Metals and metallurgy
Background (Encyclopedia of Global Resources)
Although the term “metal” is difficult to define absolutely, there are two working definitions that include almost three-quarters of the elements of the periodic table classified as metals. Chemically, metals are those elements that usually form positive ions in solutions or in compounds and whose oxides form basic water solutions. Physically, metals contain free electrons that impart properties such as metallic luster and thermal and electrical conductivity. In the periodic table, all the elements found in Groups IA and IIA and in the B groups are metals. In addition, Groups IIIA, IVA (except carbon), VA (except nitrogen and phosphorus), and VIA (except oxygen and sulfur) are classified as metals. All the metals are lustrous and, with the exception of mercury, are solids at normal temperatures. Boron (IIIA), silicon and germanium (IVA), arsenic and antimony (VA), selenium and tellurium (VIA), and astatine (VIIA) show metallic behavior in some of their compounds and are known as metalloids.
The bonding in metals explains many of their physical characteristics. The simplest model describes a metal as fixed positive ions (the nucleus and completed inner shells of electrons) in a sea of mobile valence electrons. The ions are held in place by the electrostatic attraction between the positive ions and the negative electrons, which are delocalized over the whole crystal. Because of this electron mobility, metals are good conductors...
(The entire section is 288 words.)
Natural Abundance (Encyclopedia of Global Resources)
While all the known metals are found in the Earth’s crust, the abundance varies widely, from aluminum (over 81,000 parts per million) to such rare metals as osmium and ruthenium (approximately 0.001 part per million). The metalloid silicon is the second most abundant element in the Earth’s crust, with an abundance of more than 277,000 parts per million. Some of those metals found in low concentrations, such as copper and tin, are commonly used, while many of the more abundant metals, such as titanium and rubidium, are just beginning to find uses. The metal ore most important to modern industrial society, iron, is abundant and easily reduced to metallic form. The metals that were most important to early civilizations—gold, silver, mercury, lead, iron, copper, tin, and zinc—exist in large, easily recognized deposits and in compounds that are easily reduced to elemental form.
Very few metals occur “free” in nature. The form in which a specific metal is found depends on its reactivity and on the solubility of its compounds. Many metals occur as binary oxides or sulfides in ores that also contain materials such as clay, granite, or silica from which the metal compounds must first be separated. Metals are also found as chlorides, carbonates, sulfates, silicates, and arsenides, as well as complex compounds of great variety such as LiAlSi2O6, which is a source of lithium.
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Metallurgy (Encyclopedia of Global Resources)
Metallurgy is a large field of science and art that encompasses the separation of metals from their ores, the making of alloys, and the working of metals to give them certain desired characteristics. The art of metallurgy dates from about 4000 b.c.e., when metalsmiths were able to extract silver and lead from their ores. Tin ores were obtained by 3000 b.c.e., and the production of bronze, an alloy of copper and tin, could begin. By 2700 b.c.e. iron was obtained. There is an obvious relationship between the discovery that metals could be refined and fabricated into objects such as tools and weapons and the rise of human civilizations. Early periods in the history of humankind have long been identified by the metals that became available. Throughout most of human history metallurgy was an art; the development of the science from the art has taken place gradually over the past few centuries.
The production of metals from their ores involves a three-step process: preliminary treatment in which impurities are removed, and possibly chemical treatment used to convert the metallic compound to a more easily reduced form; reduction to the free metal; and refining, in which undesirable impurities are removed and others are added to control the final characteristics of the metal.
The preliminary treatment involves physical as well as chemical treatment. Physical methods include grinding, sorting, froth flotation, magnetic separations,...
(The entire section is 694 words.)
Metals as Crystals (Encyclopedia of Global Resources)
When a metal solidifies, its atoms assume positions in a well-defined geometric pattern, a crystalline solid. The three most important patterns for metals are the body-centered cubic, the face-centered cubic, and the hexagonal. If atoms of one metal exist in the solid solution of another, the atoms of the minor constituent occupy positions in the crystal pattern of the major constituent. Since atoms of each element have characteristic size, the presence of a “stranger” atom causes distortion of the pattern and, usually, strengthening of the crystal. This strengthening is one of the major reasons that most metals are used as alloys—in solid solutions of two or more constituent metals.
Zinc is a hexagonal crystal, while copper atoms occupy the sites of a face-centered cubic lattice. As the larger zinc atoms occupy positions in the copper lattice, they distort the crystal and make it harder to deform. Brass, an alloy of copper and zinc, increases in hardness as the zinc concentration increases up to 36 percent, at which point the crystal changes to a body-centered cubic pattern with markedly different characteristics. Careful selection of various combinations of elements in differing concentrations can produce alloys with almost any desired characteristics.
The carbon steels are a good example of this variation. Various amounts of carbon and metals such as molybdenum are introduced into molten iron ore to create...
(The entire section is 254 words.)
Metals in Living Systems (Encyclopedia of Global Resources)
“Essential” metals are those whose absence will prevent some particular organism from completing its life cycle, including reproduction. These metals are classified, according to the amounts needed, as macronutrients or micronutrients. For animals the macronutrients are potassium, sodium, magnesium, and calcium. Sodium and potassium establish concentration differences across cell membranes by means of active transport and set up osmotic and electrochemical gradients. They are structure promoters for nucleic acids and proteins.
Magnesium, calcium, and zinc are enzyme activators and structure promoters. Magnesium is an essential component of chlorophyll, the pigment in plants responsible for photosynthesis. Calcium salts are insoluble and act as structure formers in both plants and animals. In muscles the calcium concentration is controlled to act as a neuromuscular trigger.
Among the important micronutrients are chromium and iron. In mammals, chromium is involved in the metabolism of glucose. The oxygen-carrying molecule in mammalian blood is hemoglobin, an iron-porphyrin protein. Many other metals are known to be important in varying amounts, but their specific activity is not yet clearly understood. This is and will continue to be an active field of research in biochemistry and molecular biology.
One of the interesting current techniques for studying the activity of metals on a cellular level is...
(The entire section is 284 words.)
Metals as Toxins (Encyclopedia of Global Resources)
Those materials that have a negative effect on metabolic processes in a specific organism are said to be toxic to that organism. Many metals fall into this category. Today toxic metals are found in the atmosphere and the waters of the Earth. Some are present because of natural processes such as erosion, forest fires, or volcanic eruptions, others because of the activities of humankind. The natural toxins are less problematic because many organisms, during the process of evolution, developed tolerances to what might be considered toxic.
Maintaining good air quality is a major problem for industrial nations. Highly toxic metals, whose long-term effects on the health of humans and the environment are of concern, have been released into the atmosphere in large quantities. The atmosphere is the medium of transfer of these toxins from the point of origin to distant ecosystems. Prior to the 1970’s, attention was focused on gaseous pollutants such as sulfur dioxide (SO2) and nitrogen oxide (NOx) and on total particulate matter. Since that time, improved analytical techniques have provided improved data on trace metals in the atmosphere, making studies on health effects possible.
The largest contributors to trace metal pollution are vehicular traffic, energy generation, and industrial metal production. For some metals, such as selenium, mercury, and manganese, natural emissions on a global scale far exceed those from...
(The entire section is 450 words.)
Further Reading (Encyclopedia of Global Resources)
Chandler, Harry. Metallurgy for the Non-Metallurgist. Materials Park, Ohio: ASM International, 1998.
Craddock, Paul, and Janet Lang. Mining and Metal Production Through the Ages. London: British Museum, 2003.
Moniz, B. J. Metallurgy. 4th ed. Homewood, Ill.: American Technical Publishers, 2007.
Neely, John E., and Thomas J. Bertone. Practical Metallurgy and Materials of Industry. 6th ed. Upper Saddle River, N.J.: Prentice Hall, 2003.
Nriagu, Jerome O., and Cliff I. Davidson, eds. Toxic Metals in the Atmosphere. New York: Wiley, 1986.
Street, Arthur, and William Alexander. Metals in the Service of Man. 10th ed. London: Penguin, 1994.
Wolfe, John A. Mineral Resources: A World Review. New York: Chapman and Hall, 1984.
(The entire section is 103 words.)