Method and system for treating lignin

1. A method for separating lignin from a lignin containing liquid medium and treating the separated lignin, wherein the method comprises:

precipitating lignin from the lignin containing liquid medium by adding a pH lowering agent to the lignin containing liquid medium,

thereafter subjecting the lignin to a hydrothermal carbonization process in a wet state such that a slurry of carbon containing material is formed,

recovering carbon containing material derived from the lignin as the result of the carbonization after the hydrothermal carbonization process, and

adjusting the pH value of the lignin in the wet state before the hydrothermal carbonization process to a value above 8 so as to control particle size of the carbon containing material.

2. The method according to claim 1 , wherein the lignin is subjected to the hydrothermal carbonization in a suspension.

3. The method according to claim 2 , wherein the method further comprises the steps of:

suspending the lignin in a liquid to make said suspension, and

subjecting said suspension to the hydrothermal carbonization process.

4. The method according to claim 3 , wherein the method comprises at least:

a precipitation stage, wherein the pH lowering agent is added to the lignin containing liquid medium and lignin is precipitated,

after the precipitation stage, a first separation stage, wherein the precipitated lignin is separated from a remaining liquid phase of the lignin containing liquid medium,

after the first separation stage, a suspending stage, wherein the precipitated and separated lignin is suspended for obtaining a lignin suspension,

after the suspending stage, a hydrothermal carbonization stage, wherein the lignin suspension is treated for obtaining a slurry of carbon-containing material, and

after the hydrothermal carbonization stage, a second separation stage, wherein the carbon containing material is separated from the slurry.

5. The method according to claim 3 , wherein the method comprises:

separating the carbon containing material from the suspension after the hydrothermal carbonization,

obtaining a liquid after separating the carbon containing material from the suspension, and

suspending the lignin in said liquid to make a suspension for the hydrothermal carbonization process.

6. The method according to claim 5 , wherein said liquid is a filtrate of a slurry of the carbon containing material.

7. The method according to claim 1 , wherein the hydrothermal carbonization stage comprises heating the lignin in a wet state to a temperature of 150 to 250° C.

8. The method according to claim 7 , wherein in the hydrothermal carbonization stage the lignin is heated at a pressure of 20 to 40 bar.

9. The method according to claim 1 , wherein the lignin is subjected to the hydrothermal carbonization in a suspension at a pH value between 8 and 10.

10. The method according to claim 9 , wherein the pH lowering agent is carbon dioxide.

11. The method according to claim 1 , wherein the lignin containing liquid medium is liquid from a biomass conversion process.

12. The method according to claim 1 , wherein the lignin containing liquid medium is black liquor.

13. The method according to claim 1 , wherein the method comprises:

making a carbon containing product by the hydrothermal carbonization process, and

using the carbon containing product as a raw material for the production of tires, steel, electrode carbon or activated carbon.

14. The method according to claim 1 , wherein the method comprises preheating the lignin in a wet state before conveying the lignin suspension to the hydrothermal carbonization stage.

15. The method according to claim 1 , wherein the method comprises:

separating the carbon containing material from the suspension after the hydrothermal carbonization,

obtaining a liquid after separating the carbon containing material from the suspension, and

circulating the liquid to the lignin containing liquid medium.