Stripping rare earth elements and iron from D2EHPA during zinc solvent extraction

[摘要] ENGLISH ABSTRACT: At Skorpion Zinc mine, in south-west Namibia, zinc oxide ore is refined through sulphuric acidleaching, solvent extraction, electrowinning and casting of the final 99.995 % Zn metal. Over thelast four years, the rare earth element concentrations, with particular reference to Y, Yb, Er andSc, have significantly increased in the circulating electrolyte and the zinc-stripped organic phasestreams in the electrowinning and solvent extraction processes. This project had two mainobjectives: firstly, the effect(s) of rare earths on the zinc solvent extraction and electrowinningprocesses were to be determined; based on these results, the second objective was to find asuitable method for removing rare earth elements from the organic phase during zinc solventextraction.The investigation into the effect of the rare earths on zinc electrowinning showed that anincrease of 100 mg/l in the electrolyte Y concentration caused a decrease of 6 % in currentefficiency. The elemental order of decreasing current efficiency was found to be: Y > Yb > Er >Sc. In the zinc solvent extraction process, it was found that an increase in the total organic rareearth elements and iron concentration from 3100 to 6250 mg/l resulted in doubled viscosity andan increase in phase disengagement time from 100 to 700 seconds. The organic zinc loadingcapacity after two extraction stages was reduced by 1 – 3 g/l depending on the pregnant leachsolution used.The detrimental effect of rare earth elements on solvent extraction and electroplating of zinctherefore justified the development of a rare earth element removal process. Stripping of lowconcentrations of rare earth elements from 40% D2EHPA diluted in kerosene to produce a cleanorganic for zinc extraction was investigated using bench-scale experiments in a glass jacketedmixing cylinder. For the rare earths, the best stripping agent was found to be H2SO4, followed byHCl and then HNO3. Hydrochloric acid achieved better Fe stripping than sulphuric acid. Acidconcentration was tested in the range of 1 to 7 M, organic-to-aqueous ratio for the range of 0.25to 6.0 and temperatures between 30 and 55 °C. More than 80% stripping of yttrium and erbiumcould be achieved at an optimum hydrochloric acid concentration of 5 M and more than 90%rare earth element (specifically Y, Er, Yb) stripping from the organic phase could be achievedwith 5 M sulphuric acid. Stripping was improved by reducing the organic-to-aqueous ratio to as low as 0.5 and increasingthe temperature. Stripping increased with increasing temperature in an S-shaped curve,flattening off at 50°C. The effect of O:A ratio was more significant than the effect of temperatureon rare earth stripping. The results showed good repeatability, and were not limited by the rareearth concentration, agitation rate or equilibrium time in the range of set points used in theexperiments.Statistical models were compiled to fit the experimental data obtained for Y, Yb, Er and Fe whenstripped with sulphuric and hydrochloric acid respectively. All models showed dependence onthe acid concentration and squared-concentration and interaction effects between the O:A ratioand temperature and stripping agent concentration were significant. The models were compiledfor the experimental data obtained from stripping synthetically prepared organic and then testedon results obtained when stripping the plant organic phase.The following three process solutions were discussed for implementation on a plant scale for theremoval of rare earths from the organic phase during zinc solvent extraction: Sulphuric acidstripping mixer settler or stripping column, improvement of available HCl stripping section andreplacement of the organic inventory. The possibility of an oxalic acid precipitation process toobtain value from the rare earths as by-product was also discussed. It was concluded that thecurrent process that uses HCl to strip off iron and rare earths would be the best practically andfinancially feasible process. Value can be gained from the rare earths if a rare earth element -oxalic acid precipitation section that is financially feasible can be established.

[发布机构] Stellenbosch University