Corundum, artificial

Emery is a mixture of granular corundum of dark color, magnetite and hematite, sometimes with spinel. Quartz may be present For a long time emery was supposed to be an ore of iron. Until the introduction of artificial abrasives, emery was much used for such purposes. 

For a large number of cases and applications, a metal oxide is more important than the metal itself. Many oxides are found native in nature or are formed spontaneously (e.g., Ti02, which is used in white paint and in the whitening of paper), while others are artificially produced from the metal itself (e.g., AI2O3, which protects aluminum from surface mechanical damage and corrosion it is produced by an electrolytic industrial process, and it also occurs in nature as the mineral corundum). 

Abrasives have evolved into an essential component of modern industry. Sandstone, emery, and corundum were the abrasives of choice until the late 1800s when artificial materials were developed. Today synthetic abrasives offer such improved performance that the natural ones have been largely replaced except for jobs where cost is paramount. In 1987 U.S. statistics (4) showed natural abrasive production to be about 7 million while that of crude manufactured abrasives was over 182 million. Total value of abrasives and abrasive products worldwide is estimated to be over 6 billion dollars. 

Aluminum is a constituent of many minerals, including clay (ka-olinite), mica, feldspar, sillimanite, and the zeolites. Some of these minerals are discussed under the chemistry of silicon, in Chapter 31. Aluminum oxide (alumina), occurs in nature as the mineral corundum. Corundum is the hardest of aU naturally occurring substances except diamond it scratches all other minerals, but is itself scratched by diamond, and also by the artificial substances boron carbide, and silicon carbide, SiC. Corundum and impure corundum (emery) are used as abrasives. 

A nickel-chrome spinel, NiO.CrjOg, has been made artificially in small green crystals. The green colour of emerald, serpentine, possibly sapphire, and other minerals is due to the presence of compounds of chromium, which also occur in rubies, corundums, and spinels. It has been shown that the peculiar spectra of ruby and diamond are due to chromium oxide which has been compelled to vibrate in an abnormal manner, leading to the production of narrow absorption bands. Traces of chromium have been detected in the soot produced by the combustion of coal obtained from deposits at Li ge. Compounds of chromium are not known to play any part in the economy of plants or animals. 

Fig. 3-9. Rietveld-fit of an X-ray diffi actogram. The sample is an artificial mixture of synthetic hematite, goethite and corundum (internal standard) in a ratio of a b c. The X-ray diffractogram (upper) is followed by the x-ray line positions of the three components (middle) and the differences between the measured and the fitted intensities (residuals). Courtesy J. Friedl and H. Stanjek).

White bauxites are chiefly used for the production of aluminium sulphate and the alums. Red bauxites form the raw material for the preparation of alumina, and therefore of aluminium. Intermediate or refractory bauxites, fused in an electric furnace, give artificial corundum. 

Altuninium Osi —Alumina—AlaOa—102—occurs in nature, nearly pure, as corundum, emery, ruby, sapphire and topaz and is formed artificially, by calcining the hydrate, or ammonia alum, at a red heat. 

As the above-mentioned formation of zirconium hydride shows, not only metals but also various compounds can be the end product of a thermitic process. Silicides, such as the compound BaSi from the peroxide and oxide with silicon, have been quoted by Ullmann from German patents. The lower oxide of titanium (TiO) has been described as obtainable from the dioxide and magnesium. A very hard form of artificial corundum (crystalline aluminum oxide) of superior quality for grinding and polishing operations is the byproduct of the above-described production of chromium metal. ... 

Abrasive materials are usually classified into two groups, natural and manufactured ones. The natural abrasives are generally referred to as those that have been produced by the uncontrolled forces of nature and because of that, they can contain many impurities and vary in quality. Emery, corundum, quartz, flint, garnet, diamond, tripoli, diatomaceous earth, sandstone, pumice, and natural sharpening stones are some of them (Krar 1995 Jacobs 1928). On the other hand, artificial abrasives were first developed in the late nineteenth century and overcame the problems of impurities and inconsistencies, since their manufactore could be carefully controlled. Some manufactored abrasives are carbide of silicon, aluminum oxide, glass, and the metallic abrasives such as steel wool and steel shot and grit (Krar 1995 Jacobs 1928) (Table 1). 

The flame fusion technique (see Figure 7.20b) was originally devised in 1904 by Verneuil for the manufacture of artificial gemstones, such as corundum (white sapphire) and ruby. This method is now used for the mass production of jewels for watches and scientific instruments. A trickle of fine alumina powder plus traces of colouring oxides is fed at a controlled rate into an oxyhydrogen flame. Fusion occurs and the molten droplets fall on to a ceramic collecting rod. A seed crystal cemented to the rod is fused in the flame and the rod is lowered at a rate that allows the top of the growing crystal (known as a boule) to remain just molten. Renewed interest has recently been shown in this method for the production of rubies for lasers. 

Kessler Abrasion Tester. Apparatus designed by the National Bureau of Standards, USA, for the determination of the abrasion resistance of floor tiles and quarries. A notched steel wheel is mounted on an overhanging frame so that a definite and constant weight bears on the test-piece as the wheel revolves No. 60 artificial corundum is fed at a specified rate between the wheel and the test-piece, which is mounted in an inclined position. (D. W. Kessler, Nat. Bur. Stand. Tech. News Bull., 34,159,1950.) Ketteler-Helmholz Formula. A formula for the optical dispersion of a glass ... 

In another variation, if vanadium ions, replace a few Al + ions in corundum, the result is a gemstone known as alexandrite. This gemstone appears green in reflected natural light and red in transmitted or artificial light. 

Figure 5.38. The photo shows natural aluminium oxide Al20g (corundum). Aluminium oxide is only surpassed in hardness by diamond. Artificially made aluminium oxide is widely used as an abrasive.

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