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

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Abstract

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 α and Tliq; and promoting bulk crystallization at temperatures of 600–700 °C. Additions of divalent cations increase density, α, 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+/ΣFe, 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.

Introduction

Alkali borosilicate glasses are currently the global material of choice for the safe immobilization of high-level radioactive waste (HLW) which is highly radioactive and heat-generating. Silicate glasses are also the obvious host matrix for the vitrification of toxic incinerator ashes given that these wastes are usually rich in SiO2. Phosphate glasses by comparison have few applications; however, they have received substantial attention over the past 40–50 years as possible host materials for the immobilization of certain specific radioactive wastes [1], [2], [3], [4], [5]. Some Russian HLW has been immobilized in sodium aluminophosphate glasses at the Mayak facility in Ozersk [4], [5]. The formation of phosphate glasses by vitrification of phosphate-rich sludges from the fabricated metal products industry has also recently been investigated [6], [7].

Sodium aluminophosphate glasses can, in some cases, provide advantages over alkali borosilicate glasses. These advantages include lower melting temperatures and higher waste loading capacities. However, phosphate glasses are generally more corrosive towards refractory melter linings and have relatively low thermal stabilities [2], [3] although recent work has demonstrated that small additions of B2O3 to sodium aluminophosphate [8] and SiO2, Al2O3 or B2O3 to iron phosphate [9], [10] glass compositions can substantially improve their thermal stability. Thermal stability is important because the incorporation of heat-generating wastes and/or the presence of high temperatures in underground repositories can cause glasses with low thermal stabilities to crystallize, which can in turn lead to volume changes and may impair the chemical durability and mechanical performance of the waste form. Published research into phosphate glasses for waste immobilization focussed largely on lead–iron phosphate glasses during the 1980s [1], [2], [3]. However, concerns remain regarding lead–iron phosphate glasses in terms of their corrosivity during melting, thermal stability and their chemical durability [3].

Iron phosphate glasses have been studied for their potential waste immobilization applications since the mid-1990s [2]. A substantial proportion of this research has dealt with formulations based on the ternary P2O5–Fe2O3–FeO system, and particularly around the familiar 60P2O5–40Fe2O3 (mol%) composition and derivatives there of. Due to the often complex chemical nature of waste-loaded glasses it can be useful to simplify their compositions in order to study individual components and their effects on properties and structure. These data can provide useful information and around which specific experiments involving actual radioactive wastes or simulants may be based. Several published studies deal with P2O5–Fe2O3–FeO–RxOy glasses, for which R = Na [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], K [19], [22], Cs [16], [17], [23], [24], [25], Ca [16], [26], [27], Sr [17], [24], [28], Ba [29], [30], [31], Zn [20], [32], [33], Pb [1], [2], [3], Al [10], [13], [21], B [9], [10] and Si [10]. Despite this apparent depth of information there exists little published data which allows direct comparisons to be made between the effects of different types and different contents of modifier cations upon the properties of the resulting materials. Nor have the effects of components less commonly associated with ‘legacy’ nuclear wastes or with HLW been researched in any great depth. Documented glass formation, structure and property information for these systems is therefore far from comprehensive.

Recently we have studied glass formation and the solubility of SO3 in 60P2O5–40Fe2O3 (mol%) glasses to which have been added oxides of monovalent (Li, Na, and K) and divalent (Mg, Ca, Ba, and Pb) cations. These oxides were supplied to the glasses through the use of sulfate batch materials. This has allowed assessments of the glass formation region (for glasses cooled in air on a 200 g scale) and sulfate solubility [34] and some effects of batch sulfate on iron redox [35] in these materials. With few exceptions, low levels (<0.5 mol%) of sulfate remained in the resulting glassy or crystalline materials. Consequently these materials may be essentially regarded as occupying the P2O5–Fe2O3–FeO–RxOy system. The research described in the present paper further describes our ongoing studies into the effects of doping iron phosphate glasses with RxOy, where R = selected R+, R2+, R3+ and R4+ cations, and their effects upon properties and structure. Primarily our motivation is the development and understanding of new glasses with potential applications in radioactive or toxic waste immobilization; however, the information that we have generated may also prove useful to those researching the use of iron phosphate glasses in other applications.

Section snippets

Experimental procedures

Glasses have been prepared from analytical grade >99 % purity NH4H2PO4, Fe2O3, Li2SO4, Na2SO4, Na2CO3, K2SO4, MgSO4, CaSO4, BaSO4 and [PbO + (NH4)2SO4]. Batches to produce 200 g of glass with nominal molar composition [(1−x)·(0.6P2O5–0.4Fe2O3)]·xR2SO4, where x = 0–0.5, were placed in mullite (3Al2O3·2SiO2) crucibles and heated overnight to 1030 °C. Crucibles were then transferred to a furnace at 1150 °C and held at this temperature for 1 h. Melts were stirred at ∼60 rpm at 1150 °C for a further 2 h

Results

Analyzed glass compositions and measured properties are shown in Table 1, Table 2. Figs. 1(a) and (b) illustrate density and molar volume, respectively, as functions of nominal modifier oxide content. The experimental errors are smaller than the data points shown. XRD analysis has confirmed that the majority of samples are X-ray amorphous; any identified crystalline phases are noted in Table 1, Table 2. Thermal analysis has been performed only on those samples which have been confirmed to be

Composition, density, molar volume and crystallinity

It is often useful to numerically represent glass composition and structure in some meaningful way when considering the effects of methodical changes in composition, as we have studied here. Several such scales exist, although all have their limitations. Previously when studying the sulfur capacity of these and other glasses [34] we have considered simple scales such as P2O5 content or [O]/[P] ratio in addition to more complex, and arguably more meaningful, scales such as theoretical optical

Conclusions

Molar additions of monovalent (Li, Na or K) or divalent (Mg, Ca, Ba, and Pb) oxides to a 60 mol% P2O5–40 mol% Fe2O3 (nominal) glass result in substantially different effects on thermal properties and chemical durability. The addition of up to 40 mol% R2O has relatively little effect on density, Tg and Td; however, addition of RO increases these parameters proportionately to the level of addition. Both monovalent and divalent additions result in increases in α50–300 and Tliq.

The iron redox ratio, Fe

Acknowledgements

The authors acknowledge with thanks EPSRC, the UK’s Engineering and Physical Sciences Research Council, for funding this research. The authors also wish to acknowledge two anonymous reviewers for their suggestions and constructive comments.

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