Recovery of Gaseous HCL from Primarily Magnesium Chloride Solutions

Abstract

Due to its many benefits over conventional H2SO4 leaching, the use of HCl for leaching has been increasing in many flowsheets, particularly for laterites and rare earth elements ores. However, HCl is more expensive than H2SO4 and an inexpensive method of regenerating it must be developed to ensure its economic viability in these flowsheets. This thesis presents a robust process which operates at low temperatures and atmospheric pressure thereby reducing operating costs. The addition of H2SO4 to MgCl2 solutions in the presence of air sparging is shown here to produce external HCl recoveries up to 64.6%. The effects of temperatures between 75 °C and 100 °C, airflow between 200 and 600 mL/min, stoichiometric H2SO4 addition between 100% and 300%, and initial chloride concentration between 4 and 10 M were investigated. Increasing chloride concentration and H2SO4 addition were found to be beneficial to HCl recovery, consistent with Le Chatelier’s Principle. The positive impact of increasing temperature and airflow were determined to be due to higher temperatures and increased airflows favouring the volatilization of HCl. Additionally, the impact of the incorporation of iron, calcium, sodium, and a combination of these ions were studied. The presence of calcium and sodium were found to be detrimental and iron beneficial to total HCl recoveries. After initial batch experiments were conducted, the process was scaled-up to a two-stage continuous mode operation which yielded a maximum HCl recovery of 56.4%. While overall recoveries were found to decrease in continuous operation, the temperature, airflow, H2SO4 addition, and initial chloride concentration trends were found to be consistent with the batch experiments. The inclusion of iron had a greater positive impact in the continuous experiments and sodium was less detrimental to the system. The resulting liquid and solid samples were analyzed using inductively coupled plasma, atomic absorption spectroscopy, x-ray powder diffraction, carbon-sulphur analysis, and thermogravimetric analysis as appropriate to examine the mass balance and precipitate formed. The precipitate was found to be MgSO4.2H2O, CaSO4.2H2O, and FeSO4.7H2O which will impact further treatment.