A method for selectively removing a tin coating from an article having a tin coating on a steel substrate

Abstract

The invention relates to a method for selectively removing a tin coating from an article having a tin coating on a steel substrate comprising the steps of preparing an alkaline solution comprising an abrasive for mechanically removing the tin coating and / or an additional layer covering the tin coating and immersing the article into the alkaline solution.

Description

[0001] A METHOD FOR SELECTIVELY REMOVING A TIN COATING FROM AN ARTICLE HAVING A TIN COATING ON A STEEL SUBSTRATE

[0002] Field of the Invention

[0003] The invention relates to a method for selectively removing a tin coating from an article having a tin coating on a steel substrate. The invention also relates to an alkaline solution suitable for use in the method.

[0004] Background of the Invention

[0005] 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, specifically tin coating applied by coating or plating on the steel substrates for example 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.

[0006] In patent document 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 alkaline 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 is inefficient in terms of cost and energy due to the requirement of keeping the temperature of the liquid bath above 110°C.

[0007] Steel scrap may have lacquer and paint layers which need to be removed from the steel substrate before / during recycling the steel. In patent document 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 the tin beneath the paint layer. However, this process also needs high amount of energy as heat for the alkaline solution. In addition, the process takes a long time.

[0008] Objectives of the Invention

[0009] It is an objective of the present invention to provide a method for selectively removing a tin coating from an article having a tin coating on a steel substrate at an improved dissolution rate.

[0010] It is another objective of the present invention to provide a method for recovering steel by selectively removing a tin coating from an article having a tin coating on a steel substrate.

[0011] It is another objective of the present invention to provide a method for recovering tin by selectively removing a tin coating from an article having a tin coating on a steel substrate.

[0012] It is another objective of the invention to provide an alkaline solution for use in a method for selectively removing a tin coating from an article having a tin coating on a steel substrate. Description of the Invention

[0013] One or more of the objectives of the invention are realized by providing a method for selectively removing a tin coating from an article having a tin coating on a steel substrate comprising the steps of

[0014] - preparing an alkaline solution comprising an abrasive for mechanically removing the tin coating and / or an additional layer covering the tin coating;

[0015] - immersing the article into the alkaline solution.

[0016] The article is immersed into the alkaline solution to selectively dissolute at least a portion of the tin coating from the steel substrate. In this context, the abrasive is suitable for at least partly removing the tin coating and / or an additional layer covering the tin coating. In a possible embodiment of the invention, the abrasive is made of ceramic, metal, any other material suitable for abrading the tin coating and / or the additional layer or combination thereof. The alkaline solution is prepared by dissolving a base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) in water and adding the abrasive into the water. By means of the abrasive, the tin coating and / or the additional layer covering the tin coating can be damaged and the removal of the tin coating / additional layer can be at least partly realized due to the frictional movement between the tin coating / additional layer and the abrasive. In addition, in case the article has additional layer(s), damaging the additional layer(s) brings the alkaline solution into direct contact with the tin coating thereby accelerating the dissolution of tin coating.

[0017] Preferably, the abrasive is made of metal. In a possible embodiment of the invention, the abrasive is made of ferrous material, nickel, platina, or iridium. In a possible embodiment, the alkaline solution can contain the abrasives made of different materials from each other. It has been found that the formation of hydrogen is one of the significant factors in the dissolution of the tin coating. The formation of hydrogen on these metal abrasives is easier than the formation of hydrogen on the tin coating. Therefore, the abrasive made of metal, specifically made of any one of nickel, platina, iridium, or ferrous material does not only strip the tin coating mechanically but also acts as a catalyst that accelerates the hydrogen evolution in the alkaline solution without being dissolved. Enhancing the formation of hydrogen in the solution using the metal abrasives accelerates the dissolution of the tin coating. Moreover, due to the attrition of the metal abrasives, fine metal particles are dispersed in the alkaline solution and thus the dissolution of tin coating is improved by increasing the catalytic surface area being in contact with the tin coating. Preferably, the abrasive is made of ferrous material, specifically made of iron as iron is cheaper than the nickel, platina, and iridium. More importantly, after the detinning, separation and recovering of tin, abrasive and the substrate are simplified in case the abrasive is made of iron.

[0018] The abrasive is added to the alkaline solution in the form of pellet, lump, stone, or a combination thereof. The abrasive may have an irregular shape such as lump or it may have a regular shape such as pyramid or prism. In a possible embodiment of the invention, the largest dimension of the abrasive is between about 5 and 50 mm. The dimension can be length, width, height, or diameter of the abrasive. Thus, adequate contact points between the tin coating and these abrasives can be obtained to damage / remove the tin coating and / or an additional layer covering the tin coating. In a possible embodiment of the invention, the abrasive is a ceramic stone. In a possible embodiment, the abrasive is a metal pellet or a metal lump. For instance, the abrasive can be iron pellet or nickel pellet or a steel part / scrap such as bolts and nuts.

[0019] In a possible embodiment of the invention, the alkaline solution further comprises metal powder, preferably iron powder suspended in the alkaline solution. Metal powder provides larger catalytic surface area than a metal pellet or any other steel part such as bolts or nuts. Therefore, preparing an alkaline solution comprising iron powder significantly increases the dissolution speed of the tin coating and reduces the process time. Moreover, the combination of iron powder, abrasive and alkaline solution provides an effective process for stripping tin coating and paint and / or lacquer layers at the same time. Therefore, articles comprising additional layers such as lacquer, varnish, polymer, and paint layers can be efficiently recycled due to this combination.

[0020] In a possible embodiment of the invention, the concentration of iron powder in the alkaline solution is between about 100 and 600 g / l, preferably between about 150 and 500 g / l, more preferably about 200 and 400 g / l, and most preferably about 225 and 300 g / l. Below the lower limit, the amount of iron powder is insufficient to reduce the process time significantly. On other hand, above the upper limit, it only increases the cost of the process in exchange for diminishing marginal benefit. In a possible embodiment of the invention, the iron powder has average particle size of between about 5 and 1000 pm, preferably between about 10 and 100 pm and more preferably between about 15 and 50 pm. In a possible embodiment of the invention, the method comprises a step of shredding the articles before immersing them into the alkaline solution. Shredding the articles avoids folding and crumbling of the articles, especially scrap parts, thereby enabling all surfaces to contact with both alkaline solution, abrasive and optional iron powder. Thus, more homogeneous and effective stripping can be obtained.

[0021] In a possible embodiment of the invention, the alkaline solution during the removal process is agitated. The alkaline solution may be agitated by techniques known in the art, including stirring, magnetic stirring, circulating by pumping around, ultrasound techniques, or combinations thereof. In a possible embodiment of the invention, the method comprises a step of stirring the alkaline solution in such a way that abrasive and / or metal particles / powder remain in suspension. Thus, the contact of the article with the abrasive and / or metal particles / powder can be ensured. Alternatively, the article is kept in motion within the alkaline solution, for example using rotatable perforated drums holding the article in a rotating device. In a possible embodiment of the invention, the method comprises a step of rotating the article in a drum of a rotating device holding the alkaline solution. The rotating device promotes the contact between the article and the abrasive and the optional iron powder. Therefore, the dissolution rate of the tin coating is significantly further increased.

[0022] In a possible embodiment of the invention, the article is 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.

[0023] In a possible embodiment of the invention, the alkaline solution comprises OH' ions in a range of 0.5 to 5 M, and preferably in a range of 1.5 to 4 M, more preferably in a range of 1.8 to 3 M, and most preferably in a range of 2 to 2.4 M. Molarity (M) is the unit concentration expressed as the number of moles of dissolved solute per litre of solution. A too high a molarity adds to the costs and makes it difficult to rinse the solution after the dissolution of the tin coating due to the increased density and viscosity of the solution. Furthermore, it has been surprisingly found that the metal abrasive, especially abrasive made of iron reduces the need for the high molarity for dissolution of the tin coating. In a possible embodiment of the invention, the alkaline solution is based on sodium hydroxide (NaOH). Sodium hydroxide is a relatively cheap source material and available in industrial quantities. An alternative alkaline solution can be based on potassium hydroxide (KOH). In a possible embodiment of the invention, the temperature of the alkaline solution is between 15°C and 90°C, preferably between 30°C and 80°C, more preferably between 40°C and 70°C, and most preferably between 50°C and 65°C. Elevating the temperature of the alkaline solution reduces the stripping time of the tin coating. However, it increases energy consumption and therefore cost. Especially, above 90°C, the elevated temperature loses its marginal effect, and it makes the process inefficient in terms of cost and energy. On the other hand, for example, tinplate may have iron-tin alloy layers such as FeSn or FeSn2, which is more resistant to most chemicals used for detinning, formed at the interface between the tin coating and steel substrate. Although the iron-tin alloy layer is the small part of the coating, the temperature of the alkaline solution can be provided between 50°C and 90°C, preferably between 60°C and 80°C, more preferably between 65°C and 75°C in order to completely remove the tin coating including layer of the iron-tin alloy. In a possible embodiment of the invention, before immersing the article into the alkaline solution, the article is heated up to the temperature of between 50°C and 90°C, preferably between 60°C and 80°C, more preferably between 65°C and 75°C in order to substantially completely remove the tin coating including any iron-tin alloy.

[0024] In a possible embodiment of the invention, the article is heated before immersing the article into the alkaline 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 alkaline 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.

[0025] In a possible embodiment of the invention, the method comprises a step of recovering the metal abrasives from the alkaline solution by magnetic separation. A bar magnet is immersed into the alkaline solution to collect metal abrasives and removed from the solution with the metal abrasives, especially metal particles / powder. Abrasive made of iron is the best option among other abrasives for the magnetic separation. In a possible embodiment of the invention, the method comprises a step of recovering dissolved tin from the alkaline solution by either evaporation, crystallization, electrolysis, electroplating, or precipitation. Preferably, tin is recovered by electroplating. Specifically, when the article contains tin, the dissolved tin can be removed from the alkaline solution by means of electroplating. Thus, the waste of the materials can be minimized by using the alkaline solution as an electrolyte source of electroplating after and / or during the removal of the tin coating from the article.

[0026] In a possible embodiment of the invention, the method comprises a step of recovering steel by removing the steel substrate from the alkaline solution after the dissolution of the tin coating is substantially completed. After being taken out of the alkaline solution, the steel substrate can be preferably rinsed with water to be prepared and may be used as clean steel scrap in for example a steelmaking process.

[0027] Moreover, one or more of the objectives of the invention are realized by an alkaline solution for selectively removing a tin coating from an article having a tin coating on a steel substrate in the above-mentioned method, comprising OH- ions in a range of 0.5 to 5 M, and preferably in a range of 1.5 to 4 M, more preferably in a range of 1.8 to 3 M and most preferably in a range of 2 to 2.4 M and an abrasive for mechanically removing the tin coating and / or an additional layer covering the tin coating. In a possible embodiment of the invention, the alkaline solution further comprises a metal powder, preferably iron powder, suspended in the alkaline solution. In a possible embodiment of the invention, the concentration of iron powder in the alkaline solution is between 100 and 600 g / l, preferably between 150 and 500 g / l, more preferably 200 and 400 g / l, and most preferably 225 and 300 g / l. In a possible embodiment of the invention, the abrasive is made of nickel, platina, iridium, or ferrous material. Preferably, ferrous material is iron.

[0028] Each possible embodiment disclosed in this text can be combined with the other possible embodiments disclosed in this text if there are no technical constraints.

[0029] The invention will now be described with reference to the following non-limiting examples and figures.

[0030] Fig.1 shows the effect of alkaline solution on the weight loss of tin from both sides of tinplate as function of processing time.

[0031] Fig.2 and Fig.3 show the effect of alkaline solutions on the weight loss of tin from the sides of tinplate having a lacquered side as function of processing time. Fig.4 shows the weight loss of tin from the sides of tinplate as function of processing time wherein the tinplates, of which starting temperature is at the room temperature, are rotated in a rotating device.

[0032] Fig.5 and Fig.6 show the weight losses of tin from the sides of tinplates having a lacquered side as function of processing time wherein the samples, of which starting temperature is at the room temperature, are rotated in a rotating device.

[0033] Fig.7 and Fig.8 show the weight losses of tin from the sides of tinplates as function of processing time wherein the examples are respectively unheated and pre-heated.

[0034] Fig.9 shows the weight loss of tin from the sides of tinplate as function of processing time and Fig.10 shows the weight loss of tin from the sides of end-of-life tin scrap as function of processing time.

[0035] Fig.11 shows the weight losses of tin from the sides of tinplates having a lacquered side as function of processing time wherein the abrasive is iron lump.

[0036] Example 1

[0037] The effect of alkaline solutions on the weight loss of tin from tinplate as function of processing time and solution temperature has been investigated on a laboratory scale of testing.

[0038] Tinplate samples with dimensions of 50x50 mm having a tin coating of 11.2 g / m2on one side and of 2.8 g / m2on the other side have been used in these test series.

[0039] Tinplate samples have been immersed in a measuring glass containing 1000 ml of 2M alkaline solution (NaOH) being stirred for immersion times up to 300 min. Separate alkaline solutions were kept at different temperatures up to 81°C. In order to observe the sole effects of the iron powder and nickel pellets, alkaline solutions comprising either nickel pellets with an average diameter in the range of between about 8 and 10 mm or iron powder were prepared in various nickel pellet / iron powder concentrations. In addition, alkaline solutions without iron pellets and nickel pellets were also prepared as comparative examples.

[0040] For each sample, the remaining amount of Sn on either side of the tinplate has been determined as well as the total weight loss of the sample. The amount of tin is measured using a handheld calibrated X-Ray Fluorescence device. The results are presented in Table 1. Results show that both iron powder and nickel pellets increase the dissolution rate of the tin. In addition, increasing the temperature of the alkaline solution significantly reduces the detinning time.

[0041] Table 1

[0042] Example 2

[0043] In the example 2, the effect of alkaline solutions on the weight loss of tin from tinplate having a lacquered side as function of processing time has been investigated on a laboratory scale of testing.

[0044] Three different tinplate samples were prepared.

[0045] - Sample A having a tin coating of 11.3 g / m2on one side and of 4.2 g / m2on the other side. - Sample B having a tin coating of 4.1 g / m2on lacquered side and of 4.5 g / m2on the non-lacquered side.

[0046] - Sample C having a tin coating of 2.7 g / m2on lacquered side and of 2.7 g / m2on the non-lacquered side.

[0047] In the first test of this example, 1.5 kg of Sample A were placed into a drum of the rotating device having 6 I of 2 M alkaline solution, 8 I ceramic stones and 200 g / l iron powder. The ceramic stones have an average diameter of about 10 mm. The residual volume of the ceramic stones is approximately 3.2 I.

[0048] In the second test of this example, 0.75 kg of Sample B and 0.75 kg of Sample C were placed into a drum of a rotating device having 6 I of 2 M alkaline solution, 8 I ceramic stones and 200 g / l iron powder. The ceramic stones have average diameter of about 10 mm. The residual volume of the ceramic stones is approximately 3.2 I.

[0049] Both alkaline solutions were at room temperature and the samples were rotated in the rotating device for 300 minutes. The rotational speed was 45 rpm.

[0050] For each sample, the remaining amount of Sn on either side of the tinplate has been determined as well as the total weight loss of the sample. The amount of tin is measured using a handheld calibrated X-Ray Fluorescence device. The results are shown in Fig.1 for Sample A, Fig.2 for Sample B and Fig.3 for Sample C.

[0051] As shown in these figures, ceramic stones having an average diameter of about 10 mm are not sufficient to abrade the lacquered surface completely. On the other hand, non- lacquered surfaces were effectively detinned.

[0052] Example 3

[0053] In the example 3, the effectiveness of the combination of the alkaline solution and the abrasive made of ferrous material has been investigated on the dissolution of tin from the tinplate having lacquered face.

[0054] Three different tinplate samples were prepared.

[0055] - Sample A having a tin coating of 11.3 g / m2on one side and of 4.2 g / m2on the other side.

[0056] - Sample B having a tin coating of 4.1 g / m2on lacquered side and of 4.5 g / m2on the non-lacquered side. - Sample C having a tin coating of 2.7 g / m2on lacquered side and of 2.7 g / m2on the non-lacquered side.

[0057] In this example, 2 kg of Sample A, 0.2 kg of Sample B and 0.2 kg of Sample C were placed into the drum of the rotating device having 8 I of 2 M alkaline solution, 1 I ceramic stones and 8 I nuts and bolts made of low carbon steel. The ceramic stones have average diameter of about 10 mm. The residual volume of the ceramic stones is approximately 0.8 I.

[0058] The alkaline solution was at room temperature and the samples were rotated in the rotating device for 300 minutes. The rotational speed was 45 rpm.

[0059] For each sample, the remaining amount of Sn on either side of the tinplate has been determined as well as the total weight loss of the sample. The amount of tin is measured using a handheld calibrated X-Ray Fluorescence device. The results are shown in Fig.4 for Sample A, Fig.5 for Sample B and Fig.6 for Sample C.

[0060] As shown in these figures, although the lacquered faces were delacquered and detinned by means of the combination of the alkaline solution, heavy steel parts and the ceramic stones, detinning of these faces were not completed within a reasonable time.

[0061] Example 4

[0062] In the example 4, the effect of the temperature has been investigated on the dissolution of tin from the tinplate having lacquered face.

[0063] In the first test of this example, 1.5 kg of Sample B (same composition as for the Example 3) was placed into the drum of the rotating device having 8 I of 2 M alkaline solution, 1 I ceramic stones and 8 I nuts and bolts made of low carbon steel. The ceramic stones have average diameter of about 10 mm. The residual volume of the ceramic stones is approximately 0.8 I. The alkaline solution was at 20 °C and the samples were rotated in the rotating device for 300 minutes. The rotational speed was 45 rpm.

[0064] In the second test of this example, the samples are heated before they are placed to the drum. Same process of the first test was repeated with same conditions. The starting temperature of the samples was 76 °C with a cooling rate of about 10 °C / h.

[0065] The samples were rotated in the rotating device for 300 minutes. The rotational speed was 45 rpm. For each test, the remaining amount of Sn on either side of the tinplate has been determined as well as the total weight loss of the sample. The amount of tin is measured using a handheld calibrated X-Ray Fluorescence device. The result of the first test is presented in Fig.7 and the result of second test is presented in Fig.8.

[0066] As shown in Fig.8, pre-heating the articles before detinning process enables the delacquering and detinning of the lacquered surface in an efficient way.

[0067] Example 5

[0068] In the example 5, the effect of the alkaline solution has been investigated on the dissolution of tin from the tinplate having iron-tin alloy layers.

[0069] 0.5 kg of Sample A (same composition as for the Example 3), 3 I mixture of shredded and non-shredded end-of-life tinplate scrap and 200 quarters of lacquered can lids were placed into the drum of the rotating device having 8 I of 2 M alkaline solution, 2 I ceramic stones and 4 I nuts and bolts made of low carbon steel and 4 I cut-off steel scrap as an abrasive. The ceramic stones have average diameter of about 10 mm. The residual volume of the ceramic stones is approximately 0.8 I. The shredded end-of-life tinplate scrap has a tin coating of 3.38 g / m2, and the non-shredded end-of-life tinplate scrap has a tin coating of 3.63 g / m2. The can lids have either tin coating of 2.70 g / m2or of 2.60 g / m2. Before the removal process, sample A is held for 24 hrs at 275°C to form iron-tin alloy layer of FeSn2 at the interface between the tin coating and steel substrate. After the heating, Sample A has a tin coating of 11.3 g / m2on one side and 4.0 g / m2on the other side.

[0070] The samples are heated before they are placed into the drum. The starting temperature of the samples was 80 °C with a cooling rate of about 10 °C / h. The samples were rotated in the rotating device for 300 minutes. The rotational speed was 45 rpm.

[0071] For each test in this example, the remaining amount of Sn on either side of the samples has been determined as well as the total weight loss of the sample. The amount of tin is measured using a handheld calibrated X-Ray Fluorescence device. The result of the Sample A is presented in Fig.9 and the result of the end-of-life scrap is presented in Fig.10.

[0072] According to the results, delacquering and detinning of the surfaces successfully realized by means of the combination of the alkaline solution and the abrasive. It was also observed that lids were completely delacquered and detinned after 30 minutes. Example 6

[0073] In the example 6, the effectiveness of the combination of the alkaline solution and the abrasive made of iron has been investigated on the dissolution of tin from the tinplate having lacquered face.

[0074] - Tinplate samples with dimensions of 50x50 mm having a tin coating of 4.1 g / m2 on lacquered side and of 4.5 g / m2 on the non-lacquered side have been used in these test series.

[0075] 5 kg of tinplate samples have been immersed in an iron barrel containing 20I of 0.5 M alkaline solution (NaOH) and 100 kg of iron lumps as abrasive. Alkaline solution was at room temperature and the samples were rotated in the rotating device for 300 minutes. The rotational speed was 45 rpm.

[0076] 10 samples were taken out of the barrel and the remaining amount of Sn on either side of the tinplate samples have been determined as well as the total weight losses of the samples. The amount of tin is measured using a handheld calibrated X-Ray Fluorescence device. The average values of these measurements are given in Fig.11 .

[0077] As shown in Fig.11 , both surfaces are detinned, but non-lacquered surfaces were effectively detinned are more effectively detinned compared to lacquered surfaces. In addition, abrasive made of iron enabled to use alkaline solution at low molarity.

Claims

CLAIMS1 . A method for selectively removing a tin coating from an article having a tin coating on a steel substrate comprising the steps of- preparing an alkaline solution comprising an abrasive for mechanically removing the tin coating and / or an additional layer covering the tin coating; and- immersing the article into the alkaline solution.

2. The method according to Claim 1 , wherein the abrasive is made of nickel, platina, iridium, or ferrous material.

3. The method according to Claim 1 or 2, wherein the abrasive is made of iron.

4. The method according to any one of preceding claims, wherein the abrasive is added to the alkaline solution in the form of pellet, lump, stone, or a combination thereof.

5. The method according to any one of preceding claims, wherein the alkaline solution further comprises a metal powder, preferably iron powder, suspended in the alkaline solution.

6. The method according to Claim 5, wherein the concentration of metal powder, preferably iron powder, in the alkaline solution is between 100 and 600 g / l, preferably between 150 and 500 g / l, more preferably 200 and 400 g / l, and most preferably 225 and 300 g / l.

7. The method according to any one of preceding claims, wherein the alkaline solution comprises OH' ions in a range of 0.5 to 5 M, and preferably in a range of 1.5 to 4 M, more preferably in a range of 1.8 to 3 M, and most preferably in a range of 2 to 2.4 M.

8. The method according to any one of preceding claims, wherein the temperature of the alkaline solution is between 15°C and 90°C, preferably between 30°C and 80°C, more preferably between 40°C and 70°C, and most preferably between 50°C and 65°C.

9. The method according to any one of preceding claims, wherein the article is heated up to the temperature of between 50°C and 90°C, preferably between 60°C and 80°C, more preferably between 65°C and 75°C, before immersing the article into the alkaline solution.

10. The method according to any one of the preceding claims, wherein the method comprising a step of shredding the articles before immersing them into the alkaline solution.

11. The method according to any one of the preceding claims, wherein the method comprising a step of recovering dissolved tin coating from the alkaline solution by either evaporation, crystallization, electrolysis, electroplating, or precipitation.

12. The method according to any one of the preceding claims, wherein the method comprising a step of recovering steel by removing the steel substrate from the alkaline solution after the dissolution of the tin coating is substantially completed.

13. An alkaline solution for selectively removing a tin coating from an article having a tin coating on a steel substrate in the method according to any one of the preceding claims comprising OH' ions in a range of 0.5 to 5 M, and preferably in a range of 1.5 to 4 M, more preferably in a range of 1.8 to 3 M, and most preferably in a range of 2 to 2.4 M and an abrasive for mechanically removing the tin coating and / or an additional layer covering the tin coating.

Images