FUNDAMENTAL INVESTIGATIONS OF THE SO2/AIR, PEROXIDE AND CARO’S ACID
CYANIDE DESTRUCTION PROCESSES
By
Paul Breuer, Coby Jeffery and Rebecca Meakin
Parker CRC for Integrated Hydrometallurgy Solutions
CSIRO Minerals Down Under National Research Flagship
CSIRO Process Science and Engineering
Australia
Presenter and Corresponding Author
Paul Breuer
[email protected]
ABSTRACT
Fundamental investigations have been conducted on the SO2/air, peroxide and Caro’s acid cyanide
destruction processes to establish a more detailed understanding of the reaction mechanisms and
kinetics. 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 CSTR’s, the DO concentration provides an indicator to the residual
oxygen capacity available in the process; zero DO in the last reactor indicates insufficient
oxygen addition for the rate of cyanide and sulfite addition.
3. The addition of hydrogen peroxide to the SO2/air process to potentially increase the cyanide
oxidation is not beneficial and is not recommended as sulfite is preferential oxidised over
cyanide.
The copper catalysed peroxide destruction of cyanide investigations found that the solution
composition (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 cyanide
oxidation.
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 Caro’s acid
addition or within the localised zone where Caro’s acid is added, and can significantly reduce
the cyanide oxidation efficiency.
Thiosulfate is detrimental to all the cyanide destruction processes due to the oxidation reaction
having a high oxidant demand. Thiocyanate has an impact only on the Caro’s acid process (high
oxidant demand), particularly with high copper and thiocyanate concentrations. Most metal
cyanides precipitate from solution once the copper cyanide has been destroyed, however zinc
precipitates before copper whilst mercury is not precipitated by any of the destruction processes.
1
INTRODUCTION
Commercially the INCO process (SO2/air) and Caro’s acid are commonly employed to treat
cyanidation tails from gold plants to achieve regulatory requirements and the ICMI Code compliance
of less than 50 mg/L weak acid dissociable (WAD) cyanide for the discharge of cyanidation tails into
a tailings storage facility (TSF). Peroxide, which is less effective on slurries, is often used to treat
the return water from the TSF to lower cyanide levels that would otherwise impact plant
performance. 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 findings
from fundamental investigations of these three cyanide destruction processes currently being
utilised by the gold industry; SO2/air, hydrogen peroxide and Caro’s acid.
SO2/AIR PROCESS
There are two patented SO2/air processes (3, 4), of which the INCO process is more commonly
adopted and used for the treatment of slurries. The sulfur dioxide dissolves into solution forming
sulfite at the pH’s typically adopted in the destruction process:
SO 2  H2 O  SO 32-  2H
(1)
The sulfite ion is the reactant in the process and thus sodium sulfite (Na2SO3) or sodium
metabisulfite (Na2S2O5) can also be used as a source of sulfite. The process is based upon
conversion of cyanide (including cyanides weakly complexed with metal ions) to cyanate using
sulfite 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:
Cu
CN-  SO 32-  O 2 
 OCN-  SO 24-
(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 transfer
of oxygen. Typical reaction times in order to achieve the required oxygen mass transfer vary from
about 30 minutes to 2 hours. Iron complexed cyanides are reduced to the ferrous state and
precipitated with copper, nickel or zinc as insoluble metal-iron-cyanide complexes. Residual metals
liberated from the WAD cyanide complexes are precipitated as their hydroxides. The process does
not 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:
2SO 32-  O 2  2SO 24-
(3)
Reaction mechanisms
An indicator that the SO2/Air process and reaction chemistry is not straight forward was evident by
INCO’s need to become involved in the process engineering as initial installations in the mid 1980’s
experienced poor field performance, losses in process kinetics, were unable to maintain continuous
operation and had difficulties restarting the process after upsets (5). The experience and learning’s
gained by INCO from these initial installations provided knowledge of the process and equipment
limitations which guided future testwork, process design and engineering of the cyanide destruction
reactor. Because of this in-house development, knowledge and experience, little fundamental
understanding of the reaction chemistry has been publicly available until recently (6). Included in
the 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 the
basic chemistry of sulfite and cyanide solutions and the oxidation of these by dissolved oxygen is
required. Figure 1 shows that for the experimental setup used by the authors to study the SO2/Air
system, the rate of sulfite oxidation without cyanide or copper present proceeds via Reaction 3 at an
appreciable rate with all the sulfite oxidised in a little over one hour; this reaction is known to occur
via a free radical mechanism. In comparison, sulfite is shown not to be oxidised in the presence of
cyanide ions, suggesting cyanide ions may act as a free radical scavenger; notably the cyanide ions
2
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 concurrent
oxidation of cyanide which is discussed further below. This suggests a reaction mechanism in
which the copper acts as a catalyst for the oxidation of sulfite by oxygen; most probably by
facilitating electron transfer from the sulfite to oxygen.
Sulfite Concentration (mM)
8
6
No cyanide or Cu
Cyanide, no Cu
4
Cyanide, 0.8 mM Cu
2
0
0
50
100
150
200
Time (min)
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 sulfate
formation) and that of cyanide (cyanate formation) occur in parallel until essentially all the cyanide
has 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 to
be oxidised (as observed in Figure 1 for the sulfite solution without cyanide or copper present). The
dissolved oxygen concentration is zero through-out and increases only once all the sulfite has been
oxidised. This indicates that the reaction rate is limited by the oxygen uptake rate (discussed
further below). The close match in final cyanate concentration with the initial cyanide concentration
indicates that cyanate is the major cyanide oxidation product.
10
Concentration (mM)
8
Sulf ite
Sulf ate
Cyanate
6
Copper
Free Cyanide
4
2
0
0
50
100
150
200
Time (min)
Figure 2: Reactant and product concentrations for SO2/air oxidation of cyanide
(4 mM NaCN, 0.8 mM CuSO4, 8 mM Na2SO3, pH 9, air sparged).
3
Notable also in Figure 2 is that the initial cyanate concentration (sample taken after just two
minutes) is greater than expected from the addition of copper sulfate (Reaction 4) and the initial
oxygen in solution. The sulfite and sulfate concentrations at this point also indicate an initial rapid
reaction. The mechanism for this initial rapid reaction is unclear, however, the extent is dependent
on conditions and reagent addition.
2Cu 2  7CN   2OH   2Cu(CN) 32  CNO   H 2 O
(4)
Presence of free cyanide
Notable for the results shown in Figure 2 is that with the presence of copper there was no
measureable free cyanide in the two minute and subsequent samples. In comparison, Figure 3
shows that when free cyanide is present (higher initial cyanide concentration) no oxidation of
cyanide occurs subsequent to some initial rapid oxidation. However, the oxidation of sulfite is
observed to occur despite the presence of free cyanide which was observed to stop sulfite oxidation
in the absence of copper (Figure 1). This suggests that copper cyanide catalyses the oxidation of
sulfite and that this mechanism does not propagate to involve the oxidation of cyanide when free
cyanide is present, but does so when there is no free cyanide.
18
16
Concentration (mM)
14
Sulf ite
12
Sulf ate
10
Cyanate
Copper
8
Free Cyanide
6
4
2
0
0
50
100
150
200
Time (min)
Figure 3: Effect of free cyanide on reactant and product concentrations for SO2/air oxidation
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 a
copper cyanide solution is that in a CSTR process it may appear from a WAD cyanide
measurement that the process is working, though perhaps not as effectively as expected. This is
shown in Figure 4 where with 8 mM NaCN in the feed the presence of free cyanide stops the
oxidation of cyanide other than the initial rapid oxidation upon addition of the sulfite in Tank 1; the
greater reduction in WAD CN than generated cyanate is due to volatilisation of HCN. In
comparison, the results for 4 mM NaCN in the feed shows greater oxidation of cyanide for the same
sulfite and oxygen addition to the reactors. As the transfer of oxygen is limited and thus oxidation
incomplete in tank 1, further oxidation is observed in tank 2. With no oxidation of cyanide occurring
in the presence of free cyanide, a greater decrease in the sulfite concentration occurs across the
reactors (not shown) due to Reaction 3 rather than Reaction 2 taking place.
4
8
WAD CN - 4 mM NaCN
Concentration (mM)
7
Cyanate - 4 mM NaCN
WAD CN - 8 mM NaCN
6
Cyanate - 8 mM NaCN
5
4
3
2
1
0
Feed
Tank 1
Tank 2
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 free
cyanide 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 cyanide
oxidation to cyanate in the first reactor is greater than the addition of free cyanide; as such the
resultant mixed solution within the CSTR will have no free cyanide. In starting up the process, or
where 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 cyanate
and complexes the free cyanide according to Reaction 4. Excess copper addition results in the
formation of some copper di-cyanide complex. Where the free cyanide addition to a CSTR is
greater than the maximum rate of cyanide oxidation (dependent on oxygen uptake rate, which is
discussed below), then the continual addition of copper sulfate is required to sustain oxidation of
cyanide.
Oxygen uptake
In practise, the SO2/Air cyanide destruction process is typically conducted in one (or more) CSTR’s
where the oxygen uptake is sufficient for the required cyanide destruction. The rate of oxygen
uptake for the system limits the quantity of WAD cyanide that can be destroyed. This is largely
dependent 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. The
interfacial transfer area is a function of the gas bubble size and the gas hold-up in the CSTR, thus
decreasing the gas bubble size or increasing the gas holdup will increase the transfer area. Studies
have shown that increasing the power per unit volume applied to the agitation breaks gas bubbles
into smaller bubbles and increases the interfacial area (7). Special nozzles also offer the ability to
introduce very fine gas bubbles into the reactor, such as the CSIRO developed GL Nozzle
technology (8). Increasing the reactor volume and/or the number of reactors, whilst maintaining the
bubble 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 reactants
entering 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 oxygen
uptake and thus the dissolved oxygen concentration is zero and the oxidation of sulfite and cyanide
is incomplete. The DO concentration thus indicates the extent of additional capacity available in the
system or whether the system capacity has been exceeded (zero DO). Similarly, the oxygen
5
uptake would be exceeded at high cyanide concentrations (with proportional increase in sulfite) in
attempting to achieve the same residual cyanide concentration.
8
25
7
Concentration (mM)
5
15
Cyanate
4
Sulf ite
3
10
Sulf ate
Copper
2
DO
5
DO Concentration (mg/L)
20
6
1
0
0
0
10
20
30
Residence Time (min)
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, O2 sparged).
Sulfite to cyanide stoichiometry
Shown in Figure 6 is that the oxidation of cyanide, with adequate oxygen uptake, closely follows the
stoichiometry of Reaction 2 when the stoichiometric addition of sulfite to cyanide is 1 or less. When
excess 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 cyanide
is all that is required to achieve essentially complete destruction of the WAD cyanide. Excess
sulfite addition is a wasted cost for the process and can result in the oxygen uptake being exceeded
by the reactant demand, in which case the DO is zero and yet all the cyanide has been oxidised.
30
12
Cyanate
25
Sulf ite
Sulf ate
8
20
Copper
DO
6
15
4
10
2
5
DO Concentration (mg/L)
Concentration (mM)
10
0
0
0
2
4
6
8
10
12
Sulfite Concentration (mM)
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, O2 sparged, 20 min RT).
6
Oxidation of other species
Cyanidation of gold ores containing sulfide minerals results in the formation of thiocyanate from the
reaction of cyanide with metal sulfides. Consistent with the literature, investigations have found no
conditions 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, which
typically 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 be
oxidised to high oxidation state oxy-sulfur species such as sulfite and sulfate), though significant
concentrations can remain after cyanidation and enter the cyanide destruction process. The
presence of thiosulfate ions reduces the overall cyanide oxidation rate and in parallel thiosulfate is
oxidised to sulfate (6). The reason for the reduced cyanide oxidation rate is that the oxidation of
thiosulfate consumes oxygen (Equation 5) and thus competes for oxygen with the oxidation of
cyanide. Thus, a high thiosulfate concentration can significantly increase the oxygen requirement
and negatively impact on cyanide destruction.
S 2 O 32-  2O 2  2OH  2SO 24-  H2 O
(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 and
subsequent to copper precipitation. Only partial destruction/precipitation was observed for
cadmium and cobalt cyanide complexes, whilst mercury cyanide complexes essentially remained
dissolved in solution.
Peroxide assisted
The use of peroxide in conjunction with the INCO process has been trialled and used at some
operations, particularly where the process is limited by oxygen uptake. There are two possible
mechanisms 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.
30
35
No H2O2
30
No H2O2
20 mM H2O2 initially
Sulfite Concentration (mM)
Calculated WAD CN Concentration (mM)
To investigate these possible mechanisms, batch SO2/Air experiments limited by the oxygen uptake
were conducted with peroxide added initially or after 4 hours. These results are compared with no
addition of peroxide in Figure 7. Notably, the addition of peroxide initially had no beneficial effect on
the oxidation of cyanide, instead it rapidly oxidised sulfite to sulfate. The addition of peroxide after 4
hours also resulted in the rapid oxidation of the remaining sulfite to sulfate, along with a notable
decrease in the WAD cyanide concentration. The reason for the increased oxidation of cyanide is
attributed to oxidation of cyanide by the residual peroxide (remaining after reaction with the residual
sulfite), which is catalysed by copper (see following section). Addition of peroxide into the SO2/Air
process is thus not recommended, though addition subsequent to the SO2/Air process could be
used to further reduce the WAD CN concentration where the SO2/Air process capacity is exceeded.
20 mM H2O2 af ter 4 hrs
25
20
15
10
25
20 mM H2O2 initially
20 mM H2O2 af ter 4 hrs
20
15
10
5
5
0
0
0
50
100
150
200
250
300
0
350
50
100
150
200
250
Time (min)
Time (min)
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).
7
300
350
HYDROGEN PEROXIDE PROCESS
DuPont and Degussa have separately developed and patented several versions of the hydrogen
peroxide process for treating cyanide tailings solutions (9, 10, 11, 12, 13). The process has limited
application in slurries due to the high reagent consumption resulting from the reactions of peroxide
with solids in the slurry. The process is based upon oxidation of WAD cyanides to cyanate using
hydrogen peroxide in the presence of a soluble copper catalyst (not added if already present) to
increase the reaction rate. The overall reaction being:
Cu
CN-  H2 O 2 
 OCN-  H2 O
(6)
Reaction periods typically range from about 30 minutes to 3 hours depending upon the copper to
cyanide 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 WAD
cyanide increases with increasing copper, thus higher peroxide to cyanide addition is required to
achieve the same residual WAD cyanide at higher copper concentrations (1). The process
operates over a wide range of pH values, with the fastest rate reported to be at pH 10 (1). The
optimal pH for metals removal after cyanide destruction is reported as about 9.0 to 9.5. Iron
cyanides are precipitated as for the SO2/air process. Similarly, the process does not oxidise
thiocyanate 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 dissolved
solids is of concern in the treated water.
2H2O 2  2H2 O  O 2
(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 being
cyanate (Figure 8). In contrast to the SO2/air system, cyanide oxidation occurs even when there is
free cyanide present (i.e. when there are insufficient metal ions, such as copper, in solution to
complex all the cyanide). Once there is no free cyanide (after ~70 minutes) there continues to be
further 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 minutes
coinciding with the solution turning yellow (formation of a fine precipitate), and bubbles being
generated. This appears to correspond to the point where the depletion of cyanide is such that the
copper starts to precipitate (CN:Cu approaching 2:1) which catalyses the decomposition of H2O2. A
yellow intermediate has been previously reported in the decomposition of H2O2 by copper in alkaline
solution (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, the
decomposition of peroxide resulted in incomplete oxidation of cyanide. Doubling the concentration
of peroxide for the same conditions resulted in a much faster reaction rate with significantly more
oxygen generation and larger DO spike after only 15 minutes. More than half the copper was also
precipitated from solution. Previous work by the authors (6) covers in more detail the effect of
copper and peroxide concentration on the reaction kinetics. Beattie and Polyblank (15) have shown
that whilst free cyanide is present the rate of cyanide oxidation is first order with respect to peroxide
and copper concentration and independent of the free cyanide concentration. The independence
with cyanide concentration suggests either a mechanism in which the copper is involved in the rate
limiting 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 added
is shown in Figure 8 to be significantly greater than that from the addition of copper sulfate to the
cyanide solution (Reaction 4). The quantity of cyanate formed increases with copper concentration
and is significantly more than that which can be accounted for via Reaction 6 in the two minutes
before sampling and the three to five minutes taken for the sample to be injected in the HPLC for
analysis. This rapid initial oxidation upon mixing appears similar to that observed for the SO2/Air
process and further investigations are required to establish the mechanism for this.
8
4
0.5
H2O2
Cyanide
0.4
Copper
DO
0.3
2
0.2
1
0.1
0
DO Concentration (mM)
Concentration (mM)
Cyanate
3
0
0
50
100
150
200
Time (min)
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 copper
concentrations. With high copper concentrations (> 5 mM), however, the reaction rate is extremely
fast (order of minutes) and thus kinetic measurements are not possible with the analysis techniques
available. To study the reaction stoichiometry a series of batch experiments were thus conducted
with increasing peroxide addition (all other conditions being the same). An example of such data is
shown 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 cannot
be removed from solution with excess peroxide addition.
3.
The close match between the Total CN and the Initial Total CN less the cyanate formed
indicates 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 less
than the cyanide which is oxidised. The reason for this is unclear and requires further
investigation; 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, the
stoichiometry increases (approximately doubles) to oxidise the third cyanide complexed with
copper (overall stoichiometry of 1.5 to oxidise 15 mM free cyanide and 15 mM cyanide
complexed as the third cyanide with copper). This indicates decomposition of peroxide takes
place when the CN:Cu ratio is less than 3 and before copper precipitates. The stoichiometry
during copper precipitation appears to be around 1.5 (ratio levels out), which is surprisingly
better than for the proceeding period. Not surprisingly the stoichiometry increases with excess
addition of peroxide above which little further cyanide is oxidised and copper precipitated.
Similar batch experiments have also been conducted with different initial solution pH’s (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) and
importantly much greater peroxide addition was required to achieve the same cyanide oxidation
once precipitation occurred. This suggests that the pH influences the catalysis of peroxide
decomposition and thus lower pH’s are possibly better. This is consistent with the increased
stability of peroxide in clear solutions at lower pH’s; HO2- ion is formed at the higher pH’s which is
less stable than H2O2.
9
Free cyanide
Total CN
Initial Total CN - OCN
Copper
H2O2/CN ox
60
2.0
40
1.5
30
1.0
H2O2/CN ox
Concentration (mM)
50
20
0.5
10
0
0.0
0
20
40
60
80
100
H2O2 (mM)
Figure 9: Concentrations and ratio of H2O2/CN oxidised for batch experiments with varying
H2O2 addition. (45 mM NaCN, 15 mM CuCN, pH 8)
60
pH 11.3
Total CN (mM)
50
pH 10
pH 8
40
30
20
10
0
0
20
40
60
80
100
120
H2O2 (mM)
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 iron
oxides, found that the rate of decomposition of peroxide is highly dependent on the precipitate
composition. Thus, the presence of other metal ions can influence the precipitate composition and
therefore the catalysed rate of peroxide decomposition. Investigations conducted with plant
solutions have found variable results as a function of pH and can be contrary to those shown in
Figure 10, highlighting that the solution composition and pH can have a significant impact on the
ratio of peroxide addition to cyanide oxidation.
Oxidation of other species
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 H2O2 was added with the pH maintained at
10. Thiosulfate was found to be oxidised more readily than cyanide (2.4 mM thiosulfate destroyed
compared to 1.1 mM cyanate formed). From this it can be calculated, assuming the peroxide was
only consumed by cyanide or thiosulfate oxidation, that the stoichiometry of peroxide to thiosulfate
10
oxidation is ~2:1 respectively. With an additional 6 mM H2O2 added (12 mM in total), all the
thiosulfate was destroyed but only 3.3 mM cyanide had been oxidised. This indicates that the
presence of thiosulfate can have a significant impact on the peroxide process efficiency. No
oxidation of thiocyanate was evident in this test.
Investigations conducted with other metal cyanides present found that the metal cyanide complexes
of nickel, silver, cobalt and zinc are only partially destroyed and the metals precipitated from
solution in conjunction and subsequent to copper precipitation. Cadmium and mercury cyanide
complexes essentially remained dissolved in solution.
CARO’S ACID PROCESS
Caro’s 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 tailings
detoxification, particularly for tailings slurry. Caro’s acid is produced from concentrated hydrogen
peroxide and concentrated sulfuric acid (0.33-0.66 mole ratio of peroxide/sulfuric) in an exothermic
reaction:
H2 O 2  H2 SO 4  H2 SO 5  H2 O
(8)
The “hot” process yields 25 - 45 % Caro’s acid, whilst a “cold” process yields 70 - 80 % (1). Due to
its instability, Caro’s acid is produced on-site and used immediately for cyanide detoxification with
only minimal intermediate storage. The reaction of Caro’s acid with cyanide (and WAD cyanides)
does not require a catalyst such as copper as the reaction is rapid and typically complete within a
few minutes.
H2 SO 5  CN  OCN  H2 SO 4
(9)
Caro’s acid will also react with thiocyanate to some extent (Castrantas et al., 1995), but the reagent
consumption is high as indicated by the reaction stoichiometry:
4H2 SO 5  SCN  H2 O  OCN  5H2SO 4
(10)
Chemistry
The reaction of Caro’s acid with cyanide and copper cyanide is very rapid with complete oxidation to
cyanate within two minutes (6). It is because of this very rapid reaction rate, only small reaction
tanks are required or the reaction can be carried out in the transfer line to the tailings storage
facility. Further oxidation of cyanate by Caro’s acid is slow at pH 10, but increases in rate with
decreasing pH (1). Thus, avoiding a significant decrease in the pH is important to assure cyanide
destruction without the need to add excess Caro’s acid. Some cyanate oxidation also occurs even
at the higher pH’s due to the localised low pH upon the addition of Caro’s acid and the extremely
fast reaction kinetics (6).
Oxidation of other species
As both thiocyanate and thiosulfate are oxidised by Caro’s acid, an investigation was conducted
with step-wise addition of Caro’s acid to a copper cyanide solution also containing thiocyanate and
thiosulfate, to establish the selectivity by which Caro’s acid reacts with each of these species. The
results of this investigation are shown in Figure 11. Notably, the thiosulfate ions are initially
oxidised in parallel with free cyanide ions. The thiosulfate oxidation products were difficult to
quantify, particularly for sulfate due to the large addition of sulfate with the Caro’s acid in
comparison to that generated. Assuming that the cyanide and thiosulfate oxidation reactions are
the only reactions consuming Caro’s acid initially, the stoichiometry of Caro’s acid to thiosulfate
oxidation is calculated to be ~3:1. This indicates that the presence of thiosulfate will have a
significant impact on the Caro’s acid process efficiency.
Figure 11 shows that thiocyanate ions are not oxidised along with the thiosulfate and free cyanide
initially, but do undergo oxidation in parallel with the copper complexed cyanide; the rate of
thiocyanate oxidation being slower than the cyanide complexed with copper. In this case, the
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addition of excess Caro’s acid results in further oxidation of thiocyanate subsequent to copper
cyanide oxidation. Clearly, the presence of a high thiocyanate concentration will have a significant
impact of the Caro’s acid efficiency, particularly if low WAD cyanide concentrations are being
targeted.
20
Free cyanide
18
Total CN
16
Concentration (mM)
Thiocyanate
14
Copper
12
Thiosulfate
10
8
6
4
2
0
0
10
20
30
40
50
60
70
Caro's Acid (mM)
Figure 11: Cyanide oxidation with increasing Caro’s acid addition
in the presence of thiocyanate and thiosulfate.
(20 mM NaCN, 3.5 mM Cu, 10 mM KSCN, 1 mM Na2S2O3, pH 10)
Investigations conducted with other metal cyanide complexes present in the cyanide solution found
that the metal cyanide complexes of nickel, silver, cobalt, cadmium and zinc are essentially
destroyed 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 Caro’s acid, each have different
mechanisms 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, though
zinc 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 free
cyanide stops the oxidation of cyanide, but sulfite is still oxidised. The use of a CSTR, or a series of
CSTR’s, 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 the
reactor. The DO concentration provides an indicator to the residual capacity available in the
process; zero DO in the last reactor indicates insufficient oxygen addition for the rate of cyanide and
sulfite addition. Upsets to the process (for example the loss of sulfite or oxygen addition) can result
in the presence of free cyanide in the reactor which stops further cyanide oxidation. In such a case
the initial rapid oxidation of some cyanide on the mixing of sulfite in the first reactor can give the
appearance that the process is still operating OK, though the WAD cyanide destruction is less than
expected from the sulfite stoichiometry. The addition of copper sulfate is typically used to
destroy/complex the free cyanide and restart the process. The addition of hydrogen peroxide to the
SO2/air process to potentially increase the cyanide oxidation was not beneficial and is not
recommended as sulfite is preferential oxidised over cyanide. The addition of peroxide subsequent
to the SO2/Air process, however, can be beneficial, particularly if the process is limited by oxygen
addition (and sulfite is not added in excess of the oxygen addition).
Due to the catalysed decomposition of peroxide by solids, the copper catalysed peroxide
destruction 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. This
can significantly impact on the efficiency of the process, particularly with high copper and
thiocyanate concentrations and targeting a low WAD cyanide concentration.
Caros’ acid rapidly oxidises free cyanide, copper cyanides, thiosulfate and thiocyanate. The free
cyanide and thiosulfate are first rapidly oxidised, with the subsequent parallel oxidation of copper
cyanide and thiocyanate (though at a slower rate than the copper cyanide). The control of pH is
also 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 Caro’s acid is added and results in
reduced cyanide oxidation efficiency.
Thiosulfate is detrimental to the cyanide destruction processes as the oxidation of thiosulfate has a
significant oxidant requirement; thiosulfate likely to be an issue with the processing of gold ores
containing sulfide minerals, particularly when pre-oxidation is used to minimise the impact on
cyanide consumption during leaching. Thiocyanate only has an impact on the Caro’s acid process
due to the parallel oxidation with copper cyanide; the impact is particularly significant for high
copper and thiocyanate concentrations when targeting a low WAD cyanide concentration.
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