粉煤灰资源特性与制备Si-Al-O-C-N系材料的研究

【作者】 张磊；

【导师】 杨久俊；

【作者基本信息】 郑州大学 ， 材料学， 2009， 博士

【摘要】 粉煤灰是排放量最大的工业废渣,对其进行资源化综合利用对我国经济和环境的可持续发展具有重要的意义。目前粉煤灰的利用相对集中于建筑材料工业,尚有大量粉煤灰未加利用而被填埋或堆放,不仅占用大量可耕地和较高维护费用,且会造成大气和地下水污染,给社会带来严重的经济和环境问题。粉煤灰中通常含有不同类型的球形颗粒,主要有漂珠、微珠（沉珠）和富铁微珠。微珠除作为低附加值用于传统的建筑材料外,还可直接用于制作隔热、耐火、保温材料,或对其进行适当的表面活化或改性处理用于耐磨、屏蔽、滤波及其它功能材料中。粉煤灰微珠的分离提取及其高附加值利用对于转变目前粉煤灰资源的浪费及占有土地、污染环境的局面,缓解资源短缺并建立新的经济增长点,具有十分重要社会和经济意义。本论文在国家自然科学基金“粉煤灰厚壁微珠活化、强化及其应用研究”（项目编号:50472030）的资助下,进行了粉煤灰微珠资源特性及其分离沉降特点的研究,并就粉煤灰微珠制备Si-Al-O-C-N系列微球及复相粉体进行了深入研究。研究工作取得的主要进展如下:1.对全国不同地区26家电厂粉煤灰进行了比较全面的化学组成、矿物组成及颗粒组成等资源特性分析,着重研究了粉煤灰中微珠含量及其影响因素,对深化粉煤灰微珠成因的认识,为其大规模的资源化以及深加工利用提供了有价值的基础数据:研究结果表明我国不同地区的粉煤灰化学成分波动很大,烧失量较10年前有很大程度的降低,反映了“十五”计划以来我国粉煤灰资源特性的变化特点。不同电厂的粉煤灰都包括微珠,富铁微珠,不定形颗粒和碳粒,微珠含量与其化学组成、锅炉燃烧温度、粉煤灰的细度有明显的相关性。所调研的超过70%的电厂的粉煤灰中含有60%以上的微珠,微珠含量最高的河南禹州电厂达到96%。数据表明,很多电厂的粉煤灰都具备微珠分离提取的潜力,和微珠深加工利用的良好前景。2.对不同密度、不同粒径的微珠与不定形颗粒的分离沉降特点进行了理论分析和实验初步研究,针对其沉降过程中的运动特点初步确定了微珠与不定形颗粒的分离方案:微珠与不定形颗粒的自由沉降实验表明微珠的沉降表现出与理论分析一致的变化趋势;不定形颗粒的沉降末速比同粒径、同密度的微珠的沉降速度小,但这种差别随着粒径的减小和密度的增大而逐渐变得不显著,粒径大于75μm的微珠和不定形颗粒具有比较大的速度差值,这也为它们的分离提供了实验依据。就微珠与不定形颗粒的集合体而言,不同密度、粒径的微珠和不定形颗粒的沉降末速存在很大的重合区间,造成了分选的困难。我们认为对于大多数电厂来说,分离出的小于45μm的粉煤灰就意味着分离出了微珠。对于大于45μm的颗粒中也富含微珠的粉煤灰,可以通过筛分的方法首先实现粉煤灰45μm以上不同粒径颗粒的分离,然后对同一粒径的微珠进行气力分选而实现微珠与不定形颗粒的分离。3.基于粉煤灰微珠的形貌特点和化学组成,首次利用粉煤灰微珠和不同碳源成功地制备出SiC/Mullite复合球体、SiC空心（实心）球体、SiC/AlN-多型体复合球体和Sialon球体等Si-Al-O-C-N系列材料:（1）以粉煤灰微珠与炭黑为原料采用微波加热和常规加热方式通过控制温度、反应时间和炭黑用量都可以制备成SiC/Mullite复合球体,其机理是将微珠表层玻璃体转化为SiC而维持内部Mullite不变。研究表明SiC晶粒在粉煤灰微珠表面原位生成,以固相扩散的方式进行。Mullite分解与否是所制备的产物能否维持球形形貌的根本所在,过量炭黑阻隔微珠之间可能发生的粘结,是维持SiC/Mullite球体的关键。同常规加热方式相比,采用微波加热制备SiC/Mullite复合球体具有降低反应温度较高,缩短保温时间,节能降耗等功效。（2）通过简单的筛分制备出炭黑球模板,粉煤灰提供SiO源,通过控制反应温度、粉煤灰用量和保温时间可以制得球壳厚度不同的宏观大孔SiC空心球乃至实心球。所制得的SiC空心球球壳由弯曲缠绕的SiC纳米线构成,线宽度约50～500nm,长度通常为十几微米,属于带有缺陷结构的β-SiC晶体。弯曲的β-SiC纳米线被认为是通过VLS和VS机制生成的,在VLS机制中粉煤灰所含有的Fe起到催化液滴作用。（3）以炭黑球为模板,粉煤灰为原料,通过微波加热碳热还原氮化反应可以制得具有梯度结构的SiC/AlN-多型体复合球体。所制备的SiC/AlN-多型体复合球体具有AlN-多型体的外壳、SiC纳米线过渡层和SiC晶须与花朵状SiC晶体构成的核心。（4）以活性炭为还原剂采用常规加热方法成功制备出了O’-Sialon和β-Sialon球体。活性炭用量高于理论配比,有利于生成更多的氮化产物,且是微珠氮化后能维持其球形形貌的重要因素。以炭黑为还原剂则在制备过程中易产生SiC,且所制备的Sialon部分以晶须的形式出现。4.以粉煤灰为原料成功地制备SiC/Al₂O₃、SiC/Mullite/Al₂O₃、SiC/AIN等微米级复相粉体,得出了制备各种复相粉体了最佳工艺,并就粉煤灰的碳热还原氮化过程及各种复相粉体的形成机理进行了分析:研究表明,粉煤灰的碳热还原氮化反应分为5步:石英的相转变,二氧化硅反应生成SiC,Mullite分解,Sialon、15R、AlON等中间产物的生成,中间产物分解出AlN。温度、碳用量、碳源种类、保温时间、气氛等都会不同程度地影响着粉体的相组成。5.对粉煤灰微珠制备Si-Al-O-C-N系材料过程中的微珠的结构与物相的演化以及部分元素的迁移规律及赋存状态的进行了研究,深化了粉煤灰微珠制备各种材料反应过程与反应机理的认识:通过粉煤灰微珠制备Si-Al-O-C-N系列球体和粉体的过程的分析,SiC/Mullite球体维持球体的原因在于Mullite的不分解,而Sialon维持球体的原因在于Mullite直接固溶参与反应,而反应过程中物相转变受温度、碳用量、气氛、保温时间等多种因素的影响。对部分元素的迁移规律研究表明,采用微波加热时,随着温度升高,莫来石晶格中Fe的固溶度减少,Fe从球体内部往表面迁移,赋存于Fe₃Si中。常规加热根据反应程度可能以Fe3_Si或单质Fe的方式存在的,Fe在表面张力的作用下凝固时收缩成球形。Ti、Ca和Mg也有往球体表面迁移的趋势,Na、K则随气体流失。6.本文对粉煤灰微珠制备Si-Al-O-C-N系列材料的探索表明,经过进一步系统深入的研究,以粉煤灰微珠为原料,通过反应条件的设计与控制,制备出指定组成和性能的复合功能材料有可能实现,显示了粉煤灰微珠制备Si-Al-O-C-N系列材料具有很高的理论研究价值和很好的应用前景。更多还原

【Abstract】 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.更多还原