WO1994008087A1 - Improved bleaching of high consistency lignocellulosic pulp - Google Patents

Abstract

The present invention pertains to a method having increased selectivity for bleaching lignocellulosic pulp having a high consistency from a first GE brightness to a second, higher GE brightness which comprises the steps of (a) fluffing the high consistency pulp having the first GE brightness to a specific surface area of at least about 90m2/kg; and (b) contacting the fluffed pulp with a gaseous bleaching agent to form a bleached pulp having the second, higher GE brightness. The present invention also pertains to a bleached lignocellulosic pulp prepared by the method of the present invention.

Description

  
 



   IMPROVED BLEACHING
 OF HIGH CONSISTENCY LIGNOCELLULOSIC PULP
 BACKGROUND OF THE INVENTION
 Field of the Invention
 This invention pertains to methods for bleaching lignocellulosic pulp having a high consistency. More particularly, this invention pertains to methods for bleaching lignocellulosic pulp with a gaseous bleaching agent which require reduced amounts of bleaching agent and yield pulps having higher viscosity, higher pulp strength, and higher GE brightness. The method comprises fluffing the lignocellulosic pulp to a specific surface area of at least about 90m2/kg prior to contacting the pulp with the gaseous bleaching agent. The invention also pertains to bleached lignocellulosic pulps prepared by the improved method.  



   Description of the Background
 Wood is composed of two main components - a fibrous carbohydrate or holocellulosic component and a non-fibrous component called lignin.



  Holocellulose is composed of about 70% alkaliinsoluble alpha-cellulose and 30% alkali-soluble hemicellulose. The non-fibrous lignin component is a three-dimensional polymeric material consisting mainly of phenylpropane units.



   For use in paper-making processes, wood must be converted to pulp. During chemical pulping, the cellulosic fibers are separated from one another in a manner to preserve the inherent fiber strength and to remove as much lignin as possible. In a chemical pulping process, wood is digested with chemical solutions to solubilize and remove a portion of the lignin. Examples of chemical pulping procedures are the soda (sodium hydroxide), sulfite, and kraft processes.



   The basic and modified kraft processes are the principle chemical processes utilized in paper manufacturing because these processes do not significantly degrade the cellulosic component of the wood. The basic kraft process involves digesting wood chips in an aqueous solution of  sodium hydroxide and sodium sulfide to form a high strength pulp. The basic kraft process is described in detail in Handbook for Pulp  & Paper
Technologists, Chapter 7, Kraft Pulping (TAPPI,
U.S.A.).



   The modified kraft processes are even milder than the basic kraft process on the cellulosic component and yield even higher strength pulp. The modified kraft processes, also known as "extended delignification" pulp processes, either involve adding the pulping chemicals in a specific sequence, adding the pulping chemicals at different locations in the digestion apparatus, adding the pulping chemicals at different time periods, or removing and reinjecting liquors in a prescribed sequence to remove lignin and reduce attack on the cellulosic fibers. The kraft-AQ process involves adding small amounts of anthraquinone to the pulping liquor to accelerate delignification and limit attack upon the cellulosic fibers. A variety of additional extended delignification techniques are known in the art and include Kamyr Modified
Continuous Cooking (MCC), described by V.A.



  Kortelainen and E.A. Backlund in TAPPI, vol. 68 (11), 70 (1985); Beloit Rapid Displacement Heating   (RDH), described by R.S. Grant in TAPPI, vol. 66 (3), 120 (1983); and Sunds Cold Blow Cooking, described by B. Pettersson and B. Ernerfeldt in
Pulp and Paper, vol. 59 (11), 90 (1985).



   The pulp formed after digestion of the wood is generally a dark colored slurry of cellulosic fibers known as "brownstock". The dark color of the brownstock is caused by chromophoric groups in the lignin remaining in the pulp formed during the digestion period. This dark lignocellulosic pulp may be used directly in the paper making operation if paper color is not important and may be bleached to a brightness consistent with the planned utilization of the pulp. Prior to bleaching, the pulp is generally transferred to a blow tank to relieve pressure and to separate the pulp material as a fibrous mass. The fibrous mass is then washed to remove residual chemicals and soluble materials such as lignin degradation products.



   To lighten the color of the brownstock pulp and make it suitable for use in printing, writing, or other white paper applications, the lignin remaining in the pulp must be chemically removed or converted into colorless compounds by bleaching and brightening. Pulp bleaching is generally a multi  stage process employing chlorine-containing compounds such as calcium hypochlorite, sodium hypochlorite, elemental chlorine, or chlorine dioxide. Bleaching of lignocellulosic pulp using chlorine containing compounds is well known in the art and is discussed in detail in United States patent no. 1,957,937, issued to   Campbell    et al.;
United States patent no. 2,975,169, issued to
Cranford et al., United States patent no.



  3,462,344, issued to Kindron et al.; and Handbook for Pulp and Paper Technologists, Chapter 11:
Bleaching (11.3) (TAPPI, USA).



   The following letter codes will be used to describe chemical reactants and process steps employed in paper making.



   C = Chlorination - Reaction with
 elemental chlorine
 in acidic medium.



   E = Alkaline - Dissolution of
 Extraction reaction products
 with NaOH.



     E0    = Oxidative - Dissolution of
 Alkaline reaction products
 Extraction with NaOH and
 oxygen.



   D = Chlorine - Reaction with
 Dioxide   C102    in acidic
 medium.



   P = Peroxide - Reaction with
 peroxides in
 alkaline medium.  



   O = Oxygen - Reaction with
 elemental oxygen in
 alkaline medium.



   0 = Modified - Uniform alkali
 Oxygen treatment of low to
 medium consistency
 pulp followed by
 reaction of high
 consistency pulp
 with oxygen.



   Z = Ozone - Reaction with
 ozone.



      Zm = Modified - Uniform reaction with   
 Ozone ozone.



   C/D = - Admixtures of
 chlorine and
 chlorine dioxide.



   H = Hypochlorite - Reaction with
 hypochlorite in an
 alkaline solution.



   Although chlorine and chlorine containing compounds are effective bleaching agents, chlorine is difficult to handle and is hazardous to personnel and machinery. Furthermore, the effluents from chlorine bleaching processes contain large amounts of chloride by-products which readily corrode paper making equipment and chlorinated compounds which can pose environmental concerns.



  Chloride ion build-up prevents the recycling of washer filtrate in a closed system operation unless expensive recovery operations are employed.  



   As a consequence, chlorine-containing bleaching agents have been replaced with nonchlorine- containing bleaching agents such as oxygen and hydrogen peroxide. The use of oxygen permits the recycling of the effluent and permits a substantial reduction in the amount of elemental chlorine used. A number of processes for bleaching and delignifying pulp with oxygen have been proposed, such as those described in United States patent no. 1,860,432, issued to Richter, United
States patents nos. 2,926,114 and 3,024,158, issued to Granaaard et al., United States patent no.

 

  3,274,049, issued to Gaschke et al., United States patent no. 3,384,533 issued to   Mevlan    et al.,
United States patent no. 3,251,730, issued to
Watanabe, United States patent no. 3,432,282, issued to Rerolle et al., United States patent no.



  3,661,699, issued to   Farley,    United States patent no. 4,619,733, issued to Kooi; and P. Christensen in "Bleaching of Sulphate Pulps with Hydrogen
Peroxide", Norsk Skogindustri, 268-271 (1973).



  Alkaline pretreatment of pulp prior to oxygen delignification is suggested by United States patent no. 4,806,203, issued to Elton and United
States patent no. 5,085,734, issued to   Griags.    A  method using oxygen as a first stage bleaching agent to solubilize a major amount of lignin and chlorine as a second stage bleaching agent to remove the remaining lignin is described by P.



  Christensen, "Bleaching of Sulphate Pulps with
Hydrogen Peroxide", Norst Skogindustri, 268-271 (1973).



   The use of oxygen as a bleaching agent is not, however, completely satisfactory. For example, oxygen is not as selective a delignification agent as elemental chlorine and only a limited number of oxygen delignification reactions can be carried out until the cellulosic fibers are attacked. In addition, the lignin remaining after oxygen delignification has typically been removed by chlorine bleaching to obtain a fully-bleached pulp using reduced amounts of chlorine. Even at reduced chlorine concentrations, however, the corrosive chloride by-products soon reach unacceptable concentration levels in a closed cycle operation.



   Ozone has also been used as a bleaching agent for pulp. The exceptional oxidative properties of ozone and its relatively high cost, however, have limited the development of satisfactory ozone bleaching processes. Ozone readily reduces the  lignin in pulp but it also aggressively attacks the cellulosic fibers to reduce the strength of the pulp. The reactivity and stability of ozone is also extremely sensitive to reaction conditions such as pH changes.



   United States patent no. 2,466,633, issued to
Brabender et al., describes a bleaching process wherein ozone is passed through a pulp having a moisture content of between 25% and 55% and a pH in the range from 4 to 7. N. Liebergott, "Paprizone
Treatment, A New Technique for Brightening a
Strengthening Mechanical Pulps" describes a single stage bleaching process which combines peroxide and ozone in a synergistic combination to brighten and strengthen mechanical pulp. United States patent no. 4,196,043, issued to Sinah, discloses a multistage bleaching process characterized by from one to three ozone bleaching stages and a final treatment with alkaline hydrogen peroxide, each stage being separated by an alkaline extraction.



  United States patent no. 4,372,812, issued to   Philips    et al., discloses a multi-stage bleaching process which comprises an oxygen bleaching step, a peroxide bleaching step, and an ozone bleaching step. United States patent no. 4,468,286, issued  to Johnsen, discloses a method for bleaching pulp with ozone in a multi-path system.



   European patent application no. 308,314, to
Coste et al., discloses a method for treating a lignocellulosic material with ozone which includes reducing a mechanical pulp to a state of division sufficient to assure maximal interface between the pulp and the ozone.



   United States patent no. 4,278,496, issued to
Fritzvold, discloses a bleaching process which comprises treating a finely divided pulp with ozone initially at a low pH value then later at a high pH value. The pulp is preferably processed before ozone treatment to a "light and fluffy consistency".



   C.A. Lindholm, PaDer   ja    Puu, (68)4, 1986 283290, discusses homogeneous bleaching and states that "an open well-fluffed pulp is needed to enable ozone to come into contact with the water layers surrounding the single fibres." C.A. Lindholm,
Proc. Int.   PulD Bleachincr Conf.,    vol. 2, Stockholm, 1991; 1-17, discusses consistency in ozone bleaching and states that the pulp must be "fluffed in order to expose the surface of the single fibres to the ozone-containing gas".  



   A. R. Proctor, Pulp Paper Maaazine Canada, 75(6), 1974; T210-214, states that "[t]rial experiments showed that pulp fluffing had no significant effects on sheet properties.



   A comparison of the ozone bleaching of medium consistency pulp and high consistency pulp showed that the medium consistency pulp had lower tensile strength and higher tear strength because of fiber deformation. For high consistency bleaching, part of the pulp was fluffed in a refiner, E.   Oltmann    et al., Heft 7 341-350, 343 (1992).



   European patent application no. 492,039 discloses a method for delignifying cellulosic material having a consistency of about 25% to about 45% which comprises the steps of pneumatically conveying the cellulosic material through a path in contact with ozone. European patent application no. 492,040 discloses a method for delignifying cellulosic material having a consistency of about 30% to about 45% which comprises the steps of tumbling the cellulosic material through a path in contact with ozone to keep the material loose and homogeneous with a high surface area to volume ratio.  



   Despite efforts carried out in this area, no satisfactory commercial high-consistency process for the manufacture of ozone bleached lignocellulosic pulp from softwood and related pulp, especially southern softwood, has been disclosed. Accordingly, methods for bleaching lignocellulosic pulp which require reduced amounts of bleaching agent and yield pulps having higher viscosity, higher pulp strength, and higher GE brightness are highly desirable. The present invention provides such novel bleaching methods without the disadvantages characteristic of previously known methods.



   BRIEF DESCRIPTION OF THE FIGURES
 Figure 1 is a graph showing the effect of fluffing on the ozone bleaching of kraft/AQ-O pulp with a nominal consistency of 42%. Ozone consumption is plotted versus ISO brightness.



   Figure 2 is a graph showing the effect of fluffing on the ozone bleaching of kraft/AQ-O pulp with a nominal consistency of 42%. ISO brightness is plotted versus intrinsic viscosity.



   Figures 3A and 3B are graphs showing the effect of fluffing on the ozone bleaching of  kraft/AQ-O pulp with a nominal consistency of 42%.



  In Figure 3A, aerodynamic specific surface area is plotted versus intrinsic viscosity. In Figure 3B, aerodynamic specific surface area is plotted versus ozone consumption.

 

   Figure 4 is a graph showing the effect of fluffing on the ozone bleaching of kraft/AQ-O pulp with a nominal consistency of 42%. Tensile index is plotted versus tear index.



   SUMMARY OF THE INVENTION
 The present invention pertains to a method having increased selectivity for bleaching lignocellulosic pulp having a high consistency from a first GE brightness to a second, higher GE brightness which comprises the steps of (a) fluffing the high consistency pulp having the first
GE brightness to a specific surface area of at least about 90m2/kg; and (b) contacting the fluffed pulp with a gaseous bleaching agent to form a bleached pulp having the second, higher GE brightness.



   The present invention also pertains to a bleached lignocellulosic pulp having a high consistency prepared by a method having increased  selectivity for bleaching lignocellulosic pulp which comprises the steps of (a) fluffing a high consistency pulp having a first GE brightness to a specific surface area of at least about 90m2/kg; and (b) contacting the fluffed pulp with a gaseous bleaching agent to form a bleached pulp having a second, higher GE brightness.



   DETAILED DESCRIPTION OF THE INVENTION
 Applicants have discovered an improved method for bleaching high consistency lignocellulosic pulp. By fluffing lignocellulosic pulp to a specific surface area of at least about 90m2/kg prior to bleaching the pulp, applicants have found that the bleaching agent consumption during the bleaching step of the pulp can be reduced. Pulps bleached according to the method of the present invention have a higher pulp viscosity, a higher pull strength, and a higher GE brightness than conventionally bleached pulps. The bleaching method of the present invention may be employed to bleach chemical or mechanical pulps and may be followed by other bleaching steps. The chemical requirement in later bleaching steps of the pulp is also reduced.

  Applicants believe that fluffing  high consistency lignocellulosic pulp to a specific surface area of at least about 90m2/kg increases the surface area of the pulp for better contact with the bleaching agent and thereby increases the rate, uniformity, and efficiency of the bleaching reaction. The enhanced uniformity of the bleaching reaction results in less bleaching agent consumption by the pulp and more selective reaction of the bleaching agent with the lignin and less degradation of the cellulosic fibers. Pulps bleached according to the selective method of the present invention have a higher pulp viscosity at a given brightness and a higher brightness at a given ozone consumption.



   The present method for fluffing high consistency lignocellulosic pulp prior to bleaching the pulp may be carried out in a refiner fluffer or other type of fluffer to expose individual fibers which were in the interior of a floc to a gaseous bleaching agent while leaving the cell walls intact. The fluffing process (or refiner fluffing process) is a low energy process employing a large gap clearance in the fluffer to physically separate but not break or mechanically modify the individual fibers. As shown below, because the individual  fibers are not physically modified, fluffing can increase the surface area of the pulp without substantially changing pulp freeness (pulp drainability).



     Example    Pulp   Freeness      (CSF)   
Unflufted Pulp 747s1   fluffed    Pulp   (2.055    plate gap) 75011
Fluffed Pulp   (O 8s    plate gap)   74      Fluffed    Pulp   (0.5-a    plats gap)   75951   
 The present method for fluffing high consistency lignocellulosic pulp prior to breaching the pulp is distinct from the refining process which is a process generally employed after bleaching and before papermaking.

  The refining process is a high energy process employing a small gap clearance to physically or mechanically modify the individual fibers by removing the primary cell wall, introducing cracks and fissures into the remaining primary and secondary cell walls, fibrillating, and introducing other imperfections into the fiber, G.A. Smook, Handbook For Paper  & BR<
Paper Technologist, TAPPI (1982), p. 183. The fibrils and other imperfections serve to increase the chemical and physical bonding sites between the  fibers during the webbing step to form the paper sheet. The refining of mechanical pulp also results in mechanical modification of the fibers; mechanical pulping causes fiber cutting and breakage while thermomechanical pulping causes rupture and break-up of the cell walls, G.A. Smook,
Handbook For Paper and Paper Technologist, TAPPI (1982), p. 55-56.

  Because the individual fibers are modified and fines are created, refining decreases pulp freeness, G.A. Smook, Handbook For
Paper and Paper Technologist, TAPPI (1982), p. 189.



  While refining improves the papermaking step, refining exposes the internal areas of the fiber making them susceptible to chemical attack, such as acid hydrolysis, to produce weaker fibers, Hartler et al., Svensk Papperst., 63, 263-271 (1960),
Bausch et al, Svensk Papperst., 63, 279-285 (1960),
Stone et al., Pulp Paper Mag. Can., 59(6), 165-173 (1958), Stone et al., Pulp Paper Mag. Can., 62(6), 317-326 (1961). The present method for fluffing high consistency lignocellulosic pulp does not include physically or mechanically modifying the individual fibers because refining prior to bleaching the pulp would be expected to produce weaker fibers.  



   In accord with the present invention, the method of having increase selectivity for bleaching high consistency lignocellulosic pulp includes fluffing the high consistency pulp without substantially changing the pulp freeness. Without substantially changing the pulp freeness of the pulp means that the fluffing step is carried out with minimal mechanical modification of the fibers of the pulp so as not to cause a deterioration in the quality of the pulp after the following bleaching   step    In a preferred embodiment, the pulp freeness of the fluffed pulp is changed less than about 15%, more preferably less than about 10%, and most preferably less than about 5%.



   Throughout this specification, the following definitions will be used. The definitions are based on those found in Rydholm,   Pulping    Processes,
Intersciences Publishers, 1965, pages 862-863 and
TAPPI Monograph No. 27, The Bleachina of Pulp,
Rapson, Ed., The Technical Association of Pulp and
Paper Industry, 1963, pages 186-187.



   The term "consistency", as used herein, means pulp concentration and refers to the amount of pulp fiber in a slurry expressed as a percentage of the weight of the oven dried fiber over the total  weight of the fiber and water. The consistency of a pulp will depend upon the type of dewatering equipment used.



   The term "low consistency", as used herein, refers to concentration ranges of pulp up to about 6%, and preferably up to about 5%. Low consistency pulp is a suspension that is pumpable by an ordinary centrifugal pump and is produced using deckers and filters without press rolls.

 

   The term "medium consistency", as used herein, refers to concentration ranges of pulp between about 6% and 20%. Below about 15%, medium consistency pulp can be produced by filters. This level of consistency is the consistency of the pulp mat leaving a vacuum drum filter in the brownstock washing system and the bleaching system. The consistency of a slurry from a washer, either a brownstock washer or a bleaching stage washer, is about 9% to 15%. Above about 15%, medium consistency pulp can be produced by press rolls.



  Rydholm states that the usual range for medium consistency pulp is from about 10% to 18% and
Rapson states that the usual range for medium consistency pulp is from about 9% to 15%. Medium consistency pulp is pumpable by special machinery.  



   The term "high consistency", as used herein, refers to concentration ranges of pulp between about 20% and about 65%. Rydholm states that the concentration range of high consistency pulp is from about 25% to 35% and Rapson states that the concentration range of high consistency pulp is from about 20% to 35%. These high consistency pulps are obtainable only by the use of presses.



  The liquid phase is largely absorbed by the fibers and the pulp can be pumped only very short distances.



   The term "pulping", as used herein, is used in its conventional sense to refer to digestion of lignocellulosic material to form brownstock.



  Pulping methods include, for example, kraft, the kraft-AQ process, and other forms of extended delignification.



   The term "modified kraft process", as used herein, refers to extended delignification processes and all other modified kraft processes with the exception of the kraft-AQ process. The kraft-AQ process has achieved a special status and acceptance in the art and is separately known by that name. The oxygen delignification step following pulping is not an extended  delignification method but is rather a first delignification step for bleaching or brightening the pulp.



   There are two principal types of measurements to determine the completeness of the pulping or bleaching process. The measurements are referred to as the "degree of delignification" and the "brightness" of the pulp. The degree of delignification is normally used in connection with the pulping process and the early bleaching stages because it is less precise when only small amounts of lignin are present in the pulp such as in the later bleaching stages. The brightness factor is normally used in connection with the bleaching process because it is more precise when only small amounts of lignin are present and the pulp is lightly colored and highly reflective.



   There are many methods for measuring the degree of delignification but most are variations of the permanganate test. The normal permanganate test provides a permanganate number of "K no." which is the number of cubic centimeters of tenth normal potassium permanganate solution consumed by one gram of oven dried pulp under specified  conditions. The K no. is determined by TAPPI
Standard Test T-214.



   There are also a number of methods for measuring pulp brightness. Pulp brightness is a measure of reflectivity and its value is expressed as a percent of a scale. A standard method for determining pulp brightness is GE brightness (GEB) which is expressed as a percentage of a maximum GE brightness. GE brightness is determined by the
TAPPI Official Method T-452.



   A second method for determining pulp brightness is ISO brightness (ISO) which is expressed as a percentage of maximum ISO brightness. ISO brightness is determined by the
TAPPI Official Method T-525.



   The viscosity of a wood pulp is a measure of the degree of polymerization of the cellulose chains which make up the individual wood fibers.



  A standard method for determining pulp viscosity is cupriethylendiamine   (1,CED")    viscosity. CED viscosity, which will be referred to simply as viscosity, is expressed in units of centripoise (cp). Viscosity is determined by the TAPPI
Official Method T-230.  



   A second method for measuring pulp viscosity is intrinsic viscosity which is expressed in units of   dm3/kg.    Intrinsic viscosity is determined by the ASTM Official Method D-1795.



   In accord with the present invention, lignocellulosic pulp is fluffed to a specific surface area of at least about 90m2/kg prior to the pulp bleaching step. The specific surface area of the pulp should be at least about 90m2/kg to provide sufficient pulp surface area to optimally react with the bleaching agent and augment the rate of the bleaching reaction. In general, the greater the specific surface area of the lignocellulosic pulp, the lower the bleaching agent consumption by the pulp during the bleaching step and the greater the pulp viscosity, pulp strength, and GE brightness of the resulting pulp. In a preferred embodiment, the lignocellulosic pulp is fluffed to a specific surface area of at least about 100m2/kg, more preferably at least about 120m2/kg, and most preferably at least about 180m2/kg.

  In another preferred embodiment, the lignocellulosic pulp is fluffed to a specific surface area from about 100m2/kg to about 1000m2/kg, more preferably from about 120m2/kg to about 500m2/kg, and most  preferably from about 180m2/kg to about 350m2/kg.



   The woods which may be employed in the present invention include both hard and soft woods. In one embodiment, the lignocellulosic pulp is prepared from a hard wood. In another embodiment, the lignocellulosic pulp is prepared from a soft wood.



  Preferably, the lignocellulosic pulp is prepared from soft wood. More preferably, the lignocellulosic pulp is prepared from pine. Most preferably, the lignocellulosic pulp is prepared from kraft/AQ-O pine feedstock. In yet another embodiment, the lignocellulosic pulp is prepared from a mixture of hard and soft wood.



   In general, any fluffing apparatus which will fluff the high consistency pulp to a specific surface area of at least about 90m2/kg may be used.



  Examples of fluffers which may be employed include the refiner fluffer and the pin fluffer. In a preferred embodiment, the fluffer is a refiner fluffer. The apparatus useful in accordance with the present invention comprises fluffing apparatuses well known in the art and therefore the selection of the specific apparatus will be apparent to the artisan.  



   The lignocellulosic pulp in the present invention is a pulp having a high consistency. In a preferred embodiment, the consistency of the pulp is from about 20% to about 65%, more preferably from about 28% to about 55%, and most preferably from about 35% to about 48%.

 

   The lignocellulosic pulp in the present invention may be a chemical pulp or a mechanical pulp. Preferably, the lignocellulosic pulp is a chemical pulp. The bleaching method of the present invention may also be employed in the final bleaching step of a pulp.



   The bleaching agent for the lignocellulosic pulp in the present invention is a fast reacting gaseous bleaching agent. In general, the more reactive the bleaching agent employed, the greater the reduction of the bleaching agent requirement of the bleaching reaction and the greater the improvement in the viscosity, strength, and GE brightness of the product pulp. Gaseous bleaching agents are well known in the art and include, for example, ozone, oxygen, and chlorine-containing compounds such as elemental chlorine and chlorine dioxide. The preferred gaseous bleaching agent is ozone. For convenience, ozone will be referred to  as the gaseous bleaching agent throughout this specification.



   A typical pulping, delignifying, and bleaching multi-stage process includes the following steps:
 (a) pulping the lignocellulosic material and recovering the pulping chemicals;
 (b) washington the pulp to remove chemical residues and residual lignin, and screening the pulp to remove fiber bundles;
 (c) delignifying the pulp with alkaline oxygen (i.e., o or o,);
 (d) washing the partially delignified pulp to remove dissolved organics, screening the pulp, and recycling a portion of the effluent;
 (e) chelating and acidifying the pulp to bind metal ions and adjusting the pH level;
 (f) thickening the pulp to highconsistency;
 (g) contacting the pulp with a gaseous bleaching agent such as ozone (i.e., Z or   Z,)      )   to further delignify and partially bleach the material;

  ;
 (h) washing the partially bleached pulp and recycling a portion of the effluent;  
 (i) extracting the pulp with alkali to remove residual lignin;
 (j) washing the extracted pulp and recycling a portion of the effluent;
 (k) contacting the pulp with a second bleaching agent (i.e., D or P) to brighten and bleach the pulp;
 (1) washing the bleached pulp to obtain a bleached product having a GE brightness from about 70% to about   90t;    and
 (m) recycling a portion of the effluent from the P bleaching stage or the D bleaching stage.



   Throughout this application, various publications have been referenced. The disclosure in these publications are incorporated herein by reference in order to more fully describe the state of the art.



   The present invention is further illustrated by the following examples which are not intended to limit the effective scope of the claims. All parts and percentages in the examples and throughout the specification and claims are by weight of the final composition unless otherwise specified.  



   Examples 1-8
 These examples demonstrate the improvement in high consistency ozone bleaching as a result of employing a fluffed pulp with a suitable specific surface area.



   Pine feedstocks were fluffed using a refiner fluffer, two pin fluffers, an impact fluffer, and a laboratory hammer mill. In addition, pulp which had been coarsely shredded but not fluffed was also used. The pulp was bleached using a range of ozone applications and selected ozonated pulp was extracted and subjected to two chlorine dioxide bleaching stages.



   A kraft/AQ-O pine feedstock, having a GE brightness of 33.7%, a viscosity of 15.9 cp, and a
K. no. of 7.7, was employed. The pulp was diluted, acidified and chelated, and then pressed. The high consistency press mat was directed to a shredder conveyor and then collected for later fluffing.



  Refiner fluffing was carried out using a Sunds
Defibrator 500 mm pilot refiner fluffer equipped with type 5120 refiner plates, and the spacing between the plates was between 0.5 mm and 6.0 mm.



  Pin fluffing was carried out using a production scale pin fluffer. Fluffing was also performed  using laboratory equipment, including a modified blender, and a laboratory pin fluffer.



   The degree of fluffing for each of these pulps was measured using the aerodynamic resistance method described by R.G. Garner and the R.J.



  Kerekes in Pulp Paper Canada Transactions,   yol.    79 (9), TR82 (1978), and using the data analysis procedure described by A.A. Robertson and S.G.



  Mason in Pulp Paper Magazine Canada, vol. 50 (12), 103 (1949). Table A shows the results obtained for several pulps tested in this study. The specific surface areas ranged from as low as 34 m2/o.d. kg pulp for relatively unfluffed pulp (shredder) to as high as 226 m2/o.d. kg for well fluffed pulp (refiner).



   The pulps were then ozone bleached using a range of ozone consumption levels. Figure 1 shows the results plotted as ozone consumption versus GE brightness. The best refiner fluffed pulp required only 4.8 kg 03/ODMT (ODMT - o.d. metric ton) to reach 53% ISO brightness, while the shredder pulp, which had the worst fluffing, required over 30% more ozone (about 6.5 kg 03/ODMT). Figure 2 shows the results plotted as GE brightness versus intrinsic viscosity. Figure 2 shows that the  refiner pulp retained the highest viscosity at a given brightness, while the shredder viscosities were among the lowest.



   For each of the pulps, a linear regression of the data in Figures 1 and 2 was performed to calculate the ozone consumption and pulp viscosity at 53% ISO brightness. The results are plotted in
Figures 3A and 3B as a function of the aerodynamic specific surface, where it can be seen that an increase in the specific surface has the beneficial effect of reducing the amount of ozone required to reach 53% ISO brightness, and increasing the resulting pulp viscosity.



   Examples 9-12
 These examples further demonstrate the relationship between the degree of pulp fluffing and optimal high consistency ozone bleaching.



   A K/AQ-O pulp was fluffed in four different ways using methods described above. The pulp had initial GE brightness of 33.7%, an 8.0 K no., and 16.2 cp viscosity. After fluffing, the specific surface was determined for each pulp. The refiner pulp had the highest specific surface, 166 m2/kg, and the shredder pulp had the lowest specific surface, 43 m2/kg. The fluffed pulps were bleached  using ozone to approximately the same GE brightness. The results are shown in Table B. The best fluffed pulp (refiner) had the highest GE brightness, the highest viscosity, and the lowest ozone consumption after ozone bleaching, while the poorest fluffed pulp (shredder) had a lower brightness, the lowest viscosity, and the highest ozone consumption.

 

   Each of the ozone bleached pulps was then extracted and bleached with two chlorine dioxide bleaching stages, and the results are shown in
Table B. The pulp which had been refiner fluffed prior to ozone bleaching, and had the highest viscosity after ozone bleaching, maintained the highest viscosity after subsequent bleaching to a
GE brightness of about 89%, and required the smallest amount of chlorine dioxide to reach this brightness level. In comparison, the pulps which were not fluffed as well had lower viscosities and required more chlorine dioxide to reach the same brightness.



   The strength of these four pulps was evaluated. A PFI mill was used and the handsheets were prepared according to the standard Tappi procedures T-205 and T-248. Figure 4 shows the  tear index plotted versus tensile index. Comparing tear index at a given tensile index, the refiner pulp is 10-20% higher than the shredder pulp, and 5-10% higher than the pulps which had been fluffed using the pin and impact fluffers.



   These results show a benefit for refiner fluffing over other types of fluffing in terms of reduced ozone requirement, reduced chemical requirement for final bleaching, and improved pulp viscosity and strength. A comparison of the estimated relative costs of a refiner fluffer, a pin fluffer, and impact fluffer, and shredder in terms of capital and operating costs was made. The reduction in ozone requirement offsets the increased capital required for a refiner fluffer resulting in essentially equal costs for all methods examined. The refiner fluffer results in a stronger product at approximately the same cost.  



   Table A
 Fluffing Characterization
By Aerodynamic Resistance Method
Example Fluffer Specific
Surface Area
 (theta,   m2/kg)   
 K/AQ-O pulp, nominal 42% consistency 1 Shredder 34 2 Impact fluffer 66 3 Pin fluffer 85 4 Refiner, plate gap A 226 5 Refiner, plate gap B 225 6 Refiner,

   plate gap C 150 7 Laboratory pin fluffer 132 8 Laboratory blender 204
 Table B
 Effect of Degree of Fluffing
 on Ozone and Subsequent Bleaching
Fluffing
 Fluffer Shredder Impact Pin
Refiner
 Specific surface area 43 66 85
 166    (m2/kg)     
Z Stage
 Ozone consumption 5.9 4.3 3.5
 2.9
 (kg/ODMT)
 GE Brightness (%) 54.5 52.9 53.9
 55.4
 Viscosity (cp) 9.5 10.6 10.4
 10.6
E Stage
 GE Brightness (%) 63.7 59.2 58.4 61.5
First D Stage
 Chlorine dioxide 7.5 7.0 7.0 5.5
 consumption (kg/ODMT)
 GE Brightness (%) 86.9 86.3 86.4 86.7
Second D Stage
 Chlorine dioxide 1.6 3.7 3.7 3.2
 consumption   (kg/ODMT)   
 Total chlorine dioxide 9.1 10.7 10.7 8.7
 consumption (kg/ODMT)
 GE Brightness (%) 88.3 89.8 89.9 89.2
 Viscosity (cp) 8.5 9.5 9.3 10.0
 Summary
 The refiner fluffer produced the best fiber separation.

  The specific surface area was found to have a significant impact on ozone bleaching results with refiner fluffing reducing the ozone requirement by 10% to 30% over other fluffing  options. Chemical requirements for final bleaching were also reduced. Improved fluffing resulted in a higher pulp viscosity after the ozone stage (by up to lcp or 35dm3/kg) as well as for the finalbleached pulp. After final bleaching, the   refiner    fluffed pulp had a higher strength. The tear factor for the refiner-fluffed pulp at a given tensile was about 5% to 10% higher than for pin or impact-fluffed pulp and 10% to 20% higher than for poorly-fluffed shredder pulp.

 

   The higher capital cost associated with better fluffing is compensated for by the reduction in ozone generation equipment costs because of the reduced ozone requirement with better fluffing.



  The result is that all four fluffing options for which an economic analysis was performed have very similar capital costs.



   The invention being thus described, it will be obvious that the same may be varied in many ways.



  Such variations are not to be regarded as a departure from the spirit and scope of the invention and all such modifications are intended to be included within the scope of the following claims. 

Claims

We Claim:

1. A method having increased selectivity for bleaching lignocellulosic pulp having a high consistency from a first GE brightness to a second, higher GE brightness which comprises the steps of: (a) fluffing the high consistency pulp having the first GE brightness to a specific surface area of at least about 90m2/kg without substantially changing the pulp freeness; and (b) contacting the fluffed pulp with a gaseous bleaching agent to form a bleached pulp having the second, higher GE brightness.

2. The method according to claim 1, wherein the consistency of the lignocellulosic pulp is from about 20% to about 65%.

3. The method according to claim 2, wherein the consistency of the lignocellulosic pulp is from about 28% to about 55%.

4. The method according to claim 1, wherein the lignocellulosic pulp is a chemical pulp.

5. The method according to claim 1, wherein the lignocellulosic pulp is prepared from a soft wood.

6. The method according to claim 5, wherein the lignocellulosic pulp is prepared from pine.

7. The method according to claim 1, wherein the lignocellulosic pulp is prepared from a hard wood.

8. The method according to claim 1, wherein the specific surface area is at least about 100m2/kg.

9. The method according to claim 8, wherein the specific surface area is at least about 120m2/kg.

10. The method according to claim 9, wherein the specific surface area is at least about 180m2/kg.

11. The method according to claim 1, wherein the specific surface area is from about 100m2/kg to about 1000m2/kg.

12. The method according to claim 11, wherein the specific surface area is from about 120m2/kg to about 500m2/kg.

13. The method according to claim 12, wherein the specific surface area is from about 180m2/kg to about 350m2/kg.

14. The method according to claim 1, wherein the gaseous bleaching agent is ozone.

15. A bleached lignocellulosic pulp having a high consistency prepared by a method having increased selectivity for bleaching lignocellulosic pulp which comprises the steps of: (a) fluffing a high consistency pulp having a first GE brightness to a specific surface area of at least about 90m2/kg without substantially changing the pulp freeness; and (b) contacting the fluffed pulp with a gaseous bleaching agent to form a bleached pulp having a second, higher GE brightness.

16. The lignocellulosic pulp according to claim 15, wherein the consistency of the lignocellulosic pulp is from about 20% to about 65%.

17. The lignocellulosic pulp according to claim 16, wherein the consistency of the lignocellulosic pulp is from about 28% to about 55%.

18. The lignocellulosic pulp according to claim 15, wherein the pulp is a chemical pulp.

19. The lignocellulosic pulp according to claim 15, wherein the pulp in step (a) is prepared from a soft wood.

20. The lignocellulosic pulp according to claim 19, wherein the lignocellulosic pulp in step (a) is prepared from pine.

21. The lignocellulosic pulp according to claim 15, wherein the lignocellulosic pulp in step (a) is prepared from a hard wood.

22. The lignocellulosic pulp according to claim 15, wherein the specific surface area is at least about 100m2/kg.

23. The lignocellulosic pulp according to claim 22, wherein the specific surface area is at least about 120m2/kg.

24. The lignocellulosic pulp according to claim 23, wherein the specific surface area is at least about 180m2/kg.

25. The lignocellulosic pulp according to claim 15, wherein the specific surface area is from about 100m2/kg to about 1000m2/kg.

26. The lignocellulosic pulp according to claim 25, wherein the specific surface area is from about 120m2/kg to about 500m2/kg.

27. The lignocellulosic pulp according to claim 26, wherein the specific surface area is from about 180m2/kg to about 350m2/kg.

28. The lignocellulosic pulp according to claim 15, wherein the gaseous bleaching agent is ozone.

29. The method according to claim 1, wherein the pulp freeness of the fluffed pulp is changed less than about 15%.

30. The lignocellulosic pulp according to claim 15, wherein the pulp freeness of the fluffed pulp is changed less than about 15.