This somewhat uncommon metal occurs in considerable proportions in several minerals, the most important being wolfram (which contains 76 1/2 per cent. of tungsten tri-oxide), scheelite (80 1/2 per cent.), and scheelitine or stolzite (51 per cent.). There are several methods by which tungsten may be prepared in a metallic form as an incoherent powder, viz:-(1) Calcining an intimate mixture of tungsten trioxide and carbon in a closed crucible. (2) Reducing tungsten trioxide in a current of hydrogen. (3) Reducing tungsten chloride in sodium vapour. (4) Mixing finely-powdered wolfram with 3 per cent. potassium carbonate and 20 to 30 per cent. common salt, heating the mixture to redness for 1/2 hour, boiling the cooled and pulverized mass with hydrochloric acid, and reducing the resultant pure trioxide in a stream of hydrogen. (5) Adding 1 part commercial sodium tungstate to 2 parts common hydrochloric acid (sp. gr. 1.18 to 1.19), passing steam in till the liquid boils, then adding nitric acid (sp. gr. 1.35) in quantity equal to 4 or 5 per cent. of the sodium salt, and continuing the boiling till the separated tungstic oxide assumes a dark-yellow colour; this is thoroughly washed, dried, mixed with 10 per cent. finely powdered charcoal and 2 or 3 per cent. rosin dust, heated to whiteness for several hours in a well-closed crucible, pulverized, and freed from charcoal by levigation; the product yields 80 to 90 per cent. metallic tungsten.
Commercially, tungsten is mainly obtained in the form of sodium tungstate, as a bye-product in the metallurgy of tin, by what is known as Oxland's process; this is as follows. The sp. gr. of wolfram is so nearly the same as that of black tin that these cannot be separated by any washing operation; therefore the object aimed at is to convert the wolfram (iron and manganese tungstates ) into sodium tungstate, this latter salt being soluble in water, and leaving the iron and manganese as oxides in a very finely divided state. The converting process is carried on in a reverberatory furnace provided with a cast-iron bed; the charge is introduced through a hopper in the crown of the arch, and is spread over the bed so as to be exposed to the flame traversing the furnace; the products of combustion pass over the firebridge and down into a flue (formed by a diagonal brickwork partition), which leads them to the front of the furnace, under the iron bed, returning along the other side to the chimney, thus enveloping the whole bed with heated vapours.
About 1/2 ton forms a charge (more or less, according as the mineral is in a granular or a pulpy condition), which consists of an admixture of soda ash with dry ore in such proportions that the soda added is rather more than is required to combine with all the tungstic acid present; the mass is stirred and raked about to ensure all portions being brought to a bright-red heat, and is known to be working well when it frizzles, assumes a moist appearance, and adheres slightly to the tools. A roasting of 2 1/2 to 3 hours' duration should suffice if properly effected, and the product is then better than when a longer period or a stronger heat is necessitated; as soon as ready the charge is withdrawn in successive quantities through a hole in the hearth, and falls into a vault, whence it is transferred as required to the lixiviating pans, and then treated, while still hot, with water; a clear solution of sodium tungstate filters off through the mass of tin ore, and runs into reservoirs, being subsequently either allowed to crystallize, or evaporated to dryness in iron pans, and affording a crude product containing about 70 per cent. of the dry tungstate.
Metallic tungsten is rapidly attacked by aqua-regia, slowly by nitric acid, and powerfully by boiling potash solution; it is unacted upon by water and by moist or dry oxygen at ordinary temperatures, but burns when heated to redness in air or oxygen, and oxidizes when subjected red hot to the action of steam; it combines with chlorine when heated in it to 482° to 572° F. (250° to 300o C), and less readily with bromine and iodine. Alloys, See pp. 32-33.