Preparation Of SiAlON Multiphase Materials From Circulating Fluidized Bed (CFB) Fly Ash Alumina Extracted Acid Slag

As a large amount industrial solid waste,the production amount of fly ash is considerable every year in China.Fly ash occupies a large amount of land resources.If not handled properly,it will cause air and water pollution,which is a typical bulk industrial solid waste.Because of its fine particle size and reactivity,fly ash can be used as a substitute for aluminosilicate mineral resources.In various ways of comprehensive utilization of fly ash,the technology of extracting aluminum from high alumina fly ash by acid and the utilization of acid slag can not only solve the problem of aluminum resource shortage in China,but also realize the high value utilization and large-scale disposal of fly ash,which is a very promising comprehensive utilization method.Compared with conventional aluminosilicate mineral raw materials or ordinary fly ash,the acid leaching slag produced in acid leaching process of aluminum from fly ash has higher unburned carbon content and good chemical reaction activity.It is a new industrial raw material with weak acidity,high silicon content,porosity and high chemical activity.But at present,the related basic research and application research on acid slag of fly ash is relatively few,and there is no in-depth discussion.There are few reports on carbothermal reduction nitridation of acid slag of fly ash.In view of this situation,this paper makes full use of the composition characteristics and high reaction activity of acid slag of circulating fluidized bed?CFB?fly ash to replace the traditional Si Al ON starting materials,and try to prepare the Si Al ON multiphase material under a more mild carbothermal reduction and nitridation condition,and analyze its influencing factors and mechanism,so as to manufacture the Si Al ON multiphase material in large scale and with low cost.At the same time,it also provides a new direction for the resource utilization and high value utilization of fly ash acid slag.In this paper,starting from the research of chemical composition and phase composition of raw materials,the characteristics of high-temperature reaction of raw materials were analyzed,and the experimental scheme was designed based on the related research on carbothermal reduction and nitridation of aluminosilicate.The influence factors such as carbon environment,atmosphere environment,reaction temperature,holding time,different silicon sources,addition of rare earth elements,and so on in this process were discussed respectively.Through the analysis of phase composition change,chemical composition transformation and morphology evolution characteristics of the products,the basic rules of preparation of Si Al ON multiphase materials from acid soluble slag of CFB fly ash were understood.The main achievements of this paper are as follows:1.After adding proper amount of external carbon source into acid slag of fly ash,the phase transition from oxides to nitrides and then to carbides can be realized.It is confirmed that the expected nitriding product?-Si Al ON phase can be obtained under suitable carbon environment.The samples buried under carbon black can realize the spontaneous stratification of the products.The upper layer can form special nano ribbon morphology with high aspect ratio,while the lower layer is mainly composed of near spherical particles and small curved fibers.Carbon environment is essential for the carbothermal reduction nitridation reaction of CFB fly ash acid slag.The influence mechanism is that,on the one hand,it participates in the carbothermal nitriding reaction as a reducing agent or protective agent to realize the regulation of the product phase composition.On the other hand,it acts as a crystal core to adsorb and capture gaseous Si O,and promotes the products to produce different morphologies by its own pore structure.2.Different atmosphere environment has a significant impact on the carbothermal reaction products of acid slag of fly ash.In the air atmosphere under carbon burial condition,the main carbothermal products are silicon carbide and corundum,which is completely different from the product phases under nitrogen atmosphere under carbon burial condition.Under the condition of carbon burial,the increase of reaction temperature and the increase of C/Si molar ratio will promote the transformation of oxide materials and the production and growth of carbonized products.The influence mechanism of atmosphere environment is that it directly changes the type of high-temperature chemical reaction,decides whether the material will undergo nitriding reaction or carbonization reaction,and then changes the phase composition and morphology of the products.3.Due to the destruction of the chemical bond between silicon and aluminum in the process of acid dissolution,CFB fly ash acid slag has advantages over similar aluminosilicate minerals in terms of reaction activity,which can realize the formation of nitriding product multiphase materials at a temperature as low as 1375°C.The results show that the reaction temperature has an effect on the phase composition and morphology of the products.Increasing the reaction temperature will promote the nitriding reaction.When the reaction temperature is increased,the products tend to produce longer straight fibers.The influence mechanism of reaction temperature is that increasing the reaction temperature will effectively promote the phase transformation and grain growth of the products.4.The silicon source materials similar to acid slag of CFB fly ash,such as pulverized coal boiler fly ash,diatomite and raw illite,can also realize carbothermal reduction and nitridation under high temperature in nitrogen atmosphere.However,the phase composition of the products will change greatly depending on the chemical composition of raw materials.The acid slag of CFB fly ash is more suitable to be raw materials for carbothermal reduction and nitridation in terms of composition and high reactivity.While the nitridation reaction related to diatomite and raw illite was rarely described in the past.The existence of aluminum and alkali metals can effectively reduce the reaction temperature,and excessive aluminum will even change the final nitriding products.By adding Fe₂O₃ into the raw material,reaction temperature of the whole system will be reduced and it make the reaction more complete.5.By changing the content of the rare earth oxide Eu₂O₃,the relative content of the phase composition of the product can be changed,and the photoluminescence performance of the product is affected.The results show that the blue violet spectrum of the multiphase powders can be excited by 325 nm light,which is different from the emission spectra of?-Si Al ON pure phase powder in previous literatures.The relative intensity of the emission spectrum of the sample with Eu₂O₃ after calcination is obviously higher than that of other samples.Lower Si C content and higher crystallinity may improve the photoluminescence properties of the samples.The addition of rare earth oxide Eu₂O₃ also has a significant effect on the morphology of the product,which makes the edge angle of the products from sharp to smooth.The mechanism of the influence of Eu₂O₃ on the photoluminescence properties of the products is that the Eu²⁺introduced provides a high active photoluminescence center and forms a good interaction with the nitride matrix under the nitriding reduction condition.However,the concentration quenching effect will be caused by the excessive concentration of Eu₂O₃,and the photoluminescence intensity will be decreased.The photoluminescence properties of samples with different C/Si molar ratio were affected by adjusting the content of inert phase in the system.The influence mechanism of the chemical composition of silicon source on the photoluminescence of the product is mainly determined by the phase composition determining by the chemical composition of the product,changing the coordination environment around the photoluminescence center of Eu²⁺and directly changing the peak position and peak intensity of the luminescence spectrum.The emission spectrum of the nitrides multiphase material can be controlled by changing the Al:Si molar ratio of the starting material.