Lauren Blum, Environmental Defence Fund, 1996
Diagrammatic representation of the production of pulp and paper using bleach processes
In a chemical pulping process, heat and chemicals are added to wood chips in a pressure cooker called the digester. In the kraft process, an aqueous solution of sodium hydroxide and sodium sulfide, known as white liquor, selectively dissolve the lignin and make it soluble in the cooking liquid. After 2 to 4 hours, the mixture of pulp, spent pulping chemicals and wood waste is discharged from the digester. The pulp is washed to separate it from the black liquor - the pulping chemicals and wood waste. Kraft pulping is a low yield process - only 45% of the wood used becomes pulp. The pulp, called brownstock at this point in the process, is ready to be bleached. Softwood pulp from a conventional cooking process contains about 4.5% lignin. This lignin will be removed and the pulp will be brightened during the bleaching process.
Efficient pulp washing is very important because it ensures the maximum recovery of the pulping chemicals and it minimizes the amount of organic waste carried over with the pulp into the bleaching process. Poorly washed pulps require higher bleaching chemical doses, thus increasing the cost and the amount of organic waste discharged in the bleach plant effluent