Grinding Samples to a Given Screen Size
It is impossible in the laboratory to imitate exactly the degree of grinding in a working scale mill by using a certain screen size as a standard. For instance, if it is decided that a 100-mesh product is desired an operator may produce several entirely different degrees of grinding though using the same mesh screen, according to the frequency of the screening. If screening is done after long intervals of grinding, on a bucking table or with closely set discs if using a disc grinder, the resulting product though nominally 100 mesh will have a far finer character as a whole than if the screening be frequent during bucking or if the discs of the grinder be set apart and gradually closed up after each screening. And probably none of the products obtained by these different procedures will correspond to the pulp of a mill where 100-mesh grinding is aimed at. In one case known to the writer the product obtained from a bucking table, putting everything through a 100-mesh sieve, only contained 50% of 200 material whereas the finished mill pulp on the same ore contained 5% of -f 100- mesh material and 75% of 200. It is thus apparent that deductions drawn from laboratory extraction results on pulps all of which pass a given screen size must be received with discrimination.
Computing Cyanide Consumption. This may be done as in the following example: Ore taken, 250 grams. Ratio of solution to ore, 3:1 = 750 cc : 250 grams. Cyanide strength, 0.3% KCN.
It should be noted that cyanide consumption as figured above represents chemical consumption only. In practice there will also be a mechanical loss of cyanide in residues, and also a loss to some extent in precipitation, if the zinc process be used. The mechanical loss in residues will depend on the strength of solution used, and also on whether the slime residue is dewatered by a filter or by decantation,
In the case of silver ores the chemical consumption indicated in a bottle test usually corresponds very closely with the actual total consumption to be expected in practice, and for this reason ; an important factor in the cyanide consumption is the amount that combines with the silver: on precipitation this silver is replaced by zinc, and this zinc solution on coming in contact with the new lime added in the mill is partly dissociated and some free or available cyanide liberated. The amount of such cyanide thus recovered (a phenomenon which is not apparent in the ordinary laboratory test) comes near to offsetting the mechanical loss of cyanide in the filter residues.
Tests made as described will be found in most cases to approach very closely to working conditions, and to give results concordant with large-scale figures. A variety of combinations will suggest themselves to the operator. Assuming the ore to be " allslimed" he will need to ascertain, among other things, the best ratio of solution to ore, the best and most economical cyanide strength, and the time of treatment necessary; and a series of bottle charges may be made up and carried through to elucidate each point separately.
The figure for the extraction obtained on such tests is usually based on the difference between the assays of the head and the residue, and this seems to be a perfectly proper procedure, since both head and residue sampling is completely under control. Some experimenters make a point of checking the extraction, calculated as above, by an assay of the solution, but on these small-scale bottle tests the fact of a portion of the solution being withdrawn each day for titration complicates the operation and renders the result less reliable than the figures based on the ore and residue assays.
Leaching Tests. If the preliminary test seemed to indicate that excessively fine grinding was not necessary, it is probable that it will be mtore profitable to separate the pulp into sand and slime, percolating the former and agitating the latter.
As already stated, a good idea of the extraction to be expected by leaching may be obtained by agitating the sample in a bottle. Actual leaching experiments on a laboratory scale, with charges of one or two kilos, may be made to confirm this fairly satisfactorily if proper precautions are taken. The usual method is to use a glass percolator or an inverted acid bottle whose bottom has been cut off, placing a filtering medium in the small end, and fitting a cork in the neck, through which passes a piece of glass tubing tipped with rubber tube and a screw clip. About 3 kilos of the ore, ground to No. 30 sieve are taken and the slime and 200 sand washed out in water through a 200 sieve, or the slime alone may be removed by panning. (This water may be eventually used for making up the cyanide solution.) The 200 material or slime is settled with lime water and decanted, and the +200 material or sand is drained as free from water as possible by continued tapping in a gold pan and pouring off the supernatant liquor. In this way sufficient moisture may be removed from the sand to permit of its being well mixed by rubbing with the hands. A sample for assay is taken of both sand and slime. Of the thickened slime one or more bottle charges are made up for an agitation, test, and the sand, or an approximately known weight of it, is mixed with a suitable amount of fine lime, and charged into the percolator, each layer being firmly pressed down with the foot of a measuring cylinder or other tamping device.
A certain amount of cyanide solution is poured on each day and allowed to percolate slowly through, the daily proportion being regulated by the number of days treatment and ratio of solution decided on. Such ratio may be from % to 1^ times the weight of ore taken for percolation.
If there is not too much fine sand in the pulp to interfere with percolation the separation into sand and slime may be made by merely washing in the gold-pan, though in this method most of the fine sand will remain in the portion to be percolated while in practice a large part of it would pass over into the agitation product.
In the case of readily oxidized ores it is sometimes more convenient to avoid handling wet products before treatment, and separation may be made by dry-screening the sample with a 200 or 150 sieve.
One of the principal difficulties in such percolation tests lies in the fact that the charge is so much shallower than a working charge that there is not sufficient head to overcome the capillarity of the interstices, so that, even with sand coarse enough to percolate very rapidly, the level of solution will not fall much below the level of the sand. The result of this is that the charge is not aerated (as it is in practice), by the air following the solution down into the interstices, between each wash. This may be overcome by applying a vacuum under the filtering medium after the solution has ceased to percolate by gravity. In this way the residual solution is drawn off and the air follows it down. The charge should then stand for several hours before the next wash is applied. This procedure is more important than it may seem, since a difference in extraction of 20 per cent, to 30 per cent, has been in some instances observed according to whether the vacuum was applied or not.
Another difficulty with small-scale leaching tests lies in the evaporation of solution, which is excessive, and out of all proportion to the size of the charge, rendering determination of- the cyanide consumption quite unreliable unless careful measurement be made of the solution coming off, and the deficit made good by addition of water. Even so, the figure obtained is to be received with caution, as it is usually considerably in excess of workingscale results.
The most reliable way to conduct a leaching experiment is to use a vertical piece of iron pipe the depth of the tank proposed to be used in the plant, say, 6 ft. high, fitted at the bottom with a sleeve and plug, on top of which is laid a filtering medium. The plug is bored in the centre to admit a small iron cock. The diameter of the pipe should be at least 6 inches, because if too small the sand will not pack to a normal density, and will also be difficult to discharge when finished. Even with a 6-in. pipe it is often advisable, if charging with damp sand, to pack each layer gently down with a broom handle so as to approximate to the density found in a large working tank. Such a leaching test will need from 50 to 100 Ib. of sand for each charge, and of course will necessitate provision for grinding and classifying several hundred pounds of ore at a time.