Kinetics of silica-assisted pyro-hydrolysis of CaCl2 waste for the recovery of hydrochloric acid (HCl): Effect of interaction between CaCl2 and SiO2

Pyro-hydrolysis of CaCl2 waste for HCl acid regeneration is a circular process enabling the sustainable development of the critical energy mineral extraction industry. However, the lack of detailed kinetic information has posed challenges in implementing this technology. An acidic additive such as silica (i.e., SiO2) is essential to make this reaction thermodynamically available, yet its reaction kinetics are still unknown. This study aims to reveal the detailed reaction kinetics of the SiO2-assisted pyro-hydrolysis of CaCl2 under two typical industrial settings for the feeding of reactants, physical mixing of powders and spray feeding where CaCl2 solution is expected to initially undertake drying and subsequently deposit onto the solid silica powders for a close contact. The isothermal experiment results revealed that the HCl release rate is strongly temperature-dependent. Comparatively, the CaCl2-deposited silica demonstrated a significantly faster release of HCl than the physically mixed sample, achieving 92% HCl release within 30 min at 800 degrees C. SEM analysis confirmed a closer interparticle contact for the CaCl2-deposited sample, which is responsive to the accelerated HCl release. In addition, the experimental results were successfully fitted by a variable activation energy model which discovered a steady increase of the activation energy with the progression of the HCl release for both feedstock modes. This is primarily due to the continuous change on the formation of intermediates. The model also confirmed a lower activation energy for the CaCl2-deposited sample than its physically mixed counterpart at any given conversion extent of CaCl2. In contrast, for the physically mixed sample, the loose inter-particle contact led to the formation of a stable intermediate, Ca(OH)Cl, which can readily reverse to CaCl2, thereby slowing down the overall HCl release rate.