EP1765727A1

EP1765727A1 - Method for processing aqueous media containing metal nitrate or sulphate salts

Info

Publication number: EP1765727A1
Application number: EP05778910A
Authority: EP
Inventors: Jean-Michel Tauzia; Marie Gaudre; François CANSELL
Original assignee: Centre National de la Recherche Scientifique CNRS; SNPE Materiaux Energetiques SA
Current assignee: Centre National de la Recherche Scientifique CNRS; Safran Ceramics SA
Priority date: 2004-06-18
Filing date: 2005-06-17
Publication date: 2007-03-28
Also published as: NO20070256L; FR2871791A1; FR2871791B1; WO2006008378A1

Abstract

The invention relates to a method for processing an aqueous medium (m) containing at least one type of metal nitrate or sulphate and making it possible to convert said soluble nitrate or sulphate into an insoluble oxide and a nitric or sulphuric acid, wherein the inventive processing method consists (a) in processing said aqueous medium (m) at a temperature ranging from 100 to 700 °C and a pressure of 1-50 MPa and, subsequently in entirely or partially recovering and/or processing the thus obtainable nitric or sulphuric acid. The use of the inventive method for converting soluble metal salts into the insoluble form thereof for purifying aqueous effluents produced by metal nitric or sulphuric acid etching is also disclosed

Description

A method of treating aqueous media comprising metal salts of nitrate or sulfate type.

The present invention relates to a process for carrying out the conversion of nitrate or sulphate metal salts present in solution in aqueous media into metal species of the oxides type. This process for converting soluble metal species into metal species of the oxides type, which are generally insoluble or easily extractable from the medium (the soluble metal oxides are easily removed from an aqueous medium by techniques known per se), may in particular be used to eliminate metal species of the nitrate or sulphate type present in aqueous effluents.

Many effluents obtained at the end of industrial processes contain metal salts of nitrate or sulfate type. By way of example, mention may in particular be made of the effluents resulting from the acid attack of metals by nitric acid or by sulfuric acid, and in particular the effluents obtained during the treatment of metals in pickling baths. nitric acid or sulfuric acid, such as those used for example for stripping sheets prior to surface treatments.

In view of the standards in force, which limit the content of metal ions and anions of sulphate and nitrate type in effluents discharged by industries, it is necessary to treat the aqueous media containing metal sulphates or nitrates obtained according to the abovementioned processes, in order to substantially eliminate these metal salts, before they can be considered for release into the environment. To achieve the elimination of metal salts in aqueous effluents of this type, various solutions have been proposed, for example implementing processes of precipitation, bioprecipitation, crystallization, ion exchange, electrolysis, or more specific techniques such as as electrodialysis for example. Among these methods of the state of the art, some do not prove sufficiently effective to eliminate quantitatively metal salts in the medium. This is particularly the case with certain crystallization or precipitation processes. Others, such as electrolysis, are certainly more efficient, but involve high treatment costs.

An object of the present invention is to provide a novel process for the removal of nitrate or sulphate metal salts present in an aqueous medium such as an industrial effluent, which is sufficiently interesting in terms of efficiency and costs for to replace the currently known salt elimination techniques, and which preferably has a more advantageous cost-effectiveness ratio than the techniques which have been described so far.

For this purpose, the subject of the present invention is a process for treating an aqueous medium (m) containing at least one metal salt of a metal M, chosen from a nitrate or a sulphate allowing the conversion of said salt of the metal M to metal oxide M. This treatment method comprises a step (a) of treating said aqueous medium (m) at a temperature of between 100 and 700 ^° C. and at a pressure of between 1 and 50 MPa (ie that is, between 10 and 500 bar), whereby the metal salt is converted to metal oxide M and nitric or sulfuric acid (respectively to nitric acid or sulfuric acid, depending on whether the salt is a nitrate or a sulphate) . More precisely, a conversion of the initial medium (m) into a medium that comprises:

an aqueous phase which contains nitric or sulfuric acid; and

- A solid phase based on metal oxide M. The treatment process then comprises a step where the aqueous phase which contains nitric or sulfuric acid is recovered and / or treated, in whole or in part.

In this context, according to one embodiment, the aqueous phase which contains the acid obtained at the end of step (a) is recovered in whole or part as an aqueous solution of acid. According to a more particular embodiment, the metal salt of the metal M is a nitrate, whereby an aqueous phase containing nitric acid is obtained at the end of step (a), and the process comprises a step (b) additional treating the nitric acid obtained with ammonia NH ₃ at a temperature of 100 to 700 ⁰ C and a pressure of 1 to 50 MPa, so as to convert the nitric acid produced in the step (a) at N ₂ and H ₂ O.

Step (a) of the process of the invention aims at converting the nitrate or sulfate of the metal M into an easily removable form, preferably an insoluble form, of this metal, namely the oxide of this metal. This conversion of a soluble form (nitrate or sulfate) of the metal M into an oxide, preferably insoluble, may advantageously be used to extract the metal M from the aqueous solution. In the case of an insoluble oxide, the oxide of the metal M obtained is extracted from the medium by a solid / liquid separation, generally by filtration. Any other solid / liquid separation process may however be envisaged, such as, for example, a separation by centrifugation, or even by nanofiltration.

Subject to conducting step (a) for a period of time sufficient for the conversion to be effective, the treatment of the medium (m) at a temperature of 100 to 700 ^° C. and under a pressure of from 1 to 50 MPa allows, in most cases, a substantial conversion of salt to insoluble oxide. Depending on the nature of the metal M, the conversion rate obtained at the end of step (a) can vary to a large extent, however, in the most general case, the above-mentioned temperature and pressure conditions are capable of allowing the conversion of at least 60% of oxide, and most often at least 70%, of the nitrates or sulphates of the metal M present in the aqueous medium (m). In most cases, a conversion rate of the nitrate or sulphates of the metal M into oxide of 80% or more is obtained, and generally at least 85%, this conversion rate being able to reach 90% or even 95% or even 99% or more in some cases. In particular, so that the conversion of the nitrates or sulphates of the metal M into oxide is as effective as possible, it is generally preferable that step (a) of the process is conducted at a temperature of at least 250 ⁰ C ₁ preferably of at least 300 ° C, and preferably at least 350 ⁰ C. Thus, in one interesting embodiment, step (a) is conducted at a supercritical temperature, ie a temperature above 374 ° C. Moreover, in particular to reduce the costs of the process, the reaction can be carried out at a temperature of less than or equal to 600 ^° C., this temperature generally remaining less than or equal to 550 ^° C. Thus, to obtain an interesting cost / efficiency ratio for the process, the temperature of implementation of step (a) is advantageously between 250 ° C. and 550 ^° C., and preferably between 300 ° C. and 500 ° C. (typically between 400 and 500 ° C.).

On the other hand, in order to effectively convert the nitrates or sulphates of the metal M into oxide, the step (a) is advantageously carried out under a pressure of at least 10 MPa (100 bar), this pressure preferably being at least 20 MPa (200 bar). Thus, for example, the process of the invention can be conducted at a supercritical pressure, ie, a pressure greater than 22.1 MPa (221 bar). In this context, according to a specific embodiment, step (a) is conducted under supercritical conditions, namely at a temperature greater than 374 ° C. and at a pressure greater than 22.1 MPa (221 bar). In particular, to limit the cost of the process, step (a) may advantageously be carried out under a pressure less than or equal to 40 MPa (400 bar), or even less than or equal to 30 MPa (300 bar). Thus, interestingly in terms of efficiency and cost, step (a) is conducted under a pressure of between 10 and 40 MPa (for example between 20 and 35 MPa, and preferably between 25 and 30 MPa). Thus, according to one embodiment of the invention which is most often advantageous, step (a) is carried out at a temperature of between 300 ^° C. and 550 ^° C. and at a pressure of between 10 and 40 MPa. and generally under supercritical conditions (for example between 400 and 500 ^° C. and under a pressure of 25 to 30 MPa).

The process of the invention is particularly advantageous for the treatment of industrial effluents resulting from the treatment of metals with acid sulfuric or nitric. However, more generally, most aqueous media (m) containing nitrates and / or metal sulfates whose oxides are stable in aqueous medium can be treated according to the method of the invention.

Most often, the metal M whose nitrate or sulfate is present in the aqueous medium (m) treated in step (a) of the process of the invention is chosen from:

certain alkaline earth metals such as beryllium (Be) and magnesium (Mg);

transition metals (group "d" of the periodic table of elements), such as scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn) ₁ - niobium (Nb), molybdenum (Mo), zirconium (Zr), or tin (Sn);

metals of the family of lanthanides or actinides (group T of the periodic table of elements) such as, in particular, thorium (Th), uranium (U) and plutonium (Pu);

- other metals such as aluminum (Al) or lead (Pb).

The process of the invention is particularly suitable for the treatment of an aqueous medium (m) containing iron nitrates, iron sulfates, or mixtures of these salts.

According to a first variant of the process of the invention, the aqueous medium

(m) treated in step (a) comprises a sulfate of the metal M as the metal salt. In this case, step (a) leads to a conversion of the sulfate of the metal to oxide and sulfuric acid. Thus, at the end of step (a), a conversion of the initial medium (m) into a medium which comprises: an aqueous phase (aq _su if) which contains sulfuric acid and which has generally a very low content of metal cations; and

a solid phase based on metal oxide M, this solid phase easily separable from the aqueous phase (aq _SU | _f ), in particular by filtration. According to this first variant, the aqueous phase (aq _su if) obtained at the end of step (a) is recovered and / or treated, wholly or partially

The method of the invention according to this first variant can advantageously be used to substantially eliminate the metal cations and sulphate anions present in a medium (m) containing metal sulphates, especially for treating effluents resulting from the acid attack. of metals by aqueous solutions containing sulfuric acid, for example for release into the environment. In this case, the process comprises, after step (a), a step of separating the oxide-based solid phase of the metal M and the aqueous phase (aq _su if) _> this step being generally carried out by filtration but any other type of solid / liquid separation can still be used. Thus, on the one hand, most of the metallic elements in the form of a solid, more easily manipulated and storable than in their soluble form in the initial medium (m), and on the other hand, the aqueous phase (aq _su if) containing the starting sulphate ions and having a reduced content of metal cations. The aqueous phase (aq _su if) thus separated is then subjected to a step of removing sulfate ions contained therein, the desulphation of the aqueous phase (aq _su if) can be conducted according to any conventional means, for example by exchange ion. In the ^J frame of such a purification method the implementation of the inventive method allows quantitative and easy removal of metal cations prior to ion exchange, which has the particular advantage of avoiding a poisoning of ion exchange devices by metallic species. According to another embodiment conceivable when the aqueous medium

(m) contains metal sulphates, all or part of the aqueous phase (aq _su i _f ) obtained at the end of step (a) can be recovered as an aqueous solution of sulfuric acid. Indeed, it should be emphasized that, given its low content of metal cations, the aqueous phase (aq _su ife) is an aqueous solution of sulfuric acid which is generally recoverable industrially, especially as a solution to achieve a attack or stripping of metals. In the context of this specific embodiment, the aqueous phase (aq _su i _f ) recovered can be recycled in a continuous process: where appropriate, the process of the invention is a process conducted continuously, in which all or part of the aqueous phase (aq _su if) obtained at the end of said step (a) is used to attack a metal, whereby an aqueous solution of a metal sulfate which is reinjected is obtained as an aqueous medium (m) in step (a).

According to a second variant of the process of the invention, the aqueous medium (m) treated in step (a) comprises a nitrate of the metal M as metal salt. In this second case, step (a) leads to a conversion of the nitrate of the metal into oxide and nitric acid, which is why, at the end of step (a), a conversion of the medium is obtained. (m) initial in a medium that includes:

an aqueous phase (aq _n itr) which contains nitric acid and which generally has a very low content of metal cations; and

- a phase solid based oxide of the metal M, this phase solid being easily separable from the aqueous phase (aq _n jtr). especially by filtration.

According to this variant, the aqueous phase (aq _n r _t r) obtained at the end of step (a) is recovered and / or treated, wholly or partially

In a similar manner to the phase (aq _SU | _f ) of the preceding variant, the aqueous phase (aq _n itr) constitutes an aqueous solution of low metal species acid which is exploitable industrially, in particular as a solution. nitric acid for etching or etching metals.

Also, according to a particular embodiment of the process of the invention, all or part of the aqueous phase (aq _n _r ) obtained at the end of step (a) can be recovered as an aqueous solution of nitric acid, the aqueous phase thus recovered may in particular be used to carry out an acid attack leading to the formation of aqueous effluents containing metal nitrates, which can be treated according to step (a) of the process of the invention.

Thus, according to a particular embodiment, the method of the invention according to this second variant can be a continuously conducted process, in which all or part of the aqueous phase (aq _n i _tr ) obtained at the end of said step (a) is used to attack a metal, whereby an aqueous solution of a metal nitrate is obtained which is reinjected, as medium aqueous (m) in step (a).

Alternatively, according to another embodiment that can be envisaged when the aqueous medium (m) contains a nitrate, it may be advantageous to eliminate the nitrate ions present in the form of nitric acid in the aqueous phase (aq _n i _t r). obtained at the end of step (a). This is particularly the case when the process implements, as aqueous medium (m), an aqueous effluent containing metal nitrates, in which it is sought to eliminate substantially the nitrate anions and the metal cations, for example with a view to rejecting in the environment. When it is desired to carry out such elimination of nitric acid, the process advantageously comprises an additional step (b) of treating the nitric acid obtained in step (a) with ammonia NH ₃ at a temperature of temperature of 100 to 700 ^° C. and under a pressure of 1 to 50 MPa, so as to convert the nitric acid produced in step (a) into N ₂ and H ₂ O. Preferably, step (b) is implemented in the preferred temperature and pressure ranges given for step (a). Moreover, in particular so as to perform the most complete conversion possible nitric acid, it is most often indicated that in step (b), the molar ratio NH ₃ / HNO ₃ of the amount of ammonia injected into the medium, relative to the amount of nitric acid present in the medium at the end of step (a) is between 4: 3 (that is to say 1, 33: 1) and 2 : 1, this ratio being advantageously at least 5: 3 (i.e. 1, 66: 1).

Most often, steps (a) and (b) take place successively, namely that the step (a) of converting the metal nitrate to oxide and nitric acid takes place at first, and that it is then followed, in a second step, by step (b). In this case, step (b) most often consists of injecting NH ₃ ammonia into the medium obtained at the end of step (a), leaving the medium under the conditions of temperature and pressure. of step (a). When steps (a) and (b) are conducted successively, it is possible, if desired, to treat only a part of the nitric acid obtained in the aqueous phase (aq _n i _t r) at the end of step (a), by deriving a portion of the aqueous phase (aq _n j _tr ) prior to the implementation of step (b). In this context, according to a specific embodiment, the process can thus be carried out under the following conditions: at the end of step (a), a first part of the nitric acid obtained in the aqueous phase (aq) _n itr) at the end of step (a) is recovered, this acid then being advantageously used to carry out the etching of a metal, thus leading to the production of an aqueous effluent containing metal nitrates, which can be reinjected in step (a); and the other part of the nitric acid obtained in the aqueous phase (aq _n i _t r) is treated with NH ₃ ammonia under the conditions of step (b) as defined above.

According to another embodiment, the steps (a) and (b) can be conducted jointly, within the same reactor. In this case, the implementation of steps (a) and (b) consists simply in conducting step (a) as defined above under an atmosphere containing NH ₃ ammonia: the imposed temperature and pressure conditions then induce a conversion of the metal nitrates present in the medium (m) into oxides and nitric acid, and the nitric acid formed is immediately consumed by a conversion reaction to N ₂ and H ₂ O in the presence of the gaseous ammonia present in the the reactor atmosphere.

As pointed out above, the process of the invention is particularly useful for removing any metal salt of the nitrate or sulfate type present in an aqueous medium. In this context, it should also be noted that the conditions for carrying out step (a) also make the process particularly suitable for treating aqueous media comprising, in addition to metal nitrates or sulphates, organic compounds. It should be emphasized that the temperature and pressure conditions of step (a) are conditions well adapted to hydrothermal degradation of most organic compounds. "Hydrothermal degradation" means a conversion of an organic species into mineral species (for example a conversion in the form of water, and CO2 and / or CO, when the degraded organic compound contains only the elements C, H and O), by putting under pressure and temperature , generally in the presence of an oxidizing agent. Such "hydrothermal degradations" of organic species in mineral species have in particular been described in the article "Hydrothermal oxidation: new concept for treatment of industrial and urban wastes" by M. Bottreau (Supercritical Fluids and Materials, Polytechnic Institute of Lorraine 2003 ). Insofar as it is carried out at a temperature of 100 to 700 ° C. and at a pressure of 1 to 50 MPa, and preferably under supercritical conditions, step (a) of the process is adapted to carry out such degradation conversion. hydrothermal organic compounds if such compounds are present in the medium (m).

Thus, when the medium (m) contains an organic compound that is to be removed, step (a) is advantageously carried out in the presence of an oxidizing agent for the degradation of said organic compound into mineral species. For this purpose, an oxidizing agent is generally introduced within the step (a) in an amount sufficient to degrade said organic compound into mineral species. The oxidizing agent used in this context may be chosen from any conventional oxidizing agent. in the field of hydrothermal degradation, such as, for example, gaseous oxygen, hydrogen peroxide, nitric acid, air or a mixture of one or more of these oxidizing agents. The amount of oxidant to be used in this context can be easily determined by a technician of the field, based on the conditions generally implemented in the field of hydrothermal degradation. However, it should be emphasized that, in the particular case where the medium (m) treated initially contains nitrates, step (a) leads to the conversion of these nitrates to nitric acid which then acts as an oxidizer. The presence of nitrates in the medium (m) is therefore particularly advantageous, insofar as it makes it possible to reduce the amount of oxidizing agent to be introduced into the medium in order to effect the hydrothermal degradation of the organic species to be eliminated. In some cases, if the nitrates are present in a sufficient amount in the initial medium, the acid nitric acid resulting from their conversion can even be sufficient, alone, to carry out the hydrothermal degradation of the organic species present.

According to one particular embodiment, the organic compound present, where appropriate, in the medium (m) may be a nitrogenous organic compound. In this case, a formation of nitrogen oxides NO _x (NO, N ₂ O ₄ , N ₂ O, NO _{2 in} particular) is obtained at the end of step (a). In this case, it is most often desirable to eliminate these NOx nitrogen oxides, especially when the process of the invention is conducted as part of an industrial effluent treatment. For this purpose, the process then advantageously comprises a step (b1) of treatment of nitrogen oxides NOx formed in the presence of ammonia, so as to convert these NOx nitrogen oxides into N ₂ and H ₂ O. The amount Ammonia to be used in this step to efficiently convert NOx to N ₂ and H ₂ O is preferably such that the molar NH ₃ / NO _X ratio is between 4: 3 and 2: 1, preferably at least equal to 5/3. In practice, step (b1) is generally conducted under the same conditions of temperature and pressure as step (a).

Thus, step (b1) may for example be conducted after step (a), by injecting ammonia NH ₃ in the medium obtained at the end of step (a), and leaving the more often the medium under the conditions of temperature and pressure of step (a).

Alternatively, step (b1) may be implemented together with step (a) within the same reactor: in this second case, the process is carried out by implementing step (a) under a atmosphere containing ammonia, whereby the NO _x formed in step (a) are converted to nitrogen and water as they are formed.

When the medium (m) used contains nitrates in addition to the nitrogenous organic compound, and a step (b) for converting the nitric acid formed in step (a) is carried out, the steps ( b) and (b1) are combined in a single step. More generally, a step of the type of step (b1) defined above can also be carried out if the medium that it is desired to treat according to the method of the invention contains NO _x nitrogen oxides. Thus, according to a particular embodiment of the invention, the method can be implemented to treat a medium containing, in addition to the aqueous medium (m), a gaseous phase containing NO _x nitrogen oxides. In this case, the medium obtained at the end of step (a) comprises such nitrogen oxides NO _x . When it is desired to eliminate these nitrogen oxides (which is most often the case), the process comprises, in addition to step (a), a step (b2) for treating the nitrogen oxides NOx in the presence of ammonia, so as to convert these nitrogen oxides NOx N ₂ and H ₂ O. In general, this step (b2) is then conducted under the same conditions as step (b1) implemented in the case where the medium (m) contains a nitrogenous organic compound. When it is desired to implement two or more of the steps (b), (b1) and (b2), these steps are combined in a single step.

According to an interesting embodiment, the medium (m) treated in the step

(a) of the process of the invention comes from the acid attack of the metal M with nitric acid. In this case, the nitrogen oxides formed during the acid attack of the metal M can be treated according to the process of the invention, together with the treatment of the aqueous phase.

Thus, according to a specific embodiment, the method of the present invention comprises the steps of:

(Ao) attacking the metal M with an aqueous nitric acid solution, whereby said aqueous medium (m) comprising a nitrate of said metal M and a gaseous phase containing nitrogen oxides is obtained;

NO _x ;

(A) treating said aqueous medium (m) at a temperature of ^~ 100 to 700 ⁰ C and under a pressure of 1 to 50 MPa, and advantageously in the preferred conditions described above for step (a), whereby the nitrate of metal M is converted to metal oxide M and nitric acid; and (B) treating the medium obtained at a temperature of 100 to 700 ° C and a pressure of 1 to 50 MPa in the presence of ammonia, so as to convert the nitric acid formed in step (A) and the oxides of nitrogen NO _x initially present in the gas phase at N ₂ and H ₂ O.

As a rule, the step (A ₀ ) is performed first, prior to step (A). However, according to one conceivable embodiment, which is particularly interesting in terms of reaction time and space, the steps (A ₀ ), (B) and (C) can be conducted jointly within the same reactor . In this case, the process generally involves contacting the metal M and the nitric acid solution (generally by immersing the metal M in the nitric acid solution) under a nitrogen-containing atmosphere. ammonia, in a closed chamber heated to a temperature of 100 to 700 ° C and a pressure of 1 to 50 MPa, and preferably under the advantageous conditions defined above for step (a).

The implementation of the steps (A ₀ ), (A) and (B), whether conducted successively or together in the same reactor, allows, in particular to convert a metal mass of the metal M an oxide of said metal, generally recoverable at the end of step (B) in the form of powder (by filtration and drying for example), without leading to the formation of harmful or harmful sub-products to the environment. In this context, the steps (Ao), (A) and (B) can be used in particular to convert metal waste metal M in the form of a metal oxide powder, more easily manipulated and storable. The metal scrap that can be processed by the steps (Ao), (A) and (B) are, in a general manner, all the waste comprising metallic elements (carcasses, household waste of the tin type, etc.), and in particular waste comprising a metal casing, such as, for example, button cells, or used nuclear fuel.

More particularly, the steps (Ao), (A) and (B) can be used for the conversion of specific metal scrap, comprising both metal elements and organic compounds, into oxide of said metal M, water, and inorganic compounds. In particular, the treated waste may for example be waste comprising an organic charge in a metal envelope based on the metal M, such as, for example, chemical weapons or button cells based on polymer electrolyte. In the context of this particular mode, the steps (A ₀ ), (A) and (B) of the process of the invention most often consist of:

(Ao) attacking the metal M with an aqueous solution of nitric acid, whereby said aqueous medium (m) comprising a nitrate of said metal M and the organic compounds initially present in the medium, and a gaseous phase containing nitrogen oxides NO _x, etching of the metal further accompanied by a release of organic compounds when treated waste include these organic compounds in a metal shell;

(A) treating said aqueous medium (m) at a temperature of 100 to 700 ^° C. and at a pressure of 1 to 50 MPa, advantageously under the preferential conditions defined above for step (a), whereby the nitrate of metal M is converted to metal oxide M and nitric acid, nitric acid then acting as oxidant and converting organic compounds into inorganic compounds (water, CO ₂ and CO in particular), this step being optionally conducted in adding to the medium an additional oxidizing agent (for example gaseous oxygen, or nitric acid) if the quantity of nitric acid produced is not sufficient to ensure a sufficient conversion of the organic compounds present; and (B) treating the medium obtained at a temperature of 100 to 700 ^° C. and under a pressure of 1 to 50 MPa in the presence of ammonia, so as to transform the nitric acid possibly remaining at the end of the step (A) and nitrogen oxides NO _x to N ₂ and H ₂ O.

Various aspects and advantages of the method of the invention will become even more explicit in view of the examples given below, in which the method of the invention has been implemented to perform the treatment of different aqueous media representative of the different possible forms of effluent treatment according to the invention.

Figure 1 appended schematically describes an apparatus used according to the invention, of the type used in Examples 1 to 3. This apparatus comprises a closed reactor 1, capable of operating under supercritical conditions (the means of putting under pressure and temperatures, of the usual type, are not detailed). Through the inlet duct 2, the aqueous medium to be treated is introduced. A conduit connected to a bottle 3 makes it possible to introduce NH ₃ ammonia into the reactor's sky, if necessary. The inlet ducts of the aqueous medium to be treated and ammonia are provided with control valves 4a and 4b for closing or opening these ducts. The gaseous effluents of the reaction are recovered by a first outlet duct 6 provided with the valve 4c, which allows the duct to be closed during the reaction. The liquid effluents are recovered by a second outlet duct 7, also equipped with a valve 4d. The solid effluents are recovered at the bottom of the reactor once the gaseous and liquid effluents have been removed.

EXAMPLE 1 Treatment of an aqueous medium comprising a metal salt

In this first example, the treated medium is a solution consisting of 2.3 g of iron nitrate dissolved in 40 ml of demineralized water. This solution therefore constitutes a medium initially containing 0.5 g of iron in the solubilized salt state.

This medium was treated for 15 minutes under supercritical conditions, namely at a temperature of 490 ^° C. and at a pressure of 26.5 MPa (265 bar). The reaction was carried out by placing the treated medium, initially at a temperature of 40 ^° C., in a glass tube placed in a closed reactor with a capacity of 100 ml, provided with a heating coil and coupled to a high bench. pressure. The pressurization of the reactor was carried out almost automatically instant and the temperature of the medium was raised from 40 ° C to 49O ⁰ C in 20 minutes.

Following this treatment at 490 ° C and 26.5 MPa, the medium was brought back to atmospheric pressure, and the medium was allowed to cool to room temperature (with a 45-minute temperature-down time).

At the end of this treatment, a quantitative conversion of iron nitrate to nitric acid and iron oxide was obtained. More precisely, at the end of the reaction, a medium consisting of: 35 g was obtained. an aqueous liquid phase containing nitric acid HNO ₃ ; and

an insoluble solid precipitate, separable from the liquid phase by filtration, based on Fe ₂ θ ₃ iron oxide, which comprises 0.45 g of iron.

The conversion yield of iron in the soluble state (nitrate) to iron in the insoluble state (oxide) obtained is therefore 90%.

EXAMPLE 2 Treatment of an aqueous medium comprising a mixture of a metal salt and a nitrogenous organic compound.

In this second example, the treated medium is a mixture of:

a solution of 2.3 g of iron nitrate dissolved in 40 ml of demineralised water;

- 2 g of a nitrogenous organic compound, namely fenuron (herbicide of formula C ₆ H ₅ -NH-CO-N (CHs) ₂ .

This medium initially contains 0.5 g of iron in the solubilized salt state and a nitrogenous organic compound corresponding to a chemical oxygen demand of 3.4 g.

This medium was treated for 15 minutes under supercritical conditions, at a temperature of 490 ^° C. and a pressure of 26.5 MPa (265 bar). The reaction was carried out in the same reactor as in Example 1. The pressurization of the reactor was carried out almost instantaneously and the temperature of the medium was raised from 40 ^° C. to 490 ^° C. in 20 minutes.

Following this first treatment, the medium was maintained under the aforementioned supercritical conditions, and ammonia NH ₃ was injected into the reactor skies at a pressure of 40 MPa. This injection was carried out for 1 minute.

Following this injection, the medium was maintained for 15 minutes at a temperature of 490 ^° C., and under a pressure of 30 MPa.

The medium was then brought back to room temperature and atmospheric pressure, with a duration of descent in temperature of 45 minutes.

At the end of these treatments, the reactor obtained a quantitative conversion of iron nitrate to nitric acid and iron oxide, as well as a conversion of most of the nitrogenous organic compound to water, CO ₂ and N ₂ . Thus, the medium obtained in fine consists of:

35 g of an aqueous liquid phase containing nitric acid HNO ₃ ; an insoluble solid precipitate, separable from the liquid phase by filtration, based on Fe ₂ O ₃ iron oxide, which comprises 0.43 g of iron; and

a gaseous phase, in which CO ₂ carbon dioxide and N ₂ nitrogen have only been detected as gases formed, and very small traces of NO _x type products. Detection of the gases formed in FIG. the gaseous phase was carried out by mass spectroscopy and chemiluminescence.

The conversion efficiency of iron in the soluble state (nitrate) to iron in the insoluble state (oxide) is thus 86%.

EXAMPLE 3 Conversion of a metal into a metal oxide

In this third example, the method of the invention has been implemented to convert a metal mass into a metal oxide, without under-production of harmful compounds in terms of environmental release such as oxides of carbon dioxide. NO _x nitrogen, salts or acids. For this purpose, an aqueous solution of nitric acid corresponding to a nitric acid mass of 20 g was introduced into the reactor of Example 1. In this solution was introduced an iron screw corresponding to an iron mass of 1.85 g. The reactor was then closed and NH ₃ ammonia was injected at a pressure of 1 MPa. At the same time, the temperature of the medium was raised from room temperature to 490 ° C in 20 minutes.

The medium was held for 15 minutes at a temperature of 490 ° C under a pressure of 25 MPa. The medium was then cooled to room temperature and atmospheric pressure with a 45-minute temperature-lowering time.

At the end of this treatment, a quantitative conversion of the iron screw initially introduced in the form of an Fe ₂ θ ₃ iron oxide precipitate (rusty precipitate) was obtained in the reactor. Fe ₂ O ₃ was performed by X-ray analysis).

Claims

A method of treating an aqueous medium (m) containing at least one metal salt of a metal M, selected from a nitrate or a sulfate, for converting said salt of the metal M to an oxide of the metal M, said method of treatment comprising a step (a) of treating said aqueous medium (m) at a temperature of 100 to 700 ° C and a pressure of 1 to 50 MPa, whereby a conversion of the initial medium (m) to a medium that includes:

an aqueous phase which contains nitric or suifuric acid; and

a solid phase based on metal oxide M, and wherein said process then comprises a stage where the aqueous phase which contains nitric or sulfuric acid is recovered and / or treated, in whole or in part.

2. The method of claim 1 wherein the aqueous phase which contains the acid obtained at the end of step (a) is recovered in whole or part as an aqueous acid solution.

3. The process according to claim 1, wherein the metal salt of the metal M is a nitrate, whereby the aqueous phase obtained contains nitric acid, and wherein the process comprises an additional step (b) of treating the nitric acid. obtained during step (a) with NH ₃ ammonia at a temperature of 100 to 700 ^° C. and at a pressure of 1 to 50 MPa, so as to convert the nitric acid produced in step (a). ) to N ₂ and H ₂ O.

4. Process according to any one of claims 1 to 3, characterized in that the oxide of the metal M obtained is extracted from the medium by a solid / liquid separation.

5. Method according to any one of claims 1 to 5, characterized in that step (a) is condμite at a temperature between 250 and 550 ⁰ C and at a pressure between 10 and 40 MPa.

6. Method according to any one of claims 1 to 5, characterized in that step (a) is conducted under supercritical conditions, namely at a temperature greater than 374 ° C and a pressure greater than 22.1 MPa (221 bars).

7. Method according to any one of claims 1 to 6, characterized in that the metal M is selected from beryllium (Be), magnesium (Mg), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb) ), molybdenum (Mo), zirconium (Zr), tin (Sn), thorium (Th), uranium (U), plutonium (Pu), aluminum (Al) and lead ( bp).

8. Process according to claim 7, characterized in that the metal M is iron.

9. Process according to any one of claims 1 to 8, characterized in that the metal salt present in the aqueous medium (m) is a sulfate of the metal M, step (a) then leading to a conversion of the metal salt to oxide of the metal M and sulfuric acid, whereby a conversion of the initial medium (m) into a medium which comprises:

an aqueous phase (aq _su i _f ) which contains sulfuric acid; and

a solid phase based on metal oxide M, in that the aqueous phase (aq _su if) obtained at the end of step (a) is recovered and / or treated, in whole or in part.

10. The method of claim 9, for substantially eliminating the metal cations and sulphate anions present in the medium (m), characterized in that it comprises, following step (a), a step of separating the solid phase based on metal oxide M and the aqueous phase (aq _su i _f ), then a step of removing the sulfate ions contained in the aqueous phase (aq _su if _f ) -

11. The method of claim 9, characterized in that all or part of the aqueous phase (aq _su i _f ) is recovered as aqueous sulfuric acid solution.

12. Process according to claim 9, characterized in that it is a process carried out continuously, in which all or part of the aqueous phase (aq _su i _f ) obtained at the end of said step ( a) is used to attack a metal, whereby an aqueous solution of a metal sulphate which is reinjected is obtained as an aqueous medium (m) in recipe (a).

13. Process according to any one of claims 1 to 8, characterized in that the metal salt present in the aqueous medium (m) is a nitrate of the metal M, step (a) then leading to a conversion of the metal salt to oxide of the metal M and nitric acid, whereby a conversion of the initial medium (m) into a medium which comprises:

an aqueous phase (aq _n i _tr ) which contains nitric acid; and - a phase solid based metal M oxide in that the aqueous phase (aq jtr _n) obtained at the end of step (a) is recovered and / or treated, in whole or part.

14. The method of claim 13, characterized in that all or part of the aqueous phase (aq _n itr) obtained at the end of step (a) is recovered as an aqueous solution of nitric acid.

15. Process according to claim 13, characterized in that it is a process carried out continuously, in which all or part of the aqueous phase (aq _n i _t r) obtained at the end of said step (a) is used to attack a metal, whereby an aqueous solution of a metal nitrate which is reinjected is obtained as an aqueous medium (m) in step (a).

16. The method of claim 13, characterized in that the method comprises a step (b) of treating the nitric acid obtained in step (a) with ammonia NH ₃ at a temperature of 100 to 700 ⁰ C and under a pressure of 1 to 50 MPa, so as to convert the nitric acid produced in step (a) into N ₂ and H ₂ O.

17. The method of claim 16, characterized in that, in step (b), the molar ratio NH ₃ / HNO ₃ of the amount of ammonia injected into the medium, relative to the amount of nitric acid present in the medium at the end of step (a) is between 4: 3 and 2: 1.

18. The method of claim 16 or claim 17, characterized in that the step (a) of converting the metal nitrate oxide and nitric acid is followed by said step (b), this step (b) of injecting ammonia NH ₃ in the medium obtained at the end of step (a), and to leave the medium under the temperature and pressure conditions of step (a).

19. The method of claim 16 or claim 17, characterized in that step (a) and step (b) are carried out jointly within the same reactor, this joint implementation of the two stages. (a) and (b) being carried out by conducting step (a) under an atmosphere containing NH ₃ ammonia.

20. Method according to one of claims 1 to 8, characterized in that the aqueous medium (m) treated in step (a) further contains an organic compound, and in that said step (a) is conducted in presence of an oxidizing agent for the degradation of said organic compound into mineral species.

21. The method of claim 20, characterized in that an oxidizing agent is introduced within the step (a), in an amount sufficient to degrade said organic compound into mineral species.

22. The method of claim 20 or claim 21, characterized in that the aqueous medium (m) initially contains nitrates, whereby step (a) leads to the conversion of these nitrates into nitric acid which then plays the role of oxidizer.

23. Process according to any one of Claims 20 to 22, characterized in that the organic compound is a nitrogenous organic compound, whereby NO _x nitrogen oxides are formed at the end of the stage. (a), and in that the process comprises a step (b1) for treating nitrogen oxides NOx formed in the presence of ammonia, so as to convert these NOx nitrogen oxides to N ₂ and H ₂ O.

24. Process according to any one of claims 1 to 17, for the treatment of a medium containing said aqueous medium (m) and a gaseous phase containing NO _x nitrogen oxides, characterized in that it comprises, more than step (a), a step (b2) of treatment of nitrogen oxides NO _x in the presence of ammonia, so as to convert these nitrogen oxides NO _x N ₂ and H ₂ O.

25. The process as claimed in claim 24, for converting a metal mass of the metal M into an oxide of said metal, characterized in that it comprises the steps of: (A ₀ ) etching the metal M with an aqueous solution nitric acid, whereby said aqueous medium (m) comprising a nitrate of said metal M and a gaseous phase containing NO _x nitrogen oxides;

(A) treating said aqueous medium (m) at a temperature of 100 to 700 ^° C and a pressure of 1 to 50 MPa, whereby the nitrate of the metal M is converted to the oxide of the metal M and nitric acid; and

(B) treating the medium obtained at a temperature of 100 to 700 ^° C. and at a pressure of 1 to 50 MPa in the presence of ammonia, so as to convert the nitric acid formed in step (A) and the oxides of nitrogen NO _x initially present in the gas phase in N ₂ and H ₂ O.

26. The method of claim 25, characterized in that the steps (A ₀ ), (B) and (C) are conducted jointly within the same reactor, by contacting the metal M and the solution of nitric acid under an atmosphere containing ammonia, in a closed chamber heated to a temperature of 100 to 700 ⁰ C and a pressure of 1 to 50 MPa.

27. A process according to claim 25 or claim 26 for converting a metal-based metal waste M into the form of a metal oxide powder.

28. The method of claim 27 for the conversion of button cells or spent nuclear fuel.

29. The method of claim 25, for the conversion for the conversion of scrap metal comprising metallic elements based on the metal M and organic compounds, oxide of said metal M, water, and inorganic compounds, characterized in that it comprises the steps of:

(A ₀ ) etching of the metal M with an aqueous solution of nitric acid, whereby said aqueous medium (m) comprising a nitrate of said metal M and the organic compounds initially present in the medium, and a gaseous phase are obtained; containing nitrogen oxides NO _x ,; (A) treating the aqueous medium (m) thus obtained at a temperature of 100 to

700 ⁰ C and under a pressure of 1 to 50 MPa ₁ whereby the nitrate of the metal M is converted to oxide of the metal M and nitric acid, the nitric acid then playing the role of oxidizing and converting organic compounds into inorganic compounds; and (B) treating the medium obtained at a temperature of 100 to 700 ^° C. and under a pressure of 1 to 50 MPa in the presence of ammonia, so as to transform the nitric acid possibly remaining at the end of the step (A) and nitrogen oxides NO _x to N ₂ and H ₂ O.

30. The method of claim 29, characterized in that step (A) is conducted by adding an additional oxidizing agent to the medium.

31. The method of claim 29 or claim 30 for converting waste comprising a charge of organic nature into a metal casing based on the metal M, oxide of said metal M, water, and inorganic compounds.

32. The method of claim 31 for the conversion of a used chemical weapon.