Revealing the Mechanism of Ultrasound-Assisted Spherical Agglomeration for Achieving Millimeter-Sized Agglomerates of 3-Nitro-1,2,4-triazol-5-one with High Sphericity and Tunable Sizes

Spherical agglomeration is a particle enlargement technique that employs an immiscible bridging liquid to transform crystals into round compact agglomerates during the crystallization process, which offers advantages in enhancing the processing efficiency and powder properties. This work innovatively introduces an ultrasonic strengthening process in the spherical agglomeration, resulting in the formation of millimeter-sized agglomerates of 3-nitro-1,2,4-triazol-5-one (NTO) with high sphericity and tunable sizes, offering a noticeable advancement over conventional agglomeration methods. Toluene was determined as the optimal bridging liquid by calculating the isosteric adsorption heat between the solvents and crystal surfaces, which was confirmed by experimental validation. The study demonstrates the synergistic effects of combining ultrasound with toluene, which not only enhances the compactness and size control of the agglomerates but also contributes to the uniformity and repeatability of the agglomeration process. In addition, the effects of various factors, including the volume ratio of bridging liquid and solute (BSR), ultrasound duration, solvent/antisolvent ratio, and stirring speed, were systematically investigated to determine optimal operating parameters. The optimized operation conditions could give compact agglomerates with controlled sizes ranging from hundreds to thousands of micrometers and sphericity exceeding 90%. The mechanism study revealed that the strong wettability of toluene on the crystals, combined with the cavitation and mechanical effects induced by ultrasound, not only improved the initial crystal quality but also enhanced the liquid bridge force between toluene and crystals. This resulted in changes in the rheological behavior of the liquid/NTO mixture, facilitating deformation and agglomeration during shear agitation. The agglomerates exhibited identical crystal structure and composition to normal NTO, while the angle of repose tests showed an improvement in their flowability. This improvement in powder properties could have implications for the material’s handling, storage, and application in industries.