Many gold operations have experienced different levels of losses when the plant treated exclusively free milling gold ores. These operations recover their gold, when subjected to the processes of crushing, amalgamating with mercury, gravity and cyanidation with high recoveries. Refractory gold ores are those which contain gold in such association as to cause much loss in the ordinary process of crushing, gravity, amalgamating, flotation or cyanidation; in this case, other minerals are present with gold, which, by their physical and chemical properties, act deleteriously and render the recovery process difficult.
The losses are associated to the presence of fine gold held in suspension by water and carried downstream. Fine gold particles encased or attached to pieces of rock and carried into tailings or downstream. Clayey ores make the battery water very muddy and holds mechanically fine gold particles in suspension, until deposited in tailings or downstream. When any artificial conditions arise, whereby certain substances may find their way into crushing mills or other appliances, practically insuperable difficulties may be occasioned; similar results may arise from natural films coating the gold. Anything which creates films and prevents perfect contact between the metals, renders the gold non-amalgamable, therefore all but the heavier particles will be swept into the tailings or downstream. Though gold be in a perfect condition for amalgamation, impure mercury will not act; in such a case, but the heavier particles would be washed into tailings or downstream.
The points mentioned in the previous paragraph tend to affect the cyanidation process. Also the lack of liberation is other important point to consider. Although both gold may be in a perfect condition for cyanidation, and have leached, excessive comminution, agitation or grinding action in the treatment process and other causes affect the performance of the process. When amalgamation was practiced, it was produced the granulation of the amalgam and left it in a condition to be readily carried off by water, either into tailings or downstream.
Regarding the refractory gold ores, fine gold held in suspension by water and carried downstream. Fine gold attached to pieces of rock or encased in pieces of base mineral and carried into tailings or downstream. Base minerals associated with the gold invariably coat the surfaces of gold particles to a large percentage of the total quantity contained in the ore, and also give rise to complicated chemical reactions; the effect of either or both actions is to prevent the recovery of gold, and allow the gold to be carried into tailings or downstream. The partial decomposition of base minerals in refractory ores produce acidulated water and its train of attendant evils which act deleteriously in cyanidation and amalgamation, causing granulation of the mercury (sickening or flouring) which prevents the particles re-uniting, leaving them in a condition to be readily carried by water. 5. The mechanical effect of heavy mineral particles in the ore, falling through the stream of water, prevents contact between the amalgamating surfaces and eventually cut off the leachability causing loss of gold. Most of the sources of loss enumerated above may be common to both classes of ore in different magnitude; they are not twice specified in every case, in order to reduce repetition.