Charles George Warnford Lock.

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

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centration, as well as the percentage of mineral left in the tailings,
he adjusted acoording to wish. The intermediate grades would then
he made to yield higher concentrates by the lower tables of the com-
pound vanner, as described before.

One compound vanner will treat about 9 tons per day of 10 hours,
giving absolutely dean tailings, besides separating the various
minerals contained in the ore, one from the other. It has been
prored to give excellent results in treating slimes of lead and blende
ores containing iron or copper pyrites, or spathic iron ; and it is
particularly well adapted for dealing with tin and copper ores, avoiding
iU the handling now so frequently necessary in consequence of the
use of imperfect machines.

In dressing auriferous ores containing a percentage of pyrites,
this vanner does closer work with less cost than any other machine.

The Stein vanner, adopted at the Himmelfahrt works of the
German Government, possesses several advantages. At Freiberg, one
manner washes 2^ (<^) tons pulp in 10 hours, yielding at once a
<:lean product, and giving poorer tailings than either the Bittinger
or the Frue. Worlang on mixed ore containing blende and pyrites
it delivers : pure galena ; galena and mispickel mixed ; iron pyrites
dean; blende and iron pyrites mixed; tailings. A^hen the pulp is
iK)t toolow sprade, galena with 70 per cent, lead can be easily obtained.
Thej need Tittle labour, one millman being able to handle 8 of them.
Tailmgs show no lead and only '005- '01 per cent, silver.

BsdcQes. — ^These machines are of several patterns.

The convex or centre-head (Fig. 75) consists of a circular pit about
22 ft, diam., and 1-1^ ft. deep at the circumference, with a raised centre
10 ft. diam., and a floor fallmg towards the outer circle at a slope of
•bont 1 in 30 for a length of 6 ft The stuff is brought to the centre
of the huddle by launders a, into which flows a constant stream
of water, and is distributed upon the raised centre from a revolving
P^ \ carrying a number of spouts, so as to spread the stream of pulp
^ery uniformly in a thin film, which flows gradually outwards over
the whole of uie sloping floor to the circumference. In its passage
^^'vn the slope, the material held in suspension by the water is
P^duaUy deposited according to its specific gravity, and the ore being
the heaviest is the first thrown down, and is consequently in greatest
F^^x^on towards the centre of the huddle. The overflow e for the
^'Mte and slime from the circumference of the huddle is regulated by

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146 ECONOMIC MINING.

a wooden partition perforated with horizontal rows of holes, which are
successively plugged up from the bottom as the height of the deposit
in the huddle rises. To facilitate the uniform spreading of the stuff
over the floor of the huddle, and prevent the formation of gutters or



\



I



channels in the deposit, a set of revolving arms d are employed, from
each of which is suspended a sweep carrying a number of brashes
or small pieces of cloth, and these being drawn round on the surface
of the deposit keep it to an even surface throughout ; the distributiBg
spouts and sweeps are driven at about 5 or 6 rev. per minute. Ai



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CONCENTRATING, 147

the deposit acoumulates in the buddle, the sweeps are snocessively
laiwd to a oorresponding extent ; and the process is thus oontinned
nntil the whole buddle is filled np to the top of the centre cone, which
usually takes about 10 hours. The contents are then divided into three
concentric portions, each about a third of the whole breadth,, irhich
are called the head, middle, and tail ; the head, or portior n >.rest
the ceotre, contains about 70 per cent, of all the metal ^ the stuff
rapplied to the buddle, the middle nearly 20 per cen;., and the tail,
or portion next the circumference, contains only a trace ; the re-
maining particles of metal are carried off by the water in the state
ofglime.

In the ooncaye buddle (Fig. 76) the stuff is supplied at the centre,
bat is conveyed thence direct to the circumference, by revolving
sponts that deliver it in a continuous stream upon a circular ledge,
from which it flows uniformly over the conical floor, falling at a slope
of about 1 in 12 towards the centre ; it is kept uniformly distributed
by means of revolving sweeps. The greatest portion of the metal is
in this case deposited round the circumference of the floor, and the
slime and waste flow away through rows of holes in the sides of a
centre wall; as the depth of deposit increases, the level of the
overflow is gradually raised by plugging up these holes in succession.

Quite in advance of the older forms of buddle is the Linkenbach
table, a circular buddle made by the Humboldt Works at Ealk, on
the Bliine. It has a fixed table and continuous action, and is made
35 ft diam. or more. I'he surface is cemented quite smooth. Work-
infiat the Maria mine, near Beuthen, Germany, on partly concentrated
slimes from fine jigs and partly old v^aste slimes, this buddle has
given good results. With 3 tables, each 33 ft. diam., about 40 tons
of clayey blende slimes are treated per 10 hours shift, and a product
containing 28-30 per cent, zinc is obtained from a slimo with origi-
nally &-10 per cent. zinc. The waste water contains only 4-5 per
cent zinc.

A necessary appendix to the common buddle is a slime-frame or
table, an effective and simple self-acting form of which is shown in
^. 77. A launder bringing the slimes from the buddies passes
between two rows of the slime-frames, set back to back, and the
delivery to each frame is dihtribnted by a fluted spreader, and then
flows uniformly in a gentle stream over the surface of the frame,
which is at a slope of 1 in 7, and is divided at the middle into two
halves by a 5-in. step ; the waste flows off at the bottom of the frame
into the launder c. The stuff deposited on the frame is then flushed
off at successive intervals of a few minutes each, by a self-acting
contrivance consisting of two rocking troughs d^ which are gradually
filled with clear water from a launder e ; when full, they overbalance,
and discharge their whole contents suddenly upon the top of each half
of the frame. The tipping movement of the troughs opens at the same
time the covers of two launders /, one at the foot of each half of the
frame, into which the stuff deposited on the frame is washed by the
discharge of the water, the two halves being kept separate because
the greater portion of the ore is retained on the upper half of the
frame.

L 2



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ECONOMIC MINING.

ret dressing ores, the treatment of fine sands and slimes forms
9t difficult part of the operation, and the one oocasiening



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CONCENTRATING, i49

float away on the water. This has led Prof. Bilharz of Himmelfahrt
to enunciate the principle that disintegration must take place step by
step. He asserts that only by gradual crushing is it possible to
guard the single grains of the separated substance against unnecessary
destruction, and to let complete pulverisation take place only where
the most intimate intermixture makes it absolutely necessary. Even
in cases where the several substances of the ore are very intimately
interwoven or finely disseminated in particles hardly visible to the
naked eye, as in auriferous quartz, he thinks it will be found advis-
able to reduce the size of the particles gradually, and to remove a
part in the coarser grain even when thereby running the risk of
obtdning a fijial product of a lower degree of enrichment. But such
ores as most silicious plumbiferous and auriferous arsenical or iron
pyrites are seldom so much intermixed tbat a large part may not be
separated in coarse grains, and they admit of very successful applica-
tion of graduated disintegration, so that the production of sHmes,
that is to say, complete pulverisation, may be limited considerably.
His method of treatment, according to these maxims, is : — The mixed
ore derived from hand-picking the so-called crushing and stamping
ore of the size of a fist, is, together with the mine smalls, thrown on
a grate consisting of an inclined plate having 30 mm. perforations.
This allows the small particles to fall through it, while the coarser
ones are taken to the rockbreaker, which is set coarse, and admits a
second hand-picking or gleaning of the ore leaving it. The material
which has fiJlen through the grate, and likewise the broken-up ore
turned out by the stonebreakers, is collected in a separating apparatus
placed immediately below the former. This apparatus assorts the
ore into particles of accurately graduated sizes and separates it from
such coarser pieces as are not sufficiently broken up. Tho sizes over
7 and under 16 mm. are turned over directly to the waste jiggers, so
called, because they are intended principally to separate the waste
matter and to produce partly enriched, although still mixed, ore,
which is reduced still nirther in the crushing mill. The pieces
which are not sized in the separation apparatus, being refused as too
cxtarse, fall on a moving belt or plane placed obliquely, and are
separated by gleaning into pure ore and pure waste, the remaining
pieces of mixed material being left on the belt to be dropped through
\ funnel into the first (coarse) crushing mill. The particles broken
ip in the crusher go directly to the separation apparatus for over
nedium-sized grains placed under it. From this, the grains graduated
mto fixed sizes flow directly on the jiggers, while the refuse of this
kecond separation apparatus has to undergo another crushing in the
lecpnd rolling milL By arranging the works in stories, without any
xitermediate transportation whatever, the gradual separation into
rrain sizes is continued in a similar manner, as is also the jigging
<HKlacted in connection with it, after which, generally another
fourth) crushing in the third rolling mill becomes necessary. Not
uDtn after this occurs the last (fifth) crushing of the still remaining
>artiole8 of intermixed material, or the complete pulverisation of the
rve in the stamp mill.

Moigmeiit Concentration. — Where a magnetic ore is associated with

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150 ECONOMIC MINING.

non-magnetic bodies, it is quite feasible under certain oonditionB to
efifect separation by electro-magnetism. This refers not only to those
ores which are naturally magnetic, but also to those which by roasting
can be rendered magnetic, provided of course that the cost of roasting
is not prohibitive. In some cases, as with low grade iron ore, it is
the iron which is sought to be concentrated by magnetic seleotioD.
In other cases, as with the auriferous beach sands of New Zealand
and Oregon, the object is to isolate the gold particles by removing
the iron grains. While the operation is always the same in fact, viz.
collection of the iron particles, the objects are reversed, and the
success of thu operation must be measured by the special features of
each case, particularly with regard to the degrees of concentration
which are required and brought about In one instance, a 10 per cent
enrichment of the iron ore may be highly satisfactory ; in another
case, removal of 50 per cent, of the iron from the auriferous sand may
be totally useless. Thus the matter resolves itself at an early stage
into a subject for special rather than general treatment. (See
Iron)

One point, however, may be insisted on h«'re, viz. that magnetic
separation can in any case only succeed with pulverulent matters,
consequently everythmg in the shape of an ore or massive mineral
must first be reduced more or less to powder, the degree depending on
various circumstances, but averaging about 16 mesh, and it is of some
importance that a granular condition be secured.

"Dry Concentration, — There are many points which render dry con-
centration a debideratum. Water for concentrating purposes is often
costly and imcertain in supply, and wet concentrated ores carry much
water which has to be got rid of before further treatment. Dry con-
centration delivers the product in the beet form for transport and
smelting. Hence the application of air instead of water as a medium
for separating gangue from ore has received much attention. All the
earlier attempts courted failure by neglecting to size the material
Later inventors have not overlooked that essential feature, but some
allow it to follow instead of preceding the classification by weight

One of the most recent processes is the Fape-Henneberg, in which
the dry pulverised ore is placed on rapidly revolving discs firom which
it is scattered in all directions. The centrifugal force acts propor-
tionately to the weight of the particles, in su(£ a way that particles
of equal weight collect radially at an equal distance round the centre.
When circular collecting troughs (called " rings ") are arranged round
the discs, the products which collect in each arrange themselves in
such a way that a small heavy ore particle will be found with a lar^re
particle of waste rock or gangue. This mixture must be separated
either by dry screening or by wet jiggiug or huddling.

In actual practice, diffictdties occur, although the process appears
very simple, and the reason is that in order to properly separate the
mineral particles a large proportion of the ore has been reduced to
fine dust or slime ; and in treating this very fine dust the centrifugal
force becomes powerless owing to the particles having practically nc
weight This dust generally becomes mixed up during the scatterinjg
of ore with the various products in the lings, and hinders the oentri-



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CONCENTRATING. 151

fngal action, and in particular the screening operation. The Pape-
Henneberg process deals with this difficulty in the following manner.
The centre of the centrifugal machine is covered over 6^ ft diam.,
and the disc hangs free. A current of air, formed by exhaustion,
plays radially from all sides of the disc in a contrary durection to the
ore particles, and carries off the dust down a funnel below the disc.
It is possible to regulate this opposing current of air, so that the
Telocity of the ore particles may be acted upon not only according to
their initial velocity but also according to pleasure. And as different
ores behave in different manners, it is only a question of varying the
intensity of the opposing blast to suit each particular case. It is,
however, of importance not to produce more fines than the ore treated
calls for.

The result obtained by the use of centrifugal force is a heavy
product in the outside ring, and a light product in the inside ring —
with middle products between the two.

All the products in the rings are capable of being screened, as they
contain no dust proper. The coarse products screen easiest, and the
fine ones with more difficulty, and the operation may be conducted
with screens of very small mesh without modifying results. Al-
though every ring -product consists of small particles of mineral
mixed with large particles of waste rock or gangue, yet every such
mixture contains '* middles " obtained by screening, and it is always
advisable to separate such ** middle " products, and submit them to a
farther treatment, which mav be, in places where there is a total
want of water, a repetition of the above treatment, or, where water
is plentiful, a treatment on tables.

Wet table treatment is to be commended for all products of the
procees, which, owing to their fineneso, cannot be screened, that is for
all products obtained from the chambers where the exhausted fine
dust settles. The value of this product is, however, with most ores
80 small, that a forther treatment, when water is scarce, may be
neglected. But in treating complex ores, and especially those in
which the different minerals have similar specific gravities, it is
absolutely necessary to combine the ordinary wet process with the



the Clarkson-Stanfield dry concentrator, which is better known
in this country, the classification by size precedes the concentrating
process, and this would seem to be the more rational method. The

ration of this machine will be better understood by reference to
illustration (Fig. 78), in which H is the feed hopper ; P, pulley
driving distributor; D, distributor; F, fan; S, sp^d regiuator;
N, speed-regulator handle ; M, air-damper handle ; K, receiver. It
is based on the principle that when a piuiicle of weight %d is revolved
at a velocity « around a vertical axis, and at a distance r from it
\§ representug the value of gravity), the centrifugal force is repre-

aented by the expression . Now for any one place, ^ is a constant

quantity; r, the radius of the distributor is constant; and r, the
velocity with which the outer edge of the distributor revolves, is
laade constant ; therefore to, the weight of the particle, is the only

*L 4

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ECONOMIC MINING.



variable quantity. Thus the centrifugal force with which each par-
ticle is ejected is proportional to its weight, and when the particles
are all of the same size the weights are proportional to the densities.
Thus a particle of galena is ejected with about 3 times the force
which carries a particle of quartz of the same size, but with less than
twice the force of a particle of blende, in accordance with their relative
specific gravities. A particle having been ejected has a kinetic or
motive energy proportional to the force with which it has been
ejected, and consequently to its weight, as already shown, or to its
density for particles of equal size. The resistance of the air impedes
the motion of each particle to an extent measured by the ratio of this
resistance to the energy of the particle, and thus causes a separation
of particles of dififerent weights. Assuming that the resistance of the
air is proportional to the area of the cross-section through a particle,
the distance to which each particle will fly will be proportional to
the ratio of its weight to this area.




Fia. 78. — Olarkson-Stanfield Conoentbatob.

Since the introduction of the machine several improvements have
suggested themselves in the direction of economy in operation, such
as minimising the wear and tear, augmenting accessibility for control
and repairs, and rendering the process as automatic as possible. Thus
in recent forms, the driving mechanism for the distributor is sus-
pended from above, where it is out of the way of the discharge and
less liable to be impaired by falling grit. So too, with the various
divisions of the receiver, instead of having to be brushed out as in
the older form, they are now prolonged downwards at the bottom,
forming self-discharging shutes. Points like these have as much to
do with the industrial success of a machine as the correctness of the
principle on which it is founded, and very satisfactory results have
followed this attention to practical details. The machine as it stands
now is much the most simple of the dry concentrators, and has done
excellent work on mixed ores in competition with wet concentrators,
with as good efficiency at much less cost.



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153



NON-METALLIFEROUS MINERALS.



ALUM.

To a Tery small extent, alum is found native, as an efflorescence or
incrustation, in regions of recent volcanic activity, where it is col-
lected, dissolved in water, and purified by re-crystallisation. This
product as found contains usually equal quantities of alumina
snlphate and some other sulphate combined with water.

Alum rock or alum stone, also found in volcanic districts, is pro-
duced by the action of sulphurous vapours upon aluminiferous earths
or rocks. It is composed principally of alumina sulphate and silica,
various samples showing the following range of percental in-
gredients: — Silica, to 62; alumina, 17J to 40; sulphuric acid, 12 J
to 35J; potash, 1 to 14; water, 3 to 10; iron oxide usually nil but
sometimes 1^. The mineral is calcined in large kilns, and then
lixiviated with boiling water. The presence cf lime carbonate or of
iron oxide is very detrimental.





o.


h.


c.


d.


Solphide of iron (pyrites)


4-20


8-50


38-48


9^63


Silica


52-25


51-16


15-41


20^47


Protoxide of iron


8-49


6-11


,,


2^18


Alumiiia ..


18-75


18-30


11-64


18-91


Lime


1-25


2-15


2-22


•40


Magnesia


•91


•90


-32


217


Oxide of manganese

Sulphnric acid (SO,)


trace


trace


,,


-55


1-37


2-50


,,


-05


PbUah


•13


trace


,,


1-26


Soda


•20


trace


,,


-21


aiorine


trace


trace


, ,




Witer


£•88


2^00


,,


8-54


Coal


4-97


8-29


, ,


8-51


Lew


4-60


(?)


3-13


1-59


Carbon or bituminous matter .. ..


••




28-80


••



Alnm shale is of much greater industrial importance. It is a kind
of clay, containing much iron pyrites and bituminous matter, and
closely resembling ordinary clay slate. Large beds are found in the
Soan£navian peninsula, Bohemia, the Hartz, and the mountainous
districts of the Lower Ehine. In Great Britain, they occur at Hurlet
and Campsie, near Glasgow, near Whitby, in Yorkshire, and in Lanca-



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154 ECONOMIC MINING.

shire and Warwickshire. For many years these places were the chief
seats of the mannfactxire, alum works at the latter place having been
established since the year 1600. The Whitby beds occur in Liassie
strata, and are overlaid by a deposit of hard stone called *' dogger."
The shale beds reach a thickness of 200-300 ft., and are of a bluish-
grey colour and general shaly character, but do not exhibit regular
features. Some portions are much more earthy and soft than others,
these being the most richly aluminous. The composition varies as
shown in the table given on the preceding page, a being Whitby top
rock ; 6, Whitby bottom rock ; c, Campsie top rock ; d, Campsie bottom
rock.

llie mineral is submitted to pile roasting under very careful
supervision, an operation occupying several months. Some 4000-6000
tons are raised yearly in Great Britain, and 500-3000 tons in Qermany,
the value being about 1 6«. a ton.*

* Full details of manufacture are given in *Spon8* Encyclopedia,' article
" Alum."



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NON-METALLIFEROUS MINERALS. 155



AMBEB.

This interestiiig fossil resin oocurs along the shores of the Baltic in
deposits of varying thickness and productiveness. Occasionallv
detached pieces of amber are gathered along the Danish and Swedish
coasts and on the eastern counties of England. Sicilian amber, from
the neighbourhood of Catania, is particularly beautifal, showing
brilliant plays of colour. Amber has been found also near Basle, in
Switzerlajid ; and in France, particularly in the Departments of Aisne,
Bas Bhin, Oard, and Loire. In England it has been found in the
London clay; and in the United States in New Jersey, Maryland,
and North Carolina.

But with the exception of the Burmah deposits, which are worked
irr^larly, and have an indefinite value, the only commercial sources
of amber are the Baltic mines. Here it is found in close association
with mineralised wood underneath a covering of sand and clay that
in places is 40 and 50 ft. deep. The amber is found in romided or
Btalactitio pieces, with pyrites and sulphate of iron. Some of the
excavations are 100 ft. deep, but there seems to be little regularity
in the course of the lead. The richest deposits are between Memel
and Konigsberg, and at other points alon^ the coast. Here the amber-
bearing deposit is about 3 ft. thick, and is mined as any other similar
itratum. Amber is obtained also by dredging and diving, and is
picked up along the shore after heavy storms. Of recent years the
greater amount has been obtained by mining, the product of the
dredgers and the divers having shown for some years past a steady
decline.

A certain amount of preparation is necessary before marketing.
The lumps are freed from adhering sand, &c., by washing, and then
wrted according to quality. That which is valuable for its beauty is
treated with great care, and worked up for pipes, <&c. The small
B^Taps are steamed with chemicals to destroy the dark colour, and
when thus rendered clear they are melted and agglutinated into
larger pieces and are thus bought by viEtmish-makers.*

The total production amounts to the very large figure of 150 to
200 tons yearly, and the average value is stated at about 12«. 6<2. a lb.,



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 17 of 76)