BENEFITS OF A CHELATING STAGE PRIOR

BENEFITS OF A CHELATING STAGE PRIOR

TO PEROXIDE BLEACHING

ABSTRACT

         The pulping industry has been phasing out  chlorine as a bleaching agent and nowadays only a minor part of the production of bleached paper pulp involves the use of chlorine gas. This development has been driven by environmental concerns, because of the formation and emission of  chlorinated organic substances that follows the use of chlorine gas in pulp bleaching. The chlorine gas has, to a large extent, been replaced by chlorine dioxide, which generates much lower amounts chlorinated organics than chlorine gas.  There are, however, even more environmentally benign bleaching chemicals available, such as hydrogen peroxide. Hydrogen peroxide is nowadays becoming a commonly used bleaching agent in chemical pulp mills. The bleaching efficiency of hydrogen peroxide is, however, dependent on the design of the bleaching sequence in which the peroxide stage is included. In this paper, the importance of a proper chelating stage (treatment with EDTA) prior to peroxide bleaching of eucalyptus kraft pulp is high-lighted. The results show that a high brightness at a high viscosity can be achieved at a relatively low consumption of peroxide, if the bleaching is preceded by a chelation stage. These results are in agreement with results found in the literature. Some results regarding the impact of the alkali addition in the peroxide stage are also included.   

INTRODUCTION

     The world's paper consumption has increased by 50 % during the last decade. Consequently, the consumption of bleaching agents for pulp bleaching has increased as well. Pulp and paper production has long been recognized as a significant source of pollution. Especially the use of chlorine based bleaching agents has had a negative impact on the environment. When gaseous Cl2 is used in bleaching, considerable amounts of organ chlorine compounds are formed (Juuti, S., 1996). Gaseous Cl2 has largely been replaced by chlorine dioxide (ClO2) in bleaching in recent years, to reduce the formation of lipophilic organ chlorine and volatile molecular chlorinated compounds. However, the use of ClO2 in bleaching does not totally eliminate the formation of some harmful material and from an environmental point of view, it could be advantageous if the use of chlorine dioxide was decreased. (Nakatama, 2004). Society awareness of the environment has urged the use of environmentally friendly compounds in bleaching processes to substitute bleaching sequences including chlorine based chemicals. Such a bleaching concept is usually referred to as totally chlorine free (TCF) bleaching. Hydrogen peroxide (H2O2) is one of the key chemicals in TCF bleaching (Van Lierop, 1994). The goals when using this chemical are to maximize delignification, brightness and selectivity towards lignin, and to minimize the consumption of hydrogen peroxide

PEROXIDE BLEACHING  

     Oxygen delignification was industrially established in the 1970's as a method for lowering the kappa number of kraft pulp prior to the bleach plant. Hydrogen peroxide, which is another oxygen based bleaching agent, was introduced at a large scale in bleach plants in the 1990's. There are many similarities between oxygen delignification and hydrogen peroxide bleaching. The pulp is treated under alkaline conditions and molecular oxygen and peroxides are present in both cases, although in different proportions. Hydrogen peroxide is formed in oxygen delignification and oxygen is generated through the decomposition of hydrogen peroxide during peroxide bleaching (Gierer and Imsgard 1977). Compared with oxygen delignification, peroxide delignification appears to provide better color removal, because of the specific action of hydrogen peroxide on chromophores (Dence and Reeve, 1996).

     There are, however, some difficulties regarding the stability of the peroxide that have to be considered when hydrogen peroxide is to be used in bleaching.   Hydrogen peroxide is very stable under acidic conditions, but under alkaline conditions, it has a propensity for decomposition. The decomposition of hydrogen peroxide is also accelerated by temperature. The active bleaching species in alkaline hydrogen peroxide systems is the perhydroxyl anion, HOO-, which is formed under alkaline conditions (Dence and Reeve, 1996). The dissociation of hydrogen peroxide (the formation of the perhydroxyl anion) is shown in equation (1).

HOOH + HO-           HOO- +H2O          (1)

This anion is a strong nucleophile and is primarily responsible for the bleaching effect of alkaline hydrogen peroxide. There are two competing reactions in hydrogen peroxide bleaching: the first leads to delignification and a brightness increase while a second parallel reaction leads to the decomposition of hydrogen peroxide into water and oxygen.  The decomposition is catalyzed by transition metal ions and  is usually claimed to involve the formation of reactive intermediates (HO•, and O2-•). The overall reaction can be written as in equation 2.

H2O2 + HO2-          H2O + O2 + HO-            (2)

   

     Hydrogen peroxide bleaching is usually performed at pH 10-11.5. Under these conditions transition metals can hardly exist as free ions. Collodete et al, (1988) have studied the solubility of Fe3+, Cu2+ and Mn2+ in the pH range 9.8 to 11.8. After aging at 500C for 120 minutes, the solution

were subjected to ultra filtration (cut off >1000 MW) and an analysis of the filtrates showed that that no amounts of iron, copper or manganese above the detection limit (10 ppb) could be found in the solution after filtration. The rate of peroxide decomposition was shown to be considerably lower in the filtrates than in the original solutions, which implies that the decomposition is surface-catalyzed by colloidal transition metal- oxides/hydroxides.     In bleaching, the decomposition is undesirable for two reasons: consumption of peroxide in non-bleaching reactions and formation of hydroxyl radicals (HO•), which oxidize and degrade cellulose. In a study of hydroxyl radical formation by hydrogen peroxide under bleaching conditions, the free hydroxyl radicals are formed by processes catalyzed by mononuclear transition metal ion complexes. Colloidal particles of transition metal oxides/hydroxides were proposed to decompose hydrogen peroxide directly into oxygen and water.

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Maria Gabriela Medina

C.I. 16779553

CRF