3
[7], MillSlicer [7], etc.). These tools can be useful to the
operator and tied into control logic for maintaining mill
filling under variable operating conditions and minimiz-
ing direct shell impacts. Significant changes in ore-specific
gravity, such as Phu Kham, make it difficult to control mill
filling. Soft sensors allowed for a mill filling control strategy
to be developed based on changing ore competency and
specific gravity [8].
CHALLENGES AND SOLUTIONS TO
MAINTAIN A STABLE LOAD IN SAG
MILLS
The common operational challenges that impact SAG mill
load stability are discussed in detail in this section.
SAG Mill Feed
Challenges—Causes of Variable Feed to the Mill
Variability in the SAG mill feed can be attributed to mul-
tiple sources including heterogeneity in the feed material
or multiple pits, primary crusher control, and recycled feed
from the pebble crushing circuit.
Ore competence refers to the ore’s ability to break
down into smaller particles under the applied force in the
SAG mill. Variability in ore competence can significantly
affect the milling process, achieving finer or coarser prod-
uct based on the operating conditions. A highly competent
ore is more resistant to grinding, requiring more energy
to break down, resulting in lower throughput and finer
product. Whereas a less competent ore leads to higher
throughput and a coarser product. Therefore, developing
a comprehensive understanding of ore competence and its
variability and its impact downstream (grind size vs recov-
ery) is crucial for optimizing SAG mill performance [9].
The causes of variable feed and its impact on the SAG
mill load are:
• Primary, secondary, or tertiary crushed product fed
SAG mills.
A significant portion of feed size variability is
the result of operating practices such as allowing drift
in the primary crusher gap, stockpile segregation,
and stockpile dozing [2].
Drift in the primary crusher gap increases
the top size of the feed to the SAG mill drastically
impacting mill filling, throughput, and grind size.
Circuit design can impact feed size variability
as well such as if the SAG mill is preceded by a feed
bin or a coarse ore stockpile (COS). Segregation
within a bin and the location of the feeders draw-
ing from the bin can cause feed size fluctuations as
the bin is drawn down, as seen at Newcrest’s Cadia
operation [10]. Segregation in a coarse ore stockpile
can impact feed size, particularly with dual train
circuits, where two SAG mill trains are fed from a
single COS, as seen in Newmont’s Penasquito opera-
tion [11]. Due to the asymmetrical draw-down
of Penasquito’s COS, one SAG mill receives a sig-
nificantly finer feed distribution depending on the
height of the COS. Penasquito’s circuit has a second-
ary crusher line, feeding an HPGR prior to the SAG
mill. Inconsistent HPGR operation led to frequent
feed size fluctuations in the SAG mill.
Dozing of the coarse ore stockpile can cause
significant fluctuation in SAG mill size due to stock-
pile segregation and should not be standard prac-
tice. Figure 4 highlights a rapidly changing feed
size’s negative impact on the SAG mill load and feed
rate. The mill rapidly overloads as the mill breakage
rates decrease with an increasing feed percentage of
coarse material. Rapid overloading is likely to occur
when dozing.
Figure 3 -Example of highly variable mill filling
Figure 4. Impact of coarse material on SAG feed rate
[7], MillSlicer [7], etc.). These tools can be useful to the
operator and tied into control logic for maintaining mill
filling under variable operating conditions and minimiz-
ing direct shell impacts. Significant changes in ore-specific
gravity, such as Phu Kham, make it difficult to control mill
filling. Soft sensors allowed for a mill filling control strategy
to be developed based on changing ore competency and
specific gravity [8].
CHALLENGES AND SOLUTIONS TO
MAINTAIN A STABLE LOAD IN SAG
MILLS
The common operational challenges that impact SAG mill
load stability are discussed in detail in this section.
SAG Mill Feed
Challenges—Causes of Variable Feed to the Mill
Variability in the SAG mill feed can be attributed to mul-
tiple sources including heterogeneity in the feed material
or multiple pits, primary crusher control, and recycled feed
from the pebble crushing circuit.
Ore competence refers to the ore’s ability to break
down into smaller particles under the applied force in the
SAG mill. Variability in ore competence can significantly
affect the milling process, achieving finer or coarser prod-
uct based on the operating conditions. A highly competent
ore is more resistant to grinding, requiring more energy
to break down, resulting in lower throughput and finer
product. Whereas a less competent ore leads to higher
throughput and a coarser product. Therefore, developing
a comprehensive understanding of ore competence and its
variability and its impact downstream (grind size vs recov-
ery) is crucial for optimizing SAG mill performance [9].
The causes of variable feed and its impact on the SAG
mill load are:
• Primary, secondary, or tertiary crushed product fed
SAG mills.
A significant portion of feed size variability is
the result of operating practices such as allowing drift
in the primary crusher gap, stockpile segregation,
and stockpile dozing [2].
Drift in the primary crusher gap increases
the top size of the feed to the SAG mill drastically
impacting mill filling, throughput, and grind size.
Circuit design can impact feed size variability
as well such as if the SAG mill is preceded by a feed
bin or a coarse ore stockpile (COS). Segregation
within a bin and the location of the feeders draw-
ing from the bin can cause feed size fluctuations as
the bin is drawn down, as seen at Newcrest’s Cadia
operation [10]. Segregation in a coarse ore stockpile
can impact feed size, particularly with dual train
circuits, where two SAG mill trains are fed from a
single COS, as seen in Newmont’s Penasquito opera-
tion [11]. Due to the asymmetrical draw-down
of Penasquito’s COS, one SAG mill receives a sig-
nificantly finer feed distribution depending on the
height of the COS. Penasquito’s circuit has a second-
ary crusher line, feeding an HPGR prior to the SAG
mill. Inconsistent HPGR operation led to frequent
feed size fluctuations in the SAG mill.
Dozing of the coarse ore stockpile can cause
significant fluctuation in SAG mill size due to stock-
pile segregation and should not be standard prac-
tice. Figure 4 highlights a rapidly changing feed
size’s negative impact on the SAG mill load and feed
rate. The mill rapidly overloads as the mill breakage
rates decrease with an increasing feed percentage of
coarse material. Rapid overloading is likely to occur
when dozing.
Figure 3 -Example of highly variable mill filling
Figure 4. Impact of coarse material on SAG feed rate