Fly Ash Resources Characteristics And Preparation Of Si - Al - O - C - N Series Materials Research

Fly ash from coal-fired power stations is the largest amount of industrial residue. Comprehensive utilization of fly ash is of great significance to economy and environment.At present,the chief consumer of fly ash is construction materials industry.A relatively large percentage of fly ash is mainly utilized for the manufacture of cement,concrete and other products,the remainder is directly stacking or landfills, which cause not only the unproductive use of land and the associated long-term financial burden of maintenance but also the air pollution and groundwater pollution. In recent years,many researchers have taken fly ash as the main raw materials to prepare high performance ceramic-based materials,such as Sialon powders, SiC/Al₂O₃ composite powders etc.Although fly ash as a low-cost raw materials can reduce the costs of ceramic,large quantity of spherical particles among fly ash——fly ash microspheres were ignored.Microspheres can be used not only as traditional building materials of low value,but also directly for the preparation of thermal insulation,fire resistance materials,or be utilized for high-strength,abrasion resistance,shielding,filtering and other functional materials through proper surface activation or modification.As a consequence,there is a growing interest in looking for avenues where the fly ash can be used of value added products,since it presents a unique nature source of the particulate material for functional materials,which have great significance to realize sustainable development and the value added comprehensive utilization of fly ash.The purpose of this thesis is to investigate the resource characteristics and precipitation process of microspheres from fly ash and its process and mechanism for preparation of Si-Al-O-N-C composite.Moreover,this thesis is financially supported by National Natural Science Found of China under Grant No.50472030.The progresses are as follows:1.Fly ashes sourced from twenty-six coal-fired power plants were characterized by chemical,mineralogy composition and particle morphology, which is helpful to enrich the database of fly ash resource characteristics and provide valuable basic data for the lager scale resource utilization of fly ash.The results show that the chemical compositions of different fly ashes have a large fluctuation and loss on ignition has decreased to a great degree,which manifest the achievements of electric power reform since tenth five year plan.Fly ashes from different area mainly are composed of four types particles including microspheres, ferrospheres,irregular particles and residual carbon.There is higher than 70%fly ashes contains microspheres content over 60%,which shows that many fly ashes have a potential prospect of microsphere separating and deep process.2.Investigation on setting process of microspheres and irregular particles with different size and different density has performed by theoretical analysis and experimental study.And separating plan is preliminary designed based the setting process data of microspheres and irregular particles.The results show that experimental data of setting process of microspheres and irregular particles is according with theoretical analysis results.Irregular particles have a lower setting speed than microspheres with same size and density.And the difference on speed of two particles decreased with size decreasing.There is an obvious difference between microspheres and irregular particles when its size is over 75μm.Moreover,there exists an overlapped area of speed range of microspheres and irregular particles, which caused of the difficulty in separating of them.For many fly ashes,separation of size less than 45μm particles that is means successfully abstained microspheres.When the particle size is more than 45μm,microspheres can be abstained through two steps, first,particles over 45μm abstained by sieve method,then,microsphere and irregular particles separated by pneumatic separation based on their different setting speed.3.Based on composition and particle morphology of microspheres, SiC/Muilite composite sphere,SiC hollow（solid） sphere,SiC/AlN-polytype sphere and Sialon sphere are successfully synthesized by carbothermal reduction between microspheres and different carbon source.（1） SiC/Mullite composite spheres prepared through carbothermal reduction reaction between fly ash microspheres and carbon black both by microwave heating and conventional heating.The resusts show that the reaction mechanism mainly associates with solid-solid reaction between C（s） and SiO₂（s） on the surface of fly ash microspheres with its spherical shape unchanging when the temperature is between 1100℃-1300℃.When the temperature is higher than 1300℃,SiC is easier orientation growth to whisker through VLS mechanics for the presence of liquid phase.The spherical morphology of prepared product is related to the survivorship of mullite.Excess content of carbon black barrier the adhesion among microspheres, which is necessary to maintain the shape of SiC/Mullite spheres.The microwave reaction synthesis of SiC/Mullite microspheres is superior to the conventional method in lower synthesis temperature and shorter holding time,which will be beneficial for the industry to save energy and time.（2） Macroscopic SiC hollow sphere has been synthesized by a microwave heating and carbothermal reduction method with commercial carbon spheres as template and fly ash as silica source.The results show that the SiC hollow spheres have retained the similar size with the original carbon sphere template,and the shell of SiC hollow sphere is composed of a lot of irregular SiC nanowires with 5-20μm in length and 50-500 nm in diameter which belongs to the.single crystallized 3C-SiC with defect.The formation of curving SiC wires should be attribute to the VLS mode and the space preventing function,the Fe catalyst is indispensable reason of it.（3） SiC/AlN-polytype sphere with obviously gradient structure has been synthesized by a microwave heating and carbothermal reduction nitridation with commercial carbon spheres as template and fly ash as silica and aluminum source. SiC/AlN-polytype composite spheres are composed of three layers:the shell （AlN-polytype）,the transition layer（SiC nanowires）,and the core（SiC whiskers and flower-like SiC crystals）.The formation process of SiC/AlN-polytype composite microspheres as follows:First,SiC hollow spheres which is composed of SiC nanowires formed.Then the Al,Al₂O steam decomposed from the Al₂O₃ in nitrogen atmosphere deposit on the surface of SiC hollow spheres to form the outer layer of AlN- polytype.After that,the surface densification result in internal and external gas exchange slowing down,which changing the gas saturation of growth of internal SiC, so that SiC grows along the different directions of the side line which result in the formation of flower-liked SiC crystals.（4） Active carbon and microspheres with hollow structure were taken as raw materials to prepareβ-Sialon hollow spheres by carbonthemral reduction nitridation. The results show that carbonthemral reduction of fly ash microspheres begins at 1300℃.Excess content of active carbon and particle size of microspheres are important factors on forming ofβ-Sialon hollow spheres.β-Sialon hollow spheres that obtained with microspheres of particle size greater than 100μm and excess 10% content of active carbon at 1500℃,have some features of rough surface and perfect hollowness.4.SiC/Al₂O₃,SiC/Mullite/Al₂O₃ and SiC/AlN composite powder are successfully synthesized by carbothermal reduction between microspheres and carbon black,the optimum technological parameters are determined as follows, temperature,carbon content,carbon source,holding time,atmosphere,etc.The results show the carbothermal reduction nitridation can be divided into five steps:first, phase transition of quartz;conversion of SiO₂ into SiC;decomposition of Mullite; then,formation of intermediate products,such as Sialon,15R,AlON,etc.The last step is intermediate products degrade to AlN.5.Finally,we discussed the structural evolution and movement behavior of some elements during the whole process of fly ash microspheres to prepare Si-Al-O-C-N series composite sphere and powder.The results show that the spherical morphology of prepared SiC/Mullite composite spheres is related to the survivorship of mullite.However,Sialon sphere is attributed to the direct take part in the reaction without decomposing.Movement behavior of some elements mostly is affected by temperature,carbon content,atmosphere,holding time,etc.Elements like Fe,Ti,Ca,Mg,always move to the surface of products,but some element is drained away with gas,such as Na and K.6.The preliminary investigation on precipitation process of microspheres from fly ash and preparation of Si-Al-O-N-C composite,the prescribed properties and functions of microspheres can be achieved by further careful composition design and process control,and preparation of Si-Al-O-N-C composite with fly ash microspheres have high theoretical research value and wide application prospect.