I
K. L. ROHDE, C. E. MAY, B. J. NEWBY, and 6. D. WITHERS Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
Fission-Product Ruthenium Volatility at High Temperatures Volatile fission-product ruthenium tetroxide is relatively stable to thermal or redox decomposition in a vapor phase of air and nitrogen oxides at 400' C., thus representing a serious design problem in waste calcination
F I s s I o T - m o D u m ruthenium is a major hazard and complicates process design in many high temperature radiochemical processes. Ll-here nitric acid or nitrate salts are decomposed, high volatility of the ruthenium is expected and in the process described, it was definitely experienced. T h e process studied was the fluidizedbed calcination of the first cycle aqueous raffinate from solvent extraction reprocessing of spent aluminum-uranium fuel elements. Superficially, aluminum nitrate is converted to alumina and oxides of nitrogen. I t was hoped that the bulk of the fission products would remain with the alumina to be stored, leaving the oxides of nitrogen essentially free of contamination. I t was recognized that ruthenium would represent a problem. Three factors were involved : oxidation-reduction equilibria, often varying significantly with minor changes in temperature and system composition; the kinetics of the ruthenium oxidationreduction; and actual volatilization of the ruthenium tetroxide. Vapor pressure data for the tetroxide ( 3 ) are available but there are few other applicable fundamental data. T h e behavior of ruthenium in nitric acid recovery processes (4-6)is not directly applicable
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to the same chemical system a t temperatures above 100' C. Batch Calcination Two duplicate experiments were performed in a batch calciner consisting of a round-bottomed stainless steel vessel of about 300-ml. capacity heated electri. cally a t the base. The solution and solid temperatures were measured by a thermocouple entering from the top of the vessel and almost touching the bottom. The calciner off-gas was passed through a water-cooled condenser and the noncondensables were scrubbed twice in caustic scrubbers. The design of this equipment was necessarily simple, as full activity level raffinate was used as feed. T h e calciner was operated batchwise Lvith a feed of solvent extraction raffinate. T h e macro constituents were 1.7.44 aluminum nitrate, 1.OMnitric acid, and 0.1M sodium nitrate. T h e total beta activity was 1.6 X 109 counts/(min.) (ml.) and the ruthenium beta 5.1 X l o 7 counts/(min.)(ml.), the fission products being 2 year-cooled. At the start of the experiments a 100ml. portion of feed was drawn into the calciner, T h e calcination was done a t a slightly reduced pressure, about 640 m m .
INDUSTRIAL AND ENGINEERING CHEMISTRY
of mercury. T o conduct a calcination, the temperature was raised rapidly to about 100' C., then slowly, about 1 ' per minute, from 100' to 200' C.. and then rapidly to 400' C. Heating was then continued a t 400' C., the nominal calcining temperature for several hours. The condensate receiver was so designed that periodic samples could be withdrawn by hypodermic needle into a n evacuated bottle.
Continuous Calcination with Air Flow T h e calciner for use with air or other simulated fluidizing media flow is shown in Figure 1. The calcination takes place on the fritted stainless steel disk, with the alumina collecting there. T h e atmosphere in which the calcination reaction (and ruthenium oxidation) takes place is a mixture of the simulated fluidizing media and the calcination reaction products. T h e design of this experiment required that a small amount of synthetic raffinate (about 35 ml. per experiment) be fed to the calciner, followed by a n inactive acid purge. The total nitrate content of the two solutions was equal. At the start of each experiment the
calciner was preheated to the operating temperature, and the flow of preheated simulated fluidizing medium started prior to feeding liquid with syringe pumps. For interpretation of these data, the accumulative volatile activity was plotted against calcination time (proportional to the accumulative calciner feed volume) and the saturation value of the volatile activity was observed. I n general, the total ruthenium content of the scrubber was small compared to the ruthenium in the condensates (Table I).
RAFF I NATE FEED TO CALCINER
CALCINER OFF-GAS7
Discussion Calcination Chemistry. Because of the interaction of the chemistry of the macro constituents of the calciner feed stream with the fission-product ruthenium chemistry, it is essential to establish the compositions of macro constituents both in the solution feed to the calciner and in the calciner atmosphere and offgas. For studying ruthenium volatility, the equilibrium composition of the calciner off-gas has been calculated as follows. Aluminum nitrate was assumed to decompose completely to alumina, oxides of nitrogen, and oxygen. ' There may be some small quantity of basic aluminum nitrate in the calcine, but this is not considered significant for this study. Although aluminum nitrate de-
SCRUBBER
SAMPLER
Figure 1.
CONTROLLED AT WOO C.
Equipment for static calcining of aluminum raffinates
Calcination takes place on fritted stainless steel disks, with alumina collecting there
composition may proceed through intermediate partial hydrolysis steps, the final products are identical. Sodium n!trate was assumed to be stable a t 400' C., the calcination temperature. Nitric acid would yield water, oxides of nitrogen, and oxygen. I t was judged that the equilibrium be-
tween nitric oxides and nitrogen dioxide in the calciner and the calciner off-gas system would be significant in determining the calciner off-gas composition; therefore, the equilibrium constants for this reaction were calculated at several temperatures. These constants were used with the experimental flowsheet air and solution feed proportions to calculate equilibrium off-gas compositions (Table 11). T h e ratio of nitrogen
Typical Detailed Data for a Ruthenium Volatility Experiment of Continuous Calcination with Air Flow Portion Portion of B Acidity, Activity, of Total @ Total Ru in Stream N C./(Min.)(Ml.) in Feed, % Feed, %
dioxide to nitric oxide, ' 2 calculated ' ,
Table l.
Rr
Active feed Inactive feed Condensates
1,2,3 4
x
2.05 6.6'
5.5
104
4.5
1.2 x 104 1.3 X 106 8.3 x 103 2.2 x 103 138
0
... 6.4
5 through 13 14 through 18 Caustic scrubbers
6.0 1.75 basic
Total a
100 0
100 0
0.6 1.2 2.0 0.3 0.4 4.5
19 36 38 4 1 98
With 1.5M Al(N03)a present total nitrate was about 6 . 5 M .
Table 11. i
STEEL PLATE
Type of Simulated Fluidization Air NOd
NO K =
Equilibrium Compositions Were Significant in Determining Calciner Off-Gas Compositions
Temp.,
' C.
25 400 550 400 400
'NO
in this manner, was used as a parameter for correlation of the ruthenium volatility data. Because of the known instability of ruthenium tetroxide, observation of volatile ruthenium in a higher temperature process would depend not alone on oxidation to the tetroxide, but also on maintaining the oxidizing atmosphere throughout the system, so that decomposition would not occur in the vapor space away from the point of volatilization.
Ka 2
x 106 4 0.5 4 4
Off-Gm Compn., Mole Fraction 0 2 NO
N2
H20
0.60 0.60 0.60 0.0 0.0
0.20 0.20 0.20 0.34 0.20
0.16 0.17
0.17 0.0006 0.00014
5
NOn
x
10-7 0.01 0.023 0.61 0.76
0.028
NOn/NO Mole Ratio 6 X lo4
0.018 '
0.005, 0.05 0.04
2 0.2 0.09 0.05
% of Feedb Proved Ru Ru retained volatility on solids
s i '(5) 29 (2) 34 (1) 1.4 (2)
... ... 3le ... 90
PNO~!PNO, X. dg3. for NO(g) j-1/2 Oz(g) +.NOz(g). Free energy data are given in literature (1). Number of experiments under similar conditions indicated in parenthesis. In these two experiments at 550' C., 40% of ruthenium fed t o calciner was found on surface of calciner components. d(Nitricoxide used at slightly under flowsheet proportions. All other experiments run at identical vapor velocities.
VOL. 51,
NO.
1
JANUARY 1959
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pretreated feeds and simulated air fluidization. EFFECTO F CALCINATION TEMPERATURE. As the volatility of ruthenium had been observed to decrease (2) when the operating temperature of a fluid bed calciner was increased above 400'C. a series of experiments was run in the laboratory calciner a t 400 and 550°C. with simulated air fluidization. A significant reduction in the amount of volatile ruthenium was noted (Table 11). Conclusions
Figure 2. Volatility was markedly reduced when nitric oxide replaced air as fluidizing medium. The volatile tetroxide was not formed or the oxidation reversed so close to the solid that the ruthenium essentially remained on the alumina
Ruthenium Volatility Demonstrated b y Batch Calcination. By heating a batch of feed slowly through the critical temperature region, 100' to 150' C., it was possible to show the ruthenium oxidation and volatilization as a function of the progress of the denitration of a discrete batch of aluminum nitratenitric acid. With slow heating below 130' C. no ruthenium was volatilized with the dilute nitric acid. However, as the temperature passed 130 to 140 'C. there was a sharp increase in condensate ruthenium concentration. T h e additional material collected following this surge was not consequential. I t is presumed that a large amount of ruthenium never reached the surface of the solid cake. While the observed volatility increase was significant, the critical temperature may be associated with several concurrent changes: The extrapolated boiling point of macro ruthenium tetroxide would be about 120' C . a t the pressure of this calciner. T h e calciner condensate became markedly richer in nitric acid, suggesting that the oxidizing power of the calciner environment was increasing rapidly. The macro phase in the calciner was changing from a liquid to a solid. Of these three, the disappearance of the macro liquid phase would appear to account best for the sudden increase in the partial pressure of the ruthenium tetroxide. Continuous Static Calcination Simulating Steam Fluidization. With the continuously fed calciner the effect of steam fluidization was studied. This operating technique represented a process alternate. For this work aqueous
70
INDUSTRIAL AND ENGINEERING CHEMISTRY
feeds containing G.bM, 1.5iM,and 0.01M total nitrate were calcined. In spite of gross dilution of the feed with water and the resultant gross dilution of the calciner atmosphere with steam, no variation in the volatility was noted a t 400' C. Sixty per cent of the ruthenium in the feed was found in the condensate in each case. Continuous Static Calcination with Simulated Atmosphere. Three variables were studied with this equipment. NITRITETREATMEKT OF FEEDSOLUTION. Feed solutions and purge acids were prepared, first 0.1M in sodium nitrite and later 1.OM. A contact time of 30 minutes was allowed prior to use. Calcination of these feeds yielded very high recovery of the ruthenium in the offgas essentially equivalent to that experienced when air fluidization was used without feed treatment. T h e calcination temperature was again 400' C. NITRICOXIDEAs A SIMULATED FLUIDIZING MEDIUM.T h e modes of oxidation of the ruthenium, as well as the early recognition of the reversibility of this oxidation, strongly suggested that a n atmosphere low in oxygen or having a mole ratio would not mainlow N02,"O tain ruthenium in the oxidized state. Therefore, a series of experiments was performed in which nitric oxide replaced air as the simulated fluidizing medium. Under these conditions, a marked reduction in volatility was noted. T h e calculated calciner atmosphere compositions, as well as the amount of ruthenium tetroxide recoved, are shown in Table 11. These results are shown in Figure 2, with the results obtained with nitrite
When a nitrate salt or acid solution containing fission-product ruthenium is decomposed thermally above 100" C., the ruthenium is oxidized in a local reaction-Le., within the liquid or on the surface of the solid being formed-and appears as a volatile material, probably the tetroxide. Appreciable concentrations of nitrite ion added to the aqueous feed did not inhibit this oxidation, and it has been concluded that it would be difficult in practice to balance large quantities of fluidizing air or other oxidizing atmospheres with aqueous reducing agents. When the atmosphere in which the decomposition took place was reducing with respect to ruthenium tetroxide-Le., in the presence of nitric oxide-the volatile tetroxide was not formed or the oxidation reversed so close to the solid that the ruthenium essenitally remained on the alumina. Where the operating temperature was higher, 550' instead of 400' C., the ruthenium was found not so much on the solid as on the equipment in the high temperature region, suggesting that the ruthenium tetroxide did not decompose adjacent to the solid, where the temperature was limited by the evaporation of liquid, but rather on the heated walls of the equipment. Literature Cited (1) International Critical Tables, vol. VII, p. 239, McGraw-Hill, New York,
1930.
(2) Jonke, A. -4., Loeding, 3. W., Chem.
Eng. Division, Argonne Natl. Laboratory, private communication. (3) Lawroski, S., others, Chem. Eng. Division, Argonne Natl. Lab., Summary Report for January, February, and March 1950, ANL-4463, p. 67 (secret). (4) Ibid., ANL-4820, p. 89 (1952) (secret). (5) Ibzd., April, May, and June 1952, ANL-4872, p. confidential). ( 6 ) Wilson, A. ("Ruthenium Behavior in Nitric Acid Distillation," HW-45620 (Sept. 1, 1956) (confidential).
z!,
RECEIVED for review April 7, 1958 ACCEPTED October 27, 1958 Division of Industrial and Engineering Chemistry, Symposium on Reprocessing Chemistry for Irradiated Fuel, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
