Art. XLII.—On Improvements in the Processes for Extracting and Saving Gold.
[Read before the Auckland Institute, November 15, 1869.]
The quartz crushing machinery at the Thames is very excellent, and the appliances used for saving the gold ore are, I have no doubt, on the whole, as effective as is possible, consistently with the rapidity and economy required.
Nevertheless one occasionally hears instances of exceedingly conflicting and unexpected results—thus I find in a paper published a few days ago:
“Clarkson's machine.—One parcel of calcined stuff from the ‘John O'Groat,’ weighing only 11 tons, yielded 11 oz. 4 dwt. retorted gold. Another parcel of 11 tons uncalcined yielded 22 oz. 14 dwt., and 2 tons uncalcined stuff, crushed in the single stamper, yielded 8 oz. of gold. The battery is now crushing a 30 ton lot for the ‘Pukehinau.’
I am informed that care was taken that these several parcels should, as far as possible, be of uniform quality. And, in this Institution, on the 5th October, last year, a discussion arose in which it was stated that the use of the water from one source, as compared with another not apparently purer, caused a difference of one-third in the quantity of gold obtained.* Nor is it at all wonderful that occasional failures to save the gold by amalgamation should take place, when it is considered that for the amalgamation to be complete, every particle of gold must be brought into actual contact with mercury, the minutest film on the surface of either metal being sufficient to prevent their union, a circumstance which may arise either from the sickening of the mercury, as it is called, or from the gold having become coated with some repelling film derived from the liquids—as in the case named by Mr. Whitaker —or from the gases evolved in calcination, as in the case referred to from the “John o' Groat.”
What then, in my opinion, is wanting is, not any improvement in the machinery and appliances now in use, so much as a ready and certain means of ascertaining by assay—which shall be trustworthy and at the same time not too expensive—whether or not the process is going on rightly, and all, or the great majority, of the gold present in the material is being saved; since if it should be found that appreciable portions of the gold are passing away with the tailings, it will generally not be difficult to ascertain the cause of the evil, and to apply an appropriate remedy, such as cyanide of potassium, or Mr. Crook's sodium amalgam, if the failure appeared to arise from the sickening of the mercury; or to a different treatment of the quartz, or the use of other waters, if the fault appeared to lie in their conditions.
The subject to which I wish to draw attention, and on which I would invite discussion, is the practical methods of assaying quartzoze and earthy matters supposed to contain small portions of gold. Now since the commonest observation shows that gold is never equally distributed through the mass, and since it is probable that after every care in pulverization the gold may still remain in particles of appreciable size, it follows that a very small sample can never be depended upon as representing the mass, and therefore that the delicate analytical processes of the chemist, which can only be used on very small quantities, are not adapted to the practical uses of the gold-miner. Now 1 oz. of gold to the ton is equivalent to 1 part in 32,666, or 1 grain in 4 ⅔ lbs. avoirdupois, therefore it is convenient to take this quantity, or an aliquot part of it, for the assay, and in practice I think one-fourth of it, or 1 ⅙ lb. will be found the smallest from which satisfactory results may be obtained, since in that quantity one-eightieth of a grain of gold will represent 1 dwt. to the ton.
[Footnote] * See “Trans. New Zealand Institute,” Vol. i., p. 72.
Assuming then that 1 ⅙ lb. is the smallest quantity upon which any assay can be satisfactorily made, it remains to be considered how such a quantity can be acted upon so as to separate any gold it may contain, with such precision as to attain certainty that the result shall be true to at least one-eightieth part of a grain.
Now the methods of assaying may be divided into three classes:—1st. Separating out the gold from the mass by a menstruum which will dissolve the precious metal, without acting on the earthy matter. 2nd. By resolving it into a form by which the earthy matter may be dissolved out, leaving the metallic matters free and in moderate compass. 3rd. Fusion of the whole with such a flux as would cause any gold or silver present to separate from the slag, either alone or in combination with lead.
This last practice, which is the dry assay proper, yields very certain and accurate results, but since the matters would require to be mixed with five or six times their weight of litharge, the quantities which I have stated as the minimum which could be used with advantage, would require the use of crucibles so large as to be exceedingly inconvenient and expensive; since a crucible could never be used twice.
The second method, as ordinarily practised by the chemist, of fluxing with carbonate of soda, would be still more inconvenient from the same causes, but a modification of the process, which I shall presently explain, appears to me the most likely one for easily obtaining reliable results.
In the use of the first method the menstrua which will dissolve the gold from the earthy matters are confined to two, viz., metallic mercury, and chlorine. Now the principal use for which assays of tailings are required, is to ascertain whether the mercury used on the ripple-tables and in the stamp-box has succeeded or not in dissolving the whole of the gold out of the material. If it has failed in doing so, the most probable cause of failure is that the particles of gold may have been coated with a film of some matter which prevented the contact of mercury with them. But if this is so, it is obvious that the same cause will be in operation to prevent combination when an assay is made by the same means, and that nothing could be more futile than to attempt to test the fact, whether the whole of the gold had been amalgamated by the mercury on the ripple beds, by repeating the very same operation on a sample, therefore I think it is demonstrable that any assay of tailings by amalgamation, is absolutely delusive and worthless.
The other method of solution, by chlorine, would be nearly perfect if the gold were in a state approaching to purity, or were it alloyed only with copper; but gold mixed with from one-third to one-half its weight of silver, as is the case generally with Thames gold, is precisely that modification which is insoluble in chlorine, the coating of chloride of silver formed, being sufficient entirely to protect the gold beneath it, from the solvent action of chlorine, unless the mechanical subdivision of the particles is absolutely infinitesimal.
We are thus left to the second method of dissolving off the earthy matter, and this must be done without the use of crucibles.
Now quartz is soluble in solution of caustic potash, at all temperatures, and at a temperature of about 300°, and upwards, the solution takes place readily and rapidly, if then the sample of earthy matters, mixed with about three times its weight of caustic potash dissolved in three or four parts of water, were placed in a clean iron vessel, in a steam chest, in which it could be subjected for two or three hours to the action of steam at a pressure of about 60 lbs. to the square inch—which corresponds to the temperature of 307° —the whole of the quartz, or at least with the exception of a few of the larger grains, would be resolved into a silicate of soda which would then readily dissolve out with hot water, leaving the gold and silver with oxide of iron and
alumina, or any other basis present, which would remain on the filter. A little dilute hydrochloric acid would readily dissolve out all except the precious metals, and these could then be either operated on analytically, or wrapped in a little lead foil and collected into a button on the cupel. The same thing might be done by placing the material in an ordinary mercury bottle, and subjecting it to heat in an oil bath; unless, however, good means were adopted for regulating the heat so as not to exceed 350°, or a pressure of 120 lbs. on the square inch, a danger of explosion might be incurred.
The solution of quartzose matters in this manner has been frequently employed, and I do not apprehend the smallest difficulty in its use; if iron pyrites were present in larger quantities, the sulphuret of sodium produced might possibly dissolve a little gold, unless a little nitre were added.