Fundamentals of the Ozone/Methanol Bleaching Process, 1996 Pulping Conference Proceedings

G. J. Kang

Y. Ni

A. R. P. van Heiningen

Dr. Jack McKenzie

Limerick Pulp and Paper Research and Education Centre

Recently, we reported a pulp ozone bleaching technique which significantly reduces the cellulose degradation. This technique involves impregnation of unbleached kraft pulp with an acidified solvent solution, such as methanol, during ozone bleaching. The consumption of methanol by ozone in this ozone/methanol bleaching process is important for three reasons, 1) expensive ozone is wasted, 2) methanol is lost and 3) the oxidation products from the ozone-methanol reaction, such as formic acid, formaldehyde and hydrogen peroxide may complicate the subsequent methanol recovery and/or waste water treatment processes. Therefore, in this paper, the consumption of methanol by ozone was studied for high consistency, well fluffed pulp which was impregnated with a 75% aqueous methanol solution of pH 2.0. It was found that the consumption rate of methanol by ozone is controlled by chemical kinetics. The oxidation of methanol in the impregnation liquor by ozone can be described by the direct attack of ozone on methanol to form formic acid and formaldehyde. The methanol oxidation product distribution has been determined as a function of temperature and ozone charge. The consumption of methanol in unbleached kraft pulp is mostly determined by oxidation with oxygen in a chain reaction initiated by radicals generated from the reactions between lignin and ozone. As a result of the oxidation the methanol consumption is about 0.1% on pulp at an ozone consumption of about 1.1% on pulp, and about 0.4-0.5% on pulp at an ozone consumption of about 2.0% on pulp, irrespective of the temperature during high-consistency ozonation of the methanol-impregnated unbleached pulp.