Rare earth elements (REE) are critical raw materials required in numerous applications for which there are no suitable substitutes [1]. Industrial extraction of REE is typically achieved by solvent extraction. However, techniques such as hollow fibre liquid membranes (HFLM) offer advantages over conventional extraction processes. HFLM systems require lower volumes of organics, have a low energy consumption, require fewer process steps and are easy to scale up [2, 3]. However, these techniques have not yet been applied on an industrial scale, and further research is required to optimise these processes for the extraction of multicomponent REE solutions.
The lower operating costs of hollow fibre liquid systems may make these techniques an interesting alternative for the extraction of REE from secondary sources, where economic feasibility may be difficult to achieve. This research has sought to understand the potential applicability of HFLM for the extraction of REE from apatite concentrates. In this context, the effect of impurities such as iron, calcium and magnesium in REE multicomponent solutions was evaluated.
Liquid-liquid extraction tests were performed to assess the extraction and stripping equilibria. The effect of different operating conditions on mass transfer and the selectivity of separating REE from impurities was investigated in HFLM tests.
The experimental tests have shown that the formation of a third phase in the organic phase can have a significant detrimental effect on the performance of the process. It is therefore essential to select operating conditions that prevent this phenomenon. The system was operated at a low feed pH, and the organic phase contained a sufficiently high concentration of extractant and the phase modifier tributyl phosphate.
The effect of various operating parameters on mass transfer and selectivity was evaluated. Mass transfer in the system increased significantly with higher REE concentrations in the feed phase, higher feed pH and higher feed flowrate. However, further increases in the pH and/or REE concentration in the feed phase would likely result in the formation of a third phase. Feed pH has a significant effect on REE selectivity, particularly at lower pH levels, where the extraction of heavy REE is favoured over light REE. Selectivity increases slightly at higher feed flows.
The presence of impurities in the REE multicomponent solution poses some challenges for the selective extraction of REE over impurities. Iron was generally extracted simultaneously with the REE and the selectivity towards iron increased as the pH of the feed phase decreased. Furthermore, heavy REE exhibited selectivity over light REE at feed pH less than 0.5. Selective extraction of REE over magnesium was observed at all feed pH levels in the range of 0 to 2.0 and over calcium in the range of 0 to 0.75.
In conclusion, HFLM techniques can be successfully applied for the simultaneous extraction and stripping of REE. The use of D2EHPA as extractant can cause third phase formation, although the system can be operated safely under defined operating conditions. The effect of various factors on the performance of the HFRLM system for REE extraction has been evaluated. Selective extraction of REE over impurities requires a comprehensive evaluation of the equilibria and the operating conditions of the system. The information obtained provided new insights that will be useful in the development of optimised REE extraction processes using hollow fibre liquid membranes.