Theory of the above operations

—At Holy well, Gra ing ton, and in Cornwall, the result of the first graduated roasting heat, is a mixture of undecomposed sulphuret of lead, with sulphate and oxide of lead, in proportions which vary with the degree of care bestowed upon the proce . After the roasting, the heat is raised to convert the sludge into a pasty ma ; in which the oxide and sulphate re-act upon the sulphuret, so as to produce a sub-sulphuret, which parts with the metal by liquation. The cooling of the furnace facilitates the liquation every time that the sub-sulphuret is formed, and the ore has pa ed by increase of temperature from the pasty into the liquid state. Cooling brings back the sludge to the pasty condition, and is therefore nece ary for the due separation of the different bodies. The drying up of the thin slags by lime is intended to liberate the oxide of lead, and allow it to re-act upon any sulphuret which may have resisted roasting or decomposition. It is also useful as a thickener , in a mechanical point of view. The iron of the tools, which wear away very fast, is also serviceable in reducing the sulphuret of lead. The small coal added along with the lime at Gra ing ton, and also sometimes at Holy well, aids in reducing the oxide of lead, and in transforming the sulphate into sulphuret. IMG:4147767755307473660_illo0756.png:Smelting furnace 3. The smelting furnace or ore hearth. —This furnace, called by the French éco ais , is from 22 to 24 inches in height and 1 foot by 1 1 ⁄ 2 in area inside; but its horizontal section, always rectangular, varies much in its dimensions at different levels, as shown in fig. 634. The hearth and the sides are of cast iron; the sole-plate A B is also of cast iron, 2 1 ⁄ 2 inches thick, having on its back and two sides an upright ledge, A C , 2 1 ⁄ 2 inches thick, and 4 1 ⁄ 4 high. In front of the hearth there is another cast iron plate M N , called the work-stone , surrounded on every side excepting towards the sole of the furnace, by a ledge one inch in thickne and height. The plate slopes from behind forwards, and its posterior ledge, which is about 4 1 ⁄ 2 inches above the surface of the hearth, is separated from it by a void space q , which is filled with a mixture of bone ash and galena, both in fine powder, moistened and pre ed down together. The melted lead cannot penetrate into this body, but after filling the basin at the bottom of the furnace, flows naturally out by the gutter (nearly an inch deep) through a groove in the work-stone ; and then pa es into a cauldron of reception P , styled the melting-pot , placed below the front edge of the work-stone . The posterior ledge of the sole is surmounted by a piece of cast iron C D , called the back-stone , 28 inches long, and 6 1 ⁄ 2 high; on which the tuyère or blast-pipe is placed. It supports another piece of cast iron E , called pipe-stone , scooped out at its under part, in the middle of its length, for the pa age of the tuyère . This piece advances 2 inches into the interior of the furnace, the back wall of which is finally crowned by another piece of cast iron E H , called also back-stone . On the ledges of the two sides of the sole, are placed two pieces of cast iron, called bearers , each of which is 5 inches in breadth and height, and 26 inches long. They advance an inch or two above the posterior and highest edge of the work-stone , and contribute effectually to fix it solidly in its place. These bearers support, through the intervention of several ranges of fire-bricks, a piece of cast iron called a fore-stone , which has the same dimensions as the piece called the back-stone , on which the base of the blowing-machine rests. This piece is in contact, at each of its extremities, with another ma of cast iron, 6 inches cube, called the key-stone , supported on the masonry. Lastly, the void spaces left between the two key-stones and the back part of the furnace are filled up with two ma es of cast iron exactly like the key-stones. The front of the furnace is open for about 12 inches from the lower part of the front cro -piece called fore-stone , up to the superior part of the work-stone . It is through this opening that the smelter operates. The gaseous products of the combustion, on escaping from this ore-hearth, are frequently made to pa through a long flue, sloped very slightly upwards, in which they deposit all the particles of ore that they may have swept along; these flues, whose length is sometimes more than 100 yards, are usually 5 feet high and 3 feet wide in the inside, and always terminate in a chimney stalk. The matters deposited near the commencement of the flue require to be washed; but not the other dusty deposits. The whole may then be carried back to the roasting furnace, to be calcined and re-agglutinated, or introduced without any preparation into the slag-hearth . IMG:4147767755307473660_illo0757a.png:Slag-hearth 4. Figs. 635 , 636. represent a slag-hearth, the fourneau à manche (elbow furnace) of the French, and the krummofen (crooked furnace) of the Germans; such as is used at Alston Moor, in Cumber land, for the reduction of the lead-slag. It resembles the Scotch furnace. The shaft is a parallelopiped, whose base is 26 inches by 22 in area inside, and whose height is 3 feet; the sole-plate a , of cast iron, slopes slightly down to the basin of reception, or the fore-hearth b . Upon both of the long sides of the sole-plate there are cast iron beams, called bearers C C , of great strength, which support the side walls built of a coarse grained sand-stone, as well as the cast-iron plate d ( fore-stone ), which forms the front of the shaft. This stands 7 inches off from the sole-plate, leaving an empty space between them. The back side is made of cast iron, from the sole-plate to the horizontal tuyère in its middle; but above this point it is made of sand-stone. The tuyère is from 1 1 ⁄ 5 to 2 inches in diameter. In front of the fore-hearth b , a cistern e , is placed, through which water continually flows, so that the slags which spontaneously overflow the fore-hearth may become inflated and shattered, whereby the lead di eminated through them may be readily separated by washing. The lead itself flows from the fore-hearth b , through an orifice, into an iron pot f , which is kept hot over a fire. The metal obtained from this slag-hearth is much le pure than that extracted directly from the ore. The whole bottom of the furnace is filled to a height of 17 inches, that is, to within 2 or 3 inches of the tuyère, with the rubbish of coke reduced to coarse powder and beat strongly down. At each smelting shift , this bed must be made anew, and the interior of the furnace above the tuyère repaired, with the exception of the front, consisting of cast iron. In advance of the furnace there is a basin of reception, which is also filled with coke rubbish. Farther off is a pit, full of water, replenished by a cold stream, which ince antly runs in through a pipe. The scoriæ, in flowing out of the furnace, pa over the coke bed in the basin of reception, and then fall into the water, whose coolne makes them fly into small pieces, after which they are easily washed, so as to separate the lead that may be entangled among them. IMG:4147767755307473660_illo0757b.png:Bellows These furnaces are urged, in general, by wooden bellows; fig. 637. But at the smelting works of Lea, near Matlock, the blowing-machine consists of two casks, which move upon horizontal axes. Each of these casks is divided into two equal parts by a fixed plane that pa es through its axis, and is filled with water to a certain height. The water of one side communicates with that of the other by an opening in the lower part attached to a crank of a bucket-wheel. At each dem i-oscillation, one of the compartments, being in communication with the external air, is filled; whilst the other, on the contrary, communicates with the nozzle, and supplies wind to the furnace. 5. Refining or cupellation furnace. See Silver . 6. Smelting by the reverberatory furnace , is adopted exclusively in Derby shire, and in some works at Alston-moor. The charge in the hopper consists commonly of 16 cwt., each weighing 120 lbs. avoirdupois, composed of an intimate mixture of 5, 6, 7 or even 8 kinds of ore, derived from different mines, and prepared in different ways. The proportions of the mixture are determined by experience, and are of great consequence to the succe of the work. The ore is rather in the form of grains than of a fine schlich ; it is sometimes very pure, and affords 75 per cent. ; but usually it is mixed up with a large proportion of carbonate and fluate of lime; and its product varies from 65 to 23 per cent. After scraping the slaggy matters out of the furnace, a fresh smelting shift is introduced at an interval of a few minutes; and thus, by means of two alternate workmen, who relieve each other every seven or eight hours, the weekly operations continue without interruption. The average product in lead of the reverberatory furnaces in Derby shire, during several years, has been 66 per cent. of the ore. Very fine ore has, however, afforded 76. 7. Smelting of the drawn slag, on the slag-mill hearth. —The black slag of the reverberatory furnace is broken by hammers into small pieces, and mixed in proper proportions with the coal cinders that fall through the grate of the reverberatory fire. The leaden matts that float on the surface of the bath, and the dust deposited in the chimney, are added, along with some poor ore containing a gangue of fluor spar and limestone, which had been put aside during the mechanical preparation. With such a mixture, the slag-hearth, already described, figs. 635 , 636. , is charged. By the action of heat and coal, the lead is revived, the earthy matters flow into very liquid scoriæ, and the whole is made to pa acro the body of fire into a basin of reception placed beneath. The scoriæ are thickened by throwing quicklime upon them, and they are then raked away. At the end of the operation the lead is cast into pigs or ingots of a peculiar form. This is called slag-lead. It is harder, more sonorous than the lead obtained from the reverberatory furnace, and is preferred for the manufacture of minium, lead shot, and some other purposes. 8. Treatment of lead ores by the Scotch furnace, or ore-hearth. —This furnace is generally employed in the counties of North umber land, Cumber land, and Durham, for the smelting of lead ores, which were formerly carried to them without any preparation, but now they are exposed to a preliminary calcination. The roasted ore yields in the Scotch furnace a more considerable product than the crude ore, because it forms in the furnace a more porous ma , and at the same time it works drier , to use the founders’ expre ion; that is, it allows the stream of air impelled by the bellows to diffuse itself more completely acro the matters contained in the furnace. The charge of the roasting furnace, figs. 631 , 632 , 633. , is from 9 to 11 cwt. of ore, put into the furnace without any addition. Three such shifts are usually pa ed through in eight hours. The fire should be urged in such a manner as to produce constantly a dense smoke, without letting any part of the ore melt and form a slag; an accident which would obstruct the principal end of the proce , which is to burn off the sulphur and antimony, and to expel the carbonic acid of the carbonate of lead. The ore must be frequently turned over, by moving it from the bridge to the other end and back again. To prevent the ore from running into ma es as it cools, it is made to fall out of the furnace into a pit full of water, placed below one of the lateral doors.