The in-situ brightening of pulp by the electro-reduction of oxygen, mediated with anthraquinone-2-sulfonic acid
Creators: name: Chen, Margaret Ying Ting
Publisher: name: University of British Columbia
Year: 2009

description: A redox cycle involving anthraquinone 2-sulfonic acid - - denoted as AQS, and oxygen in an aqueous alkaline media was investigated for its ability to promote the in-situ electroreduction of oxygen to hydrogen peroxide with the coincident promotion of the brightening of thermo-mechanical pulp. Current applied at a carbon cloth cathode was used to reduce the AQS to hydroanthraquinone - - denoted as H₂AQS. The reaction of H₂AQS with oxygen was shown to produce hydrogen peroxide. Evidence presented in this study indicates the operation of a redox cycle involving AQS/ H₂AQS, oxygen and current to produce hydrogen peroxide. It is thought hydrogen peroxide produced in this redox cycle was responsible for the larger brightness gains and yellowness reduction of the thermomechanical pulp observed in experimental runs employing the AQS/oxygen redox cycle, than in corresponding runs in the absence of the AQS/oxygen redox cycle. The batch electrochemical reactor employed in this study consisted of a platinized titanium anode and carbon cloth cathode. Major modifications of the reactor design, which included increasing the area of the working electrode (the cathode), decreasing the area of the counter electrode (anode), employing membrane/fritted glass separators and providing adequate mixing, were found to suppress undesirable electrochemical reactions and promote even current distribution in the electrochemical cell, resulting in an increased current efficiency and production rate of peroxide. All runs were conducted at 60 °C pH 10.5-11 and at atmospheric pressure. Some other important quantitative findings were: • In a solution containing 0.1 M Na₂SO₄, 4.7mM Na₂SiO₃, 0.32mM MgSO₄, the hydrogen peroxide produced in 3 hours with the use of the 5mM AQS and oxygen redox cycle was approximately 49.5(+/-) 2.5 mM, which is 28(+/-) 2.5mM more than in a corresponding run in the absence of the AQS. • In a solution containing 0.1 M Na₂SO₄, 4.7mM Na₂SiO₃, 0.32mM MgSO₄, with oxygen purged for 3 hours (at 1.5A without pulp), subsequently followed by oxygen purged for 5 hours (at 0 A with 1 wt. % thermomechanical pulp), the presence of 5mM AQS brought about 21 (%ISO) gain and 15 (%) yellowness reduction more than for a corresponding run in the absence of the AQS. • A 2³ factorial experiment was performed to illuminate the three (3) key process variables: current (0.3A to 1.5A), gas (Air-Oxygen), the AQS concentration (OmM to 5mM). From this factorial experiment, it is found that the presence of two key factors, namely oxygen and current, are essential and the presence of catalyst factor, such as AQS, is beneficial in the in-situ electrochemical generation of hydrogen peroxide in the AQS/oxygen system. Although the AQS/oxygen redox cycle, used in this project, generated hydrogen peroxide at a high current efficiency and produced a brightened pulp that is comparable to that obtained using merchant peroxide, this system cannot be realized in the industry due to the severe fouling problems associated with the use of its working electrode. It is thought that the AQS crystallizes and blocks electro-active areas of the carbon cloth electrode, which makes it unsuitable for any subsequent runs. Due to the fouling problems posed by the carbon cloth cathode with the AQS redox couple, the electroreduction of oxygen mediated by AQS is impractical for industrial use without further study.