WO2004063424A2 - Two-stage or multi-stage method for treating phosphated rinsing water using a membrane - Google Patents

Abstract

The invention relates to a method for phosphating metallic surfaces. According to said method, the metallic surfaces are rinsed with rinsing water after phosphation, and the rinsing water is subjected to a two-stage or multi-stage nanofiltration process. The concentrate of a nanofiltration stage is respectively used as a parent solution for the ensuing nanofiltration stage. The concentrate of the last nanofiltration stage is used to complete the phosphation solution.

Description

 "Two- or multi-stage membrane treatment process for phosphating rinse water"

The invention is in the field of phosphating metal surfaces, as is carried out as a widespread corrosion protection measure in the metalworking industry such as, for example, the automotive industry and the household appliance industry, but also in part in steelworks. It relates to a process for the treatment of the overflow of the phosphating baths and / or the rinsing water after the phosphating, hereinafter referred to as "phosphating rinsing water". The process improves the wastewater treatment and enables the recycling of bath contents into the phosphating bath and / or water into the overall process.

The phosphating of metals pursues the goal _k to produce firmly adherent metal phosphate layers on the metal surface, which in themselves improve corrosion resistance and, in conjunction with paints and other organic coatings, contribute to a significant increase in adhesion and resistance to infiltration when exposed to corrosion. Such phosphating processes have long been known in the prior art. For the pretreatment before painting, the low-zinc phosphating processes are particularly suitable, in which the phosphating solutions have comparatively low zinc ion contents of e.g. B. have about 0.5 to about 2 g / l. An important parameter in these low-zinc phosphating baths is the weight ratio of phosphate ions to zinc ions, which is usually in the range> 12 and can take values up to 30.

It has been shown that by using polyvalent cations other than zinc in the phosphating baths, phosphate layers with significantly improved corrosion protection and paint adhesion properties can be formed. For example, find low-zinc processes with the addition of z. B. 0.5 to 1.5 g / l of manganese ions and z. B. 0.05 to 2.0 g / l of nickel ions as a so-called trication Process for the preparation of metal surfaces for painting, for example for the cathodic electro-painting of car bodies, widely used.

A phosphating solution contains layer-forming components such as e.g. Zinc and possibly other divalent metal ions and phosphate ions. In addition, a phosphating solution contains non-layer-forming components such as, in particular, accelerators and their degradation products. The degradation products of the accelerator result from the fact that it reacts with the hydrogen formed on the metal surface by the pickling reaction. The non-layer-forming components, such as alkali metal ions, which accumulate over time in the phosphating bath, and in particular the degradation products of the accelerator, can only be removed from the phosphating solution by discharging part of the phosphating solution and replacing it continuously or discontinuously with new phosphating solution. Phosphating solution can be discharged, for example, by operating the phosphating bath with an overflow and discarding the overflow. As a rule, however, an overflow is not necessary, since the phosphated metal parts discharge a sufficient amount of phosphating solution as an adhering liquid film and transfer it into the rinsing water.

After the phosphating, the phosphating solution adhering to the phosphated parts, such as automobile bodies, is rinsed off with water. Since the phosphating solution contains heavy metals and possibly other ingredients that must not be released into the environment in an uncontrolled manner, the rinsing water must be subjected to a water treatment. This must be done in a separate step before being discharged into a biological sewage treatment plant, as otherwise the functioning of the sewage treatment plant would be endangered.

Since both the disposal of the waste water (from the phosphating bath overflow and / or rinsing water) and the supply of the phosphating system with fresh water are cost factors, there is a need to minimize these costs. DE-A-198 13 058 describes a process for the treatment of phosphating bath overflow and / or rinsing water after phosphating, the phosphating being carried out with an acidic aqueous phosphating solution which contains 3 to 50 g / l phosphate ions, calculated as Pθ4 ^{3 "} , 0.2 to 3 g / l zinc ions, optionally further metal ions and accelerators, the phosphating bath overflow and / or the rinsing water being subjected to nanofiltration, however, when the process is operated in practice, the filtration membrane becomes blocked within a few hours can be attributed to the fact that the phosphate sludge contained in the rinsing water after the phosphating and in the phosphating bath overflow cannot be completely removed from the solution by technically customary filtration methods, for example by using bag or sand filters the membrane and blocked this.

The unpublished German patent application DE-A-101 27918 tries to remedy this deficiency. It describes a process for the preparation of phosphating bath overflow or rinsing water after phosphating with a phosphating solution which contains 3 to 50 g / l phosphate ions and 0.2 to 3 g / l zinc ions, the phosphating bath overflow or the rinsing water being subjected to membrane filtration, and wherein Retentate of the membrane filtration can be carried out in a retentate circuit, characterized in that i) a reagent for delaying the membrane blocking, which is selected from a) 0.01 to 5 g / l, is added to the phosphating bath overflow, the rinsing water or the retentate circuit before the membrane filtration a complexing agent for heavy metals b) an acid in such an amount that the pH of the rinsing water is reduced to a range between 0.5 and 2.5, and / or ii) the membrane filtration is interrupted at selected times and the membrane with a treated aqueous solution of an acid b) having a pH in the range between 0 u nd 1.8.

In the prior art, attempts have been made to process phosphating rinse water according to different methods by means of a two-stage membrane filtration To improve aspects. For example, DE-A-197 54 109 describes a treatment process for rinsing water after phosphating, which involves working with a phosphating solution which contains an organic molecule with a molecular weight of at least 80 as an accelerator. The mixture is worked up by ultrafiltration, the concentrate being discarded. The filtrate is preferably subjected to nanofiltration in a second step. The concentrate of the nanofiltration is returned to the phosphating solution, the filtrate is used again as rinsing water.

DE 197 43 933 recommends a sequence of processes consisting of ultrafiltration and reverse osmosis. The ultrafiltration concentrate is added to the phosphating solution and the filtrate is subjected to reverse osmosis. The reverse osmosis concentrate is also returned to the phosphating solution. The filtrate is reused as rinse water.

EP-A-1 106 711 uses a two-stage reverse osmosis. Here, the rinsing water is first acidified after phosphating and then subjected to the first reverse osmosis. The concentrate from this is fed to the phosphating solution. The filtrate from the first reverse osmosis is neutralized by adding alkali and subjected to a second reverse osmosis. The concentrate from this is discarded, the filtrate is reused as rinsing water.

The two-stage processes mentioned above have at least one of the following disadvantages: 1) layer-forming cations of the phosphating solution, which are valuable substances, are discarded; 2) Reverse osmosis produces largely salt-free water that can be used for a final sink. This step is technically complex and therefore expensive.

The aim of the present invention is to further improve membrane processes for working up phosphating rinse water, for example in order to reduce the consumption of chemicals, to increase membrane service life or to make the process sequence more cost-effective. The object of the present invention is a method for phosphating metal surfaces, the metal surfaces being sprayed and / or dipped a) cleaned with at least one cleaning solution, b) rinsed after cleaning with at least a first rinse water, c) phosphated with a phosphating solution , which contains 3 to 50 g / l phosphate ions, calculated as Pθ4 ^{3 "} , 0.2 to 3 g / l zinc ions, optionally further divalent metal ions and optionally accelerator, and d) after the phosphating, rinsed with at least one second rinse water and wherein e) continuously or discontinuously takes part of the second rinsing water, f) acidified and g) subject to a first nanofiltration, whereby a first retentate, in which zinc ions and possibly the other divalent cations are enriched by a first enrichment factor, and a first Permeate is obtained, in which zinc ions and possibly the further one n divalent cations are depleted, h) the first permeate is at least partially transferred into the cleaning solution and / or into the first rinsing water and / or subjected to a further ion separation, i) the first retentate is subjected to at least one further nanofiltration, at least one further retentate, in which zinc ions and possibly the further divalent cations are further enriched by a further enrichment factor and contains at least one further permeate, and k) the further retentate or, in the case of several further nanofiltration stages, the last further retentate at least partially, if necessary after supplementing with further ones Active ingredients or auxiliary substances, transferred into the phosphating solution.

Rinsing is usually operated in an overflow in a phosphating process. Therefore, the easiest way to remove part of the second rinse water in sub-step e) is to catch the overflow and treat it in accordance with the further process steps. In sub-step f), preference is given to using those acids which can be returned to the phosphating bath and at least do not interfere there, but preferably are even active ingredients. Accordingly, the acids used are preferably those which are valuable substances in industrially customary phosphating baths. These are in particular phosphoric acid, hydrofluoric acid and acids, the acid residues of which are complex fluorides of boron, silicon, titanium or zircon. If the phosphating bath should or may contain nitrate ions as an accelerator or co-accelerator, nitric acid is also suitable.

If cation exchangers are used elsewhere in the phosphating process, which bind layer-forming cations from phosphating processes and which are regenerated with strong acids, an acidic aqueous solution is preferably used as the acid in sub-step f), which was obtained during the regeneration of such a cation exchanger. This leads to an additional reuse of chemicals and thus to cost savings.

According to sub-step h), the first permeate can be at least partially transferred into the cleaning solution and / or into the first rinsing water and / or subjected to a further ion separation. A further nanofiltration or a cation exchanger can, for example, be provided for this further ion separation.

Another alternative in sub-step h) is to adjust the permeate of the first nanofiltration to a pH in the range between 6 and 8 and to use it as rinsing water after cleaning the metal surfaces to be phosphated, ie between cleaning and phosphating. For economic reasons, sodium hydroxide solution is preferably used to adjust the pH.

An alternative to the procedure mentioned directly above is that the permeate of the first nanofiltration is adjusted to a pH in the range between 9 and 13 and used to supplement the cleaning solution for cleaning the metal surfaces to be phosphated. For this purpose too, sodium hydroxide is preferably used to adjust the pH, if not Potassium hydroxide solution is preferred for reasons of the solubility of salts formed.

You can also add surfactants. The surfactants are preferably selected from nonionic surfactants, for example ethoxylates and / or propoxylates of alkyl alcohols or alkylamines having 8 to 22 carbon atoms in the alkyl chain. Ethoxylates and / or propoxylates of such alkyl alcohols or alkyl amines which are obtainable from vegetable or animal fats or oils are preferably used here.

If necessary, builder substances can also be added to the permeate of the first nanofiltration in addition to the surfactants. These are preferably selected from the group of water-soluble borates, silicates, carbonates, triphosphates or pyrophosphates. The cations of these salts are preferably sodium and / or potassium ions.

The retentate of the first nanofiltration contains enriched layer-forming cations such as zinc, nickel and / or manganese ions. This retentate is subjected to at least one further nanofiltration in sub-step i) and the layer-forming cations are thereby further enriched in its retentate (“further retentate”). Since these are valuable substances for phosphating, the further retentate is preferably worked up in such a way that it at least is proportionally returned to the phosphating bath if it is not subjected to one or more further nanofiltration stages and is thereby further concentrated in further retentates. In this case, the "further" retentate obtained last is at least partially returned to the phosphating bath. In one possible embodiment, this is done in such a way that the retentate is used as the basis for the preparation of a supplementary solution for the phosphating bath and thus with active ingredients (for example with ion-forming metals, with acids selected from nitric acid, phosphoric acid, hydrofluoric acid or acids whose acid residues are complex fluorides of B, Si, Ti or Zr, or internal accelerators such as hydroxylamine) are added to form a supplemental solution for a phosphating bath. However, the respective further retentate can also be returned directly to the phosphating solution without enrichment with active ingredients. However, care must be taken that the free acid content in the phosphating bath does not drop below the permissible value. Accordingly, one embodiment of the process according to the invention consists in adjusting the pH of this further retentate to a value in the range between 2.55 to 3.2, preferably between 2.6 to 3.0, and the retentate then leads back into the phosphating solution. An alkali such as caustic soda could be used to raise the pH. However, this would have the undesirable effect that salts accumulate in the phosphating bath which are not active substances in the phosphating and unnecessarily burden the phosphating bath. For this reason, substances which contain active substances for the phosphating bath are preferably used to raise the pH. For example, the hydroxylamine or oxides or carbonates of zinc, nickel and / or manganese which acts as a phosphating accelerator can be used to raise the pH, the content of layer-forming cations being added to the phosphating solution.

By this procedure, the layer-forming divalent cations present in the rinse water after the phosphating are returned to the phosphating solution. The cycle for these cations is therefore largely closed. Phosphoric acid is preferably used to acidify the rinsing water or the phosphating bath overflow before membrane filtration, since this acid is a main component of the phosphating solution.

This two-stage or multi-stage procedure has the advantage over that in the unpublished German patent application DE-A-101 27 918 that lower enrichment factors can be used in each of the two or more nanofiltration stages. This reduces the pressures required and extends the membrane service life. A smaller amount of acid is sufficient for the acidification in sub-step f), so that less alkali is used for later neutralization. This makes the overall process more economical. When using nanofiltration in two or more stages, divalent cations are preferably retained, while monovalent ions largely pass through the membrane. In contrast, all ions are largely retained in reverse osmosis. The latter leads, in the procedure according to EP-A-1 10671 cited at the outset, that divalent groups together with monovalent cations are returned to the phosphating bath, so that the latter is salted up. This is avoided in the method according to the present invention.

The further permeate obtained in sub-step i) can be further processed or discarded as desired. The economy of the process according to the invention can be further increased by adding the further permeate at least partially to the part of the second rinse water removed in sub-step e) before subjecting it to the first nanofiltration in sub-step g). As a result, the cycle for layer-forming divalent cations still present in a low concentration in the further permeate is completely closed. Monovalent ions return to the first nanofiltration stage and in their first filtrate.

The first retentate obtained in sub-step g) can be mixed with additional acid analogous to sub-step f) (and the explanations given above for this) with acid before sub-step i) subjecting it to further nanofiltration. In general, however, this should not be necessary and is less desirable in terms of the economical use of chemicals.

The first nanofiltration g) is preferably carried out in such a way that the zinc ions contained in the feed of the nanofiltration and possibly the further divalent cations in the first retentate are concentrated by a first enrichment factor in the range from 3 to 20, preferably from 6 to 15. This is possible by choosing an appropriate pressure and adjusting the permeate flow. This first enrichment factor is less than that which is required for an economical one-step procedure according to the already cited DE-A-101 27 918. The membrane load is correspondingly lower. Analogously, the further nanofiltration i) is preferably carried out in such a way that the zinc ions contained in the feed of the further nanofiltration and possibly the further divalent cations in the further retentate can be concentrated by a further enrichment factor in the range from 3 to 15. This applies to every further nanofiltration stage.

By concentrating the divalent cations in two or more stages, a total enrichment factor in the last further retentate compared to the rinse water is obtained, which corresponds to the product of the individual enrichment factors of the two or more nanofiltration stages. A particularly economical compromise between achieved enrichment, technical effort and membrane service life is obtained by coordinating the concentration enrichment factors in the first nanofiltration g) and in each further nanofiltration i) in such a way that the zinc ions contained in the feed of the first nanofiltration and optionally the further divalent cations in the last further retentate are concentrated by a total enrichment factor in the range from 20 to 80.

It has already been mentioned above that in step h) the pH of the first permeate is preferably raised by adding alkali before it is at least partially transferred into the cleaning solution and / or the first rinsing water.

In the following it is described in more detail in connection with which phosphating solutions the method according to the invention is preferably operated.

The zinc contents of the phosphating solution are preferably in the range from 0.4 to 2 g / l and in particular from 0.5 to 1.5 g / l, as are customary for low-zinc processes. The weight ratio of phosphate ions to zinc ions in the phosphate baths can vary within wide limits, provided that it is in the range between 3.7 and 30. A weight ratio between 10 and 20 is particularly preferred

In addition to the zinc and phosphate ions, the phosphating bath can contain other components which are currently common in phosphating baths. Phosphating solutions which contain further mono- or divalent metal ions, which experience has shown to have a favorable effect on the paint adhesion and the corrosion protection of the phosphate layers thus produced, are preferably used. Accordingly, the phosphating solution according to the invention preferably additionally contains one or more of the following cations:

0.1 to 4 g / l manganese (ll),

0.05 to 2.5 g / l nickel (II),

0.2 to 2.5 g / l magnesium (ll),

0.2 to 2.5 g / l calcium (ll),

0.002 to 0.2 g / l copper (ll),

0.1 to 2 g / l cobalt (II).

For example, in addition to zinc ions, the phosphating solution contains 0.1 to 4 g / l of manganese ions and 0.002 to 0.2 g / l of copper ions as additional cations and not more than 0.05 g / l, in particular not more than 0.001 g / l, of nickel ions. However, if one wishes to adhere to the conventional trication technology, phosphating baths can be used which, in addition to zinc ions, contain 0.1 to 4, preferably up to 3 g / l manganese ions and additionally 0.05 to 2.5 g / l nickel ions. The form in which the cations are introduced into the phosphating baths is in principle irrelevant. It is particularly advisable to use oxides and / or carbonates as the cation source. Because of the risk of salting up the phosphating baths, salts of acids other than phosphoric acid should preferably be avoided.

In the case of phosphating baths which are said to be suitable for different substrates, it has become customary to use free and / or complex-bound fluoride in amounts of up to 2.5 g / l of total fluoride, of which up to 750 mg / l of free fluoride, each calculated as F ^" , In the absence of fluoride, the aluminum content of the bath should not exceed 3 mg / l. In the presence of fluoride, higher Al contents are tolerated as a result of the complex formation, provided the concentration of the uncomplexed AI does not exceed 3 mg / l. In addition to the layer-forming divalent cations, phosphating baths generally also contain sodium, potassium and / or ammonium ions to adjust the free acid.

Phosphating baths that are used exclusively for the treatment of galvanized material do not necessarily have to contain a so-called accelerator. However, accelerators, which are required for the phosphating of non-galvanized steel surfaces, are also often used in technology for the phosphating of galvanized material. Accelerating phosphating solutions have the additional advantage that they are suitable for both galvanized and non-galvanized materials. This is particularly important when phosphating car bodies, as these often contain both galvanized and non-galvanized surfaces.

In the prior art, different accelerators are available for phosphating baths. They accelerate the formation of layers and facilitate the formation of closed phosphate layers, since they react with the hydrogen generated during the pickling reaction. This process is referred to as "depolarization". This prevents the formation of hydrogen bubbles on the metal surface which interfere with the formation of the layer. In the process according to the invention, preference is given to those accelerators whose by-products or degradation products (reaction products with hydrogen) are the nanofiltration membrane This ensures that these by-products and degradation products of the accelerator do not accumulate in the phosphating bath, but are at least partially discharged from the system via the filtrate (permeate) of the nanofiltration.

Particularly suitable are those accelerators which form either water or monovalently charged ions as by-products or degradation products, which can pass through the nanofiltration membrane. For example, the phosphating solution can contain one or more of the following accelerators: 0.3 to 4 g / l chlorine 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.

In the depolarization reaction on the metal surface, chlorine ions are formed from chloride ions, nitrate ions and ammonium ions from nitrate ions, ammonium ions from nitrate ions, ammonium ions from hydroxylamine and water from hydrogen peroxide. The anions or ammonium ions formed can pass through the nanofiltration membrane, so that they are at least partially discharged from the phosphating bath overflow or from the rinsing water after the phosphating in the process according to the invention.

Together with or instead of chlorine ions, hydrogen peroxide can advantageously be used as the accelerator. This can be used as such or in the form of compounds which form hydrogen peroxide under the conditions of the phosphating bath. However, no polyvalent ions should preferably be formed as by-products, since these would be enriched in the phosphate bath when the concentrate of the nanofiltration was returned. Therefore, alkali metal peroxides are particularly suitable as an alternative to hydrogen peroxide.

An accelerator which is also preferably to be used in the process according to the invention is hydroxylamine. If this is added to the phosphating bath in free form or in the form of hydroxylammonium phosphates, hydroxylammonium nitrate and / or hydroxylammonium chloride, only degradation or by-products are also formed which can penetrate a nanofiltration membrane.

Different membrane types are available in the prior art for nanofiltration. Since phosphating baths and the corresponding rinsing water react acidic, the nanofiltration membrane used should be acid-stable. For example, inorganic membranes such as. B. ceramic membranes. Organic polymer membranes can also be used the. A polyamide membrane is particularly suitable. The membrane can be used, for example, as a winding module, tube module or plate module.

Embodiment 1, Comparative Example 1

These examples were carried out with liquids which were conducted in a pilot system with constant inflow and outflow over an 80 cm ² membrane. The membrane was in a closed circuit for inflow and retentate, the overflow rate of the liquid over the membrane (15 m ³ / hm ² ) being kept constant. The amounts of permeate and retentate given below were removed from the circuit and replaced by fresh rinsing water or permeate from the preceding nanofiltration stage (inflow)

BeispieM:

step 1

Phosphating rinse water (1) with a composition:

Ni 20 mg / l

Zn 22.5 mg / l

Mn 23.3 mg / l

Conductivity 680 μS.cm pH 3.5

Rinsing water 2 was prepared from rinsing water 1 by adding 100 g / m ^{3 of} 85% phosphoric acid in order to lower the pH to 3.

Flushing water (2) was circulated over a membrane NF TS 80 (company TRISEP) with the following parameters: membrane pressure: 12 bar working temperature: 35 ° C The rinsing water is separated through the membrane into a permeate (1) and a concentrate (retentate, taken from the circuit) (1).

The following stable conditions were reached after about 4 hours. No notable deviations were found during the further experiment. In particular, the membrane did not appear to clog slowly.

Concentrate (1)

Quantity withdrawn from the circuit: 3.0 l / h * m ²

Composition: Ni 184 mg / l

Zn 236 mg / l Mn 185 mg / l conductivity 2800 μS.cm pH 3.0

The concentrate (1) is used to apply the second membrane, see below.

Permeate (1)

Flow rate through the membrane: 30 l / h * m ²

Composition: Ni 2.5 mg / l

Zn 3.0 mg / l

Mn 2.4 mg / l

Conductivity 510 μS.cm pH 2.9

Further use of permeate (1): It can be sent to the central wastewater treatment, where the heavy metals are reduced to below the permitted limit values (eg in Germany: 0.5 mg / l for Ni). Alternatively, after neutralization, it can be used as rinsing water after cleaning in a phosphating system. Or it can continue locally up to the permitted limit values be purified, for example by ion exchange or further membrane filtration (nanofiltration or reverse osmosis)

Level 2

The concentrate (1) is passed over a membrane NF TS 80 (TRISEP) under the following conditions (in the circuit as for stage 1): membrane pressure: 12 bar working temperature: 33 ° C

The concentrate (1) removed from the circuit in the first stage is separated through the membrane into a permeate (2) and a concentrate (retentate, removed from the circuit) (2).

The following stable conditions were reached after about 4 hours. No notable deviations were found during the further experiment. In particular, the membrane did not appear to clog slowly.

Concentrate (2)

Quantity taken from the circuit: 4.5 l / h * m ²

Composition: Ni 700 mg / l

Zn 1000 mg / l Mn 700 mg / l conductivity 7200 μS.cm pH 3.0

The concentrate (2) can be used without pH adjustment to supplement a phosphating solution. It is not necessary to add sodium hydroxide solution.

Permeate (2)

Flow rate through the membrane: 19 l / h * m ² Composition: Ni 9.5 mg / l

Zn 12 mg / l

Mn 9.5 mg / l

Conductivity 800 μS.cm pH 2.85

Possible use of permeate (2): It can be sent to the central wastewater treatment facility, where the heavy metals are reduced to below the permitted limit values (e.g. in Germany: 0.5 mg / l for Ni). Alternatively, it can be returned to the rinse water (2) before the first membrane filtration. Or it can be further cleaned locally up to the permitted limit values, for example by ion exchange or further membrane filtration (nanofiltration or reverse osmosis).

Liquids treated after this process:

Fraction quantity (liter) flow direction in the process

Rinse water (2) 10 in

Concentrate (1) 0.9 in / out

Permeate (1) 9.1

Concentrate (2) 0.18

Permeate (2) 0.72 out, optional in

Comparative Example 1:

Phosphating rinse water (1) with a composition: Ni 25 mg / l Zn 35 mg / l Mn 20 mg / l pH 3.8

Rinse water 2 was prepared from rinse water 1 by adding 5000 g / m ^{3 of} 85% phosphoric acid in order to lower the pH to 1.9.

Flushing water (2) was passed over a membrane DESAL DK (company. Osmonic) with the following parameters: membrane pressure: 6 bar working temperature: 35 ° C

The rinsing water is separated through the membrane into a permeate (1) and a concentrate (retentate, taken from the circuit) (1).

The following stable conditions were reached after about 4 hours. No notable deviations were found during the further experiment. In particular, the membrane did not appear to clog slowly.

Note: Experiments with a higher pH of the rinse water (2) led to a higher pH for the concentrate (1), combined with a deviation in the working conditions, which indicates that the membrane is clogged.

Concentrate (1)

Quantity taken from the circuit: 1.1 l / h ^* m ²

Composition: Ni 800 mg / l

Zn 900 mg / l Mn 840 mg / l conductivity 7250 μS.cm pH 2.3

Use of concentrate (1): After pH adjustment to 3.0, it can be used to supplement a phosphating solution. To bring the concentrate to the pH of the phosphating solution, 1070 g / m ³ NaOH (100%) are required.

Permeate (1)

Flow rate through the membrane: 45 l / h * m ²

Composition: Ni 0.9 mg / l

Zn 1.0 mg / l

Mn 1.1 mg / l

Conductivity 1900 μS.cm pH 1, 9

Further use of permeate (1): It can be sent to the central wastewater treatment, where the heavy metals are reduced to below the permitted limit values (e.g. in Germany: 0.5 mg / l for Ni). Alternatively, after neutralization, it can be used as rinsing water after cleaning in a phosphating system. Or it can be further cleaned locally up to the permitted limit values, for example by ion exchange or further membrane filtration (nanofiltration or reverse osmosis)

Liquids treated after this process:

Fraction quantity (liter) flow direction in the process

Rinse water (2) 10 in

Concentrates (1) 0.24

Permeate (1) 9.76 Embodiment 2, Comparative Examples 2 and 3

(Concentrations in ppm = mg / l)

A rinse water after phosphating contained:

Zn (ppm) 35 H2P04 (ppm) 500

Mn (ppm) 20 NOs (ppm) 200

Ni (ppm 25 pH 3.8

Na (ppm) 120 sludge (g / l) 0.05

Preparation of rinse water:

Sludge filtration by bag filtration: Filter: Lofclear 523 D from Hayward Sludge removal: 88%

Nanofiltration:

a. Nanofiltration rinse water (level 1):

Operating conditions:

Desal DK membrane (Osmonics)

Pressure difference: 4-6 bar

Temperature: 35 ° C

Volume ratio: permeate / retentate: 10/1 to 40/1 = concentration factor:

10x to 40x, see table below. Retentate a is used according to the invention for the second stage.

b. Nanofiltration retentate a (level 2, only in example 2)

Operating conditions:

Desal DK membrane (Osmonics)

Pressure difference: 6 bar

Temperature: 35 ° C

Volume ratio: permeate / retentate: 4/1 = concentration factor: 4x Results:

Concentration factor H3Pθ4 dosage (85%, kg / m ³ rinsing water) gate (single stage): 40 x

Vergleichsbeisp. 2: comparative example 3: 5; 1, 25

retentate

pH 2.3 2.7

FS (pH 2.8) (P) ^η 2.8 0.7

Zn Conc. (ppm) 1408 1407

Mn conc. (ppm) 798 795

Ni Conc. (ppm) 998 1007

Na Conc. (ppm) 229 227

permeate

Zn Conc. (ppm) 1.8 7.8

Mn conc. (ppm) 0.9 4.1

Ni Conc. (ppm) 1.1 3.9

Na Conc. (ppm) 122 124

^*} FS = "Free acid", expressed in "points"("P"), here with the following definition: Determined by titration of 100 ml undiluted solution with 0.1 N sodium hydroxide solution up to a pH value of 2.8. The consumption in ml of sodium hydroxide solution divided by 10 (for comparison with the usual determination method, which starts from 10 ml of solution) gives the free acid score. In Comparative Example 3, acidification with 1.25 kg of 85% phosphoric acid per m ^{3 of} rinsing water proved to be insufficient to prevent the membrane from blocking at the chosen concentration factor of 40. Therefore the experiment had to be stopped because the membrane was blocked.

In comparison example 2, however, acidification with 5 kg of 85% phosphoric acid per m ^{3 of} rinsing water was sufficient to prevent the membrane from blocking at the chosen concentration factor of 40. However, an excessively acidic retentate was obtained which would have to be partially neutralized before being returned to a phosphating solution. This would lead to an increased consumption of chemicals.

Embodiment 2: H3P04 dosing before the 1st nanofiltration stage: 1.25 kg of 85% phosphoric acid per m ^{3 of} rinsing water

Concentration factor Concentration factor 1st stage: 10 x 2nd stage: 4x

Retentate 1st stage -> Retentate 2nd stage

pH 2.3 -> 2.7

FS (pH 2.8) (P) 0.8 -> 0.6

Zn Conc. (ppm) 350 → 1401

Mn conc. (ppm) 200 -> 799

Ni Conc. (ppm) 247 → 998

Na Conc. (ppm) 192 -> 287

NO 3 Conc. (ppm) 456 → 1198

Permeate 1st stage

Zn Conc. (ppm) 0.5 Mn conc. (ppm) 0.25

Ni Conc. (ppm) 0.25

Na Conc. (ppm) 121

A total concentration factor of 40 is achieved by the two-stage nanofiltration according to the invention, corresponding to comparative example 2. However, only 1.25 kg of 85% phosphoric acid per m ^{3 of} rinsing water are required for the acidification before the first nanofiltration (which, according to comparative example 3, by one for one-stage concentration Factor 40 would lead to blocking of the membrane). In the second stage, a retentate with an acid content is obtained, which can be transferred directly to a phosphating solution without partial neutralization.

Claims

claims 1. Process for phosphating metal surfaces, the metal surfaces being sprayed and / or dipped a) cleaned with at least one cleaning solution, b) rinsed after cleaning with at least a first rinse water, c) phosphated with a phosphating solution which is 3 to 50 g / l phosphate ions, calculated as PO4 ^{3 "} , containing 0.2 to 3 g / l zinc ions, optionally further divalent metal ions and optionally accelerator, and d) rinsing with at least one second rinse water after phosphating, and wherein e) continuously or discontinuously takes part of the second rinse water, f) acidified and g) subjected to a first nanofiltration, whereby a first retentate, in which zinc ions and possibly the further divalent cations are enriched by a first enrichment factor, and a first permeate, in which Zinc ions and possibly the other divalent cations are depleted, h) the first permeate is at least partially transferred into the cleaning solution and / or into the first rinse water and / or subjected to a further ion separation, i) the first retentate is subjected to at least one further nanofiltration, at least one further retentate containing zinc ions and optionally the further divalent ones Cations are further enriched by a further enrichment factor, and contain at least one further permeate, and k) the further retentate or, in the case of several further nanofiltration stages, the last further retentate at least partially, optionally after supplementing with further active substances or auxiliary substances, into the phosphating solution transferred. 2. The method according to claim 1, characterized in that one continues I) the further permeate obtained in sub-step i) is at least partially added to the part of the second rinsing water removed in sub-step e) before it is subjected to the first nanofiltration in sub-step g). 3. The method according to one or both of claims 1 and 2, characterized in that acid is added to the first retentate obtained in sub-step g) before it is subjected to further nanofiltration in sub-step i). 4. The method according to one or more of claims 1 to 3, characterized in that in the first nanofiltration g) the zinc ions contained in the feed of the nanofiltration and optionally the further divalent cations in the first retentate by a first enrichment factor in the range from 3 to 20 concentrated. 5. The method according to one or more of claims 1 to 4, characterized in that in the further nanofiltration i) the zinc ions contained in the feed of the nanofiltration and optionally the further divalent cations in the further retentate by a further enrichment factor in the range from 3 to 15 concentrated. 6. The method according to one or both of claims 4 and 5, characterized in that the concentration factors of the concentration in the first nanofiltration g) and in the further nanofiltration step (s) i) are coordinated with one another in such a way that the zinc ions contained in the feed of the first nanofiltration and, if appropriate, the further divalent cations in the last further retentate are concentrated overall by a total enrichment factor in the range from 20 to 80. 7. The method according to one or more of claims 1 to 6, characterized in that in step f) an acidic aqueous solution is used as the acid, which was obtained in the regeneration of a cation exchanger. 8. The method according to one or more of claims 1 to 7, characterized in that in step h) the pH of the first permeate is increased by addition of alkali before the first permeate is at least partially in the cleaning solution and / or in the first rinse water transferred. 9. The method according to one or more of claims 1 to 8, characterized in that the phosphating solution as further divalent cations 0.05 to 2.5 g / l nickel ions and / or 0.1 to 4, preferably up to 3 g / l manganese ions contains.