Sustainable recovery of ammonium sulfate from saline wastewater using eutectic freeze crystallization: Crystallization kinetics and thermodynamics

Abstract

This work explored the application of eutectic freeze crystallization (EFC) process as a sustainable desalination approach for recovery of ammonium sulfate (AS) salt from highly saline solution. Batch mode EFC experiments were conducted using jacketed crystallizer to investigate the influence of AS feed concentration (35–42 wt%) and agitation speed (100–400 rpm) on the AS crystallization kinetics and thermodynamic behaviour. Higher AS concentration increased the supersaturation level which enhanced both the nucleation and growth rates. Prolonged freezing duration resulted in solute depletion and a shift to growth-dominated kinetics. Increased agitation speed improved the mass and heat transfer which accelerated crystal formation. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis confirmed the formation of high-purity orthorhombic AS crystals with larger crystal size achieved at higher feed concentration and longer freezing time. Energy analysis showed that the EFC process required 0.4826 kWh/kg AS recovered, representing approximately 75% lower energy consumption as compared to the conventional evaporative crystallization process. Overall, this study demonstrated that effective control of the EFC operating conditions improved the separation efficiency and product quality, highlighting the viability of EFC process for achieving both water reclamation and resource recovery which aligned with circular economy and zero liquid discharge strategies.