Metallurgy
From Encyclopediak
Metallur"gy, that branch of applied science which deals with the extraction of metals from their ores. Some metals are found free as, for instance, gold and platinum, which usually occur as such, while silver, copper and bismuth are often obtained free. In those cases they require but little treatment. Metals such as iron, tin, lead, zinc, nickel, antimony, mercury and aluminum occur almost invariably in combination with other substances, as oxides, sulphides, sulphates, carbonates, silicates, chlorides, etc.
ORE DRESSING. When ores are mined they are usually found to contain a large quantity of material that is worthless. Most of this is disposed of by crushing the ore and washing it over screens in order to separate or concentrate the desired materials. This process is known as concentration and is performed by machinery. The concentrate is then dried and smelted to procure the metal. Metallurgy has to do not only with the extraction of metals from their ores, but also with the working of them into final products by casting, rolling or pressing. In these final operations the metals undergo changes in the structure of their molecular particles, which, with the chemical alterations assumed in the preliminary working of them, renders their appearance quite different from that in which they were found in the ore.
PROCESSES. Various metals require different treatments, and these may be divided into the following processes 1. smelting, in which heat, aided by chemical reactions, is employed to extract the metal from the ores 2. amalgamation, in which an amalgam is formed with the metal and mercury, the mercury afterwards being separated by distillation 3. chemical extraction, by which the metal is dissolved and afterwards precipitated in solid form by chemicals or by electrochemical deposition 4. electrolytic, in which the metal is extracted from the ore or refined by an electric treatment, called electrolysis. Sometimes a combination of two or more of these processes is used. Smelting in blast and reverberatory furnaces is the process most generally used, and is employed in extracting iron. copper and lead from their ores. Amalgamation is used chiefly in gold and silver ores. Zinc is heated in fire-clay retorts, and the vapors are condensed. Aluminum is produced by passing a powerful electric current through melted cryolite, in which aluminum oxide is dissolved. Ores containing a small quantity of gold, known as low-grade ore, are generally treated by the cyanide process after the ore is crushed to a fine powder, it is subjected to a solution of potassium cyanide. When silver ores have been washed with salt, a solution of sodium hyposulphite is used to dissolve the silver chloride. See ALUMINUM, AMALGAM, ANTIMONY, COPPER, IRON AND STEEL, GOLD, LEAD, MERCURY, SILVER, ZINC, BLAST FURNACE, REVERBERATORY FURNACE.
