Rare earths recovery by fractional precipitation from a sulfuric leach liquor obtained after phosphogypsum processing

The present work describes a process to recover rare earth elements (REEs) by selective precipitation with ammonia from phosphogypsum (PG) waste, a by-product of the phosphate industry. A pretreatment step was carried out with a solution of sodium chloride and sodium carbonate (60 g/L) in order to assure, respectively, a better solubilization of impurities and a conversion of PG to a calcite matrix easily attacked in an acid medium. Enriching a sulfuric acid solution (15%) with REEs was performed by double contacting the same acid solution with appropriate amounts of the pretreated PG by maintaining the operating conditions (L/S = 3, 100 degrees C and 2 h) each time. The leaching method produced a sulfuric leach liquor rich with rare earths with a final concentration of 4308 mg/L as well as two anhydrite solids. The latter, having the same level of purity, could be used safely in many industrial applications. Afterward, the progressive addition of ammonia (10%) to the REEs-rich liquor led to form three precipitates at different pH values: 1.1; 2.3 and 6. The ICP/MS analysis of the precipitation liquors showed that the optimum pH for the precipitation of REEs is 1.1 allowing their recovery with an efficiency of 98.5%; while precipitations at higher pH of 2.3 and 6 had a lower recovery yield of respectively 42.4% and 39.3%. The X-ray diffraction analysis of the precipitate obtained at pH 1.1 showed that it corresponds to an ammonium sulfate compound. Analyzes with photoluminescence technique showed the characteristic luminescence bands of Ce3+, Gd3+, Dy3+, Pr3+ and Sm3+ (after excitations at lambda(Exc) = 260 nm and lambda(Exc) = 440 nm) confirming the presence of REEs in the precipitate.