Thermodynamics of complex chemical equilibria in surfactant mixtures

A thermodynamic approach was developed to predict the precipitation conditions of surfactants using the solubility product relationship between surfactant monomer concentrations, in order to calculate the monomer-precipitate equilibrium. This approach provides an explicit equation which predicts the amount of solid phase which forms in any surfactant mixture. All calculations of the total change in Gibbs energy (ΔG) were performed for concentrations of both surfactants that were below their CMC values. The elaborated ΔG-pH diagrams offer the possibility to determine the areas of thermodynamic stability of the solid phases depending on the chemical composition and acidity of the studied system. It was shown that with increasing concentration of the surfactant and the metal ion, the range of precipitate formation, either as slightly soluble salt or as slightly soluble acid, was extended by a few pH units in all cases.

Thermodynamic analysis of the copper (I) homogeneous and heterogeneous speciation in ammonium thiosulfate leaching systems

The paper presents a new thermodynamic approach to studying mixed ligand complex formation reactions in the multi-component two-phase system “cuprous oxide-ammoniacal copper (I) thiosulfate aqueous solution” under real conditions. The quintessence of the developed approach consists in the thermodynamic analysis of concurrent reactions in the heterogeneous system Cu(I) – NH3 – S2O32− – H2O on the basis of the introduced notion of generalized reaction equations. A detailed thermodynamic analysis in ammonium thiosulfate leaching systems has considered the formation of mixed ligand complex species Cu(OH)i(NH3)j(S2O3)k. The new feature is to focus on the chemical description of the overall process of complex formation consisting of a series of concurrent reactions, where the nature and ratio of the concentrations of chemical species, formed in such reactions, depend on the ratio of the concentrations of the metal Cu+ and ligands, temperature, and other thermodynamic parameters. On the basis of the selected thermodynamic data for involved species, the thermodynamic stability areas of solid phase and the repartition of soluble and insoluble chemical species towards the solution pH and a number of total reagent concentrations in heterogeneous mixtures have been investigated. The diagrams of heterogeneous and homogeneous chemical equilibria are used for the graphical representation of complex equilibria and distribution of soluble and insoluble species in examined systems. The used approach utilizes thermodynamic relationships coupled with original mass balance constraints, where the solid phases are explicitly expressed. The obtained results are useful for optimization of the overall scheme of thiosulfate leaching in gold hydrometallurgy to minimize reagent consumption and maximize gold extraction.