The physical and chemical properties of coal are highly variable . Industries that use coal specify a range of properties that are required for their intended process. Coal suppliers try to find coals that most closely match those requirements. Coal is treated in processes called beneficiation to prepare a material that meets the customer's needs and is as homogenous as possible. Samples of coal from both cores and mines are taken to determine the treatment that must be performed. Preparation plants that perform specific beneficiation processes are constructed as near as possible to the location where the coal is mined.
Three kinds of processes may be performed at the plants: (1) sizing —controlled by a crushing and screening process, (2) increasing heating value —by removing noncombustible ash and rock by gravity separation, and (3) removing or controlling undesirable mineral and chemical components (sulfur, sodium, and trace elements) —by a combination of gravity separation and blending. Traditionally, most coal preparation was primarily concerned with sulfur and ash reduction. Today, however, much more sophisticated processes have more narrow and complex physical and chemical requirements for coal stock.