Activation of cathode

Abstract

The present invention relates to a process for production of alkali metal chlorate, and to a method of activating a cathode comprising electrolyzing an electrolyte comprising alkali metal chloride in an electrolytic cell in which at least one anode and at least one cathode are arranged wherein a) said electrolyte comprises chromium in any form in an amount ranging from about 0.01-10.sup.6 to about 500-10.sup.6 mol/dm.sup.3 b) said electrolyte comprises molybdenum, tungsten, vanadium, manganese and/or mixtures thereof in any form in a total amount ranging from about 0.1-10.sup.6 mol/dm.sup.3 to about 0.5-10.sup.3 mol/dm.sup.3.

Claims

1. Process for reducing cell voltage in an electrolytic cell during production of alkali metal chlorate comprising electrolyzing an electrolyte comprising alkali metal chloride in the electrolytic cell in which at least one anode and at least one cathode are arranged wherein a) said electrolyte comprises chromium in any form in an amount ranging from 0.01.Math.10.sup.6 to 100.Math.10.sup.6 mol/dm.sup.3 b) a cathode electrodepositing solution is provided to the electrolyte, the cathode electrodepositing solution comprising molybdenum, tungsten, vanadium, manganese and/or mixtures thereof in any form in a total amount ranging from 0.1.Math.10.sup.6 to 0.1.Math.10.sup.3 mol/dm.sup.3 to form a cathode coating during the process for production of alkali metal chlorate, and wherein the at least one anode and the at least one cathode consist of a substrate consisting of at least one of titanium, titanium suboxide, and MAX phase, and further wherein the cathode is void of iron or iron compounds and has a density from 3 to 20 g/cm.sup.3.

2. Process according to claim 1, wherein the chromium in any form is a chromium compound that is added to the electrolyte in the form of Na.sub.2CrO.sub.4, Na.sub.2Cr.sub.2O.sub.7, CrO.sub.3 and/or mixtures thereof.

3. Process according to claim 1, wherein the cell is undivided.

4. Process according to claim 1, wherein the shape of the at least one anode and/or the at least one cathode is cylindrical.

5. Process according to claim 1, wherein the chromium is present in the electrolyte in an amount ranging from 0.1.Math.10.sup.6 to 50.Math.10.sup.6 mol/dm.sup.3.

6. Process according to claim 1, wherein the molybdenum, tungsten, vanadium, manganese and/or mixtures thereof is present in the electrolyte in an amount from 0.001.Math.10.sup.3 to 0.1.Math.10.sup.3 mol/dm.sup.3.

7. Process according to claim 1, wherein the at least one cathode substrate is selected from titanium, MAX phase, and/or mixtures thereof.

8. Process according to claim 1, wherein the current density at the at least one anode ranges from 0.6 to 4 kA/m.sup.2.

9. Process according to claim 1, wherein the current density at the at least one cathode ranges from 0.05 to 4 kA/m.sup.2.

10. Process according to claim 8, wherein the current density at the at least one anode ranges from 1 to 3.5 kA/m.sup.2.

11. Process according to claim 9, wherein the current density at the at least one cathode ranges from 0.6 to 2.5 kA/m.sup.2.

12. Process according to claim 5, wherein the chromium is present in the electrolyte in an amount ranging from 5.Math.10.sup.6 to 30.Math.10.sup.6 mol/dm.sup.3.

13. Process according to claim 6, wherein the molybdenum, tungsten, vanadium, manganese and/or mixtures thereof is present in the electrolyte in an amount from 0.01.Math.10.sup.3 to 0.05.Math.10.sup.3 mol/dm.sup.3.

14. Process according to claim 1, wherein the at least one anode has a density from 3 to 20 g/cm.sup.3.

15. Process according to claim 1, wherein the density of the at least one cathode is from 4 to 5 g/cm.sup.3.

16. Process for reducing cell voltage in an electrolytic cell during production of alkali metal chlorate, the process comprising: arranging an anode and a cathode in the electrolytic cell, wherein the anode and the cathode consist of a substrate consisting of at least one of titanium, titanium suboxide, and MAX phase, and the cathode is void of iron or iron compounds; reducing voltage on the cathode while producing alkali metal chlorate by forming a cathode coating on at least a portion of the cathode, wherein the cathode coating is formed by depositing a cathode electrodepositing solution onto the cathode and is provided to an electrolyte in the electrolytic cell during said process, wherein the cathode electrodepositing solution comprises molybdenum, tungsten, vanadium, manganese and/or mixtures thereof in any form in a total amount ranging from 0.1.Math.10.sup.6 to 0.1.Math.10.sup.3 mol/dm.sup.3; and electrolyzing the electrolyte in the electrolytic cell to produce the alkali metal chlorate, wherein the electrolyte comprises the cathode electrodepositing solution, alkali metal chloride and chromium in any form in an amount ranging from 0.01.Math.10.sup.6 to 100.Math.10.sup.6 mol/dm.sup.3.

Description

EXAMPLE 1

(1) A small chlorate producing pilot plant comprising an electrolyzing cell and a reaction vessel (also acting as a gas separator) was used. The electrolyte was circulated by means of a pump. On top of the reactor vessel, gas was withdrawn; a small amount of chlorine species was absorbed in 5 Molar sodium hydroxide; water was completely eliminated by adsorption in desiccant. The oxygen content in the remaining gas was then measured continuously in % by volume. The oxygen flow (liter/s) was also measured in order to calculate the cathodic current efficiency (CCE) on the cathode. The hydrogen flow rate was determined by subtracting the oxygen part from the total gas flow rate. The CCE was then calculated from the hydrogen flow rate using the following expression CCE=(Normal liter H.sub.2 per second 122.4).Math.(2F/I), where F is Faraday's constant, and I is the current through the cell in ampere.

(2) The starting electrolyte used was a water solution containing 120 g/L NaCl and 580 g/L NaClO.sub.3. The anode in the electrolyzing cell was a PSC120 (DSA, TiO.sub.2/RuO.sub.2) available from Permascand. As cathode material a MAXTHAL 312 (Ti.sub.3SiC.sub.2) (4.1 g/cm.sup.3) available from Kanthal with a machined surface was used. The distance between the anode and the cathode was about 4 mm. The exposed geometrical surface area for electrolysis, for the anode and cathode respectively, was 30 cm.sup.2. A current density of 3 kA/m.sup.2 both on the anode and the cathode was used in each experiment. The temperature in the electrolyte during the experiments was 802 C.

(3) The activation of the cathode by addition of MoO.sub.3 as set out in table 1 is clearly seen, with low amounts of Na.sub.2Cr.sub.2O.sub.7.2H.sub.2O (9 M, corresponding to 18 M as Cr) also present in the electrolyte.

(4) In table 1, it can be noted that the experiments in which small amounts of MoO.sub.3 were used in the electrolyte resulted in oxygen evolution of 3.5-3.8%. A significant activation effect can be noticed in table 1, although the amount of MoO.sub.3 in the electrolyte is very low. The values in table 1 are taken after stable conditions has been reached, after each addition.

(5) TABLE-US-00001 TABLE 1 Amount of MoO.sub.3 in Oxygen (%) CCE (%) Cell voltage (V) electrolyte 3.8 ~100 3.27 3.8 ~100 3.21 1 mg/L (0.007 mM) 3.7 ~100 3.17 2 mg/L (0.014 mM) 3.6 ~100 3.15 5 mg/L (0.035 mM) 3.5 ~100 3.15 10 mg/L (0.07 mM)

EXAMPLE 2

(6) Long term effects were studied as 1 mg/L (0.007 mM) and 100 mg/L (0.7 mM) respectively of MoO.sub.3 were added to the electrolyte (table 2). The setup was as in example 1 (with a new MAXTHAL 312 electrode as cathode).

(7) TABLE-US-00002 TABLE 2 Oxygen (%) CCE (%) Cell voltage (V) MoO.sub.3 in electrolyte* >4 ~100 3.31 .sup.3.5* ~100** 3.15** 1 mg/L (0.007 mM) >>4** ~100** 3.11** 100 mg/L (0.7 mM) *5 h after addition of MoO.sub.3. **4 h after addition of MoO.sub.3.

(8) It is clear that the experiment with 100 mg/L MoO.sub.3 results in considerable oxygen levels. The cathode is, however, considerably activated.

EXAMPLE 3

(9) In a test to study how the cathodic current density affects the activation of the cathode (a new MAXTHAL 312), the setup and starting electrolyte of example 1 was used. After having added 50 mg/L (0.35 mM) of MoO.sub.3 to the electrolyte an activation of the cell voltage to 3.05 V was stabilized at 2 kA/m.sup.2. Then, the current density at the cathode was increased to 3 kA/m.sup.2 for about 1.5 h and then lowered again to 2 kA/m.sup.2. The cathode became further activated by about 20 mV only by the increase in current density for a period of three minutes.

EXAMPLE 4

(10) A number of small scale experiments in which molybdenum was added to the electrolyte were performed. A 5 M NaCl(aq) solution was used in all electrolytes. No chromate was present in the experiments. As a working electrode a titanium disk was used rotating at 3000 rpm at 70 C. and pH 6.5. Six experiments were performed in which the potential at the working electrode was kept at 1.5 V vs. Ag/AgCl for five minutes. After this the potential was lowered. At a certain current density, 0.5 kA/m.sup.2 on the working electrode, readings of the potential versus Ag/AgCl were sampled as set out in tables 3 (5 M NaCl) and 4 (5M NaCl, 15 mM NaClO).

(11) TABLE-US-00003 TABLE 3 No C(Na.sub.2MoO.sub.4), mM C(MoO.sub.3), mM E (V) vs. Ag/AgCl 1 0 0 1.50 2 1 0 1.25 3 0 1 1.25

(12) TABLE-US-00004 TABLE 4 No C(Na.sub.2MoO.sub.4), mM C(MoO.sub.3), mM E (V) vs. Ag/AgCl 1 0 0 1.47 2 1 0 1.19 3 0 1 1.19

(13) It is clear that small amounts of molybdenum species reduces the voltage on the titanium cathode.

EXAMPLE 5

(14) As a test to see how a tungsten species compares to molybdenum species as activator, three experiments were performed, also here using a rotating disk. In this case the electrode material was Maxphase (Maxthal 312 from Kanthal). In this experiment the disk was rotating at 3000 rpm, polarized at 2 kA/m.sup.2. The electrolyte solution contained 5 M NaCl(aq) at a temperature of 70 C., and a pH of 6.5. The experiments were performed according to table 5 and readings were performed after 15 minutes.

(15) TABLE-US-00005 TABLE 5 No Additive E (V) vs. Ag/AgCl* 1 None 1.53 2 10 mM Na.sub.2MoO.sub.4 1.39 3 10 mM Na.sub.2WO.sub.4 1.43 *Potential was corrected for iR drop

EXAMPLE 6

(16) To study the effect of chromium, four experiments were performed with electrolytes as set out in table 6. A titanium disk was used as working electrode, rotating at 3000 rpm at 70 C. and pH 6.5. The potential at the working electrode was kept at 1.5 V vs. Ag/AgCl for five minutes. After this the potential was lowered by a rate of 50 mV/s and the current density on the working electrode was monitored. In the experiments the current density was sampled at around 0.8 V vs. Ag/AgCl and used as measurement of how significant the reduction of hypochlorite is. Higher cathodic currents at this potential will point to more reduction of hypochlorite and hence a lower selectivity for the hydrogen evolution, eventually resulting in a lower cathodic current efficiency, as measured in examples 1 and 2.

(17) TABLE-US-00006 TABLE 6 Current density at 0.8 V No Electrolyte composition vs. Ag/AgCl 1 5M NaCl + 15 mM NaClO 0.33 kA/m.sup.2 2 5M NaCl + 15 mM NaClO + 20 M 0.01 kA/m.sup.2 Cr(VI) 3 110 g/dm3 NaCl + 550 g/dm3 NaClO3 + 0.02 kA/m.sup.2 15 mM NaClO + 18 M Cr(VI) 4 110 g/dm3 NaCl + 550 g/dm3 NaClO3 + 0.14 kA/m.sup.2 15 mM NaClO + 2 M Cr(VI)