Department of Chemical Engineering

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Department Process Engineering now has a new name, and will be known from March 2023, as Department of Chemical Engineering.

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Browsing Department of Chemical Engineering by Author "Alberts, Estelle"

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    Stripping rare earth elements and iron from D2EHPA during zinc solvent extraction
    (Stellenbosch : Stellenbosch University, 2011-12) Alberts, Estelle; Dorfling, C.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: At Skorpion Zinc mine, in south-west Namibia, zinc oxide ore is refined through sulphuric acid leaching, solvent extraction, electrowinning and casting of the final 99.995 % Zn metal. Over the last four years, the rare earth element concentrations, with particular reference to Y, Yb, Er and Sc, have significantly increased in the circulating electrolyte and the zinc-stripped organic phase streams in the electrowinning and solvent extraction processes. This project had two main objectives: firstly, the effect(s) of rare earths on the zinc solvent extraction and electrowinning processes were to be determined; based on these results, the second objective was to find a suitable method for removing rare earth elements from the organic phase during zinc solvent extraction. The investigation into the effect of the rare earths on zinc electrowinning showed that an increase of 100 mg/l in the electrolyte Y concentration caused a decrease of 6 % in current efficiency. 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 rare earth elements and iron concentration from 3100 to 6250 mg/l resulted in doubled viscosity and an increase in phase disengagement time from 100 to 700 seconds. The organic zinc loading capacity after two extraction stages was reduced by 1 – 3 g/l depending on the pregnant leach solution used. The detrimental effect of rare earth elements on solvent extraction and electroplating of zinc therefore justified the development of a rare earth element removal process. Stripping of low concentrations of rare earth elements from 40% D2EHPA diluted in kerosene to produce a clean organic for zinc extraction was investigated using bench-scale experiments in a glass jacketed mixing cylinder. For the rare earths, the best stripping agent was found to be H2SO4, followed by HCl and then HNO3. Hydrochloric acid achieved better Fe stripping than sulphuric acid. Acid concentration was tested in the range of 1 to 7 M, organic-to-aqueous ratio for the range of 0.25 to 6.0 and temperatures between 30 and 55 °C. More than 80% stripping of yttrium and erbium could 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 achieved with 5 M sulphuric acid. Stripping was improved by reducing the organic-to-aqueous ratio to as low as 0.5 and increasing the 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 temperature on rare earth stripping. The results showed good repeatability, and were not limited by the rare earth concentration, agitation rate or equilibrium time in the range of set points used in the experiments. Statistical models were compiled to fit the experimental data obtained for Y, Yb, Er and Fe when stripped with sulphuric and hydrochloric acid respectively. All models showed dependence on the acid concentration and squared-concentration and interaction effects between the O:A ratio and temperature and stripping agent concentration were significant. The models were compiled for the experimental data obtained from stripping synthetically prepared organic and then tested on results obtained when stripping the plant organic phase. The following three process solutions were discussed for implementation on a plant scale for the removal of rare earths from the organic phase during zinc solvent extraction: Sulphuric acid stripping mixer settler or stripping column, improvement of available HCl stripping section and replacement of the organic inventory. The possibility of an oxalic acid precipitation process to obtain value from the rare earths as by-product was also discussed. It was concluded that the current process that uses HCl to strip off iron and rare earths would be the best practically and financially 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.