From waste to functional products: Controlled recovery of calcium fluoride and lithium sulfate monohydrate from phosphogypsum

Abstract

Phosphogypsum (PG), a massive by-product of the phosphoric acid industry, remains largely stored due to impurity concerns and limited valorization routes that balance purity, safety, and scalability. This study presents a low-energy, two-product conversion strategy for Moroccan PG, yielding calcium fluoride (CaF 2 ) and lithium sulfate monohydrate (Li 2 SO 4 ·H 2 O) through selective precipitation using lithium fluoride. Optimization revealed that 0.1 M LiF at 25°C for 2 h maximizes CaF 2 purity confirmed by XRD, FTIR, SEM/EDS and TG/DTA. Higher concentrations (0.2 M) induced LiF co-precipitation, reducing the final purity of the material. The filtrate, upon evaporation, yielded Li 2 SO 4 ·H 2 O, validated by XRD, FTIR, and TG/DTA (14.3% mass loss at 123°C, matching theoretical dehydration). SEM showed characteristic plate-like crystals, while EDS confirmed dominant S and O, with trace P (1.5 wt%) but no detectable Ca or F proving effective phase separation. Thermal analysis confirmed high stability: CaF 2 retained > 98% mass up to 900°C; Li 2 SO 4 ·H 2 O dehydrated cleanly without decomposition. Energy demand is low around 3.2 kWh/kg CaF 2 , over 80% less than thermal routes, with reagent costs offset by dual-product output and potential Li⁺ recycling. The process generates only a small inert residue, meeting leaching criteria for safe disposal. This work demonstrates that Moroccan PG can serve as a viable feedstock for high-value inorganic salts not through force, but through fidelity to its chemistry.