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A coal preparation plant takes coal from a mine and cleans it for transport. When coal comes out of a mine, it is full of impurities. These impurities lower its overall value, reduce its usefulness and increase the cost to ship it. Since impure coal is so detrimental to its use, most coal mining companies own or use a coal preparation plant. In most cases, these plants remove the impurities and process the coal to a standard size and grade for shipment.
Coal that comes directly from a mine is called run-of-mine (ROM) coal. This is a very impure substance. Most ROM coal has other minerals mined along with the coal such as worthless rock, chemicals and explosive residue. Process contaminants consist of broken pieces of equipment, fabric, paper or any other material that entered the load via a miner or processor. All of these non-coal items increase the weight of the coal, which ups its transport costs and, thereby, lowers its market value.
The first step in most coal preparation processes is crushing. The coal, along with all the various impurities, is crushed to a powder. The crushed coal is extremely light and will naturally try to separate from the crushed material. When the crushed ROM is placed in water, much of the heavier material will sink to the bottom, but the light coal will float on the surface. The water also leaches chemicals from the coal powder.
The next step is removing the coal from the water. After the two are separated, the coal is dried and formed into a transportable solid. At this point, the coal is in a usable form and is ready to ship to market or move to a firing station for use.
The water containing the impurities and the water removed from the dried coal is combined into coal slurry. This mixture is often highly toxic and dangerous. Most of the time, a coal preparation plant will have a method of storing or purifying this water, often through evaporation. The dangerous material is a common point of discussion when talking about the dangers of coal mining processes.
Most of the time, a coal preparation plant will keep a stockpile of coal ready for processing. This is more of a technological concern than a financial one. The machinery used to purify and separate coal benefits from a slow and constant flow of material. By keeping a stockpile, the coal preparation plant is able to keep production at the exact level it wants, even when there are large breaks between deliveries.