Chapter Five - Hydrogen Peroxide Bleaching

Abstract

Hydrogen peroxide is a very effective bleaching chemical if applied under appropriate conditions. It decomposes to give water and oxygen only, so it is ideally suited to applications in which the effect on the environment of effluents has to be minimal. In chemical pulp bleaching, hydrogen peroxide is used mostly as a reinforcement to existing bleaching stages. The extraction process reinforced with atmospheric peroxide has gained notability due to its good effectiveness and low capital requirement for implementation. However, peroxide reinforcement can be more effective when pressurized in the so-called PHT-stage technology. The most common types of alkaline extractions today are those reinforced with oxygen and peroxide, partially pressurized alkaline extraction with oxygen and hydrogen peroxide (EOP) or pressurized all the way, the so-called (PO) stage. Use of hydrogen peroxide in the extraction stage results in more reduction in chlorine chemicals and also decreases the color of mill effluent. Other advantages of alkaline extraction with peroxide include improvement of environmental parameters such as color COD, BOD, and AOX. Significant reduction in effluent color is the greatest benefit of H2O2 addition in the extraction stage. It is common to use the alkaline extraction reinforced with oxygen (EO) or hydrogen peroxide (EP, PHT), or both (EOP, PO), to compensate for lower chlorine dioxide availability and also to make possible the bleaching in short sequences. Bleach plants that have a low availability of chlorine dioxide and require high peroxide dosages (0.8–1.0%) need more severe conditions for peroxide consumption. In these cases, pressurized peroxide stages such as (PO) or PHT are recommended because they allow use of high temperatures. A very significant increase in brightness is achieved when peroxide is applied to the alkaline extraction. Another positive effect is the peroxide effectiveness to bleach shives; even when they are not completely bleached, they are lighter and less visible. H 2 O 2 can also be used in the second alkaline extraction stage to counteract pulp darkening to reduce chlorine dioxide consumption. Use of pressurized peroxide stages (PO) makes possible to achieve a high final brightness in totally chlorine-free (TCF) bleaching. In sequences with chlorine dioxide (ECF bleaching), a powerful peroxide stage will reduce the consumption of chlorine dioxide or even replace one chlorine dioxide stage. A hot, pressurized peroxide stage operates at temperatures above 100 °C with a small amount of oxygen added. A prerequisite for successful peroxide bleaching is that the content of metal ions, for example, manganese, copper, and iron, is low. Several mills around the world are using peroxide for bleaching.