To solve the problem of treating and leaching a clay ore in which the gold was finely disseminated, the author was able to suggest a unique method which he had seen described twenty-five years previously in a paper, “Occurrence and Treatment of Gold Ore at Bidi, Sarawak, Borneo,” by T. C. Scrutton, which appeared in Trans. 15, I. M. M., 1905-1906, in which clay ore was treated by leaching in relatively large lumps. Its successful application in this instance is our justification for including a detailed description of the method in the present volume.
The auriferous deposits at Bidi consist of a series of unconnected hills lying upon the weathered surface of limestone. These hills consist of clays and earth containing boulders of stone of varying structure—pure silica, silicified sandstone and shale. The clays and earths carry from 2 to 15 dwt. gold per ton, averaging 5 dwt.; the stone, from 3 to 30 dwt. per ton, averaging 8 dwt. The proportion of stone to clay occurring in the deposits is roughly 1 to 5.
The ore is trammed to the six ore bins. The clay ore bins are four in number, and their arrangement constitutes an important feature in the direct treatment of clays.
To obtain rapid and easy leaching of the clayey material, it is necessary:
- To keep the fine earthy and sandy material separate as far as possible from the more plastic clayey material and to treat the two separately in different vats.
- To ensure that all earthy and clayey material, when charged into the vats, is in the form of balls firm enough to maintain their form when charged into the vat and of a size varying from 3 in. in the case of the clayey material to ¼ in. in the case of the finer.
The first of these conditions is attained by providing four separate clay bins and carefully selecting the material from each according to its tendency to break up and form fines or to agglomerate and form balls.
The second is provided for by running the clay from the tippler down to a fan¬shaped chute, about 40 ft. long and inclined at 60 deg. into the ore bin; thus the masses of clay are broken up and formed into balls by rolling down the chute. The wetter and more plastic the clay treated the longer and steeper must this chute be to ensure the clay’s being in a leachable condition on arriving at the ore bin. The clay ore bins are rectangular and discharge through bottom sliding doors into wagons running below.
Vats
The leaching vats are 30 in number, 18 of 100 and 12 of 50 tons’ capacity ; the former are 6 ft., and the latter 3 ft. deep, all of 27 ft. 6 in. diameter. They are constructed of 3/8-in. mild steel plates, riveted with ½-in. rivets, 1¾-in. pitch.
Charging
Side-tip wagons are employed and are filled direct from the stone and clay ore bins, whence they are run direct over the vats and tipped; two pairs of rails run over each vat, arranged at such a distance apart as to require a minimum amount of shoveling to level off the material in the vat.
The following are the principles regulating the method of charging:
- Clay must be charged in layers not more than 3 ft. deep; if this depth be exceeded, difficulties are experienced in leaching and washing, resulting in impaired extraction.
- Coarse material must be kept separate from fine to ensure good leaching.
- In the event of charging two different classes of stone and clay into the same vat, the operation must be conducted so as to leave the material as far as possible in horizontal, not vertical, layers.
In the case of the 50-ton vats, which are only 3 ft. deep, they are filled with clay from one ore bin, no further precautions being necessary.
With the 100-ton vats, in order to treat as large a quantity of clay as possible, sufficient clay is charged into the bottom of the vat to form a layer 2 ft. deep; this is then levelled off, and sufficient stone charged to form a layer 1½ ft. deep; this having been leveled off, the vat is filled to the top with clay, giving an upper layer of 2 ft.; thus the layer of stone, by forming a porous bed in the middle of the clay, prevents the formation of channels throughout the whole mass and, by separating the clay into two thin layers, renders leaching comparatively easy; by charging in this manner equally good extractions are obtained from the clay treated in the large as in the small vats.
To ensure obtaining the correct tonnage, it is necessary to fill the vats until the clay stands about 2 in. above the top of the vat, as the material when properly charged lies exceedingly loosely and, after solution has been on for a short time, sinks down to 9 in. below the top of the vat.
Discharging
This is effected in the usual manner by shovelling the material through four bottom-discharge doors into wagons running on two lines of rails below the vats.
Treatment. The usual treatment for 100-ton vats occupies 10 days and gives the following results:
Charge……………………………………………..5.4 dwt.
Discharge………………………………………….1.2 dwt.
Extraction…………………………………………78 per cent
KCy consumption…………………………….0.97 lb. per ton
The solutions used are:
Strong solution, containing……………………………………….0.07% KCy
Sump solution, containing………………………………………..0.05% KCy
These solutions have to be kept at a definite standard of alkalinity, which is effected partly by putting a certain quantity of lime into each wagon at the bottom of the main incline and partly by the addition of soda to the sumps; the quantities of each used are regulated by the working of the zinc boxes. The average consumption of lime is 8.8 lb. per ton, and the maximum amount of soda usually added 0.14 lb. per ton of solution.
So long as this standard of alkalinity is carefully maintained, the solutions give very little trouble. If, however, the alkalinity gets too high, the solutions become dirty, foul smelling, and full of arsenic and antimony; if too low, the consumption of cyanide is so great as to show no precipitate with AgNO3 after once running through a vat, extraction, of course, suffering accordingly.
The first filling of strong solution is put on by very slow upward leaching, so as to disturb as little as possible the fine material lying loosely in the interstices between the larger balls of clay. The vat is gradually filled by this means, the operation, if properly conducted, occupying about 3 hr.; as the charge in the vat becomes soaked in solution, it settles down, finally sinking to about 9 in. below the top. About 30 tons of solution is required for the first filling, but of this only about 20 tons can be drained off, the remainder being absorbed by the clay.
The vat when full is shut up and allowed to soak for 4 hr.; it is then opened and allowed to drain at such a pace as to ensure its being just dry in another 4 hr., when it is pumped up again, the solution being run on to the top of the charge, and the vat leached by gravity in the usual manner. This alternate filling by downward leaching, soaking, and draining is carried on until about 150 tons of strong solution has been put on the vat, i.e., for about four days.
Sump solution is then substituted, the first filling being put on by upward leaching, and the subsequent filling by downward leaching, in the usual manner. The effect of using slow upward leaching in the middle of the treatment is to lighten the charge and form new channels for solution by altering the direction of pressure.
The downward leaching is continued as before until about 180 tons of sump solution has been given. This usually occupies about 5 days; the actual time of treatment, however, is judged by the amount of gold extracted according to the solution assays. When the solutions coming away contain only ½ dwt. per ton of gold, a final water wash is given, and the vat discharged.
In order to ensure a regular flow of solution through the zinc boxes, it is necessary to divide the vats into two lots, one lot being drained while the other is soaking.
After repeated trials the foregoing method has been found to give the best results for clayey material charged direct into the vats. Given that the material has been properly charged into the vat, solution of the gold takes place almost immediately on contact with the fresh cyanide solution, the metal being in an extremely fine state of sub-division, and lumps of clayey material containing only about 16 per cent moisture being readily permeable by solution.
Practically, the total gold contents of the clays are dissolved by the cyanide; this was shown by taking a number of samples of the material discharged from the vats and applying repeated washings of water; by this means alone a final extraction of over 95 per cent was obtained.
The problem, then, in order to ensure good extraction, is to wash out the auriferous cyanide which has been absorbed by the balls of clay. This cannot be effected by direct washing, in the ordinary sense of the term, the lumps of clay being only very slightly pervious and the interstices forming easy channels for solutions, but it must be brought about by diffusion between the solutions rich in gold remaining in the dissolved clay and the solutions containing practically no gold being pumped into the vats.
In order to obtain the best results, the point to be aimed at is to give the charge as much fresh solution as possible, consistently with sufficient time of soaking to allow of a certain amount of diffusion between the fresh solution and the auriferous solution remaining in the clay; prolonged soaking has not been found to give good results, doubtless owing to the slower diffusion of liquids carrying nearly the same quantities of gold in solutions, as compared with those differing widely in gold values. There is, however, a decided limit to the amount of solution which can be used, as it has been found that, unless a sufficient time of soaking be given to allow the new solution to permeate the whole of the charge, the extraction suffers considerably, the new solutions coming away by certain easily formed channels and absorbing very little of the richer solutions contained in less readily permeable parts of the vat.
Likewise slow draining off, i.e., at the rate of 5 tons solution per hour, is a necessity. If this rate be exceeded, a much larger proportion of moisture remains in the vat, doubtless on account of the solution’s descending too quickly into the vat to allow the small particles of solution lying in the interstices to agglomerate and descend with the mass of liquid; they are accordingly cut off by the air and left.
Extractor House
The precipitation is effected in the usual manner by zinc shavings, the zinc boxes being 16 in number and containing in all 936 cu. ft. zinc. At the rate of flow given this allows 1 cu. ft. zinc for 1 ton solution per 24 hr.
Considering the large amounts of antimony and arsenic contained in the charge, the precipitation gives very little trouble, though from time to time the solutions become abnormally foul and the precipitation is impaired.
When working satisfactorily, 0.5 per cent cyanide solution entering at 2 dwt. per ton is reduced to 3 grains.