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

Abstract

The present invention relates to a method for recovering carbonates by treating wastewater (E) and / or residual sludge (B) 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 method comprises the steps of: - Hydroxide ions and / or OH of the residual sludge (B) - Hydroxide ions CO 3 2- The process includes converting the carbonate salt to carbonate ions and separating the water and carbonate salt.

Description

[Technical field]

[0001] The present invention relates to a method for treating wastewater and residual sludge from oxidation baths in chemical plants for nitriding in molten salt baths, to recover 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, a variant of nitriding in which carbon is diffused into the part in addition to nitrogen. 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 and corrosion 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 called effluents) and requires water, which can present significant environmental impacts and result in high production costs, especially 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 a nitriding treatment followed by an oxidation treatment, with the nitriding salts and the oxidizing salts thus being selectively 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] Solid effluents B are also called "residual sludge". They are mainly collected in the oxidation tank 3 and in the nitridation tank 2 and are usually transported to a salt mine for processing and storage. They contain insoluble species and salts. For the two types of salts (nitriding salts and oxidizing salts), metals and metal oxides (iron and alloy metals) originating from the parts being processed constitute the insolubles. For the oxidizing salts, 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 provides a method for recovering carbonates by treating wastewater (EL) from an oxidation tank and / or residual sludge (ES) in at least one chemical installation for nitriding in a molten salt tank, the method comprising the steps of: - hydroxide ions OH from wastewater (E) and / or residual sludge (B) - Carbonate ion CO3 2- converting - separating the water from the carbonate salt formed from the carbonate ions and the metal cations from the oxidizing salt; - Carbonate recovery process The present invention provides a method comprising:

[0022] According to the treatment method of the invention, the wastewater at the outlet of the stop tank and the residual sludge from the oxidation tank are treated only to recover carbonates. The carbonates, which are components of the nitriding salts, may be fed to the nitriding tank or may be enhanced. If the water recycling step described below is carried out, the water recovered after the treatment process is advantageously fed to the stop tank and / or to the aqueous washing tank.

[0023] More precisely, hydroxide ions OH from the residual sludge and / or wastewater - is removed, the hydroxide ions of which are constituents of the oxidizing bath. According to the invention, the hydroxide ions are converted into carbonate ions. The corresponding carbonates (carbonate ions in the form of a salt) are then recovered and then advantageously reintroduced into the nitriding bath.

[0024] Typically, the regenerating salt is added to the nitriding tank. Typically, the regenerating salt is added to the nitriding tank.

[0025] Therefore, the recycled carbonate ions (CO3) 2- When they are reintroduced into the nitriding tank, they are converted into cyanate ions CNO - is converted to

[0026] Water is advantageously re-charged into the stopping tank and / or the rinsing / washing tank.

[0027] In carrying it out, the effluent of the oxidation process and waste is fed to a nitriding tank and, advantageously, to a separate water tank, in particular a washing tank and / or a stopping tank.

[0028] The re-enhancement of 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 this nitriding salt raw material and by reusing the latter to replenish the process, while reducing solid and liquid waste.

[0029] The method of the present invention therefore offers both environmental and economic advantages.

[0030] It is specified that the above components can be used without oxidation only, if a 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 achieved by simply filtering the wastewater.

[0031] The nitriding facility may have multiple nitriding tanks and / or multiple oxidation tanks and / or multiple stopping tanks.

[0032] 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, hydroxide ions OH - Carbonate ion CO3 2- The decarbonation to may be carried out using a first chemical plant in a first location, and the separation of water and carbonate may be carried out using a second chemical plant in a second location separate from the first location. In addition, there may be several nitriding plants, i.e. several nitriding lines, in the same location.

[0033] In the process of separating the water and carbonates, the water corresponds to wastewater and / or sludge leachate that has been brought into contact with the sludge to extract hydroxide ions from the sludge.

[0034] The conversion step can in particular be carried out in several ways: - Hydroxide ion OH in wastewater only - Conversion of, or - by leaching of the residual sludge into water or, more generally, by introducing the above-mentioned hydroxide ions OH - The hydroxide ions OH of the residual sludge presolvated with - Conversion of, or - The hydroxide ion OH mentioned above is generated by leaching the residual sludge into wastewater. - The hydroxide ions OH of the residual sludge presolvated with- Conversion of.

[0035] 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: The conversion process is carried out by the following reaction (3): [C1] 2OH - +CO2→H2O+CO3 2- (1) According to this, hydroxide ion OH - reacts with carbon dioxide CO2 to produce carbonate ions CO3 2- and water. - The above method produces hydroxide ions (OH - Carbonate ion CO3 2- Before converting it into nitrile salt, it includes a step of separating, advantageously by filtration, any metal particles or metal oxides present in the waste water. This makes it possible to optimize the purity of the nitrile salt, since metal particles are impurities. - Hydroxide ion OH - The conversion is carried out by blowing in carbon dioxide CO2. - Hydroxide ion OH - The conversion is carried out by bubbling air. The hydroxide then reacts with CO2 in the air. Although this conversion may take additional time, the method is simpler to carry out and may be cheaper. The separation of water and carbonates is carried out by filtration and / or drying. the method further comprises the step of reintroducing the carbonate into the nitriding vessel after separating the water and the carbonate. the process further comprises a step of recycling the water to the stopping tank and / or to at least one washing tank after separation of the water and the carbonates. - the method includes a step of readjusting the amount of cations relative to the content of the nitriding tank before reintroducing the carbonates into the nitriding tank to make them compatible with the nitriding salts. This step actually consists in adding alkaline carbonates (for example lithium, sodium or potassium carbonate) to the carbonates obtained, in amounts that correspond to the experimental contents of the carbonates in the nitriding tank. It makes it possible to keep the ratios of the different cations substantially constant or, as a minimum, to limit their variations. This step is therefore particularly advantageous in particular when the carbonates are recycled by reintroducing them into the nitriding tank, since the nitriding tank maintains substantially constant physicochemical properties over time, which improves the reproducibility of the method. - The nitriding tank contains 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 produces hydroxide ions (OH - and / or nitrate ion NO3 - , and optionally carbonate ions.

[0036] Some carbonates produced by the method of the present invention may have significant enhancement potential, such as lithium carbonate, Li2CO3.

[0037] A further advantage of the method of the present invention is that it allows for the capture of CO2.

[0038] 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]

[0039] [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 detailed diagram showing the conversion reaction of hydroxide ions OH− in residual sludge and / or wastewater to carbonate ions CO3 2− by injecting carbon dioxide CO2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] 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, a nitriding tank 11, an oxidation tank 12 and a stopping tank 13.

[0041] 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 at a temperature that is usually between 500°C and 630°C.

[0042] 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.

[0043] 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 the nitriding tank in order to rapidly convert it into

[0044] The nitriding bath 11 contains alkali ions, preferably lithium ions Li + , and / or potassium ion K + , and / or sodium ion Na + Further includes:

[0045] 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. Said oxidizing salts consist, inter alia, of hydroxides.

[0046] The oxidation treatment is intended to improve the corrosion resistance of the part and to give it a uniform black appearance.

[0047] After the oxidation 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 washes, before being dried in an oven and then removed.

[0048] As can be seen from FIG. 2, the oxidation treatment of the parts produces a sludge that is extracted by the degreasing of the bath, which leads to the formation of a residual sludge B, and a stop water producing wastewater E, which constitute respectively solid and liquid wastes that the method of the invention allows to recycle.

[0049] Carbonate recovery is achieved by removing hydroxide ions (OH) from the residual sludge and / or wastewater. - Carbonate ion CO3 2- It is based on the conversion to

[0050] Preferably, the conversion step is carried out by the following reaction (1): [C1] 2OH - +CO2→H2O+CO3 2- (1) According to this, hydroxide ion OH - reacts with carbon dioxide CO2 to produce carbonate ions CO3 2- and water.

[0051] Carbonate ion CO3, referenced as 14 in Figure 2 2- The nitriding agent is then reintroduced into the nitriding tank in the form of salt, and converted to cyanate ions (CNO) by the regenerating salt. - may optionally be converted into

[0052] It is also possible to enhance certain carbonates, in particular lithium carbonate. The latter is easily further separated from the sodium and potassium carbonates by precipitation due to its very low solubility. It can be recovered, for example, by a first decantation followed by filtration, and the other carbonates are then recovered, for example, by drying.

[0053] Preferably, the hydroxide ion OH - Carbonate ion CO3 2- The step of separating out the metal particles or metal oxides present in the wastewater and / or the residual sludge prior to conversion to is advantageously carried out by filtration, optionally followed by drying.

[0054] Preferably, water and carbonate ions CO3 2- After separation from the sulphuric acid, a step of recycling the water to the stopping tank is carried out.

[0055] Preferably, carbonate ions CO3 2- Before reintroduction into said nitriding tank, a step of separating the carbonates from the water is advantageously carried out by filtration, optionally with subsequent drying.

[0056] Preferably, carbonate ions CO3 2- before being recharged into the nitriding vessel, a step is carried out in which the amount of cations is readjusted relative to the contents of the nitriding vessel to make them compatible with the nitriding salts.

[0057] Hydroxide ion (OH) from solubilized residual sludge - , and / or hydroxide ions OH from wastewater -An embodiment of the conversion reaction of is shown in detail in Figure 3. According to this embodiment, the reaction is carried out by blowing in carbon dioxide CO2.

[0058] In this figure, wastewater E and optionally solubilized residual sludge B are pre-placed in solution 15 in a first glass receptacle 16. A second glass receptacle 17 is immersed in the solution 15 upside down, leaving a space for a tube 18 to pass between the interior and exterior of the second receptacle 17. Thus, the solution 15 fills the capacity of the two receptacles 16, 17. The purpose of this installation is to capture carbon dioxide CO2.

[0059] The tube 18 has a first opening 19 opening into the solution inside the second receptacle 17, and a second opening 20 opening into a carbon dioxide CO2 supply outside the receptacle 16, 17. The tube 18 is thus partially immersed in the solution 15 and passes through the two receptacles 16, 17.

[0060] The solution located outside the first receptacle 16 contains hydroxide ions OH - and carbonate ion CO3 2- Contains:

[0061] Carbon dioxide CO2 is injected into the second receptacle 17 via a second opening 20 of the tube 18. Carbon dioxide CO2 bubbles 21 are formed in the solution 15 inside the second receptacle 17. The solution 15 located inside the second receptacle 17 is composed of carbon dioxide CO2 and hydroxide ions OH. - Carbonate ions CO3 resulting from the reaction between 2- the overlying region 22 is rich in carbon dioxide CO2, the latter being captured by the second receptacle 17.

[0062] Next, carbonate ion CO3 2-can be precipitated. The precipitation is effected by several parameters which 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.

[0063] According to the formation of the precipitate, the water 40 is filtered, for example using a filter press, and the recovered carbonate is then dried. It is conceivable to evaporate the water, for example by a dryer, to further recover the carbonate (non-precipitate).

[0064] The solid carbonate is re-introduced into the nitriding bath as a nitriding salt. To keep things simple, the nitriding salt is represented by the carbonate ion CO3, reference numeral 14 in FIG. 2- is the same as:

[0065] Preferably, the water 40 is recycled to the stopping tank 13 .

[0066] Thus, the method according to the invention not only makes it possible to reduce the nitriding and oxidation wastes, but also to reintroduce the nitriding salts into the nitriding bath, which is effected by an economical and environmentally friendly recycling in line with current environmental standards and technical considerations. [Explanation of symbols]

[0067] 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 Carbonate ion CO3 2- 15...solution 16 - First glass receptacle 17...Second Glass Receptacle 18...Tube 19...First opening 20...Second opening 21 Carbon Dioxide (CO2) Bubbles 22. Covered Area 40...water

Claims

1. A method for recovering carbonates by treating wastewater (E) from an oxidation tank and / or residual sludge (B) in at least one installation (10) for nitriding in a molten salt tank having a nitriding tank (11), an oxidation tank (12) and a stop tank (13), comprising the following steps: - hydroxide ions OH of the wastewater (E) and / or the residual sludge (B) － Carbonate ion CO ３ ２－ converting - separating the water from the carbonate salt formed from the carbonate ions and the metal cations from the oxidizing salt, - recovering carbonates; The method includes:

2. The conversion step is carried out by the following reaction (1): [Chemical formula 1] 2OH － +CO ２ → H ２ O+CO ３ ２－ (1) According to this, hydroxide ion OH － Carbon dioxide CO ２ React with carbonate ion CO ３ ２－ and water.

3. Hydroxide ion OH － Carbonate ion CO ３ ２－ 3. The process according to claim 1 or 2, comprising a step of separating metal particles or metal oxides present in the wastewater (E) and / or the residual sludge (B) before the conversion into

4. 4. The method according to any one of claims 1 to 3, further comprising the step of re-introducing the carbonate into the nitriding tank (11) after separating the water and the carbonate.

5. 5. The method according to claim 4, further comprising the step of readjusting the amount of cations relative to the content of the nitriding tank (11) before reintroducing the carbonate salts into the nitriding tank (11) to make them compatible with the nitriding salts.

6. 6. The method according to any one of claims 1 to 5, 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 separation of the water and the carbonates.

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