Concentration Tests

If the agitation tests on the sample of ore ground to pass 200 sieve are not satisfactory, it will be necessary to try the effect of concentration. For this purpose take about 500 grams of ore, crushed to No. 30 sieve, and carefully pan twice over. The tailing may then be ground in water or cyanide solution as already described and screened wet through a No. 80 or 100 sieve, and again panned. To assist the operation of panning the 100 product, a large proportion of which will be slime, the fine part may be floated off and run over a little canvas table, leaving the granular part only to be panned. In this case, however, the product of the canvas table will need to be cleaned in the pan, but this is simpler and less tedious than panning the whole. The tailing, after being sampled for assay, may be cyanided as it is, or first ground to pass a No. 200 sieve.

Lead Salts. When experimenting with silver ores some charges should always be made up with the addition of varying amounts of lead acetate or litharge as the presence of lead will often be found a considerable aid to extraction of the silver, particularly when there is no zinc present in the solution.

If litharge be used for this purpose it should be ground fine, say to pass 200 mesh.

In the case of many ores a critical point in the addition of lead compounds is reached which yields the maximum increase in extraction and beyond which the improvement due to its use declines until a point is reached when the extraction is no better and sometimes even worse than without the addition of lead so the determination of the right amount of lead compound to be used is a matter of great importance.

Test for Repeated Use of the Same Solution. In dealing with any unknown ore there is an important experiment which should never be neglected. It sometimes happens, especially with ores containing arsenic and antimony, that a test made with new solution will give a high extraction, while a similar test made up with solution which has been used over several times after precipitation with zinc will give a much lower one. In the case of such ores the presence of lead is often as inimical to a good extraction as the presence of zinc.

To make this test it is necessary to start the series with several bottle charges so as to furnish sufficient solution with which to carry through the experiment. Each time the solution is used there is a loss due to withdrawals for titration and other causes. The first of the series may be made with 4 bottles, charged in the usual way, the solution after treatment and precipitation with zinc dust being made up to cyanide strength and used to treat the next series of perhaps 3 bottle charges of the original size or 4 smaller charges, and so on until the same solution has been used over 5 or 6 times, by which time the original solution will probably have shrunk to only sufficient for one bottle charge. If by the 6th round no deterioration in extraction has been noted it is safe to assume that the activity of the solution will not be impaired by repeated use on a working scale, under the conditions of zinc precipitation. For this experiment the use of a laboratory vacuum filter is almost a necessity.

The whole amount of wash, however, necessary for displacing the dissolved metals preparatory to the assay of the ore residue should not be added to the stock solution as that would dilute it out of all proportion to milling conditions. Assuming that the filter cake when firmly set contains about 25% moisture, and working with a 250-gm. charge of ore 83 cc of water would be first put on and allowed to leach through, to be at once removed from the receiver and added to the stock solution. After this ample water washes are given to ensure the complete removal of soluble gold and silver from the cake.

Use of Working Plant Solution. When making experiments on an unknown ore using barren solution from a working plant where zinc precipitation is used a serious error in the calculation of cyanide consumption is likely to occur if the figures are based on the titrations for free cyanide, because when such solution comes in contact with the lime added in making up a bottle charge some of the zinc-potassium cyanide will be regenerated and recorded as free cyanide, and thus the consumption will appear much less than the actual figure should be. This may be avoided by making a complete set of titrations for total cyanide when making the usual ones for free cyanide, and then basing the consumption calculation on the readings for total cyanide.

When the ore, however, itself contains zinc soluble in cyanide solution it is practically impossible to estimate the cyanide consumption correctly when using barren plant solution.

Larger Agitation Charges. Where there are conveniences for grinding and classifying several hundred pounds of ore at a time it is often of advantage to check the results of bottle tests by charges of 50 to 100 Ib. For this purpose it is easy to fit up some small tubs of wood or sheet iron and fix mechanical stirring devices in them. Such mechanical agitation is sufficient to give the maximum extraction in many cases, but with ores that have a reducing tendency it is often necessary to supplement it with an air lift, to facilitate aeration of the charge. If there is a workshop available it is not difficult to have some small Pachuca tanks made out of sheet iron, and these are often very useful for experimental work. When using such tanks in dry climates it is needful to be on the watch for considerable evaporation, resulting in a decreased ratio of dilution, and consequent concentration of precious metal and cyanide content in the solution. It is also necessary, as already pointed out, to see that the chances for an accumulation of dead pulp in the cone are reduced to a minimum as this is a fertile cause of salting the residue sample.

High Cyanide Consumption. If experiments on any ore show an abnormally heavy consumption of cyanide (and assuming, of course, that protective alkali has been present in excess the whole time), it is advisable to try to ascertain the cause of the loss. By testing the solution after contact with the ore, for zinc, copper, silver, sulphocyanate, and ferrocyanide, it is often possible to account for nearly the whole amount consumed. Should there remain a considerable proportion unaccounted for it may be due to the presence and dissolution of some metal or metals other than those mentioned, or it may be due to oxidation by the use of excessive aeration, or occasionally to the presence in the pulp of organic substances.

It is only possible here to give an outline of the more common methods of preliminary experiment, and to suggest lines of investigation. Special conditions will call for variation and increase of detail, which each operator will be able to develop according to his needs.