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

s 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 CaSO 4 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 SO 4 2- cluster in the CaSO 4 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.