XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3711
Drive System Selection
Operational Flexibility Resulting into Smaller Carbon
Footprint per Ton of Concentrate
It is a well-known common practice that rather sooner than
later a concentrator will push tonnage, exceeding the origi-
nal design throughput. Even if this would be at the cost of
a lower relative recovery when not enough installed mill-
ing power is available, because the total absolute recovery
will still be higher, justifying it from an economical view-
point. Nowadays there is often a focus on the efficiency of
the comminution circuit only: how much tonnage can be
handled, resulting into a certain amount of CO2 emissions
per ton of processed ore. However, we need to keep in mind
that the ore particles are being ground only in order to lib-
erate the target minerals from the gangue, so that concen-
trate can be produced. Therefore, the focus should rather be
on CO2 emissions per ton of produced concentrate. In case of
increasing tonnage throughput, the relative recovery is typi-
cally decreasing due to a coarser grind. But it still takes a
vast amount of energy to reduce the rejected particles with
non-liberated target minerals to the “just-too-coarse” size
and the total CO2 emissions per ton of produced concen-
trate increases. From a global perspective, the unnecessary
recovery losses should be compensated by mining, trans-
porting, crushing and grinding new ore, causing additional
CO2 emissions. Furthermore, as the examples shown in
this paper, the variable speed option for a ball mill shall
not be disregarded. On the contrary, variable speed for a
ball mill should be seriously considered during equipment
selection, if this operational flexibility could later result into
potential benefits minimizing the CO2 emissions per ton of
produced concentrate.
For the larger 28 MW ball mills in this case study, the
only viable drive option is the gearless mill drive, where the
electrical rotor poles are mounted directly on the mill pole
flange, eliminating the ring-gear and pinions. The gearless
mill drive with the inherent variable speed feature provides
not only the highest electrical efficiency, but also the high-
est availability. The smaller 18.6 MW ball mills are well
within the typical range of gearless mill drives and at the
upper limit of variable low-speed dual pinion ring-geared
mill drives. For this generic comparison study variable low-
speed dual pinion ring-geared mill drives are considered for
the 18.6 MW ball mills. The typical efficiencies for these
two mill drive options are shown in Table 2.
Availability
The overall mill availability plays undoubtedly also an
important role. Around two decades ago, there have been
issues with large mills and drive systems, some of them
resulting into significant unscheduled downtime, although
in hindsight not all failures should have ended in the way
they did (Bos, van de Vijfeijken and Koponen, 2011). The
suppliers and operators have stepped up since and the avail-
ability of gearless mills has increased to the highest levels.
The gearless mill drive eliminates all mechanical transmis-
sion components in the drive train and is typically sub-
stantially better monitored than a ring-geared mill drive.
Furthermore, the gearless mill drive consists of only one
mill motor, as where the dual pinion mill drive has two mill
motors, resulting into higher overall mill availability of the
gearless mill. The typical availability values for the two mill
drive options are shown in Table 3.
The typical total mill availability of a gearless mill with
GMD is 0.77% higher than of the smaller dual pinion
ring-geared mill, which at first sight does not seem to be
significant. However, the overall expected average annual
downtime of a gearless mill is 35.5% lower than of a geared
mill. Furthermore 0.77% represents almost three days loss
of production on an annual basis, where each day of pro-
duction loss equals millions USD costs.
Total CAPEX and OPEX Comparison
A total CAPEX comparison (including the mills, drives,
foundation, cabling, transportation, etc.) for two large gear-
less versus three smaller geared driven mills resulted into
only around 6.2% higher CAPEX for the two mill option.
The exact numbers are obviously project specific, as cer-
tain costs like concrete and transportation vary depending
upon the project location. The 6.2% difference in CAPEX
Table 2. Typical mill drive train efficiencies
Efficiency GMD
Dual Pinion Variable
Low-Speed
Mechanical efficiency [%]100.0 98.5
Transformer efficiency [%]99.0 99.0
Variable speed drive efficiency [%]99.4 98.6
Motor efficiency [%]96.5 97.1
Total electrical efficiency [%]95.0 94.8
Total Mill Drive Train Efficiency [%]95.0 93.4
Drive System Selection
Operational Flexibility Resulting into Smaller Carbon
Footprint per Ton of Concentrate
It is a well-known common practice that rather sooner than
later a concentrator will push tonnage, exceeding the origi-
nal design throughput. Even if this would be at the cost of
a lower relative recovery when not enough installed mill-
ing power is available, because the total absolute recovery
will still be higher, justifying it from an economical view-
point. Nowadays there is often a focus on the efficiency of
the comminution circuit only: how much tonnage can be
handled, resulting into a certain amount of CO2 emissions
per ton of processed ore. However, we need to keep in mind
that the ore particles are being ground only in order to lib-
erate the target minerals from the gangue, so that concen-
trate can be produced. Therefore, the focus should rather be
on CO2 emissions per ton of produced concentrate. In case of
increasing tonnage throughput, the relative recovery is typi-
cally decreasing due to a coarser grind. But it still takes a
vast amount of energy to reduce the rejected particles with
non-liberated target minerals to the “just-too-coarse” size
and the total CO2 emissions per ton of produced concen-
trate increases. From a global perspective, the unnecessary
recovery losses should be compensated by mining, trans-
porting, crushing and grinding new ore, causing additional
CO2 emissions. Furthermore, as the examples shown in
this paper, the variable speed option for a ball mill shall
not be disregarded. On the contrary, variable speed for a
ball mill should be seriously considered during equipment
selection, if this operational flexibility could later result into
potential benefits minimizing the CO2 emissions per ton of
produced concentrate.
For the larger 28 MW ball mills in this case study, the
only viable drive option is the gearless mill drive, where the
electrical rotor poles are mounted directly on the mill pole
flange, eliminating the ring-gear and pinions. The gearless
mill drive with the inherent variable speed feature provides
not only the highest electrical efficiency, but also the high-
est availability. The smaller 18.6 MW ball mills are well
within the typical range of gearless mill drives and at the
upper limit of variable low-speed dual pinion ring-geared
mill drives. For this generic comparison study variable low-
speed dual pinion ring-geared mill drives are considered for
the 18.6 MW ball mills. The typical efficiencies for these
two mill drive options are shown in Table 2.
Availability
The overall mill availability plays undoubtedly also an
important role. Around two decades ago, there have been
issues with large mills and drive systems, some of them
resulting into significant unscheduled downtime, although
in hindsight not all failures should have ended in the way
they did (Bos, van de Vijfeijken and Koponen, 2011). The
suppliers and operators have stepped up since and the avail-
ability of gearless mills has increased to the highest levels.
The gearless mill drive eliminates all mechanical transmis-
sion components in the drive train and is typically sub-
stantially better monitored than a ring-geared mill drive.
Furthermore, the gearless mill drive consists of only one
mill motor, as where the dual pinion mill drive has two mill
motors, resulting into higher overall mill availability of the
gearless mill. The typical availability values for the two mill
drive options are shown in Table 3.
The typical total mill availability of a gearless mill with
GMD is 0.77% higher than of the smaller dual pinion
ring-geared mill, which at first sight does not seem to be
significant. However, the overall expected average annual
downtime of a gearless mill is 35.5% lower than of a geared
mill. Furthermore 0.77% represents almost three days loss
of production on an annual basis, where each day of pro-
duction loss equals millions USD costs.
Total CAPEX and OPEX Comparison
A total CAPEX comparison (including the mills, drives,
foundation, cabling, transportation, etc.) for two large gear-
less versus three smaller geared driven mills resulted into
only around 6.2% higher CAPEX for the two mill option.
The exact numbers are obviously project specific, as cer-
tain costs like concrete and transportation vary depending
upon the project location. The 6.2% difference in CAPEX
Table 2. Typical mill drive train efficiencies
Efficiency GMD
Dual Pinion Variable
Low-Speed
Mechanical efficiency [%]100.0 98.5
Transformer efficiency [%]99.0 99.0
Variable speed drive efficiency [%]99.4 98.6
Motor efficiency [%]96.5 97.1
Total electrical efficiency [%]95.0 94.8
Total Mill Drive Train Efficiency [%]95.0 93.4