Fluxes
Iron ore, coal and air are the main raw materials for iron and steelmaking but many other substances are used. Apart from energy sources, such as natural gas and electrical power, the majority of these supplementary materials can be broadly classified as fluxes, alloys and refractories.
Fluxes are the materials used in metallurgical processing to react with impurities to form a liquid slag at the process temperature. This allows the metal to be easily separated from the impurities. The impurities are mainly oxides, either as they occur in the feed (gangue and ash) or generated in refining, so the fluxes are also generally oxides.
Composition
Fluxes have 'acidic' or 'basic' properties. Silicon dioxide, SiO2, forms an acid slag, while calcium oxide, CaO, forms a basic slag. This is analogous to acids and bases in aqueous chemistry, such that acid and basic oxides tend to react together. Physically, this means that high melting point oxides can form lower melting point mixtures. Chemically, if there is a surplus of one type of oxide, then the mixture will more strongly hold (dissolve) an oxide of the other type in the slag. Therefore a very basic slag (high in CaO, low in SiO2) is more efficient for removing phosphorus during refining because phosphorus pentoxide P2O5 is an acid oxide. On the other hand, a basic slag will not dissolve basic oxides so readily, which enables high melting point oxides to be used as container materials for slags providing they are the same type as the slag.
Thus "basic" oxygen steelmaking employs basic slags (high in CaO and often saturated in magnesium oxide MgO) in a vessel lined with high melting point basic oxide refactory bricks (mainly MgO), referred to as magnesia bricks.
Limestone
Limestone, the most used flux, is a sedimentary rock, usually white, varying from hard and compact to soft and friable.
Limestone for use in a Bas furnace must be burnt (calcined) in kilns before charging. A fine grained lump limestone is needed to produce a sufficiently hard lime.
Dolomite
Dolomite is becoming increasingly important as a flux material. It is a combined calcium magnesium carbonate, geologically similar to limestone, reacting chemically in the same way. Lump material is calcined along with limestone as a BOS flux, and fines may be added to the sinter plant to modify the blast furnace slag composition.
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