Alumina

Large quantities of alumina are produced yearly for the manufacture of metallic aluminum. In 1980, 90 percent of the raw material, bauxite, was obtained from foreign sources. Jamaica, Haiti, the Dominican Republic, Surinam, Guyana, Guinea, and Australia are the countries from which the United States imports this bauxite. Total consumption was 15.6 x 106 t, about 96 percent of this going to alumina production; smaller uses include abrasives chemical manufacture, refractories and ceramic fibers.

The production of alumina may be divided into the following steps:

Bauxite, a mineral containing about 55% aluminum oxide and less than 7% silica, is crushed wet ground to 100 meshes. The finely divided bauxite is dissolved under pressure and heat in Bayer digesters with concentrated spent caustic soda solution from a previous and sufficient lime and soda ash. Sodium aluminates is formed, and dissolved silica is precipitated as sodium aluminum silica.

The undisolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used.

• The filtered solution of sodium aluminates is hydrolyzed to precipitated aluminum hydroxide by cooling.
•  The precipitate is filtered from the liquor and washed.
•  The aluminum hydroxide is calcined by heating to 980oC in a rotary kiln. 
•  The alumina is cooled and shipped to reduction plants.
•  The dilute caustic soda filtered from the aluminum hydroxide is concentrated for reuse.
•  The red mud may be reworked for recovery of additional amounts of alumina.

Several other process for producing alumina based on ores other than bauxite have been announced. A Mexican process (called the U.G. process) uses alunite, a hydrous sulfate of aluminum and potassium. It is claimed to be capable of producing 99% pure alumina from alunite containing only 10 to 15% alumina, compared with bauxite which assays 50% alumina. The alunite is crushed, dehydroxilated by heating to 750oC, ground and treated with aqueous ammonia. Filtration removes the alumina hydrate, and potassium and aluminum sulfate are recovered from the filtrate and sold as fertilizer. The alumina hydrate is treated with sulfur dioxide gas, and the resulting aluminum sulfate converted to alumina by heating in a kiln.

The French Pechiney Ugine Kuhlmann process treats clays and shales with concentrated sulfuric acid. Hydrochloric acid is added during the crystallization step to form aluminum chloride which crystallizes readily. Much raw material must be handled, because the clays and shales a lower alumina content bauxite.

Other methods involve the treatment of clays with nitric acid (Bureau of Mines) and the continuous electrolysis of aluminum chloride. (Aleoa).