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针对钒渣现有钠化焙烧提钒主流工艺钒资源提取率低、铬不能提取、废水废气及含铬废渣污染严重等问题，中国科学院过程工程研究所开发了钒渣亚熔盐法钒铬共提清洁生产工艺，可在低温以及近常压条件下实现钒渣中钒铬的高效同步提取，为解决我国36亿吨高铬型钒钛磁铁矿的利用提供了有效解决方案。钒渣经亚熔盐反应后获得的高碱性溶液中钒铬的有效分离是工艺的核心关键环节，但目前的钒铬分离方法多适用于酸性及中性条件，亟需开发碱介质中钒铬高效清洁分离方法。本文以NaOH体系中钒铬的高效分离为目标，在对体系溶解度系统研究的基础上提出低碱区冷却结晶钒酸钠，高碱区蒸发结晶铬酸钠的钒铬分离新方法，并对结晶分离过程的热力学基础、动力学调控规律、工艺参数的确定以及产物深度提纯等进行了系统研究，为钒渣亚熔盐法钒铬共提清洁生产工艺提供了坚实的理论支撑和应用指导。论文取得如下创新性成果：1. 建立了NaOH碱介质中钒铬高效结晶分离的新方法。通过测定40 °C和80 °C条件下四元水盐体系NaOH?Na3VO4?Na2CrO4?H2O溶解度相图，基于钒铬溶解度随温度及碱浓度变化规律的差异，结合碱介质中杂质组元CO32-和SiO2对钒铬溶解度的影响，提出钙化除杂后低碱区冷却结晶分离钒酸钠，继而高碱区蒸发结晶分离铬酸钠的钒铬分离新方法。2. 获得了完整的钒酸钠结晶过程介稳区及其成核生长规律。通过考察温度、碱浓度、转速以及降温速率等参数对介稳区宽度的影响，获得相应条件下介稳区宽度及调控规律，为碱介质中钒铬相分离方法设计提供了重要依据；通过测定钒酸钠在NaOH溶液中的结晶诱导期，确定钒酸钠结晶为单核成核机理，其生长模式随碱浓度升高由连续生长向生长传递型生长转变；以经典粒数衡算方程为基础，使用间歇动态法对钒酸钠冷却结晶过程的动力学行为进行考察，通过矩量变换法处理并计算钒酸钠结晶动力学方程，确定了其固液两相间结晶过程传递行为特性。3. 实现了NaOH体系中钒酸钠和铬酸钠的高效结晶分离。通过考察各工艺参数对钒酸钠结晶的影响，得到钒酸钠结晶工艺条件为：NaOH浓度300 g·L-1左右，搅拌转速160 rpm，从80 °C冷却至40 °C结晶，降温速率1 °C·min -1左右，保温时间105 min，此条件下钒酸钠结晶率大于54 %，晶体纯度大于93 %，所得产物为Na3VO4·3H2O，平均粒度388.21 μm。通过蒸发结晶至NaOH浓度600~700 g·L-1可实现铬酸钠的分离，铬酸钠结晶率可超过52 %，晶体纯度大于95 %，所获得晶体为Na2CrO4。4. 在高碱度条件下实现了钒铬的高效萃取分离，获得高纯钒酸钠产品。针对含有少量Na2CrO4杂质的钒酸钠，使用萃取剂Aliquat 336以及改性剂TBP实现了铬的萃取分离，考察六价铬液液萃取动力学、热力学及相关机理，实现了对初始液相pH为13.63的微量铬杂质超过75 %的单级萃取率以及钒的零萃取，分离效果显著，使用NaOH或HNO3可实现高达100 %的反萃效率。;The traditional vanadium production processes, such as sodium salt roasting technology using vanadium slag, have a number of disadvantages including low vanadium extraction rate, generation of high salinity ammonia waste water, release of harmful kiln gases, and more importantly, creation of toxic chromium-containing tailings. In order to solve the problems mentioned above, a new vanadium production process based on the sub-molten salt (SMS) technology has been developed by Institute of Process Engineering, Chinese Academy of Sciences. The SMS technology could realize a synergistic and efficient extraction of both vanadium and chromium under mild reaction condition (low temperature and near ambient pressure), providing technical supports for the effective utilization of the 3.6 billion tons of high-chromium containing vanadium titano-magnetite ores in China. After leaching of the vanadium slag using the SMS media, an alkaline solution containing both sodium vanadate and sodium chromate is obtained, and the clean separation of the salts from the multi-component system is critical for the new process. However, the currently-existed separation methods generally require acidic or neutral conditions, and therefore, it is of great importance to develop new separation method to consummate the brand new SMS technology. In this dissertation, a new separation method was proposed based on the solubility of sodium vanadate and sodium chromate in NaOH solutions, and they could be separated via cooling crystallization and evaporation crystallization from alkaline solutions sequentially. Further, fundamental researches with respect to the crystallization thermodynamics, crystal growth kinetics, parameter optimization as well as deep purification of sodium vanadate were systematically studied, providing theoretical basis and technical support for the development of new method to recovery both vanadium and chromium from the vanadium slag efficiently and cleanly using the SMS media.The main progresses are achieved as follows: 1. A new crystallization method has been established for the efficient separation of sodium vanadate and sodium chromate from strong NaOH alkaline solution. The solubility of sodium vanadate and sodium chromate at 40 °C and 80 °C in the NaOH?Na3VO4?Na2CrO4?H2O system has been measured, the changes of solubility with temperature and alkali concentration have been systematically investigated, and the influences of major impurities including sodium carbonate and sodium silicate have been analyzed. Based on fundamental study, a new separation method has been designed. Sodium vanadate and sodium chromate could be sequentially separated via cooling crystallization and evaporation crystallization in alkaline solutions after the removal of impurities by calcification.2. Comprehensive investigation on the metastable region, nucleation and crystal growth characteristics during the sodium vanadate cooling crystallization have been conducted for optimizing the separation process. By studying the effect of temperature, alkali concentration, stirring speed and cooling rate, metastable region regulating principles were obtained, allowing for a better control of the crystallization procedure. The induction time for the sodium vanadate crystallization from the NaOH solution has also been determined, and it was concluded that the crystallization of sodium vanadate followed a mononuclear nucleation mechanism, and its growth mode changed from continuous growth to transmitted growth as the alkali concentration increased. Based on the classical population balance equation, the kinetic behavior of sodium vanadate during cooling crystallization was investigated by using the intermittent dynamic method, and the kinetic equation was processed and calculated by moment transformation method.3. Effective separation of sodium vanadate and sodium chromate from concentrated NaOH solution has been achieved. The optimal conditions for sodium vanadate crystallization was determined to be: NaOH concentration around 300 g·L-1, stirring speed of 160 rpm, cooling down from 80 °C to 40 °C at cooling rate around 1 °C·min -1, and holding time of 105 min. Under the above conditions, the optimized crystallization rate of sodium vanadate was more than 54 %, and 93 % pure Na3VO4·3H2O with average particle size of 388.21 μm could be obtained. Further, the Na2CrO4 crystal could be separated by evaporation crystallization at 600~700 g·L-1 NaOH concentration, and recrystallization rate of above 52 % could be achieved with the Na2CrO4 purity higher than 95%.4. Liquid-liquid solvent extraction of vanadium and chromium at high alkalinity has been realized with high purity product of sodium vanadate. The separation of Na2CrO4 impurity was realized by solvent extraction using extractant Aliquat 336 and modifier TBP. The thermodynamics of solvent extraction, kinetics and mechanism of hexavalent chromium were investigated as well. The single-stage extraction rate was higher than 75 % with the initial liquid phase pH of 13.63 while realizing nearly no extraction of vanadium. By using NaOH or HNO3, the stripping efficiency could be achieved up to 100 %.
|冯曼. 亚熔盐多元体系中钒铬酸盐清洁分离应用基础研究[D]. 中国科学院研究生院,2018.|
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