3706 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
Detour Lake from Agnico Eagle in Canada has a similar
grinding circuit and mill sizes as Constancia, but with the
substantial difference that not only the SAG mills, but also
the ball mills are variable low-speed driven, which enabled
additional energy savings, when changing from metal to
lighter rubber liners (Torrealba-Vargas et al. 2019): “An
extra 10% to 20% in energy savings (working index effi-
ciency at approximately similar P80) could be found by
adapting the mill’s operational parameters (speed) based on
liner profile, but this energy savings cannot be evaluated
independently of the evaluation of the grinding circuit
functional performance equations (in this study, the refer-
ence was t/h of –75 μm).” This paper clearly concluded:
“Initial investment of variable speed drives for ball mills pres-
ents positive outcomes in terms of production and operational
flexibility.”
As described above, variable speed enables operation of
a ball mill with a high grinding efficiency by adjusting the
mill speed if a change to a different liner technology (dif-
ferent shape, material and/or weight) is applied in the ball
mill. Additionally, also the status of the liners (new, half-
worn, worn) has a significant impact on both the opera-
tional grinding efficiency and the plant availability as shown
in a study (Mejia and Klein 2019) on a 24' × 39' ball mill:
“It is very well known by mill operators that mill through-
put decreases as the liner wears that force them to increase
the ball load and mill speed to maintain mill throughput.”
Therefore, the grinding efficiency in fixed speed mills is
influenced by the fluctuation of the mill power draw and
the location of the shoulder, which varies with the liner sta-
tus as shown in Figure 5 and 6.
The result is that in fixed speed ball mills the ball mill
operating work index increases notably as the liners wear
down as shown in Figure 7, even if the operators would
manage a constant mill power draw by adapting the ball
charge. This results inevitably into additional CO2 emis-
sions per ton of concentrate produced, which would be
avoided if the mill speed could be adjusted over the mill
liner lifetime: “The Wio is an indicator of grinding ineffi-
ciency, which can be used to define the point in time when
the mill liners need to be changed, or when the mill speed
need to be increased.”
The conclusion of this study is very clear about the
enormous benefits of having variable speed ball mills: “Mill
throughput can be improved by changing liners more fre-
quently or by increasing mill speed to maintain throughput.
Two scenarios for increasing mill throughput were analyzed.
The first one involves changing the liner before the efficiency
drops and the second one requires increasing the mill speed from
75% to 80%. Both approaches show considerable financial
benefits. However, the benefits for scenario 1 depends on
mill down time needed for liner replacement resulting in
lost production, whereas scenario 2 does not require mill shut-
downs. Mill grinding efficiency of scenario 1, increasing the
number of liner changes, produces an increase net cash flow
of $1.1 million and scenario 2, increasing mill speed, produces
an increase net cash flow of$1.9 million.”
Source: Mejia and Klein 2019
Figure 5. Fluctuating power draw as liners wear
Detour Lake from Agnico Eagle in Canada has a similar
grinding circuit and mill sizes as Constancia, but with the
substantial difference that not only the SAG mills, but also
the ball mills are variable low-speed driven, which enabled
additional energy savings, when changing from metal to
lighter rubber liners (Torrealba-Vargas et al. 2019): “An
extra 10% to 20% in energy savings (working index effi-
ciency at approximately similar P80) could be found by
adapting the mill’s operational parameters (speed) based on
liner profile, but this energy savings cannot be evaluated
independently of the evaluation of the grinding circuit
functional performance equations (in this study, the refer-
ence was t/h of –75 μm).” This paper clearly concluded:
“Initial investment of variable speed drives for ball mills pres-
ents positive outcomes in terms of production and operational
flexibility.”
As described above, variable speed enables operation of
a ball mill with a high grinding efficiency by adjusting the
mill speed if a change to a different liner technology (dif-
ferent shape, material and/or weight) is applied in the ball
mill. Additionally, also the status of the liners (new, half-
worn, worn) has a significant impact on both the opera-
tional grinding efficiency and the plant availability as shown
in a study (Mejia and Klein 2019) on a 24' × 39' ball mill:
“It is very well known by mill operators that mill through-
put decreases as the liner wears that force them to increase
the ball load and mill speed to maintain mill throughput.”
Therefore, the grinding efficiency in fixed speed mills is
influenced by the fluctuation of the mill power draw and
the location of the shoulder, which varies with the liner sta-
tus as shown in Figure 5 and 6.
The result is that in fixed speed ball mills the ball mill
operating work index increases notably as the liners wear
down as shown in Figure 7, even if the operators would
manage a constant mill power draw by adapting the ball
charge. This results inevitably into additional CO2 emis-
sions per ton of concentrate produced, which would be
avoided if the mill speed could be adjusted over the mill
liner lifetime: “The Wio is an indicator of grinding ineffi-
ciency, which can be used to define the point in time when
the mill liners need to be changed, or when the mill speed
need to be increased.”
The conclusion of this study is very clear about the
enormous benefits of having variable speed ball mills: “Mill
throughput can be improved by changing liners more fre-
quently or by increasing mill speed to maintain throughput.
Two scenarios for increasing mill throughput were analyzed.
The first one involves changing the liner before the efficiency
drops and the second one requires increasing the mill speed from
75% to 80%. Both approaches show considerable financial
benefits. However, the benefits for scenario 1 depends on
mill down time needed for liner replacement resulting in
lost production, whereas scenario 2 does not require mill shut-
downs. Mill grinding efficiency of scenario 1, increasing the
number of liner changes, produces an increase net cash flow
of $1.1 million and scenario 2, increasing mill speed, produces
an increase net cash flow of$1.9 million.”
Source: Mejia and Klein 2019
Figure 5. Fluctuating power draw as liners wear