Thiourea Nucleation Behavior Investigation Based on Metastable Zone Width and Applied for Continuous Crystallization Designing

Batch crystallization of thiourea has been industrially employed for decades. However, studies of the continuous crystallization and nucleation mechanism of industrial thiourea, especially in the presence of synthesis-related impurities, remain scarce. Herein, the nucleation mechanism of industrial thiourea was investigated under varying conditions. Results revealed that the nucleation constant decreased with increasing saturation temperature, signifying a progressive acceleration in the nucleation rate. Notably, when the saturation temperature surpassed 301 K, the nucleation mechanism shifted from progressive to instantaneous. Furthermore, both the critical nucleation size and critical Gibbs free energy decreased as the saturation temperature rose. Leveraging these insights, a continuous crystallization apparatus was designed, which was experimentally validated to maintain operational stability and achieved a target product purity of 79 wt %, compared to 40 wt % from batch crystallization. These findings offer significant implications for optimizing the crystallization process of industrial thiourea.