Te - Tellurium

Properties

Tellurium is a semi-metal (metalloid); it is silvery-white and metallic looking, but is more usually available as a dark grey powder. Tellurium is the 72nd most abundant element within the Earth’s crust, making it a relatively rare element.  Tellurium is a member of Group 16 of the Periodic Table, which it shares with non metallic elements, such as oxygen and sulphur. It lies on the boundary of metals and non-metals and has properties of both. Tellurium does not behave like a typical metal, as it is very brittle and has poor electrical conduction. Tellurium burns in air or oxygen, is unaffected by water or hydrochloric acid, but dissolves in nitric acid.  It can act as a semi-conductor, is slightly photo-sensitive, and is also toxic.

History

Tellurium was discovered in Germany in 1782 by Franz Joseph Muller von Reichenstein, who at the time was an amateur scientist.  Whilst travelling around Europe, Muller had collected many mineral samples, one of which, found in a min in Transylvania, was of particular interest. At first Muller believed it to be a sample of uncombined antimony, and when proven wrong, he then announced that it was in fact bismuth sulfide. This was also proven wrong, and further tests showed it to be a compound of gold combined with an unknown element.  Muller was unable to extract this new element, and therefore sent a sample of it to the German chemist Martin Klaproth. Klaproth confirmed that Muller’s discovery was a new element, obtained a pure sample and decide to call it tellurium. The name tellurium derives from the Latin word for Earth, ‘tellus’.

Sources

Tellurium occurs in relatively few minerals, including calaverite (AuTe2), sylvanite (AgAuTe4) and tellurite (TeO2), but it is commercially produced only as a by-product of refining other metals. Over 80% of tellurium production comes from the anode slimes from copper refining.  These slimes can contain as much as 8% tellurium, rendering this source of tellurium the most efficient and cost effective process.  Tellurium can also be recovered as a by-product in a similar way from gold, lead, nickel, platinum and zinc mining.  The world production of tellurium is estimated around 220 mt per year, and the main mining regions are the USA (circa 50 mt per year), Canada (circa 20 mt per year), Peru (circa 30 mt per year), Russia (circa 20 mt per year) and Japan (circa 40 mt per year). Many producers do not report their production of tellurium.

Uses

The single biggest consumer of tellurium is in alloys with steel to improve machinability. It is also added to lead to increase hardness and acid-resistance for use in batteries, it can vulcanise rubber, tint glass and ceramics in electronic devices and is also used as an industrial catalyst in oil refining.  A relatively new consumer of tellurium is in rewritable CDs, DVDs and Blu-ray discs, where tellurium suboxide is used in the media layer of these discs.  Tellurium is also used in new phase change memory chips developed by Intel™.
Currently tellurium is receiving considerable attention for its role in photovoltaic solar cells. This process, using cadmium telluride (CdTe), employs tellurium’s photosensitive properties to give solar panels high efficiency. The production of CdTe solar panels has seen the demand for high-purity tellurium (up to 99.99999%) increase considerably over the last decade.  The future for tellurium looks promising with the recent focus on renewable energy sources, although it is thought that there is unlikely to be a supply shortage in the near future.

Tellurium is traded is various forms, as ingots, granules, pieces or powder.  Basic grade commercial tellurium is minimum 99.5% but many producers achieve higher purity than this.