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Susan Keane
Gustavo AngelociMarcello M. Veiga
Ludovic Bernaudat
Reducing/Replacing Hg in AGM Operations
Suriname
Control Hg Control Hg BioavailabilityBioavailability
Technical Solutions for Hg PollutionTechnical Solutions for Hg Pollution
Alternative Alternative Processes to Processes to Replace HgReplace Hg
Reduce Hg Reduce Hg Use and Use and
EmissionsEmissions
Long-term Short-termMedium-term
Replace amalgamation
with other process
Avoid methylation covering or
dredging Hg-contaminated
tailings
Avoid exposure to Hg and
eliminate bad practices
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
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)
Concentrate Grade
Recovery
Mass of concentrate
Recovery and Concentrate Grade are Antagonists
More mass in the conc. = low grade... but high recovery
Less mass in the concentrate = high grade... but low recovery
Gold Not Liberated Liberated Gold
Assuming that the dark particles are gold and
the white are other minerals, e.g. quartz
0.07 mm
Gold Recovery is Low when Gold is not Liberated
Gravity Separation
Good Grinding Does not Need Sophisticated Equipment
Mozambique
Indonesia
NO Hg ADDED in the ball millsNO Hg ADDED in the ball mills
10 kg of ore ground with 10 kg of ore ground with 14 steel balls for 45 min. 14 steel balls for 45 min.
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
Gravity ConcentrationGravity 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)
• Direct smelting of concentrates• Chlorination• Intensive Cyanidation• Other lixiviants
Grinding
Ore
Concentration
TailingConcentrateCyanidation
Other lixiviants
Direct smelting
What to Do with Concentrates?What to Do with Concentrates?
Direct SmeltingDirect 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
Direct SmeltingDirect 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
Flux creates slag with silicates and oxides
1100 oC
ConcentrateDirect SmeltingDirect Smelting
Concentrate must be rich in gold to create weight to collect
all dispersed particles of gold to the bottom of the crucible
Rehani (2010) recommends 5% Au
Low amount of concentrate
• 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
Direct Smelting is not Direct Smelting is not applicable to all oresapplicable to all ores
• 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
Good for:
But:
Direct smeltingDirect smelting
Main Problem:
Direct smeltingDirect smelting
Source: Artisanal Gold Council, 2012
ChlorinationChlorination
• 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
Chlorination iGoliChlorination iGoli
18
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
iGoli ProcessiGoli 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
iGoliiGoli
Photo: Mintek, South Africa 2001
Chlorination of ConcentratesChlorination 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
Chlorination of ConcentratesChlorination of Concentrates
• Main reasons of low Au extraction:
1. Gold in the sulphides are not available to be leached (gold is occluded in the sulphides)
2. Oxidation of sulphides is neeeded but it is slow
3. Iron and Copper goes into solution and contaminate the gold deposited on the cathode
22
• 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
Good for:
But:
ChlorinationChlorination
Main Problem:
Intensive CyanidationIntensive 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
(Photo: Rodolfo S.)
Cyanidation in a small ball mill with a cartridge of activated charcoal
• 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
Result of the Intensive CyanidationResult of the Intensive Cyanidation(Cyanidation in a small ball mill)
97% gold extracted in 12 h
Leaching concentrates from centrifuge with cyanide
Replacing Hg with CyanideReplacing Hg with CyanideIntensive Cyanidation of ConcentratesIntensive Cyanidation of Concentrates
(Field Tests in Brazil)(Field Tests in Brazil)
BrazilSousa et al (2010). J. Cleaner Production . v.18, p. 1757-1766
• 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
Intensive CyanidationIntensive Cyanidation
• More complicated for AGM
• More capital needed
• More difficult to acquire reagents
• Companies are trying to sell “magic bullets” that are indeed “black boxes
Other ReagentsOther Reagents
Other ReagentsOther ReagentsName 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-
ChlorineCl-, OCl-, Cl2
ClO3-
1-4 AuCl4-
Adapted from Trindade & Barbosa Filho. Reagentes Alternativos ao Cianeto. Chapter 9, p. 211-252. In: Extração de Ouro - Princípios, Tecnologia e Meio Ambiente. CETEM/CNPq, Rio de Janeiro, Brazil
ConclusionConclusion
Intensive cyanidationc