(85d) Comprehensive Characterization of Stressing in Mills By Particle Probes

In industry mills are the working horse for particle size reduction. However, a clear picture and a detailed understanding of the mechanisms involved in stressing and particle breakage are far from being complete. During comminution distributions of stresses are applied. Both, the type of stressing (one sided or two sided, loading by compression or impact) and the acting stress energies are distributed. So called mill functions link the acting stresses and their frequency to the used apparatus and its operational conditions. So far the aforesaid functions are widely unknown. Within this contribution a recently proposed novel approach based on stressing of fully characterized single particle probes is applied to wet comminution in stirred media mills [1]. Our approach relies on the fact that metallic and oxide probe particles deformed by single particle compression in a scanning electron microscope [2] exhibit the same morphology than particles recovered from stirred media milling, i.e. stressing in the technical apparatus can be mimicked by uniaxial compression [3]. By finite element modelling of the uniaxial compression and image analysis of geometry changes for individual particles (c.f. ratio of waist diameter to initial particle size) the energy absorption of the particle probes can be described. To access the stress number the formation of contacts upon stressing is treated by formal kinetics. Thus, the stressing history in terms of stress energy and stress number can be determined experimentally and stress energy and the stress number distributions in the mill become accessible. For the lab-scale stirred media mill operated at exemplary process conditions it will be shown that the involved stress energy and stress number distributions are broad and depend on grinding bead size, particle size and material. Furthermore, it will be shown that the grinding kinetics in the submicron size range is predominantly determined by particle capture. Our methodology is not limited to wet comminution and applicability to other types of mills will be shown. In summary, the proposed methodology will be useful for the understanding of stressing in technical mills, the development of models, the design of mills and ultimately to the optimization of comminution processes.

[1] A. Strobel, S. Romeis, S. Wittpahl, P. Herre, J. Schmidt, W. Peukert, Characterization of stressing conditions in mills – A comprehensive research strategy based on well-characterized model particles, Powder Technol. 305 (2017) 652–661.

[2] S. Romeis, J. Paul, M. Ziener, W. Peukert, A novel apparatus for in situ compression of submicron structures and particles in a high resolution SEM, Rev Sci Instrum 83 (2012).

[3] J. Paul, S. Romeis, M. Mačković, V. Marthala, P. Herre, T. Przybilla, M. Hartmann et al., In situ cracking of silica beads in the SEM and TEM - effect of particle size on structure-property correlations, Powder Technol. 270 (2015) 337–347.