Fundamental Study On Preparation Of Si-Fe-Al Alloy By Fly Ash Electrothermal Method

Si-Fe-Al alloy is mainly used as deoxidizer for steelmaking and reducer for magnesium smelting by thermal reduction method because of its high sealing degree(4.2?4.6 g/cm3,1.6 times of Al),low melting point(1060?)and good deoxidizing effect.With the rapid development of iron and steel industry,the demand for Si-Fe-Al alloy is increasing gradually.At present,the main methods of industrial preparation of Si-Fe-Al alloy are molten metal blending method,molten salt electrolysis method and electrothermal method,among which the products prepared by molten metal blending method and molten salt electrolysis method have high purity,but the process flow was long,energy consumption was large and cost was high.Electrothermal method has the advantages of short process flow and low energy consumption,it was the most potential preparation method of Si-Fe-Al alloy.Fly ash is the main solid waste from coal-fired power plants.With the development of coal-fired power industry,the emissions of fly ash is increasing year by year,and has become one of the largest industrial waste in China.The results show that fly ash is mainly composed of oxides of Al,Si,Fe and other metals,so it was a valuable secondary resource.Therefore,the direct discharge of fly ash without treatment not only brings serious environmental pollution,but also causes great waste of resources.In order to realize the secondary utilization of fly ash,this paper mainly uses fly ash as raw material and petroleum coke as carbon reductant to prepare Si-Fe-Al alloy by electrothermal method,and probes into the related reaction mechanism,to find a new way for high added value utilization of fly ash and lay a theoretical foundation for the industrial production of Si-Fe-Al alloy by fly ash.The main contents of this paper are follows:(1)The physical and chemical properties of fly ash ware studied.The results show that the particle size of fly ash is very fine D50=5.70 ?m;The total content of SiO2,Al2O3 and Fe2O3 are 80.73 wt%,and the ratio of Al to Si(A/S)was 0.46.The main phases are SiO2 and mullite(3Al2O3·2SiO2).SEM-EDS analysis shows that fly ash was composed of spherical particles with different particle sizes and there is no obvious difference in element composition between the particles with different particle sizes.(2)The thermodynamics of the reaction system was studied.The thermodynamic study of SiO2-C system shows that the reduction of SiO2 mainly takes place in the process of carbide formation and decomposition,and the intermediate products are SiO(g)and SiC(s).The thermodynamic study of Al2O3-C system shows that the most important reduction process was Al4C3.With the increase of temperature,Al4C3 begins to decompose,and as the temperature continues to rise,Al4C3 decomposes to form liquid Al and aluminum vapor.(3)The reduction experiments of pure Al2O3 under non-vacuum conditions show that the main products were Al2OC,Al4C3 and Al4O4C,and the crucible deposit is mainly Al2O3.(4)The process parameters were studied.The optimum process parameters were determined as follows:pressure 40 MPa,holding time 60 s,binder content 4 wt%,water content 13 wt%,porosity 37.27%and compressive strength 8.46 MPa.The high temperature smelting experiment shows that the smelting temperature was higher than 1900 ?,the mass ratio of A to B pellets was 3:1 and the A pellets were kept in furnace for 30 minutes and then the B pellets were kept for 20 minutes.The optimum smelting conditions were obtained.(5)The results of EAF smelting experiments shows that the content of Al in alloy sample increases obviously,and the content of Fe in the alloy sample in each region remains basically unchanged.The alloy sample was composed of Fe0.42Si2.47 Fe-Si phase and Al0.3Si0.5Fe3,Al2SiFe Si-Fe-Al phase.The prolongation of holding time was helpful to the decomposition of SiC and the homogenization of alloy..(6)Through thermodynamic analysis,it can be inferred that the possible process of carbothermal reduction of SiO2 and Al2O3 was follows:firstly,SiC and Al4C3,Al2OC and Al4O4C were formed by reaction;Then Al2O3 interacts with Al4C3 to form Al.Then SiC interacts with Al2O3,Al2OC and Al4O4C to form Al and Si.When the reaction of aluminum compounds was complete,the reduction reaction basically ends,and the remaining SiC does not decompose and remains in the metal solution.Therefore,excessive Al2O3 was helpful to improve the quality of the alloy.