Process of recovering alkali from black liquor of papermaking

摘要

This invention provides an improved process for recovering chemicals from black liquor in alkaline pulping, which is achieved by introducing depuration treatments of green liquor and lime before causticization reaction and a two-step-causticization procedure in the causticization procedure of the conventional chemical recovery process. The process of this invention not only avoids the generation of the waste residue (lime mud) from the beginning and saves the recovery and treatment of lime mud, but also can directly produce a series of precipitated calcium carbonate products with various particle sizes by regulating process conditions while the alkali is recovered, said CaCO₃ products including a CaCO₃ suitable for making coated paper, wherein ≧50% of the CaCO₃ has a particle size below 2 μm; a CaCO₃ suitable for making neutral sized paper, wherein ≧90% of the CaCO₃ have a particle size below 2 μm; a superfine CaCO₃ having an average particle size 1 μm. The present invention can be used for treatment of pollution of black liquor in the alkaline pulping.