Synthesis of titanosilicates from leucoxene ores

Natural tectotitanosilicates as compounds with ion-exchange properties are of particular practical interest in connection with the problem of radioactive waste disposal. Tectotitanosilicates are widely used in industry as selective adsorbent of radionuclides (Cs, Sr, Eu) from radioactive solutions. In the industry zeolites have been widely used now for these purposes as natural microporous materials with tetrahedral structure. Minerals with hetero-frame structures are more resistant to radiation influence and change of acid-base properties of aqueous solutions than zeolites. The unique properties of titanosilicates are caused by existence of ion-exchange functional groups and specific geometrical lattice constants. The aim of the study is the assessment of a possibility of obtaining titanosilicates with hetero-frame structure from the siliceous-titanium concentrate being a product of processing of leucoxene ores. The results of the hydrothermal synthesis of a series of tectotitanosilicates with the structure of sitinakit and natisit are presented in the paper. Hydrothermal autoclave synthesis was used as the main synthesis method. Siliceous-titanium concentrate prepared according to the original ammonium fluoride technology of leucoxene ores processing developed in the Institute of Geology of Komi Science Center was used as a template. The process of titanosilicates forming is carried out in a teflon-lined autoclave at an elevated temperature. For the synthesis a 100 ml autoclave was chosen, the degree of filling was 80 %. Sodium hydroxide solution was used as mineralizer. We investigated the influence of mineralizer concentration on the structure of the obtained titanosilicates, the molar ratio of SiO₂: TiO₂, and temperature as a key parameter of the synthesis providing stability of a crystallization. The synthesized samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC). It was found that the molar ratio of the basic components - silica and titanium oxides (SiO₂: TiO₂) in the hydrated precipitate influences the formation of the crystalline structure of the products. Formation of titanosilicates with natisit structure takes place at a molar ratio of 3:1, with a ratio of 1:1 the sitinakit phase is formed. In both cases, the alkaline environment and a relatively high temperature of 250°С provide stability of crystallization. Temperature decrease leads to a synthesis of samples with less crystallinity, but does not affect the phase composition.

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