PROCESS AND DEVICE FOR CLEANING GALVANIC BATHS TO PLATE METALS

The invention concerns a process which is used to plate functional layers of acidic or alkaline zinc or zinc alloy baths which contain nitrogenous organic additives, a soluble zinc salt and if necessary other metal salts selected from Fe, Ni, Co and Sn salts, where the bath composition for the regeneration is conveyed through an appropriate device having an ion exchanger resin to remove cyanide ions.

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Description
NATURE OF THE INVENTION

The invention concerns a process and a device for cleaning galvanic baths to plate metals, in particular alkaline zinc-nickel alloy baths, using ion exchangers in order to prolong the lifetime of electrolytes and remove any undesirable decomposition products.

BACKGROUND OF THE INVENTION

Zinc-nickel coatings are used in all applications that require high quality surface protection when subject to corrosion. The conventional field of application is the automobile manufacture for components that are used in the engine bay, on braking systems and in the landing gear bay. For this reason, alkaline zinc-nickel electrolytes have been used more recently as published in U.S. Pat. No. 4,889,602, which for example have the following electrolyte composition:

TABLE 1 Electrolyte deposit of a zinc-nickel electrolyte Zinc oxide ZnO 11.3 g/l Nickel sulphate hexahydrate NiSO4*6H2O  4.1 g/l Sodium hydroxide NaOH  120 g/l Polyethyleneamine (complexing eg. (C2H5N)n  5.1 g/l agent)

The amines in the electrolyte act as complexing agents for the nickel ions. Complexing agents are constituents of numerous galvanic and chemical processes which are used in the separation of metals. The zinc-nickel electrolyte is usually driven by insoluble nickel anodes. The zinc content is kept constant by adding a suitable zinc ion source and the nickel content is kept constant by adding a source of nickel ions. The colour of the zinc-nickel electrolyte however changes from blue-purple to brown after a certain time of operation.

After a certain time of operation, nitriles (so-called organically bonded cyanide which can contain nitriles as well as isonitriles) and cyanide ions are formed in the zinc-nickel electrolytes through anodic oxidation from the amine-containing complexing agents. The problem of cyanide pollution requires the continuous replacement of the electrolytes and a special waste water treatment which in turn significantly affects the operating costs of the electrolyte. After several days, or weeks, there is a noticeable increase in the discoloration and a separation into two phases. The top phase is dark brown. This phase causes considerable problems when the work pieces are coated, for instance the uneven distribution of the coating thickness or blistering. The continuous removal or skimming of this second brown phase is therefore absolutely essential. This operation requires a considerable amount of time and money. The formation of the second phase is traced back to the concept that the amines in an alkaline solution on the nickel anodes are transformed to nitriles (organically bonded cyanides). This however means that because of the decomposition of the amines, new complexing agents have to keep being added which in turn increases the process costs.

Several processes are described in the prior art to reduce the concentration of cyanides.

The activated carbon cleaning process is a common process that is used in electroplating to remove organic impurities in nickel electrolytes. The quantities of activated carbon used are determined in preliminary tests. The quantities most frequently used for activated carbon cleaning are 2-5 g/l. The activated carbon is added at a temperature of between 50-60° C. Once added, the electrolyte is stirred intensively. After approximately half an hour, the absorbable substances are absorbed by the activated carbon and are filtered out. The disadvantage of this process, however, is that all organic constituents are thereby removed from the electrolytes. For zinc-nickel electrolytes this would mean that not only the decomposition products, but also all other organic constituents such as for example brighteners and complexing agents, are removed.

The publication EP 1 344 850 A1 features a device to reduce the build-up of cyanide by separating the anode from the alkaline electrolyte using an ion exchanger membrane. This separation prevents a reaction of the amines on the nickel anodes and therefore also any undesirable side-reactions. The occurring side-reactions, problems of disposal, formation of a second phase and the adverse impact on the quality of the plated zinc-nickel layer, are thereby also avoided. It is therefore no longer necessary to replace the bath and spend lots of time and money on skimming the second phase which has formed. The zinc-nickel electrolyte acts as a catholyte. The medium in the anode compartment which is separated using the aforementioned ion exchanger membrane, is known as the anolyte whereby in this case either sulphuric acid or phosphoric acid can be used. The disadvantage of this process is the use of a costly and high-maintenance ion exchanger membrane, which can also not be used for all common metallization baths.

The publication EP 0 601 504 B1 describes the cleaning of galvanic baths for the separation of metals using polymer absorber resins. Similar to the activated carbon treatment, the disadvantage is that not only the decomposition products, but also all other organic constituents such as for example brighteners and complexing agents, are removed.

DESCRIPTION OF THE DRAWINGS

FIG. 1: Ion exchanger regeneration unit

FIG. 2: Hull cell set-up

FIG. 3: Procedure and regeneration effect using a combination of an ion exchanger and the freezing out of sodium carbonate

FIG. 4: Comparison of the layer thickness distribution of different zinc-nickel electrolytes

 DESCRIPTION OF THE INVENTION

The aim of this invention is to selectively remove the cyanide and nitriles that have formed during the metallization process, from the electrolytes. Surprisingly, by using ion exchange resins which are able to bind cyanide ions, it was possible to remove not only the cyanide ions but also the nitriles from the bath. The use of ion exchange resins for this specific purpose is unknown in prior art.

In alkaline zinc-nickel electrolytes with amine-containing complexing agents (eg. polyethyleneamine), a nitrile compound is formed during the operation. The disadvantage of the decomposition product is that as the lifetime of the electrolyte is extended or as the decomposition product increases, an oily and waxy second phase is formed. The formation of the decomposition product is responsible for the loss of expensive complexing agents and the formation of highly toxic cyanide. From the amine-containing complexing agents, nitriles (R—CN, this always includes isonitriles, R—NC) are formed, initially in the oxidative reaction at the anode, which then react further to form cyanide ions (CN−).

These problems lead to reduced efficiency and loss of quality of the plated layer. Here the efficiency is the percentage part of the total current introduced to plate a defined amount of metal. To counteract the reduced efficiency, the current density is usually increased, which however in turn accelerates the decomposition rate of the complexing agent to the nitrile (R—CN) and cyanide. Tests have shown that the second phase contains large quantities of cyanide, metal and sodium carbonate (Na2CO3). It can therefore be assumed that these decomposition products are influenced by the nitrile or that they exist together as the concentration continues to increase and form a second phase. From a procedural point of view, it is difficult to separate the second phase since the liquid in the bath is constantly moving. Furthermore this also means a constant loss of complexed metal ions and other precious additives which are also in this phase. It is therefore an object of the present invention to selectively remove the cyanide and organically bonded cyanide (nitrile) from the electrolyte.

The reduction in organically bonded cyanide would be noticeable in the change in content of the total organic carbon found in the process solution. This could, however, also mean that other vital organic constituents, such as brighteners or organic complexing agents are lost in the electrolyte. If it was the case that brighteners are removed from the electrolyte, this would considerably affect the optical quality of the plated layer. A reduction in the content of cyanide and nitrile compounds would subsequently increase the efficiency.

According to the present invention, the cyanide and organically bonded cyanide is to be removed using an ion exchange resin.

Ion exchange resins are used to remove toxic substances or interfering anions or cations from waste water. The advantage of this process is that it does not require a precipitation or chemical destruction since the interfering substances can be removed from the waste water without being changed. Ion exchange resins are high-molecular organic substances. The rigid and insoluble frame has easily interchangeable counterions on it. These are easily movable and interchangeable counterions, usually hydrogen ions or hydroxyl ions. The regeneration of galvanic process baths is therefore a suitable process to extend the lifetime of electrolytes by removing interfering cations or anions. The batch operation is a process for the ion exchange. The ion exchanger resins come into contact with the electrolyte solution in a receptacle. The process is finished as soon as there is an exchange equilibrium between the counterions from the exchanger and similarly charged ions from the electrolyte solution. If additional ions have to be removed from the electrolyte using the ion exchanger resins, then new resins have to be added. The resins are filtered out once the equilibrium is established.

The column process is the process most commonly used in the laboratory. Here, the ion exchanger resin is packed into a column. All necessary operations are then performed in the pack which has been created. Two different work techniques are distinguished, namely working with a decreasing and increasing liquid layer. With the decreasing liquid layer, the electrolyte flows through the column from the top down and with the increasing liquid layer from the bottom up. Filling the column is a straightforward operation. The resin in its current form is first of all transferred to a beaker containing distilled water to swell the resins. This operation is necessary to prevent the column from shattering and to avoid the column from being to densely packed as the resins swell. Two hours is usually sufficient for the resins to swell. The resin is then sludged in the column whilst making sure that the resin which is already layered, is covered with water at all times. This is necessary in order to prevent any effects from air bubbles. Any excess water is constantly removed from the column. Finally, once the resin has been filled, a piece of cotton wadding is placed across the top of the pack. The following sub-processes should be carried out during the operating cycle of a ion exchanger column:

    • 1. Load (ion exchange)
    • 2. Wash exchanger pack
    • 3. Regenerate
    • 4. Wash exchanger pack

Washing between operations is necessary to remove any residues of reagents in the ion exchanger column. During the regeneration process, the exchanger pack is transformed to its original state (non-loaded state). If the ion that was exchanged during the ion exchange is to be recovered again, it is removed by the ion exchanger by eluting with a suitable liquid. According to the invention, the process solution flows through the ion exchanger resins, whereby the cyanides are taken up on the anchor groups through interactions and the hydroxide anions are released on the electrolytes. Surprisingly, nitrile compounds can also be removed in this way.

Each ion exchanger resin that is capable of binding cyanide ions, can be used within the framework of the present invention. Suitable ion exchange resins to bind cyanide ions are for example described in Ludwig Hartinger: Handbuch der Abwasser- and Recyclingtechnik, 2nd ed. 1991 on pages 352-361, which is incorporated herein by reference. According to paragraph 5.2.3.3.4 and Table 5-1 anions like cyanide can be exchanged utilizing strongly alkaline anion exchange resins. Such resins comprise resins made from polyacrylamide possessing quaternary ammonium groups. Such resin material is commercially available and for example described in Table 13 (page 89) of: Robert Kunin, Ion Exchange Resins, reprint 1985, which is herein incorporated by reference. Quaternary strong base resins suitable comprise Amberlite IRA-400 (Rohm & Haas Co.), Amberlite IRA-401 (Rohm & Haas Co.), Amberlite IRA-410 (Rohm & Haas Co.), Dowex 1 (Nalcite SBR) (Dow Chemical Co.), Dowex 2 (Nalcite SAR) (Dow Chemical Co.).

All such resins are also capable of binding nitriles.

By way of example, tests were carried out using the ion exchanger resins Lewatit MonoPlus M600 and MonoPlus M500 produced by Lanxess Deutschland GmbH. These resins are extremely alkaline anion exchangers which as a functional group, have quaternary amines. The matrix is a cross-linked polystyrene. The bulk density is 680 g/l, the effective grain size is 0.62 mm.

FIG. 1 shows the column process with an increased liquid layer according to one embodiment of the present invention. In the bottom part of the column (4) is a glass, ceramic or plastic frit, or a spray register or spray pole or sieve (6) through which the process solution can flow evenly through the ion exchanger resin (5). The ion exchanger resin (5) is embedded in the column. At the top end of the column, there is a glass, ceramic or plastic frit or a sieve (3). This is to prevent the resins from moving upwards and to ensure that only the process solution gets through. In the collection receptacle (1) which is used for the galvanic bath for the separation of metals, is the contaminated process solution which is conveyed through the column using a hose pump (2). Once the process solution has passed through the column, it is collected in a receptacle (7) which can be identical to receptacle (1). The device used for the metallization process comprises, as shown in FIGS. 1 and 3, a receptacle (1) to take a zinc or zinc alloy bath, a connected pump system (2), which is connected to the ion exchanger device (4) to take the zinc or zinc alloy bath, which contains ion exchanger resin (5) and a collection device (7) for the zinc or zinc alloy bath passing through the ion exchanger resin (5), which can be identical to receptacle (1). The ion exchanger resin (5) in the ion exchanger device (4) can be on a spray register, spray pole or sieve.

The receptacle (1) is generally equivalent to the galvanic zinc or zinc alloy bath and consists of at least an anode, a cathode (the substrate to be coated) and a voltage source.

In addition, there can also be—as shown in FIG. 3—a freezing device (8) between the receptacle (1) and the ion exchanger device (4) to cool the solution and separate a sodium carbonate solid. The freezing device (8) includes a cooling unit (9) to cool the solution to a temperature that is preferably below 10° C., more preferably between 2-5° C. and an outlet (10) to separate the crystallised sodium carbonate. There can also be a receptacle (11) between the freezing device (8) and the ion exchanger (4) to take the zinc or zinc alloy bath that has been cleaned from carbonates.

It was not possible to regenerate the resins Lewatit MonoPlus M600 and M500 using a sodium hydroxide solution. A stronger anion is needed to exchange the bonded cyanide anions. According to the invention, the proposal is to transform the ion exchanger resins back to their chloride form. Strong acids such as for example hydrochloric acid (HCl) cannot be used as this would immediately form toxic hydrogen cyanide. During the regeneration test of the resins, sodium chloride was used to separate the cyanide from the resins and transform the resins back to the chloride form. The regeneration solution with sodium chloride was moved into the very alkaline range (pH value >10) with a 0.5% by weight sodium hydroxide, since cyanides can quickly decompose below this pH value and form toxic hydrogen cyanide. The regeneration tests were examined using three different concentrations of sodium chloride (6, 12 and 18% by weight, Tables 2-7). The regeneration operation was realised at a linear speed of 5 m/h. One litre of sodium chloride solution was used for the regeneration and conveyed through the ion exchanger pack. Four portions of sample fractions having a volume of 250 ml each were taken and the content of different electrolyte parameters was analysed, compared and assessed. Data from the analysis was used to calculate the amount of cyanide which had bonded to the resin and was able to be eluted through the regeneration process. Referring to Table 2: a total volume of 1 l of regeneration solution with 6% in weight of NaCl was used to elute the cyanide (including organic nitrile) from the column containing Lewatit MonoPlus M600. Sample 1 is an analysis of the first 250 ml of regeneration solution used to elute the cyanide from the column, Sample 2 the second portion of 250 ml, Sample 3 the third portion of 250 ml and Sample 4 the fourth portion of 250 ml, giving 1 l of total eluent. The amount of total cyanide in 1 l eluent is 1.525 mg. The same was performed for the other regeneration cycles according to Tables 3-7.

The results show that it is beneficial to use solutions with a high chloride ion content to regenerate the columns.

TABLE 2 Lewatit MonoPlus M600 - Determination of the eluted quantity of cyanide, regeneration with 6% in weight of NaCl solution Lewatit Cyanide Quantity of cyanide in MonoPlus M600 concentration 250 ml sample volume Regeneration [mg/l] [mg/sample volume] Sample 1 1.8 0.450 Sample 2 1.5 0.375 Sample 3 1.5 0.375 Sample 4 1.3 0.325 From 100 ml resin, eluted quantity 1.525 of cyanide ->

TABLE 3 Lewatit MonoPlus M500 - Determination of the eluted quantity of cyanide, regeneration with 6% in weight of NaCl solution Lewatit Cyanide Quantity of cyanide in MonoPlus M500 concentration 250 ml sample volume Regeneration [mg/l] [mg/sample volume] Sample 1 3.6 0.900 Sample 2 3.7 0.925 Sample 3 2.9 0.725 Sample 4 2.2 0.550 From 100 ml resin, eluted quantity 3.100 of cyanide ->

TABLE 4 Lewatit MonoPlus M600 - Determination of the eluted quantity of cyanide, regeneration with 12% in weight of NaCl solution Quantity of cyanide Lewatit Cyanide in 250 ml sample MonoPlus M600 concentration volume Regeneration [mg/l] [mg/sample volume] Sample 1 7 1.750 Sample 2 6.5 1.625 Sample 3 6.5 1.625 Sample 4 6.5 1.625 From 100 ml resin, eluted quantity 6.626 of cyanide ->

TABLE 5 Lewatit MonoPlus M500 - Determination of the eluted quantity of cyanide, regeneration with 12% in weight of NaCl solution Quantity of cyanide in Lewatit Cyanide 250 ml sample MonoPlus M500 concentration volume Regeneration [mg/l] [mg/sample volume] Sample 1 18.0 4.500 Sample 2 8.5 2.125 Sample 3 8.5 2.125 Sample 4 6.2 1.550 From 100 ml resin, eluted quantity 10.300 of cyanide ->

TABLE 6 Lewatit MonoPlus M600 - Determination of the eluted quantity of cyanide, regeneration with 18% in weight of NaCl solution Quantity of cyanide Lewatit Cyanide in 250 ml sample MonoPlus M600 concentration volume Regeneration [mg/l] [mg/sample volume] Sample 1 35 8.75 Sample 2 41 10.25 Sample 3 44 11.00 Sample 4 45 11.25 From 100 ml resin, eluted quantity 41.25 of cyanide ->

TABLE 7 Lewatit MonoPlus M500 - Determination of the eluted quantity of cyanide, regeneration with 18% in weight of NaCl solution Quantity of cyanide Lewatit Cyanide in 250 ml sample MonoPlus M 500 concentration volume Regeneration [mg/l] [mg/sample volume] Sample 1 17 4.25 Sample 2 18 4.50 Sample 3 17 4.25 Sample 4 17 4.25 From 100 ml resin, eluted quantity 17.25 of cyanide ->

Tests where the temperature was increased (flow temperature in the beaker 55° C., average temperature in the ion exchanger column 35° C.) have shown that achieving the correct temperature significantly reduced the regeneration medium requirement. The solution used had a sodium chloride ion concentration of 18% in weight.

TABLE 7b Lewatit MonoPlus M600 - Determination of the eluted quantity of cyanide, regeneration with 18% in weight of NaCl solution Volume Volume Concentration Elution capacity regeneration flow rate Temperature (NaCl + NaOH) [mg cyanide/per litre solution [BV/h]* [° C.] [%] regeneration solution] [BV]** 33 RT 18 + 5 42 238 2.5 RT 18 + 5 119 84 2.5 55° C. first runnings, 18 + 5 310 33 35° C. in the column 2.5 55° C. first runnings, 18 + 5 312 32 35° C. in the column 2.5 55° C. first runnings, 18 + 5 286 35 35° C. in the column *[BV/h] = bed volume per hour **[BV] = bed volume

The aged electrolyte which is to be regenerated, should if possible be as close as possible to the original state (new batch). New batches of alkaline zinc-nickel electrolytes usually have an efficiency of 70% for a current density of 1 A/dm2. In electroplating, in order to assess the regeneration effect, the Hull cell test can be used and there is the option to determine the efficiency of the electrolyte using Faraday's law. Based on the layer thickness distribution of the electrolyte, it is possible to assess how good the regeneration effect is using an ion exchange resin.

The Hull cell is used to determine the effects of the bath parameters (eg. temperature, pH value, electrolyte composition, lack of or surplus of additives, cleanliness, impurities from foreign metals) on the property of the plated layer depending on the current density.

Since in a Hull cell the cathode is diagonal to the anode (see FIG. 2), there is a distribution of current densities on the cathode. This makes it possible to examine the effect of the current density in a single experiment. Understandably the current density is higher at the edge nearest the anode than at the edge away from the anode (FIG. 2). The quality of coated surfaces, ie. the composition, thickness, evenness and other properties, therefore primarily depend on the composition of the electrolyte and the plating conditions. The key quality factors are the composition of the electrolyte and the current parameters which must be monitored to assure a high quality coating. The composition of the electrolyte plays a significant role in this instance. Each individual additive in the electrolyte influences the properties of the electrolyte and the plated layer. In order to obtain the desired layer quality, the concentration of the electrolyte constituents must be within certain limits. The majority of electrolytes contain, in addition to the inorganic constituents, additional organic-type additives. These organic constituents are designed to influence the properties of the layer that is to be plated. This includes for instance brightening, levelling, hardness, ductility and throwing power ability. The Hull cell test was carried out to examine the appearance of the plated layer and the zinc-nickel composition. Tests were carried out with the Hull cell on a new, on an aged and on an electrolyte that had been regenerated using ion exchange resins. This test is designed to give an indication as to how effective it is to come close to the original state (new batch). The Hull cell can be used to establish how losses during the ion exchange process affect the plating rate. The additives however only work effectively if they are used in a certain concentration and composition.

Qualitatively, by visually assessing the brightness of the coated plates, it can be said that the reduction in the TOC (Total Organic Carbon) content, as shown in Tables 8 and 9, is due to the reduction of the nitrile concentration and that of the amine-containing complexing agents.

The ion exchange process can preferably be carried out in conjunction with the freezing out of sodium carbonate to further increase the efficiency of the process and match the plating performance of a non-aged electrolyte. The electrolyte solution can be conveyed through a cooling device either before or after treatment in the ion exchange resin column (see FIG. 3). During cooling, a sodium carbonate phase which can be separated, is formed. The old electrolyte is preferably treated in the freezer unit first and then in the ion exchange resin unit.

Operational Examples

Tests were carried out on an electrolyte to plate zinc-nickel alloys in accordance with Table 1.

For this test, 100 ml of resin was swelled in fully desalinated water for two hours and then sludged into the column. Prior to the loading process, the Lewatit MonoPlus M600 was regenerated using a 2% by weight sodium hydroxide solution to transform the resins to the OH− form. For the Lewatit MonoPlus M500, this was done using a 3% by weight sodium hydroxide solution. The loading process is realised according to the values indicated by the manufacturer. In practice, it is customary to indicate the loading process in bed volume per hour (BV/h). This value in turn refers to the embedded quantity of resin which is embedded in the column. Loading generally takes place at 10 BV/h. Based on our quantity of resin used (100 ml), the volume flow rate is 1000 ml/h. This represents a rate of 1.51 m/h and is within the value range specified by the manufacturer. Before the tests were carried out, a reference sample was taken from the zinc-nickel electrolyte which was to be regenerated (Sample 0 in the tables corresponds to an aged electrolyte). In the preliminary test which was carried out to examine the selectivity of the ion exchanger resins, 1000 ml of alkaline zinc-nickel electrolytes were conveyed through the ion exchanger column, where 250 ml of sample fractions were taken every fifteen minutes (Samples 1-4 in Tables 8 and 9).

The content of the different constituents was then examined in the sample fractions and compared with one another. The metal content, sodium hydroxide content, sodium carbonate content, sodium sulphate content, content of the complexing agents, TOC content and the total cyanide content of the samples was examined.

Tables 8 and 9 show the test results.

TABLE 8 Test results of the loading process for Lewatit MonoPlus M600 (aged electrolyte) Sample 0 Sample 1 Sample 2 Sample 3 Sample 4 Time Time Time Time Time [min] [min] [min] [min] [min] Lewatit MonoPlus M600 0 0-15 15-30 30-45 45-60 Zinc Zn [g/l] 12.4 11.9 12.0 12.1 12.0 Nickel Ni [g/l] 1.5 1.5 1.5 1.5 1.5 Sodium NaOH [g/l] 94.5 78.9 94.7 95.5 94.1 hydroxide Sodium Na2CO3 [g/l] 56.6 58.8 59.2 57.8 58.3 carbonate Sodium sulphate Na2SO4 [g/l] 4.50 4.73 4.77 4.51 4.70 Complexing — [ml/l] 140 132 135 138 135 agent Total cyanide CN− [mg/l] 92.0 5.2 5.1 5.2 4.8 TOC — [g/l] 45.8 40.0 44.2 44.0 44.0

TABLE 9 Test results of the loading process for Lewatit MonoPlus M500 Sample 0 Sample 1 Sample 2 Sample 3 Sample 4 Time Time Time Time Time [min] [min] [min] [min] [min] Lewatit MonoPlus M500 0 0-15 15-30 30-45 45-60 Zinc Zn [g/l] 11.5 7.7 11.4 11.7 11.5 Nickel Ni [g/l] 1.4 1.4 1.4 1.5 1.4 Sodium NaOH [g/l] 147 140 148 146 149 hydroxide Sodium Na2CO3 [g/l] 72.1 60.4 71.6 71.2 72.8 carbonate Sodium sulphate Na2SO4 [g/l] 7.69 5.48 7.59 7.52 8.19 Complexing — [ml/l] 137 97 128 132 131 agent Total cyanide CN− [mg/l] 75.0 17.0 7.5 7.6 7.7 TOC — [g/l] 42.9 25.9 40.8 41.6 42.2

The values in Tables 8 and 9 show that the metal content concentrations are virtually constant and do not change significantly. The nickel concentration remains unchanged and does not fluctuate. The sodium hydroxide concentration slightly drops at first. The reason for this is that the resins could not be fully transformed to the OH− form during the regeneration process. The resins therefore were still able to absorb the hydroxide ions. The sodium hydroxide concentration, however, takes on the same order of magnitude again as that of Sample 0 and remains virtually the same. The content of sodium carbonate and sodium sulphate remain virtually unchanged.

There is a clear reduction in cyanide and a lower yet significant reduction in the TOC.

The test shows that the resin Lewatit MonoPlus M600 retains the interfering cyanide from the process solution. The test also shows that the resin's absorption capacity has by no means been reached and that the cyanide content dropped even after 60 minutes. In comparison to the Lewatit MonoPlus M500 it is clear that the cyanide concentration is initially accompanied by a reduction in the concentration of zinc, sodium hydroxide, sodium carbonate, sodium sulphate and the complexing agent in the first fraction (Sample 1). The nickel concentration remains virtually constant throughout the whole test period.

The Hull cell tests were carried out to examine the appearance of the plated layer and the zinc-nickel composition. Tests were carried out with the Hull cell on a new, on an aged and on an electrolyte that had been regenerated using ion exchangers.

This test is designed to give an indication as to how important it is to come close to the original state (non aged electrolyte). The Hull cell can be used to establish how losses during the ion exchange process effect the plating rate.

The Hull cell was filled with 250 ml of electrolyte as per Table 1. A nickel anode was used as the anode. Once the Hull cell plate had been cleaned, a 1-ampere current was applied. The coating time was fifteen minutes.

The low current density range (see FIG. 2) shows an even and bright plating result. The electrolyte which was treated using the ion exchanger resin Lewatit MonoPlus M600, revealed an even and bright surface across the whole current density spectrum. The assessment of the surface should be classed as bright. It can therefore be confirmed that the ion exchange process which is used to remove cyanide from the alkaline zinc-nickel electrolyte, significantly improves the appearance of the plated layer. More importantly however is the finding that the appearance was not in any way worsened, which indicated that no organic additives, which are responsible for the appearance of the depsit, were removed from the plating bath by the ion exchange process.

This also leads to the conclusion that the reduction of the TOC content is due to the reduction of the organic complexing agent and the organically bonded cyanide (nitrile). The same result was obtained using an electrolyte which was treated with the Lewatit MonoPlus M500 ion exchanger resin.

The high and low current density ranges shown in FIG. 2 act as measuring points for determining a layer thickness and the alloy composition of the zinc-nickel layer. After the coating process in the Hull cell, the layer thicknesses were measured using an X-ray fluorescence measurement device at the two measuring points A (high current density range) and B (low current density range). Five measurements were taken at each measuring point. In electroplating, the X-ray fluorescence analysis is a standard method used for a quick and non-destructive determination of layer thicknesses. By using this measurement method, it was possible to ascertain the layer thickness and the amount of nickel and zinc. Based on the layer thickness distribution, it was then possible to draw a conclusion concerning the effect of the ion exchange process on the electrolyte parameters. The base or reference value which is to be obtained using the regeneration process, is the layer thickness distribution of the newly included electrolyte [Table 10].

A comparison of the layer thickness distribution for a new and an aged electrolyte [Table 11] also shows how quickly the efficiency level and thereby also the separation rate of the electrolyte drops as the lifetime increases. In order to retain the same matrix relating to the batch, it is necessary to replenish the quantities of metal ions for the old electrolyte as per Table 11 as well as those lost through the ion exchange process. The initial concentration (Sample 0) is needed for this.

TABLE 10 Composition of the layer - new electrolyte Layer thickness Measuring point A Measuring point B distribution of Layer Nickel Zinc Layer Nickel Zinc the new thickness part part thickness part part electrolyte [μm] [%] [%] [μm] [%] [%] 1 5.15 13.9 86.1 1.76 13.7 86.3 2 5.22 13.8 86.2 1.77 14.2 85.8 3 5.14 14.5 85.5 1.78 13.9 86.1 4 5.10 14.2 85.8 1.87 14.5 85.5 5 5.18 13.9 86.1 1.80 13.7 86.3 Mean value 5.16 14.1 85.9 1.80 14.0 86.0

TABLE 11 Composition of the layer - aged electrolyte Layer thickness Measuring point A Measuring point B distribution of Layer Nickel Zinc Layer Nickel Zinc the aged thickness part part thickness part part electrolyte [μm] [%] [%] [μm] [%] [%] 1 3.13 14.2 84.7 1.27 15.4 84.6 2 3.14 14.5 85.5 1.25 15.0 85.0 3 3.13 15.1 84.9 1.26 14.2 85.8 4 3.16 14.5 85.5 1.26 14.5 85.5 5 3.13 14.3 85.7 1.25 14.2 85.8 Mean value 3.14 14.5 85.3 1.26 14.7 85.3

After the regeneration and replenishment of the aged electrolyte [Tables 12 and 13], the Hull cell test shows that the plated layer thickness at measuring points A and B is considerably higher and is closer to the non aged electrolyte, in comparison to the aged electrolyte [Table 11]. The result also shows that the nickel and zinc composition has not changed in the layer. It can therefore be said that removing the cyanide and organically bonded cyanide accelerates the separation rate of the alkaline zinc-nickel electrolyte and that the bath quality is significantly increased in comparison to the aged plating bath by using an ion exchanger system.

TABLE 12 Layer thickness at the measuring point/electrolyte regenerated with Lewatit MonoPlus M600 and missing quantities supplemented Layer thickness distribution Electrolyte regenerated and supplemented Measuring point A Measuring point B Lewatit Layer Nickel Zinc Layer Nickel Zinc MonoPlus thickness part part thickness part part M600 [μm] [%] [%] [μm] [%] [%] 1 3.60 14.0 86.0 1.30 13.5 86.5 2 3.59 14.8 85.2 1.33 14.0 86.0 3 3.66 14.9 85.1 1.39 14.5 85.5 4 3.65 14.7 85.3 1.38 14.0 86.0 5 3.63 13.6 86.4 1.39 14.3 85.7 Mean value 3.63 14.4 85.6 1.36 14.1 85.9

TABLE 13 Layer thickness at the measuring point/electrolyte regenerated with Lewatit MonoPlus M500 and missing quantities supplemented Layer thickness distribution Electrolyte regenerated and & supplemented Measuring point A Measuring point B Lewatit Layer Nickel Zinc Layer Nickel Zinc MonoPlus thickness part part thickness part part M600 [μm] [%] [%] [μm] [%] [%] 1 3.75 14.7 85.3 1.41 14.3 85.7 2 3.69 14.5 85.5 1.35 14.3 85.7 3 3.69 14.5 85.5 1.39 14.9 85.1 4 3.70 14.3 85.7 1.38 14.5 85.5 5 3.71 14.4 85.6 1.40 14.4 85.6 Mean value 3.71 14.5 85.5 1.39 14.5 85.5

The efficiency of the electrolyte can be increased further by freezing out the sodium carbonate. A comparison of the layer thicknesses in the aged electrolyte with the lower concentration of sodium carbonate [Table 14] after the freezing out process and the aged electrolyte with the higher concentration of sodium carbonate [Table 11] where there was no freezing out, shows that the decrease in the sodium carbonate concentration at the least affects the separation rate. There was no evidence that the metal composition was affected. An examination of the efficiency of the electrolyte once the sodium carbonate had been frozen out revealed a 7% increase in the efficiency of the electrolyte. A regeneration of the zinc-nickel electrolyte by freezing out the sodium carbonate and removing the cyanide and nitrile using the ion exchanger is particularly advantageous.

TABLE 14 Layer thickness distribution - electrolyte - sodium carbonate frozen out. Layer thickness distribution Electrolyte Na2CO3 Measuring point A Measuring point B removed Layer Nickel Zinc Layer Nickel Zinc by means thickness part part thickness part part of freezing out [μm] [%] [%] [μm] [%] [%] 1 3.22 13.8 86.2 1.23 13.8 86.2 2 3.22 14.1 85.9 1.25 14.6 85.4 3 3.22 13.8 86.2 1.24 14.6 85.4 4 3.25 14.0 86.0 1.25 13.7 86.3 5 3.22 14.5 85.5 1.24 14.9 85.1 Mean value 3.23 14.0 86.0 1.24 14.3 85.7

Using macro throwing power measurements, it was also possible to assess how effectively the ion exchange process can bring the aged electrolyte to the original state (non aged electrolyte). 250 ml of electrolyte were again filled into the Hull cell as per Table 1. The Hull cell plate was galvanized for 15 minutes. In order to be able to assess the throwing power and the regeneration effect as a result of the ion exchanger and the freezing out process, the throwing power abilities of various electrolytes were assessed. To do so, the Hull cell plate which was to be coated and was 30 mm from the lower edge of the plate, was measured at intervals of a centimeter. The measuring points were indicated by crosses on the coated plate. The measurement was taken using the X-ray fluorescence measurement process. From the layer thickness distribution of the plates it was possible to determine the effects of the regeneration process on the electrolyte. The measured layer thicknesses were applied across the length of the plate [FIG. 4]. It shows that after 15 minutes of metallization, across the whole length of the Hull cell plate, the electrolytes which were regenerated with Lewatit MonoPlusM500 and Lewatit MonoPlusM600 created a layer thickness which was significantly higher than could be achieved using the aged bath.

Claims

1. Process used to deposit functional layers of zinc or zinc alloys from alkaline zinc or zinc alloy plating baths which contain nitrogenous organic additives, a soluble zinc salt and optionally other metal salts, comprising the following steps:

(i) providing a zinc or zinc alloy bath containing the above mentioned components;
(ii) electrolytically depositing a zinc or zinc alloy layer on a substrate that is to be coated according to known processes;
(iii) removing at least a part of the zinc or zinc alloy bath and conveyancing the part that has been removed and which contains cyanide ions and nitriles that have been formed during the deposition according to step (ii) through a device that includes an ion exchange resin which is designed specifically for separating cyanide ions;
(iv) returning the conveyed part to the zinc or zinc alloy bath.

2. Process according to claim 1, characterised in that the other metal salts are selected from the group consisting of Fe, Ni, Co and Sn salts.

3. Process according to claim 1, characterised in that the following procedural step is also scheduled:

(v) Supplementing of used components from the zinc or zinc alloy bath.

4. Process according to claim 1, characterised in that the removal of the part of the zinc or zinc alloy bath and its return is a continuous or discontinuous process.

5. Process according to claim 1, characterised in that the zinc or zinc alloy bath contains organic additives selected from brighteners, surface-active agents and nitrogenous complexing agents.

6. Process according to claim 1, characterised in that the nitrogenous complexing agents are selected from the group comprising polyalkyleneamines.

7. Process according to claim 1, characterised in that it is a galvanic bath which is used to plate zinc-nickel alloys.

8. Process according to claim 1, characterised in that the ion exchange resin is selected from the group consisting of strongly alkaline anion exchange resins.

9. Process according to claim 1, characterised in that the ion exchange resin is selected from the group consisting of ion exchange resins having as a functional group quaternary amines.

10. Process according to claim 1, characterised in that the ion exchange resin is regenerated through contact with a sodium chloride solution and then by means of conditioning using sodium hydroxide.

11. Process according to claim 10, characterised in that the sodium chloride solution presents a sodium chloride concentration of 5-35% by weight.

12. Process according to claim 10, characterised in that the temperature of the sodium chloride solution is 10-70° C. during regeneration on the ion exchanger column.

13. Process according to claim 1, characterised in that the process includes the additional step iii b) cooling of the zinc or zinc alloy bath for the separation of sodium carbonate at a temperature which is below 10° C.

14. Device used to carry out the process according to claim 1, comprising a receptacle to take a zinc or zinc alloy bath, a connected pump system, which is connected to the ion exchanger device to take the zinc or zinc alloy bath, which has an ion exchanger and a collection device for the zinc or zinc alloy bath conveyed through the ion exchanger resin, which is identical to the receptacle.

15. Device according to claim 14, characterised in that the ion exchanger resin in the ion exchanger device is on a spray register, spray pole or sieve.

16. Device according to claim 14, characterised in that there is a freezing device between the receptacle and the ion exchanger device to cool the solution and separate a sodium carbonate solid.

17. Process according to claim 11, characterised in that the temperature of the sodium chloride solution is 10-70° C. during regeneration on the ion exchanger column.

18. Device according to claim 15, characterised in that there is a freezing device between the receptacle and the ion exchanger device to cool the solution and separate a sodium carbonate solid.

Patent History
Publication number: 20110210006
Type: Application
Filed: Nov 17, 2009
Publication Date: Sep 1, 2011
Inventor: Serdar Turan Karagöl (Berlin)
Application Number: 13/123,934
Classifications
Current U.S. Class: Purifying Electrolyte (205/99); Recirculation (204/234); With Filter (204/235); With Heater Or Cooler (204/236)
International Classification: C25D 21/22 (20060101); C25D 17/00 (20060101); C25D 3/56 (20060101);