Twentieth Century Inventions: A Forecast

by George Sutherland

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.