The extraction of metals from their ores is usually described as metallurgy, and metallurgical chemistry is a special branch of chemical science which is usually conducted on a large scale at high temperatures.

Ores are generally mixed with earthy matters and often contain two or more metals in various forms up to a certain stage; they are all treated in a similar manner, that is, the ores have to go through some preliminary processes such as crushing, washing, dressing, and roasting, before being smelted. Advantage is also taken of the properties which are inherent in each particular metal to assist in the extraction. The methods used are broadly classified under the following headings:-

Smelting

By mixing the ore with a flux and sometimes with fuel in various kinds of furnaces, and raising it to a high temperature by means of a heated blast of air sufficient to reduce the ore to a liquid condition and then pouring it into moulds.

The following metals can be extracted from their ores in this manner: copper, chromium, cobalt, lead, nickel, iron, magnesium, manganese, silver, tantalum, tin, tungsten.

Liquation

In this process a metal that liquefies at a low temperature is separated from a more infusible one by taking advantage of their different melting or fusing-points, and the following metals can be obtained in this way: antimony, bismuth, lead, silver.

Distillation

By heating certain ores the solid metals are driven off in the form of vapour; this is then condensed to a liquid or solid state. The following metals can be distilled from their ores: mercury, cadmium, zinc.

Amalgamation

This is accomplished by bringing the ores into contact with mercury, which has an affinity for some metals. The resulting amalgam is then heated, which drives off the mercury in the form of vapour, leaving the metal behind. The following metals can be extracted in this way: gold, silver.

Electrolytic Methods

This is where the metals are produced by fusing the ores in an electric furnace; this is owing to the great heat required to bring the metal to a fluid condition. The following metals are extracted in this way: aluminium, magnesium, tantalum, tungsten, chromium.

Wet Methods

The ores in this process are dissolved in various acids or salts and are then precipitated by the addition of another acid, salt, or gas. The following metals can be extracted by this method: nickel, cobalt, gold, palladium, platinum, silver.

Lead And Zinc Method

By fusing the ore with lead or zinc, with which they combine, then heating the alloys so that the lead and zinc are either liquated or volatilized, leaving the metal behind in a spongy mass which is then smelted in a special furnace to refine it. It is then poured into a mould. The following are treated in this manner: iridium, osmium, platinum.

Figs, i, 2, 3, and 4. Illustrations of various systems of projection.

Figs, I, 2, 3, And 4

Illustrations of various systems of projection.