Chromium Preparation

Chromium Preparation in metallic state is usually performed by reduction of the sesquioxide, Cr₂O₃, the chief source of which is chrome iron ore. The latter, after separation from gangue, is powdered, washed, mixed with lime and potassium carbonate, dried, and then roasted, the mass being continually stirred. The potassium chromate produced is extracted with water and converted into the dichromate by treatment with sulphuric acid. After recrystallisation the potassium dichromate is reduced by heating with sulphur, starch, or other suitable material, and finally washed with water; the chromium sesquioxide remains undissolved.

Deville, by strongly heating the oxide with sugar charcoal in a brasqued lime crucible, obtained small quantities of metallic chromium contaminated with carbide and silicide. The reaction is never quite quantitative. The reduction was effected by Moissan in an electric arc furnace, when a product containing a considerable proportion of carbon was obtained. This was eliminated by re-fusing with lime, calcium carbide being formed, and then again fusing the product in a crucible brasqued with the double oxide of calcium and chromium.

The reduction can be effected by the alkali metals, magnesium, and aluminium. Goldschmidt's alumino-thermic process consists in mixing the requisite proportions of chromium sesquioxide and aluminium powder, placing the mixture - covered first with a layer of mixed barium peroxide and aluminium powder, and finally with a layer of powdered fluorspar - in a magnesia or refractory clay crucible, and igniting with a "fuse" of magnesium ribbon. The temperature developed by the combustion of the barium peroxide-aluminium mixture is sufficiently high to cause a violent, often explosive, reduction of the chromium sesquioxide; it is consequently necessary for the operator to protect his face and hands. The alumina remains on the surface as a slag, which crystallises on cooling, forming corubin, while the chromium is found at the bottom of the crucible as a metallic button.

On the manufacturing scale, about 100 kilos, of chromium are produced in a single operation. It is preferable to use a mixture of chromium sesquioxide and trioxide (or potassium dichromate) instead of the sesquioxide alone for incorporation with aluminium powder. Suitable proportions are: 20 grams of a mixture containing 50 grams of fused powdered potassium dichromate with 18 grams of aluminium powder, together with 100 grams of a mixture containing 160 grams of dry aluminium powder with 450 grams of calcined chromium sesquioxide. Commercially, pure carbon-free chromium is produced only by reduction of chromium oxide with aluminium.

Chromium can also be obtained by reduction of the chloride or of chromates, from the boride, and by electrolytic methods.

Wohler obtained metallic chromium by reduction of chromic chloride with potassium, while Fremy obtained the metal in a crystalline condition by passing sodium vapour over chromic chloride in an atmosphere of pure dry hydrogen. Wohler prepared similar crystals - minute octahedra - by fusion of chromic chloride with zinc under a layer of mixed sodium and potassium chlorides, and removal of the zinc from the alloy so formed by means of nitric acid. Metallic calcium reduces chromic chloride at a dull red heat without formation of an alloy.

Electro-deposition of metallic chromium from a solution of chromic chloride was accomplished by Bunsen, who obtained a coherent deposit of the metal possessing a bright surface and in appearance somewhat resembling iron. It has been observed that in the electrolytic deposition of chromium from solutions of chromic salts, green solutions are first converted into violet solutions, the chromium being deposited from the latter and not from the former modification. Chromium is deposited from solutions of chromic acid by electrolysis, especially in presence of sulphuric acid. Chromic salts are simultaneously formed, and the solution becomes brown during the electrolysis, which proceeds more readily with impure chromic acid than with pure. The best results appear to be obtained by using a solution containing chromic acid (245 grams per litre) and chromium sulphate (3 grams per litre), with a current density of 13.4 amperes per square decimetre. Such solutions, with chromium anodes, may be used for plating iron and steel. The anodes show no tendency to become passive, and the coatings obtained are stated to be very resistant to acids and to atmospheric corrosion. Electrolysis of the fused salts is difficult because of their high melting-points.

Crystalline chromium has been obtained by reduction of lead chromate in a brasqued crucible. A concentrated solution of chromic chloride, when treated with sodium amalgam, yields a chromium amalgam which, on distilling at 350° C. in hydrogen, or at lower temperatures in a vacuum, leaves a residue of amorphous chromium which under some conditions is pyrophoric. Fusion of chromium boride with metallic copper gives metallic chromium. Reduction of the sesquioxide at 1500° C. with pure hydrogen yields the metal in a very pure condition. Various modifications of and alternatives for the above methods have been suggested.