Revealing the Mechanism\nof Ultrasound-Assisted Spherical\nAgglomeration for Achieving Millimeter-Sized Agglomerates of 3‑Nitro-1,2,4-triazol-5-one\nwith High Sphericity and Tunable Sizes

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