When an ore has a low percentage of the desired metal, a method of physical concentration must be used before the extraction process begins. In one such method, the ore is crushed and placed in a machine where, by shaking, the heavier particles containing the metal are separated from the lighter rock particles by gravity. Another method is the flotation process, used commonly for copper sulfide ores. In certain cases (as when gold, silver, or occasionally copper occur "free," i.e., uncombined chemically in sand or rock), mechanical or ore dressing methods alone are sufficient to obtain relatively pure metal. Waste material is washed away or separated by screening and gravity; the concentrated ore is then treated by various chemical processes.
Processes for separating the metal from the impurities it is found with or the other elements with which it is combined depend upon the chemical nature of the ore to be treated and upon the properties of the metal to be extracted. Gold and silver are often removed from the impurities associated with them by treatment with mercury, in which they are soluble. Another method for the separation of gold and silver is the so-called cyanide process. The Parkes process, which is based on silver being soluble in molten zinc while lead is not, is used to free silver from lead ores. Since almost all the metals are found combined with other elements in nature, chemical reactions are required to set them free. These chemical processes are classified as pyrometallurgy, electrometallurgy, and hydrometallurgy.
Pyrometallurgy, or the use of heat for the treatment of an ore, includes smelting and roasting. If the ore is an oxide, it is heated with a reducing agent, such as carbon in the form of coke or coal; the oxygen of the ore combines with the carbon and is removed in carbon dioxide, a gas (see oxidation and reduction). The waste material in the ore is called gangue; it is removed by means of a substance called a flux which, when heated, combines with it to form a molten mass called slag. Being lighter than the metal, the slag floats on it and can be skimmed or drawn off. The flux used depends upon the chemical nature of the ore; limestone is usually employed with a siliceous gangue. A sulfide ore is commonly roasted, i.e., heated in air. The metal of the ore combines with oxygen of the air to form an oxide, and the sulfur of the ore also combines with oxygen to form sulfur dioxide, which, being a gas, passes off. The metallic oxide is then treated with a reducing agent. When a carbonate ore is heated, the oxide of the metal is formed, and carbon dioxide is given off; the oxide is then reduced.
Electrometallurgy includes the preparation of certain active metals, such as aluminum, calcium, barium, magnesium, potassium, and sodium, by electrolysis: a fused compound of the metal, commonly the chloride, is subjected to an electric current, the metal collecting at the cathode.
Hydrometallurgy, sometimes called leaching, involves the selective dissolution of metals from their ores. For example, certain copper oxide and carbonate ores are treated with dilute sulfuric acid, forming water-soluble copper sulfate. The metal is recovered by electrolysis of the solution. If the metal obtained from the ore still contains impurities, special refining processes are required.
See R. E. Reed-Hill et al., Physical Metallurgy Principles (1991); H. Chandler, Metallurgy for the Non-Metallurgist (1998); D. A. Brandt et al., Metallurgy Fundamentals (1999).
The earliest recorded metal employed by humans appears to be gold which can be found free or "native". Small amounts of natural gold have been found in Spanish caves used during the late Paleolithic period, c. 40,000 BC.
Silver, copper, tin and meteoric iron can also be found native, allowing a limited amount of metalworking in early cultures. Egyptian weapons made from meteoric iron in about 3000 B.C. were highly prized as "Daggers from Heaven. However, by learning to get copper and tin by heating rocks and combining copper and tin to make an alloy called bronze, the technology of metallurgy began about 3500 B.C. with the Bronze Age.
The extraction of iron from its ore into a workable metal is much more difficult. It appears to have been invented by the Hittites in about 1200 B.C., beginning the Iron Age. The secret of extracting and working iron was a key factor in the success of the Philistines
Historical developments in ferrous metallurgy can be found in a wide variety of past cultures and civilizations. This includes the ancient and medieval kingdoms and empires of the Middle East and Near East, ancient Egypt and Anatolia (Turkey), Carthage, the Greeks and Romans of ancient Europe, medieval Europe, ancient and medieval China, ancient and medieval India, ancient and medieval Japan, etc. Of interest to note is that many applications, practices, and devices associated or involved in metallurgy were possibly established in ancient China before Europeans mastered these crafts (such as the innovation of the blast furnace, cast iron, steel, hydraulic-powered trip hammers, etc.). However, modern research suggests that Roman technology was far more sophisticated than hitherto supposed, especially in mining methods, metal extraction and forging. They were for example expert in hydraulic mining methods well before the Chinese, or any other civilization of the time.
A 16th century book by Georg Agricola called De re metallica describes the highly developed and complex processes of mining metal ores, metal extraction and metallurgy of the time. Agricola has been described as the "father of metallurgy
Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. In order to convert a metal oxide or sulfide to a purer metal, the ore must be reduced either physically, chemically, or electrolytically.
Extractive metallurgists are interested in three primary streams: feed, concentrate (valuable metal oxide/sulfide), and tailings (waste). After mining, large pieces of the ore feed are broken through crushing and/or grinding in order to obtain particles small enough where each particle is either mostly valuable or mostly waste. Concentrating the particles of a value in a form supporting separation enables the desired metal to be removed from waste products.
Mining may not be necessary if the ore body and physical environment are conducive to leaching. Leaching dissolves minerals in an ore body and results in an enriched solution. The solution is collected and processed to extract valuable metals.
Ore bodies often contain more than one valuable metal. Tailings of a previous process may be used as a feed in another process to extract a secondary product from the original ore. Additionally, a concentrate may contain more than one valuable metal. That concentrate would then be processed to separate the valuable metals into individual constituents.
The five most used metals are:
The general physical properties of metals are:
The properties of metals make them suitable for different uses in daily life.
Pure elemental metals are often too soft to be of practical use which is why much of metallurgy focuses on formulating useful alloys.
Common engineering metals include aluminium, chromium, copper, iron, magnesium, nickel, titanium and zinc. These are most often used as alloys. Much effort has been placed on understanding the iron-carbon alloy system, which includes steels and cast irons. Plain carbon steels are used in low cost, high strength applications where weight and corrosion are not a problem. Cast irons, including ductile iron are also part of the iron-carbon system.
Cupro-nickel alloys such as Monel are used in highly corrosive environments and for non-magnetic applications. Nickel-based superalloys like Inconel are used in high temperature applications such as turbochargers, pressure vessels, and heat exchangers. For extremely high temperatures, single crystal alloys are used to minimize creep.
"Cold working" processes, where the product’s shape is altered by rolling, fabrication or other processes while the product is cold, can increase the strength of the product by a process called work hardening. Work hardening creates microscopic defects in the metal, which resist further changes of shape.
Welding is a technique for joining metal components by melting the base material. A filler material of similar composition may also be melted into the joint.
Brazing is a technique for joining metals at a temperature below their melting point. A filler with a melting point below that of the base metal is used, and is drawn into the joint by capillary action. Brazing results in a mechanical and metallurgical bond between work pieces.
Soldering is a method of joining metals below their melting points using a filler metal. Soldering results in a mechanical joint and occurs at lower temperatures than brazing, specifically below 450 C (840 F).
Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. Common heat treatment processes include annealing, precipitation strengthening, quenching, and tempering. The annealing process softens the metal by allowing recovery of cold work and grain growth. Quenching can be used to harden alloy steels, or in precipitation hardenable alloys, to trap dissolved solute atoms in solution. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.
Electroplating is a common surface-treatment technique. It involves bonding a thin layer of another metal such as gold, silver, chromium or zinc to the surface of the product. It is used to reduce corrosion as well as to improve the product's aesthetic appearance.
Case hardening is a process in which an alloying element, most commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.
Metallurgists study the microscopic and macroscopic properties using metallography, a technique invented by Henry Clifton Sorby. In metallography, an alloy of interest is ground flat and polished to a mirror finish. The sample can then be etched to reveal the microstructure and macrostructure of the metal. A metallurgist can then examine the sample with an optical or electron microscope and learn a great deal about the sample's composition, mechanical properties, and processing history.
Crystallography, often using diffraction of x-rays or electrons, is another valuable tool available to the modern metallurgist. Crystallography allow the identification of unknown materials and reveals the crystal structure of the sample. Quantitative crystallography can be used to calculate the amount of phases present as well as the degree of strain to which a sample has been subjected.
Substances on the Earth's surface will come in contact with air, water or acids. A major concern for the use of metals is their corrosion. The shiny surface of many metals becomes dull in time. This is due to a slow chemical reaction between the surface of the metal and oxygen in the air, this is typically a surface coating of the metal oxide. The general word equation is:
metal+oxygen → metal oxide
For example: The dull appearance of the metal lead is due to a coating of lead oxide.
If the surface is scratched then the shiny lead metal can be seen underneath.
Heating can speed up the reaction with oxygen. If a piece of copper is heated it quickly becomes coated in black copper oxide. The word equation is:
copper + oxygen → copper oxide