2 Gold Cyanidation

Universidade de São Paulo,
Escola Politécnica, Dept Engenharia de Minas
PMI 5003
Studying Gold Ores:
Mineralogy, Cyanidation, Toxicity
and Environmental Issues
Gold Process Mineralogy
Dr. Arthur Pinto Chaves
(Universidade de São Paulo, Dept Engenharia de Minas)
Dr. Marcello Veiga
(University of Britsh Columbia, Dept Mining Engineering, Canada)
Common Gold Minerals
Gold
Electrum
Cuproauride
Porpezite
Rhodite
Iridic gold
Au
(Au,Ag)
(Au,Cu)
(Au,Pd)
(Au,Rh)
(Au,Ir)
Calaverite
Krennerite
Montbrayite
Petzite (antamokite)
AuTe 2
(Au,Ag)Te 2
(Au,Sb)2Te 3
Ag3 AuTe 2
Uytenbogaardtite
Ag3AuS2
Aurostibite
AuSb2
Fischesserite
Ag3AuSe2
Common Gold Minerals
Physical Properties of Gold
Property
% Au
SG
Mohs Hardness
Native Gold
Electrum
>75
45-75
16-19.3
13-16
2.5-3
2-2.5
Platinum gold
Bismuthian gold
Gold amalgam
Maldonite
Auricupride
Palladium Curoauride
(Au,Pt)
(Au,Bi)
Au 2 Hg 3 (?)
Au 2 Bi
AuCu3
(Cu,Pd)3 Au 2
Muthmannite
Sylvanite
Kostovite
Nagyagite
(Ag,Au)Te
(Au,Ag)Te 4
AuCuTe 4
Pb 5 Au(Te,Sb)4 S5-8
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
2.5
38.6-44.3
9.9
Petzite
19-25.2
9.1
2.5
Hessite
4.7
8.4
2.5-3
1
Physical Properties of Gold Minerals
Au%
SG
Mohs
Hardness
Nagyagite
7.4-10.2
4.5
1.5
Kostovite
25.2
Mineral
2-2.5
Aurostibnite
43.5-50.9
9.9
3
Maldonite
64.5-65.1
15.5
1.5-2
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
HENLEY, K.J,(1975). Gold-ore mineralogy and its relation to metallurgical treatment.
Minerals Sci. Engng, vol.7, n. 4, p. 289-312
Gold in the Electron Microscope
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
Image of back-scattered electrons
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)
–
–
–
–
–
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
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
2
Gold Ore Processing Practices
(Alluvial)
• USUALLY gold is liberated in
alluvial ore, but sometimes it is not
Gold Ore Processing Practices
(Alluvial)
Scrubbing
• 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
Gravity and/or Flotation
Concentration
Free gold
Gold Ore Processing Practices
(Alluvial)
Brazil, 1993 (BBC Documentary)
Dredging alluvial gold in the Amazon
Gold Ore Processing Practices
(free milling ores)
Comminution
Cyanide
Leach
Comminution
Gravity and/or Flotation
Concentration
Gold
Comminution
Gravity
Sep
Free gold
Flotation
Oxidation
Cyanide
Leach
Free gold
Cyanide
Leach
Gold
Concentration of alluvial gold by flotation. Photo H. Wotruba
Gold Ore Processing Practices
(refractory)
Brazil, 2006
Mining coluvial gold ore
in the Amazon
Typical Terminology for Gold Ores
• 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
Reflected light in optical microscope
Fine gold in pyrite grain
Gold
3
Free Gold Particle
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
25 µm
Gold Associated with sulfides
Reflected light in optical microscope
Gold Associated with sulfides
Polished-thin Section, reflected light, optical microscope
Ga
Chalcopyrite
CuFeS2
Au
Si
Gold with Galena and Silicates
Gold is usually difficult to polish
(lots of scratches)
Reflected light
Very often in the optical microscope, chalcopyrite looks like
gold, but gold is much brighter
HENLEY, K.J,(1975). Gold-ore mineralogy and its relation to metallurgical treatment.
Minerals Sci. Engng, vol.7, n. 4, p. 289-312
Gold Associated with sulfides
Gold Associated with Sulfides
To differentiate Au from Chalcopyrite
Reflected Light
Reflected Light
gold
gold
chalcopyrite
gold
sulfides (chalcopyrite and pyrite) become oxidized in
the optical microscope when the specimen is attacked
by diluted HNO3 (10%) for one day
4
Studying of Liberation of Gold Ores
•
•
•
increasing liberation.
Grain size distribution
of a “garimpo” tailing
before grinding.
2
Au 1.5
(g/t) 1
0.5
Grain size
analyses
conc.
Fire Assay
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.
#
#
#
10
0#
15
0#
20
0#
27
0#
40
0#
-4
00
#
65
25
Au 20
15
distr.10
(%) 5
0
65
#
10
0#
15
0#
20
0#
27
0#
40
0#
-4
00
#
tail.
Mesh
Gold grades in screened fractions
- 20 min.
- 30 min.
Batch
Flotation
28
20
Grinding
(various time)
- no grinding
- 5 min.
- 10 min.
28
#
grinding time
20
#
Composite
Sample
#
0
#
The average gold grade of this tailing from Crixás, Brazil is
around 0.7 to 1 g/tonne. Gold liberation was studied as follows::
3
2.5
48
#
Increasing grinding time
Example
35
#
•
Studying Liberation by Flotation or Gravity Concentration
48
Studying Liberation by Flotation or Gravity Concentration
35
Studying Gold Liberation
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
Mesh
Gold distribution in screened fractions
Studying Liberation by Flotation or Gravity Concentration
Studying Liberation by Flotation or Gravity 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.
% Mass Passing
100
100
90
%Gold
recovery
80
by
concentration 70
80
grinding time
60
40
1 min
5 min
10 min
20
20 min
D80 for 10 min of grinding
(probable gold liberation)
0
16
60
20
28
35
48
65
100
150
200
270
400
-400
Tyler Mesh
50
0
5
10
20
30
Grinding time (min.)
5
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
•
•
•
Au exposed but not liberated
Quartz particle
Hg Activation Improves Amalgamation
Hg Activation Improves Amalgamation
• 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)
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
•
•
•
•
•
•
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)
+
wire
Battery
12 V
-
It forms sodiumamalgam which is
more consistent
than pure Hg
Graphite rod
Mercury
Water with 10%
of salt NaCl or
KCl
Brazil, 2006
Zimbabwe, 2006
6
Brown Sugar in the Amalgamation Barrel
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
•
•
•
•
Indonesia, 2006
Studying of Liberation of Gold Ores
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).
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
•
Studying of Liberation of Gold Ores
Studying of Liberation of Gold Ores
Gold Ore (8g/tonne Au)
Results:
35, 48, 65, 100, 150, 200, 270
mesh
Screening
light fractions
Heavy Liquid
Separation
heavy fractions
amalgams
Amalgamation
(60% solids; Hg:solids = 1:20)
residue
Cyanidation
(*)
(*)
(10g/L NaCN,
NaCN, pH=10, 40%sol, 24 h)
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.
•
Crushing ((-28#)
solutions
Chemical Analysis
Nitric Acid
Hg--Dissolution
Hg
gold
Chemical
Analysis
gold
residue
Fire Assay
Fraction
(Tyler mesh)
-28 +35
-35 +48
-48 +65
-65 +100
-100 +150
-150 +200
-200 +270
•
(*)
gold
Just the -48 mesh fractions were leached
by cyanide. High concentration of CN was
used to avoid CN consumpion control
•
Gold in Heavy
Products (%)
32
68
57
55
62
60
n.a.
Amalgamated
Gold (%)
4
1
21
48
67
87
n.a
CN Leached
Gold (%)
n.a
n.a
n.a
88.0
93.2
95.0
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.
7
Studying of Liberation of Gold Ores
Studying of Liberation of Gold Ores
Example:
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.
Representative
sample
Store in bags
Fusion
75
-
Cyanidation
50
- +100#
- 6 Mesh (3.4 mm)
sub-samples
sub-sample
-
0
(polished thin sections)
Crushing
DAu (%)
100
25
Optical and electron
microscopy
Chips of sample
selected
Amalgamation
-200+400#
-400#
-100+200#
Grinding
- 28 M (0.6 mm)
Screening
fractions
Heavy Liquid
Separation
heavies
Liberation of Sulfides
(Microscopy)
Chemical
Analysis
Gravity Sep.
tailing
Flotation
conc.
Cyanidation
residue
Diagnosis
Leaching
Amalgamation
residues
Overall Gold recovered by:
Cyanidation
Fusion:
Cyanidation:
Amalgamation:
11.4 %
64.0 %
24.6 %
residues
Fire Assay
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.
Size of Gold
Particle (mm)
The mass of sample to
be used is a function of
the gold particle shape,
size and distribution.
= splitting
= optional
Studying of Liberation of Gold Ores
•
Grinding with
different times
kg of sample required
4 ppm Au
1 ppm Au
2.0
400
1000
1.0
50
200
30
0.5
8
0.25
1
4
0.125
0.1
0.5
0.062
0.02
0.05
0.031
0.002
0.006
0.015
0.0002
0.002
0.008
0.00002
0.0001
• 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)
History of Gold Cyanidation
Date
Name
Discovery
6th
Century
Jabir
Hayyan
Discovered aqua regia & that it dissolved gold
6HCl + 2HNO3 + 2Au → 2AuCl3 + 2NO + 4H2O
1704
Gold Cyanidation
Discovered Prussian Blue (Fe4[Fe(CN)6]3) by
accidentally combining dried blood with potash
(K2CO3) and treating with iron vitriol (FeSO4)
1774
C.W.
Schule
Discovered chlorine & that it dissolved gold
1852
K.F.
Plattner
Applied chlorination to gold recovery
2Au + 3Cl2 → 2AuCl3
Early
1700s
P.J.
Discovered potassium ferrocyanide
Macquer (K4[Fe(CN)6]) by combining Prussian Blue with
Alkali (KOH)
1782
C.W.
Schule
Discovered Blue Acid by heating Prussian Blue
with Dilute Sulphuric Acid
8
Date
Name
1811
J.L. GayLussac
1834
Potassium cyanide produced by fusing potassium
ferro cyanide with potash K4[Fe(CN)6] + K2CO3 →
6KCN + FeCO3
1. Oxygen Theory, L. Elsner (1846)
2. Hydrogen Theory, L. Janin (1888)
1840
Elkington
Patented process using KCN to prepare
electrolyte for electroplating Au and Ag
1846
L. Elsner
Recognized importance of oxygen to dissolution of
Au with cyanide
4Au + 8KCN + O2 + 2H2O → 4KAu(CN)2 + 4KOH
1887
Cyanidation Process (theories)
Discovery
Determined Blue Acid was HCN
J.S.
MacArthur,
R.W.
Forrest,
W. Forrest
4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH
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 + 2NaOH
4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 4NaOH
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.
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 + 2OHAu → Au+ + e
Au+ + CN- → AuCN
AuCN + CN- → Au(CN)2
Au + O2 + 2CN- + 2H2O + e- →Au(CN)2- + 2OH- + H2O2
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
pH of Gold Cyanidation
bitter
almond
smell
Au Dissolution Rate
CN- + H2O = HCN + OHNaOH
Ca(OH)2
10.5
pH
Au cyanidation is more efficient at pH=10.5
Ca(OH)2 precipitates on gold surface inhibiting the cyanide
attackE.avoid high pHs
9
Oxygen in Cyanidation
Cyanidation Process
Mechanisms
– 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
Normal Cyanidation
• In normal conditions of cyanidation, the minimum NaCN
concentration to extract gold is 75 mg/L.
• 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
Silver Slows Down Cyanidation
• 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)
Some Minerals Consume Cyanide
•
•
•
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.
Effect of Lead Nitrate
•
•
•
•
Lead nitrate reduces cyanide consumption and
speed up the gold leaching reaction
Pb forms insoluble PbS removing passivation
ions (S2+) from gold surface
Local galvanic cells between is also formed on
gold surface
In a cyanide solution, lead nitrate, lead sulfide
and lead sulphite react with gold to form
AuPb2, AuPb3 and metallic lead, which clearly
accelerate the gold dissolution
Deschenes et al (2000/ Minerals Engineering v.13, n.12, p.1263-1279.
10
Effect of Lead Nitrate
Some Minerals Consume Cyanide
Au Extraction (%)
50 g/t Pb(NO3)2
No Pb(NO3)2
Time (h)
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
Deschenes et al (2000/ Minerals Engineering v.13, n.12, p.1263-1279.
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 cyanideconsuming 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)-ammoniacyanide complex such as Cu(NH3)4(CN)2 that is
responsible for gold dissolution
11
Preg-Robbing Ore
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
•
•
•
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
•
•
•
•
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
Recovering Gold from Cyanide Solutions
• Gold can be recovered from CN solution by 3 methods:
Activated Charcoal (AC) Adsorption
Zinc precipitation (Merrill-Crowe
• 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.
Ore or Conc.
Ore or Conc.
Cyanidation
Cyanidation
with AC
Cyanidation
pulp
CIP
AC
Separation S/L
AC
Elution
Au rich solution
solution
Elution
solution
Resin Adsorption
• The first two are the most popular methods
Source: Marsden, J.O, House I.
2006. The chemistry of gold
extraction. 2nd ed. Littleton, CO:
SME – Society for Mining,
Metallurgy and Exploration. 651p
Columns with AC
Au rich solution
AC
Electrolysis
Electrolysis
Elution
Gold
Gold
Electrolysis
Au rich solution
Gold
AC = activated chacoal
12
Recovering Gold with Activated Charcoal
•
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
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 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
Source: Marsden, J.O, House I. 2006. The chemistry of gold extraction. 2nd ed. Littleton, CO: SME – Society for Mining,
Metallurgy and Exploration. 651p
Elution of Activated Charcoal
• 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 oC
and 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
13
Elution - Stripping of Au from AC
Recovering Gold with Activated Charcoal
• 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.
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.
Typical Carbon-in-Pulp Process Flowsheet
Ore or Conc.
Re-cycle to Process
Comminution
Cyanide Leach
pulp
CIP
Recovering Gold with Zinc
4Au + 8CN- + O2 + 2H2O = 4Au(CN)2- + 4OH2Au(CN)2- + Zn = Zn(CN)42- + 2Au (Merrill-Crowe process)
• 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.
Cyanide Destruction
solution
AC
Tailings Pond
Elution
Electrolysis
Gold
• 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
Elution columns in Portovelo, Ecuador
solution
In the case of leaching concentrates,
comminution is not always needed
Merrill-Crowe Process
(Factors Affecting Zinc Precipitation)
• 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
14
Merrill-Crowe Process
(Factors Affecting Zinc Precipitation)
Typical Merrill-Crowe Process Flowsheet
Ore or Conc.
• Cyanide concentration - minimum concentration
required otherwise rate of precipitation is reduced
(Typically > 0.05 - 0.20 g/L)
solution
• 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 Znhydroxide which passivates Zn surface. Zn added at 5 to
30 times the stoichiometric Au requirement.
Re-cycle to Process
Comminution
Cyanide Leach
pulp
pulp
S/L Separation
Settling Tank
pulp
solution
solution
pulp
Precipitation w/ Zn
Cyanide Destruction
Leach with Acid
Tailings Pond
pulp
Precipitate w/Al
Gold
In the case of leaching concentrates,
comminution is not always needed
Zinc
Electrowinning
• Instead of Zn precipitation, electrolysis can be used to treat
high-grade gold solutions (carbon eluates) to produce
loaded cathodes or cathode cell sludges
Main Cyanidation Practices
• Agitated Tanks
– CIP (Carbon-in-Pulp)
– CIL (Carbon-in-Leach)
Au(CN)2- + e- → Au + 2CN• Advantages (over Zn precipitation)
• Static leaching
No chemicals or metals introduced in process
More selective for Au and Ag over Cu
– Vat leaching with Carbon-in-Column or Zinc ppt
– Heap leaching with Carbon-in-Column or Zinc ppt
• Intensive cyanidation (for concentrates)
• Higher purity product
• Disadvantages
Low single pass efficiency per unit cell requiring
recirculation of solution to achieve acceptable extraction
– Gekko
– Acacia
– Mill-leaching (cyanidation in a small ball-mill)
It needs high Au concentration in solution
Cyanidation in Agitated Tanks in Small-scale
• Cyanidation is being used in
160 tanks in Zaruma-Portovelo
region
• Tanks range from 10 to 20 m³
processing from 4 to 20
tonnes/day
• Some plants use either MerrillCrowe or CIP (Carbon-in-Pulp)
techniques
Cyanidation in Agitated Tanks in Small-scale
• 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
Ecuador, 2009
Ecuador, 2009
15
Cyanidation in Agitated Tanks in Small-scale
• 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 Vats
Cyanidation in Agitated Tanks in Small-scale
• Merrill-Crowe process in Ecuador
• 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%.
Ecuador, 2007
Cyanidation in Vats
• Some artisanal miners leach for 30 days
Vat-leaching in the Tapajós Region,
Amazon, Brazil
Tailing pond
(baixao)
Vat-leaching
tank
Pool with CN
solution
Artisanal Gold Miners in Zimbabwe using Amalgamation
Tailings to fill the cyanidation vat
Tanks with
activated charcoal
Pool with
pregnant solution
96
16
Heap Leaching
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
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%
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, NaCN
consumption 0.06 kg/t ore
Installing the plastic pads
http://www.mining-technology.com/projects/minera/
Yanacocha Mine, Peru
http://www.mining-technology.com/projects/minera/minera2.html
• 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 concentrate
gold 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
Intensive Cyanidation
•
•
•
•
•
•
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.
17
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
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-).
Intensive Cyanidation
•
•
•
•
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 of gravity and flotation
systems have been used by GEKKO and
Acacia Systems
• The GEKKO process is a thin film in a drum
• 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
Intensive Cyanidation
• Acacia System CS 250: 375 kg conc./cycle
Acacia System:
• Knelson Centrifuge
provide concentrates
for the reactor
• 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
http://www.knelsongravitysolutions.com/page361.htm
18
Intensive Cyanidation
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
Small-scale Intensive Cyanidation implemented
in Tapajós, Brazil
+2mm
Screen 2 mm
Hammer mill
Gold ore
(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
A Simple Intensive Cyanidation System
Icon centrifuge: 2 tonnes/h of ore
-2mm
Icon centrifuge
tailing
Icon centrifuge
tailing
flotation
co
nc
.
conc.
.
nc
co
Intensive
Cyanidation
activated
charcoal
elution
final
tailing
electrolisis
meting
gold
Icon® (Falcon) centrifuge
A Simple Intensive Cyanidation System
Re-grinding the centrifuge
concentrates
Source: Falcon
website, 2009
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%
Capsule of activated
charcoal in the ball mill
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.
19
Centrifuge concentrate
[CN] 20 g/L, 0.3 g/L H2O2
grinding
Intensive Cyanidation in Ball Mill
• 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
Same but no grinding
Conventional cyanidation
Head sample, [CN] 1g/L,
pH 10-11, no grinding
Ecuador, 2009
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.
Intensive Cyanidation Tests in
Ecuador
• 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)
Result of Amalgamation
• Manual
amalgamation
• 8 hours in a batea
with mercury and
brown sugar
• Gold recovery from
the gravity
concentrate after
amalgamation was:
26%
Ecuador, 2007
Ecuador, 2007
Result of Conventional Cyanidation
Result of Intensive Cyanidation in Ball Mill
• 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
• Gold recovery after
8 hours of millleaching the
concentrate was:
95%
• Cyanide
consumption was
0.95 kg/t of
concentrate
Ecuador, 2007
20
Results of Elution of Activated Charcoal
Intensive Cyanidation (conclusion)
• For environmental and economic reasons,
intensive cyanidation of gravity and flotation
concentrates must be the future of gold mining
industry.
•
•
•
•
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)
• 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
Ecuador, 2007
Diagnosis Leaching
• This is a procedure to assess why the gold is not being
extracted by classical cyanidation (low CN concentration
and no peroxide)
Tests to Check Accessibility of
Cyanide to Gold Particles
• 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
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.
• %Au extracted = mg gold in solution (volume was 467mL)
• 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
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
• Filter the solution and send it to be analyzed by atomic
absorption. Wash, dry and keep the residue.
• Grind below 200 mesh (0.074mm) and use same leaching
with CN conditions as above
• With the result from the atomic absorption you can check
if the total gold was dissolved in cyanide
• Filter and send solution to atomic absorption. Wash, dry and
keep the residue.
21
Diagnosis Leaching
Diagnosis Leaching
• If the total gold was not extract yet, gold can be occluded
in the sulfide minerals. Weigh the dry residue.
• Leach the residue with cyanide using similar conditions
as above. Filter and keep residue.
Fe3+
• Oxidize sulfides using 2M HCl and 100 g/L
as FeCl3,
at boiling temp. for 6 to 24 h, L:S=2:1. Use an agitated
beaker.
• 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
• 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)
• 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.
• 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)
• If total gold was not extracted yetEuse filtration residue.
Probably the acetonitrile was not enough to extract all
organic matter
• Dry and weigh residue
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
Environmental Management of
Cyanide
• 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
Adapted from Lorenzen, L. (1995). Minerals Engineering, v. 8, n. 3, p. 247-256
Use 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
Gold Mining
13%
Used in
production of
Organic
Chemicals
85%
Other
2%
• 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.
Used for
NaCN
production
•
ICMI (2010). International Cyanide Management Institute. http://www.cyanidecode.org/cyanide_use.php
22
Gold Uses
Gold Uses
Source: Elma van der Lingen (2005). Gold’s Other Uses, The LBMA Precious
Metals Conference 2005, Johannesburg, p.75-80
Source: Elma van der Lingen (2005). Gold’s Other Uses, The LBMA Precious
Metals Conference 2005, Johannesburg, p.75-80
Gold Consumption
Gold Mine Production 2009
(tonnes)
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)
Total: 2572 tonnes
• 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/buyinggold/uploaded_images/indian-gold-742896.jpg
Others (854)
Source: Goldsheet. http://www.goldsheetlinks.com/production.htm
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 NaCN
per tonne of ore.
• In average 100 tonnes NaCN is used to produce 1 tonne
of 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
Species
Rainbow trout
Yellow perch
Snail
Temperature
(°C)
6
10
12
18
15
21
20
96-h LC50 (mg/L)
0.028
0.057
0.042
0.068
0.090
0.102
0.432
Higher temperature, lower toxicity
23
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
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
Source: Infoczarina, 2008
http://www.chicagostagereview.com/wp-content/uploads/2008/09/jonestown1.jpg
Cyanide Spills
Cyanide Spills
• Baia Mare, Romania, mine re-opened
in 1999.
• 1000 tonnes
of fish killed
• Intense rain in Jan 2000 = overflow
• CN in the
Danudbe
m3
• 100,000
of tailings with cyanide
released to the river
• 50 to 100 tonnes of cyanide released
• Public
reaction was
horrible
24
Cyanide Spills
• 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
Rudimentary Cyanidation Plant made by
Artisanal Miners in Zimbabwe
Amalgamation
tailings (full of
Hg) are
submitted to
cyanidation
http://www.ec.gc.ca/inre-nwri/default.asp?lang=En&n=0CD66675-1&offset=14&toc=show
Rudimentary Cyanidation Plant made by
Artisanal Miners in Zimbabwe
Misuse of Cyanide
• Increasing the use of Hg-amalgamation followed
by cyanidation in Artisanal Mining.
• Cyanidation of Hg-rich tailings forms Hg(CN)42which is a very stable and persistent compound.
This can be transformed into highly toxic
compounds, e.g. Methylmercurycyanide
...at least they know that cyanide is dangerous!!!
Artisanal Gold Miners (Sulawesi, Indonesia)
Artisanal Gold Miners (Sulawesi, Indonesia)
Rudimentary tailing pond; often CN-rich
tailings reach the streams
Recovering loaded charcoal (retained in a screen)
Only CN destruction method = NATURAL = sun light
False perception that this destroys all cyanide
complexes
25
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
Cyanide Toxicity (humans)
Cyanide Toxicity
• 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 (humans)
• In solution, 3 to 5 mg of cyanide/kg body weight is lethal
• Cyanide is readily absorbed through inhalation,
ingestion or skin contact.
If a mine uses a CN
solution of 300
mg/L
• In respiratory exposure to hydrocyanic acid (HCN
gas), death occurs at 0.1 to 0.3 g/m³
A 100-kg person
should drink 1.5 L
to die
• Cyanide is a potent asphyxiant. It induces tissue
anoxia through inactivation of cytochrome oxidase
due to the reaction of CN- and Fe3+.
• Oxygen cannot be utilized and death results from
the depression of the central nervous system.
You need to drink a lot of Cyanide solution to die
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.
Fishing with Cyanide
It’s forbidden to fish with
cyanide and explosives
• 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.
Ecuador, 2006
26
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)
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
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
(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.
• Kidneys
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.
• Hypothyroidism results in inflammation of the thyroid gland
and reduction of hormone production.
Cyanide Toxicity
(congenital hypothyroidism)
Cretinism:
• Poor length growth
Adult stature without
treatment ranges
from 1 to 1.6 m
• Neurological
impairment
• Thickened skin
goitre
Sources: http://www.endocrineweb.com/thyfunction.html
http://en.wikipedia.org/wiki/Goitre
thyroid gland
normally weighs
<30g
• Enlarged tongue
• Protruding abdomen
Sources:
• http://www.gidabilimi.com/images/kretenizm1.png
• Wikipedia
27
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.
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
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.
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)42-, Cd(CN)3-, Cd(CN)42-, Cu(CN)2-,
Cu(CN)32-, Ni(CN)42-, 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)64-. These are stable under ambient
conditions of pH and temperature
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
Main Cyanide Reactions
Hydrolysis
Oxidation of HCN/CN-
CN- + H2O = HCN + OH2HCN + O2 = 2HCNO
2CN- + O2 + catalyst = 2CNO-
Hydrolysis of CNO
HCNO + H2O = NH3 + CO2
Hydrolysis/saponification of HCN + 2H2O = NH4COOH or
HCN + 2H2O = NH3 + HCOOH
HCN
Aerobic biodegradation
2HCN + O2 + enzyme = 2HCNO
S 2x − + CN- = S 2x −−1 + CNS
-
• Under anaerobic conditions, cyanides denitrify to
gaseous nitrogen compounds and enter the
atmosphere.
Thiocyanate formation
• All cyanide complexes are subjected to (biotic and
abiotic) oxidation.
Metal-cyanide complexation
Zn2+ + 4CN- = Zn (CN ) 24 −
Anaerobic biodegradation
CN- + H2S = HCNS + H+
HCN + HS- = HCNS + H+
Cyanide compound
dissociation
S 2 O 32 − + CN- = SO 32 − + CNSNaCN = Na+ + CN-
28
Cyanide Treatment Processes
Cyanide Cycle
• Natural Degradation (Lagooning)
NH3 + CO2
HCN/CNcyanidation
tank
• Oxidation Processes
evaporation and
decomposition
– Alkaline Chlorination
spill
tailing pond
River
NH3 + HCO3or
tailing with cyanide
HCN/CN-
Fe(CN)63Fe(CN)64-
NO3-
– Electrochemical Processes
– Ozonation
– Hydrogen Peroxide
CO32-
– INCO’s SO2/Air Process
CH4 + CO2
• Precipitation (Cuprous Process)
Natural Degradation
Cyanide Treatment Processes (cont.)
• Conversion to Less Toxic Forms
• Biological Treatment
• Cyanide Recovery Processes
• 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:
– Acidification/Volatilization/Reneutralization AVR
–
–
pH
temperature
– Ion Exchange
–
–
UV light
aeration
– Activated Charcoal
• Other variables:
– Electrolytic Processes
–
–
–
Natural Degradation
• The equilibrium between free cyanide species (CNand HCN) is pH dependent: HCN = H+ + CNDissociation
constant
+
−
[ H ].[ CN ]
= 4 .93 x10 −10
[ HCN ]
• 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.
Kd =
(CN-/HCN)
• Free cyanide
is rare in mining tailings
because of the high reactivity of the CN- molecule
with metal ions.
biodegradation
precipitation
conversion to thiocyanate
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 ) 24 −
10-30.7
Ni( CN ) 2−
4
10-30
Cu( CN ) 23 −
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
29
Natural Degradation
•
3−
4−
Ferrocyanides ( Fe( CN ) 6 ) and Ferricyanides ( Fe (CN ) 6 )
are very stable compounds. They can form complexes with
trace metals. Most complexes are insoluble, e.g.:
SnFe (CN ) 6 = tin(IV) ferrocyanide
Cyanide Guidelines (Canadian Legislation)
Water Use
Strong-acid
dissociable
cyanide plus
thiocyanate
µg/L (as CN)
Strong-acid
dissociable
cyanide
µg/L (as CN)
Weak-acid
dissociable
cyanide
µg/L (as CN)
Raw Drinking Water
(maximum)
200 µg/L
Not applicable
Not applicable
Fe 4 [ Fe( CN ) 6 ]3 = ferric ferrocyanide
Co 3[ Fe( CN ) 6 ]2 = cobalt ferricyanide
Sn 3 [Fe(CN ) 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
Freshwater Aquatic Life
(30-day average)
Not applicable None proposed
< or =
5 µg/L
Freshwater Aquatic Life
(maximum)
Not applicable None proposed
10 µg/L
Marine and Estuarine
Aquatic Life
(maximum at any time)
Not applicable None proposed
1µg/L
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
• 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.
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%
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 onethousandth as toxic as hydrogen cyanide). The
oxidation of cyanide is represented by the equations:
Any pH:
High pH:
CN- + Cl2 = CNCl + Cl-
CNCl + 2OH- = CNO- + Cl- + H2O
With hypochlorite: CN- + OCl- = CNO- + Cl-
30
Oxidation by Chlorine
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
• 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
• Disadvantages:
High operating cost (reagents are costly)
Requires pH control to prevent cyanogen
chloride which is highly toxic
Ferro and Ferricyanides are not destroyed
High content of chloride and chlorine in
effluents
Some chlorination products (e.g. Nahypochlorite, 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:
Me( CN ) 2n − n + 2 e − = n CN − + Me° ↓
( Me = divalent metal)
Electrochlorination :
Anode reaction:
Cl − = Cl° + e −
2Cl° = Cl 2 (g)
Cathode reaction:
H 2 O + e − = H° + OH −
2 H ° = H 2 (g ) ↑
•
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. Ironcyanides are not destroyed
•
Cyanide can be regenerated
Oxidation with Hydrogen Peroxide
Oxidation with Hydrogen Peroxide
• Cyanide oxidation with H2O2 is a fast, one-step
reaction, forming non-toxic intermediates.
• Cu and Ni cyanides are partially destroyed and
Iron-cyanides are partially precipitated.
• H2O2 oxidizes free and Zn & Cd cyanides to
cyanate which is further hydrolyzed yielding
biodegradable ammonia and carbonate.
• Since copper and other metal ions are already in
many mining effluents, additional metallic catalyst
may not be required to enhance the reaction rate.
CN- + H2O2 = CNO- + H2O
(Cu2+
is a catalyst)
CNO- + 2H2O = NH4+ + CO32-
• Degussa process uses borate compounds as
catalysts resulting in a substantial saving of
hydrogen peroxide consumption (up to 60%).
(pH just < 7)
31
Oxidation with Hydrogen Peroxide
Oxidation with Hydrogen Peroxide
(Example)
• There are a variety of processes combining hydrogen
peroxide with other compounds, such as glycolonitrile
(Kastone process), H2SO4 (Caro’s acid), SO2, etc.
Species
• Formation of thiocyanate by H2O2 is slow (in contrast to
chlorination).
As
Cu
total CN
Fe
Se
Ag
Zn
• 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 Teck
Corona in 1985. Many plants in Canada use peroxide.
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.
-
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.
-
CN + SO2 + O2 + H2O = CNO + H2SO4
(Cu2+ is a catalyst)
Me(CN) 24− + 4 SO 2 + 2 O 2 + 4H 2 O = 4 CNO − + 4 H 2SO 4 + Me 2+
Me2+ = Zn2+, Cu2+, Ni2+, Cd2+, etc.
(neutralization): H2SO4 + Ca(OH)2 = CaSO4.2H2O ↓
(pH>8)
(precipitation): Me2+ + Ca(OH)2 = Me(OH)2 ↓ + Ca2+
2 Me 2 + + Fe(CN ) 64− = Me 2 Fe(CN ) 6 ↓
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
EPA dws
(mg/L)
0.05
9.0
0.05
0.3
0.01
0.05
5
Note: H2O2 dosage = 2.5 mL/L
dws = drinking water standard
INCO SO2/Air Process
•
Effluent (mg/L)
Before
After
0.2
<0.05
4.5
<1
280
3
16
<0.015
5
4
3.2
1
157
<1
INCO SO2/Air Process
Species
Total cyanide
WAD
cyanides
Cyanate
Thiocyanate
Cu
Fe
Ni
Liquid Effluent (mg/L)
Before
After
365.8
0.69
225
0.15
44.6
100.7
35.45
45.82
4.13
324.2
82.5
1.21
0.26
0.25
Final Tailing
(mg/L)
29.4
12.2
52.9
18.9
7.7
5.35
0.51
32
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
Combinox Process
•
•
•
•
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
Compound (mg/L)
• 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.
0.58
0.03
Free CN
<2
<0.02
0.31
<0.01
WAD CN (moderately
complexed)
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.
• 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
CN- + H+ = HCN
Acidification:
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%.
Solution after 10
days
Total CN
Biological Treatment
• Cyanide is degraded by aerobic and anaerobic
microbes
Solution after
Combinox
Cyanisorb Process
Barren solution
300 ppm HCN
HCN ladden air
H2SO4
bleed air
10%
air
pH 5 - 7.5
Ca(OH)2
• AVR technology has been developed to allow
treatment of slurry streams directly without the need for
solid-liquid separation.
stripped slurry
NaCN (recovered)
solution
Pulp to tailing pond
Metals precipitation
33
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
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
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.
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
34