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
  • 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
  • C01—INORGANIC CHEMISTRY
  • C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
  • C01D3/00—Halides of sodium, potassium or alkali metals in general
  • C01D3/14—Purification
  • C01D3/145—Purification by solid ion-exchangers or solid chelating agents
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/46—Treatment of water, waste water, or sewage by electrochemical methods
  • C02F1/461—Treatment of water, waste water, or sewage by electrochemical methods by electrolysis
  • C02F1/46104—Devices therefor; Their operating or servicing
• • 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
  • C25B13/00—Diaphragms; Spacing elements
• • 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
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/68—Treatment of water, waste water, or sewage by addition of specified substances, e.g. trace elements, for ameliorating potable water
  • C02F1/683—Treatment of water, waste water, or sewage by addition of specified substances, e.g. trace elements, for ameliorating potable water by addition of complex-forming compounds
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2201/00—Apparatus for treatment of water, waste water or sewage
  • C02F2201/46—Apparatus for electrochemical processes
  • C02F2201/461—Electrolysis apparatus
  • C02F2201/46105—Details relating to the electrolytic devices
  • C02F2201/46115—Electrolytic cell with membranes or diaphragms
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2201/00—Apparatus for treatment of water, waste water or sewage
  • C02F2201/46—Apparatus for electrochemical processes
  • C02F2201/461—Electrolysis apparatus
  • C02F2201/46105—Details relating to the electrolytic devices
  • C02F2201/4618—Supplying or removing reactants or electrolyte
  • C02F2201/46185—Recycling the cathodic or anodic feed
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F5/00—Softening water; Preventing scale; Adding scale preventatives or scale removers to water, e.g. adding sequestering agents
  • C02F5/08—Treatment of water with complexing chemicals or other solubilising agents for softening, scale prevention or scale removal, e.g. adding sequestering agents
  • C02F5/10—Treatment of water with complexing chemicals or other solubilising agents for softening, scale prevention or scale removal, e.g. adding sequestering agents using organic substances

Definitions

• the invention relates to a process for preventing the degeneration of membranes in electrolysis plants for the production of chlorine during operation using certain complexing agents.
• Chlorine is an important industrial chemical used to produce pesticides, disinfectants, chlorine bleach cleaners, swimming pool chemicals, PVC plastic, synthetic rubber, and other chlorinated chemicals. Chlorine can be prepared from an aqueous sodium chloride solution (brine) by electrolysis, giving caustic soda and hydrogen as byproducts. Three methods for the extraction of chlorine by electrolysis are used industrially. Mercury cell electrolysis was the first method used to produce chlorine on an industrial scale. Titanium anodes are located above a liquid mercury cathode and a solution of sodium chloride is positioned between the electrodes. When an electrical current is applied, chloride is released at the titanium anodes, while the sodium dissolves into the mercury cathode forming an amalgam.
• a second method is diaphragm cell electrolysis, where an asbestos diaphragm is deposited on an iron grid cathode to prevent the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing. This method uses less energy than the mercury cell, but the sodium hydroxide is not as easily concentrated and precipitated into a useful substance.
• a third method is membrane cell electrolysis, which eliminates the need for mercury and asbestos as process materials and enables greater energy efficiency. In said method, the electrolysis is performed in electrolysis cells which are divided into two by a membrane acting as an ion exchanger.
• a brine treatment is therefore to be performed to reduce the content of organic contaminations and also a treatment to reduce brine "hardness", i.e. to reduce the levels of calcium, magnesium, and strontium ions present in the brine to below about 20 ppb for calcium and magnesium ions, commonly employed upper specification limits for these alkaline earth metal ions in brine serving as feedstock in a chlor-alkali plant.
• brine preparation and treatment typically consist of the following steps.
• raw salt sodium chloride
• dechlorinated brine i.e.
• brine which after having left the electrolysis cells is dechlorinated and subsequently recycled, to produce a saturated brine solution.
• the brine solution optionally after dilution with water, is first subjected to a treatment wherein alkaline earth and transition metals are precipitated as their carbonates and/or hydroxides, followed by a filtering or settling process such as clarification.
• a further treatment is necessary wherein essentially all the remaining calcium, strontium, and magnesium impurities are removed.
• This treatment is typically performed using chelating ion exchange resins having a high affinity for alkaline earth metal ions.
• transition metal impurities present in the brine originating for example from iron-containing non-caking agents or present in the water stream used to prepare the brine, are only partially bound by the Ca, Sr, and/or Mg- removing ion exchanger.
• the brine which is to be fed to the electrolysing cells will still comprise a significant residual amount of these transition metal ions, such. t as Fe(ll), Fe(lll), and/or Al(lll) ions.
• transition metal contaminations present in the brine normally have a strong tendency to precipitate, int. al. as oxides and/or hydroxide complexes.
• the transition metal ions will normally precipitate on the surface of the membrane at the anode side and inside the membrane, causing degeneration of the membrane.
• the energy consumption will gradually increase, while the production of chlorine will gradually decrease due to a decreased current efficiency, until the membrane is fouled to such an extent that it needs cleaning or replacing.
• the overall production capacity would increase while the overall energy consumption would decrease.
• complexing agents according to the present invention decompose in the presence of chlorine into components that cause no worry in the chlor-alkali process and, as a consequence, that these complexing agents do not foul the membranes themselves.
• the complexing agents according to the present invention can even be dosed to the aqueous salt solution if dissolved active chlorine is present therein.
• the membrane needs to be cleaned less frequently, and less aggressive cleaning products can be used. Furthermore, the membrane lifetime and the current efficiency were found to increase, while the cell voltage was decreased. Moreover, the process according to the present invention has the advantage that it can be implemented in conventional electrolysis plants without necessitating extended adaptations of the installations.
• the present invention relates to a membrane cell electrolysis process to prepare chlorine from an aqueous salt solution
• aqueous salt solution comprising the steps of dissolving a sodium chloride source in water to form an aqueous salt solution comprising sodium chloride, and dosing one or more special complexing agents selected from the group consisting of monosaccharides, oligosaccharides, polysaccharides, polycarboxylic acids, polyacrylates, polymaleic acids, and derivatives thereof to said aqueous salt solution in a total amount of, on average, between 0.1 mg and 1,000 mg per litre of aqueous salt solution at a stage of the electrolysis process where the amount of active chlorine dissolved in said aqueous salt solution is less than 1.5 g per litre of aqueous salt solution and/or to the membrane cells, in order to reduce fouling and/or clogging of the membrane in said membrane cell.
• the aqueous salt solution comprising sodium chloride is hereinafter also denominated as "brine feed”
• a complexing agent selected from the group consisting of monosaccharides, oligosaccharides, polysaccharides, polycarboxylic acids, polyacrylates, polymaleic acids, and derivatives thereof and which does not comprise any carboxylic acid groups should pass test 1 or test 2, and preferably, it should pass both test 1 and test 2, in order to be suitable for use according to the present invention:
• the complexing agent is to be used in an electrolysis process which is performed under alkaline conditions, said test should be performed as follows: To 1 litre of saturated brine having a pH of 2, 150 ppb FeCU based on the amount of Fe(lll) are added. Furthermore, 100 ppm of complexing agent are added. Subsequently, a sodium hydroxide solution is added until the pH is 10. The mixture is filtered through a 0.45 micron pore size Millez- HA syringe driven filter unit and the filtered brine is analysed for Fe concentration by conventional ICP (Inductively Coupled Plasma) spectrometry. If less than 50 ppb of Fe(lll) is present in the solution, the complexing agent is considered to have passed test 1 as well.
• ICP Inductively Coupled Plasma
• a complexing agent comprises one or more carboxylic acid groups, an additional requirement is that it should also pass test 3 in order to be suitable for use according to the present invention:
• a feed brine containing 300 g/l of NaCI, 300 ppm of a carboxylic acid group-containing complexing agent, 0.72 ppm of Mg, 5.8 ppm of Ca, 1.90 ppm of Fe is prepared.
• the pH of the brine is 10.3.
• An ion exchange step is performed on this brine at 60°C to treat the brine for hardness.
• An aminomethyl phosphonic acid-functionalised chelating ion exchange resin (AMP type resin; 50 g resin) is used for this purpose.
• the ion exchange step is performed at a feed rate of 14 g/min (12.6 bed volumes / h). If after 24 hours the concentration of Fe is less than 100 ppb, the complexing agent is considered to have passed test 3.
• a complexing agent which fulfills the above-mentioned requirements is hereinafter denoted as a degeneration reducing agent.
• a complexing agent such as N,N,N',N'- ethylenediamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA) or sodium gluconate
• EDTA N,N,N',N'- ethylenediamine tetraacetic acid
• NTA nitrilotriacetic acid
• sodium gluconate sodium gluconate
• a chelating agent such as gluconate anion
• water-soluble complexes are formed with a fraction of the multivending transition metal cations such as Fe(lll), Ni(ll), and Cr(lll). Complexation thus is beneficial in that it hinders transition metal salts from precipitating in the manufacturing equipment.
• gluconate anion forms a strong iron-gluconate complex, thereby solubilising iron in the brine solution.
• a brine solution is purified for use in an electrolysis cell by subjecting it to a primary brine treatment in order to remove alkaline earth metals such as calcium and magnesium present in the brine either by passing it through an ion exchange resin or by elevating the pH to precipitate the calcium and magnesium as their carbonates and/or hydroxides.
• problems will arise.
• the fraction of a transition metal species such as iron(lll) present as a gluconate complex remains strongly chelated and will therefore not be removed in said purification step.
• an additional brine purification step should therefore be performed wherein the brine is passed through a functionalised resin to remove a fraction of the multivending metal ion contaminants present in the brine in the form of water-soluble complexes with metal chelating agent.
• conventional electrolysis plants where typically only a primary brine purification section is present, require transformation before the just-mentioned processes can be performed, which is undesirable.
• nitrogen in chelating agents such as EDTA or NTA will lead to the formation of NCI 3 in the electrolysis cells, leading to the formation of explosive gas mixtures, which is also undesirable.
• the degeneration reducing agents according to the present invention function as follows inside the membrane electrolysis cells of a chlor-alkali plant.
• the degeneration reducing agents will be decomposed due to the presence of chlorine at the anode side of the membrane.
• compounds comprising carboxylic end groups will be formed. Said acid groups will complex the transition metal ions present in the brine.
• the transition metal(s) preferably Fe(lll)
• the transition metal(s) preferably Fe(lll)
• the degeneration reducing agents of the present invention have the advantage that they can be dosed to the feed brine before the brine is subjected to brine purification steps such as purification using an ion exchanger. This is because they merely form a relatively weak complex with transition metal ions. Hence, when the brine comprising the contaminants as well as the degeneration reducing agent(s) is fed to an ion exchanger for reduction of "hardness", the degeneration reducing agent will release all alkali metal ions and/or transition metal ions. Subsequently, these ions will be removed by this treatment. For example, when in the primary brine purification the brine is passed through an ion exchanger, these ions will be absorbed by said ion exchanger to the largest possible extent.
• degeneration reducing agent(s) will remain dissolved in the brine.
• the residual amount of transition metal ions in the brine not bound by the ion exchanger will be confronted with an excess of degeneration reducing agent, which will complex these ions.
• degeneration reducing agent can be found among saccharides, polycarboxylic acids, and derivatives thereof.
• saccharides are used.
• saccharide as used throughout the specification is meant to include monosaccharides (i.e. carbohydrates which usually possess 3 - 9 carbon atoms), oligosaccharides (i.e. carbohydrates which usually possess 2 - 20 monosaccharide units), and polysaccharides (i.e. carbohydrates possessing more than 20 monosaccharide units).
• Carbohydrate is used in its usual annotation to denominate products of the formula C ⁇ (H 2 O) y , wherein x is 3 - 2,000, preferably 3 - 900, and wherein y is 3 - 2,000, preferably 3 - 900.
• derivatives of said saccharides can also be employed as degeneration reducing agents.
• Derivatised saccharides are preferably selected from the group consisting of dehydrated saccharides, esterified saccharides, saccharides bearing one or more phosphate groups, one or more phosphonate groups, one or more phosphino groups, one or more sulfate ⁇ groups, one or more sulfonate groups, and/or one or more amino groups.
• degeneration reducing agents comprising nitrogen are less preferred.
• the degeneration reducing agents preferably do not contain any CH 2 or CH 3 groups since the presence of such groups is known to result in the formation of undesired chloroform and/or other chlorinated organic compounds in electrolysis operations. More preferred derivatives of saccharides are selected from the group consisting of dehydrated saccharides and esterified saccharides.
• Suitable monosaccharides are for example fructose, ribose, erythrose, and monoglyceraldehyde.
• Suitable oligosaccharides are for example lactose, maltose, and saccharose (also called sucrose).
• Suitable polysaccharides are amylose, cellulose. Saccharides which can be used according to the invention also include (partially) oxidised saccharides and derivatives thereof.
• An example of an oxidised saccharide that can be used is tartaric acid (D, L, meso, and/or mixtures thereof).
• hydroxypolycarboxylic acids and especially meso tartaric acid are preferably not used.
• the (derivatised) saccharides can be in the open form or in the ⁇ - or ⁇ -ring form.
• the (derivatised) saccharide is a ketone or an aldehyde, generally referred to as a ketose and an aldose, respectively.
• polycarboxylic acids which can be used include but are not limited to polymaleic acid, polyacrylic acid, Belsperse ® , Belgard ® EV, and Belgard ® EV 2000.
• the one or more degeneration reducing agents can be dosed to the brine at any stage of the process, provided that the brine contains less than 1.5 g, preferably less than 1.0 g, more preferably less than 0.75 g, most preferably less than 0.5 g of dissolved active chlorine per litre of brine.
• the brine to which the degeneration reducing agent(s) is/are dosed contains no active chlorine. In practice, this means that the degeneration reducing agent(s) is/are dosed to the brine after the dechlorination step, but before entrance of the brine feed into the electrolysis cells. This is to avoid early decomposition of the degeneration reducing agents due to reaction with said chlorine as much as possible.
• Dosing the degeneration reducing agents to the brine just after dechlorination, e.g in the salt dissolver, is less preferred, since, preferably, the residence time of the degeneration reducing agents in the brine before it enters the electrolysis cells is made as brief as possible.
• the one or more degeneration reducing agents are dosed to the brine in a later stage of the process, e.g. after a primary brine treatment step.
• the degeneration reducing agents are dosed to the brine after said brine has been subjected to a purification step using an ion exchanger. It is also possible to dose the degeneration reducing agent(s) directly in the membrane electrolysis cells.
• the word "dosing" is used to describe the step of adding the one or more degeneration reducing agents to the aqueous salt solution comprising sodium chloride which serves as feed brine for the membrane cell electrolysis process to produce chlorine in order to prevent degeneration of the membrane.
• the complexing agent(s) is/are not already present in the sodium chloride compositions from which the brine is prepared.
• the dosing can be done continuously, meaning that for a certain period of time the compounds are continuously added to the feed brine.
• the degeneration reducing , agent in more than one stage of the process, for example, by dosing part of the degeneration reducing agent continuously or intermittently to the feed brine before the ion exchange purification step, while another part is dosed continuously or intermittently directly in the membrane cell.
• the degeneration reducing agent(s) can be dosed to the feed brine in any conventional manner. Preferably, they are dosed to the feed brine in the form of an aqueous solution or as a solid.
• the sodium chloride may be from any source but is preferably sodium chloride from a natural salt source such as rock salt, a subterraneous sodium chloride deposit in a well exploited by means of dissolution mining, and/or solar salt, including lake or sea salt.
• the aqueous salt solution is prepared by dissolution of said salt source in water.
• any water supply typically used in conventional chlor-alkali processes can be employed. It may for example be demineralised water, dechlorinated depleted brine, or mixtures thereof.
• the brine in the processes of the present invention comprises at least 100 g/l of sodium chloride, more preferably at least 150 g/l, even more preferably at least 200 g/l, and most preferably at least 300 g/l.
• brine leaving the electrolysis cells also denoted as depleted brine
• depleted brine is first dechlorinated and subsequently recycled into the process.
• concentration of sodium chloride in the brine solution may be increased to obtain the most efficient operation of the cell. It is furthermore possible to store depleted brine in an anolyte tank where it is mixed with fresh, purified brine, after which it is returned to the electrolysis cells.
• the concentration of the degeneration reducing agent(s) in the brine entering the electrolysis cells is kept at a constant level so that the compositionof the brine for the membrane process is not fluctuating. chorus.
• the total amount of degeneration reducing agents present in the feed brine entering the membrane electrolysis cell is, on average, less than 1 ,000 mg, preferably less than 500 mg per litre of feed brine.
• less than 200 mg and more preferably less than 50 mg of degeneration reducing agent(s) is used per litre of feed brine.
• Concentrations of degeneration reducing agent(s) higher than 1,000 mg per litre of feed brine are also possible, but are less preferred.
• more than 0.1 mg, preferably more than 1 mg, and most preferably more than 5 mg of degeneration reducing agent(s) is used per litre of feed brine.
• one or more water- soluble cellulose ethers are used to reduce or inhibit fouling and/or clogging of the membranes.
• Cellulose ethers are certain derivatives of cellulose. Cellulose is a poly- saccharide composed of individual anhydroglycose units which are linked through a 1.4 glucosidic bond (see Formula A).
• the number "n” of anhydroglucose units in the polymer chain is defined as the degree of polymerisation (DP).
• the degree of polymerisation (DP) of cellulose depends on the origin of the cellulose.
• the water-soluble cellulose ethers according to the present invention have a DP of between 250 and 30,000.
• Each anhydroglucose ring carries three OH-groups at positions 2, 3, and 6, which are chemically active. The distribution of the substituents introduced onto the polymer chain is largely determined by the relative reactivity of these three OH-groups.
• Cellulose ethers are typically made by reacting some or all of these cellulose OH-groups of highly purified and bleached cellulose with one or more alkylating agents. Since potentially three hydroxyl groups are available on each anhydroglucose ring, derivatives of cellulose are usually characterised in terms of a "degree of substitution” (DS), which is an average per anhydroglucose unit for the whole chain and can range between 0 and 3. Reaction of merely one type of alkylating agent with cellulose results in a simple cellulose ether, whereas using of two or more types of alkylating agents leads to the formation of mixed ethers.
• DS degree of substitution
• the water-soluble cellulose ethers according to the present invention can be either simple or mixed ethers, preferably having a DS of between 0.2 and 3, more preferably between 0.5 and 2.5.
• Cellulose ethers can be divided into ionic and non-ionic types.
• Ionic cellulose ethers such as for example sodium carboxymethylcellulose, contain substituents which are electrically charged.
• Nonionic cellulose ethers, such as methylcellulose and hydroxyethylcellulose carry electrically neutral substituents.
• Mixed ethers with ionic and non-ionic substituents are classified according to their predominant features.
• Cellulose ethers suitable for use according to the present invention can be either ionic or non-ionic cellulose ethers, as long as they are water-soluble. It is noted that by the term “water-soluble” is meant that the cellulose ether has a solubility in water at 20°C of at least 1 ppm, but preferably of at least 10 ppm, more preferably of at least 50 ppm, and most preferably of at least 100 ppm. Preferably, use is made of one or more cellulose ethers selected from the group consisting of alkyl, alkylhydroxyalkyl, hydroxyalkyl, and carboxyalkyl cellulose.
• the cellulose ether according to the present invention is selected from the group consisting of carboxymethyl cellulose, sodium carboxymethyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, and hydrophobically modified hydroxyethyl cellulose. It was found that these types of cellulose ethers can be used in the process to prepare chlorine from brine, because in the presence of chlorine they will decompose into components that cause no worry in the chlor-alkali process. In particular, merely carbon dioxide, hydrochloric acid, and water are formed.
• water-soluble cellulose ethers are used, preferably their total amount present in the brine feed is less than 500 mg, more preferably less than 250 mg per litre of the brine feed. If water-soluble cellulose ethers are used, preferably their total amount is at least 1 mg, more preferably at least 5 mg, and most preferably at least 25 mg per litre of the brine feed.
• membrane as used throughout this specification is meant to denote any membrane conventionally employed in an electrolysis cell. Examples of frequently used membrane types in electrolysis cells are National ® ex DuPont and Aciplex ® ex Asahi Kasei Corporation. The present invention is elucidated by means of the following non-limiting Example.
• a membrane electrolysis experiment was executed in a laboratory set-up consisting of the following equipment: • 2 cylindrical glass containers, i.e. the anolyte vessel and the catholyte vessel, with an effective volume of about 125 ml each. • A 5 cm, 1 cm diameter glass connection between the containers equipped with a construction to place a 2.2 cm effective diameter electrolysis membrane in the tubing separating the anolyte from the catholyte. • 1 reference electrode placed as close as possible to the membrane surface in the catholyte and 1 reference electrode placed as close as possible to the membrane surface in the anolyte. These electrodes are used to measure the voltage drop over the membrane. • Two magnetic stirring devices to stir the catholyte and anolyte vessels.
• a stock solution of 5 litres of nearly saturated brine (300 g/l of NaCI) was prepared by adding sodium chloride (ultrapure grade ex Merck) to distilled water.
• 100 ppm (mg/l) of Fe were added to the solution as FeCfo.
• the resulting brine is called the iron-containing electrolysis feed brine.
• a stock solution of 5 litres of caustic with a concentration of 21 wt% of NaOH was prepared by dissolving ultrapure NaOH (ex Merck) in distilled water.

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