Comparative Study On Magnetic Separation And Properties Of Fly Ash

Fly ash is the main solid waste discharged from coal-fired power plants.The emissions and stockpiles of fly ash are large.There is a large amount of fly ash need to be used urgently.In the comprehensive utilization of fly ash in China,refined and high value-added utilization is mainly to extract useful components,such as silicon,aluminum,iron and rare elements,which has the problems of low utilization level,low utilization amount and after extract the useful components there are still difficult to use.While using in building materials,it is widely used as a blending agent for mortar and concrete,which can improve workability of concrete effectively.Recently the utilization rate of fly ash has been improved but still at a relatively low level compared with developed countries.Since fly ash contains magnetic iron oxide and the content of fly ash iron oxide in the southwest is about 10?25%,it can be separated by magnetic separation.Magnetic separation of fly ash has attracted the attention of many scholars,most of them focus on the magnetic separation process of fly ash,but there is a lack of relevant research on the property change of fly ash after magnetic separation compared with the original ash.After magnetic separation the content of iron oxide in the fly ash will be reduced,so the content of the active silicon and aluminum will be increased relatively,so the pozzolanic activity of the fly ash can be improved.The change of performance of fly ash as cement and concrete blending after magnetic separation is worth exploring.Dry magnetic separation and wet magnetic separation of different types of fly ash in the Luohuang power plant were carried out.The factors affecting the magnetic separation effect were investigated,and the optimal process parameters of magnetic separation were determined.The basic physical and chemical properties of original fly ash,high iron fly ash and low iron fly ash obtained from magnetic separation were compared,including chemical composition and loss on ignition,phase composition,particle morphology,particle size distribution,setting time,water demand ratio,viscosity and pozzolanic activity.The experimental results show that(1)Under the conditions of current intensity 2.5A,magnetic tube inclination angle of 80° and magnetic separation times of 3 times by dry magnetic separation.The iron oxide content of high iron fly ash increased from 14.08% to 57.66%,and the low iron fly ash is reduced to 8.64%,the removal rate of iron is 38.64%.(2)Under the condition of 20% magnetic separation medium concentration,3% ethanol was used as the dispersing agent,four times of different magnetic intensity by wet magnetic separation,iron oxide content of the high iron fly ash can reach to 55.24%.After 4 times of wet magnetic separation under12000 Gs magnetic intensity,the iron oxide content of low iron fly ash was reduced to 4.55%,and the removal rate of iron was 67.68%.(3)The content of iron oxide and particle size in original fly ash have a great influence on the magnetic separation result.The coarser particles or the higher the iron oxide content of original fly ash,the iron oxide content of high iron fly ash is higher,and the removal rate of iron oxide about low iron fly ash is higher after magnetic separation.(4)Compared with dry magnetic separation,the recovery rate of high iron fly ash by wet magnetic separation is higher,about 20%,and the removal rate of iron is higher,otherwise the wet magnetic separation product less dust,and the magnetic separation medium can be recycled.After magnetic separation,the properties of fly ash have changed.(1)The iron oxide content in low iron fly ash(LIFA)can not be separated by magnetic separation after the iron content is reduced to a certain level when the content of iron oxide is about 5%.(2)XRD result shows the content of magnetite of high iron fly ash(HIFA)is obviously increased.(3)After wet magnetic separation,with the increase of iron oxide content in high iron fly asA,the particles become coarser.While the particle size of low iron fly ash decreased slightly.(4)High iron fly ash has good sphericity,and the surface of the particles contains different kinds of crystals.the needle-like crystal particles have the highest iron content.(5)After magnetic separation the water requirement ratio of low iron fly ash has decreased and the setting time has extended.(6)The high iron fly ash can reduces the viscosity of fly ash-cement system is better than that of original fly ash and low iron fly ash.(7)The 28 day compressive strength ratio is used to test pozzolanic activity of fly ash,the low iron fly ash was bout 14% higher than that of original fly ash,and the pozzolanic activity of fly ash increased with the curing age,the highest pozzolanic activity index of low iron fly ash is at 56 d,about93%.The trend tends to be flat in the later period.Low iron fly ash can be used as cement and concrete admixture.