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alternately to convey the gas to one or other of two ports (one shown) G, of the heating-chamber E. H, H', are the air-ports of the heating-chamber, communicating through the flues K, K', with the regenerators A, A'. I, I', are steam-jets, placed in the return flues, for directing a portion of the waste products of combustion to the grates of the converter. J is the valve for reversing the direction of the air flowing into the furnace, and of the products of combustion through the regenerators to the chimney-flue. O, O', are hinged caps for alternately admitting and shutting off the products of combustion from the heating-chamber to the converter. These caps are worked automatically by means of the connections O, O', attached to the rocking-beam, the same movement which closes D opening O', and that which closes D' opening O. Q, q, are doors for giving access to the grates of the converter for clearing them.

The mode of working the furnace is as follows:-Gas from the converter B passes through the flue C' and the valve D' to the gas-port G', and into the combustion-chamber h'g'. Air for combustion passes through the regenerator A', the air-flue K', and the air-port H' into the combustion-chamber, where it meets the gas from the converter, and combustion ensues. The horseshoe flame sweeps round the heating-chamber E, the products of combustion passing away by the second combustion-chamber hg, go partly through the regenerator A, and reversing valve J into the chimney-flue, and partly down the flue G, whence they are drawn by means of the steam jet I through the capped inlet O under the grates of the producer B, there to be converted into combustible gases. From time to time the direction of the flame in the furnace is reversed by manipulating the rocking-beam, carrying the valves D, D', and the reversing valve J in the usual manner of working regenerative gas-furnaces. An auxiliary steam-jet is provided for supplying air to start the producer when the furnace is first heated up.

Besides the advantages in the saving of fuel, labour, and metal, the simplicity of design of the new furnace renders its cost of construction not much greater than that of a solid-fuel furnace, while its cost of maintenance is very much less. The cost of construction of the new furnace is found to be about twofifths of that of the old form of regenerative gas-furnace, of the same productive capacity, with separate gas-producers and gasregenerators, and the space occupied below ground is also considerably reduced.

In order to ecomonise labour, various modifications of the reverberatory furnace have been invented. The laboratory has been

considerably increased in height, and the material allowed to fall as a shower of dust through a shaft that is traversed from bottom to top by the flame from a lateral fire-place. In some cases the dust falls freely; in others there are obstacles in the way. The most successful furnace of the former class is that

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The furnaces of

of Stetefeldt for roasting argentiferous ores. Gerstenhöfer and Hasenclever may be taken as types of the latter class.

In the Stetefeldt furnace, the pulverised ore, mixed with salt, is charged in at the top of the shaft a (Figs. 68 and 69) by the.

aid of a mechanical feeder arranged at b on the top of the shaft. The ore falls in a slow stream down the shaft and encounters the flames proceeding from the fire-places, e, e. For the ore which is carried along the flue, f, through which the gases escape, a third fire-place, l, is provided. This assists the action of the main shaft, and causes the ore to reach the flue m in a completely roasted and chloridised state, if it has not already been discharged through the door d upon the cooling floor. At h there are cast-iron plates, which form the bottom of the flue, and which allow any dust settling to fall into the chamber i, whence it is removed through the door k. The flue bridges, g, g, are of cast-iron. The fire-places and arches are of fire-brick; the remainder of the furnace is of common brick.

The vertical shaft is usually 30 to 35 feet in height, and 4 to 5 feet square at the base. These furnaces roast about 40 tons of ore in twenty-four hours. In Utah as much as 64 tons are treated daily. In the Stetefeldt furnace the chloridising roasting action is very rapid and complete, whilst the expenditure of labour and of fuel is small.*

4. Closed-vessel Furnaces.-In furnaces of this class the material to be heated is separated from the fuel by an envelope, in the form of a closed vessel. The vessel is heated by being in contact with the fuel, or by the flame developed from a fire on a grate, or lastly by the gases from a producer. The form of the vessel is determined by the process that is to be undertaken. Thus, for simple heating, the muffle is employed, whilst for fusion crucibles are used. Retorts and similar vessels are used exclusively for distillation and sublimation.

The ordinary assay furnace may be taken as typical of the wind-furnaces used by the brass-founder. These furnaces vary according to the size and number of crucibles inserted in them. The crucible gas-furnace used for melting steel consists of a long hearth on which the crucibles are placed in pairs. This hearth has a movable roof through which the crucibles may be withdrawn, and on the long sides are the flues from the regenerators, the construction of which is similar to that of other regenerative furnaces.

The best example of a retort-furnace is afforded by the furnace used in Belgium for smelting zinc ores. In this furnace the ores are reduced in a number of fire-clay retorts with a bellied fire-clay nozzle in which the zinc condenses, and a sheet-iron tube in which

*

Küstel, "Roasting of Gold and Silver Ores." 1871; "The Stetefeldt Furnace, San Francisco, 1878"; Engineering, 1885, Sept. 25.

the zinc oxide is collected. The retorts are placed in rows in a vertical arched chamber, with a fire-place at the base. In recent furnaces of this kind the fire-place is arranged for gaseous firing.

The furnaces are usually built in pairs, back to back. At Angleur* they contain 160 retorts on either side. The charge for each retort is 25 lbs. of a mixture of equal weights of powdered roasted blende and non-caking coal.

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An external view of the Belgian zinc furnace is shown in Fig. 70.

Retort-furnaces are also used for smelting mercury ores, the condensers sometimes consisting of earthen pipes, or aludels (Fig. 71), thrust one into another. The ore is roasted in a circular kiln with a system of openings connecting with a series of ranges of aludels resting on the doubly inclined surface of a

* See article Zinc, by de Lalande, in Wurtz, Dict. de Chem.

terrace. In the recent furnaces at Almaden there are sixteen of

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The tube-furnace is occasionally employed in the extraction of bismuth from its ores by liquation. Native bismuth melts at 264°, and advantage was taken of this fact to separate it from the more or less infusible materials accompanying it. This process has, however, been almost entirely supplanted by ordinary smelting and wet methods. The liquation-furnace, used at Schneeberg, in Saxony, contains a series of cast-iron tubes, oval in section, and inclining towards the front, where the ore is placed. The liquated metal is received in iron vessels heated by a separate fire.

A furnace of the closed-vessel type is used in the manufacture of steel by the addition of carbon to malleable iron, a process termed cementation. The furnace (Fig. 72) consists of a rectangular chamber supplied with chimneys, c c, and divided into two parts by a fire-place, a, on either side of which is a rectangular fire-brick vessel, or converting-pot, b, varying from 8 to 15 feet in length and 3 feet in width and depth. These pots are thoroughly heated by the flames, and the products of combustion reach the conical hood, e, some 40 feet in height, which serves to prevent loss of heat by radiation as well as to carry off the smoke. At d there is a man-hole, built up during the working of the furnace, but opened for cooling down and for the withdrawal of the charge.

The Bessemer Converter. This furnace has the shape of a pear, the form originally given it by Bessemer. Unlike all other smelting furnaces, the converter is usually not fixed but supported on standards by trunnions. To one of these a pinion is keyed, by means of which the vessel can be inoved through an angle of 170°, so that the molten metal may be poured from its mouth. The other trunnion is hollow and admits the blast to the vessel. A pipe from this trunnion passes to the tuyere-box, forming the bottom of the converter, which is perforated by ten to nineteen circular holes, into each of which is placed a conical fire-clay tuyere, perforated with twelve holes each inch in diameter.

* Revista Minera, 1889, p. 129.

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