9
Pegging Detection
Pegging occurs when rock or spalled balls get trapped in the
openings of the discharge grates. It can vary from minor
pegging to very challenging situations (more than 40%
pegging) where the entire discharge grate is affected/sub-
stantially blocked as shown in Figure 12.
As described in the previous section, LDLs greater than
70 are an indication of the undesired situation of grinding
media overthrow which in turn produces both liner frac-
tures and broken grinding media. For optimal operation,
LDL is typically desired to be around ~50.
Figure 13 shows an industrial example for the LDL sig-
nal in a 40-foot diameter SAG mill. In this case, the LDL
reached values of Extreme High LDL 200 for over 5 hours
of operation as shown in the red circle. This LDL indicates
‘metal on metal’ undesired operation that fractures balls
and liners. When a significant quantity of broken balls is
combined with fine ore, lower density and high mill speed,
pegging has a very high opportunity to develop and the
discharge grates become blinded. The blinding of the dis-
charge grates creates an imbalance or blockage of flow in
the mill.
Luckily, this can be detected with the feed and discharge
trunnion sensors. The Trunnion Vibration Difference is
shown in Figure 13 (gray). Shortly after the Extreme High
LDL event, the Trunnion Vibration Difference migrates
(blue arrow) to high limit (20). This indicates the imbal-
ance between the feed and the discharge of the mill due
to the pegging event that was confirmed by inspections
as shown in the Figure 13 with vertical dotted lines. The
pegging affected the throughput severely, especially when
the second inspection (major pegging event, 75%) was
observed.
Figure 11. Mill speed control based on optimal impact angle (IA) in a 32ft SAG mill
Figure 12. Discharge grate pegging
Pegging Detection
Pegging occurs when rock or spalled balls get trapped in the
openings of the discharge grates. It can vary from minor
pegging to very challenging situations (more than 40%
pegging) where the entire discharge grate is affected/sub-
stantially blocked as shown in Figure 12.
As described in the previous section, LDLs greater than
70 are an indication of the undesired situation of grinding
media overthrow which in turn produces both liner frac-
tures and broken grinding media. For optimal operation,
LDL is typically desired to be around ~50.
Figure 13 shows an industrial example for the LDL sig-
nal in a 40-foot diameter SAG mill. In this case, the LDL
reached values of Extreme High LDL 200 for over 5 hours
of operation as shown in the red circle. This LDL indicates
‘metal on metal’ undesired operation that fractures balls
and liners. When a significant quantity of broken balls is
combined with fine ore, lower density and high mill speed,
pegging has a very high opportunity to develop and the
discharge grates become blinded. The blinding of the dis-
charge grates creates an imbalance or blockage of flow in
the mill.
Luckily, this can be detected with the feed and discharge
trunnion sensors. The Trunnion Vibration Difference is
shown in Figure 13 (gray). Shortly after the Extreme High
LDL event, the Trunnion Vibration Difference migrates
(blue arrow) to high limit (20). This indicates the imbal-
ance between the feed and the discharge of the mill due
to the pegging event that was confirmed by inspections
as shown in the Figure 13 with vertical dotted lines. The
pegging affected the throughput severely, especially when
the second inspection (major pegging event, 75%) was
observed.
Figure 11. Mill speed control based on optimal impact angle (IA) in a 32ft SAG mill
Figure 12. Discharge grate pegging