The cross-section of this furnace at the tuyi•res is :33 X 60 in., while 10 in. higher up a bosh is begun. so that 30 in. shove the tuvjres the cross-section is enlarged to 45 X 78 in. The four lower cast-iron jackets terminate at this point, where they are slInnonnted by the other four. which still diverge slightly. so that at their upper surface, 7 ft. 6 in. above the tuyeres, the fur nace has an inside section of 54 X 87 in., which is retained to the charging door, 10 ft. 6 in. above the tuyeres. The slag top is 6 in. below the latter, and the erneible is 14 in. deep, lined with brick, and provided with a drop bottom, The object of the bosh is to increase the reducing action, with the view of obtaining cleaner slags.
In smelting sulphide ores the Amer ican practice of the present day is to do away entirely with the ordinary deep crucible, substituting for it mere ly a sloping bottom a foot or less below the tnycires, from which the entire molt en material escapes through a narrow• groove under the breast. then first en tering an outside crucible or well, in which the matte separates from the slag and is tapped into molds, while the slag flows from a spout into iron pots arranged on wheels for convenient dumping,. In this manner, chilling over the metal in the crucible and the troublesome freez ing of the tap-hole are avoided. The formation of sows is also prevented by the immediate escape of the fused are from the powerful reducing action of the fuel. Provision is made to prevent any escape of blast under the breast, either by so thoroughly covering the orifice and channel that only a minute groove exists, which is constantly filled to its utmost capacity with molten ore, which soon forms an impervious cover to its channel ; or by sit raising the terminal slag-spout, and lowering the anterior wall of the furnace. that the blast is securely trapped, just as sewer-gas is prevented from escaping in an ordinary drain. This system of exterior crucibles was introduced in this country by Mr. James Douglas, Jr., tit his Phamix ville Works in 1879. • The height of the furnace depends upon the character of the ore and the quality of the fuel : refractory, siliceous ore, and dense, strong coke requiring and permitting the employ ment of a higher furnace than the opposite conditions. With basic and easily fusible ores any height above 10 ft.. (from tuy6res to charging door) is rarely met with : even with refractory ores the danger of reducing metallic iron and the general unmanageability of a high furnace practically limits the height to 11 ft.. Dr. E. D. Peters gives the cost of smelting an easily fusible copper-ore in a circular water-jacket furnace, 42 in. in diameter. having it capacity of
50 tons per 24 hours, as :11.9S per ton in the East, and $6.40 per ton in Arizona.
The Ikrresholr Euu•uacr is a modification of the above. it has a or well. which is sometimes, for convenience of removal. placed on wheels. though more frequently IL rests oi solid ground. The bottom of the fu mince consists Inertly of a circular, concave, cast-iron plate, firmly bolted to the lower border of the water-jacket. which extends about 12 in. below the t uplres. The bottom is covered with a single course of tire-brick resting on a shallow layer of sand. The outlet of the furnace is a small circular opening in the water-jacket. There is a similar opening in the bark wall of the movable hearth. which is protected by a small, separate water-jacket. Thus is formed a short. water-cooled channel from the furnace to the tire-hearth. The slag-discharge from the fore-hearth is several inches higher than this channel. so that the ]inter is covered several inches deep with molten material, and the blast is complett•ly trapped, The slag runs out front the fore-httrth coutt0000,ly ; the matte is tapped at intervals. In the latter operation the slag-spout is plugged with a ball of plastic clay, so that the blast is tightly confined even after the molten material has descended below the top of the channel from the furnace. As it is sometimes impossible or inadvisable to close the tap hole in the fore-hearth at the exact moment when the last of the matte has escaped and the first of the slag begins to flow, a tilting launder is arranged between the matte-spout and the molds, which, when held up by a chain, conducts the liquid to the regular molds, but when released by a catch, turns upon a horizontal pivot and conveys the slag in the opposite direction,. where it is cast in proper shape for resmelting.
According to Dr. E. D. Peters, from whose _Modem American Methods of Copper-Smelling this description is taken, the cost of smelting in a large Herreshoff furnace is very low. The number of men required per furnace is 10. With gas-house coke and repairs exceptionally low, the cost per ton of ore at the Laurel Hill Chemical Works, Long Island City, N. Y., does not aggregate 80 cents per ton of ore. The average charge of ore in the 48-in, circular fur nace at those works was 56 tons per 24 hours, and of the 60-in. furnace 84 tons. At Butte, Mont., a 48-in. furnace, with 6 2-in. tuyeres and Fib. blast, smelted from 60 to 65 tons of cal cined pyritic concontrates daily.