3860 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
increasingly finer intergrowth, so that milling to below
80 µm is frequently necessary (Harder, 2022). This is partic-
ularly relevant in the case of copper deposits. These depos-
its currently have an average value mineral content of only
0.7 to 0.9 %(Norgate and Jahanshahi, 2011). Mudd et al.
showed that over 50 %of the considered copper depos-
its have less than 0.51 %copper content and only 3.2 %
of the deposits have 3 %or higher copper content (Mudd
et al., 2013). The increasingly exploitation of polymetallic
particle systems smaller than 10 µm is to be expected in the
future (Peuker et al., 2012).
On the basis of these trends, it can be stated that all of
them call for more efficient ways to liberate ores. Already
in 2012, Peuker et al. called for the development of more
efficient possibilities of wet grinding (Peuker et al., 2012).
STATE OF THE ART
Ore Liberation
The dominant machines in the field of ore grinding are
currently SAG and ball mills. These machines are reliable,
robust, and can operate at high capacities. However, SAG
and ball mills are also known for their inherent energy-effi-
ciencies in comminuting ore. In recent years, efforts have
increased to replace tumbling mills with more efficient
technologies.
In the field of dry grinding, alternative technologies
have already been investigated (Segura-Salazar et al., 2021)
or are even in industrial application, e.g., HPGR for the
milling of copper ore in Peru or of iron ore in Australia (van
der Meer, F. P. et al., 2015).
Detailed investigations were conducted by Gagnon et
al. on a possible alternative to ore dressing using the SAG-
HPGR combination. The pilot plant was compared for
this purpose with existing conventional plants for gold and
copper ore processing. The tests showed energy savings of
approximately 62 %by direct comparison of the ball mill
with HPGR. Furthermore, the comparisons also showed
that when all the necessary peripherals are considered, the
energy savings are still at least 28 %.This large difference is
due to the fact that in the SAG-HPGR circuit, wet screen-
ing occurs after the SAG mill to separate the fines. However,
according to the study, the HPGR can be operated in a
controlled manner up to a maximum moisture content of
only 10 %.This makes dewatering of the material after wet
screening unavoidable (Gagnon et al., 2021).
High Pressure Grinding Rolls (HPGR) and Vertical
Roller Mills (VRM) are currently limited in terms of mate-
rial moisture. Furthermore, Saramak and Kleiv showed that
the specific energy demand increases while the moisture
content rises up to 8 %.This is caused in particular by the
decreasing milling gap (Saramak and Kleiv, 2013).
Several sources indicate that VRM is optimally suited
for ore liberation (Leonida, 2020 Harder, 2022). However,
VRM are limited to handle material with approximately 2
to 4 %moisture, due to the internal air sifting process. As a
result of higher moisture, agglomeration of the fines could
occur and thus a distortion of the sifting process could hap-
pen. Schubert, considers that dry milling in a VRM is pos-
sible up to a feed moisture of 8 %without additional drying
(Schubert et al., 1990). In a patent, VALE S.A. describes
several routes of a comminution processes for iron ore at
natural feed moisture of 5 to 12 %using VRMs, HPGRs
or Roller Crushers (Marques and Donda, 2019).
For higher feed moistures, drying of the feed material is
necessary. This would reduce the energy savings achieved by
the alternative comminution method. This is especially the
case in climatic regions where drying is not favored by the
ambient conditions. A particularly drastic example would
be northern Sweden, where temperatures regularly drop
below –20 °C in winter. This exact case was reported by
Ballantyne and Lane. They compared a wet ball mill grind-
ing circuit with a dry milling HPGR or VRM circuit. They
highlighted the energy savings for comminution achieved
with the HPGR and VRM application. For the case inves-
tigated in this study the ball mill consumes 35 kWh/t,
whereas the HPGR and VRM requires only approximately
15.7 kWh/t. However, the ground material has a relatively
high moisture content of approximately 14 %,which is
mostly caused by ice and snow at the stockpile. Therefore
they documented an energy input of 147 kWh/t by the
hot gas generator to dry the material (Ballantyne and Lane,
2022).
The early work form Bond showed that the mass-spe-
cific energy requirement for dry grinding in ball mills is
approximately 30 %higher than that of wet grinding. This
aspect is documented as the Bond Correction Efficiency
Factor (rod and ball mills) EF1 =1.3 for dry grinding
(Rowland, 1998). More recently, Ogonowski et al. showed
a comparison between wet and dry grinding with a com-
pletely new and different mill type. In their investigations,
they demonstrated that wet grinding is more efficient than
dry grinding (Ogonowski et al., 2018). Therefore, it does
not seem appropriate to consider exclusively dry grinding
technologies when investigating the path forward for ore
grinding.
Vertical Roller Mill (VRM)
VRMs so far typically operate in airflow mode. The milling
material, on the rotating milling table, is pushed outward
increasingly finer intergrowth, so that milling to below
80 µm is frequently necessary (Harder, 2022). This is partic-
ularly relevant in the case of copper deposits. These depos-
its currently have an average value mineral content of only
0.7 to 0.9 %(Norgate and Jahanshahi, 2011). Mudd et al.
showed that over 50 %of the considered copper depos-
its have less than 0.51 %copper content and only 3.2 %
of the deposits have 3 %or higher copper content (Mudd
et al., 2013). The increasingly exploitation of polymetallic
particle systems smaller than 10 µm is to be expected in the
future (Peuker et al., 2012).
On the basis of these trends, it can be stated that all of
them call for more efficient ways to liberate ores. Already
in 2012, Peuker et al. called for the development of more
efficient possibilities of wet grinding (Peuker et al., 2012).
STATE OF THE ART
Ore Liberation
The dominant machines in the field of ore grinding are
currently SAG and ball mills. These machines are reliable,
robust, and can operate at high capacities. However, SAG
and ball mills are also known for their inherent energy-effi-
ciencies in comminuting ore. In recent years, efforts have
increased to replace tumbling mills with more efficient
technologies.
In the field of dry grinding, alternative technologies
have already been investigated (Segura-Salazar et al., 2021)
or are even in industrial application, e.g., HPGR for the
milling of copper ore in Peru or of iron ore in Australia (van
der Meer, F. P. et al., 2015).
Detailed investigations were conducted by Gagnon et
al. on a possible alternative to ore dressing using the SAG-
HPGR combination. The pilot plant was compared for
this purpose with existing conventional plants for gold and
copper ore processing. The tests showed energy savings of
approximately 62 %by direct comparison of the ball mill
with HPGR. Furthermore, the comparisons also showed
that when all the necessary peripherals are considered, the
energy savings are still at least 28 %.This large difference is
due to the fact that in the SAG-HPGR circuit, wet screen-
ing occurs after the SAG mill to separate the fines. However,
according to the study, the HPGR can be operated in a
controlled manner up to a maximum moisture content of
only 10 %.This makes dewatering of the material after wet
screening unavoidable (Gagnon et al., 2021).
High Pressure Grinding Rolls (HPGR) and Vertical
Roller Mills (VRM) are currently limited in terms of mate-
rial moisture. Furthermore, Saramak and Kleiv showed that
the specific energy demand increases while the moisture
content rises up to 8 %.This is caused in particular by the
decreasing milling gap (Saramak and Kleiv, 2013).
Several sources indicate that VRM is optimally suited
for ore liberation (Leonida, 2020 Harder, 2022). However,
VRM are limited to handle material with approximately 2
to 4 %moisture, due to the internal air sifting process. As a
result of higher moisture, agglomeration of the fines could
occur and thus a distortion of the sifting process could hap-
pen. Schubert, considers that dry milling in a VRM is pos-
sible up to a feed moisture of 8 %without additional drying
(Schubert et al., 1990). In a patent, VALE S.A. describes
several routes of a comminution processes for iron ore at
natural feed moisture of 5 to 12 %using VRMs, HPGRs
or Roller Crushers (Marques and Donda, 2019).
For higher feed moistures, drying of the feed material is
necessary. This would reduce the energy savings achieved by
the alternative comminution method. This is especially the
case in climatic regions where drying is not favored by the
ambient conditions. A particularly drastic example would
be northern Sweden, where temperatures regularly drop
below –20 °C in winter. This exact case was reported by
Ballantyne and Lane. They compared a wet ball mill grind-
ing circuit with a dry milling HPGR or VRM circuit. They
highlighted the energy savings for comminution achieved
with the HPGR and VRM application. For the case inves-
tigated in this study the ball mill consumes 35 kWh/t,
whereas the HPGR and VRM requires only approximately
15.7 kWh/t. However, the ground material has a relatively
high moisture content of approximately 14 %,which is
mostly caused by ice and snow at the stockpile. Therefore
they documented an energy input of 147 kWh/t by the
hot gas generator to dry the material (Ballantyne and Lane,
2022).
The early work form Bond showed that the mass-spe-
cific energy requirement for dry grinding in ball mills is
approximately 30 %higher than that of wet grinding. This
aspect is documented as the Bond Correction Efficiency
Factor (rod and ball mills) EF1 =1.3 for dry grinding
(Rowland, 1998). More recently, Ogonowski et al. showed
a comparison between wet and dry grinding with a com-
pletely new and different mill type. In their investigations,
they demonstrated that wet grinding is more efficient than
dry grinding (Ogonowski et al., 2018). Therefore, it does
not seem appropriate to consider exclusively dry grinding
technologies when investigating the path forward for ore
grinding.
Vertical Roller Mill (VRM)
VRMs so far typically operate in airflow mode. The milling
material, on the rotating milling table, is pushed outward