(_a_) The span of arch which can be supported in the
construction.
(_b_) The length of the tools which can be conveniently
managed.
The maximum width so far found satisfactory is about 19 feet, so
that this dimension being fixed, the furnace capacity is enlarged by
increasing the length, and this is limited only by the distance from
the fire-box to which the flame can maintain the temperature necessary
for keeping the charge in a state of perfect fluidity. For many years
the length was regarded as limited to 50 feet, smelting about 2·7 to
3·0 tons of charge per ton of coal, but E. P. Mathewson, at Anaconda,
finding the escaping gases still very hot, gradually increased the
length of the hearth, first to 60 feet, then to 80 feet, and finally up
to 116 feet, when the furnace smelted 4·83 to 5·0 tons of charge per
ton of coal. The gases then left the furnace at a temperature of about
950° C., and contained sufficient heat to fire two Stirling boilers,
each of 375 H.P. Every furnace thus provided about 600 H.P. from this
waste heat, and the gases finally escaped at a temperature of 320° C.
[Illustration: Fig. 23.—Development of the Reverberatory Furnace
(Gowland).]
The capacity of these large furnaces is about 270 to 300 tons of charge
per day, and in addition to the economy and efficiency resulting from
the treatment of such large quantities of material at once, there are
the further great advantages in that—
(_a_) Settling of matte and slag is much more perfect
when such large quantities of fluid material are
stored.
(_b_) Tapping of matte and slag is easier and more
efficiently conducted.
About 110 feet appears to be the practicable maximum for furnace
length, and reverberatories of this size are being constructed wherever
circumstances permit, several new smelters having erected such
furnaces—there are eight at Anaconda, Mont.; two at Garfield, Utah;
five at Tooele, Utah; four at Cananea, etc. The length of the hearth is
naturally dependent upon the character of the fuel, particularly the
length of flame given out on burning. Bituminous fat coals are the most
suitable for this purpose, and in localities where such fuel is not
available, the use of liquid fuel has now been successfully adopted.
4. _Maintaining a Heated Matte Pool in the Furnace._—This is probably
the most important and beneficial advance made in reverberatory
practice.
In certain stages of the old Welsh process, a store of matte was
retained in the furnace after skimming off the slag, but the object was
to collect a sufficiently large quantity of matte in the furnace for
convenient tapping out.
The modern practice has several objects and possesses enormous
advantages—
(i.) It assists efficient settling.
(ii.) It conserves the heat inside the furnace.
(iii.) It presents a highly heated surface
for the fresh charge to fall upon, and thus greatly
increases the rapidity of melting, by ensuring that
the charge is heated both from above and from below.
(iv.) It prevents the sticking of half-fused charges
to the furnace bottom, the removal of which masses
would necessitate much labour, and occasion cooling
of the furnace by the opening of working doors.
(v.) It preserves the furnace bottom.
Liquid matte has practically no action on the siliceous material of
the hearth, and so presents an inert mass between the bottom and the
charge. This charge consists of calcines (mainly oxides of iron), which
would, during the process of melting down, slag with and corrode the
furnace hearth were it not protected by the matte layer.
(vi.) It allows of continuous charging and withdrawal
of materials, and of continued high temperature
in the furnace, thus protecting the furnace
lining from much wear and tear. Nothing damages
furnace linings more than exposure to changes of
temperature, on account of the continual expansion
and contraction of the brickwork and the low
thermal conductivity of the silica. Furnace linings
wear out much more from such action than from long
exposure to continued high temperature.
(vii.) There is effected an enormous saving of time,
fuel, and labour by maintaining a constant high
temperature, instead of having to heat the furnace
up again after each tapping and charging, as was
the case with the older methods of working.
(viii.) The levelling of the charges in the furnace is
greatly facilitated. The charges would otherwise
pile up under the charging hoppers, and form
heaps which are not only difficult to melt down,
but which tend to stick to the furnace bottom,
requiring time and arduous labour for their
removal. In modern practice, charges in quantities
of 10 to 15 tons at a time maybe dropped in, these
merely spread themselves out on the bath of molten
material and float down in a thin stream towards
the skimming door at the end, and they generally
melt and disappear when half-way down the furnace.
By this means, the working doors at the side need practically never be
opened for manipulating the fresh charges.
5. _The Charging of Hot Calcines._—This improvement was also introduced
by Pearse, and possesses very many advantages; he was able to increase
the furnace output by 23 per cent. with the aid of this device.
Instead of allowing the materials from the roasters to cool down, they
are taken straight from the roaster bins to the hoppers which feed
the reverberatory furnace, where they retain much of their heat until
charged into the furnace, being then still red hot as a rule. Much time
and fuel is thus saved owing to the charge requiring less heating up,
and the cooling action of charging is diminished.
A charge of 15 tons is completely melted within an hour.
6. _Regulation of Furnace by Draft Pressure._—It has already been
pointed out that rapid combustion of fuel, and consequently rapid
melting, is greatly assisted by good draft through the furnace. In
modern practice, where the factors, such as charge composition,
nature of fuel, and furnace proportions, have been satisfactorily
arranged for independently, the actual working of the furnace is
regulated by the draft pressures. These are registered automatically by
water-manometers arranged at various points. One usually communicates
with the furnace, above the fire-bridge; another is connected to
the down-take flues. The indications of these instruments enable a
record to be kept of the various operations, and of the charging of
the furnace, as well as of the condition of the fire. The usual draft
pressure worked with corresponds to about 0·8 inch of water, registered
above the fire-bridge.
On opening the hopper for charging, the pressure drops almost to zero;
the opening of any doors causes a reduction in pressure; the charging
of coal is also rendered noticeable by a drop in the record. Reduction
of pressure also indicates “airing” of the furnace by an excess of
air entering through channels in the bed of coal; draft-pressure thus
acting as a check on the firing and also on the grating, since the
formation of excessive clinker in the fire-box is indicated by an
increase in the pressure.
Corresponding to such record over an 8-hour shift, as shown on fig. 24,
Offerhaus noted the following furnace manipulations, illustrating how
accurately the operations are checked by this method:—
a.m.
7.00–7.14 Skimming (coal charged during this period).
7.16–7.16½ Side door opened.
7.28–7.31 Coal charged.
7.52–7.57 _Charged._
8.05–8.15 Tapped.
8.15 Coal charged.
8.40 Coal charged.
8.54–8.59 Grating.
9.05 Side door opened. _Charged._
9.27 Coal charged.
9.49 Coal charged.
10.07 _Charged._
10.25 Coal charged.
10.41 Coal charged.
10.45–10.58 Skimming.
11.04 Coal charged.
11.16 _Charged._
11.16–11.35 Some grating.
11.36 Coal charged.
12.03 p.m. Coal charged.
12.04 _Charged._
12.37–12.48½ Tapped, 1½ ladles (about 11 tons).
12.45 Coal charged.
1.00 _Charged._
1.11–1.45 Grating.
1.26 Coal charged.
1.44 _Charged._
1.51 Coal charged.
2.18 Coal charged.
Total charges during shift, 16 coal, 7 calcines.
The draft record is placed close to the charging platform, in order
to be in a convenient position for the guidance of the workmen. The
draft in the main flues is 1·7 to 1·8 inches water pressure; this is
similarly recorded in the foreman’s office.
7. _Continuous Working of the Furnace._—The continuous working of the
furnace is a most important factor in modern practice, and is naturally
inseparably bound up with the principle of maintaining the heated
matte-pool in the furnace, which allows of the continuous charging of
hot “calcines,” and the continuous or regular withdrawal of slag and of
matte when required.
[Illustration: Fig. 24.—Draft Pressure Record of Anaconda Reverberatory
Furnace (Offerhaus).]
The matte (which can be efficiently settled, owing to the prevailing
high temperature and the large mass of heated material in the furnace)
is stored there until required at the converters, when the desired
quantities are tapped out. The slag which is produced by the smelting
action gradually accumulates, and at regular intervals most of it is
run out (rather than skimmed). This usually takes place every four
hours. The slag accumulates until it reaches a level some 3 or 4 inches
above the skimming plate at the end of the furnace, and the quantity
which is run out at each “skimming” amounts to some 60 or 80 tons, the
contents of the furnace being lowered to such an extent that a fresh
accumulation of material may proceed during the next four hours. No
pulling of the slag is required as in the older methods of working,
since the material is so very hot and fluid that it simply pours out of
the furnace, and twenty minutes usually suffices for the whole of the
60 or 80 tons to run off, the rabble being used chiefly to regulate and
control the stream, and to keep back siliceous crusts or floaters. The
slag is run out until the matte is seen underneath, on flapping back a
thin layer, or until the level of the skimming plate is reached, and
its removal is such a short and simple operation that there is very
little interference with the regular and continuous running of the
furnace. Similarly, the tapping of as much as 50 to 100 tons of matte
from the store of 250 tons of hot fluid material has little influence
on the continuous working. Charging of coal and calcines is performed
at regular intervals, and the charges of 15 tons of “calcines” fed
in at a time, readily melt down and settle. Practically the only
interference with continuous running is the necessity for claying and
repairing, and the use of the matte pool on the hearth has lessened
the frequency for this to a large extent, the hearth bottom itself
being protected from corrosion, owing to the sulphides exerting no
action upon it, whilst the oxides in the charge which would be capable
of attacking the siliceous bottom are slagged off before they get an
opportunity of reaching it. The hearth bottom, if properly put in, is
practically permanent.
The portion of the furnace most subject to corrosion is at the slag
line, where deep channels are gradually cut out. Every four to six
weeks the furnace is tapped dry, repaired, and fettled, as much as 20
tons of fettling sand being often required for this purpose. The sand
is thrown in and patted into place by long rabbles, the operations
occupying about eighteen hours. Every nine months or so the furnace is
repaired more fully, 20 or 30 feet of brickwork near the fire-bridge
being taken down, and the great cavities in the side walls repaired
by masons, using silica bricks. The employment of higher temperatures
in modern work allows of more siliceous slags being produced, which
lessens the tendency to the eating away of the walls.
The feeding of siliceous copper ores through a series of small hoppers
situated in the roof, near to the walls, has lately been introduced
with a view to protecting the furnace sides from the corrosive action
of the slag, and to exposing a suitable siliceous flux to this
material. This appears to have fulfilled its purpose to some extent,
but various difficulties have been encountered in practice, especially
the tendency for the cold added material to form floaters, which
require limestone additions in order that they may be fluxed off; and
the cooling effects and leakages through the openings have also given
trouble.
8. _Modified Constructional Details._—In addition to the increased
size of fire-box, hearth, and flues, and to the necessity for very
heavy staying in order to keep the enormous arch in permanent shape,
which are characteristic of modern practice, the construction of modern
furnaces involves the building of a suitable hearth to carry the heavy
burden of hot and fluid matte which is stored in the furnace.
It was formerly considered correct practice, in the smaller types of
furnace, to construct the hearth over a vault, in order to keep the
underside cool and thus prevent the corrosion and eating away of the
siliceous bottom by the oxidised charges, during the process of melting
down. In modern practice it is absolutely essential to work with a
perfectly solid structure.
[Illustration: Fig. 25.—Skimming Reverberatory Furnace, Anaconda.]
[Illustration: Fig. 26.—Transverse Section of Modern Reverberatory
Furnace, Anaconda, indicating Foundations, Hearth, and Bracing.]
(_a_) Because the hearth must be kept as hot as possible, so as to
ensure rapid melting of the charge and maintain the products in a
perfectly fluid condition. Any circumstance tending to cool the hearth
is rigorously avoided, this being the contrary of the older practice.
The protective influence of the heated matte-pool in modern work
preserves the bed from the corroding effects of fresh oxidised charges,
and in consequence, the maximum degree of heat can with safety be
maintained on the furnace hearth.
(_b_) The enormous weight of charge and the heavy arch and walls demand
the strongest possible foundations and support.
[Illustration: Fig. 27.—Reverberatory Furnace under Construction.]
In building modern reverberatories, the foundation for the hearth is
constructed of solid masonry or brickwork, or as at Anaconda, of a
solid bed of slag, some 24 inches in depth, run in from an adjacent
furnace. The I-beams used for carrying the bracing are erected in a
surrounding trench, and a further quantity of slag (4 feet thick by 2
feet deep) is run in, thus yielding a perfectly rigid and impervious
foundation (Fig. 26). On the top of this slag-foundation is built a
layer, 12 inches thick, of silica bricks, and upon this, the actual
working bottom of the furnace is constructed.
This bottom is now put in also in a manner different to the older
practice, and excellent results have accrued from the change.
The old method of constructing sand bottoms consisted of putting in
the beds of sand, layer by layer, and thoroughly fritting each one
before the addition of the next: in modern practice, it is found that
proper consolidation is not attained with beds of the enormous area now
employed, when the bottom is constructed in such layers.
The present method of working the reverberatory furnace is not to
drop the charge on to the sand hearth at all, but into the deep
pool of matte, and the sand-hearth is regarded more as a convenient
foundation for the support of this liquid working-bed, on account of
its constituting a cheap non-conducting and fire-proof material which
is unaffected by the materials resting upon it. It was found, however,
on commencing this matte-pool practice, that the older method of
putting in the bottom in successive sand layers was not suitable for
this work; after a little wear, the beds became raised in layers, this
being especially the case if any holes happened to be eaten through
in places. Moreover, the large weight of matte tended to find its way
down between the layers and raise them up bodily, or else it worked
down at the edges of the hearth and side walls, and either broke out
underneath the former or through the latter. When it was ascertained
that liquid matte itself had no corrosive action on the siliceous
hearth if the latter be kept constantly covered, and that the causes of
breakouts were principally due to mechanical weaknesses, it required
only improvements in design and construction in order to avoid them.
This is now attained by constructing the bed in a compact and perfectly
massive form, and is best accomplished by putting in the whole layer
of 26 inches of sand at once, and firing as hard as it is possible for
the brickwork to stand. The method has met with exceptional success in
practice, rigid and impervious hearths are obtained; it being found
that less than 1 inch has worn off the bed after two years’ working.
[Illustration: Fig. 28.—Sectional Plan and Elevation of Reverberatory
Furnace at Anaconda.]
=Large Reverberatory Furnaces: Details of Construction.=—The large
furnaces at Anaconda were the first of the modern type to be
constructed, they have met with enormous success in practice and
constitute the standard form. Similar furnaces are now in operation
or under construction at many of the large modern camps, and are of
similar design and construction.
The _hearth_ is 102 to 116 feet long by 19 feet wide.
_Grate_, 16 feet by 8 feet = 128 square feet grate area.
Ratio of hearth to grate area is 16 : 1.
_Distance_ from hearth to level of fire-bridge, 26 inches; hearth to
crown of arch, 6 feet 5 inches. _Walls_ are 26 inches thick. _Roof_ is
15 inches thick (except for 4 to 6 feet over the fire-bridge, where it
is 20 inches). The bracing of the furnace is necessarily particularly
strong (see Fig. 29). Lined inside with silica brick, said to be the
finest in the world. The bed is of the finest Dillon sand (97·5 per
cent. silica), ground to pass ¼-inch mesh; the bed has a slope of
8 inches towards the tap-holes, of which there are two. During the
construction of the large furnace there are left in the roof ten
expansion openings of 3 inches each, which by the time the furnace has
attained its working temperature, become closed up (see Fig. 30). The
conker plate which runs through the fire-bridge is 14 to 15 feet long,
and is made thicker near the furnace side, where it is 3 inches thick.
The air space through the plate is 2 feet 3 inches by 9 inches, and
serves the purpose of keeping the fire-bridge cool; air passes through
it continuously, and if the plate shows signs of becoming hot, a blast
of cold high-pressure air is sent through it. Still further heating of
the plate and signs of red heat are an indication that the 2 feet of
silica of the fire-bridge wall are being burnt through.
=Working of the Reverberatory Plant at Anaconda.=—The plant consists
of eight large furnaces, built parallel to one another, seven being
usually at work whilst the eighth is undergoing repair. Each furnace
treats 300 tons of hot calcines and flue-dust daily.
_Charging._—The furnaces are charged every 65 to 70 minutes with 15-ton
charges, and as soon as one charge is melted, another is added; with
average running, 150 charges are worked in the seven furnaces daily.
The charge train, consisting of an engine and three cars, each of which
carries 5 tons of charge, travels from the roasters and enters the
reverberatory building by an overhead track running above the charge
bins of the furnaces. It discharges through hoppers into the bins
which extend across the entire width of the hearth. Bins were formerly
arranged at intervals all the way down the furnace, but now only the
two bins nearest to the fire-bridge are employed. Into the back bin, 10
tons of charge are placed, and into the other, 5 tons. Each of these
bins discharges through two hopper discharge openings, feeding the
furnace through holes in the roof (Figs. 29, 30), which are closed,
when not in use, by round firebrick tiles 20 inches in diameter and 2½
inches thick; these are moved in and out of position by means of levers
operated from the fire-box platform.
The temperature maintained in the furnace is high, approximating to
1,500° C., and just previous to dropping in a fresh charge, a workman,
by means of a rabble, feels about the hearth below the charging hopper
in order to ensure that all of the previous charge has been melted,
and that none of it is sticking to the furnace hearth. By employing
only the comparatively small quantities of 15 tons, this sticking is
avoided, since such charges are not heavy enough to sink unmelted
through the 8 inches of slag and 8 inches of matte in the furnace.
The former practice of feeding charges amounting to 45 tons through
hoppers situated all the way along the furnace had given serious
trouble in that respect, and had consequently to be discarded. When the
examination of the hearth is completed, the time occupied being very
short, the side door is closed, and sealed with sand; the covers to
the holes in the roof are now withdrawn, the gates closing the hoppers
pulled back, and first the 5-ton, then the 10-ton charge is dropped
into the furnace. The whole operation, including the preliminary
opening of the door to test the furnace bottom, occupies five minutes.
Very little hand labour is required round these enormous furnaces,
except for the grating of the fires, for the charging of coal and
calcines every hour by the operation of levers from the fire-box
platform, for the skimming of slag at intervals of four hours, and for
the tapping of matte when required. The whole of this work is conducted
by the skimmer and two helpers to each furnace, one of the men also
looking after the boilers.
As soon as the charge has been dropped on to the pool of molten
material, the mass appears to spread out over the surface and float
towards the skimming door, in a thin slow-moving stream which
disappears when about half-way down, being usually melted within one
hour. The former 40-ton charges required as much as eight hours for
melting.
Owing to the great heating effect of the large bath of hot material
below, and of the intense flame above, there is but little cooling
action on adding the fresh charge; whilst with this length of furnace,
practically all the dust is settled, and very little is carried into
the flues.
_Coaling._—The quantity of coal employed amounts to 20 to 25 per cent.
of the charge, or about 50 to 60 tons per day per furnace, 1 ton of
coal smelting rather less than 5 tons of calcines.
[Illustration: Fig. 29.—Fire-box End of Reverberatory Furnace, showing
massive Bracing, Charge Bins, and Charging Levers—Anaconda.]
[Illustration: Fig. 30.—Interior of Reverberatory Furnace (looking
towards Skimming Door), showing Expansion Spaces in Roof, and Charging
Holes—Anaconda.]
Coal is charged every 40 minutes in quantities of 1½ tons at a time,
from bins which extend across the entire width of the fireplace,
feeding through four hoppers into openings 1 foot square in the roof of
the fire-box, and the withdrawing of the gates is operated by means of
levers at the platform. Over the fire-bridge are two rows of air-holes
used for regulating the length and character of the flame in the
furnace; the flame, however, plays a subordinate part in the smelting
reactions. The coal employed is from Diamondsville, Wyoming, and gives
a flame 125 feet in length, the appearance of which is gauged through
the window fixed in the off-take flue, this being visible from the
fire-box platform. The coal is run-of-mine quality, and considerable
slack is used. It possesses a high calorific power and a large
proportion of volatile constituents, but clinkers rather badly, and a
clinker grate is worked with.
_Grating._—The fire rests upon 3-inch round bars placed at 4½ to 6-inch
centres, and is maintained at a depth of about 27 inches. Grating
requires to be conducted at fairly frequent intervals, usually twice
per shift, in order to keep the fire free and to prevent channelling,
which is indicated on the draft gauge by a drop from 0·75 inch to 0·50
inch, due to airing. It serves further to prevent clinkering, which,
when taking place in the fire, causes a rise of from 0·75 up to 1·0
inch on the gauge. The operation of grating usually occupies about
half-an-hour; the work is arduous, and the heat to which the workman is
exposed is itself very trying.
_Coke Recovery._—A constant stream of half-burnt fuel and ashes falls
through the bars, and during the clinkering operations large quantities
are dropped. The material all falls down a bank inclined at 45°,
into a channel where it is met by a stream of water which washes it
along launders and through a grizzle, to a settling tank. The settled
products are subsequently jigged, the recovered coke being washed over
the tail-board to a trommel, and by this means 10 per cent. of the fuel
charged into the furnace is recovered in a useful form. This coke is
used up as a constituent of the briquettes.
TABLE VI.—DAILY REPORT—REVERBERATORY FURNACES. AUGUST 17TH, 1908 (GOOD
DAY).
─────┬─────┬──────┬─────────────────────────────────────────────────
│ │ │ Charge.
│ │ ├──────┬─────────┬─────────┬─────────┬──────┬─────
Furn-│ │ Total│ │ │ Blast │ │ │
ace │ │ Smel-│ Cal- │Macdougal│ Furnace │ Main │ │Resi-
No. │ Coal│ ted │cines │Flue-Dust│Flue-Dust│Flue-Dust│Extras│dues
─────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼─────
│ Tons│ Tons │ Tons │ Tons │ Tons │ Tons │ Tons │ Tons
│ │ │ │ │ │ │ Fine │
│ │ │ │ │ │ │ lime │
│ │ │ │ │ │ │ rock.│
1 │ 60·6│ 288·8│ 279·2│ .. │ 8·9 │ .. │ 0·7 │ ..
2 │ 57·2│ 277·7│ 262·7│ .. │ 2·9 │ 11·8 │ 0·3 │ ..
3 │ 64·1│ 286·7│ 253·2│ 12·0 │ 8·9 │ 11·8 │ 0·8 │ ..
4 │ 60·5│ 278·7│ 264·7│ .. │ 2·6 │ 3·9 │ 0·2 │ 7·3
5 │ 57·3│ 245·9│ 221·7│ 12·0 │ 11·2 │ .. │ 1·0 │ ..
6 │ 57·3│ 273·1│ 264·4│ .. │ 7·9 │ .. │ 0·8 │ ..
7 │ .. │ .. │ .. │ .. │ .. │ .. │ .. │ ..
8 │ 57·4│ 278·7│ 266·8│ 11·9 │ .. │ .. │ .. │ ..
─────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼─────
Total│414·4│1929·6│1812·7│ 35·9 │ 42·4 │ 27·5 │ 3·8 │ 7·3
─────┴─────┴──────┴──────┴─────────┴─────────┴─────────┴──────┴─────
─────┬────────┬───────┬───────────────────────┬──────┬───────┬───────
│ │ Cost │ Delays. │ │ │Ladles
│ │of Coal├─────┬────────┬────────┤ │ │ of
Furn-│Copper │ per │Wait-│ Waiting│ │ │ │ Matte
ace │Material│ Ton │ ing │ for │ Miscel-│ Total│Boilers│ in
No. │Smelted │ of │ for │Calcines│laneous │Delays│Working│Furnace
│per Ton │ Metal │Coal │ │ │ │ │at End
│of Coal │Melted │ │ │ │ │ │of Day.
─────┼────────┼───────┼─────┼────────┼────────┼──────┼───────┼───────
│ Tons │ $. │Hours│ Hours │ Hours │ Hours│ Hours │
1 │ 4·77 │ 0·95 │ ─┐ ┌─ │ 24 │ 10
2 │ 4·85 │ 0·94 │ │ │ │ 24 │ 10
3 │ 4·47 │ 1·02 │ │ │ │ 24 │ 10
4 │ 4·61 │ 0·99 │ ├────── No delays. ─────┤ │ 24 │ 10
5 │ 4·29 │ 1·06 │ │ │ │ 24 │ 10
6 │ 4·77 │ 0·95 │ │ │ │ 24 │ 10
7 │ .. │ .. │ │ │ │ .. │ ..
8 │ 4·85 │ 0·94 │ ─┘ └─ │ 24 │ 10
─────┼────────┼───────┼─────┬────────┬────────┬──────┼───────┼───────
Total│ .. │ .. │ .. │ .. │ .. │ .. │ 168 │ 70
─────┴────────┴───────┴─────┴────────┴────────┴──────┴───────┴───────
Draft, 1·7 inches. Number of furnaces running, 7·00
All furnaces working slow. Number of charges, 140
Furnace No. 5, one bad charge. Ladles matte tapped, 34
Cupriferous material smelted per furnace, 275·6 tons.
────────────────────────────────────────────────────────────────────
DAILY REPORT—REVERBERATORY FURNACES. AUGUST 19TH, 1908.
─────┬─────┬──────┬─────────────────────────────────────────────────
│ │ │ Charge.
│ │ ├──────┬─────────┬─────────┬─────────┬──────┬─────
Furn-│ │ Total│ │ │ Blast │ │ │
ace │ │ Smel-│ Cal- │Macdougal│ Furnace │ Main │ │Resi-
No. │ Coal│ ted │cines │Flue-Dust│Flue-Dust│Flue-Dust│Extras│dues
─────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼─────
│ Tons│ Tons │ Tons │ Tons │ Tons │ Tons │ Tons │ Tons
1 │ 55·4│ 143·0│ 143·0│ .. │ .. │ .. │ .. │ ..
2 │ 55·4│ 246·4│ 240·1│ .. │ .. │ .. │ .. │ 6·3
3 │ 62·1│ 250·7│ 236·9│ .. │ .. │ 13·8 │ .. │ ..
4 │ 58·9│ 262·7│ 262·9│ .. │ .. │ .. │ .. │ ..
5 │ 62·2│ 247·8│ 247·8│ .. │ .. │ .. │ .. │ ..
6 │ 59·1│ 241·9│ 241·9│ .. │ .. │ .. │ .. │ ..
7 │ .. │ .. │ .. │ .. │ .. │ .. │ .. │ ..
8 │ 55·1│ 252·9│ 252·9│ .. │ .. │ .. │ .. │ ..
───────┼─────┼──────┼──────┼─────────┼─────────┼─────────┼──────┼────
Total, │408·2│1645·6│1625·5│ .. │ .. │ 13·8 │ .. │ 6·3
───────┴─────┴──────┴──────┴─────────┴─────────┴─────────┴──────┴────
─────────────────────────────────────────────────────────────────────
DAILY REPORT—REVERBERATORY FURNACES. AUGUST 19TH, 1908.
─────┬────────┬───────┬───────────────────────┬──────┬───────┬───────
│ │ Cost │ Delays. │ │ │Ladles
│ │of Coal├─────┬────────┬────────┤ │ │ of
Furn-│Copper │ per │Wait-│ Waiting│ │ │ │ Matte
ace │Material│ Ton │ ing │ for │ Miscel-│ Total│Boilers│ in
No. │Smelted │ of │ for │Calcines│laneous │Delays│Working│Furnace
│per Ton │ Metal │Coal │ │ │ │ │at End
│of Coal │Melted │ │ │ │ │ │of Day.
─────┼────────┼───────┼─────┼────────┼────────┼──────┼───────┼───────
1 │ 2·58 │ 1·76 │ .. │ .. │ 8·00 │ 8·00 │ 22 │ 2
2 │ 4·45 │ 1·02 │ .. │ .. │ .. │ .. │ 24 │ 8
3 │ 4·04 │ 1·13 │ .. │ .. │ .. │ .. │ 24 │ 6
4 │ 4·46 │ 1·02 │ .. │ .. │ .. │ .. │ 24 │ 6
5 │ 3·98 │ 1·14 │ .. │ .. │ .. │ .. │ 24 │ 8
6 │ 4·09 │ 1·11 │ .. │ .. │ .. │ .. │ 24 │ 8
7 │ .. │ .. │ .. │ .. │ .. │ .. │ .. │ ..
8 │ 4·59 │ 1·19 │ .. │ .. │ .. │ .. │ 24 │ 8
───────┼────────┼───────┼─────┼────────┼────────┼──────┼───────┼────
Total,│ .. │ .. │ .. │ .. │ 8·00 │ 8·00 │ 166 │ 46
───────┴────────┴───────┴─────┴────────┴────────┴──────┴───────┴────
Draft, 1·7 inches.
Furnace No. 1 delayed 8 hours tapping and claying.
Furnace No. 7 down for repairs.
Bad coal on all furnaces.
Number of furnaces running, 6·67
Number of charges, 118
Ladles matte tapped, 47
Cupriferous material smelted per furnace, 246·7 tons.
_Tapping the Furnace._—Matte is usually withdrawn from these large
stores upon such occasions as it is required for the converters, though
sometimes when the supply has got ahead of the converters’ demands, the
matte is tapped and run outside the reverberatory building, being cast
into large matte-beds. The tap-holes are situated between the second
and third doors, and between the fourth and fifth; and each consists
essentially of a copper plate 2 inches thick and 25 inches square,
which at first stands back 9 inches from the outside of the wall.
Through this plate a 1-inch hole has been drilled. The tapping bar is
maintained inserted up this hole, being passed through the conical clay
plug which closes it. At the back of the plate is 21 inches of lining
material through which the tapping-hole passes. When the copper plate
shows signs of a red heat, it is an indication of the lining tending to
burn through; this part of the furnace is then cooled, the plate taken
out, a 9-inch layer of sand is rammed into position, and the plate is
thus moved forward a corresponding distance. Such a tap-hole plate
lasts for about five months.
The reverberatories are usually not tapped until they contain about 250
tons of matte. The operation of tapping is performed by withdrawing
the rod by means of a wedge and ring, when the matte flows along the
launders leading to the ladles for the converters; two ladles of about
8 tons capacity each are usually filled at once, each ladleful being
sampled at the runner. The tap-hole is then stopped with a cone of
clay, and the tapping-rod driven through it again.
Typical daily reports of the furnaces are appended in Tables VI. and
VII., and a monthly report on Table VIII.
TABLE VII.—FROM DAILY ASSAY REPORT—REVERBERATORY FURNACES. AUGUST 19,
1908.
─────────────────┬───────────────────────────────
│ Per Cent Copper in Slag.
Furnace Number. ├──────────┬──────────┬─────────
│ Shift 1. │ Shift 2. │ Shift 3.
─────────────────┼──────────┼──────────┼─────────
1 │ 0·30 │ 0·30 │ 0·30
2 │ 0·30 │ 0·35 │ 0·25
3 │ 0·30 │ 0·30 │ 0·35
4 │ 0·45 │ 0·30 │ 0·25
5 │ 0·30 │ 0·40 │ 0·35
6 │ 0·30 │ 0·20 │ 0·20
7 │ .. │ .. │ ..
8 │ 0·35 │ 0·25 │ 0·30
─────────────────┼──────────┼──────────┼─────────
Average in slag, │ 0·35 │ 0·30 │ 0·30
─────────────────┴──────────┴──────────┴─────────
┌─
│ SiO_{2}, 29·5 per cent.
│ FeO, 37·3 "
Composition of calcines ──┤ S, 7·7 "
│ CaO, 2·7 "
│ Copper, 8·6 "
└─
┌─
│ SiO_{2}, 39·4 per cent.
Composition of slag, ──┤ FeO, 40·7 "
│ CaO, 4·3 "
└─
Copper in matte, 38·6 "
TABLE VIII.—MONTHLY REPORT—REVERBERATORY FURNACES. TOTAL CHARGE—ALL
FURNACES.
───────────────────────┬───────┬────────────────┬────────────────
│Charge.│ SiO_{2}. │ FeO.
───────────────────────┼───────┼─────────┬──────┼─────────┬──────
│ Tons. │Per cent.│ Tons.│Per cent.│ Tons.
Calcines and lime rock │ 50,054│ 27·20 │13,616│ 39·40 │19,721
M‘Dougal flue-dust, │ 977│ 30·50 │ 298│ 21·90 │ 214
Blast flue-dust, │ 1,639│ 35·90 │ 588│ 22·00 │ 361
Converter flue-dust, │ 132│ 1·90 │ 2│ 6·60 │ 9
Main flue-dust, │ 1,034│ 30·2 │ 312│ 17·80 │ 184
───────────────────────┼───────┼─────────┼──────┼─────────┼──────
Total, │ 53,836│ .. │14,816│ .. │20,489
───────────────────────┼───────┼─────────┼──────┼─────────┼──────
Matte to converter, │ 10,950│ .. │ .. │ 36·70 │ 4,019
Matte chips to B.F., │ 74│ 8·20 │ 6│ 38·10 │ 28
Slag chips to B.F., │ 609│ 39·50 │ 241│ 37·40 │ 228
───────────────────────┼───────┼─────────┼──────┼─────────┼──────
Deduct from above │ │ │ │ │
total, │ 11,633│ .. │ 247│ .. │ 4,275
───────────────────────┼───────┼─────────┼──────┼─────────┼──────
Leaves for slag, │ .. │ .. │14,569│ .. │16,214
───────────────────────┴───────┴─────────┴──────┴─────────┴──────
───────────────────────┬────────────┬─────────────┬─────────────────
│ Lime. │ Sulphur. │ Copper.
───────────────────────┼──────┬─────┼──────┬──────┼───────┬─────────
│ Per │Tons.│ Per │ Tons.│ Per │ Lbs.
│ cent.│ │ cent.│ │ cent.│
Calcines and lime rock,│ 2·30 │1,150│ 8·40 │ 4,205│ 8·266 │8,274,799
M‘Dougal flue-dust, │ 1·30 │ 13│14·00 │ 137│ 7·884 │ 152,295
Blast flue-dust, │ 4·30 │ 70│ 6·70 │ 110│ 5·698 │ 186,782
Converter flue-dust, │ .. │ .. │12·10 │ 16│68·743 │ 181,482
Main flue-dust, │ 2·10 │ 22│ 8·80 │ 90│ 7·128 │ 147,405
───────────────────────┼──────┼─────┼──────┼──────┼───────┼─────────
Total, │ .. │1,256│ .. │ 4,558│ 8·305 │8,942,763
───────────────────────┼──────┼─────┼──────┼──────┼───────┼─────────
Matte to converter, │ .. │ .. │26·40 │ 2,891│38·209 │8,367,872
Matte chips to B.F., │ 0·20 │ .. │21·80 │ 16│32·811 │ 48,560
Slag chips to B.F., │ 2·30 │ 14│ 2·20 │ 13│35·597 │ 43,357
───────────────────────┼──────┼─────┼──────┼──────┼───────┼─────────
Deduct from above │ │ │ │ │ │
total, │ .. │ 14│ .. │ 2,920│ .. │8,459,789
───────────────────────┼──────┼─────┼──────┼──────┼───────┼─────────
Leaves for slag, │ .. │1,242│ .. │ .. │ .. │ ..
───────────────────────┴──────┴─────┴──────┴──────┴───────┴─────────
Analysis.
┌─────────┴──────────┐
_Slag Calculation_─ │Calculated. Actual.│
SiO_{2} in slag, 14,569 ÷ 38,538 37·8 37·1
FeO " 16,214 ÷ 38,538 42·1 43·2
CaO " 1,242 ÷ 38,538 3·2 2·8
────── ────── ────── ─────
32,025 at 83·17 = 38,538 83·10 83·10
═══════════════ ══════ ═════
=Fuels for Reverberatory Furnace Work.=—The chief requirements of the
fuel for good reverberatory work will now be apparent, particularly
with regard to length of flame. This depends to a large extent upon the
proportion of volatile hydrocarbons, but also on the conditions under
which they are given off. For instance, a coal which rapidly parts with
its hydrocarbons and leaves in the grate a dense layer of slow-burning
coke would be unsuitable for reverberatory work, though some caking is
necessary in order that the fuel should not burn away too rapidly, as
it should yield a good bed of the required depth.
The great success of large reverberatory furnaces worked under suitable
conditions, has had the tendency to tempt smelters in different parts
of the world to erect furnaces of similar size independently of the
character of the available fuel, and in several cases results have been
unsatisfactory, at least in the earlier stages.
These preliminary failures have, however, served the purpose of
developing the adaptation of other fuels for this work, and from the
employment of oil for the purpose, important extensions in practice
will undoubtedly develop in the future of reverberatory furnace working.
The device of using pulverised coal as a fuel has attracted attention
at several smelters where the local coal as mined was proved to be
unsuitable for use. In practice, however, the method has, up to the
present, given unsatisfactory results, for although a longer flame and
higher temperature have been obtained in the furnace, difficulties
in working have arisen which appear to bar its use. One of the chief
drawbacks has been due to the fine ash from the fuel, which is
deposited in the flues in large quantities and even causes considerable
slagging in them, impeding the working of the furnace and preventing
the recovery of heat from the furnace gases. Further difficulty,
though not quite so serious, was caused by the dust being blown upon
the charge and tending to settle upon it; forming a non-conducting
blanket which retarded the melting of the material by the flames. The
method does not appear at present to offer much promise of extended
application to copper smelting.
_Oil Fuel in Reverberatory Practice._—The successful application
of oil as a fuel marks a useful advance in reverberatory practice,
particularly in connection with the working of large furnaces.
On several of the smaller plants, oil fuel has been in use with
considerable success for some time, but within recent years the
building of large-sized furnaces without having at hand suitable
coal resources has led to attempts to employ oil in its place, and
the preliminary difficulties appear to have been to a large extent
successfully overcome. The work at the Cananea Smelter with oil fuel,
and the discussion on Ricketts’ first report of his experience, afford
valuable indications of the possibilities of this method. Working on
charges consisting to a large extent of flue-dust, several thousand
tons of material have been smelted in furnaces yielding 245 tons daily
output, at a cost which compares very favourably with that of ordinary
practice. This success is particularly noteworthy in view of certain
features in the preliminary system of working which will doubtless be
altered at no very distant date, and of the fact that flue-dust is
sometimes a difficult material to melt in a reverberatory furnace, even
when good coal is available as a fuel.
[Illustration: Fig. 31.—Shelby Oil-burner for Reverberatory Furnace
Use.]
The chief difficulties in working appear to have been largely in
connection with the regulation of the flame and the management of the
oil-burners. In endeavouring to obtain the requisite high temperature
over the entire length of the furnace-hearth, an intense local action
was caused near the place where the oil in the form of a spray entered
the furnace, resulting in the burning out of the roof-arch on several
occasions. These difficulties will doubtless be overcome with further
experience in the design and management of the burners constructed for
this class of work.
At Cananea, four oil burners of the Shelby type are employed on each
furnace, and this form is stated to project the flame further into the
furnace, and to prevent its impinging on the roof, more successfully
than the other types tried. The waste heat fires three Stirling boilers
of 664 H.P. Less than one barrel (42 gallons, or 310 lbs.) of oil is
consumed per dry ton of charge, and of this quantity 0·43 barrel is
chargeable to steam-raising under the boilers. The manner of working
the charges, and the furnace construction in other respects, follow
very closely the methods of operation already described.
=Costs of Oil-fired Reverberatory Working.=—Ricketts has contributed a
useful analysis of the costs of reverberatory work using oil as fuel,
under the conditions prevailing at Cananea, Mexico. He noted that
the use of too much oil should be avoided. This precaution led to a
decrease in the amount of repairs necessary. 550 barrels of oil were
required to get the furnace into fairly good condition, and 8 barrels
per furnace per hour to keep it going well. It is hoped ultimately to
reduce the oil consumption to 0·8 barrel gross per ton of charge.
Analysis of Oil-fired Reverberatory Furnace Costs—Cananea—February to
July, 1911, inclusive.
Furnace Days, 312·5.
TONNAGE CHARGED— Dry Tons. Per cent.
of Total.
Flue-dust, 21,019 34·99
Calcines, 35,533 59·15
Ores, 3,040 5·06
Limestone, 479 0·80
—————— ——————
60,071 100·00
====== ======
DISTRIBUTION OF COSTS— Amount. Per
Dry Ton.
Operating expenses, $111,687·17 $1·8593
Slag and matte expense, 5,111·07 0·0851
Boiler-house, 11,468·77 0·1909
General expense, 4,218·58 0·0702
Cost of flux, 817·46 0·0136
————————- ——————-
$133,303·05 $2·2192
Steam credit, 48,861·86 0·8134
————————- ——————-
Operating cost, $84,441·19 $1·4057
=========== =======