Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/36—Regeneration of waste pickling liquors

Definitions

• recycling plants are used to separate free acids and salts, such as acid tardation and diffusion dialysis, to reduce the nitrate pollution in the wastewater by recovering the free acids and thus to make the disposal of the waste acids more economical.
• the acid savings that can be achieved with this are considerable, but do not really solve the actual nitrate problem, since large quantities of nitrate-containing wastewater continue to be produced by the nitrate salts.
• acid recycling systems the majority of nitrate wastewater pollution no longer comes from the pickling baths, but from the connected rinsing baths and exhaust air scrubbers, which are not recycled.
• a largely complete recycling of the pickling bath concentrates is offered by a thermal process, the so-called roasting process.
• the pickling acids are combined evaporated with the water and roasted the metals to oxides.
• the acid residues of the metal salts are recovered as free acids in the roaster's distillate.
• the pickling bath concentrates can thus be treated almost wastewater and waste-free.
• the roasting process is energy-intensive, with the energy consumption being directly proportional to the feed volume and about 100 m 3 of natural gas being consumed per 1 m 3 of feed volume. Since the roasting process evaporates water and acids equally, the rinsing and waste air waste water, which is too dilute, cannot be roasted directly. Due to the high water content, the acid concentrations would be too low or the volume too large to return to the pickling bath. The rinse water must therefore still be treated in a wastewater system. Since the pollution of this wastewater, especially the nitrates, can be up to 50% of the total nitric acid consumption, the roasting process, as has been used up to now, is not the comprehensive solution, especially with regard to the nitrate pollution of the wastewater.
• the aim must therefore be to concentrate the highly diluted wastewater from the sinks and exhaust air scrubbers to such an extent that they can be introduced into the roasting process.
• concentration of the diluted wastewater is still not feasible because the techniques available cannot be used.
• membrane technologies in the form of electrodialysis and reverse osmosis systems cannot be used due to the insufficient membrane resistance.
• Evaporator systems are not usable because of the volatility of nitric acid and hydrofluoric acid into the distillate. If free hydrofluoric and nitric acids are present in the feed to the evaporator, up to 50% of these free acids can be found in the distillate, so that the distillate cannot be used as rinsing water.
• the distillate which now only contains 50% of the original nitrate load, should still be over the wastewater system would be disposed of and would in turn solve the nitrate problem in the
• the invention is therefore based on the object of avoiding the disadvantages described and developing the methods and devices known from the prior art in such a way that an economical method for recycling metal pickling baths is provided while retaining advantages.
• a method or a device should be made available which enables metal pickling to be operated largely free of waste water and waste, in particular the nitrate content should be as low as possible.
• the above object is achieved by a method for recycling metal pickling baths, including the associated rinsing baths and waste air scrubbers, characterized by a) converting the free acids present in the liquid waste streams to be treated into the metal salt form before recycling, b) separating water from the largely acid-free metal salt solution obtained in order to obtain a concentrated metal salt solution, and c) feeding the concentrated metal salt solution into a thermal process for the recovery of metal oxides and free acids.
• the invention also relates to a device for recycling metal pickling baths, including the associated rinsing baths and exhaust air washer, which comprises: at least one plant for converting the free acids present in the liquid waste streams to be treated into the metal salt form before recycling, at least one plant for Separation of water from the largely acid-free metal salt solution obtained in order to obtain a concentrated metal salt solution and at least one plant for the thermal salt splitting of the salt concentrate streams from the pickling baths and the rinses for the extraction of metal oxides and free acids.
• the method according to the invention and the device according to the invention thus shows a way, as well as for conventional pickling baths based on HNO / HF / HCl, the disadvantages of steam volatility caused by these acids when using thermal methods are avoided and the diluted waste water from the sinks and the exhaust air scrubbers is evaporated can be. On the one hand, this solves the nitrate problem in wastewater and, on the other hand, the roasting process is possible from a more economical perspective.
• the method / device of the invention uses conventional components in such a way that wastewater and waste free operation can be maintained under economic conditions.
• the last stage, which determines the economic viability due to the high energy consumption, is thermal salt splitting according to step c), such as the so-called roasting 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 natural gas per m feed.
• the roasting process therefore has the lowest possible inflow volume (corresponds to a high metal content in the pickling bath), but this is not always desirable from the pickling conditions.
• High metal contents in the pickling bath cause lower pickling capacities and higher NO x losses in the exhaust air from the pickling baths and thus a higher load on the exhaust air scrubbers.
• an evaporator in particular one with mechanical vapor compression, is used according to the invention for reducing the feed volume to the roaster and thus a particularly cost-effective procedure in step b).
• 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 removes from the inlet to the roaster saves energy costs of approximately 100 m natural gas. It is also known that the roaster's waste air losses, especially of nitric acid in the form of NO x , are noteworthy and can range from 10-15% of the feed quantity.
• the roaster is therefore preferably supplied with the smallest possible amount of nitrate or nitric acid.
• a separation system for acids and salts is used for this in a preferred variant, such as, 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.
• the NO x losses, based on the concentrate flow, are therefore only about 1% with regeneration compared to 10% without regeneration. Comparable conditions apply to hydrofluoric acid, but the absolute values are lower, since hydrofluoric acid only makes up about 20% of the nitric acid concentration.
• the free acids ENT and HF are volatile during evaporation and can be found in a high percentage in the distillate. If the roaster strives to convert 100% of the acid into the distillate, one wants to get as little acid as possible into the distillate when the rinse water is evaporated. This does not succeed if free acids are present in the feed to the evaporator.
• the distillate obtained from an evaporator can no longer be used directly as rinsing water. There would be additional process steps, e.g. B. ion exchanger circulation systems, necessary to enable use of the distillate.
• the necessary additional investments have a negative impact on profitability.
• a direct evaporation of rinse water and waste water from the exhaust air scrubbers is therefore not economically feasible. For the same reason, further evaporation of the pickling bath concentrates to save operating costs before roasting is not an advantage.
• the method according to the invention solves the problems described above, in that according to one embodiment of the invention the free acids in the feed to the evaporator are eliminated without the degree of acid recovery being impaired by the roaster. According to the invention, the degree of acid recovery and also the recovery of metal oxides can surprisingly be increased significantly, while at the same time lower operating costs.
• the free acid is separated from the recycling stream (pickling bath concentrates) in two separate steps.
• the pickling bath concentrates are preferably treated in an acid regeneration plant, such as acid retardation or diffusion dialysis.
• the acid retardation is based on an ion exchange process, in which a special resin absorbs the acid during loading, while the metal salt solution passes through the resin bed unaffected and, dissolved in water, leaves the plant.
• the free acids emerging from the acid regeneration plant preferably go back into the pickling bath, while a low-acid but metal salt-rich stream is collected for further treatment.
• the wastewater stream from the regeneration system can advantageously be mixed with the wastewater streams from the sinks and the exhaust air scrubbers. The result is a low flow of free acids and a medium flow of metal salts with a high water content.
• a relatively large amount of metal hydroxides has to be used in the above-mentioned way of joint evaporation of streams from the recycling plant and the waste water from rinsing and exhaust air scrubbers in order to eliminate the free acids. This large amount would have to be supplied from the outside and thus represents an additional logistical problem. Partial use of metal oxides which were previously produced in the roaster would be possible, but would have a negative impact on the economy.
• the metal salt, such as metal hydroxide, used in step a) for the conversion of the free acids, before the evaporation of the roaster concentrates, is preferably precipitated from the resulting rinsing and exhaust air waters under special conditions in accordance with the method according to the invention.
• a neutralization chemical is expediently used, which precipitates the metals but keeps the acid residues in solution.
• Sodium hydroxide solution and potassium hydroxide solution are possible here, although it has been found that working with potassium hydroxide solution is advantageous for the further treatment of the acid residues.
• the metals are preferably precipitated as hydroxides and filtered off.
• the filter cake obtained can then advantageously be placed in a container with a stirrer in front of the evaporator for pickling bath con- concentrates are introduced to convert the residues of free acids from the recycling plant for pickling bath concentrates into metal salts.
• the water flowing out of the neutralization contains, for example, the neutral salts potassium fluoride and potassium nitrate in a highly diluted form. Disposing of this water flow via a wastewater system would in turn increase the nitrate load in the wastewater.
• the method or the device according to the invention can therefore preferably be used to split the neutral salts present in this stream into the pickling acids HF, HNO 3 and the neutralization chemical potassium hydroxide solution. For example, cation exchanger and electro-dialysis systems come into question. The acids are then returned to the pickling bath and the potassium hydroxide solution to the neutralization. The cycle would then be closed and the sinks and air scrubbers free of waste and waste water.
• the wastewater streams from rinsing and exhaust air washers consist of more than 95% water
• Generate acids and neutralization chemical For example, reverse osmosis systems and evaporators are available as system components for water separation. Since higher concentration rates are achieved with an evaporator, an evaporator system is preferred at this point.
• the neutralization and precipitation of the metals creates a salt water stream without any free acids with a pH> 8, for example.
• VE quality demineralized water with a pH of approx. 7
• Another advantage of this procedure lies in the lower aggressiveness of the neutralized water flow compared to a flow with the metal salts as from the pickling concentrates.
• the acid-free salt stream of the pickling bath concentrates expediently has a pH of only about 2.5 to 3 and is therefore still extremely aggressive, the pH of the neutralized rinsing water is preferably pH 8 and is therefore not very aggressive.
• conventional stainless steel e.g. B. the quality
• a method or a device is made available which enables metal pickling to be operated largely free of wastewater and waste, the wastewater load in particular being as low as possible with nitrates.
• the salt separation plant such as a roasting process, can be operated from a more economical point of view.
• the present invention makes it possible for the regeneration of the pickling bath concentrates to remove about 90% of the free acids from the pickling bath solution and only add about 10% to the roaster, as a result of which the NO x losses, based on the concentrate stream, are achieved using the process according to the invention can be reduced to very low about 1%.
• Figure 1 shows a pickling device (1) with a subsequent sink (4).
• the conventional regeneration system with roaster (3) has been expanded to include an evaporator system (12) for flushing and exhaust air.
• the volume flow (2) from the pickling bath (1) should be approximately 3.5 m 3 / h and the volume flow (6) from the sinks approximately 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 rinsing water (6) is large in volume, it cannot be fed directly into the roaster (3) and must be concentrated beforehand.
• An evaporator with vapor compression is provided as the concentrator (12), since this type is the lowest
• the acids used in pickling metal are volatile in steam. Therefore, before evaporation, efforts must be made to avoid free acids.
• the free acids in the rinsing water stream (6) are converted into metal salts in a reactor (5) by adding a reagent (11).
• the reagent (11) is preferably a metal hydroxide of a species which also occurs in the pickling bath. This measure means that significantly fewer acids are found in the distillate (7); however, the quality is usually not sufficient to use it for rinsing purposes in the last rinse stage. However, it is possible to use the distillate (7) in previous rinsing stages.
• Another reason for converting the free acids into metal salts in the reactor (5) are corrosion problems in the concentrator (12). The fewer free acids in the feed (6a), the lower the corrosion attack on the stainless steel to be used. Inexpensive stainless steel can be used in the construction.
• VE flushing quality (10) of the last flushing stage it is advantageous to provide an additional device (13). Since the material load in the outlet (8) of the last rinsing stage is low, a circuit ion exchanger system (13) is suitable for this. The water losses in the last rinse stage due to overflow to the previous rinse stages can be compensated for by a deionized water flow (9).
• the metal salt-containing stream (6a) fed to the concentrator (12) is concentrated as much as possible in order to keep the volume flow (15) to the roaster (3) small.
• the currents (2 + 15) are separated into acids and metal oxides in a thermal process.
• the volume flow (16) with the acids is returned to the pickling bath (1), the metal oxides can be fed to a melting process for recycling.
• the inflow volume (2) to the roaster (3) from the pickling bath (1) depends on the 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 1 in the pickling bath (1). Furthermore, the iron content in the pickling bath (1) should not increase, since otherwise iron fluoride precipitates would occur in the pickling bath (1).
• the concentrate stream (15) of the evaporator of about 0.5 m 3 / h, so that the roaster (3) should preferably be designed for an inlet volume of 4.0 m 3 / h.
• the energy consumption of the roaster (3) will be approximately 400 m 3 / h of natural gas under these conditions, the energy consumption of the evaporator (12) will be approximately 375 kWh / h. If the flushing water flow (6) were introduced directly into the roaster (3), the energy consumption would increase to about 1500 m 3 / h of natural gas. The investment costs for the roaster (3) would be many times higher.
• Table 1 A comparative economic calculation of the variants of the method according to the invention according to Examples 1 to 3 with recycling with a method without recycling is shown in Table 1.
• Example 2 shows an optimized process compared to Example 1.
• the free acids are a hindrance to recycling.
• a system (13) is provided in Example 2 to separate free acids and metal salts.
• the volume flow (18) with the free acids is fed back into the pickling bath (1), while a volume flow (19) with the metal salts is fed to the reactor (5) for further treatment.
• the waste acid stream (2) also contains mechanical impurities (scale) in this case, filtration (7) is required for the further treatment of the volume flow (2).
• the stream (8) freed of mechanical impurities is introduced into the separation system (13).
• An acid retardation system (13) is used for the separation of metal salts and acids.
• This system requires process water (20), to which there are no particularly high quality requirements.
• a partial flow of the rinsing water flow (6) is used to operate the system (13). This has the advantage that the volume flow (23) to the evaporator is reduced.
• the metal salt stream (19) is generated with the flushing water stream (20).
• the metal salt stream (19) is low in acids and rich in metal salts.
• the metal salt stream (19) is fed to a reactor (5) together with the partial stream (21) from the filtration and the rinsing water stream (22).
• this reactor (5) residual free acid is converted into metal salt by an externally provided 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 flow (15) with the metal salts and a partial flow (10) with the distillate and a residual amount with 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 system is then fed back into the rinsing system as rinsing water (9).
• 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.
• 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 quantity (15).
• Example 3 shows a process which is further optimized compared to example 2.
• the free acids from the pickling bath stream (2) are separated with a plant (13) into a stream (18) with free acids and a stream (19) with metal salts.
• this stream (23) which is small in volume, is fed to a concentrator (12).
• the large flushing water flow (20) is fed to a separate treatment in a system (24).
• the metals are precipitated and filtered off by adding a neutralization chemical (KOH).
• KOH neutralization chemical
• the precipitated metals are transferred as streams (11) into the reactor (5) as metal hydroxides in order to convert the free acids into metal salts.
• the waste water stream (26) generated during the neutralization contains the neutral salts KOH and KF and is fed 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 volatility of the acids during evaporation.
• the distillate (9) produced in the evaporator (27) has VE quality and can be fed directly into the last sink (4) as rinsing water. An additional treatment via an ion exchanger system is no longer necessary. Furthermore, the now neutral inlet (26) to the concentrator (27) allows the construction of conventional stainless steel, which leads to cost savings in the investments.
• 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).
• the concentrator (12) produces a slightly acidic distillate (10).
• this water can easily be used as process water for the separation system (13) and does not need to be treated additionally.
• the concentrator (27) in the system (24) can be made from commercially available stainless steels. This lowers the investment costs in particular, since the concentrator (27), at around 15 m 3 / h, is many times larger than the concentrator (12).
• distillate from concentrator (27) has VE quality and does not have to be post-treated via an ion exchanger.
• the separate rinse water treatment (20) allows the feed volume to the roaster (3) to be reduced slightly again. While the feed volume (15) still makes up about 1 m 3 / h in Example 2, it is reduced to about 0.83 m 3 / h in Example 3. The roaster's energy consumption is correspondingly lower (3).

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