COBALT
Overview
Humans have been using compounds of cobalt since at least 1400 B.C. The compounds were used to color glass and glazes blue. In 1735, Swedish chemist Georg Brandt (1694-1768) analyzed a dark blue pigment found in copper ore. Brandt demonstrated that the pigment contained a new element, later named cobalt.
Cobalt is a transition metal, one of several elements found in Rows 4 through 7 between Groups 2 and 13 in the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Cobalt is located between iron and nickel and shares many chemical and physical properties with these two elements.
SYMBOL
Co
ATOMIC NUMBER
27
ATOMIC MASS
58.9332
FAMILY
Group 9 (VIIIB)
Transition metal
PRONUNCIATION
CO-balt
The United States has to import all the cobalt it uses. The most important application is in the production of superalloys. Superalloys primarily consist of iron, cobalt, or nickel, with small amounts of other metals, such as chromium, tungsten, aluminum, and titanium. Superalloys are resistant to corrosion (rusting) and retain their properties at high temperatures. Superalloys are used in jet engine parts and gas turbines.
Discovery and naming
Cobalt dyes have been used for centuries. Craftsmen used materials from the earth to color glass, pottery, glazes, and other materials. Cobalt minerals were especially prized for their rich blue color.
The word cobalt may have been first used near the end of the fifteenth century. In German, the word Kobold means "goblin" or "evil spirit." The term was used by miners to describe a mineral that was very difficult to mine and was damaging to their health. When the mineral was heated, it gave off an offensive gas that caused illness. The gas that affected the miners was arsenic trioxide (As 4 O 6 ), which often occurs with cobalt in nature.
At first, chemists were skeptical about Brandt's claims of a new element, but he continued his research on the mineral. He showed that its compounds were a much deeper blue than copper compounds. (Copper and cobalt compounds had long been confused with each other.) Eventually, Brandt was given credit for the discovery of the element. The name chosen was a version of the original German term, Kobold.
Physical properties
Cobalt is a hard, gray metal that looks much like iron and nickel. It is ductile, but only moderately malleable. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets.
Cobalt is one of only three naturally occurring magnetic metals. The other two are iron and nickel. The magnetic properties of cobalt are even more obvious in alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals.
The melting point of cobalt metal is 1,493°C (2,719°F), and the boiling point is about 3,100°C (5,600°F). The density is 8.9 grams per cubic centimeter.
Chemical properties
Cobalt is a moderately reactive element. It combines slowly with oxygen in the air, but does not catch fire and burn unless it is in a powder form. It reacts with most acids to produce hydrogen gas. It does not react with water at room temperatures.
Occurrence in nature
Cobalt is a relatively abundant element at about 10 to 30 parts per million. This places it in the upper third of elements according to their abundance in the Earth's crust.
The most common ores of cobalt are cobaltite, smaltite, chloranthite, and linnaeite. The major suppliers of cobalt in the world are Zambia, Canada, Russia, Australia, Zaire, and Cuba. No cobalt is mined in the United States.
The word cobalt, from the German word Kobold , meaning "goblin" or "evil spirit," was used by miners to describe a mineral that was very difficult to mine and was damaging to their health.
Isotopes
There is only one naturally occurring isotope of cobalt, cobalt-59. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of
Ten radioactive isotopes of cobalt are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
Cobalt-60 is one of the most widely used of all radioactive isotopes. In medicine, it is used to find and treat diseases. For example, it is used in a test known as the Schilling test. This test is a method for determining whether a person's body is making and using vitaminB 12 properly. Two other isotopes of cobalt, cobalt-57 and cobalt-58, are used for the same purpose.
Cobalt-60 is also used to treat cancer. The radiation given off by the isotope kills cancer cells. The isotope has been used for nearly 50 years to treat various forms of cancer.
A growing use of cobalt-60 is in food irradiation. Food irradiation is a method for preserving food. The food is exposed to radiation from cobalt-60. That radiation kills bacteria and other organisms that cause disease and spoilage. The food can be stored longer without going bad after being irradiated.
There is some controversy about the use of irradiation as a way of preserving food. Some people worry that harmful compounds will be produced during irradiation. So far, no proof has been found that irradiation is a dangerous method of food preservation.
Cobalt-60 is also used in industrial applications. The radiation it gives off acts like X rays from an X-ray machine. It can penetrate metals. The X-ray pattern produced by radiating a material tells about its strength, composition, and other properties.
Extraction
Cobalt is obtained by heating its ores to produce cobalt oxide (Co
2
O
3
). That compound is then heated with aluminum to free the pure metal:
Cobalt oxide can be converted to cobalt chloride (CoCl
3
). An electric current is then passed through molten (melted) cobalt
chloride to obtain the free element:
Uses
About 65 percent of cobalt used in the United States is used to make alloys, mostly superalloys. Superalloys are used in situations where metals are placed under extreme stress, often at high temperatures. A gas turbine, a device for making electricity, is a good example. It looks a bit like a large airplane propeller, with many blades. A hot, high speed gas pushes against the turbine blades, making them spin very fast. The motion generates electricity. Cobalt superalloys hold up to the high temperature stress produced in the machine.
Cobalt is also used in making magnetic alloys. These alloys are used to make devices that must hold a magnetic field, such as electric motors and generators. A magnetic field is the space around an electric current or a magnet in which a magnetic force can be observed. Another application of cobalt alloys is in the production of cemented carbides. In metallurgy, cementation is the process by which one metal is covered with a fine coating of a second metal. Cementation is used to make very hard, strong alloys, such as those used in drilling tools and dies.
Compounds
Cobalt compounds are widely used to make coloring materials. The following compounds are used to color glass, glazes, cosmetics, paints, rubber, inks, and pottery: cobalt oxide, or cobalt black (Co 2 O 3 ); cobalt potassium nitrite, or cobalt yellow (CoK 3 (NO 2 ) 6 )); cobalt aluminate, or cobalt blue (Co(AlO 2 ) 2 ); and cobalt ammonium phosphate, or cobalt violet (CoNH 4 PO 4 ).
Another important use of cobalt compounds is as catalysts. A catalyst is a substance used to speed up or slow down a chemical reaction. The catalyst does not undergo any change itself during the reaction. Cobalt molybdate (CoMoO 4 ) is used in the petroleum industry to convert crude oil to gasoline and other petroleum products. It is also used to remove sulfur from crude oil.
Cobalt compounds account for about 25 percent of cobalt used in the United States.
Health effects
Cobalt is a trace mineral in the human body. A trace mineral is an element needed by plants and animals in minute amounts. The absence of a trace mineral in the diet often leads to health problems. In animals, cobalt is used to make certain essential enzymes. An enzyme is a catalyst in a living organism. It speeds up the rate at which certain changes take place in the body. Enzymes are essential in order for living cells to function properly. Cobalt is needed for the production of vitamin B 12 . Vitamin B 12 is necessary to ensure that an adequate number of red blood cells is produced in the body.
A lack of cobalt in the soil can cause health problems, too. For example, sheep in Australia are subject to a disease known as Coast disease, due to a deficiency of cobalt in the soil.
An excess of cobalt can also cause health problems. For example, people who work with the metal may inhale its dust or get the dust on their skin. Cobalt dust can cause vomiting, diarrhea, or breathing problems. On the skin, it can cause rashes and irritation.