THE FOLLOWING INFO IS
PRESENTED FOR ITS HISTORIC VALUE. PROCESSING WITH MERCURY IS NOT
RECOMMENDED.
Loss of Gold in Amalgamation and Its Remedies. The loss of
gold in amalgamation may be due to:
1. Free gold included in or surrounded by the gangue rock.
2. Gold that is chemically combined in the tellurides or mechanically
enclosed in the sulfides.
3. 'Float' gold that is carried along on top of or suspended in the pulp,
and which does not come in contact with the amalgamated plate.
4. 'Rusty' or coated gold.
5. 'Overstamping.'
6. Poor amalgamation due to the methods in use.
7. Poor amalgamation due to deleterious substances in the ore.
First: If the loss is in the free gold included in or surrounded by the gangue rock, a sizing test will reveal it by the higher value of the coarser sands. In some cases the coarser sands can be crushed finer in a hand mortar and panned to show a 'prospect' of free gold. The correction for this is to crush finer, not the ore in general, but these coarser grains. This crushing is better accomplished by using a finer screen with the same or a lower height of discharge. Running two batteries with different size screens in competition with each other and comparing the tailing assays will determine in an empirical way, but sizing tests in connection with this is necessary for a true diagnosis of the conditions.
Second: Gold in association with tellurium is chemically combined and can only be saved by cyanidation along special lines, by chlorination, or by smelting, and rarely by concentration. Gold in the sulfides is mainly in a free state, finely divided, and mechanically held by the sulfides. This gold is usually saved by concentration, and in some cases by cyanidation of the tailing without concentration. However, a part of it can be amalgamated, as in the Gilpin County practice, by the use of -a wide mortar, deep discharge, long and slow drop, and an attempt to catch the gold inside the mortar. Here the sulfide because of its higher specific gravity sinks to the bottom and is held longer in the mortar than the balance of the pulp, allowing the gold to be liberated by the thorough sliming of the sulfide, and to be brought in long contact with the mercury and inside plates. This process has had slight application outside of the locality mentioned, where it was necessitated by a large proportion of the gold being contained in the sulfide that was of too low a grade to ship and smelt profitably. Part of the loss in the tailing may be due to sulfide crushed so fine (slimed) that it cannot be caught on the concentrators ; this will be considered under 'Over stamping.' The amalgamation of the gold in the sulfide has been accomplished by grinding it in amalgamating pans, tube-mills, or arastras, but the extraction has never been high, so that it is now preferable to cyanide it if it contains no interfering elements, or to ship it to the smelters. However, there are local conditions or peculiarities of the metallurgical practice under which it may be highly advisable to reduce the value of the sulfide to be shipped, smelted, or cyanided by preliminary fine grinding and amalgamation. And this question should be thoroughly investigated, both in existing plants and in designing new plants.
Third: 'Float' gold really refers to that gold which occurs in flakes so light and thin that it is floated along on the surface of the pulp, perhaps buoyed up by a bubble of air ; but in most cases it will be found to be gold so fine that it is carried suspended in the pulp and gets no opportunity to come in contact with the amalgamated plate. When an ore containing visible gold is pounded up in a hand mortar and panned, the gold is found to be pounded into scales or into infinitesimal bits, depending on the nature of the gold. It is doubtful if much gold is over stamped to an extent producing scales so thin that they will float on the surface of water like gold-leaf, although such gold has been found in both mills and placers; but it can be understood that gold which is powdered fine may be carried along in the pulp clear of the plates, though it really does not float. The loss attributed in a tentative way to float gold is found by assaying the flocculent slime of the tailing. Should this show a value as high or higher than the sand, it would indicate over stamping, and adjustments calculated to prevent this should be made. A part of the value lost in the tailing and assigned to float gold is in the slimed sulfide, but only a part of the loss can be rightly ascribed to this. Perhaps the easiest explanation is to say that the loss consists of gold, sulfide, and amalgam in microscopic particles, which is both coated with slime to an extent that prevents it from amalgamating and is held suspended in the pulp by the slime. Increasing the grade of the plates and using as little water as possible in the mortar to secure a better wave motion and contact between the pulp and the plates, together with making the plates longer and keeping them covered with a bed of soft amalgam, will aid in saving more of the fine and float gold. If the battery water is being re-used it should be well settled, for as the water or pulp becomes thicker and more slimy it will carry off more of this light gold. Patent amalgamators for catching this kind of gold should be tried. It is practically impossible to determine the form or condition of the gold in the slime where the amalgamation and concentration has been capably done, or to hope to promote any further extraction by laboratory and amalgamation tests; reliance must be placed on less sliming and on cyanidation.
Fourth: “Rusty” gold is free gold covered with a film of some substance other than air or the gangue rock in which it is contained. This may be due to an oily or greasy mineral peculiar to the ore, like graphite ; to an oxide of iron or copper, or to other compounds of the base metals; to silicates of magnesia or alumina; or to slime arising from crushing the ore. This film prevents the gold from coming in direct contact with the mercury. Gold to amalgamate must be clean, so that it may be readily wetted by the quicksilver. When gold is dirty, rusty, or coated, and the contaminating material does not amalgamate with quicksilver, then the quicksilver through its surface tension is negative and strongly repellant of the contaminating material and its more or less enclosed golden grain. Rusty gold can sometimes be detected by panning the tailing and examining the concentrate with a microscope. This gold should be caught by the concentrators, or in the cyanide plant if not too coarse, also by the use of riffles or blanket tables. To amalgamate this gold it must be made clean by being scoured. This can be done by using a high discharge, preferably with a coarser screen and a narrow mortar that the tendency to over stamp may be reduced. With a low-discharge, rapid-crushing mortar, the ore is in the mortar an average of four or five minutes, this length of time can be doubled or trebled by increasing the height of discharge, so that the particles of gold, especially the heavier ones, are subjected to the scouring and attrition of the stamp and the pulp for an increased length of time. Sodium amalgam in the mercury should be tried. If any of this gold is retained in the traps, they should be cleaned often, perhaps at each plate dressing, the sand being ground in the clean-up barrel with some additional mercury, to scour and amalgamate this gold. Theoretically, coarse crushing in the mortar followed by regrinding and amalgamating in a pan or roller mill should be successful. It is considered that the pan amalgamator will amalgamate gold that no other method of amalgamation will save.
Fifth: “Over stamping” is holding the pulp longer in the mortar subject to the action of the stamps than is necessary, thereby pulverizing it finer than is required or is beneficial. While the capacity is reduced, the term properly has no reference to that, but to the treatment the ore receives causing it to give a reduced extraction. Experiments have shown that a hammered gold is not readily amalgamable, and further experiments tend to prove this to be due to the gold being covered with a film of dirt and grease in the process of hammering, which, in connection with its increased density, does not allow it to be so easily wetted by the mercury. As has been observed under float gold, it is improbable that much gold which can be hammered into a scale is rendered non-amalgamable by stamping in the mortar, especially in the presence of mercury; while there is no doubt that gold in the allotropic form of being brittle can be stamped so fine that it is hard to catch in the mortar or on the plates, particularly if it is covered with slime. The danger of over stamping is augmented with increase in the grade and percentage of the sulfide. The Gilpin County, Colorado practice is an ideal illustration of over stamping sulfide. As to whether the ore is being over stamped or not is judged from the sizing-test assays taken in connection with the tonnage and operating expenses. If the assays of the slime and fine sands closely approach or are higher than those of the coarser sands, adjustments should be made that will reduce the percentage of fine material in favor of a higher tonnage. If it is an actual case of over stamping, resulting in the gold being rendered less amalgamable by being hammered, broken up, or coated with a film, or the sulfide being slimed, the proper change of adjustments- should reduce the assays of the slime and finer sands. The changes of adjustment have one object in view to get the pulp out of the mortar as quickly as possible after having been crushed to the proper size to liberate the gold and sulfide from their matrix.
Sixth: Poor amalgamation, due to the methods in use, may be caused by the following: To keeping the amalgam so hard that it becomes crystalline and breaks away, or that it reduces the tendency of the gold to catch. To keeping the plates so wet that the mercury and amalgam run down into the trap. To the gold not catching due to stains, bare spots, plates cleaned too close, too much water used, or too small a plate area. To loss of amalgam by not removing or bedding down the crumbs when dressing the plates. To use of impure mercury. To grease falling into the mortar and contaminating the plates. In fact, to bad practice in any of the various details connected with amalgamating.
Seventh: Poor amalgamation due to deleterious substances in the ore does not often occur. Arsenical and antimonial ores are the worst offenders in this regard. They tend to foul the mercury and amalgam, coating it with a film of the slimed material so that the mercury does not readily amalgamate with the gold, but is sickened and a large part of it lost. This trouble is liable to occur to some extent with any base and heavily sulfided ore. The principal remedy where the loss of mercury is great, is to practice outside amalgamation; though in cases where the outside plates become coated and fouled, it may be necessary to attempt to catch and hold the gold within the mortar. Crushing in cyanide solution may assist the amalgamation in so far as the cyanide solution will tend to keep the mercury and amalgam in better condition. Clayey, talcose, and other slimy ores sometimes give trouble in a similar way, or by coating the gold.
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