Pyrohydrolysis of CaCl₂ waste for the recovery of HCl acid upon the synergistic effects from MgCl₂ and silica

Abstract

In this paper, an efficient HCl acid recovery method from the mixture of alkaline-earth-metal chlorides waste was demonstrated via co-pyrohydrolysis in a laboratory-scale horizontal furnace in a temperature range of 700–1000 °C, and fixed additions of SiO₂ and steam. The synergistic effect of MgCl₂ on the HCl recovery from CaCl₂ was explored intensively. A double-sided effect was revealed. For the reaction temperatures below 1000 °C, the MgCl₂ addition delayed the HCl release through competing with CaCl₂ for the inclusion into silica matrix. In contrast, once the chloride mixtures were subjected to 1000 °C with a noticeable residence time (e.g., 2 h) and at a minimum molar ratio of 0.5 of MgCl₂ to CaCl₂, the MgCl₂ addition promoted the HCl release remarkably, via promoting the conversion of Ca₃(SiO₄)Cl₂ into Ca₈Mg(SiO₄)₄Cl₂. A portion of Mg²⁺ derived from the early decomposition of MgCl₂ substituted the Ca(I) site in Ca₃(SiO₄)Cl₂, thereby resulting in the formation of weak Mg–Cl bond that is in favor of the HCl release. Additionally, the remaining Mg²⁺ consumed the excessive SiO₂ so as to cause the skeleton of [SiO₄]⁴⁻ to be fully affiliated and balanced by cations to form Ca₈Mg(SiO₄)₄Cl₂ in which the weaker ionic polarization between Ca²⁺ and adjacent anions further enhanced the breakage of the Ca–Cl bonds.