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.