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Coal byproducts could be a promising feedstock to alleviate the supply risk of critical rare earth elements (REEs) due to their abundance and REE content. Herein, we investigated the economic and environmental potential of producing REEs from coal fly ash and lignite through an integrated process of leaching, biosorption, and oxalic precipitation on the basis of experimental data and modeling results. Two microbe immobilization systems (polyethylene glycol diacrylate (PEGDA) microbe beads and Si sol–gels) were examined for their efficiency in immobilizing Arthrobacter nicotianae to selectively recover REEs. Techno-economic analysis revealed that North Dakota lignite could be a profitable feedstock when Si sol–gel is used due to its high cell loading and REE adsorption capacity as well as high reuse cycles. Life cycle analysis revealed that Si sol–gel-based biosorption could be more environmental friendly than the prevailing REE production in China due to the use of less toxic chemicals. However, fly ash sourced from Powder River Basin coals was neither profitable nor environmentally sustainable, primarily due to low solubility of high-value scandium at an economical pulp density (100 g ash/L of acid solution). To further improve the proposed biotechnology, future research could focus on scandium recovery, leaching efficiency at a high pulp density, and reuse cycles of the immobilized microbes.

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.

Scandium (Sc) has great potential for use in aerospace and clean energy applications, but its supply is currently limited by a lack of commercially viable deposits and the environmental burden of its production. In this work, a biosorption-based flow-through process was developed for extraction of Sc from low-grade feedstocks. A microbe-encapsulated silica gel (MESG) biosorbent was synthesized through sol–gel encapsulation of Arthrobacter nicotianae, a bacterium that selectively adsorbs Sc. Microscopic imaging revealed a high cell loading and macroporous structure, which enabled rapid mass transport and adsorption/desorption of metal ions. The biosorbent displayed high Sc selectivity against lanthanides and major base metals, with the exception of Fe­(III). Following pH adjustment to remove Fe­(III) from an acid leachate prepared from lignite coal, a packed-bed column loaded with the MESG biosorbent exhibited near-complete Sc separation from lanthanides; the column eluate had a Sc enrichment factor of 10.9, with Sc constituting 96.4% of the total rare earth elements. The MESG biosorbent exhibited no significant degradation with regard to both adsorption capacity and physical structure after 10 adsorption/desorption cycles. Overall, our results suggest that the MESG biosorbent offers an effective and green alternative to conventional liquid–liquid extraction for Sc recovery.