Abstract:
The present invention provides a system and method for purifying vanadium pentoxide. Industrial grade vanadium pentoxide is converted to vanadium oxytrichloride by low temperature fluidizing chlorination, wherein chlorinating gas is preheated via heat exchange between fluidizing gas and chlorination flue gas, and an appropriate amount of air is added to enable a part of carbon powder to combust so as to achieve a balanced heat supply during the chlorination, thereby increasing the efficiency of chlorination and ensuring good selectivity in low temperature chlorination. The vanadium oxytrichloride is purified by rectification, and then subjected to plasma oxidation, thereby obtaining a high-purity vanadium pentoxide product and chlorine gas. The chlorine gas is returned for low temperature chlorination. The system and method have advantages of favorable adaptability to raw material, no discharge of contaminated wastewater, low energy consumption and chlorine consumption in production, stable product quality, etc.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the national phase entry of International Application No. PCT/CN2016/072520, filed on Jan. 28, 2016, which is based upon and claims priority to Chinese Patent Application No. CN201510052178.5, filed on Jan. 30, 2015, the entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the fields of chemical engineering and materials, and more particularly to a system and method for purifying vanadium pentoxide. 
       BACKGROUND OF THE INVENTION 
       [0003]    Vanadium pentoxide is one of the important industrial vanadium products, and widely applied in the production of alloy additives such as ferrovanadium and vanadium nitride, and in the fields of catalysts, colorants, cemented carbide additives and the like. With the continuous development of new energy technologies, there is a growing demand on high-purity vanadium pentoxide (with a purity of above 3N5) in the battery industry, including an all-vanadium redox flow battery (VRB) with good large-scale energy storage performance, a vanadate-based lithium-ion battery used for electric automobiles and the like. However, in general, only vanadium pentoxide with a purity of 2N5 (i.e. the product according with the specification in HGT 3485-2003) can be prepared by the existing industrial technology, which is difficult to meet requirements on vanadium pentoxide for the battery industry. Therefore, how to prepare high-purity vanadium pentoxide with low cost and high efficiency is one of the urgent issues needed to be solved in the field of new energy technologies. 
         [0004]    At present, high-purity vanadium pentoxide powder is usually obtained by the following method: a vanadium-leaching solution or a vanadium solution which is obtained by dissolving a vanadium-rich material (such as ammonium polyorthovanadate, ammonium metavanadate, industrial grade vanadium pentoxide, etc.) is used as a raw material, and purified by the method such as chemical precipitation purification and/or solvent extraction/ion resin exchange or the like, to obtain a purified vanadium solution; the purified vanadium solution is subjected to ammonium salt precipitation to obtain the purified ammonium polyorthovanadate or ammonium metavanadate precipitate; then, the precipitate is subjected to decomposition by calcination to obtain the high-purity vanadium pentoxide powder, as described in Chinese Patent Applications CN1843938A, CN102730757A, CN103145187A, CN103515642A, CN103194603A, CN103787414A, CN102181635A and CN103663557A, European Patent EP0713257B1, etc. In these methods, the process parameter for impurity removal is closely related to the content of the impurity in the raw material, thus the adaptability to the raw material is poor. Moreover, the calcium salt and magnesium salt scavengers or extractants, the acid and alkali reagents and ammonium salts for vanadium precipitation used in the purification process are also liable to introduce impurities. In order to improve the quality of the product, it is usually required to use expensive reagents with high purity, thereby leading to the following problems: the cost is too high, large-scale production cannot be implemented and the purity of the product is difficult to stabilize at above 3N5. 
         [0005]    For the problems that the scavengers or extractants are liable to introduce impurities and the cost of the reagents used is too high, the relevant agencies also propose the use of the repeated precipitation method to achieve purification and impurity removal of a vanadium solution; that is, by using the ammonium salt precipitation characteristic of the vanadium-containing solution, vanadium is selectively precipitated out, to confine a part of the impurity ions to the solution after precipitation; the resulting ammonium salt precipitate is dissolved and then multiple repeated operations are conducted, to obtain more pure ammonium polyorthovanadate or ammonium metavanadate precipitate; and the precipitate is subjected to decomposition by calcination to obtain a high-purity vanadium pentoxide powder, as described in Chinese Patent Applications CN103606694A, CN102923775A, etc. This process effectively reduces the amount of the reagents used and the possibility that the reagents introduce impurities. However, the dissolution-precipitation process still requires use of a large quantity of high-purity acid and alkali reagents and ammonium salts, therefore the cost of purification is still high; and the cumbersome multiple precipitation operations not only lower the production efficiency but also lead to a significant decline in the direct recovery rate of vanadium. In addition, in the above-mentioned solution purification methods, extraction/back extraction, precipitation, washing and other operation steps will produce a large amount of waste water mainly containing a small quantity of vanadium ions and ammonium ions and a large amount of sodium salts, which results in difficult treatment and outstanding problem of pollution and also seriously restricts the large-scale industrial application of the methods. 
         [0006]    Due to the large difference in the boiling points and saturated vapor pressures of metal chlorides, different metal chlorides are easily separated by distillation/rectification. Raw material chlorination—purification by rectification—subsequent treatment is a commonly-used preparation process for high-purity materials such as high-purity silicon (polysilicon), high-purity silicon dioxide, and the like. Because of a very large difference between boiling points of the chloride of vanadium, vanadium oxytrichloride, and the chlorides of common impurities such as iron, calcium, magnesium, aluminum, sodium, potassium and the like, high-purity vanadium oxytrichloride is easily obtained by rectification, and high-purity vanadium pentoxide can be prepared by subjecting the high-purity vanadium oxytrichloride to hydrolysis and ammonium salt precipitation, supplemented by calcination. Therefore, the use of the chlorination method for the preparation of high-purity vanadium pentoxide has a greater advantage in principle. In fact, the use of the chlorination method for the preparation of high-purity vanadium pentoxide is not only feasible in principle, but also has been implemented in the laboratory by the researchers of Iowa State University in the United States as early as the 1960s (Journal of the Less-Common Metals, 1960, 2: 29-35). They employed ammonium polyorthovanadate as a raw material, and prepared the crude vanadium oxytrichloride by chlorination with addition of carbon, then obtained high-purity vanadium oxytrichloride through purification by distillation, and conducted ammonium salt precipitation to obtain high-purity ammonium metavanadate, and finally calcined high-purity ammonium metavanadate at 500-600° C., to obtain the high-purity vanadium pentoxide powder. However, a large amount of wastewater containing ammonia and nitrogen will be produced in the precipitation and the washing processes (at least 1.8 ton of ammonium chloride waste salt is produced per ton of a vanadium pentoxide product), leading to difficult treatment; and the precipitation, drying and calcination processes of ammonium salts not only require high energy consumption, but also easily cause environmental pollution. In addition, the study only realizes the intermittent preparation of high-purity vanadium pentoxide by the chlorination method with the laboratory equipment, and cannot provide related information on how to use the chlorination method for continuous preparation of high-purity vanadium pentoxide on an industrial scale. It may be for exactly these reasons that the report on continuous preparation of high-purity vanadium pentoxide by the chlorination method is difficult to find in the decades after the study. 
         [0007]    Recently, Chinese Patent Application CN103130279A proposes a method for preparing high-purity vanadium pentoxide by using the chlorination method with a vanadium-iron magnetic iron ore, vanadium slag, vanadium-containing catalyst and other materials containing vanadium as raw materials. A mixture of chlorides of vanadium is obtained through chlorination with addition of carbon—dust removal—condensing, and vanadium tetrachloride is separated through rectification to obtain pure vanadium oxytrichloride, then the vanadium oxytrichloride is fed into an ultrapure aqueous solution or ultrapure aqueous solution of ammonia and precipitated, and the precipitate is filtered, dried and calcined to obtain vanadium pentoxide. This patent has the following deficiencies: (1) similar to the above study of Iowa State University in the United States, this patent actually provides the basic flow of chlorination only, lacking the specific operable solutions. For example, the method of chlorination comprises both boiling chlorination and molten salt chlorination, which are completely different methods of chlorination. For another example, concerning the chlorination reactor, it is proposed to use reactors such as “rotary kiln, fluidized furnace, boiling furnace, shaft furnace, multi-hearth furnace” and the like, which actually covers almost all of the commonly-used mainstream reactors in the metallurgical industry; however, different reactors&#39; requirements for raw materials differ greatly. For example, the shaft furnace can only handle “coarse” particles with a particle size more than 8 mm, and needs to conduct pelleting and sintering pretreatment when “fine” particles are processed, while boiling chlorination is generally suitable for the treatment of fine particles. Therefore, a particular vanadium raw material cannot be directly applied to rotary kiln, fluidized furnace, boiling furnace, shaft furnace, multi-hearth furnace and other reactors. Moreover, the “fluidized furnace” and “boiling furnace” are essentially the same, just different in names; therefore, since these reactors vary widely in operation mode and condition, the method cannot actually be implemented on the condition that only basic flow is provided. (2) Vanadium oxytrichloride is fed into the ultrapure aqueous solution for hydrolysis. However, because vanadium pentoxide is easily dissolved in the hydrochloric acid solution, the recovery rate of precipitation of vanadium is too low. Moreover, in the hydrochloric acid solution with an HCl concentration more than 6.0 mol/L, when vanadium pentoxide is dissolved, it will be reduced to VOCl 2  and chlorine gas is released, which will further reduce the recovery rate of precipitation of vanadium. Precipitation and washing processes will inevitably produce a large amount of hydrochloric acid solution containing vanadium, and it is difficult to effectively achieve a comprehensive treatment. 
         [0008]    In addition, for large-scale industrial applications, there still exists the following two problems in the existing technologies for chlorination of vanadium raw materials: (1) calcination for chlorination of vanadium raw materials is a strong exothermic process, and in addition to preheating the solid and gas reaction materials, the heat generated by the chlorination reaction still needs to be removed by furnace wall heat dissipation to stabilize the temperature in the chlorination; therefore, both the solid and gas are usually enters the reactor at a temperature of near room temperature, and only can participate in the reaction after been preheated by the heat produced from the chlorination reaction, resulting in too low efficiency of reaction in part of the chlorination reactor; (2) since the heat produced by the chlorination reaction needs to be removed through dissipation of a large amount of heat to maintain the operation temperature, the operating condition and environmental climate change are both liable to cause fluctuations in chlorination temperature, resulting in reduction of selectivity in chlorination and efficiency, and it is needed to use a reasonable method for balanced supply of heat and temperature regulation. Therefore, reasonable heat supply and temperature control must be provided. Only in this way, it is possible to effectively improve the efficiency of chlorination and obtain stable chlorination temperature, so as to ensure the selectivity in the chlorination to effectively inhibit the chlorination of impurities. 
         [0009]    Therefore, achieving the temperature regulation of chlorination process, improving the direct recovery rate of vanadium, reducing the amount of the waste discharged, and decreasing the energy consumption and chlorine consumption in production, are the keys to increase the economy of the technology for purifying and preparing high-purity vanadium pentoxide through the chlorination method. 
       SUMMARY OF THE INVENTION 
       [0010]    In view of the above problem, the present invention proposes a system and method for purifying vanadium pentoxide, to ensure good selectivity in low temperature chlorination, avoid the production of a large amount of polluted wastewater, and reduce the energy consumption and chlorine consumption in the production of high-purity vanadium pentoxide and the operation cost. In order to achieve these objects, the present invention adopts the following technical solutions. 
         [0011]    The present invention provides a system for purifying vanadium pentoxide, comprising feeding device  1 , low temperature chlorination fluidized bed  2 , rectification and purification device  3 , plasma oxidation device  4 , tail gas washing absorber  5 , induced draft fan  6  and chimney  7 ; 
         [0012]    wherein the feeding device  1  comprises industrial grade vanadium pentoxide hopper  1 - 1 , industrial grade vanadium pentoxide screw feeder  1 - 2 , carbon powder hopper  1 - 3  and carbon powder screw feeder  1 - 4 ; 
         [0013]    the low temperature chlorination fluidized bed  2  comprises chlorination bed feeder  2 - 1 , chlorination fluidized bed body  2 - 2 , chlorination bed cyclone separator  2 - 3 , flue gas heat exchanger  2 - 4 , flue gas condenser  2 - 5 , chlorination bed acid-seal tank  2 - 6  and chlorination bed spiral slag-discharging device  2 - 7 ; 
         [0014]    the rectification and purification device  3  comprises distilling still  3 - 1 , rectifying column  3 - 2 , distillate condenser  3 - 3 , reflux liquid collecting tank  3 - 4 , silicon-containing vanadium oxytrichloride storage tank  3 - 5 , rectification section acid-seal tank  3 - 6 , high-purity vanadium oxytrichloride condenser  3 - 7 , and high-purity vanadium oxytrichloride storage tank  3 - 8 ; 
         [0015]    the plasma oxidation device  4  comprises air filtration purifier  4 - 1 , reactant nozzle  4 - 2 , plasma reactor  4 - 3 , primary cyclone separator  4 - 4 , secondary cyclone separator  4 - 5 , rotary-vane pump  4 - 6  and gas compressor  4 - 7 ; 
         [0016]    wherein a feed outlet at the bottom of the industrial grade vanadium pentoxide hopper  1 - 1  is connected with a feed inlet of the industrial grade vanadium pentoxide screw feeder  1 - 2 ; a feed outlet at the bottom of the carbon powder hopper  1 - 3  is connected with a feed inlet of the carbon powder screw feeder  1 - 4 ; and a feed outlet of the industrial grade vanadium pentoxide screw feeder  1 - 2  and a feed outlet of the carbon powder screw feeder  1 - 4  are both connected with a feed inlet of the chlorination bed feeder  2 - 1  through a pipeline; 
         [0017]    a feed discharge opening of the chlorination bed feeder  2 - 1  is connected with a feed inlet at the upper part of the chlorination fluidized bed body  2 - 2  through a pipeline; a gas inlet at the bottom of the chlorination bed feeder  2 - 1  is connected with a nitrogen gas source main pipe through a pipeline; the chlorination bed cyclone separator  2 - 3  is provided at the center of the top of the expansion section of the chlorination fluidized bed body  2 - 2 ; a gas outlet at the top of the chlorination bed cyclone separator  2 - 3  is connected with a hot flue gas inlet of the flue gas heat exchanger  2 - 4  through a pipeline; a cold flue gas outlet of the flue gas heat exchanger  2 - 4  is connected with a gas inlet of the flue gas condenser  2 - 5  through a pipeline; a gas outlet of the flue gas condenser  2 - 5  is connected with a gas inlet of the chlorination bed acid-seal tank  2 - 6  through a pipeline; a gas outlet of the chlorination bed acid-seal tank  2 - 6  is connected with a gas inlet of the tail gas washing absorber  7  through a pipeline; a slag-discharge opening at the lower part of the chlorination fluidized bed body  2 - 2  is connected with a feed inlet of the chlorination bed spiral slag-discharging device  2 - 7  through a pipeline; a gas inlet at the bottom of the chlorination fluidized bed body  2 - 2  is connected with a hot gas outlet of the flue gas heat exchanger  2 - 4  through a pipeline; and a cold gas inlet of the flue gas heat exchanger  2 - 4  is connected with a chlorine gas source main pipe, the nitrogen gas source main pipe and a compressed air main pipe through pipelines, respectively; 
         [0018]    a liquid outlet at the bottom of the flue gas condenser  2 - 5  is connected with a feed inlet of the rectifying column  3 - 2  through a pipeline; a steam outlet of the distilling still  3 - 1  is connected with a steam inlet of the rectifying column  3 - 2  through a pipeline; a reflux inlet of the distilling still  3 - 1  is connected with a liquid reflux outlet at the bottom of the rectifying column  3 - 2  through a pipeline; a gas outlet at the top of the rectifying column  3 - 2  is connected with a gas inlet of the distillate condenser  3 - 3  through a pipeline; a liquid outlet of the distillate condenser  3 - 3  is connected with a liquid inlet of the reflux liquid collecting tank  3 - 4  through a pipeline; a reflux liquid outlet of the reflux liquid collecting tank  3 - 4  is connected with a reflux liquid inlet at the top of the rectifying column  3 - 2  through a pipeline; a feed discharge opening of the reflux liquid collecting tank  3 - 4  is connected with an inlet of the silicon-containing vanadium oxytrichloride storage tank  3 - 5  through a pipeline; an exhaust gas outlet of the silicon-containing vanadium oxytrichloride storage tank  3 - 5  is connected with a gas inlet of the rectification section acid-seal tank  3 - 6  through a pipeline; a gas outlet of the rectification section acid-seal tank  3 - 6  is connected with a gas inlet of the tail gas washing absorber  5  through a pipeline; a rectificate outlet of the rectifying column  3 - 2  is connected with a gas inlet of the high-purity vanadium oxytrichloride condenser  3 - 7  through a pipeline; a liquid outlet of the high-purity vanadium oxytrichloride condenser  3 - 7  is connected with a liquid inlet of the high-purity vanadium oxytrichloride storage tank  3 - 8  through a pipeline; and an underflow outlet is provided at the bottom of the distilling still  3 - 1 ; 
         [0019]    a gas inlet of the air filtration purifier  4 - 1  is connected with the compressed air main pipe through a pipeline; a gas outlet of the air filtration purifier  4 - 1  is connected with an air inlet of the reactant nozzle  4 - 2  and a gas inlet of the secondary cyclone separator  4 - 5  through pipelines, respectively; a liquid outlet of the high-purity vanadium oxytrichloride storage tank  3 - 8  is connected with a chloride inlet of the reactant nozzle  4 - 2  through a pipeline; the reactant nozzle  4 - 2  is provided at the center of the upper part of the plasma reactor  4 - 3 ; a feed outlet at the bottom of the plasma reactor  4 - 3  is connected with a gas inlet of the primary cyclone separator  4 - 4  through a pipeline; a gas outlet of the primary cyclone separator  4 - 4  is connected with a gas inlet of the rotary-vane pump  4 - 6  through a pipeline; a gas outlet of the rotary-vane pump  4 - 6  is connected with a gas inlet of the gas compressor  4 - 7  through a pipeline; a gas outlet of the gas compressor  4 - 7  is connected with a cold gas inlet of the flue gas heat exchanger  2 - 4  through a pipeline; a feed discharge opening at the lower part of the primary cyclone separator  4 - 4  is connected with a gas inlet of the secondary cyclone separator  4 - 5  through a pipeline; a gas outlet at the top of the secondary cyclone separator  4 - 5  is connected with a gas inlet of the tail gas washing absorber  5  through a pipeline; and a feed outlet at the bottom of the secondary cyclone separator  4 - 5  is connected with a high-purity vanadium pentoxide product hopper through a pipeline; 
         [0020]    a gas outlet of the tail gas washing absorber  5  is connected with a gas inlet of the induced draft fan  6  through a pipeline; and a gas outlet of the induced draft fan  6  is connected with a gas inlet at the bottom of the chimney  7  through a pipeline. 
         [0021]    The present invention further provides a method for purifying vanadium pentoxide based on the above system, comprising the following steps: 
         [0022]    allowing industrial grade vanadium pentoxide powder in the industrial grade vanadium pentoxide hopper  1 - 1  and carbon powder in the carbon powder hopper  1 - 3  to enter the chlorination bed feeder  2 - 1  simultaneously through the industrial grade vanadium pentoxide screw feeder  1 - 2  and the carbon powder screw feeder  1 - 4  respectively and be mixed therein, and then enter the chlorination fluidized bed body  2 - 2 ; allowing chlorine gas from the chlorine gas source main pipe, nitrogen gas from the nitrogen gas source main pipe and air from the compressed air main pipe to be preheated by exchanging heat with chlorination flue gas by the flue gas heat exchanger  2 - 4 , and then enter the chlorination fluidized bed body  2 - 2  to allow the vanadium pentoxide, the carbon powder and other powder materials to be kept at a fluidized state and chemically reacted, wherein the air enables a part of the carbon powder to combust to provide heat for maintaining the temperature of the fluid bed, and the chlorine gas and the carbon powder function together to make vanadium pentoxide and a small amount of impurities be chlorinated, to form chlorinated residues and chlorination flue gas rich in vanadium oxytrichloride; discharging the chlorinated residues through the slag-discharge opening at the lower part of the chlorination fluidized bed body  2 - 2  and the chlorination bed spiral slag-discharging device  2 - 7  in turn; and allowing the chlorination flue gas to be subjected to dust removing by the chlorination bed cyclone separator  2 - 3  and fall back to the chlorination fluidized bed body  2 - 2 , and then be precooled by the flue gas heat exchanger  2 - 4  and enter the flue gas condenser  2 - 5 , such that vanadium oxytrichloride therein is condensed to form a crude vanadium oxytrichloride liquid and the remaining tail gas enters the tail gas washing absorber  5  through the chlorination bed acid-seal tank  2 - 6 ; 
         [0023]    allowing the crude vanadium oxytrichloride liquid formed by the flue gas condenser  2 - 5  to enter the rectifying column  3 - 2  and the distilling still  3 - 1  to be subjected to rectification operation, to obtain a vanadium-rich waste rich in high-boiling-point impurity, silicon-containing vanadium oxytrichloride vapor rich in low-boiling-point impurities and high-purity vanadium oxytrichloride vapor, wherein the vanadium-rich waste is used for the subsequent recovery of vanadium; condensing the silicon-containing vanadium oxytrichloride vapor into liquid by the distillate condenser  3 - 3 , wherein a part of the liquid returns to the rectifying column  3 - 2  through the reflux liquid collecting tank  3 - 4 , and the remaining liquid enters the silicon-containing vanadium oxytrichloride storage tank  3 - 5 ; transmitting the exhaust gas produced in the silicon-containing vanadium oxytrichloride storage tank  3 - 5  to the tail gas washing absorber  5  through the rectification section acid-seal tank  3 - 6 , wherein silicon-containing vanadium oxytrichloride can be applied in the field of chemical engineering such as the field of catalysis; and condensing the high-purity vanadium oxytrichloride vapor into liquid by the high-purity vanadium oxytrichloride condenser  3 - 7  and allowing the liquid to enter the high-purity vanadium oxytrichloride storage tank  3 - 8 ; 
         [0024]    allowing the high-purity vanadium oxytrichloride in the high-purity vanadium oxytrichloride storage tank  3 - 8  to enter the plasma reactor  4 - 3  through the reactant nozzle  4 - 2 ; purifying compressed air by the air filtration purifier  4 - 1  and then allowing the compressed air to enter the plasma reactor  4 - 3  through the reactant nozzle  4 - 2 , such that the vanadium oxytrichloride is oxidized to produce vanadium pentoxide powder and oxidization flue gas rich in chlorine gas; discharging the oxidation product by the feed outlet at the bottom of the plasma reactor  4 - 3  into the primary cyclone separator  4 - 4  for gas-solid separation, and allowing the oxidation flue gas which is produced through separation to be pressurized by the rotary-vane pump  4 - 6  and the gas compressor  4 - 7  and then returned for chlorination of the industrial grade vanadium pentoxide; allowing the vanadium pentoxide powder discharged from the bottom of the primary cyclone separator  4 - 4  together with the purified air from the air filtration purifier  4 - 1  to enter the secondary cyclone separator  4 - 5 , to remove a small amount of chlorine gas entrained by the powder through thorough mixing and gas-solid separation to obtain a high-purity vanadium pentoxide product, and transmitting the product to the high-purity product hopper; allowing chlorine-containing tail gas discharged from the secondary cyclone separator  4 - 5  to enter the tail gas washing absorber  5  for treatment; and transmitting the gas discharged from the tail gas washing absorber  5  after absorption treatment with an alkali solution to the chimney  7  then to vent through the induced draft fan  6 . 
         [0025]    The first characteristic of the present invention lies in that: in the method for producing high-purity vanadium pentoxide powder, in the chlorination fluidized bed body  2 - 2 , the amount of the carbon powder added in the low temperature chlorination process is 10%-20% of the mass of the industrial grade vanadium pentoxide powder; and in the chlorination, the operation temperature is 300-500° C. and the average residence time of the powder is 30-80 min. 
         [0026]    The second characteristic of the present invention lies in that: in the rectifying column  3 - 2 , the number of trays in the rectification section is 5-10, and the number of trays in the stripping section is 10-20 in the rectification operation; and in the rectification operation, the reflux ratio (i.e., the ratio of the quantity of reflux at the top of the column to the amount of the discharged material) is kept at  15 - 40 . 
         [0027]    The third characteristic of the present invention lies in that: in the plasma reactor  4 - 3 , high-purity vanadium pentoxide is prepared directly by plasma oxidation of high-purity vanadium oxytrichloride, and in the plasma oxidation, the amount of the purified air fed is 2-50 times of the theoretical amount of the purified air. 
         [0028]    The purity of the high-purity vanadium pentoxide powder prepared by the present invention is above 4N. 
         [0029]    Compared with the prior art, the present invention has the following outstanding advantages: 
         [0030]    (1) Through heat exchange between the chlorinating gas and the chlorination flue gas, preheating of the chlorinating gas is achieved while the flue gas is cooled, which makes the temperature distribution in the chlorination reactor more uniform, thereby improving the efficiency of low temperature chlorination of vanadium raw material effectively. 
         [0031]    (2) By adding an appropriate amount of air to enable a part of carbon powder to combust, a balanced heat supply and temperature regulation during the chlorination are implemented, thereby stabilizing the operation temperature in the chlorination, increasing the efficiency of the chlorination reaction, ensuring good selectivity in the chlorination, and avoiding side reactions such as generation of vanadium tetrachloride. 
         [0032]    (3) Vanadium oxytrichloride which is purified by distillation is directly oxidized by plasma to produce vanadium pentoxide and chlorine gas. As compared to the traditional hydrolysis precipitation process, not only the production of a large amount of vanadium-containing wastewater is avoided, but also the recycling of chlorine gas is achieved, thereby reducing the consumption of chlorine gas effectively. 
         [0033]    (4) Purified air is used to further remove a small amount of chlorine gas entrained by the vanadium pentoxide product through the cyclone separator, thereby improving the quality of the product effectively. 
         [0034]    The present invention has the advantages of favorable adaptability to a raw material, good selectivity in low temperature chlorination, no discharge of contaminated wastewater, low consumption of chlorine gas, low energy consumption in production and low operation cost, stable product quality and so on, and is suitable for the large scale industrial production of the high-purity vanadium pentoxide powder with a purity of above 4N, with good economic and social benefits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The accompanying drawing is used to provide further illustration of the present invention and constitutes a part of the specification. It is used to explain the present invention together with the examples of the present invention, rather than limit the present invention. 
           [0036]      FIG. 1  is a schematic diagram illustrating the configuration of a system for purifying vanadium pentoxide according to the present invention. 
       
    
    
     REFERENCE SIGNS 
       [0000]    
       
         
           
               1  Feeding device 
               1 - 1  Industrial grade vanadium pentoxide hopper 
               1 - 2  Industrial grade vanadium pentoxide screw feeder 
               1 - 3  Carbon powder hopper 
               1 - 4  Carbon powder screw feeder 
               2  Low temperature chlorination fluidized bed 
               2 - 1  Chlorination bed feeder 
               2 - 2  Chlorination fluidized bed body 
               2 - 3  Chlorination bed cyclone separator 
               2 - 4  Flue gas heat exchanger 
               2 - 5  Flue gas condenser 
               2 - 6  Chlorination bed acid-seal tank 
               2 - 7  Chlorination bed spiral slag-discharging device 
               3  Rectification and purification device 
               3 - 1  Distilling still 
               3 - 2  Rectifying column 
               3 - 3  Distillate condenser 
               3 - 4  Reflux liquid collecting tank 
               3 - 5  Silicon-containing vanadium oxytrichloride storage tank 
               3 - 6  Rectification section acid-seal tank 
               3 - 7  High-purity vanadium oxytrichloride condenser 
               3 - 8  High-purity vanadium oxytrichloride storage tank 
               4  Plasma oxidation device 
               4 - 1  Air filtration purifier 
               4 - 2  Reactant nozzle 
               4 - 3  Plasma reactor 
               4 - 4  Primary cyclone separator 
               4 - 5  Secondary cyclone separator 
               4 - 6  Rotary-vane pump 
               4 - 7  gas compressor 
               5  Tail gas washing absorber 
               6  Induced draft fan 
               7  Chimney 
           
         
       
     
       DETAILED DESCRIPTION OF THE INVENTION 
       [0070]    In order to make the object, technical solution and advantages of the present invention be clearer, the technical solution in the examples of the present invention will be described clearly and completely below with reference to the accompanying drawing of the examples of the present invention. Obviously, the described examples are only a part of the examples of the present invention, not all examples. It is worth noting that the examples are merely used for illustrating the technical solution of the present invention, rather than limiting the present invention.  FIG. 1  is a schematic diagram illustrating a system for purifying vanadium pentoxide according to the present invention. 
         [0071]    Referring to  FIG. 1 , the system for purifying vanadium pentoxide used in this example comprises feeding device  1 , low temperature chlorination fluidized bed  2 , rectification and purification device  3 , plasma oxidation device  4 , tail gas washing absorber  5 , induced draft fan  6  and chimney  7 ; 
         [0072]    wherein the feeding device  1  comprises industrial grade vanadium pentoxide hopper  1 - 1 , industrial grade vanadium pentoxide screw feeder  1 - 2 , carbon powder hopper  1 - 3  and carbon powder screw feeder  1 - 4 ; 
         [0073]    the low temperature chlorination fluidized bed  2  comprises chlorination bed feeder  2 - 1 , chlorination fluidized bed body  2 - 2 , chlorination bed cyclone separator  2 - 3 , flue gas heat exchanger  2 - 4 , flue gas condenser  2 - 5 , chlorination bed acid-seal tank  2 - 6  and chlorination bed spiral slag-discharging device  2 - 7 ; 
         [0074]    the rectification and purification device  3  comprises distilling still  3 - 1 , rectifying column  3 - 2 , distillate condenser  3 - 3 , reflux liquid collecting tank  3 - 4 , silicon-containing vanadium oxytrichloride storage tank  3 - 5 , rectification section acid-seal tank  3 - 6 , high-purity vanadium oxytrichloride condenser  3 - 7 , and high-purity vanadium oxytrichloride storage tank  3 - 8 ; 
         [0075]    the plasma oxidation device  4  comprises air filtration purifier  4 - 1 , reactant nozzle  4 - 2 , plasma reactor  4 - 3 , primary cyclone separator  4 - 4 , secondary cyclone separator  4 - 5 , rotary-vane pump  4 - 6  and gas compressor  4 - 7 ; 
         [0076]    wherein a feed outlet at the bottom of the industrial grade vanadium pentoxide hopper  1 - 1  is connected with a feed inlet of the industrial grade vanadium pentoxide screw feeder  1 - 2 ; a feed outlet at the bottom of the carbon powder hopper  1 - 3  is connected with a feed inlet of the carbon powder screw feeder  1 - 4 ; and a feed outlet of the industrial grade vanadium pentoxide screw feeder  1 - 2  and a feed outlet of the carbon powder screw feeder  1 - 4  are both connected with a feed inlet of the chlorination bed feeder  2 - 1  through a pipeline; 
         [0077]    a feed discharge opening of the chlorination bed feeder  2 - 1  is connected with a feed inlet at the upper part of the chlorination fluidized bed body  2 - 2  through a pipeline; a gas inlet at the bottom of the chlorination bed feeder  2 - 1  is connected with a nitrogen gas source main pipe through a pipeline; the chlorination bed cyclone separator  2 - 3  is provided at the center of the top of the expansion section of the chlorination fluidized bed body  2 - 2 ; a gas outlet at the top of the chlorination bed cyclone separator  2 - 3  is connected with a hot flue gas inlet of the flue gas heat exchanger  2 - 4  through a pipeline; a cold flue gas outlet of the flue gas heat exchanger  2 - 4  is connected with a gas inlet of the flue gas condenser  2 - 5  through a pipeline; a gas outlet of the flue gas condenser  2 - 5  is connected with a gas inlet of the chlorination bed acid-seal tank  2 - 6  through a pipeline; a gas outlet of the chlorination bed acid-seal tank  2 - 6  is connected with a gas inlet of the tail gas washing absorber  7  through a pipeline; a slag-discharge opening at the lower part of the chlorination fluidized bed body  2 - 2  is connected with a feed inlet of the chlorination bed spiral slag-discharging device  2 - 7  through a pipeline; a gas inlet at the bottom of the chlorination fluidized bed body  2 - 2  is connected with a hot gas outlet of the flue gas heat exchanger  2 - 4  through a pipeline; and a cold gas inlet of the flue gas heat exchanger  2 - 4  is connected with a chlorine gas source main pipe, the nitrogen gas source main pipe and a compressed air main pipe through a pipeline, respectively; 
         [0078]    a liquid outlet at the bottom of the flue gas condenser  2 - 5  is connected with a feed inlet of the rectifying column  3 - 2  through a pipeline; a steam outlet of the distilling still  3 - 1  is connected with a steam inlet of the rectifying column  3 - 2  through a pipeline; a reflux inlet of the distilling still  3 - 1  is connected with a liquid reflux outlet at the bottom of the rectifying column  3 - 2  through a pipeline; a gas outlet at the top of the rectifying column  3 - 2  is connected with a gas inlet of the distillate condenser  3 - 3  through a pipeline; a liquid outlet of the distillate condenser  3 - 3  is connected with a liquid inlet of the reflux liquid collecting tank  3 - 4  through a pipeline; a reflux liquid outlet of the reflux liquid collecting tank  3 - 4  is connected with a reflux liquid inlet at the top of the rectifying column  3 - 2  through a pipeline; a feed discharge opening of the reflux liquid collecting tank  3 - 4  is connected with an inlet of the silicon-containing vanadium oxytrichloride storage tank  3 - 5  through a pipeline; an exhaust gas outlet of the silicon-containing vanadium oxytrichloride storage tank  3 - 5  is connected with a gas inlet of the rectification section acid-seal tank  3 - 6  through a pipeline; a gas outlet of the rectification section acid-seal tank  3 - 6  is connected with a gas inlet of the tail gas washing absorber  7  through a pipeline; a rectificate outlet of the rectifying column  3 - 2  is connected with a gas inlet of the high-purity vanadium oxytrichloride condenser  3 - 7  through a pipeline; a liquid outlet of the high-purity vanadium oxytrichloride condenser  3 - 7  is connected with a liquid inlet of the high-purity vanadium oxytrichloride storage tank  3 - 8  through a pipeline; and an underflow outlet is provided at the bottom of the distilling still  3 - 1 ; 
         [0079]    a gas inlet of the air filtration purifier  4 - 1  is connected with the compressed air main pipe through a pipeline; a gas outlet of the air filtration purifier  4 - 1  is connected with an air inlet of the reactant nozzle  4 - 2  and a gas inlet of the secondary cyclone separator  4 - 5  through pipelines, respectively; a liquid outlet of the high-purity vanadium oxytrichloride storage tank  3 - 8  is connected with a chloride inlet of the reactant nozzle  4 - 2  through a pipeline; the reactant nozzle  4 - 2  is provided at the center of the upper part of the plasma reactor  4 - 3 ; a feed outlet at the bottom of the plasma reactor  4 - 3  is connected with a gas inlet of the primary cyclone separator  4 - 4  through a pipeline; a gas outlet of the primary cyclone separator  4 - 4  is connected with a gas inlet of the rotary-vane pump  4 - 6  through a pipeline; a gas outlet of the rotary-vane pump  4 - 6  is connected with a gas inlet of the gas compressor  4 - 7  through a pipeline; a gas outlet of the gas compressor  4 - 7  is connected with a cold gas inlet of the flue gas heat exchanger  2 - 4  through a pipeline; a feed discharge opening at the lower part of the primary cyclone separator  4 - 4  is connected with a gas inlet of the secondary cyclone separator  4 - 5  through a pipeline; a gas outlet at the top of the secondary cyclone separator  4 - 5  is connected with a gas inlet of the tail gas washing absorber  5  through a pipeline; and a feed outlet at the bottom of the secondary cyclone separator  4 - 5  is connected with a high-purity vanadium pentoxide product hopper through a pipeline; 
         [0080]    a gas outlet of the tail gas washing absorber  5  is connected with a gas inlet of the induced draft fan  6  through a pipeline; and a gas outlet of the induced draft fan  6  is connected with a gas inlet at the bottom of the chimney  7  through a pipeline. 
         [0081]    The above system is used in this example to purify vanadium pentoxide. The specific method comprises the following steps. Industrial grade vanadium pentoxide powder in the industrial grade vanadium pentoxide hopper  1 - 1  and carbon powder in the carbon powder hopper  1 - 3  enter the chlorination bed feeder  2 - 1  simultaneously through the industrial grade vanadium pentoxide screw feeder  1 - 2  and the carbon powder screw feeder  1 - 4  respectively and are mixed therein, and then enter the chlorination fluidized bed body  2 - 2 ; chlorine gas from the chlorine gas source main pipe, nitrogen gas from the nitrogen gas source main pipe and air from the compressed air main pipe are preheated by exchanging heat with chlorination flue gas by the flue gas heat exchanger  2 - 4 , and then enter the chlorination fluidized bed body  2 - 2  to allow the vanadium pentoxide, the carbon powder and other powder materials at a fluidized state and chemically reacted, wherein the air enables a part of the carbon powder to combust to provide heat for maintaining the temperature of the fluid bed, and the chlorine gas and the carbon powder function together to make vanadium pentoxide and a small amount of impurities be chlorinated, to form chlorinated residues and chlorination flue gas rich in vanadium oxytrichloride; the chlorinated residues are discharged through the slag-discharge opening at the lower part of the chlorination fluidized bed body  2 - 2  and the chlorination bed spiral slag-discharging device  2 - 7  in turn; and the chlorination flue gas is subjected to dust removing by the chlorination bed cyclone separator  2 - 3  and falls back to the chlorination fluidized bed, and then is precooled by the flue gas heat exchanger  2 - 4  and enters the flue gas condenser  2 - 5 , such that vanadium oxytrichloride therein is condensed to form a crude vanadium oxytrichloride liquid and the remaining tail gas enters the tail gas washing absorber  5  through the chlorination bed acid-seal tank  2 - 6 ; 
         [0082]    the crude vanadium oxytrichloride liquid formed by the flue gas condenser  2 - 5  enters the rectifying column  3 - 2  and the distilling still  3 - 1  to be subjected to rectification operation, to obtain a vanadium-rich waste rich in high-boiling-point impurities, silicon-containing vanadium oxytrichloride vapor rich in low-boiling-point impurities and high-purity vanadium oxytrichloride vapor, wherein the vanadium-rich waste is used for the subsequent recovery of vanadium; the silicon-containing vanadium oxytrichloride vapor is condensed into liquid by the distillate condenser  3 - 3 , wherein a part of the liquid returns to the rectifying column  3 - 2  through the reflux liquid collecting tank  3 - 4 , and the remaining liquid enters the silicon-containing vanadium oxytrichloride storage tank  3 - 5 ; the exhaust gas produced in the silicon-containing vanadium oxytrichloride storage tank  3 - 5  is transmitted to the tail gas washing absorber  5  through the rectification section acid-seal tank  3 - 6 , wherein the silicon-containing vanadium oxytrichloride can be applied in the field of chemical engineering such as the field of catalysis; and the high-purity vanadium oxytrichloride vapor is condensed into liquid by the high-purity vanadium oxytrichloride condenser  3 - 7  and then enters the high-purity vanadium oxytrichloride storage tank  3 - 8 ; 
         [0083]    the high-purity vanadium oxytrichloride in the high-purity vanadium oxytrichloride storage tank  3 - 8  enters the plasma reactor  4 - 3  through the reactant nozzle  4 - 2 ; compressed air is purified by the air filtration purifier  4 - 1  and then enters the plasma reactor  4 - 3  through the reactant nozzle  4 - 2 , such that the vanadium oxytrichloride is oxidized to produce vanadium pentoxide powder and oxidization flue gas rich in chlorine gas; the oxidation product is discharged by the feed outlet at the bottom of the plasma reactor  4 - 3  into the primary cyclone separator  4 - 4  for gas-solid separation, and the oxidation flue gas which is produced through separation is pressurized by the rotary-vane pump  4 - 6  and the gas compressor  4 - 7  and then returned for chlorination of the industrial grade vanadium pentoxide; the vanadium pentoxide powder discharged from the bottom of the primary cyclone separator  4 - 4  together with the purified air from the air filtration purifier  4 - 1  enters the secondary cyclone separator  4 - 5 , to remove a small amount of chlorine gas entrained by the powder through thorough mixing and gas-solid separation to obtain a high-purity vanadium pentoxide product, and the product is transmitted to the high-purity product hopper; chlorine-containing tail gas discharged from the secondary cyclone separator  4 - 5  enters the tail gas washing absorber  5  for treatment; and the gas discharged from the tail gas washing absorber  5  after absorption treatment with an alkali solution is transmitted to the chimney  7  then to vent through the induced draft fan  6 . 
         [0084]    In this example, the industrial grade vanadium pentoxide powder was used as the raw material and its chemical composition is shown in Table 1. The throughput is 70 kg/h, and the high-purity vanadium pentoxide product was purified and prepared by low temperature chlorination, rectification of vanadium oxytrichloride and plasma oxidation. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Chemical composition of the industrial grade vanadium 
               
               
                 pentoxide raw material used in the example (wt %) 
               
             
          
           
               
                 V 2 O 5   
                 Si 
                 Ca 
                 Al 
                 Ti 
                 Fe 
                 Mn 
                 Na 
                 K 
                 S 
               
               
                   
               
               
                 98.8 
                 0.0150 
                 0.0275 
                 0.0099 
                 0.0260 
                 0.0971 
                 0.0293 
                 0.1385 
                 0.0714 
                 0.1274 
               
               
                   
               
             
          
         
       
     
         [0085]    The operation conditions are as follows: in the chlorination fluidized bed body  2 - 2 , the amount of the carbon powder added in the low temperature chlorination process is 20% of the mass of the industrial grade vanadium pentoxide powder, and in the chlorination, the operation temperature is 300° C. and the average residence time of the powder is 80 min; in the rectifying column  3 - 2 , the number of trays in the rectification section is 5, and the number of trays in the stripping section is 10 in the rectification operation, and the reflux ratio of the rectification operation is 40; in the plasma reactor  4 - 3 , the amount of the air fed is 2 times of the theoretical amount of the air in the plasma oxidation. Under such operation conditions, the direct recovery rate of vanadium reached 20%, and the purity of the high-purity vanadium pentoxide product reached 99.998 wt % (4N8). 
         [0086]    The operation conditions are as follows: in the chlorination fluidized bed body  2 - 2 , the amount of the carbon powder added in the low temperature chlorination process is 10% of the mass of the industrial grade vanadium pentoxide powder, and in the chlorination, the operation temperature is 500° C. and the average residence time of the powder is 30 min; in the rectifying column  3 - 2 , the number of trays in the rectification section is 10, and the number of trays in the stripping section is 20 in the rectification operation, and the reflux ratio of the rectification operation is 15; in the plasma reactor  4 - 3 , the amount of the air fed is 50 times of the theoretical amount of the air in the plasma oxidation. Under such operation conditions, the direct recovery rate of vanadium reached 85%, and the purity of the high-purity vanadium pentoxide product reached 99.9995 wt % ( 5 N). 
         [0087]    The details which are not illustrated in detail in the present invention belong to the well-known technologies in the art. 
         [0088]    Of course, the present invention can also provide a variety of examples. According to the disclosure of the present invention, those skilled in the art can make various corresponding changes and transformations without departing from the spirit and essence of the present invention; however, these corresponding changes and transformations shall all fall within the protection scope of the claims of the present invention.