Alexander G. Anshits

A novel layered zirconium molybdate as a precursor to a ceramic zirconomolybdate host for lanthanide bearing radioactive waste

Novel layered zirconium molybdates (Mo/Zr = 2) in Na+- and NH4+-exchanged forms with a defective crystalline structure and a specific surface area of up to 100 m2 g−1 were synthesized under mild hydrothermal treatment conditions (150 °C, 3 days) without structure-directing reagents. The NH4+- zirconomolybdate material was tested for its ability to immobilise aqueous radioactive waste containing rare earth elements by a sorption/impregnation–crystallization process using simulant Nd3+/Ln3+ bearing solutions. The motivation for tailoring (Ln,Zr,Mo)-ceramics was the utilization of sediment-forming components of spent nuclear fuel processing solutions, such as Mo and Zr, for immobilization of transuranium radionuclides or an actinide–lanthanide (An–Ln) fraction of high-level effluents. The target (Ln,Zr,Mo)-phase acceptable for incorporation of actinides and lanthanides, Ln2Zr3(MoO4)9 (Ln = La–Tb), was selected on the basis of studying the phase formation in triple oxide systems Ln2O3–ZrO2–MoO3 (Ln = La–Lu, Y, Sc) and analysis of the known promising host phases being developed in the world for the actinide immobilization. X-ray diffraction and thermal analysis methods were used in the study of the thermochemical conversion of the Nd3+/Ln3+-zirconomolybdates resulting from loading the layered NH4+-zirconomolybdate precursor with different quantities of Nd3+/Ln3+. The results on Nd3+ immobilization by the sorption–crystallization route reflected the influence of the acidity of the simulant solutions on the content of the target phase in the solidified (Nd,Zr,Mo)-ceramics. The impregnation/sorption–crystallization procedure provided poly-phase composites including Nd2Zr3(MoO4)9, ZrxNdy(MoO4)2 and ZrO2, the quantity of the target phase depending on the Nd3+ loading. The solidification of the Ln bearing simulant solution at 16% Men+ loading resulted in nearly mono-phase ceramics of the Nd2Zr3(MoO4)9 structural type.

Porous Materials Based on Cenospheres of Coal Fly Ash for Fixation of Cs-137 and Sr-90 in Mineral-like Aluminosilicates

Among the crystalline matrices being developed for immobilization of 137 Cs and 90 Sr are framework aluminosilicates like feldspars and feldspathoids. A novel approach to obtain mineral-like aluminosilicate forms of 137 Cs and 90 Sr with using porous materials based on hollow aluminosilicate microspheres of coal fly ash (cenospheres) has been demonstrated. Two modifications of microspherical porous materials have been developed, (i) moulded porous matrices based on consolidated cenospheres and (ii) zeolite sorbents obtained by hydrothermal zeolitization of cenospheres. The first includes impregnation of Cs- or Sr-containing solutions into porous matrices, drying and calcining at temperatures (700-900°C) lower than the softening point of the porous matrix material, at which solid phase crystallization takes place with formation of framework aluminosilicates, such as Cs-nepheline, pollucite or Sr-anorthite. Using zeolite sorbents, immobilization of Cs + and Sr 2+ proceeds through the step of trapping ions from the solution followed by drying and thermochemical solid-phase transformation of saturated zeolites into pollucite or Sr-anorthite over the same temperature range. Cenosphere-derived porous matrices and zeolite sorbents were tested in fixation of 137 Cs using spiked simulants of actual radioactive waste. The durability of crystalline aluminosilicate 137 Cs -forms obtained was shown to satisfy Russian (GOST P 50926-96) and international standards.

Composition and Structure of Block-Type Ferrospheres Isolated from Calcium-Rich Power Plant Ash

Polished sections of individual ferrospheres 30 to 40 μm in size, with single-block and blocky structures and a variable glass phase content, have been studied using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer system. The results demonstrate that the single-block globules consist of sintered magnetite crystallites containing Al2O3, MgO, and CaO as impurities and are formed from the pyrite of the initial coal. Characteristically, the ferrospheres with a variable glass phase content differ in the composition of local areas on polished sections of the globules, which attests to inhomogeneity of the melt droplets they formed from. We have identified groups of globules whose overall composition, as well as the composition of their local areas, meet general equations for the interrelation between the concentrations of their components: SiO2 = f(FeO) and SiO2 = f(Al2O3). Comparison of the coefficients of the SiO2 = f(Al2O3) dependence for the globules with the silicate modulus (SiO2/Al2O3) of the aluminosilicate mineral components of the coal indicates that the formation of this type of globules involves pyrite–anorthite or pyrite–albite associates containing quartz impurities. The composition of the spinel ferrite in the globules produced with the participation of anorthite comprises FeO, Al2O3, MgO, and CaO in concentrations of 85–96, 1.7–10, 0.1–1.8, and 0.3–2.8 wt %, respectively. In the albite-based globules, the respective concentrations are 81–92, 0.7–5.9, 1.0–5.7, and 2.2–5.6 wt %. The crystallite size and shape are determined by the size of the local melt areas where the total concentration of spinel-forming oxides exceeds 85 wt %.

Solid-Phase Transformation of Cs + - and Sr 2+ -Bearing Zeolite Sorbents Derived From Cenospheres to Mineral-Like Forms

The paper describes the studies of the transformation of Cs + - and Sr 2+ -containing zeolite sorbents synthesized from fly ash cenospheres to crystalline mineral composition, suitable for the long-term disposal. Series of Cs + - and Sr 2+ -exchanged NaP1-containing sorbents were subjected to the thermochemical transformation in the temperature range 40-1100°C at atmospheric pressure in air and the progress of reaction was monitored by DSC and XRD analysis. It was shown that initial sodium zeolite undergoes two-step transformation at 736-785°C and 892-982°C forming nepheline as the principle product, with the conversion temperatures being dependant on the heating rate. The thermal treatment of Cs + -bearing zeolite sorbent led to formation of a complex multiphase system, the principal components of which were nepheline and pollucite. Increasing cesium content in the samples led to a monotonous shift of crystallization peak to the higher temperature range (1005-1006°C). A more complicated behavior was observed for Sr 2+- containing samples, for which the crystallization temperature tends to increase (compared with NaP1) at lower Sr contents, but it starts decreasing parallel to the Sr 2+ content at Sr 2+ loadings >10 mg/g. The principal crystalline phases in Sr-NaP1 sample conversion were nepheline and Sr 2+ -containing feldspar, the quantity of which increased parallel to the increase of strontium content in zeolite. Apparent activation energies of thermochemical transformations were calculated and possible approaches to reduce transformation temperature are discussed and experimentally illustrated.