Research On Ferrosilicon Alloy Preparation And Alumina Enrichment From Coal Fly Ash Via Carbothermic Reduction

Despite the rapid development of renewable energy power generation technology in recent years,thermal power generation is still one of the main sources of power in China,and the installed capacity of thermal power generation is still increasing year by year.Thermal power plants discharge 600 million tons of coal fly ash every year,which has posed a threat to the environment and human health,so it is urgent to carry out comprehensive treatment and recycling.Because the coal fly ash contains 30-50%alumina,many scholars have studied the technology of extracting alumina from coal fly ash.At present,the main technologies of extracting Al₂O₃ from coal fly ash are sinter processes,acid leach processes and sinter-acid leach combination process.All processes are faced with the problem that the Al-O-Si bond in coal fly ash is not easy to break,and that the separation of aluminum and silicon is not easy and the silicon slag is produced in the process.In order to alleviate the problems,this paper proposes a method of enriching alumina in coal fly ash and preparing ferrosilicon alloy with Fe₂O₃ as additive and C as reducing agent at 1273-1673 K.In this paper,the processes of roasting,screening and magnetic separation are studied,and the effects of Fe₂O₃ addition,reaction temperature,holding time and gas pressure on the process are analyzed,which provides important theoretical basis for the process design and optimization of the carbothermic reduction of coal fly ash to enrich alumina and prepare ferrosilicon alloy.Firstly,the effect of adding iron oxide and reducing gas pressure on the carbothermic reduction process of coal fly ash was studied by thermodynamic analysis and experimental comparison.The theoretical calculation shows that the Gibbs free energy of the carbothermic reduction reaction of mullite and quartz can be reduced by adding iron and iron oxides or reducing gas pressure,which shows that they can promote the reaction in thermodynamics;besides,the carbothermic reduction experiment of coal fly ash under atmospheric pressure shows that mullite did not react when the temperature of calcination was 1523 K and Fe₂O₃ was not added,while mullite was reduced when Fe₂O₃ was added.In addition,the initial reduction temperature of mullite under vacuum is about 150 K lower than that under atmospheric pressure.Both theoretical calculation and experimental results show that adding Fe₂O₃and reducing gas pressure can promote the reduction process of mullite and quartz.Secondly,the reduction roasting process of coal fly ash,which plays a key role in the whole process,was studied.The influence of process parameters on the recovery of Al₂O₃ and the composition of ferrosilicon alloy was investigated by controlling variable method.The results show that the recovery of alumina increases with the increase of calcination temperature and reaction time,and the content of silicon element in ferrosilicon alloy increases with the increase of calcination temperature,but the change is not obvious with the increase of reaction time and Fe₂O₃ addition,which shows that the reaction temperature is the main factor limiting the content of ferrosilicon.Then,the micro separation mechanism of Al and Si elements in mullite phase was studied by SEM/EDS analysis of the original appearance and cross section of the samples.In the process of decomposition and reduction of mullite,some O combine with C and escape in the form of CO gas.At the same time,Si element and Fe element combine to form ferrosilicon alloy,which continuously converges and grows up to the gap in the samples.Due to the high melting point,the alumina produced by the reaction always stays in the original position in the form of solid state.This process enables the separation of Al and Si elements in mullite in the micro region,which provides the precondition for the physical separation of each component later.Through SEM analysis of the sieved samples,the magnetic and non-magnetic parts obtained by magnetic separation,it is found that the ferrosilicon particles with smooth surface and large size are beneficial to the separation of alumina and ferrosilicon in the process of sieving and magnetic separation.However,if the growth space of ferrosilicon particles is limited during the growth process,it will no longer be spherical,but grow along the gap in the sample.Besides,it will wrap some alumina adjacent to it,which will reduce the recovery of alumina.Under vacuum condition,when the calcination temperature is 1473 K and the holding time is 6 h,the recovery of alumina is 82.61%,and the content of alumina in the non-magnetic part is 87.02%.The theoretical analysis results and experimental data obtained can provide valuable experimental basis for optimizing process parameters and reducing energy consumption in scale-up experiments.