344
A. Strobel et al.
4 Conclusion
In this chapter, the grinding process in a lab-scale fluidized bed opposed jet mill
was investigated. The process and its three different unit operations—namely the
comminution in the lower part, the pneumatic transport in the middle section, and
the classification step at the top of the mill—were thoroughly discussed. Besides
tracking the evolution of the particle sizes in quasi-batch grinding experiments, the
morphology of the received fines was discussed. Four different breakage modes
could be assigned to the particles from image analysis. The frequency of the cases
provides hints on the stressing conditions in the jet mill: In the first minute, most
particles exhibit small cracks, small chips and debris are identified, or the particles
are unharmed. Larger fragments from high energy impacts were rarely observed.
From grinding kinetics, a medium holdup in the range of 400 g was identified as
optimum for the investigated mill size. For larger particles, faster comminution took
place. Strong influences of the particle feed size on the final product size could not be
observed. Since the holdup influences the grinding kinetics, fed-batch experiments
were performed with a constant holdup along the whole process time. In contrast to
the quasi-batch experiments, faster comminution was observed for higher holdups
in fed-batch mode. However, the accumulation of product particles in the milling
chamber strikingly increased towards higher processing times, which led to a more
detailed investigation of the classification and transport process.
The presented separation curves showed a fish-hook effect. Product-sized particles accumulated inside the mill for all investigated conditions. For increasing solid
holdups and classifier speeds, the amount of accumulated fines increases together
with the solid concentration in the transport zone. Additionally, it takes significantly
longer for higher holdups to reach a steady state. High-speed images showed the
formation of clusters and strands around the classifier: these clusters scavenge fine
particles. The fine particles trapped in the cluster are rejected and driven periodically
to the outer periphery of the classifier. The formation of clusters at the classifier
blades and in the periphery of the classifier wheel is strongly influenced by the classifier speed and solid holdup. Further, the breakage behaviour of the used glass beads
was examined by impact testing. The obtained data were in excellent agreement with
the breakage model of Vogel and Peukert. For the impact testing of particles below
20 μm, a custom-build low-pressure single particle impact device was designed and
operated.
The experimental evaluation of the solid distribution was only addressed for the
transport area in the middle section of the mill. However, in addition to the capacitance measurements, the solid distribution in the jet and the surrounding bed was
targeted with X-ray tomography. For further information on this method and conducted experiments within this priority program we recommend further works of the
authors that are not targeted in this contribution [34, 52].
To gain better insight into the stressing conditions, aluminium particles were used
to assess the relative particle impact velocity and stress frequency. The ratio of the
formed contact diameter and the particle diameter correlates with the relative particle
A. Strobel et al.
4 Conclusion
In this chapter, the grinding process in a lab-scale fluidized bed opposed jet mill
was investigated. The process and its three different unit operations—namely the
comminution in the lower part, the pneumatic transport in the middle section, and
the classification step at the top of the mill—were thoroughly discussed. Besides
tracking the evolution of the particle sizes in quasi-batch grinding experiments, the
morphology of the received fines was discussed. Four different breakage modes
could be assigned to the particles from image analysis. The frequency of the cases
provides hints on the stressing conditions in the jet mill: In the first minute, most
particles exhibit small cracks, small chips and debris are identified, or the particles
are unharmed. Larger fragments from high energy impacts were rarely observed.
From grinding kinetics, a medium holdup in the range of 400 g was identified as
optimum for the investigated mill size. For larger particles, faster comminution took
place. Strong influences of the particle feed size on the final product size could not be
observed. Since the holdup influences the grinding kinetics, fed-batch experiments
were performed with a constant holdup along the whole process time. In contrast to
the quasi-batch experiments, faster comminution was observed for higher holdups
in fed-batch mode. However, the accumulation of product particles in the milling
chamber strikingly increased towards higher processing times, which led to a more
detailed investigation of the classification and transport process.
The presented separation curves showed a fish-hook effect. Product-sized particles accumulated inside the mill for all investigated conditions. For increasing solid
holdups and classifier speeds, the amount of accumulated fines increases together
with the solid concentration in the transport zone. Additionally, it takes significantly
longer for higher holdups to reach a steady state. High-speed images showed the
formation of clusters and strands around the classifier: these clusters scavenge fine
particles. The fine particles trapped in the cluster are rejected and driven periodically
to the outer periphery of the classifier. The formation of clusters at the classifier
blades and in the periphery of the classifier wheel is strongly influenced by the classifier speed and solid holdup. Further, the breakage behaviour of the used glass beads
was examined by impact testing. The obtained data were in excellent agreement with
the breakage model of Vogel and Peukert. For the impact testing of particles below
20 μm, a custom-build low-pressure single particle impact device was designed and
operated.
The experimental evaluation of the solid distribution was only addressed for the
transport area in the middle section of the mill. However, in addition to the capacitance measurements, the solid distribution in the jet and the surrounding bed was
targeted with X-ray tomography. For further information on this method and conducted experiments within this priority program we recommend further works of the
authors that are not targeted in this contribution [34, 52].
To gain better insight into the stressing conditions, aluminium particles were used
to assess the relative particle impact velocity and stress frequency. The ratio of the
formed contact diameter and the particle diameter correlates with the relative particle