Mixed Chromate and Molybdate Additives for Cathodic Enhancement in the Chlorate Process

The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production. Graphical Abstract

pH control in sodium chlorate cell for energy efficiency using PSO-FOPID controller

Industrial sodium chlorate production is a highly energy-intensive electro-chemical process. If the pH of the chlorate cell is not controlled, the current efficiency drops from 99% to as low as 66.66%. Hence control of chlorate cell pH is very significant for energy-efficient sodium chlorate production. This study puts forward a fractional order PID controller for controlling the pH of the sodium chlorate cell. The tuning of FOPID controller variables is affected by employing particle swarm optimization. The highlight of the controller is that it is flexible, easy to deploy and the time of computation is significantly low as few parameters are needed to be adjusted in PSO. The performance analysis of the suggested FOPID-PSO controller was studied and compared with the traditional PI controller and PID controller using time-domain provisions like settling time, rise time and peak overshoot and error indicators like integral square error (ISE), integral absolute error (IAE), and integral time absolute error (ITAE). FOPID controller employing PSO proved to perform well compared to conventional controllers with 0.5 sec settling time and 0.1 sec rise time. This demonstrates that the FOPID-PSO controller has better setpoint tracking, which is very essential for the process under consideration.