Metallurgy
is the science and technology of metals, including the extraction of metals
from ores, the preparation of metals for use, and the study of the relationship
between structures and properties of metals.
Metallurgical processes consist of two operations: concentration, separating a metal or metallic compound from the useless waste rock material, or gangue, which accompanies it in the ore; and refining, producing the metal in a pure or nearly pure state suitable for use. Three types of processes are employed both for concentration and refining: mechanical, chemical, and electrical. In most cases a combination of these methods is used.
One of the simplest methods of mechanical separation is gravity separation. This process is based on the difference in specific gravity between native metals and metallic minerals, and the other rock materials with which they are mixed. When crushed ore or ore concentrates are suspended either in water or an air blast, the heavier metal or metallic mineral particles fall to the bottom of the processing chamber, and the lighter gangue is blown or washed away. The prospector's technique of panning gold from gold-bearing sand, for example, is a small-scale gravity-separation process. Similarly, by virtue of its higher specific gravity, magnetite, a mineral of iron, may be separated from the gangue rock in which it occurs.
Flotation is the most important present-day method of mechanical concentration. In its simplest form, flotation is a modified gravity process in which finely ground ore is mixed, usually with a liquid. The metal or metallic mineral floats while the gangue sinks, although the reverse is true in some instances. In most modern flotation processes, the floating of either the metal or gangue is aided by an oil or other surface-active agent. By this means, comparatively heavy substances can be made to float on water. In one typical process, a finely ground ore containing copper sulfide is mixed with water, to which small amounts of oil, acid, or other so-called flotation reagents are added. When air is blown through this mixture, a froth is formed on the surface that has the property of mixing with the sulfide but not with the gangue. The latter material settles, and the sulfide is collected from the froth. Use of the flotation process has made possible the exploitation of many ore deposits of low concentration, and even of the wastes from processing plants that used less efficient techniques. In some cases, by means of differential flotation, different minerals can be concentrated from one complex ore in a single process.
Ores, such as magnetite, that have marked magnetic properties are concentrated by means of electromagnets that attract the metal but do not attract the gangue.
Electrostatic separation employs an electric field to separate minerals of different electrical properties by exploiting the attraction between unlike charges and the repulsion between like charges.
Chemical separation methods are, in general, the most important from the economic point of view. In present-day practice chemical separation often is used as a second stage after mechanical concentration. A greater tonnage of refined metal is obtained by smelting than by any other process. In smelting, the ore, or the concentrate from a mechanical separation process, is heated with a reducing agent and a flux to a high temperature. The reducing agent combines with the oxygen in a metallic oxide, leaving pure metal; and the flux combines with the gangue to form a slag that is liquid at the smelting temperature and thus can be skimmed off or poured away from the metal. The production of pig iron in blast furnaces is an example of smelting, and the process is also used to extract copper, lead, nickel, and many other metals from their ores.
Amalgamation is a metallurgical process that utilizes mercury to dissolve silver or gold to form an amalgam. This process has been largely supplanted by the cyanide process, in which gold or silver is dissolved in solutions of sodium or potassium cyanide. Various types of aqueous solutions are employed in different leaching, or percolating, processes to dissolve metals from ores. Metallic carbonates and sulfides are treated by roasting, heating to a temperature below the melting point of the metal. In the case of carbonates, carbon dioxide is driven off in the process, leaving a metallic oxide. When sulfides are roasted, the sulfur combines with the oxygen of the air to form gaseous sulfur dioxide, leaving metallic oxides, which are subsequently reduced by smelting.
Agglomeration of ore fines (fine particles) is accomplished by sintering or pelletizing. In the sintering process, fuel, water, air, and heat are used to fuse the ore fine into a porous mass. In pelletizing, moistened fine is formed into small pellets in the presence of limestone flux and then fired.
A number of other processes, of which pyrometallurgy (high-temperature metallurgy) and distillation are the most important, are employed in further refinement stages of a variety of metals. In the process of electrolysis the metal is deposited at the cathode from aqueous solutions or in an electrolytic furnace. Copper, nickel, zinc, silver, and gold are several examples of metals that are refined by deposition from aqueous solutions. Aluminum, barium, calcium, magnesium, beryllium, potassium, and sodium are metals that are processed in electrolytic furnaces.
Metallurgical processes consist of two operations: concentration, separating a metal or metallic compound from the useless waste rock material, or gangue, which accompanies it in the ore; and refining, producing the metal in a pure or nearly pure state suitable for use. Three types of processes are employed both for concentration and refining: mechanical, chemical, and electrical. In most cases a combination of these methods is used.
One of the simplest methods of mechanical separation is gravity separation. This process is based on the difference in specific gravity between native metals and metallic minerals, and the other rock materials with which they are mixed. When crushed ore or ore concentrates are suspended either in water or an air blast, the heavier metal or metallic mineral particles fall to the bottom of the processing chamber, and the lighter gangue is blown or washed away. The prospector's technique of panning gold from gold-bearing sand, for example, is a small-scale gravity-separation process. Similarly, by virtue of its higher specific gravity, magnetite, a mineral of iron, may be separated from the gangue rock in which it occurs.
Flotation is the most important present-day method of mechanical concentration. In its simplest form, flotation is a modified gravity process in which finely ground ore is mixed, usually with a liquid. The metal or metallic mineral floats while the gangue sinks, although the reverse is true in some instances. In most modern flotation processes, the floating of either the metal or gangue is aided by an oil or other surface-active agent. By this means, comparatively heavy substances can be made to float on water. In one typical process, a finely ground ore containing copper sulfide is mixed with water, to which small amounts of oil, acid, or other so-called flotation reagents are added. When air is blown through this mixture, a froth is formed on the surface that has the property of mixing with the sulfide but not with the gangue. The latter material settles, and the sulfide is collected from the froth. Use of the flotation process has made possible the exploitation of many ore deposits of low concentration, and even of the wastes from processing plants that used less efficient techniques. In some cases, by means of differential flotation, different minerals can be concentrated from one complex ore in a single process.
Ores, such as magnetite, that have marked magnetic properties are concentrated by means of electromagnets that attract the metal but do not attract the gangue.
Electrostatic separation employs an electric field to separate minerals of different electrical properties by exploiting the attraction between unlike charges and the repulsion between like charges.
Chemical separation methods are, in general, the most important from the economic point of view. In present-day practice chemical separation often is used as a second stage after mechanical concentration. A greater tonnage of refined metal is obtained by smelting than by any other process. In smelting, the ore, or the concentrate from a mechanical separation process, is heated with a reducing agent and a flux to a high temperature. The reducing agent combines with the oxygen in a metallic oxide, leaving pure metal; and the flux combines with the gangue to form a slag that is liquid at the smelting temperature and thus can be skimmed off or poured away from the metal. The production of pig iron in blast furnaces is an example of smelting, and the process is also used to extract copper, lead, nickel, and many other metals from their ores.
Amalgamation is a metallurgical process that utilizes mercury to dissolve silver or gold to form an amalgam. This process has been largely supplanted by the cyanide process, in which gold or silver is dissolved in solutions of sodium or potassium cyanide. Various types of aqueous solutions are employed in different leaching, or percolating, processes to dissolve metals from ores. Metallic carbonates and sulfides are treated by roasting, heating to a temperature below the melting point of the metal. In the case of carbonates, carbon dioxide is driven off in the process, leaving a metallic oxide. When sulfides are roasted, the sulfur combines with the oxygen of the air to form gaseous sulfur dioxide, leaving metallic oxides, which are subsequently reduced by smelting.
Agglomeration of ore fines (fine particles) is accomplished by sintering or pelletizing. In the sintering process, fuel, water, air, and heat are used to fuse the ore fine into a porous mass. In pelletizing, moistened fine is formed into small pellets in the presence of limestone flux and then fired.
A number of other processes, of which pyrometallurgy (high-temperature metallurgy) and distillation are the most important, are employed in further refinement stages of a variety of metals. In the process of electrolysis the metal is deposited at the cathode from aqueous solutions or in an electrolytic furnace. Copper, nickel, zinc, silver, and gold are several examples of metals that are refined by deposition from aqueous solutions. Aluminum, barium, calcium, magnesium, beryllium, potassium, and sodium are metals that are processed in electrolytic furnaces.