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Ind. Eng. Chem. Res. 2010, 49, 4542–4545
Boehmite Dissolution Model Based on Simulant Data Renee L. Russell* and Reid A. Peterson Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352
Several of the Hanford waste tanks contain significant quantities of boehmite. This boehmite will be dissolved through caustic leaching as part of the Hanford Tank Waste Treatment and Immobilization Plant (WTP) currently under construction. Therefore, it is important to fully understand the nature of this dissolution process so that caustic leaching can be effectively deployed on the Hanford tank wastes. This research determined the impact of aluminate ion on the dissolution kinetics of boehmite. In addition, other parameters that impact boehmite dissolution, such reaction temperature, were also assessed and used to develop a semiempirical model of the boehmite dissolution process. A shrinking core model was used to fit data from a series of boehmite dissolution tests with an additional term added to account for the approach to saturation. This revised model provided an adequate fit to the experimental data; however, a superior fit to the experimental data was obtained when a term was added to represent the number of dissolution sites available at the start of the reaction. These results suggest that boehmite will dissolve significantly slower as gibbsite dissolves and adds aluminate to the solution. Practically, these results indicate that the blending of wastes with gibbsite and boehmite will ultimately result in either more caustic or more time required to achieve the same fraction of boehmite dissolution. Introduction Several of the Hanford waste tanks contain significant quantities of boehmite. This boehmite will be dissolved through caustic leaching as part of the Hanford Tank Waste Treatment and Immobilization Plant (WTP) currently under construction. Therefore, it is important to fully understand the nature of this dissolution process so that caustic leaching can be effectively deployed on the Hanford tank wastes. The boehmite represents the largest component in the highlevel waste (HLW) for which aggressive leaching conditions are required to achieve dissolution. As such, understanding the boehmite leaching chemistry and the impacts on the WTP flowsheet using a boehmite simulant will be critical to the WTP performance. This paper focuses on developing a model to describe the boehmite leaching kinetics during the caustic leaching step of the Hanford waste treatment. Aluminum in the wastes is believed to be present in the two most common mineralogical phases: gibbsite (monoclinic Al(OH)3) and boehmite (orthorhombic AlOOH). The dissolution rates of the two primary mineralogical phases are considerably different.1 Therefore, the leaching kinetics will depend on the relative amounts of these phases in the waste as well as particle size, crystal habit (i.e., particle size and shape), operating temperature, hydroxide activity, aluminum solubility limits, particle Reynolds number associated with the mixing system, etc. A number of studies of boehmite dissolution have been performed in the past.2-5 In addition, a number of studies have investigated precipitation kinetics for boehmite.5-7 Palmer4 found that the presence of nitrate appeared to suppress the dissolution of boehmite. The boehmite dissolution studies have indicated, as expected, that the boehmite dissolution kinetics are a strong function of both temperature and hydroxide concentration. Note, however, that none of these studies evaluated the impact of the presence of aluminate ion on the dissolution rate. Scotford and Glastonbury8 measured the dissolution of relatively large (20-40 µm) boehmite particles and found the * To whom correspondence should be addressed. Telephone: (509)3736235. Fax: (509) 376-3108.E-mail:
[email protected].
dissolution rate at 85 °C in 5 M NaOH to be relatively slowsapproximately 3% in 3 h. They also reported an apparent activation energy of 123 kJ/mol. In a subsequent study, Scotford and Glastonbury9 found that the initial dissolution rate appears to be proportional to the hydroxide activity to the 1/2 power. Packter3 measured the dissolution rate of much smaller boehmite crystals (0.07-0.1 µm) and found the dissolution rates to be much higher-approximately 60% in 6 h at 60 °C. Packter proposed a model for the dissolution of boehmite as
( )
d
M Mo M ) -k dt Mo
( )
4/3
(1)
where k is the kinetic rate constant, M is the boemite concentration, and Mo is the initial boehmite concentration. Packter found that for these small boehmite crystals, the dissolution rate increased linearly with the hydroxide activity. Packter also found the activation energy to be between 115 and 125 kJ/mol. Pereira et al.10 developed a shrinking core model with reversible dissolution reaction for the dissolution of gibbsite as
(
dCAl CAl )k 1- o dt Cgibb
) ( 2/3
COH 1 -
CAl KCOH
)
(2)
where CAl and COH are the dissolved aluminum and OH- ions concentrations, respectively. K represents the equilibrium constant of the following reaction: Al(OH)3 + OH- T Al(OH)4 and Cogibb is the initial concentration of gibbsite added. For a constant hydroxide concentration, eq 2 can be rewritten for a gibbsite material balance as
(
d
)
AlOOH AlOOHi AlOOH ) -k dt AlOOHi
10.1021/ie901841g 2010 American Chemical Society Published on Web 04/09/2010
(
)
2/3
(OH)-(1 - σ)
(3)
Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010
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Table 1. Surface Area of Boehmite Samples
Figure 1. Schematic drawing of the caustic leaching test setup.
where σ)
Al(OH)4Al(OH)4,s-
Note that for boehmite, the rate of dissolution is expected to be significantly slower and dependent upon the initial crystal surface area and, as such, a different value for k is expected. The value for k for the given boehmite materials will be determined by regression of the experimental data. Experimental Section This section describes the methods used to conduct the leach testing for both actual waste samples and simulant samples. Boehmite was obtained from APYRAL (for product information, see http://www.gmzinc.com/index.php?page)apyral-aoh20), product AOH 20. X-ray diffraction (XRD) analysis confirmed that this material is boehmite. The tests were performed in a 1-L reaction vessel as shown in Figure 1. The
Figure 2. SEM micrograph of the boehmite to be used in the simulant.
sample ID
specific surface area (m2/g)
washed tank waste solids simulant boehmite
26 10
vessel was filled with the leaching fluid and heated to the leaching temperature. The temperature was measured with a calibrated thermocouple and controlled with a calibrated temperature controller. Boehmite was added as a powder to the reaction vessel through the sample port while stirring after the leaching fluid had reached leaching temperature, which started the clock for the test. The test solution was sampled at 1, 2, 4, 8, and 24 h. Each sample consisted of 5 mL supernatant, which was filtered through a 0.45-µm filter after being drawn from the reaction vessel and then analyzed for aluminum and sodium content by inductive coupled plasma-atomic emission spectroscopy (ICP-AES). The amount of aluminate was adjusted by dissolving gibbsite before introducing the boehmite. The amount of gibbsite added is reflected in the initial aluminate concentration at time 0 for each test. Results Sample Characterization. Figure 2 shows a scanning electron microscopy (SEM) micrograph of the commercially procured boehmite that was used in the simulant tests. Note that the average crystal size for this material is approximately 0.8 µm. The material agglomerates into larger particles, so particle-size distribution measurements do not provide significant insight into the reactivity of the boehmite. Table 1 compares the surface area of the actual tank waste sample to that of the boehmite used in these tests. As might be expected from the smaller primary particle size, the surface area of the actual tank waste material was significantly larger than for the commercially procured boehmite. Testing Matrix. A set of tests were performed where the initial matrix contained varying amounts of dissolved sodium aluminate. In these tests, the initial supernate contained various
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Figure 3. Boehmite dissolution with aluminate present using 5 M NaOH at 100 °C with stirring at 120 rpm fit to the model as expressed by eq 3 with k ) 0.05.
Figure 4. Boehmite dissolution using 5 M NaOH at 100 °C with stirring at 120 rpm, with aluminate present fit to the revised model as expressed by eq 4 and k ) 0.04.
Figure 5. Boehmite dissolution using 5 M NaOH at 85 °C with stirring at 120 rpm fit to the model as expressed by eq 5 and k ) 0.01.
Figure 6. Results using 5 M NaOH with stirring at 120 rpm from 80 to 100 °C with temperature correction as expressed by eq 5.
levels of soluble aluminate before the start of leaching. The results from these tests are plotted in Figure 3 as a function of the Al concentration in solution over time at 100 °C. Figure 3 shows that the boehmite dissolves more slowly when more Al is in the solution. Note that while eq 3 provides a reasonable fit to the experimental data, it appears to under-predict the impact of the initial aluminate concentration on the reaction rate. This is evidenced by the fact that the model underpredicts at low aluminate and appears to overpredict at high aluminate. This indicates that the dissolution model should be revised. One form that was tested and found to be statistically superior was to include a term for the initial aluminate concentration shown in eq 4.
(
d
)
AlOOH AlOOHi AlOOH ) -k dt AlOOHi
(
)
2/3
-
(OH) (1 - σ)(1 - σi) (4)
This revised model provides a statistically improved fit to the data as seen in Figure 4. As seen, this revised model now provides more accurate predictions over the entire range of initial aluminate concentrations. Note that simply squaring the last term in eq 3 did not provide the same fit to the experimental data in that it again led to underprediction of the reaction rate at low initial aluminate concentrations. To fit to the 85 °C data (Figure 5), a temperature correction term is needed. From this data, it was determined that there is a 121-kJ/mol boehmite activation energy associated with eq 5. The data in Figure 5 were used to derive this activation energy.
Figure 7. Impact of fraction of aluminum as gibbsite on boehmite dissolution for various ratios of NaOH to total insoluble Al.
Then, the resultant model with the temperature correction included was applied to the data over a range of temperatures and is shown in Figure 6. Note that based on the value for k regressed from the data at 100 °C, the value of the pre-exponetial factor Ao was 2.58 × 1015. d
(
AlOOH AlOOHi dt
)
(
) -Ao e-121000/RT
AlOOH AlOOHi
)
2/3
(OH)-(1 - σ)(1 - σi)
(5)
The impact of varying amounts of gibbsite on boehmite dissolution was evaluated with the derived model. Figure 7
Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010
shows a plot of the time necessary to achieve 50% boehmite dissolution as a function of the Na:Al molar ratio. These results show that the presence of gibbsite requires either more caustic or more time to achieve the same fraction of boehmite dissolved. Conclusions A shrinking core model was used to fit data from a series of boehmite dissolution tests. An additional term was added to the shrinking core model to account for the approach to saturation. This revised model provided an adequate fit to the experimental data; however, a superior fit to the experimental data was obtained when a term was added to represent the initial saturation at the start of the reaction as shown in the following equation:
(
d
AlOOH AlOOHi dt
)
(
) -Ao e-121000/RT
AlOOH AlOOHi
)
2/3
(OH)-(1 - σ)(1 - σi)
(5)
These results suggest that boehmite will dissolve significantly slower as gibbsite dissolves and adds aluminate to the solution. Practically, these results indicate that the blending wastes with gibbsite and boehmite will ultimately result in either more caustic or more time required to achieve the same fraction of boehmite dissolution. For the proposed dissolution process, blending strategies must consider the trade-offs between using caustic, processing time, and time to prepare feed. Acknowledgment We would like to acknowledge Don Rinehart for all of his lab work in performing the tests and Brian Riley for his SEM work. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DEAC05-76RL01830. This work was funded by the U.S. Depart-
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ment of Energy through the Office of Environmental Management and under the guidance of Bechtel National, Inc. Literature Cited (1) Music, S.; Dragcevic, D.; Popovic, S.; Vdovic, N. Microstructural Properties of Boehmite Formed Under Hydrothermal Conditions. Mater. Sci. Eng. B 1998, 52, 2–3, 145-153. (2) Palmer, D. A., Be´ne´zeth, P. Wesolowski, D. J. Experimental Studies of the Solubilities of Aluminum Oxy-Hydroxy Phases to 300°C; Oak Ridge National Laboratory, Oak Ridge, TN, 2000. (3) Packter, A. Studies on recrystallised aluminum mono-hydroxide precipitates. Kinetics of dissolution by sodium hydroxide solutions. Colloid Polym. Sci. 1976, 254, 1024–1029. (4) Palmer, D. A.; Be´ne´zeth, P.; Wesolowski, D. J. Aqueous hightemperature solubility studies. I. The solubility of boehmite as functions of ionic strength (to 5 m, NaCl), temperature (100-290°C), and pH as determined by in situ measurements. Geochim. Cosmochim. Acta 2001, 65 (13), 2081–2095. (5) Panias, D. Role of boehmite/solution interface in boehmite precipitation from supersaturated sodium aluminate solutions. Hydrometallurgy 2004, 74, 203–212. (6) Skoufadis, C.; Panias, D.; Paspaliaris, I. Kinetics of boehmite precipitation from supersaturated sodium aluminate solutions. Hydrometallurgy 2003, 68, 57–68. (7) Dash, B.; Tripathy, B. C.; Bhattacharya, I. N.; Das, S. C.; Mishra, C. R.; Mishra, B. K. Precipitation of boehmite in sodium aluminate liquor. Hydrometallurgy 2009, 95, 297–301. (8) Scotford, R. F.; Glastonbury, J. R. Effect of temperature on the rates of dissolution of gibbsite and boehmite. Can. J. Chem. Eng. 1971, 49, 611– 616. (9) Scotford, R. F.; Glastonbury, J. R. The effects of concentration on the rates of dissolution of gibbsite and boehmite. Can. J. Chem. Eng. 1972, 50, 754–758. (10) Pereira, J. A. M.; Schwaab, M.; Dell’Oro, E.; Pinto, J. C; Monteiro, J. L. F.; Henriques, C. A. The kinetics of gibbsite dissolution in NaOH. Hydrometallurgy 2009, 96, 6–13.
ReceiVed for reView November 20, 2009 ReVised manuscript receiVed February 23, 2010 Accepted March 28, 2010 IE901841G
