With the mandatory requirement of recycling and reuse of electronic scraps arising from printed circuit boards (PCB), the non-polluting separation methods to separate metals and plastics are of growing importance nowadays. Mechanical recycling of electronic scrap oriented towards the overall materials recovery from obsolete electronics is being implemented worldwide. Air classification is one among the clean mechanical separation methods that can achieve reasonably good separation of metals and plastics from the PCB stuff. In the present investigation, the separation of metals and plastics from the milled PCB using a column air classifier has been studied in detail. A detailed study on the separation of species in the air classification system was analysed by using the efficiency curve approach and the classifier model parameters such as the reduced efficiency curve parameter (α), corrected cut size (Va/Vt)50c and the size selectivity increment (ΔS) with respect to the process variables. The reduced efficiency relationship for the plastics and metals for all the experimental data were analysed using one parameter (α). The accuracy of the developed model was checked with the predicted values with all the observed values. The results showed that the simulation model is able to predict the experimental results satisfactorily.
We conducted experimental tests of the classification performance and flow field measurement in a turbo air classifier. In order to investigate the effects of operating parameters on flow field characteristics, we performed 2D velocity measurements of flow field in the classifier by laser Doppler velocimeter (LDV). The results indicate that different operating parameters have different tangential and radial velocity distributions in the passageway of rotor blades. The effect of different operating parameters on the radial velocity distribution is more intense than that of tangential velocity distribution. From 2D vectorgraphs of tangential and radial velocity, it is seen that the intensity, position, and rotation direction of a vortex is different under different operating parameters too. Air inlet velocity is held constant at about 8 m/s, and the intensity of the vortex is the least at a rotation speed of 600 r/min; air inlet velocity is held constant at about 18 m/s, and the intensity of the vortex is the least at a rotation speed of 1200 r/min. Under different operating parameters, we conducted the experiment of raw materials by employing ground calcium carbonate as the raw material. The rotation speed of the rotor is held constant at a value, and the high classification sharpness index k can be achieved around a critical air inlet velocity. The classification results testify the measurement results of flow field.
Rotating wheel air classifiers are often used in many process industry applications. The internal geometry of these equipment is quite complicated and has not been investigated in detail. In the present study, the flow field inside a rotating wheel air classifier has been calculated using CFD techniques, taking full account of the internal geometrical features. The predicted overall flow rate and the flow pattern are in good agreement with measurements and flow visualization studies. The calculations show that the induced flow depends strongly on geometric parameters such as the location of the inlet and outlet ports and the type of shutters used. Trajectory calculations of single particles show that the particle motion is influenced principally by centrifugal force, air drag, and wall-rebound characteristics. The wall rebound is possibly one of the means of how large particles enter the fines stream, leading to low efficiency at high speeds or large particles. Experiments of the classification using angular and radial shutter vanes show distinct range of operability of each type. These results have been interpreted coherently in the light of the flow pattern and particle trajectory calculations.
For the traditional air classifier, the difference of terminal velocity of particles is one of the main factors to achieve effective separation. Because the terminal velocity in turn is dependent on the particle's size, shape and density etc, it is difficult to achieve an ideal separation result. To improve the separation efficiency of particles, a new type of air classifier, the passive pulsing air classifier, was introduced. In the pulsing air classifier, expansions and contractions of the throats impose a distance-varying airflow which alternately accelerates and decelerates the particles. A density-dominant separation was achieved because of the pulsing airflow. In order to evaluate the separation effect of different air classifiers, two species of the tracing particles which have similar particle size and different density were adopted. The results of the laboratory experiments indicate that the pulsing classifiers achieve more effective results of separation and get a wider range of operating velocities of the airflow than the traditional ones.
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