Cyanide Geochemistry

1
Cyanide Geochemistry
2
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

3
Introduction to Cyanide

• 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 lt0.217 g/l SCN in saliva, lt0.007 g/l
  SCN in urine and lt0.006 g/l in gastric juices.
• Cyanogenic bacteria generate cyanide from
  glycine.
• NH2CH2COOH HCN CO2 2H2

4
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- gt H2O2 2 OH-
• Anode
• At the other end, the anodic zone, the metal
  gives up electrons and undergoes an oxidation
  reaction.
• Au gt Au e-
• Au 2CN- gt Au(CN)2-
• And then form strong complexes by Elseners/
  Adamsons 1st reaction
• 4Au 8NaCN O2 2H2O 4NaAu(CN)2 4NaOH
• Or Adamsons 2nd reaction
• 2Au 4NaCN 2H2O 2NaAu(CN)2 H2O2 2NaOH

5
Heap Leach Process
6
Cyanide tank leach and CIP circuits
7
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.

8
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.

9
(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 gt1000
  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.

10
CIP circuit
11
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 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

12

• 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.

13
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.

14
Degradation mechanisms to reduce toxicity 1.
Volatilization

• Reaction between cyanide and water
• produces HCN gas
• CN- H2O HCN OH-
• At pH lt 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 (ltpH7 99 will be
  HCN), cyanide solubility, HCN vapour pressure,
  and CN concentration in solution.

15
Degradation mechanisms to reduce toxicity2.
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.

16
2a. Neutral Cyanide Compounds

• Soluble compounds
• NaCN, KCN and Ca(CN)2, Hg(CN)2 dissolve in water
  to give cyanide anions
• NaCN Na CN-
• Ca(CN)2 Ca2 2CN-
• Insoluble Neutral Cyanide Compounds
• Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN

17
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)42-, Cd(CN)3-, Cd(CN)42-
• Moderately Strong Complexes
• Cu(CN)2-, Cu(CN)32-, Ni(CN)42-, 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.

18
Strong Complexes

• Fe(CN)64-, Co(CN)64-, Au(CN)2-, Fe(CN)63-
  form at pH llt9.0 and can form insoluble salts
  with other species.
• Ferrocyanide Fe2(CN)64- (hexaferrocyanate,
  red) and ferricyanides Fe3(CN)63-
  (hexaferricyanates, yellow) are very stable in
  the absence of light (lt100s of years) but
  dissociate in UV to form CN- and hence HCN
• Fe(CN)64- H Fe(CN)53- HCN
• 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.

19
Degradation mechanisms to reduce toxicity3.
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 oxy-hydroxides
• Clays with high cation exchange capacity (CEC)
  are less effective at scavenging CN- e.g.
  montmorillonite.

20
Degradation mechanisms to reduce toxicity4.
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

21
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 with Ag, Hg, Pb, Cu, Zn.
• Complexes may react with SCN- to form even more
  stable compounds

22
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.

23
Degradation mechanisms to reduce toxicity7.
Biodegradation

• 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
• The toxic limit for effective anaerobic
  degradation is 2 mg/L.
• Bacteria can be used in a bioreactor to decrease
• CN content e.g. Landusky heap leach
  remediation

24
Cyanide degradation in a Heap Leach

• Cyanide decreases from gt250 mg/l in leach
  solution to 130 mg/l in rinsate and then decays
  to below detection limit.

25
Cyanide degradation in Mill Tailings

• 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.
• Between 3 and 6 months, WAD CN (from CIP process)
  has reduced by a factor of 100 to a few ppm.
• 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.

26
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 incident at the Port
  Kembla steel-making operation near Wollongong.
• 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.
• 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-based Cameco Corporation)
  overturned into the Barskoon River, spilling
  nearly two tonnes of deadly cyanide.

27
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.

28
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.
• 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
• 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)