| With an increase of domestic demand and consumption for stainless steel,as the two essential elements of stainless steel,the demand for Ni and Cr is remarkably increasing.However,China is seriously lack of Ni & Cr reserves,and has a lot of low grade reserves,not high grade reserves.The high price of Ni products before the economic crisis at 2008 forced the stainless steel plants in China to investigate the possibility of various raw materials,especially the possibility to utilize the low grade reserves.How to efficiently utilize these low grade Ni & Cr reserves to lower the production cost is quite important to strength the market competition of the steel plants.Furthermore,current and traditional process to produce the stainless steel is usually to mix the hot metal,Fe-Cr alloys and Fe-Ni alloys.Chromite concentrates and powders are used to prepare the Fe-Cr alloys via smelting reduction in electric furnace,and Fe-Ni alloys are produced mainly by RKEF-EAF process from the laterite.But these two processes have various problems,such as high energy consumption,severe environmental pollution and high operation cost.Under such a background,a compact process to produce the stainless steel master alloy in blast furnace by smelting reduction of Fe-Ni-Cr composite sinter is proposed in this study.This new process maybe merge the previous three process lines(Production of Fe-Cr,Fe-Ni and heat the mixing to prepare stainless steel master alloy)into one line,so the process to produce low Ni stainless steel will be sharply shortened and the energy as well as production cost will be reduced.This thesis mainly focuses on the critical physico-chemical aspects concerning about the compact process,and the main research works are as follows:(1)Feasibility analysis of smelting reduction of composite sinter to prepare stainless steel master alloy: On the basis of thermodynamics calculations,feasibility analysis to produce the stainless steel master alloy from Fe-Ni-Cr composite sinter in blast furnace has been carried out from the points of smelting reduction and separation behaviors.Then experimental studies about the influences of reduction temperature and various fluxes,such as CaO,SiO2 and Mg O,on grades and recoveries of Fe,Ni and Cr were carried out.(2)Physico-chemical properties of slag at high temperatures: Comparing with ordinary blast furnace slag,the slag produced in this new process has a high alumina content.A criteria that slag viscosity is lower than 0.5 Pa×s at 1550 ? was chosen as a measure of hearth smooth operation,and the slag forming area for this new process has been plotted.Then molecular dynamics simulations were carried out to obtain the slag structure information at high temperatures.FT-IR and Raman spectroscopy have been conducted to analyze the melt structure of amorphous high alumina slag samples.Iso-surface tension curves of CaO-SiO2-MgO-Al2O3 slag have been plotted on the basis of Butler's equation used for the oxide melts.Additionally,experiments were carried out to study the influence of slag composition on Si content within master alloy.(3)Physico-chemical properties of Fe-Ni-Cr alloys at high temperatures: On the basis of molar volumes and excess molar volumes,Chou model was applied to build the density evaluation model of Fe-Ni-Cr alloys,and iso-density curves have been given.Prediction models for surface tensions of Fe-Ni,Fe-Cr and Ni-Cr binary alloys based on Butler's equation by considering the excess Gibbs free energy item have been developed,and iso-surface tension curves of Fe-Ni-Cr ternary alloy was plotted.On the consideration of mixing Gibbs free energy,viscosities of Fe-Ni,Fe-Cr and Ni-Cr binary alloys were calculated,and iso-viscosity curves have been plotted.(4)Simulation of this compact process of preparing the stainless steel master alloy in a blast furnace: On the consideration of mass balance,heat balance and heat loss and introduction of two different coke phase types(combustion coke in the raceway and reaction coke in the hearth),a general simulation model of blast furnace has been constructed using METSIM.Influences of various operating conditions,such as blast air temperature,O2 enrichment,pulverized coal and H2 O,on the new process based on above blast furnace model were investigated,and optimal operating parameters have been given.By the mentioned above experimental studies and simulation analysis,main conclusions obtained in this work are as follows:(1)It is feasible to prepare the stainless steel master alloy by smelting reduction of Fe-Ni-Cr composite sinter in blast furnace.At 1600 ?,grades of Fe,Ni and Cr within master alloy are 73.39 %,1.77 %,16.42 %,respectively,and metal recoveries are 90.32 %,92.11%,80.46%,respectively.CaO is an effective and important slag former.SiO2 can lower the slag viscosity,but it is not in favor of the recovery of Cr.MgO is a useless and harmful slag former.(2)With an increase of the basicity(Ca O/(Al2O3+SiO2)),the slag structure becomes simple.The variations of CNSi-Si,CNSi-Al,CNAl-Al and CNAl-Si(CN: Coordination Number)show that CaO prefers to break [SiO4] tetrahedrons linked with [SiO4] or [AlO4].FT-IR results indicated that a broadening of the width of Si-O,Al-O and Si-O-Al band group was found,indicating the depolymerization of slag structure and the reduce of linkage between [SiO4] and [AlO4] tetrahedrons.By the quantitative analysis of Raman results,it can be found that Q3(Si)remarkably decreases,while Q0(Si)or Q1(Si)increases,indicating the depolymerization of slag structure.Addition of SiO2 lowers the surface tension of slag,while CaO and MgO can increase its surface tension.When MgO equals to 9%,an increase of CaO and decrease of SiO2 result in Si within master alloy drops from 2.7% down to 0.2%.But when MgO goes up to 14%,Si has a maximum value,and the tendency of its variation is contrary to that of slag viscosity.(3)An increase of Cr within ternary alloy lowers the decrease of alloy density and surface tension,and Ni can slightly increase the density and decrease the surface tension.By comparing the evaluated values and experimental data of Fe-Cr binary alloy,surface tension of pure Cr(1150 mN/m)was proposed.Ni can lower the alloy viscosity,while Cr can only increase its viscosity when Cr is above 20 mole%.When Cr is below 20 mole%,Ni 80~90 mole%,and Fe is below 20 mole%,there is a low viscosity region of ternary alloy.(4)When hearth operation temperature is set 1600 ?,optimal operating parameters obtained from the process simulation model without considering the volume effect of blast furnace were as follows: 2337 Nm3/THM blast air volume,1250 ? blast air temperature,6% O2 enrichment,365 kg(CaO:SiO2=9:1)as flux,0% H2 O addition,280 kg/THM pulverized coal and 743 kg/THM coke.On the basis of optimal operation paramaters,if the blast air temperature increases by 50 ?,the coke ratio can reduce 25 kg/THM.If O2 enrichment ratio rises to 10%,balst air volume can drop to 1995 Nm3/THM,but there is little change for coke ratio.If pulverized coal amount increases by 20 kg/THM,the coke ratio can reduce 14 kg/THM.If water content within blast air goes up to 2%,the coke ratio dramatically increases to 846 kg/THM.By summarizing the domestic and abroad research status of the processes of Ni and Cr reserves,a compact process to produce the stainless steel master alloy in blast furnace by smelting reduction of Fe-Ni-Cr composite sinter was proposed in this study based on that stainless steel plants in China to investigate the possibility of various raw materials.This thesis mainly studied the smelting reduction of Fe-Ni-Cr composite sinter,and physico-chemical properties of slag and ternary alloy at high temperatures were calculated and measured.Then the influences of various operation parameters on this new process were investigated by a simulation model of blast furnace built by METSIM.These above fundamental studies can be used to guide the application and design of the compact process. |
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