Synthesis of sodium hypochlorite solutions in coaxial flow cells in current reverse mode

Abstract

Electrodes made of platinized titanium with a surface platinum content of 2–3 mg/cm2 can be used in non-diaphragm flow- and accumulative-type electrolyzers for the electrolysis of low-concentrated NaCl solutions in order to obtain pure NaClO solutions. If electrolysis of 0.15 M NaCl solution is carried out on platinized titanium electrodes in the usual mode at current densities of 20–40 mA/cm2, then the anode surface passes into an oxidized passive state. In this case, the current efficiency of hypochlorite does not exceed 40%, and the current efficiency of chlorate is more than 20%. During a short electrolysis on a preliminarily reduced surface of platinized titanium, the current efficiency of hypochlorite reaches 90% with almost no accumulation of chlorate. Carrying out the long-term electrolysis of low-concentrated NaCl solutions in the regime of periodic polarity reverse makes it possible to significantly (up to 10 times) reduce the content of chlorate in the resulting sodium hypochlorite solutions. The most promising is the synthesis of sodium hypochlorite solutions in flow-type electrochemical reactors consisting of several series-connected electrochemical modules with an undivided electrode space in the mode of periodic current reverse. The electrolyzer of two series-connected cells in the mode without current reverse allows obtaining a solution that contains 500 mg/L of sodium hypochlorite and 130 mg/L of sodium chlorate. Carrying out the electrolysis in the reverse current mode every 30 s reduces the content of sodium chlorate to 25 mg/L, which makes it possible to produce high-purity NaClO solutions.