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Methods employed for the removal of impurities from coal depend on the physical differences between the impurity and the coal; one such example is the difference in specific gravity.

Coal preparation plants generally use gravity process equipment to separate the refuse from the coal. Most of the extraneous impurities mined with coal are much heavier than the coal itself – coal has a specific gravity between 1.35 and 1.5, while the refuse rock has a specific gravity on the order of 2.1 to 2.3 – and separation can be effected by immersing the run-of-mine coal in a fluid having a specific gravity greater than that of the coal but less than that of the impurity (heavy media process). This allows the coal to float and the heavy waste material to sink and the two products are collected separately (Figure 3.2) (Couch, 1991).

Figure 3.2 A dense-medium separation method (Speight, 2013).