Modelling the sulfate capacity of simulated radioactive waste borosilicate glasses

The capacity of simulated high-level radioactive waste borosilicate glasses to incorporate sulfate has been studied as a function of glass composition. Combined Raman, 57Fe Mössbauer and literature evidence supports the attribution of coordination numbers and oxidation states of constituent cations for the purposes of modelling, and results confirm the validity of correlating sulfate incorporation in multicomponent borosilicate radioactive waste glasses with different models. A strong compositional dependency is observed and this can be described by an inverse linear relationship between incorporated sulfate (mol% SO42−) and total cation field strength index of the glass, Σ(z/a2), with a high goodness-of-fit (R2 ≈ 0.950). Similar relationships are also obtained if theoretical optical basicity, Λth (R2 ≈ 0.930) or non-bridging oxygen per tetrahedron ratio, NBO/T (R2 ≈ 0.919), are used. Results support the application of these models, and in particular Σ(z/a2), as predictive tools to aid the development of new glass compositions with enhanced sulfate capacities

A Moessbauer and electrical study of tin containing glasses

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Doping of iron phosphate glasses with Al2O3, SiO2 or B2O3 for improved thermal stability

The effects of doping 60 P2O5–40 Fe2O3 (mol%) glasses with 5–10 mol% SiO2, Al2O3 or B2O3 on their thermal stability, iron environments and redox were investigated. Thermal stability improved markedly with 5% dopant addition in the order Al2O3 > SiO2 > B2O3 ≫ base glass. Solubility of pro rata additions when melted at 1150 °C was 5–10% SiO2, 10% B2O3. It was possible to dissolve 5% Al2O3 by replacing Fe2O3. These additions generally had little effect on dilatometric measurements and iron environments, however the Fe2+/ΣFe redox ratio increased in the order base glass < Al2O3 < SiO2 < B2O3. This behaviour was broadly consistent with the effects of glass basicity. The increased thermal stability of these glasses may improve their suitability for applications such as waste immobilisation or sealing

Vitrified metal finishing wastes II. Thermal and structural characterisation

Waste filter cakes from two metal finishing operations were heat treated and vitrified. Substantial weight loss during heating was due to emission of water, volatile sulphur-rich and chlorine-rich compounds, and the combustion of carbonaceous components. Estimations of COx, SOx and HCl emissions were based on chemical analyses. Upon cooling from molten, one sample remained amorphous but all others partially crystallised. Crystalline nature was dependent upon waste composition and the level of P2O5 addition. Thermal stabilities of the waste forms were good, but less so than MW, a borosilicate glass developed for its high temperature stability. Mössbauer and FTIR analyses showed that iron environments in the different vitrified waste forms were very similar. Iron was present predominantly as Fe3+, although the exact redox ratio varied slightly between waste forms. Iron in both redox states occupied distorted octahedral coordination polyhedra with similar levels of site distortion. Phosphate networks in the vitreous materials were highly de-polymerised, consisting largely of (PO4)3− monomer and (P2O7)2− dimer units. This explained the high chemical durability of these waste forms and their structural insensitivity to compositional change, underlining their suitability as hosts for the immobilisation of toxic and nuclear wastes

Vitrification of UK intermediate level radioactive wastes arising from site decommissioning. Initial laboratory trials

Vitrification was considered as one potential treatment option for four types of wet intermediate level radioactive waste (wet ILW) arising from decommissioning of a UK Magnox nuclear power station. Here we discuss the results of initial laboratory scale trial vitrification studies using suitable glass compositions which were previously short listed from a matrix of 80 potential candidates. The results of the initial trials have (a) demonstrated the feasibility of vitrification of these wet ILWs at 35 wt% (dry) waste loading; (b) confirmed that the candidate glasses exhibit acceptable chemical durabilities; and (c) enabled further down-selection to three final candidate glasses which have undergone detailed analysis and testing, which will be discussed in a forthcoming publication. Waste loading of 35 wt% (dry) waste has been demonstrated for all four waste permutations under consideration, and results indicate that achievable final waste loading limits may be considerably higher. For most glasses studied Cs retention was 60–75%; however, this result was partly attributable to high volatilisation rates resulting from the especially high surface area / volume ratio of the small laboratory melts studied. Reductions in melting temperature from 1200°C are possible for the majority of the studied glasses, which should also increase Cs retention and reduce melt corrosivity

Preliminary studies of sulphate solubility and redox in 60P2O5–40Fe2O3 glasses

Preliminary studies of sulphate solubility, redox, composition, refractory corrosion and density of 60P2O5–40Fe2O3 (molar percent) glasses are presented. Techniques included Mössbauer spectroscopy and energy-dispersive X-ray spectroscopy. Sulphate solubility of the glass was very low, only 0.01 wt.% SO3. Iron (II) sulphate batch addition resulted in a more reduced glass than a sulphate-free batch. Redox, considered independently of temperature, had no effect on refractory corrosion or melt volatilisation in the range studied. No differences were detected in refractory corrosion using alumina or mullite crucibles. Higher oxygen partial pressure had no effect on iron valence. Values, trends and conclusions relating to density, molar volume, iron environment and iron coordination in these glasses were consistent with the accepted view

Structure and properties of iron borophosphate glasses

Thermal stability, structure and aqueous chemical durability of glasses of nominal composition 60P2O5-40Fe2O3 (mol%) doped with up to 20% B2O3 have been investigated. B2O3 was substituted for either P2O5 or Fe2O3, or was added on a pro rata basis. No substantial melt volatilisation was detected, and refractory corrosion was minimal. Iron cations of both redox states occupied distorted octahedral sites; B2O3 additions did not affect iron coordination, although Tg and the Fe2+/ΣFe redox ratio increased. Addition of up to 10% B2O3 did not significantly affect durability but it improved thermal stability and increased liquidus temperature. Chemical durability (as measured by the product consistency test, PCT) decreased by approximately one order of magnitude for every further 5% B2O3 addition above 10% when substituted for Fe2O3. Thermal analysis indicated increasing glass stability with increasing B2O3 content. X-ray diffraction showed that one sample of nominal composition 50P2O5-40Fe2O3-10B2O3 developed crystalline B0·57Fe0·43PO4 or a similar phase during cooling. There was no evidence that this significantly affected chemical durability. The addition of B2O3 at low levels (<10%) may be useful in improving the performance of iron phosphate glasses for applications such as waste immobilisation or sealing glasses

Effects of modifier additions on the thermal properties, chemical durability, oxidation state and structure of iron phosphate glasses

Modified iron phosphate glasses have been prepared with nominal molar compositions [(1�x)�(0.6P2O5– 0.4Fe2O3)]�xRySO4, where x = 0–0.5 in increments of 0.1 and R = Li, Na, K, Mg, Ca, Ba, or Pb and y = 1 or 2. In most cases the vast majority or all of the sulfate volatalizes and quarternary P2O5–Fe2O3–FeO–RyOz glasses or partially crystalline materials are formed. Here we have characterized the structure, thermal properties, chemical durability and redox state of these materials. Raman spectroscopy indicates that increasing modifier oxide additions result in depolymerization of the phosphate network such that the average value of i, the number of bridging oxygens per –(PO4)– tetrahedron, and expressed as Qi, decreases. Differences have been observed between the structural effects of different modifier types but these are secondary to the amount of modifier added. Alkali additions have little effect on density; slightly increasing Tg and Td; increasing a and Tliq; and promoting bulk crystallization at temperatures of 600–700 �C. Additions of divalent cations increase density, a, Tg, Td, Tliq and promote bulk crystallization at temperatures of 700–800 �C. Overall the addition of divalent cations has a less deleterious effect on glass stability than alkali additions. 57Fe Mössbauer spectroscopy confirms that iron is present as Fe2+ and Fe3+ ions which primarily occupy distorted octahedral sites. This is consistent with accepted structural models for iron phosphate glasses. The iron redox ratio, Fe2+/RFe, has a value of 0.13–0.29 for the glasses studied. The base glass exhibits a very low aqueous leach rate when measured by Product Consistency Test B, a standard durability test for nuclear waste glasses. The addition of high quantities of alkali oxide (30–40 mol% R2O) to the base glass increases leach rates, but only to levels comparable with those measured for a commercial soda-lime-silica glass and for a surrogate nuclear waste-loaded borosilicate glass. Divalent cation additions decrease aqueous leach rates and large additions (30–50 mol% RO) provide exceptionally low leach rates that are 2–3 orders of magnitude lower than have been measured for the surrogate waste-loaded borosilicate glass. The P2O5–Fe2O3–FeO–BaO glasses reported here show particular promise as they are ultra-durable, thermally stable, low-melting glasses with a large glass-forming compositional range

Age Hardening in Ultrafine-Grained Al-2 Pct Fe Alloy Processed by High-Pressure Torsion

https://www.scopus.com/inward/record.url?eid=2-s2.0-84937759515&partnerID=40&md5=6c6d7635f04f5ef9de34768fe585779cA cast Al-2 wt pct Fe alloy was processed by high-pressure torsion (HPT) at room temperature and then subjected to artificial aging at temperatures of 373 K and 473 K (100 °C and 200 °C). The aging behavior was studied by Vickers microhardness measurements and by microstructural analyses using transmission electron microscopy and X-ray diffraction. The initial intermetallic structures, composed of a mixture of Al + Al6Fe and Al + Al3Fe eutectics phases, were partially dissolved in the matrix up to a supersaturation of ~1 wt pct Fe. The microstructure was refined by HPT to an ultrafine-grained level with a minimum grain size of ~120 nm in the matrix and a dispersion of particles less than 400 nm. Age hardening was achieved within 0.25 hours at 473 K (200 °C), to a maximum UTS of ~700 MPa as a result of nano-sized precipitation within the ultrafine grains. The uniform elongation exceeded ~12 pct even at intermediate levels of imposed strain by HPT, while it decreased to ~6 pct with the subsequent aging treatment. The thermal stability of the ultrafine-grained structure was verified to exceed 16 days at 373 K (100 °C) and 12 hours at 473 K (200 °C). © 2015, The Minerals, Metals & Materials Society and ASM International