Charles George Warnford Lock.

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

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tion of the tin ore with which the wolfram is associated, is effected as
follows ; — The tin ore is dressed as perfectly as possible in the ordinary
manner; then, having ascertained the proportion of wolfrum in it,
such a proportion of soda ash or crude soda is added as will proride
an equivalent of soda for the tungstic acid of the wolfram present
The mixture is then put into a reverberatory furnace (Fig. 174), and
there roasted at a low red-heat, until a combination is effected between
the tungstic acid and the soda. The iron with which the acid hsd
been previously in combination is at the same time converted into a
peroxide, and rendered sufficiently light to be washed off wit^ facility.
When the roasting is completed— which may be known by the change
of colour, and by the mass assuming a sligntly pasty condition — ^t^
charge is drawn through a hole in the bed of the furnace into the
*' wrinkle " a beneath. A fresh charge is introduced through the
hole h in the crown of the furnace from the ** dry *' e ; and as 60(hi as

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the cliarge is spread over the bed c the furnace is shut, the fire d is
made up, and it is left without further stirring, until the surface of
the charge assumes the appearance of becoming moist, with a slight
hissing or frizzling sound. In the meantime the charge, while still
red-hot in the interior, is removed from the " wrinkle and thrown
into a cistern of water. The water thus heated dissolves the soda
tongstate ; the solution is run off from a hole in the bottom, provided
with a suitable filter, to prevent the running out of the tin ore.

Fio. 174. — Wolfram Fubnaok.

Fresh water is again run on, to wash off the remainder of the soluble
matters; and the tin stuff is then removed from the tank to the
burning-house dressing- floor for final treatment. The strong solution
is evaporated in iron pans to the crystallising point, when it is drawn
off into coolers. After a few days a crop of crystallised soda tung-
Btate is obtained ; and the mother liquor is again treated in a similar
manner for the obtaining of a further quantity of crystals. The
washings of the tin stuff run off from the tank are used instead of
plain water for the lixiviation of fresh charges from the furnace.

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Though never found in large quantities, oraninm is widely dis-
tributed, and forms several mineraJs. The commonest is pitcb-bl^de,
a compound oxide, containing 81j^ per cent, uranium, 4 lead, ftiui
\ iron, with oxygen and water, and sometimes magnesia, manganese,
and silica. Usually it occurs only in small pockets, as in Annaberg,
in Saxony, but a distinct vein of it has been found and worked at the
Union mines, Grainpound Bead, ComwalL Uranmica, containing
61 per cent, uranium oxide, with phosphorus and copper; uraniniD"
ochre, uranocaloite, and trogerite, and even lees common minenk
The ore as mined in Cornwall affords 18-29 per cent of the metal,
and is by far the most important commercial source ; it is cakined,
powdered, dissolved, and precipitated ; the precipitate, filtered as^
dried, goes into the market as a yellow powder of uranium seequi-
oxide. Very small quantities have been at intervals produced in the
Black Hills, South Ehftkota. The chief application of the oxide b for
porcelain staining, though proposals have been made to substitnte
the metal for gold in electro-platinff, and to utilise its high eleotrioal
resistance in electric lighting. The market price of the metal is
about 18«. a lb.

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This useful and well-known metal occurs in a variety of forms, the
most common being : —

Per cent.

Zincite, red oxide, ZnO containing 80

Blende, black jack, or fphalerite, sulphide, ZnS „ 67

Willeroite, silicate, 2ZnO,8iO, „ 58}

Calamine, hydrated silicate. 2ZnO,8iO„H,0 „ 54

Smithsonite, carbonate, ZnO,GO, .. .. .. „ 52

Ghristophite and mannatite, ferro-zino sulphides .... „ 30-40

Franklinite, a mixture of zinc, iron, and manganese oxides „ 5}

Zinc ores are very generally an accompaniment of lead ores, and
their geological occurrence has been already in part described ; but
some remarkable instances of lead-free zinc deposits are encountered
in the United States, and will be farther discussed.

The chief British mine, Minora, near Wrexham, dates from Eoman
times, but has only of late years been a zinc producer of any import-
ance, the galena giving place to blende as depth is reached, so that the
mineral raised now affords about 1\ per cent, blende and only \\ per
cent, galena. The deposit occupies a faulted area varying from a
knife edge to 18 ft. in thickness, avera^ng perhaps 6 ft., in Car-
honiferous limestone and Millstone grit, the gangue being calcite.

The most important deposits of Germany are those of Upper
Silesia, which occur in the beds of the lower ** Muschelkalk," and
extend from Tamowitz in a south-easterly direction through Beuten
to Poland. The ores consist partlv of carbonate and silicate of zinc,
with compact blende, but principally of zindferons brown ironstones,
" red calamine," and '* white calamine," the white generally forming
the lower bed and the red the upper. The product is about 60 per
cent, calamine and 40 per cent, blende. In the Eiffel limestones
(Iserlohn, Amsber^) occur irregular fissures filled with calamine and
concretions of blende ; and similar deposits are found in the Devonian
limestone of Altenblihren, and in the magnesian limestones of the
same age at Gladbach, near Cologne. At JOiepenlinchen, near Stol-
berg, this stockwork formation occupies an area 125 yd. by 50, which
is removed bodily for treatment. The LUderich mine is on a shattered
zone in the Lenneschiefer beds (Middle Devonian^ At Wiesloch, in
Baden, is an important deposit of calamine in tne ** Muschelkalk."
Blende occurs with the lead ores of the Upper Harz, particularly in
the district of Lautenthal, and in connection with the lead and silver
<aree of Freiberg and with the lead ores of the Holzappel group.

At the New Pierrefitte mines, France, the chief product is
argentiferous galena, contained in a granulite gangue, irregularly
impregnated with magnetite intermixed with blende; the very

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fermginons varieties called oliristopliite and marmatite are often
present. The ore as mined carries about 20 per cent, blende, 20 per
cent, magnetite, and 1-2 per cent, argentiferous galena.

The Spanish mines of Reocin and TJdias in Santander and of
Linares in Temel, produce large quantities of very rich calamine
(49-64 per cent, metal).

The important beds at Ammaberg, Sweden, exhibit a miztoie of
blende, pyrite and pyrrhotite, with other non-metalliferous minerak,
impregnating a kind of gneiss, the mica of which is often replaced by
the ore. Qalena is not mentioned as an accompaniment, and is
probably absent, as the Yieille Montague Co. which works theie
deposits extensively produces a remarkably pure zinc.

In India, zinc is only found to any extent in Oodaypore, where
the Jawar mines were formerly worked on a large scale, and yielded
a yearly revenue of nearly 2^ lakhs of rupees. At present no
extensive zinc workings exist in India, though possibly the indica-
tions of the metal at both Sirmur and Tavoy might yield profitable
results to scientific development.

The United States * possess several important zinc mining centio,
sometimes in common with lead ores, sometimes remarkably free
from lead. At Saucon Yalley, Pennsylvania, occur zino-blende and
its oxidation products, calamine and smithsonite, filling innumetible
cracks and fissures in a disturbed magnesian limestone, thougbt to
belong to the Chazy stage. There are three principal mines, tbe
Ueberroth, the Hartman, and the Saucon, the first-named being in
the portion which is tilted nearly to a vertical dip and is mudi dis-
turbed, while the next is where the dip has gradually decreased to
35^. The mines are on a belt some f mile long. At the Uebenotb
an enormous quantity of calamine was found on the sorfaoe, bat it
passed in depth into blende, and was clearly an oxidation prodnei
In the others, the blende came nearer the surface. The ore follows
the bedding planes and the joints normal to these throughout a xooe
10-40 ft. across, and fills the cracks. At the intersections the largett
masses are found. Six larger parallel fissures were especially marked
at the Ueberroth. A little pyrite occurs with the Uende, and thiB.
powdery coatings of ^eenoclute sometimes appear on its surface, but h
is entirely free from lead, and a very high grade spelter is made fiom it

At Franklin Furnace and Sterling, New Jersey, is an enormoNra
continuous bed 2500 ft. long, 8-30 ft. wide above, and swelling tt»
over 125 ft. at 200 ft. in depth, consisting of franklinite, wiUemi^
and zincite^ in crystalline limestone of Cambrian, Lower Silurian, or
Archsean age, according to different geologists. The ore bodies are
interbedded in the limestone, and are associated with much magneftite.
The ore consists of franklinite in black crystals, set in a matrix ti
zincite, willemite, and calcite. The richest ore lacks the calcite, awi
consists of the other three in varying proportions. This beet ore »
in largest amount in the Buckwheat mine, beyond the trap dyke

• F. L. Clew, in * Mineral ReflonrceB,' 1882, p. 358; Kemp, *Ore D^j<wt«,'
p. 174; W. H. Case, En. and Min. Jl., Sept 16, 1892; E. C. Moxham, do., Not. '&,
1893; W. P. Blake, «• Zinc-ore Deposits of New Mexico," Trans. Amer. Inst. Ife
Engs., 1894.

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which oats it. The limestone containing the ore has a notable per-
oentage of manganese replacing the calcium — 16^ per cent. MnCOs.
An explanation of the cleposit is that the franklinite bed is an
** original manganese-zino-iron deposit in limestone, much as many
Silaro-Gambrian limonite beds are seen to-day, and that in the
general metamorphism of the region it became changed to its present
condition" (Kemp). The ore is easily mined and requires little
Bsleotion. The cost of mining and putting it on rails should not
exceed the cost of quarrying an equal amount of limestone. An
average sample would contain about 36 per cent, zinc oxide, 22 per
cent, metallic iron, and 11 per cent metallic manganese. Owing to
its low fNBTcentageof zinc and high percentage of iron and manganese,
this ore is unsuitable for the manufacture of spelter. The mechanical
separation of the zinc ore has been found impracticable on account of
the intimate chemical and mechanical mixture of the different ingre-
dienta The ore is, however, particularly adapted for the manu-
fiftotnre of zinc oxide, or *'zinc white," for which purpose it is
exclusively used. The calamine got at Sterling, bemg lead-free,
makes an excellent spelter.

In the Upper Mississippi lead region are extensive shallow deposits
of *^ dry-bone" (smithsonite, or zinc carbonate), some portions of
which are very rich, but these rich layers are ordinarily only a few
inches thick. Above and below them are more important layers
largely contaminated with limestone, too lean to work as spelter ores,
and not amenable to concentration. They have been used to make
sine oxide, and have given a yield of 30-33 per cent.

The Gagnon vein, near Butte, Montana, exhibits the remarkable
feature of very large quantities of rich silver- and copper-bearing
oino-blende, some tm>usand8 of tons of which were mined and smelted,
principally at Argo, Colorado. Smelter-returns show varying con-
tents of shipments as follows : — silver, 50-200 ob. per ton ; copper,
1-42 per cent. ; and zinc, 7-48^ per cent. The occurrence of blende
containing so lar^ a percentage of copper is remarkable. Probably,
however, the varieties which are so high in oopper have undergone
some change, the oopper being enriched by the oxidation and partial
disappearance of the zinc. Very fine specimens of goslarite (zinc
sulphate) are now to be found on the walls of the leveb in the Gagnon,
which would indicate beyond a doubt that the zinc-blende has been
oxidised to a large extent. The same influences which effected the
oxidation and removal of the zinc may have caused the disappearance
of a portion of the silver ; for as a rule, in the lots of ore smelted, an
increase in the percentage of oopper accompanies a corresponding
decrease in BilverKM>ntents ; and the presence of native silver in very
thin plates throughout the masses of the blende would indicate some
sort of secondary deposition of the silver. The gangue is quartz and
felroar, with occasional barite.

The liurgest Virginian mines are in Wythe County, and of these
the Bertha is best known. According to Boyd, there are in one sec-
tion 486 ft. of strata impregnated with leeA and zinc in vailing
amounts. Farther east, other openings of conKiderable promise have
lately been made at Bonsacks. The zinc ore bodies are at times of

2 T

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great size (40 ft. wide\ and are associated with more or less of \t^
minerals and iron pyrites. The Bertha ore, which is free from l»d
and iron (hence its high value), has a gangue of soft, unctuous day
(*' buckfat "), somewhat difficult to dissolve, and of a specific gravity
approaching that of the ore, calling, consequently, for careful treat-
ment in separation. The ore as mined contains about 26 per cent,
zinc. The min e yields 1 2,000-1 6,000 tons of unwashed ore to the acre,
and the ground is now being worked over at the rate of Sj-4 acrea
^r annum. The output is 200 tons a day, hoisted from 17 shafts,
with a working force of about 300 men.

The zinc deposits of south-western New Mexioo occur in a region
of palsBOzoic (probably Lower Carboniferous and older) limestones,
resting on granite (containing much epidote), and both tiuversed by
intrusive porphyry dykes. The zinc ores comprise the carbonate
(smithsonite), with some calamine, and were at first quarried at the
surface, and then followed downward in irregular pits and cave-Kke
• excavations, in some instances to a depth of 60 ft. or more. The ore
occurs in irregularly concentric crusts or layers, or in caverDons
masses made up of small layers or concretionary sheets of pure carbo-
nate ore, sometimes in close association with aggregations kA small
quartz crystals, the presence of which reduces the percentage of zinc
— and, consequently, the commercial value — so much as to prevent
profitable shipment. The best carbonate ores, assayed by the ctr
load, contain 35-38 per cent, metallic zinc The excavations upcm
these carbonate ores show that they occupy cavernous spaoee in the
limestone strata, and irregular openings between the beds, gradually
thinning out to mere seams. Doubtless they originally existed as
sulphide, which occurs abundantly in the same region ; it is the dark
reddish-brown variety, rich in zinc and free from arsenic and anti-
mony. It does not appear to be highly ar^ntiferous, and is an
excellent ore for making spelter. It is granular massive, and exkts
in beds sometimes 20 ft. or more thick. It is usually intermingled
with iron pyrites in grains and bodies of irregular shape disp^wd
through tne mass. The deposits are generally lenticular, and are
classed by Blake as " contact deposits, or segregations following tiie
dykes or the planes of metamorphism." The blende is cloeely assod-
ated with a garnet rock (grossularite) and with actinolite, and Blake
remarks that this *' remarkable association of large beds'of zino-l^ende
with pyrite, hematite, actinolite and grossularite in lenticular layers,
and in disseminated particles in the substance of the actinolite and
the garnet rock, forming great contact aggregations or segr^ated
beds in limestone, appears to be unique." The associations are also
highly detrimental, for the garnet is so heavy that it cannot be
mechanically separated from one portion of the blende, and the pyrite
mixed with the other portion impairs its value for either spelter osr
oxide, until transportation facilities permit of dressing by steam
power. Smithsonite carrying 35 per cent, zinc and upwarcU is profit-
ably disposed of at 4/. a ton.

Treaimeni, — The first operation with all zinc ores is a process of
dressing, to separate the zinciferous portions from the plumbifercwss
or the ferriferous, as the case may be.

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An example whicli well illnstrates the mode of procedure in the

former case is aflforded by tlie Ltiderich dressing floors of the Vieill^

Montagne Co. Here the veinstnff, as it comes from the mine, is

diyided into fine and rough. The former is at once washed, cru^ed,

sized and jigged. The latter ^oes first to the breaker. An ordinary

arrangement of trommels divides the stnff into classes according td

size (3*7, 5*2, 7*2, and 10 mm.), and each size eoes to a separate jig

which divides into: {a\ galena, (6) mixed galena and blende, (c)

blende, (d) mixed blenae and waste, (a) waste. The sand is lifted

by a centrifugal pnmp to a system of pointed boxes^ and is herd

divided into 6 different sizes of grain, clean water being supplied.

Each size is conducted to a Harz jig in which a similar separation to

that already mentioned takes place. The overflow from the pointed

boxcB carries the slimes to a set of 2 pyramidal boxes, and the two

sizes of deposited slimes are treated separately on revolving convex

tables.- These tables are 14 ft. diam. have an inclination of 1 in 12,

and revolve twice in 6 minutes. The waste sand is piled near the

works, and the fine slimes are filtered, the waste water escaping in a

remarkably clear condition. The dressing operation requires a supply

of 9 cub. m. of water per minute. This is pumped a short distance

by steam power. The engine driving the whole of the machinery is

60 h.p. ; the steam pressure is 6 atmos., and 90-95 tons veinstuff are

treated daily, this quantity producing on an average 28 metric tons

of blende a day. The galena produced is about 20 tons a month.

The ore may be classed into \ lump, \ grain and \ fine. Selection is

carefully carried on underground, tne deads being systematically

employed for filling. The veinstuff is brought from the mine by an

inclined railway, the daily quantity being 50 tons, exclusive of


Quite a different method is required at theNewPierrefitte* works,
vrhere iron is the principal ingredient to be eliminated. The ores
x>ntain blende intermixed with magnetite, which occurs minutely
disseminated through the blende and accessory galena, and gangue.
rhe intermixture is very irregular, ma^etite aggregates varying
^m minute quantities to such a proportion as to form a practically
nassive mineral. The formation also contains small quantities of
nagnetic pyrites. Very ferruginous varieties of blende, as christo-
)hite and marmatite, are also present. On the average, the ore may be
aken to contain 20 per cent, blende, 20 per cent, magnetite, and 1-2 per
«nt. argentiferous galena, the remainder being gangue. When the
>lende is massive, dressing is a simple and easy process, the magnetite
>eing in very small proportion, and chiefly with the gangue, so that jigs
nd buddies readily afford a product carrying 46 per cent, metallic zinc.
Jut when the blende is less massive, there is invariablv more magne-
ite present, both in gangue and ore. The blende itself is also more
Brmginous in composition, and consequently contains less metallic
inc, and the above operations become no longer possible. To treat
his latter class of ore, and the middlings and bye-products from the
jrmcr operations, the fwociiw operandi is to crush the ore to the

* H. L. Lawrence, " Dressing of Zinc-blende Ores and Magnetite at the New
ierrefitto Mines," Trans. Inst Min. and Met, iii. 92.

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required size, classify, treat the coarser classes on jigs, and tlie sKmes
on onddles or tables. The jigs can be made to ^oeld :— ^1) mixed
salena and magnetite, which present no difficulty in final cleausmg;
(2) mixed magnetite and blende, which can be magnetically separat^
giying-— (a) pure blende for market, (b) magnetite, (e) middlings,
consi^ing of particles of mixed blende and magnetite; also lU
magnetic pyrites which is not magnetic enough to pass with the
magnetite. The buddies yield in the first waging :y - (i) he^iingi
consisting of galena, blende, and magnetite ; (e) tailings consisting
of waste. The headings (d) can now be ms^etically separated, and
the final freeing of galena from blende can be effecSted on Bittinger
tables or other appliances.

Comparing tiie two methods of dressing before magnetic treatment
and magnetic treatment before dressing, the former has tiie adyantage,
as the valuable galena is freed from uie blende at the outset, and no
re-cmshing of these two minerals together is necessary, thus avoiding
the tedious and offcen wasteful operation of treating their slimei
Further, the middle product is minimised in quantity and confined
to the very poor, almost unprofitable, ore, consisting of highly fernt-
nnous blende and magnetic pyrites. The magnetic separator, having
less bulk to treat, can be regulated and adjusted to requiremoita of
each class of ore, which can be passed in rotation as demanded. On
this principle, the works were erected.

The ore is dumped on to a platform, and fed to a pair of Ooniish
rolls fitted with Baff wheeL The main sizing screen is wire-wove,
and has 4 holes to the inch. A bucket elevator carries the pulp to
8 classifying trommels, the first of Cornish gauge. No. 17 ; the second.
No. 28 ; the third. No. 36. A Spitz^erinne divides out a fourth dasB,
and 4 4-hutched Oreen's jiggers are arranged to receive and treat tiie
classified pulp. The overflowing slimes pass on to 2 Spitzkasten, and
are separately treated on 2 Bittinger revolving continuous buddies.
The waste is here extracted, and the headings are magneticallj
treated, the final separation of galena and blende taking place on 2
Bittinger percussion tables.

The firat hutches of all 4 jigs catch all pure galena, and all galena
mixed with magnetite. The second hutches deliver a mixed product
of galena, blende, and magnetite. This is re-jigged on the auxiHaiy
machines, which are of exactly similar pattern to the others. Here
the last remnants of ealena are freed from the remaining pulp, whidi
consists of blende and magnetite. Noe. 3 and 4 hutches give Uende
and magnetite, with most of the magnetic pyrites, a proporticm of
which passes over with the tails. The first or galena {mxlucts, both
from main and auxiliary jigs, are passed on to the ordinary fine roller
house, where they are cleansed in conjunction with the jig- middlings
from the main lead-dressing floors. The blende products (Nos. 3 and 4
hutches) from main and auxiliary jigs, consist, for the most part, of
homogeneous grains of magnetite and blende. There is, however, a
certain proportion of stuff ohieflv coming from the coaisest jigger
composed of grains which are a mixture of blende and magnetite.

To separate the pure blende from the magnetite, and prevent the
grains of mixed blende from passing into the latter, the Sauter-Harlr

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[Paris) magnetio separator (Fig. 171) is used, as it allows of making
I middle product, so necessary to the snccess of the operation, by
partitioning off at a h. The magnets are soffidentiv strong to develop
\ manietio force corresponding to 1680 watts. The catc£-board and
Hviaon were added at the mines, after careful experiment as to their
positicm and size. This machine has a capacity of 20-^0 tons mineral
n 12 honrs, and costs 17(M. The feed has to be reg^nlated according
io size of ore, nicety of work required, and proportion of magnetite
to other ingredients. This separator works on dry ores only. The
^et products from the jigs are taken to sloping platforms ontside the
Imilding, to allow excess of moisture to orain off, and thence they
3888 to drying floors. These consist of a double series of brick flues,
Ach with an ordinary brick furnace, and covered by cast-iron plates
\ in. thick. The dry ore passes to two separators, one of 900 watts

Fig. 175.— Maghbtio Sbpabatob.

eing sufficienilv powerful for treating the fines. The number of
erolutions of the drum best adapted for good work has been found

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 73 of 76)