Patent Publication Number: US-2018030651-A1

Title: Method and compositions for oxygen delignification of chemical pulp

Description:
BACKGROUND 
     1. Field of the Invention 
     The present disclosure generally relates to improvements in oxygen delignification of chemical pulp. More particularly, the disclosure relates to the use of reducing agents to remove lignin during an oxygen delignification process. 
     2. Description of the Related Art 
     Chemical pulping of lignocellulosic materials with sodium sulfide (e.g., the kraft process) and without sodium sulfide (e.g., an alkaline or soda process) yields brownstock pulp that may be further processed through delignification and bleaching stages to reach the target brightness and lignin content of the final pulp product. The kraft process is one of the major pulping processes in the pulp and paper industry. Papermaking typically includes a stock preparation stage, a pulping stage, a bleaching stage, a wet end stage, and a dry end stage. 
     Lignin is a highly complex polyphenolic compound, which is an integral part of the secondary cell walls of plants. It is one of the most abundant organic polymers and typically constitutes between a quarter and a third of the dry mass of wood. Lignin contains numerous functional groups, including carbonyls, which are highly reactive and interfere with numerous chemical processing steps in papermaking. Lignin content (commonly reported as kappa number) is negatively correlated with pulp brightness; therefore, removing lignin from pulp is a central goal of pulp processing. 
     Lignin can be removed from pulp through oxygen delignification. Oxygen delignification is achieved through exposure of pulp to pressurized oxygen at elevated temperatures under alkaline conditions. A drawback of oxygen delignification is carbohydrate depolymerization due to severe process conditions, such as temperature and alkalinity. 
     BRIEF SUMMARY 
     The present disclosure generally relates to improvements in oxygen delignification of chemical pulp. Some embodiments of this disclosure relate to methods for pulp delignification in a kraft process, which may include adding a composition to brownstock pulp. The composition added to the pulp may comprise a reducing agent in an alkaline solution. In certain embodiments, the composition may be added after a pulp digestion stage and before an oxygen delignification process. 
     In some embodiments, compositions for brownstock pulp delignification are disclosed. The compositions may include a gas and an alkaline solution. The gas may be pressurized oxygen and the alkaline solution may comprise a reducing agent and a base. The reducing agent may be stable in the composition in the presence of oxygen and under the process conditions. 
     The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter that form the subject of the claims of this application. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent embodiments do not depart from the spirit and scope of the disclosure as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A detailed description of the invention is hereafter described with specific reference being made to the drawings in which: 
         FIG. 1  shows bleaching process stabilization; 
         FIG. 2  shows pulp brightness decrease after trial period; 
         FIG. 3  shows bleaching cost/ton increase after trial period; 
         FIG. 4  shows ClO 2  dosage lb/ton increase after trial period; and 
         FIG. 5  shows calculated % O 2  delignification increase during trial period. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are described below to illustrate certain aspects of the present disclosure. However, it should be understood that the present disclosure is not limited to the embodiments explicitly described. 
     In some embodiments of the present disclosure, a composition is provided comprising a reducing agent in an alkaline solution. This composition may be added to brownstock pulp. Brownstock pulp is commonly understood to refer to pulp that may result from digesting wood chips in a kraft or soda process. A kraft process may include the treatment of wood chips with a hot mixture of water, sodium hydroxide, and sodium sulfide to break and remove lignin. The technology entails several steps, both mechanical and chemical. In certain embodiments, the composition may be added after a pulp digestion stage and before an oxygen delignification process. 
     The reducing agent is not limited and numerous reducing agents may be used in accordance with the embodiments disclosed herein. In some embodiments, the reducing agent is a hydride donor. In some embodiments, the reducing agent may be selected from the group consisting of lithium borohydride, sodium borohydride, potassium borohydride, and any combination thereof. In one particular embodiment, the reducing agent is sodium borohydride. A representative example of a composition containing sodium borohydride that may be used in accordance with the present disclosure is Borol®, which comprises 12 wt. % sodium borohydride and 40 wt. % sodium hydroxide. 
     Sodium borohydride (BH) is a strong reducing agent that reacts with carbonyl groups in lignin and provides additional avenues for lignin depolymerization. Without wishing to be bound by theory, it appears that at low dosages, such as below 0.1 wt. %, BH selectively attacks functional groups in pulp, thus activating lignin and passivating carbohydrates relative to subsequent reactions. While BH is a known bleaching chemical that may also improve pulping at high doses, its unexpected activity at low dosages, such as lower than 0.1 wt. % discovered by the present inventors can be explained, for example, by the presence of active functionalities in pulp that are small in number but high in importance in terms of directing the bulk pulping process, residual lignin preservation and carbohydrate preservation. 
     In certain embodiments of the present disclosure, the reducing agent is added to the pulp in an amount less than about 0.1%, by weight, based on oven dried pulp. The pulp may be brownstock pulp. In some embodiments, the reducing agent may be added to the brownstock pulp in an amount between about 0.005% and 0.1%, by weight, based on oven dried pulp. The reducing agent may also be added in an amount between about 0.001% and 0.005%, between about 0.001% and 0.002%, between about 0.001% and 0.003%, between about 0.001% and 0.004%, between about 0.0001% and 0.001%, between about 0.0005% and 0.001%, between about 0.005% and 0.05%, between about 0.005% and 0.01%, between about 0.05% and 0.1%, between about 0.05% and about 0.08%, between about 0.07% and about 0.1%, and/or between about 0.00005% and 0.0001%, by weight, based on oven dried pulp. In some aspects, the dosage of the reducing agent, such as sodium borohydride, may be sufficiently low to provide no bleaching to the pulp. 
     In some embodiments, the composition comprises an amount of about 1% to about 34%, by weight, of the reducing agent and an amount of about 1% to about 50%, by weight, of a base in water. Suitable bases may include, but are not limited to, sodium hydroxide, potassium hydroxide, magnesium hydroxide, sodium carbonate, calcium carbonate, and any combination thereof. Any reducing agent, or any combination of reducing agents, may be used in accordance with this embodiment. For example, the reducing agent may be sodium borohydride. In some embodiments, the composition may be added to brownstock pulp that may contain black liquor. 
     Other compounds, such as 9,10-antraquinone (AQ), used in delignification processes act as catalysts, unlike BH, which is a consumed pulp activator. This led to a universal, but incorrect, assumption that a high dose (&gt;0.1 wt. %) of BH is always required to achieve the same effect as catalysts. Considering the cost of the treatment, this led to the belief that the use of BH is economically unrealistic. Moreover, no convenient, stable product and no efficient and safe feeding methods have been proposed; without them the technology would not be feasible. Adding powdered BH at the paper mill is unsafe because the chemical reacts with water under neutral pH, thereby releasing hydrogen. The present disclosure seeks to overcome these problems by using a solution comprising the reducing agent instead of a solid. The present inventors determined that a broad range of reducing agents, such as a borohydride, can be stabilized in alkaline solution and therefore can be safely used in industrial applications. 
     In certain embodiments of this disclosure, the composition comprises an alkaline solution containing sodium borohydride, which is a stable, safe and convenient form for transportation, storage and dosing to the target process. The composition may contain a base at a concentration from approximately 10 wt. % to approximately 40 wt. % and from approximately 5 wt. % to approximately 25 wt. % reducing agent. The reducing agent may be a borohydride-containing compound and the alkaline solution may comprise sodium hydroxide. 
     In certain embodiments, the composition may consist of a reducing agent (or combination of reducing agents), a base (or a combination of bases), and water. In other embodiments, the composition may consist of sodium borohydride, sodium hydroxide and water. 
     In some embodiments, the oxygen delignification process may be improved by feeding a composition comprising BH in an aqueous alkaline solution to the pulp liquor prior to an oxygen delignification process. The composition may comprise, for example, Borol®, Venpure®, or any other liquid containing approximately 10 wt. % to approximately 25 wt. % NaBH 4  in approximately 20 wt. % to approximately 40 wt. % NaOH. 
     In particular embodiments, the reducing agent in the composition comprises sodium borohydride at a concentration of about 12 wt. % and the composition additionally comprises sodium hydroxide at a concentration of about 40 wt. %. In some embodiments, the alkaline solution may have a pH from about 10 to about 14. The pH range may be, for example, about 10 to about 13, about 10 to about 12, or about 10 to about 11. 
     In still further embodiments, the method may comprise forming a mixture of the composition with an alkaline liquor and adding the mixture to the brownstock pulp prior to a temperature increase in the oxygen delignification process. The composition may comprise a stabilized borohydride alkaline solution. Instead of adding the composition directly to the brownstock pulp, the composition may be added, for example, to an alkaline liquor prior to being added to the brownstock pulp being fed into an oxygen delignification process. The alkaline liquor may comprise sodium hydroxide. The alkaline liquor may be fed into the oxygen delignification process to maintain a NaOH to oxygen mass ratio of about 0.9. In some embodiments, the oxygen delignification process excludes adding peroxides. 
     In some embodiments, the composition may further comprise a surfactant or a combination of surfactants. The surfactant is not limited and can be, for example, cationic, anionic, zwitterionic, nonionic, or amphoteric. An illustrative example of a nonionic surfactant is a triblock copolymer of PEO-PPO-PEO, where PEO (polyethylene oxide) is hydrophilic and PPO (polypropylene oxide) is hydrophobic. Compatibility testing carried out by the inventors demonstrated that the sodium borohydride in an alkaline solution can be applied with a surfactant, or combination of surfactants, without any side reactions. 
     In some embodiments, the composition further comprises a phase transfer catalyst to aid in oxygen transfer into the liquid phase. The phase transfer catalyst is not limited and may illustratively be selected from the group consisting of tetra-n-butylammonium bromide, methyltrioctylammonium chloride, benzyltrimethylammonium chloride, hexadecyltributylphosphonium bromide, and any combination thereof. 
     In still further embodiments, the composition may comprise a chelant. The chelant may be selected from the group consisting of diethylene-triamine-pentamethylene phosphonic acid (DTMPA) and salts thereof, salts of diethylene-triamine-pentamethylene phosphonic acid (DTMPA), diethylenetriaminepentaacetic acid (DTPA), salts of diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), salts of ethylenediaminetetraacetic acid (EDTA), and any combination thereof. 
     In some embodiments, the composition further comprises a polysulfide. Polysulfides are compounds that comprise chains of sulfur atoms and can include anions and organic polysulfides. Anions may have the general formula S2-n, where “n” can be any number. These anions are the conjugate bases of the hydrogen polysulfides H2S n . Organic polysulfides may have the general formula RS n R, where R may be, for example, any alkyl group or any aryl group. The inventors discovered in a stability test that BH and polysulfides are compatible so they may be combined in an alkaline solution. The discovery was achieved by keeping a solution of the two components at elevated temperature and measuring residual active BH by means of acidification and comparing the volume of evolved hydrogen with a control sample. The polysulfide may be sodium polysulfide. The polysulfide may be mixed into the composition with sodium borohydride and/or fed separately. 
     In additional embodiments, the method may further comprise feeding the brownstock pulp and the composition into a reactor, pressurizing the reactor with oxygen, and increasing a temperature inside the reactor to about 80° C. to about 110° C. The pressure inside the reactor may be approximately 85 psi. The pressure inside the reactor may range from approximately 80 psi to approximately 100 psi. The pH inside the reactor may be between approximately 10 and approximately 14. The consistency of the pulp slurry may be between about 1% to about 30%. Consistency is a term used in the art to refer the mass fraction (or percentage) of solid or filterable material in a given slurry sample. In other embodiments, the consistency may be from about 9% to about 15%, from about 1% to about 3%, or from about 3% to about 9%. 
     It is counterintuitive to use a reducing chemical in the oxidative process of oxygen delignification. However, the present inventors unexpectedly discovered that BH does not react with oxygen under the process conditions. Reactions of pulp with BH and oxygen are separated in time while being part of the same delignification process. Furthermore, addition of BH can be done without making adjustments to standard process equipment. 
     The methods disclosed herein may further comprise bleaching the pulp material using a bleaching solution. The bleaching solution may comprise chlorine dioxide. The pulp may be whitened using doses of chlorine dioxide that would have been insufficient to reach the brightness target had no BH been added to the pulp. The methods may further comprise contacting the pulp material with one or more chelants. 
     In some embodiments, the composition added to the brownstock pulp may comprise a gas and an alkaline solution. The gas may be pressurized oxygen and the alkaline solution may comprise a reducing agent and a base. In certain embodiments, the composition may consist of pressurized oxygen, sodium borohydride, and sodium hydroxide in water. The alkaline solution may be added to the composition at low doses, such as from about 0.05 wt. % to about 0.5 wt. %. 
     In accordance with the present disclosure, an oxygen delignification process can be improved by adding an alkaline solution comprising a reducing agent to brownstock pulp. Applying the methods disclosed herein results in significant savings in process chemicals, process stability, decreased kappa number of the pulp, increased pulp brightness, and higher pulp yield. Less white liquor is needed during oxygen delignification and less chlorine dioxide is needed in later bleaching steps. In addition to chemical savings, the process may be operated at milder conditions, for example, the temperature during oxygen delignification may be lowered to prevent degradation of cellulose and hemicellulose. The alkalinity of the pulp slurry may be reduced as well. These savings and improvements are completely unexpected at the low doses of BH disclosed herein. Additionally, the embodiments of the present disclosure provide increased safety and efficiency that has never been achieved. Moreover, the embodiments of the present disclosure make this BH technology viable for kraft pulp applications. 
     EXAMPLES 
     About 10.5 g (based on oven dried pulp) samples of brownstock pulp were processed through an oxygen delignification procedure conducted at 3% consistency and with an initial pH of about 11.9 (adjusted with 10 wt. % NaOH). The BH alkaline solution (Borol®) was added to the brownstock pulp slurry at the target pH. The brownstock pulp slurry and chemicals were added into an open Parr reactor. The reactor was closed and pressurized with oxygen to 100 psi. Mixed at the “max” rate, the slurry was heated to the target temperature in about 15 to about 20 minutes. Time at the target temperature was measured from the moment the reactor temperature was within about 5° C. of the target temperature to the reactor cool-down initiation, which was about 40 min. The reactor was cooled and depressurized. The pulp was washed in a Buchner funnel lined with cheesecloth and divided in half. One half was used for making brightness pads and the other was saved for second stage bleaching with chlorine dioxide, when needed. 
     Chlorine dioxide bleaching was conducted at about 10% consistency, a pH of about 4 (adjusted with 10 wt. % H 2 SO 4 ), at about 70° C. for about 60 minutes. The pulp was washed in a Buchner funnel lined with cheesecloth with about 3 L of water. Excess water was squeezed from the samples and brightness pads were prepared. No acidification at this step was required. 
     In order to make brightness pads, the pulp samples were diluted with water to about 1 L in a plastic beaker and stirred for about 10 minutes each. To eliminate residual alkalinity, 1 drop of 5 N H 2 SO 4  was added. Then, the samples were passed through filter paper, placed on a metal plate and pressed for about 5 minutes to remove all excess water. After excess water removal, the samples were left overnight at a constant humidity (about 50%) and temperature of about 23° C. Brightness was measured with a Technodyne Color Touch 2 (Model ISO) instrument. Kappa number was measured according to TAPPI test method T 236. 
     Brownstock low-delignified kraft pulp from a Northwest mill (kappa# 60, brightness 16.48) was used in the test. The ISO brightness in the control (without BH) was 19.87 and the ISO brightness was 21.99 when treated with 0.05 wt. % BH. After bleaching with 3 wt. % chlorine dioxide, the ISO brightness in the control (without BH) was 39.96 and the ISO brightness was 44.60 using 0.05 wt. % BH. 
     In another experiment (Table 1, Experiment B), the chlorine dioxide content was 2 wt. % and the BH content was 0.5 wt. %. The ISO brightness in the control (without BH) was 19.67 and the ISO brightness was 21.29 using BH. After bleaching with 3 wt. % chlorine dioxide, the ISO brightness in the control (without BH) was 30.87 and the ISO brightness was 35.68 using BH. 
     Thus, the inventors unexpectedly discovered that the effect of BH does not depend linearly on the dose and a noticeable effect can be achieved at a low dose. This effect multiplies at the following bleaching stages. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 ISO brightness levels for different treatments of brownstock 
               
               
                 pulp. 
               
            
           
           
               
               
               
            
               
                   
                 After Oxygen 
                   
               
               
                   
                 Delignification 
                 After D0 Bleaching 
               
            
           
           
               
               
               
               
               
            
               
                 Experiment 
                 Control 
                 BH treatment 
                 Control 
                 BH treatment 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 A 0.05 wt % BH 
                 19.87 
                 21.99 
                 39.96 
                 44.60 
               
               
                 B 0.5 wt % BH 
                 19.67 
                 21.29 
                 30.87 
                 35.68 
               
               
                   
               
            
           
         
       
     
     Oxygen Delignification Mill Trial 
     Several single day trials were conducted at a Northeast mill on Northern hardwood brownstock pulp (˜50% maple, ˜20% birch, ˜20% beech, and ˜10% poplar, blended with ˜3-6% softwood) from a kraft process. The kappa number of the pulp before oxygen delignification was about 12-14. The stock temperature entering the O 2  reactor was about 71° C., and the temperature and pressure in the reactor was about 82-88° C. and about 85 psi, respectively. At a consistency of about 4% to about 5% and pH of about 10.5, the brownstock pulp was in the reactor for about 1.5 to about 2 hours. The NaOH : oxygen ratio was maintained at about 0.90. The caustic and oxygen were on a dosage control for production. A 12 wt. % solution of sodium borohydride in 40 wt. % aqueous alkaline solution was fed at about 1.0 lb/oven dried ton of brownstock pulp to the sodium hydroxide feed just before the O 2  reactor. 
     The trial data show that the proposed treatment results in process stabilization, higher brightness, lower bleaching costs, lower chlorine dioxide consumption, and improved delignification ( FIGS. 1-5 ). 
       FIG. 1  shows that before the trial period the bleach load had a range of 3.3. However, during the trial period, the variability dropped to 0.5. After the trial period, the range increased to 1.4. Applying the composition to the pulp before oxygen delignification resulted in process stabilization at the bleaching stages. It also resulted in higher brightness, lower bleaching costs, lower chlorine dioxide consumption, and improved delignification (in  FIGS. 1-5 , the middle line represents an average and the lines on either side of the middle line represent +/−1 Sigma). 
       FIG. 2  shows a decrease in pulp brightness after the trial period when the composition was applied to the process. Pulp brightness averaged 39.7 during the trial period, as measured by a Technodyne Color Touch 2 instrument. However, after the trial period, the brightness decreased to an average of 37.7 with greater variability. 
       FIG. 3  shows how bleaching costs increased after the trial period from an average of 48.1$/ton to an average of 51.3 $/ton. The disclosed inventive method produces sensitized pulp that requires less bleaching chemicals to reach target pulp brightness. 
       FIG. 4  shows an increase in chlorine dioxide after the trial period from 49.6 lb/ton to 53.7 lb/ton. Less chlorine dioxide is required when using the composition comprising sodium borohydride in an alkaline solution. 
       FIG. 5  shows that the average % delignification of the pulp increased during the trial period to an average of 23.1 from an average of 20.2. Activating lignin before oxygen delignification improved removal of lignin from the pulp. 
     All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While this invention may be embodied in many different forms, there are described in detail herein specific embodiments of the invention. The present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated. In addition, unless expressly stated to the contrary, use of the term “a” is intended to include “at least one” or “one or more.” For example, “a reducing agent” is intended to include “at least one reducing agent” or “one or more reducing agents.” 
     Any ranges given either in absolute terms or in approximate terms are intended to encompass both, and any definitions used herein are intended to be clarifying and not limiting. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all subranges (including all fractional and whole values) subsumed therein. 
     Furthermore, the invention encompasses any and all possible combinations of some or all of the various embodiments described herein. It should also be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.