Evaluation and Analysis of Trace Impurity Migration Pathways on the Design of Highly Efficient Solution Crystallization-Based Purification Process

Abstract

Separation and purification of fine chemicals with minor or trace amounts of impurity, for instance, metal ions, to produce high-end and high-pure chemicals has been of utmost importance in recent years. Understanding the key factors and impurity migration pathways is critical to designing a highly efficient crystallization-based purification process. This study combines theoretical models with experiments to analyze the impurity migration mechanism and obtain the key factors of purification for the optimization of the solution crystallization process, using a model compound ferrous sulfate. The migration and deposition of trace amounts of Mg 2+ impurity within ferrous sulfate heptahydrate crystals were examined by a general workflow, where the liquid inclusion was found to be the dominant impurity incorporation mechanism. Given that the liquid inclusions are strongly influenced by crystallization kinetics, we quantified the effects of the supersaturation ratio and temperature on inclusion formation using optical microscopy and ICP analysis. The results indicated that higher temperatures and a greater supersaturation ratio increase growth rates and promote the formation of mother liquor inclusions within the crystals, leading to poor impurity removal efficiency. Furthermore, based on the crystal growth kinetics, we found that with the increase of supersaturation ratio, the growth mechanism of ferrous sulfate heptahydrate changes, leading to a significant increase in distribution coefficient and poor separation effect. Finally, the formation mechanism and process design to reduce liquid inclusion have been discussed.