• Crystal growth rate coefficient for thiourea is in the range of 10^-7 to 10^-8 m·s^-1, with a transition from diffusion-controlled to surface reaction-controlled as temperature decreases.
• Temperature primarily affects the surface reaction step, with minimal impact on diffusion, leading to a two-stage growth mechanism with a critical temperature around 25 °C.
• Process optimization via adjusting agitation (higher at high temperatures, lower at low temperatures) significantly improves CSD, yielding 84% of crystals in the 0.7-0.9 mm range.
• The study offers a practical strategy to reduce fine crystals and improve product quality in industrial thiourea crystallization.