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

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Presentation on theme: "Susan Keane Gustavo Angeloci Marcello M. Veiga Ludovic Bernaudat Reducing/Replacing Hg in AGM Operations Suriname."— Presentation transcript:

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

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

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 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

6 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

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

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

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 Flux creates slag with silicates and oxides 1100 o C Concentrate Direct 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

14 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 applicable to all ores

15 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 o C) Miners will lose lots of gold to obtain rich gold concentrates Good for: But: Direct smelting 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 18 HCl and NaOCl are added to the concentrate pulp: NaOCl + 2HCl NaCl + Cl 2 + H 2 O Sodium metabisulfite is added to the clarified solution to precipitate gold: 3 Na 2 SO 2 O H 2 O + 2 HAuCl 4 3 NaHSO HCl + 2 Au Photo:

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 SO 2, 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: 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

23 Alluvial gold (no sulphides) Concentrates with free gold and no sulphides Concentrates with sulphides must be roasted before leaching (this creates SO 2 pollution A little complicated for small is needed Good for: But: Chlorination 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 (Photo: Rodolfo S.) Cyanidation in a small ball mill with a cartridge of activated charcoal

25 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 Cyanidation Result of the Intensive Cyanidation (Cyanidation in a small ball mill)

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

27 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 Cyanidation

28 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

29 NameReagentpHComplex formed with Au ThioureaNH 2 CSNH 2 1-4[Au(NH 2 CSNH 2 ) 2 ] + BromineBr - 1-7AuBr 4 - IodineI-I- 1-5AuI 2 - ThiocyanateSCN - 1-3[Au(SCN) 4 ] - ThiosulfateS 2 O [Au(S 2 O 3 ) 2 ] 3- Chlorine Cl -, OCl -, Cl 2 ClO AuCl 4 - 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

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