Patent ID: 8172981

Claim:
A method for separation of lignin from black liquor comprising the following steps: a) precipitation of lignin by acidifying black liquor, resulting in a first lignin suspension and thereupon performing separation, thereby forming a first cake of solid material that is a first lignin filter cake, b) suspending the lignin filter cake obtained in step a) and during the suspension operation adjusting the pH level of the suspension to approximately the pH level of the wash water of step c) below, whereupon a second lignin suspension is obtained, c) separating the second lignin suspension thereby forming a second cake of solid material, thus a second filter cake, and thereafter adding acidified wash water for displacement washing followed by enhancement of the lignin filter cake dryness whereupon an acid filtrate is obtained, and d) conveying the filtrate from step c) or selected parts of the filtrate from step c) for dilution before separation in step a) and/or addition as wash water to the filter cake produced in step a), wherein the ionic strength is adjusted before addition by adding a salt, ESP dust, pulp mill recovery boiler precipitator dust or sodium sulfate in order to control an Na/S balance in a pulp mill, and the pH is adjusted by adding ESP-dust or pulp mill recovery boiler precipitator dust, and additional use of filtrate in step d) is to the suspension step of step b) or prior to the filtrations step of step c).