WO2019241408A1 - Procédés et processus d'isolement/extraction de lignine - Google Patents

Procédés et processus d'isolement/extraction de lignine Download PDF

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Publication number
WO2019241408A1
WO2019241408A1 PCT/US2019/036802 US2019036802W WO2019241408A1 WO 2019241408 A1 WO2019241408 A1 WO 2019241408A1 US 2019036802 W US2019036802 W US 2019036802W WO 2019241408 A1 WO2019241408 A1 WO 2019241408A1
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Prior art keywords
lignin
black liquor
filtrate
wash
solids
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PCT/US2019/036802
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English (en)
Inventor
Bruno Marcoccia
Shabnam SANAEI
John Gray MARTIN, III
Rolf Ryham
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Domtar Paper Company, Llc
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Publication of WO2019241408A1 publication Critical patent/WO2019241408A1/fr

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    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C11/00—Regeneration of pulp liquors or effluent waste waters
    • D21C11/0007—Recovery of by-products, i.e. compounds other than those necessary for pulping, for multiple uses or not otherwise provided for

Definitions

  • 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.
  • a lignin source e.g., pulping by-product such as black liquor.
  • the methods and process can include optimized processes performed under conditions to minimize undesirable effects of H 2 S gas.
  • 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.
  • 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.
  • 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.
  • a lignin preparation process can comprise(a) a lignin precipitation step comprising: (i) acidifying a black liquor feed source with an acid to form an acidified black liquor having a pH of less than 7.5 and a C0 2 content of less than 5 to 25 wt% forming a black liquor lignin colloid; and (ii) degassing the black liquor during acidification, where subparts (a)(i) and (a)(ii) are performed concurrently to form an acidified/degassed black liquor; (b) subjecting the degassed colloid to a solids-liquids separation process selected from barrier filtration, sedimentation, high pressure filtration, or centrifugation forming a solids fraction and a liquids forming a solids cake and a filtrate; and (c) washing the solids cake.
  • a solids-liquids separation process selected from barrier filtration, sedimentation, high pressure filtration, or centrifugation forming a solids fraction and a liquids forming
  • the acid used for acidification can be selected from the group 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, and generator waste acid.
  • the acid used for acidification is sulfuric acid.
  • the lignin preparation process can further comprise capturing H 2 S recovered from the degassing process and converting the captured H 2 S into
  • the deep acidification lignin preparation process can further comprise mixing nucleation, coagulation, and/or flocculation (NCF) agents to the degassed acidified black liquor prior to solids-liquids separation to form a treated black liquor.
  • NCF agent is a metal, recirculated acidic lignin, spent acid, or cationic coagulants.
  • the deep acidification lignin preparation process can further comprise adding a filtration aid prior to solids-liquids separation.
  • the filtration aid is selected from diatomaceous earth, perlite, cellulose, or combinations thereof.
  • Certain embodiments are directed to an interstage wash methods for lignin preparation. Certain aspects are directed to lignin preparation processes comprising: (a) acidifying a black liquor feed source to a pH of less than 9 to form an black liquor lignin colloid; (b) subjecting a black liquor lignin colloid to a coarse solid-liquid separation process with a particle size cut-off of from 15 to 100 pm to form a first solid fraction and a first lignin filtrate; (c) washing the first solid fraction, using a displacement wash process, a dilution wash process, or hybrid displacement-dilution wash process to form a first lignin wash; (d) collecting the first washed lignin solids; (e) acidifying the first lignin filtrate, the first lignin wash, or the first lignin filtrate pooled with the first lignin wash to form a second lignin colloid and performing a nucleation, coagulation, and/or floccul
  • filters can be pretreated or pre-coated with a filtration aid.
  • Boiler feed water and/or wash from ion exchange columns (1, 5, 10, 15, 20, 25, 30, 35, 40, to 50 volume percent) can be added to a feed source or filtrates to enhance coagulation.
  • the acidification of the black liquor can be performed by 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, spent water treatment acids, or mixtures thereof.
  • the acidification can be performed by adding sulfuric acid.
  • the interstage wash lignin preparation processes can further comprise mixing nucleation, coagulation, and/or flocculation (NCF) agents to the acidified black liquor prior, the first lignin filtrate, or the second lignin filtrate prior to solids-liquids separation.
  • NCF agent is a metal, recirculated acidic lignin, spent acid, or cationic coagulants.
  • the interstage wash lignin preparation processes can further comprise adding a filtration aid to the acidified black liquor prior, the first lignin filtrate, or the second lignin filtrate prior to solids-liquids separation.
  • a filtration aid is selected from diatomaceous earth, perlite, cellulose, or combinations thereof.
  • the amount of lignin in solution during the various stages of the lignin preparation processes described in this disclosure can be adjusted by controlling the temperature, ionic strength, and water content of the colloid or wash solutions.
  • the solids cake is washed with 2 parts wash to 1 part solids cake.
  • the lignin preparation processes can further comprise oxidizing the filtrate from a solids-liquids separation to form an oxidized filtrate.
  • the oxidized filtrate can be used in washing the solids cake or solid fraction by washing the solids cake or solid fraction with a wash solution comprising 25 to 50 % v/v of the oxidized filtrate.
  • the processes can further comprise filtering the black liquor prior to acidification and using the filtrate as the black liquor feed source in the lignin precipitation step.
  • weight percent 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.
  • volume percent 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • FIG. 2 illustrates one embodiment of a lignin isolation/extraction process including a one- step acidification and gas recovery process that utilizes pre-treatment of the lignin source to filter and concentrate lignin containing solids prior to processing.
  • FIG. 3 illustrates one embodiment of a solid-liquid separation process for primary and secondary slurries with and without further fractionation.
  • FIG. 4 illustrates one embodiment of a process where gas is collected from a filtrate with and without fractionation.
  • FIG. 5 illustrates one embodiment of a multi-stage counter current and cross current wash with pH, zeta potential, and/or H 2 S profiling.
  • FIG. 6 illustrates one embodiment of a multi stage wash process.
  • Wood contains three essential components, lignin, cellulose, and hemicellulose.
  • 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.
  • Kraft alkali liquors contain appreciable amounts of hydrosulfide (HS-) and carbonate that leads to C0 2 , 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.
  • HS- hydrosulfide
  • 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.
  • 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.
  • 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.
  • NCF nucleation/coagulation/flocculation
  • 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.
  • lignin isolation methods 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.
  • 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, aluminum ions or the like.
  • the lignin feed source for example black liquor
  • the lignin feed source can be subjected to ultrafiltration (UF) which filters particles in the range of 0.001 mhi to 0.1 mhi, and/or nanofiltration (NF) which filters particles in the range of 0.1 nm to 0.001 mhi.
  • 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.
  • Ultrafiltration separates based on size exclusion or particle capture.
  • Ultrafiltration membranes are defined by the molecular weight cut-off (MWCO) of the membrane used.
  • MWCO molecular weight cut-off
  • 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.
  • the retentate can be oxidized and/or further manipulated prior to lignin precipitation.
  • 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.
  • UF ultrafiltration/nanofiltration
  • NCF nucleation-coagulation-flocculation
  • 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.
  • the pretreatment can result in a 30 to 50% reduction in initial stage acid consumption and H 2 S evolution during the precipitation process.
  • the suspended solid content 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 a thickening 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.
  • the pretreatment retentate may require further manipulation including the addition of coagulating agents and other means of reducing zeta potential (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 H 2 S and other gases.
  • Oxidation e.g ., exposure to 0 2 , hydrogen peroxide, or other oxidants
  • 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.
  • LMW relatively low molecular weight
  • HFG relatively high functional group content
  • the pretreatment filtration and other pre-precipitation steps can minimize or eliminate the use of C0 2 gas for acidification in the precipitation step.
  • generator waste acid (GWA) and sulfuric acid can be used in the precipitation step in place of C0 2 .
  • GWA generator waste acid
  • sulfuric acid can be used in the precipitation step in place of C0 2 .
  • the C0 2 is pre-dissolved in order to avoid mixing/stripping issues.
  • multivalent cations and acid washing can be used at various stages of the process to remove, recover, and reuse cations.
  • 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.
  • 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. In certain aspects these procedures can be used in conjunction with filtration aids.
  • a feed source can be subjected to a single step deep acidification to low pH with degassing.
  • the pH is about or below 7.5, 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.0, 3.5, or 3.0, including all pH values and ranges there between
  • the lignin feed source can be pretreated by filtration as described above then acidified or acidified to initiate lignin precipitation and gas release.
  • FIG. 2 illustrates an example of a single step acidification process with gas recovery.
  • a single step acidification recovery system can include one or more of an acidification/precipitation step 204 and degassing step 206 coupled to H 2 S recovery steps 216/218, and can also include a nucleation/coagulation/flocculation (NCF) step 208, solid- liquid separation step 210, wash process 212 and final drying step 214.
  • the incoming lignin feed source e.g ., black liquor or pre-filtered black liquor
  • the acidification/precipitation step is carried out in equipment that is configured to recover all or most of H 2 S gas (closed system) and convert it to H 2 S0 4 via a coupled H 2 S burner and catalytically conversion process that results in S0 2 to sulfuric acid (using for example a Haldor Tapsoe WSA process or variant thereof).
  • a combination of Cl0 2 saltcake, sulfuric acid, and C0 2 can be used as supplemental acid agents in the acidification/precipitation step as required to maintain mill sodium/sulfur balance.
  • C0 2 is not used or is used in limited amounts (5 wt% or less) as an acidifying agent due to practical issues with mixing, early H 2 S stripping, excess gas evolution (foaming), gas entrainment and collapsible particles, less efficient H 2 S combustion and sulfur recovery.
  • a combination of GWA and virgin H 2 S0 4 can be used as an acidifying agent in the acidification/precipitation step, yielding excess recovered H 2 S0 4 over what is required to acidify the lignin source to form a lignin precipitate and/or lignin particle. This H 2 S0 4 excess can be harvested and used for purposes external to the lignin isolation/extraction process.
  • the HS- in black liquor will correspond to about 30 to 40% equivalent H 2 S0 4 mass per unit lignin mass which is about equal or slightly higher than the acid requirements of the process.
  • GWA or even trim C0 2 C0 2 gas having a purity of greater than 90% C0 2
  • a net removal of sulfur from the black liquor or other lignin source can be provided, thus enhancing the mills sodium/sulfur balance resulting in significant make up chemical cost savings.
  • this front end acid can be supplemented by the brine (which will be high in multivalent cations and thus facilitate lignin coagulation).
  • the wet gas sulfuric acid (WSA) process for converting waste sulfur to acid can be used to manage waste sulfur streams.
  • WSA wet gas sulfuric acid
  • its use in a Kraft mill to create a sulfur purge is unique, especially when used in the service of an acidification process and/or lignin precipitation process.
  • 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.
  • 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.
  • an optional oxidation stage can be performed to eliminate any carryover total reduced sulfur (TRS). Oxidation can be performed before or after a first solid/liquid separation but is preferred after degassing.
  • a degassed composition can then be introduced to a coagulating/flocculating system.
  • 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).
  • the lignin particles can be isolated by using barrier filters such as an Eaton unit with 15 - 100 micron slots.
  • the single step deep acidification and degassing process can be followed by a solid-liquid separation process as described in more detail below.
  • the lignin containing materials can be washed prior to implementation of a subsequent step or stage of the process.
  • the process is controlled in a manner that results in the fractionation of lignin materials.
  • By implementing an inter-stage wash procedure different lignin streams are produced that can be used to produce various lignin fractions.
  • the various lignin materials throughout the lignin isolation/extraction process can be subjected to interstage or end stage washing steps.
  • Sodium and other carryover from the black liquor are impurities that decrease the value of the product lignin.
  • sodium and other carryover components must be washed out of the product lignin and recovered.
  • Some but not all multivalent cation coagulants can be washed off the lignin with acid treatment.
  • a counter current acid wash can be used to affect internal recovery of coagulant with minimal impact on final lignin product composition or properties.
  • the water used to wash product lignin or lignin intermediates will have costs associated with its input and eventual evaporation.
  • the wash can be designed to acidify to a minimum pH and dewater to a maximum solids content, resulting in minimization the quantity of acid needed.
  • Certain aspects of the invention can include multistage counter current clean water wash and/or weak acid wash(es) (See FIG. 5 and FIG. 6).
  • Counter current washing of solids on belt washers is a well-known practice. In this case, however, an enclosed system is preferred that facilitates gas collection and minimizes interstage cake handling.
  • solution thermodynamics are controlled to either avoid loss of colloidal stability and lignin dissolution, or use partial colloidal stability and dissolution to affect a lignin fractionation in a controlled manner.
  • Counter current processing can be coupled with cross flow extractions and cross flow addition of flocculants, recycled dissolved and suspended lignin, and acid in order to achieve desired profiles along the washing train. Profiles include pH, zeta potential, and particle size of the materials being processed. An example of a sequence is given below, but it is recognized that cross flow additions and extractions can be utilized in different sequences in order to affect the desired combination of“profiles.”
  • an optional interstage wash can be performed on dirty liquor (combination of first stage filtrate and cloth wash or equivalent) based on solubility as a function of % solids and % suspended solids. Conditions can be optimized to minimize yield loss (zeta potential can be optimized while minimizing black liquor solids).
  • an interstage wash can be performed with clean water. It is noted that the % solids and % suspended solids determine solubility. It is contemplated that the more soluble material is lower molecular weight (LMW), more highly polar (oxidized) material.
  • LMW lower molecular weight
  • Lignin fractions can be separated based on zeta potential and solubility after precipitation with the different lignin fractions having different characteristics.
  • the water soluble material can be recovered by first oxidation then further acidification, which results in an increase in polarity and a decrease in molecular weight. This can go all the way to vanillin mixtures, vanillin being a phenolic aldehyde with the molecular formula CsHgCL.
  • the lignin intermediate is washed well before exposure to acid.
  • a partial interstage wash and partial gas separation can be affected by using recycled filtrate and wash from a filter cloth (“cloth wash”) that has been oxidized and slightly acidified. Coagulants can be added here as well. This rewet and displace will prevent gassing off the first solid/liquid separator (press) when it opens while minimizing problems with cake release and yield loss.
  • subsequent acidification can be split into two stage acid addition so that a first stage is pH 7 (around) to singularly gas off H 2 S and subsequent acid to target pH 2 to 4, where C0 2 gasses off.
  • flocculating multivalents are added to the first stage.
  • the inventors found that by varying washing conditions, they not only vary the amount of lignin that re-dissolves but that they also vary the particle size of remaining lignin and the properties of the dissolved lignin. These results imply that the most easily washed material tends to have lower molecular weight and higher functional group content. The undissolved lignin in suspension is easier to filter indicating greater particle size and a more narrow size distribution.
  • controlling the washing of lignin cakes or slurries can be used to affect one or more relatively simple and cost effective fractionations of the lignin product between (i) low molecular weight, high functional group content/polarity/reactivity and (ii) high molecular weight, low functional group content/polarity/reactivity.
  • NCF Nucleation/Coagulation/Flocculation
  • 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. Lignin 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.
  • 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.
  • Lignin particles that are formed in mildly alkaline conditions 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.
  • 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.
  • 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.
  • recirculated lignin is enhancing flocculation, as opposed to seed lignin aimed at nucleation.
  • a NCF system 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.
  • LMW molecular weight
  • LFGC lower functional group content
  • 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.
  • the inventors have found that NCF is significantly improved by recycling lignin particles that have already been separated and, preferably, fully acidified and washed.
  • different fractions of the lignin solid particles are either (A) relatively LMW/HFGC (small particles) or (B) relatively HMW/LFGC (larger particles but not flocculated) or (C) average MW/FGC (unfractionated lignin and/or large flocculated particles of lignin).
  • the NCF process can produce all fractions (LMW/HFGC fraction, HMW/LFGC fraction, and average MW/FGC fraction; or a combination of LMW/HFGC fraction and HMW/LFGC fraction; or a combination of HMW/LFGC fraction and average MW/FGC fraction; or a LMW/HFGC fraction; or a HMW/LFGC fraction; or an average MW/FGC fraction (See FIG. 3b).
  • the amount of LMW/HFGC fraction and HMW/LFGC fraction can be adjusted, e.g., to within certain practical limits.
  • 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.
  • 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).
  • a primary control variable of interest i.e ., those variables ranked by their greatest degree of control at minimal cost
  • a primary control variable of interest i.e ., those variables ranked by their greatest degree of control at minimal cost
  • 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.
  • a lignin particle recycle can be used during coagulation.
  • 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.
  • FIG. 3a and 3b illustrates additional embodiments that include additional processes that follow the initial or first NCF process.
  • FIG. 3a depicts the receipt of NCF treated lignin material by solid-liquid separation system 320 wherein SLS produce a retentate the is further oxidized. The oxidized retentate can then be subjected to multistage wash process 312 forming a lignin product.
  • FIG. 3a diagrams a process for the further fractionation of a product from an initial or first NCF process. Lignin material is exposed to a course solid- liquid separation process 320 having a predetermined particle size cut-off. In this particular example the particle size cut-off is 15 to 100 pm, which produces a HMW/LFG lignin product.
  • the retentate is processed as in FIG. 3b.
  • the filtrate is further processed through second NCF process 322.
  • the filtrates can be treated with coagulation agents, steam, and/or oxidized prior to a second solid-liquid separation process 324.
  • Second SLS 324 can be conducted using a fine process with a particle size cut-off of about 1 to 10 pm producing a LMW/HFG lignin product.
  • 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.
  • SLS solid/liquid separation
  • 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.
  • barrier filters such as continuous centrifugal or gravity based (floatation or settling) systems.
  • NCF particle nucleation-coagulation-flocculation
  • 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).
  • 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.
  • a fine screen for fine particles
  • a fine NCF system followed by a fine screen, e.g., for re-dispersed or re-dissolved lignin.
  • 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.
  • 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.
  • 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.
  • 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.
  • the wash solution can comprise 25, 30, 25, 40, 45 to 50 vol. % of the oxidized filtrate.
  • 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.
  • 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.
  • 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
  • at least one of the NCF agent(s) is selected from the group of NCF agents consisting of: metals, recirculated acid
  • 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.
  • 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.
  • 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.

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  • Separation Of Suspended Particles By Flocculating Agents (AREA)

Abstract

L'invention concerne des méthodes, des procédés et des systèmes permettant d'isoler la lignine de la liqueur noire d'une manière plus efficace et rentable.
PCT/US2019/036802 2018-06-12 2019-06-12 Procédés et processus d'isolement/extraction de lignine WO2019241408A1 (fr)

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