Coal that comes from a mine is a complex mixture of materials with a large variety of physical properties. In addition to the coal itself, pieces of rock, sand, and various minerals are contained in the mixture. Thus, before coal can be sold to consumers, it must be cleaned. The cleaning process consists of a number of steps that results in a product that is specifically suited to the needs of particular consumers. Among the earliest of these steps is crushing and sizing, two processes that reduce the coal to a form required by the consumer.
The next step in coal preparation is a washing or cleaning step. This step is necessary not only to meet consumer requirements, but also to ensure that its combustion will conform to environmental standards.
Coal washing is accomplished by one of two major processes, by density separation or by froth flotation. Both processes depend on the fact that the particles of which a coal sample are made have different densities. When water, for example, is added to the sample, particles sink to various depths depending on their densities. The various components of the sample can thus be separated from each other.
In some cases, a liquid other than water may be used to achieve this separation. In a heavy medium bath, for example, a mineral such as magnetite or feldspar in finely divided form may be mixed with water, forming a liquid medium whose density is significantly greater than that of pure water.
A number of devices and systems have been developed for extracting the various components of coal once they have been separated with a water or heavy medium treatment. One of the oldest of these devices is the jig. In a jig, the column of water is maintained in a constant up-and-down movement by means of a flow of air. Clean coal particles are carried to the top of the jig by this motion, while heavier refuse particles sink to the bottom.
Another method of extraction, the cyclone, consists of a tank in which the working fluid (water or a heavy medium) is kept in a constant circular motion. The tank is constructed so that lighter clean coal particles are thrown out of one side, while heavier refuse particles are ejected through the bottom.
Shaking tables are another extraction method. As the table shakes back and forth, particles are separated by size, producing clean coal at one end and waste products at the other.
In cylindrical separators, a coal mixture is fed into a spinning column of air that throws the heavier waste particles outward. They coat the inner wall of the cylinder and fall to the bottom, where they are drawn off. The clean coal particles remain in the center of the air column and are drawn off at the top of the cylinder.
Froth flotation processes depend on the production of tiny air bubbles to which coal particles adhere. The amount of absorption onto a bubble depends not only on a particle's density, but also on certain surface characteristics. Separation of clean coal from waste materials can be achieved in froth flotation by varying factors, such as pH of the solution, time of treatment, particle size and shape, rate of aeration , solution density, and bubble size.
[David E. Newton ]
Ward, C. R. Coal Geology and Coal Technology. Melbourne, Australia: Blackwell Scientific Publications, 1984.