Patent Application
20250223213
The present invention relates to a method for treating waste waters, and residue sludge from the oxidation bath, to recover oxidation salts, carbonate salts and water, in a chemical installation for nitridation in a molten salt bath.
In automotive, aeronautical, or industrial applications, the mechanical parts are generally subjected to significant stresses in operation.
Conventionally, the mechanical parts can receive treatments of a physico-chemical nature, making it possible to improve some of their properties, among which appear, in particular, friction properties, resistance to wear, resistance to fatigue, resistance to seizing, or also resistance to corrosion.
A treatment known from the prior art is nitridation. Nitridation consists of immersing a ferrous metal part in a medium which is able to transfer nitrogen, which can be, in particular, a molten salt bath. In the present text, nitridation also includes nitrocarburising, which is a variant of nitridation, in which carbon diffuses in the part in addition to nitrogen. The ARCOR® method, designed and implemented by the Applicant, is a preferred example of the nitridation method.
In reference to
The mechanical parts are first degreased, washed, and heating by successively passing into the degreasing bath 5, into the washing baths 6, and into the drying device 7, respectively. The parts are then immersed in the nitridation bath 2, which is a molten salt bath, containing, among others, carbonates. As needed, adding regeneration salts will lead to the transformation of carbonates into cyanates, which are reactive species. Nitrogen (and optionally carbon) diffuses in the parts and precipitates in the form of nitrides, which leads to the formation of a combination layer mainly comprising iron nitrides (or other alloy elements), and an underlying diffusion zone, in which nitrogen is present between the iron atoms (solid solution) or nitrogen reacts with the alloy elements contained in steel to form nitrides.
The combined effect of the combination layer and the diffusion layer makes it possible to obtain a nitrided part having good friction properties, as well as an excellent resistance to wear.
After nitridation, the parts are immersed in the oxidation bath 3 comprising oxidising salts, typically hydroxides and/or nitrates, to improve their resistance to corrosion and give them a uniform black appearance.
After oxidation, the parts are immersed in the stop bath 4 which contains cold water, which stops oxidation, then are cleaned by several successive washes in the washing baths 8, before being dried in the drying device 9, then discharged.
For certain treatments, the oxidation step is not required, the parts are transferred from the nitridation bath directly to a dedicated stop bath. The waters from this stop bath are nitridation salt-enriched (and no longer in oxidation salts).
Although nitridation is an effective method for improving the properties of the mechanical parts, it has the disadvantage of generating liquid and solid waste (sometimes called effluents), and requires water. This represents a significant ecological impact, and can cause relatively high production costs, in particular due to the treatment of waste described below.
During an ARCOR treatment, nitridation salts are entrained by the parts and pollute the oxidation bath, then salts from the oxidation bath are entrained and transferred into the stop bath. Secondary chemical reactions also create sludge/waste even in treatment baths (nitridation and oxidation).
It is also possible that the line is used alternatively with or without oxidation and that the stop bath thus alternatively serves as a stop bath for nitridation then for oxidation. Nitridation salts and oxidation salts are thus alternatively transferred into the same stop bath.
This waste is represented in
The solid effluents B are called “residue sludge”. They are mainly collected in the oxidation bath 3 and in the nitridation bath 2 (after performing the method), and are generally sent into salt mines to be treated and stored there. They contain insoluble species and salt. For the two types of salts (nitridation salt and oxidation salt), metals and metal oxides coming from the parts to be treated (iron and alloy metals) constitute insolubles. For the oxidation salt, carbonates constitute a large portion of insolubles. Residue sludge is physically removed during regular maintenance operations.
The liquid effluents E are called “waste waters”. They are collected at the outlet of the stop bath 4, and are generally sent into purification stations or managed by companies specialising in waste treatment.
In addition, the purchase costs of raw materials are themselves also relatively high, it is therefore interesting to optimise their consumption.
Faced with this situation, the Applicant has therefore sought a solution to reduce the environmental impact and to control the costs associated with nitridation methods.
The Applicant has quite specifically focused their research on reducing waste, and limiting waste for both renewable resources such as water, and non-renewable resources, such as raw materials used in nitridation and oxidation baths, i.e. nitridation and oxidation salts.
The Applicant has arrived at developing a treatment method which makes it possible to overcome all of the abovementioned disadvantages, and this, without modifying the architecture of the nitridation chains, therefore being able to be implemented on current lines.
To this end, the invention proposes a method for treating waste waters (E), and/or residue sludge (B) from the oxidation bath, to recover oxidation salts and carbonates, in at least one installation for nitridation in a molten salt bath comprising a nitridation bath, an oxidation bath, and a stop bath,
According to the treatment method of the invention, the waste waters at the outlet of the stop bath and the residue sludge from the oxidation bath are treated in order to separately recover the oxidation salts, carbonates and water. Advantageously, the recovered water can supply the stop bath and/or the washing baths when a water recycling step as described below is carried out.
Carbonate ions CO32− come from the entrainment of the salts from the nitridation salts and are formed in the reaction medium. They constitute a “contaminating” species for the oxidation bath. The decarbonation step aims to remove the carbonate ions by precipitation, in order to be able to isolate the oxidising species (nitrates and/or hydroxides) necessary for the correct operation of the oxidation bath.
The removing of carbonate ions is advantageously used to produce enhanceable carbonates. This enhancement contributes to reducing the cost of nitridation in a salt bath. At the same time, it reduces the ecological impact.
After solid/liquid separation, the oxidation salts are recovered. The latter can thus be reinjected into the oxidation bath or be re-enhanced, and water is advantageously reinjected into the stop bath and/or the washing/rinsing baths.
In doing so, the oxidation and waste effluents supply the oxidation bath and advantageously, the different water tanks, in particular, the washing baths and/or the stop bath.
The re-enhancement of the oxidation salts and the carbonate salts, and if necessary, their recycling, drastically reduces the environmental impact of the industrial nitridation method, by reducing solid and liquid waste, while reusing the latter, in order to supply the method, due to this reducing the quantities and the costs of raw materials, which are oxidising salts.
The method of the invention therefore offers a double ecological and economic advantage.
It is specified that the parts can only undergo one nitridation, without oxidation, in which case the stop bath serves to stop the nitridation reaction. The method of the invention does not apply to a single nitridation line, without oxidation bath, as in this case, the recovery of carbonates can be done by simply filtering the waste waters.
The nitridation installation can comprise several nitridation baths and/or several oxidation baths and/or several stop baths.
The nitridation installation is used in the present text in its more general acceptance. The different baths that it comprises, can in practice, be installed in a same location or in different locations. For example, the decarbonation of waste waters and/or waste sludge can be carried out with a first chemical installation in a first place, and the separation of the liquid phase and the solid phase can be carried out with a second chemical installation in a second place different from the first. In addition, a same place may comprise several nitridation installations, i.e. several nitridation lines.
The decarbonation step can, in particular, be carried out, in several ways:
According to other aspects, the treatment method according to the invention has the following different features taken individually or according to their technically possible combinations:
[Chem. 1]
Ca(OH)2+CO32−→CaCO3+2OH− (1)
[Chem. 2]
Ca(NO3)2+CO32−→CaCO3+2NO3− (2)
Other advantages and features of the invention will appear upon reading the following description given as a non-limiting, illustrative example, in reference to the following accompanying figures:
The method of the invention will now be described in detail in reference to
The mechanical parts to be treated, previously degreased, then rinsed and dried in an oven, are immersed in the nitridation bath 11. The nitridation bath 11 is composed of molten nitridation salts which are brought to a temperature which is typically between 500° C. and 630° C.
Nitridation aims to give the parts a greater hardness, and to improve their mechanical properties, in particular resistance to seizing and to wear, by diffusion of nitrogen, and optionally carbon in steel, in the case of a nitrocarburising.
The nitridation bath 11 mainly comprises cyanate ions CNO− and carbonate ions CO32-. Regeneration salts are added if needed in the nitridation bath to rapidly convert the carbonate ions CO32− into cyanate ions CNO−.
The nitridation bath 11 further comprises alkaline ions, preferably lithium ions Li+, and/or potassium ions K+, and/or sodium ions Na+.
After the nitridation step, the mechanical parts are immersed in the oxidation bath 12. The oxidation bath 12 is composed of molten oxidising salts brought to a temperature typically of around 450° C. The oxidation salts are composed among others of hydroxides.
Oxidation aims to improve resistance to corrosion of parts, and to give them a uniform black appearance.
After the oxidation step, the parts are immersed in the stop bath 13, which contains cold water (waste water), then are cleaned in washing baths, preferably successive cascading washes, before being dried in an oven, then discharged.
As can be seen in
The recovery of oxidation salts is based on the decarbonation of residue sludge and/or waste waters by precipitation of carbonate ions CO32-.
In the preferred embodiment shown in
Carbonate ions CO32− react with lime Ca(OH)2 according to the following equation (1):
[Chem. 1]
Ca(OH)2+CO32−→CaCO3+2OH− (1)
Thus, hydroxide ions and calcium carbonate are obtained.
Carbonate ions CO32− react with calcium nitrate Ca(NO3)2 according to the following equation (2):
[Chem. 2]
Ca(NO3)2+CO32−→CaCO3+2NO3− (2)
In this case, nitrate ions and calcium carbonate are obtained.
When lime Ca(OH)2 and/or calcium nitrate Ca(NO3)2 are both added into the reaction medium, the reactions (1) and (2) both occur. Carbonate ions CO32− react therefore both with lime Ca(OH)2 and calcium nitrate Ca(NO3)2.
When only lime Ca(OH)2 is added into the reaction medium, only the reaction (1) occurs. When only calcium nitrate Ca(NO3)2 is added into the reaction medium, only the reaction (2) occurs.
After a decantation phase, the liquid phase containing oxidation salts 23 and the solid phase containing calcium carbonate precipitate CaCO3 are separated. Preferably, the separation is done by filtration, for example on a filter press.
In the liquid phase, water and oxidation salts 23 are then separated, preferably by drying, in order to obtain oxidation salts 23 in solid form.
The oxidation salts 23 in solid form can be preserved to be enhanced, or can be reinjected into the oxidation bath.
The decarbonation is illustrated in more detail in
In this figure, the residue sludge B is put into a solution under stirring in a leaching step. A leaching solution 14 is obtained.
It is specified that the leaching solution 14 can be prepared by solvation of only residue sludge B, by adding water, or by solvation of residue sludge B with waste waters E.
According to a preferred embodiment of the invention, the leaching solution 14 is formed by solvation of residue sludge B by waste waters E. This embodiment is particularly advantageous in that it makes it possible to recycle waste waters E while using the latter as a solvent to solvate the residue sludge B, which makes it possible to reduce water consumption.
According to an embodiment, this solution passes, before decarbonation, a first filtration step in order to remove the metal particles and metal oxides.
The leaching solution 14 is transferred into a reactor 15, supplied with lime Ca(OH)2 and/or calcium nitrate Ca(NO3)2, and stirred.
A calcium carbonate CaCO3 precipitate is formed in a solution. The mixture is thus composed of a solid phase, calcium carbonate precipitate CaCO3, and a liquid phase containing oxidation salts.
After decantation, a solid/liquid separation of the settled solids 16, preferably by filtration of the mixture as
The residue 18 comprising calcium carbonate precipitate CaCO3 is put aside, and can be subsequently enhanced. The filtrate 19 is preferably reinjected into the reactor in order to maximise the yield of the decarbonation.
The supernatant 20 of the reactor is thus transferred into a drying device 21, such as an oven, for example.
Preferably, the drying condensates 22 are reinjected into the leaching solution 14, or in the stop bath, or in the washing baths, in order to reduce water consumption.
Drying makes it possible to obtain recycled oxidation salts 23, which are thus preserved or reinjected into the oxidation bath.
The method according to the invention thus makes it possible, not only to reduce nitridation and oxidation waste, but also to reload the oxidation bath with oxidation salts, which is conveyed by an economic and ecological recycling, in line with the current environmental standards and technicalities.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2200854 | Jan 2022 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2022/052054 | 10/31/2022 | WO |
