Effect of Chloride Concentration on Gold Recovery in

Pressure Oxidation Process

Ida Fok & Amy Ng (speaker), HATCHOctober 24, 2007

Agenda

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

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

Overview of Gold Recovery Process

Yes

Pressure Oxidation

Refractory?

No

Gold Ores

Cyanidation

Carbon Adsorption

Reduction

NaCN

Activated Carbon

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 + O2 + 2 H2O =

4 NaAu(CN)2 + 4 NaOH

Refractory?

No

Gold Ores

Cyanidation

Carbon Adsorption

Reduction

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

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)

Pressure Oxidation Process

• Oxidation of sulfides, such as pyrite (FeS2) and marcasite (FeS2) 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

Autoclaves

Commercial-sized autoclave in a gold recovery plant

Autoclave in a copper demonstration plant

Technical Problem

• Testwork data shows that gold recovery is highly dependent on chloride concentration

• Study of dissolution of gold to gold chloride (AuCl4-) is required through:– Literature review– OLI Corrosion Analyzer Version 2.0.52

Hypothesis

• Dissolution of gold (Auo oxidized to +3 state to form AuCl4- in the presence of chloride ions) in acidic pressure oxidation is dependent on chloride concentration.

Objective

• To verify the hypothesis using Au-Cl-H2O stability diagrams generated by OLI Corrosion Analyzer Version 2.0

Au-Cl-H2O Stability Diagram from OLI

Au-Cl-H2O Stability Diagram from OLIAu Recovery (Cl = 50 ppm, Acid = 10 g/l, Ratio of Fe+2/Fe+3 = 1

Data from OLI

Effect of Chloride Concentration and Temperature

760

780

800

820

840

860

880

900

920

940

0 50 100 150 200 250

Temperature (oC)

E (m

V vs

Ag/

AgC

l)

Cl = 25 ppmCl = 100 ppmCl = 150 ppm

Findings

• The higher the Cl concentration, the lower the reduction potential (E) of AuCl4-/Au

• Oxidation of Auo is more favourable due to common ion effect, according to this reaction:

AuCl4- + 3 e- Auo + 4 Cl-

• Study extended to effect of temperature, ferrous/ferric ratio and H2SO4 concentration

Conclusions

• Reduction potential (E) of AuCl4-/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

Questions or Comments?