Product characteristics in simultaneous crystallization of NaCl and CaSO₄ from aqueous solution with seeding

Zero liquid discharge strategies for industrial wastewater treatment have become prominent in recent years. When evaporative simultaneous crystallization is applied, knowledge about the particle crystallization mechanisms and morphology are important to ease solids downstream handling and for recovery of their valuable components. In this work, batch simultaneous crystallization of sodium chloride (NaCl) and calcium sulphate hemihydrate (CaSO₄.0.5H₂O) from aqueous solution was studied. It was found that CaSO₄.0.5H₂O is not an effective substrate for NaCl heterogeneous nucleation, but agglomerates with NaCl particles instead. CaSO₄.0.5H₂O particles on the surface of NaCl crystals sterically hamper agglomeration of NaCl particles with each other and, for a sufficiently high CaSO₄.0.5H₂O seed load, cover the NaCl crystals so that NaCl supersaturation rises, inducing NaCl primary nucleation. Simultaneous crystallization of CaSO₄.0.5H₂O and NaCl yields a satisfactory product for downstream handling even for high CaSO₄.0.5H₂O content in the crystallizer. However, seeding with CaSO₄.0.5H₂O is not recommended as it reduces the mean size and increases product size dispersion. NaCl seeding may be considered if it is desirable to separate CaSO₄.0.5H₂O from NaCl downstream the crystallizer by size classification, as it favors CaSO₄.0.5H₂O to build up in product sizes of 200 μm and below.