Abstract: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.

Key words: crystallization; gypsum; KCl regeneration; calcium removal