## 3 Results and Discussion
### Effect of the Media Loading Evaluated from PEPT Experiments  
In this section, the influence of the bead loading on the flow field is discussed only for the impeller speed of 600 rpm and the clearance of 19 mm. Figure 11 shows the dimensionless velocity magnitude for the bead loading of 2.4 kg and 3.0 kg, respectively. The increase in the media loading did not affect either the maximum speed or the extent of the region at high velocity but an increase in the average machine power consumption from 0.16 KW to 0.18 KW was observed when the 3.0 kg of media were utilised. Presumably, extra machine power consumption was required to drive and lift the additional beads and provide to them kinetic and potential energy. The increase of the media loading also caused an increase of the bed height with media moving at low velocity above the top impeller pin. To gather complementary information on the effect of the bead loading on the flow field the normalised occupancy was calculated and reported in Figure 12. The occupancy plot revealed that the tracer distribution within the pot was more homogeneous for the higher than the lower bead loading. This is slightly counter-intuitive since an increase of the bed weight should increase the constraint of the media movements as well. No significant differences in occupancy were observed at the bottom of the attritor where for both the bead loadings the tracer spent a considerably higher amount of time compared to other parts of the pot. Noticeably, for both the media loading a lack of data was seen in the region defined by Z \(<\) 10 mm and R \(<\) 40 mm (white region at the bottom of the vessel), where the tracer never accessed during the entire time of the experiments. For both these two loadings, at the beginning of the experiments the tracer position was at about 2/3 of the media bed (Z\(\sim\)100 mm), hence the effect of the initial tracer position on the lack of data is excluded. Presumably, this data deficiency is not associated to the effect of the media loading, but instead, as highlighted by the Figures 9 and 10, it may be related to the selection of the clearance. Figure 11: PEPT dimensionless velocity magnitude (\(v/v_{tip}\)) for two bead loading: 2.4 kg and 3.0 kg of media. Test case using the clearance of 19 mm and the impeller speed of 600 rpm. Validation of a Static Friction Adjusted-DEM Model by Using Positron Emission Particle Tracking (PEPT) Measurements  
Contrary to the PEPT experiments where a single radioactively labelled particle, the tracer, is followed for long time to be representative of the entire system, in a DEM simulation the position and the velocity of every particle can be calculated at the same time. Therefore, due to the nature of the DEM method and the high number of beads present in the system a simulation time of five seconds, corresponding to a minimum of 25 impeller rotations, ensured the generation of steady state and high-quality data. Among the experiments reported in Table 1 two setups were selected for the comparison between PEPT and DEM: 1) bead loading of 2.4 kg, clearance of 19 mm, impeller speed of 300 rpm (experiment number 4 of Table 1) and 2) bead loading of 2.4 kg, clearance of 19 mm, impeller speed of 600 rpm (experiment number 5 of Table 1). For these two cases, the simulations were performed to replicate the exact settings (bead size, geometry, bead loading, impeller speed, clearance) used for the PEPT experiments. The static friction coefficient for the yttria-stabilised zirconia beads was not known a priori and, hence, three values of the friction coefficient (0.15, 0.35 and 0.50) were utilised in the simulations. The best friction coefficient value was then selected by comparing the probability density function (pdf) of the bead velocity calculated by DEM simulations against the one derived from PEPT experiments. As shown so far, by changing one of the operating parameters (impeller speed, bead loading and impeller clearance) different changes of the beads motion will occur in different regions of the mill.