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Candidate wasteforms for the immobilization of chloride-containing radioactive waste
Journal of Non-Crystalline Solids 348 (2004) 225–229 www.elsevier.com/locate/jnoncrysol
Candidate wasteforms for the immobilization of chloride-containing radioactive waste B.L. Metcalfe *, I.W. Donald Materials Science Research Division, AWE, Aldermaston, Berkshire RG7 4PR, UK
Abstract Immobilization of special categories of radioactive wastes containing both actinide and chloride species is of increasing interest. In the present contribution we report the results of investigations into the feasibility of immobilizing these wastes by vitrification and solid state reaction between zeolite and calcium phosphate and the waste. We also report on the reactions between the ceramic wasteforms and various glass binders used to convert the powder wasteforms into a monolithic form as these can adversely effect the properties of the monolith, e.g. leach resistance. These reactions can result in a loss of chloride from the product due to the formation of a volatile chloride or the formation of the soluble phase halite. Methods for overcoming these undesirable reactions are discussed. It was shown that vitrification was not a suitable process for these wastes but that they could be incorporated into either zeolite 4A or calcium phosphate to produce powder wasteforms with chloride leach rates below 0.2 mg m 2 day 1. Ó 2004 Published by Elsevier B.V.
1. Introduction In recent years there has been an increased interest [1] in the immobilization of wastes arising from the pyrochemical reprocessing of plutonium. These wastes differ from those produced during the reprocessing of spent nuclear fuel in that they can contain concentrations of actinide and chloride ions. Vitrification using borosilicate glass as the host, the process used for immobilizing spent nuclear fuel waste, is not suitable for pyrochemical wastes as the actinide and chloride ions have low solubilities, leading to the formation of large volumes of waste and, in the case of chloride ions, phase separation. A review of immobilization of high level wastes, HLW, [2], to our knowledge, revealed very little work had been published on the immobilization of wastes having such a chloride content. Although the review demonstrated *
Corresponding author. Tel.: 44 118 982 7941; fax: 44 118 982 4739. E-mail address: [email protected] (B.L. Metcalfe). 0022-3093/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2004.08.173
that several options existed for the immobilization of the actinide constituents, options for immobilizing chloride ions were more restricted. An intensive literature search for durable materials which could accommodate large quantities of calcium chloride into their structures led us to the conclusion that the most viable options appeared to be based on calcium phosphate ceramics, especially the phases chlorapatite, Ca5Cl(PO4)3, and spodiosite, Ca2Cl(PO4), zeolites e.g. [3–5] and possibly phosphate glasses. Although the chloride ions are not chemically bonded in zeolites and would therefore have a smaller leach resistance compared to a glassy material, the zeolite structure can be collapsed to form sodalite which would have a higher chloride retention. Phosphate glasses had been investigated as potential hosts for spent nuclear fuel waste e.g. [6] but interest quickly declined (except in former USSR) due to the combination of poor durability and the highly corrosive nature of the glass melt when compared to borosilicate glasses which outweighed their advantages of lower melt temperatures, lower viscosity and higher sulphate solubility.
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Table 1 Compositions of glasses (mol%) Code
Na2O
Al2O3
P2O5
Fe2O3
CaCl2
Ag2O
CuO
Er2O3
CaO
GT1/89 GT1/102 GT1/105 GT1/110 GT1/111 GT1/162 GT1/151 GT1/140 GT1/138
41.0 [40.8] 35.2 [34.9] [37.0] 35.0 [34.9] 34.1 [33.3] – – – –
20.5 [19.4] 21.4 [16.6] [17.7] 19.6 [16.6] 16.9 [15.9] – [10.0] – –
38.5 [39.8] 38.4 [41.3] [36.1] 39.5 [41.3] 41.0 [41.6] [60.0] [60.0] [65.0] [65.0]
– 5.0 [7.2] – 5.9 [7.2] 8.0 [9.2] [40.0] – – –
– – [9.2] – – – – – –
– – – – – – [30.0] – –
– – – – – – – [20.0] [31.0]
– – – – – – – – [4.0]
– – – – – – – [15.0] –
Figures in [ ] are nominal compositions.
However interest in phosphate glasses was renewed when new compositions based on lead iron phosphate and sodium iron phosphate having durabilities which were 2–3 orders of magnitude better than borosilicate glass were reported e.g. [7,8]. More recently glass compositions based on iron phosphate have been proposed [9] as an alternative host to borosilicate glass for the vitrification of HLW containing heavy metals and halides. A different approach has been proposed by Donze et al. [10] to convert heavy metal chlorides into phosphate glasses using ammonium dihydrogen phosphate as a precursor. In their method the chloride would be volatilized off as ammonium chloride, leaving the heavy metals behind as a vitrified wasteform.
2. Experimental Zeolite 4A, 1 a sodium aluminosilicate zeolite, was chosen for the work. Initial experiments to determine the maximum chloride loading were carried out using zeolite powder (particle size < 5 lm) and calcium chloride which had been dry ball-milled for 24 h. Samples of zeolite containing between 10% and 25% calcium chloride were placed in alumina crucibles and transferred to a pre-heated furnace at 600 °C to remove any absorbed water. After 2 h the furnace temperature was increased to 750 °C and the samples held for 5 h. When cold, the powder produced was washed in demineralized water and the washable chloride content determined. Two further samples were calcined using the same heating schedule but in these 20% of the calcium chloride was replaced by samarium chloride, used as a surrogate for the actinides in the waste. Preparation of chlorapatite and spodiosite followed the same process as that described above except calcium phosphate replaced zeolite as the precursor and the dwell times were reduced to 1 and 2 h for drying and calcining respectively.
1
Aldrich Chemical Company Inc.
Based on their thermal characteristics and durability in water determined previously by ourselves, several potential phosphate glass compositions, suitable for use as binders for converting the powders into monolithic wasteforms acceptable for long-term storage, had been identified from the large number of phosphate glasses studied and the composition of these glasses investigated are given in Table 1. In addition to the sodium aluminophosphate compositions, three alkali free glass compositions which had been reported in the literature were manufactured since we thought any reaction between them and chloride ions would lead to the formation of an insoluble chloride compound. These were a silver phosphate glass [11] and two compositions based on copper phosphate [12,13]. The binary composition was prepared from iron III oxide, Fe2O3, and phosphorus pentoxide whereas the ternary compositions were manufactured from sodium dihydrogen phosphate, ammonium dihydrogen phosphate and aluminium phosphate. Quaternary compositions were made by adding iron phosphate to the ternary composition, laboratory reference GT1/89. Melting was carried out in alumina crucibles at 1250 °C in two stages. After the first melting the glass was cast into water and the resultant frit lightly ground and then re-melted to improve homogeneity before being cast into water again. The procedure for manufacturing the silver phosphate glass differed from that reported [11] insofar as oxygen was not bubbled through the melt in order to maintain oxidizing conditions but by replacing the ammonium dihydrogen phosphate used by phosphorus pentoxide. Experiments to establish the maximum amount of calcium chloride which could be incorporated into some glasses were performed by mixing the glass frit with calcium chloride powder and melting in alumina crucibles. Melting schedules were varied depending on the viscosities of the melts and the volatilities of melts as observed prior to casting. Sometimes at the higher calcium chloride loadings the excess calcium chloride formed a clear, fluid layer on the surface of the molten glass. In these
B.L. Metcalfe, I.W. Donald / Journal of Non-Crystalline Solids 348 (2004) 225–229
cases the crucible contents were cast onto a metal plate, not into water. The sodium iron aluminium phosphate compositions were too viscous at the processing temperatures to pour easily and rather than increase the processing temperature, with the inherent risk of increased volatilization of calcium chloride, an alternative approach was used. In this the calcium chloride was dissolved in the lower melting ternary sodium aluminium phosphate glass, cast into water and the frit mixed with iron phosphate and melted. Sintering experiments were conducted on ceramic powder containing 20% simulant waste, (an 80% calcium chloride, 20% samarium chloride mixture), with varying proportions of the sodium aluminium phosphate glass to determine the extent of reaction between the glass and ceramic powder and how this would affect the durability of the monolithic wasteform, especially the release of chloride ions. Experiments were conducted using either cylinders pressed at 69 MPa or loose powder tamped lightly into a crucible. Identification of the phases present in the monolithic wasteforms was performed by X-ray diffraction (XRD) using Cu Ka radiation. Leach rates for both the zeolite and calcium phosphate wasteforms were determined over 16 weeks at 20 °C, nominally ambient temperature, using de-mineralized water at pH 7. Experiments were performed on the ceramic powders produced by calcination of simulant waste with either zeolite or calcium phosphate as described above. Samples were removed from the study at regular intervals, the powder filtered off and the concentration of Cl in the leachate determined by ion chromatography.
3. Results Zeolite 4A was found to be able to incorporate a maximum calcium chloride content of about 23.2% when calcined at 750 °C for 5 h. Within the range 12– 23.2% there was always approximately 0.4% unreacted calcium chloride present in the samples which could be removed by washing the powder in water. When the simulant waste was used at a 20% loading, exactly the same result was obtained although at 10% loading no washable chloride was detected. Calcium phosphate was found to give similar results in that there was always a small quantity of soluble chloride which had not been incorporated into the structure and was largely independent of the chloride loading, although higher loadings tended to produce higher soluble chloride values. As a result of these experiments, a standard loading of 20% simulated waste was used for all subsequent work as this amount represented a realistic compromise between maximizing the waste loading and minimizing the unreacted (washable) chloride content.
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Table 2 Nominal glass/calcium chloride compositions Glass
CaCl2 (%)
Melt temperature (°C)
Time (h)
Weight loss (%)
GT1/89 GT1/89 GT1/102 GT1/110 GT1/111 GT1/162 GT1/105
10 20 10 10 10 20a 10b
1050 1100 1050 1050 1050 800 1050
2.0 2.0 3.0 2.0 2.0 1.0 2.0
1.1 ± 0.1 Not determined Not determined 3.4 ± 0.1 3.6 ± 0.1 15.4 ± 0.1 1.8 ± 0.1
a b
Mixed CaCl2/MgCl2. FePO4.
The surface areas of the calcium phosphate and zeolite wasteforms, as determined by the BET method [14], were 7.8 ± 0.3 and 2.6 ± 0.2 m2 g 1 respectively. Using these data and the mass of Cl ions in the leachate, the normalized leach rates were calculated as 0.18 ± 0.02 and 0.02 ± 0.003 mg m 2 day 1 for the zeolite and calcium phosphate wasteforms respectively, demonstrating that both had an acceptable durability, even though XRD analysis showed the zeolite had not converted into the more durable sodalite. The glass/calcium chloride combinations which were made to determine the approximate maximum chloride concentrations possible are listed in Table 2. Major phases identified in the monolithic wasteforms produced in the sintering experiments using the ternary glass and calcium phosphate host are given in Table 3. The modified procedure for manufacturing the silver phosphate based glass proved successful for a 50 g batch of glass but a 100 g batch melted in the same crucible underwent reduction and the formation of an opaque yellow glass.
Table 3 Phases present in monolithic wasteforms Run
Glass content (%)
Pressed (Y/N)
Sinter temp. (°C)
Phases
22 24 26 27 28 30 32a 33 34 35 36 37 38b
40 40 40 40 30 20 40 40 40 40 30 30 40
Y Y N N N N N N N N N N N
700 750 800 750 800 800 800 700 650 625 650 700 650
Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap,
Ap chlorapatite, Sp spodiosite. a Quaternary glass GT1/110. b Waste loading 10.5%.
Sp Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp,
NaCl NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 AlPO4
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Preliminary trials with the silver phosphate based glass produced monoliths which were unsuitable as we observed that the glass had partially decomposed and precipitated silver and so the work was not continued. Monoliths prepared using glass GT1/138 did not densify during manufacture and work on copper phosphate systems was discontinued.
4. Discussion Although the glasses produced were not analyzed for chloride content it is apparent that as the weight losses recorded are much smaller than the mass of calcium chloride added to each glass, that most of the calcium chloride was incorporated. A weight loss of 1.1% would indicate GT1/89 + 10% CaCl2 must contain a minimum of 5.3% chloride. Attempts to increase the chloride content of GT1/89 to 20% resulted in the melt having a crust over the surface on top of which was a clear liquid, presumed to be molten calcium chloride. The contents of the crucible were cast onto a metal plate and examined. At this stage the product was distinctly two phase and so was ground and re-melted. After the second casting the product still contained large white areas which were either crystalline or phase separated. GT1/ 102 was found to be too viscous to pour and no attempt was made to re-melt at a higher temperature because of the increased risk of volatilizing the calcium chloride. The quaternary compositions GT1/110 and GT1/111 were both melted at 1050 °C but each were quite viscous and were heated to 1150 °C so that they were easier to cast. More success was obtained using the alternative method of dissolving the calcium chloride in the ternary glass and then converting that glass to a sodium iron aluminium phosphate glass as can been seen from the data for GT1/105. Part of this success may be due to the lower iron phosphate content, 10% compared to 20% and 25%, of the product because it had been established [15] that increasing the iron phosphate content increased the liquidus temperature of the glass. During the melting of the binary iron phosphate glass with a calcium chloride/magnesium chloride mixture a significant proportion of the crucible contents volatilized which resulted in the outside of the crucible and the furnace interior being coated with a brown/black deposit. To date this deposit, which is insoluble in water and nitric acid, has not been identified. No further experiments were carried out on this system. From the results of these screening experiments it can be seen that from the chloride solubility aspect, sodium iron aluminium phosphate offers no advantage over the ternary sodium aluminium phosphate compositions. The levels of chloride solubility achievable were too
low for these glasses to be used in the vitrification of the waste but were well in excess of the washable chloride content of the ceramic powders and therefore could be used as the binder when manufacturing monolithic wasteforms. With the exception of two samples, the use of sodium aluminium phosphate glass led to the formation of halite, which being soluble in water is very undesirable. Of the two processes which do not lead to the formation of halite, the use of a lower temperature during sintering would be expected to reduce the likelihood of reaction occurring but this reduction in itself is not enough. Pressing the powder prior to sintering is also required. Spodiosite, present in the powder wasteform prior to sintering, could not be positively identified in the monolithic wasteforms. We suggest that the glass is preferentially reacting with spodiosite leading to the formation of chlorapatite, aluminium phosphate and halite. If correct this explanation could explain why halite was not observed in the sample made from powder containing only 10.5% waste, i.e. corresponding to chlorapatite only.
5. Conclusions This work has shown that it is possible to include calcium chloride in sodium aluminium phosphate and sodium iron aluminium phosphate based glasses but not at a concentration which would make them viable candidates for consideration as vitrification hosts for chloride containing pyrochemical wastes. Incorporation of the waste into either zeolite 4A or calcium phosphate by a solid state calcination process offers a more promising route for the immobilization of these wastes, with calcium phosphate being the preferred option. Durability of both wasteforms in aqueous environment was very good with Cl ions leach rates of 0.18 ± 0.02 and 0.02 ± 0.003 mg m 2 day 1 for zeolite 4A and calcium phosphate respectively. The use of sodium aluminium phosphate glass as the binder for the calcium phosphate wasteform leads to the formation of halite under most of the sintering conditions investigated, although a combination of pressing at 69 MPa followed by sintering at 700 °C or less avoids this problem, as does reducing the calcium chloride loading to <11 wt%.
Acknowledgments The authors thank Dr M. Brenchley for the zeolite results and Miss R. Greedharee for assistance manufacturing the glasses. Ó British Crown Copyright 2004/MOD. Published with the permission of the Controller of Her Britannic MajestyÕs Stationery Office.
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References [1] I.W. Donald, B.L. Metcalfe, M.E. Brenchley, R.S.R. Greedharee, in: L.G. Mallinson (Ed.), Proc. Int. Conf. Ageing Studies and Lifetime Extension of Materials, Kluwer Academic/Plenum, New York, 2000, p. 647. [2] I.W. Donald, B.L. Metcalfe, R.N.J. Taylor, J. Mater. Sci. 32 (1997) 1139. [3] M.A. Lewis, D.F. Fisher, L.J. Smith, J. Am. Ceram. Soc. 76 (1993) 2826. [4] T. Koyama, Method to synthesize dense crystallized sodalite pellet for immobilizing halide salt radioactive waste, US Patent 5340506, August 1994. [5] M.A. Lewis, D.F. Fisher, C.D. Murphy, in: A. Barkatt, R. Van Konynenburg (Eds.), Scientific Basis for Nuclear Waste Management, vol. 333, MRS, Pittsburg, 1994, p. 277. [6] L.P. Hatch, G.C. Weth, E.J. Tuthill, in: S. Eklund (Ed.), Treatment and Storage of High-level Radioactive Wastes, Int. Atomic Energy Agency, Vienna, 1963, p. 531.
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[7] B.C. Sales, L.A. Boatner, in: W. Lutze, R.C. Ewing (Eds.), Radioactive Wasteforms for the Future, Elsevier Science, New York, 1988, p. 193. [8] X. Yu, D.E. Day, G.J. Long, R.K. Brow, J. Non-Cryst. Solids 215 (1997) 21. [9] D.E. Day, C.S. Ray, G.K. Marasinghe, M. Karablut, X. Fang, EMSP Project 55110, June 1998. [10] S. Donze, L. Montaigne, G. Palavit, Chem. Mater. 12 (2000) 1921. [11] Y.B. Peng, D.E. Day, Glass Technol. 32 (1991) 200. [12] C. Ananthamohan, C.A. Hogarth, C.R. Theocharis, D. Yeates, J. Mater. Sci. 25 (1990) 3956. [13] S.P. Edirisinghe, C.A. Hogarth, K.A.K. Lott, J. Mater. Sci. Lett. 8 (1989) 645. [14] A.W. Adamson, Physical Chemistry of Surfaces, Wiley Interscience, New York, 1976. [15] I.W. Donald, B.L. Metcalfe, in: Proc. 2nd International Symposium on Non-Crystalline Solids, 21–25 September 2003, Campos do Jordao, Brazil.
Candidate wasteforms for the immobilization of chloride-containing radioactive waste B.L. Metcalfe *, I.W. Donald Materials Science Research Division, AWE, Aldermaston, Berkshire RG7 4PR, UK
Abstract Immobilization of special categories of radioactive wastes containing both actinide and chloride species is of increasing interest. In the present contribution we report the results of investigations into the feasibility of immobilizing these wastes by vitrification and solid state reaction between zeolite and calcium phosphate and the waste. We also report on the reactions between the ceramic wasteforms and various glass binders used to convert the powder wasteforms into a monolithic form as these can adversely effect the properties of the monolith, e.g. leach resistance. These reactions can result in a loss of chloride from the product due to the formation of a volatile chloride or the formation of the soluble phase halite. Methods for overcoming these undesirable reactions are discussed. It was shown that vitrification was not a suitable process for these wastes but that they could be incorporated into either zeolite 4A or calcium phosphate to produce powder wasteforms with chloride leach rates below 0.2 mg m 2 day 1. Ó 2004 Published by Elsevier B.V.
1. Introduction In recent years there has been an increased interest [1] in the immobilization of wastes arising from the pyrochemical reprocessing of plutonium. These wastes differ from those produced during the reprocessing of spent nuclear fuel in that they can contain concentrations of actinide and chloride ions. Vitrification using borosilicate glass as the host, the process used for immobilizing spent nuclear fuel waste, is not suitable for pyrochemical wastes as the actinide and chloride ions have low solubilities, leading to the formation of large volumes of waste and, in the case of chloride ions, phase separation. A review of immobilization of high level wastes, HLW, [2], to our knowledge, revealed very little work had been published on the immobilization of wastes having such a chloride content. Although the review demonstrated *
Corresponding author. Tel.: 44 118 982 7941; fax: 44 118 982 4739. E-mail address: [email protected] (B.L. Metcalfe). 0022-3093/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2004.08.173
that several options existed for the immobilization of the actinide constituents, options for immobilizing chloride ions were more restricted. An intensive literature search for durable materials which could accommodate large quantities of calcium chloride into their structures led us to the conclusion that the most viable options appeared to be based on calcium phosphate ceramics, especially the phases chlorapatite, Ca5Cl(PO4)3, and spodiosite, Ca2Cl(PO4), zeolites e.g. [3–5] and possibly phosphate glasses. Although the chloride ions are not chemically bonded in zeolites and would therefore have a smaller leach resistance compared to a glassy material, the zeolite structure can be collapsed to form sodalite which would have a higher chloride retention. Phosphate glasses had been investigated as potential hosts for spent nuclear fuel waste e.g. [6] but interest quickly declined (except in former USSR) due to the combination of poor durability and the highly corrosive nature of the glass melt when compared to borosilicate glasses which outweighed their advantages of lower melt temperatures, lower viscosity and higher sulphate solubility.
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Table 1 Compositions of glasses (mol%) Code
Na2O
Al2O3
P2O5
Fe2O3
CaCl2
Ag2O
CuO
Er2O3
CaO
GT1/89 GT1/102 GT1/105 GT1/110 GT1/111 GT1/162 GT1/151 GT1/140 GT1/138
41.0 [40.8] 35.2 [34.9] [37.0] 35.0 [34.9] 34.1 [33.3] – – – –
20.5 [19.4] 21.4 [16.6] [17.7] 19.6 [16.6] 16.9 [15.9] – [10.0] – –
38.5 [39.8] 38.4 [41.3] [36.1] 39.5 [41.3] 41.0 [41.6] [60.0] [60.0] [65.0] [65.0]
– 5.0 [7.2] – 5.9 [7.2] 8.0 [9.2] [40.0] – – –
– – [9.2] – – – – – –
– – – – – – [30.0] – –
– – – – – – – [20.0] [31.0]
– – – – – – – – [4.0]
– – – – – – – [15.0] –
Figures in [ ] are nominal compositions.
However interest in phosphate glasses was renewed when new compositions based on lead iron phosphate and sodium iron phosphate having durabilities which were 2–3 orders of magnitude better than borosilicate glass were reported e.g. [7,8]. More recently glass compositions based on iron phosphate have been proposed [9] as an alternative host to borosilicate glass for the vitrification of HLW containing heavy metals and halides. A different approach has been proposed by Donze et al. [10] to convert heavy metal chlorides into phosphate glasses using ammonium dihydrogen phosphate as a precursor. In their method the chloride would be volatilized off as ammonium chloride, leaving the heavy metals behind as a vitrified wasteform.
2. Experimental Zeolite 4A, 1 a sodium aluminosilicate zeolite, was chosen for the work. Initial experiments to determine the maximum chloride loading were carried out using zeolite powder (particle size < 5 lm) and calcium chloride which had been dry ball-milled for 24 h. Samples of zeolite containing between 10% and 25% calcium chloride were placed in alumina crucibles and transferred to a pre-heated furnace at 600 °C to remove any absorbed water. After 2 h the furnace temperature was increased to 750 °C and the samples held for 5 h. When cold, the powder produced was washed in demineralized water and the washable chloride content determined. Two further samples were calcined using the same heating schedule but in these 20% of the calcium chloride was replaced by samarium chloride, used as a surrogate for the actinides in the waste. Preparation of chlorapatite and spodiosite followed the same process as that described above except calcium phosphate replaced zeolite as the precursor and the dwell times were reduced to 1 and 2 h for drying and calcining respectively.
1
Aldrich Chemical Company Inc.
Based on their thermal characteristics and durability in water determined previously by ourselves, several potential phosphate glass compositions, suitable for use as binders for converting the powders into monolithic wasteforms acceptable for long-term storage, had been identified from the large number of phosphate glasses studied and the composition of these glasses investigated are given in Table 1. In addition to the sodium aluminophosphate compositions, three alkali free glass compositions which had been reported in the literature were manufactured since we thought any reaction between them and chloride ions would lead to the formation of an insoluble chloride compound. These were a silver phosphate glass [11] and two compositions based on copper phosphate [12,13]. The binary composition was prepared from iron III oxide, Fe2O3, and phosphorus pentoxide whereas the ternary compositions were manufactured from sodium dihydrogen phosphate, ammonium dihydrogen phosphate and aluminium phosphate. Quaternary compositions were made by adding iron phosphate to the ternary composition, laboratory reference GT1/89. Melting was carried out in alumina crucibles at 1250 °C in two stages. After the first melting the glass was cast into water and the resultant frit lightly ground and then re-melted to improve homogeneity before being cast into water again. The procedure for manufacturing the silver phosphate glass differed from that reported [11] insofar as oxygen was not bubbled through the melt in order to maintain oxidizing conditions but by replacing the ammonium dihydrogen phosphate used by phosphorus pentoxide. Experiments to establish the maximum amount of calcium chloride which could be incorporated into some glasses were performed by mixing the glass frit with calcium chloride powder and melting in alumina crucibles. Melting schedules were varied depending on the viscosities of the melts and the volatilities of melts as observed prior to casting. Sometimes at the higher calcium chloride loadings the excess calcium chloride formed a clear, fluid layer on the surface of the molten glass. In these
B.L. Metcalfe, I.W. Donald / Journal of Non-Crystalline Solids 348 (2004) 225–229
cases the crucible contents were cast onto a metal plate, not into water. The sodium iron aluminium phosphate compositions were too viscous at the processing temperatures to pour easily and rather than increase the processing temperature, with the inherent risk of increased volatilization of calcium chloride, an alternative approach was used. In this the calcium chloride was dissolved in the lower melting ternary sodium aluminium phosphate glass, cast into water and the frit mixed with iron phosphate and melted. Sintering experiments were conducted on ceramic powder containing 20% simulant waste, (an 80% calcium chloride, 20% samarium chloride mixture), with varying proportions of the sodium aluminium phosphate glass to determine the extent of reaction between the glass and ceramic powder and how this would affect the durability of the monolithic wasteform, especially the release of chloride ions. Experiments were conducted using either cylinders pressed at 69 MPa or loose powder tamped lightly into a crucible. Identification of the phases present in the monolithic wasteforms was performed by X-ray diffraction (XRD) using Cu Ka radiation. Leach rates for both the zeolite and calcium phosphate wasteforms were determined over 16 weeks at 20 °C, nominally ambient temperature, using de-mineralized water at pH 7. Experiments were performed on the ceramic powders produced by calcination of simulant waste with either zeolite or calcium phosphate as described above. Samples were removed from the study at regular intervals, the powder filtered off and the concentration of Cl in the leachate determined by ion chromatography.
3. Results Zeolite 4A was found to be able to incorporate a maximum calcium chloride content of about 23.2% when calcined at 750 °C for 5 h. Within the range 12– 23.2% there was always approximately 0.4% unreacted calcium chloride present in the samples which could be removed by washing the powder in water. When the simulant waste was used at a 20% loading, exactly the same result was obtained although at 10% loading no washable chloride was detected. Calcium phosphate was found to give similar results in that there was always a small quantity of soluble chloride which had not been incorporated into the structure and was largely independent of the chloride loading, although higher loadings tended to produce higher soluble chloride values. As a result of these experiments, a standard loading of 20% simulated waste was used for all subsequent work as this amount represented a realistic compromise between maximizing the waste loading and minimizing the unreacted (washable) chloride content.
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Table 2 Nominal glass/calcium chloride compositions Glass
CaCl2 (%)
Melt temperature (°C)
Time (h)
Weight loss (%)
GT1/89 GT1/89 GT1/102 GT1/110 GT1/111 GT1/162 GT1/105
10 20 10 10 10 20a 10b
1050 1100 1050 1050 1050 800 1050
2.0 2.0 3.0 2.0 2.0 1.0 2.0
1.1 ± 0.1 Not determined Not determined 3.4 ± 0.1 3.6 ± 0.1 15.4 ± 0.1 1.8 ± 0.1
a b
Mixed CaCl2/MgCl2. FePO4.
The surface areas of the calcium phosphate and zeolite wasteforms, as determined by the BET method [14], were 7.8 ± 0.3 and 2.6 ± 0.2 m2 g 1 respectively. Using these data and the mass of Cl ions in the leachate, the normalized leach rates were calculated as 0.18 ± 0.02 and 0.02 ± 0.003 mg m 2 day 1 for the zeolite and calcium phosphate wasteforms respectively, demonstrating that both had an acceptable durability, even though XRD analysis showed the zeolite had not converted into the more durable sodalite. The glass/calcium chloride combinations which were made to determine the approximate maximum chloride concentrations possible are listed in Table 2. Major phases identified in the monolithic wasteforms produced in the sintering experiments using the ternary glass and calcium phosphate host are given in Table 3. The modified procedure for manufacturing the silver phosphate based glass proved successful for a 50 g batch of glass but a 100 g batch melted in the same crucible underwent reduction and the formation of an opaque yellow glass.
Table 3 Phases present in monolithic wasteforms Run
Glass content (%)
Pressed (Y/N)
Sinter temp. (°C)
Phases
22 24 26 27 28 30 32a 33 34 35 36 37 38b
40 40 40 40 30 20 40 40 40 40 30 30 40
Y Y N N N N N N N N N N N
700 750 800 750 800 800 800 700 650 625 650 700 650
Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap, Ap,
Ap chlorapatite, Sp spodiosite. a Quaternary glass GT1/110. b Waste loading 10.5%.
Sp Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp, Sp,
NaCl NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 NaCl, AlPO4 AlPO4
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Preliminary trials with the silver phosphate based glass produced monoliths which were unsuitable as we observed that the glass had partially decomposed and precipitated silver and so the work was not continued. Monoliths prepared using glass GT1/138 did not densify during manufacture and work on copper phosphate systems was discontinued.
4. Discussion Although the glasses produced were not analyzed for chloride content it is apparent that as the weight losses recorded are much smaller than the mass of calcium chloride added to each glass, that most of the calcium chloride was incorporated. A weight loss of 1.1% would indicate GT1/89 + 10% CaCl2 must contain a minimum of 5.3% chloride. Attempts to increase the chloride content of GT1/89 to 20% resulted in the melt having a crust over the surface on top of which was a clear liquid, presumed to be molten calcium chloride. The contents of the crucible were cast onto a metal plate and examined. At this stage the product was distinctly two phase and so was ground and re-melted. After the second casting the product still contained large white areas which were either crystalline or phase separated. GT1/ 102 was found to be too viscous to pour and no attempt was made to re-melt at a higher temperature because of the increased risk of volatilizing the calcium chloride. The quaternary compositions GT1/110 and GT1/111 were both melted at 1050 °C but each were quite viscous and were heated to 1150 °C so that they were easier to cast. More success was obtained using the alternative method of dissolving the calcium chloride in the ternary glass and then converting that glass to a sodium iron aluminium phosphate glass as can been seen from the data for GT1/105. Part of this success may be due to the lower iron phosphate content, 10% compared to 20% and 25%, of the product because it had been established [15] that increasing the iron phosphate content increased the liquidus temperature of the glass. During the melting of the binary iron phosphate glass with a calcium chloride/magnesium chloride mixture a significant proportion of the crucible contents volatilized which resulted in the outside of the crucible and the furnace interior being coated with a brown/black deposit. To date this deposit, which is insoluble in water and nitric acid, has not been identified. No further experiments were carried out on this system. From the results of these screening experiments it can be seen that from the chloride solubility aspect, sodium iron aluminium phosphate offers no advantage over the ternary sodium aluminium phosphate compositions. The levels of chloride solubility achievable were too
low for these glasses to be used in the vitrification of the waste but were well in excess of the washable chloride content of the ceramic powders and therefore could be used as the binder when manufacturing monolithic wasteforms. With the exception of two samples, the use of sodium aluminium phosphate glass led to the formation of halite, which being soluble in water is very undesirable. Of the two processes which do not lead to the formation of halite, the use of a lower temperature during sintering would be expected to reduce the likelihood of reaction occurring but this reduction in itself is not enough. Pressing the powder prior to sintering is also required. Spodiosite, present in the powder wasteform prior to sintering, could not be positively identified in the monolithic wasteforms. We suggest that the glass is preferentially reacting with spodiosite leading to the formation of chlorapatite, aluminium phosphate and halite. If correct this explanation could explain why halite was not observed in the sample made from powder containing only 10.5% waste, i.e. corresponding to chlorapatite only.
5. Conclusions This work has shown that it is possible to include calcium chloride in sodium aluminium phosphate and sodium iron aluminium phosphate based glasses but not at a concentration which would make them viable candidates for consideration as vitrification hosts for chloride containing pyrochemical wastes. Incorporation of the waste into either zeolite 4A or calcium phosphate by a solid state calcination process offers a more promising route for the immobilization of these wastes, with calcium phosphate being the preferred option. Durability of both wasteforms in aqueous environment was very good with Cl ions leach rates of 0.18 ± 0.02 and 0.02 ± 0.003 mg m 2 day 1 for zeolite 4A and calcium phosphate respectively. The use of sodium aluminium phosphate glass as the binder for the calcium phosphate wasteform leads to the formation of halite under most of the sintering conditions investigated, although a combination of pressing at 69 MPa followed by sintering at 700 °C or less avoids this problem, as does reducing the calcium chloride loading to <11 wt%.
Acknowledgments The authors thank Dr M. Brenchley for the zeolite results and Miss R. Greedharee for assistance manufacturing the glasses. Ó British Crown Copyright 2004/MOD. Published with the permission of the Controller of Her Britannic MajestyÕs Stationery Office.
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References [1] I.W. Donald, B.L. Metcalfe, M.E. Brenchley, R.S.R. Greedharee, in: L.G. Mallinson (Ed.), Proc. Int. Conf. Ageing Studies and Lifetime Extension of Materials, Kluwer Academic/Plenum, New York, 2000, p. 647. [2] I.W. Donald, B.L. Metcalfe, R.N.J. Taylor, J. Mater. Sci. 32 (1997) 1139. [3] M.A. Lewis, D.F. Fisher, L.J. Smith, J. Am. Ceram. Soc. 76 (1993) 2826. [4] T. Koyama, Method to synthesize dense crystallized sodalite pellet for immobilizing halide salt radioactive waste, US Patent 5340506, August 1994. [5] M.A. Lewis, D.F. Fisher, C.D. Murphy, in: A. Barkatt, R. Van Konynenburg (Eds.), Scientific Basis for Nuclear Waste Management, vol. 333, MRS, Pittsburg, 1994, p. 277. [6] L.P. Hatch, G.C. Weth, E.J. Tuthill, in: S. Eklund (Ed.), Treatment and Storage of High-level Radioactive Wastes, Int. Atomic Energy Agency, Vienna, 1963, p. 531.
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[7] B.C. Sales, L.A. Boatner, in: W. Lutze, R.C. Ewing (Eds.), Radioactive Wasteforms for the Future, Elsevier Science, New York, 1988, p. 193. [8] X. Yu, D.E. Day, G.J. Long, R.K. Brow, J. Non-Cryst. Solids 215 (1997) 21. [9] D.E. Day, C.S. Ray, G.K. Marasinghe, M. Karablut, X. Fang, EMSP Project 55110, June 1998. [10] S. Donze, L. Montaigne, G. Palavit, Chem. Mater. 12 (2000) 1921. [11] Y.B. Peng, D.E. Day, Glass Technol. 32 (1991) 200. [12] C. Ananthamohan, C.A. Hogarth, C.R. Theocharis, D. Yeates, J. Mater. Sci. 25 (1990) 3956. [13] S.P. Edirisinghe, C.A. Hogarth, K.A.K. Lott, J. Mater. Sci. Lett. 8 (1989) 645. [14] A.W. Adamson, Physical Chemistry of Surfaces, Wiley Interscience, New York, 1976. [15] I.W. Donald, B.L. Metcalfe, in: Proc. 2nd International Symposium on Non-Crystalline Solids, 21–25 September 2003, Campos do Jordao, Brazil.