FURNACE, METALLURGICAL. A contrivance in which metallurgical operations are carried out under the influence of heat derived either from the combustion of fuel or from the heating effect of the electric current. The temperature of the furnace may be high as in the case of the open-hearth furnace used in the manufacture of steel, or low, as in several types of calcining kilns, without affecting the accuracy of the definition.
Furnaces vary very much in shape and size and in the complex ity of their internal arrangements, but all, with the exception of the electric furnace, consist of two essential parts: (a) the fire box in which the fuel is burnt and (b) the hearth or laboratory in which the actual metallurgical operation is carried out. The air necessary for the combustion of the fuel may be introduced under pressure from a fan or some form of blowing engine (in which case the furnace is described as working under forced draught), or it may be drawn into the furnace by a chimney, when the fur nace is said to be working under natural draught. All furnaces are equipped with some type of chimney even when it is not used as a draught producer, by which to remove the products of combus tion of the fuel and the waste gases from any reaction taking place in the hearth of the furnace. In many cases the fuel, in stead of being burned in the fire-box of the furnace, is converted into gas in a separate piece of apparatus called a gas-producer and delivered to the furnace in the gaseous state, an arrangement per mitting of greater control of combustion.
The classification of furnaces is very difficult, on account of the large number of forms in use and the looseness of the terms used in their description; perhaps the most satisfactory method is to arrange them, according to the manner in which the charge is heated, in three main groups :—(i) Those in which the fuel and substance to be heated are in contact ; (2) reverberatory furnaces or those where the substance heated is in contact with the products of combustion but not with the fuel itself ; (3) those in which the substance heated is not in contact either with the fuel or with the products of combustion, these being again subdivided as follows: (ia) Shaft furnaces where the height is considerably greater than the diameter.
(lb) Hearth furnaces where the height is only a little greater than the diameter.
(2a) Roasting and calcining furnaces where the charge is not melted.
(2b) Melting furnaces.
(3a) Muffle furnaces, where the heating chamber is fixed and forms part of the furnace structure.
(3b) Crucible furnaces, where the heating chamber is moveable and independent of the furnace structure.
(3c) Retort furnaces, in which the materials are volatilized and afterwards condensed.
It is necessary to subdivide these again into (a) kilns or natural draught furnaces and (b) blast or forced draught furnaces.
Shaft furnaces are of very ancient origin ; remains in the form of blast pipes have been found in many parts of the world, and native iron-workers in India still use primitive structures of this type. In all members of this group the fire-box and hearth are combined to form a single chamber into which the fuel and material to be heated are charged.
These are used in cases where a high temperature is not necessary and where the products of the operation are not required in the molten state, as in lime burning and in the calcina tion of ores. Their forms vary according to the specific purpose of which they are to be used, but the vertical shaft is common to all. Internally the shaft may have parallel sides, or sides which gradually approach each other in the form of an inverted cone; in cross section they may be circular, elliptical or rectangular. The best example of this class of furnace is to be found in the large kilns used in the calcination of iron ores, and of these the Gjers kiln, used extensively in the Cleveland district, may be cited. The body consists of a cylindrical shell of wrought iron lined with fire brick and standing on an iron ring supported by cast iron pillars, thus leaving an open space between the bottom of the shell and the floor upon which it stands. The furnace is con tinuous in action, the mixture of ore and fuel being fed in at the top and withdrawn at the bottom; in order to cause the calcined material to pass outwards a cast iron cone is placed on the centre of the floor plates supporting the body of the furnace. The bot tom of the furnace instead of being open, as in the example just described, may be closed with a step grate and the calcined material withdrawn below the level of the grate.
This type of furnace (see illustration under BLAST FURNACE) is extensively used in the smelting of iron, cop per and lead ores. In general form it resembles a kiln, but differs from it in several details. The air is forced into it under pressure, and the charge is tapped in the liquid condition. In size and form it varies from the large iron smelting furnace to the small furnace used in lead smelting. The early furnace was generally rectangular in section, but this in time, so far as iron smelting is concerned, gave place to a circular one, though the rectangular form is still retained in copper and lead smelting. The vertical section on the other hand is subject to considerable variation and is dependent on the nature of the operation carried out in the furnace; if simple fusion is the object as in the foundry cupola the internal cross-section is only slightly smaller at the bottom than at the top, but in ore smelting, where there is a considerable change in volume of the charge as it descends in the furnace, the sides slope inwards towards the bottom of the furnace and generally termi nate in a small parallel portion of very much smaller cross-section than the upper part of the furnace. In this part called the "well" the reduced metal collects and is tapped off at intervals. For further information on this subject see IRON, LEAD and COPPER.
These resemble blast furnaces in that the air supply is under pressure, and that the fuel and substance to be heated are in contact, but differ from them, as has been stated, in the rela tion between the height and width, the height in the case of hearth furnaces being not much greater than the width. In general they consist of a shallow chamber lined with refractory or other ma terial unacted upon by the products of the operation ; the blast is introduced through one or more twyers or nozzles and impinges either downwards on the charge or in a horizontal direction just under it. On account of the method of introducing the blast the atmosphere of the furnace can be made oxidizing or reducing at will, a factor which enables it to be used in a variety of operations. Examples of its use under oxidizing conditions are to be found in the Yorkshire Refinery and the Walloon Hearth used in the re fining of pig-iron and the production of wrought iron respectively; under reducing conditions in the Catalan Forge which is used for the manufacture of wrought iron direct from the ore ; and under combined reducing and oxidizing conditions in the Scotch Hearth for the reduction of lead from galena. A familiar example of this type of furnace is also found in the smith's hearth.
Furnaces of this type are very much used in metallurgy and in principle are entirely different from those already described. The fuel is burnt in a separate chamber and the hot products of combustion led through an other, called the hearth, in which the material tc be heated is placed, passing from this to the chimney. Between the fire-box and the hearth and again between the hearth and chimney are brickwork ridges called respectively the fire and flue bridges. An arched roof covers the whole and is highest over the fire-box end, from there sloping gradually downwards towards the flue bridge, so that the flame may be deflected or reverberated towards the hearth—hence the name. The weight of the roof exerts consider able pressure on the side walls, tending to push them outwards; to counteract this it is usual to bind the furnace together with stout iron rods and plates, so arranged that they can be easily adjusted to allow for the expansion of the brickwork as the furnace heats up. The hearth is usually carried on a brickwork arch forming a vault under the furnace, but this procedure is sometimes de parted from according to the particular process in operation. Working doors are provided in the sides or at the ends of the furnace ; when the doors are in the sides, the fire-box or boxes and the chimney flues are arranged at the ends, and vice versa, the latter arrangement being very common in the large calciners and roasting furnaces used in non-ferrous metallurgy.
Reverberatory furnaces are of two types, according as they are required for low or high temperature operations ; in the former case the ratio of the area of the hearth to that of the fire-box is greater than in the latter.
In these the hearth is made of fire-brick set in fire-clay, and is on the same level as the door sills, in order to facilitate the removal of the charge, which is not molten and therefore has to be raked out. If noxious gases are given off from the material under treatment a slot is often left in the furnace hearth just inside the doors through which the charge can be raked into the vault below the furnace and there allowed to cool. The hearth must not be longer than can be uni formly heated by the flame which fixes it at about 20 ft.; the width is usually not greater than i o ft. as the charge has to be turned over continuously and this is about the greatest width which a workman can handle with a rabble. The charge may be introduced through the working doors or it may be admitted through hoppers placed in the roof of the furnace. The f urnace just described is a simple single hearthed one, but more complex forms are common in which use is made of more than one hearth in the same furnace; these may be placed in line, each hearth being raised a few inches above the succeeding one and the charge raked from one to the other as the operation proceeds, or they may be arranged one above the other the charge being raked through a hole in the upper hearth on to the one immediately below until it finally arrives at the bottom hearth of the furnace. As has been already stated the ore under treatment has to be continuously turned over, and various successful attempts have been made to accomplish' this mechanically. Thus there are very long calcining furnaces of the O'Hara type in which the ore is turned over and drawn along the hearth by mechanical rabbles, or the hearth instead of being in a straight line may be curved to form part of a circle as in the Pearce and Brown Horseshoe calciners; again, instead of using rabbles the hearth of the furnace may be made to move as in the Bruckner Cylinder. In this modification of the reverberatory furnace the hearth portion consists of a horizontal iron cylinder lined with fire-brick and resting on rollers. The ore is introduced through doors in the side of the cylinder which are afterwards closed and the cylinder slowly rotated. For further information on these mechanical furnaces the reader is referred to
