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Abstract

The electro-oxidation of sodium carbonate (Na₂C0₃) to percarbonate (C₂O₆²⁻), with its subsequent hydrolysis was investigated as a means to produce peroxide and to drive the in-situ electrochemical brightening of thermo-mechanical pulp. The conditions for the electrochemical production of peroxide and factors that affected the in-situ electrochemical brightening of TMP were studied by conducting variable level experiments. The investigations were performed using a 2-liter batch electrochemical reactor, fabricated of titanium, with a platinum anode. To promote peroxide generation, different types of cathode material and diaphragms were evaluated; and major modifications to the reactor comprising the anode, cooling system, mixer, and position of thermocouple were performed. The important quantitative findings are as follows: • Experiments with uncovered zirconium cathode generated the highest peroxide concentration than other cathode materials with or without a diaphragm. • The pH of the electrolyte dominated the process of peroxide accumulation through its effect on the concentration of CO₃²⁻. High pH (>11) resulted in high peroxide concentration. While brightening, alkali darkening will not negate the brightening responses in high pH (~11.5). • Temperature had a significant effect on peroxide generation and on brightening. Low anode coolant temperature (1°C) resulted in higher peroxide concentration in the early stage of the process; and high electrolyte temperature promoted the production of peroxide and raised the brightening responses. • The investigation of peroxide production (without pulp) indicated that under equipment limitations, the maximum peroxide concentration for 180 minutes was around 0.08M. The experimental conditions were as follows: • Electrolyte: 2M Na₂C0₃ , 400ppm MgS0₄ , 0.002M DTPA (pH = 11.6) • Electrolyte temperature: 60°C • Current set point: 30A (float voltage) • Mixing speed: 200rpm • Anode coolant: 1°C at 6 liter per minute Electro-brightening of 2.5 % consistency TMP achieved a 17.4 % ISO of brightness gain and a 19.2 % of yellowness loss with pulp (initial brightness: 41.6 % ISO; initial yellowness: 36.8 %) and experimental conditions mentioned above. The specific brightening energy for kWh this brightened TMP was 17 x 10² (kWh/ton•%ISO) , which corresponded to an operation cost of approx. $ 1,020 per ton of OD pulp for 20 % ISO brightness gain. The economic reactor sizing factor, brightening space-time yield, was 0.145 (ton•%ISO/m³•hr). A brightness reversion test showed that electro-brightened pulp has the same brightness stability as conventional peroxide brightened pulp. • The 2³ factorial experiment indicated that the combination of high current, high electrolyte temperature, and high sodium carbonate concentration has positive effects on electrobrightening responses. • Long brightening retention time (240 minutes) resulted in less brightness gain and yellowness loss, which means brightening time longer than 180 minutes is not worthwhile. Also, cycling current on/off with a 30-minute interval gave less brightness gain and yellowness loss than that of a comparable standard electro-brightening run. This indicates that the 30-minute interval was too long or there was not enough peroxide available to brighten the pulp. • High pulp consistency (4.5%) had higher brightening responses than low pulp consistency (2.5%), which demonstrates the electro-brightening process is similar to the conventional peroxide brightening process, with respect to pulp consistency. The in-situ electrochemical brightening process could not be optimized in this thesis study, due to limitations of the laboratory reactor. The Teflon coating and non-conductive glue in the reactor were deteriorated under the experimental conditions to the extent that the reactor was unsuitable for further experiments. However, in-situ electrochemical brightening of TMP using sodium carbonate as the source of brightening agent does produce a brightened pulp that is comparable to that obtained using merchant peroxide.