Spinel, formation

Spinel is a mineral with a composition MgAl204 (see Section 5.3.10). A large number of other oxides crystallise with the same structure, and these are collectively referred to as spinels. The formula of spinels is AB2O4, where A is most often a divalent cation, and B a trivalent cation, as in MgAl204 itself. 

The spinel formation reaction can be represented by the chemical equation 

It has been found that the reaction between MgO and AI2O3 follows this latter mechanism. The 

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Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

In a study of the spinel formation reaction CoO + A1203 = C0AI2O4... 

Armijo [1198] has discussed a number of features of the kinetics and mechanisms of spinel formation. Under suitable conditions, the rates of some high temperature spinel syntheses can be studied gravimetrically [1199]. Holt [1180] has been concerned with the role of oxygen diffusion in CoA1204 formation. 

Further investigations of spinel formation reactions are to be found in the literature [1], but the above representative selection illustrates a number of typical features of these rate processes. Following migration of cations from one constituent onto the surfaces of the other, the process is limited by the rate of diffusion across a barrier layer. While obedience to a particular kinetic expression is sometimes reported, the data available are not always sufficiently precise to enable the fit found to be positively... 

In principle, reaction schemes similar to that given in the preceding paragraph may be developed for other comparable rate processes, for example spinel formation. However, Stone [27] has pointed out that, where the barrier phase is not an efficient ionic conductor, the overall reaction may be controlled by the movement of a single cation and anion. In addition, there is the probability that lattice imperfections (internal surfaces, cracks, leakage paths [1172], etc.) may provide the most efficient route to product formation.]... 

While many reactions are undoubtedly of these three types, few kinetic studies are available. There are, however, close resemblances between specific rate processes in which a gas is evolved and the more intensively studied spinel formation reactions (Sect. 4.1). 

CCP close-packed structures,or pillars are placed between the layers to provide the stabilization. We reported on compounds KMn02 i o,i84 (VO)j-Mn02, which are examples of such pillared structures. The former is stable to spinel formation at low current densities, and the latter shows excellent stability but poor rate capability. The groups of Dahn and Doeff among others have pursued non-ccp structures by looking at tunnel structures such as... 

There is no clear evidence to identify the active material for SO2 removal in a MgAl20 stoichiometric system. Figure 13 shows results for a 50-50 mole% magnesia-alumina material prepared from magnesium hydroxide and alumina sol and calcined at various temperatures. An attempt was made to correlate SO2 removal with compound formation, as measured by X-ray diffraction, and surface area. As indicated in the figure, SO2 removal ability decreased with Increasing calcination temperature as did surface area. X-ray diffraction analysis showed spinel formation increases as... 

Inert markers have been used to obtain additional information regarding the mechanism of spinel formation. A thin platinum wire is placed at the boundary between the two reactants before the reaction starts. The location of the marker after the reaction has proceeded to a considerable extent is supposed to throw light on the mechanism of diffusion. While the interpretation of marker experiments is straightforward in metallic systems, giving the desired information, in ionic systems the interpretation is more complicated because the diffusion is restricted mainly to the cation sublattice and it is not clear to which sublattice the markers are attached. The use of natural markers such as pores in the reactants has supported the counterdiffusion of cations in oxide spinel formation reactions. A treatment of the kinetics of solid-solid reactions becomes more complicated when the product is partly soluble in the reactants and also when there is more than one product. 

Spinel formation is usually treated under some tacit assumptions which do not always hold. For example, it is tacitly assumed that the oxygen potential of the surrounding gas atmosphere prevails throughout the reaction product during reaction. In other words, it is assumed that d,u0 = 0. Although this inference reduces the number of variables by one and simplifies the formal treatment, the subsequent analysis will show that the assumption is normally not adequate. 

Figure 6-6. Fluxes and interlace reactions for different boundary conditions during spinel formation AO + BjOj = AB204. a) Oxygen excluded from phase boundaries, b) Oxygen has access to both boundaries. c) Only oxygen (of different potential) is available at the boundaries. d) Oxygen (of different potential) and one reactant (AO) is available at the boundaries, e) AO (but no oxygen) is available at one boundary both B203 and oxygen are available at the other boundary.

Figure6-8. The course of yVAO- , r0- , and Naq-/u0 during spinel formation. AO-f ---------Reaction of the first kind.

Experiments have shown that Aoxide spinel formation is on the order of 10 4cm at ca. 1000°C [C.A. Duckwitz, H. Schmalzried (1971)]. Using Eqns. (10.45) and (10.46) with the accepted cation diffusivities (on the order of 10 10 cm2/s), one can estimate from j% that each A particle crosses the boundary about ten times per second each way. In other words, quenching cannot preserve the atomistic structure of a moving interface which developed during the motion by kinetic processes. This also means that heat conduction is slower than a structural change on the atomic scale, unless one quenches extremely small systems. 

Sometimes, one has independent information on r. Let us consider an interface controlled spinel formation (A0+B203 = AB204). We assume that the rate limiting interface is AB204/A0 and also that the spinel product is a so-called normal spinel in which the A cations are situated on tetrahedral sites. Therefore, in the super-... 

However, during long exposures to medium-temperature operating conditions, e.g. 1000°C, spinel formation is certainly expected. Wang etal.60 demonstrated this for the Ni-alumina system, showing the diffusion of Ni atoms to the free surface of the nanocomposite, followed by the formation of a nickel spinel surface coating which then limits the kinetics of subsequent oxidation. In this case the formation of a spinel surface layer may be beneficial to mechanical properties, since the reaction results in a volume increase, and the formation of compressive residual stresses. An analogous behavior was reported for ceramic particle nanocomposites, where oxidation of SiC particles results in an increase in volume and compressive residual stresses.61... 

High-density spinel refractory brick is made by calcining (1200-1300°C for about 3 h) compacted powders and then sintering at 1700°C. The extent of spinel formation increases by calcination however, the powder mixture is not completely converted to spinel. Typically 10-15% of a-Al203 and 5-10% of MgO are observed by XRD, depending on the conditions of temperature, time and particle size of the reactants. 

Aksel, C., Spinel formation, reaction conditions and densification properties of magnesia-spinel composites , Key Eng. Mater, 2004 264-8 1071 -4. 

Sintering of the active component is another important cause of catalyst deactivation. This could also be a reason why regeneration attempts by reduction of N1O/AI2O3 catalysts that are deactivated by spinel formation may not succeed. 

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