CH620248A5 - Method for the more economical use of water in phosphating metals - Google Patents

Description

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PATENT CLAIMS

1. A method for improving the water balance in the phosphating of metals, the process comprising one or more aqueous degreasing stages, one or more subsequent rinsing stages (SpE), the phosphating stage and at least two subsequent rinsing stages (Sp Pi and Sp P2), characterized in that that the degreasing solution is fed to a preparation and the cleaned solution is returned to the degreasing stage and in the rinsing stages the rinsing water in the countercurrent to the workpiece flow is cascaded from rinsing stage Sp P2 to Sp Pi and to Sp E, taken from the latter, in a preparation system by adding Interfering dissolved anions and cations are removed from the precipitant and, after the sludge formed has been separated off, at least the rinsing stage Sp P2 is returned.

2. The method according to claim 1, characterized in that an ultrafiltration is carried out in the preparation of the degreasing solution.

3. The method according to claim 1 or 2, characterized in that in the rinse water treatment plant calcium hydroxide is used as a precipitant, optionally with the use of magnesium hydroxide.

4. The method according to any one of the preceding claims, characterized in that an inclined clarifier is used to separate the sludge formed.

5. The method according to any one of the preceding claims, characterized in that the sludge-free solution is passed through a surfactant adsorber.

6. The method according to any one of the preceding claims, characterized in that a surfactant-containing alkaline cleaner based on ammonium phosphate is used in the degreasing stage.

7. The method according to any one of the preceding claims, characterized in that in the phosphating stage a solution containing phosphate ions, layer-forming cations and peroxide accelerators and essentially free of those components which result in a water neutralization of the solution with a neutralization with Ca (OH) salts , is and is held, is used.

8. The method according to any one of the preceding claims, characterized in that the workpieces are treated with deionized water before they leave the system and this is regenerated in an isolated circuit, preferably using reverse osmosis.

A common process in the phosphating of metals comprises one or more degreasing stages using aqueous cleaning solutions, one or more subsequent rinsing stages, then the actual phosphating treatment and then at least two subsequent rinsing stages.

The production of flawless phosphate layers requires a metal surface that is free of corrosion products and on which oil and grease films - if at all - are only available in very thin layers. Before phosphating, the surfaces are degreased by single- or multi-stage treatment with aqueous cleaning solutions. The oil, grease and dirt contaminations removed from the surfaces pass into the cleaning solution and gradually accumulate therein in such a way that satisfactory cleaning is no longer carried out. So far, it has been common practice to discard such cleaning solutions after their effectiveness has been exhausted. You will then be drained and reattached. After the cleaning stage, the workpieces are rinsed with water to remove adhering cleaning solution. Impurities therefore also accumulate in the rinse water. In order not to let their concentration rise above the harmful limit, the rinsing baths are operated with a continuous flow of water or are drained from time to time and mixed with fresh water. The cleaned and rinsed workpieces are then provided with a phosphate coating by treatment with an aqueous phosphating bath. At least two further rinsing stages follow the phosphating stage in order to remove residues of the acidic phosphating solution from the surfaces. These rinsing stages are also usually carried out with a continuous supply of water in order to avoid an undesirable increase in harmful components and to ensure a good rinsing effect. It is known that in the rinsing process after cleaning and in the rinsing stages after the phosphating treatment, the rinsing water is in each case countercurrent to the workpiece flow. Although considerable water savings are already possible as a result, the total requirement for fresh water is still undesirably high. It is costly to procure and is becoming increasingly difficult in the required quantities. In addition, the accumulation of considerable amounts of wastewater and its treatment result in further problems and increasing requirements. It is therefore an important and urgent technical concern to reduce the need for fresh water in phosphating processes and to reduce the amount of waste water to a minimum.

It has now been found that this object can advantageously be achieved in a process of the type mentioned at the outset by firstly feeding the degreasing solution to a preparation and returning the cleaned solution to the degreasing stage and, furthermore, providing a circuit in the rinsing stages in which the rinsing water in countercurrent to the workpiece flow is cascaded from the rinsing stages after phosphating to the rinsing stage after cleaning, removed from the latter, freed from annoying dissolved anions and cations in a treatment plant by adding suitable precipitants and after separating the sludge formed at least the second rinsing stage is fed again after the phosphating.

The workpieces can be degreased in one or more stages. The selection of the aqueous cleaner depends on the type and extent of contamination on the surfaces to be treated. In most cases, alkaline cleaners are used. They contain an inorganic structure and suitable surfactants. Components of the inorganic framework can be di- and trialkali phosphates, tetraalkali pyrophosphates and other condensed alkali phosphates, alkali borates, alkali silicates, alkali carbonates and finally also alkali hydroxides. Additives that activate the subsequent phosphating can also belong to the components of the inorganic framework. Low foaming wetting agents are preferred as surfactants, especially for spray cleaning. The concentration of the inorganic framework used in the cleaner bath is usually between 1 and 10 g / 1, the wetting agent content in the cleaner bath is generally between about 0.03 and 0.8 g / 1.

In some applications, however, hot water can also be considered as a cleaning solution, for example by splashing, flooding and the like. A strong relative movement between the workpiece surface and the liquid should be ensured. Aqueous surfactant solutions can also be used as neutral cleaners. However, alkaline cleaners with surfactant content are usually used. In this case, it may prove advantageous to use ammonium phosphate as the alkali metal

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620 248

component. As a result, the alkali content in the overall circuit can be kept lower.

According to one of the features according to the invention, the degreasing solution is fed to a preparation and the cleaned solution is returned to the degreasing stage. The preparation is preferably carried out continuously. The way in which the cleaning solution is prepared can be done in various ways, depending on the type of soiling that has been absorbed. Since the main task of the aqueous cleaning agent solutions is generally to remove oily contaminants and the oils and fats can be emulsified to a greater or lesser extent in the solution, ultrafiltration is preferably carried out in the preparation described. The oil-containing cleaning solution, if necessary previously freed from settled insoluble substances by decantation or coarse or fine filtration, is e.g. pressed through a membrane, the pores of which are dimensioned in such a way that it allows dissolved low-molecular substances to pass through with the water and at least retains the amount of oil absorbed to such an extent that the permeate can be returned to the degreasing bath without difficulty. Ultrafiltration devices and their use for separating oil from liquids are known (cf. e.g. DE-OS 2 422 777). Suitable and preferred membrane materials are those based on inorganic oxides. The use of ultrafiltration not only has the advantage that the troublesome oil content is removed from the solution, but also the advantage that the solution fed back to the degreasing bath also contains dissolved substances of the alkaline structure and possibly also surfactants. This reduces the consumption of detergent.

The regenerated solution is returned to the degreasing bath or, in the case of multi-stage degreasing, to the last degreasing stage. This can also be done, for example, by applying the regenerated solution to the degreased workpieces after the degreasing stage or the last degreasing stage by means of a separate spray ring and then reaching the degreasing stage by a directed process. With this method of operation, the entry of contaminated degreasing solution into the subsequent rinsing stage is reduced.

The retentate obtained during ultrafiltration can be further concentrated by renewed ultrafiltration. It can also be concentrated by evaporation of the aqueous phase, for example by means of a thin-film evaporator. The volume by volume of oil to be discarded and impurities that are fed into a collecting container can accordingly be kept low.

As already mentioned, it is preferred to carry out the ultrafiltration in the preferably continuous preparation of the degreasing solution because of the particular advantages which can be achieved. In principle, however, it is also possible to use other methods. This is the case, for example, if greases are removed from the treated surfaces without substantial oil emulsification in the cleaning agent solution. For example, the methods of floating, sedimenting, centrifuging, filtering and / or floating can be used to separate off the oil, fat and solid contents.

The consumption or the losses of the degreasing bath of detergent components can be compensated for by appropriate addition with the components of the inorganic structure and surfactant. Water losses occurring in the degreasing bath can be replenished, for example, from the circuit of the regenerated rinsing water.

After degreasing, the workpieces to be phosphated are rinsed in one or more stages, then provided with the desired coating in the phosphating stage and then rinsed again with water in at least two rinsing stages. According to the invention, in the rinsing stages, the rinsing water is conducted in a countercurrent to the workpiece flow from the rinsing stages after phosphating to the rinsing stage after degreasing. The rinsing water is removed from this rinsing stage and returned to a reactor in order to precipitate undesired dissolved anions and cations. Phosphates and heavy metals can be precipitated, for example, by neutralization with alkaline earth metal hydroxide, in particular Ca (OH) 2. The reaction mixture is expediently brought to a pH of about 8.5. Dissolved alkali ions are not precipitated, but generally cause less interference, at least as long as there is no excessive accumulation in the rinsing baths. The entry of alkali ions from the degreasing stage can be reduced, for example, by using ammonium phosphate in the cleaner structure instead of alkali metal salts. In this case, it may be advisable to introduce 2 magnesium ions in addition to the Ca (OH) in the neutralization of the rinsing water removed from the rinsing step after degreasing, in order to precipitate dissolved ammonium ions as poorly soluble magnesium ammonium phosphate. The introduction of alkali ions and undesired anions into the rinsing water circuit can also be influenced in particular by avoiding the introduction of such cations and anions into the rinsing zone after the phosphating step. This can be achieved particularly advantageously by carrying out the phosphating with a solution which is essentially free of components which result in water-soluble salts when the solution is neutralized with Ca (OH) 2, and also supplements the solution in this way becomes. Such a phosphating method has been proposed in DE-OS 2 327 304. In the procedure according to the invention, it is preferably used in the phosphating stage. In such a process e.g. Peroxide is used to accelerate the formation of the phosphate coating, and the chemicals used to prepare and generate the bath are selected so that only those anions and cations are added to the phosphating bath that are too difficult to neutralize with Ca (OH) 2. or insoluble salts. The residues of the phosphating bath introduced into the rinsing stages after the phosphating stage and the return of the rinsing water to the rinsing bath after degreasing therefore do not result in any additional contamination by anions and cations, which remain in the circuit after treatment of the rinsing water by suitable precipitants.

The precipitate resulting from the neutralization of the removed rinsing bath can be removed by the known methods of sludge separation, e.g. be removed from the solution by sedimentation and / or filtration. Particularly advantageous results can be achieved by using an inclined clarifier. The separated sludge can be fed to a sludge thickener and a filter press for further concentration.

The desludged solution can then first be passed through a surfactant adsorber. This is recommended if a surfactant-containing solution is used in the degreasing stage, which will be the case in most areas of application. For example, a filter made of activated carbon can be used as the surfactant adsorber. However, a regenerable exchanger system is preferably used.

The rinsing solution prepared in the aforementioned manner is then fed to the at least second rinsing stage after the phosphating and is used in the manner described in the form of a cascade in countercurrent to the workpiece flow. The regenerated rinsing solution is preferably applied to the workpieces after the at least second rinsing stage5

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sprayed and the drain introduced into this rinse stage.

In order to achieve consistently good results, it is expedient to keep the total flow of the circuit at least high enough that the rinsing water introduced into the second or last rinsing stage has a pH in the range between 6 and 9.5.

The shortage of water that occurs in the overall circuit can be expediently adjusted by adding fresh water after the last rinsing stage, spraying it onto the workpieces and allowing it to flow into the last rinsing stage. The use of low-salt fresh water is an advantage.

Following the method of working according to the invention, a separate aftertreatment, e.g. by rinsing with deionized water or using a passivating aftertreatment agent, if desired. When using deionized water, regeneration in an isolated circuit is recommended, e.g. by reverse osmosis or ion exchange.

In the circulatory process according to the invention, there is also the possibility of including the sludge obtained in the phosphating stage in the procedure. The sludge can be separated off in the usual way, but the thin sludge is also introduced into the treatment plant for reaction with suitable precipitants. The method of operation according to the invention makes it possible to carry out phosphating processes with a considerably reduced need for fresh water and a very low amount of waste water.

A preferred process is shown schematically in the drawing.

In the example below, the significantly improved water balance compared to conventional working methods is explained in more detail.

In a 5-zone continuous spraying system for automobile bodies, 30,000 m2 of steel surface were treated daily (= 16 h) according to the following procedure:

1st stage: degreasing (aqueous alkaline detergent containing surfactant, 3 g / I; Na content 0.9 g / 1)

2nd stage: rinsing (bath volume 6 m3)

3rd stage: phosphating (2, S g / 1 Zn, 7.0 g / 1. P2O5,0.01 g / 1 Ni, 0.1 g / 1 H2O2)

4th stage: rinsing (bath volume 6 m3)

5th stage: rinsing (bath volume 6 m3)

s

Then it was re-brewed with fully salted water.

The discharge of solution per m2 of treated surface is approximately 100 ml / m2 in practice. The rinse water iii required in every zone is therefore 51 / m2 (dilution 1:50), corresponding to 150 m3 / day. Therefore 3 X150 = 450 m3 / day are required for the usual continuous flushing.

In the case of cascading of the rinsing baths (with a daily new batch), the rinsing water consumption is reduced to 150 + 18 (Neuanstaz) = about 170 m3 / day.

After using the circulation system according to the invention and precipitating disruptive dissolved anions and cations in a reactor using Ca (OH) 2 and then clarifying and recycling the solution, the water requirement (with daily new batch) was only 45 m3 / day (30 new batches + 15 supplements) . The amount of thin sludge drawn off was 0.5 l / m2 = 15 m3 / day. A further reduction in the water requirement is therefore possible by further sludge treatment, so that less fresh water is required to supplement the losses caused by sludge removal.

170 mg / 1 Ca (OH) 2 were required to precipitate anions and cations in the reactor. The concentration of non-noticeable substances in the circuit totaling approx. 30 30 m3 (3 rinsing baths, reactor, inclined clarifier, pump sump) after 1 day was below about 90 mg / 1 Na and below 50 mg / 1 surfactant. The solution used in the degreasing stage was fed to a continuous oil separation and the regenerated solution returned to the cleaning bath. It can therefore be seen that the water saving is 90% compared to the usual continuous rinsing in bodywork systems and is still very significant compared to a cascade rinsing.

In addition, the cycle 40 according to the invention eliminates the costs which normally have to be expended for the preparation of the cleaning baths which are discarded every 2-3 days.

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1 sheet of drawings