Origin and Nature of Kraft Colour: 2 The Role of Bleaching in the Formation of the Extraction Stage Effluent Colour

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Abstract
Upon chlorination and extraction, high molecular weight (>1000D) kraft lignin (KL) is degraded to a material which is structurally similar to extraction stage effluent. During bleaching there is a significant decrease in the aromaticity of KL without a concomitant decrease in colour, which would be expected if aromatic residues are the kraft chromophores. Treatment of simple sugars under conditions similar to those of kraft pulping yielded polymeric products which, on treatment with chlorine and subsequential extraction (CE), gave products with similar spectral characteristics to extraction stage effluent. It is proposed that the chromophores responsible for kraft colour are keto-enols probably derived from the degradation of carbohydrates during the kraft cook.