Iron-alumina spinel

Although the fine structure hypothesis of the promoter action of alumina on iron appears well founded, it is conceivable that, in addition to the conservation of the iron surface, the beneficial effect of alumina involves also a modification of the valence forces at the borderlines between alumina (or the iron-alumina spinel A1203 FeO), and the crystallites of metallic iron (49). 

The ease of reduction and subsequent performance of the catalyst will be strongly influenced by the extent and nature of the interaction between the metal and the oxide support. In ammonia synthesis studies the most commonly used oxides have been alumina and magnesia. " Some work has also been carried out with basic materials such as calcia, etc., although they have little commercial potential due to their low surface areas. In the case of magnesia and alumina, high and reasonably stable dispersions can be produced but complete reduction is sometimes very difficult to achieve. In the case of coprecipitated iron/alumina the spinels formed can only be reduced at temperatures above 1073 The reducibility of the supported systems also tends to be retarded to a greater extent by low vapor pressures of water than less highly dispersed catalysts. ... 

Alumina is incorporated as a sohd solution of the iron aluminate spinel, hercynite, in the crystal lattice. The alumina concentration should be less than the solubility of alumina in magnetite. This corresponds to a maximiun content of about 3% alumina. Any excess of alumina does not go into solid solution, and leads to a reduction in catalytic activity, particitlarly when using catalysts promoted with alumina. The presence of alumina as a structural promoter also leads to the formation of wustite and stabihzes the reduced catalyst. Small amounts of magnesia can also dissolve into magnetite and act as a promoter. The calcium component exists in the form of ferrites or alirminates by neutrahzing acidic components—such as silica—and protects the potash that activates the catalyst. 

Copper (Cu) and zinc (Zn), when mixed, form the alloy brass. Magnesia (MgO) and alumina (AljOj) when mixed in equal proportions form spinel. Iron (Fe) and carbon (C) mix to give carbon steel. 

Wilson (1990) has provided some details of the microstructural evolution in this fiber. A fine-grained o-AljOj fiber is obtained by seeding the high temperature a-alumina with a very fine hydrous colloidal iron oxide. The fine iron oxide improves the nucleation rate of a-Al Oj, with the result that a high density, ultra-fine, homogeneous o-Al203 fiber is obtained. The rationale for seeding with iron oxide as follows. Basic salts of aluminiun decompose into transition aluminum oxide spinels such as above 400°C. These transition cubic spinels... 

For all these reasons an increase in chromium concentration may be the best solution to increase the sulphidation resistance of iron-aluminium alloys. The present work has shown that chromium has a very beneficial effect on the sulphidation resistance in oxygen deficient gases. Such improvement of sulphidation resistance in H2S environments has also been found in nickel-aluminium alloys, where the corrosion rate decreased significantly, when 8 wt% chromium were added [22], Whether chromium increases the rate of alumina formation or contributes to the formation of protective spinell type oxides Fe(Cr,Al)203 cannot be decided from the available results. It is also possible that chromium catalyses the transition from a less to a more protective type of A1203. It was for example found that chromium enhances the transition from 0-Al2O3 to a-Al203 in Ni-Al at temperatures above 1000°C [30],... 

Fourcroy and Hatiy suggested chromium as a suitable name for the element in recognition of the various colours shown by its derivatives, Greek khroma colour. The same year Vauquelin detected chromium in the spinel ruby while Taessert showed it to be an essential constituent of chrome iron ore or chromite, FeO.Cr2Os — now the main source of chromium compounds. The colour of the ruby is now usually attributed to its chromium content and artificial rubies are manufactured by fusing pure alumina with a little oxide of chromium, to colour it, in an... 

In the original work on catalytic ammonia synthesis, Haber [41] had used an osmium catalyst, but this metal was much too expensive to be the basis of the large-scale industrial plants. In the long search for alternatives to the Mittasch catalyst, alkali-promoted ruthenium was found to exhibit specific activity, which is even superior to the iron catalyst [42] and which was subsequently developed to an industrial catalyst [43]. The Mittasch catalyst is cheap and the alumina promoter provides a high specific surface area. This situation is different with Ru catalysts that are prepared as small particles on a suitable support. Figure 1.1 was a t)q)ical electron microscopic picture from such a catalyst particle on MgAl204 (spinel) support [44]. 

The other ternary oxides also occur as spinel superstructures such as CaFe204 (ferrite) or potassium iron oxides which contain the spinel structure as a building block. These complex structures of the beta alumina type consist of the formal composition (K2O x 11 Fc203) and can be described as a layer structure with unit cells of spinel linked by layers of potassium ions octahedrally coordinated to the (4 -h 4) oxygen ions of two adjacent spinel units. 

Fireclay bricks (alumina silica bricks, alumina calcium oxide silica bricks, and other silicate bricks) are not optimal barrier materials for A1 reductirMi cells. As we have mentioned, cryolite-based electrolyte for A1 reduction is a substance that dissolves alumina better than anything else. Certainly, it will dissolve all alumina-based refractory compositions and almost all other oxides similar in chemical structure to alumina. From a chemical point of view, the effective refractory barriers against the penetration of cryolite might be tin oxide, nickel oxide, compounds of nickel oxide, iron oxide, or zinc oxide (such as spinel Fe NiOs). These oxides almost do not react with NaF and aluminium fluoride [175]. Yet the cost of these materials, which is 50-100 times higher than that of firebrick, provides the impetus to find less costly variants of alumina silica materials. 

From experimental work with precipitated cobalt-iron catalysts it appeared that the cobalt reacted with alumina giving cobalt spinels, which helped to form smaller iron and cobalt crystallites during reduction and to increase the surface area to make the catalyst more stable and less sensitive to oxide forming poisons. In the presence of potash, cobalt was able to decrease the reduction temperature. 

Sarpoolaky H, Zhang S, Lee WE. Corrosion of high-alumina and near-stoichiometric spinels in iron-containing silicate slags. J Euro Ceram Soc 2003 23(2) 293-300. 

The first-mentioned product G is a ULCC-type castable and based on fused brown alumina/SiC with approximately 3% carbon. It is applied in the casthouse of blast furnaces, especially in the pig iron trough. The second example H is a spinel-forming ULCC castable, which is used as wear lining in steel ladles. 

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