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Best Practices for Dust Control in Metal/Nonmetal Mining – Crushing Facilities

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; consequently, mine workers are potentially exposed to silica dust when rock is cut, drilled, crushed, and transported.
The stone and metal/nonmetal mining industry encompasses many types of commodities. The potential for respirable silica dust exposure to workers in the stone and metal/nonmetal mining industry is related to the percentage of silica in the product being mined or processed. For crushed and broken stone or dimension stone, silica percentages are on the high end, with sandstones and granites averaging 70% to 90%. On the low end are limestones, averaging 20% to 30%. For all metal/nonmetal ores, silica percentages average from 5% to 20% [USBM 1992]. Therefore, airborne concentrations of silica dust are dependent upon the silica percentage in the rock and ore being mined. Each commodity has common dust sources related to the mining cycle, which includes drilling, blasting, loading, hauling, and crushing.

Sampling surveys have shown that underground crushing facilities, which include the dump, the crushers, and the associated conveyor belts and transfer points, can be a significant source of silica dust generation. Airborne silica concentrations can be extremely high depending on the bulk content of silica in the rock and crusher production capacity. Occupations typically exposed to silica dust from this source obviously include the crusher operators and truck drivers, and also the mechanics, cleanup men, and laborers whose tasks require them to work in this area.
Several methods for reducing worker exposure to silica dust at crusher locations are provided below:

Isolate the facility from the general mine air circuit. Dust generated from this source can be adequately contained using brattice or permanent stoppings to isolate the entire facility (dump, crusher, and belt). Booster fans using blowing ventilation should be positioned in key locations to increase airflow around the facility and dilute and transport dust away from the location to a return entry. Booster fans may be either axial vane or propeller, but recent studies have shown that propeller fans dilute and transport dust more effectively, especially in large-opening mines [Chekan et al. 2006]. Figure 1 shows the plan view of a limestone crusher facility isolated from the other mine developments using stoppings and a blowing propeller fan to move dust-laden air into the return.
Figure 1 - Typical method used to isolate crushing facility from mine air.

Figure 1 - Typical method used to isolate crushing facility from mine air.


Ventilate with a closed ventilation system. A closed ventilation system, where a plenum is located under the crusher, may be required in cases where the facility cannot be isolated and dust cannot be directed to the return entries. Air is exhausted from under the plenum, creating an indraft at the crusher jaws to capture the dust. The dust-laden air is then directed to a nearby return, a bag house, or a fan-powered dust collector where it is captured by filters and the clean air can be discharged into the mine air [NIOSH 2003a]. Figure 2 shows a conceptual approach to control crusher dust in a stone mine using a closed ventilation system.
Figure 2 - . Closed ventilation system using plenum.

Figure 2 - . Closed ventilation system using plenum.


Use filtration/pressurization systems in mobile equipment cabs and operator booths. As mining equipment ages, many of the original components on the cab enclosure deteriorate through normal operation in harsh mine environments. As a result, the effectiveness of the air filtration system and cab seals is lessened and the protection initially afforded to operators is compromised, possibly exposing them to elevated levels of respirable silica dust. NIOSH has worked with a number of manufacturers to develop cost-effective methods to improve both filtration effectiveness and cab integrity on these older cabs with the goal of reducing silica dust levels inside the cabs. Research results show dust levels inside upgraded cabs were reduced from 65% to 95% when compared to levels outside the cabs [NIOSH 2008]. Retrofit options from several manufactures are available for both enclosed cabs and booths. Figure 3 shows an effective design of an enclosed filtration and pressurization system.
Figure 3 - Filtration and pressurization system components and design.

Figure 3 - Filtration and pressurization system components and design.


Consider five key factors for maintaining and operating enclosed cabs and booths.

(1) Ensure good cab enclosure integrity which will allow the filtration/pressurization system to achieve positive pressurization against wind penetration into the enclosure. Studies show that significant improvements in cab protection factors were achieved when cab pressures exceeded 0.01 inches of water gauge [Cecala et al. 2005].

(2) Utilize high-efficiency respirable dust filters on the intake air supply into the cab. Filter efficiency performance specifications used in the field were 95% or greater on respirable-sized dusts. Laboratory experiments showed an order of magnitude increase in cab protection factors when using a 99%-efficient filter versus a 38%efficient filter on respirable-sized particles [NIOSH 2008].

(3) Use an efficient respirable dust recirculation filter. All the cab field demonstrations used recirculation filters that were 95% efficient, or greater, on respirable-sized dusts. Laboratory experiments showed an order of magnitude increase in cab protection factors when using an 85%- to 94.9%-efficient filter on respirable-sized dusts as compared to using no recirculation filter [NIOSH 2008]. Laboratory testing also showed that the time for interior cab concentration to decrease and reach stability after the cab door is closed was cut by more than half when using the recirculation filter.

(4) Minimize dust sources in the cab by using good housekeeping practices, such as periodically cleaning soiled cab floors, using a sweeping compound on the floor, or vacuuming dust from a cloth seat [NIOSH 2001b]. Also, relocate heaters that are mounted near the floor. These units have been shown to blow air across soiled cab floors and increase dust levels inside the cab [NIOSH 2001a].

(5) Keep doors closed during equipment operation. One study showed a ninefold increase in dust concentrations inside the cab when doors were frequently opened during the sampling period [Cecala et al. 2007].

Use canopy air curtains. In many underground mines, operator enclosures cannot be used due to various mining or operational parameters. An alternative option for operators in open cabs and crusher compartments is a canopy air curtain, which filters and blows clean air over the operator’s breathing zone (Figure 4) [Goodman et al. 2006; Goodman and Organiscak 2001].
Figure 4 - Canopy air curtain blows filtered air over worker.

Figure 4 - Canopy air curtain blows filtered air over worker.
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.
Cecala AB, Organiscak JA, Zimmer JA, Moredock D, Hillis M [2007]. Opening door on drill cab during non-drilling can significantly increase operator’s dust exposure. Rock Prod J 110(10):29–32.

Chekan GJ, Colinet JF [2002]. Silica dust sources in underground limestone mines. In: Proceedings of the Thirty-Third Annual Institute on Mining Health, Safety and Research. Blacksburg, VA: Virginia Polytechnic Institute and State University, Department of Mining and Minerals Engineering, pp. 55–70.

Chekan GJ, Colinet JF, Grau RH III [2006]. Impact of fan type for reducing respirable dust in an underground limestone crushing facility. In: Proceedings of the 11th North American/Ninth U.S. Ventilation Symposium, University Park, PA, June 5–7, pp. 203–210.

Goodman GVR, Organiscak JA [2001]. Laboratory evaluation of a canopy air curtain for controlling occupational exposures of roof bolters. In: Proceedings of the 7th International Mine Ventilation Congress, Krakow, Poland.

Goodman GVR, Beck TW, Pollock DE, Colinet JF [2006]. Emerging technologies control respirable dust exposures for continuous miner and roof bolter personnel. In: Proceedings of the 11th North American/Ninth U.S. Ventilation Symposium, University Park, PA, June 5–7.

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 487: Sweeping compound application reduces dust from soiled floors within enclosed operator cabs. 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 [2003a]. Dust control in stone mines. By Kissell FN, Chekan GJ. In: Handbook for dust control in mining. 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. 2003-147, pp. 57–72.

NIOSH [2003b]. Underground hard-rock dust control. By Kissell FN, Stachulak JS. In: Handbook for dust control in mining. 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. 2003-147, pp. 83–96.

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.

USBM [1992]. Crystalline silica primer. Washington, DC: U.S. Department of the Interior, U.S. Bureau of Mines.
blasting
crushing
drilling
hauling
loading
metal/nonmetal mining
mineral processing
miners
mining
stone mining
underground mines
For operating enclosed cabs and booths; laboratory experiments showed an order of magnitude increase in cab protection factors when using an 85%- to 94.9%-efficient filter on respirable-sized dusts as compared to using no recirculation filter [NIOSH 2008].