Patent Publication Number: US-5296212-A

Title: Method for the removal of sulfur from phosphorus

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for purifying phosphorus, as a result of which the area of application of products produced from phosphorus, can be expanded due to a great reduction in sulfur content. 
     Generally, organic impurities up to 0.3% are contained in technical grade white phosphorus. These impurities interfere in some areas of application with further processing and must therefore be removed before subsequent reaction. According to GDR patent 76,482, removal of the interfering organic impurity is possible by a treatment of the phosphorus with sulfuric acid. The phosphorus to be purified is intensively mixed briefly with highly concentrated sulfuric acid in a special apparatus. During this mixing, an emulsion is formed. In order to separate the purified phosphorus rapidly from the emulsion, the latter is passed into warm water. A rapid separation of the phosphorus is required in order to avoid explosive reactions between phosphorus and sulfuric acid. To remove enclosed sulfuric acid, the phosphorus is passed through a packed column. Due to its higher specific gravity, the phosphorus sinks countercurrently to the warm water. With this method, the phosphorus is largely freed from organic impurities. 
     A relatively large increase in sulfur content of the phosphorus, purified in this manner, is due to the reaction of the sulfuric acid with the phosphorus during the purification method and is disadvantageous for some applications. In general, concentrations of 500 ppm and, in rare cases, concentrations as high as 2,000 ppm are attained. In a side reaction, a small portion of the sulfuric acid is reduced by the phosphorus and therefore leads to an increase in the sulfur content of the phosphorus. According to an older purification method, the phosphorus is also treated with concentrated sulfuric acid. The sulfuric acid, however, is separated off on the basis of its higher specific gravity and without the addition of water. This method also leads to a large increase in the sulfur content of the phosphorus. With this procedure, sulfur contents of more than 500 ppm have been detected. In certain applications, the high sulfur content leads to undesirable side reactions. For example, hydrogen sulfide is formed as a by-product when phosphorus is converted to phosphane. It is also disturbing that such phosphorus, upon contact with water, gives off poisonous hydrogen sulfide and thereby makes handling and transport more difficult. No method for removing sulfur from white phosphorus, which is suitable for industrial application, is described in the literature. Since phosphorus is an extremely reactive chemical, problems were to be expected with the removal of sulfur. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method, easily realizable technically, for removing sulfur from white phosphorus, which has been purified with sulfuric acid. 
     Surprisingly, it has been found that sulfur, remaining in phosphorus after purification of the phosphorus with concentrated sulfuric acid, can easily and rapidly be removed by a treatment of the liquid phosphorus with water, aqueous solutions or the vapors of these liquids at temperatures above the melting point of phosphorus. As expected, the reaction time can be shortened at higher temperatures. Treatment with a weakly alkaline washing liquid is particularly advantageous for the removal of the sulfur. Strongly alkaline solutions are less suitable because of the greater formation of phosphane. 
     This method of reducing the sulfur content can also be employed if, after the sulfuric acid treatment, the acid has been separated off because of its higher specific gravity and without the addition of water or if the phosphorus contains sulfur for other reasons. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is explained below by means of three examples. 
     EXAMPLE 1 
     White phosphorus (80 g), which had been treated with concentrated sulfuric acid to remove organic impurities and contained 25 ppm of oil and 1,500 ppm of sulfur, was contacted intensively for 1.5 hours at 75° C. with 150 mL of water in a sulfiting flask, which was equipped with a reflux condenser, thermometer and stirrer and had a capacity of 500 mL. After the phosphorus had cooled down, the water was removed. After the treatment, the phosphorus contained 25 ppm of oil and 40 ppm of sulfur. 
     EXAMPLE 2 
     In each case, 100 g of white phosphorus, which had been treated with concentrated sulfuric acid to remove organic impurities and contained 20 ppm of oil and 1250 ppm or 1600 ppm of sulfur, was treated in the same apparatus as in Example 1 with an aqueous solution. 
     The following cleaning effects were achieved: 
     
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                                  Sulfur                                  
                                        Sulfur Content                    
       Concen-                                                            
            Washing                                                       
                 pH of      Treatment                                     
                                  Content                                 
                                        After                             
Dissolved                                                                 
       tration                                                            
            Solution                                                      
                 Washing                                                  
                      Treatment                                           
                            Time  Before                                  
                                        Treatment                         
Material                                                                  
       %    in mL                                                         
                 Solution                                                 
                      Temp. h     Treatment                               
                                        ppm                               
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NaHCO.sub.3                                                               
       1.0  100  9    75    1.0   1600  &lt;10                               
NaHCO.sub.3                                                               
       1.0   33  9    60    0.75  1600  40                                
NaHCO.sub.3                                                               
       1.0   33  9    90    0.5   1600  25                                
NaCH.sub.3 COO                                                            
       3.0  100  10   75    1.0   1600  10                                
NaCl   3.0  100  6    75    1.5   1250  50                                
H.sub.2 O.sub.2                                                           
       0.5  100  5    75    1.0   1250  140                               
NH.sub.4 OH                                                               
       1.0  100  10   75    1.0   1250  &lt;10                               
HCl    3.0  100  1    75    1.0   1250  140                               
CH.sub.3 OH                                                               
       10.0 100  6    60    1.0   1600  60                                
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     EXAMPLE 3 
     White phosphorus (100 g), which had been treated with concentrated sulfuric acid to remove organic impurities and contained 20 ppm of oil and 500 ppm of sulfur, was continuously passed over a period of 1 hour into a packed column (diameter 32 mm, length 1,500 mm). The phosphorus, dripping down, was washed countercurrently and uniformly with steam, which was generated by evaporating 80 g of water. The phosphorus, so treated, was drawn off into a container at the bottom of the column. It had a sulfur content of 130 ppm.