Synchrotron X-ray absorption spectroscopy study of the evolution of chlorine during the pyro-hydrolysis of calcium and magnesium chloride waste

Effective liberation of chlorine (Cl) as a recoverable product such as hydrochloric acid is crucial for the clean disposal of massive Cl-bearing industrial solid waste. This study aims to clarify the evolution of Cl upon the pyrohydrolysis of CaCl₂ waste. Particularly, the use of silica and MgCl₂ to promote the breakage of Ca-Cl bonds to release HCl gas has been investigated, via synchrotron X-ray absorption spectroscopy (XAS). As confirmed, in the presence of silica, the pyrohydrolysis of CaCl₂ commences from 800 °C, lower than the minimum temperature predicted based on the existing thermodynamic database. The attraction of Ca²⁺ by SiO₄⁴⁻ breaks the Ca-Cl bond successfully. The addition of Mg²⁺ can also improve the HCl regeneration extent to nearly 100%. Upon the addition of Mg²⁺, a structure of Ca-O-Mg-Cl is very likely to form, in which the second coordination shell of Ca²⁺ is occupied by both Cl⁻ and Mg²⁺. Consequently, the incorporated Mg²⁺ bonds with Cl⁻, “pushing” the Ca²⁺ in the third shell further away, leading to a distorted and less crystalline silicate matrix from which the liberation of Cl⁻ is easier. The Cl K-edge XANES shows that the reaction residues feature a unique, long-range multi-scattering phenomenon; this differs from the fully molten Cl-bearing glasses that bear a high similarity with CaCl₂.