Intensified and Selective Recovery of Critical Metals from Aqueous Extracts: Fundamentals and System Design

Abstract

The global transition into a zero-carbon economy is spurring our demand for critical minerals such as rare earth elements (REE), yet the underlying supply chain is unable to keep up the pace. While recovering REE from wastes is a promising solution, improving metal recovery efficiency and the overall economic interest is necessary for technology development. This study developed a system to effectively concentrate and recover REE and multiple valuable metals from the aqueous leachate of a representative waste feedstock (municipal solid waste incineration ash). Leaching solution from mineral acid was compared to organic ligand citrate. This modular system consists of four steps. Eutectic freeze crystallization was used as an energy-efficient pre-treatment step to concentrate extracted metals, reduce reaction volume, while remarkably enhancing the efficiency of downstream separation and processing. Sulfide precipitation, alkaline precipitation, and oxalate precipitation were then performed to sequentially recover Cu and Zn, Al and Fe, and REE, respectively, with >95% recovery efficiency and >98% product purity for each step. Thermodynamic modeling elucidated the important roles of citrate and resulting metal-citrate complexes in protecting target metals from prematurely precipitating out. Carefully designed step-wise addition of precipitating agents outcompetes citrate to form insoluble metal precipitates, enabling sequential and selective metal recovery with high efficiency. Our results highlighted the dual role of citrate as a leaching and protecting agent in metal recovery and the importance of pre-concentration step in enhancing overall system efficiency. This system can be applied to the intensified resource extraction of other feedstocks as well.

Keywords: Rare earth elements, critical mineral, eutectic freeze crystallization, Thermodynamic modeling, metal-ligand complexation