Art. XVI.—Notes on Quartz Crushing at the Thames.*
[Read before the Auckland Institute, 27th November, 1876.]
I Have recounted on a prior occasion the chief errors of quartz crushing as practised on the Thames. I shall now endeavour to show how a great many of them may be avoided even by mechanical means of manipulation, that is without using chemical re-agents. Battery managers in general know too little of chemistry to look with favour on any process of gold saving dependent on chemical means.
That a vast amount of gold is lost yearly all over the world for want of proper means of saving it, is only too evident from the accounts contained in all metallurgical books recently published that treat on gold. It has been ascertained that 50, 60, and 70 per cent. are common losses. That even this has been exceeded on the Thames, a case that came under my notice fully proved. The directors of the “Golden Crown” mine, which at the time I speak of was a large producer, were desirous of ascertaining the relative merits of two rival batteries, so as to get their quartz crushed at the better establishment. To accomplish this they sent 50 tons of quartz to each mill. Special care was taken that the loads from the tip were sent alternately to each place, so as to equalize the quality of the quartz as much as possible. In the one battery, as well as I can now recollect, the quartz was crushed with quicksilver in the boxes, passed over quicksilver plates, then through Chilian mills, then over short blanket strakes. The blanketings and what remained in the Chilian mills were then passed through the amalgamating barrel. The total product of the 50 tons at that battery was less than 25 ounces, or under one-half ounce to the ton.
[Footnote] * For previous Paper by Author see “Trans. N. Z. Inst.,” Vol. VIII., p. 176.
At the other battery the quartz was crushed with quicksilver, passed over quicksilver plates, then over shaking tables, then through Wheeler pans, and the amalgamation finished in barrels. The results from the battery and silver plates, from the shaking tables, and from the pans, were kept separate, making three distinct parcels. Each of these gave the astounding result of over one-half ounce to the ton, the entire crushing of the 50 tons amounting to nearly 80 ounces. But this was not the entire result, as the resultant tailings were allowed to settle in pits, and being treated in Wheeler pans at a subsequent date, yielded ten ounces more. Had not such a fact come under my direct notice I would barely have believed the statements of books. A great deal of the mischief is caused by the miners themselves, who having seen crushing done in other countries perhaps a few years ago, will not submit to any new form of manipulation, however good, unless it be as cheap as the old system, or as cheap as that of any other battery on the same field. In the instance above where the battery that had shaking tables and Wheeler's pans, had saved over three times the amount of gold, the cost of crushing would have amounted to twice that of the other, but as that would not be given in general, the shaking tables and pans had to be discarded so as to be able to compete with the neighbouring mills in price. It is the price of crushing that has been looked to, not the yield.
The great fault of crushing at the Thames is, I believe, using large quantities of quicksilver in the battery boxes; but to convince a Thames miner of that fact, would, I believe, be an impossibility. They are generally very prejudiced in their opinions of gold saving. I remember once a miner wished to bet with a battery manager that the quicksilver used in his battery was light weight, and that a bottle full of it would only weigh half that of a bottle fresh from the shop, and therefore not so good for gold saving as new silver. His reason for the assertion was, that, at the battery referred to, they did not replace their quicksilver often, and that it got lighter by constant use. As the absurd bet was not taken up, the miner took his crushing to another mill, imagining that, as the bet was not taken up, he was right. This may be an extreme case, and I mention it merely to show what the quartz crusher has to contend with occasionally. In general, the crushing batteries at the Thames are well built, and are effective. They will put through as much material and last as long as any others in the world. The groundwork, therefore, is good; but, in my opinion, the manipulation is defective. I have already mentioned that the system generally adopted now is crushing with a large amount of quicksilver in the boxes, and using very fine punched gratings. I consider the use of quicksilver in the boxes and the use of fine punched gratings are both erroneous.
The quicksilver and amalgam in the boxes, being continually battered, passes away as floured quicksilver, even floating on the water, the heavier particles rolling off the plates. This is amply proved by the great loss of quicksilver that is experienced constantly, and by the quantity that has been washed out of waste tailings. What remains in the battery is sickened, and is unfit for amalgamation, and thus allows the gold to escape. The reduced material then passes over the quicksilver plates, and then over blanket strakes, which are often washed. The blanket tailings are then amalgamated in large berdans, having a loose and a drag ball. In this process gold and quicksilver are not only lost, but a great deal of power is wasted, thereby increasing the cost of the produce of gold. It is difficult to reduce any material to a fine state by stamping—beyond a certain amount: in quartz, we may presume, to less than one-sixteenth of an inch in diameter; down to that it is comparatively easy, by the use of coarser gratings than those now used. It would be an easy matter to grind the coarse tailings by a separate grinding machine. I would further recommend the use of wire gratings, instead of punched sheets. Wire gratings, although not so strong as the other, will admit of a better flood, and the extra trouble of replacement would be well compensated for by the extra amount of work done. Another great fault is the amalgamating in the berdans, where great loss is experienced in gold, quicksilver, and power. The drag grinds the berdan more than the tailings, and converts a part of the quicksilver and amalgam into floured particles that readily pass away.
Having described what, in my opinion, are the defects of the present system, I would recommend the following treatment to utilize the present appliances, with some small alterations and a few additions. Crush the quartz (after it has been puddled to get rid of the clay that exists in mullocky leaders), coarsely, without quicksilver, in the battery, using wire gratings, having, say, 81 holes to a square inch. Save as much gold as possible by the quicksilver plates and ripples. The tailings should then be gathered in pits, and passed through grinding machines, or passed direct from the silver plates through the grinding machines. When the tailings are fine enough, they should be amalgamated in barrels, or any other gentle system of amalgamation. One berdan should be sufficient for a large establishment, and it should be reserved for the cleaning of amalgam only. The whole of this could be easily arranged, and it would cause less work than is now entailed in working a battery, and I am confident a great deal less power will be necessary, and more gold saved.