, Ontario, Canada: Mining Association of Canada.

Martin Marietta Corp [1987]. Dust control handbook for minerals processing. Raleigh, NC: Martin Marietta Corporation. USBM contract no. J0235005.

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.

Yourt GR [1990]. Design principles for dust control at mine crushing and screening operations. Canadian Min J 10:65–70.
metal/nonmetal mining
mineral mining
mineral processing
transfer point
transfer point or chute