Method for removing a metal coating from an article having a steel substrate
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- TATA STEEL IJMUIDEN BV
- Filing Date
- 2024-07-26
- Publication Date
- 2026-06-03
AI Technical Summary
Existing methods for removing metal coatings from steel substrates are inefficient in terms of energy and cost, and often result in the unintended dissolution of the steel substrate, making selective removal and recycling challenging.
The use of an aqueous solution comprising trichloroacetic acid (TCA) or methane sulfonic acid (MSA) to selectively dissolve metal coatings such as tin, zinc, aluminum, or magnesium from steel substrates without significant dissolution of the steel, at room temperature and without the need for applied current or heat.
This method allows for the selective and efficient removal of metal coatings from steel substrates, minimizing energy consumption and costs, while effectively preventing the dissolution of the steel substrate, thus enabling effective recycling of both the steel and the metal coatings.
Smart Images
Figure EP2024071308_30012025_PF_FP_ABST
Abstract
Description
[0001] METHOD FOR REMOVING A METAL COATING FROM AN ARTICLE HAVING A STEEL SUBSTRATE
[0002] Field of the Invention
[0003] The invention relates to a method for selectively removing a metal coating from an article having a steel substrate.
[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 coatings 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 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 steelmaking process.
[0006] 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.
[0007] 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.
[0008] 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 conducted 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.
[0009] Besides tin, various other metals such as Zn, Al, Mg 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.
[0010] 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.
[0011] The invention provides an additional improvement, an additional advantage, or an alternative to the prior art. Objectives of the Invention
[0012] 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.
[0013] 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 costefficient manner.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] Description of the Invention
[0018] 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 trichloroacetic acid (TCA) or methane sulfonic acid (MSA), immersing the article into the aqueous solution. Both TCA and MSA are relatively strong organic acids, and they can easily dissolve the metal coating without a need for applied current and heat. Moreover, it is surprisingly found that both acids are highly selective towards metal coatings compared to the steel. 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 aqueous solution consists essentially of water and one of trichloroacetic acid (TCA) and methane sulfonic acid (MSA). Thus, the risk of the dissolution of the iron due to the other substances in the aqueous solution can be prevented.
[0019] In this method, distilled water, demineralized water, or tap water can be used to obtain the aqueous solution. 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 can be obtained.
[0020] In a possible embodiment, the aqueous solution comprises methane sulfonic acid (CH3SO3H). The following reaction takes place between tin and methane sulfonic acid:
[0021] 2CH3SO3H + Sn Sn2++ 2CH3SO3' + H2
[0022] Methane sulfonic acid (MSA) has a pKa value of -1.9 and acts as a strong acid due to its sulfonic group and that easily dissolves the metal coating. Moreover, MSA is a widely available industrial agent that enables a cost-effective method to be achieved. The concentration of MSA in the aqueous solution ranges from 15 wt.% to 70 wt.%, preferably from 40 wt.% to 60 wt.%, more preferably from 45 wt.% to 55 wt.%, most preferably from 50 wt.% to 55 wt.% based on the total weight of the solution.
[0023] In a possible embodiment, the aqueous solution comprises trichloroacetic acid (CCI3COOH). The following reaction takes place between tin and trichloroacetic acid:
[0024] 2CCI3COOH + Sn Sn2++ 2CI3COO- + H2
[0025] With a pKa value of 0.7, trichloroacetic acid (TCA) is also a relatively strong acid. It is advantageous that TCA can dissolve the metal coatings at much lower concentration than MSA can. In a possible embodiment, the concentration of TCA in the aqueous solution ranges from 3 wt.% to 25 wt.%, preferably from 5 wt.% to %20 wt.%, more preferably from 7 wt.% to 15 wt.%, most preferably from 10 wt.% to 15 wt.% based on the total weight of the solution. In a possible embodiment, the metal coating comprises one or more of Sn, Zn, Al, or Mg. The metal coating may comprise the metal alloys of Sn, Zn, Al or Mg. 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 elements can be chosen or preferred according to environmental, technical and / or economical constraints. The metals 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 steel substrate is made of carbon steel. Preferably, the steel substrate is made of mild steel also known as plain-carbon steel and low-carbon steel. In a possible embodiment of the invention, the article is a tinplate. The tinplate can be a painted or an unpainted tinplate. The tinplate can be derived from tin cans, tin can trimmings, tin can scrap, or a combination thereof. Moreover, the article may comprise a metal coating comprising the elements such as Mg, Zn, Al, or combinations / alloys thereof, for instance the article can be a component of an automobile. The article can also be a part of a household appliance or a part for buildings such as profiles, panels, or drainage systems.
[0026] In a possible embodiment, the method comprises a step of recovering dissolved metal coating from the aqueous solution by means of electroplating. Specifically, when the article contains tin, the dissolved tin 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.
[0027] According to a 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 is rinsed with water to be prepared for melting in a furnace.
[0028] 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 and / or volatilizing the organic substance thereon. The organic substance can be a polymer, plastics, paper label, oil, paint residue, leftover or any other organic 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, preferably from 20 to 45 minutes, more preferably from 25 to 35 minutes. The duration of the heat treatment is most preferably 30 minutes.
[0029] The article may have additional layers such as lacquer, varnish, polymer, and paint layers. In a possible embodiment of the invention, the method comprises 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.
[0030] Moreover, one or more of the objectives of the invention are realized by an aqueous solution for use in the above-mentioned method, comprising Trichloroacetic acid (TCA) or methane sulfonic acid (MSA). In a possible embodiment, the aqueous solution consists essentially of water and one of trichloroacetic acid (TCA) and methane sulfonic acid (MSA).
[0031] In a possible embodiment, the aqueous solution comprises Trichloroacetic acid (TCA). The concentration of TCA in the aqueous solution ranges from 3 wt.% to 25 wt.%, preferably from 5 wt.% to %20, more preferably from 7 wt.% to 15 wt.%, most preferably from 10 wt.% to 15 wt.% based on the total weight of the solution.
[0032] According to the further aspect of the invention, the aqueous solution comprises Methane sulfonic acid (MSA). The concentration of MSA in the aqueous solution ranges from 15 wt.% to 70 wt.%, preferably from 40 wt.% to 60 wt.%, more preferably from 45 wt.% to 55 wt.%, most preferably from 50 wt.% to 55 wt.% based on the total weight of the solution.
[0033] 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. Example 1
[0034] 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 methane sulfonic acid (MSA). The concentration of MSA in the aqueous solution is 50 wt.% based on the total weight of the solution.
[0035] The temperature of the aqueous solution was 20°C. The plates were periodically weighed to determine weight losses of the plates.
[0036] The weight losses of the plates as a function of the time are shown in Table 1. On the one hand MSA effectively dissolved the tin, while on the other hand the dissolutions of the other metals were hardly observed. Table 1 clearly demonstrates the selectiveness of MSA towards tin compared to other metals.
[0037] Table 1
[0038] Example 2
[0039] 50 x 50 mm tinplate was prepared and immersed into the aqueous solution containing methane sulfonic acid (MSA). The tinplate was obtained from the production scrap and each face of the tinplate contained different amount of tin. The concentration of MSA in the aqueous solution is 50 wt.% based on the total weight of the solution.
[0040] The temperature of the aqueous solution was 20°C.
[0041] Fig. 1 shows the weight losses of the tinplate and tin amounts of the back and front face of the tinplate as a function of the time. The plates were periodically weighed to determine weight losses of the plates. 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 corelated 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 calibration curve of a tinplate, is shown in the Fig.1 .
[0042] As seen in the Fig. 1 , the dissolution of the tin on the front and back faces was completed within a short time. In addition, it is clearly observed from the total weight loss values that the dissolution of the iron is significantly limited in the inventive method.
[0043] Example 3
[0044] 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 the aqueous solution containing Trichloroacetic acid (TCA). The concentration of TCA in the aqueous solution is 10 wt. % based on the total weight of the solution.
[0045] The temperature of the aqueous solution was 20°C. The plates were periodically weighed to determine weight losses of the plates.
[0046] The weight losses of the plates as a function of the time are shown in Table 2. The dissolutions of the metals excluding tin were hardly observed or limited. Therefore, T able 2 clearly demonstrates the selectiveness of TCA towards tin compared to other metals. Example 4
[0047] 50 x 50 mm tinplate was prepared and immersed into the aqueous solution containing Trichloroacetic acid (TCA). The tinplate was obtained from the production scrap and each face of the tinplate contained different amount of tin. The concentration of TCA in the aqueous solution is 10 wt. % based on the total weight of the solution.
[0048] The temperature of the aqueous solution was 20°C. Fig. 2 shows the weight losses of the tinplate and tin amounts of the back and front faces of the tinplates as a function of the time. The amount of tin on both faces was measured with an X-ray fluorescence spectrometer (XRF) in the same way as described in Example 2. The plates were periodically weighed to determine weight losses of the plates.
[0049] As shown in the Fig.2, the dissolution of the tin on the front and back faces was completed within a short time. In addition, it is clearly observed from the total weight loss values that the dissolution of the iron is significantly limited in the inventive method.
Claims
CLAIMS1. A method for selectively removing a metal coating from an article having a steel substrate comprising the steps of preparing an aqueous solution comprising trichloroacetic acid (TCA) or methane sulfonic acid (MSA), 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 Claim 1 or 2, wherein the concentration of TCA in the aqueous solution ranges from 3 wt.% to 25 wt.%, preferably from 5 wt.% to 20 wt.%, more preferably from 7 wt.% to 15 wt.% based on the total weight of the solution.
4. The method according to Claim 1 or 2, wherein the concentration of MSA in the aqueous solution ranges from 15 wt.% to 70 wt.%, preferably from 40 wt. % to 60 wt.%, more preferably from 45 wt.% to 55 wt.% based on the total weight of the solution.
5. The method according to any one of the preceding claims, wherein the metal coating comprises one or more of Sn, Zn, Al, or Mg.
6. The method according to any one of the preceding claims, wherein the method comprising a step of recovering dissolved metal coating from the aqueous solution by means of electroplating.
7. 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 aqueous solution after the dissolution of the metal coating is substantially completed.
8. The method according to any one of the preceding claims, wherein the article is tinplate.
9. The method according to Claim 8, wherein the method comprising a step of recovering dissolved tin from the aqueous solution by means of electroplating.
10. The method according to any one of the preceding claims, wherein the method comprises a step of heating the article before immersing the article into the aqueous solution.
11. The method according to any one of the preceding claims, wherein the method comprising 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.
12. The method according to any one of the preceding claims, wherein the steel substrate is made of carbon steel.
13. An aqueous solution for use in the method according to the any one the preceding claims, wherein the aqueous solution comprises Trichloroacetic acid (TCA) or methane sulfonic acid (MSA).