Chromium

Chromium is the 21st[35] most abundant element in Earth's crust with an average concentration of 100 ppm. Chromium compounds are found in the environment from the erosion of chromium-containing rocks, and can be redistributed by volcanic eruptions. Typical background concentrations of chromium in environmental media are: atmosphere <10 ng/m3; soil <500 mg/kg; vegetation <0.5 mg/kg; freshwater <10 μg/L; seawater <1 μg/L; sediment <80 mg/kg.[36] Chromium is mined as chromite (FeCr2O4) ore.[37]

About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, about a third in Kazakhstan,[38] while India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.[39] Although rare, deposits of native chromium exist.[40][41] The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental chromium and diamonds.[42]

The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location. In most cases, Cr(III) is the dominating species,[22] but in some areas, the ground water can contain up to 39 µg/L of total chromium, of which 30 µg/L is Cr(VI).[43]

Chromium minerals as pigments came to the attention of the west in the eighteenth century. On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead.[44][45] Though misidentified as a lead compound with selenium and iron components, the mineral was in fact crocoite with a formula of PbCrO4.[46] In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as a pigment in paints. After Pallas, the use of Siberian red lead as a paint pigment began to develop rapidly throughout the region.[47] Crocoite would be the principal source of chromium in pigments until the discovery of chromite many years later.[48]

In 1794, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid.[46] In 1797, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven, for which he is credited as the one who truly discovered the element.[49][50] Vauquelin was also able to detect traces of chromium in precious gemstones, such as ruby and emerald.[46][51]

During the nineteenth century, chromium was primarily used not only as a component of paints, but in tanning salts as well. For quite some time, the crocoite found in Russia was the main source for such tanning materials. In 1827, a larger chromite deposit was discovered near Baltimore, United States, which quickly met the demand for tanning salts much more adequately than the crocoite that had been used previously.[52] This made the United States the largest producer of chromium products until the year 1848, when larger deposits of chromite were uncovered near the city of Bursa, Turkey.[37] With the development of metallurgy and chemical industries in the Western world, the need for chromium increased.[53]

Chromium is also famous for its reflective, metallic luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.[54]

Approximately 28.8 million metric tons (Mt) of marketable chromite ore was produced in 2013, and converted into 7.5 Mt of ferrochromium.[39] According to John F. Papp, writing for the USGS, "Ferrochromium is the leading end use of chromite ore, [and] stainless steel is the leading end use of ferrochromium."[39]

The largest producers of chromium ore in 2013 have been South Africa (48%), Kazakhstan (13%), Turkey (11%), and India (10%), with several other countries producing the rest of about 18% of the world production.[39]

The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction.[55]

For the production of pure chromium, the iron must be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.[55]

The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.[55]

The creation of metal alloys account for 85% of the available chromium's usage. The remainder of chromium is used in the chemical, refractory, and foundry industries.[57]

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