(_a_) Grade of matte.
(_b_) Temperature of matte.
(_c_) Condition of converter lining.
(_d_) Rate of production of matte.
(_e_) Condition of affairs at the casting
and refining furnaces.
As instances of the way in which some of these circumstances affect
procedure, the following examples may be quoted.
(_a_) When working with matte of low grade, especially in small
quantities, as formerly operated, the loss of heat by radiation and
by that carried away in the large quantity of slag produced, is very
considerable, whilst towards the later part of the blow, the amount of
sulphide fuel diminishes to such an extent that the maintenance of the
desired temperature is difficult. The bulk of the final copper-product
of the operation is very small and the metal is therefore liable to
chill. In such cases, the system of “doubling” is useful. This consists
of blowing the matte to the white-metal stage, pouring off the slag and
adding a further charge of matte. This, on the resumption of blowing,
restores heat and yields a charge of white metal sufficient to maintain
the required temperature for the last stage of the blow, as well as
affording a convenient yield of metallic copper.
(_b_) i. When working with a freshly-lined converter the charge is
necessarily rather less than usual, owing to the smaller size of the
cavity, and this results in a smaller yield of white metal, which is
also colder. At the white-metal stage the slag is poured off, and the
cavity having now become larger, owing to the fluxing action upon the
lining, a fresh charge of hot matte is added, introducing fresh heat,
further enlarging the cavity, and providing for a hot and plentiful
supply of white metal for the blowing up to blister copper.
(_c_) ii. When the lining commences to wear thin, the converter may be
retained solely for the purpose of blowing successive charges of white
metal up to blister-copper, since owing to the very low iron content of
white metal, there is little fluxing action on the lining during this
stage, whilst the large quantity of white metal which can be operated
in the enlarged cavity ensures a good supply of heat.
When linings burn through, the charge is transferred to another
converter and the bessemerising finished there.
The management of the converters as thus indicated, and the
distribution of the charges among the various converters are left to
the head skimmer, who has control of the converter floor.
_Working of the Basic-lined Converter._—The actual operations of
bessemerising in the basic-lined converter differ but little from
those where the silica lining is used. One important change has,
however, been made, viz.: the introduction of the siliceous flux before
the commencement of the blow. The lining having been heated up and
“seasoned,” the charge of four or five ladles-full (30 to 40 tons) of
matte is poured into the upright converter through the throat, 3 to 4
tons of siliceous flux, which must be well dried, are added, and the
blast is turned on gently (at 5 lbs. pressure), whilst the converter
is slowly turned back—these precautions being necessary in order
to prevent excessive blowing out of the dry siliceous fines at the
commencement of the work. When the silica is fairly well incorporated,
the blast-pressure is increased to about 10 to 12 lbs. per square inch,
the blowing is continued for 30 to 45 minutes, and after the silica
has been fluxed by the iron oxides—which is tested by feeling the
charge with an iron rod inserted through an opening in the breast—the
converter is turned over and the slag poured off. A fresh charge of
matte and a further quantity of siliceous ore are added and the blowing
is resumed, these operations being repeated several times until the
desired quantity of white metal has been accumulated, which is then
blown up to blister copper in the usual manner. During the early
stages of the blow, the operation is largely controlled by judging
the quantity of iron remaining in the matte, from the appearance of
small samples which are ladled out of the converter from time to time,
and from this, the quantity of siliceous material required for the
further fluxing is deduced. This material must be quite dry, so as
to flux evenly and not form floaters. One of the advantages of the
basic process is that siliceous ores containing values (the extraction
of which may be profitable) which might not be suitable for use in
siliceous linings, can be conveniently employed as flux in conjunction
with the basic lining, though naturally the best work is done with flux
containing a maximum of free silica. The character of the slag is not
very different to that produced in the silica-lined converter, though
it is usually lower in silica contents, and owing to the methods of
frequent pouring, it is lower in copper values.
_Special Features of Basic-lined Converter Work._—The basic-lined
converter tends to lose heat by radiation and conduction more quickly
than does the silica-lined vessel, due to the walls being thinner and
the lining material a better conductor. Owing, however, to the use of
larger charges, to the increased fuel value of the low-grade mattes,
and to the larger blast-volume used, heat is retained sufficiently
well for the successful operation of the bessemerising process. The
temperature is, however, generally lower than that obtained when
using the siliceous lining, and constant punching of the tuyeres is
necessary—two men being required per shift for this work. The great
advantages of the basic lining are connected chiefly with the fact that
the frequent relining associated with the silica-lined converter is
avoided, hence an extensive relining plant is not required, smaller
building space and a lighter crane can be used. The use of basic
linings further affords a means of extracting the copper and other
values from siliceous ores which can be used as flux, but which might
otherwise be difficult to treat, and it has made possible the cheaper
treatment of low-grade mattes.