METHOD AND APPARATUS FOR TREATING CYANIDE-CONTAINING LIQUIDS

Description

[0001] The invention relates to a process for treating cyanide-containing liquids, in particular cyanide-containing wastewater, in which a) cyanide-containing liquid is subjected to a pretreatment in a pretreatment zone in which at least a predetermined pH value between 3 and 12 and a predetermined temperature are set, a base liquid being formed during the pretreatment; b) base liquid at a temperature between 40°C and 60°C is fed into at least one reaction reactor and, in this reaction reactor, is mixed with at least one oxidizing agent in the form of hydrogen peroxide, thereby initiating an oxidation reaction of the cyanides; c) liquid from the reaction reactor is transferred as an intermediate liquid into at least one process reactor in which conditions prevail under which the oxidation reaction of the cyanides initiated in the reaction reactor can take place, a process liquid being formed;d) at least temporarily maintaining a continuous liquid flow, in which da) base liquid is continuously transferred from the pretreatment zone into the reaction reactor; db) intermediate liquid is continuously transferred from the reaction reactor into the process reactor; and dc) process liquid is continuously withdrawn from the process reactor.

[0002] Furthermore, the invention relates to a plant for treating cyanide-containing liquids, in particular cyanide-containing wastewater, with a) a pretreatment zone in which cyanide-containing liquid can be subjected to pretreatment, comprising a pretreatment reactor in which at least one pH metering device is present, by means of which a pH adjusting agent (32) can be metered in so that a predetermined pH value between 3 and 12 can be set, and a predetermined temperature can be set, wherein a base liquid is formed during the pretreatment; b) at least one reaction reactor into which base liquid at a temperature between 40°C and 60°C is fed and in which the base liquid can be mixed with an oxidizing agent in the form of hydrogen peroxide, at least by means of an existing oxidizing agent metering device, thereby initiating an oxidation reaction of the cyanides;wherein c) liquid from the reaction reactor can be transferred as intermediate liquid via a connecting line into at least one process reactor in which conditions prevail under which the oxidation reaction of the cyanides initiated in the reaction reactor can take place, whereby a process liquid is formed; d) a continuous liquid flow can be maintained at least temporarily by means of a pump system, in which da) base liquid is continuously transferred from the pretreatment reactor of the pretreatment zone into the reaction reactor via a transfer line; db) intermediate liquid is continuously transferred from the reaction reactor via the connecting line into the process reactor; and dc) process liquid is continuously withdrawn from the process reactor.

[0003] Such methods and systems are described, for example, in US 5,676,846 A, US 4,416,786 A, US 5,178,775 A, US 5,093,007 A or US 3,900,555 A.

[0004] Many industrial sectors generate wastewater contaminated with high salt concentrations and toxic or persistent substances and compounds, and these undesirable components cannot be removed through biological treatment and purification processes. Such wastewater is treated using chemical-physical processes.

[0005] In this case, wastewater contaminated with cyanides is treated. The term cyanide refers to all salts and compounds of hydrogen cyanide (HCN, hydrogen cyanide). Examples include hydrogen cyanide, alkali and alkaline earth cyanides, numerous metal complexes such as Zn(CN)2 and potassium hexacyanidoferrate(III), as well as other compounds such as thiocyanates and cyanohydrins. Depending on the stability of the cyanide compounds, they are usually divided into free cyanide, readily releasable cyanide, and stable metal cyanide complexes.

[0006] Cyanides that are easily converted to hydrogen cyanide (HCN) upon acidification to a pH value below 4 are considered free cyanide. These include hydrogen cyanide, alkali metal cyanides, and alkaline earth metal cyanides. The cyanide ion reacts as a ligand with various metals to form metal cyanide complexes. The stability of the complexes depends on the oxidation number of the metal ion, which gives rise to the above-mentioned classification. The group of easily releasable cyanides includes, in addition to free cyanides, complexes with silver (Ag), cadmium (Cd), copper (Cu), mercury (Hg), nickel (Ni), and zinc (Zn). Metal cyanide complexes with low to moderate stability are referred to as "weak acid dissociable" (WAD). Far more stable are iron complexes, especially iron(II) complexes, which are not addition complexes but rather interpenetration complexes. They are stable even in concentrated sulfuric acid.Stable metal cyanide complexes are referred to as strong acid dissociable (SAD). According to DIN 38405-13, complexes with cobalt are only partially detected in the determination of readily releasable cyanide, which is why they are classified here as stable metal cyanide complexes.

[0007] In the present case, simple nitriles R-CN, such as acetonitrile or benzonitrile, as well as cyanate ions (OCN -< ), thiocyanate ions (SCN -< ) and cyanogen chloride (CICN) and derivatives thereof are also to be considered to fall under the term cyanide, although such compounds do not fall under the term cyanide according to DIN 38405-13.

[0008] The ratio between free cyanides and metal cyanide complexes in water depends on the pH value and the concentrations of heavy metals capable of forming metal cyanide complexes. In alkaline conditions, free cyanide is completely dissociated and forms stable metal complexes. In neutral and acidic environments, free cyanide is weakly dissociated, or hydrogen cyanide is preferentially formed. Weakly complexed metal cyanides decompose at pH values ​​below 4, which contributes to the increased formation of hydrogen cyanide.

[0009] Above a certain concentration, all cyanide compounds cause ecotoxicity and human toxicity. Hydrogen cyanide, or the cyanide ion, is by far the most toxic. The cyanide ion acts as a nonspecific enzyme inhibitor and achieves its high toxicity by inhibiting the enzyme cytochrome c oxidase, which stops cellular respiration and causes cell death. At low doses, symptoms include irritation of mucous membranes, headaches, dizziness, shortness of breath, palpitations, and vomiting. High doses lead to suffocation, respiratory and circulatory arrest. Furthermore, late effects on the central nervous system are known. Due to its low boiling point of 25°C, HCN is often absorbed through the respiratory tract. A dose of just 200–300 ppm is fatal. The lethal dose for oral ingestion is 1 mg CN / kg body weight. The MAK value is 1.9 ppm.

[0010] Cyanides are known to impact biological processes in wastewater treatment. Although cyanides can be broken down by some microorganisms, they have a negative impact on biological processes at elevated concentrations. The presence of cyanides often inactivates microorganisms by inhibiting cellular respiration. Furthermore, the release of heavy metals caused by cyanide degradation can further inhibit biological processes. With regard to free cyanides, concentrations greater than 0.2 mg / l for nitrification and greater than 5 mg / l for denitrification should be avoided to ensure stable plant operation. Metal complex cyanides, on the other hand, lead to partial inactivation of biological processes only at higher concentrations due to their lower toxicity.

[0011] The majority of cyanides in surface waters are attributable to industrial sources; cyanide compounds are widely used in various industries and enter surface waters through wastewater. These cyanide-containing wastewaters originate primarily from industrial areas such as metal cleaning and metal coating, electroplating, metal processing, automotive parts production, steel processing (particularly during steel tempering), mining, photography, the pharmaceutical industry, coking plants, ore leaching, and pesticide and plastics production. Cyanides are used in most industrial applications due to their tendency to form metal complexes. Cyanide-containing solutions are used, for example, in metal recovery and the mining industry to extract metals such as gold (Au) and silver (Ag) from ore.Cyanide-containing solutions are used in electroplating processes to keep metal ions such as zinc (Zn) and cadmium (Cd) in solution in a neutral to alkaline environment. In the chemical industry, cyanides are used to produce other chemicals such as acrylonitrile, adiponitrile, and methyl methacrylate. In the steel industry, cyanide-contaminated wastewater is generated during blast furnace gas treatment with gas scrubbers and during distillation processes in coking plants. Cracking and delayed coke formation in the petroleum industry also generate cyanide-containing waste and wastewater.

[0012] The object of the invention is to provide a method and a plant of the type mentioned above, by means of which an effective and resource- and energy-saving treatment of cyanide-containing wastewater is possible.

[0013] This task is solved in a method of the type mentioned above by e) base liquid and / or intermediate liquid and / or process liquid is returned to the pretreatment zone by means of an analysis circuit device, wherein the cyanide concentration in the respective liquid is determined by means of the analysis circuit device; and f) process liquid is returned to the pretreatment zone; and / or g) process liquid is at least partially transferred into a storage container (84) during withdrawal and liquid is returned from the storage container to the pretreatment zone.

[0014] According to the invention, cyanide detoxification is thus carried out in a continuous process in a reactor cascade with at least two stages, rather than in a batch process. This does not preclude the possibility of temporarily operating in batch mode, for example, in the event of malfunctions or high cyanide concentrations that must first be reduced. The process fluid withdrawn from the process reactor has a residual cyanide concentration that is lower than the cyanide concentration of the base fluid and lower than the cyanide concentration of the intermediate fluid. The cyanide concentration of the process fluid should therefore not exceed a predetermined threshold or limit value that defines a permissible cyanide concentration at which the fluid can be considered to be cyanide-detoxified. Conclusions can be drawn about the course of the reactions in the pretreatment zone, in the reaction reactor, or in the process reactor.The base liquid can be diluted if necessary.

[0015] If a catalyst and / or a pH adjuster is also added to the reaction reactor as needed, the reaction conditions can be adapted to the prevailing requirements during the process.

[0016] Preferably, copper in the form of a copper sulfate solution is used as the catalyst and / or sodium hydroxide solution is used as the pH adjuster.

[0017] If a pH adjuster, in particular sodium hydroxide solution, and / or an oxidizing agent, in particular hydrogen peroxide, is added to the process reactor as required, the reaction conditions can also be adapted to the prevailing requirements during the process.

[0018] Preferably, the pH value in the continuous liquid stream is kept largely constant, in particular in a range between 9 and 10, and / or the temperature of the liquid stream is kept largely constant, in particular in a range between 40°C and 60°C.

[0019] The above-mentioned task is solved in a system of the type mentioned above by e) base liquid and / or intermediate liquid and / or process liquid can be returned to the pretreatment zone by means of an analysis circuit device, wherein the cyanide concentration in the respective liquid can be determined by means of the analysis circuit device with the aid of a cyanide analysis device; and f) process liquid can be returned to the pretreatment zone via a return system; and / or g) a storage container is present, into which process liquid can be at least partially transferred via a storage line when withdrawn from the process reactor, and liquid from the storage container can be returned to the pretreatment zone.

[0020] Advantageously, the system also has some or all of the features specified in dependent claims 7 to 9. The advantages correspond to the advantages explained above for the method with respect to the respective corresponding features.

[0021] An exemplary embodiment of the invention will now be explained in more detail with reference to the single figure. The figure schematically shows a system 10 for treating cyanide-containing liquid 12, which may in particular be cyanide-containing wastewater 14 from industrial processes as explained above.

[0022] The plant 10 comprises a pretreatment zone 16, in which cyanide-containing liquid 12 undergoes a pretreatment in which at least a predetermined pH value and a predetermined temperature are established. During the pretreatment, a base liquid 18 is formed, which, following the pretreatment, passes from the pretreatment zone 16 via a transfer line 20 into a reaction zone 22.

[0023] The pretreatment zone 16 comprises a pretreatment reactor 24 in which the liquid present there is stirred by a stirrer 26.1, which is only indicated stylized, so that existing liquid components are intimately mixed.

[0024] The cyanide-containing liquid 12 to be treated is fed to the pretreatment reactor 24 through a feed line 28. A pH adjusting agent 32 is also metered into the pretreatment reactor 24 by means of a pH metering device 30, which has a pH storage container 34 for the pH adjusting agent 32 and a pH discharge line 36 connected thereto, which in turn opens into the pretreatment reactor 24 and includes a metering valve V1.

[0025] Apart from the metering valve V1 and the valves listed below, system 10 is equipped with corresponding valves at all points where a liquid flow must be interrupted, released, throttled or increased, as is well known.

[0026] In the present embodiment, the pH adjusting agent 32 is sodium hydroxide (NaOH). The pH value of the liquid in the pretreatment reactor 24 is monitored by sensors not specifically shown, which are known per se. The pH dosing device 30 increases, decreases, or interrupts the supply of pH adjusting agent 32 depending on the pH value determined therein compared to a target pH value.

[0027] A temperature control device 38 is provided to ensure that the base liquid produced in the pretreatment zone 16 during pretreatment reaches the reaction zone 22 at a defined temperature. In the present embodiment, the temperature control device 38 is designed as a heat exchanger 40, through which the transfer line 20 passes. In a modification not specifically shown, a heating unit or the like can also be provided at the pretreatment reactor 24, so that the base liquid is produced in the pretreatment reactor 24. The base liquid 18 should have a temperature between 40°C and 60°C, in particular a temperature of 50°C.

[0028] The liquids in the system 10 are conveyed in a manner known per se by means of a pump system 42, of which only one conveying pump arranged in the transfer line 20 is shown as an example and does not bear a separate reference symbol.

[0029] The transfer line 20 now opens into the reaction zone 22 into a reaction reactor 44 equipped with a stirrer 26.2. A catalyst 48 is metered into the reaction reactor 44 by means of a catalyst metering device 46, which has a catalyst storage container 50 for the catalyst 48 and a catalyst discharge line 52 connected thereto, which in turn opens into the reaction reactor 44. In the present embodiment, the catalyst 48 is copper sulfate CuSO 4 , and a CuSO 4 solution is used. In the present embodiment, a 10 to 20% CuSO 4 solution is used.

[0030] An oxidizing agent 56 is also metered into the reaction reactor 44 by means of an oxidizing agent metering device 54, which has an oxidizing agent storage container 58 for the oxidizing agent 56 and an oxidizing agent discharge line 60 connected thereto, which in turn opens into the reaction reactor 44 and in which a metering valve V2 is arranged. In the present embodiment, the oxidizing agent 56 is hydrogen peroxide H 2 O 2 , and a 25 to 50% H 2 O 2 solution is used.

[0031] In addition, pH adjusting agent 32 can be added to the reaction reactor 44 via a second pH dosing device 62. In the present embodiment, a pH branch line 64 leads from the dosing valve V1 in the pH discharge line 36 of the pH dosing device 30. The discharged amount of pH adjusting agent 32 can be adjusted independently of the amount added to the pretreatment reactor 24. The dosing valve V1 is designed as a multi-way valve, by means of which the volume flow through the pH discharge line 36 and the pH branch line 64 can be adjusted. Furthermore, a dosing valve V3 is arranged in the pH branch line 64.

[0032] By adding catalyst 48 and oxidizing agent 56, an oxidation reaction of the cyanides is initiated in the reaction reactor 44 and an intermediate liquid 66 is formed.

[0033] When H2O2 is used as an oxidizing agent in the presence of copper as a catalyst, cyanide ions are oxidized to cyanate according to CN -< + H2O2 -(Cu)-> OCN -< + H2O. This reaction can proceed in a relatively wide pH range between 3 and 12. Although it is known to carry out the reaction at a pH of 4 to 5, in the present case a pH between 9 and 10 is preferably set during the pretreatment, which is then maintained in the reaction reactor 44. At such pH values, the release of hydrogen cyanide HCN can be largely prevented and the stability of H2O2 can be ensured.

[0034] Cyanate is subsequently hydrolyzed and further oxidized in alkaline conditions to carbonate CO 3 -< and ammonia NH 3 . In acidic conditions, cyanate could be oxidized to carbon dioxide CO 2 and ammonium NH 4 +<.

[0035] The reaction reactor 44 is connected by a connecting line 68 to a process reactor 70 of the reaction zone 22, through which the intermediate liquid 66 is transferred into the process reactor 70. The process reactor 70 is equipped with a stirrer 26.3. Conditions prevail in the process reactor 70 under which the oxidation reaction of the cyanides initiated in the reaction reactor 44 can proceed, forming a process liquid 72.

[0036] pH adjusting agent 32 can be added to the process reactor 70 via a third pH dosing device 74. In the present embodiment, a further pH branch line 76 branches off from the dosing valve V3 in the pH branch line 64 of the second pH dosing device 62. The dispensed amount of pH adjusting agent 32 can be adjusted independently of the amount added to the pretreatment reactor 24. The dosing valve V3 is accordingly designed as a multi-way valve, through which the respective volume flow through the pH branch lines 64 and 76 can be adjusted.

[0037] Furthermore, oxidant 56 can be supplied to the process reactor 68 via a second oxidant metering device 78. In the present embodiment, an oxidant branch line 80 leads from the metering valve V2 in the oxidant discharge line 60 of the first oxidant metering device 54. The discharge amount of oxidant 56 can be adjusted independently of the amount metered into the reaction reactor 44. The metering valve V2 is accordingly designed as a multi-way valve, by means of which the respective volume flow through the oxidant discharge line 60 and the oxidant branch line 80 can be adjusted.

[0038] The process reactor 70 is connected to a storage tank 84 via a storage line 82, so that process fluid 72 can be transferred into the storage tank 84. The storage tank 84 has an outlet line 86, by means of which fluid can be discharged from the storage tank 84.

[0039] The cyanide content of the liquids present in the plant can be determined at various stages during the ongoing treatment using a cyanide analysis device 88, so that the treatment process, the temperature during pretreatment and the dosage of catalyst 48 and oxidant 56 can be adjusted depending on the analysis results.

[0040] For this purpose, the transfer line 20, the reaction reactor 44 and the process reactor 70 are connected via a respective analysis circuit line 90, 92 and 94, respectively, to the cyanide analysis device 88, in which the liquids from the analysis circuit lines 90, 92, 94 can each be analyzed separately for their cyanide concentration.

[0041] The cyanide analysis device 88 comprises a circular collection line 96, via which the partial streams are returned to the pretreatment reactor 24. Overall, the analysis circuit lines 90, 92, 94, together with the cyanide analysis device 88 and the circular collection line 96, thus form an analysis circuit 88, 90, 92, 94, 96, by means of which base liquid 18 and / or intermediate liquid 66 and / or process liquid is returned to the pretreatment zone 16, wherein the cyanide concentration in the respective liquid 66, 72, 88 is determined by means of the analysis circuit 88.

[0042] In addition, the pretreatment reactor 24 is connected to the process reactor 70 and the storage tank 84 via a return system 98. For this purpose, a return manifold 100 leads from a multi-way valve 102 to the pretreatment reactor 24, with a return line 104 from the process reactor 70 and a return line 106 from the storage tank 84 leading into the multi-way valve 102.

[0043] Each reactor 24, 44, 70 and the storage vessel 84 is connected to an exhaust air system (not shown separately), which creates a negative pressure in the reactors 24, 44, 70 and the storage vessel 84 by continuously extracting the air. The extracted air can be thermally treated in an exhaust air incineration plant (also not shown separately).

[0044] The plant 10 is operated in a continuous flow process. For this purpose, the pump system 42 is configured to maintain a continuous liquid flow, in which base liquid 18 is continuously transferred from the pretreatment zone 16 to the reaction reactor 44, intermediate liquid 66 is continuously transferred from the reaction reactor 44 to the process reactor 70, and process liquid 72 is continuously withdrawn from the process reactor 70.

[0045] The reaction parameters pH, temperature, and cyanide concentration are monitored during the treatment process. From the determined reaction parameters, the respective requirements for pH adjuster 32, catalyst 48, and oxidant 56 are determined for a smooth reaction in the pretreatment reactor 24, the reaction reactor 44, and the process reactor 70. pH adjuster 32, catalyst 48, and oxidant 56 are then added at the appropriate point as needed.

[0046] Preferably, the pH value in the continuous liquid stream is kept substantially constant and in particular in the above-mentioned range between 9 and 10. The temperature in the continuous liquid stream is also preferably kept constant, in particular in the range between 40°C and 60°C mentioned above for the base liquid 18 and preferably at 50°C.

[0047] By recycling process liquid 72 from the process reactor 70 or liquid from the storage tank 84, the liquid in the pretreatment reactor 24 can be diluted if necessary, so that the cyanide concentration of the base liquid 18 entering the reaction zone 22 is lower than without this measure.

[0048] If the analysis of the process liquid in the process reactor 70 shows that the cyanide concentration there exceeds the above-mentioned permissible threshold or limit value for a cyanide-detoxified solution, the storage line 82 is closed and the process liquid is returned to the pretreatment reactor 24 until the cyanide concentration in the process reactor 70 has dropped sufficiently that the process liquid with the permissible residual cyanide concentration can be withdrawn and fed to the storage container 84.

[0049] The plant 10 described above is designed as a two-stage reactor comprising the reaction reactor 44 and the process reactor 70. In modifications not specifically shown, further reactors may follow the process reactor 70, so that cyanide detoxification takes place in a continuous process in a three-, four-, or even higher-stage reactor cascade.

Claims

1. A method for treating cyanide-containing liquids (12), more particularly cyanide-containing wastewater (14), wherein a) cyanide-containing liquid (12) is subjected in a pretreatment zone (16) to a pretreatment wherein at least a specified pH between 3 and 12 and a specified temperature are established, the pretreatment forming a base liquid (18); b) base liquid (18) having a temperature between 40°C and 60°C enters at least one reaction reactor (44) and is admixed in that reaction reactor (44) at least with an oxidizing agent (56) in the form of hydrogen peroxide, thereby initiating an oxidation reaction of the cyanides, c) liquid from the reaction reactor (44) is transferred as intermediate liquid (66) into at least one process reactor (70), in which conditions prevail under which the cyanide oxidation reaction initiated in the reaction reactor (44) is able to proceed, forming a process liquid (72); d) at least periodically a continuous liquid stream is maintained, wherein da) base liquid (18) is transferred continuously from the pretreatment zone (16) into the reaction reactor (44); db) intermediate liquid (66) is transferred continuously from the reaction reactor (44) into the process reactor (70); and dc) process liquid (72) is withdrawn continuously from the process reactor (70); characterized in that e) base liquid (18) and / or intermediate liquid (66) and / or process liquid (72) is returned into the pretreatment zone (16) by means of an analytical circuit facility (88, 90, 92, 94, 96), the analytical circuit facility (88) ascertaining the cyanide concentration in the respective liquid (66, 72, 88); and f) process liquid (72) is returned into the pretreatment zone (16); and / or g) process liquid (72) on withdrawal is transferred at least partly into a storage vessel (84), and liquid from the storage vessel (84) is returned into the pretreatment zone (16).

2. The method as claimed in claim 1, characterized in that additionally a catalyst (48) and / or a pH modifier (32) is added to the reaction reactor (44).

3. The method as claimed in claim 2, characterized in that said catalyst (48) used is copper in the form of a copper sulfate solution and / or said pH modifier used is aqueous sodium hydroxide solution.

4. The method as claimed in any of claims 1 to 3, characterized in that a pH modifier (32), more particularly aqueous sodium hydroxide solution, and / or an oxidizing agent (56) in the form of hydrogen peroxide, is added to the process reactor (70).

5. The method as claimed in any of claims 1 to 4, characterized in that the pH in the continuous liquid stream is held largely constant, more particularly in a range between 9 and 10, and / or the temperature of the liquid stream is held largely constant, more particularly in a range between 40°C and 60°C.

6. A plant for treating cyanide-containing liquids (12), more particularly cyanide-containing wastewater (14), having a) a pretreatment zone (16), in which cyanide-containing liquid (12) can be subjected to a pretreatment which comprises a pretreatment reactor (24), wherein a pH metering facility (30) is provided by means of which a pH metering modifier (32) can be added insuch a way that a specified pH between 3 and 12 can be established, and a specified temperature can be established, the pretreatment forming a base liquid (18); b) at least one reaction reactor (44) into which base liquid (18) having a temperature between 40°C and 60°C enters, in which reaction reactor (44) the base liquid (18) can be admixed with an oxidizing agent (56) in the form of hydrogen peroxide, thereby initiating an oxidation reaction of the cyanides wherein c) liquid from the reaction reactor (44) can be transferred as intermediate liquid (66) via a connecting line (68) into at least one process reactor (70), in which conditions prevail under which the cyanide oxidation reaction initiated in the reaction reactor (44) is able to proceed, forming a process liquid (72); d) by means of a pump system (42), at least periodically, a continuous liquid stream can be maintained, wherein da) base liquid (18) is transferred continuously from the pretreatment reactor (24) of the pretreatment zone (16) into the reaction reactor (44); db) intermediate liquid (66) is transferred continuously from the reaction reactor (44) via the connection line (68) into the process reactor (70); and dc) process liquid (72) is withdrawn continuously from the process reactor (70); characterized in that e) base liquid (18) and / or intermediate liquid (66) and / or process liquid (72) can be returned into the pretreatment zone (16) by means of an analytical circuit facility (88, 90, 92, 94, 96), the analytical circuit facility (88) being able to ascertain the cyanide concentration in the respective liquid (66, 72, 88) with the aid of a cyanide analysis facility (88); and f) process liquid (72) can be returned via a recycle system (98) into the pretreatment zone (16); and / or g) there is a storage vessel (84) into which process liquid (72) on withdrawal is at least partly transferrable, and liquid from the storage vessel (84) can be returned into the pretreatment zone (16).

7. The plant as claimed in claim 6, characterized in that additionally a catalyst (48), by means of a catalyst metering facility (46), and / or a pH modifier (32), by means of a pH metering facility (62), can be added to the reaction reactor (44).

8. The plant as claimed in claim 7, characterized in that the catalyst (48) is copper in the form of a copper sulfate solution and / or the pH modifier is aqueous sodium hydroxide solution.

9. The plant as claimed in any of claims 6 to 8, characterized in that a pH modifier (32), by means of a pH metering facility (74), and / or an oxidizing agent (56) in the form of hydrogen peroxide, by means of an oxidizing agent metering facility (78), can be added to the process reactor (70).