Two principal modifications of furnace may be described according as the jackets are of cast- or of wrought-iron, the dimensions and other details of construction not necessarily differing in the two cases. Figs. 61, 62 and 63 * show a furnace with wrought jackets erected at Great Falls, Montana, in 1891. Fig. 64 shows the lower part of the most recently erected furnace at the Globe Smelting Works, Denver, 1895; while Figs. 65 and 66 show the new furnaces of the Broken Hill Proprietary Block 14 Company at Port Adelaide, South Australia (1897). * Borrowed from Hofman, Metallurgy of Lead, 1892. Dimensions. The sectional area of modern oblong furnaces has been increased during the past fifteen years by increasing the length, it being, as already pointed out, impracticable to widen these furnaces without serious disadvantage. Fifteen years ago or so, there were few furnaces in existence over 6 feet long, whereas nowadays 120 inches to 140 inches is the usual size, and some have been built even 160 inches or over 13 feet long. The width at tuyere level varies between 2 feet 6 inches and 3 feet 6 inches; a few years ago the tendency was to reduce the width to 30 or 33 inches in order to use a lower blast pressure and prevent the heat from creeping up, but of late most of the new furnaces erected are 40 or 42 inches across tuyeres, corresponding with the increase in height and with the increased rapidity of driving. As already mentioned, the height of water-jacket furnaces has been increased of late years to cope with the more refractory ores now smelted and the more infusible slags produced, the most recent furnaces-except those working on zinciferous ores with very ferruginous slags being generally 17 feet to 20 feet from tuyere level to stock line. 1. Foundation.-A good foundation is essential, and solid rock is always best, unless more than 10 feet from surface, in which case a frame work of heavy planking placed crosswise and embedded in concrete may serve instead. The excavation should be at least 3 feet larger each way than the furnace bed plate, and is filled either with good rubble masonry, brick work, or a block of concrete into which are rammed as many large stones as possible, cement being in any case mixed with the mortar. The best of all possible foundations is formed by filling the excavation with molten slag, forming, when cold, a single block; but this is only attainable for additions to plant in an already existing works where a sufficiently quick succession of slag pots can be obtained. In any case, the topmost course must be perfectly smooth and level which, in a new works, is secured by making it of brick, and when molten slag is available can be easily obtained by forming shallow dams with pieces of bar iron (the tops of which are carefully levelled) and then filling them. successively with molten slag. When a perfectly even and level foundation has been obtained, a layer of clay mortar is spread over it, and then the wrought-iron or steel "base plate," inch or inch thick is laid in position. This plate has generally a rim of 13-inch angle-iron rivetted on to assist in holding the hearth castings or "caisson plates" in position, and its function is to prevent leakage of lead from the well or crucible. 2. Crucible with Automatic Tap.-The crucible of this furnace, built up on the bed plate, of firebrick laid as closely as possible with thin fireclay mortar, is surrounded by thick "crucible castings" or caisson plates, which have to be very strong in order to resist the pressure of the lead in the crucible, frequently 20 to 25 tons dead weight of lead alone, without counting the downward pressure of the column of charge which floats upon it. These castings are commonly strengthened with projecting ribs, and bolted together. Their great weight, however, renders them very awkward for use in localities remote from railroad facilities, and at many modern works in America, as well as in the various Australian plants, they have been done away with, being replaced by bent rails enclosing a boiler plate outer casing, as shown in Figs. 61 and 62. Hofman indeed suggests that the outer casing of the crucible should be oval in form, when it could be enclosed in a rivetted boiler plate shell strengthened by means of continuous bands of flat bar iron, but, though a very practical suggestion, the author is not aware that it has ever been put into practice. Inside the plates the well is built up, a space about 2 inches wide being left between them and the firebrick, which is afterwards tamped with sand or brasque, so as to allow for expansion. The Arents automatic tap is simply an inclined channel halfOp. cit., p. 186. * |