Many definitions of the word “slime” have been attempted to Leaching Fine Gold but the one that best expresses its meaning from a practical point of view is that given by Park that it is that part of the pulverized ore that is not percolable on a commercial scale without the use of pressure. The consequences of this definition are that slime cannot be treated commercially by the same methods as sand and requires entirely different handling. The fundamental constituent of slime is the amorphous or clayey matter present in all ores to a greater or less extent and this is supplemented by varying amounts of exceedingly fine granular material produced in the process of crushing or pulverizing.

For some years after the leaching process of cyaniding was well established no feasible method was discovered for treating the slime which had been separated from the percolable part of the pulp: consequently the object of the metallurgist was to avoid as much as possible the formation of slime in milling, and in so doing there was a tendency to produce a pulp whose maximum sized particles were too coarse to liberate the precious metals and expose them to the action of the cyanide solution. With the development, however, of successful methods of treating the slime, the practice in milling was aimed more and more at the production of a pulp fine enough to yield the maximum extraction by cyanide.

This principle developed rapidly until the process of “all-sliming” came to be considered as the panacea for all the ills of the cyanide process and in discussing the erection of a new mill it was considered almost heresy to suggest the possibility that “all-sliming” might not be the most commercially profitable method of treating the particular ore in question. The expression “all-sliming” does not usually mean grinding everything to a “slime” in the sense of the definition already laid down, but merely grinding sufficiently fine to avoid mechanical difficulties in the treatment of the whole of it by slime methods. In the majority of instances the product of “all-sliming” contains from 5% to 35% of sand that will not pass a 200 sieve; moreover, a good proportion of the sand —200 sieve is perfectly leachable if properly freed from slime, as has been already explained, so it is fairly safe to say that of the usual “all-slime” pulp from 30% to 50% does not come under the terms of the definition of slime, that is, it would be perfectly practicable to leach it by gravity on a commercial scale.

In many cases it may be just as effective and more convenient to consider the ground product all as “slime” and deal with it as such, but the writer has shown instances where it was more profitable to make a separation and treat the percolable part by leaching. For example, in one “all-sliming” mill the plant capacity restricted the agitation period of each charge to 24 hours; this was sufficient to give a good extraction on the true slime, but the fine granular material required considerably longer. Here the general extraction was improved, the tonnage increased, and the costs lowered by erecting a subsidiary leaching plant and removing as a percolable product about 40% of the finished pulp.

The question of “all-sliming” any given ore should be determined solely by commercial considerations. Extraction tests will, of course, be made but it will not be sufficient for these to show an increased extraction; the amount of such increase must be weighed against the increased cost of treatment and an estimate made of the probable net gain, if any. Even when all- sliming is indicated it usually will not be profitable in practice to reduce the entire ore to accord with the definition of slime. In pulverizing an ore the first thing to separate as a finished product is the natural slime, next the most friable portions will yield to disintegration, and so on, until there is left a residuum of the toughest and most refractory material. This latter will circulate round and round the milling system, only a small portion being eliminated each time as finished product. The cost of reducing this residuum is disproportionately high and in many instances will not result in a final profit.

It sometimes happens too that this tough residuum contains a lower assay value than any other portion of the ore, making it still more unprofitable to carry the process to a conclusion.

The amount of oversize finally allowed to pass to treatment will determine the point as to whether it is worth while to separate a part to be leached or to treat the whole by agitation. As a rule when the amount of coarse material is not sufficient to interfere with the mechanical working of the agitators and yields its maximum extraction within the time available, it will pay better to treat the whole as if it were slime.

What has been said thus far deals with those ores on which all-sliming has been indicated as the ideal treatment. Statements may often be read to the effect that while gold ores can be treated advantageously by a combination of leaching and agitation, for silver ores all-sliming is the only way to obtain a satisfactory extraction. Like most generalizations, these statements are to be received with caution. While it is probably true to say that most silver ores require finer grinding than the majority of gold ores, yet in the writer’s experience the silver ores that unquestionably need “all-slime” treatment are in the minority. Very many of them show no appreciable gain in extraction when the —100 mesh mark has been passed, and with some the limit may be put at — 80 or even coarser. In cases such as these it is difficult to see any point in trying to make an “all-slime” product. If the costs are increased (as they undoubtedly will be) without resulting in any increased recovery, the commercial result will be a loss.

Even when the finer comminution does show a substantial increase in extraction the coarser grinding may result in a greater net profit on account of lower costs and larger output from any given crushing unit.

If the degree of comminution decided on results in a pulp 50% of which will remain on a 200 sieve, separation of the sand for leaching will almost always be advisable. Early in the development of the cyanide process agitation methods were tried for sand, but were abandoned on account of the expense and mechanical difficulties involved, and the same objections hold good today, though perhaps not quite to the same extent.