Comparison study of mixing effect on batch cooling crystallization of 3-nitro-1,2,4-triazol-5-one (NTO) using mechanical stirrer and ultrasound irradiation

The insensitive explosive 3-Nitro-1,2,4-trialzol-5-one (NTO) has been recrystallized from water in an effort to prepare crystals with smaller size and narrower distribution in a batch cooling crystallizer. Two mixing devices, i.e., a mechanically stirred system with and without ultrasound in aqueous media were employed to compare the mixing effect on the crystallization. Under ultrasound irradiation, the metastable zone width was significantly reduced by more than 2 fold and the crystal size was shifted from 140∼160 μm to 50∼70 μm with a narrower CSD compared to the mechanically stirred system. However in the mechanical stirrer, the mixing effect on NTO crystallization was negligible if the impeller speed was sufficient to suspend all crystals in the crystallizer. It was found that the crystal growth was not influenced by mixing. We suggest that the NTO crystals were formed by primary heterogeneous nucleation that is common in batch cooling system. Finally, the population balance model (PBM), with the empirical nucleation and growth kinetic expressions, was solved numerically to predict the crystal size and the CSD with batch time, and the results were in good agreement with the experimental data.