2
• Maintaining a stable feed size distribution and con-
sistent ore feed competency
• Recommended mill liner and discharge system
designs
• Selecting suitable grinding media
• Recommended instrumentation and optimized pro-
cess control
IMPORTANCE OF MAINTAINING A
STABLE SAG MILL LOAD
Many operating variables (combination or separately) con-
tribute to a SAG mill load’s stability or instability in SAG
mill-based comminution circuits. As the SAG mill duty
changes based on the feed size distribution, the mill inter-
nals should be changed and configured, and the operating
conditions adapted [3].
But why is maintaining a stable SAG mill load so impor-
tant? A stable SAG mill load means the SAG mill is operating
in the optimal mill filling range for efficient grinding as often
as possible.
SAG mill load-instability results in the SAG mill oper-
ating in the low load zone, resulting in liner and media
damage or in the high load zone, resulting in poor breakage
efficiency and resulting lower throughput.
The normal mill filling operating range is typically
lower than the peak mill power draw [4] when the mill
is operating at a nominal speed of 75% critical speed -as
shown in Figure 2. The SAG mill power drops off at very
high total fillings due to the shift in the center of mass of
the charge towards the center of the mill outweighing the
increased mass of the charge [5]. The shift in the center
of mass results in a reduction in mill torque, thus a lower
power draw.
For a SAG mill operating with nominal speeds, suit-
able liner and discharge system design, the feed rate fol-
lows a similar trend as the mill power draw when plotted
against total mill filling, reaching a peak feed rate, and then
decreasing with increasing mill fillings [5]. The feed rate
peak normally occurs at lower mill fillings than the power
peak [4]. The power and throughput peaks converge with
increasing mill speed therefore, using maximum SAG mill
power draw can be an appropriate measure in the scenario
that SAG mill throughput is maximized. Increasing the
SAG mill load stability and operating at the optimal mill
filling, increases the amount of operating time at the peak
of the power and feed rate curves.
It is not uncommon for operations to have a poor
understanding of the optimal mill filling and how the cor-
responding measurement of mill load, via a load cell, or
bearing pressure translates. Operations will control the mill
to a maximum mill load or bearing pressure value and allow
for significant drift during normal operation. An example
of a highly variable mill filling is shown in Figure 3. The
operation measured the mill filling monthly but did not
adjust the operating or control strategies to improve the
operating range allowing the mill filling to vary between
13–30 %v/v.
There are several soft sensors commercially available
that provide inferred mill filling measurements (LoadIQ
Figure 1. Key variables affecting the stability of the SAG mill load (after [1])
Figure 2. SAG mill operating zones
• Maintaining a stable feed size distribution and con-
sistent ore feed competency
• Recommended mill liner and discharge system
designs
• Selecting suitable grinding media
• Recommended instrumentation and optimized pro-
cess control
IMPORTANCE OF MAINTAINING A
STABLE SAG MILL LOAD
Many operating variables (combination or separately) con-
tribute to a SAG mill load’s stability or instability in SAG
mill-based comminution circuits. As the SAG mill duty
changes based on the feed size distribution, the mill inter-
nals should be changed and configured, and the operating
conditions adapted [3].
But why is maintaining a stable SAG mill load so impor-
tant? A stable SAG mill load means the SAG mill is operating
in the optimal mill filling range for efficient grinding as often
as possible.
SAG mill load-instability results in the SAG mill oper-
ating in the low load zone, resulting in liner and media
damage or in the high load zone, resulting in poor breakage
efficiency and resulting lower throughput.
The normal mill filling operating range is typically
lower than the peak mill power draw [4] when the mill
is operating at a nominal speed of 75% critical speed -as
shown in Figure 2. The SAG mill power drops off at very
high total fillings due to the shift in the center of mass of
the charge towards the center of the mill outweighing the
increased mass of the charge [5]. The shift in the center
of mass results in a reduction in mill torque, thus a lower
power draw.
For a SAG mill operating with nominal speeds, suit-
able liner and discharge system design, the feed rate fol-
lows a similar trend as the mill power draw when plotted
against total mill filling, reaching a peak feed rate, and then
decreasing with increasing mill fillings [5]. The feed rate
peak normally occurs at lower mill fillings than the power
peak [4]. The power and throughput peaks converge with
increasing mill speed therefore, using maximum SAG mill
power draw can be an appropriate measure in the scenario
that SAG mill throughput is maximized. Increasing the
SAG mill load stability and operating at the optimal mill
filling, increases the amount of operating time at the peak
of the power and feed rate curves.
It is not uncommon for operations to have a poor
understanding of the optimal mill filling and how the cor-
responding measurement of mill load, via a load cell, or
bearing pressure translates. Operations will control the mill
to a maximum mill load or bearing pressure value and allow
for significant drift during normal operation. An example
of a highly variable mill filling is shown in Figure 3. The
operation measured the mill filling monthly but did not
adjust the operating or control strategies to improve the
operating range allowing the mill filling to vary between
13–30 %v/v.
There are several soft sensors commercially available
that provide inferred mill filling measurements (LoadIQ
Figure 1. Key variables affecting the stability of the SAG mill load (after [1])
Figure 2. SAG mill operating zones