XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3709
the additional losses of the gearboxes and pinions/ring-gear,
there is no margin at all left on the installed power. As soon
as the actual BWi would be slightly higher or Constancia
would be slightly pushing tonnage over the original design
throughput of 76’000 tpd, then immediately the installed
mill power becomes the bottleneck, increasing the P80 over
the original design value of 106 μm. Lane et al. (2015)
also stated: “The ball mills were potentially slightly over-
loaded after Year 6 producing a P80 of between 120 and
130 microns compared with the target P80 of 106 microns.
The value of the increase in throughput more than coun-
tered the loss of recovery due to the coarser P80.” Indeed,
the plant is achieving higher throughput between 80’000
and 90’000 tpd, but with a significant higher new P80 tar-
get of 160 μm and a consequently relative low copper recov-
ery of around 84 -87% only, instead of the original 90%
design value, even when the ball mill motors are nowadays
operating at a slightly higher power of up to 8250 kW each
(Tavchandjian, 2021). Additionally, Klohn et al. (2016)
stated that in the design phase it was recognized that “High
zinc ores were recognized during the DFS as being poten-
tially problematic during treatment, with zinc (as sphalerite)
reporting to the copper concentrate as a penalty element.”
Indeed, the coarser P80 results into a higher lead and zinc
contamination of the final copper concentrate as reported
in the NI43-101 Technical Report (Tavchandjian, 2021):
“Optimization studies will continue with a focus on reduc-
ing levels of zinc and lead contaminants and increasing cop-
per concentrate grades.” At first sight it may be surprising
that for these large power consumers rather inefficient mill
drives (fixed high-speed ring-geared with gearboxes) were
selected, which do not even provide operating flexibility of
variable speed to maximize copper recovery and minimize
zinc and lead contamination but in those days energy effi-
ciency was not always a priority, as confirmed by Lane et al.
(2017): “The first is the Constancia Project where energy
efficiency was not a major consideration in equipment selec-
tion..” Klohn, Stephenson and Granados (2016) reported:
“Ausenco’s design approach focused on developing a capital
efficient copper molybdenum concentrator.” Gearless mill
drives would have resulted into higher CAPEX, but then
also a higher installed power would have been easily possi-
ble, resulting into a finer grind and higher copper recovery,
as well as a lower zinc and lead contamination in the final
copper concentrate due to a more selective flotation. Also,
a higher drive train efficiency and lower CO2 emissions per
ton of concentrate would have been achieved with gearless
mill drives. Interestingly on the downstream side a 3 MW
regrind mill is further reducing the rougher concentrate
particle sizes to a target P80 of 25 μm, increasing the final
copper recovery, but unfortunately this does not help to
reduce the recovery losses realized upstream at the rougher
flotation.
Advantages of variable speed known since more
than a century. The call for variable speed ball mills is
nothing new more than 40 years ago it was stated (Herbst,
1983) “Mill speed has considerable potential as an additional
manipulated variable. Over a century ago, Davis (1919)
suggested that if a grinding circuit showed a build up of
coarse discharge sizes then the mill speed should be increased.
Although this effect has been demonstrated in industrial
scale tests for step changes in mill speed (Jones, Johnson,
1976), the inability to change mill speed has prohibited its
use in grinding circuit control. However, recent advances
in solid state electronics and the development of the ring-
motor suggest that continuous variation in mill speed may
be not only feasible but also desirable.” “The advantages of
variable mill speed for the control of wet grinding are as yet
untested. Some of the possible advantages are:
• Quicker control of product size or circulating load.
• Power savings due to more efficient grinding.
• Better control of process downstream from the grind-
ing circuit due to less fluctuation in the feed rate.
• Longer liner life due to reduced speed for grinding
soft ores.”
“Two control strategies involving mill speed showed sub-
stantial improvement over the current “best” feedback con-
trol strategies which use fresh ore federate and sump water
addition rate as manipulated variables. The most attractive
strategy is one involving mill speed manipulation to control
circulating load and water addition to control product size.
Its advantage has been attributed to the fact that direct
kinetic compensation for hardness variations was possible
by manipulating mill speed.”
Mill Drives Short List
Since the target of this case study is to compare grinding
circuits with minimized CO2 emissions during the com-
plete mine lifetime, it is now obvious that variable speed
mills without gearboxes are required. Therefore, only gear-
less mills and variable low-speed dual pinion mills are being
considered.
EQUIPMENT TRADE-OFF
An important aspect to consider when comparing the fewer
quantity gearless mills to higher quantity gear-driven mills,
is the resultant equipment layout. Grinding mills should
be configured to allow adequate clearance between mills to
promote effective and efficient maintenance tasks for site
the additional losses of the gearboxes and pinions/ring-gear,
there is no margin at all left on the installed power. As soon
as the actual BWi would be slightly higher or Constancia
would be slightly pushing tonnage over the original design
throughput of 76’000 tpd, then immediately the installed
mill power becomes the bottleneck, increasing the P80 over
the original design value of 106 μm. Lane et al. (2015)
also stated: “The ball mills were potentially slightly over-
loaded after Year 6 producing a P80 of between 120 and
130 microns compared with the target P80 of 106 microns.
The value of the increase in throughput more than coun-
tered the loss of recovery due to the coarser P80.” Indeed,
the plant is achieving higher throughput between 80’000
and 90’000 tpd, but with a significant higher new P80 tar-
get of 160 μm and a consequently relative low copper recov-
ery of around 84 -87% only, instead of the original 90%
design value, even when the ball mill motors are nowadays
operating at a slightly higher power of up to 8250 kW each
(Tavchandjian, 2021). Additionally, Klohn et al. (2016)
stated that in the design phase it was recognized that “High
zinc ores were recognized during the DFS as being poten-
tially problematic during treatment, with zinc (as sphalerite)
reporting to the copper concentrate as a penalty element.”
Indeed, the coarser P80 results into a higher lead and zinc
contamination of the final copper concentrate as reported
in the NI43-101 Technical Report (Tavchandjian, 2021):
“Optimization studies will continue with a focus on reduc-
ing levels of zinc and lead contaminants and increasing cop-
per concentrate grades.” At first sight it may be surprising
that for these large power consumers rather inefficient mill
drives (fixed high-speed ring-geared with gearboxes) were
selected, which do not even provide operating flexibility of
variable speed to maximize copper recovery and minimize
zinc and lead contamination but in those days energy effi-
ciency was not always a priority, as confirmed by Lane et al.
(2017): “The first is the Constancia Project where energy
efficiency was not a major consideration in equipment selec-
tion..” Klohn, Stephenson and Granados (2016) reported:
“Ausenco’s design approach focused on developing a capital
efficient copper molybdenum concentrator.” Gearless mill
drives would have resulted into higher CAPEX, but then
also a higher installed power would have been easily possi-
ble, resulting into a finer grind and higher copper recovery,
as well as a lower zinc and lead contamination in the final
copper concentrate due to a more selective flotation. Also,
a higher drive train efficiency and lower CO2 emissions per
ton of concentrate would have been achieved with gearless
mill drives. Interestingly on the downstream side a 3 MW
regrind mill is further reducing the rougher concentrate
particle sizes to a target P80 of 25 μm, increasing the final
copper recovery, but unfortunately this does not help to
reduce the recovery losses realized upstream at the rougher
flotation.
Advantages of variable speed known since more
than a century. The call for variable speed ball mills is
nothing new more than 40 years ago it was stated (Herbst,
1983) “Mill speed has considerable potential as an additional
manipulated variable. Over a century ago, Davis (1919)
suggested that if a grinding circuit showed a build up of
coarse discharge sizes then the mill speed should be increased.
Although this effect has been demonstrated in industrial
scale tests for step changes in mill speed (Jones, Johnson,
1976), the inability to change mill speed has prohibited its
use in grinding circuit control. However, recent advances
in solid state electronics and the development of the ring-
motor suggest that continuous variation in mill speed may
be not only feasible but also desirable.” “The advantages of
variable mill speed for the control of wet grinding are as yet
untested. Some of the possible advantages are:
• Quicker control of product size or circulating load.
• Power savings due to more efficient grinding.
• Better control of process downstream from the grind-
ing circuit due to less fluctuation in the feed rate.
• Longer liner life due to reduced speed for grinding
soft ores.”
“Two control strategies involving mill speed showed sub-
stantial improvement over the current “best” feedback con-
trol strategies which use fresh ore federate and sump water
addition rate as manipulated variables. The most attractive
strategy is one involving mill speed manipulation to control
circulating load and water addition to control product size.
Its advantage has been attributed to the fact that direct
kinetic compensation for hardness variations was possible
by manipulating mill speed.”
Mill Drives Short List
Since the target of this case study is to compare grinding
circuits with minimized CO2 emissions during the com-
plete mine lifetime, it is now obvious that variable speed
mills without gearboxes are required. Therefore, only gear-
less mills and variable low-speed dual pinion mills are being
considered.
EQUIPMENT TRADE-OFF
An important aspect to consider when comparing the fewer
quantity gearless mills to higher quantity gear-driven mills,
is the resultant equipment layout. Grinding mills should
be configured to allow adequate clearance between mills to
promote effective and efficient maintenance tasks for site