Functions of the Furnace — As Melting Agent—Reduction
Smelting — Oxidation in the Furnace — The Pyritic
Principle — Features of Modern Practice:
Water-Jacketing, Increase in Furnace Size, External
Settling — Constructional Details of the Furnace.
=The Functions of the Blast Furnace.=—The functions of the blast
furnace may be considered from three points of view:—
=1. As a Melting Agent.=
=2. As a Reducing Medium.=
=3. As an Oxidising Medium.=
In modern copper smelting practice, the blast furnace is under ordinary
circumstances never employed in the capacity of a reducing medium, but
is used for a variety of work in which its operations range from those
of a melting furnace to those more particularly of an oxidising medium,
as its oxidising functions are becoming developed to a gradually
increasing extent.
In the older processes of copper smelting, when working on oxidised
charges, the melting and reducing functions of the furnace were
exercised simultaneously; when, at a later stage, sulphides were
smelted in the charge, the directly reducing function was utilised to
a very much smaller extent. In the reducing atmosphere then maintained
inside the furnace, the sulphides liquated and melted down without
causing much concentration of the copper in the product, elimination
of sulphur being effected mainly by the direct action of heat on the
pyritic constituents of the charge, and by the interactions between the
sulphides and the oxidised compounds of copper present.
When, however, increasing quantities of sulphide ore became available,
modifications in blast-furnace smelting practice were introduced
with a view to increasing the concentration of the copper, this
being attempted either by preparatory roasting or by the addition of
oxidised cupriferous materials to the charge, sulphur being thus
eliminated and some concentration resulting in consequence. In such
work the furnace chiefly exercised its melting function, allowing, as
in the case of reverberatory working, of the formation and thorough
fusion of sulphide matte and silicate slag from the mixture of oxides
and sulphides in the charge. In the latest developments of practice,
the oxidation has been carried out to a continually increasing extent
by the air blast at the tuyeres of the furnace.
1. =The Melting Functions of the Blast Furnace.=—The blast furnace
is under ordinary circumstances, usually regarded as the cheapest
of melting agents. Compared with the reverberatory, the heat in the
blast furnace is utilised more efficiently. Reverberatory working
involves the passing of a flame over the surface of the charge, and the
transference of this heat through the mass depends upon the conducting
power of the material itself, which is, however, usually poor. Although
the modern reverberatory practice of melting thin layers of preheated
charge both from above and from below has greatly increased the
efficiency of the furnace in this respect, the closer contact of charge
and fuel in the blast furnace allows of a more thorough communication
of the heat.
The principal features of blast-furnace working which tend to make it
the cheaper and more efficient agent for the treatment of cupriferous
materials—with the exception of fines—are those of construction,
working, and fuel economy.
(_a_) The construction of the furnace is comparatively
simple, and it is not excessively expensive to erect;
furnaces and accessory plant can be purchased complete
and easily set up and taken down again when required.
(_b_) The furnace is elastic in its operation, especially
where the supply of material varies from time to time,
involving changes in the composition of the charge.
(_c_) The furnace is readily started, shut down, and
restarted at will, and without much difficulty or additional
expense.
(_d_) The operation and smelting are rapid and cheap,
the capacity can be made enormously large; all classes of
material—except fines—such as ores, slags, and residues,
which accumulate to a considerable extent round a smelter,
can be conveniently dealt with directly, whilst fines can
now, where necessary, often be prepared into a suitable form
for blast-furnace treatment.
(_e_) The heat is more efficiently communicated to the
individual parts of the charge, in consequence of the more
intimate contact of charge and fuel.
(_f_) The fuel consumption is low, the natural fuel values
of the iron and sulphur on the charge can be utilised, and
the degree of oxidation (and consequent concentration) can
be controlled in the furnace operation.
(_g_) The furnace works continuously (in modern practice
the reverberatory furnace is also continuous in its action).
Owing to the great elasticity in blast-furnace operation, and its
capability of dealing with practically every class of copper-bearing
material in lump form, modern practice is of the most diverse character.
2. =The Blast Furnace as a Reducing Medium.=—In modern smelting
practice, with but a few exceptional instances, a distinctly reducing
atmosphere is avoided as far as possible. This arises largely from
the fact that the material available in modern work usually demands
oxidation in order that satisfactory concentration may be effected.
In the early days of copper smelting, however, the reducing action
was the chief function which was exercised, mainly because at that
time oxidised ores constituted an important part of the charge, and
a reducing action was required to obtain marketable products from
such material. At a later stage in the development of blast-furnace
practice, the sulphide ores which became available were roasted, and
the resulting oxidised products were subjected to reduction smelting,
in order to extract the metal. On such oxidised charges, blast furnaces
were almost universally employed, using carbonaceous fuel either in the
form of coke or charcoal, this material fulfilling the double purpose
of fuel and reducing agent, the excess carbon causing the reduction of
the metal from the oxidised ore.
This operation was known commonly as “black-copper smelting.” At the
present time such oxidised ores are rarely met with in sufficient
quantity by themselves to be worked by this method, which involves also
very serious losses in operation. Further, such oxidised materials
are in many cases valuable for smelting along with sulphide charges,
greatly assisting the concentration, and it is usually advantageous to
employ them in this manner.
The losses and difficulties in “black-copper smelting” are, however,
of interest in so far as they apply to certain analogous problems in
modern work. These difficulties in reduction smelting arose largely
from three causes:—
(_a_) Losses of copper in the slag.
(_b_) Simultaneous reduction of iron with the copper.
(_c_) Chilling in the furnace hearth.
(_a_) In the case of reduction smelting where sulphides are not present
in any appreciable quantity, the losses of copper may be either
(i.) As silicate, or
(ii.) As metal.
(i.) Sulphur is the natural protector of the copper in the furnace
charge, as, owing to their powerful affinity, a fusible, fluid and
dense product is formed, which is very slightly soluble in slag; and
on this account, a ready separation of the copper from the earthy
materials can be effected. So long as sulphur is present in moderate
quantity there is little chance of copper entering the slag as silicate.
In reduction smelting, however, and especially in black-copper smelting
where sulphur is lacking, such losses are liable to occur, since copper
oxide is itself strongly basic, and readily fluxes off with silica at
high temperatures, yielding silicates. These products are less dense,
and are markedly soluble in the other silicates which constitute the
slag; moreover, the copper oxides themselves are likewise partly
soluble in, and are readily carried in suspension by, the silicate
slags.
In order to prevent such losses as much as possible, the reducing
conditions in the furnace must be increased by the employment of more
coke, so as to ensure the reduction of the copper oxides and silicates.
These reducing conditions must not, however, be too drastic, especially
if the temperature of working be high, on account of the great tendency
to cause (_b_) a reduction of metallic iron, which results in the
formation of bears and scaffolds, with their attendant difficulties of
removal and their interference with working.
Between these opposing causes of loss and difficulty, a careful balance
has to be observed in the smelting operations. (In modern practice,
losses of copper as silicate and oxide, for reasons such as those
detailed above, occur to a marked extent in those operations where the
sulphur is present in small proportions only, and particularly where
the reactions are intensely oxidising, as in the furnace-refining
operations and the later stages in the converter process. The slags in
such cases usually carry considerable quantities of copper in the form
of silicate and oxide, not infrequently to the extent of 20 to 30 per
cent., or even more. The quantity of this slag is, however, kept as
small as possible, and copper in the material is readily recovered by
the addition of these slags to the blast-furnace charge.)
(ii.) Losses of copper as metal also, were formerly serious in
black-copper smelting, the metallic copper held in suspension in
the slag being indeed the chief source of loss in this method. The
efficient separation of copper from slag, especially in the small
quantities formerly operated, was therefore of importance. Satisfactory
settling was, however, difficult of application, since the behaviour of
metallic copper is very different from that of sulphides. It is much
less fusible, much less fluid, and the small globules, as reduced, do
not readily coalesce, whilst the high temperatures favourable to good
fluidity of the products and to good settling, promote copper losses
from the other causes noted above.
Moreover, the high melting point of the metal and its great
conductivity added to the difficulties in providing suitable
arrangements for settling, since the copper not only tended to chill
readily in any external settler, but it was also very liable to do so
in the crucible of the ordinary form of water-jacketed blast furnace,
such masses being exceedingly difficult to remove, whilst the working
of the furnace was necessarily much interfered with.
In order to conduct the necessary internal settling, the older type of
blast furnace was required, in which water-jacketing near the hearth
was dispensed with, a large crucible bottom of non-conducting brasque
or brickwork being employed instead. Such a form of furnace is not
adapted to the modern methods of smelting where enormous capacity and
output are essential, whilst such a system of working interferes with
the rapid and continuous smelting of large quantities, to a greater
extent than if the whole of the molten products are run out of the
furnace continuously and the settling performed in an external vessel.
3. =The Blast Furnace as an Oxidising Medium: Sulphide Ores in the
Blast Furnace.=—In modern blast-furnace practice, the oxidising
function of the furnace is the principal feature of working. Sulphide
ores now constitute the chief source of copper, and the smelting
operations involve the oxidation of the accompanying constituents and
the elimination of the resulting oxidised products.
Such ores when smelted in the blast furnace with carbonaceous fuel, and
under the reducing conditions characteristic of the older methods of
working, would yield a product showing low concentration of the copper,
since the reducing conditions would largely retard the oxidation of
sulphur which is an essential for the enrichment of the matte. Except
for the sulphur eliminated from the pyritic constituents by the direct
action of heat, and a certain quantity by the interactions with oxides
as already indicated, the loss of sulphur would be slight. The furnace
under such circumstances would thus tend mainly to exercise its melting
function, and the result of such working would be the melting down
and subsequent separation of the sulphides and slag, with even less
tendency to concentration than occurs in the reverberatory furnace,
where the atmosphere is less distinctly reducing.
The modern method of smelting sulphide ores being essentially an
oxidising process, it is necessary that oxygen be added to the charge
with the object of promoting the elimination of the sulphur and iron,
and the consequent concentration of the copper.