Refractories corundum

Corundum is an aluminum oxide that possesses a hexagonal crystal structure, The compound is extremely hard (2000 on the Knoop scale), sp gr 3.95, and is widely used in abrasives and refractories. Corundum is manufactured by fusing alumina or bauxite in an electric arc furnace operated at about SIOO C. 

The compounds of greatest importance are aluminum oxide, the sulfate, and the soluble sulfate with potassium (alum). The oxide, alumina, occurs naturally as ruby, sapphire, corundum, and emery, and is used in glassmaking and refractories. Synthetic ruby and sapphire are used in lasers for producing coherent light. 

Aluminum oxide, which has the mineral name corundum, is a solid that has several important uses. Because it will withstand very high temperatures, it is a refractory material, and because of its hardness it is commonly used in abrasives. Corundum often contains traces of other metals that impart a color to the crystals, making them valuable as gemstones. For example, ruby contains a small amount of chromium oxide, which causes the crystal to have a red color. By adding a small amount of a suitable metal oxide, it is possible to produce gemstones having a range of colors. 

Magnesium. Corundum-hibonite associations are what eould be the first eondensates from a solar composition gas. Mg is not a refractory element and is strongly depleted in... 

CAIs are composed of a variety of minerals, primarily hibonite, perovskite, melilite, spinel, aluminum- and titanium-rich diopside, anorthite, forsterite, and occasionally corundum or grossite. They also show significant enrichments in refractory trace elements. CAIs exhibit a host of isotopic anomalies inherited from incorporated presolar grains or from the early nebula itself. 

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

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

Corundum, a-Al203, is a hard and chemically inert material that is a useful abrasive and refractory material that has the composition A1203. When traces of other metals are present,... 

Compared to carbonaceous presolar grains much less isotope information, mostly for the major elements, is available for O-rich presolar dust (Lodders Amari 2005 Zinner 2007). This has several reasons. Other than presolar SiC and graphite, presolar oxides have only low trace-element concentrations. Moreover, it is not possible to produce chemical separates that consist mostly of presolar oxide grains. Presolar silicates can be found only in situ by ion imaging (see Appendix 2), a time-consuming task. The O-isotopic data of refractory oxides (corundum and other... 

This black, refractory substance is made by reduction of V2O5 with hydrogen or carbon monoxide. It has the corundum structure but is difficult to obtain pure, since it has a marked tendency to become oxygen deficient without change in structure. Compositions as low in oxygen as VOi35 are reported to retain the corundum structure. 

Krot A. N., Huss G. R., and Hutcheon 1. D. (2001b) Corundum-hibonite refractory inclusions from Adelaide condensation or crystallization from melt Meteorit. Planet. Set 36, A105. 

Another form of alkali metal attack on the hot faces of refractory linings involves their high temperature reaction with various components of the brick to form expansive crystalline phases which cause brick to bloat on their hot faces and, subsequently, erode or spall. An example Is the case of alumina brick exposed to sodium at temperatures from about 1700°F to 3000°F. Although sodium does not form a low temperature melt with alumina, it reacts with the alpha phase of alumina, corundum, to form beta alumina, sodium aluminate. Beta alumina has a much greater volume than the very dense corundum and, therefore, disrupts the brick bonding matrix, causing eventual bond failure. 

In this way, it is possible to explain the low thermal expansion of silica refractories in the medium temperature range and a considerable expansion at low temperatures. This i.s also why silica differs considerably in thermal shock resistance in these temperature ranges. For this type of refractory, it is thus inadequate to specify the usual value of mean thermal expansion coefficient for a certain temperature range (e.g. 20—1000 C). The polymorphic inversion also results in an expansion anomaly of stabilized Z1O2 and of fusion-cast corundum-baddeleyite refractories around 1000 C (cf also Fig. 201). 

The first type of these refractories were mullite products melted from a mixture of bauxite or AI2O3 with clay. Besides mullite crystals, the product also contained less corundum and glass. It has been proved, however, that a still superior corrosion resistance is exhibited by similar corundum and corundum-baddeleyitic materials and the manufacture of mullite refractories has been abandoned. 

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