Refractories

Chromite is an important refractory material, although production for this purpose is only approximately 1% of world production of chromite.

Refractory grade chromite requires a very low silica content, typically 0.7% SiO2. The chromite is generally produced as a fine-grained concentrate from which most of the silica, which occurs in the gangue, has been removed. The refractoriness of the chromite is also determined by the amount of combined Cr2O3 and Al2O3, which should exceed 57%.

Refractory chromite consumption has decreased over the last 35 years due to changes in steelmaking technology. However, it still has an important niche in the refractories industry.

Chromite has long been used in basic refractories in conjunction with magnesia, the 'chrome-mag' or 'mag-chrome' refractories. Mag-chrome refractories are preferred in sectors of non-ferrous metallurgy, such as copper, lead and zinc refining, because of their excellent corrosion resistance. However, there has been a downward trend in their use in this sector, partly due to the increasing use of hydrometallurgical processes.

In iron and steelmaking, the move from open-hearth furnaces to basic oxygen and electric furnaces has led to a sharp decline in mag-chrome brick usage. The bricks are, however, still used in steelmaking ladles, in argon-oxygen decarburisers (AODs) and in tap-hole plugging.

Cement and lime kilns are the second largest user of these refractories but only consume about 7% of world production. The use of mag-chrome bricks has virtually disappeared in cement kilns in Europe and North America due to the regulations and costs of disposal of the used bricks which may contain hexavalent chromium as a result of the oxidising atmosphere in the kilns. In the rest of the world, the use of mag-chrome bricks is still widespread. Advantages of these bricks in cement kilns include excellent thermal shock resistance, good corrosion resistance and high hot strength.

Mag-chrome bricks are used in high temperature furnaces in the glass industry but are also being phased out due to concerns regarding hexavalent chromium.

Refractory chromite in its granular forms, or chromite sand, is widely used in the foundry industry for both ferrous and non-ferrous castings, particularly in the automotive and heavy engineering sectors. The chromite sands have high refractoriness with a melting temperature of about 2150ºC, low wettability and good chemical stability, low thermal expansion, bloating by oxidation of FeO to Fe2O3 and Fe3O4 which helps to seal the mould, and high density and thermal conductivity which promotes rapid solidification of the castings.

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