Study On Law Of Alumina Depositing On Bag And Adsorpting HF

In aluminum electrolysis industry, bag filters are widely used to remove alumina dust and toxic HF gas in alumina electrolysis fume. The dust layer diposited on the bag can not only filter fine dust, but also adsorb HF gas. In this paper, in order to get a good effect of filter and HF adsorption within a suitable filter pressure drop range, numerical methods, theoretical analysis and experimental methods are comprehensively applied to study the laws of alumina sedimentation on the bag and HF adsorption by alumina layer. Internal flow field analysis of bag filter is studied with numerical method, on which the structural optimization program is brought forward. The law of alumina depositing on the bag and HF adsorption by alumina layer are experimentally studied; the relationship between dust layer thickness vertical distribution and the average filtration velocity, dust content, filter time and the length of filter bag are fit with equations. On theory, force analysis of alumina particles is conducted; pressure drop model of bag filter is derived; filter mechanism of bag filter, the mechanism of HF adsorption by alumina are analyzed. Comprehensive models of alumina deposition on bag and HF adsorption are set up. These models are solved with numerical method with MATLAB software, the model results are verified with experimental results, which shows a good consistency between the two.Industrial practice shows, there are several inadequacies on the bag filters widely used in aluminum industry in our country. Three optimized programs are put forward: (1) add deflector in intake in the dust collector; (2) add deflector in the side-stream channel; (3) add baffle plate at the bottom of the filter. These methods are comprehensively applied to improve the internal flow field. Each program is numerically analyzed with FLUENT software and with discrete-phase model, velocity-track of alumina particle and gas velocity distribution are obtained, optimal structure is determined by the uniformity of particle and gas flow field distribution in filter. With the improved collector structure, the airflow distribution will be improved, so that the airflow will rise uniformly without large eddy current, which is in the favor of formation of uniform dust layers and fluoride removal.In the experiment part, a new dust experimental device and a new dust layer thickness measurement method are designed in order to study the deposition law of alumina on the bag. In the experiments, a transparent plexiglass bag device and a plat bag structure are adopted and the dust layer thickness is measured on-line with mobile measuring microscope from outside of dust layer. In the paper, the influence of 1 to 6 meter bag on the dust distribution is studied with 1-meter-long flat bag, which solves the two problems that the thickness of dust layer is hard to measure and the cost is high for the experiment equipment designed with traditional way. Based on this device, the influence of the average filtration rate of the flue gas, flue gas dust content, time and the length of the bag on the dust layer thickness vertical distribution is experimentally studied. GuassAmp peak function is firstly used to fit the vertical distribution of dust layer thickness. Of the three intermediate variables of the function, A. w can be used to evaluate the distribution of the dust layer. A reflects the relationship between sediment thickness and filtration rate, dust content and the filtering time; w divided by the bag length L, which reflects the uniformity extent of dust layer on the bag. x_c denotes the degree for the location with biggest dustlayer thickness to the midpoint of the bag. Therefore, the fitting way have some innovative on the study on bag filter.In addition, on the experimental setup, the effect of average filtration velocity, alumina content and HF concentration on HF adsorption by alumina dust layer is studied. Study shows that: in the same period of the filter, the higher the dust concentration, the better the effect of fluoride removal; the best bag length is 5 meter considered by the effect of filter pressure drop and fluoride removal. with the average filtration velocity with the scope of 20mm/s-60 mm/s, the best effect of fluoride removal appears at 40 mm/s, filtration speed higher or lower will result in poorer effect on fluoride removal; the higher HF concentration at the entrance, the higher HF concentration at the exit. The study also shows that the bag couldn't be too thorough pulse cleaned, or it will result in lower fluoride removal efficiency with little dust on the bag just after cleaning.On the base of Darcy's law, the planar filter pressure drop equation associated with dust thickness is derived according to given conditions in the paper. On the basis of molecular aerodynamic theory and precious study, HF adsorption mechanism in alumina layer is analyzed with intrinsic reaction and Macrokinetics theory, and the analysis shows that it accords with Unreacted Shrinking Core Model. The corresponding adsorption rate equation is established.Finally, on the base of experiments and theoretical analysis, integrated models on alumina sediment and HF adsorption are set up and the models are numerically solved with MATLAB software. The numeric solution and experimental results have good consistency, which verifies the effectiveness of the integrated models. The integrated models can predict pressure drop and fluoride removal efficiency of bag filter according to operation conditions and length of bag, which will provide a reference for engineering design.For the reason of experimental conditions, there are some inadequacies in the study. Further study, such as different temperatures and different samples of alumina on the impact of FIF adsorption, is needed. In addition, the integrated model is needed to be revised according to industrial practice.