Coltan (short for columbite–tantalites and known industrially as tantalite) is a dull black metallic ore from which are extracted the elements niobium and tantalum. The niobium-dominant mineral in coltan is columbite (after niobium's original American name columbium), and the tantalum-dominant mineral is tantalite.
Tantalum from coltan is used to manufacture tantalum capacitors which are used for portable telephones, personal computers, automotive electronics, and cameras.Coltan mining has financed serious conflict in the Democratic Republic of Congo, including the Ituri conflict and the Second Congo War.
Coltan was discovered in 1802, one year after niobium. In 1801, Charles Hatchett (1765-1847), an English chemist, analysed a specimen of an unknown mineral from the collection of the British Museum in London. This was a heavy black mineral obtained from the collection of the first governor of Connecticut, John Winthrop, who was an alchemist, manufacturing chemist, physician, and rock collector. The mineral, later called columbite, was found near New London, Connecticut. Charles Hatchett determined that this mineral contained a 'new earth', which implied a new element had been found. He named this element columbium apparently in reference to its source being America.
The names columbium and niobium were both used to identify this element for almost another century, with columbium being preferred in the Americas. It was not until 1949 that the International Union of Pure and Applied Chemistry (IUPAC) officially adopted niobium as the name for this element. Old habits die slowly, and some metallurgists continue to use the term columbium to this day.
Typical market specifications
Coltan on the international market generally contains a minimum of 30% Ta2O5, while lower grade material with a minimum of 20% Ta2O5 may also be of interest. The commercial payable value is based on the Ta2O5 content alone, any Nb2O5 is generally ignored.
Should material contain mainly Nb2O5 and only little Ta2O5, then it would be sold as columbite and should contain a minimum of 50% Nb2O5. The payable value is based on the Nb2O5+Ta2O5 content payable entirely as Nb2O5; the Ta2O5 content is not paid a higher rate.
Coltan and niobium raw materials often contain somewhat elevated levels of naturally occurring thorium and uranium, usually high enough for them to be classified as radioactive for handling and transport. Such raw materials are then also known as Naturally Occurring Radioactive Materials (NORM).
It is incumbent on any producer or trader to also analyse for Th and U in order to demonstrate whether the material is radioactive or not. This is reflected in the T.I.C.'s Transport Policy. As a guide, these elements are of concern for the purpose of mining and handling at levels above 1 Bq/g, a level obtained from e.g. 0.013% ThO2 plus 0.0048% U3O8, whereas for transport alone the levels of concern are an order of magnitude higher, i.e.10 Bq/g or 0.13% ThO2 plus 0.048% U3O8.
Production of raw materials
Extraction or production of Coltan raw materials of all kinds is widespread, whether by primary industrial mining, artisanal mining, as a secondary mineral or as a byproduct. Brazil is currently the major producer. Significant quantities are also produced in China, the DR Congo, Russia and Rwanda. Additional quantities are produced, some intermittently or at a low level, in Australia, Burundi, France, Malaysia, Mozambique, Namibia, Nigeria, Thailand and Zimbabwe.
Processing: extraction and refining
The extraction and refining of Coltan, including the separation from niobium in these various tantalum-containing mineral concentrates, is generally accomplished by treating the ores with a mixture of hydrofluoric and sulfuric acids at elevated temperatures. This causes the Coltan and niobium values to dissolve as complex fluorides, and numerous impurities that were present also dissolve. Other elements such as silicon, iron, manganese, titanium, zirconium, uranium, thorium, rare earths, et c. are generally present. The filtration of the digestion slurry, and further processing via solvent extraction using methyl isobutyl ketone (MIBK) or liquid ion exchange using an amine extractant in kerosene, produces highly purified solutions of Coltan and niobium. Generally, the tantalum values in solution are converted into potassium tantalum fluoride (K2TaF7) or tantalum oxide (Ta2O5). The niobium is recovered as niobium oxide (Nb2O5) via neutralization of the niobium fluoride complex with ammonia to form the hydroxide, followed by calcination to the oxide.
There exist alternative methods which are used when they are better suited to particular local conditions. One used for a titanium-niobium-tantalum-rare earth mineral concentrate involves blending the crushed concentrate with coke and passing this through a chlorination stage which separates out the rare earths and other elements including most of the thorium. The resulting titanium-niobium-tantalum oxichloride gas is dropped in temperature which causes the iron, thorium and alkali metals to precipitate out. The cleaned titanium-niobium-tantalum oxichloride gas is then cooled to a liquid and distilled to separate out low-boiling titanium chloride gas, whereafter the niobium-tantalum oxichloride gas is further chlorinated to produce NbCl5 and TaCl5. These chlorides are fractionally distilled and the niobium chloride subsequently reacted with steam to produce the hydroxide which is calcined to oxide. The tantalum chloride is reacted with ammonium hydroxide to produce the oxide.
The primary tantalum chemicals of industrial significance, in addition to K2TaF7 and Ta2O5, are tantalum chloride (TaCl5), lithium tantalate (LiTaO3) and tantalum carbide (TaC).
Tantalum metal powder, including the precursor to capacitor grade powder, is generally produced by the sodium reduction of the potassium tantalum fluoride in a molten salt system at high temperature. The metal can also be produced by the carbon or aluminium reduction of the oxide or the hydrogen or alkaline earth reduction of tantalum chloride. The choice of process is based on the specific application and whether the resultant tantalum will be further consolidated by processing into ingot, sheet, rod, tubing, wire and other fabricated articles.
The consolidation of metal powder for ingot and processing into various metallurgical products begins with either vacuum arc melting or electron beam melting of metal feedstocks, comprised of powder or high purity scrap where the elements with boiling points greater than tantalum are not present. Double and triple melt ingots achieve a very high level of purification with regard to metallics and interstitials. Ingots are used to produce the various metallurgical products named earlier. Ingot stock is also used for the production of such alloys as tantalum-10% tungsten. Ingot and pure tantalum scrap are used in the production of alloys for land and air-based turbines.
Use and demand
Coltan is used primarily for the production of tantalum capacitors, used in many electronic devices. Many sources mention coltan's importance in the production of mobile phones, but tantalum capacitors are used in almost every kind of electronic device. Niobium and tantalum have a wide range of uses, including refractive lenses for glasses, cameras, phones and printers. They are also used in semiconductor circuits, and capacitors for small electronic devices such as hearing aids, pacemakers, and mp3 players, as well as in computer hard drives, automobile electronics, and surface acoustic wave (SAW) filters for mobile phones.
Coltan is also used to make high-temperature alloys for jet engines and air- and land-based turbines. More recently, the nickel-tantalum superalloys used in jet engines account for 15% of tantalum consumption, but pending orders for the Airbus and the 787 Dreamliner may increase this proportion, as well as China's pending order for 62 787-8 airplanes.