Impact comminution in jet mills

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The objective of this project is the experimental analysis of two-phase flow in fluidized bed jet mills and the development of a related flow-sheet model for implementation in Dyssol. The fluidized be opposed jet mill is particularly interesting because three key unit operations, namely impact comminution, pneumatic transport and classification with the integrated impeller wheel classifier, are combined into one apparatus. In the first funding period, the stressing conditions experienced by the particles in the multi-phase jets were characterized in detail. For this purpose, different experimental set-ups were used to investigate the fluid mechanics in the multiphase jets. The particle velocity in both the subsonic and choked flow regimes was measured in a semicircle plant using Particle Image Velocimetry. In this way, the kinetic energy required for particle breakage was calculated. The solids volume concentration in the jet was measured via capacitance probes. Moreover, flow pattern could be analyzed noninvasive by x-ray computer tomography.Additional grinding kinetics have been determined in a commercial fluidized bed opposed jet mill. Based on this, a simple model for the grinding kinetics was established, which combines fluid mechanics and material specific breakage probabilities. Furthermore, a shortcut model for the comminution unit within the mill was developed. This model will be extended by further experiments for different process and geometric parameters. Main focus is on the two most important parameters: the particle size distribution and the holdup, which continuously change during dynamic processes. The experimental characterization of stress number and stress energy distributions has been realized in a stirred media mill for two-sided stressing by a novel method based on mechanically well-defined spherical metal particles, and will be transferred to one-sided stressing in the fluidized bed mill. Mechanically fully characterized metal particles will be stressed in the mill. Subsequent image analysis of the stressed and deformed particles allows the determination of the number of stress events and the absorbed energy. Finite element simulations in combination with SEM supported compression experiments can relate the shape of the compressed particles to the volume-specific energy absorption. Material functions for one-sided impact stressing will be measured by the newly built single particle impact apparatus. The breakage probability and breakage function of limestone, glass and alumina particles (< 40 µm) will be systematically measured.