Tellurium Production

Tellurium is extracted by sulphide ores processing from semi-products of copper, lead-zinc manufacture, as well as from some gold ores. Copper electrolysis sludge which contains from 0.5 to 2% Tellurium, as well as silver, gold, selenium and copper may me used as main feed stock in Tellurium production.

For this purpose sludge first should be freed from copper and selenium; the rest, which contains noble metals with Tellurium, lead, tin and other components is fused for obtaining a gold-silver alloy. Tellurium in form of Na₂TeO₃ passes into soda-tellurium slag in which concentration of Tellurium reaches 20-35%. Slag is crushed, milled and leached by water. Te is extracted from the solution on cathode. The extracted tellurium concentrate is treated by alkaline in the presence of aluminum powder, converting tellurium into dissolved tellurides. The solution is separated from insoluble heavy metals impurities concentrates and in blown by air. 99%-pure tellurium precipitates in elementary state. High-purity tellurium is obtained after recurrent recovery process. Most pure tellurium in extracted by combination of chemical refinement, distillation and zone melting.

The processes which have been used for the extraction of tellurium from its minerals and from the residues obtained from the metallurgical treatment of telluriferous ores of other metals, are relatively numerous, a different procedure having been adopted in almost every case. In effect, however, the principles involved are few. The Tellurium Extraction may be performed as alkali telluride by heating with alkali carbonate and a reducing agent, when the aqueous solution can be submitted to atmospheric oxidation, with liberation of the tellurium. This method, usually followed for bismuth telluride ores, resembles in some respects the method of treatment of mixtures containing free tellurium; e.g. "chamber mud," from Japanese sulphur, is extracted with alkaline potassium cyanide solution, the resulting solution of unstable potassium tellurocyanide on treatment with hydrogen sulphide giving a deposit of tellurium sulphide; this deposit can again be converted into potassium tellurocyanide by fusion with potassium cyanide and the tellurium then separated by atmospheric oxidation of the solution.

On the other hand, especially in the case of the gold and silver ores, a process of oxidation may be applied. Oxidation may be effected by fusion with a mixture of potassium nitrate and alkali carbonate, by the action of aqua regia or even by merely heating with concentrated or fuming sulphuric acid. The last-named reagent, in the form of oleum containing 20 per cent. SO₃, is now generally applied for the extraction of tellurium from hessite-bearing ore, the earthy mineral containing 4 to 5 per cent, of silver telluride, which constitutes one of the two most important sources of commercial tellurium, the other source being the "anode mud" of the copper refineries. After oxidation, water is added, and silver, the main product of the Tellurium Extraction, is precipitated by the addition of hydrochloric acid. The aqueous solution of the resulting tellurous or telluric acid is then heated with a reducing agent, sulphurous acid being frequently applied, although metals such as zinc may be used, the tellurium being precipitated. If aqua regia has been used as the oxidising agent, gold is first removed by means of ferrous sulphate and oxalic acid.

The "anode mud" is now a much more important source of tellurium than any of the minerals. The mud is generally roasted in a current of air, when tellurium dioxide is formed. This condenses, together with oxides of arsenic and antimony, as small white crystals in the flues and cooling chambers, and is reduced to tellurium by smelting with charcoal at a low temperature.

The crude product from any of the foregoing processes usually contains from 70 to 85 per cent, of tellurium.

Taking into account its relatively small practical importance, no element has had more time or thought devoted to its purification than has Tellurium Purification. This arises largely from the unexpected position of its atomic weight amongst the corresponding figures for the other elements. The variation in the results obtained supplies good evidence that the entire removal of impurities is a difficult task.

Different types of methods for Tellurium Purification have been applied. Brauner in 1889, by means of aqua regia, converted crude tellurium into telluric chloride, the aqueous solution of which on precipitation by sulphur dioxide deposited the element together with selenium and traces of copper and lead as the only impurities. The precipitate was dissolved in fused potassium cyanide in an atmosphere of hydrogen and the tellurium separated from the aqueous extract by treatment with a current of air. The element was then distilled in a current of hydrogen; an alternative is distillation in a vacuum. Another method of treatment for the telluric chloride solution is to precipitate in three fractions using sulphur dioxide, when it is found that the middle fraction is of a high degree of purity.

Another group of methods involves the conversion of the tellurium into some compound, inorganic or organic, which can be purified and then made to regenerate the element. Hydrogen telluride, crystalline telluric acid, basic tellurium nitrate and diphenyl telluride have all been made use of.

Under suitable current conditions, tellurium may be deposited electrolytically in a very pure form from sulphuric acid solution.