9 Impact Comminution in Jet Mills
311
2.1.2 Fed-Batch Experiments
During quasi-batch experiments, the holdup and the PSD in the milling chamber are
changing simultaneously. Therefore, the influences of individual parameters on the
process cannot be differentiated. To minimize the influence of the changing holdup,
fed-batch experiments were performed: The process is interrupted in fixed intervals
to stock up the holdup with the feed material. Within the first 5 min of operation, an
interval of 1 min was chosen, followed by a 5 min interval for the remaining process
time. Sampling was performed according to the protocol given above.
2.2 Fluidized Bed with Secondary Gas Injection
For some case studies, a lab-scale fluidized bed [33, 34] (inner diameter of 94 mm,
circulation zone height of 570 mm) with secondary gas injection was used. Fluidization gas velocity was set to 2.4 cm s
−1 . A sintered metal plate at the bottom of the bed
was used to ensure a homogeneously distribution of the gas. For particle separation,
a cyclone and a filter are installed at the top. The diameter d 0 of the used nozzles
was 2 mm. A single nozzle setup and a two-opposing nozzle setup were investigated.
The distance to the focal point of the opposing nozzles was 42 mm. The jet velocity
was set to 200 m s
−1 (velocity at the nozzle threat). Sampling was performed after
intervals of 10 min by dismantling the lower segment of the fluidized bed: The solid
was manually mixed and samples were taken at three different spots and merged.
2.3 Material
2.3.1 Soda-Lime Glass Beads
Spherical soda-lime glass beads (Silibeads®, Type S) by Sigmund Lindner (Germany) were used as model particles. In comparison to other investigated materials
like talc [35], hydrargillite [9, 19] or ethenzamide [20, 36] the glass beads have distinct advantages for the chosen application: They show superior mechanical properties (Mohs hardness ≥6, Young’s modulus E = 63 GPa, [37]) and thus show rather
slow breakage. By purposely slowing down the comminution process, the dynamic
changes of the process can be observed in greater detail. The density of the beads
was 2500 kg m
−3 .
For the investigation of the grinding dynamics and kinetics, as-received feed fractions with Sauter diameters x 1,2 of 61, 93, and 127 μm and corresponding spans
((x 90,3 − x 10,3 )/x 50,3 )) of 0.71, 0.68 and 0.64, respectively, were used. The corresponding PSDs are given in Fig. 2a together with an SEM image of one of the feed
fractions (Fig. 2b). From the SEM images, the sphericity ψ i of the individual particles
is calculated according to Eq. 2:
311
2.1.2 Fed-Batch Experiments
During quasi-batch experiments, the holdup and the PSD in the milling chamber are
changing simultaneously. Therefore, the influences of individual parameters on the
process cannot be differentiated. To minimize the influence of the changing holdup,
fed-batch experiments were performed: The process is interrupted in fixed intervals
to stock up the holdup with the feed material. Within the first 5 min of operation, an
interval of 1 min was chosen, followed by a 5 min interval for the remaining process
time. Sampling was performed according to the protocol given above.
2.2 Fluidized Bed with Secondary Gas Injection
For some case studies, a lab-scale fluidized bed [33, 34] (inner diameter of 94 mm,
circulation zone height of 570 mm) with secondary gas injection was used. Fluidization gas velocity was set to 2.4 cm s
−1 . A sintered metal plate at the bottom of the bed
was used to ensure a homogeneously distribution of the gas. For particle separation,
a cyclone and a filter are installed at the top. The diameter d 0 of the used nozzles
was 2 mm. A single nozzle setup and a two-opposing nozzle setup were investigated.
The distance to the focal point of the opposing nozzles was 42 mm. The jet velocity
was set to 200 m s
−1 (velocity at the nozzle threat). Sampling was performed after
intervals of 10 min by dismantling the lower segment of the fluidized bed: The solid
was manually mixed and samples were taken at three different spots and merged.
2.3 Material
2.3.1 Soda-Lime Glass Beads
Spherical soda-lime glass beads (Silibeads®, Type S) by Sigmund Lindner (Germany) were used as model particles. In comparison to other investigated materials
like talc [35], hydrargillite [9, 19] or ethenzamide [20, 36] the glass beads have distinct advantages for the chosen application: They show superior mechanical properties (Mohs hardness ≥6, Young’s modulus E = 63 GPa, [37]) and thus show rather
slow breakage. By purposely slowing down the comminution process, the dynamic
changes of the process can be observed in greater detail. The density of the beads
was 2500 kg m
−3 .
For the investigation of the grinding dynamics and kinetics, as-received feed fractions with Sauter diameters x 1,2 of 61, 93, and 127 μm and corresponding spans
((x 90,3 − x 10,3 )/x 50,3 )) of 0.71, 0.68 and 0.64, respectively, were used. The corresponding PSDs are given in Fig. 2a together with an SEM image of one of the feed
fractions (Fig. 2b). From the SEM images, the sphericity ψ i of the individual particles
is calculated according to Eq. 2: