Preparation of Tungsten

Scheelite is a preferable source of tungsten, the metal being more easily obtained from this than from other tungsten minerals.

In order to obtain pure tungstic anhydride, WO₃, or salts of the metal, from which tungsten may be prepared by suitable methods of reduction, the mineral concentrates are crushed and passed through a magnetic separator in order to remove tinstone and other impurities, and then oxidised by roasting in the air, care being taken that fusion does not occur; addition of an alkali solution then takes place, the tungsten being dissolved as alkali tungstate, while any iron and manganese remain as insoluble oxides. The necessary alkali may be added to the powdered mineral before roasting, or the mineral may be decomposed by heating under pressure with caustic alkali or by the use of an alkali silicate. Wohler's method consists in fusing together wolframite with twice its weight of calcium chloride for one hour and extracting the melt with water. A solution of the chlorides of calcium, iron, and manganese is obtained, while the insoluble calcium tungstate, CaWO₄, on heating with concentrated hydrochloric acid, yields tungstic anhydride. Sodium hydrogen sulphate may be used for the decomposition of wolframite; a mixture of soda and sodium nitrate, or of the carbonates of sodium and potassium, may also be employed. Pure tungstic anhydride may then be obtained by solution in ammonia and reprecipitation by acids, or by reduction, conversion into the oxychloride, and treatment with acid.

Metallic tungsten may be obtained by a number of methods, including processes similar to those employed in the case of chromium and molybdenum.

Reduction by carbon takes place at a red heat; by employment of the electric furnace Moissan reduced 800 grams of the trioxide with 80 grams of charcoal, using a current of 900 amperes at 50 volts for ten minutes, and obtaining a product containing 0.13 per cent, of carbon. Wolframite, though not reducible with zinc or magnesium, is reduced by carbon in the electric furnace, carbon being finally removed from the product by fusion with tungstic acid and calcium fluoride.

Probably the best method available for the preparation of pure tungsten is the reduction in a porcelain tube of purified tungsten trioxide contained in nickel boats by means of pure dry electrolytic hydrogen (the desiccation being effected by phosphoric anhydride) at 1100° to 1150° C.

Tungsten for the manufacture of metallic filament electric glow lamps was formerly obtained by a modification of Delepine's process, consisting of reduction of the trioxide by heating with powdered zinc, and removal of the excess of zinc and zinc oxide by treatment with acids. If Goldschmidt's alumino-thermic method of reduction is applied, the reaction is very violent and an alloy containing 2.6 per cent, aluminium is obtained. The presence of this impurity is undesirable for filament manufacture. It is preferable to use a deficiency of aluminium and to treat the product with hydrochloric acid. Metallic calcium may also be used for reduction.

Reduction of tungsten chloride, either by heating alone at 1000° to 1500° C. when the compound dissociates, leaving a residue of pure tungsten, or by means of hydrogen or sodium. The reduction of the chloride or rather oxychloride by means of carbon and hydrogen is applied to the manufacture of filaments for tungsten glow lamps in connection with which the details of the process are presently described.

The metal is obtained in the amorphous condition by the electrolysis of fused sodium tungstate, and in globules containing 96 per cent, of tungsten when a solution of the trioxide in cryolite is electrolysed. A good deposit of very pure tungsten may be obtained by electrolysis of a solution of tungstic acid in a fused mixture of sodium and potassium chlorides if less than 1 part of the acid to 2 parts of fused chloride is present. A solution of the hexa- chloride in certain organic liquids, e.g. acetone, when submitted to electrolysis under suitable conditions, yields metallic tungsten.