ITVA970010A1 - PICKLING PROCESS OF STAINLESS STEELS WITH CONTINUOUS EXTRACTION FROM THE PICKLING SOLUTION OF THE EXCESS BIVALENT IRON - Google Patents

Description

"PROCESS OF PICKLING OF STAINLESS STEELS WITH CONTINUOUS EXTRACTION FROM THE PICKLING SOLUTION OF EXCESS BIVALENT IRON AND OTHER DISSOLVED METALS"

In the stainless steel production and processing industry, pickling / passivation baths are widely used.

These baths make it possible to remove the oxidized and decromized layers that are formed on the surface of the material following its hot transformations and jointly to form a homogeneous and protective layer of chromium oxide.

Industrial practice traditionally involved the use of aqueous mixtures of nitric acid and hydrofluoric acid.

The choice of this mixture was dictated by the need to have a strong mineral acid capable of attacking the material, an oxidant capable of maintaining the potential of the solution within the passivity range of stainless steel and a complexing agent for iron. dissolved.

The nitric / hydrofluoric mixture met this need since nitric acid is a strong, oxidizing mineral acid and hydrofluoric acid is a good complexing agent for iron.

With the development of more precise attention to protecting the environment and the operator, the regulations relating to emissions into the atmosphere and discharges into water have become increasingly restrictive.

This "tightening" of the limits imposed has made the use of nitric hydrofluoric mixtures more and more expensive, requiring the use of technologies to contain the emissions of nitrogen oxides into the atmosphere (generated by the reduction of nitric acid) and nitrates / nitrites in the waters (coming from the exhausted baths, from the rinsing water and from the abatement towers)

The increasing cost of pickling and the difficulty of meeting the requirements of the most stringent regulations in force have in recent years promoted the development of alternative and nitric acid-free pickling mixtures.

These new blends have already been well developed and currently around 20% of stainless steel produced in Europe is pickled without the use of nitric acid.

Most of these new pickling blends are based on the replacement of nitric acid with trivalent iron.

Trivalent iron carries out its pickling action by solubilizing the metallic iron and bringing it to bivalent iron.

In the reaction, the trivalent iron is also reduced to bivalent according to the scheme:

2Fe3 + Fe ° -> 3Fe2 + (1) The pickling mixture based on hydrofluoric acid and trivalent iron often uses together also strong mineral acids, mainly sulfuric, which allow to have a higher concentration of H +, to increase the potential redox of the solution and to increase the solubility of the iron reducing the danger of crystallization and / or formation of sludge.

Various methods of reoxidation of divalent to trivalent iron are known for the regeneration of the oxidizing power of the solution. Some of these use air mixed with hydrogen peroxide (French patent FR-A-2587369), others hydrogen peroxide (Italian patent N .: 1.246.252, international patent application WO-A-91 -05079, English patent GB-A- 2000196, Japanese patent JP-A-57035686) and yet another oxygen-on-bed-catalytic (European patent application No. 96830112.7).

Other proposed solutions based on the use of different oxidants such as permanganates, organic peroxides, persulfates, perborates, or electrochemical types, etc., have not found industrial application for different reasons and in any case difficult to reconcile with the need to maintain the cost of recovery. commonly acceptable.

Therefore, in parallel with the studies aimed at the elimination of nitric acid, numerous efforts have also been made in order to reduce pickling costs, to reduce purification problems and to guarantee a constant pickling result.

Different technologies have been proposed and implemented, originally studied for nitric / hydrofluoric baths and then used as a result in the new baths based on sulfuric and hydrofluoric acids and trivalent iron.

Let's briefly recall the most popular recovery technologies currently.

ION DELAY

It involves the elution of the pickling solution on a particular chromatographic resin.

On this resin a separation of the metal salts from the acids is obtained thanks to the different retention power.

In particular, the metallic salts pass through the chromatographic bed in a faster time than the free acids.

By appropriately calculating the speed and retention times, it is possible to discharge the salts and keep the acids in the resin bed, which are then recovered with a flow of water.

This technique has as a limitation the fact that only the free acids are recovered and that furthermore this recovery is not total since there is still some free acid in the stream of the discharged metal salts.

ELECTRODIALYSIS

Although the technique is different from ionic delay, the results and limits are similar.

Through the electrodialysis cells it is in fact possible to recover only and only part of the free acids by discharging the metal salts.

DIALYSIS BY DIFFUSION

It is another type of membrane process that uses the difference in acid concentration between two compartments of a cell separated by an anionic membrane as a "driving force".

The negative ion of the acid (F- NO3- 0 S042-) permeates through the membrane and, to ensure electroneutrality, carries the hydrogen ion to form acid.

The metal cations are instead stopped by the membrane and discharged as salts.

Once again it is a technique that allows to recover only and only part of the free acids.

SOLVENT EXTRACTION

A technique proposed by the German company KERAMCHEMIE involves the extraction of free acids, and their subsequent stripping, with tributyl phosphate.

The limitation of the recoverability of only the acids remains and the process is rather complex.

ROASTING

A process developed and proposed by the German company RUTHNER involves placing the pickling solution in a reactor operating at over 500 ° C.

In this reactor the metal salts are pyrolyzed to form pellets of metal oxides and gaseous acids which join the free acids already gasified to be absorbed with water and recovered during pickling. This is a complete technique which however has relatively high investment and management costs due to the need to operate at high temperatures with highly aggressive solutions and gases.

DISTILLATION

It involves the addition of sulfuric acid to the pickling bath and the distillation and recovery of nitric and hydrofluoric acids in the pickling.

However, technical (evacuation of the concentrate, absorption of gases, etc.) and economic problems have in fact limited the diffusion of the technology.

To better define the field of application of! process object of the present invention it is useful to remember what are the prevailing conditions by limiting the consideration to the new pickling mixtures based on hydrofluoric acid, and possibly sulfuric or other non-oxidizing acid and trivalent iron salts, since the invention is particularly effective in the treatment of the solution and resulting from these pickling processes.

To simplify the analysis, remember that stainless steels are mainly composed of an alloy of Fe, Cr, Ni (austenitic) or Fe, Cr (ferritic and martensitic). These must be pickled mainly when they undergo heating (due to heat treatment, welding, etc.) which causes the formation of an oxide layer.

Analyzing the composition of the alloy starting from the surface of the product, one will find a layer of chromium-rich oxide, an underlying layer of chromium-poor steel (decromized layer) and, finally, the steel with its massive composition.

The difference in the chromium contents derives from the migration of this towards the oxide layer during the period in which the material is at a high temperature.

Therefore an effective pickling solution must:

remove the oxide;

solubilizing the decromized layer;

passivate the surface as soon as the layer is reached

massive.

The oxide is removed with any pickling mixture

it is used, for partial dissolution and for partial detachment.

The fraction of complex chromium oxides is in fact unassailable - even by sulfuric / hydrofluoric solutions.

The main dissolution reactions can be like this

schematized:

Where the H + ion can come from sulfuric acid or from acid

hydrofluoric.

The main dissolution reactions of the decromized layer

instead, they provide for the following mechanisms:

To ensure the continuous conduct of pickling it is necessary

therefore restore the consumed H + ions and Fe3 + ions.

As regards the H + ions, an addition of fresh acids is carried out

while as regards the Fe3 + ions these are reformed

reoxidizing the Fe2 + ions generated.

As anticipated, oxidation is generally obtained using oxygen or hydrogen peroxide according to the reactions:

As a consequence, in addition to the consumption of oxidant, there is an increase in ferric ions in solution and, also considering the initial reactions, an increase in nickel and chromium ions.

The increase of metals in solution is proportional to the composition of the alloy as regards iron and nickel and is much lower (around 30 than the content present in the alloy) as regards chromium for the reasons highlighted above.

Although pickling baths that use sulfuric and hydrofluoric acids and trivalent iron show a certain constancy of performance in a wide range of metal concentrations, for extreme values (trivalent iron below 25 g / l or above 80 g / l) yes has a marked decay of pickling speed.

The user therefore has two possibilities:

a) Discharge part of the bath once the concentration of metals considered critical for the type of processing has been exceeded.

b) Discharge continuously a stream of solution in order to maintain the concentration of metals constantly at the optimal value.

In both cases, however, a wastewater is generated with the following simplified qualitative composition:

This wastewater must undergo purification before it can be discharged.

The purification phase is generally carried out with lime, which transforms metals into hydroxides, anions into calcium salts and neutralizes free acidity.

While generally there are no problems in respecting the discharge limits relating to the concentrations of SO42- and metals, the situation often changes when considering fluorides.

From the above, it is evident the need or the usefulness of reducing the cost of the pickling treatment intended as consumption of oxidant and consumption of acid in maintaining a constant effectiveness of the pickling solution in which there is a constant supply of divalent iron. and other metal cations deriving from the etching and dissolution reactions of the surface layers of the treated pieces.

A process has now been found and constitutes the object of the present invention which is capable of grasping the aforementioned important objective of a marked reduction in the consumption of acids and oxidants in a way that is as simple and requires a relatively modest investment as it is surprisingly effective in ensure a clear reduction in costs deriving from a corresponding reduction in the consumption of acids and oxidants.

To these surprising results, the invention adds further advantages in terms of allowing the relatively simple and inexpensive recovery of precious metals, separately from iron, benefiting through these possible recovery operations from a further reduction in waste treatment costs.

It has been found that it is possible and economically advantageous to carry out in a continuous way a sequestration treatment of metal cations from the solution of the pickling bath, contacting it with a cationic resin and adjusting the flow and / or the contact time so as to sequester the cations. metals to an extent sufficient to reduce by a value between 40% and 20% the divalent iron content that is generated during pickling both by reduction of ferric and by dissolution of the treated material, in so doing, in addition to controlling the accumulation of metals in solution in the pickling bath, the consumption of oxidant, for example of hydrogen peroxide, necessary for the reoxidation of divalent iron to trivalent iron, as well as the consumption of acids, typically hydrofluoric acid and / or sulfuric acid, are significantly reduced. add to the pickling solution to ensure the formation of fluoferric ion complexes and / or maintain the pH of the solution in ac predetermined amplitude which can generally range from 0.5 to 2.5, deriving from keeping the concentration of divalent iron and other metal cations in the solution substantially constant at the optimum level.

The percentage range of bivalent iron sequestration indicated above is instrumental to fully grasp these advantages of reducing consumption, according to the operating characteristics of the pickling process, according to the analysis reported above and relating to recurring situations in the context of this type of treatment of commercial alloys. Naturally, a maximization of the achievable savings is possible by monitoring the solution of the pickling bath and consequently adjusting the elution parameters on the cationic resin of part of the divalent iron together with the other metal cations, so as to optimize the process as a whole.

The necessary reoxidation of divalent iron to trivalent iron which is constantly consumed as an oxidant! pickling and passivation process, can be carried out on the same solution flow conducted through the elution tower on cationic resin, both upstream and downstream of this, or alternatively be carried out in parallel on a portion of recirculating flow of the pickling solution.

It has been found that a cationic resin with polystyrene polystyrene structure cross-linked with divinylbenzene, in which R-SCV type functional groups are present, is perfectly suitable.

Naturally, the cationic resin bed must be periodically regenerated by washing the resin with a strong acid solution, for example hydrochloric acid or sulfuric acid, recovering a washing solution containing the corresponding salts of the sequestered metal cations.

For this need, two or more elution towers can be used, cyclically put into operation while the resin of the one just isolated from the recirculation circuit of the pickling solution is regenerated by washing with strong acid followed by appropriate rinsing with water.

According to a further important aspect, the process of the invention lends itself to integrating an effective and simple recovery process of precious metals in separate form with respect to iron, as well as to reducing the quantity of fluoride ions in the wastewater as will be described in more detail below. this description.

In light of the foregoing, let us now move on to introduce the innovation covered by this patent which allows for the achievement of important advantages such as:

- Reduction of the pickling cost, understood as the consumption of acids and oxidants

- Maintenance of a pickling solution with constant efficacy - Reduction of fluoride ions in the exhaust

To illustrate the mechanisms of the process of the invention to achieve these surprising results, it is useful to consider in what form the ions in solution are presented.

By analyzing the solutions and constants of formation of the complexes, it can be seen that:

- The trivalent iron ions form anionic and neutral complexes with the fluoride ion such as:

- The divalent iron and nickel ions are present in free form of cations:

- The trivalent chromium ions are partly complexed with the fluoride ion and are partly also free as cations

According to the essential aspect of the process of the invention, the pickling solution is recirculated through a cationic resin capable of blocking the nickel, the divalent iron and the free cations of the trivalent chromium, returning the H + ions:

Naturally, not all the divalent iron present in solution, which is formed continuously as a result of the dissolution reaction of the metallic iron of the alloy, must be separated as it is always necessary to ensure the presence of a sufficient quantity of trivalent iron for a correct pickling action.

To better clarify the above concept, we consider, by way of example, that the only reaction of iron intake is 7), which we report:

Considering that of the three divalent iron ions formed, one is eliminated from the solution by the cationic resin and two must be reoxidized to trivalent iron:

In practice, a net saving of about a third of oxidant is obtained despite the fact that in reality the account is complex when other reactions from 2) to 9) come into play. However, these reactions as a whole tend to compensate for each other, maintaining the saving in terms of oxidant consumed in the order of one third.

The same order of magnitude of the savings obtained on the oxidant is obtained on the consumption of acid in fact, as seen, in the reoxidation reactions 10) and 11) there is always a consumption of acidity:

Furthermore, it should be remembered that the formation of a trivalent iron ion causes in parallel the "consumption" of 3 - 6 fluoride ions that are used to form the fluoferric complexes, therefore the correct oxidation reactions can be represented as follows

The metals blocked on the resin bed in the R - Me form are then easily recovered as salts by eluting the column with a strong acid solution, for example hydrochloric or sulfuric acid.

The salts of these metals can be recovered individually.

In summary, the process of the invention allows to keep the pickling solution constantly efficient through the constant maintenance, at the optimal levels, of the concentration of ferric ions so as to guarantee a correct REDOX potential of the pickling solution included in the range from about 200 mV to about 500 mV, while keeping the concentration of free metal cations in the solution low and constant, including the excess of divalent iron resulting from the dissolution of the treated alloy. Maintaining the concentration of ferric ions at optimal levels corresponds to an optimization of the consumption of oxidant used to reoxidize the divalent to trivalent iron.

The consumption of hydrofluoric acid in particular is drastically reduced until it is equal to the fraction lost by evaporation and entrainment, by virtue of the fact that the solution must never be replaced and all the free, bound and complexed hydrofluoric contained in the solution is fully recovered.

These and other advantages will become more evident through the exposition of some significant examples of embodiment of the invention.

Figure 1 shows the scheme of the regeneration cycle of the pickling solution according to the present invention;

Figure 2 is an alternative scheme for implementing the process of the invention;

Figure 3 is an overall scheme which also includes stages of separation and recovery of precious metals.

EXAMPLE 1

4.4 It of solution taken from a pickling bath operating on a strip line having the following composition was percolated onto a 92.5 cm high resin column for a total volume of 0.5 It:

Free HF 30.00 g / lt

Total F- 60.00 g / lt

Total Fe 28.70 g / lt

Trivalent Fe 23.90 g / lt

Bivalent Fe 4.80 g / lt

Bivalent Ni 4.24 g / lt

Trivalent Cr 3.58 g / lt

Fractions of 500 ml of the eluted solution were periodically taken on which quantitative analyzes were carried out, the results of which are reported in the following table

The resin was then washed using 1.8 It of running water. The analysis of the rinsing waters revealed the following composition:

Total Fe 3,240 g

Trivalent Fe 3.190 g

Bivalent Fe 0.046 g

Bivalent Ni 0.259 g

Trivalent Cr 0.390 g

Then the real regeneration of the resin was carried out using 1.8 It of a 6% solution of HCI.

The analysis of this solution gave rise to the following results:

Bivalent Fe 13.50 g or 0.484 eq

Bivalent Ni 13.00 g or 0.443 eq

Trivalent Cr 0.40 g or 0.023 eq

For a total of 1.9 eq metal / liter resin

The operation was repeated 10 times to ensure that there was no impoverishment in the quality of the separation.

The resin was then placed in contact with the solution at 80 ° C for 15 days and the test was repeated, finding similar results and therefore demonstrating the compatibility of the resin with the solution.

EXAMPLE 2

Experimentation similar to that of example 1 was performed with a solution having the following composition:

Free HF 30.0 g / l

Free H2SO4 30.0 g / l

Total Fe 35.0 g / l

Bivalent Fe 5.2 g / l

Trivalent Fe 29.8 g / l

Bivalent Ni 5.1 g / l

Trivalent Cr 4.7 g / l

In this case, the retention capacity of the resin was equal to approx. 1.5 eq / lt proving that this is only slightly influenced by the presence of modest concentrations of strong acids.

However, the efficiency calculation remains very positive in terms of the cost-effectiveness of the process.

EXAMPLE 3

Experimentation similar to that of examples 1 and 2 was carried out with a solution with the following composition:

Free HF 30.0 g / l

Free H2SO4 150.0 g / l

Total Fe 36.0 g / l

Bivalent Fe 5.2 g / l

Trivalent Fe 30.8 g / l

Bivalent Ni 5.1 g / l

Trivalent Cr 4.7 g / l

In this case, the retention capacity of the resin was much lower and equal to approx. 1.0 eq / l, however, no fluoride ions were retained.

The operator therefore faces two choices, namely to use a larger plant or to reduce the concentration of free acids by means of one of the many known techniques, commonly used for this purpose.

EXAMPLES OF ECONOMIC CALCULATION

As a practical example, we report a strip pickling line that has a fully operational productivity of 100,000 tons / year, that is of medium type.

An “ionic delay” unit is already installed on the line in question which allows, as described in the introductory part, to recover most of the free acids contained in the wastewater at the discharge.

The calculation of savings conducted by comparison with a line in which there was no recovery technology would obviously become even more significant.

The following calculations were made on a recent annual balance sheet.

PRODUCTION LINE DATA

- MATERIAL TYPE:

QUALITY: Austenitic ca. 95%

Ferritics approx. 5%

ORIGIN: Hot rolled ca. 60%

Cold rolled approx. 40% AVERAGE PRODUCTIVITY: 14 tons / hour

- TREATMENT CYCLE

ANNEALING OVEN TANK 1 PRE-PICKLING WITH H2SO4 ELECTROLYTIC TANK 2 PRE-PICKING WITH H2SO4 ELECTROLYTIC TANK 3 PICKLING / PASSIVATION WITH HF / Fe TRIVALENT TANK 4 PICKLING / PASSIVATION WITH HF / Fe TRIVALENT - COMPOSITION TANKS

Free HF from 5 to 50 g / lt

Total metals from 5 to 60 g / lt depending on the processed materials, kept constant through ionic delay

Approximate subdivision of metals Fe / Cr / Ni = 80/10/10

DATA COLLECTED DURING THE YEAR RELATING TO TANK 3 E

TO THE TANK 4

QUANTITY OF PICKLED: Approx. 85,000 tons

FLUORHYDRIC ACID CONSUMPTION: Approx. 250 tons at 40%

FRACTION HF LOST IN THE WASHING

EVAPORATION: Approx. 30 tons at 40%

QUANTITY OF DISSOLVED METALS (Fe Cr Ni): Approx. 140 tons

AVERAGE COST HF AT 40%: Lit.715 / Kg

We reasonably consider a loss of 10% of the 220 tons of hydrofluoric acid discharged (250 - 30 = 220 tons at 40%) we can quantify the acid recovery value around 140,000,000 Lit / year

The recovery of the acid also makes it possible to reduce the quantity of sludge by a value equal to approx. 320 ton derived from the following calculations:

Since the produced mud possesses approx. 60% of water, the real quantity of sludge formed in relation to CaF2, neglecting the excess lime necessary to respect the limits, will be equal to:

On the other hand, using the process of the invention, for the regeneration of the resin, approx. 420 tons of hydrochloric acid at 35% which, given a value of 100 Lit./Kg, have a cost of approx. Lit.40,000,000.

The count was made by calculating a stoichiometric consumption of HCi equal to the quantity of HF recovered:

Summarizing the process of the invention, it will allow for annual savings to be estimated at 100,000,000 Ut. and in parallel to avoid the production of approx. 300 tons of mud and to make the purification phase easier and safer.

The unit itself should allow the removal of approximately 20 kg / h of metal.

Considering the absorption capacity of the resin equal to 1.9 eq / lt and considering an average PE (equivalent weight) of the removed metals equal to 27, 390 It of resin will be needed for each hour of absorption calculated as follows:

Assuming to perform two resin regenerations per day, a plant containing approx. 4,700 It resin.

At present, it is believed that the optimal geometry of the plant includes three columns, two of which are in series operation and one in regeneration and in stand-by.

This allows to fully exploit the resin bed before it is regenerated by adopting the following cycle:

1) Columns in work 1 -> 2

Columns in regeneration 3

2) Columns in work 2 -> 3

Columns in regener. 1

3) Columns in operation 3 -> 1

Columns in regener. 2

4) Start over from 1)

Therefore, assuming 3 columns of 2,000 It of resin, you will have approximately the following investment costs

As can be seen, the investment cost is surprisingly comparable with the savings that can be achieved annually. The results of similar evaluations performed on a number of different metallurgical processes are shown in the following table.

According to a particularly advantageous embodiment of the invention, schematically exemplified in Figure 3, the regeneration washing of the resin is performed with a dilute solution, for example at 6-10%, of hydrochloric acid. The washing and possibly rinsing solution of the resin is then sent to an oxidation stage operating according to any of the known methods mentioned in the preamble of this description, such as for example that of carrying out a reoxidation with air or oxygen on a catalytic bed, such as described in European patent application No.

96830112.7, so as to substantially oxidize all the divalent iron present in the solution.

The resulting solution will still contain free cations of the other metals present in the treated steel and typically cations such as: Ni2 + Cr3 + and l-f, together with chlorine-ferric anionic complexes, FeCL4 and Cl '.

In practice, using a hydrochloric solution, most of the iron present in solution and reoxidized to ferric is complexed by chloride ions forming anionic complexes.

The washing solution thus reoxidized is contacted on a further elution tower on cationic resin so as to absorb the cations of the other metals (Ni2 +, Cr3 +) separately from the complexed trivalent iron which can be sent with the eluted solution to a treatment station and recovery.

The periodic washing of the resin with hydrochloric acid or sulfuric acid produces a washing solution containing cations of precious metals such as nickel and chromium, which are easily recoverable, even separately, through a precipitation stage conducted at different pH values, by dosing caustic soda or sodium carbonate to solution, according to the flow chart of figure 3.

Ferric chloride can also be recovered and marketed as a coagulant for purification plants.

The use of hydrochloric acid for washing the resin, in addition to allowing an easy separation of iron from other recoverable metals, also adds the advantage of reducing the quantity of sludge produced in the purification of wastewater avoiding the formation of insoluble calcium salts ( such as fluorides and sulphates), using hydrochloric acid, however, it becomes essential to perform a complete rinse of the resin bed at the end of the regeneration phase and before putting the resin back into operation as even modest quantities of residual hydrochloric acid could compromise the surface quality of pickled steel.

It is evident that the invention allows to ensure the maintenance of a constant and optimal concentration of the trivalent iron ion and to eliminate, as they dissolve, the other metal cations, as well as the excess of divalent iron, which having no useful function in the pickling process, in fact they reduce the pickling power of the solution. The process of the invention lends itself to automatic management of the entire process using automatic stations for analyzing the solutions and dosing and flow regulation systems, managed by a control processor.

A clearly possible refinement of the process of the invention is that of using selective cationic resins so as to optimally manage the extraction from the pickling solution of excess metal cations and their recovery in a separate form from each other during the treatment of the solution for regenerating (washing) the resin or resins.

Claims (5)

CLAIMS 1. Process of regeneration of an acidic pickling solution of stainless acids containing ionic complexes of fluoferric and divalent iron cations, and of other metals contained in stainless steel, comprising an oxidation step of divalent iron to trivalent iron for an amount sufficient to maintain the REDOX potential of the solution in a predetermined range and periodic additions of hydrofluoric acid and / or another mineral acid to maintain the pH value of the solution in a predetermined range and operations to remove excess divalent iron cations from the solution and of the cations of said other metals deriving from the dissolution of the treated steel, characterized in that it comprises the following operations continuously contacting said pickling solution with a cationic resin in an elution tower through a bed of said resin, adjusting the solution flow rate through said resin bed and the contact time so as to sequester said metal cations to a sufficient extent to reduce the bivalent iron content produced in the solution during pickling by a value between 40% and 20%; regenerating the sequestering properties of the resin by periodic washing with strong acid, recovering the corresponding salts of said metal cations sequestered by the resin. 2. Process according to claim 1, characterized in that said reoxidation of the divalent iron to trivalent iron in said pickling solution is carried out on the same stream of pickling solution that passes through said elution tower on resin, upstream or downstream of this one. uitima; in the case of reoxidation upstream, said reoxidation of the divalent iron to trivalent iron in the solution, occurring to a limited extent for a portion between 80% and 60% of the entire divalent iron content, the flow recirculating through a pickling bath. 3. Process according to claim 1, characterized in that said reoxidation of the divalent iron to trivalent iron is carried out on a flow of pickling solution parallel to the flow passing through said elution tower on resin, both flows recirculating through a pickling bath. 4. Process according to claim 1, characterized in that said strong acid for washing the resin is HCl. 5. Process according to claim 4, characterized in that the hydrochloric solution for washing the resin is subjected to a step of oxidation of the divalent iron to trivalent iron, generating anionic chloroferric complexes in the solution; the process further comprising a step of contacting the solution containing said chlor-ferric complexes and cations of said other metals on cationic resin suitable to sequester said metal cations of metals other than iron, present in the solution in the form of anionic complexes and a recovery step of said other metals from the acid washing solution of the resin in the form of the respective hydroxides by treating the washing solution with caustic soda and / or sodium carbonate,