WO2001072641A1

WO2001072641A1 - Method and apparatus for reducing an electrolyte containing nitric acid

Info

Publication number: WO2001072641A1
Application number: PCT/EP2000/002725
Authority: WO
Inventors: André Henri Alain Joseph RAHIER; Eric Laurent Lionel Cantrel; Alfons Frans Fonteyne; Veerle Harriet Josepha Van Alsenoy; Anne SØRENSEN; Michel Camille Léon Ghislain KLEIN
Original assignee: Sck.Cen
Priority date: 2000-03-28
Filing date: 2000-03-28
Publication date: 2001-10-04
Also published as: AU2000238151A1; EP1284931A1

Abstract

Accumulation of nitrous acid is avoided and/or production of nitrogen oxides is suppressed during an electrolytic reduction of an aqueous electrolyte containing nitric acid, and optionally organic matter to be decomposed, by adding an efficient amount of a substance reactive with nitrous acid to the catholyte in such a way that the nitrous acid formed at the cathode is entirely oxidized there to nitric acid. This method can be performed in an apparatus comprising a first container for storing and feeding an oxidable species such as an organic matter, a second container for storing and feeding a nitric acid aqueous solution and further comprising an electrolyser having an anode compartment connected to the first container and a cathode compartment connected to the second container through a nitric acid aqueous solution feeding circuit, said anode compartment and said cathode compartment being separated by a membrane, and further comprising a device for providing a mixture of water and of a substance reactive with nitrous acid and a means for injecting the said mixture into the nitric acid aqueous solution feeding circuit before said nitric acid aqueous solution enters the cathode compartment.

Description

METHOD AND APPARATUS FOR REDUCING AN ELECTROLYTE CONTAINING NITRIC ACID

The present invention relates to an improved method for the reduction of an electrolyte containing nitric acid More specifically, the present invention relates to a method for avoiding accumulation of nitrous acid and/or for suppressing production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid The present invention further relates to an improved method for the decomposition of organic matter when added to an aqueous electrolyte containing nitric acid and silver ions Additionally the present invention relates to an apparatus designed for performing the said improved method

BACKGROUND OF THE INVENTION

Nitric acid is a powerful oxidizing agent, the use of which generally involves reduction of nitric acid and results in the production of nitrogen oxides which can create serious pollution problems The nitrogen oxides most commonly present in the gaseous effluents from a nitπc acid oxidizing system are the colorless nitric oxide NO and the brown nitrogen dioxide NO₂ Since nitric oxide reacts easily and almost quantitatively with atmospheric oxygen to mainly produce nitrogen peroxide NO₂ or N,O₄, with additional traces of N_:O₃ and N,O₅ due to thermodvnamic instability under standard conditions, this mixture of oxides being generally considered as a single NO toxic pollutant Generation of concentrated NO_x fumes where concentrated acid solutions containing nitric acid are used as an oxidizing agent necessitates control and treatment, such as by water scrubbing, to prevent it from becoming an intolerable health hazard Nowadays, many processes rely on electrochemical techniques, either to produce molecules or to oxidise or reduce selectively different chemical species for specific purposes, the choice of the electrolyte generally being the consequence of the said purposes When nitric acid appears as the best choice for the electrolyte, there arises the difficult question of its reduction in the cathode compartment of the electrolytic cell Indeed, the cathodic current induces the production of nitrous acid according to the following reaction :

NO + 3 H⁺ + 2 e^" → HNO₂ + H₂O Nitrous acid is not stable in the acidic medium and, when its concentration is high enough, decomposes easily either according to the following first set of reactions :

2 HNO₂ → H₂ O + N₂O₃ then N₂O₃ → NO + NO₂ or according to the following second set of reactions : HNO₂ + HNO3 → 2 NO₂ + H₂O and

In such an electrolytic process, the massive production of nitrogen oxides in the cathode compartment is disadvantageous to the extent that adequate counter-measures must be taken in order to eliminate these NO_x products. For instance, EP-A-297,738_discloses treating waste organic matter by adding it to an electrolyte comprising nitric acid and silver ions as oxidizing species at a temperature of 50-90°C and regenerating the said oxidizing species, following its reduction by interaction with the electrolyte, by means of the electric current. This document acknowledges that reduction to nitrous acid without evolution of gas is possible only within certain limits of current density and that if nitrous acid concentration is allowed to build-up in the catholyte, the occurring reduction to NO results in problems of frothing and gas blanketing raising the resistance of the electrolyte. In the latter case, nitric acid may be recovered by scrubbing NO from the catholyte off-gas, using water or dilute acid, then concentrating it in an evaporator. Alternatively, NO may be scrubbed by the catholyte liquor continuously withdrawn from the cathode and returned through a packed column. This however results in a large non-compact equipment which does not make it possible to move the said equipment to the site of oxidable species to be treated and therefore implies the hazard of transporting the said optionally contaminated species to the site where electrolytic equipment is implanted.

US-A-3,945,865 discloses preventing the evolution of NO fumes in the dissolution of metals in a nitric acid-containing solution by adding hydrogen peroxide to said solution and maintaining the hydrogen peroxide concentration during dissolution between 1 and 20 g/1. Metals considered in this procedure are copper, zinc, titanium, zirconium and molybdenum. This document further states that the stability of hydrogen peroxide is adversely affected by the presence of certain metals such as iron, copper and lead, which catalyze its decomposition, in which case a hydrogen peroxide stabilizer must be added. US-A-4,938,838 discloses reducing the emission of NO gas in a bath for stainless steel, copper or brass containing nitric acid by measuring the redox potential in the said bath and automatically supplying hydrogen peroxide to the bath in an amount such that the redox potential is at about its maximum value. However none of the above cited documents provides an efficient solution to the problem raised, since on the one hand those skilled in the art are focused on recovering NO_x after they have been produced rather than avoiding their production and, on the other hand, the technique relying on redox potential to control the amount of hydrogen peroxide is not easy to implement in the case of a huge electrolytic production of NO_x pollutants. Therefore a first objective of the present invention is to provide a safe and efficient means for avoiding accumulation of nitrous acid and/or for suppressing production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid. A second objective of the present invention is to provide an improved method for the decomposition of organic matter when added to an aqueous electrolyte containing nitric acid and silver ions.

SUMMARY OF THE INVENTION

In order to meet the above identified objectives, the present invention is based on the principle of entirely oxidizing nitrous acid before any formation of nitrous oxides during electrolytic reduction. Therefore, in a first aspect the present invention provides a method to avoid accumulation of nitrous acid and/or suppress production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid, by adding an efficient amount of a substance reactive with nitrous acid to the catholyte in such a way that the nitrous acid formed at the cathode is entirely oxidized there to nitric acid. The substance added is an oxidant, preferably hydrogen peroxide or a material which acts like or generates hydrogen peroxide in situ when in contact with aqueous solutions, e.g. ozone or even oxygen. In a second aspect, the present invention provides the use of the above method specifically for the decomposition of organic matter by electrolytic reduction, in which an aqueous electrolyte containing nitric acid and silver ions is subjected to an electric potential and the organic matter is added to the said electrolyte. Specifically, the present invention provides a method of treating waste matter, more particularly organic waste matter or radioactively contaminated waste matter. Finally the present invention provides an apparatus for performing the above-mentioned method, comprising a first container for storing and feeding an oxidable species such as an organic matter, a second container for storing and feeding a nitric acid aqueous solution and further comprising an electrolyser having an anode compartment connected to the first container and a cathode compartment connected to the second container through a nitric acid aqueous solution feeding circuit, said anode compartment and said cathode compartment being separated by a membrane, and further comprising a device for providing a mixture of water and of a substance reactive with nitrous acid and a means for injecting the said mixture into the nitric acid aqueous solution feeding circuit before said nitric acid aqueous solution enters the cathode compartment. BRIEF DESCRIPTION OF THE DRAWING

The single attached figure is a schematic representation of an electrolytic apparatus for performing the reduction of an electrolyte containing a nitric acid aqueous solution by means of the improved method of this invention when hydrogen peroxide is used as the substance reactive with nitrous oxide. DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment, the present invention relates to a method to avoid accumulation of nitrous acid and/or suppress production of nitrogen oxides during an electrochemical reduction process of an aqueous electrolyte containing nitric acid and simultaneous oxidation of oxidable species, by adding an efficient amount of a substance reactive with nitrous acid to the catholyte in such a way that the nitrous acid formed at the cathode is entirely oxidized there to nitric acid. A preferred substance reactive with nitrous acid for use in this invention is a strong oxidant, most preferably hydrogen peroxide or any substance (such ad oxygen or ozone) able to act like or generate hydrogen peroxide in situ under the conditions of the said electrolytic reduction. As a result of this method, nitrous acid cannot decompose anymore into the polluting nitrogen oxides. According to the preferred embodiment of the present invention, nitrous acid reacts rapidly and easily with hydrogen peroxide according to the following equation: HNO₂ + H₂O₂ → HNO₃ + H₂O

This equation shows a further advantage of the present invention, i.e. nitric acid is directly and rapidly regenerated in the electrolytic cell itself. Therefore, as long as the amount of the substance reactive with nitrous acid, e.g. hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ, injected into the electrolytic cell is sufficient, nitrous acid cannot accumulate in the aqueous electrolyte and consequently the massive production of polluting gases NO_x which conventionally occurs in the absence of the substance reactive with nitrous acid, is necessarily suppressed. On the other hand, careful examination of the cathodic phenomena observed in an aqueous solution of nitric acid containing hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ reveals that at least part of the current may result from the direct reduction of the said peroxide according to the following equation: H₂O₂ + 2 H⁺ + 2e → 2 H₂O As will be readily understood from those skilled in the art, the relative contribution of this second reaction depends not only on the type of cathode used and on the current density but also on the ratio of the concentrations of hydrogen peroxide and nitric acid. Therefore, according to both above-mentioned equations, not only the production of polluting gases NO_x is suppressed, but also the reduction of nitric acid is decreased. Consideration of the various above equations also shows that the various electrochemical transformations inevitably result in the production of water. In particular, the injected hydrogen peroxide will be reduced to water, thereby causing a continuous dilution of the catholyte.

According to a preferred embodiment of the present invention, the cathode used for electrolytic reduction consists of a material which is not susceptible to be corroded by the mixture of nitric acid and of the substance reactive with nitrous acid. In particular when the substance reactive with nitrous acid is hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ, a slight excess of hydrogen peroxide in the catholyte can be tolerated provided that the cathode is suitably selected. The terms "suitably selected", as used herein, mean that the cathode must be compatible with both high concentrations of nitric acid and the presence of hydrogen peroxide. The cathode may be made for instance from any material such as zirconium, niobium, tantalum, graphite, glassy carbon, gold, SS316 stainless steel or another class of steel specifically designed for the technology of highly concentrated nitric acid solutions. On the contrary, it is not desirable that the cathode be made from titanium since the latter easily forms complexes with hydrogen peroxide. The amount of the substance reactive with nitrous acid to be used in the method according to the invention is rather critical, i.e. the said substance should be used at least in stoechiometric amount with respect to the nitrous acid susceptible to be formed during electrolytic reduction, and preferably in excess of the amount sufficient for entirely oxidizing nitrous acid to nitric acid at the cathode. For this purpose, the continuous or discontinuous addition of the substance reactive with nitrous acid is preferably controlled on the basis of a measurement of the concentration of the said substance, such as hydrogen peroxide, in the catholyte and, most preferably (for reasons related to process control technology such as the dead time resulting from the duration of the measurement), on the basis of both said hydrogen peroxide concentration in the catholyte and the electrolytic current. In turn, the measurement of said substance (such as hydrogen peroxide) in the nitric acid solution can be achieved by any suitable means already known to those skilled in the art. For instance, measuring the concentration of hydrogen peroxide in a nitric acid aqueous solution can be readily achieved by the detection of its coloured complexes by means of a spectrophotometer or colorimeter. The electrolytic current intensity is not a parameter critical to the present invention and may conveniently be comprised in a range between about 50 amps and about 200,000 amps, as already known to those skilled in the art, although a current intensity below or above this range is not believed to be detrimental to the efficiency of the method of the invention. Similarly, the concentration of nitric acid in the aqueous electrolyte is not a parameter critical to the present invention and may conveniently be comprised in a range between about 4 M and about 15 M.

In a second embodiment, the present invention relates to a method incorporating the above-described inventive feature and further comprising the decomposition of organic matter by electrolytic oxidation, in which an aqueous electrolyte containing nitric acid and silver ions as an electrochemically regenerable oxidising species is subjected to an electric current and the organic matter is added to the electrolyte. The organic matter involved in this embodiment of the invention usually is waste matter which may be in polymeric or monomeric form. The term " organic matter " as used herein refers both to a matter consisting essentially of organic moieties and to a matter comprising organic moieties together with inorganic moieties or elements such as metallic species either covalently or ionically linked to said organic moieties. For instance it may refer to polymeric salts used as flocculants in industry or as additives to polymeric formulations. When in polymeric form, the organic matter may comprise natural polymers such as cellulose and rubber, or synthetic polymers and resins such as, for instance, synthetic rubbers, thermoplastic and thermosetting resins and formulations, ion exchange resins (including, without limitation, anionic and cationic polymers such as divinylbenzene/styrene copolymers, phenol/formaldehyde copolymers and the like) and ion exchange resins loaded with cations such as heavy metals or radioactive species. The organic matter involved in this embodiment of the invention may also be or comprise a radioactively contaminated waste matter. As an example, it may be waste matter contaminated with a toxic material such as dioxine or with a radioactive species or it may comprise spent nuclear fuel such as uranium carbide fuel. As another example, the organic waste matter may comprise a solvent derived from the reprocessing of nuclear fuel or a contaminated hydraulic fluid, oil or grease or kerosene.

The form and amount of silver ions used in this embodiment of the invention is not a parameter critical to the present invention. Silver ions may be present in the anolyte in the form of, for instance, silver nitrate and their concentration may conveniently be comprised in a range between about 0.02 M and about 1 M, as already known to those skilled in the art, although a concentration below or above this range is not believed to be detrimental to the efficiency of the method.

In each of the above-described embodiments of the method according to the present invention, it is not necessary that nitric acid be the only component of the aqueous electrolyte. Rather, the said electrolyte may further comprise one or more other powerful oxidizing agent(s) such as another mineral acid easily miscible with nitric acid, for instance perchloric acid.

As will be readily understood, the method according to the present invention provides numerous advantages: first of all, it provides an efficient and easily implementable means for avoiding the presence of nitrous acid and/or polluting nitrogen oxides in the course of any industrial process comprising a step of electrolytic reduction of an aqueous electrolyte containing nitric acid. Secondly it provides a safe and cost-effective means for treating waste matter containing organic species optionally mixed together with contaminating species. Thirdly it allows to reduce the size of a regeneration device of nitric acid in the cathode compartment of the electrolytic cell, as explained hereinbelow, up to the point where the whole device becomes compact and moveable. In such way, it becomes possible to transport the waste treatment device on to the site of waste rather than running the costs, risks and hazards of transporting the optionally contaminated organic waste to be treated on to the site of the treatment device. In another aspect, the present invention therefore relates to an apparatus for performing the above-described method, comprising a first container for storing and feeding an oxidable species such as an organic matter, a second container for storing and feeding a nitric acid aqueous solution and further comprising an electrolyser having an anode compartment connected to the first container and a cathode compartment connected to the second container through a nitric acid aqueous solution feeding circuit, said anode compartment and said cathode compartment being separated by a membrane, and further comprising a device for providing a mixture of water and of a substance reactive with nitrous acid and a means for injecting the said mixture into the nitric acid aqueous solution feeding circuit before said nitric acid aqueous solution enters the cathode compartment. The mixture of water and of the substance reactive with nitrous acid should be as homogeneous as possible at the time when it is injected into and admixed with the nitric acid aqueous solution. Homogeneity of the said mixture will be obtained in different ways depending on the chemical and/or physical nature of the substance reactive with nitrous acid. When the latter is a liquid easily soluble in water, such as hydrogen peroxide, the mixture of water and hydrogen peroxide will readily be provided by a third container for storing the said mixture. On the other hand, when the substance reactive with nitrous acid is a gas which generates hydrogen peroxide in situ when in contact with aqueous solutions, e.g. ozone or oxygen, the device for providing a mixture of water and the substance reactive with nitrous acid may be a high efficiency static gas-liquid mixer. Furthermore, when the substance reactive with nitrous acid is ozone, the apparatus of the present invention may comprise, upstream of the said high efficiency static gas-liquid mixer, an ozoniser for producing ozone from oxygen and it may also include a device for destroying a possible excess of ozone before release.

According to a preferred embodiment of the invention, the said apparatus further comprises at least an electrodialyser for maintaining the concentration of nitric acid in the cathodic circuit of the electrolyser constant. According to another preferred embodiment of the invention, the said apparatus further comprises at least a reverse osmose unit for maintaining the concentration of nitric acid in the anodic and cathodic circuits of the electrolyser constant. For a better understanding of the present invention, the apparatus will now be explained by reference to the accompanying single figure. An electrolyzer (4) comprises an anode compartment (5) and a cathode compartment (6) in which nitric acid is reduced to nitrous acid. Functioning of the apparatus may be summarized as follows: in order to avoid nitrous acid to accumulate in the cathode loop and decompose into nitrogen oxides, a means is provided for injecting a substance reactive with nitrous acid, for instance hydrogen peroxide, into the said cathode loop. As mentioned herein-above, hydrogen peroxide oxidises nitrous acid to nitric acid. Water enters the cathode loop of the electrolyser (4) by means of the addition of hydrogen peroxide and its subsequent reduction and also because hydrogen peroxide is only available in aqueous diluted forms. Water entering the cathode loop of the electrolyser (4) in this way is removed by the simultaneous action of two electrodialysers (12) and (19) and a reverse osmose unit (27). Each electrodialyser (12,19) comprises a concentrate loop and a diluate loop, allowing a fraction of the catholyte to be enriched in acid and the other fraction being diluted in acid. The enriched fraction from the first electrodialyser (12) is returned to the cathode loop of the electrolyser (4) while the diluted fraction from the first electrodialyser (12) is sent to a second electrodialyser (19) where again a fraction is enriched in acid and sent back to the cathode loop of the electrolyser (4) while the other fraction is diluted again and sent to a reverse osmose unit (27). The said reverse osmose unit (27) then withdraws water from the diluate loop of the second electrodialyser (19). The overall result of this construction is that water produced by the addition and reduction of hydrogen peroxide is withdrawn by the combined action of two electrodialysers (12,19) and a reverse osmose unit (27).

To enter into more detail, an example of an apparatus for performing the second embodiment of the method according to the present invention is schematically represented on the enclosed single figure and comprises the following elements : - a storing and feeding container (1) for the organic matter to be treated / decomposed, a storing and feeding container (2) for a concentrated aqueous solution comprising nitric acid (and optionally one or more other mineral acid(s) in miscible proportions with nitric acid), a storing and feeding container (3) for an aqueous solution of a substance reactive with nitrous acid, e.g. hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ, - an electrolyser (4) comprising an anode compartment (5) connected to the storing and feeding container (1) and a cathode compartment (6) connected to the storing and feeding containers (2) and (3), - a first electrodialyser (12) comprising a concentrate compartment (13) connected with the cathode compartment (6) of the electrolyser (4) and a diluate compartment (14), a second electrodialyser (19) comprising a concentrate compartment (20) and a diluate compartment (21), the diluate compartment (14) of the first electrodialyser (12) being connected to the second electrodialyser (19), preferably to both compartments thereof, and - a reverse osmose unit (27) comprising a low pressure compartment (28) connected to the anode compartment (5) of the electrolyser (4) and a high pressure compartment (29) connected to the diluate compartment (21) of the second electrodialyser (19).

The anode compartment (5) of the electrolyser (4) constitutes a part, together with container (1), of a circulation loop further comprising a circulation pump (7) and a filter (8). The cathode compartment (6) of the electrolyser (4) is also included in a circulation loop constituted by a buffer container (9), a circulation pump (10) and a filter (11). The concentrate compartment (13) of the first electrodialyser (12) is part, together with the buffer container (9), of a circulation loop further comprising a circulation pump (15) and a filter (16). The diluate compartment (14) of the first electrodialyser (12) is connected:

- firstly, through a circulation pump (17) and a filter (18), to the concentrate compartment (20) of the second electrodialyser (19), and

- secondly, through a circulation pump (22), a filter (23), a buffer container (24), a circulation pump (25) and a filter (26), to the diluate compartment (21) of the second electrodialyser (19).

The high pressure compartment (29) of the reverse osmose unit (27) is connected to the buffer container (24) upstream of the diluate compartment (21) of the second electrodialyser (19), whereas the low pressure compartment (29) of the reverse osmose unit (27) is connected, through a circulation pump (30) and a filter (31), to the storing and feeding container (1) and, hence, to the anode compartment (5) of the electrolyser (4). Finally, the storing and feeding container (3) is connected, through a circulation pump (32) and a filter (33), to the buffer container (9) which additionally receives nitric acid from the storing and feeding container (2).

As will easily be understood, the above construction should not be considered as limiting the scope of the present invention, as far as the apparatus for performing the inventive method is concerned, and many variations thereof will readily appear to those skilled in the art. In any event, as previously indicated, this construction allows the whole apparatus to become compact and therefore easily moveable, which constitutes a sensible advantage over the organic waste treatment apparatuses known so far.

Claims

1. A method to avoid accumulation of nitrous acid and/or suppress production of nitrogen oxides during an electrolytic reduction of an aqueous electrolyte containing nitric acid, wherein an efficient amount of a substance reactive with nitrous acid is added to the catholyte in such a way that the nitrous acid formed at the cathode is entirely oxidized there to nitric acid.

2. A method according to claim 1, wherein the said substance reactive with nitrous acid is an oxidant.

3. A method according to claim 1 , wherein the said substance reactive with nitrous acid is hydrogen peroxide or any substance able to act like or generate hydrogen peroxide in situ when in contact with an aqueous solution.

4. A method according to claim 3, wherein the said substance reactive with nitrous acid is ozone or oxygen.

5. A method according to any of claims 1 to 4, further comprising the decomposition of organic matter by electrolytic reduction, in which an aqueous electrolyte containing nitric acid and silver ions is subjected to an electric current and the organic matter is added to the electrolyte.

6. A method according to any of claim 1 to 5, wherein the cathode used for electrolytic reduction consists of a material which is not susceptible to be corroded by the mixture of nitric acid and of the substance reactive with nitrous acid.

7. A method according to claim 6, wherein the cathode used for electrolytic reduction consists of a material selected from zirconium, niobium, tantalum, graphite, glassy carbon, gold and stainless steel SS316.

8. A method according to any of claims 1 to 7, wherein said substance reactive with nitrous acid is used in excess of the amount sufficient for entirely oxidizing nitrous acid to nitric acid at the cathode.

9. A method according to any of claims 1 to 8, wherein the addition of the substance reactive with nitrous acid is controlled on the basis of the measurement of the concentration of the said substance in the catholyte and/or on the basis of the electrolytic current.

10. A method according to any of claims 3 to 9, wherein the said organic matter comprises a radioactively contaminated waste matter.

11. A method according to any of claims 3 to 9, wherein the said organic matter comprises polymeric or monomeric organic waste matter.

12. An apparatus for performing a method according to any of claims 1 to 11, comprising a first container for storing and feeding an oxidable species, a second container for storing and feeding a nitric acid aqueous solution and further comprising an electrolyser having an anode compartment connected to the first container and a cathode compartment connected to the second container through a nitric acid aqueous solution feeding circuit, said anode compartment and said cathode compartment being separated by a membrane, characterized in that the said apparatus further comprises a device for providing a mixture of water and of a substance reactive with nitrous acid and a means for injecting the said mixture into the nitric acid aqueous solution feeding circuit before said nitric acid aqueous solution enters the cathode compartment.

13. An apparatus according to claim 12, further comprising at least an electrodialyser for maintaining the concentration of nitric acid in the cathodic circuit of the electrolyser constant.

14. An apparatus according to claim 12 or claim 13, further comprising at least a reverse osmose unit for maintaining the concentration of nitric acid in the anodic and cathodic circuits of the electrolyser constant.

15. An apparatus according to any of claims 12 to 14, wherein the device for providing a mixture of water and of a substance reactive with nitrous acid is a third container for storing an aqueous solution of hydrogen peroxide.

16. An apparatus according to any of claims 12 to 14, wherein the device for providing a mixture of water and of a substance reactive with nitrous acid is a high efficiency static gas-liquid mixer for mixing water and a gas which acts as or generates hydrogen peroxide in situ.

17. An apparatus according to claim 15, comprising :

- a storing and feeding container (1) for the organic matter to be decomposed,

- a storing and feeding container (2) for an aqueous solution comprising nitric acid, a storing and feeding container (3) for an aqueous solution of a substance reactive with nitrous acid, - an electrolyser (4) comprising an anode compartment (5) connected to the storing and feeding container (1) and a cathode compartment (6) connected to the storing and feeding containers (2) and (3), a first electrodialyser (12) comprising a concentrate compartment (13) connected with the cathode compartment (6) of the electrolyser (4) and a diluate compartment (14), - a second electrodialyser (19) comprising a concentrate compartment (20) and a diluate compartment (21),

- the diluate compartment (14) of the first electrodialyser (12) being connected to the second electrodialyser (19), and a reverse osmose unit (27) comprising a low pressure compartment (28) connected to the anode compartment (5) of the electrolyser (4) and a high pressure compartment (29) connected to the diluate compartment (21) of the second electrodialyser (19).

18. An apparatus according to any of claims 12 to 17, wherein the cathode used for electrolytic reduction consists of a material which is not susceptible to be corroded by the mixture of nitric acid and of the substance reactive with nitrous acid.

19. An apparatus according to claim 18, wherein the said material is selected from zirconium, niobium, tantalum, graphite, glassy carbon, gold and stainless steel SS316.