The modelling of equilibrium data for the solvent extraction of metals.

Abstract

In the design of a solvent extraction plant for metals it is necessary to treat the equilibrium data either graphically or analytically. This enables stagewise calculations to be performed to explore solvent usage and feed conditions and so optimise on the cost of recovery of the metal. The present study reviews the methods of presenting and correlating equilibrium data for solvent extraction systems involving metals; the limitations of each approach are highlighted. The most important example of metal extraction which has been studied falls into that class involving a metal species, in acid aqueous media, which displaces protons from the organic extractant during the mass transfer process. In the typical case of copper extracted by hydroxyoximes the equilibrium problem reduces to one of modelling a surface. The concentration of copper in the organic phase is a function of the concentrations of copper and sulphuric acid in the aqueous phase. Empirical models seem to be as satisfactory to the engineer as chemical ones provided the model is used between restricted regions of data. Practical work has involved the measurement of equilibrium data for the extraction of zinc and copper by di- (2-ethylhexyl) phosphoric acid (D2EHPA) in a kerosene diluent. Both the single and binary metal systems have been studied. The data reported are for feeds between

2-40 g/l zinc, 2-60 g/l copper and 20% V/v D2EHPA. A limited number of results are also presented for 10% V/v D2EHPA. Specific chemical and empirical models have been developed to correlate these data. Because the extractant has a high affinity for zinc in the presence of copper the binary data can be modelled making the assumption that the interaction of copper on zinc is negligible. However the effect of zinc on the limited extraction of copper is significant, the zinc reducing the copper loading markedly. In this way zinc can be decontaminated of copper. The methods for presentation of equilibrium data are discussed and a computer program is given which displays three dimensional equilibrium surfaces. Classical design procedures for stagewise. calculations are used against these three dimensional data plots. Computer programs which allow the investigation of a variety of operating conditions have also been written.