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Engineering Controls Database

Best Practices for Dust Control in Coal Mining – Surface Mines – Dry Dust Collection Systems for Drills

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 operators are typically exposed to the highest levels of respirable dust.
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. A typical dry dust collection system is shown in Figure 1. It is composed of a self-cleaning (compressed air back-pulsing of filters) dry dust collector sucking the dusty air from underneath the shrouded drill deck located over the hole. Ninety percent of dust emissions with this type of system are attributed to drill deck shroud leakage, drill stem bushing leakage, and dust collector dump discharge.

Practical aspects for optimizing these dust collection systems are described below:

Maintain a tight drill deck shroud enclosure with the ground. Dust emissions are significantly reduced around the drill deck shroud by maintaining the ground-to-shroud gap height below 8 in [NIOSH 2005; USBM 1987]. This can be accomplished by better vertical positioning of the drill table shroud by the operator to minimize the ground-to-shroud gap. Dust levels were significantly reduced from 21.4 to 2.5 mg/m3 next to the drill deck shroud when the drill operator changed the drill setup procedure to minimize this gap [Organiscak and Page 1999]. Also, the ground-to-shroud gap can be more tightly closed by using a flexible shroud design that can be mechanically raised and lowered to the ground via cables and hydraulic actuators. An adjustable height shroud design maintains a better seal with uneven ground and was found to keep dust emissions next to the shroud below 0.5 mg/m3 at several drill operations [NIOSH 1998, 2005]. Finally, a shroud constructed in sections with vertical gaps along sections or corners can also be a source of shroud leakage. Overlapping sections of shroud material can reduce gaps and leakage, particularly at the difficult to seal corner sections of the shroud [Page and Organiscak 1995].

Maintain a collector-to-bailing airflow ratio of at least 3:1. Dust emissions are significantly decreased around the shroud at or above a 3:1 collector-to-bailing airflow ratio [NIOSH 2005]. Dust collector airflow reductions under the shroud are generally caused by restrictions and/or leakages in the system. Loaded filters and material in the ductwork are likely causes of restrictions, whereas damaged ductwork and holes are likely causes of leakage in the system. Thus, inspection and maintenance of the dust collection system are vital to achieving and maintaining optimal collector operation and airflow.

Maintain a good drill stem seal with the drill table. A rubber drill stem bushing (see Figure 1) restricts bailing airflow from blowing dust and cuttings through the drill deck but is subject to mechanical wear. Therefore, this bushing should be examined frequently and replaced after mechanical wear allows dust to escape. An alternative sealing method involves using a nonmechanical compressed air ring seal manifold under the drill deck. This manifold consists of a donut-shaped pipe with closely spaced holes on the inside perimeter that discharges air jets in a radial pattern at the drill stem. The high-velocity air jets block the gap between the drill stem and deck, reducing respirable dust leakage through the drill deck by 41%–70% [Page 1991].

Figure -1-  Typical dry dust collection system used on surface drills.

Figure -1- Typical dry dust collection system used on surface drills.

Shroud the collector dump discharge close to the ground. Dumping dust from the collector discharge several feet above ground level can disperse significant amounts of airborne respirable dust. Dust emission reductions of greater than 63% were measured by the collector discharge dump after installing an extended shroud near ground level (Figure 1) [Reed et al. 2004; USBM 1995]. These shrouds can be fabricated quickly by wrapping brattice cloth around the perimeter of the collector discharge dump and securing it to the discharge dump with a hose clamp.

Maintain the dust collector as specified by manufacturer. Collector system components should be frequently inspected and damaged components repaired or replaced. A 51% dust emission reduction was measured at one drill after a broken collector fan belt was replaced, while another drill showed a reduction of 83% after the torn deck shroud was replaced [Organiscak and Page 1999].
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.

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

NIOSH [1998]. Hazard controls: New shroud design controls silica dust from surface mine and construction blast hole drills. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, HC27, DHHS (NIOSH) Publication No. 98–150.

NIOSH [2005]. Technology news 512: Improve drill dust collector capture through better shroud and inlet configurations. 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. 2006–108.
coal miners
coal mining
surface mines
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