SU1584752A3 - Method of producing chlorine and sodium hydroxide - Google Patents

Abstract

52-EE-O-301 A halogen, such as chlorine, is generated by electrolysis of an aqueous solution of an alkali metal halide such as sodium chloride, in a cell having anolyte and catholyte chambers separated by a solid polymer electrolyte in the form of a stabler selectively cation permeable, ion exchange membrane. One or more catalytic electrodes including at least one thermally stabilized, reduced oxide of a platinum group metal are bonded to the surface of the membrane. An aqueous brine solution is brought into contact with the anode and water or an aqueous NaOH solution is brought into contact with the cathode. The brine is electrolyzed to produce chlorine at the anode and hydrogen and caustic at the cathode. The cell membrane preferably has an anion rejecting cathode side barrier layer which rejects hydroxyl ions to block back migration of caustic to the anode thereby enhancing the cathode current efficiency of the cell and of the process.

Description

The invention relates to electrochemical production, in particular, to methods for producing chlorine and alkali.

The aim of the invention is to reduce power consumption.

The process of obtaining chlorine occurs as a result of electrolysis of a solution of NaCl with a concentration of 2.5-5 M in the anode chamber, with a concentration of 5 M being preferred.

During electrolysis, a polymeric fluorocarbon cation-exchange membrane is used, sulfo and carboxyl groups are grafted on the cathode side, and a coating of a mixture of electrocatalytic particles with a fluorocarbon electrode is coated on the cathode side of the membrane.

binder The electrocatalyst includes reduced oxides of metals of the platinum group, such as ruthenium, iridium, and their mixture. The reduced oxides are heated at 350-750 ° C for 30 minutes to 6 hours, with the preferred thermal stabilization procedure being carried out by heating the reduced oxides for 1 hour at a temperature of 550-600 ° C. An anode connected to the membrane containing reduced ruthenium oxides is further stabilized by mixing it with graphite and / or mixing with reduced oxides of other platinum group metals, such as 1GO.

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(with a content of 5-25% iridium, the preferred amount is 5%) or platinum, rhodium, and so on, & also with reduced oxides Metals such as titanium (with a preferred content of 25-50% TiO) or with reduced tantalum oxides (25% or more). It was also found that a three-component alloy of reduced oxides of titanium, ruthenium and iridium (Ru, Ti, Ir) 0x or tantalum, ruthenium and iridium (Ru, Ir, Ta) Ox, bonded to the membrane, is very effective for obtaining sustainable durable anode In the case of a three-component alloy, it is preferable to include 5 to 25% by weight of reduced iridium oxides, approximately 50% by weight of reduced ruthenium oxides, and the remainder is such a valuable metal as titanium. Instead of titanium, mo (other transition metals such as niobium, tantalum, zirconium or hafnium can be used. J Alloys from reduced oxides of the listed metals are mixed with a fluorocarbon binder, taken in an amount of 15-30% by weight of the mixture.

The cathode part of the membrane is a mixture of particles of fluorocarbon binder with platinum black, which is taken in an amount of 0.4 4 mg / cm. Equally, other electrocatalytic materials, such as palladium, gold, silver, magnesium, cobalt, nickel, graphite, as well as reduced oxides, which are used on the anode part of the membrane-electrode block, can be used. The thickness of the cathode layer in the block is 1 5 -75 microns.

Electrolytic baths with cathodes 12.5-50 microns thick from platinum black with 15% Teflon work with a current output of 80% upon receipt

5M NaOH, electrolysis temperature 88 -91 ° C. When using a cathode 75 microns thick from a mixture of ruthenium with graphite, the current efficiency is 54% when preparing a 5 M sodium hydroxide solution.

In tab. 1 shows the dependence of the current output on the cathode thickness.

The electrode is made gas-permeable so that gases released at the interface between the electrode and the membrane can come out and

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porous to allow water to penetrate to the interface between the cathode and the membrane, where sodium hydroxide is formed, and that the sodium chloride solution has quick access to the membrane and the catalytic sites of the electrode.

A cation-exchange membrane is used as a membrane, on the cathode side of which a sulfo and carboxyl groups are grafted. Perfluorocarbon sulfonic acid cation exchange membranes provide good cation transfer, are stable, characterized by high thermal stability, and are not exposed to acids and strong oxidizing agents. The higher the concentration of sulfonic acid radicals, the greater the ion-exchange capacity, and hence the ability of the hydrated membrane to transfer cations. However, as the ion exchange capacity of the membrane increases, the water content increases, and the ability of the membrane to drain the salt decreases. The rate at which caustic soda migrates from the cathode to the anode, thus, increases with increasing ion-exchange capacitance. This leads to a decrease in the cathode current output, as well as to the formation of oxygen at the anode with all the undesirable consequences that accompany this phenomenon. Therefore, the preferred ion-exchange membrane for use in brine electrolysis is a layer consisting of thin (2 mm thickness) of a film with a milliequivalent weight of 1500, a cation-exchange membrane with a low water content (5-15%), which is characterized by a high salt removal capacity, attached to 4 mm or more oh film with high ion exchange capacity (milliekvivalentny weight is 1100) using Teflon tissue. One type. Such a layered structure is marketed by DuPont Company under the trade name Nafion 315.

The ion exchange membrane is made by caustic soaking (3 to 8 M) for 1 hour to fix the content of water in the membrane, ion transfer properties and turn it into a sulfonate form. In the case of a laminated membrane connected by a Teflon tissue, it may be desirable

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clean the membrane or Teflon fabric by washing it back with a stream of 70% HNOij for 3-4 h

For the manufacture of electrodes, platinum group metal oxides (ruthenium, iridium) are used with reduced oxides of such transition metals as titanium or graphite, which are combined with particles of the cation exchange membrane to form porous, gas-permeable catalytic electrodes, which are obtained decomposition of mixed metal salts in the presence or absence of excess sodium salts, i.e. nitrates, carbonates, etc. One method of producing these materials is the method of obtaining platinum by using thermally decomposable iridium, titanium or ruthenium halide compounds, i.e. salts of these metals, such as iridium chloride, ruthenium chloride or titanium chloride. An example is the production of a double alloy (ruthenium, iridium). Oh, when finely ground ruthenium and iridium salts are mixed with one another in the same mass proportion of ruthenium and iridium, as the alloy formed requires. An excess amount of sodium nitrate or equivalent alkali metal salts is added to this mixture, after which the mixture is fused in a quartz cup at 500-600 ° C for 3 hours. The residue is thoroughly washed to remove the nitrates and halides in it. The resulting slurry of mixed and fused oxides is reduced at room temperature by electrochemical reduction or by passing hydrogen bubbles through the mixture. The resulting product is thoroughly dried, crushed and screened through a nylon sieve. Typically, after sieving, the particles have a diameter of 3.7 microns.

The alloy of reduced oxides of ruthenium and iridium is then subjected to thermal stabilization as a result of heating for 1 hour at 500-600 ° C. The electrode is formed by mixing reduced, thermally stabilized oxides of metals of the platinum group with Teflon polytetrafluoroethylene particles (one under

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a common type of these particles is sold by DuPont Kompani (under the trade name Teflon T-30).

The reduced platinum group metal oxides can be mixed with a conductive carrier such as graphite, transition metal carbides, transition metals to improve stability and reduce the use of noble metal dosages (0.5 mg / cm). When using graphite-ruthenium mixture, powdered graphite is mixed with 15-30% by weight of a graphite-teflon mixture. The reduced metal oxides are mixed with a mixture of graphite-Teflon.

Table 2 presents data on the consumption of electrical energy and current efficiency (current output), depending on the thickness of the cathode associated with the membrane.

As can be seen from Table 2, a thickness of 75 microns is a threshold beyond which the consumption of electrical energy increases rapidly. The optimum thickness is 25 microns. Data concerning the percentage of 30 fluorinated binder in the cathode layer is not less than 15%, otherwise the electrodes would have an unacceptably short service life (possibly due to mechanical instability), and not more than 30%, since otherwise In this case, the cell voltage would be very high (over 4.5 V) due to excessive hydrophobicity and insufficient electrical conductivity of the electrode associated with the membrane.

Table 3 shows the data on the voltage on the bath, depending on the performance of the cathode and the anode.

From the data of Table 3 it can be seen that the voltage in the electrolytic bath 1, in which the electrodes are completely connected, is almost 1 volt in the control electrolytic bath 6, in which the electrodes are not connected at all. The electrolytic baths 2 and 3 of the mixed type with the addition of the cathode and the electrolytic baths 4 and 5 of the mixed type with the addition of the anode show results by about 0.4–0.6 V worse than those obtained for an electrolytic bath with fully connected electrodes, but 0.3-0.5 V is better than 35

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processes carried out in an electrolytic bath without connecting any electrodes.

Creating a process for obtaining chlorine from brine, far superior to known processes, became possible as a result of the reaction of brine anolyte and aqueous catholyte on catalytic electrodes directly attached and embedded in the surface of the cation-exchange membrane, and on the cathode of hydrogen and caustic with high degree of purity. Thanks to this device, the catalytic sites on the electrodes are in direct contact with the membrane and acid-exchange radicals in the membrane, making it possible to create a process that is more efficient with respect to voltage, in which the required bath potential is much better (1 V or more) than the known ones. processes. The use of highly efficient catalysts from a reduced oxide of a noble metal combined with a fluorinated hydrocarbon, as well as catalysts from a reduced oxide of a noble metal combined with a fluorinated hydrocarbon and graphite, characterized by low overvoltages, further increases the efficiency of this process.

The electrolysis process takes place in an electrolytic cell with the supply of a solution of sodium chloride with a concentration of 2.5–5 M, with a preference given to a 5 molar solution, since the current efficiency increases directly proportional to the increase in concentration.

Table 2 contains data on a 73% current output for a cathode layer thickness of 50-75 µm and a 54% yield for

current for the cathode layer 75 microns thick. In this case, the value of 75 µm for the thickness of the cathode layer is the boundary, beyond which the current efficiency is always lower, i.e. about 50-60%. When the thickness of the cathode layer is 75 μm, the current efficiency may be more or less satisfactory, which is typical of limit values. For example, in Table 1, two current output values (78 and 54%) are given for two cathode layers having a thickness of 75 microns. The lime electrolysis cell has the following parameters: 4.2-4.4 V, 90 ° C, 3000 A / m H. Under this condition, the consumption (consumption) of electricity in the known device is 3500 kWh / t NaOH.

The electricity consumption for the proposed technical solution is 2300 - 3300 kWh / t NaOH and provides maximum energy savings of the order of 200-1200 kW (h / t NaOH.

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Claims (1)

Invention Formula A method of producing chlorine and sodium hydroxide by electrolysis of sodium chloride in an electrolyzer separated by a polymer fluorocarbon cation-exchange membrane, on the cathode side of which sulfo-and carboxyl groups are grafted, on the cathode and anode spaces, characterized in that, in order to reduce power consumption, on the cathode side of the membrane a coating was deposited from a mixture of electrocatalytic particles with a fluorocarbon binder, taken in an amount of 15-30% by weight of the mixture, 15-75 µm thick and maintains the concentration of sodium chloride during electrolysis in the anode space of 2.5 - 5 M. Platinum chern50-7564 Platinum Chern 25-5075 Platinum chern12,578 5% platinum mobile on graphite7578 15% ruthenium oxide with graphite7554 Table 1 4.0 3.1 5.5 3.0 5.0 Indicators The value of the indicators at the thickness of the cathode associated with the membrane, µm (mils) Current output,% 78 Electricity consumption at 3.3 V as average voltage, kWh2796 with membrane) Platinized niobium screen (not connected to the membrane) KiOk - graphite (connected to the membrane) RuOx (connected to the membrane) Platinized niobium screen (not connected to the membrane) Table 2 73 54 57 2660 2988 4040 3820 Table3 black (confined membrane) niobium (not connected by a membrane) niobium (not connected by a membrane) niobium (not connected by a membrane) 3.4 3.5 3.3 3.8