Silica-assisted pyro-hydrolysis of CaCl2 waste for the recovery of hydrochloric acid (HCl): Reaction pathways with the evolution of Ca(OH)Cl intermediate by experimental investigation and DFT modelling

The chlorine evolution mechanism remains unclear during the thermal treatment of CaCl2/Ca(OH)Cl-containing solid waste. In this paper, we have conducted both experimental investigation and density functional theory (DFT) calculation to elucidate the mechanism of pyro-hydrolysis of CaCl2 with and without SiO2 in the temperature ranges of 400–900 °C. It was determined that pyro-hydrolysis of CaCl2 alone generated a maximum of 12% HCl by decomposition into Ca(OH)Cl, which is a stable intermediate that can be reverted to CaCl2 at 800 °C. Upon the addition of SiO2 at an equimolar ratio to CaCl2, the HCl release extent was accelerated to 50% at 900 °C. Both experiments and DFT calculations prove that the added SiO2 can promote the dissociation of water molecules which provides hydroxyl ions that enable the conversion of CaCl2 into Ca(OH)Cl at low temperatures. The resulting Ca(OH)Cl can also quickly react with SiO2 to form Cl-bearing silicates such as Ca2SiO3Cl2 and Ca3SiO4Cl2 with weakened CaCl bond that are relatively easy to cleave into Cl-free CaSiO3 and HCl(g) from 800 °C.