Method for treating wastewater and residual sludge by carbonation and decarbonation in a chemical plant for nitriding in a molten salt bath

Abstract

The present invention relates to a method for treating wastewater (EL) and / or residual sludge (ES) from an oxidation tank in an installation (10) for nitriding in a molten salt tank having a nitriding tank (11), an oxidation tank (12) and a stop tank (13). The treatment method includes a decarbonation method A for recovering oxidizing salts and a carbonation method B for recovering carbonate salts.

Description

[Technical field]

[0001] The present invention relates to a method for treating wastewater and residual sludge from an oxidation bath in a chemical plant for nitriding in a molten salt bath to recover oxidation salts, carbonates and water. [Background technology]

[0002] In automotive, aeronautical or industrial applications, mechanical parts are typically subjected to high stresses during operation.

[0003] Conventionally, mechanical parts can be subjected to treatments having physicochemical properties making it possible to improve some of their properties, including in particular the friction properties, the wear resistance, the fatigue resistance, the seizure resistance or even the corrosion resistance.

[0004] A process known from the prior art is the nitriding process, which consists of immersing a ferrous metal part in a medium capable of transporting nitrogen, which may in particular be a molten salt bath. In this document, nitriding also includes the nitrocarburizing process, which is a variant of the nitriding process, in which carbon, in addition to nitrogen, is diffused into the part. The ARCOR® process, which has been conceived and implemented by the applicant, is a preferred example of a nitriding process.

[0005] With reference to Fig. 1, which represents an installation for nitriding according to the prior art, such an installation 1 comprises several successive tanks, among which, from upstream to downstream, a nitriding tank 2, an oxidation tank 3 and a stop tank 4, also called "water stop tank". Upstream of the nitriding tank, a degreasing tank 5, or one or more cleaning tanks 6 and a drying device 7, for example an oven, are usually provided. Downstream of the stop tank 4, one or more cleaning tanks 8 and a device 9 for drying the parts are usually provided.

[0006] The machine part is first degreased, cleaned and heated by passing it successively through a degreasing tank 5, a cleaning tank 6 and a drying device 7. The part is then immersed in a nitriding tank 2, which is a bath of molten salts containing, inter alia, carbonates. If necessary, the addition of regenerating salts can convert the carbonates to the reactivated species cyanates. Nitrogen (and optionally carbon) diffuses in the part and precipitates in the form of nitrides, resulting in the formation of a combination layer mainly containing iron nitrides (or other alloying elements) and an underlying diffusion zone, where the nitrogen is either present among the iron atoms (solid solution) or reacts with the alloying elements contained in the steel to form nitrides.

[0007] The combined effect of the combination layer and the diffusion layer makes it possible to obtain nitrided parts having good friction properties as well as excellent wear resistance.

[0008] After nitriding, the parts are immersed in an oxidising bath 3 containing oxidising salts, usually hydroxides and / or nitrates, to improve their corrosion resistance and give them a uniform black appearance.

[0009] After the oxidation treatment, the parts are immersed in a stopping bath 4 containing cold water to stop the oxidation, then cleaned by multiple successive washes in a cleaning bath 8, then dried in a drying device 9 and removed.

[0010] For certain processes, no oxidation step is required and the parts are transferred directly from the nitriding bath to a dedicated stopping bath, whose water is enriched with nitriding salts (and is no longer an oxidizing salt).

[0011] Nitriding, although an effective method for improving the properties of mechanical components, has the drawback of producing liquid and solid waste products (sometimes referred to as effluents) and requires water, which can present a significant environmental impact and result in high production costs, particularly the relatively high production costs due to waste disposal as described below.

[0012] During the ARCOR process, nitriding salts are entrained by the parts and contaminate the oxidizing tank, after which the salts from the oxidizing tank are entrained and transferred into the stop tank. Secondary chemical reactions also produce sludge / waste in the treatment tanks (nitriding and oxidizing).

[0013] It is also possible to use the line with or without an oxidation treatment, and alternatively the stop tank serves as a stop tank for a nitridation treatment followed by an oxidation treatment. The nitridation salt and the oxidation salt may therefore be transferred into the same stop tank.

[0014] This waste is represented in Figure 1. These are the solid effluents E S However, liquid effluent E L It is also.

[0015] Solid Effluent E S are also called "residual sludge". They are mainly collected in the oxidation tank 3 and in the nitriding tank 2 (after carrying out the method) and are usually transferred to a salt mine for treatment and storage there. They contain insoluble species and salts. For the two types of salts (nitriding salts and oxidizing salts), the metals and metal oxides (iron and alloy metals) originating from the parts to be treated constitute the insoluble matter. For the oxidizing salts, the carbonates constitute the majority of the insoluble matter. The residual sludge is physically removed during normal maintenance operations.

[0016] Liquid Effluent E L are referred to as "wastewaters". They are collected at the outlet of the stop tank 4 and are usually sent to a purification station or treated by companies specialized in waste treatment.

[0017] In addition, since the purchase costs of raw materials are themselves relatively high, optimizing their consumption is also of interest. Summary of the Invention [Problem to be solved by the invention]

[0018] Faced with this situation, the applicant sought measures to reduce the adverse environmental impacts associated with the nitriding process and to control the costs.

[0019] The applicant has placed a particularly strong focus on research into reducing the amount of waste and limiting the waste of both renewable resources, e.g. water, and non-renewable resources, e.g. the raw materials used in the nitriding and oxidation baths, i.e. nitrating and oxidizing salts. [Means for solving the problem]

[0020] The Applicant has now developed a process that is able to overcome all the above mentioned drawbacks and which can be integrated into current lines, since it does not require any changes to the structure of the nitriding chain.

[0021] To this end, the present invention provides: In at least one facility for nitriding in a molten salt bath having a nitriding bath, an oxidation bath, and a stop bath, wastewater (E L ) and / or residual sludge (E S ) to recover oxidizing salts and carbonates, the treatment method comprising the steps of: - The following steps: Carbonate ion CO3 2- - decarbonation of wastewater and / or residual sludge by precipitation of Separating a liquid phase containing oxidizing salts from a solid phase containing carbonate precipitate; A process for the recovery of oxidizing salts on the one hand and a process for the recovery of carbonates on the other hand. A method of decarboxylation comprising: - The following steps: Hydroxide ion (OH) in wastewater - and / or hydroxide ions (OH) of the residual sludge - Carbonate ion CO3 2- A process of converting Separation of the water from the carbonate salt formed from the carbonate ions and the metal cations from the oxidizing salt; Carbonate recovery process Carbonation method B, comprising:

[0022] By choosing between these two, Method A and Method B, it is possible to adapt the salt for recovery or recycling, which will most likely be needed at some point in the future, or at a time when purchase costs are highest.

[0023] According to the treatment method of the invention, the waste water at the outlet of the stopping tank and the residual sludge from the oxidation tank are treated and preferably recycled to supply oxidizing salts to the oxidation tank, to supply carbonates which are components of the nitriding salts to the nitriding tank, and advantageously to supply water to the stopping tank and / or the washing tank, if a water recycling step is carried out as described below.

[0024] Carbonate ion CO3 2- are formed in the reaction medium, originating from the entrainment of salts from nitrating salts. They constitute the "polluting" species of the oxidation bath. The decarbonation step aims to remove the carbonates by precipitation, in order to make it possible to isolate the oxidizing species (nitrates and / or hydroxides) necessary for the correct operation of the oxidation bath.

[0025] After solid / liquid separation, the oxidizing salts are recovered, the latter may then be re-introduced or re-enhanced in the oxidation tank, and the water is advantageously re-introduced into the stop tank and / or wash tank.

[0026] In doing so, the effluent of the oxidation process and waste is fed to an oxidation tank and, advantageously, to a separate water tank.

[0027] In carbonation method B, hydroxide ions (OH) are extracted from the residual sludge and / or wastewater. -are removed, the latter being a constituent of the oxidation bath. According to the invention, the hydroxide ions are converted into carbonate ions. The corresponding carbonates (carbonate ions in the form of salts) are then recovered and then advantageously reintroduced into the nitriding bath.

[0028] Usually, regenerating salt is added to the nitriding tank. The latter is carbonate ion (CO3) 2- Cyanate ion CNO - Convert to.

[0029] In this way, recycled carbonate ions (CO3) 2- When they are reintroduced into the nitriding tank, they are converted into cyanate ions (CNO) by the regenerating salt. - is converted to

[0030] Water is advantageously recharged into the stopping tank and / or the rinsing / washing tank.

[0031] In carrying it out, the effluent of the oxidation process and waste is fed to the nitriding tank and, advantageously, to a separate water tank, in particular a washing tank and / or a stopping tank.

[0032] The re-enhancement of oxidizing and carbonate salts, and, where necessary, their recycling, allows the present process to dramatically reduce the environmental impact of industrial nitriding processes by reducing solid and liquid waste while simultaneously reusing the latter, thereby reducing the amount and cost of the raw materials, which are the oxidizing and nitriding salts.

[0033] The method of the present invention therefore provides both environmental and economic advantages.

[0034] It is specified that the parts can go through only one nitriding step without being subjected to an oxidation step, when the stop tank acts to stop the nitriding reaction. The method of the invention does not apply to a single nitriding line without an oxidation tank, as in this case, and the recovery of carbonates can be performed by simply filtering the wastewater.

[0035] The nitriding installation may have several nitriding tanks and / or several oxidation tanks and / or several stopping tanks.

[0036] The nitriding installation is used herein with its more general latitude. The various tanks it has can in fact be located at the same or different locations. For example, process A can be carried out in a first chemical installation at a first location and process B can be carried out in a second chemical installation at a second location separate from the first one. Furthermore, it is possible to have several nitriding installations at the same location, i.e. several nitriding lines, each capable of carrying out one of process A and process B, or both of process A and process B.

[0037] In method A, the decarboxylation step can in particular be carried out in several ways: - Decarbonation of wastewater only, or decarbonation of residual sludges previously leached in water or, more generally, in an aqueous solution, for example tap water or industrial water, or, more generally, in an aqueous solution, or - Decarbonation of residual sludge previously leached into wastewater.

[0038] In method B, the conversion step can be carried out in several ways, in particular: - Hydroxide ion OH in wastewater only - Conversion of, or - leaching the residual sludge in water, or more generally in an aqueous solution, for example tap water or industrial water, to obtain hydroxide ions OH -The hydroxide ions (OH) of the residual sludge presolvated with - Conversion of, or - Hydroxide ions (OH) are released by leaching the residual sludge into wastewater. - The hydroxide ions (OH) of the residual sludge presolvated with - Conversion of.

[0039] According to further aspects, the treatment method according to the invention has the following various characteristics, individually or in any technically possible combination thereof: - Decarbonation produces carbonate ions CO3 2- and calcium ion Ca 2+ (CaCO3 precipitate), Lithium Li + (Li2CaO3 precipitates), Barium Ba 2+ (BaCO3 precipitate), Magnesium Mg 2+ (MgCO3 precipitate), Iron(II)Fe 2+ (FeCO3 precipitate), copper Cu 2+ (CuCO3 precipitate), Manganese Mn 2+ (MnCO3 precipitates), Cobalt Co 2+ (CoCO3 precipitates), or mixtures thereof. Poorly soluble species that can precipitate in solution are targeted. - in order to form carbonates of alkali metals and / or alkaline earth metals, the decarboxylation of method A is carried out using hydroxides and / or nitrates of alkali metals and / or alkaline earth metals. - Decarbonation in method A is carried out using lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2 to form calcium carbonate CaCO3 precipitate according to either of the following respective reactions (1) and (2): [C1] Ca(OH)2+CO3 2- →CaCO3+2OH - (1) [C2] Ca(NO3)2+CO3 2- →CaCO3+2NO3 - (2).

[0040] The choice of decarboxylation reaction makes it possible to select the carbonate that is desired to be produced (according to the cation used) for enhancement and / or recycling, thus making it possible to adapt the process to the carbonate requirements. - Decarbonation method A further comprises a step of leaching the residual sludge by addition of water prior to the decarbonation step. The residual sludge leaching step is carried out at least partly with wastewater. - decarbonation method A further comprises a step of separating, for example by filtration, metal particles or metal oxides present in the wastewater and / or the residual sludge, prior to the decarbonation step, which makes it possible to optimize the purity of the carbonate precipitate with a view to its re-enhancement. - The step of separating the liquid and solid phases in decarbonation method A is carried out by decantation followed by filtration. The decarbonation method A further comprises, after the step of separating the liquid and solid phases, a step of separating the water from the oxidizing salt in the liquid phase and recovering the oxidizing salt in solid form. In method A of decarbonation, the step of separating the water and the oxidizing salts in the liquid phase is carried out by drying the liquid phase, for example in a vacuum evaporator. The decarbonation method A further comprises, after the step of separating the liquid and solid phases, a step of reintroducing the oxidizing salt into the oxidation treatment tank. Preferably, if a separation of the oxidizing salt in the liquid phase from the water for recovering the oxidizing salt in solid form is carried out, the step of reintroducing the oxidizing salt is carried out after the latter. The decarbonation method A further comprises a step of recycling the water to the stop tank and / or to at least one wash tank after separating the water and the oxidizing salts. The conversion step of method B is to convert hydroxide ion OH - reacts with carbon dioxide CO2 to produce carbonate ions CO3 2- and water. [C3] 2OH -+CO2→H2O+CO3 2- (3) - Carbonation method B is the hydroxide ion OH - Carbonate ion CO3 2- This method includes a step of separating, advantageously by filtration, any metal particles or metal oxides present in the wastewater and / or residual sludge before converting it into nitrifying salt, since metal particles are impurities, which makes it possible to optimize the purity of the nitrifying salt. - Hydroxide ion OH in carbonation method B - The conversion is carried out by blowing in carbon dioxide CO2. - Carbonation method B: hydroxide ion OH - The conversion is carried out by bubbling air. The hydroxide then reacts with the CO2 present in the air. Although this conversion may take more time, this method is easier to carry out and is less expensive. In carbonation method B, the separation of water and carbonate is carried out by filtration and / or drying. The carbonation method B further comprises a step of recycling the water to the stopping tank and / or to at least one washing tank after separating the water and the carbonate salts. Carbonation method B further comprises the step of separating the water and the carbonate, and then reintroducing the carbonate into the nitriding tank. - Carbonation method B comprises a step of readjusting the amount of cations to the amount of the content of the nitriding tank before reintroducing the carbonates into the nitriding tank, in order to make them compatible with the nitriding salts. This step in fact consists in adding alkaline carbonates (for example lithium carbonate, sodium carbonate or potassium carbonate) to the carbonates obtained, in amounts corresponding to the test content of the carbonates in the nitriding tank. It makes it possible to keep the ratio of the different cations substantially constant or, as a minimum, to limit their variations. This step is therefore particularly advantageous when the carbonates are recycled by reintroducing them into the nitriding tank in order to maintain the physicochemical properties of the nitriding tank substantially constant over time, which improves the reproducibility of the method. - The nitriding tank is a cyanate ion (CNO- and carbonate ion CO3 2- Includes. The nitriding bath further contains alkali ions, preferably lithium ions Li + , and / or potassium ion K + , and / or sodium ion Na + Includes. - The oxidation treatment tank produces hydroxide ions (OH - and / or nitrate ion NO3 - , and optionally carbonate ions.

[0041] Some carbonates produced by the method of the present invention may have significant enhancement properties, such as, for example, lithium carbonate, Li2CO3.

[0042] An additional advantage of carbonation method B is that it allows for the capture of CO2.

[0043] Other advantages and features of the present invention will become apparent from reading the following description, given by way of non-limiting example and with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0044] [Figure 1] FIG. 1 shows an industrial installation for nitriding in a molten salt bath according to the prior art. [Diagram 2] FIG. 2 shows an industrial installation for nitriding in a molten salt bath according to the invention, where only the nitriding, oxidation and stop baths are shown and a recycle loop of the oxidizing salt is depicted. [Diagram 3] FIG. 3 is a detailed diagram of the decarbonation reaction of residual sludge and / or wastewater. [Figure 4] FIG. 4 shows an industrial nitriding installation similar to that of FIG. 2, in which a recycle loop for the nitriding salts is shown. [Diagram 5] FIG. 5 is a detailed diagram showing the reaction of conversion of hydroxide ions OH − in residual sludge and / or cooling water by injection of carbon dioxide CO 2 . [Figure 6] FIG. 5 shows an industrial nitriding installation similar to FIGS. 2 and 4, in which an oxidizing salt recycle loop and a nitriding salt recycle loop are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] The method of the invention will now be described in detail with reference to FIG. 2, which is a simplified representation of an installation 10 for nitriding in a molten salt bath, in which, for the purposes of brevity of the present specification, only the main elements of the installation are kept, namely the nitriding tank 11, the oxidation tank 12 and the stopping tank 13.

[0046] The machine part to be treated, previously degreased, then rinsed and dried in an oven, is immersed in the nitriding bath 11. The nitriding bath 11 consists of a molten nitriding salt brought to a temperature that is usually between 500°C and 630°C.

[0047] The nitriding treatment has the purpose of increasing the hardness of the parts by diffusing nitrogen, and optionally carbon, in the steel, in the case of nitrocarburizing, and improving their mechanical properties, in particular their seizure and wear resistance.

[0048] The nitriding tank 11 mainly produces cyanate ions CNO - and carbonate ion CO3 2- The regenerating salt contains carbonate ions CO3 2- Cyanate ion CNO - If necessary, it is introduced into a nitriding bath for rapid conversion to

[0049] The nitriding bath 11 contains alkali ions, preferably lithium ions Li + , and / or potassium ion K + , and / or sodium ion Na + Further includes.

[0050] After the nitriding step, the machine part is immersed in an oxidation bath 12. The oxidation bath 12 consists of molten oxidizing salts, typically at a temperature of approximately 450° C. The oxidizing salts consist, inter alia, of hydroxides.

[0051] The oxidation treatment is intended to improve the corrosion resistance of the part and to give it a uniform black appearance.

[0052] After the oxidation treatment step, the parts are immersed in a stop bath 13 containing cold water (waste water) and then cleaned in a washing bath, preferably by successive cascade washings, after which they are dried in an oven and removed.

[0053] As can be seen from Figure 2, the oxidation process of the parts produces a sludge that is extracted by the degreasing tank, which is the residual sludge E S and the stop water is wastewater E L which constitute respectively solid and liquid wastes which the method of the invention allows to recycle.

[0054] The processing method according to the present invention includes a method A of decarbonation and a method B of carbonation.

[0055] Method A is carbonate ion CO3 2- Residual sludge E by precipitation S and / or wastewater E L This includes decarboxylation of

[0056] According to a preferred embodiment of the invention, the decarbonation is carried out by means of lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2. S Carbonate ion CO3 2- and / or wastewater E L Carbonate ion CO3 2- is carried out by precipitation of calcium carbonate, CaCO3.

[0057] Carbonate ion CO3 2- reacts with lime Ca(OH)2 according to the following equation (1): [C1] Ca(OH)2+CO3 2- → CaCO3+2OH - (1).

[0058] In this way, hydroxide ions and calcium carbonate are obtained.

[0059] Carbonate ion CO3 2- reacts with calcium nitrate Ca(NO3)2 according to the following equation (2): [C2] Ca(NO3)2+CO3 2- → CaCO3+2NO3 - (2).

[0060] In this case, nitrate ions and calcium carbonate are obtained.

[0061] If both lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2 are introduced into the reaction medium, both reactions (1) and (2) occur. Thus, carbonate ions CO3 2- reacts with both lime, Ca(OH)2, and calcium nitrate, Ca(NO3)2.

[0062] If only lime, Ca(OH)2, is introduced into the reaction medium, only reaction (1) takes place. If only calcium nitrate, Ca(NO3)2, is introduced into the reaction medium, only reaction (2) takes place.

[0063] After the decantation step, the liquid phase comprising the oxidizing salts 23 and the solid phase comprising the calcium carbonate precipitate CaCO3 are separated. Preferably, the separation is carried out by filtration, for example by filtration in a filter press.

[0064] In the liquid phase, the water and the oxidizing salt are then separated, preferably by drying, to obtain the oxidizing salt 23 in solid form.

[0065] The solid oxidizing salt 23 may be stored for enhancement or may be reintroduced into the oxidizing bath.

[0066] Decarboxylation is illustrated in more detail in FIG.

[0067] In this figure, residual sludge E S is put into solution together with wastewater under stirring in the leaching process. A leachate 14 is obtained.

[0068] Leachate 14 is the residual sludge E by adding water. S By solvation alone or residual sludge E S Wastewater E L It is specified that it can be prepared by solvation with

[0069] According to a preferred embodiment of the present invention, the leachate 14 is a residual sludge E S Wastewater E L This embodiment is formed by solvating the residual sludge E S To solvate the waste liquid E L While using as a solvent, wastewater E L This is particularly advantageous in that it allows recycling of the water, which makes it possible to reduce water consumption.

[0070] The leachate 14 is transferred into a reactor 15, where lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2 are added and stirred.

[0071] A calcium carbonate precipitate, CaCO, is formed in the solution. The mixture is therefore composed of a solid phase, the calcium carbonate precipitate, CaCO, and a liquid phase containing oxidizing salts.

[0072] After decantation, a filtration device 17 is used to separate the settled solids 16 into solid / liquid form, preferably by filtration of the mixture, as illustrated in FIG.

[0073] The residue 18 containing the calcium carbonate precipitate CaCO3 can be set aside and then enhanced. The filtrate 19 is preferably recharged into the reactor to maximize the decarbonation yield.

[0074] The reactor supernatant 20 is transferred to a drying device 21, such as an oven.

[0075] Preferably, the dried condensate 22 is reintroduced into the leachate 14 or into a stop tank or into a wash tank to reduce water consumption.

[0076] The drying step makes it possible to obtain recycled oxidizing salts 23, which can be stored or reintroduced into the oxidation bath.

[0077] Method B is carbonate ion CO3 2- from the residual sludge to form hydroxide ions OH - and / or hydroxide ions OH from wastewater - This includes conversion reactions of

[0078] Preferably, the conversion step is carried out by converting hydroxide ions (OH) according to the following reaction (3): - reacts with carbon dioxide CO2 to produce carbonate ions CO3 2- and water by the reaction: [C3] 2OH - +CO2 → H2O+CO3 2- (3).

[0079] Carbonate ion CO3, reference number 32 in Figure 4 2- are then reintroduced in the form of salts into the nitriding bath, where they are optionally converted to cyanate ions CNO by regenerating salts. - may be converted to

[0080] Also possible is the enhancement of certain carbonates, in particular lithium carbonate. The latter, moreover, thanks to its very low solubility, is easily separated from the sodium and potassium carbonates by precipitation. It can be recovered, for example, by first decanting and then filtering, and the other carbonates are then recovered, for example by drying.

[0081] Preferably, the hydroxide ion OH - Carbonate ion CO3 2- Before the conversion to 1,000,000 sulphate, a step of separating out the metal particles or metal oxides present in the waste water and / or the residual sludge is carried out, advantageously by filtration, and optionally then drying.

[0082] Preferably, water is converted to carbonate ions CO3 2- After separation from the nitrate, a water recycling step to the stop tank is carried out.

[0083] Preferably, carbonate ions CO3 2- Before re-introducing into the nitriding tank, the step of separating the carbonates and the water is advantageously carried out by filtration, optionally followed by drying.

[0084] Preferably, carbonate ions CO3 2- Before being recharged into the nitriding vessel, a step is carried out to readjust the amount of cations relative to the content of the nitriding vessel to make them compatible with the nitriding salts.

[0085] Hydroxide ion (OH) from solubilized residual sludge - , and / or hydroxide ions OH from wastewater - One embodiment of the conversion reaction is shown in more detail in Figure 5. According to this embodiment, the reaction is carried out by infufflation of carbon dioxide CO2.

[0086] In this diagram, wastewater E L and optionally in some cases, solubilized residual sludge E S is pre-placed in solution 24 in a first glass receptacle 25. A second glass receptacle 26 is immersed upside down in solution 24, so as to create a space for a tube 27 between the inside and outside of the second receptacle 26. Solution 24 occupies the volume of both receptacles 25, 26. The purpose of this setup is to capture carbon dioxide CO2.

[0087] The tube 27 has a first opening 28 opening into the solution inside the second receptacle 26 and a second opening 29 opening into a supply of carbon dioxide CO2 outside the receptacle 25, 26. The tube 27 is thus partially immersed in the solution 24 and passes through the two receptacles 25, 26.

[0088] The solution outside the first receptacle 25 contains hydroxide ions OH - and carbonate ion CO3 2- Includes.

[0089] Carbon dioxide CO2 is injected into the second receptacle 26 through a second opening 29 of the tube 27. Carbon dioxide CO2 bubbles 30 form in the solution 24 inside the second receptacle 26. The solution 24 inside the second receptacle 26 is composed of carbon dioxide CO2 and hydroxide ions OH - Carbonate ions CO3 resulting from the reaction between 2- and the overlying region 31 is rich in carbon dioxide CO2, the latter being captured by the second receptacle 17.

[0090] Next, carbonate ion CO3 2- can be precipitated. The precipitation depends on several parameters that can be adjusted for this purpose, in particular the hydroxide ions (OH) of the solution. - , Carbonate ion CO3 2- , and the concentration of the cation.

[0091] Following formation of the precipitate, the water 40 is filtered, for example using a filter press, and the recovered carbonate salt is then dried. It is contemplated that the water may be evaporated, for example using a dryer, to further recover the carbonate salt (non-precipitate).

[0092] The solid carbonate is then reintroduced into the nitriding bath as a nitriding salt. To keep things simple, the nitriding salt is represented by the carbonate ions CO3, reference numeral 32 in Figs. 2- is the same as:

[0093] Preferably, the water 40 is recycled to the stopping tank 13 .

[0094] FIG. 6 shows two recycle loops, Method A and Method B, which are represented separately in FIGS.

[0095] Thus, the method according to the invention not only makes it possible to reduce the nitriding and oxidation wastes, but also to reintroduce oxidizing salts into the oxidation bath and nitriding salts into the nitriding bath, which is effected by economical and environmentally friendly recycling in line with current environmental standards and technical matters. [Explanation of symbols]

[0096] 1...Equipment 2. Nitriding tank 3. Oxidation treatment tank 4...Stop tank 5...Degreasing tank 6. Cleaning tank 7...Drying device 8. Cleaning tank 9. Device for drying parts 10...Equipment 11. Nitriding tank 12. Oxidation treatment tank 13...Stop tank 14. Leachate 15. Reactor 16 - Settled solids 17...filtration device 18...residue 19...filtrate 20. Supernatant 21...Drying equipment 22. Dry condensate 23. Oxidizing salts 24...solution 25...First glass receptacle 26...Second Glass Receptacle 27...Tube 28... First opening 29...Second opening 30 Carbon Dioxide (CO2) Bubbles 31 - Covered Area 32 Carbonate ion CO3 2- 40...water

Claims

1. In at least one installation (10) for nitriding in a molten salt bath having a nitriding bath (11), an oxidation bath (12) and a stop bath (13), wastewater (E Ｌ ), and / or residual sludge (E Ｓ 2. A process for treating a sulphuric acid salt (SSA) to recover oxidizing salts and carbonate salts comprising the steps of: - The following steps: Carbonate ion CO ３ ２－ The precipitation of Ｌ ) and / or residual sludge (E Ｓ ) decarboxylation; separating a liquid phase containing the oxidizing salt (23) from a solid phase containing a carbonate precipitate; recovering, on the one hand, the oxidizing salts (23) and, on the other hand, the carbonate salts; A method of decarboxylation comprising: - The following steps: Wastewater (E Ｌ ) hydroxide ion OH － and / or residual sludge (E Ｓ ) hydroxide ion OH － Carbonate ion CO ３ ２－ converting separating water from carbonate salts formed from carbonate ions and metal cations from the oxidizing salt; A process for recovering carbonate A carbonation method B comprising: The method includes:

2. In method A, decarboxylation is carried out by converting carbonate ions CO ３ ２－ and calcium ion Ca ２＋ (CaCO ３ Precipitate), lithium Li ＋ (Li ２ CaO ３ Precipitates), Barium Ba ２＋ (BaCO ３ Precipitates), Magnesium Mg ２＋ (MgCO ３ Precipitates), iron (II) Fe ２＋ (FeCO ３ precipitate), copper Cu ２＋ (CuCO ３ precipitates), manganese Mn ２＋ (MnCO ３ Precipitates), Cobalt Co ２＋ (CoCO ３ 2. The method of claim 1, wherein the reaction is carried out by reaction between a monovalent or divalent cation selected from the group consisting of cations (e.g., cations of the formula (I), cations of the formula (II ...

3. 2. The process according to claim 1, wherein in process A, the decarboxylation is carried out using hydroxides and / or nitrates of alkali metals and / or alkaline earth metals to form carbonates of these alkali metals and / or alkaline earth metals.

4. Method A decarbonation is performed using lime Ca(OH) ２ and / or calcium nitrate Ca(NO ３ ) ２ are carried out using the following reactions (1) and (2): [Chemical formula 1] Ca(OH) ２ +CO ３ ２－ → CaCO ３ +2OH － (1) [Case 2] () ３ ) ２ +CO ３ ２－ → CaCO ３ +2NO ３ － () Calcium carbonate CaCO ３ The method of claim 1 , wherein a precipitate is formed.

5. The decarbonation method A comprises the steps of: Ｓ 5. The method according to claim 1, further comprising the step of leaching (14) said precipitation being carried out in a leachate (14).

6. Residual sludge (E Ｓ The leaching process of the wastewater (E Ｌ 6. The method of claim 5, wherein the method is carried out using

7. The decarbonation method A comprises the steps of: Ｌ ) and / or residual sludge (E Ｓ 7. The method of claim 1, further comprising the step of separating metal particles or metal oxides present in the sintered product.

8. 8. The method according to claim 1, wherein the decarbonation method A further comprises, after the step of separating the liquid and solid phases, a step of separating water from the oxidizing salt (23) in the liquid phase to recover the oxidizing salt in solid form.

9. 9. The method according to claim 8, wherein the decarbonation method A further comprises recycling the water to the stopping tank (13) and / or to the at least one washing tank (6, 8) after separating the water and the oxidizing salt (23).

10. 10. The method according to claim 1, wherein the decarbonation method A further comprises a step of reintroducing the oxidizing salt (23) into the oxidation tank (12) after the step of separating the liquid and solid phases.

11. The conversion step of method B is the following reaction (3): [Chemical 3] 2OH － +CO ２ → H ２ O+CO ３ ２－ (3) According to this, hydroxide ion OH － Carbon dioxide CO ２ React with carbonate ion CO ３ ２－ 11. The process according to claim 1 , wherein the process is carried out by a reaction to form

12. Carbonation method B is the hydroxide ion OH － Carbonate ion CO ３ ２－ Before conversion to Ｌ ) and / or residual sludge (E Ｓ 12. The method of claim 1, further comprising the step of separating metal particles or metal oxides present in the sintered product.

13. 13. The method according to any one of claims 1 to 12, further comprising the step of re-introducing the carbonate into the nitriding tank (11) after separating the water and the carbonate.

14. 14. The method according to claim 13, wherein the carbonation method B further comprises the step of readjusting the amount of cations relative to the content of the nitriding tank (11) to make them compatible with the nitriding salts before re-introducing the carbonate salts into the nitriding tank (11).

15. 15. The method according to any one of claims 1 to 14, wherein the carbonation method B further comprises recycling the water to the stopping tank (13) and / or to the at least one washing tank (6, 8) after separation of the water and the oxidizing salts.

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