Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate(CaCO3). Limestone is used as a flux to assist the removal of impurities in the steel such as sulphur, phosphorous etc. Mostly, limestone is converted (calicined) to burnt lime first in a kiln before being used for steelmaking.
However smaller amounts of raw limestone may also be used. The flux, which may also include additions of dolomite, quartz, feldspar etc establishes an equilibrium with the impurities in the steel and so the amounts added needs to be carefully calculated to ensure the correct balance of flux viscosity and chemistry, and steel chemistry is obtained.
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