Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash

Abstract

Due to the growing demands of rare earth elements (REEs) and the vulnerability of REEs to potential supply disruption, there have been increasing interests in recovering REEs from waste streams such as coal fly ash (CFA). Meanwhile, CFA as a large industrial waste stream in the United States (U.S.) poses significant environmental and economic burdens. Recovery of REEs from CFA is a promising solution to the REE scarcity issue and also brings opportunities for CFA management. This study demonstrates a green system for REE recovery from Class F and C CFA that consists of three modules: REE leaching using citrate, REE separation and concentration using oxalate, and zeolite synthesis using secondary wastes from Modules I and II. In Module I, ∼10 and 60% REEs were leached from the Class F and C CFA samples, respectively, using citrate at pH 4. In Module II, the addition of oxalate selectively precipitated and concentrated REEs from the leachate via the formation of weddellite (CaC 2 O 4 •2H 2 O), while other trace metals remained in solution. In Module III, zeolite was synthesized using wastes from Modules I and II. This study is characterized by the successful recovery of REEs and upcycling of secondary wastes, which addresses both REE recovery and CFA management challenges.

Rare Earth Element Adsorption to Clay Minerals: Mechanistic Insights and Implications for Recovery from Secondary Sources

“…The REE can then be recovered as high purity end-products via solvent extraction, ion-exchange, or selective precipitation with oxalic acid to produce oxalates which are subsequently converted to rare earth oxides via roasting . Similarly, there has been significant research into different processes to recover REE from coal ash which are significantly more efficient and environmentally friendly than mining REE from ore deposits. − …”

Rare Earth Elements Recovery and Waste Management of Municipal Solid Waste Incineration Ash

“…Some particles are still irregularly shaped with round edges, possibly due to incomplete nucleation and crystal growth within such a short reaction time. By comparing the concentrations of non-REE before and after oxalate precipitation, Ca 2+ was preferentially precipitated with oxalate (Figure b), agreeing with previous thermodynamic modeling results that Ca-oxalate is the only oversaturated phase (saturation index >0) as the Ca 2+ concentration is dramatically higher than REE and other non-REE, and its reaction with oxalate is thermodynamically favorable …”

“…Pretreatment processes in many studies are designed to break down durable aluminosilicate matrixes (e.g., amorphous glass phase) and release encapsulated REE phases. , These processes involve the addition of strong alkaline agents and heating at elevated temperatures (up to 450–800 °C), , which often account for a substantial portion of the total operational cost. It is important to note that the efficient REE extraction in our study without any pretreatments might have benefited from the relatively low Si content in MSWIA (Table and Table S1) compared to CFA (especially Class F) .…”

“…Despite its effectiveness and high scalability, such a process leaves a heavy environmental footprint and requires intensive chemical/energy input . Our recent study developed a green three-step treatment system that effectively extracts REE from coal fly ash (CFA) . Thanks to the formation of a soluble metal–ligand complex, using diluted organic acids under a mild pH could achieve comparable REE extraction efficiency as concentrated mineral acids. , Unlike many mineral acids, citric acid is non-hazardous and biodegradable, forming a stable REE–citrate complex with a high stability constant (log β ≈ 6.7–11.8). − After leaching, separation of REE from other interfering metals (e.g., Mg, Al, and Fe) in the leachate was achieved by adding oxalate.…”

“…Thanks to the formation of a soluble metal–ligand complex, using diluted organic acids under a mild pH could achieve comparable REE extraction efficiency as concentrated mineral acids. , Unlike many mineral acids, citric acid is non-hazardous and biodegradable, forming a stable REE–citrate complex with a high stability constant (log β ≈ 6.7–11.8). − After leaching, separation of REE from other interfering metals (e.g., Mg, Al, and Fe) in the leachate was achieved by adding oxalate. Our PHREEQC calculations predicted that in a Ca-rich solution, oxalate would preferentially react with Ca 2+ to form calcium oxalate, coprecipitating REE while leaving other metals in the aqueous phase . These two steps produced not only a purified REE solid product but also waste byproducts at the same time.…”