Minimizing losses of gold in amalgamation will result in optimum Gold Amalgamation Recovery which may be ranged under the following heads:
- Loss of free gold contained in amalgam, due to flouring of mercury, scouring of plates. This has been dealt with above under the heads “Treatment of the Plates” and “Loss of Mercury.”
- Loss of gold which “ floats ” in water and is carried away with the slimes. See section on “Float Gold.”
The remedy for the losses due to the above two causes is the use of drops and mercury traps. - Loss of gold which is not in a condition to be directly amalgamated.
The latter heading may be subdivided into three:
- (а) Loss of gold contained in sulphides, tellurides, &c. See “Gold in Pyrites” and the treatment of tellurides described in Chapter xvi.
- (b) Loss of free gold, which is prevented from being amalgamated by being coated with a film of some mineral (“ rusty ” gold), or with grease.
- (c) Loss of “ free ” gold embedded in particles of rock. The remedy is finer crushing in the battery or in re-grinding machines.
In the preparation of gold ores for amalgamation every care must be taken that the course best suited to each particular case is being pursued. In some instances, which are not common, the whole of the gold may be present in a form in which it can be directly amalgamated. In general, however, the gold is present in two or more forms, one capable and the others not capable of amalgamation. In such cases there is no reason to be dissatisfied with the action of an amalgamating machine if it extracts a high percentage of the free gold, even though the total extraction obtained by it is comparatively low.
The following scheme of examining tailings with a view to determine the causes and amount of loss is given by M’Dermott & Duffield:
Small samples are taken at intervals from the waste outflow of the mill, until a bucketful is collected ; this is allowed to settle for several hours, the clear water is decanted, preferably through a filter, and the remainder evaporated to dryness. Care must be taken to avoid spilling anything out of the vessels containing the samples. The sample having been well mixed, portions are treated as follows :—
- One part is panned and examined for free gold, amalgam, and quicksilver. If these are present, it is probably the fault of the millman, and nothing further need be done until this state of things is remedied.
- The tailings are sized by screening, and the coarse, medium, and fine materials (the latter consisting, say, of that portion which passes a 100-mesh screen) are weighed and assayed separately, the coarser portions being reground and panned to find whether their values are in free gold or in sulphides.
- The sulphides are separated from the tailings on a vanning shovel or batea, and are weighed and assayed. It may thus be determined whether they are worth saving, and the size of the mesh used for the screens will depend largely on this, and on the nature of the sulphides, which will in many cases be badly slimed and difficult to catch if the ore is finely crushed.
- This requires much skill and patience, but can in almost all cases be successfully accomplished by the vanning shovel. The concentrates may be examined under the microscope for fine specks of gold, but these and the fine sulphides can be recovered by concentration on suitable machinery. The assay value of the tailings from the vanning shovel will give some idea of the amount of float gold which is being lost. It will usually be found to be smaller than may be expected. If it is large, the use of some system of amalgamation more perfect than that by copper plates (such as pan-amalgamation) or of a method of smelting, or of a wet method, may be considered, if the advantages appear sufficient to pay for the presumably increased cost.
“Float” Gold
The loss of finely-divided or “float” gold, particularly when it cannot be checked by the use of swinging plates, or of drops between the amalgamated plates, is often another name for the loss of slimed sulphides. Many examples have been adduced of the large percentage of the gold in the ores crushed in particular mills, which has been carried away suspended in water in a form not easily recoverable by settling. In the majority of these cases, however, no attempt seems to have been made to distinguish between the values contained in slimed sulphides and those existing as particles of free gold. Where this is not done there are no grounds for the assumption that any free gold is escaping at all. Thus G. M‘Dougal, of Grass Valley, California, found that a gallon of water in a stream, ¾ mile below two mills, contained on an average 1.18 cents worth of gold. He called this “float” gold, but did not try to find out its physical condition, and it was very likely contained in sulphides. Again at the Spring Gully Mine, in Queensland, the tailings from the battery, if settled in the ordinary way by running off the water, were found to contain 7 dwts. of gold per ton, but if carefully filtered, assayed 15 dwts. All such examples prove only that the slimes are rich, not that “float” gold is being lost, and although it is of course likely that some finely-divided gold is carried away in suspension in water during the treatment of many ores, nevertheless, if sufficient care were taken in ascertaining this loss, it would probably prove to be less than is generally believed.
