During the air classification process in the zig-zag classifiers, two dominant forces can be considered: a gravity force and an air drag force as shown below. A classifier consists of the diagonal pates where the air flow is applied towards the upper outlet. The feed is provided in the central part and the large heavy particles are flowing down while the fine light particles are lifted upwards by the air flow. The shape of the classifier tube is like a zig-zag to provide many stages of separation to get better efficiency.
Typical forces acting on particles during air classification in the zig-zag classifier.
Images show the first moments of separation of the smaller and larger particles.
The classifier outlet in the lower part refers to the coarse fraction received directly in the container. The fine fraction in the upper outlet is mixed with the air and has to be separated in a cyclone or a filter. Typical installation of the ZZX classifier is shown below.
The typical industrial ZZX classifier construction is shown below. The classifier consists of 10 separate compartments, where each of them can function like an individual ZZX classifier. The feed material is supplied to the ZZX classifier by the vibratory feeder. The feeder distributes the input material evenly at the outlet across the 1.6 m wide outlet chute. The feed material falls down to the classifier sections and it is evenly distributed into 10 separate streams by the inlet plates. The coarse fraction falls down to the common outlet and the fine fractions is removed by the upper outlet chute. In order to provide an equal material load and air flow rate along the separation channels, each channel is equipped with the pressure drop measurement device. The measuring ports are placed in the upper and lower section of each separation section. Any variation in material load or irregular air flow rate will be detected by this dp measurement. There is a natural tendency that the central part of the classifier sections will provide higher air flow than the side sections – therefore there is a separate adjustment mechanism installed at the upper outlet. It consists of the three sliding plates inside the outlet channel which can be individually adjusted to allow air flow adjustment between the central part and the side sections. This adjustment is normally done during commissioning and should not be carried out during regular operation, without a specific reason. The maximal deviation during dp measurement has to be set by the commissioning team and it is individual for each application. This procedure is described in the separate document related to the complete system operation. Inspection of the classifier can be carried out by opening the door sections in the front of the main unit.
It is possible to integrate the ZZX classifier with the dynamic classifier as shown below for the classicisation circuit operating at 50 t/h capacity. In this case the material is first classified in the ZZX unit to remove the majority of the very coarse particles. Then, the finer particles can be classified in the dynamic classifier to provide the precise cut point. This solution reduces the amount of material in the dynamic classifier and provides an optimal processing configuration for wide particle size ranges. The static classification can be combined with the dynamic one, especially during production of finer particles under 100 microns, as shown on the diagram below.
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