+--------------------+---------------+------------+------------+
| Country. | Workable | Annual | Annual |
| | Deposits. | Output. |Consumption.|
+--------------------+---------------+------------+------------+
| | tons. | tons. | tons. |
| United States | 1,100,000,000 | 35,000,000 | 35,000,000 |
| Great Britain | 1,000,000,000 | 14,000,000 | 20,000,000 |
| Germany | 2,200,000,000 | 21,000,000 | 24,000,900 |
| Spain | 500,000,000 | 8,000,000 | 1,000,000 |
| Russia and Finland | 1,500,000,000 | 4,000,000 | 6,000,000 |
| France | 1,500,000,000 | 6,000,000 | 8,000,000 |
| Sweden | 1,000,000,000 | 4,000,000 | 1,000,000 |
| Austria-Hungary | 1,200,000,000 | 3,000,000 | 4,000,000 |
| Other countries | | 5,000,000 | 1,000,000 |
+--------------------+---------------+------------+------------+
| Total |10,000,000,000 |100,000,000 |100,000,000 |
+--------------------+---------------+------------+------------+
_Note to Table._--Though this estimate seems to be near the truth as
regards the British ores, it does not credit the United States with
one-tenth, if indeed with one-twentieth, of their true quantity as
estimated by that country's Geological Survey in 1907.
52. _What Constitutes an Iron Ore._--Whether a ferruginous rock is or is
not ore is purely a question of current demand and supply. That is ore
from which there is reasonable hope that metal can be extracted with
profit, if not to-day, then within a reasonable length of time. Rock
containing 2(1/2)% of gold is ah extraordinarily rich gold ore; that
with 2(1/2)% of copper is a profitable one to-day; that containing
2(1/2)% of iron is not so to-day, for the sole reason that its iron
cannot be extracted with profit in competition with the existing richer
ores. But it will become a profitable ore as soon as the richer ore
shall have been exhausted. Very few of the ores which, are mined to-day
contain less than 25% of iron, and some of them contain over 60%. As
these richest ores are exhausted, poorer and poorer ones will be used,
and the cost of iron will increase progressively if measured either in
units of the actual energy used in mining and smelting it, or in its
power of purchasing animal and vegetable products, cotton, wool, corn,
&c., the supply of which is renewable and indeed capable of very great
increase, but probably not if measured in its power of purchasing the
various mineral products, e.g. the other metals, coal, petroleum and the
precious stones, of which the supply is limited. This is simply one
instance of the inevitable progressive increase in cost of the
irrecreatable mineral relatively to the recreatable animal and
vegetable. When, in the course of centuries, the exhaustion of richer
ores shall have forced us to mine, crush and concentrate mechanically or
by magnetism the ores which contain only 2 or 3% of iron, then the cost
of iron in the ore, measured in terms of the energy needed to mine and
concentrate it, will be comparable with the actual cost of the copper in
the ore of the copper-mines of to-day. But, intermediate in richness
between these two extremes, the iron ores mined to-day and these 2 and
3% ores, there is an incalculably great quantity of ore capable of
mechanical concentration, and another perhaps vaster store of ore which
we do not yet know how to concentrate mechanically, so that the day when
a pound of iron in the ore will cost as much as a pound of copper in the
ore costs to-day is immeasurably distant.
53. _Future Cost of Ore._--The cost of iron ore is likely to rise much
less rapidly than that of coal, because the additions to our known
supply are likely to be very much greater in the case of ore than in
that of coal, for the reason that, while rich and great iron ore beds
may exist anywhere, those of coal are confined chiefly to the
Carboniferous formation, a fact which has led to the systematic survey
and measurement of this formation in most countries. In short, a very
large part of the earth's coal supply is known and measured, but its
iron ore supply is hardly to be guessed. On the other hand, the cost of
iron ore is likely to rise much faster than that of the potential
aluminium ores, clay and its derivatives, because of the vast extent and
richness of the deposits of this latter class. It is possible that, at
some remote day, aluminium, or one of its alloys, may become the great
structural material, and iron be used chiefly for those objects for
which it is especially fitted, such as magnets, springs and cutting
tools.
In passing, it may be noted that the cost of the ore itself forms a
relatively small part of the cost even of the cruder forms of steel,
hardly a quarter of the cost of such simple products as rails, and an
insignificant part of the cost of many most important finished
objects, such as magnets, cutting tools, springs and wire, for which
iron is almost indispensable. Thus, if the use of ores very much
poorer than those we now treat, and the need of concentrating them
mechanically, were to double the cost of a pound of iron in the
concentrated ore ready for smelting, that would increase the cost of
rails by only one quarter. Hence the addition to the cost of finished
steel objects which is due to our being forced to use progressively
poorer and poorer ores is likely to be much less than the addition due
to the progressive rise in the cost of coal and in the cost of labour,
because of the ever-rising scale of living. The effect of each of
these additions will be lessened by the future improvements in
processes of manufacture, and more particularly by the progressive
replacement of that ephemeral source of energy, coal, by the secular
sources, the winds, waves, tides, sunshine, the earth's heat and,
greatest of all, its momentum.
54. _Ore Supply of the Chief Iron-making Countries._--The United States
mine nearly all of their iron ores, Austria-Hungary, Russia and France
mine the greater part of theirs, but none of these countries exports
much ore. Great Britain and Germany, besides mining a great deal of ore,
still have to import much from Spain, Sweden and in the case of Germany
from Luxemburg, although, because of the customs arrangement between
these last two countries, this importation is not usually reported.
Belgium imports nearly all of its ore, while Sweden and Spain export
most of the ore which they mine.
55. _Great Britain_ has many valuable ore beds, some rich in iron,
many of them near to beds of coal and to the sea-coast, to canals or
to navigable rivers. They extend from Northamptonshire to near
Glasgow. About two-thirds of the ore mined is clayey siderite. In 1905
the Cleveland district in North Yorkshire supplied 41% of the total
British product of iron ores; Lincolnshire, 14.8%; Northamptonshire,
13.9%; Leicestershire, 4.7%; Cumberland, 8.6%; North Lancashire, 2.7%;
Staffordshire, 6.1%; and Scotland, 5.7%. The annual production of
British iron ore reached 18,031,957 tons in 1882, but in 1905 it had
fallen to 14,590,703 tons, valued at L3,482,184. In addition
7,344,786 tons, or about half as much as was mined in Great Britain,
were imported, 78.5% of it from Spain. The most important British ore
deposit is the Lower Cleveland bed of oolitic siderite in the Middle
Lias, near Middlesborough. It is from 10 to 17 ft. thick, and its ore
contains about 30% of iron.
56. _Geographical Distribution of the British Works._--Most of the
British iron works lie in and near the important coal-fields in
Scotland between the mouth of the Clyde and the Forth, in Cleveland
and Durham, in Cumberland and Lancashire, in south Yorkshire,
Derbyshire, and Lincolnshire, in Staffordshire and Northamptonshire,
and in south Wales in spite of its lack of ore.
The most important group is that of Cleveland and Durham, which makes
about one-third of all the British pig iron. It has the great
Cleveland ore bed and the excellent Durham coal near tidewater at
Middlesbrough. The most important seat of the manufacture of cutlery
and the finer kinds of steel is at Sheffield.
57. The _United States_ have great deposits of ore in many different
places. The rich beds near Lake Superior, chiefly red haematite,
yielding at present about 55% of iron, are thought to contain between
1(1/2) and 2 billion tons, and the red and brown haematites of the
southern states about 10 billion tons. The middle states, New York,
New Jersey and Pennsylvania, are known to have many great deposits of
rich magnetite, which supplied a very large proportion of the American
ores till the discovery of the very cheaply mined ores of Lake
Superior. In 1906 these latter formed 80% of the American production,
and the southern states supplied about 13% of it, while the rich
deposits of the middle states are husbanded in accordance with the law
that ore bodies are drawn on in the order of their apparent
profitableness.
The most important American iron-making district is in and about
Pittsburg, to whose cheap coal the rich Lake Superior ores are brought
nearly 1000 m., about four-fifths of the distance in the large ore
steamers of the Great Lakes. Chicago, nearer to the Lake ores, though
rather far from the Pittsburg coal-field, is a very important centre
for rail-making for the railroads of the western states. Ohio, the
Lake Erie end of New York State, eastern Pennsylvania and Maryland
have very important works, the ore for which comes in part from Lake
Superior and in part from Pennsylvania, New York and Cuba, and the
fuel from Pennsylvania and its neighbourhood. Tennessee and Alabama in
the south rely on southern ore and fuel.
58. _Germany_ gets about two-thirds of her total ore supply from the
great Jurassic "Minette" ore deposit of Luxemburg and Lorraine, which
reaches also into France and Belgium. In spite of its containing only
about 36% of iron, this deposit is of very great value because of its
great size, and of the consequent small cost of mining. It stretches
through an area of about 8 m. wide and 40 m. long, and in some places
it is nearly 60 ft. thick. There are valuable deposits also in
Siegerland and in many other parts of the country.
59. _Sweden_ has abundant, rich and very pure iron ores, but her lack
of coal has restricted her iron manufacture chiefly to the very purest
and best classes of iron and steel, in making which her thrifty and
intelligent people have developed very rare skill. The magnetite ore
bodies which supply this industry lie in a band about 180 m. long,
reaching from a little north of Stockholm westerly toward the
Norwegian frontier, between the latitudes 59 deg. and 61 deg. N. In
Swedish Lapland, near the Arctic circle, are the great Gellivara,
Kirunavara and Luossavara magnetite beds, among the largest in Europe.
From these beds, which in some parts are about 300 ft. thick, much ore
is sent to Germany and Great Britain.
60. _Other Countries._--Spain has large, rich and pure iron ore beds,
near both her northern and her southern sea coast. She exports about
90% of all the iron ore which she mines, most of it to England. France
draws most of her iron ore from her own part of the great Minette ore
deposit, and from those parts of it which were taken from her when she
lost Alsace and Lorraine. Russia's most valuable ore deposit is the
very large and easily mined one of Krivoi Rog in the south, from which
comes about half of the Russian iron ore. It is near the Donetz
coal-field, the largest in Europe. There are also important ore beds
in the Urals, near the border of Finland, and at the south of Moscow.
In Austria-Hungary, besides the famous Styrian Erzberg, with its
siderite ore bed about 450 ft. thick, there are cheaply mined but poor
and impure ores near Prague, and important ore beds in both northern
and southern Hungary. Algeria, Canada, Cuba and India have valuable
ore bodies.
61. _Richness of Iron Ores._--The American ores now mined are
decidedly richer than those of most European countries. To make a ton
of pig iron needs only about 1.9 tons of ore in the United States, 2
tons in Sweden and Russia, 2.4 tons in Great Britain and Germany, and
about 2.7 tons in France and Belgium, while about 3 tons of the native
British ores are needed per ton of pig iron.
62. _The general scheme of iron manufacture_ is shown diagrammatically
in fig. 6. To put the iron contained in iron ore into a state in which
it can be used as a metal requires essentially, first its deoxidation,
and second its separation from the other mineral matter, such as clay,
quartz, &c. with which it is found associated. These two things are done
simultaneously by heating and melting the ore in contact with coke,
charcoal or anthracite, in the iron blast furnace, from which issue
intermittently two molten streams, the iron now deoxidized and
incidentally carburized by the fuel with which it has been in contact,
and the mineral matter, now called "slag." This crude cast iron, called
"pig iron," may be run from the blast furnace directly into moulds,
which give the metal the final shape in which it is to be used in the
arts; but it is almost always either remelted, following path 1 of fig.
6, and then cast into castings of cast iron, or converted into wrought
iron or steel by purifying it, following path 2.
[Illustration: FIG. 6.--General Scheme of Iron Manufacture.]
If it is to follow path 1, the castings into which it is made may be
either (a) grey or (b) chilled or (c) malleable. Grey iron castings
are made by remelting the pig iron either in a small shaft or "cupola"
furnace, or in a reverberatory or "air" furnace, with very little
change of chemical composition, and then casting it directly into
suitable moulds, usually of either "baked," i.e. oven-dried, or
"green," i.e. moist undried, sand, but sometimes of iron covered with
a refractory coating to protect it from being melted or overheated by
the molten cast iron. The general procedure in the manufacture of
chilled and of malleable castings has been described in SS 30 and 31.
If the pig iron is to follow path 2, the purification which converts
it into wrought iron or steel consists chiefly in oxidizing and
thereby removing its carbon, phosphorus and other impurities, while it
is molten, either by means of the oxygen of atmospheric air blown
through it as in the Bessemer process, or by the oxygen of iron ore
stirred into it as in the puddling and Bell-Krupp processes, or by
both together as in the open hearth process.
On its way from the blast furnace to the converter or open hearth
furnace the pig iron is often passed through a great reservoir called
a "mixer," which acts also as an equalizer, to lessen the variation in
composition of the cast iron, and as a purifier, removing part of the
sulphur and silicon.
63. _Shaping and Adjusting Processes._--Besides these extraction and
purification processes there are those of adjustment and shaping. The
_adjusting processes_ adjust either the ultimate composition, e.g.
carburizing wrought iron by long heating in contact with charcoal
(cementation), or the proximate composition or constitution, as in the
hardening, tempering and annealing of steel already described (SS 28,
29), or both, as in the process of making malleable cast iron (S 31).
The _shaping processes_ include the _mechanical_ ones, such as rolling,
forging and wire-drawing, and the _remelting_ ones such as the crucible
process of melting wrought iron or steel in crucibles and casting it in
ingots for the manufacture of the best kinds of tool steel. Indeed, the
remelting of cast iron to make grey iron castings belongs here. This
classification, though it helps to give a general idea of the subject,
yet like most of its kind cannot be applied rigidly. Thus the crucible
process in its American form both carburizes and remelts, and the open
hearth process is often used rather for remelting than for purifying.
64. The _iron blast furnace_, a crude but very efficient piece of
apparatus, is an enormous shaft usually about 80 ft. high and 20 ft.
wide at its widest part. It is at all times full from top to bottom,
somewhat as sketched in figs. 7 and 8, of a solid column of lumps of
fuel, ore and limestone, which are charged through a hopper at the top,
and descend slowly as the lower end of the column is eaten off through
the burning away of its coke by means of very hot air or "blast" blown
through holes or "tuyeres" near the bottom or "hearth," and through the
melting away, by the heat thus generated, both of the iron itself which
has been deoxidized in its descent, and of the other minerals of the
ore, called the "gangue," which unite with the lime of the limestone and
the ash of the fuel to form a complex molten silicate called the
"cinder" or "slag."
[Illustration: FIG. 7.--Section of Duquesne Blast Furnace.
GG, Flanges on the ore bucket;
HH, Fixed flanges on the top of the furnace;
J, Counterweighted false bell;
K, Main bell;
O, Tuyere;
P, Cinder notch;
RR', Water cooled boxes;
S, Blast pipe;
T, Cable for allowing conical bottom of bucket to drop.]
[Illustration: FIG. 8.--Lower Part of the Blast Furnace.
Lumps of Coke [symbol]
Lumps* of Iron Ore [symbol]
Lumps* of Lime [symbol]
Drops of Slag [symbol]
Drops of Iron [symbol]
Layer of Molten Slag [symbol]
Layer of Molten Iron [symbol]
* The ore and lime actually exist here in powder. They are shown in
lump form because of the difficulty of presenting to the eye their
powdered state.]
[Illustration: FIG. 9.--Method of transferring charge from bucket to
main charging bell, without permitting escape of furnace gas (lettering
as in fig. 7).]
Interpenetrating this descending column of solid ore, limestone and
coke, there is an upward rushing column of hot gases, the atmospheric
nitrogen of the blast from the tuyeres, and the carbonic oxide from the
combustion of the coke by that blast. The upward ascent of the column of
gases is as swift as the descent of the solid charge is slow. The former
occupies but a very few seconds, the latter from 12 to 15 hours.
In the upper part of the furnace the carbonic oxide deoxidizes the iron
oxide of the ore by such reactions as xCO + FeO_x = Fe + xCO2. Part of
the resultant carbonic acid is again deoxidized to carbonic oxide by the
surrounding fuel, CO2 + C = 2CO, and the carbonic oxide thus formed
deoxidizes more iron oxide, &c. As indicated in fig. 7, before the iron
ore has descended very far it has given up nearly the whole of its
oxygen, and thus lost its power of oxidizing the rising carbonic oxide,
so that from here down the atmosphere of the furnace consists
essentially of carbonic oxide and nitrogen.
But the transfer of heat from the rising gases to the sinking solids,
which has been going on in the upper part of the furnace, continues as
the solid column gradually sinks downward to the hearth, till at the
"fusion level" (A in fig. 7) the solid matter has become so hot that the
now deoxidized iron melts, as does the slag as fast as it is formed by
the union of its three constituents, the gangue, the lime resulting from
the decomposition of the limestone and the ash of the fuel. Hence from
this level down the only solid matter is the coke, in lumps which are
burning rapidly and hence shrinking, while between them the molten iron
and slag trickle, somewhat as sketched in fig. 8, to collect in the
hearth in two layers as distinct as water and oil, the iron below, the
slag above.
As they collect, the molten iron is drawn off at intervals through a
hole A (fig. 8), temporarily stopped with clay, at the very bottom, and
the slag through another hole a little higher up, called the "cinder
notch." Thus the furnace may be said to have four zones, those of (1)
deoxidation, (2) heating, (3) melting, and (4) collecting, though of
course the heating is really going on in all four of them.
In its slow descent the deoxidized iron nearly saturates itself with
carbon, of which it usually contains between 3.5 and 4%, taking it in
part from the fuel with which it is in such intimate contact, and in
part from the finely divided carbon deposited within the very lumps of
ore, by the reaction 2CO = C + CO2. This carburizing is an indispensable
part of the process, because through it alone can the iron be made
fusible enough to melt at the temperature which can be generated in the
furnace, and only when liquid can it be separated readily and completely
from the slag. In fact, the molten iron is heated so far above its
melting point that, instead of being run at once into pigs as is usual,
it may, without solidifying, be carried even several miles in large
clay-lined ladles to the mill where it is to be converted into steel.
65. The _fuel_ has, in addition to its duties of deoxidizing and
carburizing the iron and yielding the heat needed for melting both the
iron and slag, the further task of desulphurizing the iron, probably by
the reaction FeS + CaO + C = Fe + CaS + CO.
The desulphurizing effect of this transfer of the sulphur from union
with iron to union with calcium is due to the fact that, whereas iron
sulphide dissolves readily in the molten metallic iron, calcium
sulphide, in the presence of a slag rich in lime, does not, but by
preference enters the slag, which may thus absorb even as much as 3%
of sulphur. This action is of great importance whether the metal is to
be used as cast iron or is to be converted into wrought iron or steel.
In the former case there is no later chance to remove sulphur, a
minute quantity of which does great harm by leading to the formation
of cementite instead of graphite and ferrite, and thus making the
cast-iron castings too hard to be cut to exact shape with steel tools;
in the latter case the converting or purifying processes, which are
essentially oxidizing ones, though they remove the other impurities,
carbon, silicon, phosphorus and manganese, are not well adapted to
desulphurizing, which needs rather deoxidizing conditions, so as to
cause the formation of calcium sulphide, than oxidizing ones.
66. The _duty of the limestone_ (CaCO3) is to furnish enough lime to
form with the gangue of the ore and the ash of the fuel a lime silicate
or slag of such a composition (1) that it will melt at the temperature
which it reaches at about level A, of fig. 7, (2) that it will be fluid
enough to run out through the cinder notch, and (3) that it will be rich
enough in lime to supply that needed for the desulphurizing reaction FeS
+ CaO + C = Fe + CaS + CO. In short, its duty is to "flux" the gangue
and ash, and wash out the sulphur.
67. In order that the _slag_ shall have these properties its composition
usually lies between the following limits: silica, 26 to 35%; lime,
_plus_ 1.4 times the magnesia, 45 to 55%; alumina, 5 to 20%. Of these
the silica and alumina are chiefly those which the gangue of the ore and
the ash of the fuel introduce, whereas the lime is that added
intentionally to form with these others a slag of the needed physical
properties.
Thus the more gangue the ore contains, i.e. the poorer it is in iron,
the more limestone must in general be added, and hence the more slag
results, though of course an ore the gangue of which initially
contains much lime and little silica needs a much smaller addition of
limestone than one of which the gangue is chiefly silica. Further, the
more sulphur there is to remove, the greater must be the quantity of
slag needed to dissolve it as calcium sulphide. In smelting the rich
Lake Superior ores the quantity of slag made was formerly as small as
28% of that of the pig iron, whereas in smelting the Cleveland ores of
Great Britain it is usually necessary to make as much as 1(1/2) tons of
slag for each ton of iron.
68. _Shape and Size of the Blast-Furnace._--Large size has here, as in
most metallurgical operations, not only its usual advantage of economy
of installation, labour and administration per unit of product, but the
further very important one that it lessens the proportion which the
outer heat-radiating and hence heat-wasting surface bears to the whole.
The limits set to the furnace builder's natural desire to make his
furnace as large as possible, and its present shape (an obtuse inverted
cone set below an acute upright one, both of them truncated), have been
reached in part empirically, and in part by reasoning which is open to
question, as indeed are the reasons which will now be offered reservedly
for both size and shape.
First the width at the tuyeres (fig. 7) has generally been limited to
about 12(1/2) ft. by the fear that, if it were greater, the blast would
penetrate so feebly to the centre that the difference in conditions
between centre and circumference would be so great as to cause serious
unevenness of working. Of late furnaces have been built even as wide as
17 ft. in the hearth, and it may prove that a width materially greater
than 12(1/2) ft. can profitably be used. With the width at the bottom
thus limited, the furnace builder naturally tries to gain volume as
rapidly as possible by flaring or "battering" his walls outwards, i.e.
by making the "bosh" or lower part of his furnace an inverted cone as
obtuse as is consistent with the free descent of the solid charge. In
practice a furnace may be made to work regularly if its boshes make an
angle of between 73 deg. and 76 deg. with the horizontal, and we may
assume that one element of this regularity is the regular easy sliding
of the charge over this steep slope. A still steeper one not only gives
less available room, but actually leads to irregular working, perhaps
because it unduly favours the passage of the rising gas along the walls
instead of up and through the charge, and thus causes the deoxidation of
the central core to lag behind that of the periphery of the column, with
the consequence that this central core arrives at the bottom
incompletely deoxidized.
In the very swift-running furnaces of the Pittsburg type this outward
flare of the boshes ceases at about 12 ft. above the tuyeres, and is
there reversed, as in fig. 7, so that the furnace above this is a very
acute upright cone, the walls of which make an angle of about 4 deg.
with the vertical, instead of an obtuse inverted cone.
In explanation or justification of this it has been said that a much
easier descent must be provided above this level than is needed below
it. Below this level the solid charge descends easily, because it
consists of coke alone or nearly alone, and this in turn because the
temperature here is so high as to melt not only the iron now
deoxidized and brought to the metallic state, but also the gangue of
the ore and the limestone, which here unite to form the molten slag,
and run freely down between the lumps of coke. This coke descends
freely even through this fast-narrowing space, because it is perfectly
solid and dry without a trace of pastiness. But immediately above this
level the charge is relatively viscous, because here the temperature
has fallen so far that it is now at the melting or formation point of
the slag, which therefore is pasty, liable to weld the whole mass
together as so much tar would, and thus to obstruct the descent of the
charge, or in short to "scaffold."
The reason why at this level the walls must form an upright instead of
an inverted cone, why the furnace must widen downward instead of
narrowing, is, according to some metallurgists, that this shape is
needed in order that, in spite of the pastiness of the slag in this
formative period of incipient fusion, this layer may descend freely as
the lower part of the column is gradually eaten away. To this very
plausible theory it may be objected that in many slow-running
furnaces, which work very regularly and show no sign of scaffolding,
the outward flare of the boshes continues (though steepened) far above
this region of pastiness, indeed nearly half-way to the top of the
furnace. This proves that the regular descent of the material in its
pasty state can take place even in a space which is narrowing
downwards. To this objection it may in turn be answered that, though
this degree of freedom of descent may suffice for a slow-running
furnace, particularly if the slag is given such a composition that it
passes quickly from the solid state to one of decided fluidity, yet it
is not enough for swift-running ones, especially if the composition of
the slag is such that, in melting, it remains long in a very sticky
condition. In limiting the diameter at the tuyeres to 12(1/2) ft., the
height of the boshes to one which will keep their upper end below the
region of pastiness, and their slope to one over which the burning
coke will descend freely, we limit the width of the furnace at the top
of the boshes and thus complete the outline of the lower part of the
furnace.
The height of the furnace is rarely as great as 100 ft., and in the
belief of many metallurgists it should not be much more than 80 ft.
There are some very evident disadvantages of excessive height; for
instance, that the weight of an excessively high column of solid coke,
ore and limestone tends to crush the coke and jam the charge in the
lower and narrowing part of the furnace, and that the frictional
resistance of a long column calls for a greater consumption of power for
driving the blast up through it. Moreover, this resistance increases
much more rapidly than the height of the furnace, even if the rapidity
with which the blast is forced through is constant; and it still further
increases if the additional space gained by lengthening the furnace is
made useful by increasing proportionally the rate of production, as
indeed would naturally be done, because the chief motive for gaining
this additional space is to increase production.
The reason why the frictional resistance would be further increased is
the very simple one that the increase in the rate of production
implies directly a corresponding increase in the quantity of blast
forced through, and hence in the velocity of the rising gases, because
the chemical work of the blast furnace needs a certain quantity of
blast for each ton of iron made. In short, to increase the rate of
production by lengthening the furnace increases the frictional
resistance of the rising gases, both by increasing their quantity and
hence their velocity and by lengthening their path.
Indeed, one important reason for the difficulties in working very high
furnaces, e.g. those 100 ft. high, may be that this frictional
resistance becomes so great as actually to interrupt the even descent
of the charge, parts of which are at times suspended like a ball in
the rising jet of a fountain, to fall perhaps with destructive
violence when some shifting condition momentarily lessens the
friction. We see how powerful must be the lifting effect of the rising
gases when we reflect that their velocity in a 100 ft. furnace rapidly
driven is probably at least as great as 2000 ft. per minute, or that
of a "high wind." Conceive these gases passing at this great velocity
through the narrow openings between the adjoining lumps of coke and
ore. Indeed, the velocity must be far greater than this where the edge
or corner of one lump touches the side of another, and the only room
for the passage of this enormous quantity of gas is that left by the
roughness and irregularity of the individual lumps.
The furnace is made rather narrow at the top or "stock line," in order
that the entering ore, fuel and flux may readily be distributed evenly.
But extreme narrowness would not only cause the escaping gases to move
so swiftly that they would sweep much of the fine ore out of the
furnace, but would also throw needless work on the blowing engines by
throttling back the rising gases, and would lessen unduly the space
available for the charge in the upper part of the furnace.
From its top down, the walls of the furnace slope outward at an angle of
between 3 deg. and 8 deg., partly in order to ease the descent of the
charge, here impeded by the swelling of the individual particles of ore
caused by the deposition within them of great quantities of fine carbon,
by the reaction of 2CO = C + CO2. To widen it more abruptly would indeed
increase the volume of the furnace, but would probably lead to grave
irregularities in the distribution of the gas and charge, and hence in
the working of the furnace.
When we have thus fixed the height of the furnace, its diameter at its
ends, and the slope of its upper and lower parts, we have completed its
outline closely enough for our purpose here.
69. _Hot Blast and Dry Blast._--On its way from the blowing engine to
the tuyeres of the blast-furnace, the blast, i.e. the air forced in for
the purpose of burning the fuel, is usually pre-heated, and in some of
the most progressive works is dried by Gayley's refrigerating process.
These steps lead to a saving of fuel so great as to be astonishing at
first sight--indeed in case of Gayley's blast-drying process incredible
to most writers, who proved easily and promptly to their own
satisfaction that the actual saving was impossible. But the explanation
is really so very simple that it is rather the incredulity of these
writers that is astonishing. In the hearth of the blast furnace the heat
made latent by the fusion of the iron and slag must of course be
supplied by some body which is itself at a temperature above the melting
point of these bodies, which for simplicity of exposition we may call
the critical temperature of the blast-furnace process, because heat will
flow only from a hotter to a cooler object. Much the same is true of the
heat needed for the deoxidation of the silica, SiO2 + 2C = Si + 2CO2.
Now the heat developed by the combustion of coke to carbonic oxide with
cold air containing the usual quantity of moisture, develops a
temperature only slightly above this critical point; and it is only the
heat represented by this narrow temperature-margin that is available for
doing this critical work of fusion and deoxidation. That is the crux of
the matter. If by pre-heating the blast we add to the sum of the heat
available; or if by drying it we subtract from the work to be done by
that heat the quantity needed for decomposing the atmospheric moisture;
or if by removing part of its nitrogen we lessen the mass over which the
heat developed has to be spread--if by any of these means we raise the
temperature developed by the combustion of the coke, it is clear that we
increase the proportion of the total heat which is available for this
critical work in exactly the way in which we should increase the
proportion of the water of a stream, initially 100 in. deep, which
should flow over a waste weir initially 1 in. beneath the stream's
surface, by raising the upper surface of the water 10 in. and thus
increasing the depth of the water to 110 in. Clearly this raising the
level of the water by 10% increases tenfold, or by 1000%, the volume of
water which is above the level of the weir.
The special conditions of the blast-furnace actually exaggerate the
saving due to this widening of the available temperature-margin, and
beyond this drying the blast does great good by preventing the serious
irregularities in working the furnace caused by changes in the
humidity of the air with varying weather.
70. _Means of Heating the Blast._--After the ascending column of gases
has done its work of heating and deoxidizing the ore, it still
necessarily contains so much carbonic oxide, usually between 20 and 26%
by weight, that it is a very valuable fuel, part of which is used for
raising steam for generating the blast itself and driving the rolling
mill engines, &c., or directly in gas engines, and the rest for heating
the blast. This heating was formerly done by burning part of the gases,
after their escape from the furnace top, in a large combustion chamber,
around a series of cast iron pipes through which the blast passed on its
way from the blowing engine to the tuyeres. But these "iron pipe stoves"
are fast going out of use, chiefly because they are destroyed quickly if
an attempt is made to heat the blast above 1000 deg. F. (538 deg. C.),
often a very important thing. In their place the regenerative stoves of
the Whitwell and Cowper types (figs. 10 and 11) are used. With these the
regular temperature of the blast at some works is about 1400 deg. F.
(760 deg. C.), and the usual blast temperature lies between 900 deg. and
1200 deg. F. (480 deg. and 650 deg. C.).
Like the Siemens furnace, described in S 99, they have two distinct
phases: one, "on gas," during which part of the waste gas of the
blast-furnace is burnt within the stove, highly heating the great
surface of brickwork which for that purpose is provided within it; the
other, "on wind," during which the blast is heated by passing it back
over these very surfaces which have thus been heated. They are
heat-filters or heat-traps for impounding the heat developed by the
combustion of the furnace gas, and later returning it to the blast. Each
blast-furnace is now provided with three or even four of these stoves,
which collectively may be nearly thrice as large as the furnace itself.
At any given time one of these is "on wind" and the others "on gas."
[Illustration: FIG. 10.--Whitwell Hot-Blast Stove, as modified by H.
Kennedy. When "on wind," the cold blast is forced in at A, and passes
four times up and down, as shown by means of unbroken arrows, escaping
as hot-blast at B. When "on gas," the gas and air enter at the bottom of
each of the three larger vertical chambers, pass once up through the
stove, and escape at the top, as shown by means of broken arrows. Hence
this is a four-pass stove when on wind, but a one-pass stove when on
gas.]
The Whitwell stove (fig. 10), by means of the surface of several
fire-brick walls, catches in one phase the heat evolved by the burning
gas as it sweeps through, and in the other phase returns that heat to
the entering blast as it sweeps through from left to right. In the
original Whitwell stove, which lacks the chimneys shown at the top of
fig. 10, both the burning gas and the blast pass up and down
repeatedly. In the H. Kennedy modification, shown in fig. 10, the gas
and air in one phase enter at the bottom of all three of the large
vertical chambers, burn in passing upwards, and escape at once at the
top, as shown by the broken arrows. In the other phase the cold blast,
forced in at A, passes four times up and down, as shown by the
unbroken arrows, and escapes as hot blast at B. This, then, is a
"one-pass" stove when on gas but a "four-pass" one when on wind.
The Cowper stove (Fig. 11) differs from the Whitwell (1) in having not
a series of flat smooth walls, but a great number of narrow vertical
flues, E, for the alternate absorption and emission of the heat, with
the consequence that, for given outside dimensions, it offers about
one-half more heating surface than the true Whitwell stove; and (2) in
that the gas and the blast pass only once up and once down through it,
instead of twice up and twice down as in the modern true Whitwell
stoves. As regards frictional resistance, this smaller number of
reversals of direction compensates in a measure for the smaller area
of the Cowper flues. The large combustion chamber B permits thorough
combustion of the gas.
71. _Preservation of the Furnace Walls._--The combined fluxing and
abrading action of the descending charge tends to wear away the lining
of the furnace where it is hottest, which of course is near its lower
end, thus changing its shape materially, lessening its efficiency, and
in particular increasing its consumption of fuel. The walls, therefore,
are now made thin, and are thoroughly cooled by water, which circulates
through pipes or boxes bedded in them. James Gayley's method of cooling,
shown in fig. 7, is to set in the brickwork walls several horizontal
rows of flat water-cooled bronze boxes, RR', extending nearly to the
interior of the furnace, and tapered so that they can readily be
withdrawn and replaced in case they burn through. The brickwork may wear
back to the front edges of these boxes, or even, as is shown at R', a
little farther. But in the latter case their edges still determine the
effective profile of the furnace walls because the depressions at the
back of these edges become filled with carbon and scoriaceous matter
when the furnace is in normal working. Each of these rows, of which five
are shown in fig. 7, consists of a great number of short segmental
boxes.
72. _Blast-furnace Gas Engines._--When the gas which escapes from the
furnace top is used in gas engines it generates about four times as much
power as when it is used for raising steam. It has been calculated that
the gas from a pair of old-fashioned blast-furnaces making 1600 tons of
iron per week would in this way yield some 16,000 horse-power in excess
of their own needs, and that all the available blast-furnace gas in the
United States would develop about 1,500,000 horse-power, to develop
which by raising steam would need about 20,000,000 tons of coal a year.
Of this power about half would be used at the blast-furnaces themselves,
leaving 750,000 horse-power available for driving the machinery of the
rolling mills, &c.
[Illustration: FIG. 11.--Diagram of Cowper Hot-Blast Stove at Duquesne.
(After J. Kennedy.) Broken arrows show the path of the gas and air while
the stove is "on gas," and solid arrows that of the blast while it is
"on wind."
A, Entrance for blast-furnace gas.
B, B, Combustion chamber.
C, Chimney valve.
D, Cold blast main.
E, Hollow bricks.]
This use of the gas engine is likely to have far-reaching results. In
order to utilize this power, the converting mill, in which the pig
iron is converted into steel, and the rolling mills must adjoin the
blast-furnace. The numerous converting mills which treat pig iron made
at a distance will now have the crushing burden of providing in other
ways the power which their rivals get from the blast-furnace, in
addition to the severe disadvantage under which they already suffer,
of wasting the initial heat of the molten cast iron as it runs from
the blast-furnace. Before its use in the gas engine, the blast-furnace
gas has to be freed carefully from the large quantity of fine ore dust
which it carries in suspension.
73. _Mechanical Appliances._--Moving the raw materials and the products:
In order to move economically the great quantity of materials which
enter and issue from each furnace daily, mechanical appliances have at
many works displaced hand labour wholly, and indeed that any of the
materials should be shovelled by hand is not to be thought of in
designing new works.
The arrangement at the Carnegie Company's Duquesne works (fig. 12) may
serve as an example of modern methods of handling. The standard-gauge
cars which bring the ore and coke to Duquesne pass over one of three
very long rows of bins, A, B, and C (fig. 12), of which A and B
receive the materials (ore, coke and limestone) for immediate use,
while C receives those to be stored for winter use. From A and B the
materials are drawn as they are needed into large buckets D standing
on cars, which carry them to the foot of the hoist track EE, up which
they are hoisted to the top of the furnace. Arrived here, the material
is introduced into the furnace by an ingenious piece of mechanism
which completely prevents the furnace gas from escaping into the air.
The hoist-engineer in the house F at the foot of the furnace, when
informed by means of an indicator that the bucket has arrived at the
top, lowers it so that its flanges GG (fig. 7) rest on the
corresponding fixed flanges HH, as shown in fig. 9. The farther
descent of the bucket being thus arrested, the special cable T is now
slackened, so that the conical bottom of the bucket drops down,
pressing down by its weight the counter-weighted false cover J of the
furnace, so that the contents of the bucket slide down into the space
between this false cover and the true charging bell, K. The special
cable T is now tightened again, and lifts the bottom of the bucket so
as both to close it and to close the space between J and K, by
allowing J to rise back to its initial place. The bucket then descends
along the hoist-track to make way for the next succeeding one, and K
is lowered, dropping the charge into the furnace. Thus some 1700 tons
of materials are charged daily into each of these furnaces without
being shovelled at all, running by gravity from bin to bucket and from
bucket to furnace, and being hoisted and charged into the furnace by a
single engineer below, without any assistance or supervision at the
furnace-top.
[Illustration: FIG. 12.--Diagram of the Carnegie Blast-Furnace Plant
at Duquesne, Pa.
A and B, Bins for stock for immediate use.
C, Receiving bin for winter stock pile.
D, D, Ore bucket.
EE, Hoist-track.
F, Hoist-engine house.
LL, Travelling crane commanding stock pile.
M, Ore bucket receiving ore for stock pile.
M', Bucket removing ore from stock pile.
N, N, N, Ladles carrying the molten cast iron to the works, where
it is converted into steel by the open hearth process.]
The winter stock of materials is drawn from the left-hand row of bins,
and distributed over immense stock piles by means of the great crane
LL (fig. 12), which transfers it as it is needed to the row A of bins,
whence it is carried to the furnace, as already explained.
74. _Casting the Molten Pig Iron._--The molten pig iron at many works is
still run directly from the furnace into sand or iron moulds arranged in
a way which suggests a nursing litter of pigs; hence the name "pig
iron." These pigs are then usually broken by hand. The Uehling casting
machine (fig. 13) has displaced this method in many works. It consists
essentially of a series of thin-walled moulds, BB, carried by endless
chains past the lip of a great ladle A. This pours into them the molten
cast iron which it has just received directly from the blast-furnace. As
the string of moulds, each thus containing a pig, moves slowly forward,
the pigs solidify and cool, the more quickly because in transit they are
sprayed with water or even submerged in water in the tank EE. Arrived at
the farther sheave C, the now cool pigs are dumped into a railway car.
[Illustration: FIG. 13.--Diagram of Pig-Casting Machine.
A, Ladle bringing the cast iron from the blast-furnace.
BB, The moulds.
C, D, Sheaves carrying the endless chain of moulds.
EE, Tank in which the moulds are submerged.
F, Car into which the cooled pigs are dropped.
G, Distributing funnel.]
Besides a great saving of labour, only partly offset by the cost of
repairs, these machines have the great merit of making the management
independent of a very troublesome set of labourers, the hand
pig-breakers, who were not only absolutely indispensable for every
cast and every day, because the pig iron must be removed promptly to
make way for the next succeeding cast of iron, but very difficult to
replace because of the great physical endurance which their work
requires.
75. _Direct Processes for making Wrought Iron and Steel._--The present
way of getting the iron of the ore into the form of wrought iron and
steel by first making cast iron and then purifying it, i.e. by first
putting carbon and silicon into the iron and then taking them out again
at great expense, at first sight seems so unreasonably roundabout that
many "direct" processes of extracting the iron without thus charging it
with carbon and silicon have been proposed, and some of them have at
times been important. But to-day they have almost ceased to exist.
That the blast-furnace process must be followed by a purifying one,
that carburization must at once be undone by decarburization, is
clearly a disadvantage, but it is one which is far out weighed by five
important incidental advantages. (1) The strong deoxidizing action
incidental to this carburizing removes the sulphur easily and cheaply,
a thing hardly to be expected of any direct process so far as we can
see. (2) The carburizing incidentally carburizes the brickwork of the
furnace, and thus protects it against corrosion by the molten slag.
(3) It protects the molten iron against reoxidation, the greatest
stumbling block in the way of the direct processes hitherto. (4) This
same strong deoxidizing action leads to the practically complete
deoxidation and hence extraction of the iron. (5) In that carburizing
lowers the melting point of the iron greatly, it lowers somewhat the
temperature to which the mineral matter of the ore has to be raised in
order that the iron may be separated from it, because this separation
requires that both iron and slag shall be very fluid. Indeed, few if
any of the direct processes have attempted to make this separation, or
to make it complete, leaving it for some subsequent operation, such as
the open hearth process.
In addition, the blast-furnace uses a very cheap source of energy,
coke, anthracite, charcoal, and even certain kinds of raw bituminous
coal, and owing first to the intimacy of contact between this fuel and
the ore on which it works, and second to the thoroughness of the
transfer of heat from the products of that fuel's combustion in their
long upward journey through the descending charge, even this cheap
energy is used most effectively.
Thus we have reasons enough why the blast-furnace has displaced all
competing processes, without taking into account its further advantage
in lending itself easily to working on an enormous scale and with
trifling consumption of labour, still further lessened by the general
practice of transferring the molten cast iron in enormous ladles into
the vessels in which its conversion into steel takes place.
Nevertheless, a direct process may yet be made profitable under
conditions which specially favour it, such as the lack of any fuel
suitable for the blast-furnace, coupled with an abundance of cheap
fuel suitable for a direct process and of cheap rich ore nearly free
from sulphur.
76. The chief difficulty in the way of modifying the blast-furnace
process itself so as to make it accomplish what the direct processes aim
at, by giving its product less carbon and silicon than pig iron as now
made contains, is the removal of the sulphur. The processes for
converting cast iron into steel can now remove phosphorus easily, but
the removal of sulphur in them is so difficult that it has to be
accomplished for the most part in the blast-furnace itself. As
desulphurizing seems to need the direct and energetic action of carbon
on the molten iron itself, and as molten iron absorbs carbon most
greedily, it is hard to see how the blast-furnace is to desulphurize
without carburizing almost to saturation, i.e. without making cast iron.
77. _Direct Metal and the Mixer._--Until relatively lately the cast iron
for the Bessemer and open-hearth processes was nearly always allowed to
solidify in pigs, which were next broken up by hand and remelted at
great cost. It has long been seen that there would be a great saving if
this remelting could be avoided and "direct metal," i.e. the molten cast
iron direct from the blast-furnace, could be treated in the conversion
process. The obstacle is that, owing to unavoidable irregularities in
the blast-furnace process, the silicon- and sulphur-content of the cast
iron vary to a degree and with an abruptness which are inconvenient for
any conversion process and intolerable for the Bessemer process. For the
acid variety of this process, which does not remove sulphur, this most
harmful element must be held below a limit which is always low, though
it varies somewhat with the use to which the steel is to be put.
Further, the point at which the process should be arrested is recognized
by the appearance of the flame which issues from the converter's mouth,
and variations in the silicon-content of the cast iron treated alter
this appearance, so that the indications of the flame become confusing,
and control over the process is lost. Moreover, the quality of the
resultant steel depends upon the temperature of the process, and this in
turn depends upon the proportion of silicon, the combustion of which is
the chief source of the heat developed. Hence the importance of having
the silicon-content constant. In the basic Bessemer process, also,
unforeseen variations in the silicon-content are harmful, because the
quantity of lime added should be just that needed to neutralize the
resultant silica and the phosphoric acid and no more. Hence the
importance of having the silicon-content uniform. This uniformity is now
given by the use of the "mixer" invented by Captain W. R. Jones.
This "mixer" is a great reservoir into which successive lots of molten
cast iron from all the blast-furnaces available are poured, forming a
great molten mass of from 200 to 750 tons. This is kept molten by a
flame playing above it, and successive lots of the cast iron thus mixed
are drawn off, as they are needed, for conversion into steel by the
Bessemer or open-hearth process. An excess of silicon or sulphur in the
cast iron from one blast-furnace is diluted by thus mixing this iron
with that from the other furnaces. Should several furnaces
simultaneously make iron too rich in silicon, this may be diluted by
pouring into the mixer some low-silicon iron melted for this purpose in
a cupola furnace. This device not only makes the cast iron much more
uniform, but also removes much of its sulphur by a curious slow
reaction. Many metals have the power of dissolving their own oxides and
sulphides, but not those of other metals. Thus iron, at least highly
carburetted, i.e. cast iron, dissolves its own sulphide freely, but not
that of either calcium or manganese. Consequently, when we deoxidize
calcium in the iron blast-furnace, it greedily absorbs the sulphur which
has been dissolved in the iron as iron sulphide, and the sulphide of
calcium thus formed separates from the iron. In like manner, if the
molten iron in the mixer contains manganese, this metal unites with the
sulphur present, and the manganese sulphide, insoluble in the iron,
slowly rises to the surface, and as it reaches the air, its sulphur
oxidizes to sulphurous acid, which escapes. Further, an important part
of the silicon may be removed in the mixer by keeping it very hot and
covering the metal with a rather basic slag. This is very useful if the
iron is intended for either the basic Bessemer or the basic open-hearth
process, for both of which silicon is harmful.
78. _Conversion or Purifying Processes for converting Cast Iron into
Steel or Wrought Iron._--As the essential difference between cast iron
on one hand and wrought iron and steel on the other is that the former
contains necessarily much more carbon, usually more silicon, and often
more phosphorus that are suitable or indeed permissible in the latter
two, the chief work of all these conversion processes is to remove the
excess of these several foreign elements by oxidizing them to carbonic
oxide CO, silica SiO2, and phosphoric acid P2O5, respectively. Of these
the first escapes immediately as a gas, and the others unite with iron
oxide, lime, or other strong base present to form a molten silicate or
silico-phosphate called "cinder" or "slag," which floats on the molten
or pasty metal. The ultimate source of the oxygen may be the air, as in
the Bessemer process, or rich iron oxide as in the puddling process, or
both as in the open-hearth process; but in any case iron oxide is the
chief immediate source, as is to be expected, because the oxygen of the
air would naturally unite in much greater proportion with some of the
great quantity of iron offered to it than with the small quantity of
these impurities. The iron oxide thus formed immediately oxidizes these
foreign elements, so that the iron is really a carrier of oxygen from
air to impurity. The typical reactions are something like the following:
Fe3O4 + 4C = 4CO + 3Fe; Fe3O4 + C = 3FeO + CO; 2P + 5Fe3O4 = 12FeO +
3FeO,P2O5; Si + 2Fe3O4 = 3FeO,SiO2 + 3FeO. Beside this their chief and
easy work of oxidizing carbon, silicon and phosphorus, the conversion
processes have the harder task of removing sulphur, chiefly by
converting it into calcium sulphide, CaS, or manganous sulphide, MnS,
which rise to the top of the molten metal and there enter the overlying
slag, from which the sulphur may escape by oxidizing to the gaseous
compound, sulphurous acid, SO2.
79. In the _puddling process_ molten cast iron is converted into wrought
iron, i.e. low-carbon slag-bearing iron, by oxidizing its carbon,
silicon and phosphorus, by means of iron oxide stirred into it as it
lies in a thin shallow layer in the "hearth" or flat basin of a
reverberatory furnace (fig. 14), itself lined with iron ore. As the iron
oxide is stirred into the molten metal laboriously by the workman or
"puddler" with his hook or "rabble," it oxidizes the silicon to silica
and the phosphorus to phosphoric acid, and unites with both these
products, forming with them a basic iron silicate rich in phosphorus,
called "puddling" or "tap cinder." It oxidizes the carbon also, which
escapes in purple jets of burning carbonic oxide. As the melting point
of the metal is gradually raised by the progressive decarburization, it
at length passes above the temperature of the furnace, about 1400 deg.
C., with the consequence that the metal, now below its melting point,
solidifies in pasty grains, or "comes to nature." These grains the
puddler welds together by means of his rabble into rough 80-lb. balls,
each like a sponge of metallic iron particles with its pores filled with
the still molten cinder. These balls are next worked into merchantable
shape, and the cinder is simultaneously expelled in large part, first by
hammering them one at a time under a steam hammer (fig. 37) or by
squeezing them, and next by rolling them. The squeezing is usually done
in the way shown in fig. 15.
[Illustration: FIG. 14.--Puddling Furnace.]
[Illustration: FIG. 15.--Plan of Burden's Excentric Revolving Squeezer
for Puddled Balls.]
Here BB is a large fixed iron cylinder, corrugated within, and C an
excentric cylinder, also corrugated, which, in turning to the right,
by the friction of its corrugated surface rotates the puddled ball D
which has just entered at A, so that, turning around its own axis, it
travels to the right and is gradually changed from a ball into a
bloom, a rough cylindrical mass of white hot iron, still dripping with
cinder. This bloom is immediately rolled down into a long flat bar,
called "muck bar," and this in turn is cut into short lengths which,
piled one on another, are reheated and again rolled down, sometimes
with repeated cutting, piling and re-rolling, into the final shape in
which it is actually to be used. But, roll and re-roll as often as we
like, much cinder remains imbedded in the iron, in the form of threads
and rods drawn out in the direction of rolling, and of course
weakening the metal in the transverse direction.
80. _Machine Puddling._--The few men who have, and are willing to
exercise, the great strength and endurance which the puddler needs when
he is stirring the pasty iron and balling it up, command such high
wages, and with their little 500-lb. charges turn out their iron so
slowly, that many ways of puddling by machinery have been tried. None
has succeeded permanently, though indeed one offered by J. P. Roe is not
without promise. The essential difficulty has been that none of them
could subdivide the rapidly solidifying charge into the small balls
which the workman dexterously forms by hand, and that if the charge is
not thus subdivided but drawn as a single ball, the cinder cannot be
squeezed out of it thoroughly enough.
81. _Direct Puddling._--In common practice the cast iron as it runs from
the blast-furnace is allowed to solidify and cool completely in the form
of pigs, which are then graded by their fracture, and remelted in the
puddling furnace itself. At Hourpes, in order to save the expense of
this remelting, the molten cast iron as it comes from the blast-furnace
is poured directly into the puddling furnace, in large charges of about
2200 lb., which are thus about four times as large as those of common
puddling furnaces. These large charges are puddled by two gangs of four
men each, and a great saving in fuel and labour is effected.
Attractive as are these advances in puddling, they have not been
widely adopted, for two chief reasons: First, owners of puddling works
have been reluctant to spend money freely in plant for a process of
which the future is so uncertain, and this unwillingness has been the
more natural because these very men are in large part the more
conservative fraction, which has resisted the temptation to abandon
puddling and adopt the steel-making processes. Second, in puddling
iron which is to be used as a raw material for making very fine steel
by the crucible process, quality is the thing of first importance.
Now in the series of operations, the blast-furnace, puddling and
crucible processes, through which the iron passes from the state of
ore to that of crucible tool steel, it is so difficult to detect just
which are the conditions essential to excellence in the final product
that, once a given procedure has been found to yield excellent steel,
every one of its details is adhered to by the more cautious
ironmasters, often with surprising conservatism. Buyers of certain
excellent classes of Swedish iron have been said to object even to the
substitution of electricity for water-power as a means of driving the
machinery of the forge. In case of direct puddling and the use of
larger charges this conservatism has some foundation, because the
established custom of allowing the cast iron to solidify gives a
better opportunity of examining its fracture, and thus of rejecting
unsuitable iron, than is afforded in direct puddling. So, too, when
several puddlers are jointly responsible for the thoroughness of their
work, as happens in puddling large charges, they will not exercise
such care (nor indeed will a given degree of care be so effective) as
when responsibility for each charge rests on one man.
82. The _removal of phosphorus_, a very important duty of the puddling
process, requires that the cinder shall be "basic," i.e. that it shall
have a great excess of the strong base, ferrous oxide, FeO, for the
phosphoric acid to unite with, lest it be deoxidized by the carbon of
the iron as fast as it forms, and so return to the iron, following the
general rule that oxidized bodies enter the slag and unoxidized ones the
metallic iron. But this basicity implies that for each part of the
silica or silicic acid which inevitably results from the oxidation of
the silicon of the pig iron, the cinder shall contain some three parts
of iron oxide, itself a valuable and expensive substance. Hence, in
order to save iron oxide the pig iron used should be nearly free from
silicon. It should also be nearly free from sulphur, because of the
great difficulty of removing this element in the puddling process. But
the strong deoxidizing conditions needed in the blast-furnace to remove
sulphur tend strongly to deoxidize silica and thus to make the pig iron
rich in silicon.
83. The _"refinery process"_ of fitting pig iron for the puddling
process by removing the silicon without the carbon, is sometimes used
because of this difficulty in making a pig iron initially low in both
sulphur and silicon. In this process molten pig iron with much silicon
but little sulphur has its silicon oxidized to silica and thus slagged
off, by means of a blast of air playing on the iron through a blanket of
burning coke which covers it. The coke thus at once supplies by its
combustion the heat needed for melting the iron and keeping it hot, and
by itself dissolving in the molten metal returns carbon to it as fast as
this element is burnt out by the blast, so that the "refined" cast iron
which results, though still rich in carbon and therefore easy to melt in
the puddling process, has relatively little silicon.
84. In the _Bessemer or "pneumatic" process_, which indeed might be
called the "fuel-less" process, molten pig iron is converted into steel
by having its carbon, silicon and manganese, and often its phosphorus
and sulphur, oxidized and thus removed by air forced through it in so
many fine streams and hence so rapidly that the heat generated by the
oxidation of these impurities suffices in and by itself, unaided by
burning any other fuel, not only to keep the iron molten, but even to
raise its temperature from a point initially but little above the
melting point of cast iron, say 1150 deg. to 1250 deg. C., to one well
above the melting point of the resultant steel, say 1500 deg. C. The
"Bessemer converter" or "vessel" (fig. 16) in which this wonderful
process is carried out is a huge retort, lined with clay, dolomite or
other refractory material, hung aloft and turned on trunnions, DD,
through the right-hand one of which the blast is carried to the
gooseneck E, which in turn delivers it to the tuyeres Q at the bottom.
There are two distinct varieties of this process, the original
undephosphorizing or "acid" Bessemer process, so called because the
converter is lined with acid materials, i.e. those rich in silicic acid,
such as quartz and clay, and because the slag is consequently acid, i.e.
siliceous; and the dephosphorizing or "Thomas" or "basic Bessemer"
process, so called because the converter is lined with basic materials,
usually calcined dolomite, a mixture of lime and magnesia, bound
together with tar, and because the slag is made very basic by adding
much lime to it. In the basic Bessemer process phosphorus is readily
removed by oxidation, because the product of its oxidation, phosphoric
acid, P2O5, in the presence of an excess of base forms stable phosphates
of lime and iron which pass into the slag, making it valuable as an
artificial manure. But this dephosphorization by oxidation can be
carried out only in the case slag is basic. If it is acid, i.e. if it
holds much more than 20% of so powerful an acid as silica, then the
phosphoric acid has so feeble a hold on the base in the slag that it is
immediately re-deoxidized by the carbon of the metal, or even by the
iron itself, P2O5 + 5Fe = 2P + 5FeO, and the resultant deoxidized
phosphorus immediately recombines with the iron. Now in an acid-lined
converter the slag is necessarily acid, because even an initially basic
slag would immediately corrode away enough of the acid lining to make
itself acid. Hence phosphorus cannot be removed in an acid-lined
converter. Though all this is elementary to-day, not only was it
unknown, indeed unguessed, at the time of the invention of the Bessemer
process, but even when, nearly a quarter of a century later, a young
English metallurgical chemist, Sidney Gilchrist Thomas (1850-1885),
offered to the British Iron and Steel Institute a paper describing his
success in dephosphorizing by the Bessemer process with a basic-lined
converter and a basic slag, that body rejected it.
[Illustration: FIG. 16.--12-15 ton Bessemer Converter.
A, Trunnion-ring. O, Tuyere-plate.
B, Main shell. P, False plate.
C, Upper part of shell. Q, Tuyeres.
D, Trunnions. R, Keys holding lid of tuyere-box.
E, Goose-neck. S, Refractory lining.
F, Tuyere-box. U, Key-link holding bottom.
N, Lid of tuyere-box.]
[Illustration: FIG. 17.--Bessemer Converter, turned down in position to
receive and discharge the molten metal.]
85. In carrying out the acid Bessemer process, the converter, preheated
to about 1200 deg. C. by burning coke in it, is turned into the position
shown in fig. 17, and the charge of molten pig iron, which sometimes
weighs as much as 20 tons, is poured into it through its mouth. The
converter is then turned upright into the position shown in fig. 16, so
that the blast, which has been let on just before this, entering through
the great number of tuyere holes in the bottom, forces its way up
through the relatively shallow layer of iron, throwing it up within the
converter as a boiling foam, and oxidizing the foreign elements so
rapidly that in some cases their removal is complete after 5 minutes.
The oxygen of the blast having been thus taken up by the molten metal,
its nitrogen issues from the mouth of the converter as a pale
spark-bearing cone. Under normal conditions the silicon oxidizes first.
Later, when most of it has been oxidized, the carbon begins to oxidize
to carbonic oxide, which in turn burns to carbonic acid as it meets the
outer air on escaping from the mouth of the converter, and generates a
true flame which grows bright, then brilliant, then almost blinding, as
it rushes and roars, then "drops," i.e. shortens and suddenly grows
quiet when the last of the carbon has burnt away, and no flame-forming
substance remains. Thus may a 20-ton charge of cast iron be converted
into steel in ten minutes.[4] It is by the appearance of the flame that
the operator or "blower" knows when to end the process, judging by its
brilliancy, colour, sound, sparks, smoke and other indications.
86. _Recarburizing._--The process may be interrupted as soon as the
carbon-content has fallen to that which the final product is to have, or
it may be continued till nearly the whole of the carbon has been burned
out, and then the needed carbon may be added by "recarburizing." The
former of these ways is followed by the very skilful and intelligent
blowers in Sweden, who, with the temperature and all other conditions
well under control, and with their minds set on the quality rather than
on the quantity of their product, can thus make steel of any desired
carbon-content from 0.10 to 1.25%. But even with all their skill and
care, while the carbon-content is still high the indications of the
flame are not so decisive as to justify them in omitting to test the
steel before removing it from the converter, as a check on the accuracy
of their blowing. The delay which this test causes is so unwelcome that
in all other countries the blower continues the blow until
decarburization is nearly complete, because of the very great accuracy
with which he can then read the indications of the flame, an accuracy
which leaves little to be desired. Then, without waiting to test the
product, he "recarburizes" it, i.e. adds enough carbon to give it the
content desired, and then immediately pours the steel into a great
clay-lined casting ladle by turning the converter over, and through a
nozzle in the bottom of this ladle pours the steel into its ingot
moulds. In making very low-carbon steel this recarburizing proper is not
needed; but in any event a considerable quantity of manganese must be
added unless the pig iron initially contains much of that metal, in
order to remove from the molten steel the oxygen which it has absorbed
from the blast, lest this make it redshort. If the carbon-content is not
to be raised materially, this manganese is added in the form of
preheated lumps of "ferro-manganese," which contains about 80% of
manganese, 5% of carbon and 15% of iron, with a little silicon and other
impurities. If, on the other hand, the carbon-content is to be raised,
then carbon and manganese are usually added together in the form of a
manganiferous molten pig iron, called spiegeleisen, i.e. "mirror-iron,"
from the brilliancy of its facets, and usually containing somewhere
about 12% of manganese and 4% of carbon, though the proportion between
these two elements has to be adjusted so as to introduce the desired
quantity of each into the molten steel. Part of the carbon of this
spiegeleisen unites with the oxygen occluded in the molten iron to form
carbonic oxide, and again a bright flame, greenish with manganese,
escapes from the converter.
87. _Darby's Process._--Another way of introducing the carbon is Darby's
process of throwing large paper bags filled with anthracite, coke or
gas-carbon into the casting ladle as the molten steel is pouring into
it. The steel dissolves the carbon of this fuel even more quickly than
water would dissolve salt under like conditions.
88. _Bessemer and Mushet._--Bessemer had no very wide knowledge of
metallurgy, and after overcoming many stupendous difficulties he was
greatly embarrassed by the brittleness or "redshortness" of his steel,
which he did not know how to cure. But two remedies were quickly
offered, one by the skilful Swede, Goransson, who used a pig iron
initially rich in manganese and stopped his blow before much oxygen had
been taken up; and the other by a British steel maker, Robert Mushet,
who proposed the use of the manganiferous cast iron called spiegeleisen,
and thereby removed the only remaining serious obstacle to the rapid
spread of the process.
From this many have claimed for Mushet a part almost or even quite
equal to Bessemer's in the development of the Bessemer process, even
calling it the "Bessemer-Mushet process." But this seems most unjust.
Mushet had no such exclusive knowledge of the effects of manganese
that he alone could have helped Bessemer; and even if nobody had then
proposed the use of spiegeleisen, the development of the Swedish
Bessemer practice would have gone on, and, the process thus
established and its value and great economy thus shown in Sweden, it
would have been only a question of time how soon somebody would have
proposed the addition of manganese. Mushet's aid was certainly
valuable, but not more than Goransson's, who, besides thus offering a
preventive of redshortness, further helped the process on by raising
its temperature by the simple expedient of further subdividing the
blast, thus increasing the surface of contact between blast and metal,
and thus in turn hastening the oxidation. The two great essential
discoveries were first that the rapid passage of air through molten
cast iron raised its temperature above the melting point of low-carbon
steel, or as it was then called "malleable iron," and second that this
low-carbon steel, which Bessemer was the first to make in important
quantities, was in fact an extraordinarily valuable substance when
made under proper conditions.
89. _Source of Heat._--The carbon of the pig iron, burning as it does
only to carbonic oxide within the converter, does not by itself generate
a temperature high enough for the needs of the process. The oxidation of
manganese is capable of generating a very high temperature, but it has
the very serious disadvantage of causing such thick clouds of smoky
oxide of manganese as to hide the flame from the blower, and prevent him
from recognizing the moment when the blow should be ended. Thus it comes
about that the temperature is regulated primarily by adjusting the
quantity of silicon in the pig iron treated, 1(1/4)% of this element
usually sufficing. If any individual blow proves to be too hot, it may
be cooled by throwing cold "scrap" steel such as the waste ends of rails
and other pieces, into the converter, or by injecting with the blast a
little steam, which is decomposed by the iron by the endothermic
reaction H2O + Fe = 2H + FeO. If the temperature is not high enough, it
is raised by managing the blast in such a way as to oxidize some of the
iron itself permanently, and thus to generate much heat.
90. The _basic_ or dephosphorizing variety of the Bessemer process,
called in Germany the "Thomas" process, differs from the acid process in
four chief points: (1) that its slag is made very basic and hence
dephosphorizing by adding much lime to it; (2) that the lining is basic,
because an acid lining would quickly be destroyed by such a basic slag;
(3) that the process is arrested not at the "drop of the flame" (S85)
but at a predetermined length of time after it; and (4) that phosphorus
instead of silicon is the chief source of heat. Let us consider these in
turn.
91. The _slag_, in order that it may have such an excess of base that
this will retain the phosphoric acid as fast as it is formed by the
oxidation of the phosphorus of the pig iron, and prevent it from being
re-deoxidized and re-absorbed by the iron, should, according to von
Ehrenwerth's rule which is generally followed, contain enough lime to
form approximately a tetra-calcic silicate, 4CaO,SiO2 with the silica
which results from the oxidation of the silicon of the pig iron and
tri-calcic phosphate, 3CaO,P2O5, with the phosphoric acid which forms.
The danger of this "rephosphorization" is greatest at the end of the
blow, when the recarburizing additions are made. This lime is charged in
the form of common quicklime, CaO, resulting from the calcination of a
pure limestone, CaCO3, which should be as free as possible from silica.
The usual composition of this slag is iron oxide, 10 to 16%; lime, 40 to
50%; magnesia, 5%; silica, 6 to 9%; phosphoric acid, 16 to 20%. Its
phosphoric acid makes it so valuable as a fertilizer that it is a most
important by-product. In order that the phosphoric acid may be the more
fully liberated by the humic acid, &c., of the earth, a little silicious
sand is mixed with the still molten slag after it has been poured off
from the molten steel. The slag is used in agriculture with no further
preparation, save very fine grinding.
92. The _lining of the converter_ is made of 90% of the mixture of lime
and magnesia which results from calcining dolomite, (Ca,Mg)CO3, at a
very high temperature, and 10% of coal tar freed from its water by
heating. This mixture may be rammed in place, or baked blocks of it may
be laid up like a masonry wall. In either case such a lining is
expensive, and has but a short life, in few works more than 200 charges,
and in some only 100, though the silicious lining of the acid converter
lasts thousands of charges. Hence, for the basic process, spare
converters must be provided, so that there may always be some of them
re-lining, either while standing in the same place as when in use, or,
as in Holley's arrangement, in a separate repair house, to which these
gigantic vessels are removed bodily.
93. _Control of the Basic Bessemer Process._--The removal of the greater
part of the phosphorus takes place after the carbon has been oxidized
and the flame has consequently "dropped," probably because the lime,
which is charged in solid lumps, is taken up by the slag so slowly that
not until late in the operation does the slag become so basic as to be
retentive of phosphoric acid. Hence in making steel rich in carbon it is
not possible, as in the acid Bessemer process, to end the operation as
soon as the carbon in the metal has fallen to the point sought, but it
is necessary to remove practically all of the carbon, then the
phosphorus, and then "recarburize," i.e. add whatever carbon the steel
is to contain. The quantity of phosphorus in the pig iron is usually
known accurately, and the dephosphorization takes place so regularly
that the quantity of air which it needs can be foretold closely. The
blower therefore stops the process when he has blown a predetermined
quantity of air through, counting from the drop of the flame; but as a
check on his forecast he usually tests the blown metal before
recarburizing it.
94. _Source of Heat._--Silicon cannot here be used as the chief source
of heat as it is in the acid Bessemer process, because most of the heat
which its oxidation generates is consumed in heating the great
quantities of lime needed for neutralizing the resultant silica.
Fortunately the phosphorus, turned from a curse into a blessing,
develops by its oxidation the needed temperature, though the fact that
this requires at least 1.80% of phosphorus limits the use of the
process, because there are few ores which can be made to yield so
phosphoric a pig iron. Further objections to the presence of silicon are
that the resultant silica (1) corrodes the lining of the converter, (2)
makes the slag froth so that it both throws much of the charge out and
blocks up the nose of the converter, and (3) leads to rephosphorization.
These effects are so serious that until very lately it was thought that
the silicon could not safely be much in excess of 1%. But Massenez and
Richards, following the plan outlined by Pourcel in 1879, have found
that even 3% of silicon is permissible if, by adding iron ore, the
resultant silica is made into a fluid slag, and if this is removed in
the early cool part of the process, when it attacks the lining of the
converter but slightly. Manganese to the extent of 1.80% is desired as a
means of preventing the resultant steel from being redshort, i.e.
brittle at a red or forging heat. The pig iron should be as nearly free
as possible from sulphur, because the removal of any large quantity of
this injurious element in the process itself is both difficult and
expensive.
95. The _car casting_ system deserves description chiefly because it
shows how, when the scale of operations is as enormous as it is in the
Bessemer process, even a slight simplification and a slight
heat-saving may be of great economic importance.
Whatever be the form into which the steel is to be rolled, it must in
general first be poured from the Bessemer converter in which it is
made into a large clay-lined ladle, and thence cast in vertical
pyramidal ingots. To bring them to a temperature suitable for rolling,
these ingots must be set in heating or soaking furnaces (S 125), and
this should be done as soon as possible after they are cast, both to
lessen the loss of their initial heat, and to make way for the next
succeeding lot of ingots, a matter of great importance, because the
charges of steel follow each other at such very brief intervals. A
pair of working converters has made 4958 charges of 10 tons each, or a
total of 50,547 tons, in one month, or at an average rate of a charge
every seven minutes and twenty-four seconds throughout every working
day. It is this extraordinary rapidity that makes the process so
economical and determines the way in which its details must be carried
out. Moreover, since the mould acts as a covering to retard the loss
of heat, it should not be removed from the ingot until just before the
latter is to be placed in its soaking furnace. These conditions are
fulfilled by the car casting system of F. W. Wood, of Sparrows Point,
Md., in which the moulds, while receiving the steel, stand on a train
of cars, which are immediately run to the side of the soaking furnace.
Here, as soon as the ingots have so far solidified that they can be
lifted without breaking, their moulds are removed and set on an
adjoining train of cars, and the ingots are charged directly into the
soaking furnace. The mould-train now carries its empty moulds to a
cooling yard, and, as soon as they are cool enough to be used again,
carries them back to the neighbourhood of the converters to receive a
new lot of steel. In this system there is for each ingot and each
mould only one handling in which it is moved as a separate unit, the
mould from one train to the other, the ingot from its train into the
furnace. In the other movements, all the moulds and ingots of a given
charge of steel are grouped as a train, which is moved as a unit by a
locomotive. The difficulty in the way of this system was that, in
pouring the steel from ladle to mould, more or less of it occasionally
spatters, and these spatterings, if they strike the rails or the
running gear of the cars, obstruct and foul them, preventing the
movement of the train, because the solidified steel is extremely
tenacious. But this cannot be tolerated, because the economy of the
process requires extreme promptness in each of its steps. On account
of this difficulty the moulds formerly stood, not on cars, but
directly on the floor of a casting pit while receiving the molten
steel. When the ingots had so far solidified that they could be
handled, the moulds were removed and set on the floor to cool, the
ingots were set on a car and carried to the soaking furnace, and the
moulds were then replaced in the casting pit. Here each mould and each
ingot was handled as a separate unit twice, instead of only once as in
the car casting system; the ingots radiated away great quantities of
heat in passing naked from the converting mill to the soaking
furnaces, and the heat which they and the moulds radiated while in the
converting mill was not only wasted, but made this mill, open-doored
as it was, so intolerably hot, that the cost of labour there was
materially increased. Mr Wood met this difficulty by the simple device
of so shaping the cars that they completely protect both their own
running gear and the track from all possible spattering, a device
which, simple as it is, has materially lessened the cost of the steel
and greatly increased the production. How great the increase has been,
from this and many other causes, is shown in Table III.
TABLE III.--_Maximum Production of Ingots by a Pair of American
Converters._
Gross Tons per Week.
1870 254
1880 3,433
1889 8,549
1899 (average for a month) 11,233
1903 15,704
Thus in thirty-three years the rate of production per pair of vessels
increased more than sixty-fold. The production of European Bessemer
works is very much less than that of American. Indeed, the whole
German production of acid Bessemer steel in 1899 was at a rate but
slightly greater than that here given for one pair of American
converters; and three pairs, if this rate were continued, would make
almost exactly as much steel as all the sixty-five active British
Bessemer converters, acid and basic together, made in 1899.
96. _Range in Size of Converters._--In the Bessemer process, and
indeed in most high-temperature processes, to operate on a large scale
has, in addition to the usual economies which it offers in other
industries, a special one, arising from the fact that from a large hot
furnace or hot mass in general a very much smaller proportion of its
heat dissipates through radiation and like causes than from a smaller
body, just as a thin red-hot wire cools in the air much faster than a
thick bar equally hot. Hence the progressive increase which has
occurred in the size of converters, until now some of them can treat a
20-ton charge, is not surprising. But, on the other hand, when only a
relatively small quantity of a special kind of steel is needed, very
much smaller charges, in some cases weighing even less than half a
ton, have been treated with technical success.
97. _The Bessemer Process for making Steel Castings._--This has been
particularly true in the manufacture of steel castings, i.e. objects
usually of more or less intricate shape, which are cast initially in
the form in which they are to be used, instead of being forged or
rolled to that form from steel cast originally in ingots. For making
castings, especially those which are so thin and intricate that, in
order that the molten steel may remain molten long enough to run into
the thin parts of the mould, it must be heated initially very far
above its melting-point, the Bessemer process has a very great
advantage in that it can develop a much higher temperature than is
attainable in either of its competitors, the crucible and the
open-hearth processes. Indeed, no limit has yet been found to the
temperature which can be reached, if matters are so arranged that not
only the carbon and silicon of the pig iron, but also a considerable
part of the metallic iron which is the iron itself, are oxidized by
the blast; or if, as in the Walrand-Legenisel modification, after the
combustion of the initial carbon and silicon of the pig iron has
already raised the charge to a very high temperature, a still further
rise of temperature is brought about by adding more silicon in the
form of ferro-silicon, and oxidizing it by further blowing. But in the
crucible and the open-hearth processes the temperature attainable is
limited by the danger of melting the furnace itself, both because some
essential parts of it, which, unfortunately, are of a destructible
shape, are placed most unfavourably in that they are surrounded by the
heat on all sides, and because the furnace is necessarily hotter than
the steel made within it. But no part of the Bessemer converter is of
a shape easily affected by the heat, no part of it is exposed to the
heat on more than one side, and the converter itself is necessarily
cooler than the metal within it, because the heat is generated within
the metal itself by the combustion of its silicon and other calorific
elements. In it the steel heats the converter, whereas in the
open-hearth and crucible processes the furnace heats the steel.
98. The _open-hearth process_ consists in making molten steel out of pig
or cast iron and "scrap," i.e. waste pieces of steel and iron melted
together on the "open hearth," i.e. the uncovered basin-like bottom of a
reverberatory furnace, under conditions of which fig. 18 may give a
general idea. The conversion of cast iron into steel, of course,
consists in lessening its content of the several foreign elements,
carbon, silicon, phosphorus, &c. The open-hearth process does this by
two distinct steps: (1) by oxidizing and removing these elements by
means of the flame of the furnace, usually aided by the oxygen of light
charges of iron ore, and (2) by diluting them with scrap steel or its
equivalent. The "pig and ore" or "Siemens" variety of the process works
chiefly by oxidation, the "pig and scrap" or "Siemens-Martin" variety
chiefly by dilution, sometimes indeed by extreme dilution, as when 10
parts of cast iron are diluted with 90 parts of scrap. Both varieties
may be carried out in the basic and dephosphorizing way, i.e. in
presence of a basic slag and in a basic- or neutral-lined furnace; or in
the acid and undephosphorizing way, in presence of an acid, i.e.
silicious slag, and in a furnace with a silicious lining.
[Illustration: FIG. 18.--Open-Hearth Process.
Half Section showing condition of charge when boiling very gently.
Half Section showing condition of charge when boiling violently during
oreing.]
The charge may be melted down on the "open hearth" itself, or, as in the
more advanced practice, the pig iron may be brought in the molten state
from the blast furnace in which it is made. Then the furnaceman,
controlling the decarburization and purification of the molten charge by
his examination of test ingots taken from time to time, gradually
oxidizes and so removes the foreign elements, and thus brings the metal
simultaneously to approximately the composition needed and to a
temperature far enough above its present melting-point to permit of its
being cast into ingots or other castings. He then pours or taps the
molten charge from the furnace into a large clay-lined casting ladle,
giving it the final additions of manganese, usually with carbon and
often with silicon, needed to give it exactly the desired composition.
He then casts it into its final form through a nozzle in the bottom of
the casting ladle, as in the Bessemer process.
The oxidation of the foreign elements must be very slow, lest the
effervescence due to the escape of carbonic oxide from the carbon of the
metal throw the charge out of the doors and ports of the furnace, which
itself must be shallow in order to hold the flame down close to the
charge. It is in large part because of this shallowness, which contrasts
so strongly with the height and roominess of the Bessemer converter,
that the process lasts hours where the Bessemer process lasts minutes,
though there is the further difference that in the open-hearth process
the transfer of heat from flame to charge through the intervening layer
of slag is necessarily slow, whereas in the Bessemer process the heat,
generated as it is in and by the metallic bath itself, raises the
temperature very rapidly. The slowness of this rise of the temperature
compels us to make the removal of the carbon slow for a very simple
reason. That removal progressively raises the melting-point of the
metal, after line Aa of Fig. 1, i.e. makes the charge more and more
infusible; and this progressive rise of the melting-point of the charge
must not be allowed to outrun the actual rise of temperature, or in
other words the charge must always be kept molten, because once
solidified it is very hard to remelt. Thus the necessary slowness of the
heating up of the molten charge would compel us to make the removal of
the carbon slow, even if this slowness were not already forced on us by
the danger of having the charge froth so much as to run out of the
furnace.
The general plan of the open-hearth process was certainly conceived by
Josiah Marshall Heath in 1845, if not indeed by Reaumur in 1722, but for
lack of a furnace in which a high enough temperature could be generated
it could not be carried out until the development of the Siemens
regenerative gas furnace about 1860. It was in large part through the
efforts of Le Chatelier that this process, so long conceived, was at
last, in 1864, put into actual use by the brothers Martin, of Sireuil in
France.