literature Cited
(1) Christiansen, R. M., Hixqon, A. N., Znd. Eng. Chem. 49, 1017 (1957). (2) Elzinga, E. R., Banchero, J. T., Chem. Eng. Progr. Symp, Ser., No. 29, 5 5 , 149 (1959). (3) Garner, L., Skelland, B. D., Trans. Inst. Chem. Engrr., London 2 9 , 315 (1951). (4) Garwin, L., Smith, B. D., Chem. Eng. Progr. 49, 591 (1953). (5) Handlos, A. E., Baron, T., A . I . Ch. E. J . 3, 127 (1957). (6) Hughes, R. R.? Gilliland, E. R., Chem. Eng. Progr. 48, 497 (19 52). ( 7 ) Kays, W. M., London, A . L., “Compact Heat Exchangers,” National Press, Palo Alto, Calif., 1955. (8) Kramers, H., Physica 1 2 , 61 (June 1946). (9) McDowell, R. V., Myers, J. E., A . I . Ch. E . J . 2 , 384 (1956).
(10) Minard. G. LV.. Johnson, A. I., Chem. Eng. Pro~yr. 48, 62 (1952). (11) Reman, G. H., Chem. Tech. 1 2 , 128 (August 1957). (12) Rodger, LV. .4.,Trice, V. G., Rushton, J. H., Chem. Eng. Pro,gr. 5 2 , 515 (1956). (13) Sherwood, T. K., Evans. V. E., Longcor. J. V. A , . Ind. Eng. Chem. 31, 1144 (1939). (14) Vermeulen. T., LVilliams, G. M., Langlois, G. E.: Chem. Eng. Progr. 5 1 , 85-F (1935). (15) LVood\sard, T.: “Heat Transfer in a Spray Column.” FiftyThird Meeting, A.I.Ch.E., LVashington, December 1960.
RECEIVED for review August 20: 1963 ACCEPTED September 30, 1964
TRIFLUORIDE PROCESSING OF PLUTONIUM METAL RECYCLE J
. M
.
C L EV E LA N D
,
Rocky Fiats Dicision, The Dow Chemical Go., Golden Colo.
Plutonium metal recycle may be reprocessed by a one-step method, involving simultaneous dissolution and precipitation as the trifluoride with a hydrochloric acid-hydrofluoric acid mixture. After filtration and drying, the trifluoride may b e reduced with calcium to plutonium metal (using iodine booster), or converted to PuFd and reduced to metal (without booster) in yields averaging 98y0. Filtrate losses are of the order of 10 p.p.m. of the metal processed. The process achieves good purification with respect to iron and carbon, but i s only 25 to 6Oy0effective in removing aluminum.
C tinely
Plutonium trifluoride is not hydrated, and may be dried easily. and. if desired, reduced directly to the metal Lvithout fluorination.
Hydrofluoric acid is more stable than hydrogen peroxide. Filtrates contain such a low concentration of plutonium that the)- may be discarded irithout further treatment. Plutonium t*ifluoride is more stable than plutonium peroxide.
T h e disadvantages of trifluoride precipitation are the limited purification achieved and the need to use hydrofluoric acid. but in many cases these dra\vbacks are over shado\red by the advantages of the process. Although the process described by O r t h is applicable only to nitrate solutions, the advantages of trifluoride precipitation recommend it for the processing of other plutonium-containing materials, such as metal recycle. This material. consisting primarily of casting skulls, reject metal buttons. and lathe turnings, is generated in considerable quantities a t metal fabrication plants and, if sufficiently pure, may be recycled simply by melting and recasting. Since impurities-particularly iron-tend to concentrate in the recycle material, however, much of it must be purified chemically before re-use. This purification normally involves ignition of the metal to oxide, dissolution by extended refluxing in concentrated nitric acid containing approximately 0.2j.U fluoride, and treatment by one of the precipitation processes described above. This procedure suffers from the slow rate of dissolution of plutonium dioxide and the relatively large number of steps; in addition, the decontamination obtained is often unnecessary. except in the case of iron. Furthermore. the catalytic effect of iron on peroxide decomposition requires that dissolver solutions containing high concentrations of iron be purified-generally by ion exchange-before peroxide precipitation. To eliminate these disadvantages. a trifluoride process was developed for the treatment of metal recycle. Description of Process. Hydrochloric acid dissolves plutonium much more rapidly than nitric-hydrofluoric acid, and \vas therefore chosen as the dissolution agent for this process. Simple dissolution of the metal followed by precipitation of the
of plutonium nitrate solution to metal is rouaccomplished by precipitation of a n insoluble plutonium compound which is then converted to PuF4 and reduced to the metal with calcium ( 7 , 2). Three precipitation processes are in use, involving the precipitation of plutonium as the oxalate ( 7 , 2 ) , peroxide ( 7 , 2, 3 ) , or trifluoride ( I ) , respectively. Each process has specific advantages and limitations. Oxalate precipitation involves solutions and compounds that are more stable than those in the peroxide process and less corrosive than the solutions employed in the trifluoride process. I t is more satisfactory than peroxide precipitation for processing solutions which contain high concentrations of impuritiessuch as iron-that catalyze peroxide decomposition. O n the other hand, oxalate precipitation does not achieve the high degree of decontamination of the peroxide process; in particular. complete carbon removal is difficult, and thorough calcination and fluorination are necessary to obtain a product of acceptable purity. Peroxide precipitation affords the highest degree of decontamination of the three processes. because of the limited number of elements that form insoluble peroxides. Furthermore. since the excess hydrogen peroxide may be destroyed by heating, recycle is simpler. T h e peroxide process suffers from the instability of peroxides (a problem which can be controlled, however) and from the relatively high filtrate losses compared to the other two processes. T h e trifiuoride process has been described by O r t h (-0, \vho lists its advantages as follows: ONVERSIOK
230
l&EC PROCESS D E S I G N A N D DEVELOPMENT
trifluoride by the addition of hydrofluoric acid produced precipitates that \\-ere pasty and difficult to filter, unless the solutions \\-ere diluted to a point where filtrate losses and waste voliinies became excessive. T h e contradictory requirements of a readi1)- filterable precipitate and small solution volumes \\.ere met by the use of a hydrochloric-hydrofluoric acid mixture. in \vhich the metal is simultaneously dissolved by hydrochloric acid and precipitated as the trifluoride by hydrofluoric acid. 'The instantaneous concentration of plutonium in solution is small; thus in effect a homogeneous precipitation takes place. ivith the result that a precipitate \vith satisfactory filtration characteristics is obtained Liithout resorting to large sollition volumes P r o c e d u r e , Dissolution-precipitation of massive metal (casting skulls, etc.) is accomplished by the dropwise addition of a n acid mixture 3.5.11 in HC1 and 12h.1 in HF. Approximately 1750 nil. of this acid mixture are required per kilogram of metal processed. Dissolution is rapid, but may be controlled by the rate of acid addition. Once the metal becomes submerged, agitation (Lvith a n electric stirrer) is necessary to ensure adeqiiate mixing. .4fter the reaction is complete, the precipitate is digested for 30 minutes and filtered with a sintercd platinum filter. Filtration time varies: but averages about 1 hour. Because of losses by evaporation during dissolution. the filtrate has a volume (per kilogram of metal processed) ol only about 1 liter. a n d normally has such a low plutonium conten-about 0.01 graxn--that it may be discarded. T h e precipitate is tvashed \vith dilute HF. water, a n d alcohol. a n d air-dried a t room temperature. Csually drying requires 16 to 18. hours. but sometimes a longer period is necessary.
.A different procedure is necessary for the processing of metal turnings because of their larger surface area a n d consequently greater reactivity. Furthermore. because of their high percentage of void space, they are somewhat insulated from one another, thus allowing heat buildup in a local area a n d a n attendant fire hazard due to the pyrophoricity of plutonium. 'l'he safest procedure for processing turnings (which must be free of heterogeneous insoluble impurities, such as organic matter) consists in adding them. a small portion a t a time. to 12.W ml. (per kilogram of metal) of a 5 M HCI-5M H F mixture. A s the addition of turnings proceeds, t\vo 250-ml. portions of 27214 H F are added to the solution. (If the H F is not added incrementally. the high initial concentration of this acid inhibits the dissolution. and can result in a n undesirable accumiilation of metal in the solution before dissolution begins.) Subsequent treatment is the same as that described for massive metal. hietal tiirnings compressed into briquets cannot safely be treated by either procedure. There is still sufficient void space to cause a fire hazard if the acid is added to the metal, Xvhile addition of the metal briquet to the acid results in a n uncontrollable reaction. Briquets are actually more reactive than uncompressed turnings, probably because of strains introduced during compaction. Equipment. All of the process equipment is polyethylene or polyethylene-coated to minimize corrosion. T h e dissolution-precipitation is conducted in a covered polyethylene beaker. with the off-gases being passed through a caustic scrubber. T h e dissolver must be critically safe or else batch sizes must be limited to avoid nuclear hazard. Furthermore, the equipment should be adequately shielded to protect operating personnel from the neutron flux associated with plutonii.im fluorides. Because of the toxicity of plutonium, all operations must be performed in proper'ly ventilated glove boxes. D r y Chemistry. As O r t h ( 4 )has discussed, several methods exist for conversion of PuF3 to metal: P u F 3 may be roasted a t 600' C. in argon for 1.5 hours a n d then reduced directly to metal with calcium, using iodine as hoostrr.
Table I.
Purification Data Analyris of Pioduct M e t a l ,
Analysis of Feed M e t a l , P.P.4f. . Fe AI C Cr -Vi
687a 687= 687" 448 921 11 .200" 11?20Oa 11 ,200a 4. 000a 4: 000"
a
93 759 45 13 13 93 759 45 93 759 45 13 81 443 125 35 75 22 85 152 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
.
.
.
.
__
-
~
Fe
133 252" 252h 276 331 966 1395 873 707 458 745 233
~~
PPM A1 C 73 76 78 77 78 77 63 245 93 77 93 61 41 132 19 134 36 129 . . . . .
Cr 25 53 53 38 70
L $ ~
3 14 14 12 17
. . . . . . . ., .. , , ..
. . . .
. . . . . . . . . . . . . . . . . .
Two runs combinrd f o r reductton.
Same feed material.
l h e trifluoride may be oxygenated a t 400' C. for 2 hours and thereby converted to a mixture of PuF4 and PuOs which may be reduced to metal in 93 to 97y0y-ield Lvithout booster. PuF3 may be fluorinated with HF (containing a small amount of oxygen) a t 600' C. to produce PuF4. \vhich is then reduced to metal. This procedure requires the use of a fluorination furnace \Yith its attendant corrosion problems, but it eliminates the need for iodine booster Lvithout a sacrifice in yield. Reduction yields almost invariably are in the vicinity of 987&/,. Process Performance. Filtrate losses are much lower than minimum discard levels. A total of only 0.2 gram of plutonium was discarded in the filtrates from the processing of 22 kg. of metal. Purification data are reproduced in Table I . Significant decreases in concentration are noted for iron and carbon, b u t purification with respect to aluminum is poor. Removal of chromium and nickel is erratic; but these impurities are not often a problem. These values apply only to homogeneous impurities present in the metal. Insoluble heterogeneous impurities (such as organic matter) that may be present \vith metal turnings will remain with the precipitate during filtration and ultimately contribute to the impurity level of the product metal. For this reason, it is important that heterogeneous impurities be removed mechanically before procrssing. Metal with a coating of plutonium dioxide may be processed by this procedure: since the insoluble oxide \vi11 remain with the trifluoride and will largely be reduced to the metal. Obviously, the process is not suitable for the treatment of pliitonium dioxide alone. T h e filterability of the PuFa precipitate could probably be improved further by the use of a more dilute H C - H F solution. However, since filtration is acceptably rapid. any improvement in filtration \vould probably not justify the accompanying increased filtrate losses. literature Cited
(1) Harmon, K . M.; Keas: i V , H.>in "Process Chemistry," "Progress in Nuclear Energy," F. R. Bruce, J. M. Fletcher? H. H. Hyman, eds., Ser. 111, Vol. 2, pp. 184-93, Pergamon Press, New York: 1958. (2) Harmon, K. M.. Reas, LV. H., U. S. At. Energy Comm., Rept. TID-7534 (Rook 1): 332-48 (1957) ; HW-49597A (April 11 II.
1nc7\ I,,',.
(3) Mainland, E. iV., Orth, D. A., Field, E. I>., Radke, J . H., Ind. Eng. Chem. 53, 685 (1961). (4) Orth, D. A,, ISD. ENG.CHEM.PROCESS DESEXDEVELOP. 2,
121 (1963).
RECEIVED for review June 29, 1964 ACCEPTED October 19, 1964 Work performed for the U. S. Atomic Energy Commission under
Contract AT(29-1)-1106.
VOL. 4
NO.
2
APRIL
1965
231
