MXPA97000479A - Process to electrochemically dissolve a metal such as zinc or est - Google Patents

Abstract

The object of the invention is to provide an improved electrochemical dissolution process for electrochemically dissolving a first metal created simultaneously evolution of hydrogen in a second metal, the second metal is a metal having a higher current exchange density for evolution of hydrogen than The first metal, both metals are immersed in an aqueous electrolytic system where the first metal and the second are galvanically coupled. By means of the application of measures to reduce the inhibition of the evolution of hydrogen in the second metal with which the speed of dissolution of the first metal is increased. The measures to reduce the inhibition adequately comprise selecting temperatures and concentrations of the electrolyte, dividing the electrolyte into two fluids coupled by a selectively permeable device, and suitably selecting the resistance value of a connecting means electrically connecting the first and second metals. The invention is particularly useful for removing ZnóSn from scrap steel containing Znó

Description

PROCESS TO ELECTROCHEMICALLY DISSOLVE A METAL LIKE ZINC OR TIN.

D E S C R I P C T ION TECHNICAL FIELD: The invention relates to a method for electrochemically dissolving a first metal such as zinc or tin by simultaneously creating evolution of hydrogen in a second metal and a use of that method for treating scrap and an improved method for removing steel scrap.

PREVIOUS TECHNIQUE.

Examples of industrial applications for dissolving a metal, such as Zn or Sn in an aqueous electrolyte, are for preparing a solution containing zinc or tin ions for purposes of electroplating or unsealing or unsealing scrap metal, especially steel scrap.

To prepare or replenish a solution for plating purposes a metal can be fed to an aqueous electrolyte in a substantially pure form and selective dissolution with respect to other metals is not necessary. To remove the metal from, for example, steel scrap, selective, selective dissolution is desirable in which only the metal or metals to be dissolved are those that dissolve, in order to be able to separate the metal and scrap from the metal. steel practically free of that metal.

As tin and zinc can be applied as a r layer on the steel by electroplating from an electrolyte and the resulting steel has to be recirculated, both of the aforesaid dissolution applications are particularly important for steel production and recirculation. Tinned steel is a widely used packaging material and galvanized steel is used in numerous product applications for example in automotive applications. In an electrochemical process for the selective dissolution of zinc and tin, these metals and also lead and aluminum can be separated from steel scrap, to thereby provide a rery of those metals as well as clean steel scrap that can be returned to reuse in steel manufacturing.

The need for highly efficient removal of especially scrap zinc from steel has increased lately due to the fact that the production of galvanized steel has increased enormously in the last 20 years, particularly in the construction industry and in the automotive industry as well as for household goods. In the life cycle sooner or later, this zinc-containing steel will form scrap that must be reprocessed in a steelmaking process for which only a limited scrap zinc content is conceivable or desirable.

In a known process for dissolving metals in an aqueous electrolyte and for preparing a solution for plating purposes, electrical energy is fed to the electrolytic process in order to obtain an acceptable dissolution rate. In order to obviate the electrodeposition of the metal that is dissolving in the aqueous electrolyte on the cathode, a membrane separating the anolyte and the catholyte can be installed.

In the processes of descaling or de-tinning of metal scrap which is carried out in an alkaline electrolyte for selectivity, the dissolution is carried out, for example without electric power supplied, making use of intrinsic galvanic coupling between different metals in the scrap in which case the dissolution rates are very low or the electrical energy is supplied from an external source such as a rectifier to the process.

THE INVENTION It is the object of the invention to improve the efficiency of the dissolution process by reducing the consumption of energy or materials and / or obviating the need to install electric power and appliances for direct current supply.

This objective is achieved using the method in accordance with clause 1.

In the present invention a galvanic process is used, this is a process in which the first and second metals are coupled without the external electric power being fed to the process. It is notable that a galvanic process itself is known and is known to have the disadvantage that the dissolution rate almost immediately after immersion falls to a very low level, this is as soon as the concentration of the first metal in the aqueous electrolyte begins to rise.

According to the invention, in a method using this new and inventive galvanic process, measures have to be taken to remove the evolution of hydrogen in the second metal, thereby increasing the speed of dissolution of the first metal, which measures they must be of a very simple nature.

In a preferred embodiment, the aqueous electrolyte is an alkaline solution. And here you get the advantage that in the case of processing steel scrap the steel base is passivated, in other words it does not dissolve. Also the parts of the equipment can be made using steel.

It is preferred that the alkaline solution has an alkalinity greater than 8 M, preferably greater than 9 M. The rate of dissolution is increased rapidly for hydroxide concentrations. This rapid increase is unexpected because the region of hydroxide concentrations of up to 7-8 M, the rate of dissolution increases disproportionately with an abrupt decrease.

It is preferred that the alkaline solution be maintained at a temperature above 340 K, preferably above 350 K. Above this temperature the dissolution rate is substantially increased.

Furthermore, it is preferred that the mechanical abrasion be carried out on the surface of the second metal, this also promotes the evolution of hydrogen.

The evolution of hydrogen is also promoted if powder of a second metal is added to the aqueous electrolyte surrounding the second metal, by stirring the aqueous solution containing the powder.

In an embodiment wherein the first metal is in the form of separate elements, the first and second metals are coupled by a current collector making contact with the first metal. The current collector then electrically connects the first and second metals. That current collector can be a metal box containing the electrolyte.

An active surface of Mg is advantageous if it is desired that hydrogen evolution occurs as a consequence of the presence of the current collector, since under these conditions evolution of hydrogen in Mg does not occur.

In a very interesting embodiment of the method according to the present invention, the first and second metals are galvanically coupled with connection means that provide an electrical resistance that is selected such that the current flow through the connection means is practically at the value maximum that is obtained by varying the resistance. Surprisingly, an optimum resistance value is not necessarily the minimum resistance of the connection means and can be selected for maximum current flow through the connection means, which maximum current flow corresponds to the maximum dissolvable speed obtainable. If the resistance is lowered from infinity to zero, the current flow through the resistance first rises as expected. However, surprisingly, if the resistance drops below a specific resistance value, the current flow unexpectedly drops. According to the invention, an optimum resistance value can be selected for maximum current flow and therefore for maximum dissolution rate.

In a particular embodiment, the electrical circuit comprising the connection means is periodically interrupted. In cases where the inhibition of hydrogen evolution develops at a lower speed than in its interrupted parts of the circuit, by means of adequate switching on and off of the galvanic process, a higher integral efficiency can be performed.

In a more preferred embodiment the aqueous electrolyte is divided into a first fluid contacting the first metal and a second fluid contacting the second metal, with the first and second fluids being coupled by a selectively permeable device that opposes the ion passage of the first metal to the second metal.

This measure results in a remarkable increase in the rate of dissolution.

The invention is advantageously used to remove a coating of a metal substrate, for example Zn or Sn from scrap metal, preferably steel scrap.

According to the invention, a known apparatus having first and second process volumes coupled by a device capable of opposing the passage of ions can be used advantageously for galvanic dissolution of Zn or Sn.

The invention also relates to a method for treating steel scrap containing zinc by electrochemical descaling in an alkaline solution in a first process and recovering the zinc in a second process, characterized in that the dezincing in the first process is performed galvanically, is say without external electric power supply. In this method considerable savings are achieved in that the de-zipping is done without the supply of external electric power.

It is believed that in a galvanic process, the inhibition of the hydrogen evolution reaction (HER) is caused by the occurrence of a phenomenon that can be called low potential position (UPD), which means that in a method of agreement with the preamble of clause 1, although the first metal will not form a massive deposit on the second metal, it tends to form a (sub-) monolayer on the surface of the second metal, which apparently opposes the evolution of hydrogen.

As the rate of dissolution and the evolution of hydrogen correspond, reducing the inhibition of the evolution of hydrogen in the second metal, according to the invention, the dissolution of the first metal can be promoted.

BRIEF DESCRIPTION OF THE DRAWINGS Reference is now made to the accompanying drawings, in which: Figure 1 represents the dissolution speed of a first metal, both in case the first metal is isolated and in the case where it is coupled to a second metal, which has a higher exchange current density for HER than the first metal.

Figure 2 depicts the dissolution rate of zinc intrinsically coupled to the steel as a function of the dissolved zinc (7.5 M NaOH, 298 K).

Figure 3 shows the dissolution rate of zinc, both isolated and intrinsically coupled to the steel, as a function of NaOH concentration (2 g l "dissolved Zn, 343 K).

Figure 4 depicts the dissolution rate of zinc intrinsically coupled to the steel as a function of NaOH concentration (2 g l "1 Zn dissolved, 298 K).

Figure 5 depicts the dissolution rate of zinc intrinsically coupled with the steel as a function of NaOH concentration (2 g l "1 dissolved Zn, 323 and 343 K).

Figure 6 depicts the dissolution rate of zinc intrinsically coupled to the steel as a function of temperature (7.5 M NaOH, 10 g l "of dissolved zinc).

Figure 7 depicts the dissolution rate of zinc intrinsically coupled to the steel as a function of temperature (2.5 M NaOH, 2 g l "of dissolved zinc).

Figure 8 depicts the dissolution rate of zinc intrinsically coupled to the steel as a function of temperature (7.5 M NaOH, 2 g 1 of dissolved zinc).

Figure 9 represents the linear scan voltammograms (scanning speed 1 mv s) of a steel electrode (7.5 M NaOH, 298 K, various amounts of zinc dissolved as indicated).

Figure 10 shows the inhibition factor as a function of dissolved Zn (7.5 M NaOH, 298 K).

Figure 11 represents voltammogra or cyclic (scanning speed 1 mVs *) of a steel electrode (7.5 M NaOH, 3 g l "1 dissolved, at 343 K).

Figure 12 depicts a schematic presentation of a coupled current experiment without a barrier between the anodic and cathodic compartment; the resistol in the external circuit was varied as indicated; anodic compartment (7.5 M NaOH, 5 g l "of dissolved Zn, 298 K), the cathode compartment equal to the anodic compartment.

Figure 13 shows the anodic current in EZn and the cathodic current in EFe in the case of the experimental embodiment as illustrated in figure 12.

Figure 14 represents a schematic presentation of a current experiment coupled with a barrier between the anodic and cathodic compartment; the resistol in the external circuit was varied as indicated; Anodic compartment (7.5 M NaOH, 5 g l "1 dissolved Zn, 298 K), the cathode compartment (7.5 M NaOH, 298 K).

Figure 15 represents the anodic current in E Zp and the cathode current in EFe in the case of the experimental embodiment as illustrated in Figure 14.

Figure 16 depicts the cathodic current as a function of EFß for current-coupled experiments with and without a barrier between the anodic and cathodic compartments.

Figure 17 is the same as Figure 16, but now the cathodic current as a function of EFe for the current experiments coupled with a barrier between the anodic and cathodic compartment, has been extrapolated to more negative potentials using the Butler- equation. Volmer Figure 18 represents the scratching effect of the surface of a steel electrode on the HER (7.5 M NaOH, 3 g 1 dissolved, 343 K).

Where all the referred potentials are measured against Ag / AgCl, KCl (saturated) as a reference electrode, which has a potential of 0.197 V against NHE (normal hydrogen electrodes).

The invention will now be demonstrated by means of non-limiting examples comprising results of experiments.

For spontaneous dissolution of a first metal, M is indicated, for convenience, in an aqueous electrolyte for it to occur, some requirements will have to be met, which are put in what follows.

First of all, the first metal M, will act as an anode: M,? M ^ + ne where n is the number of electrons per oxidized atom. Electrons are released as a result of the anodic reaction and are rapidly consumed in a corresponding cathodic reaction. The cathodic reaction in the highlighted invention is the hydrogen evolution reaction (HER): nH * + ne? l / 2nH2 (g) (acid solution) nH20 + ne? l / 2nH2 (g) + nOH (alkaline solution) Thus, both reactions proceed at the same time. In case of spontaneous dissolution, the reversible cell Ecell potential (open circuit) that is defined as the cathodic Ec potential minus the anodic Ea potential, will be positive. This case refers in general to a galvanic cell in contrast to an electrolytic cell. In case of an Ecell electrolytic cell < 0, a potential difference has to be applied, with the help of an external device, such as a rectifier, to force the electrode reactions to proceed in a direction opposite to their spontaneous tendencies. The invention relates to galvanic cells, so EcßU > 0. This condition is valid for both Zn and Sn and more generally for all metals in negative scale in the electrochemical series, in both cases an acidic environment and an alkaline environment. These metals will spontaneously dissolve, with which HER will be carried out simultaneously on its surface. However, the last reaction proceeds very slowly on both surfaces of Zn and Sn. Consequently, the HER determines the regimen of the entire reaction. In electrochemical terms the velocity of a particular electrode reaction is expressed by its exchange current density (symbol: i0). A "slow" electrode is characterized by a slow i0 (H20? H2). The HER on a zinc surface has an exchange current density in the order of l? "10" A / cm .. The dissolution rate of M, can be significantly increased by galvanic coupling of M, to a second metal M2 , having a higher exchange current density for HER than M1f such as Pt, Pd, Ir, Co, Ni and Fe or steel, in the case of M1 being zinc or tin. In the case of Fe, i0 (H20? H2"l?" 5"5 A cm" 2.

The galvanic coupling effect of M, to a foreign metallic M2 substrate is illustrated in figure 1. If M1 is immersed in an aqueous solution, will adopt a mixed potential, called the corrosion potential, at which the anodic current equals the cathodic current, which current is called the corrosion current but, because the corrosion implies an undesirable deterioration of a metal, here this current it will be referred to as a dissolving stream. In the case of M, coupled to M2, the mixed potential is shifted in a positive direction, which results in a larger dissolution current.

In order to study the effect of galvanic coupling on the dissolution rate, experiments were performed on galvanized steel on the one hand, in which case the galvanic coupling is intrinsic. As a reference material, galvanized steel on both sides and pure zinc have also been used in some experiments. Material samples for 6.5 x 5.5 cm 2 tests were prepared and exposed to sodium hydroxide solutions. The rate of dissolution was determined by weight loss experiments. The exposure time, amount of dissolved zinc, sodium hydroxide concentration and temperature were stranded. All the experiments were carried out at least twice. The dispersion in the numerical results was marginal.

EXAMPLE 1 Samples of galvanized steel test material on one side were exposed to a solution of 7.5 M sodium hydroxide at 298 K with a different amount of dissolved zinc. As can be seen in Figure 2, the dissolution rate of zinc becomes very slow drastically once a small amount of zinc dissolves. This effect is much larger than expected from the calculations. These calculations revealed that this effect can not be explained by assuming the Butler-Volmer kinetics, correcting for the displacement in the reversible potential of the redox pair Zn (OH) 4 / Zn, which was calculated with the Nernst equation.

EXAMPLE 2 In a further experiment, samples of galvanized steel test material on one side and on the sides as well as samples of pure zinc test material were exposed to sodium hydroxide solutions of different alkalinity. From figure 3, comparing the experiments on galvanized steel on both sides, that is to say without coupling with the steel, with the experiments on galvanized steel on the one hand, that is to say the coupling to the steel being intrinsic, it is observed that the speed of dissolution it is considerably increased by galvanic coupling to steel. Experiments were also carried out on pure zinc, which were very similar to the results obtained from experiments carried out on galvanized steel on both sides. Notably, it is the inflection point at higher sodium hydroxide concentrations. When the dissolution rate of zinc only increased slowly to a concentration of approximately 8 M, it suddenly and unexpectedly increases markedly at higher concentrations. In the case of galvanized steel on both sides, the rate of dissolution is almost independent of the concentration of sodium hydroxide. The remarkable increase in the dissolution rate of zinc at higher sodium hydroxide concentrations has invariably been reproduced under various experimental conditions (see Figures 4-5).

EXAMPLE 3 In a further experiment samples of galvanized steel test material on one side were exposed to sodium hydroxide solutions at different temperatures. From figures 6-8 it can be seen that the dissolution rate of zinc increases markedly at higher temperatures. Surprisingly the "Arrhenius" graph, Ln dissolution speed against (1 / T), does not give straight lines, but indicated that at higher temperatures the increase in resolution speed is much larger than expected.

EXAMPLE 4 In view of the experiments described above, the HER was studied in greater detail on a steel surface, since this reaction, as mentioned above, determines the rate of dissolution. A steel electrode was immersed in sodium hydroxide solutions, together with a Pt counter-electrode and a reference electrode of Ag / AgCl, KCl (saturated). Using a potentiostat, it was possible to control the potential of the steel electrode with respect to the reference electrode. The potential of the steel electrode was linearly varied over time with a scanning rate of 1 mV / s in the negative direction and simultaneously the current was measured. Figure 9 shows the linear scan voltammograms to various amounts of dissolved zinc. Clearly, the presence of only a small amount of dissolved zinc, already slows the HER on a steel surface drastically. Apparently a form of low potential deposition (UPD) of zinc on steel occurs, whereby a (sub-) monolayer of zinc is deposited on the steel. The degree to which HER is inhibited has been evaluated experimentally in various concentrations of dissolved zinc (Figure 10). 5 g / l of dissolved zinc inhibits the rate of dissolution by a factor of 150, at 298 K. Also, the coverage of the steel surface has been evaluated. The coverage here means the degree to which the active surface of the second metal is covered with ions of the first metal. A logarithmic relationship between the concentration of Zn (OH) "and the covering was found, which indicates that the UPN of Zn on the steel follows the Tem in adsorption isotherm.

EXAMPLE 5 In a further experiment, similar to that of Example 4, but now the potential of the steel electrode was linearly varied with time (scanning rate 1 mV / s), first in the negative direction, then back in the positive direction and simultaneously the current was measured. Figure 11 shows a so-called cyclic voltammogram of a steel electrode in a 7.5 M sodium hydroxide solution with 3 g / l of zinc dissolved at a temperature of 343 K. The hysteresis between the forward and backward exploration causes UPD to take some time to fully develop. This was also confirmed by multiple potential stage experiments, where the potential suddenly switched from the reversible potential to a potential within the UPD region, which invariably shows that it takes some time for the current to become stationary. This opens the opportunity to decrease the effect of UPD by breaking the contact between a galvanic Fe-Zn coupling, before the (sub-) zinc monolayer has full development opportunity. Once contact is broken, both metals will adopt their reversible potential. Therefore, the (sub) onocapa of Zn will dissolve again. Then, the contact is restored and so on.

EXAMPLE 6 In another experiment, coupled current measurements were made without and with a barrier, which opposes the transfer of Zn (OH) ions from the anodic to the cathodic, but allows the passage of other ions that are not Zn ions ( OH) 2'4, in order to limit the ohmic drop on the barrier as much as possible.

MEASURES OF COUPLED CURRENT WITHOUT A BARRIER: A zinc rod was immersed in the anodic compartment of a cell H and an iron bar was immersed in the cathode compartment (see Figure 13). Cell H was filled with an aqueous electrolyte (2.5 M NaOH, 5 g / L dissolved zinc, 298 K). The bars were partially covered with an electroplated tape, 3 M (registered trademark) number 484 in order to expose a well-defined area of 2 cm to the solution. A variable resistor was inserted between the bars, on which the potential difference was measured with a high-input impedance multimeter. A reference electrode was placed in both compartments, in such a way that the potentials of the electrodes could be measured separately. The resistance was gradually reduced from R-8 O (open circuit) to R = 0 O (short circuit). The current of the cell was calculated from the Law of O. It can be seen from Figure 13 that ZD UPD manifests itself directly. By lowering the resistance, the potential of the iron rod moves from its reversible value (open circuit) to its mixed potential (short circuit). It can be easily seen that the HER is drastically inhibited, the inhibition being stronger in the mixed potential. Once the cell current has passed its maximum value and is decreased, the zinc rod is less polarized; its displacement of potential returns to the reversible potential. Surprisingly, the maximum current has been reached for R = 6 O. This resistance of Ohm will not be considered as the absolute value for which the maximum current is reached in all cases, the important conclusion is that the maximum current is not necessarily reached at Ohm's minimum resistance. In other words, there are cases in which the cell current can be increased by increasing the resistance in the circuit. In practice, this means that the cell current can be maximized, increasing the external resistance from the short-circuit situation until the condition dl / dR = 0 has been satisfied. Once dl / dR = 0, EFe will have reached a value, which will depend on the concentration of Zn (OH) 2 * 4 as can be seen from Figure 9 and EZn will have reached its most positive value.

MEASURES OF COUPLED CURRENT WITH A BARRIER: A barrier was inserted between both compartments (see figure 14). The compartments were filled with the same solution as in the previous experiment, but now the cathode compartment did not contain any dissolved Zn. The potential difference between the two reference electrodes represents the voltage drop on the barrier, from figure 15 it can be seen that inhibition of the HER no longer occurs, which leads to larger cell currents. In the short-circuit situation EZn is no longer equal to EFe, which is caused by the voltage drop over the barrier, which had a resistance of approximately 12 O. The HER now satisfies the Butler-Volmer equation: i = l? '5exp (-17.8 (E-Eeq)) A cm "2 where Ee is the equilibrium potential, which is in good agreement with the values of the literature, reducing the voltage drop on the barrier The cell current can be much larger because both the anodic and cathodic currents are exponentially dependent on the potential Suitable membranes are commercially available, for example, a naphion membrane can be used (trademark). the cathodic current against EFe, to compare both coupled current experiments In Figure 17, the cathodic current of the current experiment coupled with a barrier between the anodic and cathodic compartments has been extrapolated to more negative EFe potentials, starting from the which becomes clear that larger currents will be reached when the voltage drop on the barrier is reduced.

EXAMPLE 7 In a further experiment, the influence of a mechanical treatment of a second metal was studied. It can be seen from figure 18 that the scratching of a metal surface has a strong defect on the HER. This effect probably results from an increased activity of the surface with respect to the HER. The increase in surface activity is advantageously combined with any other measures to promote dissolution, since a synergistic effect occurs under all circumstances.

EXAMPLE 8 In another experiment, fine iron powder was added to a 1 1 beaker flask containing a 2.5 M sodium hydroxide solution with 5 g / l dissolved zinc at 353 K.

The solution was stirred continuously. 6 samples of galvanized steel test material on both sides were exposed to the solution. The thickness of the zinc layer was 8 μm. Every 5 min a sample was taken and the effectiveness of dezincification was evaluated. It appeared that the amount of iron powder had a strong effect on the dissolution rate of zinc. The time needed to complete the descaling was reduced by 24 min. to an amount of 50 g / l of iron powder less than 5 min. at an amount of 200 g / l. Adding more powder than 200 g / l did not give improvement.

EXAMPLE 9 In a further experiment it was observed that both Sn coupled to steel or Pt leads to acceptable dissolution rates. Because Sn is more noble than Zn (Esn> EZn), the dissolution rate of Sn coupled to M2 was lower than in the case of Zn coupled to M2.

EXAMPLE 10 It was observed that if Mg is used as a current collector, no HER occurs on the surface of the current collector. The specific electrical resistance of Mg is somewhat larger than that of Cu, but still small enough to conduct considerable currents with negligible ohmic losses. These qualities make Mg an ideal current collector, if evolution of hydrogen in the current collector is not desired.

As follows from the examples and experiments above, several steps can be taken to obtain favorable dissolution rates of metals such as Sn and Zn. As shown, steel scrap can be very economically deinked or unsettled by processing the scrap into a steel tank comprising two compartments separated by a membrane in order to prevent UPD in the cathode compartment. Another possibility to prevent UPD greatly is to add a metal powder of M2, for example iron powder to the tank and stir the electrolyte. By mechanisms as previously described, see example 5, very high dissolution rates are obtained. In this case the iron powder can be maintained in a confined part of the deposit by a suitable member in the form of, for example, a separation screen. In general, the cathodic / anodic surface regime will be chosen to be as large as possible for high dissolution rates or rates.

Claims (24)

R E I V I N D I C A C I O N S

1. - Method for electrochemically dissolving a first metal by simultaneously creating evolution of hydrogen in a second metal, the second metal is a metal having a greater current exchange density for evolution of hydrogen than the first metal; both metals are immersed in an aqueous electrolytic system in which the first metal and the second metal are galvanically coupled; characterized by applying at least one measure to reduce the inhibition of the evolution of hydrogen in the second metal thereby increasing the rate of dissolution of the first metal.

2. - Method according to clause 1, characterized in that the first metal is practically Zn or Sn.

3. - Method according to clause 2, characterized in that the second metal is selected from the group consisting of Pt, Pd, Ir, Co, Ni, Fe and ferrous materials including steel.

4. - Method according to any of clauses 1 to 3, characterized in that the first metal is zinc.

5. - Method according to clause 4, characterized in that the second metal is iron or steel.

6. - Method according to any of clauses 1 to 3, characterized in that the first metal is Sn.

1. - Method according to clause 6, characterized in that the second metal is Pt.

8. - Method according to any of the preceding clauses, characterized in that the aqueous electrolyte is one or more alkaline solutions.

9. - Method according to clause 8, characterized in that the or each of the alkaline solutions is a sodium hydroxide solution.

10. - Method according to clause 8 or 9, characterized in that the at least one measure comprises that the or each of the alkaline solutions is selected to have a hydroxide concentration of more than 8 M, preferably of more than 9 M.

11. - Method according to any of clauses 8 to 10, characterized in that at least the alkaline solution that is put in contact with the second metal is maintained at a temperature above 340 k, preferably above 350 K.

12. - Method according to any of clauses 1 to 11, characterized in that the at least one measurement comprises mechanical abrasion of the surface of the second metal.

13. Method according to any of clauses 1 to 12, characterized in that the at least one measure comprises adding powder of a second metal to the aqueous electrolyte surrounding the second metal and stirring the aqueous electrolyte containing the powder.

14. - Method according to any of clauses 8 to 13, wherein the first metal is in the form of separate elements, characterized in that the first and second metals are coupled through a current collector that contacts the first metal.

15. - Method according to clause 14, characterized in that the current collector has an active surface of Mg.

16. - Method according to any of the preceding clauses 1 to 15, characterized in that the at least one measure comprises that the first and second metals are galvanically coupled by connection means that provide an electrical resistance that is selected such that the flow of current through of the connection means is practically at the maximum value obtainable by varying the resistance.

17. - Method according to clause 16, characterized in that the electrical circuit comprising the connection means is interrupted periodically.

18. Method according to any of the preceding clauses 1 to 17, characterized in that the at least one measure comprises dividing the aqueous electrolyte in a first fluid to dissolve the first metal and a second fluid that comes into contact with the second metal; the first and second fluids are coupled by a selectively permeable device that opposes the passage of ions from the first metal to the second fluid.

19. - Method according to any of the preceding clauses 1 to 18, characterized in that the first metal is in the form of a coating on a metal substrate and the second metal is separated from the metal substrate.

20. - Use of the method according to any of clauses 1 to 19, to remove by electrochemical dissolution Zn and / or Sn of metal scrap.

21. - Use according to clause 20 to remove Zn from scrap steel.

22. - Use according to clause 20 to remove Sn from steel scrap.

23. - Use of an apparatus comprising a first process volume for electrochemically dissolving Zn or Sn and a second process volume for evolution of hydrogen, with the first and second process volumes coupled by a device capable of opposing the passage of metal ions which is dissolving for galvanic dissolution of Zn or Sn.

24. - Method to treat scrap steel containing Zn electrochemically disconnected in an alkaline solution in a first process and recovering the zinc in a second process, characterized in that the dezincing in the first process is performed galvanically, this is without external power supply . SUMMARY The object of the invention is to provide an improved electrochemical dissolution process for electrochemically dissolving a first metal by simultaneously creating evolution of hydrogen in a second metal, the second metal is a metal having a higher current exchange density for evolution of hydrogen than the first metal; both metals are immersed in an aqueous electrolytic system in which the first metal and the second metal are galvanically coupled. By means of the application of measures to reduce the inhibition of the evolution of hydrogen in the second metal with which the speed of dissolution of the first metal is increased. Measures to reduce inhibition suitably comprise selecting temperatures and electrolyte concentrations to divide the electrolyte into two fluids coupled by a selectively permeable device and suitably selecting the resistance value of a connecting means electrically connecting the first and second metals. The invention is particularly useful for removing Zn or Sn from scrap steel containing Zn or Sn.