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Glass ceramics for incinerator ash immobilization
Journal of Nuclear Materials 416 (2011) 230–235
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Glass ceramics for incinerator ash immobilization G.A. Malinina, O.I. Stefanovsky, S.V. Stefanovsky ⇑ SIA Radon, 7th Rostovskii lane 2/14, Moscow 119121, Russia
a r t i c l e
i n f o
Article history: Available online 26 February 2011
a b s t r a c t Calcined solid radioactive waste (incinerator slag) surrogate and either Na2Si2O5 or Na2B4O7 (borax) at various mass ratios were melted in silicon carbide crucibles in a resistive furnace at temperatures of up to 1775 K (slag without additives). Portions of the melts were poured onto a metal plate; the residues were slowly cooled in turned-off furnace. Both quenched and slowly cooled materials were composed of the same phases. At high slag contents in silicate-based materials nepheline and britholite were found to be major phases. Britholite formed at higher slag content (85 wt.%) became major phase in the vitrified slag. In the system with borax at low slag contents (25 and 50 wt.%) material are composed of predominant vitreous and minor calcium silicate larnite type phase Ca2SiO4 where Ca2+ ions are replaced by different cations. The materials containing slag in amount of 75 wt.% and more are chemically durable. The changes in the structure of anionic motif of quenched samples depending on slag loading were studied by IR spectroscopy. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction There is a wide variety of solid radioactive wastes. Some of them may contain alpha-emitters, mainly actinides (Th, U, Pu, Am). To treat burnable solids, the conventional method of waste management is incineration providing for waste volume reduction factors of up to 100 and more. The process is performed in chamber-type or shaft furnaces [1,2]. The final products of the incineration process are fly ash and bottom slag (incinerator ash). The latter concentrates heavy metals including actinides. The bottom ash as well as fly ash are non-uniform, dusting, easily leachable material not suitable for ultimate disposal. It has to be treated yielding stable solid chemically durable waste form. The most perspective method is incineration with liquid slagging with production of primarily crystalline or glass-crystalline material similar to igneous rock or stone casting [3]. For example a full-scale pilot plant with a waste capacity of up to 250 kg h 1 and liquid slagging is currently under operation at SIA Radon [2]. However, at the present time chamber-type incinerators are most commonly used and the incinerator slag formed needs to be consolidated and stabilized. This may be done by direct melting using plasma, induction or microwave heating or incorporation in matrix material such as cement, bitumen or glass [4]. Vitrified (glassy) slags normally contain crystalline constituent whose amount may be significant. Crystalline phases may incorporate various waste constituents such as strontium, transition metals, rare earths and actinides thus
⇑ Corresponding author. Tel.: +7 916 370 4257; fax: +7 495 919 3194. E-mail address: [email protected] (S.V. Stefanovsky). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.01.127
reducing their leachability, whereas cesium normally enters vitreous phase. There are numerous works on design, preparation and characterization of glasses and glass ceramics as waste forms. They were summarized in reviews [5–8]. Moreover there are original works on immobilization of incinerator slags and other solid wastes in glass and glass ceramics using both special additives and natural raw materials (see, for example, [9–16]). At waste vitrification the most convenient method is fluxing of waste with natural raw material or commercially available additives. These may be soluble (sodium silicate) glass Na2OnSiO2 (n = 23), borax Na2O2B2O2, borosilicate frit as well as naturally occurring rocks (nepheline syenite, basalt and other igneous rocks, etc.). In the present work, phase composition was studied together with structure and elemental partitioning in glass-crystalline materials (glassy slags) produced by smelting of simulated incinerator slag with either sodium disilicate or sodium tetraborate. Sm2O3 was introduced as an An2O3 surrogate.
2. Experimental Mixture of reagent-grade chemicals Na2CO3, K2CO3, CaCO3, Ca3(PO4)2, Al2O3, FeO, SiO2, and Sm2O3 was calcined at 1175 K for 6 h to obtain a solid radioactive waste (incinerator slag) surrogate with a target composition of (wt.%) 6.0 Na2O, 9.0 K2O, 15.0 CaO, 15.0 Al2O3, 10.0 FeO, 30.0 SiO2, 10.0 P2O5, 5.0 Sm2O3. Thus produced simulated slag was intermixed with either liquid sodium silicate (soluble glass) with approximate composition Na2Si2O5 or Na2B4O7 (borax) at various mass ratios (Table 1): x Slag, (100 x) Na2Si2O5 and x Slag, (100 x) Na2B4O7 (x = 25, 50, 75, 85, 100).
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G.A. Malinina et al. / Journal of Nuclear Materials 416 (2011) 230–235 Table 1 Specified chemical compositions of the materials. Oxides
Na2O K2O CaO Al2O3 FeO SiO2 P2O5 B2O3 Sm2O3 T (K) a b
Slag content (wt.%) Na–Sia
25Si
50Si
75Si
85Si
Borax
25B
50B
75B
85B
MSb
34.07 – – – – 65.93 – – – 1575
27.05 2.25 3.75 3.75 2.50 56.95 2.50 – 1.25 1575
20.03 4.50 7.50 7.50 5.00 47.97 5.00 – 2.50 1575
13.02 6.75 11.25 11.25 7.50 38.98 7.50 – 3.75 1625
10.21 7.65 12.75 12.75 8.50 35.39 8.50 – 4.25 1575
30.69 – – – – – – 69.31 – 1075
24.52 2.25 3.75 3.75 2.50 7.50 2.50 51.98 1.25 1275
18.35 4.50 7.50 7.50 5.00 15.00 5.00 34.65 2.5 1325
12.17 6.75 11.25 11.25 7.5 22.5 7.5 17.33 3.75 1525
9.70 7.65 12.75 12.75 8.50 25.50 8.50 10.40 4.25 1575
6.0 9.0 15.0 15.0 10.0 30.0 10.0 – 5.0 1775
Soluble glass with sodium disilicate composition. Melted slag.
Batches were melted in silicon carbide crucibles in a resistive furnace at temperatures from 1275 K (25 slag, 75 borax) to 1775 K (slag without additives) with keeping at maximum temperature for 1 h. Portions of the melts were poured onto a metal plate; the residues were slowly cooled in turned-off furnace. The products were examined by X-ray diffraction (XRD) using a Rigaku D/Max 2200 diffractometer (Cu Ka radiation, voltage is 40 keV, current is 20 mA), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) using a JSM-5610LV+JED-2300 analytical unit (metals, oxides, phosphates and silicates were used as standards), and infrared spectroscopy using a IKS-20 spectrophotometer (compaction of powdered samples with KBr). Leach resistance of the products was measured using a PCT procedure [17] and compared with that for standard EA glass [18].
3. Results and discussion Both quenched and slowly cooled materials were composed of the same phases but content of crystalline phases in the slowly cooled samples was much higher and crystals had larger size and more regular geometry. Slag surrogate melted at 1775 K and slowly cooled in turned-off furnace (Fig. 1) had the highest degree of crystallinity among the quenched and slowly cooled samples and was composed of glass and crystalline phases with XRD patterns close to synthetic britholite Ca10-xREx(PO4)6 x(SiO4)xO2, nepheline (Na,K)AlSiO4 and magnetite-type spinel Fe(Fe,Al)2O4. After fluxing of the slag with 15 wt.% sodium silicate (85Si) nepheline was found to be major phase, spinel was secondary in abundance phase and britholite was minor phase in the sample. Sample 75Si had similar phase composition. Sample 50Si contained nepheline and orthorhombic variety of calcium orthosilicate (larnite) Ca2SiO4. Sample 25Si was found to be amorphous. As it is seen from SEM images in backscattered electrons the melted slag contains lighter and darker areas separated by layers saturated with fine micron-sized white colored crystals (Fig. 2, 1–3). Lighter areas are also non-uniform (Fig. 2, 2) and contain zones enriched and depleted with Sm (Table 2). Due to sub-micron sizes of crystals in these zones their chemical composition cannot be determinate precisely but it may be suggested from crystal chemical behavior of the elements that lighter zones are enriched with britholite crystals whereas the darker zones contain mainly nepheline. Magnetite-type spinel is located in the border layers where it is observed as white star-like micron-sized individual or aggregated crystals (Fig. 2, 3). On SEM image of the near-border zone all three areas are composed of nepheline and britholite crystals distributed within the vitreous matrix with various ratios between them. In the border zone white elongated crystals are
Fig. 1. XRD patterns of glass ceramics produced by slow cooling.
britholite whose chemical composition (Table 2) may be recalculated to crystal chemical formula (Na1.20K2.06Ca1.79Fe1.01Al3.27Sm0.67) (Si4.46P1.10Al0.44)O26 x.
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Fig. 2. SEM images of the samples of melted slag (1–3), 85Si (4–6), 75Si (7,8), 85B (9,10), and 75B (11,12). Analytical points are indicated on Figs. 2 and 3. Scale bars in lm.
Table 2 Chemical composition (wt.%) of the silica-bearing materials in various analytical points on Fig. 2. Oxides
Melted slag Fig. 2, 2
Na2O Al2O3 SiO2 P2O5 K2O CaO Fe2O3 Sm2O3 Total
Fig. 2, 3
85Si
75Si
Fig. 2, 6
Fig. 2, 7
Light
Dark
1
2
3
4
1
1
2
3
5.66 17.58 26.94 7.69 9.99 11.56 12.04 8.49 99.95
6.26 20.36 25.25 6.36 8.55 9.84 15.19 6.44 98.25
4.16 16.14 29.60 8.11 11.74 12.13 9.68 8.24 99.80
5.82 20.35 29.40 7.85 10.12 10.85 8.59 8.32 101.30
5.39 19.26 29.51 8.12 10.77 11.23 8.92 7.43 100.63
3.83 19.29 27.46 7.97 9.99 10.27 8.17 12.01 98.99
11.74 13.8 26.45 15.08 4.99 16.45 5.19 4.26 97.96
11.05 16.49 37.15 8.36 4.65 9.16 7.29 3.68 97.83
14.05 27.46 39.35 3.40 6.02 4.19 5.59 0.98 101.02
8.86 12.88 27.57 5.01 3.29 11.54 20.58 3.23 92.96
The slowly cooled sample 85Si has a texture similar to that in the previous sample (Fig. 2, 4). It is seen at high magnification that the matrix glass is phase-separated (Fig. 2, 5). Nepheline dominates over britholite and magnetite and is present as dark irregular crystals with average chemical composition (Table 2) recalculated to formula Na0.61K0.27Ca0.12Al0.65Fe0.21Si1.08P0.04Sm0.02O4.14. Excess of oxygen in the formula is due to either occurrence of some fraction of Fe as Fe(II) or capture of surrounding material by electron probe. Britholite on SEM images occurs as white crystals (Fig. 2, 6). Its chemical composition (Table 2) is recalculated to crystal chemical formula (Na3.38K0.94Ca2.62Fe0.58Al2.26Sm0.22)(Si3.94P1.90Al0.16)O26 x.
The sample 75Si is composed of elongated darker and lighter crystals (Fig. 2, 7) distributed within the phase-separated vitreous phase (Fig. 2, 8). The darker and lighter crystals are nepheline and britholite, respectively (Table 2). Their average crystal chemical formulae are Na0.69K0.19Ca0.12Sm0.01Al0.82Fe0.11Si1.00P0.07O4.08 and (Na2.82K0.68Ca2.04Fe2.57Al1.72Sm0.17)(Si4.52P0.70Al0.78)O26 x. For the samples with borax additive britholite remains major crystalline phase at 85 and 75 wt.% slag content (Samples 85B and 75B) whereas nepheline and magnetite occur as secondary in abundance phases. Sample 50B contains larnite and sample 25B is amorphous (Fig. 1). In all the boron-containing samples size of
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crystals is smaller than electron probe diameter and their chemical compositions cannot be determined precisely. It may be concluded that britholite to nepheline ratio is somewhat variable in different analytical points (Table 3). Anyway, britholite prevails over nepheline and magnetite-type spinel (points #1, 2, 3) except point #4 in the sample 85B (Fig. 2, 10) and point #1 in the sample 75B (Fig. 2, 12), where analyses show chemical composition of magnetite with captured surrounding glass. In the latter point analytical sum significantly exceeds 100 wt.% probably due to occurrence of appreciable fraction of Fe in divalent form as Fe2+ ions which enter the structure of magnetite. XRD and SEM/EDS data show that the mechanism of interaction between slag surrogate and borax is close to normal dissolution of slag constituents in a sodium tetraborate melt: amount of crystalline phases occurring in the slag reduced with the increase of borax content, whereas in the boron free silicate system change in phase composition of the materials with the increase of sodium disilicate content in favor of nepheline occurs. In the materials studied four crystalline phases were found: britholite-type, nepheline, spinel-type magnetite, and larnite (at 50 wt.% slag content). Britholite is considered as one of the candidate actinide host phase [19]. In our materials the britholite phase accumulates Sm which was used as an analog of trivalent actinides and this may be a reason of low normalized releases of Sm from glass ceramics (Table 4). Nepheline in borosilicate glasses is usually considered as a troublesome phase depleting matrix glass with Al2O3 and SiO2 thus reducing its chemical durability although nepheline itself is quite leach resistant [20]. Nepheline formation offers some negative effect on chemical durability of borosilicate glasses as it is seen from comparison of normalized releases of the elements from quenched materials containing trace of nepheline and slowly cooled materials with much higher nepheline contents but does nearly not affect it in boron free silicate glasses studied in the present work (Table 4). In the borosilicate glasses formation of nepheline actually extracts some fraction of Al2O3 and SiO2 from residual glass, fraction of B2O3 in residual glass increases and as a result chemical durability of glass ceramics decreases. In boron free glasses crystallization of nepheline as well as britholite even though decreases molar content of Al2O3 in residual glass but increases molar content of SiO2 in it thus neutralizing negative effect of nepheline formation. Moreover for the silicatebased materials with high content of crystalline constituent and minor vitreous phase the effect of the residual glass on its properties such as chemical durability becomes negligible. In the whole, normalized releases of B, Na and Si from the slag-bearing glass ceramics are much lower than those from EA glass [18] and comparable with those from the SRNL glass designed for immobilization of high-Al SB4 waste [21]. Spinel incorporates transition elements, Mg and Al and, therefore, it may be considered as a host phase for corrosion products
(Cr, Mn, Fe, Co, Ni). As it has been demonstrated in our previous works [21] occurrence of spinel in borosilicate glasses in amount of at least 20–25 vol.% did not effect on rheological properties of glass melts and did not deteriorate chemical durability of glass. IR spectra of the quenched samples with sodium disilicate flux (Fig. 3, 1–5) consist of strong bands within the ranges of 900– 1200 cm 1, 400–600 cm 1 due to m3 stretching and m2 bending modes of SiAO bonds in SiO4 tetrahedra, and weaker band at 600–800 cm 1 due to superposition of m1 stretching modes in SiO4 tetrahedra with one (Q1, two (Q2) or three (Q3) bridging oxy5 gen ions and stretching modes of AlAO bonds in AlO4 and, in less extent, FeAO bonds in FeO4 tetrahedra [22–25]. In IR spectrum of sample 25Si the band 900–1200 cm 1 is doublet with components of 1060 and 960 cm 1 attributed to m3 stretching modes in SiAOASi bridges and SiAO bonds. Increase of slag content from 25 to 100 wt.% gradually reduces this splitting right up till formation of broad structureless band centered at 1010 cm 1 due to growth in intensity lower wavenumber component. Maxima of the bands 600–800 cm 1 and 400–600 cm 1 are shifted from 765 to 700 cm 1 and from 475 to 460 cm 1, respectively. Moreover formation and growth in intensity of the band at 570 cm 1 takes place. As follows from IR spectra of slag-silicate materials increase of 4 slag loading increases fraction of SiO4 units with higher number 2 1 of non-bridging oxygen ions (Q , Q ) but this effect is compensated 5 by increasing the number of AlO4 and, in less extent, FeO54 units capable to be built in silicon-oxygen chains or network. Thus, overall degree of connectedness of glass network remains approximately the same. Incorporation of aluminum and iron oxides into the glassy materials results in formation of additional bands in IR spectra such as 565–570 cm 1 and weak bands within the range of 400–550 cm 1 overlapping with strong band due to bending 4 modes in SiO4 tetrahedra (Fig. 3, 1–5). IR spectra of glass with a sodium tetraborate formulation and glass containing 25 wt.% slag surrogate (Fig. 3, 6 and 7) demonstrate strong absorption within the ranges of 1200–1500 cm 1 (stretching modes on BO33 triangles), 850–1100 cm 1 (superposition of the band centered at 1060 cm 1 due to stretching modes in B3AOAB4 bridges and the band centered at 930 cm 1 due to stretching modes in BO54 tetrahedra), 650–780 cm 1 and 400– 550 cm 1 (bending modes in BO33 triangles and BO54 tetrahedra, respectively) [26–28]. Increase of slag surrogate content in the boron-containing materials results in the following changes in IR spectra (Fig. 3, 6– 9): decreasing relative intensity of the bands 1200–1500 and 650–780 cm 1, increasing relative intensity of the bands 850– 1100 and 400–550 cm 1, formation and growth in intensity the bands at 1280 and 655 cm 1, narrowing the band 850– 1100 cm 1 and loosening its doublet character being transformed into a single line centered at 970 cm 1, broadening the band cen-
Table 3 Chemical composition (wt.%) of boron-containing materials in various analytical points on Fig. 2. Oxides
B2O3a Na2O Al2O3 SiO2 P2O5 K2O CaO Fe2O3 Sm2O3 Total a
Fig. 2, 10
Fig. 2, 11
12
1
2
3
Average
4
1
2
3
4
1
(1.18) 10.26 21.70 27.17 10.63 4.76 13.21 7.56 3.53 98.82
(11.39) 5.82 16.96 23.42 9.90 7.90 13.39 7.54 3.68 88.61
(6.18) 8.97 21.11 28.97 7.79 6.24 10.59 6.98 3.17 93.82
(6.25) 8.35 19.92 26.52 9.44 6.30 12.40 7.36 3.46 93.75
(15.36) 2.61 8.11 9.94 5.74 3.70 7.56 44.74 2.24 84.64
(24.69) 9.81 22.56 13.24 6.14 6.69 10.11 4.30 2.46 75.31
– 11.47 26.32 29.11 7.76 6.14 10.10 5.77 3.53 100.20
(14.00) 8.69 21.03 19.67 8.51 6.91 12.12 5.49 3.58 86.00
(18.87) 9.26 23.17 19.70 5.03 8.11 8.26 4.95 2.65 81.13
–
Calculated by difference between 100 wt.% and total.
8.46 8.16 11.05 2.56 2.00 3.09 71.15 0.66 107.13
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Table 4 PCT-A data for glass ceramics at various slag loadings. Glass ID
Slag loading (wt.%)
75Si, quenched 75Si, slowly cooled 85Si, quenched 85Si, slowly cooled 90Si, quenched 90Si, slowly cooled Melted slag, quenched Melted slag, slowly cooled 75B, quenched 75B, slowly cooled 85B, quenched 85B, slowly cooled 90B, quenched 90B, slowly cooled SB4-60 (bench-scale) [21] SB4-50 (large-scale) [21] Environmental assessment (EA) standard [18]
75 75 85 85 90 90 100 100 75 75 85 85 90 90 60 50
Normalized release (g l
1
)
B
Na
Si
Sm
– – – – – – – – 5.55 5.77 3.17 6.23 0.84 2.11 0.66 1.00 18.57
2.44 2.48 1.58 1.57 1.55 1.56 2.03 2.05 3.24 3.26 2.17 4.32 1.67 3.73 0.57 0.60 13.73
1.75 1.73 1.32 1.30 1.00 1.03 0.64 0.60 3.16 3.25 2.54 5.47 1.09 2.20 0.32 0.34 3.92
0.25 0.24 0.20 0.21 0.12 0.10 0.08 0.05 0.34 0.30 0.26 0.22 0.17 0.15 – – –
lowering of symmetry of structural units due to effect of multicharged cations and pre-crystallization processes. Annealing of the samples leads to their crystallization with segregation of britholite, nepheline, spinel and larnite phases depending on chemical composition of the materials. 4. Conclusion Solid inorganic radioactive waste such as incinerator ash (slag) may be incorporated in borosilicate or boron free aluminosilicate glass ceramics using borax or soluble glass (sodium silicates) as fluxing additives at temperatures of up to 1575 K. Waste loading in the materials reaches 85–90 wt.%. Melted slag may be also used as waste form but requires higher melting temperatures (1725–1775 K). Glass ceramics formed at spontaneous cooling of melts in turned-off furnace are composed of micron-sized crystals of britholite, nepheline and spinel (magnetite). At that, britholite is a host phase for actinides and rare earths. High chemically durable materials are formed at waste loading of as large as 75 wt.%. As follows from IR spectra, at increase of slag loading destruction of silicate-based glass network is compensated by in5 crease of content of AlO4 and in less extent FeO54 tetrahedra. In glasses with sodium tetraborate increase of slag content results in transition of boron from trigonal to tetrahedral coordination. References
Fig. 3. IR spectra of slag-bearing glass ceramics. Symbols of the materials on the right of spectra see in Table 1.
tered at 720 cm 1, formation of weak bands within the range of 500–700 cm 1. The changes observed in IR spectra of the materials are due to transition of part of boron atoms from ternary to quarterly coordination (change in ratio of intensity of the bands 1200–1500, 650– 800, 850–1100 and 400–550 cm 1 in favor of 850–100 and 400– 550 cm 1 responsible for stretching and bending modes of BAO 5 bonds in BO54 tetrahedra) and increase of content of AlO4 and, in less extent, FeO54 and PO34 units (stronger absorption and formation of multiple bands at 1100 and 450–600 cm 1). Splitting of the bands due to stretching and bending modes are typical of
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Glass ceramics for incinerator ash immobilization G.A. Malinina, O.I. Stefanovsky, S.V. Stefanovsky ⇑ SIA Radon, 7th Rostovskii lane 2/14, Moscow 119121, Russia
a r t i c l e
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Article history: Available online 26 February 2011
a b s t r a c t Calcined solid radioactive waste (incinerator slag) surrogate and either Na2Si2O5 or Na2B4O7 (borax) at various mass ratios were melted in silicon carbide crucibles in a resistive furnace at temperatures of up to 1775 K (slag without additives). Portions of the melts were poured onto a metal plate; the residues were slowly cooled in turned-off furnace. Both quenched and slowly cooled materials were composed of the same phases. At high slag contents in silicate-based materials nepheline and britholite were found to be major phases. Britholite formed at higher slag content (85 wt.%) became major phase in the vitrified slag. In the system with borax at low slag contents (25 and 50 wt.%) material are composed of predominant vitreous and minor calcium silicate larnite type phase Ca2SiO4 where Ca2+ ions are replaced by different cations. The materials containing slag in amount of 75 wt.% and more are chemically durable. The changes in the structure of anionic motif of quenched samples depending on slag loading were studied by IR spectroscopy. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction There is a wide variety of solid radioactive wastes. Some of them may contain alpha-emitters, mainly actinides (Th, U, Pu, Am). To treat burnable solids, the conventional method of waste management is incineration providing for waste volume reduction factors of up to 100 and more. The process is performed in chamber-type or shaft furnaces [1,2]. The final products of the incineration process are fly ash and bottom slag (incinerator ash). The latter concentrates heavy metals including actinides. The bottom ash as well as fly ash are non-uniform, dusting, easily leachable material not suitable for ultimate disposal. It has to be treated yielding stable solid chemically durable waste form. The most perspective method is incineration with liquid slagging with production of primarily crystalline or glass-crystalline material similar to igneous rock or stone casting [3]. For example a full-scale pilot plant with a waste capacity of up to 250 kg h 1 and liquid slagging is currently under operation at SIA Radon [2]. However, at the present time chamber-type incinerators are most commonly used and the incinerator slag formed needs to be consolidated and stabilized. This may be done by direct melting using plasma, induction or microwave heating or incorporation in matrix material such as cement, bitumen or glass [4]. Vitrified (glassy) slags normally contain crystalline constituent whose amount may be significant. Crystalline phases may incorporate various waste constituents such as strontium, transition metals, rare earths and actinides thus
⇑ Corresponding author. Tel.: +7 916 370 4257; fax: +7 495 919 3194. E-mail address: [email protected] (S.V. Stefanovsky). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.01.127
reducing their leachability, whereas cesium normally enters vitreous phase. There are numerous works on design, preparation and characterization of glasses and glass ceramics as waste forms. They were summarized in reviews [5–8]. Moreover there are original works on immobilization of incinerator slags and other solid wastes in glass and glass ceramics using both special additives and natural raw materials (see, for example, [9–16]). At waste vitrification the most convenient method is fluxing of waste with natural raw material or commercially available additives. These may be soluble (sodium silicate) glass Na2OnSiO2 (n = 23), borax Na2O2B2O2, borosilicate frit as well as naturally occurring rocks (nepheline syenite, basalt and other igneous rocks, etc.). In the present work, phase composition was studied together with structure and elemental partitioning in glass-crystalline materials (glassy slags) produced by smelting of simulated incinerator slag with either sodium disilicate or sodium tetraborate. Sm2O3 was introduced as an An2O3 surrogate.
2. Experimental Mixture of reagent-grade chemicals Na2CO3, K2CO3, CaCO3, Ca3(PO4)2, Al2O3, FeO, SiO2, and Sm2O3 was calcined at 1175 K for 6 h to obtain a solid radioactive waste (incinerator slag) surrogate with a target composition of (wt.%) 6.0 Na2O, 9.0 K2O, 15.0 CaO, 15.0 Al2O3, 10.0 FeO, 30.0 SiO2, 10.0 P2O5, 5.0 Sm2O3. Thus produced simulated slag was intermixed with either liquid sodium silicate (soluble glass) with approximate composition Na2Si2O5 or Na2B4O7 (borax) at various mass ratios (Table 1): x Slag, (100 x) Na2Si2O5 and x Slag, (100 x) Na2B4O7 (x = 25, 50, 75, 85, 100).
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G.A. Malinina et al. / Journal of Nuclear Materials 416 (2011) 230–235 Table 1 Specified chemical compositions of the materials. Oxides
Na2O K2O CaO Al2O3 FeO SiO2 P2O5 B2O3 Sm2O3 T (K) a b
Slag content (wt.%) Na–Sia
25Si
50Si
75Si
85Si
Borax
25B
50B
75B
85B
MSb
34.07 – – – – 65.93 – – – 1575
27.05 2.25 3.75 3.75 2.50 56.95 2.50 – 1.25 1575
20.03 4.50 7.50 7.50 5.00 47.97 5.00 – 2.50 1575
13.02 6.75 11.25 11.25 7.50 38.98 7.50 – 3.75 1625
10.21 7.65 12.75 12.75 8.50 35.39 8.50 – 4.25 1575
30.69 – – – – – – 69.31 – 1075
24.52 2.25 3.75 3.75 2.50 7.50 2.50 51.98 1.25 1275
18.35 4.50 7.50 7.50 5.00 15.00 5.00 34.65 2.5 1325
12.17 6.75 11.25 11.25 7.5 22.5 7.5 17.33 3.75 1525
9.70 7.65 12.75 12.75 8.50 25.50 8.50 10.40 4.25 1575
6.0 9.0 15.0 15.0 10.0 30.0 10.0 – 5.0 1775
Soluble glass with sodium disilicate composition. Melted slag.
Batches were melted in silicon carbide crucibles in a resistive furnace at temperatures from 1275 K (25 slag, 75 borax) to 1775 K (slag without additives) with keeping at maximum temperature for 1 h. Portions of the melts were poured onto a metal plate; the residues were slowly cooled in turned-off furnace. The products were examined by X-ray diffraction (XRD) using a Rigaku D/Max 2200 diffractometer (Cu Ka radiation, voltage is 40 keV, current is 20 mA), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) using a JSM-5610LV+JED-2300 analytical unit (metals, oxides, phosphates and silicates were used as standards), and infrared spectroscopy using a IKS-20 spectrophotometer (compaction of powdered samples with KBr). Leach resistance of the products was measured using a PCT procedure [17] and compared with that for standard EA glass [18].
3. Results and discussion Both quenched and slowly cooled materials were composed of the same phases but content of crystalline phases in the slowly cooled samples was much higher and crystals had larger size and more regular geometry. Slag surrogate melted at 1775 K and slowly cooled in turned-off furnace (Fig. 1) had the highest degree of crystallinity among the quenched and slowly cooled samples and was composed of glass and crystalline phases with XRD patterns close to synthetic britholite Ca10-xREx(PO4)6 x(SiO4)xO2, nepheline (Na,K)AlSiO4 and magnetite-type spinel Fe(Fe,Al)2O4. After fluxing of the slag with 15 wt.% sodium silicate (85Si) nepheline was found to be major phase, spinel was secondary in abundance phase and britholite was minor phase in the sample. Sample 75Si had similar phase composition. Sample 50Si contained nepheline and orthorhombic variety of calcium orthosilicate (larnite) Ca2SiO4. Sample 25Si was found to be amorphous. As it is seen from SEM images in backscattered electrons the melted slag contains lighter and darker areas separated by layers saturated with fine micron-sized white colored crystals (Fig. 2, 1–3). Lighter areas are also non-uniform (Fig. 2, 2) and contain zones enriched and depleted with Sm (Table 2). Due to sub-micron sizes of crystals in these zones their chemical composition cannot be determinate precisely but it may be suggested from crystal chemical behavior of the elements that lighter zones are enriched with britholite crystals whereas the darker zones contain mainly nepheline. Magnetite-type spinel is located in the border layers where it is observed as white star-like micron-sized individual or aggregated crystals (Fig. 2, 3). On SEM image of the near-border zone all three areas are composed of nepheline and britholite crystals distributed within the vitreous matrix with various ratios between them. In the border zone white elongated crystals are
Fig. 1. XRD patterns of glass ceramics produced by slow cooling.
britholite whose chemical composition (Table 2) may be recalculated to crystal chemical formula (Na1.20K2.06Ca1.79Fe1.01Al3.27Sm0.67) (Si4.46P1.10Al0.44)O26 x.
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Fig. 2. SEM images of the samples of melted slag (1–3), 85Si (4–6), 75Si (7,8), 85B (9,10), and 75B (11,12). Analytical points are indicated on Figs. 2 and 3. Scale bars in lm.
Table 2 Chemical composition (wt.%) of the silica-bearing materials in various analytical points on Fig. 2. Oxides
Melted slag Fig. 2, 2
Na2O Al2O3 SiO2 P2O5 K2O CaO Fe2O3 Sm2O3 Total
Fig. 2, 3
85Si
75Si
Fig. 2, 6
Fig. 2, 7
Light
Dark
1
2
3
4
1
1
2
3
5.66 17.58 26.94 7.69 9.99 11.56 12.04 8.49 99.95
6.26 20.36 25.25 6.36 8.55 9.84 15.19 6.44 98.25
4.16 16.14 29.60 8.11 11.74 12.13 9.68 8.24 99.80
5.82 20.35 29.40 7.85 10.12 10.85 8.59 8.32 101.30
5.39 19.26 29.51 8.12 10.77 11.23 8.92 7.43 100.63
3.83 19.29 27.46 7.97 9.99 10.27 8.17 12.01 98.99
11.74 13.8 26.45 15.08 4.99 16.45 5.19 4.26 97.96
11.05 16.49 37.15 8.36 4.65 9.16 7.29 3.68 97.83
14.05 27.46 39.35 3.40 6.02 4.19 5.59 0.98 101.02
8.86 12.88 27.57 5.01 3.29 11.54 20.58 3.23 92.96
The slowly cooled sample 85Si has a texture similar to that in the previous sample (Fig. 2, 4). It is seen at high magnification that the matrix glass is phase-separated (Fig. 2, 5). Nepheline dominates over britholite and magnetite and is present as dark irregular crystals with average chemical composition (Table 2) recalculated to formula Na0.61K0.27Ca0.12Al0.65Fe0.21Si1.08P0.04Sm0.02O4.14. Excess of oxygen in the formula is due to either occurrence of some fraction of Fe as Fe(II) or capture of surrounding material by electron probe. Britholite on SEM images occurs as white crystals (Fig. 2, 6). Its chemical composition (Table 2) is recalculated to crystal chemical formula (Na3.38K0.94Ca2.62Fe0.58Al2.26Sm0.22)(Si3.94P1.90Al0.16)O26 x.
The sample 75Si is composed of elongated darker and lighter crystals (Fig. 2, 7) distributed within the phase-separated vitreous phase (Fig. 2, 8). The darker and lighter crystals are nepheline and britholite, respectively (Table 2). Their average crystal chemical formulae are Na0.69K0.19Ca0.12Sm0.01Al0.82Fe0.11Si1.00P0.07O4.08 and (Na2.82K0.68Ca2.04Fe2.57Al1.72Sm0.17)(Si4.52P0.70Al0.78)O26 x. For the samples with borax additive britholite remains major crystalline phase at 85 and 75 wt.% slag content (Samples 85B and 75B) whereas nepheline and magnetite occur as secondary in abundance phases. Sample 50B contains larnite and sample 25B is amorphous (Fig. 1). In all the boron-containing samples size of
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crystals is smaller than electron probe diameter and their chemical compositions cannot be determined precisely. It may be concluded that britholite to nepheline ratio is somewhat variable in different analytical points (Table 3). Anyway, britholite prevails over nepheline and magnetite-type spinel (points #1, 2, 3) except point #4 in the sample 85B (Fig. 2, 10) and point #1 in the sample 75B (Fig. 2, 12), where analyses show chemical composition of magnetite with captured surrounding glass. In the latter point analytical sum significantly exceeds 100 wt.% probably due to occurrence of appreciable fraction of Fe in divalent form as Fe2+ ions which enter the structure of magnetite. XRD and SEM/EDS data show that the mechanism of interaction between slag surrogate and borax is close to normal dissolution of slag constituents in a sodium tetraborate melt: amount of crystalline phases occurring in the slag reduced with the increase of borax content, whereas in the boron free silicate system change in phase composition of the materials with the increase of sodium disilicate content in favor of nepheline occurs. In the materials studied four crystalline phases were found: britholite-type, nepheline, spinel-type magnetite, and larnite (at 50 wt.% slag content). Britholite is considered as one of the candidate actinide host phase [19]. In our materials the britholite phase accumulates Sm which was used as an analog of trivalent actinides and this may be a reason of low normalized releases of Sm from glass ceramics (Table 4). Nepheline in borosilicate glasses is usually considered as a troublesome phase depleting matrix glass with Al2O3 and SiO2 thus reducing its chemical durability although nepheline itself is quite leach resistant [20]. Nepheline formation offers some negative effect on chemical durability of borosilicate glasses as it is seen from comparison of normalized releases of the elements from quenched materials containing trace of nepheline and slowly cooled materials with much higher nepheline contents but does nearly not affect it in boron free silicate glasses studied in the present work (Table 4). In the borosilicate glasses formation of nepheline actually extracts some fraction of Al2O3 and SiO2 from residual glass, fraction of B2O3 in residual glass increases and as a result chemical durability of glass ceramics decreases. In boron free glasses crystallization of nepheline as well as britholite even though decreases molar content of Al2O3 in residual glass but increases molar content of SiO2 in it thus neutralizing negative effect of nepheline formation. Moreover for the silicatebased materials with high content of crystalline constituent and minor vitreous phase the effect of the residual glass on its properties such as chemical durability becomes negligible. In the whole, normalized releases of B, Na and Si from the slag-bearing glass ceramics are much lower than those from EA glass [18] and comparable with those from the SRNL glass designed for immobilization of high-Al SB4 waste [21]. Spinel incorporates transition elements, Mg and Al and, therefore, it may be considered as a host phase for corrosion products
(Cr, Mn, Fe, Co, Ni). As it has been demonstrated in our previous works [21] occurrence of spinel in borosilicate glasses in amount of at least 20–25 vol.% did not effect on rheological properties of glass melts and did not deteriorate chemical durability of glass. IR spectra of the quenched samples with sodium disilicate flux (Fig. 3, 1–5) consist of strong bands within the ranges of 900– 1200 cm 1, 400–600 cm 1 due to m3 stretching and m2 bending modes of SiAO bonds in SiO4 tetrahedra, and weaker band at 600–800 cm 1 due to superposition of m1 stretching modes in SiO4 tetrahedra with one (Q1, two (Q2) or three (Q3) bridging oxy5 gen ions and stretching modes of AlAO bonds in AlO4 and, in less extent, FeAO bonds in FeO4 tetrahedra [22–25]. In IR spectrum of sample 25Si the band 900–1200 cm 1 is doublet with components of 1060 and 960 cm 1 attributed to m3 stretching modes in SiAOASi bridges and SiAO bonds. Increase of slag content from 25 to 100 wt.% gradually reduces this splitting right up till formation of broad structureless band centered at 1010 cm 1 due to growth in intensity lower wavenumber component. Maxima of the bands 600–800 cm 1 and 400–600 cm 1 are shifted from 765 to 700 cm 1 and from 475 to 460 cm 1, respectively. Moreover formation and growth in intensity of the band at 570 cm 1 takes place. As follows from IR spectra of slag-silicate materials increase of 4 slag loading increases fraction of SiO4 units with higher number 2 1 of non-bridging oxygen ions (Q , Q ) but this effect is compensated 5 by increasing the number of AlO4 and, in less extent, FeO54 units capable to be built in silicon-oxygen chains or network. Thus, overall degree of connectedness of glass network remains approximately the same. Incorporation of aluminum and iron oxides into the glassy materials results in formation of additional bands in IR spectra such as 565–570 cm 1 and weak bands within the range of 400–550 cm 1 overlapping with strong band due to bending 4 modes in SiO4 tetrahedra (Fig. 3, 1–5). IR spectra of glass with a sodium tetraborate formulation and glass containing 25 wt.% slag surrogate (Fig. 3, 6 and 7) demonstrate strong absorption within the ranges of 1200–1500 cm 1 (stretching modes on BO33 triangles), 850–1100 cm 1 (superposition of the band centered at 1060 cm 1 due to stretching modes in B3AOAB4 bridges and the band centered at 930 cm 1 due to stretching modes in BO54 tetrahedra), 650–780 cm 1 and 400– 550 cm 1 (bending modes in BO33 triangles and BO54 tetrahedra, respectively) [26–28]. Increase of slag surrogate content in the boron-containing materials results in the following changes in IR spectra (Fig. 3, 6– 9): decreasing relative intensity of the bands 1200–1500 and 650–780 cm 1, increasing relative intensity of the bands 850– 1100 and 400–550 cm 1, formation and growth in intensity the bands at 1280 and 655 cm 1, narrowing the band 850– 1100 cm 1 and loosening its doublet character being transformed into a single line centered at 970 cm 1, broadening the band cen-
Table 3 Chemical composition (wt.%) of boron-containing materials in various analytical points on Fig. 2. Oxides
B2O3a Na2O Al2O3 SiO2 P2O5 K2O CaO Fe2O3 Sm2O3 Total a
Fig. 2, 10
Fig. 2, 11
12
1
2
3
Average
4
1
2
3
4
1
(1.18) 10.26 21.70 27.17 10.63 4.76 13.21 7.56 3.53 98.82
(11.39) 5.82 16.96 23.42 9.90 7.90 13.39 7.54 3.68 88.61
(6.18) 8.97 21.11 28.97 7.79 6.24 10.59 6.98 3.17 93.82
(6.25) 8.35 19.92 26.52 9.44 6.30 12.40 7.36 3.46 93.75
(15.36) 2.61 8.11 9.94 5.74 3.70 7.56 44.74 2.24 84.64
(24.69) 9.81 22.56 13.24 6.14 6.69 10.11 4.30 2.46 75.31
– 11.47 26.32 29.11 7.76 6.14 10.10 5.77 3.53 100.20
(14.00) 8.69 21.03 19.67 8.51 6.91 12.12 5.49 3.58 86.00
(18.87) 9.26 23.17 19.70 5.03 8.11 8.26 4.95 2.65 81.13
–
Calculated by difference between 100 wt.% and total.
8.46 8.16 11.05 2.56 2.00 3.09 71.15 0.66 107.13
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Table 4 PCT-A data for glass ceramics at various slag loadings. Glass ID
Slag loading (wt.%)
75Si, quenched 75Si, slowly cooled 85Si, quenched 85Si, slowly cooled 90Si, quenched 90Si, slowly cooled Melted slag, quenched Melted slag, slowly cooled 75B, quenched 75B, slowly cooled 85B, quenched 85B, slowly cooled 90B, quenched 90B, slowly cooled SB4-60 (bench-scale) [21] SB4-50 (large-scale) [21] Environmental assessment (EA) standard [18]
75 75 85 85 90 90 100 100 75 75 85 85 90 90 60 50
Normalized release (g l
1
)
B
Na
Si
Sm
– – – – – – – – 5.55 5.77 3.17 6.23 0.84 2.11 0.66 1.00 18.57
2.44 2.48 1.58 1.57 1.55 1.56 2.03 2.05 3.24 3.26 2.17 4.32 1.67 3.73 0.57 0.60 13.73
1.75 1.73 1.32 1.30 1.00 1.03 0.64 0.60 3.16 3.25 2.54 5.47 1.09 2.20 0.32 0.34 3.92
0.25 0.24 0.20 0.21 0.12 0.10 0.08 0.05 0.34 0.30 0.26 0.22 0.17 0.15 – – –
lowering of symmetry of structural units due to effect of multicharged cations and pre-crystallization processes. Annealing of the samples leads to their crystallization with segregation of britholite, nepheline, spinel and larnite phases depending on chemical composition of the materials. 4. Conclusion Solid inorganic radioactive waste such as incinerator ash (slag) may be incorporated in borosilicate or boron free aluminosilicate glass ceramics using borax or soluble glass (sodium silicates) as fluxing additives at temperatures of up to 1575 K. Waste loading in the materials reaches 85–90 wt.%. Melted slag may be also used as waste form but requires higher melting temperatures (1725–1775 K). Glass ceramics formed at spontaneous cooling of melts in turned-off furnace are composed of micron-sized crystals of britholite, nepheline and spinel (magnetite). At that, britholite is a host phase for actinides and rare earths. High chemically durable materials are formed at waste loading of as large as 75 wt.%. As follows from IR spectra, at increase of slag loading destruction of silicate-based glass network is compensated by in5 crease of content of AlO4 and in less extent FeO54 tetrahedra. In glasses with sodium tetraborate increase of slag content results in transition of boron from trigonal to tetrahedral coordination. References
Fig. 3. IR spectra of slag-bearing glass ceramics. Symbols of the materials on the right of spectra see in Table 1.
tered at 720 cm 1, formation of weak bands within the range of 500–700 cm 1. The changes observed in IR spectra of the materials are due to transition of part of boron atoms from ternary to quarterly coordination (change in ratio of intensity of the bands 1200–1500, 650– 800, 850–1100 and 400–550 cm 1 in favor of 850–100 and 400– 550 cm 1 responsible for stretching and bending modes of BAO 5 bonds in BO54 tetrahedra) and increase of content of AlO4 and, in less extent, FeO54 and PO34 units (stronger absorption and formation of multiple bands at 1100 and 450–600 cm 1). Splitting of the bands due to stretching and bending modes are typical of
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