P3-24
Synthesis of Sodalite by Hydrothermal Method and Characterizations
H. Rabiller*, F. Bart*, JL. Dussossoy*, T. Advocat,* P. Perouty*, D. Rigaud*, V. Gori*, C.Mazzocchia**, F. Martini**
*CEA/DEN/VRH/DTCD CEA VALRHO MARCOULE BP 17171 30207 Bagnols sur Cèze cedex, France [email protected]; [email protected] ** Politecnico Di Milano P.zza L. da Vinci 32, 20133 Milano - Italy [email protected]
Abstract – Sodalite is a natural chloride mineral potentially interesting as a containment matrix for waste chloride salts containing fission products. CEA, within the PYROREP European contract, aimed at assessing the intrinsic stability of pure (Na) sodalite, then substituted sodalites, provided by the Politecnico di Milano team. Na-sodalite samples were synthesized by a hydrothermal process, and the substitution for Na by K was performed by ion exchange. Soxhlet tests were carried out: the initial alteration rates ranged from 0.16 g·m-2d-1 to 21 g·m-2d-1 for Na-sodalite and 0.30 g·m-2d-1 to 32.8 g·m-2d-1 for K-sodalite. The amplitude of these ranges is due to the open porosity of the pellets and thus a high uncertainty regarding the actual material surface area in contact with water. Leach tests under saturation conditions indicated preferential release of the Na and K cations bound to chlorine in the sodalite structure.
INTRODUCTION with 6 tetrahedrons parallel to {111}. The rings Sodalite, (Na8[(Al6Si6O24)]Cl2), is a material define a series of tunnels in which the frequently cited [1],[2],[3] as a potentially intersections form large cavities occupied by interesting containment matrix for chloride salts chloride ions coordinated with sodium ions wastes containing fission products arising from (tetrahedrons). pyrometallurgical reprocessing of spent fuel. Indeed, its three-dimensional “cage” structure[4] is naturally able to contain halogen salts. Its thermal stability compared with zeolite is another advantage, making it a potential candidate material for confinement of chlorinated waste. However, substantial development work is still Fig. 1. Tetrahedral lattice in sodalite [4] necessary before sodalite can be seriously considered as a containment material. The Na-Sodalite Synthesis Method methods used to synthesize sodalite cannot The method used was a hydrothermal process prevent the formation of large quantities of free that consisted in heating a mixture of 2 g of salts. kaolinite (Al2Si2O5(OH)4), 200 mL of 4M Leach tests were performed by CEA on the sodium hydroxide, and 10 g of NaCl to 80°C. samples provided by Politecnico di Milano to The duration of the reaction seems to be highly assess the intrinsic stability of pure (Na) sodalite, dependent on certain parameters, particularly the then sodalite with other substituted cations such purity of the starting materials. The reaction was as Li, K, Cs, Sr. Politecnico di Milano followed by XRD, which showed the gradual synthesized the Na-sodalite using a hydrothermal transition from kaolinite to a kaolinite-sodalite method, followed by cation exchange for the K- mixture and finally Sodalite. It was recovered by sodalite. filtration.
SYNTHESIS OF THE MATERIALS K-substituted Sodalite Synthesis Method by Sodalite Structure[4] Ion Exchange (sodium substitution) The aluminosilicate network comprises SiO4 and This study aimed at assessing the sodalite AlO4 tetrahedrons linked by their corners. By loading capacity for other cations such as Li, K, virtue of this structure, cubic-octahedral cage Cs, Sr. Li, Cs, Sr are still to be tested. K-sodalite units (Fig. 1) are formed. These units can be was made by mixing 0.3 g of sodalite for 3 g of described as a pattern of six rings with 4 KCl in inert atmosphere for 2 hours at 800°C, tetrahedrons parallel to {100} and eight rings with an 80% exchange rate. With 0.3 g of KCl
ATALANTE 2004 Nîmes (France) June 21-25, 2004 1 P3-24 for 0.3 g of sodalite, the exchange rate was 25% C .V PMN(i) = i (g·m-2) This refers to the ratio of substitutions versus the f .S number of exchangeable sites, calculated with i where C concentration of element i in solution, respect to the two exchangeable sodium atoms in i : V : solution volume the formula Na [(Al Si O )]Cl . 8 6 6 24 2 f : the fraction of element i in the material i S : leached surface area. CHARACTERIZATION TECHNIQUES
Sample Physical Characterizations Importance of the surface area Samples were characterized by XRD and SEM- If the samples present open porosity, the actual EDS. Thermogravimetric analyses and solid surface area in contact with water and thus dilatometer measurements were also performed the expression rate (g·m-2d-1) are subject to very to determine the material sintering temperature. high uncertainty. Specific surface area measurements by the BET method were carried out on the sintered pellets. RESULTS
Characterization of the Powders Synthesized: Leaching tests The specific surface area of the powder before Description of the Soxhlet mode dynamic tests sintering measured by the BET (Ar) method was Soxhlet-mode dynamic test, during which the 9.9 m2·g-1 for both samples. sample was in contact with renewed pure water Na-sodalite to measure the initial alteration rate was XRD analysis of the Na-sodalite sample showed performed. Leaching was performed in a that the major phase was sodalite together with Soxhlet device (Fig. 2) consisting of a boiler quartz, most likely present in the starting beneath a condenser and a heating coil. materials as a kaolinite impurity (Fig. 3). The slight but systematic shift between the experimental X-ray diffraction analysis peaks and the peaks obtained with reference compounds can be attributed to differences in chemical composition, for example the presence of OH or an insufficient quantity of chlorine; this appears to be confirmed by SEM EDS chemical analysis. Fig. 2. Soxhlet leaching device The elements extracted from the material were Sodalite Italienne SI2 recovered in the boiler where they were 8000 7000
6000 concentrated during the test: the material 5000 4000
3000 Lin (Counts) Lin alteration rate was calculated by sampling a 2000 1000
0
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 fraction of the solution in the boiler. After 8000
7000 acidification, the sample was analyzed by ICP- 6000 5000
4000
3000 AES and by ionic chromatography for chlorine. (Counts) Lin 2000
1000
0 37.3 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 2-Theta - Scale SI2 sodalite italienne pastille densifiée - File: Drx02-99.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 89.950 ° - Step: 0.050 ° - Step time: 6. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 10.000 ° - Theta: Operations: Import 37-0476 (*) - Sodalite - Na4Al3Si3O12Cl - Y: 113.46 % - d x by: 1. - WL: 1.54056 - Cubic - a 8.8784 - b 8.87840 - c 8.87840 - alpha 90.000 - beta 90.000 - gamma 90.000 - Primitive - P-43n (218) - 2 - 699.849 - I/Ic P Static tests 46-1045 (*) - Quartz, syn - SiO2 - Y: 38.51 % - d x by: 1. - WL: 1.54056 - Hexagonal - a 4.91344 - b 4.91344 - c 5.40524 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P3221 (154) - 3 - 113.010 - I/Ic PD These tests were conducted at a very high S/V ratio to ensure that saturation conditions were Fig. 3: XRD spectrum of Na-sodalite rapidly obtained. For this purpose, the material sodalite (green vertical lines); quartz (red lines) was tested in powder form, and the same solution remained in contact with the powder SEM-EDS analysis of the powder showed two throughout the test. The experiments were phases. The composition of the light-colored performed in Teflon reactors at 90°C for 7 days. phase revealed resembled Na5Al6Si6O24Cl1. This formula is only indicative, as no quantitative Determination of the alteration rate analysis could be performed on the sample due The leaching results were calculated from the to the very small grain size. element concentrations in the boiler and expressed as a normalized mass loss (g·m-2). K-sodalite The normalized mass loss of an element i is XRD analysis revealed a mixture of several given by the formula: phases: sodalite (majority phase), SiO2 and
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probably leucite (KAlSi2O6). Some peaks could density calculated was about not be assigned (Fig. 4). 68 % for Na-sodalite and 72% for K-sodalite. SEM analysis showed two phases (in addition to This also revealed the existence of open porosity. aggregates containing Zr, Ti and O), one containing chlorine, the other not. EDS analysis Leach Test Results of the chlorinated phase showed a mean Na-sodalite pellet composition of Na5.45(Si5.88Al5.06O24)K1.97Cl1.9. The following curve of elemental normalized This composition is also only indicative, since mass loss versus time (Fig. 5) could be plotted. quantitative analysis was impossible with such The rates of release of the elements was almost small grains (1 to 10µm) and because sodium linear. migrated under the beam.
SI3 2400 2300 2200 2100 2000 1900 1800 1700 1600
) 1500 s
t 1400 1300
oun 1200
(C 1100
n 1000 Li 900 800 700 600 Fig. 5:. Normalized mass loss for Na-sodalite 500 400 300 200 100 0
12 20 30 40 50 60 2-Theta - Scale Based on the overall pellet mass loss and the SI3 - File: X03-93b.raw - Type: 2Th/Th locked - Start: 10.209 ° - End: 90.098 ° - Step: 0.050 ° - Step time: 3. 5 s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 10.209 ° - Theta: 5.00 Operations: Background 1.000,1.000 | Displacement -0.367 | Import 82-0517 (C) - Sodalite - Na4Cl(Al3Si3O12) - Y: 18.75 % - d x by: 1. - WL: 1.54056 - 0 - I/Ic PDF 3.1 - 85-0794 (C) - Silicon Oxide - SiO2 - Y: 10.42 % - d x by: 1. - WL: 1.54056 - 0 - I/Ic PDF 3.1 - BET surface area, the leach rate was about 38-1423 (*) - Leucite, syn - KAlSi2O6 - Y: 2.43 % - d x by: 1. - WL: 1.54056 - 0 - -2 -1 Fig. 4. XRD spectrum of sample K-sodalite 0.16 g·m d after 14 days. The low rate sodalite (red vertical lines); leucite (green lines) confirmed the low solubility of the material under these conditions (pure water, 100°C), Fabrication of Pellets and Characterizations assuming the surface area was correctly Fabrication estimated. Based on the pellet geometric surface The thermogravimetric analysis curves showed a area (which minimizes the effective surface area in contact with water) the calculated rate was 21 slight weight loss observed between 400 and -2 -1 600°C, which is generally attributed to OH g·m d . The actual initial alteration rate of the groups leaving the structure [6]. A very slight sodalite test sample was therefore between these drop can be observed near 900°C, then a two values. significant mass loss between 950°C and 1100°C due to the transformation of sodalite into K-sodalite pellet nepheline. The following curve of elemental normalized On the dilatometry curves, shrinkage was mass loss versus time (Fig. 6) could be plotted: observed near 550°C due to the disappearance of kaolinite. The nepheline transformation is seen between about 1000 and 1100°C. Some shrinkage appears to have occurred near 900°C. These preliminary results were used to determine the following material sintering conditions: after cold pressing, the pellets were submitted to a Fig. 6: Soxhlet test results for K-sodalite heating cycle (2°C/min) with a 5-hour residence time at 915°C. The resulting pellets could be Potassium was released more rapidly than the tested but remained brittle and porous. other elements. This could be an artifact if the Characterization quantity of potassium actually incorporated in X-ray diffraction analysis was performed on a the material was less than assumed (80% of the sintered pellet to check that sodalite was still the exchangeable sites). majority phase. A small quantity of nepheline Based on the overall pellet mass loss the leach was also observed. rate was about 0.30 g·m-2d-1. Once again, The BET (krypton) specific surface area of the assuming the surface area was exact, the rate was 2 -1 Na-sodalite pellet was 0.12 m ·g compared with low. A highly conservative calculation based on 2 -1 0.13 m ·g for the K-sodalite pellet. These high the pellet geometric surface area yielded a rate of values confirm that the pellets were 32.8 g·m-2d-1. The actual initial alteration rate insufficiently densified, as the geometric surface was therefore between these two values. 2 area of the pellets was about 5 cm . The relative
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Static test results on powders CONCLUSION AND OUTLOOK The 7-day normalized mass losses indicated Material densification tests resulted in samples preferential release of sodium and chlorine. The that could be tested, but were porous (relative normalized mass losses for the constituent density about 70 %), with open porosity, and elements of the sodalite structure (Si, Al) were of thus a high uncertainty regarding the actual the same order of magnitude (2.10-4 g.m-2 at 7 material surface area in contact with water. days), as were the alkali metal mass losses (about Therefore, the initial alteration rates ranged from 1.5 10-3 g.m-2 at 7 days). Chlorine appeared to be 0.16 g·m-2d-1 to 21 g·m-2d-1 for pure sodalite and the least mobile for the K-sodalite. However, 0.30 g·m-2d-1 to 32.8 g·m-2d-1 for substituted there was a chlorine deficit in the sample (SEM) sodalite. It is impossible to reach a definite that could have affected the calculated value. conclusion on the intrinsic quality of the material as a dense containment matrix. Optimization of DISCUSSION the fabrication protocol and the sintering are needed. Calculated with the BET surface area, the leach Moreover, sodalites, synthesized by the rate was about 0.16 g·m-2d-1 after 14 days for the hydrothermal method, present a large fraction of Na-sodalite and 0.30 g·m-2d-1 after 14 days for salts which did not react, generating substantial the K-sodalite. This low rate confirmed the low quantities of secondary waste. Dry routes should solubility of the material under these conditions prove to be more realistic from the standpoint of (pure water, 100°C), assuming the surface area an industrial process, although their feasibility was correctly estimated. cannot yet be assessed. Some other studies[3][2] Although it is obvious that the rate calculated evaluated sodalite-glass composites. The glass with the geometric surface area value is highly was used both to prevent the formation of pessimistic, it was not possible to assess the secondary phases, and to enhance the exact value of the initial rate of alteration in containment of some fission products. these conditions. Therefore, it was only possible to evaluate a range bracketing the initial REFERENCES alteration rate: from 0.16 g·m-2d-1 to 21 g·m-2d-1 [1]. KOYAMA, T., “Waste form development for Na-sodalite and 0.30 g·m-2d-1 to 32.8 g·m-2d-1 for immobilization of radioactive halide salt for K- sodalite. generated from pyrometallurgical In the future, an effort must be made to obtain reprocessing”, International Conference, sintered pellets providing dense samples. Future Nuclear Systems, Oct 1997, Yokohama For high S/V ratio tests, the normalized mass [2]. PEREIRA, C, Annual meeting of the losses of the constituents of the sodalite structure American Ceramic Society, Cincinnati, 1-3 (Si, Al) were of the same order of magnitude and May 1995, conf-950401-4, p389-395 were lower than the alkali metal mass losses. [3]. LEWIS, M.A., FISCHER D.F., Mat. Res. This had also been observed by Koyama [1], who Soc. Symp. Proc., vol 333, p 277-284 (1994) carried out leach tests on powder samples with a [4]. DEER, W. A., HOWWIE, R. A., high S/V ratio and showed a preferential release ZUSSMAN, J., An Introduction to Rock of K and Cl. Forming Materials, 2nd edition, p 496-500 These results also tend to be confirmed by the [5]. BRAGG, W.L. et CLARINGBULL, G.F., XRD analysis made on the soxlhet leached Crystal Structure of Minerals, London, Bell samples: at the surface, the sodalite signal had & Sons, fig 233 (1965) disappeared: the major mineral phase [6]. BUHL, J.C., HOFFMANN, W., corresponds to nepheline. The pellet was BUCKERMANN, W. A, Solid state Nuclear polished to eliminate the altered layer, and Magnetic Reson., vol 9, issue 2-4, , p.121- unaltered sodalite was again detected. This could 130 (1997) be attributable to sodium and chlorine losses resulting in the stoechiometry of nepheline. As the structure was not the same, two hypotheses can be postulated: either sodalite was leached from the sample and only the initial nepheline was observed or, more probably, a nepheline phase reprecipitated on the surface.
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