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