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The chemistry of uranium. Part XXVII. Kinetics of the dissolution of uranium dioxide powder in a solution containing sodium carbonate, sodium bicarbonate and potassium cyanide
HydrometaUurgy, 6
(1981) 197--201 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
197
THE CHEMISTRY OF URANIUM. PART XXVII. KINETICS OF THE DISSOLUTION OF URANIUM DIOXIDE POWDER IN A SOLUTION CONTAINING SODIUM CARBONATE, SODIUM BICARBONATE AND POTASSIUM CYANIDE
J.G.H. D U PREEZ*, D.C. M O R R I S and C.P.J. V A N V U U R E N
Uranium Chemistry Research Unit, Universityof Port Elizabeth,Port Elizabeth (South Africa) (Received July 18, 1979; accepted in revised form July 10, 1980)
ABSTRACT
Du Preez, J.G.H., Morris, D.C. and Van Vuuren, C.P.J.,1981. The chemistry of uranium. Part XXVII. Kinetics of the dissolutionof uranium dioxide powder in a solution containing sodium carbonate, sodium bicarbonate and potassium cyanide. Hydrometallurgy, 6: 197--201.. The dissolution of uranium dioxide in solutions containing sodium carbonate, sodium bicarbonate and potassium cyanide has been studied at temperatures in the range 30--80°C. The kinetics of the dissolution of uranium dioxide have been interpreted according to the fraction reacted, a. It is assumed that the uranium dioxide particles are spherical in shape. The results of the study indicate that the reaction boundary of the uranium dioxide particles is moving at constant velocity and that the products of the reaction are not limiting the reaction in the range a = 0 to a = 0.8. Arrhenius plots gave an activation energy, for the whole process of uranium dioxide dissolution, of 51.1 kJ mol-1.
INTRODUCTION
Previous work has shown that the rate of uranium dioxide oxidation in carbonate/bicarbonate solutions is proportional to the surface area of the uranium dioxide, and also to the square r o o t of the oxygen partial pressure. (Pearson and Wadsworth, 1958; Schortmann and De Sesa, 1958). In experiments performed by Schortmann and De Sesa (1958), where the uranium dioxide used was in a p o w d e r form, it was found that the rate was directly proportional to the surface area per unit volume. Peters and Halpern (1953), w h o used pitchblende in the form of discs of known surface area, have shown that the rate of dissolution is directly proportional to the surface area of the solid. In the present work the uranium dioxide was in the form of a fine p o w d e r whose surface area was unknown. It has been shown that, under conditions *To w h o m correspondence should be directed.
0304-386X/81/0000-0000/$02.50
© 1981 Elsevier Scientific Publishing Company
198 where the products of the reaction are soluble or, in general, leave the surface of the reacting particles, the reaction of particulates can be formalised in terms of the fraction reacted, a (Wadsworth 1975; pp. 449--453). If the products do n o t limit the reaction and if the solution concentration of the reactant remains constant, then the velocity of movement of the interface will be constant with time. This will result in linear kinetics. If it is assumed that the uranium dioxide particles are spherical, although the result is the same for any isometric shape, then the rate of reaction at the surface of the sphere can be expressed in the form dn/dt
(1)
= 4 n ra k / s
where n = number of molecules; r = radius of the sphere; s = cross sectional area of one of the reacting molecules; k = a constant containing the fraction of surface sites available, concentration terms other than surface concentration and the specific rate constant having the dimension secondThe total number of molecules, n, in the sphere is {2)
n = 4nr3/3v
where v = the volume of one molecule. This equation can be differentiated with respect to time and equated to eqn. (1), giving (3)
dr/dt = -vk/s
Equation (3) represents constant velocity movement of the reaction interface. If r0 is the initial radius of the sphere and a is the fraction reacted it can be shown that (4)
a = 1-r31ro 3
which, upon differentiation with respect to time, becomes da - 3 r 2 dr d t - r03 dt
(5)
Combining eqns. (3), (4) and (5) gives da dt
3v ( 1 - ~ ) 213 k / r o s
(6)
Imposing the conditions a =0 when t = 0 , eqn. (6) can be integrated giving
[1-(l-a)
'/3 ]
= k01
where k o 1 = v k / r o s
t
(7) (8)
Equation (7) indicates that a plot of [ 1 - ( 1 - ~ ) 1/3 ] against t should be linear with a slope of k0 ~.
199
EXPERIMENTAL DETAILS The uranium dioxide was provided by the South African Atomic Energy Board and had the composition UO2.o24 • It was f o u n d t h a t 80% of the uranium dioxide w a s - 2 0 0 mesh (U.S. Sieve Series - 0 . 0 7 4 ram). The leach solution used consisted of 4% Na2CO3,1.5% NaHCO3 and 0.025% KCN and had a pH of 10.2. The KCN was present to assess its effect on the rate of dissolution of uranium dioxide. This was part o f a larger study on the feasibility of a combined gold and uranium dioxide leach. It was found that the presence of cyanide had no effect on the rate of dissolution of uranium dioxide (Du Preez et al., 1980). The concentration of uranium VI was determined by measuring the absorbance of the uranyl--pyridylazo resorcinol complex at 530 nm using a Beckman Acta MVII spectrophotometer, as described by Florence and Farrar (1963). The molar absorption coefficient, e, of the uranyl--pyridylazo resorcinol complex was f o u n d to be 39,700 dm a mol -~ cm ~ In the experiment, accurately weighed i g samples of uranium dioxide were used together with 100 cm a of the leach solution. The temperature of the mixtures was controlled by passing water, at the desired temperature, through the hollow glass walls of the reaction vessels. The mixtures were stirred magnetically. The oxidant used was oxygen at atmospheric pressure. RESULTS The variation o f [ 1 - ( 1 - ~ ) 1/3] with time is shown in Fig. 1 for different temperatures in the range 303--353 K. In the majority o f cases good linear relationships are obtained up to a =0.8, the plot for 353 K showing the greatest deviation at values of a > 0.8. These experiments, therefore, suggest that the system gives linear kinetics in the range a = 0 to 0.8. According to the Arrhenius equation k0 = A exp (-A E / R T ) where A = pre exponential or frequency factor; k0 = rate constant at temperature T, A E = activation energy; R = gas constant -- 8.314J K -1 mol-1; T = absolute temperature. Thus a graph of In k0 against 1 / T should give a straight line of slope - A E / R . From eqn. (8), the slopes of the lines in Fig. 1, ko 1 = vk/ros
(8)
where v, r0 and s are all temperature-independent. The constant k contains the specific rate constant k0 as well as other terms. If the other components of k are temperature-independent, then a plot of In k0 ~ against 1 / T should give a straight line from whose slope the activation energy can be calculated. This plot is shown in Fig. 2 from which the activation energy, AE, was f o u n d to be 51.1 kJ mol -~.
200
0.6
353K
GID
o.II// .~
333K
o
303 K
/
A
*/
0
~
10
20
30
40
Time(h)
Fig. 1. The variation of [ 1 - ( 1 - ~ ) l/s ] with time at temperatures of 3 0 3 , 3 1 3 , 218, 333 and 353 K.
i
T
f
~o - I .5
-2.5
--
©
-35
-4.5 2.8
I
I
I
2.9
3.0
3.1
3.2
1/T x103( K-1) Fig. 2. The variation of In ko ~ with the reciprocal of the absolute temperature.
3.3
201 Peters and Halpern (1953), using pitchblende, found a similar value of 51.5 kJ mol -~. Schortmann and De Sesa (1958) q u o t e a value of 56.1 kJ mol -~, whilst Pearson and Wadsworth (1958) contend that the previous values quoted are composite activation energies. They give the real activation energy, relating to the absorption of H:CO3; as 28.0 kJ mol -~. They give a calculated composite activation energy of 43.5 kJ mol -~ and say that a direct determination from the rate plots over a wide range of pressures yield composite activation energies in the range 46.0 to 62.8 kJ mol -~, with the lower values resulting from the rates measured at high pressures. CONCLUSIONS
The results of this study indicate that the reaction boundary of the uranium dioxide particles is moving at constant velocity, giving rise to linear kinetics. The products of the reaction are ~ o t limiting the reaction over the range = 0 to 0.8. The variation of k01 with temperature enabled the activation energy, for the whole process, to be determined. This was found to be 51.1 kJ mo1-1. Considering the assumptions made, this value compares well with previously determined values. ACKNOWLEDGEMENTS
The authors wish to thank Johannesburg Consolidated Investment Company Limited who supplied the funds necessary for the above work. REFERENCES 1 D u Preez, J.G.H., Morris, D.C., Van Vuuren, C.P.J. and Oertell, M., 1980. The Chemistry of Uranium. Part XXVI. Alkaline dissolution of A u and/or UO2 powders. Hydrometall., 6: 147--158. 2 Florence, T.M. and Farrar, Y., 1963. Spectrophotometric determination of uranium with 4-(2-pyridylazo) resorcinol. Anal. Chem., 35 : 1613--1616. 3 Pearson, R.L. and Wadsworth, M.E., 1958. A kinetic study of the dissolution of U O 2 in carbonate solution. Trans. Metall. A.I.M.E., 212: 294--300. 4 Peters, E. and Halpern, J., 1953. Studies in the carbonate leaching of uranium ores. If. Kinetics of the dissolution of pitchblende. Trans. Can. Inst. Min. Metall., LVI: 350--354. 5 Schortmann, W.E. and De Sesa, M.A., 1958. Kinetics of the dissolution of uranium dioxide in carbonate-bicarbonate solutiona Proc. 2nd Int. Conf. on the Peaceful Uses of Atomic Energy, Vol. 3, United Nations, Geneva, pp. 333--341. 6 Wadsworth, M.E., 1975. Reactions at surfaces. IV. Reactions at nonmetal-solution interfaces. In : H. Eyring (Ed.), Physical Chemistry -- A n Advanced Treatise, Vol. VII, Reactions in Condensed Phases, Academic Press, N e w York, pp. 449--472.
(1981) 197--201 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
197
THE CHEMISTRY OF URANIUM. PART XXVII. KINETICS OF THE DISSOLUTION OF URANIUM DIOXIDE POWDER IN A SOLUTION CONTAINING SODIUM CARBONATE, SODIUM BICARBONATE AND POTASSIUM CYANIDE
J.G.H. D U PREEZ*, D.C. M O R R I S and C.P.J. V A N V U U R E N
Uranium Chemistry Research Unit, Universityof Port Elizabeth,Port Elizabeth (South Africa) (Received July 18, 1979; accepted in revised form July 10, 1980)
ABSTRACT
Du Preez, J.G.H., Morris, D.C. and Van Vuuren, C.P.J.,1981. The chemistry of uranium. Part XXVII. Kinetics of the dissolutionof uranium dioxide powder in a solution containing sodium carbonate, sodium bicarbonate and potassium cyanide. Hydrometallurgy, 6: 197--201.. The dissolution of uranium dioxide in solutions containing sodium carbonate, sodium bicarbonate and potassium cyanide has been studied at temperatures in the range 30--80°C. The kinetics of the dissolution of uranium dioxide have been interpreted according to the fraction reacted, a. It is assumed that the uranium dioxide particles are spherical in shape. The results of the study indicate that the reaction boundary of the uranium dioxide particles is moving at constant velocity and that the products of the reaction are not limiting the reaction in the range a = 0 to a = 0.8. Arrhenius plots gave an activation energy, for the whole process of uranium dioxide dissolution, of 51.1 kJ mol-1.
INTRODUCTION
Previous work has shown that the rate of uranium dioxide oxidation in carbonate/bicarbonate solutions is proportional to the surface area of the uranium dioxide, and also to the square r o o t of the oxygen partial pressure. (Pearson and Wadsworth, 1958; Schortmann and De Sesa, 1958). In experiments performed by Schortmann and De Sesa (1958), where the uranium dioxide used was in a p o w d e r form, it was found that the rate was directly proportional to the surface area per unit volume. Peters and Halpern (1953), w h o used pitchblende in the form of discs of known surface area, have shown that the rate of dissolution is directly proportional to the surface area of the solid. In the present work the uranium dioxide was in the form of a fine p o w d e r whose surface area was unknown. It has been shown that, under conditions *To w h o m correspondence should be directed.
0304-386X/81/0000-0000/$02.50
© 1981 Elsevier Scientific Publishing Company
198 where the products of the reaction are soluble or, in general, leave the surface of the reacting particles, the reaction of particulates can be formalised in terms of the fraction reacted, a (Wadsworth 1975; pp. 449--453). If the products do n o t limit the reaction and if the solution concentration of the reactant remains constant, then the velocity of movement of the interface will be constant with time. This will result in linear kinetics. If it is assumed that the uranium dioxide particles are spherical, although the result is the same for any isometric shape, then the rate of reaction at the surface of the sphere can be expressed in the form dn/dt
(1)
= 4 n ra k / s
where n = number of molecules; r = radius of the sphere; s = cross sectional area of one of the reacting molecules; k = a constant containing the fraction of surface sites available, concentration terms other than surface concentration and the specific rate constant having the dimension secondThe total number of molecules, n, in the sphere is {2)
n = 4nr3/3v
where v = the volume of one molecule. This equation can be differentiated with respect to time and equated to eqn. (1), giving (3)
dr/dt = -vk/s
Equation (3) represents constant velocity movement of the reaction interface. If r0 is the initial radius of the sphere and a is the fraction reacted it can be shown that (4)
a = 1-r31ro 3
which, upon differentiation with respect to time, becomes da - 3 r 2 dr d t - r03 dt
(5)
Combining eqns. (3), (4) and (5) gives da dt
3v ( 1 - ~ ) 213 k / r o s
(6)
Imposing the conditions a =0 when t = 0 , eqn. (6) can be integrated giving
[1-(l-a)
'/3 ]
= k01
where k o 1 = v k / r o s
t
(7) (8)
Equation (7) indicates that a plot of [ 1 - ( 1 - ~ ) 1/3 ] against t should be linear with a slope of k0 ~.
199
EXPERIMENTAL DETAILS The uranium dioxide was provided by the South African Atomic Energy Board and had the composition UO2.o24 • It was f o u n d t h a t 80% of the uranium dioxide w a s - 2 0 0 mesh (U.S. Sieve Series - 0 . 0 7 4 ram). The leach solution used consisted of 4% Na2CO3,1.5% NaHCO3 and 0.025% KCN and had a pH of 10.2. The KCN was present to assess its effect on the rate of dissolution of uranium dioxide. This was part o f a larger study on the feasibility of a combined gold and uranium dioxide leach. It was found that the presence of cyanide had no effect on the rate of dissolution of uranium dioxide (Du Preez et al., 1980). The concentration of uranium VI was determined by measuring the absorbance of the uranyl--pyridylazo resorcinol complex at 530 nm using a Beckman Acta MVII spectrophotometer, as described by Florence and Farrar (1963). The molar absorption coefficient, e, of the uranyl--pyridylazo resorcinol complex was f o u n d to be 39,700 dm a mol -~ cm ~ In the experiment, accurately weighed i g samples of uranium dioxide were used together with 100 cm a of the leach solution. The temperature of the mixtures was controlled by passing water, at the desired temperature, through the hollow glass walls of the reaction vessels. The mixtures were stirred magnetically. The oxidant used was oxygen at atmospheric pressure. RESULTS The variation o f [ 1 - ( 1 - ~ ) 1/3] with time is shown in Fig. 1 for different temperatures in the range 303--353 K. In the majority o f cases good linear relationships are obtained up to a =0.8, the plot for 353 K showing the greatest deviation at values of a > 0.8. These experiments, therefore, suggest that the system gives linear kinetics in the range a = 0 to 0.8. According to the Arrhenius equation k0 = A exp (-A E / R T ) where A = pre exponential or frequency factor; k0 = rate constant at temperature T, A E = activation energy; R = gas constant -- 8.314J K -1 mol-1; T = absolute temperature. Thus a graph of In k0 against 1 / T should give a straight line of slope - A E / R . From eqn. (8), the slopes of the lines in Fig. 1, ko 1 = vk/ros
(8)
where v, r0 and s are all temperature-independent. The constant k contains the specific rate constant k0 as well as other terms. If the other components of k are temperature-independent, then a plot of In k0 ~ against 1 / T should give a straight line from whose slope the activation energy can be calculated. This plot is shown in Fig. 2 from which the activation energy, AE, was f o u n d to be 51.1 kJ mol -~.
200
0.6
353K
GID
o.II// .~
333K
o
303 K
/
A
*/
0
~
10
20
30
40
Time(h)
Fig. 1. The variation of [ 1 - ( 1 - ~ ) l/s ] with time at temperatures of 3 0 3 , 3 1 3 , 218, 333 and 353 K.
i
T
f
~o - I .5
-2.5
--
©
-35
-4.5 2.8
I
I
I
2.9
3.0
3.1
3.2
1/T x103( K-1) Fig. 2. The variation of In ko ~ with the reciprocal of the absolute temperature.
3.3
201 Peters and Halpern (1953), using pitchblende, found a similar value of 51.5 kJ mol -~. Schortmann and De Sesa (1958) q u o t e a value of 56.1 kJ mol -~, whilst Pearson and Wadsworth (1958) contend that the previous values quoted are composite activation energies. They give the real activation energy, relating to the absorption of H:CO3; as 28.0 kJ mol -~. They give a calculated composite activation energy of 43.5 kJ mol -~ and say that a direct determination from the rate plots over a wide range of pressures yield composite activation energies in the range 46.0 to 62.8 kJ mol -~, with the lower values resulting from the rates measured at high pressures. CONCLUSIONS
The results of this study indicate that the reaction boundary of the uranium dioxide particles is moving at constant velocity, giving rise to linear kinetics. The products of the reaction are ~ o t limiting the reaction over the range = 0 to 0.8. The variation of k01 with temperature enabled the activation energy, for the whole process, to be determined. This was found to be 51.1 kJ mo1-1. Considering the assumptions made, this value compares well with previously determined values. ACKNOWLEDGEMENTS
The authors wish to thank Johannesburg Consolidated Investment Company Limited who supplied the funds necessary for the above work. REFERENCES 1 D u Preez, J.G.H., Morris, D.C., Van Vuuren, C.P.J. and Oertell, M., 1980. The Chemistry of Uranium. Part XXVI. Alkaline dissolution of A u and/or UO2 powders. Hydrometall., 6: 147--158. 2 Florence, T.M. and Farrar, Y., 1963. Spectrophotometric determination of uranium with 4-(2-pyridylazo) resorcinol. Anal. Chem., 35 : 1613--1616. 3 Pearson, R.L. and Wadsworth, M.E., 1958. A kinetic study of the dissolution of U O 2 in carbonate solution. Trans. Metall. A.I.M.E., 212: 294--300. 4 Peters, E. and Halpern, J., 1953. Studies in the carbonate leaching of uranium ores. If. Kinetics of the dissolution of pitchblende. Trans. Can. Inst. Min. Metall., LVI: 350--354. 5 Schortmann, W.E. and De Sesa, M.A., 1958. Kinetics of the dissolution of uranium dioxide in carbonate-bicarbonate solutiona Proc. 2nd Int. Conf. on the Peaceful Uses of Atomic Energy, Vol. 3, United Nations, Geneva, pp. 333--341. 6 Wadsworth, M.E., 1975. Reactions at surfaces. IV. Reactions at nonmetal-solution interfaces. In : H. Eyring (Ed.), Physical Chemistry -- A n Advanced Treatise, Vol. VII, Reactions in Condensed Phases, Academic Press, N e w York, pp. 449--472.