The Anglo Research Nickel (ArNi) process is a novel extractive metallurgical process that arose out of the need to develop a processing route for the recovery of nickel from lateritic ore deposits that is both economical and environmentally acceptable. Kieserite (MgSO4·H2O) crystallisation is a critical step in the process which leads to the regeneration of reagents (HCl, H2SO4 and MgO). Hence, the regeneration of reagents is dependent on the amount of magnesium sulphate that precipitates out within the limits of the operating conditions. These conditions include temperature and the ioninteractions of the background aqueous environment. Hence, by manipulating these parameters the optimal region and hence, operating conditions where the minimum solubility of the solute lies can be identified. This novel ArNi process demonstrates the power of manipulating aqueous chemical environments in order to regenerate reagents and hence, develop more sustainable processes. Thus, the ArNi process has provided the building blocks to reinventing the way mining processes are designed, implemented and perceived. Therefore, in order to develop a broader understanding of how aqueous environments can be manipulated in order to process different types of ores, the solubility of NaCl in hypersaline brines was also investigated as a 2nd model system. Temperature and ion interactions are the most important factors affecting both the solubility of slightly soluble magnesium sulphate, and highly soluble sodium chloride salts, as well as for the type of hydrate formed. However, there is a lack of data for the thermodynamic properties of these salts in multi-component systems, especially their solubilites at high temperatures. Thus, there is scope for the development of a better understanding of the ion interactions in multi-component systems under different aqueous environments and temperature conditions, and how these affect the precipitated solute. To achieve the objectives of the study, experiments were conducted in 450 ml glass reactors. The desired operating temperatures were attained using heating bands and maintained with temperature controllers. Spiral reflux condensers were fitted to condense any vapour that evolved and ensure that the volume of solvent remained constant. Face-centred central composite designs and central composite factorial designs were adopted for the FeCl3-MgCl2-HCl-MgSO4-H2O and ZnCl2-HCl-NaCl-H2O systems respectively. The factors that were varied were the concentrations of FeCl3, MgCl2 and HCl for the FeCl3-MgCl2-HCl-MgSO4-H2O system at 105°C and the concentrations of ZnCl2 and HCl at temperatures of 40°C, 80°C and 107°C for the ZnCl2- HCl-NaCl-H2O system. The measured responses were the solubility of MgSO4 and NaCl. Characterisations of the hydrates of MgSO4 that formed under different aqueous environments were also established for the FeCl3-MgCl2-HCl-MgSO4-H2O system. FeCl3-MgCl2-HCl-MgSO4-H2O system Statistical analysis of each of the factorial phases established that a second order model best fits the experimental data and accounts for 98.3%, 96.1% and 98.3% of the variation in the solubility of MgSO4. Within each phase, MgCl2 concentration had the most significant effect on the solubility of MgSO4 of all the varied factors. MgCl2 suppressed the solubility of MgSO4 due to the presence of the common Mg2+ ion. HCl had the opposite effect on the solubility of MgSO4 i.e. increasing the concentration of HCl resulted in an increase in the solubility of MgSO4 due to an increase in ionic strength. At low concentrations of MgCl2 and HCl, increasing the concentration of FeCl3 decreased the solubility of MgSO4 due to the bond formation between SO4 2- ions and ferric hydroxyl complexes. At high concentrations of MgCl2 and HCl, increasing the concentration of FeCl3 had a minimal effect on the solubility of MgSO4. MgSO4·H2O precipitated independently or with a combination of MgSO4·1.25H2O or MgSO4·6H2O at each of the different concentration limits of MgCl2, FeCl3 and HCl. The FeCl3 factor did not have an influence on the hydrate or hydrates that formed. However, the presence of MgCl2 and HCl had a dehydrating action on the formation of the hydrates with HCl having a more pronounced effect. ZnCl2-HCl-NaCl-H2O system Statistical analysis of the central composite factorial designs at each of the temperatures investigated, found that a linear order model best fits the experimental data and accounted for 69.1%, 62.7% and 55.1% of the variation in the solubility of NaCl. The concentration of ZnCl2 had the most pronounced influence on the solubility of NaCl. An increase in the concentration of ZnCl2 increased the solubility of NaCl on account of the formation of homo-polar bonds which decreases the ionization of the solution. The increase in the concentration of HCl decreased the solubility of NaCl. The effect of temperature did not have a significant effect on the solubility of NaCl because of the flat solubility line. The findings in this study have shown that ion interactions play a crucial role in the solubility of salts in hypersaline brines. In addition, each ion has a different effect (common ion effect, ionic strength effects or complex formation) on the solubility of a specific salt and is unique to its individual system. Thermodynamic modelling can predict salt solubility trends. However, in order to gain a fundamental understanding of a system, especially complex systems, experimental measurements are a necessity. The experimental measurements provide an in-depth understanding of specific systems which can lead to the manipulation of aqueous environments towards the development of more sustainable processes and hence, a whole new approach to extractive metallurgy.

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