Metallurgy of selenium and tellurium

Selenium and tellurium are relatively rare elements, belonging to the same family as oxygen, sulfur and polonium. They are semiconductors, with electrical properties somewhere between those of a metal and an insulator. In their elemental forms, they are often regarded as metals, although they are metalloids.

Selenium's chemical properties are intermediate between those of sulfur and tellurium. However, tellurium, although closely related to selenium, is more basic, more metallic and more strongly amphoteric. Selenium and tellurium give rise to inorganic and organic compounds similar to sulfur compounds, but with different properties and behavior.

Large Se and Te resources exist in base metal sulfide deposits mined for Cu, Ni, Co and Pb. However, their recovery is only incidental. Commercial quantities are recovered industrially from anodic copper electrorefining slimes and cupronickel matte etching solutions.

Selenium's main use is in electronic components and photocopiers, but it is also widely used in glassware, pigments, rubber, metal alloys, the oil and textile industries, medicine and photography.

Tellurium is used in thermoelectrics. It facilitates the machining of steels and is also used in the form of CdTe in solar cells.

Se and Te are extracted from anodic copper sludge using pyro and hydrometallurgical processes: alkaline or sulfate roasting, volatilization, dry chlorination, acid or alkaline leaching, etc. In most plants, before recovering Se and Te, the anodic sludge is pickled. In most plants, before recovering Se and Te, the anode sludge is dewatered. After extraction of selenium and tellurium, the residues are treated to recover precious metals.

Hydrometallurgy is the technology used to recover selenium and tellurium from pressurized etching solutions of nickel-copper mattes.

Separation of Se and Te is based on the relative ease of oxidation and reduction of their valence states, and on differences in the solubility or volatility of their compounds, particularly their respective oxides, oxy-acids and salts. Tellurium-containing selenium is purified by selenium distillation. Purification of selenium-containing tellurium is carried out from the oxide rather than the metal.

Recycling of selenium and tellurium is limited, involving only photoreceptor scraps and used xerography material.

Selenium and its compounds are acutely and chronically toxic to many animals and humans. Soluble tellurites are toxic, while elemental tellurium and stable tellurides are relatively inert.