EP1771387A1 - Method for processing aqueous effluents containing metal nitrate or nitrite salts - Google Patents

Abstract

The invention relates to a method for processing an aqueous effluent (E) containing in solution at least one type of metal (M) nitrite or nitrate whose oxide is stable in an aqueous medium consisting (I) in introducing cations NH4+ in aqueous solution, generally in the form of an aqueous ammonia solution, into the aqueous effluent and, subsequently (II) in processing the obtainable medium at a temperature ranging from 100 to 700 °C and a pressure of 1-50 MPa (i.e. from 10 to 500 bar), thereby converting the metal (M) nitrite or nitrate into metal (M) oxide, nitrogen and water.

Description

 A process for the treatment of aqueous effluents comprising nitrate or nitrite-type metal salts

The present invention relates to a process for the treatment of aqueous effluents containing nitrate or nitrite-type metal salts, enabling the conversion of these salts to insoluble metal species of oxides type, without the formation of by-products such as acids or nitrogen oxides. This process is particularly suitable for removing nitrate or nitrite-type metallic species in industrial aqueous effluents prior to their release into the environment.

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

In view of the standards in force, which limit the content of metal ions and anions in particular nitrite and nitrate type in effluents discharged by industries, it is necessary to treat the aqueous media containing sulphates or metal nitrates obtained according to the aforementioned methods , so as to substantially eliminate these metal salts, before they can consider their release into the environment. To achieve the elimination of metal salts in aqueous effluents of this type, various solutions have been proposed in the past, which involve, for example, processes of precipitation, bioprecipitation, crystallization, ion exchange, electrolysis, or more specific techniques such 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 effective, but induce high treatment costs. In addition, most of the processes described in the context of the treatment of effluents containing nitrates or nitrites lead ultimately to the production of nitric or nitrous acid and / or nitrogen oxides, which constitute pollutants that it is necessary to eliminate the effluent in later stages (filtration, trapping ...) before considering a rejection in the environment, which, again, translates into an increase in the cost of the process .

An object of the present invention is to provide a new process for the removal of nitrate or nitrite 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 method for treating an aqueous effluent E containing, in solution, at least one nitrate or a nitrite of a metal M whose oxide is stable in an aqueous medium, said process comprising the steps of:

(I) introducing into the aqueous effluent E NH ₄ ⁺ cations in aqueous solution, these NH ₄ ⁺ cations being introduced preferably in the form of an aqueous solution of ammonia; then

(II) treat the medium obtained at a temperature of 100 to 700 ^° C. and at a pressure of 1 to 50 MPa (that is to say between 10 and 500 bar), whereby a conversion of the nitrate or nitrite of metal M to metal oxide M, nitrogen and water.

The process of the invention makes it possible to convert the nitrate or the nitrite of the metal M into an insoluble form of this metal, namely into an oxide of the metal M

This conversion of a soluble form (nitrate or nitrite) of the metal M into an insoluble form can advantageously be used to extract the metal M from the aqueous solution. In this case, the oxide of the metal M obtained at the end of step (II) is extracted from the medium by solid / liquid separation, usually by filtration. Any other method of solid / liquid separation can however be envisaged, such as for example a separation by centrifugation, or even by decantation.

Provided that step (II) is carried out for a period of time sufficient for the conversion to be effective, treatment of the aqueous effluent E at a temperature of 100 to 700 ^° C. and under a pressure of 1 at 50 MPa allows, most often, a substantial conversion of the nitrate or nitrite of the metal M oxide, generally insoluble. Depending on the nature of the metal M, the conversion rate of the nitrate or nitrite of the metal M into oxide obtained at the end of step (II) can vary to a large extent, however, in the case Generally, the above-mentioned temperature and pressure conditions are capable of allowing a conversion in the oxide form of at least 60%, and most often at least 70% of the nitrates or nitrites of the metal M present in the aqueous effluent E initial. In most cases, a conversion rate of the nitrate or nitrites of the metal M into oxide of 80% or more is obtained, this conversion rate being generally at least 85%, and up to 90% or even 95%, and even ^" 99% or more in some cases.

In particular so that the conversion of nitrate or nitrite of the metal M oxide is as effective as possible, it is generally preferred that step (II) of the process is conducted at a temperature of at least 250 ⁰ C, preferably, at least 300 ^° C., and advantageously at least 350 ^° C. Thus, according to an advantageous embodiment, step (II) is carried out at a supercritical temperature, namely a temperature greater than 374 ° C. vs. Furthermore, in particular to reduce the costs of the process, the reaction can be conducted at a temperature of less than or equal to 600 ° C., this temperature generally remaining below or equal to 550 ° C. Thus, in order to obtain an advantageous cost-effectiveness ratio for the process, the temperature of implementation of step (II) 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 nitrites of the metal M into oxide, the step (II) 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 (II) 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 (II) may advantageously be carried out under a pressure of less than or equal to 40 MPa (400 bar), or even less than or equal to 30 MPa (300 bar). Thus, interestingly in particular in terms of efficiency and cost, step (II) may for example be conducted under a pressure of between 10 and 40 MPa (for example between 20 and 35 MPa, typically between 25 and 30 MPa ). Thus, according to one embodiment of the invention which is most often advantageous, step (II) 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 40O and 500 ⁰ C and under a pressure of 25 to 30 MPa).

It should be emphasized that, in addition to obtaining an efficient conversion of the nitrates or nitrites of the effluent E into insoluble metal oxides, the method of the invention makes it possible to carry out this transformation without leading to the formation undesirable by-products such as acids or nitrogen oxides. For this purpose, it is preferable, in the most general case, in step (I) the cations NH ₄ ⁺ in aqueous solution is introduced so as to obtain a molar ratio NH ₄ ⁺ / NO ₃ to ^~ less than 1: 3 in the medium to be treated in step (II). Preferably, for preventing or minimizing formation of byproducts type NO _x or acids, NH ₄ ⁺ cations are introduced in step (I) so that the molar ratio NH ₄ ⁺ ZNOs ^"is at least equal to 2: 3 and advantageously at least 3: 3. Notably in order to reduce the costs of the process, this molar ratio remains, however, advantageously to be less than or equal to Thus, according to an advantageous embodiment, the NH ₄ ⁺ cations are introduced in step (I) so that the NH ₄ ⁺ / N ₃ ^' molar ratio in the medium to be treated in the step (II) is between 4: 3 and 6: 3, for example between 4.5: 3 and 5.5: 3, this ratio being typically of the order of 5: 3.

In view of these different aspects, the process of the invention is particularly particularly useful for carrying out the treatment of industrial effluents prior to their discharge into the environment. Indeed, in most cases, it is possible to envisage a rejection of the liquid and gaseous media obtained at the end of stage (II), which most often contain an extremely reduced content of metal salts, acids and gases of the type nitrogen oxide. The solid phase is most often easily recoverable, for example by a simple filtration or decantation step.

In particular, the process of the invention is suitable for the treatment of effluents resulting from the treatment of metals with nitric acid. However, much more generally, most of the aqueous effluents containing nitrates and / or metal nitrites whose oxides are stable in aqueous medium can be treated according to the process of the invention.

Most often, the metal M whose nitrate or nitrite is present in the effluent E treated according to the method of the invention is chosen from: - certain alkaline earth metals such as beryllium (Be) and magnesium (Mg) or else strontium (Sr);

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),

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, or even lead or tin. In particular, the process of the invention is well suited to the treatment of an aqueous medium (m) containing iron nitrates, iron nitrites, or mixtures of these salts.

According to a particular variant of the process of the invention, the treated aqueous effluent E may contain an organic compound, in addition to the nitrates and / or nitrites present in aqueous solution.

In this case, step (II) may advantageously be used to achieve a hydrothermal degradation of the organic compound. It should be emphasized that the temperature and pressure conditions of stage (II) are conditions well adapted to hydrothermal degradation of most organic compounds. By "hydrothermal degradation of an organic compound" is meant, within the meaning of the present description, a conversion of an organic species into mineral species (for example a conversion in the form of water, and CO ₂ 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 been described in particular in the article entitled "Hydrothermal oxidation: a new concept for treatment of industrial and urban liquid waste" by M. Bottreau (Supercritical Fluids and Materials, ISBN -2-905267-39-9).

Thus, when the effluent E contains an organic compound that is to be removed, the step (11) is generally conducted with the additional presence of an oxidizing agent, in an amount sufficient to oxidize said organic compound under the conditions of step (II).

The oxidizing agent used in this context can be chosen from any oxidizing agent customary in the field of hydrothermal degradation, such as, for example, gaseous oxygen, hydrogen peroxide, nitric acid, air or a mixture of two or more of these oxidants. 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 applied in the field of hydrothermal degradation.

In a particular embodiment, the organic compound present may be a nitrogenous organic compound. Such a compound is generally led to the formation of nitrogen oxides NO _x (NO, N ₂ O ₄ , NO ₂ , or N ₂ O, in particular) under the conditions of stage (II). In this case, it is therefore most often desirable to eliminate these nitrogen oxides NO _x , especially when the process of the invention is carried out as part of a treatment of industrial effluent intended for release into the atmosphere. 'environment. To this end, the process can advantageously be carried out by introducing, in step (I), an additional quantity of NH ₄ ⁺ cations in aqueous solution, generally in the form of an aqueous ammonia solution, for converting these oxides of nitrogen NO _x in nitrogen and water in step (II).

Thus, when the effluent E treated according to the process of the invention comprises a nitrogenous organic compound, it is most often advantageous that, in step (1), NH ₄ ⁺ cations in aqueous solution are added so as to obtain, The medium to be treated in step (II) an NH4 ⁺ / (NO ₃ ^" + Norg) molar ratio (where N _or g represents the amount of nitrogen present in the nitrogenous organic compound) of at least 1: 3, and preferably at least equal to 2: 3. More preferably, especially in order to avoid as much as possible the formation of by-products, this ratio NH4 ⁺ / (NO ₃ ^' + N _O rg) is at least equal to 3: 3, and advantageously of at least 4: 3, this ratio generally remaining less than or equal to 6: 3, especially for cost reasons, Thus, interestingly in terms of efficiency / cost ratio, the ratio NH ₄ ⁺ / (NO ₃ ^' + N _org ) can be between 4: 3 and 6: 3, for example between 4.5: 3 and 5.5: 3, this ratio being typically of the order of 5: 3.

According to another variant of the process of the invention, the aqueous effluent E may also contain nitrogen oxides NO _x , in addition to the nitrate or nitrite type salts present in aqueous solution. In this case, it is here again most often indicated that the process is conducted by introducing a quantity additional NH ₄ ⁺ cations in aqueous solution in step (I), generally in the form of an aqueous ammonia solution, for converting these nitrogen oxides NO _x to nitrogen and water in step (II) ). In this case, the NH ₄ ⁺ cations in aqueous solution are generally added in step (I) so as to obtain in the medium to be treated in step (II) an NH ₄ ⁺ Z molar ratio (NO ₃ ^" + NO _x ) at least equal to 1: 3, preferably at least 2: 3, and advantageously at least equal to 3: 3, this ratio nevertheless remaining generally less than or equal to 6: 3, in particular for reasons of Thus, this ratio NH ₄ ⁺ / (NO ₃ ^' + NO _X ) is advantageously between 4: 3 and 6: 3, for example between 4.5: 3 and 5.5: 3, this ratio being typically the order of 5: 3.

According to another particular variant, the aqueous effluent E contains, in addition to the nitrates or nitrites of the metal M, both an organic compound and NO _x nitrogen oxides. In this case, the process advantageously has the following characteristics: in step (I), an additional quantity of NH ₄ ⁺ cations in aqueous solution is introduced, preferably in the form of an aqueous ammonia solution, for iesdits oxides convert NO _x into nitrogen and water in step (II); and

step (II) is conducted with the additional presence of an oxidizing agent, in an amount sufficient to oxidize said organic compound under the conditions of step (II).

According to this particular variant, when the aqueous effluent E is an organic compound which is nitrogenous as an organic compound, leading to the formation of NO _x under the conditions of stage (II), it is generally indicated that an additional quantity of NH cations ₄ ⁺ in aqueous solution is introduced in step (I) to convert these oxides NO _x to nitrogen and water in step (II). In this case, in step (I), NH ₄ ⁺ cations are generally introduced in aqueous solution so as to obtain in the medium to be treated in step (II) an NH ₄ ⁺ / (NO 3> molar ratio> NO _X + N _or g) (where N _or g represents the amount of nitrogen present in the nitrogenous organic compound) of at least 1: 3, preferably at least equal to 2; 3, and advantageously at least equal to 3: 3, this ratio nevertheless remaining more often less than or equal to 6: 3. This NH ₄ V ratio (NO ₃ ^" + NO _x + N _org ) is advantageously between 4: 3. and 6: 3, for example between 4.5: 3 and 5.5: 3, and is typically in the range of 5: 3.

Various aspects and advantages of the process of the invention will become even more explicit in the light of the examples given below.

EXAMPLE 1 Treatment of an aqueous effluent containing a metallic nitrate (iron nitrate)

9.5 g of an aqueous solution of iron nitrate of concentration equal to 111.2 g / l were treated according to the process of the invention. This solution initially contains 0.24 g of Fe in the solubilized salt state.

To these 9.5 g of aqueous solution of iron nitrate, it was first added

4.2 g of aqueous ammonia solution NH ₄ OH at 173 g / l.

The medium obtained was then introduced into a glass tube, which was placed in a closed reactor, at an initial temperature of 40 ^° C. The reactor, of closed type, has a capacity of 100 ml and is equipped with a heating coil, and is coupled to a high pressure bench. The temperature of the reactor was raised from 40 ° C. to 45 ^° C. over a period of 20 minutes, whereby a pressure of 25.5 MPa was obtained within the reactor.

The medium was treated under the supercritical conditions thus reached (temperature of 450 ^° C. and pressure of 25.5 MPa) for 15 minutes.

Following this treatment, the medium was brought back to atmospheric pressure and allowed to cool to room temperature (25 ^° C.) over 45 minutes.

At the end of this treatment, a quantitative conversion of iron nitrate to iron oxide, N ₂ and H ₂ O was obtained. More specifically, the aqueous medium obtained at the end of the treatment contains 0.33 g of Fe ₂ O ₃ , the theoretical mass expected Fe ₂ O ₃ , corresponding to the total conversion of Fe initially present in the solution, being 0.35g. In other words, a conversion of iron nitrate to Fe ₂ O ₃ oxide was obtained with a yield of 94%.

EXAMPLE 2 Treatment of an aqueous effluent containing a metallic nitrate (iron nitrate) and an organic compound (acetic acid).

Aqueous medium consisting of a mixture of 4.8 g of an aqueous solution of iron nitrate of concentration equal to 111.2 g / l and 3 g of acetic acid was treated according to the process of the invention. This solution initially contains 0.12 g of Fe in the solubilized salt state and has a COD (chemical oxygen demand) of 35 g / l.

To these 9.5 g of aqueous solution of iron nitrate was added 2.2 g of an aqueous ammonia solution NH ₄ OH at 173 g / l and 4.4 g of an aqueous solution of peroxide of hydrogen in aqueous solution at 50% by weight.

The medium thus obtained was introduced into a glass tube, which was placed in a closed reactor, at an initial temperature of 40 ^° C. The reactor, of closed type, has a capacity of 100 ml and is equipped with a heating coil, and is coupled to a high pressure bench. The temperature of the reactor was raised from 40 ° C. to 450 ^° C. over a period of 20 minutes, whereby a pressure of 25.5 MPa was obtained within the reactor.

The medium was treated under the supercritical conditions thus reached (temperature of 450 ^° C. and pressure of 25.5 MPa) for 15 minutes.

Following this treatment, the medium was cooled to atmospheric pressure and allowed to cool to room temperature (25 ° C) in 45 minutes.

At the end of this treatment, a quantitative conversion of the iron nitrate and of the organic compound was obtained in iron oxide, N ₂ , CO ₂ , and H ₂ O. More specifically, the aqueous medium obtained at the end of the treatment contains: 0.165 g of Fe ₂ O ₃ , the theoretical expected weight of Fe ₂ O ₃ , corresponding to the total conversion of Fe initially present in the solution, being 0.18 g (a conversion of iron nitrate to Fe ₂ O ₃ oxide) with a yield of 92% was obtained);

an organic compound content corresponding to a final COD of

1.75 g / l (95% COD reduction).

Claims

A method for treating an aqueous effluent E containing, in solution, at least one nitrate or nitrite of a metal M whose oxide is stable in an aqueous medium, said method comprising the steps of: (I) introducing into the aqueous effluent E NH ₄ ⁺ cations in aqueous solution; then (II) treat the medium obtained at a temperature of 100 to 700 ^° C. and at a pressure of 1 to 50 MPa (that is to say between 10 and 500 bar), whereby a conversion of the nitrate or nitrite of metal M to metal oxide M, nitrogen and water. 2. Method according to claim 1, characterized in that, in step (I), NH ₄ ⁺ cations are introduced in the form of an aqueous solution of ammonia. 3. Method according to claim 1 or claim 2, characterized in that, in step (I), the NH ₄ ⁺ cations in aqueous solution are introduced so as to obtain in the medium to be treated in the step ( II) a molar ratio NH ₄ ¹ VNO ₃ ^' at least equal to 1: 3, and preferably between 4: 3 and 6: 3. 4. Process according to any one of Claims 1 to 3, characterized in that step (II) is carried out at a temperature of between 250 and 550 ^° C. and under a pressure of between 10 and 40 MPa. 5. Method according to any one of claims 1 to 4, characterized in that step (II) is conducted under supercritical conditions, namely at a temperature greater than 374 ° C and a pressure greater than 22.1 MPa (221 bars). 6. Method according to any one of claims 1 to 5, characterized in that the metal M is selected from beryllium (Be), magnesium (Mg), strontium (Sr), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), molybdenum (Mo), thorium ( Th), uranium (U), plutonium (Pu), aluminum (Al), lead (Pb) and tin (Sn). 7. Process according to claim 6, characterized in that the metal M is iron. 8. Process according to any one of claims 1 to 7, characterized in that the aqueous effluent E further contains an organic compound, and in that step (II) is conducted with the additional presence of an agent oxidizing agent, in an amount sufficient to oxidize said organic compound under the conditions of step (11). 9. Method according to claim 8, characterized in that the organic compound is a nitrogenous organic compound capable of leading to the formation of nitrogen oxides NO _x under the conditions of stage (II), and in that, in step (I), an additional quantity of NH ₄ ⁺ cations in aqueous solution is introduced to convert these nitrogen oxides NO _x to nitrogen and water in stage (II). 10. Process according to claim 9, characterized in that, in step (I), NH / in aqueous solution cations are added so as to obtain in the medium to be treated in step (II) an NH4 molar ratio. ⁺ / (NO3 ^" + N _or g) (where N _or g represents the amount of nitrogen present in the nitrogenous organic compound) of at least 1: 3, and preferably of between 4: 3 and 6: 3. 11. Process according to any one of Claims 1 to 10, characterized in that the aqueous effluent E additionally contains oxides of nitrogen NO _x , and in that, in stage (I), a additional quantity of NH ₄ ⁺ cations in aqueous solution to convert these nitrogen oxides NO _x to nitrogen and water in step (II). 12. Process according to claim 11, characterized in that, in step (I), NH / aqueous solution cations are added in order to obtain in the medium to be treated in step (II), an NH ₄ ¹ V molar ratio (NO ₃ ^~ + NO _x ) of at least 1: 3, and preferably of between 4: 3 and 6: 3. 13. Process according to any one of Claims 1 to 12, characterized in that the aqueous effluent E contains, in addition to the nitrate or nitrite of the metal M, an organic compound and NO _x nitrogen oxides, and what: - in step (I) is introduced an additional quantity of NH ₄ ⁺ cations in aqueous solution to convert said oxides NO _x into nitrogen and water in step (II); and step (II) is conducted with the additional presence of an oxidizing agent, in an amount sufficient to oxidize said organic compound under the conditions of step (II). 14. The method of claim 13, characterized in that the organic compound is an organic compound nitrogen under the conditions of step (II), leading to the formation of NOx under the conditions of step (II), and in step (I), an additional quantity of NH ₄ ⁺ cations in aqueous solution is introduced to convert the oxides NO _x to nitrogen and water in step (II). 15. Method according to claim 14, characterized in that, in step (I), NH ₄ ⁺ cations are introduced in aqueous solution so as to obtain in the medium to be treated in step (II) a molar ratio. NH ₄ V (NO ₃ ^' + NO _x + N _or g) (where N _O rg represents the amount of nitrogen present in the nitrogenous organic compound) of at least 1: 3, and preferably of between 4: 3 and 6: 3.