Digester Process
An advanced chemical pulping method splits the addition of cooking chemicals into multiple stages within a continuous digester. In kraft pulp production, modified continuous cooking is employed to achieve a low lignin content while preserving the strength of the cellulose. This process replaces the traditional single stage chemical addition method.
Chemical Control
The process works by maintaining a uniform concentration of sodium hydroxide and sodium sulfide throughout the cook, rather than having a high initial concentration that drops off. In modified continuous cooking, the cooking liquor is introduced at several points along the height of the digester, and the spent liquor is extracted continuously. This controlled environment reduces the degradation of the hemicellulose and cellulose fibers, resulting in a higher yield of strong pulp.
The temperature profile is also kept more uniform to avoid thermal damage to the fibers.
Fiber Quality
Lower chemical usage in downstream bleaching represents a major advantage of this cooking technique. This lower lignin content reduces the volume of bleaching chemicals required in subsequent stages, which lowers operating costs and reduces the environmental impact of the bleach plant effluent. Pulps produced this way exhibit high brightness stability and excellent papermaking properties.
This makes the method highly suitable for producing high quality packaging boards and printing papers.
