History

In 1770 a lead mineral was found in the Ural Mountains and called crocoite (from the Greek krokoeis "saffron-coloured") because of its red colour. In 1797 Louis-Nicolas Vauquelin produced chromium (III) oxide from crocoite and hydrochloric acid and a year later impure chromium by reducing chromium (III) oxide with charcoal. This newly isolated element was named chromium because of the many different colours of its salts (from the Greek chroma "colour"). Chromium compounds were used in the 19th century as paint pigments and in tanning. For about 100 years now chromium has been used mainly to manufacture corrosion and heat-resistant alloys.

Occurrence

Metallic chromium is very rarely found in deposits. Chromium is almost always mined as chromite (FeCr2O4). The main deposits are located in South Africa, Kazakhstan, India, Zimbabwe, Turkey and Finland. At present South Africa mines almost 50% of marketable chromite, followed by Kazakhstan and India with about 15% each.

Approximately four million tonnes of ferrochromium were produced from a mined quantity of about 15 million tonnes of chromite ore in 2000.

Production

Chromite ore is transformed to chromate by oxidation at about 1,200 °C. The sodium chromate is extracted with hot water and transformed to dichromate with sulphuric acid. Chromium (III) oxide is then obtained by reduction with coal and thereafter chromium by an aluminothermal reduction. Ferrochromium is produced by reducing chromite with coal in an arc furnace at 2,800 °C.

Properties

Chromium is a silver-coloured, corrosion and tarnish-resistant hard metal that is ductile and malleable in its original state.

It has a density of 7.18 g/cm3, a melting point of 1,890 °C and a boiling point of 2,670 °C.

Uses

Chromium is used in the form of ferrochromium to manufacture corrosion-resistant and high-strength chromium and chromium nickel steels, pure chromium as alloying metal and for chromium plating to manufacture corrosion-resistant coatings.