Iron oxide, precipitation hematite

During other electrode reactions, however, sparingly soluble non-metallic oxides are precipitated. If the precipitation forms a dense coating it may result in passivation of the electrode. An important example of this phenomenon is passivation of iron by electrochemical precipitation of iron oxide Fe203 ( hematite ) or Fe304 ( magnetite ). 

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. ... 

Wet preparation of red iron oxides can involve either a hydrothermal process (see Hydrothermal processing) or a direct precipitation and growth of iron oxide particles on specially prepared nucleating seeds of Fe202- In the hydrothermal process, iron(II) salt is chemically oxidized to iron(III) salt, which is further treated by alkahes to precipitate a hydrated iron(III) oxide gel. The gel can be dehydrated to anhydrous hematite under pressure at a temperature around 150°C. 

Transparent red iron oxide is composed mainly of hematite, a-Ee202, having primary particles about 10 nm. It is prepared by a precipitation reaction from a dilute solution of an iron salt at a temperature around 30°C, foUowed by a complete oxidation in the presence of some seeding additives,... 

Figure 1.1.20 shows the differential thermal analysis (DTA) data for the cores, of chromium hydrous oxides particles prepared in the absence of hematite, and of coated particles. It is obvious that the latter behave as the coating material, when alone. This example clearly indicates the possibility of having the surface site characteristics of chromium hydrous oxide induced onto ellipsoidal iron oxide particles. The latter morphology cannot be achieved by diiecl precipitation of the same chromium compound. 

Transparent red iron oxides containing iron oxide hydrate can also be produced directly by precipitation. A hematite content of > 85 % can be obtained when iron(II) hydroxide or iron(II) carbonate is precipitated from iron(II) salt solutions at ca. 30 °C and when oxidation is carried out to completion with aeration and seeding additives (e.g., chlorides of magnesium, calcium, or aluminum) [5.271], Transparent iron oxides can also be synthesized by heating finely atomized liquid pentacarbonyl iron in the presence of excess air at 580-800 °C [5.272], [5.273]. The products have a primary particle size of ca. 10 nm, are X-ray amorphous, and have an isometric particle form. Hues ranging from red to orange can be obtained with this procedure, however, it is not suitable for yellow hues. 

A third chemical weathering mechanism that is of importance is oxidation/ reduction that involves mainly the elements carbon, iron, manganese and, of course, oxygen. An equilibrium reaction between dissolved C02 and bicarbonate ions can lead to the precipitation of ferrous iron, giving a hematite (ferric oxide) precipitate ... 

Can this model published in 2003 (Marion et al. 2003a) explain all the geochemical findings of the 2004 Mars Exploration Rover (MER) missions Not exactly In our model we predicted that ferrous iron would precipitate as siderite (FeCOo) early in the temporal sequence, and siderite would ultimately be oxidized to ferric minerals such as ferrihydrite [Fe(OH)3] and hematite (FeoOo) (Fig. 5.10). There is no place in this conceptual model for the precipitation of ferrous or ferric sulfate minerals as suggested by the MER missions (Squyres et al. 2004 Lane, 2004). This problem could be simply rectified by drawing an arrow from siderite through the surface acidification... 

It is also possible to obtain transparent red iron oxide with a hematite content of 85% directly by precipitation of iron(l 1) hydroxide or carbonate from iron(II) salt solution and oxidation with air at ca. 30 C in the presence of one of the chlorides of magnesium, calcium or aluminum [5.189]. 

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

Raman chemical imaging can be employed to access the homogeneity and the structural stability in terms of oxidation rates, onset of hematite, and organic contamination of as-precipitate and oxidized iron oxide samples. Oxidation-related Raman features have been established by comparative study of bulk oxides and nanoparticles attained in two different oxidation states, suggesting that the solid nanophase synthesized had a mixed magnetite-maghemite composition [52]. 

The molar ratio of Fc203/Fe0 in the BIFs is less than 1.0 in all but one of the 28 analyses of Isua iron formation reported by Dymek and Klein (1988). In the one exception the ratio is 1.17. Unless the values of this ratio were reduced significantly during metamorphism, the analyses indicate that magnetite was the dominant iron oxide, and that hematite was absent or very minor in these iron formations. This, in turn, shows that some of the hydrothermal Fe was oxidized to Fe prior to deposition, but that not enough was oxidized to lead to the precipitation of Fc203 and/ or Fe oxyhydroxide precursors, or that these phases were subsequently replaced by magnetite. 

Figure 9. Isotherm fits of the surface precipitation model for the sorption of ferrous iron on iron oxides ( ) magnetite, ( ) goethite, ( ) lepidocrocite, (A) hematite (for parameter values see Table I, Fe(II)soi = concentration of dissolved ferrous iron, rpe= concentration of surface-bound ferrous iron per total concentration of iron oxide, pH 7.2,1=20 mM, T=25 C, 25 m L l, teq=15 min) adapted from (7).

The lijima Refinery is the only refinery in the world which applies the hematite process for iron removal. This process is excellent for recovering Au, Ag, Cu, Pb and rare metals, and for the precipitation of iron oxide usable in cement production, because of its high iron content. Furthermore, we are trying to develop a new process to recover more rare metals and to purify the iron oxide to broaden its application base. 

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