Membrane-cooling coupled crystallization with zero solvent discharge for spherical HMX: Process design, growth kinetics and mechanism

The simultaneous achievement of crystal characteristics control and environmentally benign is challenging the formulation processing of energetic materials (EMs). Herein, a membrane-cooling coupled crystallization (MCr-CCr) with complete solvent recovery is demonstrated for the preparation of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) crystals with high sphericity and controlled sizes. The permeate flow rate, cooling rate and feed flow rate are found to be the key process parameters in MCr-CCr that determine the supersaturation, seeds number and population density, which collectively dictate the morphology and sizes of the crystalline products. The L 9 (3 4 ) orthogonal experiment indicates that the influential order of the aforementioned three parameters varies with the evaluation indicators (mean size and coefficient of variation). The growth kinetics and morphological evolution analysis suggest that the formation of spherical HMX requires a stable supersaturation ratio (c.a. 1.1) near the lower limit of the metastable zone. Its growth mechanism follows the agglomeration-dissolution mechanism. The agglomeration is responsible for enlarging the crystals, and the dissolution/abrasion shapes the crystals into a sphere. The obtained spherical HMX has high purity (greater than 99.5%), circularity (87.9%) and bulk density (1.17 g·cm −3 ). The proposed MCr-CCr process provides an efficient and sustainable crystallization route for EMs.