PROCESS FOR THE ELECTROCHEMICAL PURIFICATION OF CHLORIDE-CONTAINING PROCESS SOLUTIONS

Abstract

The invention relates to a method for the electrochemical purification of chloride-containing, aqueous process solutions, which are contaminated with organic chemical compounds, using a boron-doped diamond electrode at a pH value of at least 9.5.

Claims

1.-17. (canceled)

18. A process for the electrochemical purification of chloride-containing aqueous process solutions contaminated with organic chemical compounds using a boron-doped diamond electrode, wherein the purification using a boron-doped diamond electrode is carried out a potential of more than 1.4 V measured against the reversible hydrogen electrode (RHE) and a pH of the process solution of at least pH 10, in the anode zone of an electrolysis cell to a prescribed total content of organic chemical compounds (TOC).

19. The process as claimed in claim 18, wherein the purification is carried out in a plurality of passes of the process solution through the anode zone.

20. The process as claimed in claim 18, wherein the purification is carried out in a plurality of separate anode zones connected in series.

21. The process as claimed in claim 18, wherein the purification is carried out to a total content of organic chemical compounds (TOC) of not more than 500 mg/kg.

22. The process as claimed in claim 18, wherein the process solution contains an organic solvent selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons and halogenated aromatic hydrocarbons.

23. The process as claimed in claim 18, wherein the process solution contains a catalyst residue.

24. The process as claimed in claim 18, wherein the process solution contains monomers or low molecular weight polymers.

25. The process as claimed in claim 18, wherein the process solution comprises cresols, in particular from preproduction for crop protection agents, as organic chemical impurity.

26. The process as claimed in claim 18, wherein the concentration of chloride ions in the process solution at the beginning of the purification is up to 20% by weight.

27. The process as claimed in claim 18, wherein the process solution comprises essentially chloride ions from alkali metal chloride, in particular sodium chloride.

28. The process as claimed in claim 18, wherein the process solution is a process water from the production of polymers, in particular a polymer from the group consisting of polycarbonate, polyurethanes and precursors thereof, in particular of isocyanates, particularly preferably of methylenedi(phenyl isocyanate) (MDI), tolylene diisocyanate (TDI), or from the production of dyes, crop protection agents, pharmaceutical compounds and precursors thereof.

29. The process as claimed in claim 18, wherein the process solution is a process water from the production of epichlorohydrin.

30. The process as claimed in claim 18, wherein the boron-doped diamond electrode is based on a support composed of at least one material selected from the group consisting of: tantalum, silicon and niobium.

31. The process as claimed in claim 18, wherein the boron-doped diamond electrode has a multiple coating comprising finely divided diamond.

32. The process as claimed in claim 30, wherein the multiple coating comprising diamond has a minimum thickness of 10 m.

33. The process as claimed in claim 18, wherein the purified process water is subsequently subjected to an alkali metal chloride electrolysis.

34. The process as claimed in claim 32, wherein the materials chlorine and sodium hydroxide and optionally hydrogen obtainable from the alkali metal chloride electrolysis located downstream of the electrolytic purification are recirculated independently of one another to the chemical production of polymers, dyes, crop protection agents, pharmaceutical compounds and precursors thereof.

Description

EXAMPLES (GENERAL DESCRIPTION)

[0078] A cell Z divided by an ion-exchange membrane 3, as is shown schematically in FIG. 1, was used. A Diachem diamond electrode from Condias (plate electrode) or an expanded metal electrode from DIACCON was used as an anode 11 in an anode space 1. The electrode here is in each case a diamond electrode on the support material niobium. The active area, measured at the ion-exchange membrane area of the electrolysis cell, was 100 cm.sup.2. The electrode spacing was 12 mm, resulting from an 8 mm space in between anode 11 and membrane 3 and a 4 mm spacing between membrane 3 and cathode 12, A Flemion F-133 membrane from Asahi Glass was used as ion-exchange membrane 3. The laboratory cell was pressed together by means of six threaded rods and nuts (M12) tightened with a defined moment of 15 Nm. The electrolytes 14 and 15 were in each case circulated. An electrolyte pump 4 was in each case used for the anolyte 14 and a pump 5 was used for the catholyte 15. The anolyte 14 was pumped in the circuit at a volume flow of 76.8 l/h and the catholyte 15 was pumped in the circuit at a volume flow of 15.0 l/h. Before each experiment, both circuits were flushed with DI water (DI=deionized) for about one hour; the water was changed three times during the time. After introduction of the electrolytes 14, 15, these were pumped at the abovementioned volume flow through the heat exchangers 6, 7 and heated to a temperature of 60 C. The temperature was measured by means of the temperature sensors (Pt 100) in the respective circuit. Depending on the way in which the process solution to be purified is to be treated, a storage vessel 8 for stocking the process solution to be purified can be additionally installed in the circuit.

[0079] An oxygen depolarized cathode (ODC) 12 was used as cathode. The cathode space 2 is separated impermeably from the gas space (2b) by the oxygen depolarized cathode 12. To start up the electrolysis, pure oxygen or an oxygen-containing gas is introduced via an inlet 2c into the gas space 2b. Excess oxygen/oxygen-containing gas goes out from the gas space 2b again via the outlet 2d. The gas stream leaving the outlet 2d from the gas space 2b could be backed up by banking-up or by means of immersion into a liquid and the pressure in the gas chamber 2b could thus be increased. The oxygen pressure in the gas space 2b preferably more than 20 mbar and can, depending on the cell design, be increased up to 60 mbar. Possible condensate formation caused in the gas space 2b, e.g. by passage of catholyte through the ODC 12 is discharged together with excess gas via 2d from the gas space 2b. On reaching the desired electrolyte temperature, the rectifier (not shown) is switched on and the current is increased in a ramp up to the desired operating current. The rectifiers are controlled by a measuring and regulating system from Delphin. At the beginning of the experiment, samples were taken from the anolyte circuit and the catholyte circuit in order to determine the initial pH of the solutions by means of a pH meter and for monitoring by means of acid-base titration. In addition, samples were taken from the anolyte circuit at defined time intervals during the experiment in order to determine the decrease in TOC over time. The cell voltage and also the anode and cathode potentials were continually measured and monitored during the experiment.

[0080] In order to set the pH of the electrolytes and observe the course during the experiment, the pH was determined by means of a pH measuring instrument from Mettler Toledo (model FiveEasy) and monitored by means of an acid-base titration.

[0081] The TOC content of the samples was determined by means of a TOC instrument from Elementar (model vario TOC cube). The sample was here diluted with DI water by a factor 5 and brought to a pH of 1 by means of concentrated hydrochloric acid (32% by weight). The chlorine analysis used for evaluating the experiments is described in detail below.

[0082] Furthermore, the anolyte was examined for the presence of chlorine in the oxidation state zero or greater than zero and also in respect of the chloride concentration. For analysis of the chloride concentration, the Mohr chloride determination was employed. Firstly, 1 ml of the solution is taken at room temperature (Eppendorf pipette), diluted with 100 ml of distilled water and a spatula tip of sodium hydrogencarbonate (NaHCO.sub.3) (pH buffer) is subsequently added. The sample is subsequently acidified by means of 5-10 drops of 10% strength nitric acid, and 5 ml of potassium chromate solution are added. The solution is then titrated against a 0.1 M silver nitrate solution (AgNO.sub.3) until a brown coloration persists. As a result of the silver nitrate solution added during the titration, white silver chloride precipitates at the equivalence point. The persistent brown coloration arises from the equivalence point onward by formation of sparingly soluble silver chromate. The concentration of sodium chloride is thus calculated from the consumption of silver nitrate.

[0083] The analysis to determine whether chlorine in the oxidation state zero or greater than zero is present is carried out by analysis of the compounds sodium hypochlorite or hypochlorous acid and chlorate. The analysis of sodium hypochlorite/hypochlorous acid and chlorate is carried out by total chlorine determination in bleaching liqor. 1 ml of the sample solution was firstly diluted with distilled water to 300 ml and provided with a spatula tip of NaHCO.sub.3. The titration was subsequently carried out with arsenous acid (0.05 M) as spot sample on potassium iodide starch paper. In the presence of sodium hypochlorite/hypochlorous acid, chlorine and chlorate, the potassium iodide starch paper becomes violet, and the titration was carried out until the spot sample on the starch paper no longer displayed a coloration.

[0084] The proportion of the total chlorine which was present in the form of chlorate was determined as follows: the detection of chlorate was carried out directly after the total chlorine determination. To determine the chlorate concentration of the solution, it is firstly necessary to determine a blank, and subsequently determine the sample value. The blank characterizes the amount of chlorate in the solution before the sample is added. 10 ml of the sulfuric acid ammonium iron(II) sulfate solution (for the blank determination without sample) was firstly added to the 1 ml sample and the mixture was diluted with distilled water. The reagent was brought to boiling and boiled for 10 minutes. After cooling, a titration with potassium permanganate solution (KMnO.sub.4, 0.02 M) was carried out to the first persistent pink coloration both for the blank determination and also the determination of chlorate. In the detection of chlorate, the chlorate firstly reacts with the Fe.sup.2+ions of the acidic solution, and the excess of Fe.sup.2+ions is subsequently oxidized by means of potassium permanganate solution (KMnO.sub.4). The concentration of chlorate is calculated from the consumption of potassium permanganate solution by sample and blank.

[0085] The cell construction described serves merely to illustrate the process of the invention. The process to water treatment can be carried out in various cell designs with and without use of a gas diffusion electrode.

Example 1According to the InventionFormate DegradationIllustrative Imitation Process Water From Methylenedi(Phenylamine) (MDA) Production

[0086] The process water to be treated was circulated through a laboratory electrolysis cell equipped with a Condias Diachern electrode as described above, a Covestro oxygen depolarized cathode and a cation-exchange membrane of the Flemion F133 type at a current of 4 kA/m.sup.2 and correspondingly an average voltage of 4 V. The anolyte consisted of a sodium chloride-containing process solution containing 10% by weight of sodium chloride and having a pH of 14.4. The content of sodium formate impurity was, measured as TOC, 24.48 mg/kg. A 1 molar sodium hydroxide solution was used as catholyte.

[0087] During the one hour during which the experiment was carried out, the anode potential was a constant 3.0 V vs, RHE, and the average cathode potential was 0.6 V vs. RHE. Furthermore, the degradation of organics (TOC) was measured over the experiment and the anolyte was examined to determine its total chlorine content (sodium hypochlorite, hypochlorous acid and chlorate) as described above.

[0088] The TOC content was 24.48 mg/kg at the beginning and could be completely mineralized at 20 Ah/l, so that the TOC at the end of the experiment was <1 mg/kg. The formation of chlorine in the oxidation state zero or greater than zero at the anode could not be detected during the entire process procedure.

Example 2According to the InventionPhenol Degradation

[0089] A 10% strength by weight NaCl-containing solution was admixed with phenol so that a TOC of 30.55 mg/kg was measured. The pH of the solution was 14.31. The solution was treated in an electrolysis cell as described in example 1 . The current density was maintained at 3 kA/m.sup.2. After the application of 30 Ah/l, the TOC content was only 9 mg/kg. The formation of chlorine in the oxidation state zero or greater than zero at the anode could not be detected here either.

Example 3According to the InventionProcess Water from MDA Production Production

[0090] The experiment of example 1 was carried out using an NaCl-containing solution from production of MDA. The pH of the NaCl-containing solution was 14.46. The solution had an initial TOC value of 70 mg/kg and was treated at a current density of 6 kA/m.sup.2. After 30 Ah/l, the TOC content was only 7 mg/kg.

[0091] Here too, purification of the process water could be carried out successfully, with no chlorine in the oxidation state zero or greater than zero being detected.

Example 4According to the InventionDegradation of Catalyst from PolycarbonateEthylpiperidine

[0092] The experiment of example 1 was carried out using ethylpiperidine as example of a catalyst residue as organic impurity in 10% strength by weight sodium chloride solution at a pH of 14.38. The averagae cell voltage was about 4.3 V at a current density of 4 kA/m.sup.2. The TOC content at the beginning of the electrolysis was 28 mg/kg. After introduction of a total of 30 AM, the TOC content was only 15 mg/kg. Formation of chlorine, hypochlorite or chlorate was not observed.

[0093] The anolyte was additionally examined titter the end of a Gerstel PDMS Twister analysis (absorption of polydimethylsiloxane and subsequent desorption with subsequent gas chromatography/mass spectrometry) and the organic trace materials present in the anolyte were revealed and identified. Chlorinated hydrocarbons could not be determined. This demonstrates that no chlorine formation occurs at the BDD anode in combination with the absent chlorine in the oxidation state zero and greater than zero.

Example 5Comparative Example Using Standard Coating of an Anode from Chloralkali Electrolysis (DSA Coating) Compared to a BDD Anode

[0094] The experiment of example 4 was carried out using a dimensionally stable anode (DSA) provided with a coating corresponding to chloralkali electrolysis. The coating was based on a mixture of iridium oxide and ruthenium oxide from Denora.

[0095] Even when samples were taken during the experiment, an odor of chlorine could be perceived. The consequent of anodic chlorine formation is the formation of chlorinated hydrocarbons, which could be confirmed by means of a Twister analysis of the anolyte after the end of the experiment.

Example 6BDD Coatings Comparative Example pH<9.5

[0096] The experiment described under example 3 was repeated, but the pH of the anolyte was set to pH 8. The experiment was carried out at a current density of 4 kA/m.sup.2, and an average cell voltage of 4.5 V was established. After the end of the experiment, chlorinated hydrocarbons could likewise be found in the anolyte by means of a Twister analysis.

Example 7BDD Using Imitation Process Water

[0097] In a cell as described in example 1 but equipped with an expanded metal electrode from DIACCON, an NaCl-containing solution having the following composition was used as anolyte: 15 g/l of NaCl, 132 mg/kg of formate, 0.56 mg/kg of aniline, 11.6 mg/kg of MDA, 30 mg/kg of phenol. The pH of the solution was 13.4. The volume flow of the anolyte was 121 l/h. A 1 molar sodium hydroxide solution was used as catholyte and was pumped at a volume flow of 15 l/h around the circuit. The current was 1 kA/m.sup.2, and the temperature was 60 C. The initial TOC was 78 mg/kg. After 30 minutes, the pH was 13.2 and the TOC content was only 18 mg/kg. 4 Ah/l of charge were introduced for the purification. The formation of chlorine in the oxidation state zero or is greater than zero at the anode could not be detected.

Example 8MDA Degradation

[0098] A 10% strength by weight NaCl-containing solution was admixed with 0.45 millimol of methylenedi(phenylamine) (MDA) and treated in an electrolysis cell as described in example 1. The pH was 14.4, and the current density was 5.5 kA/m.sup.2. The amount of dissolved MDA, which corresponded to a measured TOC of 25 mg/kg, was completely mineralized electrochemically after only 10 AWL The TOC content of the treated solution was 0 mg/kg. The formation of chlorine in the oxidation state zero or greater than zero at the anode could not be detected.

Example 9pH 7Influence of pH Value

[0099] In a cell as described in example 1 but equipped with an expanded metal electrode from DIACCON, an NaCl-containing solution having the following composition was used as anolyte: 15 g/l of NaCl, 132 mg/kg of formate, 0.56 mg/kg of aniline, 11.6 mg/kg of MDA, 30 mg/kg of phenol. The pH of the solution was 13.4. The volume flow of the anolyte was 121 l/h. A 1 molar sodium hydroxide solution was used as catholyte and was pumped around the circuit at a volume flow of 15 /h. The current was 1 kA/m.sup.2, and the temperature was 60 C. The initial TOC was 78 mg/kg. After 20 minutes, the pH was 13.4 and the TOC content was 34 mg/kg. The formation of chlorine in the oxidation state zero or greater than zero at the anode could not be detected.

[0100] The pH was then decreased to pH 7. After introduction of only 4 Ah/l, 3.5 g/l of chlorine in the oxidation state zero or greater than zero were found. The TOC content was not reduced in this case.

Example 10Use of a Purified MDA Process Water in the Chloralkali Electrolysis Cell

[0101] Process water is purified as described in example 1 and brought by means of solid sodium chloride to a concentration of 17% by weight of NaCl. The NaCl-containing solution produced in this way is subsequently used for chloralkali electrolysis in a laboratory electrolysis cell. The electrolysis cell has an anode area of 0.01 m.sup.2 and is operated at a current density of 4 kA/m.sup.2, a temperature at the outlet from the cathode side of 88 C., and a temperature at the output from the anode side of 89 C. Commercially coated electrodes having a coating for chloralkali electrolysis from DENORA, Germany are used as electrodes. An ion-exchange membrane N982 WX from Chemours is used for separating anode space and cathode space. The electrolysis voltage is 3.02 V. A sodium chloride-containing solution is pumped at a mass flow of 0.98 kg/h through the anode chamber. The concentration of the solution fed to the anode chamber is 25% by weight of NaCl. A 20% strength by weight NaCl solution can be taken from the anode chamber. 0.121 kg/h of the 17% strength by weight purified NaCl-containing solution and a further 0.0653 kg/h of solid sodium chloride are added to the NaCl solution taken from the anode chamber. The solution is subsequently fed back into the anode chamber.

[0102] On the cathode side, a sodium hydroxide solution is pumped in the circuit at a mass flow of 1.107 kg/h. The concentration of the sodium hydroxide solution fed into the cathode side was 30% by weight of NaOH, and the sodium hydroxide solution taken from the cathode side has a concentration of 32% of NaOH, 0.188 kg/h of the 31.9% strength alkali are taken from the volume stream, and the remainder is made up with 0.0664 kg/h of water and recirculated back into the cathode element.

[0103] A negative influence of the imitation process water freed of formate by means of the BDD electrode on the performance of the cell cannot be observed.