Determination of Surface Gold Preg-Robbed on Carbonaceous Matter From Autoclave Pressure Oxidation (Ac Pox)/Carbon in Leach (Cil) Stream Products

Request for analysis:
A refractory sulphide ore containing carbonaceous matter has been processed in an AC POX/CIL test plant. The gold recovery after cyanidation leaching is low. The CIL residue was analysed for possible losses due to preg-robbing of gold from the cyanide solution onto carbonaceous material present in the sample.
Objectives:

  • Detection (“speciation”) of different forms of surface gold preg-robbed on carbonaceous matter:  metallic gold Au° and gold compounds (Au(CN)2–, AuS(CN)2—…) by ToF-SIMS.
  • Quantification of the established forms of preg-robbed surface gold by ToF-SIMS based on compound specific Au standards in activated carbon.

Methodology:

  • Various types of carbonaceous matter present in the sample are characterized with regard to their preg-robbing capacity using a set of several complementary techniques:
    • Composition established by SEM/EDX (scanning electron microscopy coupled with energy dispersive x-ray spectroscopy)
    • Structure (maturity) of the carbonaceous matter determined by laser Raman spectroscopy
    • Preg-robbing capacity of each type of carbonaceous matter evaluated by ToF-SIMS
  • A large number of individual carbonaceous particles were analysed by ToF-SIMS for the presence of surface gold. This technique provides non-destructive elemental and molecular surface analysis and allows for simultaneous detection of metallic gold and gold compounds. Due to the low molecular fragmentation during the ToF-SIMS analysis, it is possible to detect (“speciate”) simultaneously the presence of Au in both elemental (Au°) and compound forms such as Au(CN)2, AuCl2 or Au(SCN)2. The quantification of the ToF-SIMS data is based on element and compound specific standards with established detection limits for surface metallic and compound gold in the low ppm range.

Results of the study:
Separate carbonaceous particles from the CIL residue sample were selected under the optical stereoscope and mounted on a copper substrate for analysis.
Major findings:

  • The SEM/EDX analysis identified two different groups of carbon-containing particles: total carbonaceous matter particles (TCM) containing almost 100% carbon (Figure 1) and carbonaceous particles (quartz particles with variable amounts of finely disseminated carbonaceous matter, Figure 2).
  • A comparison between the laser Raman spectra of carbon standards and the carbonaceous material from the CIL residue samples shows that the TCM structure is similar to that of graphitic carbon while the Disseminated TCM particles carry the characteristics of activated carbon, Figure 3.
  • ToF-SIMS surface spectra and images of the surface Au distribution were established for each particle. The speciation of the gold preg-robbed on carbonaceous matter from the CIL tail sample showed the presence of both metallic gold and Au(CN)2 compound, Figure 4.
  • The quantified metallic gold and Au(CN)2 gold preg-robbed on carbonaceous  matter is listed in Table 1.  Concentrations measured for each grain and their average values per mineral type with corresponding 95% confidence intervals are included in the table.


Figure 1. Optical microscope and SEM images along with EDX spectra and semi-quantitative elemental analyses of total carbonaceous matter (TCM) grain.

Figure 2. Optical microscope and SEM images along with EDX spectra and semi-quantitative elemental analyses of two areas on a grain with disseminated carbonaceous matter. The arrow indicates the grain analyses in the context image.

Figure 3. Overlain Raman spectra from carbonaceous areas in several carbonaceous particles along with reference graphite and activated carbon spectra

Figure 4. Optical microscope images and ToF-SIMS elemental and compositional maps for a selected carbonaceous particle. The quantified amount of metallic gold on this particle was 4.5ppm, while the amount of Au as Au(CN)2 compound gold was 38.1ppm.
Table 1 – Measured concentrations of surface gold on C-matter from CIL tails