Integrated Mechanochemical and Hydrometallurgical Process for the Recovery of Terbium and Gallium from Garnet Waste

Abstract The increasing demand for gallium and terbium in advanced technologies necessitates sustainable recovery methods from secondary sources. This study presents a novel mechanochemical approach for recycling critical materials from end-of-life Terbium Gallium Garnet (TGG), a synthetic garnet rich in Ga and Tb. Optimized ball milling in the presence of NaOH facilitates partial decomposition of TGG, enabling selective leaching of gallium into an aqueous phase while terbium remains in the solid residue. Structural characterization confirms phase decomposition and defect formation, enhancing dissolution efficiency. Terbium is subsequently recovered via oxalate precipitation, yielding high-purity terbium oxalate (>99.5%). Gallium purification is achieved through a three-step solvent extraction using di(2-ethylhexyl)phosphoric acid (D2EHPA), effectively removing iron, chromium, and residual terbium. Final gallium recovery is achieved through urea-assisted selective precipitation, yielding gallium product with ≥99.7% purity. The integrated process demonstrates high separation factors (αGa/Tb > 4000) using only NaOH and water, and achieves significant material circularity with minimal reagent use. Preliminary life cycle assessment (LCA) and operating expenditure (OPEX) based techno-economic analysis (TEA) indicate approximately 85% lower global warming potential and 92% lower operating cost than direct aqua regia dissolution of milled TGG, largely due to reduced acid use, neutralization demand, processing time, and aqueous waste generation. This integrated mechanochemical and hydrometallurgical strategy offers a scalable, low-reagent pathway for recovering high-purity critical elements from garnet waste, supporting circular economy initiatives and enhancing the resilience of supply chains in advanced energy and electronics sectors.