Process of stripping zinc from coated steel substrates

Abstract

The invention relates to a process of selectively stripping zinc from zinc coated steel substrates according to claim 1. The invention further relates to an aqueous alkaline ammonia solution suitable for use in the process.

Description

[0001] PROCESS OF STRIPPING ZINC FROM COATED STEEL SUBSTRATES

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a process of selectively stripping zinc from zinc coated steel substrates. The invention further relates to an aqueous solution suitable for use in this process.

[0004] BACKGROUND TO THE INVENTION

[0005] For many applications steel is provided with a coating of zinc. After use it is desirable to return such coated material as scrap containing metal to an earlier stage of the production chain. However, problems can occur with the reuse of such scrap. Most coated metals such as galvanized steel cannot be reused, or cannot be reused in the desired quantities in a production process earlier in the chain, for example the steel production process, because the zinc coating has a negative effect on the quality of the new product or interferes with the manufacturing process of the new steel product. Therefore, for these and similar reasons, it is desirable that such zinc coatings are removed and separated from the underlying metal material, which can then be readily reused in a steelmaking process.

[0006] Patent document EP0727499-A1 discloses a method of treating zinc-coated steel scrap parts to remove at least partly the zinc coating layer on the parts, the method including maintaining the parts in motion in a vessel in a tumbling manner so that multiple collisions occur within the vessel, wherein an alkaline solution is present in said vessel, and wherein a plurality of abrading stones are present in said vessel, whereby said zinc coating layer is at least partly removed.

[0007] There is a demand for improved methods having a high recovery rate of selectively removing zinc from zinc coated iron or steel substrates.

[0008] DESCRIPTION OF THE INVENTION

[0009] It is an object of the invention to provide a process of selectively removing zinc from zinc coated iron or steel substrates.

[0010] It is an object of the invention to provide at least an alternative process of selectively removing zinc from zinc coated iron or steel substrates.

[0011] It is another object of the invention to provide an aqueous solution suitable for use in a process of selectively removing zinc from zinc coated iron or steel substrates.

[0012] These and other objects and further advantages are met or exceeded by the present invention providing a process according to claim 1 , and an aqueous solution according to claim 8, and with preferred embodiments in the dependent claims and the description. In order to achieve these objects, the present invention proposes, in a first aspect, a process of stripping zinc from a coated steel substrate, the process comprising the steps of: providing a coated steel substrate comprising a zinc deposit on the steel substrate; and contacting the zinc deposit with an aqueous 0.1 to 14 M ammonia solution and an oxidizing agent selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate- ion, perborate-ion, permanganate-ion, or an inorganic salt thereof, capable of oxidizing zinc to selectively leach at least a portion of the zinc deposit from the coated steel substrate.

[0013] The method according to this invention provides a process of selectively removing zinc from zinc coated iron or steel substrates. The process has a high recovery rate or leaching efficiency and can be performed at ambient temperatures and under atmospheric conditions. The process of selectively removing zinc from zinc coated iron or steel substrates is without attacking the underlying iron or steel substrate. In best practice leaching efficiency rates close to 100% can be achieved depending on the nature of the zinc deposit. As the recovery rate of the zinc is very high the steel substrate may be further used as clean scrap in for example a steelmaking process not significantly increasing the zinc impurity level. The process uses chemicals having a low environmental impact. The process according to the invention is a simple process using readily available chemical components already well accepted for use in the chemical and metallurgical industry. The process is scalable to industrial scale.

[0014] In accordance with the invention it has been found that the specific combination of an ammonia solution together with a strong oxidizing agent selected from the defined group provides for a rapid dissolving or leaching of zinc into the alkaline solution. The combination of these compounds function to dissolve zinc oxides from the metal surface by forming complex zinc compounds (i.e. , zinc tetra-amine complex). The iron or steel substrate will only to a very limited extent react with the oxidizing agent, e.g. ammonium persulfate or sodium persulfate, but since the aqueous solution due to the dissolved ammonia is highly alkaline, viz. having a pH of more than about 9.5 and more typically of more than about 10, any formed non-soluble iron- oxide / hydroxide layer will not dissolve and forms a protective layer preventing the remaining iron from reacting. Thus the process is selective in removing the zinc from a zinc coated steel substrates without attacking the underlying steel substrate.

[0015] In an embodiment the method comprises the step of contacting the zinc deposit with an aqueous solution consisting of an 0.1 to 14 M ammonia solution and an oxidizing agent selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate-ion, perborate- ion, permanganate-ion, or an inorganic salt thereof, capable of oxidizing zinc to selectively leach at least a portion of the zinc deposit from the coated steel substrate.

[0016] The ammonia solution, also known as ammonia water, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). The process is performed using an ammonia solution in a concentration range of 0.1 to 14 M. Molarity (M) is the unit concentration expressed as the number of moles of dissolved solute per litre of solution. In an embodiment the concentration is at least 0.5 M. In an embodiment the concentration does not exceed 10 M, and preferably does not exceed 5 M. A too high a molarity adds to the costs and might create safety issues in the handling of the very alkaline solution. A too high a molarity adds to the costs and might create safety issues in the handling of the very alkaline solution. With increasing molarity the viscosity of the solution is increasing and might make it more difficult to subsequently rinse the solution of the zinc stripped steel substrate.

[0017] In the process according to the invention, the concentration of the oxidizing agent present in the ammonia solution can vary depending upon the particular application, the type of zinc material being removed from the steel substrate, the amount of the zinc to be removed, temperature and process economy. For most applications, however, the concentration of oxidizing agent is in a range of 5 to 200 g / l. In an embodiment the concentration is at least 10 g / l. In an embodiment the concentration does not exceed 100 g / l, and more preferably does not exceed 80 g / L

[0018] For the leaching of the zinc, the amount of oxidizing agent is preferably in the stoichiometric equivalent amount, thus one mol zinc requires one mol of for example persulfate present. However, in order to achieve a good process economy, it is preferably that about 1 .1 to 1 .6 times the stoichiometric amount of oxidizing agent necessary is present.

[0019] In accordance with the invention, the strong oxidizing agent assisting in the leaching of the zinc is selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate- ion, perborate-ion, permanganate-ion, or an inorganic salt thereof.

[0020] These ions or salts are highly soluble in water and provide a high recovery rate of the zinc. It avoids the use of gaseous oxygen as oxidizing agent, e.g. added to the solution by injection via bubbling or sparging techniques, or forced into the solution by placing the solution in a closed pressurized oxygen atmosphere. The use of a gaseous oxidizing agent undesirably results in an accelerated removal of ammonia from the aqueous solution and thus requires constant monitoring and replenishing to maintain optimum process conditions. The supply of a gaseous oxidizing agent requires considerable capital investment and maintenance. When air would be used the leaching process is very inefficient due to the low oxygen content and thus requires very long leaching time adversely affecting the process economy, in particular when performed at ambient temperature. When oxygen gas would be used the process economy is also adversely affected as oxygen gas is a very expensive process gas and the presence of pressurized oxygen may create safety issues on a shopfloor.

[0021] More specifically the selection is made from: a persulfate-ion or persulfate containing salt, i.e. ammonium, sodium or potassium salt; a peroxide, in particular of hydrogen, sodium or potassium; a perchlorate-ion or perchlorate containing salt, i.e. ammonium, sodium or potassium salt; a percarbonate-ion or percarbonate containing salt, i.e. ammonium, sodium or potassium salt; a perborate-ion or perborate containing salt, i.e. ammonium, sodium or potassium salt; or a permanganate or permanganate containing salt, i.e. ammonium, sodium or potassium salt.

[0022] In an embodiment the oxidizing agent is a persulfate-ion or a percarbonate-ion, or an inorganic salt thereof, and assists in the leaching of the zinc. These compounds provide a high zinc recovery. These compounds are industry accepted chemical compounds having a relatively low environmental impact and are available against reasonable costs.

[0023] In a preferred embodiment the oxidizing agent is a persulfate inorganic salt selected from the group of ammonium persulfate ([NFU^Os) and also known as ammonium peroxydisulfate, potassium persulfate (K2S2O8) and also known as potassium peroxydisulfate, sodium persulfate (Na2S2O8) and also known as sodium peroxydisulfate, or a combination thereof.

[0024] Most preferably the oxidizing agent is ammonium persulfate as it has a high solubility in an ammonia solution and providing a high zinc recovery. It has also a low environmental impact.

[0025] In a preferred embodiment the oxidizing agent is a persulfate inorganic salt selected from the group of ammonium percarbonate ([NH4]2CO3), potassium percarbonate (K2H3CO6) and also known as potassium carbonate peroxide, sodium percarbonate (Na2H3CO6) and also known as sodium carbonate peroxide, or a combination thereof.

[0026] Most preferably the oxidizing agent is ammonium percarbonate as it has a high solubility in an ammonia solution and providing a high zinc recovery. It has also a low environmental impact.

[0027] The leaching time or contact time of the coated steel substrate with the alkaline solution can vary depending upon the particular application, the type of zinc material being removed from the steel substrate, the concentration of the various chemical compounds, the scale of the process, process economy, and the amount of the zinc to be removed. It has been found that a leaching time or contact time of at least 5 minutes, and preferably of at least 10 minutes, is being preferred to achieve a fair recovery of zinc, in particular when performing the process on an industrial scale. It has been found that the leaching of zinc is very rapid in the process according to the invention. In an embodiment the leaching time does not exceed about 90 minutes, and in the best practices does not have to exceed about 60 minutes, and more preferably does not exceed about 25 minutes. These process times are very short compared to many hydrometallurgical processes known in the art.

[0028] The process can be performed over a broad range of temperatures, namely in a range of 5°C to 80°C. Preferably the method is performed at a temperature in a range of 15°C to 60°C, and more preferably in a range of 15°C to 40°C, and most preferably in a range of 15°C to 35°C. It has been observed, however, that the solution starts to boil at about 50°C and boiling may become very violent at about 80°C making a good process control more difficult. At this temperature range the process can be applied on an industrial scale at ambient temperatures with no or very limited additional heating of the ammonia solution so a cost and energy efficient process is obtained. At the temperature range of 15°C to 40°C the ammonia solution maintains remains physically stable and a high zinc leaching efficiency is obtained.

[0029] In an embodiment of the process the ammonia solution containing the coated steel substrate is agitated to increase the efficiency of the leaching process. The ammonia solution may be kept in motion or is agitated by techniques known in the art, including stirring, magnetic stirring, circulating by pumping around, ultrasound techniques, or combinations thereof. Alternatively, the coated steel scrap material is kept in motion within the ammonia solution, for example using rotatable perforated drums holding the scrap metal.

[0030] In an embodiment of the invention the process comprises a step of recovering the dissolved zinc from the ammonia solution by either crystallization, electrolysis, electroplating, or precipitation. Preferably, the dissolved zinc can be recovered from the ammonia solution by means of electroplating. Thus, the waste of the materials can be minimized by using the ammonia solution as an electrolyte source of electroplating after the removal of the zinc deposit or coating from said coated steel scrap material.

[0031] In an embodiment of the invention the process further comprises a step of recovering the zinc deposit stripped steel base metal by removing it from the ammonia solution after the leaching of the zinc deposit is substantially completed. Preferably at least 80% of the zinc has been removed or stripped from the original coated steel substrate, and in the best practices at least 90%. After being taken out of the ammonia solution, the zinc deposit stripped steel base metal is preferably rinsed with water and may be used as clean steel scrap metal in for example a steelmaking process. In an aspect of the invention it relates to an aqueous alkaline composition having a pH of >9.5, and preferably pH of >10, for the stripping of zinc deposit from a zinc coated steel substrate, comprising of, and preferably consisting of: 0.1 to 14 M ammonia, preferably 0.1 to 5 M ammonia, and 5 to 200 g / l of an oxidizing agent selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate-ion, perborate-ion, permanganate-ion, or an inorganic salt thereof, and capable of oxidizing zinc to selectively leach at least a portion of the zinc deposit from the coated steel substrate; and with more preferred ranges as herein described.

[0032] In an embodiment it relates to an aqueous alkaline composition for the stripping of zinc deposit from a coated steel substrate, comprising of, and preferably consisting of: 0.1 to 14 M ammonia, preferably 0.1 to 5 M ammonia, and 5 to 100 g / l of an oxidizing agent selected from the group of ammonium persulfate, potassium persulfate, and sodium persulfate; and with more preferred ranges as herein described.

[0033] In an embodiment it relates to an aqueous alkaline composition for the stripping of zinc deposit from a coated steel substrate, comprising of, and preferably consisting of: 0.1 to 14 M ammonia, preferably 0.1 to 5 M ammonia, and 5 to 100 g / l of an oxidizing agent selected from the group of ammonium percarbonate, potassium percarbonate, and sodium percarbonate; and with more preferred ranges as herein described.

[0034] The invention also relates to the use of said aqueous alkaline composition having a pH of >9.5, and preferably a pH of >10, comprising of, and preferably consisting of: 0.1 to 14 M ammonia, preferably 0.1 to 5 M ammonia, and 5 to 200 g / l of an oxidizing agent selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate-ion, perborate-ion, permanganate-ion, or an inorganic salt thereof, and capable of oxidizing zinc to selectively leach at least a portion of the zinc deposit from the coated steel substrate; and with more preferred ranges of the chemical components as herein described, for the stripping of zinc deposit from a zinc coated steel substrate.

[0035] In another aspect of the invention it relates to the use of one of ammonium persulfate, potassium persulfate, and sodium persulfate in the stripping of zinc deposit from a zinc coated steel substrate in a process according to the invention. Preferably it relates to the use of ammonium persulfate in the stripping of zinc deposit from a zinc coated steel substrate in a process according to the invention. In a further aspect of the invention it relates to the use of one of ammonium percarbonate, potassium percarbonate, and sodium percarbonate in the stripping of zinc deposit from a zinc coated steel substrate in a process according to the invention. Preferably it relates to the use of ammonium percarbonate in the stripping of zinc deposit from a zinc coated steel substrate in a process according to the invention.

[0036] The invention will now be illustrated with reference to the example according to the invention.

[0037] EXAMPLE

[0038] To demonstrate the proof of principle for the leaching of zinc, in an experimental setup the leaching efficiency as function of leaching time for a zinc-plated steel of 50x50 mm having a pure zinc layer of 80 g / m2 per side has been measured in a measuring glass holding a stirred aqueous solution consisting of 1.5 M ammonia and 30 g / l of ammonium persulfate ([NH4]2S2O8). At ambient temperature (20°C) the zinc-plated sample has been immersed in this aqueous solution and the leaching efficiency (%) as function of the leaching time has been measured and the results are shown in Fig. 1. Although a zinc tetra-amine complex compound is formed, the aqueous solution remains colourless. The leaching efficiency, also known as the recovery rate, is the percentage of zinc removed compared to what is initially present in the zinc coating on the steel substrate.

[0039] The solution used provides rapid leaching of the zinc from the zinc-coated steel substrate and in this setup an almost complete removal of the zinc deposit was achieved after about 10 minutes.

Claims

CLAIMS1. Process of stripping zinc from a coated steel substrate, the process comprising the steps of: providing a coated steel substrate comprising a zinc deposit on a steel substrate; contacting the zinc deposit with an aqueous 0.1 to 14 M ammonia solution and an oxidizing agent selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate-ion, perborate-ion, permanganate-ion, or an inorganic salt thereof, and capable of oxidizing zinc.

2. Process according to claim 1 , wherein the concentration of the oxidizing agent is in a range of 5 to 200 g / l, and preferably of 5 to 100 g / l, and more preferably of 10 to 100 g / l.

3. Process according to claims 1 or 2, wherein the oxidizing agent is a persulfate-ion or inorganic salt thereof, and preferably the oxidizing agent is an inorganic salt selected from the group of ammonium persulfate, potassium persulfate, sodium persulfate, or a combination thereof.

4. Process according to claims 1 or 2, wherein the oxidizing agent is a percarbonate-ion or inorganic salt thereof, and preferably the oxidizing agent is an inorganic salt selected from the group of ammonium percarbonate, potassium percarbonate, sodium percarbonate, or a combination thereof.

5. Process according to claim 1 or 2, wherein the oxidizing agent is ammonium persulfate.

6. Process according to claim 1 or 2, wherein the oxidizing agent is ammonium percarbonate.

7. Process according to any one of claims 1 to 6, wherein the temperature of the ammonia solution is in a range of 5°C to 80°C, preferably of 15°C to 60°C, and more preferably of 15°C to 40°C.

8. Process according to any one of claims 1 to 7, wherein the aqueous ammonia solution has a pH > 9.5, and preferably of pH >10.

9. Process according to any one of claims 1 to 8, wherein contacting the coated steel substrate with said aqueous solution is for at least 5 min., and preferably for not more than 90 min., and more preferably for not more than 60 min.

10. Process according to any one of claims 1 to 9, wherein the process comprises a step of recovering dissolved zinc from the ammonia solution by either crystallization, electrolysis, electroplating, or precipitation.

11. Aqueous alkaline composition having a pH > 9.5 for the stripping of zinc deposit from a coated steel substrate, comprising of 0.1 to 14 M ammonia, preferably 0.1 to 5 M ammonia; and 5 to 200 g / l of an oxidizing agent selected from the group of: a persulfate-ion, peroxideion, perchlorate-ion, percarbonate-ion, perborate-ion, permanganate-ion, or an inorganic salt thereof, and capable of oxidizing zinc to selectively leach at least a portion of the zinc deposit from the coated steel substrate.

12. Aqueous alkaline composition according to claim 11 having a pH > 10.

13. Aqueous alkaline composition according to claim 11 or 12, wherein the aqueous solution comprises 5 to 100 g / l, and more preferably 10 to 100 g / l, of an oxidizing agent selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate-ion, perborate-ion, permanganate-ion, or an inorganic salt thereof, and capable of oxidizing copper to selectively leach at least a portion of the copper deposit from the coated steel substrate.

14. Use of one of ammonium persulfate, potassium persulfate, or sodium persulfate, and preferably ammonium persulfate, in the stripping zinc deposit from a coated steel substrate in a process according to any one of claims 1 to 3, or 5, or 7 to 10.

15. Use of one of ammonium percarbonate, potassium percarbonate, or sodium percarbonate, and preferably ammonium percarbonate, in the stripping zinc deposit from a coated steel substrate in a process according to any one of claims 1 to 2, or 4, or 6 to 10.

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