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

Song Zhou, Binbin Qian, Tahereh Hosseini, Anthony De Girolamo, Lian Zhang

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4 Citations (Scopus)


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 SiO 2 and steam. The synergistic effect of MgCl 2 on the HCl recovery from CaCl 2 was explored intensively. A double-sided effect was revealed. For the reaction temperatures below 1000 °C, the MgCl 2 addition delayed the HCl release through competing with CaCl 2 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 MgCl 2 to CaCl 2 , the MgCl 2 addition promoted the HCl release remarkably, via promoting the conversion of Ca 3 (SiO 4 )Cl 2 into Ca 8 Mg(SiO 4 ) 4 Cl 2 . A portion of Mg 2+ derived from the early decomposition of MgCl 2 substituted the Ca(I) site in Ca 3 (SiO 4 )Cl 2 , thereby resulting in the formation of weak Mg-Cl bond that is in favor of the HCl release. Additionally, the remaining Mg 2+ consumed the excessive SiO 2 so as to cause the skeleton of [SiO 4 ] 4- to be fully affiliated and balanced by cations to form Ca 8 Mg(SiO 4 ) 4 Cl 2 in which the weaker ionic polarization between Ca 2+ and adjacent anions further enhanced the breakage of the Ca-Cl bonds.

Original languageEnglish
Pages (from-to)3349-3355
Number of pages7
JournalACS Sustainable Chemistry & Engineering
Issue number3
Publication statusPublished - 4 Feb 2019


  • Alkaline-earth-metal chlorides
  • Cl release
  • HCl regeneration
  • Pyrohydrolysis
  • Silica

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