ROAST-REACTION SMELTING
SCOTCH HEARTHS AND REVERBERATORY FURNACES
LEAD SMELTING IN THE SCOTCH HEARTH
BY KENNETH W. M. MIDDLETON
(July 6, 1905)
In view of the fact that the Scotch hearth in its improved form is now
coming to the front again to some extent in lead smelting, it may prove
interesting to give a brief account of its present use in the north of
England.
Admitting that, where preliminary roasting is necessary, the best
results can be obtained with the water-jacketed blast furnace (this
being more especially the case where labor is an expensive item), we
have still as an alternative the method of smelting raw in the Scotch
hearth. At one works, which I recently visited, all the ore was smelted
raw; at another, all the ore received a preliminary roast, and it is
instructive to compare the results obtained in the two cases. The
following data refer to a fairly “free-smelting” galena assaying nearly
80 per cent. of lead.
When smelting raw ore in the hearth, fully 7½ long tons can be treated
in 24 hours, the amount of lead produced direct from the furnace in the
first fire being 8400 to 9000 lb.; this is equivalent to 56 to 60 per
cent. of lead, the remaining 24 to 20 per cent. going into the fume and
the slag.
When smelting ore which has received a preliminary roast of two hours,
12,000 lb. of lead is produced direct from the hearth, this being
equivalent to 65 per cent. of the ore. When the ore is roasted, the
output of the hearth is practically the same for all ores of equal
richness; but when smelting raw, if the galena is finely divided, the
output may fall much below that given herewith; while, on the other
hand, under the most favorable conditions it may rise to 12,000 lb. in
24 hours, or even more.
I had an opportunity of seeing a parcel of galena carrying 84 per cent.
of lead (but broken down very fine) smelted raw. The ore was kept damp
and the blast fairly low; but, in spite of that, a quantity of the ore
was blown into the flue, and only 5100 lb. of lead was produced from
the hearth in 24 hours.
Galena carrying only 65 per cent. of lead does not give nearly as
satisfactory results when smelted raw in the hearth; barely six tons of
ore can be smelted in 24 hours, and only 4500 to 5400 lb. of lead can
be produced directly. This is equivalent to, say, 43 per cent. of the
ore in the first fire; the remaining 22 per cent. goes into the slag or
to the flue as fume. Moreover, the 65 per cent. ore requires 1500 lb.
of coal in 24 hours, while the 80 per cent. galena uses only 1000 lb.
Turning now for a moment to the costs of smelting raw and of smelting
after a preliminary roast, we find that (in the case of the two works
we have been considering) the results are all in favor of smelting raw,
so far as a galena carrying nearly 80 per cent. is concerned.
The cost of smelting, per ton of lead produced, is given herewith:
ORE SMELTED RAW
Smelters’ wages $2.04
“ coal (425 lb.) 0.38
——-
Total $2.42
A very small quantity of lime is also used in this case for some ores,
but its cost would never amount to more than 4c. per ton of lead
produced.
ORE RECEIVING A PRELIMINARY ROAST
Roasters’ wages $0.61
“ coal (425 lb.) 0.65
Smelters’ wages 1.08
“ coal (75 lb.) 0.11
Peat and lime 0.08
——-
Total $2.53
It should be noted also that the smelters at the works where the ore
was not roasted receive higher pay. In the eight-hour shift they
produce about 1½ tons of lead; and as there are two of them to a
furnace, they make $3.06 between them, or $1.53 each. The two men
smelting roasted ore produce about two tons in an eight-hour shift, and
therefore each receives $1.08 per shift.
Coming now to fume-smelting in the hearth, we can again compare the
results obtained in smelting raw and after roasting. It is well to
bear in mind, also, that, while only 6½ per cent. of the lead goes in
the fume when smelting roasted ores in the hearth, a considerably
larger proportion is thus lost when smelting raw ores. When fume is
smelted raw, it is best dealt with when containing about 40 per cent.
of moisture. One man attends to the hearth (instead of two as when
smelting ore), and in 24 hours 3000 lb. of lead is produced, the amount
of coal used being 2100 lb. No lime is required.
When smelting roasted fume, two men attend to the hearth and the output
is 6000 lb. in 24 hours, the amount of coal used being 1800 lb. In this
latter case fluorspar happens to be available (practically free of
cost), and a little of it is used with advantage in fume-smelting, as
well as a small quantity of lime.
The cost of fume-smelting per ton of lead produced is given herewith:
FUME SMELTED RAW
Smelters’ wages $2.88
“ coal (1400 lb.) 2.13
——-—-
Total $5.01
FUME RECEIVING A PRELIMINARY ROAST
Roasters’ wages $2.08
“ coal (1450 lb.) 2.18
Smelters’ wages 2.04
“ coal (600 lb.) 0.92
Peat and lime 0.08
———--
Total $7.30
In this case, as in that of ore, the smelter of the raw fume gets
better pay; he has $1.44 per eight-hour shift, while the smelter of the
roasted ore has only $1.02 per eight-hour shift.
Fume takes four hours to roast, as compared to the two hours taken by
ore.
From these facts regarding Scotch-hearth smelting, it would seem that
with galena carrying, say, over 70 per cent. lead (but more especially
with ore up to 80 per cent. in lead, and, moreover, fairly free from
impurities detrimental to “free” smelting), very satisfactory results
can be obtained by smelting raw. Against this, however, it must be said
that at the works where the ore is roasted attempts at smelting raw
have been made several times without sufficient success to justify the
adoption of this method, although the ores smelted average 75 per cent.
lead and seem quite suitable for the purpose.
Probably this may be accounted for by the fact that the method of
running the furnace when raw ore is being smelted is rather different
from that adopted when dealing with roasted ore. Moreover, at the works
under notice the furnaces are not of the most modern construction; and,
as the old custom of dropping a peat in front of the blast every time
the fire is made up still survives, it is necessary to shut off the
blast while this is being done, and the fire is then apt to get rather
slack.
The gray slag produced in the hearth is smelted in a small blast
furnace, a little poor fume, and sometimes a small quantity of
fluorspar, being added to facilitate the process. Some figures
regarding slag-smelting may be of interest. The slag-smelters produce
9000 lb. of lead in 24 hours. The cost of slag-smelting per ton of lead
produced is as follows:
Smelters’ wages $1.60
Coke (1500 lb.) 3.42
Peat 0.06
———--
Total $5.08
Recent analyses of Weardale (Durham county) lead smelted in the Scotch
hearth, and slag-lead smelted in the blast furnace, are given herewith:
─────────┬───────────────────┬────────────────────┬──────────────────
│ FUME-LEAD FROM │ SILVER-LEAD FROM │ SLAG-LEAD FROM
│ HEARTH │ HEARTH │ BLAST FURNACE
─────────┼───────────────────┼────────────────────┼──────────────────
Lead │ 99.957 │ 99.957 │ 99.013
Silver │ 0.0035 │ 0.0200 │ 0.0142
│ (1oz. 2dwt. 21gr. │ (6oz. 10dwt. 16gr. │(4oz. 12dwt. 18gr.
│ per Long Ton) │ per Long Ton) │ per Long Ton)
Tin │ nil │ nil │ nil
Antimony │ nil │ nil │ 0.874
Copper │ nil │ nil │ 0.024
Iron │ 0.019 │ 0.019 │ 0.023
Zinc │ nil │ nil │ nil
│ ──────── │ ──────── │ ────────
│ 99.9795 │ 99.9960 │ 99.9482
─────────┴───────────────────┴────────────────────┴──────────────────
The ordinary form of the Scotch hearth is probably too well known
to need much description. The dimensions which have been found most
suitable are as follows: Front to back, 21 in.; width, 27 in.; depth
of hearth, 8 to 12 in. Formerly the distance from front to back was 24
in., but this was found too much for the blast and for the men.
The cast-iron hearth which holds the molten lead is set in brickwork;
if 8 in. deep and capable of holding about ¾ ton of lead, it is quite
large enough. The workstone or inclined plate in front of the hearth
is cast in one piece with it, and has a raised holder on either side
at the lower edge, and a gutter to convey the overflowing lead to the
melting-pot. The latter is best made with a partition and an opening
at the bottom through which clean lead can run, so that it can be
ladled into molds without the necessity for skimming the dross off
the surface. It is well also to have a small fireplace below the
melting-pot.
On each side of the hearth, and resting on it, is a heavy cast-iron
block, 9 in. thick, 15 in. high, 27 to 28 in. long. To save metal,
these are now cast hollow and air is caused to pass through them. On
the back of the hearth stands another cast-iron block known as the
“pipestone,” through which the blast comes into the furnace. In the
older forms of pipestone the blast comes in through a simple round or
oval pipe, a common size being 3 or 4 in. wide by 2½ in. high, and the
pipestone is not water-cooled. With this construction the hearth will
not run satisfactorily unless the pipestone is set with the greatest
care, so as to have the tuyere exactly in the center, and as there
is no water-cooling the metal quickly burns away when fume is being
smelted. Moreover, the blast is apt to be stopped by slag adhering to
the end of the pipe. As already mentioned, a peat is dropped in front
of the blast every time the fire is made up, with the object of keeping
a clear passage open for the blast. This old custom has, however,
several serious disadvantages; first, it prevents the blast being kept
on continuously; and, second, it makes it necessary to have the hearth
open at the top so that the smelter-man can go in by the side of it. In
this case the ore is fed from the side by the smelter-man, who works
under the large hood placed above the furnace to carry away the fume.
Even when he is engaged in shoveling back the fire from the front and
is not underneath the hood, it is impossible to prevent some fume from
blowing out; and there is much more liability to lead-poisoning than
when the hearth is closed at the top by the chimney and the smelter-men
work from the front. The best arrangement is to have the hearth
entirely closed in by the chimney, except for the opening at the front,
and to have a small auxiliary flue above the workstone leading direct
to the open air to catch any fume that may blow out past the shutter in
front of the hearth.
In an improved form of pipestone, a pipe connected to the blast-main
fits into the semicircular opening at the back and is driven tight
against a ridge in the flat side of the opening. Going through the
pipestone, the arch becomes gradually flatter, and the blast emerges
into the hearth, about 2 in. above the level of the molten lead,
through an oblong slit 12 in. long by 1 in. wide, with a ledge
projecting 1½ in. immediately above it. The back and front are similar,
so that when one side gets damaged the pipestone can be turned back to
front.
Water is conveyed in a 2½ in. iron pipe to the pipestone, and after
passing through it is led away from the other end to a water-box, which
stands beside the hearth and into which the red-hot lumps of slag are
thrown to safeguard the smelters from the noxious fumes.
On the top of the pipestone rests an upper backstone, also of cast
iron; it extends somewhat higher than the blocks at the sides. All this
metal above the level of the lead is necessary because the partially
fused lumps which stick to it have to be knocked off with a long bar,
so that if fire-bricks were used in place of cast iron they would soon
be broken up and destroyed.
With a covered-in hearth, when the ore is charged from the front,
the following is the method adopted in smelting raw ore: The charge
floats on the molten lead in the hearth, and at short intervals the
two smelters running the furnace ease it up with long bars, which they
insert underneath in the lead. Any pieces of slag adhering to the sides
and pipestone are broken off. After easing up the fire, the lumps of
partially reduced ore, mixed with cinders and slag, are shoveled on
to the back of the fire; the slag is drawn out upon the workstone
(any pieces of ore adhering to it being broken off and returned to
the hearth), and it is then quenched in a water-box placed alongside
the workstone. One or two shovelfuls of coal, broken fairly small
and generally kept damp, are thrown on the fire, together with the
necessary amount of ore, which is also kept damp if in a fine state
of division. It is part of the duty of the two smelters to ladle out
the lead from the melting-pot into the molds. In smelting ore a fairly
strong, steady blast is required, and it is made to blow right through
so as to keep the front of the fire bright. A little lime is thrown on
the front of the fire when the slag gets too greasy.
When smelting raw fume one man attends to the furnace. It does not
have to be made up nearly as frequently, the work being easier for
one man than smelting ore is for two. The unreduced clinkers and slag
are dealt with exactly as in smelting ore; and coal is also, in this
case, thrown on the back of the fire, but the blast does not blow
right through to the front. On the contrary, the front of the fire is
kept tamped up with fume, which should be of the coherency of a thick
mud. The blast is not so strong as that necessary for ore. The idea is
partially to bake the fume before submitting it to the hottest part of
the furnace, or to the part where the blast is most strongly felt. It
is only when smelting fume that it is necessary to keep the pipestone
water-cooled.
To start a furnace takes from two to three hours. The hearth is left
full of lead, and this has to be melted before the hearth is in normal
working order. Drawing the fire takes about three-quarters of an hour;
the clinkers are taken off and kept for starting the next run, and the
sides and back of the hearth are cleaned down.
THE FEDERAL SMELTING WORKS, NEAR ALTON, ILL.[7]
BY O. PUFAHL
(June 2, 1906)
The works of the Federal Lead Company, near Alton, Ill., were erected
in 1902. They have a connection with the Chicago, Peoria & St. Louis
Railway, by which they receive all their raw materials, and by which
all the lead produced is shipped.
The ore smelted is galena, with dolomitic gangue, and a small quantity
of pyrites (containing a little copper, nickel, and cobalt) from
southeastern Missouri, and consists chiefly of fine concentrates,
containing 60 to 70 per cent. lead. In addition thereto a small
proportion of lump ore is also smelted.
A striking feature at these works is the excellent facility for
handling the materials. The bins for the ore, coke and coal are made
of concrete and steel and are filled from cars running on tracks
laid above them. For transporting the materials about the works a
narrow-gage railway with electric locomotives is used.
The ores are smelted by the Scotch-hearth process. There are 20 hearths
arranged in a row in a building constructed wholly of steel and stone.
The sump (4 × 2 × 1 ft.) of each furnace contains about one ton of
lead. The furnaces are operated with low-pressure blast from a main
which passes along the whole row. The blast enters the furnace from a
wind chest at the back through eight 1 in. iron pipes, 2 in. above the
bath of lead. The two sides and the rear wall are cooled by a cast-iron
water jacket of 1 in. internal width.
Two men work, in eight-hour shifts, at each of the furnaces, receiving
4.75 and 4.25c. respectively for every 100 lb. of lead produced. The
ore is weighed out and heaped up in front of the furnaces; on the
track near by the coke is wheeled up in a flat iron car with two
compartments. The furnacemen are chiefly negroes. At the side of each
furnace is a small stock of coal, which is used chiefly for maintaining
a small fire under the lead kettle. Only small quantities of coal are
added from time to time during the smelting operation.
Over each furnace is placed an iron hood, through which the fumes and
gases escape. They pass first through a collecting pipe, extending
through the whole works, to a 1500 ft. dust flue, measuring 10 × 10
ft., in internal cross-section. Near the middle of this is placed a
fan of 100,000 cu. ft. capacity per minute, which forces the fumes and
gases into the bag-house, where they are filtered through 1500 sacks of
loosely woven cotton cloth, each 25 ft. long and 18 in. in diameter,
and thence pass up a 150 ft. stack.
The dust recovered in the collecting flue is burnt, together with the
fume caught by the bags, the coal which it contains furnishing the
combustible. It burns smolderingly and frits together somewhat. The
product (chiefly lead sulphate) is then smelted in a shaft furnace,
together with the gray slag from the hearth furnaces. The total
extraction of lead is about 98 per cent., i.e., the combined process
of Scotch-hearth and blast-furnace smelting yields 98 per cent. of the
lead contained in the crude ore.
The direct yield of lead from the Scotch hearths is about 70 per cent.
They also produce gray slag, containing much lead, which amounts to
about 25 per cent. of the weight of the ore. About equal proportions
of lead pass into the slag and into the flue dust. When working to
the full capacity, with rich ore (80 per cent. lead and more) the 20
furnaces can produce about 200 tons of lead in 24 hours. The coke
consumption in the hearth furnaces amounts to only 8 per cent. of the
ore. The lead from these furnaces is refined for 30 minutes to one
hour by steam in a cast-iron kettle of 35 tons capacity, and is cast
into bars either alone or mixed with lead from the shaft furnace. The
“Federal Brand” carries nearly 99.9 per cent. lead, 0.05 to 0.1 per
cent. copper, and traces of nickel and cobalt.
The working up of the between products from the hearth-furnaces is
carried out as follows: Slag, burnt flue dust and roasted matte from
a previous run, together with a liberal proportion of iron slag (from
the iron works at Alton), are smelted in a 12-tuyere blast furnace
for work-lead and matte. The furnace is provided with a lead well at
the back. The matte and slag are tapped off together at the front and
flow through a number of slag pots for separation. The shells which
remain adhering to the walls of the pots on pouring out the slag are
returned to the furnace. All the waste slag (containing about 0.5 per
cent. lead) is dumped down a ravine belonging to the territory of the
smeltery.
The lead from the shaft furnace is liquated in a small reverberatory
furnace, of which the hearth consists of two inclined perforated
iron plates. The residue is returned to the shaft furnace, while the
liquated lead flows directly to the refining kettle, which is filled
in the course of four hours. Here it is steamed for about one hour and
is then cast into bars through a Steitz siphon, after skimming off the
oxide. The matte is crushed and roasted in a reverberatory furnace (60
ft. long).
The power plant comprises three Stirling boilers and two 250 h. p.
compound engines, of which one is for reserve; also one steam-driven
dynamo, coupled direct to the engine, furnishing the current for the
entire plant, for the electric locomotives, etc.
The coke is obtained from Pennsylvania and costs about $4 a ton, while
the coal comes from near-by collieries and costs $1 per ton.
In the well-equipped laboratory the lead in the ores and slags is
determined daily by Alexander’s (molybdate) method, while the silver
content of the lead (a little over 1 oz. per ton) is estimated only
once a month in an average sample. When the plant is in full operation
it gives employment to 150 men. Cases of lead-poisoning are said to
occur but rarely, and then only in a mild form.
LEAD SMELTING AT TARNOWITZ
(September 23, 1905)
The account of the introduction of the Huntington-Heberlein process at
Tarnowitz, Prussia, published elsewhere in this issue, is of peculiar
interest inasmuch as it tells of the complete displacement by the new
process of one of the old processes of lead smelting which had become
classic in the art. The roast-reaction process of lead smelting,
especially as carried out in reverberatory furnaces, has been for a
long time decadent, even in Europe. Tarnowitz was one of the places
where it survived most vigorously.
Outside of Europe, this process never found any generally extensive
application. It was tried in the Joplin district, and elsewhere in
Missouri, with Flintshire furnaces in the seventies. Later it was
employed with modified Flintshire and Tarnowitz furnaces at Desloge,
in the Flat River district of Missouri, where the plant is still in
operation, but on a reduced scale.
The roast-reaction process of smelting, as practised at Tarnowitz,
was characterized by a comparatively large charge, slow roasting and
low temperature, differing in these respects from the Carinthian and
Welsh processes. It was not aimed to extract the maximum proportion of
lead in the reverberatory furnace itself, the residue therefrom, which
inevitably is high in lead, being subsequently smelted in the blast
furnace. Ores too low in lead to be suitable for the reverberatory
smelting were sintered in ordinary furnaces and smelted in the blast
furnace together with the residue from the other process. In both of
these processes the loss of lead was comparatively high. One of the
most obvious advantages of the Huntington-Heberlein process is its
ability to reduce the loss of lead. The result in that respect at
Tarnowitz is clearly stated by Mr. Biernbaum, whose paper will surely
attract a good deal of attention.[8]
LEAD SMELTING IN REVERBERATORY FURNACES AT DESLOGE, MO.
BY WALTER RENTON INGALLS
(December 16, 1905)
The roast-reaction method of lead smelting in reverberatory furnaces
never found any general employment in the United States, although
in connection with the rude air-furnaces it was early introduced in
Missouri. The more elaborate Flintshire furnaces were tried at Granby,
in the Joplin district, but they were displaced there by Scotch
hearths. The most extensive installation of furnaces of the Flintshire
type was made at Desloge, in the Flat River district of southeastern
Missouri. This continued in full operation until 1903, when the major
portion of the plant was closed, it being found more economical to ship
the ore elsewhere for smelting. However, two furnaces have been kept
in use to work up surplus ore. As a matter of historic interest, it is
worth while to record the technical results at Desloge, which have not
previously been described in metallurgical literature.
The Desloge plant, which was situated close to the dressing works
connected with the mine, and was designed for the smelting of its
concentrate, comprised five furnaces. The furnaces were of various
constructions. The oldest of them was of the Flintshire type, and
had a hearth 10 ft. wide and 14 ft. long. The other furnaces were a
combination of the Flintshire and Tarnowitz types. They were built
originally like the newer furnaces at Tarnowitz, Upper Silesia, with a
rather large rectangular hearth and a lead sump placed at one side of
the hearth near the throat end; but good results were not obtained from
that construction, wherefore the furnaces were rearranged with the sump
at one side, but in the middle of the furnace, as in the Flintshire
form. The rectangular shape of the Tarnowitz hearth was, however,
retained. Furnaces thus modified had hearths 11 ft. wide and 16 ft.
long, except one which had a hearth 13 ft. wide.
The same quantity of ore was put through each of these furnaces, the
increase in hearth area being practically of no useful effect, because
of inability to attain the requisite temperature in all parts of the
larger hearths with the method of heating employed. The men objected
especially to a furnace with hearth 13 ft. wide, which it was found
difficult to keep in proper condition, and also difficult to handle
efficiently. Even the width of 11 ft. was considered too great, and
preference was expressed for a 10 ft. width. In this connection, it may
be noted that the old furnaces at Tarnowitz were 11 ft. 9 in. long and
10 ft. 10 in. wide, while the new furnaces were 16 ft. long and 8 ft.
10 in. wide (Hofman, “Metallurgy of Lead,” fifth edition, p. 112). All
of these dimensions were exceeded at Desloge.
The Flintshire furnaces at Desloge had three working doors per side;
the others had four, but only three per side were used, the doors
nearest the throat end being kept closed because of insufficient
temperature in that part of the furnace. The furnace with hearth 11
× 14 ft. had a grate area of 6.5 × 3 ft. = 19.5 sq. ft.; the 11 × 16
furnaces had grates 8 × 3 = 24 ft. sq. The ratios of grate to hearth
area were therefore approximately 1:8 and 1:7.3, respectively. (Compare
with ratio of 1:10 at Tarnowitz, and 1:6⅔ at Stiperstones.) The ash
pits were open from behind in the customary English fashion. The grate
bars were cast iron, 36 in. long. The bars were 1 in. thick at the top,
with ⅝ in. spaces between them. The open spaces were 32 in. long,
including the rib in the middle. The bars were 4 in. deep at the middle
and 2 in. at the ends. The distance from the surface of the grate bars
to the fire-door varied in the different furnaces. Some of those with
hearths 11 × 16 ft. and grates 8 × 3 ft. had the bars 6 in. below the
fire-door; in others the bars were almost on a level with the fire-door.
The furnaces were run with a comparatively thin bed of coal on the
grate, and combustion was very imperfect, the percentage of unburned
carbon in the ash being commonly high. This was unavoidable with the
method of firing employed and the inferior character of the coal
(southern Illinois). The excessive consumption of coal was due largely,
however, to the practice of raking out the entire bed of coal at the
beginning of the operation of “firing down” (beginning the reaction
period), when a fresh fire was built with cordwood and large lumps of
coal.
Each furnace had two flues at the throat, 16 × 18 in. in size, each
flue being provided with a separate damper. Each furnace had an
iron chimney approximately 55 ft. high, of which 13 ft. was a brick
pedestal (64 × 64 in.) and the remaining 42 ft. sheet steel, guyed. The
chimneys were 42 in. in diameter. The distance from the outside end
of the furnace to the chimney was approximately 6 ft., and there was
consequently but little opportunity for flue dust to collect in the
flue. About once a month, however, the chimney was opened at the base
and about two wheelbarrows (say 600 lb.) of flue dust, assaying about
50 per cent. lead, was recovered per furnace.
The furnace house was a frame building 45 ft. wide, with boarded sides
and a corrugated-iron pitch roof, supported by steel trusses. The
furnaces were set in this house, side by side, their longitudinal axes
being at right angles to the longitudinal axis of the building. The
distance from the outside of the fire-box end of the furnace to the
side of the building was 10 ft. The coal was unloaded from a railway
track alongside of the building and was wheeled to the furnace in
barrows. Some of the furnaces were placed 18 ft. apart; others 22 ft.
apart. The men much preferred the greater distance, which made their
work easier, an important consideration in this method of smelting.
The hight from the floor to the working door of the furnace was
approximately 36 in. The working doors were formed with cast-iron
frames, making openings 7 × 11 in. on the inside and 15 × 28 in. on
the outside. On the side of the furnace opposite the middle working
door was placed a cast-iron hemispherical pot, set partially below the
floor-line. This pot was 16 in. deep and 24 in. in diameter; the metal
was ¼ in. thick. The distance from the top of the pot to the line of
the working door was 31 in.; from the top of the pot to the bottom of
the tap-door was 7 in. The tap-door was 4 in. wide and 9 in. high,
opening through a cast-iron plate 1½ in. thick. Below the tap-door
and on a line with the upper rim of the pot was a tap-hole 3½ in. in
diameter. The frames of the working doors had lugs in front, against
which the buckstaves bore, to hold the frames in position. All other
parts of the sides of the furnace, including the fire-box, were cased
with ⅝ in. cast-iron plates, which were obviously too light, being
badly cracked.
The cost of a furnace when built in 1893 was approximately $1400,
not including the chimney; but with the increased cost of material
the present expense would probably be about $2000. Notwithstanding
the light construction of the furnaces, repairs were never a large
item. Once a month a furnace was idle about 24 hours while the throat
was being cleaned out, and every two months some repairing, such as
relining the fire-boxes, etc., was required. If repairs had to be made
on the inside of the furnace, two days would be lost while it was
cooling sufficiently for the men to enter. In refiring a furnace, from
8 to 12 hours was required to raise it to the proper temperature. Out
of the 365 days of the year, a furnace would lose from 20 to 25 days,
for cleaning the throat and making repairs to the fire-box, arch, etc.
When a furnace was run with two shifts the schedule of operation was as
follows:
Drop charge 4 a.m.
Begin work 7 a.m.
Begin firing down 11 a.m.
Begin first tapping 1 p.m.
Rake out slag 2.30 p.m.
Begin second tapping 3 p.m.
Drop charge 4 p.m.
Begin working 5.30 p.m.
Begin firing down 11 p.m.
Begin first tapping 1 a.m.
Rake out slag 2.30 a.m.
Begin second tapping 3 p.m.
With three shifts on a furnace, the schedule was as follows:
Drop charge 7 a.m.
Begin firing down 12 a.m.
Begin tapping 1 p.m.
Rake out slag 2 p.m.
Begin tapping 2.30 p.m.
Drop charge 3 p.m.
Begin firing down 8 p.m.
Begin tapping 9 p.m.
Rake out slag 10 p.m.
Begin tapping 10.30 p.m.
Drop charge 11.00 p.m.
Begin firing down 4 a.m.
Begin tapping 5 a.m.
Rake out slag 6 a.m.
Begin tapping 6.30 a.m.
The hearths were composed of about 8 in. of gray slag beaten down
solidly on a basin of brick, which rested on a filling of clay, rammed
solid. The hearth was patched if necessary after the drawing of each
charge.
The system of smelting was analogous to that which was practiced
in Wales rather than to the Silesian, the charges being worked off
quickly, and with the aim of making a high extraction of lead directly
and a gray slag of comparatively low content in lead. The average
furnace charge was 3500 lb. At the beginning of the reaction period
about 85 to 100 lb. of crushed fluorspar was thrown into the furnace
and mixed well with the charge. The furnace doors were then closed
tightly and the temperature raised, the grate having previously been
cleaned. At the first tapping about 1200 lb. of lead would be obtained.
A small quantity of chips and bark was thrown into the lead in the
kettle, which was then poled for a few minutes, skimmed, and ladled
into molds, the pigs weighing 80 lb. The skimmings and dross were
put back into the furnace. The pig lead was sold as “ordinary soft
Missouri.” The gray slag was raked out of the furnace, at the end of
the operation, into a barrow, by which it was wheeled to a pile outside
of the building. Shipments of the slag were made to other smelters from
time to time, 95 per cent. of its lead content being paid for when its
assay was over 40 per cent., and 90 per cent. when lower.
Each furnace was manned by one smelter ($1.75) and one helper ($1.55)
per shift, when two shifts per 24 hours were run. They had to get their
own coal, ore and flux, and wheel away their gray slag and ashes. In
winter, when three shifts were run, the men were paid only $1.65 and
$1.50 respectively. There was a foreman on the day shift, but none at
night. The total coal consumption was ordinarily about 0.8 to 0.9 per
ton of ore. Run-of-mine coal was used, which cost about $2 per ton
delivered. The coal was of inferior quality, and it was wastefully
burned, as previously referred to, wherefore the consumption was high
in comparison with the average at Tarnowitz, where it used to be about
0.5 per ton of ore.
The chief features of the practice at Desloge are compared with those
at Tarnowitz, Silesia and Holywell (Flintshire), and Stiperstones
(Shropshire), Wales, in the following table, the data for Silesia and
Wales being taken from Hofman’s “Metallurgy of Lead,” fifth edition,
pp. 112, 113.
──────────────────────┬─────────┬────────┬─────────┬─────────┬────────
DETAIL │HOLYWELL │ STIPER-│TARNOWITZ│TARNOWITZ│ DESLOGE
│ │ STONES │ │ │
──────────────────────┼─────────┼────────┼─────────┼─────────┼────────
Hearth length, ft. │ 12.00 │ 9.75 │ 11.75 │ 16.00 │ 16.00
Hearth width, ft. │ 9.50 │ 9.50 │ 10.83 │ 8.83 │ 11.00
Grate length, ft. │ 4.50 │ 4.50 │ 8.00 │ 8.00 │ 8.00
Grate width, ft. │ 2.50 │ 2.50 │ 1.67 │ 1.67 │ 3.00
Grate area: hearth │ │ │ │ │
area │ 1:8 │ 1:6⅔ │ 1:10 │ 1:10 │ 1:7⅓
Charges per 24 hr., │ 3 │ 3 │ 2 │ 2 │ 3
Ore smelted per │ │ │ │ │
24 hr., lb. │ 7,050 │ 7,050 │ 8,800 │ 16,500 │ 10,500
Assay of ore, % Pb │ 75-80 │ 77.5 │ 70-74 │ 70-74 │ 70
Gray slag, % of charge│ 12 │ │ 15 │ 30 │ 27
Gray slag, % Pb │ 55 │ │ 38.8 │ 56 │ 38
Men per 24 hr. │ 6 │ 4 │ 4 │ 6 │ 6
Coal used per ton ore │0.57-0.76│ 0.56 │ 0.46 │ 0.50 │ 0.90
──────────────────────┴─────────┴────────┴─────────┴─────────┴────────
The regular furnace charge at Desloge was 3500 lb. The working of three
charges per 24 hours gave a daily capacity of 10,500 lb. per furnace.
These figures refer to the wet weight of the concentrate, which was
smelted just as delivered from the mill. Its size was 9 mm. and finer.
Assuming its average moisture content to be 5 per cent., the daily
capacity per furnace was about 10,000 lb. (5 tons) of dry ore.
The metallurgical result is indicated by the figures for two months
of operation in 1900. The quantity of ore smelted was 1012 tons,
equivalent to approximately 962 tons dry weight. The pig lead produced
was 523.3 tons, or 54.4 per cent. of the weight of the ore. The gray
slag produced was 262.25 tons, or about 27 per cent. of the weight of
the ore. The assay of the ore was approximately 70 per cent. lead,
giving a content of 673.4 tons in the ore smelted. The gray slag
assayed approximately 38 per cent. lead, giving a content of 99.66
tons. Assuming that 90 per cent. of the lead in the gray slag be
recoverable in the subsequent smelting in the blast furnace, or 89.7
tons, the total extraction of lead in the process was 523.3 + 89.7 ÷
673.4 = 91 per cent. The metallurgical efficiency of the process was,
therefore, reasonably high, especially in view of the absence of dust
chambers.
* * * * *
The cost of smelting with five furnaces in operation, each treating
three charges per day, was approximately as follows:
1 foreman at $3 $3.00
5 furnace crews at $9.90 49.50
Unloading 21 tons of coal at 6c. 1.26
Loading 14 tons lead at 15c. 2.10
“ 7 tons gray slag at 15c. 1.05
——————
Total labor $56.91
21 tons coal at $2 $42.00
Flux and supplies 13.00
Blacksmithing and repairs 10.00
——————
Total $121.91
On the basis of 6.25 tons of wet ore, this would be $4.65 per ton. The
actual cost in seven consecutive months of 1900 was as follows: Labor,
$1.98 per ton; coal, $1.86; flux and supplies, $0.51; blacksmithing and
repairs, $0.39; miscellaneous, $0,017; total, $4.757. If the cost of
smelting the gray slag be reckoned at $8 per ton, and the proportion
of gray slag be reckoned at 0.25 ton per ton of galena concentrate,
the total cost of treatment of the latter comes to about $6.75 per ton
of wet charge, or about $7 per ton of dry charge. This cost could be
materially reduced in a larger and more perfectly designed plant.
The practice at Desloge did not compare unfavorably, either in respect
to metal extracted or in smelting cost, with the roast-reduction method
of smelting or the Scotch hearth method, as carried out in the plants
of similar capacity and approximately the same date of construction,
smelting the same class of ore, but the larger and more recent plants
in the vicinity of St. Louis could offer sufficiently better terms to
make it advisable to close down the Desloge plant and ship the ore to
them. One of the drawbacks of the reverberatory method of smelting
was the necessity of shipping away the gray slag, the quantity of
that product made in a small plant being insufficient to warrant the
operation of an independent shaft furnace.