388
M. Weers et al.
4 Conclusion
In spite of the broad range of applications of flow and sieve classification the physical
phenomena of higher particle loadings are not completely understood. As common
departure point, the single particle model of Molerus was used here. For the sieving,
the single particle motion and acceleration was studied and the Molerus model was
modified with a selectivity factor, which accounts for particle-particle interactions
as well as the formation of particle layers on the sieve bottom. Particle accumulation
effects, which occur in steady state and in instationary operation, may hinder the
particle mobility at higher loadings. However, it was observed that along the vibrating
screen the separation curve did not change significantly.
In flow classification, the commonly used models cannot account for instationary effects. Therefore, a new system to measure the time to reach steady state was
employed. It turned out that a hold-up volume needs to be filled before steady state
is reached which may take several tens of minutes for low loadings. However, the
hold-up volume seems to be rather constant. For steady state operation, the Molerus
model was used by refining tangential and radial velocities. For particles with high
Stokes numbers, the flow field can be neglected in a first approximation and the
cut size is dependent on the particle impaction probability on the blades. With the
detailed knowledge of the mean radial airflow, a much better prediction of the separation curve can be obtained. In contrast to the radial velocity, the tangential air
velocity is of less significance for the cut size and sharpness of separation. This is
due to the formation of a particle cloud, whose tangential velocity is to a large extent
dominated by the circumferential velocity of the deflector wheel.
For particles with low Stokes numbers, the rotary symmetry of the airflow entering
the deflector wheel will gain in importance, while the particle entrance trajectories
will be less significant. Ironically, the model of Molerus gives a good prediction of
the cut size even its conception of the particle motion is not correct for particles with
high Stokes numbers.
References
1. Teipel, U.: Energetic Materials: Particles Processing and Characterization. Wiley-VCH Verlag,
Weinheim (2006). ISBN 3-527-30240-9
2. Plitt, L.R.: The analysis of solid-solid separations in classifiers. CIM Bull. 64 (1971)
3. Rogers, R.S.C.: A classification function for vibrating screens. Powder Technol. 31, 135 (1982)
4. Molerus, O., Hoffmann, H.: Darstellung von Windsichtertrennkurven durch ein stochastisches
Modell. Chem. Ing. Tech. 41(5+6), 340–344 (1969)
5. Trawinski, H.: Die mathematische Formulierung der Tromp-Kurve. Aufbereitungstechnik 17,
248–254, 449–459 (1976)
6. Soldinger, M.: Influence of particle size and bed thickness on the screening process. Miner.
Eng. 13, 297–312 (2000)
7. Deghani, A., Monhemius, A.J., Gochin, R.J.: Evaluating the Nakajim et al. model for
rectangular-aperture screens. Miner. Eng. 15, 1089–1094 (2002)
M. Weers et al.
4 Conclusion
In spite of the broad range of applications of flow and sieve classification the physical
phenomena of higher particle loadings are not completely understood. As common
departure point, the single particle model of Molerus was used here. For the sieving,
the single particle motion and acceleration was studied and the Molerus model was
modified with a selectivity factor, which accounts for particle-particle interactions
as well as the formation of particle layers on the sieve bottom. Particle accumulation
effects, which occur in steady state and in instationary operation, may hinder the
particle mobility at higher loadings. However, it was observed that along the vibrating
screen the separation curve did not change significantly.
In flow classification, the commonly used models cannot account for instationary effects. Therefore, a new system to measure the time to reach steady state was
employed. It turned out that a hold-up volume needs to be filled before steady state
is reached which may take several tens of minutes for low loadings. However, the
hold-up volume seems to be rather constant. For steady state operation, the Molerus
model was used by refining tangential and radial velocities. For particles with high
Stokes numbers, the flow field can be neglected in a first approximation and the
cut size is dependent on the particle impaction probability on the blades. With the
detailed knowledge of the mean radial airflow, a much better prediction of the separation curve can be obtained. In contrast to the radial velocity, the tangential air
velocity is of less significance for the cut size and sharpness of separation. This is
due to the formation of a particle cloud, whose tangential velocity is to a large extent
dominated by the circumferential velocity of the deflector wheel.
For particles with low Stokes numbers, the rotary symmetry of the airflow entering
the deflector wheel will gain in importance, while the particle entrance trajectories
will be less significant. Ironically, the model of Molerus gives a good prediction of
the cut size even its conception of the particle motion is not correct for particles with
high Stokes numbers.
References
1. Teipel, U.: Energetic Materials: Particles Processing and Characterization. Wiley-VCH Verlag,
Weinheim (2006). ISBN 3-527-30240-9
2. Plitt, L.R.: The analysis of solid-solid separations in classifiers. CIM Bull. 64 (1971)
3. Rogers, R.S.C.: A classification function for vibrating screens. Powder Technol. 31, 135 (1982)
4. Molerus, O., Hoffmann, H.: Darstellung von Windsichtertrennkurven durch ein stochastisches
Modell. Chem. Ing. Tech. 41(5+6), 340–344 (1969)
5. Trawinski, H.: Die mathematische Formulierung der Tromp-Kurve. Aufbereitungstechnik 17,
248–254, 449–459 (1976)
6. Soldinger, M.: Influence of particle size and bed thickness on the screening process. Miner.
Eng. 13, 297–312 (2000)
7. Deghani, A., Monhemius, A.J., Gochin, R.J.: Evaluating the Nakajim et al. model for
rectangular-aperture screens. Miner. Eng. 15, 1089–1094 (2002)