Method for the membrane electrodialysis treatment of an effluent from a nitriding line

Abstract

The present invention relates to a method for treating an effluent from a nitriding line by membrane electrodialysis, the method comprising the application of an electric voltage between the anode and the cathode of an electrodialysis device for separating alkali metal cations and anionic species selected from nitrates, hydroxides, nitrites, carbonates and mixtures thereof.

Description

[0001] Description

[0002] Title of the invention: Process for treating an effluent from a nitriding line by membrane electrodialysis

[0003] Technical field of the invention

[0004] The present invention relates to a process for treating an effluent from a nitriding line by membrane electrodialysis, the process comprising the application of an electrical voltage between the anode and the cathode of an electrodialysis device to separate alkali metal cations and anionic species selected from nitrates, hydroxides, nitrites, carbonates and their mixtures.

[0005] Prior state of the art

[0006] The treatment and recovery of waste from the chemical industry undoubtedly represents one of the major challenges of the 21st century. emecentury, with paramount ecological and environmental challenges. In France and Europe, regulations prioritize the principles of the circular economy, based on the famous five "Rs": reduce, reuse, repair, recycle, and reinvent. Recycling thus occupies a key position.

[0007] Mechanical parts from the automotive, aeronautical or industrial sectors are generally subjected to significant stresses during their use.

[0008] They therefore undergo a preliminary treatment called nitriding, which improves their physicochemical properties, including their friction properties, wear resistance, fatigue resistance, seizing resistance, and corrosion resistance.

[0009] Nitriding involves immersing a ferrous metal part in a nitrogen-releasing medium, such as a molten salt bath ('nitriding bath' at temperatures of around 600°C or higher). This treatment creates a unique surface structure specific to the nitriding process. Nitrogen diffusion generates surface layers with enhanced properties compared to the core of the part, particularly in terms of wear, corrosion, and fatigue resistance. In this document, nitriding also encompasses nitrocarburizing, a variant of nitriding in which carbon diffuses into the part in addition to nitrogen. The ARCOR® process, described in patents FR2972459 and FR2812888 of the Applicant, is a preferred example of a nitriding process.

[0010] The industrially used nitriding baths in which mechanical parts are immersed are baths based on cyanates and carbonates.

[0011] In practice, cyanates decompose to form, among other things, cyanides, carbonates, and nitrogen, which is then available to diffuse into the part being nitrided. Due to the consumption of cyanates and the resulting enrichment in carbonates, the nitriding bath must be regenerated, notably by adding supplements to bring the cyanide and cyanate levels back within acceptable ranges while ensuring effectiveness.

[0012] Once nitrided, the parts generally undergo a post-oxidation treatment to passivate the surface of the nitride layer present on the treated parts, to improve their corrosion resistance and to give them a uniform black appearance.

[0013] This post-oxidation is achieved by immersing the nitrided parts in a bath of molten salts ('oxidation bath') containing carbonates, hydroxides and nitrates of alkali metals.

[0014] In general, the treatment of mechanical parts by nitriding involves quenching these parts in a nitriding bath, then in an oxidation bath, and finally in a stop bath containing cold water, which stops the oxidation of the parts.

[0015] The enrichment of the nitriding bath with carbonates, the dissolution of metals from the parts to be treated, as well as the successive passage of hardened mechanical parts between the different baths, creates waste in these baths over time.

[0016] These wastes occur and accumulate in both solid and liquid forms. In an industrial process, these wastes are discharged as an effluent containing solid and / or liquid matter. The solid matter is typically sludge, known as 'scouring sludge', which must be removed regularly, thus also requiring the regeneration of the salt baths.

[0017] Similarly, the liquids typically correspond to the holding water contained in the holding bath and therefore require regular changes. The descaling sludge is generally sent to salt mines for storage and treatment, while the holding water is sent to wastewater treatment plants or handled by specialized waste management companies. However, both the sludge and the holding water contain salts from the treatment process, which are thus lost.

[0018] In addition to the costs, pollution and waste of resources involved in disposing of this waste, its management is responsible for significant greenhouse gas emissions.

[0019] In addition, the constant regeneration of salt baths represents a significant financial cost.

[0020] Given the current economic and environmental challenges, it is therefore essential to develop effective solutions to address these problems and to be able to treat and recover value from this waste of used salt.

[0021] The holy grail, from a financial and environmental point of view, is to be able to recycle this waste and reintroduce the treated materials and liquids into an industrial process operating in a closed circuit.

[0022] While separating salts from insolubles (metals, oxides, nitrides) in oxidation bath sludge by leaching may seem straightforward, this approach is very water-intensive. For industrial applications, and to optimize resource use, wastewater is used for sludge leaching. However, this sludge contains a mixture of oxidation and nitriding salts, and simply drying the leachate allows for the recovery of salts, but not in directly usable compositions.

[0023] The Applicant has thus focused its research in recent years on the treatment of effluents from nitriding lines and containing waste in order to recover salts from oxidation and nitriding baths and purified water in order to reinject them into a nitriding circuit, with the aim of saving energy, water and raw materials.

[0024] The Applicant has thus developed a process for recovering carbonate salts from cleaning sludge and / or shutdown water, as described in its application WO2023 / 144457 A1. This process comprises a first step of transforming the hydroxide ions contained in this sludge and / or shutdown water into carbonate ions by injecting carbon dioxide, followed by separating the carbonate ions into a precipitate by filtration. The carbonate salts thus recovered can then be reused for nitriding. The Applicant's application WO2023 / 144458 A1 also describes an alternative process for recovering salts, which are both raw materials, from nitriding and oxidation baths. This process involves adding metal hydroxides to precipitate carbonate ions contained in the sludge and / or shutdown water of the nitriding process and separately collecting the carbonate precipitate and the oxidation salts by filtration.

[0025] However, these processes have a limited yield of recycled salts. Furthermore, the cationic composition of the oxidation salt resulting from recycling is not adjustable, and the released lithium is not recovered, ultimately constituting an undesirable and valuable component of the recycled oxidation salt.

[0026] These processes do indeed allow the recovery of carbonate and oxidation salts used to feed nitriding and oxidation baths. However, the recovered salts contain a mixture of lithium (Li), sodium (Na), and potassium (K) cations that cannot be separated.

[0027] Lithium, which has become an expensive element nowadays, is not necessary in oxidation salts and is not present in commercial oxidation salts.

[0028] Thus, there is a need to find alternative solutions to these processes to treat, valorize and above all recycle efficiently, continuously or semi-continuously, the salts and waters of these effluents from nitriding processes and also to recover lithium in order to valorize it.

[0029] The process according to the present invention is based on the ionic separation of the species contained in the waste generated in the various baths used in a nitriding process. Ionic separation is achieved by electrodialysis, thus enabling the production and reuse of these salts as raw materials for the nitriding and oxidation baths.

[0030] This process makes it possible to overcome the drawbacks mentioned above, in particular:

[0031] - to recover the lithium contained in this waste,

[0032] - to produce nitriding and oxidation salts simultaneously,

[0033] - improve the yield of recycled salts,

[0034] - to control the composition of recycled salt.

[0035] Finally, the recovery and, where applicable, recycling of these oxidation and nitriding salts drastically reduces the environmental impact of the industrial nitriding process. Furthermore, by eliminating solid and liquid waste while reusing it to feed the process, it is possible to reduce the quantities and costs of raw materials such as oxidizing and nitriding salts.

[0036] The process of the invention therefore offers a double ecological and economic advantage.

[0037] It is ultimately part of an environmental approach to reducing greenhouse gas and energy emissions and to valorizing raw materials.

[0038] Description of the invention

[0039] The present invention relates to a membrane electrodialysis treatment process for effluent from a nitriding line, the process comprising applying an electrical voltage between the anode and cathode of an electrodialysis device to separate alkali metal cations and anionic species selected from nitrates, hydroxides, nitrites, carbonates, and mixtures thereof. In particular, the anionic species are in the form of a mixture of nitrates, hydroxides, nitrites, and carbonates.

[0040] Figures

[0041] Figure 1 illustrates an example of an electrodialysis device comprising 4 ion separation compartments and 1 anodic and cathodic compartment for implementing the process of the present invention.

[0042] Figure 2 illustrates a second embodiment of the electrodialysis device comprising 6 ion separation compartments and 1 anodic and cathodic compartment.

[0043] Description of the invention

[0044] By "effluent(s) from a nitriding line" we mean the water from the leaching of waste produced in the molten salt baths of the nitriding process.

[0045] By "waste" we mean liquid and solid waste, including cleaning sludge and / or stop water produced in baths used to treat a mechanical part by nitriding, namely nitriding, oxidation and stop baths of a nitriding line of an industrial process.

[0046] "X and / or Y" means, according to the invention, "X", or "Y", or "X and Y".

[0047] Also included in the invention are all possible combinations of the various disclosed embodiments, whether preferred or given by way of example. Furthermore, where ranges of values ​​are indicated, the bounds are part of those ranges. The disclosure also includes all combinations of the bounds of these ranges of values. For example, the ranges of values ​​"1-20, preferably 5-15" imply the disclosure of the ranges "1-5", "1-15", "5-20", and "15-20", and the values ​​1, 5, 15, and 20.

[0048] The effluent to be treated

[0049] The present invention relates to a membrane electrodialysis treatment process for effluents from a nitriding line. The process comprises applying an electrical voltage between the anode and cathode of an electrodialysis device to separate alkali metal cations and anionic species selected from nitrates, hydroxides, nitrites, carbonates, and mixtures thereof. In particular, the anionic species are in the form of a mixture of nitrates, hydroxides, nitrites, and carbonates.

[0050] The waste formed in the various nitriding baths, i.e. oxidation baths, nitriding and stop water, typically undergoes pre-treatment such as leaching to dissolve the salts contained in the solid waste in pure water or water already containing liquid waste to produce a leachate constituting the effluent to be treated.

[0051] The effluent to be treated contains cleaning sludge and / or stop water which comes from the different baths of a nitriding line and is then conveyed to be treated in an electrodialysis device.

[0052] These baths are nitriding and oxidation baths containing molten salts. The stop bath contains an aqueous saline solution. Stop baths generally contain between 50 and 300 g / L of solids.

[0053] The nitriding bath generally contains carbonate salts such as a mixture of sodium, potassium and lithium carbonate and cyanates.

[0054] The oxidation bath contains salts based on nitrates, hydroxides and carbonates.

[0055] As is well known to those skilled in the art, salts are ionic compounds resulting from the association of cations and anions.

[0056] The anions of the salts composing these baths are:

[0057] - carbonates (CO3 2 ), - cyanates (OCN ),

[0058] - hydroxides (OH) and

[0059] - nitrates (NO3 2 )

[0060] - nitrites (NO2).

[0061] The salts in these baths are typically alkali metal salts.

[0062] Advantageously, the cations of the salts composing these baths are:

[0063] - lithiums (Li + ),

[0064] - potassiums (K + ), And

[0065] - sodiums (Na + ).

[0066] These different ionic species are contained in the cleaning sludge and / or the stop water.

[0067] The ionic concentration in these sludges and / or stop waters is at least 4%, preferably between 20% and 100% by weight.

[0068] The cationic concentration in these sludges and / or stop waters is at least 2%, preferably between 4% and 45% by weight.

[0069] The anionic concentration in these sludges and / or stop waters is at least 2%, preferably between 5% and 55%.

[0070] Sludge typically contains 40% cationic species and 60% cationic species by weight.

[0071] Stop waters at 200 g / L can, for example, contain 8% cationic species and 12% cationic species by weight.

[0072] A person skilled in the art will be able to determine the various parameters (voltage, conductivity, temperature, pressure, flow rate, current) of the device according to the concentration

[0073] The electrodialysis device

[0074] In the context of the invention, the nitriding process refers to an industrial nitriding process carried out in a semi-continuous manner.

[0075] Referring to Figure 1 by way of non-limiting example, in one embodiment, the electrodialysis device comprises two electrodes, an anode (A) and a cathode (C), and between these two electrodes a series of compartments separated by membranes alternately exchanging anions (MA) and cations (MC). The device also includes n-1 membrane spacers (EM).

[0076] The internal space between two adjacent membranes constitutes a compartment C.

[0077] The device includes, at its ends, an anodic compartment (CA) containing the anode and a cathodic compartment (CC) containing the cathode.

[0078] The compartments are permeable to the fluid. The n-1 membrane spacers (EM) and the membranes (MA and MC) are arranged alternately between the electrodes.

[0079] In some embodiments, the electrodialysis device comprises the following successive compartments:

[0080] - a cathode compartment CC placed at one end of the device, this compartment being intended to contain an aqueous electrolytic solution and a cathode,

[0081] - an anodic compartment CA placed at the other end of the device, this compartment being intended to contain an aqueous electrolytic solution and an anode,

[0082] - a CE compartment intended to contain the effluent to be treated, positioned between the cationic compartment CC and the anionic compartment CA,

[0083] - a cationic compartment Ccatioi positioned between compartment CE and compartment CC, this compartment being intended to contain the alkali metal cations separated from the effluent of compartment CE

[0084] - a Canio anionic compartment positioned between the CE compartment and the CA compartment, this compartment being intended to contain the anionic species separated from the effluent of the CE and

[0085] - a Ccarbi compartment positioned between the Ccatioi compartment and the CC compartment, this compartment being intended to contain a solution of alkali metal carbonates, and said compartments being delimited alternately by a cationic membrane MC and by an anionic membrane MA in the following manner from the CC compartment towards the CA compartment: CC / MC / CCarbi / MA / Ccatioi / MC / CE / MA / Canio / MC / CA.

[0086] According to another embodiment, as illustrated in Figure 2, the device comprises the following successive compartments:

[0087] - a cathode compartment CC placed at one end of the device, this compartment being intended to contain an aqueous electrolytic solution and a cathode, - an anodic compartment CA placed at the other end of the device, this compartment being intended to contain an aqueous electrolytic solution and an anode

[0088] - a CE compartment intended to contain the effluent to be treated, positioned between the cationic compartment CC and the anionic compartment CA,

[0089] - a first cationic compartment Ccatioi positioned between the CE compartment and the CC compartment, this compartment being intended to contain the cations separated from the effluent of the CE,

[0090] - a Canio anionic compartment positioned between the CE compartment and the CA compartment, this compartment being intended to contain the anions separated from the effluent of the CE,

[0091] - a first compartment CCarbi positioned between compartment Ccatioi and compartment CC, this compartment being intended to contain a solution of alkali metal carbonates,

[0092] - a second compartment Ccarb2 positioned between the Canio compartment and the CA compartment, this compartment being intended to contain a solution of alkali metal carbonates to adjust the Canio compartment by counter-ion and

[0093] - a second cationic compartment Ccatio? positioned between compartment CCarb2 and compartment CA, this compartment being intended to contain the cations separated from the electrolytic solution of the anodic compartment CA, and the anions separated from the alkali metal carbonate solution of compartment CCarb2, the different compartments delimited alternately by cation exchange membranes (MC) and anion exchange membranes (MA) in the following manner from compartment CC towards compartment CA:

[0094] CC / MC / CCarbi / MA / Ccatioi / MC / CE / MA / Canio / MC / CCarb2 / MA / Ccatio2 / MC / CA.

[0095] Alkali metal cations are advantageously chosen from lithium Li, potassium K, sodium Na and their mixtures.

[0096] Typically, the CE compartment containing the effluent to be treated is a dilution compartment which becomes depleted in ionic species to be separated, i.e. in cations and anions in the process of the invention.

[0097] Conversely, the cationic compartments Ccatio and Canio are concentration compartments that become enriched in ionic species to be separated, i.e., cations in Ccatio and anions in Canio. Thus, generally, under the application of an electrical voltage, the cations of the effluent to be treated migrate towards the cathode, leaving the CE compartment, through a cationic membrane MC to concentrate in the adjacent Ccatio compartment, which they cannot leave due to the presence of the following anionic membrane MA.

[0098] Simultaneously, the anions in the EC migrate towards the anode by crossing an anionic membrane MA and pass into the adjacent Canio compartment where they concentrate and cannot leave due to the presence of the following cationic membrane MC.

[0099] The electrodialysis device may consist of a stack of several electrodialysis cells. For example, the number of electrodialysis cells can advantageously vary between 1 and 80 cells.

[0100] In the context of the invention, an electrodialysis cell comprises 4 compartments: Ccarb, CE, Canio and Ccatio.

[0101] In preferred embodiments, the device comprises 24 electrodialysis cells.

[0102] The electrodes, anode and cathode, used are classic electrodes and are known to a person skilled in the art as part of their general knowledge.

[0103] The anode can, for example, be made of graphite, titanium coated with precious metals or precious metal oxides, notably platinum-plated titanium. The cathode is, for example, made of graphite, stainless steel, or nickel.

[0104] The electrolytic solution contained in the anodic and cathodic compartments is an ionizable compound that is identical in both compartments. A solution of caustic soda (NaOH) or potassium hydroxide (KOH) can be used, for example.

[0105] This electrolyte solution ensures sufficient conductivity.

[0106] Preferably, the concentration of the electrolyte solution shall be equal to or greater than 0.5 mol / L, although this lower limit is not considered critical for the implementation of this process. Advantageously, the concentration shall not exceed 2 mol / L.

[0107] The electrolytic solution is advantageously a saturated solution of electrolyte, for example potassium hydroxide.

[0108] The electrodialysis device is supplied with the waste to be treated in a CE compartment.

[0109] The feed rate is advantageously between 500 and 3,000 L / h, preferably between 500 and 2,000 L / h.

[0110] Cation exchange membranes (MC) and anion exchange membranes (MA) are classic membranes well-known to those skilled in the art, who will be able to select them appropriately. Preferably, cationic membranes are selectively permeable to monovalent cations, and anionic membranes are selectively permeable to both monovalent and divalent anions. They are generally made of polymer.

[0111] Under the application of electrical voltage, the cations of the effluent migrate from the CE compartment into the Ccatio compartment by crossing the cationic membrane.

[0112] Similarly, the anions in the effluent migrate from the CE compartment into the Canio compartment by passing through the anion exchange membrane.

[0113] Advantageously, at least one of the compartments of the device includes a power inlet and an outlet for evacuating the contents of at least one compartment.

[0114] Preferably, compartments Ccarbi, Ccarb?, Ccatioi, Ccatio?, CE, Canio have an inlet and an outlet.

[0115] In this case, compartments Ccatioi, Ccatio? and Canio are typically supplied with softened water, i.e. water free of calcium and magnesium ions, allowing the obtaining in compartment Ccatioi and, possibly Ccatio2, of an aqueous solution enriched in cations called 'cationic concentrate' and in compartment Canio of an aqueous solution enriched in anions called 'anionic concentrate'.

[0116] The circulation of cations and anions from the EC to the respective compartments Ccatioi and Canio occurs until concentrations of these species close to saturation are reached, corresponding to a predefined value of conductivity in these compartments.

[0117] Thus, the separation is considered complete when the cationic concentrate in Ccatio1, and possibly Ccatio2, reaches a conductivity value between 80 and 250 mS / cm measured using a conductivity probe, preferably between 120 and 200 mS / cm.

[0118] Similarly, separation is considered complete when the anionic concentrate in Canio reaches a conductivity value between 150 and 400 mS / cm, measured using a conductivity probe, preferably between 250 and 350 mS / cm. Thus, the enrichment of cations and anions in the Ccatioi, and possibly Ccatio2, and Canio compartments, respectively, can be continuously measured.

[0119] The anionic concentration in the Canio compartment is advantageously between 150 and 400 g / l, preferably between 200 and 350 g / l.

[0120] The cationic concentrations in the Ccatioi and Ccatio? compartments are advantageously between 70 and 200 g / l, preferably between 100 and 150 g / l.

[0121] Anionic concentrate typically contains nitrates, nitrites, carbonates and hydroxides. Cationic concentrate typically contains alkali metals selected from lithium, potassium, sodium and mixtures thereof.

[0122] In order to obtain and reuse some of the raw materials of interest from the nitriding and oxidation baths, the cationic and anionic concentrates are adjusted with a counter-ionic solution of carbonate salts.

[0123] Preferably, it is a solution of potassium carbonate (K2CO3) or sodium carbonate (TSfeCCL), either alone or in a mixture. Preferably, the carbonate solution is a mixture of sodium and potassium carbonates.

[0124] This carbonate solution is injected into a Ccarbi compartment, and possibly also into Ccart>2 if present.

[0125] It provides the counter-ions necessary for balancing the species in the anionic and cationic concentrates. The proportions of the two carbonate salts allow for adjusting the Na / K molar ratios of the recycled oxidation salts.

[0126] The Na / K molar ratio of the targeted oxidation salts is the same as that used in nitriding baths; indeed, this ratio is important because it determines the eutectic point of the nitriding bath.

[0127] Carbonate anions CCl 2' migrate from the Ccarbi compartment to the Ccatioi compartment containing the separated cations, typically lithium Li + potassium K + and sodium Na + The effluent contained in the CE is used to form carbonate salts, typically a mixture of lithium carbonate (Li2Cu3), sodium carbonate (Na2Cu3), and potassium carbonate (K2CO3), which are then reused as feedstock for the nitriding bath in the nitriding process. Thus, lithium is found only in the nitriding salts, where its presence is desired.

[0128] In one embodiment, as illustrated in Figure 1, the solution contained in the Canio compartment is adjusted in counter-ions by the electrolytic solution of the anodic compartment CA.

[0129] Thus, the cations of the CA electrolytic solution migrate through the MC membrane into the Canio compartment to form nitrate, nitrite, carbonate and hydroxide salts ('anionic concentrate').

[0130] According to one embodiment, the device includes a second compartment Ccart>2, as illustrated in Figure 2, positioned between Canio and CA, to adjust in counterions the solution contained in the Canio compartment.

[0131] When the carbonate salt used to adjust the anionic concentration to counterions is Na2Cu3, then the sodium cations Na + migrate from the CCart>2 compartment to the Canio compartment containing the separated anions, typically nitrates NOf, carbonates CCl 2 ' and hydroxides HO', from the effluent to form salts such as NaNOs, NaNCh, Na2COs and NaOH.

[0132] By analogy, when the carbonate source is potassium carbonate, alone or mixed with sodium, the anionic concentrate is enriched in ionic species such as KOH, KNO3, KNO2 and K2CO3.

[0133] The counter-ion adjustment step allows obtaining an anionic concentrate in Canio and a cationic concentrate in Ccatioi, and possibly Ccatio2 if present.

[0134] Effluent treatment process from the nitriding line

[0135] The present invention relates to a method for treating effluent from an electrodialysis nitriding line, comprising applying an electrical voltage between an anode and a cathode to separate, in an electrodialysis device, alkali metals and anionic species selected from nitrates, nitrites, hydroxides, and carbonates. In particular, the anionic species are in the form of a mixture of nitrates, hydroxides, nitrites, and carbonates.

[0136] In some embodiments, the process comprises the following steps:

[0137] (a) Introduction of the effluent to be treated into a CE compartment of the electrodialysis device, (b) Separation of alkali metal cations and anionic species from the effluent into compartments adjacent to the CE compartment, by applying an electrical voltage between the anode and the cathode of the electrodialysis device,

[0138] (c) Obtaining a cationic concentrate containing alkali metal cations and an anionic concentrate containing anionic species.

[0139] The voltage applied between the anode and cathode is advantageously between 10 and 100 V.

[0140] During electrolysis, the power supply delivers a current density of between 5 and 500 mA / cm2, preferably between 20 and 200 mA / cm2 and more preferably between 10 and 100 mA / cm2.

[0141] This current density is applied continuously throughout the duration of the treatment, provided that all process parameters remain within normal operating conditions.

[0142] The electronic charge is typically between 100 and 500 C / eq, preferably between 150 and 400 C / eq, more preferably between 200 and 300 C / eq.

[0143] The electrodialysis device is advantageously as described above.

[0144] In some embodiments, the process is carried out in a semi-contained state within an open circuit.

[0145] The temperature at which the process of the invention is implemented must be compatible with the stability of the membranes. Indeed, while in principle high temperatures are favorable, by increasing electrolytic mobility and reducing the viscosity of the solution to be treated, an increase in temperature can decrease the lifespan of the membranes.

[0146] Thus, the temperature of electrodialysis is advantageously less than or equal to 45 °C, more advantageously between 10 °C and 40 °C.

[0147] The effluent to be treated is preferably obtained after leaching the waste from the nitriding baths. Before treatment by electrodialysis, the waste may undergo one or more pretreatments to make the process more efficient. For example, it may undergo a first leaching stage consisting of dissolving the solid waste in water or in water containing liquid waste, producing a leachate that constitutes the effluent to be treated. The process advantageously includes, before ion separation by electrodialysis, a pretreatment stage of the waste by leaching.

[0148] The residence time of the effluent to be treated in the device for a separation cycle is between 5 and 15 min, preferably between 5 and 10 min.

[0149] During step b), the cations and anions are separated from the effluent under the effect of the electric field and migrate into the adjacent compartments Ccatioi, for the cations and through an MC membrane, and Canio, for the anions through an MA membrane leading to the obtaining of a cationic and anionic concentrate respectively in these compartments.

[0150] Migration continues until an ionic concentration close to saturation is reached in these compartments.

[0151] This concentration predefines a threshold value of conductivity in the Ccatioi and Canio compartments.

[0152] Preferably, in step c), the anionic concentrate and / or the cationic concentrate has a conductivity between 50 and 400 mS / cm, preferably 100 and 350 mS / cm.

[0153] In some embodiments, the process further includes a step of treating the cationic concentrate and / or the anionic concentrate by distillation to recover a cationic and / or anionic condensate. In particular, the cationic condensate can be recovered after a distillation step of the cationic concentrate. The anionic condensate can be recovered after a distillation step of the anionic concentrate.

[0154] The cationic concentrate and / or the anionic concentrate can also be concentrated or dried.

[0155] Advantageously, the cationic and / or anionic concentrate is evaporated and the resulting vapors are recovered by condensation forming a cationic condensate and / or an anionic condensate.

[0156] Distillation of the cationic concentrate yields, on the one hand, a cationic condensate ('distillate') consisting of water, and on the other hand, a solid residue containing carbonate salts. The solid residue containing the carbonate salts is optionally dried.

[0157] The dry carbonate salts can therefore be reused as raw materials for nitriding baths in an industrial nitriding process.

[0158] In addition to the presence of water, the cationic condensate may contain ionic species present in the cationic concentrate.

[0159] Distillation of the anionic concentrate leads to the recovery, on the one hand, of an anionic condensate ('distillate') consisting of water, and on the other hand of a solid residue comprising salts of hydroxide, nitrite, nitrate and possibly carbonate.

[0160] The solid residue comprising these different salts constituting the oxidation salts is possibly dried.

[0161] The dry oxidation salts can therefore be reused as raw materials for the oxidation baths of an industrial nitriding process.

[0162] In addition to the presence of water, the anionic condensate may contain ionic species present in the anionic concentrate.

[0163] The aqueous mixture of the cationic condensate and / or the anionic condensate can typically be reinjected into the electrodialysis device to serve as an aqueous source supplying the cationic compartment(s) Ccatioi, and possibly Ccatio? if present, and the anionic Canio.

[0164] According to some embodiments, at least one of the compartments includes a power inlet and an outlet.

[0165] Preferably, compartment Ccatioi, Ccatio? and / or Canio is supplied with softened water and / or cationic condensate.

[0166] After ion separation by electrodialysis, a purified effluent is also recovered; that is, an effluent free of ions. By "free" we mean that the ion content is less than 50 mS / cm in conductivity.

[0167] This effluent can also be reinjected into the closed circuit for a second separation by electrodialysis. In some embodiments, the effluent can thus be subjected to several purification cycles in the electrodialysis device or through several devices arranged in series.

Claims

Demands 1. Process for treating by membrane electrodialysis an effluent from a nitriding line, the process comprising the application of an electrical voltage between the anode and the cathode of an electrodialysis device to separate alkali metal cations and anionic species in the form of a mixture of nitrates, hydroxides, nitrites and carbonates.

2. A treatment process according to claim 1, wherein the process comprises the following steps: (a) Introduction of the effluent to be treated into a CE compartment of the electrodialysis device, (b) Separation of alkali metal cations and anionic species from the effluent into compartments adjacent to the CE compartment, by applying an electrical voltage between the anode and cathode of the electrodialysis device, (c) Obtaining a cationic concentrate containing alkali metal cations and an anionic concentrate containing anionic species.

3. A treatment method according to any one of the preceding claims, wherein the electrodialysis device comprises the following successive compartments: - a cathode compartment CC placed at one end of the device, this compartment being intended to contain an aqueous electrolytic solution and a cathode, - an anodic compartment CA placed at the other end of the device, this compartment being intended to contain an aqueous electrolytic solution and an anode, - a CE compartment intended to contain the effluent to be treated, positioned between the cationic compartment CC and the anionic compartment CA, - a cationic compartment Ccatioi positioned between compartment CE and compartment CC, this compartment being intended to contain the alkali metal cations separated from the effluent of compartment CE - a Canio anionic compartment positioned between the CE compartment and the CA compartment, this compartment being intended to contain the anionic species separated from the effluent of the CE and - a compartment Ccarbi positioned between compartment Ccatioi and compartment CC, this compartment being intended to contain a solution of alkali metal carbonates, and said compartments being delimited alternately by a cationic membrane MC and by an anionic membrane MA in the following manner from compartment CC towards compartment CA: CC / MC / CCarbi / MA / CCatioi / MC / CE / MA / Canio / MC / CA.

4. A treatment method according to claim 1 or claim 2, wherein the electrodialysis device comprises the following successive compartments: - a cathode compartment CC placed at one end of the device, this compartment being intended to contain an aqueous electrolytic solution and a cathode, - an anodic compartment CA placed at the other end of the device, this compartment being intended to contain an aqueous electrolytic solution and an anode - a CE compartment intended to contain the effluent to be treated, positioned between the cationic compartment CC and the anionic compartment CA, - a first cationic compartment Ccatioi positioned between the CE compartment and the CC compartment, this compartment being intended to contain the cations separated from the effluent of the CE, - a Canio anionic compartment positioned between the CE compartment and the CA compartment, this compartment being intended to contain the anions separated from the effluent of the CE, - a first compartment CCarbi positioned between compartment Ccatioi and compartment CC, this compartment being intended to contain a solution of alkali metal carbonates, - a second compartment Ccarb2 positioned between the Canio compartment and the CA compartment, this compartment being intended to contain a solution of alkali metal carbonates to adjust the Canio compartment by counter-ion and - a second cationic compartment Ccatio? positioned between compartment CCarb2 and compartment CA, this compartment being intended to contain the cations separated from the electrolytic solution of the anodic compartment CA, and the anions separated from the alkali metal carbonate solution of compartment CCarb2, the different compartments delimited alternately by cation exchange membranes (MC) and anion exchange membranes (MA) in the following manner from compartment CC towards compartment CA: CC / MC / CCarbi / MA / Ccatioi / MC / CE / MA / Canio / MC / CCarb2 / MA / Ccatio2 / MC / CA.

5. A treatment method according to any one of the preceding claims, wherein the alkali metal cations are selected from lithium Li, potassium K, sodium Na and mixtures thereof.

6. Processing method according to any one of the preceding claims wherein the electronic charge is between 100 and 500 C / eq, preferably between 150 and 400 C / eq, more preferably between 200 and 300 C / eq.

7. A treatment process according to any one of the preceding claims wherein the residence time of the effluent in the electrodialysis device is between 5 and 15 min, preferably between 5 and 10 min.

8. Processing method according to any one of the preceding claims wherein the process is carried out in semi-contained mode in an open circuit.

9. Processing method according to any one of the preceding claims wherein the process is carried out at a temperature less than or equal to 50 °C, preferably between 10 and 45 °C.

10. A treatment process according to any one of the preceding claims, wherein the process includes, prior to purification by electrodialysis, a pre-treatment step of the effluent by leaching.

11. Processing method according to any one of the preceding claims, wherein the process includes an adjustment step, of the cationic and anionic concentrates with a counter-ion solution of carbonate salts, necessary for balancing the species of the anionic and cationic concentrates.

12. Processing method according to any one of claims 2 to 10 wherein in step c) the anionic concentrate and / or the cationic concentrate has a conductivity between 50 and 400 mS, preferably 100 and 350 mS.

13. A processing method according to any one of claims 3 to 12, wherein at least one of the compartments comprises a feed inlet and an outlet 14. Processing method according to claim 13 in which compartment Ccatioi, Ccatio2 and / or Canio is supplied with softened water and / or cationic condensate recovered after a distillation step of the cationic concentrate.

Images