4
• Pebble crusher product control
There are two considerations to SAG mill stabil-
ity when operating recycle pebble crushers, the closed
side setting (CSS) of the recycle pebble crusher and
the recycle rate back to the SAG mill.
Failing to maintain a CSS, at or below the SAG
mill trommel or discharge screen aperture will result
in a build-up of critically sized material in the SAG
mill. Critically sized material is typically 25–50 mm
and is too small for effective impact breakage and
too large for attrition. The presence of critically sized
reduces the ability to feed fresh material to the SAG
mill [12].
Intermittently operating a recycle pebble
crusher, resulting in abruptly fluctuating recycle rates
to the SAG mill decreases mill stability and through-
put [13]. The impact of the on-off operation of the
pebble crushers is exacerbated when the CSS is not
tightly controlled.
• Changes in the ore density
A change in the ore-specific gravity can impact
the ability to control the SAG mill discharge den-
sity. Water addition is typically controlled by milled
tonnes, so transitions between low and high SG ores
can cause disturbances in the mill load, as seen at
Phu Kham [8]. Phu Kham installed a bulk volumet-
ric sensor on the SAG mill feed conveyor just before
the feed weightometer to infer the online ore density
measurement. This information was used to control
SAG mill water addition by volume, instead of mill
tonnes to maintain optimum slurry density.
Solution—Operating Load Setpoint
The first step to optimizing SAG mill performance is deter-
mining the optimal operating parameters for the mill, par-
ticularly mill filling. The performance of the SAG mill is
very sensitive to the mill filling, which influences through-
put, power draw and grind size [4]. The objective of the
control strategy in most operations should be to maintain
the optimum mill filling for the given ore properties over
the entire life of the mill liners.
How to determine SAG mill performance curves:
• Step 1: Crash stop and grind out the mill
Crash stop the SAG mill by abruptly cutting the feed
rate, mill power and water addition during stable
operation, typically by tripping the feed conveyor
pull cord.
Measure the mill filling level by pulling the feed
chute and counting the exposed lifters, measuring
the chord length, and measuring the height of the
charge. Molycop provides simple tools to conduct
these measurements [7]. To develop a good relation-
ship, at least two different mill filling levels should be
measured plus the ball charge [4].
The rock charge is then carefully ground out
until just the ball charge remains. The ball charge is
then measured in a similar fashion. The three data
points can be used to determine the relationship
between mill load and mill filling.
Due to liner wear, this procedure should be
conducted with new and worn liners to develop the
relationship over the life of the liners.
• Step 2: Collect power vs mill filling data
Operate the SAG mill at a range of mill fillings and
mill speeds. The range of mill fillings is typically
between 20–40 %v/v but should be adjusted based
on each operation not to exceed the SAG mill oper-
ating or structural limits. Mill speeds should be in
the range of 60–75 %critical. Mill speed should be
limited to not cause direct shell impacts.
The mill should be stabilized at multiple inter-
vals within the mill filling and mill speed ranges,
by adjusting the mill feed rate. After a minimum
of 30 minutes of stable operation, a discharge
sample should be taken to determine product size
distribution.
Determine the average power draw and feed
rate for each mill filling and mill speed interval.
• Step 3: Construct the performance curves
Plot mill power draw, feed rate and grind size against
mill filling for each mill speed interval. The perfor-
mance curves can be used to determine the opti-
mal mill filling based on throughput and grind size
targets.
Performance curves can be developed for major
ore types to develop operating strategies for ore
variability.
An example of a performance curve is shown in
Figure 5.
A similar performance curve, constructed with the
same methodology, can determine the optimal SAG mill
operating density for a given ore type. A throughput peak
will present itself within an ideal mill density range for
given mill speed. Maintaining this density, typically in the
70–78 %w/w range, can reduce the likelihood of slurry
pooling due to excessive mill dilution and increase the
breakage efficiency.
• Pebble crusher product control
There are two considerations to SAG mill stabil-
ity when operating recycle pebble crushers, the closed
side setting (CSS) of the recycle pebble crusher and
the recycle rate back to the SAG mill.
Failing to maintain a CSS, at or below the SAG
mill trommel or discharge screen aperture will result
in a build-up of critically sized material in the SAG
mill. Critically sized material is typically 25–50 mm
and is too small for effective impact breakage and
too large for attrition. The presence of critically sized
reduces the ability to feed fresh material to the SAG
mill [12].
Intermittently operating a recycle pebble
crusher, resulting in abruptly fluctuating recycle rates
to the SAG mill decreases mill stability and through-
put [13]. The impact of the on-off operation of the
pebble crushers is exacerbated when the CSS is not
tightly controlled.
• Changes in the ore density
A change in the ore-specific gravity can impact
the ability to control the SAG mill discharge den-
sity. Water addition is typically controlled by milled
tonnes, so transitions between low and high SG ores
can cause disturbances in the mill load, as seen at
Phu Kham [8]. Phu Kham installed a bulk volumet-
ric sensor on the SAG mill feed conveyor just before
the feed weightometer to infer the online ore density
measurement. This information was used to control
SAG mill water addition by volume, instead of mill
tonnes to maintain optimum slurry density.
Solution—Operating Load Setpoint
The first step to optimizing SAG mill performance is deter-
mining the optimal operating parameters for the mill, par-
ticularly mill filling. The performance of the SAG mill is
very sensitive to the mill filling, which influences through-
put, power draw and grind size [4]. The objective of the
control strategy in most operations should be to maintain
the optimum mill filling for the given ore properties over
the entire life of the mill liners.
How to determine SAG mill performance curves:
• Step 1: Crash stop and grind out the mill
Crash stop the SAG mill by abruptly cutting the feed
rate, mill power and water addition during stable
operation, typically by tripping the feed conveyor
pull cord.
Measure the mill filling level by pulling the feed
chute and counting the exposed lifters, measuring
the chord length, and measuring the height of the
charge. Molycop provides simple tools to conduct
these measurements [7]. To develop a good relation-
ship, at least two different mill filling levels should be
measured plus the ball charge [4].
The rock charge is then carefully ground out
until just the ball charge remains. The ball charge is
then measured in a similar fashion. The three data
points can be used to determine the relationship
between mill load and mill filling.
Due to liner wear, this procedure should be
conducted with new and worn liners to develop the
relationship over the life of the liners.
• Step 2: Collect power vs mill filling data
Operate the SAG mill at a range of mill fillings and
mill speeds. The range of mill fillings is typically
between 20–40 %v/v but should be adjusted based
on each operation not to exceed the SAG mill oper-
ating or structural limits. Mill speeds should be in
the range of 60–75 %critical. Mill speed should be
limited to not cause direct shell impacts.
The mill should be stabilized at multiple inter-
vals within the mill filling and mill speed ranges,
by adjusting the mill feed rate. After a minimum
of 30 minutes of stable operation, a discharge
sample should be taken to determine product size
distribution.
Determine the average power draw and feed
rate for each mill filling and mill speed interval.
• Step 3: Construct the performance curves
Plot mill power draw, feed rate and grind size against
mill filling for each mill speed interval. The perfor-
mance curves can be used to determine the opti-
mal mill filling based on throughput and grind size
targets.
Performance curves can be developed for major
ore types to develop operating strategies for ore
variability.
An example of a performance curve is shown in
Figure 5.
A similar performance curve, constructed with the
same methodology, can determine the optimal SAG mill
operating density for a given ore type. A throughput peak
will present itself within an ideal mill density range for
given mill speed. Maintaining this density, typically in the
70–78 %w/w range, can reduce the likelihood of slurry
pooling due to excessive mill dilution and increase the
breakage efficiency.