METHOD AND SYSTEM FOR DEPOSITING A ZINC-NICKEL ALLOY ON A SUBSTRATE

Abstract

The present invention relates to a method for depositing a zinc-nickel alloy onto a substrate, the method comprising the steps: (a) providing the substrate, (b) providing an aqueous zinc-nickel deposition bath as a catholyte in a deposition compartment wherein - the deposition compartment comprises at least one anode with an anolyte, and - the anolyte is separated from the catholyte by at least one membrane, and the catholyte comprises (i) nickel ions, (ii) at least one complexing agent for nickel ions, and (iii) zinc ions, (c) contacting the substrate with the catholyte in the deposition compartment such that the zinc-nickel alloy is electrolytically deposited onto the substrate, thereby obtaining a zinc-nickel coated substrate, wherein after step (c) the nickel ions in the catholyte have a lower concentration than before step (c),(d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, so as to obtain a rinsed zinc-nickel coated substrate and rinse water, the rinse water comprising a portion of at least one ion complexing agent for nickel ions and a portion of the nickel ions, characterized in that (i) at least a portion of the rinse water and / or at least a portion of the catholyte is treated in a first treatment compartment such that the water is separated from the at least one complexing agent for the nickel ions and the nickel ions, (ii) at least a portion of the at least one complexing agent separated from the water is returned to the catholyte, and (iii) a source of nickel ions is added to the catholyte,provided that the source of nickel ions does not comprise said at least one complexing agent for nickel ions or any other complexing agent for nickel ions.

Description

METHOD AND SYSTEM FOR DEPOSITING A ZINC-NICKEL ALLOY ON A SUBSTRATE FIELD OF INVENTION The present invention, according to a first aspect, relates to a method for depositing a zinc-nickel alloy onto a substrate, in particular to a method for electrolytically depositing a zinc-nickel alloy onto a substrate. In accordance with a second aspect, the present invention further relates to a system for depositing a zinc-nickel alloy onto a substrate, in particular to a system for electrolytically depositing a zinc-nickel alloy onto a substrate. BACKGROUND OF THE INVENTION Electrolytic deposition of a metal alloy, sometimes also called coating, onto other metals or metal-coated plastics, usually called substrates, is a well-established technique for increasing the corrosion resistance of these substrates. Deposition is typically carried out using anodes, with the substrate acting as the cathode, by applying an electric current in a respective electrolyte. In some cases, it is advantageous to separate the electrolyte using a semipermeable membrane into a catholyte compartment, comprising the catholyte (the electrolyte in the cathode space), and an anolyte compartment, comprising the anolyte (the electrolyte in the anode space). Often, the anolyte is different from the catholyte. By applying an electrical potential, a current flows through the anolyte, across the membrane, and into the catholyte to initiate electrolytic deposition onto the substrate. US patent 2011 / 031127 A1, Hillebrand, describes an alkaline electroplating or electroplating bath of this type for metallization or electroplating of zinc-nickel coatings, having an anode and a cathode, wherein the anode is separated from the alkaline electrolyte by an ion exchange membrane. US patent 2013 / 0264215 A1, Umicore, discloses an anode system configured to be suitable for use in electroplating cells for the deposition of electrolytic coatings by simple immersion in the catholyte. In this system, after immersion in the catholyte, the catholyte is separated from the anode by a swollen polymer membrane permeable to cations or anions. The polymer membrane is in direct contact with the anode and not with the cathode. The membrane is attached to the anode by electrolyte-permeable supports and pressing devices using a multi-layer structure, ensuring good contact of the membrane with the anode. Document DE 20 2015 002 289 U1 describes an electrodialysis cell with an anion and cation exchange membrane for use as an anode in alkaline zinc electrolytes and zinc alloys for electrodeposition in galvanic systems. Document EP 1 533 399 A2 refers to a method for alkaline zinc nickel plating with reduced wastewater. caq / nn / zznz / E / YiAi Zinc-nickel plating baths are typically used continuously for extended periods, such as weeks or even months, to enable efficient deposition of the zinc-nickel alloy onto a variety of substrates. When a zinc-nickel plating bath is used for such a long time, undesirable compounds (particularly the degradation products of organic compounds, such as complexing agents, including cyanides) begin to accumulate in the bath over time. This often significantly impairs the plating process after a certain period and may ultimately necessitate at least partial replacement of the zinc-nickel plating bath. In many cases, this is avoided by continuously removing at least a portion of the plating bath (e.g.,(through entrainment), as wastewater. However, because nickel ions and often cyanides are involved, intensive wastewater treatment is required before disposal. Therefore, there is a continuous demand for further improvements to existing deposition methods, particularly in light of environmental considerations. Given increasingly stringent legal limits worldwide, especially regarding nickel ions, there is an urgent need for a more sustainable method for depositing a zinc-nickel alloy onto a substrate—one that produces less or no wastewater, or at least reduced contamination with critical metal ions. Furthermore, such a method must be cost-effective and not compromise current corrosion protection standards. BRIEF DESCRIPTION OF THE INVENTION Objectives of the invention Therefore, the objective of the present invention is to provide a very environmentally friendly method and system for depositing a zinc-nickel alloy onto a substrate, which does not produce wastewater, or at least minimizes contamination with critical metal ions, such as nickel ions and cyanide ions, but at the same time can operate economically for a long time. The aforementioned objective is achieved, in a first aspect, by means of a method for depositing a zinc-nickel alloy onto a substrate, the method comprising the following stages or steps: (a) provide the substrate, (b) provide an aqueous zinc-nickel catholyte deposition bath in a deposition compartment, wherein: - the deposition compartment comprises at least one anode with an anolyte, and - the anolyte is separated from the catholyte by at least one membrane, and the catholyte comprises: (i) nickel ions, (ii) at least one complexing agent for nickel ions, and (iii) zinc ions, caq / nn / zznz / E / YiAi (c) contacting the substrate with the catholyte in the deposition compartment, so that the zinc-nickel alloy is electrolytically deposited onto the substrate, thereby obtaining a zinc-nickel coated substrate, wherein: after step (c), the nickel ions in the catholyte have a lower concentration than before step (c), (d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, such that a rinsed zinc-nickel coated substrate and rinse water are obtained, wherein: The rinse water comprises a portion of at least one complexing agent for nickel ions and a portion of the nickel ions, characterized in that: (i) at least a portion, (preferably all), of the rinse water and / or at least a portion of the catholyte are treated in a first treatment compartment, such that the water is separated from the at least one complexing agent for nickel ions and nickel ions, (ii) at least a portion, (preferably all), of the at least one complexing agent separated from the water is returned to the catholyte, and (iii) a source of nickel ions is added to the catholyte, provided that the source of nickel ions does not comprise said at least one complexing agent for nickel ions, or any other complexing agent for nickel ions. The method of the present invention excellently achieves the objective defined above, as it allows for closed-loop operation, theoretically for an unlimited period of time, but at least for weeks and particularly for months. During that time, the water is removed substantially free of nickel and cyanide ions (and is therefore not considered wastewater). During closed-loop operation, only the nickel and zinc ions, which are deposited onto the substrate during deposition, should preferably be replenished. All other compounds included in the deposition bath, preferably in the catholyte, are recycled. By returning at least a portion (preferably all) of the at least one complexing agent, which has been separated from the water in the first treatment compartment, to the catholyte (either directly or indirectly), the concentration of the at least one complexing agent for the nickel ions in the catholyte is maintained at a constant concentration. As defined in the method of the present invention, little or none of the complexing agent should be replenished. This is achieved by using at least one anode with at least one membrane. Such membranes prevent the anodic degradation of organic compounds, for example, of the complexing agent. The complexing agent carried over to the rinse compartment is recycled by means of the first treatment compartment. This allows the nickel ions to be replenished free of any complexing agent. In particular, it is sufficient to provide an initial concentration of at least one complexing agent for nickel ions when establishing the zinc-nickel deposition aqueous bath, caq / nn / zznz / E / YiAi preferably the catholyte, wherein no additional complexing agent should be added during the deposition method. Furthermore, when the rinse water is treated in the first treatment compartment in such a way that the water is separated, very pure water is usually obtained, which can be used again. The aforementioned objective is further achieved in accordance with a second aspect by means of a system for depositing a zinc-nickel alloy onto a substrate, the system comprising: (I) optionally, a pre-rinse compartment for pre-rinsing the substrate, (II) a deposition compartment for electrolytically depositing the zinc-nickel alloy onto the substrate in a catholyte to obtain a zinc-nickel coated substrate, wherein the deposition compartment comprises at least one anode with at least one membrane, (III) a rinse compartment for rinsing the zinc-nickel coated substrate to obtain a rinsed zinc-nickel coated substrate and rinse water, (IV) a first treatment compartment for treating the rinse water and a portion of the catholyte to separate the water from nickel ions and nickel ion complexing agents, and (V) optionally, a second treatment compartment for treating the catholyte to separate dissolved anions from the catholyte, wherein: The first treatment compartment is adapted in such a way that: - the separated water is returned to the pre-rinse compartment and / or the rinse compartment, and - The separated nickel ions and the separate complexing agents for the nickel ions are returned to the deposition compartment, preferably through a mixing compartment. BRIEF DESCRIPTION OF THE FIGURE Figure 1 shows a schematic representation of a system for depositing a zinc-nickel alloy onto a substrate, preferably for carrying out the method of the present invention. The system comprises several compartments. Most of them are fluidly connected to each other. Further details are given in the Examples section below. DETAILED DESCRIPTION OF THE INVENTION In the context of the present invention, the term at least one, one or more of one and one or more denotes (and is interchangeable with) one, two, three or more of three. In the context of the present invention, the anolyte is normally an electrolyte that is in direct contact with at least one anode, wherein the catholyte is an electrolyte or at least part of an electrolyte that is in contact with the cathode, i.e., the substrate, at least for the time that the catholyte is in a deposition compartment. As previously mentioned, a major advantage of the method of the present invention is that no degradation products are formed due to the anodes having at least one membrane. This preferably means that at least one anode and at least one membrane are adapted to form the anolyte, which is separated from the catholyte, and selective ion impregnation between the catholyte and anolyte is only possible through this membrane. The membrane is adapted to prevent the complexing agent from passing through it (from the catholyte to the anolyte). This enables closed-loop operation, which constantly recycles the initial concentration of the complexing agent for the nickel ions. Most preferably, the membrane allows only the impregnation of hydrogen ions (formed in the anolyte) into the catholyte. Therefore, a method of the present invention is preferred, wherein the at least one complexing agent for nickel ions is not in contact with the at least one anode, most preferably not in contact with any of the at least one anode. Furthermore, a method of the present invention is preferred, wherein the catholyte comprises only an initial concentration of at least one complexing agent for nickel ions for at least one nickel ion exchange, more preferably for at least two nickel ion exchanges, even more preferably for at least three nickel ion exchanges, most preferably for the entire lifetime of the catholyte. Said at least one membrane preferably allows only proton diffusion between the anolyte and the catholyte, which ensures an efficient distribution of charges between the anolyte and the catholyte. During the method of the present invention, water is normally introduced into the catholyte, for example, by means of the nickel ion source to replenish the nickel ions. However, in the first treatment compartment, the excess water is separated and subsequently removed from the method of the present invention so as to maintain a substantially constant volume of catholyte over time. If such excess water cannot be used in the method of the present invention, it is preferably readily discarded because it is substantially free of nickel ions and preferably also of zinc ions; substantially no complexing agents are present. In summary, the method of the present invention allows for economical, sustainable, and continuous operation over an extended period, i.e., for several weeks or even several months. During this extended period, no nickel-contaminated wastewater is produced, and no valuable metal ions or complexing agents are lost due to entrainment. Essentially, only the amount of deposited nickel and zinc ions needs to be replenished by a respective nickel and zinc ion source. With regard to the method of the present invention, more preferably at least a portion of the rinse water (preferably all of it) and at least a portion of the catholyte are treated in the first treatment compartment so that the water is separated from the at least one complexing agent for nickel ions and the nickel ions. Treating a portion of the catholyte as well (in addition to the rinse water, preferably in addition to all of the rinse water) allows for maintaining a basically constant volume of catholyte. caq / nn / zznz / E / YiAi By separating at least one complexing agent for nickel ions and the nickel ions from water, the complexing agent thus recycled and the nickel ions have a desired concentration before returning them to the catholyte. A preferred method of the present invention is one in which the complexing agent separated from the water is returned to the catholyte as a concentrated aqueous solution. More preferably, the complexing agent separated from the water is returned directly or indirectly to the catholyte as a concentrated aqueous solution via the mixing unit. The mixing unit is preferably used to mix the separated complexing agent with, for example, the nickel ion source and / or a zinc ion source; more preferably, the mixing unit provides a freshly mixed aqueous zinc-nickel deposition bath ready to be transferred to the deposition compartment in order to supplement the catholyte. By returning the complexing agent and thus maintaining a basically constant concentration of the complexing agent, a continuously constant stabilization of the nickel ions in the catholyte is achieved, which in turn provides good catholyte stability. When the complexing agent is returned indirectly to the catholyte through the mixing unit, the complexing agent is preferably used to form complexes with the nickel ions freshly introduced from the nickel ion source in the mixing unit (see Figure 1). Therefore, a method of the present invention is preferred, wherein the nickel ion source is added directly or indirectly to the catholyte, preferably indirectly through a mixing unit (preferably as described above). A method of the present invention is preferred, wherein a source of zinc ions is added directly or indirectly to the catholyte, preferably indirectly through a mixing unit (preferably as described above). More preferably, the zinc ions are obtained by dissolving metallic zinc in sodium hydroxide to obtain zinc hydroxo complexes, thereby enabling efficient stabilization of the zinc ions in the catholyte. Adding the nickel and zinc ion source to the catholyte replenishes the nickel and zinc ions. Preferably, the nickel and zinc ion source is added indirectly through the mixing unit so that a thoroughly mixed composition is prepared before transferring it to the deposition compartment. A method of the present invention is preferred, wherein the anolyte is water, preferably water comprising sulfuric acid, more preferably water comprising from 5% by volume to 40% by volume of sulfuric acid. A method of the present invention is preferred, wherein the catholyte comprises more than 50% by volume of water, based on the total volume of the catholyte, more preferably comprising 75% by volume or more of water, even more preferably comprising 85% by volume or more of water, and most preferably comprising 92% by volume or more of water. Preferably, water is the only solvent in the catholyte. A preferred method of the present invention is one in which the source of nickel ions is an aqueous solution comprising water and a nickel salt dissolved therein. A preferred method of the present invention is one in which the nickel salt is an inorganic salt. This preferably means that the nickel salt does not comprise a carboxylic acid anion, more preferably does not comprise an organic acid anion, and most preferably does not comprise an organic anion. By excluding organic anions, particularly carboxylic anions, the accumulation of potentially undesirable organic anions in the catholyte over time can be avoided. Furthermore, potential complexing agents for nickel ions are thus essentially excluded. A method of the present invention is preferred, wherein the nickel salt comprises nickel sulfate, preferably nickel sulfate hexahydrate. A method of the present invention is preferred, wherein the nickel salt does not comprise nickel chloride. By excluding nickel chloride, the concentration of chloride ions in the catholyte can be minimized or, more preferably, even eliminated, thereby eliminating the need to remove excessive amounts of chloride from the catholyte during the method of the present invention (which is often difficult due to the high solubility of chloride salts). A method of the present invention is preferred, wherein the nickel salt does not comprise nickel nitrate. By excluding nickel nitrate, the concentration of nitrate ions in the catholyte is avoided. In many cases, nitrate disrupts the entire electrolytic deposition process and is highly undesirable. The preferred source of nickel ions is an aqueous solution comprising water and nickel sulfate, preferably nickel sulfate hexahydrate, dissolved therein. Such a preferred source of nickel ions is excellently suited for replenishing nickel ions. Regarding any accumulation of sulfate anions, see the text below. A method of the present invention is preferred, wherein in the nickel ion source, the nickel ions have a concentration in the range of 70 g / L to 140 g / L, based on the total volume of the nickel ion source, preferably 80 g / L to 120 g / L, more preferably 90 g / L to 110 g / L, even more preferably 95 g / L to 105 g / L. As mentioned above, the nickel ion source does not comprise said at least one complexing agent for nickel ions or any other complexing agent for nickel ions. This means that the at least one complexing agent for nickel ions is not replenished by means of the nickel ion source. More preferably, the at least one complexing agent for nickel ions is not replenished at all. Furthermore, no complexing agent other than the at least one complexing agent for nickel ions is added to the catholyte, for example, the complexing agent used to initially set up the aqueous zinc-nickel deposition bath. Therefore, a method of the present invention is preferred, wherein the catholyte comprises only one complexing agent for nickel ions (and, therefore, not a mixture of two or more complexing agents).This is useful for controlling the total amount of complexing agent in the catholyte over a long period of time. A method of the present invention is preferred, wherein the nickel ion source is essentially free of or does not comprise tetraethylenepentamine, preferably is essentially free of or does not comprise a diamine, and most preferably is essentially free of or does not comprise an amine. This is preferred above all because such compounds are normally used as complexing agents for nickel ions in an aqueous zinc-nickel plating bath (for further details on complexing agents, see the text below). In particular, these compounds are therefore undesirable in the nickel ion source to avoid their accumulation. A method of the present invention is preferred, wherein the nickel ion source is essentially free of or does not comprise an amine having one or more of one, preferably two, primary amine groups, and one or more of a secondary amine group. In the method of the present invention, the catholyte comprises at least one (preferably one) complexing agent for nickel ions. A preferred method of the present invention comprises, in the catholyte, at least one complexing agent for nickel ions, including a chelating complexing agent, and preferably, the chelating complexing agent is the only complexing agent for nickel ions in the catholyte. The use of a chelating complexing agent ensures efficient stabilization of the nickel ions in the catholyte. In particular, the at least one complexing agent is provided only once when the aqueous zinc-nickel deposition bath is initially established, and no further complexing agent is added thereafter. A preferred method of the present invention comprises, in the catholyte, at least one complexing agent for nickel ions, comprising an amine, preferably a diamine, and more preferably tetraethylenepentamine. The amine, diamine, and tetraethylenepentamine, respectively, as complexing agents for nickel ions, provide excellent stabilization of the nickel ions in the catholyte, particularly at an alkaline pH. A method of the present invention is preferred, wherein the amine, preferably the diamine, more preferably tetraethylenepentamine, is the sole complexing agent for the nickel ions in the catholyte. A method of the present invention is preferred, wherein the diamine is selected from the group consisting of ethylenediamine, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine. Generally, a method of the present invention is preferred, wherein in the catholyte the at least one complexing agent for nickel ions comprises an amine having one or more, preferably two, primary amino groups and one or more secondary amino groups. A method of the present invention is preferred, wherein the amine having one or more, preferably two, primary amine groups and one or more secondary amine groups, is the sole complexing agent for the nickel ions in the catholyte. caq i nn / zznz / E / YiAi A method of the present invention is preferred, wherein the nickel ions from the nickel ion source added to the catholyte do not form complexes before being in contact with an alkaline environment, preferably an environment having a pH ranging from 10.0 to 14.0, more preferably from 11.0 to 13.3, even more preferably from 11.5 to 13.0, even more preferably from 12.0 to 12.9, most preferably from 12.3 to 12.8. In other words, the nickel ions from the nickel ion source added to the catholyte preferably form complexes for the first time when they are brought into contact with an alkaline environment, preferably an environment having a pH as defined above, which is more preferably the catholyte. Furthermore, this text refers to an alternative method for depositing a zinc-nickel alloy onto a substrate, the method comprising the following stages or steps: (a) provide the substrate, (b) provide an alkaline aqueous zinc-nickel deposition bath as a catholyte in a deposition compartment, wherein - the deposition compartment comprises at least one anode with an anolyte, and - the anolyte is separated from the catholyte by at least one membrane, and the catholyte comprises: (i) nickel ions, (ii) at least one complexing agent for nickel ions, and (iii) zinc ions, (c) contacting the substrate with the catholyte in the deposition compartment so that the zinc-nickel alloy is electrolytically deposited onto the substrate and a zinc-nickel coated substrate is thereby obtained, wherein: after step (c) the nickel ions in the catholyte have a lower concentration than before step (c), (d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, such that a rinsed zinc-nickel coated substrate and rinse water are obtained, wherein the rinse water comprises a portion of at least one nickel ion complexing agent and a portion of the nickel ions, characterized in that: (i) at least a portion of the rinse water and / or at least a portion of the catholyte is treated in a first treatment compartment such that the water is separated from the at least one complexing agent for the nickel ions, (ii) at least a portion of the at least one complexing agent separated from the water is returned to the catholyte, and (iii) nickel ions are added to the catholyte from a nickel ion source to replenish the nickel ions, wherein the nickel ions from the nickel ion source added to the catholyte do not form a complex with a complexing agent before being in contact with an alkaline environment, preferably an environment having a pH ranging from 10.0 to 14.0, more preferably from 11.0 to 13.3, even more preferably from 11.5 to 14.0. 13.0, even more preferably between 12.0 and 12.9, most preferably from 12.3 to 12.8. The features of the method of the present invention as defined throughout this text, (including preferred features, etc.), are also preferably applicable to the alternative method, (if technically applicable). A method of the present invention is preferred, wherein step (a), prior to step (c), comprises the step of: (a-1) pre-rinsing the substrate in a pre-rinse compartment comprising water, so as to obtain a pre-rinsed substrate and pre-rinse water. By pre-rinsing the substrate in the pre-wash or pre-rinse compartment, any potential contamination is removed before transferring it to the deposition compartment. Preferably, the pre-wash or pre-rinse compartment contains an aqueous sodium hydroxide solution as the pre-rinse solution. In the method of the present invention, in step (d), the zinc-nickel coated substrate is rinsed in a rinsing compartment. A method of the present invention is preferred, wherein the rinse compartment comprises 2 to 5 fluidly connected rinse sub-compartments forming a rinse cascade. Such a rinsing cascade is particularly efficient in rinsing, as the concentration of rinsed ions from the zinc-nickel coated substrate is effectively reduced step by step, so that the lowest rinsing subcompartment comprises a significantly lower ion concentration compared to the rinsing subcompartment further upstream of the rinsing cascade. The deposition compartment contains at least one anode and at least one membrane, where at least one membrane separates the anolyte from the catholyte. Ideally, the at least one membrane is a semipermeable membrane. This means that the at least one membrane is selectively permeable. A method of the present invention is preferred, wherein at least one membrane is a cation-exchange membrane. By using a cation-exchange membrane, any disadvantageous impregnation of the at least one complexing agent from the catholyte to the anolyte is effectively avoided. A method of the present invention is preferred, wherein in the deposition compartment at least one anode is an insoluble anode, preferably an insoluble mixed metal oxide anode, more preferably an insoluble iridium / tantalum oxide on a titanium anode. A method of the present invention is preferred, wherein at least one anode is located at a distance from at least one membrane in the range of 0.5 mm to 5.0 mm, preferably from 0.75 mm to 4 mm, and more preferably from 1.0 mm to 3.0 mm. This advantageously allows the anolyte volume to be kept low, which in turn results in low amounts of anolyte wastewater. caq / nn / zznz / E / YiAi In the method of the present invention, at least a portion of the rinse water and / or at least a portion of the catholyte is treated in a first treatment compartment so that the water is separated from at least one complexing agent for nickel ions and the nickel ions. A method of the present invention is preferred, wherein the first treatment compartment comprises an evaporator, preferably a vacuum evaporator. A method of the present invention is preferred, wherein a vacuum is applied to the evaporator in a range of 1 mbar to 100 mbar, preferably from 5 mbar to 70 mbar, more preferably from 10 mbar to 50 mbar, most preferably from 15 mbar to 35 mbar. A method of the present invention is preferred, wherein, in the first treatment compartment, preferably in the evaporator, more preferably in the vacuum evaporator, the water is separated at a temperature in a range of 18°C ​​to 50°C, preferably from 23°C to 46°C, more preferably from 28°C to 42°C, most preferably from 31°C to 40°C. Using an evaporator, preferably a vacuum evaporator, efficient water evaporation can be achieved, particularly by reducing atmospheric pressure. This allows for efficient separation of water from nickel ions and at least one complexing agent. Since the boiling point of water is significantly lower than the boiling point of at least one complexing agent, nickel ions, and / or zinc ions, efficient water separation is achieved. By operating the vacuum evaporator at a temperature between 18°C ​​and 50°C, unwanted heating or even thermal degradation of at least one complexing agent is avoided. A method of the present invention is preferred, wherein the vacuum evaporator is operated and controlled based on the measurement of the density of the concentrated aqueous solution, preferably the density of the concentrated aqueous solution being in the range of 1.08 kg / L to 1.30 kg / L, based on the total volume of the concentrated aqueous solution, preferably from 1.10 kg / L to 1.26 kg / L, more preferably from 1.15 kg / L to 1.24 kg / L, and most preferably from 1.20 kg / L to 1.23 kg / L. Control based on density measurement is excellently suited for automatically operating the first treatment compartment, preferably the evaporator, and more preferably the vacuum evaporator. The density ranges mentioned above are the most preferred. However, in some cases, a higher maximum density is acceptable provided that the concentrated aqueous solution does not form a phase separation. This possibly includes maximum densities of, for example, 1.28 kg / L, 1.30 kg / L, in some cases even 1.32 kg / L. Phase separation also normally depends on the total amount of, for example, sulfate, carbonate and hydroxides (for example, sodium and / or potassium), which vary over time. As defined above, the concentrated aqueous solution is aqueous. Therefore, a method of the present invention is preferred, wherein the concentrated aqueous solution is homogeneous. This preferably means that the concentrated aqueous solution forms only a single phase; in other words, the concentrated aqueous solution preferably does not form a phase separation. Most preferably, the concentrated aqueous solution does not comprise an organic phase separate from an aqueous phase. Thus, even more preferred is a method of the present invention, in which the concentrated aqueous solution is completely aqueous. By not exceeding the maximum density mentioned above, preferably 1.26 kg / L (or even higher as mentioned above), phase separation is preferably avoided. As a result of the treatment in the first treatment compartment, very pure water and concentrated aqueous solution are obtained. A method of the present invention is preferred, wherein at least a portion of the separated water obtained in the first treatment compartment is returned to the pre-rinse compartment and / or the rinse compartment. Preferably, at least a portion of the separated water obtained in the first treatment compartment is returned to the rinse compartment, more preferably to a rinse sub-compartment of the rinse cascade. By returning the separated water, which is very pure, water is recycled and wastewater is avoided because the separated water is substantially free of complexing agents, nickel ions, and zinc ions. Therefore, it is very suitable for reuse in the pre-rinse and rinse compartments. With this type of circuit, fresh water is preferably not needed for rinsing for a relatively long time. A preferred method of the present invention is one in which water is separated from at least one nickel ion complexing agent and nickel ions, such that the catholyte in the deposition compartment has a substantially constant volume, preferably a constant volume. This is achieved in particular if, in the first treatment compartment, in addition to the rinse water, at least a portion of the catholyte is further treated. Typically, more water is introduced into the catholyte (for example, by adding the nickel ion source and forming anodically produced hydrogen ions) than is separated from the rinse water. A method of the present invention is preferred, wherein at least a portion (preferably all) of the at least one complexing agent separated from water and at least a portion (preferably all) of the nickel ions separated from water are returned to the catholyte, preferably as a concentrated aqueous solution (preferably as described throughout). Preferably, the concentrated aqueous solution is returned directly or indirectly, preferably indirectly through the mixing unit. Rinse water typically also contains zinc ions. Therefore, a method of the present invention is preferred, wherein the rinse water comprises a portion of the zinc ions. A method of the present invention is preferred, wherein in the first treatment compartment the water is separated from nickel ions, from at least one complexing agent for nickel ions and from zinc ions. crq / ηη / ζζηζ / Ε / γίΛΐ A method of the present invention is preferred, wherein the nickel ions, the zinc ions and at least one complexing agent for the nickel ions are returned together to the catholyte, preferably as a concentrated aqueous solution (preferably as described throughout the text). As mentioned above, a preferred source of nickel ions comprises nickel sulfate. This means that sulfate anions are introduced into the catholyte, where they typically accumulate over time. In addition, the catholyte typically tends to form and accumulate carbonate anions. Both anions are usually highly soluble in the catholyte. Although a certain concentration can be tolerated, excessive accumulation of such anions should be avoided. Therefore, a method of the present invention is preferred, wherein said method comprises the step of: (e) treating at least a portion of the catholyte in a second treatment compartment, such that the dissolved anions are separated from the catholyte, preferably by precipitation and / or ion exchange, most preferably by precipitation. A method of the present invention is preferred, wherein the dissolved anions comprise sulfate, carbonate and / or chloride, preferably at least sulfate and carbonate. By applying step (e) in addition to steps (a) through (d), the concentration of dissolved anions in the catholyte is significantly reduced, and overaccumulation is prevented. As a result, the method of the present invention can operate for an extremely long time. Preferably, step (e) is applied when the dissolved anions have reached an undesirable concentration, either individually or in total. Preferably, step (e) comprises precipitation to remove one or more of these anions from the catholyte, most preferably by reducing the temperature of at least a portion of the catholyte in the second treatment compartment and thereby reducing the solubility of the respective salts. Thus, preferably, the sulfate and carbonate anions are separated from the catholyte by means of precipitated salts comprising sulfate anion and carbonate anion. Ideally, the treatment in step (e) forms a solid precipitate. If the solid precipitate coprecipitates more catholyte ingredients, then replacement of the catholyte is recommended (for example, with at least one complexing agent for nickel ions). In some cases, such coprecipitation appears unavoidable. A method of the present invention is less preferred, wherein in the second treatment compartment the dissolved anions are separated by ion exchange. Normally, ion exchange is not sufficiently specific for such dissolved anions. A preferred method of the present invention is one in which precipitation is carried out at a temperature in the range of -5°C to 11.0°C, preferably in the range of 0.5°C to 10.0°C, more preferably in the range of 1.0°C to 8.0°C, even more preferably in the range of 1.5°C to 6°C, and most preferably in the range of 2.0°C to 4.0°C. As mentioned above, by significantly reducing the temperature in the second treatment compartment, salts comprising sparingly soluble anions are typically formed, thereby at least partially removing these anions from the catholyte. Most preferably, the salts comprising sparingly soluble anions are sodium salts. An alternative preferred temperature ranges from -3°C to 5°C, preferably from -2.5°C to 4°C, most preferably from -2°C to 3°C. Therefore, a method of the present invention is preferred, wherein the dissolved anions comprise at least sulfate anions, and wherein the sulfate anions are preferably separated by precipitated sodium sulfate. Furthermore, a method of the present invention is preferred, wherein the dissolved anions comprise at least sulfate anions and carbonate anions, and wherein the sulfate anions and carbonate anions are preferably separated by precipitated sodium sulfate and sodium carbonate, respectively. Sodium salts are particularly preferred because sodium hydroxide is primarily used to maintain the catholyte pH. Since hydrogen ions are constantly formed anodically (resulting in chemically formed water), the hydroxide must be constantly replenished, which also introduces significant amounts of sodium. Therefore, the sodium is removed by treatment in the second treatment compartment. A method of the present invention is preferred, wherein the catholyte is alkaline, preferably with a pH in the range of 10.0 to 14.0, more preferably from 11.0 to 13.3, even more preferably from 11.5 to 13.0, even more preferably from 12.0 to 12.9, most preferably from 12.3 to 12.8. As mentioned previously, the formation of degradation products in the catholyte is essentially prevented because at least one anode and at least one membrane separate the anolyte from the catholyte. This means that unwanted cyanides are essentially not formed in the catholyte. Therefore, a method of the present invention is preferred, wherein the catholyte comprises cyanide ions in the range of 0 mg / L to 2.5 mg / L, based on the total volume of the catholyte, preferably from 0 mg / L to 1.5 mg / L, more preferably from 0 mg / L to 1 mg / L, and most preferably from 0 mg / L to 0.5 mg / L. Most preferably, the catholyte is essentially free of cyanide ions, i.e., from 0.001 mg / L to 0.05 mg / L; even more preferably, it comprises no cyanide ions. A preferred method of the present invention is one in which the catholyte comprises oxalate ions in the range of 0 mg / L to 2.5 mg / L, based on the total volume of the catholyte, preferably from 0 mg / L to 1.5 mg / L, more preferably from 0 mg / L to 1 mg / L, and most preferably from 0 mg / L to 0.5 mg / L. Most preferably, the catholyte is essentially free of oxalate ions, i.e., from 0.001 mg / L to 0.05 mg / L; even more preferably, it comprises no oxalate ions. Oxalate ions are also typical degradation products, which are essentially avoided in the method of the present invention. Since cyanide and oxalate ions are not formed in the catholyte, no specific wastewater treatment is required to address these ions. As mentioned above, the zinc ions in the catholyte are replenished by means of a zinc ion source. A method of the present invention is preferred, wherein the zinc ions in the catholyte are present as hydroxo complexes. Preferably, the zinc ion source comprises water, hydroxide ions (preferably sodium hydroxide), and metallic zinc. Such hydroxo complexes are preferably obtained by dissolving metallic zinc under alkaline conditions. caq / ηη / ζζηζ / Ε / γίΛΐ A preferred method of the present invention is one in which the zinc ions in the catholyte do not form a complex with at least one complexing agent for nickel ions, preferably do not form a complex with a diamine, and more preferably do not form a complex with an organic complexing agent. Most preferably, the zinc ions in the catholyte are strongly stable as hydroxo complexes such that no formation of zinc ion complexes with at least one complexing agent for nickel ions is observed under alkaline conditions. A method of the present invention is preferred, wherein in the catholyte the zinc ions have a concentration below 10 g / L, preferably in a range of 5.0 g / L to 9.0 g / L, more preferably from 5.2 g / L to 8.5 g / L, even more preferably from 5.4 g / L to 8.0 g / L, even more preferably from 5.7 g / L to 7.5 g / L, most preferably from 5.9 g / L to 7.3 g / L. A method of the present invention is preferred, wherein the nickel ions in the catholyte have a concentration below 2.0 g / L, preferably in the range of 0.5 g / L to 1.9 g / L, more preferably from 0.6 g / L to 1.7 g / L, even more preferably from 0.7 g / L to 1.6 g / L, even more preferably from 0.8 g / L to 1.5 g / L, most preferably from 0.9 g / L to 1.4 g / L. Advantageously, in the method of the present invention, the concentrations defined above for nickel and zinc ions are typically lower than those commonly used in methods known in the art. Since the nickel ions and preferably the zinc ions are recycled in the method of the present invention, no significant quantities of nickel and zinc ions are wasted. As mentioned above, the excess water (which is very pure) is separated and removed from the method of the present invention. A method of the present invention is preferred, wherein at least a portion of the separated water obtained in the first treatment compartment is removed, wherein the removed water comprises nickel ions in a concentration range of 0 mg / L to 1.0 mg / L, based on the total volume of the removed water, preferably from 0 mg / L to 0.5 mg / L, even more preferably from 0.01 mg / L to 0.11 mg / L and most preferably from 0.01 mg / L to 0.1 mg / L. A method of the present invention is preferred, wherein at least a portion of the separated water obtained in the first treatment compartment is removed, wherein the removed water comprises zinc ions in a concentration range of 0 mg / L to 1.0 mg / L, based on the total volume of the removed water, preferably from 0 mg / L to 0.5 mg / L, more preferably from 0.01 mg / L to 0.11 mg / L and most preferably from 0.01 mg / L to 0.1 mg / L. Preferably, only a pH adjustment is required before the excess water is removed. In other cases, it is highly preferable to use the discarded water (preferably the excess water) for the pre-rinse, that is, in a rinse step performed before steps (b) and (c). Preferably, this means discarding (or removing) this water in a pre-rinse compartment. This is the most preferred method. In this case, no water is wasted, but rather used to the greatest extent possible. caq / ηη / ζζηζ / Ε / γίΛΐ Also preferred is a method of the present invention, wherein the discarded water, (preferably the excess water), is used in additional pre-treatment steps before steps (b) and (c), more preferably in the cleaning steps, most preferably in one or more of a degreasing step, (e.g., a soaking cleaning step, an electro-cleaning step, etc.). Also preferred is a method of the present invention, wherein the discarded water (preferably the excess water) is used in one or more of a subsequent treatment step, preferably in a passivation step to passivate the zinc-nickel coated substrate. By using excess water in one or more of the aforementioned applications, water is used optimally and wastewater is reduced as much as possible. The present invention according to the second aspect provides a system for depositing a zinc-nickel alloy onto a substrate, the system comprising: (I) optionally, a pre-wash compartment for pre-rinsing the substrate, (II) a deposition compartment for electrolytically depositing the zinc-nickel alloy onto the substrate in a catholyte, such that a zinc-nickel coated substrate is obtained, wherein the deposition compartment comprises at least one anode with at least one membrane, (III) a rinse compartment for rinsing the zinc-nickel coated substrate to obtain a rinsed zinc-nickel coated substrate and rinse water, (IV) a first treatment compartment for treating the rinse water and a portion of the catholyte so that the water is separated from the nickel ions and nickel ion complexing agents, and (V) optionally, a second treatment compartment for treating the catholyte so that the dissolved anions are separated from the catholyte, wherein: The first treatment compartment is adapted so that: - the separated water is returned to the pre-rinse compartment and / or the rinse compartment, and - The separated nickel ions and the separated complexing agents for nickel ions are returned to the deposition compartment, preferably through a mixing compartment. With respect to (I), (II), (III), (IV), and (V) of the system of the present invention, the foregoing applies with respect to the method of the present invention. Thus, preferably, the foregoing applies with respect to the method of the present invention, and preferably what is described as preferred, also applies to the system of the present invention. The present invention is described in more detail by the following non-limiting examples. Examples Test coating installation (according to the invention) In a test coating installation according to the invention, a zinc-nickel deposition bath is installed as a catholyte in a deposition compartment (approx. 20,000 L) to deposit a zinc-nickel alloy on small metal parts (e.g., screws; approx. 40 kg of load per barrel). The catholyte initially comprises 0.9 g / L to 1.4 g / L of nickel(II) ions, 5.9 g / L to 7.3 g / L of zinc(II) ions, and a diamine, additionally with at least one secondary amino group as a chelating complexing agent for the nickel ions. The pH is strongly alkaline, around 12.5, and is adjusted with sodium hydroxide. A plurality of iridium / tantalum-insoluble oxide is used on titanium anodes with cation-exchange membranes. For each anode, the distance between the anode and the respective membrane is less than 5 mm. Each anolyte, comprising water with sulfuric acid, is separated from the catholyte by said membranes, so that the complexing agent is never in contact with the anodes. The metal parts are brought into contact with the catholyte in the deposition compartment (at approximately 25°C), and a current density of less than 1 A / dm2 is applied for electrolytic deposition for variable times between 130 min and 170 min. The test plate setup was used for 4 months and water consumption, chemical compounds and water disposal were closely monitored. During the four-month process period, nickel ions are replenished using a nickel ion source, which is an aqueous solution comprising dissolved nickel sulfate without any complexing agents for the nickel ions, and which has a nickel ion concentration of approximately 100 g / L. Zinc is replenished from metallic zinc dissolved under alkaline pH conditions. No additional complexing agents are used for the zinc ions due to the formation of zinc hydroxide complexes under alkaline conditions. After the zinc-nickel alloy is deposited, the metal parts are rinsed with water in a rinse compartment comprising five fluidly connected rinse subcompartments forming a 5-step rinse cascade. Portions of the rinse water and portions of the catholyte are repeatedly combined and transferred to a vacuum evaporator (40°C, approximately 50 mbar, capacity: approximately 150 L / h) to separate the water from the complexing agent, nickel ions, and zinc ions, respectively. A portion of the separated water is returned to the rinse cascade. The excess water (nickel and zinc concentration below 0.1 mg / L) is for disposal or other industrial purposes, particularly for a pre-rinse stage as used in this example. In each case, the separated water has a conductivity of less than 200 pS / cm.Nickel ions, zinc ions, and the complexing agent are enriched as a concentrated aqueous solution (density between 1.20 kg / L and 1.23 kg / L; completely aqueous with no phase separation) and returned to the catholyte. During the approximately four-month operating period, less than 500 L / week of excess water (<200 pS / cm) is removed, preferably for pre-rinsing. Even after a four-month operating period, the catholyte does not contain decomposition products such as cyanide and oxalate ions. This confirms that the complex-forming agent Cao / nn / zznz / E / YiAi does not decompose in either the deposition compartment or the vacuum evaporator. This justifies the repeated use of the water. After an operating time of approximately four months, a portion of the catholyte is treated in a second treatment compartment (freezing unit) at a temperature between 2°C and 4°C, or between -2°C and 2°C, to precipitate at least a portion of sulfate and carbonate anions. However, even after four months, a critical concentration of carbonate and sulfate in the catholyte was still not reached. During the four-month operating period, no complexing agent was added to the catholyte. Instead, the concentration of the complexing agent in the catholyte remained constant, with a variation of + / -2.5% due to measurement inaccuracies and variable catholyte volumes. Nickel and zinc ions were replenished to maintain their concentration within the initially established ranges. Furthermore, no nickel-contaminated water was produced for disposal. Furthermore, the cathodic current efficiency (CCE) was approximately 15% to 30% higher than in a comparative test coating configuration (see below). Comparative test coating configuration (not according to the invention): In a comparative trial coating setup (not according to the invention), a deposition bath is established that is essentially identical to the catholyte used in the trial coating setup according to the invention (also similar in terms of volume). However, the anodes are not separated by membranes. Therefore, the complexing agent decomposes at least partially at the anode and must thus be replenished along with nickel ions. Although the rinse water (i.e., wastewater) is subjected to a vacuum evaporator to reduce its volume before disposal, the wastewater comprises significant quantities of decomposition products, including cyanide. This necessitates professional and costly disposal. The volume of (concentrated) wastewater amounted to approximately 1000 L / week, with a nickel concentration of at least 1 g / L, zinc of at least 8 g / L, and cyanide of at least 0.1 g / L, and significant amounts of complexing agent. Therefore, a significant amount of nickel and zinc is lost, which must be replenished in the plating bath. In addition, the complexing agent must be added regularly to the plating bath. In contrast, the method of the present invention (see the example according to the invention) not only reduces the amount of water to be discarded, but also ensures that the discarded water is substantially free of nickel and zinc ions. These ions, transferred during rinsing, are recycled back into the catholyte along with the complexing agent. As a result, the method of the present invention is a very economical and environmentally friendly method and a significant improvement over existing methods. System for depositing a zinc-nickel alloy onto a substrate, (according to the invention): cao / nn / zznz / E / YiAi Figure 1 shows a schematic representation of a system 1 for depositing a zinc-nickel alloy onto a substrate, wherein an aqueous zinc-nickel deposition bath is provided as a catholyte 3-1 in the deposition compartment 3. System 1 optionally includes a pre-rinse compartment 2 for pre-rinsing the substrate. Since the substrate to be coated is often contaminated with unwanted contaminants, a pre-rinse of the substrate in the pre-rinse compartment 2, for example, with an alkaline pre-rinse solution, is generally recommended. However, if the substrate is already clean, a pre-rinse is preferably omitted. System 1 further comprises the deposition compartment 3 for electrolytically depositing the zinc-nickel alloy onto the substrate in the catholyte 3-1. The catholyte provided in the deposition compartment comprises nickel ions and at least one complexing agent for nickel and zinc ions. At least one anode with at least one membrane 3-2, separating the catholyte from an anolyte, is provided in the deposition compartment 3. The volume of the anolyte is defined by the space formed by the at least one anode and the at least one membrane. When the substrate, preferably the previously rinsed substrate, is transferred to the catholyte 31 in the deposition compartment 3 and a current is applied, the zinc-nickel alloy is electrolytically deposited onto the substrate, so that the zinc-nickel coated substrate is obtained. System 1 further comprises a rinse compartment 4 for rinsing the zinc-nickel coated substrate to obtain a rinsed zinc-nickel coated substrate and rinse water. Rinsing the zinc-nickel coated substrate removes the catholyte residue, so that the resulting rinse water comprises a portion of the catholyte, which in turn comprises nickel ions, at least one complexing agent for nickel ions and zinc ions. The rinse water is transferred (preferably pumped) from rinse compartment 4 via rinse water line 4-1 to a first treatment compartment 5 of system 1 for treatment. Additionally, a portion of the catholyte is transferred (preferably pumped) from deposition compartment 3 to the first treatment compartment 5 via catholyte removal line 3-3. This latter transfer is necessary to maintain a constant volume of catholyte. The treatment compartment 5 is preferably an evaporator, more preferably a vacuum evaporator, which allows for efficient separation of water by evaporation. At least some of the separated water, preferably evaporated, is returned from the first treatment compartment 5 to the rinse compartment 4 via the water return line 4-2. Additionally, and optionally, another portion of the water is returned to the pre-rinse compartment (not shown). Excess water is removed via the water disposal line 5-2 and is preferably used for other industrial purposes, as this water is very pure. After separating water from nickel ions, from at least one complexing agent for nickel ions, and zinc ions in the first treatment compartment 5, the separated nickel ions, the at least one complexing agent for the separated nickel ions, and the separated zinc ions are returned to the deposition compartment 3 as a concentrated aqueous solution, either directly or as shown in Figure 1, preferably indirectly by transferring them from the first treatment compartment 5 to the optional mixing unit 6 via the separation line 5-1. The optional mixing unit 6 is fluidly connected to the nickel ion source 7-1, which is preferably an aqueous solution comprising water and nickel sulfate dissolved therein, and to the zinc ion source 7-2, preferably as described earlier in the text of the method of the present invention. In the mixing unit 6, the replenished nickel and zinc ions are thoroughly mixed with the concentrated aqueous solution before being returned to the deposition compartment 3 via the return line 6-1, thereby closing the circuit. Thus, the nickel ions, zinc ions, and at least one complexing agent for the nickel ions are maintained at a substantially constant concentration in the catholyte. System 1 further comprises an optional second treatment compartment 8 for treating catholyte 3-1 so that dissolved anions, such as sulfate and carbonate anions, are separated from the catholyte 3-1. When the system operates for an extended period, for example, several months, the concentration of dissolved anions reaches an undesirable level, requiring at least partial removal of these anions in the second treatment compartment, preferably by precipitation. These precipitated anions are then removed via anion removal line 8-1. Reference signals: system for depositing a zinc-nickel alloy onto a substrate pre-rinse or pre-wash compartment deposition compartment 3-1 space for the catholith 3-2 at least one anode with at least one membrane 3-3 catholith removal line rinse compartment 4-1 rinse water line 4-2 water return line first treatment compartment 5-1 separation line 5-2 water disposal line mixing unit 6-1 return line 7-1 nickel ion source 7-2 Zinc ion source second treatment compartment 8-1 anion removal line

Claims

CLAIMS 1. A method for depositing a zinc-nickel alloy onto a substrate, the method comprising the steps of: (a) providing the substrate, (b) providing an aqueous zinc-nickel deposition bath as a catholyte in a deposition compartment, wherein: - the deposition compartment comprises at least one anode with an anolyte, and - the anolyte is separated from the catholyte by at least one membrane, and the catholyte comprises: (i) nickel ions, (ii) at least one complexing agent for nickel ions, and (iii) zinc ions, (c) contacting the substrate with the catholyte in the deposition compartment, such that the zinc-nickel alloy is electrolytically deposited onto the substrate and a zinc-nickel coated substrate is thereby obtained, wherein: after step (c) the nickel ions in the catholyte have a lower concentration than before step (c),(d) rinsing the zinc-nickel coated substrate in a rinsing compartment comprising water, so as to obtain a rinsed zinc-nickel coated substrate and rinse water, wherein: the rinse water comprises a portion of at least one complexing agent for nickel ions and a portion of the nickel ions, characterized in that: (i) at least a portion of the rinse water and / or at least a portion of the catholyte are treated in a first treatment compartment such that the water is separated from the at least one complexing agent for the nickel ions and the nickel ions, (ii) at least a portion of the at least one complexing agent separated from the water is returned to the catholyte, and (iii) a source of nickel ions is added to the catholyte, preferably directly or indirectly,provided that the source of nickel ions does not comprise said at least one complexing agent for nickel ions or any other complexing agent for nickel ions.

2. The method according to claim 1, further characterized in that the at least one complexing agent for the nickel ions is not in contact with the at least one anode.

3. The method according to claim 1 or 2, further characterized in that the source of nickel ions is an aqueous solution comprising water and a nickel salt dissolved therein.

4. The method according to any of the preceding claims, further characterized in that the nickel ion source is essentially free from or does not comprise tetraethylenepentamine, preferably is essentially free from or does not comprise a diamine, most preferably is essentially free from or does not comprise an amine.

5. The method according to any of the preceding claims, further characterized in that in the catholyte the at least one complexing agent for nickel ions comprises an amine, preferably a diamine, more preferably tetraethylenepentamine.

6. The method according to any of the preceding claims, further characterized in that step (a), prior to step (c), comprises the step of: (a-1) pre-rinsing the substrate in a pre-rinse compartment comprising water, so as to obtain a pre-rinsed substrate and pre-rinse water.

7. The method in accordance with any of the preceding claims, further characterized in that the at least one anode has a distance to the at least one membrane in the range of 0.5 mm to 5.0 mm, preferably from 0.75 mm to 4 mm, more preferably from 1.0 mm to 3.0 mm.

8. The method in accordance with any of the preceding claims, further characterized in that the first treatment compartment comprises an evaporator, preferably a vacuum evaporator.

9. The method in accordance with any of the preceding claims, further characterized in that at least a portion of the separated water obtained in the first treatment compartment is returned to the pre-rinse compartment and / or the rinse compartment.

10. The method according to any of the preceding claims, further characterized in that it comprises the step of: (e) treating at least a portion of the catholyte in a second treatment compartment such that the dissolved anions are separated from the catholyte, preferably by precipitation and / or by ion exchange, most preferably by precipitation.

11. The method according to claim 10, further characterized in that the precipitation is carried out at a temperature in the range of -5°C to 11.0°C, preferably in the range of 0.5°C to 10.0°C, more preferably in the range of 1.0°C to 8.0°C, even more preferably in the range of 1.5°C to 6°C, most preferably in the range of 2.0°C to 4.0°C.

12. The method according to claim 10 or 11, further characterized in that the dissolved anions comprise at least sulfate anions, and wherein the sulfate anions are preferably separated by precipitated sodium sulfate.

13. The method in accordance with any of the preceding claims, further characterized in that the zinc ions in the catholyte are present as hydroxo complexes.

14. The method according to any of the preceding claims, further characterized in that at least a portion of the separated water obtained in the first treatment compartment is discarded, wherein the discarded water comprises nickel ions in a concentration range of 0 mg / L to 1.0 mg / L, based on the total volume of the discarded water, preferably from 0 mg / L to 0.5 mg / L, even more preferably from 0.01 mg / L to 0.11 mg / L, and most preferably from 0.01 mg / L to 0.1 mg / L.

15. A system (1) for depositing a zinc-nickel alloy onto a substrate, the system (1) comprising: (I) optionally, a pre-rinsing compartment (2) for pre-rinsing the substrate, (II) a deposition compartment (3) for electrolytically depositing the zinc-nickel alloy onto the substrate in a catholyte (3-1) to obtain a zinc-nickel coated substrate, wherein the deposition compartment (3) comprises at least one anode with at least one membrane (3-2), (III) a rinsing compartment (4) for rinsing the zinc-nickel coated substrate to obtain a rinsed zinc-nickel coated substrate and rinse water, (IV) a first treatment compartment (5) for treating the rinse water and a portion of the catholyte (3-1) to separate the water from the nickel ions and complexing agents for the ions of nickel, and (V) optionally,a second treatment compartment (8) for treating the catholyte in such a way that the dissolved anions are separated from the catholyte, characterized in that: the first treatment compartment (5) is adapted in such a way that: - the separated water is returned to the pre-wash compartment (2) and / or the rinse compartment (4), and - the separated nickel ions and the separated nickel ion complexing agents are returned to the deposition compartment (3), preferably via a mixing compartment (6).