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

Abstract

The present invention relates to a method for recovering oxidizing salts by treating wastewater (E) and / or residual sludge (B) from an oxidation tank in a plant (10) for nitriding in a molten salt tank having a nitriding tank (11), an oxidation tank (12) and a stop tank (13). The method comprises a step of decarbonating the wastewater (E) and / or the residual sludge (B) in order to recover the oxidizing salts, and a step of separating a liquid phase containing the oxidizing salts (23) from a solid phase containing carbonate precipitates.

Description

[Technical field]

[0001] The present invention relates to a method for treating wastewater and residual sludge from oxidation baths in chemical installations for nitriding in molten salt baths to recover oxidizing 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 may be subjected to treatments of a physico-chemical nature 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 in the prior art is the nitriding process, which consists of immersing a ferrous metal part in a medium allowing the migration of nitrogen, which may in particular be a molten salt bath. In this specification, nitriding also includes nitrocarburizing, which is a variant of nitriding, in which carbon, in addition to nitrogen, is diffused into the part. The ARCOR® process, devised 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, the 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 there is usually a degreasing tank 5, or one or more cleaning tanks 6 and a drying device 7, such as an oven. Downstream of the stop tank 4 there is usually one or more cleaning tanks 8 and a device 9 for drying the parts.

[0006] The machine part is first degreased, cleaned and heated by passing 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, among other things, carbonates. If necessary, the addition of regenerating salts can convert the carbonates into the reactivated species cyanates. Nitrogen (and optionally carbon) can diffuse in the part and precipitate in the form of nitrides, forming a combination layer mainly containing iron nitrides (or other alloying elements) and an underlying diffusion zone where the nitrogen is present between 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 oxidation bath 3 containing oxidizing 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, after which they are dried in a drying device 9 and removed.

[0010] In some processes, the oxidation step is not necessary and the parts are transferred directly from the nitriding bath to a dedicated stopping bath, the water of which is rich in 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 effluent) and requires water, which can present significant environmental impacts and result in relatively high production costs, particularly due to waste disposal as described below.

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

[0013] It is also possible, alternatively, to use the line with or without an oxidation treatment and for the stop tank to thus act as a stop tank for the nitriding treatment followed by the oxidation treatment: the nitriding salts and the oxidizing salts are thus transferred into the same stop tank.

[0014] The waste products are represented in Figure 1. These are solid effluents B and also liquid effluents E.

[0015] The solid effluents B are also called "residual sludge". They are mainly collected in the oxidation tank 3 and in the nitridation tank 2 (after carrying out the method) and are usually transported to a salt mine for processing 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) coming from the parts being processed constitute the insolubles. For the oxidizing salts, the carbonates constitute the majority of the insolubles. The residual sludges are physically removed during normal maintenance operations.

[0016] The liquid effluents E 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 a company 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 environmental impact and to control the costs associated with the nitriding process.

[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 salts and oxidizing salts. [Means for solving the problem]

[0020] The Applicant has now arrived at the development of a processing method capable of overcoming all the above mentioned drawbacks, which can be implemented in current lines without the need to modify the structure of the nitriding chain.

[0021] To achieve this goal, the present invention relates to a method for treating wastewater (E) from an oxidation tank and / or residual sludge (B) in at least one installation for nitriding in a molten salt tank, comprising a nitriding tank, an oxidation tank and a stop tank, to recover oxidizing salts and carbonates, the method comprising 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 precipitates; - recovery of the oxidizing salts on the one hand and the carbonates on the other hand, The present invention provides a method comprising:

[0022] According to the treatment method of the present invention, the wastewater at the outlet of the stop tank and the residual sludge from the oxidation tank are treated to recover separately the oxidizing salts, carbonates and water. Advantageously, the recovered water can be fed to the stop tank and / or the washing tank if the water recycling step described below is implemented.

[0023] Carbonate ion CO3 2-are formed in the reaction medium, resulting from the entrainment of salts from nitrating salts. They constitute a "contaminating" species of the oxidation bath. The decarbonation step described above aims to remove the carbonate ions 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.

[0024] The carbonate ion removal process is advantageously utilized to produce enhanceable carbonates. This enhancement contributes to reducing the cost of the nitriding process in the salt bath. At the same time, it also reduces the environmental impact.

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

[0026] In carrying it out, the effluent of the oxidation process and waste is fed to an oxidation tank and, advantageously, to a separate water tank, in particular a washing tank and / or a stopping tank.

[0027] The re-enhancement of the oxidizing salts and carbonates, and, if necessary, their recycling, dramatically reduces the environmental impact of industrial nitriding processes due to the reduction in the amount and cost of the oxidizing salt raw material and by reusing the latter to replenish the process, while reducing solid and liquid waste.

[0028] The method of the present invention therefore offers both environmental and economic advantages.

[0029] It is specified that the above components can be used for one nitriding process without oxidation, if the stop tank acts to stop the nitriding reaction. The method of the present invention does not apply to a single nitriding line without an oxidation tank, as in this case, and the recovery of carbonates can be achieved by simply filtering the wastewater.

[0030] The nitriding facility may have multiple nitriding tanks and / or multiple oxidation tanks and / or multiple stopping tanks.

[0031] The above-mentioned nitriding equipment 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, the decarbonation of the wastewater and / or waste sludge can be carried out in a first chemical equipment at a first location, and the separation of the liquid and solid phases can be carried out in a second chemical equipment at a second location separate from the first location. In addition, there may be multiple nitriding equipment, i.e. multiple nitriding lines, at the same location.

[0032] The decarboxylation process can be carried out in several ways, in particular: - 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 This can be carried out by:

[0033] According to other aspects, the processing method according to the invention has the following various characteristics, taken 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 capable of precipitating in solution are targeted. - Decarboxylation is carried out using hydroxides and / or nitrates of alkali metals and / or alkaline earth metals in order to form carbonates of these alkali metals and / or alkaline earth metals. - Decarbonation is carried out using lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2 to form calcium carbonate CaCO3 according to either of the following respective reactions (1) and (2): [C1] Ca(OH)2+CO3 2- →CaCO3+2OH - (1) [Case 2] Ca(NO3)2+CO3 2- →CaCO3+2NO3 - (2).

[0034] The choice of decarboxylation reaction makes it possible to select the desired carbonate to be produced (depending on the cation used) for enhancement and / or recycling, thus allowing the method to be adapted to the desired carbonate. The process further comprises, prior to the decarbonation step, a step of leaching the residual sludge by addition of water. The residual sludge leaching step is carried out at least partly with wastewater. said process further comprises, prior to the decarbonation step, a step of separating, for example by filtration, metal particles or metal oxides present in the wastewater and / or the residual sludge, which makes it possible to optimize the purity of the carbonate precipitate with a view to its re-enhancement. The separation of the liquid phase containing the oxidizing salts from the solid phase is carried out by decantation followed by filtration. the process further comprises, after the step of separating the liquid and solid phases, a step of separating the water and the oxidizing salt in the liquid phase and recovering the oxidizing salt in solid form. 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 method further comprises, after separation of the water and the oxidizing salts, recycling the water to the stopping tank and / or to the at least one washing tank. the method 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 water in the liquid phase and the oxidizing salt is carried out in order to recover said oxidizing salt in solid form, the step of reintroducing the oxidizing salt is carried out after the latter. - The above nitriding tank is used to treat cyanate ions (CNO - and carbonate ion CO3 2- Includes. - The nitriding bath contains alkali ions, preferably lithium ions Li + , and / or potassium ion K + , and / or sodium ion Na + Further includes. - The oxidation treatment tank above produces hydroxide ions (OH - and / or nitrate ion NO3 - , and optionally carbonate ions.

[0035] Description of the drawings 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]

[0036] [Figure 1] FIG. 1 is a diagram illustrating an industrial installation for nitriding in a molten salt bath according to the prior art. [Diagram 2] FIG. 2 is a diagram illustrating an industrial installation for nitriding in a molten salt bath according to the invention, where only the nitriding bath, the oxidation bath and the stopping bath are shown. [Diagram 3] FIG. 3 is a diagram illustrating in detail the reaction of decarbonation of residual sludge and / or wastewater, according to certain embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] 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 held, namely the nitriding tank 11, the oxidation tank 12 and the stopping tank 13.

[0038] 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 molten nitriding salts brought to a temperature that is usually between 500°C and 630°C.

[0039] The nitriding treatment has the purpose of increasing the hardness of the parts by diffusing nitrogen, and optionally also carbon, in the steel, in the case of nitrocarburizing, and improving their mechanical properties, in particular their seizure and wear resistance.

[0040] 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 added to the nitriding bath to rapidly convert it to

[0041] The nitriding bath 11 contains alkali ions, preferably lithium ions Li + , and / or potassium ion K + , and / or sodium ion Na + Further includes.

[0042] After the nitriding step, the machine part is immersed in an oxidation bath 12. The oxidation bath 12 is composed of molten oxidizing salts, typically at a temperature of approximately 450° C. Said oxidizing salts are composed, inter alia, of hydroxides.

[0043] The oxidation treatment is intended to improve the corrosion resistance of the part and to give it a uniform black appearance.

[0044] 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, and then dried in an oven before being removed.

[0045] As can be seen from FIG. 2, the oxidation treatment of the parts leads to the formation of residual sludge B, producing a sludge that is extracted by the degreasing of the bath, and the stop water producing wastewater E, which constitute respectively solid and liquid wastes that the method of the invention allows to recycle.

[0046] The recovery of oxidizing salts is done by converting carbonate ions (CO3) 2- It is based on the decarbonation of residual sludge and / or wastewater by precipitation of

[0047] In a preferred embodiment shown in Figs. 2 and 3, decarbonation is carried out by decarbonating the residual sludge E with lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2 by decarbonating the residual sludge E with carbonate ions CO3 2- and / or carbonate ions CO3 in wastewater B 2- to form calcium carbonate CaCO3. For example, calcium carbonate can be enhanced as a base material for building materials.

[0048] Carbonate ion CO3 2- is expressed by the following formula (1): [C1] Ca(OH)2+CO3 2- →CaCO3+2OH - (1) It reacts with lime Ca(OH)2 according to:

[0049] In this way, hydroxide ions and calcium carbonate are obtained.

[0050] Carbonate ion CO3 2- is expressed by the following formula (2): [Case 2] Ca(NO3)2+CO3 2- →CaCO3+2NO3 - (2) It reacts with calcium nitrate Ca(NO3)2 according to:

[0051] In this case, nitrate ions and calcium carbonate are obtained.

[0052] If both lime Ca(OH)2 and / or calcium nitrate Ca(NO3)2 are introduced into the reaction medium, both reactions (1) and (2) will occur. Thus, carbonate ions CO3 2- reacts with both lime, Ca(OH)2, and calcium nitrate, Ca(NO3)2.

[0053] If only lime, Ca(OH)2, is added to the reaction medium, only reaction (1) takes place. If only calcium nitrate, Ca(NO3)2, is added to the reaction medium, only reaction (2) takes place.

[0054] After the decantation phase, the liquid phase containing the oxidizing salts 23 and the solid phase containing the calcium carbonate precipitate CaCO3 are separated. Preferably, this separation is performed by filtration, for example by a filter press.

[0055] In the liquid phase, the water and oxidizing salt 23 are then separated, preferably by drying, to obtain the oxidizing salt 23 in solid form.

[0056] The solid oxidizing salt 23 may be stored for enhancement or may be reintroduced into the oxidation bath.

[0057] Decarboxylation is illustrated in more detail in FIG.

[0058] In this diagram, during the leaching process, the residual sludge B is put into solution under stirring. A leachate 14 is obtained.

[0059] It is specified that the leachate 14 can be prepared by solvation of the residual sludge B alone by addition of water or by solvation of the residual sludge B with wastewater E.

[0060] According to a preferred embodiment of the invention, the leachate 14 is formed by solvation of the residual sludge B by the wastewater E. This embodiment is particularly advantageous in that it allows recycling of the wastewater E whilst using the latter as a solvent to solvate the residual sludge B, which makes it possible to reduce water consumption.

[0061] According to one embodiment, the solution is passed through a first filtration step prior to decarbonation to remove metal particles and metal oxides.

[0062] 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.

[0063] 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.

[0064] After decantation, solid / liquid separation of the settled solids 16 is preferably performed by filtration of the mixture using a filtration device 17 as shown in FIG.

[0065] The residue 18 containing the calcium carbonate precipitate CaCO3 can be set aside and then enhanced. The filtrate 19 is preferably recharged to the reactor to maximize the decarbonation yield.

[0066] The reactor supernatant 20 is then transferred into a drying device 21, such as an oven.

[0067] Preferably, the dried condensate 22 is reintroduced into the leachate 14 or into a stop tank or wash tank to reduce water consumption.

[0068] The drying step makes it possible to obtain recycled oxidizing salts 23, which can then be stored or reintroduced into the oxidation bath.

[0069] Thus, the method according to the invention not only makes it possible to reduce the nitriding and oxidation wastes, but also to reintroduce the oxidizing salts into the oxidation bath, which results in an economical and environmentally friendly recycling in line with current environmental standards and technical considerations. [Explanation of symbols]

[0070] 1 equipment 2. Nitriding tank 3. Oxidation tank 4 Stop tank 5 Degreasing tank 6 Cleaning tank 7 Drying equipment 8. Cleaning tank 9. Equipment for drying parts 10 Equipment 11 Nitriding tank 12 Oxidation tank 13 Stop tank 14 Exudate 15 Reactor 16 Settled solids 17 Filtration device 18 Residue 19 Filtrate 20 Supernatant 21 Drying equipment 22 Dry Condensate 23 Oxidizing Salts

Claims

1. A method for treating wastewater (E) from an oxidation bath and / or residual sludge (B) 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) to recover oxidizing salts and carbonates, comprising the following steps: - Carbonate ion CO ３ ２－ decarbonation of the wastewater (E) and / or the residual sludge (B) by precipitation of - separating the liquid phase containing the oxidizing salt (23) from the solid phase containing the carbonate precipitate, - recovery of oxidizing salts on the one hand and carbonates on the other hand; A method comprising:

2. Decarbonation produces 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 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. Decarbonation occurs when lime Ca(OH) ２ and / or calcium nitrate Ca(NO ３ ) ２ and the following reactions (1) and (2): [Chemical formula 1] Ca(OH) ２ +CO ３ ２－ →CaCO ３ +2OH － (1) [Case 2] CK(NO ３ ) ２ +CO ３ ２－ → ３ +2NO ３ － (2) Calcium carbonate CaCO ３ The method of claim 1 , further comprising forming

5. 5. The method according to claim 1, further comprising a step of leaching the residual sludge (B) prior to the precipitation step, said precipitation being carried out in the leachate (14).

6. 6. The method according to claim 5, wherein the leaching step of the residual sludge (B) is carried out with wastewater (E).

7. 7. The method according to claim 1, further comprising a step of separating metal particles or metal oxides present in the wastewater (E) and / or the residual sludge (B) prior to the decarbonation step.

8. 8. The method according to claim 1, further comprising, after the step of separating the liquid and solid phases, separating the 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, further comprising the step of 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 any one of claims 1 to 9, further comprising the step of re-introducing the oxidizing salt (23) into the oxidation tank (12) after the step of separating the liquid and solid phases.

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