EP0968322B1 - Method and device for operating milling baths - Google Patents

Description

The invention relates to a method and apparatus for operating milling baths, in which a to be milled immersing a metallic workpiece in a milling medium containing a milling medium, oxidized by a chemical reaction between metal and milling medium, the metal and in a soluble complex is transferred, subjected to the resulting mixture of a separation is separated by the excess milling medium from the complex, the recovered Milling medium is used again for operating milling baths, and the Complex is subjected to a treatment process.

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

The milling bath described there is an alkaline milling bath in which workpieces made of aluminum are dissolved by means of sodium hydroxide solution according to the following equation: 2 Al + 2 NaOH + 6 H ₂ O -> 2 NaAl (OH) ₄ + 3 H ₂

The aluminum was thus oxidized and converted into a water-soluble aluminate complex AL (OH) ₄ .

In the process for recovering., Described in EP-A-0 465 822 Sodium hydroxide and to obtain aluminum hydroxide becomes the metal ion complex treated highly concentrated Fräsbadgemisch so that initially performed a dialysis step becomes. In this dialysis step, a large portion of the sodium hydroxide from Aluminate complex separately. The caustic soda obtained in the dialysis step is still sufficiently concentrated to be returned to a milling bath. The However, aluminate solution has been significantly diluted by the dialysis step. The Concentration was about halved.

The treatment process of the complex consists of splitting the aluminate complex into insoluble aluminum hydroxide and sodium hydroxide solution by adding water, which is carried out by the following equilibrium reaction:

The precipitated aluminum hydroxide is a valuable substance, for example, the Aluminum industry is fed, it corresponds approximately to the substance, after the usual Bayer process arises.

A disadvantage of the aforementioned method is that in the dialysis step, ie at the so-called diffusionsdialytischen removal of caustic soda large amounts of water needed to maintain the concentration gradient for operating the dialysis. The dialysis speeds are extremely low and it becomes a very high membrane area needed to be able to implement appropriate quantities.

In order to be able to carry out any technically sensible treatment of the complex, must be achieved in the long milling very high aluminate concentrations, because otherwise the dilution solution during the dialysis step will be so little concentrated that a meaningful work-up is no longer possible.

But it was found that it is unfavorable for the milling result, if in the milling after and after very high concentrations of metal complex are present, since thereby the Milling speed is greatly reduced. If you go near the operation of the Saturation limit of the metal complex in the milling bath, there is a risk that at Fault controls already in the milling pool Fällvorgänge take place, which is not desirable, because the workpieces to be treated are then contaminated and costly cleaning steps must be followed.

From US-A-5 141 610 has become known in the workup of acid or alkaline milling baths to carry out the separation of the milling medium by electrodialysis. In electrodialysis, an electric field is applied across the membrane, at the dialysis membrane on the one hand, for example, receives sodium hydroxide, on the returns to another side. Electrolysis processes take place at the electrodes.

Although this method requires less membrane area, but requires a high Energy expenditure and the necessary electrodes for electrolysis of the strong corrosive bath medium attacked. This method is not suitable for large quantities From Fräsbadflüssigkeit in the range of several hundred cubic meters makes economic sense work up.

But a large-scale economic work-up is an important aspect of present invention.

WO 85/01 670 A1 discloses a process for the treatment of aqueous, metal-ion-containing Abfaillösungen described. The separation of the process products and their separation The solvent is water by a combination of reverse osmosis and a "water-splitting" process, such as electrodialysis. By means of ion-selective Electrodialysis, the process products are separated from each other and by means of the reverse osmosis, the water is separated from the process products, the Order is arbitrary. The various process streams become at least partially recycled.

The reverse osmosis is not ion selective and can therefore only in combination with another method such as electrodialysis for the separation of Process products are used. The combination of two procedures requires an increased procedural effort. On the problem of electrodialysis has already been pointed out.

WO-A-9527681 discloses a method and apparatus for purification contaminated lyes and recovery of hydroxide from production plants of Food, biotechnology or pharmaceutical industries. there The contaminated solution is pressurized via a nanofiltration membrane guided. The membrane holds low molecular weight organic molecules of molecular weight back from at least 150.

In US-A-5587083 a method for nanofiltration of concentrated aqueous Salt solutions described. This is a salt solution under pressure over a Nanofiltration membrane out, with monovalent and multivalent ions separated become.

For the purposes of the present invention, the term milling is not only a understood minor superficial etching or pickling a piece of metal, but Also, a well-defined significant material removal, as otherwise only by cutting Processing is done.

The aircraft industry uses large aluminum components For example, missiles are essentially constructed of an aluminum skin. Due to manufacturing tolerances in the forming processes, for example for Formation of aircraft skin panels, it is necessary to use the already preformed parts to undergo a chemical etching or milling to be quite defined, often too only in certain areas significant amounts of material, ie aluminum, ablate. In the aircraft industry, the weight of the components plays a significant role Role, so it is common to molded components due to manufacturing tolerances hard to chemically mill to a certain weight.

Accordingly, the milling baths are very large, contents of 60 m ³ are possible to treat such large components. As a result, very high amounts of reprocessed milling baths are incurred.

Here is where the invention and it is therefore an object of the invention, an alkaline To operate aluminum milling bath such that each cheap and easy to control Fräsbadbedingungen present, yet a simple apparatus and energetically little consuming preparation of the milling is possible.

According to the invention the object is achieved in that the alkaline milling a Mixture of dissolved complexed aluminum ions and milling medium in one Concentration of dissolved complexed aluminum ions is taken that far below its saturation limit is that the separation is a nanofiltration, in which the milling medium using the principle of reverse osmosis of Mixture is separated, at the same time a concentration of the residue on complexed aluminum ions takes place.

It is thus according to a first important aspect of the invention, the mixture from dissolved complexed aluminum ions and milling medium already at a relative taken from low concentration of complexed aluminum ions. As already mentioned, it was found that well controllable and defined etching or Milling conditions at relatively low concentrations of dissolved aluminate complex can be achieved, which become increasingly unfavorable with increasing concentration and control technology are difficult to detect.

The second important aspect of the invention now opens the possibility of a downstream economically meaningful processing because of the separation Concentration of the residue on the same time via nanofiltration complexed aluminum ions takes place.

In nanofiltration, the medium to be filtered is subjected to a pressure and there is an ion-selective membrane separation. For the term nanofiltration There are also the synonyms hyperfiltration and low pressure reverse osmosis. little one Molecules such as sodium hydroxide or water can easily membrane larger molecules, such as the aluminate complex, can pass through Membrane only difficult to happen. This allows both water and that Milling medium, such as sodium hydroxide, are separated from the Fräsbadgemisch, which then increases the concentration of aluminate complex in the remaining solution becomes.

It is not waited in the operation of the invention the Fräsbad, up in the Fräsbad such a high concentration of aluminate complex has arisen, the one later economically meaningful preparation allows, but it becomes from the Milling bath already taken at very low concentrations. In the Downstream nanofiltration, the milling medium and water is separated and Thus, at the same time the aluminate complex concentrated so that an economical meaningful preparation of the complex is feasible.

Thus, the task is completely solved.

The invention also relates to a device according to claim 9 for Implementation of the procedure.

In a further embodiment of the invention, wherein in the processing of the aluminate complex Milling medium is released again, the resulting solution also fed to the nanofiltration.

Taking again the example of the workup of a Natriumaluminatkomplexes Back, it is again caustic soda. That is, in the aqueous filtrate are Sodium hydroxide and also residual amounts of Natriumaluminatkomplex included. By the feeding of this solution to the solution, which is fed to the nanofiltration can the sodium hydroxide, which in the previous nanofiltration still in the Aluminate complex was bound, now also be separated, and the milling bath to be led back. This makes it possible, as the whole necessary Caustic soda to lead in the circuit, which is a significant in large-scale facilities economic and ecological effect.

In a further embodiment of the invention, in which the treatment of the complex by diluting with water and precipitating the aluminum as hydroxide takes place, the addition of water is controlled so that the resulting in the separation Filtrate compensates for the evaporation loss of the milling bath.

Milling baths are often at high temperatures, for example in the range of 75-80 ° C guided, so that a considerable amount of the water of the milling bath gradually evaporated. By adding a correspondingly dilute filtrate from the Processing process of Aluminatkomplexes nanofiltration corresponding amounts of water be fed, which together with the caustic soda through the membrane of the Nanofiltration pass and thus for a sufficient tracking of water to the milling bath.

In a further embodiment of the invention, the milling bath, the mixture of dissolved complexed aluminum ions and milling medium either running or taken in batches. Depending on the size of the system and the control technology Conditions, the mixture can be removed accordingly.

In a further embodiment of the invention is the mixture of lye and Aluminate complex at aluminum bath concentrations in the range of 5-20 g / l Taken from aluminum. This measure has the advantage that the bath at extremely low concentrations of aluminum is operated. At extremely low Aluminum concentrations in the bath is practically pure caustic soda, the so Concentrated and aggresive is that, for example, when immersing very large Sharing no uniform etch or milling result can be achieved because the first immersed areas are longer and therefore milled more than the later submerged areas. At 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 precipitations take place.

In a further embodiment of the invention, the concentration of alkali is in the Milling bath in the range of 110-150 g / l sodium hydroxide. It will be at the Nanofiltration resulting sodium hydroxide continuously recycled so that in this In favorable areas the concentration is kept, so that an optimal and defined, 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 a concentration of aluminum up to 100 g / l takes place.

Concentrations in these orders of magnitude create a supersaturated Solution of aluminate complex, which is accessible for economic processing. In This solution already spontaneously crystallization processes take place.

It is understood that the above and the still to explanatory features not only in the specified combinations, but also in Other combinations or alone can be used without the scope of to leave the present invention. The invention will be described below with reference to some Embodiments in conjunction with the accompanying drawings closer described and explained.

Fig. 1

schematisch eine Anlage mit der das erfindungsgemäße Verfahren betrieben wird, und

Fig. 2

ein Diagramm, bei dem die Konzentration an Fräsmedium gegenüber der Konzentration an gelöstem Aluminatkomplex aufgetragen ist, zur Erläuterung der Verfahrensführung.

Show it:

Fig. 1

schematically a plant with which the inventive method is operated, and

Fig. 2

a diagram in which the concentration of milling medium is plotted against the concentration of dissolved aluminate complex, to illustrate the process.

In Fig. 1, a milling bath 10 can be seen, in which an aqueous solution of sodium hydroxide is contained as a 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 already escape considerable amounts of water vapor 12.

In the milling 10 metal workpieces are immersed in aluminum, For example, large-scale curved aircraft fairing elements to an extent of several meters.

The aluminum of the workpiece reacts with the caustic soda to form hydrogen and a soluble sodium aluminate complex, NaAl (OH) ₄ .

In the retracted state of the plant is already at a bath concentration of about 10 g of aluminum per liter of bath liquid taken this and a nanofilter 14th fed, in which a nanofiltration is performed.

The nanofilter 14 is constructed so that an inner tube has a tubular support structure has, in which a membrane is placed. This pipe is from another pipe surround.

Such nanofilters are used as modular units, as for example under the Description SelRO Tubula Modul TM 1228 from Membrane Products Kiryat Weizmann Ltd., Rehovot / Israel. The inserted membrane has the Specification MPT-34.

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

Such a module is guided in the cross-flow principle, i. the milling bath solution becomes, over one not shown here pump with a pressure of 1 - 6 MPa applied and in one Direction through the inner tube at a speed of 1 - 2 m / s out. Under In these conditions, sodium hydroxide and water molecules pass through the membrane radial direction and will be converted into the so-called NaOH permeate, the the milling bath 10 is recycled. The impoverished in water and sodium hydroxide Milling bath solution is concentrated after leaving the nanofilter 14 of aluminate complex. The conditions are chosen so as to concentrate in the range of about 50-100 g / l of aluminum.

This liquid, referred to as Al concentrate, can now be subjected to a treatment process be supplied either on site or at a company that uses aluminum can be performed.

The Al concentrate is diluted according to the known Bayer method with water, and optionally with the addition of appropriate germs, the resulting aluminum hydroxide Al (OH) _{3 is} precipitated and centrifuged and fed as Al product, for example, the aluminum production.

The filtrate remaining in the separation of the precipitated Al (OH) ₃ by filtration or centrifugation contains the sodium hydroxide solution formed during the decomposition of the sodium aluminate complex and, if appropriate, still dissolved unsplit sodium aluminate complex. This filtrate can be fed again to the nanofilter 14, where it is converted into the NaOH permeate and returned to the milling bath 10.

Thus, it is possible in large-scale systems on site, the sodium hydroxide (NaOH) in to lead a closed cycle. Considering that at an aircraft manufacturer About 1,200 t of sodium hydroxide per year are used in milling baths, the economic will be Dimension of recovery and recycling of caustic soda significantly.

On the basis of the diagram of Fig. 2, the advantageous process control is even clearer seen. The milling bath 10 of Fig. 1 is operated so that the concentration of Sodium hydroxide solution is about 120 g / l, the concentration of dissolved aluminum in about Range of 10 g / l. Through nanofiltration and continuous recycling incurred NaOH permeate these bath conditions can be maintained permanently become. In this area, optimum etch or milling results are to be achieved, i. the Milling speed is not so high that large parts during the dipping process are milled differently, but the milling speed is still sufficient and desirable high.

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

It is first carried out a dilution step with water, so that this solution correspondingly relative to sodium and aluminum (in the form of the dissolved aluminate complex) impoverished.

Then, by adding germs, the aluminum hydroxide is precipitated and the precipitated aluminum hydroxide filtered off or, for example, centrifuged. The resulting Filtrate contains in addition to residual amounts of the still uncleaved Natriumaluminatkomplexes Caustic soda, which has formed during the cleavage of the complex, that is, it will be again previously bound caustic soda free.

This filtrate is fed back to the nanofiltration, it is controlled so that the Sodium hydroxide permeate is diluted so that the evaporation losses of the Milling baths are compensated.

The described dilution step is therefore carried out first, since a direct Splitting the highly concentrated aluminate solution to a sodium hydroxide concentration in the filtrate, which is much 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 caustic soda solution contained in the milling and recovered the Circuit are returned, so that ultimately no sodium hydroxide is consumed.

It is obvious that this method is very easy to grasp control technology, All you need to do is measure the aluminum concentration and then either continuously or batchwise, the mixture be removed from the milling.

In Fig. 2 a conventional bath guide is shown with the dashed sawtooth curve.

If you start at the upper left end of the sawtooth curve, it can be seen that at Bath concentrations with about 150 g of sodium hydroxide per liter of solution is assumed. By reaction with the aluminum workpiece, the concentration of sodium hydroxide solution decreases and the concentration of dissolved aluminum to, for example, to a value in Range of 20 g / l aluminum. By external addition of sodium hydroxide to the milling bath increases its concentration again leaps and it goes on the milling process, where Caustic soda is consumed again and continuously the concentration of aluminum in the milling bath increases. When reaching a concentration of 30g of aluminum per liter of milling bath is again Sodium hydroxide added, in this state, such a bath is already as a bad bath called It can then be approached to the saturation curve and thereby at these conditions, a maximum concentration of almost 50 g / l Aluminum can be achieved. But there is already the danger that precipitations take place, since it is already working very close to the boundary of saturation. It is It can be seen that the control over the sawtooth curve considerable metrological Requires equipment that are much more expensive than in the detection according to the method of the invention.

With the described bath guide can work at each optimal Milling conditions doubling the removal rate of about 1 mm per hour about 2 mm per hour of material thickness can be achieved.

Furthermore, a significant reduction in downtime for bath renewal is found in the operation according to the sawtooth curve is about 30% downtime, namely to replace the entire bath and replace it with a fresh, remove the bottom body from Already incurred Al (OH) ₃ and order Add sodium hydroxide solution cyclically.

Due to the exact operation in ideal conditions, it is possible to be very close to the optimum depth of cut was added to the control system, at most 90% of them had previously been used to achieve optimal milling depth, whereby iterative measuring methods are performed had.

Claims (10)

1. Method for operating aluminium milling baths,

   a) in which an aluminium workpiece which is to be milled is immersed in a milling bath containing an alkaline milling medium,

   b) a chemical reaction between aluminium and milling medium oxidizes the aluminium and converts it into a soluble aluminate complex,

   c) the mixture formed is subjected to a separation step, by means of which excess milling medium is separated from the aluminate complex,

   d) the milling medium which has been recovered is reused for operating milling baths, and

   e) the aluminate complex is subjected to a treatment process,
   characterized

   f) in that a mixture of dissolved complexed aluminium ions and milling medium is removed from the milling bath at a concentration of dissolved complexed aluminium ions which is well below its saturation limit,

   g) in that the separation step is a nanofiltration step in which the milling medium is separated from the mixture using the reverse osmosis principle,

   h) with the residue of complexed aluminium ions being concentrated at the same time.
2. Method according to Claim 1, in which during the treatment of the aluminate complex milling medium is released again, characterized in that the resulting solution is likewise fed to the nanofiltration step.
3. Method according to Claim 2, characterized in that the treatment of the aluminate complex involves diluting with water and precipitating and separating off the aluminium as a hydroxide, and the addition of water is controlled in such a way that the filtrate which results during the separation compensates for evaporation losses from the milling bath.
4. Method according to one of Claims 1-3, characterized in that the mixture of dissolved complexed aluminium ion and milling medium is removed continuously or batchwise from the milling bath.
5. Method according to one of Claims 1-4, characterized in that the mixture of lye and aluminate complex is removed from alkaline milling baths for dissolution purposes at Al bath concentrations in the range from 5-20 g/l.
6. Method according to Claim 5, characterized in that the concentration of lye is kept in the range from 110-150 g/l.
7. Method according to one of Claims 1-6, characterized in that the nanofiltration step is carried out in such a way that aluminium is concentrated up to 100 g/l.
8. Method according to one of Claims 1-7, characterized in that the nanofiltration step is carried out at pressures in the range from 1-6 MPa.
9. Device for carrying out the method according to one of Claims 1-8,

   a) having a milling bath (10),

   b) having a device for removing and separating the mixture formed in the milling bath (10),

   c) having a device for returning the milling medium which has been separated off into the milling bath,

   d) having a device for treating the mixture residue which has been separated off,

   e) the separation device being a nanofilter (14), in which the milling medium can be separated from the mixture using the reverse osmosis principle,
   characterized

   f) in that the nanofilter is configured in such a manner that it retains aluminate complexes which are formed when an aluminium workpiece reacts with an alkaline milling medium.
10. Device according to Claim 9, characterized
    m) in that the nanofilter is configured as a membrane tube which is surrounded by a second tube for returning the permeate.

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