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Osmium Production


Osmium is taken out from nickel-copper and copper-molybdenum platinum containing deposits. South Africa is one of the biggest suppliers. Platinoids are concentrated in copper and nickel electrolytic refining sludge from which 0.1 mass %. Osmium containing concentrate is obtained. Osmium is extracted by roasting on air at 800-900 °C. The gaseous phase which contains OsO4, SO2 and SeO2, is absorbed by 10% NaOH solution. It is neutralized until pH 8 and then precipitated by SO2. The sediment contains Na8[Os(SO3)6], it is treated by H2SO4 for SO2 removing and by oxidizer, e.g. Cl2 in the presence of NaOH. The yielded OsO4 is sublimated with its vapours absorbed by 20% alkali solution with Osmium concentration 60 g per liter. The solution is treated by methanol for reduction of yielded perosmate to the osmate, then by saturated solution KOH, after which K2[OsO2(OH)4] is precipitated. This salt is reduced by hydrogen then roasted at 930 °C and milled until fine powder containing 99.98% Osmium and iron, nickel and silicon impurities.

By another method the OsO4 alkali solution is treated by NH4Cl with Fremy's salt [OsO2(NH4)4]Cl2 as the output of the reaction. The salt then is roasted in hydrogen atmosphere at 700-800 °C which results with Osmium sponge. The sponge is milled and is purified from contaminations until 99.95% by exposing to fluohydric acid.

Preparation of Osmium

Osmium may be isolated in a fairly pure state from the distillate, rich in that metal, obtained during the process of extracting ruthenium from osmiridium. The distillate is redistilled and collected in aqueous ammonia. Saturation with hydrogen sulphide yields a precipitate of the brown tetrasulphide, OsS4, which is separated by filtration and heated in a closed carbon crucible to a high temperature, when the osmium is found on the upper part of the crucible as a brilliant metal, bluish in colour, somewhat resembling zinc.

Osmium may also be obtained in an amorphous, but pure, condition by reduction of the vaporised tetroxide, OsO4, by passage through a red-hot porcelain tube together with a mixture of the dioxide and monoxide of carbon. The osmium separates out as a powder.

Other methods consist in reducing aqueous solutions of osmium tetroxide with an alkali formate, when the metal is precipitated as a blue powder; by decomposing acidulated solutions of the tetroxide with zinc or with mercury, in which latter case the resulting amalgam is distilled.

Osmium may be obtained in a crystalline form by heating to redness with seven or eight times its weight of tin in a carbon crucible and allowing to cool slowly. The osmium separates on solidification and may be isolated by treatment with hydrochloric acid, which dissolves away the tin.

Recovery of Osmium from Residues

Osmium residues containing organic matter do not yield up the whole of their osmium content as chloride when mixed with sodium chloride and ignited in chlorine. To recover the metal from such sources, therefore, it is convenient to ignite the mass in a current of oxygen, whereby the organic matter is oxidised to carbon dioxide, and the osmium to tetroxide, which is absorbed in alkaline reducing agents. As examples of the last named, alcohol in dilute caustic alkali solution, and alcoholic hydrazine hydrate, may be mentioned. The reduced solution is evaporated to dryness, and the residue ignited in hydrogen and cooled in carbon dioxide.

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