The increasing industrial demand for nanoparticles challanges the application of stirred media mills to grinding in the sub-micron size range. It was shown recently [1] that the grinding behavior of particles in the sub-micron size range in stirred media mills and the minimum achievable particle size is strongly influenced by the suspension stability and thus the agglomeration behavior of the suspension. Therefore, an appropriate modeling of the process must include a superposition of the two opposing processes in the mill i.e. breakage and agglomeration which can be done by means of population balance models. Modeling must now include the influence of colloidal surface forces and hydrodynamic forces on particle aggregation and breakup.
In this report the modeling of the sub-micron grinding is done by a superpo- sition of the population balance models for agglomeration and grinding with the appropriate kernels. This leads to a system of partial differential equations, which can be solved in various ways numerically. Here a modified h-p Galerkin algorithm which is implemented in the commercially available software package PARSIVAL developed by CiT (CiT GmbH, Rastede, Germany) and the moment methodology according to Diemer [2], [3] are used and compared. This includes a comparison of the derived particle size distributions, moments and its accuracy depending on the starting particle size distribution and the used agglomeration and breakage ker- nels. Finally, the computational effort of both methods in comparison to the prior mentioned parameters is evaluated in terms of practical application.
Furthermore the fundamental work on the agglomeration process and its mecha- nism described in the IFPRI report 2002 was continued and improved. Experiments are performed on a well-characterized, model system of monodisperse primary nanoparticles that are destabilized and aggregated under various milling conditions. Conditions spanning Brownian to turbulent collision aggregation in model stirred media mills are explored to study the effects of colloidal stability on the aggregation process. The agglomeration kinetics are measured using dynamic light scattering (DLS) as a function of particle and electrolyte concentrations. Further information on the agglomeration process and the structure of the agglomerates are also ob- tained from small angle neutron scattering (SANS) and rheo-optical light scattering experiments (ROA). Theoretical predictions based on independently measured par- ticle and solution properties as well as mill characteristics are compared against the experimental results to demonstrate that particle aggregation kinetics in a stirred media mill can be controlled through the colloidal interactions and the milling con- ditions. This research provides a theoretical basis for understanding stirred media milling of nanoparticle slurries and as such, is a step towards a predictive model of sub-micron stirred media milling.
Finally the paper reports about mechanochemical changes during wet grinding of alumina in stirred media mills. The results show the formation of alumina hydroxide that can be analysed by means of X-ray diffraction analysis, thermogravimetry and dynamic scanning calorimetry. The formation of hydroxide is strongly dependent on pH value which influences the grinding mechanism in the nanometer size range.