Since the beginning of the last century, many studies have been performed in
order to improve our understanding on the milling process. Recently, Mishra and
Rajamani (1992) applied the Discrete Element Method (DEM) to solve the
milling problem. Since then, this method gained considerable success due to its
ability to predict load motion and power draw by tumbling mills as affected by
operating conditions. The application of this method at an industrial stage requires
a more rigorous validation in order to produce realistic output.
Lifter profiles play a key role in the performance of tumbling mills since they
influence the motion of mill charge. Since lifters change profiles during their
useful life due to wear, the performance of tumbling mills will correspondingly
vary as a function of time. There is therefore a need to predict forces and wear on
mill lifters in order not only to chose or design an initial lifter profile which
optimizes tumbling mills performance over the lifters’ useful life but also to
evaluate lifter replacement time and type and also modifications which can be
performed on lifters and/or operating mill conditions in order to extend the lifters’
useful life. Despite the importance related to this subject, few works has been
done in this field.
In this thesis, we firstly assess the ability of the Discrete Element Method to
model the tangential and normal forces exerted by the mill charge on lifters. Data
from an experimental two-dimensional mill designed in order to record the normal
and tangential forces exerted on an instrumented lifter were available. The
measured results obtained at different speeds and percentages of filling have been
compared to the Discrete Element Method simulated results in the same
conditions. A good agreement has been found between the experimental and the
simulated results in terms of toe, shoulder positions and amplitude of forces.
After this validation of the DEM, we secondly assess the ability of this method to
predict the wear of lifters in dry milling conditions. We derived a mathematical
wear equation describing the removal of materials from lifters which takes into
account all types of wear occurring in dry milling environment. We introduce a
new approach to implement this equation in the DEM code in order to produce
realistic simulated profiles. Our new method developed has been tested against
laboratory and industrial data of evolving lifter profiles due to wear. Good
agreement has been found between the simulated and the measured profiles.
The variation of the load behaviour as a function of lifter wear in industrial
tumbling mills studied was also investigated in this thesis. The objectives were to
improve the understanding of the grinding process and quantify the variation of
load behaviour as a function of lifter wear. Lifter modifications were also
explored in order to extend lifters useful life.
An attempt was also made in this thesis to derive, from the description of the load
behaviour, equations in order to predict the wear of lifters without using the
Discrete Element Method. Equations derived show the difficulty to use this
approach. Success in this case was achieved only in a particular case where no
significant changes occur in the load behaviour as a function of lifters wear. This
finding confirms the DEM as the adequate tool to model forces and wear of
tumbling mill lifters.
The results obtained are of great economical significance since they can improve
the profitability of mineral processing plants. A step forward in the use of the
DEM not only to design milling equipments but also to improve the
understanding, optimise and quantify the change occurring as a function of lifters
wear was achieved.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/6031 |
| Date | 10 February 2009 |
| Creators | Kalala, Johnny Tshibangu |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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