Chromium Chemicals

In 2006, only 2% of the world's production of chromite was used for chromium chemicals. Nevertheless, these play an important part in the world's chemical industry.

Sodium chromate is the primary product in the manufacture of chromium chemicals. Approximately 825,000 tonnes of sodium dichromate were produced in 2006.

It is produced by reaction in a rotary kiln to which the chromite is fed together with soda ash (sodium carbonate) and sometimes ground lime, limestone or dolomite.

The sodium chromate is then converted into a variety of chemicals including sodium dichromate, ammonium and potassium dichromate, chromic acid, chromic oxide and basic chromium sulphate.

The earliest use of chromium chemicals was during or before the 19th century for colour and pigment applications, due to their very bright colours.

Currently, the largest use of chromium chemicals is as basic chromium sulphate in the leather tanning industry.

The second largest use of chromium chemicals is in the metal finishing industry. Main applications include decorative chromium plating, hard chromium plating for engineering requirements, and pickling of plastics.

Pigment applications are still important for chromium chemicals, sometimes mixed with other elements. The pigments are all very brightly coloured in clear yellow, orange, green, turquoise and blue. They are used in paints, plastics, ceramics and surface finishes.

Pure chromium oxide is used alone or together with alumina, zirconia and silica for high temperature and attack resistant refractories.

Other, smaller, applications include the use of chromium chemicals catalysts and oxidants in commercial chemical synthesis.

Chromium Metal

Chromium metal consists of nearly 100% chromium. Impurities include Fe, Al, Si.

Approximately 40,000 tonnes of chromium metal were produced in 2007.

There are two ways of producing chromium metal, the aluminothermic process chrome oxide and the electrolytic process using ferrochromium or chromic acid.

Chromium metal standard grades range from 99% to 99.4%. Higher grades are available as 99.6% Cr and degassed quality with 99.8% Cr and low gas.

Chrome metal is mainly used in the production of specialty alloys, nickel and cobalt -based alloys (super alloys) where low iron is required. Due to their unique high temperature and corrosion resistance properties, these high performance alloys are used in the most critical environments, such as aeronautic, oil & gas production, land based turbines, petrochemical and chemical processing.

In addition, chromium metal powder is used in the production of welding electrodes and cored wires, aluminium briquettes and master alloys.

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.