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Transcript
Cyanide Geochemistry
Outline
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Introduction to Cyanide
Cyanide in the beneficiation of gold
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Heap Leach Process
Cyanide tank leach and CIP circuits
Optimum Conditions for CN leaching
Extraction of gold from the CN solution
• (a) Merrill Crowe Process
• (b) CIP Process
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Cyanide Analysis
Toxicity
Degradation mechanisms to reduce toxicity
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1. Volatolization
2. Complexation
3. Adsorption
4. Oxidation to Cyanate
5. Formation of Thiocyanate, SCN6. Hydrolysis
7.Biodegradation
Cyanide degradation in a Heap Leach
Cyanide degradation in Mill Tailings
Examples of Cyanide Spills
Summary
References
Introduction to Cyanide
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1.4 m tonnes CN produced annually
13% CN is used for the extraction of Au and Ag: 460 of 875 Au/Ag mines use CN
87% used in production of paint, adhesives, cmputer electronics, fire retardants,
cosmetics, dyes, nylon, Plexiglas, rocket propellant and pharmaceuticals
Cocaine
CuCN.9(C17H19O3N.HCN).7HCN
Novocaine
CuCN.9(C17H20O2N.HCN).HCN
Codeine
CuCN.4(C18H21O3N.HCN).3HCN
Nicotine
CuCN.2(C10H14 N2.HCN).1.5HCN
Morphine
CuCN.9(C17H19O3N.HCN).7HCN
Caffeine
4CuCN.(C8H10O2N4.HCN)
Natural Cyanide
• Cyanide is naturally produced by both fauna and flora.
• Humans have <0.217 g/l SCN in saliva, <0.007 g/l SCN in urine and <0.006 g/l in gastric
juices.
• Cyanogenic bacteria generate cyanide from glycine.
NH2CH2COOH = HCN + CO2 + 2H2
Cyanide in the beneficiation of gold
• 0.05% NaCN solution is used to extract Au and Ag from ore
• Au dissolves by two processes occurring simultaneously on its surface.
Cathode
• At one end of the metal, the cathodic zone, oxygen takes up electrons
and undergoes a reduction reaction.
O2 + 2 H2O + 2 e- => H2O2 + 2 OHAnode
• At the other end, the anodic zone, the metal gives up electrons and
undergoes an oxidation reaction.
Au => Au+ + eAu+ + 2CN- => Au(CN)2-
• And then form strong complexes by Elsener’s/ Adamson’s 1st reaction:
4Au + 8NaCN + O2 + 2H2O = 4NaAu(CN)2 + 4NaOH
Or Adamson’s 2nd reaction
2Au + 4NaCN +2H2O = 2NaAu(CN)2 + H2O2 + 2NaOH
Heap Leach Process
Cyanide
tank leach
and CIP
circuits
Optimum Conditions for CN leaching
• The rate of Au dissolution is determined by the rate at which the
dissolved oxygen and/or the cyanide ions permeate or diffuse
through the Nernst layer (~0.05 mm) which surrounds the surface of
Au.
– CN tanks must be aerated by agitation or by pumping air through.
• Increasing the temperature of the leach solution will promote the
dissolution of Au, but as the temperature increases, the solubility of
oxygen decreases.
– The optimal temperature is 60 to 80º C.
• Other metallic species from ore minerals, e.g. sphalerite (ZnS),
chalcocite (Cu2S), chalcopyrite (CuFeS2), bornite (FeS.2Cu2S.CuS),
will form complexes with CN.
– Therefore more CN is needed than for just Au complexation.
– The tailings will contain these complexes.
Extraction of gold from the CN solution
(a) Merrill Crowe Process
• Merrill Crowe process discovered and patented by Charles
Washington Merrill around 1900, thenrefined by Thomas B. Crowe,
working for the Merrill Company
• Zinc replaces Au in the NaAu(CN)2 complex, as it has a higher
affinity for CN- than gold
NaAu(CN)2+ Zn = NaZn(CN)2 + Au
• Au precipitates as a solid.
• Early zinc precipitation systems simply used a wooden box filled
with zinc chips. They were very inefficient and much of the dissolved
gold remained in solution.
• The Merrill-Crowe process works better than the early zinc boxes
because it uses zinc powder and reduces the amount of dissolved
oxygen.
(b) Carbon in Pulp (CIP)
• Carbon in Pulp was introduced in 1985,
• Granular activated carbon particles (burnt coconut shells) have a high
porosity, each pore is about 10-20 Å and the surficial area is >1000
m2/g.
• The carbon particles are much larger than the ground ore particles.
• The activated carbon and cyanided pulp are agitated together.
• Au(CN)2 becomes adsorbed onto the charged surface of the activated
carbon.
• The loaded activated carbon is mechanically screened to separate it
from the barren ore pulp
• The gold adsorbed on the activated carbon is recovered from the carbon
by elution with a hot caustic aqueous cyanide solution.
• The carbon is then regenerated and returned to the adsorption circuit
• The gold is recovered from the eluate using either zinc cementation or
electrowinning.
• The gold concentrate is then smelted and refined to gold bullion that
typically contains about 70 - 90% gold.
• The bullion is then further refined to either 99.99% or 99.999% fineness
using chlorination, smelting and electro-refining.
CIP
circuit
Cyanide Analysis
CN is difficult to analyze because of the difference in solubility of the
various complexes.
1. Weak acid dissociable (WAD) cyanide.
• Most often used as it measures the cyanide which would be easily
leached in mildly acidic conditions including free cyanide and weakly
complexed cyanide (with Cd and Ni).
• The WAD technique is least susceptible to interference and overestimation.
• There are two methods of analysis:
• a) Reflux distillation for one hour in mild acid, buffered with acetate to
pH of 4.5. HCN collected and measured by titration
• b) Picric Acid titration
• 2. Cyanide amenable to chlorination
• Analyses the same compounds as WAD and is accepted by the US
EPA.
• A two step process measures CN evolving before and after
chlorination
3. Total Cyanide:
• Reflux for one
hour in strong
acid which
dissociates most
complexes and
measure HCN
which is absorbed
in NaOH solution.
• Analytical
interferences from
oxidizing agents,
sulphides,
sulphates,
thiocyanate,
nitrate, nitrite,
carbonate,
thiosulphates.
TOXICITY
• Cyanide binds to the active Fe atom in cytochrome c oxidase and
inactivates oxidative respiration.
• Cyanide may be inhaled ingested or absorbed through the skin but
does not accumulate in the body.
• HCN and CN- are acutely toxic if inhaled or ingested and result in
convulsions, vomiting, coma and death.
• Lethal doses (LD 50) of KCN or NaCN: 1.1-1.5 mg/kg of body weight.
• Lower long term concentrations result in neuropathy, optical atrophy,
pernicious anaemia.
• Cyanide complexes are not as toxic as free cyanide and their toxicity
depends on ability of the gut to break down the complex and absorb
the free cyanide.
• Ferric ferrocyanide is used as an antidote to thallium poisoning.
Degradation
mechanisms to
reduce toxicity
1. Volatilization
Reaction between cyanide and water
produces HCN gas
CN- + H2O = HCN + OH-
•
•
At pH < 8.3 HCN is the dominant species.
Therefore cyanide leaching operation is
kept at a pH over 10.
•
HCN is a colourless liquid or gas: with a
boiling point of 25.7oC.
•
Reaction is dependant on pH (<pH7 99%
will be HCN), cyanide solubility, HCN
vapour pressure, and CN concentration in
solution.
Degradation mechanisms to reduce toxicity
2. Complexation
72 complexes with varying solubilities are
possible from 28 elements. These rapid reactions
immediately remove CN- from solution.
• Complexes may absorb on organic and inorganic
surfaces or precipitate as insoluble salts with Fe,
Cu, Ni, Mn, Pb, Zn, Cd, Sn, Ag.
• Complex may dissociate in acid conditions but
may persist for hundreds of years.
2a. Neutral Cyanide Compounds
Soluble compounds
NaCN, KCN and Ca(CN)2, Hg(CN)2 dissolve in
water to give cyanide anions
NaCN = Na+ + CNCa(CN)2 = Ca2+ + 2CNInsoluble Neutral Cyanide Compounds
Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN
2b Charged metal CN complexes
Cyanide complexes form in order of increasing
number of CN ligands with successively higher CN
concentration
• Weak Complexes:
• [Zn(CN)4]2-, [Cd(CN)3]-, [Cd(CN)4]2• Moderately Strong Complexes:
• [Cu(CN)2]-, [Cu(CN)3]2-, [Ni(CN)4]2-, [Ag(CN)2]• The rate of dissolution depends on pH, temperature,
intensity of light, and bacteria
• Weak and moderately strong cyanide complexes will
break down at pH 4.5 so will register in the weak
acid dissociable (WAD) cyanide analysis.
Strong Complexes
• [Fe(CN)6]4-, [Co(CN)6]4-, [Au(CN)2]-, [Fe(CN)6]3- form at pH l<9.0 and
can form insoluble salts with other species.
• Ferrocyanide [Fe2+(CN)6]4- (hexaferrocyanate, red) and ferricyanides
[Fe3+(CN)6]3- (hexaferricyanates, yellow) are very stable in the
absence of light (<100s of years) but dissociate in UV to form CNand hence HCN
Fe(CN)64- + H+ = Fe(CN)53- + HCN
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•
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The transformation of Fe3+ to Fe2+ leaves CN content constant.
This oxidation/reduction couple is pH dependent.
Reaction is very slow so most mine wastes have both species.
When both Fe2+ and Fe3+ are present
the compound is a deep “Prussian” blue.
Degradation mechanisms to reduce toxicity
3. Adsorption
• Adsorption of CN- on Fe, Al and Mn oxides
and hydroxides and on clays.
• Clays with high anion exchange capacity
are most effective e.g. clays containing
kaolinite, chlorite, gibbsite or Al or Fe oxyhydroxides
• Clays with high cation exchange capacity
(CEC) are less effective at scavenging CNe.g. montmorillonite.
Degradation mechanisms to reduce toxicity
4. Oxidation to Cyanate
•
•
Cyanide can be oxidized to
less toxic cyanate
HCN + 0.5O2 = HCNO
•
From the phase diagram,
cyanate should be the
dominant form under
environmental conditions but
this requires strong oxidants
e.g. ozone, H2O2, plus UV,
bacteria or a catalyst.
•
Adsorption onto organics or
carbonaceous material which
causes CN to become
oxidized
Degradation mechanisms to reduce toxicity
5. Formation of Thiocyanate, SCN• In neutral to basic solution
• From oxidation products of sulphides such as chalcopyrite,
chalcocite, pyrrhotite not pyrite and sphalerite.
• From polysulphides
Sx2- + CN- = Sx-12- + SCN-
• From thiosulphates
S2O32- + CN- = SO32- + SCN• SCN- behaves like a pseudohalogen and forms insoluble salts with
Ag, Hg, Pb, Cu, Zn.
• Complexes may react with SCN- to form even more stable
compounds
Degradation mechanisms to reduce toxicity
6. Hydrolysis
HCN + 2H2O = NH4COOH (ammonium formate)
HCN + 2H2O = NH3 + HCOOH (formic acid)
• Slow reaction, 2% per month
• Dependent on pH.
Degradation mechanisms to reduce toxicity
7. Biodegradation
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Aerobic degradation in unsaturated zones is 25 times more effective than in saturated
zones
HCN + O2 = 2 HCNO
HCNO + 0.5 O2 + H2O = NH3 + CO2
•
Anaerobic degradation in the saturated zones
CN + H2S = HCNS + H+
HCN + HS = HCNS + H+
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The toxic limit for effective anaerobic degradation is 2 mg/L.
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Bacteria can be used in a bioreactor to decrease
CN content e.g. Landusky heap leach remediation
Cyanide degradation in a Heap Leach
• Cyanide decreases from >250 mg/l in leach solution to 130 mg/l in
rinsate and then decays to below detection limit.
Cyanide degradation in Mill Tailings
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Most CN is degraded by volatilization
of HCN because the pH is lowered
immediately from 10 by rainwater and
uptake of CO2 from air and more
slowly by oxidation of sulphides.
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Between 3 and 6 months, WAD CN
(from CIP process) has reduced by a
factor of 100 to a few ppm.
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There are slight difference between
surface and deep waters and between
winter and summer.
•
There is a need to consider
transformation of CN between solid,
liquid and gas phases. This may be
dependent on type of soil, cations,
weather, bacteria, depth and degree of
oxygenation of pond.
Examples of Cyanide Spills
•
Hungary-Romania-Slovakia-Ukrain: 1-11 February 2000cyanide spill in Szamos and
Tisza rivers polluted the Danube
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Australia February 8, 2000: BHP fined over cyanide pollution incident
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Ghana: 23rd October 2004, and 16 June 2006 BHP fined over cyanide pollution incident
at the Port Kembla steel-making operation near Wollongong.
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Honduras: 3rd May 2006 In the Siria Valley in Honduras, are extensive. Cyanide and
heavy metal contamination of several water sources in the area of the San Martin mine
has been confirmed.
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Romania: 30 January 2000 Baia Mare Mine
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Kyrgystan: May 20 1998, a truck carrying sodium cyanide to Kyrgyzstan's Kumtor Gold
Company (one-third owned and operated by a subsidiary of the Saskatchewan-based
Cameco Corporation) overturned into the Barskoon River, spilling nearly two tonnes of
deadly cyanide.
Summary
• Cyanide/ CIP is an efficient method to extract Au and Ag.
• Most CN will convert to HCN in tailings ponds or heap
leach and volatilize under increasing acidic conditions or
be consumed by bacteria.
• CN forms complexes of varying strengths and longevity
with metals
• The major environmental issues relate to spills from
tailings ponds, trucks pipes before CN has decomposed.
Cyanide spill kills fish and wildlife immediately but the
major long term problems relate to heavy metal
contamination, some coming from the decomposition of
metal cyanide complexes.
References
•
Filipek, L H., (1999) Determination of the Source and Pathway of Cyanide
Bearing Mine Water Seepage, in The Environmental Geochemistry of
Mineral Deposits Part B Case Studies and Research Topics Eds Filipeck,
L.H. and Plumlee, G.S.
•
Meehan, S.M. (2000) The fate of cyanide in groundwater at gaswork sites in
SE Australia, PhD thesis, University of Melbourne.
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Smith, A.,(1994) The Geochemistry of Cyanide in Short Course Handbook
on Environmental Geochemistry of Sulphide Mine-Wastes Ed. Jambor, J.L.
and Blowes, D.W. MAC
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Smith, A.C.S & Mudder, T.I. (1998) The Environmental Geochemistry of
Cyanide in The Environmental Geochemistry of Mineral Deposits Part A
Processes, Techniques and Health Issues, eds Plumlee and Logsdon.
Review in Economic Geology Volume 6A, Society of Economic Geologists.
(all 11. figures and tables)
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