Undesirable iron oxides

The synthesis route chosen for a particular product is governed by all the above considerations. There are three major synthesis methods for industrial pigments  

Solid state transformations including thermal decomposition of iron salts and oxide-hydroxides. This method produces red, black and brown pigments.  

The organic reduction process also termed the aniline, laux or nitrobenzene process. This method leads to black, yellow or red pigments. 

Precipitation of soluble Fe salts with alkali followed by oxidation. There are two variations of this method. The pigments obtained may be yellow, red, orange or black. 

Other methods including hydrothermal precipitation, flame hydrolysis, thermal decomposition of Fe(CO)s and high temperature reaction of Fe chloride with iron, are used only on a small scale to obtain specialty products (see Chap. 19). 

---

Thermal decomposition of iron pentacarbonyl. Very finely divided red iron oxide is obtained by atomizing iron pentacarbonyl, Fe(CO)5, and burning it in excess of air. The size of the particles depends on the temperature (580-800 °C) and the residence time in the reactor. The smallest particles are transparent and consist of 2-line ferri-hydrite, whereas the larger, semi-transparent particles consist of hematite (see Chap. 19). The only byproduct of the reaction is carbon dioxide, hence, the process has no undesirable environmental side effects. Magnetite can be produced by the same process if it is carried out at 100-400 °C. Thermal decomposition of iron pentacarbonyl is also used to coat aluminium powder (in a fluidized bed) and also mica platelets with iron oxides to produce interference or nacreous pigments. 

Iron oxides may be either beneficial or undesirable. Everyone is familiar with rust, a mixture of various Fe oxides, as the end product of corrosion of iron. The modem steel industry, on the other hand, relies on the huge deposits of hematite and magnetite (iron ores) found in many parts of the world. 

The inhibition of adsorbed nucleic acids after the desorption process can be attributed to the following factors (1) the possible release of undesirable impurities originating from the particles, such as bare iron oxide nanoparticles, ferric or ferrous ions, surfactant, etc., and (2) the desorption of adsorbed inhibitor initially present in the biological sample under study. 

CsHg/NOx molar ratio of consumed propane and reduced NO was between 0.5 and 1.0 for all three iron zeolites up to this temperature. Moreover, the seleetivily of the SCR reaction over Fe-FER and Fe-BEA was nearly stable up to 450 C, while steeply decreasing with temperature for Fe-MFI. Thus, at 450 °C nearly 7 molecules of CaHg are formally consumed for 1 molecule of converted NOx over Fe-MFI, and stiU only about 1.5 molecule of CaHg for Fe-FER. That is obviously due to unselective oxidation of the propane at higher temperatures, connected to the undesirable activity of the iron oxide species evidenced in Fe-MFI catalyst by EPR and Mossbauer spectroscopy. 

This catalyst has been optimized over the years [80-82], and the best support was found to be acetylene black due to its highly olefinic nature. Palladium was initially chosen as the main catalytic metal, due to its high activity and low cost. This was improved by promoting it with a small amount of platinum however, this catalyst was too active and yielded unwanted side products via reactions such as ring hydrogenation. The selectivity of this catalyst was then corrected by the addition of iron oxide, which impeded the undesired reactions. Iron has also been proven to be a promoter for the hydrogenation of aliphatic nitro compounds [83]... 

Where neither of these two methods may be applied, a third and technically undesirable process is sometimes used. This involves solvent extraction of the iron oxide cake, which brings with it filtration problems and costly solvent recovery. Where a cheap solvent, such as naphtha, can be used, these difficulties are somewhat mitigated. Where a water-miscible solvent can be used, it is usually added to the reduction mixture initially, as discussed above. 

As some impurities may remain in the glass, namely iron oxides FeO or Fc203, an undesirable coloration can be observed, which can be eliminated in two steps ... 

Many phosphate ores have high iron contents that render them undesirable for chemical processing into fertilizers. Iron minerals are often present as iron oxides coating the surface or interlocking with the phosphate grains. Complex iron-containing silicates (such as am-phiboles, micas, and clays) are another major source of iron in most phosphate ores. Iron may also be present as iron phosphates. 

Inviscid melt spinning is considered to be a potentially viable alternative to wire drawing [51] for making steel wires for radial automobile tires, but a prior product development did not reach beyond the pilot plant level. Using silica steels, the complex chemistry (Equations 4-6) produce also minute amounts of iron oxides which were detected by ESCA [51], and are a potentially undesirable trace byproduct. The challenge [4] remains to fine tune the chemistry of this process before commercial development. 

Welding fluxes are used to prevent, dissolve or facilitate removal of oxides and other undesirable surface substances. Welding rod is a form of filler metal used that does not conduct an electric current. Welding alloys may be aluminium powder with iron oxide, nickel, manganese or steel. 

The catalytic data also indicated a strong dependence of the low hydrocarbons conversion on the catalyst elemental composition. For the Lai, s Fe03 g perovskites, a progressive Fe20s enrichment of the surface was evidenced by XPS characterization when decreasing the lanthanum content of the solid. Such additional undesirable surface iron oxide enrichment induced an inhibiting effect on the catalytic activity. As a result, the most efficient lanthanum iron-based perov-sldte was the stoichiometric LaFe03 mixed oxide [40]. 

The oxide-layer network is present essentially in the bulk of the catalyst. The significant amount of surface iron oxides found in some samples of the present study can be attributed in part to a thick layer (=incomplete reduction) of network oxides and must be ascribed in part to individual segregated oxide crystals. These segregates, which have been identified by electron microscopy as well as by differential charging in the XPS (see, e.g.. Fig. 2.42), may be binary or ternary oxides. Whatever their structure, they play the role of spectator species, which have been formed as an undesirable consequence of an inappropriate choice of reduction conditions. 

A different approach for the preparation of iron oxide powders has been presented by Liu et al. first, an iron hydroxide precipitate was obtained after addition of aqueous ammonia to a solution of iron nitrate in water. Then the precipitate was dialyzed to remove undesired ions, and after peptization a stable sol was obtained. Crystalline a-Fe20s was prepared by evaporating the solution and annealing the resulting gel at high temperatures. These powders were then used to prepare conductometric sensing devices that show good sensitivity and selectivity for ethanol detection at temperatures lower than 400 °C [27]. 

When the steam to carbon ratio in the reforming section is reduced, the conditions in the shift section must be carefully evaluated. If the steam to dry gas ratio becomes too low, severe problems may arise due to conversion of the iron oxide in the high temperature shift catalyst to iron carbide, which will promote formation of undesirable by-products (hydrocarbons and oxygenates) (see Sect. 6.3.3.1). 

One further point about the need to keep the iron oxide content of MgO grains low. In the presence of carbon in magnesia-carbon refractories, iron oxide reacts with carbon therefore, this carbon oxidation-iron oxide reduction reaction contributes to the loss of carbon from the refractory, an undesirable effect that is explored in a later section. 

Caro s acid has been used ia AustraUa as an oxidant ia the acid-leaching of uranium ores. It acts by oxidising the iron present ia the solution from Fe " to Fe ". This Fe " then oxidizes the uranium. Alternative oxidants that have been used iaclude pyrolusite and chlorate ion. These are both undesirable because their effluents, containing Mn " or CF, contaminate watercourses. 

---