Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, conce...

Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

Selectivity between oxygen and chlorine evolution in the chlor-alkali and chlorate processes : a comprehensive review

The chlor-alkali process has come a long way from the days of the diaphragm and mercury cell process to the present membrane cell process, with huge reduction in power consumption and hence its carbon footprint. Although there is reduction in the release of highly toxic mercury to the environment, there is increased release of less harmful substances such as chloride and chlorate because the membrane cell is less tolerant to contaminants, and hence requires higher purity brine. The technology providers have continued to improve upon the process to reduce power consumption and to reduce the plant’s footprint. This review looks briefly at the three technologies and attempts to look at where we currently are at. All new chlor-alkali plants being built are of the membrane process. This review mentions some of the areas where improvements can be made to the membrane process.

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The chlor-alkali process: Work in Progress

Effects of process conditions on cell voltage, current efficiency and voltage balance of a chlor-alkali membrane cell

Analytical relations between reaction order and tafel slope derivatives for electrocatalytic reactions involving chemisorbed intermediates

To obtain higher chlorine purity hydrochloric acid can be added to the feed brine of membrane cells in alkali chloride electrolysis. During the electrolytic process hydroxide ions migrate from the cathode compartment into the anode compartment. Hydrochloric acid neutralizes these hydroxide ions and, hence, formation of the by-products (oxygen in the anode gas and sodium chlorate in the anolyte) is reduced. With laboratory membrane cells the effects of varied amounts of hydrochloric acid on concentrations and current efficiencies of these by-products have been studied. Under normal operating conditions (with pH of feed brine between 2 and 11) the formation of by-products is not influenced by the addition of acid. Effects can only be observed at brine pH values<1. Maximum effects occur if the brine pH is 0.1 and the anolyte pH is 2. The latter value is the limiting pH given by the membrane suppliers. At this point 6.3 dm3 hydrochloric acid (37% HCl) per 1 m3 of the feed brine have to be added in order to obtain an anode gas with 0.4% oxygen by volume. The formation of sodium chlorate is completely suppressed. Problems connected with this process and its application to industrial electrolysis are discussed.

D. Bergner

Papers

Reduction of by-product formation in alkali chloride membrane electrolysis