A noval salt effect coupled vacuum distillation process for high efficient ultra-azeotropic reuse of spent HCl

The escalating challenge of recycling and treating spent hydrochloric acid (HCl) in various industries, driven by stringent environmental regulations, necessitates urgent solutions for the reuse of spent HCl in industrial processes. However, the concentration and reuse of spent HCl have been impeded by azeotropism. In this study, an innovative salt effect coupled vacuum distillation process was introduced to enable the rapid and convenient recovery of HCl. Salts and vacuum environment both can be found to break the azeotropism effectively, augment the disparity in the dissociation abilities of HCl and H2O in clusters, and enhance the evaporation rate. Specifically, MgCl2 and CaCl2 salt systems were selected, achieving recovered HCl concentrations of 9.66 and 9.85 mol/L, with corresponding recovery rates of 86.45 % and 80.65 %, respectively. For the reuse of spent HCl from chlorinated distillation for extracting germanium (Ge), the presence of As(V) poses a risk to Ge recovery and potential contamination of the Ge product. Excellent resistance to As(Ⅴ) mobilization (<2%) was obtained in the CaCl2 system. To comprehensively understand the salt’s role in breaking azeotropism, molecular simulations were employed. The Mg2+/Ca2+ ions were observed to bind with H2O molecules in the solution, weakening the force between H2O and HCl, decreasing the dissociation energy of HCl molecules by 35 times (from 102.82 to 2.94 kJ/mol), and increasing the dissociation energy of H2O molecules from 52.5 to 61.5 kJ/mol. The proposed process integrates high HCl recovery with an ultra-azeotropic concentrating effect, offering a novel approach for reusing spent HCl in industrial applications.