Patent Publication Number: US-6033642-A

Title: Method for producing silicon tetrafluoride from uranium oxyfluoride

Description:
FIELD OF INVENTION 
     This invention relates to a method for producing silicon tetrafluoride and more particularly to a method for producing silicon tetrafluoride from uranium oxyfluoride without radioactive carryover. 
     BACKGROUND OF INVENTION 
     Silicon tetrafluoride is widely used in the manufacturer of semi-conductor chips, pyrogenic silica, and other industrially important chemicals. 
     Silicon tetrafluoride can be produced in several ways all of which are based on reacting silica (SiO 2 ) with either hydrofluoric acid (HF) or fluorosilicic acid (H 2  SiF 6 ). Thus to produce SiF 4  from SiO 2 , the production of either hydrofluoric or fluorosilicic acid intermediate is required. 
     Current processes to produce SiF 4 , in varying degrees of purity, include the reaction of silica with hydrogen fluoride gas according to the reaction: 
     
         SiO.sub.2 (s)+4HF(g)→SiF.sub.4 (g)+2H.sub.2 O 
    
     See U.S. Pat. No. 4,382,071. 
     Purity of the SiF 4  is dependent on the source of the silica and hydrogen fluoride reagents. The reaction is typically carried out at 25°-55° C. in concentrated sulfuric acid (&gt;80% H 2  SO 4 ) in order to diminish the reverse reaction through capture of the product H 2  O. This process also uses large amounts of HF which raises concerns for corrosion, safety and environmental management. 
     Production of SiF 4  from fluorosilicic acid can be accomplished according to the reaction: 
     
         SiO.sub.2 (s)+2H.sub.2 SiF.sub.6 (aq)→3SiF.sub.4 (g)+2H.sub.2 O 
    
     See U.S. Pat. No. 4,470,959. This reaction is also carried out in concentrated sulfuric acid (&gt;80% H 2  SO 4 ), but usually at a slightly higher temperature, between 25°-95° C. 
     It is also possible to produce SiF 4  directly from fluorosilicic acid by thermal decomposition: 
     
         H.sub.2 SiF.sub.6 (aq)→SiF.sub.4 (g)+2HF(g) 
    
     However, typical input fluorosilicic acid (20-30% aqueous) comes from fertilizer and phosphoric acid/super phosphate manufacturing waste tails. The fluorosilicic acid is generally low grade containing many impurities such as phosphorous, nitrogen and sulfur, all of which are detrimental to producing high purity SiF 4 . 
     Yet another multi-step process for producing SiF 4  utilizes the reaction of fluorosilicic acid with sodium fluoride and silicon dioxide according to the reaction: 
     
         2H.sub.2 SiF.sub.6 (aq)+6 NaF+SiO.sub.2 →3Na.sub.2 SiF.sub.6 (s)+2H.sub.2 O 
    
     followed by thermal treatment of the fluorosilicate salt at 600° C. to release SiF 4  according to the reaction: 
     
         Na.sub.2 SiF.sub.6 →SiF.sub.4 (g)+2NaF 
    
     See U.S. Pat. No. 4,615,872. 
     As with the processes discussed above, this introduces impurities through use of low grade fluorosilicic acid diminishing the purity of the silicon tetrafluoride produced. 
     However, uranium is a naturally occurring element which is comprised of approximately 0.7% 235  U and 99.3% 238  U.  235  U is used to produce Nuclear Energy, while  238  U is not. Because of the low percentage of  235  U found in naturally occurring uranium, naturally occurring uranium must be enriched in order to obtain sufficient amounts of  235  U which will support nuclear fission. This enrichment process, aside from producing high concentrations of  235  U, produces huge amounts of depleted uranium hexafluoride (UF 6  by-product which is a very hazardous compound posing a serious health threat. Since depleted uranium metal is radioactive and expensive to produce, it is used in limited quantities for highly specialized applications. Accordingly, alternative uses are needed in order to avoid having to dispose of the UF 6  at great expense by storing it indefinitely. 
     One solution to reducing the large stores of UF 6  is to reduce UF 6  to UO 2  F 2  and convert the UO 2  F 2  into SiF 4 , and an oxide of uranium, e.g. UO 2 ,UO 3  or U 3  O 8 . 
     One use for uranium oxide is to add it to the concrete which is used to build bunkers in which radioactive waste is stored in order to provide high density shielding for the stored high level radioactive waste. Uranium oxide possesses outstanding radioactive shielding properties, and when added to concrete in the form of aggregate makes a very effective, low cost shielding material. 
     Thus, the use of UO 2  F 2  as the fluorine source for production of silicon tetrafluoride eliminates the problems of impurities introduced by the use of fluorosilicic acid. UO 2  F 2  is a relatively inert solid that is easy to handle with proper precaution for containment of low level radioactive materials. UO 2  F 2  is also produced in very high purity, being derived from the highly refined UF 6 . Thus, the use of UO 2  F 2  has both technical and economic advantages in the production of high purity SiF 4 . 
     SUMMARY OF INVENTION 
     It is therefore an object of this invention to provide an improved method for producing silicon tetrafluoride. 
     It is a further object of the present invention to provide such a method which produces silicon tetrafluoride from uranium oxyfluoride. 
     It is a further object of the present invention to provide such a method which produces silicon tetrafluoride from uranium oxyfluoride without radioactive carryover. 
     It is a further object of the present invention to provide such a method which produces silicon tetrafluoride which is essentially pure. 
     It is a further object of the present invention to provide such a method which completely reacts the uranium oxyfluoride into uranium oxide. 
     It is a further object of the present invention to provide such a method which is cost effective to implement. 
     The invention results from the realization that a truly efficient and cost effective method for producing silicon tetrafluoride can be achieved by combining uranium oxyfluoride and silicon dioxide as an oxidizing agent, silicon dioxide having a lower thermodynamic stability than any oxide of uranium produced, in stoichiometric proportion so that when the combination is heated to a temperature below the melting point of the uranium oxyfluoride, the uranium oxyfluoride completely reacts with the silicon dioxide to produce silicon tetrafluoride which is free from radioactive carryover and is essentially pure. 
     The invention features a method for producing silicon tetrafluoride including combining uranium oxyfluoride and silicon dioxide, heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium and removing the silicon tetrafluoride. 
     In a preferred embodiment combining may include combining the uranium oxyfluoride and the silicon dioxide in stoichiometric proportion. An oxidizing co-reagent may be introduced to the reaction. Removing may include scrubbing the silicon tetrafluoride. Scrubbing may include reacting the silicon tetrafluoride with sodium fluoride. Scrubbing may include reacting the silicon tetrafluoride with an alkaline metal fluoride the alkaline metal fluoride may include potassium fluoride, lithium fluoride or barium fluoride. Heating may include heating the combination in the temperature range of 400° C. to 1000° C. The oxide of uranium oxide produced may include U 3  O 8 . The oxide of uranium may include UO 2 . The uranium oxyfluoride may include powdered uranium oxyfluoride. The silicon dioxide may include fumed silica. The silicon dioxide may include a mixture of quartz, cristobalite, tridymite, lechatelierite, stishovite, chalcedony, or coesite. The silicon dioxide may include silica glass. 
     The invention also features silicon tetrafluoride produced by combining uranium oxyfluoride and silicon dioxide, heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce non-radioactive silicon tetrafluoride and an oxide of uranium, and removing the silicon tetrafluoride. 
     The invention also features silicon tetrafluoride produced by combining uranium oxyfluoride and silicon dioxide, heating the combination below the melting point of the uranium oxyfluoride to sufficiently react the uranium oxyfluoride and the silicon dioxide to produce a non-radioactive fluorine compound and an oxide of uranium, and removing the uranium oxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
     FIG. 1 is a block diagram of the method for producing silicon tetrafluoride according to the present invention; 
     FIG. 2 is a flow chart of the method for producing silicon tetrafluoride according to the present invention; 
     FIG. 3 is a schematic diagram of the apparatus used to produce the silicon tetrafluoride according to the present invention; 
     FIG. 4A is a representation of an x-ray diffraction analysis performed on sodium hexafluorosilicate obtained from the trap residue produced by the apparatus of FIG. 3 according to the method of the present invention; 
     FIG. 4B is a representation of a standard x-ray diffraction pattern for Na 2  SiF 6  to verify the presence of silicon tetrafluoride in the residue of FIG. 4A; 
     FIG. 5A is a representation of an x-ray diffraction analysis, similar to FIG. 4, performed on the reaction residue produced by the apparatus of FIG. 3; 
     FIG. 5B is a representation of a standard x-ray diffraction pattern for U 3  O 8  to verify the presence of uranium oxide in the residue of FIG. 5A; 
     FIG. 6 is a schematic diagram, similar to FIG. 3, of the apparatus used to produce SiF 4  with the addition of oxygen (as dry air) to the reaction; 
     FIG. 7A is a representation of an x-ray diffraction analysis, similar to FIG. 4A, performed on the trap residue produced by the apparatus of FIG. 6 according to the method of the present invention; 
     FIG. 7B is a representation of a standard diffraction pattern for Na 2  SiF 6  to verify the presence of SiF 4  in the residue of FIG. 7A; 
     FIG. 8A is a representation of an x-ray diffraction analysis, similar to FIG. 5A, performed on the reaction residue produced by the apparatus of FIG. 6; and 
     FIG. 8B is a representation of a standard x-ray diffraction for U 3  O 8  to verify the presence of uranium oxide in the residue of FIG. 8A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Uranium hexafluoride (UF 6 ) is reacted with H 2  O to produce uranium oxyfluoride (UO 2  F 2 ) 10, FIG. 1. The UF 6  reacts with the H 2  O, not necessarily steam, when placed in close proximity to one another such that UO 2  F 2  precipitates from the air directly as the UF 6  reacts with the water vapor. The UO 2  F 2  10 is combined with silicon dioxide (SiO 2 ) 12 as an oxidizing agent. SiO 2  is common silica sand and is very abundant and relatively inexpensive. An oxidizing co-reagent 13 may optionally be added to scavenge free oxygen. The combination is heated 14 below the melting point of UO 2  F 2  to produce silicon tetrafluoride (SiF 4 ) and an oxide of uranium 16 without uranium carryover into the SiF 4 . The non-radioactive SiF 4  is then removed 18 leaving only the completely reacted uranium oxide 20. The SiF 4  is then scrubbed and collected 22. 
     The UO 2  F 2 , or uranyl fluoride, which may be granular, is combined in stoichiometric proportion to the SiO 2  oxidizing agent, block 24, FIG. 2. This ensures that the UO 2  F 2  is completely reacted. The SiO 2  may, however, be mixed in slight excess of its stoichiometric proportion. The SiO 2  has a lower thermodynamic stability than any oxide of uranium, such as UO 2 , UO 3  or U 3  O 8 , which may be produced. Thus, when the combination is heated to sufficiently react the SiO 2  with the UO 2  F 2 , block 26, the SiO 2  readily gives up its oxygen and an oxide of uranium and SiF 4  are produced, block 28. Depending on the oxide of uranium desired, an oxidizing co-reagent such as C, CO, or even silicon metal (Si) may be added to the reaction, block 30, shown in phantom. By using Si as the co-reagent the Si reacts with the oxygen produced to make additional SiO 2  which is reacted with the UO 2  F 2 . 
     The combination is heated, block 26, only to a temperature below the melting point of the UO 2  F 2 , typically within a temperature range of 400° C. to 1000° C. Heating the combination below the melting point of UO 2  F 2  prevents vaporization of the uranium, ensuring that the uranium does not enter the gaseous phase and thus contaminate the SiF 4  with radioactive uranium, and SiF 4  produced remains essentially pure because of the purity of the products combined. 
     Heating the stoichiometric combination below the melting point of UO 2  F 2  further ensures that the UO 2  F 2  completely reacts with the SiO 2  oxidizing agent, because no unreacted uranium is lost to the SiF 4 . The gaseous SiF 4  and solid oxide of uranium are easily separated so that the nonradioactive SiF 4  may be removed, block 32. 
     Once the SiF 4  has been removed, block 32, for example by scrubbing the SiF 4  using an absorbent bed or combining the SiF 4  with a reactive agent to separate out the SiF 4 , the SiF 4  can be collected, block 34. The SiF 4  may be recovered by heat decomposition of the post scrubbing residue, well known in the art. Scrubbing is not a necessary limitation of the present invention as the SiF 4  may be directly collected. Scrubbing is done primarily to verify the presence and purity of the SiF 4 . 
     Thus, by heating UO 2  F 2  below its melting point ensures not only that the SiF 4  is not contaminated by unreacted uranium, but that all the UO 2  F 2  is reacted due to the stoichiometric proportions in which the combination was mixed, producing a valuable oxide of uranium. The SiO 2 , has a lower thermodynamic stability than any oxide of uranium produced, as determined by the enthalpy of formulation, as shown in table 1. 
     
                       TABLE 1                                                     
______________________________________                                    
          ENTHALPY OF FORMATION                                           
  OXIDE     (-ΔH.sub.f °298) KCAL/MOLE                       
______________________________________                                    
U.sub.3 O.sub.8                                                           
          854.1                                                           
  UO.sub.2                           259.0                                
  SiO.sub.2                          217.0                                
  UO.sub.3                           294.0                                
______________________________________                                    
 
    
     Because SiO 2  has a lower enthalpy of formation than either U 3  O 8 , UO 3  or UO 2 , it readily gives up its oxygen which is readily accepted by the uranium, which has an affinity for oxygen, to produce UO 2 , UO 3  or U 3  O 8 , according to the reactions: 
     
         6UO.sub.2 F.sub.2 +3SiO.sub.2 →2U.sub.3 O.sub.8 +3SiF.sub.4 +O.sub.2(1) 
    
     
         2UO.sub.2 F.sub.2 +(s)+SiO.sub.2 (s)+C(g)→2UO.sub.2 (s)+SiF.sub.4 (g)+CO.sub.2 (g)                                          (2) 
    
     
         2UO.sub.2 F.sub.2 +SiO.sub.2 +2CO→2UO.sub.2 +SiF.sub.4 +2CO.sub.2(3) 
    
     
         6UO.sub.2 F.sub.2 +2SiO.sub.2 +Si→2UO.sub.3 O.sub.8 +3SiF.sub.4(4) 
    
     
         2UO.sub.2 F.sub.2 +2SiO.sub.2 →2UO.sub.3 +SiF.sub.4 (5) 
    
     Heating the combination below the melting point of UO 2  F 2  ensures no radioactive carryover into the fluorine compound, as well as no contamination of the fluorine compound. 
     EXAMPLE 1 
     An amount of 20.1572 grams of UO 2  F 2  was combined with 2.1461 grams of fumed silicon dioxide (Alfa AESAR SiO 2  --99.8%, -325 mesh, 400 m 2  /gm). The combination was vigorously shaken for ten minutes to thoroughly mix the combination. The combination 38, FIG. 3, was transferred to a high density, non-porous, alumina (99.8%) ceramic crucible 40. The crucible was then placed at the bottom of larger vessel 42, made of Haynes® HR 160 Alloy material and sealed with corrosion resistant metal cap 44 and Teflon® gasket seal 46 to provide a gas tight enclosure. Cap 44 has inlet ports 48 and outlet port 50 for allowing the gas to flow through vessel 42. 
     Sealed vessel 42 was placed in a crucible furnace, not shown. Two of inlet ports 48 were capped and the side entry port 48&#39; was used to introduce dry air to the combination at 200-250 cc/min. This flow was maintained throughout the reaction. Vessel 42 was heated to 700° C. in 10°/min. increments and maintained for three hours thereafter. Vessel 42 was cooled to ambient temperature in 20°/min. increments and crucible 40 removed from vessel 42. Post reaction residue 54, shown in phantom, was dark gray and flowed easily from crucible 40. Conversion of UO 2  F 2  to U 3  O 8  was essentially complete, nearly 100% yield as determined by x-ray diffraction analysis of the post reaction residue. 
     Verification of the SiF 4  may be done in several ways to include collection of the gaseous SiF 4  and performing gas analysis, passing the SiF 4  over an absorbent bed and analyzing the bed material, or chemically reacting the SiF 4  in a trap and analyzing the product using x-ray diffraction. 
     The reaction occurring in the trap in this particular example is given by the reaction: 
     
         2NaF(s)+SiF.sub.4 (g)→Na.sub.2 SiF.sub.6 (s)        (6) 
    
     Similar reactions can also be produced by using other alkaline metal fluorides such as potassium fluoride (KF), lithium fluoride (LiF) and barium fluoride (BaF 2 ) in the trap. 
     Verification of SiF 4  was done indirectly using x-ray powder diffraction analysis of the sodium hexafluorosilicate (Na 2  SiF 6 ) produced in trap 52. X-ray diffraction pattern 62, FIG. 4A, of the trap residue was compared to theoretical reference pattern 64, FIG. 4B, for Na 2  SiF 6 . The correlation of peaks 101-116 of trap residue pattern 62 and peaks 101&#39;-116&#39; of reference pattern 64 indicates the presence of Na 2  SiF 6  in the residue. Moreover, the absence of non-correlating peaks confirms that no uranium contaminates the SiF 4 . 
     Accordingly, verification of Na 2  SiF 6  indirectly verifies the presence of SiF 4  based on equation (6) above and confirms that there is no uranium carryover. 
     Verification of complete reaction of the products to produce oxides of uranium was achieved using x-ray powder diffraction analysis on post reaction residue 54. X-ray diffraction pattern 66, FIG. 5A, of the product residue was compared to standard x-ray diffraction reference pattern 68, FIG. 5B, for U 3  O 8 . The correlation of peaks 201-216 of reaction residue pattern 66 with peaks 201&#39;-216&#39; of U 3  O 8  reference pattern 68 indicates the presence of U 3  O 8  in the post reaction residue. Moreover, the absence of non-correlating peaks indicates the absense of unreacted uranium. 
     EXAMPLE 2 
     An amount of 20.0115 grams of UO 2  F 2  was combined with 2.1070 grams of SiO 2  The SiO 2  used was Celite® brand pool filter sand consisting of a mixture of silica quartz (CAS#91053-39-3) and cristobalite (CAS#68855-54-9). The SiO 2  may, however, include other naturally occurring mineral forms of SiO 2  including quartz, cristobalite, tridymite, lechatelierite, stishovite, chalcedony, coesite, silica glass or mixtures thereof. The combination was vigorously shaken for ten minutes to thoroughly mix the combination. The combination 36&#39;, FIG. 6, was transferred to high density, non-porous, alumina (99.8%) ceramic crucible 40&#39;. The crucible was then placed at the bottom of larger vessel 42&#39;, made of Haynes® HR160 Alloy material and sealed with a metal cap 44&#39; and Teflon® gasket seal 46&#39;. 
     Sealed vessel 42&#39; was placed in a crucible furnace, not shown. For this particular example, one of inlet ports 48&#39; was capped and one was connected to a supply of high purity, dry air including O 2 . Trap vessel 52&#39;, containing sodium fluoride (NaF) for scrubbing the SiF 4  reaction effluent, was connected to outlet port 50&#39;. 
     Prior to heating, the flow of air was initiated at 50-100 cc/min and maintained throughout the experiment. Vessel 42&#39; was heated to 700° C. in 10°/min. increments and maintained for four and one half (4.5) hours thereafter. Vessel 42&#39; was cooled to ambient temperature in 20°/min. increments and crucible 40&#39; removed from vessel 42&#39;. Post reaction residue 54&#39;, shown in phantom, was brown in color and flowed easily from crucible 40&#39;. Conversion of UO 2  F 2  to U 3  O 8  and was again essentially complete, nearly 100% yield as determined by x-ray diffraction analysis of the post reaction residue. 
     Verification of SiF 4  was done indirectly using x-ray diffraction on the sodium hexafluorosilicate (Na 2  SiF 6 ) produced in trap 52&#39;. X-ray diffraction pattern 70, FIG. 7A, of the trap residue was compared to reference pattern 72, FIG. 7B, for Na 2  SiF 6 . The correlation of peaks 301-316 of trap residue pattern 70 and peaks 301&#39;-316&#39; of reference pattern 72 indicates the presence of Na 2  SiF 6  in the residue. 
     Accordingly, verification of Na 2  SiF 6  indirectly verifies the presence of SiF 4  based on equation (6) above and confirms that there is no uranium carryover. 
     Verification of complete reaction of the products to produce oxides of uranium was achieved using x-ray powder diffraction analysis on post reaction residue 54&#39;. X-ray diffraction pattern 74, FIG. 8A, of the product residue was compared to standard x-ray diffraction reference pattern 76, FIG. 8B, for U 3  O 8 . The correlation of peaks 401-409 of reaction residue pattern 74 with peaks 401&#39;-409&#39; of U 3  O 8  reference pattern 76 indicates the presence of U 3  O 8  in the post reaction residue. Moreover, the absence of non-correlating peaks confirms the absence of unreacted uranium. 
     Thus, the method according to the present invention not only reacts all of the uranium oxyfluoride, but produces a silicon tetrafluoride compound of very high purity by heating the stoichiometric combination below the melting point of the uranium oxyfluoride. The uranium oxyfluoride completely reacts to produce uranium oxide without radioactive carryover into the silicon tetrafluoride compound by-product producing a commercially valuable product without requiring additional processing or generating other low level radioactive wastes. Moreover, producing uranium oxide from uranium oxyfluoride, rather than directly from uranium hexafluoride, produces less radioactive and diluted HF than if all the fluorine in UF 6  was converted to HF. 
     Although specific features of this invention are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. 
     Other embodiments will occur to those skilled in the art and are within the following claims: