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.