(_a_) Fusible at the temperature of furnace working.
(_b_) Fluid and run easily.
(_c_) Of sufficiently low specific gravity as will allow
of good settling and separation of the matte or
metallic products.
It is well known that within certain broad limits of silica content,
slags will fulfil these conditions to a greater or less extent, whilst
the most suitable and economic slag under any particular circumstances
is decided, as stated above, by the composition of the charge, the
quantity and character of the available fluxes, and the previous
experience with the furnace. The limits of the silica content for
suitable slags as just indicated are fixed by several well-known
general properties of the silicates.
Speaking broadly, and from the point of view of the more or less
ferruginous silicates constituting copper-smelting slags, the
more basic silicates—such as the subsilicate class (oxygen in
acid : oxygen in base < 1 : 1)—are generally characterised by high
formation-temperature, and by being very fluid, thin and fiery, dense
and corrosive. On the other hand, the more acid silicates, such as
those of the multi-silicate class (oxygen in acid : oxygen in base >
2 : 1) are characterised by lower formation-temperature and low density,
and by being thick and viscous.
As the silica content within this range of silicates increases, the
melting point is lowered and the specific gravity is reduced, features
which are very advantageous from the point of view of the production
of clean slags. Their fluidity, however, decreases, and a very high
temperature is thus required in order to render them sufficiently
limpid to run freely from the furnace. On this account the highest
proportions of silica usually considered feasible in a slag, correspond
to the bisilicates of the representative composition, MO. SiO_{2}.
With high temperature conditions in the furnace and rapid working,
such slags can be dealt with successfully, and if the charges are
necessarily highly siliceous, it may be advantageous from the economic
point of view to work with this class of slag.
In proportion as the silica content gradually decreases and as they
become more basic, the silicates are more and more corrosive and fiery,
and especially in the case of the iron silicates, they gradually attain
such a high specific gravity that efficient settling of the matte is
not possible. In addition, the more basic the silicate the greater is
its dissolving power for sulphides, hence high copper losses in the
slags result from these combined causes. Such basic silicates possess,
however, the advantage of marked liquidity, and of flowing from the
furnace in a thin limpid stream. The high density and the solvent power
of basic slags thus fix a limit to the composition which is considered
economically suitable, and the lowest proportions of silica usually
worked with correspond to the mono-silicates represented by the formula
2MO. SiO_{2}. Slags containing a greater proportion of base (usually
iron) possess too high a density to permit of clean settling. In
practice, therefore, the majority of slags are mixed silicates of a
composition ranging between the limpid but somewhat dense mono-silicate
and the lighter but more viscous bisilicate, corresponding to silica
contents of from 30 to 48 per cent. of silica, and within the limits of
35 to 45 per cent. of silica most copper blast-furnace slags will be
found. The composition roughly corresponds in a large number of cases
to that of the sesqui-silicates of the general formula 4MO. 3SiO_{2}
(oxygen in base : oxygen in acid :: 4 : 6 :: 1 : 1½).
As is well known, mixed silicates—_i.e._, silicates of two or more
bases—are generally characterised by the properties of increased
fusibility, and often of increased fluidity, and their employment is
usual and generally advantageous in smelting practice. The relative
proportion between the various bases in such mixed silicates is largely
a matter depending upon the prevailing conditions at the smelter.
In modern smelting, particularly where partial pyritic work is
conducted, and where fairly siliceous charges are worked, a slag
running about 40 per cent. SiO_{2} is aimed for, iron and earth oxides
constituting the remaining 60 per cent. or so. In cases where this
quantity of iron is present in the charge, the slag may be constituted
chiefly of iron silicate, but even in such instances the advantages of
lime additions are marked. When iron is not available in sufficient
quantity, the extra fuel costs and working difficulties of running
with more siliceous slags would render their production undesirable,
and the purchase of limestone or similar earthy flux is particularly
advantageous. The purely iron silicates are usually dense, and thus
tend to hold up copper values both in mechanical suspension as well
as in solution; the addition of lime, which has a marked effect in
reducing the specific gravity, permits of more basic slags being worked
with, where necessary, without such heavy losses in the slag.
The presence of lime silicate with the iron silicates has a marked
influence on the fluidity of the slags, even when they are more highly
siliceous, whilst on account of the lower atomic weight of calcium,
lime will, weight for weight, flux off a greater quantity of silica
than will ferrous oxide. In forming a slag of similar oxygen ratio,
thus—
Mono-silicate of lime, 2CaO . SiO_{2},
Lime : silica :: 112 to 60, or 1 part to 0·54 part.
Mono-silicate of iron, 2FeO . SiO_{2},
Iron oxide : silica :: 144 to 60, or 1 part to 0·42 part;
hence for the production of a slag of the same oxygen ratio, less
weight of lime would be required to flux off the same weight of silica;
in other words, the replacing values of the two oxides are as 112 to
144, or 7 to 9.
Of the other bases which are occasionally present in slags, the
proportions of the oxides of magnesium and zinc are sometimes
considerable, the calculations being analogous to the previous
cases. The case of alumina is anomalous, and its behaviour in slag
production is not definitely understood. Many experienced workers
hold the view that it tends to act either as acid or base, according
to the proportions of silica. Thus, in a very siliceous slag, alumina
in moderate quantity behaves as a basic oxide, forming aluminium
silicates, and in very basic or low silica slags the alumina appears
either to neutralise some of the excess base, acting as an acidic
oxide, or to dissolve as such in the slag, whilst in intermediate cases
it possibly behaves partly as an acid and partly as base. This view
has recently been questioned, and it has been suggested by Shelby that
alumina always acts as an acid in the formation of slags. The matter is
thus one which requires further considerable investigation.
Usually neither alumina nor zinc oxide behave very satisfactorily in
the furnace when present in large quantities, tending to thicken the
slags and to promote viscosity.
=Anaconda Practice in Charge Calculations.=—An example of some of the
practical considerations which enter into the calculation and making up
of charges is well illustrated in certain particulars of the practice
as conducted at Anaconda. Details of the materials charged over a
period of one month are indicated in Table X. The important charge
constituents available in large quantity include:—
Cu. SiO_{2}. Fe(O). S.
% % % %
First-class smelting ore, 8·6 54·0 13·6 14·0
Concentrates, 10·9 26·0 32·0 32·0
Briquettes, 5·0 50·0 13·0 13·0
Lime-rock (flux), .. .. .. ..
Old converter slags and residues, .. .. .. ..