## 3 Results and Discussion
### Effect of Impeller Speed Evaluated from PEPT Experiments  
The effect of the impeller speed on the grinding media motion is discussed for the bead loading of 2.4 kg and the impeller clearance of 12.7 mm. The impeller speed was varied from 300 to 600 rpm corresponding to tip speeds of 1.1 m/s and 2.1 m/s, respectively. Figure 4 shows the colour maps of the normalised velocity (magnitude of velocity divided by the corresponding impeller tip speed) within the attrort vessel. For a better comprehension of the velocity field, the positions of the impeller arms with respect to the vessel are showed in Figure 4. The colour scale indicates in red and blue high velocity and low velocity, respectively. For all the three impeller speeds, the normalised velocity was significantly higher around the impeller-pin locations than in the area beneath the impeller shaft. In the latter area of the equipment the contact between the beads and the impeller pins was indirect and it resulted in a poor exchange  
Figure 4: PEPT dimensionless velocity magnitude (\(\
icefrac{{\
u}}{{v_{tip}}}\)) at the different impeller speeds of 300 rpm, 450 rpm and 600 rpm. Test case using the bead loading of 2.4 kg and the bottom clearance of 12.7 mm. of momentum. This caused the formation of a stagnant region where the media motion was feeble. Comparing the three velocity flow fields of Figure 4 two observations can be emphasised. Firstly, the position of the region of the highest dimensionless velocity (around the pins' location) and its size did not change noticeably when increasing the impeller speed. This agrees with the findings of [11] who observed an increase in media velocity only within the impeller swept zone (where the pins are located) and markedly little variation outside of it. Secondly, from 300 rpm to 600 rpm the media bed was gradually rising upwards towards the top of the vessel due to the increasing centrifugal force generated by the impeller rotation. In fact, as reported by previous PEPT investigations on wet stirred media mills [9; 11] an increase of the impeller speed results in the media bed expansion and in the net movement of the media along the vertical attritor axis. The effect of the impeller speed on both the media bed expansion and the flow recirculation patterns was better illustrated by the velocity vectors. Figure 5 displays the velocity vector maps, calculated by using the axial and the radial components of the velocity, at different impeller speeds for the bead loading of 2.4 kg and the bottom clearance of 12.7 mm. For example, at 300 rpm the maximum bed height was about 125 mm (76% of the pot height), at 450 rpm was 139 mm (85% of the pot height) whereas at 600 rpm it was 145 mm (88% of the pot height). Hence, an expansion of the media along the vertical axis of approximately 20 mm occurred when the impeller speed was doubled. The velocity vector maps also highlight the presence of two media flow patterns: an upper recirculation loop, visible for all impeller speeds, and a lower recirculation loop more evident at the higher impeller speeds than the lower. The upper recirculation loop clearly developed and reinforced with the increase of the impeller speed as a major fraction of media was lifted up by the impeller pins and then recirculated back to the impeller zone. This suggests that a better dispersion and distribution of the material to be ground across the media bed may be achieved at high impeller speeds, leading to potential positive effects on the grinding efficiency. Even  
Figure 5: PEPT velocity vector maps for the impeller speed of 300 rpm, 450 rpm and 600 rpm highlighting the recirculation patterns within the attritor. Test case using the bead loading of 2.4 kg and the bottom clearance of 12.7 mm. the size of the lower recirculation loop increased with the impeller speed, but its pattern became evident only at the maximum speed of 600 rpm.