Study on the crystallization of ammonium sulfate in the presence of additives

Abstract

Large-particle ammonium sulfate crystals possess higher commercial value due to their superior physicochemical properties; however, their industrial-scale production remains challenging. Initially, the effects of various operational factors on the crystal size distribution (CSD) of ammonium sulfate in an additive-free system were systematically investigated, and the basic process parameters were determined. Subsequently, two optimized crystallization processes were developed by incorporating composite additives. The results show that the two target systems (constant-rate and deceleration processes) significantly enhance the yield of large crystals, increasing the mass fraction of particles ≥2.0 mm to above 27.00%, compared to less than 5.00% in the conventional process. Crystallinity is also improved from 80.06% in the conventional system to over 87.00%, with a further reduction in crystal aspect ratio. Further crystallization thermodynamics and nucleation kinetics analyses revealed the action mechanism of additives: the additives increase the solid–liquid interfacial energy and widen the metastable zone width (MSZW) of the solution, which inhibits nucleation and promotes crystal growth, thereby providing favorable kinetics for the formation of larger crystals. Moreover, the additives help guide solute deposition in a more ordered manner into the crystal lattice, thus increasing crystallinity. Additionally, the adsorption of additives at active sites on crystal surfaces modifies the relative growth rates of different crystal faces, leading to regulated crystal morphology.