Metallurgical coal is a special type of coal used to make metallurgical coke. There are two types of metallurgical coal used to make coke: hard coking coal and semisoft coking coal. According to Grande Cache Coal, these types of coal are ideal for coke because they melt, swell and re-solidify when placed into a superheated furnace. These types of coal also have have low levels of impurities. A third type of metallurgical coal, PCI, is sometimes used in steel or iron making to replace more-expensive coke, indicates Grande Cache Coal.
Coke is created by heating metallurgical coal to around 2,000 degrees Fahrenheit. The coking process takes about 12 to 36 hours in the oven to complete, according to the World Coal Association. After being taken out of the oven, the coke is cooled with water or air to before it is either placed into storage or is put directly to the blast furnace. The finished product is about two-thirds the weight of the original raw material, according to the U.S. Energy Information Administration.
Factors Affecting Quality
High quality coke is normally made from high quality coking coals. Coke makers often use a mix of coal to improve the quality of the coke they produce. A quality coal should not generate high pressure on the coke oven wall and should shrink enough to allow easy removal from the coking oven. The properties of coke and its performance in the process are influenced by moisture content, density, weathering of coal, and several other factors, according to the American Iron and Steel Institute.
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Coke is mainly used to make iron, which, in turn, is used as the main ingredient in steel. The World Coal Association outlines the iron-making process roughly as follows. During a typical iron-making procedure, iron ore, coke and small quantities of minerals used to collect impurities are placed into a blast furnace. Superheated air is then blown into the furnace, making the coke burn. The burning produces carbon monoxide -- which reacts with the iron ore -- and heat to melt the iron. A tap at the bottom of the furnace allows molten iron, as well as impurities, to flow out of the furnace.