Method and system for treating lignin

Abstract

The invention provides a method and a system for separating lignin from a lignin containing liquid medium, such as pulp mill black liquor, and treating the separated lignin. The method comprises at least the following steps: a) a precipitation stage (1), wherein a pH lowering agent (A) is added to the lignin containing slurry for precipitating lignin, b) followed by a first separation stage (2), wherein the precipitated lignin is separated as a lignin cake from the remaining liquid phase of the lignin containing slurry, c) a suspending stage (3), wherein the lignin cake is suspended for obtaining a lignin suspension, d) a hydrothermal carbonization stage (4), wherein the lignin suspension is treated for obtaining a slurry of carbon containing material, and e) a second separation stage (5), wherein the carbon containing material is separated from the slurry.

Claims

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.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will be described in more detail with reference to the appended drawings, in which

(2) FIG. 1 shows a process flow chart according to an embodiment of the invention, and

(3) FIG. 2 shows a detailed process flow chart of a hydro thermal carbonization stage according to an embodiment of the invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

(4) Although the following example concerns mainly the processing of black liquor that is a part of the chemical cycle of a kraft pulp mill, the details of the example can be applied in processing of other spent liquors of chemical pulping as well.

(5) FIG. 1 shows a process flow chart, wherein lignin containing liquid medium 10 is fed to a lignin separation and treating process according to the invention for obtaining a carbon containing material 60 as an end product (carbon containing product). The lignin containing liquid medium is preferably concentrated black liquor, spent liquor from a cooking process that has already gone through several evaporation stages in its course to a recovery boiler where it will be burnt. The vertical arrow from bottom to top on the left side of the figure represents the flow of black liquor from a digester to the recovery boiler and the successive evaporation stages where the dry solids content of the black liquor increases are represented by squares. The evaporation stages can be a series of evaporators that are normally used for concentrating black liquor before the recovery boiler.

(6) The black liquor is taken from the flow to the precipitation when it has not yet reached the final dry solids content where it is burnt in the boiler. The black liquor is taken to the precipitation at a dry solids content of about 30-45%.

(7) However, it is understood that the black liquor, which contains lignin, can be taken at any point between the digester and the recovery boiler.

(8) The process comprises a precipitation stage 1, wherein lignin is precipitated from the lignin containing liquid medium 10 by adding pH lowering agent A, followed by a first separation stage 2, wherein the precipitated lignin is separated as a lignin cake 30 from the remaining liquid phase of the lignin containing liquid medium 20 with lowered pH. The lignin cake 30 is suspended in a suspending stage 3 using water, wherein a lignin suspension 40 is obtained. Additional substances B, such as acid for adjusting the pH for the HTC stage can be added. For example sulphuric acid can be added. The lignin suspension 40 is treated in a hydrothermal carbonization (HTC) stage 4, wherein slurry of carbon containing material 50 is obtained, and the carbon containing product 60 (HTC carbon) is separated from the slurry in a second separation stage 5. A filtrate 70 of the second separation stage is circulated to the suspending stage 3 to be used as the suspending water and/or to a flow of the lignin containing liquid medium 10 coming from the digester. The filtrate is preferably circulated to the flow of lean black liquor before the evaporation stages. It is possible to use part of the filtrate for the suspending stage 3 and circulate part of the filtrate to the black liquor flow, as shown by FIG. 1. Sodium is separated from the lignin in the HTC process, and it is returned back to the black liquor with the filtrate.

(9) The carbon containing product 60 can be washed after the separation of the filtrate 70 and dried.

(10) The hydrothermal carbonization works in a wide variety of liquid/solid ratios of the lignin suspension.

(11) FIG. 1 also shows the circulation of black liquor from the first separation stage 2 back to the flow of the black liquor in the evaporation stages.

(12) Before the HTC stage 4, the pH of the lignin suspension is preferably adjusted to control the particle size of the carbon containing product 60. It is of particular importance to adjust the pH value of the lignin suspension to suppress the formation of too coarse particles, which is due to the polymerization of lignin during the HTC by the effect of H.sup.+ ions. Thus, the pH of the suspension is preferably adjusted to be above 7, more preferably above 8. Further, it is advantageous that the pH during the HTC process is between 7 and 11, preferably between 8 and 10. The pH of the lignin suspension 40 is preferably adjusted so that it will be within these ranges during the HTC stage 4.

(13) The particle size of the carbon containing product can be influenced by adjusting the H.sup.+ ion concentration in the lignin suspension that is subjected to the HTC. By reducing the H.sup.+ ion concentration of the lignin suspension the particle size of the product can be reduced. Thus, the choice of the pH for the HTC is dependent on the desired particle size. The particle size can be expressed for example by D90 value of the particle size distribution.

(14) The pH can be adjusted to a suitable range for the HTC with acid (additional substance B) added in the suspending stage. However, either acids or bases can be used for the pH adjustment, depending on the initial pH of the suspension and the target pH value.

(15) FIG. 2 is a detailed process flow chart of a hydro thermal carbonization stage 4 according to an embodiment of the invention. The HTC stage may comprise a first heat exchanger 6 and a steam heater 7, through which a lignin suspension from the suspending stage 3 is conveyed before feeding it to a HTC reactor 8. Thus, the lignin suspension 40 can be preheated to the required temperature before the hydrothermal carbonization. At least one cooler 9 can be arranged after the HTC reactor 8 for cooling the slurry of carbon containing material 50. The slurry 50 from the HTC reactor 8 is conveyed through the heat exchanger 6 before the cooler 9, wherein the heat of the slurry of carbon containing material can be utilized in the preheating of the lignin suspension.

(16) The carbon containing product can be dried to remove the remaining liquid, for example by evaporating the remaining liquid.

(17) As can be seen in the preceding disclosure, the process of producing carbon containing material can be well integrated in the treatment of black liquor in a chemical pulp mill.

(18) Many variations of the present invention will suggest themselves to those skilled in the art in light of the above detailed description. Such obvious variations are within the full intended scope of the appended claims.