Engineering Controls Database
Best Practices for Dust Control in Metal/Nonmetal Mining –
Mineral Processing Operations – Conveying
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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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At mineral processing operations, conveyors are the major component used to transfer ore from one process to another. A conveyor can generate significant quantities of respirable dust and be one of the greatest sources of dust emissions within an operation. There are four main areas of dust generation and liberation from conveyors: • When ore is dumped onto the belt. • As ore travels on the belt. • From the underside return idlers due to carryback on the belt. • When ore is dumped or transferred to another belt or process. One of the challenges with conveyors is the number of belts used and the total distance traveled throughout a mineral processing plant. Some belts are located outside where dust liberation is not as critical as when they are within a facility. Another challenge particular to conveyors is their ability to generate or liberate dust while operating, whether they are loaded with ore or empty. |
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Controlling dust from conveyors requires constant vigilance by the maintenance staff to repair and replace worn and broken parts. There are a number of techniques to reduce dust liberation from conveyors, as follows: Suppress. When properly designed and installed, water sprays are a cost-effective method of controlling dust from conveyors. The most common and effective practice for conveyor sprays is to wet the entire width of product on the belt. The amount of moisture applied should be varied and tested at each operation to determine the optimum quantity, but 1% moisture added to product ratio is a good starting point. A number of studies have indicated that wetting the return side of the conveyor belt also helps reduce dust liberation. This is effective because it reduces dust generation from the idlers as well as at the belt drives and pulleys. In many cases, water sprays located on the top (wetting the product) and the bottom (reducing dust from the idlers) at the same application point can be an effective strategy [Courtney 1983; Ford 1973]. These locations are also beneficial from an installation and cost standpoint. When considering nozzle type for these suppression systems, fan spray nozzles are normally the most common design because they minimize the volume of water added for the amount of coverage. For these types of applications, it is more advantageous to locate the water sprays at the beginning of the dust source (i.e., the dump or transfer location), because as the water and ore continually mix together, the amount of wetted surface area of the ore increases, thus increasing the suppression potential and reduction in dust liberation. Using sprays at higher flow rates can sometimes create air turbulence, which makes it more difficult to contain/suppress the dust. Because of this, it is normally recommended to use more spray nozzles at lower flow rates and position them at locations closer to the ore [NIOSH 2003]. Enclose. Enclosures are an effective dust control technique for many applications within mineral processing plants if they are correctly designed and installed, and this principle also applies to conveyors. Enclosures for conveyor and transfer points can be either full or partial type [Zimmer 2003]. One of the most common partial types of enclosures is with the use of skirting, which keeps the material on the belt, especially immediately after it exits a loading chute. An inclined skirting design, in which the skirting belt is angled at approximately 30 degrees from vertical, is more advantageous over a standard vertical design because of wear issues. This skirting design improves the loading of ore onto the conveyor and reduces the amount of dust generated. Enclosures at head and tail ends of the conveyor are a common practice because they are effective at controlling dust at these locations. Designing the proper size enclosure is a critical factor because, as the ore is dumped onto the conveyor, it entrains a measurable amount of air (venturi effect) and this can pressurize the enclosure if it is undersized. Dust curtains are another form of enclosure used to contain dust within a conveyor and are very cost effective to install. These curtains are normally installed at the head and/or tail ends of the conveyor. In many instances, local exhaust ventilation (LEV) is tied into the enclosure at these conveyor dump or transfer locations to capture generated dust. It has been shown that the takeoff port to the LEV system should be a least 6 feet from the dump point to minimize the pickup of oversized particles [MAC 1980]. The air velocity at this exhaust port should also be kept below 500 feet per minute (fpm) to avoid the pickup of larger particles [Yourt 1990]. Belt scraper. An effective method to reduce dust being liberated from conveyors is with belt scrapers. Although belt scrapers come in many different styles, types, and trade names sold by numerous commercial manufacturers, their function remains the same, which is to reduce the amount of carryback on the belt once the ore is discharged. Carryback is the material that sticks or clings to the conveyor belt after the material is discharged at the head pulley. As this material dries and passes over the return idlers, it falls from the belt and the respirable portion of this dust becomes airborne. The goal is to remove this carryback product before it is released into the air and becomes a source of contamination to the workers. When dust levels are high, a common practice is to use two or three belt scrapers at different locations in an effort to further reduce the amount of carryback material on the belt [Roberts et al. 1987]. Belt wash. Some studies have shown that the oversized material is more easily removed through scraping but the smaller respirable-sized particles tend to remain adhered to the conveyor. When this occurs, a belt wash should be installed. A belt wash sprays the conveyor belt with water while simultaneously scraping it to remove the product. In a number of published studies in this area, this technique has been shown to increase the cleaning effectiveness by approximately 14% [Planner 1990]. Effective belt loading. Providing an effective belt loading area helps to reduce the amount of dust generated during belt loading. The first design goal of an effective system is to reduce belt sag and vibration during ore loading. To do this, some operations are using slider-bar cradles which stabilize this area [Stahura and Marti 1995]. These cradles can be made to provide a shock-absorbing action to cushion the impact during belt loading. This goal can also be achieved with low-friction bars that provide a flat support area for loading, which minimizes belt sag. Effective belt loading should also include side shields which contain the fine-sized particles and help to minimize the liberation of dust Figure 1 shows a combination of the various techniques discussed that help enclose and minimize dust generated during conveying. This includes the use of plastic stripping to minimize the opening size, a rockbox, belt skirting, and the dust collection pick-up point. In addition, wet suppression is also shown, which will be discussed in another data base report.  |
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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. Courtney WG [1983]. Single spray reduces dust 90%. Coal Min Proc June:75–77. Ford VHW [1973]. Bottom belt sprays as a method of dust control on conveyors. Min Tech (UK) 55(635):387–391. 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. 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 [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. Planner JH [1990]. Water as a means of spillage control in coal handling facilities. In: Proceedings of the Coal Handling and Utilization Conference. Sydney, Australia: Institution of Engineers Australia, pp. 264–270. Stahura R, Marti A [1995]. Conveyor trends. World Min Equip 19(5):12–21. Roberts AW, Ooms M, Bennett D [1987]. Bulk solid conveyor belt interaction in relation to belt cleaning. Bulk Solids Handling 7(3):355–362. Yourt GR [1990]. Design principles for dust control at mine crushing and screening operations. Canadian Min J 10:65–70. Zimmer W [2003]. Dust and water—do they really clash? Bulk Solids Handling 23(5):302–307. |
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conveyer belt workers conveyers metal/nonmetal mining mineral mining mineral processing miners |