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VI COAL PPEPARATION TECHNIQUES Practically all coal produced in the United States is crushed and sized before being shipped, and in 1975, 41.2 percent was processed through a preparation or teneficiation or a washing plant.1 Coal beneficiation refers to the removal of mineral matter from the coal whether it be rock of shale partings, roof and floor contamination, or pyrite. In recent years the reduction of sulfur content, especially in steam coals, has become a major objective (see Appendix B). In the West, the coal generally is crushed and sized only (except for metallurgical coal). Single- and double- roll crushers are used for the initial reduction from run- of-mine to a nominal 6- to 8-inch size. A secondary crusher, often of the hammer-mill type, reduces it to less than 2 inches for shipping. In the central and eastern coal regions, all coal is similarly crushed and sized before shipping or processing. In some cases, single-stage crushing or rotary breakers are used for sizing. Vibrating screens, pan, apron, and other types of feeders are necessary in the sizing and material-handling process. Automatic sampling and belt weighing and rail weigh-in- motion scales are common. All metallurgical coal traditionally has been washed to improve its quality to the maximum economic extent possible. Coal preparation may be an important factor in coal gasification since particle size and distribution as well as moisture, ash, sulfur, and heat content may be important. The underground coal now being mined generaly is finer (because of increased mechanization) and wetter (because of the greater quantity of water being used in dust control) than that mined in the past and therefore plant design and processing equipment must be given special attention. Coal beneficiation plants use a variety of equipment to separate the lighter coal from the heavier rock and clayey materials and heavy media cyclones, jigs, tromp vessels, hydrocyclones, tables, and froth flotation are common. After the coal and waste material are separated, various cleaning, water-removal, and clarifying processes (e.g.. 47
screening [static and vibrating], centrifuging, filtering, thickening, settling, and drying) are used. Metallurgical coal frequently is dried after washing to reduce the moisture content and freight costs and to prevent its freezing in the freight cars, but steam coal usually is left as it is. If finer sizes are recovered, however, drying may be necessary. When drying western subbiturainous coal and lignite, inherent rather than extraneous moisture is removed. However, to date, spontaneous combustion, dust, and overall economic considerations have discouraged the application of commercial drying installations. A number of commercial driers are available for removing extraneous and inherent moisture. They range from fixed-bed moving grates to rotating louvers and fluidized-bed dryers, and each has special applications. The disposal of wastes from wash plants has received much attention in recent years. If waste disposal dams are used, they must be constructed in accordance with strict requirements to meet both environmental and safety standards. More and more waste is being disposed of in solid (as opposed to slurry) form. Coarse wastes are centrifuged and trucked to disposal areas. Fine wastes either are pumped in slurry form or are filtered and conveyed or trucked to disposal areas. Excess water is decanted and pumped back to the processing plant. New and better processes for dewatering coal and for clarifying process and waste waters are being investigated, and new developments are continuously being put into practice, while water control systems for these processes have high capital and operating costs, they also maximize the recirculation of plant processing water and minimize or completely eliminate the discharge of waste waters into the natural streams. Sulfur control is now and will continue to be a prime factor in the development of new coal properties, and western coal with its low sulfur content will play a big part in new steam coal markets. Even given its lower heat content (about 25 percent less than the average eastern coal), there are large reserves of western coal that, without preparation, will meet the present emission standard of 1.2 pounds of SO2 per million Btu in new plants. Low- sulfur eastern coals generally have been reserved for metallurgical purposes. It generally is possible to reduce the pyrite sulfur content in high-sulfur coals by washing. The two-stage flotation process of the 0.S. Bureau of Mines is reasonably successful in maximizing pyritic sulfur reduction of some coals and is being tested on a commercial basis; it deserves a continuous and expanded research effort.
REFERENCE 1. U.S. Bureau of Mines, Meekly Coal Report, April 2i* 1977, Mineral Industries Survey (Washington, D.C.: U.S. Bureau of Mines, 1977). 49