Field of the Invention

The present invention relates to an aqueous stripping composition for electrolytically removing a metal deposit from a substrate, the composition comprising

(i) nitrate anions,

(ii) one or more than one carboxylic acid and/or salts thereof, and

(iii) one or more than one kind of halogen anions, wherein in the aqueous stripping composition (i) is the only nitrogen-containing species. The present invention furthermore relates to a respective use thereof and a respective method utilizing said aqueous stripping composition.

Background of the Invention

Stripping of a metal deposit is generally important in the plating industry, particularly in the electroplating industry. Typically, the substrate to be plated needs to be somehow fixed in, or at least restricted to, a certain place (i.e. a fixation), e.g. on a rack or in a barrel, respectively, before it is contacted with an electroplating composition. As a matter of fact, in many cases the metal layers are not only deposited on the substrate but also, at least partly, on the respective fixation. Since the fixation is re-used, the metal layers are accumulating over time, forming an undesired and comparatively thick metal deposit. This is particularly a problem for electrical contact areas in the fixation (also often named contact tips). Thus, a stripping of such a metal deposit is desired from time to time, particularly for maintaining the fixation in reliable conditions and free of undesired metal layers/metal deposits. In many cases, the fixation comprises or is made of stainless-steel as a base material.

Furthermore, when exposed to the various aggressive chemicals typically used in a plat- ing/electroplating composition and during loading/discharging of the substrates to the fixation, it has been observed for stainless-steel that this material does suffer certain surface modification. In many cases, increased roughness implicates a risk of increased dendrite formation during electroplating and reduced rinsing properties is observed, which are not desired. Deeper investigations have shown that reliable stainless-steel contact areas are essential in modern processes to ensure optimum current flow to the substrate. It has been found that a good plating without defects (i.e. expected thickness, no shadings, and/or no undesired roughness), as well as an effective/safe attaching to the fixation significantly depends on the condition of those contact areas. However, keeping the fixation and particularly the contact areas free from the undesired metal deposit, dendrites, and undesired roughness, all at the same time, is demanding. Typically, a high degree of selective stripping is required to remove the undesired metal deposit, typically without harming the base material of the fixation. This means that, on the one hand, a respective stripping composition must be highly selective for typical kinds of metals such as copper, nickel, and chromium, while, on the other hand, removal of stainless-steel must not occur.

Generally, stripping compositions are known in the art.

US 6,332,970 B1 refers to electrolytic stripping solutions comprising oxoacids and/or oxoacid salts, and hydrogen peroxide for the rapid removal of electroless nickel from iron, steel, aluminum, and titanium alloys as well as other selected electrically conductive substrates.

EP 3 168 332 B1 refers to the use of a jig electrolytic stripper for removing palladium, comprising for example nitric acid, a further nitrogen-containing compound, and a bromide.

Although effective stripping compositions are known, it has been observed that typically they cannot fulfill all of the aforementioned requirements, i.e. removal of the undesired metal deposit and, at the same time, reducing/preventing an undesired surface roughness.

In addition to a good stripping quality, in the past years, sustainability improvements became more and more important. As a matter of facts, a fully utilized aqueous stripping composition comprising not only the original compounds but additionally the stripped metal cations, must be disposed. Therefore, wastewater treatment is an important issue.

There is therefore an increasing demand to further improve existing stripping compositions, particularly in view of sustainability aspects, and the above-mentioned requirements. of the Invention It is the objective of the present invention to provide a more sustainable stripping composition without compromising the stripping quality and stripping selectivity. It is particularly an objective to reduce wastewater treatment steps/efforts and most preferably to even allow at least partially a recycling of at least some ingredients, primarily of stripped metals. Furthermore, the stripping composition should be able to reduce/prevent an undesired surface roughening particularly on stainless-steel, e.g. stainless-steel contact areas.

Summary of the Invention

These objectives are solved by an aqueous stripping composition for electrolytically removing a metal deposit from a substrate, the composition comprising

(i) nitrate anions,

(ii) one or more than one carboxylic acid and/or salts thereof, and

(iii) one or more than one kind of halogen anions, wherein in the aqueous stripping composition (i) is the only nitrogen-containing species.

Furthermore, the objectives are solved by a method for removing partly or fully a metal deposit from a substrate as explained in further detail below in the text.

In the context of the present invention, it is an essential feature that the aqueous stripping composition contains nitrate anions as the only nitrogen-containing species (or compound). This is essential in view of wastewater treatment because nitrogen-containing compounds such as ammonia and/or other amines are typically problematic in view of wastewater treatment due to their stable metal-complex formation. Typically, stripped metals must be removed beforehand to comply with common wastewater regulations. If ammonia and/or other amines are used, typically various treatment steps are included to remove them, such as precipitation, decantation, filtration, chemical oxidation, neutralization, and/or chemical reduction. As of today, most aqueous stripping compositions, particularly for removing copper and nickel, do require all or almost all of these treatment steps. On the contrary, the aqueous stripping composition of the present invention generally conforms to legal regulations already after a single neutralization and precipitation step, without the need of further chemical oxidation and/or reduction. This is accomplished due to being free of nitrogen-containing compounds other than nitrate anions.

Furthermore, the aqueous stripping composition of the present invention allows a selective removal of nickel (including very noble semi-bright to less noble bright nickel), copper, chromium, palladium and alloys comprising at least one thereof without a substantial removal of stainless-steel, which is a typical base material for substrates such as e.g. for fixations. In this context, it is not relevant whether said metals and alloys thereof are present in single layers, combinations, or mixtures thereof. They will be removed, most preferably with very similar stripping rates (in particular for nickel and copper), irrespective of how they are organized in the metal deposit. Thus, in the context of the present invention, the metal deposit comprises at least one or more than one metal selected from the group consisting of copper, nickel, chromium, and palladium; alternatively the metal deposit preferably comprises one or more than one metal selected from the group consisting of copper, nickel, chromium, palladium, and tin. This preferably includes alloys comprising at least one of the aforementioned metals. Most preferably, copper and/or (preferably and) nickel are present in a higher amount than palladium, preferably individually as well as in combination. Also preferably, copper and/or (preferably and) nickel are present in a higher amount than chromium, preferably individually as well as in combination. Most preferably, palladium is present in the lowest amount compared to the individual amounts of copper, nickel, and chromium.

Furthermore, own experiments have shown that the aqueous stripping composition of the present invention shows a particular well balanced base material removal. This means that the removal of the base material is not entirely zero but rather very low. It was first assumed that even a very low removal is not desired. However, surprisingly it turned out that such a very low removal contributes to a desired surface smoothing. This means that undesired pits and scratches are leveled or even prevented, cavities are opened and smoothened, and a superior surface cleaning effect is obtained, which reduces contamination of a respective subsequent electroplating composition. Furthermore, an improved rinsing property was observed. It has been found out that even a repeated application of the aqueous stripping composition does not lead to a significant loss of base material over time but rather provides above-mentioned advantages. It was very surprising that the aqueous stripping composition of the present invention is showing such a well-balanced individual removal.

In addition to all this, it was unexpected that the aqueous stripping composition of the present invention allows an electrowinning application. This means that utilizing the aqueous stripping composition of the present invention in a respective electrolytic stripping method, the stripped metals (which are anodically dissolved in the aqueous stripping composition) can be deposited on the cathode. This most particularly applies to copper and palladium, most preferably to copper. However, this is also applicable to stripped nickel ions. Thus, stripped metals do not need to be discarded fully but rather are preferably recycled, at least partly. Even a selective cathodic metal deposition is most preferably possible in order to separate stripped metals from each other prior to recycling. It is assumed that the absence of nitrogen-containing species other than nitrate anions prevent an overly strong complexation of the stripped metal cations, which in turn allows a cathodic deposition. A respective deposit is cathodically formed, which preferably mainly comprises copper, as copper is present in a higher amount than palladium in an utilized aqueous stripping composition.

Detailed Description of the Invention

In the context of the present invention, the term “removing” and “removed” denotes a dissolving (i.e. a dissolution), preferably an electro-chemical dissolving and dissolution, respectively. It likewise means a stripping, preferably an electro-chemical stripping. In each case, the removal primarily depends on the electrical current as a driving force accelerating and primarily pushing the dissolution. Preferably it includes a transition from an insoluble state into a soluble state, most preferably by electro-chemical anodic oxidation. This includes that metals of the metal deposit form an anode connected to a rectifier while a counter electrode is used as a cathode to allow said electro-chemical dissolution. As a result, the removed metals preferably form metal ions.

The aqueous stripping composition of the present invention comprises a solvent. Preferably, the solvent is electroconductive to support said anodic dissolution.

The aqueous stripping composition of the present invention is aqueous, i.e. it comprises water, preferably at least 55 vol.-% or more is water, based on the total volume of the aqueous stripping composition, more preferably 65 vol.-% or more, even more preferably 75 vol.-% or more, yet even more preferably 85 vol.-% or more, still more preferably 90 vol.-% or more, most preferably 95 vol.-% or more. Most preferably, water is the only solvent.

Preferred is an aqueous stripping composition of the present invention, wherein the aqueous stripping composition is alkaline, acidic or neutral, preferably acidic or neutral, most preferably acidic.

More preferred is an aqueous stripping composition of the present invention, wherein the aqueous stripping composition has a pH ranging from 4 to 9, preferably from 4.5 to 8.5, more preferably from 5 to 8, more preferably from 5.3 to 7.5, even more preferably from 5.5 to 7, most preferably from 5.7 to 6.5. Preferred is an aqueous stripping composition of the present invention, wherein (i) has a concentration ranging from 10 mmol/L to 2600 mmol/L, based on the total volume of the aqueous stripping composition, preferably from 100 mmol/L to 2000 mmol/L, more preferably from 200 mmol/L to 1600 mmol/L, even more preferably from 300 mmol/L to 1400 mmol/L, yet even more preferably from 400 mmol/L to 1200 mmol/L, most preferably from 500 mmol/L to 900 mmol/L, yet most preferably from 550 mmol/L to 750 mmol/L.

If the concentration is significantly below 10 mmol/L, in many cases the metal deposit is insufficiently removed, in particular the removal of nickel and its alloys is insufficient. If the concentration is significantly exceeding 2600 mmol/L, often it was observed that the removal is excessive, including that even stainless-steel is too strongly removed. Therefore, aforementioned well-balanced individual removal is deteriorated.

In some cases, preferred is an aqueous stripping composition of the present invention, wherein (i) has a total concentration of at least 100 mmol/L, based on the total volume of the aqueous stripping composition, preferably of at least 140 mmol/L, more preferably of at least 200 mmol/L. This applies most preferably in combination with the upper concentration limits mentioned in the ranges above for (i). In a few cases, if (i) has a total concentration below 100 mmol/L an undesired stainless-steel removal was observed.

A preferred source of said nitrate anions is an alkaline nitrate salt, preferably sodium nitrate.

It is assumed that said nitrate anions primarily interact with hydronium cations generated from anodic water hydrolysis. This leads to a local generation of nitric acid, which, on the one hand, efficiently dissolves the metal deposit, particularly copper and nickel. On the other hand, nitric acid forms a protective passivation layer on stainless-steel. Upon dissolution of the metal deposit, dissolved metal cations form soluble nitrate salts.

It is essential that (i) is the only source of nitrogen in the entire aqueous stripping composition. This particularly includes that in (ii) no compounds are included that comprise a nitrogen atom.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) has a total concentration ranging from 25 mmol/L to 3000 mmol/L, based on the total volume of the aqueous stripping composition, preferably from 70 mmol/L to 2000 mmol/L, more preferably from 120 mmol/L to 1500 mmol/L, even more preferably from 230 mmol/L to 1000 mmol/L, yet even more preferably from 300 mmol/L to 900 mmol/L, most preferably from 370 mmol/L to 800 mmol/L, even most preferably from 450 mmol/L to 700 mmol/L.

If the concentration is significantly below 25 mmol/L, in a number of cases the conductivity of the aqueous stripping composition is too low such that the dissolution of the metal deposit is too slow. Under such conditions, the dissolution typically requires very high amounts of energy and a respective method is not economic anymore. If the concentration is significantly exceeding 2500 mmol/L, it was often observed that a blocking salt layer was formed on the anode, stopping or at least diminishing the current flow and therefore deteriorating the complete stripping process.

In the context of the present invention, the one or more than one carboxylic acid and/or salts thereof, preferably serves as conductivity agent and/or complexing agent.

Preferred is an aqueous stripping composition of the present invention, wherein (i) to (iii) has a molar ratio of 1 or more, based on the total concentration of (iii), preferably of 1.1 or more, more preferably of 1.3 or more, even more preferably of 1.5 or more, most preferably of 2 or more. If this molar ratio is below 1 , in some cases an undesired stain- less-steel removal was observed.

More preferred is an aqueous stripping composition of the present invention, wherein (i) to (iii) has a molar ratio ranging from 1 to 20, based on the total concentration of (iii), preferably from 1 .1 to 15, more preferably from 1.3 to 11 , even more preferably from 1.5 to 9, yet even more preferably from 1.6 to 7, most preferably from 1.7 to 4. In some cases, very preferred is a ratio ranging from 1.7 to 2.8. Very preferred is a ratio of (i) to (iii) of below 10 (but more than zero), preferably below 9. In some cases, an undesired surface roughening was observed if this ratio is significantly exceeding 10.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) to (iii) has a molar ratio of 1 or more, based on the individual total concentration of (ii) and (iii), preferably of 1.3 or more, more preferably of 1 .5 or more, even more preferably of 1.7 or more, most preferably of 1.8 or more.

More preferred is an aqueous stripping composition of the present invention, wherein (ii) to (iii) has a molar ratio ranging from 1 to 8, based on the individual total concentration of (ii) and (iii), preferably from 1.3 to 6, more preferably from 1.5 to 4, even more preferably from 1.7 to 3.5, most preferably from 1.8 to 2.3.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) to (i) has a molar ratio of more than 0 but less than 10, based on the total concentration of (ii), preferably ranging from 0.1 to 9, more preferably from 0.2 to 8, even more preferably from 0.3 to 6, yet even more preferably from 0.4 to 4, most preferably from 0.5 to 2. If this ratio is significantly exceeding 10, in some cases an undesired surface roughening was observed.

The aqueous stripping composition of the present invention comprises one or more than one carboxylic acid and/or salts thereof, preferably in a total concentration and/or molar ratio as defined above. The one or more than one carboxylic acid and salts thereof do not comprise nitrogen.

In some cases, it is preferred that said one or more than one carboxylic acid and/or salts thereof are the only organic compounds in the aqueous stripping composition.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises (ii-a) one or more than one C1-C4 carboxylic acid and/or salts thereof, preferably one or more than one C1-C3 carboxylic acid and/or salts thereof, most preferably one or more than one C2-C3 carboxylic acid and/or salts thereof. This preferably applies to a monocarboxylic acid.

Preferably, carboxylic acids according to (ii-a) primarily serve as conductivity agents in the context of the present invention and preferably serve in addition as buffer agent.

Preferred is an aqueous stripping composition of the present invention, wherein (i) to (ii- a) has a molar ratio of 0.1 or more, based on the individual total concentration of (i) and (ii-a), preferably of 0.4 or more, more preferably of 0.7 or more, even more preferably of 1 or more, most preferably of 1.2 or more.

Preferred is an aqueous stripping composition of the present invention, wherein (ii-a) has a total (i.e. the total of all individual (ii-a)) concentration ranging from 50 mmol/L to 2000 mmol/L, based on the total volume of the aqueous stripping composition, preferably from 100 mmol/L to 1500 mmol/L, more preferably from 150 mmol/L to 1000 mmol/L, even more preferably from 200 mmol/L to 900 mmol/L, yet even more preferably from 250 mmol/L to 800 mmol/L, most preferably from 300 mmol/L to 700 mmol/L, even most preferably from 400 mmol/L to 600 mmol/L.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises (ii-b) one or more than one C5-C14 carboxylic acid and/or salts thereof, preferably one or more than one C6-C10 carboxylic acid and/or salts thereof, more preferably one or more than one C6-C9 carboxylic acid and/or salts thereof, most preferably one or more than one C6-C8 carboxylic acid and/or salts thereof, yet most preferably one or more than one C6-C7 carboxylic acid and/or salts thereof. This preferably applies to a hydroxycarboxylic acid. This more preferably applies to a monocarboxylic acid, a dicarboxylic acid, and/or a tricarboxylic acid, most preferably to a monocarboxylic acid. This most preferably applies to a hydroxy-monocarboxylic acid and/or a hydroxy-tricarboxylic acid, most preferably to a hydroxy-monocarboxylic acid. In some cases, a tricarboxylic acid, preferably a hydroxy-tricarboxylic acid, most preferably citric acid, and/or salts thereof are preferred. Preferably, (ii-b) is present in addition to (ii-a), which is in most cases significantly improving the efficiency of the aqueous stripping composition.

Preferably, carboxylic acids according to (ii-b) primarily serve as complexing agents in the context of the present invention.

Preferred is an aqueous stripping composition of the present invention, wherein (i) to (ii- b) has a molar ratio of 1 or more, based on the individual total concentration of (i) and (ii- b), preferably of 2 or more, more preferably of 5 or more, even more preferably of 8 or more, most preferably of 10 or more. Very preferred is a range from 1 to 25, preferably from 2 to 20, most preferably from 5 to 15, even most preferably from 10 to 14.

Preferred is an aqueous stripping composition of the present invention, wherein (ii-b) has a total (i.e. the total of all individual (ii-b)) concentration ranging from 1 mmol/L to 1000 mmol/L, based on the total volume of the aqueous stripping composition, preferably from 10 mmol/L to 500 mmol/L, more preferably from 20 mmol/L to 250 mmol/L, even more preferably from 30 mmol/L to 150 mmol/L, yet even more preferably from 35 mmol/L to 120 mmol/L, most preferably from 40 mmol/L to 90 mmol/L, even most preferably from 45 mmol/L to 75 mmol/L.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises at least one monocarboxylic acid and/or salts thereof.

More preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises only monocarboxylic acids and/or salts thereof. In some rare cases it is preferred that (ii-b) comprises only tricarboxylic acids and/or salts thereof.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises two or more than two, preferably two, carboxylic acids and/or salts thereof.

Preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises two or more than two, preferably two, monocarboxylic acids and/or salts thereof.

More preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises formic acid, acetic acid, and/or salts thereof, preferably acetic acid and/or salts thereof. Preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises at least one hydroxy carboxylic acid and/or salts thereof.

More preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises citric acid, gluconic acid, heptagluconic acid, and/or salts thereof, most preferably gluconic acid and/or salts thereof.

Most preferred is an aqueous stripping composition of the present invention, wherein (ii) comprises

(ii-a) a first mono-carboxylic acid and/or salts thereof, preferably a C1-C4 monocarboxylic acid and/or salts thereof, more preferably formic acid, acetic acid, and/or salts thereof, most preferably acetic acid and/or salts thereof, and additionally

(ii-b) a second mono-carboxylic acid and/or salts thereof, preferably a C5-C14 mono-carboxylic acid and/or salts thereof, more preferably a C5-C14 hy- droxy-monocarboxylic acid and/or salts thereof, most preferably gluconic acid, heptagluconic acid, and/or salts thereof.

Furthermore, the aqueous stripping composition of the present invention comprises (iii) one or more than one kind of halogen anions.

The one or more than one kind of halogen anions primarily support removal of oxides, particularly of nickel oxides. This supports the removal of the metal deposit.

Preferred is an aqueous stripping composition of the present invention, wherein (iii) comprises fluoride, chloride, bromide and/or iodide, preferably chloride, bromide, and/or iodide, more preferably chloride and/or bromide, most preferably bromide. Preferably, (iii) comprises only chloride and/or bromide, most preferably only bromide. Most preferably, bromide is the only kind of halogen anions in the aqueous stripping composition.

Preferred is an aqueous stripping composition of the present invention, wherein (iii) has a total concentration ranging from 1 mmol/L to 1250 mmol/L, based on the total volume of the aqueous stripping composition, preferably from 40 mmol/L to 1000 mmol/L, more preferably from 80 mmol/L to 880 mmol/L, even more preferably from 120 mmol/L to 760 mmol/L, yet even more preferably from 160 mmol/L to 640 mmol/L, most preferably from 200 mmol/L to 520 mmol/L, yet most preferably from 280 mmol/L to 400 mmol/L. This most preferably applies to bromide. Generally preferred is an aqueous stripping composition of the present invention, wherein (iii) has a total concentration of at least 100 mmol/L, based on the total volume of the aqueous stripping composition, preferably of at least 180 mmol/L. This applies most preferably to the upper concentration limits mentioned in the ranges above for (iii).

If the concentration is significantly below 1 mmol/L, in many cases the removal of the metal deposit is retarded, particularly for nickel. If the concentration is significantly exceeding 1250 mmol/L, it was surprisingly observed that the well-balanced base material removal (i.e. the surface smoothening) of stainless-steel is too low, particularly if bromide is involved.

As mentioned above, in the aqueous stripping composition (i) is the only nitrogen-con- taining species. Thus, the aqueous stripping composition of the present invention is preferably substantially free of, preferably does not comprise, ammonium ions.

Preferred is an aqueous stripping composition of the present invention, wherein the aqueous stripping composition is substantially free of, preferably does not comprise, sulfuric acid, preferably is substantially free of, preferably does not comprise, sulfate anions.

Preferred is an aqueous stripping composition of the present invention, wherein the aqueous stripping composition is substantially free of, preferably does not comprise, phosphoric acid, preferably is substantially free of, preferably does not comprise, phosphate anions.

The present invention furthermore relates to a use of an aqueous stripping composition as defined above, preferably as defined above as being preferred, for electrolytically removing partly or fully a metal deposit comprising palladium, copper, nickel, chromium, and/or alloys comprising at least one thereof from a substrate, wherein the substrate is different from the metal deposit. Alternatively, the metal deposit comprises palladium, copper, nickel, chromium, tin, and/or alloys comprising at least one thereof.

The aforementioned regarding the aqueous stripping composition of the present invention, including features defined as being particularly preferred, preferably applies likewise to the use according to the present invention.

The present invention furthermore relates to a method for removing partly or fully a metal deposit from a substrate, the method comprising the steps

(a) providing as an anode the substrate comprising the metal deposit, the metal deposit comprising palladium, copper, nickel, chromium, and/or alloys comprising at least one thereof, (b) providing an aqueous stripping composition comprising

(i) nitrate anions,

(ii) one or more than one carboxylic acid and/or salts thereof, and

(iii) one or more than one kind of halogen anions, wherein in the aqueous stripping composition (i) is the only nitrogen-contain- ing species,

(c) providing at least one cathode,

(d) contacting the substrate with the aqueous stripping composition and applying an electrical current to the anode and the at least one cathode such that the metal deposit is partly or fully removed from the substrate, wherein the substrate is different from the metal deposit.

The aforementioned regarding the aqueous stripping composition of the present invention, including features defined as being particularly preferred, preferably applies likewise to the method of the present invention.

Alternatively, in step (a) the metal deposit comprises palladium, copper, nickel, chromium, tin, and/or alloys comprising at least one thereof.

Preferred is a method of the present invention, wherein the substrate comprises iron, preferably is a stainless-steel substrate.

Preferred is a method of the present invention, wherein the substrate is a fixation, preferably a rack, most preferably a fixation and rack, respectively, for holding substrates to be plated in a wet-chemical plating process. However, the aqueous stripping composition of the present invention as well as the method of the present invention may not be limited to such specific substrates.

As mentioned above, the aqueous stripping composition of the present invention shows a particular well balanced base material removal. As a result thereof, a desired surface smoothing is obtained as well as maintained. This likewise applies to the method of the present invention. Preferred is a method of the present invention, wherein the substrate has a surface roughness R _a ranging from 0.5 pm to 1.5 pm, preferably ranging from 0.7 pm to 1.3 pm, most preferably from 0.9 pm to 1.1 pm. More preferred is a method of the present invention, wherein said roughness R _a is maintained after step (d) is repeatedly carried out. As also mentioned already above, the present invention allows an electrowinning application. This is a huge advantage over conventional stripping compositions and allows a very sustainable application because removed (i.e. stripped) metals can be recycled.

Very preferred is a method of the present invention, wherein at least one of palladium, copper, nickel, and chromium (alternatively at least one of palladium, copper, nickel, chromium, and tin) from the metal deposit is simultaneously deposited onto the at least one cathode, preferably the metal deposit comprises at least copper and during step (d) copper is simultaneously deposited onto the at least one cathode. The term “simultaneously” denotes while step (d) is carried out and takes place.

More preferred is a method of the present invention, wherein during step (d) more than one of palladium, copper, nickel, and chromium (alternatively more than one of palladium, copper, nickel, chromium, and tin) from the metal deposit are selectively and individually deposited onto more than one cathode. This is a very preferred selective electrowinning application. Own experiments indicate that on the basis of selective and individual cathodic current densities palladium, copper, nickel, and chromium (being removed in step (d) from the metal deposit) are individually deposited onto the respective cathode. While removed nickel preferably requires a comparatively high current density (i.e. a cathode with a comparatively small surface area), palladium preferably requires a comparatively low cathodic current density (i.e. a cathode with a comparatively large surface area). Typically, removed copper requires a cathodic current density between the cathodic current density defined for palladium and nickel.

However, in some cases it is preferred that during step (d) more than one of palladium, copper, nickel, and chromium (alternatively more than one of palladium, copper, nickel, chromium, and tin) from the metal deposit are jointly deposited onto one cathode.

Generally preferred is a method of the present invention, wherein the at least one cathode comprises iron, lead, carbon, titanium, platinum, copper, nickel, mixtures, and/or oxides thereof. This preferably refers to the at least one cathode without a deposited metal thereon, i.e. in its blank condition.

More preferred is a method of the present invention, wherein the at least one cathode comprises a steel cathode, a lead cathode, a graphite cathode, a titanium cathode, a platinum cathode, a copper cathode, a nickel cathode, and/or a mixed metal oxide (MMO) cathode.

A preferred steel cathode comprises a stainless-steel cathode. A preferred titanium cathode comprises a platinized titanium cathode.

In some cases, preferred is a method of the present invention, wherein the at least one cathode comprises copper, preferably is copper. In such a preferred case, the at least one copper cathode is furthermore deposited with comparatively pure metallic copper such that a homogeneous cathode body with additional copper is obtained. Such a cathode body is of high value and can be easily recycled.

In other cases, preferred is a method of the present invention, wherein the at least one cathode comprises iron, preferably comprises (most preferably is) stainless-steel.

Preferred is a method of the present invention, wherein in step (d) the aqueous stripping composition has a temperature ranging from 20°C to 80°C, preferably ranging from 21 °C to 65°C, more preferably ranging from 22°C to 55°C, even more preferably ranging from 23°C to 45°C, most preferably ranging from 24°C to 36°C.

Preferred is a method of the present invention, wherein in step (d) the metal deposit is partly or fully removed with a rate ranging from 0.1 pm/min to 30 pm/min, preferably ranging from 1 pm/min to 26 pm/min, more preferably ranging from 3 pm/min to 22 pm/min, even more preferably ranging from 5 pm/min to 18 pm/min, yet even more preferably ranging from 7 pm/min to 15 pm/min, most preferably ranging from 11 pm/min to 13 pm/min. This most preferably applies to a current density as defined hereafter. Furthermore, this most preferably applies to copper and nickel in the metal deposit.

Preferred is a method of the present invention, wherein in step (d) the electrical current has a current density ranging from 10 A/dm ² to 100 A/dm ² , preferably ranging from 20 A/dm ² to 90 A/dm ² , more preferably ranging from 30 A/dm ² to 80 A/dm ² , even more preferably ranging from 40 A/dm ² to 70 A/dm ² , most preferably ranging from 45 A/dm ² to 60 A/dm ² , even most preferably ranging from 47 A/dm ² to 53 A/dm ² .

Preferred is a method of the present invention, wherein in step (d) the contacting is carried out by immersing the substrate into the aqueous stripping composition.

In step (d), the contacting is preferably carried out for a time period ranging from 10 seconds to 120 minutes, preferably ranging from 30 seconds to 100 minutes, more preferably ranging from 1 minute to 80 minutes, even more preferably ranging from 1.5 minutes to 60 minutes, most preferably ranging from 2 minutes to 45 minutes. Typically, the time period depends on the total thickness of the metal deposit, its composition, and the applied conditions. In some cases, a very preferred time period is ranging from 30 seconds to 30 minutes, preferably from 60 seconds to 20 minutes. The spirit of the present invention is further illustrated in the following examples without limiting the scope of the invention as herein defined in the claims.

Examples

1. Aqueous stripping compositions: The following examples “E1” to “E6” (according to the invention) were prepared by dissolving the respective compounds in water. Further details are summarized in Table 1; concentrations are given in mmol/L if not stated otherwise. When required, a pH correction has been carried out using sodium hydroxide.

Table 1

* based on total

In table 1 , acetic acid and sodium acetate are considered as (ii-a), wherein sodium gluconate is considered as (ii-b).

Due to a lower total amount of (i) and (iii), E2 exhibits an increased electrical resistance compared to the other compositions. Although this is slightly less preferred, E2 provides still very acceptable results (see below).

2. Electrolytically removing a metal deposit

Stripping compositions according to examples E1 to E6 were tested at a temperature ranging from 25°C to 35°C for electrolytically stripping metal deposits. The anodic current density was about 50 A/dm ² .

The tested metal deposits were subjected to the respective aqueous stripping compositions for about 2 minutes, except for stainless-steel, which was so mildly attacked that exposure of about 10 minutes were applied.

The tested metal deposits, typically in form of a respective panel or wire thereof, and the respective stripping rates are summarized in Table 2. The striping rates were determined based on weight loss.

Table 2, stripping rates in pm/min

As shown in Table 2, an excellent stripping rate for the noblest (semi-bright) Ni and for Cu was obtained, wherein the loss for stainless-steel is not zero but insignificant. In the context of the present invention this is desired to obtain or maintain a smooth surface of the stainless-steel substrate.

In order to evaluate the surface roughness for stainless-steel, the surface roughness of a stainless-steel substrate was determined before subjecting it to the respective aqueous stripping compositions and was characterized with S _a of about 1 pm. After above mentioned 10 minutes the measurement was repeated with the result that for all examples the S _a was again about 1 , in case of E3 even 0.8 pm and of E6 0.9 pm. Thus, the tested aqueous stripping compositions upon slightly etching the stainless-steel surface did not significantly increase the surface roughness upon stripping but rather can even decrease surface roughness. S _a denotes, as an absolute value, the difference in height of each point compared to the arithmetical mean of the surface and is commonly used to evaluate surface roughness. This is a key pre-requisite for the surface smoothing effect obtained by the present invention. As a result, an initial surface roughness of a respective stain- less-steel substrate can be maintained.

In further test (data not shown), the following stripping rates for further metals and metal alloys have been experimentally determined: Nickel-phosphorous: ranging from 2 pm/min to 15 pm/min depending on phosphorous content in the alloy (high phosphorous leads to a decreased stripping rate)

Bright nickel: 15 pm/min

Chrome (from trivalent chromium): 4 pm/min Chrome (from hexavalent chromium): 6 pm/min

Palladium (galvanic layer) 0.1 pm/min

Tin (galvanic layer, acidic) 4 pm/min up to 25 pm/min