Process Modelling of Gravity Induced Stirred Mills 397120/s42997650_phd_the¢ power, feed...
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Process Modelling of Gravity Induced Stirred Mills
Md Maruf Hasan
M.Sc. (Minerals and Environmental Engineering)
B.Sc. (Mechanical Engineering)
A thesis submitted for the degree of Doctor of Philosophy at
The University of Queensland in 2016
Sustainable Minerals Institute
Julius Kruttschnitt Mineral Research Centre
Fine grinding is becoming an integral part of mineral processing plants to liberate valuable minerals
from fine-grained low-grade ore bodies. Stirred mills are becoming recognized by the industry as a
more efficient technology selection compared to the ball mill for fine grinding (< 100 µm). The
increasing number of stirred mill installations in mineral processing concentrators has necessitated a
process model development that enables full circuit modelling and simulation. Many researchers
have developed mathematical models to assist in the design and optimization of the stirred mill
operation including gravity induced stirred mills such as the Metso Vertimill and the Nippon Eirich
Most of the developed models lack sufficient responses to changes in process conditions.
Furthermore, there is an internal classification effect that is previously mentioned in several
publications but was not quantified through experiments. Therefore, a research program was
designed and carried out to describe and model different sub-processes related to the operation of
the gravity induced stirred mill. The sub-process models (i.e. grinding and internal classification)
were combined into a single model structure to represent the operation in a gravity induced stirred
During this research, comprehensive test work was carried out in a batch gravity induced stirred
mill using Cu-Au ore and limestone to evaluate the effect of operating conditions (i.e. specific
energy consumption, slurry density, grinding media size and stirrer tip speed) on particle size and
fines generation (-75μm). The test data showed a finer product size when the mill operates at higher
specific energy, lower slurry density, smaller grinding media size and higher stirrer tip speed. The
test work also identified that attrition breakage mechanism is predominant in the gravity induced
mill. Sub-process models that relate the selection function to the process operating conditions were
developed. The internal classification effect on mill operation was also measured through test work
by using industrial grade silica to identify its effect on the mill operation. The result showed that
particle classification takes place inside the gravity induced mill in certain favorable conditions.
Models were developed to link the classification parameters and the mill operating conditions to be
included in the combined model structure.
A process model (using a time-based population balance technique) was developed, integrating
individual sub-processes models such as breakage, selection and classification functions. The model
was validated with industry survey data in different process conditions. The validation result
showed that the developed model was capable to predict the mill product size distribution when mill
power, feed rate, slurry solids concentration and grinding media size are varied.
The thesis has successfully developed a process model of the gravity induced stirred mill with
predictive capability. Moreover, it has developed an in-depth understanding of the laboratory scale
gravity induced mill behaviour and its breakage mechanism. The inclusion of the classification with
the breakage process into a single model structure is novel in comminution process modelling.
Declaration by author
This thesis is composed of my original work, and contains no material previously published or
written by another person except where due reference has been made in the text. I have clearly
stated the contribution by others to jointly-authored works that I have included in my thesis.
I have clearly stated the contribution of others to my thesis as a whole, including statistical
assistance, survey design, data analysis, significant technical procedures, professional editorial
advice, and any other original research work used or reported in my thesis. The content of my thesis
is the result of work I have carried out since the commencement of my research higher degree
candidature and does not include a substantial part of work that has been submitted to qualify for
the award of any other degree or diploma in any university or other tertiary institution. I have
clearly stated which parts of my thesis, if any, have been submitted to qualify for another award.
I acknowledge that an electronic copy of my thesis must be lodged with the University Library and,
subject to the policy and procedures of The University of Queensland, the thesis be made available
for research and study in accordance with the Copyright Act 1968 unless a period of embargo has
been approved by the Dean of the Graduate School.
I acknowledge that copyright of all material contained in my thesis resides with the copyright
holder(s) of that material. Where appropriate I have obtained copyright permission from the
copyright holder to reproduce material in this thesis.
Publications during candidature
Journal publication relevant to this thesis:
Hasan, M., Palaniandy, S., Hilden, M., Powell., M. 2016. Investigating internal classification
within gravity induced stirred mills. Minerals Engineering 95: 5-13.
Conference publication relevant to this thesis:
Hasan, M., Palaniandy, S., Hilden, M., Powell, M., 2014. The Investigation of Particle
Classification in Vertical Type Stirred Mill. International Conference of Mineral Processing
Technology – MPT 2014, Vishakhapatnam, India, p.160.
Publications included in this thesis
No publications included
Contributions by others to the thesis
Professor Malcolm Powell, Dr. Sam Palaniandy, and Dr. Marko Hilden were responsible for setting
up this thesis project and establishing the initial research goal. They provided the initial ideas for
developing the process model and experimental devices and reviewed the experimental design.
They also made great contributions to the interpretation of the data.
Dr. Sam Palaniandy organized the experimental device and helped in analyzing the experimental
Dr. Sam Palaniandy and Dr. Marko Hilden assisted in developing the thesis structure and critically
reviewed the draft thesis. Professor Malcolm Powell reviewed the final draft for necessary
Metso Minerals constructed and supplied the batch vertical stirred mill and the limestone sample for
the test work.
Mr. Jonathan Worth, Mr. Michael Kilmartin and Mr. John Wedmaier modified and made necessary
adjustments and repairs to the experimental devices.
Naren Vijayakumar and Daniel Mitchell provided the ground Cu-Au (-4.75 mm) sample from their
pilot SAG mill test work.
Statement of parts of the thesis submitted to qualify for the award of another degree
Firstly, I thank Allah the Almighty, for giving me the opportunity to commence and complete this
thesis successfully. This research work is a part of larger project funded by Metso Minerals under
the name of “Collaborative Research on Stirred Milling Technology”. I am greatly thankful to the
following persons and institutions for their contributions in accomplishing this thesis.
o To Julius Kruttschnitt Mineral Research Centre (JKMRC) and The University of
Queensland (UQ) for offering me the scholarship and provide required support and
environment to conduct the Ph.D. work. It is an honour to become alumni of both JKMRC
and UQ, and I will remain proud for the rest of my life for this achievement.
o To my principle supervisor, Dr Sam Palaniandy for his exceptional support throughout my
PhD life. He provided me the freedom to conduct this research and guided me through all
the way towards the end of completing this research work. He always gave me the
confidence and provided me timely feedback for the thesis I wrote. I am also thankful to him
for his time and patience towards me.
o To my associate supervisors Dr Marko Hilden and Professor Malcolm Powell for their
academic support, guidance and time while conducting the research work. They were also
very helpful by providing me valuable advice for conducting the test work. The weekly
meeting with all my academic supervisors was very helpful and made me work hard to
accomplish the goal.
o To Metso Minerals for the financial support and opportunity to conduct this thesis work. I
am also thankful to them for providing me the opportunity to conduct experiments in their
laboratory in York, Pennsylvania during the initial period of this research work.