Metals and metallurgy

Enormous amounts of mineral resources are mined each year to supply society’s requirements for metals. In addition, large amounts of carbon, oxygen, and electricity are consumed in the various metallurgical processes by which the raw materials are converted for use.

Background

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 of electricity and thermal energy. This electron sea also shields neighboring layers of positive ions as they move past one another. Therefore most metals are ductile (capable of being drawn into wires) and malleable (capable of being spread into sheets). The absorption of electromagnetic radiation by the mobile valence electrons and its reemission as visible light explains the luster that is characteristic of metals.

Natural Abundance

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.

Metallurgy

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, and gravity concentration. Chemical reactions may also be used for concentration. The use of cyanide solution to extract gold from its ores is an example of chemical concentration. In 1890, Karl Bayer devised a process which is based on the fact that aluminum trihydrate dissolves in hot caustic soda but other materials in bauxite do not. The result is almost pure Al₂O₃. Frequently, many metals present in small percentages are found in ores with more abundant metals. The processes used to concentrate the primary metal also concentrates the minor ones as well and makes their extraction possible. Most ores are mined and processed for more than one metal. Iron is a notable exception.

Large-scale redox reactions are the means by which metals from ores are reduced to free metals. The particular method used depends on the reactivity of the metal. The most active metals, such as aluminum, magnesium, and sodium, are reduced by electrolytic reduction. Metal oxides are usually reduced by heating with carbon or hydrogen. This age-old process produces by far the greatest volume of free metals such as iron, copper, zinc, cadmium, tin, and nickel. Sulfides are usually roasted in air to produce oxides, which are then reduced to the free metal. Some sulfides, such as copper sulfide, produce the free metal directly by roasting.

The refining step encompasses an array of processes designed to remove any remaining impurities and to convert the metal to a form demanded by the end user. The major divisions of refining are pyrometallurgy, or fire refining, and electrometallurgy, or electrolysis. There are a few processes that do not fall into either of these major divisions such as the gaseous diffusion of uranium hexafluoride molecules to produce isotopically enriched uranium for the nuclear power industry.

Pyrometallurgy is a general name for a number of processes, including, but not limited to, roasting (heating to a temperature where oxidation occurs without melting, usually to eliminate sulfides); calcining (heating in a kiln to drive off an undesirable constituent such as carbon, which goes off as CO₂); and distilling (heating the mineral containing the metal to decomposition above the melting point of the metal, which is collected in a condenser).

Electrolytic refining involves immersing an anode of impure metal and a cathode of pure metal in a solution of ions of the metal and passing an electric current through it. Metal ions from the solution plate out on the cathode and are replaced in the solution by ions from the anode. Impurities either drop to the bottom as sludge or remain in solution. These by-products, often containing gold, silver, and platinum, are later recovered by additional processes. Electrolytic refining is expensive in terms of the electricity required and of the often toxic solutions remaining to be safely disposed of.

Metals as Crystals

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 desired strength, ductility, or malleability in the finished steel product. Another example is the intentional doping of the semiconductor silicon with boron or phosphorus to create different conduction capabilities.

Metals in Living Systems

“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 fluorescent imaging. Metals such as calcium interact with fluorescent dyes. The dyes have different fluorescent characteristics in the presence or absence of specific metal. Special cameras, called charge coupled devices (CCDs), are mounted on microscopes and feed electrical signals directly to a computer, which creates an image. Metal concentrations inside and outside cells can be studied in the presence and absence of other nutrients to establish relationships among the various materials that are needed to sustain viable cell activity.

Metals as Toxins

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 (NO_x) 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 anthropogenic sources. However, local manganese emissions from human-made sources in Europe far exceed those from natural sources. This illustrates the problem facing humankind. Emission patterns must be studied for local, regional, and global effects. Global emission patterns have been studied and compared with statistical information of the world’s use of ores, rocks, and fuels and to the production of various types of goods. These studies allow the major sources of various toxic metals to be identified.

Coal combustion has been identified as the chief emission source of beryllium, cobalt, molybdenum, antimony, and selenium. Nickel and vanadium come mainly from oil firing. Smelters and other noniron refining plants emit most of the arsenic, cadmium, copper, and zinc. Chromium and manganese are released as side products of iron refining and steel production. Finally, gasoline combustion is the main cause of lead pollution. Identification of the main culprits should point the way to the changes needed to reduce emission levels of these metals and to choices regarding future industrial growth. Installation of scrubbing devices for removal of toxic materials from gaseous emissions and replacement of old boilers will reduce some emissions. New coal technologies such as coal pyrolysis and in situgasification should also reduce the contamination of the environment to some degree. Much more data on regional and local patterns are necessary to restore the health of the atmosphere.

Bibliography

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