Cyanide Destruction Methods MINE 292 - Lecture 19

1. Cyanide Destruction MethodsMINE 292 - Lecture 19 John A. Meech

2. Acknowledgement • Marcello Veiga • Terry Mudder http://www.belgeler.com/blg/2ng4/chemistry-and-treatment-of-cyanidation-wastes-by-terry-i-mudder#

3. Types of Cyanide 1.Free cyanide (HCN/CN-). Free cyanide is the active form to leach gold 2.Weak and moderately strong cyanide complexes Zn(CN)42-, Cd(CN)3-, Cd(CN)42-, Cu(CN)2-, Cu(CN)32-, Ni(CN)42-, Ag(CN)2- - Decomposed in weak acid solution (pH 3 to 6). 3.Strongly-bound cyanide complexes Co(CN)64-, Au(CN)2-, Fe(CN)64- Stable under ambient conditions of pH & temperature

4. Cyanide Analyses (forms of cyanide) • total cyanide, • weak acid dissociable (WAD), and • free cyanide

5. Cyanide Detection Limits Auto analysis (tot) – segmented flow with in-line UV digestion and McLeod micro-still reflux Auto analysis (WAD) - segmented flow using auto ASTM method and McLeod micro-still reflux

6. Cyanide Guidelines Canadian MMER (Metal Mining Effluent Regulation, 2002) Maximum totalcyanide in mining effluent Monthly average = 1.0 mg/L Composite sample = 1.5 mg/L Grab sample = 2.0 mg/L World Bank Guidelines (1995): Total Cyanide = 1.0 mg/L WAD cyanide = 0.5 mg/L Free Cyanide = 0.1 mg/L In no case should concentration in receiving water outside a designated mixing zone exceed 0.022 mg/L

7. Stability of Cyanide Complexes Stability of the complex

8. Treatment and Recovery of Cyanide • Natural Attenuation or Degradation • Alkaline Chlorination • Hydrogen Peroxide – H2O2(Dupont / Degussa) • INCO SO2/Air • Biological Treatment - active - passive • Activated Carbon Adsorption

9. Treatment and Recovery of Cyanide Other Methods • Caro's Acid (H2SO5) • Ozone Oxidation (O3) • Cyanide Recovery - tailings washing - stripping and adsorption • Precipitation of Cyanide (NaCN or KCN) • Ion Exchange • Reverse Osmosis • Removal of Metals, Thio-cyanate (CNS-), Cyanate (CNO-), Ammonia (NH3), and Nitrates (NO3-)

10. Natural Degradation • Dominant mechanism is volatilisation of HCN from solution • Pond pH is lowered by CO2 uptake from air and acidic rainwater influx • Equilibrium pH from CO2 uptake is from 7.0 to 9.0. • Changes free cyanide/HCN and WAD cyanide/HCN equilibria • Also mitigated by temperature increase, UV exposure, and aeration • Freeze-thaw cycles also affect cyanide in northern Canadian climates

11. Freeze-Thaw Cycle- pure cyanide solution J.A. Meech, 1986. Cyanide effluent control by freeze/thaw processing, Environmental Geochemistry and Health, 7(2), 80-84.

12. Thaw Cycle Cyanide Distribution- Cullaton Lake Gold Mine, NwT Free Cyanide Iron Cyanide Complexes J.A. Meech, 1986. Cyanide effluent control by freeze/thaw processing, Environmental Geochemistry and Health, 7(2), 80-84.

13. Natural Degradation • Dome, Cullaton Lake, and Lupin mines designed their TSFs for primary treatment • Giant Yellowknife is using it for partial treatment • Others consider it a pre-treatment process

14. Natural Degradation Natural degradation tests: Temperature = 26°C pH = 11.0

15. Natural Degradation Results for Canadian Mines * Used a two pond sequential system

16. Natural Degradation Examples of Natural Cyanide Attenuation in Tailings Impoundments in Australia Source: Minerals Council of Australia, 1996. “Tailings Storage Facilities at Australian Gold Mines”, February.

17. Copper Cyanide Complex Stability Cu(CN)2- = Cu2+ + 2CN- Cu2+ + 2OH- = Cu(OH)2 For a CN- concentrate = 10-3 M

18. Cyanide Stability CN- + H2O = HCN + OH-

19. Natural Degradation Cyanide Natural degradation in Northern Canadian mine J.W. Schmidt, L. Simovic, and E.E. Shannon, 1981. "Development studies for suitable Technologies to removal cyanide and heavy metals from gold milling effluents", Proc. 36thIndustrial Waste Conf., Purdue University, Lafayette, Indiana, p. 831-849.

20. Natural Degradation Advantages: • Relatively inexpensive • Total and WAD cyanide levels < 5.0 mg/L • Iron complexes decomposed if sunlight is sufficient • Process is suitable for batch or continuous process • Concentrations of trace metals can also be reduced • Process is suitable as primary or pre-treatment

21. Alkaline Chlorination • Chemical treatment process to oxidize free & WAD cyanide • Oldest and most widely recognized • Used in metal plating and finishing plants • Still used in a few mines but trend is toward other oxidation processes • Best applied on clear solutions when WAD cyanide, thiocyanate, and/or ammonia require removal

22. Alkaline Chlorination Process Chemistry STAGE 1a: free and WAD cyanide converted to cyanogen chloride (CNCl) using chlorine or hypochlorite (pH 10.5-11.5) Cl2 + CN- = CNCl + Cl- very rapid OCl- + CN- = CNO- + Cl- very rapid STAGE 1b: CNCl chloride hydrolyses to yield cyanate (CNO-) CNCl + H2O = CNO- + Cl- + 2H+ 15 minutes STAGE 2: Hydrolysis of CNO- in the presence of excess chlorine OCN- + OH- + H2O = NH3 + CO32- 1-1.5 hours

23. Alkaline Chlorination Process Chemistry In presence of excess chlorine or hypochlorite, ammonia will react further to yield nitrogen gas (very expensive) 3Cl2 + 2NH3 = N2 + 6Cl- + 6H+ Thiocyanate (SCN-) contributes to overall chlorine demand Oxidized in preference to cyanide 4Cl2 + SCN- + 5H2O = SO42- + CNO- + 8Cl- + 10H+

24. Alkaline Chlorination Process Flowsheet – Mosquito Creek, 1987

25. Alkaline Chlorination Process Flowsheet – Baker Lake, 1987

26. Alkaline Chlorination Process Flowsheet – Carolin Mine, 1987

27. Alkaline Chlorination Process Performance

28. Alkaline Chlorination Operating Costs (1983) $ per m3 4 to 9 $ per kg Tot CN 5 to 13 $ per tonne ore 0.65 to 1.31 In 2012, multiply these figures by 2 to 3

29. Hydrogen Peroxide • Used at steel hardening and plating operations • Investigated by DuPont and and Degussa • Several versions of this process have been patented • First continuous test at Homestake Mine in early 80s • First full-scale H2O2 plant at Ok Tedi, Papua New Guinea • Currently many plants in operation worldwide • Process can achieve low levels of free and WAD cyanide • Process is limited to treat effluents rather than slurries • High consumption of H2O2 from reaction with solids

30. Hydrogen Peroxide Process Chemistry Oxidation of free and WAD cyanides (i.e.,cadmium, copper, nickel and zinc cyanides): CN- + H2O2 = CNO- + H2O M(CN)42- + 4H2O2 + 2OH- = 4CNO- + 4H2O + M(OH)2(s) Soluble copper catalyst increases reaction rate. Catalyst can be copper present in solution or added as copper sulfate (very expensive).

31. Hydrogen Peroxide Process Chemistry Highly stable iron cyanide complexes are not converted to cyanate by hydrogen peroxide Removed through precipitation of insoluble copper-iron-cyanide complex 2Cu2+ + Fe(CN)64- = Cu2Fe(CN)6 (s)

32. Hydrogen Peroxide Process Chemistry • ~ 10 to 20% of the cyanate is converted to ammonia CNO- + H+ + 2H2O = HCO3- + NH4+ • Typically, H2O2 added at 200 to 450 % of theoretical • Commonly available at 35, 50, and 70% strength • 70% H2O2 is rarely used due to safety concerns

33. Hydrogen Peroxide Process Flowsheet – Degussa Plant at Ok Tedi

34. Hydrogen Peroxide Process Flowsheet – H2O2 Plant at Teck-Corona Mill

35. Hydrogen Peroxide Process Performance

36. Hydrogen Peroxide Advantages • Capital costs lower or equal to other chemical processes • Relatively simple in design and operation • All forms of cyanide including iron complexes forms can be removed if copper is added • Heavy metals are significantly reduced • Adaptable to batch and continuous operations • Close pH control is not required • Low quantity of sludge • No license fees required • Automation is not necessary, but available

37. Hydrogen Peroxide Disadvantages • High reagent costs • High concentrations of cyanate >>> increased ammonia • Process does not remove ammonia or thiocyanate • Additional treatment may be required for ammonia/thiocyanate • Cyanide is not recovered • Process is not suitable for treatment of tailings slurries

38. Oxidation with Hydrogen Peroxide • Some Artisanal Miners in Portoveloattempt to destroy cyanide effluents with peroxide but some add reagent to slurry (poor practice) • Process takes more than one week to reach the total cyanide level of 1 mg/L before discharging into the river or re-circulating to the process • No filtration is used to remove precipitated solids

39. Oxidation with Hydrogen Peroxide • There are a variety of processes combining hydrogen peroxide with other compounds, such as glycolonitrile (Kastone process), H2SO5(Caro’s acid), SO2, etc. • Destruction of thiocyanateby H2O2 is slower than Chlorination • H2O2 consumption is around 3 kg/kg CN-. Theoretical dosage is 1.5 kg H2O2/kg CN- • Process is not suitable for slurries (too long a time)

40. Oxidation with Hydrogen Peroxide(Example) Note: H2O2 dosage = 2.5 mL/L dws = drinking water standard

41. Cyanide Destruction with H2O2 • Cyanide destruction tank in Portovelo. Peroxide was added to the tank and slurry was agitated for 5 to 7 days. • The red color of the suspended solids is from sulfide oxidation Ecuador

42. INCO SO2-Air • Two patented versions of the SO2-Air process • First patented and marketed by INCO • INCO process converts WAD cyanide to cyanate with mixture of SO2 & air with a soluble copper catalyst at a controlled pH • Conversion of WAD cyanide directly to cyanate. • Iron complexes reduced to ferrous state and precipitated as insoluble copper-iron-complexes • Residual metals are precipitated as hydroxides • Second process developed at Heath Steel Mines with patent assigned to Noranda • Noranda process uses pure SO2 rather than mixing with air • INCO process is used at over 80 mines worldwide

43. INCO SO2-Air – connection to the Super-Stack (370 m high) • Came up with this process to find a market for SO2 • Forced in 1970s to recover SO2 and reduce Acid Rain

44. INCO SO2-Air Process Chemistry Free and WAD cyanides are oxidized to cyanate by SO2 and air in the presence of soluble copper catalyst CN- + SO2 + O2 + H2O = CNO- + SO42- + 2H+ M(CN)42- + 4SO2 + 4O2 + 4H2O = 4CNO- + 8H+ + 4SO42- + M2+ Reaction normally carried out at pH 8.0 to 9.0 Formation of acid means lime is required for pH control Decrease in performance can occur if pH fluctuates Optimal pH determined experimentally Temperature has little effect from 5 to 60°C

45. INCO SO2-Air Process Chemistry • Theoretical SO2 is 2.46 g SO2 / g WAD cyanide • In practice, usage ranges from 3.0 to 5.0 g • SO2 can be either liquid SO2 sodium sulphite (Na2SO3) or sodium metabisulphite (Na2S2O5). • Ammonium bisulphite (NH4HSO3) has also been used but impact of ammonia on treated effluent is a concern • Choice of reagent is determined by cost and availability

46. INCO SO2-Air Process Flowsheet

47. INCO SO2-Air Process Performance Source: G.H. Robbins, 1996. “Historical Development of INCO SO2/AIR Cyanide Destruction Process”, CIM Bulletin, pp. 63-69.

48. INCO SO2-Air Process Performance Source: E. Devuyst, B. Conard, G. Robbins, and R. Vergunst, R., 1989a."The Inco SO2/Air Process", Gold Mining Effluent Seminars, Vancouver, B.C. E. Devuyst, B. Conard, R. Vergunst, and B. Tandi, 1989b. "Cyanide Removal Using SO2 & Air", J. Minerals, Metals, and Materials, 41(12), 43-45. E. Devuyst, G. Robbins, R. Vergunst, B. Tandi, and P. Iamarino, 1991. "Inco's Cyanide Removal Technology Working Well", Mining Engi, 207-8.

49. Activated Carbon Adsorption • Both granular and powdered carbon can be used • Initial work (cyanide adsorbed, then oxidized by catalysis) • Presence of metal ions, particularly copper, enhance removal • Removes low levels of WAD cyanide, i.e., complexed metals. • Cyanide can be removed for possible reuse without oxidation • Process Steps • add metal ions • form cyanide complexes • adsorb onto granular activated carbon • Effluent WAD levels below 0.5 mg/L from influent levels of 75 mg/L • Cost of fresh carbon and regeneration too high at elevated WAD levels • Very effective at WAD trace levels (<2.0 mg/L)

50. Activated Carbon Adsorption Process Steps