Charles George Warnford Lock.

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

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lying upon a false bottom of iron or steel dies. As the ore becomes
sufficiently fine, it is ejected from the mortar through a grating or
screen of perforated metal or wire cloth. The chief advantages of
stamps are :— (a) Simplicity of working parts and tolerance of ill-



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REDUCING. 127

sage ; (fr) most repairs can be effected by an ordinary smith, and do
£>t entail stoppage of a whole mill ; (c) wearing parts are simple
Bstings; (d) eyery millman understands them, or thinks he does.
Lgainst these advantages it mnst be said : — (a) that they require
eiy costly foundations ; (2>) that they are not adapted to dry crash-
Dg though sometimes so employed ; (c) that they turn out a very
meven product owing to the impossibility of ejecting each particle
B 80(m as it is small enough ; (c2) which also means that much power
B oonsiuned in doing useless and for some purposes injurious work.

The effective capacity of a stamp battery depends upon many
onditions, ari^ing from the nature of the mineral and the operations
rhich are to succeed the crushing, as well as from peculiarities of the
Nittery itself. Chief among the latter are the nature of the blow (due
the weight of the stamp, the height it falls, its speed, whether
t rotates or not), the rate and regularity of feed, the character of
ibe wearing surfaces (shoes and dies), the gauge and inclination of
ihe screens, and the supply of water for carrying out the " pulp " or
TQshed ore. The product of every mine may be said to possess special
^MitTires demanding a certain combination of qualities in the stamp
»tterjr in order to give a maximum economy, yet it is very seldom
thought worth while to experiment with an ore before erecting the
fattteiy, a stereotyped pattern being adopted without any thought.

To overcome some of the faults inherent in the ordinary falling
V gravitation stamp various devices have been introduced. One of
tbese, due to Harvey & Co., of Hayle, is a second cam shaft to
hasten the blows of the stamps, by which they can be increased from
^ to 250 a minute. Automatic feeders are essential for good work,
>od must be adapted to the nature of the ore.

The so-called ** steam-stamp " consists of a single stamp head
^ked by a direct-acting vertical engine — a steam hammer in fact
—in a mortar having a discharge screen on all sides. Its capacity
» great, where coarse crushing is all that is desired, but it compares
nry nnfavourably with fine jaw-breakers even in this application ; it
ii alt(^ether unsuited for fine work, and notably increases the pro-
portion of " slimes."

Ab the stamp is only effective at the moment of its fall, firstly in
cfwhing the ore, and secondly in splashing the pulp over the screen,
tti obvious method of increasing its effectiveness is to multiply the
nnmher of drops per minute. One pi-actical way of doing this is
with the double cam, already alluded to ; another is by providing an
ftir-cQ^on to raise the stamp and drive it down ; ana a third is by
kttachmg a steel spring. In Husband's stamp the rapidity of the
UowB is thus augmented. The stems of the stamps, usually a pair,
work in pneumatic cylinders. Each cylinder is pierced with holes
*^t ) in. diam., above and below the central position of the piston ;
*o that when the piston momentarily occupies the mid-position, air
»t atmospheric pressure filb both ends of the cylinder. The cylinder,
^*emg coupled to the crank-shaft by connecting-rods, is raised by
^ts rotation, the air in it below the piston is compressed, and the
■^p is thereby pushed up. Similarly, the downward motion of the
'Tlinder causes a compression of the air above the piston, which

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128 ECONOMIC MINING.

urges the stamp down with greater velocity than it would ha'
by virtue of its weight alone. This arrangement gives the Btam
a &11 several inches greater than that given by the crank,
contrivance may be added for rotating the stamps, so as to bdm
even wearing of shoes and dies. The effective capacity of a sing
Husband stamp is stated to be about 6 times as great as that of %
ordinary gravitation stamp of the same weight, and under prop
management their working generally is attended with satis^ctoi
results. The air-cushions formed between the ends of the cylinde
and the pistoDS, besides increasing the effective capacity of the stam
reduce the jar on the machine. Other advantages are portability
small consumption of power. But it seems best suited for coarse
medium pulverising, owing to the difficulty attending the use of thin'
screens in machines of such large capacity.

A four-head battery of Husband stamps, working on hard tin ore
at Tregurtha Downs mine, Cornwall, during 1889 stamped 2ftJ tons
a day per head including all stoppages, to pass a No. 36 grating.
consnming 23^ indicated h.p. per head, and costing 4^. per ton for
renewals and repairs. These figures are about six times as favour-
able as for gravitation stamps, to say nothing of the reduced size of
building requirefl, and lessened wear and tear. Punched metal screens
last much better than wire cloth. Of course, the introduction of the
air-cushion means an additional complication of the machine, which
would be a disadvantage in some cases, as for instance where skilled
labour was not procurable, or where low temperature would be likely
to create trouble by formation of ice in the pneumatic cylinders.
Much lubrication is also necessary, and may contaminate the product
unless care be taken.

The peculiar principle of the Elephant stamp lies in the intro-
duction of semicircular bow springs, compressed at their ends by
helical springs, between the crank-shaft and the lever, which,
receiving the force of the recoil from the blows of the stamps, store
it up, and give it out at the next descent of the crank, effecting a
certain saving of power. The springs also act as cushions, and take
up a great part of the wear and tear of the machine. Two heads of
Elephant stamps are stated to be capable of stamping 12 to 15 tons of
hard quartz per 24 hours fine enough to pass wet through SO-mesh
.screens. They are considerably cheaper, lighter, and more easily
transported and erected than ordinary stamps, and require less driving
power for the same effective capacity ; being less simple they mn
greater risk of injury, and are by no means so easy to repair, while
the lateral wear on the top, caused by the levers carrying the heads,
is excessive, and that of the heads themselves is uneven.

It is very interesting to note that while engineers generally point
with pride to the increased capacity gained by the introduction of
circular rotating stamps, the most recent development in Grermany,
where stamp-batteries were invented, is a stamp-head rectangular in
section. These are placed in batteries of 11 instead of 5, occupying
only the same space by reason of their being closer together — less
than \ in. apart — and this arrangement is said to have given very good
and economical results for fine crushing.



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129



Among edge-nmner mills, the most familiar and widely used is
16 simple yet cnmbersome Chili mill, of whiph it may be said that
maohine has a better record in practice, though it is the fashion to
espise it by reason of its rudeness. Properly used, the Chili mill is
most oBefol machine. It is urged against it: (a) that the runners
re 80 bulky and heavy that they cannot be transported up countiy ;
») that the material bein^ crushed is apt to run away in front of tne
mner ; (c) that a reduction to impalpable powder results rather than
granular comminution ; (d) that much power is wasted by reason
fthe excesBive friction at the end collars of the spindles owing to
M centrifugal motion of the runners.

Against all these indictments it will be sufficient to quote one
umple of the highly successful introduction of a Chili mill (by




FiQ. 69. — Cmu MUiL.

ffiake of New Haven) into a works for reducing very hard mineral
^m \ in. till it would pass a 40-mesh screen. Each runner of the
Bill, as shown in cross-section in Fig. 69, weighed about a ton and
MMured 4 ft. diam. and 8 in. across the face, the distanoe between
Mitsides being 4 ft 2 in. The central spindle carrying horizontal
Kxis on which the runners revolved, made 40 rev. a minute. The
kyns of the runners were of hard white iron 8 in. thick ; and the bed
^ the pan carried segmental dies of best chilled iron, and was sur-
nmnded bv an endoeure of sheet iron 4 ft. hi^h to retam the splashes
of ore and water. The crushed ore was discharged from the pan
Vy & current of water on to the inner peripherv of revolving screens of
^t^-mesh, quite separate from the mill. The output reckoned on
tlu)iitazids of tons was at the rate of Sf tons of the hardest and tough-
«it quartdte per hour per t"i11, at a cost for power far less than by
n>UA. For 3000 tons, the wear on the runners was \ in., and on the



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130 ECONOMIC MINING.

segmental diee ^ to | in., the total amount being equal to *12 lb. o(
iron per ton of ore.

An excellent form of Chili mill is made by the Hnmboldt Engi-
neering Works, of Ealk, near Cologne, containing some improTementi
of their own introduction. One of the most important of these is that
the runners are carried on cranked axles, which pivot in bearings o&
the head of the vertical shaft, so that each runner can rise indepen-
dently of the other, remaining at the same time perpendicular. In
the older patterns the runners are carried on a common axle, depen-
dent on each other, and as it does not often happen that they an
passing over stones of equal size, they are the greater part of the time
indin^ from the vertical. A fannel rotates with the vertical shaft
and feeds the material equally, while a scraper continually spreadi
the material ; the capacity of &e mill has been thereby considerably
increased, as compared with the older pattern. The cast-iron runnen
have renewable chilled iron tyres ; the grinding bed of the pan ak)
consists of a renewable chilled iron ring, in two, three, or four parti,
according to size. Chilled iron lasts longer than cast steel. To avdd
as much as possible the diffasion of dust which occurs in dry grinding
most materials, the runners have a casing of sheet iron, through th»
top of which only the feeding hopper protrudes; these casings aie
made in two parts for ease of removal, and are further provided wi^
tightly closing doors.

The Bryan roller mill is a modification of the Chili without anj
obvious superiority. Neate's "dynamic grinder" appears to difiisr
chiefly in having the runners inclined inwards until the centrifugal
force, at the predetermined speed, exactly balances the tendency to
fall inwards, thus relieving the end collars* This machine has given
great satisfaction in cement-grinding.

Another development of the roller mill is the Schranz fine-omshec,
employed at Lauremburg, on the Lahn, for reducing the mixed pro*
ducts from the fine jigs. This mill consists of a revolving pan ani
3 conical rollers. The rolls revolve on spindles fixed radially at at
angle to each other of 120^ The inner ends of the spindles an
rigidly fixed. The outer ends move in a frame, and are capable of
being pressed down by means of set-screws and rubber buffers on to
the revolving pan, or annular crushing ring. As the pan revdvoi^
the rolls are driven by frictional contact, the fact that they an
conical preventing any grinding action and consequent production d
slimes. The ore, which has been reduced in a rocking crusher, a
passed successively beneath each of the 3 rolls, under increasiog
pressure, and on escaping from the third is washed into a launder foi
further treatment.

The Huntington mill has gained a prominent position as a c(R»
petitor with stamps in wet-orushing gold ores. It coDsists of a seriei
of circular pestles hung by yokes so as to revolve inside a casing, tbi
centrifugal action bringing them into close contact with a circnlai
steel ring-die, and the ore being pulverised by being thrown into thi
palii of the rollers. For soft or argillaceous material the maoh'



^oes well, but its success with really hard ores is very doubtful, uk
in any case it demands skilled attendance. Though nkuoh rnotf^

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REDUCING, 131

quickly and cheaply erected than stamps, and consuming less power
in working, the Huntington mill is more liable to injury and more
iitacoessible for repairs, besides doing less work, and being much more
difficult to feed. Its true sphere seems to be in fine-grinding for
finishing an already ground material, as the abundant area available
for screens provides a rapid discharge. Highly sulphuretted ores will
not iBRie.

Becent years have witnessed the introduction of a number of r^
indng madhines depending on the action of a revolving ball or series
^balls.

One of the earliest to claim attention was the Olobe mill, since
Dodified and renamed the Cyclops. In this a single free ball of hard
Bwtal is carried round in a vertical plane by frictional contact with a
)air of flexible discs fitted on a horizontally revolving shafb, the
Ending being done between the ball and a grooved circular path,
rhis machine has considerable capacity, but it is heavy and compli-
ftted, and much power is wasted in raising the ball during half of
ach revolution, entailing also uneven wear.

Jordan's fine-reduoer is on a much better principle, a revolving
«n carrying 3 large white iron balls, while the oenl^ of the pan is
bmed and fitted with screens, over which the pulp is constantly
rashed, and can thus readily escape as soon as it is fine enough, avoid-
Dg that perpetual grinding and regrinding which is such a glaring
lolt in most machines. Excellent work has been done by the Jordan
nlveriser in reducing hard mineral to pass through an dO-mesh
ereen.

The Cravrford mill is a rotating pan carrying a series of 8 small
alls, and is remarkable for dispensing entirely with screens and rely-
ig upon a current of water for overcoming the specific gravity of the
lineral and floating it away as soon as fine enough. For operations
a a limited scale this seems to be a useful little machine, simple,
>mpact, self-contained, cheap, and easily erected and set to work,
t operates efficiently down to 120 mesh, which few other machines
uk do.

A multiple ball mill made at the Oruson works, Magdeburg, and
iToured in Germany for dry crushing, depends partly upon a stamp-
ig action, the balls being picked up and dropped during each rotation,
be power required is smaU, and tiie output is relatively large, but
16 wear and tear of the crushing surface is excessive. The machine
IS been nsed chiefly on cement clinker and basic slag.

Probably the best form of ball mill using a number of balls is that
ade by the Humboldt Engineering Works, in which the screening
»plianoe6 and the arrangements for renewing the most rapidly wear-
g parts are greatly improved. .

Quite a number of so-called '* pneumatic pulverisers " have been
trodnoed of late years, in which currents of air are supposed to
arge themselves with the material to be pulverised, and bring
out mutual comminution of the particles by attrition among them-
Lveo. These machines display great ingenuity, but consume much
met for a small capacity of work, and, the '' attrition " being impar-
klhr distributed, the wear and tear on the interior of the machines

k2

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132 ECONOMIC MINING.

is excessiye. They may haye a useful sphere with sabstanoes of oiilj
moderate hardness.

One of the principal economic features of all reducing machinerj
must necessarily be the durability of the wearing surfaces, and apaj
from considerations determined by the special needs of the minen^
under treatment, the manufacturer should be guided by this circnn^
stance in his choice of the metal, alloy, or other substance oompoebj
the actual grinding surfaces. In the United States, chrome steel ha{
largely come into use for stamp-shoes, and in this country Had£eld
haye adopted manganese steel with great success in roUs, omshei
jaws, and stamp castings.

Experiments made at the Bobinson mill, Johannesburg, in 1892
showed that chrome steel shoes and dies lasted on the ayerage abon
80 to 85 days longer than Fraser & Ohalmers' Bessemer steel, o
Sandycroft forged steel, when crushing about 5 tons per head per 2
hours.

Another part of the mill, whateyer its kind, which has to wii
stand excessiye wear and tear, is the screen or grating through whii
the pulyerised material must pass. These are either of ptmchi
metal or of wire cloth, and a« much delay is caused in replacing woi
screens, it is essential to haye the best quality procurable and of
suitable kind. An excellent article is turned out by Greening,
Warrington, Lancashire.



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133



CONCENTRA TING.

Thi object of oonoentiatiDg ores is to separate the useful from the
QselesB portionsy and to sort the valuable minerals from each other.
It should be carried out as near the mine as possible, to avoid carriage
&f worthless materiaL Water is essential, and dressing-floors shomd
be arranged so that the matters can be moved forward in a measure
by their own specific gravity, otherwise extra cost will be incurred for
^yidng.

The specific gravities of minerals commonly met with are as
follows: —



Gold (always containing

nmenlfer) .. .. 12-7-19-8

Meraury 10-6

Sitfer 10-5

Copptf 8-4-9

Iroo 7-5

Galena 7-5

CMritflrite 6-4-7*1

Chmabar 6*7-8*2

Mapiekel 60-6*2

Pnostita • 5*5

Iioa pyrites 4*8-5*2

FiUert 5-0-5]

Pvpla copper oie .. 4*9-5*1



Magnetite 4*8-5*2

Stibnite 4*6

Barytes 4*3-4*7

Oppper pyrites .. ,. .. 4*1-4*8

Zincblende 4*1

Chalybite 8*6-8*9

Calamine 8-8-8*6

FlnoTspar 8*1

G^tlcite 2*6-8*0

Felspar 2*5-2*9

Quarts 2*5-2*7

Gypsum 2*2-2*4

Coal 1-2-1-5

Lignite 1*2-1*4



Deoompoeed silver and other ores are difficult to dress, especially
if easily powdered, e. g. malachite, argentiferous cemssite, cinnabar,
and spangles of native silver. It is difficult to separate zincblende,
copper pyrites^ iron pyrites, mispickel, and barytes from silver ores ;
wi&am from tin ores; chlorite and epidote from copper ores; and
chahrbite from copper pyrites and galena.

The following minerals are affected by the association of those in
l)rackets: —

Hematite, limonite, and chalybite (iron and copper j^rites and
apatite are injurious).

Caadterite (iron pyrites, copper pyrites, mispickel, and zincblende
are injurious; bismuth makes the colour dull: copper makes it
brittle).

Lead (arsenic makes it brittle ; antimony makes it hard ; fluor-
ipar promotes its fusibility; chalybite aud barytes are also advan-
tagecrtw).

Zinc (lead spoils it)»

Copper (lead must be separated from it if the coppeFis to be
treated by precipitation process.)



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134 ECONOMIC MINING.

Silver (lead and antimony are injurious for the amalgamation
process ; also talc for chlorination.)

Cobalt (for bine paint: calcspar, brownspar, manganese spar,
bomstone, fermginons qnartz, and galena are injtirions ; nickel, wnen
predominant, imparts a red tinge ; arsenic intensifies the bine colour,
and renders it more agreeable).

Magnetite (mica, lime, garnet, angite, and hornblende promote
its fasibility).

Anriferons pyrites (for chlorination process, talc and tellnridee
are injnrions).

"Wlien pieces of different minerals of the same size are allowed to
settle in water, the heavier particles fall first. When pieces of dif-
ferent minerals of the same weight are subjected to a flow of water
sufficient to move them along, the specifically lighter material, having
a larger surfietce exposed, is washed away quickest.

Jig: — The process of "jigging" or "hutching" is resorted to
chiefly in the dressing of minerals in fragments of comparatively
large grain, such for instance as are delivered by jaw-crushers. The
charge of ore is placed in a sieve or frame having a bottom of wire
gauze or perforated metal plate, where it is subjected to a series of
small lifts or jerks in rapid succession from water being forced
upwards, by which means tiie lighter earthy fragments are gradually
collected at the top, while the clean ore accumulates on the bottom^
The jigging motion is produced either by jerking the sieve up and
down in a cistern of water, or by forcing water up mrough the bottom
by means of a pisten. In its crudest form, the operation is inter-
mittent, being suspended during charging and discharging, but it
may easily be made continuous.

Within its proper sphere no substitute for the jig has been found
that does the wonc as well or as cheaply. Ite capacity is large, it
requires but little attention, and the losses of ore are small. In one
form or another, it is the principal machine used in the concentration
of low-grade ores, and for the separation of one metallic mineral from
another ; and it is almost the only machine used for the purification
of coaL

There are two recognised systems of jigging, the English and the
Continental or German. The English system is a development of
the hand-jigging formerly employed in Cornwall and other metal-
liferous districts of England. In this method the crushed ore, coarse
and fine together, is ^t jigged on a hand-sieve with coarse meah ;
and the fine stufl^ somewhat concentrated by passing through the
bed of coarse mineral-grains on the sieve, is again jigged on a sieve
of finer mesh. In the adaptetion of the system to machine-jigging,
many modifications of detau have been necessary. The general treat-
ment, however, is the same, and includes a preliminary jigging
on roughing-jigs, followed by a concentration of the hutdhwork on
finishing-jigs of finer mesh. In both roughing- and finishing-jigs, a
bed of mineral is maintained on the sieve, and the concentration is
mainly effected by jigging through this bed. The Continental or
German system starts with a size-dassifioation by screens, after which
the different sizes are treated on separate jigs.



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CONCENTRATING. 135

There are also oombinations of the two systems. The use of a
bed of mineral-grams, and oonoentration by jigging through the sieve,
hfts been adopted in Continental practice for fine jigging* At the
Lake Superior dressing-works, a somewhat imperfect size-classification
by water has been introduced as a preliminary to jigging* In the
Continental dressing- works, again, the tendency of late years has
been to reduce the number of sizes jigged, and to abandon the very
close sizing which formerly characterised the method.

The arguments for close sizing are drawn from the well-known
laws goYeming the fall of bodies in water, which may be stated as
Ibllows : — A body falling in still water moves at first.with accelerated
relocity, as in a vacuum. The resistance of the water, however,
^creases with the square of the velocity, and finally equalises and
learnlisefl* the accelerating force. Thereafter the grains move with
miform falling velocity.

It is argued that, in order to e&ct a separation by the water-
srrents of a jig, the range of size in the grains treated must not
Exceed the size-ratio of equal-falling grains. But the conditions
rhich obtain in jigging are not the same as in the case of bodies
ailing freely in water. We have to deal not with single isolated
[Tains, but with numbers of these grains moving together. The
maUer grains move in the interspaces between the large grains, and,
onaequently, in constricted channels. The large grains, also, so far .
B their movements are independent of the surrounding grains, as in
he separation of gangue from ore, and of one mineral from another,
lOve in the interstitial channels between the other grains. These
liannels must have great influence on the movement of the grains,
feverthelees many have been content to assume the formula for free-
diling grains as of universal application, and have drawn therefrom
K arguments and data for the dose and accurate sizing characterising
le Continental system of ore-dressing. It is, however, possible to
feet a very satisfactory separation on jigs, with material sized
stween wide limits only, and in a very imperfect manner, or even
itirely without sizing.

The princi^l advantage of the English method of jigging is that
dispenses with the operation of sizing by screens. The plant is
us simpler and cheaper, and the expense of treatment is less. If
« jigging be conducted entirely without size-classification, it has



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 15 of 76)