Extraction Process of Gold (Au) and Silver (Ag)

Generally, extraction process of all metal always include the Comminution (Size Reduction)to liberate achieve mineral. The next process of gold extraction from Hydrometallurgy route is Leaching. There are reagents that can be used to gold leaching process: Mercury (Hg), Cyanide (CN-) usual as KCN or NaCN, Thiosulphate (Na2S2O3), and Thiourea.
Reagent vote for leaching process depend on:
1. Kind of Ore
2. Grade of Gold in the Ore
3. Cost
4. Material Handling
5. Government Regulation
Until now, the most reagent for gold leaching process in Industry is Cyanide (CN-) although hazardous material. It’s caused by highest recovery of gold (>95%), short time process, and more economize.

In the next time, the position of cyanide will be changed by another reagent that environmental friendly but the recovery of gold still high. many reagents being tried by researcher and Thiosulphate is the best reagent to change cyanide. Based on our research, thiosulphate give high gold recovery for Oxide and Sulphide Gold Mineral although the consumption of this reagent higher than cyanide. But thiosulphate is more environtmental friendly and cash cost can be lower because there is not additonal cost such as cyanide destruction. The problem for this reagent is about stabilisation. Thiosulphate is not stable hence need a good process controlling.

Below is the flowsheet of gold extraction process in industry:

Concentration

The Concentration process will be done when many native gold in the ore. Native gold will not break by grinding / milling, it will be changed to another shape, float on the slurry and go together to the tailing dam before be leached perfectly.

There are many concentration method, the most method that often be used: Humphreys Spiral, Shaking Table, or Jigging. This method work by the difference of density between gold and impurities hence cost of production lower. The Advance of gravity concentration is Knelson Concentrator

Leaching

There are many theory of the gold leaching: Elsner’s Oxygen Theory, Janin’s Hydrogen Theory, Bodlanders’s Hydrogen Peroxide Theory, Boonstra’s Corrosion Theory, until Kinetic proving Theory by Habashi. Elsner’s Oxygen Theory is The most of theory that used, reaction of gold and silver leaching:

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

Below is the mechanism of reaction in a electrochemical cell :

The process parameter of leaching :
1. Particle Size
Depend on mineral, usually 80-90% -200 mesh (-74 micron)

2. Cyanide Strength
> Cyanide strength make > kinetics of reaction. Usually 750-850 ppm NaCN, depend on mineral.
3. Dissolved Oxygen (DO)
Generally, > DO make > kinetic of reaction depend on mineral. But based on limiting theory, the cyanide-DO ratio is 6 (constant). So if we use cyanide excess, the kinetic of reaction is controlled by dissolved oxygen. Usually, standard parameter for dissolved oxygen is 6-8 ppm.

4. pH 10-10,5
If pH<10>10,5, the possibility is: H2O2 will be formed and recovery also decrease.

5. %-Solid
> %-solid make gold and silver recovery decrease. If %-solid is too low, maybe gold recovery will increase but cyanide consumption will increase significantly. Beside that production capacity will decrease. Usually, many industry operate %-solid between 40-50%-weight.

6. Temperature
> Temperature make > kinetic of reaction, but limited by Dissolved oxygen. Because >Temperature make dissolved oxygen decrease. Usually, process temperature is not controlled.

7. Retention Time
> Retention time make gold recovery increase but production capacity decrease. Usually, retention time for gold processing about 48 hours.

Route 1
Adsorption

This process is the first process from gold recovery. There are many kind of adsorbans that can be used, activated carbon, zeolit, or resin. Common adsorban in many gold processing industry is activated carbon. Several factor to choose activated carbon are:
1. Hardness/attrition resistant
2. Activity
3. Total gold capasity adsorption
4. Shape and size distribution
5. Ash content
6. Bulk Density
7. Moisture
8. Surface area
9. %-Carbon Tetrachloride (CCl4)
10. %-w/wt Benzene adsorption
Hardness/attrition resistant is the most important, beside that activity and total gold capacity adsorption also important.

Elution

Elution process is divorce of complex Au(CN)2- and Ag(CN)2- from activated carbon. There are many standard than can be used, depend on mineral and adsorban. Common elution method is AARL (Anglo American Research Laboratory). This elution standard is divided to 6 stage:
1. Acid Wash
The aim of acid wash is solute of carbonate (CO32-), usually in calcium carbonate (CaCO3) form. HCl 3% is used at this stage and the reaction is :

CaCO3 + 2HCl =CaCl2 + CO2 + H2O

Beside this acid, we also can used the other acid for example: HNO3. because this acid more oxidative than HCl, so we must control well in order that the carbon (CO) is not oxidized to CO2.

2. Water Wash
This stage only for Carbon cleaning from HCl.

3. Pretreatment /Presoak
Absolutely, pretreatment/presoak is the first process for divorcing Au and Ag from activated carbon (C-Au(CN)2-). NaOH 3% and NaCN 3% are used in this process at 80-90 C.

4. Recycle Elution
after C-Au(CN)2- is divorced, water recycle is flowed at 100-120oC and pressure 300-400 Kpa. Water from elution column is Pregnant Solution and ready to next process (Electrowinning)

5. Water Elution
6. Water Cooling

At stage 5 and 6, water from elution column enter to recycle tank for next recycle elution.

Electrowinning
The Principle of electrowinning is metal sedimentation from pregnant solution by electricity. Direct current is used in this process hence resulting reduction-oxidation reaction at electrode:
1. Anode
Oxidation reaction always:

2H2O =O2 + 4H+ + 4 e

Because of H+ formed, pH-solution decrease and HCN gas will be formed. This gas is corrosive and the consumption of anode will increase hence we must control pH-solution more than 12,5.

2. Cathode
Reduction reaction always:

Au+ + e = Au and Ag+ + e = Ag

Sum total of cathode is more than anode:

Cathode = Anode +1

Voltage and current for electrowinning can be got by Nerst’s equation. The product of electrowinning process is cake and then ready to be smelted.

Route 2
Precipitation
Precipitation can be done for clarified pregnant solution. Before precipitation, solid-liquid separation is done at thickener. Usually, we use CCD Thickener (Counter Current Decantation).

Precipitation can be done by many method:
1. Gas precipitation
2. Ion exchange precipitation
3. Cementation
Cementation is the most popular method. In the cementation method, metal powder is used to reduction of achieve metal. Commonly, Zn powder is used for gold cementation. Metal with lower potential reduction can be used in cementation process:

Li, K, Ba, Ca, Na, Mg, Al, Zn, Cu, Fe, Pb, Ag, Pt, Au.

For example : if we want to get Au and Ag, we must add Pb, Fe, Cu, Zn etc. Al can not be used for cyanide complex Au(CN)2- because Al oxide is protective hence reduction reaction finished. The reaction of gold cementation by Zn is :

Au+ + Ag+ + Zn = Au + Ag + Zn2+

The product of cementation is cake and ready to smelting process.

Smelting

Smelting process work at 1200 C, borax (Na2B4O7•10H2O) added as flux to retain slag and increase of basicity hence tapping can be done easier. The product of smelting process is Dore bullion (Au-Ag alloy).

Refining
Refining process is done to get pure gold (99,99%). Usually, there are two method that used by refinery industry:
1. High Ag Bullion
Electrorefining is the first process, the principle of electrorefining is the same as electrowinning. But anode for this process is Bullion, and AgNO3 is the solution. Silver will be at cathode and gold at anode, then smelting process is done to get gold and silver bar.

We must do the electrolysis to get 99,99%, the solution of gold electrolysis is Au(Cl)2-.

2. High Au Bullion
Bullion is smelted directly with flowing Cl2 gas, chloride gas will retain Au and we will get Au and Ag bar.

We also have to do the electrolysis to get 99,99%, the solution of gold electrolysis is Au(Cl)2-.