Electrofilter powders, a by-product of the Bayer process for the production of alumina from bauxite, were leached with sulphuric acid to dissolve gibbsite and transition aluminas, thus obtaining a commercial aluminium sulphate solution and a solid residue. This residue is treated again under more drastic conditions with sulphuric acidic in a furnace at a higher temperature, is then leached with water and filtered, a small amount of solid remaining (alpha-alumina). The liquid is a highly acidic aluminium sulphate solution which does not fulfil commercial grade specifications: the liquor is accordingly treated with potassium hydroxide or ammonium hydroxide to obtain potassium or ammonium alum. Experimental tests were conducted to investigate the synthesis of alum by crystallization. The effects on alum formation of various operating conditions, including the amount of potassium or ammonium hydroxide, temperature and seed alum dosage, were examined. The crystallization process was found to be quite effective in obtaining alum. (C) 2010 Elsevier B.V. All rights reserved.