Current efficiency of individual electrodes in the sodium chlorate process: a pilot plant study

Current efficiency in the sodium chlorate process is a key issue in the evaluation of the power consumption. A pilot cell unit for executing the sodium chlorate process was constructed to study the current efficiency of the anode and cathode separately. The effects of sodium dichromate and sodium sulphate concentrations and the electrolyte temperature on the anode and cathode current efficiencies were studied. Corrosion products formed on the mild steel cathodes after their removal from the cell were characterised using X-ray diffraction and infrared spectroscopy. The results show that the cathodic current efficiency increases with increasing dichromate concentrations in the electrolyte until approximately 5 g dm(-3) is reached. At this optimum concentration of dichromate, the presence of sulphate ions decreases the cathodic current efficiency. For moderate increases in temperature, the cathodic current efficiency increases, but oxygen evolution is promoted, and the power consumption also increases. Surface characterisation of the electrodes after their exposure to air shows two primary types of behaviour, depending on the process parameters. At low dichromate concentrations, amorphous corrosion layers are formed, while at higher concentrations, reduced forms of iron hydroxides, i.e., green rust, are identified. Although the electrodes were positioned at the open circuit potential for 40 min before their removal from the cell, chromium remains on the cathode surface. This result might explain the corrosion-inhibiting effect of the addition of chromate to the electrolyte. The results from this study can be used to optimise operating procedures in real plants, decrease the energy consumption and minimise the environmental impact of these processes. [GRAPHICS] .