332
A. Strobel et al.
In conclusion, higher grinding pressures lead to higher impact numbers. Accordingly, we attribute the widely observed higher grinding efficiency at higher pressures
to a significant increase in the impact numbers rather than to the impact velocity.
Considering the approximated residence times from Sect. 3.2 (40 min of processing,
feed-batch mode, x 1,2 = 93 μm, between 75 min at 100 g holdup, and 110 min for
the 700 g holdup), the influence of the impact number becomes apparent. The number of impacts would add up to several thousand per particle. Considering the high
impact numbers and the unexpectedly low relative impact velocities, a fatigue-like
behaviour of the stressed particles must be considered as the main driving force of
this dry comminution process.
3.4 Transport Zone
For the discharge of the fine solids produced in the grinding zone at the bottom of
the grinding chamber, transport to the classifier at the top of the machine is required.
Therefore, to participate in the transport, the diameter of the particles must be below
the single grain settling diameter in the up-flow: For the used limestone particles
the settling diameter was calculated to be ~120 μm [31]. Since already 90% of the
initial feed particles are smaller than the single grain settling diameter, the transport
of the solids material in the chamber towards the classifier should be promoted.
The knowledge of the solid concentration and, therefore, the mass flow towards the
classifier is of essential knowledge for the modelling of the apparatus. (Note: For
the results discussed in Sects. 3.4 and 3.5, limestone was processed at a grinding
pressure of 3 bar in quasi-batch mode. Holdup and classifier speed were changed
according to the notifications).
The solid concentration (1 – ε) in the transport zone (the section between the
grinding chamber and the classifier) for different holdups was measured during the
comminution process by capacitance probes. Results are shown in Fig. 15a. A linear
increase from 0.01 to 0.03 with progressing time is detected for the lowest investigated
holdup of 100 g. For the other cases, a maximum in solid concentration is observed
right after the start. Besides, the maximum value scales with the initial holdup.
With proceeding time, the solid concentration decreases, passes a minimum and
rises again, while the differences between the holdups vanish. The rising values
following the start indicate an increased production and subsequent transport of fine
and intermediate particles towards the classifier. Thus, the load in the transport zone
is decreased. Passing the minimum, a significantly higher amount of intermediate
particles, which are not discharged but only recirculated in the milling chamber, has
to be present. Towards the end of the depicted experiments, the solid concentrations
approach a steady-state value. From the processing of glass beads and for the here
shown data for limestone, the same overall trend is found: Higher holdups lead to
higher solid concentrations in the transport region [29].
A. Strobel et al.
In conclusion, higher grinding pressures lead to higher impact numbers. Accordingly, we attribute the widely observed higher grinding efficiency at higher pressures
to a significant increase in the impact numbers rather than to the impact velocity.
Considering the approximated residence times from Sect. 3.2 (40 min of processing,
feed-batch mode, x 1,2 = 93 μm, between 75 min at 100 g holdup, and 110 min for
the 700 g holdup), the influence of the impact number becomes apparent. The number of impacts would add up to several thousand per particle. Considering the high
impact numbers and the unexpectedly low relative impact velocities, a fatigue-like
behaviour of the stressed particles must be considered as the main driving force of
this dry comminution process.
3.4 Transport Zone
For the discharge of the fine solids produced in the grinding zone at the bottom of
the grinding chamber, transport to the classifier at the top of the machine is required.
Therefore, to participate in the transport, the diameter of the particles must be below
the single grain settling diameter in the up-flow: For the used limestone particles
the settling diameter was calculated to be ~120 μm [31]. Since already 90% of the
initial feed particles are smaller than the single grain settling diameter, the transport
of the solids material in the chamber towards the classifier should be promoted.
The knowledge of the solid concentration and, therefore, the mass flow towards the
classifier is of essential knowledge for the modelling of the apparatus. (Note: For
the results discussed in Sects. 3.4 and 3.5, limestone was processed at a grinding
pressure of 3 bar in quasi-batch mode. Holdup and classifier speed were changed
according to the notifications).
The solid concentration (1 – ε) in the transport zone (the section between the
grinding chamber and the classifier) for different holdups was measured during the
comminution process by capacitance probes. Results are shown in Fig. 15a. A linear
increase from 0.01 to 0.03 with progressing time is detected for the lowest investigated
holdup of 100 g. For the other cases, a maximum in solid concentration is observed
right after the start. Besides, the maximum value scales with the initial holdup.
With proceeding time, the solid concentration decreases, passes a minimum and
rises again, while the differences between the holdups vanish. The rising values
following the start indicate an increased production and subsequent transport of fine
and intermediate particles towards the classifier. Thus, the load in the transport zone
is decreased. Passing the minimum, a significantly higher amount of intermediate
particles, which are not discharged but only recirculated in the milling chamber, has
to be present. Towards the end of the depicted experiments, the solid concentrations
approach a steady-state value. From the processing of glass beads and for the here
shown data for limestone, the same overall trend is found: Higher holdups lead to
higher solid concentrations in the transport region [29].