Copper was one of the first metals used by humans, and it has made vital contributions to sustaining and improving society since the dawn of civilization. Copper is easily stretched, molded, and shaped; is resistant to corrosion; and conducts heat and electricity efficiently. As a result, it continues to be a material of choice for a variety of domestic, industrial, and high-technology applications today.
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What is copper?
In its pure form or as an alloy, copper (Cu) is one of the most important metals in society. The pure metal has a face-centred cubic crystal structure, and there is no critical temperature at which this crystal structure changes. Consequently, it is ductile and possesses a high level of electrical and thermal conductivity, making it attractive for a wide range of ornamental and practical applications. With cold-working, copper becomes harder, but it can be made soft again with the heat treating process known as annealing.
Principal forms in which copper ores are found include native copper, porphyry copper, massive deposits, and mixed ores. Native copper is simply the metal found unadulterated in nature. Occasionally copper is still found in its native form, but more frequently it is mixed with other minerals, some of which may have value themselves. The amount of copper in an ore can vary from 0.4 percent to more than 12 percent. Porphyry copper deposits, in which the copper materials are more or less uniformly scattered throughout the rock, account for the greatest tonnage of metal in the producing areas of the world. The copper minerals in the upper portions of such deposits are in general oxides (copper chemically combined with oxygen), those in the lower levels sulfides (copper with sulfur). The host rock is porphyry, schist, or other rock. Massive deposits are of higher metal content but of more limited extent; they may be oxidized in the upper portion with sulfides lower down. In mixed ores, nickel, zinc, or lead can accompany the copper; when such ore is mined, these other metals also are refined and sold as by-products.
Although commercial deposits of copper ores occur in almost every continent, 70 percent of the world’s known reserves are found in seven countries: Chile, the United States, Russia, Congo (Kinshasa), Peru, Zambia, and Mexico. The greatest known reserve of copper ore in one body is the deposit at El Teniente mine in Chile.
When and where copper was discovered ?
Copper was discovered and first used during the Neolithic Period, or New Stone Age. Though the exact time of this discovery will probably never be known, it is believed to have been about 8000 bce. Copper is found in the free metallic state in nature; this native copper is the material that humans employed as a substitute for stone. From it they fashioned crude hammers and knives and, later, other utensils. The malleability of the material made it relatively simple to shape implements by beating the metal. Pounding hardened the copper so that more durable edges resulted; the bright reddish colour of the metal and its durability made it highly prized.
The search for copper during this early period led to the discovery and working of deposits of native copper. Sometime after 6000 bce the discovery was made that the metal could be melted in the campfire and cast into the desired shape. Then followed the discovery of the relation of metallic copper to copper-bearing rock and the possibility of reducing ores to the metal by the use of fire and charcoal. This was the dawn of the metallic age and the birth of metallurgy.
The early development of copper probably was most advanced in Egypt. As early as 5000 bce, copper weapons and implements were left in graves for the use of the dead. Definite records have been found of the working of copper mines on the Sinai Peninsula about 3800 bce, and the discovery of crucibles at these mines indicates that the art of extracting the metal included some refining. Copper was hammered into thin sheets, and the sheets were formed into pipes and other objects. During this period bronze first appeared. The oldest known piece of this material is a bronze rod found in the pyramid at Maydūm (Medum), near Memphis in Egypt, the date of origin being generally accepted as about 3700 bce.
Bronze, an alloy of copper and tin, is both harder and tougher than either; it was widely employed to fashion weapons and objects of art. The period of its extensive and characteristic use has been designated the Bronze Age. From Egypt the use of bronze rapidly spread over the Mediterranean area: to Crete in 3000 bce, to Sicily in 2500 bce, to France and other parts of Europe in 2000 bce, and to Britain and the Scandinavian area in 1800 bce.
About 3000 bce copper was produced extensively on the island of Cyprus. The copper deposits there were highly prized by the successive masters of the island—Egyptians, Assyrians, Phoenicians, Greeks, Persians, and Romans. Cyprus was almost the sole source of copper to the Romans, who called it aes cyprium (“ore of Cyprus”), which was shortened to cyprium and later corrupted to cuprum, from which comes the English name copper. The first two letters of the Latin name constitute the chemical symbol Cu.
The Copper Age in the Americas probably dawned between 100 and 200 ce. Native copper was mined and used extensively and, though some bronze appeared in South America, its use developed slowly until after the arrival of Columbus and other European explorers. Both North and South America passed more or less directly from the Copper Age into the Iron Age.
As man learned to fashion weapons from iron and steel, copper began to assume another role. Being a durable metal and possessed of great beauty, it was used extensively for household utensils and water pipes and for marine uses and other purposes that required resistance to corrosion. The unusual ability of this metal to conduct electric current accounts for its greatest use today.
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What are its characteristics?
- Strength: Copper is a weak metal with a tensile strength about half that of mild carbon steel. This explains why copper is easily formed by hand, but is not a good choice for structural applications.
- Toughness: Copper may not be strong, but it is not easy to break due to its high toughness. This property comes in handy for piping and tube applications, where a rupture can be dangerous and expensive.
- Ductility: Copper is very ductile and also very malleable. The electrical and jewelry industries benefit from the ductility of copper.
- Conductivity: Second only to silver, copper is not only an excellent conductor of electricity, but also of heat. As a result, copper serves well in applications such as cookware, where it quickly draws heat to the food inside.
- Bronze: 88-95% Cu by weight. Used in coins, cymbals and artwork.
- Aluminum bronze: 74-95% Cu by weight. Higher corrosion resistance than regular bronze and useful in marine applications.
- Brass: a wide range of alloys containing 50-90% Cu by weight. Made into everything from ammunition cartridges to doorknobs.
- Cupronickel: 55-90% Cu by weight. Used in coins, marine applications and in musical instrument strings.
- Nickel silver: 60% Cu by weight. Contains no silver, but has a similar appearance. Often made into musical instruments and jewelry.
- Beryllium copper: 97-99.5% Cu by weight. Incredibly strong but toxic copper alloy that does not spark, making it safe for use in dangerous gas environments.
What are the various stages of metal processing?
The extraction of copper from ore is normally carried out in three major steps.
- The first step, mineral processing, is to liberate the copper minerals and remove waste constituents—such as alumina, limestone, pyrite, and silica—so that the copper minerals and other nonferrous minerals of value are concentrated into.
In the ore-dressing plant, the material received from the mine is crushed in several stages and finely ground to a size which ensures that copper minerals are liberated from the waste materials, or gangue. In cases where the next step is leaching (most frequently in the case of oxide ores), complete liberation of the copper minerals is not always necessary; the ore needs to be crushed and ground only to the extent required to expose the surface of the minerals to the leaching agent. For sulfide ores, on the other hand, selective flotation normally follows the crushing and grinding stage and requires an optimal degree of liberation.
In the flotation process, the finely ground ore, mixed with water and special reagents, is agitated by mechanical and pneumatic devices. These produce air bubbles in the ore-water mixture, or slurry. The reagents provide an attraction between the surface of the copper minerals and the air bubbles. As the bubbles rise to the surface, they carry the copper minerals with them, leaving gangue minerals in the cell to be discarded as tailings. Collection of the froth from the surface of the flotation cell yields a copper concentrate. To increase the recovery of copper and reduce losses, the tailings are frequently reground and passed through a second flotation, the concentrate from which is combined with the initial production. The flotation concentrate is then dewatered and filtered to produce a filter cake that is sent to a copper smelter.
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- The second step, involving either smelting or leaching, removes a large proportion of impurity elements—in particular iron and, in the case of sulfide ores, sulfur.
Once a concentrate has been produced containing copper and other metals of value (such as gold and silver), the next step is to remove impurity elements. In older processes the concentrate, containing between 5 and 10 percent water, is first roasted in a cylindrical, refractory-lined furnace of either the hearth or fluidized-bed type. As concentrate is fed into the roaster, it is heated by a stream of hot air to about 590 °C (1,100 °F). Volatile impurities such as arsenic, mercury, and some of the sulfur are driven off, the sulfur being removed as sulfur dioxide. What remains is an oxidized product containing a percentage of sulfur that is sufficiently low for smelting. This is traditionally done in a reverberatory or electric-arc furnace, into which concentrate is fed along with a suitable amount of flux, usually silica and occasionally limestone. These are heated by combusted fuel or electric current to a temperature of 1,230–1,300 °C (2,250–2,370 °F), producing an artificial copper-iron sulfide that settles in a molten pool at the bottom of the furnace. The sulfide material, known as matte, contains from 45 to 70 percent copper, depending on the particular process. Gangue minerals and oxidized impurities, including most of the iron, react with the flux and form a light, fluid layer of slag over the matte. A certain percentage of the volatile impurities, such as sulfur, is oxidized and leaves with the process gas stream.
The traditional two-stage process described above has to a large extent been replaced by newer flash or bath smelting processes. These begin with a dry concentrate containing less than 1 percent water, which, along with flux, is contacted in a furnace by a blast of oxygen or oxygen-enriched air. Iron and sulfur are oxidized, and the heat generated by these exothermic reactions is sufficient to smelt the concentrate to a liquid matte and slag. Depending on the composition of the concentrate, it is possible to carry out smelting autogenously—that is, without the use of auxiliary fuel, as is required in reverberatory or electric-arc smelting. In addition to reducing the consumption of fuel, the new processes produce relatively low volumes of gas, which, being high in sulfur dioxide, is well suited to the production of sulfuric acid. New smelters are designed to capture 90 percent or more of the sulfur contained in the feed materials.
After the slag, which contains a large percentage of the impurity elements, is removed from the matte, the remaining iron and sulfur are removed in the conversion process. The converter is a cylindrical steel shell, normally about four metres in diameter and lined with refractory brick. After being charged with matte, flux, and copper scrap (to control temperature), the converter is rotated in order to immerse tuyeres in the molten bath. Air or oxygen-enriched air is then blown through the tuyeres into the fluid. Iron and sulfur are converted to oxides and are removed in either the gas stream or the slag (the latter being recycled for the recovery of remaining values), leaving a “blister” copper containing between 98.5 and 99.5 percent copper and up to 0.8 percent oxygen. The converter is rotated for skimming the slag and pouring the blister copper.
The conversion of liquid matte in a rotating converter is a batch operation, but newer continuous processes utilize stationary furnaces similar to those used in smelting. Continuous systems have the advantage of reducing the gaseous and particulate emissions normally produced during conversion.
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- The final step, refining, removes the last traces of the impurity elements and produces a copper product of 99.99 percent purity.
The final step consists of fire refining the blister copper to reduce the sulfur and oxygen to even lower levels. This oxidation-reduction process is usually carried out in a separate furnace to ensure that the final smelter product reaches the level of 99.5 percent copper that is required for electrolytic refining. At this point, the copper is cast into anodes, the shape and weight of which are dictated by the particular electrolytic refinery.
In the electrolytic process, copper anodes and starting sheets are immersed in an electrolytic solution made up of copper sulfate and sulfuric acid. An electric current is passed through the solution, and copper from the positively charged anode is deposited in a pure form on the negatively charged starting sheet, which acts as a cathode. Minor impurities, including precious metals, settle at the bottom of the cell as anode slimes for further processing. Copper in solution from a hydrometallurgical process is recovered in a similar electrolytic cell using a lead anode. Here, the electric current removes the copper from solution rather than from the anode, for deposition on a cathode starter sheet (when the metal is plated from solution in this manner, the process is known as electrowinning). Both processes are capable of producing cathode copper of more than 99.9 percent purity.
How is it used today?
- Wire and cable: Despite competition from other materials, copper remains the preferred electrical conductor in nearly all categories of electrical wiring except overhead electric power transmission where aluminium is often preferred. Copper wire is used in power generation, power transmission, power distribution, telecommunications, electronics circuitry, and countless types of electrical equipment. Electrical wiring is the most important market for the copper industry. This includes structural power wiring, power distribution cable, appliance wire, communications cable, automotive wire and cable, and magnet wire. Roughly half of all copper mined is used for electrical wire and cable conductors. Many electrical devices rely on copper wiring because of its multitude of inherent beneficial properties, such as its high electrical conductivity, tensile strength, ductility, creep (deformation) resistance, corrosion resistance, low thermal expansion, high thermal conductivity, ease of soldering, malleability, and ease of installation.
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- Electric motors: Copper’s superior conductivity enhances the efficiency of electrical motors. This is important because motors and motor-driven systems account for 43%–46% of all global electricity consumption and 69% of all electricity used by industry.Increasing the mass and cross section of copper in a coil increases the efficiency of the motor.
- Architecture: Copper has been used since ancient times as a durable, corrosion resistant, and weatherproof architectural material. Roofs, flashings, rain gutters, downspouts, domes, spires, vaults, and doors have been made from copper for hundreds or thousands of years. Copper’s architectural use has been expanded in modern times to include interior and exterior wall cladding, building expansion joints, radio frequency shielding, and antimicrobial and decorative indoor products such as attractive handrails, bathroom fixtures, and counter tops. Some of copper’s other important benefits as an architectural material include low thermal movement, light weight, lightning protection, and recyclability.
The metal’s distinctive natural green patina has long been coveted by architects and designers. The final patina is a particularly durable layer that is highly resistant to atmospheric corrosion, thereby protecting the underlying metal against further weathering. It can be a mixture of carbonate and sulfate compounds in various amounts, depending upon environmental conditions such as sulfur-containing acid rain.Architectural copper and its alloys can also be ‘finished’ to embark a particular look, feel, and/or color. Finishes include mechanical surface treatments, chemical coloring, and coatings.Copper has excellent brazing and soldering properties and can be welded; the best results are obtained with gas metal arc welding.
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