EP0968322A1

EP0968322A1 - Method and device for operating milling baths

Info

Publication number: EP0968322A1
Application number: EP98916915A
Authority: EP
Inventors: Karsten LÖHR
Original assignee: DaimlerChrysler AG
Current assignee: Mercedes Benz Group AG
Priority date: 1997-03-14
Filing date: 1998-03-11
Publication date: 2000-01-05
Anticipated expiration: 2018-03-11
Also published as: DE19710563C2; DE59810404D1; WO1998041672A1; EP0968322B1; DE19710563A1; US6454958B1

Abstract

The invention concerns a method of operating a milling bath (10) wherein a metal workpiece to be milled is immersed in a milling bath (10) containing a milling medium, the metal oxidizing owing to a chemical reaction between the metal and milling medium and being transformed into a soluble complex. The resultant mixture is subjected to a separation process which separates the excess milling medium from the complex, the recovered milling medium being used for the further operation of milling baths and the complex being subjected to a preparation process. According to the invention, a mixture of dissolved complexed metallic ions and milling medium is removed from the milling bath at a concentration of dissolved complexed metallic ions which is far below the saturation limit thereof. The separation process is a nanofiltration process in which the milling medium is separated from the mixture according to the principle of reverse osmosis, the residue simultaneously being concentrated with complexed metallic ions.

Description

description

METHOD AND DEVICE FOR OPERATING MILLING BATHS

The invention relates to a method for operating milling baths, in which a metal workpiece to be milled is immersed in a milling bath containing a milling medium, the metal is oxidized by a chemical reaction between the metal and the milling medium and converted into a soluble complex, the resulting mixture of a separation is subjected to, by which excess milling medium is separated from the complex, the recovered milling medium is used again to operate milling baths, and the complex is subjected to a preparation process.

Such a method is known from EP-A-0 465 822.

The milling bath described there is an alkaline milling bath in which aluminum workpieces are dissolved using sodium hydroxide solution according to the following equation:

2 AI + 2 NaOH + 6 H2O -> 2 NaAI (OH) ₄ + 3 H ₂ The aluminum was thus oxidized and converted into a water-soluble aluminate complex AL (OH).

In the process described in EP-A-0 465 822 for the recovery of sodium hydroxide and for the recovery of aluminum hydroxide, the milling bath mixture, which is highly concentrated in the metal ion complex, is treated in such a way that a dialysis step is carried out first. In this dialysis step, a large part of the sodium hydroxide is separated from the aluminate complex. The sodium hydroxide solution obtained in the dialysis step is still sufficiently concentrated to be able to be returned to a milling bath. However, the aluminate solution has been considerably diluted by the dialysis step. The concentration was cut in half.

The processing process of the complex now consists in splitting the aluminate complex into water-insoluble aluminum hydroxide and sodium hydroxide solution by adding water, which takes place according to the following equilibrium reaction:

NaAI (0H) ₄ ^ -> NaOH + AI (OH) ₃

The precipitated aluminum hydroxide is a valuable substance that is used, for example, in the aluminum industry. It roughly corresponds to the substance that is produced by the usual Bayer process.

A disadvantage of the process mentioned at the outset is that large amounts of water are required in the dialysis step, that is to say the so-called diffusion dialytic removal of the sodium hydroxide solution, in order to maintain the concentration gradient for the operation of the dialysis. The dialysis speeds are extremely low and a very large membrane area is required in order to be able to convert corresponding amounts.

In order to be able to carry out a technically sensible preparation of the complex at all, very high aluminate concentrations must be achieved initially in the milling bath, because otherwise the solution is so little concentrated due to the dilution during the dialysis step that a sensible workup is no longer possible. However, it was found that it is unfavorable for the milling result if very high concentrations of metal complex are gradually present in the milling bath, since this greatly reduces the milling speed. If you approach the saturation limit of the metal complex in the milling bath during operation, there is a risk that in the event of incorrect control, precipitation processes will already take place in the milling bath, which is not desirable in any case, since the workpieces to be treated are then contaminated and complex cleaning steps have to be followed.

From US-A-5 141 610 it is known to carry out the separation of the milling medium by electrodialysis when working up acidic or alkaline milling baths. In electrodialysis, an electrical field is created across the membrane, in which the dialysis membrane absorbs, for example, sodium hydroxide solution on the one side and releases it again on the other side. Electrolysis processes take place on the electrodes.

Although this method requires less membrane area, it requires a large amount of energy and the electrodes required for electrolysis are attacked by the strongly caustic bath medium. This process is not suitable for economically meaningful processing of large quantities of milling bath fluid in the range of several hundred cubic meters.

However, a large-scale economic processing is an important aspect of the present invention.

WO 85/01 670 A1 describes a process for treating aqueous waste solutions containing metal ions. The process products are separated and separated from the solvent water by a combination of reverse osmosis and a “water splitting” process, for example electrodialysis. The process products are separated from one another by means of ion-selective electrodialysis and the water is separated from the process products by means of reverse osmosis, the The various process streams are at least partially recycled. Reverse osmosis is not ion-selective and can therefore only be used in combination with another process such as electrodialysis for the separation of process products. The combination of two processes requires increased process engineering. The problem of electrodialysis has already been pointed out.

In the sense of the present invention, the term milling is understood not only as a slight surface etching or pickling of a metal piece, but also as a very defined, substantial amount of material removal, which is otherwise only achieved by machining.

Large components made of aluminum are used in the aircraft industry, for example missiles are essentially constructed from an aluminum skin. Due to manufacturing tolerances in the forming processes, for example for the formation of aircraft skin linings, it is necessary to subject the preformed parts to chemical etching or milling in order to remove defined amounts of material, i.e. aluminum, often only in certain areas. In the aircraft industry, the weight of the components plays a significant role, so that it is common to chemically mill molded components that are too heavy due to manufacturing tolerances to a certain weight.

Accordingly, the milling baths are very large, contents of 60 m ³ are possible in order to be able to treat such large components. As a result, very high amounts of milling baths are processed.

This is where the invention comes in and it is therefore the object of the invention to operate a milling bath in such a way that favorable and easy-to-control milling bath conditions are present, but apparatus-simple and energy-saving preparation of the milling bath is possible.

According to the invention the object is achieved in that a mixture of dissolved complexed metal ion and milling medium is removed from the milling bath at a concentration of dissolved complexed metal ion which is far below its saturation limit that the separation is a nanofiltration, in which the milling medium is separated from the mixture using the principle of reverse osmosis, with concurrent concentration of the residue of complexed metal ion taking place.

According to a first important aspect of the invention, the mixture of dissolved complexed metal ion and milling medium is thus removed even at a relatively low concentration of complexed metal ion. As already mentioned, it was found that well-controlled and defined etching or milling conditions can be achieved with relatively low concentrations of dissolved metal complex, which become increasingly unfavorable with increasing concentration and are difficult to grasp in terms of control technology.

The second important aspect of the invention now opens up the possibility of a subsequent economically meaningful preparation because the separation by means of nanofiltration simultaneously results in a concentration of the residue on the complexed metal ion.

With nanofiltration, the medium to be filtered is pressurized and ion-selective membrane separation takes place. The terms hyperfiltration and low-pressure reverse osmosis also exist for the term nanofiltration. Small molecules such as sodium hydroxide or water can easily pass through the membrane, larger molecules such as the metal complex can only pass through the membrane with difficulty. As a result, both water and the milling medium, for example sodium hydroxide solution, can be separated from the milling bath mixture, which then increases the concentration of metal complex in the remaining solution.

When operating the milling bath in accordance with the invention, it is not waited until such a high concentration of metal complex has been created in the milling bath that enables later economically expedient processing, but it is removed from the milling bath even at very low concentrations. In the subsequent nanofiltration, the milling medium and water are separated off and, at the same time, the metal complex is concentrated in such a way that an economically sensible preparation of the complex can be carried out.

The task is thus completely solved. In a further embodiment of the invention, in which milling medium is released again when the complex is prepared, the resulting solution is also fed to the nanofiltration.

If you use the example of working up a sodium aluminate complex again, sodium hydroxide solution is produced again. This means that the aqueous filtrate contains sodium hydroxide solution and also residual amounts of sodium aluminate complex. By feeding this solution to the solution that is fed to the nanofiltration, the sodium hydroxide, which was still bound in the aluminate complex during the previous nanofiltration, can now also be separated off and returned to the milling bath. This makes it possible to circulate the entire necessary sodium hydroxide solution, which means a considerable economic and ecological effect in large-scale plants.

In a further embodiment of the invention, in which the complex is prepared by diluting with water and precipitating and separating the metal as hydroxide, the water addition is controlled so that the filtrate resulting from the separation compensates for the evaporation loss of the milling bath.

Milling baths are often carried out at high temperatures, for example in the range of 75-80 ° C., so that a considerable amount of the water in the milling bath evaporates gradually. By adding an appropriately diluted filtrate from the preparation process of the complex, appropriate amounts of water can be added to the nanofiltration, which together with the sodium hydroxide solution pass through the membrane of the nanofiltration and thus ensure adequate water supply to the milling bath.

In a further embodiment of the invention, the mixture of dissolved complexed metal ion and milling medium is removed from the milling bath either continuously or in batches. Depending on the size of the system and the control technology, the mixture can be removed accordingly.

In a further embodiment of the invention, namely when alkaline milling baths are used to dissolve aluminum, the mixture of alkali and aluminate complex is added Aluminum bath concentrations in the range of 5 - 20 g / l aluminum taken. This measure now has the advantage that the bath is operated at extremely favorable concentrations of aluminum. With extremely low aluminum concentrations in the bath, there is practically pure sodium hydroxide solution, which is so concentrated and aggressive that, for example, immersing very large parts does not result in a uniform etching or milling result, since the areas immersed first are milled longer and therefore more than the areas later immersed. With very high concentrations of aluminum in the milling bath, the milling speed drops sharply and there is a risk of over-concentration points, so that precipitation takes place.

In a further embodiment of the invention, the concentration of alkali in the milling bath is in the range from 1 10-150 g / l sodium hydroxide. The sodium hydroxide created during the nanofiltration is continuously recycled in such a way that the concentration is maintained in these favorable ranges, so that an optimal and defined, and thus easily controllable milling result can be achieved.

In a further embodiment of the invention, the nanofiltration is carried out in such a way that the aluminum is concentrated up to 100 g / l.

Concentrations in this order of magnitude result in a supersaturated solution of metal complex that is accessible for economical processing. Crystallization processes already take place spontaneously in this solution.

Although alkaline milling baths for dissolving aluminum have essentially been described in order to explain aluminum, it goes without saying that other metals can also be milled that dissolve with alkaline media and form corresponding complexes, such as zinc.

Likewise, acidic operation of milling belts is also possible in which the complexation takes place with the anion of the acid, for example so-called chloro complexes are formed when dissolved with hydrochloric acid. Such complexes can also be worked up, and the corresponding metal hydroxides can then be precipitated again by dilution with water or by alkalizing. It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the specified combinations but also in other combinations or on their own without departing from the scope of the present invention. The invention is described and explained in more detail below on the basis of a few exemplary embodiments in conjunction with the accompanying drawings.

Show it:

Fig. 1 shows schematically a system with which the inventive method is operated, and

Fig. 2 is a diagram in which the concentration of milling medium compared to the

Concentration of dissolved metal complex is applied to explain the procedure.

1 shows a milling bath 10 in which an aqueous solution of sodium hydroxide is contained as the milling medium. The milling bath has a volume of about 60 m ³ and is operated at a temperature of 75-80 ° C., so that considerable amounts of water vapor 12 already escape.

Metallic workpieces made of aluminum are immersed in the milling bath 10, for example large-area curved aircraft trim elements with a dimension of several meters.

The aluminum of the workpiece reacts with the sodium hydroxide solution to form hydrogen and a soluble sodium aluminate complex, NaAI (OH) ₄ .

In the retracted state of the system, a bath concentration of approximately 10 g aluminum per liter of bath liquid is removed and fed to a nanofilter 14, in which a nanofiltration is carried out. The nanofilter 14 is constructed such that an inner tube has a tubular support structure in which a membrane is placed. This tube is surrounded by another tube.

Such nanofilters are used as module units, such as those offered by Membrane Products Kiryat Weizmann Ltd., Rehovot / Israel under the name SeIRO Tubula Modul TM 1228. The membrane used has the specification MPT-34.

The membrane tube of a tube module has a diameter of approximately 10 mm, the support body consists of a perforated, porous tubular support body on which the membrane film MPT-34 is placed.

Such a module is based on the cross flow principle, i.e. the milling bath solution is pressurized with a pressure of 1-6 MPa by a pump (not shown here) and passed in one direction through the inner tube at a speed of 1-2 m / s. Under these conditions, sodium hydroxide and water molecules pass through the membrane in the radial direction and are converted into the so-called NaOH permeate, which is returned to the milling bath 10. The milling bath solution, which is depleted in water and sodium hydroxide, is concentrated in aluminate complex after leaving the nanofilter 14. The conditions are chosen so that a concentration in the range of about 50-100 g / l aluminum takes place.

This liquid, which is referred to as Al concentrate, can now be fed to a preparation process, which can be carried out either on site or at a company that produces aluminum.

The Al concentrate is diluted with water according to the known Bayer process, and the aluminum hydroxide Al (OH) _{3 formed is} precipitated and centrifuged, if appropriate with the addition of appropriate germs, and fed as an Al product, for example to aluminum production. The filtrate which remains when the precipitated Al (OH) _{3 is} separated off by filtration or centrifugation contains the sodium hydroxide solution formed when the sodium aluminate complex is split up and any undissolved sodium aluminate complex which is still dissolved. This filtrate can be fed again to the nanofilter 14, transferred there into the NaOH permeate and returned to the milling bath 10.

It is therefore possible to conduct the sodium hydroxide (NaOH) in a closed circuit on site in large-scale industrial plants. If you consider that around 1,200 l of sodium hydroxide are used in milling baths every year by an aircraft manufacturer, the economic dimension of the recovery and recycling of sodium hydroxide solution becomes clear.

The advantageous process control can be seen more clearly on the basis of the diagram in FIG. 2. The milling bath 10 of FIG. 1 is operated in such a way that the concentration of sodium hydroxide solution is approximately 120 g / l, the concentration of dissolved aluminum is approximately in the range of 10 g / l. These bath conditions can be maintained continuously through the nanofiltration and the constant recycling of the NaOH permeate formed. Optimal etching or milling results can be achieved in this area, i.e. the milling speed is not so high that large parts are milled differently during the immersion process, but the milling speed is still sufficient and desirably high.

The concentration of aluminum is increased during nanofiltration, as indicated by the horizontal arrow pointing to the right. In the illustrated embodiment, one ends up with an aluminum concentration of about 75 g / l. In this state, approximately 70-80% of the sodium hydroxide solution contained in the milling bath has already been removed and converted into the NaOH permeate.

A dilution step with water is first carried out, so that this solution is relatively poor in sodium and aluminum (in the form of the dissolved aluminate complex).

The aluminum hydroxide is then precipitated by adding germs and the precipitated aluminum hydroxide is filtered off or, for example, centrifuged off. That arose The filtrate contains, in addition to residual amounts of the still undissolved sodium aluminate complex, sodium hydroxide solution which has arisen during the cleavage of the complex, that is to say that previously bound sodium hydroxide solution is released again.

This filtrate is returned to the nanofiltration, it is controlled so that the sodium hydroxide permeate is diluted to such an extent that the evaporation losses of the milling bath are compensated for.

The dilution step described is first carried out because a direct splitting of the highly concentrated aluminate solution would lead to a sodium hydroxide concentration in the filtrate which is significantly higher than the solution desired in the bath.

By recycling the filtrate after the precipitation of the aluminum hydroxide, the remaining 20-30% of the sodium hydroxide solution contained in the milling bath can be recovered and returned to the circuit so that ultimately no sodium hydroxide solution is used.

It is obvious that this method is very easy to grasp in terms of control technology, only the aluminum concentration needs to be measured and the mixture is then removed from the milling bath either continuously or in batches.

2 shows a conventional bath guide with the dashed sawtooth curve.

If one starts at the upper left end of the sawtooth curve, it can be seen that bath concentrations start with about 150 g of sodium hydroxide per liter of solution. By reaction with the aluminum workpiece, the concentration of sodium hydroxide solution decreases and the concentration of dissolved aluminum increases, for example to a value in the range of 20 g / l aluminum. The external addition of sodium hydroxide to the milling bath increases its concentration again suddenly and the milling process continues, whereby sodium hydroxide solution is used again and the concentration of aluminum in the milling bath increases continuously. When a concentration of 30g aluminum per liter milling bath is reached, sodium hydroxide solution is again added, in this state such a bath is already referred to as a bad bath. It can then be brought up to the saturation curve and thus a maximum concentration of just under 50 g / l under these conditions Aluminum can be achieved. Here, however, there is already a risk that precipitations will take place, since work is already very close to the saturation limit. It can be seen that the control via the sawtooth curve requires considerable measuring equipment, which is considerably more complex than when recording according to the method according to the invention.

With the bath guide described, working with optimal milling conditions can double the removal rate from approximately 1 mm per hour to approximately 2 mm per hour material thickness.

Furthermore, a considerable reduction in downtimes for bath renewal is found, when operating in accordance with the sawtooth curve there is approximately 30% downtimes, namely to replace the entire bath and replace it with a fresh one, to remove the body from AI (OH) ₃ that has already accumulated, and so on Add sodium hydroxide solution cyclically.

Due to the exact mode of operation under ideal conditions, it is possible to get very close to the optimum milling depth in terms of control technology, previously only 90% of the optimal milling depth had to be reached, whereby iterative measuring methods had to be carried out.

Claims

claims

1. A method for operating milling baths, in which a metal workpiece to be milled is immersed in a milling bath containing a milling medium, the metal is oxidized by a chemical reaction between metal and milling medium and converted into a soluble complex, the resulting mixture is subjected to a separation , through which excess milling medium is separated from the complex, the recovered milling medium is used again to operate milling baths, and - the complex is subjected to a preparation process, characterized in that the milling bath is a mixture of dissolved complexed metal ion and milling medium at a concentration of dissolved complexed metal ion is taken, which is far below its saturation limit, - that the separation is a nanofiltration, in which the milling medium under

Application of the principle of reverse osmosis is separated from the mixture, with concurrent concentration of the residue of complexed metal ion taking place.

2. The method according to claim 1, in which during the preparation of the complex milling medium is released again, characterized in that the resulting solution is also fed to the nanofiltration

3. The method according to claim 2, characterized in that the preparation of the complex is carried out by dilution with water and cases and separation of the metal as a hydroxide, and the water addition is controlled so that the resulting filtrate in the separation evaporation losses of

Compensates milling bath.

4. The method according to any one of claims 1-3, characterized in that the mixture of dissolved complexed metal ion and milling medium is removed continuously or in batches from the milling bath.

5. The method according to any one of claims 1-4, characterized in that the operation of alkaline milling baths for dissolving aluminum

Mixture of lye and aluminate complex is taken at Al bath concentrations in the range of 5-20 g / l.

6. The method according to claim 5, characterized in that the concentration of alkali is kept in the range of 110 - 150 g / l.

7. The method according to any one of claims 1-6, characterized in that the nanofiltration is carried out so that a considerable concentration of the metal, in the case of aluminum up to 100 g / l, takes place.

8. The method according to any one of claims 1-7, characterized in that the nanofiltration is carried out at pressures in the range of 1-6 MPa.

9. Device for performing the method according to one of claims 1-8, with a milling bath (10), with a device for removing and separating the mixture formed in the milling bath (10) for separating excess milling medium from the complex, characterized in that the The device for separating is a nanofilter (14) in which the milling medium can be separated from the mixture using the principle of reverse osmosis and at the same time the residue of complexed metal ion is concentrated.