File:Reverberatory furnace for tungsten refining.png

English: The process consists in taking tin ores mixed with wolfram, dressed as completely as possible by the old process, and having ascertained by analysis the quantity of wolfram contained therein, then mixing therewith such a quantity of soda ash of known value as shall afford an equivalent of soda for combination with the tungstic acid of the wolfram, which is the tungstate of iron and manganese; the objject of the process being by calcination to convert the insoluble tuugstate of iron and manganese into the soluble tungstate of soda, leaving the oxides of iron and manganese in a very finelyivided state and of low specific gravities, so that they can be easily washed off" with water. The mixture, in cliarges of five to ten cwt., is roasted in a reverberatory furnace on a cast-iron bed of the construction shown in figs. 670 and 671. The 670 use of the cast-iron bed is attended with considerable economy in the consumption of fuel, and it is admirably well adapted for the calcination of the raw ores, for the evolution of the sulphur and arsenic contained in them ; but it is especially necessary instead of fire brick or tile, to avoid the loss which would accrue from the reaction of the soda ash on the silica of the brick, and the formation of sodasilicate of tin which would consequently take place. The mixture is introduced to the bed through a hole in the crown of the furnace ; from a side door it is eciually distributed over the bed, and from time to time it is turned over by the furnace man until the whole mass is of a dull red heat, emitting a slight hissing sound, and in an incipient pasty condition. In successive quantities the charge is then drawn through a hole in the bed of the furnace into the wrinkle or arch beneath, whence it is removed to cisterns, in which it is lixiviated with water, and the tungstate of soda is drawn off in solution. The residuary mass left in the cisterns, — the whole of the soluble matter having been washed out, — is removed to the burning-house floors, and is there dressed over again in the usual manner, the final product of the operations being very nearly pure black oxide of tin. The liquid obtained is either evaporated sufficiently for crystallization when set aside to cool, or is at once dried down to powder. The crystals of tungstate of soda thus obtained are colorless, translucent, of a beautiful pearly lustre, having the form of rhombic prisms or of four-sided laminae.