(1) In the melting down of a furnace charge, the copper has first claim
on any sulphur which may be present.
(2) Only such sulphur as remains in excess after the copper has been
satisfied, is free to combine with other constituents of the charge.
These fundamental principles can best be illustrated by following the
reactions during the smelting of a typical charge. Thus—
Copper—-+
\
+ ➡ Cu_{2}S—+
/ \
Sulphur—+ + ➡ × Cu_{2}S. yFeS ... _Matte_.
\ /
+ ➡ FeS—-—-+
/
Iron——-+
\
+ ➡ FeO——+
/ \
Oxygen—-+ + ➡ × FeO. ySiO_{2} ... _Slag_.
/
Silica ➡ SiO_{2}—+
The copper takes up sufficient sulphur to form Cu_{2}S; the remaining
sulphur combines with any iron which is available, forming FeS. These
two sulphides, dissolving in all proportions, constitute the _matte_
product of smelting.
The iron in excess of that required by the sulphur becomes oxidised,
and the resulting oxide combines with silica in the charge, forming the
silicate _slag_ of the smelting operation.[5]
It will thus be apparent that, in general, the larger the amount of
sulphur present in a furnace charge, the more FeS will there be in the
matte after melting, and the smaller will be the proportion of copper.
In consequence, the grade of the matte will be lower.
The proportion of sulphur in the charge thus controls the concentration
of the copper by the smelting operation, and, in order to effect the
desired concentration, oxygen is required in order to burn off sulphur
and to oxidise iron. There are two general methods of supplying this
necessary oxygen.
(1) By a preliminary oxidation of the charge outside the smelting
furnace—_Roasting_.
(2) By oxidation inside the smelting furnace itself—_The pyritic
principle_ (to be considered later).
=Modern Practice as regards Roasting.=—In modern copper smelting, the
tendency is to do away with roasting as much as possible.
_Objections to Roasting._—(1) Expense involved by a separate
preliminary process. This includes
(_a_) Preparation of the ore for roasting.
(_b_) Extra ground, and plant required for handling.
(_c_) Labour, fuel, etc., required.
(_d_) Extra handling of material before and after roasting.
(2) Heavy mechanical and other losses during the process.
(3) Loss of the fuel value of the iron and sulphur for smelting.
(4) Necessity, in the majority of cases, of having the ore in a
fine state of division in order to conduct efficient roasting, thus
militating against its subsequent use in the blast furnace, unless the
product receives preliminary agglomeration.
Thus at Tennessee, the cost of roasting was about 40 cents, or 1s. 8d.
per ton of ore (equivalent to ½d. on every pound of copper produced).
The cost for the year 1903 amounted to £19,000, employing 170 men
out of a total staff of 900 at mines and smelters. The conditions
for roasting were here exceptionally favourable. The closing of the
roast-yards set at liberty £34,000, which had been tied up in this
manner.
_Advantages of Roasting._—Illustrative of the conditions under which
roasting is advantageously conducted in modern practice, the case of
the Butte second-class ores may be quoted.
These ores contain about 5 per cent. of copper in the form of
sulphides, finely disseminated through large quantities of siliceous
gangue. Direct smelting in a blast furnace would not yield a matte
of the desired “converter” grade, except at very heavy expense
and difficulty. The ore is, therefore, wet-dressed up to 9 to 10
per cent. copper, and the coarse concentrates now help to yield a
good matte, when smelted in the blast furnace. By the wet-dressing
treatment, however, a considerable quantity of fine material is
unavoidably produced, for which the most convenient treatment in
such large quantities, under prevailing conditions, is in the
reverberatory furnace. The atmosphere of this type of furnace being
to a great extent neutral, the charge would tend simply to melt down
without very much reduction of sulphur, resulting in the production
of very low-grade matte. Roasting of these fine concentrates is,
therefore, desirable for reducing the sulphur to such an extent as
will yield a high-grade converter matte.[6] Roasting being thus often
advisable as a preliminary, its inclusion in a smelting scheme under
suitable conditions entails the following advantages over the direct
reverberatory treatment of unroasted ores:—
(1) It ensures satisfactory concentration on smelting.
(2) It leaves reverberatory furnace smelting practically
a remelting operation, and so affords exact control of
the concentration effected.
(3) The roaster gases may be utilised for making acid.
In modern practice the work of the reverberatory plant is controlled at
the roasters. The reverberatory foreman smelts whatever mixture is sent
from the roasting plant, and if the grade of the resulting matte is not
satisfactory, it is in the roasting operations that the required change
is made for the correct adjustment of the sulphur and for controlling
the consequent tenor of the matte.
=The Reactions of Roasting.=—The operation of roasting is the exposing
of a substance to the effects of heat and air, in order to oxidise it,
and to render it more suitable for subsequent smelting operations.[7]
In the case of the ordinary sulphide copper ores, roasting not
only (_a_) reduces sulphur, and so ensures good concentration on
smelting, but (_b_) by oxidising the iron, provides a ready flux for
siliceous gangues. The more important reactions occurring to the usual
constituents of the copper ores which are roasted, may be summarised as
follows:—
_Iron Pyrites._—First loses free sulphur at a low temperature: it is
generally assumed that FeS is left, but the residual sulphide rarely
attains this composition—
FeS_{2} ➡ FeS + S.
_Iron Sulphide._—Sulphur has a great affinity for oxygen, to form
SO_{2} and it may be assumed that this reaction first takes place thus—