Abstract:
In order to provide a method for producing lithium hexafluorophosphate capable of producing lithium hexafluorophosphate of a higher purity than in the related art without the necessity for after-treatment for removal of impurities, a method is characterized by filtering lithium hexafluorophosphate coexisting with a solvent and then carrying out after-filtering drying in a gas atmosphere containing PF 5 .

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing lithium hexafluorophosphate, and specifically relates to a method for producing lithium hexafluorophosphate useful as an electrolyte for lithium secondary batteries and a catalyst for organic synthesis. 
     2. Description of the Related Art 
     Conventionally, lithium hexafluorophosphate is produced by the following steps: 
     1) Lithium fluoride (LiF) is dissolved in hydrofluoric acid anhydride to give a solution. 
     2) PF 5  gas is blown into this solution. This operation causes LiF to react with PF 5  to produce lithium hexafluorophosphate. 
     3) The solution is cooled to deposit lithium hexafluorophosphate. 
     4) The solution is filtered to separate the deposited lithium hexafluorophosphate. 
     5) The separated lithium hexafluorophosphate is then dried to yield lithium hexafluorophosphate as a product. 
     6) Following this step, a product of lithium hexafluorophosphate is dissolved in an organic solvent, for example, to for use as an electrolyte for a lithium battery. 
     However, according to the conventional related method for producing lithium hexafluorophosphate, many kinds of impurities such as lithium fluoride and oxyfluoride acid compound are contained in hexafluorophosphate. 
     When lithium hexafluorophosphate containing such impurities is dissolved in an organic solvent for use as an electrolyte for a lithium battery, lithium fluoride contaminate is not soluble in the organic solvent, thereby requiring filtering of the electrolyte. 
     Examples of methods for removing these impurities are a method using an ion exchange resin where lithium hexafluorophosphate is dissolved in organic solvent to remove impurities (Japanese Published Unexamined Patent Application No. 1984-87774) and a method where neutralization treatment is carried out in an organic solvent (1984-81870.) However, these methods are complex in operation and provide low productivity. 
     As described above, related methods for producing lithium hexafluorophosphate require complex after-treatment. Even if after-treatment is performed, lithium hexafluorophosphate of high purity cannot be produced, which is problematic. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for producing lithium hexafluorophosphate capable of yielding lithium hexafluorophosphate of high purity without after-treatment. The method for producing lithium hexafluorophosphate according to the present invention is characterized by filtering lithium hexafluorophosphate coexisting with the solvent and then drying after filtering in a gas atmosphere containing PF 5 . 
     Organic solvents that may be used as the solvent are solvents that do not react with hydrofluoric acid anhydride or lithium hexafluorophosphate. 
     A state where lithium hexafluorophosphate coexists with the solvent is a state in which, the following steps are completed: 
     1) Lithium fluoride (LiF) is dissolved in hydrofluoric acid anhydride to give a solution. 
     2) PF 5  gas is blown into this solution. This operation causes LiF and PF 5  to react to produce lithium hexafluorophosphate. 
     3) The solution is cooled to deposit lithium hexafluorophosphate. 
     In the present invention, the step of filtering the solution is performed in a gas atmosphere containing PF 5 . 
     Gases that may be employed as the gas containing PF 5  (atmosphere gas, hereafter) are gases that are prepared by diluting PF 5  with a gas (for example, HF gas, HCl gas, nitrogen gas, argon gas, helium gas, xenon gas, dried air, a gas that does not react with PF 5  or lithium hexafluorophosphate.) 
     The concentration of PF 5  in the atmosphere gas is preferably 1 to 50 mol %, with 1 to 20 mol % being more preferable, and 1 to 10 mol % being still more preferable. A concentration less than 1 mol % may result in the effects being insufficient. A concentration of over 50 mol % causes an increase in PF 5  consumption and is therefore expensive. 
     On the other hand, 0.003 to 0.3 kg of PF 5  is preferable for 1 kg of lithium hexafluorophosphate, with of 0.003 to 0.03 kg of PF 5  being more preferable. Less than 0.003 kg of PF 5  may not cause effects that are sufficient. An amount over 0.3 kg causes an increase in PF 5  consumption which is expensive. 
     Meanwhile, the concentration of impurities, particularly moisture, contained in the atmosphere gas is preferably 100 ppb (parts per billion) or smaller, more preferably 10 ppb or smaller, and still more preferably 1 ppb or smaller. Such control brings about a yield of lithium hexafluorophosphate of high purity. 
     The pressure of the atmosphere gas may be normal pressure. The temperature may also be normal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a conceptual diagram of the filtering apparatus used in the present invention; and 
         FIG. 2  is a conceptual diagram of the drying apparatus used in the present invention. 
       Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 of the Present Invention 
     Hydrofluoric acid anhydride solution containing 22% lithium hexafluorophosphate was cooled at −20° C. to deposit crystals of lithium hexafluorophosphate. 
     Next, this slurry was transferred to a filtering apparatus (20 L (liter) volume) with inlet and outlet orifices for the flow of the atmosphere gas. The slurry was filtered for 10 minutes in a flow of an atmosphere gas consisting of 3 mol % of PF 5  and 97 mol % of nitrogen at a flow rate of 15 L/min., dried in the same atmosphere gas at room temperature for 25 min., and finally heated at 105° C. overnight (for example, about 6 to 8 hours). 
     The amount of the lithium hexafluorophosphate crystal yielded was 11.3 kg and impurities were 50 ppm in HF and 70 ppm in LiF. 
     The amount of PF 5  used was 0.008 kg per 1 kg of lithium hexafluorophosphate. 
     COMPARATIVE EXAMPLE 
     Hydrofluoric acid anhydride solution containing 22% lithium hexafluorophosphate was cooled at −20° C. to deposit a crystalline form of lithium hexafluorophosphate. 
     Next, this slurry was transferred to the filtering apparatus (20 L (liter) volume) with inlet and outlet orifices for the atmosphere gas. The slurry was filtered for 10 minutes in the flowing atmosphere gas. The atmosphere gas consisted exclusively of nitrogen and was fed at a flow rate of 15 L/min. The slurry was then dried in the atmosphere gas at room temperature for 25 min., and finally heated at 105° C. overnight. 
     The amount of the lithium hexafluorophosphate crystal yielded was 11.1 kg and impurities were 58 ppm in HF and 930 ppm in LiF. 
     The slurry decomposed during filtering and drying, resulting in an increase in LiF as an impurity. 
     Embodiment 2 
     Hydrofluoric acid anhydride solution containing 22% of lithium hexafluorophosphate was cooled at −20° C. to deposit a crystalline form of lithium hexafluorophosphate. 
     Next, this slurry was transferred to the filtering apparatus (100 L (liter) volume) with inlet and outlet orifices for the atmosphere gas. The slurry is filtered for 60 minutes with the atmosphere gas consisting of 15 mol % of PF 5  and 85 mol % of nitrogen flowing thereover at a flow rate of 12 L/min., dried in the same atmosphere gas at room temperature for 30 min., and further dried for 90 min. in the flow of an atmosphere gas consisting of 5 mol % of PF 5  and 95 mol % of nitrogen at a 35 L/min. flow rate, and finally heated at 105° C. overnight. 
     The amount of the lithium hexafluorophosphate crystal yielded was 57 kg and impurities were 52 ppm in HF and 90 ppm in LiF. 
     The amount of PF 5  used was 0.022 kg per 1 kg of lithium hexafluorophosphate. 
     COMPARATIVE EXAMPLE 2 
     Hydrofluoric acid anhydride solution containing 22% lithium hexafluorophosphate was cooled at −20° C. to deposit a crystalline form of lithium hexafluorophosphate. 
     Next, this slurry was transferred to filtering apparatus (100 L (liter) volume) with inlet and outlet orifices for the atmosphere gas. The slurry was filtered for 60 minutes in the flow of the atmosphere gas. The atmosphere gas consisted exclusively of nitrogen at a flow rate of 12 L/min. Then the slurry was dried in the atmosphere gas at room temperature for 30 min., and finally heated at 105° C. overnight. 
     The amount of the lithium hexafluorophosphate crystal yielded was 54 kg and impurities were 54 ppm in HF and 1190 ppm in LiF. 
     The slurry decomposed during filtering and drying, resulting in an increase in LiF as an impurity. 
     A filtering apparatus  10 , used in performing the method of the present invention, is shown in FIG.  1 . Filtering apparatus  10  for treating lithium hexafluorophosphate crystals  12  includes a filter housing  14 , a filter plate  16 , a solvent  18 , an atmospheric gas inlet  2 , an exhaust gas outlet  22 , a crystal outlet orifice  24 , and a filtrate outlet  26 . 
     A drying apparatus  30 , used in performing the method of the present invention is shown in FIG.  2 . Drying apparatus  30  for drying lithium hexafluorophosphate crystals  12  includes a dryer housing  32 , an atmospheric gas inlet  34 , an exhaust gas outlet  36 , a crystal outlet orifice  38 , a heating vessel  40 , a heating medium inlet  42 , and a heating medium outlet  44 . 
     INDUSTRIAL APPLICABILITY 
     According to the present invention without after-treatment, lithium hexafluorophosphate of a higher purity than that produced conventionally can be yielded. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.