Cyanide Destruction Methods

Report
Cyanide Destruction Methods
MINE 292 - Lecture 19
John A. Meech
Acknowledgement
• Marcello Veiga
• Terry Mudder
http://www.belgeler.com/blg/2ng4/chemistry-and-treatment-ofcyanidation-wastes-by-terry-i-mudder#
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)64Stable under ambient conditions of pH & temperature
Cyanide Analyses (forms of cyanide)
• total cyanide,
• weak acid dissociable (WAD), and
• free cyanide
Cyanide Detection Limits
Method
Practical
Analytical Method
Detection Limit Quantifiable Limit
(mg/L)
(mg/L)
Total by Distillation
0.02
0.10
WAD by Distillation
0.02
0.05
WAD by Picric Acid
0.10
0.50
WAD by CAC Distillations
0.10
0.50
Free CN- by AgNO3 Titration
>1
>1
Free CN- by Ion-Selective Probe
0.10
0.50
Total by auto analyser
0.01
WAD by auto analyser
0.01
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
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 0.022 mg/L
Stability of Cyanide Complexes
Cyanide Complex
Co(CN ) 4
6
Dissociation Constant
10-50
Fe(CN ) 4
6
10-47
Hg(CN) 2
4
10-39
Au(CN ) 2
10-37
Cr (CN ) 3
6
10-33
Cu(CN ) 2
4
10-30.7
Ni(CN ) 2
4
10-30
Cu(CN ) 2
3
10-29.2
Cr (CN ) 4
6
10-21
Zn(CN ) 2
4
10-21
Ag(CN ) 2
10-20.4
Cd (CN) 2
4
10-19
Treatment and Recovery of Cyanide
1.
2.
3.
4.
5.
Natural Attenuation or Degradation
Alkaline Chlorination
Hydrogen Peroxide – H2O2 (Dupont / Degussa)
INCO SO2/Air
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 Exchange
13. Reverse Osmosis
14. 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
Free Cyanide
Iron Cyanide
Complexes
J.A. Meech, 1986. Cyanide effluent control by freeze/thaw processing, Environmental
Geochemistry and Health, 7(2), 80-84.
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
Temperature = 26°C
Natural degradation tests:
surface area
to volume ratio
(m-1)
Time (days)
0
2
4
I0
I2
16
I8
0.67
1.86
0.67
1.87
0.67
pH = 11.0
1.87
<-----------------ppm NaCN----------------------->
10.0 10.0
50
50
100
100
6.9
2.2
50
42
100
96
1.9
0.5
40
2.0
87
29
1.0
34
6.8
17
0.6
3.6
1.0
-
Natural Degradation
Results for Canadian Mines
Barren Bleed (mg/L)
Mine
Final Effluent (mg/L)
Location
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
* Used a two pond sequential system
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
186
150
125
99
82
57
48
13
20
20
22
9
12
0.5
10
94
89
87
82
91
85
99
79
Source: Minerals Council of Australia, 1996. “Tailings Storage Facilities at Australian Gold Mines”, February.
Copper Cyanide Complex Stability
Cu(CN)2- = Cu2+ + 2CNCu2+ + 2OH- = Cu(OH)2
For a CN- concentrate = 10-3 M
Cyanide Stability
CN- + H2O = HCN + OH-
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.
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
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 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 + ClOCl- + CN- = CNO- + Cl-
very rapid
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 + CO321-1.5 hours
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+
Alkaline Chlorination
Process Flowsheet – Mosquito Creek, 1987
Alkaline Chlorination
Process Flowsheet – Baker Lake, 1987
Alkaline Chlorination
Process Flowsheet – Carolin Mine, 1987
Alkaline Chlorination
Process Performance
Mine
Stream
Influent
Baker Lake
Effluent
% Removal
Influent
Mosquito
Effluent
Creek
% Removal
Influent
Carolin Mine Effluent
% Removal
Total Cy
(mg/L)
WAD Cy
(mg/L)
Cu
(mg/L)
Fe
(mg/L)
Zn
(mg/L)
Residual
Chlorine
2,000
8.3
99.6
310
25
91.9
1,000
170
83.0
1,900
0.7
99.9
226
0.5
98.8
710
0.95
99.9
290
5.0
98.3
10.0
0.33
96.7
97
0.38
99.6
2.4
2.8
9.4
8.0
14.9
150
53
64.7
740
3.9
99.5
93
1.4
98.5
110
5.8
94.7
2,800
320
190
-
Alkaline Chlorination
Operating Costs (1983)
$ per m3
$ per kg Tot CN
$ per tonne ore
4 to 9
5 to 13
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 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).
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
Case Study 1
Pond Overflow
Case Study 2
Barren Bleed
Case Study 3
Heap Leach
Solution
Stream
Total Cy
(mg/L)
WAD Cy
(mg/L)
Cu
(mg/L)
Fe
(mg/L)
Influent
Effluent
% Removal
Influent
Effluent
% Removal
Influent
Effluent
% Removal
19
0.7
96.3
1,350
<5
99.6
353
0.36
99.9
19
0.7
96.3
850
<1
99.9
322
0.36
99.9
20
0.4
98.0
478
<5
99.0
102
0.4
99.6
<0.1
<0.1
178
<2
98.9
11
<0.1
99.1
Hydrogen Peroxide
Advantages
1. Capital costs lower or equal to other chemical processes
2. Relatively simple in design and operation
3. All forms of cyanide including iron complexes forms can be
removed if copper is added
4. Heavy metals are significantly reduced
5. Adaptable to batch and continuous operations
6. Close pH control is not required
7. Low quantity of sludge
8. No license fees required
9. Automation is not necessary, but available
Hydrogen Peroxide
Disadvantages
1.
2.
3.
4.
5.
6.
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
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
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)
Oxidation with Hydrogen Peroxide
(Example)
Species
Effluent (mg/L)
Before
After
EPA dws
(mg/L)
As
Cu
total CN
Fe
0.2
4.5
280
16
<0.05
<1
3
<0.015
0.05
9.0
0.05
0.3
Se
Ag
Zn
5
3.2
157
4
1
<1
0.01
0.05
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 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
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
Total Cyanide (mg/L)
Mine
SO2
Lime
Cu2+
Before
After
%Removal
(g/g CNTOT)
(g/g CNTOT)
(g/g CNTOT)
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
-
-
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.
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
• Aerobic
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)
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
Advantages
1.
2.
3.
4.
5.
6.
7.
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
Biological Methods
Disadvantages
1. Additional treatment may be required
2. Cyanide is not recovered
3. 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 ASM
4. INCO SO2/Air is effective but needs source of SO2
5. Activated carbon is something to consider since it is
being used today by ASM for Au recovery
6. Biological techniques are not widely used but do
show promise
Conclusions
7.
8.
9.
10.
11.
12.
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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