In this work we developed an approach for the simultaneous crystallization of NaCl and KCl from their aqueous solution by concomitantly seeding both salts. Experiments were designed based on the ternary system phase diagram. The mother liquor concentration was chosen near the eutonic condition at its boiling temperature. The seeds for each compound had different sizes but the amounts were calculated to provide nearly equal surface areas for each material. Elementary phenomena were assessed and the chemical composition in different particle size ranges within the products was analysed. Despite the presence of seeds from each component in the suspension under crystallization, the formation of mixed composition polycrystalline particles was observed. A seeding protocol involving much larger KCl than NaCl seeds resulted in an easily separable product with two distinct populations. In general, segregation of the salts through the size ranges followed the trend induced by seeding, highlighting the possibility of retrieving relatively pure solid fractions of each salt.@en

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.

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