Study of the separation of didymium from lanthanum using liquid-liquid extraction: Comparison between saponification of the extractant and use of lactic acid

Abstract

The liquid-liquid extraction (LLE) technique is widely used for industrial-scale separation of rare-earth elements (REEs). However, the low selectivity of LLE in separating REEs, mainly adjacent elements, has prompted the investigation of modifications of this technique to find new routes that are more efficient. The selectivity of REEs separation increases when saponified extractants are used. However, saponification generates effluents containing sodium or ammonium ions that need to be removed before discharge into water bodies or be reused. As an alternative to saponification, the conditioning of the feed liquor with complexing agents has been proposed. This study compares the efficiency of didymium (Pr + Nd) and lanthanum separation with saponified and non-saponified organophosphorus extractants and with addition of lactic acid to the feed liquor. The extractants investigated were D2EHPA, P507 and Cyanex 272. The effect on the didymium/lanthanum separation of initial pH, saponification degree and lactic acid concentration and extractant were evaluated. The La and didymium separation were better when the feed solution was conditioned with lactic acid than when the extractants D2EHPA and P507 were saponified. Conversely, La and didymium separation was better when Cyanex 272 was saponified than when the feed liquor was conditioned with lactic acid. These results indicate that conditioning the feed liquor with lactic acid has the potential to replace the practice of saponification of the extractants D2EHPA and P507.