Abstract:
Mixing is an important operation that is carried out in food, paint, pharmaceutical and
mineral processing industries. Ball mills are one of the many mixing vessels used in a
mineral processing industry. During grinding, the mill’s efficiency depends on
particle presentation to the grinding media and the adequate utilisation of the applied
forces to effect breakage of particles (ore). Utilisation of applied forces is affected by
how well particles and grinding media are mixed. The study of charge mixing is
important as it affects the mill’s production rate and accelerates media wear, thus
relevant to the cost reduction for the milling process.
The kinetics of mixing in a batch ball mill were quantified both radially and axially.
Experiments were conducted in a laboratory batch ball mill and two experimental
programs were used to study the mixing process. Radial mixing of particles was
observed to increase with increasing mill speed. For a mill used in this study, mixing
of particles at Nc=90% took almost half the total time taken at Nc=75% to reach
completion. A simplified mathematical model is presented, which can be used to
predict the radial mixing of particles in a ball mill. Axial mixing of particles was
observed to be affected by both the charge system used and segregation of particles
from the grinding media. It took a minute for mixing to reach 80% completion for a
mill used in the experiments. Mixing of particles was faster in a steel balls/plastic
powders charge system than in a glass beads/quartz charge system.
The distribution of particles in a batch mill was observed to vary along the axis of the
mill. The centre of the mill was overfilled with particles, U>1, while the regions near
the mill ends were underfilled, U<1. The opposite was true for the grinding media.
The data reported was based on measurements of particle distribution along the mill
as affected by different charge systems.
The work presented in this thesis is a contribution to the continuing research on
mixing of particles in ball mills.
