| 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 ? and 80 ? in the NaOH-Na3VO4-Na2CrO4-H20 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 ? to 40 ? at cooling rate around 1 ? 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 ?m 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 Na2Cr04 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%. |
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