MINING.
Exploratory telegraphy seems likely to claim a position in the
twentieth century economics of mining, its particular role being to
aid in the determination of the "strike" of mineral-bearing lodes. One
main reason for this conclusion consists in the fact that the
formations which carry metalliferous ores are nearly always more moist
than the surrounding country, and are therefore better conductors of
the electrical current. Indeed there is good ground for the belief
that this moistness of the fissures and lodes in which metals chiefly
occur has been in part the original cause of the deposition of those
metals from their aqueous solutions percolating along the routes in
which gravitation carries them. In the volumes of _Nature_ for 1890
and 1891 will be found communications in which the present writer has
set forth some of the arguments tending to strengthen the hypothesis
that earth-currents of electricity exercise an appreciable influence
in determining the occurrence of gold and silver, and that they have
probably been to some extent instrumental in settling the distribution
of other metals.
The existence of currents of electricity passing through the earth's
crust and on its surface along the lines of least resistance has long
been an established fact. Experiments conducted at Harvard, U.S.A., by
Professor Trowbridge have proved beyond a doubt that, by means of such
delicate apparatus as the telephone and microphone, it is possible for
the observer to state in which direction, from a given point, the best
line of conductivity runs. Under certain conditions the return current
is so materially facilitated when brought along the line of a
watercourse or a moist patch of the earth's crust, that the words
heard through a telephone are distinctly more audible than they are at
a similar distance when there is no moist return circuit. Deflections
of the compass, due to the passing of earth-currents along the natural
lines of conductivity in the soil or the rocks, are so frequently
noticed as to be a source of calculation to the scientific surveyor
and astronomer. It can thus be shown not only that definite lines of
least electrical resistance exist in the earth, but also that natural
currents of greater or less strength are almost constantly passing
along these lines.
Some of the curious and puzzling empirical rules gained from the
life-long experience of miners in regard to the varying richness and
poorness of mineral lodes, according to the directions in which they
strike--whether north, south, east or west--may very probably be
explained, and to some extent justified, by the fuller light which
science may throw upon the conditions determining the action of
earth-currents in producing results similar to those of electro
deposition. If, in a given region of a mineral-bearing country, the
geological formation is such as to lend itself to the easy conduction
of currents in one direction rather than in another, the phenomenon
referred to may perhaps be partially explained. But, on the other
hand, the origin of the generating force which sets the currents in
motion must first be studied before the true conditions determining
their direction can be understood. In other words, much that is now
obscure, including the true origin of the earth's magnetism, must be
to some extent cleared up before the reasons for the seemingly erratic
strike of earth-currents and of richness in mineral lodes can be fully
explained.
Practice, however, may here get some distance ahead of science, and
may indeed lend some assistance to the latter by providing empirical
data upon which it may proceed. When once it is clearly seen that by
delicate electrical instruments, such as the telephone, the microphone
and the coherer as used in wireless telegraphy, the line of least
resistance on any given area of the earth's surface or any given piece
of its crust may be determined, the bearing of that fact in showing
the best lines of moisture and therefore the likeliest lines for
mineral lodes will soon be recognised in a very practical manner.
No class of men is keener or more enterprising in its applications of
the latest practical science to the getting of money than mining
speculators. Nor have they at all missed the significance of moist
bands occurring in any underground workings as a very favourable
augury for the close approach of highly mineralised lodes. If, then,
moisture be favourable, first to the presence of mineral-bearing
country and secondly to the conductivity of electrical lines, it is
obvious that there is a hopeful field for the exercise of ingenuity in
bringing the one into a practical relation to the other.
The occult scientific reasons for the connection may not be
understood; but it is sufficient for practical purposes to know that,
in a certain line from the surface outcropping of a mineral lode,
there has been given a demonstration of less electrical resistance
along that line than is experienced in any other direction; also to
know that such a line of least resistance is proved to have been, in
almost innumerable instances, coincident with the best line of
mineral-bearing country. The case is similar to that of the rotation
of crops in its relation to scientific microbiology. The art of mining
may get ahead of the science of physiography in respect of
earth-currents and lines of least resistance, as showing where mineral
lodes may be expected. Yet there is no doubt whatever that science
will not in the one case lag so far behind as it has done in the
other.
The first notable service rendered by systems of the kind indicated
will no doubt be in connection with the rediscovery of very valuable
lodes which have been followed up for certain distances and then lost.
In an instance of this description much fruitless exploration drives,
winzes and "jump-ups" may have been carried out in the surrounding
country rock near the place where the lode last "cut out"; but, in the
absence of anything to guide the mine manager and surveyor as to the
direction which the search should take, nothing but loss has been
involved in the quest. Several properties in the same neighbourhood
have, perhaps, been abandoned or suspended in operation owing to very
similar causes.
The whole group may perhaps have then been bought by an exploration
company whose _modus operandi_ will be as follows: The terminal of the
electrical exploration plant is fixed at the end of the lode where it
gave out, or else immersed in the water of the shaft which is in
connection with the lode system; and another similar terminal is fixed
by turns in each shaft of the contiguous group. The electrical
resistances offered to the return currents, or to the wireless
vibrations, are then carefully measured; and the direction of the lost
lode is taken to be that which shows the least resistance in
proportion to the distance traversed. The work of carrying out such an
investigation must of necessity be somewhat elaborate, because it may
be necessary to connect in turn each shaft, as a centre, with every
one of the others as subsidiaries. But the guidance afforded even of a
negative character, resulting in the avoidance of useless cutting and
blasting through heavy country, will prove invaluable.
Many matters will require attention, in following out such a line of
practical investigation, which are to some extent foreign to the usual
work of the mining engineer. For example, the conditions which
determine the "short-circuiting" of an earth-current require to be
carefully noted, because it would be fallacious to reason that because
the line of least resistance lay in a certain direction, therefore an
almost continuous lode would be found. Moreover, the electrical method
must only be relied upon as a guide when carefully checked by other
considerations. Other kinds of moist formations, both metalliferous
and non-metalliferous, may influence the lines of least electrical
resistance, besides those containing the particular metal which is
being sought for.
The water difficulty has enforced the abandonment of very many
valuable mines in which the positions of the lodes are still well
known. Sunken riches lying beneath the sea in old Spanish galleons
have excited the cupidity and the ingenuity of speculators and
engineers; but the total amount of wealth thus hidden away from view
is a mere insignificant fraction of the value of the rich
metalliferous lodes which lie below the water level in flooded mines.
The point in depth at which the accumulation of the water renders
further following of the lode impracticable may vary in different
countries. In China, throughout whole provinces, there is hardly a
mine to be found in which the efforts of the miners have not been
absolutely paralyzed directly the water-level was reached. But in
Western lands, as well as in South Africa and Australia, the immense
capacity of the pumps employed for keeping down the water has enabled
comparatively wet ground to be worked to a very considerable depth.
The limit, nevertheless, has been reached in many rich mining
districts. Pumps of the most approved type, and driven by the largest
and most economical steam-engines, have done their best in the
struggle against the difficulty; and yet the water has beaten them.
Rich as are the lodes which lie beneath the water, the mining engineer
is compelled to confess that the metal value which they contain would
not leave, after extraction, a sufficient margin to pay for the
enormous cost of draining the shafts. In some instances, indeed, it
remains exceedingly doubtful whether pumps of the largest capacity
ever attained in any part of the world would cope with the task
entailed in draining the abandoned shafts. The underground workings
have practically tapped subterranean rivers which, to all intents and
purposes, are inexhaustible. Or it may be that the mine has penetrated
into some hollow basin of impermeable strata filled only with porous
material which is kept constantly saturated. To drain such a piece of
country would mean practically the emptying of a lake.
Subaqueous mining is therefore one of the big problems which the
mining engineer of the twentieth century must tackle. To a certain
extent he will receive guidance in his difficult task from the
experiences of those who have virtually undertaken submarine mining
when in search of treasure lost in sunken ships. The two methods of
pumping and of subaqueous mining will in some places be carried out
conjointly.
In such instances the work assigned to the pumping machinery will be
to keep free of water those drives in which good bodies of ore were
exposed when last profitable work was being carried on. All below that
level will be permitted to fill with water, and the work of boring by
means of compressed air, of blasting out the rock and of filling the
trucks, will all be performed under the surface. For the shallower
depths large tanks, open at the top, will be constructed and slung
upon trucks run on rails along the lowest drives. Practically this
arrangement means that an iron shaft, closed at the sides and bottom,
and movable on rails laid above the surface, will be employed to keep
the water out. Somewhat similar appliances have been found very
useful in the operations for laying the foundations of bridges.
The details requiring to be worked out for the successful working of
subaqueous systems of mining are numerous and important. Chief among
these must be the needful provision for enabling the miner to see
through strong glass windows near the bottom of the iron shaft, by the
aid of electric lights slung in the water outside, and thus to
estimate the correct positions at which to place his drills and his
explosives. For this reason the work of the day must be systematically
divided so that at stated intervals the clay and other materials held
in suspension by the disturbed water may be allowed to settle and the
water be made comparatively clear.
Specially constructed strainers for the mechanical filtration of the
water near the ore face, and probably, also, chemical and other
precipitates, will be largely resorted to for facilitating this
important operation. Beside each window will be provided strong
flexible sleeves, terminating in gloves into which the miner can place
his hands for the purpose of adjusting the various pieces of machinery
required. Beyond this, of course, every possible application of
mechanical power operated from above will be resorted to, not only for
drilling, but also for gripping and removing the shattered pieces of
rock and ore resulting from the blasting operations.
From the unwatered drive or tunnel downwards, the method of working as
just described may be characterised as an underground application of
the "open-cut system". No elaborate honeycombing of the country below
the water-level will be economically possible as it is when working in
dry rock. But then, again, it is becoming plain to many experts in
mining that, in working downwards from the surface itself, the future
of their industry offers a wide field for the extension of the
open-cut system. In proportion as power becomes cheaper, the expense
attendant upon the removal of clay, sand, and rock for the purpose of
laying bare the cap of a lode at a moderate depth becomes less
formidable when balanced against the economy introduced by methods
which admit of the miner working in the open air, although at the
bottom of a kind of deep quarry. While the system of close mining will
hold its own in a very large number of localities, still there are
other places where the increasing cheapness of power for working an
open-cut and the coincident increase in the scarcity and cost of
timber for supporting the ground, will gradually shift the balance of
advantage on to the side of the open method.
At the same time great improvements are now foreshadowed in regard to
the modes of working mines by shafts and drives. Some shafts will in
future be worked practically as the vertical portions of tramways,
having endless wire ropes to convey the trucks direct from the face or
the stope to the reduction works, and thus an immense saving will be
effected in the costs incidental to mining. From the neighbourhood of
the place at which it has been won, the ore will be drawn in trucks,
attached to the endless wire rope, first along the drive on the
horizontal, and then up an incline increasing in sharpness till the
shaft is reached, where the direction of motion becomes vertical. Near
the surface, again, there is an incline, gradually leading to the
level of the ground, or rather of the elevated tramway from which the
stuff is to be tipped into the mill, or, if it be mullock, on to the
waste heap. The return of each truck is effected along the reverse
side of the endless wire-rope cable.
Ventilation is an incidental work of much importance which it becomes
more practicable to carry out in a satisfactory manner when an endless
system of truck conveyance has been provided, reaching from the
ore-face to the mill, and thence back again. The reason is mainly that
the same routes which have been prepared for this traffic are
available for the supply of air and for the return current which must
carry off the accumulated bad gases from the underground workings.
Fans, operated by the cable at various places along the line of
communication, keep up a brisk exchange of air, and the coming and
going of the trucks themselves help to maintain a good, healthy
atmosphere, even in the most remote parts of the mine. In very deep
mines, where the heat becomes unbearable after a few minutes unless a
strong wind be kept going underground, the forward and backward
courses for traffic and ventilation together are specially
advantageous.
Prices during the twentieth century will depend more definitely upon
the cost of gold-mining than they have ever done at any former time in
the world's history. In spite of all the opposition which fanaticism
and ignorance could offer to the natural trend of events in the
commercial and financial life of the world, the gold standard now
rests on an impregnable base; and every year witnesses some new
triumph for those who accept it as the foundation of the civilised
monetary system. This being the case, it is obvious that the
conditions affecting the production of gold must possess a very
peculiar interest even for those who have never lived within hundreds
of miles of any gold mine. To all intents and purposes the habit of
every man is to measure daily and even hourly the value of his efforts
at producing what the economist calls "utilities," against those of
the gold miner.
If, therefore, the latter successfully calls to his aid mechanical
giants who render his work easier and who enable him to throw into the
world's markets a larger proportion of gold for a given amount of
effort, the result must be that the price of gold must fall, or, in
other words, the prices of general commodities must rise. If, on the
other hand, all other industries have been subjected to the like
improved conditions of working, the effect must be to that extent to
balance the rise and keep prices comparatively steady.
From this point of view it will be seen that the interests of all
those who desire to see a rise in general prices are to a large extent
bound up in the improvement of methods for the extraction of gold. The
question of cheap power does not by any means monopolise the data upon
which such a problem can be provisionally decided; and yet it may be
broadly stated that in the main the increased output of gold in the
future depends upon the more economical production and application of
power. Measured against other commodities which also depend mainly
upon the same factor, gold will probably remain very steady; while, in
contrast with those things which require for the production taste and
skill rather than mere brute force or mechanical power, gold will fall
in value. In other words, the classes of articles and services
depending upon the exercise of man's higher faculties of skill, taste,
and mental power will rise in price.
Getting gold practically means, in modern times, crushing stone. This
statement is subject to fewer and fewer exceptions from one decade to
another, according as the alluvial deposits in the various
gold-producing countries become more or less completely worked out. A
partial revival of alluvial mining has been brought about through the
application of the giant dredger to cheapening the process of
extracting exceedingly small quantities of gold from alluvial drift
and dirt. Yet on the whole it will be found that the gold-mining
industry, almost all the world over, is getting down to the bed-rock
of ore-treatment by crushing and by simple methods of separation. Thus
practically we may say that the cost of gold is the cost of power in
those usually secluded localities where the precious metal is found in
quantities sufficient to tempt the investment of capital.
From this it may be inferred that the cheap transmission of power by
the electric current will effect a more profound revolution in the
gold-mining industry than in almost any other. The main deterrent to
the investing of money in opening up a new gold mine consists in the
fact that a very large and certain expense is involved in the
conveyance of heavy machinery to the locality, while the results are
very largely in the nature of a lottery. When, however, the power is
supplied from a central station, and when economical types of crusher
are more fully introduced, this deterrent will, to a large extent,
disappear. The cables which radiate from the central electric
power-house in all directions can be very readily devoted to the
furnishing of power to new mines as soon as it is found that the older
ones have been proved unprofitable.
No one will think of carrying ore to the power when it is far more
economical and profitable to carry power to the ore. In this
connection the principle of the division of labour becomes very
important. In its bearing upon the mining industry generally, whether
in its application to the precious metals or to those which are termed
the baser, and even in the work of raising coal and other
non-metalliferous minerals, the fact that nearly all mines occur in
groups will greatly aid in determining the separation of the work of
supplying power, as a distinct industry from that of mining.
Ore-dressing is an art which was in a very rudimentary state at the
middle of the nineteenth century, when the great discoveries of gold,
silver and other metals began to influence the world's markets in so
striking a manner. The ancients used the jigger in the form of a
wicker basket filled with crushed ore and jerked by hand up and down
in water for the purpose of causing the lighter parts to rise to the
top, while the more valuable portions made their way to the bottom. In
this way the copper mines of Spain were worked in the days of the
Roman Empire, and probably the system had existed from time
immemorial.
Fifty or sixty years ago the miner had got so far as to hitch his
jigging basket or sieve on to some part of his machinery, generally
his pumping engine, and thus to avoid the wearing muscular effort
involved in moving it in the water by hand. It was not until the
obvious mistake of using a machine which permitted the finest, and
sometimes the richest, parts of the ore to escape had been for many
years ineffectually admitted, that the "vanner," or moving endless
band with a stream of water running on it, was invented with the
special object of treating the finer stuff.
Jiggers and vanners form the staple of the miner's ore-dressing
machinery at the present day. The efficiency of the latter class of
separating machines, working on certain kinds of finely crushed ore,
is already so great that it may be said without exaggeration that it
could hardly be much improved upon, so far as percentage of extraction
is concerned; and yet the waste of power which is involved is
something outrageous. For the treatment of a thin layer of slimes,
perhaps no thicker than a sixpence, it is necessary to violently
agitate, with a reciprocating movement, a large and heavy framework.
Sometimes the quantity of stuff put through as the result of one
horse-power working for an hour is not more than about a
hundredweight. The consequence is that in large mines the nests of
vanners comprise scores or even hundreds of machines. When shaking
tables are used, without the addition of the endless moving bands,
good work can also be done; but the waste of power is still excessive.
The vanning spade and shallow washing dish are the prototypes of this
kind of ore-dressing machinery. Let any one place a line of
finely-crushed wet ore on a flat spade and draw the latter quickly
through still water, at the same time shaking it, and the result on
inspection, if the speed has not been so great as to sweep all the
fine grains off the surface, will be that the heavier parts of the ore
will be found to have ranged themselves on the side towards which the
spade was propelled in its progress through the water. A sheet of
glass serves for the purpose of this experiment even better than a
metal implement; but the spade is the time-honoured appliance among
miners for testing some kinds of finely crushed ore by mechanical
separation.
It is to be observed that, besides the shaking motion imparted to the
apparatus, the only active agency in the distribution of the particles
is the sidelong movement of the spade relatively to the water. But it
makes little or no difference whether the water moves sidelong on the
spade or the latter progresses through the liquid; the ore will range
itself accurately all the same. Consequently, if a circular tank be
used, and if the water be set in rotary motion, the ore on a sheet of
glass, held steady, will arrange itself in the same way. If the ore be
fed in small streams of water down the inclined surfaces of sloping
glass, or other smooth shelves set close to and parallel with one
another near the periphery of such a vessel of moving water, the
resultant motions of the heavy and of the light particles
respectively, in passing down these shelves, will be found to be so
different that the good stuff can be caught by a receptacle placed at
one part, while the tailings fall into another receiver which is
differently situated at the place where the lighter grains fall.
The main essential in this particular application of the art of
vanning is simply that the water should move or drift transversely to
lines of ore passing, while held in suspension with water, down a
smooth sloping surface. In dealing with some very light classes of
ore, and especially such as may naturally crush very fine--that is to
say, with a large proportion of impalpable "slimes"--there is a
decided advantage in causing the water to drift sidelong on the smooth
shelf by other means than the motion in a circular tank.
Adopting nearly the form of the "side delivery manner," in which the
moving band is canted to the side and the stuff runs off sideways, the
sloping smooth shelf can be worked for ore separation with merely the
streams of water holding the fine sand in suspension running down at
fixed intervals. A glass covering is placed very close to this surface
on which the streams run; and between the two is driven laterally a
strong current of wind by means of a blast-fan, which causes each
stream of water to drift a little sidewards, carrying with it the
lighter particles, but leaving on its windward side a line of nearly
pure ore. These small runlets can be multiplied, on a shelf measuring
six or eight feet in length, to such an extent that the machine can
put through as much ore as a dozen vanners, consuming only a mere
fraction of the power necessary to drive one machine of the older
type.
Cyanide solution, instead of water, is very advantageously employed
for this kind of operation in the case of extracting gold from crushed
ore. The method is to pump the liquid from the tanks in which it is
stored and to allow it to flow back by way of the vanning apparatus,
thus providing not only for catching the grains of gold by the
concentrating machine, but also for the dissolving of the fine
impalpable gold dust, or natural precipitate, by the action of the
cyanide of potassium.
Upon the use of this latter chemical will be based the main
improvements in the gold-mining industry during the twentieth century;
and, conversely, the applications of the old system of amalgamating
with mercury, in order to catch the golden particles, will be
gradually restricted. Fine concentrators, worked with cyanide
solution, perform three operations at once, namely, first, the
catching of the free gold grains; second, the production of a rich
concentrate of minerals having gold in association and intended for
smelting; and, third, the dissolving of the finest particles by the
continual action of the chemical.
In fact it is in the treatment of complex and very refractory ores
generally, whether of the precious or of the baser metals, that the
finer applications of the art of the ore-dresser will receive their
first great impetus. The vanner, as well as the jigger, will become an
instrument of precision; and in combination with rushing appliances
operated by cheap power in almost unlimited quantities it will
materially assist in multiplying the world's supply of metals. This
again will aid in promoting the further extension of machinery. Gold
will be produced in greater abundance for what is called the machinery
of commerce; and the base metals, particularly the new alloys of steel
and also copper and aluminium, will be more largely produced for
engineering and electrical purposes.
The importation--particularly to England and Scotland--of large
quantities of highly-concentrated iron ore will cause one of the first
notable developments in the mining and ore-treatment of the twentieth
century so far as the United Kingdom is concerned. The urgent
necessity for an extension in the manufacture of Bessemer steel, and
of the new and remarkable alloys in which very small quantities of
other metals are employed in order to impart altogether exceptional
qualities to iron, must accentuate the demand for those kinds of ore
which lend themselves most readily to the special requirements of the
works on hand. Hence the question of the transport of special kinds of
iron ore over longer distances will have to be faced (as it has been
already to a limited degree), and not only in reference to ores
containing a low percentage of phosphorus and therefore exceptionally
suitable for the Bessemerising process, but also in regard to ores
which are amenable to magnetic separation.
Magnetite, indeed, must bulk more largely in the future as a source of
iron, particularly because it is susceptible of magnetic separation, a
process which as yet is only in its infancy. Containing, as it does, a
larger percentage of iron than any other source from which the metal
is commercially extracted, its employment as an ore results in great
economy of fuel, as well as a reduction in the proportionate costs of
transport. When ores of iron require to be brought from oversea
places, it is obvious that those which will concentrate to the purest
product possible, and which are in other respects specially applicable
to the production of grades of steel of exceptional tensile strength,
will have the preference.
Magnetic concentration, or the separation of an ore from the waste
gangue by the attraction of powerful electro-magnets, must therefore
occupy a much more prominent place in the metallurgy of the future
than it has in that of the past. Not only may ironstone containing
magnetite be separated from other material, but several important
minerals acquire the property of becoming magnetic when subjected to
the operation of roasting, sometimes through a sulphide being
converted into a magnetic oxide.
By the use of powerful electro-magnets, the poles of which are brought
to a point or to a nearly sharp knife-edge, the intensity of the
magnetic field can be so enormously increased that even minerals which
are only feebly magnetic can readily be separated by being lifted away
from the non-magnetic material. In some systems the crushed ore is
simply permitted to fall in a continuous stream through a strong
magnetic field, and the magnetic particles are diverted out of the
vertical in their descent by the operation of the magnets.
Nor is it only those minerals that actually become themselves magnetic
on being roasted which can be so differentiated from the material with
which they are associated as to be amenable to magnetic separation.
Even differences in hygroscopic properties--that is to say, in the
degree of avidity with which a mineral takes up moisture from the
atmosphere--may be made available for the purpose of effecting a
commercially valuable separation. This is especially the case with
some complex ores in which one constituent, on being roasted, acquires
a much greater hygroscopic power than the others, the grains of the
crushed and roasted ore becoming damp and sticky while those of the
other minerals remain comparatively dry. By mixing with an ore of this
kind--after it has been allowed to "weather" for a short time--some
finely-powdered magnetite the strongly hygroscopic constituents can be
made practically magnetic, because the magnetic impalpable dust
adheres to them, while it remains separate from the grains of the
other minerals.
Hardness--as well as magnetic attraction--is a property of ore which
has as yet been made available to only a very slight extent as the
basis of a system of separation. If a quantity of mixed fragments of
glass and plumbago be pounded together in a mortar with only a
moderate degree of pressure, so as to avoid, as far as possible, the
breaking of the glass, there will soon come a stage at which the
softer material can be separated from the harder simply by means of a
fine sieve. There are many naturally-existing mineral mixtures in the
crushing of which a similar result occurs in a very marked degree;
and, indeed, there are none which do not show the peculiarity more or
less, because the constituents of an ore are never of exactly the same
degree of hardness. When the worthless parts are the softer and
therefore have the greater tendency to "slime," the ore is very
readily dressed to a high percentage by means of water.
But when the reverse is the case, and the valuable constituents
through their softness get reduced to a fine pulp long before the
other parts, the ordinary operations of the ore-dresser become much
more difficult to carry out. Most elaborate ore-reduction plants are
constructed with the view to causing the crushing surfaces, whether of
rolls or of jaws, to merely tap each piece of stone so as to break it
in bits without creating much dust. This operation is repeated over
and over again; but the stuff which is fine enough to go to the
concentrator is removed by sieving after each operation of the kind;
and the successive rolls or other crushers are set to a finer and
finer gauge, so that there is a progressive approach to the conditions
of coarse sand, which is that specially desired by the ore-dresser.
Much of this elaboration will be seen to be needless, and, moreover,
better commercial results will be obtained when it is more clearly
perceived that the recovery of a valuable ore in the form of a fine
slime may be economically effected by the action of grinders specially
constructed for the purpose of permitting the hard constituents of the
ore to remain in comparatively large grains, while the other and
softer minerals are reduced to fine slimes or dust. In other words, a
grinding plant, purposely designed to carry out its work in exactly
the opposite way to that which has been described as the system aimed
at in ordinary crushing machinery, has its place in the future of
metallurgy. Light mullers are employed to pound, or to press together,
the crushed grains for a given length of time, and then sieving
machinery completes the operation by taking out the dust from the more
palpable grains.
In some cases it will be found that an improvement can be effected by
bringing about the separation of a finer grade of dust than could be
taken out by any kind of sieve which is commercially practicable on
the large scale. This is more particularly the case in regard to
sulphide ores containing very friable constituents carrying silver. A
fine dry dust-separator may then be employed constructed on the
principle of a vibrating sloping shelf which moves rhythmically,
either in a horizontal circle or with a reciprocal motion, and which
at the same time alters its degree of inclination to the horizontal.
When the shelf is nearly level its vibration drives the coarser
particles off; but the very finest dust does not leave it until it
assumes nearly a vertical position. A large nest of similar shelves,
set close to, and parallel with, one another, can separate out a great
quantity of well-dried slimes in a very short space of time.