Gypsum crystallization and potassium chloride regeneration by reaction of calcium chloride solution with potassium sulfate solution or solid

Gypsum crystallization along with the simultaneous regeneration of KCl was investigated by the reaction of CaCl2 solution with K2SO4. Well developed sheet structure gypsum crystals were produced when K2SO4 solution was added into the CaCl2 solution by slow titration or in multiple stages over 2–8 h followed by 2 h equilibration. In order to regenerate KCl solution as concentrated as possible, K2SO4 solid was added into the given CaCl2 solution instead of K2SO4 solution, obtaining gypsum crystals with almost the same quality by multistage addition with [SO42–]/[Ca2+] molar ratio no larger than 0.8. However, impurity of K2SO4·CaSO4·H2O was detected by XRD and was further confirmed by SEM-EDS in the produced crystals when the [SO42–]/[Ca2+] ratio increased to 1.1. It is proved that appearance of the double sulfate is attributed to the relatively high concentration of K2SO4. So, it is essential to properly control the [SO42–]/[Ca2+] ratio and make sure [Ca2+] in excess to suppress the solubility of CaSO4 even at the expense of low calcium removal rate.