Characterisation of organic phase species in the extraction of nickel by pre-neutralised Versatic 10

Abstract

The extraction equilibrium of nickel from sulphate solutions using the carboxylic acid-based metal extractant Versatic 10 (Shell Chemicals) was studied. Versatic 10 is a mixture of branched isomers of predominantly C 10 monocarboxylic acids. The detailed composition of the extractant was analysed by gas chromatography (GC) and mass spectrometry (MS). The extraction of nickel was studied at 25°C using two different techniques to adjust the equilibrium pH: neutralisation of the liquid–liquid dispersion with aqueous ammonia in the mixer cell and neutralisation of the feed organic solution with concentrated aqueous ammonia prior to phase contact (pre-neutralisation). The two methods result in the same final equilibrium under equal conditions. The phase behaviour and kinetics are, however, quite different. When using pre-neutralised extractant, it was found that the composition of the loaded organic phase (Ni, H 2O, NH 3 content) and the structure of the nickel carboxylate complex depend on the degree of pre-neutralisation relative to the amount of nickel in the feed solution and the extractant concentration. The lowest residual ammonia and water concentrations in the loaded organic phase were obtained when nickel extraction was quantitatively based on the cation exchange reaction with a stoichiometry of 2NH 4 +(org)=Ni 2+(aq). If the degree of pre-neutralisation is too high for the extractable amount of nickel, the organic phase remains micellar and both water and ammonia are found in the organic phase.