Insight into the high-temperature reaction characteristics of CaS with CO2: Experimental study and theoretical calculation

To address the challenge of optimizing the utilization of industrial by-product gypsum, this study proposes a twostep reductive decomposition process for the production of quicklime from industrial by-product gypsum. In the initial step, a blend of CaS and CaO is generated through the low-temperature reductive decomposition of gypsum. Subsequently, the decomposition products undergo oxidation in a carbon dioxide environment, yielding activated quicklime. While the reductive decomposition of gypsum has been extensively documented, there exists a significant research gap concerning the reaction between CaS and CO2 at elevated temperatures. This study investigates the high-temperature reaction characteristics and reaction mechanism of the CaS and CO2 system through a combination of experiments, isothermal kinetic analysis, and density functional theory (DFT) calculations. Furthermore, it offers a comparative analysis of the physicochemical properties of quicklime produced via three different methods, using desulfurization gypsum as an illustrative example. It was observed that the reaction rate of CaS with CO2 markedly increased with rising temperature and CO2 concentration. The reaction of CaS in a CO2 atmosphere adhered to the phase boundary reaction model. DFT calculations were employed to determine the reaction pathway for the interaction between CaS and CO2, which can be summarized as follows: CaS + CO2 -> CaSO + CO, CaSO + CO2 -> CaSO2 + CO, CaSO2 + CO2 -> CaSO3 + CO, CaSO3 -> CaO + CO2. The lime generated through the two-step process of desulfurization gypsum (FGD) exhibited a superior pore structure and reactivity when compared to lime produced via the one-step process of FGD gypsum and that obtained through limestone calcination.