Marcello M. Veiga Associate Professor of University of British Columbia Vancouver, Canada  Reducing/Replacing Hg in ASM Op...
Control Hg Bioavailability Technical Solutions for Hg Pollution Alternative Processes to Replace Hg Reduce Hg Use and Emis...
Technical Solutions Alternative Processes Cyanidation Electrolytic Control Hg Bioavailability Polluted  Sites  Mercury  Di...
Amalgamation of the Whole Ore Huge Hg losses, large environmental problem Burning Amalgams in Pans Health problem for mine...
Amalgamation of Whole Ore is the Main Cause of Hg Loss <ul><li>There is a false perception that the main Hg loss in ASM is...
Hg is retained in the riffles giving the impression that amalgamation occurred in the pool Hg is spread in the pool and pu...
Hg goes with tailings Brazil, 1999 Amalgamation of the Whole Ore  (Mercury in the Sluice Box) Hg is added here
<ul><li>Between 1860-1895: 6,350 tonnes Hg lost to Carson River, Nevada  </li></ul><ul><li>12,000 tonnes Hg lost in Califo...
Plates: attrition remove Hg from plates = Hg is lost Amalgamation of the Whole Ore  (Cu-Plates with Hg) Brazil, 2008 Venez...
Venezuela, 2003 Amalgamation of the Whole Ore  (Cu-Plates with Hg)
Venezuela, 2003 Amalgamation of the Whole Ore  (Cu-Plates with Hg)
Zimbabwe, 2005 Use of Copper Amalgamation Plates generates tailings highly contaminated with Hg (usually 50-200 ppm Hg) Am...
Amalgamation of the Whole Ore  (Grinding with Hg) Indonesia, Talawaan, 2001 Adding Hg into the Grinding Circuit 25 to 30% ...
<ul><li>Colombia: Hg in the “cocos” (ball mills) </li></ul><ul><li>About 4 ounces of Hg is added in each “coco” (ball mill...
Additives used in Amalgamation  by Artisanal Miners <ul><li>Miners use these methods to reduce Hg flouring and consequentl...
Brown Sugar in the Ball Mills with Hg Ecuador, 2004
Amalgamation in Antioquia, Colombia “ coco”” elutriator “ coco” (with balls) elutriator cyanidation 70kg ore 30L water 120...
<ul><li>Tailings (up to 5000 ppm Hg) are later leached with cyanide </li></ul><ul><li>50 - 70% of Au recovered by amalgama...
Using Urine to Amalgamate  Chile, 2008
Forming Sodium-amalgam  (increase coalescence = reduces Hg flouring = less Hg loss with tailing) sodium-amalgam is more co...
Brazil, 2006 Zimbabwe, 2006 3800 miners in the GMP site in the Amazon adopted this technique
<ul><li>Amalgamate only gravity concentrates  </li></ul><ul><li>Promote good contact between Hg and gold (particles must b...
Tanzania, 2005 Manual Amalgamation Must Be Avoided
<ul><li>One part of Hg to 100 parts of concentrate </li></ul>Glass Amalgamation Barrel (3 L) Using an Amalgamating Barrel ...
<ul><li>Excavated pool lined with a plastic trap </li></ul><ul><li>When the pool is full, cover it </li></ul><ul><li>CONTA...
Reducing %Hg in amalgam from 40% (manual squeeze) to 20% (centrifuge) filtered mercury PVC cups amalgam clamps piece of cl...
Manual Centrifuge Brazil, 2006 Filtering Amalgam
Alternative to Reduce/Replace Hg <ul><li>Direct smelting of  concentrates </li></ul><ul><li>Electrolytic Process </li></ul...
Gold Not Liberated Liberated Gold Assuming that the dark particles are gold Gold Concentration is Fundamental Gold Recover...
In order to concentrate it’s necessary to LIBERATE gold particles <ul><li>Liberation is obtained by comminution (crushing ...
Good Grinding Does not Need Sophisticated Equipment Mozambique, 2005 Indonesia, 2004 NO Hg ADDED in the ball mills 10 kg o...
Panning in a closed pool (away from rivers) Some Hg-free gold can be obtained but in most cases Hg is introduced at the en...
Zig-zag Sluice Indonesia, 2006 Zig-zag sluices increase chances of capturing gold Suriname, 2007
Concentrating Gold in Sluice Box Indonesia, 2006
Centrifuges <ul><li>Good for coarse and fine gold (0.05 mm) </li></ul><ul><li>Much more efficient than sluices </li></ul><...
Why Are Centrifuges More Efficient? <ul><li>S.G. quartz = 2.7 </li></ul><ul><li>S.G. gold = 19 </li></ul>= quartz gold Whe...
Why Are Centrifuges More Efficient? When the particles are subjected to G = 60, the  difference in fall velocity is much h...
<ul><li>Introducing new centrifuges developed with UBC and the company Falcon Concentrators(Langley) </li></ul><ul><li>Pri...
Cleangold ®  Sluice Box Magnetic Sluice Boxes Venezuela, 2003 <ul><li>Ore must have magnetite </li></ul><ul><li>Iron bits ...
Sudan, 2004 Using Small Cleangold Panning Tailings with Cleangold
Concentrating Gold Using Fridge Magnets Mozambique, 2005
Gravity Concentration of a Gold Ore from Talaawan, Indonesia HMMS = Homemade magnetic sluice P80 = 0.25  m m Conclusion: g...
Concentration <ul><li>Gravity Concentration </li></ul><ul><ul><li>Depends on gold liberation (grinding) and how fine is th...
Concentration <ul><li>Flotation </li></ul><ul><ul><li>Depends on gold liberation (grinding)  </li></ul></ul><ul><ul><li>Co...
Flotation (Chile) <ul><li>Homemade flotation cells </li></ul><ul><li>Use xanthate and pine oil </li></ul><ul><li>Miners se...
Concentration <ul><li>Agglomeration </li></ul><ul><ul><li>Depends on gold liberation (grinding)  </li></ul></ul><ul><ul><l...
Direct Smelting <ul><li>Concentrates must be very rich </li></ul><ul><li>Smelting of low grade concentrates implies in Au ...
Electrolytic Process <ul><li>SALTEM Process (devised by CETEM, Brazil) </li></ul><ul><ul><li>Gold ore (or concentrate) is ...
Electrolytic Process - + Cl 2 ClO - AuCl 4 - NaCl solution Pulp of gold ore
Cyanidation <ul><li>ASM are familiar with the process </li></ul><ul><li>Problems with NaCN in ASM: </li></ul><ul><ul><li>P...
Ecuador, 2006 <ul><li>Zinc is used to precipitate gold from the cyanide solution </li></ul><ul><li>Zinc is evaporated cont...
Zimbabwe, 2005 <ul><li>Cyanidation of Hg-contaminated tailings increases the bioavailability of mercury </li></ul><ul><li>...
<ul><li>Abandoned cyanidation heap in São Chico, Brazil near the water stream </li></ul><ul><li>Tailings are still full of...
<ul><li>Carnivorous fish, Ave = 4.16  ppm Hg </li></ul><ul><li>Non-carnivorous, Ave = 1.33 ppm Hg </li></ul><ul><li>60% of...
Ecuador, 2007 Amalgamation of a Pre-concentrate  Followed by Cyanidation - Ecuador A pre-concentrate is obtained in sluice...
Ecuador, 2007 Some operations use pH = 7 in Cyanidation Amalgamation of a Pre-concentrate  Followed by Cyanidation - Ecuad...
Portovelo, Ecuador, 2007 <ul><li>Tailings with Hg cyanide are dumped into the Amarillo River, Ecuador </li></ul><ul><li>92...
Photo AJ Gunson China, 2002 <ul><li>Brazil </li></ul><ul><li>China </li></ul><ul><li>Colombia </li></ul><ul><li>Ecuador </...
Cyanidation Gold Ore from Talaawan, Indonesia HMMS = Homemade magnetic sluice  Au (ppm or g/t) P80 = 0.25 mm 6 hours of le...
Improving Cyanidation for ASM <ul><li>Instead of using Hg in ball mills, miners could use NaCN </li></ul><ul><li>No additi...
Mill-leaching (Cyanide in Ball Mill) Gold Ore from Talaawan, Indonesia  Grinding with NaCN for 2h + Leaching Time 14.8 93....
<ul><li>Ore ground in Chilean mill  80% passing 0.150mm </li></ul><ul><li>Pre-concentrate from sluice box (17.3 g Au/t): <...
<ul><li>Amalgamation of the pre-concentrate was manual </li></ul><ul><li>Use of brown sugar </li></ul><ul><li>8 hours of a...
<ul><li>Miners are familiar but not with Carbon in Leaching </li></ul><ul><li>Activated carbon was added after 7 hours of ...
Cyanidation in Agitated Tank Ecuador, 2007
<ul><li>95%  of gold extracted in 8 h (2h grinding with CN and 6 hours leaching) </li></ul><ul><li>Use of activated carbon...
<ul><li>Elution of Activated Carbon (AC)   </li></ul><ul><li>NaCN = 2g/L, NaOH = 10 g/L </li></ul><ul><li>Alcohol = 20% </...
<ul><li>Faster than agitated tank leaching: </li></ul><ul><ul><li>8h of mill-leaching = 31 h of agitated tank   </li></ul>...
Other Reagents Adapted from Trindade & Barbosa Filho. Reagentes Alternativos ao Cianeto.  Chapter 9, p. 211-252. In:  Extr...
Other Reagents <ul><li>iGoli Process (Mintek, South Africa) </li></ul><ul><ul><li>Gold from gravity concentrates (>1000 g ...
iGoli Photo: Mintek, South Africa 2001
<ul><li>There is no panacea </li></ul><ul><li>There are many processes to replace Hg  but all cases are site-specific (lev...
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Marcello M. Veiga, Associate Professor of University of British Columbia, Vancouver, Canada, Reducing/Replacing Mercury in ASM Operations

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Marcello M. Veiga, Associate Professor of University of British Columbia, Vancouver, Canada, Reducing/Replacing Mercury in ASM Operations

  1. 1. Marcello M. Veiga Associate Professor of University of British Columbia Vancouver, Canada Reducing/Replacing Hg in ASM Operations Suriname, 2008 Presented at the 8 th CASM – Community and Small-scale Mining Annual Conference Brasilia October 6-12, 2008
  2. 2. Control Hg Bioavailability Technical Solutions for Hg Pollution Alternative Processes to Replace Hg Reduce Hg Use and Emissions Long-term Short-term Medium-term Replace amalgamation with other process Avoid methylation covering or dredging Hg-contaminated tailings Avoid exposure to Hg and eliminate bad practices
  3. 3. Technical Solutions Alternative Processes Cyanidation Electrolytic Control Hg Bioavailability Polluted Sites Mercury Dispersed Re-suspension of Sediments Covering Dredging & Treatment Bioaccumulation Occurring ? Monitoring N Y Y Change of Food Habits Fish Hair Process Reduction of Hg Emissions Systemic Solutions Individual Solutions Processing Centers Organization of Associations Law Enforcement Permanent Biological Monitoring Education Retorts & special Fume hoods Activated Hg Use Hg far from people & Technical Assistance Bioaccumulation Occurring ? Amalgamate concentrates Other Lixiviants Direct Smelting
  4. 4. Amalgamation of the Whole Ore Huge Hg losses, large environmental problem Burning Amalgams in Pans Health problem for miners, family and neighbors Hg o CH 3 Hg in fish Cyanidation of Hg-contaminated tailings Hg o vapor lungs and/or Reduction of Hg Emissions Depends on the Amalgamation Procedure
  5. 5. Amalgamation of Whole Ore is the Main Cause of Hg Loss <ul><li>There is a false perception that the main Hg loss in ASM is when miners burn the amalgam in open air </li></ul><ul><li>The main losses occur when the whole ore is amalgamated: </li></ul><ul><ul><li>Hg is spread on the ground or in sluice boxes </li></ul></ul><ul><ul><li>Cu-Hg plates </li></ul></ul><ul><ul><li>Grinding with mercury </li></ul></ul>
  6. 6. Hg is retained in the riffles giving the impression that amalgamation occurred in the pool Hg is spread in the pool and pumped to the sluice box Venezuela, 1995 Amalgamation of the Whole Ore (Mercury Spread on the Ground)
  7. 7. Hg goes with tailings Brazil, 1999 Amalgamation of the Whole Ore (Mercury in the Sluice Box) Hg is added here
  8. 8. <ul><li>Between 1860-1895: 6,350 tonnes Hg lost to Carson River, Nevada </li></ul><ul><li>12,000 tonnes Hg lost in California and Nevada </li></ul><ul><li>Archives from British Columbia: 13kg of Hg/day/sluice used by miners at Cariboo, Canada (1856) </li></ul>Amalgamation of the Whole Ore (Mercury in the Sluice Box) (it was a common practice in North America during gold rush) Canada, 1856 This is 20,000 x more Hg than used by a Brazilian ASM
  9. 9. Plates: attrition remove Hg from plates = Hg is lost Amalgamation of the Whole Ore (Cu-Plates with Hg) Brazil, 2008 Venezuela, 2003
  10. 10. Venezuela, 2003 Amalgamation of the Whole Ore (Cu-Plates with Hg)
  11. 11. Venezuela, 2003 Amalgamation of the Whole Ore (Cu-Plates with Hg)
  12. 12. Zimbabwe, 2005 Use of Copper Amalgamation Plates generates tailings highly contaminated with Hg (usually 50-200 ppm Hg) Amalgamation of the Whole Ore (Cu-Plates with Hg)
  13. 13. Amalgamation of the Whole Ore (Grinding with Hg) Indonesia, Talawaan, 2001 Adding Hg into the Grinding Circuit 25 to 30% of the Hg added is lost with tailings (“flouring”)
  14. 14. <ul><li>Colombia: Hg in the “cocos” (ball mills) </li></ul><ul><li>About 4 ounces of Hg is added in each “coco” (ball mill) </li></ul>Colombia, 2007 Grinding with Hg Antioquia Province - Colombia
  15. 15. Additives used in Amalgamation by Artisanal Miners <ul><li>Miners use these methods to reduce Hg flouring and consequently Hg losses with Tailings: </li></ul><ul><li>Clean Hg with boiling water (Indonesia) </li></ul><ul><li>Lime juice (Laos, Colombia) </li></ul><ul><li>Caustic soda (Colombia) </li></ul><ul><li>Cyanide (Zimbabwe) </li></ul><ul><li>Brown sugar (Ecuador) </li></ul><ul><li>Molasse (Colombia) </li></ul><ul><li>Urine (Chile) </li></ul><ul><li>Sodium-amalgam (Colombia, Brazil) </li></ul>
  16. 16. Brown Sugar in the Ball Mills with Hg Ecuador, 2004
  17. 17. Amalgamation in Antioquia, Colombia “ coco”” elutriator “ coco” (with balls) elutriator cyanidation 70kg ore 30L water 120g Hg 50kg balls (1/2 volume) 200g Ca(OH) 2 4 hours Excess Hg + amalgam 90g Hg 10mL molasse 10L lime juice NaHCO 3 pH 5 pH 11 3 hours Excess Hg + amalgam
  18. 18. <ul><li>Tailings (up to 5000 ppm Hg) are later leached with cyanide </li></ul><ul><li>50 - 70% of Au recovered by amalgamation </li></ul><ul><li>25 - 30% of Hg is lost </li></ul><ul><li>50 to 100 tonnes Hg/a lost in the Antioquia Province </li></ul>Colombia, 2007 Grinding with Hg Antioquia Province - Colombia
  19. 19. Using Urine to Amalgamate Chile, 2008
  20. 20. Forming Sodium-amalgam (increase coalescence = reduces Hg flouring = less Hg loss with tailing) sodium-amalgam is more consistent than pure Hg Also called “ Hg Activation” Battery 12 V wire Mercury Water with NaCl (10%) + - Graphite rod Dr Pantoja’s Method
  21. 21. Brazil, 2006 Zimbabwe, 2006 3800 miners in the GMP site in the Amazon adopted this technique
  22. 22. <ul><li>Amalgamate only gravity concentrates </li></ul><ul><li>Promote good contact between Hg and gold (particles must be cleaned, e.g. detergent) </li></ul><ul><li>Avoid severe grinding that causes “flouring” </li></ul><ul><li>Additives to reduce Hg surface tension </li></ul><ul><li>Use activated Hg (Na or K-amalgam). K-amalgam is used in Colombia to capture alluvial Pt (Mineros de Antioquia SA) </li></ul><ul><li>Centrifuge amalgam after amalgamation to remove excess mercury </li></ul>What Can Be Done to Improve Amalgamation and Reduce Hg Emissions
  23. 23. Tanzania, 2005 Manual Amalgamation Must Be Avoided
  24. 24. <ul><li>One part of Hg to 100 parts of concentrate </li></ul>Glass Amalgamation Barrel (3 L) Using an Amalgamating Barrel to Improve Amalgamation Indonesia, 2006
  25. 25. <ul><li>Excavated pool lined with a plastic trap </li></ul><ul><li>When the pool is full, cover it </li></ul><ul><li>CONTAMINATED TAILINGS MUST NOT BE RECYCLED </li></ul>Brazil, 2007 After Amalgamation, the Amalgam Must Be Separated from the Heavy Minerals
  26. 26. Reducing %Hg in amalgam from 40% (manual squeeze) to 20% (centrifuge) filtered mercury PVC cups amalgam clamps piece of cloth Indonesia, 2001
  27. 27. Manual Centrifuge Brazil, 2006 Filtering Amalgam
  28. 28. Alternative to Reduce/Replace Hg <ul><li>Direct smelting of concentrates </li></ul><ul><li>Electrolytic Process </li></ul><ul><li>Cyanidation </li></ul><ul><li>Other lixiviants </li></ul>Grinding Ore Concentration Tailing Concentrate Cyanidation Other lixiviants Direct smelting
  29. 29. Gold Not Liberated Liberated Gold Assuming that the dark particles are gold Gold Concentration is Fundamental Gold Recovery is Low when Gold is not Liberated Gravity Separation 0.07 mm
  30. 30. In order to concentrate it’s necessary to LIBERATE gold particles <ul><li>Liberation is obtained by comminution (crushing and grinding) </li></ul><ul><li>Gold occurs occluded in other minerals </li></ul><ul><li>Manual crushing is inefficient </li></ul>Tanzania, 2007
  31. 31. Good Grinding Does not Need Sophisticated Equipment Mozambique, 2005 Indonesia, 2004 NO Hg ADDED in the ball mills 10 kg of ore ground with 14 steel balls for 45 min.
  32. 32. Panning in a closed pool (away from rivers) Some Hg-free gold can be obtained but in most cases Hg is introduced at the end of the panning step to capture fine Au Mozambique, 2005 Gold Concentration by Panning
  33. 33. Zig-zag Sluice Indonesia, 2006 Zig-zag sluices increase chances of capturing gold Suriname, 2007
  34. 34. Concentrating Gold in Sluice Box Indonesia, 2006
  35. 35. Centrifuges <ul><li>Good for coarse and fine gold (0.05 mm) </li></ul><ul><li>Much more efficient than sluices </li></ul><ul><li>Used by mining companies </li></ul><ul><li>High cost </li></ul><ul><li>High maintanance and control </li></ul><ul><li>Most common: Knelson and Falcon, both from B.C., Canada </li></ul>
  36. 36. Why Are Centrifuges More Efficient? <ul><li>S.G. quartz = 2.7 </li></ul><ul><li>S.G. gold = 19 </li></ul>= quartz gold When the particles are subjected to the gravity force (G =1), one gold particle of 0.02 mm fall in water as fast as a quartz particle of 0.07 mm Then there is no way to separate them
  37. 37. Why Are Centrifuges More Efficient? When the particles are subjected to G = 60, the difference in fall velocity is much higher than with G = 1 0 1 2 3 4 5 6 0 20 40 60 Gravity (G) Difference in Fall Velocity (mmsec)
  38. 38. <ul><li>Introducing new centrifuges developed with UBC and the company Falcon Concentrators(Langley) </li></ul><ul><li>Price <US$4000 </li></ul><ul><li>1-2 tonnes/h </li></ul><ul><li>Good for fine gold </li></ul>New Falcon Centrifuge for ASM Canada, 2006
  39. 39. Cleangold ® Sluice Box Magnetic Sluice Boxes Venezuela, 2003 <ul><li>Ore must have magnetite </li></ul><ul><li>Iron bits from hammer mills can also be attracted </li></ul><ul><li>Like any other gravity concentrator, uliberated gold is lost with middlings </li></ul>
  40. 40. Sudan, 2004 Using Small Cleangold Panning Tailings with Cleangold
  41. 41. Concentrating Gold Using Fridge Magnets Mozambique, 2005
  42. 42. Gravity Concentration of a Gold Ore from Talaawan, Indonesia HMMS = Homemade magnetic sluice P80 = 0.25 m m Conclusion: gold is not liberated and it’s very fine 5% 8% 7% Au Recov ery (%) 11.31 11.40 11.95 Tailings 16.00 117.00 64.00 Concentrate 11.50 12.30 12.60 Feed Knelson ® HMMS Cleangold ® Au (ppm or g/t)  
  43. 43. Concentration <ul><li>Gravity Concentration </li></ul><ul><ul><li>Depends on gold liberation (grinding) and how fine is the gold (size of the particle) </li></ul></ul><ul><ul><li>Gold in the middling product (unliberated) in a problem </li></ul></ul><ul><ul><li>For primary ore, gold recovery rarely goes beyond 30-40% (even in industries) </li></ul></ul>
  44. 44. Concentration <ul><li>Flotation </li></ul><ul><ul><li>Depends on gold liberation (grinding) </li></ul></ul><ul><ul><li>Concentrates are usually poor (200 g Au/t) </li></ul></ul><ul><ul><li>Concentrate produced must be submitted to amalgamation or cyanidation </li></ul></ul><ul><ul><li>Control of pH and reagents is needed </li></ul></ul><ul><ul><li>Little investment in flotation cell is needed </li></ul></ul>
  45. 45. Flotation (Chile) <ul><li>Homemade flotation cells </li></ul><ul><li>Use xanthate and pine oil </li></ul><ul><li>Miners sell the gold and copper concentrate </li></ul><ul><li>Sun-dried </li></ul>Chile, 2008
  46. 46. Concentration <ul><li>Agglomeration </li></ul><ul><ul><li>Depends on gold liberation (grinding) </li></ul></ul><ul><ul><li>Agglomerates (5 mm) of coal and oil are formed and put in contact of a pulp of gravity concentrate in one or more cycles. </li></ul></ul><ul><ul><li>Recoveries of 90% were obtained in one step. </li></ul></ul><ul><ul><li>Envi-Tech Inc of Edmonton, Canada has its own proprietary agglomerate (gold absorbent) </li></ul></ul><ul><ul><li>Melted paraffin to collect gold in an acidic medium </li></ul></ul><ul><ul><li>Encouraging results but none of these methods provide a simple, cheap and quick alternative for unskilled artisanal miners. </li></ul></ul>
  47. 47. Direct Smelting <ul><li>Concentrates must be very rich </li></ul><ul><li>Smelting of low grade concentrates implies in Au losses to slag and high amounts of borax used </li></ul><ul><li>Lab tests show that the threshold is around 5,000 g Au/t in a concentrate </li></ul><ul><li>To increase Au in concentrates, Au recovery decreases, i.e more Au is lost in middling </li></ul>
  48. 48. Electrolytic Process <ul><li>SALTEM Process (devised by CETEM, Brazil) </li></ul><ul><ul><li>Gold ore (or concentrate) is mixed with NaCl (1 Mol/l) which is transformed by electrolysis into a mixture of sodium hypochorite-chlorate </li></ul></ul><ul><ul><li>Seawater can also be used </li></ul></ul><ul><ul><li>More than 95% of the gold dissolves within 4 hours and is collected on a graphite cathode </li></ul></ul><ul><ul><li>Solution is recycled minimizing effluent discharge </li></ul></ul><ul><ul><li>NaCl = 100 kg/tonne of ore </li></ul></ul><ul><ul><li>Energy = 170 kwh/kg of Au </li></ul></ul><ul><ul><li>Plastic tanks are used, reducing investment cost </li></ul></ul><ul><ul><li>Very good potential but a bit complicated for ASM </li></ul></ul>
  49. 49. Electrolytic Process - + Cl 2 ClO - AuCl 4 - NaCl solution Pulp of gold ore
  50. 50. Cyanidation <ul><li>ASM are familiar with the process </li></ul><ul><li>Problems with NaCN in ASM: </li></ul><ul><ul><li>Poor control of the pH </li></ul></ul><ul><ul><li>Use of Zn precipitation followed by burning </li></ul></ul><ul><ul><li>High investment in agitated tanks </li></ul></ul><ul><ul><li>No CN destruction is applied </li></ul></ul><ul><ul><li>Cyanidation of Hg-contaminated material makes Hg more bioavailable </li></ul></ul><ul><ul><li>Poor management of tailings with CN </li></ul></ul>4Au + 8CN - + O 2 + 2H 2 O = 4Au(CN) 2 - + 4OH - 2Au(CN) 2 - + Zn = Zn(CN) 4 2- + 2Au (Merrill-Crowe process)
  51. 51. Ecuador, 2006 <ul><li>Zinc is used to precipitate gold from the cyanide solution </li></ul><ul><li>Zinc is evaporated contaminating the whole area </li></ul>Cyanidation
  52. 52. Zimbabwe, 2005 <ul><li>Cyanidation of Hg-contaminated tailings increases the bioavailability of mercury </li></ul><ul><li>Mercury-cyanide species are formed </li></ul>Cyanidation
  53. 53. <ul><li>Abandoned cyanidation heap in São Chico, Brazil near the water stream </li></ul><ul><li>Tailings are still full of Hg when submitted to cyanidation </li></ul>Brazil, S ã o Chico, 2003 Hg-contaminated Tailings are Submitted to Cyanidation - São Chico, Brazil
  54. 54. <ul><li>Carnivorous fish, Ave = 4.16 ppm Hg </li></ul><ul><li>Non-carnivorous, Ave = 1.33 ppm Hg </li></ul><ul><li>60% of fish >0.5 ppm Hg (guideline for fish consumption) </li></ul><ul><li>One fish sample = 22 ppm Hg </li></ul>Hg-contaminated Tailings are Submitted to Cyanidation - São Chico, Brazil
  55. 55. Ecuador, 2007 Amalgamation of a Pre-concentrate Followed by Cyanidation - Ecuador A pre-concentrate is obtained in sluice boxes (discharge every hour)… … and amalgamated in barrels
  56. 56. Ecuador, 2007 Some operations use pH = 7 in Cyanidation Amalgamation of a Pre-concentrate Followed by Cyanidation - Ecuador Tailings (with or without Hg) are leached with NaCN
  57. 57. Portovelo, Ecuador, 2007 <ul><li>Tailings with Hg cyanide are dumped into the Amarillo River, Ecuador </li></ul><ul><li>92 cyanidation tanks in the town of Portovelo </li></ul>Amalgamation of a Pre-concentrate Followed by Cyanidation
  58. 58. Photo AJ Gunson China, 2002 <ul><li>Brazil </li></ul><ul><li>China </li></ul><ul><li>Colombia </li></ul><ul><li>Ecuador </li></ul><ul><li>Indonesia </li></ul><ul><li>Peru </li></ul><ul><li>Philippines </li></ul><ul><li>Venezuela </li></ul><ul><li>Zimbabwe </li></ul>Misuse of Cyanidation with Amalgamation Hg is added while grinding the ore Cu plate Cyanide
  59. 59. Cyanidation Gold Ore from Talaawan, Indonesia HMMS = Homemade magnetic sluice Au (ppm or g/t) P80 = 0.25 mm 6 hours of leaching in agitated tank 84% 5% 8% 7% Au Recov ery (%) 1.96 11.31] 11.40 11.95 Tailings 32.00 16.00 117.00 64.00 Concentrate 12.20 11.50 12.30 12.60 Feed Cyanidation Knelson ® HMMS Cleangold ®
  60. 60. Improving Cyanidation for ASM <ul><li>Instead of using Hg in ball mills, miners could use NaCN </li></ul><ul><li>No additional investment needed </li></ul><ul><li>Training is fundamental </li></ul><ul><li>Quick leaching time </li></ul><ul><li>High Au recovery (>90%) </li></ul>
  61. 61. Mill-leaching (Cyanide in Ball Mill) Gold Ore from Talaawan, Indonesia Grinding with NaCN for 2h + Leaching Time 14.8 93.6 0.86 10.7 10.7 24 15.0 93.3 0.88 11.1 11.4 8 17.5 93.1 0.78 11.9 12.1 6 16.2 85.5 0.88 12.2 12.3 4 14.7 77.9 0.88 12.2 12.3 2 Au in the sample, g/tonne %Au Recovered Final Na CN g/L pH Initial Final Leaching Time, h
  62. 62. <ul><li>Ore ground in Chilean mill 80% passing 0.150mm </li></ul><ul><li>Pre-concentrate from sluice box (17.3 g Au/t): </li></ul><ul><ul><li>Amalgamation (160 kg) </li></ul></ul><ul><ul><li>Cyanidation in agitated tank (695 kg) </li></ul></ul><ul><ul><li>Field tests of mill-leaching (80kg) </li></ul></ul>Field Tests in Ecuador Ecuador, 2007
  63. 63. <ul><li>Amalgamation of the pre-concentrate was manual </li></ul><ul><li>Use of brown sugar </li></ul><ul><li>8 hours of amalgamation in a batea </li></ul><ul><li>Amalgam was burned in a retort </li></ul><ul><li>Gold recovery was 26% </li></ul>Result of Amalgamation Ecuador, 2007
  64. 64. <ul><li>Miners are familiar but not with Carbon in Leaching </li></ul><ul><li>Activated carbon was added after 7 hours of leaching </li></ul><ul><li>pH = 11 </li></ul><ul><li>41% solids </li></ul><ul><li>Gold dissolution in 7 hours of leaching was 62% </li></ul><ul><li>Total gold recovered after 31 hours was 94% </li></ul><ul><li>Gold grade of the AC was 1235 mg/kg </li></ul><ul><li>NaCN consumption of 4.5 g/kg of ore. </li></ul>Result of Cyanidation in Agitated Tank
  65. 65. Cyanidation in Agitated Tank Ecuador, 2007
  66. 66. <ul><li>95% of gold extracted in 8 h (2h grinding with CN and 6 hours leaching) </li></ul><ul><li>Use of activated carbon </li></ul><ul><li>Residual NaCN=1.7 g/L </li></ul><ul><li>Free cyanide was destroyed with bleach before being discharged </li></ul><ul><li>The NaCN consumption was 0.95g/kg of ore </li></ul>Results of Mill-leaching (Cyanide in the Grinding Circuit) Ecuador, 2007
  67. 67. <ul><li>Elution of Activated Carbon (AC) </li></ul><ul><li>NaCN = 2g/L, NaOH = 10 g/L </li></ul><ul><li>Alcohol = 20% </li></ul><ul><li>Temperature: 90 °C, 3-4 hours </li></ul><ul><li>97% Au removed from AC </li></ul><ul><li>Gold was precipitated with Zinc </li></ul><ul><li>Loaded zinc shavings were leached with nitric acid and melted with borax to produce the gold bullion </li></ul><ul><li>Zinc in the nitric solution was precipitated with lime </li></ul>Results of Mill-leaching (Cyanide in the Grinding Circuit) Ecuador, 2007
  68. 68. <ul><li>Faster than agitated tank leaching: </li></ul><ul><ul><li>8h of mill-leaching = 31 h of agitated tank </li></ul></ul><ul><li>All gold remaining in the moisture in the tailings is recovered </li></ul><ul><li>This is almost 30% more gold which is usually lost with tailings in agitated tanks </li></ul><ul><li>No need for investment in equipment (in the case of Ecuador, Colombia and Indonesia) </li></ul>Advantages of Mill-leaching (Cyanide in the Grinding Circuit)
  69. 69. 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 AuCl 4 - 1-4 Cl - , OCl - , Cl 2 ClO 3 - Chlorine [Au(S 2 O 3 ) 2 ] 3- 8-11 S 2 O 3 2- Thiosulfate [Au(SCN) 4 ] - 1-3 SCN - Thiocyanate AuI 2 - 1-5 I - Iodine AuBr 4 - 1-7 Br - Bromine [Au(NH 2 CSNH 2 ) 2 ] + 1-4 NH 2 CSNH 2 Thiourea Complex formed with Au pH Reagent Name
  70. 70. Other Reagents <ul><li>iGoli Process (Mintek, South Africa) </li></ul><ul><ul><li>Gold from gravity concentrates (>1000 g Au/t) is leached with hypochlorite and HCl </li></ul></ul><ul><ul><li>Gold is precipitated with sodium metabisulfite, or ferrous sulphate or SO 2 , etc. </li></ul></ul><ul><ul><li>Solution is filtered </li></ul></ul><ul><ul><li>Gold powder is hammered to become yellow </li></ul></ul><ul><ul><li>Many field tests in Africa </li></ul></ul><ul><ul><li>Great potential and open technology </li></ul></ul><ul><ul><li>Hard to find reagents in remote areas </li></ul></ul>
  71. 71. iGoli Photo: Mintek, South Africa 2001
  72. 72. <ul><li>There is no panacea </li></ul><ul><li>There are many processes to replace Hg but all cases are site-specific (level of education, level of investment, labor organization, type of ore, access to reagents, etc.) </li></ul><ul><li>Any method must be transparent and simple </li></ul><ul><li>Gravity concentration is key to reduce Hg emissions and promote leaching of concentrates </li></ul><ul><li>Gravity concentration alone is efficient in alluvial ores or when grinding is efficient (rarely more than 30% gold recovery is obtained) </li></ul>Conclusion

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