K.R. Barnard

Manganese in copper solvent extraction and electrowinning

Abstract In the copper solvent extraction–electrowinning (SX–EW) process, Mn2+ entrained in the organic solution may be transferred to the loaded electrolyte. It will then be oxidised during copper EW. The high-oxidation state manganese formed may in turn return to the SX circuit. The presence of high-oxidation state manganese has been associated with deterioration in the phase separation characteristics of the organic and aqueous mixture, resulting in increased phase disengagement times and the formation of stable mixed phases and emulsions. In the current work, recycle of manganese from copper EW to SX was simulated on a laboratory scale in continuous trials to investigate the mechanism of organic degradation via recycle of manganese from EW. During copper EW trials, Mn2+ in the electrolyte was primarily oxidised to Mn3+, which was further oxidised to MnO4−. Solid MnO2 particles also formed. The existence of high-oxidation state manganese species was consistent with the high redox potential in the solution. Manganese species Mn2+, Mn3+ and MnO4− were identified by their characteristic visible spectra. No evidence was found for the existence of Mn4+ in solution. The Mn2+ and Mn3+ or MnO4− concentration and the amount of MnO2 solids in the solution were determined by a combination of chemical analysis and redox titration. During copper SX–EW trials, the high-oxidation state manganese species oxidised some organic components of the organic phase during stripping. This oxidation correlated with poor phase separation and the formation of stable emulsions in both extraction and stripping stages. In this study, most of the observed organic oxidation and consequent emulsion formation was associated with the presence of Mn3+ rather than MnO4−. A number of degradation products of the hydroxyoxime extractants were detected by a combination of gas chromatography (GC), high performance liquid chromatography (HPLC) and pre-concentration techniques. The observed deterioration in phase separation characteristics correlated with the presence of 5-nonyl salicylic acid and a further hydroxyoxime degradation product which eluted in the most polar of three column chromatography fractions used to separate the degraded organics. Degradation products which eluted in the less polar fractions, to which the undegraded hydroxyoxime extractants reported, were shown not to be contributing to the observed deterioration in phase separation characteristics. Further work is in progress to verify that these more polar species do in fact inhibit phase separation processes. If correct, analysis for species more polar than the extractant could be used as a tool for diagnosis of phase separation problems due to organic degradation in SX systems.