5677154 Production of ethanol from biomass

Abstract

Secondary fibres are valuable feedstock for the paper industry, constituting about one-third of the total amount of raw materials used, because of favourable prices compared with primary fibrous pulp, and as a result of the promotion of waste paper recycling in many countries. Bleaching is an important step in the processing of recovered paper as it improves the optical characteristics of deinked pulp. Many chemicals have been used for bleaching deinked pulp. Besides the chlorine-free bleaching chemicals, hydrogen peroxide and sodium dithionite, chlorine-containing chemicals such as chlorine dioxide and sodium hypochlorite have also been used. These types of bleaching chemical are used only for wood-free recycled fibre pulp because of their lignin-degrading effects. The oxygen bleaching stage is usually reinforced with hydrogen peroxide. This is especially true for processing mixed office paper. The use of ozone has not played any significant role in the bleaching of deinked pulp so far. However, it is the only chlorine-free bleaching chemical that efficiently destroys optical brighteners in deinked pulps. Many plants around the world are using two-stage bleaching: oxidative followed by reductive stages. Some plants run these stages sequentially. The oxidative agents used today include peroxide, oxygen and even some ozone; the reductive agents are typically sodium hydrosulphite or formamidine sulphinic acid. Formamidine sulphinic acid has become a key chemical used for brightening and colour stripping of secondary fibre.Nowadays, plants commonly combine peroxide and oxygen in a pressurised stage, whereas some plants still use just a single peroxide stage. Some old newspaper plants use only reductive bleaching. Ozone use with recycled fibres is uncommon, but some plants do use it, typically in an over-the-fence arrangement with an ozone supplier. A few plants use chlorine dioxide, whereas sodium hypochlorite has fallen out of favour for environmental reasons. The most efficient bleaching of deinked pulps today seems to involve a pressurised oxygen–peroxide stage. This chapter describes bleaching of secondary fibres.