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The pulp is bleached to remove colour, leaving white cellulose fibres. To achieve satisfactory brightness, various bleaching chemicals are used in sequence. The early stage of bleaching, using oxygen, is a continuation of the delignification process started in cooking. The oxygen stage is followed by a chlorine dioxide stage and an extraction stage to remove remaining lignin and other coloured compounds from the pulp. This is followed by two final chlorine dioxide stages to ensure optimum whiteness. Bleaching sequences typically are described using letters to represent chemicals. Our modern elemental chlorine free bleach plant uses a ODEopDnD sequence. The pulp is then sent to the Pulp Machine. |
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