An insight into the behaviour of aspen CTMP in peroxide bleaching

The objective in the bleaching of mechanical pulps is to selectively remove colour-contributing components (chromophores) while simultaneously preserving pulp yield. In current practice, this involves mainly the use of hydrogen peroxide, especially when high brightness is targeted. The reactions of hydrogen peroxide bleaching, usually performed in alkaline medium, result in chromophore removal through lignin modification and/or solubilization, accompanying an inevitable removal of carbohydrates. The dissolution of lignin and carbohydrates occurs to varying extents that are determined by bleaching parameters, such as peroxide charge and alkalinity. Generally, high peroxide charge coupled with high alkalinity is essential to attaining high pulp brightness. However, this common bleaching practice causes the loss of a considerable amount of organic substances from pulp fibres, which effects the bleaching operation in many respects. For instance, removal of the materials is directly reflected in pulp yield and, in turn, becomes the source of a bleach plant's production of COD (i.e., chemical oxygen demand). Aspen is an important wood species, especially in Western Canada, in the production of market BCTMP. This manufacturing industry contributes, to a great extent, to the regional economy. Technological advances in mechanical pulping and bleaching allow the production of aspen BCTMP products of flexible properties that meet the end use requirements for a broad range of paper products [1]. One advantage of aspen CTMP is its good bleachability. The pulp is currently bleached by peroxide to a brightness of over 85 ISO, which can be used as a furnish for high brightness paper grades. Our earlier studies have shown that in bleaching brightness response is rapid, then slows down, and finally reaches a plateau from which further increasing peroxide charge only slightly increases brightness [2]. It is the final brightness increase of several units that consumes the majority of an applied bleaching chemical and causes a ...