Thiourea crystal growth kinetics, mechanism and process optimization during cooling crystallization

In the cooling crystallization process of thiourea, a significant issue is the excessively wide crystal size distribution（CSD） and the abundance of fine crystals. This investigation delves into the growth kinetics and mechanisms governing thiourea crystals during the cooling crystallization process. The fitting results indicate that the crystal growth rate coefficient, falls within the range of 10-7 to 10-8 m·s-1.Moreover, with decreasing crystallization temperature, the growth process undergoes a transition from diffusion-controlled to surface reaction-controlled, with temperature primarily influencing the surface reaction process and having a limited impact on the diffusion process. Comparing the crystal growth rate, and the diffusion-limited growth rate, at different temperatures, it is observed that the crystal growth process can be broadly divided into two stages. At temperatures above 25℃, 1/qd（qd is diffusion control index） approaches 1, indicating the predominance of diffusion control. Conversely, at temperatures below 25℃, 1/qd increases rapidly, signifying the dominance of surface reaction control.To address these findings, process optimization was conducted. During the high-temperature phase（35-25℃）, agitation was increased to reduce the limitations posed by bulk-phase diffusion in the crystallization process. In the low-temperature phase（25-15℃）, agitation was reduced to minimize crystal breakage. The optimized process resulted in a thiourea crystal product with a particle size distribution predominantly ranging from 0.7 to 0.9 mm, accounting for 84% of the total. This study provides valuable insights into resolving the issue of excessive fine crystals in the thiourea crystallization process.