LOSS OF GOLD IN MILL TAILINGS.

Mount Ida Chronicle, Volume V, Issue 273, 29 May 1874, Page 3

LOSS OF GOLD IN MILL TAILINGS.

MY. MacKellar lately made some rather startling statements in the Provincial Council with reference to the'loss of gold in the quartz mills of Otago through want of better scientific knowledge and sufficicient apparatus. It is of some interest to note the results of actual experiment, made by Mr. Skey, analyst, in 1870, on several samples of tailings taken from 31 mills at the Thames. We regret we cannot give the full report, with the very valuable tables interpolated. Those of our readers who wish further in forma lien can no doubt obtain, without much trouble, the small volume of " Reports of Geological F.xplnrntions, 1870-71," edited by the Director, Dr. Hector. Mr. Skey says, in the course of his enquiries, " it was found that great difference of opinion prevailed as to the actual amount of. gold that escapes with tho tailings from the mills,' and that: this, and several collateral questions, could only be determined by experiment." For this purpose a small laboratory was erected at Grahamsfown The points specially watched were— -."•, 1. The actual loss of gold in the course of the extraction of the metal from the ore at the different mills. ~ " 2. The cause of this loss, and how far it can be remedied. 3. The manner of combination of the gold in different forms of matrix. ' 4. The effect of the use of sea water, and water pumped from the mines, upon the machinery employed. lOSS OP MILLING. Mr. Skey notes :—Tho of'gold in the tailings from the mills I thought best to divide into that portion which, a more thorough amalgamation would readily catch, and that which could only be extracted by assay. So much 7 epends upon the manner in which the tailings are sampled, that it is proper to state first with what precautions the selection was made : , MODE, OF SAMPLING. All the samples of tailings examined, or here reported on,' were, with two or three exceptions, laken under my immediate personal superintendence. Only two or t!hree were taken from the settling pits, as tailings so deposited would be more than usually concentrated, and so vitiate the result of their analysis. The rest of the samples were collected either from the larger heaps of tailjngs which had accumulated in the yards round the various batteries, or else directly from the shoots discharging these while.the batteries were at work. In those cases where the samples were collected from the heap, each heap was sampled in twenty or thirty different places, as nearly uniform as possible, on all the exposed sides, five or six inches below Ihc surface. Where taken as issuing from the tailings shoot of the battery they were collected in the follow-, ing manner.—A well-cleaned enamelled bucket was placed under the shoot just long enough to get filled, or nearly so; the contents were then allowed to settle for two minutes, when the water and suspended matter was gently poured off, and the residue secured.' This process was repeated until enough had been obtained. When the stuff was crushed fine and a large head of water used sampling from the shoot was a very tedious operation, sometimes occupying for a single sample two or three hours. The value of the after results, however, for such cases will, of course, be correspondingly enhanced, since such samples more correctly represent the average of what passes' from the batteries day by day than samples which are more hastily taken from the heaps." All samples were well mixed and dried before examination; the results given, therefore, refer to the respective samples * when air-dried.. As regards the choice of sampling from the heap or from the shoot, perhaps the latter is the best, as it admits of the more accurate determination of. the actual loss of gold upon the stuff as it goes into the stampers, since we can allow with more certainty for the proportion of fine substances carried away in suspension by the water employed.

MODE OF DETERMINING THE YIELD OF GOLD AFFOEDED BY EE-AMALGAMATION OF TAILINGS.

For this a hand burdan was used, similar in shape and construction to the ordinary ones, and fixed at about the usual inclination. The rattf of delivery of the tailings to tho burdan,''and also the flow of water employed, was made as uniform as possible in ewry case. The following are the more important particulars relating to this process : Weight of sample ... 2to 41bs. Speed of delivery; ... l|lb. per hour Head of water ...;.;. 4 sails, per liour . Weight of mercury used., loz. per lb. of tnilings No chemical agent was used to assist in amalgamation by the burdan. It was found necessary to retort off the whole of the quicksilver from the several amalgams obtained, and not to use leather at all, otherwise the loss of gold which would have ensued must have seriously affected the correctness of the returns. Where : returns were either very large or notably small, and also in several other cases, their was verified by repeated experiment. I n a few cases the tailings from this re-amalgamation were collected and weighed and put back through the burdan, and ground up with |;a further supply of mercury in the same : I way as before,; when it was found that I stuff .yielding to the burdan .gold at the rate of 2dwts. lOgrs. per ton, yielded to : this second process not more than llgrs. [per ton, calculating upon the normal quantity taken. The, weight of tailings remaining in the setting box for the second amalgamation being out of 4lbs. . ; . . . ■".-:•• f We must now make a few extracts': briefly from Mr. Skey's . SUMMARY. OF RESULTS. 1. The amount of gold alloy separable by an almost exhaustive amalgamativc process from numerous samples of tailings taken quite indiscriminately frcm this field, has varied from a minimum of ldwt. 14grs.:to a maximum of loz. I9dwt. 18grs., as calculated upon a ton of the sample, while the assay yield has been from 2d wt. to loz. 9dwt. 2grs. of pure gold per ton for the same samples respectively. 2. The yield does not bear any fixed or definite ratio to that obtained in the first instance from the samp stuff at the batteries.

3. In respect to quality, the gold given up to this ye-amalgamation is generally! poorer than that which lias been taken afi the mills by the first amalgamation.', ■:;:;: J . . It would be difficult'to do more than make a vague allowance for this loss, and this is stated by several of the managers ufc from thirty In fifty per cent, upon' thej stuff'before crushing. ... to the reductions consequent upon making these allowances .(we have not had space to indicate these parties

culars) tlie loss of gold for many of these batteries amounts to. thirty or forty per cent, upon that,capable of being extracted by., careful and prolonged amalgamation, or to thirty-five and forty-five per cent, on the quantity actually present in the stone as shown by assay. Compared to the loss experienced on other Ooldfields where amalgamation is the process used, this does not appear excessively high. Still, it is very unsatisfactory.as regards our present, gold-saving arrangements to know that we are frequently throwing away nearly as much of the precious metal as is saved. In : nother article we will deal with the causes of this loss.