Charles George Warnford Lock.

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

. (page 45 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 45 of 76)
Font size
QR-code for this ebook

xtraction, only comparatively few mines, affording a rich ore, can be
rofitably worked.

Foremost in antimony production stands Portugal, due principally
) the mining district of Oporto, and especially to the area lying
etween the town of Vallonco and the river Douro, where some 100
ODceesions of 1000 metres oy 500 or 50,000 sq. metres each have
een taken up.* In 1880 the discovery was made that almost every
Qtimoniferous vein carried also gold.

The geological formations of Portugal are chiefly igneous and old
idimentary. The Oporto mining district lies practically in a basin
r granite, the enclosed rocks being Gambro-Silurian, Carboniferous,
ad Lauren tian slates and shales, with occasional dykes of quartzite.
"he shales course N. 10°. W., with a dip towards the east varying
t)in 50° to 80°. The most favourable rocks for good antimony ore
re bluish-grey argillaceous Silurian shales. Two main systems of
des intersect these shales : (1) coursing north-south ; and (2) coursing

The N.-S. lodes in the Gondomar district course practically in the
irection of the magnetic needle, with a dip varying from 30° to
[f W., which is opposite to that of the enclosing rock. The richest
iDeral is found in the E.-W. lodes, the most favourable course
Bing N. 75° E., the dip varying from 50° to 80° N. The lodes
BDcurally have well-defined walls, and vary greatly in thickness,
tmi a few inches to as many feet. The mineral in the N.-S. lodes is
imeminated through the gangue, whilst that in the E.-W. lodes is
mdensed into solid bancb which run in chimneys dipping about
^ E. The B.-W. lodes are generally bigger. The district is very
inch cut up by faults, which in some places throw the lodes many
«t The course and dip are very near those of the stratification of
)e shales. They carry a small amount of antimony ore and quartzose
frites, the latter often being fairly rich in gold. Occasionally the
des are bent by faults, which, in the bent part, assume more the

* The information here given is derived from a valoable paper by F. Merrioki
*Tiana. Init Min. and Met.,' 1898.

Digitized by



character of an elvan carrying much clay, but no mineraL Th
country rock in this neighbourhood is of a dry and barren-lookmj
nature, and much softer. Fig. 108 illustrates the E.-W., Fig. 109 th
N.-S. lodes and faults. The dotted lines represent cross-cuts. J

While the greatest depth yet reached is less than 1000 ft., j
appears from experience with many of the lodes that good mineral J
met with near the surface, which extends to a certain depth and thd
appears to pinch out. In the case of the Tapada lode, rich minen
has been struck below this apparent barren stratum, and such wi
probably be the case with other lodes when depth is attained.

Figs. 108, 109.— Antimont Lodes.

The Vallongo district is practically free from faults. The norti
south are the principal and paying lodes, and differ from those of tl
Gondomar district, in that the ore occurs more in the form of irn
gular pockets. Other lodes with different directions are not mufl
good and are not worked, the mineral in these being much dissem

The principal mineral worked in the Oporto district is snip]
only occasional pockets of oxide of antimony being met with,
gangue is principally quartz, rarely crystalline, with some iron
arsenical pyrites. Enclosed country rock is also associated with
ganeue. Calcspar is often met with, more frequently, however^
the lodes of the Vallongo district. Scheelite was onoe found in
Tapada lode. Tellurides are sometimes met with.

Among the other European centres of productioD, the Bohemift
mines are in granite and mica schist, the Hungarian in grami
(sometimes auriferous), the Styrian in dolomite, and the Turkish ab
in granite. Victoria, New South Wales and Western Australia tf
all large producers of auriferous stibnite; at Hill Grove the vein
occur in metamorphosed Devonian strata near a granite dyke. I
New Brunswick antimony is mined in a quartz and caldte gftogo
in clay-slates and sandstones of Cambro-Silurian age.

Digitized by



Within the United States stibnite has been found in a nnmber of
places, all in the West. At San Emigdio, Eem County, California, it
IB contained, with qnartz gangue, in a vein in granite. The vein
varies in thickness from a few inches to several feet, and has produced
some ore whioh has been smelted to regulus and shipped to market.
Sevetral other small deposits occur in San Benito County, and else-
where in California. Stibnite has also been discovered in Humboldt
[kranty, Nevada^ and in Lander County, not far fix)m Austin, in a
luartz gangue. Some remarkable deposits occur in Iron County,
Utah, as masses of radiating needles which follow the stratification
)lane8 of sandstone and fill the interstices of a conglomerate. No
attempts to work these Utah mines have been successnil. Stibnite is
bond in Sevier County, Arkansas, filling veins, with a quartz gangue,
n sandstone. An interesting deposit of senarmontite was found in
lie Mezioan province of Sonora, just south of the Arizona line, but
he q^uantity did not prove great.

The vein at San Emigdio reaches 40 ft. in thickness, but the mass
f the ore is too low grade to work. Published analyses of the best
[nudity give 62^ per cent, antimony, 6j^ bismuth, 12^ silica, 1^ alu-
linay entire freedom from lead and copper, and 3 to 15 oz. silver per
on. At present the only productive mines in America are in Nevada,
lie Beulah mine near Austin, from a vein 2 ft wide, produced 600
cms in 1892, averaging 60 per cent, metal; and the Sutherland, 200
>ii8 at 5^ per cent. ; the aggregate value being about 10,000Z. On
Mdne Creek, California, a ledge of '* quartzite and porphyritic rock "
ras remarkable for yielding several tons of nodules of metallio
[latiTe) antimony, of all sizes up to 300 lb. each.

Borneo is one of the largest shippers of antimony to Europe. Boli-
ia contains much stibnite associated with ffold and silver. One
line in Potosi is turning out 100 tons a month of 65 per cent, grade.
\ is said, however, that the veins all impoverish in depth.

Though the metallurgy of antimony is simple in theory it is dif-
oult in practice, owing to the ready volatility of the metal. Hence
le ore must be extremely pure to admit of profitable mining. Even
I Portugal, which is comparatively near the English market (the
neat centre), crude ores below 45 per cent, were not profitable in
)91. In purification by liquation, in the present stage of the metal-
rgy, it seems practically impossible successfully to separate the
ibnite from the gangue when the ores run below some such extreme
oiit as 30 per cent. Such considetations are important in their
tarings on tne value of a new enterprise. Stibnite is poorly adapted

w^ methods of dressing, because it is extremely brittle and is
[ually disseminated through hard quartz in fine needles or blades,
hich break up into slimes.

At the most progressive of the Portuguese mines, the ore on being
ised to tiie surfiice is passed over a li in. screen ; the smalls from
lis are conveyed to rolls, trommels and jigs. The stu£f passing over
deposited in bins, and ^ence is gradually drawn on to a table pro-
ded with water from a hose, and hand-picked by women, the waste
Uing into shoots for transfer to the waste-heap. The first quality

practically pure mineral is picked out and put by for barrelling ;

2 p

Digitized by VjOOQIC


the rest is hand-cobbed and separated into three qualities, the beit of
which goes with the previous selection, and contains 65-70 per cent
metal. Seoond quality runs from 56 to 60 per cent, metal, and is bar-
relled separately. The third or last quality is sent to t^e breaker,
rolls, and trommels, and divided into five classes (a) that paancg
ihroueh the trommel ; (6) that passing f in. holes ; (c) -A^ in« ; (<2) ^
in. ; ^) ^ in. The first class is again h^nd-picked and crushed with
tiie second ; the third and fourth are seiit to the jigs ; while the fiftli
goes to the settling tanks, pointed boxes, strakes, buddies, ^. Third
quality ore carries about 48 per cent, of metal. Only about 10 per
cent, of the total lode stuff raised is marketable ore.

The English method of smelting antimony ores has been well de-
scribed by E. Bodger, from whose paper * tiie fcdlowing aoooimt is

The ores must be free from lead and arsenic, neither of whidi ou
be eliminated. The ore is ground under edge runners and pMsed
through a coarse screen, the bogest pieces which are allowed to pi»
being about the size of haz^-nuts. After grinding, a sample ii
assayed to ascertain how much iron is requii^d for reduction. Tbe
process of smeltine consists in reducing the sulphide of antimony bj
means of metallic iron, the fusion taking place in crucibies meMuring
16 in. high, 11 in. wide at top, tapering to the bottom, and contaiinng
10 per cent, graphite in their composition, which are heated in a
very long reverberatory furnace. This furnace consists of a bed 54 fi
long, including the fireplaces, and 7 ft. 4 in. broad (inside site),
covered by a low arch which springs almost from the surfiaoe of tk
ground, the bed itself being sunk below. It is heated by a fireplftos
at each end, drawing into a common flue in the middle of the floor of
the furnace. The sides and top of the arch are covered with l-in
cast-iron plates, the whole anchored in the usual way. The famAoe
is sunk into the ground, so that it is quite easy to step on to its in^
covered roof.

The crucibles are lowered into their places through 42 drcak!
holes, 14 in. diam. in the arch, 21 on each side. Two 4-in. hdes ii
the furnace roof at each end of the bed are used for renxmi^

The pair of crucibles nearest the fireplaces at each end is kept &f
'* starring," or refining the crude metal. The charge for each crociUi
consists of 42 lb. ground ore, 16 lb. wrought-iron scrap, 4 lb. oomsoi
salt, and 1 lb. skimmings frcm the next operation, or uie same weight
of impure slag from a previous melting. These weights vary ww
every ore, but the above will be true for .an ore of 52 per oest

The scrap must not be cast iron. Tixmed scrap is preferred frn
its convenient form, and the small trace of tin being gener&Ol
believed to benefit the antimony. Part of the tinned scrap is beatM
up into a round ball, large enough to fit the top of the crucible looiify^
Such a ball weighs about 13 lb., and one is used for each charge, tki
remaining iron required being added in the form of turning! ^
borings, mixed through the ore, along with the salt, in the wei^ii^
• Jt 8oo. Chem. Ind., xL 16, xii. 1075.

Digitized by



Bcoop. Hie miztnre of ore, salt, and iron is dropped into the ornoible
yinmgli an iron funnel, the lump of beaten scrap being thrown in
last to form a kind of lid ; the fiiTnace hole is then olosed for about
\ hour, when the cmcible is again examined. In the meantime a
firesh charge is weighed out ready for the ornoible the moment it is
empty. As the charge melts, the ball of iron on the top falls down
and is gradually absorbed, the iron reducing the antimony to the
metallio state, it being itself oonverted into sulphida The salt assists
the separation of the slag, and tends to promote the fusion of the
fiilicious matters of the ore. The length of time required for fusion
and decomposition varies with the position occupied by the crucible.
As a rule, about 4 meltings are got irom each crucible per 12 hours,
so that, fidlowing for charging and occasional changing of crucibles,
b^ a little less than 3 hours may be taken as an average ; the richer
the ore the shorter the time required to melt it. Opposite to each
crucible, except those used for tiie final refining, is placed a conical
«a8t-iron mould close by, the furnace side, large enough to hold the
contents of the crucible, and furnished with a cast-iron lid. The
^srocible is balanced on the edge of the furnace wall, and the contents
are poured into the mould, which is at once covered with the lid ; the
cmcible is examined, scraped out if need be, replaced, and at once

The mould has at the bottom a circular |-in. hole. The first por-
tion which reaches the mould chills, and prevents the escape of the
renudnder. The fused mass, when cool, is knocked out by a hammer
and punch. When the mass is removed the reduced antimony is
knocked off the slag, which should be quite clean enough to be thrown
away. The metal obtained is known as *' singles, and contains:
antimony, 91*63 per cent.; iron, 7*23 per cent.; sulphur, 0*82 per
cent. ; insoluble matter, 0*32 per cent. An excess of iron is used to
reduce the whole of the antimony in the ore, and the next operation
consists in removing this by melting the '^singles" with a small
quantity of pure sulphide of antimony, the liquated sulphide being
lued for this purpose.

The charge for the second fusion consists of 84 lb. singles broken
fimall, 7 to 8 lb. liquated sulphide of antimony, with 4 lb. salt added
as a flux. Sometimes kelp salt is used in place of ordinary salt in this
fosion, and is found to be very siiitable. The reaction in this fusion
IB similar to that in the last operation, the excess of iron in the metal
reducing the pure sulphide of antimony to the metallic state, being
itself converted into sulphide of iron. The fusion is dbsely watched,
and great care is taken that the metal and the sulphide of antimony
shall mix thoroughly ; but much stirring with iron tools should be
avoided at this stage, as the object is to remove iron so far as possible.
When stirring is required, it is done as quickly as possible, in order
to expose the iron stirrer as little as may be to the action of the sul-
phide of antimony. When fusion is complete, the mass is skimmed
Dj means of a cast-iron ladle placed on a long shaft, and the metal is
poured into moulds identical with those used in the previous opera-
tion. The metal resulting from this melting is known as **Btar
howls," and each fusion yields a lump of about 80 lb. The skimmings

2 D 2

Digitized by



go to the first operation. An analysis of tliis second metal showed
antimony, 99 '63 per cent.: iron, 0'18 per cent.; sulphur, 0*16 pc
cent. — total, 99*87 per cent.

The surface of the crystals of this metal is covered with tin]
hright specks, which are a certain sign of the presence of sulphur
this appearance is known as ** flouring," metal showing these speck
heing said to be " floured." As in the first melting it is necessary ti
add an excess of iron in order to remove all the antimony, so in thi
case it is necessary to add an excess of sulphide of antimony in orda
to remove all the iron, and hence the presence of sulphur in the anti
mony obtained. In order to remove this sulphur, and finally ti
purify the metal, another melting is required ; and the cnstom of thf
trade being that antimony shall be sold in flat ingots, each ** starred'
or crystallised on the upper surface, it is necessary to take precautiom
so as to obtain this " star " or crystallised appearance, by means o
which the buyer judges of the purity of the metaL

These two results are achieved by melting the metal along with \
peculiar flux known as " antimony flux," a body often difficnlt to pre
pare, but easily kept in order. The rule-of-thumb process of niakin|
this flux is carried out as follows : — 3 parts ordinary American potasl
are melted in a crucible and 2 parts ground liquated solplude o
antimony are mixed in. When the mixture is complete and th«
fusion is quiet, the mass is poured out and tried on a small scale t<
see whether it yields a good *' star " ; if it does, the ingot of metal
obtained is broken, and the metal is examined to judge whether it i^
free from sulphur. If free, then the flux is considered satisfactoi]
and may be put in use ; but otherwise the flux is remelted and mor^
of one ingredient or the other is added as experience dictates.

The process of refining and restarring the star-bowls is as follows : J
The lumps of metal when cold are removed from the mould amj
thorough^ cleaned from the adhering skin of slag by chipping witi
sharp hammers, this paii; of the work being sometimes done by women]
who become ve^ expert in rapidly and completely removing everj
trace of slag. Unless this cleaning process is carefully carried ou^
it is hopeless to attempt to obtain a good star on the finished metal
The chippings are, of course, collected and returned to the second
melting. The star-bowls, having been cleaned, are broken small, and
a charge is weighed out for refining. The charge used is 84 lb. stai>
bowls and a sufficiency of the antimony flux. Enough flux is added
to surround the ingots completely, less or more according to theii
shape and thickness — ordinarily' about 8 lb. The melting takes place
in the crucibles next the fireplaces — that is to say, in those whid^
are hottest and in which the fusion will be most rapid. The charj
of metal is thrown into the crucible and narrowly watched, an
whenever it begins to melt the flux is added. As soon as the fu8i<|
appears to be complete, the fumaceman stirs the mixture once roun
only with an iron rod, and the charge is at once poured out. T)
ingot moulds are placed side by side, having between them a wedgl
shaped frame of cast iron, called a " saddle," the edge of which poial
upward, and upon which the charge is poured, when the utreal
divides, one half finding its way into each mould. These moulds af

Digitized by



ift to cook nndifltnrbed, and as they cool the flux which covers the
irface cracks, and when cold can be easily knocked off. The flux is
Bed over and over again, a piece of carbonate of potash being thrown
ito each fusion when old flux is used. In this way it will be seen
lat the flux keeps on increasing as a little potash is added and a little
liphor and antimony are picked up at each fusion. The ingots must
3 completely surrounded by flux ; there must be a thin layer of it
etween the mould and the metal, and also the whole surface of the
igot must be covered to the depth of \ in. Under the circumstances
»e metal should always give a good star and preserve a good colour.
he traces of flux which adhere are removed by washing in warm
ater with the assistance of a little sharp sand, water by itself
mig insufficient to remove the flux, which is practically insoluble
I water.
The personnel of such a furnace consists of about 36 men and
women, this total being made up as follows : — 2 flremen, one each
re, day and night, 4 ; 8 fumacemen, 4 on each side, day and night,
> ; 2 men cleaning metal, day and night, 4 ; 2 men breaking metal,
kj and night, 4 ; 1 man weighing charges, day and night, 2 — total,
I. On day shift only there are : — 3 men grinding ore, &c. ; 1 smith,
pairing tools, &c. ; 3 women packing and washing ; and 1 engine
id boiler man — ^total, 38. This does not include the making of cruel-
68, but, generally speaking, one crucible-maker and one labourer can
ike enough crucibles, working during the day only, to keep the
mace going. The coals consumed, including those for firing the
Ins, amount to about 22 tons per week, or a little more than 1^ ton
ch shift. About 11 crucibles are used per ton of reflned metal
oduoed, but this might be reduced by careful working. The yield
finished metal from such a furnace, working a 52 per cent, ore, is
ont 14J tons per week.

A great deal of volatilisation takes place from the melted metal in
e pots, and the fume thus produced is condensed in the flues of the
rziace, which are built for that purpose in a winding manner, passing
ckward and forward under the floor of the crucible drying stoves,
as to dry the pots at the same time that the fume is condensed. The
tal amount of fume varies very much ; the richer the ore the less
me there is in proportion to the antimony produced, although the
solute amount is greater than when a poorer ore is worked. About
per cent, of the total antimony contained in the ore is volatilised
oxide, and of this the greater part .is condensed in the flues. The
me is whitish, heavy, and rather crystalline, in appearance not very
dike white arsenic. It contains about 70 per cent, to 12\ per cent, of
Btallic antimony. The smelting of this fume is conducted as follows : —
test experiment is made in order to ascertain the amount of carbon
the form of coke or anthracite necessary to reduce all the antimony
■esent in the fume. This having been found, the fume is mixed by
inding under edge runners with the proper quantity of carbonaceous
atter, and of the mixture so produced a few pounds' weight is added
each charge of ore and iron when melting for singles. As the gases
ven off in the process are apt to cause the mixture in the pots to
erflow, the " boiling ore," as the workmen term the mixture of fume

Digitized by



and coke, is therefore looked upon by them with great dis&vonr ; kt
beyond the mechanical difficulties, there is no trouble whatever in
smelting the fume. The flues require cleaning out at int^rals^
sometimes once cTcry 2 or 3 months. About 3 to 6 per oent. of the
metal is lost in the slags.

The ingots, which are known in the trade as ^ French metal"
after being wrapped in straw, are packed in kegs holding about
6 cwt. net.

Continental smelting methods have been described by B. Helm-
hacker * and by C. A. Hering,* who suggest some improTements.

Where the reverberatory furnace is used without crucibles, a solid
hearth is indispensable, as the very fluid metal penetrates all crevioe&,
and where no artificial bed is sufficiently durable, recourse is had to
granite or some similar stone, formed into a trough of one piece, and
made not from the hardest stone, which will probably crack on heatbg,
but of softer, half-weathered varieties. Onroasted ore and scrap iio&
may now be plftced upon the furnace bed, where the former melts
readily and is decomposed. All the metal cannot be thus recov^ed,
since a part volatilises and a part forms a double sulphide with the

In the process by which antimonite is fused with sodium ctrbo-
nate, the mixture froths very considerably, and is found to attack
the furnace.

In the third process, the partially-roasted ore, containing Sb^
SbsOs, and Sb204, is charged into the furnace with small coal, scrap
iron, and soda (or with coal and soda alone). Where it is possibk
8 to 13 per cent, of coal and 9 to 11 per cent, of soda are used witboit
the addition of iron, since with the latter the slags are less fusible asA
do not entirely cover the bath of metal, and a regulus rich in iron is aleo
obtained. A furnace with a cavity on one of the longer sides, in whick
to collect regulus during the liquation and metal in the subsequent
treatment, is best suited for this purpose.

The great expenditure of fuel and loss by volatiliaation in theee
processes render a method of smelting by the blast furnace desirable.
Experiments in this direction have, however, been only partially
satisfactory. Helmhaoker^s early experiments failed, owing to the is6
of a coke with 12 per cent, of ash and of powdered oxides of antimonj.
He therefore made up the oxide into lumps with 10 per cent, of sodinn
sulphate and a little water, and charged into the furnace with 33 per
cent, of chaicoal. At first all went well, the charges sank regultfly
down, a flne rain of metallic globules fell into the crucible, and a good
red heat was maintained in front of the tuyers ; but in about 6 hoius
a quantity of blackish balls of slag (consisting of Na^S reduced from
the sulphate) accumulated before the tuyers; and from that tiicd
the temperature gradually fell at this point, though the heat was
maintained and reduction still proceeded in the upper part of tbe
furnace. Apparently the only difficulty consisted in finding a snf-
ciently fusible slag. Attempts to find a substitute for solid granite
hearths have failed : fireclav is more rapidly attacked by the soda ; ste^
tite exfoliates under the heat ; and magnesite is insufficiently sobi
• Dingl. Polyt. Jl., 1883.

Digitized by VjOOQ IC

Digitized by




This metal oocnrs in a number of forms, but chiefly as native bs-
muth, as a sulphide (81 per cent, metal), as an alloy with gold (mal-
donite), and as a carbonate (bismutite).

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 45 of 76)