Electrochemical brightening of pulp with sodium dithionite generated in-situ
Creators: name: Hu, Hong-Liang
Publisher: name: University of British Columbia
Year: 2009

description: This study consisted of three parts, an investigation of potential reducing agents, an investigation of conventional dithionite brightening at 1% pulp consistency and an investigation of in-situ electrochemically generated dithiomte brightening. Preliminary brightening experiments by four reducing agents, hydrazine sulfate, hydroquinone, hydroxylamine sulfate and sodium hypophosphite were conducted. At 60 °C, with 1 hour brightening period, 4 to 16 wt% brightening agent charge and 0.5 wt% chelating agent charge on od pulp under nitrogen purge, these agents could only achieve a brightness gain of up to 4 % ISO. They were less effective than sodium dithionite in mechanical pulp brightening. Sodium dithionite brightening of softwood TMP at 60 °C and 1% pulp consistency was done with a single addition of dithionite, multiple addition of dithionite and addition of dithionite in multiple stages with interstage washing. With a single addition of the same dithionite charge, the brightness gain at 1 % pulp consistency was equal to that in conventional brightening at 3 to 4% pulp consistency. For a given total sodium dithionite charge on od pulp, addition of clithionite in multiple stages with interstage washing could achieve about 1 % ISO higher brightness gain than multiple addition of dithionite in a single run, while multiple addition of dithionite in a single run could achieve about 1 % ISO higher brightness gain than a single addition of dithionite. The time and amount of each charge did not seem to affect the brightness gain for multiple additions of dithiomte. In dithionite brightening with a large amount of (bi)sufflte (13 to 131 wt% sodium sulfite on od pulp), the brightness gain was about 3 % ISO lower than that with only dithionite, but about 4 % ISO higher than that with only sulfite. Sodium bisulfite and dithionite reaction products seemed to decrease dithionite brightening capability. The addition of 2-propanol or methanol could alleviate this negative effect of sodium bisuffite on dithionite brightening by about 1% Iso. The electrochemical synthesis of sodium dithionite (without pulp) was conducted in a 1 L mixed tank batch reactor evenly divided by a cation exchange membrane with cathode area of 5500 to 13000 mm². Runs were carried out at 40 °C and 60 °C. High time average dithionite concentration (5 to 10 g/l) and fairly good current efficiency (35 - 80 %) were obtained at high current ( ≥2.3 A) together with high sodium sulfite dose (≥25.3 g). Dithionite concentration became very low ( 0 to 1 g/l) at low sulfite dose (4.6 g). Cathode area, current and sodium sulfite dose were very significant variables in this process. Increases in these variables raised the time average dithionite concentration. Increases in cathode area as well as increases in sulfite dose raised the final current efficiency for dithionite generation. Decreases in current raised final current efficiency with the cathode of 5500 mm² area, but did not have significant effect with cathode of 13000 mm² area. In-situ electrochemically generated sodium dithionite brightening at ambient temperature was briefly investigated in Plexiglas reactor of the same dimensions as the reactor given above, but with a cathode of 1000 mm² area. These runs gave a maximum brightness gain of about 5 % ISO. In-situ electrochemically generated sodium dithionite brightening at elevated temperature was extensively investigated with the same reactor and cathodes used in the electrosynthesis of dithionite. When electrochemical dithionite brightening was carried out at 0.8 % consistency in the following ranges: pH (4.0 to 5.5), sodium sulfite dose (4.6 to 46.0 g), current (0.5 to 4.0 A) and temperature (40 to 60 °C), the highest range of brightness gain was 8 to 9 % ISO with time average dithionite concentration equivalent to a charge of 10 to 103 wt% on od pulp. This highest range of brightness gain was obtained at any pH with a combination of high sulfite dose (46.0 g), high temperature (60 °C) and high current (4 A). At pH 5.5, this highest range of brightness gain was also obtained with a combination of high sulfite dose (46.0 g), high temperature (60 °C) and low current (0.5 A). Temperature had the most significant effect on brightness gain in this process, increases in temperature raised brightness gain but lowered the final net current efficiency for dithionite generation. Increases in cathode area, addition of 2-propanol and chromium did not raise this highest range of brightness gain in the ranges of operating variable studied. In the subsequent optimization of brightness gain for in-situ electrochemically generated dithionite brightening at 2% consistency, the highest range of brightness gain obtained was 11 to 12 % ISO, which was only obtained at the highest range of temperature (74 to 83 °C), with time average dithionite concentration equivalent to a charge of 7 to 10 wt% on od pulp along with a very poor final current efficiency for dithionite (0 to 13 %). Yellowness decreased in electrochemical dithionite brightening in the same way as in conventional dithionite brightening. Comparing the brightness gain of the blank experiment (no current) to the highest brightness gain obtained from the electrochemical dithionite brightening at the same sodium suffite dose, temperature, pH and pulp consistency, the maximum further brightness gain contributed by the brightening species generated by current was only about 4 % ISO. For a brightness gain of 11.4 % ISO, the fraction of the brightness gain contributed by the brightening species generated by current was about 26 % of the total brightness gain.