Process for recovering the chromium contained in a bath for pickling metallic materials and installation for its implementation

The present invention relates to the pickling of metallic materials containing Cr, in particular Cr-rich steels such as stainless steels. More specifically, 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 performance of said process, and to valorize the recovered metals as much as possible.

Chemical pickling is an essential operation in the steel manufacturing process. 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, HNO3, H2SO4, 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 pickling is often carried out, in particular, on running strips. It allows the strip, in particular, to be able to be rolled without risking that oxides become embedded in the surface of the strip and thus deteriorate its appearance and its quality, or to undergo a surface treatment in good conditions.

During this pickling operation, the baths are enriched with metallic 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 many metal cations in the bath shifts the chemical and electrochemical balances that exist in new baths, and thus reduces the effectiveness of pickling. The second is that when these dissolved metals reach their limit concentration of solubility, they precipitate and form sludge inside the baths. This sludge can then induce surface quality problems if it settles on the processed product or on the rollers that guide the product's trajectory. They can also settle in the tanks, tanks and pipes of the pickling installation, and thus affect the proper functioning of this installation.

Faced with this change in the metal content of pickling baths and the associated loss of efficiency, manufacturers have no simple way other than to regenerate the spent bath completely, or partially by adding new bath to it. . Before regeneration, the spent bath is, in whole or in part, 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 bisulphite 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 on a filter press. The metal hydroxide sludge cakes thus obtained are then sent to landfill, as there is no revaluation treatment for this sludge that would be economically viable.

Indeed, this sludge consists of the mixture of metals dissolved in the various surface treatment baths, as well as anions from at least some of these baths. For example, the high S and F contents of the sludge, coming from the acid baths when H2SO4 and HF have been used there, prevent the revalorization of this waste by pyrometallurgical process. Faced with this drawback, one of the solutions consists in separating the neutralization flows 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 in use. These solutions, such as roasting by spraying or "roasting spray" for hydrochloric baths, generally make it possible to regenerate the acid on one side and to recover the metals on the other. These methods are often expensive and do not always make it possible to regenerate an acid with a concentration that can be used for an industrial process. In addition, these processes are generally energy intensive and/or consumers of large quantities of water and/or users of resins or ion exchange membranes requiring cleaning cycles, during which the installation is not usable.

The object of the invention is to propose a method, both efficient and economically viable, for the regeneration of spent electrolytic pickling baths, and recovery of the Cr present in these spent electrolytic pickling baths, containing relatively high amounts of one or more sulfur compounds such as sulphates.

To this end, the subject of the invention is a process for recovering the Cr contained in a bath for pickling metallic materials present in a container such as a

pickling tank, said pickling bath being an aqueous solution containing at least one sulphate and Cr originating from the pickled metal, characterized in that:

- an aqueous biphasic system is formed from at least a portion of said pickling bath and a polymer comprising an unhindered ether function, said polymer preferably being a polyethylene glycol, typically a PEG-200, PEG -400 or PEG-600, optimally PEG-400, the proportion of polymer in the ternary mixture constituted by the pickling bath, which is considered to be a single chemical component, the water and the polymer being such that it is located between the straight line with the equation "weight% of polymer = 100% - weight% of the pickling bath" and the binodal curve of the pickling bath-polymer mixture at the temperature of the bath, said aqueous biphasic system comprising a polymeric phase , in which the majority of the Cr is found, and a non-polymeric phase;

- Said polymeric and non-polymeric phases are separated;

- Cr-containing precipitates are allowed to form in the polymer phase;

- a solid/liquid separation is carried out on said polymeric phase, so as to separate the polymer and the precipitates containing the Cr;

- and said precipitates are treated to recover the Cr they contain.

Preferably, to accelerate the kinetics of formation of the precipitates containing the Cr and the polymer, said polymeric phase is irradiated before said solid/liquid separation, by exposing it to at least one light source with a wavelength between 340 and 860 n.

Prior to its exposure to said light source, the polymer phase can be put in the form of a liquid film.

After said solid/liquid separation on said polymeric phase, the liquid fraction resulting from this separation can be dehydrated to recover the polymer present therein and said polymer is reused to form said two-phase aqueous mixture.

Said dehydration can be carried out by distillation.

The water resulting from said dehydration can be sent to the container that 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 comprise the following steps:

- a fraction of the pickling bath is extracted from a container which contains the pickling bath;

- an aqueous biphasic system is formed from at least a portion of said pickling bath and a polymer comprising an unhindered ether function, said polymer preferably being a polyethylene glycol, typically a polyethylene glycol of the PEG type -200, PEG-400 or PEG-600, optimally PEG-400, the proportion of polymer in the ternary mixture constituted by the pickling bath, which is considered to be a single chemical component, water and the polymer being such that it lies between the straight line of the equation "weight% of polymer = 100% - weight% of the pickling bath" and the binodal curve of the pickling bath-polymer mixture at the temperature of the bath, said aqueous biphasic system comprising a polymeric phase, in which the majority of the Cr is found, and a non-polymeric phase;

- And said non-polymeric phase of said aqueous biphasic mixture is returned to the container containing the pickling bath.

The container containing the pickling bath can be supplied with pickling bath continuously.

Said pickled metal can be a stainless steel.

The invention also relates to an installation for recovering the Cr contained in a metal pickling bath, said pickling bath being an aqueous solution containing at least one sulphate and Cr(VI) originating from the pickled metal, characterized in that 'she understands :

- A container containing said pickling bath;

- a mixing reactor connected to said pickling tank:

- a container containing a polymer intended to be introduced into said mixing reactor to form with said pickling bath an aqueous two-phase system;

- a device for separating the phases of said aqueous two-phase system into a polymeric phase and a non-polymeric phase;

- and a solid-liquid separation installation acting on said polymeric phase, so as to separate from said polymer the precipitates containing Cr(VI) present in the polymeric phase.

Installation little t also include an installation for irradiating said polymeric phase by radiation from at least one light source whose wavelength(s) is (are) in the 340-860 nm wavelength range, so 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 340-860 nm wavelength range.

The installation may also comprise an installation for recovering Cr from said precipitates containing Cr.

Said Cr recovery facility may make use of a pyrometallurgical process such as a pyrometallurgical reduction process of oxides by carbon.

The installation may comprise an installation for dehydrating 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 container containing the pickling bath can be a pickling tank.

The installation may include at least one of the following means:

- Means for recycling the non-polymeric phase in the pickling bath;

- Means for returning the used polymer, after its dehydration, to said container containing the polymer;

- and means for returning the water resulting from the dehydration of the used polymer to the pickling tank.

As will have been understood, the invention, for the recovery of Cr from a spent pickling bath, in particular electrolytic pickling, containing one or more sulphates, makes use of a liquid-liquid extraction via the use of polymers inducing the formation of aqueous biphasic systems capable of extracting the Cr(VI) from the pickling bath in a very preferential manner.

It turns out that the polymers that can be used in the context of the invention have the characteristic of having an unencumbered ether function, that is to say devoid of groups grafted onto the chain of the polymer in the immediate vicinity of the ether function.

Polyethylene glycol, in particular PEG-200, PEG-400 and PEG-600, is the preferred polymer that can be used in the context of the invention. PEG-400 is the preferred example.

General information on aqueous biphasic systems based on polymers 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, A. M. Azevedo, Separation and Purification Review 45 (2016), pp 68-80;

"Influence of Different Phase-Forming Parameters on the Phase Diagram of Several PEG-Salt Aqueous Two-Phase Systems", by B.A. Glyk, T. Scheper, S. Beutel, J. Chem. Eng. Data, 59 (2014), pp. 850-859.

The invention will now be described with reference to the following appended figures:

FIG. 1 which schematizes the succession of operations to be carried out for optimum implementation of the method according to the invention;

Figure 2 which shows in which ranges of mass concentrations of PEG-400 in a mixture consisting of PEG-400, the pickling bath and water it is necessary to be located for a satisfactory implementation of the invention, for a bath having a certain composition and depending on its temperature;

Figure 3 which shows, in mass percentages, the distribution of the elements in the phase containing the polymer after extraction, according to 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;

It should first be recalled that electrolytic pickling baths based on sulphate(s), known as “electro-sulphate baths”, are mainly used at the start of the pickling sequence for the final annealing of stainless steels. Sodium sulphate, which is typically used in these baths, is not active towards the surface of stainless steels, but only serves to carry the electrolysis current. The latter is used to transform the Cr(III), present in the oxide layer on the surface of the strip, into highly soluble Cr(VI) which will therefore be released into the bath. Thus, as pickling progresses, the bath becomes loaded with Cr(VI). This loading will induce changes in the balance of the chemical and electrochemical reactions implemented in this bath to lead to a loss of pickling efficiency when the bath reaches high concentrations of dissolved metals.

In addition, Cr(VI) is a chemical species classified as CMR (carcinogenic, mutagenic and reprotoxic) according to European REACFI regulations. It therefore presents a great danger for the people who are exposed to it. Although this bath is treated and

neutralized accordingly, the formation and accumulation of large quantities of Cr(VI) in the bath constitute a health and safety risk for operators working around 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 as, in particular, to then carry out the recovery of the Cr under suitable health and safety conditions, and at a non-prohibitive cost.

It is also recalled that the extraction of elements contained in a solution by means of the formation of a two-phase system is a process already known in the revaluation sectors. However, in most cases, these processes use organic solvents or ionic liquids, which have the disadvantage of being expensive and/or toxic.

A first evolution of this method is set out in document WO-A-2018/087364. By bringing the solution to be treated containing the inorganic salt and ionic liquid in certain proportions into contact, it is possible to form an aqueous two-phase system, in which the species present will be distributed between one and the other of the two phases. However, no application of this development to metal and alloy pickling baths is envisaged in this document.

It has also been shown that it is possible to form aqueous biphasic systems with certain polymers.

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, compared to 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 discharge 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 method can preferably be used continuously, for example on the installation represented 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 lifetime of the bath 1 compared to the case where it would be treated discontinuously, 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. Preferably, the pickling tank is supplied with new pickling bath 1 continuously.

In general, the inventors have observed that the polymer which can be used for the implementation of the invention must be a polymer whose chain comprises an unencumbered ether function. By "unhindered", we understand that no group must be grafted onto the chain in the vicinity of the O atom of the ether function, which could interfere with its action.

Polyethylene glycol PEG is a preferred example of such a polymer that can be used in the context of the invention. We recall that its general formula is:

Tetraethylene glycol dimethyl ether of general formula:

is also usable in the invention, as it proves capable of extracting Cr(VI). It can be assimilated to a PEG of low molecular weight (222 g/mol) and having no terminal OH groups.

Triethylene glycol dimethyl ether of formula:

is also part of the usable polymers.

Conversely, polypropylene glycol (PPG), whose general formula is:

is not suitable for the implementation of the invention, because its ether function is hindered by the CH3 group which is close to its ether function.

On the other hand, the inventors have found that the "Pluronic® 10R5" block copolymer from the firm BASF, of 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.

It is concluded that the presence of an unencumbered ether function does not necessarily have to concern the entire chain of the polymer, but that it suffices that at least certain portions of the chain have such an unencumbered ether function so that the Cr(VI) extraction properties of the stainless steel pickling baths by the polymer, as a component of a two-phase aqueous mixture, are present.

The expression "comprises an unhindered ether function" must therefore be understood as also relating to polymers which may not have such an unhindered ether function que on a portion of their chain and not only on their entire chain, and that the presence of hindered ether functions coexisting, inside the chain, with unhindered ether functions is not a prohibitive characteristic.

It is also concluded from all of these observations that the presence of terminal OH groups does not seem to play a significant role in the properties of the polymers which the invention takes advantage of.

Regarding PEG, there are commonly different types, which differ in their average molecular weight. The increase in the average molecular mass induces an increase in the surface of the biphasic zone in the phase diagram of the mixture and the viscosity of the mixture, for a given temperature, and a too high molecular mass can decrease the effectiveness of the polymer for the extraction of Cr(VI), under otherwise identical operating conditions. This increase in molecular mass makes mixing, settling and separation operations 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 is found 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.

The prime example of the use of PEG-400 will be discussed in detail.

Generally speaking, the proportions of PEG-400 to fluid should meet the following criteria.

The proportion of PEG-400 in the ternary mixture consisting of the pickling bath (which is considered to be a single chemical component), water and PEG-400 must be such that it is between the line d the equation "weight% of PEG-400 = 100% - weight% of pickling bath" and the binodal curve of the pickling bath-PEG-400 mixture at the temperature of the bath.

This criterion relating to the proportion of polymer used is also valid for any polymer other than PEG-400.

It is also preferable that the proportion by weight of the PEG-400 relative to the pickling bath is not greater than 50%, so that the quantity of PEG-400 to be used is not too high compared to the quantity of Cr which will be recovered, and that the process certainly remains attractive from an economic point of view.

Figure 2 shows said straight line and said binodal curves of a bath of PEG-400 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 considered range.

During the formation of the aqueous two-phase system, the metals present in the bath will be distributed between the two phases and the Cr(VI) will mainly go into the phase containing the largest part (by mass) of the polymer. Figure 3 shows, in mass percentages, the distribution of the elements in the phase containing the polymer after extraction, according to the proportion of polymer added in the original bath.

The composition of the bath tested is as follows: total Cr: 11.7 g/l; Fe: 0.5 g/l; Ni: 0.4 g/l; Na: 47 g/l; S042-: 106 g/l; pH=1.8; density: 1.14.

The conditions under which this diagram was established are as follows.

CLAIMS

1.- Process for recovering the Cr contained in a metallic material pickling bath (1) present in a container such as a pickling tank (2), said pickling bath (1) being an aqueous solution containing at least one sulphate and Cr from the pickled metal, characterized in that:

- an aqueous biphasic system is formed from at least a portion of said pickling bath (1) and a polymer comprising an unhindered ether function, said polymer preferably being a polyethylene glycol, typically a PEG- 200, PEG-400 or PEG-600, optimally PEG-400, the proportion of polymer in the ternary mixture constituted by the pickling bath, which is considered to be a single chemical component, the water and the polymer being as it lies between the straight line of the equation "weight% of polymer = 100% - weight% of the pickling bath" and the binodal curve of the pickling bath-polymer mixture at the temperature of the bath, said aqueous two-phase system comprising a polymeric phase, in which the majority of the Cr is found, and a non-polymeric phase;

- Said polymeric and non-polymeric phases are separated;

- Cr-containing precipitates are allowed to form in the polymer phase;

- a solid/liquid separation is carried out on said polymeric phase, so as to separate the polymer and the precipitates containing the Cr;

- and said precipitates are treated to recover the Cr they contain.

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

3.- Method according to claim 2, characterized in that, prior to its exposure to said light source, the polymeric phase is placed in the form of a liquid film.

4.- Method according to one of claims 1 to 3, characterized in that, after said solid/liquid separation on said polymeric phase, dehydration of the liquid fraction resulting from this separation is carried out to recover the polymer therein. present and said polymer is reused to form said aqueous biphasic mixture.

5.- Method according to claim 4, characterized in that said dehydration is carried out by distillation.

6.- Method according to claim 4 or 5, characterized in that the water resulting from said dehydration is sent to the container (2) which contains the pickling bath (1) ·

7.- Method according to one of claims 1 to 6, characterized in that the recovery of Cr from the precipitates containing it, after their separation from the remainder of the irradiated polymeric phase, is carried out by a pyrometallurgical process.

8. A process according to claim 7, characterized in that said pyrometallurgical process is a process for the pyrometallurgical reduction of oxides by carbon.

9.- Method according to one of claims 1 to 8, characterized in that:

- a fraction of the pickling bath (1) is extracted from a container (2) which contains the pickling bath (1);

- an aqueous biphasic system is formed from at least a portion of said pickling bath (1) and a polymer comprising an unhindered ether function, said 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 polymer in the ternary mixture formed by the pickling bath, which is considered to be a single chemical component, water and the polymer being such that it lies between the straight line of the equation "weight% of polymer = 100% - weight% of the pickling bath" and the binodal curve of the pickling bath-polymer mixture at the temperature of the bath, said system aqueous biphasic comprising a polymeric phase, in which the majority of the Cr is found, and a non-polymeric phase;

- And said non-polymeric phase of said aqueous biphasic mixture is returned to the container (2) containing the pickling bath.

10.- Method according to claim 9, characterized in that the pickling bath (1) is fed continuously to the container (2) containing the pickling bath.

11.- Method according to one of claims 1 to 10, characterized in that said pickled metal is a stainless steel.

12.- Cr recovery plant contained in a metal pickling bath (1), said pickling bath (1) being an aqueous solution containing at least one sulfate and Cr (VI) from the pickled metal, characterized in that that it includes:

- a container (2) containing said pickling bath (1);

- a mixing reactor (3) connected to said pickling tank (2):

- a container (4) containing a polymer intended to be introduced into said mixing reactor (3) to form with said pickling bath (1) a two-phase aqueous system them ;

- a device (5) for separating the phases of said aqueous two-phase system into a polymeric phase and a non-polymeric phase;

- and a solid-liquid separation installation (7) acting on said polymer phase, so as to separate from said polymer the precipitates containing Cr(VI) present in the polymer phase.

13.- Installation according to claim 12, characterized in that it also comprises an installation (6) for irradiating said polymeric phase by the radiation of at least one light source whose wavelength is in the range of wavelengths 340-860 nm, so as to accelerate the formation of precipitates containing Cr.

14.- Installation according to claim 13, characterized in that it comprises 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 wavelengths 340-860 nm.

15.- Installation according to one of claims 12 to 14, characterized in that it also comprises an installation (10) for recovering Cr from said precipitates containing Cr.

16.- Installation according to claim 15, characterized in that said installation (10) for the recovery of Cr makes use of a pyrometallurgical process such as a process for the pyrometallurgical reduction of oxides by carbon.

17.- Installation according to one of claims 12 to 16, characterized in that it comprises an installation (9) for dehydrating said polymer after its separation from said precipitates containing Cr.

18.- Installation according to one of claims 12 to 17, characterized in that said mixing reactor (3) and said device (5) for separating the phases of said two-phase aqueous system are constituted by the same device.

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

20.- Installation according to one of claims 12 to 19, characterized in that it comprises at least one of the following means:

- means for recycling the non-polymeric phase in the pickling bath

(1);

- Means for returning the used polymer, after its dehydration, to said container (4) containing the polymer;

- and means for returning the water resulting from the dehydration of the used polymer to the pickling tank (2).