Characterization of passivating coatings on gold grains from carbon in leach (CIL) residue sample by TOF-SIMS

Request for analysis:
Recovery efficiency during the leaching process of an ore sample containing free gold is low. Examine the surface of unleached free gold grains found in the CIL residue tails for the presence of passivating coatings.
Objectives:

• To establish the presence of passivating coatings on the surface of free gold grains from CIL residue that could impede the process of cyanide leaching.
• To gain insight on the origin of coatings.

Methodology:
Individual free gold grains were selected under the optical stereo microscope from both feed and CIL residue samples and mounted on an indium substrate. Surface spectra and images of selected surface species present on these gold grains were collected using an ION-TOF TOF-SIMS IV instrument. Depth profiles through the surface coating on selected gold grains were produced using a combination of two separate ion sources:  a Cs+ sputter ion gun and Bi3+ ion cluster analytical source.
Results of the study:
Comparative ToF-SIMS surface analysis between gold grains from the feed and cyanide leach residue samples showed significant differences in the amount of surface silver (Figure 1).
Examples of free gold grains from the CIL residue sample are presented in Figure 2. Images of surface species on the gold grains before (as received) and after sputtering of their surfaces with the Cs+ sputter ion gun are presented in Figures 3 and 5. The relative change of surface Au and Ag on these grains is presented in Figures 4 and 6.
Major findings:

• The unleached free and exposed gold grains from the CIL residue samples exhibit passivation surface coatings of Ag that slow down and inhibit further leaching of the grains in a cyanide solution.
• These coatings are related to the extraction process and not to the ore mineralogy of the feed sample. Comparison between the surface silver present on gold grains from feed and residue samples indicates that the silver passivation layer is developed on the surface of the gold grains during the leaching process.

Figure 1. Surface Ag measured on free gold grains from a feed and CIL residue by TOF-SIMS

Figure 2. Optical microscope images of gold grains from CIL residue examined by TOF-SIMS. Grain 1 is partially locked in a sulphide grain.

Figure 3. ToF-SIMS ion maps for selected surface species detected on gold grain 1 before (original “as received” surface) and after removing the upper surface layer by sputtering with a Cs+ ion source. This gold grain was attached to a pyrite grain. The ion maps show that the surface Ag coating is present on the gold grain, but not on the pyrite mineral phase.

Figure 4. Relative change of surface Au and Ag measured by TOF-SIMS on gold grain #1 before and after sputtering with Cs+ ion source

Figure 5. ToF-SIMS ion maps for selected surface species detected on gold grain 2 before (original “as received” surface) and after removing the upper surface layer in two consecutive sputter steps with a Cs+ ion source

Figure 6. Relative change of surface Au and Ag measured by TOF-SIMS on gold grain 2 before and after sputtering with Cs+ ion source