Relative distribution enhancement: a new factor for the evaluation of synergistic solvent extraction

BACKGROUND: Solvent extraction is one of the most used processes worldwide for metal extraction at an industrial and laboratory scale. As a process that requires an external component, the selection of extractants is one of the most studied topics to find the most suitable ones for a particular case. The use of combined reagents, or so-called synergistic extractants, is a branch of extraction studies that have proven better results than the use of solvents on their own. However, a comparison between synergistic extractants cannot be made with the currently reported factors. This paper provides a guide on clarifying the implications of the conventional synergistic factor, R, for the evaluation of the best extractant combinations. Moreover, a new indicator is proposed, named relative distribution enhancement factor, H, that will quantitatively measure the efficiency of the extraction and compare extraction enhancement among different experimental research. RESULTS: Four references were selected to illustrate the use of the H factor in contrast with the R factor. The usefulness of the new enhancement factor H was demonstrated by evaluating the synergistic solvent extraction of rare earth elements. It was found that the use of R factors can mislead the selection of extractants ratio, and although synergism exists this does not imply an enhanced extraction. CONCLUSION: The main finding of this work is that the newly developed H factor can easily detect extraction enhancement or diminishment and is not limited to the evaluation of two extractants.