Metallurgy

Summary

Starting as an art and a craft thousands of years ago, metallurgy has evolved into a science concerned with processing and converting metals into usable forms. The conversion of rocky ores into finished metal products involves a variety of activities. After the ores have been mined and the metals extracted from them, the metals need to be refined into purer forms and fashioned into usable shapes such as rolls, slabs, ingots, or tubing.

Another part of metallurgy is developing new types of alloys and adapting existing materials to new uses. The atomic and molecular structure of materials are manipulated in controlled manufacturing environments to create materials with desirable mechanical, electrical, magnetic, chemical, and heat-transfer properties that meet specific performance requirements.

Definition and Basic Principles

Metallurgy is the science of extracting metals and intermetallic compounds from their ores and working and applying them based on their physical, chemical, and atomic properties. It is divided into two main areasextractive metallurgy and physical metallurgy.

Extractive Metallurgy. Extractive metallurgy, also known as process or chemical metallurgy, deals with mineral processing, or converting metal compounds to more treatable forms and then refining them. Mineral processing involves separating valuable minerals of an ore from other raw materials. The ore is crushed to below a certain size and ground to powder. The mineral and waste rock are separated using a method based on the mineral's properties. After that, water is removed from the metallic concentrate or compound. Because metallic compounds are often complex mixtures, such as carbonates, sulfides, and oxides, they need to be converted to other forms for easier processing and refining. Carbonates are converted to oxides; sulfides to oxides, sulfates, and chlorides; and oxides to sulfates and chlorides. Depending on the type of metallic compound, either pyrometallurgy or hydrometallurgy is used for conversion. Both processes involve oxidation and reduction, or redox, reactions. In oxidation, the metallic element is combined with oxygen. In reduction, a reducing agent is used to remove the oxygen from the metallic element. The difference between pyrometallurgy and hydrometallurgy, as their names imply, is that the former uses heat, while the latter uses aqueous (water-based) chemical solutions. These two processes also include refining the metallic element in the final stage of extractive metallurgy when heat and chemicals are used. Electrometallurgy refers to the use of electrolytic processes to refine metal elements, precipitate dissolved metal values, and recover them in solid form.

Physical Metallurgy. Physical metallurgy deals with making metal products based on knowledge of the crystal structures and properties (chemical, electrical, magnetic, and mechanical) of metals. Metals are mixed together to make alloys. Heat is used to harden metals, and their surfaces can be protected with a metallic coating. Through a process called powder metallurgy, metals are turned into powders, compressed, and heat treated to produce a desired product. Metals can be formed into their final shapes by such operations as casting, forging, or plastic deformation. Metallography is a subfield of metallurgy that studies the microstructure of metals and alloys by various methods, especially by light and electron microscopes.

Background and History

Metallurgy came into being in the Middle East around 5000 BCE with the extraction of copper from its ore. The discovery of the first alloy, bronze—the result of melting copper and tin ores together—initiated the Bronze Age (4000–3000 BCE). Melting iron ore with charcoal to obtain iron marked the beginning of the Iron Age in Anatolia (2000–1000 BCE). Gold, silver, and lead were separated from lead-bearing silver in Greece about 500 BCE. Mercury was produced from cinnabar around 100 BCE, and it was later used to recover and refine various metals. Around 30 BCE, brass, the second alloy, was made from copper and zinc in Egypt, and another alloy, steel, was produced in India.

From the sixth to the nineteenth centuries, metallurgy focused on the development and improvement of the processes involved in obtaining iron, making steel, and extracting aluminum and magnesium from their ores. The blast furnace was developed in the eighth century and spread throughout Europe. During the sixteenth century, the first two books on metallurgy were written by Vannoccio Biringuccio, an Italian metalworker, and by Georgius Agricola, a German metallurgist.

Modern metallurgy began during the eighteenth century. Abraham Darby, an English engineer, developed a new furnace fueled by coke. Another English engineer, Sir Henry Bessemer, developed a steelmaking process in 1856. Great Britain became the greatest iron producer in the world, and Spain and France also produced large amounts of iron. About 1886, American chemist Charles Martin Hall and French metallurgist Paul-Louis-Toussaint Héroult independently developed a way to extract aluminum from its ore, which became known as the Hall-Héroult process. Aluminum soon became an important metal in manufactured goods.

Metallurgy did not emerge as a modern science with two branches, extractive and physical, until the twentieth century. The development and improvement of metallurgy were made possible by the application of knowledge of the chemical and physical principles of minerals.

How It Works

Crushing and Grinding of Ores. In the first step of mineral processing, two kinds of mechanized crushers are used to reduce ores. Jaw crushers reduce ores to less than 150 millimeters (mm) and cone crushers to less than 10–15 mm. Different kinds of grinding mills are used to reduce crushed ores to powdercylinder mills filled with grinding bodies (stones or metal balls), autogenous mills (coarsely crushed ores grinding themselves), semi-autogenous mills using some grinding bodies, and roll crushers, which combine crushing and grinding.

Separating Valuable and Waste Minerals. The process used in the next step of mineral processing depends on the properties of the minerals. Magnetic separation is used for strongly magnetic minerals such as iron ore and iron-bearing ore. Gold, tin, and tungsten ores require gravity separation. A process called flotation separation is widely used for hydrophilic (water-attracting) intergrown ores containing copper, lead, and zinc. Electrostatic separation works best with particles of different electric charges, such as mineral sands bearing zircon, rutile, and monazite.

Pyrometallurgy. Pyrometallurgy is a method of converting metallic compounds to different forms for easier processing and refining by using oxidation and reduction reactions. The first conversion process, roasting, has two main types. One type changes sulfide compounds to oxides, and the other reduces an oxide to a metal. Other types of roasts convert sulfides to sulfates or change oxides to chlorides. These processes are carried out in different kinds of steel roasters.

The second conversion process, smelting, separates a metallic compound into two partsan impure molten metal and a molten slag. The two types of smelting are reduction and matte, and the processes are done in many kinds of blast furnaces. Coke is used for fuel and limestone as a flux for making slag. Reduction smelting converts an oxide feed material to a metal and an oxide slag. Matte smelting converts a sulfide feed material to a mixture of nickel, copper, cobalt, and iron sulfides as well as an iron and silicon oxide slag.

Refining, a process of removing any impurities left after roasting or smelting, also can be done in a blast furnace. Iron, copper, and lead can be refined in an oxidation reaction that removes impurities as an oxide slag or an oxide gas. Fire refining can separate copper from its impurities of zinc, tin, iron, lead, arsenic, and antimony. Similarly, lead can be separated from such impurities as tin, antimony, and arsenic, and zinc from impurities of cadmium and lead.

Hydrometallurgy. Another method of converting metallic compounds to different forms is hydrometallurgy. It uses several types of leach solventsammonium hydroxide for sulfides and carbonates; sulfuric acid, sodium carbonate, or sodium hydroxide for oxides; and sulfuric acid or water for sulfates. The dissolved metal values are then recovered from the leaching solution in solid form. Although numerous recovery processes exist, they usually involve electrolysis. By a process called precipitation, gold that has been dissolved in sodium cyanide and placed in contact with zinc is separated from the solution and gathered on zinc. In another process, called electrolytic deposition or electrowinning, an electric current is passed through the leach solution with dissolved metals, causing metal ions to deposit at the cathode. Copper, zinc, nickel, and cobalt can be obtained this way.

Electrometallurgy.Electrolysis can be used to refine metallic elements as well as to recover them after hydrometallurgical treatment. Copper, nickel, lead, gold, and silver can be refined this way. In this method, for example, impure copper is used as the anode. When the electric current passes through the solution, atoms of pure gold travel to the cathode, acquire electrons and become neutral copper atoms. Electrolysis is also the process of recovering copper, aluminum, and magnesium in hydrometallurgy.

Alloys. Alloys are made by mixing pure metals together to obtain a substance with increased strength, increased corrosion resistance, lower cost, lower melting points, or desirable magnetic, thermal, or electrical properties. They are usually made by melting the base metals and adding alloying agents. Stainless steel, a mixture of steel, nickel, and chromium, is stronger and more chemically resistant than the base metals from which it is formed.

Powder Metallurgy. Powder metallurgy is the process of reducing metals and nonmetals to powder and shaping the powder into a solid form under great heat and pressure. Metal powders are usually produced by the atomization of streams of molten metal with a spinning disk or with a jet of water, air, or inert gas. After the powders are cold pressed for initial adhesion, they are heated to temperatures about 80 percent below the melting point of the major component. Friction between powders and pressing dies is reduced by adding lubricants, and porosity in the final product is eliminated by applying pressure.

Metal Forming. Metals are usually cast into ingots in iron molds. Casting is also carried out in molds made of sand, plaster of paris, or glue. Permanent casting uses pressure or centrifugal action. Plastic deformation is performed on metals to change their properties and dimensions. If done below the recrystallization temperature, the process is called cold working. Above this temperature but below the melting or burning point, it is called hot working. Techniques involved include rolling, pressing, extrusion, stamping, forging, and drawing. Surface treatments of metals include protective coating and hardening. In metallic coating, zinc and other metals such as chromium, cadmium, lead, and silver are often used. Surface hardening of metals is usually done with heat in a gas rich in carbon or in ammonia and hydrogen.

Applications and Products

The most important applications of metallurgy involve common metals and alloys and powder metallurgy technology.

Copper.Copper is ductile and malleable, and it resists corrosion and conducts heat and electricity. Copper and its alloy brass (copper plus zinc) are used to make coins, household fixtures (doorknobs, bolts), and decorative art objects (statues, sculptures, imitation-gold jewelry). It is also used in transportation vehicles and has many electrical applications (transformers, motors, generators, wiring harnesses). Its alloy bronze (copper plus tin) is used in plumbing and heating applications (water pipes, cooking utensils). Aluminum bronze is used to make tools and parts for aircraft and automobiles. Manganese bronze is used to make household fixtures and ship propellers.

Iron. Iron is ductile, malleable, and one of the three magnetic elements (the others are cobalt and nickel). Cast iron is resistant to corrosion and used to make manhole covers and engine blocks for gasoline and diesel engines. Wrought iron is used to make cooking utensils and outdoor household items such as fencing and furniture. Most iron is used to make steel. Steel is used as a structural material in the construction of large, heavy projects (bridges, ships, buildings) and automobile parts (body frames, radial-ply tires). When chromium and nickel are combined with steel, steel becomes stainless, and it is used to make flatware and surgical tools. Steel combined with cobalt is used to make jet engines and gas turbines.

Gold. Applications of gold are based on such properties as its electrical and thermal conductivity, ductility, malleability, resistance to corrosion, and infrared reflectivity. Gold serves as a medium of exchange and money. Its decorative applications include jewelry, golf leaf on the surfaces of buildings, and flourishes on ceramics or glassware. More practical applications include components for electronic devices (telephones, computers), parts for space vehicles, and dental fillings, crowns, and bridges.

Silver.Silver is ductile and very malleable, conducts heat, and has the highest electrical conductivity of all metals. It is used to make cutlery, jewelry, coins, long-life batteries, photographic films, and electronic components (circuits, contacts), and in dentistry. Its alloy, sterling silver (silver plus copper) is also used to make jewelry and tableware. German silver (silver plus nickel) is another alloy used for silverware.

Platinum.Platinum is one of the densest and heaviest metals. This ductile and malleable material is resistant to corrosion and conducts electricity well. It is used to make jewelry, electronic components (hard disk drive coatings, fiber-optic cables), and spark plug components. It is important in making the glass for liquid crystal displays (LCDs), in the petrol industry as an additive and refining catalyst, in medicine (anticancer drugs, implants), and in dentistry. Its alloys (platinum plus cobalt or metals in the platinum groups) are mostly used to make jewelry.

Mercury. Sometimes called quicksilver, mercury is the only common metal that is liquid at ordinary temperatures. It is a fair conductor of electricity and of high density. It is used in barometers and thermometers to recover gold from its ore and to manufacture chlorine and sodium hydroxide. Its vapor is used in street lights, fluorescent lamps, and advertising signs. Mercury compounds have various uses, such as insecticides, rat poisons, disinfectants, paint pigments, and detonators. Mercury is easily alloyed with silver, gold, and cadmium.

Lead.Lead is malleable, ductile, resistant to corrosion, and of high density. Its softness is compensated for by alloying it with such metals as calcium, antimony, tin, and arsenic. Lead is a component in lead-acid batteries, television and computer screens, ammunition, cables, solders, and water drains, and is used as a coloring element in ceramic glazes.

Magnesium.Magnesium is the lightest structural metal, with low density (two-thirds that of aluminum), superior corrosion performance, and good mechanical properties. Because it is more expensive than aluminum, its applications are somewhat limited. Magnesium and its alloys are used in the bicycle industry, racing car industry (gearbox casings, engine parts), and aerospace industry (engines, gearbox casings, generator housings, wheels).

Manganese.Manganese is a hard but very brittle, paramagnetic metal. Mostly it is used in steel alloys to increase strength, hardness, and abrasion resistance. It can be combined with aluminum and antimony to form ferromagnetic compounds. It is used to give glass an amethyst color, in fertilizers, and in water purification.

Cobalt.Cobalt has a high melting point and retains its strength at high temperatures. It is used as a pigment for glass, ceramics, and paints. When alloyed with chromium and tungsten, it is used to make high-speed cutting tools. It is also alloyed to make magnets, jet engines, and gas turbine engines.

Tungsten.Tungsten has the highest melting point and the lowest thermal expansion of all metals, high electrical conductivity, and excellent corrosion resistance. It is used to make lightbulb filaments, electric contacts, and heating elements; as an additive for strengthening steel; and in the production of tungsten carbide. Tungsten carbide is used to make dies and punches, machine tools, abrasive products, and mining equipment.

Chromium.Chromium is a hard but brittle metal of good corrosion resistance. It is mostly alloyed with other metals, especially steel, to make final products harder and more resistant to corrosion. It is also used in electroplating, leather tanning, and refractory brick making, and as glass pigments.

Cadmium.Cadmium is resistant to corrosion, malleable, ductile, and of high electrical and thermal conductivity. It is mostly used in rechargeable nickel-cadmium batteries. It is also used to make electronic components and pigments for plastics, glasses, ceramics, enamels, and artists' colors.

Nickel.Nickel is a hard, malleable, and ductile metal that is highly resistant to corrosion. Like chromium, it is used to make stainless steel. Alloyed with copper, it is used for ship propellers and chemical industry plumbing. Other uses include rechargeable batteries, coinage, foundry products, plating, burglar-proof vaults, armor plates, and crucibles.

Aluminum.Aluminum has a density about one-third that of steel, high resistance to corrosion, and excellent electrical and thermal conductivity. Moreover, this nontoxic metal reflects light and heat well. This versatile metal can be used to replace other materials depending on the application. It is widely used in such areas as food packaging and protection (foils, beverage cans), transportation (vehicles, trains, aircraft), marine applications (ships, support structures for oil and gas rigs), and buildings and architecture (roofing, gutters, architectural hardware). Other applications of aluminum include sporting goods, furniture, road signs, ladders, machined components, and lithographic printing plates.

Special Alloys. Fusible alloys are mixtures of cadmium, bismuth, lead, tin, antimony, and indium. They are used in automatic sprinklers and in forming and stretching dies and punches. Superalloys are developed for aerospace and nuclear applicationscolumbium for reactors, tantalum for rocket nozzles and heat exchangers in nuclear reactors, and a nickel-based alloy for jet and rocket engines and electric heating furnaces. The alloy of tin and niobium has superconductivity. It is used in constructing superconductive magnets that generate high field strengths without consuming much power.

Powder Metallurgy Applications and Products. Powder metallurgy was developed in the late 1920s primarily to make tungsten-carbide cutting tools and self-lubricating electric motor bearings. The technique was then applied in the automobile industry, where it is used to make precision-finished machine parts, permanent metal filters, bearing materials, and self-lubricating bearings. It is useful in fabricating products that are difficult to make by other methods, such as tungsten-carbide cutting tools, super magnets of aluminum-nickel alloy, jet and missile applications of metals and ceramics, and wrought powder metallurgy tool steel.

Careers and Course Work

High school students who wish to work as metallurgical technicians must take at least two years of mathematics and two years of science, including a physical science. Shop courses of any kind are also helpful. Positions in the metallurgical industry are typically in the areas of production, quality control, and research and development, which share many concerns and often require similar skills from prospective metallurgical technicians. Two years of study in metallurgy or materials science at a community college or technical college is therefore strongly recommended. Metallurgical technicians occupy a middle ground between engineers and skilled trade workers. Representative entry-level jobs include metallurgical laboratory technicians, metallographers, metallurgical observers, metallurgical research technicians, and metallurgical sales technicians. Students who are interested in these kinds of jobs should have an interest in science and average mathematical ability. Prospective technicians must be willing to participate in a wide variety of work and must be able to communicate well. Companies employing metallurgical technicians can be found in a wide variety of industries. Working environments vary depending on the area of activities.

A number of colleges and universities offer four-year programs in metallurgy. If students wish to become metallurgical engineers, they will need a bachelor's degree in materials science or metallurgical engineering. The first two years of college focus on subjects such as chemistry, physics, mathematics, and introductory engineering. In the following years, courses will focus on metallurgy and related engineering areas. Students who wish to become metallurgical engineers should be interested in nature and enjoy problem-solving. They also need to have good communication skills. There are basically three areas in which metallurgical engineers workextractive, physical, and mechanical metallurgy. Their work environment varies depending on their area of specialty. Companies employing metallurgical engineers include metal-producing and processing companies, aircraft companies, machinery and electrical equipment manufacturers, the federal government, engineering consulting firms, research institutes, and universities.

Social Context and Future Prospects

Metallurgy faces the challenges of reducing the effect of its processes on the air and land, making more efficient use of energy, and increasing the amount of recycling. To this end, industries increasingly use clean technologies and develop oxygen combustion methods that drastically reduce carbon dioxide and other emissions. For example, zinc, copper, and nickel are recovered from their ores through a technique in which pressure leaching is performed in an acid medium, followed by electrolysis in a conventional sulfuric acid medium. The method produces no dust, slag, or sulfur dioxide and is environmentally acceptable. It has been applied to zinc sulfide concentrates, nickel sulfides in Canada, and copper sulfide concentrates in the United States. Additionally, the steel industry has significantly reduced its release of chemicals into the air and water in the twenty-first century. However, manganese, chromium, and lead pollution continue to be problematic. Some facilities have employed carbon capture technologies to reduce their emissions.

For decades, the industry has aimed to increase energy efficiency and reduce environmental impact. North America's steel industry has reduced energy consumption by more than 60 percent since World War II. Manufacturing facilities increasingly generate power using environmentally friendly methods rather than purchasing it. Energy consumption was further decreased with the trend toward using scrap steel instead of natural resources to produce steel. Metallurgy companies have successfully increased recycling in the twenty-first century. In the United States, more steel is recycled yearly than paper, aluminum, plastic, and glass combined.

As technologies become available, more energy-efficient equipment and techniques are substituted. In addition to developing ways to lessen metallurgy's ecological impact, engineers continually improve processes for extracting and processing materials. As demand for metals with specific qualities increases, the industry must adapt while conserving, reusing, and recycling metals and finding uses for by-products. 

Steel production more than doubled in the first two decades of the twenty-first century. With the growth of clean energy products, metallurgy will continue to expand and develop. Producing energy-efficient technology requires more minerals than traditional products. For example, electric vehicles require six times the mineral products as conventional diesel and gasoline cars.

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