Recovery of critical materials from mine tailings: A comparative study of the solvent extraction of rare earths using acidic, solvating and mixed extractant systems

Abstract

Our society heavily depends on the availability of raw materials. Technology metals such as rare earth elements (REEs) are vital in many applications. Because their virgin mining and production is constrained by a multitude of factors, future exploitation of secondary sources is strongly considered. Tailings from past and present mining activities are important sources of REEs and other critical raw materials, e.g., tungsten and phosphate. The possibility of processing such tailings was thoroughly investigated in the ENVIREE European Project (2015–2018). In this paper, we assess the use of solvent extraction to recover REEs from tailings originating from New Kankberg (Sweden) and Covas (Portugal). Extraction of REEs from common mineral acid solutions was carried out using solvating (Cyanex 923 and TODGA) and acidic extractants (DEHPA and Cyanex 572). Extraction was studied in the presence of high amounts of phosphate, iron and copper in solution. This was to identify bottlenecks in the separation process and ways to mitigate them. While copper and phosphate didn't pose significant issues, iron was co-extracted with the REEs in several systems, e.g., DEHPA – sulfuric acid. Co-extraction was reduced by using a blended DEHPA – Cyanex 923 organic phase. At the same time, the extraction efficiency of REEs improved. Control of the contact time between the aqueous and organic phase, and selective stripping were also used to effectively mitigate the extraction of iron.