(513b) Visualization of Particle Movements in the Periphery of a Deflector Wheel Classifier

Although the deflector wheel classifier is the dominating separation device in the industrial separation of fine-grained particle fractions, the classifying mechanisms at high particle loadings are still not described and understood appropriately. All theoretical models for the calculation of the separation efficiency for deflection wheel classifiers are based on the solution of the simplest equation for separation processes. Here is always the one-dimensional case by neglect of source and sink-terms used, which describes the so-called Markov process (1):

(1)

where c_i (y, t) is the concentration at the site y at time t of a size class i, v_i is the speed of the particles in this size category and D_i is their diffusion coefficient. The existing models for the calculation of the separation efficiency elaborated e.g. by Molerus, Rumpf, Sender, Schubert and Husemann, which are mostly based on single particle fate, fail to recover the behavior at high particle loading where particle-particle and particle-wall collisions are encountered. Comparisons between these models and experimental data are showing that there must be some previously unknown effects which have a significant effect on the separation. To overcome this knowledge gap a deflection wheel classifier was modified in a way which allows an optical access onto the classifying wheel during the classification process. Using this modified classifier, a limestone feed and a high-speed camera it was possible to make the particle movement in the periphery of the deflection wheel visible. To represent the particle movement a limestone feed material with an X₅₀of 59.86 µm was used. The peripheral speed of deflector wheel during these experiments could be changes from 7.86 to 31.42 m/s. To get a better impression of the movement and the velocity of the particles high-speed pictures are analyzed using a PIV-software.

The high-speed camera analyses during the present study have proved that from the deflection wheel deflected particles are producing particle streaks. These particle streak formations are influenced by the peripheral speed of the deflection wheel as well as the feed mass flow. The experiments have shown that at different peripheral speed values different streak formations are appearing. Additionally, could be investigated that the viscosity of these streaks increases with the peripheral speed. The formation of a streak with an arched form can be seen in figure 1 at a peripheral speed of 7.86 m/s. It can be seen in that the feed particles are deflected from the pursuing paddle and are forming particle streak into the periphery of the deflector wheel. Figures 1 (B) to (F) are showing that the particle streak flows tangential into the rotational direction of the deflection wheel and is charged gradually with deflected particles. Additionally, can be seen that the particle streak have a filtration effect on the counter-flowing feed particles. This particle streaks are colliding with feed and deflected particles and building agglomerates. This agglomeration forces small particles to be separated into the coarse particle fraction. However, it is apparent that the particle streak formation has a big influence on the separation efficiency of a deflection wheel classifier. Because of these reasons a deviating view onto the classic separation models for a deflection wheel classifier must be considered.

Fig. 1: Formation of a particle streak with an arched form at peripheral speed of 7.86 m/s.