WO2025045703A1

WO2025045703A1 - A method for selectively removing a metal coating from an article

Info

Publication number: WO2025045703A1
Application number: PCT/EP2024/073500
Authority: WO
Inventors: Adrianus Jacobus Wittebrood; Dammes Hans Van Der Weijde
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2023-08-25
Filing date: 2024-08-21
Publication date: 2025-03-06
Anticipated expiration: 2026-02-25

Abstract

The invention relates to a method for selectively removing a metal coating from an article having a steel substrate comprising the steps of preparing an aqueous solution saturated with respect to the ferrous ions, comprising ferric ions; and immersing the article into the aqueous solution.

Description

A METHOD FOR SELECTIVELY REMOVING A METAL COATING FROM AN ARTICLE

Field of the Invention

The invention relates to a method for selectively removing a metal coating from an article having a steel substrate.

Background of the Invention

Steel recycling saves resources and reduces CO2 emissions, energy consumption and water usage. However, steel wastes usually contain various other metal elements resulting from the metal coating applied by coating or plating on the steel substrates to improve corrosion resistance or to provide a better adhesion or a better aesthetic appearance. The coating materials and steel substrate must be properly separated from each other in order to be recovered and reintroduced into the manufacturing cycle. For example, tinplate consisting of a steel substrate coated with a thin tin layer is widely used in food and beverage industries. The significant amount of steel waste as a result of high consumption and efficient waste collection makes the recycling of the tinplate inevitable for a sustainable world. However, despite its favourable effects as a tin layer on the steel substrate, tin has a detrimental effect on the steel substrate properties and the steelmaking process. Tin is also not easy to remove from a steel melt during the steelmaking process. So, it is important to remove tin from the steel waste prior to recycling the steel waste in the steelmaking process. Therefore, the tinplate scrap is subjected to a detinning process to remove tin from a steel substrate of tinplate before using the steel substrate as clean scrap in the steelmaking process.

In US4164542 (A) one of the conventional detinning methods is disclosed. The method for rapidly detinning a tin-plated scrap metal to produce a detinned base metal having a shiny metallic surface comprises the steps of immersing the scrap metal into a vessel containing an aqueous detinning solution including essentially 18-30% sodium hydroxide and 2-10% sodium nitrate or sodium nitrite for a time period up to about 20 minutes so as to completely dissolve the tin-plating from the base metal. The solution is heated to a temperature above 110°C to increase its initial effectiveness to dissolve the tinplating. With this relatively high concentration and temperature the tin reacts with the sodium salts in the solution to form sodium stannate, which precipitates out of the solution and is continuously separated therefrom in a centrifuge or filter press. The detinned scrap is rinsed with water as it emerges from the detinning bath and the used rinse water drains into the detinning bath.

However, this process does not allow selective removal of the tin from the tinplate because some of the steel substrate is also dissolved in the liquid bath during this process. Moreover, it is inefficient in terms of cost and energy due to the requirement of keeping the temperature of the liquid bath above 110° C.

Steel scrap may have lacquer and paint layers which are needed to be removed from the steel substrate before recycling the steel. In EP0105551 (B1) a method for the detinning of painted tinplate waste is disclosed, in which the waste is arranged as an anode in a bath containing NaOH and subjected to electrolytic treatment. In this method, the tinplate waste is compressed and is then arranged in a bath liquid which has an NaOH concentration in the range 6 to 15% to achieve a substantial softening of the paint. The softening of the paint is carried out at a temperature of 70 to 90°C for 12 to 16 hours. Then, electrolytic treatment is carried out to remove tin beneath the paint. However, this process also needs high amount of energy as heat for the alkaline solution. In addition, the process takes a long time.

Another patent document EP0023729 (A1) discloses the recovery of tin from tin bearing material such as old cans, tin plate, lacquered tin sheet waste, and the like, i.e., in general tin plated or tin coated iron material. The process comprises a step of contacting the iron scrap with an aqueous or organic solution containing an effective amount of ferric ions, thereby enabling the iron scrap to be detinned without external heating of the detinning medium. As shown in this patent document, ferric ion is an ideal reagent to remove tin from a tinplate as tin easily reacts with ferric ions (Fe³⁺) according to the following reaction.

Sn + 2Fe³⁺ 2Fe²⁺ + Sn²⁺

However, ferric ion reacts with iron quite easily as well according to the following reaction.

Fe + 2Fe³⁺ — > 3Fe²⁺ Therefore, it is almost impossible to prevent iron to be dissolved in the solution after dissolving of the tin in this process.

Besides tin, various other metals such as Cu, Ni, Zn, Al, Mg, Cr, Cd are also widely used as coating/plating materials on the steel substrate based on the requirements of the different industries. It is also vital to remove these metals from the steel substrate before using the steel substrate in steelmaking processes. However, selectively removing these metals and preventing the dissolution of the steel substrate create a problem similar to the problem with tin. For example, the ferric ion reacts with both nickel and iron effectively according to the reactions as shown below.

Fe + 2Fe³⁺ — > 3Fe²⁺

In conclusion, it is environmentally and economically important to selectively remove the metal coating from the steel substrate and to recover both iron and the coating metal elements in reusable quality.

The invention provides an additional improvement, an additional advantage, or an alternative to the prior art.

Objectives of the Invention

It is an objective of the present invention to provide a method for removing a metal coating from an article having a steel substrate in a selective manner.

It is also an objective of the present invention to provide a method for selectively removing a metal coating from an article having a steel substrate in an energy and cost efficient and environmentally friendly manner.

It is another objective of the present invention to provide a method for recovering steel by selectively removing a metal coating from an article having a steel substrate. It is another objective of the present invention to provide a method for recovering coating metal elements by selectively removing a metal coating from an article having a steel substrate.

It is another objective of the invention to provide an aqueous solution for use in a method for removing a metal coating from an article having a steel substrate in a selective manner.

Description of the Invention

One or more of the objectives of the invention are realized by providing a method for selectively removing a metal coating from an article having a steel substrate comprising the steps of preparing an aqueous solution comprising ferrous ions and ferric ions saturating the aqueous solution with respect to the ferrous ions immersing the article into the aqueous solution.

Saturating the aqueous solution with respect to the ferrous ions is simply achieved by adding the sufficient amount of ferrous salt into the solution. Therefore, preparing the aqueous solution and saturating the aqueous solution can be realized in one step. In other words, one or more of the objectives of the invention are realized by providing a method for selectively removing a metal coating from an article having a steel substrate comprising the steps of

- preparing an aqueous solution saturated with respect to ferrous ions, comprising ferric ions;

- immersing the article into the aqueous solution.

In this process, the aqueous solution contains ferric ions for effectively removing the metal coating. The ferric ions surprisingly only react with the coating metal elements instead of also reacting with iron thanks to the saturated solution with ferrous ions. Thus, a self-regulating process can be obtained, and the metal coating can be selectively removed from the steel substrate. The article is kept in the aqueous solution until the dissolution of the metal coating is substantially completed. After fully dissolving the metal coating, the reaction stops by itself so the dissolution of the iron in the aqueous solution can be prevented, or at least the dissolution of the iron occurs at insignificant level.

In a possible embodiment, the metal coating comprises one or more of Sn, Cu, Ni, Zn, Al, Mg, Cr or Cd. Preferably, the metal coating comprises tin. The metal coating may serve a variety of purpose such as corrosion resistance, paint adhesion, magnetism, hardness, conductivity, surface roughness or decoration. The metal elements mentioned above, or any other suitable metal elements can be chosen or preferred according to environmental, technical and/or economical constraints. The metal coating can be applied on the steel substrate by plating, coating, or any other suitable methods. The article may have a plurality of metal coating layers.

In a possible embodiment of the invention, the ferric ions in the aqueous solution are obtained by adding a ferric salt to water. Thanks to the good solubility of the ferric salt, it is easy to obtain the desired concentration of the solution. In a possible embodiment of the invention, the ferric salt is selected from the group comprising ferric sulphate, ferric chloride, and ferric nitrate. Preferably, the ferric salt is ferric chloride. More than one type of ferric salt may also be added to the water.

In a possible embodiment of the invention, the aqueous solution comprises ferric ions in a range of 0.2 to 3 M, preferably in a range of 0.3 M to 2 M, more preferably in a range of 0.4 M to 1 .6 M, most preferably in a range of 1 M to 1.5 M. Thus, the aqueous solution has sufficient amount of ferric ions to remove the metal coating within a reasonable time. An excess of ferric ions increases density and viscosity of the solution so the solution may adhere to the steel substrate that makes it difficult to rinse the solution from the substrate after the dissolution of metal coating. In a possible embodiment of the invention, the amount of the ferric salt, preferably ferric chloride, dissolved in the aqueous solution is between 0.2M and 3M, preferably 0.3M and 2M, more preferably 0.4M and 1.6M, most preferably 1 M and 1.5 M. Molarity (M) is the unit concentration expressed as the number of moles of dissolved solute per litre of solution. Although the amount of the ferric salt is given based on pure anhydrous ferric salt, the ferric salt can be provided in anhydrous or hydrated form into the water.

In a possible embodiment of the invention, the ferrous ions in the aqueous solution are obtained by adding a ferrous salt into a water. Saturating the aqueous solution with respect to the ferrous ions can be achieved simply by adding a ferrous salt into the solution. In a possible embodiment of the invention, the ferrous salt is selected from the group comprising ferrous sulphate, ferrous chloride, and ferrous nitrate. Preferably, the ferrous salt is ferrous chloride. More than one type of ferrous salt may also be added into the water. In this method, the water can be distilled water, demineralized water, or tap water.

In a possible embodiment of the invention, the article is a tinplate. The tinplate can be a painted or an unpainted tinplate or coated with a polymer layer. The tinplate can be derived from tin cans, tin can trimmings, tin can scrap, or a combination thereof. On the other hand, in a possible embodiment of the invention, the article can be a nickel-plated steel, for instance a battery case of an electrical vehicle. The article can also be a part of a household appliance or a part for buildings such as profiles, panels, or drainage systems.

In a possible embodiment of the invention, the temperature of the aqueous solution is between 5 and 50°C, preferably between 15 and 30°C, more preferably between 18 and 25°C. The entire process can be performed at room temperature without heating of the aqueous solution so the cost and energy efficient method is obtained.

In a possible embodiment of the invention, after immersing the article into the aqueous solution, the ferrous ions are oxidized to ferric ions by means of supplying hydrogen peroxide or ozone into the aqueous solution during the dissolution of the metal coating in the aqueous solution. During the dissolution of the metal coating in the aqueous solution, ferric ions are reduced to ferrous ions. On one hand, the depletion of ferric ions is prevented by means of supplying hydrogen peroxide or ozone into the aqueous solution to oxide the ferrous ions to the ferric ions, on the other hand, the aqueous solution is kept being saturated with ferrous ions by the ferrous ions generated as a result of the dissolution of metal coating. In order to keep this balance, the amount of hydrogen peroxide or ozone supplied to the aqueous solution is adjusted according to the dissolution rate of the metal coating. As a result of the one of the reactions as shown below, the depletion of the ferric ions in the aqueous solution is prevented and the process can be continued without additional ferric salt being supplied.

The ferrous ions are oxidized to ferric ions preferably by ozone as ozone can be generated on-site by using only air and electricity. The reaction takes place in an acidic environment including hydrogen ions as a result of the previous reactions. According to a further aspect of the invention, the article is heated before immersing the article into the aqueous solution. Thus, the article can be thermally cleaned by degrading, burning off, thermolysis, pyrolysis and/or volatilizing any organic substance thereon. The organic substance can be a polymer, plastics, paper label, oil, paint residue, leftover or any other contaminants. In a possible embodiment, the article is heated in air before immersing the article into the aqueous solution. In a possible embodiment, the temperature of the heat treatment is between 400°C and 540°C, preferably between 450°C and 500°C, more preferably between 470°C and 490°C, most preferably about 480°C. In a possible embodiment of the invention, the duration of the heat treatment may range from 10 to 60 minutes, preferably from 20 to 45 minutes, more preferably from 25 to 35 minutes. The duration of the heat treatment is most preferably 30 minutes.

The article may have additional layers such as lacquer, varnish, polymer, and paint layers. In a possible embodiment of the invention, the method may comprise a step of removing lacquer, varnish, polymer, or paint layers of the article by applying a solvent before immersing the article into the aqueous solution. Thus, the efficiency of the method can be increased. The solvent can be applied by immersing the article into a bath containing a solvent-based paint stripper. The article can be kept in the bath until the additional layers are fully or almost fully removed. Then, the article is taken out of the bath, rinsed, and immersed into another bath containing the aqueous solution. The solvent can be methyl ethyl ketone or ethyl acetate.

According to the further aspect of the invention, the method comprises a step of recovering dissolved coating metal from the aqueous solution by either evaporation, crystallization, electrolysis, electroplating, or precipitation. Preferably, the coating metal is recovered by electroplating. Specifically, when the article contains tin, the dissolved tin or nickel can be recovered from the aqueous solution by means of electroplating. Thus, the waste of the materials can be minimized by using the aqueous solution as an electrolyte source of electroplating after and/or during the removal of the metal coating from the article.

According to the further aspect of the invention, the method comprises a step of recovering steel by removing the steel substrate from the aqueous solution after the dissolution of the metal coating is substantially completed. After being taken out of the aqueous solution, the steel substrate can be preferably rinsed with water to be prepared for melting in a furnace.

Moreover, one or more of the objectives of the invention are realized by an aqueous solution for use in the above-mentioned method, the aqueous solution comprises ferrous ions and ferric ions. The aqueous solution is saturated with respect to the ferrous ions. In a possible embodiment, the source of the ferrous ions is a ferrous salt such as ferrous sulphate, ferrous chloride, or ferrous nitrate, dissolved in the aqueous solution. In a possible embodiment, the source of the ferric ion is a ferric salt such as ferric sulphate, ferric chloride, or ferric nitrate, dissolved in the aqueous solution. In a possible embodiment of the invention, the aqueous solution comprises ferric ions in a range of 0.2 to 3 M, preferably in a range of 0.3 M to 2 M, more preferably in a range of 0.4 M to 1.6 M, most preferably in a range of 1 M to 1.5 M. In a possible embodiment of the invention, the amount of the ferric salt, preferably ferric chloride, dissolved in the aqueous is between 0.2M and 3M, preferably 0.3M and 2M, more preferably 0.4M and 1.6M, most preferably 1 M and 1.5 M.

The invention will now be described with reference to the following non-limiting examples and figures.

Example 1

Four 50 x 50 mm different pure metal plates, each consisting of one of Sn, Fe, Cu or Ni, were prepared. The metal plates were immersed into an aqueous solution containing 0.5M FeCh. The solution was saturated with FeCh as claimed in the invention.

In order to observe the effectiveness of the method of the invention, a comparative test was conducted. Another four 50 x 50 mm different pure metal plates, each consisting of one of Sn, Fe, Cu or Ni, were prepared. The metal plates were immersed into a comparative aqueous solution containing 0.5M FeCh.

The temperature of the aqueous solution was 20°C. The plates were periodically weighed to determine weight losses of the plates.

The weight losses of the plates as a function of the time are shown in the Table 1 . It is clearly observed that the method of the invention accelerates the dissolution of Sn, and Cu while decelerating the dissolution of the Fe compared to the comparative method. Although the method of the invention affects the dissolution of nickel in a slightly negative manner, as a result of the limited dissolution of the iron it is still considered an effective method for selectively removing the metal coating.

Table 1

Example 2

50 x 50 mm tinplate was prepared and immersed into the aqueous solution containing 0.5M FeCh. The solution containing 0.5M FeCh was saturated with FeCh as claimed in the invention.

In order to observe effectiveness of the method of the invention, the comparative test was performed. Another 50 x 50 mm tinplate was prepared and immersed into the aqueous solution containing 0.5M FeCh.

Both specimens for the tests were obtained from the production scrap and each face of the specimens contained different amount of tin.

The temperature of both aqueous solutions was 20°C.

The results of the comparative test are shown in Fig.1a and the results of the test according to the method of invention are shown in Fig.1 b. The figures show the weight losses of the tinplates and tin amount of the back and front faces of the tinplates as a function of the time.

The tinplates were periodically weighed to determine weight losses of the tinplates. The amount of tin on both faces was measured with an X-ray fluorescence spectrometer (XRF). During the XRF measurement, the coated article is exposed to X-rays emitted from XRF and thus the article produces fluorescence. The fluoresced X-rays are counted by XRF, and the intensity of the fluoresced X-rays is correlated with the thickness and material type of the coating layer.

The mass of tin per unit area on both surfaces, which was converted from the intensity by using a calibration curve of a tinplate, is shown in the Figs. 1a and 1 b.

As seen in the Figures 1a and 1 b, the time to fully dissolve the tin on the front and back faces are almost the same in both solutions. However, it is clearly observed from the total weight loss values that the dissolution of the iron is significantly limited in the inventive method.

Example 3

50 x 50 mm nickel plated steel substrate was prepared and immersed into the aqueous solution containing 250 g/l FeCh. The solution was saturated with FeCh as claimed in the invention.

In order to observe the effectiveness of the inventive method, a comparative test was conducted. Another 50 x 50 mm nickel plated steel substrate was prepared and immersed into the aqueous solution containing 250 g/l FeCh.

Both specimens for the tests were obtained from the production scrap and each face of the specimens contained different amount of nickel.

The temperature of both aqueous solutions was 20°C.

The results of the comparative test are shown in Fig.2a and the results of the test according to the method of invention are shown in Fig.2b. The figures show the weight losses of the nickel plated steel substrate and nickel amount of the back and front faces of the nickel plated steel substrate as a function of the time. The plates were periodically weighed to determine weight losses of the plates. The amount of nickel on both faces was measured with an X-ray fluorescence spectrometer (XRF). The figures 2a and 2b shows the intensity of the fluoresced X-rays that is directly proportional to the coating thickness.

Although the results of both tests are similar in terms of dissolution of the nickel, in the method of the invention, the dissolution of iron is slowed down by a factor of ten. It is clearly observed that the solution saturated with ferrous ions surprisingly provides selective removal of the coating materials from a steel substrate without significant dissolution of iron.

Claims

1. A method for selectively removing a metal coating from an article having a steel substrate comprising the steps of

- preparing an aqueous solution saturated with respect to ferrous ions, comprising ferric ions

- immersing the article into the aqueous solution.

2. The method according to Claim 1, wherein the temperature of the aqueous solution is between 5 and 50°C, preferably between 15 and 30°C, more preferably between 18 and 25°C.

3. The method according to Claims 1 or 2, wherein the ferric ions in the aqueous solution are obtained by adding ferric salt into a water.

4. The method according to Claim 3, wherein the ferric salt is selected from the group comprising ferric sulphate, ferric chloride, and ferric nitrate.

5. The method according to any one of the preceding claims, wherein the ferrous ions in the aqueous solution are obtained by adding ferrous salt into a water.

6. The method according to Claim 5, wherein the ferrous salt is selected from the group comprising ferrous sulphate, ferrous chloride, and ferrous nitrate.

7. The method according to any one of the preceding claims, wherein the aqueous solution comprises ferric ions in a range of 0.2 to 3 M, preferably in a range of 0.3 M to 2 M, more preferably in a range of 0.4 M to 1.6 M, most preferably in a range of 1 M to 1.5 M.

8. The method according to any one of the preceding claims, wherein the amount of the ferric salt, preferably ferric chloride, dissolved in the aqueous solution is between 0.2 M and 3 M, preferably 0.3 M and 2 M, more preferably 0.4 M and 1.6 M, most preferably in a range of 1 M to 1.5 M.

9. The method according to any one of the preceding claims, wherein the method comprises a step of heating the article in air before immersing the article into the aqueous solution.

10. The method according to Claim 9, wherein the duration of heating treatment ranges from 10 to 60 minutes, preferably from 20 to 45 minutes, more preferably from 25 to 35 minutes.

11. The method according to any one of the preceding claims, wherein the method comprises a step of removing lacquer, varnish, polymer, or paint from the article by applying a solvent before immersing the article into the aqueous solution.

12. The method according to any one of the preceding claims, wherein the article is a tinplate.

13. The method according to any one of the preceding claims, wherein the method comprises a step of recovering dissolved coating metal from the aqueous solution by either evaporation, crystallization, electrolysis, electroplating, or precipitation.

14. The method according to any one of the preceding claims, wherein the method comprises a step of recovering steel by removing the steel substrate from the aqueous solution after the dissolution of the metal coating is substantially completed.

15. An aqueous solution for use in the method according to the any one of the preceding claims, comprising ferrous ions and ferric ions wherein the aqueous solution is saturated with respect to the ferrous ions.