Development of the Process — The Converter — Converter
Linings — Grade of Matte — Operation of the
Process — Systems of Working.
In modern copper smelting practice, matte of “converter grade,”
containing from 30 to 50 per cent. of copper, is bessemerised for the
production of metallic copper. Successful practice depends upon a
regular and continuous output of matte from the furnace plant being
available, and upon a capitalisation and resources on a sufficiently
large scale for continuous operation of the whole of the smelting plant.
=Development of the Process for Bessemerising Copper Mattes.=—The
success of Bessemer’s process, which was applied in 1856 to the
production of steel by blowing air through molten cast-iron, led to a
suggestion for its application to copper mattes and to some experiments
on the subject by Semenikow, a Russian engineer, ten years later. It
was not until 1878 that any further work was conducted on a practical
scale. In that year John Holway suggested and worked out the scheme
already referred to, the principles of which as outlined by him, form
the foundation of the pyritic and converter practice of the present
time. Air was blown through heated Rio Tinto pyrites in an ordinary
Bessemer steel converter and the experiments met with considerable
success. The apparatus was, however, not deemed convenient, as the
process worked very intermittently and large quantities of slag were
produced which required to be poured off at intervals, whilst the
position of the tuyeres in this form of converter was found to be
unsatisfactory. There are many practical difficulties in employing the
same kind of apparatus for the converting of copper mattes as for the
bessemerising of cast-iron into steel. In the first instance, the final
steel product differs but little in weight or bulk from the original
charge, whilst the process produces but little slag, owing to the
comparatively small proportions of silicon and manganese which require
to be oxidised—whereas in copper converting, the quantity of slag
produced is almost equal in weight to the amount of matte originally
charged, whilst the resulting copper product amounts to less than
one-half of this weight. Further, in bessemerising cast-iron, the blow
is of very short duration; in copper matte converting, it occupies
more than two hours, and the relative heat losses are, in consequence,
markedly different. Finally, the lining of the steel converter chiefly
serves to protect the shell; its function in the copper converter was
to act also as flux for the iron oxides produced on blowing.
In Holway’s final form of apparatus for the pyritic smelting of copper
ore to metal, the introduction of siliceous material as a flux for the
iron oxide and the use of basic lining were arranged for, with the
object of overcoming the difficulties caused by the corrosion of the
siliceous lining which acted as flux.
Though several years elapsed before the pyritic treatment of ore was
successfully conducted, the process of bessemerising the fluid matte to
metal was successfully applied on a commercial scale by Manhès in 1880,
although it was not until the following year that David’s device of
placing the tuyeres horizontally and at such a height above the bottom
as not to interfere with the metal which is obtained, solved the final
difficulties of operation on a practical scale. In 1883–4 the Manhès
converter was introduced into the United States, and at about the same
time the barrel form was designed by Manhès and David, and was also
readily adopted. Both forms developed in size, increasing in capacity
from 1 ton to that of 7 to 10 tons.
Until comparatively recent years, the chief modifications in practice
were concerned with operating and constructional details rather than
with radical changes in the principles of work. Experiments and
research have meanwhile been in constant progress with the object of
overcoming several of the grave defects connected with the apparent
necessity for the destruction of the siliceous converter-lining by
using it as flux, which was due to the difficulties of causing the iron
oxide to flux with silica when introduced in any other way.
The most vital improvement introduced into converting practice, and
that with which the future developments are most closely bound,
is the successful adaptation of basic material for the purpose of
lining the converter. This achievement, together with recent success
in the introducing of siliceous flux, promises to solve many of the
difficulties connected with the bessemerising of low-grade matte by a
continuous process.
Suggested by Holway, basic linings were tried at the Parrott Smelter,
Butte, in 1890, by Keller and others, but under the conditions of
working at that time they were found to be unsuccessful when operated
on an industrial scale. Valuable pioneer work was undertaken by
Baggaley in Montana, and after many trials, his method was successfully
operated for some months at the Pittsmont Smelter under Heywood’s
direction in 1906. Visits of inspection to this smelter in 1908
proved disappointing, it being found that most of the plant which had
promised the solution of such difficult problems had been dismantled,
largely owing to economic difficulties connected with its operation,
and the works were in process of re-organisation for the older system
of working. Meanwhile, since 1903, Knudsen, at Sulijtelma, Norway,
has successfully employed a small basic-lined converting furnace for
the combined pyritic smelting and converting of heavy sulphide ores.
The process consists usually of pyritic liquation of the sulphides,
followed by a further concentration of the matte up to ordinary
converter grade by bessemerising, the higher grade matte being then
transferred to a silica-lined vessel and blown to metal in the usual
way.
The successful operating of the basic-lined converter on the large
scale and under the conditions of working at great modern plants
was first established by Smith and Pierce at the Baltimore Copper
Company’s Smelter, and the method has since been installed and worked
with success at Garfield, Utah (five converters in operation, one in
reserve); at Perth Amboy, N.J.; at the Washoe Smelter at Anaconda—where
the whole plant is being adapted for basic-converting—and at several
other works.
A recent and promising development has been the reported successful
blowing of fine siliceous concentrates through the tuyeres of
converters at the Garfield Smelter, a method by which it might be
possible to effect the rapid and efficient extraction of values from
fine material otherwise difficult to deal with, affording at the same
time a means of conveniently supplying siliceous flux in a manner
possessing many advantages.
=Principles of the Bessemerising Process.=—The principles underlying
the converter process are those which form the basis of pyritic
smelting practice—of which bessemerising is but a phase. The reactions
involve the very rapid oxidation of iron and sulphur under practically
ideal conditions, and the fluxing by silica of the iron oxide so
produced. The heat of oxidation keeps the materials in a thoroughly
molten state, and maintains the temperature well above that required
for slag formation and perfect fluidity. The heat derived by the
combination of oxygen with the iron and sulphur and that of the iron
oxide with silica is developed so rapidly and in such quantity, owing
to the large masses now worked with, as to cause a reaction-activity
sufficient to make the process independent of heat from external
sources.
It will be noted how markedly the more recent developments of copper
smelting have taken advantage of the factors of the _time element_ and
_mass influence_ in obtaining enormous heat intensities and consequent
high temperatures, by conducting oxidation of sulphides as rapidly and
in as large mass as possible. The same absolute quantities of heat per
unit weight of charge were liberated in the older smelting methods
involving roasting, but the more leisurely manner of operating allowed
the dissipation and dispersion of much of this heat, thus necessitating
the employment of supplementary carbonaceous fuel.
=The Converter.=—The converter is a lined steel vessel in which the
molten matte is contained, and which allows of air being blown through
the material by means of tuyeres which pass through the walls.
The early form of converter was bottom-blown, and similar to that
invented by Bessemer, but it was not successful in operation on the
small quantities of copper matte worked with, owing to the chilling
effect of the cold air on the copper, which, when produced, sank to the
bottom and set above the tuyeres, stopping the air blast, and causing
much loss of metal in the slag.
The later form of converter was barrel-shaped, with a horizontal row of
tuyeres situated at some distance above the bottom so as to allow the
copper to settle, protected from the action of the blast, and also to
allow of the punching of the tuyeres as required.
The modern forms of converter comprise both the vertical and the barrel
types, modified largely as regards size and constructional details,
and although the vertical form is still in use and is even preferred
at several smelters, it has been largely superseded at most plants by
the barrel-shaped variety, whilst the possibilities of greatly enlarged
vessels using basic linings are likely to favour this replacement still
further.
1. _The Upright Bessemer Vessel_ is used, and found satisfactory
at Great Falls and at Mt. Lyell. The general size has been 8 feet
diameter and 16 feet height, with a capacity ranging from 5 to 12
tons, according to the condition of the lining, though at Great Falls
converters of 12 feet diameter with corresponding capacity are now
in use. The advantages of the vertical form are, that, owing to the
greater depth of matte through which the air passes, the oxidation is
more rapidly conducted, the lining is more efficiently supported, and
the wear by abrasion upon the lining is found to be considerably less
in amount and to be more uniformly distributed.
On the other hand, the greater depth of matte necessitates a greater
blowing pressure in order to force the air through the material,
whilst control over the operations becomes a matter of greater
difficulty.
[Illustration: Fig. 62.-Sectional Elevation and Plan of Barrel-Shaped
Silica-Lined Converter (Peters).]
2. _The Barrel Form of Converter_ is the type in common use. Among the
advantages claimed for this form are those which accrue from being able
to operate the same weight of matte in more shallow layers, as compared
with the upright form—thus requiring lower blast pressures. Another
advantage is the greater ease of regulating the depth of material
blown through, by tilting the converter and thus altering the relative
position of the tuyeres.
Owing to the successful adoption of the basic lining, the barrel type
of converter has now to be divided into two classes, since the basic
converter differs from the silica-lined type in constructional details,
and is usually of much larger dimensions. Its operation is also
conducted on somewhat different lines.
[Illustration: Fig. 63.—Latest Form of Silica-Lined Barrel Converter.]
(_a_) _The silica-lined barrel converter_ varies somewhat in size,
the Anaconda converters were, however, representative of the most
convenient dimensions.
The shell consists of ¾-inch boiler plate, 8 feet in diameter, and
12 feet 6 inches long. The converter is constructed in two portions,
the body and the hood, in order to facilitate removal, relining, and
general repairs. The ends are lined with 9 inches of firebrick, and
the body with 4 inches; it is then rammed with lining material to a
thickness of about 18 inches in all parts. There are 16 1-inch tuyeres
placed horizontally, and in the latest forms of converter, the air
is supplied by individual tuyeres which are connected to the blast
box, and which are provided with ball-valves to prevent leakages and
back-running during the necessary punching. The cavity is about 8 feet
× 4 feet by 6 feet deep when first made, and the converter then holds
conveniently about 7 tons of matte. The weight of lining is about 16
tons, and it lasts six to nine blows. The blast-pressure used is 16
lbs. per square inch.
The hood is bolted on to the body, and is furnished with conical
safety-pieces to give notice of the wearing through of the lining. The
converters tilt upon rails, which are strapped round the body, and
which travel upon rollers. Motion is communicated to the converter
either by connection with an electrical drive, or very often by
hydraulic power connecting through a rack to a pinion attached to one
of the trunnions. The air supply is usually from piston-driven blowing
engines, communicating through a blast pipe to the hollow supporting
trunnion of the converter, from which the air passes to the blast box.
[Illustration: Fig. 64.—Longitudinal Section of Basic-Lined Converter.]
(_b_) _The Basic-lined Converter._—The adoption of basic linings is of
such recent date that although the present form appears to have given
satisfaction, later developments in basic practice may cause further
modifications in design. R. H. Vail gives the following details:—
As at present operated, the basic-lined converters are long
barrel-shaped vessels consisting of a ¾-inch steel shell, 23 feet
long and 10 feet in diameter, lined with magnesite materials so as to
leave a cavity about 20 feet × 7 feet × 6 feet. Air is supplied from
thirty-two 1¼-inch tuyeres, each separately connected with the blast
box and controlled by a valve. Provision has to be made for the marked
expansion of the basic lining-material by leaving the top of the steel
shell open, joining-up the free ends by tie-rods (13, Fig. 65), whilst
the tuyere-pipe connections are flexible. The main opening or throat,
for the charging of matte and flux, is situated in the arch at one
end of the converter; it is 40 inches in diameter, and surmounted by
a short chimney-cap of iron, which is 30 inches high and lined inside
with clay. The vessel is charged through this opening. Metal and slag
are poured from the converter through an opening in the side opposite
the tuyeres, which is kept closed by bricks during the operations. An
oil-burner is provided at one end, for the purpose of supplying such
extra heat as might be required, in consequence of undue cooling of
the copper towards the end of the blow or for heating up the lining
after repairs. The converter is supported as in acid practice, though a
tilting device employing wire ropes attached to hydraulic plungers is
now being introduced in place of the rack and pinion method.
[Illustration: Fig. 65.—Basic-Lined Converter, indicating Tuyeres,
Lining, etc.]
=Converter Linings.=—The question of the lining has been the most
important consideration in copper matte converting-practice.