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A. Strobel et al.
materials [13]. Through the autogenous comminution by particle-particle impacts
induced by the gas jets, a high degree of fragmentation can be achieved for the production of fine powders <10 μm. Since no moving parts for impaction are needed, the
comminution proceeds without any wear of the machinery, avoiding also the contamination of the product. The processing of different materials in fluidized bed opposed
jet mills is addressed in several different publications [14–18]. These approaches
propose empirical models to describe the two-phase flow or just illustrate experimental effects. Other authors describe batch processing in fluidized bed opposed jet
mills while global grinding kinetics are employed to model size reduction [19–21].
The complex two-phase flow, the high gas velocities, and the difficulties to directly
assess the stressing conditions comprise major challenges. The classification process
is studied both experimentally and numerically in stand-alone classifiers, including
the fluid mechanics inside the vanes of the rotating classifier wheel [6, 22–28].
The following chapter is dedicated to the study of particle-particle interactions,
revealing the overall stressing conditions, and the dynamics of the two-phase flow
during non-stationary fine grinding. For this purpose a lab-scale fluidized bed
opposed jet mill is investigated in detail: Besides the grinding behaviour during
different operational modes, a novel approach for the direct determination of relative
particle velocities will be presented. Any comprehensive description of the comminution process requires the knowledge about the materials properties and the materials
response to the determined stresses. For this reason the single particle breakage
behaviour is studied and analysed by single particle compression and impact tests.
The presented observations are then finally used for a model of the product mass
flow.
2 Materials and Methods
2.1 Fluidized Bed Opposed Jet Mill
The experiments were performed in a lab-scale fluidized bed opposed jet mill (AFG
100, Hosokawa Alpine AG, Germany). A scheme of the setup is depicted in Fig. 1
[29]. The mill consists of a cylindrical milling chamber (inner diameter of 100 mm).
Three Laval nozzles [exit diameters of 1.9 mm (1)] are arranged in a 3D configuration
at the bottom of the mill chamber and are directed towards the central focal point.
The nozzles are supplied with pressurized air, which leads to a gas flow directed
upward towards the classifier wheel (2), whose outer diameter is 50 mm. The mill
was operated at a pressure level of 10–20 mbar below ambient conditions. An online
laser diffraction system (3) (Insitec, Malvern Panalytical, UK) is installed in the
product stream to continuously record the product particle size distributions (PSDs)
(sample rate of 1 Hz). Before the online particle size measurement, the product
particles are dispersed by a ring nozzle (4). Particle separation after the measurement
was ensured by a cyclone and a filter. For the characterization of the material inside
A. Strobel et al.
materials [13]. Through the autogenous comminution by particle-particle impacts
induced by the gas jets, a high degree of fragmentation can be achieved for the production of fine powders <10 μm. Since no moving parts for impaction are needed, the
comminution proceeds without any wear of the machinery, avoiding also the contamination of the product. The processing of different materials in fluidized bed opposed
jet mills is addressed in several different publications [14–18]. These approaches
propose empirical models to describe the two-phase flow or just illustrate experimental effects. Other authors describe batch processing in fluidized bed opposed jet
mills while global grinding kinetics are employed to model size reduction [19–21].
The complex two-phase flow, the high gas velocities, and the difficulties to directly
assess the stressing conditions comprise major challenges. The classification process
is studied both experimentally and numerically in stand-alone classifiers, including
the fluid mechanics inside the vanes of the rotating classifier wheel [6, 22–28].
The following chapter is dedicated to the study of particle-particle interactions,
revealing the overall stressing conditions, and the dynamics of the two-phase flow
during non-stationary fine grinding. For this purpose a lab-scale fluidized bed
opposed jet mill is investigated in detail: Besides the grinding behaviour during
different operational modes, a novel approach for the direct determination of relative
particle velocities will be presented. Any comprehensive description of the comminution process requires the knowledge about the materials properties and the materials
response to the determined stresses. For this reason the single particle breakage
behaviour is studied and analysed by single particle compression and impact tests.
The presented observations are then finally used for a model of the product mass
flow.
2 Materials and Methods
2.1 Fluidized Bed Opposed Jet Mill
The experiments were performed in a lab-scale fluidized bed opposed jet mill (AFG
100, Hosokawa Alpine AG, Germany). A scheme of the setup is depicted in Fig. 1
[29]. The mill consists of a cylindrical milling chamber (inner diameter of 100 mm).
Three Laval nozzles [exit diameters of 1.9 mm (1)] are arranged in a 3D configuration
at the bottom of the mill chamber and are directed towards the central focal point.
The nozzles are supplied with pressurized air, which leads to a gas flow directed
upward towards the classifier wheel (2), whose outer diameter is 50 mm. The mill
was operated at a pressure level of 10–20 mbar below ambient conditions. An online
laser diffraction system (3) (Insitec, Malvern Panalytical, UK) is installed in the
product stream to continuously record the product particle size distributions (PSDs)
(sample rate of 1 Hz). Before the online particle size measurement, the product
particles are dispersed by a ring nozzle (4). Particle separation after the measurement
was ensured by a cyclone and a filter. For the characterization of the material inside