THE AMOUNT OF MOISTURE IN PULP
The amount of water in the pulp going to a tube mill depends upon the object of grinding. If we want a granular product for concentrating, more moisture is required than for the production of a slime.
While 38.5 to 40 per cent, of moisture has been considered correct for regrinding gold and silver ores treated by the cyanide process, we find in other cases the percentage of water to be as high as 300 per cent. But for the fact that the regrinding of the steam-stamp product for copper concentration in the Lake Superior region (1914) by Hardinge mills required as high as 3 of water to 1 of ore, the limit of 300 per cent, would appear facetious, as the extreme percentage for sliming gold ores may be placed at 50 per cent. Walter Neal, using a 5-ft. by 24-ft. tube mill, found that 39 per cent, of water gave the best results for the ore he was treating, any variation from this percentage causing a falling off in the capacity of the mill. The Neal curve, Fig. 18, is here reproduced and shows very clearly the effect of increasing or decreasing the amount of moisture in the pulp from that which gives the best result.
"It is surprising to note what a marked difference in grinding, a comparatively small difference in dilution makes, and it immediately suggests that not enough attention has been paid to this point in the past. To bring the matter home in another way it may be stated that approximately three tube mills working on 39 per cent, pulp will do the work of four mills working on 36 or 48 per cent. pulp.
"It is probable that the 'critical dilution' would vary at different plants on account of different ores and various makes of tube mills, but certainly the matter is worth investigating. Undoubtedly it is more than a coincidence that V. B. Sherrod, experimenting with tube mills at the Guerrero mill, Real del Monte, finds that the grinding efficiency increases with the percentage of solids in the feed up to about 55 or 60 per cent."
Operators should not be lead into the belief that because Neal found 39 per cent, moisture to be the best dilution for the ore he has under treatment or Sherrod found 40 to 45 per cent, to be the best in his case, that this is the correct amount for any ore. The following table will give an idea of the amount of moisture contained in the pulp at various mills where a tube mill is used for sliming gold ores. It may be that those mills showing a relatively high degree of dilution are unable by reason of mechanical defects in the mill to obtain a thicker pulp. Some ores when ground to 200-mesh appear thick with 40 per cent, moisture while others containing 32 per cent, moisture are more fluid, so that as the pulp must be thick enough for the particles of ore to cling to the pebbles the per cent, of moisture necessary in the ore is a variable quantity according to the physical characteristics of the ore.
