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FUNDAMENTAL INVESTIGATIONS OF THE SO2/AIR, PEROXIDE AND CAROS ACIDCYANIDE DESTRUCTION PROCESSES

By

Paul Breuer, Coby Jeffery and Rebecca Meakin

Parker CRC for Integrated Hydrometallurgy Solutions

CSIRO Minerals Down Under National Research FlagshipCSIRO Process Science and Engineering

Australia

Presenter and Corresponding Author

Paul [email protected]

ABSTRACT

Fundamental investigations have been conducted on the SO2/air, peroxide and Caros acid cyanidedestruction processes to establish a more detailed understanding of the reaction mechanisms andkinetics. The major findings are summaries below.

For the SO2/air process it was found that:1. Upsets to the process (for example the loss of sulfite or oxygen addition) which result in the

presence of free cyanide in the reactor will stop the oxidation of cyanide.2. In a CSTR or series of CSTRs, the DO concentration provides an indicator to the residual

oxygen capacity available in the process; zero DO in the last reactor indicates insufficientoxygen addition for the rate of cyanide and sulfite addition.

3. The addition of hydrogen peroxide to the SO2/air process to potentially increase the cyanideoxidation is not beneficial and is not recommended as sulfite is preferential oxidised overcyanide.

The copper catalysed peroxide destruction of cyanide investigations found that the solutioncomposition (especially metal ions) and pH have a significant impact on the reaction chemistry,particularly the inception of precipitation and the subsequent stoichiometry of peroxide to cyanideoxidation.

Investigations of the Caros acid process found that:1. Free cyanide and thiosulfate are preferentially oxidised prior to the oxidation of copper cyanide

and thiocyanate which occur in parallel.2. The control of pH is important since at low pH, HCN forms which is not readily oxidised and the

rate of cyanate oxidation increases. This can occur if pH control is subsequent to Caros acidaddition or within the localised zone where Caros acid is added, and can significantly reduce

the cyanide oxidation efficiency.

Thiosulfate is detrimental to all the cyanide destruction processes due to the oxidation reactionhaving a high oxidant demand. Thiocyanate has an impact only on the Caros acid process (highoxidant demand), particularly with high copper and thiocyanate concentrations. Most metalcyanides precipitate from solution once the copper cyanide has been destroyed, however zincprecipitates before copper whilst mercury is not precipitated by any of the destruction processes.


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INTRODUCTION

Commercially the INCO process (SO2/air) and Caros acid are commonly employed to treatcyanidation tails from gold plants to achieve regulatory requirements and the ICMI Code complianceof less than 50 mg/L weak acid dissociable (WAD) cyanide for the discharge of cyanidation tails intoa tailings storage facility (TSF). Peroxide, which is less effective on slurries, is often used to treatthe return water from the TSF to lower cyanide levels that would otherwise impact plantperformance. Peroxide is sometimes also added into or subsequent to the INCO process. Detailed

descriptions of these processes can be found in the literature (1, 2). This paper presents findingsfrom fundamental investigations of these three cyanide destruction processes currently beingutilised by the gold industry; SO2/air, hydrogen peroxide and Caros acid.

SO2/AIR PROCESS

There are two patented SO2/air processes (3, 4), of which the INCO process is more commonlyadopted and used for the treatment of slurries. The sulfur dioxide dissolves into solution formingsulfite at the pHs typically adopted in the destruction process:

2HSOOHSO -2322 (1)

The sulfite ion is the reactant in the process and thus sodium sulfite (Na2SO3) or sodiummetabisulfite (Na2S2O5) can also be used as a source of sulfite. The process is based uponconversion of cyanide (including cyanides weakly complexed with metal ions) to cyanate usingsulfite and air in the presence of a soluble copper catalyst (not added if copper is already present)at a controlled pH. The overall reaction is:

-24

-Cu2

-23

- SOOCNOSOCN (2)

The reaction is normally carried out at a pH of 8.0 to 9.0, with lime normally required for pH control,particularly when sulfur dioxide is used. Reaction rate is extremely fast and is limited by the transferof oxygen. Typical reaction times in order to achieve the required oxygen mass transfer vary fromabout 30 minutes to 2 hours. Iron complexed cyanides are reduced to the ferrous state andprecipitated with copper, nickel or zinc as insoluble metal-iron-cyanide complexes. Residual metalsliberated from the WAD cyanide complexes are precipitated as their hydroxides. The process doesnot preferentially attack thiocyanate, with generally less than 10 % oxidised in the process (1).Inefficiency in the process results from the direct oxidation of sulfite rather than cyanide:

-242

-23 2SOO2SO (3)

Reaction mechanisms

An indicator that the SO2/Air process and reaction chemistry is not straight forward was evident byINCOs need to become involved in the process engineering as initial installations in the mid 1980s

experienced poor field performance, losses in process kinetics, were unable to maintain continuousoperation and had difficulties restarting the process after upsets (5). The experience and learningsgained by INCO from these initial installations provided knowledge of the process and equipmentlimitations which guided future testwork, process design and engineering of the cyanide destructionreactor. Because of this in-house development, knowledge and experience, little fundamentalunderstanding of the reaction chemistry has been publicly available until recently (6). Included inthe discussions below are further advancements in the understanding of the reaction mechanisms.

Role of sulfite and copper

In order to better understand the role of sulfite and copper in this process, an understanding of thebasic chemistry of sulfite and cyanide solutions and the oxidation of these by dissolved oxygen isrequired. Figure 1 shows that for the experimental setup used by the authors to study the SO2/Airsystem, the rate of sulfite oxidation without cyanide or copper present proceeds via Reaction 3 at anappreciable rate with all the sulfite oxidised in a little over one hour; this reaction is known to occurvia a free radical mechanism. In comparison, sulfite is shown not to be oxidised in the presence ofcyanide ions, suggesting cyanide ions may act as a free radical scavenger; notably the cyanide ions


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are not oxidised. However, when copper is present with cyanide, the oxidation of sulfite does occur,but at a slower rate to that observed in the absence of copper and cyanide; due to the concurrentoxidation of cyanide which is discussed further below. This suggests a reaction mechanism inwhich the copper acts as a catalyst for the oxidation of sulfite by oxygen; most probably byfacilitating electron transfer from the sulfite to oxygen.

0

2

4

6

8

0 50 100 150 200

SulfiteConcentration(mM)

Time (min)

No cyanide or Cu

Cyanide, no Cu

Cyanide, 0.8 mM Cu

Figure 1: Effect of cyanide and copper on the oxidation of sulfite

(4 mM NaCN, 8 mM Na2SO3, pH 9, air sparged).

Figure 2 shows that with copper and cyanide present, the oxidation of sulfite (mirrored by the sulfateformation) and that of cyanide (cyanate formation) occur in parallel until essentially all the cyanidehas been oxidised to cyanate in accordance with Reaction 2. At this point the majority of the copper

has also precipitated from solution, presumably as a hydroxide, and the residual sulfite continues tobe oxidised (as observed in Figure 1 for the sulfite solution without cyanide or copper present). Thedissolved oxygen concentration is zero through-out and increases only once all the sulfite has beenoxidised. This indicates that the reaction rate is limited by the oxygen uptake rate (discussedfurther below). The close match in final cyanate concentration with the initial cyanide concentrationindicates that cyanate is the major cyanide oxidation product.

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0 50 100 150 200

Concentration(mM

)

Time (min)

Sulfite

Sulfate

Cyanate

Copper

Free Cyanide

Figure 2: Reactant and product concentrations for SO2/air ox idation of cyanide

(4 mM NaCN, 0.8 mM CuSO4, 8 mM Na2SO3, pH 9, air sparged).


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Notable also in Figure 2 is that the initial cyanate concentration (sample taken after just twominutes) is greater than expected from the addition of copper sulfate (Reaction 4) and the initialoxygen in solution. The sulfite and sulfate concentrations at this point also indicate an initial rapidreaction. The mechanism for this initial rapid reaction is unclear, however, the extent is dependenton conditions and reagent addition.

OHCNO2Cu(CN)2OH7CN2Cu 223

2

(4)

Presence of free cyanide

Notable for the results shown in Figure 2 is that with the presence of copper there was nomeasureable free cyanide in the two minute and subsequent samples. In comparison, Figure 3shows that when free cyanide is present (higher initial cyanide concentration) no oxidation ofcyanide occurs subsequent to some initial rapid oxidation. However, the oxidation of sulfite isobserved to occur despite the presence of free cyanide which was observed to stop sulfite oxidationin the absence of copper (Figure 1). This suggests that copper cyanide catalyses the oxidation ofsulfite and that this mechanism does not propagate to involve the oxidation of cyanide when freecyanide is present, but does so when there is no free cyanide.

0

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12

14

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18

0 50 100 150 200

C

oncentration(mM)

Time (min)

Sulfite

Sulfate

Cyanate

Copper

Free Cyanide

Figure 3: Effect of free cyanide on reactant and product concentrations for SO2/air ox idation

of cyanide (8 mM NaCN, 0.8 mM CuSO4, 16 mM Na2SO3, pH 9, air sparged).

Initial rapid oxidation of cyanide to cyanate

An implication of the initial rapid oxidation of some cyanide to cyanate upon the mixing of sulfite to acopper cyanide solution is that in a CSTR process it may appear from a WAD cyanidemeasurement that the process is working, though perhaps not as effectively as expected. This isshown in Figure 4 where with 8 mM NaCN in the feed the presence of free cyanide stops theoxidation of cyanide other than the initial rapid oxidation upon addition of the sulfite in Tank 1; thegreater reduction in WAD CN than generated cyanate is due to volatilisation of HCN. Incomparison, the results for 4 mM NaCN in the feed shows greater oxidation of cyanide for the samesulfite and oxygen addition to the reactors. As the transfer of oxygen is limited and thus oxidationincomplete in tank 1, further oxidation is observed in tank 2. With no oxidation of cyanide occurringin the presence of free cyanide, a greater decrease in the sulfite concentration occurs across thereactors (not shown) due to Reaction 3 rather than Reaction 2 taking place.


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0

1

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3

4

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6

7

8

Feed Tank 1 Tank 2

Concentration(mM)

WAD CN - 4 mM NaCN

Cyanate - 4 mM NaCN

WAD CN - 8 mM NaCN

Cyanate - 8 mM NaCN

Figure 4: Effect of free cyanide on cyanide oxidation (cyanate formation) for a CSTR process(0.8 mM CuSO4, 8 mM Na2SO3, pH 9, air sparged, 20 min RT/tank).

Process start-up and reaction sustainability

The SO2/Air cyanide destruction process was demonstrated above to require the absence of freecyanide subsequent to the initial rapid oxidation for the continued oxidation of cyanide to cyanate.Thus, in a CSTR process the feed solution can contain free cyanide as long as the rate of cyanideoxidation to cyanate in the first reactor is greater than the addition of free cyanide; as such theresultant mixed solution within the CSTR will have no free cyanide. In starting up the process, orwhere there is a loss of sulfite or oxygen addition to the process, the free cyanide in the CSTR is

typically depleted through the addition of copper sulfate. This oxidises some cyanide to cyanateand complexes the free cyanide according to Reaction 4. Excess copper addition results in theformation of some copper di-cyanide complex. Where the free cyanide addition to a CSTR isgreater than the maximum rate of cyanide oxidation (dependent on oxygen uptake rate, which isdiscussed below), then the continual addition of copper sulfate is required to sustain oxidation ofcyanide.

Oxygen uptake

In practise, the SO2/Air cyanide destruction process is typically conducted in one (or more) CSTRswhere the oxygen uptake is sufficient for the required cyanide destruction. The rate of oxygenuptake for the system limits the quantity of WAD cyanide that can be destroyed. This is largelydependent on the liquid/gas interfacial area which is dependent on the CSTR size and design,

impeller design and power input, and the system used for air/oxygen addition to the CSTR. Theinterfacial transfer area is a function of the gas bubble size and the gas hold-up in the CSTR, thusdecreasing the gas bubble size or increasing the gas holdup will increase the transfer area. Studieshave shown that increasing the power per unit volume applied to the agitation breaks gas bubblesinto smaller bubbles and increases the interfacial area (7). Special nozzles also offer the ability tointroduce very fine gas bubbles into the reactor, such as the CSIRO developed GL Nozzletechnology (8). Increasing the reactor volume and/or the number of reactors, whilst maintaining thebubble size and gas holdup, will also increase the oxygen uptake.

To illustrate the oxygen uptake limitation, experiments were conducted with increasing throughput(reduced residence time). The results shown in Figure 5 show that with sufficient residence time(10 minute or more) the oxygen uptake of the system is greater than the demand of the reactantsentering the CSTR and there is measurable DO within the reactor. However, at high throughput

(5 minutes residence time) the demand of the reactants entering the reactor exceeds the oxygenuptake and thus the dissolved oxygen concentration is zero and the oxidation of sulfite and cyanideis incomplete. The DO concentration thus indicates the extent of additional capacity available in thesystem or whether the system capacity has been exceeded (zero DO). Similarly, the oxygen


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uptake would be exceeded at high cyanide concentrations (with proportional increase in sulfite) inattempting to achieve the same residual cyanide concentration.

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0 10 20 30

DOConcentration(mg/L)

Concentration(mM)

Residence Time (min)

Cyanate

Sulfite

Sulfate

Copper

DO

Figure 5: Effect of CSTR throughput on reactant and product concentrations for SO2/air

oxidation of cyanide (8 mM NaCN, 0.8 mM CuSO4, 8 mM Na2SO3, pH 9, O2sparged).

Sulfite to cyanide stoichiometry

Shown in Figure 6 is that the oxidation of cyanide, with adequate oxygen uptake, closely follows thestoichiometry of Reaction 2 when the stoichiometric addition of sulfite to cyanide is 1 or less. Whenexcess sulfite is added (stoichiometric addition of sulfite to cyanide is greater than 1), the excess

sulfite is oxidised according to Reaction 3. Thus, stoichiometric addition of sulfite to WAD cyanideis all that is required to achieve essentially complete destruction of the WAD cyanide. Excesssulfite addition is a wasted cost for the process and can result in the oxygen uptake being exceededby the reactant demand, in which case the DO is zero and yet all the cyanide has been oxidised.

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0 2 4 6 8 10 12

DOConc

entration(mg/L)

Concentration(mM)

Sulfite Concentration (mM)

Cyanate

Sulfite

Sulfate

Copper

DO

Figure 6: Effect of sulfite to cyanide ratio for SO2/air oxidation of cyanide in a CSTR

(8 mM NaCN, 0.8 mM CuSO4, pH 9, O2sparged, 20 min RT).


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Oxidation of other species

Cyanidation of gold ores containing sulfide minerals results in the formation of thiocyanate from thereaction of cyanide with metal sulfides. Consistent with the literature, investigations have found noconditions for the SO2/Air process that result in the oxidation of thiocyanate.

Pre-oxidation before cyanidation is often used to reduce the reactivity of sulfide minerals, whichtypically generates thiosulfate ions in solution. There is typically some loss of thiosulfate during

cyanidation (thiosulfate ions react slowly with cyanide during the cyanidation process and can beoxidised to high oxidation state oxy-sulfur species such as sulfite and sulfate), though significantconcentrations can remain after cyanidation and enter the cyanide destruction process. Thepresence of thiosulfate ions reduces the overall cyanide oxidation rate and in parallel thiosulfate isoxidised to sulfate (6). The reason for the reduced cyanide oxidation rate is that the oxidation ofthiosulfate consumes oxygen (Equation 5) and thus competes for oxygen with the oxidation ofcyanide. Thus, a high thiosulfate concentration can significantly increase the oxygen requirementand negatively impact on cyanide destruction.

OH2SO2OH2OOS 2-2

42-2

32

(5)

Investigations conducted with other metal cyanides present found that the metal cyanide complexes

of nickel, silver and zinc are destroyed and the metals precipitated from solution in conjunction andsubsequent to copper precipitation. Only partial destruction/precipitation was observed forcadmium and cobalt cyanide complexes, whilst mercury cyanide complexes essentially remaineddissolved in solution.

Peroxide assisted

The use of peroxide in conjunction with the INCO process has been trialled and used at someoperations, particularly where the process is limited by oxygen uptake. There are two possiblemechanisms by which peroxide could assist the oxidation of cyanide:

1. Copper catalysed oxidation of cyanide by the peroxide (see Hydrogen Peroxide section below).

2. Decomposition of the peroxide to oxygen which increases available oxygen for Reaction 2.

To investigate these possible mechanisms, batch SO2/Air experiments limited by the oxygen uptakewere conducted with peroxide added initially or after 4 hours. These results are compared with noaddition of peroxide in Figure 7. Notably, the addition of peroxide initially had no beneficial effect onthe oxidation of cyanide, instead it rapidly oxidised sulfite to sulfate. The addition of peroxide after 4hours also resulted in the rapid oxidation of the remaining sulfite to sulfate, along with a notabledecrease in the WAD cyanide concentration. The reason for the increased oxidation of cyanide isattributed to oxidation of cyanide by the residual peroxide (remaining after reaction with the residualsulfite), which is catalysed by copper (see following section). Addition of peroxide into the SO2/Airprocess is thus not recommended, though addition subsequent to the SO 2/Air process could beused to further reduce the WAD CN concentration where the SO2/Air process capacity is exceeded.

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25

30

35

0 50 100 150 200 250 300 350

CalculatedWADCNConcentration(mM)

Time (min)

No H2O2

20 mM H2O2 initially

20 mM H2O2 af ter 4 hrs

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

SulfiteConcentration(mM)

Time (min)

No H2O2

20 mM H2O2 initial ly

20 mM H2O2 aft er 4 hrs

Figure 7: Effect of peroxide addition to SO2/air oxidation of cyanide

(20 mM NaCN, 10 mM CuCN, 30 mM Na2SO3, pH 9, Air sparged).


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HYDROGEN PEROXIDE PROCESS

DuPont and Degussa have separately developed and patented several versions of the hydrogenperoxide process for treating cyanide tailings solutions (9, 10, 11, 12, 13). The process has limitedapplication in slurries due to the high reagent consumption resulting from the reactions of peroxidewith solids in the slurry. The process is based upon oxidation of WAD cyanides to cyanate usinghydrogen peroxide in the presence of a soluble copper catalyst (not added if already present) toincrease the reaction rate. The overall reaction being:

OHOCNOHCN 2-Cu

22-

(6)

Reaction periods typically range from about 30 minutes to 3 hours depending upon the copper tocyanide ratio, the untreated and treated cyanide levels, and the quantity of hydrogen peroxide used;reaction rate increases with increasing copper and peroxide concentration. The residual WADcyanide increases with increasing copper, thus higher peroxide to cyanide addition is required toachieve the same residual WAD cyanide at higher copper concentrations (1). The processoperates over a wide range of pH values, with the fastest rate reported to be at pH 10 (1). Theoptimal pH for metals removal after cyanide destruction is reported as about 9.0 to 9.5. Ironcyanides are precipitated as for the SO2/air process. Similarly, the process does not oxidisethiocyanate to any appreciable extent. Excess hydrogen peroxide added for cyanide oxidation will

decompose to yield oxygen and water, which is an advantage when the concentration of dissolvedsolids is of concern in the treated water.

2222 OO2HO2H (7)

Reaction Mechanisms

Role of copper

In the absence of copper the rate of cyanide oxidation by hydrogen peroxide is extremely slow (6).However, in the presence of copper the oxidation of cyanide occurs with the oxidation product beingcyanate (Figure 8). In contrast to the SO2/air system, cyanide oxidation occurs even when there isfree cyanide present (i.e. when there are insufficient metal ions, such as copper, in solution tocomplex all the cyanide). Once there is no free cyanide (after ~70 minutes) there continues to befurther oxidation of cyanide to cyanate and a continuing decrease in peroxide up to 120 minutes.After 120 minutes, the DO concentration begins to increase and spikes at around 160 minutescoinciding with the solution turning yellow (formation of a fine precipitate), and bubbles beinggenerated. This appears to correspond to the point where the depletion of cyanide is such that thecopper starts to precipitate (CN:Cu approaching 2:1) which catalyses the decomposition of H 2O2. Ayellow intermediate has been previously reported in the decomposition of H2O2by copper in alkalinesolution (14, 15). The yellow compound is thought to be a Cu(I)peroxide complex.

With only stoichiometric addition of peroxide to cyanide for the test presented in Figure 8, thedecomposition of peroxide resulted in incomplete oxidation of cyanide. Doubling the concentrationof peroxide for the same conditions resulted in a much faster reaction rate with significantly moreoxygen generation and larger DO spike after only 15 minutes. More than half the copper was alsoprecipitated from solution. Previous work by the authors (6) covers in more detail the effect ofcopper and peroxide concentration on the reaction kinetics. Beattie and Polyblank (15) have shownthat whilst free cyanide is present the rate of cyanide oxidation is first order with respect to peroxideand copper concentration and independent of the free cyanide concentration. The independencewith cyanide concentration suggests either a mechanism in which the copper is involved in the ratelimiting step or only the copper co-ordinated cyanide is oxidised.

The amount of cyanate present in the sample taken two minutes after the peroxide had been addedis shown in Figure 8 to be significantly greater than that from the addition of copper sulfate to thecyanide solution (Reaction 4). The quantity of cyanate formed increases with copper concentrationand is significantly more than that which can be accounted for via Reaction 6 in the two minutesbefore sampling and the three to five minutes taken for the sample to be injected in the HPLC foranalysis. This rapid initial oxidation upon mixing appears similar to that observed for the SO2/Airprocess and further investigations are required to establish the mechanism for this.


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0

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0 50 100 150 200

DOCon

centration(mM)C

oncentration

(mM)

Time (min)

H2O2

Cyanide

Cyanate

Copper

DO

Figure 8: Reactant and product concentrations for H2O2/Cu oxidation of cyanide(4 mM NaCN, 0.63 mM CuSO4, 4 mM H2O2, pH 10)

Reaction stoichiometry

To accurately study the reaction stoichiometry, investigations were conducted with higher copperconcentrations. With high copper concentrations (> 5 mM), however, the reaction rate is extremelyfast (order of minutes) and thus kinetic measurements are not possible with the analysis techniquesavailable. To study the reaction stoichiometry a series of batch experiments were thus conductedwith increasing peroxide addition (all other conditions being the same). An example of such data isshown in Figure 9. From this data a number of important observations can be drawn:

1. Copper begins precipitating from solution once the CN:Cu ratio reaches ~2:1.

2. There appears to be a residual ~4 mM of copper and associated ~8 mM of cyanide that cannotbe removed from solution with excess peroxide addition.

3. The close match between the Total CN and the Initial Total CN less the cyanate formedindicates that up until copper begins to precipitate the cyanide is oxidised to cyanate.However, once the copper begins to precipitate the increase in cyanate is significantly lessthan the cyanide which is oxidised. The reason for this is unclear and requires furtherinvestigation; further oxidation of cyanate is one possibility.

4. The ratio of peroxide addition to oxidised cyanide indicates that whilst free cyanide is present

(Total CN greater than 45 mM) the stoichiometry matches that of Reaction 5. Interestingly, thestoichiometry increases (approximately doubles) to oxidise the third cyanide complexed withcopper (overall stoichiometry of 1.5 to oxidise 15 mM free cyanide and 15 mM cyanidecomplexed as the third cyanide with copper). This indicates decomposition of peroxide takesplace when the CN:Cu ratio is less than 3 and before copper precipitates. The stoichiometryduring copper precipitation appears to be around 1.5 (ratio levels out), which is surprisinglybetter than for the proceeding period. Not surprisingly the stoichiometry increases with excessaddition of peroxide above which little further cyanide is oxidised and copper precipitated.

Similar batch experiments have also been conducted with different initial solution pHs (10 and 11.3)which produced similar results for the oxidation of free cyanide at low peroxide addition (Figure 10).At the higher pH the copper precipitation commenced much sooner (~20 mM H2O2 addition) andimportantly much greater peroxide addition was required to achieve the same cyanide oxidation

once precipitation occurred. This suggests that the pH influences the catalysis of peroxidedecomposition and thus lower pHs are possibly better. This is consistent with the increasedstability of peroxide in clear solutions at lower pHs; HO2

-ion is formed at the higher pHs which is

less stable than H2O2.


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0.0

0.5

1.0

1.5

2.0

0

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0 20 40 60 80 100

H

2O2/CNox

Concentration

(mM)

H2O2 (mM)

Free cyanide

Total CN

Initial Total CN - OCN

Copper

H2O2/CN ox

Figure 9: Concentrations and ratio of H2O2/CN oxidised for batch experiments w ith varyingH2O2addition. (45 mM NaCN, 15 mM CuCN, pH 8)

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Total

CN

(mM)

H2O2 (mM)

pH 11.3

pH 10

pH 8

Figure 10: Cyanide oxidation as a function of pH for batch experiments with varying H2O2

addition. (45 mM NaCN, 15 mM CuCN)

Studies of the peroxide decomposition rate in the presence of other precipitates, such as ironoxides, found that the rate of decomposition of peroxide is highly dependent on the precipitatecomposition. Thus, the presence of other metal ions can influence the precipitate composition andtherefore the catalysed rate of peroxide decomposition. Investigations conducted with plantsolutions have found variable results as a function of pH and can be contrary to those shown inFigure 10, highlighting that the solution composition and pH can have a significant impact on theratio of peroxide addition to cyanide oxidation.


An investigation was conducted with a solution containing 6 mM total cyanide, 0.8 mM copper,

4 mM thiocyanate and 4 mM thiosulfate to which 6 mM H2O2was added with the pH maintained at10. Thiosulfate was found to be oxidised more readily than cyanide (2.4 mM thiosulfate destroyedcompared to 1.1 mM cyanate formed). From this it can be calculated, assuming the peroxide wasonly consumed by cyanide or thiosulfate oxidation, that the stoichiometry of peroxide to thiosulfate


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oxidation is ~2:1 respectively. With an additional 6 mM H2O2 added (12 mM in total), all thethiosulfate was destroyed but only 3.3 mM cyanide had been oxidised. This indicates that thepresence of thiosulfate can have a significant impact on the peroxide process efficiency. Nooxidation of thiocyanate was evident in this test.

Investigations conducted with other metal cyanides present found that the metal cyanide complexesof nickel, silver, cobalt and zinc are only partially destroyed and the metals precipitated fromsolution in conjunction and subsequent to copper precipitation. Cadmium and mercury cyanide

complexes essentially remained dissolved in solution.

CAROS ACID PROCESS

Caros acid, also known as peroxymonosulphuric acid (H2SO5), is a strong oxidising agent (E0 =1.85V; DuPont, 2008) and has recently been applied at a few mining operations for tailingsdetoxification, particularly for tailings slurry. Caros acid is produced from concentrated hydrogenperoxide and concentrated sulfuric acid (0.33-0.66 mole ratio of peroxide/sulfuric) in an exothermicreaction:

OHSOHSOHOH 2524222 (8)

The hot process yields 25 - 45 % Caros acid, whilst a cold process yields 70 - 80 % (1). Due toits instability, Caros acid is produced on-site and used immediately for cyanide detoxification withonly minimal intermediate storage. The reaction of Caros acid with cyanide (and WAD cyanides)does not require a catalyst such as copper as the reaction is rapid and typically complete within afew minutes.

4252 SOHOCNCNSOH

(9)

Caros acid will also react with thiocyanate to some extent (Castrantas et al., 1995), but the reagentconsumption is high as indicated by the reaction stoichiometry:

42252 SO5HOCNOHSCNSO4H

(10)

Chemistry

The reaction of Caros acid with cyanide and copper cyanide is very rapid with complete oxidation tocyanate within two minutes (6). It is because of this very rapid reaction rate, only small reactiontanks are required or the reaction can be carried out in the transfer line to the tailings storagefacility. Further oxidation of cyanate by Caros acid is slow at pH 10, but increases in rate withdecreasing pH (1). Thus, avoiding a significant decrease in the pH is important to assure cyanidedestruction without the need to add excess Caros acid. Some cyanate oxidation also occurs evenat the higher pHs due to the localised low pH upon the addition of Caros acid and the extremelyfast reaction kinetics (6).


As both thiocyanate and thiosulfate are oxidised by Caros acid, an investigation was conductedwith step-wise addition of Caros acid to a copper cyanide solution also containing thiocyanate andthiosulfate, to establish the selectivity by which Caros acid reacts with each of these species. Theresults of this investigation are shown in Figure 11. Notably, the thiosulfate ions are initiallyoxidised in parallel with free cyanide ions. The thiosulfate oxidation products were difficult toquantify, particularly for sulfate due to the large addition of sulfate with the Caros acid incomparison to that generated. Assuming that the cyanide and thiosulfate oxidation reactions arethe only reactions consuming Caros acid initially, the stoichiometry of Caros acid to thiosulfateoxidation is calculated to be ~3:1. This indicates that the presence of thiosulfate will have asignificant impact on the Caros acid process efficiency.

Figure 11 shows that thiocyanate ions are not oxidised along with the thiosulfate and free cyanideinitially, but do undergo oxidation in parallel with the copper complexed cyanide; the rate ofthiocyanate oxidation being slower than the cyanide complexed with copper. In this case, the


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addition of excess Caros acid results in further oxidation of thiocyanate subsequent to coppercyanide oxidation. Clearly, the presence of a high thiocyanate concentration will have a significantimpact of the Caros acid efficiency, particularly if low WAD cyanide concentrations are beingtargeted.

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Concentration

(mM)

Caro's Acid (mM)

Free cyanide

Total CN

Thiocyanate

Copper

Thiosulfate

Figure 11: Cyanide oxidation w ith i ncreasing Caros acid addition

in the presence of thiocyanate and thiosul fate.(20 mM NaCN, 3.5 mM Cu, 10 mM KSCN, 1 mM Na 2S2O3, pH 10)

Investigations conducted with other metal cyanide complexes present in the cyanide solution foundthat the metal cyanide complexes of nickel, silver, cobalt, cadmium and zinc are essentiallydestroyed with the metals being precipitated from solution along with copper. The mercury cyanide

complexes, however, essentially remained dissolved in solution.

CONCLUSIONS

The three cyanide destruction processes, SO2/air, peroxide and Caros acid, each have differentmechanisms and offer different benefits depending on the properties of the stream to be treated.Most metal cyanides precipitate from solution once the copper cyanide has been destroyed, thoughzinc precipitates before copper and mercury is not precipitated by any of the destruction processes.

For the SO2/air process it is necessary that all the cyanide is complexed with metal ions, as freecyanide stops the oxidation of cyanide, but sulfite is still oxidised. The use of a CSTR, or a series ofCSTRs, allows the destruction of cyanide for a feed stream that contains free cyanide providing the

rate of cyanide oxidation in the first CSTR exceeds the feed rate of free cyanide entering thereactor. The DO concentration provides an indicator to the residual capacity available in theprocess; zero DO in the last reactor indicates insufficient oxygen addition for the rate of cyanide andsulfite addition. Upsets to the process (for example the loss of sulfite or oxygen addition) can resultin the presence of free cyanide in the reactor which stops further cyanide oxidation. In such a casethe initial rapid oxidation of some cyanide on the mixing of sulfite in the first reactor can give theappearance that the process is still operating OK, though the WAD cyanide destruction is less thanexpected from the sulfite stoichiometry. The addition of copper sulfate is typically used todestroy/complex the free cyanide and restart the process. The addition of hydrogen peroxide to theSO2/air process to potentially increase the cyanide oxidation was not beneficial and is notrecommended as sulfite is preferential oxidised over cyanide. The addition of peroxide subsequentto the SO2/Air process, however, can be beneficial, particularly if the process is limited by oxygenaddition (and sulfite is not added in excess of the oxygen addition).

Due to the catalysed decomposition of peroxide by solids, the copper catalysed peroxidedestruction of cyanide is typically used only for clear solutions. However, the solution composition(especially metal ions) and pH have a significant impact on the reaction chemistry, particularly the


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inception of precipitation and the subsequent stoichiometry of peroxide to cyanide oxidation. Thiscan significantly impact on the efficiency of the process, particularly with high copper andthiocyanate concentrations and targeting a low WAD cyanide concentration.

Caros acid rapidly oxidises free cyanide, copper cyanides, thiosulfate and thiocyanate. The freecyanide and thiosulfate are first rapidly oxidised, with the subsequent parallel oxidation of coppercyanide and thiocyanate (though at a slower rate than the copper cyanide). The control of pH isalso important since at low pH HCN is not readily oxidised and the rate of cyanate oxidation

increases. This can also occur within the localised zone where Caros acid is added and results inreduced cyanide oxidation efficiency.

Thiosulfate is detrimental to the cyanide destruction processes as the oxidation of thiosulfate has asignificant oxidant requirement; thiosulfate likely to be an issue with the processing of gold orescontaining sulfide minerals, particularly when pre-oxidation is used to minimise the impact oncyanide consumption during leaching. Thiocyanate only has an impact on the Caros acid processdue to the parallel oxidation with copper cyanide; the impact is particularly significant for highcopper and thiocyanate concentrations when targeting a low WAD cyanide concentration.

REFERENCES

1. J. Lorsch, Process and environmental chemistry of cyanidation, Degussa-AG, Frankfurt amMain, 2001.

2. T. Mudder, M. Botz, A, Smith, The cyanide compendium, [CD-ROM] Mining Journal BooksLtd, London, 2001.

3. G. Borbely, E. Devuyst, V. Ettel, M. Mosoiu, K. Schitka, Cyanide removal from aqueousstreams, US Patent 4,537,686 (INCO Limited), 1985.

4. R. Ferguson, H. Walker, Cyanide Destruction Process, Canadian Patent 1,183,617 (HeathSteele Mines Limited), 1985.

5. G. Robbins, Historical development of the INCO SO2/Air cyanide destruction process, CIMBulletin, September 1996, pp 63-69.

6. P. Breuer, C. Sutcliffe and R. Meakin, Comparison of Industrial Cyanide DestructionProcesses, XXV International Mineral Processing Congress (IMPC) 2010 Proceedings, TheAustralasian Institute of Mining and Metallurgy, Publication Series No 7/2010, pp 1483-1493.

7. K. Vant Riet, Review of measuring methods and results in nonviscous gas-liquid masstransfer in stirred vessels, Industrial Engineering and Chemical Process Design andDevelopments, 18 (3):357-364, 1979.

8. CSIRO, A new jet aeration nozzle, [Online]. Available from: http://www.csiro.au/multimedia/pf5i.html [Accessed: 14 April 2011].

9. O. Mathre, Destruction of Cyanides in Aqueous Solutions, US Patent 3,617,567 (E I du Pontde Nemours and Company), 1969.

10. J. Zumbrunn, Destruction of Dissolved Cyanides, US Patent 3,510,424 (Liquide Air), 1970.11. A. Harrison, Process for Detoxification, US Patent 4,417,987 (Interox Chemicals Limited)

1983.12. A.Griffiths, R. Norcross, G. Scherer, F. Merz, S. Gos Process for the Treatment of Effluent

Containing Cyanide and Toxic Metals Using Hydrogen Peroxide and Trimercaptotriazine, USPatent 4,822,496 (Degussa Aktiengesellschaft), 1989.

13. H. Castrantas, M. Fagan, Detoxification of Aqueous Cyanide Solutions, US Patent 5,137,642(FMC Corporation), 1992.

14. Y. Luo, K. Kustin and I. Epstein, Kinetics and mechanism of H2O2decomposition catalyzed byCu

2+in alkaline solution, Inorganic Chemistry 27: 2489-2496, 1988.

15. J. Beattie and G. Polyblank, Copper-catalysed oxidation of cyanide by peroxide in alkalineaqueous solution, Australian Journal of Chemistry, 48:861-868, 1995.

16. DuPont, DuPont Oxone Monopersulfate Compound general technical attributes, 2008.[online]. Available from: http://www2.dupont.com/Oxone/en_US/assets/downloads/K20102_Oxone_Technical_Bulletin.pdf [Accessed: 14 April 2011].

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