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Cyanide Geochemistry
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Outline Introduction to Cyanide Cyanide in the beneficiation of gold
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
Cyanide Analysis Toxicity Degradation mechanisms to reduce toxicity
1. Volatolization 2. Complexation 3. Adsorption 4. Oxidation to Cyanate
5. Formation of Thiocyanate, SCN-
6. Hydrolysis 7.Biodegradation
Cyanide degradation in a Heap Leach Cyanide degradation in Mill Tailings Examples of Cyanide Spills Summary References
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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 OH-
Anode At the other end, the anodic zone, the metal gives up electrons and
undergoes an oxidation reaction.Au => Au+ + e-
Au+ + 2CN- => Au(CN)2
-
And then form strong complexes by Elseners/ Adamsons 1st reaction:4Au + 8NaCN + O2 + 2H2O = 4NaAu(CN)2 + 4NaOH
Or Adamsons 2nd reaction2Au + 4NaCN +2H2O = 2NaAu(CN)2 + H2O2 + 2NaOH
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Heap Leach Process
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Cyanidetank leach
and CIPcircuits
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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 diffusethrough the Nernst layer (~0.05 mm) which surrounds the surface ofAu. 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 ofoxygen decreases. The optimal temperature is 60 to 80 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.
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Extraction of gold from the CN solution(a) Merrill Crowe Process
Merrill Crowe process discovered and patented by CharlesWashington 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 higheraffinity 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 dissolvedgold remained in solution.
The Merrill-Crowe process works better than the early zinc boxes
because it uses zinc powder and reduces the amount of dissolvedoxygen.
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(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 and the surficial area is >1000m2/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.
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CIPcircuit
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Cyanide AnalysisCN 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 weaklycomplexed cyanide (with Cd and Ni).
The WAD technique is least susceptible to interference and over-estimation.
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
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3. Total Cyanide: Reflux for one
hour in strong
acid whichdissociates mostcomplexes andmeasure HCNwhich is absorbedin NaOH solution.
Analyticalinterferences fromoxidizing agents,sulphides,sulphates,thiocyanate,nitrate, nitrite,carbonate,thiosulphates.
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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 toxicitydepends on ability of the gut to break down the complex and absorbthe free cyanide.
Ferric ferrocyanide is used as an antidote to thallium poisoning.
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Degradationmechanisms to
reduce toxicity1. Volatilization
Reaction between cyanide and waterproduces 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 aboiling point of 25.7oC.
Reaction is dependant on pH (
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Degradation mechanisms to reduce toxicity2. Complexation
72 complexes with varying solubilities arepossible from 28 elements. These rapid reactionsimmediately remove CN- from solution.
Complexes may absorb on organic and inorganicsurfaces or precipitate as insoluble salts with Fe,Cu, Ni, Mn, Pb, Zn, Cd, Sn, Ag.
Complex may dissociate in acid conditions butmay persist for hundreds of years.
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2a. Neutral Cyanide Compounds
Soluble compoundsNaCN, KCN and Ca(CN)2, Hg(CN)2 dissolve inwater to give cyanide anions
NaCN = Na+ + CN-Ca(CN)2 = Ca
2+ + 2CN-
Insoluble Neutral Cyanide Compounds
Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN
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2b Charged metal CN complexesCyanide complexes form in order of increasingnumber of CN ligands with successively higher CNconcentration
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 weakacid dissociable (WAD) cyanide analysis.
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Strong Complexes
[Fe(CN)6]4-, [Co(CN)
6]4-, [Au(CN)
2]-, [Fe(CN)
6]3- form at pH l
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Degradation mechanisms to reduce toxicity3. Adsorption
Adsorption of CN- on Fe, Al and Mn oxidesand hydroxides and on clays.
Clays with high anion exchange capacityare most effective e.g. clays containingkaolinite, chlorite, gibbsite or Al or Fe oxy-hydroxides
Clays with high cation exchange capacity(CEC) are less effective at scavenging CN-
e.g. montmorillonite.
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Degradation mechanisms to reduce toxicity4. Oxidation to Cyanate
Cyanide can be oxidized toless toxic cyanate
HCN + 0.5O2 = HCNO
From the phase diagram,cyanate should be thedominant form underenvironmental conditions butthis requires strong oxidantse.g. ozone, H2O2, plus UV,bacteria or a catalyst.
Adsorption onto organics orcarbonaceous material whichcauses CN to becomeoxidized
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Degradation mechanisms to reduce toxicity5. 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 withAg, Hg, Pb, Cu, Zn.
Complexes may react with SCN- to form even more stablecompounds
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Degradation mechanisms to reduce toxicity6. Hydrolysis
HCN + 2H2O = NH4COOH (ammonium formate)HCN + 2H2O = NH3 + HCOOH (formic acid)
Slow reaction, 2% per month Dependent on pH.
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Degradation mechanisms to reduce toxicity7. Biodegradation
Aerobic degradation in unsaturated zones is 25 times more effective than in saturatedzones
HCN + O2 = 2 HCNOHCNO + 0.5 O2 + H2O = NH3 + CO2
Anaerobic degradation in the saturated zones
CN + H2S = HCNS + H+HCN + HS = HCNS + H+
The toxic limit for effective anaerobic degradation is 2 mg/L.
Bacteria can be used in a bioreactor to decreaseCN content e.g. Landusky heap leach remediation
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Cyanide degradation in a Heap Leach
Cyanide decreases from >250 mg/l in leach solution to 130 mg/l inrinsate and then decays to below detection limit.
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Cyanide degradation in Mill Tailings
Most CN is degraded by volatilizationof HCN because the pH is loweredimmediately from 10 by rainwater anduptake of CO2 from air and moreslowly by oxidation of sulphides.
Between 3 and 6 months, WAD CN
(from CIP process) has reduced by afactor of 100 to a few ppm.
There are slight difference betweensurface and deep waters and betweenwinter and summer.
There is a need to considertransformation of CN between solid,liquid and gas phases. This may bedependent on type of soil, cations,weather, bacteria, depth and degree ofoxygenation of pond.
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Examples of Cyanide Spills Hungary-Romania-Slovakia-Ukrain: 1-11 February 2000cyanide spill in Szamos and
Tisza rivers polluted the Danube
Australia February 8, 2000: BHP fined over cyanide pollution incident
Ghana: 23rd October 2004, and 16 June 2006 BHP fined over cyanide pollution incidentat the Port Kembla steel-making operation near Wollongong.
Honduras: 3rd May 2006 In the Siria Valley in Honduras, are extensive. Cyanide andheavy metal contamination of several water sources in the area of the San Martin minehas been confirmed.
Romania: 30 January 2000 Baia Mare Mine
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-basedCameco Corporation) overturned into the Barskoon River, spilling nearly two tonnes ofdeadly cyanide.
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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 orbe consumed by bacteria.
CN forms complexes of varying strengths and longevitywith metals
The major environmental issues relate to spills fromtailings ponds, trucks pipes before CN has decomposed.Cyanide spill kills fish and wildlife immediately but the
major long term problems relate to heavy metalcontamination, some coming from the decomposition ofmetal cyanide complexes.
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References
Filipek, L H., (1999) Determination of the Source and Pathway of CyanideBearing Mine Water Seepage, in The Environmental Geochemistry ofMineral 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 inSE Australia, PhD thesis, University of Melbourne.
Smith, A.,(1994) The Geochemistry of Cyanide in Short Course Handbookon Environmental Geochemistry of Sulphide Mine-Wastes Ed. Jambor, J.L.and Blowes, D.W. MAC
Smith, A.C.S & Mudder, T.I. (1998) The Environmental Geochemistry of
Cyanide in The Environmental Geochemistry of Mineral Deposits Part AProcesses, Techniques and Health Issues, eds Plumlee and Logsdon.Review in Economic Geology Volume 6A, Society of Economic Geologists.
(all 11. figures and tables)