3872 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
may relate to the coarser feed material during wet grinding.
The results for the specific energy consumption are shown
in Figure 12.
There are two different levels of the specific energy
consumption recognizable. The higher mass specific energy
level varies between 12 and 14 kWh/t and was measured
at 1.32 m/s. At a milling table speed of 0.66 m/s in gen-
eral a lower mass specific energy consumption between 8
and 10 kWh/t is measured. This means, if the milling table
speed is higher, then the specific energy consumption rises.
This indicates that, the throughput of the fresh feed mate-
rial does not increase linearly with the mechanical power
consumption. More interesting, it appears that there is no
remarkable difference between the milling pressure levels.
Only the highest milling pressure of each milling table
speed is conspicuous, but each of them show different
trends. This is in contrast to the other results. It could indi-
cate that the specific energy consumption is more related
to the processed material and/or the mill throughput. The
investigations of Reichert et al. show a behavior similar to
the graph of the 1.32 m/s milling table speed. There, as the
milling pressure increases the specific energy consumption
rises as well (Reichert et al., 2015).
The dry milling has a specific energy consumption
of about 12.6 kWh/t while in comparison the wet mill-
ing only reaches 11.3 kWh/t. According to this, a factor of
approximately 1.12 can be calculated between dry and wet
milling. It is much lower compared to Bond’s correction
factor. A multiplier of 1.3 is required to convert the Bond-
Work-Index for dry grinding (Rowland, 1998). This obser-
vation is definitely influenced by the coarser feed material.
Nevertheless, the wet milling has still a lower specific energy
consumption.
Particle Size Distribution
The particle size distribution of the fresh feed material has a
relatively slight deviation during the dry milling tests. The
wet milled material was remarkably coarser, especially in
the important smaller particle size fractions. This devia-
tion was recognized after the performed tests and of course
influences the observed differences between dry and wet
milling. This may be due to segregation during the han-
dling of the material. Furthermore, this also confirms the
requirement for continuous sampling of the fresh material
to determine such variations. Because of the coarser feed
material, it is no surprise that the milling product of the wet
milling test is also coarser, when compared to the same dry
milling parameters. Within the dry milling tests the observ-
able particle size distributions of the milling product show
reasonable effects, as the pressure and table speed increases,
then the product becomes finer.
The diagrams in Figure 14 display the two products
of the installed screening machine. The coarse material is
showing ambiguous trends. Furthermore, also the screen-
ing fines seem to have no clear trend. This may refer to
the constant mill throughput of 1.625 kg/h which is also
related to the screen. Hence, the screening results should
be relatively constant. This leads to the conclusion that the
differences could relate to the slightly different PSDs of
the milling product which represents also the screen feed
material. More conspicuous is the much finer PSD of the
wet screening fines. This can mainly due to the wet pro-
cess, which tends to bind the fines in the material flow. In
contrast, during dry milling also dust can be released or is
getting exhausted which may not be captured during sam-
pling. In order to ensure proper comparison between dry
and wet milling process this needs further investigation.
CONCLUSION AND OUTLOOK
Tumbling mills have a relatively high energy demand.
A promising solution for a more efficient wet grinding
method is described. The presented milling-classification
pilot circuit is capable of dry and wet milling. For the first
step, mostly dry milling was investigated using granodiorite
as the test material. The results of one wet milling test are
presented for comparison.
The first milling tests show reasonable results. During
dry milling, the feed throughput rises from 150 kg/h to
325 kg/h at constant mill throughput if a higher milling
pressure is set (550 kN/m2 to 1408 kN/m2). Vice versa,
the circulating factor decreases from 11 to 5. The influ-
ence of the milling pressure on the mass specific energy
consumption seems to be small in the investigated range,
contrary to the milling table speed, which has a signifi-
cant influence. Hence at a milling table circumferential
speed of 1.32 m/s, the mass specific energy consumption
is between 12 and 14 kWh/t while at a lower milling table
circumferential speed of 0.66 m/s the mass specific energy
consumption is between 8 and 10 kWh/t. Compared to
this results the wet milling reached a specific energy con-
sumption of 11.3 kWh/t at a milling table circumferential
speed of 1.32 m/s and a milling pressure of 947 kN/m2.
However, while the same mill throughput is set, the wet
process reaches a lower solid material mill throughput. This
is caused by the throughput measurement of the recircula-
tion material and needs to be investigated further to pro-
vide equality for dry-wet-comparison.
may relate to the coarser feed material during wet grinding.
The results for the specific energy consumption are shown
in Figure 12.
There are two different levels of the specific energy
consumption recognizable. The higher mass specific energy
level varies between 12 and 14 kWh/t and was measured
at 1.32 m/s. At a milling table speed of 0.66 m/s in gen-
eral a lower mass specific energy consumption between 8
and 10 kWh/t is measured. This means, if the milling table
speed is higher, then the specific energy consumption rises.
This indicates that, the throughput of the fresh feed mate-
rial does not increase linearly with the mechanical power
consumption. More interesting, it appears that there is no
remarkable difference between the milling pressure levels.
Only the highest milling pressure of each milling table
speed is conspicuous, but each of them show different
trends. This is in contrast to the other results. It could indi-
cate that the specific energy consumption is more related
to the processed material and/or the mill throughput. The
investigations of Reichert et al. show a behavior similar to
the graph of the 1.32 m/s milling table speed. There, as the
milling pressure increases the specific energy consumption
rises as well (Reichert et al., 2015).
The dry milling has a specific energy consumption
of about 12.6 kWh/t while in comparison the wet mill-
ing only reaches 11.3 kWh/t. According to this, a factor of
approximately 1.12 can be calculated between dry and wet
milling. It is much lower compared to Bond’s correction
factor. A multiplier of 1.3 is required to convert the Bond-
Work-Index for dry grinding (Rowland, 1998). This obser-
vation is definitely influenced by the coarser feed material.
Nevertheless, the wet milling has still a lower specific energy
consumption.
Particle Size Distribution
The particle size distribution of the fresh feed material has a
relatively slight deviation during the dry milling tests. The
wet milled material was remarkably coarser, especially in
the important smaller particle size fractions. This devia-
tion was recognized after the performed tests and of course
influences the observed differences between dry and wet
milling. This may be due to segregation during the han-
dling of the material. Furthermore, this also confirms the
requirement for continuous sampling of the fresh material
to determine such variations. Because of the coarser feed
material, it is no surprise that the milling product of the wet
milling test is also coarser, when compared to the same dry
milling parameters. Within the dry milling tests the observ-
able particle size distributions of the milling product show
reasonable effects, as the pressure and table speed increases,
then the product becomes finer.
The diagrams in Figure 14 display the two products
of the installed screening machine. The coarse material is
showing ambiguous trends. Furthermore, also the screen-
ing fines seem to have no clear trend. This may refer to
the constant mill throughput of 1.625 kg/h which is also
related to the screen. Hence, the screening results should
be relatively constant. This leads to the conclusion that the
differences could relate to the slightly different PSDs of
the milling product which represents also the screen feed
material. More conspicuous is the much finer PSD of the
wet screening fines. This can mainly due to the wet pro-
cess, which tends to bind the fines in the material flow. In
contrast, during dry milling also dust can be released or is
getting exhausted which may not be captured during sam-
pling. In order to ensure proper comparison between dry
and wet milling process this needs further investigation.
CONCLUSION AND OUTLOOK
Tumbling mills have a relatively high energy demand.
A promising solution for a more efficient wet grinding
method is described. The presented milling-classification
pilot circuit is capable of dry and wet milling. For the first
step, mostly dry milling was investigated using granodiorite
as the test material. The results of one wet milling test are
presented for comparison.
The first milling tests show reasonable results. During
dry milling, the feed throughput rises from 150 kg/h to
325 kg/h at constant mill throughput if a higher milling
pressure is set (550 kN/m2 to 1408 kN/m2). Vice versa,
the circulating factor decreases from 11 to 5. The influ-
ence of the milling pressure on the mass specific energy
consumption seems to be small in the investigated range,
contrary to the milling table speed, which has a signifi-
cant influence. Hence at a milling table circumferential
speed of 1.32 m/s, the mass specific energy consumption
is between 12 and 14 kWh/t while at a lower milling table
circumferential speed of 0.66 m/s the mass specific energy
consumption is between 8 and 10 kWh/t. Compared to
this results the wet milling reached a specific energy con-
sumption of 11.3 kWh/t at a milling table circumferential
speed of 1.32 m/s and a milling pressure of 947 kN/m2.
However, while the same mill throughput is set, the wet
process reaches a lower solid material mill throughput. This
is caused by the throughput measurement of the recircula-
tion material and needs to be investigated further to pro-
vide equality for dry-wet-comparison.