(1) To preserve the steel shell and form a permanent
receptacle for the molten materials; by reason of
its refractory character.
(2) To prevent undue losses of heat from the materials;
by reason of its low conducting power.
The employment of the lining material as a provider of suitable
siliceous flux for the iron oxide, though until recently of vital
importance for the practical operation of the bessemerising process,
has been a necessary evil in many cases, and although it might have
been a source of considerable profit under certain conditions, this
function is unlikely in the future to be the consideration of greatest
moment.
The vital requirements in modern converter practice are permanence
of the lining and efficient means of effecting the fluxing of the
iron oxide produced in the converting operation. The necessity for
the frequent relining of converters involves not only heavy direct
expenses, but it occasions waste of heat in the old linings, waste of
material, loss of time, interruption of the processes, liabilities to
outbreaks from the converters, and necessitates much heavy machinery
for the conveying of vessels for relining, as well as large capital
outlay in relining shops, plant, and appliances. In consequence, the
employment of siliceous lining material as flux is usually a most
expensive method of supplying the requisite silica; and so much is this
the case, that an arbitrary limit to the iron contents of the matte has
been rendered necessary, in order to prevent too much of the lining
material being used up at a single blow. It was found cheaper to use
other means of concentrating low-grade matte to a suitable grade for
bessemerising—_i.e._, to flux off the excess of iron by means of silica
in the blast-or the reverberatory-furnace processes.
_Siliceous Linings._—Until recently, the only method for fluxing the
iron in bessemerising, found practicable on a commercial scale, has
been by the destruction of the siliceous lining, minimising the dead
losses as much as possible by employing for the purpose siliceous
materials from which values in the form of gold, silver, or copper
could be simultaneously extracted and collected in the products of the
operation.
Numerous attempts were made to effect combination of the iron oxides
with silica introduced by some other method, but none met with success.
Manhès blew sand through the tuyeres, and obtained as result a spongy
unfused mass in the converter—whilst silica introduced in the form
of lumps rose to the surface unchanged. In each case what silica was
required for flux, was taken up from the siliceous lining. Experiments
of a similar nature, in which basic linings were worked with, resulted
in the fluxing silica being unabsorbed as before, whilst the iron
which was in process of oxidation, not finding a suitable flux, became
super-oxidised, resulting in the production of very infusible masses
of magnetic or ferric oxides which rendered the process unworkable.
Baggaley and others in Montana devoted much attention to experiments on
different methods for introducing silica which would flux successfully,
methods such as superheating or introducing silica held in suspension
in fused silicates being tried, but without marked success, and for
many years siliceous linings were necessarily worked with.
Owing to the large quantities consumed, the siliceous material must
be obtainable cheaply and in abundant quantities. It should be high
in free silica contents, since this constituent alone is effective
as flux; it should have the property of binding well with clay or
other material, so as to yield a rigid and impervious lining; and
most important of all from the economic standpoint, it should carry
values, since by this means only, could its destruction become an
actual source of profit. At first barren quartz and barren clay were
largely used for linings, but practice gradually developed in the
direction of employing more profitable materials, and especially those
from which the extraction of the values might present difficulties, in
treatment by ordinary smelting methods. The practice as followed until
recently at Anaconda is typical of such progress. Until 1908 the lining
was chiefly made from highly siliceous ore obtained from Snowstorm,
Idaho, carrying 80 to 85 per cent. of SiO_{2}, 4 per cent. copper, as
well as gold and silver, and a little iron and sulphur. This ore was
crushed in mills and mixed with sufficient slime from the slime ponds
of the concentrating plant to make a binding mixture. The slime, which
carries about 60 per cent. of silica and also 2·5 per cent. of copper
has excellent binding properties, owing to its clayey consistency. The
proportions employed were 3 of siliceous rock to 1 of slime—no water
was used, the mixture being almost dry to the touch. Since May, 1909,
instead of employing ore obtained from outside sources, siliceous
second-class Butte ore, which was formerly concentrated, has been
very largely incorporated in the mixture used as lining material, it
contains 65 per cent. silica, about 3·5 per cent. copper, a little gold
and silver, and also iron and sulphur. The lining mixture consisted
of 2·9 parts of this material with 1 part of slime. It was thought
at first that owing to the greater proportion of sulphides and the
lower silica content of the Butte ore, this lining mixture might prove
inefficient compared with the former material, but with somewhat
greater care in lining, it was found that very little more ore was
required, and that tested by comparative silica contents it was more
effective. Thus, where the former linings lasted for an average of six
7½-ton charges, equal to 20½ tons of copper per lining, the new ones
last 5¼ such charges, equivalent to 17¾ tons of copper per lining,
showing that although the efficiency per lining was reduced to 90 per
cent., yet, calculated on comparative silica content, the new lining
proved to be the more efficient.
The operation of lining is conducted with much care; the old lining
is knocked away where necessary, rods are placed through the tuyere
holes, and lining mixture is dumped in; 6-inch layers of material at a
time being stamped down hard by means of an Ingersoll-Sargent tamping
machine, until the lining reaches within 6 inches of the tuyeres. The
wooden mould for the cavity, made up of a number of jointed pieces, is
then placed in position, and the ramming of layer after layer round
the sides is continued as before. The hood, inverted, is lined in a
similar manner, it is then placed in position on the converter body
and bolted down, a joint being made of moistened lining material. The
whole operation takes about 1½ hours. The converter is then slowly
dried by a wood fire, coal being subsequently added and kept burning
under the action of a low blast for five or six hours; it is conveyed
to the stand when required, dropped into position on the trunnion
bearings, and the connections and adjustments very readily made.
The manipulation of relining at the Tennessee smelter is conducted in a
very similar manner.
_Basic Linings._—The all-important feature of the basic lining is its
permanence, which, rendering the frequent relining of the converter
unnecessary, allows of many economies in connection with capital
outlay on plant and in operating costs. Further, owing to the lessened
need for lining repairs, the frequent hauling of converters to the
repair-shops situated at the further end of the buildings is avoided.
This allows the employment of much larger converter units, with obvious
attendant advantages, whilst it increases the ultimate possibility of
continuous operation. Thus, the size at present employed, though the
process has been in operation but a short time, is 26 feet by 12 feet,
with a capacity of 35 to 45 tons of matte, and a daily output of 33
tons of copper from 40 per cent. matte. Such a converter, lined with
9 inches of basic material, will operate for 2,000 to 3,000 tons of
copper before requiring repairs.
Keller’s report on basic linings in 1890 stated that they could not
be employed successfully, because (_a_) basic material, being a good
conductor, caused the outside of the converter to become too hot and
the inside too cold; (_b_) such material broke up easily and so was
unsuitable for use in permanent linings; and (_c_) even when basic
linings were employed, the silica which was added as flux, refused
to combine with the iron oxides. These views were very generally
accepted for some years, until Baggaley’s persistent efforts and
finally those of Pierce and Smith showed that by perfecting the
constructional methods and details, by preventing heat losses as much
as possible, and by operating on very large masses of hot material,
the above difficulties could all be overcome and the basic lining
successfully employed. The lining is of magnesia brick, and is 9 inches
in thickness, except at the tuyeres, where the bricks are 18 inches
thick. In the bottom of the converter and extending to within 18 inches
of the tuyere level is placed a filling of ordinary firebrick, which
is 13½ inches thick in the middle and 4 inches thick at the sides. The
magnesite bricks are laid in dry magnesite powder, except near the
tuyeres, where a mixture of magnesia and linseed oil is used. Expansion
cushions of wood are inserted at intervals along the side of the fresh
linings which are then “seasoned” with molten copper.
The required quantity of siliceous flux, as calculated, is now
successfully introduced by dumping it into the converter, and pouring
the matte charge upon it.
=The Grade of Matte for Converting.=—The grade of matte which is
economically the most profitable to treat in the converter is a factor
of great importance, since, if limits be fixed, the preliminary
smelting stages for matte production are made less flexible, whilst in
order to obtain matte of the correct grade, the smelting operations may
require to be conducted at greater cost, or else additional smeltings
for further concentration of the first matte may be necessitated—as is
the case, for instance, in pyritic smelting at present.
The grade of a matte is usually expressed in percentages of copper,
but from the standpoint of the practical converter operations, the
proportion of iron is the factor which decides the suitability or
otherwise of the matte for treatment, and since mattes may be regarded
as mixed sulphides of iron and copper, a matte rich in copper is
correspondingly low in iron contents, whilst a low-grade matte is high
in iron.
The importance of the iron contents of the matte from the viewpoint of
converter practice is due to iron being the chief source of heat in the
operations, and to the fact that the iron oxide produced from it is the
constituent which requires a supply of flux in order that the reactions
may proceed and the process be successfully operated. The economic
limit to the grade of matte suitable for the converter process is
reached when it becomes less costly and more profitable to supply the
required siliceous flux for the iron in the ordinary smelting furnace
rather than in the converter. So long as the destruction of the lining
was practically the only medium by which silica could be efficiently
supplied, the limit to the iron contents of the matte was fairly rigid.
The bessemerising of a low-grade matte (low in copper contents, high in
iron) entails the great advantage that a high temperature is obtained,
owing to the fuel-value of the iron. On the other hand, however, grave
disadvantages attend such practice, especially when working with the
comparatively small quantities of material usually operated, and when
employing siliceous linings. These disadvantages include the factors
that—
(_a_) Large quantities of iron oxide are formed, which
require siliceous flux.
(_b_) Large quantities of highly ferruginous slag are
produced which carry copper values, and which also
demand special attention in operating.
(_c_) The quantity of copper obtained is comparatively
small, thus increasing the proportionate losses
and working difficulties.
In bessemerising a high-grade matte, the heat production is much
smaller, owing to the decrease in the quantity of iron, which is the
chief fuel of the process, and the limiting grade is quickly reached
above which the bessemerising operation upon the matte ceases to be
self-supporting.
In consequence, up to a comparatively recent date, a compromise has
necessarily been effected, and the grade of matte operated upon has
been such as to cause as much heat production as possible, together
with the smallest practicable amount of fluxing action.
On these grounds, a matte containing from 40 to 50 per cent. of copper
(equivalent to 32 to 22 per cent. of iron) has been found generally the
most suitable. At several smelters, lower-grade mattes of from 32 to 40
per cent. copper-contents are converted most profitably, owing to such
special circumstances as the profits resulting from the destruction of
lining material, or in consequence of the fact that greater operating
costs would be involved in concentrating the matte to a higher grade by
the ordinary furnace-smelting methods.
In this connection, the successful adaptation of the basic lining by
permitting the supplying of flux by means other than from the linings,
has very important application and possibilities.
Owing to the frequent relining of these converters being then no longer
necessary, mechanical difficulties of conveying the converter bodies
to the relining shops are lessened, and larger converter units can
now be employed, treating, even at the present stage of development,
between six and seven times as large an amount of matte as formerly.
By operating on such big charges, pouring off slag as produced, and
adding fresh matte and flux without fear of destroying the lining, the
difficulties attending the converting of low grade mattes have been
successfully overcome.
The limit to the grade of matte economically suitable for the process
will depend, in the future, chiefly upon the comparative costs of
effecting the required concentration up to any desired grade, in the
blast-or reverberatory-furnace, or in the converter.
The modern smelting scheme appears, therefore, likely to develop into
the preliminary smelting of the ores by the cheapest method available,
for matte of a grade best suited economically to the running of the
furnace, the grade being independent of any rigid limit for the
subsequent converting operations—the matte being then bessemerised as
usual.
=The Converting Process=—_Acid Lining_.—There are two main stages in
the converting of copper mattes. The first is essentially elimination
of iron sulphide; the second, elimination of the remaining sulphur.
The product of the first main stage is a white metal, practically pure
copper sulphide, the iron of the matte having been slagged off in the
form of silicate, and the corresponding sulphur eliminated as SO_{2}.
The reactions during this stage are well known: the oxygen of the air
blown in, yields oxides of iron and of sulphur, as well as some copper
oxide. The latter, immediately reacting with iron sulphide which still
remains, re-forms copper sulphide, with the production of more iron
oxide. The iron oxides are fluxed by the siliceous materials present,
forming ferrous silicate slags. The iron oxidation is productive of the
greater part of the heat in the operation, and high temperature usually
marks this stage of the process, which may be termed “the slagging
stage.”
The flame which issues from the converter during this period is usually
characterised by a green colour, caused apparently by the formation of
iron-silicate slag.
When this stage is completed and the slag poured off, the white metal
is blown up to blister copper—this constituting the second main stage
of the process. The chief reactions are those of sulphur elimination
and the production of metallic copper, caused by the action of some of
the copper oxide first produced, upon the copper sulphide still present.
The flame during this period is small, thin, and fairly non-luminous,
usually of a red-purple to bronze purple colour.[16]
The progress of the blowing from copper matte to white metal and thence
to blister copper is usually indicated and controlled at the smelter by
the appearance of the flame which issues from the nose of the converter
during the first periods, and by the character of emitted shots during
the later stages. This is particularly the case with mattes of moderate
purity worked in the silica-lined converter. The successive changes
in these indications are gradual, but are easily followed by the
experienced skimmer, who is thus able to judge readily as to the manner
in which the blow is progressing, and also as to the temperature,
composition, and nature of the metal in the converter.