The catalyzing effect of chromate in the chlorate formation reaction

Electrolyte used in the manufacturing process of sodium chlorate contains chromate primarily to inhibit cathodic loss reactions. Chromate also accelerates the chlorate formation reaction which reduces the concentration of hypochlorite species in the chlorate cell leading to enhanced anodic current yield, reduced risk of oxygen explosions and reduced cost for the purification of the cell gas. Laboratory scale trials show that the rate of chlorate formation can be described as an `uncatalyzed' third order reaction with respect to hypochlorite species and a parallel chromate catalyzed reaction of apparent reaction order 2.2. Validation is presented based on data from electrolysis trials in pilot plant and bench scale at technically relevant conditions. If the dichromate concentration is increased from 0 to 5 g/L, more than 50% of the chlorate may be formed via the chromate catalyzed path and the apparent reaction order changes from 3 to about 2.4. The kinetic effect of the chromate species also lowers the optimum reaction pH below that of the uncatalyzed reaction. Furthermore, anionic hypochlorite does not seem to be necessary to generate chlorate via the chromate catalyzed path. oOther hypochlorite consuming reactions such as anodic and homogeneous oxygen formation as well as cathodic reduction and desorption of chlorine species in the cell gas only have a marginal effect on the hypochlorite concentration in the chlorate cell. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.