3712 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
includes the mills with their complete drive trains, estima-
tions for the required foundations, as well as the installa-
tion and commissioning costs. It must be noted that for
this comparison study an outdoor installation has been
considered. In case due to challenging ambient conditions
a concentrator building would be considered, then due to
the significant smaller footprint as shown in Figure 9 there
would be additional significant CAPEX savings for the
gearless option with two mills
On the OPEX side, the gearless mill drive with its
1.6% better drive train efficiency and 35.5% less down-
time is clearly the favorized solution, resulting in a relative
short Return Of Investment (ROI) of the additional 6.2%
CAPEX difference. This ROI depends upon several factors
such as the considered project specific energy price, which
has the potential to increase non-linear over the life of mine
and the loss of production costs. In this study a relative
low energy price of only 0.08 USD/kWh was used. Rather
low foss of production costs of USD 2’000’000.- per day
for the complete concentrator were considered. With these
rather conservative assumptions the ROI for the grinding
circuit with two large gearless mills versus three smaller
geared drives mills is still slightly less than one year, only
11.2 months.
Total CO2 emissions comparison
Both grinding line options consider an overall installed
power of 56,000 kW. When evaluating a 1.6% higher
electrical efficiency, the ball mills utilizing gearless grind-
ing mills will draw 896 kW less power from the electrical
supply grid. The process design criteria for this trade-off
study considered a grinding mill availability of 92% which
equates to 7,221,043 kWh of energy savings for a full year
of operation. Many mines are designed for an operational
life of 20+ years. Considering 20 years of operation as a
minimum, the energy savings realized over the life of the
mine equates to 144.4 Gigawatt-hour. This energy savings
over the life of the mine equates to 118,000 tonnes of CO2
emissions (IPCC, 2014), the equivalent of 26,300 passen-
ger vehicles driven for one year in the United States (EPA,
2023), based on a approach where coal is the marginal elec-
tricity source (Electricity Maps, 2019) .This approach has
been taken because coal is still the largest fuel used glob-
ally for electricity (IEA, 2023) and so it is assumed to be
the short-term marginal source of electricity, meaning that
if a product were to use less electricity, less coal would be
required to be burnt. Renewables like wind and solar will
however almost always provide as much to the grid as they
produce. It should be noted that this is at a global level and
marginal national electricity sources are dependent on the
local mix among other factors.
CONCLUSIONS
Gearless grinding mill drives have a higher CAPEX than
equivalent power gear driven mills, however per the
detailed evaluation conducted, the return on investment
was less than a year. Additionally, it has been identified that
the overall grinding line width (and if applicable the respec-
tive concentrator building width) could be reduced by 14%
when considering fewer but larger gearless grinding mills.
Grinding mill sizing and motor technology selec-
tion plays a critical role in the overall minerals processing
concentrator carbon footprint, as the grinding mill is the
leading power consumer. With large grinding mills, the
prominent drive technologies, variable low-speed synchro-
nous motors and gearless mill drive, provide inherent vari-
able speed capability, which allow the end user to have a
fine level of power output optimization to improve energy
efficiency. In order to truly minimize CO2 emissions per
ton of concentrate produced, no compromises shall be
Table 3. Typical availability
Availability GMD
Dual Pinion Variable
Low-Speed
Mill availability excl. drive train [%]
(Mainly mill relining)
98.0 98.0
Transformer Availability [%]99.97 99.98
Converter related Availability [%]99.99 99.98
Motor related Availability [%]99.97 99.92
Girth gear related Availability [%]100.00 99.82
Pinion related Availability [%]100.00 99.64
Gearbox related Availability [%]100.00 100.00
Special services related Availability [%]
(additional to the mill relining)
99.91 99.73
Mill Availability [%]TOTAL 97.84 97.08
includes the mills with their complete drive trains, estima-
tions for the required foundations, as well as the installa-
tion and commissioning costs. It must be noted that for
this comparison study an outdoor installation has been
considered. In case due to challenging ambient conditions
a concentrator building would be considered, then due to
the significant smaller footprint as shown in Figure 9 there
would be additional significant CAPEX savings for the
gearless option with two mills
On the OPEX side, the gearless mill drive with its
1.6% better drive train efficiency and 35.5% less down-
time is clearly the favorized solution, resulting in a relative
short Return Of Investment (ROI) of the additional 6.2%
CAPEX difference. This ROI depends upon several factors
such as the considered project specific energy price, which
has the potential to increase non-linear over the life of mine
and the loss of production costs. In this study a relative
low energy price of only 0.08 USD/kWh was used. Rather
low foss of production costs of USD 2’000’000.- per day
for the complete concentrator were considered. With these
rather conservative assumptions the ROI for the grinding
circuit with two large gearless mills versus three smaller
geared drives mills is still slightly less than one year, only
11.2 months.
Total CO2 emissions comparison
Both grinding line options consider an overall installed
power of 56,000 kW. When evaluating a 1.6% higher
electrical efficiency, the ball mills utilizing gearless grind-
ing mills will draw 896 kW less power from the electrical
supply grid. The process design criteria for this trade-off
study considered a grinding mill availability of 92% which
equates to 7,221,043 kWh of energy savings for a full year
of operation. Many mines are designed for an operational
life of 20+ years. Considering 20 years of operation as a
minimum, the energy savings realized over the life of the
mine equates to 144.4 Gigawatt-hour. This energy savings
over the life of the mine equates to 118,000 tonnes of CO2
emissions (IPCC, 2014), the equivalent of 26,300 passen-
ger vehicles driven for one year in the United States (EPA,
2023), based on a approach where coal is the marginal elec-
tricity source (Electricity Maps, 2019) .This approach has
been taken because coal is still the largest fuel used glob-
ally for electricity (IEA, 2023) and so it is assumed to be
the short-term marginal source of electricity, meaning that
if a product were to use less electricity, less coal would be
required to be burnt. Renewables like wind and solar will
however almost always provide as much to the grid as they
produce. It should be noted that this is at a global level and
marginal national electricity sources are dependent on the
local mix among other factors.
CONCLUSIONS
Gearless grinding mill drives have a higher CAPEX than
equivalent power gear driven mills, however per the
detailed evaluation conducted, the return on investment
was less than a year. Additionally, it has been identified that
the overall grinding line width (and if applicable the respec-
tive concentrator building width) could be reduced by 14%
when considering fewer but larger gearless grinding mills.
Grinding mill sizing and motor technology selec-
tion plays a critical role in the overall minerals processing
concentrator carbon footprint, as the grinding mill is the
leading power consumer. With large grinding mills, the
prominent drive technologies, variable low-speed synchro-
nous motors and gearless mill drive, provide inherent vari-
able speed capability, which allow the end user to have a
fine level of power output optimization to improve energy
efficiency. In order to truly minimize CO2 emissions per
ton of concentrate produced, no compromises shall be
Table 3. Typical availability
Availability GMD
Dual Pinion Variable
Low-Speed
Mill availability excl. drive train [%]
(Mainly mill relining)
98.0 98.0
Transformer Availability [%]99.97 99.98
Converter related Availability [%]99.99 99.98
Motor related Availability [%]99.97 99.92
Girth gear related Availability [%]100.00 99.82
Pinion related Availability [%]100.00 99.64
Gearbox related Availability [%]100.00 100.00
Special services related Availability [%]
(additional to the mill relining)
99.91 99.73
Mill Availability [%]TOTAL 97.84 97.08