Charles George Warnford Lock.

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

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e further advantage that very fine material can be treated success-
lly <m jigs, which would otherwise have to be treated on tables,
le balk of the gangue, both fine and ooarse, is thus at onoe sepa-
ted by the ^ roughing-jigs," leaving but a small amount of rich
iff to be treated on the finishing-jigs and tables. As the treatment
the fine stuff by itself is troublesome, and the capacity of slime-jigs
d tables is small, the plant and method of treatment are still furUier
nplified and cheapened.

The English method is especially well suited to the oonoentration
low-grade ores on a large scale.

The principal objection in practice to the English system of jig-
Dg Is the uQoperfect oonoentration of the material passing l^irough
e jig-bed. The hutchwork is much richer when the stim is sized



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136 ECONOMIC MINING.

before jigging. This difficulty is overcome by the re-treatment of
thifl hutohwork on finiehing-jigs. At Bonne Terre, the hutch work is
chissified in pointed boxes, ^e sands are re-treated on jigs, and the
slimes are concentrated on side-bump tables.

Munroe's investigations suggest means whereby the hutohwork
can be enriched without the necessity for this second treatment. By
treatment of fine and coarse material together on the same jig, int^-
Btitial channels will be formed between the coarse grains, in 'which
the fine stuff can be very perfectly concentrated. Again, by decreasing
the mesh of the jig-sieve, the size of the interstitial channels may \k
reduced , and still finer material be successfully concentrated. Experi-
ments made to test the possibility of reducing the number of grades
of sand, developed the unexpected result that the hutchwork was
much improved by jigging coarse and fine stuff together, the reason
for which is now clear. ^ also at Bonne Terre, where, however, the
problem is complicated by the necessity of running the roughing-jigs
80 as to suck as much fine stuff through the sieve as possible.

The problem to be solved in oonnection with the English method
of jigging is to determine the best method of treating the stuff too
fine to be concentrated successfully in the interstitial channelB of the
jig-bed.

The first and most obvious method is to separate the fine stuff by
water-classification before jigging the sands. The objections to this
course are, first, the quantity of water required to effect the separation
of the slime ; and, second, the fact that much fine stuff will be sent to
the tables that could be treated successfully on the jigs.

A second plan is to separate the fine stuff from the tailings of the
jigs by proper classifiers. This is perfectly feasible, and has the
advantage that the jigs can be so run as to produce a rich hutchwork.
This plan requires large settling-tanks, and a considerable amount of
clean water to effect the separation of the slime. It has the advantage
over the plan of separating before jigging that the fine mineral is
partly concentrated and saved by the jigs.

A third plan, which will effect a partial solution of the problem,
is to run the jigs so as to take full advantage of the interstitial action.
By reducing the mesh of the jig-sieve, finer material can be concen-
trated. With a given maximum size for the coarse sand-grains,
however, a limit will be found beyond which the mesh of the sieve
cannot be reduced. Possibly it may be found practicable to use a
three or four-sieved jig, with different mesh on each sieve, and, by
varying the stroke of the pistons, to adapt each sieve to the saving of
a certam grade of sand. At Lake Superior the tail-sieves have finer
mesh than the head-sieves. Or the roughing-jigs may be run with
little under-water, so as to suck all, or nearly all, of the fine stuff
through, thus ensuring poor tailings, and the hutchwork may then
be treated again on jigs of finer mesh. These finer jigs should be run
in the same way, and their hutchwork should be treated on tc^bles.

By one or another of the above methods, it may be fourfd prac-
ticable to save and treat the slime without using classifiers to separate
it from the sands.

A fourth plan is to conduct the crushing so as to produce a

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CONCENTRATING. 137

minimnm amonnt of slime; for example, by a system of gradual
cTQshing, tusmg two or more sets of roUs; or by coarse-cnishing,
followed by jigging and fine-crushing of *' raggings " only ; or, again,
by calcining the ore so as to make it more friable. By one or another
of these methods it may be possible to limit the production of fine
stnfi^ so that in many cases it may be allowed to escape without
Beriousloss.

The following are the main points developed by Munroe's
iiiTettigations : —

1. Bodies falling through water in a tube do not attain as high a
Telocitir as in falling through the same medium in larse vessels.

2. The falling velocity is but little affected when uie diameter of
the body is less than ^ that of the tube.

3. The falling velocity is the more retarded as the diameter of the
body approximates that of the tube.

4. A sphere -f^ the size of the tube will develop the greatest
falling velocity, and will require a current of maximum velocity to
sapport or raise it.

5. Grains falling en fMUM are really moving in confined channels,
and follow the law of the movement of bodies in tubes. The falling
velocity, and the velocity of the current necessary to support or raise
ths mass of grains, increase and diminish with the distance apart of
the grains.

6. The diameter of the channel in which the single grain moves
equals the cube root of the volume of the grain with its proportion of
the interstitial space, or, in other words, the cube root of the space
oocapied by the grain.

7. In a mass of grains of different sizes, the large grains move
nlatively in smaller channels than the small grains. The ratio of
the diameters of equal-falling grains of quartz and galena, under such
conditions, is 31 to 1, instead of 4 to 1, which latter ratio holds good
for free-fJEJling grains only.

8. The formulaB for grains moving in tubes, when applied as above
to gndns moving en masse^ enable us to compute the velocity of jig-
eorrents, and thus determine the proper length and number of strokes
of the jig-piston. The old formulce gave results many times too large.

9. Close sizing is not necessary for the separation of different
minerals by jigging, unless the difference in specific gravity is small.

10. Downward currents are apparelTcly necessary to success in
j^gg;uig through a bed. This requires confirmation by experiments
on a larger scale.

11. \ery fine material, less than ^ mm. diam., can be treated
loooessfuUy on jigs, if treated with coarse stuff*, the concentration
taking place in Uie small interstitial channels between the grains
fimning the mineral bed. For the treatment of fine stuff on jigs,
dose sizing is a positive disadvantage. Jigs work well on mixed
stuffy and very bsialy on fine stuff alone. Stuff less than ^ the size
of the smallest interstitial channels cannot be treated successfully in
this way.

12. The size of the mesh of the jig-sieve has a very important
influence, and must be proportioned to the work to be done.



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138



ECONOMIC MINING,



In the common hiand-leYer Cornish jig, the sieve is rectangular ;
to each of the short sides is attached a vertical iron bar, perforated at
the upper end with 3 long holes, through which a pair of bolts are
passed, linking them to the parallel arms of an oscillating frame. By
altering the holes through which the suspension bolts pass, the sieve
is made to hang at a greater or less depth in the rectangular w^ter
cistern or hutch in which it is worked. The suspension frame is an
unequal-armed lever ; the sieve is attached to the shorter side, while
the longer arm is terminated by a slotted part, in which works a
T-headed fixed link or connecting rod attached to the shorter arm of
a second lever placed below it. The motive power is supplied by a
boy, who jerks the longer arm of the second lever, moving it through
a height of 48 in., while the sieve is only moved through 8 in. Clean




Fig. 70. — RrmNGEB's Jig.

water is introduced through a pipe on one side of the cistern to
replace the muddy waste which is carried o£f through a similar pipe
fitted with discharging apertures at different depths on the oppoette
side. The sieve is emptied by scraping out the contents, which are
usually classified into three parts, the uppermost being thrown away ;
the middle, containing mixed ore with earthy matter, requires farther
treatment, while the bottom is clean ore. The hutch work or fine
stuff passing through the sieve collects in the cistern, and is sub-
sequently treated on the round huddle or some other sHme-wa^iing
machine.

Bittinser's jig. Fig. 70, is characterised by the inclination of the
grates and the lowness of the front partition, over which the poor
and lighter stuff falls continuously, and with very little water, iraile
the heavier and rich portions fall through the opening or slit, at the

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139



b«8e of the partition. This partition is the segment of a cylinder,
and is supported upon the lever or arm a, so as. to be movable back-
ward and forward in such a manner that the slit may be increased or
diminished at pleasure. The heavy staff, passing ^ongh the open-
ing, falls into the box 6, from which it is removed as required. The
inclination of the grate in this machine is 5^-8°. It is fed through
the hopper e, which plunges below the surface of the stuff accumulated
on the grate. The Joss of water which occurs at each stroke of the
piston is replaced from a reservoir c2 at the back of the apparatus.
According to Bittinger, experience has shown that the duty of self-
acting machines of diis kind is generally three times as great as that
from the ordinary intermittent working apparatus.

The peculiarity of the Frongoch jig (Figs. 71, 72), as described by
Prof. Le Neve Foster, is that the piston is vertical, and works in the
partition between two tubs or hutches; c is the middle partition,
and (2 is the piston working between two plates of iron e. The piston
oooapiee the whole length of the jig, as shown by/, and is worked by
the rod ^, guided at \ and passing through a stuffing-box t. The




F108. 71, 72.— Frongooh Jig.

reciprocating motion is given by a crank through the connecting rod I
and lever w, which traverses the head of the piston-rod ». The crank
baa a long loop, which enables the stroke to be varied. The same
end can be attained by an eccentric with a slot, which allows the
eccentricity to be altered at pleasure, o shows where the ore is fed
on, and f is the place of discharge of the waste or impoverished ore.
9 is the sieve, and r are holes with plugs manipulated by handles
(not shown) by which the concentrates that pass through the sieve
are drawn off. % is the pipe bringing in fresh water. These machines
are doing good work with ores containing blende and galena at
Frongoch, and have been favourably spoken of for the treatment
of tin ore.

Many other forms of jig will be found described at length in
the author's ' Mining and Ore-dressing Machinery.'

Pyramidal Troughs, — Pyramidal separators, like so many other
apparatus used in mining and metallurgy, originated in Germany,
and are often known by tiie German name of l^nizkdsten. They are
hollow, generally rectangular, pyramids, constructed of strong boards.



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I40 ECONOMIC MINING.

well joined together (strong sheet-iron may be employed also). The
sides are inclined at angles of not less than 50 , and there is a
small hole in one side close to the apex. They are fixed horizontally,
in an inverted position, and the crushed material is introduced at one
of the narrow sides, a few inches below the top, by means of a launder.
The result is that, as soon as the box is filled, a certain portion of the
crushed matter — i. e. the coarsest and heaviest, which tne water, on
account of its diminished velocity, is not able to carry farther — sinks
and slides down the inclined sides of the pyramid, and escapes
through a small hole near the apex, whilst the finer and lighter
matter passes o£f at the top by an outlet a in the centre of the side,
opposite to the point of entrance. If now a second larger box be
attached to the first, a third still larger to the second, and so on
— each succeeding box at a slightly lower level, in order to prevent
any settlement of stuff in the passage-ways — it follows not only that
the same process of settling and escaping of the particles from the
apex will take place in every box, but also that their size will decrease
nearly in inverse proportion as the surface of a succeeding box is
larger than that of the preceding one, or directly as the velocity of
the water is diminished in it.

According to this principle, if the boxes were made of only very
gradually increasing size, and the apex holes proportionately small,
it would be possible to classify the stuff into a great number of
portions, different in size of gram, before it had entirely settled, L e.
till dear water passed off from the last box. Experience has, however,
shown, that for fine ore-dressing in general, classification into 4
different sizes by an apparatus ot 4 boxes is quite sufficient. The
sizes of the different boxes, in order to ensure the most perfect classifi-
cation, depend both on the amount of material which has to pass
through them per second, and the size and character of the grains ;
and it has been found, that for the supply of every cubic foot of
material, the width of the first or smallest box must be -^y ^ — ^ ®-
for instance, for 20 cub. ft., 2 ft. — and for every succeeding box it
ought to be about double that of the preceding one, or, generally, the
widths of tiie boxes must increase nearly in geometrical progression,
2:4:8, &c., and their lengths in an arithmetical one, 3, 6, 9, d^c.

Their depths depend on the angle of inclination of the sides,
which, as already stated, is generally 50°, because if less, the stuff
would be liable to settle finmy and choke the central orifice, and
if larger, unnecessarily ereat height of the boxes would be required.
The form of the two smcQler boxes is commonly sueh that the two
short sides are inclined at the above angle, and the two long ones,
which would become far steeper, are broken — L e. are for a certain
depth from the top vertical, and afterwards inclined at the normal
angle. This modification has, however, no influence upon the action
of the boxes, but simply facilitates somewhat their construction
and firm fixing. The sides of the larger boxes are generally even
throughout.

The way in which the apex-holes are constructed has an im-
portant bearing on the operation of the boxes. At these points,
the hydrostatic pressure is considerable, and the holes shotild naturally



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CONCENTRATING. 141

be kept small, in order to prevent too mncli water passing with the
particles of stnflF; such small outlets are, however, especially in the
treatment of coarser material, very liable to become choked. This
difficulty has been met by the holes being made of conveniently large
size, but connected with pipes \ in. diam., which rise up the side of
the boxes, that of the smallest box to within 3-3]^ ft., and of others
to within 2-2^ ft. from the top, and are there furnished with small
mouthpieces supplied with taps for regulating the outflow. This
arrangement, on account of the outlets being so much higher, has the
farther advantage that a considerable amount of fall is gained
(especially as regards the large boxes), which for the subsequent
treatment of the material, is in some cases of special value.

Two more points require attention, to ensure good action of the
apparatus, namely, the introduction of the material into the different
loxes equally and without splashing, and prevention of the entrance
of chips of wood, gravel, or other impurities that are likely to stop or
obstruct the outlets. The first point is met either by having the
SQpply-laimders expanded fan-like and furnished with dividing-ledges,
or by the interposition of small troughs, the sides of which nearest
the box to be supplied are perforated near the bottom by equidistant
nnall holes. The cleaning of the material, ]previous to its entering
the first box, is generally effected by the mam supply-launder being
made a little wider near the point of entrance, and &e insertion at
this place of a fine wire-sieve across the launder and somewhat
mclined against the stream. This sieve must be occasionally looked
after, to remove any impurities collected in front ; and this, in fact,
is the chief attention tne whole apparatus requires, for otherwise it
needs hardly any supervision. If once in proper working order, its
action is constant and uniform, provided the material introduced does
not change in amoxmt and quality ; and it has this further advantage,
as compared with the slime labyrinths, that the classified stuff can,
from the outlets, be directly conveyed in small launders to other con-
centration-maohines for further treatment. One point, however, not
in &vour of the apparatus, is that a great fall of ground is required,
to permit the direct introduction of the material and allow sufficient
&11 for the tailings ; and thus, where local circumstances are un-
&Tourable, it has to be erected at a higher level, and necessitates the
nse of elevators or pumps for lifting the stuff.

There are many forms of pyramidal trough in use, their dimensions
varying according to the duty, but space only admits of typical
examples being described here. One of these is when it is desired to
settle all the pulp, including the slimes, by reason of there being too
mu<^ water present for subsequent concentration. The pointed box
ihonld tlien oe about 6 ft. deep, and 3 ft. by 7 ft. at the top, the
lonsest sides sloping till they meet at the bottom. Such a box will
settle and save about 6 tons of ore in 24 hours, discharging it auto*
matioally and continuously from the bottom by a siphon hosci with
the proper amount of water for subsequent concentration. This form
it used when the tailines ^m pan amalgamation are to be concen-
trated, after leaving the settlers and agitators, for they contain a
Itfge excess of water, which must be got rid of, so that the tailings



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142 ECONOMIC MINING,

may be of the proper consistency for concentration. Fig. 72 shows a
form of pointea box recommended by J. M. Adams in cases where the
slimes are to be separated from the battery pidp and saved. Each
box is 40 in. square at the top, and 40 in. deep, coming to a point at
the bottom ; and one box will handle 6-10 tons of pulp in 24 hours,

making a good separa-
tion. The pulp from the
battery, entering the box
a at the top, is oonfined
by partition 6, until it
passes into the box proper
c, near its bottom. Clear
water is oonyeyed from a
launder <2 above, through
a \ in. pipe e, which de-
livei-s it into the box at
the bottom. Care must
be taken that this pipe is
kept full, so that no air
bubbles may be carried
through it, creating agita-
tion, and causing sand, Ac^
to pass o£f with the slimes.
The quantity of clear
water needed varies, so it
is a good plan to have a
Fio. 72.— Pointed Box. oock in the pipe just below

the clear water box <i ; or
to partially close, with a wooden plug, the opening of the pipe in the
clear water box. At / is a hollow plug, and to it is attached a
piece of hose ^, which is used as a siphon, se that the pressure is
lessened, and too violent discharge of the pulp is prevented. With-
out the siphon hose, \ in. opening would not be too small, yyhile
with it \ in. opening is about right, and the end of the hose is
plugged accordingly. Inasmuch as foreign coarse material occasion-
ally gets into the box (prevented as much as possible by a screen
over the top), it is advisable to use in place of the hollow wooden
plug shown, a 1 J in. iron T with one end plugged, and with } in. side
outlet, attaching the siphon hose by nipple. The launder lb carries
off slimes, and the launder t conveys rich matters to the concentrators.
The Frongoch classifier (Fig. 73) is a simplification of, and im-
provement on, the German double troughs, or Sj^zhktUn^ folly
described and illustrated in ' Mining and Ore-dressing Machinery.'
It consists of an inverted wooden cone a, which can be more or less
completely closed at the bottom by a plug 6, controlled by a handle
and screw-nut c The cone a stands upon a wooden box d, which
receives water under pressure from a pipe e, and is provided with a
discharge-valve /, a mere flat plate of iron working on a pin, which
can be pushed sideways so as to close the orifice more or leas
entirely. Inside the w(X)den cone a is a sheet-iron funnel ^, which
receives the stream of ore and water from a launder \ and causes

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CONCENTRATING.



143



it to descend to the level t. There it meets with the upward
cnrrent of clear wat^r, and a separation is eflfeoted. The coarse and
heavy particles which can overcome the stream pass into the box d
below, and flow ont continuously at /, while the fine and light par-
ticles are lifted by the current and carried over the top edge of the
wooden oone a, which is
surrounded by a circular
laimder k. By altering the
flow of the upward current
of clear water and the size
of the discharge orifice, the
eeparator can be adjusted
to the special requirements
of any particular case, giv-
ing it a distinct advantage
over the older forms, which
were useless for any but the
purpose for which they were
designed. At Frongoch this
separator is used for classi-
fjmg a mixture of galena
and blende, just as it comes
from the crusher, through a
screeD with 12 holes (3 by
4) per sq. in., the coarse
going to the jigs, and the
fine to the buddies.

YonmerB, — The material
having been classified ac-
cording to size, the next

step is to submit each e^

separate size to a process of J"!

concentration, with the ob- "^^*^

ject of eliminating the valu-
able portion, or separating
the several valuable por-
tions where such occur and ^^^^
require subjection to differ-
ent treatments at a later
stage. Of the various ap-
paratus in use for this purpose, all working upon the principle of
taking advantage of the differences in specific gravity of the several
rabstanoes encountered, no class is more important than the *' vanners "
or ** shaking tables," which, in every conceivable variety, provide a
amooth inclined or horizontal surface, over which the ore is spread
by a small quantity of water, and subjected to repeated and regular
diaturbance by an " end shake," or " side shake," or other sudden
movement. For a description of numerous examples of these machines,
the reader is referred to * Practical Gold Mining.' It will suffice here
to mention the Lfihrig system, the striking feature of which, as will
be seen from Fig. 74, is the combination of 3, 4, or 6 (according to



-i_



Fig. 73.— Fbonooch GLAflsnnaB.



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144 ECONOMIC MINING.

the nature of the ore) single tables of an improyed construction,
arranged in such a way as to allow of automatically feeding tlie
'' middle products " (or not yet sufficiently clean products) from the



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upper tables on to the lower tables. By this subsequent treatmei
of the middle products on the same machine, high oonoentrati
are obtained without requiring manual labour or incurring loss <
mineral



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CONCENTRATING. US

The siiiele table oondflts in the main of a trayelling band, with
tdjiutable lateral inclination, suspended from a frame reoeiving an
end-ehake, which, combined with tne action of a spray-pipe extending
diagonally across the table, effects the separation of the various
minerals in the pnlp, according to their specific weights. These
Marions products are Washed off by the spray into receptacles in front
of the table. The material is fed on to the table by a siphon discharge
from a hydraulic classifier or pyramidal trough.

The advantages of the single table over others consist, firstly, in
tiie circumstance that one clean product is always obtained in one
operation ; and, secondly, any number of grades of concentrates of
diffigrent ores may be obtained on one table, and the degree of con-



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 16 of 76)