Comprehensive recovery of rare earth elements and gypsum from phosphogypsum: A wastewater free process combining gravity separation and hydrometallurgy

Comprehensive utilization of phosphogypsum (PG) has attracted much attention, especially for the recovery of rare earth elements (REEs) and gypsum due to the issues of stockpile, environmental pollution, and waste of associated resources. Traditional utilization methods suffered the issues of low REEs leaching efficiency, huge amount of CaSO₄ saturated wastewater and high recovery cost. To solve these issues, this study investigated the occurrence of REEs in PG and the leaching of REEs. The results show that REEs in PG are in the forms of (1) REEs mineral inclusions, (2) REEs isomorphous substitution of Ca²⁺ in gypsum lattice, (3) dispersed soluble REEs salts. Acid leaching results demonstrate that (1) the dissolution of gypsum matrix is the control factor of REEs leaching; (2) H₂SO₄ is a promising leachant considering the recycle of leachate; (3) the gypsum matrix suffers a recrystallization during the acid leaching and releases the soluble REEs from PG to aqueous solution. For the recovery of the undissolved REEs mineral inclusions, wet sieving concentrated 37.1 wt% of the REEs in a 10.7 wt% mass, increasing REEs content from 309 to 1071 ppm. Finally, a green process combining gravity separation and hydrometallurgy is proposed. This process owns the merits of wastewater free, considerable REEs recovery (about 10% increase compared with traditional processes), excellent gypsum purification (>95 wt% CaSO₄·2H₂O, with <0.06 wt% of soluble P₂O₅ and <0.015 wt% of soluble F) and reagent saving (about 2/3 less reagent consumption than non-cyclical leaching).