XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3703
Drive System Selection—Drive Train &Operational
Grinding Efficiency
For the 28 MW ball mills, gearless mill drives are the only
feasible option. Various drive systems, each with its own
features, are possible for the 18.6 MW mills: fixed and
variable speed mill drives, high-speed (with a gearbox) and
low-speed (without a gearbox, motor direct coupled to the
pinion) ring-geared mill drives, and gearless mill drives.
Since the mill drives are typically the largest electrical power
consumers in the plant, and often even in the region, effi-
ciency is nowadays pivotal in their selection: every percent
of efficiency increase has a noteworthy impact on the CO2
emissions of the complete concentrator. There are two dif-
ferent kinds of efficiencies to be considered:
1. The drive train efficiency
2. The operational grinding efficiency
Drive Train Efficiency
High-speed ring-geared mill drives with gearboxes, wound
rotor induction motors and liquid resistor starters have
a rather low overall drive train efficiency of around 93%
for fixed speed, and even lower for variable speed, which
is too low for these enormous power consumers, as they
would produce too many unnecessary CO2 emissions.
Additionally, their inherent torque spikes (Hamilton et
al. 2006) during starting would cause considerable stress
on the mechanical drive train. Low-speed dual pinion mill
drives have a considerable better drive train efficiency, since
the relative high gearbox losses are eliminated. Gearless
mill drives have by far the best drive train efficiency, as all
mechanical elements in the drive train such as ring-gear,
pinions and gearboxes are removed, while the typical cyclo-
converter efficiency is high due to minimized components
in this direct converter.
Operational Grinding Efficiency: Variable Speed
Ball Mills
Variable speed is commonly accepted as a must for SAG
mills. But ball mills, providing the last size reduction step
before concentration, are still today sometimes equipped
with fixed speed, based upon the crude argument that
in an ideal world the ball mill should just be operated at
maximum speed and maximum power draw, because in the
closed circuit the cyclone takes anyway care of the classifica-
tion. Cyclones are relative economical, occupy a rather small
footprint, but they are not a perfect classifier (Jankovic et
al. 2013). Additionally, the world in a concentrator is often
not ideal and variable speed on ball mills, the last process
step before concentration, would enable several grinding
efficiency optimization opportunities. Globally numerous
ball mills (both gearless and ring-geared) are equipped with
variable speed drives from ABB with remote monitoring
Figure 2 show some histograms of different variable speed
ball mills on four different sites, and, for some reason, the
variable speed feature is obviously being used. Some are
using a wider speed range than others, but it is interesting
that most of the time the speed of all four ball mills is a bit
slower than the maximum operational speed, also indicat-
ing that the operators are not just simply running the ball
mills flat out.
Literature Study Variable Speed Ball Mills
Orebodies are intrinsically variable in composition and
physical properties by the virtue of their heterogeneous
nature (Bueno, Foggiatto and Lane, 2015), implying that
some operational flexibility should be desirable. But are
there practical examples where variable speed on ball mills
have indeed resulted in energy savings and carbon footprint
reductions? A brief literature study of papers in the pub-
lic domain shows that from a process perspective, variable
speed on a ball mill does indeed provide significant opera-
tional benefits in the last size reduction step before concen-
tration, potentially leading to considerable energy savings
and carbon footprint reduction.
Feed Rate and Ore Hardnesss. During ramping up
of a new concentrator or new process line, it typically takes
a significant amount of time (months if not years) until
the design throughput capacity is achieved. Additionally,
the ore hardness is often increasing over time when ore is
mined from deeper levels. The Merian Gold plant from
Newmont in Suriname consists of an SABC circuit with
gravity concentration and leaching. Both the SAG and ball
mill are low-speed dual pinion mills, each equipped with a
2 × 6.5 MW variable speed drive system. Operation started
in 2016 and the following was presented (Davies et al.
2019): “Initial operation of the Merian plant was planned
to have a high proportion of saprolite for several years prior
to the transition to predominantly fresh rock feed. With
limited competent material, retaining an ore charge within
the SAG mill was not possible. During initial operations at
up to 75% critical speed led to ball/liner impacts and evi-
dence of peening on the liners. Both the SAG and Ball mills
were installed with ABB synchronous variable frequency drives
(VFD) allowing for complete control of mill speed on both
mills. Operation of the SAG mill at as low as 50% critical
speed was possible without material transfer becoming a
limiting factor to throughput. The fine nature of the saprolite
feed also meant the ball mill could initially be operated as low
as 60% critical speed without impacting the grind size targets.
Drive System Selection—Drive Train &Operational
Grinding Efficiency
For the 28 MW ball mills, gearless mill drives are the only
feasible option. Various drive systems, each with its own
features, are possible for the 18.6 MW mills: fixed and
variable speed mill drives, high-speed (with a gearbox) and
low-speed (without a gearbox, motor direct coupled to the
pinion) ring-geared mill drives, and gearless mill drives.
Since the mill drives are typically the largest electrical power
consumers in the plant, and often even in the region, effi-
ciency is nowadays pivotal in their selection: every percent
of efficiency increase has a noteworthy impact on the CO2
emissions of the complete concentrator. There are two dif-
ferent kinds of efficiencies to be considered:
1. The drive train efficiency
2. The operational grinding efficiency
Drive Train Efficiency
High-speed ring-geared mill drives with gearboxes, wound
rotor induction motors and liquid resistor starters have
a rather low overall drive train efficiency of around 93%
for fixed speed, and even lower for variable speed, which
is too low for these enormous power consumers, as they
would produce too many unnecessary CO2 emissions.
Additionally, their inherent torque spikes (Hamilton et
al. 2006) during starting would cause considerable stress
on the mechanical drive train. Low-speed dual pinion mill
drives have a considerable better drive train efficiency, since
the relative high gearbox losses are eliminated. Gearless
mill drives have by far the best drive train efficiency, as all
mechanical elements in the drive train such as ring-gear,
pinions and gearboxes are removed, while the typical cyclo-
converter efficiency is high due to minimized components
in this direct converter.
Operational Grinding Efficiency: Variable Speed
Ball Mills
Variable speed is commonly accepted as a must for SAG
mills. But ball mills, providing the last size reduction step
before concentration, are still today sometimes equipped
with fixed speed, based upon the crude argument that
in an ideal world the ball mill should just be operated at
maximum speed and maximum power draw, because in the
closed circuit the cyclone takes anyway care of the classifica-
tion. Cyclones are relative economical, occupy a rather small
footprint, but they are not a perfect classifier (Jankovic et
al. 2013). Additionally, the world in a concentrator is often
not ideal and variable speed on ball mills, the last process
step before concentration, would enable several grinding
efficiency optimization opportunities. Globally numerous
ball mills (both gearless and ring-geared) are equipped with
variable speed drives from ABB with remote monitoring
Figure 2 show some histograms of different variable speed
ball mills on four different sites, and, for some reason, the
variable speed feature is obviously being used. Some are
using a wider speed range than others, but it is interesting
that most of the time the speed of all four ball mills is a bit
slower than the maximum operational speed, also indicat-
ing that the operators are not just simply running the ball
mills flat out.
Literature Study Variable Speed Ball Mills
Orebodies are intrinsically variable in composition and
physical properties by the virtue of their heterogeneous
nature (Bueno, Foggiatto and Lane, 2015), implying that
some operational flexibility should be desirable. But are
there practical examples where variable speed on ball mills
have indeed resulted in energy savings and carbon footprint
reductions? A brief literature study of papers in the pub-
lic domain shows that from a process perspective, variable
speed on a ball mill does indeed provide significant opera-
tional benefits in the last size reduction step before concen-
tration, potentially leading to considerable energy savings
and carbon footprint reduction.
Feed Rate and Ore Hardnesss. During ramping up
of a new concentrator or new process line, it typically takes
a significant amount of time (months if not years) until
the design throughput capacity is achieved. Additionally,
the ore hardness is often increasing over time when ore is
mined from deeper levels. The Merian Gold plant from
Newmont in Suriname consists of an SABC circuit with
gravity concentration and leaching. Both the SAG and ball
mill are low-speed dual pinion mills, each equipped with a
2 × 6.5 MW variable speed drive system. Operation started
in 2016 and the following was presented (Davies et al.
2019): “Initial operation of the Merian plant was planned
to have a high proportion of saprolite for several years prior
to the transition to predominantly fresh rock feed. With
limited competent material, retaining an ore charge within
the SAG mill was not possible. During initial operations at
up to 75% critical speed led to ball/liner impacts and evi-
dence of peening on the liners. Both the SAG and Ball mills
were installed with ABB synchronous variable frequency drives
(VFD) allowing for complete control of mill speed on both
mills. Operation of the SAG mill at as low as 50% critical
speed was possible without material transfer becoming a
limiting factor to throughput. The fine nature of the saprolite
feed also meant the ball mill could initially be operated as low
as 60% critical speed without impacting the grind size targets.