A. Addition of oxygen to the charge previous to the blast furnace
smelting operation (_Roasting_).
B. Addition of oxygen to the charge during the smelting operation
itself.
i. By adding oxidised materials to the charge
(_Blast-furnace smelting with carbonaceous fuel_).
ii. By using the air blast of the furnace for
oxidising the iron and sulphur, thus at the
same time utilising these elements as fuel
and proportionately diminishing the amount of
carbonaceous fuel required (_The pyritic principle
of smelting_).
A. _Roasting practice_ has already been discussed, and the reasons for
avoiding the operation where practicable, on account of the expenses of
an extra process, the losses involved, the fineness of the product, and
the loss of fuel values, have been indicated (Lecture IV., pp. 66–80).
B. i. _Addition of Oxidised Charges in the Blast Furnace._—The tendency
for oxidised cupriferous materials to interact with sulphides
finds useful application in copper smelting, since it assists the
concentration of the copper in the resulting mattes. The principal
reactions involved in this method are—
2CuO + Cu_{2}S ➡ 4Cu + SO_{2}
2Cu_{2}O + Cu_{2}S ➡ 6Cu + SO_{2}
CuSO_{4} + Cu_{2}S ➡ 3Cu + 2SO_{2}
whereby copper is produced and sulphur is eliminated as SO_{2}. The
liberated copper interacts with the excess of iron sulphide usually
present in the furnace charge, and enters the matte as sulphide, whilst
the iron which is thus set free is oxidised and carried into the slag
as silicate, the ultimate reactions being indicated approximately by
the equation—
2Cu + FeS + _x_FeS ➡ Cu_{2}S . _x_FeS (matte) + Fe
(oxidised and enters slag).
Copper silicates readily interact with iron sulphides in the charge,
producing copper sulphides and iron silicates, thus—
Cu_{2}O . _x_SiO_{2} + FeS ➡ Cu_{2}S (enters matte)
+ FeO . _x_SiO_{2} (enters slag).
6(CuO . _x_SiO_{2}) + 4FeS ➡ 3Cu_{2}S (enters matte)
+ 4(FeO . _x_SiO_{2}) + 2_x_SiO_{2}(enters slag) + SO_{2}.
All the above reactions lead to an enrichment of the matte in copper
contents, and at the same time, to the transference of iron from the
matte to the slag, and although the conditions in the more reducing
atmosphere of the coke-fed blast furnace are not so favourable to the
fullest operation of these reactions as are the more neutral conditions
of the reverberatory, the addition of oxidised materials constitutes
a valuable means of increasing the concentration in this method of
smelting.
The blast furnace is thus also particularly suited for the recovering
of the copper from the oxidised residues, such as converter slags and
scrap, “calcine-barrings,” and the like, which accumulate in very
considerable quantities at a smelter, and which by reason of their
carrying much copper as oxide or silicate, not only add their quota of
copper to the products, but materially assist the concentration and the
furnace operation generally.
B. ii. _The Pyritic Principle in Blast-Furnace Smelting._—This is the
most important principle introduced into modern blast-furnace smelting
practice.
It has been evolved by the application of the results of experiments
conducted from two different points of view—one series mainly on a
laboratory scale, the other from actual industrial practice.
Starting from theoretical considerations, John Holway demonstrated
by experiment that the heat of oxidation of the iron and the sulphur
of pyritic copper ores was so great as to make their smelting a
self-supporting operation under suitable conditions. On the other hand,
within comparatively recent years, smeltermen as a result of working
practice, have found that an increase of sulphides on the furnace
charge has led to less and less carbonaceous fuel being necessary for
the smelting operations, providing that the conditions in the blast
furnace be sufficiently oxidising.
In utilising these results for general blast-furnace practice, the
extended and successful application of this pyritic principle has led
to marked advance in modern working.
The results obtained in a series of trials at the Keswick smelter,
California, are typical of such experiments on a practical scale, and
in spite of the two anomalous instances, the general effects of the
increase of sulphides in the charge are strongly marked (see Table IX.,
p. 120).
TABLE IX.—EFFECT ON COKE CONSUMPTION OF INCREASED SULPHUR IN THE
FURNACE CHARGE (Keswick Smelter, Cal.).
┌────────────────────┬───────────────────┐
│ Sulphur in Charge. │ Coke Consumption. │
├────────────────────┼───────────────────┤
│ 6·8 per cent. │ 15·7 per cent. │
│ 7·7 " │ 16·3 " │
│ 13·6 " │ 10·2 " │
│ 17·0 " │ 7·7 " │
│ 19·5 " │ 8·5 " │
│ 22·8 " │ 7·1 " │
│ 24·5 " │ 6·8 " │
└────────────────────┴───────────────────┘
Recent practice at Anaconda affords another instance of the utilisation
of the pyritic principle. A large quantity of the ore available (known
as second-class ore) requires wet dressing before it can be treated
most profitably at the furnaces, and the operation thus produces
considerable quantities of sulphide concentrate, of which a moderate
proportion is coarse—well suited for blast-furnace treatment. The
charge if submitted to reduction smelting with carbonaceous fuel,
would yield a matte too low in copper contents for immediate converter
treatment, since there is not available a sufficient supply of oxidised
cupriferous material to effect a high enough concentration for the
direct production of a converter-grade matte. Instead of roasting so
as to reduce the sulphur contents to the required degree, and then
smelting with the usual amount of carbonaceous fuel, the pyritic
principle has been utilised to the fullest possible extent, by smelting
the raw charge containing as much of the coarse concentrate as is
available, with a strongly oxidising blast, thus effecting the desired
concentration, and occasioning the use of a lower coke proportion
than would otherwise have been necessary. By gradually increasing the
sulphide on the charge until the sulphur proportion reached 8 to 9 per
cent., the coke consumption was reduced to about 11 to 12 per cent.
During the past two or three years the advantages of introducing more
and more sulphide have become so apparent, that increasing quantities
of ⅜ inch concentrates are being included in the charge, and although
such material is exceedingly difficult to deal with in the blast
furnace, the advantages arising from its use outweighs the trouble it
causes in actual working. By this further increase of the sulphur
proportion, from the former 8 to 9 per cent. up to 11 to 12 per cent.,
the coke consumption has been steadily reduced until it now amounts to
about 9 per cent. only.
The fuel value of the iron and sulphur is augmented at a rate much
greater than their actual increase in numerical proportion would
suggest, on account of the much higher calorific intensity of large
and massive quantities of fuel burned at once than that resulting from
smaller amounts disseminated throughout a mass of inert material such
as gangue.
The practical application of the pyritic principle to blast-furnace
practice thus involves the employment of the furnace as a medium
for conducting the required oxidation of the charge, as a result of
which, the heat of this combustion proportionately reduces the amount
of carbonaceous fuel required for the smelting and separation of
the products, whilst at the same time the desired concentration is
also effected. The basis of such working is, therefore, the powerful
oxidising action within the furnace itself, and the fullest utilisation
of the heat resulting from this oxidation of the sulphides.
In order to supply the heat necessary for the reactions and fusions
of smelting, a definite quantity of fuel is essential in the furnace.
In those cases where the proportions of sulphide are not sufficient
to supply the required amount, a supplementary quantity of coke fuel
becomes requisite.
The extent to which coke is necessary for the smelting operations
decides whether the process may be termed “true pyritic” or “partial
pyritic” smelting. In the former case, the coke allowance may be
reduced to such small proportions that its influence in the smelting
zone of the furnace is practically negligible.
In partial pyritic smelting, coke is necessary to the extent of
supplementing the heat derived from the sulphide fuel, and the
proportion employed in modern work is reduced to the lowest possible
quantity. Not only is economy in coke allowance one of the chief
essentials in furnace management, but the presence of a larger amount
than is absolutely necessary decreases the efficiency of the smelting
operations, since, owing to its reducing action and its consumption of
the oxygen in the air blast which is to be utilised for the combustion
of the iron and sulphur, the concentration of the copper in the
resulting matte would be decreased.
The extent to which the pyritic principle may be operated in actual
working depends in the first instance upon the nature of the charge
itself, especially upon the relative proportions of copper, iron, and
sulphur, and on the quantity of gangue. Since these vary in the ore
supply of different localities, the extent to which the principle may
be applied and the coke consumption be reduced, will be subject to
alteration accordingly.
Thus in the case of an ore which contains such proportions of these
constituents as would on simple melting yield a matte of converter
grade, the pyritic effect in the furnace would necessarily be very
small, and the smelting would be almost entirely a melting operation
requiring from 10 to 15 per cent. of coke on the charge, even though
the sulphur contents of the charge be high. Ores and charges of such a
composition are, however, rarely met with in modern practice, the ratio
of copper to iron sulphides usually being low.
On the other hand, in the case of an ore consisting largely of iron
sulphides with but little copper—_i.e._, a massive low-grade pyritic
ore—the pyritic effect in the furnace might reach a maximum, and the
coke required on the charge be reducible to very small proportions.
Such material is well suited for true pyritic smelting.
Hence modern practice ranges from the true pyritic smelting, where
pyritic fuel is principally employed, through varying degrees of
partial pyritic smelting, where the pyritic fuel is supplemented to
the required degree by coke, to reduction smelting, relying mainly on
carbonaceous fuel for the necessary heat supply.
In all cases, the object of the operation is to oxidise inside the
furnace so much sulphur and iron as is necessary to yield a matte
product of converter grade, utilising the natural sulphide fuel values
of the material so as to reduce to the lowest possible proportion the
quantity of coke required.
=Features of Modern Practice.=—Apart from the applications of pyritic
smelting, which will be considered separately, three features of great
importance have been introduced into modern blast-furnace working.
These involve:—
A. The practice of water-jacketing the furnace.
B. The development in the size of the furnace.
C. The practice of external settling.
[Illustration: Fig. 32.—Modern Blast-Furnace Shell of Sectioned Jackets
(P. & M. M. Co.).]
A. _The Practice of Water-jacketing._—The evolution of the blast
furnace from the primitive hole-in-the-ground form to the modern type
may be rapidly sketched. In its early stages, the development was
carried out mainly on the Continent of Europe, following the course
of the enclosing of the charge in shafts which became of gradually
increasing height, the introduction of blast through tuyeres near
the bottom of the shaft, and the arrangements for collecting the
molten materials in the hearth, and for tapping. By the year 1850
a typical form of furnace was represented by the Mansfeld pattern,
which consisted of a rectangular firebrick shaft enclosed by massive
stonework. At the lower extremity was a hearth constructed of
refractory material, usually of brasque—a mixture of fireclay and
coke—well tamped down. The dimensions were from about 2 feet to 2 feet
6 inches broad, 14 feet to 16 feet high, with two tuyeres of 1½ to 2
inches diameter, supplying blast at 4 to 10 inches water pressure; the
capacity of such a furnace being about 4 tons per twenty-four hours. It
is of interest to note that this form of furnace possessed arrangements
both for internal or external settling of the products, the usual
practice being, however, to allow the smelted material to collect and
settle in the hearth. In endeavouring to increase the capacity of the
furnace and the rapidity of working, as well as to ensure efficient
settling of the products, it became necessary to maintain a high
temperature in the lower parts; but in consequence of the excessive
heat and the corrosive nature of the molten materials, the most
refractory brasquing available was rapidly attacked, and the necessity
for adopting means to prevent the destruction of the furnace linings
became apparent.
The use of water-jacketing for this purpose had long before been
applied to certain branches of cast-iron refining, and in 1875 the
Piltz water-jacketed blast furnace was introduced for the smelting of
lead ores. This form of furnace was circular in horizontal section, and
the boshes consisted of two concentric shells between which a stream of
water circulated. This principle was quickly adopted for the purposes
of copper smelting furnaces, although modifications were found to be
necessary in certain particulars before perfectly successful working
was achieved. Owing to the higher temperatures prevailing in the
furnace, the height to which the water-jackets were carried required
to be increased, and it was chiefly when the rectangular form of
furnace was introduced that the thoroughly successful application of
water-jacketing was accomplished. This feature in blast-furnace work
was rapidly and very successfully developed by the American copper
smelters when the new establishments in the West were opened up, and
the substitution of the older form of lining by metallic water-cooled
jackets, which in comparison are practically indestructible,
immediately led to an enormous improvement in smelting practice.
The modern blast furnace is essentially a water-jacketed shell from
charging floor to base plate, rectangular in plan, and completely
sectionised.
Many of the advantages of such a furnace construction are apparent, and
have been referred to in discussing the furnace as a melting agent. The
salient features of the modern water-jacketed furnace are:—
(i.) Water-jacketed furnaces are planned, constructed,
and erected simply and with ease.
(ii.) The first cost of the furnace, making allowance
for excavation and foundations, is not unfavourable to
the water-jacketed furnace, whilst the ease of fitting
and the interchangeability of parts due to sectioning,
reduce the costs of erection.
(iii.) The convenience and simplicity in operation of
the water-jacketed furnace are very marked, whilst the
permanence in the shape tends to greater uniformity of
working and to ease of management.
(iv.) Accretions and the general difficulties of
working are readily dealt with and controlled, barring
and other operations being more conveniently conducted.
(v.) The repairing of water-jacketed furnaces is
rendered very simple, cheap, and rapid in operation,
the principle of sectionising, allowing of the ready
removal or replacement of the jackets for repairs; the
saving in time, labour, and general expense being
particularly marked.
(vi.) The elasticity of the furnace, both as regards
size and management, has been enormously increased,
and the successful extension and working of the large
modern furnaces have only become possible with the
adoption of this feature.
(vii.) Water-jacketing has allowed of the rapid driving
of furnaces, leading to an enormous increase in the
output per square foot of hearth area, by permitting
intense heating inside the furnace, and rapid
withdrawal of the molten products.
The chief consideration affecting the adoption of water-jacketing
in any locality might be the scarcity or unsuitability of the water
supply, which may necessitate a choice between the employment of brick
furnaces, or the crushing, roasting and reverberatory treatment of the
ore. In cases where the water supply is not well suited for jacketing
purposes, settling or other preliminary treatment of the water might be
required.
The former objection to water-jacketing on the assumption of valuable
heat being carried away by the jacket water, thus involving a waste of
fuel, has proved to be groundless in practice; with good management
such heat losses are smaller in amount and less damaging in effect than
those due to radiation from highly heated brick walls, quite apart from
the actual necessity for such jacketing in modern furnace construction,
even had such losses been marked.
B. _The Development in Furnace Size._—The blast furnace increased
but slowly in size during the nineteenth century up to 1850, and the
dimensions of the most advanced type did not exceed 4 feet by about 2
feet 6 incites internally at the tuyere level, the capacity being about
4 tons per day. Furnaces at this period were usually square or circular
in section.
[Illustration: Fig. 33.—Blast Furnaces under Construction, showing
Fixing of Jackets, Bottom Plate, Method of Support, Sectioning, etc.
(T. E. Co.).]
The size of such furnaces was largely dependent on the penetrating
power of the blast, and a slight increase in cross-section resulted
gradually, as improvements in the mechanical contrivances for producing
blast were developed. This, however, soon reached a limit, owing to
the difficulties in making the blast penetrate to the centre of the
charge in the wider furnaces, and to the disproportionate costliness
and increased working difficulties attendant on such practice. It was
further found that the high pressure required in order to force the
blast through an increased width of charge produced an intense local
heating effect against the tuyeres, resulting in high slag losses and
low concentration on smelting, whilst the consumption of fuel was much
increased.
An important modification in blast-furnace design was introduced in
1863, when the principle of increasing the size of the furnace in
direction of its length, whilst maintaining the width which had been
found best suited to economical working, was applied by Rachette. This
was first intended for the purposes of lead smelting, but the principle
was quickly recognised as having important applications to copper
smelting practice, and was readily adopted and developed. It has become
the basis of all subsequent modern copper blast-furnace design, and the
gradual increase in dimensions up to the enormous blast furnaces with
huge outputs of the present day has been made by extending the length
whilst maintaining a relatively small width.
For some time development proceeded along these lines slowly and with
much caution, chiefly owing to the difficulties anticipated in the
management of such large units. Up to 1885, the largest blast furnace
(at the Parrott Smelter, Butte) was but 8 feet long by 36 inches wide;
by the year 1900 the dimensions had reached 10 feet by 42 inches.
Subsequently, under the direction of the remarkably enterprising
management of the Washoe Smelter at Anaconda, a wonderful era of
furnace extensions was commenced, and is indeed, still undergoing
development.
[Illustration: Fig. 34.—Development of the Blast Furnace (Gowland).]
Here in 1902, blast furnaces 15 feet long by 56 inches wide were
erected, the plant eventually consisting of seven such furnaces built
in a straight line, and situated 21 feet apart from each other. A
largely augmented ore supply subsequently coming to the smelter for
treatment, an increased furnace capacity was required, for which only
a very limited suitable space was available. Mr. E. P. Mathewson, the
smelter superintendent, determined upon attempting the revolutionary
idea of joining up two of the 15-foot furnaces by bridging over
the 21-foot space between them, and continuing the vertical side
water-jackets across this space, thus forming a furnace 15 + 21 + 15,
or 51 feet in length. No work on such a large and boldly conceived
scale had ever been attempted before, and many difficulties in
construction and operation were anticipated.
Mathewson first conducted a series of constructional trials, and found
in the first instance that by taking suitable precautions, it would be
possible to carry out these changes whilst the furnaces themselves were
running. It was found that it was possible to remove or replace single
jackets without shutting down the furnace, by the device of forming a
crust against such a jacket, of sufficient thickness to bear the weight
of the charge for the short period of time during which the change
was being made. Such a crust is readily obtained by shutting off the
tuyeres in the particular jacket and in its neighbours, and maintaining
a rapid stream of cold water through these jackets. Further, it was
found that any desired portion of the sides or hearth of such a long
furnace could be well barred and cleaned whilst the rest of the furnace
was in operation, whereas such barring and cleaning on a small furnace
seriously interrupted the working, and reduced the capacity.
The preliminary tests being satisfactory, the necessary constructional
work was carried out whilst the two furnaces were in blast; the inner
end jackets of these furnaces were taken down, and in a short time the
new 51-foot furnace was in regular operation, and proved so remarkably
successful that two other pairs of furnaces were similarly joined
up. In the following year a still further great extension was made
by joining up in a like manner the end 51-foot furnace to the last
remaining 15-foot furnace, by again bridging over the intervening
21-foot space, thus constructing a furnace of the enormous length of 51
+ 21 + 15, or 87 feet.
It was at one time intended to carry this progress still further by
joining up the other two 51-foot furnaces, so as to make a single
one 123 feet in length, but certain difficulties in the matter of
bringing coke supplies to the two sides, under the special conditions
of available floor space, and the disastrous effects of the financial
panic of October, 1907, stopped all extension work for the time. Such
extensions would however, present no real difficulties either in
construction or in subsequent furnace management or operation.
Figs. 35 and 36 indicate in plan and elevation the arrangement of the
plant and accessories for these extended furnaces. Each 15-foot furnace
had its own settler situated in front, and these have been retained
without any change of position or any further additions. The hearth of
the newly bridged portion slopes from the middle of the bridge to the
tap-holes of the old furnaces, which still serve this purpose for the
larger ones, and from which a continuous stream of matte and slag flows
through a slag spout to the settler in front. The side water-jackets of
the old furnaces remain, being built up in two sets of panels, each 7
feet 6 inches wide, whilst the new bridge portions are constructed of
three sets of jackets, each 7 feet wide. [Illustration: Fig. 35.—Plan
of 51-foot Blast Furnace, Anaconda, indicating Position of Crucibles,
Spouts, and Connecting Bridge between Old Furnaces.]
[Illustration: Fig. 36.—Longitudinal Section and Part Elevation of
87-foot Blast Furnace, Anaconda, indicating Crucibles of Old Furnaces,
Bridge, and Jacketing.]
The furnaces in their lengthened form have proved a tremendous success,
far indeed beyond the anticipation of the designers and managers. This
is largely due
(_a_) To the increased efficiency and economy of replacing a
number of smaller furnaces situated end to end by a
single large furnace;
(_b_) To the increased intensity of heat and reactions owing
to large massed quantities of fuel burned at once, and
to large masses of material being smelted and in a state
of chemical activity.
The advantages which result from such lengthening of blast furnaces
are:—
(i.) Gain in hearth area without extension of the blast-furnace floor
and building.
(ii.) Increase in smelting or hearth area and in consequent capacity,
at a rate very much superior to the extra water-jacketing involved.
Thus, in the 51-foot furnace, the capacity has been increased in the
proportion of 3·8 to 1, the jacketed surface has increased only at the
rate of 2·4 to 1. The output has increased at a much greater speed than
was actually anticipated from the additional hearth area.
(iii.) A very marked saving of fuel. The amount of coke required for
similar charges has been reduced by one-tenth; more than 11 per cent.
was required formerly on a charge, only 10 per cent. was necessary
under the new conditions.
(iv.) The rapidity of working of the furnace has increased owing to the
effect of the narrow width and small crucible dimensions as compared
with the length. This has caused a more rapid flow through the furnace
slag-holes, thus preventing the formation of obstructions, and tending
to wash out any which might threaten to stick.
(v.) Higher furnace temperatures result, and both slag and matte are
hotter than in smaller furnaces. In consequence more siliceous slags
can be run, thus saving the cost of the fluxes which might otherwise be
necessary.
(vi.) Marked decrease in incrustation. Crusting is most likely to occur
at points where the smelting activity is lowest, and in the cooler
parts of the furnaces, such conditions being usually prevalent at the
corners, where the shape also assists in the holding up of material.
Crusting is one of the chief troubles to be prevented and overcome in
operating the blast furnace.
The elongated furnace of 87 feet length practically takes the place
of five shorter ones, representing no less than 20 corners and 10 end
jackets; the new furnace thus reduces the opportunities for crusting
at least five-fold. In this way the hearth area has been very
greatly increased, with still but two ends to hold crusts. The long
furnace-walls with their ends so far apart, in addition, offer much
less opportunity for the formation of crusts than do the side walls
of shorter furnaces, accretions obtain little support, and often tend
to break down under their own weight, whilst they can be more readily
removed by barring, on lowering the height of the furnace charge for a
time.
(vii.) The elasticity of the furnace operations has been much
increased. In short furnaces, cleaning and barring for the removal of
obstructions, etc., necessitate the shutting down of the unit, often
a complete taking down of the furnace-walls and their subsequent
replacement, followed by a re-starting of the furnace work. The ideal
in modern work is continuous running of the unit. The larger furnaces
allow of such practice, since they can be kept in operation whilst
a particular portion is undergoing cleaning or repair. As stated
above, the elongation of the furnaces themselves was conducted whilst
the older 15-foot portions were working. Leaky or worn-out jackets
or spouts are readily removed without serious interference with the
working of the rest of the furnace, and this operation usually requires
a few hours only.
(viii.) The charge may be varied in different parts of the furnace
to suit special requirements, without interfering with the general
operations. Thus, suitable additions for the smelting out of crusts, or
variations in the charge to reduce corrosion near the 21-foot bridge,
can be effected whilst the furnace is running as usual.
(ix.) Increased flow of material through the settlers is effected
without decreasing the efficiency of the settling. Each settler now
serves 25 feet of furnace-hearth length, instead of the 15 feet of
the smaller furnaces, and in spite of the more rapid passage of the
materials, the settling is actually better and the resulting slag
cleaner, owing to the higher temperatures of working and the consequent
greater liquidity of the products, whilst the settler is also hotter.
Thus the greater output of material has required no extra labour
or construction on the tapping floor, though tappings are now more
frequent.
(x.) The labour costs per ton of furnace capacity are greatly reduced,
as are also the operating and management costs, since such labour
and control are to a large extent dependent on the number of units
comprising the plant.
(xi.) The initial cost, per ton of furnace capacity, is also much
reduced. In the elongated furnace, the settlers have not been added
to, the old slag notches only are required to do duty as before, and
the older equipment for bracing and trussing provides for much of that
required in the extensions whilst the original building itself served
for the housing of the increased furnace area.
(xii.) Further extension of the furnace length is readily possible if
desired.
The older 15-feet furnaces had a smelting capacity of 5·6 tons per
square foot of hearth area per day, those of 51 feet length smelt on
an average 6·72 tons per square foot daily, whilst the output of the
87-foot furnace amounts to 3,000 tons of material daily, corresponding
to 3,000 ÷ 87 feet × 4 feet 8 inches, or about 7·5 tons per square foot
of hearth area. Whilst this particular smelter is of course unique in
the dimensions, equipment, organisation and management of its plant and
the magnitude of its operations, and though at most modern smelters the
ore supplies and smelting conditions do not admit of the introduction
of such enormous units; at the same time the principles which underlie
the great advantages of the longer form of blast furnace have had an
important influence on blast-furnace equipment and design generally.
The constructional details of these large furnaces are, for the most
part, common to all modern blast furnaces; it is mainly the size and
capacity which are exceptional. The usual length adopted at smelters
with more modest output varies from about 15 to 25 feet, with a
smelting capacity of from about 400 to 800 tons per twenty-four hours,
depending naturally on the working conditions.
C. _The Practice of External Settling._—In connection with modern
blast-furnace practice, the feature of external settling is of much
importance, its adoption having had a marked influence on:—
(_a_) The efficiency of separation of the smelted products,
and the production of clean slags.
(_b_) The output, and rapidity of working of the furnace.
(_c_) The control and organisation of the smelting processes.
(_a_) The function of the blast-furnace plant is the concentration
of the values into a matte of correct grade for further treatment,
and the production of a slag which is sufficiently clean—that is,
free from copper and other values—to allow of its being disposed of
as waste, immediately. Numerous factors decide the copper contents
of the slag which is economically the cleanest—the general average
is about 0·25 to 0·35 per cent. of copper. The actual condition of
the copper in the slags is a matter of some uncertainty, and it does
not appear improbable that very small quantities of sulphides may
actually be in solution in the silicate slags. The general consensus
of opinion, however, favours the view that much of the copper which
is present exists in the form of minute shots of the matte, actually
held in mechanical suspension, and this is certainly the case when the
copper contents exceed the limits stated above. In consequence, it is
frequently noted in practice that the copper in the slag increases
with the grade of the matte. The question has been reviewed by L. T.
Wright who suggests some actual solubility of matte-products in the
slag. Wright’s curve indicating the connection between matte-grade and
slag values is reproduced in Fig. 37. This connection might however,
possibly result from the fact that the individual shots of matte are
themselves higher in copper contents, since it may be assumed that in
fairly clean slags practically the same number of shots are held up,
owing to the forces of capillary attraction and surface tension, and
that the increased density of the higher grade mattes would influence
but slightly their downward settling when in such a fine state of
division.[11]
[Illustration: Fig. 37.—Copper Contents in the Slags accompanying
Mattes of Various Grade.]
The molten products of the blast-furnace operation are separated by
the settling of the matte and slag under the action of gravity, and
the production of the economically cleanest slag depends upon the
fulfilment of those conditions which allow of the most perfect downward
settling of the small particles of matte. The three main requirements
for efficient settling, apart from the composition of the slag, are:—
(i.) Sufficiently high temperature.
(ii.) Opportunities as regards time, rest,
and space for quiet settlement.
(iii.) Large masses of heated products.
In each of these essentials, the method of external settling, as now
conducted at modern smelters, best satisfies the conditions required
for successful work.
The present practice is to make no attempt to conduct settling in
the blast furnace, but to run the products through and out of the
furnace with the greatest speed attainable, and to allow the matte and
slag sufficient time and opportunity to settle and separate in some
independent and external vessel, which stores the matte and allows the
clean slag to run straight away to waste.
The former method of inside settling gave rise to many difficulties
in practice, but objections were urged against the external settler,
to the effect that heat might be wasted by the abstraction of hot
materials from the furnace to an exterior vessel, and that the settling
would not be efficiently conducted outside, as in the very hot interior
of the smelting furnace. Modern practice has proved conclusively that
both objections are groundless. Such heat as is carried away by the
continual stream of molten material can usually be well spared in the
modern plant, which is driven so rapidly that an abundant supply of
exceedingly hot matte and slag pass through to the settler, whilst the
results of every-day working demonstrate the efficiency of the external
settler, which cannot be equalled, far less surpassed, by any method of
inside settling, under modern smelting conditions. Thousands of tons
of slag pass daily through the settlers, clean enough to discharge
straight to the dump, the copper contents rarely exceeding 0·40 per
cent.
(_b_) The modern conditions of rapid working and large output render
the use of external settlers practically essential, owing to the double
work of smelting and separating being no longer confined to one and the
same vessel. The aim in present practice is to exercise the smelting
function only of the furnace, and to do so to its fullest capacity,
smelting for matte of the desired grade as rapidly as possible, and
therefore running the products through the furnace in a constant
rapid stream and allowing them to settle quietly outside. Under
these circumstances the furnace itself smelts most economically and
efficiently.
It will be recalled that present-day practice involves the subsequent
treatment of the fluid matte—product in the converter, so that whilst
the former methods of working might have possessed certain advantages
for the settling and storing of matte in the small furnaces, and then
tapping out and casting into cakes for subsequent treatment, such
methods have practically no application to modern systems of working.
Internal settling almost invariably leads to the accumulation of
debris, of chills and of any infusible masses of material which may be
produced in the furnace, occasioning delay in the operations, waste and
difficulty in working, and so interfering seriously with the speed and
continuity of the smelting, as well as decreasing the output of the
furnace. On the other hand, a rapid flow of hot molten material through
the furnace not only tends to prevent this formation of chills or
accretions, but greatly assists in the dissolution or removal of such
as might be formed. Should the production or collection of such masses
be transferred to the settler instead, they are more readily attacked
and remedied without interfering with the continued operation of the
furnace.
Further, the nature of the hearth which would be most satisfactory for
internal settling is not at all suited for modern smelting conditions.
The ordinary water-jacketing would have too marked a cooling effect on
the hearth for the materials to remain sufficiently hot and fluid to
allow of proper settling, whilst a brasque or similarly lined hearth
suitable for such settling would, under the present conditions of
rapid driving and intense reactions, be unable to withstand the highly
corrosive and abrasive action to which it would be subject, so that
breakouts, necessitating delays and repairs, would constantly occur.
Water-jacketing in this portion of the furnace is indeed an essential
for modern conditions, and consequently rapid driving and quiet
internal settling in the same area are quite incompatible. The modern
fore-hearth, on the other hand, is accessible and easy of repair, and
in the event of any trouble occurring therein, the furnace itself
can continue its smelting activity to the full, since other suitable
arrangements can readily be made for temporarily dealing with the
products.
[Illustration: Fig. 38.—Water-Jacketed Blast Furnace (48 inches by 240
inches). Lower Portion, indicating Air and Water Connections, Bottom
Supports, End Slag Spouts, etc. (P. & M. M. Co.).]
(_c_) The functions of the blast furnace in the modern smelting
scheme are particularly dependent upon the employment of the external
settler in conjunction with it. The work of the furnace plant is to
produce as rapidly as possible, a supply of suitable grade matte for
the converters; large quantities of hot fluid matte must be available
at a moment’s notice, and such demands are often very erratic, being
dependent on the working of the converter plant and the refining
furnaces. It is essential to the successful operation of the blast
furnaces that the manager should be in a position to work his furnace
as rapidly and continuously as possible, which is best attained by
making the output independent of irregular tappings of matte just when
required by the converter department. The settlers, in exercising the
function of reservoirs for matte, from which the converter department
may draw at will, allow of regularity of working and rapidity of
output in a manner possible in no other way. The only alternative,
using internal settling, would consist of tapping out matte at regular
intervals and casting such material when it is not immediately
required, a wasteful and unnecessary practice incompatible with modern
ideas of smelting work.
During the early stages of the development of smelter plant, the use
of reverberatory fore-hearths received considerable attention, the
principle being to build a fire-box in communication with the settler,
so as to ensure a sufficient supply of heat in the vessel for efficient
settling. Modern furnaces however, usually supply a large enough
quantity of very hot and fluid matte and slag as to allow of very
efficient separation without the use of extra heating, providing the
position and construction of the settler is suitably planned, as will
be described in due course.
The Construction of the Blast Furnace.
_Dimensions._—The modern blast furnace is a long, narrow, water-cooled
shell, rectangular in plan. The dimensions, particularly the length,
vary greatly, being regulated according to the anticipated output of
the furnace-unit. The size is generally expressed in terms of the
internal dimensions at the tuyere level, which represents the smelting
area. The width of the modern furnace varies usually from 44 to 56
inches, according to the blast pressure, method and speed of working,
concentration to be effected, and so forth. The length in many cases
is between 15 and 25 feet, when the furnace may be conveniently worked
in connection with one large settler. The capacity of such a unit
naturally depends on the conditions of working; it may be taken roughly
as from 4 to 6 tons of material per square foot of hearth area per
twenty-four hours.
_Foundations._—The furnaces are built upon a foundation which is
necessarily very strong, being usually either of solid rock or of
concrete.
_Bottom Plate._—The bottom plate of the furnace usually carries part
of the weight of the lower tier of water-jackets as well as the
furnace burden, and is supported, some distance above the ground,
on screw-jacks leaving an air-space below the furnace, which allows
of convenient access for repairs or adjustment. The height of the
construction is thus raised to a convenient distance for adjustment
to the discharge to the settlers. The bottom plate should consist of
sectionised water-cooled cast-iron plates bolted together, with a thin
layer of brickwork placed above, to protect them from the corrosive
influences to which they are subject. There is a slight slope towards
the slag-notch. The actual working bed of the furnace is however, a
chilled crust of material which sets on this bottom owing to radiation
below, and which, when suitable precautions have been taken, usually
adjusts itself naturally whilst the furnace is in operation, by what
may be termed automatic radiation. Thus, apart from the water-cooling
devices, if the working bottom wears down towards the metal plates, the
loss of heat by radiation through the thin layer of material causes a
chilling effect which leads to a thickening of the crust. Should the
crust thicken unduly and so threaten to interfere with the discharge,
the radiation is decreased owing to the thickness; and the high
temperature which prevails upon this layer causes a partial melting so
that it gradually becomes thinner again—thus regulating itself for the
most part automatically.
[Illustration: Fig. 39.—Tapping Breast of Blast Furnace, Cananea (see
p. 139).]
[Illustration: Fig. 40.—Riveted Steel Water-Jacket, showing Tuyere
Holes and Water Inlets, etc. (P. & M. M. Co.).]
[Illustration: Fig. 41.—Transverse Section through Modern Blast
Furnace, showing Arrangements of Boshed Lower Jackets, Upper Jackets
and Plates, Stays and Supports, etc.]
[Illustration: Fig. 42.—Interior of Anaconda Blast Furnace, showing
Jacketing, Tuyere Holes, and Bridge.]
_Water-Jackets._—The usual height of the modern furnace, as reckoned
from tap-hole to charge floor, is roughly from 14 or 16 feet up to 20
feet, water-jacketed all the way. The sides and ends of the furnace are
constructed of sectionised water-jackets arranged horizontally in tiers
and vertically in panels. There are usually two, occasionally three,
tiers, suitably stayed and supported. The practice as regards the
shape and arrangement of the jackets varies greatly. It was formerly
not uncommon to work with three tiers of jackets for the sides; of
these the lower tier extended only from the sole-plate to the level of
the slag-notch, forming practically the crucible jackets, the height
varying from 2 feet 6 inches to 4 feet. These were most used when the
discharge to the settler was situated at the side wall of the furnace.
Above these jackets was situated the second tier through which the
tuyeres passed; these build up the boshes of the furnace, and are
termed the “bosh” or “tuyere” jackets. In most modern furnaces these
two tiers of lower jackets are replaced by one set of panels of from 7
to 10 feet in height, the jackets being given a slight slope towards
each other at the bottom, so as to form a very small bosh angle; the
contraction is about 8 inches. This improvement does away with a good
deal of the jointing otherwise necessary near the hottest parts of
the furnace, and thus lessens the danger of leakage at these points.
The water-cooled breast-plate containing the opening for the escape
of the products is now put in position as a separate piece, well
secured to the rest of the jacketing (Fig. 39). Above the lower tier of
jackets is placed the upper series, often from 7 to 9 feet in height,
which carries the walls of the furnace up to within a few feet of the
charging platform. These jackets are parallel, and no bosh is given
(see Fig. 41).
The end jackets are usually built in two tiers only, the upper, 7
feet to 7 feet 6 inches, as a rule, and the lower, 8 feet to 9 feet 6
inches, according to circumstances; in the smaller furnaces the end
wall may sometimes consist of a single jacket only. They are vertical,
no end bosh being allowed. The end jackets are each single panels,
whilst the side walls are built up in panel sections, the width of
which vary, but are often 7 feet to 7 feet 6 inches wide, the panels
being bolted or clamped together and strongly stayed.
The water-jackets are constructed of flanged steel plate, the inner
sides of which are 5/16 to ⅜ inch thick, the outside ¼ to 5/16 inch.
The seams are flanged outwards, so as to prevent joints, etc., being
exposed to the inside of the furnace. The water space between the two
plates of the jacket is from 3 to 4 inches.
It is usual to support the weight of these jackets on I-beams carried
by the upright columns; very strong bracing and tieing is also
necessary in order to prevent the side walls from bulging by the great
pressure to which they are subjected. In order to protect the jackets
themselves from buckling by the forces acting upon them, they are
strengthened inside the water space by a series of ? bands, which run
vertically downwards between the plates, and are rivetted to the outer
side—this device is found not to interfere unduly with the proper
circulation of the water. Leakage between the joints of the separate
jackets is prevented by asbestos packing. In spite of the strong
binding and bracing of the walls in this manner, the connections are
so devised as to allow of their being unfastened very easily, so that
jackets may be readily disconnected and taken down when it becomes
necessary to do so.
Arrangements for the water supply to the jackets vary considerably.
In localities where a plentiful supply is available, each jacket has
its independent outlet and inlet pipes; in other cases it is common
to arrange an independent feed to each set of panels, water being
supplied first to the jackets of the lower tier, and being discharged
from them to the jackets situated above. The supply pipes for the
various jackets branch from water main pipes running at the sides of
the furnace.
The tuyere or bosh jackets are pierced horizontally at intervals of
about 1 foot, with a line of 5-to 7-inch holes for the fitting in of
the tuyere pieces. These are formed of steel thimbles, of ⅜-inch metal,
which have a slight taper, fitting secured against the inner plate and
rivetted to the outer one, thus allowing of ready replacement when
necessary (see also Fig. 40). Above the side jackets of the furnace
there is usually a heavy mantel-plate, 2 feet to 2 feet 6 inches high,
with a sloping front, and surmounting this are apron plates, 1 foot 6
inches to 2 feet high, inclined at 45°, constituting a hopper which
directs the charge towards the centre of the furnace in such a way as
to keep the fines nearer to the middle line, and thus leave the sides
of the charge more open, in order to ensure more regular working.
_Superstructure._—The jacketing, together with the apron and mantel
plates carry the structure up to the charging floor. Above this is
the superstructure with the arrangements for taking off the furnace
gases, and for the feeding of material for the charge. In many cases
the general practice still prevails of constructing the walls of this
portion of brickwork, often about 14 feet high, surmounting this with a
hood of metal from the top or sides of which large off-takes carry the
furnace gases to the dust chambers, and thence to the flue system and
stack. Modifications in the design of the blast-furnace superstructure
have been, however, in course of progress at many works, particularly
in connection with the employment of automatic or mechanical charging
appliances and the taking-off of the gases below the feed-floor level.
This is specially the case at plants operating the pyritic process and
where the gases are to be utilised for acid manufacture, as well as in
connection with the treatment of smelter fume. Several furnaces are
also at work using either metallic water-cooled or air-cooled tops,
from which the removal of accretions is stated to be very readily
effected.
Some of the most recent developments in the design of blast-furnace
superstructure have been described by Emmons in reviewing the
experiments at the Copperhill Smelter, Tennessee. The gases here
are used for acid-making, and are sent to Glover towers under some
pressure. The furnace top consists of cast-iron corner-posts and
dividers, the walls and ends laid up with brickwork, surmounted by
a tubular top of the Shelby type from which the gas off-takes lead.
The horizontally pivoted doors open inwards and fit tightly. These
arrangements are stated to be very satisfactory.
[Illustration: Fig. 43.—Showing Upper Jackets, Apron and Mantel Plates,
and Superstructure of Blast Furnace, Anaconda.]
[Illustration: Fig. 44.—Charging Blast Furnaces at Anaconda.]
The charging platform, suitably supported on vertical columns, runs
at the upper level, being provided, on either side of the furnace,
with tracks of rails for the charge cars. The charging doors usually
correspond in position to the panels of water-jackets, and are situated
along the whole length of each side furnace-wall, the bottom of the
charging opening being flush with the floor. They are generally moved
up and down in the grooved guides of the upright columns between them,
and are of sheet steel suitably strengthened, from 6 to 7 feet wide
and 4 feet 6 inches to about 5 feet high, supported by wire-rope and
chains, and operated by compressed air cylinders.
_The Air Supply to the Blast Furnace._—The quantity of air required by
the blast furnace varies very widely with the class of work, rapidity
of output, character of charge, and general smelting conditions. It may
be stated roughly as being from 300 to 500 cubic feet of air per minute
per square foot of hearth area, at a pressure of about 40 to 50 ozs.
per square inch.
The rotary blower of the Roots or Connersville type is very well
suited for the supply of these enormous quantities of air at moderate
pressures, but for blast at higher pressures the air leakage becomes
excessive, and piston-driven blowing engines become almost a necessity.
Such improvements have, however, been made in rotary-blowing appliances
within recent years that most blast-furnace plants are equipped with
blowers of the rotary type, which are found highly satisfactory. The
air is brought along blast mains of considerable size—about 30 inches
diameter—to the furnace building, thence to the bustle pipes of 24
inches diameter, which surround the furnace, from which branch off
the pipe connections (5 or 6 inches diameter) for the tuyeres. The
practice of equipping each furnace with its own blowing unit is fairly
general, making the necessary reserve connections in case of temporary
breakdown; many smelters, however, adopt the system of delivering
the air from all the engines into one large common air main, making
the necessary connections from this to each separate furnace. The
importance of avoiding leakages is recognised, and the requisite valves
for regulating and controlling the air supply are arranged for.
From the bustle pipe the air passes down the pipe connections which
are attached by flanged joints, thence to the tuyere pipes, which are
of cast-iron, the blast being regulated by valves. The actual form of
tuyere employed varies considerably, each smelter usually having its
own special devices for the convenience of repair, renewal, and fixing,
as well as for valve regulation and punching. The tuyere is held
against the face of the jackets by bolts, leakages being prevented by
asbestos packing.
[Illustration: Fig. 45.—Blast-Furnace Shell, with Air Connections (P. &
M. M. Co.).]
[Illustration: Fig. 46.—Details of Tuyere, Cananea Blast Furnace.]
The tuyeres are usually 4½ to 5 inches in diameter, and are generally
placed about 12 inches apart. Air is supplied only through the side
jackets, and not at the ends of the furnace.
_Heating the Air Blast._—The advisability of heating the air-supply for
copper blast-furnace smelting has been the subject of very considerable
discussion, the question requiring consideration both with respect to
its influence on the rationale of the smelting operation as well as
from the economic standpoint. The matter is dealt with more fully in
connection with pyritic practice, from which point of view Peters has
reviewed the subject exhaustively. It may be here stated that there
appears to be no advantage in preheating the air when the true pyritic
process is operated, and actual trial has resulted in the rejection of
the method at the smelters practising this work.
Where, however, coke fuel to any considerable extent is employed on
the charge, a supply of heated air through the tuyeres may result in
an increased rapidity of smelting, as well as in the production of
hotter and more fluid slags. Especially in partial pyritic smelting
and more particularly when working charges which contain but little
sulphide and where the employment of much coke is not advantageous,
the use of preheated blast may be economically very useful. In such
cases, the heat production in the furnace is not so fundamentally
bound up with the thermo-chemical reactions of slag formation as it
is in true pyritic smelting, and therefore the enhanced intensity of
combustion of coke-fuel at the tuyere-zone by the use of hot air may
exert an important influence in improving the furnace operation and in
decreasing the amount of coke-fuel required. In many such instances
indeed it has been chiefly the economic factor with reference to the
cost of installing and operating suitable devices for warming the
air-supply which has determined the question of adopting this system.
As is well known, the use of a supply of heated air causes a largely
increased calorific intensity from the combustion of coke, resulting in
higher temperature at the tuyere-zone, under which circumstances the
charge materials are smelted more rapidly, and the resulting products
are more fluid, whilst slags of higher silica content (sometimes
economically advisable) can be conveniently worked with.
The devices employed for the preheating of the blast vary
considerably—cheapness, capacity, simplicity in design and operation
being the main essentials.
The utilisation of the waste heat from the smelting furnaces or
products would suggest itself as an economical method for accomplishing
the warming of the blast, but in practice several difficulties are
encountered in efficiently making use of this heat. Heat is available
from two sources, either from the furnace gases or from the hot slag.
The very successful operation in cast-iron smelting, of hot-blast
stoves worked by the “waste gases,” cannot, however, be applied to
copper blast-furnace smelting, since the gases in this case do not
possess similar calorific value owing to the small proportions of
carbon monoxide present. Further, the temperature of these gases is
not sufficiently high to allow of the effective application of the
regenerative principle using brickwork chambers. In consequence, the
use of metal pipe-stoves offers the only method of utilising the
heating values of the furnace gases, but their comparatively low
temperature does not afford sufficient heat for the warming of the
large quantities of air which are required at the tuyeres.
The much higher temperature of the reverberatory furnace gases offers,
however, much greater scope for their utilisation in this respect, if
both classes of furnace happen to be in operation at the plant and if
they are conveniently situated for the purpose.
At several smelters, blast furnaces have been equipped with
hot-blast “tops” for the purpose of preheating the air supply, the
air-heating pipes being exposed to the gases in the upper portions
of the furnace. The Giroux blast-heating device has been installed
on furnaces at smelters in Mexico and Arizona, whilst at others in
the same localities, the Mitchell system of baffle passages has been
successfully used. The Kiddie system of running the blast pipes through
the dust chambers has been tried at Tyee, B.C. The advantages of thus
utilising the heat of waste gases have generally, however, been found
to be more than balanced by the extra costs involved.
Efforts have been made to use the heat contained in molten slag for
warming the air, but owing to the low conducting power of these
materials, and the difficulty of bringing extended surfaces in close
contact, the method has not proved itself very efficient. Blast is
occasionally warmed by passing the air through tunnels in which bogies
of molten slag are allowed to remain for some time.
When methods of utilising waste heat from the furnace products fail,
the fuel-heated iron pipe-stove is generally employed. Since the
temperatures required are comparatively low, and the margin of profit
involved by the use of hot blast is usually small, the use of the
cheapest class of fuel available is imperative; but many classes of
fuel unsuitable for other purposes may find useful application for this
work.
The stove is of the usual [shape: alternating up-down pipe] cast-iron
pipe form, designed to give the maximum exposing surface, suitably
strengthened and protected from direct action of the fire. Much
valuable information on the advantages, disadvantages, and appliances
for blast heating was afforded by the smeltermen who contributed to
the symposium on “Pyrite Smelting,” which Rickard edited for the
_Engineering and Mining Journal_.
References.
Mathewson, E. P., “The Development of the Modern Blast Furnace.”
_Eng. and Min. Journ._, May 27, 1911, p. 1057.
Wright, Lewis T., “Metal Losses in Copper Slags.”
_Bulletin Amer. Inst. Min. Eng._, 1909, Sept., No. 33, p. 817.
Shelby, Geo. F., “Cananea Blast Furnaces.”
_Engineering and Mining Journal_, April 25th, 1908.
Emmons, N. H., “Copper Blast-Furnace Tops.”
_Bulletin Amer. Inst. Min. Eng._, Feb., 1911, p. 119.
“Heating Blast.” _Engineering and Mining Journal_,
June 16 and Sept. 15 and 29, 1906.
“Pyrite Smelting,” T. A. Rickard.
Also the Authors already referred to, Austin (p. 80),
Gowland (p. 17), Peters (p. 80).