WO2024187276A1

WO2024187276A1 - Improved process for the extraction of kraft lignin from hardwood and eucalyptus black liquors

Info

Publication number: WO2024187276A1
Application number: PCT/CA2024/050298
Authority: WO
Inventors: Kurt WOYTIUK; George Sacciadis; Michael Paleologou
Original assignee: Fpinnovations
Priority date: 2023-03-14
Filing date: 2024-03-12
Publication date: 2024-09-19

Abstract

It is provided a process of increasing the filterability of a lignin slurries such those derived from the acidification of hardwood (HW) and eucalyptus (EW) black liquors, to a level of filterability comparable to softwood (SW) black liquors. The process of increasing filterability of a lignin slurry as described herein comprises the steps of adjusting the residual effective alkali (REA) of a HW or EW black liquor to minimum level of about 2.8-5 wt% as Na₂O on a black liquor solids basis; and applying a heat treatment to said black liquor to achieve a cumulative H-factor of 1800-2400 which increases filterability of the precipitated black liquor allowing filtration in conventional industrial filtration equipment such as belt, vertical, or horizontal tray filters at high rates.

Description

IMPROVED PROCESS FOR THE EXTRACTION OF KRAFT LIGNIN FROM HARDWOOD AND EUCALYPTUS BLACK LIQUORS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is claiming priority from U.S. Provisional Application No. 63/490,025 filed March 14, 2023, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] It is provided a process of increasing the filterability of lignin slurries derived from the acidification of hardwood (HW) and eucalyptus (EW) black liquors, to a level of filterability comparable to softwood black liquor slurries during the extraction of lignin.

BACKGROUND

[0003] Softwood (SW), hardwood (HW) and eucalyptus (EW) wood species are pulped in conventional kraft processes globally and the latter is fast becoming the dominant market pulp due to the relatively fast growth rate of EW plantations and the uniformity of the pulp produced. Three processes have been used commercially for the production of kraft lignin from black liquor, namely: LignoForce™, LignoBoost and the Westvaco processes (Kienberger et al., Systematic review on isolation processes for technical lignins, Processes, 9, 804-822 (2021 ), the content of which is incorporated by reference). Despite some differences between them, in all these processes, kraft black liquor is acidified with carbon dioxide to a pH of about 10 at which pH lignin comes out of solution to form a lignin slurry. After optionally allowing some time for lignin particle coagulation, the slurry is filtered in an effort to separate the lignin particles from the residual black liquor. The lignin cake is then washed in place using dilute sulphuric acid and water or immersed in a dilute sulphuric acid solution to produce purified lignin in the acid form.

[0004] Even though these processes appear to work quite well in the processing of softwood kraft liquors, HW and EW black liquors could be quite challenging. In fact, in certain cases, the lignin slurry of these liquors does not filter at all.

[0005] Typically, lignin is separated from the precipitated black liquor slurry, using a batch or continuous horizontal or vertical filter under pressure or vacuum. All commercial processes utilize such filters to separate and wash kraft lignin. The flow rate through the porous filter media in the filtration system impacts primarily the initial few seconds while the lignin cake is formed and, subsequently, the filtrate and wash flow rate through the cake itself becomes rate limiting. The separation of lignin from black liquor filtrate is, therefore, dependent on the properties of lignin particles and their interactions in forming lignin agglomerates. Filtration area is consistently the bottleneck in commercial lignin extraction processes due to the relatively high capital cost and complexity of filtration technologies. As a result, commercial production of kraft lignin is not economical when filtration area exceeds a certain design threshold.

[0006] HW and EW black liquors have been shown to have a more variable composition and processing behaviour compared to SW black liquors. As a result, once precipitated from black liquor, HW and EW lignin particles are often slow to filter in conventional industrial filtration equipment (belt, vertical, or horizontal tray filters) (please see, Tomani et al., Integration of lignin removal into a kraft pulp mill and use of lignin as a biofuel, Proceedings of the 5^th International Colloquium on Eucalyptus Pulp, Porto Seguro, Bahia, Brazil, May 9-12, 2011 ). The lignin slurry in the case of HW and EW black liquors typically requires more than twice the filtration area to produce an equivalent mass of lignin to that of a SW lignin slurry. Depending on the HW or EW species and the process conditions, some of these kraft lignin slurries have been observed to be completely unalterable using the conventional lignin slurry filtration processes. In fact, to avoid this problem, some researchers chose to use centrifugation to separate the lignin from the residual black liquor.

[0007] It is thus highly desired to be provided new means to process HW and EU lignin slurries.

SUMMARY

[0008] It is provided a process of increasing the filterability of lignin slurries derived from the acidification of HW or EW or soda black liquors comprising the steps of adjusting the minimum residual effective alkali (REA) level to about 2.8-5 wt% Na₂O on a black liquor solids basis; and applying a heat treatment of an intensity and duration that is dependent on the previous thermal history of any given black liquor to achieve an overall H-factor between about 1800 to 2400 H.

[0009] In an embodiment, the lignin polymers in black liquor are comprised of syringyl, guaiacyl and p-hydroxyphenyl monomers. [0010] In a further embodiment, the black liquor comprises free residual hemicelluloses and lignin-carbohydrate complexes (LCC).

[0011] In another embodiment, the black liquor is a hardwood (HW) black liquor or a eucalyptus (EW) wood black liquor.

[0012] In an embodiment, the black liquor is heated at a temperature between 140 and 180 °C.

[0013] In an embodiment, the black liquor is heated to achieve an overall H-factor between 1800 and 2400, the cumulative H-factor being the sum of the H-factor at which the lignin in wood chips was subjected to in pulping plus the applied H-factor on the residual pulping liquor.

[0014] In an additional embodiment, the REA level of the black liquor is firstly adjusted to a level of about 2.8-5 wt% Na₂O on a black liquor solids basis prior to the heat treatment, preferably by adding for example sodium hydroxide.

[0015] In a supplemental embodiment, the REA level of the black liquor is adjusted by adding white liquor.

[0016] In an embodiment, the white liquor is partially oxidized white liquor or fully oxidized white liquor.

[0017] In a further embodiment, the black liquor with increased filterability is filterable through a belt, a vertical tray filter or a horizontal tray filter.

[0018] It is also provided a process for removing lignin from HW, EW or soda black liquors comprising the steps of adjusting the minimum residual effective alkali (REA) level to about 2.8-5wt% Na₂O on a black liquor solids basis; applying a heat treatment to said black liquor to achieve an overall H-factor of 1800-2400; acidifying the black liquor with an acidifier forming a lignin slurry; and filtering the lignin slurry to remove the lignin.

[0019] In an embodiment, the black liquor is acidified to a pH of about 10 forming a lignin slurry.

[0020] In an embodiment, the black liquor is acidified to a pH of about 2-3 forming a lignin slurry. [0021] In another embodiment, the process for removing lignin from a black liquor as described herein further comprises the step of oxidizing with an oxidizing agent the black liquor before, after, or during the heat treatment step thus eliminating total reduced sulphur (TRS) compounds in the black liquor with oxidation of sulphides, mercaptans, and organic sulphides by said oxidising agent at a temperature effective for oxidation to thiosulphate- and other oxidized sulphur compounds and oxidation of said thiosulphate to sulphate, and such that hemicelluloses and other organics in the black liquor are oxidized by said oxidising agent at said temperature to form an acidifying agent selected from isosaccharinic acids, acetic acid, formic acid, lactic acid, oxalic acid, carbon dioxide and acidic lignin degradation products and mixtures thereof, with generation of heat through said oxidation, the generated heat leading to the creation of nucleation sites for the formation of lignin particles through lignin colloid agglomeration and coagulation.

[0022] In an embodiment, the oxidizing agent is oxygen.

[0023] In another embodiment, the acidifier is selected from carbon dioxide and sulphuric acid.

[0024] In a further embodiment, the acidifier is a strong acid such as for example sulphuric acid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Reference will now be made to the accompanying drawings.

[0026] Fig. 1 illustrates the average specific filtration rates of lignin slurries derived from untreated HW (aspen) and SW (SPF) black liquors from the same swing mill processed at the demonstration plant level under similar conditions.

[0027] Fig. 2 illustrates a flowsheet of the process described herein in accordance to an embodiment in which the alkali addition and black liquor heat treatment steps precede the oxidation step of the LignoForce™ process.

[0028] Fig. 3 illustrates a flowsheet of an alternative of the process described herein in accordance to an embodiment in which the alkali addition and black liquor heat treatment steps follow the oxidation step of the LignoForce™ process. [0029] Fig. 4 illustrates a flowsheet of an alternative of the process described herein in accordance to an embodiment in which the alkali addition and black liquor heat treatment steps are during the oxidation step of the LignoForce™ process

[0030] Fig. 5 illustrates the average specific filtration rates of lignin slurries derived from the HW heat-treated black liquor from the mill of Example 1 at the laboratory level with no alkali addition.

[0031] Fig. 6 illustrates the average specific filtration rate of lignin slurries derived from the HW heat-treated black liquor from the mill of Example 2 at various adjusted REA levels and treated at two levels of H-factor (900 and 1200); H=0 is the control corresponding to no black liquor heat treatment.

[0032] Fig. 7 illustrates the Mn (number average) and Mw (weight average) molar mass of HW lignin cakes produced from HW black liquor from the mill of Example 2 with and without heat treatment. H12 & H9 = 1200 and 900 H factor heat treatment, respectively; 2pct, 3pct, 4pct, and 5 pct = 2, 3, 4, and 5 weight percent REA expressed as Na₂O on black liquor solids; Cntrl = Control, HW black liquor with no heat treatment.

[0033] Fig. 8 illustrates the average filtration rate of the slurry dewatering process in accordance to an embodiment, wherein an increase in filtration rate is consistent with pre- or post-oxidation heat treatments.

[0034] Fig. 9 illustrates the average specific filtration rates of lignin slurries derived from untreated (H Factor = 0) and heat-treated HW black liquor at two applied H-factors (900 H and 1650 H) from the mill of Example 2 with a fixed initial REA of 2.87 wt% as Na₂O on black liquor solids at the pilot plant level.

[0035] Fig 10 illustrates the average specific filtration rates of lignin slurries derived from heat-treated EW black liquor after adjusting the REA to 3wt% as Na₂O and applying a heat treatment of 2000H (Mill 1 ) compared to SW black liquor (Mill 6, not heat-treated and no alkali added).

DETAILED DESCRIPTION

[0036] In accordance with the present disclosure, it is provided a process of increasing the filterability of lignin slurries derived from HW and EW black liquors, to a level of filterability comparable to lignin slurries derived from softwood black liquors. [0037] In an embodiment, it is provided a process of increasing filterability of a lignin slurry derived from the acidification of HW and EW black liquors comprising the steps of adjusting the residual effective alkali (REA) level of said black liquor to about 2.8-5 wt%, preferably to about 2.8-3.8 wt%, as Na2O on a black liquor solids basis; and applying a heat treatment to said black liquor to achieve an overall H-factor of 1800-2400.

[0038] It is provided herein that by applying a heat treatment to black liquors under certain conditions and in the presence of suitable amounts of alkali, the filtration of lignin slurries derived from the acidification of said black liquors is improved significantly and the filtration rate is as high as in the case of softwood lignin slurries.

[0039] HW and EW black liquors are characterized by a lower molecular weight (MW) lignin of variable composition and morphology, and a higher carbohydrate content compared to SW black liquors. The weight average molecular weight (M_w) of HW and EW kraft lignins is typically half that of SW kraft lignins resulting in lower relative viscosities after being dissolved in alkaline aqueous solutions or organic solvents. The monomers of SW lignin are almost exclusively guaiacyl in nature whereas HW and EW lignins consist of both syringyl and guaiacyl units in variable ratios (S/G ratio). HW kraft lignins also contain less condensed phenolics (i.e., less C-C linkages) and less crosslinking compared to SW kraft lignins. The existence of free residual hemicelluloses (e.g., xylan) and hemicelluloses covalently bonded to HW and EW lignin reactive sites (in the form of so-called Lignin-carbohydrate complexes or LCC) in such black liquors is a major contributor to the variable filtration properties of HW and EW lignin slurries compared to SW lignin slurries for which the filtration properties are quite uniform. Fig. 1 shows a comparison between HW (aspen) and SW (a mixture of spruce, pine and fir usually referred to as SPF) lignin slurry filterability for black liquors collected from the same kraft pulp mill and precipitated at the demonstration plant level (50 kg/day) under similar conditions. The lignin slurry filtration rate for this system in kilograms per hour per square meter of filtration area is measured as the total oven dry mass of lignin produced for each batch press divided by the cumulative pumping time for precipitated slurry sent to the press per square meter of filter area. For this kraft mill, commercial recovery of HW kraft lignin would require significantly more filtration area for the same lignin production compared to the production of SW kraft lignin.

[0040] Even though treatments have previously been applied in the prior art to both SW and HW black liquors, these were mostly used to reduce black liquor viscosity, depolymerize the lignin in black liquor or to remove calcium from black liquor in the form of calcium carbonate. For example, Kiiskila and Virkola in US Patent No. 4,929,307, describe a method of decreasing the viscosity and improving the evaporability of kraft black liquor which involves raising the temperature of the black liquor above the cooking temperature so as to split the macro-molecular lignin fraction contained in the liquor. The temperature of the black liquor was maintained at the raised level for 1 to 60 minutes, preferably for 1 to 5 minutes. Similarly, Paleologou et al., US Patent Publication No. 2020/0148835, applied a heat treatment at temperatures ranging from 150-250°C to softwood black liquor in the presence of added sodium hydroxide or white liquor and, optionally a capping agent and/or a solvent in an effort to depolymerize lignin in such black liquors before extraction using the LignoForce™ or other processes. Furthermore, Nasman and Bjork, WO 2013/036190, applied a heat treatment to hardwood black liquor at a temperature above 160°C in an effort to precipitate out calcium compounds in the bulk of the black liquor thereby minimizing calcium carbonate scaling on evaporator surfaces and improving black liquor processability. Wallmo et al., Nordic Pulp & Paper Research Journal, 24(2), 165-171 (2009) evaluated the effect of black liquor hemicellulose content on filtration properties by lowering its content through ultrafiltration, a combination of ultrafiltration and nanofiltration and heat treatment. However, the latter treatment was conducted at one set of conditions (170°C for 3 hours) without paying any attention to the previous thermal and chemical reaction history of the black liquor to be treated or the residual effective alkali (REA) in the black liquor prior to determining the extent of the thermal treatment to be applied.

[0041] In contrast, it is provided specific black liquor heat treatment conditions based on the previous thermal and chemical reaction history of any given HW or EW black liquor under which the filterability of lignin slurries is improved without depolymerizing the lignin in such black liquors, carbonizing the lignin or reducing lignin production yield. The process described herein applies to the case of slow-filtering or unalterable HW or EW precipitated black liquors. In particular, conditions were identified for the treatment of black liquor prior to lignin precipitation relating to black liquor alkalinity, temperature and duration of the treatment that can significantly improve the filterability of HW and EW lignin slurries. A peak temperature of between 140 and 180 °C and a heat treatment time of sufficient duration to provide overall H-factors (historical plus new) of 1800-2400 H were found to be most effective if a minimum level of Residual Effective Alkali (REA) content was present in the black liquors to be treated. The heating and cooling exposure time were accounted for using the integral of the temperature-time curve (the area under the temperature-time curve in pulping is referred to as the H-factor as defined by Vroom (please see Vroom, K.E., Pulp Pap. Mag. Can., 58, 228 (1957). In particular, applying H- factors between 800 and 1600 typically results in filtration rates equivalent to commercial softwood lignin recovery processes when the REA content in black liquor is sufficient for effective heat treatment. In general, applied H-factors exceeding 2000 resulted in declining filterability and temperatures exceeding 180°C (globally or locally) were found to degrade the quality of the lignin by either darkening the lignin or carbonizing/charring the smaller lignin agglomerates and reducing the lignin yield for the process.

[0042] The LignoForce™ process (please see US Patent No. 8,771,464, the content of which is incorporated by reference) was used to produce lignin from heat-treated HW and EW liquors at the laboratory and pilot plant levels. This process involves the following steps: a) Black liquor oxidation using oxygen; b) Black liquor acidification using carbon dioxide to a pH close to 10; c) Lignin slurry coagulation; d) Lignin slurry filtration to dewater the lignin cake; e) Lignin washing with dilute sulphuric acid; and f) Lignin washing with water.

[0043] In addition to the LignoForce™ process, other lignin production processes can benefit from the process described herein, including the conventional WestVaco process as well as the more recently developed LignoBoost® process (see WO 2006/031175, the content of which is incorporated by reference). In both these cases, the filtration rate during the first lignin slurry filtration step can be increased significantly when HW and EW lignin slurries are processed using the black liquor heat treatment step as provided herein. It should also be understood that by improving the first filtration step in all three of these commercial processes, the subsequent lignin cake wash steps will improve as well since there will be less black liquor carryover into the lignin cakes. In all cases, it is necessary to emphasize the importance of removing suspended solids (e.g. , fiber) from the black liquors to be treated since these can plug up the filter media used for the separation of the lignin cake from the black liquor filtrate. Hence, for any valid comparisons to be made on the effect of heat treatment on lignin filterability, it is important that most suspended materials in black liquor are removed using preferably a pressure filter with a particle size cut-off of 20-50 micron.

[0044] Lignin slurry filterability is defined as the average rate of accumulation of filtrate up to 400 grams reported in kg / hr / m² of filtration area. Heat treatment time was varied to the extent needed to achieve the desired H-factors for applied black liquor heat treatment in consideration of the thermal history of any given black liquor.

[0045] Three approaches were used successfully as described herein (see Figs. 2, 3, and 4). As shown in Fig. 2, the black liquor heat treatment step was applied ahead of the black liquor oxidation step while, as shown in Fig. 3, the black liquor heat treatment step was applied after the black liquor oxidation step. In Fig. 4, the black liquor heat treatment step was applied at the same time as the oxidation step.

[0046] In Fig. 2, a typical lignin recovery assembly comprises a weak black liquor tank 1 , a multiple effect evaporator (MEE) assembly 2, a black liquor oxidation reactor 3, a lignin precipitation reactor 4, a lignin coagulator 5 and a filter 6. Weak black liquor (e.g. , 20% solids) is fed from black liquor tank 1 along flow line 1a to MEE assembly 2. The black liquor is heated in MEE assembly 2 to generate concentrated black liquor (e.g., 50% solids) which is directed to the mill recovery boiler along flow line 2b after being further concentrated (e.g. to 70-80% solids) in concentrators. In the present process, a side stream of black liquor is fed from assembly 2 (at about 30-40% solids) along flow line 2a to a mixing tank 7 where an alkali source 8 (such as but not limited to NaOH) is added. The black liquor admixed with the alkali source is subsequently heated in the heater 9. The heated black liquor admixed with the added alkali is then fed through line 9a to the black liquor oxidation reactor 3. Oxygen gas is fed to the black liquor oxidation reactor 3 for oxidation of totally reduced sulphur (TRS) and other compounds in the black liquor. As seen in Fig. 2, pre-treated black liquor is fed along flow line 3a to a lignin precipitation reactor 4, and CO₂ is fed as acidifying agent to the lignin reactor 4 to precipitate lignin in the liquor. As provided herewith, the oxidized black liquor can be further heated through line 3c before being fed along flow line 3a to the lignin precipitation reactor 4. The acidified liquor is fed along flow line 4a to lignin coagulator 5 in which the precipitated lignin particles are allowed to coagulate. Liquor with coagulated lignin is fed along flow line 5a to a filter 6 where lignin is filtered out of the liquor. Lignin retained by filter 6 is optionally washed with H₂SO₄ and water and washed lignin is recovered. [0047] Alternatively, as illustrated in Fig. 3, black liquor is fed from assembly 2 (at about 30-40% solids) along flow line 2a to the black liquor oxidation reactor 3. The oxidized black liquor is then fed into a mixing tank where an alkali source 8 (such as but not limited to NaOH) is added. The black liquor admixed with the alkali source is subsequently heated in the heater 9. The heated black liquor admixed with the added alkali is then fed through line 3a to the lignin precipitation reactor 4.

[0048] One advantage of the latter over the former approach is the reduced energy requirements - this is a result of the fact that the black liquor oxidation reaction is highly exothermic thereby raising the temperature to a level close to what is needed for black liquor heat treatment, especially when the black liquor is rich in sodium sulphide. It should be emphasized here that black liquor oxidation is unique to the patented LignoForce™ process but can benefit other lignin recovery processes as well. It should also be understood here that the mixing of the alkali with black liquor can conducted in-line.

[0049] Alternatively, as illustrated in Fig. 4, black liquor is fed from assembly 2 (at about 30-40% solids) along flow line 2a to a mixing tank 7 where an alkali source 8 (such as but not limited to NaOH) is added. The black liquor admixed with the alkali source is, subsequently, heated in the high temperature oxidizer 9 in the presence of oxygen. The heated black liquor admixed with the added alkali is then fed to the lignin precipitation reactor 4.

EXAMPLE 1

[0050] The black liquor used resulted from the cooking of maple wood chips that were pulped at a Canadian kraft pulp mill under the following conditions:

Wood type: Maple

Digester type: Valmet Compact Cooking

Chemical charge (EA/Wood ratio): ~22 wt%

WL Sulfidity: 31 wt% on AA

Average H-Factor applied in digester: 650-800

Kappa no. out of digester: 18 [0051] Table 1 shows the composition of the HW black liquor obtained from the mill of Example 1 . It should be noted here that this liquor was not oxidized by the mill and it contained 4.24 wt% sulphide (on a black liquor solids (BLS basis)) as well as an REA of 2.87wt% on Na2O on a black liquor solids basis.

T able 1 . Composition of unoxidized HW black liquor from the mill of Example 1

[0052] Heat treatment was carried out in advance of the oxidation step of the LignoForce™ process (i.e. , using the approach shown in Fig. 2) without any further alkali addition. Fig. 5 shows the effect of applied H-factor on lignin slurry filtration rate. It should be noted that the filtration rate does not increase linearly or non-linearly with applied H- factor but displays a local maximum in filterability at approximately an applied H-factor of 1600 H. For the two trials exceeding 1600 H, the filterability is lower than the peak and equivalent to the filterability of the precipitated slurry treated at 900H. In addition to the decline in filterability, the lignin content of the final lignin cake (Klason lignin) in the latter two trials was lower than the lignin cakes obtained from the black liquors treated at 0, 900H and 1600H suggesting that larger lignin molecules were depolymerized into their oligomers and passed into the filtrate rather than being retained with the filter cake (i.e., this result suggests that excessive heat treatment reduces the lignin yield). As shown below, treatment of the black liquor at 900 H or 1600 H led to a lignin product with relatively high purity (low ash content) compared to the control (no heat treatment). Table 2. Lignin composition produced under different conditions

[0053] When one considers the thermal history of the black liquor (i.e., H-factor applied in pulping) that was heat-treated in this example, it appears that the maximum filtration rate is achieved at an overall H-factor of: 650 to 800 + 1600 = 2250-2400.

EXAMPLE 2

[0054] The black liquor used resulted from the cooking of maple and birch wood chips that were pulped at a Canadian kraft pulp mill under the following conditions:

Wood type: 50% maple and 50% birch

Digester type: Kamyr Single Vessel Hydraulic Digester

Chemical charge (EA/Wood ratio): 15.8 wt%

WL Sulfidity: 18-22 wt%

Average H-Factor applied in digester: 900-1100

Kappa no. out of digester: 15

[0055] It should be noted here that the black liquor from this mill was oxidized using oxygen - this the main reason that the sulphide content (see Table 3) is below the detection limit of the method used for its measurement. Black liquor oxidation also partially explains the low REA of this liquor given that black liquor oxidation is known to consume alkali. Table 3. Composition of oxidized HW black liquor from the mill of Example 2

[0056] In this case, the untreated black liquor (H factor = 0) was treated with alkali and heat as shown in Fig. 3. Fig. 6 shows the effect of increasing the effective alkali by adding NaOH to the black liquor prior to heat treatment at 2 levels of H-factor at the laboratory level. The untreated black liquor with an REA content of 0.019 wt% (expressed as Na₂O on a black liquor solid basis) and the black liquor with 2 wt% REA content have comparable filterability that is too low for future scale-up. However, once the REA was increased to 3 wt% Na₂O on a black liquor solids basis, the filterability improved. The maximum filterability occurred at 5 wt% REA and 900H (applied H-factor).

[0057] When one considers the thermal history of the black liquor (i.e., H-factor applied in pulping) that was heat-treated in this example, it appears that the maximum filtration rate was achieved at an overall H-factor of: 900 to 1100 + 900 to 1200 = 1800- 2300. When one compares this overall H-factor to the one calculated in Example 1 , it appears that, despite all differences in wood species, pulping conditions, black liquor oxidation and evaporation conditions, the optimum overall H-factor in the two cases is about 1800-2400 as long as a minimum REA of about 2.8-3.8 wt% as Na₂O on BLS solids is present in the black liquor to be heat treated. The REA of the black liquor to be heat treated can be adjusted by either adding sodium hydroxide to the black liquor within the limits dictated by the caustic make-up requirements of the recovery cycle (usually 10 to 20 tonne/day in a typical 1000 tonne/d Canadian kraft mill). To avoid this limitation, a mill has the option of adjusting the REA of the black liquor to be heat-treated using cooking liquor (white liquor), partially oxidized white liquor or fully oxidized white liquor. Partially oxidized white liquor is derived from white liquor whose sulphide content is mostly oxidized to thiosulphate while fully oxidized white liquor is derived from white liquor whose sulphide content is mostly oxidized to sulphate.

[0058] Fig. 7 shows the effect of the black liquor heat treatment as provided herein under different conditions on lignin MW. As clearly shown, under all applied conditions for which the filterability of the lignin slurry improved, the hardwood lignin weight average molar mass (Mw) is about 3000 g/mole as measured by gel permeation chromatography using an on-line refractive index (Rl) detector. In other words, no reduction in Mw was observed compared with the control. There are two outliers shown in Fig. 7 which are the black liquors treated at 1200 H and 900 H at 2wt% residual effective alkali in black liquor. However, these two liquors demonstrated poor filterability and washability resulting in lignins with a very high ash content (>30wt%).

[0059] Based on the above results, the effect of heat treatment of black liquor should not be confused with processes aiming to depolymerize lignin in black liquor through a heat treatment process. The mild thermal treatment as proved herewith aims at breaking down the hemicelluloses and lignin carbohydrate complexes in HW and EW black liquors to avoid having these chemical compounds interfere with lignin particle precipitation and agglomeration to form larger lignin particles. It should be noted here that it is well-known in the prior art that larger lignin particles filter and wash much better than smaller particles.

EXAMPLE 3

[0060] Finally, a control lignin slurry was derived from untreated aspen black liquor with the composition shown in T able 4. The filtration rate of the control slurry was low but the dewatering process was successful and produced a reasonably good lignin cake with an ash content of 1 .2 wt% and a pH of 4.2. To evaluate the oxidation and heat treatment in a single process unit, a 2L Parr reactor was employed. Samples of HW black liquor were generated with either as-received residual effective alkali, or 4.9wt% effective alkali. The black liquors were then loaded into the reactor and heated to 90 °C. Once at temperature, 0.6 moles of oxygen was loaded into the reactor and the black liquor was heated to 170°C until the target H factor was achieved (900 or 1600H) under continuous agitation. Once cooled, the Na₂S content of the treated black liquor was measured. The heat-treated and control samples were then oxidized to less 0.05 wt% sulfide in accordance with the LignoForce™ process. The average filtration rate of the slurry dewatering process is shown in Figure 8. Fig. 8shows an increase in filtration rate consistent with pre- or post-oxidation heat treatments. In the presence of >2.8% alkali, the filtration rates were approximately double the control and low alkali conditions.

Table 4. Composition of HW black liquor from the mill of Example 3

EXAMPLE 4

[0061] Following the evaluation of the heat treatment conditions from 500-1800 H in a benchtop lignin recovery set-up, as described in Example 2, pilot level trials were conducted using a lignin recovery system with a nominal production capacity of 4-5 kg of dry lignin per day, produced from approximately 70 L of black liquor. Fig. 9 shows the filterability of the same HW black liquor discussed in Example 2 (produced from a mixture of birch and maple) at an adjusted REA content of 2.87 wt% as Na₂O and after being heat-treated at two levels of H-Factor. The filterability is defined here as the average filtration rate to recover 60% of the initial slurry mass reported in kilograms per hour per square meter of filter area. As seen in Fig. 9, it shows excellent improvement in filterability after heat treatment at 900H and no improvement over the control condition (untreated black liquor represented by H factor = 0) when heat-treated to 1650 H. Given the thermal history of this black liquor before the applied heat treatment at 1650 H, it is clear that an overall heat treatment at an H-factor of 900 to 1100 H + 1650H = 2550-2750 H is outside the range of the recommended overall H-factors required for achieving optimum filtration rates and lignin quality.

EXAMPLE 5

[0062] Black liquor from a EW kraft pulp mill was evaluated. Table 5 shows the properties of the black liquor prior to the applied heat treatment.

Table 5. Composition of EW black liquor from the mill of Example 4

[0063] The control lignin slurry (derived from untreated black liquor) of Example 5 was unalterable and, therefore, no lignin composition could be measured. Based on the black liquor composition shown in Table 5, NaOH solution was added to increase the REA to 3 wt% as Na2O on black liquor solids and the black liquor was heat treated to 2000H. Fig. 10 compares the lignin slurry filtration rate of the heat-treated black liquor of Example 5 with the lignin slurry filtration rate of an untreated softwood black liquor with a low initial REA (<0.01 wt% as Na₂O) at the pilot plant level. As shown in Fig. 10, the filterability of the EW lignin slurry is equivalent to the SW black liquor lignin slurry. T able 6 shows the composition of the lignin produced from the lignin slurry of the Example 5 heat-treated black liquor. It should be noted here that the ash content of this lignin is quite low as is the case with the lignins produced in Examples 1-4 from HW black liquors. The relatively high residual carbohydrate content of the lignin produced from the black liquor of Example 5 suggests that a somewhat more severe heat treatment would be required to achieve lower carbohydrate contents.

Table 6. Lignin composition of EW black liquor treated with a 3 wt% of Na₂O and 2000 H heat treatment.

0064] The commercial production of kraft lignin is limited to mostly SW kraft pulp mills due to the slow filtration rates exhibited by HW and EW lignin slurries obtained following the acidification of the corresponding black liquors to a pH of around 10. The discovery of a relationship between the lignin slurry filtration rate and the extent of overall HW or EW black liquor heat treatment at a minimum REA level allows for the commercial production of high-quality HW and EW kraft lignin (e.g., low ash content) at industrially realistic filtration and washing rates. This discovery was validated by producing good quality kraft lignin at the laboratory, pilot, and demonstration plant levels using HW and EW kraft black liquors from several kraft pulp mills around the world. It appears from the data that, for each HW and EW black liquor, there is an optimum REA and H-factor at which black liquor heat treatment leads to high lignin slurry filtration rates. The optimum H-factor and REA adjustment to be applied depends on the thermal and chemical reaction history of any given black liquor (i.e. , the H-factor to which it was heated in the pulping and other processes), the chemical charges applied during the pulping process (e.g., EA/wood ratio and white liquor sulphidity) and whether the black liquor was subjected to an oxidation step.

[0065] It should be understood here that the black liquor pre-treatment approach of this patent disclosure does not only apply to difficult-to-filter HW and EW black liquors but also to difficult-to-filter black liquors derived from other pulping processes (e.g., soda process) as applied to wood or non-wood furnishes (e.g. wheat straw or bagasse straw). Such liquors usually contain very high levels of hemicellulose (e.g., xylan) and lignin carbohydrate complexes (LCC) which, as discussed previously in this patent disclosure, are known to interfere with lignin cake dewatering and washing. In the case of such black liquors, it is preferable that the black liquor acidification be conducted to a pH of 2-3 using sulphuric acid in order to maximize lignin yield.

[0066] While the present disclosure has been described with particular reference to the illustrated embodiment, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative and not in a limiting sense, that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1 . A process of increasing the filterability of lignin slurries derived from the acidification of hardwood (HW), eucalyptus (EW) or soda black liquors comprising the steps of a) adjusting the minimum residual effective alkali (REA) level to about 2.8-5 wt% Na₂O on a black liquor solids basis; and b) applying a heat treatment of an intensity and duration that is dependent on the previous thermal history of any given black liquor to achieve an overall H- factor between 1800-2400 H.

2. The process of claim 1 , wherein the lignin polymers in the black liquor are comprised of syringyl, guaiacyl and p-hydroxyphenyl monomers.

3. The process of claim 1 or 2, wherein the black liquor comprises free residual hemicelluloses and lignin-carbohydrate complexes (LCC).

4. The process of any one of claims 1-3, wherein the black liquor is heated at a temperature between 140 and 180 °C.

5. The process of any one of claims 1-4, wherein the black liquor is heated at a cumulative H-factor of 1800-2400, the cumulative H-factor being the sum of the H-factor at which the lignin in wood chips was subjected to in pulping plus the applied H-factor on the residual pulping liquor.

6. The process of any one of claims 1-5, wherein the REA level of the black liquor is firstly adjusted to a level of about 2.8-5 wt% Na₂O on black liquor solids using sodium hydroxide prior to the heat treatment.

7. The process of claim 6, wherein the REA level of the black liquor is adjusted by adding white liquor.

8. The process of claim 7, wherein the white liquor is partially oxidized white liquor or fully oxidized white liquor.

9. The process of any one of claims 1-8, wherein the black liquor with increased filterability is filterable through a belt, a vertical tray filter or a horizontal tray filter.

10. A process for removing lignin from hardwood (HW) or eucalyptus (EW) black liquors comprising the steps of: adjusting the minimum residual effective alkali (REA) level of the HW, EW or soda black liquors to about 2.8-5 wt% Na₂O on a black liquor solid basis; applying a heat treatment to said black liquor to achieve a cumulative H-factor which is in the range of 1800-2400; acidifying the black liquor with an acidifier forming a lignin slurry; and filtering the lignin slurry to remove the lignin.

11. The process of claim 10, wherein black liquor is acidified to a pH of about 10 forming a lignin slurry.

12. The process of claim 10, wherein black liquor is acidified to a pH of about 2-3 forming a lignin slurry.

13. The process of claims 11 and 12, further comprising the step of oxidizing with an oxidizing agent the black liquor before, after or during the heat treatment step, eliminating total reduced sulphur (TRS) compounds in the black liquor with oxidation of sulphides, mercaptans and organic sulphides by said oxidising agent at a temperature effective for oxidation to thiosulphate- and other oxidized sulphur compounds and oxidation of said thiosulphate to sulphate, and such that hemicelluloses and other organics in the black liquor are oxidized by said oxidizing agent at said temperature to form an acidifying agent selected from isosaccharinic acids, acetic acid, formic acid, lactic acid, oxalic acid, carbon dioxide and acidic lignin degradation products and mixtures thereof, with generation of heat through said oxidation, the generated heat leading to the creation of nucleation sites for the formation of lignin particles through lignin colloid agglomeration and coagulation.

14. The process of of claims 13, wherein the oxidizing agent is oxygen.

15. The process of of claim 11 , wherein the acidifier is selected from carbon dioxide and sulphuric acid

16. The process of claim 12, wherein the acidifier is a strong acid such as sulphuric acid.