废煤基活性炭再生制备载铁复合材料及除砷机理研究

【作者】 段昕辉；

【导师】 彭金辉；

【作者基本信息】 昆明理工大学 ， 有色金属冶金， 2012， 博士

【摘要】 在多晶硅材料生产过程中,材料质量的好坏,取决于氢气气体净化的好坏,而生产过程中尾气的净化与回收通常会用到大量的活性炭,由于现有技术仍然存在不足,使得活性炭的消耗量巨大且同时伴随有大量的废弃活性炭产生。废弃活性炭是一种典型的可再利用固废资源,为此本文提出将多晶硅生产过程产生的废弃煤基活性炭进行再生利用制备优质活性炭,同时用再生活性炭载铁制备复合材料制备高附加值产品,并对废弃活性炭再生以及制备铁／炭复合材料去除水中砷的技术和相关理论进行了系统深入的研究,取得了一些有意义的结果。1.热再生废弃煤基活性炭的技术研究采用不同活化剂,常规加热与微波加热等不同方法再生废弃煤基活性炭,采用基于中心组合设计的响应面法(RSM),研究对再生活性炭性能有关键性影响的再生温度,再生时间以及活化剂用量等工艺参数并进行优化。结合优化工艺参数和再生活性炭结构的研究,讨论废活性炭的再生反应机理。结果表明：工艺参数对活性炭碘吸附值和得率的影响规律符合两因子交互效应二次方程模型,通过对各个因素的显著性和交互作用进行分析,得到四种废弃煤基活性炭再生优化工艺参数,分别为：a.常规水蒸气热再生,再生温度983℃,再生时间135min,水蒸气流量2g/min,再生活性炭碘吸附值1053mg/g,再生得率61%；b.常规二氧化碳热再生,再生温度985℃,再生时间120min,二氧化碳流量600ml/min,再生活性炭碘吸附值1070mg/g,再生得率67%；c.微波水蒸气热再生,再生温度950℃,再生时间60min,水蒸气流量2.5g/min,再生活性炭碘吸附值1101mg/g,再生得率69%。；d.微波二氧化碳热再生,再生温度951℃,再生时间70min,二氧化碳流量553ml/min,再生活性炭碘吸附值1111mg/g,得率70%。四种方法再生获得的活性炭产品吸附能力都能达到国家标准GB/T7701-2008优级品的要求,其中,微波加热再生所得产品的吸附性能更佳,且再生过程的能量消耗有显著降低。通过氮气吸附与SEM对不同方法对再生活性炭孔隙结构进行研究,并分析不同活化剂再生机理,再生活性炭的比表面积均在1200m2/g以上。与常规热再生相比,微波加热再生利用加热速度快,选择性加热等特点,可迅速打开废弃活性炭被堵塞的孔道,能够进一步提高再生活性炭的吸附性能,实现活性炭的高效再生。以四种再生活性炭为原料,系统研究亚甲基蓝溶液浓度和吸附时间对饱和吸附量的影响,采用Langmuir和Freundlich吸附等温式对不同条件下获得的吸附平衡数据进行分析,再生活性炭吸附亚甲基蓝等温式可用Langmuir等温线描述,吸附过程可以用准二级动力学模型描述。结果表明：再生活性炭吸附亚甲基蓝的Langmuir饱和吸附量最低为370.37mg/g,这表明再生活性炭产品的具有比较发达的孔隙结构,可以用作液相吸附用途方面的商业活性炭用于处理染料废水,并且具有较好的效果。采用人工神经网络(Artificial Neural Networks, ANN),建立神经网络微波加热再生废弃煤基活性炭预测模型,研究结果表明,微波加热水蒸气和微波加热二氧化碳再生废弃煤基活性炭实验的改进BP神经网络预测模型,具有较好的预测效果,预测值与实际值有较好的拟合度,其均方误差为分别为0.0060和0.0043,该模型可以用于再生实验的指导预测,具有较高的可信度和实用意义,同时,以上的研究结果也表明,响应曲面优化与神经网络预测,二者结合互为辅助的实验设计方法有较好的参考价值和应用前景。2.再生活性炭载铁制备复合材料吸附去除水中的砷采用微波水蒸气热再生获得的活性炭载铁制备氧化铁/活性炭复合材料,采用氮气吸附、XRD、SEM等方法对样品的孔结构、物相组成、微观形貌等进行研究,结果表明：活性炭表面负载铁主要为磁铁矿(Fe3O4)以及赤铁矿(Fe203),载铁后再生活性炭的比表面积有明显的降低。将微波水蒸气再生活性炭和活性炭复合材料对水中砷的吸附效果进行了对比研究,并采用Langmuir和Freundlich吸附等温式对不同条件下获得的吸附平衡数据进行分析,再生活性炭与活性炭复合材料吸附砷的等温式可用Langmuir等温线描述,吸附过程可以用准二级动力学模型描述。负载过氧化铁后的活性炭复合材料对砷的饱和吸附量达到1.91mg/g,是再生活性炭饱和吸附量的2.8倍,这表明通过在活性炭中添加氧化铁制备复合材料,使二者的吸附作用相结合能够获得良好的除砷效果,与现有的载铁活性炭复合材料除砷文献报道过的饱和吸附量相比,本实验中获得的氧化铁／活性炭复合材料在载铁量较低(4.05%)的情况下获得了除砷性能较好的产品。采用密度泛函理论(DFT) Dmol3程序和CASTEP程序计算了几种经典的铁氧化物FeO、Fe203和Fe304以及FeAsO4数种晶体结构进行计算,获得了晶体参数以及结构特性、电子结构、能量特征、热力学常数的数据。通过比对结构稳定性,电子结构以及能量特征可以证明氧化铁在吸附砷后转变为砷酸铁的可能性,各种计算都表明砷酸铁与铁的氧化物相比,结构稳定性更高,从热力学上分析出砷酸铁的形成是一个自发过程,其方向都是向着生成砷酸铁的方向进行。通过第一性原理计算,肯定了用氧化铁除砷在理论上的可行性,验证了砷酸铁的生成理论,同时也为氧化铁体系除砷具体的机理研究打下基础,具有一定的参考价值和借鉴意义。综上,论文系统研究了废弃煤基活性炭资源化处置与载铁活性炭复合材料制备除砷的理论和工艺,前者采用不同热再生方法,利用不同活化剂再生制备优质活性炭,并首次提出微波加热多晶硅生产用废煤基活性炭再生新工艺技术,获得产品质量好,并且资源利用率高、无二次污染,实现了固废资源的再利用。后者为氧化铁／活性炭复合材料的制备开辟了新的原料来源,并借助量子化学和分子力学等手段,从微观角度探讨复合材料的除砷反应,阐明其内在化学机制,验证了砷酸铁生成理论,并进一步完善了不同种类氧化铁／活性炭复合材料除砷研究的理论基础。本研究为废弃煤基活性炭的再生及综合利用提供了一种有效的手段。更多还原

【Abstract】 In the production process of polysilicon material, the quality of the material depends on the purification of hydrogen gas, also in the production and recycling process, the exhaustion of activated carbon is very large. However, the technology existed is still inadequate, which makes the activated carbon consumption is huge and there is a large number of spent activated carbon produced. Waste activated carbon is a typical solid waste, thus, this paper presents the regeneration of spent coal based activated carbon for high-quality activated carbon and preparation of iron/AC composites from the regenerated activated carbon for high-value-added products, meanwhile, the regeneneration technology of activated carbon, the adsorption technology of arsenic in drinking water by iron/AC and some related theories are studied in-depth, some meaningful conclusion are obtained.1. The key technology on thermal regeneration of waste coal based activated carbonDifferent activation agents, conventional heating and microwave heating methods were used for thermal regerneration of spent coal based activated carbon. Basing on central composite design of response surface methodology (RSM), the key parameters such as regeneration temperature, regeneration time and the amount of activation agent, which had important influences on the performance of activated carbon products, were studied and optimized. The reaction mechanism of regenerating activated carbon was investigated along with the optimized proeess parameters and researeh of the carbon’s structures. The results indicated that:the influence of technological parameters on iodine value and yield conformed to two factor interactions and the quadratic equation model separately. By analyzing each factors’significance and the correlation, the optimized conditions for the regerneation of waste activated carbon were obtained as follows:a. Conventional steam thermal regeneration, regeneration temperature of 983℃, regeneration time of135min, steam flow rate of2g/min and iodine number of regenerated activated carbon for1053mg/g with regeneration yield of61%; b. Conventional thermal regeneration of carbon dioxide, regeneration temperature of985℃, regeneration time of120min, carbon dioxide flow rate of600ml/min and iodine number of regenerated activated carbon for1070mg/g with regeneration yield of67%; c. Microwave steam regeneration, regeneration temperature of950℃, regeneration time of60min, steam flow rate of2.5g/min and iodine number of regenerated activated carbon for1101mg/g with regeneration yield of69%; d. Microwave regeneration of carbon dioxide, regeneration temperature of951℃regeneration time of70min, carbon dioxide flow rate of553ml/min, and iodine number of regenerated activated carbon for1111mg/g with regeneration yield of70%. The adsorption capacity of activated carbon products obtained were all able to meet the national standard of China, GB/T7701-2008product requirements, among which, the absorption properties of the regenerated products by microwave perfromed better. The pore suructure of the products and mechanism of the regeneration process were ayalyed by nitrogen adsorption and SEM, the surface area of regenerated activated carbon were all abobe1200m2/g. Oweing to characteristics of selective heating, fast heating speed of microwave, the blocked pore of spent activated carbon was quickly opened, comparing with the conventional regeneration, it further improved the adsorption capacity of the regenerated activated carbon to achieve an efficient regeneration.The regenerated activated carbons were taken as raw materials for the adsorption of methylene blue, the influence of the methylene blue solution concentration and the adsorption time on the saturated adsorptive capacity were studied, Langmuir and the Freundlich adsorption uniform temperature type were used for the analysis of the adsorption equilibrium data, the adsorption uniform temperature type of methylene blue by regeneration activated carbon could be described by Langmuir isothermal. The second-level dynamics model described the adsorption process well. The result indicated that:the minimum adsorption capacity of methylene blue of Langmuir model for the regenerated activated carbon adsorption was3.70.3.7mg/g, which indicated that the activated carbon products with more developed pore structure could be used for commercial liquid adsorption for the treatment of dye wastewater, and the effect was good.Basing on the Artificial Neural Network (ANN), the neural network model was established for the prediction of microwave regeneration of spent coal based activated carbon, the results indicated that, the BP neural network prediction model of microwave regeneration with steam and CO2was reliable, the forecast and actual values fitted well, the mean square error were0.0060and0.0043, respectively. The model could be used to predict the regeneration experiments with high credibility and practical significance, while the above results also indicated that the optimization by response surface methodology and prediction by neural network could be combined, which had good reference value and application prospect.2. Research on arsnic removal by iron loaded activated carbon compositesThe activated carbon obtained by microwave steam thermal regeneration was taken as raw materials for the preparation of iron oxide preparation/activated carbon(AC) composite. The nitrogen adsorption, XRD, SEM were used for analysis of the sample pore structure, phase composition, microstructure, etc., the results showed that:the iron loaded on the carbon surface were mainly magnetite (Fe3O4) and hematite (Fe2O3), the surface area of the composite reduced obviously. The regenerated activated carbon and carbon composites were compared for adsorption of arsenic in drinking water, Langmuir and the Freundlich adsorption uniform temperature type were used for the analysis of the adsorption equilibrium data, the adsorption uniform temperature type of aresenic by regeneration activated carbon and carbon composites could be described by Langmuir isothermal, the second-level dynamics model described the adsorption process well. The arsenic adsorption capacity of carbon composite was1.91mg/g, which was2.8times of the regenerated activated carbon, which indicates that by adding iron oxide into the regenerated activated carbon, the carbon composites had a good adsorption performance on arsenic. Comparing with the the amount of saturated adsorption of arsenic reported in the existed literature, the carbon composites obtained in this experiment had a good ablity for the adsorption of arsenic with low iron content in weight(4.05%).Program of Dmol3and CASTEP basing on density functional theory (DFT) were used for the calculation of selected classic iron oxide FeO, Fe2O3, Fe3O4and FeAsO4for crystal structures. The data of the crystal parameters, structural properties, electronic structure, energy characteristics and the thermodynamic constants were obtained. By comparing the stability of the structure, the transformation of iron oxide into iron arsenate after adsorption could be proved by the electronic structure and energy characteristics. A variety of calculation results indicated that comparing with iron oxide, the atoms of iron arsenate were more stable, the energy was lower in electronic structure, the formation of iron arsenate is a spontaneous process basing on the thermodynamic analysis.In summary, the theory and techniques of recycling technology for spent activated carbon and the preparation of carbon composite material containing iron for the removal of the arsenic in water were studied systematicly. The former utilized thermal regeneration methods such as conventional heating and microwave heating, different activation agents for the preparation of high-quality activated carbon, which could obviously reduce the energy consumption, get high resource utilization, improve work condition and lower the second pollution. The latter of iron oxide/carbon composites has opened up new sources of raw materials and methods, by quantum chemistry and molecular mechanics, mechanism of arsenic removal was studied at microscopic view to clarify the intrinsic chemical mechanism of the preparation of the high-quality multi-functional composite materials to lay a solid theoretical basis. In this study, an effective way was provided for the regeneration and comprehensive utilization of spent activated carbon.更多还原