Basic Research On Ferrosilicon Alloy Prepared By Carbothermal Reduction Of Fly Ash

Fly ash is an industrial solid residue produced by the combustion of pulverized coal in coal-fired boilers of thermal power plants.The annual emissions of fly ash are greater than the utilization,so the accumulated reserves are very large.According to statistics from relevant departments,by 2020,fly ash emissions can reach 920 million tons.At present,the treatment methods of fly ash are gradually becoming diversified.The method of preparing ferrosilicon alloy based on carbothermic reduction of fly ash and vortex stirring proposed in this paper provides a choice for the high value-added utilization of fly ash.This paper analyzes the thermodynamics of the fly ash carbothermal reduction system,using chemical analysis,XRD,SEM and other methods to study the fly ash composition and mineral phases,using single factor experiments to study the fly ash carbothermal reduction process.The ferrosilicon alloy products were characterized,and the main conclusions are as follows:(1)Through the thermodynamic calculation of the fly ash carbothermal reduction system:in the temperature range of 500 K～2500 K,the oxide reduction order is Fe2O3>K2O>Na2O>SiO2>TiO2>MgO>CaO>Al2O3,and the Gibbs free energy change of the reaction of oxides to carbides is more negative than that of the The Gibbs free energy change of the reaction of oxides to the corresponding metals.The reactions between Al2O and C and SiC and Al2O3 at high temperature are the two main reactions to produce metallic aluminum.SiC and SiO2 produce metallic silicon at high temperatures,and the increase in silicon dioxide content is conducive to the decomposition of carbides and the formation of elemental silicon.The thermodynamic calculation of the carbothermic reduction of the fly ash system shows that Fe2O3 has a greater effect on the reduction of the theoretical carbothermic reduction temperature of mullite than Fe,FeO,and Fe3O4.Mullite in fly ash is relatively stable and not prone to decomposition and carbothermal reduction.Under standard conditions,the theoretical carbothermal reduction temperature of the mullite phase in fly ash is 1835 K.However,with the participation of Fe2O3,the theoretical carbothermal reduction temperature of mullite in fly ash under standard conditions is 1425 K,which is reduced by 410 K.(2)The chemical analysis of fly ash shows that fly ash mainly contains SiO2 and Al2O3,and the mass fractions of the two are 58.95%and 25.56%,respectively.The mineralogical analysis of the raw materials shows that the main phase of fly ash is mullite and free silica,and the particle size is about 100 μm.(3)Experimental study on carbothermal reduction of fly ash showed that 1%polyvinyl alcohol solution was used as the binder for pelletizing,without adding CaO,the melting temperature was 1873 K,the melting time was 2.0 h,and the 60%of theoretical carbon content was a suitable smelting process condition.The total metal recovery was 66.0%under this condition,and a ferrosilicon alloy with titanium was obtained in which Al 1.52 wt%、Si 38.7 wt%、Fe 59.56 wt%、Ti 1.63 wt%、C 1.06 wt%.(4)The ferrosilicon alloy and slag are well separated.SEM-EDS analysis shows that the composition of the alloy is not uniform and the phase interface is clear.There are mainly dark gray phase Si+FeSi2,light gray phase as FeSi+FeSi2 phase and black carbonization.(5)The analysis of the mechanism of the carbothermal reduction of fly ash shows that when the temperature is lower than 1623 K,the Fe2O3 in the fly ash has already started the carbothermal reduction reaction;When the temperature is between 1673 K-1723 K,the reaction at this time is the decomposition of mullite phase and the formation of Fe3Si;When the temperature is 1773 K,the mullite phase disappears and SiC begins to form;When the temperature is between 1773 K and 1823 K,Al2O3,SiO2 produced by the decomposition of the mullite phase continues to decompose,and continues to generate SiC and Fe3Si until the SiO2 phase completely disappears.(6)Through the XRD analysis of the slag,it can be found that under the best single factor experimental conditions,the diffraction peaks in the slag correspond to Al2O3·MgO,SiC and TiC.No diffraction peaks of Fe,FeSi and Fe3Si are found.This shows that the slag gold is completely separated.This is inseparable from the enhanced reduction of mechanical stirring and the role of directional sedimentation.