The liquid surfactant membrane process: Effect of the emulsifier type on copper extraction by LIX65N-LIX63 mixtures

Abstract

In order to investigate the role of the emulsifier type in the liquid surfactant membrane (LSM) extraction process, a comparative study of the performance of four different emulsifiers, Span 80 (active ingredient: sorbitan monooleate), Arlacel 83 (active ingredients: sorbitan mono- and dioleates), Span 85 (active ingredient: sorbitan trioleate), and Brij 72 (active ingredient: polyoxyethylene(2)stearyl ether) in LSM copper extraction systems has been undertaken. The experimental techniques employed included interfacial tension, membrane-phase viscosity, emulsion viscosity, membrane stability (as determined by K + transfer from the internal to the external phase) and LSM copper transfer measurements. Interfacial tension measurements conducted both in the presence and in the absence of the extractant (LIX65N) indicated that the interfacial activity of the emulsifiers increased in the order Brij 72 < Span 85 ⪡ Arlacel 83 ⋍ Span 80 . The emulsifier concentration required to achieve good stability varied widely with the emulsifier type (e.g., 0.1 kmol m −3 Span 80, 0.32 kmol m −3 Span 85, and 0.37 kmol m −3 Brij 72 at 5% membrane breakdown) and correlated with the relative interfacial activities. With the exception of Brij 72 LSMs which gave negligible copper transfer, metal extraction at first increased, passed through a maximum, and then decreased as the emulsifier concentration was increased. The initial increase was attributed to a decrease in membrane breakdown. On the other hand, the subsequent decrease in copper extraction with increase in emulsifier concentrations was attributed to a number of possible factors including a decrease in the rate of copper complexation at the membrane phase/external phase interface, an increase in the interfacial viscosity, a decrease in the movement of the internal phase droplets within emulsion globules, and a decrease in the external interfacial area available for metal extraction. The emulsifier concentration giving the extraction maximum increased in the order: Span 80 ( 23.2 g L −1, 0.054 kmol m −3) ⋍ Arlacel 83 (23.2 g L −1) ⪡ Span 85 (155 g L −1 , 0.162 kmol m −3). The corresponding copper extraction (at 2 h) increased in the order: Span 85 (28%) ⪡ Arlacel 83 (78%) < Span 80 (89%). Interfacial tension studies indicated that the very poor extraction performance of Brij 72 LSMs was a result of the complete displacement of LIX65N from the membrane phase/external phase interface/external phase interface. In addition, viscosity measurements indicated that the low copper extraction by Span 85 LSMs might be a result of a relatively low membrane phase/external phase interfacial area or relatively low values for the diffusivities of the carrier and the metal-carrier complex within the bulk membrane phase.