WO2018204061A1

WO2018204061A1 - Treatment of brown stock

Info

Publication number: WO2018204061A1
Application number: PCT/US2018/027782
Authority: WO
Inventors: Bruno Marcoccia; Rowan S. ATALLA; Harshad PANDE; Rajai H. Atalla
Original assignee: Cellulose Sciences International, Inc.; Domtar Paper Company, Llc
Priority date: 2017-05-02
Filing date: 2018-04-16
Publication date: 2018-11-08

Abstract

Embodiments of the invention provide methods of treating brown stock from a pulp mill to improve its delignification and/or bleaching response, including treating brown stock with an alkali alcohol/water co-solvent to yield a post-treatment solution, neutralizing the post-treatment solution, and recovering the treated brown stock.

Description

TREATMENT OF BROWN STOCK

INTRODUCTION

Lignocellulose is a complex substrate composed of a mixture of carbohydrate polymers (namely cellulose and hemicellulose) and lignin. Even though cellulose is by far the most common naturally occurring polymer, its chains are virtually locked up by lignin, hemicelluloses and, particularly, adjacent chains of cellulose. Many technical difficulties are associated with the separation of the lignocellulosic components due their complex nature. Although numerous methods exist for freeing cellulose molecules from their surroundings, these methods are typically expensive, and generally done under severe conditions involving high temperatures, long residence times and a variety of more or less troublesome chemicals.

The pulping of wood is one such well-known process in which cellulosic fibers are separated from their surroundings. Several types of pulping processes, e.g., chemical, mechanical, chemi- mechanical) are known, and each subjects the wood chips (the most typical form of feedstock for pulping mills) to severe and expensive conditions.

Two chemical pulping processes are common. The first is the sulfite process, usually carried out under acid, neutral or mildly alkaline conditions usually at temperatures between 125°C and 150°C. The second is the kraft process wherein the raw lignocellulosic, usually a chipped wood or an agricultural residue such as sugar cane bagasse, is subjected to treatment in alkaline conditions at 150°C to 190°C in an aqueous solution containing a mixture of sodium hydroxide and sodium hydrosulfide.

The operation of a pulping mill involves many production stages, usually coupled with intermediate storage tanks. Each stage has different decision variables, such as steam / water / chemical input, etc. The overall aim is maximizing efficiency and aspects of production while minimizing costs. Such aspects include, for example, recycling and recovery of reagents, the productivity and selectivity of the reaction, the quality of the product as well as aspects related to the safety and to environmental protection.

BRIEF DESCRIPTION

This application provides treatment of brown stock, which is an intermediate product in pulping technologies, to deaggregate the cellulose fibers and make the brown stock more accessible to chemical and biochemical reactions, e.g., bleaching. Brown stock is subjected to bleaching to make it suitable for making certain paper products such as personal hygiene products. It has been surprisingly found that in the pulping process, despite the fact that brown stock has been subjected to digestion to separate the cellulose fibers, the instant method provides a brown stock that has deaggregated celluloses, and reduces the amounts of bleaching chemicals that must be used on brown stock while at the same time producing a bleached pulp that is nanoporous and that has superior properties for many applications in the production of paper as well as personal hygiene products.

The present treatment of brown stock provides a method of increasing the accessibility of cellulose chains for biochemical and chemical reaction in which novel forms of cellulose, collectively known as nanoporous celluloses or laterally expanded celluloses, are formed by treating conventional sources of cellulose with an alkali in a co-solvent. The co-solvent suitably includes water and a second solvent that is polar and fully water-miscible, typically a lower alcohol or a polyol. The method includes stabilizing these nanoporous celluloses so that the alkali does not facilitate conversion to cellulose II. The process opens up the aggregated domains, making the celluloses more accessible for further chemical reaction, e.g., bleaching.

The process also provides a post- treatment of these nanoporous celluloses that is especially advantageous for plant operation wherein recycling and recovery of reagents are crucial for efficient and cost-effective operation of a pulping mill. The post- treatment is based on diminishing the alkali (e.g., sodium hydroxide) content of the post-treatment solution by converting it to a salt (e.g., a sodium salt). This is suitably accomplished by adding a neutralizing acid to reduce the pH of the solution sufficiently so that the alkali, e.g., sodium hydroxide, can no longer catalyze the conversion of cellulose to cellulose II and the nanoporosity of the cellulose under treatment is preserved. The neutralizing acid is suitably carbon dioxide. BRIEF DESCRIPTION OF THE DRAWING

The present invention may be better understood and appreciated by reference to the detailed description of specific embodiments presented herein in conjunction with the accompanying drawings of which:

FIG. 1 is a schematic diagram of a typical pulping operation; FIG. 2 is a schematic diagram of a typical bleaching plant; and FIG. 3 is a flowchart illustrating the various steps of the treatment and post-treatment of brown stock including the recycling of reagents.

DETAILED DESCRIPTION

Embodiments of the invention provide a method for treating brown stock to deaggregate its cellulosic fibers. The method also includes a post-treatment that includes diminishing the reagents in the treatment solution and recycling other reagents. The method begins with treating the brown stock with a col-solvent, typically an alkali/alcohol solution, and then reducing the alkali concentration to prevent conversion of the cellulose to cellulose II. The alkali, e.g., sodium hydroxide, is converted to an alkali salt. This is suitably accomplished by a post- treatment, i.e., adding a neutralizing acid, e.g., carbon dioxide (C0₂), to reduce the pH of the solution sufficiently so that the alkali, e.g., sodium hydroxide, can no longer catalyze the conversion to cellulose II, and the nanoporosity of the cellulosic content of the brown stock under treatment is preserved.

In a typical pulping operation, a lignocellulosic feedstock (usually wood chips) of the pulping process start with a lignin content of approximately 25%. After pulping, the lignin content is reduced below 5%. In the raw feedstock, the lignocellulosic fibers are strongly bonded to each other. After the pulping or digesting step, followed by washing to remove residues of the pulping solutions and a screening process to remove knots and shives, the cellulosic fibers are largely separate from each other. At this step, the pulp fibers are referred to as "brown stock", the coloration due to remaining lignin. Before these fibers can be used for many papermaking applications or in the manufacture of personal hygiene products or specialty celluloses, they have to be bleached in a separate step of the overall pulp production process. As demonstrated in the Example below, it was surprising that brown stock which is already the result of rather severe digestion is suitably made nanoporous in accordance with the invention. The exposure to elevated temperatures during pulping causes the polysaccharides in the fiber walls to aggregate very tightly and often occlude lignin within them. As a result, the bleaching process must usually be carried out in multiple steps and relies on the use of strongly oxidative compounds such as chlorine dioxide and hydrogen peroxide in some cases. In some instances, ozone is also used as an oxidant. Before any embodiments of the invention are explained in detail, however, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description, illustrated in the following drawings or exemplified by the Examples. Such description, drawings, and Examples are not intended to limit the scope of the invention as set forth in the appended claims. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Further, no admission is made that any reference, including any patent or patent document, cited in this specification constitutes prior art. In particular, it will be understood that, unless otherwise stated, reference to any document herein does not constitute an admission that any of these documents form part of the common general knowledge in the prior art in the United States or in any other country. Any discussion of the references states what their authors assert, and the applicant reserves the right to challenge the accuracy and pertinence of any of the documents cited herein.

Throughout this disclosure, various aspects of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity, and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, as will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof, as well as all integral and fractional numerical values within that range. As only one example, a range of 20% to 40% can be broken down into ranges of 20% to 32.5% and 32.5% to 40%, 20% to 27.5% and 27.5% to 40%, etc. Any listed range is also easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third, and upper third, etc. Further, as will also be understood by one skilled in the art, all language such as "up to," "at least," "greater than," "less than," "more than" and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. In the same manner, all ratios disclosed herein also include all sub ratios falling within the broader ratio. Further, the phrases "ranging/ranges between" a first indicate number and a second indicate number and "ranging/ranges from" a first indicate number "to" a second indicate number are used herein interchangeably. The foregoing are only examples of what is specifically intended.

Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "comprising," "including," "having," and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. "Comprising" encompasses the terms "consisting of" and "consisting essentially of." The use of "consisting essentially of" means that the composition or method may include additional ingredients and/or steps, but only if the additional ingredients and/or steps do not materially alter the basic and novel characteristics of the claimed composition or method. Unless specified or limited otherwise, the terms such as "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings.

Unless otherwise noted, technical terms are used according to conventional usage. However, as used herein, the following definitions may be useful in aiding the skilled practitioner in understanding the invention:

As used herein, the terms "cellulosic material", "cellulosic feedstock", or "cellulosic substrate" are meant to refer to any type of biomass that contains cellulose or any product containing cellulose from a process, e.g., pulping, e.g., brown stock. Cellulosic materials may include biomass in the form of grasses such as switch grass, cord grass, rye grass, miscanthus, or a combination thereof; sugar-processing residues such as sugar cane bagasse and sugar beet pulp; agricultural wastes such as soybean stover, corn stover; oat straw, rice straw, rice hulls, barley straw, corn cobs, wheat straw, canola straw, oat hulls, and corn fiber; and forestry wastes, such as recycled wood pulp fiber, sawdust, hardwood, softwood, or any combination thereof. Further, the cellulosic material may include relatively low value cellulosic or forestry materials such as recycled newsprint, recycled cardboard and the like. Cellulosic material may also include one or more species of fiber that originate from different cellulosic feedstocks. Wheat straw, barley straw, corn stover, soybean stover, canola straw, switch grass, reed canary grass, sugar cane bagasse, cord grass, oat hulls, sugar beet pulp and miscanthus are particularly advantageous as cellulosic materials due to their widespread availability and low cost.

The terms "lignocellulose", "lignocellulosic material" and "lignocellulosic biomass" are meant to refer to any type of biomass comprising the three polymers of cellulose, lignin and hemicellulose. The term "degree of polymerization" (abbreviated as D.P.) refers to the number of D-glucose monomers in a cellulose molecule. Thus, the term "average degree of polymerization", or "average D.P.", refers to the average number of D-glucose molecules per cellulose polymer in a population of cellulose polymers. As used herein, the terms "treatment," or "treating," in respect of cellulose or a substance containing cellulose are meant to refer to a process or treatment in accordance with embodiments of the invention in which cellulose is made more accessible for chemical reaction, e.g., bleaching, and/or is stabilized so the it remains nanoporous,.

"Treatment solution" in reference to treating cellulosic materials is meant to refer to a solution of an alkali in a co-solvent, typically water and a polar miscible solvent, e.g., sodium hydroxide in ethanol/water co-solvent. "Post-treatment solution" is meant to refer to the treatment solution after reaction with the cellulosic material.

"Modification or degradation" in reference to cellulose is used to refer to the biological, e.g., enzymatic, or chemical-induced alteration of the native structure of cellulose. Such changes and alterations are known to those in the art and include those involved in enzymatic degradation and/or enzymatic or chemical hydrolysis of cellulose, as well as chemical modifications involved in a variety of commercial cellulose-based products, production of paper products, production of alcohols by fermentation of biomass, and generation of hydrogen-rich biofuels. The term "stable" or "stabilizing" in regard to decrystallized cellulose refers to decrystallized cellulose that does not change materially over a selected period of time and under selected conditions.

The term "Kappa number" is an indication of the lignin content or bleachability of a pulp, and has a range of 1-100. The Kappa number of pulp is determined by a standard test (TAPPI T 236 or ISO 302:2004; incorporated herein by reference) in which the Kappa number is a measurement of the amount of a standard potassium permanganate (KMn0₄) solution that the pulp will consume. It is almost proportional to the residual lignin content of the pulp. As lignin is removed from a sample, the Kappa number is reduced. Low Kappa numbers require less KMn04 for sample analysis, and the pulp is a lighter color. The Kappa number is used to estimate the required amount of bleach chemicals to achieve targeted brightness. If the kappa number value is high, the required bleaching chemicals are also high. On the other hand, low kappa pulps are easier to bleach. The Kappa number is widely used as an in-process test in the pulp and paper mill.

Embodiments of the invention include novel methods for treating brown stock from the pulping process to form nanoporous cellulosic fibers, stabilizing the nanoporous celluloses and recycling of reagents. The methods include reacting a brown stock with a treatment solution, which includes an alkali dissolved in a co-solvent, under mild conditions of temperature and pressure that may be optimized for economic feasibility. Subjecting the brown stock to such treatment in accordance with embodiments of the invention makes the cellulose more accessible for chemical reaction, e.g., bleaching, by opening up the aggregated domains, which are also the source of recalcitrance of cellulose. The resulting nanoporous cellulose in accordance with embodiments of the invention also allows for much more uniform substitution along the cellulose chains, thus minimizing problems of quality control currently inherent in producing many cellulose derivative products.

As described above, the treatment solution for brown stock in accordance with embodiments of the invention includes an alkali dissolved in a co-solvent. Suitably, the alkali is dissolved in a co-solvent of water plus a second water-miscible solvent. In one aspect, the second solvent is suitably an alcohol which may include, e.g., methanol, ethanol, propanol, isopropanol, butanol, isobutanol, or a polyol. In another aspect, the second solvent may include other protic solvents as well as aprotic solvents that are miscible in water. In an illustrated embodiment, the co- solvent is ethanol and water.

In some embodiments of the invention, the alkali is suitably sodium hydroxide (NaOH), although other alkaline agents, such as other alkali metal hydroxides or alkali earth hydroxides, may be used, such as lithium hydroxide (LiOH) or potassium hydroxide (KOH). The concentration of, e.g., NaOH, needed in the treatment solution depends on the nature of the cellulose to be treated, as different celluloses may have their lattice forms disrupted at different concentrations of alkali. For example, the threshold for mercerization of most pulps is approximately 8% NaOH in water; for cotton, it is about 11 to 12%, depending on prior pretreatment; and for bacterial cellulose, it is about 14%. The alkali concentration in the treatment solution is suitably greater than 1 M, suitably can range from about 1 M to about 2 M. The treatment of brown stock forms novel forms of cellulose that are disordered compared to the structure of cellulose in biomasses and even compared to the structure of cellulose after the digestion process. In these novel forms of cellulose, substantial lattice order is retained by the cellulosic structure with the molecular chains remaining organized in a specific pattern, apparently maintaining the spatial relationship of the chain molecules aligned parallel to each other; but with significant internal disorder of the anhydroglucose units within individual chains. After transformation to these novel forms of cellulose, the internal organization of individual chains is less ordered than it is in the cellulosic source material and even after digestion, but the molecular chains appear to retain their organization parallel to each other in a manner not unlike that prevailing in the source celluloses. Thus, while the previously known crystalline cellulosic substances retain substantial organization at both the macroscopic and microscopic levels, organization in the novel celluloses is altered at the nanoscale level with the result being such that the space between adjacent molecular chains is increased, i.e., laterally expanded.

The degree of displacement or increased spacing between adjacent chains is on a nano-scale order, so it is not yet known whether an actual increase in fiber diameter can be easily measurable; but there is strong evidence of increased lateral separation between the chains of cellulose molecules leading to what is termed a "nanoporous structure" based upon: (i) the X- ray diffraction patterns; (ii) the Raman spectra; (iii) the surprisingly high opacity of the fibers; (iv) the smooth surfaces of the fibers observable in high magnification micrographs; and (v) the increased accessibility of the cellulose molecules to reactants, especially large molecule enzymatic reactants. (See, U.S. Patent No. 8,617,851 ; U.S. Published Application No. 2013/0172514, U.S. provisional patent application no. 60/042133, all of which are incorporated by reference.)

These novel celluloses can also be aptly referred to as "highly disordered cellulose", as "nanoporous cellulose" or as "laterally expanded cellulose", but the most significant differences between the novel celluloses and previously known celluloses have become apparent that these celluloses may be aptly described as "laterally expanded cellulose" as the individual chains appear to remain more or less coherent but spaced apart so that they are far more accessible than in conventional crystalline celluloses. Whether previously referred to as "highly disordered cellulose" or as "nanoporous cellulose", the novel celluloses described in these applications are neither amorphous, nor mercerized nor completely disordered. For convenience, these novel celluloses are hereinafter referred to either as nano-deaggregated or nanoporous cellulose or laterally expanded cellulose as, for purposes of this application, as this better describes the most significant differences between these fibers and previously known forms of cellulose. In accordance with the invention, a novel post-treatment is also provided after the initial exposure to the NaOH solution in the co-solvent system. The post-treatment includes removal of sufficient sodium hydroxide to avoid mercerization or conversion to cellulose II upon exposure to water. This post-treatment is optimal in certain special contexts wherein the possibility of sodium hydroxide catalyzing the conversion to cellulose II or the mercerized form of cellulose is prevented.

The prevention of conversion to cellulose II may be simply accomplished through washing the alkali, e.g., sodium hydroxide, out of the treated cellulose with the co-solvent. This reduces the concentration of sodium hydroxide sufficiently so that when the cellulose is eventually washed with water, the concentration of NaOH is not sufficient to catalyze the conversion to cellulose II, which represents re-aggregation into a different polymorph of cellulose that lacks the nano- porosity that is the result of the instant process.

While washing as a post-treatment procedure is effective in preventing conversion to cellulose II, it requires the use of at least twice the amount of co-solvent used in the treatment step. Embodiments of the invention now provide neutralization of the alkali in the post-treatment solution. While a number of different acids may be used for this purpose, one of the most obvious is hydrochloric acid, which effectively converts significant amounts of, e.g., sodium hydroxide to sodium chloride. While this post-treatment is effective, re-causticizing the sodium chloride to sodium hydroxide to be reused in the process is complex and requires high energy. For certain applications that are more readily scaled to larger capacity for commercial use, use carbon dioxide for the neutralization is suitably provided. Carbon dioxide, when bubbled into the co-solvent system containing sodium hydroxide, reacts with the sodium hydroxide to form first sodium carbonate, which will precipitate out of the co-solvent system at a nominal pH of 10, because sodium carbonate is not soluble in ethanol, the dominant component of the co-solvent. If appropriate, bubbling carbon dioxide can be continued until the sodium carbonate is converted to sodium bicarbonate, which also is insoluble in ethanol, at a nominal pH of 8. So carbon dioxide can be used to neutralize the sodium hydroxide after the first process or treatment step in which the cellulosics are treated with sodium hydroxide in the ethanol-water co-solvent. After neutralization of the sodium hydroxide, the nanoporous celluloses are quite stable in water. Reference is now made to FIGS. 1 and 2 which depict the stages of production of brown stock and bleached pulp in a pulp mill. As seen in FIG. 1 , a lignocellulosic feedstock, e.g. wood, suitably wood chips, enters a digestion or pulping process. Digestion produces a brown stock pulp which is washed and screened, and may optionally also be oxygen delignified in an Oxygen Delignification stage as shown in FIG. 1. In general, the first use of a chorine- containing oxidizer denotes the division between the brown stock area and the bleach plant area. Prior to the first use of chorine-containing reagents, all filtrates are countercurrently processed and treated in the mill chemical recovery loop. Chloride-containing filtrates create corrosion issues in the recovery loops, so any filtrates containing appreciable amounts of chlorides (i.e. those from the bleaching process or bleach plant) are not sent to the chemical recovery loop.

For pulping/digestion, any suitable chemical or mechanical pulping process can be used to pulp biomass in the invention. In some embodiments, the pulping step uses a chemical pulping process such as a kraft process, a sulfite process or a soda process. In the embodiment shown in FIG. 1 , the process includes a kraft pulping process (the block labeled "digestion") in which the lignocellulosic material is cooked with an alkaline cooking liquor to allow the wood chips to separate into pulp fibers without much mechanical action. Any suitable effective alkali charge can be used during the kraft process, such as an effective alkali charge within a range of from about 10 to about 20 weight percent on a dry biomass basis. The brown pulp from the digester enters brown stock washing and screening stages, where large fractions, such as knots are removed and remaining spent cooking chemicals are washed from the pulp. The brown stock then may (or may not) undergo oxygen delignification. Oxygen delignification is used in some, but not all bleached pulp grade facilities. The process results in reduced bleach plant chemical usage, but is somewhat capital intensive. Washed brown stock enters one or more stages of bleaching with chlorine dioxide, signified in FIG. 1 as the "bleaching plant". As an option, in the bleach plant, the pulp can be subjected to one or a plurality of bleaching, caustic extraction, and washing operations, which can result in further delignified and bleached pulp of an increased brightness. The bleaching treatment chemicals can be, for example, oxygen gas, ozone, chlorine dioxide, chlorine, peroxide, pure acid or a suitable alkali, or a mixture of these, and possibly other bleaching chemicals or additives. For example, pairs of chlorine dioxide and caustic extraction towers followed by pulp washing stages may be used for bleaching, or other conventional pulp bleaching arrangements may be applied to the pulp.

Thus, the bleach plant may have two or more stages of bleaching depending on the brightness target for the final pulp, each bleaching stage is typically followed by an extraction stage. FIG. 2 illustrates a typical 3-stage bleach plant with a washing treatment after each stage. The primary chlorine dioxide (CI0₂) bleaching is often referred to as the Do stage. The main purpose of the primary chlorine dioxide stage is to reduce the lignin content of wood pulp in the bleaching process. This stage forms chlorolignin compounds that are then dissolved in a subsequent stage which provides an alkali solublization medium. The primary chlorine dioxide (CI0₂) stage is an acid stage, which oxidizes lignin compounds. When followed by an alkali extraction stage, the combination treatment in the two stages can reduce the kappa number of pulp up to 80%. The stage is normally run at a moderate temperature (30-60°C) for up to an hour retention time at between 3.5% and 10% consistency. At the end of the reaction, the pulp is washed and then sent to the caustic extraction stage (denoted E) for removal of soluble cholorolignin compounds. The alkali extractive stage can be fortified with oxidants such as oxygen or hydrogen peroxide (typically denoted with subscripts o and p respectively) or a combination of both. Thus, the alkali extractive stage can be an E, Eo, Ep, or Eop stage.

Alkali extraction, like other delignifying and bleaching processes, is followed by pulp washing and thereafter almost always followed by another chlorine dioxide (D) stage. The main purpose of the next chlorine dioxide stage, often referred to as D1 , is the brightening of wood pulp in the bleaching process. Chlorine dioxide's high oxidizing properties promote an increase in brightness. This chlorine dioxide stage is an acid stage. Unlike the primary chlorine dioxide stage, the purpose is not to remove lignin, but to oxidize thus brighten the cellulose portion of the pulp. Chlorine dioxide brightening stages are most effective on pulps with a Kappa number below 5. The stage is normally run at a temperature between 60 and 80°C for between 2 to 4 hours' retention time at between 10% and 12% consistency. At the end of treatment, the pulp is, once again, washed.

Depending on the brightness target, a Di stage may be suitably followed by an extraction stage (E₂) and a final D₂ stage. It is envisioned that brown stock pulp can be treated in accordance with the present invention either before or after an oxygen delignification step shown in FIG. 1.

Reference is now made to FIG. 3. The process starts with a brown stock from a pulping process. The first step of the process is a treatment step. The brown stock is treated with a treatment solution of an alkali, such as sodium hydroxide, in a co-solvent of water and an alcohol, such as ethanol. It is at this step that the cellulose in the brown stock becomes nano- deaggregated as discussed above. This step is suitably carried out at ambient temperature and pressure. This step yields a brown stock slurry in a post-treatment solution.

The treatment solution contains alkalizing compounds, e.g., NaOH, KOH, LiOH, or combinations thereof. The alkali concentration is generally greater than 1 M, suitably can range from about 1 to about 2.0 M in a co-solvent of water and water-miscible solvent, e.g., an alcohol such as ethanol.

A second step is a post-treatment step, which suitably includes neutralization of the treatment solution after reaction with the brown stock. Neutralization may be accomplished through dilution by displacement or by use of an acidifying agent, e.g., acetic acid or carbon dioxide. It has been found that use of carbon dioxide is an efficient method of neutralizing the post- treatment solution. As Illustrated in FIG. 3, carbon dioxide (C0₂) is suitably bubbled into the post-treatment solution to precipitate the sodium carbonate and/or sodium bicarbonate within the brown stock slurry. Such neutralization precipitates the alkali as a carbonate/bicarbonate, e.g., sodium carbonate/bicarbonate.

A third step suitably includes separating the treated brown stock and precipitated alkali salts from the neutralized post-treatment solution. This is done by any one of the many processes available for separating solids from liquids, including filtration or centrifugation.

A fourth step suitably includes separating the treated brown stock fibers from the precipitant alkali salt(s) which may be accomplished with the addition of water to dissolve the salt(s). The fibers are thus suitably washed with water to dissolve the sodium carbonate and/or sodium bicarbonate. The fibers can then be separated by one of the traditional methods used in pulp mills for washing pulps, such as brown stock washers prior to transferring them to the bleaching section of the plant. A fifth step suitably includes optionally drying the treated brown stock which may be optionally hydrolyzed via enzymatic reaction or chemically reacted to form other cellulose-derived products.

A sixth step suitably includes recovering the alkali via a recaustization treatment, for example, with quicklime and recycling the recovered alkali to form treatment solution. Depending on the design of a particular pulping mill, the sodium carbonate and/or sodium bicarbonate can be re- causticized by treatment with quicklime from the limekiln. It can then be recycled to prepare the treatment solution of sodium hydroxide in the co-solvent to be reused in step 1 of the overall process. A seventh step suitably includes distilling the neutralized post-treatment solution to recover the alcohol and recycling the alcohol to form treatment solution. The neutralized post-treatment solution can be taken into a distillation column to recover the ethanol. The bottoms from this distillation, which may contain some solubilized lignins, can be transferred to a recovery boiler of the mill. The instant treatment of the brown stock makes the brown stock nanoporous, thus making the interior of the fibers and the lignin within them more accessible for further reaction. The treatment thus provides an opportunity to reduce the intensity of the bleaching process and reduce the amount of chemicals required. Furthermore, the resulting pulp will retain much of its nanoporous character, which can be advantageous in many of the products made from bleached pulps.

It is understood that the detailed design for the application of the present process in a pulp mill will have to be adapted for integration into the specific design of the particular pulp mill.

EXAMPLES

It is envisioned that the brown stock treated in accordance with the present invention, either before or after oxygen delignification, will be more labile to delignification and bleaching and provide reduced need for delignifying and bleaching reagents due to the nanoporosity of the brown stock after treatment in accordance with the present invention.

Example 1 : Pulp Brown Stock This example uses a neutralization step without any intent to recover and re-causticize the sodium salt formed as a result of neutralization. As a result, the neutralization was carried out with acetic acid so that the sodium acetate formed could be easily washed out of the treated brown stock pulp. A sample of loblolly pine brown stock pulp was provided by a pulp mill in the Southeastern United States.

400 gm of brown stock on an oven dry basis were placed in a 5-gallon reactor. The treatment solution consisted of 300 gm of NaOH dissolved in 5 liters of co-solvent consisting of 1250 ml_ of water and 3750 ml_ of ethanol. The moisture already in the pulp represented an additional 1400 ml_ of water. The final consistency was 8%.

The brown stock was stirred in the treatment solution for 20 minutes. It was then filtered and returned to the reactor. 5 Liters of water were added and this was followed by addition of glacial acidic acid to bring the pH down to 7.5. The brown stock was then washed.

The 400 gm sample of treated brown stock, together with an equal amount of untreated brown stock, was sent to an independent laboratory for assessment of their relative performance in bleaching. The tests carried out were based on standards used in the pulping industry.

The results provided by the independent laboratory are shown in Table 1 below. The test included three stages with both chemical and brightness measurements included. The general function of each of the three stages used, as set forth by the independent laboratory, is presented below.

The key results noted in Table 1 are:

The results showed that the treated pulp had a slightly lower lignin content and required a significantly reduced amount of bleaching chemicals to achieve comparable delignification. The lignin content, as determined on the basis of kappa number, declines from 2.5% to 2.3% as a result of the treatment, indicating that some of the lignin previously trapped within the fiber walls of the brown stock has become accessible for solubilization prior to any oxidative bleaching.

The D₀ stage of the treated brown stock required application of less CI0₂ than required for the untreated brown stock. Furthermore, the residual amount of CI0₂ is lower in the treated brown stock indicating that more lignin was accessible enough to react in the treated sample. The Ep stage required less NaOH for the treated sample and the final brightness was 3.4 points higher.

The Di stage resulted in brightness that is 2.1 points higher. This represents a significant increase in bleaching response. The foregoing description is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes may readily occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation shown and described, and accordingly, all modifications and equivalents are considered as falling within the scope of the invention.

Table 1

Comparison of Treated and Untreated Southern Softwood Pulps

Southern Softwood Kraft Pulp

Starting Material Untreated Treated

Kappa No. (never Dried) 19.3 17.4

Approximate lignin (%), kappa* 0.13 2.5 2.3

Do Stage: 60 min, 50°C, 10% cons.

CI02, as CI2. % 4.83 4.35

H2S04, % 0.2 0.2

Final pH 4.1 2.1

Residual, g/L as avail. CI2 0.07 0.01

Ep Stage: 60 min, 75°C, 10% cons.

NaOH, % 1.5 I .4

H202, % 0.3 0.3

Final pH 12.4 I I .9

Residual H202, % trace 0.02

Kappa No. (SCAN modified) 2.48 2.39

ISO Brightness, % 63.4 66.8

D1 Stage: 150 min. 75°C. 10% cons.

CI02, as CI02, % 1.0 1.0

NaOH, % 0.32 0.35

Final pH 4.0 4.2

Residual CI02, % 0.07 0.10

ISO Brightness, % 85.7 87.8

Claims

1. A method of treating and stabilizing a brown stock from a pulping stage in a pulp mill prior to transfer to a delignification or bleaching stage, comprising reacting/treating a brown stock pulp with a treatment solution of an alkali in an alcohol/water co-solvent to yield a treated brown stock pulp slurry in a post- treatment solution;

reducing the alkalinity of the post-treatment solution through dilution with displacement or with an acidifying agent or both to yield a neutralized post- treatment solution having the treated brown stock pulp;

separating the treated brown stock and any precipitated alkali salt, resulting from the use of acidifying agents, from the neutralized post-treatment solution; and subsequently separating any precipitated salt from the brown stock pulp by pulp washing.

2. The method of claim 1 , further comprising recovering the alkali by recaustizing the alkali salt to yield recovered alkali.

3. The method of claim 2, further comprising recycling the recovered alkali to form treatment solution.

4. The method of claim 1 , further comprising distilling the neutralized post-treatment solution from step (c) to recover the alcohol, and recycling the recovered alcohol to form treatment solution.

5. The method of claim 1 , further comprising delignifying and/or bleaching the treated brown stock pulp.

6. The method of claim 1 , wherein the alkali is an alkali metal hydroxide and the alkali salt is an alkali metal salt.

7. The method of claim 6, wherein the alkali metal hydroxide is sodium hydroxide and the alkali metal salt is sodium carbonate/sodium bicarbonate.

8. The method of claim 7, wherein the sodium carbonate/sodium bicarbonate is recaustized by treatment with quicklime.

9. The method of claim 1 , wherein the acidifying agent is carbon dioxide (C0₂).

10. The method of claim 1 , wherein the post-treatment solution has a pH in the range of 8 - 10 at the end of neutralization.

11. A method for processing a brown stock comprising: (i) treating a brown stock with a treatment solution of sodium hydroxide in an ethanol/water co-solvent to produce a composition having a treated brown stock in a post-treatment solution; (ii) adjusting the pH of the post- treatment solution with carbon dioxide to produce a neutralized post-treatment solution including the treated brown stock, the neutralized post-treatment solution having a pH between about 8 and about 10, the carbon dioxide reacting with the sodium hydroxide dissolved in the post- treatment solution to precipitate sodium carbonate/bicarbonate; (iii) separating the treated brown stock and the precipitated sodium carbonate/bicarbonate from the neutralized post- treatment solution; (iv) separating the treated brown stock from the precipitated sodium carbonate/bicarbonate salt; (v) recaustization the sodium carbonate/bicarbonate to yield recovered sodium hydroxide; and (vii) recovering the ethanol from the neutralized post- treatment solution.

12. The method of claim 1 1 , further comprising recycling the recovered sodium hydroxide to form treatment solution.

13. The method of claim 12, further comprising recycling the recovered ethanol to form treatment solution.

14. A system for treating brown stock to improve its delignification and/or bleaching response, comprising a treatment stage, a post-treatment stage and a recycling/recovery stage;

A) the treatment stage comprising reacting a brown stock with a treatment solution of sodium hydroxide in an ethanol/water co-solvent to produce a composition including a treated brown stock in a post-treatment solution; B) the post-treatment stage comprising treating the post-treatment solution with an acidifying agent to produce a neutralized post-treatment solution including the treated brown stock, the acidifying agent neutralizing the alkali and precipitating any alkali salt:;

C) the recycling/recovery stage comprising (i) separating the treated brown stock and any precipitated salt from the neutralized post-treatment solution, (ii) separating the alkali salt from the treated brown stock, (iii) recovering the alkali by recaustizing the alkali salt to yield recovered alkali, and (iv) recovering the ethanol from the neutralized post-treatment solution.

15. The system of claim 14, further comprising a delignification and/or bleaching stage including delignifying and/or bleaching the treated brown stock.

16. The system of claim 14, wherein the acidifying agent is carbon dioxide (C0₂).

A brown stock produced by the system of claim 14.