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J. R. LACHER, JOHN D. SALZMAN,I and J. D. PARK University of Colorado, Boulder, Colo.
Dissolving Uranium in Nitric Acid Dissolution rate can be increased by considering these factors
URANIUM
METAL exhibits a rather Alinteresting behavior in " 0 3 . though it is a rather active metal which reacts vigorously or even explosively with H N 0 3 when finely divided, reaction with a metal mass occurs at moderate rates and is accompanied by formation of nitrogen oxides. Although "0% is probably the most commonly used reagent for dissolving uranium, comparatively few publications have appeared on this subject; these are often concerned with dissolution of uranium alloys ( 7 , 7) rather than pure metal (2, 3, 6). Miles (5) found that dissolution of uranium in HSO, could be accomplished without the emission of oxides of nitrogen, on a laboratory scale, by introducing oxygen into the dissolver. Methods for dissolving uranium metal and its alloys, with emphasis on the preparation of solution for analysis, have been reviewed ( 4 ) . The present investigation was undertaken to obtain quantitative information concerning the influence of various factors on the dissolution rate of relatively pure uranium metal. It was found that depended the dissolution rate in " 0 3 on the presence of impurities and metallurgical treatment. If the sample contained 400 to 500 p.p.m. of carbon, the metallurgical process of rolling caused the metal to dissolve rapidly; however, if the carbon content was around 35 p.p,m., the amount of work had little effect on the rate. Samples cut from rolled rods were anisotropic, and the rate on a surface perpendicular to the rod was four times faster than on a surface parallel to it. Samples rotated in Hh-03 dissolved more slowly than those held still. The rate increased with increasing temperature, "03, and H S O z concentration. In dilute H S 0 3 the metal became passive.
Samples which received the most work showed the smallest grain size, whereas those which received no work showed the largest grain size. The metal samples themselves were of various shapes. Samples from Lots 5, 3, and 1 were pie-shaped wedges, resulting from the quartering of disks which were approximately 0.5 inch thick and which had been cut from rods 1 inch in diameter. Samples from Lots 2, 4, and 6 were similar, except that they were obtained from rods 1.4 inches in diameter. Samples from Lot 10 were obtained from a rod 1.6 inches in diameter. Samples from Lots 7, 8, and 9 were in the shape of cubes approximately 0.5 inch along an edge. Procedure. The initial measured surface area of the metal sample was obtained by means of a micrometer. T o remove oxide from the metal surface, it was placed in concentrated " 0 3 for approximately 1 minute, then removed, rinsed with watcr and acetone, dried, and weighed. The sample was then reacted with " 0 3 at the desired temperature and concentration. The 1 liter of acid used provided a sufficient excess to prevent an appreciable change in concentration during the course of the reaction. At definite time intervals. the sample was removed from the acid, and again rinsed, dried. and weighed. The weight loss in milligrams per square centimeter of initial measured surface
bath. The metal samples (cubes or pieshaped wedges weighing 20 to 50 grams) were supported in the acid by a sample holder consisting of two Teflon disks (approximately 1 inch in diameter) between which two opposite corners of a metal sample were clamped and to which a Chrome1 rod was fastened. The rod was passed through a Teflon stopper and connected to a stirring motor, which provided continuous rotation of the metal sample in the acid. Materials. The uranium samples (furnished by the Atomic Energy Commission through the Uranium Division, Mallinckrodt Chemical Works) were from 10 different lots of varying analyses and metallurgical treatments (see table). The lots were classified according to metallurgical treatment. Lots 5, 3, and 1 received a large amount of work (rolling) and Lvere then allowed to cool in air. Lots 2, 4, and 6 received a moderate amount of work. Thereupon, they were heated to 600' to 700' C. for a a period and then quenched rapidly. Lots 7, 8, and 9 were not rolled and did not receive heat treatment. Lot 10 received only a small amount of work and no heat treatment. Photomicrographs of the surfaces of samples from the various lots, taken under polarized light after the metal was etched indicated in HC1 and rinsed in "03, that the grain size of the samples varied with the amount of work received.
Experimental
Equipment. The reaction vessel consisted of a 1-liter round-bottomed flask immersed in a constant temperature 1 Present address, Chemistry Department, Northern Illinois University, DeKalb, Ill.
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20
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0
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TIMEZ HOURS
Figure 1 . Effect of impurities on dissolution rate is apparently augmented by metallurgical treatment the sample received
INDUSTRIAL AND ENGINEERING CHEMISTRY
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area was plotted us. time in hours. The dissolution rate was obtained from the slope of this curve.
Massive uranium metal dissolves in concentrated " 0 3 to produce U02(NO3)z and NO2. At lower acid concentrations the gaseous product is mainly NO. Factors in this investigation which influenced dissolution rate of massive can be conuranium metal in " 0 8 veniently divided into two groups: 0
Factors inherent in the sample being dissolved-impurities, treatment, grain size, shape, and surface area Factors easily varied experimentally -acid concentration, temperature, factors influencing concentration of reaction products
Effect of Impurities a n d Metallurgical Treatment. Samples from the various lots of uranium metal were rotated a t 0' C. Disin concentrated " 0 3 solution rate (Figure 1) was easily correlated with the the carbon and nitrogen contents of the samples. Within any work class the samples containing relatively large amounts of carbon and nitrogen dissolved more rapidly than those in which the carbon and nitrogen contents were relatively small. Since dissolution rates of the more rapidly dissolving samples tended to increase as the reactions proceeded, the initial dissolution rates are shown in the table along with the carbon and nitrogen contents of the various lots. Since these impurities are present in very qmall quantities with respect to the amount of uranium present, their effect upon the reaction is assumed to be catalytic. Samples of uranium from lots which had been worked tended to dissolve more rapidly in the direction of the axis from which they were cut. By protecting certain surfaces from reaction with the acid, it was found that the initial dissolution rate of samples from a t 25' C. was apLot 3 in 8N " 0 3 proximately four times faster on the top and bottom than on the sides of the pie-shaped wedges. Samples from lots with low carbon and nitrogen analyses (Lots 1, 6, 9, 10) dissolved slowly regardless of whether or not they had been worked (Figure 1). However, samples containing larger amounts of carbon which had received moderate amount of work (Lots 2 and 4) dissolved more rapidly than samples with similar analysis which had not been worked (Lots 7 and 8). Likewise, samples which had received a large amount of work dissolved even more rapidly, provided their carbon and nitrogen content was relatively large (Lots 3 and 5). Since grain size of the metal decreased as the amount of work
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Uranium Samples H a d Varying Analyses and Metal Treatments
Density, b a l Y S i S * Grams/
Results and Discussion
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P.P.MC N
Lot
CC.
5 1
18.93 18.87 18.98
440 350 35
109 73 9
1 4 6
18.88 18.90 19.04
491 430 15
70 44 26
7
18.94 19.00 19.04 19.03
560 285 20 19
43 13 13 6
'3
8 9 10
Amount of
Worka High High High Moderate Moderate Moderate None None None Little
Heat Treatment None None None Yes Yes Yes None None None None
Grain Size
Initial Dissolution Rate, Mg./Sq. Cm./Hr
Small Small Small Intermediate Intermediate Intermediate Large Large Large Large
0.65 0.75 0.11 0.65 0.50 0.13 0.83 0.30 0.26 0.12
hletallurgical process of rolling.
increased and samples which had been worked dissolved more rapidly along the axis of the rod from which they were cut, the effect of metallurgical treatment on dissolution rate is probably due to size and orientation of metal grains. Effect of Shape a n d Surface Area. The anisotropic dissolution of uranium metal which had been worked also caused the shape of the metal sample to be a factor influencing dissolution rate. The rate for samples cut from a rolled rod increased as the fraction of the total surface area perpendicular to the axis of the rod was increased. Thus, in investigating the dissolution rate of metal which has been worked, it is necessary that the samples have similar shapes. The total surface area of a metal sample obviously increases as it is attacked by the acid and cannot be easily measured. Dissolution rates, expressed in terms of measixed initial surface area, become less valid as the reaction proceeds, and only initial rates of dissolution are of value for comparison purposes. The apparent
Figure 2.
increase in dissolution rate of uranium metal as the reaction proceeds is a t least partially due to an increase in total surface area. Effect of Concentration of Products of t h e Reaction. The reaction of uranium metal with " 0 3 is apparently autocatalytic, since factors which tended to increase the concentration of reaction products also increased the reaction rate; likewise, factors which decreased the concentration of products also decreased the rate. Agitation of the solution decreased the dissolution rate considerably. After 90 hours in concentrated H T u - 0 3 a t 0' C., the average dissolution rate of a sample from Lot 8 which was allowed to remain a t rest during that period was nearly eight times greater than that of a similar sample rotated a t 360 r.p.m. Increasing. the rotation rate further decreased the rate of dissolution. The effect of agitation became more pronounced as the reaction proceeded and the concentration of the products increased. Initial dissolution rates were not affected by rotation rate.
Nitrite addition enhances dissolution rate VOL. 53,
NO. 4
APRIL 1961
283
0.c
0
= 25OC.
0'
2 W
ooc.
-1.c
a a ,
s - 2r
-3.c 0.50
I
I
I
I
I
I
0.60
0.70
0.80
0.90
1.00
1.10
I.,
LOG (ACID)
Figure 3.
Dissolution rate i s strongly dependent on acid concentration
The presence of urea also decreased the dissolution rate. After 8 hours of rotation in concentrated " 0 3 at 25' C., the average dissolution rate of a sample from Lot 2 was 13.5 times greater than that of a similar sample rotated in acid saturated with urea (0.5M). The dissolution rate of uranium metal can be increased considerably by the Figure addition of nitrite to the "03. 2 shows the effect of added KNOz on the initial dissolution rates of a sample and a sample from Lot 9 in 15.6N " 0 3 from Lot 7 in 6.0N "03, both a t 25' C. Between nitrite concentrations of approximately 0.25 and 0.2M the curves appear to be linear. In this region, for the reaction in 15.6N HNO; the slope is 0.5; for the reaction in 6 . O N acid, the slope is 0.6. The first point
on each curve represents the initial dissolution rate in acid to which no Kh-02 had been added. The O.OO1M nitrite had very little effect on the initial rate in concentrated "03. However, this concentration of nitrite had a very definite effect on the initial rate in 6.011'acid. Uranium apparently becomes somewhat passive, at least initially, when placed in dilute HNOa. The presence of small amounts of HXOz tends to destroy this passivity. By the addition of sufficient KNO2, the dissolution rate in 6 S H N 0 3 could be increased to a value equal to the initial rate in 15.6NHNO3. Varying the concentration of UOZ(NO3)a affected the dissolution rate only slightly; hence, the reduction products of H N 0 3 must be largely responsible
for the autocatalytic effect that occurs when uranium is dissolved in H;\;03. Since the presence of urea decreased the rate of the reaction and the addition of KSOz increased it, the substance responsible for the catalytic effect probably is H S 0 2 . The effect of agitation is thus to prevent the accumulation of Hh-02 on the surface of the metal. The increase in dissolution rate of uraas the reaction proceeds is nium in " 0 3 then a result of two factors: increase in surface area of the metal and increase in concentration of reaction products. Effect of Acid Concentration. The effect of acid concentration at 25" and 0 " C. on initial dissolution rates of samples from Lot 10 is shown in Figure 3. The dependence of the rate on acid concentration at 25 " was very similar to that at 0 " C. The maximum initial rate was obtained in 13 to 1411' acid. At higher concentrations it decreased sharply. The curve drawn for the reactions a t 25' C. has a slope of 5.5, ' C. has a and that for the reactions a t 0 slope of 6.5. The highest value of acid concentration included on the curves is 12-47, The passivity of uranium in was even more pronounced dilute " 0 3 for samples from Lot 4 when the oxide was removed from the metal by reaction with 6.0X acid than !\Then it was removed by reaction with 15.6N acid. Effect of Temperature. The initial dissolution rates of samples from Lor 10 and of samples from in 1 5 . 6 N " 0 3 Lot 6 in 8.12%' " 0 3 were measured at OD, 2jo,and 50' C. Figure 4 sholvs plots of the logarithm of the initial ratc us. the reciprocal of the absolute temperature. From the slopes of these lines the values of 15.9 and 12.2 kcal. per mole were calculated for the activation energy of the reaction in 15.6 and 8.1N "03, respectively. Acknowledgment The authors thank Karl Kuhlman for chemical analysis and photomicrographs of the metal samples.
0
5
8.1 N HNO:,
0=15.6 N HNO,
Figure 4. Activation energies of the reaction were calculated for dissolution in 15.6 and 8.1 N "03 from slopes of these lines
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literature Cited (1) Boeqlin, 4. F., Buckingham, J . A . , Chajson, L., Lemon, R. B., Paige, M. D , Stoops, C. E., A . I. Ch. E. Journal 2, 190-4 (1956). (2). Cunningham, T. R., Manhattan Project, Chicago Rept. N-42,undated. (3) Hyde, A. C., Zbid, CN-1751, 1944. (4) Larsen, R. P , Anal. Chem. 31, 545-9 (1959). (5) Miles, G. L., At. Energy Research Estab. (Gt. Brit.), Rept. C/R 1804, 1955. (6) Sutton, J. B . Manhattan Project, Chicago Rept. CN-566, 1943. (7) Wymer, R. G., Blanco, R. E., IND. ENG.C H E b i . 49, 59-61 (1957). RECEIVED for review October 7, 1960 ACCEPTED January 9, 1961 Work supported in part by U. S. Atomic Energy Commission through Uranium Division, Mallinckrodt Chemical Works.
