Process for treating standstill water and sludge removal by carbonation in a chemical plant for nitration in a molten salt bath

Description

Technical field of the invention

[0001] The present invention relates to a method for treating stop water and sludge from the oxidation bath, to recover carbonate salts and water, within a chemical nitriding installation in a molten salt bath. Technological background

[0002] In automotive, aeronautical, or industrial applications, mechanical parts are generally subjected to significant stresses in service.

[0003] Traditionally, mechanical parts can receive physicochemical treatments to improve some of their properties, including friction properties, wear resistance, fatigue resistance, seizure resistance, and corrosion resistance.

[0004] A known state-of-the-art treatment is nitriding as described in WO 9634127 A1. Nitriding consists of immersing a ferrous metal part in a medium capable of releasing nitrogen, which may in particular be a bath of molten salts. In the present text, nitriding also encompasses nitrocarburizing, which is a variant of nitriding, in which carbon diffuses into the part in addition to nitrogen. The ARCOR ®< process, designed and implemented by the Applicant, is a preferred example of a nitriding process.

[0005] In reference to the figure 1which represents a nitriding installation in accordance with the state of the art, such an installation 1 comprises several successive baths, among which, from upstream to downstream, a nitriding bath 2, an oxidation bath 3, and a stop bath 4, also called a “water stop bath”. Upstream of the nitriding bath are usually provided a degreasing bath 5, one or more washing baths 6, and a drying device 7 such as an oven. Downstream of the stop bath 4 are usually provided one or more washing baths 8, and a drying device 9 for the parts.

[0006] The mechanical parts are first degreased, washed, and heated by passing successively through the degreasing bath 5, the washing baths 6, and the drying device 7 respectively. The parts are then immersed in the nitriding bath 2, which is a bath of molten salts, containing among other things carbonates. Depending on the needs, the addition of regenerating salts will lead to the transformation of the carbonates into cyanates which are the reactive species. The nitrogen, (and possibly the carbon), diffuses into the parts and precipitates in the form of nitrides, which leads to the formation of a combination layer essentially comprising iron nitrides (or other alloying elements), and an underlying diffusion zone in which the nitrogen is present between the iron atoms (solid solution) or the nitrogen reacts with the alloying elements contained in the steel to form nitrides.

[0007] The combined effect of the combination layer and the diffusion layer results in a nitrided part with good friction properties, as well as excellent wear and corrosion resistance.

[0008] After nitriding, the parts are immersed in the oxidation bath 3 comprising oxidizing salts, typically hydroxides and / or nitrates, to improve their corrosion resistance and give them a uniform black appearance.

[0009] After oxidation, the parts are immersed in the stop bath 4 which contains cold water, which stops the oxidation, then are cleaned by several successive washes in the washing baths 8, before being dried in the drying device 9 and then unloaded.

[0010] For some treatments, the oxidation step is not required; the parts are transferred from the nitriding bath directly to a dedicated stopping bath. The water in this stopping bath is enriched with nitriding salt (and no longer with oxidation salts).

[0011] Although nitriding is an effective process for improving the properties of mechanical parts, it has the disadvantage of generating liquid and solid waste (sometimes called effluent), and requires water. This not only represents a significant ecological impact, but can also lead to relatively high production costs, particularly due to the waste treatment described below.

[0012] During an ARCOR treatment, nitriding salts are carried by the parts and pollute the oxidation bath, then salts from the oxidation bath are carried and transferred into the stop water. Secondary chemical reactions also create sludge / waste within the treatment baths (nitriding and oxidation).

[0013] It is also possible for the line to be used alternately with or without oxidation and for the stop bath to then serve alternately as a stop bath for nitriding and then for oxidation. Nitriding salts and oxidation salts are then transferred alternately into the same stop bath.

[0014] These wastes are represented on the figure 1 These are solid effluents B but also liquid effluents E.

[0015] Solid effluent B is called "sludge removal". It is collected mainly in oxidation bath 3 as well as in nitriding bath 2, and is generally sent to salt mines for treatment and storage. It contains insoluble species and salt. For both types of salt (nitriding salt and oxidation salt), metals and metal oxides from the parts to be treated (iron and alloy metals) constitute insolubles. For oxidation salt, carbonates constitute a large part of the insolubles. Sludge removal is physically removed during regular maintenance operations.

[0016] Liquid effluents E are called “stop water”. They are collected at the outlet of the stop bath 4, and are generally sent to treatment plants or handled by companies specializing in waste treatment.

[0017] In addition, the purchasing costs of raw materials are also relatively high, so it is worth optimizing their consumption. Brief description of the invention

[0018] Faced with this situation, the Applicant therefore sought a solution to reduce the environmental impact and control the costs associated with nitriding processes. The Applicant particularly focused its research on reducing waste and limiting the waste of both renewable resources such as water and non-renewable resources such as the raw materials used in nitriding and oxidation baths, i.e. nitriding and oxidation salts.

[0019] The Applicant has succeeded in developing a treatment process which makes it possible to resolve all of the aforementioned drawbacks, without modifying the architecture of the nitriding chains, and which can therefore be implemented on existing lines.

[0020] To this end, the invention proposes a method for treating the stop water (E), and / or the cleaning sludge (B) from the oxidation bath, to recover carbonate salts, within at least one chemical nitriding installation in a molten salt bath comprising a nitriding bath, an oxidation bath, and a stop bath, the method comprising the following steps: transformation of hydroxide ions OH -< of the stop water (E) and / or of the cleaning sludge (B) by reduction of said ions with carbon dioxide CO 2 to form carbonate ions CO 3 2-< and water, separation of the water and recovery of the carbonate salts formed by the carbonate ions and the metal cations from the oxidation salts.

[0021] According to the treatment method of the invention, the stop water leaving the stop bath and the sludge from the oxidation bath are treated in order to recover only carbonate salts. The carbonate salts, which are a component of the nitriding salts, can feed the nitriding bath or be recovered. The water recovered after the treatment method of the invention advantageously feeds the stop bath and / or the washing baths with water when a water recycling step as described below is carried out.

[0022] More specifically, the sludge and / or the stop water are freed from the hydroxide ions OH -< , the latter being components of the oxidation bath. According to the invention, the hydroxide ions are transformed into carbonate ions. The corresponding carbonate salts (carbonate ions in the form of salts) are then recovered and then advantageously reinjected into the nitriding bath.

[0023] Usually, regenerating salts are injected into the nitriding bath. These convert the carbonate ions CO 3 2-< into cyanate ions CNO -< .

[0024] Thus, when the recycled carbonate ions CO 3 2-< are reinjected into the nitriding bath, they are converted into cyanate ions CNO -< by the regenerating salts.

[0025] The water is advantageously reinjected into the stop bath and / or the rinse / wash baths.

[0026] In doing so, the oxidation and stopping effluents feed the nitriding bath and advantageously the various water tanks, in particular the washing baths and / or the stopping bath.

[0027] The revalorization of carbonate salts, and where appropriate their recycling, drastically reduces the environmental impact of the industrial nitriding process, by reducing solid and liquid waste while reusing the latter to feed the process, thereby reducing the quantities and costs of the raw materials that are the nitriding salts.

[0028] The process of the invention therefore offers a double ecological and economic advantage.

[0029] It is specified that the parts may only undergo nitriding, without oxidation, in which case the stop bath serves to stop the nitriding reaction. The method of the invention does not apply to a nitriding line alone, without an oxidation bath, because in this case the recovery of the carbonates can be done by simple filtering of the stop water.

[0030] The nitriding installation may include several nitriding baths and / or several oxidation baths and / or several stopping baths.

[0031] The nitriding installation is used in this text in its most general sense. The different baths it comprises can in practice be installed in the same place or in different places. For example, the transformation of hydroxide ions OH -< into carbonate ions CO 3 2-< can be carried out with a first chemical installation in a first place, and the separation of water and carbonate salts can be carried out with a second chemical installation in a second place different from the first. In addition, a single place can include several nitriding installations, i.e. several nitriding lines.

[0032] In the water and carbonate salt separation step, the water corresponds to the stop water and / or the leaching water from the sludge which has been brought into contact with the sludge in order to extract the hydroxide ions.

[0033] The transformation step can be carried out in several ways: transformation of the hydroxide ions OH -< of the stop water alone, or transformation of the hydroxide ions OH -< of the cleaning sludge, with prior solubilization of said hydroxide ions OH -< by leaching of the cleaning sludge in water, or more generally in an aqueous solution, for example mains water or industrial water, or transformation of the hydroxide ions OH -< of the cleaning sludge, with prior solubilization of said hydroxide ions OH -< by leaching of the cleaning sludge in the stop water.

[0034] According to other aspects, the treatment method according to the invention has the following different characteristics taken alone or according to their technically possible combinations: the process comprises, before the transformation of the hydroxide ions OH -< into carbonate ions CO 3 2-< , a step of separation of the metal particles or metal oxides present in the stop water, advantageously by filtration. This makes it possible to optimize the purity of the nitriding salt, since the metal particles are impurities; the transformation of the hydroxide ions OH -< is carried out by blowing in carbon dioxide CO 2; the transformation of the hydroxide ions OH -< is carried out by blowing in air. The hydroxides then react with the CO 2 present in the air.The transformation will take longer, but the process will be simple to implement and less expensive; the step of separating the water and the carbonate salts is carried out by filtration and / or drying; the process further comprises, after the step of separating the water and the carbonate salts, a step of re-injecting the carbonate salts into the nitriding bath; the process further comprises, after the separation of the water and the carbonate salts, a step of recycling the water to the stop bath and / or at least one washing bath; the process comprises, before the re-injection of the carbonate salts into the nitriding bath, a step of readjusting the contents of the cations to the contents of the nitriding bath, to make them compatible with the nitriding salts.This step consists in practice of adding alkali carbonates (for example lithium, sodium or potassium carbonates) to the carbonate salts obtained, to make their contents correspond to the experimental contents of the carbonate salts in the nitriding bath. It makes it possible to maintain a ratio of the different cations that is substantially constant, or at least to limit its variations.This step is therefore particularly advantageous when the carbonate salts are recycled by reinjecting them into the nitriding bath, since the nitriding bath retains substantially constant physicochemical properties over time, which improves the reproducibility of the process; the nitriding bath comprises cyanate ions CNO -< and carbonate ions (CO 3 ) 2-<; the nitriding bath further comprises alkali ions, preferably lithium ions Li +< , and / or potassium ions K +< , and / or sodium ions Na +<; the oxidation bath comprises hydroxide ions OH -< and / or nitrate ions NO 3 -< , and possibly carbonate ions.

[0035] Some carbonate salts produced by the present process may have significant recovery potential, such as lithium carbonate Li 2 CO 3 .

[0036] An additional advantage of the present process is that it allows CO2 to be trapped. Description of figures

[0037] Other advantages and characteristics of the invention will appear on reading the following description given by way of illustrative and non-limiting example, with reference to the following appended figures: [ Fig. 1 ] There figure 1 is a diagram illustrating a state-of-the-art industrial molten salt bath nitriding installation. Fig. 2 ] There figure 2 is a diagram illustrating an industrial molten salt bath nitriding installation according to the invention, in which only the nitriding, oxidation, and stopping baths are shown. Fig. 3 ] There figure 3 is a diagram which illustrates in detail the reaction of transformation of hydroxide ions OH -< of sludge removal and / or stop water into carbonate ions CO 3 2-< by insufflation of carbon dioxide CO 2 . Detailed description of embodiments of the invention

[0038] The method of the invention will now be described in detail with reference to the figure 2 which is a simplified representation of a molten salt bath nitriding installation 10, in that only the main elements of the installation are retained, namely a nitriding bath 11, an oxidation bath 12, and a stop bath 13, for the purpose of simplifying the present text.

[0039] The mechanical parts to be treated, previously degreased, then rinsed and dried in an oven, are immersed in the nitriding bath 11. The nitriding bath 11 is composed of molten nitriding salts which are brought to a temperature which is typically between 500°C and 630°C.

[0040] The aim of nitriding is to give parts greater hardness and to improve their mechanical properties, in particular resistance to seizure and wear, by diffusing nitrogen into the steel, and possibly carbon in the case of nitrocarburizing.

[0041] The nitriding bath 11 mainly comprises cyanate ions CNO -< and carbonate ions CO 3 2-< . Regenerating salts are added as needed to the nitriding bath to quickly convert the carbonate ions CO 3 2-< into cyanate ions CNO -< .

[0042] The nitriding bath 11 further comprises alkali ions, preferably lithium ions Li +< , and / or potassium ions K +< , and / or sodium ions Na +< .

[0043] After the nitriding step, the mechanical parts are immersed in the oxidation bath 12. The oxidation bath 12 is composed of molten oxidizing salts brought to a temperature, typically of the order of 450°C. The oxidation salts are composed, among other things, of hydroxides.

[0044] The purpose of oxidation is to improve the corrosion resistance of parts and to give them a uniform black appearance.

[0045] After the oxidation step, the parts are immersed in the stop bath 13, which contains water (stop water), then are cleaned in washing baths, preferably successive cascade washes, before being dried in an oven, then unloaded.

[0046] As can be seen on the figure 2, the oxidation of the parts generates sludge which is extracted by cleaning the bath, leading to the formation of cleaning sludge B, and the water stop generates stop water E, which constitutes respectively solid and liquid waste which the process of the invention allows to be recycled. The recovery of carbonate salts is based on the transformation of hydroxide ions OH -< from the stop water and / or cleaning sludge into carbonate ions CO 3 2-< .

[0047] According to the invention, the transformation step is carried out by reaction of the hydroxide ions OH -< with carbon dioxide CO 2 , to form carbonate ions CO 3 2-< and water, according to the following reaction (1):

[0048] Carbonate ions CO 3 2-< , referenced 14 on the figure 2 , are then reinjected in the form of salts into the nitriding bath, where they can possibly be converted into cyanate ions CNO -< by the regenerating salts.

[0049] It is also possible to recover certain carbonates, notably lithium carbonate. The latter is easily separated from sodium and potassium carbonates by precipitation thanks to its very low solubility. It can, for example, be recovered after an initial decantation followed by filtering, then the other carbonates are recovered, for example, by drying.

[0050] Preferably, before the transformation of the hydroxide ions OH -< into carbonate ions CO 3 2-< , a step of separating the metal particles or metal oxides present in the stop water is carried out, advantageously by filtration possibly followed by drying.

[0051] Preferably, after the separation of water and carbonate ions CO 3 2-< , a step of recycling the water to the stop bath is carried out.

[0052] Preferably, before re-injecting the carbonate ions CO 3 2-< into the nitriding bath, a step of separating the carbonate salts and the water is carried out, advantageously by filtration, possibly followed by drying.

[0053] Preferably, before re-injecting the carbonate ions CO 3 2-< into the nitriding bath, a step is carried out to readjust the cation contents to the contents of the nitriding bath to make them compatible with the nitriding salts.

[0054] An embodiment of the transformation reaction of hydroxide ions OH -< of solubilized sludge and / or stop water is illustrated in more detail in the figure 3 . According to this embodiment, the reaction is carried out by blowing in carbon dioxide CO 2 .

[0055] In this figure, the stop water E and possibly the solubilized cleaning sludge B are previously put into solution 15 in a first glass container 16. A second glass container 17 is immersed in the solution 15 in an upside-down position, so as to provide an opening for the passage of a tube 18, between the inside and the outside of the second container 17. The solution 15 thus occupies the volumes of the two containers 16, 17. The objective of this installation is to trap carbon dioxide CO 2 .

[0056] The tube 18 has a first opening 19 opening into the solution inside the second container 17, and a second opening 20 opening onto means for supplying carbon dioxide CO 2 outside the containers 16, 17. The tube 18 is thus partially immersed in the solution 15, and passes through the two containers 16, 17.

[0057] The solution outside the first container 16 contains hydroxide ions OH -< and carbonate ions CO 3 2-< .

[0058] Carbon dioxide CO 2 is injected into the second container 17 via the second opening 20 of the tube 18. Bubbles 21 of carbon dioxide CO 2 are formed in the solution 15 inside the second container 17. The solution 15 located inside the second container 17 is enriched with carbonate ions CO 3 2-< resulting from the reaction between the carbon dioxide CO 2 and the hydroxide ions OH -< , and the overlying region 22 is enriched with carbon dioxide CO 2 , the latter being trapped by said second container 17.

[0059] Subsequently, carbonate ions CO 3 2< can precipitate. Precipitation depends on several parameters that are adjusted for this purpose, including the concentration of hydroxide ions OH -< in the solution, carbonate ions CO 3 2< , and cations.

[0060] Depending on the formation of precipitates, the water 40 is then filtered, using a filter press for example, and then the recovered carbonates are dried. To recover more carbonates (not precipitated), it is possible to evaporate the water using a dryer for example.

[0061] The solid carbonates are then reinjected as nitriding salts into the nitriding bath. For the sake of simplification, the nitriding salts are, as for the carbonate ions CO 3 2-< , referenced 14 on the figure 2 .

[0062] Preferably, the water 40 is recycled to the stop bath 13.

[0063] The process according to the invention thus makes it possible not only to reduce nitriding and oxidation waste, but also to recharge the nitriding bath with nitriding salts, which results in economical and ecological recycling, in line with current environmental standards and procedures.

Claims

1. Method for treating waste waters (E) or residue sludge (B) from the oxidation bath or waste waters (E) and residue sludge (B) from the oxidation bath to recover carbonate salts, in at least one installation (10) for nitridation in a molten salt bath comprising a nitridation bath (11), an oxidation bath (12), and a stop bath (13), the treatment method comprising the following steps: - transformation of hydroxide ions OH- of waste waters (E) or residue sludge (B) or waste waters (E) and residue sludge (B) from the oxidation bath by reaction of said ions with carbon dioxide CO2 to form carbonate ions CO32- and water, - separation of water and recovery of carbonate salts formed by carbonate ions and of metal cations from oxidation salts.

2. Method according to claim 1, comprising, before the transformation of hydroxide ions OH- into carbonate ions CO32-, a step of separating metal particles or metal oxides present in the waste waters (E) and / or the residue sludge (B).

3. Method according to any one of the preceding claims, further comprising, after separating water and carbonate salts, a step of reinjecting carbonate salts into the nitridation bath (11).

4. Method according to claim 3, further comprising, before reinjecting carbonate salts into the nitridation bath (11), a step of readjusting contents of cations to the contents of the nitridation bath (11), to make them compatible with nitridation salts.

5. Method according to any one of the preceding claims, further comprising, after separating water and carbonate salts, a step of recycling water to the stop bath (13) and / or at least one washing bath (6, 8).