At the end of one hour, moat of the lead is run off; after 1 1/2 hour, the ore is thoroughly stirred of from both aides ; and at the end of 2 noun, the first fire is complete. (2) the heat is urged to bright redness, when the sulphide present reacts on the oxde and sulphate, and liberates much of the metal; the working and calcining endure about 1 boor, a little quicklime being introduced to help stiffen the unreduced portion. (3) The fire is driven still more fiercely for about J hour, when the slage are again spread, and more lime is thrown in, combining with any silica present, and diminishing the fusibility of the mass. (4) The. fire is driven to the utmost for 3/4 hour, when the lead la tapped out, and the slags are dosed with lime to cause their partial solidification, when they are withdrawn. A little coal being thrown in to deoxidize any lead oxide remaining, the furnace is ready for another charge. The whole operation lasts about 5 hours, and consumes about 12 cwt. of coal per ton of ore.
Dr. Ballard describes the use of the reverberatory furnace under two distinct headings - the Flintshire process and the Cornish process. The former is adopted in North and South Wales, Yorkshire, Shropshire, and Derbyshire. The latter is apparently peculiar to Par, Cornwall, and differs in some respects. It requires 2 reverberatory furnaces: the first, with a flat floor, is used for calcining the ore, which, after calcination, with addition of a little lime to prevent clotting in the early stage, is drawn out and introduced into the second or "flowing" furnace. This latter resembles the ordinary Flintshire furnace (Fig. 136), but the charge, mixed with culm (small anthracite), is introduced through doors opposite the tap - hole, instead of by a hopper in the roof. The charge is at once melted down with closed doors and strong heat. At a certain stage of the smelting, scrap - iron is introduced into the well of the furnace. The products, which are all run at one time from the tap - hole, are : first, lead; then a regulus or matte, consisting chiefly of iron proto - sulphide, but containing other metals, such as copper and silver, present in the ore; and lastly, a slag, which, since it is "clean" (i.e. contains only 1 to 1 1/2 Per cent. of lead), is thrown away.
At some works, the first stage of the smelting, in which the greater part of the lead is got out, is performed by the Flintshire process, and the grey slag drawn out is then treated by the Cornish process. (Dr. Ballard's Report.)
A characteristic of working by fusion is the necessity of producing as far as possible only proto - silicates, and not bisilicales, as in working by roasting and reaction. On roast - ing a quartzose ore, a great part of the lead is already in the state of silicate, and its precipitation will be more difficult according as the proportion of silica definitively combined is higher. As to the redaction, it is obtained by either iron or lime. The lime has a more energetic action, and the introduction of a ferrous flux renders the ashes more corrosive; but, on the other hand, slightly ferrous fluxes are more fusible, consequently the working can be done at a lower temperature, and the volatilization of lead is diminished. Finally, in treating blendic ores, the ferrous element combats the decomposition of the zinc silicates formed in the roasting, and as a consequence reduces the volatilization of this metal, which always carries away a certain amount of lead with it. In any case, very corrosive scoriae are produced, necessitating energetic measures for preserving the twyers and sides. On this account it will bo interesting to describe the successful plan adopted by Baron at the Pisa works.
These treat quartzose and pyritous ores coming from the mines of Pallieres, mixed with others from Sardinia. He commences by roasting in reverberatory furnaces, 25 to 40 ft. long and 6 ft. wide, with doors on one side only. This roasting is pushed to the point of cementing the matters, but not to fusion, which would render the ultimate reduction more difficult. For the fusion, after employing a Castilian furnace 6 ft. high and 3 ft. in diameter, blown by 2 or 3 twyers, Baron was led to increase the height of the furnace, and to close the throat by a charging door, as well as to replace the refractory bricks at the level of the twyers with cast - iron sides cooled by a current of water.
The only kind of high - blast furnace seen in use in England by Dr. Ballard is the Piltz furnace, shown in Fig. 138. It is adapted for highly argentiferous ores, and is much employed in the United States. It is charged through the top opening with ore, fuel, and an appropriate flux (iron in some form being essential), quite up to the top, into which an iron chimney a is introduced 3 or 4 ft,, to catch any fume and conduct it into the flues. The reduction of the metal takes place a little above the level of the twyers b, and the metal and 'lag. with a regulus or matte, collect in the well c below, whence they are run oft. At the Sheffield Smelting Co.'a works, a form of blast furnace, in use for smelting rich argentiferous ores, is ranch lower than the Piltz, and is closed at the top, whence proceeds a flue for carrying off the "fume." A peculiar form of water - jacket is provided to prevent the destruction of the sides in the neighbourhood of the twyers. The jacket is made of cast iron, and has a number of inclined shelves, over which flows a stream of water. The water enters cold by a pipe at the top, and flows away nearly boiling by another pipe at the bottom, available for feeding a steam boiler.
Omitting the water - jacket, a similar furnace is employed at Snailbeach for smelting slags. (Dr. Ballard's Report.)
This method consists in precipitating the lead in a metallic state by another metal capable of taking its place in the chemical combinations. Iron complies with this requirement, and the operation is conducted in a blast furnace. The iron is added either in the form of scrap, more or less oxidized, or as cast - iron, or even as ore; but whatever the material which furnishes the iron as an element of precipitation, the process is costly, besides producing a certain quantity of cinders,which increases the loss of lead and silver. It is therefore only in special cases, for treating very quartzese, slightly argentiferous and cupriferous ores, or where fuel is scarce, that the precipitation method could be preferred to those of roasting and reaction, or roasting and reduction.