Encyclopedia of Global Resources part 117 pdf

tended the same lithographic patterning that helps put millions of transistors on a single computer chip but uses it to make micromachines such as gears, beams, and motors. These microelectromechanical (MEM) structures are entrenched in automotive applications that requirepressureandaccelerationsens- ing. Other novel industrial applications (such as microrobots) and biomedical applications (such as microinjection of drugs and remote microsurgery) are coming to fruition through the development of sophisticated microsensors and actuators that can be integrated with silicon electronics on the same chip for signal processing and amplification. Lower-grade silicon metals, in the form of ferrosilicon and other silicon metals, are used in other industrial applications. Ferrosilicon is used as an alloy- ing element in aluminum, steel, brass, and bronze, as well as in the chemical industry. Demand for this form was on the rise in 2008, and in fact ferrosilicon constitutes about 80 percent of world production of silicon. China leads the pack in all silicon metal production: Top producers of ferrosilicon are, in descending order, China, Russia, Norway, the United States, and South Africa; leaders in other silicon metal production are China, Brazil, France, and Norway. S. Ashok Further Reading Cerofolini, G. F., andL. Meda. Physical Chemistry of, in, and on Silicon. New York: Springer, 1989. Chatterjee, Kaulir Kisor. “Silicon and Its Minerals.” In Uses of Industrial Minerals, Rocks, and Freshwater. New York: Nova Science, 2009. Datnoff, L.E.,G.H.Snyder, and G. H. Korndörfer. Sil- icon in Agriculture. New York: Elsevier, 2001. Grayson, Martin, ed. Encyclopedia of Semiconductor Tech- nology. New York: Wiley, 1984. Greenwood, N. N., and A. Earnshaw. “Silicon.” In Chemistry of the Elements. 2d ed. Boston: Butterworth- Heinemann, 1997. Lin, Wen, and Howard Hoff. “Silicon Materials.” In Handbook of Semiconductor ManufacturingTechnology, edited by Robert Doering and Yoshio Nishi. 2d ed. Boca Raton, Fla.: CRC Press, 2008. Massey, A. G. “Group 14: Carbon, Silicon, Germa- nium, Tin, and Lead.” In Main Group Chemistry.2d ed. New York: Wiley, 2000. Siffert, P., and E. F.Krimmel,eds.Silicon:Evolutionand Future of a Technology. Berlin: Springer, 2004. Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geological Survey Silicon: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/silicon See also: Earth’s crust; Semiconductors; Silicates. Silver Category: Mineral and other nonliving resources Where Found Silver has an average crustal abundance of 0.07 part per million (0.000007 percent) but is widely distrib- uted and occurs in recoverable amounts in many ore deposits throughout the world. It may occur as native silver, as silver sulfide(acanthite), alloyed with gold as electrum, or as complex copper, lead arsenic, and/or antimony sulfides. Primary Uses Previously, the most important uses of silver were in photographic materials; light-sensitive silver salts served to produce negatives and prints. However, beginning in 2000, the use of silver in photography declined significantly because of the emergence of digital technology. Silver is also widely used in electrical and electronic products, jewelry, dental amal- gams, and mirrors, and was widely used in the past for coinage. Technical Definition Silver (abbreviated Ag), atomic number 47, belongs to Group IB of the periodic table of the elements and is similar in some chemical and physical behavior to copper. It has two naturally occurring stable isotopes, with masses of 107 and 109,andhasanaverage atomic weight of 107.87. Its physical properties are malleabil - ity, ductility, reflectivity, and electrical conductivity. 1088 • Silver Global Resources Description, Distribution, and Forms Pure silver is a malleable, white, highly reflective metal with a densityof 10.49 grams per cubiccentime- ter. It has a melting point of 961.9° Celsius and a boiling point of approximately 2,212° Celsius; it crystal- lizes in a face-centered cubic structure. World production of silver was relatively constant at about15,000 metric tons per yearbetween the mid- 1980’s and the late 1990’s; production rose to about 20,000 metric tons by the mid-2000’s. The largest producers have been Mexico, Peru, the UnitedStates,Po- land, China, Australia, and Chile, but there has been significant production reported in nearly sixty countries. World reserve figures have remained at more than 250,000 metric tons, thus ensuring a stable supply for a number of years. The reserve base, which in- cludes ores that will very likely become economic to mine in future years, was 570,000 metric tons in 2008. Reserve figures are, however, subjectnotonly to changes in the price of silver but also to changes in the prices of the other primary metals in the deposits where silver is recovered only as a by-product. Silver itself has little or no adverse environmental impact; however, the mines that produce silver may create local problems attributable to the mining and disposal of wastes. The principal impact in older mining districts has been the release of acid generated by the weathering of pyrite that is usually present in the ores. Silver, when released because of acid mine dis- charge, is rapidly adsorbed onto iron and manganese oxides and hydroxides that precipitate in such situa- tions. Silver poisoning of humans is extremely rare, but prolonged absorption of silver compounds can cause argyria, a grayish-bluediscolorationoftheskin. The purity of silver in jewelry,coinage,bullion,and ornamental pieces is usually expressed as fineness or parts perthousand. Pure silver is “1000fine” (100 percent silver); sterling silver, widely used for decorative Global Resources Silver • 1089 Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 800 2,000 2,600 3,000 3,600 1,300 70 1,120 4,600 Metric Tons of Silver Content 5,0004,0003,0002,0001,000 United States Peru Mexico China Chile Canada Poland South Africa Other countries 1,800Australia Silver: World Mine Production, 2008 items and tableware, is 925 fine (92.5 percent silver and 7.5 percent copper). History Silver was one of the earliest known metals because it commonly occurs as a free metal. It was easily shaped by pounding and melting into ornaments, amulets, and utensils. Silverwas in use innorthern Europe and around the Mediterranean by 4000 b.c.e.,butthefirst known mining sites were in Cappadocia, in eastern Asia Minor. From there silver mining spread throughout the Mediterranean countries and the metal was obtained from manysmallmines.Byabout1000b.c.e. the most important mines were the silver-lead mines at Laurion, near Athens, Greece. These mines made Athens wealthy and financed the Greek wars; the ulti- mate production has been estimated to have reached 250 million troy ounces (7,800 metric tons). Because the silver was extracted by open roasting of the lead- rich ores, this mining also resulted in widespread lead pollution. With the rise of the Roman Empire, silver mining expanded throughout Europe and across the Middle East. Even throughout the Dark Ages, when many of European mineral industries severely declined, silver mining continued to expand. By 1300 c.e. the largest silver mine was at Joachimsthal (now Jáchymov, in the Czech Republic). A large silver coin produced there was called the Joachimsthaler, abbreviated to thaler in German; it became daalder in Dutch and ultimately “dollar” in English. Colonization of the New World in the early six- teenth century led to the discovery of major silver deposits in Mexico and Peru. In fact, the quantity of silver (16,000 metric tons) shipped back to Spain from the New World between 1520 and 1660 was about ninety times greater than the quantity of gold (181 metric tons) shipped back in the same time span. The abundance of silver led to its use as a primary coinage metal in the New World. The eight real coin, commonly called the Spanish “piece of eight,” was widely mintedand used throughout the Americas and Europe; it remained a viable coin in the United States until the mid-1800’s. Silver also became one of theprin- cipal coinage metals in the early years of the United States; it was used in the first noncirculated test coins in 1792 and then appeared in half-dollars and silver dollars in 1794 and in dimes and quarters in 1796. The minting of silver dollars was termi- nated in the 1930’s. The other coins remained silver-bearing until 1964, when clad nickel-copper dimes were introduced and when the silver be- gan to be phased out of quarters and half-dollars. The silver was removed because coinage use was outstripping supplies, industrial use was rising, and the cost of silver, long held constant, was allowed to rise. Silver is used to- day in coinage only in the produc - tion of special bullion and commem - orative coins. 1090 • Silver Global Resources Townsfolk stand beside a tower of silver ingots in Leadville, Colorado, around 1880. (The Granger Collection, New York) Obtaining Silver Silver occurs in many types of ore deposits and in minerals as diverse as the native metal (silver, Ag; electrum, Au, Ag), silver sulfide (acanthite, Ag 2 S), silver chloride (cerargyrite, AgCl), and sulfosalts (such as polybasite, Ag 16 Sb 2 S 11 , and proustite, Ag 3 AsS 3 ). Ap- proximately one-third of world silver comes from veins associated with gold as the primary commodity. Two- thirds of modern production is as a by-product from ores mined primarily for copper, lead, zinc, or gold. These ores also exist primarily as hydrothermal vein deposits, but the silver is generally present as sulfosalts, especially tetrahedrite, which is intimately intermixed with sulfides of iron, copper, lead, and zinc. Primary gold ores, both lode and placer types, generally also produce significant amounts of silver that has been held in solid solution in the native gold or electrum. Uses of Silver For many years, approximately 50 percent of silver usage in the United States was for photographic materials, especially film and printing paper. These were prepared by depositing thin layers of silver salts along with gelatin and dyes on sheets of plastic or paper. However, beginning in 2000, silverusage in photography was waning because of digital technology. Silver has the greatest thermal and electrical conductivity of any metal and hence finds broad usage in electrical and electronic products. Because silver is also quite resistant to corrosion and oxidation and is easily welded, it is especially sought for high-quality electronics. Commonly, silver is alloyed with copper, gold, nickel, or platinum group metals to provide working properties that better serve specific applications. Silver has also found considerable use in batteries, where silver compounds are coupled with zinc compounds. Such batteries are expensive and relatively short-lived, but they have a high energy out- put per unit size and weight. Consequently they find their primary usage inmilitary and space applications where high reliability for a short time is essential. They have also been used in the production of button batteries for calculators, watches, and hearing aids. Silver has found wide use in special “silver solders,” which serve effectively in air-conditioning and refrig- eration applications because they bond so well to a wide variety of metals. Silver has been used in jewelry and other decorative items since prehistoric times. Theoccurrence of native silver and the relative ease of silver refining allowed for the early use of silverthatwasquitepure.Modern silver usage for decorative purposes is usually as sterling silver (92.5 percent silver and 7.5 percent copper); sterling silver objects are made entirely of silver-copper alloys. In addition, there is much electroplated silver, which consists of a thin coating of silver, or silver- copper alloy,overabaseofsomeothermetal. Because of the high reflectance of silver in the visible spec- trum, it haslongbeenusedinthemakingofmirrors. Silver finds considerable usage in the chemical industry because it catalyzes oxidation reactions and produces organic compounds, such as formaldehyde from methanol, and oxygen and ethylene oxide from ethylene and oxygen. Silver has long served as a primary metal in dental amalgam fillings. The addition of mercury to the silver- and tin-based alloys provides dentists with an amalgam that is malleable at first but soon becomes very hard and corrosion resistant. The amalgam consists of a series of alloy phases that very effectively incorporate and contain the mercury. However, recent studies suggest these types of fillings may not be safe. Medical applications of silver include the use of sol- uble silver salts of nitrates and citrates. These have proven very effective in the treatment of bacterially caused medical conditions. Furthermore, silver ni- trate reacts rapidly with skin and mucous tissue and thus serves asa low-temperature means of cauterizing where the additional strength of scar tissue is useful. James R. Craig Further Reading Evans, Anthony M. Ore Geology and Industrial Minerals: An Introduction.3d ed. Boston: Blackwell Scientific, 1993. Greenwood, N. N., and A. Earnshaw. “Copper, Silver, and Gold.” In Chemistry of the Elements. 2d ed. Bos- ton: Butterworth-Heinemann, 1997. Guilbert, John M., andCharles F.Park, Jr.The Geology of Ore Deposits. Long Grove,Ill.:WavelandPress,2007. Howe, P. D., and S. Dobson. Silver and Silver Com- pounds: EnvironmentalAspects. Geneva, Switzerland: World Health Organization, 2002. Mohide, Thomas Patrick. Silver. Toronto: Ministry of Natural Resources, 1985. Smith, Ivan C., and Bonnie L. Carson. Silver.Vol.2in Trace Metals in the Environment. Ann Arbor, Mich.: Ann Arbor Science, 1977. White, Lawrence H., ed. The History of Gold and Silver. 3 vols. Brookfield, Vt.: Pickering & Chatto, 2000. Global Resources Silver • 1091 Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geological Survey Silver: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/silver See also: Gold; Metals and metallurgy; Mineral resource use, early history of; Native elements; Open-pit mining; Peru; Underground mining. Silviculture. See Forest management Slash-and-burn agriculture Categories: Environment, conservation, and resource management; obtaining and using resources Slash-and-burn agriculture—clearing land by burning existing vegetation and then using theashes to fertilize the land for growing crops—is one of the oldest farming techniques. The ecolog- ical costs and benefits of slash-and- burn farming in the modern world have prompted debateamongscientists. Background Also referred to as swidden agriculture or shifting cultivation, slash- and-burn agriculture is perhaps the simplest method employed by farmers. In its most basic form, slash-and- burn agriculture refers to a method of clearing land for cultivation by burning off the existing wild vegetation. The ash from the burned un- derbrush and standing timber pro - vides a quick infusion of nutrients into the soil. Generally no other fer - tilizer is applied, so the soil may be exhausted quickly, forcing farmers to move on to a new site. As the yields from crops become progressively smaller, additional farmland is cleared through burning while the depleted area is allowed to lie fallow, or unused. As long as the number of people practicing slash-and-burn agriculture in a given area remains relatively constant, any damage done to the environment tends to be both localized and temporary. As populations grow, however, larger areas must be cleared while the depleted land is allowed to rest for shorter and shorter periods of time. Slash-and-Burn Techniques In general terms, slash-and-burn agriculture refers to the practice of moving garden sites periodically to al- low depleted soilto lie fallow and rejuvenate itself naturally. Although slash-and-burn evokes an image of forestlands being burned over, the term also refers to the practice of burning off grass or other native vegetation as a way to prepare soil for gardening or farming. Swidden agriculture has been practiced in many different regions of theworld,rangingfrom northern Europe to the tropical rain forests of South America. Shifting cultivation was common, for example, in the Scandinavian nation of Finland until the early 1092 • Slash-and-burn agriculture Global Resources In Madagascar, a woman walks by a portion of forest decimated by slash-and-burn agri - culture. (AP/Wide World Photos) twentieth century. Shifting cultivation remains a key method of farming throughout much of Africa, Asia, and Latin America. In the simplest form of slash-and-burn agriculture, a fire is set and is allowed to burn the vegetation on a tract of land. After the fire has died down, food crops are planted in the burned-over area. More typically, the farmers will cut the brush and trees growing on the garden site andpilethem for burning. Large trees may be left standing to be felled later as needed for firewood, or they may be cut and burned along with the brush. After the stacked brush is burned, the ashes are raked to distribute them evenly over the garden site. Crops will be planted as quickly as possi- ble following the burn so that weeds will not have a chance to sprout. Slash-and-burn agriculture, which exposes bare soil for planting food crops without re - quiring backbreaking labor, has served as an efficient and effective method of farming for millennia. The Yanomami Anthropologists and cultural ecologists oncebelieved that soil depletion was the primary reason farmers practicing slash-and-burn would move their fields. However, studies of the indigenous peoples of South America, such as the Yanomami in the Amazonian basin, have revealed a more complex picture. The Yanomami and other rain-forest farmers follow a reg- ular pattern in rotating their gardens through the forest. Scientists learned that a typical village, which generally had apopulation of about two hundred per- sons, moves its fields every five or six years. Studies of both the soil and the crop yields indicated that a lack of nutrients in the soil was not the probable cause for the move. Rather, the farmers would move their gardens in response to thorny weeds springing up in the field. A lack of efficient tools for weeding, combined with the light or nonexistent clothing favored in the tropical climate, made it less work to clear a new gar - Global Resources Slash-and-burn agriculture • 1093 0 10 20 30 40 50 60 70 80 90 100 Relative yield (percentage) 12345 12121212312123 Corn (S. Sudan) Corn (Belize) Cassava (Zaire) Rice (Malaysia) Corn (Guatemala) Rice (Zaire) Peanuts (Zaire) Years after clearing Declining Crop Yields with Successive Harvests on Unfertilized Tropical Soils Source: Diversity and the Tropical Rain ForestData adapted from John Terborgh, . New York: W. H. Freeman, 1992. den site than to attack the nuisance plants invading the existing one. By the fourth or fifthyear after clearing an area by burning, the new brush springing up made it impractical to continue gardening in that lo- cation. This discovery has led some agronomists to suggest that the rotation of garden sites from one lo- cation to another in tropical climates could be slowed through the relatively simple means of training indigenous peoples in the use of metal hoes and scythes. Environmental Debate Slash-and-burn agriculture has sparked a fierce debate within the environmental movement. Many envi- ronmentalists believe that slash-and-burn agriculture is ecologically more sound than what some analysts term “industrialized agriculture,” or farming using chemical fertilizers; they are consequently opposed to encouraging farmers in developing nations to shift to other farming methods. While it is true that slash-and- burn does not pollute in the same way that farming with chemicals does, other analysts argue that growing populations that depend strictlyon swidden farming will require more and more land to be cleared for use as cropland. Swidden farming may cause the least direct pollution, but through the deforestation of crit- ical watersheds it can lead to other serious environmental problems, such as flooding. In addition, because there are generally no nutrients added to the soil other than the ashes of the burned vegetation, yields per hectare are considerably lower than those of farmland fertilized with chemicals. Consequently, slash-and-burn requires more hectares of cleared land than does chemically fertilized farming to produce the same amount of food. Nancy Farm Männikkö Further Reading Chagnon, Napoleon A. Yanomamö: The Fierce People.3d ed. New York: Holt, Rinehart and Winston, 1983. Food andAgriculture Organization of the UnitedNa- tions. Improved Production Systems as an Alternative to Shifting Cultivation. Rome: Author, 1984. Garrity, Dennis P., and Asmeen Khan, comps. Alterna- tives to Slash-and-Burn: A Global Initiative. Nairobi, Kenya: International Centre for Research in Agro- forestry, 1994. Haney, Emil B. The Nature of Shifting Cultivation in Latin America. Madison, Wis.: Land Tenure Center, University of Wisconsin, 1968. Palm, Cheryl A., et al., eds. Slash-and-Burn Agriculture: The Search for Alternatives. New York: Columbia Uni - versity Press, 2005. Peters, William J.,andLeonF.Neuenschwander. Slash and Burn: Farming in the Third World Forest. Moscow: University of Idaho Press, 1988. Stewart, Omer C. Forgotten Fires: Native Americans and the Transient Wilderness. Edited by Henry T. Lewis and M. Kat Anderson. Norman: University ofOkla- homa Press, 2002. Wojtkowski, Paul A. Introduction to Agroecology: Principles and Practices. NewYork:FoodProductsPress,2006. See also: Deforestation; Farmland; Fertilizers; Mono- culture agriculture; Rain forests. Slate Category: Mineral and other nonliving resources Where Found Slate is found worldwide, generally in areas where older rocks from beneath the surface of the Earth have been exposed or in areas where younger rocks have been subjected to the pressure of mountain building. The most important areas of slate production are Pennsylvania, Vermont, and Wales. Primary Uses Cut slate is used for paving, roofing, electrical panels, blackboards, and tabletops. Crushed slate is used in aggregates (materials mixed with cement to make concrete). Technical Definition Slate is formed when clay or smooth, fine-grained rock such as shale or basalt is subjected to relatively low temperature and pressure beneath the Earth’s surface. This low-grade metamorphic process results in the formation of very small, often microscopic, crystals. The pressure inside the Earth causes these crystals to beflattened and elongated, resulting in the ability of slate to be split into thin slabs. Description, Distribution, and Forms Slate is a smooth, hard, metamorphic rock that easily splits into thin, strong slabs. Slate isusuallyred, green, gray, or black, but some slate is blue or purple. The color of slate is often mottled, streaked, or spotted. A 1094 • Slate Global Resources few large grains of various minerals may be found within the otherwise homogeneous material. Thin lines may appear, which reveal the structure of the rock from which the slate was formed. Slate varies widely in color because of the presence of small particles of various minerals within it. Red and purple slates contain hematite (iron oxide). Green slates contain chlorites (various green materials composed of aluminum silicates). Gray and black slates may contain carbon in the form of graphite, organic matter, or iron sulfide. Other rocks that can be split into thin slabs are often loosely called slates. True slates can be distin- guished by the fact that the angle at which they split is different fromthe angle at which theylie.Other rocks known as “slate” usually split into slabs parallel to the way they lie. History Slate has beenusedasaroofing materialintheUnited States since early colonial times. Slate was also used in pencils until the 1930’s. Obtaining Slate Slate is obtained by quarrying from shallow open-pit mines. When an area containing slate is discovered, the material covering the slate is removed with power shovels and bulldozers. Large blocks of slate are then removed bydrillingandbyusingthenaturaltendency of the rock to split. The blocks of slate, which are about 7.5 centimeters thick, are split with chisels and mallets into slabs about 0.5 centimeter thick. The slabs are then cut to the desired size by rotating blades. Uses of Slate Cut slate, known as dimension slate, is used to form panels for roofs, floors, and pavement. It is also used to provide a smooth, strong surface for electrical panels, blackboards, billiard tables, and laboratory tabletops. Slate that is not smooth enough to be used in the form of slabs may be mined using drills and ex- plosives. The blasted slate is crushed and used in filler, in aggregate, or in roofing. Web Sites Natural Resources Canada Stone http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/content/2006/56.pdf U.S. Geological Survey Stone, Dimension http://minerals.usgs.gov/minerals/pubs/ commodity/stone_dimension/myb1-2007- stond.pdf Rose Secrest See also: Aggregates; Clays; Dimension stone; Meta- morphic processes, rocks, and mineraldeposits; Open- pit mining; Quarrying; Shale. Smelting Category: Obtaining and using resources Smelting is a process that frees metals from their ores so that the metals then can be refined and worked into a useful form. Definition Smelting is a metallurgical process in which ores are melted in a specially designed furnace in order to extract metals from the ores. Techniques vary, de- pending onthe metal and the ore, but some common principles apply. Ore is generally processed before smelting in order to remove gangue (the unwanted rock or mineral aggregates that are mined along with the ore). After processing, the resulting ore concen- trate is smelted. During smelting, ore is reduced— that is, oxygen is removed from metal oxides to yield the metal—and impurities are separated from the metal. Additives placed in the smelting furnace along with the ore help the metal to melt and physically sep- arate from the impurities. Overview An example of smelting is the blast-furnace process that produces pig iron from iron ore. A blast furnace is a vertical, chimneylike structure made of iron or steel and lined with firebrick, a specialized building material able to withstand high temperatures and extremely corrosive molten mixtures. Preheated com- pressed air is carried through tuyeres (pipes) into the bottom of the blast furnace from a gas-fired heater. A charge ofiron ore, coke,and sand or limestone flux is introduced through a valve system at the top of the blast furnace. (The flux is added to lower the fusion, or melting, temperature of the metal.) As the charge Global Resources Smelting • 1095 descends through the shaft of the furnace, the ascending hot air passes through it. The coke, which is almost pure carbon, oxidizes in the hot-air blast to become carbon monoxide. The carbon monoxide reduces the iron ore, removingits oxygen and in the process reverting to carbon dioxide. Waste gases are captured and used to heat additional fresh air for the process. The heat from the reduction reaction causes the mass within the furnace to melt and descend into the lowermost portion of the furnace, called the crucible. The molten iron sinks to the bottom of the crucible. Slag—the waste material that results when flux combines with in- combustible impurities from the ore and ash from thecoke—floats on top ofthe iron. The slag is drawn off from a hole in the up- per portion of the crucible, while the iron is tapped off from below and channeled into sand molds. Here it hardens to form pig iron, also called cast iron. Pig iron can be con- verted to steel throughtheBessemerprocess. Some copper ores can also be treated in a blast furnace, particularly high-grade oxi- dized ores. When copper sulfide ores are smelted in a blast furnace, however, the resulting product is an impure metallic sulfide called a copper matte. The matte requires further treatment to produce a form of metallic copper fit for electrolytic refining. Likewise, other metal orescaninvolvediffer- ent smelting procedures, furnace designs, fluxes, and additives. Karen N. Kähler See also: Alloys; Bessemer process; Copper; Iron; Metals and metallurgy; Steel; Steel industry. Soda ash Category: Mineral and other nonliving resources Where Found While soda ash can be manufactured from salt, it can also be found in nature in the form of deposits of the minerals trona (Na 2 CO 3 C NaHCO 3 C 2H 2D O) and nat - ron (Na 2 CO 3 C 10H 2 O) and in alkaline brines associ - ated with dry lakes. Soda ash is mined from large natural trona deposits in Wyoming and recovered from lake brines in California. It is also produced from natural deposits in South America, Africa, China, Russia, Europe, and India. In 2008, leading producers of natural soda ash, in descending order, were the United States, Kenya, and Botswana. Primary Uses One of the most fundamental industrial chemicals, soda ash is used in the manufacture of glass, chemicals, soaps and detergents, and paints.Itisalsousedin processing wood pulp to make paper, refining alumi - num, desulfurizing pig iron, purifying petroleum, and softening water. 1096 • Soda ash Global Resources This wood engraving depicts the process of iron smelting, in which molten iron pours from a blast furnace and cools in molds. (The Granger Collection, New York) Technical Definition Soda ash (also known as soda) is a commercial term for the chemical compound sodium carbonate (Na 2 CO 3 ). Its average molecular weight is 105.99. Pure soda ash is a white, odorless powder thatishygro- scopic (absorbs moisture from theair) and thatforms a strongly alkaline water solution (a 1 percent aque- ous solution hasa pH of11). Its specific gravityis 2.53. It has a melting point of853° Celsius anddecomposes before reaching a boiling point. Description, Distribution, and Forms Soda ash,or soda, is an important inorganic chemical used widely in industry. Soda ash is an alkali—that is, it is a caustic substance that dissolves in water to produce a solution with a pH substantially greater than 7. While soda ash can be manufactured from salt, it can also be obtained fromnaturallyoccurring sodium carbonate deposits and alkaline lake brines. Soda ash is one of the most fundamental industrial chemicals and is an essential part of many industrial processes. Total world production of soda ash is approximately 40 million metric tons; the United States produces about one-third of the world total. Soda ash derives its name from the earliest method for producing the substance: sea plants were burned, their ashes were boiled with water, and the resulting solution was allowed to evaporate,leavingbehindsodiumcarbonate. Soda ash is found in nature in the form of deposits of hydrated sodium carbonate minerals such as trona and natron and in alkaline brines associated with dry lakes. Most of the world’s supply of natural soda ash comes from theGreen River region ofsouthwest Wyo- ming, the largest known depositof trona. These trona beds were deposited approximately 50 million years ago in the early to mid-Eocene epoch. The Green River basin is estimated to contain 115 billion metric tons oftrona deposits. In California, soda ash isrecov- ered from lake brines at Searles Lake in San Bernar- dino County and at Owens Lake in Inyo County. Other commercially significant natural deposits are found in Chile, Venezuela,SouthAfrica, Kenya, China, Russia, Germany, Hungary, and India. Trona, the ore from which soda ash is commonly obtained, is awhite or yellow-whitemineral found as a powder on the soil surface or in fibrous or columnar layers and thick beds in saline residues. Trona deposits are formed by the drying of alkaline bodies of water in arid regions. Soda ash is also foundin brines and in saline lake water. History Soda ash has been used since the days of the earliest Egyptian dynasties. The first glass containers made with it wereproducedbyEgyptiansaround3500b.c.e. Natron from the dry lakes of theWadiNatrun, a valley in Egypt, was used as a drying agent in mummifica- tion; it may also have been mixed with malachite to create a blue glaze. In his Meteorology, Aristotle de- scribes how Umbrians in the fourth century b.c.e. produced soda ash from the ashes of reeds and rushes. Four centuries later, the Roman historian Pliny the Elder writes of soda ash being used in glass, in medi- cines for colic pains and skin disorders, and in breadmaking. Plant ash remained an important source of soda ashfor manufacturing glassand soap until the early nineteenth century. In 1791, French chemist Nicolas Leblanc devel- oped a commercial process for making soda ash from salt; however, the French Revolution impeded the development of the process, and more than thirty years passed before the process became a commercial suc- cess in Liverpool, England. The Leblanc process, which played an important role in the Industrial Rev- olution, involved decomposing salt with sulfuric acid to produce sodium sulfate and hydrochloric acid, then heating the sulfate with coal and limestone to yield soda ash. In the early 1860’s, Belgian industrial chemist Ernest Solvay and his brother Alfred devel- oped another method for producing soda ash from salt, coke, and limestone, using ammonia as a catalyst. The Solvay process, which remains an important part of modern industry, came to provide most of the soda ash used commercially; however, by the late twentieth century, antipollution legislation and the high cost of energy, labor, and materials associated with the process had made natural deposits the preferred source of soda ash for the United States. Obtaining Soda Ash Natural soda ash is commonly obtained from trona, with approximately 2 metric tons of trona producing 1 metric ton of soda ash. Some soda ash is also produced from brines and by electrolytic methods. Soda ash can be synthesized using the Solvay process, in which soda ash is manufactured from salt, ammonia, carbon dioxide, and limestone. The limestone is heated to produce quicklime andcarbon dioxide; the carbon dioxide is dissolved in a solution of water, am - monia, and salt; and sodium bicarbonate precipitates and is filteredout,dried,andheatedtoform soda ash. Global Resources Soda ash • 1097 . entirely of silver-copper alloys. In addition, there is much electroplated silver, which consists of a thin coating of silver, or silver- copper alloy,overabaseofsomeothermetal. Because of the. black, but some slate is blue or purple. The color of slate is often mottled, streaked, or spotted. A 1094 • Slate Global Resources few large grains of various minerals may be found within the otherwise. may appear, which reveal the structure of the rock from which the slate was formed. Slate varies widely in color because of the presence of small particles of various minerals within it. Red and