Analysis of high temperature reduction process of phosphogypsum by coal gasification fine slag in fluidized bed

Phosphogypsum (PG) is one of the important industrial solid waste in the world, and its traditional storage method occupies a large area of land and damages the environment. The reduction of PG to CaS and CaO by thermochemical methods not only turns waste into treasure, but also alleviates environmental pollution. The common reducing agents are mainly lignite and sulfur, etc. However, the above reducing agents have the disadvantage of high cost. Therefore, this paper proposed to use coal gasification fine slag (CGFS) to reduce PG. The reduction behavior and reaction mechanism were explored by thermodynamic calculations, fluidized bed experiments and kinetic calculations. First, thermodynamic calculations showed that the reduction of PG by CGFS was fully feasible. The fluidized bed experiments revealed that when the target product was CaS, the optimized reaction conditions were that the temperature should be kept at 850—900℃ and the C/Ca molar ratio was 2—3, under which the PG can be completely decomposed. When the target product was CaO, the temperature should be kept at 950—1000℃ and the C/Ca molar ratio was 0.5—1, but it was difficult to decompose the PG completely under this condition. In addition, the mineral fraction in CGFS can significantly affect the decomposition rate of PG and the process was mainly related to the reactivity of CGFS. Then, by comparing four common carbon-based reducing agents, it was found that lignite had the highest decomposition efficiency for PG and the fine slag and coke had higher reactivity compared to graphite, which also facilitated the decomposition process of PG. In addition, increasing the C/Ca molar ratio and reaction temperature was able to reduce the gap between graphite and the other three reducing agents. Finally, the kinetic study of the CGFS reduction process of PG indicated that the reduction process was consistent with the shrinkage nucleation reaction model with the kinetic mechanism function G(α)=−ln(1−α) and apparent activation energy of 415.78—456.83kJ/mol. It was found by SEM-EDS that the reaction started from the boundary, gradually diffused to the core and finally formed a honeycomb structure. This study would provide a theoretical basis for the development of environmentally friendly reductant decomposition of PG. © 2024 Chemical Industry Press Co., Ltd.. All rights reserved.