MEMBRANE ELECTROLYSIS PROCESSES FOR AKALINE CHLORIDE SOLUTIONS, USING A GAS-DIFFUSION ELECTRODE

Abstract

The invention relates to processes for the electrolysis of alkali chlorides by means of oxygen-depolarized electrodes, said processes having specific operating parameters for shut-down and restarting.

Claims

1.-8. (canceled)

9. A process for chloralkali electrolysis using an electrolysis cell in a gap arrangement, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell comprises at least one anode space with anode and an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode which comprises a silver-containing catalyst and an in particular from 0.01 mm to 3 mm thick sheet-like porous element through which catholyte flows between gas diffusion electrode and membrane, the electrolysis process comprises at least the following steps in this order: a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero, b) and setting of the pH of the anolyte to a value in the range from pH 2 to pH 12 during step a), c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter), and optionally for emptying of the electrolysis cell the further steps: d) cooling of the anolyte to a temperature below 70° C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V, e) switching off of the electrolysis voltage at an electrolyte temperature of <55° C., f) emptying of the cathode gap, g) emptying of the anode space, h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space, i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.

10. The process as claimed in claim 9, wherein the alkali metal chloride is sodium chloride or potassium chloride.

11. The process as claimed in claim 9, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

12. The process as claimed in claim 9, wherein the gas diffusion electrode is supplied with oxygen gas on its side facing away from the catholyte.

13. The process as claimed in claim 9, wherein the oxygen gas flow to the gas diffusion electrode is maintained when the electrolysis is switched off.

14. A process for chloralkali electrolysis using a membrane electrolysis cell in a gap arrangement between ion exchange membrane and gas diffusion electrode, in particular with a spacing of from 0.01 mm to 3 mm between ion exchange membrane and gas diffusion electrode, where the cell has at least one anode space with anode for accommodating an anolyte containing alkali metal chloride, an ion exchange membrane, a cathode space with a gas diffusion electrode as cathode, which comprises a silver-containing catalyst, and a sheet-like, porous element in the gap between ODE and membrane, which element has a thickness of, in particular, from 0.01 mm to 3 mm and through which catholyte flows during operation, wherein, for start-up of the electrolysis process, at least the following steps are carried out in this order: j) filling of the anode space with anolyte having a temperature of at least 50° C. and passage of the anolyte through it, k) preheating of catholyte to a temperature of at least 50° C., l) filling of the cathode space and the porous element with preheated catholyte having a concentration of from 7.5 to 10.5 mol/l and passage of the catholyte through them, m) setting of the electrolysis voltage to a value in the range from 0.1 to 1.4 V, n) setting and maintenance of the temperature of the catholyte and anolyte leaving the cell independently of one another to a temperature in the range from 70 to 100° C., o) setting of the concentration of the catholyte in the feed to the cell so that an alkali metal hydroxide concentration in the range from 7.5 to 12 mol/l is obtained in the output, p) setting of the concentration of the anolyte in the feed to the cell so that an alkali metal chloride concentration in the range from 2.9 to 4.3 mol/l is obtained in the output, q) setting of the production current density to a value of at least 2 kA/m.sup.2, preferably at least 4 kA/m.sup.2.

15. The process as claimed in claim 14, wherein the increase in the current density to the production current density in step q) is carried out at a rate of from 0.018 kA/(m.sup.2*min) to 0.4 kA/(m.sup.2*min) until the current density at the electrolysis element is at least 2 kA/m.sup.2.

16. The process as claimed in claim 14, wherein the start-up is a restarting of an electrolysis cell which has been shut down according to a process comprising at least the following steps in this order: a) lowering of the electrolysis voltage and removal of chlorine from the anolyte so that less than 10 mg/l of active chlorine is present in the anolyte by maintaining an electrolysis voltage per element of from 0.1 to 1.4 V and a current density which is greater than zero, b) and setting of the pH of the anolyte to a value in the range from pH 2 to pH 12 during step a), c) residence under these conditions as long as electrolyte is present in the catholyte gap (or electrolyte flows through the latter), and optionally for emptying of the electrolysis cell the further steps: d) cooling of the anolyte to a temperature below 70° C. with maintenance of the electrolysis voltage in the range from 0.1 to 1.4 V, e) switching off of the electrolysis voltage at an electrolyte temperature of <55° C., f) emptying of the cathode gap, g) emptying of the anode space, h) preferably renewed filling of the anode space with one of the following liquids: dilute alkali metal chloride solution having a maximum concentration of 4 mol/l or deionized water, and subsequent emptying of the anode space, i) filling of the cathode space with one of the following liquids: dilute alkali metal hydroxide solution having a maximum concentration of 10 mol/l or deionized water, with subsequent emptying of the cathode space.

Description

EXAMPLES

[0105] The gas diffusion electrode used in the examples was produced as described in EP 1728896 B1, as follows: a powder mixture consisting of 7% by weight of PTFE powder, 88% by weight of silver(I) oxide and 5% by weight of silver powder was applied to a gauze made of nickel wires and pressed to give an oxygen-depolarized electrode.

[0106] The electrode was installed in an electrolysis unit having an area of 100 cm.sup.2 with a DuPONT type N982 ion exchange membrane (manufactured by Chemours) and a spacing between gas diffusion electrode and ion exchange membrane of 3 mm.

[0107] The electrolysis unit has, in the assembled state, an anode space having an anolyte inlet and outlet and an anode consisting of titanium expanded metal which was coated with a commercial DSA coating for chlorine production from Denora, consisting of a mixed oxide of ruthenium oxide/iridium oxide, and a cathode space having the gas diffusion electrode as cathode and having a gas space for the oxygen and oxygen inlets and outlets, a liquid outlet and an ion exchange membrane, which are arranged between anode space and cathode space. A lower pressure prevailed in the anode space than in the cathode space, so that the ion exchange membrane was pressed onto the anode structure with a pressure of about 30 mbar as a result of the higher pressure in the cathode chamber.

[0108] The electrolysis cell was operated at a brine concentration of about 210 g/l (3.58 mol/l) of NaCl and a sodium hydroxide concentration of about 31% by weight (10.4 mol/l) at electrolyte temperatures of about 85° C. The cell voltage was corrected to 32% by weight (10.79 mol/l) of sodium hydroxide and 90° C. by customary standard methods.

[0109] The electrolytes were each introduced into the cell from below and taken off again from the top of the cell.

[0110] Oxygen was fed to the gas space of the cathode. An oxygen having a purity of more than 99.5% by volume of oxygen was used here. The oxygen was humidified with water at room temperature before being introduced into the gas space of the cathode half shell. The amount of oxygen was regulated so that a 1.5-fold stoichiometric excess over the amount of oxygen required based on the current strength set was always introduced. The oxygen is fed from the top into the gas space and discharged at the bottom.

[0111] The electrolysis unit had a gap of about 3 mm between oxygen-depolarized electrode and ion exchange membrane. This gap was filled with a porous PTFE woven fabric as percolator and spacer.

[0112] The production current density was 6 kA/m.sup.2.

Example 1—Start-Up

[0113] Before start-up of the catholyte circuit, oxygen saturated with water was fed at room temperature into the cathode space so that the pressure in the cathode gas space was 59 mbar. The hydrostatic pressure of the sodium hydroxide solution at the lowest point in the cell was 32 mbar.

[0114] After this, an external catholyte circuit containing an about 31% strength by weight (10.4 mol/l) sodium hydroxide solution was started up and the sodium hydroxide solution was heated, but the sodium hydroxide solution was not yet conveyed through the cell.

[0115] In the next step, the anolyte circuit was, according to the invention, started up and the anode space was filled with an anolyte having a concentration of about 210 g of NaCl/l (3.58 mol/l). While the anode circuit was maintained and the anolyte was conveyed through the cell, the anolyte was heated to 50° C. by means of a heat exchanger present in the anode circuit.

[0116] After the sodium hydroxide solution had attained a temperature of 50° C., the sodium hydroxide solution having a temperature of 50° C. was fed into the cell and, after filling of the cathode gap within 30 seconds, an electrolysis voltage of 1.08 V was applied. This resulted in a current density of 10 mA/cm.sup.2 being established.

[0117] The pH of the outflowing anolyte was 8.

[0118] The electrolyte was heated from 50° C. to 70° C. within 1 hour. After the temperature of the outflowing anolyte and catholyte of 70° C. had been attained, the electrolysis voltage was increased, with the electrolysis voltage being increased such that the current density was raised every 2 minutes by 50 mA/cm.sup.2 up to a current density of 600 mA/cm.sup.2.

[0119] The concentrations were regulated after start-up so that the concentration of the outflowing brine was about 210 g/l (3.59 mol/l) and that of the sodium hydroxide solution was about 31.5% by weight (10.6 mol/l).

[0120] The cell was operated for at least 24 hours under these conditions.

Example 2—Shutdown—According to the Invention

[0121] The electrolysis unit was operated at a current density of 600 mA/cm.sup.2.

[0122] For shutdowns, the current density was reduced to 1.5 mA/cm.sup.2. For this purpose, the main rectifier was disconnected and the polarization rectifier was switched in. The polarization rectifier then takes over maintenance of a current density of 1.5 mA/cm.sup.2. The operation at the low current density was maintained for 1.5 hours. After this, the anolyte is chlorine-free. This process is carried out in industrial electrolyzers for safety reasons. One of the reasons is that chlorine or chlorine compounds such as hypochlorite do not diffuse from the anolyte through the ion exchange membrane into the catholyte and lead there to corrosion of cell components or the gas diffusion electrode. On the basis of experience, the phase of chlorine-free flushing takes about 1.5 hours in industrial electrolyzers.

[0123] Electrolyte circuits remained in operation with the same volume flows as in electrolysis operation at 600 mA/cm.sup.2. The O.sub.2 supply was likewise maintained.

[0124] During the phase of chlorine-free flushing, the temperature of the anolyte and of the catholyte was reduced from 85° C. to 70° C. The cell voltage during this phase was about 1.16 V and the pH of the outflowing anolyte from the cell was pH 8.2.

[0125] After 1.5 hours, the temperature of anolyte and catholyte is reduced to 50° C., with the polarization rectifier being operated potentiostatically. Here, the voltage of 1.16 V is maintained and the current is appropriately decreased.

[0126] After cooling of anolyte and catholyte, the polarization rectifier is disconnected and the catholyte is immediately drained from the cathode space. This occurs over a time of about 30 seconds. After emptying of the cathode space, the anode space is drained within 1 hour.

[0127] The anode space is filled with deionized water from below up to a height of max. 50% of the cell height and immediately drained off again.

[0128] The cathode gap was likewise flushed by renewed switching-on of the catholyte pump and feeding of catholyte into the cathode space. For this purpose, the catholyte pump was switched on for about 10 seconds. The catholyte gap ran empty within 15 seconds.

[0129] The cell was then allowed to stand for 10 hours.

[0130] The start-up was then carried out as described in Example 1.

[0131] A total of 32 downtimes (shutdown processes) were carried out.

[0132] At the beginning of the experiment, the cell voltage at a current density of 600 mA/cm.sup.2 was 2.48 V.

[0133] After 32 downtimes, the cell voltage at a current density of 600 mA/cm.sup.2 was 2.48 V.

[0134] The cell voltage remained unchanged and damage to the gas diffusion electrode and further components did not occur.

Example 3—Shutdown—Comparative Example

[0135] An electrolysis unit was started up as in Example 1. Shutdown was carried out according to the prior art, as follows: [0136] reduction of the electrolysis current to 1.8 mA/cm.sup.2 [0137] electrolyte circuits remained in operation with the same volume flow as in electrolysis operation, likewise the O.sub.2 supply [0138] the temperature of the electrolytes was reduced to 75° C. within 1.5 hours while a current density of 1.8 mA/cm.sup.2 is maintained. [0139] the voltage supply was switched off [0140] immediately after switching off of the voltage supply, the anode space was firstly emptied over a time of about 1 hour. [0141] after emptying of the anode space, the cathode space was emptied. [0142] the anode space was then filled from below with deionized water, with the anode space being filled only halfway and immediately drained again. [0143] the cathode gap was flushed further with catholyte. After draining off of the anolyte, the catholyte was also drained from the cathode gap. [0144] the cell was then allowed to stand for 10 hours. [0145] start-up was carried out as described in Example 1. [0146] 5 downtimes according to the above-described procedure for shutdown were carried out [0147] at the beginning of the experiment, the cell voltage at a current density of 400 mA/cm.sup.2 was 2.11 V [0148] after 5 downtimes, the cell voltage at a current density of 400 mA/cm.sup.2 was 2.14 V.

[0149] The cell voltage increased by 30 mV, and damage to the gas diffusion electrode occurred.