BUILDING, BLOWING IN, PRACTICAL REMARKS UPON THE MANAGEMENT.
The Building of a Blast Furnace.
The following remarks by Mr. Overman are important, and although made for 1845 are eminently useful and practical, when modified by the circumstance of the time when written.
A furnace should be located on a dry spot, free from springs and water of any kind, and not exposed to floods after heavy rains. The ground should be then excavated, until the bottom is sufficiently solid to bear the heavy weight of the stack. The foundation should be at least one foot larger in each direction than the base of the furnace; that is to say, if the furnace is thirty feet at the base, the foundation ought to be thirty-two feet square. Any kind of hard, large stones may be used to fill the excavation. No mortar should be used in the stone work. We should be careful to leave some channels through which rain or spring water, in case it should penetrate the foundation, may flow off. Such a drain should be carefully walled up and covered. The cavities or channels for the blast pipes are to be placed level with the ground ; and the four pillars of the furnace then laid out.
Fig. 87 shows the Arrangement of the pillars and that of the channels for the blast pipes a, a. If the stack is thirty feet at the base, the work arch b may be fourteen feet wide. Eight feet are thus left on each side of the pillars. The tuyere arches c, c, c measure ten feet, which leaves ten feet pillars. The size of the room in the centre is to correspond with the diameter of the boshes; that is, nine and a half feet. This is to be measured from the centre of the stack by drawing a circle of four feet and three-quarters radius. The inside of the pillars is to be built plumb; on this the lining rests. The walls towards the arches should also be plumb; but the outside should be furled according to the general tapering of the stack. The height of this stack is thirty-five feet; its width is fifteen feet seven inches at the top, and thirty feet at the base, thus leaving a slope of two and a half inches to the foot.
Fig. 88. Front view of a blast furnace.
The material of which a stack is built has but little influence.in the operation in the furnace. Building stones of any kind, as granite, graywacke, sandstone, or even slate, will answer; but limestone is not adapted for this purpose. The pillars are to be built with great solidity, with good mortar, and may be raised to the place in which the arches are set. The arches are turned of brick, which ought to be hard-burned. Fig. 88 represents the work arch; this commences seven feet above the ground, and forms just the half of a circle. The arch, from fourteen feet at its outside, contracts to five feet at the tymp.* The tuyere arches are but ten feet wide, and twelve feet high; they contract, towards the interior, to three feet. The binder a may be walled in at the height of ten feet, and of-course crosses the arch. The stone work above the brick arches should be arched, that some of the pressure on the latter may be relieved. The brick arches have some advantage over other arrangements. Stone arches are very apt to crack and split; and if, as often happens, the blast works out at the tymp or tuyeres, the stones crack and fly in such a manner that it is dangerous to go near the fire. Iron joists are very expensive; besides, by their expansion and contraction, they weaken the stack. The brick arch is very strong, safe, and durable. When the pillars all around are seven feet high, the arches may be commenced ; also the rough-in-wall, which must be four feet wider than the lining at the widest part of the boshes, that is, thirteen feet and a half in diameter. This in-wall is to be carried to a height of five feet, plumb, whence the contraction commences. From this point to the top the contraction is uniform, and is 1 7/10 inch to the foot, thus leaving the top seven feet wide. The stone work above the arches, or the place at which the binders commence, ought to be very open. Care should be taken not to use too much mortar; besides, the mortar must not be strong, but should consist mostly of coarse sand and spalls, for fragments of stones. Pannels should be left, at such width that the binders may, at any time after the furnace is built, be pushed through them. These channels ought to be at least six inches wide; and from each a branch channel should lead in a radial line towards th e interior. In this way they serve as drains for the watery vapors from the interior of the masonry.
When .the rough walls are finished, the lining or in-wall is to be put in. This must be constructed of fire bricks; or where these cannot be obtained, or where they are too expensive, of fine-grained white sandstone, which stands the fire well, does not crack, and is an excellent material. Where the former are used, the work is very simple, for fire-bricks are moulded to the proper bevel. A long board or scantling, or a sapling, is cut of the proper length, reaching from the pillars to the top of the furnace, that is, twenty-eight feet. A round wooden pin is fastened on each end, on which this pole may be turned. Upon the pillars, as well as upon the top, a plank is fastened; in each of these planks, an auger hole just in the centre of the stack or lining is bored. The pole is set in the centre, and made to turn round its axis. To this pole some pieces of board may be fastened, in a radial direction, on which an upright, giving the proper bevel of the lining, is fixed. By turning the whole round its axis, the interior form of the in-wall is moulded. The mortar used in the lining should be fire-clay, mixed with some sand, or, what is better, with a little of the very fine riddlings from the ore yard; these riddlings make the clay very tough, arid prevent its shrinking and crumbling. Fire-brick linings are undoubtedly preferable to stone linings; but they are more expensive. Where stones are used, they should be cut and dressed, according to bevel and circle, and laid in courses of equal thickness. The mortar to be used is the same as that just described. The lining should rest upon the pillars and arches; and, where stones are used, the last five feet at the top should be built of fire-brick. If fire-brick cannot be obtained, well-burnt common bricks, which do not shrink, and which are not brittle, may be used. Between the lining and the rough wall, a space of eight inches is left, because the width of the fire-brick or stone wall seldom exceeds sixteen inches. This space is to be filled either with fragments of stone, or broken«furnace cinders, and at intervals of four or five feet may be covered with a layer of lime mortar, to prevent, in case a stone of the lining should give way, the penetration of the blast. In the mean time, that is, while the lining is raised, the binders may be put in and secured. The strongest and most secure binders are wrought-iron bars, three inches wide, and three-fourths of an inch thick. They can be rolled in one length, and should be two feet longer than the actual length across the stack; each end of such a binder is to be bent round to form an eye, as shown in Fig. 89. A flat bar of the same dimensions as the binder is pushed through this eye; a,nd sufficient room is left for a key or wedge, as shown in Fig. 90. Fig. 89. Fig. 90
Eye of a binder. End and key of a binder. To protect this end of the bar against burning, in the blacksmith's fire, the eye is formed by simply bending the bar round, and by riveting it in two places. A slight welding heat may be applied to the joint. There should be five binders on each side of the furnace, making twenty binders in all; as well as eight bars reaching from the top to the lowest binder, as shown in Fig. 88. The top of the stack should be covered with a cast-iron circular plate, as wide inside as the lining, and about twenty-four inches broad; this plate should be large enough to cover the lining and space, as well as a small part of the rough wall. It is advisable to cast this plate in halves; for if in one piece, it will warp and crack, and thus disturb the tunnel head chimney which is to be put upon it. Upon this plate it was usual to build the chimney seen in Figs. 89 and 90. This chimney was commonly square, because this form is better adapted for binding; the inside should be as wide as the top of the furnace, and its height from ten to twelve feet. On one side, a square opening is left, for filling the furnace; this opening must be secured by an iron door, which is shut after every charge. Many objections have been raised against these chimneys, and much has been said in their favor. An objection against them is, that a careless filler will throw the stock, and particularly the ore and limestone, mostly towards the door; by this means the ore is brought to the back of the hearth ; the result is bad work. Another objection is, that the fillers are very apt to be negligent, because the stock is not easily thrown in, and because great attention is required in levelling it. These disadvantages are merely imaginary, and regularity and order will overcome them.
The advantages, however, are of a highly important character, and deserve our attention. These chimneys, if properly managed, maintain a uniform temperature at the top; and it is in the power of the manager to regulate the warmth of the top, by simply attending to the opening or shutting of the door. To be able to lower or raise the temperature is very convenient, because some ores bear a high heat at the top, while to others a high heat is injurious. Another advantage is, that, in any kind of weather, the flame is not troublesome to the filler. These chimneys are built and secured by binders similar to those used in the stacks of puddling furnaces, which will be shown hereafter. The construction of the hearth is the business in which the iron master himself takes an active part. Still, as this is governed by general rules, we shall give a statement, sufficient to serve our present purpose, of the principles by which we should be guided. It is a mistaken notion that every sandstone which resists fire will make good hearthstone. It is not the heat which destroys the hearth, but the chemical action of the materials in the furnace. The durability of a hearth is determined by the skill of the manager. Any refractory material constitutes a good hearthstone, particularly silex, clay, and some modifications of lime; but a mixture of these three' substances must not be applied. The form of aggregation has considerable influence; but of this we shall speak hereafter. Sandstones are sometimes used in this country, while in other countries, the material varies according to the nature of the ore and fuel. Limestone, sandstone, gneiss, granite, plastic clay, or fireclay is employed, as circumstances require. But sandstone will answer in all cases, if the ore and fuel are properly prepared. Any sandstone which is free from iron, or from lime, or from matter which towards silex acts as a strong alkali, may be used for hearthstones. Its refractory quality must be proved by some previous test. This test consists in drying a fragment of the rock in question by a very low heat on the top of a stove, or near a fire grate, for twelve or twenty-four hours ; and then exposing it to the gradual heat of a blacksmith's fire. If the stone is friable, after a good red or white heat, or if it fall to pieces by being moistened with water, we may conclude that the rock contains lime, and that it is not good for hearthstones. But if the fragment resists the first heat well, and if it is still hard and compact, we may expose it to a welding heat in the blacksmith't fire, urging the bellows strongly for half or three-quarters of an hour. If the stone resist this heat, and if its color is not altered to brown, we may conclude that it is perfectly safe to construct a hearth of it Some specimens assume a reddish hue; but we must not thence infer that their nature is not refractory. When heated in the blacksmith's fire, the fragment becomes glazed; this glazing is produced by the fuel. Stonecoal occasions a black, and charcoal a white glaze: the former is the result of sulphate of iron; the latter of the alkalies of the wood ashes.
Fig. 91 shows the method by which hearthstones are commonly prepared and arranged, a is the bottom stone,
made of a fine cross-grained sandstone; it is from twelve to fifteen inches thick; at least four feet wide, and six
Section of a blast furnace hearth through the damstone. feet long; it reaches underneath at least half of the damstone b. This bottom stone is well bedded in fire- clay, mixed with three-fourths sand. If possible, the transverse section of the stratification ought to correspond with its upper and lower surface ; that is, if the stratification, in its native position, is horizontal, it here ought to be vertical. This arrangement affords the advantage of saving the bottom, of keeping it smooth, and of lessening its liability to injury. After the bottom stone is placed, the upper part of which must be three-fourths of an inch lower at the damstone than at the back, the two sidestones c are laid, imbedded in fireclay. These stones must be at least six feet and a half long, reaching from eighteen inches behind the crucible to the middle of the damstone. Their form is commonly square, that is, a prism of four rectangular sides: if the tuyeres are eighteen inches from the bottom, the stone is eighteen inches high and eighteen inches wide: if twenty inches from the bottom, the stone is twenty inches on each side. The transverse section of the grain is placed towards the fire, which must be the case with all the hearthstones. The sidestones are sometimes perpendicular to the bottom; but they are often bevelled according to the slope of the hearth. Upon these stones the tuyere stones d are bedded; the latter suffer much from heat, and, therefore, ought to be of the best quality. They should be from twenty to twenty-four inches square ; and even larger dimensions would be advantageous. The tuyere holes f, a kind of tapered arch, are to be cut out before the stones are bedded. These stones do not reach farther than to the front or tympstone g, and are, therefore, scarcely four feet long. The topstone e, of no particular size, is generally sufficiently high to raise at once the crucible to its designed height. After both sides are finished, the backstone h is put in, which, in case three tuyeres are used, is an almost cubical block; but where only one tuyere, or two opposite each other, are used, this backstone is frequently made sufficiently large to reach from the bottom to the top of the crucible.
The tympstone g is then put in its place ; this stone is from four to five feet in length, so as to overlap both side tuyere stones ; it should be of good quality. The tympstone is generally raised from three to four, sometimes even six or seven inches above the tuyere, by putting at i, on both sides of the sidestones, a small block of sandstone, or, what is better, fire-brick. The raising of the tympstone has this advantage. In cases of difficulty, and of hard work in the furnace, the keeper is enabled to reach with a crowbar above the tuyere. Where argillaceous or clay ores of gray iron are smelted, this is necessary. The opening left by raising the tymp is easily kept tight by a good stopper; for this purpose, a flanch, which reaches under the stone, is cast to the tympplate k. The tympstone is protected by the tymp- plate, which must be two inches thick, imbedded in fire: clay, and secured by two uprights L These angular iron plates protect the stones or bricks on each side of the tymp; they are more distinctly shown in Fig. 92.
Besides holding the tymp-plate, they afford the advantage
of keeping the forehearth clean ; for the hot cinders will not adhere to the iron plates, but are very apt to stick
tenaciously to stones or brick. At this stage of our work Horizontal section of a furnace hearth through the tuyeres during the whole of which great care must be taken to form good joints, and to employ good refractory mortar,that is, fire-clay mixed with river quartz sand, or what is preferable, with sand from pounded furnace cinders the boshes may be builded in.
In charcoal furnaces, if steep, these are generally made of fire-brick, but if built at an angle of less than 50°, good sand, mixed with a little fire-clay, is an excellent material. In the latter case, the mixture should be well stirred and worked, and every pains should be taken that the compound is well prepared before it is used. It should be well pounded in, and, to prevent cracking, should be gradually dried. If fire-bricks are used, made in proper form, and of the largest possible size, there will be no difficulty in putting in good boshes. The damstone b is very seldom laid in its place, before the furnace is properly dried, and ready for the blast. In protecting plate m, the dam plate, can be laid at any time after the furnace is in operation. The space between the hearthstones and the rough wall of the furnace stack, is filled and walled up with common brick or stones; the former are preferable, because they are softer, and have less tendency to move the rough wall, by the expansion of the hearthstones. The expense of building a stack of the foregoing size varies according to locality, and to the facilities we have at our command.
The following estimates were supposed to be correct in 1850): The rough stonework of the foundation will amount, at twenty-four and three-quarters cubic feet to the perch, to 200 perches. This foundation may be laid at twenty cents a perch. If stones can be quarried and hauled to the spot at forty cents, as is generally the case, the stonework may be laid at a cost of one hundred and twenty dollars. The excavation, estimating the expense of removing one cubic yard of earth at fifteen cents, will cost twenty-four dollars and seventy-five cents. The expense of the rough wall, assuming it to contain nearly 600 perches, will be one dollar a perch; if stones are
included, one dollar and forty cents. Masons, at that price, will make a smooth, if not a hewn, outside. An in-wall of stones will cost nearly 100 dollars; one of fire-brick, 350 dollars. The cost of hearth and boshes may be calculated at 150 dollars, if the latter are of firebrick ; but if of sand, at 100 dollarsprovided, of course, that the materials are close at hand. Binders, tymp-plate, dam-plate, and chimney binders at the top, will cost 350 dollars. Therefore, a stack of the size stated may be assumed to cost, on an average, from 1300 to 1600 dollars. Furnace stacks may be built more cheaply with bricks than with stones, where bricks can be made and laid at a reasonable cost. The rough walls of such brick stacks are generally not so thick as those of stone; but, even though they were, they would not be more expensive than stone, if a thousand can be laid at four dollars; and this may be done without much difficulty. Furnace stacks of brick have been built at various places; and their form above, the boshes is generally round; they are then called cupola furnaces, from their resemblance to the cupola of the foundry. The Great Western Iron Works, in Western Pennsylvania, erected two such stacks; but these are partly built of stone, that is, the lower or square part beginning at the ground, and terminating at the work and the tuyere arches. This kind of furnace does not bear a high reputation in Europe.
We observed them in England and France, where the general complaint against them is, that they work irregularly and consume a greater amount of fuel than square stacks. The cause of these evils may have been too thin and too rough walls, which may easily be avoided. But these furnaces have another disadvantage, that is, they nearly always break the strongest binders. In addition to this, they require too many binders; so that, on an
Fig. 93. Fig. 94.
Section and interior of a cupola blast furnace. Front view of a cupola blast furnace, at the Great Western Works. average, a round stack is not cheaper than the square stack. There may be instances, some of which we shall produce hereafter, in which a round stack is preferable. Still, for the sake of those who may be disposed to build a round stack, we will present a drawing of one in operation at the Great Western Works.* Fig. 93 represents a vertical cross section, and Fig. 94 a front view, of a cupola furnace built of brick. The drawing is so distinct as to need no particular description. The whole stack can be built altogether of brick; or partly of brick and partly of stones, as is the case at the Western Works; or altogether of stone. Stone, however, would be very expensive, on account of the dressing necessarily required. Through the lower or square part, four binders are laid ; the hoops, of wrought iron of good fibrous quality, of the upper or round part, must not be more than six inches apart, and should be two inches wide, and three-fourths of an inch thick. Below each hoop the last layer of bricks projects at least half an inch; upon this layer the hoop rests. If the stack is built of stones, pieces of iron bars are walled in to support the hoops. Between these hoops are left airholes, through which moisture has vent.
Blowing in a Furnace.
When a furnace is erected and ready to be fired, a small fire maybe put in the hearth. We should always be cautious to give the interior of the hearth a lining of common brick. This will prevent, in a great measure, the cracking and scaling of the hearth-stones. The fire is fed from below. Any kind of fuel will serve for this purpose, because the fire is only designed to dry the masonry. If the stack is new, or if it is one which has been for a long time unused, it is necessary to cover the throat by iron plates, and to leave but a small hole; this hole may be so regulated that we may burn just as much fuel as we choose. Seven weeks, and if the season is cold, eight or ten weeks of constant firing are considered necessary to dry a new stack, so that it can be charged with charcoal. But before the furnace is charged, the temporary lining or brick in the hearth must be removed. The lower part of the furnace, or the hearth, is to be filled gradually, and the fire must be permitted to rise in a blue flame on the top of the coal before the furnace is filled higher than the boshes. From this point half coal and half brands are to be used; the latter addition causes a more liberal draft of air in the furnace. If the furnace is quite warm before putting the charcoal in, arid if we are confident that no moisture exists in the masonry, ore may be charged after the furnace is half filled with charcoal; but if we doubt that moisture is wholly expelled, the whole stack should be filled with coal, and the fire kept up until we are satisfied that the walls are perfectly dry. Where everything is ready for the start, repeated grates may be formed to facilitate the burning of coal, as well as to heat the furnace. Grates are formed by laying across the tymp a short iron bar, as high up as the damstone; by resting upon this bar six or seven other bars, or ringers (crowbars), and by pushing
their points against the backstone of the hearth. A grate thus formed increases draft and heat to a considerable degree, and very soon brings the top charges down into the hearth. Where ore is charged to the top, the descent can be accelerated by leaving the grate most of the time in the hearth; but care should be taken that too much coal does not remain at the bottom, for this will injure the bars. In this way the ore charges may be brought down within twenty-four or thirty hours. But if we are not to put the blast in, and to commence smelting^ the descent of the ore charges may be delayed three, even four days, without any injury to the following operations; when everything is in order, the sinking of the ore may be hastened. This will be indicated by melting drops, often drops of iron, before the tuyeres. When these are seen, the damstone is to be laid, imbedded in clay; also its protector, the cinder platem. The hearth is once more .cleaned; the hot coal then drawn towards the dam, and covered with moist coal dust; after which a gentle blast may be let into the furnace.
During the first twenty-four hours, but little iron is made; most of the ore is transformed into slag; and the iron which comes down gets cold on the bottom stone, where it is retained. At this early stage, it would not be prudent to urge the blast machine too fast, for great caution is required to prevent those troubles which result from a cold furnace. These troubles are, generally, cold iron in the bottom, and, in consequence of that, cold tuyeres. Gentle blast, small burden, and great attention alone will prevent these evils. Where a furnace has been for a week in blast, having in that time produced from nine to ten tons of metal, and where the hearth is clean, that is, where it is perfectly free from cold iron, or clinkers, the burden may be increased, and the blast urged more strongly. A well-regulated furnace will, during the second week, make from sixteen to eighteen tons; and the same amount during the third and fourth weeks. A furnace, just started, should not receive so heavy a burden of ore as a furnace which has for some time been in operation. About half the regular burden should, as a general rule, be taken; that is, if a full charge of ore is assumed to be 700 pounds, the starting charge should be 350 pounds. This amount should not be increased for at least three or four days, or one week. During this time, while the light charges last, an abundance of brands along with the coal may be used for the purpose of keeping a clean, open furnace.
The application of blast in the furnace deserves investigation in every instance. We
will notice some leading points; but these are not presented as infallible rules. Soft and
weak charcoal cannot bear strong blast, and a pressure of from half a pound to
five-eighths of a. pound to a square inch, may be considered sufficient; strong blast
would be likely to choke the furnace above the tuyere, by depositing charcoal dust in the
boshes. Strong, coarse charcoal will bear a pressure of from three-quarters of a pound to one pound. A weaker blast is very apt to be troublesome, besides
using more coal, and producing white metal. Ore, considered as an oxide of iron, free from
foreign matter, has no relation whatever to the quality of the blast; but it is different
with ore considered as a mixture of oxide of iron and foreign matter. The kind of blast
that should be applied depends very much on the fusibility of the foreign matter. But this
question we shall discuss in another place. The
form of the interior of the blast furnace is of considerable importance. A high, narrow
hearth requires stronger blast than a furnace without a hearth, or a furnace with a low
hearth ; but the width of the top, in proportion to the diameter of the boshes, is of more
importance than the quality or pressure of the blast. It may be laid down as a rule, that
the larger the throat, in proportion to the boshes, the stronger ought to be the blast;
and that a narrow top and wide boshes, while they permit a weaker blast, involve the loss
of much fuel.
The air introduced by the blast machine into the furnace should be as dry as possible. The main reason that blast furnaces do not work so well during summer and clear, warm weather, as during winter, and cold, rainy days in summer, is, that a large amount of watery vapors is mixed with the atmospheric air in hot weather. This water is very injurious in a furnace, as we shall hereafter see. To keep the air dry, the blast machine should be erected at the coldest and driest spot we can possibly select. We should take especial care that it is not exposed to the hot air around the furnace, and that it is beyond the reach of the steam-engine; for the air will be more moist around the engine and the heated furnace than anywhere else. The best means of making a furnace work well during summer would be to put the blast machine in an ice-cellar.
With charcoal hot blast may be, under some circumstances, advantageous; but in others, it is decidedly injurious. It is, at best, a questionable improvement; and it may be doubted whether the manufacture of bar iron has derived any benefit from it; qualitatively, it has not. Hot blast is quite a help to imperfect workmen. It melts refractory ores, and delivers good foundry metal with facility. The furnace should be carried on for three or four weeks with cold blast, that the hearth and lining should be heated thoroughly before the application of hot air. The quantity of air required to be blown into a stack depends on the quantity of metal produced in the furnace. But there is a limit to the amount which the furnace produces; if we attempt to exceed that limit, loss, instead of gain, is the consequence. A narrow top, high stack, soft coal, and imperfectly roasted ores, require quantitatively more blast than where opposite conditions exist; but the blast must be weak. A wide throat, low stack, hard coal, and ores well roasted, require stronger pressure but a less volume of blast. The changing of nozzles and tuyeres is, therefore, a matter of considerable importance, and the effect of this change should be
clearly appreciated before it is attempted.