Art. XXXIV.—On the Application of Iodine and Bromine, for the detection of Gold when in minute quantities.
[Read before the Wellington Philosophical Society, Novermber 13, 1869.]
The large number of non-auriferous, or but slightly auriferous, specimens of quartz and pyritous rocks, which have lately been submitted here for examination for gold, has rendered it very desirable that some quicker, less laborious, and, if possible, more exhaustive, method of analysis, than the current one (that by amalgamation), should be employed.
In recognition of this I have frequently been urged by the Director of this Department to attempt some other process, and after several preliminary experiments I turned my attention, especially, to the use of iodine or bromine for this object.
Both of these substances differ from chlorine especially in their relatively feebler affinities for hydrogen, so there would be the less to fear, that from the generation of hydra-acids, any great preponderance of other matters would be dissolved along with the gold we wish to separate from the sample under examination.
Iodine, indeed, has already been used with advantage in the analysis of certain meteorites, for the separation of the iron and nickel existing therein in a metallic state; these it combines with, leaving the associated silicates, iron-oxides and sulphides intact.
It was this comportment of iodine with other substances, that determined me to the trial of both it and bromine for the purpose named.
The results of my experiments certainly show that either of these agents may be safely and advantageously employed for the separation of gold from its matrices.
The following are the particulars of a few of these experiments, which besides their present use, will, I think, be useful in showing what is, approximately, the smallest quantity of gold that can be positively separated and identified, by a certain course of analysis operating upon a limited quantity. The first time, I believe, anything of this kind has been attempted.
1st. 2 grammes of roasted “buddle heading” from a quartz mine at the Thames, known from previous analysis to contain gold at the rate of one ounce, or so, to the ton, was well shaken for a little while with its volume of alcoholic solution of iodine (tincture of iodine, of chemists), then allowed to subside. A piece of Swedish filter-paper was then saturated with the clear supernatant liquid, and afterwards burned to an ash; the ash, in the place of being white, as it would be if pure, was coloured purple; the colouring matter was quickly removed by bromine—a clear indication of the presence of gold. The time occupied by the whole process was twenty minutes.
2nd. 1 gramme of the same “buddle headings,” mixed with such a quantity of soil as to reduce the proportion of gold present to 2 dwts. per ton, was allowed contact with its volume of the tincture for two hours, with occasional stirring; a piece of filter-paper was then saturated with the tincture, and dried, five times consecutively, and finally burnt off as before; in this case, also, the colour of the residual ash was purple, and it gave the reaction of gold.
3rd. 32 grammes of siliceous hematite, finely-pounded, was thoroughly mixed with precipitated gold to the amount of 2 dwts. per ton; then ignited, and
treated with bromine water. After two hours the solution was filtered, and evaporated to a bulk of 20 minims; this gave a good reaction of gold to the “chloride of tin” test.
4th. 100 grammes of the hematite, with precipitated gold at the rate of ½ dwt. per ton, treated as before, but this time well washed, at the expiration of the two hours, and the washings evaporated along with the first filtrate, gave a fainter, but still decided, reaction of gold to the same test.
5th. Iodine, as tincture, substituted for bromine in experiments 3 and 4, gave similar results; the only variation made was, that as a precautionary measure allowing for its feebler, or rather slower, action, I gave contact for twelve hours.
To compare the results of the common amalgamating process with the foregoing, I have made some careful experiments; and I find that it is not certain, with the same expenditure of labour, to get reliable indications of gold, when present in less quantity than 2 dwts. per ton, operating upon about 100 grammes of material, which is about the quantity I usually take.
In summing up the results of these experiments, it appears then, that for qualitative examinations for gold, or for quantitative determinations in certain cases, iodine and bromine are each superior to mercury. It also appears that a proportion of gold equal to ½ dwt. per ton, upon a bulk of 100 grammes (about 4 ozs) of ferruginous matters, can be easily and rapidly detected.
Of course, by operating upon larger quantities, gold could be discovered by this process, were it present in far less quantities, but this is sufficiently near for the majority of cases.
These processes are especially adapted for the separation of gold from sulphides, as the preliminary roasting is extremely favourable to them, not so much chemically as mechanically, I think; the loss in the substitution of oxygen for sulphur, amounting to 25 per cent., by weight, while the volume remains constant (or nearly so); hence there is a corresponding porosity in the product, by which it is certain every atom of it is thrown open to contact with the solution of these agents.
This mechanical accessibility obviously cannot be taken advantage of by mercury.
With sulphides these processes are practically exhaustive, while, at the same time, the simultaneous extraction of other matters is avoided, or, at any rate, is so trifling, that the proper tests for gold can be safely applied directly to the concentrated solution.
Regarding the choice between iodine and bromine, I would prefer the former, when mere traces of gold and supposed to be present; or if the ore is in a finely divided state, as is generally the case when the matrix is iron pyrites.
In the roasting of such pyrites it is necessary to raise the temperature towards the end to a full-red heat, in order to decompose the ferruginous sulphates, since if these remained much iron would get into the solution.
In the case of much carbonate of lime being present, it is proper to gently reignite the roasted mineral, etc., with carbonate of ammonia, or much lime might get into the iodine or bromine solution.
On the other hand a very high temperature is to be avoided, for, from my own experience, I find a considerable quantity of fine gold can escape detection in this way, by the partial vitrification of the more fusible of the silicates.
The identification of gold by the combustion of its salts with filter-paper, as suggested in this paper, seems to promise a rapid method of estimating it, comparatively, by the aid of a series of prepared test-papers, representing gold in different degrees of dilution.