Experiments with cooling crystallization of ammonium alum, (NH4Al(SO4)(2).12H(2)O), were performed with concentrated multicomponent acidic solutions (originating from underground uranium leaching in Straz pod Ralskem area, Czech Republic, and containing as the principal components Al3+, NH4+, and SO42- ions) as well as with similar solutions prepared in the laboratory. The yield of NH4Al(SO4)(2).12H(2)O crystals increased significantly with the increasing NH4+/Al3+ molar ratio, in accordance with pertinent solubility data. The purifying effect of crystallization was quantified by means of the distribution coefficients, characterizing the uptake of ionic impurities to alum crystals; the tendency of cationic impurities to crystallize with NH4Al(SO4)(2).12H(2)O decreased in the following order: K+ >> Cr3+ > Na+ approximate to Fe3+ > Mg2+ approximate to Zn2+ > Fe2+. Additionally, gypsum (CaSO4.2H(2)O) solubilities at 25 degrees C, in mother liquors after NH4Al(SO4)(2).12H(2)O crystallization, were determined. (C) 2007 Elsevier B.V. All rights reserved.