Black Liquor

Black Liquor

White liquor is a solution of caustic chemicals, such as sodium hydroxide, that is added to wood chips in Kraft and sulfite pulping processes to dissolve lignin and release cellulose fibers from wood. The fibers are then washed, and the resulting slurry of water, caustic chemicals and dissolved lignin is called black liquor. Black liquor is recycled in a process that gives off heat energy, carbon dioxide, and recoverable chemicals. These chemicals need only a 10 percent supplement of additional caustic chemicals to become white liquor again (Larson and Raymond 1997).

Nearly 47 million dry tonnes of black liquor is produced annually and is typically recovered as part of the pulping process (BEES 2001). Since the liquor is created and consumed in recovery boilers on-site, it is not readily available for off-site utilization. The steam that is generated during the black liquor recovery process contributes significantly to the energy needs of pulp and paper mills. In addition, some chemicals can be recovered from the liquor and through recycling can reduce the pulping processs chemical needs by almost 90 percent.