| The hot-dip galvanizing steel sheet has been widely used in automotive exposed and unexposed body panel application, which requires premium surface quality. The most important factor that impairs the surface quality of the hot dip galvanizing steel sheet is the dross defect, causing a dramatic economical loss. So far, the method to eliminate the dross defect is to restrict the dross formation by processing control and equipment upgrade. Recently, with the rapid progress of electromagnetic processing of materials (EPM), electromagnetic separation technology has been developed and become a novel melt purification method with much prospect in application. It is of a great value to make electromagnetic separation technology applied in removing zinc dross from hot dip galvanizing zinc melts, clarifying the separation mechanism and distribution evolution of dross phases under electromagnetic field.Based on the state of the art of zinc dross removing from zinc melts, the following research works have been carried out on the new technology of removing zinc dross by using electromagnetic separation: (1) The zinc dross separation mechanism has been investigated by using alternating magnetic field; (2) Study on influence factors of zinc dross removal efficiency through numerical analysis; (3) The laboratory-scale experimental apparatus of electromagnetic separation have been set up, and experiments to static and continuous separation deleterious zinc dross from zinc bath were conducted on a laboratory-scale apparatus; (4) The establishment of a series of pilot-plant-scale equipment, GA and GI zinc melts purification were carried out on this equipment; (5) By numerical simulation of the pilot-plant-scale and industrial zinc pot, the change laws of velocity field, temperature field and concentration field of zinc dross were studied when the zinc melts pumped out the zinc pot. All of these investigations provided experimental and theoretical basis for hot-dip galvanizing electromagnetic purification technology applied in industry.Through the establishment of the electromagnetic separation calculation model, their kinematic behaviors of zinc dross in the electromagnetic separation process, zinc dross distribution and the relation between the removal efficiency and the main operation parameters have been investigated. The conductivity of zinc dross is far less than the zinc melts, so the zinc dross experienced the repulsive force opposite to the electromagnetic force on the melt under electromagnetic field, and the repulsive force is 3/4 times that of the electromagnetic force for same volume of zinc melts. Accordingly, those particles can be successful separated from the metal melt under electromagnetic force. Removal efficiency of zinc dross is related to the electromagnetic force and the imposing time, magnetic flux density, magnetic field frequency and size of the separator. The zinc dross removal efficiency increased with an average residence time in the ceramic pipe and magnetic flux intensity increases when square separator used for zinc melts purification. It can achieve the highest removal efficiency when the frequency of magnetic field fixed and the ratio of the separator size and skin depth is about 2 times, the removal efficiency of zinc dross will be decreased with the separator size further increase; The removal efficiency increases with the frequency of magnetic filed increased when the separator size is fixed. The electromagnetic parameters and operation parameters of the electromagnetic separation apparatus are calculated, the skin depthδshould be 2.5mm when we selected suitable magnetic frequency in accordance with the size of single-channel separator, therefore, the actual frequency of the magnetic field is set to 15-20 kHz. When the separator size, frequency of the magnetic field, and the average residue time inside the separator are set as fixed value, the magnetic flux density is no less than 0.06T, the calculated rated power of the electromagnetic apparatus is 50kW. According to all above calculated results, we established electromagnetic purification equipment. Experiments to static separate deleterious zinc dross causing surface defects of galvanizing steel sheets from zinc bath were conducted on this apparatus by using alternating magnetic field. Experimental results show that for the zinc dross particles with diameter larger than 10μm can be removed when the magnetic frequency is 17.5 kHz, the effective magnetic flux density is 0.05-0.1 T, imposed time is 15 s, and the cross section of the ceramic square pipe is 5×5 mm.Experiments to continuously separate Fe-Al-Zn dross phase from hot dip galvanizing zinc melt were conducted on a laboratory-scale apparatus by using high-frequency alternating magnetic field too. When the magnetic frequency is 17.5 kHz, effective magnetic flux intensity is 0.1T, the cross section of the ceramic square pipe is 10×10 mm, and the processing time is 0.6-2.5 s, the separation efficiency of zinc dross varies from 43.76% to 98.63%, and the experimental results are almost in good agreement with theoretical results. In order to improve the removal efficiency and make full use of the secondary disturbance inside the ceramic pipe, a new multi-stage separator is developed and the removal efficiency of multi-stage separator is calculated. As the center of each channel of the first stage separator is now occupied by the wall, and zinc dross was originally located in a "weak areas" of the first stage separator successfully changed to the "strong areas" when these dross entered the second stage separator, the zinc dross electromagnetic separation efficiency can be increased by 10% or so, and these numerical calculation results has been verified through experiments.A series of pilot-plant-scale equipment which owns independent intellectual property rights for zinc melt electromagnetic purification was developed. A new cardiopulmonary bypass zinc melts purification technology was first introduced in this patent equipment, the successful implementation of a continuous electromagnetic purification outside the zinc pot, this equipment can be seen as the prototype of the equipment for industrial applications. The GA zinc bath purification experiment results show that through extending the process time, optimize the process parameters and change the structure of ceramic separator can significantly improve the purification effect, the average zinc dross removal efficiency is up to 89.52% for all samples, the removal efficiency after separation in the launder was about 75%, zinc dross can be removed completely which particle size larger than 20μm; GI zinc bath purification experiment results show that zinc dross removal efficiency up to 70%, the removal efficiency of zinc dross less than 15μm was only 42.1% in the launder, and the removal efficiency of dross particle size larger than 15μm was 91.67%, but zinc dross can be removed completely which particle size larger than 20μm as same as in GA purification.Numerical simulation results of pilot-plant-scale zinc pot show that the return pipe should be shorted when the return flow rate of zinc is low, and vice versa. The removal efficiency increases with the increased of the flow rate of zinc bath. The particle size and concentration fields of zinc dross have little effect on the separation results.Numerical simulation results of zinc pot applied in industry show that the location of return pipe and pump pipe has effect on the"V"shaped area, but has little effect on the bulk flow. The dross concentration in the bath decreases with the processing time in the case of circulating purification. Compared to zinc pot in general, the introduction of circulating purification can significantly improve the surface quality of galvanizing steel sheet.
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