Crystal Creeping for Spatial Gradient Partitioning: A New Pre-Enrichment Strategy Assisting Multicomponent Crystallization Separation

Abstract

Abstract Based on the significant difference in interfacial creeping behavior between ammonium chloride (NH4Cl) and hexamethylenetetramine (C6H12N4, HMTA), this work proposes a spatial gradient-based pre-enrichment strategy for multicomponent mixtures. Unlike conventional sequential fractional crystallization, which relies on precise temporal control of process conditions to induce successive precipitation of different solutes, our approach enables preferential accumulation of distinct components in different regions of a substrate during a single evaporation step, thereby generating a compositional gradient. Using the NH4Cl−C6H12N4 binary system derived from glycine production mother liquor as a model, experiments demonstrate that crystal creeping produces concentric ring patterns with clear spatial differentiation: the inner region is enriched in NH4Cl, while the outer creeping zone is relatively enriched in C6H12N4. A single creeping cycle achieves only pre-enrichment rather than complete separation; however, repeated cycles progressively improve purity, raising the NH4Cl content from 80.11% to 92.87%. This gradient-partitioning approach is mild in conditions and simple to operate, offering a preliminary enrichment strategy for multicomponent mixed salt systems.