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Reactions of uranium mononitride, thorium monocarbide and uranium monocarbide with nitric acid and other aqueous reagents
J. inorg,nucl.Chem.,1968,Vol.30, pp. 2661 to 2669. PergamonPress, Printedin Great Britain
REACTIONS OF URANIUM MONONITRIDE, THORIUM MONOCARBIDE AND URANIUM MONOCARBIDE WITH NITRIC ACID AND OTHER AQUEOUS REAGENTS* LESLIE M. FERRIS Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
(Received 13 March 1968) Abstract-Uranium mononitride was inert at 80 ° in water, 3-12 M HCI, 1 M H2SO4, 1 M NaOH, 1 M NaNOz, and 5 M NHaNO3; however, it did react in 1-2.5 M AI(NO3)3. The nitride reacted with 0.5-15-8 M HNO3 to give gaseous mixtures of N2, N20, NO, and NO2 (NO2 was produced only when the acid concentration was 6 M or higher). In reactions of thorium monocarbide with 0.1 to I M HNO3 at 90 °, both hydrolysis (yielding mainly methane).and oxidation (yielding CO2, N2, and nitrogen oxides) of the carbide took place; at higher HNO3 concentrations, only oxidation occurred. The principal reaction of UC or ThC with 4 and 6 M N H4F at 80 ° was: (U or Th)C~ + 5 N H4F(a,.~
~ (U or Th)F4.N H ~FI~+ 4 N H:~gl+ CH4~I.
INTRODUCTION
REACTIONS of uranium and thorium carbides and nitrides with various aqueous reagents are being investigated as part of a general study of the chemistry of these compounds. This paper summarizes the results of studies of the reactions of uranium mononitride and thorium monocarbide with nitric acid, and of uranium monocarbide and thorium monocarbide with ammonium fluoride solutions. The behavior of the heavy-metal compounds in several other types of solution was also investigated. Previously, the reactions of uranium mononitride with aqueous reagents had received only cursory attention [1,2], although studies of the general behavior of thorium mono- and dicarbides in nitric acid have been reported [3-5]. N o reference to the reactivity of the uranium and thorium carbides in ammonium fluoride solutions was found in the literature. EXPERIMENTAL
Preparation and characterization o f the nitride. High-purity uranium nitride was prepared by allowing nitrogen (1 atm) to react with finely powdered uranium metal (obtained from the thermal decomposition of uranium hydride); nitridation was initiated at about 250°, but the temperature was ultimately increased to about 1000 °. Complete reaction gave a product having the approximate composition UN,.rs. This nitride was decomposed to uranium mononitride powder in vacuum at 1000*Research sponsored by the U.S. Atomic Energy Commission under contract with lhe Union Carbide Corporation. 1. R. M, Dell, V. J. Wheeler, and E. J. Mclver, The hydrolysis o f uranium mononitride, A E R E - R 4984 (November 1965). 2. J. E. Antill and B. L. Myatt, Corros. Sci. 6, 17 (1966). 3. Y. Sasaki, F. Ichikawa, H. lmai, and S. Uruno, Nature, Lond. 195,267 (1962). 4. P. L. Pauson, J. McLean, and W. J. Clelland, Nature, Lond. 197, 1200 (1963). 5. H. lmai and S. Uruno, Nature 206, 691 (1965). 2661
2662
L.M. F E R R I S
1200 °. Pellets (0.27-in.-dia. × 0.30-in.-long) were prepared from the powder as follows: The powder was prepressed at a pressure of 5 t.s.i, and then cold-pressed at a pressure of 50 t.s.i., using camphor as a lubricant. The cold-pressed pellets were then heated in vacuum (about 10-6 torr) to 1450°; then, purified nitrogen (1 arm) was admitted and the system was heated to 2230 ° where it was maintained for about 1 hr. The system was then cooled, first, to 1450 ° in nitrogen and, finally, to room temperature in vacuum. The resulting pellets had bulk densities that were about 83 per cent of theoretical. The average uranium and nitrogen concentrations were 94.3 and 5.47 per cent, respectively, corresponding to an average composition of UN0.9sr. Oxygen and carbon were present as impurities at levels of 6001200 and 40-100 ppm, respectively. X-ray diffraction analysis showed only the presence of U N; the lattice parameter was 4.889 A., which is in good agreement with values reported by others [6, 7]. Preparation and characterization of the carbides. Uranium monocarbide buttons were prepared by arc melting high-purity uranium metal and spectroscopic-grade carbon, using nonconsumable tungsten electrodes, as described previously [8, 9]. Chemical analyses are given in Table 1. X-ray diffraction analysis showed only the presence of the monocarbide. The thorium monocarbide specimens were prepared by arc melting high-purity thorium metal and spectroscopic-grade carbon in a helium atmosphere, using graphite electrodes[10[. Chemical analyses are shown in Table 1. X-ray diffraction analysis indicated that only ThC (lattice parameter, 5.3457 A.) was present; however, metallographic examination revealed the presence of thorium dicarbide, at a concentration of less than 5 per cent in each specimen. The oxygen and nitrogen concentrations in the carbides were less than 0.11 per cent and 110 ppm, respectively. Table 1. Analyses of the carbides used Analyses (wt. %)
Carbide UC ThC ThC
Specimen
U or Th
Total carbon
ORNL-IB ThC-1 D ThC- 11A
95"3 94.9 94.1
4'64 4'87 4'81
Free carbon
CombinedC : metal atom ratio
0"02 0" 15 0,05
0.96 0-96 0-98
Procedure. The apparatus and general procedure were similar to those used in related hydrolysis studies [8-10]. Small samples (1-4 g) of the nitride or the carbides Were allowed to react with an excess of reagent in a thermostated system that was filled with pure helium. The system was maintained at atmospheric pressure by continually adjusting the level of the mercury in the gas buret which - was attached to the reaction flask. This technique also permitted measurement of the volume of gas evolved as a function of time. After the evolution of gaseous reaction products had ceased, the gases in the system were mixed thoroughly, using the gas buret as a Toepler pump, before a homogeneous sample was withdrawn for analysis. Analytical. The carbides were analyzed by procedures that have been described previously[8-10]. Uranium, oxygen, and carbon in the mononitride were determined by the same procedures used with the carbides; the analysis for nitrogen consisted in dissolving a sample in a solution of phosphoric and sulfuric acids, removing the NH.~ by distillation, and titrating [11]. Uranium, thorium fluorine, total nitrogen, and ammonium ion in solution were determined by conventional methods. Carbon, as organic compounds, in solution was analyzed by a wet oxidation method [ 12]. P. E. Evans and T. J. Davies, J. nucl Mater. 10(1), 43 (1963). F. Anselin, J. nucl. Mater. 10(4), 301 (1963). M.J. Bradley and L. M. Ferris, lnorg. Chem. 1,683 (1962). M.J. Bradley and L. M. Ferris, lnorg. Chem. 3, 189 (1964). M.J. Bradley and L. M. Ferris, J. inorg, nucl. Chem. 27, 1021 (1965). Z. M. Turovtseva and L. L. Kunin, Analysis of gases in metals (English translation). Consultants Bureau Enterprises, Inc., New York, p. 211 (1961). 12. J. Katz, S. Abraham, and N. Baker, Analyt. Chem. 26, 1503 (1954).
6. 7. 8. 9. 10. 11.
Reactions of uranium mononitride, thorium monocarbide
2663
Gas samples were analyzed by gas-chromatographic methods. Mixtures composed solely of hydrogen and hydrocarbons were analyzed by the procedure described previously [13]. The system used for analyzing mixtures containing carbon oxides and/or nitrogen oxides in addition to hydrocarbons had, in series, a l-meter silica gel column, a liquid nitrogen cold trap, and a stream-splitter that led to both a Linde 5A Molecular Sieve column and a dimethylsulfoxide (DMSO) column. A sample of the gas to be analyzed was carried in helium through the silica gel column at room temperature to sorb any NO o present. The effluent gases passed into the cold trap where NO, N20, and CO~ were condensed. Nitrogen, methane, and CO were analyzed in the portion of gas that passed through the Molecular Sieve column. After completion of these analyses, the cold trap was heated to room temperature and the evolved gases were fed via the stream-splitter to the DMSO column for the analysis of NO, N20, and CO2. Finally, the NO~ was eluted from the silica gel column by heating the column to 300 ° in 2 rain, and was then condensed in the cold trap. The NO., was ultimately released directly to the detector by warming the cold trap to room temperature. Some of the gas samples consisted of mixtures of ammonia, hydrogen, and hydrocarbons. Ammonia was determined by allowing it to react with standardized hydrochloric acid; after the ammonia had been removed, hydrogen and the hydrocarbons were analyzed by gas chromatography. Gas-chromatographic analyses of many of the gas samples were corroborated by mass spectroscopic analysis. RESULTS
Reactions of uranium mononitride with nitric acid. The reactions of uranium mononitride with nitric acid gave yellow solutions, due to the formation of uranyl nitrate, and gaseous products composed of nitrogen and nitrogen oxides. Complete reaction resulted in the consumption of about three equivalents of H + per mole of UN; the amount of ammonium ion produced was nearly negligible. As expected, the rate of reaction was dependent on the acid concentration. Although the nitride was practically inert in 0-1 M HNOa at 80°, reactions with solutions with higher acid concentrations proceeded smoothly: the time required for complete reaction decreased from about 90 to 0.5 hr as the acid concentration was increased from 0.5 to 4 M. When the acid concentration was greater than 4 M, the reactions at 80 ° were so rapid that gas evolution could not be followed with the manually operated gas buret. Consequently, for experimental convenience. reactions with 4 to 16 M HNO3 were conducted at 35 °, where complete dissolution could be achieved in 6 hr or less (except with 4 M HNO:~ where dissolution required about 24 hr). The volume of gas evolved per gram of nitride varied between 120 and about 146 mI(STP), and could not be correlated with either the acid concentration or the reaction temperature (Table 2). The stoichiometry of the reaction varied with acid concentration; the nitrogen (NO concentration decreased with increasing acid concentration, the N20 concentration generally increased with acid concentration, and the NO concentration was maximum at about 6 M HNO:~ (Table 2). Nitrogen dioxide was detected in the off-gases when the HNO3 concentration was 6 M or higher. The results from three of the experiments (Nos. 7-9, Table 2) indicated that the stoichiometry was unaffected by the reaction temperature. The amount of elemental nitrogen found in the off-gas usually did not correspond to the amount of nitrogen that originally was present in the nitride (Table 3). As the acid concentration was increased from 0.5 to 15.8 M, the amount of elemental nitrogen found in the off-gas decreased from about 2 to 0-75 rag-atoms per mg-atom of nitride nitrogen. At the lower acid concentrations, some N(V) 13. A. D. Horton and J. L. Botts, Nucl. Sci. Engng. 18, 97 (1964).
2664
L.M. F E R R I S Table 2. Gaseous products from the reaction of UNo.986 with nitric acid
Expt.
HNO3 conc., (M)
1 2 3 4 5 6 7 8 9 10 11 12 13
0.52 0.52 1.0 2.0 2.0 3.1 4.1 4.1 4.1 6.1 10.0 10.0 15.8
Temp., (°C)
Gas evolved. ml (STP) per g of nitride
N2
N20
NO
NO2
80 80 80 80 80 80 80 80 35 35 35 35 35
120 128 133 120 134 145 135 146 134 134 132 128 138
82 68 76 66 59 37 47 41 42 27 31 30 24
1.5 4.0 3.9 7.0 10 27 21 22 24 32 38 36 31
17 28 20 27 31 36 32 37 34 41 24 23 13
nd* nd nd nd nd nd nd nd nd trace 6.7 10 32
Off-gas composition, vol. %
*Not detected. Table 3. Distribution of nitrogen in gaseous products from the reaction of UN0.986 with nitric acid
Expt.
Nitride nitrogen, rag-atoms
Nitrogen expected in gaseous products,* rag-atoms
Total
1 2 3 4 5 6 7 8 9 10 11 12 13
2"30 3"13 2"18 4.05 4.30 3"97 3"99 3-37 14.15 15"63 4.57 3"10 2"81
7"51 7.75 5.83 14.5 10.4 8.81 9.38 7.34 31.6 37.7 --5.00
5.78 7.86 5"95 9.64 11" 1 10.8 10.3 9.17 36"0 38.1 11'6 7.51 6.86
Nitrogen found in gaseous products, mg -atoms As N2 5' 16 6.22 5.04 7'38 7.73 4.90 5.75 4.61 18.3 12.9 4'30 2.76 2.10
As N20 0.096 0.366 0"256 0.776 1.32 3-55 2'54 2'46 10.4 15.4 5"21 3.22 2"75
As NO
As NO~
0.527 1-27 0"655 1'48 2.06 2"35 1"99 2' 10 7'25 9'82 1"67 1"05 0'587
---------0.464 0-476 1-42
* Difference between sum of nitride nitrogen and nitrogen in nitric acid initially and amount of nitrogen found in final solution.
(from nitrate ions) apparently was reduced to elemental nitrogen, whereas, at the higher acid concentrations, some of the nitride nitrogen was oxidized beyond elemental nitrogen. In each experiment, the total amount of nitrogen found as gaseous products, based on the gas-chromatographic analyses and the volume of gas evolved, should have been equal to the sum of the nitride nitrogen and t h e n i t r o g e n i n t h e n i t r i c a c i d i n i t i a l l y , m i n u s t h e n i t r o g e n f o u n d i n t h e final
Reactions of uranium mononitride, thorium monocarbide
2665
solution. The values obtained usually agreed within 20 per cent (Table 3, columns 3 and 4). Reactions of uranium mononitride with other reagents. The reactivity of uranium mononitride in several other aqueous reagents was tested in 24-hr. experiments at 90 °. As shown in Table 4, the nitride was inert in most of the solutions tested; reaction did occur, however, in aluminum nitrate solutions when the pH of the solution was 2 or less, due to hydrolysis of the aluminum nitrate. Table 4. Reactivity of U N 0.98~in aqueous solutions at 90 ° Solution
pH of solution
Observations
Water 1 M NaOH
--
1 M H2SO4 2.7 M H C I 12 M H C I 1 M NaNO~ 5 M NH4NO:~ 1 M AI(NO3)a 2.5 M AI(NO.j:~
---6.5 5.4 2.0 0-4
N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. Very slow reaction. Rapid reaction; reaction complete in 2 h r giving yellow solution and no residue.
--
Reactions of thorium monocarbide with nitric acid. The reactions of thorium monocarbide (specimen ThC-11 A) with nitric acid at 90 ° were very complicated, yielding gaseous products, solutions containing thorium nitrate and soluble organic compounds, and, in some cases, insoluble residues. Complete reaction of the carbide required only a few hours in water or dilute acid; however, with 2-4 M HNOa the evolution of gaseous products did not cease in less than 300 to 500 hr. The decrease in reaction rate with increase in acid concentration is illustrated in Fig. I. The 3-hr. time increment, although chosen somewhat arbitrarily, lO0~
I
I
I
I t .0
T
I
I
7--
I
I
I
-o I 4.0
--
d o
7~3
80
I2) w O ~o~ 40L9 LL 0 ~207 o 2~ '~
O - 0
I.___
I [ [ 2.0 5.0 N I T R I C ACID C O N C E N T R A T I O N , M
F i g . I. D e c r e a s e in rate of reaction of ThC with increase in nitric acid concentration at 90 °.
__J
i
2666
L.M. FERRIS
corresponded to the time that was required for complete reaction of the carbide with water. The volumes and compositions of the gases produced by complete reactions at 90° are given in Table 5. In water, only hydrolysis of the carbide carbon Table 5. Compositions of the gases evolved in reactions of ThC with nitric acid at 90°
Expt. 14 15 16 17 18 19 20 21
Vol. of gas HNO3 evolved, Conc., mI(STP) pel (M) gofThC
0.0 0.I 0.3 0.6 1"0 2.0 2.0 4.1
91 67 71 89 117 -234 273
Gas composition, vol. % CH4
CO
88T 34 19 16 8.8 nd nd nd
nd~: nd 2.0 2.7 1.6 nd nd nd
CO2
N2
N20
NO
nd 15 23 26 27 22 23 21
nd 13 18 17 19 4-2 3-3 1.7
nd 36 38 32 28 nd 3.3 2.8
nd 2-4 nd 7.3 16 73 70 75
Tin addition to methane, the gas contained 8.75 per cent H2, 2"38 per cent ethane, and lesser amounts of higher-molecular-weight hydrocarbons. :~Not detected.
occurred; in 0.1 to 1 M HNO3, both hydrolysis and oxidation took place. Hydrolysis was the predominating reaction at the lower acid concentrations, although the extent to which oxidation occurred was significant even in 0.1 M HNO3. When the acid concentration was equal to, or greater than, 2 M, the reaction apparently involved only oxidation, as evidenced by the absence of methane in the gaseous products. Hydrogen, NO2, and hydrocarbons other than methane were not detected in the off-gases from reactions with nitric acid. In the reaction of thorium monocarbide with water, practically all of the carbide carbon was found as methane and other hydrocarbons in the gaseous products. With nitric acid, however, only 50-70 per cent of the carbide carbon was found as gaseous products (Table 6). The remainder was distributed among compounds that were soluble in the solution and those that were present as insoluble reaction residues. In reactions with the more dilute (0.1-1 M) acid solutions, large quantities of foam were generated, especially in the early stages of the reactions. The solutions resulting from complete reaction of the carbide were yellow in color and produced suds on agitation. Although the organic compounds in solution were not isolated and identified, they obviously possessed some detergent properties. The solutions produced by reaction with 2 M, and more concentrated, HNO3 were usually red-brown in color and did not give the soapy effect on being shaken. In general, the amount of carbide carbon found in solution increased with increasing acid concentration, from about 15 to 35 per cent as the HNO3 concentration increased from 0-1 to 4.1 M (Table 6). Solid residues (yellow to brown in color) remained after gas evolution had ceased in reactions of thorium monocarbide with 0.1 to 2 M HNO3; no residues were detected after complete reaction with solutions that had HNO3 concentrations of 4 M or higher. The amount of carbide carbon c~ontained in the insoluble
Reactions of uranium mononitride, thorium monocarbide
2667
solids decreased f r o m about 20 to 9 per cent as the H N O 3 concentration was increased f r o m 0.1 to 2 M (Table 6). Thorium, in amounts corresponding to 1 to 2 per cent of the thorium f r o m the m o n o c a r b i d e , was also found in the residues. T h e thorium was probably present mainly as ThO2 since this oxide was detected in the residues by X - r a y diffraction analysis and since the amounts of thorium found by chemical analysis were nearly those e x p e c t e d from the oxide content of the carbide. T h e carbide analyzed about 1000 p p m oxygen, which corresponds to about 1 per cent ThO2. T h e fact that the amounts of thorium found in the residues were slightly higher than were e x p e c t e d could have been the result of additional oxidation of the carbide during handling. T h e residues obviously were not T h C because the C / T h a t o m ratios were in the range of 5-7. It is also doubtful that free graphite was f o r m e d since the residues analyzed only 15-25 per cent carbon. In other experiments, samples of thorium m o n o c a r b i d e were allowed to react with 2 to 16 M H N O 3 solutions at 80 ° for only 24 hr. Undissolved T h C (identified by X-ray diffraction analysis) remained in each case. Chemical analyses showed the C / T h a t o m ratio in the residues to be about 1 ; therefore, it is unlikely that any significant amounts of free carbon were f o r m e d during these reactions. Reactions of UC and ThC with ammonium fluoride solutions. Reactions of 4 g samples of uranium m o n o c a r b i d e (specimen O R N L - 1 B ) and thorium monocarbide (specimen T h C - 1 D ) with 4 and 6 M N H 4 F at 80 ° were complete in 5 - 1 0 hr. T h e primary reactions were: UC(s) + 5NH4F(aq) ~ N H4"UF4~s~+ 4NH3(g) + CH4(g) and ThC(s) + 5NH4F(aq) ~ NH4F'ThF4(s) + 4NHacg)+ CH4(g). Table 6. Approximate carbon distribution in reactions of ThC with nitric acid at 90°
Expt. 14 15 16 17 18 20 21
Carbon foundt%) HNO:~ Conc., In final In insoluble M As CH4 As CO As CO2 solution residue 0.0 0.1 0.3 0.6 1.0 2.0 4.1
90* 45 28 17 15 nd nd
nd + nd 3.0 2-9 2.7 nd nd
nd 20 35 28 45 70 66
nd 16 15 45 32 22 34
nd 20 18 7.5 5.6 8.7 $
*An additional 8-7 per cent of the carbide carbon was found as ethane and higher-molecular-weight hydrocarbons. ?Not detected. SNo residue in this experiment. Reactions with uranium m o n o c a r b i d e resulted in nearly quantitative precipitation of the uranium as a - N H 4 F . U F 4 . T h e composition of this green solid was confirmed b y both chemical (Table 7) and X - r a y diffraction analyses. Reactions with thorium m o n o c a r b i d e gave almost complete precipitation of thorium as a
2668
L.M. FERRIS
white solid with the approximate composition NH4F.ThF4 (Table 7). The X-ray diffraction pattern of this compound was practically identical to that of a-NH4F.UF4. The gaseous reaction products were composed mainly of methane and ammonia, although small amounts of hydrogen and hydrocarbons other than methane were also produced (Table 7). Within the limits of experimental error, the amounts of methane (and the other hydrocarbons) and hydrogen produced per mole of carbide were the same as those obtained by reaction with water. These results, in conjunction with analyses of the solutions and solid products, showed that all of the carbide carbon was converted to gaseous species. The absence of hydrogen in the products of the reaction with 6 M NH4F-1 M NH4NO3 (Expt. 24) was undoubtedly due to the reaction 2H~+½NH4NO3 ~ N H 3 + ~H20; the amount of NH3 produced by this route was negligible compared with that produced by the primary reaction. During each reaction, the pH of the solution increased from about 5 to 9. This obviously was the result of the buildup of NH3 in the portion of the system that was below the condenser, since only a small fraction of the total NH3 produced passed through the condenser and was collected with the other gaseous products. The amount of NH3 in the gas that passed through the condenser was calculated from the volume of the gas evolved and its NH3 concentration as determined by scrubbing an aliquot of the gas with standardized hydrochloric acid. The amount of NHz not present in the gas, i.e., the NH3 that was refluxed between the solution and the condenser, was estimated by the following procedure: Hydrochloric acid was pipetted into the system via the condenser until the solution was definitely acidic; then, the acidic solution was stirred for several hours to ensure that the NH3 in the vapor above it was converted to NH4C1 and dissolved. Finally, the amount of NH3 present in the final solution as NH4CI was calculated by difference from the analyses of the solution for total nitrogen (NH4F + NH4C1) and total fluorine (NH4F). In each experiment, about 4 moles of NH3 were produced for each mole of carbide that reacted (Table 7). Table 7. Products of the reactions of ammonium fluoride solutions with UC and ThC at 80 °
Expt.
Carbide
NH~F Conc., M
22 23 24 25 26 27
UC UC UC UC ThC ThC ThC
0* 4 6 6~ 0 4 4
Primary gaseous products, moles/mole of carbide H2
CH4
C2H6
NH3
0"12 0' 15 0'095 frO§ 0" 15 0"27 0"26
0"90 1"0 1"0 0"97 0"80 0"87 0"88
0"019 0"037 0"010 0"013 0"026 0"027 0'026
. 3"7 3"8 -. 4'2 4"4
U or Analyses of solid products,t Th in wt. % soln., (%) U or Th F N .
. 0"59 0"42 2"5
.
. 0"03 0"02
. 66' 1 65'6 64" 1 . 62"7 64"4
. 27'3 25"6 26"5 . 26"5 26"2
4"5 4"7 4"4 5"4 --
*See [9]. tCalculated for NH4F.UF4: U, 67.8; F, 27.0; N, 3.99; and for NH4F'TbF4: Th, 67.2; F, 27.5; N, 4.06. :~Solution was also 1 M in NH4NO3. Hydrogen consumed by reaction with NH4NO3.
Reactions of uranium mononitride, thorium monocarbide
2669
Reactions of ThC and UC with sodium hydroxide-sodium nitrate solution. At 90°, ThC (specimen ThC-11A) reacted readily with an excess of 2 M N a O H 2 M NaNO3 giving solid thorium oxide and an off-gas containing hydrocarbons. Under the same conditions, UC (specimen O R N L - I B ) was practically inert in a 24 hr test. DISCUSSION
The results of this study, which showed uranium mononitride to be inert in water, hydrochloric acid, and sulfuric acid, but reactive in nitric acid are in complete agreement with those of other studies [1, 2]. Products of the reactions with nitric acid were not given in the earlier reports; therefore, no further comparison of results is possible. lmai and Uruno[5] reported that the rate of reaction of thorium monocarbide with dilute nitric acid at 25 ° decreased markedly with increasing acid concentration, with the carbide becoming nearly passive in 2 M HNO3. In the present study, nearly identical behavior was observed at 90°. Imai and Uruno did not detect hydrocarbons in the gaseous reaction products when the acid concentration was 2 M or higher, and postulated that reactions of the carbide with solutions of lower acid concentration primarily involved hydrolysis of the carbide, yielding hydrocarbons and hydrogen. The present investigation has shown that both hydrolysis and oxidation of the carbide occur in 0-1 to 1 M HNO3 and has confirmed that only the oxidation reaction is significant at higher acid concentrations. Although quantitative results were not obtained for reactions with acid more concentrated that 4 M, the observation that red-brown solutions containing organic compounds were produced is in accord with the observations made by others[4,5]. Mellitic (benzene hexacarboxylic) acid has been isolated from solution after dissolving ThC in 6 M HNO3 [4]. The behavior of ThC in dilute nitric acid was strikingly different from that of UC. Thorium monocarbide reacted rapidly in very dilute solutions, whereas UC was nearly inert when the acid concentration was below about 0-5 M[14]. Reactions of UC with 0.5-2 M HNO3 appeared to involve only oxidation and yielded no hydrocarbons [ 14]; as noted above, ThC underwent both oxidation and hydrolysis. In 4 M HNO3, where oxidation was the predominant reaction, the compositions of the reaction off-gases were about the same for both UC and ThC. Causes for the differences in the behavior of these two carbides are not readily apparent. Obviously, more experimentation will be required before the mechanisms of the reactions are understood. Acknowledgements-The author thanks J. F. Land for conducting the experimental work, and is especially indebted to Mildred B. Sears for her many helpful suggestions. Carbide and nitride specimens were prepared by R. A. Potter, L. Queener, and R. E. McDonald of the O R N L Metals and Ceramics Division. The O R N L Analytical Chemistry Division provided analyses: chemical analyses by the groups of W. R. Laing and L. J. Brady; gas-chromatographic analyses by F. Rogers and C. M. Boyd; and X-ray diffraction analyses by R. L. Sherman.
14. L. M. Ferris and M. J. Bradley, J. Am. chem. Soc. 87, 1710 ~1965).
REACTIONS OF URANIUM MONONITRIDE, THORIUM MONOCARBIDE AND URANIUM MONOCARBIDE WITH NITRIC ACID AND OTHER AQUEOUS REAGENTS* LESLIE M. FERRIS Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
(Received 13 March 1968) Abstract-Uranium mononitride was inert at 80 ° in water, 3-12 M HCI, 1 M H2SO4, 1 M NaOH, 1 M NaNOz, and 5 M NHaNO3; however, it did react in 1-2.5 M AI(NO3)3. The nitride reacted with 0.5-15-8 M HNO3 to give gaseous mixtures of N2, N20, NO, and NO2 (NO2 was produced only when the acid concentration was 6 M or higher). In reactions of thorium monocarbide with 0.1 to I M HNO3 at 90 °, both hydrolysis (yielding mainly methane).and oxidation (yielding CO2, N2, and nitrogen oxides) of the carbide took place; at higher HNO3 concentrations, only oxidation occurred. The principal reaction of UC or ThC with 4 and 6 M N H4F at 80 ° was: (U or Th)C~ + 5 N H4F(a,.~
~ (U or Th)F4.N H ~FI~+ 4 N H:~gl+ CH4~I.
INTRODUCTION
REACTIONS of uranium and thorium carbides and nitrides with various aqueous reagents are being investigated as part of a general study of the chemistry of these compounds. This paper summarizes the results of studies of the reactions of uranium mononitride and thorium monocarbide with nitric acid, and of uranium monocarbide and thorium monocarbide with ammonium fluoride solutions. The behavior of the heavy-metal compounds in several other types of solution was also investigated. Previously, the reactions of uranium mononitride with aqueous reagents had received only cursory attention [1,2], although studies of the general behavior of thorium mono- and dicarbides in nitric acid have been reported [3-5]. N o reference to the reactivity of the uranium and thorium carbides in ammonium fluoride solutions was found in the literature. EXPERIMENTAL
Preparation and characterization o f the nitride. High-purity uranium nitride was prepared by allowing nitrogen (1 atm) to react with finely powdered uranium metal (obtained from the thermal decomposition of uranium hydride); nitridation was initiated at about 250°, but the temperature was ultimately increased to about 1000 °. Complete reaction gave a product having the approximate composition UN,.rs. This nitride was decomposed to uranium mononitride powder in vacuum at 1000*Research sponsored by the U.S. Atomic Energy Commission under contract with lhe Union Carbide Corporation. 1. R. M, Dell, V. J. Wheeler, and E. J. Mclver, The hydrolysis o f uranium mononitride, A E R E - R 4984 (November 1965). 2. J. E. Antill and B. L. Myatt, Corros. Sci. 6, 17 (1966). 3. Y. Sasaki, F. Ichikawa, H. lmai, and S. Uruno, Nature, Lond. 195,267 (1962). 4. P. L. Pauson, J. McLean, and W. J. Clelland, Nature, Lond. 197, 1200 (1963). 5. H. lmai and S. Uruno, Nature 206, 691 (1965). 2661
2662
L.M. F E R R I S
1200 °. Pellets (0.27-in.-dia. × 0.30-in.-long) were prepared from the powder as follows: The powder was prepressed at a pressure of 5 t.s.i, and then cold-pressed at a pressure of 50 t.s.i., using camphor as a lubricant. The cold-pressed pellets were then heated in vacuum (about 10-6 torr) to 1450°; then, purified nitrogen (1 arm) was admitted and the system was heated to 2230 ° where it was maintained for about 1 hr. The system was then cooled, first, to 1450 ° in nitrogen and, finally, to room temperature in vacuum. The resulting pellets had bulk densities that were about 83 per cent of theoretical. The average uranium and nitrogen concentrations were 94.3 and 5.47 per cent, respectively, corresponding to an average composition of UN0.9sr. Oxygen and carbon were present as impurities at levels of 6001200 and 40-100 ppm, respectively. X-ray diffraction analysis showed only the presence of U N; the lattice parameter was 4.889 A., which is in good agreement with values reported by others [6, 7]. Preparation and characterization of the carbides. Uranium monocarbide buttons were prepared by arc melting high-purity uranium metal and spectroscopic-grade carbon, using nonconsumable tungsten electrodes, as described previously [8, 9]. Chemical analyses are given in Table 1. X-ray diffraction analysis showed only the presence of the monocarbide. The thorium monocarbide specimens were prepared by arc melting high-purity thorium metal and spectroscopic-grade carbon in a helium atmosphere, using graphite electrodes[10[. Chemical analyses are shown in Table 1. X-ray diffraction analysis indicated that only ThC (lattice parameter, 5.3457 A.) was present; however, metallographic examination revealed the presence of thorium dicarbide, at a concentration of less than 5 per cent in each specimen. The oxygen and nitrogen concentrations in the carbides were less than 0.11 per cent and 110 ppm, respectively. Table 1. Analyses of the carbides used Analyses (wt. %)
Carbide UC ThC ThC
Specimen
U or Th
Total carbon
ORNL-IB ThC-1 D ThC- 11A
95"3 94.9 94.1
4'64 4'87 4'81
Free carbon
CombinedC : metal atom ratio
0"02 0" 15 0,05
0.96 0-96 0-98
Procedure. The apparatus and general procedure were similar to those used in related hydrolysis studies [8-10]. Small samples (1-4 g) of the nitride or the carbides Were allowed to react with an excess of reagent in a thermostated system that was filled with pure helium. The system was maintained at atmospheric pressure by continually adjusting the level of the mercury in the gas buret which - was attached to the reaction flask. This technique also permitted measurement of the volume of gas evolved as a function of time. After the evolution of gaseous reaction products had ceased, the gases in the system were mixed thoroughly, using the gas buret as a Toepler pump, before a homogeneous sample was withdrawn for analysis. Analytical. The carbides were analyzed by procedures that have been described previously[8-10]. Uranium, oxygen, and carbon in the mononitride were determined by the same procedures used with the carbides; the analysis for nitrogen consisted in dissolving a sample in a solution of phosphoric and sulfuric acids, removing the NH.~ by distillation, and titrating [11]. Uranium, thorium fluorine, total nitrogen, and ammonium ion in solution were determined by conventional methods. Carbon, as organic compounds, in solution was analyzed by a wet oxidation method [ 12]. P. E. Evans and T. J. Davies, J. nucl Mater. 10(1), 43 (1963). F. Anselin, J. nucl. Mater. 10(4), 301 (1963). M.J. Bradley and L. M. Ferris, lnorg. Chem. 1,683 (1962). M.J. Bradley and L. M. Ferris, lnorg. Chem. 3, 189 (1964). M.J. Bradley and L. M. Ferris, J. inorg, nucl. Chem. 27, 1021 (1965). Z. M. Turovtseva and L. L. Kunin, Analysis of gases in metals (English translation). Consultants Bureau Enterprises, Inc., New York, p. 211 (1961). 12. J. Katz, S. Abraham, and N. Baker, Analyt. Chem. 26, 1503 (1954).
6. 7. 8. 9. 10. 11.
Reactions of uranium mononitride, thorium monocarbide
2663
Gas samples were analyzed by gas-chromatographic methods. Mixtures composed solely of hydrogen and hydrocarbons were analyzed by the procedure described previously [13]. The system used for analyzing mixtures containing carbon oxides and/or nitrogen oxides in addition to hydrocarbons had, in series, a l-meter silica gel column, a liquid nitrogen cold trap, and a stream-splitter that led to both a Linde 5A Molecular Sieve column and a dimethylsulfoxide (DMSO) column. A sample of the gas to be analyzed was carried in helium through the silica gel column at room temperature to sorb any NO o present. The effluent gases passed into the cold trap where NO, N20, and CO~ were condensed. Nitrogen, methane, and CO were analyzed in the portion of gas that passed through the Molecular Sieve column. After completion of these analyses, the cold trap was heated to room temperature and the evolved gases were fed via the stream-splitter to the DMSO column for the analysis of NO, N20, and CO2. Finally, the NO~ was eluted from the silica gel column by heating the column to 300 ° in 2 rain, and was then condensed in the cold trap. The NO., was ultimately released directly to the detector by warming the cold trap to room temperature. Some of the gas samples consisted of mixtures of ammonia, hydrogen, and hydrocarbons. Ammonia was determined by allowing it to react with standardized hydrochloric acid; after the ammonia had been removed, hydrogen and the hydrocarbons were analyzed by gas chromatography. Gas-chromatographic analyses of many of the gas samples were corroborated by mass spectroscopic analysis. RESULTS
Reactions of uranium mononitride with nitric acid. The reactions of uranium mononitride with nitric acid gave yellow solutions, due to the formation of uranyl nitrate, and gaseous products composed of nitrogen and nitrogen oxides. Complete reaction resulted in the consumption of about three equivalents of H + per mole of UN; the amount of ammonium ion produced was nearly negligible. As expected, the rate of reaction was dependent on the acid concentration. Although the nitride was practically inert in 0-1 M HNOa at 80°, reactions with solutions with higher acid concentrations proceeded smoothly: the time required for complete reaction decreased from about 90 to 0.5 hr as the acid concentration was increased from 0.5 to 4 M. When the acid concentration was greater than 4 M, the reactions at 80 ° were so rapid that gas evolution could not be followed with the manually operated gas buret. Consequently, for experimental convenience. reactions with 4 to 16 M HNO3 were conducted at 35 °, where complete dissolution could be achieved in 6 hr or less (except with 4 M HNO:~ where dissolution required about 24 hr). The volume of gas evolved per gram of nitride varied between 120 and about 146 mI(STP), and could not be correlated with either the acid concentration or the reaction temperature (Table 2). The stoichiometry of the reaction varied with acid concentration; the nitrogen (NO concentration decreased with increasing acid concentration, the N20 concentration generally increased with acid concentration, and the NO concentration was maximum at about 6 M HNO:~ (Table 2). Nitrogen dioxide was detected in the off-gases when the HNO3 concentration was 6 M or higher. The results from three of the experiments (Nos. 7-9, Table 2) indicated that the stoichiometry was unaffected by the reaction temperature. The amount of elemental nitrogen found in the off-gas usually did not correspond to the amount of nitrogen that originally was present in the nitride (Table 3). As the acid concentration was increased from 0.5 to 15.8 M, the amount of elemental nitrogen found in the off-gas decreased from about 2 to 0-75 rag-atoms per mg-atom of nitride nitrogen. At the lower acid concentrations, some N(V) 13. A. D. Horton and J. L. Botts, Nucl. Sci. Engng. 18, 97 (1964).
2664
L.M. F E R R I S Table 2. Gaseous products from the reaction of UNo.986 with nitric acid
Expt.
HNO3 conc., (M)
1 2 3 4 5 6 7 8 9 10 11 12 13
0.52 0.52 1.0 2.0 2.0 3.1 4.1 4.1 4.1 6.1 10.0 10.0 15.8
Temp., (°C)
Gas evolved. ml (STP) per g of nitride
N2
N20
NO
NO2
80 80 80 80 80 80 80 80 35 35 35 35 35
120 128 133 120 134 145 135 146 134 134 132 128 138
82 68 76 66 59 37 47 41 42 27 31 30 24
1.5 4.0 3.9 7.0 10 27 21 22 24 32 38 36 31
17 28 20 27 31 36 32 37 34 41 24 23 13
nd* nd nd nd nd nd nd nd nd trace 6.7 10 32
Off-gas composition, vol. %
*Not detected. Table 3. Distribution of nitrogen in gaseous products from the reaction of UN0.986 with nitric acid
Expt.
Nitride nitrogen, rag-atoms
Nitrogen expected in gaseous products,* rag-atoms
Total
1 2 3 4 5 6 7 8 9 10 11 12 13
2"30 3"13 2"18 4.05 4.30 3"97 3"99 3-37 14.15 15"63 4.57 3"10 2"81
7"51 7.75 5.83 14.5 10.4 8.81 9.38 7.34 31.6 37.7 --5.00
5.78 7.86 5"95 9.64 11" 1 10.8 10.3 9.17 36"0 38.1 11'6 7.51 6.86
Nitrogen found in gaseous products, mg -atoms As N2 5' 16 6.22 5.04 7'38 7.73 4.90 5.75 4.61 18.3 12.9 4'30 2.76 2.10
As N20 0.096 0.366 0"256 0.776 1.32 3-55 2'54 2'46 10.4 15.4 5"21 3.22 2"75
As NO
As NO~
0.527 1-27 0"655 1'48 2.06 2"35 1"99 2' 10 7'25 9'82 1"67 1"05 0'587
---------0.464 0-476 1-42
* Difference between sum of nitride nitrogen and nitrogen in nitric acid initially and amount of nitrogen found in final solution.
(from nitrate ions) apparently was reduced to elemental nitrogen, whereas, at the higher acid concentrations, some of the nitride nitrogen was oxidized beyond elemental nitrogen. In each experiment, the total amount of nitrogen found as gaseous products, based on the gas-chromatographic analyses and the volume of gas evolved, should have been equal to the sum of the nitride nitrogen and t h e n i t r o g e n i n t h e n i t r i c a c i d i n i t i a l l y , m i n u s t h e n i t r o g e n f o u n d i n t h e final
Reactions of uranium mononitride, thorium monocarbide
2665
solution. The values obtained usually agreed within 20 per cent (Table 3, columns 3 and 4). Reactions of uranium mononitride with other reagents. The reactivity of uranium mononitride in several other aqueous reagents was tested in 24-hr. experiments at 90 °. As shown in Table 4, the nitride was inert in most of the solutions tested; reaction did occur, however, in aluminum nitrate solutions when the pH of the solution was 2 or less, due to hydrolysis of the aluminum nitrate. Table 4. Reactivity of U N 0.98~in aqueous solutions at 90 ° Solution
pH of solution
Observations
Water 1 M NaOH
--
1 M H2SO4 2.7 M H C I 12 M H C I 1 M NaNO~ 5 M NH4NO:~ 1 M AI(NO3)a 2.5 M AI(NO.j:~
---6.5 5.4 2.0 0-4
N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. N o reaction noted. Very slow reaction. Rapid reaction; reaction complete in 2 h r giving yellow solution and no residue.
--
Reactions of thorium monocarbide with nitric acid. The reactions of thorium monocarbide (specimen ThC-11 A) with nitric acid at 90 ° were very complicated, yielding gaseous products, solutions containing thorium nitrate and soluble organic compounds, and, in some cases, insoluble residues. Complete reaction of the carbide required only a few hours in water or dilute acid; however, with 2-4 M HNOa the evolution of gaseous products did not cease in less than 300 to 500 hr. The decrease in reaction rate with increase in acid concentration is illustrated in Fig. I. The 3-hr. time increment, although chosen somewhat arbitrarily, lO0~
I
I
I
I t .0
T
I
I
7--
I
I
I
-o I 4.0
--
d o
7~3
80
I2) w O ~o~ 40L9 LL 0 ~207 o 2~ '~
O - 0
I.___
I [ [ 2.0 5.0 N I T R I C ACID C O N C E N T R A T I O N , M
F i g . I. D e c r e a s e in rate of reaction of ThC with increase in nitric acid concentration at 90 °.
__J
i
2666
L.M. FERRIS
corresponded to the time that was required for complete reaction of the carbide with water. The volumes and compositions of the gases produced by complete reactions at 90° are given in Table 5. In water, only hydrolysis of the carbide carbon Table 5. Compositions of the gases evolved in reactions of ThC with nitric acid at 90°
Expt. 14 15 16 17 18 19 20 21
Vol. of gas HNO3 evolved, Conc., mI(STP) pel (M) gofThC
0.0 0.I 0.3 0.6 1"0 2.0 2.0 4.1
91 67 71 89 117 -234 273
Gas composition, vol. % CH4
CO
88T 34 19 16 8.8 nd nd nd
nd~: nd 2.0 2.7 1.6 nd nd nd
CO2
N2
N20
NO
nd 15 23 26 27 22 23 21
nd 13 18 17 19 4-2 3-3 1.7
nd 36 38 32 28 nd 3.3 2.8
nd 2-4 nd 7.3 16 73 70 75
Tin addition to methane, the gas contained 8.75 per cent H2, 2"38 per cent ethane, and lesser amounts of higher-molecular-weight hydrocarbons. :~Not detected.
occurred; in 0.1 to 1 M HNO3, both hydrolysis and oxidation took place. Hydrolysis was the predominating reaction at the lower acid concentrations, although the extent to which oxidation occurred was significant even in 0.1 M HNO3. When the acid concentration was equal to, or greater than, 2 M, the reaction apparently involved only oxidation, as evidenced by the absence of methane in the gaseous products. Hydrogen, NO2, and hydrocarbons other than methane were not detected in the off-gases from reactions with nitric acid. In the reaction of thorium monocarbide with water, practically all of the carbide carbon was found as methane and other hydrocarbons in the gaseous products. With nitric acid, however, only 50-70 per cent of the carbide carbon was found as gaseous products (Table 6). The remainder was distributed among compounds that were soluble in the solution and those that were present as insoluble reaction residues. In reactions with the more dilute (0.1-1 M) acid solutions, large quantities of foam were generated, especially in the early stages of the reactions. The solutions resulting from complete reaction of the carbide were yellow in color and produced suds on agitation. Although the organic compounds in solution were not isolated and identified, they obviously possessed some detergent properties. The solutions produced by reaction with 2 M, and more concentrated, HNO3 were usually red-brown in color and did not give the soapy effect on being shaken. In general, the amount of carbide carbon found in solution increased with increasing acid concentration, from about 15 to 35 per cent as the HNO3 concentration increased from 0-1 to 4.1 M (Table 6). Solid residues (yellow to brown in color) remained after gas evolution had ceased in reactions of thorium monocarbide with 0.1 to 2 M HNO3; no residues were detected after complete reaction with solutions that had HNO3 concentrations of 4 M or higher. The amount of carbide carbon c~ontained in the insoluble
Reactions of uranium mononitride, thorium monocarbide
2667
solids decreased f r o m about 20 to 9 per cent as the H N O 3 concentration was increased f r o m 0.1 to 2 M (Table 6). Thorium, in amounts corresponding to 1 to 2 per cent of the thorium f r o m the m o n o c a r b i d e , was also found in the residues. T h e thorium was probably present mainly as ThO2 since this oxide was detected in the residues by X - r a y diffraction analysis and since the amounts of thorium found by chemical analysis were nearly those e x p e c t e d from the oxide content of the carbide. T h e carbide analyzed about 1000 p p m oxygen, which corresponds to about 1 per cent ThO2. T h e fact that the amounts of thorium found in the residues were slightly higher than were e x p e c t e d could have been the result of additional oxidation of the carbide during handling. T h e residues obviously were not T h C because the C / T h a t o m ratios were in the range of 5-7. It is also doubtful that free graphite was f o r m e d since the residues analyzed only 15-25 per cent carbon. In other experiments, samples of thorium m o n o c a r b i d e were allowed to react with 2 to 16 M H N O 3 solutions at 80 ° for only 24 hr. Undissolved T h C (identified by X-ray diffraction analysis) remained in each case. Chemical analyses showed the C / T h a t o m ratio in the residues to be about 1 ; therefore, it is unlikely that any significant amounts of free carbon were f o r m e d during these reactions. Reactions of UC and ThC with ammonium fluoride solutions. Reactions of 4 g samples of uranium m o n o c a r b i d e (specimen O R N L - 1 B ) and thorium monocarbide (specimen T h C - 1 D ) with 4 and 6 M N H 4 F at 80 ° were complete in 5 - 1 0 hr. T h e primary reactions were: UC(s) + 5NH4F(aq) ~ N H4"UF4~s~+ 4NH3(g) + CH4(g) and ThC(s) + 5NH4F(aq) ~ NH4F'ThF4(s) + 4NHacg)+ CH4(g). Table 6. Approximate carbon distribution in reactions of ThC with nitric acid at 90°
Expt. 14 15 16 17 18 20 21
Carbon foundt%) HNO:~ Conc., In final In insoluble M As CH4 As CO As CO2 solution residue 0.0 0.1 0.3 0.6 1.0 2.0 4.1
90* 45 28 17 15 nd nd
nd + nd 3.0 2-9 2.7 nd nd
nd 20 35 28 45 70 66
nd 16 15 45 32 22 34
nd 20 18 7.5 5.6 8.7 $
*An additional 8-7 per cent of the carbide carbon was found as ethane and higher-molecular-weight hydrocarbons. ?Not detected. SNo residue in this experiment. Reactions with uranium m o n o c a r b i d e resulted in nearly quantitative precipitation of the uranium as a - N H 4 F . U F 4 . T h e composition of this green solid was confirmed b y both chemical (Table 7) and X - r a y diffraction analyses. Reactions with thorium m o n o c a r b i d e gave almost complete precipitation of thorium as a
2668
L.M. FERRIS
white solid with the approximate composition NH4F.ThF4 (Table 7). The X-ray diffraction pattern of this compound was practically identical to that of a-NH4F.UF4. The gaseous reaction products were composed mainly of methane and ammonia, although small amounts of hydrogen and hydrocarbons other than methane were also produced (Table 7). Within the limits of experimental error, the amounts of methane (and the other hydrocarbons) and hydrogen produced per mole of carbide were the same as those obtained by reaction with water. These results, in conjunction with analyses of the solutions and solid products, showed that all of the carbide carbon was converted to gaseous species. The absence of hydrogen in the products of the reaction with 6 M NH4F-1 M NH4NO3 (Expt. 24) was undoubtedly due to the reaction 2H~+½NH4NO3 ~ N H 3 + ~H20; the amount of NH3 produced by this route was negligible compared with that produced by the primary reaction. During each reaction, the pH of the solution increased from about 5 to 9. This obviously was the result of the buildup of NH3 in the portion of the system that was below the condenser, since only a small fraction of the total NH3 produced passed through the condenser and was collected with the other gaseous products. The amount of NH3 in the gas that passed through the condenser was calculated from the volume of the gas evolved and its NH3 concentration as determined by scrubbing an aliquot of the gas with standardized hydrochloric acid. The amount of NHz not present in the gas, i.e., the NH3 that was refluxed between the solution and the condenser, was estimated by the following procedure: Hydrochloric acid was pipetted into the system via the condenser until the solution was definitely acidic; then, the acidic solution was stirred for several hours to ensure that the NH3 in the vapor above it was converted to NH4C1 and dissolved. Finally, the amount of NH3 present in the final solution as NH4CI was calculated by difference from the analyses of the solution for total nitrogen (NH4F + NH4C1) and total fluorine (NH4F). In each experiment, about 4 moles of NH3 were produced for each mole of carbide that reacted (Table 7). Table 7. Products of the reactions of ammonium fluoride solutions with UC and ThC at 80 °
Expt.
Carbide
NH~F Conc., M
22 23 24 25 26 27
UC UC UC UC ThC ThC ThC
0* 4 6 6~ 0 4 4
Primary gaseous products, moles/mole of carbide H2
CH4
C2H6
NH3
0"12 0' 15 0'095 frO§ 0" 15 0"27 0"26
0"90 1"0 1"0 0"97 0"80 0"87 0"88
0"019 0"037 0"010 0"013 0"026 0"027 0'026
. 3"7 3"8 -. 4'2 4"4
U or Analyses of solid products,t Th in wt. % soln., (%) U or Th F N .
. 0"59 0"42 2"5
.
. 0"03 0"02
. 66' 1 65'6 64" 1 . 62"7 64"4
. 27'3 25"6 26"5 . 26"5 26"2
4"5 4"7 4"4 5"4 --
*See [9]. tCalculated for NH4F.UF4: U, 67.8; F, 27.0; N, 3.99; and for NH4F'TbF4: Th, 67.2; F, 27.5; N, 4.06. :~Solution was also 1 M in NH4NO3. Hydrogen consumed by reaction with NH4NO3.
Reactions of uranium mononitride, thorium monocarbide
2669
Reactions of ThC and UC with sodium hydroxide-sodium nitrate solution. At 90°, ThC (specimen ThC-11A) reacted readily with an excess of 2 M N a O H 2 M NaNO3 giving solid thorium oxide and an off-gas containing hydrocarbons. Under the same conditions, UC (specimen O R N L - I B ) was practically inert in a 24 hr test. DISCUSSION
The results of this study, which showed uranium mononitride to be inert in water, hydrochloric acid, and sulfuric acid, but reactive in nitric acid are in complete agreement with those of other studies [1, 2]. Products of the reactions with nitric acid were not given in the earlier reports; therefore, no further comparison of results is possible. lmai and Uruno[5] reported that the rate of reaction of thorium monocarbide with dilute nitric acid at 25 ° decreased markedly with increasing acid concentration, with the carbide becoming nearly passive in 2 M HNO3. In the present study, nearly identical behavior was observed at 90°. Imai and Uruno did not detect hydrocarbons in the gaseous reaction products when the acid concentration was 2 M or higher, and postulated that reactions of the carbide with solutions of lower acid concentration primarily involved hydrolysis of the carbide, yielding hydrocarbons and hydrogen. The present investigation has shown that both hydrolysis and oxidation of the carbide occur in 0-1 to 1 M HNO3 and has confirmed that only the oxidation reaction is significant at higher acid concentrations. Although quantitative results were not obtained for reactions with acid more concentrated that 4 M, the observation that red-brown solutions containing organic compounds were produced is in accord with the observations made by others[4,5]. Mellitic (benzene hexacarboxylic) acid has been isolated from solution after dissolving ThC in 6 M HNO3 [4]. The behavior of ThC in dilute nitric acid was strikingly different from that of UC. Thorium monocarbide reacted rapidly in very dilute solutions, whereas UC was nearly inert when the acid concentration was below about 0-5 M[14]. Reactions of UC with 0.5-2 M HNO3 appeared to involve only oxidation and yielded no hydrocarbons [ 14]; as noted above, ThC underwent both oxidation and hydrolysis. In 4 M HNO3, where oxidation was the predominant reaction, the compositions of the reaction off-gases were about the same for both UC and ThC. Causes for the differences in the behavior of these two carbides are not readily apparent. Obviously, more experimentation will be required before the mechanisms of the reactions are understood. Acknowledgements-The author thanks J. F. Land for conducting the experimental work, and is especially indebted to Mildred B. Sears for her many helpful suggestions. Carbide and nitride specimens were prepared by R. A. Potter, L. Queener, and R. E. McDonald of the O R N L Metals and Ceramics Division. The O R N L Analytical Chemistry Division provided analyses: chemical analyses by the groups of W. R. Laing and L. J. Brady; gas-chromatographic analyses by F. Rogers and C. M. Boyd; and X-ray diffraction analyses by R. L. Sherman.
14. L. M. Ferris and M. J. Bradley, J. Am. chem. Soc. 87, 1710 ~1965).