Study Of Multilevel Multi-Scale Particulate Fluidization Precision Classification Tower

In the process of powders preparation technique, Classification was the final program to determine particle average diameter, particle efficiency and particle size distribution. For narrow-level or extreme narrow-level distribution ultra fine powders manufacturing technology, classification technology was even more important than grind technology. Because of the importance of classification technology, there were a number of theoretical and experimental studies at home and abroad, also there were a large number of patents, technologies and equipments. The majority researches were concentrated on the dry type classification, in order to solve the lower limit of classification, particle efficiency and energy efficiency issues. Because traditional dry type classification was confined by the flow stability and material dispersion, there were many real difficulties in solving the particle size distribution and particle composition. Although the stabilization of wet type classification was more than the dry type classification, there were a follow-up dry and a second problem of comminution, so the economic augmentation was brought. Except to classify some valuable powders, the wet type classification was not been used in mass classification process.Based on the actuality of classification, relied on the foundation of the physical properties and the entrainment and elutriation theory of particles, used for reference of ascending flow classification and rectification principles, a new multilevel multi-scale particulate fluidization precision classification tower was designed. Multi-trays were equipped in the tower, which could be formed several fluidization layers. Underside of upper layer was particle dilute phase. Upside of lower layer was particle dense phase. So the multi-fluidization and classification effect was produced. The feed particles were dispersed by air flow before entered into the tower. Accumulation of little particles and schlepped little particles by large particles were shattered by axial-flow blowers in the drop process, so the particulate fluidization effects was produced. Different diameter particles could be obtained through adjusting the air rate in tower. After a series of tests and improving, testified in experiments that the classification equipment had high particle efficiency, narrow-level particle size distribution and lower limit of classification.Based on the forces among the ultra fine particles, an agglomeration mathematical model of ultra fine particles in dry condition was established, and the qualitative and quantitative calculation results were made out. Agglomerated equivalent diameters of different diameter SiO2 particles were calculated out in the first. The results indicated that the agglomerated equivalent diameter of 1μm particles could reached 1480μm, while the agglomerated equivalent diameter of 10μm particles was 279μm. For the particles of diameters from 1μm to 10μm, diffusion force was so small that could be neglected, electrostatic force and Van Der Waals force were the master forces. It could be explained the experimental result of particle efficiency's"fish hook effect"through this theory. Based on the entrainment and elutriation theories, the separation mathematical model of C type particles was established. The results of calculation were in concordance with the experimental results.Numerical simulation of flow and particle tracks in classification tower were been made depending on the advanced simulating software FLUENT6.2. These simulation results of flow revealed that the distribution of static pressure was a ladder line. The magnitude of static pressure was presented to a decreasing trend from the bottom to the crest of classification tower, which was similar to the grads of temperature in distillation column. This phenomena was in concordance with the distillation principle which was adopted by us. The turbulent intensity in the board was stronger than any other position. The turbulent intensity of air flow reached to the max. when it was passed the distribution sheet. The simulating results of particle tracks showed that the big particles subsided on the bottom of the tower, while the small particles escaped from the outlet at the air rate of 0.100m/s. A little portion of small particles and the medium of particles were pulled to the next sheet by circulation air flow. The bigger particles in the processing of sedimentation had bigger momentum, but some taken place collision with the boards and allured by circulation air flow whose momentum were decreased. The fine particles hold the similar momentum, which indicated that the fine particles had better following behaviors for air flow. The Reynolds number became big when the diameter of particle became big, the maximum turbulent intensity taken place in the board, which indicated the force between the coarse particle and the air flow became bigger than the fine particle. That confirmed the separating effect of the board. The numerical simulation results also validated the design validity of the sheets, circulation system of bigger particle and the rate gradient in the tower.This paper presented experimental data classified for classification tower. Previous laboratory results with an experimental tower classifying talc and quartz sand powder showed its superiority of classification. The grade efficiency of the diameter of 10μm talc particle was achieved 78.78% at velocity of 0.122 m/s while d95 of fine talc particle reached 12.84μm. Also the grade efficiency of diameter of 10μm quartz sand particle was achieved 92.80% at velocity of 0.139 m/s while d95 of fine quartz sand particle was reached 12.84μm. Grade efficiency became large while the feed rate reduced. Circular air rate was suitably at 0.5m/s,1.0m/s,1.5m/s,2.0m/s和2.5m/s from upside to underside. Grade efficiency had a decrease trend while regurgitant rate increased. The calculation air rate of diameter 10μm particle was smaller than the actual air rate. These results illustrated the distinctness between the optimal experiment velocity and the calculation velocity. And also could be interpreted that different materials had different properties (density, shape of particle, several combined force et al.), so the errors were different. It was indicated clearly that the new equipment had a good future in classification application field.