Membrane-based configurations for rare earth elements recovery from coal processing residue leachate: A comparative study

Rare earth elements (REE) are essential for high-tech industries, and their sustainable recovery from secondary resources is increasingly critical. Coal processing residues - specifically coal washing waste (CWW) from coal preparation represent a promising secondary source of REE. This study investigates, for the first time, the recovery of REE from CWW leachates using nanofiltration (NF) and supported liquid membranes (SLM), both individually and in a sequential NF + SLM configuration. NF experiments were conducted at different pH ranges and recovery ratios (RR). Results showed that low pH–low RR conditions favoured REE concentration by suppressing precipitation and surface retention, whereas high pH–high RR promoted major ion deposition, reducing REE selectivity. Direct SLM experiments revealed that REE transport is jointly governed by feed pH and strip acidity. Using 15% D2EHPA as the carrier and nitric acid as the strip solution, 4 M HNO₃ yielded the optimum balance, achieving total REE purities above 50% with low losses. In the sequential NF + SLM configuration, NF effectively increased REE concentrations in the feed. However, co-concentration of major ions led to competitive transport in SLM, affecting REE recovery and purity compared to direct SLM. Mass balance analysis confirmed that major ions accumulated within the membrane, suppressing selectivity. Overall, this work establishes CWW as a viable secondary source for REE and introduces NF + SLM as a potential process. While direct SLM from MF effluent is highlighted as a robust baseline, when NF + SLM parameters are optimized, higher purity and sustainable REE recovery within circular economy strategies can be achieved.