Tumbling mills are considered to be among the most energy intensive devices in comminution circuits and are designed to achieve size reduction and transport. These functions are influenced by the mill speed, filling and the configuration of the lifter bars attached to the mill shell (height, width, face angle, etc.). The lifter height has been shown to influence the charge motion and power draw of the mill. Most of the predictive power models used in industry do not consider lifter heights as a variable. In comminution, Positron Emission Particle Tracking (PEPT) is used to track the charge motion of particles in tumbling mill systems. The data has been used to derive a velocity profile for particle motion between the mill shell and centre of circulation (COC), a region that highlights the distinct particle motion in tumbling mills. In prior work, a laboratory scale mill and PEPT was used to develop a velocity profile model incorporating the lifter height influence on charge motion. The agreement between PEPT and the model predictions was limited to regions closest to the mill shell and the analysis was restricted for conditions where the lifter height was significantly smaller than the particle diameter. The current study aimed to supplement the PEPT results by using the Discrete Element Method (DEM) to simulate the collective effect of individual particle interactions on the charge motion. The DEM results were used to analyse the influence of the lifter height on the charge motion, velocity profile and power draw for an identical tumbling mill system. The PEPT and DEM results agreed on charge motion and power draw changes. The assumption of a constant axial pressure drop (dP/dx) along the cross-section of the mill resulted in deviations between DEM and model predictions of the velocity profile. The calculated axial pressure drop varied non-linearly along the mill radius and followed a similar trend at all operating conditions. The relationship between the velocity profile and axial pressure drop was found to vary non-linearly and followed a similar trend to the stress-strain relation of granular media. It is recommended that further research be conducted on the axial pressure drop and its influence on the velocity profile.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/30087 |
| Date | 14 May 2019 |
| Creators | Uys, Adri Mari |
| Contributors | Mainza, Aubrey |
| Publisher | Faculty of Engineering and the Built Environment, Department of Chemical Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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