Advances in research on the activation and recovery of strategic and critical valuable metals from coal fly ash

Background Coal fly ash, a bulk solid waste byproduct from coal-fired power generation, has a global annual output exceeding 0.75 billion tons. Its disordered piling poses severe environmental risks, such as the generation of heavy metal leachate and PM_2.5 emissions. However, coal fly ash is rich in aluminum (Al 100‒200 mg/g), gallium (Ga 10‒300 μg/g), lithium (Li 1 300‒3 700 μg/g), and rare earth elements (REEs 200‒800 μg/g), establishing it as a potential alternative resource of strategic metals.

Methods By analyzing the current domestic and international technologies for metal recovery from coal fly ash, this study expatiates on the mechanisms behind the occurrence of various valuable metals in coal fly ash. Specifically, Li occurs in aluminosilicates through isomorphous replacement of Al³⁺ or Si⁴⁺; Ga occurs in aluminosilicates in the form of Ga³⁺ by replacing Al³⁺; germanium (Ge) is dispersed in the glass phase as GeO₂, and REEs are mainly trapped in the aluminosilicate glass phase by the Si-O-Al network structure.

Advances This study focuses on advances in the activation, recovery, and separation techniques for strategic and critical valuable metals such as Li, Ga, and REEs. Li is recovered through acid/alkaline leaching, adsorption by adsorbents, and elution, with Li₂CO₃ crystals being obtained. Ga can be recovered through acid leaching, solvent extraction, and back-extraction. REEs can be recovered through acid/alkaline leaching, separation and enrichment, and precipitation and calcination. In combination with experimental data and industrial cases, the study reveals that key factors controlling the techniques’ efficiency include calcination temperature, promoter type, and concentration of acids/alkalis. This helps reduce the dissolution rates of impurities.

Prospects This study proposes the development directions of multi-metal synergistic recovery and low-carbon techniques, including (1) establishing the technique of phased pleaching and selective separation of multiple metals, (2) developing activation and selective enhancement based on mixed promoters, (3) replacing high-energy consumption steps with green techniques such as microwave/ultrasonic waves to reduce calcination temperature and energy consumption, and (4) developing waste heat recovery and renewable energy technologies including preheating leaching solution and preparing zeolite molecular sieves using residues. In response to the complexity and secondary pollution associated with the activation and extraction techniques of strategic and critical metals, it is advisable to develop reusable leaching solvents using green, low-carbon techniques and smart material design. This will promote the large-scale application of the technologies for reutilization of coal fly ash.