A bewildering variety of furnaces is in use in the metallurgical industries. Almost every metal, as well as each process for the same metal, has its own particular furnace. Only in the broadest sense can furnaces be classified. They cover a great range of working temperatures, of capacities from ounces to thousands of tons, and of chemical influences from the strongest oxidizing to the most powerfully reducing. Classification according to fuel and mode of receiving the heat is as follows:
(A) Direct Contact with Solid Fuel.
(a) Open hearths or crucibles (blowpipe melting).
(B) Charge Separated from Fuel (either solid, liquid, or gas fuel); radiation heating mainly.
(1) Reverberatory furnaces. This type of furnace is used for more purposes and on more metals than any other sort of furnace; variations are equally numerous.
(C) Charge Inclosed; heated by conduction through walls.
(1) Crucible furnaces (for melting steel, etc).
(2) Retort furnaces (for smelting zinc, etc).
(3) Tube furnaces (for liquating bismuth).
(4) Muffle furnaces (for roasting sulphides and the rich sulphur-dioxide gas used to make sulphuric acid).
(A) Converters (for blowing matte to blister copper, and iron to steel).
(B) Aluminothermic Crucibles; in which oxides are reduced to metals with metallic aluminum.
(A) Pure Arc Heating (Stassano steel furnace).
(B) Induction (Roechling-Rodenhauser type).
(C) Resistance (aluminum furnaces).
(D) Arc and Resistance, Combined (Heroult and Girod).
Electric furnaces, converters, crucible furnaces, and reverberatories are made tilting as well as stationary. Almost any combination of the above furnaces is possible and many such are in use.
What substance shall restrain and hold the reacting chemicals in the multitude of furnaces just described? What materials will keep the heat in? What materials will conduct the heat away fast enough? What will withstand very high temperatures? What will resist excess of acid influence? What will resist hot and powerful bases? No subject is more important to the metallurgist than refractories, for possibly it is not only high temperatures and chemical action but changing temperature which must be withstood.
Fig. IS. Welded Water Jackets for 48 by 192 Inch Copper Furnace Courtesy of Allis-Chalmeri Manufacturing Company, Miloauker, Wisconsin.
(1) Chilled Substance Itself. This is a very neat and absolute solution when it can be applied. It is used most in the electro-thermic furnaces but borders on our next refractory, water-cooled jackets.
(2) Water -Cooled Metal Jackets. Here a very thin layer of the charge will be frozen on the metal which is cooled by an abundant supply of water. All blast furnaces now are cooled with water blocks or large flat or annular jackets about the fusion zone. Fig. 15 shows a large rectangular jacket assembled in the shop. Hand holes for cleaning out are at the very base; tuyere thimbles are in each jacket just below the bosh; about the center of the bosh are the water inlets; the overflow is at the very tip top of each jacket. The jackets are firmly bolted together to keep from being squeezed apart by the charge.
(3) Fireclay. Hydrous aluminum silicate, as found abundantly in nature, not only lends itself well to molding and baking into strong shapes, but admirably resists temperatures up to some 1500° C, and is neither decidedly acid nor basic in its character. This is a very widely used refractory.
(4) Silica and Siliceous Materials. This refractory also is easily worked and fritted into .suitable shapes. It is used much for acid hearth linings, for roofs, and for side walls.
(5) Carbon and Graphite. The crucible of the iron blast furnace is essentially a graphite-lined receptacle, and is automatically so. This refractory will stand any obtainable temperature, but cannot be used in the presence of air or reducible oxides.
(6) Magnesium Oxide. Magnesium oxide, which has been carefully calcined and shrunk, is a widely used brick and hearth lining. It is for use with the bath basic or metallic. Particular care must be taken that steam does not get a chance at any time to slack and ruin this material.
(7) Bauxite (A1203).
(8) Carborundum (CSi).
(9) Chromite (FeCr204).
(10) Brasque (mixture of sand, fireclay, and coke).
(11) Zirconia (Zr02).
(12) Boron Nitride (BN).
Brasque is an ancient refractory, now little used for more than backing some primary lining. The others of the last six are all promising materials which doubtless will receive greater application when they become better known and more available.