Abstract:The thermodynamics in zinc hydrometallurgical process was studied using a chemical equilibrium modeling code (GEMS) to predict the zinc solubility and construct the species distribution and predominance diagrams for the Zn(II)-NH₃-H₂O and Zn(II)-NH₃-Cl^--H₂O system. The zinc solubilities in ammoniacal solutions were also measured with equilibrium experiments, which agree well with the predicted values. The distribution and predominance diagrams show that ammine and hydroxyl ammine complexes are the main aqueous Zn species,  is predominant in weak alkaline solution for both Zn(II)-NH₃-H₂O and Zn(II)-NH₃-Cl^--H₂O systems. In Zn(II)-NH₃-Cl^--H₂O system, the ternary complexes containing ammonia and chloride increase the zinc solubility in neutral solution. There are three zinc compounds, Zn(OH)₂, Zn(OH)_1.6Cl_0.4 and Zn(NH₃)₂Cl₂, on which the zinc solubility depends, according to the total ammonia, chloride and zinc concentration. These thermodynamic diagrams show the effects of ammonia, chloride and zinc concentration on the zinc solubility, which can provide thermodynamic references for the zinc hydrometallurgy.

Key words: thermodynamics; predominance diagram; zinc hydrometallurgy; ammonia leaching