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

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

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)

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

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

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

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 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)

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

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

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

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

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

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:

Direct smelting Source: Artisanal Gold Council, 2012

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

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

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

iGoli Photo: Mintek, South Africa 2001

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

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

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:

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.)

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

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. 1757-1766

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

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

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. 211-252. In: Extração de Ouro - Princípios, Tecnologia e Meio Ambiente. CETEM/CNPq, Rio de Janeiro, Brazil

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