COPPER ; ZINC and LEAD.
It is clear that in the reverberatory furnace the heating effect is entirely due to the flame from the burning fuel and the heated products of combustion; it is therefore necessary to have a flame with a high radiative power, and in this respect the luminous flame from burning coal is probably the most satisfactory. A very hot non-luminous flame could be produced from a deep bed of burning coke in the fire-box, but this flame on account of its want of luminosity has very little power of radiation and is on this account very inefficient. The most suitable coal for use in this class of furnace is one with moderate caking power and yielding a considerable quantity of gas on distillation thus producing a large luminous flame through the hearth of the furnace.
A very important feature of the reverberatory furnace is its great adaptability; the atmosphere can be made oxidizing or re ducing at will. If a large excess of air is admitted over the fuel, combustion will be complete and the atmosphere in the furnace strongly oxidizing; on the other hand if the amount of air passing through the furnace is restricted, combustion will be incomplete and the atmosphere in consequence reducing.
Another type of furnace belonging to this group is the Stete veldt furnace used in the chloridizing roasting of silver ores. It consists of a vertical shaft kept hot by fires arranged around the bottom ; the ore mixed with salt is admitted at the top and allowed to fall down the shaft, and as it is in the form of a sulphide no extra fuel is necessary. The roasting takes place under the influ ence of the hot products of combustion from the fires at the base as the ore falls down, and the treated ore collects in a hopper at the bottom from which it is withdrawn when cold. (See SILVER.) Muffle, Crucible and Retort Furnaces.—This group is com posed of the closed vessel type of furnace, in which the material to be heated is separated from the fuel and from the products of combustion by some form of envelope or closed vessel. The con fining vessel is heated either by being placed in the fire or by the flame produced by the burning fuel, the form of the vessel being governed by the nature of the process about to be carried out. If the material undergoing treatment has only to be protected from the products of combustion, and dust carried mechanically by the draught, a muffle furnace is used. This consists of a cham ber of brickwork or other suitable material, heated by a fire placed under it, or by the circulation round it of the products of combustion. If actual melting is the object, then a crucible furnace is employed, the material to be melted is placed in a crucible, which in turn is placed in a deep fire and completely surrounded by the burning fuel until the charge is melted, when the crucible is withdrawn from the furnace and the charge poured into a mould of some form or other, as in the manufacture of crucible steel and many other metallic alloys. Retort furnaces are used when the charge has to be distilled or sublimed as in the metallurgy of mercury and zinc. The best example of this type of furnace is to be found in the Belgian furnace for the reduction of zinc. In this the ore to be reduced is placed in fireclay retorts fitted with clay condensers, in which the zinc vapour condenses, any vapour passing out of the condensers being caught in a wrought iron tubular extension to the retort. The retorts are arranged in rows in the upper portion of a vertical arched cham ber with a fireplace at the base. Furnaces of this type may con tain as many as 30o to 400 retorts.
Another type of furnace, not included in the above classifica tion and hence forming a class by itself, is the Bessemer con verter, which although it is in principle a blast furnace differs from the type in that it is capable of rotation about a horizon tal axis, and also in the fact that the charge is introduced in the liquid condition. The heat necessary for the maintenance of the liquid state is derived from the oxidation of certain unwanted elements in the charge, brought about by a powerful stream of air introduced through the side or bottom of the converter. This type of furnace is extensively used in the metallurgy of steel and copper (see BESSEMER STEEL).
In metallurgical furnaces the greater part of the heat given out by the burning fuel escapes through the chimney, and considerable economies can be effected by using the waste heat for steam raising (by passing the escaping gases through waste heat boilers), or for raising the temperature of the air supply to the furnace. The calorific intensity of fuel is very much increased if the air used in burning it has been previously heated, and especially is this the case if the fuel is gaseous.
Two types of recuperators are in general use. The first con sists of two chambers filled with brick-work arranged in such a way that the hot gases can circulate freely between the bricks; one of these chambers is fitted in between the out-going end of the furnace and the chimney, the other between the air supply and the fire-grate. When the first mentioned chamber has attained a temperature approximating to that of the waste gases it is switched round by a suitable arrangement of valves into the second place, and these reversals are made at such intervals that a steady supply of heated air is maintained for burning the fuel. In the second type of recuperator the hot gases from the furnace are made to circulate round pipes which supply air to the fire grate.
The principles underlying the use of refractories in furnace construction are most clearly exemplified in the open-hearth fur nace. This is used in the manufacture of steel by the Acid and Basic processes ; in the former method the slags produced are very siliceous and would readily attack any basic material with which they came in contact ; in consequence the hearth and banks of the furnace, which do come in contact with the slag must be made of material unattacked under these conditions, viz., siliceous sand fritted together under the intense heat of the furnace. In the Basic process the slag is very largely composed of calcium sili cates and phosphates carrying large proportions of lime; it is very basic and would rapidly destroy any siliceous matter, hence in this case the hearth and banks must be made of basic material, cal cined dolomite being that most generally employed. The roof and ports of the furnace which have to withstand high temperature only, are always built of silica brick whether the furnace is acid or basic, but in the construction of the side walls not only the high temperature but the fluxing action of the slag has to be taken into consideration. In the Acid process, where the slag is siliceous, side walls must be siliceous also, otherwise serious fluxing will take place which in a very short time will bring about the collapse of the inside walls of the furnace ; and for the same reason a basic furnace must have side walls of basic material.
The roof as has already been mentioned is always constructed with silica bricks and precautions must be taken in the case of basic furnaces to prevent fluxing at the joint between the roof and side walls. Formerly this was done by interposing a course of chromite bricks between them, but later it was discovered that the same result could be achieved by relieving the pressure on the joint, and this latter course is the one most usually followed. Re fractory materials, according to the part they are to play in fur nace construction, may be used in their natural condition or in the form of bricks; in the latter case it is often necessary to add some foreign body to act as a binder, many of the basic refractory materials having little or no binding power.
Of the acid materials fire-clay is by far the most important ; it is essentially a hydrated silicate of alumina containing an excess of silica and small quantities of lime, soda, potash and oxide of iron. The latter elements, with the exception of oxide of iron, when present in quantities greater than a few tenths of i %, reduce the refractory properties of the clay and render it unsuit able for use at high temperatures. The effect of oxide of iron is variable and depends on the conditions prevailing in the furnace. If the atmosphere is oxidizing it is converted into ferric oxide, in which form it has no tendency to form a silicate, but if the atmos phere is reducing the ferric oxide is reduced to the ferrous state and this readily combines with silica to form a very fusible slag. Fire-clay readily absorbs water and becomes very plastic, in which form it can be moulded into any desired shape ; on burning it con tracts considerably, and this contraction, though not serious in the case of small bricks, has to be provided against when large slabs are being made. This is done by adding a proportion of burnt clay.
Ganister, an argillaceous sandstone, is also largely used for lining Bessemer converters. When made into bricks the clayey matter, naturally present, is sufficient to act as a binder. Ganister bricks are largely used on account of their strength and very re fractory nature. Some materials containing even more silica than ganister are used for the manufacture of silica bricks. When employed in this way they are generally mixed with about I% of lime as binding material, since they have no cohesive power of their own. Among such materials may be mentioned Dinas rock, calcined flints and white sand. Silica bricks are weak and friable but capable of withstanding very high temperatures, hence their exclusive use for the roofs of open-hearth furnaces. As they expand very much on heating provision must be made to take up this expansion as the furnace heats up. Of the neutral ma terials graphite and chromite are the most frequently used. Graphite, which is almost pure carbon, is used with an admixture of clay in the manufacture of crucibles. Chromite, a very infusible double oxide of chromium and iron, is mainly used in the form of bricks.
Most important amongst the basic refractories are lime, dolo mite, magnesite and bauxite. Lime is an exceedingly refractory substance, but it has little binding power and readily absorbs water, and its place in furnace construction has been taken by dolomite, a carbonate of lime and magnesia of varying compo sition. For furnace construction it ought not to contain less than 2o% magnesia and not more than about 4% ferric oxide and alumina. It is first calcined at a high temperature to expel carbon dioxide and render it less liable to deteriorate on exposure to air. It may be used either in the rough ground condition or in the form of bricks ; in the former case it is mixed with a small amount of tar, rammed and burnt in position ; in the latter, after mixing with tar, it is moulded into bricks under hydraulic pressure and after careful drying fired at a high temperature.
Magnesite, a naturally occurring carbonate of magnesia, is used chiefly in the form of bricks; for this purpose it is first calcined at a very high temperature, withdrawn from the calciner, watered, and allowed to weather for a few days; after this it is ground, mixed with tar and moulded into bricks under pressure, then after very slow drying for some weeks these are fired again at a very high temperature. The bricks are generally chocolate coloured, on account of the presence of a small amount of oxide of iron which is just sufficient to cause the brick to bind without the addition of any extraneous substance.
Bauxite is a hydrated double oxide of iron and alumina, though some samples contain very little iron; when calcined it shrinks considerably, and in this form, of ter admixture with small per centages of fire-clay, sodium silicate or lime, can be made into bricks. Well made bauxite bricks are more refractory than those made of magnesite but do not appear to have come into general use, as yet, for lining open-hearth furnaces.
In this article only the general principles of furnace construction have been treated. Another basis of classification would have been according to the nature of the fuel employed—solid, liquid or gaseous—but this was avoided as being likely to cause confusion, since any one of the furnaces can often be fired by all three kinds of fuel (see FUEL) . For the electric and open-hearth furnaces, see the articles on these subjects.
(T. BA.)