Knowledge Management System Of Institute of process engineering,CAS
|Keyword||粉煤灰, 结构陶瓷, 碱活化, 物相重构|
我国粉煤灰年排放量约达5.6亿吨，由于其利用率仅70 %，导致粉煤灰堆积总量已超过30亿吨，给我国带来了巨大的环境压力。粉煤灰主要组分为SiO2和Al2O3，与传统陶瓷原料在化学成分和物相组成上具有相似性，且其粒径细小，与传统陶瓷原料粒度相当，可作为潜在的优质新型陶瓷原料。本论文首先分析了粉煤灰与传统陶瓷原料的理化性质和烧结性能差异，研究了粉煤灰在建筑陶瓷基体中微观作用机制与碱活化机理，在此基础上开展了全煤灰建筑陶瓷制备工艺研究。基于碱介质物相重构思路，又进一步开展先进氧化物结构陶瓷和先进非氧化物结构陶瓷制备新工艺等的开发研究，为粉煤灰高值化利用提供了新思路。本论文取得如下创新性结果：（1）通过解析粉煤灰微观结构特性及元素赋存规律，揭示了其不同赋存形式在陶瓷基体中的作用机制。褐煤灰颗粒按照化学成分和烧结作用可划为三类：（I）富含酸性氧化物（SiO2和Al2O3）的不规则大颗粒，作用类似于陶瓷配方中黏土；（II）不规则石英大颗粒，作用类似于陶瓷配方中石英；（III）富含碱性氧化物（特别是CaO，MgO和Fe2O3）的球形微珠，作用类似于陶瓷配方中长石。（2）阐明了碱活化对粉煤灰杂质相剥离、物相重构、表面结构改性和硅酸盐结构激活的作用机制。碱活化过程中，三类颗粒经历不同反应历程：I类和II类颗粒发生脱硅反应，使晶体骨架作用得到改善；III类颗粒表面被新生成的羟基方钠石和P沸石相包覆，使助熔作用得到提升。同时，羟基被接枝到粉煤灰颗粒表面，使得颗粒之间产生氢键作用，粉煤灰由脊性料转变为塑性料。另外，粉煤灰硅酸盐网络逐渐解聚，部分Al（VI）转化为Al（IV）并置换[SiO4]中Si4+，使其硅酸盐结构得到激发，反应活性显著提升。（3）开发了新型全煤灰建筑陶瓷制备技术。通过控制碱活化深度制备了两种类型的活化粉煤灰：均匀P沸石相包覆的类黏土活化灰和均一羟基方钠石相的类长石活化灰。通过原始粉煤灰与I类和II类活化灰复配可制备全煤灰陶瓷砖，粉煤灰掺入量达100 %。新型全煤灰建筑陶瓷性能远优于国标GB/T 4100-2015要求，烧成温度比陶瓷工业常规烧结温度低100 °C左右，烧结温度窗口宽达100 °C，无重金属浸出和放射性风险。（4）提出了粉煤灰碱介质物相重构制备先进氧化物结构陶瓷（硅灰石、莫来石）新思路。以褐煤灰为硅源在NaOH碱介质中合成了纳米纤维状托贝莫来石晶须，纤维直径小于200 nm，且大部分纤维长径比大于25，是制备硅灰石陶瓷的优质原料。托贝莫来石晶须烧结活性高，且其微观形貌经高温烧结后仍有效保留；铝硅置换效应可以促进托贝莫来石晶须生长，晶须长径比提高，有利于提高硅灰石陶瓷力学性能。研究同时发现，碱活化过程可以调控高铝灰化学成分，使其与莫来石成分接近，且可在高铝灰颗粒表面形成P沸石包覆相，促进陶瓷烧结和莫来石晶粒生长，两阶段表观活化能分别仅为90.39 kJ/mol和168.86 kJ/mol，低于文献报道值。硅灰石陶瓷最优烧结温度仅为900 °C，产品抗折强度为52.47 MPa，体积密度为2.15 g/cm3。莫来石陶瓷最优烧结温度仅为1300 °C，产品相对密度为90.85 %，抗折强度为109.67 MPa。（5）建立了水玻璃碱活化粉煤灰制备SiC基先进陶瓷新方法。在碱活化反应中，褐煤灰硅酸盐结构和水玻璃网络通过[AlO4]交联形成具有许多局部缺陷的更大网络结构。碳热还原时，褐煤灰中活性Al2O3和Fe2O3可以诱导莫来石中间相形成，从而促进SiC晶须生长。晶须产品结晶度和纯度高，平均长径比达18.26，产率超过70 %。并在此基础上开发了高性能SiCw/SiC复合陶瓷。SiCw交替进行H2O2氧化/HF酸洗改性后形貌更加均匀，且长径比增加了39.10 %。再经Al2O3包覆后，纤维具备SiC-SiO2-Al2O3三层芯壳结构，Al2O3壳层和SiO2壳层在热压烧结中可形成莫来石界面相，发挥出显著的增韧作用。产品相对密度为93.8 %，抗折强度为533.30 MPa，断裂韧性可达13.60 MPa·m1/2，维氏硬度为20.60 GPa。;The emission of coal fly ash (CFA) in China has reached about 560 million tons per year. Since the utilization rate of CFA is only 70 %, its accumulation has exceeded 3 billion tons, bringing huge environmental pressure to China. CFA mainly comprising of SiO2 and Al2O3 has similar chemical and phase compositions with traditional ceramic raw materials. In addition, its particle size is equivalent to that of traditional ceramic raw materials, making it a promising ceramic raw material. This research first analyzed the differences between the physical and chemical properties and sintering behaviors of CFA and traditional ceramic raw materials, and then studied the micro-functional mechanism of CFA in ceramic matrix and alkali activation mechanism. On this basis, the preparation of 100 % CFA-based ceramic tiles was carried out. Based on the phase reconstruction effect of alkaline medium, further research and development of new preparation processes of the advanced oxide structural ceramics and advanced non-oxide structural ceramics were carried out, providing new insights for the high-value utilization of CFA. The innovative results are as follows:(1) The action mechanism of different CFA occurrence forms in the ceramic matrix have been revealed by analyzing its microstructure characteristics and element occurrence law. The lignite CFA particles can be divided into three categories according to their chemical compositions and sintering behaviors: (I) large irregular particles with a high acidic oxide (SiO2 and Al2O3) content, acting like common clay; (II) large irregular quartz particles; (III) small spherical particles with a high alkaline oxide (especially CaO，MgO, and Fe2O3) content, acting like common feldspar.(2) The functional mechanism of alkali-activation on CFA impurity phase separation, phase reconstruction, surface structure modification and silicate structure activation has been clarified. In alkali-activation, CFA particles undergo different processes: excess SiO2 in class-I and class-II particles is removed, improving the crystal skeleton effect; class-III particles are coated by freshly generated hydroxysodalite and P zeolite phases, improving the fluxing effect. At the same time, the hydroxyls are grafted onto the surfaces of CFA particles, producing hydrogen-bonding interaction between CFA particles. Therefore, CFA can be transformed to a plastic material. In addition, the silicate network of CFA is gradually depolymerized, and part of Al (VI) is converted into Al (IV), which can substitute for Si4+ in the tetrahedral, rendering the silicate structure unstable and improving its reactivity significantly.(3) A preparation technology of novel 100 % CFA-based ceramic tiles has been developed. Two types of activated CFA were prepared by controlling the depth of alkali-activation: clay-like activated CFA with a uniform P zeolite coating and feldspar-like activated CFA with a uniform hydroxy sodalite phase. The 100 % CFA-based ceramic tiles can be prepared by mixing raw CFA with these two activated CFA. Their performance meets the standard GB/T 4100-2015. The sintering temperature is about 100 °C lower than the conventional one, and the sintering temperature window is up to 100 °C. The products have no risks of heavy metals leaching and radioactivity.(4) A novel strategy for preparing advanced oxide structural ceramics (wollastonite and mullite ceramics) by CFA phase reconstruction in alkali medium has been proposed. Nano-fibrous tobermorite whiskers were synthesized in NaOH solution using lignite CFA as the silicon source. The fibers have a diameter less than 200 nm, and the aspect ratio of most fibers is larger than 25, making them high-quality raw materials for wollastonite ceramics. Tobermorite whiskers have high sintering activity, and their micromorphology can be effectively retained in the sintered matrix. The substitution of Al3+ for Si4+ can promote the growth and aspect ratio increasement of tobermorite whiskers, which is beneficial to the mechanical properties of wollastonite ceramics. It has also been found that the alkali-activation can make the chemical composition of high-alumina CFA closer to that of mullite and form a P zeolite coating on the surface of high-alumina CFA, promoting ceramic densification and mullite grain growth. The apparent activation energies of the two stages are only 90.39 kJ/mol and 168.86 kJ/mol, respectively, which are lower than those reported in the literature. The optimal sintering temperature of wollastonite ceramics is only 900 °C, the flexural strength of the product is 52.47 MPa, and the bulk density is 2.15 g/cm3. The optimal sintering temperature of mullite ceramics is only 1300 °C, the relative density of the product is 90.85 %, and the flexural strength is 109.67 MPa.(5) A new method for preparing SiC-based advanced ceramics from water glass-activated lignite CFA was established. In the alkali-activation, the CFA silicate structure and the water glass network are crosslinked by [AlO4] to form a larger network with many defects. During carbothermal reduction, the active Al2O3 and Fe2O3 in CFA induce the formation of mullite mesophase, thereby promoting the growth of SiC whiskers. The whiskers have high crystallinity and purity, with an average aspect ratio of 18.26 and a yield of more than 70 %. Based on these, a high-performance SiCw/SiC ceramic composite has been developed. After the alternating H2O2-oxidation/HF-leaching modification, the morphology of the SiCw appears more uniform, and their aspect ratio increases by 39.10 %. After the Al2O3 coating, the fibers have a SiC-SiO2-Al2O3 triple-layered structure. The Al2O3 shell and the SiO2 shell form a mullite interface phase during hot-press sintering, which plays a significant toughening effect. The product has a relative density of 93.8 %, a flexural strength of 533.30 MPa, a fracture toughness of 13.60 MPa·m1/2, and a Vickers hardness of 20.60 GPa.
|罗扬. 碱活化粉煤灰制备结构陶瓷应用基础研究[D]. 中国科学院大学,2020.|
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