No abstract

In spite of many studies of kaolinite synthesis, questions remain as to the transformation of gel into kaolinite, the kinetics of the reaction, and the influence of solution chemistry. The purpose of the present study was to perform a hydrothermal synthesis in order to understand better the transformation from boehmite to kaolinite. Kaolinite was synthesized from amorphous SiO2 and Al(OH)3·xH2O at fixed temperature (250°C) and pressure (30 bar). The initial pH of the solution was 2. The reaction time for the synthesis was varied from 2 to 36 h. The physical properties of synthesized kaolinite were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), nuclear magnetic resonance (NMR) spectroscopy, field-emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and energy dispersive spectrometry (EDS).The early stage of kaolinite synthesis followed activation of amorphous Al(OH)3·xH2O to initiate the reactions, i.e. ionization and subsequent crystallization of boehmite. The boehmite reacted continuously with Si4+ dissolved in solution and gradually transformed to disordered, lath-shaped boehmite. In XRD and IR patterns, the typical peaks of boehmite were weakened or disappeared following the reaction.Structural transformation from boehmite to kaolinite occurred when the Al/Si ratio of the aluminosilicate was 1.0. The kaolinite formed was in the form of curved flakes and its crystallinity increased with reaction time. In the final stage of reaction the morphology of kaolinite changed from flaky to polygonal. The hexagonal, platy kaolinite was therefore developed to allow the gradual variation of the chemical composition, crystal structure, and morphology.

Iron incorporated zeolites were prepared from Na-A type zeolite synthesized from melting slag and FeCl 3 solution and applied as adsorbents for NH 3 and H 2 S gases. Iron incorporated zeolite was pelletized, calcined and used for gas adsorption experiment. XRD analyses of the zeolite revealed that Fe 3þ concentration of solution more than 90 mM could destruct the zeolite network structure. It was observed that the gas adsorption capacities of these zeolite pellets depend significantly on iron concentration of solution and calcination temperature. The type of binders affected a little on gas adsorption capacities. From adsorption results, adsorption capacity for NH 3 was proportional to Fe 3þ concentration and it was increased with calcination temperature up to 500 C, but it was decreased over 600 C. Pellets prepared from 56 mM Fe 3þ solution calcined at 500 C showed highest ammonia adsorption capacity (3.7%). This iron incorporated Na-A type zeolites showed much higher adsorption capacities for NH 3 (2:4$3:7%) than commercially available activated carbons (0:16$0:44%) and zeolites (0:23$0:60%). In the case of adsorption capacities for H 2 S, adsorption capacity was proportional to Fe 3þ concentration and inversely proportional to calcination temperature. Pellets prepared from 78 mM Fe 3þ solution calcined at 200 C showed highest hydrogen sulfide adsorption capacity (1.5%). Adsorption capacity (0:2$1:5%) was found to be lower than that of the commercial activated carbons (1:2$2:4%) and higher than that of commercial zeolites 4A (0.15%) and 13X (0.91%). Higher ammonia adsorption in iron incorporated Na-A zeolites could be possibly due to development of number of acid sites on the zeolites surface due to incorporation of Fe 3þ ion.

Dioctahedral smectite was prepared hydrothermally from dickite [Al2Si2O5(OH)4] as a starting material by autoclaving in a closed stainless steel vessel with variable temperature, pressure, time and pH conditions. Highly crystalline smectite can be obtained at 290°C under a pressure of 69 bar for 48 h. The pH of the solution was an important factor and should be maintained at 10 to 11 for the successful formation of smectite. Characterization by X-ray powder diffraction, scanning electron microscopy, infrared spectroscopy, thermogravimetric analysis, differential thermal analysis and the Greene-Kelly test showed that the smectite synthesized was Na-beidellite, mostly because of the heat treatment of the starting material and the stoichiometric batch composition.

No abstract