General Information

Cobalt(Co), chemical element, ferromagnetic metal of Group VIII of the periodic table, used especially for heat-resistant and magnetic alloys.

The metal was isolated (c. 1735) by a Swedish chemist, Georg Brandt, though cobalt compounds had been used for centuries to impart a blue colour to glazes and ceramics. Cobalt has been detected in Egyptian statuettes and Persian necklace beads of the 3rd millennium BC, in glass found in the Pompeii ruins, in China as early as the T'ang dynasty (ad 618-907) and later in the blue porcelain of the Ming dynasty. The name kobold was first applied (16th century) to ores thought to contain copper but eventually found to be poisonous, arsenic-bearing cobalt ores. Brandt finally determined (1742) that the blue colour of these ores was due to the presence of cobalt.

Cobalt is a hard, bluish-white metal and, along with iron and nickel, is one of the three metals that are ferromagnetic at room temperature. It dissolves slowly in dilute mineral acids, does not combine directly with either hydrogen or nitrogen, but will combine, on heating, with carbon, phosphorus, or sulfur. Cobalt is also attacked by oxygen and by water vapour at elevated temperatures, with the result that cobalt(II) oxide, CoO, is produced.

Occurrence, uses, and properties.

Cobalt, though widely dispersed, makes up only 0.001 percent of the Earth's crust. It is found in small quantities in terrestrial and meteoritic native nickel-iron, in the Sun and stellar atmospheres, and combined with other elements in natural waters, in nodules beneath the oceans, in soils, in plants and animals, and in such minerals as cobaltite, linnaeite, skutterudite, smaltite, heterogenite, and erythrite. Traces of cobalt are present in many ores of iron, nickel, copper, silver, manganese, zinc, and arsenic, from which it is often recovered as a by-product.

For information on the mining, refining, and recovery of cobalt, see Industries, Extraction and Processing: Cobalt.

Cobalt is a trace element essential in the nutrition of ruminants (CATtle, sheep) and in the maturation of human red blood cells in the form of vitamin B12, the only vitamin known to contain such a heavy element.

Polished cobalt is silver-white with a faint bluish tinge. Two allotropes are known: the close-packed-hexagonal structure stable below 417 C (783 F) and the face-centred-cubic, stable at high temperatures. It is ferromagnetic up to 1,121 C (2,050 F, the highest known Curie point of any metal or alloy) and may find appliCATion where magnetic properties are needed at elevated temperatures. Natural cobalt is all stable isotope cobalt-59, from which the longest lived artificial radioactive isotope cobalt-60 (5.3-year half-life) is produced by neutron irradiation in a nuclear reactor. Gamma radiation from cobalt-60 has been used in place of X rays or alpha rays from radium in the inspection of industrial materials to reveal internal structure, flaws, or foreign objects; in cancer therapy; in sterilization studies; and in biology and industry as a radioactive tracer. It is in turn being replaced in both industrial and medical radiology by cesium-137 because of the long (30-year) half-life of the latter.

Most of the cobalt produced is used for special alloys. A relatively large percentage of the world's production goes into magnetic alloys such as the Alnicos for permanent magnets. Sizable quantities are utilized for alloys that retain their properties at high temperatures and superalloys that are used near their melting points (where steels would become too soft). Cobalt is also employed for hard-facing alloys, tool steels, low-expansion alloys (for glass-to-metal seals), and constant-modulus (elastic) alloys (for precision hairsprings). Cobalt is the most satisfactory matrix for cemented carbides.

Finely divided cobalt ignites spontaneously. Larger pieces are relatively inert in air, but above 300 C (570 F) extensive oxidation occurs.

The abundance of cobalt in the Earth's crust is relatively low, and the element is widely distributed; even the best cobalt ores contain only low concentrations of the element and require complex processing to concentrate and extract it. In general, little cobalt ore is mined for the cobalt content: the metal is recovered as a by-product from the ores of other metals, particularly copper and nickel.

Chemical compounds

History