Single-phase Zirconolite Research Articles

Immobilization of CeO2 using single-phase zirconolite and the chemical stability analysis

In this study, zirconolite-based ceramic powder was firstly fabricated from combustion reaction route. CeO2 was introduced as the surrogate of tetravalent actinides, which was directly immobilized by the single-phase zirconolite waste matrice. The results demonstrate that the crystalline incorporation of CeO2 is highly depended on the sintering temperature. 10 wt% CeO2 was directly immobilized into the zirconolite crystal structure under 1250 °C sintering, where Ce concurrently occupies the Ca and Zr sites. Higher CeO2 incorporation (15–25 wt%) leads to the formation of Ce-rich impurity phase and phase transformation from 2M-zirconolite to 3T-zirconolite. The Ce-bearing zirconolite waste forms exhibit the highest density and hardness after sintering at 1400 °C for 12 h. The aqueous durability was evaluated, where the 42 days LRCe values are 1.68 × 10–6 and 6.85 × 10–6 gm−2d−1 before and after the leachate treatment using nitric acid.

Reactive spark plasma synthesis of CaZrTi2O7 zirconolite ceramics for plutonium disposition

Near single phase zirconolite ceramics, prototypically CaZrTi2O7, were fabricated by reactive spark plasma sintering (RSPS), from commercially available CaTiO3, ZrO2 and TiO2 reagents, after processing at 1200 °C for only 1 h. Ceramics were of theoretical density and formed with a controlled mean grain size of 1.9 ± 0.6 μm. The reducing conditions of RSPS afforded the presence of paramagnetic Ti3+, as demonstrated by EPR spectroscopy. Overall, this study demonstrates the potential for RSPS to be a disruptive technology for disposition of surplus separated plutonium stockpiles in ceramic wasteforms, given its inherent advantage of near net shape products and rapid throughput.

The synthesis and chemical durability of Nd-doped single-phase zirconolite solid solutions

Nd-doped single-phase zirconolite solid solutions was synthesized by solid-state reaction and following two steps of acid treatment. The phase composition, microstructure, and chemical durability of the zirconolite solid solutions were investigated. About 15 at% Nd was successfully stabilized into the zirconolite. The element mapping images of Ca, Zr, Nd and Ti show that all the elements are almost distributed homogeneously in the zirconolite waste forms. Product Consistency Test (PCT) was conducted under different pH values (pH = 5, 7 and 9) to evaluate the chemical durability of the Nd-doped zirconolite waste forms. The normalized element release rate of Ca (LRCa) in pH = 5 medium is higher than that of pH = 7 and 9, while the LRNd value remains almost unchanged under different pH values. The LRNd value is as low as 10−5 g m−2 d−1 after 42 days.

Novel method to synthesis of single phase zirconolite ceramic

Single phase zirconolite (CaZrTi2O7) was synthesised by a novel method using acid corrosion to treat zirconolite–diopside mixture powders (2CaZrTi2O7–Mg2Si2O6). The mixture powders were prepared by solid state reaction method at 1200°C for 2 h. Single phase zirconolite powders were obtained by acid corrosion method with 5% hydrofluoric acid on the zirconolite–diopside Synroc mixture powders and washed with 10% acetic acid.

Crystal Chemistry and Cation Ordering in Zirconolite 2M

ABSTRACTStructural studies of single phase or nearly single phase zirconolite ceramic samples have been conducted using electron microscopy and microanalysis, X-ray diffraction, neutron diffraction, and spectroscopic methods. We show that it is possible to produce a complete series of zirconolite 2M samples with substitution of 2Ti by Nb+Fe in the HTB layer. The samples are single phase up to about 80% Nb +Fe substitution, with the appearance of a minor perovskite phase at higher Nb+Fe levels. Electron probe microanalysis reveals that the samples are homogeneous and close to their nominal compositions, except for those containing perovskite, which have a slight excess of Zr and a deficiency in the Fe content. The lattice parameters and the positions of certain Raman bands are non-linear as a function of composition, suggesting the possibility of cation ordering over the three available Ti sites within the HTB layer. Rietveld refinement of Synchrotron X-ray powder data for the Nb+Fe end-member have been conducted for the disordered case and for six trial models each with a different ordering scheme. Results of this exercise indicate that Fe preferentially occupies the Ti2 (split) site with partial ordering of Nb and the remaining Fe over the Ti1 and Ti2 octahedra. The preference of Fe for the five coordinated Ti2 site has been confirmed by 57Fe Mossbauer spectroscopy.

Leaching Behaviour of Zirconolite in 0.001M Citric Acid at 90°C Under Various Flow Regimes

ABSTRACTNd-bearing zirconolite was leached at 90°C for 157 days in 0.001M citric acid under single-pass-flow-through conditions (modified MCC-4 protocol). Three different flow rates were used, ranging in an order of magnitude from 10 mL per day to 100 mL per day, to determine the effect of the rate of leachant replenishment on the durability of the zirconolite. Results of previous studies on the role of complexing agents on the leaching behaviour of single-phase zirconolite have been included in the discussion.The pH of the citric acid solution was adjusted to 5 using KOH, mimicking that of the water in the parallel tests, to avoid the influence of pH on chemical durability of the zirconolite.Simulated groundwater containing 0.001M citric acid at 90°C led to congruency in elemental releases and a diminution of release rate with time of about an order of magnitude, reaching virtual constancy after about 50 to 60 days to a level of about 10−5 g m−2 day−1. The most significant finding was that the elemental release rates of Nd, Ti and Zr (and Ca and Al where detected) were similar for all flow rates. Clearly, varying flow rate by up to an order of magnitude had no effect on elemental releases i.e. there is no solubility limit control on releases at 0.001M citric acid concentration.An important finding of previous studies using identical leaching protocols with 0.001M citric acid, and inferred in our latest investigations reported here, was that there is no secondary layer development at the surface of the zirconolite to affect leach rates. In contrast, parallel tests carried out in deionised water instead of citric acid showed that hydroxides form in situ on the zirconolite surface, effectively forming hydrolysed zirconolite. This controls further dissolution of the zirconolite matrix due to the solubility limit being reached with respect to the hydrolysed phases rather than with zirconolite. Complexation by citrate ions prevents such control by hydrolysed species on zirconolite solubility.Even under the more aggressive conditions imposed in these studies (0.001M citric acid), and regardless of flow rate of the leachant, elemental releases from zirconolite are very low for a candidate wasteform and demonstrate its attributes as a ceramic-based wasteform for the containment of actinides.

Kinetics of uranium release from Synroc phases

This paper presents experimental studies on the kinetics of U release from near single-phase zirconolite, pyrochlore, brannerite and pyrochlore-rich titanate ceramic materials. The dissolution tests were conducted at 20–75°C with initial pHs from 2 to 12, and flow rates from 10 to 80 ml d −1 in the open atmosphere. The U releases from these titanate materials are controlled by initial fast process and then followed by linear kinetics. The close-to-stoichiometric U release from zirconolite and pyrochlore-rich materials and preferential U release from brannerite are consistent with the alterations observed for the natural samples. The rate constants for U releases were determined and the effects of pH and temperature were examined. For each material, the U release vs. pH exhibits a V-shape with a minimum near pH 8. The measured activation energies suggest surface reaction controlled dissolution mechanism. Pyrochlore-rich materials and zirconolite demonstrated higher chemical durability and more resistance to aqueous attack than brannerite. However, impurities and minor brannerite inclusions do not appear to have a detrimental effect on U releases from pyrochlore-rich multi-phase ceramics.

Zirconolite transformation under reducing conditions

The structural behavior of zirconolite (CaZrTi2O7) under reducing conditions at high temperature has been studied, mainly by scanning electron microscopy (SEM) and x-ray diffraction (XRD), but also with x-ray absorption spectroscopy, thermogravimetry, and electron paramagnetic resonance. The partial reduction of Ti4+to Ti3+, associated with a reducing atmosphere heat treatment, led to the initial formation of perovskite (CaTiO3) as a second phase. As the concentration of Ti3+in the zirconolite increased, so did the amount of perovskite until the zirconolite was totally transformed into a fluorite structured phase. Analysis of the reduced zirconolites showed them to be consistently deficient in Ca and enriched in Zr, in proportion to the concentration of Ti3+. To determine how electroneutrality was preserved in these reduced zirconolites, a series of zirconolites were prepared in air using In3+and Ga3+as models for Ti3+. These samples were then investigated by neutron and x-ray diffraction, SEM, solid state nuclear magnetic resonance (NMR), and nuclear quadrupole resonance (NQR).71Ga MAS NMR studies of the Ga substituted zirconolite exhibited a narrow resonance at ˜13 ppm which was attributed to six-coordinate Ga incorporated in a trace perovskite phase. Broadline71Ga NMR and69/71Ga NQR were required to characterize the Ga incorporated in the zirconolite. The resultant quadrupolar parameters ofCQ= 30.0 ± 0.05 MHz and η = 1.0 ± 0.03 indicate that the Ga site is in a highly distorted environment which would suggest that it is located on the five-coordinate Ti site within the zirconolite lattice. These results were complemented by Rietveld refinement of the neutron diffraction data from the In-doped zirconolite sample, which was optimal when all the In was located on the five-coordinate Ti site with the excess Zr located on the Ca site. It would therefore appear that charge compensation for the presence of Ti3+in zirconolite is effected via the substitution of an appropriate amount of Zr on the Ca site. The Ti3+-stabilized fluorite structure was readily oxidized back to a single phase zirconolite upon heating in air.

The CaO-TiO2-ZrO2 System at 1200 °C and the Solubilities of Hf and Gd in Zirconolite

AbstractPhase equilibria are established in the CaO-TiO2-ZrO2 system at 1200 °C, using X-ray diffraction and electron probe microanalysis. The existence of two previously reported ternary phases, zirconolite (CaZrTi2O7) and calzirtite (Ca2Zr5Ti2O16), is confirmed. Each of these phases exhibits a significant range of homogeneity between TiO2 and ZrO2 while maintaining a nearly constant concentration of CaO. The ternary solubilities of the constituent binary phases are found to be negligible, with the exceptions of the perovskites, which display mutual solubility of at least 22 mol.% and may in fact form a series of continuous solid solutions. The solubilities of Hf and Gd in zirconolite are also investigated. While Hf-bearing samples did not reach thermodynamic equilibrium under the experimental conditions employed, the existence of a Hf analog to zirconolite, CaHfTi2O7, is conclusively demonstrated. The phase is stable at the stoichiometric composition, and its lattice parameters are very close to those reported in the literature for stoichiometric zirconolite. A Gd-bearing sample of the composition Ca0.88Zr0.88Gd0.24Ti2O7 is found to be essentially single phase zirconolite, in agreement with previous investigations at higher temperatures.

Zirconolite-Rich Titanate Ceramics for High-Level Actinide Wastes

ABSTRACTNew diffraction and microstructural data on the incorporation of rare earths, Np and Pu in nominally single-phase zirconolite (CaZrTi2O7) are presented. Such data are vital for the crystal-chemical design of zirconolite-rich titanate ceramics of the Synroc family, for the immobilization of up to 30 wt% of actinide wastes from advanced reprocessing/partitioning schemes. Some trial zirconolite-rich ceramics were made with U and Nd simulating tetravalent and trivalent actinides respectively. Some preliminary results to examine chemical flexibility with regards to variations in waste/precursor are given, for different actinide waste compositions.