Gypsum crystallization and hydrochloric acid regeneration by reaction of calcium chloride solution with sulfuric acid

In this work, the crystallization of gypsum (CaSO4·2H2O) in the temperature range of 20–60 °C is investigated along with the simultaneous regeneration of HCl by the reaction of concentrated CaCl2 solution (3–3.5 M) with H2SO4 at a concentration range of 2.6–8.0 M. Fast addition of H2SO4 proved counter productive as it yielded a gel-like fine crystal suspension. Slower addition via titration or in multiple steps over 2 to 4 h followed by 2 h of equilibration proved successful in producing well developed columnar gypsum crystals of ~ 50+ µm long and 5+ µm thick.It was determined that for the purpose of producing high strength HCl (4 M), with minimum soluble CaSO4 (5 g/L at 40 °C), 8.0 M H2SO4 has to be used at a molar ratio (SO4/Ca) = 0.8. Upon the elevation of temperature or by using seed and recycling, crystal growth was promoted yielding crystals as large as 100+ µm in length and 20+ µm in thickness. Crystallization tests in the presence of several metal chlorides showed that metal impurity uptake is not a problem except for lead, which was present at 95 ppm (or mg/kg). Filtration rates higher than 4000 kg of solids deposited m− 2 h− 1 were obtained.