The sedimentation of "red mud" impurities in aqueous sodium aluminate (obtained by digesting bauxite or other aluminiferous ore with caustic soda at a high temperature) is promoted by the addition of a water-soluble polymer of a vinyl aromatic sulphonate CH2 = CH-Ar-SO3M, where M may be H or a salt-forming monovalent cation, e.g. alkali metal, ammonium or amine. The polymer, in which preferably at least 85% of the monomer units constitute the polymerized form, is added in an amount of 0.05 to 4 pounds per ton of solid in suspension, (0.025-2 gms. per kilogram) for example as an aqueous solution containing 0.005-1% by weight, preferably 0.05-0.2% by weight, of the polymer, the sodium aluminate solution being agitated during the addition. As an activator for the polymer, a lignosulphonate obtained from waste wool pulp liquors may be added to the suspension in its free acid form or as a salt, before, during or after the addition of the polymer. Polymers of sodium styrenesulphonate, potassium styrenesulphonate and sodium or vinyltoluenesulphonate are disclosed.
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