Chapter XIII: Part 13

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."

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.

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.

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.

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.

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.

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.]

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.