Note 37 bis — Chapter XVII · BORON, ALUMINIUM, AND THE ANALOGOUS METALS OF THE THIRD GROUP

The cost of working this process can be brought as low as 20 cents per lb. or about 2½ fcs. per kilo. In England, Castner, prior to the introduction of the electric method, obtained Al by taking a mixture of 1,200 parts of the double salt NaAlCl4, 600 parts of cryolite, and 350 parts of Na, and obtained about 120 parts of Al, so that the cost of this process is about 1½ time that of the electric method.

Buchner found that sulphide of aluminium, Al₂S₃, is more suitable for the preparation of Al by the electrolytic method than Al₂O₃, but since the formation of Al₂S₃ by heating a mixture of Al₂O₃, and charcoal in sulphur vapour proceeds with difficulty, Gray (1894) proposed to prepare Al₂S₃ by heating a mixture of charcoal, sulphate of aluminium, and sodium fluoride. The resultant molten mixture of NaF and Al₂S₃ gives aluminium directly under the action of an electric current.