FR2871791A1 - PROCESS FOR TREATING AQUEOUS MEDIA COMPRISING METAL SALTS OF THE NITRATE OR SULFATE TYPE - Google Patents

Abstract

The subject of the present invention is a process for treating an aqueous medium (m), containing at least one nitrate or a metal sulphate, enabling the conversion of said soluble nitrate or sulphate to an insoluble oxide and to nitric or sulfuric acid, said process process 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.The invention also relates to the use of this method of converting soluble metal salts of insoluble species for purifying aqueous effluents from attacking metals with nitric acid or sulfuric acid.

Description

2871791 i

  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 nitrates or sulphates 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 carry out the elimination of metal salts in aqueous effluents of this type, various solutions have been proposed, for example using 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.

  To this end, 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 selected from a nitrate or a sulphate enabling the conversion of said metal salt M This method of treatment 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) .

  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 metal oxide M obtained is extracted from the medium by a solid / liquid separation, generally by filtration. Any other method of solid / liquid separation can 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 a pressure of 1 to 50 MPa allows, more often than not, 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 able to allow the conversion in oxide form of at least 60%, 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 efficient as possible, it is generally preferable for step (a) of the process to be carried out at a temperature of at least 250.degree. preferably, at least 300 ° C., and advantageously at least 350 ° C. Thus, according to an advantageous embodiment, step (a) is carried out at a supercritical temperature, namely a temperature greater than 37 ° 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, in order to obtain an advantageous cost / efficiency ratio for the process, the The operating temperature of step (a) is advantageously between 250 ° C. and 550 ° C., and preferably between 300 ° C. and 500 ° C. (typically between 400 ° C. 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 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 40 MPa and (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 sulfuric or nitric acid. 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), niobium (Nb), molybdenum (Mo), zirconium (Zr), or tin (Sn); metals of the family of lanthanides or actinides (group "f" of the periodic table of elements) such as, in particular, thorium (Th), uranium (U) and plutonium (Pu); - other metals such as aluminum (AI) 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 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 (aqsulf) which contains sulfuric acid and which generally has 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 (aqsulf), in particular by filtration.

  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 (agsuif), this step being generally carried out by filtration, but any other type of solid / liquid separation may nevertheless be used. Thus, on the one hand, most of the metal elements in the form of a solid, more easily manipulated and storable than in their soluble form in the medium (m) initial, and, on the other hand, the aqueous phase (aqsulf) containing the starting sulphate ions and having a reduced content of metal cations. The aqueous phase (agsuif) thus separated is then subjected to a step of removing the sulfate ions it contains, this desulfation of the aqueous phase (aqsulf) can be conducted by any usual means, for example by ion exchange. In the context of such a purification process, the implementation of the process of the invention allows a quantitative and easy elimination 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 that can be envisaged when the aqueous medium (m) contains metal sulphates, all or part of the aqueous phase (agsuif) 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 (agsuif) is an aqueous solution of sulfuric acid which is generally recoverable industrially, particularly as a solution to carry out an attack or stripping of metals. In the context of this specific embodiment, the aqueous phase (agsuif) recovered can be recycled in a continuous process: where appropriate, the process of the invention is a continuous process in which all or part of the aqueous phase (agsuif) obtained at the end of said step (a) is used to attack a metal, whereby an aqueous solution of a metal sulphate is obtained which is reinjected, 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 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, whereby, at the end of step (a), a conversion of initial medium (m) in a medium which comprises: - an aqueous phase (agnitr) which contains nitric acid, and which generally has a very low content of metal cations; and a solid phase based on metal oxide M, this solid phase being easily separable from the aqueous phase (agnitr), in particular by filtration.

  In a similar manner to the (agsuif) phase of the preceding variant, the aqueous phase (agnitr) constitutes an aqueous solution of low metal species acid which is exploitable industrially, in particular as a nitric acid solution for perform an attack or stripping of metals. Also, according to a particular embodiment of the process of the invention, all or part of the aqueous phase (agnitr) obtained at the end of step (a) can be recovered as an aqueous solution of nitric acid, the aqueous phase thus recovered can 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 process of the invention according to this second variant can be a process conducted continuously, in which all or part of the aqueous phase (agn; tr) 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).

  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 (agnitr) obtained at the same time. the outcome of step (a). This is particularly the case when the process uses, as an aqueous medium (m), an aqueous effluent containing metal nitrates, in which it is sought to substantially eliminate the nitrate anions and the metal cations, for example with a view to rejection. 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 100 to 700 ° C and a pressure of 1 to 50 MPa, so as to convert the nitric acid produced in step (a) into N2 and H2O. Preferably, step (b) is carried out 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 NH3 / HNO3 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 ammonia NH3 into the medium obtained at the end of step (a), leaving the medium under the conditions of temperature and pressure of step (a).

  When the steps (a) and (b) are carried out successively, it is possible, if desired, to treat only a part of the nitric acid obtained in the aqueous phase (aqnitr) at the end of step (a), bypassing part of the aqueous phase (agnitr) 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 ( agnitr) 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 into step (a); and the other part of the nitric acid obtained in the aqueous phase (agnitr) 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 of conducting step (a) as defined above under an atmosphere containing ammonia NH3: the imposed temperature and pressure conditions induce then 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 N2 and H2O in the presence of gaseous ammonia present in the atmosphere of the reactor.

  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 to 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 pressurization 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 suitable for carrying out such conversion by hydrothermal degradation. of 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 2871791 i0 which then acts as an oxidant . 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 certain cases, if the nitrates are present in a sufficient quantity in the initial medium, the nitric acid resulting from their conversion may even be sufficient, on its own, 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, formation of NOx nitrogen oxides (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 to N2 and H2O. The amount of ammonia to be used in this step for efficiently converting NOx to N2 and H2O is advantageously such that the molar ratio NH3INOX is between 4/3 and 2/1, this ratio being preferably at least 5 / 3.

  In practice, step (b1) is generally conducted under the same conditions of temperature and pressure as step (a).

  Thus, step (b1) can for example be conducted after step (a), by injecting ammonia NH3 into the medium obtained at the end of step (a), and leaving the most often the medium under the temperature and pressure conditions of step (a).

  Alternatively, step (b1) may be carried out in conjunction with step (a) in the same reactor: in this second case, the process is carried out by carrying out step (a) under a atmosphere containing ammonia, whereby the NOx 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 which it is desired to treat according to the process of the invention contains NO oxides of nitrogen. Thus, according to a particular embodiment of the invention, the process can be carried out to treat a medium containing, in addition to the aqueous medium (m), a gaseous phase containing NO oxides of nitrogen. In this case, the medium obtained at the end of step (a) comprises such nitrogen oxides NO 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 nitrogen oxides NOx in the presence of ammonia, so as to convert these NOx nitrogen oxides to N2 and H2O. In general, this step (b2) is then carried out 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 steps (b), (b1) and (b2), these steps are combined in a single step.

  According to an interesting embodiment, the medium (m) treated in 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) etching the metal M with an aqueous solution of nitric acid, whereby said aqueous medium is obtained (m) comprising a nitrate of said metal M and a gaseous phase containing NO oxides of nitrogen; (A) treating said aqueous medium (m) at a temperature of 100 to 700 ° C. and at a pressure of 1 to 50 MPa, and 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; and (B) treating the obtained medium 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 NOX nitrogen present initially in the gas phase in N2 and H2O.

  As a rule, the step (Ao) is carried out initially, prior to step (A). However, according to a feasible embodiment, which is particularly interesting in terms of reaction time and space, the steps (Ao), (B) and (C) can be conducted jointly within the same reactor. In this case, the method generally comprises contacting the metal M and the nitric acid solution (generally by immersing the metal M in the nitric acid solution) under an atmosphere containing ammonia, in a closed chamber brought 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 (Ao), (A) and (B), whether conducted successively or together in the same reactor, makes it possible in particular to convert a metal mass of the metal M into an oxide of said metal, generally recoverable at the end of step (B) in powder form (by filtration and drying for example), without leading to the formation of harmful or environmentally damaging by-products. In this context, the steps (A0), (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 (A0), (A) and (B) can be used for the conversion of specific metal scrap, comprising both metallic 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 a charge of an organic nature in a metallic 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 (A0), (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 by which is obtained 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 NO oxides, the attack of the metal being accompanied further release of the organic compounds when the treated waste includes 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 the metal M is converted to the oxide of the metal M and nitric acid, the nitric acid then acting as an oxidant and converting the organic compounds into inorganic compounds (water, CO2 and CO in particular), this step being optionally carried out by adding to medium an additional oxidizing agent (eg oxygen gas, or nitric acid) if the amount of nitric acid produced is not sufficient to ensure sufficient conversion of organic compounds present; 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 transform the nitric acid possibly remaining at the end of the step ( A) and nitrogen oxides NOX to N2 and H2O.

  Various aspects and advantages of the process of the invention will appear even more clearly in the light of the examples given below, in which the process of the invention has been implemented to effect the treatment of different aqueous media representative of the different forms. possible 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 duct connected to a bottle 3 makes it possible to introduce ammonia NH3 into the sky of the reactor, 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 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, equipped with a heating coil and coupled to a high-pressure bench. . 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 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 of an aqueous liquid phase containing nitric acid HNO 3 was obtained; and an insoluble solid precipitate, separable from the liquid phase by filtration, based on Fe 2 O 3 iron oxide, which comprises 0.45 g of iron.

  The conversion efficiency of soluble iron (nitrate) to iron in the insoluble state (oxide) obtained is thus 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 demineralized water; 2 g of a nitrogenous organic compound, namely fenuron (herbicide of formula C6H5-NH-CO-N (CH3) 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 NH3 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 cooled to room temperature and atmospheric pressure, with a 45-minute temperature descent time.

  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 into water, CO2 and N2. Thus, the medium obtained in fine consists of: - 35 g of an aqueous liquid phase containing nitric acid HNO3; an insoluble solid precipitate, separable from the liquid phase by filtration, based on iron oxide Fe 2 O 3, which comprises 0.43 g of iron; and a gaseous phase, in which carbon dioxide CO 2 and nitrogen N 2, and very small traces of NOX-type products have only been detected as gases formed. Detection of gases formed in the gas phase was performed 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 to a Metal Oxide In this third example, the process of the invention was implemented to convert a metal mass into a metal oxide, without underproduction of harmful compounds in terms of rejection. in the environment such as NOx nitrogen oxides, salts or acids.

  For this purpose, an aqueous solution of nitric acid corresponding to a mass of nitric acid 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 ammonia NH 3 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 maintained 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 transformation of the iron screw initially introduced in the form of a precipitate of iron oxide Fe 2 O 3 (precipitate of rust color) was obtained in the reactor. The Fe 2 O 3 identification was carried out by X-ray analysis).

Claims (30)

  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 the metal salt is converted to the oxide of the metal M and nitric or sulfuric acid.   2. Method according to claim 1, characterized in that the oxide of the metal M obtained is extracted from the medium by a solid / liquid separation.   3. Process according to claim 1 or claim 2, characterized in that step (a) is carried out at a temperature of between 250 and 550 ° C and a pressure of between 10 and 40 MPa.   4. Method according to any one of claims 1 to 3, characterized in that step (a) is conducted under supercritical conditions, namely at a temperature above 374 C and at a pressure greater than 22.1 MPa ( 221 bars).   5. Method according to any one of claims 1 to 4, 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).   6. Method according to claim 5, characterized in that the metal M is iron.   7. Method according to any one of claims 1 to 6, 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 (aqsulf) which contains sulfuric acid; and a solid phase based on metal oxide M. 8. Method according to claim 7, for substantially eliminating the metal cations and sulphate anions present in the medium (m), characterized in that it comprises, after step (a), a step of separating the solid phase based on metal oxide M and the aqueous phase (agsulf), then a step of removing the sulfate ions contained in the aqueous phase (agsulf) 9. Process according to claim 7, characterized in that all or part of the aqueous phase (aqsulf) obtained at the end of step (a) is recovered as aqueous sulfuric acid solution.   10. Process according to claim 7, characterized in that it is a process carried out continuously, in which all or part of the aqueous phase (aqsulf) 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).   11. Process according to any one of claims 1 to 6, 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 metal oxide M and nitric acid, whereby a conversion of the initial medium (m) into a medium which comprises: an aqueous phase (agn; tr) which contains nitric acid; and a solid phase based on metal oxide M. 12. Process according to claim 11, characterized in that all or part of the aqueous phase (agn; tr) obtained at the end of step (a) is recovered as an aqueous solution of nitric acid.   13. Process according to claim 11, characterized in that it is a process carried out continuously, in which all or part of the aqueous phase (agn; tr) 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).   14. The method of claim 11, characterized in that the method further comprises a step (b) of treating the nitric acid obtained in step (a) with ammonia NH3 at a temperature of 100. at 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.   15. Method according to claim 14, characterized in that, in step (b), the NH3 / HNO3 molar ratio 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.   16. The method of claim 14 or claim 15, 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 NH3 in the medium obtained at the end of step (a), and to leave the medium under the temperature and pressure conditions of step (a).   17. The method of claim 14 or claim 15, 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, 18. Method according to one of claims 1 to 6, 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.   19. The method of claim 18, characterized in that an oxidizing agent is introduced within the step (a), in an amount sufficient to degrade said organic compound into mineral species.   20. The method of claim 18 or claim 19, 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.   21. Method according to one of claims 18 to 20, characterized in that the organic compound is a nitrogenous organic compound, whereby a formation of nitrogen oxides NO at the end of step (a) is obtained. ), and in that the process comprises a step (b1) for treating NOx nitrogen oxides formed in the presence of ammonia, so as to convert these NOx nitrogen oxides to N2 and H2O.   22. Process according to any one of claims 1 to 15, for the treatment of a medium containing said aqueous medium (m) and a gaseous phase containing NO oxides nitrogen, characterized in that it comprises, in addition of step (a), a step (b2) for treating the nitrogen oxides NO in the presence of ammonia, so as to transform these nitrogen oxides NO to N2 and H2O.   Process according to claim 22, for the conversion of a metal mass of the metal M into an oxide of said metal, characterized in that it comprises the steps 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 a gaseous phase containing nitrogen oxides NO; (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 metal oxide M and nitric acid; and (B) treating the obtained medium 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 NOX initially present in the gaseous phase in N2 and H2O.   24. Process according to claim 23, characterized in that the steps (Ao), (B) and (C) are carried out jointly within the same reactor, by contacting the metal M with the solution of nitric acid under an atmosphere containing ammonia, in a closed chamber heated to a temperature of 100 to 700 C and a pressure of 1 to 50 MPa.   25. A process according to claim 23 or claim 24 for converting a metal-based metal waste M into the form of a metal oxide powder.   26. The method of claim 25 for the conversion of button cells or spent nuclear fuel.   27. Process according to claim 23, for the conversion for the conversion of metal scrap comprising metallic elements based on the metal M and organic compounds, into oxide of said metal M, water, and inorganic compounds, characterized in that it comprises the steps 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 NOX nitrogen oxides; (A) treating the aqueous medium (m) thus obtained at a temperature of 100 to 700 ° C. and at a pressure of 1 to 50 MPa, whereby the nitrate of the metal M is converted to the oxide of the metal M and to nitric acid, the nitric acid then acting as an oxidant and converting the organic compounds into inorganic compounds; 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 transform the nitric acid possibly remaining at the end of the step ( A) and nitrogen oxides NOx to N2 and H2O.   28. The method of claim 27, characterized in that step (A) is conducted by adding an additional oxidizing agent to the medium.   io 29. The method of claim 27 or claim 28 for converting waste comprising an organic filler in a metallic shell metal M, oxide of said metal M, water, and inorganic compounds.   30. The method of claim 29 for converting a used chemical weapon.