Hematite shape control

The seeding technique can also be used for controlling the size of the monodis-persed hematite particles. If we combine seeding technique with a shape controller such as sulfate or phosphate ions, the size and shape with different aspect ratios of... 

Hematites with a particular crystal shape (plates, needles, spindles, pseudocubes, peanuts) and a narrow size distribution (monodisperse) can be obtained by adding various chemicals (shape controllers) to the system. The mechanism behind the control of shape is most likely to be the adsorption of impurities on certain crystal faces thereby reducing their growth rate in favor of that of the other faces. Internally these crystals may be either mono- or polydomainic, depending on the type and concentration of the additive. It must be kept in mind that higher additive concentrations may lead to product contamination. Some examples are summarized in the following section. 

Iron Oxide Reds. From a chemical point of view, red iron oxides are based on the stmcture of hematite, a-Fe202, and can be prepared in various shades, from orange through pure red to violet. Different shades are controlled primarily by the oxide s particle si2e, shape, and surface properties. Production. Four methods are commercially used in the preparation of iron oxide reds two-stage calcination of FeS047H2 O precipitation from an aqueous solution thermal dehydration of yellow goethite, a-FeO(OH) and oxidation of synthetic black oxide, Fe O. ... 

The liquid-phase reduction method was applied to the preparation of the supported catalyst [27]. Virtually, Muramatsu et al. reported the controlled formation of ultrafine Ni particles on hematite particles with different shapes. The Ni particles were selectively deposited on these hematite particles by the liquid-phase reduction with NaBFl4. For the concrete manner, see the following process. Nickel acetylacetonate (Ni(AA)2) and zinc acetylacetonate (Zn(AA)2) were codissolved in 40 ml of 2-propanol with a Zn/Ni ratio of 0-1.0, where the concentration of Ni was 5.0 X lO mol/dm. 0.125 g of Ti02... 

Once particles have been produced, it is possible to elucidate reasons for the observed variations in their habits. Specifically, different geometries of hematite particles were explained by the complex mechanisms of their formation, which consist of first precipitating akageneite precursors, subsequently undergoing controlled aggregation and structural transformation into hematite (95). Indeed, it was indicated that the morphological properties of the precursors were responsible for different shapes of the final products. 

Early catalysts were produced from calcined ferric oxide, potassium carbonate, a binder when required, and usually chromium oxide. Subsequently a wide range of other oxides replaced the chromium oxide typical compositions are shown in Table 7.5. The paste was extruded or granulated to produce a suitable shape and then calcined at a high temperature in the range 900°-950°C. Solid solutions of a-hematite and chromium oxide (the active catalyst precursors) were formed and these also contained potassium carbonate to inhibit coke formation. Catalyst surface area and pore volume were controlled by calcination conditions. It has been confirmed by X-ray diffraction studies that a-hematite is reduced to magnetite and that there is some combination of potash and the chromium oxide stabilizer. There is little change in the physical properties of the catalyst during reduction and subsequent operation. 

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