Method for separating lignin from black liquor, a lignin product, and use of a lignin product for the production of fuels or materials
The method is for controlling a sodium and sulphur balance of a pulp mill while separating lignin from black liquor. Lignin is precipitated by using an acid followed by filtration. The lignin filter cake thus obtained is re-suspended in acidic liquid and dewatered to form a second cake. The filtrate obtained after dewatering of the second cake is returned for washing and suspension of the first cake. Sodium sulphate-rich ESP (electrostatic precipitator) dust produced in the recovery boiler is used in the washing of the precipitated lignin cake.
1. A method for separation of lignin from black liquor, comprising:
precipitating lignin by acidifying black liquor with carbon dioxide resulting in a first lignin suspension;
separation to form a lignin cake of a solid material of a first filter cake and a first filtrate,
suspending the first filter cake to obtain a second lignin suspension,
separating the second lignin suspension to form a second lignin cake of a solid material of the second filter cake and a second filtrate,
adding a washing liquid for displacement washing of the second filter cake,
enhancing a dryness of the second filter cake,
obtaining filtrate from the displacement washing, the filtrate forming a part of the second filtrate,
increasing an ionic strength by adding recovery boiler precipitator dust to at least a first portion of the second filtrate and adjusting the pH of the second filtrate to a value below 6,
washing the first filter cake with the first portion of the second filtrate,
the recovery boiler precipitator dust of the second filtrate keeping lignin in the first filter cake in solid form, and
sending the first portion of the second filtrate to recovery after use as washing liquid, and
adding a second portion of the second filtrate to the washed first filter cake to form the second lignin suspension, adjusting the pH level of the second lignin suspension to a value below 6 and approximately the pH level of the washing liquid added to the second filter cake.
2. The method according to claim 1 wherein the method further comprises conveying the second filtrate either for use in a displacement washing during the separation, or for diluting the first lignin suspension before separation.
3. The method according to claim 1 wherein the method further comprises adding a sulphuric acid and/or residual acid from chlorine dioxide manufacture for washing or suspension of the first filter cake.
4. The method according to claim 1 wherein the method further comprises adding a virgin sulphuric acid and/or sulphurous acid, both obtained from gaseous SO2, to the second lignin suspension.
5. The method according to claim 4 wherein the virgin sulphuric acid and/or sulphurous acid obtained from SO2 is added to the second filtrate.
6. The method according to claim 1 wherein the method further comprises conveying a portion of the second filtrate to perform a soap acidulation.
7. The method according to claim 1 wherein the method further comprises conveying a portion of the second filtrate to an external treatment.
8. The method according to claim 1 wherein the method further comprises adding a salt before the separation to form the first filter cake.
9. The method according to claim 2 wherein the method further comprises adjusting the pH of the second filtrate to be equal to or below the pH of the first precipitation stage.
10. The method according to claim 1 wherein the method further comprises blowing through the first or second filter cake with a gas to dispose of any remaining liquid.
11. The method according to claim 1 wherein the method further comprises re-circulating the first filtrate obtained from separation of the first lignin suspension to a pulp mill recovery system.
12. The method according to claim 1 wherein the method further comprises maturing the first lignin suspension.