Discrete Element Method (DEM) modelling of the motion of balls and rocks in grinding mills allows the flow structure and material transport to be understood. It also provides information on the energy dissipated in every collision of resolved particles (which here are 25-200 mm). These energy dissipation events control the nature and intensity of the grinding. This grinding environment can be characterised by the collisional energy spectra. In this paper we explore the quantitative structural change in the collisional environment in SAG (Semi-Autogenous Grinding) mill as measured by the energy spectra predicted using DEM. The simulations show that increasing mill speed has little effect the collision energy distribution between the different types of collisions. Three other mill operating conditions (separately) all lead to decreases in impact breakage and increases in grinding activity for decreasing lifter height, increasing charge fill level, and decreasing rock-to-ball ratio.