The present invention relates to a process for pickling and/or passivating a stainless steel.

In general, a steel is defined as stainless if it is such that it prevents the formation of rust on its surface under normal environmental conditions, for example in the presence of oxygen and atmospheric humidity or in aqueous solutions. Stainless steels are made of iron-based alloys containing at least 10 % by weight of chromium. The formation of chromium oxide on the surface of the material gives stainless steels their characteristic corrosion resistance property. When stainless steel is subjected to thermal treatments (e.g. hot rolling, annealing, etc.) a layer of surface oxide (so-called "scale") is formed on its surface. The removal of the scale is necessary to restore the basic chemical composition of the steel on the surface thereof, so as to give the steel the required corrosion resistance properties.

The oxide forming the surface scale, in addition to iron oxides, contains oxides of the steel binding elements, for example chromium, nickel, aluminium, titanium or niobium. In particular, in hot rolling, an accumulation of chromium oxide takes place in the surface layer. The oxide layer is therefore enriched with chromium instead of iron. On the contrary, the steel layer immediately below the oxide layer is depleted in chromium (so-called "chrome-depleted" layer). A pickling process preferably dissolves the chrome-depleted layer under the oxide layer, so that the surface scale is also completely removed.

After pickling, the surface of the steel, if left exposed to atmospheric air, is covered again with a layer of opaque oxide. This phenomenon can be avoided by subjecting the surface of the just pickled steel to passivation. The passivation treatment can be carried out in treatment solutions similar to the pickling solutions, but having a higher redox potential than the redox potential of the pickling solutions. The passivation treatment forms an optically invisible passivation layer that maintains the surface of the steel polished and rustproof, too.

The pickling processes of stainless steel are well known in the state of the art. Conventional processes use pickling baths containing nitric acid, often in combination with hydrofluoric acid, which thanks to its complexing action with respect to iron ions favours the dissolution process of iron of the chrome-depleted layer and of the scale. Although these pickling baths are efficient and technically satisfactory, they have the disadvantage of having a significant environmental impact, mainly because of the considerable quantities of gaseous effluents containing nitrogen oxides produced and the large quantities of nitrates, nitrites, fluorides and sulphates released into the wastewater of the process.

In order to try to overcome the aforesaid disadvantages, in the state of the art alternative pickling and passivation processes have been developed which do not use nitric acid (so-called nitric-free process), but rather a combination of one or more acids, such as HC1, H ₂ SO ₄ , HF, and Fe ¹¹¹ ions. In nitric-free processes, Fe ¹¹¹ ions replace nitric acid in the oxidizing action. During the process the Fe ¹¹¹ ions are converted into Fe ¹¹ ions. At the same time, other Fe ¹¹ ions are eroded from the surface of the pickled steel and dissolved in the treatment solution during the operation. The bath is then progressively depleted of

Fe ¹¹¹ ions, while it is enriched with Fe ¹¹ ions. As the ratio of Fe ^III /Fe ¹¹ ions decreases, the redox potential of the bath decreases progressively, thus the effectiveness of the treatment solution being also reduced. In order to prolong the bath effectiveness, the divalent iron ions are oxidized in the trivalent state by the continuous or discontinuous addition of oxidizing agents, such as for example hydrogen peroxide, perborates, peracids and organic peroxides.

Examples of processes that limit or eliminate the use of nitric acid are described for example in: EPO792949, EP0501867, EP0236354, EP0769574, EP1552038,

US6250314, EP0795628, EP0582121. As the treatment process continues, the total concentration of metals in the bath increases and therefore the pickling and/or passivating capacity thereof decreases, until a replacement, at least partially, of the exhausted treatment solution with a fresh treatment solution becomes necessary.

In order to reduce the consumption of raw materials, the costs of the process and to reduce the environmental impact, the exhausted solution of the pickling processes, both with nitric acid and nitric-free, can be subjected to a regeneration process to recover the acid and the metals present therein, for example by pyrohydrolysis treatments. The regeneration system, generally, collects the exhausted treatment solutions coming from the different treatment lines of a same production plant or of several production plants. The regeneration product is an aqueous solution of the acid or acids initially present in the exhausted solutions. This regenerated solution can be recycled in the same process or in other processes.

An example of a process for regenerating the acids coming from pickling or passivation solutions is described in US 2013/259793 A1.

In the state of the art, the processes for pickling and regenerating the exhausted treatment solutions, even when carried out in a same production plant, are managed independently of each other. The aim for each process is to maximise production yield and to minimise consumption of raw materials and energy. This approach, however, has disadvantages. For example, the aqueous solution of the regenerated acid cannot be recycled directly to the process that generated it if this process uses acid solutions at a higher concentration than that obtainable through the regeneration process.

Although the known nitric-free pickling and passivation processes allow obtaining high-quality products with a limited environmental impact, thanks to the elimination of nitrogen oxides (NOx) in the fumes and nitrates and nitrites in the liquid discharges, in the state of the art there is always a need to identify alternative processes that allow obtaining high-quality products, with a low environmental impact and reduced consumption of raw materials, in particular the consumption of mineral acids and energy.

The Applicant has now found that it is possible to achieve the aforesaid object by operating a pickling and/or passivation process in an integrated manner with a process for regenerating the treatment solutions. The integration of the two processes is obtained by carrying out the pickling and/or passivation process under operating conditions that are such to maximise the recovery yield of the acids in the regeneration system of the exhausted treatment solutions and, at the same time, to allow the recycling thereof in the same process that generated it.

In particular, the aforesaid technical effect is obtained by means of a nitric-free pickling and/or passivation process in which a stainless steel is treated in sequence with at least two treatment solutions, contained in at least a first and a second bath, which are kept at two different redox potential values and wherein the exhausted treatment solution of the first bath is fed to a regeneration treatment to obtain an aqueous solution of regenerated acid having a chemical composition compatible with that of the second bath; in the process, moreover, a part of the treatment solution of the second bath is transferred to the first bath to replace the treatment solution sent for regeneration, while the aqueous solution of regenerated acid is fed to the second bath to replace the part of treatment solution that has been transferred to the first bath.

The first treatment solution contained in the first bath is kept at a redox potential higher than that of the second treatment solution contained in the second bath by suitably regulating the ratio of the Fe ^III /Fe ¹¹ ions in solution by oxidation. The total concentration of iron ions (Fe _tot = Fe ¹¹¹ + Fe ¹¹ ) in the first treatment solution is kept at a higher value than the total concentration of iron ions in the second treatment solution. The second treatment solution, instead, is not sent for regeneration. By operating under the aforesaid conditions, it is possible to obtain a high-quality product, significantly reducing the consumption of raw materials, in particular mineral acids and additives for bath oxidation, and energy. In this way it is avoided sending the acids to the neutralization/sedimentation plants and consequently introducing wastewaters containing soluble residues of sulphates, chlorides and fluorides into the environment. Moreover, the disposal of the sludge containing metals (Fe, Cr, Ni, Mn and others) in a special landfill is avoided, the metals can even be recovered in the form of oxides and reused.

The treatment of steel in a first bath having a relatively low redox potential (i.e. low Fe ^III /Fe ¹¹ ion ratio) allows a more rational use of the oxidizing compounds since an averagely more concentrated solution of iron ions (Fe _tot) and having a relatively high concentration of Fe ¹¹ ions is fed to the process for regenerating the acids. The regeneration of solutions that are relatively more concentrated in iron, in particular by means of regeneration processes comprising a heat treatment, is more effective since it allows a significant energy saving compared to the regeneration of diluted solutions, where most of the thermal energy is required to evaporate a higher amount of water.

The treatment of the steel in the first bath with lower redox potential, moreover, allows a more rational regeneration of the exhausted solution, since the maintenance of a lower redox potential value entails a lower consumption of oxidizing additives, since a less forced oxidation of the Fe ¹¹ ions into Fe ¹¹¹ ions is required.

The treatment in a solution having a lower redox potential implies a lower pickling efficiency of the first bath and, consequently, the possibility that the surface of the steel leaving the first bath may still present scale residues and chrome-depleted material. This disadvantage, however, is compensated by the higher efficacy and selectivity of the second treatment solution which is kept at a higher redox potential than the first solution, also due to the recycling of the regenerated acid solution. The high oxidizing capacity of the second solution allows to dissolve the aforesaid scale residues and chrome-depleted material, leaving the layers with the correct chromium content (selective action) untouched and finally to passivate the surface thereof, thus allowing to obtain a final product of high quality. With the process described herein it is possible to obtain a stainless steel with a quality comparable to that of the steels treated in conventional processes with treatment solutions containing nitric acid, with a significant reduction in the consumption of raw materials (acids, additives, oxygen, etc.), energy and environmental impact with respect to the latter.

Therefore, in accordance with a first aspect, the present invention relates to a process for pickling and/or passivating a stainless steel with an aqueous treatment solution comprising Fe ¹¹¹ ions which are turned into Fe ¹¹ ions during pickling, comprising the following steps: a. putting said stainless steel (1) in contact with a first aqueous treatment solution (30, 1000), comprising: at least one acid selected from HF and HC1; 10-65 g/1 of Fe ¹¹¹ ions,

5-60 g/1 of Fe ¹¹ ions; b. putting said steel treated in at least said step a in contact with a second aqueous treatment solution (40, 1001) comprising: - at least one acid selected from HF and HC1, said acid being identical to that of said first treatment solution (30, 1000);

10-50 g/1 of Fe ¹¹¹ ions,

0-40 g/1 of Fe ¹¹ ions; where said steps a and b are carried out by keeping:

- the weight ratio between the Fe ^III /Fe ¹¹ ions of said second treatment solution (40, 1001) at a value higher than the weight ratio between the Fe ^III /Fe ¹¹ ions of said first treatment solution (30, 1000); - the total concentration of Fe ¹¹¹ and Fe ¹¹ ions in the first treatment solution (30, 1000) at a value higher than the total concentration of Fe ¹¹¹ and Fe ¹¹ ions in the second treatment solution (40, 1001); c. when the first treatment solution (30, 1000) is exhausted, taking a part of said first solution (30,

1000) and feeding it to a regeneration process (90, 94) to obtain a regenerated aqueous solution of said acid (34, 340); d. taking a part (32, 320) of said second treatment solution and mixing it with said first treatment solution

(30, 1000); e. recycling said regenerated aqueous solution of said acid (34, 340) to at least said second aqueous treatment solution (40, 1001).

Further characteristics of the present invention are described in the appended dependent claims 2 - 14.

For the purposes of the present description and of the relative claims, the verb "to comprise" and all the terms deriving therefrom, as used herein in the description and in the claims, also include the meaning of the verb "to consist of" and the terms deriving therefrom.

The numerical limits and intervals expressed in the present description and appended claims also include the numerical value or numerical values mentioned. Furthermore, all the values and sub-intervals of a limit or numerical interval must be considered to be specifically included as though they had been explicitly mentioned.

Description of the figures

The features and advantages of the present invention will be more evident from the following description, in which reference will also be made to the attached Figures 1 - 3, each of which schematically represents a possible embodiment of the process according to the invention.

Detailed description of the invention

The following description of the process according to the present invention refers to a method for carrying out the process in a steady state condition.

In step a of the process according to the present invention, the treatment of stainless steel is carried out with a first aqueous acid treatment solution comprising at least one mineral acid and iron ions. The acid is selected from: HF, HC1 and H2SO4 and relative mixtures. The selection of the acid depends on various factors, such as the type of steel to be treated (e.g. ferritic, martensitic, austenitic, etc.), the quantity of and degree of adhesion of the scale present, as well as possible pre- and post-treatments of the steel.

For example, with reference to the treatment solution 30 of Figures 1-3, when the acid is hydrofluoric acid (HF), preferably the concentration thereof in the first treatment solution is in the range 3 - 50 g/1, preferably 5 - 40 g/1.

For example, with reference to the treatment solution 1000 of Figures 2-3, when the acid is hydrochloric acid (HC1), preferably the concentration thereof in the first treatment solution is in the range 30 - 60 g/1.

In one embodiment, the acid present in the first treatment solution is a mixture of HF and H2SO4. For example, with reference to the treatment solution 30 of Figures 1-3, when the acid is a mixture of HF and H2SO4, preferably the concentration of HF acid in the first treatment solution is in the range 5 - 40 g/ and the concentration of H2SO4is in the range 50-100 g/1. In the present description, the concentration values of the aforesaid acids refer to the concentration of free acid in the aqueous solution, as measurable, for example, by means of an acid-base, manual or automatic, titration. The term "free acid" refers to the acid not yet used to form salts with metal cations, i.e. the acid still available in the bath to form salts with the metal ions just dissolved. The pH of the first treatment solution varies as a function of the type and concentration of the acid present.

If the acid is HC1, the pH is preferably less than

1.

If the acid is HF, the pH is preferably less than

4.

If the acid is a mixture of HF and H2SO4, the pH is preferably less than 1.

In step a, the first treatment solution contains Fe ¹¹¹ ions in a concentration in the range 10- 65 g/1.

For example, with reference to the treatment solution 30 of Figures 1-3, when the acid is HF, preferably the first treatment solution contains Fe ¹¹¹ ions in a concentration in the range 10-40 g/1 and Fe ¹¹ ions in a concentration in the range 5-40 g/1.

For example, with reference to the treatment solution 30 of Figures 1-3, when the acid is HC1, preferably the first treatment solution contains Fe ¹¹¹ ions in a concentration in the range 30-65 g/1 and Fe ¹¹ ions in a concentration in the range 20-60 g/1.

In general, in the first treatment solution, the

Fe ^III /Fe ¹¹ ion ratio, hereinafter also indicated Kl, is higher than 1, preferably equal to or higher than 1.1. Except for the process start-up step, where the treatment solution essentially contains Fe ¹¹¹ ions only, the Kl value under steady operating conditions does not generally exceed the value of 10. In one embodiment, the Kl value is kept in the range 1.1 - 5.

In general, in the first treatment solution used in step a, the total concentration of iron ions, hereinafter also referred to as Fe _tot (a), is preferably in the range 30 - 100 g/1.

For example, with reference to the treatment solution 30 of Figures 1-3, when the acid is HF, preferably the concentration Fe _tot (a) is in the range 30 - 70 g/1.

For example, with reference to the treatment solution 1000 of Figures 2-3, when the acid is HC1, preferably the concentration Fe _tot (a) is in the range 40- 100 g/1. Said concentration Fe _tot (a) is kept at a value higher than the concentration of Fe _tot in the treatment solution used in step b, hereinafter also referred to as Fe _tot (b).

Preferably, the ratio Fe _{tot (} a)/Fe _tot (b), is equal to or higher than 1.1, more preferably equal to or higher than 1.2. The aforesaid ratio, in general, is less than 10, preferably less than 7. In particular, in the case of ferritic steels, the aforesaid ratio is preferably higher than 1.5.

The ratio Fe _tot (a)/Fe _tot (b) is selected on the basis of the type of steel treated and the type and mass of scale to be removed. In general, the value of the aforesaid ratio is kept in the range 1.2 - 7.

Step a is carried out by keeping the treatment solution at a temperature in the range 45-65°C. In step b, the steel that has undergone the treatment of step a is placed in contact with a second acid treatment solution comprising at least one mineral acid and iron ions. In the second treatment solution the same acid or mixture of acids as the first treatment solution is used. In this way, when the first exhausted treatment solution is sent for regeneration, at least part of the second solution can be transferred to the first bath to replace the first exhausted solution.

For example, with reference to treatment solution 40 of Figures 1-3, when the acid is hydrofluoric acid (HF), preferably the concentration thereof in the second treatment solution is in the range 3-50 g/1, preferably 4 - 40 g/1.

For example, with reference to the treatment solution 1001 of Figure 3, when the acid is hydrochloric acid (HC1), preferably the concentration thereof in the second treatment solution is in the range 60 - 100 g/1.

For example, with reference to the treatment solution 40 of Figures 1-3, when the acid is a mixture of HF and H ₂ SO ₄ , preferably the concentration of HF acid in the second treatment solution is in the range 4 - 40 g/ and the concentration of H ₂ SO ₄ is in the range 60-110 g/1.

The pH of the treatment solution in step b varies as a function of the type and concentration of the acid present. If the acid is HC1, the pH is preferably less than

1.

If the acid is HF, the pH is preferably less than

4.

If the acid is a mixture of HF and H2SO4, the pH is preferably less than 1.

In general, in step b the second treatment solution contains Fe ¹¹¹ ions in a concentration in the range 10- 50 g/1 and Fe ¹¹ ions in a concentration in the range 0- 40 g/1. For example, with reference to the treatment solution 40 of Figures 1-3, when the acid is HF, preferably the second treatment solution contains Fe ¹¹¹ ions in a concentration in the range 10-40 g/1 and Fe ¹¹ ions in a concentration in the range 0-30 g/1.

For example, with reference to the treatment solution 1001 of Figure 3, when the acid is HC1, preferably the second treatment solution contains Fe ¹¹¹ ions in a concentration in the range 30-50 g/1 and Fe ¹¹ ions in a concentration in the range 0-40 g/1.

In general, in the second treatment solution, the Fe ^III /Fe ¹¹ ion ratio, hereinafter also referred to as K2, is higher than 1, preferably higher than 1.1.

For example, with reference to the treatment solution 40 of Figures 1-3, the K2 value may tend to infinity if a treatment solution with a high redox potential and therefore substantially free of Fe ¹¹ ions or with a very low concentration of these ions is used. This treatment solution can be advantageously used, for example, when the steel is ferritic (AISI 430, 409, 441, etc.).

In one embodiment, the concentration of Fe ¹¹ ions in the second treatment solution can be kept in the range 1-30 g/1, more preferably 1-20 g/1.

Generally, the value of the ratio K, corresponding to the ratio between the K2 value and the K1 value, under steady operating conditions does not exceed the value of 6, especially of 5. In one embodiment (austenitic steels), the K value is kept in the range 1.1-2. The K value can however tend to infinity if K2 tends to infinity, for example under the aforesaid described operating conditions. In the second treatment solution used in step b, the total concentration of iron ions, hereinafter also referred to as Fe _tot (b), is preferably in the range 10 - 60 g/1.

Analytical methods for determining the concentration of Fe ¹¹¹ and Fe ¹¹ ions, acids and the pH in treatment solutions are known in the art, for example acid-base titrations, conductometric methods and density measurements.

Step b is carried out by keeping the treatment solution at a temperature in the range of 40-65°C.

In order to adjust the ratio between the Fe ¹¹¹ and Fe ¹¹ ions in the first and second treatment solution such that K1 and K2 are kept in the desired ranges for each bath, it is possible to use the oxidation methods known in the state of the art.

Preferably, the ratio between the Fe ^III /Fe ¹¹ ions in the treatment solutions is adjusted by one or more of the following operations:

- adding at least one oxidizing reagent to at least one of the aforesaid first and second aqueous treatment solution to oxidize the Fe ¹¹ ions to Fe ¹¹¹ ions; - subjecting at least one of the aforesaid first and second aqueous treatment solution to catalytic oxidation with a gas containing oxygen in the presence of a homogeneous or heterogeneous oxidation catalyst;

- oxidizing at least one of the aforesaid first and second aqueous treatment solution by electrochemical oxidation.

The oxidizing reagent may be selected for example from: hydrogen peroxide, compounds releasing hydrogen peroxide, oxygenated acids of the chlorine, permanganate salts, persulphate salts and relative mixtures.

In one embodiment, the oxidizing reagent is fed into the first and/or second treatment solution by means of a venturi-type mixer.

The oxidation of the treatment solutions can be carried out in a continuous or discontinuous way.

In step c, a part of the first treatment solution is taken and fed to a regeneration process to obtain a regenerated aqueous solution of the acid or mixture of acids used.

The treatment solution to be regenerated is preferably taken when the solution is exhausted: this condition occurs when an excessive quantity of iron ions accumulates in the treatment solution due to the progressive dissolution of the scale and of the dechromised layer. The total iron concentration (Fetot) at which a solution can be considered exhausted varies as a function of the composition of the treatment solution, the type of steel treated, the composition of the scale, the number of treatment baths present as a whole, the temperature and other parameters. In general, the value of Fetot is known to those skilled in the art or it can be determined, case by case, by experimental tests.

In general, the first treatment solution is considered exhausted when the concentration Fetot is higher than a value comprised in the range 40 - 100 g/1. Preferably, when the acid is HF the first treatment solution is considered exhausted when the concentration Fetot is higher than a value in the range 40 - 70 g/1.

Preferably, when the acid is HC1 the first treatment solution is considered exhausted when the concentration Fetot is higher than a value in the range 70 - 100 g/1.

The methods known in the prior art can be used to regenerate the treatment solution.

In one embodiment, the regeneration can be carried out by pyrohydrolysis.

In a preferred embodiment, the process of regeneration by pyrohydrolysis comprises the following steps: i. spray-drying at least a portion of the aqueous treatment solution by means of a gaseous heating fluid having a temperature within the range 180-500°C, preferably 300-400°C, to obtain in gaseous form the acid contained in said solution and dried metal salts; ii. absorbing said acid in gaseous form obtained in said step i. in water to form a regenerated aqueous solution of said acid.

Advantageously, the aforesaid regeneration process can further comprise the steps: iii. pyrolizing the dried metal salts leaving step ii, for example at a temperature in the range 400-900°C, to obtain metal oxides and further acid in gaseous form; iv. absorbing said further acid in gaseous form in water to form a further regenerated aqueous solution of said acid.

According to the present invention, independently of the type of regeneration process used, the yield of the regeneration process of an exhausted treatment solution can reach values higher than 95% by weight, said percentage being referred to the total content of anions of the acid present in the solution fed for regeneration.

The regeneration process also enables the metals extracted from the solution, in particular chromium, nickel and molybdenum, to be recovered, which can be recycled effectively in the context of the processes made in a steel manufacturing plant.

An example of a process for regenerating the acid treatment solutions suitable for the purposes of the present invention is described, for example, in US 2013259793 A1.

In step d, a part of the second treatment solution used in step b is taken and transferred into the bath containing the first treatment solution of step a, so as to partially or totally replace the volume of the first treatment solution fed for regeneration. Since the redox potential K2 of the second treatment solution is higher than the redox potential K1 of the first treatment solution, the transfer of the second solution contributes to controlling the K1 value in the first solution.

The transfer of the second treatment solution into the bath containing the first solution can be carried out continuously or discontinuously. In step e, the aqueous solution of the regenerated acid obtained in step c is recycled at least in part in the bath containing the second treatment solution, so as to partially or totally replace the volume of the second treatment solution transferred to the first bath. In the process according to the invention, the composition of the regenerated aqueous solution of acid (hereinafter also referred to as "regenerated acid"), in particular the concentration of the acid or mixture of acids used, is compatible with that of the second treatment solution and therefore it is recycled at least to it. The term "compatible" means that the concentration of acid in the regenerated solution is equal to or higher than the concentration of the acid expected in the second treatment solution. The regenerated acid, if compatible with the first treatment solution or with other treatment solutions used in the process, can also be recycled in the baths containing the aforesaid treatment solutions.

The treatment solutions, if necessary, can be maintained under the desired acid and iron ion concentration conditions through the periodic or continuous addition of fresh (i.e., non-regenerated) acids, water and oxidizing compounds.

The process according to the present invention is based on the use of at least two separate treatment solutions, in step a and in step b, to treat a stainless steel. However, in some cases, between step a and step b, other intermediate treatment solutions comprising the same acid or mixture of acids as the solutions of steps a and b together with iron ions can also be used. The number of the intermediate treatment solutions could be 1, 2, 3 or more. The intermediate treatment solutions have chemical composition and redox potential in the ranges described above for the treatment solutions of steps a and b. The chemical composition and the redox potential of the intermediate treatment solutions are selected so as to allow the transfer of each of them to the treatment solution preceding it in the sequence, so as to create a reverse cascade treatment solution flow, i.e. in the opposite direction to that in which the steel advances. In the presence of intermediate treatment solutions, the treatment solution which is sent for regeneration when exhausted is at least the treatment solution of step a, that is, the first of the sequence, and the regenerated acid is recycled at least in the bath containing the treatment solution of step b, that is the last of the sequence.

The treatment solutions used in the process according to the invention, in addition to the mineral acids and metal ions mentioned above, can also comprise the additives typically used in pickling and passivation baths known in the art, such as compounds having the function of improving the wettability of the steel surface, brighteners, accelerating agents or inhibitors of pickling reactions.

The process according to the present invention is generally a sub-step of a treatment sequence of a stainless steel. The treatment sequence may comprise one or more further steps of steel surface treatment, which are carried out before and/or after the process according to the present invention. Examples of preliminary treatments are the removal of the scale by mechanical methods, such as sandblasting or shot blasting and/or chemical and electrochemical methods. The process according to the invention may optionally comprise at least one final finishing/passivation treatment step dedicated to particular materials or finishing grades, which is carried out after step b. The process according to the invention may further comprise conventional steps of washing the surface of the laminate with water or aqueous solutions optionally containing surfactants or other additives. In particular, the washing steps, which can be carried out by immersion, spraying with water jets and possibly with the aid of metal brushes, aim at eliminating the residues of the previous treatments from the surface and avoiding contamination of the treatment solutions.

In order to carry out the process according to the present invention it is possible to use the apparatuses and the devices commonly used in the field of the processes for treating the surfaces of iron and steel products, in particular processes for treating the surfaces of steel.

In order to better understand the features and advantages of the process according to the present invention, some embodiments will be described below with reference to the accompanying drawings. The following examples of embodiment are provided for the sole purpose of illustrating the present invention and are not to be understood in a sense limiting the scope of protection defined by the appended claims.

With reference to Figure 1, a system for pickling and/or passivating a stainless steel, for example in the form of a strip of a steel laminate, comprises a series of treatment solutions 10, 20, 30 and 40, each contained in a corresponding tank, inside which the strip 1 is made to advance along a direction and in the advancement direction indicated by the arrow F. The position of the strip 1 in the treatment solutions 10, 20, 30 and 40 is determined by the rollers 81 and 82, any other intermediate rollers positioned between the tanks or inside the tanks themselves (not shown in the figures) and by the pull applied to the strip. The treatment solutions are made to circulate inside each bath, for example by means of recirculation pumps (not shown in the figures).

In general, in the bath 10 the strip 1 is subjected to a preliminary chemical or electrochemical descaling treatment.

The strip 1 leaving the treatment solution 10 is then subjected to a rinsing treatment with water in the treatment solution 20 in order to eliminate the residues of the treatment solution 10 which may be present on the surface. The strip 1 can also be subjected to brushing.

The strip 1, after the preliminary descaling treatment, is then subjected to the pickling and passivation process according to the present invention in the treatment solutions 30 and 40. For this purpose, the strip 1 is placed in contact with a first treatment solution 30 comprising HF and iron ions. Subsequently, the strip 1 is placed in contact with a second treatment solution 40 also comprising HF and iron ions. The first 30 and second 40 treatment solution have chemical composition, redox potential and pH according to the present invention.

When the first treatment solution 30 is exhausted, a portion 32 thereof is fed to a regeneration system 90 to produce a regenerated aqueous solution of HF 34. The regeneration system 90 comprises a spray-drying unit 50, a water absorption unit 60 and a pyrolysis unit 70.

The solution to be regenerated 32 is fed to the spray-drying unit 50 to obtain HF in gaseous form 55 and dried metal salts 56. The gaseous HF 55 is separated from the dried metal salts 56 and sent to the absorption unit 60 (e.g. a water absorption column) where a regenerated aqueous solution of HF 34 is produced. The dried metal salts 56 are fed to the pyrolysis unit 70 to obtain metal oxides 75 and further acid in gaseous form 76, which can be sent to the absorption unit 60 to produce further regenerated solution of HF 34. To compensate for the removal of the aliquot 32 of the first treatment solution 30, a portion 35 of the second treatment solution 40 is transferred to the tank containing the first treatment solution 30, where it is mixed with the remaining portion of the first treatment solution 30. This can be achieved through at least one duct in hydraulic communication between the two baths, for example by gravity or through active means, such as pumps and/or bleeding from the recirculation systems of the treatment solution.

The regenerated aqueous solution of HF 34 is recycled to the tank containing the treatment solution 40 to restore or maintain the desired operating conditions for the second treatment solution 40. A part 36 of the regenerated aqueous solution of HF can also be sent to the tank containing the treatment solution 30 to maintain or restore the HF and iron ion concentration conditions in this first solution.

A second embodiment of the process according to the present invention is exemplified in Figure 2. This embodiment is particularly suitable for treating steel laminates obtained by hot rolling.

The strip 1 is subjected to a preliminary descaling treatment. In the case of laminates which have undergone hot rolling processes, the descaling treatment can be advantageously carried out in an aqueous treatment solution 1000 comprising HC1 and iron ions (descaling solution). Preferably, the descaling solution 1000 comprises: - at least 30 g/1 of free HC1,

- total HC1 not higher than 200 g/1,

- 30-65 g/1 of Fe(III) ions, - 5-60 g/1 of Fe(II) ions.

Preferably, the descaling solution 1000 has a pH less than or equal to 1.

With respect to the sequence of operations described with reference to Figure 1, the sequence of Figure 2 differs mainly in that it provides for a step of regeneration of the exhausted descaling solution 1000 and for recycling the regenerated acid to the same solution, in addition to the regeneration of the treatment solution 30 containing HF and iron ions and relative recycling to the treatment solution 40.

When the descaling solution 1000 is exhausted (for example, when the concentration Fe _tot in the treatment solution 1000 exceeds a value in the range 70 - 100 g/1), a part 320 of said solution 1000 is fed to a regeneration system 94 to produce a regenerated aqueous solution of HC1 340. The regeneration system 94 comprises a spray drying unit 500, a water absorption unit 600 and a pyrolysis unit 700. The solution to be regenerated 320 is fed to the spray-drying unit 500 to obtain HC1 in gaseous form 550 and dried metal salts 560. The gaseous HC1 550 is separated from the dried metal salts 560 and sent to the absorption unit 600 (e.g. a water absorption column) where a regenerated aqueous solution of HC1 340 is produced. The dried metal salts 560 are fed to the pyrolysis unit 700 to obtain metal oxides 750 and further acid in gaseous form 760, which can be sent to the absorption unit 600 to produce further aqueous solution of HC1.

The regenerated aqueous solution of HC1 340 is recycled to the treatment solution 1000. If the descaling treatment is carried out with a single descaling solution 1000, said descaling solution is preferably regenerated with a batch process. For this purpose, it is possible to advantageously provide for the arrangement of one or more tanks for storing (not shown in the figure) the exhausted solution 320 before it is sent to the spray drying unit 500, and one or more tanks for storing (not shown in the figure) the regenerated aqueous solution of HC1 340 leaving the absorption unit 600, so as to be able to discharge the exhausted descaling solution 1000 and replace it with the regenerated solution 340, when desired.

The strip 1 leaving the descaling treatment in the treatment solution 1000 is sent to the successive tanks containing the treatment solutions 20, 30 and 40, where it undergoes the treatments previously described with reference to Figure 1.

A third embodiment of the process according to the present invention is exemplified in Figure 3. With respect to the sequence of operations described with reference to Figure 2, the sequence of Figure 3 differs mainly in that it provides for carrying out a preliminary descaling step in which at least two treatment solutions 1000 and 1001 are used in sequence, each containing HC1 and iron ions, operating under ion concentration conditions and redox potential selected in accordance with the present invention.

The chemical descaling step comprises a first treatment of the strip 1 in the descaling solution 1000 (step al) and a second treatment in the subsequent descaling solution 1001 (step bl).

Preferably, the first descaling solution 1000 comprises:

- 35-60 g/1 of free HC1,

- 30-65 g/1 of Fe(III) ions,

- 20-60 g/1 of Fe(II) ions Preferably, the second descaling solution 1001 comprises:

- 60-100 g/1 of free HC1,

- 30-50 g/1 of Fe(III) ions,

- 10-40 g/1 of Fe(II) ions. In both solutions, preferably the pH is less than

1.

The aforesaid steps al and bl are carried out by keeping the weight ratio between the Fe ^III /Fe ¹¹ ions of the second descaling solution 1001 at a value higher than the ratio between the Fe ^III /Fe ¹¹ ions in the aforesaid first descaling solution 1000. When the first descaling solution 1000 is exhausted, for example when the total concentration of Fe ¹¹¹ and Fe ¹¹ ions is higher than 70 g/1, a part 320 of said solution is taken and fed to a regeneration system 94 to obtain a regenerated aqueous solution of HC1 340. The regeneration system 94 can be substantially the same system described above with reference to Figure 2.

In the sequence of Figure 3, a part 350 of the second descaling solution 1001 present in the bath 1001 is taken and transferred to the descaling solution 1000 to replace the part 320 of said first descaling solution sent for regeneration.

A part 360 of the regenerated aqueous solution of HC1 can also be sent to the descaling solution 1000 to maintain or restore the HC1 and iron ion concentration conditions in this solution. The strip 1 leaving the descaling treatment in the treatment solutions 1000 and 1001 is sent to the successive treatment solutions 20, 30 and 40, where it undergoes the treatments previously described with reference to Figure 1.

The embodiments illustrated with reference to Figures 1 - 3 can also be used in plants which process cold rolled products or combined plants which process both hot rolled and cold rolled products. With reference to Figure 2, in the case of plants for cold rolled products or combined plants for cold and hot rolled products, the preliminary descaling treatment in the treatment solution 1000 can be advantageously carried out, according to the prior art, by electrolytic treatment in an aqueous treatment solution comprising sulphuric acid at low concentration (for cold rolled products, the sulphuric acid concentration is preferably in the range 40-100 g/1; for hot-rolled products, the sulphuric acid concentration is preferably in the range 100-250 g/1) at a controlled temperature (40-80 °C).

Preferably, the pH of the aforesaid descaling solution is kept at a value equal to or less than 3. The aforesaid concentration and pH conditions of the electrolytic descaling solution favour turning the hexavalent chromium dissolved by the scale of the treated material into trivalent chromium.

The descaling solution 1000 containing sulphuric acid and iron ions, when exhausted, can also be subjected to regeneration and the regenerated acid solution can be recycled to the same descaling step. The regeneration can be carried out by pyrolysis, with systems known in the art similar to those illustrated above. Alternatively, at least the recovery of the free acid from the exhausted solution can be carried out, for example by means of ion-delayed resin systems, optionally preceded by scale decantation systems. The strip 1 leaving the descaling treatment in the treatment solution 1000 is sent to the successive tanks containing the treatment solutions 20, 30 and 40, where it undergoes the treatments previously described with reference to Figure 1.