Method for lignin separation from black liquor having multiple acidification steps

摘要

The method is for separation of lignin from original black liquor (BLIN) having the following phases in sequence; a first precipitation phase (PR1) for precipitation of lignin by a first acidification of the original black liquor by adding a first acid or mixture of acids (G1a); followed by a second precipitation phase (PR2) for precipitation of lignin by a further acidification of the original black liquor by adding a second acid or mixture of acids (G1b). The first phase is implemented to lower the pH while avoiding any larger amount of precipitation, i.e. less than 2-5%, while the second phase is implemented in order to obtain the larger part of the precipitation. In total is 40-70% of the total lignin content of the original black liquor precipitated while the residual black liquor after precipitation is still strongly alkaline.

主权项

1. A method for separation of lignin from original black liquor (BLIN) having a first pH value, comprising the following phases in sequence:
a first precipitation phase (PR 1) wherein a first acidifier charge is added to the original black liquor in a first reaction chamber in order to decrease the pH value of the original black liquor to a second pH level whereby less than 10% of the total lignin content is precipitated, said second pH level being at least 1 pH unit below that of the first pH value, a second precipitation phase (PR 2) wherein a second acidifier charge is added to the acidified original black liquor from the first precipitation phase in a second reaction chamber in order to decrease the pH value to a third pH level whereby more than 20% of the total lignin content is additionally precipitated as growth of nucleus particles formed in the first precipitation phase and growth of nucleus particles precipitated in the second precipitation phase, said third pH level being at least 0.1 pH units below that of the second pH value, the first precipitation phase (PR 1) being distinct from the second precipitation phase (PR 2) and followed by a separation phase wherein the precipitated lignin is separated from the remaining liquid phase of the acidified original black liquor.