A theoretical and practical treatise on the manufacture of sulphuric acid and alkali, with the collateral branches
Lunge, Georg, 1839-1923
381 r ^ I I § I 3 I 8 r e CO o» CO CO feo B $ g ^ S 8| eg fcO K) fc9 H*g 8 $ S 8 8| ^ 5 2 2 K0 $ 8 6 ^ S| § § S § SI" CO »-« ^ ^ 4k. »U M wEl' <p 59 cj» fco ^ o 0» Oft fcO 0» 0» i I 1^ Oft E I I si $$ s ^ Ss^ X^ M) pj ? ■5 K y « §  side, for serving those parts of the furnace not reached by the tools from the other doors ; E E, chai^g-holes in the roof, protected by cast-iron lining ; F G, snore-holes for con- Teying the fire-gases into the flues 6* G', through which the waste heat can be utilized for boiling-down pans &c, ; H, the roof of the fiimace, on which the mixture is spread and warmed for some hours previously to being tipped through the openings E E into the furnace. Payen states the following details of work in these furnaces. On the average each charge consists of 935 kilt^. sulphate, 986 chalk, 480 small coal, and 145 mother salts from refining. The fuel for 24 hours amounts to 2800 kilt^. coal. The labour is done by 6 THB BLACK-ASH FIJENACE8. 383 fdmacemen and 6 assistants^ who take their turns in three shifts of 8 hours each. 30 minutes are reckoned for charging and spreading the batch ; 30^ 35^ and 20 minutes for the firsts second^ and third turning-over ; 20 minutes for the last raking through and pulling out the batch. The whole labour takes 135 minutes ; the pauses for resting between^ 105 minutes ; the total time for a batchy 240 minutes =4 hours. In 24 hours 6 batches of 1525 kilog. each^ together 9150 kilog. pf black ash are made^ testing 38° Decroiz. These large furnaces^ which formerly were much more frequently found on the Continent than now^ have considerable drawbacks, which have caused their almost total abolition in Germany, and their partial abandonment in France, in favour of the smaller English ball-furnaces. Th<B only advantage attached to the large furnaces, a saving in fuel, is now attained with the smaller furnaces in a rational way by employing the waste heat for evaporating the lye, heating water, &c. In the large furnaces it is actually impos- sible to heat the bed any thing like uniformly all over ; the batch will therefore in some places be finished and get burnt whilst in other places the reaction is still far behind. Nor can such very large charges be worked through evenly and ultimately fluxed to a homogeneous mass, at least not without heating them beyond their proper time. Certainly labour is not saved: 12 men in 24 hours get through 15276 kilog. = 15 tons 5 cwt. of materials ; whilst in a small English furnace with a 12 hours' shift two men manage 9 tons, with an 8 hours' shift three men 10 tons 2 cwt., in 24 hours. Another description of furnace, figured in some treatises, hardly deserves to be mentioned, viz. that in which the flame of a small black-ash furnace travels over another dish-shaped furnace-bed, in which common salt is decomposed by sulphuric acid, so that all the hydrochloric acid escapes into the air along with the smoke I Such a barbarous apparatus is not imaginable in our times. Kopp (Wurtz, Dictionn. ii. p. 1578) mentions a seemingly rational arrangement, in which the flame of the black-ash furnace heated a sulphate-furnace consisting of a pan and blind roaster; the flame first travelled round the muffie and at last underneath the pan. This plan has been given up again at the works where Kopp had found it, and that justly ; for it belongs to the number of too finely drawn attempts at saving which in the end efiect the contrary of the object they are intended to serve. Owing to the insufficient heat of the furnaces heated by the waste heat, the work rf'. 1 384 60PA. will take much more time than with direct firing; moreover one furnace perhaps requires a strong fire^ whilst the other is not yet ready for it^ and the first one must thus wait for the second ; in shorty ultimately for the same quantity of work turned out more time^ labour^ fdmace-room^ and mostly even more fuel are con- sumed than by other manufacturers who drive each apparatus by a separate fire^ and consequently can force the work through. Of course these considerations are not intended to warn against uti- lizing the waste heat at all^ but against irrational and exaggerated attempts at utilization. According to Knapp (Chem. Technol. i. 2, p. 416)^ in Germany [formerly?] furnaces with three oval beds occur, the lowest of which serves for finishing the charges, the two others, situated at a higher level alongside of each other, for a preliminary heating ; the contents of these back beds are alternately pulled down into the working-bed. The furnaces combine a complicated conJBtruction with great liability to need repairs. A furnace patented in 1868 by Perret is foimded upon the same principle as his pyrites-burner (Vol. I. p. 193) . It contains several shelves of fireclay slabs, through which the flame travels in a ser- pentine path firom bottom to top ; the heat given out by the mixing- coal assists the work. The lowest bed, which is fired directly, is only 4 ft. 7 in. wide. A practical application of this furnace is not probable ; it cannot be expected to stand any length of time ; and its object, better utilization of the heat, can be attained by employ- ing the latter for evaporating the liquors. In France, and also in Germany, sometimes large-bedded black- ash furnaces are found with one working-door on the same long side of the furnace as the fire-door, the other at right angles to it at the other end of the furnace, opposite the fire-door. We will now describe the English black-oih furnaces {baU^Jur" fuices), such as are now more and more found on the Continent as well. On the whole they are built on the same plan ; they always have two (rarely three) beds, of which that more distant from the fire-bridge is a little higher than the front bed. The beds are comparatively very small, and only intended for a charge of 8 cwt., which is first put on the back bed, afterwards on the front or working-bed. Otoly quite exceptionally are larger charges worked in similar furnaces, e.g. 14 cwt. at Aussig. The Lancashire furnace, of which fig. 168 gives a perspective THE BLACK-ASH FUBNACE8. Rg. 168. Tiew, differs from the Tyneaide furnace both in its lai^er size (it is from 8 to 12 inches wider and from 12 to 18 inches longer) and in its fireplaces. Instead of one lai^e fireplace, with the grate-bars running from side to side of the furnace and charged from the front, the Lancashire furnace haa two small fires, with bars running the long way of the fiimace and charged from one end. They are also usually leas substantially built than Tyneside furnaces. Even outwardly this difierence is noted — that Tyneside frumaces are always cased in solid cast-iron frvnt plates, whilst Lancashire furnaces are only bound by wrought-iron strips 3 to 6 inches broad and | inch thick, about 6 inches apart from one another ; these are pressed against the brickwork by the usual uprights and tie-rods. Some- times even the working-doors are only protected by frames ; but frequently metal plates are provided for these, reaching from the roof down to the bottom of the door, and held in their places by the two nearest uprights. The tie-rods of these are scarcely ever held on the uprights by screws and nuts, but by loops and wedges. Inside, the fire-bridges often have no bridge-plates and air-channels, which are never dispensed with on the Tyne. Sometimes in Lan- cashire there is fixed in the fire end of the furnaces, above the fire- doors, a cast-iron plate perforated by many holes, through which air is admitted into the furnace above the coals. This is intended to bum the smoke and save fuel, but often does the reverse; and consequently the holes are sometimes found plastered up again. 886 SODA. The construction of furnaces usual on the Tyne is shown in figs. 169 to 174 (working-drawings^ of a Tery good furnace in actual work^ all the dimensions being indicated in metrical measures). Here at the same time the boiling-down pan is shown^ as it always accompanies the furnace. Fig. 169 is a sectional plan along the line A B of fig. 170^ passing through the hollow fire-bridge^ but showing the beds and the pan from the top; fig. 170 is a sectional elevation along CD of the plan; fig. 172^ one through the fire^ along EF^ looking towards the fire-bridge ; fig. 173^ one along G H^ also looking towards the fire-bridge; fig. 174^ a section through the pan^ looking at the furnace; fig. 171^ a front elevation^ showing the firing-caye and the drainers in section. Such a furnace serves for working 3-cwt. charges of sulphate along with the requisite quantity of chalk and coal; a '^ball^' is finished in 45 to 60 minutes (after having passed a similar time on the back bed) ; and, with the necessary pauses for cleaning the fire, changing shifts, &c., 24 to 27 balls are made daily. a is the fireplace with its grates, consisting of wrought-iron bars, resting on three strong bearing-bars, set edgewise, so that there ia not too much contact between them and the grate. Besides, a bar b is let into the brickwork lower down, to serve as a fulcrum for the large pokers with which the fire is broken up and cleared from below ; c is a cave for facilitating the firing ; this permits the ash-* pit to be much lower than otherwise. Such a " cave-fire '^ consumes less fuel than an ordinary fireplace ; perhaps, owing to the larger size of the ash-pit, the supply of air is more uniform ; but probably it is more owing to the greater facility for minding the fire, so that not so much unbumt coal is raked through the bars. The fireplace must be lined with the best procurable firebricks, which must also be able to resist mechanical wear and tear by the tools. This holds good even more of the bricks composing the working-beds and the surrounding walls, but less of the arch. All of these must also resist the fluxing alkali for some time, of course only for a limited period. It is sufficient if the fireplace-lining lasts for three months, the working-bed five months, and the arch twelve months. The fireplace at the back, at d, is stepped backwards, so that the flame ranges all across the width of the furnace over the bridge e. The latter must be most carefolly built ; it is always very strong, 2 feet 6 inches to 8 feet 3 inches wide, and on the Tyne always hollow. ,f THE BLACK-ASH FURNACES. 387 A cast-iron plate /^ 1^ to 2 inches thick^ reaches right across the bridge, half above and half below the level of the working-bed, and is protected by an air-channel g from burning or melting, on the side next to the fireplace. The air-channel is open at both ends ; or if two furnaces are built back to back, it is provided "^ith a small upright shaft to procure a constant draught in it. The object of the bridge-plate is principally to prevent fluxing of the black ash through the bed and the lower part of the bridge ; this evil is much felt in Lancashire, where the plates are often wanting. Of course the air-channel must always be kept open : after some time it gene- rally gets filled with fluxed black ash ; and the fire-bridge must then be renewed. K this be n^lected, the bridge-plate will get burnt, whilst otherwise it can serve many times over again. h and X are the two furnace-beds. The charge is put through the hopper i, or, if none is provided, through the door, on the back bed t, which is 3 inches higher/ than the front bed ; from this it passes onto the front or proper working-bed A, and is drawn out through its door. Three-bedded furnaces are but rarely met with. The beds are made in this way : — First the base of the furnace is walled up with common bricks to 12 inches below the level of the working- bed, or an 18-inch wall is made all round, the inner space filled up with broken bricks, and the whole paved on the top. Upon this foundation a firebrick wall, 9 inches thick, is made round the sides of the future beds and carried up to the abutment of the arch ; this waU follows the outlines of the beds, and consequently has rounded comers at the back ; in the front it slopes towards the two doors, between which there is a wedge-shaped centrepiece /. Now the site of the bed h is filled up 3 inches high with ground fireclay or ground chalk, and these are beaten down hard. Upon this the bed itself is made of -bricks on end — ^that is, 9 inches deep; the bricks are set dry, as closely together as possible, and are driven in with the mallet, so as to leave the closest possible joints. The top is " grouted '^ with a thin paste of ground fireclay, in order to fill up the crevices between the stones. For the back bed, first three courses of common bricks are put on, some ground chalk is put on the top ; and the bed itself is made of bricks on edge (t. e. ^ inches deep), also wedged in as tightly as possible and grouted with fire- clay. The drawing clearly indicates how the joints run, so that the tools shall meet with the smallest possible resistance. The working-bed h slopes about 2 inches from the back to the 2c2 \ 1 388 SODA. front door, in order to facilitate the working of the tools and espe- cially the raking-out ; the bed i requires very little or no slope. The back bed lasts a very long time, but the working-bed, at the best, four or five months ; after this time it must be renewed, be- cause too much stuff gets lost in it and because its inequalities do not allow the tools to work properly. Then the troublesome and expensive labour has to be undertaken of removing piecemeal the bricks of the bed, which are fluxed along with black ash into a very hard mass, and then putting in a new bed. In Lancashire^ often only ^^ false beds " are made; that is, the space hollowed out by wear and tear is filled up with fire-bricks whilst the ftunace is at its highest heat, the bricks are levelled with a rake through the working- door as well as may be, a few shovelfuls of sulphate are thrown in, and these are fluxed down with closed doors. A false bed of this sort lasts one or two months. Last of all the arch is made, 9 inches thick, for which specially shaped bricks have been placed previously as an abutment on the top course of the surrounding wall. In the arch a hole over the back bed i is left for putting in the hopper *, of sheet iron or very thin cast iron. The furnaces on the Tyne are cased in solid cast-iron plates, 1 to li inch thick, both back and front, and tied together by a number of uprights, made of strong railway-plates or oblong wrought-iron bars, 4x3 inches ,* at the bottom these are let into the ground 12 inches, and are there kept from turning over by stones of a foot cube ; some- times grooves are left in the foundation of the furnace for tie-rods, which are fastened like the upper ones. Above the furnace the up- rights are always kept together by tie-rods of 1-inch round iron or |-inch square iron, usually provided with a screw-thread on either end, which pass through holes in the uprights and are tightened up by nuts. The latter must be several times tightened up subse- quently. In some places it is preferred to weld strong loops to the ends of the tie-rods which pass round the uprights ; the tightening- up is done by driving iron wedges between the loop and the upright. This skeleton of casing and binders Qf course remains, even when the brickwork of the furnace is entiriely worn out ; the furnace- walls are consequently much thinner t^an is usual on the Continent, viz. 15 to at most 18 inches. This certainly causes rather more heat to be lost by radiation ; but that is more than counterbalanced THE BLACK-ASH FURNACES. 389 by the longer duration of the furnace-lining, which does not become so hot; and the working of the furnace is easier if the front wall is thinner. The uprights are placed in such a way as to protect the parts sufPering the greatest strain. The casing-plates are in one piece from top to bottom ; but sideways of course the casing must be divided into several plates, whose joints are so arranged that they are always covered by an upright. Moreover they are connected inside by screwed-on patches or in some other way. In order to facilitate the work, a small cast-iron plate is put in front of the working-door of A, where there is most wear and tear ; there are also supporting-rods for the tools fixed in front of both doors, ending in loops which are put upon hooks bolted to the two nearest uprights. The closing of the doors is best effected by means of fireclay slabs surrounded by an iron frame, and hung from cast-iron standards by means of a chain, pulley, and balance- weight. These are indicated in the elevation at one of the doors only ; the cross sections show the doors (fig. 171) themselves. Adjoining the furnace, but separated &om it by a small air- channel s, is the boiling-down pan, p, which everywhere on the Tyne, and in many places elsewhere, is arranged for top heat. For hand- furnaces there is usually only one pan ; but sometimes even here two pans are found side by side, as with revolving furnaces, each of which can be shut ofi^ from the draught by a damper during the cleaning-out. The pans are made of |-inch sheet iron, riveted in such a way that the rivets and seams are as little as possible in the way of the tools for working the pan. For this purpose the plates run in whole lengths from back to front (towards the doors), with- out any cross seams. The joints are either butt joints, with a strap at the bottom riveted to both plates, or, in the more usual construction of lap joints, the plates are laid so that the laps run on each side to- wards the doors ; if, therefore, the rake is drawn towards a door, it slides from one plate to the other, and is not brought up by the seam. Sometimes the rivets are countersunk, especially in the " fishing pans " employed in Lancashire for bottom fire ; for top-fired pans it is more usual simply to place the round rivet- heads on the top side and do the riveting on the bottom side. 890 SODA. The shape of the pan is so designed that its whole contents can be easily drawn out^ as m&j be seen in the plan. Some- times pans of that length are made with three doors ; but usually they are made longer (a trifle above 20 feet for hand-furnaces)^ and then always with three doors^ in order to better utilize the heat and to do more work. Round the top of the pans runs an angle-iron^ turned inside^ partly for stiffening it^ and partly as an abutment for the arch. But as it is hardly strong enough for this pur- pose^ frequently a special angle-bar r (figs. 171 & 174) is put on, which permits the arch to be made 9 inches thick instead of 4^ inches. In the comers^ where the shape of the pan would cause an inconvenience^ the wall is carried up to the top of the pan ; the angle-bar is put on the top; and the arch is also sprung here of the same width as over the principal part of the pan. Fig. 170 shows how the arch slopes down from the furnace towards the pan ; sometimes the sides of the latter are shaped to suit this slope. This slope of the arch cannot be easily avoided, because on the one hand the pan-roof most be near the surface of the liquid so as to draw down the flame^ for which reason the arch is sprung directly &om one top edge of the pan to the other ; but on the other hand there should be a sufficiently wide opening both for the flame to enter the space above the pan and for the fireclay slabs 0, which form the bottom of the snore-hole and at the same time cover the air-channel s between furnace and pan, to project some way into the latter. This is done to protect the front end of the pan from burning and warping, to which it would be very much exposed if it were not cooled by the air-course and its top covered by the slabs o. Similar air-courses are left between the brick comers and the pan, sometimes even at the back end, although this is the coolest; the remainder of the pan-sides is in any case open to the air outside. In order to carry the slabs o more safely, some- times a: cast-iron saddle is put upon the pan-edge. The flue t is provided with a vertical or horizontal damper, and either leads directly into the main flue, or it is carried so as to make use of the remaining heat of the gases, which certainly must never interfere with the draught. This can be done in several ways; and we shall have more than one occasion to point out the waste heat of the black-ash furnaces as a source of heating other apparatus. The pan rests upon pillars, q q, not quite going from side to side, so as to leave a free passage in the centre ; they should leave the THE BLACK-A8H FURNACES. 391 seams accessible. Thus any leaks can be discovered and repaired hj caulking without disturbing the working of the pan; moreover the pan-bottom is kept cooler in this way. It is well to cover the floor underneath the pan and the drainer with a layer of cement, and give this a little slope towards a catch -well in case of leakage. Such cases, however, ought not to occur at all if the pan is properly riveted and fixed. In order to discharge the pan its front wall, is interrupted in two or more places, and bent outwards for about 6 inches. The pan- bottom is continued into the necks formed in this way ; the angle- iron connecting it with the pan-sides also turns round these comers, as seen in the plan fig. 169, whilst the upper angle-iron is interrupted in these places. Another angle-iron is riveted all round the openings formed in this way, as shown most clearly at «' in the elevation fig. 171. Against this leans the door u of ^-inch boiler-plate, without which the whole contents of the pan (which is sloped a little forward) would run out of the openings (door- frames). The doors are made fast in the following way: — ^The door-frame on each side has a slot (fig. 171, v!) through which bolts are put, each ending in front in a long loop. Through these loops a strong cross bar is put, strengthened in the centre and perforated by a screw-hole ; through this passes a strong (2-inch) bolt upon which a corresponding worm is cut, and which in front is turned by a handle. Thus the door u is pressed against the frame; and lest it should sufier in the place where the bolt acts, it is strengthened there. It also carries one or two handles. The doors are not carried up to the full height of the pan ; so that it is possible to work in the latter and observe its interior; in the meantime the loose cover i/ (fig. 174) of sheet iron, with a handle, prevents the cold air from entering. Before putting on a door, its edge, where it will be in contact with the frame, is plastered with a thick paste of clay or lime-putty, most of which is forced out on turning the screw; the portion remaining in makes the joint good. Any small leakage does not much harm, as the liquor is caught in the drainer. Figs. 175 to 177 show a somewhat different kind of door, which may be of cast iron; fig. 176 shows also a different mode of sup- porting the pan-arch. In any case the arch must be bound by a skeleton of 2-incb square uprights, kept together by tie-rods with screw ends both above and below the pan. In front of the pau stands the drainer, w, made of -^inch iron, which is seen in Gg. 170 from above, in lig. 174 in cross section, and in fig. 171 in longitudinal section. It has a perforated false bottonij X, consisting of plates 2 feet square, with holes | to ^ inch wide, resting on a framework, of angle- and T-bars. The true bottom of the drainer slopes to one side, and ends in a well w, in which the liquor-pump y stands. The latter is shown on a laiger scale by fig. 178. It is made entirely of iron, cast and wrought; Rg- 176- Fig. 177. the clacks are ground to their seats ; and the turned piston travels tightly in the bored-out barrel, since neither leather, nor hemp, nor oil is applicable for the caustic mother liquor. The pnmp is here drawn only high enough for pumping back the mother liquor into the pan ; it may of course be made higher, if necessary, in order to keep the mother liquor by itself. The work in the black-ash furnace is carried on in the following THX BLACK-ABH FUENACBS. Rg. 178. 394 SODA. manner : — ^The mixture is always first put on the back bed and levelled to some extent ; during its stay there it is twice turned over with the paddle and a small rake^ so as to expose fresh por- tions to the action of the fire. Here it is only thoroughly dried and heated ; but it would be difficult to work it through properly, nor would this do much good, since the materials only begin to react upon one another when one of them has begun to flux. The mixture remains on the back bed till the working-bed, upon which in the meantime the preceding charge has been finished, is ready to receive it. This is not the case immediately after the preceding ball has been drawn ; but the fire is allowed for some time to heat the empty ftimace very thoroughly; then only the charge is pushed over from the back bed, through its working-door, by means of the paddle and rake. Often this is not done all at once, but about one third of the charge is left behind till the other two thirds have become very hot and are partially fiuxed ; then the last third is pushed over. In this way the mixture can be better turned over and overheating of portions of it avoided. On the Tyne this style of working (there called '^ splitting a ball'') is quite usual. At the first pause after the back bed is empty, it is charged with a fresh batch, either from above by the hopper, or, in the absence of this, from the side through the door. In the latter case much cold air enters the furnace during the charging, and cools it down to some extent. If there are two back beds, the charge on the middle Hied is first moved onto the &ont bed, then that on the back bed onto the middle bed. The principal work of the ball-fumacemen takes place on the proper working-bed, the temperature of which gets up to a white heat ; it is usually stated to be between the melting-heat of silver and that of copper. Ferd. Fischer (Deutsch. chem. Ges. Ber. ix. p. 1559) made exact observations of temperature by means of a Siemens electrical pyrometer, and found: — 10 minutes after freshly charging, 713° C. ; 30 minutes later, 779° C. ; 15 minutes later, 874° C; again 15 minutes later, just before drawing, 932° C. This refers to a ftimace in which a charge of 3 cwt. sul- phate is just finished in 1^ hour, whilst in England this only takes from 45 to 50 minutes ; the temperature here must be accordingly higher. Kolb {loc. cit.) has made a series of experiments upon the temperature required for black-ash making, not by means of a pyrometer, like Fischer, but still sufficiently illustrating the influ- THE BLACK-ASH FURNACES. 395 ence of a rise of temperature; we shall report upon them later ou^ when describing the composition of ball soda^ and will only remark here that Kolb considers a temperature near the melting-point of silver the most favourable one for soda-making. We shall also refer in that place to Hill's observations^ according to which in the regular black-ash process the melting-heat of sodium sulphate is not attained.. Shortly after the mixture has been exposed to the intense heat of the working-bed^ it alters its nature by softening at the surface. First of all the sulphate on the top^ where it is touched by the flame^ softens^ and small clots are formed ; so far as it is converted into sulphide^ it fuses completely. Now begins the proper work of the furnaceman. He must with his tools (at this stage he works only with his '^ paddle'' or ''slice*') turn over the mass so that no large lumps of sulphate, coke, &c. shall be formed, but the fused mass constantly brought into contact with unfused materials, especially sodium sulphide with calcium carbonate. Further, he must gradually raise all portions of the charge to the requisite temperature, which is done by moving with his paddle the upper portions towards the bottom and fetching up the portions lying first on the bed. The former, once soft, remain so on the bed as weU, whilst the latter, now directly touched by the fire, attain the temperature required for their partial fusion. During this work, of course the door must be open, which causes a large but unavoidable loss of heat ; for this reason, though more especially because the labour is too heavy to be carried on unin- terruptedly, the furnaceman only works a few minutes at a time in the furnace, and then leaves it about 10 minutes to itself with closed doors; it gets up its heat again during this interval. A second and third turnings-over with the paddle follow the first; during the two latter, if the ball has been ''split," the remainder of the mixture is moved down. In order to facilitate handling the large and heavy tools, there are always fixed in front of the working-doors either rollers or easily movable iron bars ; the latter have three or four projections on the top, between which the shank of the tool is laid, and which serve as fUlcra for its lever-like action. The men like to put some waggon-grease upon these bars, which makes the tools slide more easily upon them. The latter must be 5 feet longer than the width of the fomace, lest the men should have to approach too near. Their weight is supported behind by a hook attached to a ch&ia hanging down from the rafters ; ho that, white the man has prin- cipally to do the moving backwards and forwards of the tool and the turning-over of the stuff, the weight of the tool is almost entirely taken off him. On the Tyue each ball-fumaceman has the following tools :— let. A paddle (called slice in Laucashire) with a cast-iron head (fig. 180) ; the shank is 12 feet long exclusive of the handle ; its Fig. 179. Ilg.180. iront part (next to the head) is, for the length of a yard, made of l|-inch, the remainder of IJ-inch round iron. 2nd. Two paddles with wrought-iron beads, otherwise like No. 1 : these are more easily handled, but do not last so long in the fluxed mixtnre. 8rd. A rake with caet-iron bead (fig. 179), shank as above. 4th. A light rake with wrought-iron head of 11 inches width and 4 inches high ; its shank is of only f -inch round iron. Besides, for firing, a rake with a shank 8 feet long and ^ inch thick ; a poker 9 feet long, 2 inches thick, flattened at the end like a crow- bar, for hreakjng up the scars ; a scar-book with a shank like that of the rake ; and a shovel. The chemical action of the constituents of the soda mixture upon one another probably only begins after the mass has become partially fused. It never becomes a thin liquid, but always remains THE BLACK-ASH FURNACES. 397 pasty^ principally because the lime does not fase at all and the coal only cokes. There is consequently no danger of the charge running out at the door; at the worst a spadeful of ashes may be thrown onto the door-plate. About half an hour from beginning the labour on the working-bed the mass has become sufficiently liquid; and now the reaction must be carried through in as short a time as possible^ by assiduous working. The fiimaceman therefore exchanges the paddle for a rake (because now the mass does not offer so much resistance) and continuously turns the mass over^ mixes it all, and gradually pulls it more and more towards the door. At this stage a large number of gas-bubbles (of carbon dioxide) escape from the mass^ which sometimes presents the appearance of boiling. In the beginning no flame is seen arising from the mass ; but gradually the latter^ after being rather thin^ becomes again more pasty or stiffs probably because the sodium sulphate has now been transformed into the less-fasible carbonate^ and the gas-bubbles^ which now also contain carbon monoxide, burst through with much more difficulty^ owing to the toughness of the mass. This is the cause of continu- ous small jets of flame issuing from the places once opened by the rents in the surface of the batch — ^which are probably burning carbon monoxide, though coloured intensely yeUow by the soda contained in the charge. We shall see later on^ when speaking of the theory of the process, that carbon monoxide alone now appears, whilst the gas evolved previously was exclusively carbon dioxide, and thus could not produce a flame. These long pointed flames in Lanca- shire are known by the very appropriate name of '^ candles ; " on the Tyne they are called *^ pipes.'' Their appearance in large numbers, combined with the fact of the mass getting stiffer, shows that the moment has come when the ball must be drawn out with- out any further delay. It would be quite wrong to wait till the appearance of the candles begins to ceascy as some treatises direct. Scheurer-Kestner's explanation quite agrees with the opinion stated here (see Chapter VI.). . As during the last ten minutes the fumaceman cannot at aU get away from the furnace, but must constantly work his rake in it, either he must previously run the bogie for receiving the ball to the furnace-door, and place it just underneath its ledge, or an assistant must do it for him. Usually the fumaceman has just before exchanged his hot rake for a cold one, and has greased both the shank of the latter and the bearing-rod ; he now^ whilst again 398 SODA. assiduously working the pasty mass through^ pulls it into the bogie. If the operation has been a good one^ if^ therefore^ the mass is stiff and not yet overheated^ it will swell up greatly by the evolution of gas^ similarly to a fermenting loaf of breads and a large number of those pointed yellow flames will issue from it. It, ac- cording to the prescription given in some books^ the drawing were delayed till the flame reaction is nearly finished^ a burnt red ball would be got. On the contrary^ if the ball has been fired too little, usually owing to insufficient heat in the furnace, it always remains too thin or '^soft;'^ and this state is equally disadvanta- geous, because there is then much undecomposed sulphate in the mass. Pechiney has also pointed out that a high temperature at the finish yields much better ash in every way. The mass is stirred up even in the bogie with a small rake. At Aussig (where in 24 hours 23 balls of the unusually large size of 5 cwt. of sulphate are made) a portion of the process is purposely left to go on in the bogie, by not merely thoroughly stirring up the mass, but, accord- ing to its appearance, adding one or more spadefuls of coal and stirring it in. Everywhere else also a considerable portion of the reaction takes place in the bogies after drawing. It goes without saying that the description here given does not exhaust the matter. The furnaceman must have a sort of intuitive judgment when a ball is finished; two or thrcQ minutes too much or too little can spoil it, as well as too much or too little fire, too much or too little draught ; success consequently depends upon the experience, skill, and goodwill of the fumacemen. It is not at all indispensable to have many years' experience; the author has met with instances in which a furnaceman of 15 or 20 years' standing drew much worse balls than an intelligent, willing, and strong man who had only been placed as mixer or assistant with a furnaceman in order to learn the work. Working with chalk requires more heat than with limestone. Consequently men who are used to chalk, mostly " bum '^ their balls the first time on coming to a factory where limestone is used ; in the opposite case they do not work hot enough, and get " soft '^ balls — ^that is, balls hard when cold, and not porous enough for lixiviation. One of the most important matters in black-ash making is the assiduous and thorough turning-over of the mass,^-fir8t, in order to heat it uniformly ; secondly, to mix the several parts as well as possible, to renew their points of contact, and to make the reaction THE BLACK-ASH FURNACES. 399 complete. That the results of the work are better with assiduous turning-over than otherwise^ is not only a matter of course, but has also been proved by direct experiments made by Eolb (/. c.). With the same mixture of 106 sulphate (= lOONagSOJ, 101 chalk (=94CaC08), and 53 coal (=44C), he obtained the following results : — A. Well-wrought ball. 162*2 parts black ash of 42"" ; 73*8 soda ash of SS"" Decroiz. Composition of soda ash — 112 Na^S 206 Na8S04 B. Badly wrought ball. 160 parts of black ash of 42^ Decroiz. 82 parts of soda ash of 82^ Decroiz, consisting of m n::o°'} "^-^ ^•'~. 4-58 Na^S 7-48 NaaS04 The turning-over of the mass has certainly some unavoidable draw- backs. During the time the doors are open much cold air gets into the furnace ; the furnace-beds are very much worn out by the heavy tools ; the labour is exceedingly heavy, and can only be done by very strong men and with restricted quantities. This makes the manufacture very dependent upon the ball-fumacemen, who in case of strikes cannot at once be replaced by fresh men. It is true that this labour is a purely mechanical one, and by men with sufScient bodily strength is easily learned, as we have said before; but just this suggests strongly the idea of replacing this expensive human labour altogether by cheaper and more reliable mechanical power. This has been realized, as we shall see, by the revolving ball-fur- naces. Thefumacemen are not paid by the day, but by the number and quality of the balls drawn. As far as the latter is concerned, in moat cases a deduction is made for bad balls ; but in some instances 400 SODA. the payment is regulated directly by the alkalimetrical assay of the black ash. Where larger batches than those containing 3 cwt. of sulphate are made^ in the large French furnaces^ several men must be put to one furnace. On p. 382 we have given the details^ and seen that this arrangement differs^ in every respect to its disadvantage^ from the English work with smaU but frequent batches^ even in regard to the consumption of fuel. Instead of 6 large charges^ in England with two 12 hours' shifts 24 balls^ with three 8 hours' shifts 27 balls • are made in twenty-four hours. On the Tyne these are regularly made with 3 cwt. of sulphate^ in Lancashire often with only 2^ cwt. ; but then a few more of them are turned out. Instances of the continental style of working are the following^ seen by the author in 1878 : — Ist^ 18 balls with 5 cwt. sulphate ; 2nd^ 24 balls with 8f cwt. ; 3rd^ ? balls with 4 cwt. sulphate ; 4th^ 16 balls with 6^ cwt. ; 5th, 10 balls with lOf cwt., of which half is drawn out at one time. In each case ''per 24 hours'' is under- stood. Several improvements have been proposed in the style of working the black-ash furnaces. The first of these was Leighton's patent of April 12th, 1836, for heating small coal and chalk in a furnace on a cast-iron pan lined with bricks, through which steam was introduced; upon this the sulphate was charged subsequently. Stevenson and Williamson's patent of Dec. 31st, 1855, also pre- scribes heating chalk and coal first, till some lime is formed, and introducing the sulphate with more coal afterwards. This process is not used in hand-furnaces ; but it has been the means of making the introduction of revolving furnaces possible (see below). 