Nitrate-mediated kinetic leaching for rare earth elements recovery from phosphogypsum

Abstract

Phosphogypsum (PG) is a byproduct of the decomposition of phosphate rock with sulfuric acid during phosphoric acid synthesis. Various techniques such as X-ray diffraction (XRD), Scan electron microscope with electron dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), and Thermal gravimetric analysis (TGA) are utilized to characterize PG and rare earth concentrate. Heir, both nitric acid (HNO3) and ammonium nitrate (NH4NO3), are used as leaching agents for rare earth elements (REEs) from Egyptian PG. The ideal working conditions for leaching about 85% of REEs from PG are 350 mg/kg REEs in PG, 149 μm grain size, 0.5M NH4NO3, 2M HNO3, 1/4 S/L, 30 minutes leaching time, 300 rpm, and 80°C. The data on the kinetic REEs leaching are matched to the diffusion control model. The kinetics of the applicable leaching activity and the reaction mechanism between the mixture of nitric acid and ammonium nitrate with REEs have been explored in detail. After concentration by vaporization to one liter, a final concentration of REEs reached ≈ 300 mg/L with a leachability of 85%. The REEs were precipitated using 20% oxalic acid at pH 2.5 as oxalate, and then the precipitate was calcined at 900°C in an electric oven, given a mixture of REEs oxides and CaO.