I
R.
J. ALLEN, H. G. PETROW',
and P. J. MAGNO
National l e a d Co., Inc., Winchester, Mass.
Precipitation of Uranium Tetrafluoride from Aqueous Solution by Catalytic Reduction
DURING
dioxide gas was bubbled through the solution during the reaction so as to maintain a saturated solution and keep the sulfur dioxide coccentration constant throughout the reaction period. The effect of uranium concentration on the precipitation rate was determined by precipitating UF4 from a solution originally 0.21M in uranium. The fluoride concentration was sufficiently high so that there was no appreciable change of fluoride concentration during the reaction. The temperature and the concentration of all other reactants were maintained constant during the reaction. A plot of the log of the uranium concentration us. time yields a straight line, indicating that the reaction rate is first-order with respect to uranium concentration. The effect of fluoride concentration on the reaction rate was studied. No precipitation was noted for a solution 0.018M in uranium, 0.03M in copper, 0.55M in chloride, and 0.071M in fluoride. the stoichiometric fluoride requirement. A slight excess of fluoride is necessary before the reaction can proceed in a reasonable period of time. Increasing the fluoride concentration over a range from 0.33 to 1.64M increased the precipitation rate, which revealed a first-power dependence with respect to fluoride (Figure 1). Figure 1 shows an induction period prior to the precipitation of UF4, the length of which is a function of fluoride concentration. This induction period also depends on the concentrations of uranium, copper, and chloride. I t is believed that the primary reason for the induction period is the necessity of producing a finite cuprous ion concen-
were all identified as UF4.3/4Hz0. They all had tap densities falling between 3.1 and 3.4 grams per ml., and were extremely pure. Sulfur dioxide would not reduce uranium, even in the presence of excess fluoride. However, it reduced cupric ion to cuprous ion a t elevated temperatures in the presence of a small quantity of chloride. Cuprous ion had already been shown to be a n effective reductant for uranium. Because the uraniumbearing solution used in this work contained chloride, it was apparent that if a small quantity of copper ion were added and sulfur dioxide gas passed into the hot solution, UF4 would be precipitated. A dark green, crystalline material, identified as UF4. 3/4Hz0, was obtained. The tap density of salt was 3.2 grams per ml. Once again, the salt was very pure. Temperature and the concentrations of cupric ion, chloride, sulfur dioxide, fluoride, and uranium were important variables. The two reactions, reduction of cupric ion to cuprous ion by sulfur dioxide and reduction of uranyl ion by cuprous ion to precipitate UF4, are as follows :
an extensive program directed toward the production of dense, high purity uranium tetrafluoride (UF4) from uraniferous ores, a catalytic process was discovered for the reduction of uranyl ion in the presence of fluoride to yield UF4. Most of the research was conducted with solutions prepared from sulfuric acid leach liquors of uraniferous ores. The uranium was solvent-extracted from the leach liquor into a tertiary amine. The uranium was stripped from the amine into 3M hydrochloric acid to yield a solution which contained (per liter) 120 grams of uranium, 100 of sulfate, 25 of chloride, 1 of molybdenum, and traces of phosphate, arsenate. sodium, calcium, zirconium, and magnesium. The solvent extraction process used was essentially the Amex process developed a t Oak Ridge ( I ) , although extensive changes were made in the stripping circuit to accommodate the use of hydrochloric acid. Hydrofluoric acid was added to the concentrated uranium solution, generally in slight excess of the stoichiometric requirement, and various substances were examined as possible uranium reductants. The reduction tests were performed in hot solutions, generally a t 90' C., because a lower hydrate and a denser salt are formed at elevated temperatures. Among the reducing agents studied were stannous chloride, cuprous chloride, chromous chloride, titanous chloride, and sulfur dioxide. All but sulfur dioxide were effective reducing agents, and quantitative uranium reduction was obtained. The salts obtained 1 Present address, Ionics Inc., Cam. bridge, Mass.
2CuCld--
+ SO2 + 2H20 + sod-- + 4Hf + 4C1+ UOz++ + 4HF + 4C1UF4 f 2CUClA-- + 2H20 +
2cuc12-
2CuC12-
(1)
(2)
The studies were carried out in a glass reaction vessel lined with Kel-F. The solution was stirred continuously during the Drecipitation and samples were removed periodically for analysis. Sulfur
. . 4
Figure 1. Rate of precipitation of UF4 has a first-power dependence on fluoride concentration
b
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2 3 Time in Hours
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 2. The precipitation rate and induction period are a function of copper concentration
0 1
0
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2 Time
in
3 Hours
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40
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Figure 3. Increase in chloride concentration significantly increases rate of uranium precip-
0
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tration before the reaction can proceed. The cuprous ion concentration necessary before precipitation of UFI commences is a function of the other variables involved. Increasing the copper concentration from 0.0079 to 0.063M indicated the reaction was first order with respect tb the copper concentration. As can be seen in Figure 2, the induction period is a function of the copper concentration. Reduction of cupric ion will not take place in the absence of chloride, presumably the chloride is needed to stabilize the cuprous ion. Figure 3 shows the effect of chloride concentration on the rate of UF* precipitation. Increasing the chloride concentration from 0.028 to 1.1M significantly increased the uranium precipitation rate. However, no consistent power dependency was shown for the chloride concentration. Increasing the temperature increases the rate of precipitation until the solution approaches the boiling point, where a definite decrease in the rate is noted (Figure 4). This results from the low solubility of sulfur dioxide in the boiling solution. The density and hydration of the salt are a function of the temperature. For optimum reaction rate, salt density, and degree of hydration, the reaction should be carried out between 85' and 95' C. The effect of temperature on rate of the reduction of cupric to cuprous ion by sulfur dioxide was measured. The reaction proceeds very slowly at room temperature. If the temperature increases (Figure 5), the rate ofi copper
reduction also increases. However, even a t 90* C., only 60% of the copper is reduced in 6 hours. The curve in Figure 5 indicates a comparatively rapid initial rate of reduction, after which the reaction rate levels off and proceeds very slowly, if a t all. It appears that the system is approaching equilibrium. The rate of copper reduction was independent of fluoride concentration. Increasing chloride concentrations increased the rate of copper ion reduction, although the data revealed no consistent power of dependency. No precipitation occurred when stoichiometric concentrations of cuprous chloride and hydrofluoric acid were added to a 0.018M solution of uranyl ion. The use of excess concentrations of cuprous chloride and hydrofluoric acid
Table 1. Chemical and Spectrographic Analysis of UF4.3/4 HzO Chemical Analysis Total U, % 72.38 LOI(llOo C.), % 0.58 U+4, % 72.66 Apparent density, g.(ml. 2.50 UOz, yo < 0 . 0 5 Tapdenstty, g./ml. 3.15 Spectrographic Analysis, P.P.M. on U Basis
