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Cyanide Destruction Methods MINE Lecture 19

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1 Cyanide Destruction Methods MINE 292 - Lecture 19
John A. Meech

2 Acknowledgement Marcello Veiga Terry 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 Total Cyanide by Distillation
Apparatus

6 Different Analytical Procedures
Total Cyanide Distillation (Manual and Auto-analysis) WAD Cyanide Distillation Cyanide Amenable to Chlorination (CAC) Picric Acid Zinc Dust and Ammonia Free Cyanide Silver Nitrate Titration (manual and auto-analysis) Ion Selective Electrode

7 Different Analytical Procedures
Separation of Cyanide Metal Complexes (gold, silver, copper, cobalt, iron (II & III), chromium, nickel) Ion Chromatography Analysis of Cyanide in Solids On-Line Process Monitoring of Cyanide Segmented Flow Auto-Analysis Thio-cyanate Spectrophotometry of low pH solution after reaction with Fe3+ Cyanate Cyanate >> ammonia by hydrolysis Measurement of ammonia

8 Different Analytical Procedures
Interfering Compounds Effect Oxidizing agents (negative) Sulfides (positive) Thiocyanate (positive) Nitrite and Nitrate (mainly positive) Carbonates (pH changes) (negative) Thio-sulfates and related sulfur compounds (negative) Metals - Fe, Co, Hg, Cu (negative)

9 Cyanide Detection Limits
Analytical Method Method Detection Limit (mg/L) Practical Quantifiable Limit Total by Distillation 0.02 0.10 WAD by Distillation 0.05 WAD by Picric Acid 0.50 WAD by CAC Distillations Free CN- by AgNO3 Titration >1 Free CN- by Ion-Selective Probe Total by auto analyser - 0.01 WAD by auto analyser 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

10 Cyanide Guidelines Canadian MMER
(Metal Mining Effluent Regulation, 2002) Maximum total cyanide 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 mg/L

11 Stability of Cyanide Complexes
Stability of the complex

12 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

13 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-)

14 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

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

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

17 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

18 <-----------------ppm NaCN----------------------->
Natural Degradation Natural degradation tests: Temperature = 26°C pH = 11.0 surface area to volume ratio (m-1) 0.67 1.86 1.87 Time (days) < ppm NaCN > 10.0 50 100 2 6.9 2.2 42 96 4 1.9 0.5 40 2.0 87 29 I0 - 1.0 34 6.8 I2 17 0.6 16 3.6 I8

19 Natural Degradation Results for Canadian Mines Mine Location
* Used a two pond sequential system Mine Location Barren Bleed (mg/L) Final Effluent (mg/L) Total Cyanide WAD Cyanide Dome Porcupine, ON 100 98.6 0.04 0.02 Lupin Contwoyto, NwT 223 186 0.20 Cullaton Lake* Keewatin District, NwT 800 140 - < 0.1

20 Natural Degradation Examples of Natural Cyanide Attenuation in Tailings Impoundments in Australia WAD Cyanide in Mill Tailings (mg/L) Discharge WAD Cyanide in Tailings Decant Solution (mg/L) WAD Cyanide Reduction (%) 210 13 94 186 20 89 150 87 125 22 82 99 9 91 12 85 57 0.5 48 10 79 Source: Minerals Council of Australia, “Tailings Storage Facilities at Australian Gold Mines”, February.

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

22 Cyanide Stability CN- + H2O = HCN + OH-

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

24 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

25 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

26 Alkaline Chlorination
Process Chemistry STAGE 1a: free and WAD cyanide converted to cyanogen chloride (CNCl) using chlorine or hypochlorite (pH ) 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 + CO hours

27 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+

28 Alkaline Chlorination
Process Flowsheet – Mosquito Creek, 1987

29 Alkaline Chlorination
Process Flowsheet – Baker Lake, 1987

30 Alkaline Chlorination
Process Flowsheet – Carolin Mine, 1987

31 Alkaline Chlorination
Process Performance Mine Stream Total Cy (mg/L) WAD Cy Cu Fe Zn Residual Chlorine Baker Lake Influent 2,000 1,900 290 2.4 740 - Effluent 8.3 0.7 5.0 2.8 3.9 2,800 % Removal 99.6 99.9 98.3 99.5 Mosquito Creek 310 226 10.0 9.4 93 25 0.5 0.33 8.0 1.4 320 91.9 98.8 96.7 14.9 98.5 Carolin Mine 1,000 710 97 150 110 170 0.95 0.38 53 5.8 190 83.0 64.7 94.7

32 Alkaline Chlorination
Operating Costs (1983) $ per m to 9 $ per kg Tot CN to 13 $ per tonne ore to 1.31 In 2012, multiply these figures by 2 to 3

33 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

34 Hydrogen Peroxide Process Chemistry Oxidation of free and WAD cyanides (i.e.,cadmium, copper, nickel and zinc cyanides): CN- + H2O2 = CNO- + H2O M(CN) H2O2 + 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).

35 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)

36 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

37 Hydrogen Peroxide Process Flowsheet – Degussa Plant at Ok Tedi

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

39 Hydrogen Peroxide Process Performance Mine Stream Total Cy (mg/L)
WAD Cy Cu Fe Case Study 1 Pond Overflow Influent 19 20 <0.1 Effluent 0.7 0.4 % Removal 96.3 98.0 - Case Study 2 Barren Bleed 1,350 850 478 178 < 5 < 1 < 2 99.6 99.9 99.0 98.9 Case Study 3 Heap Leach Solution 353 322 102 11 0.36 99.1

40 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

41 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

42 Oxidation with Hydrogen Peroxide
Some Artisanal Miners in Portovelo attempt 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

43 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 thiocyanate by 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)

44 Oxidation with Hydrogen Peroxide (Example)
Species Effluent (mg/L) Before After EPA dws (mg/L) As 0.2 <0.05 0.05 Cu 4.5 <1 9.0 total CN 280 3 Fe 16 <0.015 0.3 Se 5 4 0.01 Ag 3.2 1 Zn 157 Note: H2O2 dosage = 2.5 mL/L dws = drinking water standard

45 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

46 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

47 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

48 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- + SO H+ M(CN) SO2 + 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

49 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

50 INCO SO2-Air Process Flowsheet

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

52 INCO SO2-Air Process Performance Mine Total Cyanide (mg/L) SO2
(g/g CNTOT) Lime Cu2+ Before After %Removal Colosseum 364 0.4 99.9 4.6 0.12 0.04 Ketza River 150 5.0 96.7 6.0 0.3 Equity 175 2.3 98.7 3.4 0.03 Casa Berardi 1.0 99.3 4.5 - 0.10 Westmin Premier <0.2 5.8 Golden Bear 205 2.8 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), E. Devuyst, G. Robbins, R. Vergunst, B. Tandi, and P. Iamarino, "Inco's Cyanide Removal Technology Working Well", Mining Engi,

53 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)

54 Activated Carbon Adsorption
Process Steps

55 Biological Methods Aerobic Anaerobic Anoxic
Reactions occur in the presence of dissolved O2 - Cyanide (CN-) to Cyanate (CNO-) - Ammonia (NH4+) to Nitrate (NO3-) Anaerobic Reactions occur in the absence of dissolved O2 - Sulfate (SO42-) to Sulfide (S2-) Anoxic Reactions occur through aerobic pathway, but dissolved O2 not used There is low to no dissolved O2 levels - Denitrification: microorganisms use nitrate (NO3-) for growth, reducing nitrate to nitrogen gas (N2)

56 Biological Methods Homestake Mine Biological Treatment
Nickel Plate Mine Biological Treatment Santa Fe Mine Passive Bio-Treatment

57 Homestake Mine Biological Treatment

58 Nickel Plate Mine Biological Treatment

59 Santa Fe Mine Passive Bio-Treatment

60 Biological Methods Advantages Simple in design and control
Reagent costs low All forms of cyanide are treatable Heavy metals removed through absorption/precipitation Thiocyanate, cyanate, ammonia, nitrate & sulfate removed Can be active or passive system Effluent shown to be environmentally acceptable

61 Biological Methods Disadvantages Additional treatment may be required
Cyanide is not recovered In the below ground passive system, organic food source may require periodic replacement

62 Conclusions Natural Degradation in tailings pond is cheapest and a very effective method Alkaline chlorination is too expensive and leaves chlorine /chloride in the water Hydrogen peroxide is used effectively in ASM INCO SO2/Air is effective but needs source of SO2 Activated carbon is something to consider since it is being used today by ASM for Au recovery Biological techniques are not widely used but do show promise

63 Conclusions Key issue involves analytical difficulties
Need to develop expertise on CN assaying Hg use with CN can be extremely problematic Need to prevent invasions of the tailing dam Need to conduct regular (daily) analysis of tailing waters Tailing Treatment options Mill tailing slurry Double pond system to isolate effluent discharge and treat


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