Research progress on the activation and extraction of valuable strategic key metals from coal fly ash

Background Coal fly ash, a major solid waste produced from coal-fired power generation, has a global annual output exceeding 0.75 billion tons. Its disorderly disposal leads to severe environmental risks, such as heavy metal leachate and PM2.5 emissions. However, coal fly ash is rich in aluminum (Al₂O₃ 15%~50%), gallium (Ga 10~300 μg/g), lithium (Li 100~800 μg/g), and rare earth elements (REEs 100~1500 μg/g), making it a potential alternative resource for strategic metals. Methods By analyzing the current metal extraction technologies from coal fly ash both domestically and internationally, this paper describes the form of occurrence of various valuable metals in coal fly ash: Li is present in the silicate-aluminate as a replacement for Al³⁺ or Si⁴⁺ in the form of a similar ion substitution; Ga replaces Al³⁺ in the silicate-aluminate in the form of Ga³⁺; Germanium (Ge) is dispersed in the glass phase in the form of GeO₂; REEs are mainly encapsulated within the Si-O-Al network structure in the silicate-aluminate glass phase. Progress This paper focus on the activation and extraction technologies and separation processes for valuable strategic key metals such as gallium, lithium, and rare earth elements: extraction of Li can be carried out through acid/alkaline leaching, adsorption by adsorbents, and elution to obtain Li₂CO₃ crystals; Ga can be extracted through acid leaching, solvent extraction, and back-extraction; REEs can be extracted through acid/alkaline leaching, separation and enrichment, as well as precipitation and calcination. Combined with experimental data and industrial cases, the paper reveals the key control factors (e.g. calcination temperature, types of additives, acid/base concentration, etc.) for process efficiency, thereby reducing impurity leaching efficiency. Perspectives This work also proposes directions for multi-metal synergistic extraction and low-carbon development including establishing a multi-metal stepwise leaching-selective separation process, developing mixed additive activation and selective enhancement, replacing high-energy consumption steps with green processes such as microwave/ultrasonic methods to decrease the calcination temperature and energy consumption, and developing waste heat recovery and renewable energy technologies including the preheating of leaching liquid and zeolite synthesis from residue. Given the complexity of activation extraction processes for strategic key metals and the issue of secondary pollution, the development of green, low-carbon processes and smart materials and reusable leaching solvents could promote the large-scale application of coal fly ash resource recovery technologies.