When coarse free gold occurs in ores, the usual practice is to remove it by means of gold traps, jigs, blankets, etc. ahead of cyanidation. Otherwise, these coarse particles might not be completely dissolved in the time available for cyanidation. Another practice which reduces the size of the gold particles going to cyanidation is the grinding and classification of gold ores in closed circuit. This practice keeps returning the heavier gold particles to the grinding mill until they are small enough or thin enough to overflow the classifier into the cyanidation circuit.
Under what might be considered ideal conditions with respect to aeration and agitation, researchers found the maximum rate of dissolution of gold to be 3.25 mg/cm2/hr hour. By calculation this is equal to a penetration of 1.68 microns on each side of a flat gold particle, or a total reduction in thickness of 3.36 microns/hour. Thus, a piece of gold 44 microns thick (equal in thickness to 325 mesh Tyler Standard) would take not longer than 13 hours, and a piece 149 microns thick (100 mesh) not longer than 44 hours to dissolve. Metallic silver of the same thickness as gold would take twice as long to dissolve. With an actual ore and under plant conditions the rate of dissolution is affected by such factors as the association of the gold, i.e. whether or not it is completely liberated, coatings on the gold, and the efficiency of the cyanide solution. Not infrequently, however, the gold in an ore is so fine that 80 to 85% of it will dissolve in the grinding and classification circuit.
As in any heterogeneous reaction, the rate of gold and silver dissolution is directly proportional to the surface area. Kameda (1949) found a linear relation between the size of a gold particle (10 to 100um) and the time required for its dissolution. Lund (1951) found a linear relation between the dissolution rate constant and the surface area of the silver, which was in the form of a plate.