Iron oxide, precipitation properties

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 different oxidation states of a metal can have dramatically different chemical properties, which in turn affect their biogeochemical forms and significance. For example, almost 4 g/L ferrous iron, Fe(II), can dissolve in distilled water maintained at pFi 7.0. However, if the water is exposed to air and the iron is oxidized to Fe(III) essentially all the iron will precipitate, reducing the soluble Fe concentration by more than eight orders of magnitude. Oxidation state can also affect a metal ion s toxicity. For instance, the toxicity of As(III) results from its ability to inactivate enzymes, while As(V) interferes with ATP synthesis. The former is considerably more toxic to both aquatic organisms and humans. 

Andreeva and coworkers518 improved on the preparation of Au/a-Fe203 by using a deposition-precipitation method. The preparation involved deposition of Au hydroxide with a solution of sodium carbonate at 60 °C and pH = 8.0 on a fresh precipitate of iron hydroxide. The precipitate was aged for 1 hour at 60 °C, filtered, washed, dried under vacuum at 80 °C, and calcined in air at 400 °C for 2 hours. A comparison of properties and activities is provided in Table 122. The iron oxide crystallites were about 10 nm in both samples investigated. 

The precipitation process serves to produce soft, yellow, orange, black or red iron oxides with very pure hues and good wetting properties. In contrast to the solid state... 

Hydrothermal Method. Iron [Fe(III)], barium, and the dopants are precipitated as their hydroxides and reacted with an excess of sodium hydroxide solution (up to 6 mol/L) at 250-350 °C in an autoclave. This is generally followed by an annealing treatment at 750-800°C to obtain products with the desired magnetic properties. Many variations of the process have been described [5.36]-[5.40], the earliest report being from 1969 [5.41], In later processes, hydrothermal synthesis is followed by coating with cubic ferrites, a process resembling the cobalt modification of iron oxides (see Section 5.1.2). The object is to increase the saturation magnetization of the material [5.42]-[5.44],... 

Since the pH directly controls the degree of super-saturation, at least if hydroxides arc precipitated, this should be one of the crucial factors in precipitation processes. As for many other parameters, the influence of pH is not a simple one and it has to be investigated experimentally for a specific system. Even in a relatively well-known system such as iron oxide [30] the effect which the precipitation pH has on the properties of the final product is not yet clear. There is no clear relationship between the precipitation pH and the textural and catalytic properties of the precipitate. It is relatively well known what phases are formed under specific conditions, due to the importance of iron... 

Research on the chemical properties of humic substances was extended by the Swedish investigator Berzelius (1839). One of his main contributions was the isolation of two light-yellow-colored humic substances from mineral waters and a slimy mud rich in iron oxides. They were obtained from the mud by extraction with base (KOH), which was then treated with acetic acid containing copper acetate. A brown precipitate was obtained ctilled copper apocrenate. When the extract was neutralized, another precipitate was obtained, called copper crenate. The free acids, apocrenic and crenic acids, were then brought into solution by decomposition of the copper complexes with alkali. These newly described humic substances were examined in considerable detail, including isolation, elementary composition, and properties of their metal complexes (Al, Fe, Cu, Pb, Mn, etc). 

Iron (Fe, at. mass 55.85) occurs in solution in the II and III oxidation states. Compounds of Fe(III) are generally the more stable. Fe(OH)2 is precipitated at above pH 7.5, and Fe(OH)3 at above pH 2-3. Neither of the hydroxides shows acidic properties. Iron(II) exhibits properties similar to those of Ni(II) and Zn(II), forming a stable cyanide complex. Iron(III) forms fluoride, chloride, cyanide, EDTA, tartrate, and oxalate complexes. In acid media, iron(III) acts as an oxidant. 

Bulk iron oxide was prepared by adding an ammonium hydroxide solution over an aqueous solution containing Fe(N03)3. A colloidal precipitate was obtained (Fe203.3H20), which was dried at 100°C for 12 h. The precursor was then calcined at a fixed temperature (500, 600 or 800 C) for 6 hours. The catalysts prepared in this way were subjected to sintering either during reaction or in an air atmosphere (pre-sintering). In the latter case, 1 g of catalyst was placed in a tubular quartz reactor under 90 ml(STP)/min at 600°C. At certain time intervals, catalyst samples were extracted to measure their activity and physical properties. 

The metal is produced on a massive scale by the Hall-Heroult method in which alumina, a non-electrolyte, is dissolved in molten cryolite and electrolyzed. The bauxite contains iron, which would contaminate the product, so the bauxite is dissolved in hot alkali, the iron oxide is removed by filtration, and the pure alumina then precipitated by acidification. Molten aluminum is tapped off from the base of the cell and oxygen evolved at the anode. The aluminum atom is much bigger than boron (the first member of group 3) and its ionization potential is not particularly high. Consequently aluminum forms positive ions AP. However, it also has non-metallic chemical properties. Thus, it is amphoteric and also has a number of covalently bonded compounds. 

Classification Syn. iron oxide Definition A precipitated pigment of finer particle size and greater tinctorial str. than naturally occurring oxides Empirical Fe20s H2O Properties Yel. 

The effect of pigment shape on the properties of an industrial latex coating was studied in detail. Two red iron oxide pigments with similar properties were studied - one rhombohedral (produced by precipitation) and the other spheroidal (produced by calcination). They were added in the same volume concentration to the coating formulations and several liquid, dry-film, wet film and free-film properties were determined. 17 refs. 

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