DESCRIPTION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application No. 62/683,918, filed June 12, 2018, the contents of which is incorporated into the present application by reference.

FIELD OF INVENTION

[0002] The invention generally concerns improved methods and processes for isolation and/or extraction of lignin from a lignin source, e.g., pulping by-product such as black liquor. In particular the methods and process can include optimized processes performed under conditions to minimize undesirable effects of H ₂ S gas.

BACKGROUND

[0003] Wood has three primary components cellulose, hemicellulose, and lignin. During the process of chemical pulping, cellulose and hemicellulose fibers are isolated from the lignin. Lignin is a three-dimensional amorphous polymer consisting of methoxylated phenylpropane structures About 92%-94% of the fiber lignin can be removed by the Kraft cooking process, which results in spent alkali liquor, also known as black liquor (BL), that contains the lignin. Processes have been developed for isolating lignin from the BL. Lignin isolation was first proposed by Tomlinson and Tomlinson in the l940s. A comprehensive review of lab, pilot, and commercial experience was performed in the l980s by Uloth and Wearing (Pulp and Paper Canada, 90(9):T310-T314, 1989) and in 2016 by Kannangra et al ( Chem . Eng. Res. Des. 105:94-106, 2016).

[0004] Three general processes for lignin isolation have been described: (1) acidification- precipitation- solid/liquid separation; (2) ultrafiltration; and (3) electrochemical separation. Due to cost and technological maturity, commercial scale operations have all been acidification-precipitation- solid/liquid separation types. Lignin isolation from black liquor via acidification-precipitation- solid/liquid separation is generally quite complex and may be constrained by various practical considerations. Typically, such processes require Kraft black liquor to have 30 to 40% solids to achieve reasonable yield and easily-filtered materials. The often-reported tradeoff is that lowering solids results in hardening and improved uniformity of lignin particles, which in turn can be more-effectively filtered out of the liquid; whereas increasing the solids results in a higher yield. To date, the commercial-scale precipitation of lignin from BL is performed within a very narrow range of process conditions.

[0005] Current commercial processes typically initiate lignin isolation by acidifying and precipitating solids contained in the black liquor in mild-alkaline conditions, for example at a pH of 8 to 10. The black liquor typically has an initial pH of 10 to 14. Typically, acidification is performed by treatment with C0 ₂ gas to minimize the negative impact of sulfuric acid (H ₂ S0 ₄ ) on pulp mills, particularly the chemical balance of the pulp mill process, i.e., the sodium (Na) and sulfur (S) balances within the mills chemical recovery cycle. In certain instances, current commercial processes use oxidized black liquor or minimal acidification of the black liquor to avoid H ₂ S gas formation and emissions, and reduce acid consumption. Current commercial process typically follow the first precipitation stage with aggressive dewatering to a high solids cake in order to minimize conversion of HS- into H ₂ S gas.

[0006] In certain instances, current commercial practices acidify to mildly alkaline conditions of pH 9 or greater to minimize C0 ₂ consumption, even though a lower pH may precipitate harder particles that can be more-easily filtered. There are two reasons for acidifying black liquor to the mildly alkaline condition of pH 9 or greater: (1) lower overall acid consumption because less acid is needed to achieve the target pH of 9 or greater, and (2) avoidance of a pH lower than 9 because lower pH values approach the pKa of hydrogen sulfide (between 7 and 8 pH) and thus risk wholesale release of hydrosulfide present in Kraft liquors resulting in the generation of problematic H ₂ S gas.

[0007] Interestingly, it has been shown that more H ₂ S and S0 ₂ gases are released at pH 10, with and without pre-oxidation, than would be predicted by equilibria calculations, thus presenting what appears to be non-ideal behavior. One possible explanation is that the non ideal mixing of C0 ₂ gas can cause a combination of localized pH gradients (low) and/or entrained H ₂ S in excess C0 ₂ , i.e., H ₂ S drawn in and transported by C0 ₂ gas. Under any circumstance, H ₂ S gas is quite problematic with respect to metallurgical and containment requirements. In addition, the S0 ₂ formed when thiosulfates are dissolved in strong acid is also problematic. [0008] Notwithstanding the current lignin-isolation processes and prior study thereof, there remains a need for a better understanding of the solubility and/or stability of lignin molecules and colloidal particles (respectively) as they apply to the lignin isolation process. There also remains a need for improved lignin removal processes and more-valuable lignin products. Various improvements of the lignin isolation/extraction process are described herein.

SUMMARY

[0009] This disclosure includes various processes and improvements to certain methods of extracting lignin from black liquor, including lignin-preparation processes and alternative methods of separating the lignin in such extraction processes. The present methods and processes provide solutions to the efficiency and cost problems associated with acidification of black liquor and the unwanted generation of problematic and/or undesirable gases such as H ₂ S gas, and to address the difficulty of solid/liquid separation, i.e., to improve solid/liquid separation. By way of example, the present improvements and processes can reduce, e.g. minimize, or compartmentalize the gas-generation step, which can result in better-handling of gas generation and enhancing the lignin isolation/extraction process. As a result, the present processes can result in more cost efficient extraction of lignin from black liquor.

Degassing/NCF Improvements

[0010] Certain embodiments of the present lignin-preparation processes comprise: (a) a lignin precipitation step comprising acidifying a black liquor feed source to a pH of about or less than 9 to form an acidified black liquor; (b) degassing the acidified black liquor to form a degassed acidified black liquor; (c) mixing one or more nucleation, coagulation, and/or flocculation (NCF) agent(s) to the degassed acidified black liquor to form a treated black liquor; (d) separating solids from the treated black liquor to form a solids cake and a filtrate; and (e) washing the solids cake. The process can further comprise oxidizing the filtrate to form an oxidized filtrate, which may be used in downstream washing steps. In some embodiments, these processes can further comprise: conducting ultrafiltration on the black liquor prior to acidification and using the ultrafiltration retentate as the black liquor feed source in the lignin precipitation step. The acidification of the black liquor may include adding spent acid from another process, S0 ₂ (g), organic acids, HC1, HN0 ₃ , carbon dioxide, sulfuric acid, acid from acid/bleach plant effluent, generator waste acid, or a mixture of any two or more of such acids. The NCF agent can be a metal, recirculated acidic lignin, spent acid, or cationic coagulants.

[0011] In certain embodiments, these processes can further comprise: adding a filtration aid prior to separating solids from the treated black liquor. The filtration aid can comprise diatomaceous earth, perlite, cellulose, or a combination of any two or more of such filtration aids. In certain aspects separating the solids comprises subjecting the degassed acidified black liquor to a solids-liquids separation process to form a solids fraction and a liquids fraction, the solids-liquids separation process selected from the group of separation processes consisting of: flotation separation, barrier filtration, sedimentation, high-pressure filtration, or centrifugation forming. In a particular aspect, the solids separation may be performed at least partially with a barrier filter. The resulting solids cake can be washed under various conditions, for example with a wash solution having 2 parts wash to 1 part solids cake. These conditions may include: controlled temperature, such as from 20 to 90 °C; zeta potential, such as from 0, +/- 10, +/- 20, to +/- 30 mV, preferably less than +/- 30, +/- 20, +/- 10 mV; pH, such as from 2, 3, 4, 5, 6, 7, to 8; water content, such as 10 to 60 vol. % H ₂ 0; and/or the like. In certain aspects the wash solution can comprise oxidized filtrate (20 to 50 vol. %). As used herein, ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. One of the main characteristics of a solution with dissolved ions is the ionic strength. Ionic strength can be molar (mol/L) or molal (mol/kg water).

Oxidized Wash Improvements

[0012] Certain embodiments of the present lignin-preparation processes comprise: (a) a lignin-precipitation step comprising acidifying a black liquor feed source to a pH of about or less than 9 to form an acidified black liquor; (b) mixing one or more nucleation, coagulation, and/or flocculation (NCF) agents to the acidified black liquor to form a treated black liquor;

(c) separating solids from the treated black liquor to form a solids cake and a filtrate;

(d) oxidizing the filtrate to form an oxidized filtrate; and (e) washing the solids cake with a wash solution comprising 25 to 50 % v/v of the oxidized filtrate. The process can further comprising degassing the acidified black liquor to form a degassed acidified black liquor; where the NCF agent(s) are mixed into the degassed acidified black liquor to form the treated black liquor. These processes can further comprise: prior to acidifying the black liquor feed source, subjecting a black liquor to ultrafiltration, and using the ultrafiltration retentate as the black liquor feed source in the lignin-precipitation step. The acidification of the black liquor may include adding: spent acid from another process, S0 ₂ (g), organic acids, HC1, HN0 ₃ , carbon dioxide, sulfuric acid, acid from acid/bleach plant effluent, generator waste acid, or a mixture of any two or more of such acids.

[0013] In certain embodiments, these processes can further include adding a filtration aid prior to separating solids from the treated black liquor. The filtration aid can comprise diatomaceous earth, perlite, cellulose, or a combination of any two or more of such filtration aids. In certain aspects separating the solids comprises subjecting the degassed acidified black liquor to a solids-liquids separation process to form a solids fraction and a liquids fraction, the solids-liquids separation process selected from the group of separation processes consisting of: flotation separation, barrier filtration, sedimentation, high-pressure filtration, or centrifugation forming. In a particular aspect, the solids separation may be performed at least partially with a barrier filter. The resulting solids cake can be washed under various conditions, for example with a wash solution having 2 parts wash to 1 part solids cake. These wash conditions may include: controlled temperature, such as from 20 to 90 °C; zeta potential, such as from 0, +/- 10, +/- 20, to +/- 30 mV, preferably less than +/- 30, +/- 20, +/- 10 mV; pH, such as from 2 to 8; water content, such as 10 to 60 vol. % H ₂ 0; and/or the like.

Alternative Separation Modification

[0014] Certain embodiments of the present lignin-preparation processes comprise: (a) a lignin precipitation step comprising acidifying a black liquor feed source to a pH of about or less than 9 to form an acidified black liquor; (b) mixing nucleation, coagulation, and/or flocculation (NCF) agents to the acidified black liquor forming a treated black liquor; (c) subjecting the treated black liquor to a solids-liquids separation process to form a solids fraction and a liquids fraction, the solids-liquids separation process selected from the group of separation processes consisting of: flotation separation, barrier filtration, sedimentation, high- pressure filtration, or centrifugation forming; and (d) washing the solids fraction. These processes can further comprise: prior to the acidifying the black liquor feed source, subjecting a black liquor to ultrafiltration and using the ultrafiltration retentate as the black liquor feed source in the lignin-precipitation step. The acidification of the black liquor may include adding: spent acid from another process, S0 ₂ (g), organic acids, HC1, HNO ₃ , carbon dioxide, sulfuric acid, acid from acid/bleach plant effluent, generator waste acid, or a mixture of any two or more of the listed acids. [0015] In certain embodiments, these processes can further include adding a filtration aid prior to separating solids. The filtration aid can comprise: diatomaceous earth, perlite, cellulose, or a combination of any two or more of the listed filtration aids. In certain aspects separating the solids comprises subjecting the degassed acidified black liquor to a solids- liquids separation process to form a solids fraction and a liquids fraction, the solids-liquids separation process selected from the group of separation processes consisting of: flotation separation, barrier filtration, sedimentation, high-pressure filtration, or centrifugation forming. In a particular aspect, the solids separation may be performed at least partially with a barrier filter. The resulting solids cake can be washed under various conditions, for example with a wash solution having 2 parts wash to 1 part solids cake. These conditions may include: controlled temperature, such as from 20 to 90 °C; zeta potential, such as from 0, +/- 10, +/- 20, to +/- 30 mV, preferably less than +/- 30, +/- 20, +/- 10 mV; pH, such as from 2 to 8; water content, such as 10 to 60 vol. % H ₂ 0; and/or the like. The wash solution can comprise a oxidized filtrate at 20 to 50 vol %.

[0016] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to all aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa.

[0017] The term“weight percent” or“wt. refers to the weight percentage of a component, respectively, based on the total weight of material that includes the component. In one non-limiting example, 100 lbs of material that includes 10 lbs of a component may be said to include 10 wt. % of the component.

[0018] The term“volume percent” or“vol. refers to the volume percentage of a component, respectively, based on the total volume of material that includes the component. In one non-limiting example, 100 liters of material that includes 10 liter of a component may be said to include 10 vol. % of the component.

[0019] The term“about” or“approximately” are defined as being close to what is specified (and includes what is specified; e.g., a pH of about 7.5 includes 7.5), as understood by a person of ordinary skill in the art. In any disclosed embodiment or claim, the term “about” or“approximately” may be substituted with“within [a percentage] of’ what is specified, where the percentage includes 10 percent or 5 percent.

[0020] The term“substantially” is defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., a pH of substantially 7.5 includes 7.5), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the term “substantially” may be substituted with“within [a percentage] of’ what is specified, where the percentage includes 5 percent, 1 percent, or 0.1 percent.

[0021] The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean“one,” but it is also consistent with the meaning of“one or more,”“at least one,” and“one or more than one.”

[0022] The use of the term“or” in the claims is used to mean“and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and“and/or.”

[0023] As used in this specification and claim(s), the words“comprising” (and any form of comprising, such as“comprise” and“comprises”),“having” (and any form of having, such as“have” and“has”), “including” (and any form of including, such as“includes” and “include”) or“containing” (and any form of containing, such as“contains” and“contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0024] The compositions and methods of making and using the same of the present invention can“comprise,”“consist essentially of,” or“consist of’ particular ingredients, components, blends, method steps, etc., disclosed throughout the specification.

[0025] Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of the specification embodiments presented herein.

[0027] FIG. 1 illustrates one embodiment of a hybrid lignin isolation process that utilizes pre-treatment of the lignin source to filter and concentrate lignin containing solids prior to processing.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0028] Wood contains three essential components, lignin, cellulose, and hemicellulose. In chemical pulping lignin is removed from the cellulose and hemicellulose. A by-product of the certain pulping processes is black liquor, a basic aqueous solution containing dissolved lignin obtained from the cooking of pulp. This lignin containing by-product of wood pulping can be used as a feed source for lignin isolation/extraction.

[0029] Practical issues, such as those discussed above, have shaped the current commercial process for lignin isolation/extraction resulting in the creation of various process constraints. The practical issues and inefficiencies of the current commercial processes include, but are not limited to:

(1) Impact of acidifying agent on the host mill's liquor cycle sulfur/sodium balance. This constrains the type of acidifying agents used in commercial operations.

(2) Kraft alkali liquors contain appreciable amounts of hydrosulfide (HS-), resulting in H ₂ S and C0 ₂ gas release, in many cases leading to foaming when Kraft black liquors are acidified to pH 8 or lower. This constrains target pH for initial acidification to a pH of 9 or greater and constrains the subsequent solid liquid separation processes.

(3) Complex and non-ideal colloidal and particle properties of lignin in aqueous media prevent and constrain both solid-liquid separations, re-wetting, and washing operations for commercial processes. (4) Commercial processes integrated into an existing pulp mill have to minimize use of water, minimize use of acidifying agents, and minimize loss of the non lignin components of the input black liquor.

[0030] From this practical point of view what is desired is a lignin isolation/extraction process that allows for: (i) control of product properties, e.g., property enhancement or customization by fractionation; (ii) minimal acid use; (iii) minimal impact on the pulp mills sodium-sulfur (Na/S) balance, (iv) minimal capital and labor requirements, and (v) minimal off-gas handling problems.

[0031] Certain embodiments and aspects of the processes, methods, and improvements described herein address one or more of the practical issues and inefficiencies associated with the current lignin isolation/extraction processes by providing improved lignin precipitation and/or precipitated lignin isolation processes.

A. Variations of Lignin Processing

[0032] Various embodiments of the invention can incorporate one or more processes or steps for lignin precipitation and/or the processing of the lignin colloid. The processes can include, but are not limited to various combinations of (i) pre-treating the feed source prior to processes such as acidification, (ii) using alternative acidification materials and methods, (iii) degassing liquor and, in some instances, capturing off gases, and (iv) controlling the nucleation/coagulation/flocculation (NCF) processes to regulate lignin particle characteristics. Improved processes and systems may avoid or minimize gaseous agents such as C0 ₂ and/or 0 ₂ , and/or use continuous, relatively-simple solid/liquid separation systems.

1. Modified Lignin Precipitation Process

[0033] Commercial scale operations for lignin isolation are predominantly acidification- precipitation- solid/liquid separation types. Lignin isolation from black liquor via acidification-precipitation- solid/liquid separation is generally quite complex and may be constrained by various practical considerations. The lignin isolation method acidifies the black liquor so that the lignin is precipitated to form a solid phase or colloid. The solid phase is separated from the liquor or colloid, and can thereafter be cleaned or modified. In certain embodiments the pH level adjustment is combined with an adjustment of the ion strength, preferably by using alkali metal ions or multivalent alkaline earth metal ions, most preferred calcium ions. In certain embodiments, prior to lignin precipitation the lignin feed source, for example black liquor, can be subjected to ultrafiltration (UF) which filters particles in the range of 0.001 mih to 0.1 mih, and/or nanofiltration (NF) which filters particles in the range of 0.1 nm to 0.001 mih. Ultrafiltration (UF) is a membrane filtration process that separates suspended solids from a solution through a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained in the retentate, while water and low molecular weight solutes pass through the membrane in the permeate or filtrate. This separation process is used in industry and research for purifying and concentrating macromolecular solutions, especially protein solutions. Ultrafiltration separates based on size exclusion or particle capture. Ultrafiltration membranes are defined by the molecular weight cut-off (MWCO) of the membrane used. In certain embodiments the MWCO is between 1 kDa to 4 kDa. UF/NF can remove a substantial amount of inorganic, organic, and ionic components of the feed source. This filtration step can be used to increase the percent lignin in the retentate. In certain aspects the retentate can be oxidized and/or further manipulated prior to lignin precipitation.

[0034] FIG. 1 illustrates a flow diagram of a lignin isolation process that utilizes ultrafiltration/nanofiltration (UF/NF) prior to lignin precipitation. The process includes a pre-treatment process 102 where black liquor, preferably weak black liquor that includes 15 to 30 weight percent (wt. %) dissolved solids, is fractionated by ultrafiltration (UF) or nanofiltration (NF) producing a retentate and a filtrate. The retentate, which can be approximately 20 to 30 weight percent of the feed source, is exposed to one or more precipitation step 104, degassing step 106, nucleation-coagulation-flocculation (NCF) step 108, solid-liquid separation step 110, and/or wash process 112.

[0035] Pretreatment filtration can be used to increase the percentage of dissolved or suspended solids in the retentate relative to other organic, inorganic, and ionic species. In certain aspects, the pretreatment can result in a 30 to 50% reduction in initial stage acid consumption, and H ₂ S and C0 ₂ evolution during the precipitation process. During pretreatment filtration the thickness of the feed source is minimized to enhance efficiency and cost effectiveness. Pretreatment of a feed source with a controlled or predetermined suspended solids content creates unique advantageous thermodynamic conditions in the retentate solubility and colloidal properties (colloidal properties include much higher lignin concentrations, less dissociation interference, and more stable ionic charge distribution). Pretreatment filtration can be followed by an increase in suspended solids content or dewatering stage under alkaline conditions prior to the precipitation step, which may result in further reductions in total acid consumption and H ₂ S evolution.

[0036] The pretreatment retentate may require further manipulation including the addition of coagulating agents and other means of reducing zeta potential (e.g., zeta potential is as close to 0 as possible and less than +/- 20 or +/- 30 mV) (finer filtration media, multivalent cations, increased temperature and retention time, lignin solute and particle recirculation, etc.) if a subsequent alkaline precipitation or a low pH more aggressive acidification, likely below pKa of H ₂ S (for details see below), are used in the precipitation step. Also, since the initial UF stage displaces up to 70% or more initial HS-, an optional oxidation stage after pretreatment can be performed in conjunction with at least one post-oxidation washing stage to avoid formation and release of S0 ₂ . Oxidation (e.g., exposure to 0 ₂ , hydrogen peroxide, or other oxidants) at this stage is less expensive, less disruptive, and more feasible than oxidation of the feed source prior to any processing or pretreatment. This optional oxidation stage is especially useful in the case where relatively low molecular weight (LMW) and relatively high functional group content (HFG) material is desired as an end product.

[0037] The pretreatment filtration and other pre-precipitation steps can minimize or eliminate the use of C0 ₂ gas for acidification in the precipitation step. In certain embodiments generator waste acid (GWA) and sulfuric acid can be used in the precipitation step in place of C0 ₂ . For instances in which C0 ₂ is required, the C0 ₂ is pre-dissolved in order to avoid mixing/stripping issues.

[0038] In certain variations of the modified lignin precipitation process multivalent cations and acid washing can be used at various stages of the process to remove, recover, and reuse cations.

[0039] In certain aspects the filtrate can be collected, oxidize, and used as a wash solution, in particular to wash out black liquor from cakes formed later in the process.

[0040] In still a further aspect, alternative solid-liquid separation techniques, such as flotation separation, barrier filter, sedimentation, high pressure filter, centrifuge, and the like can be used due to the more refined and controlled process of forming lignin particles.

[0041] Gas separation from the acidified lignin source reduces gas entrainment and entrapment by lignin floes and clusters, which leads to easily collapsed particles that are difficult to filter. To this end, acidification with intense mixing (generating small but hard particles) followed immediately or simultaneously by gas/liquid separation can be achieved using combinations of tangential entry, vortex finder, demister sections, vacuum separation, falling thin film elements, and operation under vacuum. After gas/liquid (G/L) separation, an optional oxidation stage can be performed to kill any carryover total reduced sulfur (TRS). Oxidation can be performed before or after a first solid/liquid separation but is preferred after degassing.

[0042] A degassed composition can then be introduced to a coagulating/flocculating system. In certain aspects the coagulating/flocculating system generates large (> 1 to 100 pm) and hard lignin particles (minimal entrained gas/liquid so as to minimize collapsibility). The lower pH creates harder, less sticky lignin particles, which are easier to separate with continuous unit operations such as barrier filters or centrifuges or clarifiers (versus batch filter presses or candle filters). In certain aspects the lignin particles can be isolated by using barrier filters such as an Eaton unit with 15 - 100 micron slots.

2. Nucleation/Coagiilation/Flocculation (NCF)

[0043] The term“flocculation” refers to a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters. NCF conditions can be adjusted to manipulate the resultant mean particle size and size distributions. In certain aspects, the processed material (e.g., after degassing, etc.) can be subjected to NCF. Fignin in black liquor and in acidified black liquor behave both as a colloidal suspension and as a solute/solvent system, in many cases both simultaneously. The lignin polymer moiety is poly dispersed with 3 dimensional conformations. It consists of different types of monomeric units and linkage arrangements. The basic monomeric building blocks are syringyl or guaicyl aromatics in an aryl-aliphatic arrangement often described as“phenol propane” or C6-C3 units. The lignin moiety will contain varying amounts of polar functional groups such as aromatic phenolics and hydroxlys, aromatic methoxyls, aliphatic hydroxyls, aliphatic ether linkages, carboxlic acids, and aldehydes. Within black liquor suspensions and solutions, these functional groups play an important role in acid-base reactions, solubility, and colloidal stability.

[0044] The functional groups are weak acids that will have pKa values that range from neutral to mildly alkaline pH of 7 to 8 (e.g., carboxylic acid groups) all the way to pH of 11 to 12 (phenolic hydroxides attached to high molecular weight polymer). It is reported that pKa will vary as a function of molecular weight in addition to the type of functionality. In general, low molecular weight fractions contain higher functional group content and stronger acids, i.e., they are more highly charged and more polar than high molecular weight fractions. In black liquor solutions and suspensions, the weak acids are deprotonated and conjugated with Na (or K) in the cooking liquor. As the pH is reduced by acidification the weakest acids sequentially start to neutralize in lignin, causing both a reduction in solubility and also a reduction in surface charge that would disperse surface charged particles into a stable colloidal suspension. The lower the pH the more protonation of weak acids and the more lignin de-salts and precipitates. Note it has been reported that sequential acidification results in sequential precipitation of lignin based on molecular weight and functional group content. This would suggest a lignin fractionation could be achieved by utilizing such a process. The inventors found this not to be feasible due to pH control issues on both a micro and macro level, but importantly also due to non-idealities in the system.

[0045] Lignin particles that are formed in mildly alkaline conditions (pH 8 to 10) actually shrink upon further acidification from mildly alkaline to neutral to acidic conditions. A lower pH results in the formation of smaller, harder particles due to lower molecular weight material being precipitated but does not describe actual particle transformation. Smaller, but tougher and denser acidic particles separate better by gravity than do those particles formed in mildly alkaline conditions. These particles are also less prone to plug thin film filtration systems. Use of gaseous reactants promotes larger, less tough and less dense particles that are hard to separate. Use of coagulants comprising multivalent cations also promotes denser and tougher particles, albeit larger than otherwise. The‘cleaner’ the system, i.e., the less ionic“trash” or strength, the greater the relative impact of such cationic coagulants. Of importance, the inventors have found that the conditions of initial precipitation and any subsequent treatment can affect differences in particle density that will influence the suitability of different solid liquid separation unit operations.

[0046] Seeding or use of lignin“germs” to improve initial nucleation is only partially effective, i.e., only partially accomplishes the desired or intended result. Seeding with acidic lignin is much more effective than seeding with intermediate, mildly alkaline lignin. But neither is as effective in increasing particle size and improving filterability as the use of well- known multivalent cations such as soluble calcium, magnesium, iron, or aluminum. Surprisingly, doping mildly alkaline lignin slurries with relatively large amounts of lignin, including mildly alkaline lignin, has a significant effect on net yield and filterability. In certain instances adding 20, 30, 40, to 50 % recirculated lignin, even after nucleation, results in improved filterability that may allow for a net improvement in throughput and yield. In this case, recirculated lignin is enhancing flocculation, as opposed to seed lignin aimed at nucleation.

[0047] A NCF system’s operating conditions can be adjusted and controlled in order to create a target mean particle size and size distribution. The degree of NCF is a function of zeta potential, hydrodynamic conditions, and residence time. Zeta potential and/or particle surface charge can be minimized and NCF thus maximized by increasing: ionic strength, especially with multivalent anions; reducing particle surface charge and reducing lignin solubility by decreasing pH; increasing dissolved lignin and suspended solid lignin particle concentrations; increasing cluster formation and flocculation, with agents such as polycations; and increasing temperature. The inventors studies indicate that for a lignin sample with mean, deflocculated particle diameter of 9 pm the smaller particles, namely particles < 10 pm, consist of lignin moiety with lower molecular weight (LMW) and higher functional group content (HFGC) than larger particles. Conversely, the larger particles, namely particles > 9 pm, consist of lignin moiety with higher molecular weight (HMW) and lower functional group content (LFGC) than smaller particles. Thus a simple size fraction performed after nucleation but before flocculation is used to fractionate the LMW and HFGC lignin with smaller particles. The flocculation process can be further enhanced by recirculating dissolved and solid lignin to the point where downstream solid/liquid separation net performance can be improved. Surprisingly, the inventors have found that NCF is significantly improved by recycling lignin particles that have already been separated and, preferably, fully acidified and washed.

[0048] For example, by minimizing or eliminating flocculation and operating NCF to target a given separation ratio in a fixed separation unit, the amount of LMW/HFGC fraction and HMW/LFGC fraction can be adjusted, e.g., to within certain practical limits. In order to recover the LMW/HFGC fraction, a second stage of filtration is used. The second stage would have finer filtration cutoff dimensions, e.g., in the 1 to 2 pm range. A second stage can be supplemented by using a finer filter with enhanced NCF conditions, e.g., pH, temp, cations, and/or the like. The less LMW/HFGC produced, the lower the MW and higher FGC in that fraction, and vice versa. This approach to fractionation of lignin is unique. Alternatively, by maximizing flocculation in NCF, overall yield of average MW/FGC can be maximized and a simplified, single process system with simplified (high particle size cut off) solid liquid separation system can be used.

[0049] Variables effecting colloidal stability and coagulation kinetics can be adjusted to a target, control zeta potential and/or particle surface charge density. The control scheme can be based on process testing of particle surface charge or size, or for a given set of coarse filter conditions, on coarse filter product yield (which can be measured by on line suspended solids or turbidity meters). In certain aspects a primary control variable of interest ( i.e ., those variables ranked by their greatest degree of control at minimal cost) would be (a) the use of multivalent cations, (b) the recirculation of either dissolved or partially dewatered solid lignin (especially acidified lignin), (c) additional reduction of pH, (d) increased process temperature, and (e) increased retention time.

[0050] In general, a continuous filtration process is preferred over a batch filter process. In certain instances a course filter, e.g., a barrier screen type filter, is preferred over a fine filter. Thus, feed tank level and single pass retention time can and is preferably held relatively constant. Aggregate lignin retention time as well as dissolved lignin and suspended solids lignin concentrations can be adjusted (upwards) by adjusting the amount of recirculated lignin. The dissolved lignin can come from downstream wash filtrate recirculation and/or solid lignin from any downstream solid/liquid separators.

[0051] In certain embodiments a lignin particle recycle can be used during coagulation. For example a barrier filter can be used to thicken the slurry, recirculate part of slurry back to the coagulation stage to seed, increase retention time, and increase % solids. This recycle aspect can use either alkaline or acid lignin.

3. Solid-Liquid Separation

[0052] Certain embodiments of the invention also use continuous, relatively simple and inexpensive solid/liquid separation (SLS) systems that have low capital and operating costs, These SLS can be configured in a closed environment facilitating off gas handling. One example of a continuous solid/liquid separation system is the use of barrier filters in place of batch methods requiring filtration media, even if more unit operations and water are required to affect a given amount of washing with barrier filters. The same could be said of other, potentially less selective separation systems such as continuous centrifugal or gravity based (floatation or settling) systems. Use of a less selective separation requires methods to control lignin particle size and durability, that is the processes institute better control of the particle nucleation-coagulation-flocculation (NCF) mechanisms.

[0053] Solid-liquid separation can be achieved by any number of fundamental unit operations knows to those practiced in the art such as; flotation separation; dead end filtration; cross flow filtration; centrifugal force separation (with various types of centrifugal force separators such as cyclones and centrifuges), and gravity induced separation ( e.g ., gravity columns, sedimentation systems, or flotation systems).

[0054] System extraction or extractions can either go to a fine screen, for fine particles, or to a fine NCF system followed by a fine screen, e.g., for re-dispersed or re-dissolved lignin. In a simple, but special case the re-dispersion nature of lignin can be used to facilitate separation. An alkali slurry is separated in a course screen to remove some or even most of the lignin, but small particles can be recovered, if so desired, in micron scale filters. Off loading these filter cartridges or bags diminishes overall capacity demand and cost versus using them alone. However, a common problem thereafter is cake release and recovery. In this case clean hot water can be used to re-disperse the lignin in a back flush mode. The resultant dispersion can be mildly oxidized to remove small amounts of residual TRS or aggressively oxidized if LMW/HFG products are desired. Alternatively, the dispersed colloidal system can be added back to the main train but in the acidic part of the train profile.

[0055] Cross flow extraction can be particularly useful in segregating the filtrate flows based on those that are useful and valuable to the pulp mill, e.g., to be returned for example to first wash stage filtrate, to those that are not, e.g., the downstream filtrates. The washing and coagulation systems in downstream, i.e. not returned to liquor loop, units are not subject to liquor loop considerations, so chemicals and waste streams can be used that would not normally be put back into the liquor loop, for example excess generator saltcake, precipitator catch, spent boiler feedwater treatment acid, spent acids, and acidic bleach plant effluent. Many of these are rich in polyvalent cations and can be effective in coagulating lignin.

[0056] Continuous, less- selective and perhaps coarser mechanically cleaned barrier filters can be preferred over batch methods requiring filtration media, even if more unit operations and water may be required to perform a given amount of washing. The same could be said of other, potentially less selective separation systems such as continuous centrifugal or gravity- based systems, e.g., floatation or settling systems. Use of a less-selective separation will require means and methods to control particle size and durability, or better control of the particle nucleation-coagulation-flocculation mechanisms.

[0057] Flotation separation use flotation separators that work on the principle that the various species within the slurry interact differently with bubbles formed in the slurry. Gas bubbles introduced into the slurry attach, either through physical or chemical means, to one or more of the hydrophobic species of the slurry. The bubble-hydrophobic species agglomerates are sufficiently buoyant to lift away from the remaining constituents and are removed for further processing to concentrate and recover the adhered species.

[0058] Filtration Aid. Filtering can be enhanced by adding a filtration aid. These filter aids can be used as a precoat before the colloid is filtered. This will prevent gelatinous -type solids from plugging the filter medium and also give a clearer filtrate. Filtration aids can also be added to the colloid before filtration. This increases the porosity of the cake and reduces resistance of the cake during filtration. In certain aspects a filtration aid can be added prior to a filtration step. The cake properties and filtering efficiency can be enhanced by maintaining the permeability of the accumulating cake by using a filtration aid. A filtration aid can include, but is not limited to fibers, diatomaceous earth, perlite, polyethylenimine (PEI), cellulose, cellulose containing chemical agents, chemical pulp or combination thereof. Other suitable filtration aid material also can be used in place of or in combination with the previously listed filtration aids. The filtration aid enhances the filtration of lignin particles. Once filtered the filtration aid can be removed. In certain aspects the filtration aid is removed or dissociated from lignin particles by washing the filtration aid/lignin particle complex with hot water. Water can be at least 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 98, to 100 °C.

[0059] In any of the processes described herein the wash solution can comprise 25, 30, 25, 40, 45 to 50 vol. % of the oxidized filtrate.

[0060] Any of the processes described herein can further comprise subjecting the black liquor to ultrafiltration prior to acidifying the black liquor feed source, and using the ultrafiltration retentate as the black liquor feed source in the lignin-precipitation step. [0061] In any of the processes described herein the acidification of the black liquor can include adding one or more acids selected from the group of acids consisting of: spent acid from another process, S0 ₂ (g), organic acids, HC1, HN0 ₃ , carbon dioxide, sulfuric acid, acid from acid/bleach plant effluent, generator waste acid, and/or mixtures of any two or more of the listed acids; and, optionally, where, at least one of the NCF agent(s) is selected from the group of NCF agents consisting of: metals, recirculated acidic lignin, spent acid, and/or cationic coagulants.

[0062] Any of the processes can further comprise, prior to separating solids from the treated black liquor or lignin colloid, adding a filtration aid to the black liquor or lignin colloid; optionally where the filtration aid is selected from the group of filtration aids consisting of the group of filtration aids consisting of: diatomaceous earth, perlite, cellulose, and/or combinations of any two or more of the listed filtration aids.

[0063] In any of the processes described herein, the separation of solids may be performed by a solids-separation process selected from the group of solids- separation processes consisting of: flotation separation, dead-end filtration, cross-flow filtration, centrifugal-force separation, gravity-induced separation, and separation with a barrier filter.

[0064] Any of the processes described herein, can include washing the solids cake with a solution having 0.5, 1, 2, 3, or 4 parts wash to 1 part solids cake.

B. Lignin Composition [0065] Ultimately, particular lignin compositions are the end goal. The presence of trace multivalents is tolerable, but only to a degree. In order to remove and reuse them, dilute acid wash can be used. This will dissolve many of the cations and counter currently reintroduce them as diluent in upstream.