Selective removal of calcium, lead, zinc and magnesium from evaporative crystallization mother liquor of high-ammonium and high-salinity wastewater from rare earth smelting

The selective precipitation method was adopted to remove Ca(II), Pb(II), Zn(II), Mg(II) from the evaporative crystallization mother liquor of ammonia-containing high-salinity wastewater from rare earth smelting (containing Ca(II) 18.80 g/L, Pb(II) 0.12 g/L, Zn(II) 2.34 g/L, Mg(II) 22.85 g/L, Cl− 186.51 g/L). Firstly, the predominant areas and component distribution of the systems including [Ca2+–SO42−–NH4+–H2O], [Pb2+–Zn2+–S2−–NH4+–H2O], [Ca2+–CO32−–NH4+–H2O], [Mg2+–PO43−–NH4+–H2O] were analyzed, and the boundary conditions for the highly selective stepwise removal of Ca(II), Pb(II), Zn(II), Mg(II) were determined. On the basis of relevant reaction thermodynamics theories, a stepwise removal strategy was formulated as follows: preliminary calcium removal at pH 5.0 with the dosage of (NH4)2SO4 being 0.6 times the theoretical dosage-synchronous removal of Pb(II) and Zn(II) at pH 8.0 with the dosage of (NH4)2S being 1.0 time the theoretical dosage-deep Ca(II) removal at pH 8.0 with the dosage of NH4HCO3 being 0.4 times the theoretical dosage-deep Mg(II) removal at pH 7.0 with the dosage of (NH4)2HPO4 being 1.0 time the theoretical dosage. The experimental results demonstrate that the removal efficiencies of Ca(II), Pb(II), Zn(II) and Mg(II) reach 99.98%, 100%, 99.98% and 99.99%, respectively, and the main precipitates obtained sequentially are CaSO4·2H2O, ZnS and PbS, CaCO3, MgNH4PO4. After the selective removal of Ca(II), Pb(II), Zn(II), Mg(II),the final filtrate contains Ca(II) 1.21 mg/L, Pb(II) 0 mg/L, Zn(II) 0.28 mg/L, Mg(II) 2.19 mg/L, which can be processed to produce NH4Cl products via secondary evaporative crystallization. This study provides a novel approach for the resource utilization of the evaporative crystallization mother liquor of ammonia-containing high-salinity wastewater from rare earth smelting.