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Mercury-Catalyzed Dissolution of Aluminum in Nitric Acid Richard W. Rice* and Dhananjay V. Sarode Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634
The role of mercury in promoting the dissolution of aluminum in nitric acid was investigated experimentally. The rates at which a broad range of Al sample sizes and shapes dissolved in HNO3 were determined for various combinations of HNO3 and Hg(NO3)2 molarities. The effects of temperature and agitation were also studied, as was the product gas composition. The kinetics of dissolution appeared to shift from first- to nearly zero-order with respect to the concentration of HNO3 as the concentration of Hg was lowered, reflecting a change in rate-controlling step. As the surface-to-volume ratio of the samples was increased, the rate of dissolution per area decreased, and the rate in terms of mass per unit time changed only modestly. Mercury appears to promote Al dissolution via a “bulk” effect involving amalgam formation rather than by a surface catalytic effect. Introduction
expression
The dissolution of spent nuclear fuel elements in nitric acid is the first step in the aqueous process for recovery of fissionable materials. One of the most commonly used fuel elements consists of an aluminum/uranium alloy housed in an aluminum sheath. Because of its passivating oxide coating, aluminum is resistant to dissolution in nitric acid; thus, until recently, mercury has been added to the dissolving solution to accelerate the dissolution. However, because mercury is highly toxic, the U.S. Department of Energy has pursued research to find either a more environmentally friendly alternative or a way to minimize the amount of mercury required. In either case, it was deemed important to acquire a better understanding of the mechanism by which mercury catalyzes the dissolution of aluminum in nitric acid. This paper describes a study of that topic. The dissolution in this case is a rare example of a liquid-phase catalyst being used for a heterogeneous reaction. Furthermore, the system of reactions that occurs is complex, involving the generation of numerous gaseous species. Relatively little has been written on the subject in the open literature.1,2 In an early study, Wymer and Blanco1 reported that the overall stoichiometry of the reaction for 4 M HNO3 was
Al + 3.75HNO3 f Al(NO3)3 + 0.225NO + 0.15N2O + 0.1125N2 + 1.875H2O (1) The stoichiometric ratio of HNO3 to Al decreased from 4 for 2 M HNO3 to 3.25 for 8 M HNO3. The dissolution rate was reported to increase with the concentration of Hg(NO3)2 up to 0.005 M, but a further increase in the concentration of Hg did not increase the rate. A maximum dissolution rate was observed for HNO3 concentrations in the 6-8 M range. The only known previous study of the kinetics of Hg(NO3)2-catalyzed Al dissolution in HNO3 is that of Schlea,3 which involved dissolution of Al/U alloys. Schlea correlated his data using the * Author to whom correspondence should be addressed. Department of Chemical Engineering, Earle Hall, Clemson University, Clemson, South Carolina 29634-0909. E-mail:
[email protected]. Phone: 864-656-5428. Fax: 864656-0784.
r ) kR[Hg][HNO3]/µ
(2)
where r is the dissolution rate of Al (mg of Al per cm2 per min), k is the rate constant [(mg of Al) cP L per cm2 per min per mol2], R is the fractional dissociation of HNO3, [Hg] and [HNO3] are the respective concentrations (mol/L), and µ is the solution viscosity (cP). The viscosity term in this empirical expression was inserted to (indirectly) account for the inhibiting effect of Al(NO3)3 accumulation. No previous study explicitly addressed temperature dependence, but Posey4 reported that dissolution begins only when a threshold in the 7085 °C range is reached. Once this threshold is crossed, the highly exothermic nature of the reaction(s) causes the solution to rapidly reach its boiling point. Previous researchers have concluded that the uniqueness of Hg as an accelerating agent arises from the formation of an amalgam. Karraker5 has speculated that the following sequence of steps occurs: (i) penetration of the surface Al2O3 layer by Hg2+ or Hg2+2 ions; (ii) reduction of these ions to Hg0 by Al at the oxidemetal interface; (iii) amalgamation of the Al at the interface; and (iv) oxidation of the amalgam by HNO3, regenerating Hg2+ and resulting in the formation of NO, N2O, N2, and H2. Wymer and Blanco1 explained their results in terms of competition between the reduction of Hg2+ to Hg0, i.e., metallic or “free” mercury, and the dissolution of the amalgam, which ultimately involves the oxidation of Hg as well as Al. Experimental Section Apparatus. The reactor used was made of thickwalled Pyrex glass in the form of a nominal 2-L cylinder (10 cm id) with a rounded bottom and an integral top containing five ball-and-socket ports. The central port was used for a glass stirrer shaft/sample holder rotated by an electric motor connected to a speed controller/ tachometer. A Teflon stirrer blade was fitted to the bottom of the shaft. Depending on the type of experiment involved, the other ports were used for introducing the flushing gas, inserting a thermowell, mounting the Al sample, sampling the liquid, and connecting the exit gas condenser. The bottom half of the reactor was
10.1021/ie000763v CCC: $20.00 © 2001 American Chemical Society Published on Web 03/16/2001
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Figure 1. Schematic diagram of reaction rystem.
heated by a heating mantle. A flow of helium, metered using a mass flow controller, was used as the flushing gas for the vapor space above the liquid; thus, the mode of operation was semibatch. Figure 1, an overall schematic of the main system, shows that the reactor outlet gas was routed through a condenser, a gas/liquid separator, and a Permapure membrane dryer and was then diluted with additional He before entering a Rosemount/Beckman model 951A chemiluminescence NOx analyzer or being directly vented to a hood. Glass or plastic tubing was used for the transfer lines. A chart recorder was used to continuously monitor the NOx analyzer signal. The temperature in the reactor was measured using a thermocouple inside a glass thermowell. A Perkin-Elmer Sigma 3 gas chromatograph equipped with a thermal conductivity detector and a 152 cm × 3.2 mm Carbosphere 80/100 column was operated at 120 °C using a He carrier gas to obtain N2O, N2, and H2 concentration measurements for gas samples taken via syringes from a septum tee in the reactor exit line. Liquid samples were analyzed for dissolved Al (and Hg in some cases) using atomic absorption spectroscopy. Colorimetric determination of Hg was made using the dithizone method. Surface analysis of several Al samples was performed using Auger spectroscopy and energydispersive X-ray spectroscopy/scanning electron microscopy (EDXS). The surface areas of the powdered Al samples were determined using N2 adsorption. Materials. The standard Al sample used in most dissolution experiments was in the form of a thin rectangular slab (48 × 13 × 3 mm, approximately 4.5-5 g) with a composition in mass percent of 98.7% Al, 1.19% Ni, 0.06% Si, 0.048% Fe, and 0.026% Cu. These samples were supplied by the Westinghouse Savannah River Co. Other Al samples were obtained from Aesar in the form of 99+% 20 µm spherical powder, 99.999% 8 × 4 mm “shot”, and 99.999% annealed 0.25-, 0.5-, 1-, and 2-mmdiameter wire. Distilled/deionized water, nitric acid (Mallinckrodt, 70 mass %), and mercuric nitrate (Ricca, 0.282 N solution) or mercurous nitrate dihydrate crys-
tals (Mallinckrodt AR) were used in the dissolving solution. For the colorimetric determination of Hg the following reagents were used: a 0.001% solution of dithizone in chloroform (Ricca), potassium iodide (Baker AR), and potassium acid phthalate (Mallinckrodt AR). The gases used were H2, N2, O2 (all >99.995%), N2O (>99%), and a 7500 vppm NO in He certified blend (Matheson). Dissolution Run Procedure. For most experiments involving agitation, the Al slab was tied to the stirrer using Teflon string after two drilled holes were positioned in the slab over glass protrusions on the shaft. In runs involving periodic removal of the Al sample, the slab was similarly attached to a hooked glass rod. Once the Al sample had been inserted into the reactor and submerged in 1 L of HNO3 solution of the desired molarity, the system was flushed with He and then leaktested at 140 kPa with the agitator operating at the desired speed. The solution was then rapidly heated under a flow of 2.5 LPM of He to the desired temperature. The latter was typically the boiling point at either 102 or 140 kPa. Next, Hg(NO3)2 was added, and a stopwatch was started as soon as signs of vigorous reaction were observed. In most runs, the progress of the reaction was measured using both continuous monitoring of NO in the exit gas and frequent sampling of the liquid. As indicated in Figure 1, the exit gas was further diluted with He before it entered the NOx analyzer. Liquid samples were obtained by opening a valve connected to a small-bore glass dip tube. Each time, the first 5 mL were discarded, and then the next 3 mL were retained for analysis. For runs involving determination of N2O, N2, and H2, the He flush gas flow was 0.15 LPM, and 100-µL exit gas samples were taken and injected into the GC. Once reaction was complete, the final solution was sampled, cooled, and properly discarded. To minimize the possibility of Hg accumulation, the reactor was cleaned for 4 h using boiling 6 M HNO3 after every 2-4 runs. Further details concerning the experimental work are given elsewhere.6 Results Preliminary Tests. Early runs were made to test the suitability of various methods and approximations. In runs conducted with 0.001 M Hg(NO3)2 in boiling 2, 4, and 8 M HNO3, the temperature measured by a thermocouple tightly fitted into a hole drilled in an Al slab was found to be no more than 4 °C higher than the temperature of the surrounding liquid during the period of maximum dissolution rate and negligibly higher thereafter. This confirmed that, despite the highly exothermic nature of the reaction, the temperature at the solid-liquid interface could be reasonably well approximated by the liquid temperature. Runs were also made in which the Al slab was removed after various reaction time intervals for measurement of mass and nominal surface area. The resulting correlation between area and mass supported the assumption of equal reaction rates on all faces. These runs also confirmed that atomic absorption measurements of dissolved Al agreed with values based on gravimetric measurement within 4% at the 95% confidence level. Likewise, several runs were made to study the overall stoichiometry of the reaction, particularly the molar ratio of NO generated to Al dissolved. This information was later used in determining the reaction rate as a function of time from the exit gas NO concentration versus time curve. The trend found for the NO/Al ratio agreed with that
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Figure 2. Plot of dissolution rate vs percent dissolved for aluminum slab samples for 4 M HNO3, 0.005 M Hg, 104 °C.
reported in the literature.1 For 4 M HNO3 and 0.005 M Hg(NO3)2, the ratio ranged from 0.18 to 0.225, averaging about 0.2, and for 0.0005 M Hg(NO3)2, the ratio was virtually the same, 0.21. By combining various NO concentration vs time plots with corresponding mass of Al vs time data, a model was developed that related the dissolution rate at a given instant to the slope of the NO concentration vs time curve, the gas flow rate, the cumulative amount of NO generated, etc. It was found that modeling the vapor space in the reactor as if it were a continuous stirred tank (CST) and assuming plug flow in the transfer line (dead volume) gave good agreement between dissolution rates determined using gas monitoring and liquid sampling. The primary advantage of the gas monitoring method was that it could be used for very rapid dissolution cases, e.g., runs in which complete dissolution occurred in 1-2 min. After these preliminary tests, the main experimental plan was carried out. This consisted of a series of runs examining the separate effects of the concentration of HNO3, the concentration of Hg(NO3)2, the temperature, the agitation rate, and the Al sample geometry. Concentration Effect. This phase of the work consisted of a grid of experiments involving nearly all permutations of three variables, the concentration of HNO3, the concentration of Hg(NO3)2, and pressure. Given that the runs were made at boiling conditions, changing the pressure was the means of changing the reaction temperature. Three different initial HNO3 concentration values, 2, 4, and 8 M, were used for two Hg(NO3)2 concentration values, 0.0005 and 0.005 M, and for two pressures, 102 and 140 kPa. Because of consumption, the acid molarity decreased by roughly 0.65 M when the standard thin Al slab used in most runs completely dissolved in 1 L of solution. Thus, the concentration of HNO3 for a nominal 4 M run averaged about 3.67 M. Figure 2 is a representative plot of the dissolution rate in milligrams of Al per square centimeter per minute vs the percent mass loss for 4 M HNO3 runs for two mercuric nitrate molarities. Most runs showed this general trend, i.e., a maximum early in the run followed by a very gradual decline when the concentration of Hg(NO3)2 was 0.005 M and a somewhat steeper decline when the concentration of Hg(NO 3)2 was 0.0005 M. Roughly the same trends were observed for unagitated runs, regardless of boiling point or sample geometry. For agitated runs, the only difference was
Figure 3. Effect of dissociated nitric acid molarity on the average dissolution rate for aluminum slab samples.
that the rate maximum occurred very early (
