Study On Aggregation Mechanism Of Submicron Fe-based Fine Particles Captured By Magnetic Fiber

The iron and steel industry is a typical heavy polluting industry,which has also caused serious harm to the environment.Solid particles are common pollutants in the iron and steel industry.Among them,inhalable particles(PM_2.5)can enter the respiratory tract and deposit in the lungs,which can lead to many diseases such as industrial pneumoconiosis.The domestic iron and steel industry is based on the blast furnace-converter steelmaking long process technology.This production process has many pollution nodes,long industrial chains,and complex internal processes.As a result,the emission intensity of atmospheric pollutants in the domestic iron and steel industry is higher than that in foreign developed countries.Therefore,it is urgent to solve the problem of particulate pollutant emissions in heavy polluting industries such as iron and steel.The particles emitted by the iron and steel industry in the main production processes such as sintering,coking,ironmaking,steelmaking,and steel rolling are distributed in a polydisperse phase.The particle diameter ranges from nanometer to micrometer,and the particle shape includes spherical particles and irregular non-spherical particles.In the process of capturing submicron particles in the range of0.1 0.5μm,the diffusion effect,interception effect and inertia effect are not obvious,and the dynamic characteristics of particles and air are different,which will inevitably lead to the diffusion and escape of some fine particles.At present,the methods to improve particulate matter capture include fiber,filter media structure improvement,and selection of filter media types from micro to macro,but these methods also increase the difficulty of manufacturing and the uncertainty of particulate pollutant control.By studying the physical properties of particulate pollutants in the production process of the iron and steel industry,it is found that under high temperature conditions,the iron ore raw materials in the iron and steel industry have ferromagnetic substances such as Fe₂O₃ and Fe₃O₄ in the particulate matter after crushing and steam condensation.The particles produced in the production process are easily magnetized in the magnetic field and removed by magnetic force.To highlight the magnetic properties of the particles,the particles are defined as Fe-based fine particles.The introduction of magnetic field effect can make up for the problem that the fibers have different capturing mechanisms for particles with different particle diameters.The magnetic field effect is realized by the magnetic field force of the magnetic fiber near-wall acting on the Fe-based fine particles.Therefore,the in-depth study of magnetic field effect enhanced fiber capturing mechanism of Fe-based fine particles and analysis of the multi-field coupling（flow,magnetic and gravitational fields）aggregation process of Fe-based fine particles will provide new ideas for scientific researchers to develop new high-efficiency fiber filters to reduce particle pollutant emissions.Based on this background,this paper conducted the following research on the utilization of the magnetic field reinforced fiber to capture Fe-based fine particles in the iron and steel industry to achieve ultra-low emissions:Firstly,taking the capture process of Fe-based fine particles by magnetic fibers under a high gradient magnetic field as the research object,a physical model of Fe-based fine particles captured by magnetic fibers was established.Based on the computational fluid dynamics-discrete phase model（CFD-DPM）,the magnetic force of the magnetic fibers on the Fe-based fine particles was added through UDF custom programming,the dynamic process of magnetic fiber capturing Fe-based fine particles in a high gradient magnetic field was numerically simulated.The trajectory and capturing efficiency of magnetic fibers capturing Fe-based fine particles under the same or different magnetic forms were calculated.The dynamic characteristics of Fe-based fine particles near the wall of magnetic fiber were studied under the influence of inlet velocity,dust particle diameter,external magnetic field strength,the saturation magnetization of magnetic fiber and magnetic susceptibility of particles.In addition,based on the characteristics of dust particles discharged from the steel industry containing non-spherical particles,the influence of non-spherical particles on the capture of magnetic fibers under different magnetic fields was also calculated.Secondly,when the research object was transferred from magnetic fiber to the magnetic filter medium,the arrangement of the fibers became one of the key factors affecting the particles captured by the magnetic fibers.Therefore,the process of Fe-based fine particle capture by magnetic filter medium in two typical structure arrangements（staggered/parallel）was numerically simulated.The simulation method was the same as that of magnetic fiber capturing Fe-based fine particles.The difference was that the position coordinates of magnetic fiber were added to the UDF of magnetic force,and the interaction between magnetic fibers was considered.When the uniform magnetic fields were added to the domain where the magnetic fibers were located along the X-,Y-,and Z-axes,the external uniform magnetic field could be superimposed with the magnetic field vector generated by each magnetic fiber in the magnetic filter medium to form a composite magnetic field.The movement trajectories and filtration efficiencies in two structure arrangements of magnetic filter medium under different magnetic fields were compared,and the optimal direction of the external uniform magnetic field and the arrangement of the magnetic fibers structure were determined.When u=0.1 m/s,and 0.5μm≤d_p≤2.5μm,the results were relatively stable when the external uniform magnetic field was along the X-,and Y-axes,and the filtration efficiency of the staggered structure was higher than that of the parallel structure.The results fluctuated greatly when the external uniform magnetic field was along the Z-axis.Within the same fibrous structure arrangement,when the direction of the uniform magnetic field was the same as the direction of the inlet velocity(H//u⊥l_fiber),the performance of Fe-based fine particle capture by the magnetic fibers was the best.At the same time,the fibers in magnetic filters exhibited strong magnetic interactions through the magnetic field.When Fe-based particles are captured by magnetic fibers,they were inevitably affected by the magnetic field of adjacent magnetic fibers.Therefore,the influence of magnetic interaction between magnetic fibers on the process of capturing Fe-based fine particles was studied.When a uniform magnetic field is applied along the X-(H⊥l_fiber1⊥l_fiber2),Y-(H//l_fiber1⊥l_fiber2),and Z-axes(H//l_fiber2⊥l_fiber1),different combinations of magnetic fields could form around the magnetic fibers,which change the magnetic interaction between orthogonal magnetic fibers.By comparing the capturing process of Fe-based fine particles by orthogonal magnetic fibers under three working conditions,the capturing trajectory and filtration efficiency of each magnetic fiber capturing Fe-based fine particles in the orthogonal magnetic fiber structure under magnetic interaction were calculated,and the calculation results were compared with the capturing trajectory and filtration efficiency without magnetic interaction to determine the influence degree of magnetic interaction.However,particles in the flow field have a certain aggregation effect due to collision.To study the aggregation process of particles in the flow field during fiber capture,the Brownian aggregation behavior in the process of the sub-micron particles（0.1-0.5μm）captured by fiber was numerically studied based on a computational fluid dynamics-population balance model（CFD-PBM）,and the partition method was used to solve the population balance equation（PBE）.Moreover,by comparing the difference between the numerical simulation of fiber and filter medium capturing efficiency considering turbulent aggregation and the empirical formula,it was verified that there is turbulent aggregation in the capture process.If the magnetic field was added to the flow field,for the magnetic aggregation of Fe-based fine particles,although there was currently a formula for calculating the magnetic aggregation core,the application scope is limited,and applicable particles need to be in the continuous region or free molecular region.In the range of 0.5-2.5μm,the magnetic aggregation core cannot be used together with other aggregation cores.Therefore,for the aggregation of Fe-based fine particles,the CFD-DPM method was used to analyze the force of the two particles colliding with each other,the particle motion equilibrium equation was established,and the theoretical expression applicable to multi-field coupling（flow,magnetic and gravitational fields）aggregation was derived,and the relationship between the aggregation coefficient and aggregation was given.It provides a reference for the subsequent study of particle aggregation in fiber capturing.Finally,the dust pollutants in the typical production process of the steel industry were characterized and tested to determine the physical characteristics of the particulate pollutants,and the magnetic filter capture model suitable for engineering application was established.The magnetic polyimide filter material was prepared by impregnation method,and by detecting the filtration efficiency and filtration pressure drop of the static dust removal,it was found that the filtration efficiency of magnetic filter material after magnetization was much higher than that of the non-magnetic filter material and the magnetic filter material,especially in the range of particle sizes less than 2.0μm,and the filtration efficiency was increased by about 20%.For the filtration pressure drop of four kinds of filter media,when the cobalt ferrite nanoparticles were loaded on the surface of the filter media during the preparation of the magnetic filter material,Because the decrease in the pore size of the filter material surface,the initial filtration pressure drop of the magnetic filter material was only about 9.0 Pa larger than the filtration pressure drop of the non-magnetic filter material.