In this country, smelting is effected in a reverberatory furnace, as shown in Fig. 176, where a is the fire-door for cleaning the fire-bars; b, the draught-hole, sometimes opened during the skimming of the charge; c, a bed of fire-brick, supported on an arch, or on iron bars; d, the door for charging; and e, the door fur working the charge, which is placed at the back under the S chimney, for the purpose of preventing the oxidizing effect of the stream of air, which would obtain access to the charge, if, as is usually the case with reverberatory furnaces, the working-door were in the side; /, iron pans, with fire beneath them, into which the charge is run, when melted, through the tap-hole g; h, air passage under the hearth. The chimney is usually 40 to 59 ft. in height. The ores, as rendered to the smelter, vary in quality from 12 to 15 in 20 for white tin, or 60 to 75 per cent.; but the assays are always spoken of in the proportions of 20. For the ordinary size furnace, the charge consists of 20 to 25 cwt. of black tin, mixed with 12 to 18 per cent. of powdered culm or anthracite, and a small quantity of slaced lime or fluor-spar, varied according to the pro. portion of silica contained in the ore.
The materials are damped, well mixed, and thrown into the furnace through the door d, which is immediately closed, 2 G and the fire la maintained as gently as possible for 6 to 8 hours, or until the whole mass is fused. The charge is then welt mixed up, to ensure the complete fusion of any ore remaining unreduced. The doors are again closed for a short time, to recover the heat of the furnace, and to complete the fusion of the charge. They are again opened, and the charge is worked off, through the door g, first raking off the top scoria, which is previously chilled by throwing a little damp email coal over the charge. The cautions heating in the early part of the smelting is necessary to prevent the tin oxide from combining with the silica and lime to form a slag which would cause a great loss of tin. When the heat has reached its maximum and the fusion is complete, the mass is well stirred up, and some additional culm is introduced. The doors are closed and the fire is urged for about a further 45 minutes, when another stirring is followed by 1/4 hour's rest, which permits the heat to increase to the degree necessary for tapping. This is done by knocking out the fire coarse-grained porphyritic syenite.
The crucible of the furnace c is built of very refractory fire-brick, of oval form, and having the lower portion lined with . very stiff brosque of charcoal and tire-clay tightly rammed down. From the bottom of the furnace a channel conduct the fused charge into the iron basing whence the slag is let off at the side down the inclined plane e into the water tank f,the metal beneath being run out by the channel g into the tank h measuring about 18 in. square and 2 ft. deep. The large furnace stands under a dome indicated by the dotted line a 20 ft. long, 12 ft. wide, and 16 ft. high above the top of the furnace; and so constructed as to catch and prevent the waste of the finely powdered ore driven off by the blast. This last is produced by the bellows b, driven by a cogwheel on a shaft turned by water power. The furnace is charged through a door on the left about 3 ft. above the hearth. The smaller furnace l, about 5 ft. high, and constructed in the same manner as the other, is used only for fusing the slag from the larger furnace. Both furnaces are slowly dried before use.
The large one is first charged with fuel and slag; a moderate blast is commenced with, to reduce the most fusible part of the slag before the rest fuses; then the heat is increased till the furnace is fit for working the ore, which is then introduced. After about i hours the metal begins to show. The slag is skimmed off the surface of the basin in the bottom of the furnace as fast as it forms, until (in 20 to 2. hours) the basin is full of metal, when it is run out into the front basin. Fresh charges of raw ore are added as required.
The shape of the blast furnace varies somewhat, as may be seen from the accompanying illustrations. In Fig. 178, the furnace a Is cylindrical, 10 ft. high, and with a series of compartments above for catching the fine matters forced up by the blast entering at b; the aides and sole e are of granite, as is also the fore-hearth d, which is coated inside with clay and charcoal, for the reception of the metal and slag escaping from the shaft. The stream tin and wood charcoal are repeatedly replenished from the top so as to keep the shaft a full; the combustion of the charcoal abstracts the oxygen from the tin oxide, allowing the metallic tin to flow away at the bottom. A flux of some kind is generally charged iron oxide, or lime or " finery cinder " if there is already sufficient silica in (be ore. The flux helps to form a slag, which escapes with the liquid metal into the fore-hearth d, whence the slag is transferred to a tank of water leaving the metal to gradually fill the fore-hearth, when It is run through a tap-hole into the pan e ready for refining.
The furnace employed at Altenberg is shown in Fig. 179: a is the blast pipe (twyer); b, the furnace; c, the inclined plane for the slag; d, the fore-hearth.
Although the metal produced from the blast furnace is usually better than that obtained from the reverberatory furnace, yet the expense of fuel and the loss of ore are so much greater than with the latter, that the use of the 2 g2 blast furnace is nearly abandoned. With the reverberatory furnace, for 1 ton of tin produced, the consumption of fuel amounts to 1 3/4 tons, with a loss of 5 per cent. of metal. With the blast furnace, the fuel consumed amounts to about 3 tons of coals, with a loss of metal equal to 15 per cent.