Enhanced phosphogypsum thermal reduction by carbon in presence of sodium chloride at high temperature

This paper reports an enhanced approach of thermal reduction of phosphogypsum (PG) in the presence of sodium chloride (NaCl) in its molten phase. The thermodynamic together with in-situ thermogravimetric-Fourier transform infrared spectroscopy (TG-FTIR) result indicates the yield of CaS (solid) at the investigated temperature range (800-850 °C). Addition of NaCl not only appreciably reduces the activation energy of this reaction (800-850 °C) on average from 315 to 175 (kJ·mol-1) materially, but also significantly improve the conversion from CaSO4 to CaS. Additionally, the NaCl based waste salt (NaCl-WS) containing organic compounds (0.21 wt%) was found to produce similar results when compared with the pure NaCl addition at investigated temperature range. The optimal conditions via parametric optimization: i.e., 800-850 °C, 15 wt% (NaCl-WS), and 6 (ratio of PG/C (carbon) wt/wt), were determined. The mechanistic study together with using density functional theories (DFT) shows that the synergistic effect by NaCl addition was caused by the coupled-vacancy diffusion mechanism when NaCl melts at high temperature and C atom shows proximity to oxygen (O) atom from the SO42- cluster in the CaSO4 crystallite structure. The evaluation of the process shows appealing advantages over the conventional PG thermal reduction. By carefully manipulating temperature range together with the NaCl-WS addition, the proposed new process simultaneously realizes PG thermal reduction and environmental utilization of NaCl-WS.