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Dr. Arthur Pinto Chaves(Universidade de São Paulo, Dept Engenharia de Minas)
Dr. Marcello Veiga(University of Britsh Columbia, Dept Mining Engineering, Canada)
PMI 5003
Universidade de São Paulo, Escola Politécnica, Dept Engenharia de Minas
Studying Gold Ores:Mineralogy, Cyanidation, Toxicity
and Environmental Issues Gold Process Mineralogy
Common Gold Minerals
Gold AuElectrum (Au,Ag)Cuproauride (Au,Cu)Porpezite (Au,Pd)Rhodite (Au,Rh)Iridic gold (Au,Ir)
Calaverite AuTe2Krennerite (Au,Ag)Te2Montbrayite (Au,Sb)2Te3Petzite (antamokite) Ag3AuTe 2
Uytenbogaardtite
Aurostibite
Fischesserite
Ag3AuS2
AuSb2
Ag3AuSe2
Common Gold Minerals
Platinum gold (Au,Pt)Bismuthian gold (Au,Bi)
Gold amalgam Au2Hg3 (?)
Maldonite Au2Bi
Auricupride AuCu
Palladium Curoauride (Cu,Pd)3Au2
Muthmannite (Ag,Au)Te
Sylvanite (Au,Ag)Te4Kostovite AuCuTe4Nagyagite Pb5Au(Te,Sb)4S5-8
3
Physical Properties of Gold
Property Native Gold Electrum
% Au >75 45-75
SG 16-19.3 13-16
Mohs Hardness 2.5-3 2-2.5
Physical Properties of Gold Tellurides
Mineral %Au SG Hardness
Calaverite 39.2-42.8 9.2 2.5-3
Krennerite 30.7-43.9 8.6 2.5
Sylvanite 24.2-29.9 8.2 1.5-2
Montbroyite 38.6-44.3 9.9 2.5
Petzite 19-25.2 9.1 2.5
Hessite 4.7 8.4 2.5-3
2
Physical Properties of Gold Minerals
Mineral Au% SG Mohs
Hardness
Nagyagite 7.4-10.2 4.5 1.5
Kostovite 25.2 2-2.5
Aurostibnite 43.5-50.9 9.9 3
Maldonite 64.5-65.1 15.5 1.5-2
HENLEY, K.J,(1975). Gold-ore mineralogy and its relation to metallurgical treatment. Minerals Sci. Engng, vol.7, n. 4, p. 289-312
Gold Mineral Associations and Size
• Gold occurs in several forms
– Free gold is liberated
– Attached to gangue along grain boundaries or completely occluded within a particle grain
– Finely disseminated within a mineral structure.
GOLD GANGUE
Gold in the Electron Microscope
Image of back-scattered electrons
History of Gold Ore Processing
• 1700 to early 1900’s– Mostly alluvial gold
– Gold pans, sluice boxes and mercury amalgamation
• Late 1800’s to mid 1900’s– Cyanidation Invented
– Depletion of alluvial gold
– Increase in hardrock gold mining
– Processing evolved towards cyanidation, although mercury still used in some large scale operations
• 1970’s to present– Development of heap cyanidation
– New technologies for refractory gold ores
– Mercury use in large scale mines almost completely disappears
– Centrifugal gravity concentrators developed
– Escalation of ASM in developing countries
Process Design for Large Scale Gold Mines
• Process designs are based on ore specific mineralogy
• Processes are kept as simple as possible with minimum number of stages
• Usually coarse gold is removed first from the leaching circuit
• Cyanidation is usually applied for gold particles finer than 0.15 mm (100 mesh)
Typical Terminology for Gold Ores
• Alluvial (usually free naturally gold)
� sands and gravels, rivers, beaches
� usually gold particles are liberated
• Free milling
� surface and underground hard rock minesEgold associated with silicates
• Refractory
� not amenable to conventional cyanidation
� usually associated with sulfides
3
Gold Ore Processing Practices(Alluvial)
Gravity and/or FlotationConcentration
Scrubbing
Free gold
• USUALLY gold is liberated in alluvial ore, but sometimes it is not
• Sometimes gold is aggregated with clayEscrubbing is needed
• In Coluvial and Eluvial ores, gold IS PARTIALLY liberated and this is the reason why the Au recovery is low, as many miners do not grind the ore before concentration
Gold Ore Processing Practices(Alluvial)
Brazil, 1993 (BBC Documentary)Dredging alluvial gold in the Amazon Brazil, 2006
Mining coluvial gold orein the Amazon
Gold Ore Processing Practices(Alluvial)
Concentration of alluvial gold by flotation. Photo H. Wotruba
Gold Ore Processing Practices(free milling ores)
Comminution
Cyanide
LeachGravity and/or Flotation
Concentration
Comminution
Cyanide
Leach
Free gold
Gold
Gold
Gold Ore Processing Practices(refractory)
Gravity
Sep
Comminution
Free gold
Flotation Oxidation
CyanideLeach
Gold
• Attached (occluded) gold
– sulfides
– Silicates
• Invisible gold
• Au grains sizes range from:
– very coarse >1 mm
– coarse >0.15 mm
– fine <0.074 mm
– to very fine <0.010 mm
25 µm
Typical Terminology for Gold Ores
Reflected light in optical microscope
Fine gold in pyrite grain
4
Free Gold Particle
25 µm
Gold Associated with sulfides
• Gold rarely forms solid solution with sulfides but it is possible
• Gold can be occluded in sulfides in grains <0.02 µm
• Invisible Au can be sub microscopic gold or gold in the sulfide lattice
• Invisible Au is preferentially concentrated in arsenopyrite which is apparently related to crystal chemistry
• When an ore has fine and coarse grained arsenopyrite, Au concentrates in finer grained
Gold Associated with sulfides
HENLEY, K.J,(1975). Gold-ore mineralogy and its relation to metallurgical treatment. Minerals Sci. Engng, vol.7, n. 4, p. 289-312
Ga
Au
Gold with Galena and Silicates
Si
Gold is usually difficult to polish (lots of scratches)
Reflected light in optical microscope
Gold Associated with sulfides
Reflected light
Chalcopyrite
CuFeS2
Polished-thin Section, reflected light, optical microscope
Very often in the optical microscope, chalcopyrite looks like gold, but gold is much brighter
Reflected Light
gold
chalcopyrite
gold
Gold Associated with sulfides
gold
sulfides (chalcopyrite and pyrite) become oxidized in
the optical microscope when the specimen is attacked
by diluted HNO3 (10%) for one day
Gold Associated with Sulfides
To differentiate Au from Chalcopyrite
Reflected Light
5
Studying Gold Liberation
Studying of Liberation of Gold Ores
• It is difficult to establish gold liberation as this is usually a trace element in most ores, hardly visible in microscope.
• Liberation of gold can be obtained by indirect methods, such as heavy liquid separation, gravity concentration, flotation, etc..
• One technique (very common) to establish the gold liberation is grinding in different times followed by concentration
• Increasing grinding time increasing liberation.
Example
The average gold grade of this tailing from Crixás, Brazil is around 0.7 to 1 g/tonne. Gold liberation was studied as follows::
Sample
Grinding
Fire Assay
Batch
conc.tail.
- no grinding
- 10 min.
- 20 min.
- 30 min.
Grain size
analyses(various time)
Composite
Flotation
- 5 min.
grinding time
Studying Liberation by Flotation or Gravity Concentration
0
0.5
1
1.5
2
2.5
3
20#
28#
35#
48#
65#100#150#
200#
270#
400#
-400#
Mesh
Au
(g/t)
Gold grades in screened fractions
0
5
10
15
20
25
20#28#35#48#65#100#150#200#270#400#
-400#
Mesh
Au
distr.
(%)
Gold distribution in screened fractions
Grain size distribution
of a “garimpo” tailing
before grinding.
It is noticeable that
the Artisanal Miners
could not recover the
fine gold (they used
sluice boxes) and the
unliberated gold in
the coarse fractions.
Studying Liberation by Flotation or Gravity Concentration
50
60
70
80
90
100
0 5 10 20 30
Grinding time (min.)
%Gold
recovery
by
concentration
Concentration of the different ground sub-samples indicates
that the liberation must be around 10 minutes of grinding.
Taking this information to the grain size distribution graphic,
we can obtain the liberation size of the gold particles.
Studying Liberation by Flotation or Gravity Concentration
0
20
40
60
80
100
16 20 28 35 48 65 100 150 200 270 400 -400
Tyler Mesh
% M
ass Passing
1 min
5 min
10 min
20 minD80 for 10 min of grinding
(probable gold liberation)
grinding time
Studying Liberation by Flotation or Gravity Concentration
6
Studying of Liberation of Gold Ores
• Very often the concept of gold liberation is usually replaced by accessibility of gold to reagents such as mercury, cyanide, etc.
• In other words: submit the sample (or concentrates) to cyanidation and amalgamation.
• The grain size distribution must be knownE.or make cyanidiation or amalgamation of the sized fractions
Studying of Liberation of Gold Ores
• For amalgamation a gold particle must be at least >90% liberated
• Amalgamation is difficult for -200 mesh fractions unless the mercury is activated
• For cyanidation gold does not need to be liberated; it can be exposed
Au exposed but not liberated
Quartz particle
• Usually Hg becomes “oxidized” (dirty) when it is old
• The efficiency of amalgamation is low (usually 70%)EHg doesn’t grab Aut
• Hg forms droplets and loses coalescence
• Lots of Hg is lost to amalgamation tailings (usually have 60-300 ppm Hg)
Hg Activation Improves Amalgamation Hg Activation Improves Amalgamation
• Boil water (Indonesia)
• Detergent (Brazil, Zimbabwe, Tanzania)
• Lime juice (Laos)
• Dilluted nitric acid (Peru)
• Brown sugar (Ecuador)
• Molasse, guava leaves, lime juice, bicarbonate
(Colombia, Antioquia)
• Urine (Chile)
• Electrolytic formation of Na- or K-amalgam
(Colombia, Nariños)
Activating Hg before amalgamation
(increase coalescence = reduces Hg
flouring = less Hg loss with tailing)
It forms sodium-
amalgam which is
more consistent
than pure Hg
Battery12 V
wire
Mercury
Water with 10%
of salt NaCl or
KCl
+ -
Graphite rod
Brazil, 2006
Zimbabwe, 2006
7
Brown Sugar in the Amalgamation Barrel
Ecuador, 2004
It is not know the role of brown sugar in improving
amalgamtion. This is widely used in Ecuador and some
parys of Colombia
Amalgamation Barrel
Indonesia, 2006
Studying of Liberation of Gold Ores
• A Simple Methodology: screened fractions are submitted to heavy liquid separation.
• The floats are assayed for gold. The sinks are submitted to amalgamation. The mineral residue is submitted to cyanidation.
• This will determine the gold portion accessible to mercury (almost free gold) and accessible to cyanide (not quite free).
• The final residue, after cyanidation is analyzed by fire assay, in which all ore is melted. This determines the portion of mineral-locked gold (not amalgamated or leached).
Studying of Liberation of Gold Ores
ATTENTION:
• This is just a lab test
• This must not be used in an actual
processing plant
• Hg-contaminated tailings must not be
leached with cyanide since this forms
Hg-cyanide which is very bioavailable
ScreeningScreeningScreeningScreeningHeavy LiquidHeavy LiquidHeavy LiquidHeavy LiquidSeparationSeparationSeparationSeparationheavy fractionsheavy fractionsheavy fractionsheavy fractions Chemical AnalysisChemical AnalysisChemical AnalysisChemical AnalysisAmalgamationAmalgamationAmalgamationAmalgamation(60% solids; Hg:solids = 1:20)(60% solids; Hg:solids = 1:20)(60% solids; Hg:solids = 1:20)(60% solids; Hg:solids = 1:20) Nitric AcidNitric AcidNitric AcidNitric AcidHgHgHgHg----DissolutionDissolutionDissolutionDissolutionlight fractionslight fractionslight fractionslight fractionsamalgamsamalgamsamalgamsamalgams goldgoldgoldgoldCyanidationCyanidationCyanidationCyanidation(*)(*)(*)(*)(10g/L (10g/L (10g/L (10g/L NaCNNaCNNaCNNaCN, pH=10, 40%sol, 24 h), pH=10, 40%sol, 24 h), pH=10, 40%sol, 24 h), pH=10, 40%sol, 24 h)residueresidueresidueresidueresidueresidueresidueresidue solutionssolutionssolutionssolutions ChemicalChemicalChemicalChemicalAnalysisAnalysisAnalysisAnalysis goldgoldgoldgoldFire AssayFire AssayFire AssayFire AssaygoldgoldgoldgoldStudying of Liberation of Gold OresCrushing (Crushing (Crushing (Crushing (----28#)28#)28#)28#) 35, 48, 65, 100, 150, 200, 270 35, 48, 65, 100, 150, 200, 270 35, 48, 65, 100, 150, 200, 270 35, 48, 65, 100, 150, 200, 270 meshmeshmeshmesh
(*)Just the -48 mesh fractions were leached
by cyanide. High concentration of CN was used to avoid CN consumpion control
Gold Ore (8g/tonne Au) Results:
• All fractions finer than 48 mesh had similar Au recovery (~60%) in heavy liquid. Amalgamation showed that particles coarser than 48 mesh did not show enough exposure to be amalgamated.
Fraction (Tyler mesh)
Gold in Heavy Products (%)
Amalgamated Gold (%)
CN Leached Gold (%)
-28 +35 32 4 n.a -35 +48 68 1 n.a -48 +65 57 21 n.a -65 +100 55 48 88.0 -100 +150 62 67 93.2 -150 +200 60 87 95.0 -200 +270 n.a. n.a 99.0
• We should expect to extract up to 60% of the gold from this ore ground -65 mesh and subjected to gravity separation.
• Cyanidation of -200 mesh recovers almost all gold.
Studying of Liberation of Gold Ores
8
Studying of Liberation of Gold OresExample: Gold liberation of an ore from Texada Island, BC, Canada As the ore had more than 30% of heavy minerals (pyrite, bornite, iron oxides) (sg>2.89) and high gold grade (7.3 g/t), the heavy liquid separation was not used, just amalgamation followed by cyanidation and fire-assay (fusion) of final residues. It is possible to notice the gold liberation increasing with particle size decreasing.
Fusion
Cyanidation
Amalgamation
Overall Gold recovered by:
Fusion: 11.4 %
Cyanidation: 64.0 %
Amalgamation: 24.6 %
+100#-100+200#
-200+400#-400#
DAu (%)
100
50
75
25
0
-
-
-
-
-
Representativesample
Heavy LiquidSeparation
heavies
Amalgamation
Cyanidation
residues
residues
Fire Assay
Chips of sampleselected
Optical and electron
microscopy(polished thin sections)
Crushing
Screening
Flotation
Grinding withdifferent times
Cyanidation
conc.
- 6 Mesh (3.4 mm)
Gravity Sep.
tailing
Diagnosis
Leaching
ChemicalAnalysis
residue
Liberation of Sulfides
(Microscopy)fractions
= splitting
= optional
Grinding
Store in bagssub-sample sub-samples
- 28 M (0.6 mm)
Studying of Liberation of Gold Ores
Studying of Liberation of Gold Ores
• Clifton et al. (1969) calculated the size of sample required to contain 20 particles of gold (spheres) as a function of gold particles and grade, assuming that Au particles are uniform and randomly distributed.
0.00010.000020.008
0.0020.00020.015
0.0060.0020.031
0.050.020.062
0.50.10.125
410.25
3080.5
200501.0
10004002.0
1 ppm Au4 ppm Au
kg of sample requiredSize of Gold
Particle (mm)
The mass of sample to
be used is a function of
the gold particle shape,
size and distribution.
Studying of Liberation of Gold Ores
• According to Clifton et al. (1969) a precision of ± 50% is achieved at 95% certainty when a sample for analysis contains a minimum of 20 gold particles
• Usually 100 kg of a representative sample crushed below 1/4" (6.35 mm) is a good starting material.
• If gold particles are not larger than 0.25 mm (no nuggets), material can be stored in 1 kg bags after crushed below 6 Mesh (3.4 mm).
• When there is indication of presence of >1mm nuggets the entire process becomes complex as we cannot work with small samples.
• When the amount of sulfides in the ore is low (<2%) it is convenient to concentrate the screened fractions in heavy liquids before the microscopy (Gaudin Method)
Gold Cyanidation
Discovered Blue Acid by heating Prussian Blue with Dilute Sulphuric Acid
C.W. Schule
1782
Discovered potassium ferrocyanide (K4[Fe(CN)6]) by combining Prussian Blue with Alkali (KOH)
P.J. Macquer
Early 1700s
Applied chlorination to gold recovery
2Au + 3Cl2 → 2AuCl3
Discovered chlorine & that it dissolved gold
Discovered Prussian Blue (Fe4[Fe(CN)6]3) by accidentally combining dried blood with potash (K2CO3) and treating with iron vitriol (FeSO4)
Discovered aqua regia & that it dissolved gold
6HCl + 2HNO3 + 2Au → 2AuCl3 + 2NO + 4H2O
Discovery
K.F. Plattner
C.W. Schule
Jabir Hayyan
Name
1852
1774
1704
6th
Century
Date
History of Gold Cyanidation
9
Patented process for dissolution of Au from ore using weak cyanide solution and to precipitate Au with zinc shavings. Process was considered a significant improvement over amalgamation and chlorination.
Recognized importance of oxygen to dissolution of Au with cyanide
4Au + 8KCN + O2 + 2H2O → 4KAu(CN)2 + 4KOH
Patented process using KCN to prepare electrolyte for electroplating Au and Ag
Potassium cyanide produced by fusing potassium ferro cyanide with potash K4[Fe(CN)6] + K2CO3 →
6KCN + FeCO3
Determined Blue Acid was HCN
Discovery
J.S. MacArthur,
R.W. Forrest,
W. Forrest
L. Elsner
Elkington
J.L. Gay-Lussac
Name
1887
1846
1840
1834
1811
Date
1. Oxygen Theory, L. Elsner (1846)
4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH
2. Hydrogen Theory, L. Janin (1888)
2Au + 4NaCN + 2H2O → 2NaAu(CN)2 + 2NaOH + H2
3. Hydrogen Peroxide Theory, G. Bodlander (1896)
2Au + 4NaCN + O2 + 2H2O → 2NaAu(CN)2 + 2NaOH + H2O2
H2O2 + 2Au + 4NaCN → 2NaAu(CN)2 + 2NaOH4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 4NaOH
Cyanidation Process (theories)
4. Cyanogen Formation, S.B. Christy (1896)
O2 + 4NaCN + 2H2O → 2(CN)2 + 4NaOH
5. Corrosion Theory, B. Boonstra (1943)
O2 + 2H2O + 2e → H2O2 + 2OH-
Au → Au+ + eAu+ + CN- → AuCNAuCN + CN- → Au(CN)2Au + O2 + 2CN
- + 2H2O + e- →Au(CN)2
- + 2OH- + H2O2
Cyanidation Process (theories)Factors Affecting Cyanidation
• Hedley and Tabachnick (1968) listed the main factors affecting cyanidation:
� dissolved oxygen
� pH
� stability of cyanide solutions
� cyanide concentration
� gold particle size and shape
� accessibility of cyanide to gold (exposure)
� silver content (silver dissolves slower than gold)
� temperature
� presence of minerals consuming oxygen
� presence of cyanicides
Hedley, N. and Tabachnick, H., 1968. Chemistry of Cyanidation. Mineral Dressing Notes, n. 23.
American Cyanamid Co.
Veiga, M.M. and Klein, B. (2005). Cyanide in Mining. Edumine.com.
Cyanide Stability
CN- + H2O = HCN + OH-
bitter almond smell
pH of Gold Cyanidation
Au cyanidation is more efficient at pH=10.5
Ca(OH)2 precipitates on gold surface inhibiting the cyanide attackE.avoid high pHs
Au Dissolution Rate
pH
Ca(OH)2
NaOH
10.5
10
Cyanidation ProcessMechanisms
– Oxygen adsorption into solution
– Transport of dissolved O2 and CN- to S/L
interface
– Adsorption of O2 and CN- onto gold surface
– Electrochemical reaction
– Desorption of soluble Au-CN complexes (e.g. Au(CN)2
-) and other reaction products from solid surface
– Transport of soluble products into solution
Oxygen in Cyanidation
• The level of dissolved oxygen controls the kinetics of gold cyanidation
• The concentration of cyanide does not usually determine the rate of the gold cyanidation reaction
• The level of dissolved oxygen required for the cyanidation process depends on the cyanidation reactions and cyanide consumption by other substances (cyanicides).
• The concentration of dissolved oxygen also depends on:- pressure (altitude)- temperature- agitation- ionic strength of the solution
Normal Cyanidation• In normal conditions of cyanidation, the minimum NaCNconcentration to extract gold is 75 mg/L.
• 4 Au + 8 NaCN + O2 + 2 H2O = 4 Na[Au(CN)2] + 4 NaOH
• Gold cyanidation employs diluted sodium or potassium cyanide solutions containing 100 to 1000 mg/L.
• In average 100 tonnes of NaCN are needed to extract 1 tonne of gold
• Most plants operate at between pH 9.5 and 11.5.
• Cyanide consumption usually is between 0.05 and 5 kg NaCN/tonne of oreE.for cyanidation of concentrates, the cyanide consumption can be higher (>5 kg/t)
Silver Slows Down Cyanidation
• Some of the cations that form stable complexes with cyanide, and in the process consume cyanide, are: Cu+, Cu2+, Fe2+, Fe3+, Mn2+, Ni2+, Zn2+.
• One way to eliminate the cyanide consumption problem is to remove cyanicide, e.g. pyrrhotite, by magnetic separation or flotation (assuming it does not contain a significant amount of associated gold) prior to cyanidation.
• Other methods include adding lead nitrate, or oxidation of the sulfides by pre-aeration, oxygen or peroxide.
Some Minerals Consume Cyanide
• Lead nitrate reduces cyanide consumption and speed up the gold leaching reaction
• Pb forms insoluble PbS removing passivationions (S2+) from gold surface
• Local galvanic cells between is also formed on gold surface
• In a cyanide solution, lead nitrate, lead sulfideand lead sulphite react with gold to form AuPb2, AuPb3 and metallic lead, which clearly accelerate the gold dissolution
Effect of Lead Nitrate
Deschenes et al (2000/ Minerals Engineering v.13, n.12, p.1263-1279.
11
Effect of Lead Nitrate
Deschenes et al (2000/ Minerals Engineering v.13, n.12, p.1263-1279.
Ore: 4.2 g/t Au, 0.9 g/t Ag, 3.1 pyrrhotite, 0.4% pyrite, Conditions: 0.38 g/L NaCN, pH 10
50 g/t Pb(NO3)2
No Pb(NO3)2
Time (h)
Au Extraction (%)
Some Minerals Consume Cyanide
Some Minerals Consume Cyanide
Pyrrhotites range from Fe5S6 to Fe16S17
Some Minerals Consume Cyanide
Some Minerals Consume Cyanide
• When high concentrations of cyanide-consuming minerals (in particular Cu minerals) exist in a ore, cyanidation may become uneconomical as this results in poor gold recovery and high operating costs
• The ammonia-cyanide leaching system is an alternative approach. It is proposed that the ammonia stabilizes a copper(II)-ammonia-cyanide complex such as Cu(NH3)4(CN)2 that is responsible for gold dissolution
Some Minerals Consume Cyanide
12
Preg-Robbing Ore
• Gold in solution can also be adsorbed by some mineralogical components of the ore.
• These are known as 'preg-robbing' substances.
• The most common preg-robbing substances are carbonaceous materials which are organic substances with high surface area.
• Not all carbonaceous material is activated
Preg-Robbing Ore
• One way to eliminate this effect, in milder cases, is addition of diesel or kerosene to the leach to deactivate the carbonaceous matter,
• This has to be done with caution otherwise the posterior carbon-in-pulp (CIP) process (adsorption of gold on activated charcoal) could be adversely affected.
• The addition of kerosene can effectively passivate the carbonaceous material by coating it. This process is suitable for ore with less than 1% carbonaceous material.
Preg-Robbing Ore• In other circumstances, the organic matter must
be eliminated prior to cyanidation by flotation or by oxidization / burning
• Most common procedure to overcome this effect is the CIL (carbon-in-leach) process.
• AC is introduced into the leaching process and soluble gold is immediately adsorbed by the charcoal.
• The process takes advantage of the fact that the adsorption kinetics of gold on charcoal is faster than it is on the preg-robbing mineralogical species
Preg-Robbing Ore
Source: Marsden, J.O, House I.
2006. The chemistry of gold
extraction. 2nd ed. Littleton, CO:
SME – Society for Mining,
Metallurgy and Exploration. 651p
Recovering Gold from Cyanide Solutions
• Gold can be recovered from CN solution by 3 methods:
� Activated Charcoal (AC) Adsorption
� Zinc precipitation (Merrill-Crowe
� Resin Adsorption
• The first two are the most popular methods
• Adsorption with Activated Charcoal (AC)
� Carbon-in-Pulp (CIP)
� Carbon-in-Leach (CIL)
� Carbon-in-Columns (CIC)
Recovering Gold with Activated Charcoal
Ore or Conc.
Cyanidation
CIP
Elution
Electrolysis
Gold
Ore or Conc.
Cyanidationwith AC
Elution
Electrolysis
Gold
AC = activated chacoal
Ore or Conc.
Cyanidation
Columns with AC
Elution
Electrolysis
Gold
solutio
nAC
AC
AC
Au rich solutionAu rich solution
Au rich solution
pulp
solution
Separation S/L
13
• Activated Charcoal (AC) Properties:
- Adsorptive Capacity (porous)
- Adsorption Rate
- Mechanical Strength and Wear Resistance
- Reactivation Characteristics
- Coarse Grain Size
• Most AC are manufactured from coconut shell
• Other materials can also be used anthracite, peat, etc.
Recovering Gold with Activated Charcoal
• The CIP method (Carbon-in-Pulp ) makes use of the physical affinity that activated charcoal (AC) has for gold (it can attract 7% of its weight in gold), which it readily attracts to its surface in cyanide solution.
• Usually the industry does not load the AC with more than 8000 g Au/t charcoal = 0.8%
• The following figure shows that less gold stays in solution when the max load is 5000 ppm
• Adsorption rate on AC is rapid initially as Au adsorbs to most accessible sites but then decreases as rate controlled by diffusion along pores.
Recovering Gold with Activated Charcoal
Source: Marsden, J.O, House I. 2006. The chemistry of gold extraction. 2nd ed. Littleton, CO: SME – Society for Mining,
Metallurgy and Exploration. 651p
Recovering Gold with Activated CharcoalRecovering the AC
• As the particles of AC are coarser than the ground ore, the AC is screened after the cyanidation
• The fine material is sent to cyanide destruction
• Some fine AC particles (with Au) can be lost if the charcoal does not have good physical properties
Ecuador, 2009
• After loading the AC, gold must be extracted (eluted) from the charcoal surface
• Elution procedures are very variable. Just a high pH solution can strip Au from the AC, but this takes time.
• Usually companies use a cyanide solution 1-2 g/L and 10 g/L of NaOH and temperature 90-100 oCand up to 72 hours to strip >97% gold from AC
• Elution is not done everyday. Wait for the charcoal to be loaded (5000 – 8000 gAu/t AC)
Elution of Activated Charcoal
• Solvents such as ethanol (10 to 20% vol) can speed up the process to be completed in 8 hours
• The elution solution, now with 1000 to 3000 mg Au/L (ppm) is submitted to electrolysis
• Some people use Zn to precipitate Au from the elution solution
• Excess zinc is dissolved with acid, and gold is melted
Elution of Activated Charcoal
14
Elution - Stripping of Au from AC
• Figure: the elution solution (7 m3) is heated in a diesel oil burning column and pumped through two columns filled with the loaded activated charcoal
• Stainless steel columns work in series to the electrolysis cell and then to a large holding tank, insulated with Styrofoam in a wooden box which in turn feeds the heating vessel.
Elution columns in Portovelo, Ecuador
Recovering Gold with Activated Charcoal
• After elution, the AC must be treated with nitric or hydrochloric acid to remove contaminants.
• Usually the charcoal is treated after 10 to 12 times of use
• After acid treatment, the AC is heat to 500 °C.
• Impurities are removed as gases or remain as a tar-like residue.
• Then the charcoal is exposed to an oxidizing atmosphere of steam containing CO2 and O2 at 700 –1000 °C. This burns off the tar-like residue, develops internal pore structure, and the carbon atoms at the edges of crystallites become 'active sites' due to unsaturated valencies.
Recovering Gold with Activated CharcoalTypical Carbon-in-Pulp Process Flowsheet
Comminution
Cyanide Leach
CIP
Tailings Pond
Cyanide Destruction
Gold
Elution
Ore or Conc.
Electrolysis
AC
pulp
Re-cycle to Process
solution
In the case of leaching concentrates,
comminution is not always needed
solution
Recovering Gold with Zinc• The Merrill-Crowe process is usually preferred over the charcoal process when the pregnant solution is high in Ag, or other metals like Hg, as these metals very quickly plug the pores of the charcoal
• Pulp is filtered and the clarified solution is then passed through a vacuum de-aeration tower where oxygen is removed from the solution.
• Zinc powder is added to the solution with a dry chemical feeder. The reaction of the special fine powder zinc with the solution is almost instantaneous. Precipitated gold is recovered by filtering.
4Au + 8CN- + O2 + 2H2O = 4Au(CN)2- + 4OH-
2Au(CN)2- + Zn = Zn(CN)4
2- + 2Au (Merrill-Crowe process)
• Suspended solids - interfere with the process possibly by coating the Zn particles and contaminate the final concentrate. Therefore, leach slurries are filtered to remove solids prior to Zn precipitation.
• Dissolved Oxygen - oxygen reduction competes with Au reductions, therefore, dissolved oxygen reduces precipitation kinetics. De-aeration is required to lower dissolved oxygen levels to <0.5 - 1.0 mg/l prior to precipitation.
• pH - the process is not very sensitive over pH range of 9 to 12. Above and below this range, precipitation decreases
Merrill-Crowe Process(Factors Affecting Zinc Precipitation)
15
• Cyanide concentration - minimum concentration required otherwise rate of precipitation is reduced (Typically > 0.05 - 0.20 g/L)
• Temperature - precipitation kinetics are accelerated at elevated temperatures
• Gold concentration - precipitation rate increases with Au grade of solution
• Zinc concentration - at high Zn concentrations, the dissolution rate of Zn can be slow. Also, high Zn concentrations can lead to formation of insoluble Zn-hydroxide which passivates Zn surface. Zn added at 5 to 30 times the stoichiometric Au requirement.
Merrill-Crowe Process(Factors Affecting Zinc Precipitation)
Typical Merrill-Crowe Process Flowsheet
Comminution
Cyanide Leach
S/L Separation Settling Tank
Tailings Pond
Ore or Conc.
Cyanide Destruction
Re-cycle to Process
Gold
Precipitation w/ Zn
solution
pulp
solution pulp
solutionpulp solutionsolution
Re-cycle to Process
solution
pulp
pulp
In the case of leaching concentrates,
comminution is not always needed
Leach with Acid
Precipitate w/Al
Zinc
• Instead of Zn precipitation, electrolysis can be used to treat high-grade gold solutions (carbon eluates) to produce loaded cathodes or cathode cell sludges
Au(CN)2- + e- → Au + 2CN-
• Advantages (over Zn precipitation)
� No chemicals or metals introduced in process
� More selective for Au and Ag over Cu
• Higher purity product
• Disadvantages
� Low single pass efficiency per unit cell requiring recirculation of solution to achieve acceptable extraction
� It needs high Au concentration in solution
ElectrowinningMain Cyanidation Practices
• Agitated Tanks– CIP (Carbon-in-Pulp)
– CIL (Carbon-in-Leach)
• Static leaching– Vat leaching with Carbon-in-Column or Zinc ppt
– Heap leaching with Carbon-in-Column or Zinc ppt
• Intensive cyanidation (for concentrates)– Gekko
– Acacia
– Mill-leaching (cyanidation in a small ball-mill)
• Cyanidation is being used in 160 tanks in Zaruma-Portovelo region
• Tanks range from 10 to 20 m³processing from 4 to 20 tonnes/day
• Some plants use either Merrill-Crowe or CIP (Carbon-in-Pulp) techniques
Cyanidation in Agitated Tanks in Small-scale
Ecuador, 2009
• CIP: 3 tanks = 9 hours, 100 tonnes/d, 2 g Au/t
• 0.7 g NaCN/L, consumption = 2.5 kg NaCN/t of ore
Cyanidation in Agitated Tanks in Small-scale
Ecuador, 2009
16
• Merrill-Crowe process in Ecuador
• The concrete tanks are loaded with 3,600-4,500 kg of sluice (“canalone”) concentrate and filled with about 11 m3 of cyanide solution that ranges from 2 to 4 g NaCN/L.
• After 12 hours, agitation is stopped to allow the pulp to settle. The semi-clarified solution flows by gravity to a holding tank and then through 4 clusters of 4 PVC columns containing a total of 11 kg of zinc shavings.
• Gold precipitation is not efficient as it is not conducted in vacuum.
• Miners do not leach Zn with acid to obtain gold, they simply evaporate the zinc (bad practice!!!)
Cyanidation in Agitated Tanks in Small-scale
• Merrill-Crowe process in Ecuador
Ecuador, 2007
Cyanidation in Agitated Tanks in Small-scale
• Merrill-Crowe process in Ecuador
Ecuador, 2007
Cyanidation in Agitated Tanks in Small-scale Cyanidation in Vats
• Vat leaching: cyanide solution slowly percolates through ore in a static tank.
• Ore can be coarser than in agitation leaching, with sizes of <1 cm.
• Ore is not agitated and coarse material must have good permeability.
• Very fine grain sizes of <1 mm may tend to block the free circulation of the leaching agent.
• The leaching period is 2 to 4 days and gold recovery is approximately 70 to 80%.
• Some artisanal miners leach for 30 days
Cyanidation in Vats
Artisanal Gold Miners in Zimbabwe using Amalgamation Tailings to fill the cyanidation vat
96
Vat-leaching in the Tapajós Region,Amazon, Brazil
Vat-leaching
tank
Pool with CN
solution
Tanks with
activated charcoal
Pool with
pregnant solution
Tailing pond
(baixao)
17
Cyanidation in Heaps
• Heap Leaching: the cheapest but slowest technique of gold cyanidation
• It is a process usually applied to low-grade gold ore.
• The ore is piled to a given height on an inclined impermeable surface, a so-called leach pad.
• A sprinkler system provides a continuous spray of alkaline cyanide solution that percolates through the ore, dissolving the gold.
• The gold-bearing or pregnant solution is collected and pumped to a gold recovery plant.
http://www.sulliden.com/projects/shahuindo-gold-project.aspx
Heap Leaching
Cyanidation in Heaps
• The amount of material contained in a heap can reach 400 million tonnes (Yanacocha). Leaching time ranges from several weeks to a few months. Gold recovery rarely exceeds 70%
Installing the plastic padshttp://www.mining-technology.com/projects/minera/
Yanacocha Mine, Peruhttp://www.mining-technology.com/projects/minera/minera2.html
Cyanidation in Heaps
• Yanacocha Mine, the 4th gold producer in the world.
• Location: Cajamarca, Peru, 4700 m above sea level
• Started in 1993. In 2005, produced: 3.3 Moz gold
• Ore Production: 544,000 tonnes/d @ 0.9 g Au/t
• Rock is crushing and agglomerated with cement before going to the heap. Au recovery = 74%
• Waste:Ore Ratio ~0.40
• Solution: 0.1 g/L NaCN, pH 10.5, 10 L/h/m2, NaCNconsumption 0.06 kg/t ore
• Au is adsorbed on activated charcoal, and then eluted solution is precipitated with zinc or submitted to electrolysis
http://www.geomineinfo.com/Complimentary%20Downloads/Yanacocha.pdf
Intensive Cyanidation
• Many mining companies do not concentrategold before leaching it with cyanide
• The whole ore cyanidation is a common practice usually when gold is very fine, hard to liberate but the rock is prorous to allow penetartion of cyanide solution
• The operating cost of whole ore leachingis higher than cyanidation of concetrates also called intensive cyanidation
• The cost of destroying cyanide is high when the whole ore is leached
• Coarse gold takes a long time to be dissolved in normal cyanidation solutions.
• In normal conditions of cyanidation the solubility rate of pure metallic gold is 3.25 mg/cm2/hour.
• Pure silver dissolution rate is 1.54 mg/cm2/hour.
• The more silver, the slower is the dissolution of the Au-Ag alloy
• Then a pure gold grain of 44 µm (400 mesh) would take about 13 hours to dissolve.
• A 150 micron (100 mesh) diameter pure gold particle would need almost 44 hours to dissolve.
Hedley, N. and Tabachnick, H., 1968. Chemistry of Cyanidation. Mineral Dressing Notes, n. 23.
American Cyanamid Co.
Intensive Cyanidation
18
Intensive Cyanidation
• In the past many companies used gravity concentration followed by amalgamation of the gold in the concentrate.
• Amalgamation was used to remove the “coarse” (100 µm) gold
• The tailings from amalgamation was leached with cyanide.
• Problem: this forms mercury cyanide which is very toxic and also consumes cyanide.
• Most organized mining companies no longer use amalgamationEartisanal miners use 1000 tonnes Hg/annum
• If the ore has coarse gold (>100 µm), companies use gravity concentration and/or flotation.
• They concentrate gold until >3000 g Au/tonne and melt it with borax.
• Problem: gold can be in the slag and this must be leached with cyanide or all middling products in the gravity sep. must be recyled
• Recently, mining companies are concentrating gold (gravity or flotation) followed by leaching the concentrate: Intensive Cyanidation
Intensive Cyanidation
Intensive Cyanidation
• To leach coarse gold a strong oxidant is needed.
• The concentration of the oxidizing agent is more important than a high cyanide concentration.
• High concentration of cyanide is used because, in an oxidant environment, cyanide will be oxidized as well, forming cyanate (CNO-).
• 5 to 20 g/L of NaCN is used
• Use of catalysts Leachwell® or LeachAid® or hydrogen peroxide (up to 0.5 g/L) is common
• Temperature can also increase cyanidation rate
• Intensive cyanidation of gravity and flotation systems have been used by GEKKO and Acacia Systems
• The GEKKO process is a thin film in a drum
Intensive Cyanidation
• Acacia System CS 250: 375 kg conc./cycle
Intensive Cyanidation
• Knelson Centrifuge provide concentrates for the reactor
http://www.knelsongravitysolutions.com/page361.htm
Acacia System:
Intensive Cyanidation
• Pre-washing of the gold concentrate to remove ultra-fine solids (slimes)
• NaOH, NaCN and LeachAid®
• Agitation for 16 hours with warm CN solution
• Decanting and wash the solids with water
• Electrowinning
• Tailings sent to the normal cyanidation circuit
http://www.knelsongravitysolutions.com/page361.htm
19
Intensive Cyanidation
• A good methodology to investigate intensive cyanidation of gravity concentrates can be:
• Concentrate is ground with different times and leached with cyanide (5 to 20 g/L) for 1 to 12 hours, 40% solids and pH 10.5 to 11 and presence of H2O2 (0.3 g/L = 100mL of 3% v.v. H2O2 solution in 1 L of water)
• It is always useful to do CIL – Carbon-in-Leach process to avoid any preg-robbing effect
Veiga,M.M.; Nunes,D.; Klein,B.; Shandro,J.A.; Velasquez,P.C.; Sousa,R.N. (2009). Replacing Mercury Use
in Artisanal Gold Mining: Preliminary Tests of Mill-Leaching. J. Cleaner Production, v.17, p.1373–1381
Intensive Cyanidation(Mill-leaching)
• The ore is not primarily finely ground (this can be conducted in a hammer-mill)
• Gold concentration in centrifuge or in sluice boxes or jig or flotation
• Unliberated gold is also concentrated = pre-concentration
• Concentrate is taken to a small batch ball-mill with cyanide and a capsule of activated charcoal
• Leached for 8 hours with 10-20 g/L NaCN and 0.3 g/L H2O2, pH 10.5.
• Remove the capsule of activated charcoal
• Elution
• Electrolysis or precipitation with zinc and leaching with acid
Screen 2 mm Hammer mill
+2mm
-2mm
Icon centrifuge
Icon centrifuge
tailing
flotation
tailing
Intensive
Cyanidation
conc.
conc
.
conc.
elution
activated
charcoal
electrolisis
gold
meting
final
tailing
Gold ore
Small-scale Intensive Cyanidation implemented in Tapajós, Brazil A Simple Intensive Cyanidation System
Icon centrifuge: 2 tonnes/h of ore
Icon® (Falcon) centrifuge
Capacity: 2t/h
Pulp: 30% solids
Cycle: 15 to 30 minutes
Bowl: 1kg concent. (dry)
Day: 40 to 80kg conc
Au recovery: 50 to 65%
Source: Falcon website, 2009
A Simple Intensive Cyanidation System
Sousa,R.N.; Veiga,M.M.; Klein,B.; Telmer,K.; Gunson,A.J.; Bernaudat,L. (2010). Strategies for Reducing the
Environmental Impact of Reprocessing Mercury-contaminated Tailings in the Artisanal and Small-scale Gold
Mining Sector: Insights from Tapajos River Basin, Brazil. In press by Journal of Cleaner Production.
Capsule of activated charcoal in the ball mill
Re-grinding the centrifuge concentrates
20
• This is an easy procedure
• Replace the use of Hg in these small ball-mills
• Capsule of activated charcoal can be added together with rods or ballsEbut this depends on the resistance of the capsule
• If necessary grind first, remove rods and add the AC capsule later
Ecuador, 2009
Intensive Cyanidation in Ball MillCentrifuge concentrate
[CN] 20 g/L, 0.3 g/L H2O2
grinding
Same but no grinding
Conventional cyanidation
Head sample, [CN] 1g/L,
pH 10-11, no grinding
Sousa,R.N.; Veiga,M.M.; Klein,B.; Telmer,K.; Gunson,A.J.; Bernaudat,L. (2010). Strategies for Reducing the
Environmental Impact of Reprocessing Mercury-contaminated Tailings in the Artisanal and Small-scale Gold
Mining Sector: Insights from Tapajos River Basin, Brazil. In press by Journal of Cleaner Production.
• Concentrate from sluice boxes (17,3 g Au/t)
was split in 3 subsamples:
– Amalgamation (160 kg)
– Conventional cyanidation in tank (695 kg)
– Intensive cyanidation in ball mill (80kg)
Intensive Cyanidation Tests in
Ecuador
Ecuador, 2007Ecuador, 2007
• Manual amalgamation
• 8 hours in a bateawith mercury and brown sugar
• Gold recovery from the gravity concentrate after amalgamation was: 26%
Result of Amalgamation
Ecuador, 2007Ecuador, 2007
• In agitated tank
• pH = 11
• Gold recovery from the gravity concentrate after 31 horas was: 94%
• NaCN consumption was 4.5 kg/t of concentrate
Result of Conventional Cyanidation
• Gold recovery after 8 hours of mill-leaching the concentrate was:95%
• Cyanide consumption was 0.95 kg/t of concentrate
Ecuador, 2007
Result of Intensive Cyanidation in Ball Mill
21
• NaCN = 2g/L, NaOH = 10 g/L
• Alcohol = 20%
• Temperature: 90 °C, 8 h
• 97% of gold removed from the activated charcoal
• Gold precipitated with small amount of zinc
• Zinc leached with HNO3
• Zinc can also be precipitated with aluminum (cementation)
Ecuador, 2007
Results of Elution of Activated CharcoalIntensive Cyanidation (conclusion)
• For environmental and economic reasons, intensive cyanidation of gravity and flotation concentrates must be the future of gold mining industry.
• Coarse gold particles (up to 1 mm) can be leached in intensive cyanidation circuitsEthe use of catalysts (oxidants) is fundamental.
• This reduces liability and operating costs
• Capital costs is not much higher but depends on flotation and gravity concentration circuits
Tests to Check Accessibility of Cyanide to Gold Particles
• This is a procedure to assess why the gold is not being extracted by classical cyanidation (low CN concentration and no peroxide)
• First of all, the sample must be ground below 0.1 mm, homogenized and 30g obtained to make a fire assay
• Fire assay is a procedure to melt (~1200 oC) the whole material with borax, lead oxide and a source of carbon (e.g. flour). The carbon reduces the PbO and the lead carries the precious metals to the bottom of the crucible.
• The lead button is placed into a cupel, which is a small dish made from fishbone ash, and placed into a furnace. Lead volatilizes and soaks into the cupel, leaving a small "bead" of precious metals.
• The bead is leached with aqua regia and analyzed by atomic absorption spectrometry
Diagnosis Leaching
• Another subsample of the ground material (200 g) is leached with cyanide 1 g/L (467 mL) for 24-48 hours (rolling bottles or agitated beaker) using 30% of solids, pH 10.5, ambient temperature. During the test, check free cyanide consumption (titration with AgNO3) and eventually add more cyanide if needed.
• Some people keep the rolling bottles for 72 h, but this depends on how much coarse gold has the material. You can pan a bit of the material to check the presence of coarse (>0,1mm) gold
• Filter the solution and send it to be analyzed by atomic absorption. Wash, dry and keep the residue.
• With the result from the atomic absorption you can check if the total gold was dissolved in cyanide
Diagnosis Leaching• %Au extracted = mg gold in solution (volume was 467mL)
mg of gold in sample (weight was 200g)
• If total gold was not extractedEuse the filtration residue (now it its dried).
• Regrind it, weigh it and leach it again with cyanideEin most cases this solve the problemEbut the question is how fine you must grind the material (ore or concentrate)?
• It is not suggested to grind too fine since this is a very expensive operation in a real plant
• Grind below 200 mesh (0.074mm) and use same leaching with CN conditions as above
• Filter and send solution to atomic absorption. Wash, dry and keep the residue.
Diagnosis Leaching
22
• If the total gold was not extract yet, gold can be occluded in the sulfide minerals. Weigh the dry residue.
• Oxidize sulfides using 2M HCl and 100 g/L Fe3+ as FeCl3, at boiling temp. for 6 to 24 h, L:S=2:1. Use an agitated beaker.
• Filter the sample, wash it and keep solution (measure the volume). If you notice in the wet solids some sulfides, add on the wet sample nitric acid 1:1 (HNO3 55%: distilled water) L:S=10:1 (approximately)
• Filter it, wash it well (measure the volume of solution). All acid solutions must be analyzed as acid + oxidizing agent can dissolve some gold (some authors found gold in solution)
• Dry and weigh residue
Diagnosis Leaching
• Leach the residue with cyanide using similar conditions as above. Filter and keep residue.
• Send solution to atomic absorption
• If the total gold was not extracted yet and the sample has organic matter (dark color), use the filtration residue from the previous leaching step
• Check if gold is adsorbed on organic matter. Extract gold using 40% v/v acetonitrile in distilled water, 10 g/L sodium cyanide, and 2 g/L NaOH, 16 hours, L:S = 5:1Efilter, wash and analyze solution. Dry residue.
• If total gold was not extracted yetEuse filtration residue. Probably the acetonitrile was not enough to extract all organic matter
Diagnosis Leaching
• An ultimate test is to burn carbonaceous material off in a furnace at 700 °C for 6 hours and repeat cyanidation. This can also roast any residual sulfide
• Leach silicates with HF and filter. L:S=10:1
• Residual gold: leach the residue with aqua regia: 3 HCl + 1 HNO3. L:S=10:1
• Analyze solutionEnow the total gold was definitely extracted. Based on the weight of each leaching step calculate the % gold extracted in each step.
• The sequence of leaching can have more steps depending on the mineralogical phases to be investigated
Diagnosis Leaching
Adapted from Lorenzen, L. (1995). Minerals Engineering, v. 8, n. 3, p. 247-256
Environmental Management of Cyanide
Use of Cyanide
• About 1.4 million tonnes of hydrogen cyanide (HCN) are produced annually worldwide.
• Cyanide price is around US$ 1.1 and 1.3 per kilogram but now increased to near US$ 3/kg
Used in production of Organic Chemicals
85%
Used for NaCN
production
Gold Mining
13%
Other 2%
Use of Cyanide
• According to the ICMI – International Cyanide Management Institute, 1.4 million of HCN is produced annually in the the world. This is equivalent to 2.5 million tonnes of NaCN.
• About 13% is used in the gold mining industry.
• This is equivalent to 325,000 tonnes/a of NaCN.
• ICMI (2010). International Cyanide Management Institute. http://www.cyanidecode.org/cyanide_use.php
23
Gold Uses
Source: Elma van der Lingen (2005). Gold’s Other Uses, The LBMA Precious
Metals Conference 2005, Johannesburg, p.75-80
Gold Uses
Source: Elma van der Lingen (2005). Gold’s Other Uses, The LBMA Precious
Metals Conference 2005, Johannesburg, p.75-80
Gold Mine Production 2009
1. China (314)
2. Australia (227)
3. United States (216)
4. South Africa (205)
5. Russia (205)
6. Peru (180)
7. Canada (95)
8. Ghana (90.2)
9. Indonesia (90)
10. Uzbekistan (80)
Others (854)
(tonnes)
Total: 2572 tonnes
Source: Goldsheet. http://www.goldsheetlinks.com/production.htm
Gold Consumption
• The total gold produced in the world: 161,000 tonnes; enough to fill 2 Olympic swimming pools
• India is the largest gold consumer. In 2008 consumed 660.2 tonnes Au
• In 2008 China consumed 395.6 tonnes of gold Photo: http://goldprice.org/buying-
gold/uploaded_images/indian-gold-742896.jpg
Use of Cyanide
• Cyanidation employs diluted sodium or potassium cyanide solutions containing 50 to 500 mg/L (free cyanide is typically 50 to 100 mg/L).
• Cyanide consumption is typically 300 to 2000 g NaCNper tonne of ore.
• In average 100 tonnes NaCN is used to produce 1 tonneof gold.
• Cyanide concentration in effluents (Canadian gold industry) range from 0.3 to 30 mg/L (ppm).
• Cyanide is also used as a depressant to separate multiple sulfide ores by flotation (e.g.: depress chalcopyrite to float galena). Concentration of total cyanide in flotation effluents is <0.03 mg/L (ppm).
Cyanide Toxicity
0.43220Snail
0.10221
0.09015Yellow perch
0.06818
0.04212
0.05710
0.0286Rainbow trout
96-h LC50 (mg/L)Temperature
(°C)Species
Higher temperature, lower toxicity
24
Cyanide Toxicity
• Adverse effects on fish swimming and reproduction usually occur between 0.005 and 0.007 mg of free cyanide/L.
• Free cyanide concentrations between 0.05 and 0.2 mg/L are fatal to the more-sensitive species
Cyanide Toxicity• Public perception is that cyanide is very dangerous...
� chemical warfare
� judicial executions
� mass suicides
� the Tylenol affair
• A large majority of the public believes that cyanide is more dangerous than mercury...however cyanide is not persistent in the environment and Hg is.
• Problems:
� spills
� misuse of cyanide by Artisanal Gold Miners
Cyanide Toxicity
• Jim Jones was the founder and leader of the Peoples Temple, in Jonestown, Guyana
• On November 18, 1978, 909 Temple members, including 276 children, drunk cyanide with Cool Aid
Cyanide Toxicity
http://www.chicagostagereview.com/wp-content/uploads/2008/09/jonestown1.jpg
Source: Infoczarina, 2008
Cyanide Spills
• Baia Mare, Romania, mine re-opened in 1999.
• Intense rain in Jan 2000 = overflow
• 100,000 m3 of tailings with cyanide released to the river
• 50 to 100 tonnes of cyanide released
Cyanide Spills
• 1000 tonnes of fish killed
• CN in the Danudbe
• Public reaction was horrible
25
• 1995 – Omai Mine in the Esequibo region in Guyana
• 4 millions m3 of cyanide contaminated tailings entered the Essequibo River
• Until now the use of cyanide in mining is not allowed in Guyana
• The use of Hg has increased
http://www.ec.gc.ca/inre-nwri/default.asp?lang=En&n=0CD66675-1&offset=14&toc=show
Cyanide Spills Rudimentary Cyanidation Plant made by Artisanal Miners in Zimbabwe
Amalgamation tailings (full of
Hg) are submitted to cyanidation
Rudimentary Cyanidation Plant made by Artisanal Miners in Zimbabwe
...at least they know that cyanide is dangerous!!!
• Increasing the use of Hg-amalgamation followed by cyanidation in Artisanal Mining.
• Cyanidation of Hg-rich tailings forms Hg(CN)42-
which is a very stable and persistent compound. This can be transformed into highly toxic compounds, e.g. Methylmercurycyanide
Misuse of Cyanide
Recovering loaded charcoal (retained in a screen)
Artisanal Gold Miners (Sulawesi, Indonesia) Artisanal Gold Miners (Sulawesi, Indonesia)
Only CN destruction method = NATURAL = sun lightFalse perception that this destroys all cyanide
complexes
Rudimentary tailing pond; often CN-rich tailings reach the streams
26
Cyanide Toxicity• Free cyanide criteria currently proposed for the protection of natural resources:
� Free cyanide (CN- or HCN) criteria for the protection of aquatic life of natural resources: <0.005 mg/L (Canadian Water Quality Guidelines, 2001).
� for human protection: • drinking water in USA and Canada: MAC = 0.2 mg/L
• in drinking water in Sweden: 0.05 mg/L
• WHO Drinking water quality guideline: 0.07 mg/L (12 µg/kg body weight)
• <50mg/kg in diet
• <5mg/m³ in air
• The cyanide toxicity depends on the cyanide species. The predominance of free cyanide (CN- and HCN) depends on the pH
• In solutions with pH above 10, free cyanide is as CN- and not as HCN gas
• The gold cyanidation is usually conducted at pH 10.5 to 11
Cyanide Toxicity
Cyanide Toxicity (humans)
• In solution, 3 to 5 mg of cyanide/kg body weight is lethal
You need to drink a lot of Cyanide solution to die
If a mine uses a CN solution of 300
mg/L
A 100-kg person should drink 1.5 L
to die
Cyanide Toxicity (humans)
• Cyanide is readily absorbed through inhalation, ingestion or skin contact.
• In respiratory exposure to hydrocyanic acid (HCN gas), death occurs at 0.1 to 0.3 g/m³
• Cyanide is a potent asphyxiant. It induces tissue anoxia through inactivation of cytochrome oxidasedue to the reaction of CN- and Fe3+.
• Oxygen cannot be utilized and death results from the depression of the central nervous system.
Cyanide Toxicity (humans)
• Some plants such as sorghum, cassava, bamboo and lima beans can contain over 2,000 mg/kg total cyanide. At this level, ingestion of 170 g of the plant can be lethal to a 80 kg-human being.
• Congenital hypothyroidism (cretinism) is present in 15% of newborns in certain areas of Zaire where cassava is a staple food. This incidence is approximately 500 times that observed in industrial
countries.
Fishing with Cyanide
It’s forbidden to fish with cyanide and explosives
Ecuador, 2006
27
Cyanide Toxicity (humans)
• Cyanide is detoxified by an enzyme called
rhodanase forming thiocyanate CNS
• The major route of cyanide elimination from the body is via urinary excretion of thiocyanate (CNS).
• The toxicity of thiocyanate is significantly less than that of cyanide, but chronically elevated levels of thiocyanate in blood can inhibit the uptake of iodine by the thyroid gland, thereby reducing the formation of thyroxine.
Cyanide Toxicity (humans)
Cyanide does not accumulate in the blood and tissues following oral exposure to inorganic cyanide and no cumulative effect on the organism during repeated exposure has
been demonstrated.
There is a cumulative effect of exposure to thiocyanate resulting in thyroid toxicity,
including goitre and cretinism.
Cyanide Toxicity (humans)
Organs affected by Chronic intoxication with cyanide
(oral exposure):
• Central nervous system: neuropathies and amblyopia (partial or complete loss of vision in one eye caused by conditions that affect the normal development of vision)
• Thyroid: increase thyroid weights and depress thyroid function
• Reproduction and Development: congenital hypothyroidism was reported in human newborns
• Kidneys
Cyanide Toxicity (humans)Organs affected by Chronic intoxication with cyanide
(inhalation):
• Central nervous system: vertigo, equilibrium disturbances, nystagmus, nervousness, headache, weakness, loss of appetite, changes in smell and taste
• Cardiovascular and/or respiratory system: breathing difficulties
• Gastrointestinal tract: nausea, and gastritis
• Thyroid: Enlarged thyroids
• Reproduction and Development: risk of giving birth to low body weight infants and of perinatal death.
Cyanide Toxicity (hypothyroidism)• The main purpose of thyroid hormone is to "run the body's metabolism”. It takes iodine, found in many foods, and convert it into thyroid hormones.
thyroid gland normally weighs
<30g
• Hypothyroidism results in inflammation of the thyroid gland and reduction of hormone production.
goitre
Sources: http://www.endocrineweb.com/thyfunction.html
http://en.wikipedia.org/wiki/Goitre
Cyanide Toxicity (congenital hypothyroidism)
Cretinism:
• Poor length growth Adult stature without treatment ranges from 1 to 1.6 m
• Neurological impairment
• Thickened skin
• Enlarged tongue
• Protruding abdomen Sources:
• http://www.gidabilimi.com/images/kretenizm1.png• Wikipedia
28
Important Toxicological Facts
• The greatest source of cyanide exposure to humans and some animals is cyanogenic food plants and forage crops, not mining operations.
• Free cyanide is the primary toxic agent
• Cyanides are not mutagenic or carcinogenic.
• There is no indication that cyanide is biomagnified in food web.
• Cyanide has low persistence in the environment
• In general, the toxicity of free cyanide is
reduced when a complex is formed.
Types of Cyanide
1. Free cyanide (HCN/CN-) and simple cyanide salts (NaCN, KCN) which dissolve in water to form free cyanide. Free cyanide is the active form to leach gold
2. Weak and moderately strong cyanide complexes such as cyanides of Zn(CN)4
2-, Cd(CN)3-, Cd(CN)4
2-, Cu(CN)2-,
Cu(CN)32-, Ni(CN)4
2-, Ag(CN)2- These cyanides are known
as Weak Acid Dissociable Cyanides (WAD) as they can be decomposed in weak acid (pH 3 to 6).
3. Strongly bound cyanide complexes such as Co(CN)64-,
Au(CN)2-, Fe(CN)6
4-. These are stable under ambient conditions of pH and temperature
Free Cyanide
• Determined by tritation with AgNO3
• Silver will complex with free CN and excess of Ag+ is detected by p-dimethylaminobenzalrhodanine or by potassium iodide (KI) whihc are indicators
• AgNO3 + 2NaCN = NaAg(CN)2 + NaNO3
• Color will change from yellow to blue (dimetyl) or a white bluish color in the case of KI
• Free cyanide is determined based on the volume of silver nitrate used in the titration
• Total CN is determined by another method... distillation
Complexes with Heavy Metal
• In the leaching of gold, many other metals can be dissolved in cyanide forming complexes.
• Example of minerals soluble in cyanide: – Pyrrhotite - FeS
– Argentite - AgS
– Malachite - CuCO3.Cu(OH)2– Chalcocite - Cu2S
– Bornite - Cu5FeS4– Orpigment - As2S3– Galena - PbS, partially soluble
– Sphalerite - ZnS, partially soluble
– Pyrite - FeS2, sparingly soluble
Fate of Free Cyanide• Free cyanide seldom remains biologically available in soils and sediments because it is either complexed by trace metals, metabolized by various microorganisms, or lost through volatilization.
• Under aerobic conditions, cyanide salts in the soil are microbiologically degraded to nitrites or form complexes with metals.
• Under anaerobic conditions, cyanides denitrify to gaseous nitrogen compounds and enter the atmosphere.
• All cyanide complexes are subjected to (biotic and abiotic) oxidation.
Main Cyanide Reactions
Hydrolysis CN- + H2O = HCN + OH-
Oxidation of HCN/CN- 2HCN + O2 = 2HCNO
2CN- + O2 + catalyst = 2CNO-
Hydrolysis of CNO HCNO + H2O = NH3 + CO
2
Hydrolysis/saponification of
HCN
HCN + 2H2O = NH4COOH or
HCN + 2H2O = NH3 + HCOOH
Aerobic biodegradation 2HCN + O2 + enzyme = 2HCNO
Thiocyanate formation −2
xS + CN- = −−
21xS + CNS
-
−232OS + CN- = −2
3SO + CNS-
Cyanide compound
dissociation NaCN = Na+ + CN-
Metal-cyanide complexation Zn2+ + 4CN- = −2
4)CN(Zn
Anaerobic biodegradation CN- + H2S = HCNS + H+
HCN + HS- = HCNS + H+
29
Cyanide Cycle
cyanidation
tank
tailing pond
HCN/CN-
NH3 + CO2
evaporation and decomposition
HCN/CN-
Fe(CN)63-
Fe(CN)64-
tailing with cyanide
CH4 + CO2
NO3-
NH3 + HCO3-
or
CO32-
River
spill
Cyanide Treatment Processes
• Natural Degradation (Lagooning)
• Oxidation Processes
– Alkaline Chlorination
– Electrochemical Processes
– Ozonation
– Hydrogen Peroxide
– INCO’s SO2/Air Process
• Precipitation (Cuprous Process)
Cyanide Treatment Processes (cont.)
• Conversion to Less Toxic Forms
• Biological Treatment
• Cyanide Recovery Processes
– Acidification/Volatilization/Reneutralization -AVR
– Ion Exchange
– Activated Charcoal
– Electrolytic Processes
Natural Degradation
• It is the oldest treatment method used by Canadian gold mines to remove cyanide from effluents.
• Volatilization is the main mechanism. The most important variables are:
– pH
– temperature
– UV light
– aeration
• Other variables:
– biodegradation
– precipitation
– conversion to thiocyanate
Natural Degradation
• The equilibrium between free cyanide species (CN-and HCN) is pH dependent: HCN = H+ + CN-
• Molecular HCN has high vapor pressure and therefore can readily be volatilized to the atmosphere. At pH 7, 99.5% of free cyanide exists as molecular HCN.
• Free cyanide (CN-/HCN) is rare in mining tailings because of the high reactivity of the CN- molecule with metal ions.
1010x93.4]HCN[
]CN].[H[Kd −
−+
==Dissociation constant
Stability of Cyanide Complexes
Cyanide 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
30
Natural Degradation• Ferrocyanides (Fe CN( )6
4−) and Ferricyanides (Fe CN( )6
3−)
are very stable compounds. They can form complexes with trace metals. Most complexes are insoluble, e.g.:
SnFe CN( )6 = tin(IV) ferrocyanide
Fe Fe CN4 6 3[ ( ) ] = ferric ferrocyanide
Co Fe CN3 6 2[ ( ) ] = cobalt ferricyanide
Sn Fe CN3 6 2[ ( ) ] = tin(II) ferricyanide
• Major environmental concern:
ferrocyanides and ferricyanides (usually in the sediments) can form free cyanide by
influence of ultraviolet light This reaction can take hundreds of years
Cyanide Guidelines (Canadian Legislation)
1µg/LNone proposedNot applicable
Marine and Estuarine Aquatic Life
(maximum at any time)
10 µg/LNone proposedNot applicableFreshwater Aquatic Life
(maximum)
< or = 5 µg/L
None proposedNot applicableFreshwater Aquatic Life
(30-day average)
Not applicableNot applicable200 µg/LRaw Drinking Water
(maximum)
Weak-acid dissociable cyanide
µg/L (as CN)
Strong-acid dissociable cyanide
µg/L (as CN)
Strong-acid dissociable cyanide plus thiocyanate µg/L (as CN)
Water Use
British Columbia Approved Water Quality Guidelines (Criteria) 1998 Edition
Cyanide Guidelines
• The Canadian MMER (Metal Mining Effluent Regulation, 2002): maximum level of total cyanide to be released in a mining effluent is 1.0 mg/L (ppm) as a monthly average concentration, 1.5 mg/L in a composite sample and 2 mg/L in a grab sample.
• In Yukon Territory, the discharge limits are 0.5 mg/L of total cyanide and 0.2 mg/L of WAD (weakly acid dissociable)
• The World Bank guidelines (1995) for discharge into the environment are:
– Free Cyanide: 0.1 mg/L
– Weak Acid Dissociable: 0.5 mg/L
– Total Cyanide: 1.0 mg/L.
– In no case should the concentration in the receiving water
outside of a designated mixing zone exceed 0.022 mg/L.
Natural Degradation
• Before the mid 1970s, natural degradation was the only treatment method used by the Canadian mining industry
• Photodegradation is affected by turbidity, color and depth, intensity and wavelength of light, angle of light incidence and cloud cover.
• Degradation: free cyanide in the concentration range of 0.1 to 0.5 mg/L is volatilized at a rate of 0.021 mg CN/ft2.hr in still waters.
• Rates in agitated water are up to 3 times as great.
• A temperature increase of 10°C causes the free cyanide removal rate to increase by more than 40%
Natural Degradation
• Natural degradation is:
� suitable for removal of “free cyanide”,
� removes partially Zn and Cd cyanide complexes and thiocyanate (CNS)
� does not remove Cu and Ni complexes and Iron-cyanides.
• Natural degradation is more effective when barren solutions (“barren bleed ponds”) are stored separately from solid tailings in shallow ponds.
Oxidation by Chlorine
• Alkaline chlorination is the oxidation of cyanide in an alkaline solution by chlorine or hypochorite.
• The complete oxidation of cyanide proceeds in two stages at different pH’s:
• In the first stage, cyanide is transformed to cyanate(CNO) (which is considered approximately one-thousandth as toxic as hydrogen cyanide). The oxidation of cyanide is represented by the equations:
Any pH: CN- + Cl2 = CNCl + Cl-
High pH: CNCl + 2OH- = CNO- + Cl- + H2O
With hypochlorite: CN- + OCl- = CNO- + Cl-
31
Oxidation by Chlorine
• The second stage consists of cyanate being converted to bicarbonate and nitrogen:
2CNO- + 3Cl2 + 4H2O = (NH4)2CO3 + 3Cl2 + CO32-
(NH4)2CO3 + 3Cl2 + 6OH- + CO32- = 2HCO3
- + N2 + 6Cl- + 6H2O
Oxidation by Chlorine
• Advantages of the process:
� Easy to operate
� Reactions reasonably rapid
� Free and WAD cyanides as well as thiocyanates are destroyed
� Chlorine or hypochlorite readily available in several forms
� Capital outlay relatively low
Oxidation by Chlorine
• Disadvantages:
� High operating cost (reagents are costly)
� Requires pH control to prevent cyanogenchloride which is highly toxic
� Ferro and Ferricyanides are not destroyed
� High content of chloride and chlorine in effluents
� Some chlorination products (e.g. Na-hypochlorite, chloro-lime, etc.) are degradable: storage problems in remote regions.
Electrochemical Processes
Electroreduction: Complex metal cyanide ions reduce at the cathode to deposit or precipitate the metal, regenerating free cyanide:
)metaldivalentMe(MeCNne2)CN(Me n2n =↓°+=+ −−−
Electrochlorination : Anode reaction: Cathode reaction:
)g(ClCl2
eClCl
2=°
+°= −−
↑=°
+°=+ −−
)g(HH2
OHHeOH
2
2
• Introducing NaCl into the solution, chlorine is produced by electrolysis. Hypochlorite and chlorate can also be formed. Free cyanide, WAD cyanides and thiocyanates are destroyed. Iron-cyanides are not destroyed
• Cyanide can be regenerated
Oxidation with Hydrogen Peroxide
• Cyanide oxidation with H2O2 is a fast, one-step reaction, forming non-toxic intermediates.
• H2O2 oxidizes free and Zn & Cd cyanides to cyanate which is further hydrolyzed yielding biodegradable ammonia and carbonate.
CN- + H2O2 = CNO- + H2O
(Cu2+ is a catalyst)
CNO- + 2H2O = NH4+ + CO3
2-
(pH just < 7)
Oxidation with Hydrogen Peroxide
• Cu and Ni cyanides are partially destroyed and Iron-cyanides are partially precipitated.
• Since copper and other metal ions are already in many mining effluents, additional metallic catalyst may not be required to enhance the reaction rate.
• Degussa process uses borate compounds as catalysts resulting in a substantial saving of hydrogen peroxide consumption (up to 60%).
32
Oxidation with Hydrogen Peroxide
• There are a variety of processes combining hydrogen peroxide with other compounds, such as glycolonitrile(Kastone process), H2SO4 (Caro’s acid), SO2, etc.
• Formation of thiocyanate by H2O2 is slow (in contrast to chlorination).
• H2O2 consumption is estimated to be around 3 kg/kg CN-. Theoretical dosage: 1.5 kg H2O2/kg CN
-
• The process is not very suitable for slurries.
• High operating cost.
• First used at Ok Tedi in 1984 and in Canada at TeckCorona in 1985. Many plants in Canada use peroxide.
Oxidation with Hydrogen Peroxide(Example)
5<1157Zn
0.0513.2Ag
0.0145Se
0.3<0.01516Fe
0.053280total CN
9.0<14.5Cu
0.05<0.050.2As
EPA dws
(mg/L)
Effluent (mg/L)
Before After
Species
Note: H2O2 dosage = 2.5 mL/Ldws = drinking water standard
INCO SO2/Air Process
• Invented by INCO in 1984 as a result of a research to destroy cyanide used to depress pyrrhotite during flotation of pentandite (NiFe9S8) and Chalcopyrite (CuFeS2).
• A mixture of sulphur dioxide and air rapidly oxidizes free cyanide and WAD metal cyanide complexes in the presence of Cu2+ as catalyst.
CN- + SO2 + O2 + H2O = CNO
- + H2SO4
(Cu2+ is a catalyst)
+−− ++=+++ 242222
24 MeSOH4CNO4OH4O2SO4)CN(Me
Me2+ = Zn2+, Cu2+, Ni2+, Cd2+, etc.
(neutralization): H2SO4 + Ca(OH)2 = CaSO4.2H2O ↓
(pH>8) (precipitation): Me2+ + Ca(OH)2 = Me(OH)2 ↓ + Ca
2+
↓=+ −+62
46
2 )CN(FeMe)CN(FeMe2
INCO SO2/Air Process
• Copper should be present in minimum concentration of 50 mg/L and can be added as copper sulphate. Cu2+ additions depend on the iron content of the effluent and are in the range 0 to 0.5 g/g total CN.
• Free cyanide, Zn, Cd, Cu, Ni cyanides are destroyed. Iron-cyanides are partially precipitated. Thiocyanide is partially (10 to 20%) destroyed.
• The process is usually performed in one or two stages, bubbling SO2-air or adding sodium metabisulphite. Air flowrate is around 1 L/min per liter of solution. In practice 3-4 kg SO2 (or 5-8 kg of sodium metabisulphite) is required per kg of cyanide. Stoichiometrically the reactions require 2.46 kg SO2 per kg of WAD cyanides.
• Retention time ranges from 20 min to 2 hours.
INCO SO2/Air Process
• The process has been licensed at over 100 project sites worldwide.
• The process has been used to treat effluents containing > 200 mg/L total cyanide and routinely reduces this to <1 mg/L
• Capital cost for an installation to destroy 3,000 tonnes of slurry/d is Cn$ 1.2 million (installed)
• Typical operating cost to treat tailings from a plant using CIP is Cn$0.8/tonne
• Patent expired in 2004 0.510.254.13Ni
5.350.2645.82Fe
7.71.2135.45Cu
18.982.5100.7Thiocyanate
52.9324.244.6Cyanate
12.20.15225WAD cyanides
29.40.69365.8Total cyanide
Final Tailing
(mg/L)
Liquid Effluent (mg/L)
Before After
Species
INCO SO2/Air Process
33
Combinox Process
• Developed in Germany by CyPlus, this is a variation of the INCO Process, when the patent expired (May 2008)
• This process is a combination of SO2/Air and Peroxide (Degussa Process)
• Thiocyanate (CNS) is destroyed
• It destroys free and Zn, Ag, Cd, Cu, Ni cyanides
• Fe cyanides are precipitated in presence of copper:
Fe(CN)64- + 2Cu2+ → Cu2Fe(CN)6 (solid)
• Final product: CO2 + NH4+
• It is considered one of the most effective cyanide destruction processes
• Applied in Las Crucitas Project, in Costa Rica
• 7500 tonnes/day of saprolite
• In the future 5000 tonnes/d hard rock (1.5-2 g/t)
• All material is leached with 150-200 mg/L CN: CIP
<0.010.31WAD CN (moderately
complexed)
<0.02<2Free CN
0.030.58Total CN
Solution after 10
days
Solution after
Combinox
Compound (mg/L)
Combinox Process
Biological Treatment
• Cyanide is degraded by aerobic and anaerobic microbes
• First applied in 1984 at Homestake Lead Mine, South Dakota to treat 800 t/h of mixed mine water and tailings.
• Sensitive to temperature (optimum: 30 °C, pH 7 – 8.5).
• Process requires gradual acclimatization of mutant strains of bacteria (Pseudomonas) to the high concentration of cyanides and thiocyanate.
• Oxidation rate of cyanide to cyanate is increased by the bacteria.
Cyanide Recovery
• Acidification - Volatilization - Reneutralization – AVR Processes was used successfully at Flin Flon (Canada) from 1930 to 1975 and in several other commercial operations.
• The process consists of acidification of the alkaline cyanide leach solution producing hydrogen cyanide (HCN) which is removed by volatilization in a stream of air to be reabsorbed into an alkaline solution.
Acidification: CN- + H+ = HCN
Absorption: 2HCN + NaOH = NaCN + 2H2O
Cyanide Recovery
• Golconda (Australia) has reduced the total cyanide concentration in effluents from >200 mg/L to < 5 mg/L. Reagent consumption is 0.6 kg H2SO4/t solution and 0.45 kg NaOH/t. Cyanide recovery 50 to 85%.
• The Cyanisorb is an AVR process that uses high efficiency packed towers, low pressures and moderate pH levels. It recovers about 90% of the cyanide from tailings: Gold mine in Waihi (New Zealand), AngloGold in Argentina, DeLamar (U.S.), Marlin Mine (Guatemala) are using Cyanisorb method
• AVR technology has been developed to allow treatment of slurry streams directly without the need for solid-liquid separation.
Cyanisorb Process
pH 5 - 7.5
Barren solution
H2SO4
Ca(OH)2
300 ppm HCN
NaCN (recovered) solution
Pulp to tailing pond
Metals precipitation
10%
air
HCN ladden air
bleed air
stripped slurry
34
Cyanide Recovery: Ion Exchange• Developed in the mid-1950s for the recovery of cyanide from electroplating solutions.
• Resin impregnated with copper (to tie up all free cyanide as complex) precipitates cuprous cyanide in the resin matrix.
• Strong-base anion exchange resins remove cyanide complexes of iron, zinc, copper, nickel, cobalt, gold and silver can be removed effectively followed by elution of the resin.
• Practical problems:
� Efficient elution of all species that load onto the resin is hard to achieve
� Resin must be regenerated or new resin must be used to maintain process efficiency
� High costs involved
Cyanide Recovery: Activated Charcoal
• Activated Charcoal has been used for recovery of metals from cyanide solutions.
• Activated charcoal is capable of adsorbing up to 5 mg CN-/g from aerated alkaline cyanide solutions. This can reach 25 mg CN-
/g in the presence of a catalyst such as copper.
• Cyanide can be recovered (elution) from the AC into low ionic strength solution at elevated temperature.
You and me, we used to mine together
Cyanide together, always
I really felt, a bitter almond smell
When the pH fell, and I ran
You don’t know, where the pH goes
10.5 is good to leach the gold
Don't leach if you don’t have control
pH cannot be low
Be careful ‘cause it hurts
Don't leach if you forgot the lime
the smell can be sublime, but
Be careful ‘cause it hurts
In long term, the cyanide effect,
A lump in your neck, you will cry
Thyroidism, this can appear
And them it will be to late you’re gonna die
Don't leach gold or even silver,
Dumping cyanide in rivers
Be careful ‘cause it hurts
Don't leach if you think the fish won’t feel
Cyanide in their gills, Be careful ‘cause it hurts
You don’t agree, but with 5 ppb, fish can die
You told me, you’ve never had destroyed
The cyanide you have employed, you fool
You’ve just trust, what the sun can do
You’ve just left the tailings in a pool
Don't leach what you’re doing is quite insane
Cyanide still remains
Be careful ‘cause it hurts
Don't leach I don’t need your reasons
Telling this is sunny season
Be careful ‘cause it hurts
THE END