GB637578A - Methods for extracting alumina from raw anhydrous calcium aluminates - Google Patents

Abstract

In extracting alumina from anhydrous raw calcium aluminates with dilute alkali carbonates, the raw material is treated in one or more stages with a leaching solution at a high "Attack Ratio" (i.e. the ratio of the molecules of sodium carbonate per litre of solution to the molecules of alumina, in the solid raw material to be treated, per litre of solution) and the resulting solution is enriched and purified by at least one leaching operation at lower "Attack Ratio" during which fresh raw material is added and stirred into the solution, the residue from the operation being thereafter subjected to a high "Attack Ratio" treatment with fresh solution and so on; the enriched and purified solution being treated in known manner for the production of alumina. The process is preferably carried out at 50 DEG to 60 DEG C. Two grades of raw material may be used, the lower grade material serving principally for leaching at high "Attack Ratio," and the other for enriching and purifying the leaching liquors. In one modification, the residues are not immediately separated but are allowed to collect from successive operations and suitable amounts are removed regularly when a selected amount of residue has accumulated. By regulating the conditions, such as the charge of raw material, "Attack Ratio," time and temperature of solution, solutions may be obtained which contain at the end of the operation: (a) no sodium carbonate and no lime in solution, or (b) lime in solution, in which case the lime is precipitated by addition of sodium carbonate leaving a solution of sodium aluminate from which pure alumina may be obtained, or if the lime is present in small amount, the solution may be treated with excess carbon dioxide.