Metallurgical coal, also known as coking coal, is used to produce coke, the primary source of carbon used in steelmaking. Coal is a naturally occurring sedimentary rock formed over millions of years as plants and other organic materials are buried and subjected to geological forces. Heat and pressure cause physical and chemical changes that result in carbon-rich coal. Metallurgical coal differs from thermal coal, which is used for energy and heating, by its carbon content and its caking ability. Caking refers to the coal's ability to be converted into coke, a pure form of carbon that can be used in basic oxygen furnaces. Bituminous coal-generally classified as metallurgical grade-is harder and blacker and contains more carbon and less moisture and ash than low-rank coals.
The grade of coal and its caking ability are determined by the coal's rank-a measure of volatile matter and degree of metamorphism-as well as mineral impurities and the ability of the coal to melt, swell and resolidify when heated. The three main categories of metallurgical coal are:
1. Hard coking coals (HCC)
2. Semi-soft coking coal (SSCC)
3. Pulverized coal injection (PCI) coal
Hard coking coals like anthracite have better coking properties than semi-soft coking coals, allowing them to garner a higher price. Australian HCC is regarded as the industry benchmark. While PCI coal is not often classified as coking coal, it still is used as a source of energy in the steelmaking process and can partially replace coke in some blast furnaces.