Hydrothermal transformation

In the wetted condition, hydrothermal transformations of the waste form may occur at the temperature of service. Changes may occur either by dissolution and re-precipitation or by assimilation of water into the internal structures of the oxide matrices. The thermochemical essentials for hydrothermal recrystallisation have already been referred to previously in this paper. We also need to develop an adequate knowledge of the water-catalysed structural transformations. 

Magnetite transforms to maghemite (and thence to hematite) in water or alkali under hydrothermal conditions. Conversion to maghemite also involves outward migration of cations via cation vacancies (Swaddle Oltmann, 1980). The hydrothermal transformation is slower than that in air at the same temperature (180 °C) and it has been suggested that this is because the cation vacancies which assist cation diffusion are reduced or eliminated by the large excess of water. 

Hydrothermal transformation of ferrihydrite in a teflon bomb at 180 °C for several days yields platy crystals up to several pm in size (Schwertmann and Cornell, 2000). 

Wolska, E. Szajda.W. (1987) Temperature effects on coprecipitated Al, Fe-hydroxides during hydrothermal transformation. J. Therm. Anal. 32 797-805... 

The hydrothermal transformation process, described with the crystallization curves shown in Figure 3.10, was carried out using zeolite Na-HC (the chemical and phase composition of this sample was given in Tables 4.1 and 4.2 besides, in Figure 3.11, the SEM micrograph corresponding to sample HC [119] is shown) with the following procedure the Na-HC was hydrothermally treated with a 7.5M NaOH and 0.6M NaCl (liquid to solid ratio = 1) solution and statically heated at 373 K in methyl polypropylene bottles [119]. 

FIGURE 3.12 SEM micrograph of the Na-Y Zeolite formed after 16h of hydrothermal transformation (bar = 1 pm). 

In Figure 3.13, the XRD powder patterns of the transformation products, Na-FCSW(6h) (Figure 3.13a), Na-FCSW(12h) (Figure 3.13b), and the pattern of the Na-Y commercial sample CBV100 (Figure 3.13c), with Si02/Al203 = 5.2, provided by the PQ Corporation Malvern, PA, USA, which was used as a standard for comparison, are shown [126], The XRD powder patterns show that the transformation products are Na-faujasite zeolites, with 80% crystallinity compared with the standard (CBV-100). The analysis by x-ray diffraction of all the samples lets us confirm that the maximum yield of the hydrothermal transformation procedure was achieved at approximately 9h. 

At the beginning of the hydrothermal transformation, the spectrum consists of a single, broad low-field-shifted resonance line centered at 100 ppm (chemical shifts were measured from natrolite). As the hydrothermal treatment proceeds, the spectra show five well-resolved lines at -85.2, -89.3, -94,0, -98.9, and -103.5ppm, corresponding to Si surrounded by 4, 3, 2,1, and 0 Al, respectively [81]. 

These samples were previously studied with the help of MAS-NMR (see Section 4.6.6). It was shown that at the beginning of the hydrothermal transformation, the spectrum consists of a single,... 

It should be borne in mind that the conditions ol extrusion sometimes lend themselves well to certain hydrothermal transformations... 

In the 1950s, de Boer and his co-workers (de Boer et al., 1954, 1956 de Boer, 1957) used a variety of techniques in their studies of the thermal dehydration of gibbsite and bayerite and a more detailed picture was obtained of the conditions under which the two decomposition routes were manifested. For example, it was shown that relatively well-crystallized boehmite could be produced by the treatment of gibbsite or bayerite in saturated steam at temperatures of c. 165°C. These and other findings provided qualitative confirmation that the formation of boehmite involved an intragranular hydrothermal transformation. 

Solvothermal dehydration can avoid this limitation, and dehydration may proceed at a temperature much lower than that required by the hydrothermal reaction. However, thermal dehydration may compete with the solvothermal reaction. When dehydration of hydroxide starts, water formed by dehydration of the starting material may facilitate hydrothermal transformation of the starting material. Therefore complicated reactions may occur simultaneously. As an example, the reaction of gibbsite (a polymorph of aluminum hydroxide, Al(OH)3) in alcohols at 250°C is explained. ... 

Cubic and platelet hematite particles are produced by transformation from other particles in aqueous solutions. In this procedure, preformed particles are recrystallized from other precipitates into their hnal forms. Large cubic-like hematite particles were produced through conversion of previously generated a-FeOOH in acidic solutions of HCl at 373 K. Plate-like hematite particles were formed from a-FeOOH or Fe(OFl)3 under strong basic conditions at elevated temperatures. " There was a critical temperature for each alkaline concentration in the hydrothermal transformation. The platelets produced were single crystals with (001) flat planes. ... 

Hydrothermal transformation of various Fe oxides. Ferrihydrite (2-line), lepidocrocite, akaganeite and goethite (if poorly crystalline) can be converted to large (1-3 am) hexagonal plates of hematite if kept under water in a teflon bomb at 180 °C for 10 days. 

Almost 50 zeolite types occur naturally, of which several have attained industrial importance. Natural zeolites result indirectly from volcanic activity. They are formed by hydrothermal transformation of basalts, volcanic ash and pumices and are found e.g. in basalt cavities and in large sedimentary deposits. The industrially most important natural zeolites are ... 

Eberl, D. and Hower, J., 1977. The hydrothermal transformation of sodium and potassium smectite into mixed-layer clay. Clays Clay Miner., 25 215—227. 

Facile methods have been developed to synthesize single-crystalline manganese oxide and oxyhydroxide nanowires/nanorods. Pure phase of single-crystalline y-MnOOH nanowires/nanorods have been synthesized by hydrothermal transformation of granular y- or p-MnOa in water at 180 - 200 °C, and pure P-MnOa and a-MnaOa nanowires/nanorods formed after subsequent calcination at... 

P-36 - Hydrothermal transformation of a layered silicate, Na-magadiite, into mordenite zeolite... 

Hydrothermal transformation of a layered silicate, Na-magadiite, into a commercially important mordenite zeolite is achieved, at 175 °C, using tetraethylammonium hydroxide as the intercalating and/or structure directing agent. Samples were characterized by XRD, IR, SEM, DTA and N2 adsorption measurements. The results indicate that the mordenite samples obtained by this new route are highly crystalline, more uniform and smaller in size (0.1 -0.2 pm). 

Clinoptilolite, hydrothermal transformation 32-P-07 Clinoptilolite for lead removal 31-P-06... 

Diaspore, which is thermodynamically more stable than boehmite, can also be obtained by hydrothermal transformation of gibbsite or boehmite, but its formation is slow. Higher temperatures (i.e., >200 °C) and pressures (>15 MPa) are required for the synthesis, and the presence of diaspore seed crystals helps to avoid boehmite formation. Methods to produce weU-crystaUized diaspore have been reported these include hydrothermal synthesis at 300 °C and 3.45 x 10 Pa (5000 psi) over a 72-h period (67) or high-pressure calcination of boehmite (68). 

Autoclaving is performed in a batch reactor under high pressure and different temperatures. There are thermal and hydrothermal transformations in the presence of steam water, resulting in structural changes. In the presence of a solvent, the precipitated solid suffers structural and textural modifications, but in principle the main steps are ... 

Chermak, J.A., Rimstidt, ID. (1990). Hydrothermal transformation rate of kaolinite to muscovite/illite. Geochimica et Cosmochimica Acta, 54, 2979-2990. 

Thereafter, the sodium aluminosilicate hydrogel is hydrothermally transformed into the highly-ordered crystalline sodium zeolite 4A (see Equation 22.6) [103,109,119,122,142]. 

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