Ash Furnace
For Smelting Lead Ashes. Missouri.
One of the principal points to be attended to in building an ash-furnace is the elevation of the flue. It should rise 5-½ feet in 10; some prefer 5-½ in 11. If the ascent be too steep, the ore will run down into the basin before it gets hot, which is detrimental. If the ascent be too low, the bottom of the flue next to the basin will soon be eaten away by the heat, and thus in a short time undermine and destroy the furnace.
The flux employed is also a matter of moment. Sand, and pulverized flinty gravel, are mixed with the lead-ashes before smelting. The object of this is to promote the vitrification of the slag, which would otherwise remain stiff; the particles of revived lead would not sink through to the bottom, but remain entangled with it, and thus be lost. Lime is also sometimes employed for the same purpose; and indeed any earth would operate as a flux to the scoriaceous part of the lead-ashes, if added in a due proportion, particularly the alkaline earths. Lime and barytes, both of which are afforded in plenty at the mines, might therefore be advantageously employed, when no sand or easy-melting silicious gravel could be obtained. Good fusible sands are readily attacked and liquefied by submitting to heat with oxides of lead, alkaline salts, or any other alkaline or metallic flux; hence their extreme utility in glass, enamels, and all other vitrescent mixtures. When, therefore, silicious sand can be obtained, it will be found a more powerful flux to lead-ashes than either gravel, lime, spars, or any other substance, if we except the fluor spar. This is probably better adapted as a flux than even silicious sands; but it has not yet been brought to light at the lead-mines. Perhaps the lower strata of the earth may afford it. It is found at a lead-mine near Cave-in-Rock, on the right bank of the Ohio river, in the State of Illinois, and, with the exception of a little found at Northampton, Massachusetts, is the only place where this rare, useful, and beautiful mineral, occurs in the United States.[17]
The situation for an ash-furnace is always chosen on the declivity of a hill, as represented in the plate. The inside work, or lining, consists of slabs of hewn limestone, laid in clay-mortar, and backed by solid masonry. Although a stone less adapted for furnaces could hardly be found, yet it is made here to answer the purpose, and is an evidence of the ingenuity of men in making a bad material answer when a good one cannot be found. No sandstone or freestone, of that refractory kind used in glass and iron furnaces, is afforded in this vicinity; and the smelters seem to prefer rebuilding their furnaces often, to incurring the expense of transporting good infusible sandstones from a distance. It is not perhaps duly considered, that a furnace built of refractory materials, although expensive in the erection, would be sufficiently durable to warrant that expense, and outlast several built of limestone, which burn out every blast, and have to be rebuilt from the foundation.
Limestone is a combination of the pure earth lime with carbonic acid and water; it is a carbonate of lime. When subjected to a red heat, it parts with its carbonic acid and water, and, if the operation be continued long enough, is converted into quicklime. This effect, therefore, takes place as well in the lead-furnace as in the limekiln, and with this difference only—that in the former it is laid in a wall, protected in some degree from the heat, and will not part with its carbonic acid readily; while in the latter it is broken into comparatively small lumps, exposed to the heat on all sides, and is easily and readily converted into quicklime.
Nevertheless, although this calcination is constantly progressing, an ash-furnace will last from fifteen to twenty days, according to the skill which has been displayed in its construction, and the particular quality of the stone employed. When the stone partakes of clay (alumina), it runs into a variety of argillaceous limestone, and is manifestly better adapted to resist the effects of fire. Whenever the furnace is cooled, so that the stone can attract moisture from the atmosphere, it falls into quicklime. This change does not, however, take place rapidly; for the burning has seldom been uniform, and the stones have either been over-burned, or not burned enough; so that it requires several days, and even weeks, to assume the powdery state.
An ash-furnace, built of limestone, is estimated to cost a hundred dollars. This includes every expense, and such a furnace lasts during one blast, say fifteen or twenty days; perhaps, with great care, it will run a month. During this time, from sixty to ninety thousand pounds of lead ought to be made.
When a furnace is completed, it requires several days to dry it, and bring it to the proper state for smelting. About ten days are usually spent in this. The fire is begun very moderately at first, being only the warmth of a hot smoke, and is kept so for the first five days, by which means the moisture of the mortar and stone is gradually expelled, and without any danger of cracking the stone, or otherwise injuring the furnace. It is then raised a little every day until the furnace is brought up to a full red heat, when it is ready for the first charge of ashes.
The operation begins by shovelling a layer of ashes on the mouth of the flue, then adding a thin layer of sand or flinty gravel as a flux, and then more ashes; and so adding gravel and ashes alternately, until the required quantity is shovelled up. This is suffered to lie here and grow thoroughly hot before it is shoved down the flue into the basin; for, if introduced cold, it would check the heat too suddenly, and prove injurious in the result. When hot, the charge is shoved down the flue with a long-handled iron hoe, and another portion of ashes and gravel immediately shovelled on the mouth, suffered to heat, and then pushed down as before. This operation of heating and charging is continued until the furnace has a full charge, which may require about six hours, and in two hours more the furnace is ready for tapping. The slag, which is in a very fluid state on the top of the lead, is first drawn off, and the aperture closed up with stone and mortar. The smelter then goes to the opposite side of the furnace, and prepares for drawing off the lead by driving a stout sharp pointed iron bar through the side of the furnace, at a particular place contrived for this purpose. On removing the bar, the metallic lead flows out into a large iron pot set in the ground, and accompanied by a considerable quantity of a semi-metallic substance, called zane. This is lead not perfectly revived, being combined with some earthy particles, and oxide of lead. The zane occupies the top of the pot, and is first ladled out into hemispherical holes dug in the clay near by. This substance is of the consistence of the prepared sand used by brass-founders when hot, but acquires considerable solidity when cold. The metallic lead is then ladled into iron moulds of about eighteen inches in length, and yielding a pig of lead of about fifty pounds each. The quantity of zane made at each tapping is about equal to that of metallic lead. This is afterwards taken to the log furnace, and readily converted into lead. The lead made at the ash-furnace is not thought to be of so pure a quality as that of the first smelting made at the log furnace. It undoubtedly contains any other metals that may be combined with the ore, and is therefore more refractory. Such lead is thought to be a little harder, and some pretend to discover a lighter color.
The lead-ashes are reckoned to yield fifteen per cent. of lead (zane and all), which, added to the first smelting, makes an average product of sixty-five per cent. This estimate will hold good uniformly, when the ores have been properly dressed, and the smelting well performed. Any spar adhering to the ore, renders it refractory; blende and pyrites have the same effect. The latter is particularly injurious, as it consists chiefly of sulphur; a substance known to render all ores refractory.