Abstract:
Disclosed are electrochemical cells and methods for producing a halide of a non-alkali metal and for electrorefining the halide. The systems typically involve an electrochemical cell having a cathode structure configured for dissolving a hydrogen halide that forms the halide into a molten salt of the halogen and an alkali metal. Typically a direct current voltage is applied across the cathode and an anode that is fabricated with the non-alkali metal such that the halide of the non-alkali metal is formed adjacent the anode. Electrorefining cells and methods involve applying a direct current voltage across the anode where the halide of the non-alkali metal is formed and the cathode where the non-alkali metal is electro-deposited. In a representative embodiment the halogen is chlorine, the alkali metal is lithium and the non-alkali metal is uranium.

Description:
GOVERNMENT RIGHTS 
     The U.S. Government has rights to this invention pursuant to contract number DE-AC05-00OR22800 between the U.S. Department of Energy and Babcock &amp; Wilcox Technical Services Y-12, LLC. 
    
    
     FIELD 
     This disclosure relates to the field of electrolytic chemistry. More particularly, this disclosure relates to the production of metal halides for electrorefining of metals. 
     BACKGROUND 
     Metal halides are useful for electrorefining metals. However, the production of many metal halides is difficult. In particular, current methods for the production of uranium trichloride (UCl 3 ) on a large scale require handling of highly pyrophoric uranium/uranium hydride fines or the use of toxic cadmium chloride as an oxidizer in a molten salt bath. It is desirable to eliminate the need for both of these reagents. Moreover, it is desirable in some circumstances to provide in-situ production of metal halides such as UCl 3 . Consequently, improved systems and methods are needed for making metal halides, and in particular for making UCl 3  for electrorefining uranium. 
     SUMMARY 
     In some embodiments, the present disclosure provides an electrochemical cell for producing a metal halide. A typical electrochemical cell includes a container, a source of an acid of a halogen, and an electrolyte in the container. The composition of the electrolyte includes a molten salt of (a) the halogen and (b) an alkali metal. The electrochemical cell typically also includes an anode in the electrolyte where the anode includes a non-alkali metal. There is an anolyte portion of the electrolyte adjacent the anode. Generally there is a tube in the electrolyte, and the tube establishes a catholyte portion of the electrolyte and the tube has a permeable portion for ionic transportation. Typically a cathode is in the catholyte portion, and the cathode has a chemical feed passageway for flowing the hydrogen halide gas into the catholyte portion of the electrolyte. It is generally important that a portion of the hydrogen halide dissolves in the electrolyte that is in the catholyte portion of the electrolyte. The electrochemical cell typically includes a direct current power source that has an anode terminal that is in electrical connectivity with the anode and has a cathode terminal that is in electrical connectivity with the cathode. With this configuration, the hydrogen halide is electrolyzed adjacent the cathode to produce hydrogen and to produce anions of the halide that migrate to the anode and form the metal compound as a halide of the non-alkali metal adjacent the anode. 
     Another embodiment provides an electrochemical cell for producing an electrorefined non-alkali metal. This embodiment has a container and an electrolyte is in the container. The composition of the electrolyte includes a molten salt of (a) a halogen and (b) an alkali metal. In this embodiment there is an anode disposed in the electrolyte. An anolyte portion of electrolyte is adjacent the anode, and a halide consisting of (a) the halogen and (b) a non-alkali metal is disposed in the anolyte portion. There is a cathode disposed in the electrolyte. Further in this embodiment there is a direct current power source having an anode terminal that is in electrical connectivity with the anode and there is a cathode terminal that is in electrical connectivity with the cathode such that cations of the non-alkali metal migrate from the anolyte portion and are electro-deposited adjacent the cathode as the electrorefined non-alkali metal. 
     Method embodiments are provided for producing a non-alkali metal halide that includes a halogen and a non-alkali metal where the hydrogen halide has a solubility of at least 1 mmol/L in a molten salt of (a) the halogen and (b) an alkali metal. A typical method involves electrolytically dissociating at a cathode the hydrogen halide dissolved in the molten salt such that halogen anions and gaseous hydrogen are formed at the cathode. Such methods typically further involve electrolytically charging a metal at an anode in the molten salt such that cations of the non-alkali metal are formed at the anode. Such methods typically further involve combining the halogen anions and the cations of the non-alkali metal to form the metal compound adjacent the anode as a non-alkali metal halide. 
     Method embodiments are provided for producing an electrorefined non-alkali metal. Such methods generally involve disposing in a electrochemical cell having an anode and a cathode a mixture of (1) a halide consisting of a halogen and a non-alkali metal and (2) a molten salt of the halogen and an alkali metal. Then, typically, the methods involve applying a direct current potential across the anode and the cathode wherein cations of the non-alkali metal migrate from a region adjacent the anode and are electro-deposited adjacent the cathode as the electrorefined non-alkali metal. 
     In the various embodiments disclosed herein the halide is chlorine, the alkali metal is lithium and the non-alkali metal is uranium, such that UCl 3  is produced and/or electrorefined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various advantages are apparent by reference to the detailed description in conjunction with the figures, wherein elements are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  is a somewhat schematic view of an electrochemical cell for production of a metal halide. 
         FIG. 2  is a somewhat schematic view of a cell for production of a metal halide and electrorefining of the metal halide. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the preferred and other embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration the practice of specific embodiments of an electrochemical cell for making a metal halide and embodiments of methods for making metal halides. It is to be understood that other embodiments may be utilized, and that structural changes may be made and processes may vary in other embodiments. 
     Various embodiments disclosed herein provide systems and methods for the electrolysis of a hydrogen halide in a molten salt of (a) an alkali metal and (b) the halogen, to produce that halide of a non-alkali metal. For example, anhydrous hydrogen chloride may be electrolyzed in a molten lithium chloride salt in order to convert elemental uranium metal to uranium trichloride. 
     As used herein the term “halogen” refers to any of the elements of Table 1. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Atomic 
                   
               
               
                 Number 
                 Element 
               
               
                   
               
             
             
               
                  9 
                 Fluorine 
               
               
                 17 
                 Chlorine 
               
               
                 35 
                 Bromine 
               
               
                 53 
                 Iodine 
               
               
                 85 
                 Astatine 
               
               
                   
               
             
          
         
       
     
     As used herein the term “alkali metal” refers to any of the elements in Table 2. 
                         TABLE 2               Atomic           Number   Element                    3   Lithium       11   Sodium       19   Potassium       37   Rubidium       55   Cesium       87   Francium        4   Beryllium       12   Magnesium       20   Calcium       38   Strontium       56   Barium       88   Radium                    
Note that the “alkali metals” of Table 2 include elements that are sometimes elsewhere referred to as “alkaline earth metals.”
 
     As used herein the term “non-alkali metal” refers to any of the elements in Table 3. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Atomic 
                   
               
               
                 No. 
                 Name 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 89 
                 Actinium 
               
               
                 90 
                 Thorium 
               
               
                 91 
                 Protactinium 
               
               
                 92 
                 Uranium 
               
               
                 93 
                 Neptunium 
               
               
                 94 
                 Plutonium 
               
               
                 95 
                 Americium 
               
               
                 96 
                 Curium 
               
               
                 97 
                 Berkelium 
               
               
                 98 
                 Californium 
               
               
                 99 
                 Einsteinium 
               
               
                 100 
                 Fermium 
               
               
                 101 
                 Mendelevium 
               
               
                 102 
                 Nobelium 
               
               
                 57 
                 Lanthanum 
               
               
                 58 
                 Cerium 
               
               
                 59 
                 Praseodymium 
               
               
                 60 
                 Neodymium 
               
               
                 61 
                 Promethium 
               
               
                 62 
                 Samarium 
               
               
                 63 
                 Europium 
               
               
                 64 
                 Gadolinium 
               
               
                 65 
                 Terbium 
               
               
                 66 
                 Dysprosium 
               
               
                 67 
                 Holmium 
               
               
                 68 
                 Erbium 
               
               
                 69 
                 Thulium 
               
               
                 70 
                 Ytterbium 
               
               
                 5 
                 Boron 
               
               
                 14 
                 Silicon 
               
               
                 51 
                 Antimony 
               
               
                 52 
                 Tellurium 
               
               
                 84 
                 Polonium 
               
               
                 32 
                 Germanium 
               
               
                 33 
                 Arsenic 
               
               
                 34 
                 Selenium 
               
               
                 13 
                 Aluminum 
               
               
                 31 
                 Gallium 
               
               
                 49 
                 Indium 
               
               
                 50 
                 Tin 
               
               
                 81 
                 Thallium 
               
               
                 82 
                 Lead 
               
               
                 83 
                 Bismuth 
               
               
                 41 
                 Niobium 
               
               
                 76 
                 Osmium 
               
               
                 21 
                 Scandium 
               
               
                 22 
                 Titanium 
               
               
                 23 
                 Vanadium 
               
               
                 24 
                 Chromium 
               
               
                 25 
                 Manganese 
               
               
                 26 
                 Iron 
               
               
                 27 
                 Cobalt 
               
               
                 28 
                 Nickel 
               
               
                 29 
                 Copper 
               
               
                 30 
                 Zinc 
               
               
                 39 
                 Yttrium 
               
               
                 40 
                 Zirconium 
               
               
                 42 
                 Molybdenum 
               
               
                 43 
                 Technetium 
               
               
                 44 
                 Ruthenium 
               
               
                 45 
                 Rhodium 
               
               
                 46 
                 Palladium 
               
               
                 47 
                 Silver 
               
               
                 48 
                 Cadmium 
               
               
                 71 
                 Lutetium 
               
               
                 72 
                 Hafnium 
               
               
                 73 
                 Tantalum 
               
               
                 74 
                 Tungsten 
               
               
                 75 
                 Rhenium 
               
               
                 77 
                 Iridium 
               
               
                 78 
                 Platinum 
               
               
                 79 
                 Gold 
               
               
                 80 
                 Mercury 
               
               
                   
               
             
          
         
       
     
       FIG. 1  illustrates one embodiment of an apparatus for electrolysis of a hydrogen halide in a molten salt of (a) an alkali metal and (b) a halogen, to produce that halide of a non-alkali metal. In  FIG. 1 , an electrochemical cell  10  includes a container  12  containing an electrolyte  14 . The electrolyte includes the molten salt of (a) the alkali metal and (b) the halogen. For example, the alkali metal may be lithium and the halogen may be chlorine, and then the electrolyte  14  contains lithium chloride (LiCl). The electrochemical cell  10  has a cathode  18  and an anode  22 . The cathode  18  is generally an inert material such as graphite that is shaped into a hollow tube. In the embodiment of  FIG. 1 , the cathode  18  has an open end, but, in other embodiments, the cathode may be a hollow tube with a closed end, provided that the tube has sufficient porosity to permit the flow of a gas through the walls of the tube. The anode  22  is a corrosion resistant mesh basket made from a material such as stainless steel or titanium. One or more bulk pieces or a powder of a non-alkali metal  26  is disposed in the mesh basket of the anode  22 . For example, the non-alkali metal  26  may be uranium. In other embodiments, an anode for the electrochemical cell  10  may be fabricated integrally from a non-alkali metal. The advantage of using the mesh basket arrangement of  FIG. 1  is that the non-alkali metal that is consumed during the operation of the electrochemical cell  10  may be easily replaced in the mesh basket, whereas an anode fabricated integrally from a non-alkali metal would have to be replaced in its entirety. 
     A direct current (DC) power supply  30  is provided. An anode terminal  34  of the DC power supply  30  is in electrical connectivity with the anode  22 , and a cathode terminal  38  of the DC power supply  30  is in electrical connectivity with the cathode  18 . 
     A catholyte portion  50  of the electrolyte  14  is proximate to the cathode  18 , and an anolyte portion  54  of the electrolyte  14  is proximate to the anode  22 . The anolyte portion  54  is not isolated from the bulk of the electrolyte  14  by any physical barrier, but the catholyte portion  50  and the cathode  18  are isolated from the anolyte portion  54  and the anode  22  and by a tube  70 . Typically, the tube  70  is fabricated from quartz. The tube  70  has a permeable portion  74  for ionic transport, as subsequently described herein. Typically, the permeable portion  74  is formed with porous frits. A source  90  of a hydrogen halide is provided. For example, if the halogen is chlorine then the hydrogen halide may be anhydrous hydrogen chloride (HCl). 
     To operate the electrochemical cell  10 , gas bubbles  94  of the hydrogen halide (e.g., bubbles of anhydrous HCl) are flowed into the catholyte portion  50  through the hollow tube  70  of the anode  18 . Some of the hydrogen halide (from source  90 ) is dissolved into the electrolyte  14 . In order for the process to operate, the solubility of the acid of the halogen into the molten salt (i.e., the molten salt of (a) the alkali metal and (b) the halogen) should be at least 1 mmol/L. Then, with the DC power supply  30  energized, the following reactions occur:
 
Cathode: 3HHn→3H + +3Hn −   (Reaction 1a)
 
3H + +3 e   − →3/2H 2 (g)   (Reaction 1b)
 
Anode: M+3Hn − →MHn 3 +3 e   −   (Reaction 2)
 
where the symbols “M”=the non-alkali metal and “Hn”=the halogen.
 
Thus, when the non-alkali metal is uranium and the halogen is chlorine, Reactions 1a, 1b and 2 are:
 
Cathode: 3HCl→3H + +3Cl −   (Reaction 3a)
 
3H + +3 e   − →3/2H 2 (g)   (Reaction 3b)
 
Anode: U+3Cl − →UCl 3 +3 e   −   (Reaction 4)
 
The net reaction is:
 
M+3HHn→MHn 3 +3/2H 2 (g)   (Reaction 5)
 
such that when the non-alkali metal is uranium and the halogen is chlorine, Reaction 5 is:
 
U+3HCl→UCl 3 +3/2H 2 (g)   (Reaction 6)
 
A halide of a non-alkali metal (e.g., UCl 3 ) is formed at the anode and hydrogen gas is formed at the cathode. The halide of the non-alkali metal (e.g., UCl 3 ) is produced as a mixture with molten salt of (a) the alkali metal and (b) the halogen (e.g., LiCl).
 
     It is important to note that the same halogen is used in the hydrogen halide (from source  90 ) and in the molten salt of the alkali metal that is the electrolyte  14 . Thus, if the non-alkali metal is uranium and the molten salt of the alkali metal is LiCl, then the hydrogen halide that is used is HCl such that UCl 3  is produced as the halide of the non-alkali metal. 
       FIG. 2  illustrates an embodiment of an electrochemical cell  100  where the halide of the non-alkali metal (e.g., UCl 3 ) may be electrorefined in-situ. The electrochemical cell  100  of  FIG. 2  includes many of the same components of the electrochemical cell of  FIG. 1 . One exception is that the non-alkali metal  26  that was disposed in the mesh basket of the anode  22  in  FIG. 1  has been electrochemically converted to a halide of the non-alkali metal (such as by operation of the electrochemical cell  10 ). Consequently, in the embodiment of  FIG. 2  the halide of the non-alkali metal (e.g., UCl 3 ) and a molten salt of (a) an alkali metal and (b) the halogen (e.g., LiCl) form a mixture  104 . Typically, the halide of the non-alkali metal is at an overall concentration of about 5-10 wt % of the mixture  104 . There is natural convection in the molten salt that mixes the molten salt fairly well, albeit more slowly than mechanical stirring. 
     The electrochemical cell  100  of  FIG. 2  has two cathodes. The cathode  18  of electrochemical cell  10  in  FIG. 1  is designated as a first cathode  120  in  FIG. 2 , and the other cathode in  FIG. 2  is designated as a second cathode  124 . The second cathode  124  is typically formed from a material such as graphite, stainless steel or titanium. 
     The electrochemical cell  100  has two DC power sources. The DC power source  30  in  FIG. 1  is designated as a first DC power source  130  in  FIG. 2 , with the first DC power source  130  having a first anode terminal  134  and a first cathode terminal  138 . The other DC power source for electrochemical cell  100  is designated as a second DC power source  150 . The second DC power source  150  has a second anode terminal  154  and a second cathode terminal  158 . 
     The electrochemical cell  100  has an electrical switching system  170  that includes a first electrical switch  174  and a second electrical switch  178 . These switches permit the electrochemical cell  100  to be operated in either production mode (for producing a halide of the alkali metal) or a refining mode (for electrorefining the halide of the alkali metal). 
     When the electrochemical cell  100  is in the electrorefining mode, the first electrical switch  174  is open and the second electrical switch  178  is closed. In this configuration the second anode terminal  154  is in electrical connectivity with the anode  22  and the second cathode terminal  158  is in electrical connectivity with the second cathode  124 , and the following reactions occur:
 
Anode: M+3Hn − →MHn 3 +3 e   −   (Reaction 7)
 
Cathode: MHn 3 +3 e   − →M+3Hn −   (Reaction 8)
 
     where the symbol “M”=the non-alkali metal and “Hn”=the halogen. 
     Thus, when the non-alkali metal is uranium and the halogen is chlorine, reactions 7 and 8 are:
 
Anode: U+3Cl − →UCl 3 +3 e   −   (Reaction 9)
 
Cathode: UCl 3 +3 e   − →U+3Cl −   (Reaction 10)
 
The net reaction is:
 
M+MHn 3 →MHn 3 +M  (Reaction 11)
 
such that when the non-alkali metal is uranium and the halogen is chlorine, Reaction 11 is:
 
2U+UCl 3 →3U+3Cl −   (Reaction 12)
 
     In other words, cations of the non-alkali metal in the anolyte portion  108  of the mixture  104  migrate from the anolyte portion  108  and are electro-deposited adjacent the second cathode  124 . The halogen ions act as a mechanism for transporting ions of the non-alkali from the anode to the cathode. When the non-alkali metal is deposited on the cathode, the halogen ions are released back into the salt so that they are free to grab another non-alkali metal ion from the anode. In the case where the halogen is chlorine and the non-alkali metal is uranium, U 3+  ions migrate from the anolyte portion  108  and are electro-deposited adjacent the second cathode  124  as uranium metal while the chlorine items shuttle back and forth between the anode and the cathode. 
     When the electrochemical cell  100  is in the non-alkali metal halide production mode, a non-alkali metal (such as the non-alkali metal  26  of  FIG. 1 ) is disposed in the wire mesh anode  22  and the first electrical switch  174  is in the closed position and the second electrical switch  178  is in the open position. In this configuration the electrochemical cell  100  operates in the same fashion as described hereinbefore with regard to the electrochemical cell  10  of  FIG. 1 . 
     It is important to note that the net reaction in Reaction 6 (shown above) is spontaneous at elevated temperatures. However, that reaction is kinetically slow due to the formation of UCl 3  that presents a barrier to the HCl reactant. In a molten salt bath the UCl 3  is dissolved, so uranium may be converted to UCl 3  in a molten salt bath by simply bubbling HCl over the uranium metal. A key advantage of making the UCl 3  using methods described herein is the ability to keep the HCl contained in the catholyte compartment. By equipping the catholyte compartment with a low porosity membrane that allows primarily ionic conduction, the HCl will remain confined. This also mitigates potential corrosion of the electrorefiner structural materials without a need to remove dissolved HCl from the molten salt prior to electrorefining. 
     While the electrochemical cell  100  is depicted with two DC power supplies  130  and  150 , in some embodiments a single power supply may be used with an electrical switching system that switches its anode terminal and cathode terminal to the configurations described for the production mode and the electrorefining mode. 
     In summary, embodiments disclosed herein provide systems and methods for producing a halide of a non-alkali metal and for electrorefining the halide of the non-alkali metal. The foregoing descriptions of embodiments have been presented for purposes of illustration and exposition. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of principles and practical applications, and to thereby enable one of ordinary skill in the art to utilize the various embodiments as described and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.