Recovery of Rare Earth Elements and Co-Products from Eudialyte-Group Minerals: Mineralogical Controls, Silica Management, and Integrated Hydrometallurgy (2010–2025): A Critical Review

Eudialyte-group minerals (EGMs) are unconventional zirconosilicate resources that host rare earth elements (REEs), yttrium, zirconium, hafnium, niobium, manganese, and other high-field-strength elements within a chemically reactive but process-complex matrix. Their relevance as alternative REE feedstocks depends not on bulk grade alone, but on deposit-specific heavy-REE distributions, comparatively low uranium and thorium contents in many occurrences, and the potential to recover multiple co-products. This critical review evaluates processing literature published from 2010 through 2025 and examines the causal links among crystal chemistry, feed mineralogy, beneficiation, acid decomposition, silica behavior, solid-liquid separation, downstream purification, co-product recovery, scale-up, and environmental performance. Across the evidence base, the principal technical challenge is not simply achieving high elemental dissolution; it is controlling silicon release and polymerization while maintaining filterability, limiting acid demand, minimizing adsorption and coprecipitation losses, and generating liquors suitable for selective separation. Direct acid leaching can achieve substantial extraction under relatively mild conditions, whereas dry digestion, staged contact, and other low-water strategies provide stronger control when silica gelation governs operability. Nitric-acid and energy-assisted routes expand the design space, while bioleaching and organic-acid routes remain less mature for high-throughput primary processing. Downstream design increasingly shifts the flowsheet objective from a single REE product toward coordinated recovery of REEs, Zr–Hf, and Nb, with impurity and residue management integrated from the outset. The central research need is therefore systems-level optimization of mineral liberation, water activity, reaction pathway, filtration, recycle, impurity bleed, and co-product specifications. A route that maximizes batch extraction may still underperform at plant scale if it produces unfilterable silica, dilute liquors, excessive recycle burden, or unrecoverable metal losses.