Investigating the novel process for thorough removal of eutectic phosphate impurities from phosphogypsum

Phosphogypsum is the major component in solid waste of phosphoric acid manufacturing. After purification, phosphogypsum can be used as an alternative material to natural gypsum. Phosphate impurities, especially eutectic phosphorus, are challenging to be removed thoroughly from phosphogypsum. In this work, we reported a novel phosphogypsum purification process, in which the phase transition activation by calcination was combined with sulfuric acid dissolution during hydration. The two critical parameters, activation temperature and H2SO4 concentration were optimized for this process. Moreover, powder X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to determine the purification efficiency for removing eutectic phosphate impurity from phosphogypsum. The comprehensive characterization results suggested that this novel process can remove eutectic phosphate impurities through the mutual transformation of different crystalline forms of calcium sulfate combined with sulfuric acid dissolution. Under the optimal purification condition (165 °C activation and 1.0 wt% H2SO4), the removal rate of eutectic phosphorus reached up to 98.3%, corresponding to overall eutectic phosphorus content decreasing from 0.58% to 0.01%. Meanwhile, the water-soluble phosphorus and insoluble phosphorus were also completely removed. Thus, 99.4% of phosphate impurity (as P2O5) was removed from phosphogypsum after the two-step treatment. The findings of this paper indicated that this novel process could separate phosphate impurity in an efficient and thorough manner, enabling sustainable utilization of phosphogypsum.