GOLD RECOVERY
There has always been something very fascinating about gold. Even in
ancient times it was prized above all other things, and apparently it
was comparatively plentiful. It is estimated, for example, that King
Solomon possessed over L4,000,000 worth of it, while the little gift
which the Queen of Sheba brought him was of the handsome value of
L600,000, so that she too must have been plentifully supplied with it.
Probably it was more easily come by in those days, owing to the richness
of the primitive deposits, the best of which, perchance, have been
worked out. In one respect gold differs from all other metals (with the
single exception of platinum, which is scarcer still) in that it appears
naturally as gold, not as ore. The little pieces of gold lie in the mine
ready to be picked out, and so if the deposit in which it occurs be near
the surface, and the particles be of any considerable size, they are
sure to be found. A savage may be, and often is, very anxious to secure
weapons and tools of iron, little knowing that the very ground upon
which he stands is possibly of iron ore. He covets the single article of
iron, and in some cases is willing to give much gold for it, or ivory,
or some such treasure, while thousands or millions of tons of iron lie
at his feet, only he does not recognise it, nor would he know how to
utilise it if he did.
For iron, like all other metals except the two just referred to, is
found naturally in combination with something else, generally oxygen,
and the combination bears no resemblance at all to the metal. The red
rust so familiar to us on iron is a combination of iron and oxygen, and
it is fairly typical of the kind of state in which iron is found in the
earth. Nor would anyone recognise copper ore, lead ore, tin ore, or any
of the ores, any better than iron ore. All are difficult to recognise.
It is said that the highest compliment that a Cornish miner--the finest
metalliferous miners in the world come from Cornwall, or are the product
of Cornish influence--the highest compliment that such a man can pay to
another is to say that "he knows tin," meaning that he can tell tin ore
when he sees it.
Contrasted with these other metals, gold is easy to find. It does, it is
true, under certain conditions, form chemical compounds with other
things, as, for instance, in gold chloride, which is present in
sea-water, but it does not oxidise as the others do, and so when it is
in the earth it is in the bright yellow grains such as (if they be large
enough) can easily be recognised at sight.
And it is often found in beds of loose gravel, alluvial deposits, as
they are termed. In such cases the gold is to be had simply for the
picking up. Sometimes a lucky find occurs in the form of a big nugget,
but more often the metal lies in tiny grains at long distances apart, so
that a ton of gravel has to be sorted over to find a paltry ounce or so
of gold. Yet so desired is it that gold will always fetch its price, and
an ounce to the ton (even less) is sometimes worth getting.
But in the early history of the world there were possibly particularly
generous deposits with plenty of gold in good-sized pieces, and such
would be quickly discovered and worked by primitive man. No doubt the
chieftains of those days took much, if not all, of the gold that their
people found, and more powerful chiefs and kings would, in turn, either
by force or in trade, take it from the weaker, so that it is not
surprising to learn that some of the mighty kings and potentates of long
ago were well supplied with gold.
Yet there are few things more useless. Its value in the first instance
was probably entirely due to its beautiful colour, and the fact that it
does not easily tarnish. For this reason, coupled with the fact that it
was by no means plentiful, men liked to deck themselves with it, not
only adding to their "beauty" by so doing, but advertising to their
fellows the fact that they were men of wealth, men who possessed what
few others had, or at all events possessed it more abundantly. These
three basic facts about gold, its beauty, its freedom from deterioration
and its comparative scarcity, give it its peculiar status among the
commodities of commerce, in that for it, and for it alone, there is a
continuous and universal demand. No gold-mining company ever shut down
its properties because of the falling off in the demand for gold. No one
ever had to hawk gold about to find a purchaser; it is always saleable.
And hence its value to humanity as the great medium of exchange. When a
tailor wants bread, as has been pointed out by a great political
economist, he does not go searching for a baker who happens to need a
coat. If he did, he might starve before he found one. Instead, he gives
his coat to anyone who needs one, no matter what his trade may be,
taking gold in exchange. Then he goes with confidence to the baker,
knowing full well that he, in turn, will be perfectly ready to give
bread in exchange for gold. That is the principle upon which gold, and
in a few cases silver, has become the foundation of trade. We use it for
toning photographs and a few other things, but, practically speaking, it
is useless stuff, yet certain special circumstances have given it a
special function in civilised society, and so governments now make it up
into little flat discs, putting their own special stamp upon them as a
guarantee of size and quality, and it is by handing those little discs
about that we carry on our trade. Or even where we use no actual disc,
we pretend that we do, and use a piece of paper the value of which we
say is so many discs, but that value depends entirely upon the fact that
someone has guaranteed, on demand, to give so many discs for it.
And the strange thing about it is that although this usefulness of gold
depends upon its rarity, we lose no opportunity of looking for new
sources of supply, and so diminishing that rarity. As has been said,
gold is present in sea-water, although no one knows how to get it out,
except at a cost which makes it not worth while. But suppose that some
genius found a way, and gold thus became twice as plentiful as it is
now, the world would be no better off. Everything would cost twice as
much as it does now; that is all. A pound is merely so much gold. If
gold be twice as plentiful people will want twice as much of it in
exchange for what they have to sell. Yet, all the same, the man who
could solve that problem of getting gold from sea-water, or from
anywhere else, in fact, would be hailed as a benefactor, and for a time
at least he would reap a generous harvest.
Even as it is, science has done much for the production of gold. Not, as
in other metals, in finding ways for extracting it from its ores, for,
strictly speaking, it has none, but in finding ways of catching the tiny
particles of metal from the "gangue," as it is called, the rock or earth
in which they are embedded. The trouble is that they are so small, so
infinitesimally small, almost.
There are two great types of place where gold is found. In the alluvial
deposits, the beds of old rivers, the gold is quite loose. The
convulsions of ages ago have, in many cases, elevated these beds, until
now they are on the sides of mountains. In such cases the loose,
gravelly stuff of which they are composed is washed down by a powerful
stream of water from a huge hose-pipe terminating in a nozzle called a
"monitor." This process, called "hydraulicing," brings down everything
into a pond formed at the foot of the hill, and in some cases a boat or
raft is floated upon the pond with machinery on board for dredging up
the material. Often a powerful centrifugal pump sucks up the water
through a pipe reaching to the bottom of the pond, bringing gravel and
gold with it. Arrived in this way upon the raft, it all goes on to
separating tables, by which the gold, being heavier, is divided from the
gravel, which is lighter. These tables will be referred to again later.
In non-alluvial workings the gold is embedded in rock of some kind, such
as that called quartz. This is hard, somewhat of the nature of granite,
and before the gold can be liberated it has to be crushed to the
likeness of fine sand, so that the tiny grains of gold can be captured.
The quartz is found in veins or lodes, fissures, evidently, in the
original crust of the earth, produced probably as the earth cooled.
These have been gradually filled up by hot volcanic streams of water,
which carried not only the gold in solution but also the materials of
which the quartz is formed. It used to be thought that the veins were
the result of hot liquids forced up from below by volcanic action, the
rock and metal being themselves in the liquid state through intense
heat. It is now more generally held that water was the vehicle by which
the materials were brought in, and the vein formed. The gold in the
alluvial deposits, too, is now thought to have come there in solution in
water, and not by the erosion and washing down of rocks higher up the
original river.
However that may be, and it is the subject of discussion among
geologists and metallurgists, there the gold is to-day, firmly fixed in
the hard rock, and the problem which confronts the metallurgist is to
get it out with the least expense. The old historic way of breaking up
the quartz rock is with what are called "stamps," pestles and mortars on
a huge scale. There are a number of vertical beams of wood, each shod
with iron, fixed in a wooden frame, so that they are free to slide up
and down. Running along behind these stamps is a horizontal shaft with
projections upon it called cams. There is one cam for each stamp, and as
the shaft turns slowly round this projection catches under a projection
on the stamp, and after lifting it up a short distance drops it
suddenly. Thus, as the machine works, the stamps are lifted and dropped
in rapid succession. The rock is fed into a box into which the feet of
the stamps fall, and thus it is pounded until it is quite small.
Meanwhile a stream of water flows through the box and carries away the
finely broken particles through a kind of sieve which forms the front
of the box, and which allows the fine, small pieces to escape, while
holding back the larger ones and keeping them until they too have been
crushed.
An average stamp will weigh 600 to 700 lb., and the repeated blows of
such a hammer are enough to pulverise the hardest rock.
Machines such as these have been employed since the sixteenth century,
at all events, and the improvements of modern times are only as regards
details. It may well be wondered, then, why such an old device is still
in use and how it comes about that it has not been displaced by
something newer and better. The answer, which is an instructive one,
well worth bearing in mind by many inexperienced inventors, is that it
is so simple. It can be shipped in comparatively small parts, and so
taken cheaply to any outlandish place. A good deal of it can be made
roughly of wood, so that if native timber is available it can be made
partly at the mine, and carriage costs saved. Finally, it is so easy to
work and to understand that the most inexperienced workman can handle
it, and there is so little that can go wrong that the most careless
attendant cannot damage it.
In the bottom of the boxes there is placed some mercury, for which gold
has a curious affinity. If a particle of gold once gets into contact
with the surface of the mercury it will not get away again easily. Thus
the mercury catches and holds many of the gold particles which are
liberated when the rock is broken up.
As it reaches the required fineness, then, the crushed rock escapes from
the stamp machine and flows away in the stream of water, and although
much gold is caught by the mercury, it is by no means all. The stream is
therefore directed over tables formed of copper sheets coated with
mercury, so that additional opportunities are given to mercury to catch
the grains of gold. Moreover, the table, which, by the way, is placed at
a slight incline, is broken at intervals by little troughs of mercury
called riffles, which assist in the depositing and catching of the metal
particles.
But even then all the gold is not captured. The crushed rock is now like
sand, and some of the grains still contain gold, which has not been
detached by the crushing. The gold, however, makes such grains slightly
heavier than the others, and because of that they can be separated. The
old way is to use a blanket table, a table, that is, covered with coarse
flannel or baize, the hairs of which catch these heavier particles as
the water stream carries them along, the lighter particles escaping. The
grains so caught form what are known as "concentrates," since in them
the gold is concentrated.
The concentrates are subsequently treated as we shall see later.
Now we can see how modern scientific methods have supplemented the old
ways. Take first the case of the stamp mill or stamp battery. In spite
of that prime virtue of simplicity which has kept it at work almost
unchanged for centuries, it has its weaknesses, and no doubt for some
purposes crushing mills are better. Of these there are a great variety,
several of which depend for their action upon centrifugal force, or, as
it is more correctly termed, "centrifugal tendency." In these crushing
mills there is a ring, generally of steel, inside which are suspended
one or more heavy iron rollers. The shafts which carry these rollers are
attached by their upper ends to the driving mechanism on the top of the
mill, and when that is set in motion the rolls are carried round and
round inside the ring. Because of the centrifugal tendency, they swing
outwards, pressing heavily against the inner surface of the ring. The
rock is fed in in such a way that the rollers, as they roll round the
inside of the ring, repeatedly travel over it and crush it.
In another type of mill, called the ball mill, the principle is
different. There you have a cylinder of steel which turns upon a
horizontal axis. This cylinder is partly filled with steel balls of
various sizes, and as the mill turns, the rock, being mixed with these
balls, is pounded and broken up. As the mill turns over and over the
balls fall upon the pieces of rock, thus producing a fine powder. Other
mills, again, are but refined editions of the common mortar mill so
often seen where building operations are going on, in which heavy iron
rollers travel over the material to be crushed as it lies in a round
pan.
The blanket table, too, gives place at the modern mine to the "vanner,"
of which there are several varieties. Essentially they are much the
same, and a description of two will serve to give an idea of them all.
Let us take the "Record" vanner.
Imagine a large table formed of wood, the upper surface covered with
linoleum. It is fixed on slides so that it can move to and fro endwise.
It is given a slight slope in the direction at right angles to its
length--that is to say, one edge is a little lower than the other. The
material is fed on at one end, at the higher edge, and naturally tends
to run down and off at the lower edge. It is restrained somewhat from
doing this by the presence of rows of riffles or ridges running
lengthwise. Nevertheless it does in a short time find its way off the
table at the lower end. But all the time that it is at work the table is
being slidden backwards and forwards on the slides. By a simple but
curious mechanism it is arranged so that it moves quickly in one
direction and slowly in the other, with the result that the heavier
particles of sand--those which contain gold--are carried to the farther
end of the table. Thus, as has been said, all the stuff is fed on to the
higher edge and carried down by the water, until it falls off at the
lower edge, but during the journey from edge to edge the peculiar motion
of the table causes the different kinds of sand to separate themselves,
so that the concentrates fall off near one end, and the rest near the
other end.
Another interesting example of ingenuity is the well-known "Frue"
vanner. In this the table is a broad, endless band of india-rubber,
extended upon two rollers, one of which is slightly higher than the
other. The stream of water and crushed ore flows on at the upper end,
and runs down to the lower, the lighter particles being carried down
and dropped off at the _lower_ end, while the heavier rest upon the
band. Meanwhile the turning of the rollers carries the band slowly
along, so that the heavier particles gradually ascend and are carried
over at the _upper_ end. To assist in the separation, the whole concern
is given a side-to-side shaking motion while it is at work.
We have seen so far how the ore is crushed, and the coarser grains of
gold got out of it by the aid of mercury. The mixture of mercury and
gold is termed amalgam, and the process of extracting gold by mercury is
called amalgamation. The gold is actually dissolved in the mercury, and
so when the amalgam has been (as it is periodically) collected from the
plant, it has to be filtered and then evaporated in a retort. The
mercury vapour is caught and condensed back into a liquid, while the
gold is left in the retort. In fact the amalgam is distilled in order to
separate the gold and mercury.
But when all that is done we still have the concentrates from the
vanners, or whatever be used, to deal with. Mercury is useless with
them, for the gold is covered probably with a coating of the other
substances, whatever they may be, with which it has been associated, or
else there is mixed with the gold some substances which make
amalgamation impossible, or at least difficult.
Often roasting is necessary before anything more can be done. If arsenic
or sulphur be present, for example, they interfere with the recovery of
the gold, and roasting will disperse them. So the concentrates are
passed through great furnaces, in which they are heated in contact with
air until these objectionable matters have been oxidised or burnt.
Then finally we come to some process by which the remaining gold is
dissolved out from its admixtures in some solvent liquid from which it
can be subsequently precipitated. This is rather interesting, because it
means that man has adopted, to recover this gold from the ore, the very
method which it is believed nature employed to put it there. As already
said, the latest idea is that the gold was carried into and deposited in
the lodes where it is now to be found by water--that the gold was
actually dissolved in water at the time. But, of course, gold in its
metallic state will not dissolve in water. Salts of gold, however (the
meaning of the term salt, as applied to a metal, has been explained
earlier), will dissolve in water, as every photographer who makes up his
own toning solution knows from experience. Gold will not dissolve in
water, but chloride of gold will. And so the gold must have been carried
to its resting-place as a salt, and converted into the metallic form
after arrival. In the same way, to recover these finest particles of
all, it has to be converted back into a salt; then that salt must be
dissolved and drained away from the other stuff; and, finally, the gold
must be thrown out of solution again in some way. The great example of
this operation is the familiar "cyanide" process.
The word familiar is appropriate to this matter in only one way,
however. Holders of shares in mining companies, for example, may hear
about it repeatedly at shareholders' meetings and in prospectuses, but
very few have any clear idea as to what it is. So I cannot be accused of
telling an oft-told tale if I devote a short space to its consideration.
The combination of one atom of carbon and one atom of nitrogen is called
cyanogen.
If cyanogen be given the chance it will take unto itself an atom of
hydrogen, producing the deadly hydrocyanic or prussic acid.
Alternatively, if potassium be brought into combination with it, there
results potassium cyanide, which, with the assistance of water and
oxygen, can dissolve gold.
In applying this scientific fact to the purpose of recovering gold from
the concentrates, the latter are placed in vats with a weak solution of
the cyanide in water. The time during which they are allowed to remain
depends upon the size of the gold particles. If they be comparatively
large, it stands to reason that it must be longer than if they be
small, for they will take longer to dissolve. After the proper time,
which is found by experiment, the liquid is drawn off, and in some cases
the concentrates are given a second dose to ensure that the gold shall
be thoroughly removed and none left undissolved. If the material being
operated upon be very fine, as it often is, forming what the mining
people call "slimes," then mechanical stirrers have to be used in the
vats to keep the stuff moving, as otherwise the cyanide would not get to
all the particles and some would not be acted upon.
The liquid, having been the appropriate time in the vat, is drawn off,
placed in wooden tanks or boxes, and fine shreds of zinc are added to
it. Discs of sheet zinc are put into a lathe and a fine shaving taken
off them, and it is these fine shavings which are used. Now zinc, as we
know from the fact that it is the essential part in electric batteries,
has very pronounced electrical properties, and it is believed that these
come into play here. At all events the gold becomes deposited upon the
zinc, while the zinc itself is to a certain extent eaten away by the
solution. The result is (_a_) a solution weaker than it was before,
(_b_) the remains of the shavings, and (_c_), at the bottom of the box
in which this process takes place, _a dark mud_. That black mud, on
being heated, produces the bright metallic gold, and the object of the
whole operation is achieved. The solution is then led to another tank,
brought up to its proper strength again and is ready to be used once
more, while the remains of the shavings are used for the next batch of
material to be treated.
In some cases the crushed ore straight from the crushing mill is
cyanided, in others it is simply the remains left over from the previous
amalgamating process which is thus treated. All depends upon the nature
of the material in question.
There are other chemical methods besides the cyaniding, but it is the
chief. It has been found specially useful with the Johannesburg ores,
and to it the South African goldfields owe a great deal of their
success.
There is a more modern form of it, although the whole process is quite
novel, having been introduced only in the nineties of last century. This
development, it is almost wearying to repeat, is electrical. Instead of
the zinc shavings being used to precipitate the gold out of the
solution, the process is electrolytic. A lead anode is used while the
process is carried on in a box the bottom of which is covered with
mercury, which forms the cathode. The precipitated gold is thus
amalgamated, the amalgam being removed at intervals, retorted, and the
gold recovered.
The idea of recovering gold from the waters of the sea is certainly a
most attractive one. To some, it is true, the suggestion may bring
thoughts the reverse of pleasant, for there have been several partially
successful attempts to delude the public with specious promises of vast
dividends to be gathered in the form of pure gold from the inexhaustible
sea. Still, there is something in it, and some day the dreams may be
realised.
The quantity of gold dissolved in sea-water is so small that in 200
cubic centimetres it is impossible to detect it, even by the most
delicate tests known. The quantity needs to be multiplied threefold
before the quantity of gold becomes even detectable, to say nothing of
being recoverable.
A writer in _Cassier's Magazine_, a few years ago, related how he had
actually obtained gold from the water of Long Island Sound. But whereas
he got two dollars' worth, it cost him over 4000 dollars to do it. No
company will ever be floated on results such as that. From the mud of a
creek near New York, however, he did a little better, for there ten
dollars' worth of gold only cost 379 dollars. A company promoter would
still look askance at even that comparatively successful undertaking.
As usual, authorities differ, but there is a consensus of opinion that
in every ton of sea-water there is from one-half to one grain of gold,
besides silver and iodine.
It seems as if the water were able to dissolve that amount and no more.
If, as has been suggested earlier in this chapter, all the gold which is
now found in mines and in gravel beds was carried there in water, it is
probable that the sea obtains its gold from the same original sources,
and that, just as the hot ocean of ages ago carried its burden of gold
in solution, so the colder water of to-day has its share, the cold water
naturally carrying less than the hot did.
It is quite likely, then, that, could we find out how to rob the sea of
its precious metal, it could replenish its store from some secret hoard
of its own. But even if it could not, it would make little difference to
us, since what it holds is far more than we could ever use. Put it at
half-a-grain per ton: there are 4205 million tons in every cubic mile of
ocean, and 300 million cubic miles of water in the ocean. If all the
gold that man has ever handled were to be dissolved in the sea, no
chemist would be able to discover the fact. On the other hand, if that
half-grain per ton which we believe to be in the ocean now were to be
recovered we should have about 40,000 million tons of gold, a prospect
which is enough to make the political economist turn pale with
apprehension.
What is required is some substance which, on being added to sea-water,
will combine with the gold, and then be precipitated--that is to say,
fall to the bottom. The precipitate--that which falls to the
bottom--would need to be heavy, so that it would fall quickly and not
necessitate the water being left standing for long periods. It would
need to be cheap, too, or easily recoverable, so that it could be used
over and over again. And, finally, it would need to be such that the
gold, having been captured by it, could be easily obtained from it.
Given such a precipitant, the process of recovering the gold would be
simple and cheap. Tanks would be formed in sheltered bays and inlets. At
every tide these would be filled, and when full the precipitant would be
added. The tide falling, the water would run out again and leave the
precipitate on the floor of the tanks, whence it could be removed by
scraping. Simple treatment would release the gold from its partner,
which would then be returned to the tanks to act as the precipitant once
more. Thus by simple means, the tide itself assisting, the gold could
be obtained from the sea.
And there is nothing inherently impossible about this suggestion. The
necessary precipitant may exist, awaiting discovery. A large works
operating in this manner would produce, it is estimated, about thirteen
tons of gold per annum. It looks as if it would be a bad day for the
Rand when that discovery is made.
And there is yet another possibility, though less alluring than what has
just been described. The American writer mentioned a little while back
got a better return from the mud of a creek than from the water itself.
In all probability this is due to the action of organic matter carried
down by streams, or in some other way introduced into the waters of the
creek whence the mud was obtained. This organic matter would possibly
have an effect as a precipitant upon the dissolved gold, causing it to
be thrown out of solution and deposited in the mud. Thus the mud around
our shores, and particularly in the creeks and estuaries, may be
potential gold mines whence in time to come we may draw supplies of the
precious metal. The cyanide or some similar process may be needed in
order that we may extract the metal from its enclosing mud, but the time
may not be so very far distant when dredging for gold may be a regular
occupation at, for example, the mouths of the Thames and the Hudson.