1. Effect of Chloride Concentration on Gold Recovery in Pressure Oxidation Process Ida Fok & Amy Ng (speaker), HATCH October 24, 2007

2. Agenda Background Overview of Gold Recovery Process Pressure Oxidation Process Technical Problem Hypothesis & Objective of Study Findings from OLI Conclusions

3. Background Ida Fok –Senior Process Engineer –Over 32 years of experience in process design, control and simulation in mining and petroleum refining –Hatch global consultant for METSIM Amy Ng –Process Engineer –Over 3 years of experience in basic and detailed design of alumina, potash, and high-pressure nickel/cobalt and gold recovery plants HATCH –Global engineering consulting company specializing in mining, metallurgical, energy and infrastructure industries –Autoclave Technology Group, a multi-discipline design group specializing in the design of high-pressure metals recovery plants

4. Overview of Gold Recovery Process Yes Pressure Oxidation Refractory? No Gold Ores Cyanidation Carbon Adsorption Reduction NaCN Activated Carbon

5. Cyanidation Cyanide leaching Alkaline ore slurry is combined with sodium cyanide Cyanide forms a complex with gold which dissolves into solution: 4 Au + 8 NaCN + O 2 + 2 H 2 O = 4 NaAu(CN) 2 + 4 NaOH Refractory? No Gold Ores Cyanidation Carbon Adsorption Reduction

6. Carbon Adsorption Refractory? No Gold Ores Cyanidation Carbon Adsorption Reduction Gold-rich solution is mixed with activated carbon pellets Loaded carbon is washed with hot cyanide solution to strip gold from pellets

7. Reduction Refractory? No Gold Ores Cyanidation Carbon Adsorption Reduction Gold stripped from carbon pellets is recovered by electrolysis Gold accumulates on steel wool cathodes, which are dissolved in HCl acid Gold is ready for smelting (extraction process)

8. Pressure Oxidation Process Oxidation of sulfides, such as pyrite (FeS 2 ) and marcasite (FeS 2 ) at high T and P in autoclaves Release of encapsulated gold grains Yes Pressure Oxidation Refractory? No Gold Ores Cyanidation Carbon Adsorption Reduction Pre-treatment of refractory gold ores for cyanidation to be effective

9. Autoclaves Commercial-sized autoclave in a gold recovery plant Autoclave in a copper demonstration plant

10. Technical Problem Testwork data shows that gold recovery is highly dependent on chloride concentration Study of dissolution of gold to gold chloride (AuCl 4 - ) is required through: –Literature review –OLI Corrosion Analyzer Version 2.0.52

11. Hypothesis Dissolution of gold (Au o oxidized to +3 state to form AuCl 4 - in the presence of chloride ions) in acidic pressure oxidation is dependent on chloride concentration.

12. Objective To verify the hypothesis using Au-Cl-H 2 O stability diagrams generated by OLI Corrosion Analyzer Version 2.0

13. Au-Cl-H 2 O Stability Diagram from OLI

14. Au Recovery (Cl = 50 ppm, Acid = 10 g/l, Ratio of Fe +2 /Fe +3 = 1

15. Data from OLI

16. Findings The higher the Cl concentration, the lower the reduction potential (E) of AuCl 4 - /Au Oxidation of Au o is more favourable due to common ion effect, according to this reaction: AuCl 4 - + 3 e - Au o + 4 Cl - Study extended to effect of temperature, ferrous/ferric ratio and H 2 SO 4 concentration

17. Conclusions Reduction potential (E) of AuCl 4 - /Au decreases, i.e. dissolution tendency increases, with increasing temperature and chloride concentration OLI Corrosion Analyzer proves to be a useful tool in solving this technical problem Findings based on OLI stability diagram are consistent with those from literature

18. Questions or Comments?