GB2085478A - Alkaline anodic activation of chlorine anodes - Google Patents

Abstract

The present invention relates to a process for activation of chlorine anodes comprising as electrocatalyst mixed crystals of the oxides of ruthenium and titanium. The activation procedure comprises anodically polarising the inactive electrode by passing a current through the cell in an electrolyte which has a pH value above 12. The activated anodes thus produced operate at a lower electrode potential than normal unused anodes and enables the anodes to attain steady state more quickly than hitherto.

Description

SPECIFICATION Alkaline anodic activation of chlorine anodes The present invention relates to a process for activation of chlorine anodes comprising mixed crystals of the oxides of ruthenium and titanium as electrocatalysts.

Chlorine anodes are anodes used for the production of chlorine by electrolysis. Whilst several such anodes are availabie, the anodes most widely used commercially are those in which a base or core of an electrode is coated with a mixed crystal material comprising the oxides of titanium and ruthenium. Such anodes and methods of preparation thereof are described for example in US Patent Nos: 3632498 and 3616445.

These anodes appear to suffer a voltage rise in use due to poisoning by organic material and the activity of such anodes is conventionally restored by (i) cleaning with an organic solvent or (ii) by alkaline cathodic cleaning.

(i) The method of cleaning with an organic solvent consists of placing the poisoned anode in solvent, e.g. dichloromethane or chloroform, for a period of time and then rinsing thoroughly with distilled water. This technique has the disadvantage of requiring (a) removal of electrodes from the industrial cells and (b) large tanks filled with potentiaily dangerous organic solvents which have to be maintained at their boiling point.

(ii) The cathodic cleaning method relies on the short-circuiting of the mercury cells (in which such anodes are usually used) which results in reversal of the polarity of the electrode, i.e. the anode becomes the cathode. Thereafter the polarity is reversed again to restore the electrode to its anode status. This procedure of double reversal of polarity may have to be repeated several times to enable the anode to recover its initial potential. Whilst this technique does not necessitate the removal of the electrode from the cell, the restoration of the electrode potential to its original value is short-lived and the activity soon falls off again.Moreover, it is not advisable to subject anodes which have ruthenium oxide and titanium oxide as electrocatalysts to such a cathodic treatment because there is a risk of changing the oxidation state of the active electro-catalyst species which may in part account for the shortened life of the anode. In addition, the duration of the activation procedure is relatively long.

It has now been found that by a modified anodic treatment, it is not only possible to improve the activity of fresh anodes supplied commercially, hereafter termed as "unused" anodes, and to restore the activity of poisoned anodes, but it is also possible to reduce the duration of the activation treatment significantly.

Accordingly, the present invention is a method of lowering the electrode potential of unused or poisoned electrodes such as anodes in chlorine cells, said electrodes having an electrocatalytic coating comprising the oxides of titanium and ruthenium thereon, said method comprising anodically polarising said electrode by passing a current through the cell containing an electrolyte which has a pH value above 12.

As mentioned previously, the anodes used in the present invention maybe prepared for example by the process described in US Patent Nos: 3616445 or 3632498. The anodic polarisation maybe achieved "in situ" in the cell containing the unused or poisoned electrode or in a separate cell. Where the polarisation is carried out "in situ," the pH value of the electrolyte in the cell may be brought to the desired value, suitably a pH value of between 12 and 15, by dosing the electrolyte with a solution of caustic soda or caustic potash. A current is thereafter passed through the cell for a short duration which could be as low as five minutes. Thereafter, the reactivated anode is ready for use. However, it will be necessary to restore the pH of the electrolyte to its original value prior to the anodic polarisation for producing chlorine.This may be achieved by dosing the electrolyte with an appropriate amount of acid, e.g. hydrochloric acid, to bring the pH value down to the required level.

The current density used for achieving the anodic polarisation may vary over a wide range, preferably between 0.1 and 20 KA/m2. The temperature of anodic polarisation may be the same as that at which the anode was originally being used, suitably between 60 and 800 C. The process of the present invention is particularly suitable for activating anodes poisoned by hydraulic oils.

The present invention has the following advantages over conventional methods.

1) The activated anodes thus produced operate at lower electrode potentials than normal unused anodes.

2) The process is capable of activating both poisoned and unused anodes.

3) The duration of the activation procedure in the present process is shorter than conventionally experienced.

4) The process cleans and activates the surface of newly installed anodes so that steady state is attained quickly.

The process of the present invention is further illustrated with reference to the following Examples.

In the Examples and comparative tests reported below the following materials and procedures were used: 1. Anode The anode having a surface area of 0.5 cm2 was a dimensionally stable anode of titanium provided with an electrocatalytic coating of ruthenium oxide as supplied by Permelec s.p.A. of Italy under the Registered Trade Mark of "DSA(R)".

2. Cathode Two types of cathodes were used as counter electrodes.

(a) Teflon Bonded Graphite Cathodes 2 g of graphitized carbon (graphitized at 26000 C) were mixed with 5 g of PTFE dispersion (Fluon, Registered Trade Mark, supplied by'lCI) in an ultrasonic bath for 10 minutes. The resuiting paste was then applied to a 2 x 5 cm, 80 mesh Pt gauze. The pasted Pt gauze was finally sintered at 3500C for one hour under an atmosphere of N2.

(b) Graphite 5 x 2 x .3 cm piece of denuder graphite was used as the cathode. The graphite was immersed in concentrated HCI for about one hour before it was used as the cathode.

In all cases titanium wires provided the electrical contacts.

3. The Cell The test cells used were two compartment cells, one housing a reference electrode and the other for housing the working electrodes, namely the cathode and anode, for electrolysis.

A saturated Calomel electrode, SCE, was situated in one compartment of the test cell and was employed as an external reference electrode. The potential of the SCE at 300C against platinized platinum in a chlorine saturated 25 percent NaCI solution at 700C was 1.065 + 0.005 volts. This represents the reversible chlorine potential under the experimental conditions employed.

The other compartment housing the working electrodes was a one litre vessel with flanges. The reference electrode compartment was connected to the working electrode compartment by a Luggin capillary. The distance between the Luggin capillary and the DSA was adjusted to about 2 mm.

The test cell was heated on a hot plate and a magnetic follower was used to stir the solution. All experiments were carried out at the specified temperature in 25 percent "Analar" sodium chloride solution.

4. Electrochemical Measurements The DSA polarization curves were measured galvanostatically by passing a constant current through the cell and measuring the anode potential against the external reference electrode. The electrode potential was IR corrected by introducing a mains operated current interrupter which employed solid state relays, into the circuit. The relay interrupts the DC current at mains frequency producing a train of square wave current through the cell. Ohmic overvoltage is instantaneous and can therefore be measured by monitoring the variation of electrode potential with time on an oscilloscope screen.

EXAMPLE 1-4 Initially the new, unused anodes were standardised in a test cell containing 700 ml of saturated brine at about 700 C. In this case the graphite cathode was used. A current of 6 kA!m2 was then switched on and the electrode potential monitored until it reached a constant value as shown in Table 1.

This showed the variation of electrode potential of the new anode with time.

The anode was then transferred to another cell containing 700 ml of saturated brine at 700 C.

Predetermined volumes of 15 percent NaOH were then added to the electrolyte to bring the calculated pH value above 12 as indicated in Table 2, and a current of 6 kA/m2 was passed through the cell for five minutes to activate the anode. The activated anode was then rinsed with water and transferred back to the original test cell containing only saturated brine. Finally a 6 kA!m2 current was passed through the cell and the electrode potential measured after 30 minutes of continuous electrolysis. The results of a series of tests (Examples (1-4) obtained at different pH values are summarised in Table 2.

At the end of the experiment the activity of the electrode was tested again for 1 8 hours in a fresh saturated brine solution. During that period the iR corrected electrode potential of 1.16 volts vs SCE was maintained. The results in Table 2 were reproduced twice.

The results show that activation by anodically polarizing the anode for five minutes in saturated brine containing only 0.25 percent NaOH enables the anode to function at a lower potential than hitherto.

TABLE 1 VARIATION OF ELECTRODE POTENTIAL OF ANODE WITH TIME Curent = 6 kA/m, T = 70 C

Potential vs SCE Time not iR corrected (minutes) (volts) 0 1.95 8 1.65 20 1.5 30 1,44 120 1.42 240 1.42 TABLE 2 ALKALINE ANODIC TREATMENT IN SATURATED BRINE CONTAINING VARYING CONCENTRATIONS OF NaOH Time of activation - 5 minutes Current - 6 kAim2 Temperature - 70 C

iR iR Corrected % Calculated Potential vs SCE Correction Potential (vs SCE) Examples NaOH pH value Volts Volts Volts 0 1.415 0.220 1.195 1 0.21 12.7 1.38 0.22 1.16 2 0.42 13.02 1.38 0.22 1.160 3 0.81 13.3 1.370 0.22 1.155 4 1.0 13.4 1.38 0.22 1.16 EXAMPLES 5 AND 6 In order to test the effectiveness of the activation procedure on poisoned anodes, the same anodes and procedure as discussed in Example 1 to 4 above were used both for standardisation and activation, except that a 25% sodium chloride solution was used as electrolyte and Teflon-bonded graphite was used as the cathode. The pH of the electrolyte was adjusted to between 24 and aliquots of hydraulic oil (saturation, (2 ml in 700 ml) in Example 5 and 200 ppm in Example 6) were introduced into the cells as poisons to deactivate the anode.The anodic potential was then monitored as before. The effect of the poisoning is shown in Table 3 below, monitored-again. The results are summarised in Table 3 below.

The poisoned, deactivated anodes were then subjected to anodic polarization at 8kA/m or 10 kA/m in the presence of 10% sodium hydroxide (pH above 14) and 1 5% sodium chloride. Oxygen gas evolution was the main reaction at the anode under such conditions. The treated anode was then transferred to another test cell containing 25% sodium chloride. The current was switched on and finally the electrode potential monitored again. The results are summarised in Table 3 below.

It is clear from the results in Table 3 that alkaline anodic polarization reactivates the anode. It was shown in Examples 1-4 that 5 minutes of alkaline anodic polarization will activate new anodes.

TABLE 3 ANODIC ACTIVATION IN 10 PERCENT NaOH AND 15 PER CENT NaCI

Potential v (not iR corrected) Potential After Example Oil Current Before Oil After Oil Activation Activation No. ppm kAim2 Addition Addition Procedure (not iR corrected) 5 Sat i 8 ise 3.4 Anodic 1.65 for 22 h (2ml) after polarization 24 h for 1 h at 8kA/m 6 200 10- 1.57 1.71 Anodic 1.51 - 1.53 after polarization for 30-h 28 h for I h at 10kA/m2

Claims (6)

1. A method of lowering the electrode potential of unused or poisoned electrodes used as anodes in chlorine cells, said electrodes having an electrocatalytic coating comprising the oxides of titanium and ruthenium thereon, said method comprising anodically polarising said electrode by passing a current through the cell containing an electrolyte which has a pH value above 12.

2. A method according to claim 1 wherein the anodic polarisation is carried out "in situ" in the cell containing the unused or poisoned anode.

3. A method according to claim 1 or 2 wherein the pH of the electrolyte during anodic polarisation is between 12 and 15.

4. A method according to any one of the preceding claims wherein the current density used for the anodic polarisation is between 0.1 and 20 KA/m2.

5. A method according to any one of the preceding claims wherein the temperature at which the anodic polarisation is carried out is between 60 and 800C.

6. A method according to any one of the preceding claims wherein the electrode is a dimensionally stable anode of titanium provided with an electrocatalytic coating of ruthenium oxide and titanium oxide.