ReducingReplacing Hg in AGM Operations Susan Keane Gustavo
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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 Replace amalgamation with other process Long-term Control Hg Bioavailability Reduce Hg Use and Emissions Avoid methylation covering or dredging Hgcontaminated tailings Avoid exposure to Hg and eliminate bad practices 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 Recovery Concentrate Grade Mass of concentrate More mass in the conc. = low grade. . . but high recovery
Gold Recovery is Low when Gold is not Liberated Gold Not Liberated 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 Other lixiviants Direct smelting Tailing
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
Concentrate Direct Smelting Low amount of concentrate Flux creates slag with silicates and oxides 1100 o. C 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
Direct Smelting is not applicable to all ores • Test conducted according to Appel and Na. Oy, (2012) with 50 g of concentrate: • Ratio 1: 1 of Concentrate: Borax • High grade Au (3300 ppm) in a concentrate from Ecuador with sulphides • Result: No extraction (No slag/bead interface) • All gold stayed in the slag
Direct smelting Good for: • Small amount of concentrate • Concentrates must be very rich in gold • No sulphides But: • Slag must be very fluid (less viscous) • Energy consumption is high (110 o. C) Main Problem: • Miners will lose lots of gold to obtain rich gold concentrates
Direct smelting Source: Artisanal Gold Council, 2012
Chlorination • Used in the late 1800 s and early 1900 s • Applicable to low-silver ores (Ag. Cl forms a passivation layer) • Not used after introduction of cyanidation • Old procedure: vat leaching with chlorineacid-rich solution • Addition of Bromine speeds up gold dissolution • MINTEK in South Africa devised the i. Goli 17
Chlorination i. Goli HCl and Na. OCl are added to the concentrate pulp: Na. OCl + 2 HCl ↔ Na. Cl + Cl 2 + H 2 O Sodium metabisulfite is added to the clarified solution to precipitate gold: Photo: Mintek. co. za/igoli 3 Na 2 SO 2 O 5 + 3 H 2 O + 2 HAu. Cl 4 ↔ 3 Na. HSO 4 + 8 HCl + 2 Au 18
i. Goli 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
i. Goli Photo: Mintek, South Africa 2001
Chlorination of Concentrates • Tests with Ecuadorian gravity concentrate from centrifuge (1200 g. Au/t) • Gold extraction obtained in 45 tests were very low, close to zero: – p. H from 1. 5 to 3 – Na. Cl from 10 to 60 g/L – Na. Br from 0 to 5 g/L – Voltage from 1(copper colored deposition) to 4 V (heavy deposition with iron) 21
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
Chlorination Good for: • Alluvial gold (no sulphides) • Concentrates with free gold and no sulphides But: • Concentrates with sulphides must be roasted before leaching (this creates SO 2 pollution Main Problem: • A little complicated for small miners. . . training is needed
Intensive Cyanidation Tests with the same centrifuge conc. (1200 g. Au/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 Na. CN • Use of activated carbon • Residual Na. CN = 1. 7 g/L • Free cyanide was destroyed with bleach before being discharged • The Na. CN consumption was 0. 95 g/kg of conc. Ecuador
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 p. H Complex formed with Au Thiourea NH 2 CSNH 2 1 -4 [Au(NH 2 CSNH 2)2]+ Bromine Br- 1 -7 Au. Br 4 - I- 1 -5 Au. I 2 - Thiocyanate SCN- 1 -3 [Au(SCN)4]- Thiosulfate S 2 O 32 - 8 -11 [Au(S 2 O 3)2]3 - Cl-, OCl-, Cl 2 Cl. O 3 - 1 -4 Au. Cl 4 - Iodine Chlorine 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 Cyanide cyanidationc Borax Chlorine Concentration • Most suitable technology • Restrict to small amounts of high grade concentrate. Concentration ratio is too high. • Complicated and restricted to ores with no sulphides • The best way to reduce emissions and releases
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