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MSHA SUGGESTS DUST CONTROL METHODS
Editorial focus by Harold Hough Feb/March 2009

Most people don’t realize that effective dust control is quite complex and requires an understanding of several disciplines including mine engineering and aerodynamics. With that in mind, the Mining Safety and Health Administration (MSHA) has gone to a great deal of effort to give mine operators an idea of methods that work.

Some dust control practices may only require a minor change in mining practices. For instance, in longwall mining, shearers are designed to mine the face in either or both direction. Consequently, unless the mine has automated shields, shield setters must work downwind of the shearer during half of the mining cycle. Simply allowing the shear to free wheel during the return will reduce the amount of dust inhaled by the shield setters.

Another way to reduce dust in longwall mining is to use deep cutting with slow speed drums. Not only is less surface area exposed in deep cutting, the deep cuts create larger coal fragments. There is also less “fanning” from the fast spinning drums, which creates more airborne dust.
A good dust control program is more than adopting one or two practices. But, here are some suggestions for reducing dust in your operation.

REPLACE BITS. Regular bit replacement is an essential part of any dust control program. A sharp bit acts as a wedge to split the coal while a worn bit merely crushes the coal at the point of contact, increasing coal fines and dust. Since bit wear is a function of machine operation practices, and the property of the rock, it’s critical to frequently check and replace bits because even a seven percent wear on the tool bit can cause considerable amounts of coal dust.

SPRAYING. Upgrading your water supply system can significantly reduce your dust problems. Some improvements can include increasing pump capacity, increasing line sizes, and installing a no-clogging filtration system.

But none of these will help if you don’t understand that your nozzles are critical to effective dust control. Two important factors are the size of the dust that must be controlled and the size of the droplets of water. For the system to work, the water droplets must hit the dust particles. That is where size is critical. Obviously, the larger the drops, the fewer the number of droplets per gallon of water, and the less chance there is of it hitting most of the dust in the air. In addition, scientists studying spray systems have discovered that if the droplet is much larger than the dust particle, the dust particle simply gets caught in the air stream surrounding the water and avoids contact with it totally.

If you use a spray system that isn’t designed for your dust control purposes, you are in trouble. First, the larger drops will not come in contact with the dust. Second, larger drops mean more water consumption and more runoff.

One critical factor in dust suppression is the type of material being mined or processed. While beach sand is unlikely to become suspended in the air, Cement dust is more likely to hang in the air, while coal dust is even more difficult to control.

Since the dust that causes the greatest lung damage is about 10 microns in size, conventional water spraying systems with droplets diameters of up to 600 microns will be ineffective, even though they consume much more water. The only practical solution is to use a system that can produce droplets smaller than 200 to 300 microns.

CURTAINS. Curtains are effective dust control systems. They can be used where the material is being mined and at other points in the processing cycle. In longwall mining, a cut out curtain at the face of the mine can direct the coal dust from away from the operator and reduce dust exposure by 50% to 60%.
Dust control curtains are also ideal around crushers and on conveyors, especially in areas where the material is being transferred from one conveyor to another. They can also be combined with misting systems to not only further reduce airborne dust, but dampen the material as it moves from one place to another.

IMPROVED VENTILATION. Ventilation is an ideal way to control dust underground. Adequate amounts of air can dilute the airborne dust and carry it away from operators. However, this dust control method is production dependant. If your mine increases its production, your dust control problems will increase unless you increase your ventilation system. Don’t forget to factor this into any planned production increase.

Although every mine’s dust control program is unique, MSHA’s ideas are a good place to start if you are serious about reducing your dust problem. If you want more information, go to www.msha.gov