EP2352859B1 - Pickling method and pickling plant - Google Patents

Description

The invention relates to a pickling process for workpieces made of aluminum or aluminum alloys and a pickling plant suitable for such a pickling process.

Prior to electrolytic oxidation of aluminum or aluminum alloy workpieces or prior to further processing of such workpieces, surface preparation is required. In this surface treatment, for example, the workpieces are degreased, pickled, rinsed and dekapiert. By pickling a uniform matte workpiece surface is to be achieved, with surface defects, such as drawing depths are compensated. Such a matt, comparatively decorative surface is also referred to as an E6 finish in accordance with DIN EN 12373. With lower requirements for the external appearance of only existing oxide layers are removed, with the above-mentioned drawing or pressing grooves are largely retained - such a surface quality is also referred to as E0 finish.

The surface pretreatment by pickling takes place predominantly in alkaline baths, for example in sodium hydroxide solution. In such a pickling process, the aluminum reacts on the surface of the workpiece with the sodium hydroxide solution according to the formula:

2Al + 6H ₂ O + 2NaOH → 2NaAl (OH) ₄ + 3H ₂ ↑

The aluminum goes into solution, which does not precipitate as Al (OH) ₃ in the presence of complexing agents and free NaOH. A certain concentration of aluminum in the pickling solution is helpful to slow down the pickling process, as it can provide a high quality and matte finish.

Such a long-term stain is in equilibrium if the proportion of the aluminum going into solution corresponds approximately to the mass fraction which is removed from the pickling bath by the workpieces during removal - ie in this state of equilibrium the aluminum content of the pickling bath remains approximately constant. Such a Equilibrium state, for example, at an aluminum concentration of about 140 to 180 g / l aluminum. Among other things, the pickling quality is adversely affected by the fact that different heavy metals, silicates, etc. accumulate in the pickling solution - depending on the alloy content of the various workpieces. In this case, an increased content of zinc has a particularly negative effect, since the zinc precipitates at the grain boundaries and forms so-called "zinc flowers", which adversely affect the surface quality. To improve the pickling process, pickling additives are added to the pickling solution. Through this, for example, the flow behavior of the pickling solution can be improved by the workpiece so that less pickling solution is removed when transferring the workpieces from the pickling solution in a subsequent rinse. Other pickling additives, for example sulfides, form metal sulfides which are sparingly soluble with the heavy metals and precipitate out as sludge in the bath.

In order to prevent precipitation of aluminum hydroxide, complexing agents such as gluconate, sorbitol or other derivatives from sugar chemistry are added in such pickling baths, can be prevented by the precipitation of aluminum hydroxide.

In addition, in the alkaline pickling solution, insoluble elements such as copper, silicon, etc., remain in the mud. In conventional systems, the sludge obtained in the pickling bath, which is colored black due to the alloy constituents (for example Si, Zn, Mn and Cu compounds), is withdrawn from the plant at regular intervals, for example twice a year, and disposed of. In addition, the alkaline pickling is also removed in order to prevent an excessive increase in the Al concentration and to compensate for losses caused by carry-over (NaOH, additives).

In the US Pat. No. 3,607,482 an aluminum pickling method is disclosed in which the density of a pickling solution is measured by means of a density meter and in the case where this density exceeds a limit, a partial amount is supplied to a precipitating container. In these, solids are precipitated by supplying a precipitating agent and Permeate remaining in the precipitation vessel is then returned to the pickling bath.

In the US Pat. No. 2,975,041 For example, a treatment method for a pickling solution is described with the aim of keeping the aluminum concentration in the pickling solution within a predetermined range. For this purpose, a subset of the pickling solution in centrifuges is partially freed of solids and then cooled to precipitate aluminum hydroxide and corresponding alloying ingredients. The remaining clear phase is freed of solids in a further process step and fed to a tank in which the concentration of sodium aluminate can be adjusted by adding an aluminum hydroxide-rich filter cake obtained during the filtration.

Aluminum hydroxide is thus precipitated in both of the above-described known processes, with alloy constituents, such as, for example, zinc, being precipitated.

In the DE 100 54 128 A1 discloses a method in which the pickling liquid is mixed with a rinsing liquid of a subsequent to the pickling stationary rinse and processed with the addition of sodium hydroxide solution to a sodium aluminate solution, which can be used for example in a sewage treatment plant for phosphate precipitation, It is thus a pure recovery process that creates an intermediate from the "spent" pickling liquor.

From the EP 0157 190 B1 and under the product name "Epal®" a process is known in which a specific equilibrium between free sodium hydroxide, dissolved aluminum, crystallized aluminum hydroxide and bath stabilizer is produced in the pickling bath. Aluminum hydroxide accumulating in this process is filtered off and washed out and sent for further utilization. In this Epal process no additives are added to precipitate the heavy metals and the heavy metals (eg zinc) remain mostly in solution. This has a negative influence on the pickling result (coarse grain formation by grain boundary separation). A precipitation of the heavy metals would result in this process, that the precipitated in the bath and subsequently obtained by solids separation aluminum hydroxide would be contaminated with heavy metals - the desired utilization of Aluminimhydroxids would be severely limited. Pickling additives are not mandatory in this process because precipitation of aluminum hydroxide is desired. The aluminum then precipitates at low concentrations as aluminum hydroxide - as stated - such a low aluminum content in the pickling bath, however, deteriorates the surface quality, so that the cost savings by the accumulation of aluminum hydroxide and / or sodium hydroxide at the expense of surface quality.

In the GB 1 340 505 discloses a method by which the washing liquid used in the above-described stationary rinse can be prepared. With regard to the problems described above for pickling solution, this document does not contain any proposed solutions.

The DE 40 15 141 A1 discloses a process in which a pre-treatment bath - ie a pickling bath - continuously fresh pickling liquid is supplied and a corresponding amount of "spent" pickling liquid withdrawn and fed either a supply or a further processing.

Even with such a solution, the resulting, heavy metal-containing sludge must be removed at regular intervals and, furthermore, a considerable effort for further processing or disposal of the withdrawn pickling liquid is required.

In contrast, the invention has for its object to provide a pickling bath and a pickling plant, which allow a long-term pickling with minimal effort.

According to the solution of this problem by a pickling with the features of claim 1 and a pickling plant with the features of the independent claim 12th

According to the invention, a continuous and discontinuous preparation and concentration adjustment of a pickling solution of a pickling bath are provided in the pickling process for workpieces made of aluminum or aluminum alloys. For this purpose, a subset of the pickling solution is withdrawn continuously or in sections from the pickling bath, and solids are separated from the withdrawn subset by means of a solids separation / solids separation.

The composition of the remaining, low-solids permeate taken up in a receiver tank and the actual composition of the pickling solution of the pickle bath are analyzed and a permeate target composition adjusted by adding pickling solution and additives depending on the result of this analysis. From this permeate in the feed tank then permeate is fed back into the pickling bath to keep the pickling solution contained therein to a predetermined target composition. Due to the quasi-continuous extraction of solids from the pickling solution and precipitation of the heavy metals outside the pickling bath, a seepage of the sludge at the bottom of the pickling bath is reliably prevented, so that a long-term pickling with high efficiency is made possible.

With this approach, one turns away from conventional solutions in which a withdrawn from the pickling bath subset of the pickling solution disposed of or separately prepared and possibly had to be replaced by fresh pickling solution (sodium hydroxide plus additive).

The pickling plant according to the invention for carrying out the pickling process according to the invention is correspondingly carried out with a pickling bath which has a process for discharging a partial amount of a pickling solution from the pickling bath. The pickling plant also has a solids separation in which the heavy metals described in the form of solids removed from the stripped pickling solution become. The low-solids permeate is temporarily stored in a storage tank. The pickling plant also has an analysis for analyzing the composition of the low-solids permeate received in the receiving container.

This analysis can also be used to record the composition of the pickling solution of the pickling bath. Depending on the actual composition of the pickling solution and the permeate buffered in the storage container, pickling solution (for example sodium hydroxide solution) and additives are then supplied by means of a metering unit in order to adjust a composition of the permeate in the storage container which is dependent on the actual composition of the pickling solution. In conventional processes, the pickling solution is frequently fed to a special chemistry in which all the necessary pickling additives are contained. Such a procedure has the disadvantage that even if there is a shortage with a single pickling additive, the special chemistry consisting of a multiplicity of pickle additives is metered in, so that the other pickling additives are rather overdosed. In the inventive concept, a targeted adjustment by addition of the individual pickling additives and sodium hydroxide solution, so that such overdose is avoided.

The permeate taken up with a defined target composition is passed from the feed tank into the pickling bath in order to keep the pickling solution at a predetermined desired composition and to compensate for volume losses or to keep the Al content constant.

Volume losses can also be compensated by a downstream EO stain, floor sink or 1st sink. This bath can be integrated into the process according to the invention, wherein the filling of the sedimentation tank does not have to take place directly from the pickling bath 1. In this case, solid is separated, the concentration determined, metered accordingly and filled the pickling bath.

In one embodiment, the separation of solids takes place by means of a membrane filtration and subsequent sedimentation in the sedimentation tank. Of course, other suitable solid separation processes can be used with which heavy metals in the form of solids can be separated from an alkaline or acid pickling solution.

In a particularly preferred variant, the solids separation contains a dynamic filtration, preferably a micro- or ultrafiltration.

The retentate produced during the filtration can be circulated through the filter in a variant according to the invention in the batch mode until the retentate has reached the desired concentration (for example 10 times). In this case, the sub-quantity withdrawn from the pickling bath can be added to the circulated volume flow. Alternatively, instead of the batch operation, a continuous procedure (feed and bleed) can be selected.

The concentration preferably takes place in the sedimentation tank - as soon as the desired concentration is reached, the membrane filtration is terminated, so that flocculation and sedimentation of the undissolved heavy metal compounds takes place in the form of solids in the sedimentation tank.

The resulting in this sedimentation clear phase is subtracted in a variant of the pickling process according to the invention and fed to the storage container. According to the invention, it is preferred if the solids concentration of this clear phase corresponds approximately to the concentration of the permeate from the first filter operation.

The turbidity phase (sediment) obtained during the sedimentation is withdrawn from the sedimentation tank and fed to a solids separator, for example a static filter, and the filtrate, which has largely been freed of solids, is then fed to the feed tank. This filtrate also has about the same solids concentration of the aforementioned permeate and the aforementioned clear phase. The accumulating on the static filter cake, which has a high content of heavy metals, is preferably disposed of as waste. Through this downstream filter, the waste volume can be reduced to a minimum.

This addition of additives / caustic soda takes place as a function of the actual composition of the pickling solution taken up in the pickling bath and / or of the aluminate concentration in the feed / metering tank or of the desired concentration of an aluminate intended for further use. This composition is determined in a preferred embodiment after removing the solids, or from the permeate.

In a preferred variant of the invention, the solids separation takes place approximately at the temperature of the pickling bath, so that a correspondingly high volume flow is adjustable. The storage of the concentrated permeate solution takes place at a low temperature.

Alternatively, to achieve a minimum heavy metal concentration in the permeate, the pickling solution can also be cooled, taking advantage of the fact that the residual solubility of the sparingly soluble heavy metal compounds decreases with the temperature.

In a variant of the pickling process according to the invention, for example, in the case where the aluminum concentration in the feed tank or in the pickling solution is too high, a partial stream is withdrawn from the feed tank and fed to a collecting tank. This highly concentrated aluminate solution can then be recycled. As stated above, the aluminate solution can be optimized by the addition of NaOH or additives such as complexing agents for further processing.

The entire process and also the pickling plant can be designed as a closed system (reactor), in which case the process liquids are conducted from the feed tank to the reactor or a coating cell and not, as usual, the work piece or the "product" to the pickling solution ("Ware to chemistry ").

The method can also be used for pickling by spraying. The use of the low solids pickling solution prevents the nozzles from clogging.

According to the invention, the solids separation can be fed in addition to or as an alternative to the pickling solution removed from the pickling solution a solution from another process stage, for example an EO pickling.

In accordance with the method steps described above, the pickling plant in the area of solids separation is designed with a dynamic filter and a sedimentation tank. For concentration of the obtained at the filter retentate the pickling plant can be designed with a circulation pump for recycling retentate from the sedimentation to the inlet of the filter. About this circulation pump, the turbidity phase can be deducted from the sedimentation after sedimentation.

The pickling plant can furthermore be designed with a feed pump for conveying the clear phase arising during the sedimentation from the sedimentation tank into the feed tank.

The pickling plant is advantageously carried out with a filter over which the sedimentation occurring in the sedimentation (sediment) is freed from solids and the concentrated filtrate is then also fed to the reservoir.

This storage container can be made in two parts in a development of the invention, wherein in a permeate the above-described, resulting from the dynamic filtration, sedimentation and filtration volume flows (permeate, clear phase, filtrate) are cached. This permeate container is connected via a feed pump and a suitable valve device with a dosing. Permeate can be withdrawn from the permeate container via this feed pump and fed to the metering container. According to the online analysis performed, the composition of the permeate taken up in the dosing container is then adjusted to the desired composition by adding sodium hydroxide / additives. Permeate is then withdrawn from this template thus adjusted and fed to the pickling bath to set the desired concentration. That is, in this variant, the intermediate storage of the permeate and the setting of the dosing template are carried out separately.

In an embodiment according to the invention, the volume of the sedimentation tank approximately corresponds to the volume of the storage container, so that the storage container can also be filled with a filling of the sedimentation container.

As can be seen from the above explanations, in one embodiment of the invention, the sedimentation container can first be filled with retentate by circulation through the filter, wherein already permeate is branched off from the filter and fed directly into the feed container. After filling the sedimentation tank, the cycle to the dynamic filter is shut off and the sedimentation process begins. The resulting clear phase is fed to the feed tank, in which the metered addition of the sodium hydroxide solution (or other pickling solution) and the additives takes place in order to set a predetermined composition in the reservoir. By removing permeate from this storage tank, the concentration of the pickling solution in the pickling bath can then be kept constant, with losses and consumption of pickling solution being compensated.

The pickling plant according to the invention can be designed with a collecting container for receiving permeate withdrawn from the feed container. This highly concentrated, set, for example, with regard to the reuse aluminate can then be supplied from the cache of the mentioned further processing.

In the inventive concept, as described above, solution from another process step, for example an EO pickling or a first rinse, can also be fed to the solids separation. This solution can also - be fed directly to the pickling bath - similar to the prior art.

By including the EO pickle or rinse or first rinse, the target aluminum concentration in the aluminate, which is reused, can be adjusted independently of the desired concentration in the pickling bath. The condition is that the aluminum concentration in the pickling bath is greater than in the stabilized for further use aluminate. This is usually the case. Typically, an aluminum concentration of about 150 g / l in the aluminate solution is desired. The pickling bath usually has an Al concentration of 160-180 g / l.

• Figur 1 ein Anlagenschema einer erfindungsgemäßen Beizanlage und
• Figur 2 ein Ausführungsbeispiel einer Beizanlage mit zweiteiligem Vorlagebehälter.

Preferred embodiments of the invention are explained in more detail below with reference to schematic drawings. Show it:

• FIG. 1 a plant schematic of a pickling plant according to the invention and
• FIG. 2 An embodiment of a pickling plant with two-part storage tank.

As explained above, when anodising workpieces, they are first degreased and then pretreated in a pickling bath, wherein, depending on the quality of the pickling bath (E6 / E0), oxide layers are removed and surface irregularities caused by the production, for example extrusion, are eliminated. In order to prevent a pickling of pickling agent in subsequent processes for anodization of the workpiece, the pickling is followed by a rinsing bath, is removed in the adhering to the workpiece pickling solution. FIG. 1 shows the basic structure of a pickling plant 1, as used in a pickling process for pickling workpieces made of aluminum or aluminum alloys.

The actual pickling is carried out in a pickling bath 1, wherein alkaline pickling solutions can be used. In the described embodiment, as a pickling solution 2 sodium hydroxide solution is used to which the additives described above, for example, to prevent unwanted precipitation of aluminum hydroxide, for binding heavy metals, to improve the pickling behavior and to improve the flow behavior are added. Common additives include, for example, sodium gluconate, sodium sulfide, sodium thiosulfate or sodium nitrate. Further suitable additives can be taken from the prior art described at the outset. According to the composition of the pickling solution 2 is kept constant, wherein for the pickling detrimental solid fractions are continuously or discontinuously withdrawn. According to the invention, a partial amount of pickling solution 2 is withdrawn from the pickling bath 1, freed of solids via a solids separation / separation 4 and then fed to a feed tank 6 in which the permeate present after the solids separation by adding sodium hydroxide solution and additives by means of a metering unit 8 a predetermined composition is set, which composition is variable depending on the actual composition of the pickling solution during the pickling process. The analysis of the actual composition of the pickling solution and the composition of the permeate in the reservoir 6 via an analytic 10. Where before the above-described metered addition of permeate from the reservoir 6, this must first be filled with treated permeate. This step is explained below.

1. Fill the original container 6

About a pump 12 with the solids described above (heavy metal, sludge, etc.) laden pickling solution 2 is withdrawn from the pickling bath 1 and a valve assembly 14 of in FIG. 1 indicated by dashed lines solid separation 4 supplied. As in FIG. 1 indicated, can also be diverted via the valve assembly 14, a recirculation flow 16, which is returned to the pickling bath 1. The valve 14 can be variably adjusted so that the volume flow 3 can be divided flexibly (eg 100% bath circulation or other conditions, depending on the respective cycle (filling of 20, sedimentation in 20, etc.).) By suitable adjustment of the valve arrangement 14 Thus, the volume flow set via the pump 12 can be divided again.

When filling the storage tank 6 can be adjusted via the valve assembly 14, the pressure and the flow rate, which is the solids separation 4 is supplied. This consists in the illustrated embodiment of a dynamic filter 18 and a sedimentation 20. However, in principle, all other Feststoffabscheideverfahren can be used to separate solids from the pickling solution. For example, the separation of solids can be achieved by sedimentation alone, by centrifugation or by a static filter or by combination of known solid separation processes.

In the method of the invention, microfiltration is used wherein a membrane 22 is used to separate the solid. For the microfiltration principle, two operating modes are distinguished: the "cross-flow" and the "dead-end" flow control. In cross-flow filtration, the crude solution flows along the membrane surface, with only a relatively small portion passing through the membrane 22 as permeate. By means of such filtration, liquids with a high turbidity / solids content can be clarified. The concentrate or retentate 26 flowing along the membrane 22 is conducted into the sedimentation tank 20 and from there returned via a circulation pump 28 and a circulation valve arrangement 30 as circulation flow 32 to the input of the dynamic filter 18 and summed to the subset 3 withdrawn via the pump 12. This circulation of the retentate 26 concentrates this, so that its solids content increases continuously, since the permeate 24 is discharged into the feed tank 6. Once the desired concentration is reached, by switching the circulation valve assembly 30 and by switching off the circulation pump 28 and by switching the valve assembly 14 to circulation (or switching off the pump 12), the circulation of the retentate 26 is interrupted, so that in the sedimentation 20, the flocculation and the sedimentation process can begin. After sedimentation, settling in the sedimentation tank 20 clear phase 34 is withdrawn via a feed pump 37 from the sedimentation tank 20 and conveyed into the reservoir 6. The sedimentation present on the bottom side turbidity phase (sediment) 36 is withdrawn via the circulation pump 28 and the corresponding switched valve assembly 30 and fed to another filter, such as a static filter. In the illustrated embodiment, this static filter is designed as a chamber filter press 38, in which the solids contained in the turbidity phase 36 are retained as a filter cake. This filter cake 40 is then disposed of as waste or sent for further use. The freed from solids filtrate 42 then also flows into the feed tank 6. As already mentioned, instead of the described batch operation, a continuous flow through the sedimentation without recycling of the retentate (feed and bleed) can be selected.

In the described embodiment, the volume of the sedimentation tank 20 corresponds approximately to the volume of the receiver 6, so that after completion of sedimentation and deposition of the filter cake, the receiver tank 6 with permeate, i. filled with solids-free pickling solution is filled.

From time to time, cleaning of the membrane is required to prevent a decrease in permeate performance. This cleaning is in for simplicity FIG. 1 not shown. The cleaning can be done, for example, during sedimentation, wherein in the so-called CIP process [Cleaning in Place] first rinsed with water (neutral), then cleaned with acid and then rinsed again with water (neutral).

After the storage container 6 is now filled with permeate, the composition of the pickling solution 2 in the pickling bath 1 can be continuously adjusted or maintained at a desired value.

2. Setting the target composition of pickling solution 2

As in FIG. 1 indicated, a comparatively small amount of analysis 46 is withdrawn from the reservoir 6 and fed to the analysis 10 via a feed pump 44. About this can then the permeate in the reservoir 6, ie the proportion of aluminum, caustic soda and additives are determined. The analysis 10 is designed to be able to detect the individual components with sufficient accuracy. In this way, the actual composition of the permeate in the reservoir 6 is determined.

By briefly controlling the pump 12 and corresponding reversal of the valve assembly 14, a small proportion of pickling solution can be withdrawn from the pickling bath 1 and led to the filter 18. The permeate 24 penetrating the membrane 22 is then likewise fed to the analytics 10, so that the actual composition of the pickling solution 2 (freed from solids) is detected.

In the case in which the actual composition of the pickling solution 2 deviates from the desired composition, sodium hydroxide solution and additives are metered into the feed container 6 via the metering unit 8, so that a predetermined desired permeate composition is established. Permeate is then withdrawn from the feed tank 6 with the preset permeate target composition via the feed pump 44 and the corresponding reversed feed valve arrangement 48. This template volume flow 50 is then returned to the pickling bath 1, so that the pickling solution 2 is brought to the desired desired composition and any losses due to removal, evaporation etc. are compensated.

After the above-described solids separation, the solids content of the streams 24, 34 and 42 is substantially at the same low level near 0%. As soon as the filling level of the storage container 6 falls below a predetermined level, the pickling unit is switched back to the mode described above under 1. to fill the feed tank 6 with permeate / aluminate.

The additives metered in via the dosing unit 8 can also be adapted to the individual substance separation steps described above, so that, for example, optimized filtration or sedimentation is made possible.

In the described method, the temperature in the reservoir 6 is lower than the temperature of the pickling bath 1. Thus, the pickling bath 1, for example, be operated at a temperature of 70 ° C - the temperature in the reservoir 6 may then be for example about 40 ° C. In the filter 18 and also in the sedimentation tank 20 is the comparatively high temperature (70 ° C), so that a higher flow through the filter 18 can be realized due to this comparatively high temperature. Alternatively, the sedimentation tank 20 may be provided with a cooling 21 to cool the retentate 26 to aid in the precipitation of heavy metals.

As in FIG. 1 further indicated, via the feed pump 44 and the valve assembly 48, an unused volume fraction of permeate (consisting essentially of sodium aluminates) 52 withdrawn and fed to an industrial utilization become. Such sodium aluminate solutions are used for example for drinking water treatment, wastewater treatment or as a raw material for zeolite production. However, the withdrawal also takes place in order to prevent an excessive increase in the Al content in the bath and to keep the Al concentration within a defined target range.

The concentration of pickling solution 2 in pickling bath 1 can be kept at a defined composition by means of the method described above, the resulting solids being separated off continuously or quasi-continuously. Due to this minimum solids content in the pickling solution 2 and the avoidance of temperature and concentration fluctuations in the pickling solution 2, the risk of stone formation or the formation of sludge in the reservoir 6 or in the pickling bath 1 is practically nonexistent. Since no sludge accumulates during operation, expensive cleaning work and the concomitant shutdown of the system can be omitted.

The process described above also opens up the possibility of recovering caustic soda from the pickling or permeate solution and reuse in the pickling process.

Because of the analysis, which is integrated in the process, continuous or section-wise, by means of the analysis 10, elaborate manual analysis methods, as required in the conventional solutions described at the outset, for example a manual titration, are not required.

In the method described above, the permeate 24, the clear phase 34 and the filtrate 42 are introduced together into the reservoir 6. The metered addition to the adjustment of the original concentration also takes place in this storage tank 6.

In FIG. 2 an embodiment of a system according to the invention is shown, in which the storage container 6 is designed in two parts with a permeate 54 and a dosing 56. These two containers 54, 56 are connected to each other via a feed pump 58 and a feed line 60, so that the dosing 56 is filled via the feed pump 58 and the feed line 60 from the permeate 54 can be. The permeate tank 54, the permeate 24, the clear phase 34 and the filtrate 42, ie, the low-solids fractions from the solids separation fed.

The setting of the permeate target composition (dosing) is carried out by metering in caustic soda and / or other suitable additives, such as complexing agents, inhibitors, precipitants for heavy metals, optionally from additive containers 62, 64, 66, 68 via a Dosierventilanordnung 70 and a metering 72nd be supplied. By appropriate activation of the metering valve arrangement, depending on the analysis, sodium hydroxide solution or one or more of the additives can be added selectively in order to adjust the desired composition.

Alternatively, the metering from the additive containers 62, 64, 66 and 68 can also take place via individual metering pumps and metering lines, so that the additive containers are each connected to the metering container 56 via their own line and metering pump. This eliminates the metering valve assembly 70th

In the FIG. 2 illustrated variant has the advantage that the filling of the permeate tank 54 and the metered addition of the original volume flow 50 from the metering 56 into the pickling bath 1 for setting the desired Beizlösungszusammensetzung can be done in parallel, while in the embodiment described above, the filling and metering takes place in sections.

In the case where the pickling solution contains too high a concentration of aluminum / aluminate, aluminate should be removed to avoid precipitation. In the illustrated embodiment, this is done via the feed valve assembly 48, deducted over the concentrated permeate / aluminate 52 from the dosing 56 and a collecting 74 is supplied - so that the aluminate concentration in the dosing 56 and / or in the pickling solution 2 is lowered. The aluminate solution received in the collecting container 74 can be circulated via a circulating pump 76 and a circulating line 78. By means of a trigger valve assembly 80 aluminate can be withdrawn from the sump 74 and fed to a recycling.

A special feature of this variant of the method is that the concentration of the aluminate / permeate received in the dosing container can be detected via the analytics 10 and a targeted target concentration can be adjusted by suitable control of the dosing valve arrangement 70, which then - for example not with regard to the pickling process - is optimized for reuse. In other words, when the aluminate is withdrawn from the dosing tank 56, the aluminate concentration can be deliberately adjusted to a concentration by addition of sodium hydroxide solution and additives, for example complexing agents, which ensures optimal reuse with a stabilized solution and thus is also of interest to the company which is decreasing the aluminate.

FIG. 2 contains yet a further embodiment of the method according to the invention or the system according to the invention.

In the embodiments described above, pickling solution 2 is fed to the solids separation 4 and the resulting low-solids fraction (solids content almost 0%) is then fed to the one- or two-part feed tank 6, in which the desired concentration is set and from which the permeate feed is then fed to the pickling solution 2 becomes.

FIG. 2 shows a variant, of course, in the system diagram according to FIG. 1 is applicable and in which a solution from another upstream or downstream process stage, such as an EO stain 82 or a first sink (see above-mentioned prior art) via a feed pump 84, a Zuführventilanordnung 86 and a supply line 88 downstream of the valve assembly 14th the material flow is supplied. That is, the original volume flow 50 not only consists essentially of a subset of the pickling bath 1 but can partially or completely - depending on the setting of the valve assemblies 14 and 86 and the other process step, for example, the EO pickling 82 are removed. In principle, it is also possible to feed the pickling bath 1 directly from the EO pickling 82 via the feed valve arrangement 86. Such a feed is in the plant scheme according to FIG. 2 indicated with the direct feed line 90. Of course, the two valve assemblies 14, 86 also to a single valve assembly be summarized. Depending on the concentration of the E0 pickling solution, the supply line 88 may also open before or after one of the individual solid separation steps.

According to the above explanations, a heavy metal precipitation takes place within the pickling bath 1. In one variant, this precipitation can also take place outside the pickling bath 1. For example, it is possible to carry out the heavy metal precipitation in the sedimentation tank 20 (thickener), in which case the required amount of precipitant / sodium sulfide is first determined via process analysis 10 and then metered in from containers 92, 94. As a result, an overdose of sodium sulfide can be excluded. In this case, the additive container containing the precipitant (for example the additive container 66) will then be connected to the sedimentation container 20 via the metering valve arrangement 70.

The permeate 24 can - as in FIG. 2 indicated - even with the bypass of the analysis 10 via a line 96 and a valve assembly 98 directly into the storage container 6 ( FIG. 1 ) or the permeate container 54 (FIG. FIG. 2 ).

• Aufgrund des Löslichkeitsproduktes schwer löslicher Metallsulfide bzw. Schwermetallverbindungen ist bei niedrigeren Temperaturen eine vollständigere Fällung möglich. Die Konzentration an gelösten Schwermetallen kann dadurch gegenüber herkömmlichen Verfahren deutlich verringert werden;
• die Fällung kann gezielt erfolgen, eine Überdosierung des Fällmittel ist nicht erforderlich;
• die Auswahl an Schwermetallfällmitteln ist bei der herkömmlichen Fällung im Bad begrenzt, da Störionen den Beizprozess negativ beeinflussen können. Die Auswahl von Fällmitteln ist bei der Fällung im Beizbad darüber hinaus noch begrenzt, da dieses negativen Einfluss auf den Beizprozess haben kann. Außerdem begrenzt die Gefahr einer Verschleppung in nachfolgende Spülbäder und die Möglichkeit der Kontaminierung im Abwasser die Auswahl des Fällmittels. Die Sulfidkonzentration ist laut WHG, Anhang 40 begrenzt auf 1 mg/l. Dieses ist bei Fällung im Beizbad (durch Zugabe von Additivgemisch) nicht einzuhalten, da eine Überdosierung erfolgt und die ausgeschleppte Menge entsprechend hoch ist;
• eine Entstehung von Schwermetallschlämmen im Beizbad ist bei der erfindungsgemäßen Lösung nicht zu befürchten. Dadurch entfallen aufwendige Reinigungsarbeiten im Beizbad, des weiteren wird die Viskosität reduziert und dadurch das Spülverhalten verbessert. Eine Verschleppung von Schwermetallen bzw. Schwermetallsulfiden in nachfolgende Spülbäder tritt nicht auf.

For the sake of good order, the advantages of the invention over the prior art are again emphasized:
The inventive method makes it possible for the first time to reduce the dissolved heavy metal concentrations in the stain as well as in aluminate to a minimum, which is not possible in a precipitation in the pickling bath, as is the case in the prior art, due to a temperature dependence. The precipitation of heavy metals outside the pickling bath has the following advantages:

• Due to the solubility of poorly soluble metal sulfides or heavy metal compounds, a more complete precipitation is possible at lower temperatures. The concentration of dissolved heavy metals can be significantly reduced compared to conventional methods;
• the precipitation can be targeted, an overdose of the precipitant is not required;
• the selection of heavy metal precipitants is limited in conventional precipitation in the bathroom, since interfering ions can adversely affect the pickling process. In addition, the selection of precipitants during precipitation in the pickling bath is still limited, since this can have a negative influence on the pickling process. In addition, the risk of carryover into subsequent rinsing baths and the possibility of contamination in the wastewater limits the choice of precipitant. The sulfide concentration is limited to 1 mg / l according to WHG, Appendix 40. This can not be adhered to when precipitating in the pickling bath (by adding additive mixture), since overdosing takes place and the amount removed is correspondingly high;
• a formation of heavy metal sludge in the pickling bath is not to be feared in the inventive solution. This eliminates expensive cleaning work in the pickling bath, the viscosity is further reduced and thereby improves the flushing. Carryover of heavy metals or heavy metal sulfides into subsequent rinsing baths does not occur.

Disclosed are a method for treating workpieces of aluminum or aluminum alloys and a system that can be operated by such a method. According to the invention, a partial amount of a solution is withdrawn from a bath for the treatment of the workpieces, freed from solids and fed back to the bath at a predetermined concentration and a predetermined volume flow.

LIST OF REFERENCE NUMBERS

1

pickling bath

2

pickling solution

3

subset

4

Solid separation

6

storage container

8th

dosing

10

analytics

12

pump

14

valve assembly

16

circulation

18

dynamic filter

20

sedimentation

21

cooling

22

membrane

24

permeate

26

retentate

28

Circulation pump

30

Circulation valve assembly

32

circulating current

34

clear phase

36

cloudy phase

37

feed pump

38

Chamber filter press

40

filter cake

42

filtrate

44

template pump

46

analysis amount

48

Template valve assembly

50

Templates Flow

52

aluminate

54

permeate

56

dosing

58

feed pump

60

feeder

62

additive tank

64

additive tank

66

additive tank

68

additive tank

70

metering valve

72

metering

74

Clippings

76

circulating pump

78

circulation line

80

Vent valve arrangement

82

1. sink or EO-stain

84

feed pump

86

Zuführventilanordnung

88

feed

90

direct supply

92

container

94

container

96

management

98

valve assembly

Claims (15)

1. Pickling method for workpieces made of aluminium or aluminium alloys, which are supplied for stripping to a pickling bath (1) comprising an alkali pickling solution (2), comprising the steps:

   - continual or section-by-section removal of a partial amount (3) of the pickling solution (2) from the pickling bath (1);

   - separation of heavy metals from the partial amount (3), wherein a permeate (24) is supplied to a storage container (6);

   - analysis of the permeate (24; 46), that is low in heavy metals, contained in the storage container (6) and of the actual composition of the pickling solution (2) of the pickling bath (1);

   - supplying pickling solution (2) and additives for adjusting a permeate nominal composition as a function of the actual composition of the pickling solution (2) and the composition of the permeate (24) stored temporarily in the storage container (6) and

   - returning the permeate store (50) having a defined composition from the storage container (6) into the pickling bath (1) for adjusting a nominal composition of the pickling solution (2).
2. Pickling method according to claim 1, wherein the heavy metal separation (4) includes filtration (18) and subsequent sedimentation in a sedimentation container (20).
3. Pickling method according to claim 1 or 2, wherein a clear phase (34) occurring during the sedimentation in the sedimentation container (20) is supplied to the storage container (6).
4. Pickling method according to claim 3, wherein the retentate (26) occurring during filtration is supplied to the sedimentation container (20) and is supplied from the latter in the circuit to the filter (18) and totalled up to the partial amount (3).
5. Pickling method according to claim 3 or 4, wherein during the sedimentation in the sedimentation container (20) a cloudy phase (36) with a high content of heavy metal is removed and supplied to a heavy metal separator, a static filter (38).
6. Pickling method according to claim 3, 4, or 5, wherein the filtrate (42) is supplied to the storage container (6) and the filter cake (40) is supplied for waste disposal or further use.
7. Pickling method according to any of the preceding claim, wherein as a function of the analysis caustic soda for adjusting the nominal permeate composition is added to the permeate (24, 34, 42) contained in the storage container (6).
8. Pickling method according to claim 7, wherein heavy metals are separated from the pickling solution (2) prior to the analysis of the actual composition of the pickling solution.
9. Pickling method according to any of the preceding claims, wherein a portion of the permeate is removed and supplied to a collecting container (74).
10. Pickling method according to any of the preceding claims, wherein the latter is performed in a closed system.
11. Pickling method according to any of claims 2 to 10, wherein the filtration (18) includes micro or ultrafiltration.
12. Pickling plant for performing the pickling method according to any of the preceding claims, comprising a pickling bath (1), an outlet for removing a partial amount (3) of a pickling solution (2) of the pickling bath (1), a heavy metal separation (4), a storage container (6), analytics (10) for analysing the composition of the permeate admitted to the storage container (6) and the actual composition of the picking solution (2), with a metering unit (8) for supplying additives/pickling agents as a function of the result of the analysis and with a return of storage volume flow (50) into the pickling bath (1).
13. Pickling plant according to claim 12, wherein the heavy metal separation (4) has a dynamic filter (18) and a sedimentation container (20).
14. Pickling plant according to claim 13 comprising a circulation pump (28) for returning retentate from the sedimentation container (20) to the filter inlet and/or a feed pump (37) for conveying a clear phase (34) from the sedimentation container (20) to the storage container (6).
15. Pickling plant according to any of claims 12 to 14, comprising a filter (38) for filtering the cloudy phase (36) occurring in the sedimentation container (20) and a supply line for supplying a filtrate (42) that is free of heavy metals to the storage container (6).

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