Process of stripping copper from coated steel substrates

Abstract

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

Description

[0001] PROCESS OF STRIPPING COPPER FROM COATED STEEL SUBSTRATES

[0002] FIELD OF THE INVENTION

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

[0004] BACKGROUND TO THE INVENTION

[0005] Steel plated or steel electroplated with copper is utilized in numerous electrical and automative applications. The copper layer offers high electrical conductivity and resistance to corrosion. It is also highly scratch-resistant, making the products suitable for decorative casings in addition to functional components. Copper-plated steel has strong brazing (self-welding) properties and is used amongst others in automotive products such as brazed double-walled tubes and bearings.

[0006] Steelmaking requires the use of iron-containing material, such as steel scrap, direct reduced iron or pig iron. In order to reduce the carbon footprint of the steel industry, the use of steel scraps is seen as key. However, steel scrap contains residual elements, notably Cu, Cr, Mo, Ni, Sn and / or Zn. The use of steel scrap is therefore not widely spread for all steel grades as those residual elements can have a detrimental effect on the steel properties. During steel production by means of direct reduced iron and / or pig iron, small amounts of residual elements are inevitably left behind in the liquid steel. When using steel scrap, the amount of residual elements is much greater compared to pig iron coming from a blast furnace or direct reduced iron. Too high an amount of residual elements may adversely affect subsequent production steps such as hot rolling, annealing and coating processes. It may also adversely affect engineering properties of the resultant steel product such as for example cold forming properties and machinability. Hence there is a need to avoid the introduction of too high an amount of residual elements into the steelmaking process.

[0007] With this in mind, the importance for stripping or recovering copper from copper (electro) plated or copper coated steel scrap parts is increasing also, and represents an opportunity in urban mining and supply chain circularity. Copper-coated or copper-plated steel scrap is accumulated at different stages, including (i) off-specification copper-plated steel sheet generated in the (electro-) plating plants, (ii) reject copper-plated steel sheets and tubes at tube making facilities or other facilities manufacturing items of copper-plated steel, and (iii) old scrap of copper-plated steel products, such as end-of-life automotive tubes, collected by industrial and municipalities collection programmes. There is a demand for improved methods having a high recovery rate of selectively removing copper from copper coated iron or steel substrates.

[0008] 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.

[0009] There is a demand for improved methods having a high recovery rate of selectively removing copper from copper coated iron or steel substrates.

[0010] DESCRIPTION OF THE INVENTION

[0011] It is an object of the invention to provide a process of selectively removing copper from copper coated iron or steel substrates.

[0012] It is an object of the invention to provide at least an alternative process of selectively removing copper from copper coated iron or steel substrates.

[0013] It is another object of the invention to provide an aqueous solution suitable for use in a process of selectively removing copper from copper coated iron or steel substrates.

[0014] 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.

[0015] In order to achieve these objects, the present invention proposes, in a first aspect, a process of stripping copper from a coated steel substrate, the process comprising the steps of: providing a coated steel substrate comprising a copper deposit on the steel substrate; and contacting the copper 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 copper to selectively leach at least a portion of the copper deposit from the coated steel substrate. The method according to this invention provides a process of selectively removing copper from copper 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 copper from copper 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 copper deposit. As the recovery rate of the copper is very high the steel substrate may be further used as clean scrap in for example a steelmaking process not significantly increasing the copper 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.

[0016] 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 copper into the alkaline solution. The combination of these compounds function to dissolve copper oxides from the metal surface by forming complex copper compounds (i.e., copper 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 9.5 and more preferably of more than 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 copper from a copper coated steel substrates without attacking the underlying steel substrate.

[0017] In an embodiment the method according to the invention is to selectively dissolve copper from copper coated steel substrates.

[0018] In an embodiment the method comprises the step of contacting the copper 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 copper to selectively leach at least a portion of the copper deposit from the coated steel substrate.

[0019] 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 copper stripped steel substrate.

[0020] 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 copper material being removed from the steel substrate, the amount of the copper 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.

[0021] For the leaching of the copper the amount of oxidizing agent is preferably in the stoichiometric equivalent amount, thus one mol copper requires one mol of for example persulfate present. However, in order to achieve a good process economy, it is preferably that 1.1 to 1.6 times the stoichiometric amount of oxidizing agent necessary is present.

[0022] In accordance with the invention, the strong oxidizing agent assisting in the leaching of the copper is selected from the group of: a persulfate-ion, peroxide-ion, perchlorate-ion, percarbonate-ion, perborate-ion, permanganate-ion, or an inorganic salt thereof.

[0023] These ions or salts are highly soluble in water and provide a high recovery rate of the copper. 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. 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.

[0024] 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 copper. These compounds provide a high copper recovery. These compounds are industry accepted chemical compounds having a relatively low environmental impact and are available against reasonable costs.

[0025] In a preferred embodiment the oxidizing agent is a persulfate inorganic salt selected from the group of ammonium persulfate ([NH4]2S2O8) 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.

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

[0027] 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.

[0028] Most preferably the oxidizing agent is ammonium percarbonate as it has a high solubility in an ammonia solution and providing a high copper recovery. It has also a low environmental impact.

[0029] 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 copper 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 copper 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 copper , in particular when performing the process on an industrial scale. It has been found that the leaching of copper is very rapid in the process according to the invention. In an embodiment the leaching time does not exceed 90 minutes, and in the best practices does not have to exceed 60 minutes, and more preferably does not exceed 25 minutes. These process times are very short compared to many hydrometallurgical processes known in the art.

[0030] The process can be performed over a broad range of temperatures, namely in a range of about 5°C to 80°C. Preferably the method is performed at a temperature in a range of about 15°C to 60°C, and more preferably in a range of about 15°C to 40°C, and most preferably in a range of about 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 copper leaching efficiency is obtained.

[0031] 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.

[0032] In an embodiment of the invention the process comprises a step of recovering the dissolved copper from the ammonia solution by either crystallization, electrolysis, electroplating, or precipitation. Preferably, the dissolved copper 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 copper deposit or coating from said coated steel scrap material.

[0033] In an embodiment of the invention the process further comprises a step of recovering the copper deposit stripped steel base metal by removing it from the ammonia solution after the leaching of the copper deposit is substantially completed. Preferably at least 80% of the copper 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 copper 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.

[0034] 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 copper deposit from a copper 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 copper to selectively leach at least a portion of the copper deposit from the coated steel substrate; and with more preferred ranges as herein described.

[0035] In an embodiment it relates to an aqueous alkaline composition for the stripping of copper 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.

[0036] In an embodiment it relates to an aqueous alkaline composition for the stripping of copper 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.

[0037] 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 copper to selectively leach at least a portion of the copper deposit from the coated steel substrate; and with more preferred ranges of the chemical components as herein described, for the stripping of copper deposit from a copper coated steel substrate.

[0038] 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 copper deposit from a copper coated steel substrate in a process according to the invention. Preferably it relates to the use of ammonium persulfate in the stripping of copper deposit from a copper coated steel substrate in a process according to the invention.

[0039] 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 copper deposit from a copper coated steel substrate in a process according to the invention. Preferably it relates to the use of ammonium percarbonate in the stripping of copper deposit from a copper coated steel substrate in a process according to the invention.

[0040] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

[0041] EXAMPLE 1.

[0042] In an experimental setup the weight loss as function of leaching time of a sample of pure copper sheet of 50x50 mm has been measured in a measuring glass holding a stirred aqueous solution consisting of 1.5 M ammonia. At ambient temperature (20°C) the copper sample has been immersed in this aqueous solution and after more than about 10 minutes the colour of the solution changed from colourless to very light blue indicating the formation of a minor amount copper tetra-amine complex. However, the dissolution rate was extremely slow. This experiment does show that copper is leaching into the ammonia solution but only extremely slow.

[0043] EXAMPLE 2.

[0044] In another experiment the weight loss as function of leaching time of a sample of pure copper sheet of 50x50 mm has been measured in a measuring glass holding a stirred aqueous solution consisting of 1.5 M ammonia and 1 g / l of sodium thiosulfate (Na2S2O3). At ambient temperature (20°C) the copper sample has been immersed in this aqueous solution and after more than about 10 minutes the colour of the solution changed from colourless to very light blue indicating the formation of a copper tetra-amine complex. This experiment illustrates that copper is leaching into the ammonia solution having also a thiosulfate, but only extremely at a very low rate.

[0045] The same lack of effect occurs when using for example potassium thiosulfate, ammonium thiosulfate, ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, ammonium thioglycolate, potassium thioglycolate, sodium thioglycolate, or a combination thereof.

[0046] EXAMPLE 3.

[0047] To demonstrate the proof of principle for the leaching of copper, in an experimental setup the weight loss as function of leaching time of a sample of pure copper sheet of 50x50 mm 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 ([NFU^Os). At ambient temperature (20°C) the copper sample has been immersed in this aqueous solution and the weight loss (g) as function of the leaching time has been measured and is shown in Fig. 1. The leaching of Cu could also be observed as the solution turns dark blue due to the formation of a copper tetra-amine complex compound.

[0048] This illustrates that the process according to the invention can be used also for the selective stripping or removal of copper deposits from steel substrates.

Claims

CLAIMS1. Process of stripping copper from a coated steel substrate, the process comprising the steps of: providing a coated steel substrate comprising a copper deposit on a steel substrate; contacting the copper 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 copper.

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 claim 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 claim 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 copper from the ammonia solution by either crystallization, electrolysis, electroplating, or precipitation.11 . Aqueous alkaline composition having a pH > 9.5 for the stripping of copper deposit from a coated steel substrate, comprising of 0.1 to 14 M ammonia, and more 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 copper to selectively leach at least a portion of the copper 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 copper 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 copper 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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