Recovery of ammonium alum from waste solutions with a varying ratio of NH4 to Al in groundwater remediation after underground uranium leaching

Experiments with cooling crystallization of ammonium alum, (NH4Al(SO4)2·12H2O), were performed with concentrated multicomponent acidic solutions (originating from underground uranium leaching in Stráž 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)2·12H2O 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)2·12H2O decreased in the following order: K+ ≫ Cr3+ > Na+ ≈ Fe3+ > Mg2+ ≈ Zn2+ > Fe2+. Additionally, gypsum (CaSO4·2H2O) solubilities at 25 °C, in mother liquors after NH4Al(SO4)2·12H2O crystallization, were determined.