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Effect of Chloride Concentration on Gold Recovery in Pressure Oxidation Process

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Effect of Chloride Concentration on Gold Recovery in Pressure Oxidation Process. Ida Fok & Amy Ng (speaker), HATCH. October 24, 2007. Agenda. Background Overview of Gold Recovery Process Pressure Oxidation Process Technical Problem Hypothesis & Objective of Study Findings from OLI

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Effect of Chloride Concentration on Gold Recovery in Pressure Oxidation Process

Ida Fok & Amy Ng (speaker), HATCH

October 24, 2007

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

Gold Ores

Refractory?

No

Yes

Pressure Oxidation

Cyanidation

Carbon Adsorption

Reduction

Overview of Gold Recovery Process

NaCN

Activated Carbon

cyanidation
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

Gold Ores

Refractory?

No

Cyanidation

Carbon Adsorption

Reduction

carbon adsorption

Gold Ores

Refractory?

No

Cyanidation

Carbon Adsorption

Reduction

Carbon Adsorption
  • Gold-rich solution is mixed with activated carbon pellets
  • ‘Loaded’ carbon is washed with hot cyanide solution to strip gold from pellets
reduction
Reduction

Gold Ores

  • 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)

Refractory?

No

Cyanidation

Carbon Adsorption

Reduction

pressure oxidation process

Gold Ores

Refractory?

No

Yes

Pressure Oxidation

Cyanidation

Carbon Adsorption

Reduction

Pressure Oxidation Process
  • Pre-treatment of “refractory” gold ores for cyanidation to be effective
  • Oxidation of sulfides, such as pyrite (FeS2) and marcasite (FeS2) at high T and P in autoclaves
  • Release of encapsulated gold grains
autoclaves
Autoclaves

Commercial-sized autoclave in a gold recovery plant

Autoclave in a copper demonstration plant

technical problem
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
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
Objective
  • To verify the hypothesis using Au-Cl-H2O stability diagrams generated by OLI Corrosion Analyzer Version 2.0
au cl h 2 o stability diagram from oli1
Au-Cl-H2O Stability Diagram from OLI

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

findings
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
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