(487e) Experimental Study of Wet Milling in a Rotor-Stator Mixer
Based on the literature, it was reasoned that along with size and shape, stress intensity factor (or fracture toughness) and hardness are the important physical properties affecting breakage rate. To vary these properties, several crystalline materials were milled in antisolvents (e.g. sugar in isopropyl alcohol) to allow discrimination between mechanical size reduction and solubility effects. Collision rates were varied by changing particle concentration; and coatings and small geometric changes were used to consider interactions with solid surfaces. Rotor speed and volumetric throughput were adjusted to independently vary energy input and mill head residence time. Crystal hardness and stress intensity factor were measured with a Hysitron TriboIndenter. Particle size distributions were measured by several methods. A Lasentec focused beam reflectance measurement (FBRM) probe and Particle View Microscope (PVM) were placed in the holding tank to continuously monitor particle size and suspension uniformity. Grab samples, acquired in the holding tank at well-defined time intervals, were analysed using a Horiba laser diffraction instrument and by microscopy with an automated image analysis technique. The results from the various measurement techniques will be compared and discussed. Mechanistic models that account for particle properties and milling conditions will be presented for the various fracture mechanisms. The mechanistically correlated experimental results will be discussed in detail, as well as application to process scale up. One intended use of the correlations is as mechanistic inputs (breakage kernels) to population balance models.