Magnesia-alumina spinel

Wilson, D.R., Evans, R.M., Wadsworth, I. and Cawley, J., Properties and applications of sintered magnesia alumina spinels , UN1TECR 93 CONGRESS, Sao Paulo, Brazil, 1993. 

Evans, R.M., Magnesia-alumina spinel raw materials production and preparation , Am. Ceram. Soc. Bull., 1993 72(4), 59-63. 

Cortes, G., Magnesia alumina spinels for the refractory industry , Ceramic Technology International, 1994 109-12. 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

The dynamic membranes originally developed by Union Carbide are protected by three core patents U.S, 3977967, 4078112, and 4412921 (Trulson and Litz, 1976 Bibeau, 1978 and Leung and Cacciola, 1983) and their foreign equivalents. Those patents cover a broad range of metal oxides such as zirconia, gamma alumina, magnesia>alumina spinel, tantalum oxide and silica as the membrane materials and carbon, alumina, aluminosilicates, sintered metals, fiberglass or paper as the potential porous support materials. However, their marketed product, trade named Ucarscp membranes, focused on dynamic membranes of hydrous zirconium oxide on porous carbon support. 

Windle CJ, Bentley VK. Rebonded magnesia-alumina spinel products for oxy-fuel and alkali saturated atmospheres. Proc. Unified Inti. Tech. Conf on Refractories, Berlin, Germany, 1999 219-225. 

Silica from zeolite migrates less readily. In the magnesia-alumina system, spinel, as identified by X-ray diffraction, is inactive for SO2 removal. The effect of temperature on steam stability, oxidative adsorption and reductive desorption of SO2 are described. Five commercial catalyst types are ranked for SOx removal. 

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... 

Commercial sintered magnesia-spinel refractory materials are divided into three categories magnesia rich, stoichiometric, and alumina rich.1 Typical properties of magnesia rich spinel bricks are given in Table 14.1.2... 

Alumina, silica and mullite, magnesia and spinel, zirconia... 

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. 

In magnesia-stabilised / "-alumina, the formula is Na, (Mg2 Al5 (08 and X is essentially fixed at 0.175. An alternative way of representing it is in terms of a derivation from the stoichiometry NaAlnOi, with the formula Nai+2Mg2Aln jOi7. Various values of 2 are quoted in the literature, between 0.5 and 0.8. The value 0.747 which is in this range coincides with that obtained using the alternative, spinel-like formula given above, with x = 0.175. 

Properties Magnesia-spinel brick with alumina Magnesia-spinel brick with sintered spinel... 

MgO-spinel clinker is obtained by the same procedure, but with a substantial proportion of free MgO (75-85%). MgO-spinel is produced by sintering at temperatures of 1600-1800°C and a dense product is obtained. MgO-spinel refractory has better high temperature mechanical properties in arc furnace roof application compared to magnesia-chrome refractories, but it has lower strength than a high-alumina refractory. 

Gabis, V. and Graba, L., Microstructure of reaction-sintered spinel/corundum refractories prepared from various alumina-magnesia mixtures , Euro. Ceramics., 1991 2593-8. 

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