Metallurgy << MEHT uh `lur` jee >> is the science of separating metals from their ores (the minerals or rocks in which they are found), preparing them for use, and improving their performance. Metallurgy falls into two major divisions: extractive, or process, metallurgy; and physical, or alloy, metallurgy.
Extractive metallurgy
Extractive metallurgy deals with taking metals from their ores and refining (purifying) them. It includes a wide variety of specialized commercial processes, such as mineral dressing, roasting, sintering, smelting, leaching, electrolysis, and amalgamation.
Mineral dressing
occurs between the mining of the ore and the extraction of metals from it. Mineral dressing removes as much of the waste materials as possible from the ore. This process usually begins by grinding the ore so that the metals in it, along with certain non metallic materials, separate from the waste. Then the various waste materials may be floated or washed away. In this flotation process, crushed ore is agitated (set in motion) in water with bubbles of air or gas. Various chemicals or oils that have been added to the water cause the mineral particles to stick to the bubbles. The minerals are then removed in a froth. The waste materials that remain are called gangue << gang >>. The removal of the gangue reduces the amount of ore that must be handled when the metal is extracted. Because this ore has a higher concentration of metal, it can be more economically refined into the final metal.
Roasting
removes sulfur and other impurities from the ore. When the ore is heated in air, the sulfur and certain other impurities combine with the oxygen in the air, and pass off as gases. The remaining solid material contains a metallic oxide (combination of metal and oxygen) that must be further purified or reduced to yield the pure metal.
Sintering
may occur when the temperature at which ores are roasted becomes very high. In this process, fine particles in contact with one another join to form coarse lumps. The joining is caused by surface tension. It is the same force that causes small water drops to combine into larger drops. Sintering is sometimes accompanied by partial melting of the fine particles, but the particles often remain entirely solid during the process. The coarse lumps produced by sintering are easier to handle and use in later processes.
Smelting.
After the ores have been subjected to such preliminary processes as dressing, roasting, or sintering, processors begin the actual work of extracting the metal. The usual method of metal extraction is smelting (melting the ore in such a way as to remove impurities). In the case of iron, for example, the ore is placed in a huge, brick-lined furnace called a blast furnace, and subjected to high heat. Quantities of coke and limestone also are placed in the furnace. As the heat of the furnace is raised, the coke begins to burn and give off carbon monoxide. This gas takes oxygen from the ore, helping to purify the iron.
Many of the other impurities of the ore melt and combine with the limestone to form a liquid collection of refuse (waste materials) which is usually lighter than the iron. This refuse rises to the top of the molten metal, and is taken from the furnace as slag. The slag is drawn off from holes in the side of the furnace at a height above the level of the molten iron. The molten iron is still not completely free of impurities. But almost all the iron has been taken from the ore. The metal must now be refined further, usually by causing it to react with oxygen in a furnace.
Leaching.
Some metals can be effectively separated from their ores by hydrometallurgy (leaching). This is a method of dissolving the metal out of the ore with a chemical solvent. The metal may then be recovered from the chemical solution by a process called precipitation. For example, gold is usually separated from its ore by treating the ore with a dilute alkaline solution of sodium cyanide. After the gold is dissolved in the sodium cyanide, it is placed in contact with metallic zinc. This causes all the gold to precipitate (separate) from the solution and gather on the metallic zinc.
Electrolysis.
After the metal has been taken from its ore by leaching, it is sometimes recovered from the leaching solution by electrolysis. For example, copper is leached from some ores with sulfuric acid. Then it is placed in an electrolytic cell. There, electric current flows from a lead anode (positive pole) through the solution to a copper cathode (negative pole). The copper particles in the solution have a positive charge. These particles then seek their opposites, or the negatively charged copper cathode. Aluminum and magnesium also are recovered by electrolysis.
Electrolysis is also used to purify the metal. Copper is one of the metals that can be refined by electrolysis. The impure metal is used as the anode. When electric current is passed through the solution, the atoms of pure copper on the anode give up electrons and pass into solution as positively charged particles. These particles pass through the solution toward the cathode. There, they acquire the necessary electrons to become neutral copper atoms. Most impurities are left behind, and a plating of purified copper forms on the cathode.
Amalgamation
is a method that is sometimes used to recover gold and silver from their ores. A solution carries finely ground particles of ore over plates covered with mercury. The mercury attracts the metal and combines with it. The mercury forms an alloy, called an amalgam, with the gold or silver. Then the amalgam is heated. The heat causes the mercury to come to a boil and pass off as a gas, which is recovered and recycled. This process leaves behind a metallic sponge of pure gold or silver.
Physical metallurgy
Physical metallurgy is the branch of metallurgy that adapts metals for their final use and improves their performance. It includes any operation used to process a refined commercial metal into a finished product.
Different metals may be mixed to create a metal with special properties. Metal mixtures are called alloys. For example, mixing steel with nickel and chromium produces a strong, chemically resistant material called stainless steel. Heat treatments may improve such properties as strength and ductility (ability to be shaped). New metallic alloys include nickel base super alloys, which are used in jet engines. Shape memory alloys change shape in response to changes in temperature.
Metal is formed into its final shape by casting, rolling, forging, welding, pressing, extrusion, drawing, stamping, and other methods. Surface treatment may include heating and carburizing (combining with carbon). A coating may also be applied to the metal’s surface. For example, in a process known as galvanizing, a thin layer of zinc is applied to iron or steel to prevent rust.
New combinations of metallic and nonmetallic materials, called composites, are replacing traditional metallic alloys for many uses. Some composites are made of nonmetallic materials only. These include fiberglass and carbon epoxy, which is used in sports equipment and jet fighters. Another new approach involves the production of powders of metals and nonmetals. For example, in a process called gas atomization, droplets of molten metal are sprayed with gas to form very fine solid particles. These particles can be combined at high temperatures and under high pressure to form alloys with special properties.
History
Metallurgy is one of the oldest sciences. The people of prehistoric times knew something of physical metallurgy. For example, the ancient Chinese and Egyptians found gold and silver in their pure state as grains and nuggets, and hammered the metal into many different kinds of ornaments and other objects. The American Indians found large amounts of pure copper in the area near Lake Superior and hammered the metal into weapons, implements, and jewelry.
Sometime before written history began, some of the ancient peoples discovered the simplest principles of smelting metals from their ores. Lead was probably the first metal ever to be separated from its ore by smelting, because this ore is very easy to smelt. But as long as 4,000 years ago, the Egyptians knew how to separate iron from its ore–and iron ore is considered one of the hardest to smelt. By the time of the Assyrian civilization, the smelting of iron ore was a highly developed art. The ancient Assyrians even knew how to change iron into steel. In the Middle Ages, when alchemists were studying ways to make gold from other substances, great advances were made in metallurgy. The alchemists laid the foundations of the modern science of metallurgy. See Metal (Metals through the ages).
Careers in metallurgy
A growing demand for new alloys and composites and new treatments for pure metals has increased the importance of metallurgy as a career. Metallurgists, also called metallurgical engineers, can find jobs in industries that extract, adapt, and use metals. The mining industry also employs large numbers of metallurgists. Other openings are available in other industries, in government, and in research.
Metallurgists and other persons interested in materials production have increased their efforts to explain complex metallurgical behavior in terms of the basic laws of physics and chemistry. They also have extended the use of metallurgical research methods and skills to such nonmetallic materials as ceramics, semiconductors, plastics, organic solids, and glass. The name materials science has been given to the field that deals with both metals and nonmetals.
People interested in metallurgy or materials science as a career should have an aptitude in science and should take as many high-school science and mathematics courses as possible. Most jobs require a bachelor of science degree in metallurgical or materials engineering. Most research positions require an advanced degree.
