Auriferous deposits present different mineralogical and metallurgical characteristics that affect the concentration process to be selected. Some basic parameters to be considered are to identify gold mineralogy, gangue, gold content, gold particle size, and mineralogical associations. Based on these factors, free and coarse gold particles are recovered by gravity, and fine particles are recovered by flotation and/or cyanidation. The latter is perhaps the last option and the evaluation must be done methodically.
In this way, to obtain a high gold extraction by cyanidation, the testwork program must evaluate the particle size, leaching time, the presence of substances or compounds that retard the cyanidation such as oxidized copper minerals or organic matter.
There are several cyanidation processes and all the testing programs start with agitated leaching tests by using a mechanical stirrers or roller drivers to perform bottle tests. Typically these tests are performed at 80-85% passing 75µm and the ore is leached in slurry form (30-33% solids). The variables to be evaluated are sodium cyanide dosage, pH, leaching time, and oxygen level. For example the program can evaluate 0.5, 1.0, 1.5, 3.0, and 6 gpl of NaCN, the pH can vary from 10.0 to 11.5. Tests can last 48 or 72 hours, and the effect of chemical reagents such as hydrogen peroxide or lead nitrate on the leaching rate.
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| Leaching test by agitation | Checking leaching pH |
During the tests is common to take leaching solution samples in order to know the initial gold dissolution. At this moment, the researcher must determine the free cyanide and make the adjustments in order to have the initial dosage. The same idea is valid for lime or other chemical reagent.
At the end of the tests, the pregnant solution and residue are separated by filtration, and the residue is weighed and dried. Solution and residue are assayed for gold and silver. The solution is analyzed by atomic absorption and the residue by fire assay. With these results the balance is calculated. If during the tests was noted something strange such as the presence of copper minerals, assays must include copper so that we can know its effect on cyanidation. Low gold recoveries are due to the presence of oxidized copper minerals or gold liberation was not obtained. If the gold is refractory to cyanidation, recovery is low and the ore is a problem because needs special treatment previous to cyanidation.