1 / 30

Susan Keane Gustavo Angeloci Marcello M. Veiga Ludovic Bernaudat

Reducing/Replacing Hg in AGM Operations. Susan Keane Gustavo Angeloci Marcello M. Veiga Ludovic Bernaudat. Suriname. Technical Solutions for Hg Pollution. Control Hg Bioavailability. Reduce Hg Use and Emissions. Alternative Processes to Replace Hg.

sorcha
Download Presentation

Susan Keane Gustavo Angeloci Marcello M. Veiga Ludovic Bernaudat

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. Technical Solutions for Hg Pollution Control Hg Bioavailability Reduce Hg Use and Emissions Alternative Processes to Replace Hg Avoid methylation covering or dredging Hg-contaminated tailings Avoid exposure to Hg and eliminate bad practices Replace amalgamation with other process 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 Recovery Concentrate Grade Mass of concentrate More mass in the conc. = low grade... but high recovery

  6. 0.07 mm Gold Recovery is Low when Gold is not Liberated Liberated Gold Gold Not Liberated 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. 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 Centrifuges Are the Most Efficient Gravity Concentrators

  9. 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) Gravity Concentration

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

  11. 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

  12. 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

  13. Direct Smelting 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

  15. Direct smelting 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. 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 Process

  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 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. Replacing Hg with CyanideIntensive 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

  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 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

  30. Conclusion Intensive cyanidationc

More Related