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 minewastes and combustion ash arepotential sources of critical metals such as rare-earth elements (REEs).Major challenges in the efficient recovery of REEs are the matrixinterferences in the waste extracts that impede subsequent purificationsteps. This study evaluated feedstock matrix variables such as aqueousaluminum (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 hypothesizedthat REE mass transfer would be lower at low [REE]/[Fe] and [REE]/[Al]molar ratios due to increased competition for chelation sites at themembrane interface. However, the results showed that the absoluteFe and Al concentrations, not the molar ratios, controlled Nd andEr mass transfer. The permeability coefficients of Nd and Er weremost sensitive to the feedstock concentration of Fe3+ relativeto Al3+ and Fe2+. The threshold Fe3+ concentration that resulted in reduced Nd and Er permeability wasmore than 100 times lower than the concentrations required for Alor Fe2+ to decrease REE permeability. REE recovery ratesalso increased with increasing pH of the feedstock. Separations performedwith excess Fe3+ did not result in observable fouling atthe membrane interface. Instead, the pH gradient across the membraneand the relative cation affinity for the chelator were the major driversof mass transfer. These results provide insights for predicting REEmass transfer rates and SLM separation performance for extractionsof low-grade feedstocks.

Automated summary

Low-grade residuals such as minewastes and combustion ash arepotential sources of critical metals such as rare-earth elements (REEs). Major challenges in the efficient recovery of REEs are the matrixinterferences in the waste extracts that impede subsequent purificationsteps. This study evaluated the impact of feedstock matrix variables such as aqueous aluminum (Al), iron (Fe), and pH on the recovery of neodymium (Nd) and erbium (Er) by supported liquid membrane (SLM) separations. It was found that the absolute concentrations of Fe and Al, rather than their molar ratios, controlled the mass transfer of Nd and Er. The permeability coefficients of Nd and Er were most sensitive to the concentration of Fe3+ in the feedstock. Increasing the pH of the feedstock also increased the REE recovery rates. The results provide insights for predicting REE mass transfer rates and SLM separation performance for extraction of low-grade feedstocks.