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

Abstract

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.Electronic supplementary material The online version of this article

Understanding Selectivity in the Chlorate Process: A Step towards Efficient Hydrogen Production

“…This is also supported by results obtained for polished titanium and steel substrates, where it was shown that μM levels of chromate efficiently suppress hypochlorite reduction . However, in the commercial chlorate process, concentrations between 2–8 gL −1[48–49] are required to obtain 90–95% cathodic current efficiency . In a recent pilot study, it was shown that the cathodic current efficiency increased with increasing chromate concentration up to 5 gL −1 and then levelled off .…”

“…However, in the commercial chlorate process, concentrations between 2–8 gL −1[48–49] are required to obtain 90–95% cathodic current efficiency . In a recent pilot study, it was shown that the cathodic current efficiency increased with increasing chromate concentration up to 5 gL −1 and then levelled off . These concentrations are higher than required to form a monolayer on the electrode surfaces (see SI for a rough estimate), The excess of chromate in the electrolyte is needed to assure that the surface is completely covered with chromium oxide/hydroxide and for mild steel electrodes some of it is consumed by incorporation in the corrosion layer formed during shut downs.…”

“…The reduced layer consists of Fe(II), which readily gets oxidised by chromate to form Cr(OH) 3 or mixed Cr x Fe 1‐x (OH) 3 . This is supported by the fact that chromium remains for a substantial time on the surface of the cathode after shut down …”

Understanding selective thin films in the chlorate process

“…However, in the industrial process much larger concentrations of 2 to 8 g dm -3 are needed [50]. In a pilot plant, the cathodic current efficiency scaled with chromate concentration up to 5 g dm -3 [51]. These concentrations are much higher than required for the formation of a few monolayers, even when taking into account that some chromate is incorporated into the Fe during shutdowns [8], [51].…”

“…In a pilot plant, the cathodic current efficiency scaled with chromate concentration up to 5 g dm -3 [51]. These concentrations are much higher than required for the formation of a few monolayers, even when taking into account that some chromate is incorporated into the Fe during shutdowns [8], [51]. These discrepancies in concentration indicate that the film growth process is influenced by the process conditions, and possibly also that the film formation by itself is not sufficient to achieve state-of-the-art efficiency.…”