Abrasives

Abrasives comprise a large number of both naturally occurring minerals and rocks and manufactured products. In many cases, these manufactured products have largely replaced their natural counterparts. Some, such as diamond, are rare; others, including sand and sandstone, are found abundantly in nature. All find uses in the home or in industry because of their characteristic hardness.

Background

Because the abrasives category encompasses a great variety of materials, their worldwide distributions are highly varied. Some, such as garnet and emery, are obtained from only a few localities. Others, such as sand and sandstone, are found on all continents, in all geologic settings, and in rocks representing all geologic ages.

Use of all the abrasives reflects in some manner the characteristics of hardness. That property is utilized in cutting and drilling tools, surface polishing materials, and blasting media. The largest user of abrasives is the automobile industry. Abrasives, both natural and synthetic, are used to perform one of four basic functions: the removal of foreign substances from surfaces (“dressing”), cutting, drilling, and comminution (or pulverizing) of materials. Most abrasives lie toward the upper end of the Mohs scale. With respect to one another, however, they can be categorized as hard, moderate (or “siliceous”), or soft.

Hard Abrasives

The hard abrasives are diamond, corundum, emery, and garnet. Diamond, the hardest naturally occurring substance (10 on the Mohs scale), is normally used in three size categories: stone, bort, and powder. Only a small fraction of the diamond stones produced by mining are of gem quality. All others, as well as those produced synthetically (together referred to as industrial diamonds), are used in various industrial applications, including diamond saws, rock-drilling bits, and other abrasive tools. Bort consists of fragments and small, flawed stones. Most bort, as well as synthetic diamond, is crushed to powder and mixed with water or oil to form a that is used to polish gems. The United States has no exploitable diamond deposits, but it is the world’s leading producer of diamond dust, easily satisfying its industrial needs.

Corundum, the second-hardest naturally occurring substance (9 on the Mohs scale), is used principally in crushed form for the polishing and finishing of optical lenses and metals. Its abrasive quality is enhanced because when it breaks, it forms sharp edges. As it wears, it flakes, which produces new edges. Corundum occurs in contact rocks, pegmatites, and placer deposits. The United States has no significant deposits of corundum.

Emery is a natural mixture of corundum and magnetite, with minor amounts of spinel, hematite, or garnet. Its value as an abrasive is largely a function of the amount of corundum present. In the United States, commercial emery deposits occur near the town of Peekskill, New York, where it is mined from contact metamorphic deposits. Important production also comes from Greece and Turkey. The principal uses of emery are as abrasive sheets, grinding wheels, and nonskid surfaces on stairs and pavements. Both corundum and emery have been replaced in large measure by synthetic alumina (Al₂O₃).

Of the fifteen varieties of garnet that occur in nature, almandite is the one most commonly used as an abrasive. Uses of garnet include sandblasting, finishing hard woods, the hydrojet cutting of rocks, and (in powder form) the finishing of optical lenses. Garnet has been replaced in metalworking by synthetic materials because they can be made harder and less friable. The United States, which possesses the world’s largest of garnet (mostly in the Adirondack Mountains), accounts for half of the world’s production and is also the world’s largest consumer.

Siliceous Abrasives

The term “silica sand” is taken to mean sand of almost pure quartz content, and sandstone (or quartzite) is the lithified version of that sand. Both are examples of siliceous abrasives of moderate hardness. Silica sand is used for sandblasting and for glass grinding. Historically, sandstone has been shaped into grindstones, whetstones, and millstones. Because high-quality sandstones were deposited in shallow seas during virtually all the geological periods, the reserves of silica sand and sandstone of commercial quality in the United States are enormous. Nevertheless, siliceous material for polishing and pulverizing has been replaced to a large extent by steel balls. The market share of silica sand as a sandblasting medium has declined because of health concerns related to the breathing of silica dust, which can lead to a condition called silicosis.

Other siliceous abrasives include diatomite, pumice, tripoli, flint, and chert. Diatomite, or diatomaceous earth, is an accumulation of the siliceous remains of shell-secreting freshwater and marine algae (diatoms). Because it is lightweight and porous, diatomite finds its most important uses as a filtering medium in water purification and waste treatment plants and as a filler (extender) in paint and paper. As an abrasive, it is used in scouring soaps and powders, toothpaste, and metal-polishing pastes. The United States possesses the world’s most important reserves of diatomite. Tripoli is the remains of siliceous limestones and is similar to diatomite in composition, characteristics, and uses. Pumice, porous volcanic glass, finds its principal market as building block. A small but significant amount of pumice, however, is used as an abrasive, for scouring and stonewashing. Chert and flint, two of the many varieties of quartz, have been used in pellet form in ball mills for the comminution of metallic ores.

Soft Abrasives

The soft abrasives include feldspar, clay, dolomite, chalk, and talc. They are primarily used for the polishing and buffing of metals. Feldspar, mined from granite pegmatites, is also crushed and used in soaps and scouring powders.

Synthetic Abrasives

Beginning in about 1900, a variety of manufactured abrasives were developed that have gradually replaced natural abrasives in the marketplace. In addition to lower cost, manufactured abrasives have the advantages of being tailored to meet specific industrial needs and of being produced in uniform quality. Among the important manufactured abrasives are synthetic diamond, cubic boron nitride, fused aluminum oxide, silicon carbide, alumina-zirconia oxide, and steel shot and grit. Synthetic diamonds were first produced in 1955, the result of a process that fuses graphite and metallic catalysts at extremely high temperature and pressure. Cubic boron nitride, first synthesized in 1957, is the next hardest substance after diamond and has challenged synthetic diamond as an abrasive in many industrial applications. Fused aluminum oxide is formed at high temperatures in an electric furnace by the fusing of either or corundum. Uses include tumbling, polishing, and blasting. It is also used in coated abrasives. Silicon carbide is fused from a mixture of quartz sand and coke; it finds its primary uses as a coated abrasive, in polishing and buffing media, and in wire saws for the cutting of stone. One of the primary uses of steel shot and grit is as a blasting medium. The automobile industry is the largest consumer of artificial abrasives, and the economic fortunes of the two industries are closely tied together.

Bibliography

Elaissi, Arwa, et al. "Development of Abrasives from Non-woven Based on Used Textiles." Journal of Natural Fibers, vol. 19, no. 6, 2022, pp. 2189-2203, doi.org/10.1080/15440478.2020.1807439. Accessed 26 Dec. 2024.

Giese, Edward, and Thomas Abraham. New Abrasives and Abrasives Products, Technologies, Markets. Norwalk, Conn.: Business Communications, 1997.

Hayes, Teresa L., Debra A. Celinski, and Rebecca Friedman. Abrasives Products and Markets. Cleveland, Ohio: Freedonia Group, 2000.

Jensen, Mead Leroy, and Alan M. Bateman. Economic Mineral Deposits. 3d ed. New York: Wiley, 1979.

Kogel, Jessica Elzea, et al., eds. “Abrasives.” In Industrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006.

Manufactured Abrasives: Statistics and Information. US Geological Survey,