Art. XXXVI.—On the Absorption of Sulphur by Gold, and its Effects in Retarding Amalgamation.
[Read before the Wellington Philosophical Society, September 17, 1870.]
A Part of the duty assigned to me while recently on the Thames gold-field, was to investigate into the causes of the loss of gold experienced by the proprietors of the several batteries in that district, when working the auriferous material.
From an acquaintance with the various mechanical arrangements adopted there for securing the gold, and from a knowledge of the nature and amount of such loss, I was soon led to infer that much of it could scarcely be referred to any of the causes severally supposed to be operative.
The affinity of gold and its richer alloys for mercury is so great, and their negativeness, or unaffectibility, to and by the various chemical substances naturally having contact with them, so universally and so authoritatively affirmed, that it is generally supposed all we have to do in order to obtain an amalgam of gold and mercury, is to present them to each other, with their surfaces free from dust or stain, and to all appearance chemically clean.
But, in experimenting upon a few samples of the Thames gold, I found that though apparently quite free from any such stains, etc., they would not uniformly amalgamate over their entire surfaces; some, indeed, would not amalgamate in the least, though to all appearance as bright, as clean, and as chemically similar as those which did; the action of boiling water upon these did not in the least affect their negativeness to the mercury. This could not therefore be owing to the intervention of air mechanically adherent to them.
Observations taken since my return, show besides, that several of the cleanest-looking samples of the Otago gold which have been deposited in the Colonial Museum, manifest a similar negativeness to mercury—even to whole samples of fine alluvial gold, and those surfaces of small nuggets so situated that it was impossible they could have been hand-soiled.
All these specimens were rendered readily amalgamable by cold solutions of cyanide of potassium, nitric acid, chromic acid, or chloride of lime acidified, and, with the exception of the more cupreous of these, they were also rendered amalgamable by roasting them in an open fire for a few minutes.
Obviously, therefore, some foreign substance occupied these non-amalgam able surfaces, excluding the mercury from contact with the auriferous alloy, which substance was dissipated or decomposed by heat, and capable of removal by the several re-agents specified; the question then arose, What could this substance be? and I began to suspect that sulphur, in some form or other, was this substance, since its deportment under these conditions would certainly be very similar.
I therefore well cleaned the surfaces of several samples of Thames and Otago gold, and one of pure gold, and placed them for a féw seconds in sulphuretted hydrogen gas, well washed them afterwards, and dipped them into clean mercury, when, in the place of being instantly whitened over their whole surfaces by the mercury, they absolutely refused to amalgamate with it upon any part, and even seemed to exercise a repulsive force upon it.*.
The same effects, or rather non-effects, in relation to the mercury, followed when alkaline sulphides were substituted for the surphuretted hydrogen, also when the samples of gold were kept a short time in boiling water, in contact with sulphur—a platinum crucible being used as a precautionary measure against the introduction of alkaline matters. As in the case of the first series of specimens, these were rendered amalgamable by treatment with cyanide of potassium, nitric acid, chromic acid, or chloride of lime, also by the application of heat to the less cupreous ones.† † The effects of heat on those samples containing 6 per cent of copper (and perhaps much less) being to produce such a film of the oxide upon their surfaces that the mercury is still excluded from them, though from a different cause from that above mentioned. These samples also readily amalgamated with sodium amalgam.
Gold thus treated with sulphuretted hydrogen, sulphur, or alkaline sulphides, and thoroughly washed, then put in pure cyanide of potassium, gave a good reaction of sulphur to the nitro-prusside test; and I have also obtained, in the same manner, very distinct reactions of sulphur upon several of the native gold samples in the Museum.
These results clearly show that gold, even when in its purest state, is by no means so negative to sulphur and its compounds as is supposed, but that, on the other hand, it absorbs sulphur with great avidity; and they further show, that when this sulphur is thus absorbed by gold, even when only in very minute quantity, the metal refuses to amalgamate, although there is no visible change induced in its appearance.
Taken in connection with the presence of sulphur (or a compouud of it)
[Footnote] * This was illustrated at the close of the meeting
[Footnote] † The chlorides of gold and mercury, also nitrate of silver, have been found lately to have the same effect. See paper “On the reduction of various metals from ther solutions by metallic sulphides, etc.”
upon many samples of native gold, and the certainty that one or other of the sulphurizing agents specified does frequently occur naturally in metaliferous rocks, it seems highly probable that a large area of the natural surfaces of native gold is sulphurized, and thus rendered, according to the degree of this, non-amalgamable.
If this is so, a very sufficient cause appears for the heavy loss in the precious metal, experienced by the mill-owners in working their auriferous reefs by the amalgamation process, since it would only be those portions or grains thus sulphurized, which have chanced to get their surfaces abraded during their extraction or milling, that would be at all likely to adhere to the mercury used.
Whether this absorption is a purely mechanical one, as is assumed for that where platinum acts upon sulphuretted hydrogen and other gases, or whether it is a truly chemical one, is a matter of some interest to enquire into, particularly in connection with the, as yet, unsolved problem relative to the mode of solution and deposit of native gold; also, in relation to the question of absorption generally.
By what can properly be deduced from the facts above stated, and those which have manifested themselves to an investigation carried on especially to determine this point, it certainly appears that this absorption is the effect of chemical action.
Thus, if this absorption is mechanical, the sulphur must be in one of the two following conditions:—
As free sulphur.
Combined with hydrogen, as sulphuretted hydrogen.
I. That it is not as free sulphur was evidenced by the fact that boiling ether or bi-sulphide of carbon—two liquids having considerable affinities for sulphur—would not remove it from the gold; for, after long contact with these solvents and an after thorough washing, the gold still refused to amalgamate.
Neither of these liquids had any effect upon clean gold in regard to its behaviour with mercury.
II. That it is not combined with hydrogen, and thus condensed on the gold surface as sulphuretted hydrogen, appeared from the circumstance that sulphurous acid effected no apparent change on it; the action of this acid on sulphuretted hydrogen being a rapidly decomposing one, sulphur being liberated in a free state, capable of being detected and identified as in this state, which I could not accomplish.
Not appearing to be in either of these forms, therefore we must assume it to exist in chemical union with the metal as a sulphide of gold, forming a film of true auriferous pyrites, as was first suggested to me by Dr. Hector.
Independent of the proof derived from experiment, it may be expected that sulphur brought into close contact with a metal which we know does
form chemical union with it in a wet way, and at common temperatures, would then be in an extremely favourable condition for the exercise of chemical affinities; and the same argument applies for absorption being generally chemical, wherever there are affinities existing at the temperatures we employ between the absorbants and the absorbed substances.
Indeed, so far as these experiments and these arguments are deemed con clusive in favour of the absorption of sulphur by gold being chemical, by so much are we compelled to diverge from the received opinion that the absorption of the common gases by platinum is always a mechanical one, and are com pelled to distinguish varieties of absorption.
The affinities of sulphur, also oxygen, for platinum, are superior to their affinity for gold, why not therefore suppose sub-sulphides, or sub-oxides, to form, when these substances are respectively absorbed; but the whole question of these minute actions of metallic surfaces requires rigorous investigation, and for this I doubt not the mercury test here used for proving absolute cleanliness of surface will often prove very useful.
Reverting to that which more strictly falls within the scope of this paper, as sulphur has been found upon native gold, I should be quite prepared to find the metals tellurium and selenium also upon the natural surfaces of the gold of Transylvania and the Thames (these substances being isomorphous with sulphur and more fixed), as telluric gold is found in Transylvania, and the character of the two golds assimilates.
In conclusion, I would beg to observe the necessity of fully establishing the character of this absorption of sulphur by gold. If it is a chemical act, as present evidence tends to show, and sulphides of gold and platinum are so easily and so rapidly formed, we cannot doubt but that sulphur plays a very important part in the solution and translation of these metals from rock masses to intersecting quartz reefs, or from deep strata to superficial positions.
In whatever form, however, sulphur is thus absorbed by gold, it is certainly the greatest obstacle to a thorough and complete amalgamation which we have to contend with; no doubt other substances occasionally intervene to prevent or retard this process, such for instance as the oxides of iron and organic matter, but sulphur and its isomorphs must, I think, be the most actively and the most frequently concerned in this refusal of natural gold surfaces to amalgamate. In case further investigation should prove this sulphurization of the natural surfaces of gold to be general, it will be easy to look for a remedy against its effects; but, as yet, it would be useless to speculate as to what this should be. As stated, there are several ways of removing these films: perhaps chloride of lime in conjunction with muriatic acid would prove serviceable, but, unfortunately, it could only be applied to the “stuff” before amalgamation, and there might be a loss of gold occasioned by solution.
The sodium amalgam of Crooke would be a safe and certain remedy, and
easily applied; its effect would be in relation to this sulphide, to decompose it, and so expose the encrusted gold to the action of the mercury.
Doubtless the great benefit which has often attended the use of this amalgam has been principally due to the exercise of this kind of action.* *
[Footnote] * For further discussion of this subject, see N. Z. Geol. Survey Reports, 1870, p. 70.