Preparation is the term used to describe the production of sized coals for different markets, with the use of separation processes to minimise the presence of mineral matter. Cleaning implies optimising the separation processes to remove the maximum amount of unwanted impurities. Raw coal is a complex and heterogenous fuel that contains a variety of impurities, many of which have a negative impact on the mine-to-end-user chain. Coals may contain from below 3% to over 40% of ash-forming constituents and from below 0.5% to over 5% of sulphur, and while in the past, the principal focus has been on the behaviour and treatment of the ash-forming mineral matter and sulphur, in the USA in particular there is increasing interest in the role of coal cleaning in the removal of other perceived contaminants, such as mercury and other trace elements. Water can be regarded as a major impurity in the sense that it reduces the heating value of a coal. Some low-rank coals can contain as much as 60% water. However, technologies for drying and water removal are only considered in this Chapter in the context of conventional coal-cleaning procedures. Impurities present in coal may increase the cost of transport; cause operating problems in end-users’ plants or harm the environment if released in an uncontrolled way. The materials are mainly inorganics that are present either as mineral matter or as chemically combined elements within the organic structure. In higher-rank coals, the inorganics are mainly present as mineral matter, but in lower-rank coals there is an increasing proportion of organically bound elements. Tiny amounts may be held in water in the pore structure. Some impurities may be picked up during the handling or treatment of coal before its use and, for example, there will be small quantities of magnetite left after dense-medium washing. Sulphur is present in varying proportions in both mineral matter and organically-bound forms.

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