Recovery of zinc and the regeneration of the complexation agent from the NH4Cl-NH3-NTA system

In this work, the leaching behavior of the major components, i.e., Zn, Pb and Fe, of refractory hemimorphite in an ammonium chloride-ammonia-nitrilotriacetic acid (NH4Cl-NH3-NTA) system was investigated in detail. The thermodynamic analysis of the Zn2 +-NTA3--Cl--NH3-H2O and Pb2 +-NTA3--Cl--NH3-H2O systems demonstrated that the predominant zinc and lead species in the leaching solution were the ZnNTA(NH3)2- and PbNTA- complexes, respectively. The results of the leaching experiments indicated that more than 85% of Zn, 60% of Pb and less than 3% of Fe were leached from refractory hemimorphite using 2 mol/L NH3-2 mol/L NH4Cl-0.15 mol/L NTA as the leaching agent at 40 °C with a liquid-solid ratio of 10 mL/g. Moreover, a feasible method for zinc recovery and ligand regeneration was proposed: the ZnNTA(NH3)2- complex leaching solution was transformed into a zinc chloride solution through ammonia distillation and an acidity adjustment process. More than 96.0% of the NTA was recovered when the leaching solution pH was reduced to 1.0.