Process for detoxification

When aqueous solutions of a metal cyanide complex are detoxified with hydrogen peroxide, a residue of cyanide remains, which in the case of nickel for example may be present as Ni(CN).sub.2. In a process according to the present invention the detoxification of aqueous alkaline solutions of a metal cyanide complex is carried out using a restricted amount of certain complexing agents together with the hydrogen peroxide. A particularly suitable example of such complexing agents is ethylenediaminotetraacetic acid. It is especially preferred to use the complexing agent in a mole ratio to metal of from 0.8:1 to 1.5:1, together with hydrogen peroxide in a mole ratio to cyanide of from 2:1 to 3:1. The process is well suited to the treatment of concentrated solutions of cyanide, in many cases at least 4000 ppm, for example those resulting from the stripping of nickel plating.

The present invention relates to a process for the detoxification of 
aqueous solutions of cyanide and, in particular, to solutions of metal 
cyanide complexes by reaction with hydrogen peroxide. 
The principle of using hydrogen peroxide to detoxify aqueous solutions of 
inorganic cyanides has been known for some time. Thus, for example, J. 
Broucek indicated in Koroce a Ochrana Materialu, 1962, pp 95-96 that 
sodium cyanide could be oxidised, and he thought that it was mainly to 
ammonia and that turbidity due to copper hydroxide could be readily 
measured. The use of a catalyst such as copper sulphate to accelerate 
cyanide destruction using hydrogen peroxide was disclosed by F. Oehme in 
1966 in Tech. Eau (Brussels) No 237. Naturally it will be recognised that 
in practice the hydrogen peroxide used commercially is stabilised against 
decomposition during storage by addition of a small amount generally in 
the region of 50 to 100 ppm of conventional stabilisers, for example those 
referred to in French patent specification No. 1 564 915. Herein ppm 
indicates g/m.sup.3 unless otherwise stated. 
Although the above-mentioned methods are comparatively effective against 
most simple inorganic cyanides, when they are applied to the 
detoxification of metal cyanide complexes a relatively high proportion of 
the cyanide remains, despite the addition of levels of copper catalyst 
disclosed in the literature to be adequate for accelerating cyanide 
detoxification. As a result of the present investigations which had as 
their objective the lowering of the residual cyanide level from metal 
cyanide complexes it is now believed that the detoxification using the 
methods outlined above quite possibly terminates with the formation of a 
normal metal cyanide salt, for example in the case of nickel, 
Ni(CN).sub.2. However, it will be understood that the present invention is 
not based specifically upon any particular theory or explanation. 
It has been found in the course of the present investigations that the 
extent of detoxification of metal cyanide complexes can be improved by 
carrying out the detoxification in the presence of a restricted amount of 
selected complexing agents. Where other than a restricted amount of the 
appropriate complexing agents or inappropriate complexing agents are 
employed, less or no substantial improvement was detectable.

According to the present invention, there is provided a process for the 
detoxification of an aqueous alkaline solution of a metal cyanide complex 
by introduction therein of hydrogen peroxide in the presence of a 
complexing agent in a mole ratio to the metal of from 0.5:1 to 3:1 which 
can form with nickel cyanide a mixed ligand complex having a stability 
constant as herein defined of not greater than 9.5. 
The suitability of the complexing agent is determined by reference to the 
value of the stability constant of the nickel mixed ligand complex. The 
term "stability constant" as used herein means the constant .beta..sub.12, 
expressed a logarithm to the base of 10 for the dissociation reaction 
EQU NiL(CN).sub.2.sup.n- .revreaction.NiL.sup.2-n +2CN.sup.- 
wherein L represents a molecule of the complexing agent and 
##EQU1## 
The stability constant, together with a method for its measurement for such 
complexes is given in an article entitled "Determination of Stability 
Constants of Mixed Ligand Complexes by Kinetic Method: Part II--Mixed 
Ligand complexes of Aminocarboxylates & Cyanide Ion with Nickel(II)" by K 
Kumar and P C Nigam and published in the Indian Journal of Chemistry 
Volume 18A, September 1979 pp 247-251. 
One suitable class of complexing agents comprises aminocarboxylic acids 
which form metal complexes having a stability constant as defined herein 
of not more than 9.5. It will be recognised that under alkaline conditions 
the aminocarboxylic acids will be wholely or partly in carboxylate form 
and can be introduced in acid form or in the form of a salt, usually a 
soluble salt. In many cases, the complexing agent is selected within the 
class of aminoacetic acids/salts. 
EXAMPLES OF SUITABLE COMPLEXING AGENTS 
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Stability 
Constant Of 
Example Abbreviation 
Complex 
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Nitrilotriacetic acid 
NTA 7.72 
Diethylenetriaminopentaacetic 
DTPA 3.36 
acid 
Ethylenediaminediacetic acid 
EDDA 8.18 
1,2-diaminopropanetetraacetic 
1,2 PDTA 4.55 
acid 
1,3-diamino-2-hydroxypropane- 
HPDTA 6.18 
tetraacetic acid 
Ethylenediaminetetraacetic acid 
EDTA below 0 
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It will be recognised that the stability constant can have a negative 
value. It is preferable to select complexing agents which form complexes 
that have as low a value as possible of the stability constant. In 
consequence, preferred complexing agents form complexes, the stability 
constant for which is below 8 such as DTPA and especially preferred 
complexing agents form complexes the stability constant for which has a 
negative value, such as EDTA. 
EXAMPLES OF UNSUITABLE COMPLEXING AGENTS 
______________________________________ 
Stability 
Constant Of 
Example Abbreviation 
Complex 
______________________________________ 
1,2-diaminocyclohexane- 
CDTA na 
tetraacetic acid 
Iminodiacetic acid 
IDA 11.21 
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When unsuitable complexing agents are used, such as those above-exemplified 
or others such as ethylene-diaminotetramethylenephosphonic acid, the 
residual cyanide content is markedly higher, often an order of magnitude 
or greater than when using the selected complexing agents according to the 
present invention under practical working conditions. 
In the course of the investigation, it was found that the extent of 
detoxification increases as the mole ratio of complexing agent to metal is 
increased until a mole ratio of 1:1 is attained and thereafter gradually 
decreases. Thus, it is preferable for the mole ratio of complexing 
agent:metal to be selected within the range of 0.6:1 to 2:1 and 
especially from 0.8:1 to 1.5:1. In many instances the complexing 
agent:metal mole ratio will be 1:1 (+/-5%). 
It is preferable to employ at least a stoichiometric amount of hydrogen 
peroxide, i.e. a mole ratio of H.sub.2 O.sub.2 :CN.sup.- of at least 1:1. 
In practice, the mole ratio is usually selected within the range of from 
1.5:1 to 5:1, and in many instances the mole ratio is selected in the 
range of 2:1 to 3:1. Although it is possible to introduce the hydrogen 
peroxide in a single addition, more effective utilisation of the hydrogen 
peroxide can be achieved under many circumstances if it is introduced 
either in a plurality of incremental additions, for example 3 or 4 times 
at intervals during the reaction period often substantially regularly, or 
by progressively introducing the reagent throughout all or the greater 
part of the reaction period, such as 50-80% or by some combination in 
which, for example, part of the hydrogen peroxide is introduced initially 
and the remainder is introduced progressively. 
Although it is possible to carry out the reaction in the absence of added 
copper catalyst, it is preferable that a catalytic amount of a copper salt 
is employed, taking advantage of the fact that the copper catalyst is 
believed to act by forming a copper cyanide complex which reacts with the 
hydrogen peroxide. In fact, it has been found that the acceleration of the 
rate increased by the addition of a copper salt to other metal cyanide 
complexes complements the enhanced extent of detoxification obtained by 
addition of the complexing agent. Indeed the addition of the suitable 
complexing agents enhances even the detoxification of copper cyanide 
complexes. As a consequence, it is desirable for the cyanide solutions to 
contain at least 10 ppm copper. If insufficient copper is present it can 
be added, preferably to a concentration of from 50 to 200 ppm copper, 
advantageously in the form of a readily water soluble cupric salt, such as 
cupric sulphate, cupric chloride, cupric nitrate or cupric acetate. Higher 
concentrations of copper, even in the region of 1000 to 10000 ppm can be 
present in this process, but it will be recognised that if the detoxified 
solution is to be discharged into a watercourse, subsequent removal of the 
copper for example using an ion exchange resin should preferably be 
carried out. 
In practice, it is desirable to maintain the aqueous alkaline cyanide 
solution at a pH greater than that at which hydrogen cyanide gas is 
evolved, so that a pH of at least pH 9 and often in the range of from pH 
9.5 to pH 12 is normally selected. A convenient pH is often at around pH 
10. 
The pH of the alkaline solution can be controlled by conventional means, 
e.g. by linking a pH electrode to control the introduction of an aqueous 
alkali such as sodium hydroxide solution, should the solution fall below a 
predetermined pH, such as pH 9.5. 
The process of the present invention is well suited to the treatment of 
metal cyanides complexes in which the complex has the formula 
M(CN).sub.4.sup.2- where M represents the metal. Specific examples of 
such complexes include nickel and the group 1B (i.e. copper, silver, gold) 
cyanide complexes. 
The process of the present invention is especially suitable for the 
detoxification of comparatively concentrated solutions of the metal 
cyanide. Such solutions often have a content of at least 4000 ppm cyanide 
and for example at least 1000 or 2000 ppm nickel or other metal such as 
copper. In consequence, the present invention is of particular value in 
the detoxification of nickel stripping solution, which in many cases have 
a nickel content in the range of from 3000 to 7000 ppm and the cyanide 
content in the range of from 1000 to 4000 ppm. Such stripping solutions 
often contain very high concentrations, such as 25000 to 30000 ppm, of 
sodium meta-dinitrobenzenesulphonate which renders impractical the 
detoxification of such solutions with hypochlorite or by electrolysis. The 
sulphonate appears to have no detrimental effect, in a process according 
to the present invention. Copper cyanide solutions which can be treated 
effectively in the present invention typically contain copper in the range 
of 2000 to 10000 ppm and cyanide 4000 to 20000 ppm. 
It will be readily understood, that although the process of the present 
invention is extremely well suited for treating concentrations of cyanide 
within the aforementioned ranges, higher concentrations can be treated 
merely by employing the appropriate mole ratio of complexing agent to 
metal and hydrogen peroxide to cyanide, but in preference also dividing 
the hydrogen peroxide addition into a slightly larger number of 
incremental additions. Naturally, the present invention copes with lower 
concentrations of metal and cyanide of e.g. 500 to 1000 ppm nickel or 
copper without undue difficulty. 
The reaction between the metal cyanide and hydrogen peroxide is exothermic. 
It is preferable to maintain the cyanide solution after introduction of 
the hydrogen peroxide at a temperature of not greater than 70.degree. C., 
and especially from 60.degree. to 70.degree. C. and it will be recognised 
that cooling of the solution may be needed. It will be recognised that the 
extent of cooling is dependent on the initial concentration of cyanide in 
the solution and on the mode of introduction of the hydrogen peroxide, 
i.e. whether it is single stage or incremental or progressive. The 
reaction time depends at least partly upon the residual concentration of 
cyanide in the solution that can be tolerated by the user of the invention 
process. Broadly speaking, the extent of detoxification continues to 
increase with the effluxion of time. It is desirable to employ a reaction 
period of at least 30 minutes, and in practice the reaction period is 
often at least 60 minutes, in many cases being up to 150 minutes. It will 
be understood, though, that a reaction period in excess of 150 minutes can 
be employed, if desired, and can continue to show an increase in 
detoxification, especially where the complexing agent used forms a metal 
complex which has a stability constant at the upper end of the range, for 
example in the range of from 4 to 9.5, such as NTA or EDDA. 
Having described the invention in general terms, specific embodiments will 
be described hereinafter more fully by way of example only. 
In the Examples and comparisons, the concentrations of nickel were 
determined in most cases by a standard atomic absorption 
spectrophotometric technique and in the remaining measurements by 
difference between the total cyanide and free cyanide measurements. The 
cyanide concentrations were measured as described in "Standard Methods for 
the Examination of Water and Waste Water" (13th Edition) published by the 
American Public Health Association et al. Comparisons are designated by 
the prefix C. 
In each of the Examples and comparisons, C1 to C15, for the detoxification 
of nickel cyanide, and C16 to 19 for the detoxification of copper cyanide, 
a sample (100-200 ml) of solution having the characteristics shown in the 
Table below was introduced into a reaction vessel into which thereafter 
first, the specified complexing agent was introduced in a mole ratio to 
nickel specified in the Table as mole ratio CA:Ni or CA:Cu respectively 
and then copper sulphate was introduced into the nickel cyanide solutions, 
but not into the copper cyanide solutions, to give a concentration as Cu 
of 100 ppm, except in Example 6 and comparison C1, where no copper was 
present. In each case, the pH of the solution was then adjusted to and 
maintained at pH 10 by introduction of caustic soda as required, during 
the incremental addition of hydrogen peroxide to a total amount of the 
latter expressed in the Table as a mole ratio of H.sub.2 O.sub.2 :CN. The 
hydrogen peroxide was introduced in the form of the standard 35% by weight 
aqueous solution commercially available from Interox Chemicals Limited. In 
all the Examples and comparisons except Example 9 the hydrogen peroxide 
solution was added in three equal portions at 30 minute intervals and in 
Example 9 the hydrogen peroxide was added in four equal portions, again at 
30 minute intervals. The residual cyanide content of the solution was 
measured 30 minutes after the final portion of hydrogen peroxide had been 
introduced and the result is expressed in the Table both as measured and 
as a percentage of the initial total cyanide content. 
The complexing agents are referred to by their initials and the key to them 
will be found in the text hereinbefore, with the exception of EDTMPA in 
comparison C4 which is ethylenediaminetetramethylenephosphonic acid, 
hexapotassium salt. In each of comparisons C5 and C7 and Examples 6, 8, 9, 
10 and 11, the cyanide solution contained sodium 
meta-dinitrobenzenesulphonate at an initial concentration in excess of 
20000 ppm. 
THE TABLE 
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Residual 
CN 
Initial Mole Mole % of 
Concentration 
Complex- Ratio Ratio ini- 
Ex Metal CN ing CA: H.sub.2 O.sub.2 
Conc tial 
No ppm ppm Agent Metal :CN ppm Conc 
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Nickel CA:Ni 
C1 7900 14200 -- -- 1.5 12200 
85.9 
C2 7900 14200 -- -- 1.5 9400 66.2 
3 7900 14200 EDTA 1:1 1.5 760 5.3 
C4 7900 16760 EDTMPA 1:1 1.5 4430 26.4 
C5 3100 18100 -- -- 2.5 850 4.5 
6 3600 14100 EDTA 1:1 2.5 620 4.4 
C7 3600 14100 EDTA 0.28:1 
2.5 550 3.9 
8 2600 14100 EDTA 1:1 2.5 20 0.14 
9 3400 39240 EDTA 1:1 2.5 20 0.05 
10 7700 24250 EDTA 1:1 2.5 20 0.08 
11 3200 22170 EDTA 1:1 2.5 13 0.06 
12 7900 16760 NTA 1:1 2.5 35 0.2 
13 7900 16760 DTPA 1:1 2.5 105 0.6 
C14 7900 16760 IDA 1:1 2.5 1910 11.4 
C15 7900 16760 CDTA 1:1 2.5 5000 30 
Copper CA:Cu 
C16 2800 5000 -- -- 2.5 329 6.6 
C17 8500 15000 -- -- 2.5 2883 19.2 
18 2800 5000 EDTA 1:1 2.5 11 0.2 
19 8500 15000 EDTA 1:1 2.5 23 0.15 
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From the Table above, it will be observed that a marked improvement was 
obtained using complexing agents NTA, DTPA, and EDTA in a mole ratio to 
metal according to the present invention but that if the mole ratio to 
metal was outside the range specified, then the improvement in residual 
cyanide was markedly less. Similarly, it will be observed that complexing 
agents IDA, CDTA and EDTMPA produced markedly inferior results to using 
complexing agents selected according to the present invention. Thirdly, it 
will be observed that the residual content of cyanide in solution was not 
affected to a great extent by the initial ratio of metal to total cyanide 
in the solution. Fourthly, it will be seen that the use of the complexing 
agents without copper catalyst agents for detoxifying nickel cyanide, 
could provide a result of similar quality to that provided by a copper 
catalyst, but that addition of a copper catalyst led to an accelerated 
removal of cyanide from solution. Finally it can be seen that a mole ratio 
of hydrogen peroxide to cyanide of 2.5 to 1 gave better detoxification 
than if a mole ratio of 1.5 to 1 was used.