Phase- and morphology-controlled crystallization of gypsum by using flue-gas-desulfurization gypsum solid waste

In this work, we studied the phase- and morphology-controlled crystallization of gypsum by using flue-gas-desulfurization (FGD) gypsum source materials in HCl–H2O solution system at atmospheric pressure. The influence of the crystallization regulator sodium dodecyl sulfate (SDS) on the conversion process was examined. The results indicate that oxide impurities in FGD gypsum are removed by the treatment in this designed solution-phase system. Without the addition of SDS surfactant, calcium sulfate hemihydrate with fiber morphology is obtained and as the addition amount of SDS increases from 0.6 wt% to 5.4 wt%, the prepared sample transforms from column-like calcium sulfate hemihydrate to layer-structured calcium sulfate anhydrite. It is proposed that SDS selectively adsorbs on top (204) facet at a low SDS concentration (0.6 wt%), resulting in the decrease of aspect ratio compared with fiber-like calcium sulfate hemihydrate (without SDS addition). Layered SDS micelles are formed when 5.4 wt% SDS is added, and the soft template function, as well as the hydrophobic interaction of alkyl group in the surfactant, generates the layer-structured calcium sulfate anhydrite. The study provides a facile method for treating solid waste FGD gypsum.