Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/36—Regeneration of waste pickling liquors
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/30—Obtaining chromium, molybdenum or tungsten
  • C22B34/32—Obtaining chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
  • C22B3/20—Treatment or purification of solutions, e.g. obtained by leaching
  • C22B3/26—Treatment or purification of solutions, e.g. obtained by leaching by liquid-liquid extraction using organic compounds
  • C22B3/41—Treatment or purification of solutions, e.g. obtained by leaching by liquid-liquid extraction using organic compounds using a solution of normally solid organic compounds, e.g. dissolved polymers, sugars, or the like
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
  • C22B3/20—Treatment or purification of solutions, e.g. obtained by leaching
  • C22B3/44—Treatment or purification of solutions, e.g. obtained by leaching by chemical processes
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B5/00—General methods of reducing to metals
  • C22B5/02—Dry methods smelting of sulfides or formation of mattes
  • C22B5/10—Dry methods smelting of sulfides or formation of mattes by solid carbonaceous reducing agents
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F1/00—Etching metallic material by chemical means
  • C23F1/46—Regeneration of etching compositions
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the present invention relates to the pickling of metallic materials containing Cr, in particular steels rich in Cr such as stainless steels. More precisely, it relates to the recovery of the metals which are found in the dissolved state in the pickling baths containing sulphates, with a view to regenerating these baths thus allowing their reuse or their inclusion in a continuous treatment process without rapid degradation of the salts. performance of said process, and to enhance the recovered metals as much as possible.
• Chemical pickling is an essential operation in the manufacturing process of steels. It consists in removing the oxidized layer on the surface of the part by immersing it in an aqueous solution, most often very acidic (in particular a solution containing one or more acids chosen, typically, from HCl, HNO 3 , H 2 SO 4 , HF), but may also contain neutral salts, for example sulphates, such as Na2SC> 4, K2SO4, (NH4) 2SO4-
• Baths rich in sulphates are particularly used in electrolytic pickling processes.
• the oxidized layers to be stripped are often formed during heat treatments carried out in oxidizing atmospheres (air, in particular). This stripping is often carried out, in particular, on moving belts. It allows the strip, in particular, to be able to be laminated without the risk of oxides becoming embedded in the surface of the strip and thus deteriorating its appearance and quality, or undergoing a surface treatment under good conditions.
• the baths are enriched in metal salts resulting from the dissolution of the oxide layers and / or the base metal.
• This loading of the baths with metallic salts is harmful from two points of view.
• the first is that the presence of too much metal cations in the bath displaces the chemical and electrochemical equilibria which exist in the new baths, and thus reduces the efficiency of the stripping.
• the second is that when these dissolved metals reach their limit solubility concentration, they precipitate and form sludge inside the baths. This sludge can then induce surface quality problems if it settles on the treated product or on the rollers which guide the trajectory of the product. They can also settle in bins, tanks and piping of the pickling installation, and thus interfere with the proper functioning of this installation.
• the spent bath Before regeneration, the spent bath is, in part or in whole, sent to a neutralization station, where it is mixed with the other spent baths, then it is treated to reduce the Cr (VI) ions present (by reduction with using Fe (ll) or sodium bisulfite (NaFISC> 3 for example), then to precipitate the metal cations by adding NaOFI or Ca (OFI) 2 . Then, the solid elements are separated from the liquid by flocculation / decantation, then filtration through a filter press. The metal hydroxide sludge cakes thus obtained are then sent to landfill, as there is no upgrading treatment for this sludge which would be economically viable.
• this sludge consists of the mixture of metals dissolved in the various surface treatment baths, as well as anions from at least part of these baths.
• the high S and F sludge contents coming from acid baths when H 2 SO 4 and HF were used there, prevent the revaluation of this waste by pyrometallurgical process.
• one of the solutions consists in separating the neutralization streams in order to have on one side the spent baths containing F and S, and on the other the baths whose by-products would be less difficult to recycle.
• Treatment solutions targeting a particular type of bath already exist and are being used. These solutions, such as roasting by spraying or "roast spray" for hydrochloric baths, generally regenerate the acid on one side and recover the metals on the other. These processes are often expensive and do not always make it possible to regenerate an acid with a concentration which can be used for an industrial process. In addition, these processes are generally energy-intensive and / or consume large quantities of water and / or use resins or ion exchange membranes requiring cleaning cycles, during which the installation is not usable.
• the aim of the invention is to provide a process, which is both efficient and economically viable, for regenerating spent electrolytic pickling baths, and for recovering the Cr present in these spent electrolytic pickling baths, containing relatively high amounts of one or more sulfur compounds such as sulphates.
• the subject of the invention is a process for recovering the Cr contained in a pickling bath for metallic materials present in a container such as a pickling tank, said pickling bath being an aqueous solution containing at least one sulfate and Cr from the pickled metal, characterized in that:
• a solid / liquid separation is carried out on said polymeric phase, so as to separate the polymer and the precipitates containing Cr;
• said polymeric phase is irradiated before said solid / liquid separation, by exposing it to at least one light source with a wavelength of between 340 and 860 nm.
• the polymeric phase Prior to its exposure to said light source, the polymeric phase can be put in the form of a liquid film.
• Said dehydration can be carried out by distillation.
• the water resulting from said dehydration can be sent to the container which contains the pickling bath.
• the recovery of Cr from the precipitates containing it, after their separation from the remainder of the irradiated polymeric phase, can be carried out by a pyrometallurgical process.
• Said pyrometallurgical process may be a process for the pyrometallurgical reduction of oxides by carbon.
• Said method may include the following steps:
• the pickling bath can be fed continuously to the receptacle containing the pickling bath.
• Said pickled metal can be stainless steel.
• the subject of the invention is also an installation for recovering the Cr contained in a metal pickling bath, said pickling bath being an aqueous solution containing at least one sulfate and Cr (VI) originating from the pickled metal, characterized in that 'she understands :
• the installation may also include an installation for irradiating said polymeric phase by the radiation of at least one light source, the wavelength (s) of which is (are) in the range of wavelengths 340-860 nm. , so as to accelerate the formation of precipitates containing Cr.
• the installation may also include means for putting said polymeric phase in the form of a film and for exposing said film to at least one light source whose wavelength is in the range of wavelengths 340-860 nm.
• the installation may also include an installation for recovering Cr from said precipitates containing Cr.
• Said Cr recovery plant can make use of a pyrometallurgical process such as a process for the pyrometallurgical reduction of oxides by carbon.
• the installation may include an installation for dehydration of said polymer after its separation from said precipitates containing Cr.
• Said mixing reactor and said device for separating the phases of said aqueous two-phase system can be constituted by the same apparatus.
• the receptacle containing the pickling bath can be a pickling tank.
• the installation may include at least one of the following means:
• the invention for the recovery of Cr from a spent pickling bath, in particular electrolytic pickling, containing one or more sulphate (s), makes use of a liquid-liquid extraction via the use of polymers inducing the formation of two-phase aqueous systems capable of extracting Cr (VI) from the pickling bath in a very privileged manner.
• the polymers which can be used in the context of the invention have the characteristic of having an unhindered ether function, that is to say devoid of groups grafted onto the polymer chain in the immediate vicinity of the ether function.
• Polyethylene glycol in particular PEG-200, PEG-400 and PEG-600, is the preferred polymer which can be used in the context of the invention.
• PEG-400 is the preferred example.
• FIG. 1 which shows schematically the succession of operations to be carried out for optimal implementation of the method according to the invention
• FIG. 2 which shows the ranges of mass concentrations of PEG-400 in a mixture consisting of PEG-400, the pickling bath and water it is necessary to be situated for a satisfactory implementation of the invention, for a bath having a certain composition and depending on its temperature;
• Figure 3 which shows, in percentages by weight, the distribution of the elements in the phase containing the polymer after extraction, as a function of the proportion of polymer added in the original bath;
• Figure 4 which shows the kinetic evolution of the Cr content of the phase containing the polymer during light exposure, depending on the thickness of the liquid film to be treated;
• electrolytic pickling baths based on sulphate are mainly used at the start of the pickling sequence for the final annealing of stainless steels.
• Sodium sulfate which is typically used in these baths, is not active on the surface of stainless steels, but serves only to carry the electrolysis current.
• the latter is used to transform the Cr (III), present in the oxide layer at the surface of the strip, into very soluble Cr (VI) which will therefore be released in the bath.
• the bath becomes loaded with Cr (VI). This loading will induce changes in the equilibrium of the chemical and electrochemical reactions carried out in this bath to lead to a loss of pickling efficiency when the bath reaches high concentrations of dissolved metals.
• Cr (VI) is a chemical species classified as CMR (carcinogenic, mutagenic and reprotoxic) according to the European REACFI regulation. It therefore presents a strong danger for the people who are exposed to it. Although this bath is treated and neutralized accordingly, the formation and accumulation of large amounts of Cr (VI) in the bath constitutes a health and safety risk for operators working around the pickling baths.
• the process according to the invention makes it possible, on the one hand, to extract the Cr dissolved in the pickling bath containing one or more sulphates, and to make a sludge poor in S (that is to say containing less than 0 , 03% of S, against about 8% when, all other things being equal, a conventional method of precipitation of metal cations by CaO then filtration) is used, which can therefore be treated, without risk of releases of sulfur compounds, by pyrometallurgical means, and on the other hand, as is preferable, to recycle the bath thus depleted in Cr and to reuse it.
• This process can preferably be used continuously, for example on the installation shown schematically in FIG. 1, by taking, continuously or intermittently, a fraction of the pickling bath 1 present in the pickling tank 2 itself. same or in any other container in which the pickling bath to be regenerated 1 would have been transferred. This then allows an extension of the effective life of the bath 1 compared to the case where it would be treated batchwise, since the stability of its composition over time is better ensured.
• the extraction process according to the invention consists, in a first step, in the formation, in a mixing reactor 3, of an aqueous two-phase system between the fraction of the pickling bath 1 taken from the pickling tank 2 and a polymer stored in a dedicated container 4, such as polyethylene glycol (PEG) for example.
• a dedicated container 4 such as polyethylene glycol (PEG) for example.
• PEG polyethylene glycol
• the pickling tank is supplied with a new pickling bath 1 continuously.
• the polymer which can be used for carrying out the invention must be a polymer whose chain comprises an unhindered ether function.
• unhindered it is understood that no group should be grafted onto the chain in the vicinity of the O atom of the ether function, which could hinder its action.
• Polyethylene glycol PEG is a preferred example of such a polymer which can be used in the context of the invention.
• its general formula is:
• Tetraethylene glycol dimethyl ether of the general formula: can also be used in the invention, as it proves to be capable of extracting Cr (VI). It can be assimilated to a low molecular weight PEG (222 g / mol) and not having terminal OH groups.
• Triethylene glycol dimethyl ether of formula: is also one of the polymers which can be used.
• PPG polypropylene glycol
• block copolymer "Pluronic ® 10R5" from BASF, formula: which mixes blocks of PEG and blocks of PPG and comprises terminal OHs such as PEG and PPG, can be used for the extraction of Cr (VI) according to the invention.
• PEG there are commonly different types, which differ in their average molecular mass.
• the increase in the average molecular weight induces an increase in the area of the two-phase zone in the phase diagram of the mixture and in the viscosity of the mixture, for a given temperature, and too high a molecular weight can decrease the efficiency of the polymer. for the extraction of Cr (VI), under otherwise identical operating conditions.
• This increase in molecular weight makes mixing, settling and separation more difficult.
• PEG-200, PEG-400 and PEG-600 (the latter is solid at room temperature, like PEGs of higher molecular masses; it is therefore more difficult to handle than PEGs of lower molecular mass) represent the preferred examples of PEGs that can be used. within the scope of the invention.
• PEG-400 has proven to be the most effective.
• PEG-200 allows good extraction of Cr (VI), but leads to respective volumes of polymeric and non-polymeric phases which are not optimal for carrying out the process according to the invention.
• This criterion relating to the proportion of polymer used is also valid for any polymer other than PEG-400.
• the proportion by weight of PEG-400 relative to the pickling bath is not more than 50%, so that the amount of PEG-400 to be used is not too high relative to the amount of Cr which. will be recovered, and that the process certainly remains interesting from an economic point of view.
• Figure 2 shows said straight line and said binodal curves of a PEG-400 bath for temperatures of 25 to 80 ° C. it turns out that the influence of the bath temperature on the binodal curve is relatively minimal in the range considered.
• FIG. 3 shows, in percentages by weight, the distribution of the elements in the phase containing the polymer after extraction, depending on the proportion of polymer added to the original bath.
• the diagram was obtained for an industrial pickling bath sample as defined above, to which different proportions of PEG-400 were added.
• the results presented in the diagram (expressed in% by mass) did not vary significantly during these various experiments.
• the mixture is transferred to a separation device 5 such as a gravity settling tank, to obtain an upper polymeric phase and a lower non-polymeric phase.
• a separation device 5 such as a gravity settling tank
• the separation device 5 can quite well be integrated into the mixing reactor 3, for example by using a double-flow reactor.
• the polymer phase represents, at the end of the separation operation, 78% of the total volume and the non-polymeric phase 22% of the total volume.
• the polymeric phase contains 7.9 g / l of Cr (VI), substantially no Fe and no Ni, 6.5 g / l of Na + , 8.2 g / l of SO4 2 , 570 g / l of PEG-400. Its pH is 1.8 and its specific gravity is 1.12. It represents 0.55 times the initial mass of pickling bath and 0.65 times the initial volume of pickling bath.
• the non-polymeric phase contains 3.7 g / l of Cr (III), substantially all of the Fe and Ni initially present in the pickling bath, 108 g / l of Na + , 228 g / l of SO4 2 , 22 g / l of PEG-400. Its pH is 2.6 and its specific gravity is 1.29. It represents 0.45 times the initial mass of pickling bath and 0.35 times the initial volume of pickling bath.
• this Cr recovered is not very pure, in that it would contain other metals initially present in the steels treated by the pickling installation, this is not necessarily a big drawback since these metals are also , intended to enter significantly into the composition of the steel which will be produced with this recovered Cr, or to be tolerable therein up to levels that, more generally, the use of the Cr recovered according to the invention will not run the risk of to overtake.
• FIG. 3 shows that the SO4 2 ions are found, in the example described, at more than 90% in the non-polymeric phase, and their residual presence in the polymeric phase therefore remains acceptable.
• the Cr to be recovered is to be used for some purpose other than recycling to the steel plant, it may be desirable that it has a better purity than what has been seen. In this case, it will be necessary to verify that the polymer phase has a concentration of compounds other than Cr (VI) low enough for the intended use for Cr to be effectively possible, taking into account the treatment methods used subsequently. This could possibly lead to:
• the Cr obtained by the process according to the invention is only used in the case where the pickling baths have treated steels with low levels of alloying elements other than Cr and Ni and whose ions are likely to pass significantly into the polymeric phase; baths which have repeatedly treated steels with a high Mo content could therefore be unsuitable for some of these uses, which those skilled in the art concerned can easily assess on a case-by-case basis;
• the phase not containing the polymer namely the spent bath which has been stripped of all of its Cr (VI)
• the phase not containing the polymer can be reused as a pickling bath without the latter being very significantly polluted by the polymer. It is reintroduced into the pickling bath 1 remaining in the tank 2 if the process is carried out continuously. This reintroduction of the non-polymeric phase into the pickling bath can take place as long as it does not significantly impair the effectiveness of the pickling bath.
• the polymeric phase is treated, with a view to extracting therefrom one or more compounds containing the Cr that it contains, or even also to be able to optionally isolate the polymer in order to reuse it in the first stage of 'extraction.
• This treatment consists of allowing precipitates containing Cr to form naturally in the polymer phase without the addition of reagents being necessary. This can be done by simply allowing said polymeric phase to stand, optionally while heating it to accelerate precipitation. But, preferably, this precipitation is accelerated by means of irradiation of the polymeric phase.
• this Cr-rich polymer phase By exposing, in a suitable installation 6, this Cr-rich polymer phase and, preferably, formed in the form of a liquid film with a thickness of the order, for example, of a few cm, to irradiation by radiation at least one light source whose wavelength (s) of the radiation it emits is (are) typically located in the range of wavelengths 340-860 nm, that is to say say in the UV spectrum or the visible spectrum, Cr precipitates, typically in the form of Cr (OH) 3 . It is then possible to recover, by simple filtration (or any other suitable solid-liquid separation process) in a dedicated installation 7, a solid 8 rich in Cr, the liquid phase notably containing the polymer.
• the forming of a liquid film of the Cr-rich phase with a view to its irradiation is a preferred method, since the solution is very dark, and the photons emitted by the light source (s) can only pass through a layer of light. bath of limited thickness.
• Another solution could be to put the solution in a tank fitted with a stirring device, so that the solution exposed to the radiation is constantly renewed. It is, moreover, entirely possible to combine the two operating modes by irradiating the stirred bath before it is formed into a film. Care must also be taken that the precipitate containing Cr does not settle on the light source, for example by placing it at a sufficient distance from the bath and / or by cleaning it frequently, in particular if it is integrated into the walls of the bath. container.
• One solution may be to have a large tank provided with a multiplicity of light sources regularly distributed, and which each is cleaned in turn while the other sources remain operational.
• the optimizations of the wavelength and of the illumination power necessary for an optimal execution of the process on a given installation can be carried out by routine experiments, depending in particular on the composition and the opacity of the pickling baths. that the user has to process, the thickness of the liquid that the radiation has to pass through, etc.
• Reheating the polymer phase can improve this kinetics. This reheating can, moreover, also take place simultaneously with irradiation of the polymer phase.
• the precipitate containing Cr is allowed to form, that is to say that the formation is carried out naturally, without additions of chemicals.
• the photochemical mechanisms for obtaining precipitates containing Cr by irradiation are exposed, for example, in the article “An ecological new approach for treating Cr (VI) -containing industrial wastewater: Photochemical reduction” by Jie Liu, Kun Huang, Keng Xie, Ying Yang, Huizhou Liu, Water Research, 93 (2016) pp. 187-194.
• the photochemical treatment of extracts from stainless steel pickling baths is not envisaged.
• the kinetics of this decrease in the concentration of Cr in the liquid therefore depends on the light intensity received by the medium, as shown in FIG. 4, which reports on experiments carried out on the polymer phase of PEG-400 loaded with Cr. obtained following the test shown in Figure 1.
• the PEG was preheated to the same temperature as the bath, so that this introduction did not cool the bath and that the kinetics of the decrease in the concentration of Cr in the film liquid was optimal.
• the decrease in the Cr concentration of the liquid film is all the more rapid as the thickness of the treated film is low (5 or 2 cm), at the same light source emission intensity. .
• a given quantity by mass of polymeric phase, formed in the form of a 2 cm thick film receives a quantity of light energy greater than that received by the same quantity by mass of polymeric phase formed in the form of a 5 cm thick film subjected to the same light source with the same irradiation intensity, and, logically, this is reflected in the effectiveness of the treatment.
• a Cr concentration in the liquid film of 1.5 g / l is reached after 4 h for a film 2 cm thick, and after about 6.5 h for a film thickness of 5 cm.
• the Cr concentration of the liquid film becomes practically zero after about 5.5 hours for a 2 cm thick film, and after 8 hours for a 5 cm thick film.
• the solid phase containing Cr it is treated, after an optional drying so useful, by a process allowing the recovery of Cr, or even of the other metals that it could contain, in a dedicated installation 10, for example by using everything.
• pyrometallurgical process known in itself and the use of which would be compatible with the composition and the physical properties of the solid phase.
• a process for the pyrometallurgical reduction of oxides by carbon can be used.
• the various installations mentioned above are preferably, or even necessarily, on the same site as the pickling installation, in particular when the process is carried out continuously, with recycling or reuse, in the plant.
• pickling installation the regenerated polymer and some of the by-products, and that the pickling bath 1 remains in the pickling tank 2 without being transferred to another container.
• the Cr recovery plant 10 may well be located on a different site, to which the solid phase 8 containing Cr can be transported, since the products resulting from this recovery are not intended to be reinjected into the plant. installation according to the invention.
• a variant of the invention may also consist in carrying out a treatment of the non-polymeric phase before it is returned to the pickling bath 1 in order to extract therefrom, at least partially, the chemical species which it contains, and which would cause an excessive modification. and too rapid and / or unwanted in the composition of the pickling bath 1 if the non-polymeric fraction was reintroduced into it without treatment.
• the Fe and Ni contained in the non-polymeric phase are generally not a problem if they are reintroduced into the pickling bath 1. But we cannot completely exclude that other Metal ions can have, by accumulating in the pickling bath 1, more harmful effects if they are significantly present in the non-polymeric phase.
• the material balance of an implementation of the method according to the invention can be described as follows, in which the PEG-400 used comes entirely from the recycling of the used PEG-400 carried out in the installation. dehydration 9.
• the polymer phase is sent to the irradiation installation 6, and the precipitates and the liquid phase which result from this irradiation (it being understood, as has been said, that this irradiation only serves to accelerate the formation of the precipitates. , which would take place anyway, without the need to add chemicals) are then separated in the solid-liquid separation plant 7.
• the solid phase 8 which results therefrom represents 0.5 kg and contains 54 g of Cr in the form of Cr (VI). After drying the solid phase (which sees its mass decrease to 0.2 kg) and treatment of said solid phase in the pyrometallurgical installation 10, 54 g of Cr are recovered.
• the liquid phase containing the polymer represents 7.1 kg and contains 3 g of Cr in the form of Cr (VI). It is then dehydrated in the distillation installation 9.
• the distillate which represents 3.0 kg and does not contain Cr, is returned to the pickling bath 1.
• the 4 kg of PEG-400 which are recovered thanks to the distillation and which contain 3 g of Cr (VI) are returned to the mixing reactor 3 to start a new cycle of recovery of the Cr contained in the pickling bath 1.
• the liquids and the polymeric phase poor in Cr are then recycled to make them constitute all or part of a new pickling bath 1, and the solid phase rich in Cr is sent to the metal recovery installation 10, so as to recovering the metals present in said phase, in particular Cr, which makes it possible to exploit these metals and to avoid discharging into the environment waste loaded with these metals, and therefore potentially harmful.

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