Cyanide Destruction MethodsMINE 292 - Lecture 19

John A. Meech

Acknowledgement

• Marcello Veiga

• Terry Mudder

http://www.belgeler.com/blg/2ng4/chemistry-and-treatment-of-cyanidation-wastes-by-terry-i-mudder#

Types of Cyanide1. Free cyanide (HCN/CN-). Free cyanide is the active form to leach gold2. 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

Cyanide Analyses (forms of cyanide)

• total cyanide, • weak acid dissociable (WAD), and • free cyanide

Cyanide Detection Limits

Analytical MethodMethod

Detection Limit(mg/L)

Practical Quantifiable Limit

(mg/L)Total by Distillation 0.02 0.10WAD by Distillation 0.02 0.05WAD by Picric Acid 0.10 0.50WAD by CAC Distillations 0.10 0.50Free CN- by AgNO3 Titration >1 >1Free CN- by Ion-Selective Probe 0.10 0.50Total by auto analyser - 0.01WAD by auto analyser - 0.01

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

Cyanide GuidelinesCanadian 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/LWAD cyanide = 0.5 mg/LFree Cyanide = 0.1 mg/L

In no case should concentration in receiving water outside a designated mixing zone exceed 0.022 mg/L

Stability of Cyanide ComplexesCyanide Complex Dissociation Constant

Co CN( )64 10-50

Fe CN( )64 10-47

Hg CN( )42 10-39

Au CN( )2 10-37

Cr CN( )63 10-33

Cu CN( )42 10-30.7

Ni CN( )42 10-30

Cu CN( )32 10-29.2

Cr CN( )64 10-21

Zn CN( )42 10-21

Ag CN( )2 10-20.4

Cd CN( )42 10-19

Treatment and Recovery of Cyanide

1. Natural Attenuation or Degradation2. Alkaline Chlorination3. Hydrogen Peroxide – H2O2 (Dupont / Degussa)

4. INCO SO2/Air

5. Biological Treatment - active - passive

6. Activated Carbon Adsorption

Treatment and Recovery of Cyanide

Other Methods

8. Caro's Acid (H2SO5)

9. Ozone Oxidation (O3)

10. Cyanide Recovery - tailings washing - stripping and adsorption

11. Precipitation of Cyanide (NaCN or KCN)12. Ion Exchange13. Reverse Osmosis14. Removal of Metals, Thio-cyanate (CNS-), Cyanate (CNO-), Ammonia

(NH3), and Nitrates (NO3-)

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

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.

Thaw Cycle Cyanide Distribution- Cullaton Lake Gold Mine, NwT

J.A. Meech, 1986. Cyanide effluent control by freeze/thaw processing, Environmental Geochemistry and Health, 7(2), 80-84.

Free Cyanide Iron Cyanide Complexes

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

Natural Degradation

surface areato volume ratio

(m-1)0.67 1.86 0.67 1.87 0.67 1.87

Time (days) <-----------------ppm NaCN----------------------->0 10.0 10.0 50 50 100 1002 6.9 2.2 50 42 100 964 1.9 0.5 40 2.0 87 29I0 - - 1.0 - 34 6.8I2 - - - - 17 0.616 - - - - 3.6 -I8 - - - - 1.0 -

Natural degradation tests: Temperature = 26°C pH = 11.0

Natural Degradation

Results for Canadian Mines

* Used a two pond sequential system

Mine LocationBarren Bleed (mg/L) Final Effluent (mg/L)

Total Cyanide WAD Cyanide Total Cyanide WAD Cyanide

Dome Porcupine, ON 100 98.6 0.04 0.02

Lupin Contwoyto, NwT 223 186 0.20 0.02

Cullaton Lake* Keewatin District, NwT 800 140 - < 0.1

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 94186 20 89150 20 87125 22 8299 9 9182 12 8557 0.5 9948 10 79

Source: Minerals Council of Australia, 1996. “Tailings Storage Facilities at Australian Gold Mines”, February.

Copper Cyanide Complex StabilityCu(CN)2

- = Cu2+ + 2CN-

Cu2+ + 2OH- = Cu(OH)2

For a CN- concentrate = 10-3 M

Cyanide StabilityCN- + H2O = HCN + OH-

Natural DegradationCyanide 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.

Natural DegradationAdvantages:

• 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

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

Alkaline ChlorinationProcess 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 chlorineOCN- + OH- + H2O = NH3 + CO3

2- 1-1.5 hours

Alkaline ChlorinationProcess 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+

Alkaline ChlorinationProcess Flowsheet – Mosquito Creek, 1987

Alkaline ChlorinationProcess Flowsheet – Baker Lake, 1987

Alkaline ChlorinationProcess Flowsheet – Carolin Mine, 1987

Alkaline ChlorinationProcess Performance

Mine Stream Total Cy(mg/L)

WAD Cy(mg/L)

Cu(mg/L)

Fe(mg/L)

Zn(mg/L)

Residual Chlorine

Baker LakeInfluent 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

Influent 310 226 10.0 9.4 93 -Effluent 25 0.5 0.33 8.0 1.4 320

% Removal 91.9 98.8 96.7 14.9 98.5 -

Carolin MineInfluent 1,000 710 97 150 110 -Effluent 170 0.95 0.38 53 5.8 190

% Removal 83.0 99.9 99.6 64.7 94.7 -

Alkaline ChlorinationOperating 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

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

Hydrogen Peroxide

Process ChemistryOxidation 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).

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)

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

Hydrogen Peroxide

Process Flowsheet – Degussa Plant at Ok Tedi

Hydrogen Peroxide

Process Flowsheet – H2O2 Plant at Teck-Corona Mill

Hydrogen Peroxide

Process Performance

Mine Stream Total Cy(mg/L)

WAD Cy(mg/L)

Cu(mg/L)

Fe(mg/L)

Case Study 1 Pond Overflow

Influent 19 19 20 <0.1Effluent 0.7 0.7 0.4 <0.1

% Removal 96.3 96.3 98.0 -

Case Study 2 Barren Bleed

Influent 1,350 850 478 178Effluent < 5 < 1 < 5 < 2

% Removal 99.6 99.9 99.0 98.9

Case Study 3 Heap Leach

Solution

Influent 353 322 102 11Effluent 0.36 0.36 0.4 <0.1

% Removal 99.9 99.9 99.6 99.1

Hydrogen Peroxide

Advantages1. Capital costs lower or equal to other chemical processes2. Relatively simple in design and operation3. All forms of cyanide including iron complexes forms can be

removed if copper is added4. Heavy metals are significantly reduced5. Adaptable to batch and continuous operations6. Close pH control is not required7. Low quantity of sludge8. No license fees required9. Automation is not necessary, but available

Hydrogen Peroxide

Disadvantages1. High reagent costs2. High concentrations of cyanate >>> increased ammonia3. Process does not remove ammonia or thiocyanate4. Additional treatment may be required for ammonia/thiocyanate5. Cyanide is not recovered6. Process is not suitable for treatment of tailings slurries

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

• 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)

Oxidation with Hydrogen Peroxide

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 0.05

Fe 16 <0.015 0.3

Se 5 4 0.01

Ag 3.2 1 0.05

Zn 157 <1 5

Note: H2O2 dosage = 2.5 mL/L dws = drinking water standard

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

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

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

INCO SO2-Air

Process ChemistryFree 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+ + 4SO4

2- + M2+

Reaction normally carried out at pH 8.0 to 9.0Formation of acid means lime is required for pH controlDecrease in performance can occur if pH fluctuatesOptimal pH determined experimentallyTemperature has little effect from 5 to 60°C

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

INCO SO2-Air

Process Flowsheet

INCO SO2-Air

Process Performance

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

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.

Mine

Total Cyanide (mg/L)SO2

(g/g CNTOT)Lime

(g/g CNTOT)Cu2+

(g/g CNTOT)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 0.3

Equity 175 2.3 98.7 3.4 0 0.03

Casa Berardi 150 1.0 99.3 4.5 - 0.10

Westmin Premier 150 <0.2 99.9 5.8 - 0.12

Golden Bear 205 0.3 99.9 2.8 - -

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)

Activated Carbon Adsorption

Process Steps

Biological Methods

• AerobicReactions occur in the presence of dissolved O2

- Cyanide (CN-) to Cyanate (CNO-) - Ammonia (NH4

+) to Nitrate (NO3-)

• AnaerobicReactions occur in the absence of dissolved O2

- Sulfate (SO42-) to Sulfide (S2-)

• AnoxicReactions 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)

Biological Methods

• Homestake Mine Biological Treatment• Nickel Plate Mine Biological Treatment• Santa Fe Mine Passive Bio-Treatment

Homestake Mine Biological Treatment

Nickel Plate Mine Biological Treatment

Santa Fe Mine Passive Bio-Treatment

Biological Methods

Advantages1. Simple in design and control2. Reagent costs low3. All forms of cyanide are treatable4. Heavy metals removed through absorption/precipitation5. Thiocyanate, cyanate, ammonia, nitrate & sulfate removed 6. Can be active or passive system7. Effluent shown to be environmentally acceptable

Biological Methods

Disadvantages1. Additional treatment may be required2. Cyanide is not recovered3. In the below ground passive system, organic food source

may require periodic replacement

Conclusions

1. Natural Degradation in tailings pond is cheapest and a very effective method

2. Alkaline chlorination is too expensive and leaves chlorine /chloride in the water

3. Hydrogen peroxide is used effectively in ASM4. INCO SO2/Air is effective but needs source of SO25. Activated carbon is something to consider since it is

being used today by ASM for Au recovery6. Biological techniques are not widely used but do

show promise

Conclusions

7. Key issue involves analytical difficulties8. Need to develop expertise on CN assaying 9. Hg use with CN can be extremely problematic10. Need to prevent invasions of the tailing dam11. Need to conduct regular (daily) analysis of tailing waters12. Tailing Treatment options

– Mill tailing slurry– Double pond system to isolate effluent discharge and treat