Song Zhou is a PhD student in Monash University, and a member of Clean Solid Fuel Laboratory (CSFL). His present research interest is mainly chloride waste treatment and atomic structure analysis based on X-ray adsorption fine spectroscopy (XAFS) analysis.          


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 lab-scale horizontal furnace at 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 favour 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 polarisation between Ca2+ and adjacent anions further enhanced the breakage of the Ca-Cl bonds.