_A. Concentration or Wet Dressing._
_B. Agglomeration of Fines—(a) Briquetting, (b) Sintering._
_C. Roasting._
_A._ =Concentration or Wet Dressing.=—In treating the ores of copper,
it may be noted that in general—
_Native Ores_, unless very massive, are usually dressed in a special
manner peculiar to themselves—_e.g._, stamp-milling.
_Oxide Ores_ are rarely wet-dressed. They present much difficulty in
treatment on account of their comparatively low density, which makes
efficient wet concentration almost impossible, whilst heavy losses in
the tailings generally accompany such operations.
_Sulphide Ores._—No definite rules can be laid down as to whether the
ore should be wet-dressed or not; the treatment depends altogether
on attendant circumstances, such as—(_a_) the character of the ore,
(_b_) the concentration of the copper desired in the first smelting
operation, and (_c_) the smelting method and furnaces adopted.
Wet concentration is only profitable when the copper ore is of low
grade, and then only under suitable conditions. Thus the low tenor
may be due to admixture with much gangue or with other sulphides, or
both. A massive low-grade pyritic ore carrying but little gangue is
not suitable for such treatment, since the mixed sulphides are not
separated from one another by wet dressing, and consequently but little
enrichment of the copper in the dressed product would be possible;
apart altogether from other considerations. Such is the case, for
instance, with the Tennessee ores carrying about 2·0 per cent. of
copper and only 25 to 35 per cent. of gangue.
An ore with a self-fluxing or almost self-fluxing gangue might allow of
its copper being concentrated more cheaply and conveniently by direct
smelting than by wet dressing, this depending, of course, on the local
conditions.
In other cases a balance has to be struck as to whether the
circumstances are more favourable for removing the excess of gangue by
means of crushing and treatment in a stream of water, or by slagging it
off in a furnace with the addition of suitable fluxes. In many cases,
with low-grade ores, the former treatment is the cheaper.
The case of the low-grade ores of the Butte, Montana, district, affords
a good example of these considerations. This ore contains 5 to 5½ per
cent. copper, with a large quantity of highly siliceous gangue. It
was found that the purchase and carriage of sufficient flux, and the
cost of carrying out this fluxing operation was so expensive that it
was cheaper to build a concentrator and smelter at Anaconda, 30 miles
away—in a locality where a suitable water supply was available for the
dressing—and to convey the ore this distance in order to concentrate it
by a wet method. The dressed ore assays 9 to 10 per cent. of copper.
It is important to note that the process of wet dressing involves
crushing the ore, and yields the product in a more or less finely
divided form. Most copper sulphide minerals are exceedingly brittle,
and break up to a very small size on crushing for concentration, so
that the copper concentrates usually include a large quantity of fine
material.
There are two general types of furnace available for
smelting—reverberatory furnaces and blast furnaces—and the questions
of the desirability and of the degree of crushing and concentration
depend to a large extent on the plant and furnaces adopted or proposed.
Blast-furnace treatment has hitherto often been considered the most
economical process for smelting copper ores, especially with regard to
fuel costs, but for many reasons it is not a convenient or efficient
furnace for the direct treatment of fine material. When it is desired
to employ the blast furnace, it is necessary to make up charges
consisting, to as great an extent as possible, of coarse material.
In consequence, when concentrating ores with a view to subsequent
blast-furnace treatment, the degree of crushing and dressing has to be
modified with these factors in view; otherwise a further preliminary
manipulation of the fine concentrates that are produced is rendered
necessary. Such modified dressing schemes involve a maximum of coarse
breaking and screening, the crushing and separating stages being thus
very gradual, and the units in the plant are multiplied, whilst the
process is rendered complex in consequence. With the greatest care,
moreover, large quantities of fines are bound to be produced, and have
to be dealt with by some means other than immediate blast-furnace
treatment.
Dressing schemes and plant for sulphide copper ores are thus often
complicated, particularly for the recovery of the values from the finer
material, and cannot be discussed at any length here. Reference should
be made to Richards or other standard works on the subject.
As representative of wet-dressing practice, the Anaconda scheme may be
noted, as summarised below.
There are eight mills, each treating 1,000 tons of ore per day, and
conducting the—
Coarse crushing in Blake crushers.
Coarse sizing by trommels.
Coarse separation on Harz jigs of 1¼-inch and ⅞-inch feed.
Middlings crushing in rolls.
Middlings sizing by trommels.
Middlings separation on fine jigs of 7, 5, 2½, 1½, and
1 millimetre feed.
Finest crushing in Huntingdon mills.
Fines settling by spigot settlers.
Fines separation on Wilfley tables (471 are in use on the plant).
The muddy water goes to enormous settling ponds, where the slime
settles down, gradually drains, and dries, and it is afterwards used
for various purposes during the smelting operations; being dug out in
the form of a fine clay. A new form of centrifugal apparatus (the Peck)
is now being installed for the separation of this material. The
subsequent treatment of the products from the concentrating operation
is indicated in the diagram (fig. 12), from which it will be seen that
the—
_Coarse Concentrates_, 1¼, ⅞ (and ⅜) inch size, are smelted
in the blast furnaces.
_Fine Concentrates_, 7, 5, 2½, 1½, and 1 millimetre size,
pass to the roasters, and thence to the reverberatory
furnaces.
_Slimes_ are used for briquetting, and several other operations.
_Tailings_ pass to the dump.
[Illustration: Fig. 12.—Outline of Smelting Scheme at the Anaconda
Smelter, Montana, U.S.A.]
_B._ =Agglomeration of Fines.=—It has just been seen that the wet
concentration of ores (considered advisable in a large number of
cases) results in the production of a considerable quantity of
fine concentrate, a form of material not well suited for immediate
blast-furnace treatment.
In addition, smelters often receive considerable amounts of fines in
the smelting-ore supply, which it is not unusual to screen out and to
treat separately from the coarser materials.[4]
The alternatives for the treatment of fines, and more particularly of
fine concentrate, include smelting in reverberatory furnaces (usually
after roasting); blowing into the converter (a new process still in
the experimental stage); and blast-furnace treatment after suitable
preparation.
Blast furnaces have many advantages which lead to their extended use in
copper smelting practice, but one important feature, which also applies
to the smelting of other metals, has always to be borne in mind in
this connection—viz., _that material in a finely divided state cannot
be treated directly in a blast furnace without heavy losses_, and the
working of the furnace on such charges is not efficient.
No material less than ¼ to ⅜ inch in size, especially when in the form
of sulphides, should be fed as such into a modern blast furnace. Fines
in the furnace lead to—
(_a_) _Accretions_,
(_b_) _Irregular working of the furnace and the charge_,
(_c_) _Clotting and low concentration_,
(_d_) _Heavy flue-dust losses_,
and their presence is often the cause of much trouble at many of
the modern smelters. The agglomerating of the fines is, therefore,
a very important preliminary in any scheme of treatment involving
the employment of the blast furnace on such material. Agglomerating
is usually performed by one of two methods—(1) briquetting, (2)
sintering. Of these, briquetting has hitherto been in very general
use, but several advantages connected with the sintering process and
the resulting product are leading to its adoption with much success in
several localities, and attracting for it considerable attention at
present.
(_a_) =Briquetting=.—Among the advantages of briquetting is the fact
that it utilises large quantities of the copper-bearing slime produced
at the concentrating plant, this material often possessing good binding
properties which render it very suitable for briquette-making.
[Illustration: Fig. 13.—Sketch Plan of Briquetting Plant.]
[Illustration: Fig. 14.—Section through Auger-Former, showing
Briquetting Mechanism, of Chambers’ Machine.]
[Illustration: Fig. 15.—Chambers’ Briquette-making Machine.]
The type of plant in use at different smelters varies considerably, the
method adopted being either the stamping out of the briquettes, or by
the application of steady pressure, the production of bars which are
then cut up to convenient size.
The constituents used depend naturally on the materials available at
the smelter, briquettes, both with lime and without, being made.
_The Briquetting Plant at Anaconda._—The operation of this plant
affords a good example of the process. Its working is very successful
in using up much fine concentrate, as well as the slime from the
ponds, which acts as binding material and at the same time supplies
copper. Briquette, indeed, constitutes one of the biggest items of
the charge for the Anaconda blast furnaces. There are four Chambers’
machines in use, making 840 tons of briquettes daily. The briquettes
consist of slime, fine first-class ore screenings (< ⅜-inch size),
fine concentrate from the dressing plant, and coke (which is recovered
from the reverberatory furnace gratings). The quantities used daily
are somewhat as follows, though they are naturally subject to some
variation, depending on supplies:—
Slime, 500 tons.
First-class ore screenings, 300 "
Fine concentrate, 200 "
Coke, 70 "
and the composition of the briquettes is about—
Copper, 5·0 per cent.
Ferrous Oxide, 16 "
Silica, 45 to 50 "
Sulphur, 15 "
Lime, 0·7 "
Moisture, 15·0 "
Coke, 5·0 "
The different materials are stored in bins, and fed through doors to
conveyors, which discharge on to an elevator leading to a divided
hopper, each division of which feeds a pug-mill. The pug mills are long
troughs in which inter-moving bladed spindles rotate, churning up the
materials; the mixing being assisted by a water supply from above. The
mixture passes down a chute to one end of an auger machine, from which
it issues, through a steel ring, in the form of a continuous slab,
6 inches × 4 inches in section, to a cutter 10 feet distant, which
slices off bricks 10 inches long, each of which weighs about 10 lbs.
The bricks pass to a traveller, thence by another to feed bins. The
briquettes are not dried, but are used just as made with 15 per cent.
of moisture, and are generally the last item of the charge to be added
on the car. They crumble slightly, but are sufficiently strong to stand
the handling during charging.
Many similar methods, including hand processes, are employed.
(_b_) =Sintering Processes.=—This method of treating fines involves
roasting reactions, as well as the mechanical process of agglomerating.
Whilst it thus furthers the concentration obtained in the subsequent
furnace operation, since it eliminates some sulphur, it also utilises
the fuel value of the fines, and yields a product which works well in
the blast furnace. Several processes have been introduced, and the
M‘Murty-Rogers method installed at Wallaroo, S. Australia, illustrates
very well the principles upon which this class of treatment depends.
It is a sintering and roasting process similar in type to the
Huntingdon-Heberlein method for lead smelting, but lime is not used
as a rule. It is employed primarily for fine concentrates which are
somewhat siliceous.
=Charge.=—Must contain 15 to 35 per cent. silica, and 15 to 25 per
cent. sulphur.
=Pots.=—8 feet 6 inches in diameter, when used for ore, and 4 feet 6
inches deep; with vertical sides. There is a false grate 10 inches
above the bottom, pierced with ⅝-inch holes.
=Blast.=—1,000 cubic feet per minute at 13 to 20 ozs. pressure per
square inch.
=Capacity=, 8 to 10 tons. =Time=, 8 to 10 hours.
=Method.=—Cover the grate with a layer of roasted material, light small
fire of wood, blow, and gradually charge in the ore whilst the blast
is on. Lime is unnecessary, but water is essential in the process, and
the ore must be very wet; 6 to 9 per cent. water being used for ore
charges, and 3 to 4 per cent. with rich mattes, otherwise working is
not uniform, and the losses by dusting are great. With the requisite
quantity of water present, the working is regular and uniform, there is
little dust, and the roasting is efficiently performed.
=Products.=—If ore is charged, a sintered mass of matte and ferrous
silicate results; if poor matte is used, the product is a rich matte
and ferrous silicate; and if rich matte is used, metallic copper and
ferrous silicate are obtained. At the end of the blow the charge is
tipped out and fed into the blast furnace.
=Costs.=—The method as employed at Wallaroo to treat 400 to 500 tons of
material per week, operated at a cost of 3s. 6d. per ton, or about 1s.
more per ton than for ordinary roasting.
Though this particular process is only, to the author’s knowledge,
employed at a few smelters, sintering or blast-roasting methods on
the same principle have been introduced at several other works, and
their adoption promises to lead to very successful results, being
particularly suited for the class of material indicated above. The
advantages claimed for the process are that—
(_a_) It saves heavy mechanical losses, such as those of
the dust resulting from calcining operations and from the
charging of hot calcines into reverberatory furnaces.
(_b_) It gives a product suitable for blast-furnace
smelting—often the cheapest and most convenient method of
working.
(_c_) It results in efficient roasting and good reduction
of sulphur, yields the product in an advantageous form for
subsequent smelting, and promotes a satisfactory removal of
impurities in the slag.
In addition, the process offers the possibility in the future of being
so modified as to leave in the adequately compacted products so much
sulphide that their fuel values can be realised in the blast furnace.
In other words, after the preliminary sintering process, to smelt the
(fine) sulphide-concentrates pyritically in the blast furnace.
Of the more recent types of machine for conducting the process of
sintering, that of Dwight and Lloyd is in operation at several
smelters. The moistened ore falls on to an endless chain conveyor,
composed of separate grids carried on wheels. The conveyor carries the
ore through the flame from a small furnace which starts its ignition,
and it is then drawn over a long suction chamber where air is sucked
through the hot mass, thus effectually roasting and sintering it. The
chamber has special devices which ensure the drawing in of the air
through the charge only, and so prevent inward leakage (see Fig. 16).
The sintered cakes are finally discharged automatically into cars.
Details regarding the machine vary at different smelters; at one works
the length is 30 feet, the rate of travel 8 inches per minute, and the
vacuum in the suction chamber 6 ozs.
The size of the particles should not exceed ¼ inch, and not more than
25 per cent. of the charge should be so large. Some 3 to 5 per cent.
tends to pass through the grids, and so be drawn into the suction
chamber; this is cleared out at intervals through special doors. Water
is necessary, and from 6 to 10 per cent. must be employed in uniformly
moistening the charge, which, by the addition of suitable fluxes,
is often made of such proportions that in subsequent blast-furnace
smelting a satisfactory slag is produced without further additions. The
sulphur reduction by the process is very considerable.
[Illustration: Fig. 16.—Dwight-Lloyd Sintering Machine.]
Such blast-roasting methods, with suitable modifications, promise to
assume considerable importance in the developments of modern smelting
practice.
_c._ =Roasting.=—Roasting is often a very important preliminary stage
in the scheme of treatment of copper ores. It was formerly considered
an essential operation in smelting processes for sulphide ores, the
material being crushed and concentrated largely with a view to such
subsequent treatment. This is not the practice in modern smelting.
Roasting is now only conducted where the necessity for it arises, as
in the case where wet dressing, having been considered advisable, has
resulted in the production of large amounts of fine concentrate, and
where reverberatory furnaces are installed for the smelting of this
material. Preliminary roasting of the concentrates then conduces to the
production of a matte of converter grade in one smelting operation.
=The Principles of Copper Smelting.=—Copper extraction from sulphide
ores is essentially an oxidation process, the iron and sulphur being
oxidised and the oxide of iron slagged away. All such smelting
processes, both the older and the more modern ones, are based on this
fact, and underlying all of them are certain fundamental principles
which it is essential to keep in mind in considering every phase of the
subject.