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...