Nanotechnology applications in the pharmaceutical, materials, and chemical industries has renewed interest in the use of wet grinding in stirred media mills for the production of nanoparticles. However, challenges arise in the production of sub-micron particles that are, in part, due to colloidal surface forces influencing slurry stability and rheology. As often observed in the literature, a grinding limit in the range of 0.5 μm is reached despite high energy inputs and aggressive milling conditions. Furthermore, the product size can even increase with increased energy input, a seemingly counterintuitive result that may be attributed to aggregation of fine particles during the comminution process. In this work we postulate that colloidal stability and rheology must be considered in wet grinding to understand these results and to surmount limitations on the production of nano-sized particles. 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 a model stirred media mill are explored to study the effects of colloidal stability on the ag- gregation 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 obtained from small angle neutron scattering (SANS) experiments both at rest and under flow. Theoretical pre- dictions based on independently measured particle 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 by tailoring colloidal interac- tions and the milling conditions. Furthermore it is shown that the concept of electrostatic stabilization during wet grinding of nanoparticles can also be applied to the system of tin oxide. It is shown that in contrast to alumina no mechanochemical changes occur for the system of tin oxide during the wet grinding process. Thus the obtained median particle sizes are the result of pure mechanical grinding. In addition the suspension rheology in a stirred media mill as function of grinding time and inter particle interactions is studied.
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 nanogrinding in stirred media milling.