Treatment of nickel-containing waste water on phosphating

Abstract

A process for treating a nickel-bearing aqueous solution consisting of phosphating-bath overflow and/or of rinsing water after phosphating, phosphating being effected with an acidic aqueous phosphating solution that contains 3 to 50 g/l phosphate ions, reckoned as PO43−, 0.2 to 3 g/l zinc ions, 0.01 to 2.5 g/l nickel ions, optionally further metal ions as well as, optionally, accelerators, whereby the phosphating-bath overflow and/or the rinsing water after phosphating is conducted across a weakly acidic ion-exchanger, characterised in that the acid groups of the ion-exchanger are neutralised with alkali-metal ions to an extent amounting to no more than 15% and in that when it is fed to the ion-exchanger the nickel-bearing aqueous solution has a pH value within the range from 2.5 to 6.0.

Description

[0001] This invention pertains to the field constituted by the phosphating of metallic surfaces, as is implemented as a widespread corrosion-preventive measure in the metalworking industry, such as in the automobile industry and the household-appliance industry, for example, but sometimes also in steelworks. It relates to a process for treating the overflow of the phosphating baths and/or the rinsing water after phosphating with nickel-bearing phosphating solutions. In preferred embodiments the process enables the recycling of bath ingredients into the phosphating bath, the re-use of active substances for the purpose of producing replenishing solutions for phosphating baths, and the use of the solution that has been depleted of metal ions as rinsing water.

[0002] The phosphating of metals pursues the aim of generating layers of metal phosphate which are firmly fused on the surface of the metal and which, in themselves, already improve corrosion resistance and, in conjunction with lacquers and other organic coatings, contribute to a substantial enhancement of the adhesion and the resistance to infiltration in the event of corrosive stress. Such phosphating processes have been known for a long time in the state of the art. Suitable in particular for the pretreatment prior to lacquering are the low-zinc phosphating processes, in which the phosphating solutions have comparatively low contents of zinc ions amounting to, e.g., 0.5 to 2 g/l. A significant parameter in these low-zinc phosphating baths is the weight ratio of phosphate ions to zinc ions, which conventionally lies in the range >12 and may take values up to 30.

[0003] It has become evident that, through the concomitant use of multivalent cations other than zinc in the phosphating baths, phosphate layers can be formed having distinctly improved corrosion-prevention and lacquer-adhesion properties. For example, low-zinc processes with addition of, e.g., 0.5 to 1.5 g/l manganese ions and, e.g., 0.3 to 2.0 g/l nickel ions find wide application as so-called tri-cation processes for the preparation of metallic surfaces for lacquering, for example for the cathodic electrophoretic lacquering of automobile bodies.

[0004] A phosphating solution contains layer-forming components such as, e.g., zinc ions and optionally further divalent metal ions as well as phosphate ions. In addition, a phosphating solution contains non-layer-forming components such as alkali-metal ions for neutralising the free acid and, in particular, accelerators and decomposition products thereof. The decomposition products of the accelerator arise by virtue of the fact that the latter reacts with the hydrogen that is formed on the metallic surface by corrosive reaction. The non-layer-forming components accumulating with time in the phosphating bath—such as, for example, alkali-metal ions and, in particular, the decomposition products of the accelerator—can only be removed from the phosphating solution by a portion of the phosphating solution being discharged and discarded and being continuously or discontinuously replaced by new phosphating solution. Phosphating solution can, for example, be discharged by the phosphating bath being operated with an overflow and by the overflow being discarded. As a rule, however, an overflow is not required, since by virtue of the phosphated metal parts a sufficient quantity of phosphating solution is discharged in the form of adherent liquid film.

[0005] After the phosphating, the phosphating solution adhering to the phosphated parts, such as automobile bodies for example, is rinsed off with water. Since the phosphating solution contains heavy metals and, optionally, further ingredients that are not permitted to be released into the environment in uncontrolled manner, the rinsing water has to be subjected to a water treatment. This has to take place in a separate step prior to introduction into a biological clarification plant, since otherwise the operational capability of the clarification plant would be endangered.

[0006] Since both the disposal of the waste water (from phosphating-bath overflow and/or rinsing water) and the supply of the phosphating plant with fresh water are cost factors, there is a need to minimize these costs. German Patent Application DE 198 13 058 describes a process for treating phosphating-bath overflow and/or rinsing water after phosphating, wherein the phosphating-bath overflow and/or the rinsing water is subjected to a nanofiltration.

[0007] The concentrate of the nanofiltration can be resupplied to the phosphating bath. The filtrate of the nanofiltration constitutes waste water which has to be subjected to further treatment, optionally prior to being introduced into a biological clarification plant. German Patent Application DE 198 54 431 describes a process for saving rinsing water in the course of phosphating. In this process the phosphating-bath overflow and/or the rinsing water after phosphating is subjected to a treatment process such as, for example, a reverse osmosis, an ion-exchange process which is not characterised in any detail, a nanofiltration, an electrodialysis and/or a heavy-metal precipitation, and the aqueous phase which in each given case has been depleted of metal ions is employed as rinsing water for the purpose of rinsing the metal parts to be phosphated after they have been cleaned. The treatment of rinsing water after phosphating by ion-exchange processes is known from DE-A-42 26 080. In this case, use is made of strongly acidic cation-exchange resins on the basis of sulfonic-acid groups. Said cation-exchange resins bind all cations in non-selective manner. Since it also contains non-layer-forming cations in addition to the layer-forming cations, the regenerated material cannot be used for replenishing the phosphating solution, as this would lead to an excessive increase in salinity of the phosphating solution.

[0008] DE 199 18 713 describes an improved process for treating phosphating-bath overflow and/or rinsing water after phosphating. In this process it is at least intended to be guaranteed that a waste water for disposal ultimately arises, the contents of which in respect of zinc ions and/or nickel ions lie below the permissible waste-water limits. However, instead of a disposal by virtue of a clarification plant, the waste water is also intended to be capable of being used for the purpose of rinsing the metal parts to be phosphated after the degreasing thereof. The process is preferably to be operated in such a way that layer-forming components of the phosphating bath, in particular zinc ions and/or nickel ions, can be recovered and employed again for phosphating purposes.

[0009] The object that is formulated in the aforementioned patent is achieved by a process for treating phosphating-bath overflow and/or rinsing water after phosphating, phosphating being effected with an acidic aqueous phosphating solution that contains 3 to 50 g/l phosphate ions, reckoned as PO43−, 0.2 to 3 g/l zinc ions, optionally further metal ions as well as, optionally, accelerators, whereby the phosphating-bath overflow and/or the rinsing water after phosphating is conducted across a weakly acidic ion-exchanger after a membrane filtration or without upstream membrane filtration.

[0010] An example of a weakly acidic ion-exchanger is LewatitR TP 207 or TP 208 produced by Bayer AG. In a company publication relating to this product (Bayer AG: LewatitR—Selektivaustauscher, Eigenschaften und Anwendung von Lewatit TM 207) it is reported that in the majority of cases Lewatit TP 207 is employed after pre-exhaustion (conditioning) with alkali ions or alkaline-earth ions. In a few exceptional cases, which do not involve nickel, the use of the hydrogen form is also possible. The decomplexing pH value for nickel is specified as 2.1. This pH value indicates the hydrogen-ion concentration at which the metal ion is just desorbed from the Lewatit TP 207. This company publication further states that the maximum of the exchange capacity is attained in general if the pH value of the exhausting solution is at least 2 units above the decomplexing pH value. Accordingly, as reported in this statement, nickel is only bound to a sufficient extent at a pH value above 4.1. Consequently in the embodiment examples of the already cited DE-A-199 18 713 the ion-exchanger is employed in the monosodium form. According to the aforementioned company publication produced by Bayer AG, the outflow of the ion-exchanger in the monosodium form has a pH value that lies between 6 and 9.

[0011] Japanese Patent Application P 62287100 (cited as stated in Derwent Abstract 1988-0-25811) describes the binding of nickel ions from phosphoric-acid solution to an ion-exchanger, the acidic groups of which are neutralised with sodium ions to an extent amounting to 25 to 75%.

[0012] On the other hand, Japanese Patent Application JP 63057799 A2 (cited as stated in Patent Abstracts of Japan) discloses that nickel from a plating solution can also be bound to the H-form of an ion-exchanger with chelating iminodiacetic-acid groups (which constitute weakly acidic groups). This cannot be applied to the problem as formulated in the present invention, since plating solutions have substantially higher contents of metal ions than phosphating-bath overflow diluted with rinsing water or rinsing water after phosphating. The nickel contents of the last-named solutions lie, as a rule, within the range between 5 and 100, in particular between 10 and 50 ppm. These solutions have to be treated in such a way that the nickel contents of the treated solutions are below 1 ppm.

[0013] This is possible with the process according to DE-A-199 18 713. However, the use of a weakly acidic ion-exchanger, preferably one having chelating iminodiacetic-acid groups, in the monosodium form, which is disclosed therein entails several disadvantages. On the one hand, for the regeneration of the ion-exchanger after eluting the bound metals with acid it is necessary to convert the ion-exchanger into the monosodium form with caustic-soda solution. This contributes to the chemical consumption of the overall process and compels the user of this process to hold supply vessels and pipelines in store for the caustic-soda solution. This complicates the overall process and makes it more expensive. Moreover, this process has the disadvantage that when the ion-exchanger is employed for the purpose of treating the stated phosphating-bath waste waters a waste water arises that has been subjected to an increase in salinity by virtue of sodium salts and that can only be re-used to a limited extent. In the course of regeneration of the exhausted ion-exchanger with acid, in which a nickel-bearing solution of valuable material is preferably to be ejected, residual sodium in the ion-exchanger is likewise eluted. The nickel-bearing solution of valuable material is therefore contaminated with sodium ions and so can only be re-used to a limited extent.

[0014] The present invention sets itself the object of avoiding the aforementioned disadvantages. It is based on the surprising perception that weakly acidic ion-exchangers of the type represented by LewatitR TP 207, contrary to what is stated by the manufacturer, bind nickel from dilute solutions (nickel contents between 5 and 100, in particular between 10 and 50 ppm) to a sufficient extent and in particular selectively in relation to manganese and, partially, zinc, also at a pH value no higher than 4.

[0015] The invention accordingly provides a process for treating a nickel-bearing aqueous solution consisting of phosphating-bath overflow and/or of rinsing water after phosphating, wherein phosphating is effected with an acidic aqueous phosphating solution that contains 3 to 50 g/l phosphate ions, reckoned as PO43−, 0.2 to 3 g/l zinc ions, 0.01 to 2.5 g/l nickel ions, optionally further metal ions as well as, optionally, accelerators, whereby the phosphating-bath overflow and/or the rinsing water after phosphating is conducted across a weakly acidic ion-exchanger, characterised in that the acid groups of the ion-exchanger are neutralised with alkali-metal ions to an extent amounting to no more than 15% and in that when it is fed to the ion-exchanger the nickel-bearing aqueous solution has a pH value within the range from 2.5 to 6.0, preferably from 3 to 4.1.

[0016] Thus in accordance with the invention a weakly acidic ion-exchanger is to be employed, the acid groups of which are neutralised with alkali-metal ions to an extent amounting to no more than 10%. However, the aim is that the acid groups of the ion-exchanger are neutralised with alkali-metal ions to an extent amounting to no more than 5%, preferably no more than 3% and in particular no more than 1%. In the optimal case the ion-exchanger contains no alkali-metal ions at all. However, since equilibrium processes play a role in the regeneration of an exhausted ion-exchanger, this desired ideal state of the ion-exchanger cannot always be obtained.

[0017] A simple criterion as to whether the acid groups have been neutralised with alkali-metal ions to a sufficiently small extent is constituted by the bed volume (abbreviated as BV in the following) of the ion-exchanger. The term ‘bed volume’ is to be understood to mean the total volume of the ion-exchange particles together with the liquid between the particles. The bed volume of weakly acidic ion-exchangers usually depends on the degree of neutralisation of the acid groups. For example, if the disodium form of a weakly acidic ion-exchanger with iminodiacetic-acid groups—LewatitR TP 207 for example—with a bed volume of 500 ml is washed out with acid to such an extent that the sodium ions are removed as extensively as possible, the bed volume shrinks to 400 ml. The bed volume of the monosodium form amounts to around 450 ml. Such an ion-exchanger is in the state to be used in accordance with the invention if the bed volume of the ion-exchanger, which in the disodium form amounts to 500 ml, is not above 415 ml.

[0018] Described below are phosphating baths which are conventional in the state of the art, the bath overflow or rinsing water of which can be treated with the process according to the invention:

[0019] The zinc contents preferably lie within the range from 0.4 to 2 g/l and in particular from 0.5 to 1.5 g/l, as conventional for low-zinc processes. The weight ratio of phosphate ions to zinc ions in the phosphating baths may fluctuate within wide limits, provided that it lies within the range between 3.7 and 30. A weight ratio between 10 and 20 is particularly preferred. Moreover, the phosphating baths contain 0.01 to 2.5 g/l, preferably 0.3 to 2.0 g/l, nickel ions. In addition, the phosphating solution may contain 0.1 to 4 g/ml, in particular 0.5 to 1.5 g/l, manganese ions, as is conventional for tri-cation processes. Moreover, in addition to the zinc ions and nickel ions and optionally manganese ions, the phosphating solution may contain by way of further metal ions: 1 0.2 to 2.5 g/l magnesium (II), 0.2 to 2.5 g/l calcium (II), 0.002 to 0.2 g/l copper (II), 0.1 to 2 g/l cobalt (II).

[0020] The form in which the cations are introduced into the phosphating baths is basically of no importance. One possibility which presents itself in particular is to use oxides and/or carbonates as a cation source. On account of the risk of an increase in salinity of the phosphating baths, salts of acids other than phosphoric acid should preferably be avoided.

[0021] In the case of phosphating baths that are to be suitable for differing substrates, it has become conventional to add free and/or coordinated fluoride in quantities up to 2.5 g/l of total fluoride, thereof up to 750 mg/l of free fluoride, in each case reckoned as F−. In the case where fluoride is absent, the aluminium content of the bath is not to exceed 3 mg/l. In the case where fluoride is present, higher Al contents are tolerated as a consequence of the complexing, provided that the concentration of the non-complexed Al does not exceed 3 mg/l.

[0022] Besides the layer-forming divalent cations, phosphating baths additionally contain, as a rule, sodium ions, potassium ions and/or ammonium ions for the purpose of adjusting the free acid.

[0023] Phosphating baths that serve exclusively to treat galvanised material do not necessarily have to contain a so-called accelerator. However, accelerators that are required in the phosphating of non-galvanised steel surfaces are also frequently employed concomitantly in the state of the art in the phosphating of galvanised material.

[0024] Accelerator-containing phosphating solutions have the additional advantage that they are suitable both for galvanised materials and for non-galvanised materials. This is particularly important in the phosphating of automobile bodies, since the latter frequently contain both galvanised and non-galvanised surfaces.

[0025] Various accelerators are available in the state of the art for phosphating baths. They accelerate the formation of layers and facilitate the formation of closed phosphate layers, since they react with the hydrogen arising in the course of the corrosive reaction. This process is described as “depolarisation”. The formation of hydrogen bubbles on the metallic surface, which interfere with the formation of layers, is prevented by this means. If, within the scope of the process according to the invention, a membrane process (reverse osmosis or nanofiltration) is employed prior to the ion exchange, those accelerators are preferred, the by-products or decomposition products of which (reaction products with hydrogen) are able to penetrate the membrane. By this means it is guaranteed that these by-products and decomposition products of the accelerator do not accumulate in the phosphating bath but are discharged from the system at least partially via the filtrate of the membrane filtration.

[0026] Particularly suitable are those accelerators which form as by-products or decomposition products either water or monovalently charged ions which are able to penetrate a nanofiltration membrane. For example, the phosphating solution may contain one or more of the following accelerators: 2 0.3 to 4 g/l chlorate ions 0.01 to 0.2 g/l nitrite ions 0.1 to 10 g/l hydroxylamine 0.001 to 0.15 g/l hydrogen peroxide in free or bound form 0.5 to 80 g/l nitrate ions.

[0027] In the course of the depolarisation reaction on the metallic surface, chlorate ions are formed from chloride ions, nitrate ions and ammonium ions are formed from nitrite ions, ammonium ions are formed from nitrate ions, ammonium ions are formed from hydroxylamine, and water is formed from hydrogen peroxide. The anions or ammonium ions that are formed are able to pass through a nanofiltration membrane, so that in the process according to the invention they are discharged at least partially from the phosphating-bath overflow or from the rinsing water after phosphating.

[0028] Together with, or instead of, chlorate ions, use may advantageously be made of hydrogen peroxide by way of accelerator. This can be employed as such or in the form of compounds that form hydrogen peroxide under the conditions of the phosphating bath. However, preferably no multivalent ions are to arise as by-products in this case, since they would be enriched in the phosphating bath in the event of recycling of the concentrate of the nanofiltration. Therefore alkali-metal peroxides, in particular, present themselves as an alternative to hydrogen peroxide.

[0029] An accelerator that is likewise preferably to be used within the scope of the process according to the invention is hydroxylamine. If the latter is added to the phosphating bath in free form or in the form of hydroxylammonium phosphates, hydroxylammonium nitrate and/or hydroxylammonium chloride, likewise only decomposition products or by-products are formed that are able to penetrate a nanofiltration membrane.

[0030] The process according to the invention can be operated in such a way that the phosphating-bath overflow and/or the rinsing water after phosphating is conducted directly (optionally after removal of sludge and/or of organic constituents, which can be effected, for example, by a screen filtration or bag filtration or a filtration across a particle bed such as a sand filter, for example) across the weakly acidic ion-exchanger. As an alternative to this, the phosphating-bath overflow and/or the rinsing water after phosphating (likewise optionally after the removal of sludge and/or of organic constituents) may be subjected to a membrane filtration in the form of an ultrafiltration, a nanofiltration or a reverse osmosis. After the filtration the aqueous solution is subsequently conducted across the weakly acidic ion-exchanger. By virtue of the weakly acidic ion-exchanger, metal ions that constitute valuable materials of a phosphating solution are removed selectively from the aqueous solution. By this means, on the one hand the reliability is increased that the waste-water limits for these cations will be observed.

[0031] Moreover, these cations can be employed again for phosphating purposes after regeneration of the ion-exchanger.

[0032] Various types of membrane are available in the state of the art for an ultrafiltration, a nanofiltration or a reverse osmosis. Since phosphating baths and also the corresponding rinsing waters react acidically, the membrane that is employed should be acid-resistant. Suitable, for example, are inorganic membranes such as, e.g., ceramic membranes. Moreover, organic polymer membranes can be employed. In particular, a polyamide membrane is suitable as a nanofiltration membrane.

[0033] If one of the stated membrane-filtration processes is employed prior to the ion exchange, the process is preferably operated in such a way that the retentate of the membrane filtration is recycled into the phosphating solution. By this means, some of the layer-forming cations that are present in the overflow of the phosphating bath or in the rinsing water are already recycled into the phosphating solution. This results in a more economical mode of operation of the phosphating bath, since fewer ingredients have to be freshly supplied.

[0034] Irrespective of whether the phosphating-bath overflow and/or the rinsing water after phosphating is conducted directly to the ion-exchanger or whether one of the stated membrane-filtration processes is employed beforehand, it is preferred to free the phosphating-bath overflow and/or the rinsing water after phosphating from sludge and/or from organic constituents. Blocking of the filtration membranes or of the ion-exchanger is prevented by this means. Sludge can be removed by bag filtration, for example. The filter Lofclear 523 D produced by Loeffler GmbH, for example, is suitable here by way of filter. It removes 95% of the particles having a size below 1.5 &mgr;m and 99.9% of the particles having a size below 5.5 um. Organic constituents in the phosphating bath (for example, organic accelerators and/or decomposition products thereof or any organic polymers that are present in the phosphating bath) can be removed by activated carbon or by synthetic resins. Suitable by way of activated carbon is, for example, the type Lofsorb LA 40 E-3-01 produced by Loeffler GmbH. By way of organic resins, use may be made of Lewatit VP 0C 1066 or Dowex OPTL 285, for example, with a view to removing organic constituents.

[0035] A Desal DK membrane, for example, is suitable for the step of nanofiltration. With a pressure difference of 7 bar and at a temperature of 35° C. it provides a membrane flow of the order of 35 to 401 per m2 per hour at a volume ratio of concentrate:filtrate=1:1. A Filmtec SW 30 membrane produced by Rochem, for example, can be employed for the step of reverse osmosis. With a pressure difference of 25 bar and at a temperature of 45° C. it yields a membrane flow of approximately 301 per M2 per hour at a volume ratio of concentrate:filtrate=5:1.

[0036] By way of weakly acidic ion-exchanger, preferably such a type is employed that is selective in respect of nickel ions and/or zinc ions. Under operational conditions the weakly acidic ion-exchanger preferably binds nickel ions more strongly than zinc ions. This means that nickel ions from the solution that has been fed are able to displace zinc ions from the ion-exchanger. Monovalent cations are to be bound as little as possible. For this purpose, in particular such weakly acidic ion-exchangers are suitable that bear chelate-forming iminodiacetic-acid groups. A suitable product is LewatitR TP 207 or TP 208 produced by Bayer. Other suitable ion-exchangers are IRC 718/748 produced by Rohm & Haas, as well as S-930 produced by Purolite.

[0037] The process is preferably operated in such a way that the weakly acidic ion-exchanger is regenerated with a weakly acidic acid after exhaustion. The selectively bound nickel ions, optionally together with zinc ions still remaining, are eluted in the process and can be re-used for phosphating purposes. Through the use of the process according to the invention these cations do not have to be disposed of in the form of heavy-metal-containing sludge but can—optionally after suitable treatment—be employed again for phosphating. As a result, resources are spared.

[0038] For the regeneration of the exhausted weakly acidic ion-exchanger it is particularly preferred to make use of an acid that constitutes a valuable material for the phosphating solution. Phosphoric acid is particularly suitable. Phosphoric acid may contain, relative to the overall quantity of acid, up to a total of 10 mol. % nitric acid, hydrochloric acid and/or hydrofluoric acid.

[0039] In order, after the regeneration with acid, to keep the ion-exchanger in the acid form but largely to wash out the free acid that was used for the purpose of regeneration, after the regeneration with a strong acid the ion-exchanger is washed with water or with a quantity of lye that corresponds to a maximum of 0.5 bed volumes of 4-% caustic-soda solution. This rinsing process is carried out until such time as the pH value of the rinsing solution running off from the ion-exchanger lies between 2.1 and 4.5, preferably between 3.0 and 4.1. In this connection a rinsing water is employed, the temperature of which lies within the range between approximately 5 and approximately 50° C. and in particular between approximately 15 and approximately 45° C. For the rinsing, caustic-soda solution may be dispensed with entirely. However, this presupposes an appropriately long rinsing with water. The rinsing operation can be shortened if a quantity of lye is admixed to the rinsing water that corresponds to a maximum of 0.5 bed volumes of 4-% caustic-soda solution. With this quantity of lye the residual acid in the free volume between the ion-exchange particles is neutralised, but the acid groups of the exchanger itself are not. This means that sodium ions scarcely bind to the ion-exchanger with this low quantity of lye. Rather, sodium ions are predominantly present between the ion-exchange particles in the form of dissolved salts in the aqueous phase and are therefore rapidly displaced in the course of feeding the solution to be treated to the exchanger.

[0040] For the regeneration of the exhausted ion-exchanger the procedure is preferably such that a concentrate fraction is ejected that contains at least 0.5 wt. % nickel ions, and said fraction is re-used immediately or after replenishment with further active substances for the purpose of replenishing a phosphating solution. In this connection it is particularly preferred to replenish the regenerated material with further zinc ions and/or nickel ions and also with further active substances of a phosphating solution in such a way that a conventional replenishing solution for a phosphating bath is formed. This replenishing solution can then be used as conventionally for the purpose of replenishing the phosphating bath.

[0041] The solution that has been depleted of cations and that leaves the weakly acidic cation-exchanger in the exhaustion phase thereof can, depending on ingredients, be supplied to a simplified waste-water treatment or introduced directly into a biological clarification plant. However, it is more economical to use this solution as rinsing water for the metal parts to be phosphated after the degreasing thereof.

[0042] This embodiment of the process according to the invention has the additional advantage that rinsing water is saved.

[0043] If the phosphating-bath overflow or the rinsing water after phosphating is subjected to a membrane filtration prior to being fed to the weakly acidic ion-exchanger, the outflow from the ion-exchanger can be employed directly for rinsing purposes. If the membrane filtration arranged upstream is dispensed with, it is advisable to subject the outflow from the ion-exchanger to a membrane filtration before it is used as rinsing water. A nanofiltration is particularly suitable for these processes.

EMBODIMENT EXAMPLES

Example 1

[0044] The activity of the weakly acidic cation-exchanger according to the invention Lewatit TP 207 (Bayer) in the H+ form was examined in comparison with a fully synthetic aqueous phosphating rinsing solution. To this end, ion-exchange columns were filled with, in each case, 500 ml of resin (in the supplied form as the disodium form; in the course of exchange with acid for the purpose of forming the H+ form the volume shrinks to 400 ml) and charged with 648 bed volumes of rinsing solution, and the eluate solution emerging from the columns was continuously analysed in respect of its residual metal content. The fully synthetic rinsing solutions that were employed (pH value 4.0) contained 25 ppm nickel, 25 ppm manganese and 50 ppm zinc.

[0045] Table 1 indicates the exhaustion volumes and the corresponding nickel concentrations. 3 TABLE 1 Eluate BV (bed volume = 0.5 1) ppm Nickel  24 0.33  48 0.30  72 0.25  96 0.28 120 0.30 144 0.29 168 0.27 192 0.34 216 0.38 240 0.21 264 0.26 288 0.30 312 0.44 336 0.44 360 0.56 384 0.69 408 0.88 432 1.08 456 1.32 480 1.51 504 1.84 528 2.01 552 2.31 576 2.63 600 2.95 624 3.41 648 3.77

Comparative Example 1

[0046] In a manner analogous to Example 1, the activity of the weakly acidic cation-exchanger according to the invention Lewatit TP 207 (di-Na+ form) was investigated. To this end, the resin was conditioned after the regeneration with 2.4 BV NaOH (4%) and subsequently rinsed with 2.0 BV of desalinated water (in each case with 4 BV/h). The phosphating rinsing solutions that were employed corresponded to the data in Example 1. Table 2 indicates the exhaustion volumes and the corresponding nickel concentrations. The breakthrough behaviour for nickel is virtually identical in both Examples. 4 TABLE 2 Eluate BV (bed volume = 0.5 1) ppm Nickel  24 0.10  48 0.11  72 0.14  96 0.18 120 0.28 144 0.24 168 0.21 192 0.36 216 0.30 240 0.33 264 0.28 288 0.31 312 0.39 336 0.55 360 0.69 384 0.85 408 1.05 432 1.23 456 1.32 480 1.44 504 1.68 528 1.86 552 2.17 576 2.54 600 2.71 624 3.16 648 3.46

Example 2

[0047] In order to document the regenerative power of the exhausted ion-exchange resins, after exhaustion with 648 bed volumes of rinsing water (corresponds to 8.1 g nickel) said ion-exchange resins were re-extracted with phosphoric acid. To this end, the resins that were exhausted in accordance with Example 1 were eluted with 40-% phosphoric acid, and a concentrate fraction having the following composition was collected: nickel 1.8 wt. %, phosphate 10 wt. %.

Example 3

[0048] The activity of the weakly acidic cation-exchanger according to the invention Lewatit TP 207 (Bayer) in the H+ form was examined in comparison with a fully synthetic aqueous phosphating rinsing solution. To this end, ion-exchange columns were filled with, in each case, 500 ml of resin (supplied form: di-Na+, in the H form that was employed approximately 400 ml) and charged with 480 bed volumes of rinsing solution, and the eluate solution emerging from the columns was continuously analysed in respect of its residual metal content. The fully synthetic rinsing solutions that were employed (pH value 3.5; for comparison, in Ex. 1 pH value 4.0) contained 25 ppm nickel, 25 ppm manganese and 50 ppm zinc. Table 3 indicates the exhaustion volumes and the corresponding nickel concentrations. 5 TABLE 3 (rinsing solutions pH value 3.5; column in H+ form) Eluate BV (bed volume = 0.5 1) ppm Nickel  24 0.19  48 0.07  72 0.10  96 0.15 120 0.20 144 0.34 168 0.41 192 0.76 216 0.78 240 0.85 264 1.09 288 1.24 312 1.47 336 1.74 360 2.06 384 2.42 408 2.89 432 3.31 456 3.83 480 4.35

Comparative Example 2

[0049] In a manner analogous to Example 3, the activity of the weakly acidic cation-exchanger Lewatit TP 207 (di-Na+ form) was investigated. To this end, after the regeneration the resin was conditioned with 2.4 BV NaOH (4%) and subsequently rinsed with 2.0 BV of desalinated water (in each case with 4 BV/h). The phosphating rinsing solutions that were employed corresponded to the data in Example 3. Table 4 indicates the exhaustion volumes and the corresponding nickel concentrations. The breakthrough behaviour for nickel is very similar in both examples. 6 TABLE 4 (rinsing solutions pH value 3.5; column in di-Na+ form) Eluate BV (bed volume = 0.5 1) ppm Nickel  24 0.16  48 0.19  96 0.28 144 0.35 168 0.55 192 0.71 216 0.93 240 1.10 264 1.28 288 1.18 312 1.50 336 1.45 360 1.72 384 1.95 408 2.35 432 2.67 456 3.02 480 3.82

[0050] The following Example 4 shows that nickel is more firmly bound to LewatitR TP 207 than zinc and manganese. (The initially high nickel contents are based on residual nickel in the experimental arrangement, as a result of a preceding experimental cycle.) Manganese is only bound initially by the column but runs freely through after exhaustion with approximately 500 bed volumes of solution. In the course of this exhaustion the breakthrough of zinc also begins, whereas nickel is still almost completely bound up to approximately 1,000 bed volumes. Although nickel is bound increasingly poorly above this degree of exhaustion, it is still clearly bound, whereas the zinc content of the emerging solution is higher than that of the solution that has been fed. This means that not only no further zinc is bound but that nickel in the solution displaces the zinc that is bound to the exchanger.

Example 4

[0051] Two columns in series, both columns in H+ form, the column in position 2 from a preceding cycle already partially exhausted with Ni. The fully synthetic rinsing solutions that were employed (pH value 4.0) contained 25 ppm nickel. 25 ppm manganese and 50 ppm zinc. 7 BV ppm Ni ppm Zn ppm Mn  48 0.6 1.1 3.5  192 0.2 0.5 17  384 0.1 0.2 21  576 0.1 2.1 23  720 0.1 7.8 25  864 0.1 18 27 1056 0.2 34 28 1248 0.9 50 29 1440 3.2 59 28 1632 6.6 61 28

Claims

1. A process for treating a nickel-bearing aqueous solution consisting of phosphating-bath overflow and/or of rinsing water after phosphating, phosphating being effected with an acidic aqueous phosphating solution that contains 3 to 50 g/l phosphate ions, reckoned as PO43−, 0.2 to 3 g/l zinc ions, 0.01 to 2.5 g/l nickel ions, optionally further metal ions as well as, optionally, accelerators, whereby the phosphating-bath overflow and/or the rinsing water after phosphating is conducted across a weakly acidic ion-exchanger, characterised in that the acid groups of the ion-exchanger are neutralised with alkali-metal ions to an extent amounting to no more than 15% and in that when it is fed to the ion-exchanger the nickel-bearing aqueous solution has a pH value within the range from 2.5 to 6.0.

2. Process according to claim 1, characterised in that the phosphating-bath overflow and/or the rinsing water after phosphating is subjected to a membrane filtration in the form of an ultrafiltration, a nanofiltration or a reverse osmosis or to a different filtration process which is selected from a screen filtration or bag filtration or a filtration across a particle bed and the aqueous solution is conducted across a weakly acidic ion-exchanger after the filtration.

3. Process according to one or both of claims 1 and 2, characterised in that the weakly acidic ion-exchanger binds nickel ions more strongly than zinc ions.

4. Process according to claim 3, characterised in that the weakly acidic ion-exchanger bears chelate-forming iminodiacetic-acid groups.

5. Process according to one or more of claims 1 to 5, characterised in that the weakly acidic ion-exchanger is regenerated with a strong acid after exhaustion.

6. Process according to claim 5, characterised in that the strong acid is constituted by phosphoric acid which, where desired, may contain a total of up to 10 mol. % nitric acid, hydrochloric acid and/or hydrofluoric acid, relative to the overall quantity of acid.

7. Process according to one or both of claims 5 and 6, characterised in that after the regeneration with a strong acid the ion-exchanger is rinsed with water or with a quantity of lye that corresponds to a maximum of 0.5 bed volumes of 4-% caustic-soda solution for such time until the pH value of the rinsing solution running off from the ion-exchanger lies between 2.1 and 4.5, preferably between 3.0 and 4.1.

8. Process according to one or both of claims 5 and 6, characterised in that the regeneration of the ion-exchanger is carried out in such a way that a concentrate fraction is ejected that contains at least 0.5 wt. % nickel ions and said concentrate fraction is re-used immediately after replenishment with active substances for the purpose of replenishing a phosphating solution.

9. Process according to one or more of claims 1 to 4, characterised in that the solution obtained after passing through the weakly acidic ion-exchanger is used as rinsing water for the metal parts to be phosphated after the degreasing thereof.

10. Process according to claim 2, characterised in that a membrane filtration is carried out and in that the retentate of the membrane filtration is recycled into the phosphating solution.