Fundamental Study On The Formation And Crystallization Of Calcium Ferrite Assisted By Ultrasonic Wave

Sintering ore is the main raw material for blast furnace ironmaking,and its quality directly determines the technical and economic indicators during the blast furnace ironmaking.At present,domestic steel enterprises encounters the problem of frequent fluctuations in the composition of the sintering mixture,which increases the difficulty of sintering production.As the gangue mineral in the sinter increases,the fluidity of the initial liquid phase during the sintering process deteriorates,which inhibits the assimilation during sintering.Therefore,how to improve the economic and technical indicators of the sinter under complex raw material has become an urgent problem for steel companies.Theoretically,to form large sufficient amount of binding phase and achieve the rapid and uniform assimilation during the sintering process are the key to solving the problem.This thesis proposes to introduce ultrasonic waves into the iron ore sintering process,to accelerate the movement of particles,improve the reaction kinetics conditions,promote the uniform nucleation by applying the ultrasonic field during the solidification,and refine the calcium ferrite grains,then improve the strength and reducibility.This work focuses on the main physical and chemical behaviors involved in the ultrasonic assisted iron ore sintering process,the main research works include:?1?Formation of solid calcium ferrite with ultrasonic treatment:effects of temperature,ultrasonic treatment time,and ultrasonic power on the formation of solid calcium ferrite under ultrasonic treatment were studied experimentally;?2?Cavitation behavior of ultrasonic wave in calcium ferrite melt:The movement model of cavitation bubbles of calcium ferrite melt was established,and the effects of ultrasonic frequency,ultrasonic power and cavitation bubble radius on the cavitation bubble movement were studied by simulation method.?3?Dissolution of oxide into calcium ferrite under ultrasonic treatment:The acoustic field and flow field formed during the propagation of ultrasonic wave in the dissolution process of oxide into calcium ferrite was solved by COMSOL Multiphysics software.The dissolution behavior of Al₂O₃,SiO₂ and MgO into calcium ferrite was investigated experimentally.The dissolution interface was observed and the oxide content in the final slag was analyzed.?4?Numerical simulation of solidification process of calcium ferrite and propagation of ultrasonic wave in calcium ferrite melt:the temperature distribution during the solidification of calcium ferrite was explored using QuikCAST,the effect of ultrasonic wave on the physical field distribution of calcium ferrite was simulated by COMSOL.?5?Solidification behavior of calcium ferrite melt:The microstructure and grain distribution of calcium ferrite after solidification were observed by experimental.The effect of ultrasonic power and immersion depth on solidification of calcium ferrite,including phase composition,microstructure,grain size,density,strength and reduction;The microstructure and phase composition characteristics of the sample treated by 300W ultrasonic wave after solidification were studied,and the mechanism of ultrasonic wave was analyzed.The main conclusions are as follows:?1?Application of ultrasonic wave is beneficial to the formation of solid phase calcium ferrite.Without ultrasonic vibration,the amount of CaFe₂O₄?CF?and Ca₂Fe₂O₅?C2F?are 47.76 wt%and 40.66 wt%at 850?,respectively.With ultrasonic vibration,the temperature at which CaO starts to react with Fe₂O₃ is reduced by about50?.When the temperature reaches 850?,about 98.73 wt%of CF is formed,which is much larger than the CF content without ultrasonic treatment?47.76 wt%?.Increasing the ultrasonic power and prolonging the ultrasonic time are also beneficial to the formation of solid phase CF.?2?Smaller ultrasonic frequencies and larger ultrasonic power are beneficial to the cavitation of bubbles in calcium ferrite melt.Increasing of ultrasonic power,the acoustic pressure amplitude increases.The initial radius of smaller cavitation bubble facilitates the generation of cavitation effects and avoids steady-state cavitation.When the ultrasonic power increased,the amplitude of the acoustic pressure increased,and the amplitude of the cavitation bubble radius is larger,the period of the cavitation effect is shorter,which is favorable for cavitation.?3?Application of ultrasonic waves accelerates the dissolution of the oxide into calcium ferrite melt.When the ultrasonic wave propagates in the melt,the acoustic pressure and intensity at the ultrasonic probe are the largest,the flow velocity at the corner of the ultrasonic probe is the largest in the melt.The acoustic field and flow field formed by ultrasonic wave in the calcium ferrite improves the restrictive step of oxide into calcium ferrite,thereby accelerating the dissolution rate of oxide into calcium ferrite.Compared with the experiment without ultrasonic treatment,the dissolved amount of Al₂O₃,SiO₂ and MgO rods in calcium ferrite was significantly increased after the application of ultrasonic waves.?4?The acoustic field and flow field formed during the propagation of ultrasonic wave.The acoustic field and flow field formed during the propagation of ultrasonic wave improves the restrictive step for oxide into calcium ferrite.In regular condition,the surface is the first to solidify during the solidification of calcium ferrite,then wall of crucible starts to solidify,the center of melt is the last to solidify.When the ultrasonic wave with different power applied,the acoustic pressure and acoustic intensity directly below the ultrasonic probe in the melt are largest.When the ultrasonic power is increased to 300 W,the acoustic streaming at the corner of ultrasonic probe is the largest,about 0.002 m/s,and the flow field in the melt is symmetrically distributed in a ring shape.When the ultrasonic wave with different immersion depths are applied,the maximum acoustic pressure and acoustic intensity are the same in each melt,but its distribution in the melt is affected by the immersion depth of ultrasonic probe.As the immersion depth increases,the acoustic pressure near crucible bottom increases.?5?Increasing ultrasonic power and the immersion depth will refine the grains of calcium ferrite,reduce the area of pores,and increase the density,compressive strength and reduction of calcium ferrite.In regular condition,the calcium ferrite melt showed a hole of about 22 mm in length at the center after solidification.The composition of the phase after solidification at the center and edge of the calcium ferrite melt is similar,but the the grain size of different parts is obviously uneven.The solidified phase can be divided into calcium-rich calcium ferrite and iron-rich calcium ferrite.The calcium-rich is denser on the surface,while iron-rich calcium ferrite is looser.The microstructure of each part of the slag after 300 W ultrasonic treatment was analyzed,the main phases of different parts in the slag were basically the same,CaFe₂O₄,Ca₈?Fe,Al?₈O₂₀?Ca_3.18Fe_15.48Al_1.34O₂₈.The grain sizes of calcium ferrite are similar at the center and at the edge of different parts,indicating that the ultrasonic treatment is favorable for the uniform distribution of crystal grains inside the slag sample.