Enhanced reduction of smelting gypsum by H2S and in-situ DRIFTs investigated on the mechanisms of S-O bond activation

Abstract

The gypsum from flue gas desulfurization or the neutralization (D/N gypsum) process of acid wastewater, is an industrial by-product in nonferrous metal smelters. The S and Ca sources of D/N gypsum have significant application prospects. However, the S(IV) reduction is difficult due to the electronic inactivity of CaSO4 and the stability of the S-O band. Our experimental results indicated that the start-up temperature of CaSO4 reduced by CH4 is 900 ℃. However, adding H2S gas with a H2S/CaSO4(S/Ca) ratio of 0.2, the start-up temperature of CaSO4 reacted with CH4 decreases to 650 ℃. With the increase of the S/Ca ratio from 0.04 to 0.2, the yield of CaS increased to 94.97%, and the selectivity toward CO2 formation increases to 71.67%. In-situ diffuse reflectance Fourier transformations spectroscopy (in-situ DRIFTs) results indicate that adding H2S generates the new sulfide groups on the surface of CaSO4, resulted from the S(-II) in H2S changing the distribution of valence electrons of O and activating O. Moreover, the density functional theory (DFT) calculation results exhibit that the activation energy for the first O dissociation decreases to 2.89 eV in the presence of H2S. Furthermore, H2S optimized the reaction path of CH4 and improved the selectivity toward CO2 formation. Comparing to 1000 ℃ without H2S, under 800 ℃ with H2S, the consumption of CH4 reduces to 45%, and the CO2 selectivity increases to 71.67%. Thus, we have developed a method of CaSO4 low-temperature reduction and provide a theoretical basis for its application in engineering.