XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3747
configurations. From preliminary studies, the optimal mill
operation parameters were identified as a mill speed rang-
ing from 65% to 77% of the critical speed (equivalent to
50–60 rpm), solid loadings from 70% and 80% by weight,
a combination of varying steel ball sizes, and a lifter height
of 2 cm.
EFFECT OF DIFFERENT MILL
OPERATING VARIABLES IN BATCH
MODE AND ACOUSTICS
Following the mill test methodology, the acoustic emis-
sion responses of different mill operating conditions (both
practical and unpractical conditions) were investigated
(Owusu et al., 2021c Owusu et al., 2021h). This study
provided fundamental knowledge of the acoustic sensitivity
of mill charge at varying compositions and process vari-
ables (Owusu et al., 2022a Owusu et al., 2020b Owusu et
al., 2021h). Figure 3 presents a summary of different mill
conditions and related acoustic responses. Detailed infor-
mation can be found in an article by Owusu et al., 2021c.
The acoustic emissions recorded from the mill operated
with no load (empty mill) increased with increasing the mill
speed from 50 rpm to 70 rpm (64.5–90.3 critical speed).
However, the intensity of the noise was extremely low and
negligible. The emitted acoustics appear to originate from
the motor and mechanical movement of the mill. The result
obtained was similar when water (1000 ml) was fed into
the mill at different speeds. At 50 rpm mill speed, the mill
acoustic response reduced with an increase in the volume of
water added (500 ml, 1000 ml, and 1000 ml). When only
rock sample (model calcite) was introduced at different
speeds (50 –70 rpm or 65% to 77% of the critical speed),
an improved mill noise, which was higher (nearly constant)
than that of an empty mill and water-only, was observed
due to the collision of rocks with the mill shell/liner/lifter
and the rocks themselves. The acoustic intensity did not
vary much when the different masses of the rock sample
were tested at 50 rpm. Due to the smaller mill diameter (30
cm) and low density of the rock sample, it was noted that
the rock breakage characteristics of coarser feed size distri-
bution were negligible by impact, with only a small fraction
of finer size produced by abrasion and attrition processes.
The low breakage rate demonstrates that the mill’s diameter
is insufficient to support an AG mill system. In the scenario
of only steel ball loading, remarkable acoustic emission
was produced (varying at different mill speeds) from the
interaction of the steel balls themselves and steel balls and
liner/lifters. Additionally, increasing the steel ball percent-
age filling (5, 8, and 10% of the mill volume) resulted in
a continuous increase in mill noise. The direct collisions of
steel balls themselves and steel balls mill liner/lifters are not
recommended in operating AG/SAG mills because it can
cause rapid wear of the steel balls and liner/lifter, reducing
their lifespan and increasing production costs unnecessar-
ily (Almond and Valderrama, 2004). Furthermore, direct
interactions of steel balls and liner/lifters in the presence of
rock samples and water could suggest poor ore size reduc-
tion efficiency as well as wastage of power. The ability to
Empty mill
Only water
Only rock
Only steel balls
Rock-water
Balls-water
Rock-balls
Rock-balls-water
0.00 0.01 0.02 0.03 0.04
RMS amplitude
Figure 3. Mill acoustic response of different mill operating conditions, modified after
(Owusu et al., 2021h)
Different
mill
operatingc
iti on s
[
r
configurations. From preliminary studies, the optimal mill
operation parameters were identified as a mill speed rang-
ing from 65% to 77% of the critical speed (equivalent to
50–60 rpm), solid loadings from 70% and 80% by weight,
a combination of varying steel ball sizes, and a lifter height
of 2 cm.
EFFECT OF DIFFERENT MILL
OPERATING VARIABLES IN BATCH
MODE AND ACOUSTICS
Following the mill test methodology, the acoustic emis-
sion responses of different mill operating conditions (both
practical and unpractical conditions) were investigated
(Owusu et al., 2021c Owusu et al., 2021h). This study
provided fundamental knowledge of the acoustic sensitivity
of mill charge at varying compositions and process vari-
ables (Owusu et al., 2022a Owusu et al., 2020b Owusu et
al., 2021h). Figure 3 presents a summary of different mill
conditions and related acoustic responses. Detailed infor-
mation can be found in an article by Owusu et al., 2021c.
The acoustic emissions recorded from the mill operated
with no load (empty mill) increased with increasing the mill
speed from 50 rpm to 70 rpm (64.5–90.3 critical speed).
However, the intensity of the noise was extremely low and
negligible. The emitted acoustics appear to originate from
the motor and mechanical movement of the mill. The result
obtained was similar when water (1000 ml) was fed into
the mill at different speeds. At 50 rpm mill speed, the mill
acoustic response reduced with an increase in the volume of
water added (500 ml, 1000 ml, and 1000 ml). When only
rock sample (model calcite) was introduced at different
speeds (50 –70 rpm or 65% to 77% of the critical speed),
an improved mill noise, which was higher (nearly constant)
than that of an empty mill and water-only, was observed
due to the collision of rocks with the mill shell/liner/lifter
and the rocks themselves. The acoustic intensity did not
vary much when the different masses of the rock sample
were tested at 50 rpm. Due to the smaller mill diameter (30
cm) and low density of the rock sample, it was noted that
the rock breakage characteristics of coarser feed size distri-
bution were negligible by impact, with only a small fraction
of finer size produced by abrasion and attrition processes.
The low breakage rate demonstrates that the mill’s diameter
is insufficient to support an AG mill system. In the scenario
of only steel ball loading, remarkable acoustic emission
was produced (varying at different mill speeds) from the
interaction of the steel balls themselves and steel balls and
liner/lifters. Additionally, increasing the steel ball percent-
age filling (5, 8, and 10% of the mill volume) resulted in
a continuous increase in mill noise. The direct collisions of
steel balls themselves and steel balls mill liner/lifters are not
recommended in operating AG/SAG mills because it can
cause rapid wear of the steel balls and liner/lifter, reducing
their lifespan and increasing production costs unnecessar-
ily (Almond and Valderrama, 2004). Furthermore, direct
interactions of steel balls and liner/lifters in the presence of
rock samples and water could suggest poor ore size reduc-
tion efficiency as well as wastage of power. The ability to
Empty mill
Only water
Only rock
Only steel balls
Rock-water
Balls-water
Rock-balls
Rock-balls-water
0.00 0.01 0.02 0.03 0.04
RMS amplitude
Figure 3. Mill acoustic response of different mill operating conditions, modified after
(Owusu et al., 2021h)
Different
mill
operatingc
iti on s
[
r