Art. V.—On a new mode for the direct Desilvering of Argentiferous Gold.
[Read before the Wellington Philosophical Society, August 25, 1868.]
The tedious nature of the method at present employed to desilver argentiferous gold, was particularly forced upon my attention a few weeks ago, on reading an account of a new patent process by which the silver is removed from the alloy by chlorine, ejected into the mass while in a molten state; dependence being placed on borax, as a preventitive to the volatilization of the chloride of silver thus formed. Having doubts as to the complete prevention of this volatilization by borax, doubts which grew into positive disbelief before the results of experiments made in this direction; and thinking that there must be a want felt for a shorter method than that now in use, I sought for one which should, as in this patent process instanced, also effect a direct desilvering, but without producing a silver compound so volatile as to necessitate similar precautionary measures.
The most promising course apparent, was in some way to take advantage of the powerful basic properties of oxide of silver, as compared to those of oxide of gold. Silver is however, I believe, still considered to be quite unoxidisable in the dry way; its oxide, indeed, and its salts with volatile acid, suffering complete decomposition at elevated temperatures, but it occurred to me, if, together with nascent or feebly combined oxygen a free or loosely combined unvolatile acid were presented to the molten alloy, that the oxidation of the silver would be then effected.
The well, known fact of the absorption of oxygen by molten silver,
and its liberation as the metal cools, was assuring; for though the quantity thus absorbed is so small, as to forbid the idea of this absorption being a chemical one (27 equivalents of silver only absorbing 1 of oxygen), still I was convinced, from this, that any oxygen I might liberate in contact with the silver, would, to a certain extent, be retained by it, and in a condensed form; consequently, in a manner most favourable for subsequent chemical combination therewith.
Acting on these suggestions, after several preliminary trials, I selected bichromate of potash, as being able, singly, to supply both these desiderata: namely, nascent oxygen, and a loosely combined acid of some fixity; this salt being slowly decomposed at a white heat, with liberation of oxygen, sesquioxide of chromium, and chromate of potash.
A piece of stout silver wire was ignited with this salt, for one quarter of an hour, the fused mass allowed to cool, and then examined, when an abundance of chromate of silver was discovered to have been formed, while the wire had been greatly corroded and reduced to half its bulk.
A second piece of silver wire was completely dissolved in a subsequent fusion with the chromate, for an equal time.
Gold (and platinum) treated in like manner, refused to dissolve, at least to any perceptible extent.
It is therefore certain that in bichromate of potash we have a means of separating silver from native gold, at least to some extent, but to what extent could not well be determined in the laboratory. In any case, the method is well worth a trial by assayists and bank managers.
The manner of application is, simply to place the salt upon the surface of the molten metal. I should not consider it at all necessary to attempt their mechanical admixture, as the silver being taken up from the top, currents would be produced in the metallic mass, and kept up with gradually decreasing force, until the whole of the silver was removed.—possibly an intermittent agitation of the saline portion might be found advantageous.
To prevent loss of silver by volatilization, it is indispensable that the bichromate should be free from chlorides.
Besides the separation of silver from auriferous alloys, this method is equally effective in removing copper and iron, when present. To ensure, however, the continuous fluidity of the saline stratum, the application of borax might be continued, but nitrates, or caustic alkalies, must be rigorously excluded.
The silver is easily recoverable, as the chromate being soluble in water, it may be dissolved therein, the silver precipitated by hydrochloric acid, and the resulting chloride of silver finally decomposed by soda.
The quantity of bichromate theoretically required to oxidize the silver, and form with it this salt, is 5 equivalents to every 3 equivalents of silver—or, weight for weight, one pound of silver would require two and thirty-four-hundredths of a pound of the bichromate; but in practice the quantity required would no doubt be more, as some of the oxygen would be liberated out of contact with the alloy. For this reason, it would be best to add the salt in successive portions; and for the same reason, it is probable that the process would be found more adapted to the separation of silver, when present, in proportionally small quantities.
Allowing, however, an equal weight of the salt for loss of this kind,
and taking its retail price at one shilling and sixpence per pound,—as per Jackson and Lownsen's Price List,—every pound of the silver would incur a cost, on this head, of seven shillings for its removal.