Smelting of lead ores

—The lead ores of Derby shire and the north of England were antiently smelted in very rude furnaces, or boles , urged by the natural force of the wind, and were therefore placed on the summits or western slopes of the highest hills. More recently these furnaces were replaced by blast hearths, resembling smith’s forges, but larger; and were blown by strong bellows, moved by men or water-wheels. The principal operation of smelting is at present always executed in Derby shire in reverberatory furnaces , and at Alston Moor in furnaces similar to those known in France by the name of Scotch furnaces. Before entering into the detail of the founding proce es, we shall give a description of the furnaces e ential for both the smelting and acce ory Operations. 1. The reverberatory furnace called cupola, now exclusively used in Derby shire for the smelting of lead ores, was imported thither from Wales, about the year 1747, by a company of Quakers. The first establishment in this county was built at Kalstedge, in the district of A shover. In the works where the construction of these furnaces is most improved, they are interiorly 8 feet long by 6 wide in the middle, and two feet high at the centre. The fire, placed at one of the extremities, is separated from the body of the furnace by a body of masonry, called the fire-bridge , which is two feet thick, leaving only from 14 to 18 inches between its upper surface and the vault. From this, the highest point, the vault gradually sinks towards the further end, where it stands only 6 inches above the sole. At this extremity of the furnace, there are two openings separated by a triangular prism of fire-stone , which lead to a flue, a foot and a half wide, and 10 feet long, which is recurved towards the top, and runs into an upright chimney 55 feet high. The above flue is covered with stone slabs, carefully jointed with fire-clay, which may be removed when the deposit formed under them (which is apt to melt), requires to be cleaned out. One of the sides of the furnace is called the labourers’ side. It has a door for throwing coal upon the fire-grate, besides three small apertures each about 6 inches square. These are closed with movable plates of cast iron, which are taken off when the working requires a freer circulation of air, or for the stirring up of the materials upon the hearth. On the opposite side, called the working side, there are five apertures; namely, three equal and opposite to those just described, shutting in like manner with cast iron plates, and beneath them two other openings, one of which is for running out the lead, and another for the scoriæ. The ash pit is also on this side, covered with a little water, and so disposed as that the grate-bars may be easily cleared from the cinder slag. The hearth of the furnace is composed of the reverberatory furnace slags, to which a proper shape has been given by beating them with a strong iron rake, before their entire solidification. On the labourers’ side, this hearth rises nearly to the surface of the three openings, and falls towards the working side, so as to be 18 inches below the middle aperture. In this point, the lowest of the furnace, there is a tap-hole, through which the lead is run off into a large iron boiler (lea-pan), placed in a rece left outside in the masonry. From that lowest point, the sole gradually rises in all directions, forming thus an inside basin, into which the lead runs down as it is smelted. At the usual level of the metal bath, there is on the working side, at the end furthest from the fire, an aperture for letting off the slag. In the middle of the arched roof there is a small aperture, called the crown-hole , which is covered up during the working with a thick cast iron plate. Above this aperture a large wooden or iron hopper stands, leading beneath into an iron cylinder, through which the contents of the hopper may fall into the furnace when a trap or valve is opened. 2. The roasting furnace. —This was introduced about 30 years ago, in the neighbourhood of Alston Moor, for roasting the ore intended to pa through the Scotch furnace, a proce which greatly facilitates that operation. Since its first establishment it has succe ively received considerable improvements. IMG:4147767755307473660_illo0753.png:Cupola furnace Figs. 631 , 632 , 633. , represent the cupola furnace at the Marque of Westminster’s lead smelting works, two miles from Holy well. The hearth is hollowed out below the middle door of the furnace; it slopes from the back and ends towards this basin. The distance from the lowest point of this concavity up to the sill of the door, is usually 24 inches, but it is sometimes a little le , according to the quality of the ores to be smelted. This furnace has no hole for running off the slag, above the level of the top hole for the lead i , like the smelting furnace of Lea, near Matlock. A single chimney stalk serves for all the establishments; and receives all the flues of the various roasting and reducing furnaces. Fig. 633. gives an idea of the distribution of these flues. a a a , . are the furnaces; b , the flues, 18 inches square; these lead from each furnace to the principal conduit c , which is 5 feet deep by 2 1 ⁄ 2 wide; d is 6 feet deep by 3 wide; e is a round chamber 15 feet in diameter; f is a conduit 7 feet high by 5 wide; g another, 6 feet high by 3 wide. The chimney at h has a diameter at bottom of 30 feet, at top of 12 feet, including the thickne of its sides, forming a truncated cone 100 feet high; whose base stands upon a hill a little way from the furnaces, and 62 feet above their level. a , figs. 631 , 632. , is the grate; b , the door of the fire-place; c , the fire-bridge; d , the arched roof; e , the hearth; f f f , ., the working doors; g g , flues running into one conduit, which leads to the subterranean condensing chamber, e , and thence to the general chimney; h , a hopper-shaped opening in the top of the furnace, for supplying it with materials. This magnificent structure is not destined solely for the reduction of the ores, but for di ipating all the vapours which might prove noxious to the health of the work-people and to vegetation. The ores smelted at Holy well are very refractory galenas, mixed with blende, calamine, pyrites, carbonate of lime, ., but without any fluate of lime. They serve mutually as fluxes to one another. The coal is of inferior quality. The sole of each furnace is formed of slags obtained in the smelting, and they are all of one kind. In constructing it, 7 or 8 tons of these slags are first of all thrown upon the brick area of the hearth; are made to melt by a brisk fire, and in their stiffening state, as they cool, they permit the bottom to be sloped and hollowed into the desired shape. Four workmen, two at each side of the furnace, perform this task. The ordinary charge of ore for one smelting operation is 20 cwt., and it is introduced through the hopper; see Copper , fig. 304. An a istant placed at the back doors spreads it equally over the whole hearth with a rake; the furnace being meanwhile heated only with the declining fire of a preceding operation. No regular fire is made during the first two hours, but a gentle heat merely is kept up by throwing one or two shovelfuls of small coal upon the grate from time to time. All the doors are closed, and the register-plate of the chimney is lowered. The outer basin in front of the furnace is at this time filled with the lead derived from a former proce , the metal being covered with slags. A rectangular slit above the tap hole is left open, and remains so during the whole time of the operation, unle the lead should rise in the interior basin above the level of that orifice; in which case a little mound must be raised before it. The two doors in front furthest from the fire being soon opened, the head-smelter throws in through them, upon the sole of the furnace, the slags swimming upon the bath of lead, and a little while afterwards he opens the tap-hole, and runs off the metallic lead reduced from these slags. At the same time his a istant turns over the ore with his paddle, through the back doors. These being again closed, while the above two front doors are open, the smelter throws a shovelful of small coal or coak cinder upon the lead bath, and works the whole together, turning over the ore with the paddle or iron oar. About three quarters of an hour after the commencement of the operation, he throws back upon the sole of the hearth the fresh slags which then float upon the bath of the outer basin, and which are mixed with coaly matter. He next turns over these slags, as well as the ore with the paddle, and shuts all the doors. At this time the smelter runs off the lead into the pig-moulds. The a istant now turns over the ore once more through the back doors. A little more than an hour after the operation began, a quantity of lead proceeding from the slag last remelted, is run off by the tap; being usually in such quantity as to fill one half of the outer basin. Both the workmen then turn over the ore with the paddles, at the several doors of the furnace. Its interior is at this time of a dull red heat; the roasting being carried on rather by the combustion of the sulphurous ingredients, than by the action of the small quantity of coal in the grate. The smelter, after shutting the front doors, with the exception of that next the fire-bridge, lifts off the fresh slags lying upon the surface of the outside bath, drains them, and throws them back into the furnace. An hour and a half after the commencement, the lead begins to ooze out in small quantities from the ore; but little should be suffered to flow before two hours have expired. About this time the two workmen open all the doors, and turn over the ore, each at his own side of the furnace. An hour and three quarters after the beginning, there are few vapours in the furnace, its temperature being very moderate. No more lead is then seen to flow upon the sloping hearth. A little coal being thrown into the grate to raise the heat slightly, the workmen turn over the ore, and then close all the doors. At the end of two hours, the first fire or roasting being completed, and the doors shut, the register is to be lifted a little, and coal thrown upon the grate to give the second fire , which lasts during 25 minutes. When the doors are now opened, the inside of the furnace is of a pretty vivid red, and the lead flows down from every side towards the inner basin. The smelter with his rake or paddle pushes the slags upon that basin back towards the upper part of the sole, and his a istant spreads them uniformly over the surface through the back doors. The smelter next throws in by his middle door, a few shovelfuls of quicklime upon the lead bath. The a istant meanwhile, for a quarter of an hour, works the ore and the slags together through the three back doors, and then spreads them out, while the smelter pushes the slags from the surface of the inner basin back to the upper parts of the sole. The doors being now left open for a little, while the interior remains in repose, the metallic lead, which had been pushed back with the slags, flows down into the basin. This occasional cooling of the furnace is thought to be nece ary for the better separation of the products, especially of the slags from the lead bath. In a short time the workmen resume their rakes, and turn over the slags along with the ore. Three hours after the commencement, a little more fuel is put into the grate, merely to keep up a moderate heat of the furnace during the paddling. After three hours and ten minutes, the grate being charged with fuel for the third fire , the register is completely opened, the doors are all shut, and the furnace is left in this state for three quarters of an hour. In nearly four hours from the commencement, all the doors being opened, the a istant levels the surfaces with his rake, in order to favour the descent of any drops of lead; and then spreads the slags, which are pushed back towards him by the smelter. The latter now throws in a fresh quantity of lime, with the view not merely of covering the lead bath and preventing its oxidize ment, but of rendering the slags le fluid. Ten minutes after the third fire is completed, the smelter puts a new charge of fuel in the grate, and shuts the doors of the furnace to give it the fourth fire . In four hours and forty minutes from the commencement, this fire being finished, the doors are opened, the smelter pierces the tap-hole to discharge the lead into the outer basin, and throws some quicklime upon the slags in the inner basin. He then pushes the slags thus dried up towards the upper part of the hearth, and his a istant rakes them out by the back doors. The whole operation of a smelting shift takes about four hours and a half, or at most five hours, in which four periods may be distinguished. 1. The first fire for roasting the ores, requires very moderate firing, and lasts two hours. 2. The second fire , or the smelting, requires a higher heat, with shut doors; at the end the slags are dried up with lime, and the furnace is also allowed to cool a little. 3, 4. The last two periods, or the third and fourth fires , are likewise two smeltings or foundings, and differ from the first only in requiring a higher temperature. The heat is greatest in the last. The form and dimensions of the furnace are calculated to cause a uniform distribution of heat over the whole surface of the hearth. Sometimes billets of green wood are plunged into the metallic lead of the outer basin, causing an ebullition which favours the separation of the slags, and consequently the production of a purer lead; but no more metallic metal is obtained. Ten cwts. of coal are consumed at Holy well in smelting one ton of the lead-ore schlich or sludge; but at Gra ing ton, near Skipton in Yorkshire, with a similar furnace worked with a slower heat, the operation taking from seven hours to seven hours and a half, instead of five, only 7 1 ⁄ 2 cwts. of coal are consumed. But here the ores are le refractory, have the benefit of fluor spar as a flux, and are more exhausted of their metal, being smelted upon a le sloping hearth.