Classification is based on the different settling velocities of the particles in the moving medium to divide the particle group into two or more granularity levels, generally applicable to fine grained materials. The movement types of the medium in the classifying equipment include vertical ascending motion, approximate horizontal motion and gyratory motion.
The medium can be water and air. When materials is grading in the ascending stream, these particles whose settling velocity is greater than the velocity of the ascending stream will sink in the classifying equipment, becoming sand setting. And those whose settling velocity is smaller than the velocity of the ascending stream will be taken out by stream from the top of classifier, becoming overflow products. When the setting velocity is equal to the velocity of the ascending stream, these particles will be averagely distributed in sand setting and overflow products on a fifty-fifty basis. This granularity is called separation granularity.
The settlement forms of particles in the classifying equipment generally depend on their concentration in the equipment. These particles with smaller concentration fall into free sedimentation, and those with larger concentration belong to hindered settling. Because the hindered settling velocity of particle is smaller than free settling velocity, therefore, the hindered settling needs less ascending stream than free sedimentation, which can save water and get sand setting with larger concentration. Besides, the production capacity of the classifying equipment will be be improved accordingly.
However, due to the ratio of equilibrium settling velocity of hindered setting is larger than that of free sedimentation, in hindered setting classification there are more fine and heavy particles mixed in sand setting, thus influencing the accuracy of classification. In consequence, when the classification accuracy is higher, it is unfavorable to adopt hindered settling. The accuracy of particle grading can be also influenced by particle density and shape in addition to particle size.
