By Eric Olin
A diagnostic leach test can be very useful to qualitatively assess how gold occurs within the ore and the extent to which it may be refractory. Several different permutations of diagnostic leach tests depend on specific objectives, but they generally involve the sequential leaching of a gold ore sample with progressively stronger reagents, producing a qualitative assessment of the gold deportment within the sample. A typical 5-Stage diagnostic leach procedure includes:
Stage 1: Gravity concentration to remove the gravity recoverable gold from the sample, followed by cyanidation of the tailing to determine the cyanide leachable gold. Typically, a sample is ground and then subjected to gravity concentration with a centrifugal gravity concentrator, followed by hand-panning, or on a Mozley-type table. The concentrate is fire assayed for gold. The gravity tailing is subjected to cyanidation and the residue is assayed for gold. Gold recovered during Stage 1 is not considered refractory. A portion of the cyanidation leach residue is advanced to Stage 2.
Stage 2: Leach residue is reacted with hydrochloric acid to dissolve labile sulfide minerals such as pyrrhotite and liberate any gold that may be associated with them. The residue is then subjected to cyanidation and a sample of the residue is assayed for gold. A portion of the leach residue is advanced to Stage 3.
Stage 3: Leach residue is reacted in a hot sulfuric acid leach to dissolve acid soluble minerals not dissolved by hydrochloric acid, such as sphalerite and reactive pyrite and other labile copper sulfide minerals, and liberate any gold that might be locked within these minerals. The sulfuric acid leach residue is then subjected to cyanidation and a sample of the residue is assayed for gold.
Stage 4: Leach residue from Stage 3 is reacted in a hot nitric acid leach to dissolve more resistant sulfide minerals, such as pyrite and arsenopyrite, and liberate any gold that might be locked in these minerals. The residue is then subjected to cyanidation and a sample of the residue is assayed for gold.
Stage 5: Leach residue from Stage 4 is subjected to a high temperature roast to remove any carbonaceous material and then subjected to cyanidation and the residue is assayed for gold. Any gold remaining is assumed to be locked in silicates or associated with fine sulfides.
After the 5-Stage test is completed, a metallurgical balance is calculated to determine the distribution of gold recovered during each stage of the test. Gold recovered during Stage 1 would be considered non-refractory and could be recovered by standard gravity concentration. Gold extracted during the next four stages indicates minerals that the gold occurs with and suggests process options, such as pressure oxidation, biooxidation or roasting that could lead to improved recovery of the refractory gold values. Gold remaining in the Stage 5 leach residue is most likely locked or encapsulated and probably not recoverable unless adequate liberation can be achieved by finer grinding.
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