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A study of in-situ brightening of mechanical pulp via the electro-oxidation of sodium carbonate

University of British Columbia

This thesis project consists of investigative work on the in-situ electrochemical brightening of thermo-mechanical pulp (TMP) using sodium carbonate (Na₂C0₃) as the source of brightening agents. The conditions for the electro-oxidation of Na₂C0₃ to percarbonate (C₂O₆²⁻) and its eventual decomposition to hydrogen peroxide (H₂O₂) were studied. Next, the factors that affected the in-situ electrochemical brightening of TMP were investigated, followed by the sequential simplex optimization of the electrochemical brightening process. To further study the role of Na₂C0₃ and the active oxygen (C₂O₆²⁻ and/or H₂O₂) on the brightening process, several simulated brightening experiments using merchant H₂O₂ were performed. Finally, a rough cost comparison between the conventional H₂O₂ and the electrochemical brightening method was performed. In most of the electro-brightening experiments, a platinized titanium hollow U - tube anode and a tungsten rod cathode were used. The anode was water-cooled to raise the oxygen overvoltage at the anode and to suppress the hydrolysis of the oxidation product. The cathode area was approximately ten times smaller than the anode area to suppress the reduction of active oxygen on the cathode. Using a range of current densities (0.04 - 0.25 A/cm²), it was found that the highest time-average current efficiency for the production of active oxygen over 30 minutes without pulp at 46°C was 41% at 0.14 A/cm² and an anode voltage of 3.2 V vs SHE. The maximum active oxygen concentration produced by electrolysis of sodium carbonate in the presence of pulp was about 50% higher than that without pulp. The investigation of the in-situ brightening of TMP using Na₂CO₃ began with a full 2³ factorial experiment with approximately 750 ml of pulp slurry in 1 M Na₂CO₃ at 1% pulp consistency. The variables were temperature ( 46°C and 66°C), anode area (14.5 cm² and 29 cm²), and current (5 A and 10 A). It was found that the combination of temperature, current density and current concentration appears to have a positive effect on the active oxygen concentration, brightness gain and yellowness loss. However, this result may be unreliable due to the deterioration of the anode. Glassy carbon plate anode apparently gave 50% higher concentration of active oxygen than platinized titanium anode, but was not a good anode material for the in-situ electro-brightening reaction because its surface was eroded, introducing traces of carbon powder in the pulp suspension. Furthermore, the loose carbon powder might penetrate into the fiber network, giving an inaccurate brightness or yellowness reading on the spectrophotometer. The post-treatment of platinized titanium anode, by submersing it in warm ≈ 1 M sodium hydroxide (NaOH) solution followed by reverse-polarizing of the electrodes in ≈ 1 M of Na₂CO₃ solution, was very important in suppressing the deterioration of the anode and ensuring the reproducibility of the brightening results. Experiments on platinized titanium anodes with different electrode configurations show that good current distribution improved brightness gain and lowered specific energy. A 10 %ISO brightness gain can be achieved by brightening pulp at 1% consistency at 46°C with a specific energy of 20,000 kWh/ton of pulp. The brightness increase & yellowness loss in a single-stage 3-hour run was about 14 %ISO & 7%, whereas a single-stage 6-hour run and a two-stage run at 3 hours/stage produced brightness gain & yellowness loss of 13 %ISO & 8% and 12 %ISO & 8% respectively. This means that brightening time longer than 3 hours may not be worthwhile. The presence of sodium carbonate in the brightening suspension actually decomposed hydrogen peroxide quite rapidly compared to the rate of decomposition in conventional peroxide brightening liquor. This means that a continuous supply of active oxygen has to be available for a brightness gain to occur in the presence of sodium carbonate. A cost comparison between the electrochemical and the conventional brightening method shows that the electrochemical method cost approximately 20% more in annual operating cost than the conventional brightening method. The capital cost of the electrochemical method is approximately 140% more than that of the conventional brightening method.

Thesis/Dissertation

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2009-05-25

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http://hdl.handle.net/2429/8170

Chemical and Biological Engineering

University of British Columbia

UBCV

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