Numerical Simulation And Separation Characteristics Of Wide-Size-Range Medium-Solids Fluidized Beds

Gas-solid fluidized bed separation expands the choices of highly-efficient beneficiation methods for world coals located in areas deficient in water resources. However, the following problem occurred with the medium solids (i.e. magnetite powder) during industrial scale separation experiments: it is difficult and costly to prepare a great deal of medium solids of narrow size range. To decrease the construction cost a modularized 40-60 ton per hour KZX40 dry coal beneficiation system was constructed by the workers of China University of Mining and Technology (CUMT) and Tangshan Shenzhou Manufacturing, Co. Ltd (TSM). Therefore, it is necessary to expand the suitable size range of medium solids for an increase in applicability of the system. The hydrodynamics of medium solids of various size ranges were studied using a combination of experimental and numerical methods. Furthermore, the separation performance of wide-size-range medium-solids fluidized bed was investigated. The design of medium solids was optimized thus expanding the applicable size range of medium solids. The characteristics of fluidization and beneficiation of the industrial-scale bed were studied. Moreover, some problems were detected and studied during industrial-scale experiment. This is beneficial to commercialization of the technique.The fluidization characteristics of large Geldart B magnetite powder were studied. The results show that Syamlal-O'Brien drag model is suitable for simulating the bed at a particle restitution-coefficient of 0.9. Moreover, for the static bed height Hs≤300mm there is little effect of the static bed height on the fluidization characteristics. The simulated hydrodynamic results are consistent with the experimental data. The superficial gas velocity should be adjusted to no more than 2.0Umf.Based on coal from South Africa, the separating performance of a pilot gas-solid fluidized bed beneficiation system using a compounded medium solids of mixed the large Gelart B particles and <1mm fine coal was investigated . The separating quality of the fluidized bed drops gradually as the feed-coal particle size decreases. The cause of the differences in separating characteristics was analyzed by particle dynamics and numerical modeling. Although the Geldart B bed has excellent separation performance for 50~6mm coal the bed activity is relatively low and it is difficult and costly to prepare a great deal of the large Geldart B magnetite powder for an industrial scale separation experiment. Then the study on decrease of the lower size limit of the magnetite powder was performed.The mixing mechanism of particles in bed was studied. The experimental and simulated results show that when the content of fine Geldart B particles increases the fluidization performance is improved. Furthermore, 0.3-0.06mm particles distributes uniformly overall with no strasfication. The magnetite powder having a major component, 0.3-0.06 mm particles (≥80% by wt.), can be, hence, used as medium solids for coal separation. The gas velocity should be adjusted in a range of 1.5Umf-1.8Umf to maintain the stability of the bed. Moreover, the models for prediction of the height and average-density of bed were proposed. This laid a foundation for the development of automatic control system of bed.The separation characteristics of the compounded medium solids having a wide size range and distribution of fine coal were studied. The effect of fluidization number, distributor-to-bed pressure drop ratioΔPd/ΔPb and 1.5-0.5mm fine coal content of medium solids on the bed was investigated. The experimental results show that, atΔPd/ΔPb≥1, the ratio is not a significant factor influencing the bed fluidization and separation performance. A mixture of 1.5-0.9mm coal and the magnetite powder is not suitably used as medium solids. The 1.5-0.9mm coal content in the medium solids should be controlled to no more than 3.09%. A mixture of 0.9-0.5mm coal and the magnetite powder is suitable for coal separation. Moreover, based on the magnetite powder having a major component, 0.3-0.06 mm particles, a method to adjust the bed density is reasonable by changing <1mm fine coal content in medium solids. During industrial-scale experiments screens of a large aperture diameter (3mm) were used to separate the medium solids from the products. The effect of the < 1mm fine coal on the performance of the fluidized beds has been reported. This paper emphasizes the effect of 3-1mm fine coal on the bed performance. To maintain good fluidization performance and separation quality the value of 3-1mm coal content in medium solids should be controlled to less than 4.5% during dry beneficiation processing of coal.The suitability of the medium solids, consisting of the magnetite powder having a major component (0.3-0.06 mm particles) and <1mm fine coal, for the modularized system was investigated. The industrial experimental results show that the fluidization quality of bed was good. The separation performance of the system was excellent at a low or high separating density. Effective separation of 50-6mm coal can be implemented by the system at a high or low separating density, with an E value in the range of 0.05-0.07g/cm3.