AMALGAMATION OF GOLD.

Tuapeka Times, Volume III, Issue 209, 1 February 1872, Page 7

AMALGAMATION OF GOLD.

CONTINUATION OF MR. SKETS REPORT.

CAUSES OF LOSS.

The principal cause of loss may shortly be stated, for this and other goldfields where amalgamation is relied on, as the want of perfect chemical contact between the gold alloy and the mercury employed.

However much of this may be due to the defects in mechanical arrangements adopted for securing the gold, the result of my investigations certainly show that most of it is fairly attributable to causes of a chemical nature, and which the very best mechanical manipulation could but partially overcome.

These may be best discussed and explained so far as possible, with our present imperfect knowledge, by dividingthem into those which especially relate to the quicksilver, and those which relate to the gold alloy.

1. Regarding the quicksilver employed for amalgamation. That this substance generally sickens or "flours" when ground up with pyritoua rocks, is so well known and frequently experienced by all managers of batteries, that it would be superfluous to point out that, the true chemical contact we seek is much impeded or even entirely prevented by such a circumstance.

But as several bodies naturally occurring in such rock are capable of inducing .this unfavourable state of the mercury, it may be useful, with a view to the adoption of remedial measures, to explain what is, I believej the general cause upon this goldfield.

Whatever has been affirmed about the metallic arsenides or sulphides, sulphur and sulphuretted hydrogen, being frequently or directly active in producing this flouring, I feel convinced the greatest enemy that has to be contended with on this account on this goldfield is protosulphate of iron, or green vitriol.

I do not know that these arsenides or sulphides exercise any direct effect upon mercury ; it is only the products of their decomposition which appear so to act. And this decomposition is so slow comp&red to the ordinary speed of amalgamation, that ere it has advanced far enough to practically affect the yield, the whole of the auriferous stuff would have been passed through the mill., *

As regards sulphur and sulphuretted hydrogen, the former has not been identified at the Thames, I believe yet, in a free state, and even if it were present, by reason of its insolubility, and comparative non-volatility, its effects upon mercury would bellow. The latter is r on the other band, very rapid in its effects ; but its existence is incompatible with the presence of sulphate of iron, a salt pretty generally diffused iv welk and shafts, and porous pyritous rocks occupying superficial situations.

Taking all this into consideration, I incline to the opinion, that not only in this, .but in other gold fields, the greater part of the flouring or sickening of the mercury used is due to the action of sulphate of iron.

Another obstacle to amalgamation is sulphate of copper (blue vitriol), but it occurs far less'frequently. The Silver Crown, the Golden Crown, and Long Drive claims, and other claims around these, being the most likely places for it. This salt, however, is not more to be dreaded than the sulphate of iron, indeed, where the copper salt occurs, the iron salt will also occur, and in preponderating quantity.

Both these salts act, I believe, by forming sulphates of mercury and iron or copper, which persistently adheres to its surfaces. 2. Want of contact, owing to the composition and properties of the gold alloy itself.

In regard to the average composition of these alloys, and analysis of four Thames specimens, from so many different localities, showed that in three of them the metal antimony occured iv very sensible quantity.

Now I find by direct experiment, that antimony is not a metal which readily amalgamates, it is pretty certain, therefore, that it would com"municate something of its negativeness in this respect, to whatever gold alloy it might occur in; indeed, I have observed from iny*bwn experiments, the very slow rate at which these kinds of alloys do amalgamate.

Regarding the pioperties of these gold alloys, themselves, in more intimately affecting the chance of true chemical contact, a hitherto quite unsuspected one has been discovered in. the course of these investigations. It has become apparent that the surface of native or artificially-prepared gold x even of extreme fineness, energetically absorbs sulphur orsulphuretted "hydrogen.

When these surfaces are so affected, they absolutely fail to have such contact witli mercury as to enable the metals to amalgamate, at least such, is the case at common temperatures, but they again do so readily when treated with cyanide of potassium, free chromic acid, or chloride of lime.

Raising their temperature to a i*ed heat, also renders them amalgamable, whether or not cooled before the introduction to the mercury, but the heat of boiling water does restore the affinity of the surface for quicksilver. several specimens of native

gold from this district, and also from others where the gold is exceedingly pure, show no indisposition to amalgamate on contact with mercury, while others are only very partially affected. Nor is .it promoted or facilitated in any instance by giving them contact for a lengthened time with boiling water. So that this nonamalgamation cannot be referred to the intervention of bubbles of air adherent to their surfaces.

In these cases, as in those previously cited, contact with solutions of cyanide of potassium renders them easily and quickly amalgamable, and so also heating them, except where very cupreous, when the oxide of copper forming by contact with air at high tempera cure, acts as an invisible varnish.

All the specimens chosen for experiment were free from any discernible ferruginous or other stain,

Comparing -the re-action of artificially sulphurized gold with that of various specimens of (native gold, lead us to assume, that in 'many cases where the amalgamation of these specimens is slow, or does not takeplace, the obstacle is sulpher, most probably existing in combination with gold as an auriferous sulphide.

The bearing of the results upon the subject under investigation is obviously •important, as the exercise of this hitherto unsuspected property of gold must have a very prejudicial effect upon the amalgauon processes as practised in all reef workings.

Sulphnretted hydrogenis too common a substance, or at least the materials required for its generation are too widely spread, to allow us to hope that any larger portion of the exposed superfices of our native gold has not been affected by it to a greater or less extent, and so rendered to a corresponding extent unamalgamable.

REMEDIES AGAINST LOSS.

1. Relative to the general mechanical arrangements of the various batteries ; it would not be easy to propose any improvements in these ; and any new ideas on this subject, the fruit, of such a cursory or superficial inspection as I had an opportunity of making, could scarcely have any practical utility.

The absorbent property of gold for sulphuretted hydrogen, and the great likelihood that much of the Thames gold is as it were thus tarnished with sulphur, suggests the prolonged grinding of the auriferous rock, as a partial remedy, not so much to ensure fineness, as to ensure ahrision of the enclosed gold granules.

The results sbown by the use of the hand burdan, informs us not only how can be extracted by prolonged grinding certain conditions, but that most of the residual gold from the batteries is in a free state.

Doubtless the good results are mostly attributable to the greater extent of new clean surface thus exposed to the mercury. Whether any extensive change In favour of apparatus having greater grinding action could be profitably made, is a question which experience alone can certainly decide, but I think it very probable.

The Chilian mill seems too slow, besides, much complaint is made that the loss of mercury is very great by this mill. With the burdan, though certainly an excellent apparatus for righ hand samples or blanketings, there is evidently on the part of the few managers who use them, such a disposition to run them either too fast or with too much water, in fact to give them too much work, that one does not like to recommend them at all. They are rather deficient iv grinding power for profitable use on a large scale as at present constructed; but if some means could be devised for increasing this considerably, say by fixed shoes instead of balls, or a mixture of the two, they might be found to work well.

Concentration of the heavier portions of the auriferous rock, and the treatment of the residues, by special process, certainly merits great attention.

2. Losses occasioned by the condition of mercury used for the amalgamation. ' .

As the principal cause of the flouring or sickening of mercury upon these goldfields is the presence of sulphate of iron in the reef stuff, and as this salt is freely soluble iv water, it will naturally occur that one way of preventing the mercury being affected iv this manner, is to wash the auriferous stuff impregnated with this salt, prior to stamping. Whether such a process could be profitably used is a question for experience to determine. It could only be applied with any considerable saving effect in those cases where the reef is porous and incoherent. To a certain extent, where good water is used for the stampers and tables, the ill effects of the metallic sulphates present in the reefs are diminished.

In reference to the kind of water used for these purposes, I would desire here to remark how necessary it is to abstain from the use of mine or shaft water for the crushing or amalgamating apparatus of the battery, except, indeed, it is quite, or at least nearly, free from metallic sulphates, a circumstance rarely occurring. The use of water for these purposes containing any notable quantity of sulphates, acts even more prejudicially than the sulphates naturally existing in the stuff itself, even were they ever so abundant j . flouring in such cases

becomes, as it were, a chronic complaint. The greatest possible quantity of good wafer that the incline of the copper plates will safely allow, should be employed, since the solution of these metallic salts is thereby weakened, and so correspondingly less productive of harm. In a manner more strictly chemical, it is easy to destroy or overpower this tendency of mercury to flour in presence of these or any other substances, but for stampers and tables, where such immense quantities of matter are used, it would scarcely pay to manipulate the water itself.

(To be continued.)