ELECTRO-METALLIIRGY. Under this term are included the processes of extracting metals from their ores, puri fying them, and dealing with them by such special processes as annealing, welding, plating, etc.
Where electric power is cheap, or where very high temperatures are re quired, metals are extracted from their ores in the electric furnace, and under that heading will be found brief descrip tions of the manufacture of graphite, carborundum, and steel by this process. Calcium carbide is also made in the elec tric furnace, a mixture of lime and coke dust being heated to fusion by passing through the mixture an alternating cur rent of 4,000 amperes at 110 volts. In a resistance furnace of a somewhat dif ferent type, aluminum is extracted from its ores. In this case, a direct current is used, and the furnace, which consists of a metal case lined with aluminum, is filled with molten ore. Metallic alumi num is separated by electrolytic action (v. ELECTRO-CHEMISTRY) and accumu lates upon an iron or carbon plate at the bottom of the furnace, this plate forming the negative pole. Magnesium, sodium, and potassium are made in fur naces of a smilar type, while steel and zinc are made in arc furnaces.
Electric welding is carried out by one of three processes. The first de pends upon the production of an elec tric arc between the metallic surfaces to be welded and a rod of carbon. The metal is connected to the positive pole of a generator, and the carbon to the negative pole. The carbon, held in in sulated tongs, is brought into contact with the metal and then drawn back a few inches, an arc being thus produced, the heat from which melts the metallic surfaces together. In the second proc ess, the metal is connected to the nega tive pole, while the positive pole is formed by a lead-lined vat. This vat is filled with dilute sulphuric acid, into which the metal is plunged. Electrolytic action is set up, and the metal becomes covered with bubbles of hydrogen, thus offering powerful resistance to the pas sage of the current. The metal soon reaches the necessary temperature, when it can be removed from the bath and welded on the anvil. The third process is known as the incandescent process. The two pieces of metal to be welded are connected to opposite poles of a genera tor, and then brought together. The re sistance offered raises the surfaces to incandescence, when they can be welded.
Electroplating depends upon the electrolysis of a solution of a metallic salt, and by its means a coating of the metal in solution is deposited upon an other metal which forms the electrode of the cell in which the electrolysis takes place. For silver plating a solution of potassium silver cyanide is used; for nickel plating, nickel ammonium sul phate; and for copper plating, a solu tion of copper sulphate. In all cases the general procedure is the same. The metal to be plated forms the kathode of the cell, and must be perfectly clean and free from all traces of grease. The solution must also be kept moving, either by means of paddles or by mov ing electrodes. The current required is small, and seldom exceeds 12 amperes per square foot, being, in the case of nickel plating, as low as four amperes per square foot.
Electric refining is a similar process to electroplating, but in this case the impure metal forms the anode of the cell, the pure metal being first brought into solution and then deposited on the kathode. For instance, in the refining of copper, a solution is made containing about 2 pounds of copper sulphate and 6 ounces of sulphuric acid per gallon of water. This solution forms the elec trolyte. The crude copper to be purified forms the anode, while the kathode con sists of a graphitized plate of pure copper. When the current is passed, copper is dissolved from the anode and deposited in very pure form upon the kathode.
Electrotyping is the process by Which reproductions are made of such articles as medals, engraved plates, busts, and so on. A reversed cast is first obtained in the usual manner, wax or gutta-percha being used for the purpose. An impression of the cast is then made in a mixture of bees-wax, Venice tur pentine and plumbago, and the surface of this impression is covered with gra phite, and carefully cleaned with alcohol and water. It is then submitted to a process very similar to that of electro plating, a deposit of metal being pro duced all over the surface of the wax, until a sufficient thickness is obtained. The metal shell, thus formed, is removed from the wax and "backed" with metal —an alloy of 1-atl, antimony, and tin being commonly used. See also ELEC TRIC FURNACE, ELECTROCHEMISTRY.