Rare Earths Recovery from Calcium-Rich Coal Fly Ash: Secondary Phase Formation and Mitigation Approaches

Rare earth elements (REEs) extraction from legacy coal ash enables simultaneous waste valorization and remediation. Ashes enriched in basic compounds (e.g., CaO) lend particularly well to REEs extraction but are prone to precipitate formation that hinders upscaled implementation. This study characterizes the secondary phase precipitation that occurs during mild acid leaching and develops an approach to circumvent their formation while maintaining effective REEs recovery. Notably, precipitate characteristics vary with leachant acidity: very acidic solutions (pH < 0) condense silica gels but achieve ∼ 90 % REEs recovery, whereas mild pH values (∼ 3.5) precipitate aluminosilicate flocs and are limited to low REEs recoverability (∼ 30 to 55 %). Gel precipitates, however, alter the rheology of the leachate via their transformation to a semicondensed phase, and are problematic in upscaled operations. To minimize gel formation, we evaluate alternative extraction methods, including an acid baking (i.e., dry digestion) approach that circumvents secondary precipitates while yielding high REEs extractability of ∼ 74 % and low co-extraction of secondary elements. Together, the results here provide key insights critical to controlling acid leaching of ash feedstocks and provide alternative methods for REEs extraction that bypass gel and floc precipitation to enable effective REEs recovery from coal ash wastes.