Silica-assisted pyro-hydrolysis of CaCl 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 CaCl/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 CaCl with and without SiO in the temperature ranges of 400-900 °C. It was determined that pyro-hydrolysis of CaCl alone generated a maximum of 12% HCl by decomposition into Ca(OH)Cl, which is a stable intermediate that can be reverted to CaCl at 800 °C. Upon the addition of SiO at an equimolar ratio to CaCl, the HCl release extent was accelerated to 50% at 900 °C. Both experiments and DFT calculations prove that the added SiO can promote the dissociation of water molecules which provides hydroxyl ions that enable the conversion of CaCl into Ca(OH)Cl at low temperatures. The resulting Ca(OH)Cl can also quickly react with SiO to form Cl-bearing silicates such as CaSiOCl and CaSiOCl with weakened CaCl bond that are relatively easy to cleave into Cl-free CaSiO and HCl from 800 °C.