Chandra S. Ray

Chemically durable iron phosphate glass wasteforms

Up to 40 wt% of a simulated high level waste, whose major components were 54.6 wt% Na2O, 14.9 wt% P2O5 and 8.3 wt% Fe2O3, was successfully vitrified into iron phosphate wasteforms whose chemical durability was equivalent to that of borosilicate glass wasteforms. Because of their high fluidity, the iron phosphate wasteforms could be melted in as little as 30 min at temperatures between 1015°C and 1200°C. The addition of 3–7 wt% CaF2 to the batch decreased the melting time and temperature, by as much as 100°C, and improved the chemical durability, especially for crystallized iron phosphate wasteforms. Iron phosphate wasteforms are concluded to be a practical alternative for vitrifying those nuclear wastes not well suited for borosilicate glasses.

Structural features of iron phosphate glasses

The structures and valence states of iron ions in several iron phosphate glasses with batch compositions similar to 40Fe2O3-60P2O5 (mol%) have been investigated using Mossbauer spectroscopy, X-ray absorption fine-structure spectroscopy (XAFS), X-ray photoelectron spectroscopy (XPS), differential thermal (DTA) and thermo-gravimetric (TGA) analysis and X-ray and neutron diffraction. Mossbauer spectra show that a redox equilibria corresponding to an Fe(II)/[Fe(II) + Fe(III)] ratio of 0.2–0.4 is reached under processing conditions described in this paper. Even though the valence state of iron ions in the glass appears to be insensitive to the oxygen content in the melting atmosphere, the Fe(II) content can be increased within the observed range of redox equilibria by increasing the partial pressure of a reducing gas in the melting atmosphere. Large amounts of Fe(II), Fe(II)/[Fe(II) + Fe(III)] ≥ 0.4, appear to be detrimental to the glass-forming ability of the iron phosphate melts. The local structure of the iron phosphate glasses appears to be related to the short range structure of crystalline Fe3(P2O7)2 which consists of a network of (Fe3O12)−16 clusters. These clusters consist of one iron(II) ion and two iron(III) ions in sixfold coordination with near-neighbor oxygen ions. The (Fe3O12)−16 clusters are interconnected via (P2O7)−4 groups. Compared to other phosphate glasses, the proposed structure for iron phosphate glasses contain a smaller number of POP bonds, a feature which is believed to be responsible for the unusually good chemical durability of iron phosphate glasses.

Effect of Melting Temperature and Time on Iron Valence and Crystallization of Iron Phosphate Glasses

The effect of melting temperature and time on iron valence, dissolution rate (DR) in deionized water, and crystallization of iron phosphate glasses was investigated using a 40Fe2O3–60P2O5, mol%, batch composition. The concentration of Fe2+ ions in these glasses increased from 17% to 57% as melting temperature increased from 1150°C to 1450°C, but remained nearly constant at about 20% for melting times longer than 1 h at 1200°C. Measurements by differential thermal analysis (DTA) combined with X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showed that these glasses crystallized to Fe3(P2O7)2 and Fe4(P2O7)3 when heated in nitrogen between 600°C and 820°C, but with continued heating in air at 820°C the Fe3(P2O7)2 changed to Fe(PO4), which produced a weight gain in the sample associated with the oxidation of Fe2+ to Fe3+ ions. The DR (in deionized water) of these glasses was generally very low (∼10−9 g cm−2 min−1) and nearly independent of the relative concentration of Fe2+ or Fe3+ ions, but decreased with total iron content.

An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

Abstract The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe 2 O 3 –60P 2 O 5 (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mossbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mossbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mossbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron–oxygen coordination is in the 4–5 range indicating that iron ions have mixed tetrahedral–octahedral coordination.

Chemically Durable Iron Phosphate Glasses for Vitrifying Sodium Bearing Waste (SBW) Using Conventional and Cold Crucible Induction Melting (CCIM) Techniques

A simulated sodium bearing waste (SBW) was successfully vitrified in iron phosphate glasses (IPG) at a maximum waste loading of 40 wt% using conventional and cold crucible induction melting (CCIM) techniques. No sulfate segregation or crystalline phases were detectable in the IPG when examined by SEM and XRD. The IPG wasteforms containing 40 wt% SBW satisfy current DOE requirements for aqueous chemical durability as evaluated from their bulk dissolution rate (DR), product consistency test, and vapor hydration test. The fluid IPG wasteforms can be melted at a relatively low temperature (1000 °C) and for short times (<6 h). These properties combined with a significantly higher waste loading, and the feasibility of CCIM melting offer considerable savings in time, energy, and cost for vitrifying the SBW stored at the Idaho National Engineering and Environmental Laboratory in iron phosphate glasses.

Identifying internal and surface crystallization by differential thermal analysis for the glass-to-crystal transformations

Abstract A differential thermal analysis (DTA) method has been developed that identifies and distinguishes surface and internal (bulk) crystallization that occurs during the crystallization of a glass. This method is rapid, convenient and requires only a few (about 6–8) DTA experiments to identify the dominant crystallization mechanism (bulk vs. surface) in the glass. In this method, either the maximum height of the DTA crystallization peak, (δT)p, or the ratio T 2 p (ΔT) p where Tp is the temperature at (δT)p and (ΔT)p is the peak half-width, is determined as a function of size of the glass particles used for the DTA measurements. When analyzed by this technique, an as-quenched lithium disilicate (LS2) glass was found to crystallize predominantly by surface crystallization. The tendency for surface crystallization was enhanced when the glass particles were exposed to moisture prior to DTA. Internal or bulk crystallization dominated over surface crystallization when this LS2 glass was doped with small amounts of platinum. The DTA curves in the literature for several soda-lime-silica glasses as a function of particle size were analyzed by the present method. The analysis showed that Na2O.CaO.2SiO2 and Na2O.2CaO.3SiO2 glasses crystallized by internal crystallization, but surface crystallization was the dominant crystallization mechanism for an Na2O.CaO.3SiO2 glass. These results agree with those obtained from an analysis of the apparent activation energy for crystallization as a function of particle size for these glasses.

Mechanical and Structural Properties of Phosphate Glasses

Abstract Mechanical and structural properties of sodium (NAFP) and zinc (ZAFP) iron–aluminum–phosphate bulk glass and fibers have been investigated. Youngs modulus of the fibers was measured by a three-point bending method while the strength was measured by a two-point bending method. In general, the tensile strength of the ZAFP fibers (4.2–7.2 GPa) was higher than the tensile strength of the NAFP fibers (2.8–4.2 GPa). After exposing the fibers to air for 10 days, the strength decreased by 15–34%. The structure of bulk glass as well as fibers, studied by Mossbauer and IR spectroscopy, was very similar for all the compositions studied.

Non-isothermal calorimetric studies of the crystallization of lithium disilicate glass

The influence of preannealing treatments on the polymorphic crystallization of lithium disilicate glasses is examined. As expected, glasses heated at different rates through the temperature range where there is significant nucleation develop widely different numbers of nuclei. This can dramatically influence the stability and transformation characteristics of the annealed glass. Non-isothermal differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements are demonstrated to be useful to probe the nucleation behavior. The first systematic investigations of particle size effects on the non-isothermal transformation behavior are presented and discussed. Based on DTA and microscopy experiments, we show that small particles of lithium disilicate glasses crystallize primarily by surface crystallization. The relative importance of surface versus volume crystallization is examined by varying particle size, by introducing nucleating agents and by exposing glasses to atmospheres of different water content. These data are analyzed quantitatively using a numerical model developed in a second paper following in this volume.

Properties of mixed Na2O and K2O iron phosphate glasses

Abstract Glass formation and properties of iron phosphate glasses containing Na 2 O and K 2 O have been investigated to determine the suitability of these glasses for disposing of certain types of nuclear wastes that contain one or more alkali oxides. Two series of glasses of the general composition x K 2 O–(20− x )Na 2 O–20Fe 2 O 3 –60P 2 O 5 and x K 2 O–(20− x )Na 2 O–32Fe 2 O 3 –48P 2 O 5 , mol%, with x =0, 5, 10, 15 and 20, were chosen for this investigation. The properties that have been measured include density, molar volume, chemical durability, electrical resistivity, dielectric constant and IR spectra. No typical mixed alkali effect, namely, a minimum or maximum in the properties as a function of the ratio of two alkali ions, was observed for these glasses. The present results show that iron phosphate glasses can be suitable for vitrifying nuclear wastes that contain amounts of one or more alkali oxides ⩽20 mol%. An Fe/P ratio of the compositions at about 0.67 and the O/P ratio between 3.5 and 3.7 are desirable to obtain the maximum chemical durability of the wasteforms.

Electrical Conductivity in Mixed-alkali Iron Phosphate Glasses

Abstract The electrical conductivity of mixed-alkali, sodium and potassium, iron phosphate glasses has been studied in the frequency range from 0.1 Hz to 10 kHz and over a temperature range from 303 to 423 K. The dc conductivity of the alkali-free iron phosphate glasses was 5–10 times higher than that of the single- or mixed-alkali iron phosphate glasses containing a total of 20 mol% alkali. The dc conductivity for the mixed-alkali, sodium and potassium, iron phosphate glasses is independent of the Na/K ratio and there is no evidence of any mixed-alkali effect. The sodium and potassium ions have such a low mobility in both single- and mixed-alkali iron phosphate glasses that they make no detectable contribution to the total conductivity that is electronic in origin. The Raman spectra for all the glasses are identical which indicates that the structure of the single- or mixed-alkali glasses are the same.