EP0756582A1

EP0756582A1 - Destroying metal cyanide complexes by combined chemical oxidation and electrolysis

Info

Publication number: EP0756582A1
Application number: EP95918031A
Authority: EP
Inventors: Claude Gavach; Françoise PERSIN; Jean-Christophe Aubanel; Jacques Thauront; Thierry Cohen
Original assignee: EMC Service
Current assignee: EMC Service
Priority date: 1994-04-19
Filing date: 1995-04-19
Publication date: 1997-02-05
Also published as: CA2188274A1; AU2412295A; WO1995028358A1; FR2718731A1; FR2718731B1

Abstract

A method for destroying metal cyanide complexes in a cyanide-containing effluent having a variable composition, wherein the free anions are chemically oxidised (20) and the metals are reduced at the cathode by electrodeposition (60). The chemical oxidation and electrolysis steps can be carried out simultaneously or one after the other. A three-compartment membrane electrolysis device (62) comprising an anode, two cation exchange membranes and a cathode is also disclosed. The cation exchange membranes used are commercially available membranes having good strength properties and good selectivity in alkaline media. Moreover, they are mechanically supported by frames on a polymeric support, e.g. a polyvinyl chloride (PVC) support.

Description

DESTRUCTION OF METAL CYANO COMPLEXES BY THE COMBINATION OF CHEMICAL OXIDATION AND ELECTROLYSIS.

The invention relates to a process for the oxidation of oxidizable free anionic species and for the electrolytic separation of metals in solution in an effluent in the form of metal complexes. More particularly, the invention relates to a process for destroying cyano-metallic complexes in a cyanide effluent of variable composition. The process includes the chemical oxidation of free anions and the reduction of metals at the cathode by electro¬ deposition. The chemical oxidation and electrolysis steps can be carried out simultaneously or successively. The invention also relates to a membrane electrolysis device comprising three compartments, namely an anode compartment, a cathode compartment and a median compartment separating the anode and cathode compartments and formed by two cation or anion exchange membranes as appropriate.

State of the art

The treatment of industrial effluents comprising metal complexes constitutes a major environmental problem. Indeed, the release of metals into rivers, both in ionic form and in complexed form, can have serious consequences on the surrounding ecosystems.

However, considerable amounts of this type of industrial effluent ¹ are produced each year. French industry annually produces several thousand tonnes of effluents contaminated by metal complexes and sometimes very anions toxic such as free cyanide ions. On the other hand, most of the effluent treatment methods of this type are either very expensive, or not very effective or produce other types of contaminants such as metal sludge.

For example, metallic cyanide solutions are of particular interest, especially because of their very high toxicity. Cyanides are among the most dangerous compounds for living things. In an acidic environment, they form hydrocyanic acid (HCN), lethal gas, which binds in particular to the ferric ion of mitochondrial cytochrome oxidase and thus blocks cellular respiration causing general anorexia.

Despite their very high toxicity (a concentration of 0.05 mg / 1 leads to the death of a trout in five days), cyanides are widely used in the industry for their complexing and reducing properties. Cyanide consumption in France amounts to several thousand tonnes, most of which is reserved for metal treatment operations: quenching and electroplating. As a result, surface and heat treatment workshops, gold mines, coal mills and the steel industries, the photographic industry as well as the chemical industry are all susceptible to _. have effluents contaminated with cyanide, although their cyanide content is very variable: from a few mg / 1 to several tens of g / 1. These effluents contain either only free cyanides, or mixtures of free cyanides and complexed cyanides, most often metallic.

Most effluent decontamination treatments containing anions or metallic impurities are based on chemical or electrochemical oxidation processes. Other routes less conventional exist but remain very little developed industrially.

In the case of cyanides, most of the decyanidation treatments are based on processes of chemical or electrochemical oxidation of cyanides to cyanates. In general, if the destruction of alkaline (free) cyanides is easy and total, that of the cyano-metallic complexes is much more difficult and depends on the stability of the complexes.

Among the chemical oxidation processes of cyanides, that with sodium hypochlorite (bleach) is the most used. It takes place in two stages, the first forming toxic cyanogen chloride (CNC1), and the second consisting in its hydrolysis to cyanate.

Although this process is economical, it has a number of drawbacks. The concentration of the baths must not exceed 2 g / 1 of free cyanides because the reaction being exothermic, there would be the risk of giving off cyanogen chloride, significant quantities of metallic sludge are produced, oxidation is not selective and in addition there is an increase in salinity.

To reduce the risk of cyanogen chloride release, it is possible to replace bleach with oxygenated water which is a non-polluting oxidant and which does not increase the salinity of the treated solutions. This type of process makes it possible to oxidize metallic and organic cyanides in the presence of copper sulphate, with however a significant formation of sludge. In addition, free cyanides are hardly destroyed.

Decyanidation processes with Caro acid and dipersulfates have been developed to avoid these drawbacks. But costs high supply levels for barely better performance limit their industrial uses, especially since these super oxidants are extremely sensitive explosives.

While ozonation can be advantageous for weakly concentrated and low-volume pollution flows, it is not economically viable to treat effluents loaded with cyanides and oxidizable organic species, due to the lower energy yields of ozone production at 5%.

An alternative to the chemical oxidation processes of free cyanides and associated metal complexes consists in using electrochemical methods. In terms of decyanidation, the methods suggested so far are either effective for solutions concentrated in cyanide but not very effective for low contents, or even effective for treating solutions weakly concentrated but not economically viable for concentrated solutions.

Reference may be made to the following documents which describe methods for the electrolytic decontamination of effluents containing cyanides: "Treatment of Cyanide astes by Electrolysis" (MR Hilliε, "Transactions of the Institute of Metal Finishing", 1975, vol. 53 p. 65 to 73), "Electrochemical Destruction of Complex Cyanide" (TC Tan et al. Chem. Eng. Commun, vol. 38, pages 125 to 133), "Le Cyaniseur" (JP Divin - Surface treatment n ^c 124 , September 1973, pages 35 to 62).

There are two main types of electrochemical decyanidation. The first type consists in carrying out the anodic oxidation of the cyanides directly. This type of process, very efficient for concentrated cyanide solutions, is no longer effective for contents lower than 2 g / 1. The second type of electrochemical decyanidation process performs oxidation from oxidizing agents formed in situ. Very efficient for treating weakly concentrated solutions, this process is no longer economically viable for solutions more concentrated in cyanide and oxidizable species. In addition, the precipitation of heavy metals at the cathode disrupts the proper functioning of the process.

The decyanidation process by acidification and combustion marketed under the name of "Cyan-Cat" process, very economical for solutions of concentrations higher than 5 g / 1 is not very efficient in the presence of metals. In addition, it presents a high "potential risk" requiring numerous and very reliable controls.

It is also possible to treat cyanides by incineration. This is an interesting route insofar as the ultimate form of the waste is obtained, but which remains limited to the treatment of solutions with little metal content. In addition, the risk of emitting toxic fumes or solid particles such as cadmium oxide limits these applications. Finally, high operating and installation costs make it a relatively expensive process.

Description of the invention

The invention relates to a process for the chemical oxidation of oxidizable free anionic species, more particularly of free cyanides and of cathodic deposition of metals present in solution in an effluent in the form of metal complexes. The method includes the chemical oxidation of the free anions of the effluent and the electrolysis at the cathode of the metal complexes, the electrolysis causing the deposition of metals at the cathode and the release into Effluent from the anions contained in the metal complexes. The reduction in the number of free ligand anions facilitates the deposition of metals and leads to the release of new free anions which will in turn be destroyed by a second oxidation and so on so as to repeat the oxidation-electrolysis cycle if necessary. The method comprises the use of means preventing the movement towards the anode of the anions released in the effluent during electrolysis. Preferably, the chemical oxidation of the free anions is carried out before and after the electrolysis of the metal complexes.

The effluents whose treatment by implementing the method of the invention is envisaged include one or more types of metal complexes, for example nickel, zinc or copper. More . in particular, the process of the invention is implemented during the treatment of effluents comprising cyanide ions and complexes of metallic cyanides, preferably complexes of copper cyanide.

Still according to the process of the invention, the chemical oxidation of the easily oxidizable free anions and the electrolysis of the metal complexes can be carried out successively or batchwise, in which case the effluent can be filtered after the chemical oxidation free anions so as to eliminate most of the solid precipitates before the electrolysis of the metal complexes. Alternatively, the chemical oxidation of the easily oxidizable free anions and the electrolysis of the metal complexes can be carried out simultaneously or continuously. This second approach constitutes one of the preferred embodiments of the invention.

The invention also relates to a device for the chemical oxidation of oxidizable free anionic species and for the electrolytic separation of metals into solution in an effluent in the form of metal complexes. The device comprises a reaction vessel comprising an anode and a cathode on which the metals are deposited by electrolysis of the metal complexes of the effluent. An anion or cation exchange membrane separates the reaction vessel into two compartments, either an anode compartment comprising the anode and intended to receive the electrolyte solution, and a cathode compartment comprising the cathode and intended to receive the effluent. In situations where the chemical oxidation of the free anions and the electrolysis of the metal complexes are carried out continuously, the chemical oxidation of the free anions and the electrolysis of the metal complexes are carried out simultaneously in the cathode compartment.

The cation exchange membrane prevents movement towards the anode of the anions released in the effluent during the electrolysis of the metal complexes of the effluent.

The device of the invention also comprises membrane supports connected to two opposite positions of the interior wall of the reaction tank so as to maintain the cation exchange membrane in position in the reaction tank by means of leaktight seals formed between the membrane and supports. An effluent outlet orifice is connected to the cathode compartment of the reaction vessel so as to evacuate the effluent once the chemical oxidation of the free anions and the electrolytic separation of the metals in solution have been carried out.

In certain situations, more particularly in situations where the effluent to be treated comprises cyanide ions, the device of the invention comprises two cation exchange membranes separating the reaction vessel into three compartments, either an anode compartment comprising the anode and intended to receive an electrolyte solution, a middle compartment, between the anode and the cathode, intended for receiving an electrolyte solution and a cathode compartment comprising the cathode and intended for receive the effluent.

The device can also include an oxidation reactor connected to an effluent inlet orifice and in which the chemical oxidation of the free anions in solution in the effluent is carried out. The oxidation reactor connected to the reaction tank so as to transfer the effluent thereon once the chemical oxidation of the free anions is complete.

The process and the device of the present invention allow the improvement of the chemical oxidation performance of free anions in solution in an effluent in addition to reducing the rejection of metallic sludges usually generated by this type of process. More particularly, the method and the device of the invention can be adapted to all types of cyanide effluents whatever their concentrations. Still in the case of cyanides, the implementation of the process of the invention using a device comprising two cation exchange membranes makes it possible to avoid the formation of very toxic hydrocyanic acid.

The rest of the presentation will be made with reference to the attached figures, the captions of which follow:

- Figure 1: schematic representation of a preferred embodiment of the device of the invention provided with oxidation reactors separate from the reaction vessel;

- Figure 2: schematic representation of the reaction tank of the device of Figure 1 and the migration of ions in solution in the effluent during 1 electrolysis; - Figure 3: schematic representation of the supports holding cation exchange membranes in position in a preferred embodiment of a reaction vessel of the device of the invention;

- Figure 4: schematic representation of a preferred embodiment of the device of the invention in which the oxidation and electrolysis are carried out in the reaction tank;

FIG. 5: evolution of the copper concentration in the cathode compartment of a preferred embodiment of a reaction vessel of the device of the invention as a function of time during the electrolysis of an effluent containing metallic complexes of copper,

FIG. 6: evolution of the copper concentrations in the cathode compartment of a preferred embodiment of a reaction tank of the device of the invention as a function of the electrolysis time of an effluent containing metallic complexes of copper,

- Figure 7: evolution of the oxidation-reduction potential during electrolysis of an effluent containing metallic complexes of copper.

Detailed description of preferred embodiments of 1 • invention

The preferred embodiments of the invention described below relate to a method and a device for treating a metallic cyanide effluent. It is important to note that other types of metal complexes which can be separated by electrolysis can also be treated using the method and the device of the invention. The modifications of experimental conditions required such as for example the determination of the redox potential for stopping the electrolysis can be derived by the person skilled in the art, from the teachings of the invention.

Device for discontinuous treatment of a metallic cyanide effluent

Referring now to Figure 1, the device of the invention, generally designated by the reference numeral 10 mainly comprises an oxidation system 20 surrounded by solid lines connected to an electrolysis system 60 surrounded by dotted lines.

The oxidation system 20 comprises two oxidation reactors 30 and 40 connected to an oxidant reservoir 21 via a conduit 24. A valve 26 directs the oxidant either to the reactor 30 via the line 22, or towards the reactor 40 via the line 27. Agitators 32 and 42 are respectively placed in the oxidation reactors 30 and 40 so as to be able to agitate the effluent to be oxidized. The oxidation reactors 30 and 40 have an effluent outlet orifice, respectively the orifices 34 and 44 from which the effluent can either be conveyed to a downstream treatment via the conduits 36 and 46, and the valve 35, or be routed to the cathode compartment 70, through the conduits 38 and 48 and the valve 37, to be subjected to electrolytic treatment.

The electrolysis system 60 comprises as main element a reaction tank 62 including a cathode compartment 70, a median compartment 90 and an anode compartment 100 each comprising an outlet orifice, namely the orifices 72, 92 and 102 respectively.

Referring now to Figure 2 in which the reaction vessel 62 is illustrated in more detail, the cathode compartment 70 comprises a cathode 74 and is separated from the middle compartment 90 by a cation exchange membrane 76. Several types of cathodes can be used in the context of the present invention. By way of non-limiting example, the use of cathodes made up of a metal plate such as stainless steel or copper cathodes or else platinized titanium plates as well as large area cathodes, preferably made of polyurethane foam, preferably covered with a layer of nickel, can be considered. As regards the cation exchange membrane that can be used in the context of the present invention, it is important, in addition to its physical durability, that it has good selectivity in an alkaline medium.

The middle compartment 90 is generally defined by the space not occupied by the cathode 70 and anode 100 compartments. The limits of the middle compartment 90 are therefore the walls of the reaction vessel 60 as well as the cation exchange membranes 76 and 106.

The anode compartment 100 comprises an anode 104. The anodes used in the context of the present invention are preferably of the DSA type, either platinum titanium or ruthenium titanium anodes. A cation exchange membrane 106 of a similar nature to the cation exchange membrane 76 constitutes the wall adjacent to the middle compartment 90. It is important to note that in the reaction vessel 62 illustrated in FIG. 2, the cathode 70 and anode 100 compartments are separated by two ion exchange membranes 76 and 106. This type of construction, preferable when the effluent to be treated contains cyanide ions, is not necessary for other types of effluents. In the case of cyanide effluents, the median compartment acts as a buffer to avoid the presence of cyanide ions at the anode and the reaction which would follow by which toxic gases of cyanhidric acid type would be released. The invention is however not limited to this type of construction since in several situations, it will suffice simply to have a reaction tank comprising an anode compartment and a cathode compartment separated by a single cation or anion exchange membrane. Consequently, although the device as well as the method according to the invention are described according to a three-compartment system, the present invention also includes devices with two compartments having only one cation or membrane exchange membrane. anions.

Referring again to FIG. 1, the outlet orifice 72 of the cathode compartment 70 is connected to the oxidation reactors 30 and 40 via a conduit 73 connected to a valve 75 making it possible to orient the treated solution either to the oxidation reactor 30 via the conduit 74 or to the oxidation reactor 40 via the conduit 79.

With regard to the central compartment, the outlet orifice 92 is connected directly to an electrolyte buffer tank 110 via the conduit 94. The electrolyte buffer tank 110 is also connected to the inlet orifice 96 from the middle compartment 90 through the conduit 98.

The anode compartment 100 is connected via the outlet orifice 102 to the electrolyte buffer tank 120 via the conduit 108. The electrolyte buffer tank 120 is also connected to the anode compartment 100 by the through conduit 99. The electrolyte buffer tanks 110 and 120 are mainly used to regulate and control the pH and the concentration of electrolytes in the anode tanks 100 and median 90 during electrolysis and, for the electrolyte buffer tank 120 , to introduce in the electrolysis system 60 of the required quantities of electrolyte or else to evacuate the solutions present in the anode compartments 100 and median 90 once the electrolysis reaction has ended. As illustrated in FIG. 1, the electrolyte buffer tank 120 comprises, in its upper part, an overflow orifice 122 connected to the conduit 124 through which the water can be evacuated from the electrolysis system 60.

Located above the reaction tank 62, an electrolyte storage tank 130 is connected to the electrolyte buffer tank 120 via the conduit 132. The electrolyte is therefore introduced indirectly into the anode compartment 100 by l 'through the water make-up tank 120.

Referring now to Figure 3 which shows a plan view of a portion of the reaction vessel 62, the position of the cation exchange membrane 76 is demonstrated in more detail. The cation exchange membrane 76 is held in position in the reaction tank 62 by means of membrane supports 67 and 68 with an average thickness of approximately 0.25 mm and consisting of a polymer film 64 on polymeric fabric 66. The membrane supports 67 and 68 are fixed to the walls of the reaction tank 62. The cation exchange membrane 76 is installed in the tank 68 and placed between the membrane supports 67 and 68. The membrane supports 67 and 68 are then welded to the membrane cation exchanger 76 so as to form tight seals between the cation exchange membrane 76 and the membrane supports 67 and 68. The same type of assembly is carried out for the cation exchange membrane 106.

The materials used for the preparation of the membrane supports should preferably be flexible and have good thermal properties. As an example, silicone or chloride polyvinyl can be mentioned. As far as the structure of the membrane supports is concerned, it can be modified insofar as the membrane is securely fixed and the seal between the supports and the membrane is ensured.

Batch treatment of a metallic cyanide effluent

Cyanides and metallic ions form cyano-metallic complexes whose formation balance is as follows:

mCN- _fibers + M ^{+ n>} M (CN) _m ^{(n "m)}

Depending on the stability of these complexes and the excess of CN ions ^" , the above equilibrium is more or less shifted to the right and the reduction of metal ions is more or less difficult. It is almost zero if the ratio of CN / M ^{n +} concentrations are high. We must therefore first consider the oxidation of CN ions ^"in order to reduce the CN / M ^{n +} ratio, and allow a more effective reduction of metal ions.

The discontinuous treatment of a metallic cyanide effluent, which mainly consists in carrying out the oxidation of free cyanide ions, followed by the electrolysis at the cathode of the metal complexes and the oxidation of the cyanide ions released during the electrolysis is mainly implemented through the device of Figure 1. However, the device of Figure 4, which will be described in detail below, can also be used for discontinuous treatment of the effluent.

a) Oxidation of free cyanides.

At t = 0, that is to say in the raw effluent, the species present are: CN ^" , and M (CN) ^m (nm). 15

The addition of an oxidizing agent, for example bleach, leads to the oxidation of free cyanides according to reaction (1): CN ^" + CIO ^" + H ₂ 0> CNC1 + 2 OH ^" > CN0 ^" + H ₂ 0 + C1 ^"

At the end of the oxidation, the species present in solution are therefore CNO ^' and M (CN) _m (nm).

Referring again to FIG. 1, the effluent to be treated 50 which contains free CN ions as well as cyano-metallic complexes, is normally conveyed either to the oxidation reactor 30 via the conduits 51, 52 and 54 and towards the oxidation reactor 40 via the conduits 51, 52 and 45 and the valve 56. The two oxidation tanks 30 and 40 perform the same function but the two tanks work alternately, namely when the 'one of the tanks is in the cyanide oxidation phase, the other is in the metal reduction phase.

Although it is difficult to carry out the determination of free cyanides in the presence of metals, the effluent will generally have a concentration in total cyanides which can vary between 2 to 50 g / 1 and a metallic concentration which will vary from 2 to 30 g / 1 in the case of copper. The metal concentration will usually be much lower for metals other than copper, such as nickel and zinc.

A sufficient quantity of oxidizing agent to effect the oxidation of the free cyanides initially present in the effluent 50 is introduced in small fractions into the oxidation reactor 30 via the conduits 51, 52 and 54. Several types of oxidants can be used. One of the preferred oxidants seems to be bleach in concentrations varying between 20 and 50 chlorometric degrees. The effluent is optionally agitated with a more or less high speed during the oxidation reaction by means of the agitators 32 and 42 for a period of time sufficient to oxidize most of the free cyanide ions. The oxidation conditions (that is to say quantity and concentration of the oxidant, stirring period) are adjusted so that the redox potential of the treated effluent is brought to a level allowing its electrolysis. For example, in the case of a cyanide effluent, the redox potential is preferably around - 0 to - 500 mV, more particularly around - 100 to 0 mV once the oxidation is complete.

Once the oxidation is complete, the effluent is evacuated from the oxidation reactor 30 via the outlet orifice 34 and conveyed to the cathode compartment 70 via the conduit 38 and the valve 37. It is note that the pH of effluent 50 should normally be between 11 and 14 once the oxidation reaction is complete. Indeed, a pH below 11 would cause the formation of unwanted protons during the electrolysis of the effluent 50 in the reaction tank 62. It should also be noted that in certain situations, it may be necessary to carry out a filtration of the effluent once the oxidation has been completed. However, this step of filtering the oxidized effluent is not always required. Indeed, it is possible to avoid the formation of precipitates by adjusting the concentration of oxidant added to the effluent.

The necessary quantity of the appropriate electrolyte solution is also introduced into the anode compartment 100. Various types of electrolytes can be used in the context of the invention. Sodium hydroxide has been shown to be particularly effective. The important factor is the maintenance of a sufficient electrolyte concentration to obtain an appropriate conductivity on either side of the cation exchange membranes. More particularly, a concentration of hydroxide 17 sodium of approximately O.IN in the middle compartment and IN in the anode compartment has been shown to be effective.

b) Electrolysis of the metal ions of the oxidized solution.

At the cathode, we observe the reduction of metal ions:

(m + l) M (CN) _m (nm) + ne ^" > M + mM (CN) ^" _] ¹

At the anode a gas evolution appears:

4 OH ^' > 0 ₂ + 2H ₂ 0 + 4 e ^"

Referring to FIG. 2, the electrolysis of the metal complexes of the effluent is carried out in the cathode compartment 70, by means of an electric current applied to the cathode 74. The electrolysis can be carried out at a density of current varying between 5 and 50 mA / cm ² , more particularly at a current density of approximately 10 mA / cm ² . It is not essential that the electrolysis is carried out at a fixed current density. For example, it is possible to use current density steps when metal cathodes are used. In this situation, the electrolysis can start at a low current density, to avoid the formation of dendrites on the cathode, and then be followed by an increase in the current density. In the situation where a polyurethane foam electrode is used, the use of bearings is not necessary. As previously mentioned, the important factor is the maintenance of good conductivity of the medium in order to prolong the life of the membranes. This results in a low current density on the membranes and high on the electrode. It is also possible to carry out the electrolysis of metal complexes using pulsed electrodes insofar as the solution is sufficiently conductive.

As the electrolysis of the metal complexes takes place, the proportion of metallic cyanides increases as well as the concentration of free cyanides. The presence of the cation exchange membranes 76 and 106 makes it possible to maintain these newly formed cyanide ions in the cathode compartment 70 while avoiding the migration of the anionic cyanide complexes in the middle compartment 90.

The use of two cation exchange membranes seems desirable at least in the case of metallic cyanide effluents, to avoid the formation of hydrocyanic acid. Indeed, although the first cation exchange membrane 76 retains most of the anions inside the cathode compartment 70, small quantities of anions pass through the first cation exchange membrane 76 and are found in the middle compartment 90. Without the middle compartment 90, there would be a risk of formation of hydrocyanic acid at the anode 104.

When the value of the CN ratio ^"/ metal is too high in the cathode compartment 70, one electrolysis becomes difficult and must be stopped. In fact, the efficiency of the process seems to be managed by controlling the electrochemical potential. The value of potential for stopping electrolysis depends on the nature of the metal. The more complex and difficult to reduce the metal, the lower the redox potential that can be reached before stopping electrolysis. in the case of cyanide effluents, the redox potential of the effluent when the electrolysis is stopped is around - 100 mV to - 400 mV.

The value of the redox potential not to be exceeded after oxidation also depends on the metal. he must be such that the formation of precipitates, generally hydroxides of metallic cyanide, can be minimized. Particular attention to the redox potential is required in the case of mixtures of metals. Indeed, if the oxidation is too deep, metal ions could be released and form precipitates, except for zinc (zincate). Control of the redox potentials of the effluent during the various treatment stages therefore constitutes an important aspect of the present invention.

Once the electrolysis has been stopped, the effluent in the cathode compartment 70 is then transferred to the oxidation reactor 30 or to the oxidation reactor 40 to chemically oxidize the cyanide ions released during the electrolysis. The intensity of the oxidation is a function of the redox potential of the effluent when the electrolysis is stopped.

Once this oxidation has been carried out, the treatment of the effluent is generally completed. In certain situations, which depend mainly on the initial concentration of metals in the effluent, it may prove necessary to repeat the treatment cycle described above a certain number of times, namely the oxidation of the effluent, followed by its electrolysis and supplemented by oxidation of the cyanide ions released during electrolysis. The device of the invention has great flexibility in this regard. At the end of the treatment, when a metal cathode is used, the concentration of total cyanides should then be between 0.5 and 1 mg / 1. The concentration will be lower if a polyurethane foam cathode, preferably covered with a layer of nickel, is used. Device for continuous treatment of a metallic cyanide effluent

Referring to FIG. 4, the continuous treatment device, generally designated by the reference numeral 150, comprises an electrolysis system practically similar to the electrolysis system of the discontinuous oxidation device of FIG. 1. However , the continuous treatment device 150 does not include a separate oxidation system as is the case in FIG. 1, the effluent 50 and the oxidant being in the tank 21 being directly introduced into the cathode compartment 70 through the conduits 152 for the oxidant and 154 for the effluent and the valve 156.

The stirring system 32 that was found in FIG. 1 for the batch oxidation device 10 can also be present in the cathode compartment 70 of the continuous treatment device 150, although in the case of a treatment in continuous agitation is not absolutely necessary.

The cation exchange membranes 76 and 116 are fixed to the reaction tank 62 as described above and demonstrated in FIG. 2. The tightness of the membrane supports is particularly important here in order to avoid the formation of hydrocyanic acid which the displacement of the CN ions ^" towards the anode. It is important to note that the membrane supports as illustrated in FIG. 3 were manufactured according to the specific use of the reaction vessel. Indeed, the adaptation of these decyanidation membranes in the process had not been considered previously. so it took into account the special ^• constraints imposed by the reaction medium. Continuous treatment of a metallic cyanide effluent

The physico-chemical parameters to be controlled remain the same as those followed during the batch processing, namely pH between 11 and 14, redox potential preferable between - 200 and - 400 mV and temperatures preferably below 40 ° C. However, it is important in this continuous step to avoid excess oxidant (in particular chlorine) during the oxidation step with bleach so that the metal extraction yields are the most high possible.

Referring briefly to FIG. 4, the effluent to be treated 50 is discharged directly into the cathode compartment 70 via the conduit 154 and the valve 156. A quantity of oxidizing agent contained in the reservoir 21, sufficient to carry out the oxidation of the free cyanides initially present in the effluent 50 is also introduced into the cathode compartment 70 via the conduit 152 and the valve 156.

An electric current is then immediately applied to the cathode ^" 74 (see FIG. 2) and the electrolysis of the metal complexes present in the effluent is carried out. However, unlike the batch process, the equilibrium of the reaction does not shift towards the free forms because the CN- ions released during the electrolysis are immediately oxidized by the oxidant present in the reaction tank 70. This immediate oxidation has the effect of substantially increasing the speed of the treatment of the effluent, while eliminating the need for a second oxidation step once the electrolysis is completed.

The oxidation of the anions and the electrolysis of the metal complex are therefore carried out simultaneously in the anode compartment 70 until the desired quantity of total cyanides is obtained, ie between 0.5 and 1 mg / 1.

The yields of metal deposits at the cathode are slightly less important when the treatment is carried out continuously, but this method nevertheless obtains an unexpected success, taking into account the problems of precipitation of the solids which could be envisaged in such a situation.

Example 1

Batch processing of a semi-industrial solution

a) Solution to be treated

It is a semi-industrial solution with respective copper and total cyanide concentrations of 16 and 55 g / l, prepared from an industrial bath with little cyanide content to which potassium cyanide has been added.

b) Oxidation of the solution

The oxidation device includes:

1 glass tank

1 Tacussel XR 600 saturated calomel electrode

1 simple metal electrode in Tacussel gold

XM 500 1 milli-voltmeter Tacussel PHN 81 1 plotter Tacussel Ecoscript 1 portable pH-meter Hanna instruments HI 8424

100 ml of the solution to be treated are oxidized by slowly pouring 160 ml of chlorine recovery bleach at 65 °. A precipitate of copper hydroxide is thus formed. When the potential has a negative value ^• close to zero and stable, the solution is introduced into the cathode compartment of the electro-electrodializer of the electro-lysis device described below.

c) Electrolysis of the solution

The electrolysis device consists of the following elements:

1 current generator: Fountain, regulated supply U / I-0-80 V

0-5 A FTN 8050

1 plexiglass electro-electrodialyzer comprising:

3 compartments 8 mm thick 2 electrode holders 2 platinum titanium 2 MEC electrodes, active surface

20 cm ² 6 silicone seals 1.5 mm thick 3 Asti pumps

2 Metrix

1 portable pH meter

1 redox potential measuring device.

Nature of the solutions circulating in the electro-electrodialysis cell:

Cathode compartment: 260 ml of oxidized solution

Central compartment: 250 ml of potash

0.1 N

Anode compartment: 2 1 of potash 1 N

An electrical current of 0.2 A intensity (10mA / cm ² ) is applied. The electrolysis is stopped when the value of the redox potential of the cathode solution is between -100 and -200 mV / DHW. The evolution of the copper concentration in the cathode compartment during the electrolysis is shown in FIG. 5. During the tests, 3 ml are taken from each compartment to follow the evolution of the concentrations. The cyanides are assayed by precipitation of silver nitrate in the presence of potassium iodide as an indicator for the end of the assay. That of metals is carried out by atomic absorption spectrophotometry at the wavelength 324.8 mm for copper. The device is a "Spectra 2000 varian".

Due to the intense brown coloration, the concentration of free cyanides can only be determined at the instant before oxidation and at the end of electrolysis (clear solution). However, it is possible at the end of the tests to measure the total cyanides by distillation. For this, 100 ml of the solution to be analyzed are acidified so as to obtain the hydrocyanic acid then entrained with steam in an alkaline solution where it is trapped. The sample is then assayed by the free cyanide assay method.

Cyanide composition of the initial solution:

[CN ^" ] free = 30 g / 1 or 3 g in 100 ml

(initial volume)

[CN ^" ] total = 55 g / 1 or 5.5 g in 100 ml

Cyanide composition of the treated solution (Oxidation + Electrolysis)

[CN ^" ] free = 1.5 g / 1 or 390 mg in 260 ml

(final volume)

[CN ^" ] total = 2.8 g / 1 or 728 mg in 260 ml

Cathodic deposit mass (copper): 1 g

To complete the treatment and neutralize the remaining 13% cyanides, a second oxidation followed by electrolysis is necessary. The solution is then removed from the cathode compartment and placed in a glass beaker where 40 ml of bleach of recovery at 65 ^th chlorometric is slowly added. When the potential is stabilized, close to zero and negative, the solution is again placed in the electro-electrodialyzer.

In this case, due to the low copper concentration, the platinum titanium plate constituting the cathode is replaced by a larger surface electrode of polyurethane foam covered with a layer of nickel.

The evolution of the copper concentration in the cathode compartment is shown in Figure 6. As noted above, the content of free cyanides cannot be determined.

At the end of the test, after adding 5 ml of bleach, the concentration of free cyanide is less than 0.1 g / l and 280 mg of copper are recovered on the cathode. The concentrations of copper and free cyanides in the central and anode compartments remained practically zero throughout the duration of the tests.

The evolution of the redox potential of the treated solution is shown in Figure 7.

Claims

1. A method of chemical oxidation of oxidizable free anionic species and of electrolytic separation of metals in solution in an effluent in the form of metal complexes, said method comprising: chemical oxidation of free anions of said effluent;

Electrolysis of the metal complexes of said effluent, said electrolysis causing the deposition of metals at the cathode and the release into the effluent of the anions contained in said metal complexes, and

- The oxidation of the anions released in said effluent during electrolysis, said process comprising the use of means preventing the movement towards the anode of the anions released in effluent during electrolysis.

2. Method according to claim 1, characterized in that the chemical oxidation of the free anions is carried out before and after 1 electrolysis of the metal complexes.

3. Method according to claim 1 or 2, characterized in that the chemical oxidation of the anions is carried out by means of bleach.

4. Method according to one of claims 1 to 3, characterized in that the electrolysis of metal complexes is carried out by means of a device comprising:

- a reaction vessel (62) comprising an anode (104), and a cathode (74) at which metals are accumulated following the electrolysis of the metal complexes;

- a cation exchange membrane (76) separating the reaction vessel (62) into two compartments, namely an anode compartment (100) comprising the anode (104) and intended to receive a electrolysis solution and a cathode compartment (70) comprising the cathode (74) and intended to receive the effluent (50), said membrane (76) preventing the migration towards the anode of the anions released in the effluent during 1 electrolysis of the metal complexes of the effluent and the deposition of the metals at the cathode (74);

- membrane supports (67, 68) connected to two opposite positions of the inner wall of the reaction tank (62) so as to keep the membrane (76) in position in the tank (60) by means of tight seals formed between the membrane (76) and the supports (67, 68).

5. Method according to claim 3, characterized in that the said membrane supports (67, 68) consist of polymer films (64) on polymeric fabric (66).

6. Method according to one of claims 1 to

5, characterized in that the metal complex comprises nickel, zinc or copper.

7. Method according to one of claims 1 to

6, characterized in that the free anions comprise cyanide ions and the metal complexes are metal cyanide complexes, more particularly copper cyanide complexes.

8. Method according to claim 5 or 6, characterized in that when the effluent to. to treat comprises cyanides and complexes of metallic cyanide, more particularly complexes of copper cyanide, the anode (104) and the cathode (74) are separated by the installation of two cation exchange membranes (76, 106) forming a median compartment (90) between the two electrodes making it possible to avoid the displacement of cyanide ions towards the anode (104) and the formation of hydrocyanic acid.

9. Method according to one of claims 1 to 8, characterized in that the chemical oxidation of free anions and electrolysis at the cathode of the metal complexes of the effluent are carried out successively.

10. Method according to claim 9, characterized in that the effluent is filtered after the chemical oxidation of the free anions so as to remove the major part of the solid precipitates before electrolysis at the cathode of the metal complexes.

11. Method according to one of claims 1 to 8, characterized in that the chemical oxidation of the free complexes in solution and the electrolysis at the cathode of the metal complexes are carried out simultaneously.

12. Method according to one of claims 1 to 11, characterized in that 1 * alkaline electrolyte used during 1 electrolysis of metal complexes is chosen from the group comprising sodium hydroxide O.IN to IN and potash O.IN to IN.

13. Method according to claim 12, characterized in that when two cation exchange membranes (76, 106) are used, the concentration of the electrolyte may be lower in the compartment m dian formed by the two cation exchange membranes ( 76, 106) than in the anode compartment.

14. Method according to one of claims 1 to 13, characterized in that the cathode is a metal plate, preferably a platinum-plated titanium plate, a stainless steel plate or a copper plate or a nickel plate, a large area electrode, preferably made of polyurethane foam, preferably covered with a layer of nickel, and the anode is a DSA type anode, preferably a platinum titanium anode or a ruthenium titanium anode.

15. Method according to one of claims 8 to 14, characterized in that the potential The electrochemical value of the effluent is maintained between - 200 and - 400 mV in order to avoid the formation of precipitates of metal hydroxides.

16. Method according to one of claims 7 to 14, characterized in that the current density applied during electrolysis at the cathode varies between 5 and 50 mA / cm ² , preferably between 8 and 12 mA / cm ² .

17. Method according to one of claims 7 to 16, characterized in that the electrolysis reaction temperature is below about 40 ° C.

18. Method according to one of claims 7 to 17, characterized in that the pH of the effluent before the electrolysis varies between 11 and 14.

19. Device (10) for the chemical oxidation of oxidizable free anionic species and for the electrolytic separation of metals in solution in an effluent in the form of metal complexes, said device comprising:

- a reaction tank (62) comprising an anode (104) and a cathode (74) to which metals are deposited by electrolysis of the metal complexes of the effluent,

- a cation exchange membrane (76) separating the reaction vessel (62) into two compartments, namely an anode compartment (100) comprising the anode (104) and intended to receive an electrolyte solution and a cathode compartment

(70) comprising the cathode (74) and intended to receive the effluent (50), and in which the chemical oxidation of the free anions can be carried out, said membrane

(76) preventing the migration of anionic complexes from the cathode compartment (70) to the median compartment (90).

- membrane supports (67, 68) connected to two opposite positions of the inner wall of the reaction tank (62) so as to keep the cation exchange membrane (76) in position in the reaction tank (62) by means of tight seals formed between the membrane (76) and the supports (67, 68), and

- An effluent outlet (72) connected to the cathode compartment (70) of the reaction tank (62) so as to evacuate the effluent once the electrolytic separation of the metals in solution has been carried out.

20. Device according to claim 19, characterized in that it comprises two cation exchange membranes (76, 106) separating the reaction vessel (62) into three compartments, namely an anode compartment (100) comprising the anode (104) and intended to receive an electrolyte solution, a median compartment (90) between the anode (104) and the cathode (74) intended to receive an electrolyte solution and a cathode compartment (70 ₎ comprising the cathode (74 ) and intended to receive the effluent (50), and in which the chemical oxidation of the free anions can be carried out, said membrane (76) preventing the migration of the anionic complexes from the cathode compartment (70) to the median compartment (90) and said membrane (106) serving as an additional barrier to the possible passage of free anions towards the anode capable of causing the formation of toxic gas such as hydrocyanic acid.

21. Device according to one of claims 19 or 20, characterized in that the membrane supports (67, 68) consist of polymeric films (64) on polymeric fabrics (66).

22. Device according to claim 21, characterized in that the polymer films (64) and the polymeric fabrics (66) consist of silicone or polyvinyl chloride preferably having a thickness of approximately 0.25 mm.

23. Device according to one of claims 20 to 22, characterized in that the cathode (74) is a metal plate, preferably a platinum-plated titanium plate, a stainless steel plate or a copper plate or a nickel plate or a large surface electrode, preferably made of polyurethane foam, preferably covered with a layer of nickel, and the anode (104) is a DSA type anode, preferably a platinum titanium anode or a ruthenium titanium anode .

24. Device according to claim 20, wherein the concentration of one electrolyte is lower in the middle compartment formed by the two cation exchange membranes (76, 106) than in the anode compartment.

25. Device according to one of claims 20 to 24, characterized in that it comprises an oxidation reactor (30) connected to an effluent intake pipe (54) and in which the chemical oxidation of free anions in solution in said effluent are carried out, said oxidation reactor (30) being in communication with said reaction tank (62) via a conduit (38) so as to be able to transfer the effluent (50) ) once the chemical oxidation of the free anions is complete.

26. Device according to claim 25, characterized in that it comprises a filter between the oxidation reactor (30) and the reaction tank (62).

27. Device according to one of claims 20 to 26, characterized in that one electrolyte is sodium hydroxide or potassium hydroxide.

28. Device according to claim 20, characterized in that the middle compartment and the anode compartment comprise potassium hydroxide from 0.1N to IN or sodium hydroxide 0.1N to IN.