Critical Anode Potential in the Chlorate Process

Abstract

Anodic polarization curves on dimensionally stable anodes ͑DSAs͒ of RuO 2 /TiO 2 in chlorate electrolyte bend to a higher Tafel slope at the critical potential ͑E cr ͒ of approximately 1.2 V vs Ag/AgCl. Operating the chlorate process above E cr leads to increased oxygen evolution and higher potential losses. In this study the impact of different electrolyte parameters and electrolyte impurities on the risk of reaching/exceeding E cr was investigated. A dependency of Cl − concentration on E cr of about −90 mV/dec C Cl − was found at pH 2. An addition of Na 2 Cr 2 O 7 to chlorate electrolyte is necessary in order to keep a high current efficiency on the cathode but was found to increase the anode potential and thereby increase the risk of exceeding E cr at galvanostatic operation. Additions of impurities as 30 g/L Na 2 SO 4 or 100 ppm Si ͑added as SiO 2 ͒ resulted in increased anode potentials, but adding 1.4 g/L KH 2 PO 4 or 1 g/L HF did not have significant short-term impact on the potential. The anode potential as well as E cr decreased with increased temperature. A high temperature is beneficial in terms of the decreased anode potential, which outweighs the negative effect of a decrease in E cr .

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

“…The magnitude of the decrease in anodic potential is in line with earlier studies [37]. The remaining potential dependence can be explained by effects at the cathode and are in the range of what has been reported earlier, 1.6-1.9 mV°C -1 [10].…”

“…The addition of sodium sulphate does not seem to affect the cell voltage since all of the values are within the standard deviation. However, an increase in the anode potential with sodium sulphate in the electrolyte has been reported by Nylén et al [37]. According to the present results, such an increase in the overpotential must be compensated for by a lowering of the overpotential at the cathode, since the cell potential is the same in the absence and presence of sulphate ions in the electrolyte (Fig.…”

“…Cornell et al [36] and Nylén et al [37] discuss how the addition of chromate increases the buffer capacity and thereby hinders the decrease of the pH at the anode surface. They further discuss that the chromate adsorbed at the active sites leads to a higher overpotential, which in turn results in increased oxygen formation according to Eberil et al [38].…”

Effects of electrolyte parameters on the iron/steel cathode potential in the chlorate process

“…The electrolyte parameters are often interconnected and one parameter may have both positive and negative impacts. This is the case for the addition of chromate, which is necessary for keeping a high cathodic current efficiency, but is negative in terms of increased overpotential for both electrodes [1,15,16]. An additional positive effect of chromate is the enhanced buffer capacity in the pH range 5-7, where formation of chlorate (Eq.…”

“…Ref. [2,[18][19][20][21][22][23][24], and a few focus on the DSA-anode potential [15,16,[25][26][27]. For the steel and iron cathodes, polarisation curves at chlorate-like conditions are scarcely presented [1,28,29].…”