Method for selectively removing a metal coating from metal coated steel scrap

Abstract

The invention relates to a method for selectively removing a metal coating (Sn or Zn) from metal-coated steel scrap, the method comprising the steps of: immersing the metal-coated steel scrap into an alkaline solution comprising an oxidizing agent and solid carbon-based particles suspended in the alkaline solution. The invention also relates to the use of an alkaline solution in the method of the invention.

Description

[0001] METHOD FOR SELECTIVELY REMOVING A METAL COATING FROM METAL COATED STEEL SCRAP

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method for selectively removing a tin or zinc metal coating from metal-coated steel scrap. The invention also relates to the use of an alkaline solution use in the method of the invention.

[0004] BACKGROUND TO 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 amongst others from the metal coating applied by coating or plating on the steel substrates to improve corrosion resistance or to provide a better adhesion or other engineering properties. 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 efficient recycling of the tinplate an important issue. However, despite its favourable effects as a tin layer on the steel substrate, tin as impurity element has a detrimental effect on the steel substrate properties itself and in the steelmaking process. Tin is also not easy to remove from a steel melt during a steelmaking process. So, it is important to remove tin from the steel substrate waste prior to recycling the steel waste in the steelmaking process. Therefore, the tinplate scrap is subjected to a detinning process to remove the tin from a steel substrate of tinplate before using the steel substrate as clean scrap in the steelmaking process.

[0006] In the chemical industry leaching is the process of a solute becoming detached or extracted from its carrier substance by way of a solvent. This is achieved by dissolving the given substance in a liquid. For the leaching of tin from a steel substrate a highly acidic or alkaline solution can be used. This solution occasionally contains other additives, such as tin- complexing agents and corrosion. Hazardous substances and troublesome waste liquids (e.g., wastewater containing high amounts of nitrogen and heavy metals) are often generated. Given the strongly amphoteric nature of tin, alkaline media are advantageous for the selective extraction of tin without using corrosion inhibitors for other elements such as copper and iron. Furthermore, inexpensive steel containers and steel equipment can be used for leaching.

[0007] In patent document US-4,164,542 one of the conventional detinning methods is disclosed. The method for rapidly detinning tin-plated scrap metal to produce a detinned steel base metal having a shiny metallic surface comprises the steps of immersing the scrap metal into a vessel containing an aqueous detinning solution including 18-30% sodium hydroxide and 2-10% sodium nitrate or sodium nitrite for a time period of up to about 20 minutes so as to completely dissolve the tin-plating from the steel base metal. The solution is heated to a temperature above 110°C to increase its initial effectiveness to dissolve the tinplating layer. 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 base metal is rinsed with water as it emerges from 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, and 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] Another known batch method for the detinning of painted tinplate waste is disclosed in patent document US-4,474,655. The method comprising the steps of (a) compressing the painted tinplate waste into bundles which have a predetermined range of weightA / olume; (b) immersing the compressed waste bundles in a bath containing NaOH at a temperature in the range of 70°C to 90°C for a period of time sufficiently long, preferably for a period of 12 to 16 hours, that the NaOH in the bath liquid acts on the paint so as to achieve a substantial softening but not complete dislodging of the paint such that the tin under the paint can be removed subsequently by electrolysis, without completely dislodging the paint; (c) after step (b), effecting electrolytic treatment of said bundles in a bath containing NaOH so as to remove tin from beneath the softened paint. This process concerns a time-consuming batch process performed at elevated temperatures.

[0009] The efficiency of tin leaching can be increased using an alkaline solution containing a suitable oxidizing agent. The oxidizer enables the formation of tin oxide which dissolves much easier into an alkaline solution. A known oxidizing agent is hydrogen peroxide, however it significantly increases the costs of the overall detinning process. Other known oxidizing agents to oxidize tin include the ions of nitrate (NO3, nitrite (NO2, persulfate (S2O82, thiosulfate (S2O32, iodate (IO3j, and chlorite (CIO2. Some of these oxidizing agents are only effective at elevated temperatures or have been demonstrated only in small laboratory scale experiments. Another way to improve tin leaching is to saturate the alkaline solution with oxygen for example by passing an oxygen containing gas through the solution. However, oxygen has a limited solubility in the alkaline solution. Increasing the temperature of an alkaline solution would accelerate the dissolution of tin. But the oxygen solubility decreases with increasing temperatures, thereby counteracting any benefit of elevating the temperature.

[0010] There is a demand for a more efficient and environmental friendly leaching process for the de-coating of metal-coated steel scrap material, in particular of tin or zinc coated steel scrap material.

[0011] DESCRIPTION OF THE INVENTION

[0012] It is an object of the present invention to provide a method for selectively removing a metal coating, notably tin or zinc, from metal-coated steel scrap by means of leaching.

[0013] It is an object of the invention to provide a method for the detinning of tinplate by means of leaching.

[0014] It is an object of the invention to provide a method for the dezincifying of zinc coated steel scrap by means of leaching.

[0015] It is another object of the invention to provide at least an alternative method for selectively removing a metal coating, notably tin or zinc, from metal-coated steel scrap.

[0016] It is another object of the invention to provide an alkaline solution for use in a method of selectively removing a metal coating, notably tin or zinc, from metal-coated steel scrap material.

[0017] These objects and further advantages are met or exceeded by the present invention according to independent claim 1 and with preferred embodiments set out in the dependent claims and the description.

[0018] In order to achieve these objects, the present invention proposes, in a first aspect, a method for selectively removing a tin or zinc metal coating from metal-coated steel scrap to produce a decoated steel base metal, preferably for the detinning of tin coated steel scrap or to dezincify zinc coated steel scrap, the method comprising the steps of: immersing the tin or zinc coated steel scrap into a vessel containing an alkaline solution further comprising an oxidizing agent and solid carbon-based particles suspended in the alkaline solution.

[0019] In accordance with the invention surprisingly it has been found that the use of carbonbased particles suspended in the alkaline de-coating solution in the presence of an oxidizing agent significantly increases the dissolution rate of the tin or zinc coating from the metal-coated steel scarp metal thereby producing a steel base metal substantially free from the tin or zinc coating. Experiments have shown that solid carbon-based particles, in particular graphite-based particles, significantly increase the amount of oxygen available for the tin or zinc to oxidize and to dissolve into the alkaline solution. There appears to be a kind of synergetic effect. This unexpected effect is achieved already at low temperatures, e.g. at ambient temperature. The solid carbon-based particles are chemically unreactive and can be re-used to promote the metal oxidation. The effect is obtained without the use of further chemicals. Thus, the alkaline leaching method using an oxidizing agent in combination with solid carbon-based particles provides a very efficient technique for the rapid removal of a metal coating from metal-coated steel material and for the subsequent recovery of the valuable metal of the removed tin or zinc metal coating, e.g. for the recovery of tin or zinc dissolved into the alkaline solution. The method can be readily implemented on an industrial scale, without the necessity of discharging hazardous waste liquids or supplying additional chemicals. The essential chemicals are of low cost and readily available. The method can be carried out in a continuous way. The method can be carried out with a high yield at low temperatures making it a low energy consuming method and thus very cost effective.

[0020] In an embodiment the method is performed using an alkaline solution comprising OH - ions in a concentration range of 0.1 to 10 M, and preferably in a range of 0.5 to 10 M, and more preferably in a range of about 1 to 5 M. A too high a molarity adds to the costs and might create safety issues in the handling of the alkaline solution. Molarity (M) is the unit concentration expressed as the number of moles of dissolved solute per litre of solution.

[0021] In a preferred embodiment the alkaline solution is based on caustic soda (i.e. NaOH, sodium hydroxide). Sodium hydroxide is a relatively cheap source material and available in industrial quantities. An alternative alkaline solution can be based on potassium hydroxide (KOH).

[0022] In a preferred embodiment of the method the tin or zinc metal-coated steel scrap is immersed into the alkaline solution wherein the oxidizing agent comprises of is or created by an oxygen containing gas passing through the alkaline solution. In this embodiment the oxidizing agent is provided by passing an oxygen containing gas through the alkaline solution to saturate as much as feasible the alkaline solution with oxygen to increase or to accelerate the selective leaching of the metal from the metal coating, e.g., leaching of tin as tin oxide from a tin coating into the alkaline solution. In an embodiment the oxygen containing gas contains at least 10 vol.% of oxygen, and preferably at least 18 vol.% of oxygen. Pure oxygen gas (viz. 90 vol.% or more of oxygen) may be used for this purpose. In a preferred embodiment air is used as oxygen containing gas as oxidizing agent. Air is readily available at extremely low cost and does not cause any hazardous issues when used in an industrial environment. In this embodiment the use of further oxidizers, like e.g. the use of hydrogen peroxide, ozone, potassium iodate, or potassium permanganate, can be avoided.

[0023] It is a key feature of the invention that the oxidized alkaline solution is combined with solid carbon-based particles to increase or to accelerate the leaching of the metal from the metal coating, e.g. tin as tin oxide, into the alkaline solution. The solid carbon-based particles are preferably selected from the group: coke, graphite, graphene, soot, carbon black, peat, coal, biomass, amorphous carbon, charcoal, and biochar. Coal may be any grade of coal, including lignite, sub-bituminous coal, bituminous coal, steam coal, or anthracite. The carbon-based particles may be coal powder, and more preferably anthracite powder or coke breeze.

[0024] In a preferred embodiment the solid carbon-based particles are made from graphite material. A wide range of graphite materials can be used, including natural graphite flakes, artificial graphite, highly ordered pyrolytic graphite (HOPG), pitch-based graphite, and graphene. Graphite material has proven to be very effective, is non-toxic, regenerable, and unreactive in this method. Furthermore, it is an industry accepted material and is readily available in large quantities.

[0025] To be most effective in the decoating method the solid carbon-based particles are kept in dispersion in the alkaline solution. As known to the skilled person the size of the solid carbonbased particles should not be too small as the handling might create safety issues and these particles may have an adverse effect of the viscosity of the alkaline solution. And the particles should not be too large as this would lead to uneven dispersion and potentially settling of the solid particles and thereby reducing the efficiency. In an embodiment the solid carbon-based particles have an average particle size (APS) of at least 1 micron, and preferably of at least 5 micron. In an embodiment the average particles size is less than 250 micron, and preferably less than 150 micron. The average particle size of a powder can be analysed by laser diffraction methods known in the art.

[0026] In an embodiment of the method the alkaline solution comprises solid carbon-based particles in a concentration of at least about 5 g / l, and preferably of at least about 10 g / l. In an embodiment the concentration of solid particles does not exceed about 100 g / l. A too high an amount of dispersed solid particles has an adverse effect at least on the viscosity of the alkaline solution.

[0027] The method can be performed over a broad range of temperatures, namely in a range of about 5°C to 110°C. Preferably the method is performed at a temperature in a range of about 15°C to 50°C, and more preferably in a range of about 15°C to 35°C. At this temperature range the method can be applied on an industrial scale at ambient temperatures with no or very limited additional heating of the alkaline solution so a cost and energy efficient method is obtained. At this temperature range the alkaline solution maintains a high degree of oxygen saturation thereby increasing the yield of the decoating process. The presence of the solid carbon-based particles in accordance with the invention provides for an increased decoating efficiency by reducing the leaching time without a need to increase the temperature.

[0028] When processing flow-melted tinplate at a temperature below 50°C some tin might remain on the surface. Flow-melting means that after a tin plating operation, the tinplated material is brought for a very short time above the melting temperature of tin (232°C). The benefit for flow melting is that it provides a very smooth shiny surface which is appealing for certain applications. A downside of this flow-melting is that some of the tin at the interface between the tin coating and steel substrate starts to form the intermetallic phase FeSn2. This intermetallic phase is much more resistant to most chemicals used for detinning. For the recycling of flow melted tinplate at this temperature range below 50°C on an industrial scale this does not need to be any issue as the bulk (e.g. more than 80%, and preferably more than 90%) of the tin layer can be removed and only an amount of less than about 1 g / m2may remain. For the detinning of tinplate not having been heat treated or flow-melt treated the removal of the complete tin layer from the tin coated steel scrap material can be achieved in this temperature range below 50°C.

[0029] In an embodiment the detinning of flow-melted tinplate or heat-treated tinplate in accordance with the invention is performed at a temperature of at least 50°C enabling the complete removal of the tin layer including said layer comprising the intermetallic phase FeSn2.

[0030] In a particular favourable embodiment of the method, the method comprises the steps of: immersing tin or zinc metal-coated steel scrap into a vessel containing an alkaline solution having ions of OH- in a range of 0.5 to 10 M, preferably of 0.5 to 5 M, and being at a temperature in a range of 15°C to 35°C, the alkaline solution further comprising an oxidizing agent by passing air through the alkaline solution, and further comprising solid carbon-based particles in the form of graphite-based particles suspended in the alkaline solution. This method of selectively removal of a tin or zinc metal coating from metal-coated steel scrap offers significantly reduced leaching times and can be performed on an industrial scale at ambient temperature without the requirement for high temperatures of the alkaline solution. The method uses readily available and low cost components like air and graphite particles without the necessity of discharging hazardous waste liquids or supplying additional chemicals.

[0031] In an embodiment of the method the alkaline solution during the removal process is kept in motion to increase the efficiency of the leaching process and to keep the solid carbon-based particles in suspension. The alkaline solution may be kept in motion by techniques known in the art, including stirring, magnetic stirring, circulating by pumping around, ultrasound techniques, or combinations thereof. Alternatively, the tin or zinc metal-coated steel scrap material is kept in motion within the alkaline solution, for example using rotatable perforated drums holding the scrap metal.

[0032] The method according this invention is ideally suited for the detinning of tinplate to recover the tin and to produce a detinned steel substrate that may be further used as clean steel scrap in for example a steelmaking process.

[0033] In another embodiment the method according this invention is ideally suitable for the dezincification of zinc-coated steel scrap to recover the zinc and to produce a dezinced steel substrate that may be further used as clean steel scrap in for example a steelmaking process. Zinc-coated metal scrap includes steel that has been coated with pure zinc or a zinc alloy (e.g. Zn-AI alloy, Zn-Mg alloy, Zn-Fe alloy, Zn-AI-Mg alloy or Zn-Mg-AI alloy) produced via hot-dip coating, galvanising or galvannealing.

[0034] The decoating method can be applied on an industrial scale. The tin or zinc metal-coated steel scrap, such as for example tinplate, can be painted or unpainted metal coated steel scrap or coated with a polymer layer. The tinplate can be derived from tin cans, tin can trimmings, tin can scrap, or a combination thereof. The tinplate can be in-house tinplate scrap created in the process of manufacturing tinplate material itself. Though the tin leaching method according to this invention can be used also as an efficient recycling route for waste printed circuit boards (PCBs) to recover the valuable tin.

[0035] The tin or zinc metal-coated steel scrap material may have additional layers such as lacquer, varnish, polymer, and paint layers. In an embodiment of the invention, the method comprises a step of removing the lacquer, varnish, polymer, or paint layers from the metal- coated steel scrap material by applying a solvent before immersing said steel scrap into the alkaline solution. Thus, the efficiency of the method can be increased. The solvent can be applied by immersing said steel scrap into a bath containing a solvent-based paint stripper. The metal-coated steel scrap can be kept in the bath until the additional layers are fully or almost fully removed. Next, said metal-coated steel scrap is taken out of the bath, rinsed, and immersed into another bath containing the alkaline solution for removal of the metal coating in accordance with the invention. The solvent can be e.g. methyl ethyl ketone or ethyl acetate.

[0036] In an embodiment of the invention, the tin or zinc metal-coated steel scrap is heated before immersing the metal-coated steel scrap material into the alkaline solution. Thus, the metal-coated steel scrap material 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 an embodiment, the metal-coated steel scrap is heated in air before immersing said steel scrap into the alkaline solution in accordance with this invention. In an embodiment, the temperature of the heat-treatment is between 300°C and 540°C, preferably between 400°C and 500°C. In an embodiment the duration of the heat-treatment may range from 5 to 60 minutes, preferably from 10 to 45 minutes.

[0037] In an embodiment of the invention the method comprises a step of recovering the dissolved tin metal from the alkaline solution by either crystallization, electrolysis, electroplating, or precipitation. Preferably, the tin metal is recovered by electroplating. Specifically the dissolved tin can be recovered 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 layer from the tin coated steel scrap material.

[0038] In an embodiment of the invention the method further comprises a step of recovering the detinned steel base metal by removing it from the alkaline solution after the dissolution of the tin layer is substantially completed. After being taken out of the alkaline solution, the detinned steel base metal is preferably rinsed with water and may be used as clean steel scrap metal in for example a steelmaking process.

[0039] In an embodiment of the invention the method comprises a step of recovering the dissolved zinc metal from the alkaline solution by either crystallization, electrolysis, electroplating, or precipitation. Preferably, the zinc metal is recovered by electroplating. Specifically the dissolved zinc can be recovered 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 zinc layer from the zinc-coated steel scrap material.

[0040] In an embodiment of the invention the method further comprises a step of recovering the dezinced steel by removing it from the alkaline solution after the dissolution of the zinc layer is substantially completed. After being taken out of the alkaline solution, the dezinced steel base material is preferably rinsed with water and may be used as clean steel scrap metal in for example a steelmaking process.

[0041] In an embodiment of the method it further comprises a step of recovering the solid carbon-based particles from the alkaline solution using known liquid-solid separation techniques, including filtering, centrifugal forces to precipitate the solid particles, and sedimentation or flotation. The solid carbon-based particles, in particular graphite based particles, are chemically inert within the decoating method and are regenerable.

[0042] To avoid interference by the suspended the solid carbon-based particles in the recovery of dissolved metal, e.g. tin or zinc, from the alkaline solution, it is preferred to recover or to remove the solid carbon-based particles from the alkaline solution prior to the recovery the dissolved metal.

[0043] In a further aspect the invention relates to the use of an alkaline solution in a method of selectively removing a metal coating from metal-coated steel scrap as herein claimed and described, preferably for the detinning of tinplate or the dezincification of zinc-coated steel scrap. The alkaline solution comprises an oxidizing agent and solid carbon-based particles suspended in the alkaline solution. In a preferred embodiment the oxidizing agent is substantially oxygen saturating the alkaline solution in combination with the solid carbon-based particles suspended in the alkaline solution. The oxygen saturation is achieved by passing an oxygen containing gas through the alkaline solution. Preferred embodiments for the oxygen containing gas are set out in the description for the method of the invention and the claims.

[0044] In an embodiment the alkaline solution comprises OH -ions in a range of 0.1 to 10 M, and preferably in a range of 0.5 to 10 M, and more preferably in a range of 1 to 5 M. Preferably the alkaline solution is based on sodium hydroxide (NaOH). In an embodiment of the alkaline solution comprises solid carbon-based particles, preferably graphite particles, in a concentration of at least about 5 g / l, and preferably of at least about 10 g / l. In an embodiment the concentration of solid carbon-based particles does not exceed about 100 g / l.

[0045] In a further aspect of the invention it relates to the use of solid carbon-based particles, in particular graphite-based particles, dispersed in an alkaline solution in combination with an oxidizing agent for the selective removal of a metal coating from metal-coated steel scrap material, in particular from tinplate scrap metal or zinc-coated scrap metal, as herein set out and claimed. The solid carbon-based particles, in particular graphite particles, achieve the effect of increasing the dissolution rate of the metal from the metal-coated steel scrap material thereby producing a de-coated steel base metal. This unexpected effect is achieved already at low temperatures, e.g. at ambient temperature.

[0046] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention. EXAMPLE 1.

[0047] The effect of aerated alkaline solutions on the weight loss of tin from tinplate as function of processing time and bath temperature has been investigated on a laboratory scale of testing.

[0048] Samples of industrial produced flow melted tinplate of dimensions 50x50 mm having a tin coating of 11.8 g / m2on one side and 2.8 g / m2on the other side have been used in this tests series.

[0049] Tinplate samples have been immersed in a measuring glass containing 1000 ml of 2M alkaline solution (NaOH) being stirred for immersion times up to 60 min. Separate alkaline solution were kept at two different temperatures, viz. 20°C and 78°C. The alkaline solutions were purged with either pure nitrogen gas or air, each at a flow rate of 25 l / h.

[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 Xray Fluorescence device. The results are presented in Fig.1 A to 1 D.

[0051] Fig.1 A and Fig.l B show the results at 20°C for respectively the nitrogen and ambient air saturated alkaline solutions. Fig.1 C and Fig.1 D show the results at 78°C for respectively the nitrogen and ambient air saturated alkaline solutions.

[0052] From the results of these figures it can be seen that saturating the alkaline solution with nitrogen has no effect of the removal rate of tin from the tinplate. However, when the alkaline solution is purged with air the removal rate is significantly increased compared to nitrogen purging. Thus adding an oxidizing agent, in this case in the form of purging with an oxygen containing gas, to the alkaline detinning solution does have a notable effect on the tin removal rate from the tinplate. From the comparison between Fig. 1 B and Fig.1 D it can be seen that increasing the temperature of the alkaline solution does not provide any significant effect on the dissolution of the tin. As known, with increasing temperature the solubility of oxygen in an alkaline solution is reduced and the potential benefit of a temperature increase is minimised.

[0053] EXAMPLE 2.

[0054] In a similar set-up as in example 1 , the effect of the addition of graphite particles has been investigated on the dissolution of tin from the tinplate. Flow melted tinplate samples (same dimensions and composition as for Example 1) have been immersed in a measuring glass containing 1000 ml of 2M alkaline solution (NaOH) for immersion times up to 30 min. The alkaline solution was kept at 20°C. The alkaline solutions were stirred and purged with either pure nitrogen gas or ambient air, each at a flow rate of 25 l / h. Carbon-based particles in the form a graphite particles in an amount of 50 g / l have been added to each alkaline solution. The graphite particles used are commercially available by ThermoFischer (Kandel) GmbH, natural, microcrystal grade, APS 2-15 micron, 99.9995% (metals basis).

[0055] Fig.2A and 2B show the results for respectively the nitrogen and air saturated alkaline solution.

[0056] From the results of Fig ,2B it can be seen that the addition of graphite particles had no or at best a marginal effect on the dissolution of the tin from the tinplate when processed in an alkaline solution saturated with nitrogen. However, the addition of graphite particles to the alkaline detinning solution purged with ambient air has a very significant effect on the dissolution of tin from the tinplate. The dissolution rate is significantly increased and reaches a plateau after about 5 min. For comparison the results of Fig.l B from Example 1 having similar process conditions apart from the presence of the graphite particles show a progressing tin dissolution but no plateau was reached when processed up to 60 min.

[0057] Very surprisingly the combination of graphite particles in an oxygen saturated alkaline detinning solution appears a very effective measure for increasing the dissolution rate of tin.

[0058] EXAMPLE 3.

[0059] In a similar set-up as in example 1 , the effect of the amount of graphite particles on the dissolution of tin has been investigated using the same flow melted tinplate samples immersed in a measuring glass containing 1000 ml of 2M alkaline solution (NaOH) at 20°C, being stirred and purged with ambient air at a flow rate of 25 l / h. The graphite particles originating from the same source as with example 2 and varying amounts of 0, 30 and 50 g / l graphite have been investigated. The results are shown in Figs.3A to 3C.

[0060] The results shown in Fig.3A are comparative as no graphite particles have been added to the oxidized alkaline solution. Fig.3A shows a favourable progressing tin dissolution when processed up to 60 min. The results of Fig.3B show that in the presence of about 30 g / l of graphite particles the tin dissolution is significantly accelerated when compared to the results of Fig.3A. In Fig.3B a plateau is reached after about 10 min processing time suggesting that the tin removal is approaching about completion within these process settings. The results of Fig.3C show that increasing the amount of graphite particles up to about 50 g / l further increases the tin dissolution when compared to Fig.3B. In Fig.3C a plateau is reached already after about 5 min processing time. The amount of tin dissolution shown in Fig.3B and 3C are about the same within experimental accuracy. Thus these experiments show that the addition of solid carbon-based particles, in this particular set-up graphite particles, to an oxidized alkaline detinning solution surprisingly significantly increases tin dissolution from the scrap metal into the alkaline solution. Depending on the amount of solid particles added the dissolution of tin can be completed in a matter of minutes whereas in an oxidized alkaline solution devoid of these particles a processing time of more than 60 min is required. These results are achieved at room temperature conditions and without the use of further chemicals. Thus, the alkaline leaching method according to the invention using an oxidizing agent in combination with solid carbon-based particles provides a very efficient technique for the very rapid removal of tin from tin coated scrap metal and for the recovery of the valuable tin from the alkaline solution. The method can be readily implemented on an industrial scale, without the necessity of discharging hazardous waste liquids or supplying additional chemicals.

[0061] EXAMPLE 4.

[0062] In a similar set up as for example 2 the effect of the amount of graphite particles on the dissolution of tin has been investigated using the same flow melted tinplate samples immersed in a measuring glass containing 1000 ml of 2M alkaline solution (NaOH) at 20°C, being stirred and purged with ambient air at a flow rate of 25 l / h. The graphite particles originating from the same source as for example 2 and varying amounts of 5, 10, 20, 30, 40, and 50 g / l graphite have been investigated. The dissolution speed S in mg / min.m2of the linear part of dissolution profile was determined, viz. the slope coefficient of the weight loss curve for example in Fig ,3A to 3C.

[0063] The results are shown in Fig.4 from which it can be seen that the method of this invention is effective already at very low concentrations of graphite particles and the dissolution speed increases progressively with increasing concentration of the graphite particles.

[0064] EXAMPLE 5.

[0065] From Fig.1 to Fig.3 it can be seen that for the detinning of flow-melted tinplate at ambient temperature the removal of tin is not necessarily a fully 100% as a remaining tin layer of about 1 g / m2may remain. This is due to tin locked in the intermetallic phase FeSn2formed at the interphase between the steel substrate and the tin coating layer. This intermetallic phase is much more resistant to most chemicals used for detinning. As a consequence the complete removal of the tin layer for re-flow melted tinplate is difficult to achieve at ambient temperatures. In a similar set up as for example 2 the effect of graphite particles on the dissolution of tin has been investigated using the flow-melted tinplate material having similar concentrations per side as for example 2. The tinplate material was immersed in a measuring glass containing 1000 ml of 2M alkaline solution (NaOH) at 60°C, being stirred and purged with ambient air at a flow rate of 25 l / h. The concentration of graphite particles was 50 g / l and originating from the same source as for example 2. Compared to example 2 the temperature of the alkaline solution was 60°C instead of 20°C. The results are shown in Fig.5. From the results of Fig.5 it can be seen that the tin layer is fully removed when processed in the method at elevated temperature. For the detinning of flow-melted tinplate in accordance with the invention it is preferred to apply a temperature of the alkaline solution of at least 50°C to fully remove the tin layer from the tinplate material.

Claims

CLAIMS1. Method for selectively removing a tin or zinc coating from metal-coated steel scrap, the method comprising the steps of: immersing the tin or zinc coated steel scrap into an alkaline solution comprising an oxidizing agent and solid carbon-based particles suspended in the alkaline solution.

2. Method according to claim 1 or 2, wherein the alkaline solution comprises OH -ions in a range of 0.1 to 10 M, and preferably in a range of 0.5 to 10 M, and more preferably in a range of 1 to 5 M.

3. Method according to claim 1 or 2, wherein the alkaline solution is based on sodium hydroxide.

4. Method according to any one of claims 1 to 3, wherein the oxidizing agent comprises an oxygen containing gas passing through the alkaline solution.

5. Method according to claim 4, wherein the oxygen containing gas contains at least 10 vol.% of oxygen, and is preferably air.

6. Method according to any one of claims 1 to 5, wherein the solid carbon-based particles are selected from the group: coke, graphite, graphene, soot, carbon black, peat, coal, biomass, amorphous carbon, charcoal, and biochar.

7. Method according to any one of claims 1 to 5, wherein the solid carbon-based particles are graphite particles, and preferably formed from natural graphite flakes, artificial graphite, highly ordered pyrolytic graphite, pitch-based graphite, or graphene.

8. Method according to any of one claims 1 to 7, wherein the amount of solid carbon-based particles in the alkaline solution are in a concentration of at least 5 g / l, preferably at least 10 g / l, and preferably at most 100 g / l.

9. Method according to any one of claims 1 to 8, wherein the temperature of the alkaline solution is in a range of 5°C to 110°C, and preferably between 15°C to 50°C.

10. Method according to any one of claims 1 to 9, wherein the method comprises a step of removing lacquer, varnish, polymer, or paint from the tin or zinc coated steel scrap metal by applying a solvent before immersing the metal coated steel scrap metal into the alkaline solution.

11. Method according to any one of claims 1 to 10, wherein the method comprises a step of heating the tin or zinc coated steel scrap before immersing into the alkaline solution.

12. Method according to any one of claims 1 to 11 , wherein the method comprises a step of recovering dissolved tin or zinc from the alkaline solution by either crystallization, electrolysis, electroplating, or precipitation.

13. Method according to any one of claims 1 to 12, wherein the method comprises a step of recovering the solid carbon-based particles from the alkaline solution by a liquid-solid separation technique.

14. Method according to any one of claims 1 to 13, wherein the metal-coated steel scrap is tinplate.

15. Method according to claim 14, wherein the method comprises a step of recovering the detinned base metal by removing the detinned base metal from the alkaline solution after the dissolution of the tin coating is substantially completed.

16. Method according to any one of claims 1 to 13, wherein the metal-coated steel scrap is zinc or zinc-alloy coated steel scrap.

17. Method according to claim 16, wherein the method comprises a step of recovering the dezinced base metal by removing the dezinced base metal from the alkaline solution after the dissolution of the zinc coating is substantially completed.

18. Use of an alkaline solution comprising an oxidizing agent and solid carbon-based particles suspended in the alkaline solution, and preferably comprises OH- -ions in a range of 0.1 to 10 M, and preferably in a range of 0.5 to 10 M, and wherein preferably the amount ofsolid carbon-based particles in the alkaline solution are in a concentration of at least 5 g / l, preferably at least 10 g / l, and preferably at most 100 g / l, in the method according to any one of claims 1 to 17.

19. Use of solid carbon-based particles, in particular graphite-based particles, dispersed in an alkaline solution in combination with an oxidizing agent for the selective removal of tin or zinc metal coating from metal-coated steel scrap material in the method according to any one of claims 1 to 17.

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