9 Impact Comminution in Jet Mills
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other extreme case: the bed height is above the nozzles and back mixing of small
particles might be decreased. Due to the high mass concentration, impacts are more
likely to happen after shorter acceleration times, whereas the impact probability is
comparatively high. For an intermediate holdup of 300 g, the nozzles are covered
and the best trade-off between acceleration path lengths and stressing probability is
found.
Figure 7b shows the influence of the feed particle size on the Kapur function for
an initial holdup of 400 g: For all tested feed materials, a maximum in the grinding
kinetics is observed for sizes close to the Sauter mean diameter of the individual fraction. The determined maximum comminution efficiency increases with the particle
size. For smaller particles, the entrainment into the jet and the pneumatic transport
from the grinding zone is promoted, whereas large particles experience less acceleration inside the jets. Therefore, a higher number of larger particles around the jets is
expected which result in an increased impact probability. As will be shown later in
Sect. 3.6, the breakage probability is increased for larger particles.
3.2 Fed-Batch Comminution
A changing holdup has a significant influence on the grinding performance of the
mill for larger process times. Therefore, the holdup is now kept constant to eliminate
this influencing variable. Thus, fed-batch experiments were performed to simulate a
constant holdup: During sampling, feed material was added to replace the discharged
product. In Fig. 8a x 50,3 in the milling chamber is shown for different holdups. Similar
to the results from the quasi-batch experiments, during the whole process (40 min)
no constant particle size is reached. When looking at x 50,3 for the discharged product,
no difference between the holdups was observed. In general, the trend for the discharged fines was found to be similar for the quasi-batch experiment with different
holdups shown in Fig. 5a. An increased holdup leads to a faster size reduction of the
largest particles in the mill: For more particles being stressed in the active grinding
zone, a larger amount of smaller sized fragments is produced. As stated by Fukanaka,
comminution is faster at lower holdups due to an increased grinding energy per unit
mass [36]. When increasing the pressure and thus the jet speed, an excess of available
energy may exist: By increasing the holdup again, the number of impacts inside the
jets increases and a faster comminution results. For an increased amount of fines
inside the milling chamber, the overall flowability of the solid is decreasing due to
agglomeration and adhesion. In consequence, the fluidization behaviour and with
it the two-phase flow inside the jets changes. However, this affects the stressing
conditions of the particles. Figure 8c gives the discharged product mass flow. The
observations are in agreement with Fukanaka: Initially, the mass flow rate is rapidly
increasing until reaching a maximum value followed by a steady-state phase. For
higher holdups, the time until a steady product mass flow is reached increases. The
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other extreme case: the bed height is above the nozzles and back mixing of small
particles might be decreased. Due to the high mass concentration, impacts are more
likely to happen after shorter acceleration times, whereas the impact probability is
comparatively high. For an intermediate holdup of 300 g, the nozzles are covered
and the best trade-off between acceleration path lengths and stressing probability is
found.
Figure 7b shows the influence of the feed particle size on the Kapur function for
an initial holdup of 400 g: For all tested feed materials, a maximum in the grinding
kinetics is observed for sizes close to the Sauter mean diameter of the individual fraction. The determined maximum comminution efficiency increases with the particle
size. For smaller particles, the entrainment into the jet and the pneumatic transport
from the grinding zone is promoted, whereas large particles experience less acceleration inside the jets. Therefore, a higher number of larger particles around the jets is
expected which result in an increased impact probability. As will be shown later in
Sect. 3.6, the breakage probability is increased for larger particles.
3.2 Fed-Batch Comminution
A changing holdup has a significant influence on the grinding performance of the
mill for larger process times. Therefore, the holdup is now kept constant to eliminate
this influencing variable. Thus, fed-batch experiments were performed to simulate a
constant holdup: During sampling, feed material was added to replace the discharged
product. In Fig. 8a x 50,3 in the milling chamber is shown for different holdups. Similar
to the results from the quasi-batch experiments, during the whole process (40 min)
no constant particle size is reached. When looking at x 50,3 for the discharged product,
no difference between the holdups was observed. In general, the trend for the discharged fines was found to be similar for the quasi-batch experiment with different
holdups shown in Fig. 5a. An increased holdup leads to a faster size reduction of the
largest particles in the mill: For more particles being stressed in the active grinding
zone, a larger amount of smaller sized fragments is produced. As stated by Fukanaka,
comminution is faster at lower holdups due to an increased grinding energy per unit
mass [36]. When increasing the pressure and thus the jet speed, an excess of available
energy may exist: By increasing the holdup again, the number of impacts inside the
jets increases and a faster comminution results. For an increased amount of fines
inside the milling chamber, the overall flowability of the solid is decreasing due to
agglomeration and adhesion. In consequence, the fluidization behaviour and with
it the two-phase flow inside the jets changes. However, this affects the stressing
conditions of the particles. Figure 8c gives the discharged product mass flow. The
observations are in agreement with Fukanaka: Initially, the mass flow rate is rapidly
increasing until reaching a maximum value followed by a steady-state phase. For
higher holdups, the time until a steady product mass flow is reached increases. The