=MODERN BLAST-FURNACE PRACTICE= (_Continued_).
Charge Calculations — Charging — Working — Disposal
of Products — Pyritic Smelting — Sulphuric Acid
Manufacture from Smelter Gases.
=Charge Calculations.=—Modern practice aims at the production of a
matte of converter grade, containing usually from 40 to 50 per cent. of
copper, and preferably in a single smelting operation; except in true
pyritic work.[12]
Full analysis of the whole supply of material available at the smelter
is essential, as well as a report on the quantities of each separate
constituent.
The first step in the charge-calculation is the computation of the
total weights of copper, iron, and sulphur available for the smelting
campaign; from these quantities the losses of copper and sulphur to
be allowed for during the operation itself, as based on previous
experience, are deducted. The balance indicates the quantities of these
elements from which the matte and slag can be produced. The copper is
transformed into matte, in which product it may be regarded as existing
in the form of copper sulphide, Cu_{2}S, and the sulphur required for
this combination with the copper is calculated from the relation—
Cu_{2}S = Cu_{2} : S :: 2 × 63·5 : 32
:: 127 : 32
:: 4 : 1 approximately.
Thus every unit of copper combines with one-quarter of its own weight
of sulphur.
A matte of converter grade containing, say, 44 per cent. of copper is
constituted as follows:—Copper, 44 per cent. + sulphur, 11 per cent.,
or copper sulphide, 55 per cent., the remaining portion of the matte
being iron sulphide, which amounts to 100 − 55, or 45 per cent.
Assuming as a first approximation that this iron sulphide has the
formula FeS,[13] the proportions of iron to sulphur in this material are
Fe : S :: 56 : 32
:: 7 : 4
hence 7/11 of the remaining 45 per cent. of the matte is iron and 4/11
is sulphur—that is, the matte contains in addition, iron 28 parts,
sulphur 17 parts. Hence the composition of the converter matte is
approximately—Copper 44 parts, iron 28 parts, and sulphur 11 + 17 = 28
parts.
The amount of copper for the matte is fixed by the available ore
supply; the quantity of sulphur is controlled by the furnace operation
and charges, as judged from previous experience—the oxidation being so
regulated that the proper grade of matte is produced. The iron required
for the matte is next considered. Every 44 parts of copper require 28
parts of iron for the production of a matte of the correct grade. If
the quantity of iron in the materials available at the stock-bins be
not sufficient to furnish the amount required, as just calculated,
ferruginous material must be added as flux, if, on the other hand,
there is a superabundance of iron available in the charges for this
purpose, the excess must be fluxed off.
In this manner the amounts of the constituents for the matte production
are determined, and the composition and making up of the slag-forming
constituents are next considered. In this connection the local
conditions with respect to proximity and cost of suitable flux, as well
as experience with the previous working of the furnace and ore charges
are important factors in determining the type and composition of the
slag to be made, whilst in true pyritic practice the special conditions
of working fix certain limits to the composition of the slag, as will
be indicated later—the pyritic furnace “tending to make its own slag.”
In partial pyritic smelting, the coke allowance and the furnace
conditions allow of fairly wide latitude in making up the charges
for the production of suitable slags with which the furnace can
deal efficiently, since the heat production is not dependent on the
formation of any particular slag. It is always possible to add extra
coke for the purpose of melting the slag desired.
The scientific principle governing the calculations for slag
composition is the proper proportioning of acid and basic constituents.
This is based upon the oxygen ratio—_i.e._, the proportion of oxygen
in the acid constituents compared with that in the bases. With the
doubtful exception of alumina in certain cases, silica constitutes the
entire acid portion of most copper-smelting slags.