Understanding selective oxide films in the chlorate process

Chlorate, used in the paper pulp bleaching industry, is produced electrochemically. The process is performed in a membrane-less reactor, meaning the oxidized intermediates and product can be reduced at the cathode, leading to a large loss of efficiency. Sodium dichromate is added to the electrolyte, where it performs several functions, chief amongst which is the formation of a cathodic chrome oxide film, which inhibits the unwanted reduction. Because chromate is present in its 6-valent, highly carcinogenic state, its use is controlled by the European Union’s REACH legislation. In order to maintain sufficient efficiency to make chlorate production economically viable, a safer alternative must be found. This dissertation aims to help pave the way to this on the basis of experiments performed using rotating ring-disc electrodes, in-situ Raman spectroscopy, and electrochemical quartz crystal microbalance. First, it describes an investigation into the nature of the chrome-based film, for in order to find a replacement it is required to understand what properties must be recreated. Next, a study into a decrease of chromate concentration is shown, to determine the critical concentration required to operate the current process, so as to avoid unnecessary chromate use. It is shown that the decreased effectiveness at low concentration can be largely compensated by using molybdate as a co-additive. These results were also scaled up to a pilot scale reactor, in order to determine their applicability to the industrial scale process. Finally, the use of permanganate as a chromate-free alternative is explored. It is shown that the rich redox chemistry of manganese and the variety of possible oxides allow for myriad films, each with different properties. The dissertation concludes with a perspective and outlook on this field.