TUNGSTEN. The name tungsten means "heavy stone" (Sw. tung, heavy, sten, stone), and directs attention to the high specific gravity of the element and its ores. (Symbol W, atomic number 74, atomic weight 184.o.) Up to the middle of the 18th century the mineral then known as tungsten, but now called scheelite, together with wolfram, were both considered to be ores of tin. In 1781, K. W. Scheele showed that this mineral con tained a peculiar acid, which he named tungstic acid, combined with lime as a base, and in the same year Bergmann advanced the opinion that the new acid was a metallic calx. Two years later the Spanish chemists, the brothers d'Elhuyar, proved that this acid was present in wolfram, and by heating the acid with carbon pro duced metallic tungsten for the first time.
Tungsten occurs in nature in the form of tungstates of iron, manganese and calcium, which may be divided according to their crystalline form into two groups. The iron and manganese tung states, crystallizing in the monoclinic system, form the wolfram. group, whilst calcium tungstate or scheelite belongs to the tet ragonal system. Pure iron tungstate is known as ferber ite and pure manganese tungstate as hilbnerite. The mineral wolfram or wolframite is normally a mixture of these two compounds in varying proportions. Wolfram generally occurs in columnar, blade-like or massive forms, though sometimes it is found in well formed prismatic or tabular crystals. It breaks up readily into thin flakes and its specific gravity ranges from 7.2-7.5. The colour may vary from nearly black to a brownish tinge char acteristic of specimens rich in manganese. Scheelite or calcium tungstate may occur in the form of well developed crystals which are usually double tetragonal pyramids, but fre quently it is found in a massive form with four good cleavages. The specific gravity is about 6 and the colour variable. It is gen erally grey, pale-yellow, or pale-brown, rarely green or reddish, with a waxy lustre. The world's principal supplies of tungsten ores have come from China, Burma, Japan, Australia, Bolivia and the United States.
remove cassiterite (SnO,) which is present in a large proportion of wolframite ores, magnetic methods are employed. Wolfram, containing iron, is fairly magnetic, whilst cassiterite is not at tracted as a rule even by powerful magnets. With scheelite, for tunately, the association of cassiterite is not common, otherwise (since scheelite is non-magnetic) chemical processes would have to be employed to effect the separation.
Many methods are available for the extraction of tungsten from its ores but the general principles involved in the extraction from wolfram may be illustrated by a brief summary of the sodium carbonate fusion process. The concentrate, after removal of cas siterite and other deleterious impurities, is mixed with excess of sodium carbonate and heated in a reverberatory furnace to about 1,000° C in an oxidizing atmosphere. Thereby sodium tungstate is produced, and the iron and manganese present are converted to oxides: The soluble sodium tungstate is extracted with boiling water and on treatment with boiling hydrochloric acid an amorphous yellow precipitate of tungstic acid is obtained which yields on calcination. In the preparation from scheelite the finely ground ore is decomposed with hydrochloric acid, whereby cal cium chloride passes into solution, and tungstic acid, together with insoluble impurities like silica, remains as a sludge: To purify the tungstic acid produced in these processes, it may be dissolved in ammonia and crystallized out as ammonium para tungstate, This is decomposed by hy drochloric acid to yield purified tungstic acid, or ignited in air to tungstic oxide Freedom from impurities and a suitable physical condition are absolutely necessary if tungsten is to be used for incandescent lamps or for wireless work. In this case, the method universally adopted for the preparation of the metal is reduction of the oxide by hydrogen, whereby the tungsten is obtained as a powder. For alloying purposes, however, where greater latitude is allowed in the amount of impurity and where the physical condition of the metal is of minor importance, the oxide is reduced by carbon and the resulting powder used for tungsten steels and other alloys.