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Engineering Controls Database

Best Practices for Dust Control in Coal Mining – Surface Mines – Wet Suppression Systems

Respirable dust exposure has long been known to be a serious health threat to workers in many industries. In coal mining, overexposure to respirable coal mine dust can lead to coal workers’ pneumoconiosis (CWP). CWP is a lung disease that can be disabling and fatal in its most severe form. In addition, miners can be exposed to high levels of respirable silica dust, which can cause silicosis, another disabling and/or fatal lung disease. Exposure to coal mine dust may also increases a miner’s risk of developing chronic bronchitis, chronic obstructive pulmonary disease, and pathologic emphysema.

Once contracted, there is no cure for CWP or silicosis. The goal, therefore, is to limit worker exposure to respirable dust to prevent development of these diseases.
U.S. mine workers continue to be at risk of exposure to excessive levels of silica dust. The percentage of Mine Safety and Health Administration (MSHA) dust samples during 2004–2008 that exceeded the applicable or reduced respirable dust standard because of the presence of silica were: 12% for sand and gravel mines, 13% for stone mines, 18% for nonmetal mines, 21% for metal operations, and 11% for coal mines [MSHA 2009]. At surface mining operations, occupations most frequently exceeding the applicable respirable dust standard are usually operators of mechanized equipment such as drills, bulldozers, scrapers, front-end loaders, haul trucks, and crushers.
Drill dust is generated by compressed air (bailing airflow) flushing the drill cuttings from the hole. Dry or water-based dust collection systems are available for controlling this drill dust. Dry dust collection systems are the most common type of dust control incorporated into the drilling machine by original equipment manufacturers because of their ability to be operated in freezing temperatures. Wet suppression is another drill dust collection method and involves injecting water into the bailing airflow traveling down the drill stem. The process of the bailing airflow, water droplets, and cuttings mixing together captures the airborne dust as they travel back up the hole. However, wet suppression is infrequently used because of operational problems in cold climates, lack of a readily accessible water supply, and shorter bit life.

Practical aspects for optimizing wet suppression systems are described below:

Add small amounts of water into the bailing air until the visible dust cloud has been significantly reduced. Drill dust emissions are significantly reduced by increasing the water flow rate from 0.2 to 0.6 gallons per minute (gal/min) [USBM 1987]. A needle valve and water flow meter installed on the water supply line provides adjustable control for wet suppression systems. However, adding excessive water down the hole can cause operational problems with no appreciable improvement in dust control.

Minimizing water flow to a rolling cutter bit can increase bit life. Wet drilling with rolling cutter bits can cause premature bit wear. A drill stem water separator installed upstream of a rolling cutter bit can increase bit life without adverse affects on dust control [Listak and Reed 2007; USBM 1988]. The water separator is a bit stabilizer with an internal cyclonic or impaction water droplet classifier, removing most of the water from the bailing airflow before it is flushed through the drill bit. The water removed by the internal separator is released through external holes in the bit stabilizer (Figure 1).
Figure 1 - Water separator discharging water before it reaches the drill bit

Figure 1 - Water separator discharging water before it reaches the drill bit
NIOSH [2010]. Information circular 9517. Best practices for dust control in coal 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. 2010-110.
Listak JM, Reed WR [2007]. Water separator shows potential for reducing respirable dust generated on small-diameter rotary blasthole drills. Int J Min Reclam Environ 21(3):160–172.

MSHA [2009]. Program Evaluation and Information Resources, Standardized Information System. Arlington, VA: U.S. Department of Labor, Mine Safety and Health Administration.

USBM [1987]. Technology news 286: Optimizing dust control on surface coal mine drills. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines.

USBM [1988]. Technology news 308: Impact of drill stem water separation on dust control for surface coal mines. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines.
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respirable dust
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