Ionic activities in strong salt solutions and mixed solvents of hydrometallurgical importance

Gamini Senanayake

This thesis presents a comparative study and analysis of ionic activities in aqueous acid and salt solutions of high ionic strength, and in mixed aqueous-organic solutions as used or proposed for the hydrometallurgical processing of complex sulfides, precious metal residues, or by-products. The representative ions or solvents chosen for this study are as follows: simple cations : H+,Na+,Ca2+,Mg2+ ,Ag+,Cu+,Cu2+,Pb2+,Zn2+,Ni2+ simple anions: CT, SO2-4, CIO-4 complex anions: AgCl-2, CuCl-2 solvents: acetonitrile, ethanol, dimethylsulfoxide The various experimental and empirical methods of estimating ionic activities are discussed, focussing on the measurement of emf of appropriate cells as the most convenient and practical approach. Because this method requires an accurate calculation or measurement of liquid junction potentials (Ej) associated with a salt bridge and the test solution, the origin and basis of determining Ej is analysed for various salt bridges using high ionic strength and mixed solvent test solutions. The measured values of Ej are presented and recommended for different salts and solvent systems which allow emf values to be corrected and provide more accurate ionic activities. The measured ionic activities are rationalised in terms of the various competing ion-water, ion-solvent, ion-ion and waters-olvent interactions. It is shown that in concentrated acid and salt solutions, the ionic activity coefficients are related to the activity of water in solution according to their ionic hydration numbers. Thus for simple non-associated electrolytes, like HCl or NiCl2, hydration numbers and hydration models were readily determined. For those metal ions which form chloro-, hydroxo- or sulfato-complexes, the changes in activity are related to the formation of complex ions. In these cases the species distribution and relevant Eh-pH or Eh-log Cl diagrams are discussed and presented. Mixed solvent systems are shown to increase the activity of simple anions, but increase or decrease the activity of metal ions in perchlorate media according to the relative strength of the ion-water and ion-solvent interactions. However with mixed solvent systems containing chloride ions, the solvent also promotes complex ion formation, leading to even greater changes in activity of those metal ions with comparable ion-chloride and ion-solvent interactions. Finally, a comparison is made between the changes in ionic activities and reduction potentials brought about by adding salts or solvents to aqueous solutions. It is shown that solvent effects are generally much greater than salt effects. The practical applications of using such systems for the recovery or separation of metals or metal salts from hydrometallurgical leach solutions and residues are briefly reviewed and further possible applications are discussed.

Ionic activities in strong salt solutions and mixed solvents of hydrometallurgical importance

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