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.