GB2039959A - Electrolytic process for the preparation of sodium chlorate - Google Patents

Abstract

A process for producing sodium chlorate by electrolysis of a sodium chloride solution to which hydrochloric acid has been added in a cell containing metal anodes coated with platinum- iridium or ruthenium oxide, wherein phosphoric acid is added to the hydrochloric acid, the quantity of phosphoric acid being from 1 to 2 kg of 85% acid per tonne of sodium chlorate produced.

Description

SPECIFICATION Electrolytic process for the preparation of sodium chlorate This invention relates to an electrolytic process for the production of sodium chlorate.

The electrolytic production of sodium chlorate is effected essentially by electrolysis of a sodium chloride solution.

The reactions at the electrodes are as follows: At the anode, chlorine is formed as follows: 6 Cl- < 3CI2 + 6e (1) the chlorine formed at the anode reacts with water as follows:

and the hypochlorite formed is converted into the chlorate by the reaction: 2 HCIO + ClO- < ClO3 + 2 H+ + 2 C1- (5) In the same period, there is some electrochemical formation of chlorate ions from hypochlorite ions as follows: 6010 + 3 H20 < 2CI03- + 4CI- + 6H+ + 3/202 + 6e (6) This reaction is responsible for the release of oxygen which is observed during electrolysis and which is the essential cause of the reduction in the Faraday yield.At the cathode, hydrogen is formed as follows: 6H++6e3H2 (7) The hypochlorite formed in the cell may be reduced by the hydrogen as follows: ClO- + H2~ Cl + H20 (8) This parasitic reaction (8), which also leads to a lowering of the Faraday yield, is generally counteracted by adding sodium dichromate to the electrolyte in a quantity of from 2 to 10 g/l.

Another reason for the reduction in the Faraday yield stems from the fact that the chlorine formed at the anode can escape directly from the cell without being absorbed. Because of the following equilibria:

there is a certain chlorine vapour pressure above the solution and chlorine is therefore able to escape from the entire surface of the liquid.

Reaction (5) is of prime importance in the formation of chlorate ions and this reaction is promoted by a given pH which may vary from 6 to 6.5, the optimum value depending on the operating temperature. To maintain this optimum pH, an acid solution, usually hydrochloric acid, is therefore added to the electrolyte, and this also compensates for any chlorine losses which may occur.

This addition is carried out according to the conventional methods and is performed either by adding the acid to the solution fed into the cell, or by adding the acid directly into the cell, or both, and is generally made automatic by adjustment of the pH.

Under the operating conditions normally encountered, i.e.

NaC103 Oto 700 g/l NaCI 320 to 120 9/ sodium bichromate 2 to 10 g/l operating temperature 55 to 85"C operating pH 6 to 6.5 obtained by adding 25 to 40 kg of 33% HCI per tonne of sodium chlorate produced current density 1500 to 6000 Alum2 anodes titanium support with coating of Pt/lr or RuO2, the gases released have the following composition: H2 96% 02 3.5% Cl2 0.2 to 0.5% and the Faraday yield is greater than 92%.

Attempts have been made to improve this yield. Various solutions have been proposed, all of which rely on improving the coatings on the anodes, as for example in French Patent No. 2,187,416.

The present invention aims to provide an electrolytic process for the production of sodium chlorate which has an improved Faraday yield.

Accordingly, the present invention provides a process for the production of sodium chlorate by the electrolysis of a sodium chloride solution in the presence of hydrochloric acid used to maintain an optimum pH, wherein phosphoric acid is added to the said hydrochloric acid in an amount of from 1 to 2kg of 85% phosphoric acid per tone of sodium chlorate produced.

The increase in the yield depends on the yield of the electrolyte, the technology of the cell and the operating conditions used but the increase in yield is usually found to be from 1 to 3%.

The following Examples illustrate the present invention.

Example 1 a) Electrolysis is carried out, in accordance with conventional methods, in a 35000A industrial cell fitted with titanium anodes having a coating based on ruthenium dioxide. The cell is supplied at a rate of 120 I/h with the solution of the composition: NaCI 210 g/l NaClO3 325 git dichromate 7 g/l NaCIO 0.3 g/l Ca + Mg 35 mg/l Fe, Co, Ni 2 mg/l 550 ml of 33% HCI are continuously added to the cell.

The cell operates 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: NaCI 110 g/l NaCIO3 520 g/l dichromate 7 g/l NaCIO 1.5 9/l Ca + Mg 15 mg/l Fe, Co, Ni 2 mg/l The gases formed during electrolysis have the following composition: H2 96.7% 2 2.9% C12 0.4% and the Faraday yield is 92.7%.

b) The process described in (a) is repeated, but a continuous flow of 220 ml of 10% H3PO4 of thermal origin is added to the hydrochloric acid. The composition of the gases formed is then: H2 97.6% 2 2% C12 0.4% and the Faraday yield is 94.5%.

There is thus an improvement in yield of 1.8%.

Example 2 The electrolysis of Example 1 is repeated but using 10% phosphoric acid obtained by the wet method to give a content of 0.15 g of acid per litre of electrolyte.

The oxygen level in the gases is lowered by 0.3% thus raising the Faraday yield from 92.7% to 93.32%.

Claims (3)

1. A process for producing sodium chlorate by electrolysis of a sodium chloride solution to which hydrochloric acid has been added in a cell containing metal anodes coated with platinum-iridium or ruthenium oxide, wherein phosphoric acid is added to the hydrochloric acid, the quantity of phosphoric acid being from 1 to 2 kg of 85% acid per tone of sodium chlorate produced.

2. A process according to Claim 1, substantially as hereinbefore described in Example 1 or 2.

3. Sodium chlorate when prepared by a process as claimed in Claim 1 or 2.