Leaching means Main advantages Main disadvantages
H2SO4, HCl, HNO3 20-80% REE extraction. Possibility to modify the recovery ratio of REE and Fe depending on acid concentration and leaching steps. By-products production (Orbite Patent, and HNO3 Pilot NTUA). Low REE’s recovery. Large amounts of effluents and acid wastes. As acid concentration increases, selectivity decreases. Special equipment for corrosion resistance is needed in case of HCl. Silica gel formation at low acid concentrations.
H2SO3 Digestion in 2-3 stages of selective dissolution of REE, up to 85% Y recovery (Patent Alcan). No data available for the Sc extraction.
Carboxylic acids such as formic, citric, acetic, methanesulfonic acid, etc. Selective REE’s leaching (similar to mineral acids), Ca and Al versus Fe. Bio-leaching (Rusal Patent). Difficulty in recycling organic acids by distillation, producing large amounts of calcium sulphate. In some cases, low REE extraction. More expensive solvents than inorganic acids.
CO2, NaΗCO3
Selective metals recovery such as Sc, Ti and Zr producing carbonate complexes in the caustic solution. Large intake of CO2 in pulp (Rusal pilot plant for production of 2 tons per year Sc2O3).
Low Sc recovery (<26%) No REE recovery.
Ionic Liquid (HBET-TF2N) High selectivity of REE against Fe Si and Ti; Similar to mineral acid recoveries; low acid consumption (regeneration of IL), and purification and up concentration of REE solution. High cost of IL (non commercial commodity-synthesized on demand), IL losses in aqueous phases.