3778 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
drives (GMD), the poles of the electrical motor are directly
mounted on the mill, the stator is built around the mill,
hence the name of ring motor, and the torque is transmit-
ted magnetically through the air gap between the poles and
the stator. The first GMD had a power of 6.4 MW, turning
a 16 ft diameter mill and was commissioned by ABB in
1969 in a cement plant. Although it is unlikely that a drive
with such a relative low power would be gearless nowadays,
it is worth mentioning that it is still reliably in operation.
Since the start of operation of this first ring motor, GMDs
have steadily increased in power and diameter. Most of the
GMDs remain limited to the range where the power and
torque cannot be transmitted confidently by the mechani-
cal components of a ring geared mill drive (RMD). This
limit between RMD and GMD has evolved along with
the manufacturing capabilities of gearboxes, pinions, and
ring-gears. In the 70s, 4 MW per pinion constituted the
mechanical limit, nowadays RMDs are in operation with as
much as 9 MW per pinion.
GMDs entered the mineral processing industry in
the 80s. Throughput and mill size remained the driver for
continuously developing larger GMDs. However unlike in
the cement industry, mining processing plants often have
daily productions of several millions USD and the mills
constitute their heart and largest power consumers. Hence
operating expenses (OPEX), mainly related to availability
and efficiency, grew in importance. Gearbox failures, align-
ment issues, and damaged gearing are common causes of
unplanned downtime for RMDs. Such failures cannot
occur with GMDs due to the absence of mechanical com-
ponents, and the availability of GMDs is higher than for
RMDs as a result. GMDs have a higher efficiency than
RMDs for the same reason. Summing up the electricity
savings and increased production uptime, GMDs typically
achieve yearly OPEX savings of several millions USD com-
pared to their ring geared counterparts.
The 90s witnessed the largest progress in terms of power
and diameter of GMDs. At the beginning of the decade the
largest gearless mill had a diameter of 36 ft with only 11.2
MW rated power. At the end of the decade the power had
almost doubled to 20 MW, while the diameter had reached
40 ft and GMD manufacturers were optimistic about push-
ing the boundary further. In the paper “How big is big -
exploring today’s limits of SAG and ball mill technology”
Riezinger et al. (2001) concluded that a 44 ft GMD would
be feasible. This optimism was dampened when several fail-
ures on large GMDs recently put into operation affected
both motor manufacturers. As addressed in “How big is
big? Revisited” by Meimaris et al. (2006) it appeared that
several design weaknesses needed to be fixed before the next
step could be reliably made. Increasing the size was not the
priority anymore, instead several fundamental electrical
and mechanical design changes were implemented prevent-
ing any new failures of operating and new GMDs. The eco-
nomic crisis of 2008 further slowed down the development
of large mining projects, so that the development of bigger
GMDs was put on hold for several years.
The largest GMDs in operation nowadays are driv-
ing SAG 40 ft, 28 MW and ball mills 28 ft, 22 MW. The
lessons from the early failures have been learnt and these
GMDs have accumulated thousands of running hours with
availability of typically 99.9%. ABB also retains the record
of the largest manufactured GMD, awarded in 2010 for
a 42 ft diameter SAG mill for a project that was unfortu-
nately stopped for economic reasons (Orser, Svalbonas, and
van de Vijfeijken, 2011).
Productivity and throughput remain key drivers for
large GMDs, but the mining industry faces many new
challenges such as the necessity to reduce greenhouse gas
emissions, lower ore grades, mining at high altitude and
unfriendly ambient conditions, or increased shareholder
pressure for a fast return on investment. With its unri-
valled efficiency, availability, connectivity, and throughput
allowing to reduce the number of processing lines, GMDs
become part of the solution.
SAG AND BALL MILLS
For many years grinding mill size development has remained
relatively static at the long-established combination of 40 ft
SAG and 28 ft ball mill. However, in recent years there
has been increasing interest in larger SAG and ball mills.
Correspondingly, this has fueled research and investment
into progressively more-detailed evaluations of design and
manufacturing limits, oftentimes driven by early-stage flow
sheet development.
Following a detailed review of the published literature,
state-of-the art technologies, design tools, and manufactur-
ing capabilities, Bordi and Green (2023) presented recom-
mendations for the next increment in mill size development
at the 2023 SAG Conference. For SAG mills the next step
is clearly defined as 44 ft with power up to approximately
35 MW. For ball mills, power is the preferred metric with
a 29 to 30 MW target using a diameter in the range 30 to
32 ft depending on power and application.
Notably, these proposed sizes are not at the absolute
limit of engineering or manufacturing capabilities, but
rather the sizes which represent a sensible and logical next
step with a quantifiable and manageable risk profile. Bordi
and Green (2023) outlined a series of capabilities, technolo-
gies and engineering methods that facilitate risk mitigation.
drives (GMD), the poles of the electrical motor are directly
mounted on the mill, the stator is built around the mill,
hence the name of ring motor, and the torque is transmit-
ted magnetically through the air gap between the poles and
the stator. The first GMD had a power of 6.4 MW, turning
a 16 ft diameter mill and was commissioned by ABB in
1969 in a cement plant. Although it is unlikely that a drive
with such a relative low power would be gearless nowadays,
it is worth mentioning that it is still reliably in operation.
Since the start of operation of this first ring motor, GMDs
have steadily increased in power and diameter. Most of the
GMDs remain limited to the range where the power and
torque cannot be transmitted confidently by the mechani-
cal components of a ring geared mill drive (RMD). This
limit between RMD and GMD has evolved along with
the manufacturing capabilities of gearboxes, pinions, and
ring-gears. In the 70s, 4 MW per pinion constituted the
mechanical limit, nowadays RMDs are in operation with as
much as 9 MW per pinion.
GMDs entered the mineral processing industry in
the 80s. Throughput and mill size remained the driver for
continuously developing larger GMDs. However unlike in
the cement industry, mining processing plants often have
daily productions of several millions USD and the mills
constitute their heart and largest power consumers. Hence
operating expenses (OPEX), mainly related to availability
and efficiency, grew in importance. Gearbox failures, align-
ment issues, and damaged gearing are common causes of
unplanned downtime for RMDs. Such failures cannot
occur with GMDs due to the absence of mechanical com-
ponents, and the availability of GMDs is higher than for
RMDs as a result. GMDs have a higher efficiency than
RMDs for the same reason. Summing up the electricity
savings and increased production uptime, GMDs typically
achieve yearly OPEX savings of several millions USD com-
pared to their ring geared counterparts.
The 90s witnessed the largest progress in terms of power
and diameter of GMDs. At the beginning of the decade the
largest gearless mill had a diameter of 36 ft with only 11.2
MW rated power. At the end of the decade the power had
almost doubled to 20 MW, while the diameter had reached
40 ft and GMD manufacturers were optimistic about push-
ing the boundary further. In the paper “How big is big -
exploring today’s limits of SAG and ball mill technology”
Riezinger et al. (2001) concluded that a 44 ft GMD would
be feasible. This optimism was dampened when several fail-
ures on large GMDs recently put into operation affected
both motor manufacturers. As addressed in “How big is
big? Revisited” by Meimaris et al. (2006) it appeared that
several design weaknesses needed to be fixed before the next
step could be reliably made. Increasing the size was not the
priority anymore, instead several fundamental electrical
and mechanical design changes were implemented prevent-
ing any new failures of operating and new GMDs. The eco-
nomic crisis of 2008 further slowed down the development
of large mining projects, so that the development of bigger
GMDs was put on hold for several years.
The largest GMDs in operation nowadays are driv-
ing SAG 40 ft, 28 MW and ball mills 28 ft, 22 MW. The
lessons from the early failures have been learnt and these
GMDs have accumulated thousands of running hours with
availability of typically 99.9%. ABB also retains the record
of the largest manufactured GMD, awarded in 2010 for
a 42 ft diameter SAG mill for a project that was unfortu-
nately stopped for economic reasons (Orser, Svalbonas, and
van de Vijfeijken, 2011).
Productivity and throughput remain key drivers for
large GMDs, but the mining industry faces many new
challenges such as the necessity to reduce greenhouse gas
emissions, lower ore grades, mining at high altitude and
unfriendly ambient conditions, or increased shareholder
pressure for a fast return on investment. With its unri-
valled efficiency, availability, connectivity, and throughput
allowing to reduce the number of processing lines, GMDs
become part of the solution.
SAG AND BALL MILLS
For many years grinding mill size development has remained
relatively static at the long-established combination of 40 ft
SAG and 28 ft ball mill. However, in recent years there
has been increasing interest in larger SAG and ball mills.
Correspondingly, this has fueled research and investment
into progressively more-detailed evaluations of design and
manufacturing limits, oftentimes driven by early-stage flow
sheet development.
Following a detailed review of the published literature,
state-of-the art technologies, design tools, and manufactur-
ing capabilities, Bordi and Green (2023) presented recom-
mendations for the next increment in mill size development
at the 2023 SAG Conference. For SAG mills the next step
is clearly defined as 44 ft with power up to approximately
35 MW. For ball mills, power is the preferred metric with
a 29 to 30 MW target using a diameter in the range 30 to
32 ft depending on power and application.
Notably, these proposed sizes are not at the absolute
limit of engineering or manufacturing capabilities, but
rather the sizes which represent a sensible and logical next
step with a quantifiable and manageable risk profile. Bordi
and Green (2023) outlined a series of capabilities, technolo-
gies and engineering methods that facilitate risk mitigation.