ORIGIN OF SELENIUM & TELLURIUM

Se and Te are rare elements almost never found in the native state. Tellurium was discovered in 1782 by an Austrian chemist, Von Reichenstein, while examining gold ore from Transylvania. Selenium was discovered in 1817 by a Swedish chemistry professor, Berzelius. They are widely dispersed in the earth's crust at average levels less abundant than silver. They rank 69th and 75th, respectively, in order of abundance.

There are no ores from which Se can be mined as a primary product. It is found principally in sulfide minerals of copper iron, and lead and is most common in chalcopyrite, bornite and pyrite. It occurs most abundantly in the North American porphyry copper deposits of the western U.S., and in the copper-nickel and copper-zinc ores of central and eastern Canada. Se values also occur in copper ores in Africa, Asia, Europe, South America, Oceania and Australia. Occasionally it is found in conjunction with native sulfur and in the form of selenites of other metals.

Te rarely occurs in the native state, usually being associated with gold, silver, copper, lead, mercury and bismuth ores. One of the most common tellurium minerals is sylvanite, a complex gold-silver telluride.

Extraction of Se and Te from ore is a complex process involving numerous hydro- and pyro-metallurgical treatments. Flotation concentrates are smelted to remove sulfur, iron and other impurities. Treatment of 200 tons of copper ore typically yields one pound of Se. Treatment of 500 tons of copper ore typically yields one pound of Te.

Se and Te are undesirable impurities in copper. Separation takes place in the electrolytic copper refinery where impure Cu anodes are dissolved by electrolysis and redeposited as high purity copper cathodes. The impurities dissolve in the electrolyte or settle as slimes to the bottom of the cells. Present in the slimes as selenides and tellurides of copper, gold, and silver, Se and Te are separated by various processes, and are further refined into commercial product forms and compounds.

Both Se and Te are economically recoverable from industrial scrap and chemical process residues. For example, worn out and damaged photoreceptor drums are returned to manufacturers for recycling, where the Se-Te alloy is stripped off and shipped to refineries for separation and purification. An estimated 15% of refined selenium production comes from secondary sources.