| The demand for medium and low carbon ferrochrome in metallurgicalindustry is increasing year by year, with the rapid developmentof low carbon alloy structure steel and detailed classification of steel.The Perrin process and electric-silicothermic method can meet the industrialproduction of medium, low and micro-carbon ferrochrome. But as traditionalmethods, they all belong to liquid-phase decarburization process and willproduce chromium slag about5%~10%, which cause serious environmentpollution. Vacuum method is now widely used as solid-phase decarburizationprocess. The method has the following characteristics: without adding flux,obtaining products at lower temperature, with high chromium recovery and lowrefractory consumption, while with many links and long decarburization period.Based on thermodynamics calculation, the solid-phase decarburization ofhigh-carbon ferrochrome powders (HCFP) can be achieved in microwaveheating field and the industrial calcium carbonate powders (ICCP) asdecarburizer. The microstructure, absorbing performance of HCFP, the carbon content, electromagnetic properties and oxidation degree of decarburizedmaterial in microwave field are also analyzed, and compare with muffle furnaceheating in this thesis, trying to find a new solid-phase decarbonization method ofhigh-carbon ferrochrome. The results are as follows:(1) Theory research on the solid-phase decarburization of HCFP inmicrowave field. The thermodynamic conditions of gas-solid phasedecarburization of chromium and iron carbides are obtained with O2, CO2andwater vapor by thermodynamics calculation. The lattice of carbides wasactivitated in microwave field, which changed the free energy of mixture, and allthese activitated the reactions being in chemical equilibrium. So thedecarburization reactions, which spontaneously occur by the thermodynamicsanalysis, can also occur spontaneously in microwave field, and even with lowerstarting reaction temperature.(2)The mechanism analysis of HCFP heated in microwave field. Thefirst-principles calculations suggest that, significant changes in its chemicalbonds make dielectric properties of carbides change when iron doped in o-Cr7C3.The dielectric properties differ from its doping positions, which with highervalue when doping in Cr3. The microwave loss way of HCFP mainly containsthe thermal defects, doping defects generated during cooling process, thevacancy dipoles existed in carbide crystals, and the interfacial polarization lossin the lower microwave frequencies, etc. And its magnetic moment mainlycomes from electron spin, while diamagnetic features from chromium and carbon.(3) Experimental study on the solid-phase decarburization of HCFP inmicrowave field. The impacting factors on the decarburization are discussed,such as molar ratio, holding time and heating temperature, etc. The results showthat, the bigger molar ratio, the longer holding time and the higher heatingtemperature under certain conditions will result in the better decarburization.The optimum conditions for experiment are, with molar ratio of1:1.4,temperature of1100℃in microwave field, holding time of60min and with thedecarburization rate of79.04%.(4) The decarburization effect is obviously better in microwave field thanthat of muffle furnace. It is pointed out that the decarburization rate and thedegree of oxidation should be comprehensively considered so as to measure thedecarburization. Under the same conditions, higher decarburization rate andlower degree of oxidation can be achieved in microwave field. When heated inmuffle furnace, the oxide film in decarburized material outer is thicker, and thevalence of metal element of oxide is higher than that of microwave field.CaCr2O7in decarburized material can be found at1100℃in muffle furnace,and the particle diameter of oxidation material was up to36.2μm when hold for60min. Excessive oxidation of decarburized material in this condition made littlesense to the decarburization in muffle furnace. While CaCr2O7in decarburizedmaterial was found at1200℃and hold for60min, the particle diameter ofoxidation of completely decarburized was smaller than32.0μm in microwave field.(5) Study on the microstructure of the material. The microstructure ofHCFP was related with the cooling rate. Nonequilibrium eutectic phase can beobtained by high cooling rate destroyed the balance of crystallization conditions.Meanwhile, high cooling rate can accelerate the precipitation of flake graphite inhigh-carbon ferrochomre, which could be easily decarburized by CO2inmicrowave field. The higher heating temperature will result in the increaseddissociation proportion and the reduced content of primary phase indecarburized material. The influences of microwave heating temperature andholding time on the electromagnetic properties, in fact, due to the change of thecarbon content in decarburized material, carbon vacancies and crystal structure.(6) Kinetics study on the solid-phase decarburization of HCFP inmicrowave field. The decarburization reaction process in microwave field was inline with the unreacted core shrinking model. The kinetics of decarburization ofHCFP could be well described by second order kinetics equation and thereaction rate was dominated by internal diffusion of gas, with the reactionactivation energy of69.32kJ·mol-1. The activation energy is smaller than that ofBoudouard reaction of carbon, which proves that microwave can lower thedecarburization activation energy. The solid-phase decarburization of carbides inHCFP and oxidation of its products, with ICCP as decarburizer, were all carriedout step by step in microwave field. |
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