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SATRIO HERBIROWO3334090728 – METALLURGICAL

ENGINEERING

GOLD CYANIDATION

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Outline

Preface

Basic Theory

Thermodynamics Aspect

Kinetics Aspect

Application

Conclusion

References

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Gold Cyanidation ? Answer :

“leaching of gold ores with NaCN leaching agents and the presence of oxygen”

INTRODUCTION

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BASIC THEORY

Kinetics evidence

Corrosion theory

Thermodynamics evidence

Cyanate formation

Cyanogen formation

Bodlander’s hydrogen peroxide theory

Mclaurin and Christy

Janin’s hydrogen theory

Elsner’s oxygen theory

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Elsner’s oxygen theory

Elsner (1846) was the first to recognize that oxygen was essential for the dissolution of gold in cyanide solution

4 Au+8NaCN +O2+2H2O 4NaAu(CN)2+4 NaOH

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Janin’s hydrogen theory

Janin (1888,1892) was convienced that hydrogen gas must be evolved during the process and oxygen isn’t required

4 Au+4NaCN +2H2O 2NaAu(CN)2+2NaOH+H2

Maclaurin (1893) and Christy (1896), that oxygen was essential for cyanidation

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Bodlander’s hydrogen peroxide theory

Bodlander (1896) suggested that dissolution should proceed through two steps

2 Au + 4 NaCN + O2 + 2 H2O→ 2 NaOH + 2 NaAu[CN]2 + H2O2

2 Au + 4 NaCN + H2O2→ 2NaOH + 2 NaAu[CN]2

Hydrogen peroxide is formed as an intermediate product

Total reaction is equal to elsner’s theory

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Cyanogen formation

Christy (1896) suggested the oxygen necessary for dissolution liberated cyanogen gas, which to be the active agent for attacking gold

½ O2 + 2NaCN + H2O→ (CN)2 + 2 NaOH2 Au + 2 NaCN + (CN)2 →2 NaAu(CN)2

Skey (1897) and Park (1898), gave conclusive evidence that cyanogen gas not able to dissolve gold in aqueous solutions of cyanogen

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Cyanate formation

MacArthur (1905), oxygen were necessary for cyanidation

Potassium cyanate to be formed by oxidation of cyanide for dissolution gold

Green (1913), cyanate not able to dissolve gold

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Thermodynamics evidence

Barsky et al. (1934), the free energy were in favor Elsner and Bodlander’s equation

Whereas Janin’s equation wasn’t feasible

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Corrosion theory

Boonstra (1943), cyanidation=metal corrosion process

Oxygen dissolved reduced to H2O2 and hydroxyl ion

O2 + 2e + 2H2O→ H2O2 + 2 OH-

H2O2 + 2e → 2 OH-

Au → Au+ + e

Au+ + CN- → AuCN

AuCN + CN- → Au[CN]-

This was experimentally by Thompson (1947)

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Kinetic evidence

Habashi (1966), the dissolution process is electrochemical

2 Au + 4 NaCN + O2 + 2 H2O→ 2NaOH + 2 NaAu[CN]2 + H2O2

Without oxygen, cyanidation is a slow reaction

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Kinetic Effect

Cyanideconcentrati

onpH Temperatur

e

Oxygen pressure

Foreign ions

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Cyanideconcentration

The rate of dissolution if [CN] concentration

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pH effect

pH Cyanidation rate

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Temperature effect

Temperature cyanidation rate

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Oxygen pressure effect

At [CN] <<<, cyanidation rate depends only [CN]

At [CN] >>>, cyanidation rate depends PO2

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Accelerating effect Retarding effect

(+) Pb, Hg, Bi, Tl salt dissolution

Consumption of oxygen

Consumption of free cyanide from solution

Foreign ion effect

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Anodic Reaction Cathodic Reaction

Au → Au+ + eAu+2 CN-

→Au(CN)2+

O2 + H2O + 2e → H2O2 + 2 OH-

Mechanism (Habashi’s kinetics aspect)

“Occur in the electrochemical dissolution”

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2A ϑCN- ϑO2 [CN-][O2]Rate = --------------------------------

{ϑCN- [CN-] + 4 [O2] ϑO2 }

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Application

PT Aneka Tambang (Persero) Tbk(London Bullion Market Association certificated)Capacity : 75 tons

General Flowsheet

• Zinc Powder (Merril Crowe)

• Carbon in Leach (CIL)

• Carbon in Pulp (CIP)

• Carbon in Column (CIC)

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Conclusion

Cyanidation is leaching of gold ores with NaCN leaching agents and the presence of oxygen

Cyanidation kinetics follow the rate of dissolution kinetics Habashi

The factors affecting the rate of reaction is Cyanide concentration, pH, temperature, oxygen pressure, and foreign ions

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References

Habashi, F., 1997. Principle of Extractive Metallurgy Vol III Hydrometallurgy. New York. Wiley-VCH

Pehlke R. D., 1973 . Unit Process of Extractive Metallurgy. The University of Michigan Ann Arbor. American Elsevier Pub. Co

Logsdon M. J., 1999. The Management of Cyanide in Gold Extraction. International Council on Metals and the Environment. Ontario

Srithammavut W., 2008. Modeling of gold cyanidation. Master of Science (Technology) Thesis

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Revision

1. Why added salt more than 5 mg/l makes retarding effects?

Answer :Kondos et al. (1995) also presented that

higher concentrations of lead nitrate can reduce gold extraction

The retarding effect of Pb2+ is thought to be due to an insoluble film of Pb(CN)2 which deposits on the surface of gold. (Habashi, 1997)