CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of co-pending U.S. Patent Application Serial No. 10/932,604, filed September 2, 2004, which is a continuation in-part-of Patent Application Serial No. 10/618,119, filed July 11, 2003, which is a continuation application of U.S. Patent Application Serial No. 09/747,469, filed December 20, 2000, now U.S. Patent No. 6,627,784, which claims priority to U.S. Provisional Patent Application Serial No. 60/204,838, filed May 17, 2000; all of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD The present invention generally relates to methods for processing lignocellulosic pulp, and more particularly to delignifying and bleaching lignocellulosic pulp. BACKGROUND Wood is comprised of two main components, a fibrous cellulose and a non- fibrous component. The polymeric chains forming the fibrous cellulose portion of the wood are aligned with one another and form strong associated bonds with adjacent chains. The non-fibrous portion of the wood comprises a three-dimensional polymeric material formed primarily of phenylpropane units, known as lignin. The lignin is interspersed both between and in the cellulosic fibers, bonding them into a solid mass. Processes for the production of paper and paper products generally includes a pulping stage in which wood, usually in the form of wood chips, is reduced to a fibrous mass by removing a substantial portion of the lignin. Some of these processes include digestion of the wood by a Kraft or modified Kraft process resulting in the formation of a dark colored slurry of cellulose fibers known as "brownstock." The dark color of the brownstock is attributable to the presence in the pulp after digestion of lignin that has been chemically modified during pulping to form chromophoric groups. In order to lighten the color of the brownstock pulp sufficiently to make it suitable for use in various paper applications, it is necessary to remove much of the remaining lignin. Further reduction of the concentration of lignin in the lignocelluosic pulp is carried out in specific delignification processes, bleaching processes, or combinations of the two. Delignification processes include, for example, chemical treatment with chlorine-containing compounds, such as sodium hypochlorite in a caustic medium, or oxygen delignification with oxygen-containing compounds. Both types of delignification processes typically are followed by bleaching operations in which the delignified pulp is bleached or brightened with ozone, chlorine, or chlorine dioxide. The use of chlorine or chlorinated compounds in paper making processes is common. The use of such compounds usually result in the production of effluent containing substantial quantities of color, BOD (biological oxygen demand), COD (chemical oxygen demand) and chlorides, which require additional processing before being discharged. Therefore, reductions in the amount of chlorinated compounds used the paper making processes can reduce the amount of pollutants produced by the processes. Conventional oxygen delignification processes, in some cases, have produced smaller amounts of chlorinated organic compounds and reduced levels of pollutant discharge, as compared with other types of delignification processes. However, conventional oxygen delignification processes typically require significant amounts of oxygen-containing materials and significant processing time to produce desired levels of delignification. The overall delignification rate for conventional oxygen delignification processes is controlled by the mass transfer limitations associated with the three-phase nature of the system. In order to react with the lignin inside the lignocellulosic fibers, oxygen must cross the gas-liquid interface, diffuse through the liquid film surrounding the fiber and then through the fiber wall itself. Conventional processes have been found to be ineffective at sufficiently dissolving oxygen, even at the highest power input, to overcome these mass transfer limitations. Furthermore, these oxygen delignification processes usually require that the concentration of lignocellulosic pulp in the pulp slurry that is produced in the initial stages of the paper making process be increased so that the oxygen-containing materials can diffuse sufficiently through the pulp to effect delignification. The concentration of pulp usually must be lowered again for further processing. The concentration manipulations are time-consuming and energy intensive. Conventional ozone bleaching processes are typically costly as compared to other bleaching processes due to the high cost of ozone and the large amounts of ozone needed to achieve the desired pulp brightness. Accordingly, alternative delignification processes are needed that have the potential to reduce or eliminate some or all of the above disadvantages.

SUMMARY Methods for treating lignocellulosic pulp with cavitation are disclosed. The methods generally include delignifying and/or bleaching lignocellulosic pulp in the presence of cavitation. In one aspect of the present invention, a method of processing lignocellulosic pulp is provided that comprises contacting an oxidizing agent with a slurry comprising pulp in the presence of cavitation to produce a delignified pulp. In another aspect of the present invention, a method for processing lignocellulosic pulp is provided that comprises delignifying a lignocellulosic pulp in a cavitation zone to produce a delignified pulp. In a further aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing a slurry comprising pulp with a non-alkaline oxidizing agent in the presence of cavitation to produce a delignified pulp. In still a further aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing an oxidizing agent and a slurry containing lignocellulosic pulp in the presence of cavitation to produce a delignified pulp, wherein the mixture of the oxidizing agent and the slurry exhibits a pH above 7. In another aspect of the present invention, a method of making paper is provided that comprises pulping a lignocellulosic feedstock to produce a lignocellulosic pulp and preparing a slurry of the lignocellulosic pulp. The method also comprises delignifying the lignocellulosic pulp in the presence of cavitation to produce a delignified pulp, and forming paper from the delignified pulp. In one aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing a bleaching agent and delignified pulp in a cavitation region to produce a bleached, delignified pulp. In another aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing a bleaching agent and a lignocellulosic pulp in a cavitation region to produce a bleached pulp. In yet another aspect of the present invention, a method of treating lignocellulosic pulp is provided that comprises mixing a bleaching agent and a secondary pulp in a cavitation region to produce a bleached secondary pulp. In a further aspect of the present invention, a method of making paper is provided that comprises delignifying a lignocellulosic pulp to form a delignified pulp, bleaching the delignified pulp in a cavitation region, and forming paper from the bleached, delignified pulp. In yet another aspect of the present invention, a method of making paper is provided that comprises delignifying a lignocellulosic pulp in a cavitation region to form a delignified pulp, bleaching the delignified pulp in a cavitation region, and forming paper from the bleached, delignified pulp. Li yet another aspect of the present invention, a method of making paper is provided that comprises bleaching a secondary pulp in a cavitation region to form a bleached secondary pulp. These and other aspects of the present invention are set forth in greater detail in the description below and in the accompanying drawings which are briefly described as follows.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow diagram showing a typical processing scheme for making paper from wood chips. Figure 2 illustrates a system in which lignocellulosic pulp can be delignified in the presence of cavitation. Figure 3 is a cross-sectional view of the reactor shown in Figure 2. Figure 4 is a diagram showing results of delignification trials charted as percent delignification versus residence time. Figure 5 is a diagram showing results of delignification trials charted as percent delignification versus residence time. Figure 6 is a diagram illustrating ozone delignification efficiency as a function of reactor speed at ambient temperatures (about 2O0C to about 3O0C) for different ozone charges. Figure 7 is a diagram illustrating ozone delignification efficiency as a function of reactor speed at higher temperatures (about 5O0C to about 6O0C) for different ozone charges. Figure 8 is a diagram illustrating the reduction of pulp viscosity as a function of Kappa number reduction.

DETAILED DESCRIPTION The present invention includes methods for delignifying lignocellulosic pulp. The delignification generally is carried out by oxidizing the lignocellulosic pulp contained within a slurry in the presence of cavitation to produce a delignified pulp. As used herein, the term "delignification" refers to a process of reducing the amount of lignin contained within a material, particularly by a chemical reaction, such as oxidation, which can be carried out in conjunction with one or more other mechanical process or chemical reactions. The term "delignified pulp" refers to a pulp derived from lignocellulosic material that, as a result of delignifying, exhibits a smaller Kappa Number than the pulp exhibited prior to delignification. The amount of delignification that occurs typically is determined by comparing the Kappa number of the pulp slurry before delignifϊcation and the Kappa Number of the delignified pulp. As used herein, the term "Kappa Number" refers to the volume (in millimeters) of 0.1N potassium permanganate solution consumed by one gram of moisture free pulp. The amount of delignification can be expressed as a percent reduction in the two Kappa Numbers. The methods of the present invention can effectuate delignifications of greater than 5% and, in some instances, about 5% to about 55% and above. While a variety of oxidants, such as sodium hydroxide (NaOH), sodium hydrosulfite, chlorine, chlorine dioxide, and hydrogen peroxide (H2O2), can be used in delignifying the pulp, the methods particularly are directed to the use of gaseous oxidizing agents, such as air, molecular oxygen (O2), ozone (O3) and combinations thereof to carry out the delignification. The methods of the present invention generally are directed to delignifying lignocellulosic pulp in a slurry at any consistency, but particularly at medium or low consistencies. As used herein, the term "consistency" refers to the concentration by weight of pulp in a pulp slurry on a dry weight basis. The term "high consistency" refers to pulp slurry containing greater than about 15% by weight pulp. The term "medium consistency" refers to pulp slurry containing about 6% to about 15% by weight pulp. The term "low consistency" refers to pulp slurry containing about 0.1% to about 6% by weight pulp. Consequently, pulp being processed to make paper can be maintained at or near the consistency level in previous steps, delignified and further processed without raising the consistency prior to delignification and then lowering the consistency for further processing. The methods generally include contacting an oxidizing agent with a slurry comprising pulp in the presence of cavitation to produce a delignified pulp. The oxidizing agent can be a non-alkaline agent, such as gaseous oxidants, air, molecular oxygen, and ozone, an alkaline agent such as sodium hydroxide, or a combinations thereof. The oxidizing agent can be added to the pulp slurry during mixing, immediately before mixing, or can be remaining in the slurry from a previous process. Contacting the slurry and oxidizing agent in the presence of cavitation can be done under pressure. In one example, contacting is done under pressure in the range of about 480 kPa to about 1035 kPa. Furthermore, the contacted oxidizing agent and slurry also can be heated, either in the presence of cavitation or before being exposed to cavitation, to further effectuate delignification. hi one aspect, either one or both of the oxidizing agent and slurry, or the mixture of the two, can be heated to a range of about 5O0C to about 120°C. In another aspect, either one or both of the oxidizing agent and slurry, or the mixture of the two, can be heated to a range of about 800C to about 1000C. This contacting also can take place in a pH range of about 9 to about 12. The methods also can include directing the delignified pulp to a retention tank and holding the oxidant/slurry mixture containing delignified pulp in the retention tank for a predetermined period of time to effectuate additional delignification. The oxidant/slurry mixture can be held under pressure, for example in the range of about 480 kPa to about 1035 kPa, for a period of time. The time period in which the mixture can be held, either under pressure or otherwise to effectuate further delignification, can be in the range of about 1 minute to about 2 hours. In another aspect of the present invention, the time period is about 1 minute to about 30 minutes. In yet another aspect of the present invention, the time period is about 1 minute to about 20 minutes. The amount of oxidant used to delignify the pulp depends on the type of lignocellulosic material that is being treated. For example, when molecular oxygen is being used to delignify pulp before washing, molecular oxygen is combined with the slurry in a range of about 3% to about 15% by weight based on the weight of the dry pulp fiber in the slurry. For delignification of pulp after washing, molecular oxygen is added in a range of about 1% to about 4% by weight. The specific amount of oxidant that is used to delignify the lignocellulosic pulp can be affected by the type of material it is, whether softwood or hardwood, and the amount of sodium sulfide (Na2S) that remains in the slurry from previous process steps. In one aspect, the methods of the present invention generally include adding oxidant to the slurry in the range of about 1 % to about 20% by weight on a dry pulp fiber basis. In another aspect of the present invention, the methods include adding oxidant to the slurry in the range of about 5% to about 15% by weight on a dry pulp fiber basis. In yet another aspect of the present invention, the methods include adding oxidant to the slurry in the range of about 1% to about 4% by weight of the dry pulp fiber. The slurry to which the oxidant is added can have a consistency in the range of about 0.1% to about 15%. In another aspect of the present invention, the slurry can have a consistency in the range of about 0.1% to about 12%. In yet another aspect of the present invention, the slurry can have a consistency of less than about 6%. In still another aspect of the present invention, the slurry can have a consistency of about 0.1 to about 6%. A variety of bleaching agents, such as air, oxygen, ozone, hydrogen peroxide, sodium hydrosulfite, chlorine, chlorine dioxide, or any combination thereof can be used in bleaching the delignified pulp. In one aspect of the present invention, the bleaching agent may be contacted with a delignified pulp slurry of any consistency, for example, a pulp slurry containing from about 0.5 to about 15 weight % pulp on a dry weight basis. Accordingly, delignified pulp exiting the delignifying stage can be introduced into the bleaching stage without changing the consistency of the pulp. Alternatively, the consistency of the delignified pulp can be raised or lowered before the pulp is introduced into the bleaching stage. In another aspect of the present invention, a lignocellulosic pulp is contacted with a bleaching agent in the presence of cavitation. In yet another aspect of the present invention, a secondary pulp is contacted with the bleaching agent in the presence of cavitation. As defined herein, secondary pulp comnprises any fibrous material that has undergone a manufacturing process and is being recycled as the raw material to produce a new manufactured product. The pulp (delignified, lignocellulosic, or secondary pulp) and bleaching agent may be contacted together in the presence of cavitation under any range of reactor conditions. For example, the pulp and bleaching agent may be contacted for about one second to about 120 seconds. The bleaching agent can be added to the pulp during mixing or immediately before mixing. Typically, the bleaching agent, for example, as ozone, is introduced into the reactor in an amount up to 16 kg/ton of pulp. The contacting can take place at any pH, for example, a pH range of about 1.5 to about 4. Suitable reactor temperatures include, but are not limited to, ambient temperatures, for example, temperatures ranging from about 70 to about 850F, or higher and lower temperatures, for example, temperatures ranging from about 60 to about 1350F. Suitable reactor pressures include, but are not limited to, from about 30 to about 100 psig. Referring now in more detail to the drawings, in which like numerals refer to like parts throughout the several views, FIG. 1 is a flow diagram of a typical processing scheme for producing paper from wood chips 105. First, the wood chips 105 are converted into lignocellulosic pulp 115 in a digesting stage 110. The lignocellulosic pulp 115 is subsequently delignified in a delignifying stage 120 to produce a delignified pulp 125. The delignified pulp 125 can optionally be bleached or brightened in a bleaching stage 130 to form a bleached, delignified pulp 135. The bleached, delignified pulp 135 can subsequently be rolled into sheets at a paper plant 140. The digesting stage 110 typically consists of a conventional wood chip cooker where the wood is reduced to a fibrous mass. Examples of conventional digesting processes are mechanical, chemical, and semichemical pulping processes. Mechanical pulping comprises a process wherein wood is pressed lengthwise against a rough, revolving grinding stone or other suitable material for reducing the wood into a fibrous mass. Other typical mechanical pulping processes involve the shredding or grinding of wood chips between the rotating discs of a refiner device. The resulting fibrous mass is typically known as a refined mechanical pulp (RMP). Optionally, a thermal pre-softening step is employed before the grinding step to reduce the amount of energy required to grind the wood and to modify the resultant pulp properties. The combination of pre- softening and mechanical grinding is known in the art as thermo-mechanical pulping. Chemical pulping comprises a process to degrade and dissolve away the lignin in wood such that the cellulose and hemicellulose remain in the form of substantially intact fibers. Chemical pulping typically involves cooking wood chips with the appropriate chemicals in an aqueous solutaion at elevated temperatures and pressures. Kraft chemical processing comprises cooking the wood chips with sodium hydroxide (NaOH) and sodium sulfide (Na2S). Alternative chemical processes comprise cooking the wood chips with sulphurous acid (H2SO3) and hydrogen sulfite (HSO3). Semichemical pulping comprises a combination of chemical and mechanical pulping processes. For example, the wood chips are partially softened or digested with chemicals and then further digested using a mechanical pulping device. The delignifying stage 120 and bleaching stage 130 may comprise a cavitation reactor or a conventional reactor. For example, the lignocellulosic pulp 115 may be delignified in the presence of cavitation, followed by a conventional bleaching stage 130. In another aspect, the lignocellulosic pulp 115 may be processed in a deligniiϊcation stage 120 comprising a conventional delignification reactor, followed by a bleaching stage 130 comprising a cavitation reactor. In yet another aspect, the lignocellulosic pulp 115 may be delignified in the presence of cavitation, followed by bleaching in the presence of cavitation. FIG. 2 illustrates a system 100 comprising an apparatus in which a lignocellulosic pulp slurry can be delignified and/or bleached. The system 100 includes a reactor 11 in which the slurry is exposed to cavitation. The system 100 also includes a feed tank 50 which contains the pulp slurry that is to be delignified and/or bleached. The pulp slurry can be low or medium consistency. The feed tank 50 is in flow communication with the reactor 11 by delivery line 55, which has a flow meter 60 disposed therein for monitoring the flow rate and/or amount of slurry flowing through the delivery line 55. A feed pump 65 also is provided in flow communication with the delivery line 55 to pump the slurry from the feed tank 50 to the reactor 11. A gas inlet 28 is provided in flow communication with the delivery line 55 to allow the introduction of gaseous oxidizing and/or bleaching agents into the slurry stream as it flows to the reactor 11. An electric motor 70 is operably connected to the shaft 18 of the cavitator 20 so as to provide the driving force for rotating the rotor 17 of the cavitator 20. As used herein, the term "cavitator" refers to a device that can induce cavitation in a fluid. Also, as used herein, the term "mechanical cavitator" refers to a device that can induce cavitation in a fluid by moving a body through the fluid. An exit line 73 is in flow communication with the reactor 11 and routes the mixture of slurry and oxidizing and/or bleaching agent to a retention tank 80. The mixture of slurry and oxidizing and/or bleaching agent can be retained in the retention tank for a predetermined period of time or simply until an appropriate amount of delignification and/or bleaching has occurred, as can be calculated from determining the Kappa Numbers of the slurry over time. Sample lines 77 can be provided in-line with the exit line 73 and the product line 75 to allow samples to be taken to determine Kappa Numbers of the slurry and monitor quality. As shown in FIGS. 2 and 3, the reactor 11 comprises a cylindrical housing 12 defining an internal cylindrical chamber 15. hi the figures, the housing 12 is formed of a wall 13 capped by end plates 14 secured to each other by bolts 16. The wall 13 is sandwiched between the plates 14. The cylindrical rotor 17 is disposed within the cylindrical chamber 15 of the housing and is mounted on the axially extending shaft 18. The shaft 18 is journaled on either side of the rotor within bearing assemblies 19 that, in turn, are mounted within bearing assembly housings 21. The bearing assembly housings 21 are secured to the housing 12 by means of appropriate fasteners such as bolts 22. The shaft 18 projects from one of the bearing housings 21 and is coupled to the electric motor 70 or other motive means. It will thus be seen that the rotor 17 may be spun or rotated within the cylindrical chamber 15 in the direction of arrows 23 by activating the motor 70 coupled to the shaft 18. The rotor 17 has a peripheral surface that is formed with one or more circumferentially extending arrays of irregularities in the form of relatively shallow holes or bores 24. As shown in FIG. 3, the rotor 17 is provided with five arrays of bores 24 separated by voids 26, the purpose of which is described in more detail below. It should be understood, however, that fewer or more than five arrays of bores may be provided in the peripheral surface of the rotor as desired depending upon the intended fluids and flowrates. Further, irregularities other than holes or bores also may be provided. The rotor 17 is sized relative to the cylindrical chamber 15 in which it is housed to define a space, referred to herein as a cavitation zone 32, between the peripheral surface of the rotor and the cylindrical chamber wall 13 of the chamber 15. An inlet port 25 is provided in the housing 12 for supplying from the delivery line 55 the slurry to be delignified and/or bleached in the interior chamber 15. Gaseous oxidizing and/or bleaching agents, such as air, molecular oxygen, ozone, chlorine, chlorine dioxide, or any combination thereof, can be introduced into the delivery line 55 through the gas supply conduit 28 and entrained in the form of bubbles within the stream of slurry flowing through the delivery line 55, if desired. Alternatively, the oxidizing and/or bleaching agent can be introduced into the slurry in liquid form. Oxidants such as sodium hydrosulfite, chlorine, chlorine dioxide, hydrogen peroxide, or any combination thereof can be introduced into the delivery line 55. At the junction of the delivery line 55 and the gas supply conduit 28, the slurry and oxidizing and/or bleaching agent form a gas/slurry mixture in the form of relatively large gas bubbles 31 entrained within the flow of slurry 29. This mixture of slurry and gas bubbles is directed into the cylindrical chamber 15 of the housing 12 through the inlet port 25 as shown. An outlet port 35 is provided in the housing 12 and is located in the cap 14 of the housing opposite to the location of the inlet port 25. Location of the outlet port 35 in this way ensures that the entire volume of the gas/slurry mixture traverses at least one of the arrays of bores 24 and thus moves through a cavitation zone prior to exiting the hydrosonic mixer 11. The outlet port 35 is in fluid communication with the exit line 73, which directs the gas/slurry mixture to the retention tank 80. hi operation, the reactor 11 functions to mix the pulp slurry with the oxidizing and/or bleaching agent and induce cavitation in the slurry to effectuate thorough mixing. A slurry containing lignocellulosic pulp exhibiting a first Kappa number is pumped from the feed tank 50 through the delivery line 55. A gaseous oxidant is supplied through the gas supply conduit 28 to the slurry stream, which then form a mixture comprised of relatively large gas bubbles 31 entrained within the slurry 29. The slurry/gas bubble mixture moves through the delivery line 55 and enters the chamber 15 through the supply port 25. From the supply port 25, the mixture moves toward the periphery of the rapidly rotating rotor 17 and enters the cavitation zones 32 in the region of the bores 24. As described in substantial detail in U.S. Pat. No. 5,188,090, the disclosure of which is hereby incorporated by reference, within the cavitation zones 32, millions of microscopic cavitation bubbles are formed in the mixture within and around the rapidly moving bores 24 on the rotor. Since these cavitation bubbles are unstable, they collapse rapidly after their formation. As a result, the millions of microscopic cavitation bubbles continuously form and collapse within and around the bores 24 of the rotor, creating cavitation induced shock waves that propagate through the mixture in a violent albeit localized process. As the mixture of slurry and relatively large gas bubbles moves into and through the cavitation zones 32, the gas bubbles in the mixture are bombarded by the microscopic cavitation bubbles as they form and further are impacted by the cavitation shock waves created as the cavitation bubbles collapse. This results in a "chopping up" of the relatively large gas bubbles into smaller gas bubbles, which themselves are chopped up into even smaller gas bubbles and so on in a process that occurs very quickly. Thus, the original gas bubbles are continuously chopped up and reduced to millions of tiny microscopic gas bubbles within the cavitation zone. The dispersement and random flow patterns within the cavitation zone 32 provide a high degree of mixing of the oxidant and slurry. Some conventional systems do not achieve a thorough mixing of the oxidant and slurry, thus requiring the addition of substantially more oxidant into the slurry, resulting in increased costs and still not guaranteeing even mixing of the combination. The turbulence of the fluids within the cavitation zone 32 leads to more complete mixing of the oxidant with the slurry. The term "cavitation zone" is used herein to refer to any region in which cavitation is induced in the lignocellulosic pulp slurry, and, more particularly, a region specifically established for the generation of cavitation within the slurry. In regards to the reactor 11, shown in FIGS. 2 and 3, the term "cavitation zone" refers to the region between the outer periphery of the rotor wherein the bores are formed and the cylindrical wall of the housing chamber. This area is where the most intense cavitation activity occurs. It should be understood, however, that cavitation may occur, albeit with less intensity, in regions other than this space such as, for example, in the reservoir or region between the sides or faces of the rotor and the housing. The term "cavitation region" is used herein to refer to a region comprising a plurality of cavitation zones, wherein each cavitation zone has a void zone adjacent thereto. For example, in the apparatus shown in FIGS. 2 and 3, the cavitation zone may be the area between the outer periphery of the rotor 17 where the bores 24 are formed and the cylindrical wall of the housing chamber, while the void zone may be the area between the outer periphery of the rotor 17 where the voids 26 are formed and the cylindrical wall of the housing chamber. However, any apparatus which provides a cavitaiton region as described herein is intended to be within the scope of the present invention. The process of cavitating the oxidant/slurry mixture can be on a substantially continuous basis in that a continuous flow of slurry is pumped into the hydrosonic mixer 11, treated by cavitation and then discharged from the reactor 11. Alternatively, the process can be conducted on a batchwise basis, wherein a specified amount of slurry and oxidant is charged to the reactor 11 , cavitated, and then discharged before any additional material is charged to the mixer. Delignification and/or bleaching occurs within the cavitation zone of the reactor 11 and can continue in the retention tank 80 if so desired. The oxidizing and/or bleaching agent and slurry are mixed within the reactor 11 under pressure, typically in the range of about 480 kPa to about 1035 kPa. The residence time of the slurry within the reactor generally is within the range of about 20 seconds to about 60 seconds, although this range can vary depending upon the flowrate and size of the mixer. The pulp contained within the slurry is delignified and/or bleached within the mixer. In one example, a delignified pulp exhibiting about 20% to about 25% delignification can be produced within the reactor. If additional delignification is desired, the delignified oxidant/slurry mixture can be discharged from the reactor 11 through outlet port 35, exit line 73 and into retention tank 80. The mixture can be retained in the retention tank for a period of time under pressure to further delignify the pulp. In one example, further delignification in the range of up to about 52% can be effectuated by retaining the mixture in the retention tank 80 for a time period in the range of about 5 to about 30. When the desired amount of delignification is achieved, the slurry can be directed through the product line 75 for further processing, such as washing, bleaching, etc. Although the methods of the present invention have been illustrated being carried out using a reactor as shown in FIGS. 2 and 3 and additionally described in the incorporated references, the methods for delignifying lignocellulosic pulp in the presence of cavitation are not limited to being carried out only with such devices. Rather, any apparatus or system that can generate cavitation in the pulp slurry can be used in conjunction with the methods of the present invention. For example, systems employing venturiz nozzles, sonic wave generators, or other mechanical mixers that produce cavitation in the slurry can be used.

EXAMPLES Example 1

Oxygen Delignification Softwood slurry with consistency of 5% was heated to 195° F. Either black liquor or sodium hydroxide was added to the slurry to adjust the pH of the slurry to about 12, so as to mimic process conditions for before and after washing, respectively. Hot slurry was

pumped through the cavitating reactor and the line pressure was increased to about 95

psig. Oxygen was added to the slurry before it entered the cavitating reactor. The rotor

of the reactor included thirty holes or bores per row and was driven by a variable electric

motor. The cavitating reactor was made of 316 stainless steel and was sixteen inches in

length and three inches in width. The rotor was set at about 1200 rpm during the trials.

The slurry went to a retention tank for residence time after leaving the reactor. Samples

were taken at the feed tank and after the retention tank.

Table 1

Example 2

Oxygen Delignification of Softwood Pulp Before Washers

Trials were run in the cavitating reactor of Example 1 to determine the amount of

oxygen necessary to achieve desired delignification. Before washing of lignocellulosic

pulp slurry using cavitation, pulp slurries typically carry black liquor solids that are oxidizable by the oxidant intended for delignification. Consequently, the amount of

oxygen provided to delignify the pulp should be sufficient to allow for the competing

reaction of the oxidation of the black liquor.

Table 2 shows the process conditioning for the trials to determine oxygen

requirement for other oxidizeable compounds in black liquor. FIG. 4 shows the results of

the trial.

Table 2

Results show that black liquor solids (excluding fiber) oxidize in the presence of

oxidizing agent as does the lignin. The results shown in FIG. 4 indicate that the rate of

delignification is independent of the competing reaction as long as there is enough

oxidant for all the reactions to occur.

Example 3

Oxygen Delignification

In this example about 2.5% hardwood slurry after washers was sent through the

reactor and then to the retention tank. The slurry temperature was about 90°C and the pH was about 11.8. The starting Kappa Number was about 13. Oxygen was added to the slurry prior to the slurry entering the cavitating reactor, which was the same as described in Example 1. Samples were taken at the feed tank, after the reactor and before the retention tank, and after the retention tank. The results shown in FIG. 5 indicate that during the brief time period of about one minute in the reactor, the slurry exhibited up to about 20% of delignification. Example 4 Ozone Delignification A well-washed kraft hardwood pulp slurry with about a 3% pulp consistency and an initial Kappa number of about 13.5 was obtained and used at ambient temperatures and an initial pH of about 2.5. The pH was adjusted with sulfuric acid and the temperature was adjusted using direct steam. The slurry was pumped through a cavitation reactor at a rate of about 2.5 to about 3 Tons/Day and the speed of the cavitation rotor was varied from 900 to 3600 rpm. The cavitation reactor was a ShockWave Power Generator with a 15.4" x 2" rotor, 150 HP motor, and an adjustable-frequency AC drive. The ozone charge was kept constant at about 0.53%. Ozonated pulp from the reactor went into an upflow column (0.25 x 2.7 m) with a pressure control valve at the top. Pulp was discharged from the column to an enclosed tank with a gas separator. The ozone concentration of the separated gas was determined with a PCI monitor. Pulp samples for testing were collected from a sampling valve in the discharge piping following the upflow column. The ozonated pulp samples were caustic extracted at 10% consistency with about 1.25% NaOH for about 60 minutes at about 7O0C in plastic bags in a water bath. The Kappa number was measured using standard

method TAPPI UM246. The delignifϊcation efficiency from the ZE (ozone

delignification "Z" followed by caustic extraction "E") partial sequence was calculated as

the decrease in Kappa number per kg of ozone applied. Inlet and outlet temperatures and

pressures were measured in the feed and discharge lines. The results are shown in Table

3.

The delignification efficiency was in a range of about 1.8 to about 2.1 kappa unit

decrease per kg ozone charged. A systematic effect on the efficiency was not observed

with an increase in reactor speed, since the lowest efficiency appeared to be obtained at

the intermediate speed of 1800 rpm.

Table 3

Example 5 Ozone Delignification

A well-washed kraft hardwood pulp slurry with about a 2.9% pulp consistency, an

initial Kappa number of about 11, and an initial pH of about 2.5 was processed as in

Example 4. The rotor speed was varied from 900 to 3600 rpm, and the temperature was

maintained at either about 280C or about 5O0C. Black liquor carryover was about 2.1 kg

COD/Oven Dried Metric Ton. These results are shown in Table 4. An increase in

efficiency was observed as the reactor speed was increased from 900 to 3600 rpm. There

also appeared to be a decrease in efficiency at higher temperatures for a given ozone

charge.

Table 4

Example 6 Ozone Delignification

A well-washed kraft hardwood pulp slurry with about a 2.7% pulp consistency, an

initial Kappa number of about 14.9, an initial viscosity of about 33.0 cps (as determined

by standard method TAPPI T230), and an initial pH of about 2.5 was processed as in

Example 4. The rotor speed was varied from 900 to 3600 rpni, and the temperature was

maintained at about 280C. Black liquor carryover was about 14 kg COD/ODMT. These

results are shown in Table 5.

Table 5

Example 7 Ozone Delignification

A well-washed kraft hardwood pulp slurry with about a 2.8% pulp consistency, an

initial Kappa number of about 12.7, and an initial pH between about 2.0 to about 2.8 was

processed as in Example 4. The rotor speed was varied from 900 to 3600 rpm, and the

inlet temperature ranged from about 210C or about 550C. These results are shown in

Table 6.

Table 6

A statistical analysis of the ozone delignifiaction data, excluding the data set from

Example 4, showed that the delignification efficiency increased with the reactor speed.

There was no clear trend in the effect of ozone charge or temperature on delignification efficiency at any given speed. The delignification efficiency as a function of speed at ambient temperatures (about 2O0C to about 3O0C) for different ozone charges is shown in Figure 6. The delignification efficiency at higher temperatures (about 5O0C to about 600C) is shown in Figure 7.

Example 8 Ozone Delignification A well-washed kraft hardwood pulp slurry with about a 2.0% pulp consistency and an initial Kappa number of about 14.4 was obtained and used at temperatures ranging between about 51 to about 560C and an initial pH of between about 2.2 to about 2.6. Black liquor carryover was increased to either about 30 or about 73 kg COD/Metric Ton. The slurry was pumped through a cavitation reactor and the speed of the cavitation rotor was maintained at about 1800 rpm. A reinforced extraction (EPo) stage was used following the ozone delignification stage. EPo stages were carried out in a pressurized peg mixer at about 10% consistency for about 60 minutes at about 770C with about 1.50% NaOH and about 0.25% H2O2. The initial pressure was about 60 psig O2 which was decreased to about 0 psig over the course of the reaction. The delignification efficiency from the Z(EPo) partial sequence was calculated as the decrease in Kappa number per kg of ozone applied. The results are shown in Table 7. The higher ozone charges used and use of the (EPo) stage indicates that the ozone is somewhat less efficient with the higher carryover. Table 7

Example 9 Pulp Quality Improvement

The pulp viscosity was measured for each of the hardwood pulps in Examples 6

and 7, before and after ozone delignification. A reduction in viscosity following the ZE

stages was observed as a function of the reduction in Kappa number, as shown in Figure

8. Handsheets were prepared from the hardwood pulps in Examples 6 and 7, before and

after ozone delignification using standard method TAPPI T205. The reduction in pulp

viscosity did not result in a loss in handsheet tear and tensile strengths (as determined by

standard method TAPPI T220), as shown in Table 8. A refining effect was observed

based on the 25-30% reduction in freeness. There was a corresponding increase in tear

strength of about 10% and in tensile strength of 15-20%. Table 8

Example 10

Improved Gas/Liquid Mixing

Oxygen and water were mixed together using various types of reactors and the

KLa constant (volumetric gas/liquid mass transfer coefficient) was measured. As shown

below in Table 9, the cavitating Shockwave Power Generator exhibits significantly higher

KLa values (i.e., improved mass transfer properties) as compared to conventional reactors. Although not wishing to be bound by theory, it is believed that the improved KLa values obtained using the Shockwave Power Generator are due to the reactor's ability to significantly reduce the size of the oxygen bubbles within the oxygen/water mixture.

Table 9

Example 11 Ultrasonic Effect Softwood slurry with consistency of 4 wt% pulp and an initial Kappa number of about 27.2 was pumped through the cavitating reactor with a rotor set at about 1200 rpm during the trials. In Trial A, the slurry was pumped through the cavitating reactor in the absence of additional components. NaOH was added to the slurry in an amount sufficient to reach a pH of about 11.5, and this mixture was pumped through the cavitating reactor in the presence of NaOH for Trial B. In Trial C, oxygen and NaOH were added to the slurry to obtain a mixture and the mixture was pumped through the cavitating reactor. The mixture contained about 1.5 wt% oxygen and had an initial pH of about 11.5. Samples were taken for each trial and the final Kappa number was measured. The results are shown below in Table 10.

Table 10

As shown in Table 10, the slurry exhibits a decrease of about 1.5 in the Kappa number, even in the absence of oxidizing agents. This decrease can be attributed to the ultrasonic effect of the cavitation on the lignocellulosic fiber, resulting in the opening of the fiber pores and the removal of fragmented lignin from the fiber. Further, because of the pore-opening effect, when oxidizing and/or bleaching agents are used in combination with the cavitating reactor, the chemicals have easier access to the lignin in the lignocellulosic fiber. Although certain aspects of the invention have been described and illustrated, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.