Lignin recovery using separation by heat

Abstract

Disclosed herein are methods for separating lignin from a lignin-containing feedstock. In some embodiments, the method comprises incorporating the acidifying agent into an alkaline feedstock by laminar mixing and heating an acidic feedstock which contains lignin. Various procedures may be carried out thereafter including washing or purification.

Description

Lignin Recovery Using Separation by HeatCross Reference to Related Application

[0001] This application claims filing benefit of United States Provisional Patent Application Serial No. 63 / 595,395 having a filing date of 11 / 2 / 2023, which is incorporated herein by reference for all purposes.Federal Research Statement

[0002] This invention was made with government support under grant number DE-EE0008502 awarded by the Department of Energy. The government has certain rights in the invention.Background

[0003] Agricultural residues are an abundant and currently underutilized resource available to advance both energy security and sustainability. Com stover, for example, is the most abundant agricultural residue in the United States, with an estimated 250 million tons / yr available in the U.S. alone. Being renewable carbon, such residues can decrease our reliance on fossil fuels without impacting food supplies - in contrast with today’s bioethanol plants that use corn. Next-generation biorefineries have been developed that use non-food lignocellulosic feedstocks such as corn stover and wheat straw, but the complexity (and cost) of isolating the cellulose from the lignin matrix, along with the structural disruptions to the biomass necessary to improve enzymatic access, has made competition with corn-based biorefineries a challenge.

[0004] To date, lignocellulosic biorefineries have focused on the cellulose and hemicellulose (polysaccharides) portion of the biomass feedstock, but value must also be obtained from the lignin byproduct to improve economic viability.Furthermore, because of its aromaticity and abundance, lignin would be preferred as the feedstock for applications such as phenol-formaldehyde resins, polyurethane foams, carbon fibers, platform chemicals, and even high-value sustainable aviation fuels.

[0005] A significant advantage of agricultural residues is that delignification (i.e. , separation of lignin from the cellulose) is possible with only a mild alkaline pretreatment — as opposed to woody biomass, which requires the aggressive kraftprocess to produce an adequately delignified cellulose stream with desirable properties. Mild alkaline pretreatment with sodium hydroxide has become one of the most relevant pretreatment methods for agricultural (ag) residuals, as it produces a solid, minimally degraded carbohydrate stream, along with an alkaline liquor phase containing largely unaltered (i.e. , native state) lignins with little or no odor (unlike sulfurous kraft lignin). Today, the most commonly practiced method for lignin recovery from ag residues is to acidify the dilute alkaline liquor phase (i.e., with no evaporation of the liquor beforehand) with H2SO4 or HCI, in order to precipitate the solid lignin from the spent alkaline liquor. Filtration or centrifugation is then used to separate the lignin precipitate from the spent alkaline liquor. However, centrifugation is expensive and both capital and energy-intensive, and filtration of lignin suffers from fouling issues.Summary

[0006] In some embodiments of the present disclosure, a method for separating lignin from a lignin-containing feedstock is described. In some embodiments, a method for separating lignin from an alkaline feedstock comprising lignin comprises acidifying the alkaline feedstock and heating the acidified feedstock.

[0007] Further, in some embodiments, a method for separating lignin from a lignin-containing feedstock comprises raising the pH of the lignin-containing feedstock to form an alkaline feedstock, diluting or concentrating the alkaline feedstock, acidifying the alkaline feedstock to form an acidified feedstock, and heating the acidified feedstock.

[0008] In some embodiments of the present disclosure, a lignin agglomerate that floats in the acidified feedstock may be formed after incorporating an acidifying agent using gentle mixing, such as laminar mixing. As an example, the present disclosure describes methods such as those described above, wherein the step of acidifying the alkaline feedstock comprises adding an acidifying agent to the alkaline feedstock. In some embodiments of the present disclosure, the acidifying agent is incorporated into the alkaline feedstock by laminar mixing. Further, incorporating the acidifying agent into the alkaline feedstock may form a lignin agglomerate within the acidified feedstock. This lignin agglomerate may float in the acidified feedstock, and the step of heating, as described above, may cause the lignin agglomerate to contract to form a lignin cake which floats on the acidified feedstock.

[0009] These and other embodiments of the present disclosure will be further described.Brief Description of the Figures

[0010] A full and enabling disclosure of the present subject matter, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which:

[0011] FIG. 1 is a schematic showing how the process of heating after acidification under gentle stirring creates a floating, microbubble-containing, agglomerated lignin, which upon further heating will coalesce to form a microbubblecontaining, floating lignin cake.

[0012] FIG. 2A-2D are images showing the formation of a floating lignin cake after heating an acidified, microbubble-containing feedstock which comprises lignin.

[0013] FIG. 3A shows the increase in wt % dry solids in the lignin-containing phase created by acidification to pH 2 (triangle marked line) or pH 4 (square marked line) upon heating compared to the initial black liquor wt % dry solids (dashed line).

[0014] FIG. 3B shows the increase in wt % lignin in the lignin-containing phase created by acidification to pH 2 (triangle marked line) or pH 4 (square marked line) upon heating compared to the initial black liquor wt % lignin (dashed line).Detailed Description

[0015] Reference will now be made in detail to various embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each embodiment is provided by way of explanation of the subject matter, not limitation thereof. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment may be used in another embodiment to yield a still further embodiment.

[0016] As used herein, the terms “black liquor” and “alkaline feedstock” are used interchangeably to refer to a lignin-containing solution which has a basic pH.

[0017] Generally, the method relates to separating the lignin from a lignincontaining feedstock. Said feedstock can be derived from a variety of sources;however, in some embodiments, the feedstock may be derived from bio-based materials. The method taught herein has several advantages, including, but not limited to, reduced impurity content, increased ease of separation, and reduced separation times. For instance, the presently described method allows for the separation of lignin from a lignin-containing feedstock without having to employ timeconsuming and / or expensive centrifugation and / or filtration steps.

[0018] In some embodiments of the present disclosure, the lignin-containing feedstock can be derived from a range of lignocellulosic bio-based source materials, including both woody and non-woody sources. Woody lignocellulosic biomass can be sourced from forests, agriculture, or any other source and can encompass hardwood and / or softwood source materials. For example, fast-growing tree species such as hybrid willow (Salix) and poplar as have been developed for production in agricultural settings can be utilized. Perennial and annual grasses can provide non- woody lignocellulosic source materials. Examples of grass source materials can include, without limitation, switchgrass (Panicum virgatum), miscanthus (Miscanthus spp. Anderss.), canary grass (Phalaris arundinacea), giant reed (Arundo donax L.), alfalfa (Medicago sativa L.), sorghum (Sorghum bicolor) and Napier grass (Pennisetum purpureum).

[0019] In some embodiments of the present disclosure, agriculture systems can be a source of agricultural-derived feedstock. Agricultural systems can produce several different types of non-woody lignocellulosic biomass materials including primarily cellulosic materials such as plant leaves and higher lignin-content materials such as stems and stalks. Harvesting of cereals, vegetables, and fruits can provide lignocellulosic biomass source materials. Agricultural residues including field residues and processing residues can provide lignocellulosic source materials. Field residues include materials left in an agricultural field after harvesting the crop, and can include, without limitation, straw and stalks, leaves, and seed pods. Processing residues, such as husks, seeds, bagasse and roots, include those materials left after the processing of the crop into a desired form. Examples of agricultural residue source materials can include, without limitation, rice straw, wheat straw, com stover, and sugarcane bagasse.

[0020] Other waste streams such as municipal waste, industrial waste, construction waste, sawmill waste, etc., can provide a lignocellulosic biomass sourcematerial. For instance, yard waste, holiday waste, etc. can provide a lignocellulosic source material in some embodiments.

[0021] In some embodiments, the lignin-containing feedstock can be derived from a source material by preprocessing. In one embodiment, the lignin-containing feedstock can include lignin that has been previously separated from other components of a lignocellulosic biomass source material, and the cohesive structure of the natural biomass source material can have been altered and / or damaged by the pretreatment. The lignin-containing feedstock can include other components of a lignocellulosic biomass source material in combination with the lignin. However, one or more other components of a lignocellulosic biomass source material may have been removed from the source material or altered as compared to the original source material during one or more pretreatment processes. In one embodiment, the lignincontaining feedstock can include lignin in an amount of 5 wt.% or greater, such as 20 wt.% or greater, such as 30 wt.% or greater, such as 40 wt.% or greater, such as 55 wt.% or greater, in some embodiments. Processes described herein are not limited to high lignin-content feedstocks, however, and in other embodiments the feedstock can include lignin in an amount of about 30% or less. For instance, the feedstock may comprise lignin in an amount of between 5 wt.% and 40 wt.%, such as between 10 wt.% and 30 wt.%, such as between 15 wt.% and 25 wt.%.

[0022] Said pre-processing may be used to convert the lignocellulosic to a particulate. For instance, pre-processing may include removing debris from the lignocellulosic biomass, chopping, grinding, shredding, tearing, mulching or pulverizing the lignocellulosic biomass, and adding it to a solvent.

[0023] Because lignin-containing feedstocks may be sourced from a variety of places, such as agricultural and municipal ones as discussed above, the lignincontaining feedstock can contain components other than lignin, such as ash and carbohydrates.

[0024] In some embodiments of the present disclosure, the lignin-containing feedstock may undergo pre-treatment in order to make the lignin-containing feedstock have an alkaline pH. Without wishing to be limited by any particular theory, making the lignin-containing feedstock have an alkaline pH can increase the solubility of lignin in a polar solvent.

[0025] The pre-treatment processes which can be used are not particularly limited. In some embodiments of the present disclosure an alkaline feedstock maybe obtained by an alkaline pre-treatment process. An alkaline feedstock can be obtained from the alkaline pre-treatment process and can be further treated prior to the herein described separation process. Generally, however, the pre-treatment may include adding a base to the lignin-containing feedstock. Examples of potential bases for use in the process of pre-treating the lignin-containing feedstock include, but are not limited to, hydroxides of sodium, potassium, calcium, magnesium, aluminum, ammonium or mixtures thereof. Further, pre-treating may be performed to increase the pH of the lignin-containing feedstock to be greater than 9, such as greater than 11 , such as greater than 13. The lignin-containing feedstock, when treated to be alkaline, may form an alkaline feedstock.

[0026] While the alkaline pre-treatment process may be used, the present disclosure is not limited to alkaline pre-treatment processes as described above. For instance, the pre-treatment may comprise a soda ash pulping process, a kraft process or other alkaline pre-treatments known in the art.

[0027] In some embodiments of the present disclosure, following the formation of the alkaline feedstock, the method may comprise the step of physically separating the alkaline feedstock from undissolved solids. For instance, undissolved solids may comprise a large amount of carbohydrates, such as cellulose.

[0028] In some embodiments of the present disclosure, the method may comprise diluting or concentrating the alkaline feedstock. For instance, the alkaline feedstock may have additional solvent added so as to change the concentration of the lignin within the feedstock. Alternatively, the alkaline feedstock may have solvent removed to concentrate the alkaline feedstock.

[0029] In some embodiments, following the separation of the alkaline feedstock from remaining solids, a treatment that can cause the precipitation of a portion of the alkaline feedstock can be carried out. In such an embodiment, the precipitate can include lignin, which can then be further treated according to the disclosed methods. Such an alkaline feedstock precipitation process can separate a portion of the impurities contained in the alkaline feedstock such as ash, metals, hemicellulose, etc., from the alkaline feedstock.

[0030] Generally, the step of precipitating lignin may be applicable to alkaline feedstocks which comprise lignin, regardless of whether they were obtained from the alkaline pre-treatment process as described above. For instance, the method of precipitation generally includes the steps of first supplying an alkaline feedstockwhich comprises lignin, and acidifying the alkaline feedstock. Acidification may be carried out in a variety of methods, such as by the addition of an acidifying agent which may comprise liquid or aqueous acids, such as, but not limited to, inorganic acids such as sulfuric acid, hydrochloric acid or nitric acid, or mixtures thereof, or organic acids, such as acetic acid, formic acid, citric acid or lactic acid, or mixtures thereof. During the acidification of the alkaline feedstock, the alkaline feedstock may be subject to stirring to promote even distribution of the acidifying agent. After the alkaline feedstock has been acidified, the precipitated lignin may form a lignin agglomerate.

[0031] In some embodiments, the step of acidifying the alkaline feedstock as is described above may entail adding acid in an amount sufficient to obtain an acidified feedstock with a pH of less than 6, such as less than 4, such as less than 2.

[0032] The present inventors have found that the manner of stirring used to distribute the acidifying agent within the alkaline feedstock may affect the formation of a floating, microbubble-containing lignin agglomerate. Without wishing to be limited to any particular theory, the present inventors have found that strong stirring while incorporating the acidifying agent prevents formation of the floating, microbubble-containing lignin agglomerate. Instead, under strong stirring, particles of lignin may become finely dispersed within the feedstock or sink. Rather, the present inventors have found that gentle mixing may promote the formation of a floating, microbubble-containing lignin agglomerate, such that the lignin particles do not become finely dispersed within the feedstock or sink. Such gentle mixing may be characterized by mixing which causes laminar or low-transitional flow. Such laminar or low-transitional mixing may have a mixing Reynolds number of less than 2100, such as less than 1500.

[0033] In some embodiments of the present disclosure, the acidified feedstock may be heated in order to facilitate separation of the agglomerate from the acidified feedstock. Heating the acidified feedstock may aid in both the separation and the purification of the lignin agglomerate, as the step of heating may excise a portion of the residual acidified feedstock entrained within the lignin agglomerate.

[0034] The step of heating may be performed in a variety of manners, though in some embodiments the step of heating comprises heating the acidified feedstock and lignin agglomerate to greater than 40 degrees Celsius, such as greater than 50 degrees Celsius, such as greater than 60 degrees Celsius, such as greater than 80degrees Celsius, such as greater than 90 degrees Celsius. Stated differently, the acidified feedstock comprising the lignin agglomerate may be heated to less than 100 degrees Celsius, such as less than 80 degrees Celsius, such as less than 60 degrees Celsius. In some embodiments of the present invention, the acidified feedstock and lignin agglomerate may be heated between 40 and 99 degrees Celsius, such as between 60 and 85 degrees Celsius.

[0035] In some embodiments of the present disclosure, the step of heating and the step of acidifying the alkaline feedstock may be reversed. For instance, the alkaline feedstock may be heated to the temperature as described above, and then it may be acidified as described above to create the desired floating, microbubblecontaining lignin agglomerate. In some embodiments of the present disclosure, the alkaline feedstock may be heated before and after acidification.

[0036] As described above, a portion of the acidified feedstock may be excised from the lignin agglomerate. As may be seen in FIG. 2A, the initially formed floating, microbubble-containing lignin agglomerate may be present in the entire volume of a container before any heating is applied. Thereafter, in FIGS. 2B-2D, the lignin agglomerate takes up a progressively smaller fraction of the volume of the container as temperature is increased and the acidified feedstock is excised. For instance, the lignin agglomerate may take up a quarter of its original volume when heated to 65 degrees Celsius.

[0037] Without wishing to be limited by any particular theory, it is believed by the present inventors that the step of heating may allow for the lignin agglomerate to contract, thereby expelling the majority of the residual acidified feedstock contained therein. With the expulsion of the majority of the acidified feedstock, the majority of the impurities contained within the initial alkaline feedstock are expelled from the lignin agglomerate, and instead remain in the acidified feedstock. For instance, the acidified feedstock may comprise impurities due to the source from which it was derived and / or impurities due to the pretreatment process(es) carried out on the lignocellulosic biomass source material prior to the separation process. Impurities may include sodium and potassium as stated above, as well as other metals, collectively known as ash, and carbohydrates. Said impurities may be present in the alkaline feedstock in an amount greater than 10 wt. %, such as greater than 20 wt. %, such as greater than 40 wt.%, such as greater than 60 wt.% or more. Of course, the acidified feedstock is not limited to any particular level of impurities, andexamples are provided merely for the sake of description of possible embodiments encompassed herein. The lignin agglomerate contracting may decrease the weight percent of impurities within the lignin cake. For instance, the lignin cake may have an impurity content of between 0 and 50 wt.%, such as between 1 and 25 wt.%, such as between 5 and 20 wt.%.

[0038] Without wishing to be limited by any particular theory, the present inventors have found that acidified feedstocks derived from different sources may benefit from different heating step temperatures. For instance, when the acidified feedstock comprises softwood the acidified feedstock and lignin agglomerate may be heated to temperatures that may exceed 55 degrees, but if the acidified feedstock comprises miscanthus, the acidified feedstock and lignin agglomerate may be heated to a temperature that may have a maximum of 55 degrees Celsius.

[0039] The step of heating the acidified feedstock may involve the temperatures described above. However, it has been noted by the present inventors that heating the acidified feedstock above the boiling temperature of the solvent, in this case 100 degrees Celsius for a water-based solvent, can lead to the lignin agglomerate becoming dispersed in the acidified feedstock, making removal therefore more difficult. Thus, the step of heating the acidified feedstock may comprise heating the acidified feedstock to a temperature below its boiling point.

[0040] In some embodiments, the step of heating the acidified feedstock may be performed at elevated pressures, such as pressures greater than 1 atmosphere. For instance, elevated pressures may be greater than 1 atmosphere, such as 2 atmospheres, such as 3 atmospheres, such as 4 atmospheres. Elevated pressures may allow for the step of heating the acidified feedstock to exceed temperatures of 100 degrees Celsius, such as greater than 120 degrees Celsius, such as greater than 150 degrees Celsius.

[0041] After the step of heating the acidified feedstock and the contraction of the floating, microbubble-containing lignin agglomerate, the lignin agglomerate may form a floating, micro-bubbling containing lignin “cake”, as shown in FIG. 2D. This lignin cake has a relatively lower content of impurities, as is discussed above. Without wishing to be bound to any particular theory, the lignin cake may float because of the generation of gaseous carbon dioxide resulting from the acid-base reaction when the alkaline feedstock is acidified, and the microbubbles formed may have the propensity to associate with the lignin, and thereby may float the lignin agglomerate or cake.However, as discussed above, the lignin agglomerate and the succeeding lignin cake are able to form only if gentle stirring is employed during acidification. The flotation of the lignin cake is an unexpected result-lignin by itself has a density of about 2 g / cc; thus, the self-forming, floating nature of the initially formed lignin agglomerate - and of the final lignin cake - is unexpected.

[0042] The self-forming and floating nature of the lignin cake presents a facile route to separating the lignin from the feedstock from which it is derived. The lignin cake may be recovered by removing the acidified feedstock, or by directly removing the floating lignin cake by mechanical means. Regardless of the method of removal, the step of removing the floating lignin cake from the acidified feedstock may be performed without the use of centrifugation or filtration.

[0043] Thereafter, the lignin cake may be subject to a washing step. This washing step generally comprises washing the lignin cake with a solvent to remove remaining ash and carbohydrates. For example, the lignin cake may be washed with water, which can remove a large amount of ash. Alternatively, the washing step may be similar as to what is described in U.S. Patent Number 10,053,482, which is incorporated by reference herein. Briefly, the process described in the previously named patent may comprise a lignin recovery process which utilizes one or more separation stages within which a lignin-containing feed is combined with a solvent solution. Upon combination and optional heating, the mixture comprising the lignin and the solvent solution separates to form two liquid phases: a first solvent-rich liquid phase in a first fraction and a second lignin-rich liquid phase in a second fraction.The two phases may have different compositions in terms of lignin content, impurity content, density and viscosity, allowing for the separation of the same.Detailed Description of the Figures

[0044] FIG. 1 is a schematic showing the contraction of a floating, microbubblecontaining lignin agglomerate to form a floating, microbubble-containing lignin cake when the lignin agglomerate is subjected to heat. The acidified feedstock 110 contained within vessel 120 is subject to heating as described above. After heating, lignin cake 140 is formed. Additionally, lignin cake 140 may be separated from spent feedstock 130 contained within vessel 120 by any of the means as described above. As seen in FIG. 1 , acidified feedstock 110 comprises a microbubble-containing lignin agglomerate dispersed throughout.

[0045] FIG. 2A-D is a series of photographs showing the formation of a floating, microbubble-containing lignin agglomerate immediately after the acidification of an alkaline feedstock under gentle stirring. First, in FIG. 2A, the lignin agglomerate immediately forms as a separate phase present as a bubbly foam throughout the acidified feedstock liquid. Next, after heating to 45 degrees Celsius in FIG. 2B, the lignin agglomerate begins to contract and floats on top of the denser, acidified feedstock phase. This process of contraction of the lignin agglomerate to form a floating lignin cake, due to excision of the acidified feedstock, is carried out further in FIGS. 2C at 55 degrees Celsius and 2D at 65 degrees Celsius.

[0046] Further, during the process of heating, the lignin agglomerate may take up a progressively smaller fraction of the volume of the container as it contracts to form a cake. As shown in FIG. 2A, the acidified feedstock may comprise lignin particles dispersed throughout said phase. Upon heating to 45 degrees Celsius, the lignin cake may be present in 60 vol.% of the container. This percent may further decrease upon heating to 55 and 65 degrees Celsius, where the lignin cake may be present in 40 and 25 vol.% of the container respectively.

[0047] FIG. 3A shows the increase in wt.% of dry solids in the cake upon acidification to pH 2 (triangle marked line) or pH 4 (square marked line) and heating compared to the initial black liquor wt.% of dry solids (dashed line). As can be seen in FIG. 3A, the wt.% of dry solids in the cake increase to 18 wt.% when heated to 55 degrees Celsius for the pH 2 feedstock. The cake derived from the pH 2 feedstock increases in wt.% of dry solids to 26 wt.%, 30 wt.% and 31 wt.% when heated to 65, 75 and 85 degrees Celsius respectively. The pH 4 cake increases in wt.% of dry solids to 11 wt.%, 19 wt.%, 25wt.% and 30 wt.% when heated to 55, 65, 75 and 85 degrees Celsius respectively.

[0048] FIG. 3B shows the increase in wt.% of lignin in the cake upon acidification to pH 2 (triangle marked line) or pH 4 (square marked line) and heating compared to the initial black liquor wt.% lignin (dashed line). As can be seen in FIG. 3B, the wt.% of lignin in the cake increases to 57 wt.% when heated to 55 degrees Celsius for the pH 2 feedstock. The same cake increases in wt.% of lignin to 68 wt.%, 72 wt.% and 74 wt.% when heated to 65, 75 and 85 degrees Celsius respectively. The pH 4 cake changes in wt.% lignin to 64 wt.%, 67 wt.%, 73wt.% and 72 wt.% when heated to 55, 65, 75 and 85 degrees Celsius respectively.

[0049] The present invention may be better understood with reference to the examples, set forth below.Example 1 : Separation of Lignin

[0050] Raw corn stover and wheat straw biomass in this study were obtained from both Idaho National Laboratory (Idaho Falls, ID) and Clemson University’s Simpson Station Agronomic Unit (Pendleton, SC), respectively; the softwood pine chips were obtained from a commercial supplier. The sodium hydroxide pellets for alkaline pretreatment were purchased from Fischer Scientific. Deionized water for alkaline pretreatment was produced via either a Milli-Q Advantage A10 or a Culligan deionization system combined with a Milli-Q reference system (Millipore Z00QSV0WW). For both the reference titrations and the present method, sulfuric acid (95-98% ACS, cat. no. BDH3072-2.5LG) was purchased from VWR and diluted to 20 wt % with deionized water. Deionized water was used for all experiments and procedures, onwards. The acetone (>99.5%, cat. no. BDH1101-19L) for ALPHA experiments and the 72% sulfuric acid (RICCA R8191600-1A) for sugars analyses were obtained from VWR. For heteronuclear single quantum coherence (HSQC) NMR, methyl sulfoxide-d6 (99.9% atom D, cat. No. AC166290250) was purchased from Fischer Scientific.

[0051] The pretreatment procedure was adapted from previous work, and is described in brief next: biomass was pretreated in either a 20-L digester (RegMed AU / E-20, Sao Paulo, Brazil) for corn stover, or a 2-L Parr reactor (Moline, IL) for wheat straw and softwood. All were treated at a 9:1 alkaline liquor: biomass (dry) ratio (wt / wt), with sodium hydroxide added at a ratio of 130mg NaOH / g dry biomass. The digester was heated to either 140 °C (corn stover) or 160 °C (wheat straw and softwood) and held at temperature for 30 min before being allowed to cool to <50 °C (overnight for the 20-L digester, ~2-3 h for the 2-L Parr reactor). The liquor was then separated from the solids and stored prior to use.

[0052] The method for determining lignin content for both lignin derived from the present process and the spent liquor was adapted from literature. In particular, 0.3 g of lignin dried at 105 °C for 4-5 h was weighed into a glass pressure tube (Ace Glass cat. no. 8648-113, max 150 psig @ 120 °C) to which 3 mL of 72% sulfuric acid was added. The combination was macerated with a glass stirring rod, and the test tube was submerged into a 30 °C water bath for 1 h, stirring every 15 min with the same glass stirring rod. Subsequently, 84 mL of water was added, and the capped tubeswere autoclaved for 1 h at 121 °C using the “liquids” setting. The resulting solutions were then vacuum-filtered through a Whatman #4 filter, and the residue was dried and weighed to determine the acid-insoluble lignin content. A sample of filtrate was collected and diluted by a factor of 60-85 to obtain an absorbance value between 0.25 and 1 ; the sample was then filtered through a 22 pm syringe filter (VWR, cat. no. 76479-010) prior to analysis on a UV spectrophotometer (VWR UV-3100PC). dm Absorbance values were taken at 280 nm, and an absorptivity coefficient of 110 -3g*cm was used for calculating the acid-soluble lignin content.

[0053] In order to determine carbohydrate content, 100 mg of lignin and 1 mL of 72% sulfuric acid were held at 30 °C for 1 h in a glass pressure tube (Ace Glass cat. no. 8648-09, 150 psig @ 120 °C max), stirring every 15 min. Following this, 28 mL of DI water were added to the tubes which were then capped and placed in an autoclave set at 121 °C for 1 h on the liquids setting. An HPLC unit equipped with an Aminex HPX-87H column and a Waters 2414 Refractive Index detector was used for analysis of the samples, following the cooling of the autoclaved samples and filtration through a 22 pm syringe (VWR, cat. no. 76476-010) directly into the autosampler vials.

[0054] Ash content was determined using a bulk ash method. In brief, 0.5-1 g of lignin was weighed into a crucible and heated in air for 4 h at 750 °C. The crucibles were then cooled to room temperature and weighed to determine ash content.

[0055] A glass burette was used to rapidly add the appropriate acid volume to 250 g of alkaline liquor at room temperature in a 1000-mL beaker so as to achieve a pH of 2 or 4. The solution was stirred with a spatula by hand for up to 10 s, whereupon it achieved a smooth, foam-like appearance, with the total volume increase upon acidification approaching 40%. The beaker was then placed in a gravity convection oven set at 115 °C for the heating step. Once the desired temperature was reached (55-85 °C in 10 °C increments), the two phases (a floating, cleaned lignin cake and a clear, impurities-rich and denser, spent-liquor phase) were separated by pouring off the spent-liquor phase while restraining the floating, lignincake phase with a spatula. This lignin-rich cake was first air-dried in a fume hood overnight and was then placed in an oven at 95°C until no mass loss occurred, indicating dryness.

[0056] As shown in FIGS. 2A-2D, the process of acidifying and heating the alkaline feedstock allowed for a lignin cake to form that, because it floats, was easily removed from the remaining acidified feedstock. Additionally, since most of the impurities remain within the acidified feedstock, impurity content in the cake was reduced. Various impurity contents are shown below in Table 1.

[0057] The above table demonstrates that lignin cakes produced by the method disclosed herein have low impurity content, especially compared to the alkaline feedstocks from which they are derived. As compared the alkaline feedstock from which they are derived, which typically have an ash content of about 40 wt.% and a carbohydrate content of 20 wt.%, the lignin cakes have a much lower content of impurities, ranging from 5 wt.% to 7 wt.% for ash and carbohydrates, respectively.Example 2: Purification by Washing Step

[0058] The lignin cakes, particularly those derived from com stover, were subject to further purification by water washing as described above. The results of said water washing are shown below in Table 2.Water: Wet Ash Content (wt %) Lignin Content (wt Lignin Cake Ratio %)2: i3.1 ± 0.1 74.0 ± 1.65: 1 1.5 =1= 0 1 76.7 ± 1.810: 1 0.4 ± 0.3 79.2 ± 4.3

[0059] As can be seen in Table 2 above, water washing reduced the content of ash within the corn stover cake dramatically, with only a 2:1 ratio of water to wetlignin cake. Further washing decreased the ash content even further. Carbohydrates were also reduced upon washing, but to a smaller extent (between 10-20% for the three washing ratios tested).

[0060] While certain embodiments of the disclosed subject matter have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the subject matter.

Claims

WHAT IS CLAIMED IS:1 . A method for separating lignin from an alkaline feedstock comprising lignin, the method comprising: acidifying the alkaline feedstock; and heating the acidified feedstock.

2. The method of claim 1 , wherein the step of acidifying the alkaline feedstock comprises adding an acidifying agent to the alkaline feedstock.

3. The method of claim 2, wherein the acidifying agent is incorporated into the alkaline feedstock by laminar mixing.

4. The method of claim 3, wherein the step of incorporating the acidifying agent into the alkaline feedstock forms a lignin agglomerate in the acidified feedstock.

5. The method of claim 4, wherein the lignin agglomerate floats in the acidified feedstock.

6. The method of claim 5, wherein the step of heating causes the lignin agglomerate to contract to form a lignin cake that floats on the acidified feedstock.

7. The method of claim 1 , further comprising removing the acidified feedstock.

8. The method of claim 1 , further comprising removing the lignin from the acidified feedstock.

9. The method of claim 1 , wherein the alkaline feedstock comprises black liquor.

10. The method of claim 1 , wherein the acidified feedstock is heated to between40 and 95 degrees Celsius.11 . The method of claim 1 , wherein the alkaline feedstock comprises an agriculture-derived feedstock.

12. The method of claim 1 , wherein the alkaline feedstock comprises a corn stover derived feedstock.

13. The method of claim 1 , wherein the alkaline feedstock comprises a wheat straw derived feedstock.

14. The method of claim 1 , wherein the alkaline feedstock comprises a hardwood derived feedstock.

15. The method of claim 1 , wherein the alkaline feedstock comprises a softwood derived feedstock.

16. The method of claim 1 , wherein the step of heating the acidified feedstock comprises heating the acidified feedstock to below its boiling point.

17. The method of claim 1 , wherein the step of heating the acidified feedstock occurs at greater than one atmosphere.

18. The method of claim 1 , wherein the alkaline feedstock comprises sodium hydroxide.

19. The method of claim 1 , wherein the acidified feedstock has a pH less than 4.

20. The method of claim 1 , wherein the acidified feedstock has a pH less than 2.21 . A method for separating lignin from a lignin-containing feedstock, the method comprising: raising the pH of the lignin-containing feedstock to form an alkaline feedstock; diluting or concentrating the alkaline feedstock; acidifying the alkaline feedstock to form an acidified feedstock; and heating the acidified feedstock.

22. The method of claim 21 , wherein the lignin-containing feedstock comprises a corn stover derived feedstock.

23. The method of claim 21 , wherein the lignin-containing feedstock comprises a wheat straw derived feedstock.

24. The method of claim 21 , wherein the step of acidifying the alkaline feedstock comprises adding an acidifying agent to the alkaline feedstock.

25. The method of claim 24, wherein the acidifying agent is incorporated into the alkaline feedstock by laminar mixing.

26. The method of claim 25, wherein the step of incorporating the acidifying agent into the alkaline feedstock forms a lignin agglomerate in the acidified feedstock.

27. The method of claim 26, wherein the lignin agglomerate floats in the acidified feedstock.

28. The method of claim 27, wherein the step of heating causes the lignin agglomerate to contract to form a lignin cake that floats on the acidified feedstock.

29. The method of claim 21 , wherein the acidified feedstock has a pH less than 4.

30. The method of claim 21 , wherein the step of heating the acidified feedstock comprises heating the acidified feedstock to below its boiling point.31 . The method of claim 21 , wherein the acidified feedstock is heated to between 40 and 95 degrees Celsius.

32. The method of claim 21 , wherein the lignin-containing feedstock comprises an agriculture-derived feedstock.

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