Leaching titanate

In 1990, appioximately 66,000 metric tons of alumina trihydiate [12252-70-9] AI2O2 3H20, the most widely used flame retardant, was used to inhibit the flammabihty of plastics processed at low temperatures. Alumina trihydrate is manufactured from either bauxite ore or recovered aluminum by either the Bayer or sinter processes (25). In the Bayer process, the bauxite ore is digested in a caustic solution, then filtered to remove siUcate, titanate, and iron impurities. The alumina trihydrate is recovered from the filtered solution by precipitation. In the sinter process the aluminum is leached from the ore using a solution of soda and lime from which pure alumina trihydrate is recovered (see Aluminum compounds). 

The overall development program included the study of other exchange materials such as niobates, zirconates, and tantalates, some of which had superior ion exchange and leaching properties, but were initially economically unattractive as compared to the titanates. These alternate materials will be briefly discussed along with applications to nuclide stabilization in other areas of nuclear processing. 

Wide variations in Cs and Na leaching observed in some of the first samples produced were found to result from the formation of the corresponding molybdate compounds which are highly water soluble. This problem was eliminated by the addition of 1-2% by weight of elemental silicon which served as a reducing agent and prevented molybdate formation. The silicon was added during the preparation of the sodium titanate material. 

During development, evaluation of the consolidated materials was based primarily on two criteria, leachability and the concentration factor, i.e., the concentration of waste oxides on a volume basis. The concentration factor is directly affected by the residual porosity in a consolidated waste as well as by the dilution caused by the addition of consolidation aids. This factor can be as high as 1.2 g/cm3 for a fully dense ( 5 g/cm ) titanate waste prepared from the projected Barnwell plant solution composition. The factor is slightly lower for a titanate waste containing silicon and zeolite additions, which has a typical density of U.2 g/cm3. The leachability was determined by an "instantaneous leach test developed for fast, comparative evaluations of materials, the details of which are described elsewhere (l6). 

A long term leaching study to compare the behavior of a titanate waste with glassy waste forms and to attempt to elucidate a leaching mechanism has been in progress for l6 months (15) ... 

Based on dissolved ions only, the titanate waste showed an overall leach rate of x 10 5 g/cm day and a rate of 5 3 x 10 7 g/cm day for the fission waste oxides only. The results indicate that the leaching which is occurring is associated with the silicate phases in the ceramic, i.e., the Si02 formed from the silicon and the zeolite. The glass samples showed overall leach rates of 6-15 x 10 5 g/cm day and fission waste oxide leach rates of 1.8-2.7 x 10 g/cm day, where the higher rates in both cases were observed in the phosphate-containing glass. 

The preparation, composition, structure and leaching characteristics of a crystalline, ceramic radioactive waste form have been discussed, and where applicable, compared with vitrified waste forms. The inorganic ion exchange materials used such as sodium titanate were prepared from the corresponding metal alkoxide. The alkoxides were reacted in methanol with a base containing the desired exchangeable cation and the final powder form was produced by hydrolysis in an acetone-water mixture followed by vacuum drying the precipitate at ambient temperature. 

Leaching of contaminants Eission products Tc, partitioned from high-level waste tanks, debris from contaminated pipes from K-25 plant at Oak Ridge Cs, from salt supernatant and sludge, silico-titanates, and wastewater Radioactive components Ra, Fernald silo waste, transuranics, simulated and actual Rocky Flats ash waste, wastewater... 

An unusual precipitation uses a molten salt as solvent. For the preparation of lead zirconate titanate, Ti02, Zr02, and excess PbO are mixed with a NaCl-KCl mixture and reacted at 1000 C. Lead zirconate titanate is less soluble in the molten salts than is Ti02 or Zr02 and precipitates. Finally, the salt and excess PbO are leached out using an acetic acid solution. 

Titanates of alkaline earth metals undergo selective leaching of alkaline earth metal cations at acidic pH. The composition of commercial products is usually different from the idealized formula. 

Results of another experiment support this interpretation. An attempt was made to extract the titanium from a cogelled catalyst and from a titanated silica, each containing the same amount of Ti (2 wt%) and calcined at the same temperature, 600 °C. Presumably, if the titanium were in the bulk it would not be leached out readily. An acidic solution of hydrogen peroxide was used to perform the extraction. More than 90% of the titanium could be removed from the titanated silica, but only 58% could be extracted from the cogel. Thus, cogelled titania was less accessible, but even in the cogel most of it could be removed. Apparently, even in cogel, the titanium has some preference for the surface. 

A second-generation immobilization material, synroc, is in development. This synthetic rock, based on mixed titanate phases such as zirconolite, hollandite, or perovskite, incorporates the HLW elements into its crystal structure, yielding excellent chemical stability. Synroc features leach rates more than an order of magnitude lower than borosilicate glass. 

The application of nuclear methods to studies of ceramic surfaces and interfaces is a well-developed methodology, although not as yet applied extensively to mineral surfaces. Relevant applications of RBS, PIXE, NRA, ERDA and high-resolution a-spectroscopy have been described in the review by Matzke [9.1]. Analyses by RBS, used in the channeling mode, and NRA have revealed structural defects in U Ov-y RBS is sensitive mainly to the metal atom sublattice. whereas NRA can be used selectively to study the nonmeial sublattice. It was shown that the U sublattice is nearly identical to that of UO2, whereas significant cluster formation occurs in the O sublattice these observations have implications for surface sites in the material. Investigations of surface reactions (e.g., leaching and dis.soIution) of titanates and UO2 by nuclear techniques have... 

Recently [211], a post-synthesis modification of zeolite beta consisting of separate dealumination and titanation steps has been reported. First hydroxyl nests were formed by removal of up to 90% of the aluminum by leaching with oxalic or nitric acid, than up to 2 wt.% titanium was inserted into the lattice vacancies without formation of Ti02 as a second phase by treatment with gaseous TiCl4 at 500 °C. 

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