Metal Production

views updated

Metal Production

The term metal production refers to all of the processes involved in the conversion of a raw material, such as a metallic ore, to a final form in which the metal can be used for some commercial or industrial purpose. Within the periodic table there are some 90 elements that can be described as metals. They all have various characteristics in common ranging from bonding to chemical nature. Broadly speaking the metals are elements that conduct electricity, are malleable, and are ductile.

In some instances, metal production involves relatively few steps since the metal already occurs in an elemental form in nature. Such is the case with gold, silver, platinum, and other so-called noble metals. These metals normally occur in nature uncombined with other elements and can therefore be put to some commercial use with comparatively little additional treatment.

In the majority of cases, however, metals occur in nature as compounds, such as the oxide or the sulfide, and must first be converted to their elemental state. They may then be treated in a wide variety of ways in order to make them usable for specific practical applications.

Mining

The first step in metal production always involves some form of mining. Mining refers to the process of removing the metal in its free or combined state from Earth’s surface. The two most common forms of mining are surface and subsurface mining. In the former case, the metal or its ore can be removed from the upper few meters of Earth’s surface. Much of the world’s copper, for example, is obtained from huge open-pit mines may range in depth to as much as nearly a 0.6 mi (1 km) and in width to as much as more than 2.25 mi (3.5 km). Subsurface mining is used to collect metallic ores that are at greater depths below Earth’s surface.

A few metals can be obtained from seawater rather than or in addition to being taken from Earth’s crust. Magnesium is one example. Every cubic mile of seawater contains about six million tons of magnesium, primarily in the form of magnesium chloride. The magnesium is first precipitated out of seawater as magnesium hydroxide using lime (calcium hydroxide). The magnesium hydroxide is then converted back to magnesium chloride, now a pure compound rather than the complex mixture that comes from the sea. Finally, magnesium metal is obtained from the magnesium chloride by passing an electric current through a water solution of the compound.

Purification

In most cases, metals and their ores occur in the ground as part of complex mixtures that also contain rocks, sand, clay, silt, and other impurities. The first step in producing the metal for commercial use, therefore, is to separate the ore from waste materials with which it occurs. The term ore is used to describe a compound of a metal that contains enough of that metal to make it economically practical to extract the metal from the compound.

One example of the way in which an ore can be purified is the froth flotation method used with ores of copper, zinc, and some other metals. In this method, impure ore taken from the ground is, first, ground into a powder and, then, mixed with water and a frothing agent such as pine oil. Next, a stream of air is blown through the mixture, causing it to bubble and froth. In the frothing process, impurities such as sand and rock are wetted by the water and sink to the bottom of the container. The metal ore does not adsorb water but does adsorb the pine oil. The oil-coated ore floats to the top of the mixture, where it can be skimmed off.

Reduction

Metals always occur in their oxidized state in ores, often as the oxide or sulfide of the metal. In order to convert an ore to its elemental state, therefore, it must be reduced. Reduction is a chemical reaction that is the opposite of oxidation. Metals can be reduced in a variety of different ways.

With ores of iron, for example, reduction can be accomplished by reacting oxides of iron with carbon and carbon monoxide. One of the common devices used for this purpose is the blast furnace. The blast furnace is a tall cylindrical vessel into which is fed iron ore (consisting of oxides of iron), coke (nearly pure carbon) and limestone. The temperature in the blast furnace is then raised to more than 1,832^°F (1,000^°C). At this temperature, carbon reacts with oxygen to form carbon monoxide, which in turn, reacts with oxides of iron to form pure iron metal. The limestone in the original mixture added to the blast furnace reacts with and removes silicon dioxide (sand), an impurity commonly found with iron ore.

Some metallic oxides do not readily yield to chemical reduction reactions like those in the blast furnace process described above. The reduction of aluminum oxide to aluminum metal is an example. Until 1886, no economically satisfactory method for carrying out this process had been discovered. Then, as a young college chemistry student, American inventor and engineer Charles Martin Hall (1863–1914) invented a simple and inexpensive electrical method for reducing aluminum oxide. Because of Hall’s invention, aluminum gained widespread use throughout the world.

In the first step in this process, aluminum oxide is separated from other oxides (such as oxides of iron) with which it also occurs by the Bayer process. In the Bayer process, the naturally occurring oxide mixture is added to sodium hydroxide, which dissolves out aluminum oxide, leaving other oxides behind. The aluminum oxide is then dissolved in a mineral known as cryolite (sodium aluminum fluoride) and placed in an electrolytic cell. When electric current passes through the cell, molten aluminum metal is formed, sinks to the bottom of the cell, and can be drawn off from the cell.

In some instances, an ore is treated to change its chemical state before being reduced. The most common ores of zinc, for example, are the sulfides. These compounds are first roasted in an excess of air, converting zinc sulfide to zinc oxide. The zinc oxide is then reduced either by reacting it with coke (as in the case of iron) or by electrolyzing it (as in the case of aluminum).

Alloys

Pure metals themselves are often not satisfactory for many practical applications. For example, pure gold is too soft for most uses and is combined with other metals to form harder, more resistant mixtures. Mixtures that contain two or more metals are known as alloys. Perhaps the best known and most widely used of all alloys is steel.

The term steel refers to a number of different substances that contain iron as their major component along with one or more other elements. Stainless steel,

KEY TERMS

Alloy— A mixture of two or more metals with properties distinct from the metals of which it is made.

Bayer process— A process in which sodium hydroxide is added to a mixture of naturally occurring oxides so that aluminum oxide is dissolved out of the mixture.

Hall process— A process for the production of aluminum metal by passing an electric current through a mixture of aluminum oxide dissolved in cryolite (sodium aluminum fluoride).

Noble metal— A metal that does not readily react with other elements and that, therefore, normally occurs in nature in a free, or uncombined, state.

Ore— A compound of a metal from which the metal can be extracted at an economically feasible cost.

Reduction— The process by which an atom’s oxidation state is decreased, by its gaining one or more electrons.

as an example, contains about 18% chromium, 10% nickel, and small amounts of manganese, carbon, phosphorus, sulfur, and silicon, along with iron. When niobium is added to a steel alloy, the final product has unusually great strength. The addition of cobalt produces a form of steel that withstands the high temperatures of jet engines and gas turbines, and silicon steels are used in making electrical equipment.

In the final stages of metal production, the finished product is formed into some shape that can be used in other industries to make final products. Thus, steel can be purchased in the form of flat sheets, rings, wire rope and thread, slabs, cylinders, and other shapes.