Product characteristics in simultaneous crystallization of NaCl and CaSO4 from aqueous solution with seeding
G. P. Zago (Universidade de São Paulo)
F. Marques Penha (TU Delft - Complex Fluid Processing, TU Delft - Intensified Reaction and Separation Systems, Universidade de São Paulo)
M. M. Seckler (Universidade de São Paulo)
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Abstract
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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