Smelt′ing-fur′nace.
A furnace for disengaging the metal from its gangue or the non-metalliferous portions of the ore. The furnaces differ much for treating different metals.
See list under metal-Lurgy; furnace.
The smelting-furnace for iron is in the form of a truncated quadrilateral pyramid about 50 or 55 feet high.
The outer part is of brick or squared stone, with contrivances to obviate the danger of its cracking by the expansion that takes place when it is heated, and it is lined with two courses of
fire-bricks having a layer of pounded coke between them to prevent the escape of the heat.
The interior or cavity may be divided into the following parts from below upward.
First, the hearth, about two feet high; its base and sides are formed of massive blocks of coarse, pebbly gritstone, as being the most infusible of all common building-stones.
Upon this is erected the crucible, a four-sided cavity between 6 and 7 feet high, slightly enlarging upward, so as to be at top about 2 1/2 feet wide.
The part above, called the
boshes, is in the shape of a funnel or inverted cone, about 8 feet high and 12 feet wide at top. On this is placed the great cavity of the furnace, of an irregular conical form, about 30 feet high, and gradually narrowing so as to be only about 3 feet in diameter at the top. From this part it enlarges into a funnel-shaped chimney, about 8 feet high, in which is cut a large square aperture, through which the charge is thrown from time to time into the furnace.
About two feet above the hearth is an aperture through which the blast-pipe or tuyere is introduced.
Sometimes there are two opposite tuyeres, and occasionally even three.
The character of the iron is affected by the nature of the fuel, and, by the choice of the latter, metal may be rendered more suitable for the purpose for which it is intended.
The effect in the smelting-furnace is due to the high temperature, and this is produced by the action of the oxygen of the atmosphere, which enters at the tuyere-hole, excites intense heat by combination with the carbonaceous particles of the fuel, the other constituents of the air passing out, in company with certain gases evolved, at the top of the furnace.
The air may be hot or cold, but is driven by a machine of some description.
Varieties of the original forms of blowers may be found under
bellows, but the larger kinds of blast apparatus are associated under the caption blower (which see). In a furnace working under high pressure and delivering 6,292 cubic feet per minute (estimated at atmospheric pressure), the weight of the air thrown in is calculated at 693,504 pounds, while the charge of coke, ore, and limestone in the same time amounts to 74,648 pounds.
The heat is developed, as has been said, by the combination of the oxygen of the air with the carbon of the fuel; but part of the carbon is required to reduce the magnetic oxide to the metallic state, and some carbon is also required to unite with the iron to form cast-iron, which is a compound of iron and carbon.
The amount of air required will therefore be the quantity necessary to combine with the remainder of the carbon of the fuel, after deducting the amount of carbon required to reduce the oxide, and to unite with the metal.
The charge, placed on top of the furnace, descends gradually, the iron becoming gradually carbonized.
As the carbon penetrates the fragments of ore, the limestone parts with its carbonic acid, which passes off The fuel loses some of its combustible ingredients.
As the charge comes under the direct action of the blast, the reactions are more energetic, the fuel burns rapidly, its carbon uniting with the oxygen and with the metal, which becomes melted, while the lime unites with the earthy particles to form a fusible slag; the fused matters descend from the boshes into the crucible, the metal, by its superior gravity, taking the lower position on the hearth, from whence it is drawn off from time to time, either into ladles to form castings, or into furrows made in sand, where it is run into pigs.
The vitreous scoriae or
slag floats on the iron and overflows at an aperture.
See slag; blast-furnace.
The appearance of the slag indicates the cooking condition of the furnace.
Here the skill of the smelter will watch the healthy working of his furnace, detect the signs of disorder, and determine upon the appropriate remedies.
An authority gives the following indications: If the color of the slag be pale yellow, the sign is favorable.
Green color indicates oxide of iron and a deficiency of lime.
Streaks of blue indicate protoxide of iron, and show a deficiency of fuel or excess of blast.
Dark-colored, heavy slag shows that iron is going to waste, and suggests that the iron produced will be deficient in carbon.
It indicates either a deficiency of fuel or a too rapid working of the furnace, so that the iron was imperfectly carbonized on arriving within the action of the blast.
Great economy of fuel, with a generally admitted deterioration of quality of the metal, is effected by using a blast, heated artificially.
The heat attained varies from 200° to 600° Fah. This was invented by
Neilson.
See hot-blast.
The primitive smelting-furnace by which the “iron is taken out of the earth” (Job XVIII. 2), and which the Hebrews learned to use while in
Egypt, was probably like the ancient Indian furnace yet used in
Asia, and thus described by
Dr. Ure:—
The furnace or blomary in which the ore is smelted is from 4 to 5 feet high; it is somewhat pear-shaped, being about 5 feet wide at bottom and 1 at top. It is built entirely of clay.
There is an opening in front about a foot or more in hight, which is filled with clay at the commencement, and broken down at the end of each smelting operation.
The bellows are usually made of goat's skin, and the nozzles are inserted into tubes of clay which pass into the furnace.
The furnace is filled with charcoal, and a lighted coal being introduced before the nozzles, the mass in the interior is soon kindled.
As soon as this is accomplished, a small portion of the ore, previously moistened with water to prevent it from running through the charcoal, but without any flux whatever, is laid on top of the coals, and covered with charcoal to fill up the furnace.
In this manner ore and fuel are supplied, and the bellows urged for three or four hours. When the process is stopped, and the temporary wall in front broken down, the bloom is removed with a pair of tongs from the bottom of the furnace.
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It was said of the land of
Canaan (Deuteronomy VIII. 9), “a land whose stones are iron, and out of whose hills thou mayest dig brass” (copper). The hills of
Palestine furnished the ore in the time of the
Judges, and do to this day. It was used for making the bedstead of Og, king of
Bashan (see bedstead), for the axes and sickles of the Egyptians from time immemorial, and for axes in
Palestine in the times of
Samson and Elisha; for chains in the time of Jeremiah; harrows in the time of Samuel and David; for mattocks, files, goads, swords, spears, shares, colters, forks, etc., previous to the time of
Saul, say about 1100 B. C., and no doubt long before.
The
Israelites worked in the iron-furnaces of
Egypt during their captivity.
The rigidity and strength of iron afford a basis for several metaphors in that most ancient and wonderful poem, the
Book of Job.
The iron-smelting furnaces of
Africa are thus described by
Dr. Livingston:—
“At every third or fourth village (in the regions near Lake Nyssa) we saw a kiln-looking structure, about 6 feet high and 2 1/2 feet in diameter.
It is a clay, fire-hardened furnace for smelting iron.
No flux is used, whether the specular iron, the yellow hematite, or magnetic iron ore be used, and yet capital metal is produced.
Native manufactured iron is so good that the natives declare English iron ‘rotten’ in comparison, and specimens of
African hoes were pronounced at
Birmingham nearly equal to the best
Swedish iron.”
Dr. Barth makes a similar statement.
The articles produced by these peoples are hammers, tongs, hoes, adzes, fish-hooks, needles, and spear-heads.
The
assagais of the Caffres are made of iron similarly procured, and of excellent quality.
The
wootz of
India is still produced in the manner partially described by Aristotle when speaking of
India, and by
Diodorus Siculus, referring to the iron ores of the island of Ethalia.
The
Hottentots, though so far below the average of what may be classed as savages, have pottery, iron manufactures, sheep, and oxen.
Their iron-furnace is a hole in a raised ground, large enough to contain a good quantity of iron stones, which are plentiful on the surface in some parts of their country.
About 18 inches from the upper hole they make a smaller one, connecting with the former by a narrow channel.
A hot fire is made in the upper hole, sufficiently long to heat the earth thoroughly, and it is then charged with fuel and iron.
Fuel is added, and the fire urged until the metal runs into the receiver.
When it is cool it is broken into pieces, heated, and hammered out with stones.
It is almost exclusively used for making weapons.
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Kol smelting-furnace, Hindostan. |
Fig. 5220 represents a blast-furnace of the Kols, a tribe of iron-smelters in Lower Bengal and Orissa.
The men are nomads, going from place to place, as the abundance of ore and wood may prompt them.
The charcoal in the furnace being well ignited, ore is fed in alternately with charcoal, the fuel resting on the inclined tray, so as to be readily raked in. As the metal sinks to the bottom, slag runs off at an aperture above the basin, which is occupied by a viscid mass of iron.
The blowers are two boxes with skin covers, which are alternately depressed by the feet and raised by the spring-poles.
Each skin cover has a hole in the middle, which is stopped by the heel as the weight of the person is thrown upon it, and is left open by withdrawal of the foot as the cover is raised.
Variously modified in detail and increased in size, these simple furnaces are to be found in several parts of
Europe, the Catalan and
Swedish furnaces resembling in all probability those of the Chalybes, so famous in the time of
Marathon (490 B. C.), and those of the
fabrica or military forge established in
England by Hadrian (A. D. 120) at
Bath, in the vicinity of iron ore and wood.
The brave islanders met their Roman invaders with scythes, swords, and spears of iron, and the export of that metal from thence shortly afterward is mentioned by
Strabo.
During the
Roman occupation of
England some of the richest beds of iron ore were worked, and the
debris and cinders yet exist to testify to two facts, — one, that the amount of material treated was immense; the other, that the plans adopted were wasteful, as it has since been found profitable to work the cinders over again.
During the
Saxon occupation the furnaces were still in blast, especially in
Gloucestershire.
The direct method of obtaining wrought-iron from the ore prevailed until the commencement of the fifteenth century, and then gradually gave way to a less direct process, but one more convenient in the handling of large quantities.
Furnaces, operating by the aid of a strong blast, to
melt the iron and obtain
cast-iron, which is carbureted in the process, were in use in the neighborhood of the
Rhine about 1500.
A second process in a
forge hearth was used to eliminate the carbon and other impurities, and the result was
wrought-iron.
It took several centuries to accomplish this with wood, and several other centuries to devise means for substituting pit-coal for charcoal.
In the reign of Elizabeth blast-furnaces were of sufficient size to produce from two to three tons of
pig-iron per day by the use of charcoal.
In the small works the iron was made malleable before being withdrawn from the blast-furnace, and in larger works was treated by the refinery furnace.
Wood becoming scarce, and a number of furnaces having gone out of blast, in 1612
Simon Sturtevant was granted a patent in
England for 31 years for the use of pit-coal in smelting iron.
Failing in his proposed plans, he rendered up his patent in the following year.
Successive persons applied for a patent for the same, the government continuing desirous of encouraging the development of home resources.
Dudley, in 1619, succeeded in producing three tons of iron per week in a small blast-furnace by the use of coke from pit-coal.
The parties who yet possessed plenty of wood, and with whom the production of iron was fast becoming a monopoly, urged the charcoal-burners to destroy the works of
Dudley, which was done.
Dudley's patent was granted for 31 years, which would bring it to 1650, the time of the Protectorate, when
England had a ruler fit to succeed Queen Bess.
The celebrated statute of King James, limiting the duration of patents to 14 years, was passed in 1624.
Dudley's petition for an extension was refused.
Iron of poor quality continued to be made in districts where wood was scarce, and of good quality from charcoal in places where forests yet remained.
The demand for iron continuing to grow, — a natural effect of advancing civilization, — iron was imported from
Sweden and
Russia in large quantities and of excellent quality.
The forests of these countries gave them a natural advantage over
England, whose forests had by this time become thinned out, so that the use of wood for iron smelting had been forbidden by act of Parliament in 1581 within 22 miles of the metropolis, or 14 miles of the
Thames, and eventually was prohibited altogether.
The art of making iron with pit-coal and of casting articles of iron was revived by
Abraham Darby, of Colebrookdale, about 1713, and was perseveringly followed, although it was but little noised abroad.
In the “Philosophical Transactions” for 1747 it is referred to as a curiosity.
See casting, p. 449.
The extension of the iron manufacture dates from the introduction of the steam-engine, which increased the power of the blast; and the blowing engines, driven by manual, horse, or ox power, were henceforth operated by
steam-engines.
The dimension of the blast apparatus was increased from time to time, and about 1760 coke was commonly used in smelting.
In 1760
Smeaton erected at the
Carron Works the first large blowing cylinders, and shortly afterward
Boulton and
Watt supplied the
steam-engines by which the blowers were driven.
Neilson, of
Glasgow, introduced the hot blast in 1828.
Aubulos, in
France, in 1811, and
Budd, in
England, in 1845, heated the blast by the escaping hot gases of the blast-furnace.
In the smelting of iron four tons weight of gaseous products are thrown off into the air for each ton of iron produced.
See under iron; malleable iron; and other titles, for which see list under metallurgy.
As a means of estimating by comparison the value of the hot blast, some facts may be mentioned.
Mushet states that at the
Clyde Iron Works, before the introduction of the hot blast, the quantity of materials necessary for the production of one ton of
pig-iron was, —
| Calcined ore | 1 3/4 tons |
| Coke | 3 tons |
| Limestone | 1/2 ton. |
In 1831, when the system was coming into use, the blast being
warm, —
| Calcined ore | 2 tons |
| Coke | 2 tons |
| Limestone | 1/2 ton. |
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In 1839, with a hot blast, —
| Calcined ore | 1 3/4 tons |
| Coke | 1 3/4 tons |
| Limestone | 1/2 ton. |
the saving in fuel being nearly one half.
In addition may be mentioned the fact that anthracite coal and black band ore are intractable under the cold blast, but the former yields an intense heat, and the latter a rich percentage of good iron with the hot blast.
The Calder Works, in 1831, demonstrated the needlessness of coking when the hot blast is employed.
Experiments in smelting with anthracite coal were tried at
Mauch Chunk in 1820, in
France in 1827, and in
Wales successfully by the aid of
Neilson's hot-blast ovens in 1837.
The experiment at
Mauch Chunk was repeated, with the addition of the hot blast, in 1838-39, and succeeded in producing about two tons per day. The Pioneer furnace at
Pottsville was blown in July, 1839.
The first iron-works in
America were established near
Jamestown, Virginia, in 1619.
In 1622, however, the works were destroyed, and the workmen, with their families, massacred by the Indians.
The next attempt was at
Lynn, Massachusetts, on the banks of the
Saugus, in 1648.
The ore used was the bog ore, still plentiful in that locality.
At these works
Joseph Jenks, a native of
Hammersmith, England, in 1652, by order of the Province of
Massachusetts Bay, coined silver shillings, sixpences, and threepences, known as the
pine-tree coinage, from the device of a pine-tree on one side.
Early in the eighteenth century, a smeltingfur-nace was erected in
Virginia by
Sir Alexander Spottswood, governor of
Virginia, who lived at the
Temple Farm, near
Yorktown, Va. He had been wounded at
Blenheim, where he served with
Marlborough.
He was the first to cross the
Blue Ridge and see the Shenandoah Valley.
He was appointed commander of the expedition to
Carthagena, but died at
Annapolis, Md., June, 1740, as the troops were about to embark.
He was buried in the mausoleum from which the
Temple Farm derived its name.
In this expedition the elder brother of
George Washington served, and on his return named his estate on the
Potomac “
Mount Vernon,” after the
English admiral.
The blast-furnace for reducing iron from its ores is shown at
Fig. 5221,
A.
It consists of an interior lining of
fire-bricks a a, forming a doubly conical chamber, surrounded by a packing of broken scoriae or refractory sand, and incased within a construction of masonry
b b, from the upper part of which the charge of fuel and ore is delivered through a suitable opening into the furnace.
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Smelting-furnace. |
The portion from
c to
d is termed the
shaft; d to
e, the
boshes; the widest part being the
belly or upper part of the boshes; the narrow part
f the throat, below which is the crucible or hearth
g, which receives the molten metal; the lower part of this is prolonged toward the front, forming the breast-pan, which is closed by the dam-stone
h, between which and the side of the furnace wall is a slit, called the tap-hole, closed by fireclay, which is removed to withdraw the molten metal.
The dam-stone is protected by an iron plate.
The top of the open side of the hearth is formed by a large slab of stone, termed the
tymp, supported by a massive piece of iron, termed the
tympiron.
i is one of the tuyeres, usually two in number, through which compressed air is forced, to assist combustion and promote fusion of the metal.
The furnace is charged first with fuel, and as this burns down alternate layers of fuel and of mixed ore and limestone or other flux, according to the nature of the ore employed, are added (
B).
The iron collects on the hearth, while the slag produced by the combination of the flux with the foreign matters in the ore floats on top and is drawn off over the dam-stone.
As the hearth becomes filled with metal, usually about twice in 24 hours, the tap-hole is opened and the metal allowed to flow.
The interior of the furnace may be divided into five zones: the first heating zone
a b (
Fig. 5221,
C); the reduction zone
b c; the carburation zone
c d; the melting zone
d e; the combustion zone
e f. In the first, the materials become thoroughly dried and are brought to a low red heat; in the second, the ore is reduced to a protoxide, and finally to metallic iron, by the various gases, carbonic oxide, carbureted hydrogen, and hydrocyanic-acid gas or vapors of cyanide of potassium; in a certain part of this zone the iron is present in a malleable state.
In the carburation zone the metal becomes combined with carbon, producing a steely and caky iron, which, on falling into the lower or melting zone
d e, becomes fully charged with carbon, by which it is brought into the condition of
pig-iron.
The figures indicate the temperatures at the respective parts of the furnace.
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Smelting-furnace. |
Buttgenbach's blast-furnace is so arranged that the base is independent of the stack, which is a mere shell of firebricks, about equal in thickness to the lining of the ordinary blast-furnace.
The base is formed either of brickwork with open arches (
A) or of cast-iron standards (
B). In the first case the shaft rests on a crown-ring above the tops of the arches, and in the latter upon a cast-iron ring-plate supported by the standards.
The boshes join the stack just above the basering, and both are hooped at intervals; they and the tuyeres are protected by water-boxes.
The gases are led off by a central tube, and through lateral openings, which communicate with the hollow columns, which serve as downtakes, and also support the gallery.
Fig. 5223 is a vertical section of the smeltingfurnace commonly used in the
Pacific States.
Four cast-iron columns, from 8 to 11 feet high, support a square cast-iron plate
d with a circular hole about 4 feet in diameter.
On this plate is built the stack
b of the furnace, with an opening
c through which the furnace is charged.
Under the plate and inside the columns is built the cylindrical shaft of the furnace, filling the space from the plate to the ground.
d is a tuyere, and
e the stirring and discharging hole.
The smelting-furnace of
Shropshire, England, is a stone and
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brick structure of a truncated conical form, 55 feet high, and 38 wide at bottom.
Its cost, there, is about £ 1,800; and it requires in its construction 160,000 bricks, 3,900
fire-bricks, and 825 bosh-bricks.
Its production is about 60 tons of iron per week.
The furnaces are built larger and smaller than the size mentioned.
Including the coal of calcination, it is estimated that 3 1/2 tons of coal are required to obtain a ton of cast-iron.
The proportions of the materials dumped into the furnace are 14 1/2 tons of coke, 16 of roasted ore, 6 3/4 tons of limestone, every 24 hours, producing 7 tons of
pig-iron every 12 hours. Advantage is taken of a side-hill to make a convenient access for charging and delivering.
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Smelting-furnace of the Pacific States. |
In the illustration,
a represents the regulating-cylinder, 8 feet in diameter and hight;
b, the floating piston, loaded with weights, proportionate to the power of the machine;
c, a valve 26 inches long, 11 inches wide, by which the air is passed from the pumping-cylinder into the regulator;
d, the aperture at which the blast is forced into the pipe leading to the tuyere.
The pipe is 18 inches in diameter; the wider this can be made, the less is the friction and the more powerful the blast;
e is the blowing or pumping cylinder, 9 feet high, and 6 feet in diameter, the piston within it having a stroke of from 5 to 7 feet;
f, the blowing piston, with its valve or valves, of which there are sometimes several distributed over the surface of the piston, the area of each being proportioned to the number;
g is a pier of stone or masonry supporting the regulating-cylinder, to which is attached the flange and blowing-cylinder;
h is the safety-valve or cock, by the simple turning of which the blast may be admitted to or shut off from the furnace, passing to a collateral tube on the opposite side;
i, the tuyere, by which the blast enters the furnace; the end of the taper pipe which approaches the tuyere receives small pipes of various diameters, from 2 to 3 inches, called nose-pipes; these are applied at pleasure, as the strength and velocity of the blast may require.
k, the bottom of the hearth, 2 feet square;
l, the top of the hearth, 2 feet 6 inches square;
k l, the hight of the hearth, 6 feet 6 inches;
l is also at the bottom of the boshes, and where they terminate is of the same size as the top of the hearth, only the former is round and the latter square.
m, the top of the boshes, 12 feet diameter and 8 feet perpendicular hight.
n, the top of the furnace, at which the materials are charged, commonly 3 feet diameter;
m n, the internal cavity of the furnace from the top of the boshes upward, 30 feet high;
n k, total hight of the internal parts of the furnace, 44 1/2 feet.
o o, the lining; this is done in the nicest manner, with firebricks made on purpose, 13 inches long and 3 inches thick.
p p, a vacancy round the outside of the first lining, 3 inches broad, and filled with coal-dust; this space is allowed for the expansion which might take place in consequence of the swelling of the materials by heat when descending to the bottom of the furnace.
q q, the second lining, similar to the first.
r, cast-iron lintel on which the bottom of the arch is supported.
r s, the rise of the arch; the arch on the outside is 14 feet high and 18 feet wide.
v v are the extremes of the hearth, 10 feet square; this and the bosh-stones are always made from a coarse gritted freestone, whose fracture presents large rounded grains of quartz, connected by a cement less pure.
The description by an iron-master, as given by
J. R. Chapin, gives a sensible idea of the process:—
You must know that there are about 140 tons of material in the furnace, in the proportion of 60 to 75 tons of ore, 60 tons of coal, and 15 to 20 tons of limestone, fed into the furnace at the opening above.
The furnace is 40 feet square at bottom, and 40 feet high, with a hollow space or “flask” in the center, lined with fire-brick, and about 14 feet in diameter.
The material dumped into the furnace becomes melted, and the iron, being the heaviest, sinks to the bottom, while the flux, like oil upon water, floats upon the surface, and, having an affinity for the dross of the coal and iron, it grasps and holds it separately from the metal, until it is drawn off in what is called slag. This is done once every hour.
The gases evolved pass out at the chimney.
The trouble is, the iron also has an affinity for the dross, and does, and will, retain some of it, notwithstanding all we can do.
The floor of the building is of fine sand, divided into two parts by a track, on either side of which gutters, or runners, are formed leading from the mouth of the furnace.
At equal distances are 8 branch gutters, or sows, as they are technically called, which conduct the molten ore to feed the pigs in the bed. All these are nicely formed by each set of hands after the previous cast has been cooled and removed.
See blast-fur-Nace.
“You see, there are 26
pigs in a
bed, and 4
pigs in the
sow; that is, they break the
sow into 4 pieces, each the size of a
pig. There are 16 beds, and consequently there are 480
pigs, or about 11 tons in each cast.
At each of the branch gutters, or
sows, a man is stationed with a spade, with which he prevents the metal flowing into his bed until the bed below him is filled, when he suddenly transplaces it, and, cutting off the flow downward, turns it into his own bed. The next man does the same in succession, and when all the beds on one side of the track are filled, the flow is turned in the same manner into the other
runner, and the process is repeated until all are filled, when the opening in the flask is closed by day prepared for that purpose.
New supplies of coal, ore, and limestone are dumped in above, and the operation of smelting goes on for the next 12 hours.”
The
pig-iron is used either for casting, or for conversion into wrought-iron by puddling, etc. See casting; puddling.
Howell's furnace for making malleable iron direct from the ore with stone coal, patented in the
United States about 1831, was thus described by the inventor: “This furnace combines within itself the advantages of a close furnace and an open fire.
In the upper or close portion, being all that above the hearth, with anthracite coal, excited by a proper blast, a degree of heat is generated much greater than can possibly be obtained in the ordinary fire with charcoal; while the lower portion, opening into the hearth and permitting the free action of the blast upon the burthen, performs all the offices of the open or forge fire.
The ore, descending to the region of the tuyeres, becomes perfectly fused, and, passing below the influence of the blast, a part is driven out at the open front.
The burthen in the furnace being temporarily supported by bars, the masses are gathered into a
loup, which is removed by tongs and taken to the forgehammer.”
