SODA.— Black Ash Furnaces and Boiling Down Pans. 807
more than charging by hopper, which is alwaysdecidedly preferable where it can be applied.
The size of bed shown in Plate III. is that mostusual on the Tyne; in Lancashire the bed is gen-erally much larger, and is especially deeper from backto front. It is also often preferred in Lancashire tobuild two small fire-places at the end of the furnace,their bars running parallel with its longest dimension,instead of the one large fire-place with bars runningacross, as shown in the diagrams. In that case thereis often a metal plate put in across the fire-end ofthe furnace, 2 feet deep, having a number of per-forations 3 inches in diameter, with a correspondingnumber of holes (“ pigeon holes ”) in the inner fire-brick casing. These holes serve to admit a furtherquantity of air to the burning fuel, apart from whatenters through the grate-bars; and this arrangementis found to cause a very sharp and steady heatjust over the fire-bridge, besides saving a good dealin wear and tear of the fire-bars.
It is hardly necessary to say that all parts of thefurnace which come into contact with the fire orball-mixture must be made of very good fire-bricks.It is usual to make this fire-brick lining 9 inches thickall over; even with this thickness it does not last manymonths in the more exposed places, such as the fire-hearth, the fire-bridge, and the working bed. It isnot only the heat, but the action of the alkali, and,in the case of the bed, that of the tools, whichdestroys the fire-bricks. They ought, therefore, tobe as hard and well burnt as possible; porous bricksare more easily penetrated and dissolved by thealkali (at the same time making the latter impure),and worn away by the tools. The less sand theycontain the better, as it is more liable to be actedupon by the fused alkali than the aluminium silicate.
The ball-furnace requires for its proper work adegree of heat approximating (although not quitereaching) white heat, and naturally a vast amount ofit escapes unused through the “snore-hole,” or overthe end bridge, behind the back bed. This wasteheat is utilized in boiling down the solution of soda"obtained in the process of lixiviating the black-ash ;but not always in the same manner. In all factorieson the Tyne, and in some of those in Lancashire , theliquors are contained in “open" pans, where the fireis carried along the surface of the liquid, and the eva-poration consequently is increased by the draughtcarrying away the vapour; whilst the salt, as it crystal-lizes out Of the more and more concentrated solution,falls to the bottom and accumulates till it reaches tothe top, and the whole is converted into a mortar-like mass, which is pulled bodily out of the pan andallowed to drain in an iron drainer furnished with aperforated false bottom, as shown in Plate III. Thepans are made of three-eighth inch iron, of about thesame width as the furnace, from 16 to 20 feet long andfrom 1 foot 6 inches to 2 feet 6 inches high; some-times they are made to slope towards the front. Theyare supplied with two or three drawing doors, theform of which can be seen from the diagrams. Thedoors themselves are pressed against the angle-ironrim, which runs all round the opening of the pan,
by means of a strong screw or by wedges, after alayer of good clay, or sometimes of lime, has beenput in between. This is sufficient to make the jointtight, and a slight leakage is not of very much con-sequence, as the necks of the drawing openings reachover into the drainer, so that any escaping liquid issaved there. The angle-iron must not stand up inthe opening at all, as it would otherwise be impos-sible to drain all the contents of the pan. Theplates are put together in such a manner that thejoints all run at right angles to the doors (acrossthe width of the pan), and they are overlappedon each side towards the doors; the object beingthat the men’s tools when raking out the pan shallnot be stopped by any joints. For the same pur-pose it has been sometimes thought necessary tocountersink the rivets; but this is rarely done now.An angle-iron runs round the top of the pan,which not only stiffens the same, but also servesas a hold for the arch covering the pan, betweenwhich and the liquor the flame has to travel. Thearch is often only 5 inches thick; but it is muchbetter to make it 9 inches, as in the Plate, al-though this involves some extra plates along thesides of the pan to strengthen the “spring” ofthe arch. A 9-inch arch is both stronger and keepsthe heat much better together than a 5-inch one.The pan is not set immediately at the end of theball-furnace—it would in that case very soon burndown at that side; but an air course is left along itsends, and fire-clay lumps are laid over this, projectingseveral inches into the inside of the pan, so that thefire shall not destroy the top of the plates where theyare bare of liquor. The pans ought always to beset on pillars in such a manner that the space under-neath is accessible. In that case any leakage can bedetected before much harm is done, and there ismuch greater accessibility for repairs when thesehave become inevitable.
This, then, is the system of open pans. The fireis more efficient in them; they do more work andlast longer than pans heated from below; but thesalt in them is necessarily contaminated by theproducts of combustion, and it does not pay towork them except by boiling down the whole con-tents to a pasty mass. In several Lancashire pansthis is avoided by allowing the furnace gas to heatpans from below. These are now generally made ofthe shape called “ boat pans,” from the fact of theirtransverse section having some similarity to that ofa boat with sloping sides, as will bo seen from Figs16 to 22. Fig. 16 is a section of Wrong’s iron boatpan ; Fig. 17 a front elevation of the same; Fig. 18