In" the early days of the war, shipping losses cut imports of bauxite, the principal aluminum-bearing ore, to a point that seriously endangered the aluminum industry. Funds were made available by the War Department for the construction of a pilot plant at the Bureau to extract alumina from abundant American clays. Difficulty in obtaining equipment hampered construction, but by May I943 a small plant capable of producing about 50 lbs. of alumina a day had been built. Since then the plant has been operated and improved, but certain phases, such as calcination of the oxide and recovery of acid, require further work.
The process consists in (I) roasting clay at about 7oo ยฐ C. to render the alumina soluble, (2) leaching the roasted product with dilute hydrochloric acid, (3) filtering to separate the insoluble siliceous matter from the solution containing the aluminum and soluble impurities such as iron and alkali salts, (4) concentrating the solution, (5) precipitating the aluminum as the hydrated chloride from the concentrated solution, (6) removing the crystals of hydrated aluminum chloride by centrifuging in a rubber-coated centrifuge, (7) washing the crystals to remove adhering impurities, (8) calcining the hydrated chloride to obtain alumina and to expel combined hydrochloric acid which is used in the next cycle, and (9) recovering hydrochloric acid from the waste products at the end of the process.
The alumina obtained in this pilot plant has an average purity of about 99.6 per cent., the significant impurities being o.25 per cent. of chlorine, 0.02 per cent. of iron, and o.05 per cent. of silicon. This compares favorably with alumina produced from high-grade bauxite ores. For some uses, the alumina possesses exceptional properties, for example, as a polishing material for metallographic specimens and as the raw component of certain heat-resisting enamels.
No recent cost estimate has been made, but about one year ago it was indicated that alumina made by this process would be approximately twice as expensive as the regular commercial product. However, reduction in price of hydrochloric acid when bought in large quantities, and improvement in plant operation should materially reduce this figure if a shortage of bauxite ore ever again threatens the industry.