The New Student's Reference Work/Metallurgy

Metallurgy (mĕt′al-lûr′jy) is the science which deals with the extraction of metals from the ores in which they are found in mining (q.v.). Some metals are found as such, and are then said to be free, native or virgin metals. Gold and platinum usually occur free; silver, copper and bismuth often so. The other metals, as iron, lead, tin, zinc, nickel, mercury, antimony and aluminum invariably, or almost invariably, occur mineralized, that is, in combination with other elements as oxides, sulphides, sulphates, carbonates, silicates or chlorides.

Ores, as they are found and mined, usually contain large quantities of worthless materials, called gangue, consisting of common rocks and minerals. Such ores often are washed by machines called jigs, vanners, etc. to separate or concentrate the valuable material. In this process the lighter gangue is carried away by water and the heavier valuable minerals are collected. Usually it is necessary to crush and sort the ore by sieves before the operation is carried out. Where free gold occurs in sand or gravel a simpler method of washing is used. A stream of water carries the material through a long trough or sluice, the bottom of which is provided with grooves or riffles in which quicksilver (mercury) is placed. The heavy gold sinks to the bottom, and is there held by the mercury as an amalgam.

Treatment with mercury or amalgamation is often applied to compact ores of gold and silver in stamp-mills, which consist of arrangements like mortars and pestles worked by machinery. The stamping is usually done in the presence of water, and as fast as the ore becomes fine enough it passes with water through screens and over copper plates covered with a layer of mercury, where the valuable metals are caught. Mercury is also placed on plates within the mortar. Ores containing sulphides, tellurides etc. require heating in contact with air, or roasting, to burn off sulphur and other impurities before they are treated by amalgamation. Common salt is usually added to silver ores of this kind before roasting.

Certain gold and silver ores are powdered and leached, that is, lixiviated, with solutions of chemicals to dissolve out the precious metals. For instance, a weak solution of potassium cyanide is used to dissolve free gold in the cyanide process, and solution of sodium hyposulphite is employed to dissolve silver chloride from ores that have been roasted with salt.

The most important metallurgical process is smelting. In this operation the ore, often after it has been concentrated, and frequently after it has been roasted, is mixed with a flux, if necessary, and melted at a high temperature. The earthy materials of the ore with the flux form a fluid slag, while the action of the fuel, or other chemical action, produces a molten metal in some cases or, in other cases, a fused mixture of sulphides, called matte, which is much richer than the original ore in the amount of valuable metal it contains. In iron-smelting the ore, mixed with coke or coal as fuel and with limestone as flux, is fed in at the top of an enormous blast-furnace, a structure somewhat like a barrel in shape, but slenderer and much narrower at the bottom than elsewhere. Hot air, forced in through pipes near the bottom of the furnace, burns the fuel and produces great heat. Metallic iron and slag are formed, and are allowed to flow out from time to time through holes near the bottom; the metal, being heavier, runs out through a hole that is lower than the one used for the slag. The metal is called pig-iron, and is used in foundries for making articles of cast iron, as well as for making wrought iron and steel. (See Iron and Steel.)

Copper-ores are smelted in blast-furnaces that are much smaller than those used for iron. They are also sometimes smelted in reverberatory furnaces, in which the heating is accomplished by the flame from a coal-fire made in a separate compartment of the furnace or by means of gas. With certain copper-ores, as those containing the oxides or the carbonates, metallic copper may be the direct product of smelting; but the very common ores containing sulphides are at first smelted for copper matte, which consists chiefly of the sulphides of copper and iron. The metal is extracted from the matte by operations in which air at high temperatures burns out or oxidizes the sulphur and iron. When gold and silver are present in copper-ores, the precious metals are found in the metallic copper produced from them and are recovered by electrolytic refining, which consists in dissolving the metal and redepositing it by means of an electric current acting in an appropriate solution. The gold and silver and some other impurities are left undissolved in a finely-divided condition.

Lead-smelting is carried out both in reverberatory furnaces and in blast-furnaces. When galena, lead sulphide, the most common lead-ore, is obtained nearly pure, either directly from the mine or by concentration, it frequently is smelted by roasting it on the bed of a reverberatory furnace until it is partly changed to oxide and sulphate, then raising the heat to fusion and thus causing the oxidized part to act upon the remaining lead-sulphide with the formation of metallic lead and sulphur dioxide gas. The smelting of lead-ores in blast-furnaces is carried out particularly for the sake of obtaining the silver that the ores usually contain and also for obtaining the silver from ores containing little or no lead, which are purposely mixed with lead-ores with this end in view. If a sufficient proportion of lead is produced, practically all the silver (and the gold also) in the ores goes into the lead.  The greater part of the silver produced in the world is extracted by lead in this way.  If the ores contain sulphides, they are roasted before they are smelted in the blast-furnace.  Coke is the usual fuel, and limestone and iron-ore are generally used as fluxes.  The silver which lead contains is usually extracted by dissolving small quantities of zinc in the hot, molten metal.  As the metal cools, the zinc becomes solid; then it rises to the surface, bringing the silver with it, and is skimmed off.

Zinc cannot be obtained by ordinary smelting processes, because it boils and is vaporized at the temperature at which it is reduced to the metallic state.  This metal is therefore obtained by first roasting the ore, if it is the usual sulphide, and then heating it with coal in retorts made of fire-clay.  The zinc distills and is condensed and collected.

A distillation process is also used for obtaining the volatile metal mercury from its ores.

Aluminium is not reduced to the metallic state from its compounds by the ordinary smelting processes.  The principal method of producing it consists in passing a powerful electric current through melted cryolite in which aluminium oxide is dissolved.

The processes used for obtaining several other metals are similar to those that have been mentioned.

Metallurgy is a very ancient art that has been gradually developed and improved during historical times, but the greatest improvements were made during the 19th century, and they have been largely due to the assistance afforded by advancing knowledge of engineering and of chemical and physical sciences.