Knowledge Management System Of Institute of process engineering,CAS
|Place of Conferral||北京|
|Keyword||高铝粉煤灰 高碱体系 硅基材料 碳酸钙 结晶控制|
高铝粉煤灰年排放量超过2500万吨，综合利用率低，大量堆存造成严重的环境污染。高铝粉煤灰中氧化铝含量可达50%左右，同时富含大量的非晶态氧化硅组分。当前，研发经济可行的氧化铝提取技术是高铝粉煤灰综合利用的战略需求，而通过碱法预脱除非晶态氧化硅、提高高铝粉煤灰铝硅比是其中的关键环节。本文主要针对高铝粉煤灰预脱硅过程中产生的高碱性脱硅液高值利用问题，开展了高碱体系中硅基材料可控制备与介质循环的基础研究，提出了硅酸钙晶须及保温材料制备、高比表面二氧化硅联产超细碳酸钙两种工艺路线，重点研究高碱体系中硅基材料的晶型和形貌变化规律、控制方法、杂质离子赋存形式及动力学等，将为高铝粉煤灰中伴生非晶态氧化硅高值化利用提供技术支撑。主要研究内容和结论如下：（1）针对高碱体系中硅酸钙晶须的水热制备过程，考察了钙硅比（C/S）、碱浓度、反应温度等条件对硅酸钙晶型和形貌的影响，建立了反应条件与不同形貌及晶型的硅酸钙晶须对应关系，通过工艺条件控制，可以合成长径比大于100且分散性良好的变针硅钙石型晶须；根据硅酸钙晶型和形貌的对应关系，发现硅灰石族硅酸钙均具有晶须形貌，通过晶面表面能计算得到变针硅钙石晶须的轴向和径向晶面的晶面能分别为0.057 eV/?3和0.027 eV/?3，确定晶体轴向的优先生长是晶须形貌的成因。（2）以制备用于保温材料的硬硅钙石产品为目标，提出低温苛化制备低碱含量无定型水合硅酸钙（C-S-H），并进一步水热制备硬硅钙石的新工艺路线；分析表明C-S-H为硅氧四面体链状结构，C-S-H中残留的Na+包括吸附钠和层间结合钠；基于此，开发了离子交换三级逆流洗涤，5倍洗水下C-S-H中Na+可降低至0.35%；以此C-S-H为原料水热得到硬硅钙石产品，可用于生产硬硅钙石型保温材料；进一步研究发现反应体系中最大Al/Si比2.69%，过量铝会促使晶型转变为托贝莫来石，但通过配入硅或加入螯合剂EDTA的方式进行抑制。（3）开展了高碱低模数脱硅液碳化法制备低密度高比表面二氧化硅研究，提出了两段碳化法制备白炭黑，开展了工艺优化，白炭黑比表面积最高420.82 m2/g；针对碳化法制备气凝胶过程，考察了硅浓度、老化过程、表面改性及溶剂置换等工艺条件的影响，结果表明，优化条件下制备得到的气凝胶密度最低为0.34 g/mL，比表面积最高为700.72 m2/g；系统考察了反应过程中体系pH值、表面基团的变化规律，发现表面改性及溶剂置换过程中溶剂分层以致无法有效表面改性；基于此，采用溶剂预置换可有效改性，形成疏水的表面结构，二氧化硅密度可降至0.25 g/mL。（4）针对碳化残液循环利用问题，开发了苛化过程碱回收和碳酸钙形貌的协同控制技术；考察了反应温度、搅拌速率、浓度等条件对转化率和碳酸钙形貌的影响，通过工艺条件控制，碳酸根转化率为95.70%，并得到形貌均一、粒径约100 nm超细碳酸钙产品；深入研究了反应过程中碳酸钙的形貌变化规律，结果表明反应初始阶段碳酸钙是晶体和无定型结构的复合结构，随着反应时间的延长，碳酸钙形貌主要为规整的正方体颗粒，然后出现其他形状的碳酸钙颗粒，在高碱性体系下随着反应的进一步进行，大部分立方体形颗粒开始溶解，形成团状物质。（5）针对超细碳酸钙晶体颗粒分散性差、易团聚的特点，利用混合悬浮混合产品排料技术开展了高碱体系中碳酸钙结晶动力学研究；结果表明该体系中，碳酸钙晶体生长是表面反应控制，而成核速率则是受粒径控制，体系中过量的OH-会与Ca2+结合导致生长速率常数和成核速率常数均较低；团聚因子与平均停留时间呈正比例关系，氢氧化钙浓度升高后，悬浮密度的增加则会导致颗粒间碰撞概率增加从而破坏碳酸钙颗粒的团聚；基于此，通过降低平均停留时间可以有效减少碳酸钙团聚，而提高悬浮密度则会使碳酸钙粒径分布两极分化，从而有利于大小粒径的分离。
High alumina fly ash (HAFA) is a typical solid waste whoseemission amount is about 25 million tons per year, and the emission leads to serious pollutions. HAFA presents about 50 wt% alumina content, which is the same with the low grade bauxite. Therefore, it is seen as potential alternative to bauxite. Its utilization can resolve the environment pollution, and can effectively relieve the strain on China's aluminum resources. The extraction of alumina with alkali is widely studied. In this paper, in view of the utilization of the desilication solution generated in the utilization process of HAFA, the routes to prepare calcium silicate and silica-calcium carbonate were developed. In the recycle process of alkali, phase and morphology change of the silicon-based materials were investigated. The structure and morphology of the crystals in the reaction were well controlled. Through the researches, the preparation of high-value silicon materials can be prepared, silicon treatment in HAFA can also contribute to environment-friendly utilization of low-grade ore and other kind of solid waste containing high silicon contents. The main contents and results are as follows: (1) The preparation of calcium silicate nanofibers in the alkaline system was studied, and the effects of the conditions were investigated. The optimum conditions are as follows: Liquid/solid ratio is 7, initial concentration of alkali is 2 mol/L, Ca/Si (C/S) ratio is 1, temperaute is 240 °C, and reaction time is 5 h. Under these conditions, L/D ratio of the nanofiber can reach 100. Meanwhile, the effects of the reaction temperature, C/S ratio, and initial concentration of alkali were systematically investigated, and the correlation between the conditions and the phases is built. Accoding to the relation between the phase and morphology, calcium silicate in wollastonite group possess the morphology of nanofiber. Based on the transmission electron microscope (TEM), selected-area electron diffraction (SAED), and surface energy calculation of foshagite, the formation of nanofiber was explained through the crystal growth.(2) A two-step route to prepare calcium solicate hydrate (C-S-H) and xonotlite was reported, and the effects of C/S ratio, concentration of sodium ions, and temperature were investigated. The single-chain tetrahedron structure in C-S-H obtained in the alkaline system with short reaction time was determined through 29Si magic-angle spinning(MAS)nuclear magnetic resonance(NMR). The X-ray photoelectron spectroscopy(XPS) analysis results confirmedthe existence of sodium ions as Na-OSi and Na-OH in the synthesized C-S-H.Moreover, Na+combinations in the interlayer can be removed through ion exchange with 3-order countercurrent washing. The content of sodium ions can be declined to 0.35 % with 5 times washing water, and xonotlite can be prepared under hydrothermal conditions. In addition, the phase can be affected by alumimun, and tobermorite can be obtained with the Al/Si ratio of over 2.69 %. The synthesis of tobermorite can be prohibited through the addition of silicon or EDTA.(3) A two-step route to prepare silica with high surface area and superfine calcium carbonate was reported. Focusing on the preparation process of silica through carbonation, the effects of the conditions on the surface area of silica were investigated. Under the optimal conditions, the surface area of silica can get to 420.82 m2/g. The carbonation route for silica areogel synthesis with high surface area was studied, and the effects of the silicon concentration, aging temperature, surface modification, solvent exchange et al on silicaareogel properties were also concerned. Under the optimum conditions, the bulk density of silica can be 0.34 g/mL, and the surface area can be 700.72 m2/g. Through potentiometric titration and in-situ infared spectroscopy, the change in the system was investigated. In order to prevent the separation of the mixed solvent, pre-exchange was adopted to make sure the reaction between trimethylchlorosilane (TMCS) and silica. Then, the hydrophobic structure on the suface can be formed to reduce the density of silica areogel to 0.25 g/mL.(4) In the highly alkaline causticization system, the coupling control on the CO32- conversion and the CaCO3 crystallization depended on the complicated reaction conditions. The causticization conditions in HAFA utilization process were determined as the reaction temperature of 50 °C, stirring rate of 200 rpm, feeding time of 30 min, Ca2+/CO32- ratio of 1.1, and Ca(OH)2 concentration of 2 mol/L, respectively. With these conditions, the conversion of carbonate can reach 95.67%. The alkali can be well recycled and superfine CaCO3 with the single crystal size of about 100 nm is obtained. In addition, the influences of alkali and Ca(OH)2 on crystallization process was investigated by the study of morphology change with TEM image and SAED patterns.(5) In causticization, the crystallization kinetics of CaCO3 was investigated in a continuousMixed-suspension-mixed-product removal (MSMPR) crystallizer. The volume growth rate, nucleation rate, and agglomeration kernel of CaCO3 were each obtained. Crystal growth is surface-integration-limited, nucleation is size-limited, and relatively low growth and nucleation rate coefficients are obtained due to the combination effect of excess OH- with Ca2+. The positive order of mean residence time for CaCO3 agglomeration in the reaction system indicates that agglomeration increases with increasing mean residence time. Meanwhile, the increase in magma density induces greater agglomeration at 0.011 mol/L, but increasingly frequent and energetic collisions break down the agglomerates at a high solid concentration of CaCO3.
|朱干宇. 高铝粉煤灰非晶态氧化硅高值化利用基础研究[D]. 北京. 中国科学院研究生院,2016.|
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