the quantification of cyanide and its reaction products ... · cyanide destruction studies with...

THE QUANTIFICATION OF CYANIDE AND ITS REACTION PRODUCTS 287

IntroductionThe implementation of more stringent regulations and therecent development of the International CyanideManagement Code (ICMC) by the industry is aimed atavoiding future environment incidents with cyanidationplants. To meet the ICMC discharge to tailingsrequirements and/or regulations, many operations nowadopt cyanide recovery or destruction processes. InAustralia, there is also the reporting requirement of theNational Pollution Inventory (NPI), whereby all cyanideused in the gold recovery process must be accounted for.Thus, there is a need to clearly understand the cyanidedeportment in gold processing operations, particularly inAustralia.

The recovery of gold from sulphide ores and concentratesis becoming more widespread as the supply of oxide ore-bodies diminishes. Many sulphide minerals are problematicin the cyanidation of these ores and concentrates, as theyare known to consume oxygen and cyanide (Hedley andTabachnick, 1968; Nicol, Fleming and Paul, 1987;Deschenes, 2005; Dai and Jeffrey, 2006; May et al., 2005).Most metal sulphides decompose in aerated, alkalinecyanide solution to form metal ions, metal oxides or metalcyanide complexes and various sulphur-containing speciesincluding thiocyanate, sulphide and thiosulphate ions, withpolythionates and polysulphides having also been detected(Marsden and House, 2006). Gold processing plants todaytypically have the ability to measure ‘free’ cyanide andweak acid dissociable (WAD) cyanide on site and in someoperations online analyses are used to control reagentaddition. However, these analyses provide little informationon the deportment of cyanide within the processing plantand thus samples must be sent out to analysis laboratories toattain this information. This typically requires the

application of several analysis techniques to quantify themetal cyanides, thiocyanate and cyanate; atomic adsorptionspectroscopy or ion coupled plasma for elemental analysisand wet chemistry techniques for other species. Ionchromatography has also been used to analyse for thevarious cyanide species and metal cyanides (Fagan, 1995;Huang, Paull and Haddad, 1996; Pohlandt, 1984; Fagan etal., 1998; Muir, 1994; Miura and Kawaoi, 2000; Fagan etal., 1997; Otu, Byerley and Robinson, 1996; Haddad, 1988;Hilton, 1986; Zvinowanda, Okonkwo and Gurira, 2008;Sumiyoshi, Yagi and Nakamura, 1995).

Sulphur dioxide (or sulphite salt) / air or Caro’s acid(sulphuric acid + hydrogen peroxide) are commonly used inthe destruction of cyanide. However, the understanding andmechanisms of such reactions are unclear or have not beenpublished in the open literature. Thus, there is a clear needto be able to measure the deportment of sulphur species, inaddition to cyanide, to better understand the chemicalprocesses taking place and the reaction products generatedwithin a gold processing plant. This information isimportant in evaluating and monitoring the environmentalrisk of an operation.

This paper describes the methodology developed utilizingion chromatography to analyse various cyanide and sulphurspecies present during cyanidation of complex gold oresand cyanide destruction processes. The application of thismethod to enhance the understanding of the chemistryrelated to the deportment of cyanide in gold processing isdemonstrated.

Experimental proceduresCyanide destruction studies with sulphite were carried outin a 500 mL reactor fitted with baffles and an overheadstirrer rotating at 600 rpm. Experiments were conducted

BREUER, P.L., HEWITT, D.M., SUTCLIFFE C.A., and JEFFREY, M.I. The quantification of cyanide and its reaction products during leaching and cyanidedestruction processes. World Gold Conference 2009, The Southern African Institute of Mining and Metallurgy, 2009.

The quantification of cyanide and its reaction products duringleaching and cyanide destruction processes

P.L. BREUER, D.M. HEWITT, C.A. SUTCLIFFE and M.I. JEFFREYParker Centre, (CSIRO)

One of the key issues facing gold producers is the management of cyanide and the implementationof the International Cyanide Management Code. Many operations now use cyanide destructionprocesses to ensure that the material being discharged to the tailings dam is compliant withregulatory requirements and the International Cyanide Management Code. One of the problemsfor operations is the quantification of both cyanide species and their reaction products throughoutthe leach and destruction circuits. Therefore methods have been developed utilizing highperformance liquid chromatography (HPLC) to measure free cyanide, cyanide complexes ofmetals such as copper and iron, and other species formed from reactions in leaching and cyanidedestruction, including cyanate, thiocyanate, and sulphur species (eg. sulphite, thiosulphate andsulphate). This enables a complete mass balance for cyanide and sulphur in both leaching andcyanide destruction. This paper outlines the methodology which has been developed and discussesthe application of these methods in the fundamental research currently being conducted on theimpact of sulphide minerals in cyanidation and cyanide destruction chemistry.

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WORLD GOLD CONFERENCE 2009288

with a solution volume of 300 mL at room temperature (20°C) and atmospheric pressure, with the air sparge rate intothe reactor maintained at 100 mL min-1. This approach wasadopted to provide a constant rate of oxygen addition to thereactor. The pH was initially adjusted by either using HClor NaOH. The pH and DO were both monitored, with thepH automatically controlled with NaOH addition as thereaction proceeded. Samples were taken at regular timeintervals, filtered where needed and analysed by HPLC asdescribed below.

Development of analysis methodologyThe authors have previously developed a HPLCmethodology for the analysis of some cyanide and sulphuranion species generated during the cyanidation of mineralsulphides (Breuer et al., 2007). This method utilized agradient method with strongly adsorbing perchlorate ions inthe eluent to facilitate the intial separation of the weaklyadsorbed anions and the elution of the strongly adsorbedmetal cyanides within a reasonable analysis timeframe of15 minutes. However, this method is limited to UV activespecies for detection/quantification as the eluent containingperchlorate ions cannot be suppressed to measureconductivity. By UV cyanate was detectible, thoughquantification was poor due to weak UV absorbance andthe separation between chloride and cyanate was poor withperchlorate eluent. In addition, the non-UV active cyanideand sulphate species could not be quantified using thisanalysis method, and as such necessitated the developmentof a method that can effectively quantify all species ofinterest.

Non-UV active speciesThe first step was to change the eluent from perchlorate tohydroxide, which can be suppressed in order for theconductivity to be used in quantifying species present insolution. Nonetheless, to undertake this successfully, ananalytical column capable of handling high hydroxideconcentrations was also required. Initially the DionexIonPac AS20 anion exchange column was chosen becauseof its capability of separating chloride and cyanate with thehydroxide eluent. However, the system presented threeproblems that required to be addressed; these consisted of:

• Separation of sulphate and sulphite ions was notachievable by adjusting the hydroxide eluentconcentration using the AS20 anion exchange column.

• The elution time for the strongly adsorbed species suchas thiocyanate and the metal cyanides becameexcessive (> 30 minutes) because of limited hydroxideconcentration that could be suppressed, particularly as ashort analysis time was needed for the study ofreactions with metastable intermediate species(discussed later in this paper).

• Cyanide ion is not UV active and forms HCN(aq) in thesuppressed eluent, and therefore, could not be detectedusing the conductivity method.

The first issue was initially resolved by quantifying thesulphite using UV detection, and therefore, correcting theconductivity detection for sulphite concentration in order tocalculate the sulphate concentration. However, a betterresolution was achieved by use of an alternative anionexchange column (Dionex IonPac AS17 column) thatallowed separate quantification of sulphate and sulphite

ions using a hydroxide eluent. It was also found that byplacing the suppressor before the UV detector, the baselineinterference of hydroxide at low UV wavelengths wasreduced significantly.

The second issue was addressed through truncating thisnew method which then required the analysis of thestrongly adsorbed species to be conducted separately usingthe perchlorate eluent. To achieve this, a means of rapidlyflushing the strongly adsorbed species from the systemwithout the need for a significant equilibration time beforethe next sample analysis was required. To avoidoverloading the suppressor when the hydroxideconcentration was raised to rapidly flush from the columnthe strongly retained species, a switching valve was insertedinto the circuit to divert the eluent flow away from thesuppressor. However, this approach was unsuccessful insignificantly reducing the time between analyses, as thetime required to flush and re-equilibrate the separationcolumn after the truncated analysis was long.

Analysis of the separation and retention of stronglyadsorbed species by guard columns (analytical columnremoved) indicated that combining the AG17C and AG20in series provided adequate capacity and retention time forseparation of the strongly adsorbed metal cyanide species;the AG17C had the longest retention time, but since it has alower capacity, the AG20 was included to ensure that wheninjecting metal cyanide ions at high concentration that thesespecies did not reach the analytical column. Whencombined with the appropriate timing for the switchingvalve, the guard columns could be switched out of theanalysis stream such that the strongly adsorbed species didnot enter into the analytical column. This allowedsimultaneous removal of the metal cyanide species from theguard columns while the guard column were switched outof the analysis stream. The elution of the guard columnswas achieved using a separate pump and eluent. As theguard columns had to be switched back into the analysisstream at the end of the analysis process ready for the nextsample, a 100 mM sodium hydroxide was used for trialpurposes as a flushing eluent. This was the highestconcentration that could be used without overloading thesuppressor when the guard columns were switched backinto the analysis stream and also required minimalequilibration time. Due to the small capacity of the guardcolumns this eluent was found to be sufficient to elute thestrongly adsorbed species within five minutes. Thus, theguard columns could be switched back into the analysisstream before the end of the sample analysis to equilibratewith the low hydroxide concentration used in the rest of thesystem. The instability of copper cyanide at high pH in theabsence of free cyanide (Dai and Breuer, 2009) also made itnecessary to have 0.5 mM NaCN in the flushing eluent toavoid copper hydroxide formation during the flushingprocess as this was found to adversely affect subsequentanalyses.

The long retention time for the strongly adsorbedthiocyanate and thiosulphate ions was solved throughaddition of acetonitrile into the eluent. For instance, byincreasing the acetonitrile concentration reduced theretention time of anion species in the analytical column,especially thiosulphate and thiocyanate. However, thedownside to using acetonitrile was its UV absorbance atlow wavelengths. This was addressed through varying the

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acetonitrile concentration in the eluent such that thebackground UV absorbance remained constant during thedetection of species of interest.

The third issue of analysis for cyanide was to add anamperometric detector at the end of the analysis stream.The amperometric detector with a silver working electrodemeasures the current at a constant applied potential due tosilver oxidation to silver cyanide. As the analysis streamhad been suppressed for conductivity detection anelectrolyte containing perchlorate had to be mixed with theanalysis stream to increase the solution conductivity.Sodium hydroxide was added to the electrolyte to raise thepH to ensure that all cyanide was present as cyanide ionsand not HCN.

The developed system consisted of a Waters 2695 HPLCseparation module using a Dionex IonPac AS17 anionexchange column equipped with a Dionex IonPac AG17Cand AG20 guard columns for the separation and analysis ofthe weak anion exchange species. Detection was by UV(Waters 2996 PDA detector), suppressed conductivity(Dionex AMMS 300 suppressor and Waters 432conductivity detector) and amperometric (amperometricmodule from CNSolution 3202) detection. A flowsheet ofthe analysis set-up is shown in Figure 1 and explained indetail below. An analysis flow rate of 1 mL min-1 was usedwith a sodium hydroxide/acetonitrile mixture chosen as themobile phase with a step change gradient elution methodadopted as indicated in Figure 2. Strongly adsorbed anionsare trapped in the guard column and back-flushed offlineusing a switching value during the analysis (from 7.5 to12.5 minutes) with 100 mM sodium hydroxide and 0.5 mMsodium cyanide. The electrolyte added to the analysis

stream after the conductivity detector contained 0.5 Msodium perchlorate and 0.1 M sodium hydroxide; added at0.2 mL min-1. The Empower™ software package (WatersCorporation) was used to control the HPLC and calculatepeak areas from each of the detector response profiles(absorbance, conductivity or current vs. time).

Figure 3 shows the chromatograms for an injection ofsolution containing chloride, cyanate, cyanide, sulphite,sulphate, thiosulphate, and thiocyanate. Clearly, there isgood separation of these species allowing for easyquantification, and all are eluted within 14 minutes. Somespecies were quantified by both UV and suppressedconductivity thus providing a check for the analyticalresults. In terms of quantification, the PDA detectorproduced a 3D matrix of data for absorbance vs.wavelength vs. time, and hence the chromatograph can be

Figure 1. Flowsheet of HPLC configuration used for anion analysis

Figure 2. Eluent profile utilized to achieve separation of thevarious anion species



displayed for any wavelength. The quantification ofsulphite, thiosulphate and thiocyanate was thus conductedat 192, 214 and 194 nm, respectively.

An important consideration in the cyanide measurementis how the measured value relates to metal cyanides presentin the sample. Commonly, copper is present in leachsolutions and is added as a catalyst in some cyanidedestruction processes. Both copper tricyanide and coppertetracyanide species are present when there is free cyanidein a sample, although the tricyanide complex is dominantunder typical cyanidation conditions (Lu, Dreisinger andCooper, 2002). However, it is unknown if the coppercyanide speciation changes as the copper is trapped on theguard columns and separated from the free cyanide. Thesilver nitrate titration using a potentiometric endpointmeasures the ‘free’ cyanide as the point at which 3 moles ofcyanide remain complexed with each mole of copper(Breuer and Rumball, 2007). Comparison of titration andHPLC measurements of cyanide in the presence of coppershown in Table I indicates that any copper tetracyanidepresent, undergoes re-equilibration to the tricyanide andfree cyanide; either as a result of dilution in the HPLC or

after adsorption as the free cyanide is separated away(Equation [1]). Notably, the HPLC results are all slightlyhigher than the theoretical and titration values (and theextent appears related to the copper concentration)suggesting the equilibration may also result in the formationof a small quantity of Cu(CN)2

- in the column due to theequilibrium shown in Equation [2] which lies far to theright.

[1]

[2]

Strongly adsorbed anion speciesAs the new HPLC method described above does notanalyse the strongly adsorbed metal cyanide species, thepreviously developed perchlorate based method was used toquantify them. The latter method was modified to be asimple isocratic elution which allowed a shorter analysistime (as separation and quantification of the less stronglyadsorbed anions was no longer needed). Figure 4 shows thechromatogram at 200 nm for an injection of solutioncontaining copper, nickel and iron cyanide. Thequantification of copper, nickel and iron cyanide wasconducted at 230, 267 and 214 nm, respectively.

Figure 3. HPLC chromatograph for anion species using DionexIonPac AG17C, AG20 and AS17 columns with the eluent profile

shown in Figure 2; UV (top), conductivity (middle) andamperometric (bottom)

Figure 4. HPLC chromatograph for metal cyanide species usingDionex IonPac AG16 and AS16 columns and UV detection(Waters 2998 PDA detector) with isocratic elution (255 mM

sodium perchlorate, 0.5 mM sodium cyanide)

Table IComparison of AgNO3 titration and HPLC measurements for

cyanide in the presence of copper

Cu (mM) CN/Cu ‘Free’ cyanide (mM)

Theoretical Titration HPLC

3 3 0.1 below detection 0.40

5 3 0.1 below detection 0.7

10 3.5 4.6 4.7 5.5

20 3.5 10.2 10.4 12.1

2.5 5 5.9 5.9 6.0

5 5 10.3 10.0 10.6

1.5 10 10.2 10.2 10.8



Application of analysis methodology

Impact of sulphide minerals during cyanidationThe developed analysis methodology was applied toquantify the dissolution/oxidation products of sulphideminerals formed in solution during cyanidation and to studythe effect of pre-oxidation and/or lead(II) addition inreducing the impact of sulphide minerals on cyanidation(Breuer et al., 2007; Breuer, Hewitt and Meakin, 2008).The experiments conducted in caustic cyanide solutionfound that pyrite is passive compared to pyrrhotite andchalcopyrite in the presence of cyanide. On the other hand,all minerals were found to be reactive during pre-oxidationin the absence of cyanide, with pyrite and chalcopyrite notshowing any evidence of passivation. Pre-oxidation hadvery little effect on the reactivity of pyrite duringcyanidation; however, it significantly reduced the reactivityand reagent consumption of chalcopyrite, and to a lesserextent that of pyrrhotite during subsequent cyanidation.Iron and copper were not found to dissolve into solutionduring pre-oxidation and thiosulphate was the main sulphurspecies formed. Thiocyanate was the main sulphur speciesformed in the presence of cyanide. The addition of lead(II)significantly reduced the reactivity and reagentconsumption of pyrrhotite but increased the reactivity ofchalcopyrite.

Some sulphide minerals such as chalcocite undergo non-oxidative dissolution in cyanide solution resulting in thepresence of sulphide ions in solution (Breuer, Dai andJeffrey, 2005). The oxidation of sulphide ions in thepresence of cyanide and lead(II) ions has been studied andthe mechanism elucidated to involve a lead sulphide surfacecatalyzed oxidation of sulphide ions by oxygen (Breuer,Jeffrey and Hewitt, 2008). A similar surface catalyzedoxidation of sulphide ions by oxygen also occurs in thepresence of pyrite (Hewitt et al., 2009) and thus potentiallyother semi-conductive surfaces present in the ore may becatalytic. The oxidation mechanism in the presence ofpyrite was shown to involve the production ofpolysulphides as an intermediate oxidation product. In theabsence of cyanide, the polysulphides are further oxidizedto thiosulphate, while with cyanide present, thiocyanate andsulphite are also formed from the reaction of polysulphideswith cyanide and dissolved oxygen. Polysulphide chainlength was shown to affect the final reaction products ofpolysulphide oxidation by dissolved oxygen.

Cyanide destruction and recycle processesA significant focus of an environmental managementproject within the Gold Market of The Parker CRC forIntegrated Hydrometallurgical Solutions is on variouscyanide destruction technologies adopted within the goldindustry. The aim is to establish the cyanide destructionproducts and reaction mechanisms and to evaluate theresidual environmental risks associated with the dischargesfrom these processes. The initial experimental workfocused on the sulphite (SO2 or sulphite salt) / air oxidationof cyanide (Equation [3]). In this system the cyanideoxidation is catalyzed by the presence (or addition) ofcopper. The results from a batch test are shown in Figure 5which illustrates the information generated. Initially, somecyanide is oxidized to cyanate after adding copper (addedas copper sulphate) and the remaining cyanide complexes

with copper (Equation [4]). The formation of cyanateindicates that the cyanide complexed with copper isoxidized to cyanate within the first 40 minutes. Thedecrease in copper indicates that the copper precipitatesfrom the solution due to the oxidation of cyanide. As theformed cyanate is stable within this system and the quantityof cyanate formed does not match the cyanide added, itwould appear that the copper precipitate is potentiallyCuCN. Sulphate is the only oxidation product of sulphitefound in solution and its generation matches closely the lossof sulphite. The additional sulphite is consumed by thehomogeneous side reaction with oxygen and thus the DO isobserved to rapidly decrease initially to zero when sulphitewas added and only increases once all the sulphite has beenoxidized.

[3]

[4]

Investigations are currently continuing into the effect ofvarious parameters on the reaction mechanism and kineticswith the use of a continuous stirred tank reactor (CSTR)set-up consisting of two reactors in series. Future work willseparately investigate the use of peroxide and Caro’s acidas alternative oxidants for cyanide destruction. With thisnew methodology it is also planned to investigate thechemistry in the sulphidization–acidification–recycle–thickening (SART) process which recovers copper sulphideand cyanide from copper cyanide solutions), andparticularly for plant solutions where the chemistry appearsmore complex.

Cyanide and sulphur deportment studiesOne of the major drivers of developing this newmethodology was the necessity to measure cyanidedeportment in gold processing plants to meet cyanide codecompliance and NPI reporting requirements. For example,the information derived using this methodology is shown inFigure 6. It is clearly evident that the addition of cyanide toLeach Tank 1 causes a significant quantity of cyanide to

Figure 5. Concentration of various species for a batch destructionof cyanide by sulphite. Experimental conditions: 8 mM Na2SO3, 4

mM NaCN, 50 ppm Cu, Air sparged, pH 7

SulphiteSulphateCyanateCopperDO (ppm)



react forming thiocyanate and cyanate. A considerablequantity of copper is also dissolved which is complexed bycyanide. The loss of free cyanide over the 8 leach tanks isnot accounted for by the increase in other cyanide species,indicating losses due to volatilization in the form of HCN.The plant in this case used Caro’s acid for cyanidedestruction for which it is evident that the residual cyanide(and copper cyanide) is oxidized firstly to cyanate in thefirst tank (Destruct T1 in Figure 6) which is furtheroxidized in the second tank (Destruct T2 in Figure 6). Asignificant portion of the thiocyanate is also destroyed byCaro’s acid. There is only a small further decrease inthiocyanate and cyanate in transfer to the tailings storagefacility (TSF). Some further natural degradation of cyanateoccurs within the tailings facility.

ConclusionsAn analysis methodology has been developed to quantifycyanide and sulphur species typically present in goldprocessing solutions using HPLC anion exchange with UV,conductivity and amperometric detection. The requirementfor a short analysis time to study reaction kinetics wasachieved by implementing an offline flushing procedure forthe guard columns in order to remove the strongly adsorbedmetal cyanides which were analysed via an alternativemethod.

The application of this methodology aided in studyingand providing greater insights into the impact of sulphideminerals during cyanidation and the cyanide destructionand recycling processes. The analysis information providedon plant samples by this new methodology assistsoperations in meeting ICMC compliance and NPI reportingrequirements. It also improves the quantification andunderstanding of the cyanide deportment within goldprocessing operations and with regular analysis helpsidentify the cause for changes in reagent consumption.

Acknowledgements

The support of the Parker CRC for IntegratedHydrometallurgy Solutions (established and supportedunder the Australian Government’s Cooperative ResearchCentres Program) is gratefully acknowledged

References

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Figure 6. Cyanide deportment measured in a gold processing plant



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Paul Leslie BreuerSenior Research Scientist, Parker Centre, (CSIRO)

Paul Breuer was awarded a Doctor of Philosophy, from Monash University in 2002 during a periodof 11 years in research at Monash University where he studied hydrometallurgical processes andparticularly gold dissolution in thiosulfate solutions. Paul joined CSIRO in 2004 as a seniorresearch scientist and currently leads research activities involving cyanidation for gold recovery.Paul is also the project leader of a Parker Centre gold project on Environmental Management inGold Processing addressing the deportment of cyanide and toxic trace elements. Paul’s expertiseand research interests include:

• Cyanidation of copper-gold ores and concentrates and the impact of sulfide minerals• Deportment and speciation of cyanide and toxic trace elements• Ion chromatography (for determining cyanide and sulfur speciation)• Cyanide destruction and recycle processes• Electrochemistry in leaching and electrowinning processes.

Matthew Ian JeffreyMarket Leader—Gold, Parker Centre (CSIRO)

Matthew Jeffrey has been awarded a Bachelor of Engineering (Chemical) with first class honoursfrom the University of New South Wales in 1994, and a Doctor of Philosophy, from CurtinUniversity (AJ Parker CRC for Hydrometallurgy) in 1998. Matthew has extensive researchexperience in Hydrometallurgy, being based at Monash University for 8 years as a senior lecturerprior to joining CSIRO. He has published over 70 papers in the field, and holds a patent. Matthewis currently the Parker Centre gold market leader. Matthew’s expertise and research interestsinclude:

• Processing of gold using cyanide or thiosulfate• Leaching and electrochemistry of gold and base metals• Electrowinning• Resin and carbon adsorption and elution• Modelling and solution speciation• Ion chromatography (for determining sulfur and cyanide speciation).


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