Separation of Rare-Earth Elements by Supported Liquid Membranes: Impacts of Soluble Iron, Aluminum, and pH in Low-Grade Feedstocks
Low-grade residuals such as mine wastes and combustion ash are potential sources of critical metals such as rare-earth elements (REEs). Major challenges in the efficient recovery of REEs are the matrix interferences in the waste extracts that impede subsequent purification steps. This study evaluated feedstock matrix variables such as aqueous aluminum (Al), iron (Fe), and pH for their impact on neodymium (Nd) and erbium (Er) recovery flux by supported liquid membrane (SLM) separations, a type of liquid-liquid extraction method. We initially hypothesized that REE mass transfer would be lower at low [REE]/[Fe] and [REE]/[Al] molar ratios due to increased competition for chelation sites at the membrane interface. However, the results showed that the absolute Fe and Al concentrations, not the molar ratios, controlled Nd and Er mass transfer. The permeability coefficients of Nd and Er were most sensitive to the feedstock concentration of Fe3+ relative to Al3+ and Fe2+. The threshold Fe3+ concentration that resulted in reduced Nd and Er permeability was more than 100 times lower than the concentrations required for Al or Fe2+ to decrease REE permeability. REE recovery rates also increased with increasing pH of the feedstock. Separations performed with excess Fe3+ did not result in observable fouling at the membrane interface. Instead, the pH gradient across the membrane and the relative cation affinity for the chelator were the major drivers of mass transfer. These results provide insights for predicting REE mass transfer rates and SLM separation performance for extractions of low-grade feedstocks.
