The production of submicron and nanometer sized particles by wet grinding in stirred media mills was investigated. Small grinding media and, thus, small stress energies are required for an effective grinding, i.e. for a minimization of the specific energy. Moreover, the relations, particularly the so-called stress model, derived for grinding of coarser particles in stirred media mills are also valid for grinding of nanoparticles. However, for the production of alumina particles with sizes below approximately 200 nm an effective stabilization of the particles against agglomera-tion is necessary, especially, if yttrium-stabilised zirconium oxide grinding media are employed. An electrostatic stabilization with anorganic acids is effective if the surface charge is near the one of neutral chloride. This behaviour can be described by the so-called Hofmeister series. In case of organic acids the hydrocarbon chain should be as short as possible.
Beside a high grinding efficiency a low product contamination by grinding media wear is important. As known from grinding coarser particles grinding media wear at different operating conditions can be described by a so-called wear energy. More-over, the grinding media wear can be minimized by optimizing the suspension vis-cosity and, thus, by adjusting the pH-value: If the viscosity is too low, the collisions of the grinding media are not damped and, thus, the wear is high. If the viscosity is too high, high concentrations and packing of the grinding media in front of the separation device occur causing high wear of the grinding media. Besides optimiz-ing the viscosity the product contamination can be minimized by using coarse alu-mina particles as grinding media. First results show that nanoparticles can be pro-duced. by such an autogenous grinding process.
Besides the investigations on nanogrinding the mechanism of capturing particles between two grinding media were investigated using a specially designed model apparatus. Among others the measurements with this model apparatus showed that adhesion of product particles at the grinding media surface and decreasing product particle sizes increase the number of particles caught at one stress event.