mete 112 - cyanidation

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GOLD LE CHING

Prepared by:

Engr. Suzette T. Escalera

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Hydrometallurgy

•   The art and science of aqueous methods of 

extracting metals from their ores

•  Generally, it involves two distinct steps:

 Leaching   –   selective dissolution of metal values

from an ore

 Precipitation   –   selective recovery of the metal

values from the solution

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Hydrometallurgy

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Gold Production

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- Involves the use of aqueous solutions containing a

lixiviant which is brought into contact with a material

containing a valuable metal

•   Gold cyanidation is called MacArthur-Forrest

Process

-   a metallurgical technique for extracting gold from

low-grade ore by converting the gold to a water

soluble coordination complex

Leaching

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Gold dissolution

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• Cyanide

- refers to a group of compounds made of carbon (C)

and nitrogen (N)

- Cyanide solutions readily bond with gold, silver andother metals, which is why the mining industry uses

it.

Reagents

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  It will be converted to hydrocyanic acid, HCN, if it

contacts acid.

  Cyanide occurs naturally in many plants. It is found

combined with the sugar glucose to form a chemicalknown as amygdalin.

 Cyanide reacts with the hemoglobin in your blood. It

takes the place of oxygen.

Facts about Cyanide

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  Cyanide is destroyed by sodium thiosulfate or

sodium nitrite.

 Cyanide dissolves both gold and silver from alkaline

solution.   Cyanide is a very "clean" extraction system, as it does

not extract many of the base metals such as iron.

This is in contrast to halide extraction, which

dissolves large quantities of iron, aluminum, and

other metals.

Facts about Cyanide

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  Free cyanide – most toxic form (CN- and HCN)

•   @ pH 11: over 99% of cyanide remains in sol’n

•   @ pH 9.3-9.5: CN- and HCN are in equilibrium

•   @ pH 7: over 99% of cyanide exists as HCN

Cyanide Chemistry

Total Cyanide

Weak AcidDissociable

(WAD) Cyanide

Free Cyanide CN and HCN

Weak andmoderately strong

metal-cyanidecomplexes

Ag, Cd, Cu, Hg,

Ni and Zn

Strong metalcyanide

complexes

Fe, Co, Au, Pt,

Pd

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• Oxygen

-   one of the reagents consumed during

cyanidation, and a deficiency in dissolved

oxygen slows leaching rate.

- can also be added by dosing the pulp

with hydrogen peroxide solution

Reagents

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• Lime (CaO)

- for pH modification; added prior to milling

which makes the slurry alkaline (high pH).

Reagents

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• Lead Nitrate (Pb(NO3)2)

- added to the leach circuit and has the effect

of accelerating the gold dissolution rate

Reagents

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Size of Gold Particles

 – The gold must be in a fine particulate form(from the grinding circuit).

 – The feed slurry must have an 80% passing size of 

75µm to achieve suitable gold recovery.

Factors Affecting Cyanidation

Reaction Rate

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Free Cyanide Concentration

 – Increasing cyanide concentration drives thecyanidation reaction to the right.

 – There must be sufficient free cyanide ions in

solution to dissolve all the gold, otherwise it will

be lost to tailings.

Factors Affecting Cyanidation

Reaction Rate

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Dissolved Oxygen

 –  There must be sufficient entrained oxygen in theslurry.

 – Increasing the oxygen concentration drives the

reaction to the right.

Factors Affecting Cyanidation

Reaction Rate

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Slurry pH

 – pH modification is achieved by adding lime whichmakes the slurry alkaline (high pH).

Factors Affecting Cyanidation

Reaction Rate

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Slurry Density

 – Reagent consumption is minimised by maximisingslurry density, since optimal concentrations can be

achieved at lower dosages, due to the smaller

volume of solution per unit mass of material.

Factors Affecting Cyanidation

Reaction Rate

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Residence Time

 –  The longer the gold particles are in contact withthe cyanide in the slurry the more gold that will be

leached.

Factors Affecting Cyanidation

Reaction Rate

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Agitation

 – Effective agitation allows the reactants tointimately mix and prevents the solids from

settling out.

 – Agitation also ensures that the gold cyanide

complex ions are removed so as to allow   ‘access’

on the gold  particles’  surface for more unreacted

cyanide ions to leach more gold from the particle.

Factors Affecting Cyanidation

Reaction Rate

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Temperature

 – Higher temperatures will increase the rate of golddissolution, however it is not economical to heat

the slurry.

 – High temperatures also reduce the capacity of 

carbon to adsorb gold and lower the solubility of 

oxygen in the slurry.

Factors Affecting Cyanidation

Reaction Rate

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Cyanocides and Oxygen Consumers

 – Many other metals and minerals besides gold alsodissolve in alkaline cyanide solution producing

substances which can reduce the efficiency of gold

leaching.

 – copper, nickel, iron and sulphide minerals

(cyanocides); arsenic (oxygen consumer)

Factors Affecting Cyanidation

Reaction Rate

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Gold Production

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- a term used to describe the attraction of a mineral

compound to the surface of another material.

  Activated carbon is used to adsorb the gold out of solution. Because the cyanide ion forms very strong

complexes with gold, it is the gold cyanide complex

that is loaded onto the carbon rather than being

deposited as metallic gold.

Adsorption

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• Activated Carbon

- The dissolved gold in solution is adsorbed onto

activated carbon

- The method of recovery of gold from the processslurry is by carbon adsorption.

Reagents

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Gold Production

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1 mL AgNO3 = 100 ppm CN-

% =

−

. /

−  

% = .

% Free Cyanide

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MW (H2C2O4 2H2O) = 126 g/mol

MW(CaO) = 56 g/mol

, ( 

2

2

O

4

2

2

O)=

.  

2

2

O

4

2

2

O

′

2

2

O

4

2

2

O

=

Lime content

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[−

] =

   +

−

 +

−

=

−

pOH = -log [OH-]

pH = 14- pOH

Protective Alkalinity

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… end