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 £4,000,000 worth of it, while the little gift which the Queen of Sheba brought him was of the handsome value of £600,000, 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.