A satisfactory laboratory method for the amalgamation test must secure the following conditions: The individual attention necessary in the separation of mercury must be reduced to a minimum, and the entire manipulation must be as simple as possible. To this end a mechanical agitator must be used, and preferably one which allows of the bottles being introduced or removed while in motion; to avoid mechanical loss bottles with some rapid but tight closing device must be adopted. Time is saved on the whole by assaying the mercury rather than the tailing, provided sufficiently pure mercury can be prepared or obtained. Sufficient mercury must be used to allow of quick action and to avoid serious error in case a few centigrams are lost. The addition of chemicals is generally to be avoided, and sodium amalgam should be used only when absolutely necessary. The cost of materials and labor must not be excessive.
The method finally to be described was evolved after much experimentation with the above considerations in view. It was intended purely as a laboratory method, the shaker requiring power; it may, however, be used at any assay office, and at a pinch can be carried out wherever nitric acid and a blowpipe are available, as two bottles can be shaken at a time by hand. The cost for materials per assay, including gasoline or fuel oil. is well under 10c.
A convenient agitator is a stoutly built wooden box with 24 or more compartments, each 3 in. square and 6 in. deep, driven from a shaft running about 250 RPM, giving a horizontal travel of about 2 in. The entire bottom, or each section, should be covered with heavy belting or linoleum. The throw may be varied by means of a wrist-pin adjustable in several holes in a disk. The box may be supported by four vertical springs of steel, 1/8″ x 1.5″, 20 to 24 in. long, bolted to it and to two sills. One of this description has been in use over five years without repairs except for the replacing of a few bolts. The connecting rod is of oak, 2 by 3/4″. Where a Frue vanner is available, or any similar mechanism making 200 to 270 horizontal oscillations per minute of about 2″, it may be utilized for this purpose. In the case of a Frue vanner this is best done by attaching supports to the oscillating frame and carrying the bottle rack across the belt.
The average percentage amalgamated in some thousands of 100 gram tests differs very little from the average stamp-mill percentage of recovery for, the same period of years. Provided the samples are ground fairly fine at least to pass 80 mesh. The results, so far as my experience goes, have differed but little with the fineness, even if ground to 200 mesh. Presumably, with the ores thus examined, the free gold is mostly exposed by a moderate degree of crushing, occurring largely, as is the case with so many gold ores, along former fracture surfaces. ‘
This agitator and the magnesium citrate bottles have been found exceedingly useful for such work as preliminary cyanide tests; when the capacity of the bottle is too small it is convenient to take a number of portions and either make parallel tests or mix the products for assay.
Equipment Required for Systematic Amalgamation Tests:
- Agitator with 24 compartments, as described above.
- Twenty-four “citrate magnesia” bottles, with spring clamps and rubber washers; These have a capacity about 350 to 370 cc.
- Twenty-four Griffin beakers, capacity about 250 cc.
- Two enameled-iron pans, rectangular, with straight sides, to hold 12 beakers each.
- Filtering rack with 12 x 2 3/4″ funnels and 12 extra beakers.
- Hot plate.
- Glass-stoppered burette for mercury, standing over an enameled pan.
- Robervahl scale, turning with 0.5 gr.
- Weighing scoop and steep-sided copper funnel or hopper for charging ore into bottles.
- Cylinder, graduated to 100, 150, and 200 cc.
- Six-inch funnel for water.
- A few porcelain dishes about 3 in. diameter, and some enameled pans about 8 in. diameter and about 2 in. deep.
- Filter paper, 12.5 cm., S. and S. No. 595 or similar quality.
- Scorifiers, 2 in. or less in diameter; these may be used repeatedly and are best glazed before first using.
- Test lead.
- Nitric acid, diluted with about double its volume of water.
- Mercury, pure or previously assayed.
- Silver foil, cut in pieces of about 0.5 gr each.
Routine Method for Gold Only
One hundred grams of crushed ore or tailing are weighed out, charged into a bottle by means of the metal funnel, 150 cc. of water added, then 1.5 to 2 cc. of pure mercury from a burette. The stopper is then clamped, the bottle folded in a thick piece of cloth or flannel and put into the moving “shaker” for 2 hr. It is then removed, opened, covered with the thumb, shaken and inverted over a 3″ porcelain dish, and as much as possible of the clean mercury allowed to run out. It is then shaken again for a moment, and if necessary a little more water is added, and more mercury allowed to run out into a second dish, and so on, as long as any mercury comes out. If not floured the mercury is nearly all removed in two operations. The clean mercury thus obtained is transferred to a beaker. The bottle is again shaken well and inverted so as to let a little sand run out into the dish; this is then panned into an enamel dish. Usually only a few globules are obtained; if much is found the whole charge must be panned. If “floured” a small globule of liquid sodium amalgam should be added at this point.
To the mercury, after collecting in a 250-cc. beaker, is added g. of pure silver foil. If this has been carefully prepared and cleaned by treating with cyanide solution or weak nitric acid, or by slightly amalgamating the surface, it may be used to pick up the small globules of mercury obtained in panning. Nitric acid (about 150 cc. of sp. gr. 1.14 or 1.15) previously warmed to about 70° C. is now poured into the beaker; all the beakers are set together in a pan, which is placed on the hot plate and left until all the mercury has dissolved. If not too hot it is unnecessary to use covers. As soon as the mercury disappears the liquid is filtered, the residue is rinsed on to the paper, washed once or twice with very dilute nitric acid (not over 5%.) and once with water. The paper is then sprinkled with test lead, folded, and placed in a small glazed scorifier, and covered with lead enough to make up the total to 15 or 20 g. Enough pure silver is added to insure ready parting, and 2 or 3 g. of borax glass. The scorifiers are then charged into a muffle, the paper carefully burned, the lead scorified a few minutes, poured, and cupelled. The cupel buttons are parted in porcelain cups, and the gold annealed and weighed.
- A correction is subtracted for gold contained in the silver and mercury.
- Using a 100-g. sample, each milligram of gold represents you can calculate your costs.
If preferred 4 assay tons may be used, when each milligram indicates exactly 0.25 troy ounce; or 120.5 g., when each milligram indicates exactly the cost per ton. With these amounts the water must be proportionally increased, and the manipulation in the bottles is not quite as easy as with 100 g.
In ordinary routine work the tailings are discarded. For special purposes, as when they are to be tested by cyaniding, concentration, etc., or if the original assays are unsatisfactory by reason of the presence of coarse gold, they are carefully panned free of mercury and caught in one of the larger pans, allowed to settle completely, decanted, and if necessary dried for further tests or for assay.
In special work, and particularly in case there is a tendency to flouring, the mercury is weighed out to the nearest centigram (any sodium amalgam being similarly weighed and added to it), and the recovered mercury is also weighed after drying. The recovered gold, multiplied by the former weight, and divided by the latter, gives the total gold amalgamated. In ordinary work with finely ground ore the loss of mercury runs between 1 and 2 per cent.; and is less if the entire amount of tailing is panned.
The ore should be ground to a fairly constant degree of fineness. The product of a disk grinder, set for about 100 mesh for use in assay work, has been found to give sufficiently uniform results for most purposes. For careful comparative tests it would be preferable to sift to a definite size.
Using 100 g., 150 cc. water is preferable to 200 cc.; with less water the pulp is too thick. Material of unusually high or low specific gravity may require special attention as regards thickness of pulp. Reducing the time from 2 hr. to 1.5hr. generally gives good results; a less time is not safe; 1.75 to 2 hr. appear to be safe limits.
The temperature should be nearly constant from day to day; if much above the normal the results are somewhat higher; if it falls nearly to the freezing point they may be several per cent. low.
The concentration of acid recommended (one volume of concentrated nitric acid to two of water, giving about sp. gr. 1.14) was found advantageous after a number of tests. It allows of warming in advance so as to expedite action, without becoming too violent at any stage. The amount recommended leaves a sufficient excess to prevent separation of mercury salts. If concentrated during the heating it may be necessary to dilute slightly before filtering.
Addition of silver to the mercury reduces the time required for solution by about one-half, as determined by a large number of comparative tests. Commercial proof silver invariably contains gold; samples examined have carried about 0.01 mg. per half gram; or about the same amount as found in 20 g. of some of the best samples of mercury tested.
The correction for gold in mercury and silver is best determined by taking five times the amount to be used per assay, parting them together and carrying out the whole procedure exactly as in the regular work, but using five times the standard amount of test lead, scorifying off the greater portion. To avoid repeating this blank test unnecessarily it is well to have a considerable reserve stock of both mercury and silver. A thousand determinations using 1.5 to 2 cc. (20 to 27 g.) require 45 to 60 lb. of mercury, and about 16 oz. of silver.
With ores carrying less than a few $ of gold per ton it is advisable to weigh out two or more parallel lots and combine the gold after parting.
If amalgamable silver is to be determined in the ore it is best to make duplicate tests, parting one and scorifying the other.
- Stamp Milling of Gold Ores,
- Trans., m, 645 (1895).
- Trans., xxvi, 187 (1896),
- Metallurgy of Gold.
- Pacific Coast Miner, vol. vii, No. 2, p. 30 (Jan. 10, 1903).
- Mining and Scientific Press, vol. xcv, No. 10, p. 300 (Sept. 7, 1907). Mining World (Chicago), vol. xxxii, No. 6, p. 319 (Feb. 5, 1910). Testing for Metallurgical Processes.
- Cyanid.e Handbook (1910).
- Fulton’s Manual of Fire Assaying, 2d ed. (1911).
- Rand Metallurgical Practice, vol. 1(1912).
- Sampling and Assay of the Precious Metals (1913).
- Elements of the Art of Assaying Metals (Mortimer’s translation, 1764). Fire Assaying (1907).
- Notes on Assaying.
- Manual of Practical Assaying (1893).
- Notes on Assaying (1904).