METHOD FOR RECOVERING CHROMIUM CONTAINED IN A PICKLING BATH FOR METALLIC MATERIALS AND PLANT FOR IMPLEMENTING THE SAME

Abstract

A method is described for recovering Cr from a pickling bath of metallic materials (1), the bath (1) being an aqueous solution containing at least one sulfate and Cr from the pickled metal, the method comprising the steps of: - forming an aqueous two-phase system from at least a portion of the pickling bath (1) and a polymer comprising a hindered ether function, the proportion of the polymer in the ternary mixture consisting of the pickling bath being considered a single chemical component, water and the polymer, being such that it ranges between the line of the equation "% wt polymer = 100% - % wt pickling bath" and the bimodal curve of the pickling bath / polymer mixture, the aqueous two-phase system comprising a polymer phase, in which most of the Cr is found, and a non-polymer phase; - separating the polymer and non-polymer phases; - allowing precipitates containing the Cr to form in the polymer phase;- Perform solid / liquid separation of the polymer phase to separate the polymer and the precipitates containing Cr; - and process the precipitates to recover the Cr contained therein. Installation for the implementation of the method.

Description

METHOD FOR RECOVERING CHROMIUM CONTAINED IN A PICKLING BATH FOR METALLIC MATERIALS AND INSTALLATION FOR IMPLEMENTING THE SAME Field of invention The present invention relates to the pickling of metallic materials containing chromium, particularly high-chromium steels such as stainless steels. More specifically, it relates to the recovery of materials found in a dissolved state in pickling baths containing sulfates, for the purpose of regenerating these baths and thus allowing their reuse or inclusion in a continuous processing method without rapid degradation of the method's performance, thereby improving the quality of the recovered metals as much as possible. Background of the invention Chemical pickling is an essential operation in the steelmaking process. It consists of removing the rust from the surface of the part and immersing the part in an aqueous solution, most often with a high acid content (particularly a solution containing one or more acids typically selected from HCl, HNO3, H2SO4, HF), but also possibly containing salts such as sulfates like Na2SO4, K2SO4, (NH4)2SO4. Sulfate-rich baths are particularly used in electrolytic pickling processes. The oxidized layers that are pickled form during heat treatments carried out in an oxidizing atmosphere, particularly air. This pickling is frequently performed on strips being transported in motion. It allows the strips to be rolled without the risk of oxides becoming embedded in the strip's surface, thus deteriorating its appearance and quality, or it allows for surface treatment under favorable conditions. Through this entire pickling process, the baths are enriched with metal salts derived from the dissolution of the oxide layers and / or the base metal. This loading of baths with metal salts is detrimental from several perspectives. First, the presence of large quantities of metal cations in the bath alters the chemical and electrochemical equilibria, thus reducing the effectiveness of the pickling process. Second, when these dissolved metals reach their solubility limit, they precipitate and form sludge within the baths. This sludge can lead to surface quality problems if it deposits on the treated product, particularly on the rollers that guide its path. It can also accumulate in the vats, tanks, and piping of the pickling system, consequently hindering its proper operation. Faced with this change in the metal content of pickling baths and the associated loss of efficiency, the only simple means for industrial processes is to regenerate the used bath, either partially or completely, by adding fresh bath. Before regeneration, all or part of the used bath is sent to a neutralization station where it is mixed with other used baths and then treated to reduce the Cr(VI) ions it contains (by reduction with Fe(II) or sodium NaHSO3, for example), followed by precipitation of the metal cations through the addition of NaOH or Ca(OH)2. The solid elements are then separated from the liquid through flocculation / settling and filtered in a pressure refiller. The resulting metal hydroxide sludge cakes are disposed of as waste since there is no longer an economically viable upgrading treatment for this sludge. This sludge is composed of a mixture of metals dissolved in different surface treatment baths, and of the anions from at least some of these baths. For example, the high sulfur (S) and fluoride (F) content of this sludge, derived from acid baths using hydrogen sulfide (H₂SO₄) and hydrofluoride (HF), prevents the reuse of this residue via pyrometallurgy. To overcome this drawback, one solution is to separate the neutralization streams, obtaining on one side the spent baths containing F and S, and on the other side the baths with byproducts that are difficult to recycle. Treatment solutions for specific types of baths already exist and are in use. These solutions, such as spray roasting for hydrochloride baths, generally allow for acid regeneration on one hand and metal recovery on the other. These methods are often expensive and do not always allow for acid regeneration at a concentration suitable for industrial processes. Furthermore, these methods are generally energy-intensive, consume large quantities of water, and / or use ion-exchange resins or membranes that require cleaning cycles during which the facility cannot be used. Brief description of the invention The objective of the invention is to propose a method that is both sufficient and economically viable for regenerating spent electrolytic pickling baths and for recovering the Cr contained in these spent electrolytic pickling baths that contain relatively high amounts of one or more sulfur-containing compounds such as sulfates. For this purpose, the object of the invention is a method for recovering Cr from a pickling bath of metallic materials contained in a receptacle such as a pickling tank, the pickling bath being an aqueous solution containing at least one sulfate and Cr derived from the pickling metal, characterized in that: - an aqueous two-phase system is formed from at least a portion of the pickling bath and a polymer comprising a hindered ether function, the polymer being preferably a polyethylene glycol, typically PEG-200, PEG-400 or PEG-600, optimally PEG-400, the proportion of the polymer in the ternary mixture - formed by the pickling bath considered to be a single chemical component, water and the polymer - is such that it lies between the line of the equation « wt% polymer = 100% - wt% pickling bath» and the bimodal curve of the pickling bath / polymer mixture at the bath temperature, the aqueous two-phase system comprising a polymer phase in which most of the Cr is found, and a non-polymer phase; - the polymer and non-polymer phases separate; - In the polymer phase, precipitates containing Cr are allowed to form; - Solid-liquid separation is carried out in the polymer phase, to separate the precipitates containing polymer and Cr; - the precipitates are treated to recover the Cr contained in them. / ζ / ζηη / ζζηζ / Β / γι Preferably, to accelerate the formation kinetics of the Cr-containing polymer precipitates, the polymer phase is irradiated prior to solid-liquid separation by exposure to at least one light source with a wavelength between 340 and 860 nm. Before exposure to the light source, the polymer phase can be placed in the form of a liquid film. After the solid-liquid or polymer phase separation, the liquid fraction resulting from this separation can be dehydrated to recover the polymer contained therein, and the polymer is reused to form an aqueous two-phase mixture. Dehydration can be carried out by distillation. The water resulting from dehydration can be sent to the receptacle containing the pickling bath. The recovery of Cr from the precipitates in which it is contained, after its separation from the rest of the irradiated polymer phase, can be carried out through a pyrometallurgical process. The pyrometallurgical process can be a pyrometallurgical reduction of oxides using carbon. The process may include the following steps: - a fraction of the pickling bath is drawn from a receptacle containing the pickling bath; - an aqueous two-phase system is formed from at least a portion of the pickling bath and a polymer comprising a hindered ether function, the polymer being preferably a polyethylene glycol, typically a polyethylene glycol of the PEG-200, PEG-400 or PEG-600 type, optimally PEG400, the proportion of the polymer in the ternary mixture - formed by the pickling bath considered to be a single chemical component, water and the polymer - being such that it lies between the line of the equation « wt% polymer = 100% - wt% pickling bath» and the bimodal curve of the pickling bath / polymer mixture at the bath temperature, the aqueous two-phase system comprising a polymer phase in which most of the Cr is found, and a non-polymer phase; - the non-polymer phase of the aqueous two-phase mixture is returned to the receptacle containing the pickling bath. The receptacle containing the pickling bath can be continuously fed with the pickling bath. The pickling metal can be stainless steel. A further objective of the invention is an installation for recovering Cr contained in a metal pickling bath, the pickling bath being an aqueous solution containing at least one sulfate and Cr(VI) derived from the pickling metal, characterized in that it comprises: - a receptacle containing the pickling bath; - a mixing reactor connected to the pickling tank; - a receptacle containing a polymer proposed to be added to the mixing reactor to form an aqueous two-phase system with the pickling bath; - a device for separating the phases of the aqueous two-phase system into a polymer phase and a non-polymer phase; - and a solid-liquid installation acting on the polymer phase to separate the polymer from the Cr(VI)-containing precipitates present in the polymer phase. The installation may also comprise an installation for irradiating the polymer phase through radiation by at least one light source of wavelength(s) in the range of 340-860 nm, to accelerate the formation of Cr-containing precipitates. The installation may also comprise means for placing the polymer phase in film form and exposing the film to at least one light source having a wavelength in the wavelength range of 340-860. The installation may also include an installation for recovering Cr from Cr-containing precipitates. The Cr recovery facility may have resources for a pyrometallurgical process such as pyrometallurgical reduction of oxides using carbon. The installation may include a polymer dehydration installation after the separation of the polymer from the Cr-containing precipitates. The mixing reactor and the device that separates the phases of the aqueous two-phase system can be composed of the same piece of equipment. The receptacle that contains the pickling bath can be a pickling tank. The installation may comprise at least one of the following means: - means for recycling the non-polymer phase in the pickling bath; - means for returning the depleted polymer after dehydration to the receptacle containing the polymer; - and means for returning the water, which results from the dehydration of the spent polymer, back to the pickling tank. As will have been understood, for the recovery of Cr from an exhausted pickling bath, electrolytic pickling in particular, which contains one or more sulfates, the invention has means for liquid-liquid extraction through the use of polymers that lead to the formation of aqueous two-phase systems capable of extracting Cr(VI) much more preferably from the pickling bath. It appears that the polymers capable of being used in the invention have the characteristic of possessing a hindered ether function, i.e., free of infected groups in the polymer chain in the immediate vicinity of the ether function. Polyethylene glycol, in particular PEG-200, PEG-400, and PEG-600, is the polymer of choice for use in the invention. PEG-400 is the preferred example. General information on aqueous two-phase systems containing polymer can be found, for example, in the following documents: « Partitioning in Aqueous Two-Phase Systems: Fundamentals, Applications and Trends », by A. Lima Grilo, M. Raquel Aires-Barros, AM Azevedo, Separation and Purification Review 45 (2016), / ζ / ζηη / ζζηζ / Β / γι pages 68-80; « Influence of Different Phase-Forming Parameters on the Phase Diagram of Several PEG-Salt Aqueous Two-Phase Systems », by BA Glyk, T. Scheper, S. Beutel, J. Chem. Eng. Data, 59 (2014), pages 850-859. Brief description of the drawings The invention is now described with reference to the following attached FIGS. FIG. 1 outlines the sequence of operations to be carried out for the optimal implementation of the method of the invention. FIG. 2 shows the weight concentration ranges of PEG-400 in a mixture composed of PEG-400, pickling bath and water that must be taken into account for the satisfactory implementation of the invention, for a bath having a certain composition and as a function of the bath temperature. FIG. 3, weight percentages, gives the distribution of elements in the polymer-containing phase after extraction, as a function of the proportion of polymer added to the initial bath. FIG. 4 shows the kinetic change in the Cr content of the polymer-containing phase during exposure to light, according to the thickness of the liquid film being treated. Detailed description of the invention First, it is important to remember that electrolytic pickling baths containing sulfate(s), called "electrosulfate baths," are primarily used at the beginning of the pickling sequence for the final annealing of stainless steels. The sodium sulfate typically used in these baths has no effect on the surface of the steel but serves only to carry the electrolysis current. Electrolysis is used to dissolve the highly soluble Cr(VI) contained in the oxide layer on the surface of the strip, which is then released into the bath. Therefore, as pickling progresses, the bath is charged with Cr(VI). This charge induces changes in the equilibria of the chemical and electrochemical reactions taking place in the bath and leads to a loss of pickling efficiency when the bath reaches high concentrations of dissolved metals. Furthermore, Cr(VI) is a chemical species classified as CMR (carcinogenic, mutagenic, toxic to reproduction) according to the European REACH regulation. Therefore, it poses a significant danger to individuals exposed to it. Although this bath is treated and neutralized accordingly, the formation and accumulation of large quantities of Cr(VI) in the bath itself represents a risk to the safety and health of workers operating near the pickling baths. For all these reasons, it is important to be able to recover this Cr(VI) in a form that makes it harmless, so that subsequent Cr recovery can be carried out under suitable hygiene and safety conditions, and at a non-prohibitive cost. It is also worth noting that extracting elements from a solution through the formation of a two-phase system is a well-established method in the recycling sector. However, in most cases, these methods use organic solvents or ionic liquids, which are expensive and / or toxic. / ζ / ζηη / ζζηζ / Β / γι A preliminary application of this method is described in WO-A-2018 / 087364. By contacting the solution containing the inorganic salt with the ionic liquid in certain proportions, it is possible to form an aqueous two-phase system in which the species will divide between one or the other of the two phases. However, the application of this method to pickling baths for metals and alloys is not foreseen in this document. It has also been shown that it is possible to form aqueous two-phase systems with some polymers. With the method of the invention, it is possible first to extract dissolved Cr from the pickling bath containing one or more sulfates, and obtain a S-depleted sludge from it (i.e., containing less than 0.03% S, compared to approximately 8% when a conventional method of precipitation of the metal cations by CaO followed by filtration is used), which can therefore be treated by pyrometallurgical means and the risk of discharge of sulfur-containing compounds, and secondly, as is preferable, the bath thus depleted of Cr can be recycled for reuse thereof. This method is preferably a continuous method implemented, for example, in the installation schematically illustrated in FIG. 1, with continuous or intermittent extraction from a portion of the pickling bath 1 contained in pickling tank 2 alone or in any other vessel into which the regenerating pickling bath 1 may have been transferred. This allows for a longer effective lifespan of bath 1 compared to other bath treatments, since the stability of its composition over time is better ensured. The extraction method of the invention, in a first step, allows the formation in a mixing reactor 3 of a two-phase aqueous system between the fraction of the pickling bath 1 extracted from the pickling tank 2 and a polymer stored in a dedicated receptacle 4, for example, polyethylene glycol (PEG). Preferably, the pickling tank is continuously fed with the fresh pickling bath 1. In general, the inventors have ensured that the capable polymer used to implement the invention must be a polymer having a chain comprising a hindered ether functional group. By "hindered," it is understood that no group may be grafted onto the chain in the vicinity of the O atom of the ether functional group that could impede its action. Polyethylene glycol (PEG) is a preferred example of the polymer capable of being used in the invention. It is recalled that it has the general formula: / z / znn / zznz / B / Yi Tetraethylene glycol dimethyl ether of the general formula: i7.i ζηη / ζζηζ / Ε / γΐΛ It can also be used in the invention while proving capable of extracting Cr(VI). It can be compared to a low molecular weight PEG (222 g / mol) that does not have any terminal OH groups. Triethylene glycol dimethyl ether of the formula: It also belongs to the polymers that can be used. Conversely, polypropylene glycol (PPG), which has the general formula: It is not suitable for implementing the invention since its ether function is hindered by the CH3 group which is close to the ether function. Furthermore, the inventors have noted that the block copolymer "PluronicMR10R5" by BASF, of the formula: A mixture of PEG blocks and PPG blocks comprising end OH groups similar to PEG and PPG can be used for the extraction of Cr(VI) according to the invention. It is concluded that the presence of a hindered ether function does not necessarily need to refer to the entire polymer chain, but it is sufficient that at least some portions of the chain must have a hindered ether function so that the polymer, as a component of an aqueous two-phase mixture, has the properties of extracting Cr(VI) from pickling baths of stainless steels. The expression "comprises a hindered ether function" is therefore understood to also refer to polymers that can only exhibit the hindered ether function in a portion of their chain and not just the entire chain, and that the presence of coexisting hindered ether functions within the chain is not a prohibitive feature. It is also concluded from all these observations that the presence of the terminal OH groups does not play a significant role in the properties of the polymers from which the invention derives benefit. With respect to PEGs, several types currently exist that differ in average molecular weight. An increase in average molecular weight leads to an increase in the surface area of ​​the two-phase region in the mixture's phase diagram and in the mixture's viscosity at a given temperature. A very high molecular weight can reduce the polymer's efficiency in extracting Cr(VI) under the same operating conditions. This increase in molecular weight makes mixing, settling, and separation operations more difficult. PEG-200, PEG-400, and PEG-600 (the latter being a solid at room temperature similar to higher molecular weight PEGs; therefore, it is more difficult to handle than lower molecular weight PEGs) represent the preferred examples of PEGs suitable for use in the invention. PEG-400 proves to be the most efficient.PEG-200 allows good extraction of Cr(VI), but leads to respective volumes of polymer and non-polymer phases that are not optimal for implementing the method of the invention. The preferred example of PEG-400 usage is now described in detail. In general, the proportions of PEG-400 with respect to the fluid must meet the following criteria. The proportion of PEG-400 in the ternary mixture - formed by the pickling bath (considered to be an individual chemical component), water and PEG-400 - must be such that it lies between the line of the equation "% by weight of PEG-400 = 100% - % by weight of pickling bath" and the bimodal curve of the pickling bath / PEG-400 mixture at the bath temperature. This criterion relates to the proportion of polymer used, which is also valid for any polymer other than PEG-400. It is also preferable that the weight ratio of PEG-400 to the pickling bath should not be higher than 50%, so that the amount of PEG-400 used is not too high in relation to the amount of Cr to be recovered, and the method is certainly advantageous from an economic point of view. Figure 2 shows the line and bimodal curves of a PEG-400 bath at temperatures from 25 to 80°C. The impact of the bath temperature on the bimodal curve proves to be relatively minimal within the range under consideration. When the aqueous two-phase system forms, the metals contained in the bath will divide between the two phases, with Cr(VI) migrating entirely to the phase containing the larger amount (by weight) of polymer. Figure 3 shows the weight percentage distribution of the elements in the polymer-containing phase after extraction, as a function of the proportion of polymer added to the initial bath. The composition of the tested bath was as follows: Total Cr: 11.7 g / L; Fe: 0.5 g / L; Ni: 0.4 g / L; Na: 47 g / L; SO42': 106 g / L; pH = 1.8; density: 1.14. The conditions under which this diagram is drawn are as follows. The diagram was obtained for a sample of industrial pickling bath as defined above, to which different proportions of PEG-400 were added. The temperature was controlled at T = 80°C ± 3°C and the total volumes tested ranged from 10 mL to 1.5 L. In one experimental variant, the total Cr content was modified by reducing it to 5 g / L and adjusting the temperature to T = / ζ / ζηη / ζζηζ / B / γι 67°C for a total volume of 1.5 L. The results given in the diagram (expressed in % by weight) showed no significant variation by all these different experiments. The proportion of Cr that passes into the phase in which most, if not all, of the polymer (at this point, PEG-400) will be mixed with water (this mixture is called the "polymer phase") increases with the weight proportion of polymer in the treated bath. Conversely, the proportion of the other measured elements and ions (Ni, Fe, Na, SO42j) decreases with the same weight proportion of polymer in the treated bath. These other elements and ions are therefore found in the vast majority in the other phase (called the "non-polymer phase"). In a test example, an amount of PEG-400, with a density of 1.13, equal to 0.66 times the weight of the pickling bath, or in other words, 0.58 times the volume of the pickling bath, was added to the pickling bath mentioned above. This PEG-400 was drawn from receptacle 4 in FIG. 1. The mixing reactor 3 therefore contained a bath, which was left under agitation, having a weight 1.66 times the initial weight of the pickling bath and a volume 1.58 times the initial volume of the pickling bath. The mixture was then transferred to a separator device 5, such as a gravity settler, to obtain an upper polymer phase and a lower non-polymer base. It will be understood that it is entirely possible for the separator device 5 to be integrated with the mixing reactor 3. For example, using a double flow reactor. In the test example described, the polymer phase at the end of the separation operation represents 78% of the total volume and the non-polymer phase 22% of the total volume. The polymer phase contains 7.9 g / L of Cr(VI), substantially free of Fe and Ni, 6.5 g / L of Na+, 8.2 g / L of SO42', and 570 g / L of PEG-400. It has a pH of 1.8 and a density of 1.12. It represents 0.55 times the initial weight of the pickling bath and 0.65 times the initial volume of the pickling bath. The non-polymer phase contains 3.7 g / L of Cr(III), substantially all of the Fe and Ni initially contained in the pickling bath, 108 g / L of Na+, 228 g / L of SO42', and 22 g / L of PEG-400. It has a pH of 2.6 and a density of 1.29. It represents 0.45 times the initial weight of the pickling bath and 0.35 times the initial volume of the pickling bath. As can be seen in Figure 3, after a PEG-400 concentration of 40 wt% in the bath, almost all of the PEG-400 is in the polymer phase (approximately 1 wt% remains in the non-polymer phase). Conversely, virtually all of the Fe and Ni, the metal ions primarily present in the pickled steel bath, are in the non-polymer phase (the Na+ in the SO42' ions are also entirely in the non-polymer phase). Regarding Cr, approximately 85% is found in the polymer phase. However, a more detailed analysis of the results shows that the Cr found in the polymer phase is entirely Cr(VI), which was the primary target for extraction from the pickling bath. The Cr found in the non-polymer phase is Cr(III). The inventors verified during these tests that Cr(VI) is the only metal ion, among those searched for, that is very significantly present in the polymer phase. Cr(lll), which is residually present in the bath after electrolytic pickling, was not significantly extracted by PEG-400. Metal ions other than the various Fe, Cr, and Ni ions (the latter being present if conventional grades of austenitic steel are treated, in which this element is abundant) derived from pickled steel, and which are not oxonium ions as opposed to Cr(VI)-containing ions, can also be found in the pickling bath in relatively small quantities depending on the specific steel grade being treated. As shown in Figure 3, these ions are more closely bound to the non-polymer phase and are therefore not significantly extracted from the bath. This is not detrimental to the spirit of the invention. The objective is to recover the Cr(VI) contained in the electrolytic pickling bath, typically through pyrometallurgical means, so that it can be recycled in the form of Cr that can be optimally returned to the steelmaking furnace that produced the pickled steel. In this way, the production of Cr(VI)-contaminated waste is avoided, and additionally, the Cr derived from the pickling process can be returned to the steel production circuit, thus providing substantial savings in raw materials.If this recovered Cr is not very pure, in that it may contain other metals initially present in the steels treated by the pickling facility, this is not necessarily a major disadvantage since these metals are also intended to be significantly included in the composition of the steel that will be produced with this recovered Cr, or are tolerable up to levels that most are generally not at risk of being exceeded through the use of the recovered Cr of the invention. One possible exception could be Mo, a portion of which, as will be observed, can be found in the polymer phase when the pickling bath contains treated stainless steels that contain Mo. However, in practice, and more frequently, the addition of Cr recovered using the method of the invention is rarely sufficient to cause the Mo content to exceed a tolerable value in a stainless steel that is not intended to contain a significant amount of Mo. The recovered Cr must have a very high Mo content for use in large quantities in the preparation of non-alloyed Mo steel, which can be expected to be used as a raw material in a smelting furnace in addition to other low-Mo raw materials that will dilute the Mo contributed by the recovered Cr. Significant contamination of the polymer phase by sulfur could be detrimental, as it would compromise the possibility of recovering chromium through pyrometallurgical processes and lead to the release of very high levels of sulfur-containing compounds. However, Figure 3 shows that the SO42' ions in the described example are more than 90% in the non-polymer phase, and their residual presence in the polymer phase is therefore still acceptable. If the recovered Cr is intended for a purpose other than recycling for steelmaking, its purity may need to be higher than that just described. In this case, it must be verified that the concentration of compounds other than Cr(VI) in the polymer phase is sufficiently low so that the proposed use of the Cr is as efficient as possible, taking into account the subsequent treatment processes. This may possibly lead to: Use of Cr obtained by the method of the invention only when the pickling baths have steels treated with low contents of alloying elements other than Cr and Ni, the ions of which could significantly pass into the polymer phase; baths having steels repeatedly treated with high Mo content could therefore not be suitable for some uses, as can be easily appreciated on a case-by-case basis by persons skilled in the art; Or the application to the polymer phase and / or the products derived from them of additional metal separation processes that would improve the purity of the Cr obtained after recovery. As shown in FIG. 3, increasing the proportion of PEG-400 in the bath to 50% causes little or no change in the polymer, Cr(VI), and Fe distributions between the two phases, but the migrations of Na+, SO42', and N2+ in the non-polymer phase increase slightly. Almost all of the polymer is found in only one of the phases, particularly the polymer phase, which also contains almost 85% of the initial Cr, which, as mentioned, corresponds to all the Cr(VI) contained in the pickling bath. The remaining 15% of the initial Cr(III) is found entirely in the non-polymer phase. Approximately 1% of the polymer may exist in the non-polymer phase. The explanation for this distribution of the various metal ions between the polymer and non-polymer phases is evidently that PEG-400, like the other polymers suitable for use in the invention described above, only extracts oxonium ions from these metals, i.e., those containing oxygen. With respect to Cr, the HCrO4' and CrO42- ions present in the pickling bath are extracted in the polymer phase, and these are precisely the ions in which Cr has a valence of VI. On the other hand, the Cr3+ ions remain in the non-polymer phase, along with the Fe2+ and Ni2+ ions in particular. The test also shows that when Cu2+, Mo2+, and Mn2+ ions are present in the pickling bath, they are also found, at least in large part, in the non-polymer phase. However, MoO4' ions, if present, are found in the polymer phase since they are oxonium ions. Mo therefore behaves like Cr in the invention. It can be seen that this is not necessarily a major disadvantage and can be remedied if necessary. After separating the two phases, polymer and non-polymer, in a suitable device 5 by filtration, settling, or any other conventional method adapted to the respective properties of the phases, the phase not containing the polymer—that is, the spent bath from which all the Cr(VI) has been removed—can be reused as a pickling bath without significant polymer contamination. It is returned to the pickling bath 1, which remains in tank 2, if the method is continuous. This return of the non-polymer phase to the pickling bath can be carried out as long as it does not significantly impair the effectiveness of the pickling bath. Through all the experimentation, the user can set the tolerable content threshold of / ζ / ζηη / ζζηζ / Β / γι Cr(VI) in the pickling bath, after which the bath's effectiveness can no longer be considered sufficient. In this case, tank 2 must be emptied for complete renewal of the pickling bath, or pickling bath 1 must be diluted with fresh components to reduce the Cr(VI) content to a suitable level. In all cases, the near-total removal of Cr(VI) by the method of the invention allows for a significantly longer service life for pickling bath 1 before tank 2 needs to be emptied and bath 1 significantly diluted earlier than necessary. As already mentioned, the continuous feeding of tank 2 with fresh pickling bath allows for a delay when this emptying becomes required. Experimentation has also shown that the return of Fe and Ni through the recycled non-polymer phase does not significantly impair the effectiveness of the pickling bath. Therefore, the Cr(VI) content of the pickling bath should be monitored periodically to determine when the recycling of the non-polymer phase should be discontinued. Without any special precautions, adequate effectiveness is restored to the pickling bath. In a second step of the process, the polymer phase is treated to extract one or more Cr-containing compounds present within it, or optionally to isolate the polymer for reuse in the first extraction step. This treatment involves allowing the Cr-containing precipitates to form naturally without the addition of reagents. This can be achieved simply by letting the polymer phase stand, optionally while heating it to accelerate precipitation. However, preferably, this precipitation is accelerated by irradiating the polymer phase. By exposing this Cr-rich polymer phase, preferably in the form of a liquid film approximately a few centimeters thick, to irradiation in a suitable facility 6 by at least one light source having radiation wavelengths typically emitted within the 340-860 nm range (i.e., in the UV or visible spectrum), the Cr typically precipitates as Cr(OH)3. Subsequently, it is possible, by simple filtration (or any other suitable solid-liquid separation method) in a dedicated facility 7, to recover a Cr-rich solid 8, the liquid phase of which contains the polymer. Applying the Cr-rich phase as a liquid film for irradiation is a preferred method, as the solution is very dark and the photons emitted by the light source(s) can only penetrate a limited-thickness bath layer. Another possible solution is to place the solution in a vessel equipped with a stirring device so that the solution exposed to radiation is constantly renewed. The combination of both operating methods can also be achieved by irradiating the bath under stirring before applying it as a film. Care must also be taken to prevent the Cr-containing precipitate from depositing on the light source, for example, by placing the source at a sufficient distance from the bath and / or by frequently cleaning the source, particularly if it is integrated into the vessel walls. One solution is to provide a large container fitted with multiple regularly spaced light sources, each of which is cleaned in turn while the other sources continue to operate. The optimization of the wavelength and lighting power required for the optimal implementation of the method in a given installation can be obtained through routine experiments, particularly based on the composition and opacity of the pickling baths that the user must treat, the thickness of the liquid that the radiation must penetrate, etc. However, experiments have shown that it is possible to obtain precipitates of substantially the same type without irradiation in the UV-visible spectrum. Therefore, the recovery reaction of Cr(VI) from the polymer phase can also take place in installation 6 without the irradiation described. However, in this case, the reaction kinetics are less favorable. Heating the polymer phase can improve the kinetics. Heating can also be applied simultaneously with irradiation of the polymer phase. Another seemingly simpler way to form the Cr-containing precipitate is to add sodium hydroxide or lime, Ca(OH)₂, to the polymer solution. It is also possible, before adding the sodium hydroxide, to add NaHSO₃ to reduce the Cr(VI) to Cr(III), causing the sodium hydroxide to precipitate Cr(III). However, this would entail the use of additional chemicals that would degrade the ecological and economic balance of the operation, contaminate the polymer, and make its reuse impossible, at least without further treatment. Furthermore, if sodium hydroxide or lime is added alone, the yield would be very low because there is not enough Cr(III) in the polymer phase. With a prior addition of NaHSO3, the yield is higher, but the precipitate becomes enriched with sulfate, and therefore with sulfur. This significant sulfur content has been shown to be incompatible with the preferred method of Cr recovery via pyrometallurgy. Moreover, the addition of these chemicals causes a pH shift, making it alkaline, which must be corrected if the reaction products are recycled back into the bath.In general, this method is not advisable, since the reaction for the formation of the precipitate containing Cr does not require the addition of chemicals to take place, and whenever necessary the reaction can be accelerated by heating and / or irradiation in the UV and / or visible range. Therefore, according to the invention, the Cr-containing precipitate is allowed to form; that is, its formation occurs naturally without the addition of chemicals. If desired, the kinetics of this formation can be accelerated by the addition of heat and / or irradiation in the UV or visible range. The photochemical mechanisms for obtaining Cr-containing precipitates through irradiation are described, for example, in the article “A novel ecological approach to the treatment of industrial wastewater containing Cr(VI): Photochemical reduction” by Jie Liu, Kun Huang, Keng Xie, Ying Yang, and Huizhou Liu, Water Research, 93 (2016), pp. 187–194. However, the photochemical treatment of extracts from stainless steel pickling baths is not considered. Furthermore, contrary to the opinion of the authors of this article, the inventors' experience has shown that precipitation of Cr-containing compounds is possible without irradiation, at least in the pickling baths of sulfate-containing stainless steels, provided sufficient time is available and / or the medium is heated to accelerate the reaction. Therefore, the kinetics of this reduction in Cr concentration in the liquid depends on the intensity of the light received by the medium, as shown in Figure 4, which illustrates the experiments performed on the Cr-containing PEG-400 polymer phase obtained after the test illustrated in Figure 1. Before being added to the bath, the PEG was preheated to the same temperature as the bath so that this addition would not cool the bath and thus obtain optimal kinetics for the reduction of Cr concentration in the liquid film. In Figure 4, it can be seen that the reduction of Cr concentration in the liquid film is faster the thinner the treated film is (5 or 2 cm), at the same emission intensity of the light source. The light source in the described tests is a continuous spectrum lamp. Its illuminance measured in the visible spectrum at a distance of d = 250 mm is 9000 Lux; at ad = 200 mm it is 12000 Lux. It has a power of 11.6 W / m², of which 25% is in the ultraviolet (< 392 nm). In other words, a given weight of polymer phase applied as a 2 cm thick film receives more light energy than the same weight of polymer phase applied as a 5 cm thick film exposed to the same light source at the same intensity. This naturally affects the treatment's effectiveness. If, in both cases, the initial Cr concentration in the polymer phase is 8.5 g / L, a Cr concentration of 1.5 g / L is reached in the liquid film after 4 hours for a 2 cm thick film and after approximately 6.5 hours for a 5 cm thick film. The Cr concentration in the liquid film becomes practically zero after approximately 5.5 hours for a 2 cm thick film and after 8 hours for a 5 cm thick film. Once most of the Cr has precipitated and separated from the liquid in which it was contained in the step just described, the polymer contained in the now-released polymer phase, depleted of most of the Cr it has captured, must be dehydrated if this polymer is to be reused in the first step of Cr extraction from the pickling bath, as is preferred. Since the boiling points of the polymer and water are very different, the dehydrated PEG-400 can be recovered by means of a distillation step, for example, carried out in a dedicated unit 9. This dehydrated PEG-400 is sent to the vessel 4 containing the polymer used in the method of the invention. The water extracted in the distillation unit 9 and then recondensed can preferably be reused at the beginning of the method by returning it to the pickling tank 2. As for the solid phase containing the Cr, it is treated, after optional drying where necessary, with a process that allows the recovery of the Cr and even any other metals it may contain, in a dedicated facility, for example, by applying any known pyrometallurgical process inherently compatible with the composition and physical properties of the solid phase. In particular, a pyrometallurgical reduction process of oxides using carbon can be applied. / ζ / ζηη / ζζηζ / Β / γι In general, the various installations mentioned are preferably, or even necessarily, located at the same site as the pickling installation, particularly when the method used is a continuous process with recycling or reuse of the regenerated polymer and some byproducts within the pickling installation, and when the pickling bath 1 remains in pickling tank 2 without being transferred to another container. However, it is entirely possible for the Cr 10 recovery installation to be located at a different site, to which the Cr-containing solid phase 8 can be transported, since the products derived from this recovery are not intended for reinjection into the installation of the invention. It will be noted that the operations of recycling the non-polymeric phase in pickling bath 1, reusing the polymer after dewatering, and returning the resulting dewatering water to pickling tank 2 can be performed or omitted independently, as desired by the user. The continuous implementation version of the described and illustrated method is the version in which a maximum amount of recovered materials, which do not contain large quantities of Cr and even other recoverable metals that can be captured in the polymer phase, is recycled back into the installation by means adapted for this purpose. In this way, the addition of external materials is minimized, provided that the composition of pickling bath 1 remains compatible with the proper execution of the pickling process.However, it is possible to choose not to resort to some of these recycling and reuse operations, particularly if the method of the invention as a vahante is implemented as a batch method. An aspect of the invention may also consist of treating the non-polymeric phase before returning it to the pickling bath 1 to remove at least some of the chemical species it contains that would cause an excessively strong, rapid, and / or undesirable change in the composition of the pickling bath 1 if the non-polymeric fraction were returned untreated. As mentioned above, the Fe and Ni contained in the non-polymeric phase are generally not detrimental if returned to the pickling bath 1. However, it cannot be entirely ruled out that other metal ions, by accumulating in the pickling bath 1, may have more detrimental effects if they are present in significant quantities in the non-polymeric phase. This could refer specifically to the residual presence of spent polymer in the recycled non-polymeric phase, which would most significantly impair the effectiveness of the invention's method. The optional treatment of the non-polymeric phase before returning it to the pickling bath should therefore have as its primary objective: - Or extract and regenerate the residual polymer to restore the same characteristics as the polymer contained in container 4, i.e., fresh or dehydrated polymer obtained from the polymer phase in distillation installation 9; - Or to degrade the residual polymer, in particular through an advanced oxidation process. By way of example, the mass balance of an implementation of the method of the invention can be described as follows, wherein the PEG-400 used is derived entirely from the recycling of spent PEG-400 carried out in the dehydration facility 9. / ζ / ζηη / ζζηζ / Β / γι The starting product is a pickling bath 1 for stainless steels, which essentially contains Na2SO4 in aqueous solution and has the following characteristics Total Cr. 11.7 g / L; Fe: 0.5 g / L; Ni: 0.4 g / L; Na: 47g / L; SO42-: 106 g / L; pH = 1.8; density: 1.14. From pickling tank 2 containing the bath, 6 kg of this bath 1 are removed and introduced into the mixing reactor 3, to which 4.0 kg of recycled PEG-400 from the last stage of the method, containing residual 3 g of Cr, are also added. The assembly is subjected to agitation. After stirring, the components of the mixture are transferred to the separator device 5 to separate into a polymer and a non-polymer phase, as described previously. Alternatively, the separation can take place in the mixing reactor 3 if it is capable of performing both functions. This results in: - 2.4 kg of non-polymeric phase containing 4 g of Cr in the form of Cr(III) that is returned to pickling tank 2; - 7.6 kg of liquid polymer phase containing 57 g of Cr in the form of Cr(VI). The polymer phase is sent to the irradiation facility 6, and the precipitates and liquid phase resulting from this irradiation (understanding, as stated above, that this irradiation is only used to accelerate the formation of precipitates which, however, would occur without the need to add any chemicals) are separated in the solid-liquid separator facility 7. The resulting solid phase 8 weighs 0.5 kg and contains 54 g of Cr in the form of Cr(VI). After drying the solid phase (whose weight is reduced to 0.2 kg) and treating the solid phase in the pyrometallurgical plant 10, 54 g of Cr are recovered. The liquid phase comprising the polymer represents 7.1 kg and contains 3 g of Cr in the form of Cr(VI). It is dehydrated in distillation unit 9. The distillate, representing 3.0 kg and containing no Cr, is returned to pickling bath 1. The 4 kg of PEG-400 recovered by distillation, containing 3 g of Cr(VI), are returned to mixing reactor 3 to begin a new cycle of Cr recovery from pickling bath 1. There is no loss of the 61 g of Cr extracted from the pickling bath 1 during the treatment, since both the 4 g of Cr(III) derived from the separator installation 5 and the 3 g of Cr(VI) in the recycled PEG-400 after the dehydration stage 9 of the PEG-400 by distillation are returned to the pickling bath treatment circuit, and 54 g of Cr are recovered after the pyrometallurgical treatment 10. A continuous implementation of the method of the invention has been described and illustrated in detail, in which the polymer and the spent Cr liquid phases are recycled back into the pickling bath. Optionally, the continuous nature of the method can be enhanced by constantly feeding the pickling tank 2 with fresh pickling bath 1. It would, of course, be within the spirit of the invention to implement the method in batch mode. Under these conditions, as soon as it is determined that the composition of the pickling bath 1 is about to become unusable due to excessively high concentrations of Cr, polymers, and / or other elements or substances that govern the chemical and electrochemical equilibria, the entire pickling bath is treated in a single operation using the method of the invention.Subsequently, the liquids and the Cr-depleted polymer phase are recycled to form all or part of a new pickling bath 1, and the Cr-rich solid phase is sent to the metal recovery facility 10 to recover the metals contained in that phase, Cr in particular, thus enabling the reuse of these metals and preventing the release into the environment of waste containing these metals and therefore potentially harmful. Obviously, the list of equipment used to apply the method of the invention is not exhaustive. Other accessories may be added, as is customary, to perform common functions for processing materials comparable to those of the invention, or to optimally perform the functions of each of the aforementioned pieces of equipment. Furthermore, although the invention is preferably applied to the treatment of pickling baths for steels containing significant amounts of Cr, such as stainless steels, the application of the invention to the treatment of pickling baths for other Cr-containing alloys, for which the aforementioned problems would also arise, can be fully considered.

Claims

1. A method for recovering Cr contained in a pickling bath for metallic materials (1) contained in a vessel such as a pickling tank (2), the pickling bath (1) being an aqueous solution containing at least one sulfate and Cr derived from the pickled metal, characterized in that: - a two-phase aqueous system is formed by at least a portion of the pickling bath (1) and by a polymer comprising a hindered ether function, the polymer preferably being a polyethylene glycol, typically PEG-200, PEG-400 or PEG-600, optimally PEG-400, the proportion of the polymer in the ternary mixture - formed by the pickling bath considered to be a single chemical component, water and the polymer - is such that it lies between the line of the equation "% by weight of polymer = 100% - % by weight of pickling bath" and the bimodal curve of the pickling bath / polymer mixture at temperature from the bathroom,The two-phase aqueous system comprises a polymer phase, in which most of the Cr is located, and a non-polymer phase; - the polymer and non-polymer phases are separated; - Cr-containing precipitates are allowed to form in the polymer phase; - solid-liquid separation is carried out in the polymer phase to separate the polymer and the Cr-containing precipitates; - the precipitates are treated to recover the Cr contained therein.

2. The method according to claim 1, characterized in that, to accelerate the formation kinetics of the Cr-containing precipitates and the polymer, the polymer phase is irradiated prior to the solid-liquid separation by exposing it to at least one light source with a wavelength between 340 and 860 nm.

3. The method according to claim 2, characterized in that, before exposure to said light source, the polymer phase is placed in the form of a liquid film.

4. The method according to any of claims 1 to 3, characterized in that, after solid-liquid separation in the polymer phase, the liquid fraction resulting from this separation is dehydrated to recover the polymer it contains and the polymer is reused to form the aqueous two-phase mixture.

5. The method according to claim 4, characterized in that the dehydration is carried out by distillation.

6. The method according to claim 4 or 5, characterized in that the water resulting from dehydration is sent to the tank (2) containing the pickling bath (1).

7. The method according to any of claims 1 to 6, characterized in that the recovery of Cr from the precipitates in which it is contained, after its separation from the rest of the irradiated polymer phase, is carried out by a pyrometallurgical process.

8. The method according to claim 7, characterized in that the pyrometallurgical process is a pyrometallurgical reduction process of oxides using carbon. / ζ / ζηη / ζζηζ / Β / γι 9. The method according to any of claims 1 to 8, characterized in that: - a fraction of pickling bath (1) is extracted from a tank (2) containing the pickling bath (1);- an aqueous two-phase system is formed from at least a portion of the pickling bath (1) and a polymer comprising a hindered ether function, the polymer preferably being a polyethylene glycol, typically a polyethylene glycol of type PEG-200, PEG-400 or PEG-600, optimally PEG-400, the proportion of the polymer in the ternary mixture - formed by the pickling bath considered to be a single chemical component, water and the polymer - being such that it lies between the line of the equation "% wt polymer = 100% - % wt pickling bath" and the bimodal curve of the pickling bath / polymer mixture at the bath temperature, the aqueous two-phase system comprising a polymer phase in which most of the Cr is found, and a non-polymer phase; - and the non-polymer phase of the aqueous two-phase mixture is sent to tank (2) containing the pickling bath.; 10. The method according to claim 9, characterized in that the tank (2) containing the pickling bath (1) is continuously fed with pickling bath (1).

11. The method in accordance with any of claims 1 to 10, characterized in that the pickled metal is a stainless steel.

12. An installation for recovering Cr contained in a metal pickling bath (1), the pickling bath (1) being an aqueous solution containing at least one sulfate and Cr(VI) derived from the pickled metal, characterized in that it comprises: - a vessel (2) containing the pickling bath (1); - a mixing reactor (3) connected to the pickling tank (2); - a vessel (4) containing a polymer intended to be placed in the mixing reactor (3) to form a two-phase aqueous system with the pickling bath (1); - a device (5) for separating the phases of the two-phase aqueous system into a polymer phase and a non-polymer phase; - and a solid-liquid installation (7) acting on the polymer phase to separate, from the polymer, the precipitates containing Cr(VI) present in the polymer phase.

13. The installation according to claim 12, characterized in that it also comprises an installation (6) for irradiating the polymer phase with radiation from at least one light source with a wavelength between 340 and 860 nm, to accelerate the formation therein of precipitates containing Cr.

14. The installation according to claim 13, characterized in that it comprises means for placing the polymer phase in film form and for exposing the film to at least one light source in the wavelength range of 340-860 nm.

15. The installation according to any of claims 12 to 14, characterized in that it also comprises an installation (10) for recovering Cr from precipitates containing / z / znn / zznz / B / Yi Cr.

16. The installation according to claim 15, characterized in that the installation (10) for the recovery of Cr uses a pyrometallurgical process such as the pyrometallurgical reduction of oxides using carbon.

17. The installation according to any of claims 12 to 16, characterized in that it comprises an installation (9) for dehydrating the polymer after separating it from the Cr-containing precipitates.

18. The installation according to any of claims 12 to 17, characterized in that the mixing reactor (3) and the separator device (5) of the phases of the aqueous two-phase system are composed of the same equipment.

19. The installation according to any of claims 12 to 18, characterized in that the container (2) containing the pickling bath (1) is a pickling tank.

20. The installation according to any of claims 12 to 19, characterized in that it comprises at least one of the following means: - means for recycling the non-polymer phase in the pickling bath (1) - means for returning the spent polymer, after dehydration, to the container (4) containing the polymer - and means for returning the water, resulting from the dehydration of the polymer, to the pickling tank (2).