Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

. (page 60 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 60 of 76)
Font size
QR-code for this ebook


5^9



expiration of abont 2 hoars the doors are all closed, the damper is
fully raised, and the whole charge is melted down into the well, which
then contains lead at the bottom and slag at the top. A sbovelfal of
lime is throwrn npon this and mixed with the slag, with the object, it
« said, of thickening it, and the slag is with any unrednced portions
)f ore pushed back on to the bed of the furnace and remelted. Wben
t has all run down, more lime is added, and the thickened slag is
igain poshed back to drain the lead ont The lead is then tapped




Fio. 147. — FLnrrsmsB Fubnace.



Fio. 148.— Mkxioan Sbrpenton.



hrongh a hole made by driving an iron bar into the well through
be day stopping in 6. The slag, known as '' grey slag,*' is then raked
int. The time occupied in working off a charge is about 5 hours,
gnited coal with slack is thrown upon the surface of the lead in the
)ot, and subsequently skimmed off with the scorias and thrown bcM^k
nto the furnace for more lead to sweat out.

The Derbyshire ore carries much barytes, which necessitates the
addition of fluorspar or calcspar. The slag in that case (called " run
lag ") is chiefly tapped off before the lead, for which purpose the
nmace is provided with two tap-holes instead of one.

The Cornish method requires two reverberatory furnaces. The
irst, with a flat floor, is used for calcining the ore, which, after cal-
ination with the addition of a little lime to prevent clotting in the
arly stage, is drawn out and introduced into what is termed a " flow-
Qg furnace." This is a reverberatory furnace similar to that used in
he Flintshire process, hut the charge, mixed with culm (small an-
hracite) is introduced not by a hopper in the roof, but through the
oors opposite the tap-hole. The cnarge is at once melted down with
losed doors and strong heat. At a certain stage of the smelting,
per cent, scrap iron is introduced into the well of the furnace. The
Tcducts, which are all run at one time from the tap-hole, are first
904^ theu a regulus or matte (consisting chiefly of iron protosulphide,
>ut .ntaining other metals such as copper or silver present in the
re), and lastly a slag which contains only 1-lJ^ per cent, lead, and
I thrown away. This method is used on ore yielding 60-70 per cent,
^ad, composed of galena, blende, spathic iron, grey and black copper
res. quartz and fluorspar.

The derpehUm of the Sierra Mojada, Mexico,* is a reverberatory
timaoe, built of adobe, with a stone lining to the chimney and the
iclined part of the hearth. The fire-box a (Fig. 148) is 3 ft. long

* R. E. Ghism, Trans. Amer. Inst Min. Engs., xv. 560.

Digitized by VjOOQIC



S30 ECONOMIC MINING.

and 1^ ft wide, Tanlted over, and at the inner end has a fire-bndge b
about 1 ft. high. The badn e is about 36 X 18 in., lined with x»-
fractory day and well fettled with slag, the lowest point being besidB
the tap-hole cL Behind the basin, and doping upwards at about l(f , ii
the hearth e, 12 ft. long by 18 in. sq. section, floored and partial^
lined with refractory stone; it opens directly into chimney J^ whico
rises 16 ft. above iLe highest part of the hearth e. The oharging
door g has the same sectional area as the hearth ; the latter is axdied
over in its entire length, and is provided with working doors A about
8 in. square at intervals. The charge is 10 arrobas ore with 1 d
litharge and 3 of lead scum from the remelting furnace, thrown is
at ^, each shovelful of diarge being mixed with 4 shovelsfVil of diar-
coal dust. Metallic lead and litharge are soon formed, malring a skg
with the small amount of silica present, and forming a crust on tof
of the ore layer. This being the case, the charge is skimmed, tfai
crust being moved downward on the hearth towards the fire-box ii
fragments, while the under portion of the charge is rabbled, so as U
be better exposed to the action of heat. These operations are repeat«>^
in succession, fresh ore being charged whenever neoessary, and ihi
former charge being moved downward until, in 3-4 hours aftd
cbareing, the basin is full of melted lead, on top of which floats i
highly basic slag, only made fusible by the large quantity of lea^
(sometimes 35 per cent) which it contains. The lead is run off froii
time to time into a basin or receptacle dug in the ground at the vl^
of the furnace, and cools into ruae slabs. The furnace lining is ooo^
tinually used up to provide silica for the slag. The slabs need le
melting and moulding. The cost is stated * at 28«. a ton. The prc^
duct is an excellent lead, carrying over 90 oz. silver to the toi^
The slags carry 30 per cent, lead, the object being a large saving o
silver at lowest cost for reducing and cupelling the lead.

The Carintbian reverberatory for galena slunes is 10 f^. long, 5 ft
wide, and contracts towards one end, sloping also uniformly in tb^
same direction, and ending in a narrow gutter which conducts tb
reduced metal to a well just inside. The fireplace runs parallel to tfai
longer axis, and is provided with air holes; the grate is of stone, il
for wood fuel, and of iron bars if for brown coaL The hearth bed »
6 in. thick, made of a mixture of fireclay, old beds, poor slimes, ant
slags, all fused into a mass. The complete treatment of about 750 lb
of ore occupies 21 hours; about 11 cub. ft. of wood are burned p«
cwt. of charge ; and 2\ per cent, lead is lost. Blast furnaces are sup
planting this method.

A great variety of ores are smelted at Freiberg, mostly in th(
form of slimes, and containing (a) galena mixed with arsenical aD<|
iron pyrites, blende, calcspar, bary tee, quartz, and brownspar ; (ft) ala
a Kilver ore carrying much eartny matter and a little pyrites anj
galena ; (c) and a copper ore carrying silver ; (d) as w^ as a fluxing
ore giving pyrites, biendo. calcspar, galena, copper, and *03 per cent
silver. The galena a is divided into 1st and 2nd classes, aooording »i
it does or does not carry 30 per cent. lead. The preliniinary step ii
roasting in the reverberatory (Fig. 149) in which the flames pass fivH
♦ J. N. JudsoD, op. dt., XV. 587.



Digitized by



Google



METALLIFEROUS MINERALS. 531

be fireplace a over the lower bed 6, up to and oyer the upper b^ e,
nd carry the snlphurous acid through the flue d to a condenser. The
barge, conaieting of 50 per cent Ist class a, 30 per cent. 2nd class a,
nd 20 per cent. 6 or e, is introduced on to the upper bed and gradu-
lly poshed towards a hole communicating with the lower bed,
brough which it falls, and is raked forward ; it is thus exposed to
iradually increasing heat till fusion is reached. The charge is re-
ewed continuously at each move forward, 1 ton of ore requiring 8-16
ours. The metals are mostly oxidised, but 4 per cent, sulphur re-
lains. The roasted and agglutinated ore is smelted with 5 per cent.
Ime and some roasted matter in a blast furnace ; and the regulus is
oasted and resmelted seyeral times till rich enough in copper for
pecial extraction of that metal. Lead slags, and poor dry ores, are
melted in other reverberatories (Fig. 150) ; charges of 20 cwt. lead
lags, 5 cwt. raw ore, 5 cwt. roasted ore, and 2 cwt. quartz, are intro-



FiG0. 149, 150. — Frbibebo Reverberatories.

Inced by the movable hopper a, and uniformly spread over the
learth 6, making the layer somewhat hij^her near the fire-bridge c.
Imelting is conducted without air for about 3 hours, when the almost
luid mass is stirred and exposed for 20 minutes to a still higher tem-
leratore, after which the slag is drawn off. When 2 or 3 charges
lave been smelted, the regulus is tapped off into moulds and cooled,
nd finally smelted again in a Pilz furnace.

At Clausthal the preliminary calcining to remove sulphur used to
e done by piling ore and pine logs in heaps, igniting, and leaving
hem to bum for 3 or 4 weeks, after the manner of the tdera at Rio
Pinto (see p. 431). The oxides and sulphates thus formed are after-
rards smelted in blast furnaces. Pile roasting has also been common
n Utah, using the lighter woods (pinon pine gives too much heat), at
I cost of 2b, 6d. a t(m for raw ore, and 9«. a ton for matte.* At Las
Trojes, too, Michoacan, Mexico, the highly pyritic ore is calcined in
jpen heaps called ealeraa^ but the operation is only partially por-
brmed in wet or windy weather, and occupies a long time, so that
ttalls are coming into favour as a substitute, notably on account of
he reduced loss of silver.

In France, highly silicious ores are roasted in a reverberatory like

• B. H Terhune, *♦ Ore and Matte Roaeting in Utah," Trans. Amer. Inst. Miii.
^gB., Jaly 1887.

Digitized byVjOOQlC



53^ ECONOMIC MINING.

the Flintshire fumaoe, except that it is filled to the level of thewoik-
ing doors with black slag, so as to form a flat hearth. The opentioD
is made intermittent, rabbling taking place alternately with periods
of heating with closed doors. When sufficient sulphur has been driTen
off (in about 6 hours), the heat is forced till the charge runs together,
when it is drawn out on the floor, and cooled ready for smelting.

In the south of France an effort is made to get uniformity is
silver contents by weighing out and spreading in equal layers, one c«
top of the other, the required quantities of the several orw and
mattes to form a roasting bed of about 20 tons, and this stratified bed
is cut down vertically into 1 ton charges of equal oomposition. If
very silicious, a little coke dust is added ; the matte serves as flm.
and is usually about 10 per cent, of tiie whole. The furnaces measore
about 40 ft. long and 15 ft. across widest part, built of lava or stone.
and lined with tirebrick. The ore is dropped from a hopper at tk
farthest end from the fire on to the drying bed, where it is stirred fur
6 hours ; then pushed nearer the fire for another 6 hours, and fiiuilij
raked on to the fluxing bed, next the fire, and a few inches below th<?

level of the other two beds.
^ till a further 6 hours' heatiii^

results in fusion, when it s
drawn. The work is m&de
continuous by adding a frei^
charge as each is moved od,
and thus 8 tons are roafctedl
and fluxed in 24 hours at »
cost of 2 tons coaL The flm |
is 6 per cent, lime and 7 ftr
cent iron slags. Pure oreei
are treated in the rerer-

resting on iron supporta v^
formed of firebricks on edge, with the usual slag covering. TtA\
tap-hole and lead well are placed near the flue end to reduce 1««
by volatilisation. The aim is to convert about one-half the wJ
phide into sulphate or oxide first, and then, by increasing the tea-'
perature, to get a reaction between the oxidised and unoridL^i
portions, a^he furnace being red hot, the charge is dropped from tkj
hopper and evenly spread over the bottom. The heat is slowly J
creased, and air is freely admitted to cause oxidation. Wheneverl
crust of oxidised material forms on the surface of the charge, a d«|
surface is exposed by rabbling. r!in*lA«i ai^ «o-^ #v.« ^«v^ ;« nrJ
ference to coal at this stage of th
heat, do not yield any gaseous hy
tion, and cost less. After 4-6 hoi
desulphurised, when the temperati
to the fire, and the second stage (
menced. Care has to be taken tha
as that would cause loss of lead bj?
lime is thrown on the mass whenc



Digitized by



Go ogle



METALLIFEROUS MINERALS. 533

imd is thorongbly worked into it. Tlie consumption of lime in this
operation amounts to about 2 per cent, of the charge. The reduced
metal, which first appears in globules on the surface, drains down the
slope of the hearth into the lead well. After about 3 hours enough
will have collected to justify tapping. The lead flows into a pot under
which a fire burns to keep it liquid. The dross is skimmed off and
thrown back into the furnace. Coal dust, cinders, and powdered lime
are stirred into the lead, and the impurities are once more skimmed
off, when the lead is ladled into moulds. When as much lead as
possible has been extracted from the charge, the heat is increased in
order to Completely oxidise the remaining material, but not sufficiently
to fuse it. After this object has been attained, the pot skimmings,
oonsisting chiefly of cinders and sulphides, are thrown on the charge,
when a further yield of lead is tapped off, and the slags are raked out
through a door at the back of the furnace. The entire process
requires about 5 hours. Before introducing the next charge, the
famace bottom has to be thoroughly examined, and, if necessary,
repaired, as it is of great importance to keep it perfectly smooth and
sloping evenly towards the tap-hole. About 40 per cent, of ooal is
required. The total loss amounts to about 3*5 per cent., mainly
caused by volatilisation, but a considerable portion of this is recovered
from the flues in which it condenses. The slags retain about 20 per
cent of the original amount of lead,
and are resmelted in a blast fur-
nace, when the greater part of this
is also recovered.

The Spanish boZicAc (Tig. 152) as
used at Linares, is built of rubble
and clay, and lined throughout with
refractory clay. The fireplace a,
5 ft. 6 in.*long and 2 ft. 2 in. wide,
has no grate, and is fed with
brushwood by the door 6. The
hearth c measures 7^ by 6 ft, and

connects by flues d. with a ch<im- Ym. 152.— Spanish Bolichb.

her «, which is regarded as highly

important in controlling the draft, and certainly serves to retain some
mechanically suspended mineral. The flue / leads to chimney ^,
about 30 ft. high. The bottom of hearth c slopes gently towards the
working door A, immediately within which is a well t for collecting
the molten metal, connectea with a receptacle Jc outside. The charge
is thrown into the furnace, evenly spread on the hearth, and fre-
quently rabbled for \\ hours, during calcination. Then the tempera-
ture is raised and the charge is smelted, yielding about 80 per cent, of
tiie metal in the ore, while the grey slags contain 40-50 per cent and
are resmelted in a blast furnace.

A sort of intermediate between the reverberatory and the blast
furnace is the ore hearth (Fig. 153). It affords one of the simplest
lead smelting methods, and recommends itself by its small consump-
tion of fuel, quick operation, and inexpensiveness in general, as
compared with the reverberatory and blast furnace processes. It also

Digitized by VjOOQIC



534 ECONOMIC MINING.

permitB an intermption of the operation at any stage withoitt great
lo68 of temperatore ; and peat or wood may be used as fbel where ooal
or coke is not obtainable.

These reasons recommend the hearth fomaoe in places where
another kind might otherwise be nsed, and particularly to small
smelting establishments. Very pnre galena may be treated direct in
the hearth furnace ; impure ores have generally to be roasted pre-
Tionsly. In the Scotch hearth, a cold blast is nsed ; in the Americsa,
a saving of fael is effected by nsing a hot one.




Fio. 153.— Scotch Obs Hbabth.

It consists of an oblong cast-iron tank or well a, abont 2 ft. 6 in.
wide, 2 ft. from front to back, and 1 ft. deep, capable of containing
about 2 tons oflead, with which it is filled to the brim, the snr&oeof
the lead forming, in fact, the floor of the hearth. The floor thu
formed is enclosed at the sides with blocks of cast-iron, and another
block of cast iron is placed behind, and is perforated for the pasBSge
of the tuyer h that conducts the blast into the furnace about 2 in. above
the surface of the lead in the well. A shaft of brickwork proceeds
upwards from the hearth to the flue, and there is behind it a Imnd flue
or pit into which the " hearth ends," or dusty matter which oomes off
with the fume, may fall, and from which it is removed as requisite.
The front opening to the hearth is sometimes provided with a sliding
shutter, which by means of a counterpoise can be raised or let down
in its groove so as nearly to close in the front of the hearth. Extend-
ing forwards from the front oif the hearth, and inclining down waids tt
an angle from it, is a plate of iron c called the *' forestone," in which
there is a groove <l, that leads towards an iron pot e, kept hot bj t
little fire beneath. The ore is fed in either from the front or through
a hopper at the side. A fire of coal being made upon the hearth, and
heaped up chiefly behind, a moderate blast is put on and Uie oxe
(sometimes previously calcined) is thrown on, and if there be a
shutter tiiis is put down. After the lapse of a few minutes, the
workman introduces a poker and stirs up the fuel and ore, and from
time to time repeats the above process with fresh small quantities of
ore, adding fuel as it appears requisite. At intervals of a few minuteB
he raises the shutter and draws forward a portion of the charge on to
the forestone and picks out from it portions of **grey slag" whidi he
pushes aside, and ultimately throws off on to the floor of the workshop
at the side of the hearth. As the lead forms, it runs into the well,
\ overflows alon^ the channel of the forestone into the pot e set to
*ve it, from which it is ladled into the moulds. It is a prooea



Digitized by



Google



METALLIFEROUS MINERALS. 535

which requires constant manipulation of the charge, two workmen
being continnally occupied in addii^ ore or fuel, poking np the
charge, &c., at intervals of a few minutes. Lime is used, as in the
Flintehire process, to thicken the slags.

The&vonred type of reverberatory (or "roaster") in America* is
the " 4-h6arth,'* so called because the length of the hearth is roughly
4 times its width, though the slope may he continuous from end to
end, and no step may mark the division between the hearths. To
each «* hearth ** belong 4 working doors, 2 on each side, generally
opposite, bnt better alternating. A furnace with 4 heaiihs, each
14-1 6 ft. square, should deiBd with 8-10 tons ore a day if only roasting ;
bnt if the charge has to be fused or slagged in addition, the qnantity
will not exceed 6 tons, and the expense is proportionately increased.
The fusion hearth is nearly circular in plan, the radius being 10 ft. 8 in.
From the last calcining-hearth to the bed of the fusion-hearth is a drop
of 26 in., the object being to get the end of the arched roof of this
portion of the furnace below the level of the calcining-hearths, so as
to ensure the flames being completely reflected upon the mass under-
going fusion, before being spread over the wider calcining hearths
beyond. The vaults beneatn the calcining hearths are best flUed
solid, and should be used as dust chambers only when lack of space
compels it. Under the fusion hearth an air space is necessary. The
object of fusion is to obtain a slagged mass in lumps that can be
hajidled, and will help to keep the blast furnace open. But while in
simple roasting the lead loss does not exceed 5 per cent. — say 2 per
cent, on a 40 per cent galena— in fume, half of which should be re-
covered, and the silver loss is not more than 1 per cent, (unless much
chloride is present), in fusion, on the contrary, the lead loss will be
15-20 per cent, and the silver loss 3 per cent, under ordinary con-
ditions. Hence it is advisable to use the maximum of roasted ore
which the furnaces will bear, and to fuse
only enough to keep the charge open,
■electing ores low in lead and fine in size,
such as concentrates from gold mills and
antimonial, arsenical, and zinciferous
parcels.

The Mexican Jbmo OGMfettano f or up-
right furnace (Fig. 154) is an exceedingly
simple structure, being little more than
a niche in an adobe wall, more or less

completely lined with refractory stone. piQ. 154.— Msxican Hobno
It is always of approximately square Oastellano.

section, about 14 in. on each side at

the mouth, and tapers gradually from the mouth to the tuyer-Ievel,
where the section is about 12 in. on each side. It is about 3^ ft. high,
from the mouth to the tap-hole. The single tuyer a, about 1 in.
diam., is 5-7 in. above the tap-hole, and the bottom of the furnace has
a slant forward, commencing just below the tuyer and ending at the
tap-hole. The tap-hole is some 2^-3 ft. above the floor of the casting-



♦ H. F. Collins, ** Smelting ProooBses/* op. cit.
t R. £. Chism, op. oit, p. 555.



Digitized by



Google



536 ECONOMIC MINING.

room, and a sort of bench of stone and dirt, witli a mde iMflin in its
centre, is formed in front of the furnace, to reoeiye the molten pco-
ducts and give them a chance to separate before going farther. When
the furnace is to be operated, the lower fotirth of the shaft or nidie s
dug out to a depth of 3 in., lined with refracUny clay mixed with
charcoal-dost, and well rammed. The bottom is made of the same
material, and the fonrth or open side of the niche is dosed np from
top to bottom with adobes luted with clay, leaving the tap-hole about

2 in. diam. The charging of the furnace is done at the open mouth,
the feeder carrying the materials and fuel in a little tray, and monnt-
ing up on the bendi just mentioned to throw them in. Two fnmaoes
are built side by side, with an interval of 4} ft. clear between them.
The slag is led off on the side of each furnace fiBui;her from the oentre-
line ; but the lead from both furnaces runs towards the centre into ft
rongh, oval depression in the floor of the casting-room, where it con-
soliaates into a rough slab. The tuyers, one to each fomaoe, ard
poked through the refractory lining, and wedged around with
refractory clay. Generally, each furnace is supplied with wind by a
large blacksmith-bellows, worked by hand-power through a system of
levers, which allows the weight of the men to do most of the work.
Sometimes a Sturtevant blower is used. On beginning to work with
a new or newly-lined fumaoe, the shaft is slowly heated up by a small
charcoal or wood fire, without blast. When the whole structure has
thoroughly dried out> more charcoal is thrown in, the blast is turned
gently on, and a small charge of slag and lead scum from the r&-
molting furnaces is exhibited. The cbarges are gradually increased
in size and mixed with ore, until the full burden of the fumaoe is
reached.

The CastiUan blast furnace measures about 8 ft. high and 3 ft
diam., and is constructed of firebricks moulded into the shape required,
the shaft thus constructed being surmounted by a box-shaped hood,
in the sides of which are the flue and feeding door, and on top an arch
of brickwork laid in clay forms a dome. The breast is formed by an
iron pan, having on its upper edge a lip to allow the slag to flow oC
and on one side a long narrow slot for tapping or drawing off the
reduced metal. The bottom of this fumaoe is made in the following
way : — A mixture of fire-clay and coke-dust is slightly moistened and
stamped or beaten into the hearth bottom until it reaches the top of
the breast pan. This is hoUowed out in the usual way to form a
cavity for the collection of the reduced metal, and allowed to dry
thoroughly before the apparatus is used. The blast is applied l^

3 tuyers having a diameter of 5j^ in. at the receiving ends and 3 in at
the nozzles. Tne blast is conducted to them through brick channels
placed under the floor of the furnace house. The structure is secured
by iron bands encircling it, and the hood is supported by iron columns.
In working, the charge should never contain over a third of its
volume in lead. If richer ore is to be treated, it must be reduced to
this proportion by the addition of poor slags. To prevent the walls
from getting too hot, and preserve the bricks from burning or melting,
- •*^ has to be taken in charging to throw the fuel towards the centre

'he ore towards the walls. Attention has to be paid to the proper



Digitized by



Google



METALLIFEROUS MINERALS. 537

Bgnlation of the tem^ratnre, as a too high degree of heat will canse
068 of lead by yolatilisation. Ab long as the slag flows liquid and
-eadilj, the cooler the fiimace is kept the better. Some fermginoos
>re is usaally added at intervals during the operation. The slag flows
toDtinuously into east-iron wagons, from which it is dumped after
taving cooled down. The advantage of this is that if at any time
he furnace should run lead or matte, it can easily be recovered,
ilxtensive condensing apparatus should be provided, as a considerable
onount of lead volatilises even when every precaution is taken.*

The typical European blast furnaces are the Pilz and the Bas-
hette, differing little but in shape, the former being circular or
diagonal and l£e latter an elongated parallelogram in section. The
^Iz has 7 tuyers, and its upper portion is sustained by a cast-iron



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 60 of 76)