Cyanide Geochemistry

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

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 <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 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 Elsener’s/ Adamson’s 1st reaction:4Au + 8NaCN + O2 + 2H2O = 4NaAu(CN)2 + 4NaOH

Or Adamson’s 2nd reaction2Au + 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 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 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

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. VolatilizationReaction 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 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 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.

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

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 CN- and hence HCN

Fe(CN)64- + H+ = Fe(CN)5

3- + 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.

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.

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

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 Sx

2- + CN- = Sx-12- + SCN-

• From thiosulphatesS2O3

2- + 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 toxicity6. Hydrolysis

HCN + 2H2O = NH4COOH (ammonium formate)

HCN + 2H2O = NH3 + HCOOH (formic acid)

• Slow reaction, 2% per month• Dependent on pH.

Degradation mechanisms to reduce toxicity7. Biodegradation

• Aerobic degradation in unsaturated zones is 25 times more effective than in saturated zones

HCN + O2 = 2 HCNOHCNO + 0.5 O2 + H2O = NH3 + CO2

• Anaerobic degradation in the saturated zonesCN + 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

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• 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.

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.

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.

• 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)