chromium

Chromium (lat. Chromium) is a chemical element in the Periodic Table of Elements that has the symbol Cr and proton number 24.

History
In 1761, Johann Gottlob Lehmann discovered orange-red Lead Chromate (PbCrO4) in the Urals, which he called Red Lead Ore. He identified it as a lead-iron-selenium compound and thus chromium remained undiscovered.

In 1770, Peter Simon Pallas found a red lead mineral in the same place, which he called Krokoite (from the Greek krokos, saffron) after its red color. The use of red lead rod as a colored clay grew rapidly. A brilliant yellow pigment made from crocoite called chrome yellow became fashionable. Many remember him as the yellow mail.

In 1797, Louis Nicolas Vauquelin obtained chromium oxide (Cr2O3) from crocoite and hydrochloric acid. In 1798, he produced impure elemental chromium by reducing chromium oxide with charcoal. This newly isolated element received the name chromium (from the Greek chroma, color) because of the many colors of its salts. Trace amounts of the new element were also demonstrated by Vauquelin in gems such as ruby and emerald. In the 19th century, chromium compounds were mainly used as dyes. At the end of the 20th century, chromium and its compounds are used mainly for the production of corrosion-resistant and heat-resistant alloys.

Occurrence
Chromium has been mined almost exclusively as chromite (FeCr2O4) or rarely as chromiferide, surface or only at shallow depth. Chrome metal is obtained by reducing this ore with aluminum or silicon.

South Africa supplies roughly half of the world's chromite consumption. Countries such as Kazakhstan, India and Turkey are also worth mentioning.

In 2000, approximately 15 million tons of chromite ore were delivered to the market. It yielded 4 million ferrochrome with a market value of $2.5 billion. Metallic chromium is rarely found in deposits. One such occurrence is the Udachnaya mine in Russia, where kimberlite diatreme is mined. Diamonds and metallic chromium are found in the reduced groundmass of this kimberlite, apparently originating from a deeper part of the Earth's mantle.

Acquisition
The supplied chromite ore is separated from the tailings. In the second step, oxidative fusion takes place under heat to dichromate. Subsequently, it is reduced by carbon to chromium oxide and by aluminum to elemental chromium. Chromium cannot be obtained by reduction with carbon from oxide ores, because chromium carbide is formed. Pure chromium is produced by electrolysis of Cr3+- ions from a sulfuric acid solution. The corresponding compounds are produced by dissolving chromium oxide or ferrochromium in sulfuric acid. As a starting material, ferrochrome naturally requires the prior removal of iron.

Extremely pure Chromium is produced using additional purification steps by the van Arkel de Boer method according to van Arkel and de Boer.

Ferrochrome is produced by reducing chromite in an arc furnace at 2,800°C.

Features
Chromium is a steel-like, non-corrosive and non-oxidizing hard metal that is tough, malleable and malleable in its basic state.

The usual oxidation states of chromium are +2, +3 and +6, with +3 being the most stable. In oxidation state +6, it is a strong but also poisonous oxidant.

The use
Chromium and its compounds are used in a variety of ways:

Hard chrome plating
Galvanic application of up to 500 µm (0.5 mm) abrasion- and rust-resistant layer directly on steel, cast iron, copper, ... Even aluminum can be chrome-plated after the intermediate layer is applied (hard chrome-plated aluminum cylinders in engines).
Decorative chrome plating.
Electroplating of a less than 1 µm (0.001 mm) thick Cr-layer for decorative purposes with an anti-corrosion intermediate layer of nickel or nickel-copper. Very often, parts made of plastic are also chromed.
Alloy in stainless and heat-resistant noble steels and non-ferrous alloys.
as a catalyst
as chromite in the manufacture of molds for firing bricks
glass coloring or painting pigments.
CrIII-compounds color the glass emerald green, CrVI-compounds yellow (zinc yellow).
Restriction
The RoHS directive restricts the use of hexavalent chromium and 5 other substances (cadmium, lead, mercury, polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) in the production of electrical and electronic equipment that ends up in landfills in large quantities, thereby contributing to the protection of human health and the environment.The restriction therefore does not apply to long-life industrial, telecommunications, medical, scientific, etc. equipment that is manufactured in small quantities and that can be assumed not to end up in landfill.