(1) _Operating_— Amount. Per
Dry Ton.
Labour, $17,829·42 $0·2968
Power, 592·36 0·0099
Fuel oil, 88,028·99 1·4654
Coal, 243·61 0·0041
Water, 91·68 0·0015
Transportation, 380·45 0·0063
Sundries, 315·64 0·0053
Flux, 817·46 0·0136
————————- ——————-
$108,299·61 $1·8029
=========== =======
(2) _Repairs_—
Labour, $11,063·93 $0·1842
Supplies, 12,425·30 0·2068
Shop expense, 1,514·21 0·0252
———————— ——————-
$25,003·44 $0·4162
———————— ——————-
Total, $133,303·05 $2·2191
Steam credit, 48,861·86 0·8134
————————- ——————-
Net total, $84,441·19 $1·4057
=========== =======
_Gaseous Fuel._—The proposal to employ gaseous fuel in copper smelting
dates from the introduction of this method of furnace-firing by Siemens
50 years ago. It is, however, not in general use, although at several
smelters gas-firing is employed in furnaces for the refining of the
metal.
The chief difficulties have been in connection with the control of the
flame, burning-out of the roof having been a not infrequent occurrence
when employing gaseous fuel, and the method has been tried and given up
at the Great Falls Smelter in Montana, and at several other works.
The practical difficulties ought not, however, to be insuperable should
gas-firing be otherwise found most practicable for the particular
conditions at the smelter, although there appear to be certain physical
characteristics of such flames which may be responsible for some of the
difficulties met with in employing this type of fuel for the working of
very large reverberatory furnaces.
=The Condition of the Charge for Good Reverberatory Work.=—The
considerations which decide the advisability or otherwise of installing
at a smelter, any particular types of furnace, whether reverberatory
or blast furnace or both, cover a very wide field, and will be more
apparent when blast-furnace practice has been reviewed in detail. It
is clear that the blast furnace is unsuited for the direct smelting
of fine materials as such, and that the reverberatory form of furnace
is best fitted for their treatment when large quantities of this
material require to be dealt with. Actual practice has shown, however,
that the reverberatory does not give equally satisfactory results on
all classes of fines, and that there are certain physical and chemical
conditions of the charge which appear to be necessary for the most
successful and rapid smelting. When such conditions are not adhered to,
less satisfactory working has resulted. Recent experience has, to some
extent, defined more clearly the nature of these requirements, and has
indicated the procedure which is necessary in order to avoid an undue
supply of the less suitable material for the reverberatory charge.
It is usual to smelt in the reverberatory furnaces, where such are
available, the greater portion of the dust which accumulates in very
large quantities in the flues at the smelter. The reverberatory is the
only type of furnace in which such material could be treated directly,
under the present conditions of working. In practice, however, it has
been found in several instances, though not universally, that such dust
is considerably more difficult to treat in the furnace, and entails
considerably more expense in smelting than does the ordinary roasted
concentrate. It is estimated by Ricketts that this extra cost is
practically equivalent to the expense of roasting an equal weight of
concentrate.
Flue-dust, as a rule, consists mainly of material in a minute state
of division, in which condition, as is well known, a much higher
temperature is required for its fusion than if it were in the form of
coarser particles. This is largely due to the poor conductivity for
heat which generally characterises such dust, and to the insulation
by the air envelopes surrounding the individual grains, which thus
prevents the heat passing from particle to particle, and retards
their clotting, even when the prevailing temperature would otherwise
be sufficient to cause fusion. The particles of flue-dust moreover,
have been blown from the surface of the charge, especially in the
blast-furnace process, and are thus rapidly and often almost completely
oxidised in passing through the oxidising atmosphere which prevails
above the charge and in the flues. Such oxides clot only with the
greatest difficulty, and are characterised by comparative infusibility
and poor conducting power, and hence are found to melt with considerable
difficulty when treated in the reverberatory furnace.[10]
Roasted fine concentrate, on the other hand, constitutes an ideal
material for the reverberatory furnace charge, and the system of
passing both the concentrate and the flux through the roasters has
been shown to possess numerous advantages. In addition to the thorough
mixing and the preheating of the furnace charge, it was found that
its chemical and physical conditions were particularly well suited
for the subsequent reverberatory furnace treatment. The particles of
concentrate, being gradually heated and constantly stirred in the
presence of the small proportion of flux usually required, roast well,
and lose the desired quantity of sulphur without an undue amount
of preliminary clotting which would otherwise interfere with the
operation, whilst any residual sulphide in the product is uniformly
distributed through the roasted charge. In addition, at the higher
temperatures which prevail in the later stages of the roasting process
when almost as much sulphur as was desired has been driven off, the
materials are raised to a point approaching incipient fusion and
slagging. The heat in the reverberatory furnace is sufficient to
complete this effect, and enable the necessary chemical combinations
and physical separations to be readily accomplished.
The roasted concentrate should therefore form the main proportion of
the reverberatory charge, working in with it, in moderate quantities,
such flue-dust as is made at the smelter. Of this flue-dust, it is
naturally desirable to produce as small an amount as possible, not only
on account of the difficulties in subsequent treatment, but also on
account of the actual losses in the economy of the furnace processes
and the cost of rehandling, etc. In modern smelting, naturally, every
effort is made to reduce the quantity of dust to the lowest practicable
limit.
The greater portion of the dust results from the treatment of
unsuitably fine material in the blast furnace, and by decreasing the
quantity of this constituent the flue-dust problem will be largely
overcome. The smelting of fine concentrate in the blast furnace has
up to the present been considered judicious where circumstances
have rendered imperative the addition of sulphides to the charge
irrespective of their physical condition (either to act as a base for
the matte, or on account of their fuel values), though naturally the
proportion of fines has been kept as low as possible.
The recent developments in sintering processes, however, suggest the
possibility of the future successful treatment, after preliminary
agglomeration, of fine concentrate in the blast furnace, and if it
be found possible to conduct the sintering by utilising the heat of
oxidisation of the more free sulphur atom of the pyrites, and thus
leave the bulk of the iron-sulphide fuel values in the sintered
product, as suggested by Peters, the difficulties in connection with
excessive flue-dust production from the above causes will be largely
overcome, and the reverberatories will thus be relieved of this
difficult constituent of their charge.
It therefore appears desirable, when circumstances permit, either to
agglomerate fine concentrates and then treat them in the blast furnace,
or else to roast them and smelt the product in the reverberatories.
So far as present experience has gone, it appears that—other
circumstances being equally favourable—the correct scheme of treatment
depends almost entirely upon the composition of the concentrate, there
being for each process a particular class of fines for which it is best
suited. The sintering process deals most satisfactorily with one class
of concentrate, whilst the roasting process seems more particularly
suited for a different type of material.
Thus the higher the iron and sulphur values, and the lower the silica
content, the more successful, cheap, and efficient is the roasting
process—the Anaconda material for example roasts well, requires
practically no external fuel or heating, and with the added flux, works
very successfully in the reverberatories.
As the silica content increases, however, and the iron and sulphur
contents diminish, there is a consequent decrease in the natural fuel
values of the material, and as a result, the roasting is neither so
efficient nor so cheaply operated, owing to the need of external fuel
for giving the required roasting temperatures. On the other hand, it
appears to be just this class of material which is best suited for
blast-roasting.
It is found in actual working practice that material which does not
contain a certain proportion of silica does not work well in the
blast-roasting or sintering processes, the resulting product being
found to be more irregular in composition and more difficult to operate
in the sintering plant. It would therefore appear that a certain class
of fine concentrate higher in silica and lower in iron and sulphur
contents, which is not quite so suitable for ordinary roasting (owing
to the necessity for external heating, due to lower fuel values) is
eminently suited for blast roasting or sintering processes, yielding
lump products very suitable for subsequent blast-furnace treatment.
The reverberatory furnace thus deals most successfully with fine
table concentrates high in iron and sulphur, moderately low in
silica; roasted, with its required flux, to the necessary extent, and
then charged whilst still red hot into the furnaces. To relieve the
reverberatories of the greater bulk of the blast-furnace flue-dust,
which it treats with more difficulty, fine concentrates, as such,
require to be kept out of the blast-furnace charge, either by
subjecting the more siliceous material to a preparatory sintering
process, or by reserving the highly pyritic variety for roasting and
subsequent reverberatory treatment.
References.
Peters, E. D., “Principles of Copper Smelting.”
Offerhaus, C., “Modern Reverberatory Smelting of Copper Ores.”
_Eng. and Min. Journ._, June 13, 1908, pp. 1189–1193;
June 20, 1908, pp. 1234–1236.
Ricketts, L. D., “Experiments in Reverberatory Practice at Cananea,
Mexico,” and discussion,
_Trans. Inst. Min. and Met._, vol. xix., 1909–10, pp. 147–185.
Ricketts, L. D., “Developments of Cananea Practice.”
_Engineering and Mining Journal_, Oct. 7th, 1911, p. 693.