Method for lignin separation from black liquor involving multiple acidification steps

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 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 as nucleus particles, said second pH level being at least 1 pH unit below that of the first pH value,
• a first storage phase (ST) wherein the original black liquor is kept at or below the second pH level for a retention time of at least 25 minutes during which storage phase the precipitated lignin particles increase in size thus increasing the filterability of the precipitated lignin,
• 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 order to decrease the pH value to a third pH level whereby more than 20% of the total lignin content is additionally precipitated and as growth of nucleus lignin particles formed after the first storage phase, said third pH level being at least 1 pH units below that of the second pH value, and
• followed by a separation phase wherein the precipitated lignin is separated from the remaining liquid phase of the acidified original black liquor.