Chemistry of Peroxide Bleaching of Kraft Pulps, 1996 International Pulp Bleaching Conference Proceedings

The reactions between alkaline hydrogen peroxide and kraft pulps, have been studied by peroxide oxidation of isolated residual lignin as well as by isolation of residual and dissolved lignin from pulps after different bleaching stages. The resulting lignin samples have been subjected to different analytical methods, like 13-C NMR, UV/VIS and FTIR spectroscopy and conductometric titration. The object of the study is to get further knowledge of the chemistry involved INTRODUCTION Traditionally bleaching has been carried out using chlorine containing chemicals, but environmental concern and legislation have forced the pulp industry towards non chlorine bleaching and the production of totally chlorine free pulps (TCF pulp). Among several new techniques great interest is focused on hydrogen peroxide and today the bleaching sequence OQP is used in several Swedish mills, with good results. In the OQP sequence the lignin undergoes several different oxidative reactions, bringing out changes in the structure and the functional groups, leading to a brighter and more water soluble lignin. A better knowledge of these structural changes could help us in understanding the reactions leading to them and thus help us to enhance and improve the bleaching. In order to study the reactions between alkaline hydrogen peroxide and kraft pulps, residual lignin, isolated by acid hydrolysis of oxygen bleached kraft pulp [l], have been oxidised with alkaline hydrogen peroxide under different conditions, i.e., temperature and time (50-90°C, l0-160 min). In some experiments magnesium sulphate and/or DTPA have been added to stabilise the hydrogen peroxide against decomposition. The resulting lignin samples have been..