The thermodynamic chemistry of the aqueous copper-ammonia thiosulfate system

Silvia Beatriz Black

A fundamental thermodynamic study was undertaken in order to establish the speciation of copper(I) and copper(II) during the leaching and recovery of gold from thiosulfate-copper-ammonia solutions. Despite considerable research into this complex leaching system, the lack of important fundamental chemistry has delayed the implementation of the thiosulfate process as an alternative to cyanidation. Over the last two decades, research in this field has focused on the kinetics and electrochemistry of leaching, which involves the use of copper(II) as an oxidant. However, the fundamental thermodynamic data for copper(I) and copper(II) in this system is limited. Ion association was found to significantly affect the dissociation constant of the ammonium ion in solutions containing sodium sulfate and/or sodium thiosulfate, thus influencing the free ammonia concentration in solution. These findings highlight the importance of using the correct dissociation constant value in thermodynamic studies that involve ammonia in order to obtain precise stability constants. It has been established that the mixed-ligand complexes Cu(NH3)(S2O3)23- and Cu(NH3)(S2O3)- exist in solution and they are more stable than the other species Cu(S2O3)35-, Cu(NH3)2+ and Cu(NH3)3+ at high concentrations of ammonia and/or thiosulfate. The relative proportions of each two species is dependant on the [NH3]:[S2O32-] ratio in solution. This is reflected in two- and three- dimensional speciation diagrams that have been constructed for typical leaching and recovery processes using the stability constants obtained in this study. The 3-dimensional diagrams reveal subtle speciation trends that are not easily discernable from the 2-dimensional diagrams. An investigation into the effect of high sulfate and chloride concentrations showed that these anions are not involved in the complexation with copper(I)-ammonia or copper(I)-ammonia-thiosulfate species under the experimental conditions studied. However, these anions and perchlorate formed relatively stable species with the copper(II)-ammine complexes in the absence of thiosulfate. Stability constants were obtained for the species Cu(NH3)4SO40, Cu(NH3)4Cl+ and Cu(NH3)4ClO4+ and it is suggested that these anionic ligands form outer-sphere complexes with the Cu(NH3)42+ ion. Various methods of predicting stability constants for mixed-ligand complexes from those for the corresponding single ligand systems have been evaluated for this copper(I) system. Although the results have not been quantitatively accurate, the trends suggest that an appropriate method may serve as a useful qualitative tool to predict the possible existence of mixed-ligand complexes. The combined application of 2- and 3-dimensional speciation and potential diagrams could be used as a hydrometallurgical tool in the design, optimization and control of possible future processes for the extraction of gold using thiosulfate in the presence of copper ions and ammonia. The work presented in this thesis adds to our understanding of the chemistry of copper(I) and copper(II) in this leaching system.

The thermodynamic chemistry of the aqueous copper-ammonia thiosulfate system

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