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black liquor is the waste product from the kraft process when digesting pulpwood into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers.
Kraft is the two major alkaline processes to produce chemical pulps. However, in the process, cellulose fibers are disassociated from lignin by chemical reactions. These reactions occur in a pressurized digester, where wood chips or fibers are heated and cooked with the cooking liquor, composed basically of NaOH. Specially in the Kraft process, Na2S is added to the digester for improving the disassociation of lignin from cellulose fibers, accelerating the wood cooking operation and increasing the mechanical resistance of the pulp. Note that the products resulting from the digester reactions are the cellulose pulp and the black liquor .
In chemical pulp mills, about half of the original raw wood material is dissolved in the spent pulping liquor. Most modern pulp mills burn the liquor in order to recover the cooling chemicals and utilize the heat value of the organic material, whereas in a forest biorefinery, wood components such as liginin and hemicelluloses are extracted form the pulping liquor and used in high-value-added chemicals.
Using membrane separation technology, lignosulfonate and reducing sugar from acid waste liquid can be separated , the organic load of BOD can be greatly reduced. The separated lignosulfonate can be used to produce surfactant and other by-products The reducing sugar can be used to produce xylose, alcohol and feed protein.
Alkali lignin recovered from alkali waste liquid can be modified to produce cement alkali water agent, etc. After ultrafiltration treatment, the removal rate of COD and BOD can reach 70%, and the removal rate of chroma can reach 90% or more.
2.recovery of alkali
The purpose of alkali recovery is to convert the sodium salt of black liquor into NaOH and re-use it for cooking, which can be recycled continuously to reduce the consumption of alkali and reduce the pollution to the water area.