Recycling an electroplating bath of trivalent chromium cations

Abstract

A method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process, wherein a chromium layer is deposited on a substrate, is disclosed. The method comprises subjecting the electroplating bath to an electrodialysis treatment in an electrodialysis arrangement comprising a cathode compartment, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other. The electrodialysis treatment comprises: - providing the electroplating bath into the cathode compartment, - providing a solution of a chromium(III)salt into the anode compartment, and - applying an electric voltage over the cation exchange membrane for transporting trivalent chromium cations from the anode compartment to the cathode compartment. Further is disclosed the use of a cation exchange membrane.

Description

[0001] RECYCLING AN ELECTROPLATING BATH OF TRIVALENT CHROMIUM CATIONS

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process. The present disclosure further relates to the use of a cation exchange membrane in an electrodialysis treatment of an electroplating bath of trivalent chromium cations.

[0004] BACKGROUND

[0005] An electroplating bath of trivalent chromium cations is commonly used for producing a chromium layer in an electroplating process. As such a process continues, the trivalent chromium cation content of the electroplating bath decreases and there may become a need to change the bath to a fresh one due to salt accumulation. There remains a need for an efficient way how to provide additional amount of trivalent chromium cations to an electroplating bath.

[0006] SUMMARY

[0007] Disclosed is a method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process, wherein a chromium layer is deposited on a substrate. The method comprises providing the electroplating bath from the electroplating process and subj ecting the electroplating bath to an electrodialysis treatment in an electrodialysis arrangement comprising a cathode compartment, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, wherein the electrodialysis treatment comprises:- providing the electroplating bath into the cathode compartment,

[0008] - providing a solution of a chromium ( I I I ) salt into the anode compartment,

[0009] - applying an electric voltage over the cation exchange membrane for transporting trivalent chromium cations from the anode compartment to the cathode compartment,

[0010] wherein the cation exchange membrane is configured to allow transportation of trivalent chromium cations from the anode compartment to the cathode compartment but to prevent transportation of the anion part of the chromium ( I I I ) salt from the anode compartment to the cathode compartment, to reduce accumulation of the anion part of the chromium ( I I I ) salt in the electroplating bath in the cathode compartment.

[0011] Further is disclosed is a method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process, wherein a chromium layer is deposited on a substrate. The method comprises providing the electroplating bath from the electroplating process and subj ecting the electroplating bath to an electrodialysis treatment in an electrodialysis arrangement comprising a cathode compartment, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, wherein the electrodialysis treatment comprises:

[0012] - providing the electroplating bath into the cathode compartment,

[0013] - providing a solution of a chromium ( I I I ) salt into the anode compartment,

[0014] - applying an electric voltage over the cation exchange membrane for transporting trivalent chromium cations from the anode compartment to the cathode compartment,wherein the cation exchange membrane is configured to allow transportation of trivalent chromium cations from the anode compartment to the cathode compartment but to prevent transportation of the anion part of the chromium(III)salt from the anode compartment to the cathode compartment, to reduce accumulation of the anion part of the chromium(III)salt in the electroplating bath in the cathode compartment,

[0015] wherein the method comprises reusing the electroplating bath after the electrodialysis treatment in an electroplating process, wherein a chromium layer is deposited on a substrate.

[0016] Further is disclosed the use of a cation exchange membrane in an electrodialysis treatment of an electroplating bath of trivalent chromium cations recycled from an electroplating process, where the electroplating bath has been used to deposit a chromium layer on a substrate, in an electrodialysis arrangement comprising a cathode compartment containing the electroplating bath, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, for allowing transportation of trivalent chromium cations from the anode compartment to the cathode compartment but preventing transportation of the anion part of the chromium ( 111 ) salt from the anode compartment to the cathode compartment when adding a solution of chromium ( I I I ) salt into the anode compartment during the electrodialysis treatment, to reduce accumulation of the anion part of the chromium ( I I I ) salt in the electroplating bath.

[0017] DETAILED DESCRIPTION

[0018] The present disclosure relates to a method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process, wherein a chromium layer is deposited on a substrate. The method comprises providing the electroplating bathfrom the electroplating process and subj ecting the electroplating bath to an electrodialysis treatment in an electrodialysis arrangement comprising a cathode compartment, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, wherein the electrodialysis treatment comprises:

[0019] - providing the electroplating bath into the cathode compartment,

[0020] - providing a solution of a chromium ( I I I ) salt into the anode compartment,

[0021] - applying an electric voltage over the cation exchange membrane for transporting trivalent chromium cations from the anode compartment to the cathode compartment,

[0022] wherein the cation exchange membrane is configured to allow transportation of trivalent chromium cations from the anode compartment to the cathode compartment but to prevent transportation of the anion part of the chromium ( I I I ) salt from the anode compartment to the cathode compartment, to reduce accumulation of the anion part of the chromium ( I I I ) salt in the electroplating bath in the cathode compartment.

[0023] The present disclosure further relates to a method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process, wherein a chromium layer is deposited on a substrate. The method comprises providing the electroplating bath from the electroplating process and subj ecting the electroplating bath to an electrodialysis treatment in an electrodialysis arrangement comprising a cathode compartment, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, wherein the electrodialysis treatment comprises:

[0024] - providing the electroplating bath into the cathode compartment,- providing a solution of a chromium ( I I I ) salt into the anode compartment,

[0025] - applying an electric voltage over the cation exchange membrane for transporting trivalent chromium cations from the anode compartment to the cathode compartment,

[0026] wherein the cation exchange membrane is configured to allow transportation of trivalent chromium cations from the anode compartment to the cathode compartment but to prevent transportation of the anion part of the chromium(III)salt from the anode compartment to the cathode compartment, to reduce accumulation of the anion part of the chromium(III)salt in the electroplating bath in the cathode compartment,

[0027] wherein the method comprises reusing the electroplating bath after the electrodialysis treatment in an electroplating process, wherein a chromium layer is deposited on a substrate.

[0028] The present disclosure further relates to the use of a cation exchange membrane in an electrodialysis treatment of an electroplating bath of trivalent chromium cations recycled from an electroplating process, where the electroplating bath has been used to deposit a chromium layer on a substrate, in an electrodialysis arrangement comprising a cathode compartment containing the electroplating bath, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, for allowing transportation of trivalent chromium cations from the anode compartment to the cathode compartment but preventing transportation of the anion part of the chromium ( I I I ) salt from the anode compartment to the cathode compartment when adding a solution of a chromium ( I I I ) salt into the anode compartment during the electrodialysis treatment, to reduce accumulation of the anion part of the chromium ( I I I ) salt in the electroplating bath.In the context of this specification, the terms "electroplating process", "electrolytic plating" and "electrodeposition" may be understood as synonyms. A chromium layer may be produced on a substrate, e. g. a metal substrate, by an electroplating process.

[0029] In the context of this specification, the term "recycling" may be understood as referring to treating an electroplating bath that has been used in an electroplating process for adding trivalent chromium cations therein, whereafter the electroplating bath may be reused in an electroplating process.

[0030] The electrodialysis treatment is carried out in an electrodialysis arrangement that is separate from the electroplating process, where the deposition of a chromium layer is taking place. Thus, the electrodialysis treatment does not need to be carried out physically in the same place as the electroplating process. The electrodialysis arrangement is thus a separate unit from the electroplating unit.

[0031] In the context of this specification, the term "chromium layer" may be understood as referring to layer formed from an electroplating bath of trivalent chromium cations. The chromium cations present in the electroplating bath are thus present substantially only in the trivalent form, not in the hexavalent form. In this connection, the wording electroplating "from an electroplating bath of trivalent chromium cations" is used to define a process in which the deposition is taking place from an electroplating bath in which chromium cations are present substantially only in the trivalent form.

[0032] Thus, in an electroplating process of forming a chromium layer, an electroplating bath of trivalent chromium cations is used. As the electroplating process continues, the amount of trivalent chromium cations is decreased and additional trivalent chromium cations need to be added into the electroplating bath.Chromium ( I I I ) sulphate solution is commonly used as the source for trivalent chromium cations. However, adding chromium ( I I I ) sulphate solution directly into the electroplating bath has the adverse effect that sulphate ions are accumulated in the electroplating bath during the electroplating process. As a result of this the electroplating process needs to be stopped and the electroplating bath used therein changed at regular intervals. This is time consuming and non-cost effective for a large-scale chromium plating process. The inventors surprisingly found out that recycling the electroplating bath from the electroplating process and subj ecting the same to the separate electrodialysis treatment as disclosed in the current specification removes the need to interrupt the chromium plating process as sulphate ions are not accumulated in the electroplating bath. The electrodialysis treatment as disclosed in the current specification has the added utility of one being able to selectively add trivalent chromium cations into the electroplating bath without the addition of sulphate ions that are not consumed in the electroplating process but on the contrary result in undesired accumulation. Thus, the lifetime of the electroplating bath is increased.

[0033] The adverse effect of accumulation of the anion part of the chromium ( I I I ) salt in the electroplating bath during the electroplating process, relates especially to the use of the trivalent form of chromium. This is due to the fact that the trivalent form of chromium is to be added e. g. as chromium(III)sulphate, where the sulphate part is not used by the electroplating process. When using hexavalent form of chromium in the electroplating bath one may not face the same effect as the hexavalent form of chromium may be added to the electroplating bath as chromium (VI ) oxide that will react with the water present in the electroplating bath.There may also be a need to add other components into the electroplating bath before being reused in the electroplating process. Such components may be added directly in to the electroplating bath before, during, or after the electrodialysis treatment. The electroplating bath may be an aqueous electroplating bath.

[0034] In one embodiment, the method comprises reusing the electroplating bath after the electrodialysis treatment in an electroplating process, wherein a chromium layer is deposited on a substrate. Thus, recycling of the electroplating bath may comprise subj ecting the electroplating bath from an electroplating process, where it has been used for forming a chromium layer, to the electrodialysis treatment as disclosed in the current specification, and then to provide the same back to an electroplating process. The electroplating bath that has underwent the electrodialysis treatment may be provided back to the same electroplating process from where it was originally taken or to another electroplating process. The method as disclosed in the current specification thus has the added utility of enabling a continuous process, where there is no need to interrupt the electroplating process for changing the electroplating bath to a fresh one.

[0035] The electroplating bath that has been sub ected to electrodialysis treatment thus contains an amount of trivalent chromium cations needed for the electroplating process. However, as the accumulation of the anion part of the chromium ( I I I ) salt in the electroplating bath in the cathode compartment is reduced, the electroplating bath does not contain said anion part to an extent that would adversely affect the following electroplating process. Instead, the electroplating process may be continued for a longer period of time without the need to exchange the electroplating bath to a fresh one, as thee. g. sulphate is not accumulated in the electroplating bath during the electroplating process.

[0036] In one embodiment, providing the electroplating bath into the cathode compartment is carried out in a continuous manner during the electrodialysis treatment. When providing electroplating bath into the cathode compartment in a continuous manner, treated electroplating bath may simultaneously be recovered or removed from the cathode compartment in a continuous manner to be reused in an electroplating process. Alternatively, providing the electroplating bath into the cathode compartment may be carried out in a batchwise manner. When providing the electroplating bath into the cathode compartment in a batchwise manner, the electroplating bath is provided into the cathode compartment, then subj ected to the electrodialysis treatment for a predetermined period of time, after which the electroplating bath is recovered or removed from the cathode compartment to be reused in an electroplating process.

[0037] A solution of a chromium ( 111 ) salt is used in the electrodialysis treatment. In one embodiment, the chromium(III)salt is chromium(III)sulphate or chromium(III)chloride. These chromium ( I I I ) salts are readily available and can be provided at low cost. The chromium ( 111 ) salt to be used is mixed with a solution, e. g. water, whereby the chromium ( I I I ) salt dissolves therein and a solution of chromium ( I I I ) salt is formed.

[0038] From the chromium ( III ) salt is formed trivalent chromium cation (Cr3+) and its counterion, which may vary depending on which salt is used. In case of chromium ( 111 ) sulphate the anion part of the salt, or the counterion, is sulphate ion (SO42-). In the case of chromium ( 111 ) chloride the anion part, or counterion, is chloride ion (Cl-).

[0039] In one embodiment, providing a solution of the chromium ( I I I ) salt into the anode compartment is carried out in a continuous manner during the electrodialysistreatment. Alternative, providing a solution of the chromium ( I I I ) salt into the anode compartment may be carried out in a batchwise manner.

[0040] In one embodiment, the concentration of trivalent chromium cations in the electroplating bath in the cathode compartment is increased to a value of 5 - 30 g / 1, or 7.5 - 25 g / 1, or 10 - 20 g / 1, or 9 - 14 g / 1, during the electrodialysis treatment.

[0041] In one embodiment, the solution of the chromium ( 111 ) salt has a chromium ( I I I ) salt concentration of 1 - 50 %, or 3 - 45 %, or 5 - 40 %, or 10 - 36 %, or 15 - 34 %, or 20 - 32 %, or 25 - 30 %.

[0042] In one embodiment, the chromium content of the solution of the chromium ( I I I ) salt is 0.5 - 20 weight-%, or 3 - 17 weight-%, or 5 - 15 weight-%.

[0043] In one embodiment, the solution of the chromium ( III ) salt further comprises sulphuric acid. The solution of chromium ( I I I ) salt may comprise 0 - 10 weight-%, or 1 - 9 weight-%, or 2 - 8 weight-%, or 3 - 7 weight-%, or 4 - 6 weight-%, of sulphuric acid. Sulfuric acid or an acidic environment has the added utility of improving the solubility of the chromium salt, such as chromium sulfate, and stabilizes its concentration in the solution, thus preventing possible precipitation due to hydrolysis.

[0044] The electrodialysis treatment may be carried out by applying an electric voltage. Applying an electric voltage over the cation exchange membrane during the electrodialysis treatment allows the transportation of the ions, and especially the trivalent chromium cations, formed in the solution, from the anode compartment to the cathode compartment. The transportation rate or speed may be affected by the selected electric voltage value. The electric voltage value may be e. g. about 9 V. The electrodialysis treatment is carried out by using an external power supply. Thus, a power supply differentor separate from the one used in the electroplating process is used in the electrodialysis treatment.

[0045] The cation exchange membrane may be any cation exchange membrane suitable to be used for allowing the transportation of trivalent chromium cations therethrough but preventing or reducing the transportation of the anion part of the chromium ( I I I ) salt therethrough. In one embodiment, the cation exchange membrane is a perfluorosulfonic acid (PFSA) membrane.

[0046] In one embodiment, the electrodialysis treatment is carried out at a current density value of 0.1 - 20 A / dm2, or 0.3 - 18 A / dm2, or 0.5 - 17 A / dm2, or 1 -15 A / dm2, or 3 - 13 A / dm2, o r 5 - 11 A / dm2, o r 7 - 9 A / dm2. In one embodiment, the electrodialysis treatment is carried out at a constant current density value selected from 0.1 - 20 A / dm2, or 0.3 - 18 A / dm2, or 0.5 - 17 A / dm2, or 1 - 15 A / dm2, or 3 - 13 A / dm2, or 5 - 11 A / dm2, or 7 - 9 A / dm2. In the electrodialysis treatment the unit for the current density is " A / dm2", wherein the "dm2" refers to the area of the cation exchange membrane. The current density value has the added utility of one being able to carry out the electrodialysis treatment without chromium being deposited at the same time on the cathode but to remain the trivalent chromium cations in the electroplating bath until the electroplating bath is reused. The method as disclosed in the current specification has the added utility of one being able to adjust the current density values of the electrodialysis treatment separately from the actual electrodeposition process, where different current density values may be used in order to carry out deposition of the chromium layer from the electroplating bath.

[0047] In one embodiment, the cathode compartment comprises a cathode made of a material that inhibits or prevents chromium to be deposited thereon. By using a cathode made of material that may prevent chromium to be deposited thereon, has the added utility of furtherhindering the possibility of chromium deposition taking place during the electrodialysis treatment. In addition to the material of the cathode also the other processing conditions used in the method as disclosed in the current specification have the added utility of preventing chromium from precipitating.

[0048] In one embodiment, the cathode compartment comprises a cathode made of titanium-based multi-metal oxide or of platinized metal. In one embodiment, the anode compartment comprises an anode made of titanium-based multi-metal oxide. Such a material is mechanically durable and allows the electrodialysis arrangement to be built in a smaller size scale. In the context of this specification, the term "titanium-based multi-metal oxide" may be understood as referring to a type of compound that includes titanium (Ti) as one of the metals, along with other metals, in oxide form. Such other metals may include transition metals like iron (Fe), copper (Cu), cobalt (Co), vanadium (V), and others. Examples of titanium-based multi-metal oxides are ruthenium-iridium oxide coated titanium and iridium-tantalum oxide coated titanium. Example of platinized metal is platinized titanium.

[0049] In one embodiment, the content of trivalent chromium cations in the anode compartment is higher than the content of trivalent chromium cations in the cathode compartment. In one embodiment, the content of trivalent chromium cations in the anode compartment is at least one and a half time, or at least two-times, or at least three-times, the content of trivalent chromium cations in the cathode compartment. Ensuring a higher content of trivalent chromium cations in the anode compartment compared to the cathode compartment has the added utility that the electrodialysis treatment may be carried out in a more efficient manner resulting in faster electrodialysis treatment. However, in some embodiment, the content of trivalent chromium cations in the anodecompartment may be less than the content of trivalent chromium cations in the cathode compartment. In such a case, the electrodialysis treatment is working but may not be as efficient as when the content of trivalent chromium cations is higher in the anode compartment compared to the cathode compartment.

[0050] In one embodiment, the temperature of the solution of the chromium ( I I I ) salt in the anode compartment is kept at a value selected from 20 - 70 °C, or 30 - 60 °C, or 45 - 55 °C. Such a temperature of the chromium (III) salt solution may hinder the crystallisation of the chromium ( III ) salt during the electrodialysis treatment.

[0051] In one embodiment, the temperature of the electroplating bath in the cathode compartment is kept at a value selected from 20 - 70 °C, or 30 - 60 °C, or 45 -55 °C. Keeping the temperature of the electroplating bath at a temperature of 20 - 70 °C has the added utility of ensuring that the electroplating bath remains in soluble form and that no precipitation takes place during the electrodialysis treatment. If the temperature of the electroplating bath is too high de-complexation or decomposition may take place.

[0052] In one embodiment, the pH of the solution of the chromium ( I I I ) salt in the anode compartment is 0.5 -6.5, or 1 - 6.0, or 2 - 5, or 3 - 4. In one embodiment, the pH of the electroplating bath in the cathode compartment is 2 - 6.5, or 3 - 6, or 3.5 - 5.5, or 4.0 -5.0. The pH may be adjusted by the use of e. g. sodium hydroxide, ammonium hydroxide, or formic acid. Keeping the pH at these values has the added utility of hindering precipitation.

[0053] The method as disclosed in the current specification has the added utility of enabling a continuous process where the concentration of trivalent chromium cations (Cr3 +) in the electroplating bath can be kept constant during its use in an electroplating process.The method as disclosed in the current specification has the added utility that by the electrodialysis treatment one is able to prolong the lifetime of the electroplating bath. The method as disclosed in the current specification has the added utility of reducing the need for maintenance treatments of the electroplating process while keeping the composition of the electroplating bath stable.

[0054] The method as disclosed in the current specification has the added utility of being a cost-effective method for adding trivalent chromium cations into the electroplating bath. Being able to hinder the accumulation of sulphate ions in the electroplating bath during the electroplating process has a large impact on the production costs in industrial use. With the method as disclosed in the current specification there is thus no need to stop the electroplating process at frequent time intervals for changing the electroplating bath to a fresh one.

[0055] EXAMPLES

[0056] The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

[0057] Example 1 Electrodialysis treatment of an electro¬ plating bath

[0058] In this example addition of trivalent chromium cations from a chromium (III) sulphate solution into an electroplating bath via electrodialysis treatment was tested.The electroplating bath was recovered from an electroplating process, wherein a chromium layer was: formed on a substrate. The electroplating bath was placed in a cathode compartment of a electrodialysis arrangement and chromium (III) sulphate solution was added into the anode compartment and an electric voltage of 6 - 10 V was applied. The following testing parameters were used in the electrodialysis treatment:

[0059] 30 % chromium (III) sulphate solution

[0060] Current density: 4 A / dm2

[0061] Temperature in both the anode and cathode compartment: 55 °C

[0062] Distance between anode and cathode: 24 cm

[0063] Volume in both the anode and cathode compartment: 1 1 (after the test the volume of the electroplating bath in the cathode compartment was 0.95 1 (5 % evaporation) ) Area of the cation exchange membrane: 0.5 dm2Tested cation exchange membrane: Hubei CM

[0064] Testing time: 22 h

[0065] The results are presented in the below table 1:

[0066] Table 1. The content of the electroplating bath Before elecAfter elecChange trodialysis trodialysis treatment treatment

[0067] pH 4. 9

[0068] Cr+9.5 g / 1 14. 6 g / 1 + 53 % SO42~ 8. 6 g / 1 8. 9 q / 1 + 3 o p 1 " 14.0 g / 1 13.2 g / 1 “5 % Br 14.9 g / 1 13.5 g / 1 "9 %

[0069] ! 2

[0070]

[0071] K+13.8 g / 1 14.2 g / 1

[0072] From the above results one may see that the content of trivalent chromium cations highly increased in the electroplating bath, which evidences that thetrivalent chromium cations were efficiently transported through the cation exchange membrane. No crystallization or precipitation of the trivalent chromium cations were noticed.

[0073] The pH decrease suggests that acidic H+ions were transferred through the cation exchange membrane into the electroplating bath during the electrodialysis treatment. This may be due to the dissociation of hy¬ drogen and the accumulation of H* ions in the electro¬ plating bath under the influence of the electric field.

[0074] The concentration of chlorine ions decreased, which may be due to the migration of anions through the cation exchange membrane, whereby Cl~-ions were transferred to the anode side. Alternatively, this may be due chemical reaction or evaporation occurred that reduced the concentration of Cl~-ions in the electroplating bath.

[0075] The decrease in Br~ concentration may be related to the transfer of anions through the cation ex¬ change membrane, as bromine ions are negatively charged and can move to the anode side.

[0076] During the test, 5.04 g / 1 trivalent chromium cations were added into the electroplating bath during the 22 hours testing time. Thus 0.23 g / 1 was transported per hour, i. e. 0.46 g / dm2 / hour of trivalent chromium cations were transported through the cation exchange membrane

[0077] Example 2 - Testing the effect of the current density on the cation exchange membrane

[0078] In this example addition of trivalent chromium cations from a chromium (III) sulphate solution into an electroplating bath via electrodialysis treatment using a higher current density was tested.

[0079] The electroplating bath was recovered from an electroplating process, wherein a chromium layer wasformed on a substrate. The electroplating bath was placed in a cathode compartment of the electrodialysis arrangement and chromiu (III) sulphate solution was added into the anode compartment and an electric voltage was applied. Applied voltage from the rectifier was 16 - 21 V. The following testing parameters were used in the electrodialysis treatment:

[0080] 30 % chromium (III) sulphate solution

[0081] Current density: 12 A / dm2

[0082] Temperature in both the anode and cathode compartment: 44 °C

[0083] Distance between anode and cathode: 24 cm

[0084] Volume in both the anode and cathode compartment: 1 1 (after the test the volume of the electroplating bath in the cathode compartment was 0.98 1 (2 % evaporation) ) Area of the ion exchange membrane: 0.5 dm2

[0085] Tested ion exchange membrane: Hubei CM

[0086] Testing time: 4 h

[0087] The results are presented in the below table 2:

[0088] Table 2. The content of the electroplating bath Before elecAfter elecChange trodialysis trodialysis treatment treatment

[0089] pH 4. 9 5. 6

[0090] Cr⁺ 11.6 g / l 13.5 g / l +16 %SO₄²⁻ 8.9 g / l 9.9 g / l +10 %Cl⁻ 10.4 g / l 9.8 g / l -5 %Br⁻ 12.5 g / l 11.6 g / l -7 %

[0091]

[0092] K+11.0 g / l 11.8 g / l +7 %

[0093] From the above results one may see that the content of trivalent chromium cations increased in the electroplating bath, but so did also the sulphate ion concentration. One may conclude that the cation exchangemembrane may lose its selectivity when a higher current density was used.

[0094] During the test, 1.8 g / 1 trivalent chromium cations were added into the electroplating bath during the 4 hours testing time. Thus 0.44 g / 1 was transported per hour, i. e. 0.88 g / dm2 / hour of trivalent chromium cations were transported through the ion exchange mem¬ brane. However, the rate of sulphate leakage was also rising from 0.026 g / dm2 / hour to 0.45 g / dm2 / hour.

[0095] Example 3 Electrodialysis treatment without using electric voltage

[0096] In this example a similar type of electrodial¬ ysis treatment as described in examples 1 and 2 was tested but without using any external current or volt¬ age. Thus, the electroplating bath and testing arrange¬ ments were similar to the ones in examples 1 and 2 with the exception that no external electric voltage was applied. The following testing parameters were used in the electrodialysis treatment:

[0097] 30 % chromium (III) sulphate solution

[0098] Current density: 0 A / dm2

[0099] Temperature in both the anode and cathode compartment: 44 °C

[0100] Distance between anode and cathode: 24 cm

[0101] Volume in both the anode and cathode compartment: 1 1 (after the test the volume of the electroplating bath in the cathode compartment was 0.97 1 (3 % evaporation) ) Area of the ion exchange membrane: 0.5 dm2

[0102] Tested ion exchange membrane: Hubei CM

[0103] Testing time: 22 h

[0104] The results are presented in the table 3 below:Table 3. The content of the electroplating bathBefore elec- After elec- Changetrodialysis trodialysistreatment treatment

[0105] pH 4. 6 5. 9

[0106] Cr+11.2 g / l 11.6 g / l +4 %SO42-6.4 g / l 7.2 g / l +13 %Cl-7.6 g / l 9.1 g / l +19 %Br-12.1 g / l 12.2 g / l +1 %

[0107]

[0108] K+9. 9 g / 1 10. 0 g / 1 + 1 %

[0109] From the above results one may see that the content o f trivalent chromium cations increased much les s than in el ectrodia lys i s treatment experiments in exampl e 1 and exampl e 2. In addition, one can see that the content o f sulphate ions highly increased when no current was used in the electrodialys i s treatment. Al so the content o f chloride ions was increased. This may be taken to evidence that the method as di sclosed in the current speci f ication may not wor k as intended without any cu rrent or voltage being used.

[0110] During the test, 0. 5 g / 1 o f trivalent chromium cations were added into the el ectroplat ing bath during the 22 hours test ing time. Thus 0. 022 g / 1 was trans¬ ported per hour, i. e. 0. 044 g / dm2 / hour o f trivalent chromium cat ions were transported through the cation exchange membrane. The rate i s only 10 % o f the rate o f e lectrodialys i s and i s too low for practical appl ica¬ tions. Concurrently the rate o f sulphate ion trans fer was 1. 6 g / dm2 / hour, which means that the electroplating bath would be enriched with sulphate salts and eventu¬ al ly would cause the solution to be rej ected.

[0111] I t i s obvious to a person s ki l led in the art that with the advancement of technology, the bas ic idea may be implemented in various ways. The embodiments arethus not limited to the examples described above; instead, they may vary within the scope of the claims.

[0112] The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method and use disclosed herein may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to ' an' item refers to one or more of those items. The term "comprising" is used in this specification to mean including the feature (s) or act (s) followed thereafter, without excluding the presence of one or more additional features or acts.

Claims

CLAIMS1. A method for recycling an electroplating bath of trivalent chromium cations that has been used in an electroplating process, wherein a chromium layer is deposited on a substrate, wherein the method comprises providing the electroplating bath from the electroplating process and subj ecting the electroplating bath to an electrodialysis treatment in an electrodialysis arrangement comprising a cathode compartment, an anode compartment, and an cation exchange membrane separating the cathode compartment and the anode compartment from each other, wherein the electrodialysis treatment comprises:- providing the electroplating bath into the cathode compartment,- providing a solution of a chromium(III)salt into the anode compartment,- applying an electric voltage over the cation exchange membrane for transporting trivalent chromium cations from the anode compartment to the cathode compartment,wherein the cation exchange membrane is configured to allow transportation of trivalent chromium cations from the anode compartment to the cathode compartment but to prevent transportation of the anion part of the chromium(III)salt from the anode compartment to the cathode compartment, to reduce accumulation of the anion part of the chromium(III)salt in the electroplating bath in the cathode compartment,wherein the method comprises reusing the electroplating bath after the electrodialysis treatment in an electroplating process, wherein a chromium layer is deposited on a substrate.

2. The method of any one of the preceding claims, wherein the concentration of trivalent chromium cations in the electroplating bath in the cathode compartment is increased to a value of 5 - 30 g / 1, or 7.5- 25 g / 1, or 10 - 20 g / 1, or 9 - 14 g / 1, during the electrodialysis treatment.

3. The method of any one of the preceding claims, wherein the solution of the chromium(III)salt has a chromium(III)salt concentration of 1 - 50 %, or 3 - 45 %, or 5 - 40 %, or 10 - 36 %, or 15 - 34 %, or 20 - 32 %, or 25 - 30 %.

4. The method of any one of the preceding claims, wherein the chromium content of the solution of the chromium(III)salt is 0.5 - 20 weight-%, or 3 - 17 weight-%, or 5 - 15 weight-%.

5. The method of any one of the preceding claims, wherein the chromium(III)salt is chromium(III)sulphate or chromium(III)chloride.

6. The method of any one of the preceding claims, wherein the solution of the chromium(III)salt further comprises sulphuric acid.

7. The method of any one of the preceding claims, wherein the electrodialysis treatment is carried out at a constant current density value selected from 0.1 - 20 A / dm2, or 0.3 - 18 A / dm2, or 0.5 - 17 A / dm2, or 1 - 15 A / dm2, or 3 - 13 A / dm2, or 5 - 11 A / dm2, or 7 - 9 A / dm2.

8. The method of any one of the preceding claims, wherein the cathode compartment comprises a cathode made of a material that inhibits or prevents chromium to be deposited thereon.

9. The method of any one of the preceding claims, wherein the anode compartment comprises an anode made of titanium-based multi-metal oxide.

10. The method of any one of the preceding claims, wherein the content of trivalent chromium cations in the anode compartment is at least one and a half time, or at least two-times, or at least three-times, the content of trivalent chromium cations in the cathode compartment.

11. The method of any one of the preceding claims, wherein the temperature of the solution of the chromium(III)salt in the anode compartment is kept at a value selected from 20 - 70 °C, or 30 - 60 °C, or 45 - 55 °C.

12. The method of any one of the preceding claims, wherein the temperature of the electroplating bath in the cathode compartment is kept at a value selected from 20 - 70 °C, or 30 - 60 °C, or 45 - 55 °C.

13. The method of any one of the preceding claims, wherein the pH of the solution of the chromium(III)salt in the anode compartment is 0.5 - 6.5, or 1 - 6.0, or 2 - 5, or 3 - 4.

14. The method of any one of the preceding claims, wherein the pH of the electroplating bath in the cathode compartment is 2 - 6.5, or 3 - 6, or 3.5 - 5.5, or 4.0 - 5.0.

15. The use of a cation exchange membrane in an electrodialysis treatment of an electroplating bath of trivalent chromium cations recycled from an electroplating process, where the electroplating bath has been used to deposit a chromium layer on a substrate, in an electrodialysis arrangement comprising a cathode compartment containing the electroplating bath, an anode compartment, and a cation exchange membrane separating the cathode compartment and the anode compartment from each other, for allowing transportation of trivalent chromium cations from the anode compartment to the cathode compartment but preventing transportation of the anion part of the chromium(III)salt from the anode compartment to the cathode compartment when adding a solution of a chromium(III)salt into the anode compartment during the electrodialysis treatment, to reduce accumulation of the anion part of the chromium(III)salt in the electroplating bath.

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