UBC Theses and Dissertations
Electrochemical generation of hydrogen peroxide with in-situ brightening of mechanical pulp Srinivasan, Rajagopal
Generation of hydrogen peroxide by electroreduction of oxygen on a slurry graphite cathode in an alkali medium was investigated to brighten mechanical pulp in-situ in the reactor. Four different reactors comprising both divided and undivided configurations were evaluated for peroxide generation and two of these were used for in-situ brightening experiments. Several different types of particulate graphite as the oxygen sparged slurry cathode and stainless steel, graphite and platinised titanium as anode materials were evaluated in different electrolytes for the generation of hydrogen peroxide, all of which contained the standard additives (MgSO₄, Na₂SiO₃ and DTP A) for peroxide brightening. Diaphragms of three different materials were investigated for hydrogen peroxide production in a diaphragm reactor of 1.9 liters volume. Results from these experiments with a diaphragm sheathed stainless steel anode indicate the loss of hydrogen peroxide by oxidation on the anode due to the high permeability of diaphragm material. Experiments in a divided membrane reactor without pulp show hydrogen peroxide concentrations of up to 0.025 M, comparable to the concentration of peroxide employed in conventional brightening of TMP could be obtained with a current efficiency of 20% at 2 Amps current with 500 ml UCAR 1 graphite of size -0.42 + 0.29 mm as cathode slurried in 500 ml 0.5 M Na₂SO₄ catholyte containing NaOH at 60°C and pH around 11. The efforts to brighten pulp in the divided membrane reactor with UCAR 1 and pyrolytic graphite cathodes were hampered by the generation of graphite fines of less than five micron in size and subsequent graphite loading of fiber lumen. Consequently, the brightness of the in-situ brightened pulp declined to 32% ISO from an unbrightened value of 47% ISO. Agitation imparted by the impeller to the graphite-pulp mixture appear to cause the generation of graphite fines and the consequent loss of brightness. Hydrogen peroxide generation and in-situ brightening were investigated in an undivided reactor of 12.5 cm diameter incorporating a fritted SS sparger plate and a water cooled anode. Six different types of graphite as particulate cathode with platinised titanium or SS as anodes were evaluated for peroxide production. Oxygen was sparged through the cathode graphite bed and an impeller at 60 rpm was used to rake the graphite bed to achieve good oxygen transfer without disturbing the integrity of the bed. With the platinised titanium anode and pyrolytic graphite (-1.7 + 0.5 mm) cathode up to 0.043 M H₂O₂ was obtained at 2 Amps and 5.7 volts with 44% current efficiency in 1 M Na₂CO₃ at 60°C and pH 11.15 with an oxygen flow of 0.31 liters/min @ STP. Peroxide generation was not favored on the SS anode. The effect of alkali darkening of TMP in in-situ brightening was investigated. The results indicate the undesirable alkali induced darkening could be avoided by the introduction of pulp into the reactor after sufficient peroxide is generated and present in the electrolyte. Consequently, TMP of 1% consistency in 1 M Na₂CO₃ was utilised for in-situ brightening with pyrolytic graphite cathode and platinised titanium anode in the undivided reactor and the pulp was introduced to the reactor after 60 minutes of run, when the peroxide concentration was 0.036 M. The electrolyte pH ranged from 11.15 to 10.7, the cell voltage and current were 6.9 volts and 2 amps respectively and oxygen flow was maintained at 0.31 liters/min @ STP. In this case brightness of the pulp increased from an initial value of 43% ISO to 47% ISO at the end of 150 minutes run while yellowness declined from 34% to 19%. The successful in-situ brightening was repeated twice and the brightness increased from 43% ISO to 53% ISO and yellowness declined from 34% to 21% consistently. Increasing the run duration from 150 to 240 minutes resulted in final pulp brightness of 53% ISO and yellowness of 23%. Preliminary cost calculations indicate the relatively high cost of US$3800 to brighten a ton of TMP in-situ as compared to US$33 in conventional brightening. The cost of oxygen and Na₂CO₃ reflect 95% of the cost of in-situ brightening. The disparity between the cost of in-situ and conventional brightening could be significantly reduced by further investigation and redesigning the process to recover and recycle the oxygen and Na₂CO₃.
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