During characterization, the composition of the different-size raw coal particles is identified. The composition of the raw coal and the required clean coal specifications dictate the type of equipment that must be used to remove the mineral matter. Crushing liberates mineral matter. Complete liberation can only be approached by reducing the mined coal to very fine sizes, since particles containing both coal and mineral matter, called middlings, are also produced during crushing. Separation involves partitioning of the individual particles into their appropriate size groupings—coarse, intermediate, and fine fractions—and separating the mineral matter particles from the coal particles within each size fraction. Separation techniques for larger-size raw coal particles generally depend on the relative density difference between the organic coal and inorganic mineral matter particles. Separation techniques for fine raw coal particles utilize the difference in the surface properties of the particles in water. Disposition is the dewatering and storage of the cleaned coal and the disposal of the mineral matter.
Coal preparation began simply as a means of controlling the size of raw coal, but mechanized mining led to mechanized cleaning and the subsequent evolution of coarse, intermediate, and fine coal cleaning defined in terms of raw coal particle size ranges. All coals for the metallurgical and export markets are beneficiated, as well as coals sold for other industrial purposes. For most of its history, the primary objective of steam coal cleaning has been to reduce ash levels rather than sulfur content. The introduction of environmental requirements in the 1970s increased the interest in more extensive cleaning of coal to remove larger amounts of sulfur. Today, fine coal (less than 0.5 mm) cleaning is being further subdivided. Coal quality specifications have become more restrictive as a result of environmental regulations and as the impact of coal quality on boiler operating problems, such as slagging and deposition on tubes, has become better understood.
Coal preparation technology was first developed for the European coal industry and was licensed as needed by American companies. Mineral processing technology was also adapted for coal preparation. Significant technology development was conducted by the U.S. steel industry, since the coal used as feedstock for coke is required to meet very stringent specifications, particularly for sulfur content. The U.S. Bureau of Mines established internationally recognized in-house expertise in coal cleaning; this effort was continued under the DOE at the Pittsburgh Energy Technology Center (PETC). As a result of the tightening of coal specifications to comply with environmental regulations, EPRI (Electric Power Research Institute) established a Clean Coal Testing Facility (spun off in 1994 as an independent company, CQ, Inc.). In addition, a number of states, including Pennsylvania, West Virginia, Illinois, Ohio, and Kentucky, established research programs to improve the quality of their coals. Some research on coal