EP1521864B1 - Method and device for recycling metal pickling baths - Google Patents

Abstract

The invention relates to a method for recycling metal pickling baths, including rinsing baths and air washers. Said method is characterized in that a) before the recycling process, free acids in the liquid waste flows to be treated are converted into a metallic salt form, b) water is separated from the largely acid-free metallic salt solution produced in order to obtain a concentrated metal salt solution, and c) the concentrated metal salt solution is subjected to a thermal method in order to obtain metal oxides and free acids. The invention also relates to a corresponding device. The inventive method and device enable the degree of acid recovery and also the production of metal oxides to be significantly increased, with lower operating costs.

Description

Conventional metal pickling baths are operated on the basis of nitric, hydrofluoric and / or hydrochloric acid. In addition to the economic aspects, the problem of these pickling is an undesirably high amount of nitrate in the wastewater to be treated. To reduce this nitrate pollution pickling with replacement acids for nitric acid, z. For example, sulfuric acid with a greatly reduced proportion of nitric acid, but which are extremely disadvantageous from the pickling quality and capacity.

Furthermore, recycling plants are used for separating free acids and salts, such as acid regeneration and diffusion dialysis, in order to reduce the nitrate load in wastewater by recovering the free acids and thus also to make the disposal of waste acids more economical. The resulting savings in acids are considerable, but do not really solve the actual nitrate problem, as continue to large amounts of nitrate wastewater are produced by the nitrate salts. When using acid recycling plants, the majority of nitrate wastewater pollution no longer comes from the pickling baths, but from the associated rinsing baths and exhaust air scrubbers, which are not recycled.

A proposal for the treatment of spent pickling acid, for example, from the doctrine of DE 38 25 857 A1 according to which the spent pickling acid with a certain iron content and fluoride / iron ratio with alkali is adjusted to pH 4 to 6 to form a crystalline precipitate and the liquid phase, if appropriate after removal of the precipitate, is evaporated to dryness.

From the revelation of DE 39 06 791 A1 Furthermore, a process for the preparation of metal-containing, nitric acid, hydrofluoric acid containing waste stains, in which the Abfallbeize is limited in a permselective membranes and between a pair of electrodes, in whose anode and cathode space contained sulfur, arranged dialysis cell is introduced.

A largely complete recycling of pickling bath concentrates offers a thermal process, the so-called roasting process. This is where the pickling acids come together evaporated with the water and the metals roasted to oxides. The acid residues of the metal salts are recovered as free acids in the distillate of the roaster. This means that the pickling bath concentrates can be treated almost without wastewater and waste.

In the article "Industrial Oxide Raw Materials - Production according to the Andritz-Ruthner Spray Grate Process" by Dr. Ing. Wolfgang Kladnig, Speech Room, Vol. 124, no. 11/12, 1991 , A process for the industrial production of raw materials is described in which first by the addition of hydrochloric acid, a metal chloride solution is prepared. The metal salt solution prepared in this way is subsequently purified and subjected to pyrohydrolysis in which the metal oxides to be recovered and also hydrogen chloride gas are formed. While the metal oxides are subjected to further purification steps, the hydrogen chloride gas is recycled back to hydrochloric acid using water. The hydrochloric acid thus obtained is reused for the re-preparation of a metal chloride solution.

From the EP-A-0 578 537 is a method for recycling Metallbeizbädem known to be removed from the Metallbeizbad unwanted compounds such as silicon, aluminum and chromium compounds. For this purpose, metal scrap is added to the metal pickling bath to be cleaned in a first step in order to neutralize the free acidity of the metal pickling bath, wherein the neutralization must be carried out under a neutral gas atmosphere in order to prevent unwanted side reactions of iron compounds contained in the metal pickling bath into trivalent iron compounds. Subsequently, the solids contained in the neutralized metal pickling bath are filtered out. By specifically reducing the acidity of the metal pickling bath, ie increasing the pH, unwanted compounds, such as silicon, aluminum and chromium compounds, precipitate, which are dissolved at a lower pH, whereby the metal pickling bath can be cleaned of them. Subsequently, the cleaned metal pickling bath is returned to the pickling process.

Thus, according to the prior art according to the EP 0 296 147 A1 describes a process for the recovery or recovery of acids from metal-containing solutions of these acids, after which the solutions are spray-roasted in a reactor at temperatures of 200 to 500 ° C and a subsequent absorption and condensation of the resulting gases in columns at temperatures from 0 to 70 ° C. be subjected.

However, the roasting process is energy intensive, with the energy consumption being directly proportional to the feed volume and consuming about 100 m ^{3 of} natural gas per 1 m ³ feed volume. Since the roasting process vaporizes water and acids equally, the rinsing and wastewater effluents that are too dilute can not be roasted directly. Due to the high proportion of water, the acid concentrations would be too small or the volume would be too large to return it to the pickling bath. The rinsing waters must therefore still be treated in a wastewater system. Since the material load of these effluents, especially nitrates, may well account for 50% of the total nitric acid consumption, the roasting process per se, as used hitherto, is not the comprehensive solution, especially with regard to the nitrate load of the wastewater.

The aim must therefore be to concentrate the highly diluted wastewater from the rinses and fume scrubbers so far that they can be introduced into the roasting process. The concentration of dilute wastewater is still not feasible, since the available techniques are not applicable. Thus, membrane technologies in the form of electrodialysis and reverse osmosis systems can not be used due to insufficient membrane resistance. Evaporator plants are not useful because of the steam volatility of nitric acid and hydrofluoric acid into the distillate. In the presence of free hydrofluoric and nitric acids in the feed to the evaporator can be found in the distillate up to 50% of these free acids, so that use of the distillate is not possible as rinse water. The distillate, which now contains only 50% of the original nitrate load, would still have to the sewage plant be disposed of and would thus in turn solve the nitrate problem in the wastewater not comprehensive.

The invention is therefore based on the object to avoid the disadvantages described and to further develop the methods and devices known from the beach of technology so that while retaining advantages, an economical method for recycling Metallbeizbädem is provided. A method or a device should be made available which makes it possible to operate metal pickling as far as possible without wastewater and waste, in particular the wastewater load with nitrates should be as low as possible.

According to the invention, the above object is achieved by a method for recycling metal pickling baths according to patent claim 1.

With the method according to the invention, particularly good results are achieved in the recycling of steel pickling baths, in particular of stainless steel pickling baths.

The invention also provides an apparatus for recycling metal pickling baths, including the associated rinsing baths and fume scrubbers according to claim 15.

The method according to the invention or the apparatus according to the invention thus shows a way as well as for conventional pickling baths based on HNO ₃ / HF / HCl which avoids the disadvantages of steam volatility arising from these acids when using thermal processes and the diluted wastewaters from the rinses and the exhaust air scrubbers can be evaporated. Thus, on the one hand the nitrate problem in the wastewater is solved and on the other hand, the roasting process from a more economical point of view possible.

The method / apparatus of the invention uses conventional components in the form that effluent and waste-free operation can be maintained under economical conditions. The last and because of the high energy consumption the economy determining stage is the thermal salt cleavage according to step c), such as the so-called roasting process. In this process, the liquid phases, such as water and acids, are evaporated and then the vapor phase is condensed again, thereby recovering the acids. The metals are oxidized at high temperatures and accumulate as solids. The energy consumption and thus the operating costs of the roaster largely depend on the feed volume to the roaster and amount to about 1000 kWh or 100 m ³ natural gas per m ³ inlet. For reasons of energy, the roasting process therefore has the lowest possible feed volume (corresponding to a high metal content in the pickling bath), which is not always desirable, however, because of the pickling conditions. High metal contents in the pickling bath cause lower pickling capacities and higher NO _x losses in the exhaust air of the pickling baths and thus a higher load on the scrubbers.

For the reduction of the feed volume to the roaster and thus a particularly cost-effective mode of operation in step b), according to the invention, according to a preferred embodiment, an evaporator, in particular one with mechanical vapor compression, is used. This type of evaporator has an energy consumption of only about 20-25 kWh per ton of feed. Every ton of water that the evaporator draws from the inlet to the roaster saves energy costs of approximately 100 m ^{3 of} natural gas.

Furthermore, it is known that the exhaust air losses of the roaster, especially of nitric acid in the form of NO _x , are noteworthy and can move in the range of 10-15% of the feed rate. The roaster is therefore preferably supplied with the smallest possible amount of nitrate or nitric acid. According to the invention, a separation plant for acids and salts is used for this purpose, for example retardation or diffusion dialysis in order to keep the free acids away from the roaster. The free acids are returned directly to the pickling bath. Due to the regeneration of the pickling bath concentrates, about 90% of the free acids are separated from the pickling bath solution and only about 10% are fed to the roaster. The NO _x losses, based on the concentrate stream, therefore amount to only about 1% with regeneration compared to 10% without regeneration. For hydrofluoric acid, comparable conditions apply, but the absolute values are lower since hydrofluoric acid accounts for only about 20% of the nitric acid concentration.

As already noted for the roaster, the free acids HNO ³ and HF are volatile in the evaporation and found to a high percentage in the distillate again. If the roaster is anxious to transfer the acid 100% into the distillate, one would like to get as little acid as possible into the distillate during the evaporation of the rinsing water. This does not succeed in the presence of free acids in the feed to the evaporator. The distillate obtained from an evaporator can no longer be used directly as flushing water. It would be additional process steps, eg. B. Ion exchanger circulation systems, necessary to allow use of the distillate. The additional investment required will weigh on profitability. A direct evaporation of rinse water and wastewater from the exhaust air scrubbers is therefore not economically feasible. For the same reason, further evaporation of pickling bath concentrates to save operating costs before roasting is not an advantage.

As already stated for the roaster, the more optimal operating conditions for operating an evaporator are the largely absence of free acids in the feed. It is thus disadvantageous to supply pickling bath concentrates directly to the evaporator. Reduction of the free acids by a separation plant as described above provides considerable advantages, but can be further improved because there are still enough free acid residues in the salt solution which are the distillate of the evaporator contaminate. The direct supply of rinse water to the evaporator fails because the evaporator from the concentrate ago in the range of Beizbadkonzentration (high content of free acids).

The process of the present invention solves the problems outlined above by eliminating, according to one embodiment of the invention, the free acids in the feed to the evaporator without affecting the degree of acid recovery by the roaster. Surprisingly, according to the invention, the degree of acid recovery and also the recovery of metal oxides, at a lower operating cost, can be significantly increased.

According to a particularly preferred embodiment, the separation of the free acid from the recycling stream (pickling bath concentrates) takes place in two separate steps. Preferably, the pickling bath concentrates are treated in an acid regeneration plant, such as acid regeneration or diffusion dialysis. The acid regeneration is based on an ion exchange process in which a particular resin absorbs the acid during loading while the metal salt solution passes through the resin bed unaffected and leaves the plant dissolved in water.

The free acids resulting from the acid regeneration system preferably go back into the pickling bath, while a low-acid but high-metal salt stream is collected for further treatment. The wastewater stream of the regeneration plant can be advantageously mixed with the wastewater streams from the sinks and the exhaust air scrubbers. The result is a lower free acids and metal salts medium flow with a high water content.

Operating the evaporator with the above-mentioned feed brings advantages, which, however, can be further improved. The small proportion of free acids would concentrate during the evaporation process, with much of the free acids entering the distillate. Only when the free acids are almost completely converted to metal salts, the vapor volatility of the acids is suppressed and gives an acid-free distillate, which can be used directly as rinse water again.

Neutralization chemicals, such as sodium hydroxide solution, lime etc., are usually used for the reaction of the free acids. This simple and conventional method is not advantageous for the process according to the invention, since the metals sodium, calcium, etc. also enter the roaster, but are not desirable here. The process according to the invention therefore preferably uses metal hydroxides, metal carbonates or metal oxides with metals which are likewise used in the pickling bath in step a).

In the above-mentioned way of co-evaporation of streams from the recycling plant and the effluents from rinses and fume scrubbers, a relatively large amount of metal hydroxides must be used to eliminate the free acids. This large amount would have to be supplied from the outside and thus represents an additional logistical problem. A partial use of metal oxides, which were previously produced in the roaster, would be possible, but weighs on the economy.

It is more economical, as in a particularly preferred embodiment of the method according to the invention, to supply the rinsing water together with the wastewater to a separate treatment. The aim is not to introduce the acids from the rinsing and waste water into the roasting circuit. By this measure, the amount of free acids is drastically reduced before evaporation. Thus, the consumption of metals for the implementation of the free acids is reduced accordingly at this point and it is no longer dependent on an external supply of metals.

The metal salt used for the reaction of the free acids, before the evaporation of the roasting concentrates, in step a), such as metal hydroxide, is precipitated according to the inventive method, preferably from the resulting rinsing and waste water under special conditions. It is expedient to use a neutralization chemical which precipitates the metals but keeps the acid residues in solution. Sodium hydroxide solution and potassium hydroxide solution are possible here, and it has been found that working with potassium hydroxide solution is advantageous for the further treatment of the acid residues.

By neutralizing the rinsing and waste water, the metals are preferably precipitated as hydroxides and filtered off. The filter cake obtained can then advantageously in a container with stirrer before the pickling bath for evaporators be initiated to convert here the remains of the free acids from the recycling plant for pickling bath concentrates to metal salts.

The effluent from the neutralization of water contains, for example, the neutral salts of potassium fluoride and potassium nitrate in highly diluted form. Disposal of this water flow via a wastewater treatment plant would in turn cause the nitrate pollution in the wastewater to skyrocket. Accordingly, the method or device according to the invention can preferably be used to split the neutral salts present in this stream into the HF, HNO ₃ and potassium hydroxide neutralization chemicals. For example, cation exchange and electrodialysis systems are possible. The acids are then returned to the pickling bath and the potassium hydroxide solution into the neutralization. Thus, the cycle would be closed and the sinks and fume scrubbers waste and sewage free.

Since the wastewater streams from rinses and scrubbers consist of more than 95% water, it is preferred that before the salt cleavage according to step c) a separation of the water in step b) takes place, so in the plant for salt splitting a sufficiently high concentration of free To produce acids and neutralization chemical. Reverse osmosis systems and evaporators are available as plant components for water separation, for example. Since higher Aufkonzentrationsraten be achieved with an evaporator, an evaporator system is preferred at this point.

The neutralization and precipitation of the metals produces a salt water stream without any free acids, for example with a pH> 8. This means for the evaporation that no volatile acids are present in this stream, the distillate produced has a high quality and is directly again Rinse water with VE quality (demineralized water with pH of about 7) can be used. Another advantage of this procedure is the lower aggressiveness of the neutralized water stream over a stream with the metal salts as from the pickling concentrates. While the acid-free salt stream of pickling bath concentrates expediently has a pH of only about 2.5 to 3 and thus is still extremely aggressive, the pH of the neutralized rinse waters is preferably at pH 8 and is thus less aggressive. Both for the reverse osmosis and for an evaporator, this results in consequences in the selection of materials for the respective Investment. While for the stream with pH 8 conventional stainless steels, eg. For reasons of corrosion resistance, for example, quality 1.4571.bzw V4A should be sufficient for the acidic stream special high-alloyed steels for plant construction. Since the neutral stream usually requires the much larger evaporator, can be saved by separate evaporator systems considerable investment costs through the choice of materials.

The advantages associated with the invention are complex. A method or a device is provided which makes it possible to operate metal pickling as far as possible without wastewater and waste, the wastewater load with nitrates in particular being as low as possible. At the same time, the salt separation plant, like a roasting process, can be operated under more economical aspects.

According to the invention a way is thus shown, as well as for conventional pickling baths based on HNO ₃ / HF avoided by these acids disadvantages of steam volatility when using thermal processes and the diluted wastewater from the sinks and the exhaust air scrubbers can be evaporated.

By converting the free acids into metal salts in the reactor, there are no problems of corrosion in the concentrator, such as an evaporator, and cheaper stainless steels can be used in the design. By appropriate optimization, for example, control of the volume flows, smaller dimensions can be used in the devices, such as a smaller sized concentrator, which is associated with a significant reduction in costs.

Furthermore, the present invention makes it possible that by the regeneration of the pickling bath concentrates about 90% of the free acids are separated from the pickling bath solution and only about 10% are fed to the roaster, whereby the NO _x losses, based on the concentrate stream, with the inventive method can be reduced to very low about 1%. Accordingly, according to the present invention, the degree of acid recovery as well as the recovery of metal oxides can be significantly increased while having lower operating costs.

The invention will be described in detail below with reference to three examples which are not intended to limit the teaching according to the invention. Within the scope of the disclosure according to the invention, further embodiments are obvious to the person skilled in the art.

Examples Example 1 (not according to the invention)

FIG. 1 shows a pickling device (1) with subsequent sink (4). The conventional replenishment system with roaster (3) has been extended by an evaporator system (12) for rinsing and waste water. The volume flow (2) from the pickling bath (1) should be about 3.5 m ³ / h and the volume flow (6) from the rinses about 15 m ³ / h. These values apply to all 3 examples.

The pickling bath concentrates (2) are fed directly to the roaster (3). Since the resulting rinsing waters (6) are large in volume, they can not be introduced directly into the roaster (3) and must be concentrated in advance. As a concentrator (12), an evaporator with vapor compression is provided, since this type has the lowest energy consumption with about 25 kwh / m ³ distillate.

It is known that the acids used in pickling of metal (HNO ₃ , HF and HCl) are steam volatile. It must therefore be sought before evaporation to avoid free acids. According to the invention, the free acids in the rinsing water stream (6) are converted into metal salts in a reactor (5) by addition of a reagent (11). The reagent (11) is preferably a metal hydroxide of a species which also occurs in the pickling bath. By this measure significantly less acids are found in the distillate (7); However, the quality is usually not enough to use it for rinsing in the last rinse stage. However, use of the distillate (7) in previous rinsing stages is possible. Another reason for the conversion of the free acids into metal salts in the reactor (5) are corrosion problems in the concentrator (12). The less free acids in the feed (6a), the lower the corrosive attack on the stainless steels to be used. Less expensive stainless steels can be used in the construction.

In order to achieve the desired demineralised rinsing quality (10) of the last rinsing stage, it is advantageous to provide an additional device (13). Since the material load in the course (8) of the last rinsing stage is low, a cycle ion exchanger system (13) is suitable for this purpose. The water losses of the last rinse stage through overflow to the previous rinsing stages can be compensated by a demineralised water flow (9).

The metal salt-containing stream (6a) supplied to the concentrator (12) is concentrated as much as possible in order to keep the volume flow (15) to the roaster (3) small.

In the roaster (3) the streams (2 + 15) are separated in a thermal process into acids and metal oxides. The volume flow (16) with the acids is returned to the pickling bath (1) and the metal oxides can be sent to a melting process for recycling.

The feed volume (2) to the roaster (3) from the pickling bath (1) depends on pickling capacity and metal concentration in the pickling bath. In the present case, a volume flow of about 3.5 m ³ / h is assumed, which maintains an iron content of about 35 g / l in the pickling bath (1). Furthermore, the iron content in the pickling bath (1) should not rise, since otherwise iron fluoride precipitations would occur in the pickling bath (1). To this stream (2) comes the concentrate stream (15) of the evaporator of about 0.5 m ³ / h, so that the roaster (3) should preferably be designed for a feed volume of 4.0 m ³ / h.

The energy consumption of the roaster (3) under these conditions will be about 400 m ³ / h natural gas, the energy consumption of the evaporator (12) about 375 kWh / h. In a direct introduction of the flushing water stream (6) in the roaster (3), the energy consumption would rise to about 1500 m ³ / h of natural gas. The investment costs for the roaster (3) would be many times higher.

A comparative calculation of economic efficiency of the variants of the process according to the invention of Examples 1 to 3 with recycling by a process without recycling is shown in Table 1.

Example 2

Example 2 shows an optimized method over Example 1. As can be seen from Example 1, the free acids are a hindrance in recycling. Since the pickling bath concentrates of pickling (1) produce the highest acid concentrations, in Example 2 a plant (13) is provided to separate free acids and metal salts. The volume flow (18) with the free acids is passed back into the pickling bath (1), while a volume flow (19) with the metal salts is fed to the reactor (5) for further treatment. Since the waste acid stream (2) in this case also contains mechanical impurities (scale), filtration (7) is required for the further treatment of the volume flow (2). The freed of mechanical impurities stream (8) is introduced into the separation plant (13).

For the separation of metal salts and acids, an acid regeneration system (13) is used. This plant requires process water (20), to which no particularly high quality requirements are placed. A partial flow of the resulting flushing water flow (6) is used for the operation of the system (13). This has the advantage that the volume flow (23) is reduced to the evaporator. With the flushing water stream (20) of the metal salt stream (19) is generated. The metal salt stream (19) is low in acids and rich in metal salts.

The metal salt stream (19) is fed together with the part stream (21) from the filtration and the rinsing water stream (22) to a reactor (5). In this reactor (5), residual free acid is converted into metal salt by an externally prepared reagent (11) (see also Example 1).

The largely acid-free volume flow (23) is fed to a concentrator (12) as in Example 1 and separated into a partial stream (15) with the metal salts and a partial stream (10) with the distillate and a residual amount of free acid. The Distillate (10) again has no VE quality and can be supplied as raw water to an existing demineralization plant. The raw water (10) treated in the VE plant is then fed back into the flushing system as flushing water (9).

Due to the very low content of free acids in the inlet (23) to the concentrator (12) high concentration factors in the concentrator (12) can be realized. As a result, the volume flow (15) to the roaster (3) can be reduced from about 4 m ³ / h to about 1 m ³ / h compared to example 1. This measure reduces the energy consumption in the roaster (3) compared to Example 1 by about 300 m ³ / h of natural gas. The energy consumption of the concentrator (12) remains approximately the same as in Example 1.

Further advantages of the method according to Example 2 are:

The capacity (investment costs) of the roaster (3) can be reduced due to the reduced volume flow (15).

The exhaust gas losses of the roaster (3) of free acids is a percentage constant of the feed rate (15). By recycling the free acids in Appendix (13) passes only a subset of acids in the roaster (3), with correspondingly lower exhaust gas losses.

The economy of this process is shown in Table 1.

Example 3

Example 3 shows a comparison with Example 2 further optimized method. As in Example 2, in Example 3, the free acids from the Beizbadstrom (2) with a system (13) in a stream (18) with free acids and a stream (19) with metal salts are separated. In example 3, however, only this small volume (23) of volume is fed to a concentrator (12). The large volume of flushing water stream (20) is supplied to a separate treatment in a plant (24). In Appendix (24), by adding a neutralization chemical (KOH), the metals are precipitated and filtered off. The Precipitated metals are transferred as metal hydroxides as stream (11) in the reactor (5), here to convert the free acids to metal salts.

The waste water stream (26) produced during the neutralization contains the neutral salts KOH and KF and is supplied to the concentrator (27). Since only neutral salts are present in the inlet (26) to the concentrator (27), there is no longer any risk of the vapor being volatile in the evaporation process. The distillate (9) produced in the evaporator (27) has VE quality and can be introduced directly into the last sink (4) as rinse water. Additional treatment via an ion exchanger system is no longer required. Furthermore, the now neutral inlet (26) to the concentrator (27) allows this conventional in the construction of stainless steels, resulting in cost savings in investment.

The concentrate (28) of KF and KNO ₃ produced by the evaporator (27) is fed to an electrolysis cell (29) in which the salts are separated into acids and lye. The lye stream (25) is used again in the neutralization (24) and the acids (30) are used again in the pickling bath (1).

• Im Beispiel 2 gehen beide Volumenströme (2/8/19) und (6/22) in einer Größenordnung von etwa 15 m³/h über den Konzentrator (12). Da der pH-Wert des Zulaufes (23) zum Konzentrator (12) nicht neutral sondern sauer ist, werden für die Konstruktion hochwertige Edelstähle erforderlich was die Investitionskosten steigert.
• Im Beispiel 3 wird lediglich der Volumenstrom (2/8/19) in der Größenordnung von etwa 3,5 m³/h in den Konzentrator (12) eingeleitet. Obwohl auch dieser Konzentrator in hochwertigen Edelstählen gebaut werden muß, reduzieren sich die Investtitionskosten, da wesentlich kleiner gebaut werden kann.
• Wie in Beispiel 2 erzeugt der Konzentrator (12) ein leicht saures Destillat (10). Dieses Wasser kann aber ohne weiteres für die Trennanlage (13) als Prozesswasser verwendet werden und braucht nicht zusätzlich behandelt werden.
• Ferner kann durch die Neutralisation des Spülwasserstromes (20) der Konzentrator (27) in der Anlage (24) aus handelsüblichen Edelstählen gefertigt werden. Das senkt insbesondere die Investitionskosten, da der Konzentrator (27) mit etwa 15 m³/h um ein Vielfaches größer ist als Konzentrator (12).
• Weiterhin hat das Destillat von Konzentrator (27) VE-Qualität und muß nicht über einen Ionentauscher nachbehandelt werden.
• Das in der Neutralisation (24) erzeugte Metallhydroxid (11) wird für die Umsetzung der freien Säure im Reaktor (5) verbraucht. Reaktor (5) wird daher von einer externen Versorgung durch das Reagenz (11) in Beispiel 2 befreit.
• Durch die separate Spülwasserbehandlung (20) kann das Zulaufvolumen zum Röster (3) noch einmal geringfügig reduziert werden. Während das Zulaufvolumen (15) im Beispiel 2 noch etwa 1 m³/h ausmacht, reduziert es sich im Beispiel 3 auf etwa 0,83 m³/h. Entsprechend niedriger ist der Energieverbrauch des Rösters (3).

Tabelle 1 Wirtschaftlicher Vergleich der Beispiele Investitionen Betriebskosten Einsparungen pay back (Mio €) (Mio €/a) (Mio €/a) (Jahre) ohne Recycling 0 4,4 0 >> Beispiel 1 9,0 0,7 3,7 2,4 Beispiel 2 8,0 0,4 4,0 2,0 Beispiel 3 7,0 0,3 4,1 1,7 While the energy consumption is comparable in Examples 2 and 3, there are advantages in Example 3 in the investment costs, which can be described as follows:

• In Example 2, both volume flows (2/8/19) and (6/22) go on the order of about 15 m ³ / h via the concentrator (12). Since the pH of the feed (23) to the concentrator (12) is not neutral but acidic, high-grade stainless steels are required for the construction which increases the investment costs.
• In Example 3, only the volume flow (2/8/19) in the order of about 3.5 m ³ / h is introduced into the concentrator (12). Although this concentrator must be built in high-grade stainless steel, the investment costs are reduced, since it can be built much smaller.
• As in Example 2, the concentrator (12) produces a slightly acidic distillate (10). However, this water can be readily used for the separation plant (13) as process water and does not need to be treated additionally.
• Furthermore, by neutralizing the flushing water flow (20), the concentrator (27) in the system (24) can be manufactured from commercially available stainless steels. This reduces in particular the investment costs, since the concentrator (27) at a rate of about 15 m ³ / h is many times greater than the concentrator (12).
• Furthermore, the distillate of concentrator (27) has VE quality and does not have to be aftertreated via an ion exchanger.
• The metal hydroxide (11) produced in the neutralization (24) is consumed for the reaction of the free acid in the reactor (5). Reactor (5) is therefore freed from an external supply by the reagent (11) in Example 2.
• The separate flushing water treatment (20), the feed volume to the roaster (3) can be slightly reduced again. While the feed volume (15) in Example 2 is still about 1 m ³ / h, it reduces in Example 3 to about 0.83 m ³ / h. The energy consumption of the roaster (3) is correspondingly lower.

Table 1 Economic comparison of the examples investments operating cost savings pay back (€ million) (€ million / a) (€ million / a) (Years) without recycling 0 4.4 0 >> example 1 9.0 0.7 3.7 2.4 Example 2 8.0 0.4 4.0 2.0 Example 3 7.0 0.3 4.1 1.7

Claims (23)

1. A process for recycling metal pickling baths including the associated rinsing baths and waste air washers comprising the steps of

   a) converting the free acids present in the liquid waste flows to be treated into the metallic salt form prior to recycling;

   b) separating water from the obtained largely acid-free metallic salt solution in order to obtain a concentrated metallic salt solution;

   c) supplying the concentrated metallic salt solution to a thermal process to produce metal oxides and free acids; and

   wherein the waste flow from the pickling baths to be recycled is split in a suitable separating unit into a first portion of the flow with the metallic salts to be recycled and a second portion of the flow with free acids, which are returned to the pickling bath.
2. A process according to claim 1, characterized in that the acidic waste flows from the pickling baths and the rinsing baths/waste air washers are each subjected to a separate treatment.
3. A process according to claim 1 or claim 2, characterized in that the separated water is returned to the process to be reused.
4. A process according to claim 1, characterized in that the residual free acids present in the first portion of the flow are converted into metallic salts in accordance with step a) with metal hydroxides, metal oxides or metal carbonates of the metals used in the pickling bath.
5. A process according to claim 4, characterized in that the first portion of the flow treated with metallic salt is converted in a unit for separating water in accordance with step b) into a concentrated metallic salt solution close to the solubility limit of the metallic salts.
6. A process according to claim 5, characterized in that the water separated in step b) in the form of a slightly acidic distillate is returned to the separating unit as process water.
7. A process according to any of claims 1 to 6, characterized in that the first portion of the flow is mixed with the acidic waste flow from the rinsing baths/waste air washers prior to step a).
8. A process according to claim 5, characterized in that the concentrated metallic salt solution from the pickling baths and optionally from the rinsing baths and waste air washers is supplied to a thermal process to separate the salts into metal oxides and free acids in accordance with step c).
9. A process according to claim 1, characterized in that the rinsing water and/or waste water of the rinsing baths/waste air washers is neutralized with a chemical, in particular caustic soda solution or caustic potash solution, whereby the acid residues remain in dissolved form but the metals are precipitated.
10. A process according to claim 9, characterized in that the precipitated and filtered metallic salts, in particular as metal hydroxides, are supplied to step a) to convert the free acid into metallic salts.
11. A process according to claim 9, characterized in that the neutralized waste water is converted in a unit for separating water into a concentrated salt solution close to the solubility limit of the metallic salts and the produced distillate is reused for rinsing purposes.
12. A process according to claim 11, characterized in that the concentrated salt solution is converted in a unit for salt separation into acids and bases, in particular a cation exchanger or electrodialysis unit, for reuse in the process.
13. A process according to at least one of the preceding claims, characterized in that a steel pickling bath is used as metal pickling bath.
14. A process according to claim 13, characterized in that a stainless steel pickling bath is used as steel pickling bath.
15. A device for recycling metal pickling baths (1) including the associated rinsing baths/waste air washers (4) comprising:

    - at least one unit (5) for converting the free acids present in the liquid waste flows to be treated (2, 6) into the metallic salt form prior to recycling;

    - at least one unit for separating water (12, 27) from the obtained largely acid-free metallic salt solution in order to obtain a concentrated metallic salt solution;

    - at least one unit for the thermal salt separation (3) of the salt concentrate flows from the pickling baths (1) and rinsing baths/waste air washers (4) to produce metal oxides and free acids; and

    - a separating unit (13) to split the metal flow to be recycled from the pickling baths (1) into a first portion of the flow (19) with the metallic salts to be recycled and a second portion of the flow (18) with free acids, which are returned to the pickling bath (1).
16. A device according to claim 15, characterized in that the separating unit (13) is an acid regeneration unit, in particular an acid retardation or diffusion dialysis unit.
17. A device according to any of claims 15 to 16, characterized in that the unit for the thermal salt separation is a roaster (3).
18. A device according to any of claims 15 to 17, characterized by pipes for the first portion of the flow (19) and/or the produced rinsing and waste air water (22, 26, 6a) to a concentrator (12, 27), in particular an evaporator.
19. A device according to claim 18, characterized in that upstream of concentrator (12), a reactor (5) is provided in which the free acids present can be converted into metallic salts by adding a reagent (11).
20. A device according to claim 19, characterized in that reagent (11) is a metal hydroxide of the metal that is also present in the pickling bath.
21. A device according to at least one of the preceding claims, characterized in that a unit (24) is provided in which the metals can be precipitated from waste flow (6) of the rinsing baths/waste air washers (4) by adding a neutralizing chemical and filtered off, and the obtained reagent (11) can be supplied to reactor (5).
22. A device according to at least one of claims 15 to 21, characterized in that the feed volume into the unit for the thermal salt separation (3) is adjusted via concentrator (12) to keep the volume flow (15) to unit (3) small.
23. A device according to at least one of claims 15 to 22, characterized in that a unit for separating water (29), in particular an electrolysis unit, is provided for the metallic salt solution concentrated in concentrator (27) from rinsing and waste air water (26).

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