WO2001037937A1

WO2001037937A1 - Destruction of metal halides using solvated electrons

Info

Publication number: WO2001037937A1
Application number: PCT/US2000/032161
Authority: WO
Inventors: Gerry D. Getman; Robert W. Mouk; Albert E. Abel
Original assignee: Commodore Applied Technologies, Inc.
Priority date: 1999-11-23
Filing date: 2000-11-22
Publication date: 2001-05-31
Also published as: AU3073401A

Abstract

Methods for the destruction of metal halides, and particularly transition metal halides, are performed by creating a reaction mixture prepared from raw materials which include at least one metal halide, a nitrogenous base and an active metal, such as sodium in an amount sufficient to dehalogenate, reduce or otherwise decompose the metal halide; and then react the mixture. Methods of the invention subject transition metal halides, including uranium hexafluoride, tungsten hexafluoride, etc., to a 'dissolving metal reduction' to produce a composition comprising salts and oxides of the metals, including elemental metals, like uranium, tungsten, etc. Thus, potentially hazardous transition metal halides, like UF6 and depleted UF6 are fully converted to their base metals, and less volatile metal oxides. While uranium metal would still be radioactive it is no longer in such a volatile format.

Description

DESTRUCTION OF METAL HALIDES USING SOLVATED ELECTRONS

TECHNICAL FIELD

This invention relates generally to methods for the destruction of metal halides, and more specifically, to the chemical decomposition of large bulk quantities of transition metal halides, and particularly toxic and reactive uranium fluorides, like depleted uranium hexafluoride and other such hazardous halogenated compounds employing solvated electron chemistry.

BACKGROUND OF THE INVENTION Depleted UF₆ results from the process of making uranium suitable for use as fuel for nuclear reactors or military applications. The use of uranium in these applications requires increasing the proportion of the uranium-235 isotope found in natural uranium, which is approximately 0.7 percent, through an isotopic separation process called uranium enrichment. That process currently used in the United States for uranium enrichment is called -gaseous diffusion-, wherein UF₆ gas is separated into two parts: one enriched in uranium-235 and the other depleted in uranium-235. The enriched UF₆ is used for manufacturing commercial reactor fuel, which typically contains 2 to 5 percent uranium-235, or for military applications, such as navel reactor fuel, which requires further enrichment to 95 percent or more uranium-235.

The depleted UF₆ (hereinafter "DU< ") which typically contains from 0.2 to 0.4 percent uranium-235, is stored as a solid in large metal cylinders at gaseous diffusion facilities. The vast majority of the cylinders have a 14-ton (12 metric-ton) capacity. During storage, these cylinders normally contain DUF₆ in a solid state at the bottom of the vessel and DUF₆ in gaseous state at less than atmospheric pressure at the top of the storage cylinder. However, some DUF₆ can also be present in a liquid phase. At atmospheric pressure, DUF₆ is a solid at temperatures below 57 °C and a gas above that temperature. Solid, liquid and gaseous phases can exist together in equilibrium at 22 psia and at a temperature of about 147 °F.

Thus, it will be seen that UF₆ is used for two principal reasons. First, it can be conveniently used as a gas for processing, as a liquid for feeding and withdrawing, and as a solid for storage. Each of these states is achievable at relatively low temperatures and pressures. Secondly, because fluorine has only one natural isotope, all the isotopic separative capacity of the diffusion plant is used to enrich the concentration of the lighter uranium isotopes.

Environmentally, because storage of DUF₆ began in the early 1950' s, many of the cylinders now show evidence of external corrosion, and potentially hazardous leakages have been reported. Needless to say, the characteristics of DUF₆ pose substantial potential health risks. Uranium is radioactive, and DUF₆ in storage emits low levels of gamma and neutron radiation. The radiation levels measured on the outside surface of filled DUF₆ storage cylinders are typically about 2 to 3 millirem per hour (mrem/h) , decreasing to about 1 mrem/h at a distance of 0.3 m. In addition, if DUF₆ is released to the atmosphere, it reacts with water in the air to form hydrogen fluoride and a uranium fluoride compound, uranyl fluoride (U0₂F₂) . All are toxic. Uranium is a heavy metal that, in addition to being radioactive, can have toxic effects (primarily on the kidneys) if it enters the blood stream by means of ingestion or inhalation. Furthermore, accompanying HF is an extremely corrosive gas which can damage the lungs and cause death if inhaled at high concentrations . A number of the proposed strategies for treating and converting DUF₆ result in the generation of unwanted by-products. For example, steam or moisture can be used to hydrolyze DUF₆, but in so doing generates hydrogen fluoride as a by-product. Hydrogen fluoride is very corrosive and difficult to handle and store, and as previously mentioned, highly toxic to biological systems. Accordingly, it would be desirable to have low cost, reliable methods for the decomposition of hazardous metal halides, such as uranium fluorides, and particularly DUF₆, which methods are capable of dehalogenating, reducing or decomposing such metal halides to less volatile substances which are less hazardous and more environmentally manageable.

SUMMARY OF THE INVENTION It is therefore a principal object of the invention to provide methods for the destruction of metal halides. One such embodiment includes creating a reaction mixture prepared from raw materials which include at least one metal halide, a nitrogenous base and an active metal in an amount sufficient to dehalogenate, reduce or otherwise decompose the metal halide; and then react the mixture. Accordingly, methods of this invention, in their preferred embodiments, subject transition metal halides, e.g., uranium fluorides, tungsten fluorides, and so on, to a "dissolving metal reduction" to produce an elemental composition consisting of the dissolving metal used in the reaction mixture, the metal, e.g., uranium, tungsten, etc.; oxygen, halogen and carbon.

Most of the metal halide in the reaction mixture is decomposed in this reduction reaction. However, as an optional but preferred step, any remaining metal halide can be converted, first by removing the nitrogenous base to yield a solid residue, and as a final step reacting the solid residue in an aqueous solution, such as water, to provide the elemental metal, e.g., uranium metal; the dissolving metal, e.g., sodium metal, and various less volatile metal oxides. Alternatively, oxidants, such as peroxides, ozone, persulfates, perchlorates, hypohalites, such as hypochlorite, and so on, can be used in converting any residual metal halide.

Thus, according to the immediate invention potentially hazardous transition metal halides, like UF₆ and DUF₆ are fully converted to their base metals, and less volatile metal oxides, for example. While the uranium metal would still be radioactive it is no longer in such a volatile format like UF₆ which at room temperature has a vapor pressure comparable to methanol of over 100 mm Hg. Hence, the by-products of the methods of this invention, namely uranium metal and their oxides are in a more manageable format for more reliable and safer storage.

Dissolving metal reduction chemistry is not new; it is embodied in the well known "Birch Reduction, " which was first reported in the technical literature in 1944. The Birch Reduction is a method for reducing aromatic rings by means of alkali metals in liquid ammonia to give mainly the dihydro derivatives; see, e.g., "The Merck Index," 12th Ed., Merck & Co., Inc., Whitehouse Station, NJ USA, 1996, p. ONR-10.

Such dissolving metal reductions have been the subject of much investigation and numerous publications. Reviews include the following: G. W. Watt, Chem. Rev. , 46_/ 317-379 (1950) and M. Smith, "Dissolving Metal Reductions," in "Reduction : Techniques and Applications in Organi c Synthesis, " ed. R. L. Augustine, Marcel Decker, Inc., New York, NY, 1968, pages 95-170. Dissolving metal reduction chemistry is applicable to compounds containing a wide range of functional groups. For example, the reaction of pesticides with sodium and liquid ammonia was reported some years ago; M. V. Kennedy and coworkers, J. Environ . Quali ty, 1 , 63-65 (1972) . It is believed that the dissolution of an active metal, such as sodium, in a nitrogenous base, such as liquid ammonia, produces "solvated electrons," which are responsible for the intense blue color of the resultant solutions; that is:

(I) Na +(NH₃)_X- Na⁺ + e^" (solvated)

According to the present invention, the preferred method for destroying metal halides comprises, in a broad sense, treating the metal halides with solvated electrons. The method for decomposing metal halides can be performed by the steps of: (i) forming a solution of solvated electrons by dissolving an active metal in a nitrogenous base in a sufficient amount to decompose the metal halide, and

(ii) reacting the metal halide with the solvated electrons to yield a reaction product in the solution comprising a substantially reduced amount of the metal halide.

Any residual metal halide remaining can be converted by the steps of:

(iii) separating the nitrogenous base from the reaction product to provide a solid residue, and

(iv) treating the residue with an agent suitable for decomposing any residual metal halide, typically to substances of substantially less volatility (and toxicity to a biological system through the elimination of hydrogen fluoride by-products) . This may include the step of reacting with water, or to any of the aforementioned oxidants, such as hydrogen peroxide, ozone, and the like. Hence, by the reduction of UF₆ or stored DUF₆, for example, with solvated electrons, and a final treatment with water would provide a highly economic, and more storable format for radionuclide products, and also allow for the recovery of free metals, including uranium and sodium metals for recycling.

The methods of this invention are especially useful in decomposing or dehalogenating transition metal halides as a group of materials, and includes such representative examples as the uranium fluorides, i.e., UF₃, UF , UF₆ , and more specifically, their depleted species, such as DUF₆. Other representative examples included within the meaning of the expression

"transition metal halides" are tungsten halides, e.g., WF₆; MoF₆ and other volatile metal fluorides and chlorides, such as plutonium halides, e.g., PuF₃, PuF , PuF₆.

For purposes of this invention the expression "DUF₆.. or

"depleted uranium hexafluoride" as appearing in the specification and claims is intended to include any compound of hexavalent uranium and fluorine that has been stripped of at least a portion of the fissionable isotope ²³⁵U that the uranium once contained, so that it has a lower proportion on a weight percent basis than the ²³⁵U found in nature.

It is therefore still a further object of the invention to provide a method for decomposing transition metal halides by the steps, which comprise:

(i) reacting the transition metal halide with a nitrogenous base to form a nitrogenous metal halide intermediate, and

(ii) reducing the intermediate of (i) with solvated electrons. The solvated electrons can be formed in-si tu by the introduction of an active metal in an amount sufficient to decompose the transition metal halide. In the event of residual transition metal halide the nitrogenous base can be separated to form a uranium-containing residue, followed by treating the residue with water. These inventors also found the foregoing methodologies of employing solvated electron chemistry are also useful in decontaminating containers, vessels and other processing equipment, including cylinders used in the storage of DUF₆, for instance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the methods of this invention are applicable to the decomposition of a wide range of metal halides, including their respective fluorides, chlorides, bromides and iodides, they are especially useful in treating halides of transition metals, namely Group Illb through Group IB of the Periodic Table. More specifically, this includes the First transition series of elements from scandium through copper; the Second transition series of yttrium through silver; the Third transition series from lanthanum; and hafnium through gold; the rare earth metals from cerium to lutetium, and finally the actinide series beginning with thorium, uranium, plutonium, and so on. Particularly included are the more volatile uranium halides, like UF₆, and particularly DUEg generated in the gaseous diffusion process for the enrichment of uranium. While not intending to be bound by this representation, it is nevertheless believed that the decomposition of uranium fluoride according to the present invention with solvated electrons produced by dissolving an active metal, represented by sodium, in a nitrogenous base, represented by liquid ammonia, proceeds as follows:

(NH₃) UF₆ + Na > Na₂C0₃ + NaU oxides + U fluorides + U oxides + U°

The evaporation of the ammonia leaves a solid residue which undergoes further reaction in the presence of water to yield a low volatility product comprising mainly uranium and sodium metals, and a mixture of sodium and uranium oxides. Fluorine in the reaction is converted to salts, namely sodium fluoride and ammonium fluoride. The process may also be performed by ammonolysis reaction wherein, for example, UF₆ (or depleted DUF₆) is initially treated with a nitrogenous base, represented by liquid ammonia:

UF₆ + NH₃ > ammonobasic uraniumfluorides + NH F

The foregoing ammonolysis products can undergo further reaction with the introduction of an active metal, represented by sodium metal, wherein solvated electrons are formed in-si tu :

Na° ammonobasic uraniumfluorides + NH₄F + e > ammonobasic product of uranium-sodium

The ammonia is evaporated off to yield a residue. When the residue is treated with water, rather surprisingly lower volatility oxides of uranium and of sodium-uranium form without the generation of hydrogen fluoride, rather than hydrolysis occurring, the reaction apparently proceeding according to following reaction:

(NH₂)₂UNa₄ + (NH₂)₄UNa₂ + HOH > Na₄U0₂ + Na₂U0₄ WO 01/37937 PCTYUSOO/32161

With regard to the active metal to be employed in the preferred embodiments of this invention, whereas the literature reports the use of a number of other metals, such as Mg, Al, Fe, Sn, Zn, and alloys thereof, in dissolving metal reductions, in the methods of this invention, it is preferred that the active metal be selected from one or a combination of the metals found in Groups IA and IIA of the Periodic Table of the Elements; that is, the alkali and alkaline earth metals. Largely for reasons of availability and economy, it is most preferred that the active metal be selected from Li, Na, K, Ca, and mixtures thereof. In most cases, the use of sodium, which is widely available and inexpensive, will prove to be satisfactory.

The nitrogenous base which is required in this process can be selected from ammonia, amines, and the like, or mixtures thereof. Anhydrous liquid ammonia is readily available, since it is widely employed as a fertilizer in agricultural applications. Consequently, it is also relatively inexpensive and so is the preferred nitrogenous base. However, ammonia boils at about -33°C, requiring that solutions of liquid ammonia be cooled, that the solution be pressurized, or both. In those cases where this is inconvenient, a number of amines are readily available and can be employed as the nitrogenous base.

Representative classes of useful amines include primary amines, secondary amines, tertiary amines, and mixtures thereof. Specific examples of such amines include alkyl amines, like methyl amine, ethyl amine, n-propyl amine, iso-propylamine, 2- methylpropylamine, and t-butylamine, which are primary amines; as well as dimethylamine and methylethylamine, which are secondary amines; and tertiary amines, such as triethyl amine. Di- and trialkylamines can also be employed, as can saturated cyclic amines such as piperidine. Amines which are liquids at the desired reaction temperature are preferred and, among these amines, methylamine (bp -6.3°C), ethyl amine (bp 16.6 C), prσpylamine (bp 49° C) , isopropyl-amine (bp 33.0°C), butylamine (bp 11 . 8° C) , and ethylene-diamine (bp 116.5°C), are especially useful.

In some cases it may be advantageous to combine the nitrogenous base with another solvating substance such as an ether; for example, tetrahydrofuran, diethyl ether, dioxane, or 1, 2-dimethoxyethane, or a hydrocarbon; for example, pentane, decane, and so forth. In selecting the nitrogenous base and any cosolvents to be included therewith, it should be borne in mind that solvated electrons are extremely reactive, so it is preferred that neither the nitrogenous base nor any cosolvent included therewith contain groups which compete with the transition metal halide and react with the solvated electrons. Such groups include, for example, aromatic hydrocarbons groups which may undergo the Birch Reduction, and acid, hydroxyl, peroxide, sulfide, halogen, and ethylenic unsaturation, and they should, in general, be avoided so as to prevent undesirable side reactions .

Although other conditions can sometimes be employed to advantage, the methods of this invention are preferably carried out at temperatures in the range of about -33°C to about 70°C

, and at atmospheric pressures, or in standard pressure vessels.

In carrying out the methods of this invention, the ratio of nitrogenous base to metal halide or intermediate ammonolysis product in the reaction mixture is preferably between about 20:1 to about 4:1 on a weight/weight basis.

The reaction of solvated electrons with transition metal should require a single electron per halogen. Thus, for the decomposition of UF₆, the metal requirements should be about 0.4 pounds of sodium metal per pound of UF₆ for the decomposition of UF₆. The amount of active metal in the reaction mixture, should preferably be in the range of about 0.1 percent to about 8.0 percent by weight based on the weight of the mixture, and more preferably, between about 2.0 percent and about 7.0 percent.

The course of the reaction involving solvated electrons can be followed readily by monitoring the blue color of the reaction mixture which is characteristic of solutions of nitrogenous base and active metal. When the blue color disappears, it is a signal that metal halide has reacted with all of the solvated electrons, and more active metal or solution containing solvated electrons can be added to the reaction mixture. In many cases it is preferred that the addition of active metal or additional solvated electrons be continued until the metal halide has completely reacted with the solvated electrons, a state which is signaled when the blue color of the mixture remains. The rate of the reaction between the metal halide and the solvated electrons is rapid, the reaction in most cases being substantially complete in a matter of minutes to a few hours.

The process may include an optional, but often preferred step following the initial decomposition of the metal halide with solvated electrons. That is, subsequent to the application of the solvated electrons, the residual product mixture is optionally (but desirably) oxidized, preferably by non-thermal means, by reacting the products of the metal halide decomposition with a chemical oxidant. Preferably, however, before introducing the oxidant, residual nitrogenous base is removed, e.g., ammonia is removed from the reactor by allowing remaining vapors to evaporate. Representative oxidants and mixtures of oxidants which may be employed include hydrogen peroxide, ozone, dichromates and permanganates of alkali metals, persulfates, perchlorates, and so on. In carrying out this additional step optimally, the process requires introducing into the reactor system a sufficient amount of a suitable oxidizing agent to completely react with any residual metal halide remaining from the initial reaction with the solvated electrons. The purpose of this oxidation step is to take any residual metal halide to its highest possible oxidation state reasonably achievable.

Hence, if post-destruction oxidation is to be employed, the metal halide is first reacted with nitrogenous base, preferably including solvated electrons, followed by a secondary treatment step which comprises reacting the residuals with an oxidizing agent.

EXAMPLE I

To demonstrate the initial reaction of a metal halide and a nitrogenous base a laboratory set-up was assembled consisting of a 500 ml round bottom, 4 necked flask equipped with a mechanical stirrer and a hexane/dry ice cooled bath; a hexane/dry ice cooled reflux condenser, and a 1 liter pressure equalizing addition funnel.

3.3 g of tungsten hexafluoride (WF₆) in a liquid/gaseous state from a cylinder was condensed as a solid in 300 ml of anhydrous liquid ammonia added rapidly to the flask at atmospheric pressure and at a temperature of -33 °C. A vigorous reaction occurred with brownish fumes developing in the apparatus. While still cooling the stirrer was initiated for a period of about 15 to 30 minutes. Some of the fumes contacted the wall of the flask and turned white shortly thereafter. The hexane/dry ice cooling bath for the flask was removed which allowed the flask and its contents to warm gradually to room temperature over several hours. This resulted in the evaporation and venting of the ammonia from the flask via the condenser. The bottom of the flask contained most of the solid residue. Melting point determinations on residue samples were performed and found to parallel those recited in the literature.

The residue may be further reacted with solvated electrons by dissolving sodium metal or other alkali or alkaline earth metal in a nitrogenous base followed by introduction of the solution to the flask containing the solid reside of the ammonolysis reaction followed by oxidation with water or an oxidizing agent, such as hydrogen peroxide.

EXAMPLE II

A further experiment was conducted to demonstrate the reduction of WF₆ with solvated electrons. Using the apparatus of Example I, a new sample of WF₆ weighing about 3.0 g was introduced into the R.B. flask and then reacted with solvated electrons prepared by dissolving 3.0 g of sodium metal in 250 ml of anhydrous liquid ammonia, which was added to the flask through the addition funnel. The blue color solution gradually faded after a period of several minutes. The ammonia in the reaction mixture was allowed to warm to ambient temperature conditions and evaporate over a period of several hours leaving a solid residue. This residue was recovered and allowed to remain in an open petri dish for one day while exposed to ambient air and moisture conditions to allow oxidation of the material before submission for analysis. X-ray diffraction analysis (XRD) performed by Southwest Research Institute, San Antonio, TX, U.S.A., determined the material to comprise disodium tungstate.

EXAMPLE III A further experiment was performed to demonstrate the ability of solvated electrons to decompose UF₆. The same laboratory set-up described in Example I was used. In attempting to transfer a sample of pure UF₆ from a cylinder of the material to the R.B. flask positioned in the hexane/dry ice cooling bath, 1.97 g of solid UF₆ material that had collected in the Teflon transfer tube from the cylinder to the flask was used. 2 g of sodium metal was dissolved in 250 ml of anhydrous liquid ammonia in the addition funnel and introduced very quickly into the R.B. flask containing the UF₆. Initial fuming was observed, but quickly subsided. After all the blue color from the solvated electrons faded, an additional 0.5 g of sodium metal was introduced into the reaction mixture. The blue color persisted for about 5 minutes, and then turned gray in color. The cooling bath was removed from the R.B. flask allowing the temperature of the reaction mixture to rise, thereby causing the ammonia in the mixture to evaporate. The solid residue remaining was collected for analysis.

Southwest Research Institute using x-ray diffraction analysis determined the solid material prior to treatment with water consisted of 45% sodium carbonate; 15% sodium uranium oxides; 15% uranium fluorides, 15% uranium oxides, 5% uranium metal and 5% uranyl hydroxide.

A portion of the solid material submitted to Southwest Research Institute was reacted with water and then analyzed. XRF analysis showed the elemental composition to consist of sodium, uranium, oxygen, carbon and silicon. Hydrogen, which may have been present, would not have been detected using XRF. Uranium was the main metallic constituent followed by sodium. XRD analysis showed the water treated sample consisted of 65% sodium uranium oxides and 35% uranium oxides, confirming further reaction of the metal halide and decomposition to products of a higher oxidation state.

Claims

WE CLAIM:

1. A method for decomposing a metal halide, characterized by the steps which comprise:

(i) forming a solution of solvated electrons by dissolving an active metal in a nitrogenous base in a sufficient amount to decompose a transition metal halide, and

(ii) reacting said transition metal halide with said solvated electrons to yield a reaction product in said solution comprising a substantially reduced amount of said transition metal halide.

2. The method of claim 1 characterized by the steps which further include:

(iii) separating the nitrogenous base from said reaction product to provide a solid residue, and

(iv) treating said residue with an agent suitable for decomposing any residual transition metal halide.

3. The method of Claim 2 characterized by said agent which is an oxidant or water.

4. The method of Claim 1 characterized by said metal halide which is a transition metal halide.

5. The method of claim 4 characterized by said transition metal halide which is uranium fluoride or depleted uranium fluoride.

6. The method of claim 3 characterized by said oxidant which is a member selected from the group consisting of a peroxide, ozone, a persulfate and a perchlorate.

7. The method of Claim 5 characterized by the depleted uranium fluoride which is depleted uranium hexafluoride.

8. The method of claim 1 characterized by said active metal which is a member selected from the group consisting of lithium, sodium, potassium, calcium and mixtures thereof, and the nitrogenous base is liquid ammonia.

9. A method for decomposing a transition metal halide, characterized by the steps which comprise:

(i) creating a reaction mixture prepared from raw material which include:

(a) a nitrogenous base;

(b) a metal halide, and (c) an active metal in an amount sufficient to decompose the metal halide, and (ii) reacting said mixture.

10. The method of claim 9 characterized by the further steps which include: (iii) separating the nitrogenous base after reacting said mixture to form a metal-containing residue, and (iv) treating the residue with an oxidant.

11. The method of claim 9 characterized by the reaction mixture which comprises solvated electrons and the transition metal halide is uranium fluoride.

12. The method of claim 11 characterized by the uranium fluoride which is UF₆.

13. The method of claim 11 characterized by the uranium fluoride which is depleted uranium hexafluoride.

14. The method of claim 9 characterized by said active metal which is a member selected from the group consisting of lithium, sodium, potassium, calcium and mixtures thereof.

15. The method of claim 9 characterized by said nitrogenous base which is a member selected from the group consisting of ammonia, amines and mixtures thereof.

16. The method of claim 15 characterized by the ammonia which is anhydrous liquid ammonia, and the amines are selected from the group consisting of ethylamine, ethylamine, propylamine, isopropylamine, butylamine and ethylenediamine .

17. A method for decomposing a transition metal halide, characterized by the steps which comprise:

(i) reacting a transition metal halide with a liquid ammonia to form an ammonia-metal halide intermediate, and

(ii) reducing the intermediate of (ii) with solvated electrons .

18. The method of claim 17 characterized by the further steps which include:

(iii) separating the liquid ammonia after reducing with solvated electrons to form a uranium-containing residue, and (iv) treating the residue with water.