Extensive research with HIGmill grinding rotors has shown that flat grinding discs in stirred media mills in hard rock applications experience poorer process performance compared to castellated rotors. The first industrial size equipment to benefit from this is Kevitsa Cu-Ni mine, where the energy improvement from flat to castellated discs was 28-50%. This observation was investigated further through different size pilot scale units, which clearly confirmed a step change shift in the specific grinding energy versus the product particle size at 80% passing. The design of the rotors was further developed and fine-tuned with DEM (Discrete Element Method). The validated results have allowed the castellated rotor design to be applied as a standard to all HIGmills.