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Engineering Controls Database

Best Practices for Dust Control in Coal Mining – Continuous Mining Operations – Water Spray 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.
CWP contributed to the deaths of 10,406 U.S. miners during 1995–2004 [NIOSH 2008]. Pneumoconiosis continues to be a very serious health threat to underground coal mine workers.

The greatest source of respirable dust at continuous mining operations is the continuous mining machine. Dust generated by the continuous miner has the potential to expose the miner operator and anyone working downwind of the active mining. Also, continuous miner and roof bolter operators are often exposed to elevated silica levels as a result of cutting or drilling into rock. For operations on reduced dust standards, MSHA inspector samples from 2004–2008 show that 20% of miner operator samples and 10% of bolter operator samples exceeded their applicable reduced dust standard [MSHA 2009].
As with any dust source, air and water are used to dilute, suppress, redirect, or capture dust. Suppression of dust is the most effective means of dust control. Suppression is achieved by the direct application of water, usually at the point of attack, to wet the coal before and as it is broken to prevent dust from becoming airborne. Once dust is airborne, other methods of control must be applied to dilute it, direct it away from workers, or remove it from the work environment. Redirection of dust is achieved by water sprays that move dust-laden air in a direction away from the operator and into the return entry or behind the return curtain. Capture of dust is achieved either by water sprays that impact with the dust in the air to remove it or with mechanical dust collectors.

There are several types of water sprays available for use on continuous miners to control dust. Spray nozzle type, location, pattern, flow, and pressure are all factors to consider when designing a spray system. The type of spray used at a particular location depends on the desired application. For example, for suppression of dust, high flow at low pressure close to the source is most effective. For airborne dust capture, smaller high-velocity droplets are required to impact with dust and remove it from the air. For redirection, higher pressure is required. Figure 1 shows the sprays most commonly used for controlling dust.

Figure 1 - Spray types used for dust control in mining.

Figure 1 - Spray types used for dust control in mining.

Figure 2 shows the airborne capture performance of the different spray nozzles at different operating pressures and compares the relative effectiveness of each spray type. Although air-atomizing nozzles produce the best airborne dust capture, their use in mines is impractical due to high maintenance requirements (i.e., they are prone to clogging) and the need to supply compressed air to each nozzle.

Figure 2 - Relative spray effectiveness of four spray nozzles used in mining.

Figure 2 - Relative spray effectiveness of four spray nozzles used in mining.

Most continuous miners use a combination of spray types to achieve the best control. Although higher water pressure will raise the effectiveness of water sprays, a marked disadvantage is that it entrains large volumes of air and subsequently dust. This can result in dust rollback. The earliest water sprays on a continuous miner were used for bit lubrication, bit cooling, and dust control. Although these sprays controlled respirable dust exposure to a limited extent, they also created large quantities of turbulence and dust rollback. Dust rollback over the continuous miner infiltrated the operator’s position, resulting in dust overexposure. To control rollback, sprays were relocated atop and beneath the cutting drum. The top sprays operated at a pressure of 100 pounds per square inch (psi) and a flow rate of 0.95 gallons per minute (gpm) per spray. Two large-orifice, deluge-type sprays were mounted on the left and right underside of the boom and directed to spray into the cutting bits. These sprays operated at a low pressure of about 7 psi and a higher flow rate of 5 gpm per spray. Dust rollback was decreased as a result because the spray droplets moved only a short distance before impacting on the cutting bits. The short distance also increased coal surface wetting capabilities while minimizing turbulence (Figure 3). In-mine evaluations of these boom sprays show that miner operator dust exposures are reduced by 40% compared to the factory-installed spray system [Schroeder et al. 1986].

Figure 3 - Spray location impact on dust rollback

Figure 3 - Spray location impact on dust rollback

High-pressure sprays, installed at the rear corner of the shovel on the side opposite from the exhausting ventilation curtain, can sweep underboom dust toward the curtain. Extensive underground testing showed that the shovel sprays reduced coal dust exposures by 60% at the miner operator’s location while virtually eliminating exposures to respirable quartz dust [Schroeder et al. 1986]. The following practices have been shown to reduce dust exposures on continuous mining operations:

• Dust rollback over the miner can be caused by high-pressure (>100 psi), wide-angle cone sprays [Jayaraman et al. 1984]. A typical miner spray does most of its airborne dust collection in the first 12 in. Thus, top and side nozzles should be arranged for “low” reach and no overspray (Figure 4, A and B). Past research has shown that flat-fan sprays at a horizontal orientation with high flow and low pressure (<100 psi) across the boom, located as close to the cutter head as possible, provide uniform coal wetting across the cutter head during mining while limiting rollback. Large-orifice, low-pressure deluge throat sprays should be used under the boom on flight conveyors at 5 gpm. Broken material should be wetted as it is gathered and conveyed (Figure 4, C) [Schroeder et al. 1986].

Figure 4 - Antirollback spray system for miner

Figure 4 - Antirollback spray system for miner

• For dust containment at the face, flat-fan sprays, located 1 ft back from the cutter head on both sides of the miner with a vertical spray pattern that is oriented 30° from the miner body, act as blocking sprays and will help contain dust, thus enabling improved dust capture by the scrubber inlets [Goodman 2000]. Laboratory testing has shown that increasing spray nozzle pressure and/or the width of the spray angle will increase the airflow induced by that nozzle, potentially increasing its effectiveness as a blocking spray [Pollock and Organiscak 2007].

• High-pressure sprays are recommended for redirecting of dust. High pressure (>150 psi) raises the efficiency per unit of water [Jayaraman and Jankowski 1988] and is effective for air moving (Figure 5). However, care must be taken when determining location and direction because high pressure can cause turbulence, leading to rollback.

Figure 5 - Air-moving effectiveness of different spray types

Figure 5 - Air-moving effectiveness of different spray types

• A directional spray system design (spray fan) is a water-powered ventilation system originally designed to sweep methane gas toward the return. This system contains several spray manifolds placed on the continuous mining machine to direct fresh intake air to the cutting face, sweep contaminated air across the face, and direct this airflow into the return airway. In practice, the spray fan system is used only with an exhaust face ventilation scheme. The spray fan design has been successful for methane control at the face, but is not as effective for dust control [Goodman et al. 2004].

• Recently developed wet-head continuous miners have water sprays mounted on the cutter head directly behind the cutter bits. Water sprays in this position cool bits, which reduce frictional ignitions and have the potential for reducing dust generation during cutting. Available spray nozzles are either solid-cone spray patterns with flow of 0.4 gpm at 100 psi, or hollow-cone patterns with a flow of 0.2 gpm at 100 psi. However, wet-head technology has not yet consistently demonstrated dust reduction benefits in reported studies. These studies have shown variation in dust reductions under its current configuration [Strebig 1975; Goodman et al. 2006]. One benefit that has been reported by mining machine operators is increased visibility at the cutter head when the wet head is used. This may lead to better control of the cutting head and have a beneficial impact on required maintenance (reduced bit changes) and subsequent dust generation.

• Good water filtration greatly aids in spray system effectiveness. Dirt and rust particles in the water line can cause frequent clogging of spray nozzles. A simple, nonclogging water filtration system is available and should be used to replace conventional spray filters [Divers 1976].

• Operators should examine, clean, and/or replace sprays if necessary before each cut.
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.

NIOSH [2008]. Work-related lung disease surveillance report, 2007. 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. 2008143a.

Divers EF [1976]. Nonclogging water spray system for continuous mining machines: installation and operating guidelines. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, IC 8727. NTIS No. PB 265 934

Fields KG, Atchison DJ, Haney RA [1991]. Evaluation of dust control for deep cut coal mining systems using a machine mounted dust collector. In: Proceedings of the Third Symposium on Respirable Dust in the Mineral Industries. Littleton, CO: Society for Mining, Metallurgy, and Exploration, Inc., pp. 349–353.

Goodman GVR, Pollock DE, Beck TW [2004]. A comparison of a directional spray system and a flooded-bed scrubber for controlling respirable dust exposures and face gas concentrations. In: Ganguli R, Bandopadhyay S, eds. Mine ventilation: Proceedings of the 10th U.S./North American Mine Ventilation Symposium (Anchorage, AK, May 16–19, 2004). Leiden, Netherlands: Balkema, pp. 241–248.

Goodman GVR, Beck TW, Pollock DE, Colinet JF, Organiscak JA [2006]. Emerging technologies control respirable dust exposures for continuous mining and roof bolting personnel. In: Mutmansky JM, Ramani RV, eds. Proceedings of the 11th U.S./North American Mine Ventilation Symposium (University Park, PA, June 5–7, 2006). London: Taylor & Francis Group, pp. 211–216.

Jayaraman NI, Kissell FN, Schroeder W [1984]. Modify spray heads to reduce dust rollback on miners. Coal Age 89(6):56–57.

MSHA [2009]. Standardized Information System: Respirable coal mine quartz dust data. Arlington, VA: U.S. Department of Labor, Mine Safety and Health Administration.

Pollock DE, Organiscak JA [2007]. Airborne dust capture and induced airflow of various spray nozzle designs. Aerosol Sci Technol 41(7):711–720.

Schroeder WE, Babbitt C, Muldoon TL [1986]. Development of optimal water spray systems for dust control in underground mines. Foster-Miller, Inc. U.S. Bureau of Mines contract H0199070. NTIS No. PB 87-141537.

Schultz MJ, Fields KG [1999]. Dust control considerations for deep cut mining sections. SME preprint 99-163. Littleton, CO: Society for Mining, Metallurgy, and Exploration, Inc.

Strebig KC [1975]. “Wet-head” tests on miners concluded. Coal Min & Process 12(4): 78–80, 88.
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