CH641841A5 - Method for improving the performance in the faraday electrolytic preparation of sodium chlorate. - Google Patents

Description

641841

2

CLAIMS Process for improving the Faraday yield during the electrolytic manufacture of sodium chlorate on metal anodes coated with platinum-iridium or ruthenium oxide, characterized in that hydrochloric acid used for maintain the optimal operating pH, phosphoric acid at a rate of 1 to 2 kg of 85% acid per tonne of sodium chlorate produced.

CIO + H "1

: H CIO

, 15

10

The present invention relates to an improved process for the manufacture of sodium chlorate by electrolysis; this meadow '

yielded improves the Faraday yield in the case where electrolysis cells are used which are equipped with metal anodes, in particular made of titanium, coated with an electro-active layer such as platinum-iridium or ruthenium oxide.

The electrolytic manufacture of sodium chlorate is carried out essentially by electrolysis of a solution of sodium chloride.

The reactions at the electrodes are as follows:

- at the anode, the following chlorine is formed:

ÓC1 ~ -> 3C1, + 6

(1) '

the chlorine formed at the anode reacts with the following water:

3C12 + 3H20? ± 3H + + 3C1 + 3HC10 (2) 30

3 HCIO 3 H + + 3C10 (3)

35

3 H20 3 OH- + 3 H + (4) '

and the formed hypochlorite transforms into chlorate according to the reaction:

2 HCIO + CIO "- CIO3- + 2 H + + 2 Cl- (5)

40

At the same time, there is electrochemical formation of chlorate from the following hypochlorite ions:

6 CIO- + 3HzO - 2 CIO3- + 4 Cl- + 6 H + + 2 02 + 6 a (6) 45

This reaction would be responsible for the release of oxygen that is observed during electrolysis, release which is the essential cause of the decrease in the FARADAY yield. - at the cathode, the following hydrogen is formed: 50

6H + + 6th -> 3H,

(7)

CIO- + H2 -► Cl- + H2O

(8)

This parasitic reaction (8), also generating a drop in the FARADAY yield, is generally combated by adding sodium dichromate at a rate of 2 to 10 g / l in the electrolyte.

Another cause of the decline in FARADAY yield comes from the fact that the chlorine which forms at the anode can escape directly from the cell without being absorbed, as a result of the equilibria.

HCIO + H + + Cl ^ C12 + H20

there is a certain chlorine vapor pressure above the solution and chlorine can therefore escape from the entire surface of the liquid.

The reaction (5) is essential in the genesis of the chlorate and this reaction is favored by a given pH variable from 6 to 6.5, the optimal value depending in fact on the operating temperature. To maintain this optimum pH, an acid solution is therefore added to the electrolyte, generally hydrochloric acid, which also makes it possible to compensate for any losses of chlorine.

This addition is made according to conventional methods

- either in the cell supply solution

- either directly in the cell

- either in two ways simultaneously and it is generally automated thanks to a pH regulation.

Under the operating conditions usually encountered

NaC103 g / 10 to 700 NaClg / 1320 to 120 sodium bichromate g / 12 to 10 operating temperature 55 to 85 ° C operating pH 6 to 6.5 obtained by adding 25 to 40 kg of HCl 33% per tonne of sodium chlorate manufactured current density A / m2 1500 to 6000 titanium support anodes with Pt / Ir or Ru02 coating the gases released have the following composition:

H2

o2

Cl2

96% 3.5% 0.2 to 0.5%

The hypochlorite formed in the cell can be reduced by the following hydrogen:

60

65

and the FARADAY yield is greater than 92%.

Attempts have been made to improve this yield. Various solutions have been proposed which all boil down to improving the anode coatings, as indicated in French Patent No. 2,187,416.

The licensee has developed a process that allows a 1 to 3 percent increase in Faraday yield by direct intervention on the electrolyte.

This process consists in adding to the hydrochloric acid used to maintain the optimum pH, phosphoric acid at a rate of 1 to 2 kg of 85% phosphoric acid per tonne of sodium chlorate produced.

The increase in yield depends on the yield of the electrolyte, the cell technology and the operating conditions used.

The following examples illustrate the process according to the invention.

Example 1

1) Electrolysis is carried out according to the conventional operating mode in an industrial cell of 35,000 A equipped with titanium anodes with a coating based on ruthenium dioxide. The cell is supplied with a solution of the following composition:

NaClg / 1,210

NaC103 g / 1,325 bichromate g / 1 7 NaClOg / 1 0.3

Ca + Mgmg / 1 35 Fe, Co, Ni mg / 1 2 the feed rate is 1201 / h.

3

641841

550 ml of 33% HCl are continuously added to the cell. The cell works with a current density of 2000 A / m2 at 70 ° C. The pH is 6.3

The composition of the solution leaving the cell is: NaClg / 1110

NaC103 g / 1,520 bichromate g / 1 7 NaClOg / 1 1,5

Ca + Mgmg / 1 15 Fe, Co, Ni mg / 1 2

the gases formed during electrolysis have the following composition

H2 96.7%

02 2.9%

Cl2 0.4%

the FARADAY yield is equal to 92.7%

2) under the same operating conditions and according to the process of the present invention, a continuous flow rate of 220 ml of H3PO4 of thermal origin at 10% is added to the hydrochloric acid. The composition of the gases formed is then:

s H2 97.6%

02 2%

Cl2 0.4%

and the FARADAY yield reaches 94.5%.

io There is therefore an improvement in yield of 1.8%.

Example 2

Under the same electrolysis operating conditions as in Example 1, 10% phosphoric acid obtained by the wet method is added, at a content of 0.15 g of acid per liter of electrolyte.

The oxygen level in the gases is lowered by 0.3%, which brings the FARADAY yield from 92.72% to 93.32%.

VS