A method for separating lignin in a pulp mill process and a system for carrying out the method

Abstract

A method for separating lignin (9) from an alkaline aqueous slurry (5) containing lignocellulosic biomass material, the method being carried out in a pulp mill and comprising an alkaline first lignin precipitation step (20) and an acidic second lignin precipitation step (22), wherein the alkaline first lignin precipitation step (20) involves lowering an initial alkaline pH value of the aqueous slurry by means of a first acidifying agent to a lower alkaline pH value The first acidifying agent comprises or consists of sulphuric acid (19) produced from sulphur, which has been at least partly derived from a green liquor stripping process (15) in the pulp mill. A system for carrying out the method is also disclosed.

Description

[0001] A METHOD FOR SEPARATING LIGNIN IN A PULP MILL PROCESS AND A SYSTEM FOR CARRYING OUT THE METHOD

[0002] TECHNICAL FIELD

[0003] The disclosure pertains to a method for separating lignin from an aqueous slurry containing lignocellulosic biomass material, such as black liquor, the method being carried out in a pulp mill and comprising an alkaline first lignin precipitation step and an acidic second lignin precipitation step, wherein the alkaline first lignin precipitation step involves lowering an initial pH level of the aqueous slurry by means of a first acidifying agent. A system for carrying out the method is also disclosed.

[0004] BACKGROUND

[0005] An efficient process for lignin separation from black liquor is the Lignoboost™ process, in which the basic concept includes a two-stage acidic wash process. The Lignoboost™ process has been described in several earlier published patent applications, such as WO2 006 / 031175, W02006 / 038863, and WO2014 / 116150.

[0006] The lignin separation method involves a first step of lowering the pH of black liquor from about pH 13 in the original black liquor down to a pH value in the order of from 8 to 12. This first precipitation step is commonly carried out using carbon dioxide, CO2, and is followed by a first separation step. The low-purity lignin obtained after the first separation step is then subjected to a re-slurry step and a second, acidic precipitation step, commonly performed using sulphuric acid, H2SO4, to lower the pH to a value in the order of from 1 to 4.

[0007] An important aspect of the process is that the required charge of chemicals for the acidification may be high. If this is the case, the costs of fresh chemicals constitute a large part of the operational costs and commercial viability of the process will thereby be lower.

[0008] To reduce the requirement of charges of fresh chemicals, it has been suggested, e.g., in WO2014 / 116150 and WO2023 / 277774, to use acidifying agents in the form of mill generated waste flows as it may alleviate waste disposal problems and reduce environmental impact. As the precipitation of lignin requires acidification of alkaline black liquor flows, much of the total amount of acidifying agent is used to decrease the pH level down to a point where lignin starts to precipitate. The first precipitation phase typically reduces the pH level from about pH 13 in the original black liquor down to a pH of about 11.5, and normally does not involve any nucleation of lignin particles. The amount of acidifying agent needed is nevertheless relatively high for this first phase as the pH follows a logarithmic curve, and any following additional lowering a pH from 11.5 requires far less acidifying agent for the same order of lowered absolute pH value. Furthermore, the volume of liquid in the first precipitation step is much greater than the volume of liquid in the second precipitation step, which also contributes to the demand for acidifying agent being considerably greater in the first precipitation step than in the second precipitation step.

[0009] For these reasons, CO2 has been generally considered to be the most practical and economically viable option for use as an acidifying agent in the first precipitation step. However, CO2 sourcing has been found to present problems in terms of investments and operational costs. Commonly, the CO2 which is used in present-day processes is produced from natural gas and requires purchase and installation of a new process area at the pulp mill. Furthermore, CO2 usage may be problematic from a sustainability perspective.

[0010] In WO2014 / 118441 , it has been suggested to produce sulphuric acid from H2S which is derived from spent pulping liquor produced by kraft-type pulping at very high sulphidity. The sulphuric acid may be used for recovery of by-products such as lignin. However, in order to produce a sufficient amount of sulphuric acid it is necessary that the sulphidity in the process is considerably higher than in a conventional kraft pulping process such as above 50 %, and preferably around 80 %.

[0011] Accordingly, there remains a need for an improved method for recovering lignin from alkaline flows containing lignocellulosic biomass material, such as black liquor, from a pulping process. An object of the present disclosure is to provide a lignin recovery method using acidifying agents generated from flows in the pulping process, with minimum influence on the conditions of the pulping process.

[0012] SUMMARY

[0013] The above and further objects can be achieved with a lignin separation method in accordance with claim 1 and with a system for separating lignin in accordance with claim 6. Variations of the disclosure are set out in the dependent claims and in the following description. The present invention offers a method for separating lignin from an aqueous slurry containing lignocellulosic biomass material, the method being carried out in a pulp mill and comprising an alkaline first lignin precipitation step and an acidic second lignin precipitation step, wherein the alkaline first lignin precipitation step involves lowering an initial pH level of the aqueous slurry by means of a first acidifying agent. The first acidifying agent comprises or consists of sulphuric acid, wherein the sulphuric acid is produced from sulphur which has at least partly been derived from a green liquor stripping process in the pulp mill.

[0014] In addition to the sulphur which has been derived from a green liquor stripping process in the pulp mill the first acidifying agent can comprise sulphur which has been derived from other process flows in the pulp mill, such as from a black liquor heating process or from concentrated non-condensable gas (CNCG).

[0015] By applying an additional process loop to an existing chemical recovery system in a pulp mill, it has been made possible to produce sufficient sulphuric acid, H2SO4, to be able to substitute at least a major part of the conventionally used CO2 as an acidifying agent in a first precipitation step in a process for recovering lignin from an aqueous slurry containing lignocellulosic biomass material, typically black liquor in a kraft pulping process.

[0016] The method combines the use of systems and technologies which may already be present in a pulp mill, including a green liquor stripping arrangement, a sulphuric acid plant and a two-step lignin recovery process, such as the LignoBoost™ process. By use of the method as disclosed herein, it has proven possible to precipitate medium-thick original black liquor with above 30 % dry solids, such as 40% dry solids using sulphuric acid instead of CO2. The black liquor from the lignin recovery system which is returned to evaporation will contain more inorganics (sulphates) than in a lignin recovery system using CO2 as a first acidifying agent, thereby creating a higher sulphur load in the recovery boiler. The excess sulphur is subsequently removed from the green liquor with green liquor H2S-stripping. Accordingly, the new extra sulphur loop creates a higher load of S being carried from medium consistency black liquor to green liquor than with a lignin recovery process using CO2 as a first acidifying agent. The Na / S balance in the rest of the pulp mill remains the same as in a pulp mill with a lignin recovery process utilizing CO2 as first acidifying agent in the lignin recovery process. A typical two-step lignin precipitation process as disclosed herein can be generally described as involving in sequence:

[0017] Precipitation of lignin by lowering the pH of an aqueous alkaline slurry containing lignocellulosic biomass material, such as black liquor, by adding a first acidifying agent in any suitable precipitation reactor to reach a lower alkaline pH;

[0018] Dewatering the alkaline slurry to form a first filter cake with a high lignin content, the dewatering being carried out in any suitable filter press. A first filtrate is withdrawn from the lignin suspension. Residual black liquor can be displaced from the filter cake by means of a pressurized gas;

[0019] Suspending (re-slurrying) the first lignin cake in a second pH lowering step, using a second acidifying agent, the second step being an acidic step and being carried out in any suitable suspension vessel;

[0020] Dewatering the acidic suspension and washing the lignin to form a second filter cake with high lignin content. The dewatering can be carried out using any suitable filter press which can produce a second filtrate from the lignin suspension. At least a portion of this second filtrate may be re-circulated back to the suspension stage. Washing of the second filter cake can be carried out using any suitable washing equipment.

[0021] By eliminating or at least partly substituting CO2 as an acidifying agent in the first acidification step of the lignin separation process, cost efficiency as well as sustainability of the pulp mill can be improved.

[0022] A recovery boiler is part of a recovery system in the pulp mill. In the recovery boiler, black liquor from the pulping process is burnt to recover chemicals and thermal energy generated in the burning process.

[0023] In addition to a recovery boiler, a recovery system in a pulp mill generally comprises an evaporator or other means for concentrating black liquor from the pulping process before introducing the concentrated black liquor from the evaporator into the recovery boiler. A dissolving tank is also commonly provided for dissolution of smelt from the recovery boiler. The dissolved smelt leaves the dissolving tank as green liquor, which is recausticized and recycled as white liquor to the digester of the pulp mill. A part of the white liquor may also be used in processing of the pulp after the digester e.g., in oxygen delignification. The initial pH value in the alkaline first lignin precipitation step may be from 12 to 13.5 and the lower alkaline pH value may be from 8 to 11 .5, preferably from 10 to 11 .5.

[0024] It may be preferred that the pH value is lowered by at least 0.5 in the first lignin precipitation step.

[0025] The pH value in the acidic second lignin precipitation step may be from 1 to 4, typically in the range of from 2 to 3.

[0026] It is to be understood that each of the first and second precipitation steps can be performed in one or more sub-steps.

[0027] In the method as disclosed herein, precipitated lignin from the alkaline first lignin precipitation step can be separated from the alkaline aqueous slurry and the lignin which is obtained after separation can be re-slurried and treated with a second acidifying agent in the acidic second lignin precipitation step, whereafter the re-slurried and treated lignin from the second lignin precipitation step is separated from the acidic slurry.

[0028] The method can further comprise a washing step for washing the lignin obtained from the acidic precipitation stage.

[0029] The second acidifying agent can comprise or consist of sulphuric acid, wherein the sulphuric acid can have been produced from sulphur which has been derived from a green liquor stripping process in the pulp mill.

[0030] It may, thus, be preferred that the second acidifying agent is the same as the first acidifying agent.

[0031] Disclosed herein is also a system for separating lignin from an aqueous alkaline slurry containing lignocellulosic biomass material, the system comprising at least a first precipitation vessel configured to carry out at least a first precipitation step, wherein the first precipitation step is performed during a reaction time and includes adding at least a first acidifying agent to the aqueous slurry, so that a decrease in the alkaline pH value of the aqueous slurry to a lower alkaline pH value is obtained; a lignin separation arrangement for separating precipitated lignin from the first precipitation step, a re-slurry arrangement for re- slurrying and treating the precipitated lignin from the first precipitation step with a second acidifying agent at an acidic pH value; a second lignin separation arrangement for separating the re-slurried and treated lignin from the acidic slurry; a green liquor stripping arrangement configured for removing sulphur and sodium from green liquor and a sulphuric acid plant configured to produce sulphuric acid from hydrogen sulphide, H2S, gas derived from a green liquor stripping process performed in the green liquor stripping arrangement. The system further comprises an arrangement for supplying sulphuric acid to the first precipitation vessel as a first acidifying agent, the sulphuric acid being produced in the sulphuric acid plant from hydrogen sulphide, H2S, gas derived from a green liquor stripping process performed in the green liquor stripping arrangement.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The lignin separation method as disclosed herein will be further explained hereinafter with reference to the appended drawings wherein:

[0034] Fig. 1 shows a schematic diagram of a Kraft pulping process; and

[0035] Fig. 2 shows a schematic diagram of a lignin separation process.

[0036] DETAILED DESCRIPTION

[0037] Different aspects of the present disclosure will be described more fully hereinafter with reference to the drawings. The lignin recovery method disclosed herein should not be construed as being limited to the aspects set forth herein but can be varied within the scope of the appended claims.

[0038] With reference to Fig. 1 , there is shown a schematic diagram of a kraft pulping process, also referred to as a sulphate process. The pulp mill, which is shown in Fig. 1 in a highly schematic and simplified way, is a kraft pulp mill. Hence, it is to be understood that further equipment and process steps may be included in the pulping process and the recovery system of the pulp mill. In the kraft pulping process, wood chips 1 are treated in a digester 2 with an aqueous cooking liquor, commonly referred to as “white liquor” and containing sodium hydroxide (NaOH) and sodium sulfide (Na2S). The cooking process breaks the bonds that link lignin, hemicellulose, and cellulose. After cooking, the cellulose fibres are further treated in a pulp production process 3 involving washing, bleaching, and drying to produce cellulose pulp 4. Process liquid 5, commonly referred to as “black liquor”, is derived from the pulp production process 3, following on the digester 2. The black liquor 5 is subjected to a two-step lignin precipitation process 6, such as the LignoBoost™ process, for recovering lignin 9 from the black liquor 5. As disclosed herein, the process for separating lignin 9 from the black liquor 5 comprises an alkaline first lignin precipitation step and an acidic second lignin precipitation step, wherein the alkaline first lignin precipitation step involves lowering the initial alkaline pH value of the black liquor 5 by means of a first acidifying agent to a lower alkaline pH value.

[0039] The delignified black liquor 5 is subjected to evaporation in an evaporator 7, where the black liquor 5 from the pulping process is concentrated. The concentrated black liquor 5 is then transferred to a recovery boiler 8.

[0040] The liquor flows in the kraft pulp mill may be referred to as a “cyclic liquor flow system” comprising a main liquor cycle where at least a part of liquor which is used in the digester in the pulp mill is reconditioned and recycled to the digester. The cyclic liquor flow system comprises sub-cycles of liquor flow from different process equipment in the pulp mill. It is also to be understood that water and chemicals are added and withdrawn from the cyclic liquor flow system as part of a chemical recovery system in the pulp mill.

[0041] The recovery boiler 8 is part of the recovery system. In the recovery boiler 8, black liquor from the pulping process is burnt to recover chemicals and thermal energy generated by the burning process.

[0042] The chemicals are recovered from smelt 25 which is withdrawn from the recovery boiler 8 and is transferred to a dissolving tank 10. The dissolved smelt 25 leaves the dissolving tank as green liquor 11 , which is subsequently recausticized in a causticizer 12 and recycled as white liquor 13 to the digester 2 of the Kraft pulp mill.

[0043] After leaving the dissolving tank 10, the green liquor 11 is subjected to a green liquor H2S- stripping process. In the H2S stripper 15, sodium and sulphur are separated from the green liquor 11 in the form of sodium sulfide Na2S, sodium carbonate Na2COs, and a hydrogen sulfide H2S gas stream 16 that is used to produce sulphuric acid H2SO4 19 in a sulphuric acid plant 17. Na2S and Na2COs are used in the caustizising process and a part of the Na2CC>3 from the H2S stripper 15 is converted to sodium bicarbonate NaHCOs.

[0044] At least a part of the sulphuric acid H2SO4 19, which is produced in the sulphuric acid plant 17 from sulphur, which has been derived from the green liquor H2S-stripper 15, is used as the first acidifying agent in the lignin recovery process 6.

[0045] As shown in Fig. 1 , prior to the H2S stripping 15, the green liquor 11 may be subjected to an optional pre-carbonation step in a pre-carbonator 18. In the pre-carbonator 18, carbon dioxide CO2 reacts with sodium sulfide Na2S and sodium hydroxide NaOH to produce sodium bisulfide NaHS and sodium carbonate Na2CO3. After pre-carbonation, the green liquor 11 contains mainly Na2COs, NaHS, and a negligible amount of NaHCCh, which is conveyed to the H2S stripper 15.

[0046] The general two-step process 6 for recovering lignin 9 from original black liquor is shown in more detail in Fig. 2.

[0047] The separation 6 of lignin 9 from original black liquor 5 comprises the following steps in sequence:

[0048] Precipitation 20 of lignin by lowering the pH of the original black liquor 5 by adding a first acidifying agent, which comprises or consists of sulphuric acid 19, which is produced at least partly from sulphur which has been derived from the green liquor H2S-stripping process in the pulp mill.

[0049] Dewatering 21 the lignin from the first precipitation step while forming a first filter cake. Dewatering may be carried out using any suitable filter press.

[0050] Reslurrying the first filter cake and performing a second precipitation step 22 using a second acidifying agent. The second acidifying agent preferably comprises or consists of sulphuric acid 19, which is produced at least partly from sulphur which has been derived from the green liquor H2S-stripping process in the pulp mill.

[0051] Dewatering 23 the precipitated lignin from the second precipitation step to produce a second filter cake. Dewatering may be carried out using any suitable filter press.

[0052] Washing 24 the lignin in the second filter cake using any suitable washing equipment to obtain recovered lignin 9.

Claims

CLAIMS1. A method for separating lignin (9) from an alkaline aqueous slurry (5) containing lignocellulosic biomass material, the method being carried out in a pulp mill and comprising an alkaline first lignin precipitation step (20) and an acidic second lignin precipitation step (22), wherein the alkaline first lignin precipitation step (20) involves lowering an initial alkaline pH value of the aqueous slurry by means of a first acidifying agent to a lower alkaline pH value, characterized in that the first acidifying agent comprises or consists of sulphuric acid (19), wherein the sulphuric acid (19) is produced from sulphur, which has been at least partly derived from a green liquor stripping process (15) in the pulp mill.

2. A method according to claim 1 , wherein the initial pH value in the alkaline first lignin precipitation step (20) is from 12 to 13.5 and the lower alkaline pH value is from 8 to 11.5, preferably from 10 to 11.5.

3. A method according to claim 1 or 2, wherein the pH value in the acidic second lignin precipitation step (22) is from 1 to 4.

4. A method according to any one of the preceding claims, wherein precipitated lignin from the alkaline first lignin precipitation step is separated from the alkaline aqueous slurry and the lignin, which is obtained after separation is re-slurried and treated with a second acidifying agent in the acidic second lignin precipitation step (22), whereafter the re-slurried and treated lignin from the second lignin precipitation step is separated from the acidic slurry.

5. A method according to claim 4, wherein the second acidifying agent comprises or consists of sulphuric acid (19), wherein the sulphuric acid is produced from sulphur, which has been at least partly derived from a green liquor stripping process in the pulp mill.

6. A System for separating lignin from an aqueous alkaline slurry containing lignocellulosic biomass material, the system comprising at least a first precipitation vessel configured to carry out at least a first precipitation step, wherein the first precipitation step is performed during a reaction time and includes adding at least a first acidifying agent to the aqueous slurry, so that a decrease in the alkaline pH value of the aqueous slurry to a lower alkaline pH value is obtained; a lignin separation arrangement for separating precipitated lignin from the first precipitation step, a re-slurry arrangement for re-slurrying and treating theprecipitated lignin from the first precipitation step with a second acidifying agent at an acidic pH value; a second lignin separation arrangement for separating the re-slurried and treated lignin from the acidic slurry; a green liquor stripping arrangement configured for removing sulphur and sodium from green liquor and a sulphuric acid plant configured to produce sulphuric acid from hydrogen sulphide (H2S) gas derived from a green liquor stripping process performed in the green liquor stripping arrangement, characterized in that the system comprises an arrangement for supplying sulphuric acid to the first precipitation vessel as a first acidifying agent, the sulphuric acid being produced in the sulphuric acid plant and comprising or consisting of the sulphuric acid, which has been produced from hydrogen sulphide (H2S) gas derived from the green liquor stripping process performed in the green liquor stripping arrangement.

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