Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes

Definitions

• the invention relates to a process for the production of high-purity chlorine. It relates more particularly to the production of high-purity chlorine by the technique of the membrane electrolysis of sodium chloride. Numerous applications using chlorine require a high-purity product. The tolerated contents of impurities are, for example, commonly less than 10 ppm and sometimes even lower still.
• chlorine is produced by electrolysis of a sodium chloride brine, the major source of impurities is found in the starting material of the process. This is because sodium chloride in the natural state comprises bromides in proportions varying significantly according to the origins of the sodium chloride used, in particular when it originates from the mining of ores. These bromides are reencountered in the brine and contaminate the chlorine produced.
• Membrane electrolysis is a process for the production of chlorine and sodium hydroxide employing an array of cells each comprising an anode, a cathode and a separator composed of an ion-exchange membrane permeable to cations.
• the membrane separates the anode compartment, through which the brine passes and in which the chlorine is given off by the anode, from the cathode compartment, towards which the sodium cations migrate through the membrane to form sodium hydroxide therein.
• certain impurities present in the brine can, in addition, damage the membranes and destabilize the process.
• sodium chloride brines contaminated by bromides it is known (Modern Chlor Alkali Technology, vol. 7, p. 157-161) to oxidize the bromides using active chlorine, the bromine obtained subsequently being removed from the brine by stripping with air or passing over anionic ion- exchange resins.
• the formation of bromates has to be avoided.
• these known techniques are very inelegant and necessitate high capital costs.
• the invention is targeted at providing a process for the production of high- purity chlorine by membrane electrolysis of brines which is simple, efficient and economical, h this description, the term "high-purity chlorine” is understood to mean chlorine comprising less than 20 ppm, advantageously less than 10 ppm, preferably less than 5 ppm and very particularly preferably less than 1 ppm of bromine.
• the invention relates to a process for the production of high- purity chlorine by electrolysis in a membrane cell of a sodium chloride brine contaminated by bromides, the process being characterized in that the brine is oxidized so as to convert the bromides therein to bromates.
• the production of chlorine by electrolysis in a membrane cell of a sodium chloride brine is a very widespread technique.
• the cells can be monopolar or bipolar. This technique is based on the use of an ion-exchange membrane selectively permeable to cations and theoretically impermeable to anions.
• the selectivity ofthe membrane makes it possible to obtain, in conjunction with the chlorine, a concentrated sodium hydroxide solution. However, this selectivity ofthe membrane is not total.
• the concentration of he sodium hydroxide solution obtained is for this reason limited by the imperfect selectivity of he membrane. This is because certain anions, such as, for example, OH " ions can pass through it. A certain degree of contamination ofthe sodium hydroxide solution by chloride ions also occurs due to this limited selectivity ofthe membrane.
• the concentrated brine is introduced, preferably after purification from calcium and magnesium, into the anode compartment, which it leaves in the form of a depleted brine. After the brine has passed through a dechlorination unit, sodium chloride comprising bromides in the form of impurities is then introduced therein to reconcentrate the brine, which is finally reintroduced into the anode compartment, which closes the loop ofthe brine circuit.
• the release of chlorine takes place at the anode, while the Na + ions migrating through the ionic membrane rejoin the cathode compartment, where the sodium hydroxide (NaOH) solution is formed.
• NaOH sodium hydroxide
• the bromides present in the brine are oxidized to form bromates.
• the inventors have found that the bromate ions formed do not to a measurable extent pass through the ion-exchange membrane and are therefore not reconverted to bromides at the cathode. For this reason, they pass through the electrolyser without affecting the quality ofthe chlorine and ofthe sodium hydroxide produced.
• the oxidation can be carried out by any appropriate technique, such as the use of ozone.
• the oxidation is carried out using active chlorine.
• the source of active chlorine can be Cl 2 , sodium hypoc oriteiNaClO) or chlorinated brine.
• the introduction of active chlorine into the brine can be obtained, for example, using a metering pump (case of NaCIO), by stripping (case of Cl 2 ) or by adjusting a bypass placed on the unit for the dechlorination ofthe brine.
• the introduction of active chlorine can be carried out upstream or downstream of the membrane electrolysis cell, the brine circuit operating in a closed or open loop. However, it is preferable to introduce it upstream, immediately before the cell.
• the fittings used which are in contact with the brine are advantageously made from a substance resistant to a chlorinated brine at high temperature, such as titanium, steel treated with hard rubber or heUcally-reinforced PVC-C.
• a sufficient temperature for example of greater than 40°C.
• the oxidation is carried out at a temperature of greater than 60°C. It is preferable for the temperature to be greater than 70°C, indeed even greater than 75°C. Temperatures having a value of at least 80°C are particularly advantageous.
• an excessively low value ofthe pH ofthe brine has a negative effect on the conversion ofthe bromides to bromates. For example, at a pH of between 1 and 2, it was observed in some cases that bromates are converted to bromic acid, which itself decomposes to bromine, which can contaminate the chlorine produced.
• the pH of he brine is maintained at values of at least 2.5, preferably of at least 3, more preferably still of at least 4. However, it is recommended to avoid values of greater than 9. The values from 6 to 8.5 are particularly recommended, those varying from 7 to 7.5 being the most preferred.
• the bromates obtained by oxidation according to the invention pass through the ion-exchange membrane electrolyser without contaminating either the chlorine or the sodium hydroxide solution produced a some cases, to avoid an increase in the concentration ofthe bromates in the brine loop, it is advantageous to use a bleed. The bromates can then either be discharged or treated.
• a portion at least ofthe bromates produced is bled off and reduced by treatment of the brine in the ' presence of hydrogen over a catalyst made of noble metal on a support.
• the bromates which leave the brine loop in the bleed are reduced to bromides, which are better tolerated in the environment.
• Details relating to the catalytic reduction of bromates by treatment in the presence of hydrogen over a noble metal catalyst may be found in the Solvay document EP 0779880B1. Numerous supports are mentioned in this document, such as inorganic oxides (Al 2 O 3 , SiO 2 , ZrO 2 , MgO or TiO 2 ), aluminium silicate, magnesium aluminosilicate or active carbon.
• the support is active carbon.
• active carbon having a specific surface (BET) of greater than 1000 m 2 /g and a particle size such that at least 80% ofthe particles have a diameter of between 0.5 and 2.5 mm.
• BET specific surface
• the noble metal is preferably selected, alone or in combination with a metal from the copper group, from the metals of he eighth subgroup of the Periodic Table of the Elements.
• the noble metal is rhodium.
• Example 1 (in accordance with the invention) An electrolysis room comprising an array of monopolar electrolysers each comprising approximately 40 m 2 of Flemion® F8020 cation-permeable membrane (Asahi Glass) was fed with 200 m 3 h of brine comprising from 290 to 310 g/1 of sodium chloride.
• the brine having a temperature of approximately 80°C, comprised between 5 and 10 ppm of bromine in the form of bromides.
• the concentration of bromides in the brine was measured by oxidation of the bromides to bromates using sodium hypochlorite in a buffered medium, then reduction ofthe bromates by KI in a highly acidic medium and, finally, assaying by thiosulphate of I 2 formed.
• the unit for the dechlorination ofthe brine was equipped with a bypass which can be adjusted so as to control the presence of active chlorine in the brine loop in order to oxidize the bromides to bromates.
• the amount of active chlorine in the brine was adjusted in order to obtain an excess of approximately 400% with respect to the stoichiometric amount neceissary for the theoretical conversion to bromates of 100% ofthe bromides present in the brine.
• the residence time granted to the oxidation reaction was 20 minutes (holding tank).
• the true degree of conversion obtained was approximately 90%.
• the pH was maintained above 2.5, at a value of approximately 3, during the test.
• the presence of bromine in the chlorine produced was measured. An amount of approximately 15 mg/kg was obtained.
• the measurement technique used is based on the absorption of a given amount of chlorine (comprising bromine) in sodium hydroxide solution in the presence of hydrogen peroxide to prevent the formation of hypohalites. After the hydrogen peroxide has been completely decomposed by refluxing, the alkaline solution is neutralized and then buffered. The bromides are then oxidized to bromates using sodium hypochlorite. The excess of this reagent is destroyed with formic acid.
• Example 2 (in accordance with the invention " ) The procedure was the same as in Example 1, except that the brine flow rate was reduced to 120 m /h. For this reason, the residence time was increased, which further improved the oxidation reaction. The presence of bromine in the chlorine produced was measured under the same conditions as in Example 1. An amount of approximately 5 mg/kg was obtained.
• Example 3 (not in accordance with the invention) The procedure was the same as in Example 1 but the brine dechlorination bypass was closed, which very significantly reduced the presence of chlorine in the brine. The presence of bromine in the chlorine produced was measured, still under the same conditions as in Example 1. An amount of approximately 100 mg kg was obtained. The comparison of Examples 1 to 3 illustrates the advantage ofthe invention in the production of high-purity chlorine.

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• Chemical Kinetics & Catalysis (AREA)
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• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Treatment Of Water By Oxidation Or Reduction (AREA)

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• 2004
  • 2004-01-12 FR FR0400212A patent/FR2864969B1/fr not_active Expired - Fee Related
  • 2004-12-24 EP EP04805034A patent/EP1706520A2/en not_active Withdrawn
  • 2004-12-24 EA EA200601308A patent/EA012069B1/ru not_active IP Right Cessation
  • 2004-12-24 WO PCT/EP2004/053708 patent/WO2005068686A2/en active Application Filing

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