(626e) Processing Techniques for Improved Batch Cooling Crystallization of RDX

RDX particle microstructure plays a major role in the shock sensitivity of RDX explosive formulations. Poor crystal quality of RDX characterized by crystals with high proportions of defects, results in an increased potential for hot spot formation and high sensitiveness that are responsible for initiation of detonation. In recent years there is a strong demand for improved quality of RDX which includes crystals with less structural defects (internal voids, inclusions, cracks), uniform size distribution, less surface roughness, and better purity. Most of these properties can be controlled by changing the processing conditions during crystallization.

An experimental study to improve the crystal quality of RDX was carried out using a Mettler Toledo RC1 reaction calorimeter coupled with in-situ FTIR spectroscopy. In this work several techniques to control or eliminate the nucleation of RDX during cooling crystallization were explored. The present work demonstrates several useful methods that can be applied to control nucleation and crystal growth kinetics for achieving the desired product.

Techniques aimed at improving crystallization must necessarily address nucleation and crystal growth kinetics. Nucleation is the onset of precipitation that occurs once an adequately high level of supersaturation has been reached. Stable nuclei consequently grow to full size crystals. The product size distribution is thus dependent on the nucleation. There is also evidence that the crystal habit may also be impacted by the early stage precipitation during and shortly after nucleation.

The influence of ultrasound on the crystallization of RDX was investigated. Ultrasonic power was found to significantly lower the level of supersaturation necessary for nucleation, thereby allowing a significantly better control over the onset of nucleation. It was found that the average crystal size decreases with an increase of ultrasonic power. It was also found that the presence of ultrasound during the nucleation stage influenced the crystal morphology.

Another technique employed was the usage of external seeding. Seeding eliminates spontaneous nucleation, thereby enabling better control of the product size distribution, which is dependent on the amount of loaded seeds, a quantity fairly easy to control. Seeds can be added at low supersaturation levels, therefore spontaneous nucleation can be avoided.

Upon the onset of crystal growth phase, which follows either nucleation or seeding, the ability to control the supersaturation level during this phase becomes critical since spontaneous nucleation can occur, adversely impacting the particle size distribution. With cooling crystallization, the cooling rate must therefore be controlled, preferably to maintain a constant level of supersaturation such that the rate of spontaneous nucleation is not significant. Rapid linear cooling profiles produce a supersaturation peak in the early stages of the process that result in uncontrolled nucleation. A non-linear cooling profile is thus required. For the case of RDX this was obtained by measuring the crystal growth kinetics as a function of supersaturation and applying the kinetic data to an empirical crystal growth model.

RDX crystal growth kinetics during batch cooling crystallization experiments with seeding were obtained from the analysis of desupersaturation curves measured by in situ FTIR spectroscopy. RDX crystal growth rate was calculated using constant and time dependent total surface area approaches. Using mathematical calculations, the crystal growth rate coefficient and growth order were determined from existing empirical expression for crystal growth. Knowledge of these kinetic parameters allows the calculation of crystal growth rate at different supersaturation levels. The optimal cooling profile that maintains a constant low level of supersaturation within the metastable limit was determined from the numerical solution of the supersaturation balance equation.