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Thallium Production


Industrially thallium is obtained as a by-product of iron and non-ferrous metal sulphide ores processing. It is also extracted from copper, lead and zinc production half-products. The processing method depends on feedstock chemical composition. For example, thallium and other valuable metals are extracted from lead production dust by suplhadizing it in boiling bed at 300-350°C, after which the sulphate mass is leached by water. Thallium is extracted by iodine containing 50% kerosene solution of tributyl phosphate, and then the element is reextracted by sulphuric acid (300 g/l) with 3% hydrogen peroxide. The metal is separated from the reextarcting products by cementation on zinc sheets. 99.99% pure thallium is obtained after remelting the products of the processing under caustic soda layer. Electrolytic refining and crystallizing purification is applied for obtaining more high-pure metal.

Original purification of Thallium

The best source of thallium is thalliferous iron pyrites. All deposits of pyrites do not contain thallium in appreciable quantity. Lamy mentions the Belgian pyrites from Theux, Namur, and Philippeville and certain Spanish pyrites as being particularly rich in thallium.

Methods for extracting thallium from thalliferous pyrites have been described, but are rarely, if ever, used. Thallium is best obtained from the flue dust or chamber deposits from sulphuric acid works in which thalliferous pyrites is burnt.

Flue dust, which contains thallium, lead, arsenic, selenium, etc., is repeatedly boiled with water slightly acidified with sulphuric acid. The filtered solution is concentrated and the thallium precipitated by introducing metallic zinc into the solution. Thallium is thereby precipitated in the form of needles or glittering plates. Alternative procedures are to precipitate the thallium as thallous chloride by adding a soluble chloride, or as thallous sulphide by adding sodium carbonate, filtering from precipitated impurities, adding potassium cyanide, and then saturating with hydrogen sulphide.

Lead chamber deposit is neutralised with lead oxide or lime and extracted with hot water. The solution is filtered and concentrated, and the thallium precipitated as metal, chloride, or sulphide as described above.

The zinc sulphate mother liquors obtained from a white vitriol factory at Goslar in the Harz were found by Bunsen to yield 0.05 per cent, of their weight of thallous chloride. The thallium in such a solution may be precipitated by means of zinc, the deposit, containing a little cadmium and copper, treated with dilute sulphuric acid to dissolve the thallium and cadmium, and the thallium precipitated as thallous iodide from the filtered solution.

The crude metallic thallium is purified by converting it into thallous sulphate and treating the slightly acid solution with hydrogen sulphide to eliminate traces of mercury, silver, arsenic, antimony, and bismuth. The filtered solution is treated with ammonia to remove traces of iron and aluminium, and the filtrate concentrated until thallous sulphate crystallises out. Crude thallous chloride is slowly added to hot, concentrated sulphuric acid and the mass heated until all hydrochloric acid is expelled. The solution is, as in the preceding case, purified by successive hydrogen sulphide and ammonia treatments, and thallous sulphate then crystallised from the solution. A similar method of treatment may be applied to crude thallous sulphide. From the pure thallous sulphate, thallium is most conveniently obtained in a state of purity by electrolysis, as described by Foerster. From pure thallous iodide (chloride or bromide), thallium may be prepared by fusion with potassium cyanide and sodium carbonate (Werther). Thallium may be also prepared by heating thallous oxalate in a covered crucible (Willm).

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