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  The cobalt of electronic structure [Ar] 4s2 3d7 is the second element of the eighth secondary group, this transition metal is part of the iron group. The simple cobalt body has physical properties quite similar to those of iron and nickel. From a chemical point of view, it is less reactive than iron. Cobalt is also an element of group 9, of which the first three Co, Rh and Ir constitute the group of cobalt.

Natural cobalt minerals, in the form of paints, enamels or varnishes, have been used since ancient times, in particular around 2600 BC. AD in Egypt or in the civilizations of Mesopotamia which gave birth to Babylon. So the archaeologist finds blue decorations on pottery or terracotta dishes. Glass beads tinted with cobalt blue are common around 2200 BC. AD in present-day Iran. These techniques for coloring the glass of an intense blue or obtaining enamels are common in the Mediterranean world from the second century BC. They are generally silicates and / or aluminates of cobalt and potassium, formerly called safre or smalt recognized in Western Europe in certain luxury Romanesque stained glass windows, or later of cobalt oxides called "cobalt blue" .

Das Kobold or das Kobalt is the traditional name that the Saxon miners in search of precious metals attribute to these deceptive derivatives. These generally sulfur and / or arsenic ores too rich in cobalt give metallurgical treatment smelly gases or toxic oxide fumes based on arsenic trioxide, poisonous fumes and, if heated excessively, a flow of molten material , brittle when cold, speiss or hard and brittle metals with high sulfur or arsenic contents that metallurgists can neither process nor forge. The miners then believed the mining district bewitched by evil and falsifying dwarfs, facetious and thieves, the kobolds of bluish appearance and deadly which had stolen the precious metal, very often copper, and replaced by this improper and confused material a.

These worthless minerals or their by-products are named in vernacular German by a neutral noun Kobelet, Kobolt or Kobelt. Paracelsus mentions the cobolt in 1522 with a more Latin script. The Latin coboltum or cobaltum then established itself in learned Europe. Note that the technical term cobolt also designated in French the metallic arsenic reduced to powder b.

Until the eighteenth century and the following century, the term "cobalt" and its cobolt variant, denotes the ore or its residue which was formerly mostly unusable. However, from the seventeenth century with the Germanic fashion of Blaufarbung, that is to say, coloring in blue, cobalt ore finds greater utility because men of the chemical art and glassmaker manufacture with this waste of 'excellent blue pigments, in particular the smalt of glassmakers or ceramists, the "blue of charron pencils" ... Georg Brandt, specialist in arsenic minerals, strives to recognize the specific element underlying its ores or - mining products between 1733 and 1738.

In Venice, the master glassmakers of Murano asked the Venice dwarfs to source cobalt ore and produce their famous works tinted in Murano blue. The dwarves brought this ore from mines north of the Alps, where they were decked out with the term kobold (goblin, or goblin, supernatural creature) 8.

In 1735, the Swedish chemist Georg Brandt (1694-1768) claims to have extracted from this mineral a reddish, hard and brittle white "semi-metal", in reality samples of impure metal which he calls "cobalt" 9. In 1745, he demonstrated that cobalt was the cause of the blue color that cobalt ore imparted to glasses10. This clarification is not useless because the scholarly community most often attributed the color blue either to bismuth, or to iron and arsenic11.

But most of this work goes almost unnoticed. It was not until 1780 that Torbern Olof Bergman prepared pure metal and belatedly recognized the pioneering role of Georg Brandt in the discovery of an element, then confirmed. In 1798, chemists note that the ammoniacal solutions of cobalt chloride CoCl2 allow orange crystals stable to heat to precipitate, up to 180 ° C, a body which transforms into another darker stable body beyond this temperature.

Its main minerals are in the form of arsenides, arseniosulphides, sulfides and oxides, arsenates. Cobalt contents are generally low in these specific deposits, but they are sometimes enriched by geochemical processes or concentrated by natural erosion.

The main constituent minerals of these ores are cubic smaltite CoAs2, formerly smaltine, orthorhombic cobaltite CoAsS, formerly cobaltine or "gray cobalt", or oxidation products such as erythritis Co3 (AsO4) 2 · 8H2O, formerly "flower cobalt ". Let us also cite the orthorhombic CoAs modderite, the CoAs2 orthorhombic safflorite and the CoAs2 monoclinic clinosafflorite, the CoAs3-x cubic skutterudite, the CoAsS monoclinic alloclasite .... Smaltines generically designated arsenic cobalt ores, while cobaltins corresponded to sulfo-arsenic ores.


Skutterudite crystals, Ouarzazate, Morocco
Its extraction is mainly done from copper or nickel ores, and even lead ores. Carrolite CuS.Co2S3 is a mixed sulfide of copper and cobalt.

The most exploited or most promising minerals are in Africa, in particular in the Democratic Republic of the Congo which produces 65% of world production14, in Zambia and Morocco, in North America, for example in Canada and the United States, but also in Russia, Australia and New Caledonia, formerly known in the 19th century for its cobalt-rich manganese deposits.

The concentrated ores from the old gray cobalt, smaltine and cobaltine exploitations, or today the rare sulphide or arsenic ores, allow by air roasting to obtain cobalt oxide, in a generally hydrated form called asbolane. This is reduced either by hydrogen gas or by charcoal, made of cobalt metal.

During the 1990s, the cobalt-bearing sulphide ores were to provide a content of 0.1 to 0.03% by mass. But most of the cobalt is a byproduct of the copper mining industry. Enrichment is carried out by flotation using detergents.


cobalt obtained by electrolysis
If the extraction of sulphides goes through pyrolysis, the extraction of oxides uses the hydrometallurgy processes. The separation of copper and cobalt is carried out during the electrolysis processes. 500 tonnes of minerals yield one tonne of cobalt. Annual cobalt production in the early 1990s was around 40,000 tonnes.

Physical and chemical properties of the single metal body, preparation and alloys

Cobalt pieces in test tube
The simple body is a bright white, steel gray or silver white metal with red reflection, quite brittle, not very malleable, with a density between 8.8 and 8.9 at 25 ° C and a Mohs hardness of around 5.5 g. It is harder and more brittle than iron. But pure metal is sufficiently ductile and malleable to be able to be forged, hot and cold rolled, drawn into wire.

There are two allotropic varieties of cobalt. Coα is a metallic crystal with a hexagonal mesh, with the parameters a = 2.507 Å and c = 4.069 Å, with a density of 8.9 at 20 ° C, the most stable at room temperature, and Coβ with a cubic face-centered mesh, a = 3.544 Å, less compact and significantly less stable below 400 ° C. The phase transition between Coα and Coβ with volume increase of 0.3% takes place at 417 ° C ± 7 ° C, it is sensitive to certain specific impurities15. But this transition is slow, and there is quite commonly a mixture of the two varieties over a wide range of temperatures, which explains the slight variations in physical properties of the cobalt metal material. Coβ is a metastable form at room temperature, however it can be obtained by quenching or by certain electrolysis processes. It is likely that the delayed transition plays a role in the bursting of certain cathodic deposits of cobalt by electrolysis.

This body is ferromagnetic (capable of retaining a strong magnetization) at ordinary temperature. The Curie point is around 1,121 ° C. The permeability is around 68, or two thirds of that of iron, with a maximum of 245. The coercive field is around 707 A / m.

The electrical conductivity is 26% IACS, that is to say a little quarter that of pure copper.

It is not as fusible as iron. It melts above 1490 ° C, pure at 1495 ° C. It boils around 2,927 ° C, in practice between 2,600 ° C and 3,100 ° C
　

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