Recovery of ammonium alum from waste solutions with a varying ratio of NH 4 to Al in groundwater remediation after underground uranium leaching

Abstract

Experiments with cooling crystallization of ammonium alum, (NH 4Al(SO 4) 2·12H 2O), 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 Al 3+, N H 4 + , and S O 4 2 − ions) as well as with similar solutions prepared in the laboratory. The yield of NH 4Al(SO 4) 2·12H 2O crystals increased significantly with the increasing N H 4 + / A l 3 + 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 NH 4Al(SO 4) 2·12H 2O decreased in the following order: K + ≫ Cr 3+ > Na + ≈ Fe 3+ > Mg 2+ ≈ Zn 2+ > Fe 2+. Additionally, gypsum (CaSO 4·2H 2O) solubilities at 25 °C, in mother liquors after NH 4Al(SO 4) 2·12H 2O crystallization, were determined.