1. Reducing/Replacing Hg in AGM Operations
Susan Keane
Gustavo Angeloci
Marcello M. Veiga
Ludovic Bernaudat
Suriname

2. Technical Solutions for Hg Pollution
Alternative Processes to Replace Hg
Control Hg Bioavailability
Reduce Hg Use and Emissions
Replace amalgamation with other process
Avoid methylation covering or dredging Hg-contaminated tailings
Avoid exposure to Hg and eliminate bad practices
Long-term
Medium-term
Short-term

3. Replacing Hg CONCENTRATION IS THE SOLUTION TO REDUCE OR REPLACE Hg
Any process to leach (or even amalgamate) gold must be applied to small amount of concentrates
Concentration = mass reduction
Gravity concentrates have usually 0.01 to 1% of the original mass
Flotation usually generates 5 to 10% of the original mass

4. Replacing Hg: Main Problem
THE MORE YOU CONCENTRATE THE MORE GOLD YOU LOSE
There are very few situastions in which you can concentrate to high grade (of the concentrate) and high gold recovery...this is rare
Gold particles need to be totally LIBERATED from the other minerals and have all the same size (which is very rare to occur)

5. Recovery and Concentrate Grade are Antagonists
Less mass in the concentrate = high grade... but low recovery
More mass in the conc. = low grade... but high recovery
Concentrate Grade
Recovery
Mass of concentrate

6. Gold Recovery is Low when Gold is not Liberated
Gold Not Liberated
Liberated Gold
0.07 mm
Gravity Separation
Assuming that the dark particles are gold and the white are other minerals, e.g. quartz

7. Good Grinding Does not Need Sophisticated Equipment
Mozambique
Indonesia
10 kg of ore ground with
14 steel balls for 45 min.
NO Hg ADDED in the ball mills

8. Centrifuges Are the Most Efficient Gravity Concentrators
Good for coarse and fine gold (0.05 mm)
Much more efficient than sluices
Used by mining companies
High cost
High maintanance and control
Most common: Knelson and Falcon, both from B.C., Canada

9. Gravity Concentration
Gravity concentration is usually good for “coarse” gold (0.1 mm)
Tailings from gravity concentratiuon are usually subjected to flotation
Flotation is good for fine gold
In industry: gravity concentration + flotation recover > 90% of gold in the original material (but good gold liberation is fundamental)

10. What to Do with Concentrates?
Direct smelting of concentrates
Chlorination
Intensive Cyanidation
Other lixiviants
Ore
Grinding
Concentration
Cyanidation
Concentrate
Tailing
Other lixiviants
Direct smelting

11. Direct Smelting
Used by ALL mining companies to remove “free” (liberated) and coarse gold before flotation or leaching with cyanide
Why?
Because “coarse” gold does not float and takes long time to be leached with cyanide under normal conditions

12. Direct Smelting Concentrates must be very rich
Smelting of low grade concentrates implies in Au losses to slag and high amounts of borax used
Lab tests show that concentrates must have >5,000 g Au/t
To increase Au in concentrates, Au recovery decreases, i.e more Au is lost in middling

13. Direct Smelting Flux creates slag with silicates and oxides
Concentrate
Low amount of concentrate
Flux creates slag with silicates and oxides
Concentrate must be rich in gold to create weight to collect all dispersed particles of gold to the bottom of the crucible
1100 oC
Rehani (2010) recommends 5% Au

14. Direct Smelting is not applicable to all ores
Test conducted according to Appel and Na-Oy, (2012) with 50g of concentrate:
Ratio 1:1 of Concentrate:Borax
High grade Au (3300ppm) in a concentrate from Ecuador with sulphides
Result: No extraction (No slag/bead interface)
All gold stayed in the slag
Silica, potassium nitrate and sodium bicarbonate, in order to form a flux that oxidize the sulphides in order to free the metals and promote separation between the bead and the slag. 14

15. Direct smelting Small amount of concentrate
Good for:
Small amount of concentrate
Concentrates must be very rich in gold
No sulphides
Slag must be very fluid (less viscous)
Energy consumption is high (110 oC)
Miners will lose lots of gold to obtain rich gold concentrates
But:
Main Problem:

16. Direct smelting
Source: Artisanal Gold Council, 2012

17. Chlorination Used in the late 1800s and early 1900s
Applicable to low-silver ores (AgCl forms a passivation layer)
Not used after introduction of cyanidation
Old procedure: vat leaching with chlorine-acid-rich solution
Addition of Bromine speeds up gold dissolution
MINTEK in South Africa devised the iGoli 17

18. Chlorination iGoli HCl and NaOCl are added to the concentrate pulp:
NaOCl + 2HCl ↔ NaCl + Cl2 + H2O
Sodium metabisulfite is added to the clarified solution to precipitate gold:
3 Na2SO2O5 + 3 H2O + 2 HAuCl4 ↔
3 NaHSO4 + 8 HCl + 2 Au
Photo: Mintek.co.za/igoli 18

19. iGoli Process Mintek, South Africa
Gold from gravity concentrates (>1000 g Au/t) is leached with hypochlorite and HCl
Gold is precipitated with sodium metabisulfite, or ferrous sulphate or SO2, etc.
Solution is filtered
Gold powder is hammered to become yellow
Many field tests in Africa
Great potential and open technology
Hard to find reagents in remote areas
Lots of training needed
Problems when the concentrate has sulphides

20. iGoli
Photo: Mintek, South Africa 2001

21. Chlorination of Concentrates
Tests with Ecuadorian gravity concentrate from centrifuge (1200 gAu/t)
Gold extraction obtained in 45 tests were very low, close to zero:
pH from 1.5 to 3
NaCl from 10 to 60 g/L
NaBr from 0 to 5 g/L
Voltage from 1(copper colored deposition) to 4V (heavy deposition with iron) 21

22. Chlorination of Concentrates
Main reasons of low Au extraction:
Gold in the sulphides are not available to be leached (gold is occluded in the sulphides)
Oxidation of sulphides is neeeded but it is slow
Iron and Copper goes into solution and contaminate the gold deposited on the cathode 22

23. Chlorination Alluvial gold (no sulphides)
Good for:
Alluvial gold (no sulphides)
Concentrates with free gold and no sulphides
Concentrates with sulphides must be roasted before leaching (this creates SO2 pollution
A little complicated for small miners...training is needed
But:
Main Problem:

24. Intensive Cyanidation
Tests with the same centrifuge conc. (1200 gAu/t):
20 g/L Sodium Cyanide
0.3 g/L Hydrogen Peroxide
50% solids
Extraction of 98.5% in 24 h leaching
Cyanidation in a small ball mill with a cartridge of activated charcoal
(Photo: Rodolfo S.)

25. Result of the Intensive Cyanidation (Cyanidation in a small ball mill)
95% of gold extracted in 8 h with 6 g/L NaCN
Use of activated carbon
Residual NaCN = 1.7 g/L
Free cyanide was destroyed with bleach before being discharged
The NaCN consumption was 0.95 g/kg of conc.
Ecuador

26. Leaching concentrates from centrifuge with cyanide
Replacing Hg with Cyanide Intensive Cyanidation of Concentrates (Field Tests in Brazil)
Leaching concentrates from centrifuge with cyanide
97% gold extracted in 12 h
Brazil
Sousa et al (2010). J. Cleaner Production . v.18, p

27. Intensive Cyanidation
Easy to transfer the technique to AGM
Cyanide is already being used in most AGM sites... then can be used only for concentrates
Oxygenated water speeds up the reaction
It can be replaced by Oxyclean (or Vanish) used to clean clothes
Residual cyanide is destroyed in the process

28. Other Reagents More complicated for AGM More capital needed
More difficult to acquire reagents
Companies are trying to sell “magic bullets” that are indeed “black boxes

29. Other Reagents Name Reagent pH Complex formed with Au Thiourea
NH2CSNH2
1-4
[Au(NH2CSNH2)2]+
Bromine
Br-
1-7
AuBr4-
Iodine I-
1-5
AuI2-
Thiocyanate
SCN-
1-3
[Au(SCN)4]-
Thiosulfate
S2O32-
8-11
[Au(S2O3)2]3-
Chlorine
Cl-, OCl-, Cl2
ClO3-
AuCl4-
Adapted from Trindade & Barbosa Filho. Reagentes Alternativos ao Cianeto. Chapter 9, p In: Extração de Ouro - Princípios, Tecnologia e Meio Ambiente. CETEM/CNPq, Rio de Janeiro, Brazil

30. Conclusion Intensive cyanidationc Cyanide Most suitable technology
Borax
Restrict to small amounts of high grade concentrate. Concentration ratio is too high.
Chlorine
Complicated and restricted to ores with no sulphides
Concentration
The best way to reduce emissions and releases
Intensive
cyanidationc