Process for Producing Ammonium Diuranate

United States Patent im [in 3,758,664 Gerrald [45] Sept. 11, 1973

[54] PROCESS FOR PRODUCING AMMONIUM Primary Examiner—Carl D. Quarforth DIURANATE Assistant Examiner—F. M. Gittes Attorney—F. Shapoe [75] Inventor: Lonnie D. Gerrald, Columbia, S.C. [73] Assignee: Westinghouse Electric Corporation, [57] ABSTRACT Pittsburgh, Pa. A process for the recovery of uranium as ammonium [22] Filed: Dec. 29, 1970 diuranate from aqueous solutions having uranyl and fluoride ions such as produced by hydrolyzing UF6, and [21] Appl. No.: 102,451 for reducing soluble uranium losses in the liquid waste stream, by forming a first neutralized solution of uranyl [52] U.S. CI 423/15, 423/253, 423/261 fluoride (U02F2), with concentrated ammonium hy- [51] Int. CI COlg 43/00 droxide, at a pH of about 4.5 to 6.0, and then by precip- [58] Field of Search 23/335, 346, 355 itating essentially all of the uranium, as ammonium di- uranate, [(NH4)2U207], (ADU), from the solution by [56] References Cited adding concentrated ammonium hydroxide under con- UNITED STATES PATENTS trolled conditions preventing formation of any soluble fluoride complexes that would otherwise be ordinarily 3,579,311 5/1971 McCoy.. 23/335 formed during the precipitation of the ammonium di- 3,394,997 7/1968 Hollander 23/335 uranate, to produce an ADU slurry which is calcinable 3,272,602 9/1966 Suehiro et al 23/335 2,466,118 4/1949 Miller et al 23/335 to U02 having good ceramic pelletizing properties. 2 Claims, 1 Drawing Figure

22^ CONCENTRATED NHjOH SUPPLY t.g 24-29% NH3

-58.6 LBS.

-20

FIRST NEUTRALIZED SOLUTION a STORAGE 100 LBS. PH45-6D x(50 °F

26-,

U02F2 + HF+H20 U02F2+NH4F+HF+HgO Patented Sept. 11, 1973 3,758,664

. UJ Q li. i_ CaO o io«

WITNESSES: INVENTOR Lonnie D. Gerrold

ATTORNEY 3,758,664 1 2 PROCESS FOR PRODUCING AMMONIUM mation of the soluble uranium complexes, i.e., DIURANATE UOIFI-3NH4F. Once these uranium complexes are . formed they remain in solution and the uranium values BACKGROUND OF THE INVENTION are ^ Jcause they cannot be readjly recovered 1. Field of the Invention 5 therefrom and are carried away in the waste stream or This invention relates to a process for the recovery of require costly ion exchange or other treatments for uranium values from aqueous solutions containing ura- their recovery. nyl and fluoride ions, and, more particularly, it pertains In addition, the process of U.S. Pat. No. 2,466,118 to the conversion of uranium hexafluoride to ammo- employs a concentrated solution of ammonia (8 to IS nium diuranate by a two stage treatment with concen- 10 molar), because in that fluoride system the solubility of trated ammonium hydroxide. uranium is lowest when the ratio of ammonia to ura- 2. Description of the Prior Art nium is highest. Thus, the initial addition of concen- One method of preparing uranium dioxide (UOt) trated ammonium hydroxide is sufficient to precipitate from uranium hexafluoride (UFS), involves an initial a major portion of uranium, and suitably constitutes an reaction of the UF( with water to form an aqueous hy- IS excess over the stoichiometric quantity of ammonium drolysis solution of uranyl fluoride (UOjF,) and hydro- hydroxide required to neutralize any acidity and to pre- fluoric acid (HG). Thereafter, it has been one conven- cipitate all of the uranium in the absence of fluoride tional practice, as set forth in U.S. Pat. No. 2,466,118, ions. The resulting product, UOt, is finely divided and to add a concentrated solution of ammonium hydrox- is difficult to dewater and dry. ide greatly in excess of the necessary stoichiometric 20 The process of U.S. Pat. No. 3,394,997 employs a amount of ammonium hydroxide, preferably 2 to 3 large volume of dilute aqueous ammonium solution times greater than stoichiometric, to react with the ura- substantially less than 2 molar, and preferably 0.7 to 1 nyl fluoride solution in order to precipitate the uranium molar, to precipitate ammonium diuranate [ in the form of ammonium diuranate [ (NH4),Ui07 ]. (NH4)1Ul07 ], which is relatively crystalline and granu- The latter compound was then digested at a specific 25 lar in nature. While the resulting product UOj readily temperature after which additional ammonium hydrox- dewaters and dries rapidly it does not possess optimum ide was added, at least 5 times and preferably 7 to 13 ceramic sinterability properties. The process is still sub- times stoichiometric, to prevent substantial complexing ject to a complex recovery procedure to minimize ura- of the uranyl ions by any fluoride ions. It will be under- nium loss. stood that these processes required large excesses of 30 jn a copending application Ser. No. 865,232, as- ammonium hydroxide. Another variation thereof is dis- signed to the assignee of the present invention, there is closed in U.S. Pat. No. 3,394,997, wherein large vol- disclosed a process for converting a hydrolysis solution umes of dilute, less than 1.2 molar, ammonium hydrox- of uranium hexafluoride by adding thereto only dilute ide to provide from 7 to 12 moles of NH3 for each mole ammonium hydroxide (about 1 molar). The dilute am- of uranium. 35 monjum hydroxide necessarily introduce very large The precipitation of ammonium diuranate [ volumes of water which not only require larger reaction (NH4)zU207 ] with ammonium hydroxide is not com- and holding or storage vessels, and larger piping and plete at the stoichiometric point because of the com- pumps but also require more equipment to separate the plex nature of the uranyl fluoride solutions. A large ex- ADU precipitate from the water with dissolved NH4F. 40 cess of the ammonium hydroxide is used to drive the The large volumes of NH4F solution impose a disposal reaction to completion. An example of the reaction is problem, because of its large NH4OH content, set forth in the following formula: Although the specific reaction for the precipitation 2UOjFj + 4HF + 10 NH4OH - (NH4),U,07 + of UOjF, under equilibrium conditions with NH4OH 8NH4F + 7HjO (Ti should lead, at least as an initial step, to hydrated UOt. 45 (OH),, it is believed that other additional reactions A disadvantage of the foregoing one step procedure occur (such as dimerization, hydration, and ammonia- is that it requires a great excess of ammonium hydrox- tion) to preclude the exclusive specific reaction in- ide. A more critical shortcoming is that the ammonium tended. Stoichiometric ammonium diuranate has never diuranate precipitate formed does not completely sepa- been reported as being directly produced from aqueous rate and recover all of the uranium from the uranyl flu- medium directly in a single step so that polymerization oride. These reaction products often include from 1.5 most probably must occur. to 2.0 percent of uranium fluoride present as a mixed Ammonium diuranate is believed to be a more corn- fluoride, particularly UOtF,-3NH4F, which is relatively plex compound than is indicated by the commonly ac- soluble and which therefore leads to a loss of uranium J5 cepted formula (NH,),Ut07. There are indications that in the waste stream. Such fluoride complexes are ex- some molecules may have many atoms of uranium, up tremely difficult to break in order to recover the ura- to 7 or more, and that more than two (NH4) groups nium. Also it is desirable to produce ADU precipitate may be present. However, the formula (NH4)2U207 with the smallest possible traces of fluorides. represents quite closely the average composition of this From extended investigation it has been found that 6Q complex uranium compound. formula (1) defines an ideal reaction which in fact does SUMMARY OF THP IMVPNTION not occur quantitatively even under the most ideal con- SUMMARY OF THE INVENTION ditions. In practice it has been found that the one step In accordance with this invention it has been found reaction of formula (1) results in a number of side reac- that the foregoing disadvantages may be overcome by tions which occur so rapidly that the HF does not pref- 6S first partially neutralizing with concentrated ammo- erentially react to completion with NH4OH to produce nium hydroxide the hydrolysis solution of uranium hex- NH4F. Rather, other reactions occur simultaneously afluoride which contains uranyl fluoride (UOtFt) to between HF, UO,F2> and NH4OH, which result in for- form the compound ammonium fluoride (NH4F) with 3,758,664 3 4 the solution being at a pH of from about 4.5 to about ent. However, it is critical that the resulting first neu- 6.0, a good average being a pH of 5.0, which prevents tralized solution be acidic with a slight amount of HF the uranyl ion from entering fluoride complexing reac- remaining unneutralized so that the pH of the solution tions at that time or during subsequent reactions. The at equilibrium is from about 4.5 to about 6.0, so that no process requires an initial addition of a solution of con- 5 ADU precipitate forms. Accordingly, in formula (2) centrated ammonium hydroxide at least about 3N, for only the NH4OH and HF interact to form NH4F and example, about 24 to 29% NH3, to the uranyl fluoride H20, the UO^ being essentially unreacted at this solution to form ammonium fluoride with about stage. 85-95% of the HF, with only 5 to 15 percent of the HF By limiting the amount of the ammonium hydroxide being unreacted. After equilibrium is reached, more 10 added, as well as by controlling the resulting solution concentrated ammonium hydroxide (from 3 to 6 mo- pH, only water soluble ammonium fluoride is preferen- lar) is thoroughly and rapidly admixed with the first tially formed. Under these conditions any undesirable neutralized solution in a second step to form ammo- reactivity of the uranium compound is essentially zero nium diuranate precipitate almost quantitatively. The so that the formation of undesirable complexes, such as reaction is carried out with the first neutralized solution 15 U02F2'3NH4F, does not occur. Thus, as shown in for- being at about 150°F and the ammonium hydroxide mula (2) nearly all of the fluoride ions of the HF are being below 100°F so that the exothermic reaction does combined with the ammonium ions to form ammonium not raise the temperature above about 175°F. The fluoride, leaving the uranyl compound free to react quantity of ammonium hydroxide is such as to provide with subsequently added dilute ammonium hydroxide, from two to about three times the stoichiometric quan- 20 Because of the use of concentrated ammonium hy- tity needed to react with the uranyl fluoride to produce droxide, the volume of solution increases only slightly, ammonium diuranate therefrom. The pH of the solu- as compared to reactions using dilute ammonium hy- tion should be at least 9.5, and may reach a pH of 10 droxide of 1 to 1.2 normality. After equilibrium is or slightly more. A large excess, beyond this point, of reached the resulting first neutrali2ed solution is then ammonium hydroxide solution is to be avoided. Under 25 rapidly and thoroughly admixed with additional con- these conditions the ammonium diuranate reaction is centrated (about 3 to 6 Normal) ammonium hydrox- essentially complete and substantially no complexes of ide, by adding from at least two and up to three or uranium and fluoride result. A minimum volume of slightly more stoichiometric equivalents to react with water per mol of uranium is employed in both steps. the UOJFJ plus enough to neutralize the residual HF After digestion at from 40°C to 70°C for from 5 to 30 30 present. The first neutralized solution at from 125°F to minutes, the ammonium diuranate forms a readily sep- 175°F is rapidly mixed, as by spraying it into a spray of arable precipitate product which is flowable or pump- relatively cold ammonium hydroxide. The reaction able as a slurry in a small amount of water. When sub- proceeds in accordance with the following formula: sequently reduced to uranium dioxide by calcining at 2U02F2 + 7NH4OH 4- any HF -<• 2U02(0H)2 + 35 400°C to 1000°C, this ADU product results in uranium 5NH4F + 2NH4OH + H2Q - (NH4)2U207 + dioxide with excellent ceramic pelletizing properties 5NH4F + 4H20 3 for fuel element applications.

__ In this formula the qeauation is balanced at the assumed BRIEF DESCRIPTION OF THE DRAWINGS basis ^ one mole of HF is pr^ent for two moles of 40 For a better understanding of the invention reference U02F2 though in fact substantially far less HF would is made to the drawings, in which usually be present. The reaction-solution will be at FIG. 1 is a flow sheet of an embodiment of the pro- about 150°F. As shown, the intermediate product, ura- cess of this invention. nyl hydroxide, immediately reacts; with additional am-

„„ _IT_ monium hydroxide to form ammonium diuranate DESCRIPTION OF THE PREFERRED 45 (ADU) ^ precipitates out of soIution. Substantially complete precipitation of the uranium as ADU occurs In accordance with the present invention, the process and essentially no dissolved uranium compound is pres- for recovering uranium from an aqueous solution con- ent in the liquid. taining uranyl and fluoride ions, which aqueous solu- The two-step process involves a time relation. If the tion may be produced by introducing UF6 into water total reaction is carried out in a one step reaction by for example, in a ratio of one pound of UFg per 10 the addtion of NH4OH in substantial excess of the stoi- pounds of water thereby forming a hydrolysis product chiometric amount required to neutralize HF, the reac- thereof, involves two key steps. The first step proceeds tions are so rapid that the NH4F being formed cannot according to the following formula: be prevented from entering into the formation of the JJ U02F2 + 4NH4OH + 4HF - U02F2 + 4NH4F -f undesired uranium complexes before the critically nec- 4H20 + slight amount of HF, 2 essary U02(0H)2 forms. As a result of the two-step process less uranium is lost having an equilibrium pH of about 4.5 to 6.0. About 85 in the waste stream in the form of fluoride complexes to 95 percent of the HF is neutralized to NH4F, with which are difficult and costly to separate and to re- from 5 to 15 percent HF remaining. The ammonium cover. Relatively minimal amounts of waste liquid need hydroxide is concentrated, at least 12N, the preferred to be disposed of and pollution is minimal. The waste range being from about 24 to 29 percent NH3. As liquid is passed through an ion exchange column to ab- shown in the formula (2) the amount of ammonium hy- sorb any traces of uranium in the solution, droxide added to a solution containing uranyl fluoride 65 The ADU forms a fine, non-colloidal precipitate. In and'hydrogen fluoride is limited to slightly less than the order to improve the precipitate, it is pumped as a stoichiometric amount of ammonium hydroxide slurry to a digestor where it is held at from 40°C to needed to react with all of the hydrogen fluoride pres- 70°C for from 5 to 30 minutes. The digestor output 3,758,664 5 6

comprises a slurry with larger ADU particles which can (29% NH3) per hour and admixed rapidly with the hy- be readily treated in a centrifuge to extract a major pro- drolysis solution entering via conduit 18. This quantity portion of the liquid, leaving a flowable or pumpable of ammonium hydroxide will be sufficient to react with slurry containing the ADU as a major component. approximately 90 percent of the HF present in the hy- The latter slurry is then introduced into a calcining 5 drolyzate. The pH of the resulting first neutralyzed so- kiln where three successive reactions occur: first, in an lution will be about 5 ± 0.5. The temperature of the so- initial relatively cool zone the water is driven off leav- lution will be about 150°F. ing only ADU powder; secondly the ADU is thermally The first neutralized solution, comprising an aqueous decomposed at a higher temperature to U03 and UsOg, solution of UOjFj, NH4F, and HF, is pumped through and NH3 gas as well as steam are given off, and finally, 10 conduit 26 to an ADU precipitation tank 30 into which a reducing gas such as hydrogen is passed over the ura- it is sprayed at the rate of 1,168 pounds per hour. Since nium oxides while at from about 400°C to 1,000°C to the first neutralized solution is at about 150°F and the convert the uranium to uranium dioxide powder. reaction with ammonium hydroxide is exothermic so Traces of mechanically adsorbed or otherwise present that NHS would tend to flash off, the concentrated am- fluorides also are evaporated in the kiln. 15 monium hydroxide solution carried by pipe 36 from The fineness and structural nature of the ADU parti- Supply tank 22 is diluted with cold water carried into cles in the slurry fed to the calcining kiln determine the pipe 36 by pipe 34 from a tank 32. However, the most ceramic pelletizing qualities of the uranium dioxide rapid and complete reaction occurs when the solution powder. The ADU product of the process of this inven- mixture is between 125°F and 175°F. A desirable quan- tion has shown outstanding ceramic pelletizing quali- 20 tity is 1.5 gallons of cold water per gallon of concen- ties. When pelleted properly and fired, pellets of en- trated ammonium hydroxide. The diluted ammonium riched uranium dioxide of excellent quality for nuclear hydroxide solution is below 100°F, for example at 70°F reactor purpose are obtained. to 90°F, and is at least 3 molar, and preferably up to In the second step (formula 3), for each mole of ura- about 5N to 6N. About 650 to 700 pounds an hour of nyl fluoride present at least 7 moles (and preferably 15 25 5N solution may be used in this case. By spraying at 40 to 25 moles) of ammonium hydroxide are added. The the diluted ammonium hydroxide so that the spray im- ammonium hydroxide should be at least 3N, and is pinges upon and commingles with the spray of first neu- preferably about 6 Normal. tralized solution from spray head 38 rapid admixing It has been found that the uranium in the waste with essentially complete reaction takes place. There stream by the two step procedure of this invention is 30 results a fine precipitate of yellow ammonium diura- greatly reduced as compared with the losses in the best nate comprising substantially all of the uranium in the previously known single step procedure of conven- solution. The sprayed solutions form a slurry or suspen- tional methods wherein a 7 to 13 fold stoichiometric sion of fine ADU particles carried in the liquid in the excess of ammonium hydroxide to uranyl fluoride is tank 30. The pH in the tank 30 is at least 9.5, and pref- employed. This results because the process of the pres- 35 erably about 10.0 to 10.5. ent invention overcomes the disadvantages of prior In order to keep the ADU slurry suspended and the procedures; namely, the use of the minimal amounts of reaction completed, it is rapidly withdrawn from the water in the ammonium hydroxide, and the substan- bottom of tank 30 by a pipe 42 leading to a pump 44. tially negligible formation of relatively stable, water A major proportion of the slurry is returned from the soluble fluoride complexes with uranium, that lead to 40 pump outlet by conduit 46 to tank 30. A portion of the the presence of uranium in the waste stream. Once the pumped slurry is conveyed by conduit 48, branching complexes are formed they are exceedingly difficult to from conduit 46, to an ADU slurry digestor tank 50 recover. As a result, the present process enables the re- where it is held for from 5 to 30 minutes at from 40°to covery of nearly all of the uranium involved in the reac- 708C. After being held in the digestor, the fine ADU tion. 45 particles agglomerate to larger, readily separable parti- The following example illustrates the practice of the cles. invention: An outlet 52 conveys the digested ADU slurry to a suitable dewatering means, preferably a centrifuge EXAMPLE wherein most of the liquid comprising water having dis- Referring to the drawing, gaseous uranium hexafluo- solved therein NH«F and NH4OH, is separated. The ride is passed via conduit 12 at the rate of 100 pounds centrifuge is set so that the solid output of ADU has per hour into a hydrolysis tank 10. The gaseous ura- sufficient liquid so that it is pumpable or flowable so nium hexafluoride may be produced by heating a tank that it can be metered at a desirable rate into the cal- of UF„ to a temperature of, for example, 225°F to ss cining kiln. In the kiln the water is evaporated, the dry 250°F. Pure water is introduced at the rate of 1,010 particles of ADU are thermally decomposed and the pounds per hour into tank 10 from a supply tank 14 via resulting uranium oxides are reduced to a uranium di- pipe 16 by spraying or trickling over a packing where oxide powder. it absorbs and hydrolyzes the UFS as follows: It is difficult to examine any specific ADU precipitate UFS + 2HjO —> UOjFj + 4HF 60 and judge its ultimate ceramic pellet sinterability char- The reaction is exothermic (and steam is added) so that acteristics. It is known that an extremely fine precipi- the resulting*hydrolyzate is at about 150°F. tate has poor ceramic properties, as does a very coarse The hydrolyzate from tank 10 is conveyed by conduit particle. The best test is to actually prepare compacts 18, at the rate of 1,110 pounds per hour, to a first neu- of the UO, powder under standard conditions of from tralizing tank 20. Concentrated aqueous ammonium 65 about 5,000 to 50,000 psi pressure and to sinter at hydroxide at least 12N, and preferably having from 24 from 1,600°C to 1,800°C, and testing the pellets for to 29 percent NH3 concentration, is carried from sup- their strength, shape, homogeneity and freedom from ply tank 22 by a conduit 24 at the rate of 58.6 pounds cracks and other flaws, that one can characterize the 3,7 i >8,664 7 8 ceramic pelletizing characteristics of the ADU 2. The process of converting uranium hexafluoride to particles. The ADU product of the present invention uranium dioxide powder having good ceramic pelletiz- has shown excellent ceramic properties and pellets ing properties, comprising the steps of: of enriched uranium dioxide, for example 3 percent a. contacting water with the uranium hexafluoride to enrichment, have been made therefrom into fully 5 produce an acidic aqueous hydrolysis solution con- acceptable nuclear fuel elements. taining uranyl fluoride and hydrogen fluoride, I claim as my invention: b. adding aqueous ammonium hydroxide of a con- 1. A process for the substantially complete recovery centration of at least 6 molar to the acidic aqueous of the uranium in uranium hexafluoride as ammonium hydrolysis solution to produce a first neutralized 10 diuranate, comprising the steps of: solution having a pH of from 4.5 to 6.0, the added a. contacting water with the uranium hexafluoride to ammonium hydroxide being sufficient to react with produce an acidic aqueous hydrolysis solution con- only from about 85 to 95 percent of the hydrogen taining uranyl fluoride and hyrogen fluoride, fluoride in he acidic solution and with substantially b. adding aqueous ammonium hydroxide of a con- none of the uranyl fluoride, centration of at least 6 molar to the acidic aqueous 15 c. thereafter rapidly admixing the first neutralized so- hydrolysis solution to produce a first neutralized lution while it is at a temperature of from 125°F to solution having a pH of from 4.5 to 6.0, the added 175°F with aqueous ammonium hydroxide at a ammonium hydroxide being sufficient to react with temperature below 100°F and of a molarity of at only from about 85 to 95 percent of the hydrogen least 3 in an amount to provide at least two times fluoride in the solution and with substantially none 20 the stoichiometric quantity needed to convert all of of the uranyl fluoride, the uranium in the first neutralized solution to am- c. thereafter rapidly admixing the first neutralized so- monium diurante, the resulting solution having a lution while it is at a temperature of from 125 °F to pH of at least 9.5, and the ammonium diuranate so 175°F with aqueous ammonium hydroxide at a 25 produced precipitating rapidly as a fine precipitate temperature below 100°F and of a molarity of at to form a slurry, least 3 in an amount to provide at least two times d. digesting the slurry at a temperature of about 40°C the stoichiometric quantity needed to convert all of to 70°C for a period of about 5 to 30 minutes, the uranium in the first neutralized solution to am- e. separating a major portion of the liquid from the monium diuranate, the resulting solution having a pH of at least 9.5, and the ammonium diuranate so 30 digested ammonium diuranate particles, only suffi- produced precipitating rapidly as a fine precipitate cient liquid being present to produce a flowable or to form a slurry, pumpable ammonium diuranate slurry end prod- uct, and d. digesting the slurry at a temperature of about 40°C f. calcining the ammonium diuranate slurry at tem- to 70°C for a period of about 5 to 30 minutes, and 35 peratures up to about 1,000°C whereby to evapo- e. separating a major portion of the liquid from the rate all the liquid, and decompose the ammonium digested ammonium diuranate particles, only suffi- diuranate in the presence of a reducing atmosphere cient liquid being present to produce a flowable or to produce a pure uranium dioxide powder. pumpable ammonium diuranate slurry end prod- * * * * * uct. 40

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