Effects of the emulsifier in copper extraction by LIX64N-span 80 liquid surfactant membranes

Span 80-LIX65N-LIX63 liquid surfactant membranes (LSMs) were used to extract copper from sulfate solutions at a pH of 2.0. The physico-chemical effects of Span 80 (sorbitan monooleate) were investigated with the aid of measurements of emulsion stability (as determined by leakage of K⁺ ions from the internal to the external phase), membrane phase viscosity, emulsion viscosity, and interfacial tension. As the concentration of Span 80 was increased, copper extraction rate at first increased, rose through a maximum, and then decreased. The initial increase corresponded to a significant improvement in the stability of emulsion globules. Interfacial tension measurements indicated that in order to achieve stable emulsion globules, a high adsorption density of the emulsifier at the emulsion interfaces in a LSM system was needed. It was further shown that a bulk phase emulsifier-extractant interaction at high extractant concentrations had an adverse effect on the stability of emulsion globules by removing emulsifier molecules from the interface. However, the high emulsifier adsorption density required for emulsion stability led to a decrease in the extractant concentration at the interface and therefore reduced the rate of copper complexation at the membrane phase/external phase interface. In addition, the viscosity measurements suggested that the decrease in metal extraction at emulsifier concentrations above the optimum might also be partly a result of the restricted movement of the internal phase droplets, an increase in the interfacial viscosity, and/or a decrease in the membrane phase/external phase interfacial area.