WO1992012289A1 - Chlorine-free process for bleaching lignocellulosic pulp - Google Patents

Abstract

A process for delignifying and bleaching a lignocellulosic pulp without the use of elemental chlorine or chlorine-containing compounds by oxygen delignifying (40) the pulp to a K No. of about 10 or less, a viscosity of greater than about 12 cps and a GEB of about 33-43; and thereafter further delignifying (78) the partially delignified pulp by lifting, displacing and tossing the pulp in a radial direction while advancing it in an axial direction in a plug flow-like manner with an effective amount of ozone (88) for a sufficient time to obtain a substantially delignified pulp having a K No. of about 5 or less, a viscosity of at least about 9-10 cps and a GE brightness of at least about 59. The substantially delignified pulp (102) may then be brightened to a final product having a GE brightness of at least about 75, or alternately up to about 83+ by contacting the ozonated pulp with a sufficient amount of a peroxide compound for a time of up to about three hours, with optional mixing of the pulp/peroxide mixture.

Description

CHLORINE-FREE PROCESS FOR BLEACHING LIGNOCELLULOSIC PULP

Field of the Invention

The invention relates to methods of bleaching lignocellulosic materials and more particularly, to a multi-stage, chlorine-free process for bleaching wood pulp.

Background of the Invention

The processing of chemical and semi-chemical cellulosic pulps in the manufacture of various grades of paper and paper products generally requires that such pulps be subjected to several successive bleaching treatments. These bleaching treatments are optionally interspersed with various washing, dilution, extraction and/or concentration stages in order to arrive at a final product having a desired lignin content and a desired brightness.

It has been conventional for many years to delignify and bleach wood pulp with elemental chlorine ("C") and/or chlorine-containing compounds such as chlorine dioxide ("D") . This process is described, for example, in U.S. patents No. 1,957,937 to Campbell et al.; 2,975,169 to Cranford et al., and 3,462,344 to Kindron et al., as well as in the Handbook For Pulp and Paper Technologists - Chapter 11: Bleaching (§11.3), TAPPI, USA.

However, although compounds such as those described above have proven to be effective bleaching agents, they suffer from several deficiencies, e.g., they are difficult to handle and they cause corrosion of the processing equipment within the mill. In addition, concern about the possible environmental effects of disposing of chlorine-containing effluents from pulp bleaching mills by sewering these effluents has led to significant changes in government requirements and permits for such mills. These new rules mandate more stringent standards for the handling of such effluents. Moreover, the recycle of these effluents is not in itself a satisfactory answer since the build-up of chlorides within the mill over time precludes the operation of such a closed system without employing recovery techniques requiring extensive, and therefore expensive, modifications.

In an effort to overcome these disadvantages, those working in this field have extensively examined numerous alternative bleaching processes designed to reduce or eliminate the use of elemental chlorine and chlorine-containing compounds from multi-stage bleaching processes for lignocellulosic pulps. These alternative processes utilize, for example, various combinations of oxygen ("0") , ozone ("Z") , alkaline extraction ("E") and peroxides ("P") , to name but a few of the chemicals used. Complicating these efforts, however, is the requirement that high levels of pulp brightness are necessary for many of the applications for which such pulp is to be used. The prior art processes which utilize these materials in various combinations are, however, often unable to achieve these high pulp brightness levels without an unacceptable loss in pulp strength, as evidenced by a corresponding decrease in viscosity of the pulp product.

One chlorine-free bleaching sequence is disclosed in U.S. patent No. 4,196,043 to Singh. This process is characterized by the use of from one to three ozone bleaching stages and a final treatment by alkaline hydrogen peroxide, with each bleaching stage being separated by an alkaline extraction step. One such sequence is designated, for example, in the chemical shorthand prevalent in the paper industry, as ZEZEP. Moreover, further in accordance with this process, the effluent from each bleach treatment stage may be collected, recycled and reused in bleaching the pulp. Another example of a bleaching system which has been disclosed for use in place of the aforementioned chlorine-containing systems is designated as OZEP. This multi-step process, which is described for example in Liebergott et al., "Bleaching a Softwood Kraft Pulp Without Chlorine Compounds", TAPPI Journal. 76 (August 1984) and N. Soteland, "Bleaching of Chemical Pulps With Oxygen and Ozone", Pulp and Paper Magazine of Canada. T153-58 (1974) subjects the lignocellulosic pulp to an initial oxygen delignification stage, followed by a treatment of the partially delignified pulp with a gaseous ozone bleaching agent. Subsequent to the ozone treatment, the pulp is subjected to an alkaline extraction to remove a substantial portion of any remaining lignin and to prevent redeposition upon the pulp of the by-products from the previous reactions. This treatment is then followed by a final bleaching treatment with an alkaline peroxide solution.

A major goal of those working in this industry has been to simplify the bleaching process by reducing both the volume of bleaching chemicals used as well as the time required for obtaining the bleached pulp. One such modified sequence, i.e., designated as OZP (oxygen, ozone, peroxide) , eliminates the extraction ("E") step between the ozone and the peroxide bleaching stages of the OZEP process described above. OZP sequences are discussed, for example, in U.S. patents No. 4,459,174 to Papageorges et al. and 4,450,044 to Fritzvold et al., as well as in Liebergott et al., supra; Soteland, "Bleaching of Chemical Pulps With Ozone and Oxygen", 75 Pulp and Paper Magazine of Canada, 4, p. T-153 (April 1974) and Norsk Skogindustri, p. 199 (September 1978) ; Soteland, "Comparison Between Oxygen and Ozone Delignification of Sulphite Pulps", Report of 1984 TAPPI Symposium on Oxygen Delignification, p. 71 and Patt et al., "Use of Ozone For Pulp Bleaching", Papier 42 (10A) : p. 14-23 (October 1988) . Despite all of the research conducted in this area, however, applicants are not aware of any commercially feasible process utilizing an OZP sequence, such as that disclosed and claimed herein, which is capable of producing semi-bleached pulps having a GE brightness of at least up to 75, as well as pulps having a GEB of 83 or greater, without a corresponding unacceptable loss in pulp strength.

Summary of the Invention The process of the present invention provides novel combinations of delignification and bleaching steps which eliminate the problems encountered in the prior art by increasing the efficiency and reducing the cost and duration of pulp bleaching. The multi-stage process of the invention also eliminates the use of elemental chlorine and/or chlorine-containing bleaching agents, thus substantially reducing or eliminating pollution of the environment while optimizing the physical properties of the resultant pulp product in an energy efficient, cost effective manner. The present process can work on virtually all wood species, including the difficult-to- bleach southern U.S. softwoods, as well as the more readily bleached hardwoods.

Applicants¹ process includes several stages, i.e., a pulping stage, an oxygen delignification stage and an ozone delignification/bleaching stage which together comprise what will be referred to herein as the "front end" of the process. In addition, the process further comprises a hydrogen peroxide bleaching stage, which follows directly upon completion of the ozone treatment. The peroxide bleaching stage is referred to in the present specification as the "back end" of the process.

The present process does not require or utilize an extraction stage between the ozone delignification/ bleaching treatment and the peroxide bleaching stage, thus reducing both its cost and duration in contrast to prior art bleaching processes, while obtaining pulp with GE brightness values comparable to those in the prior art without sacrificing pulp strength.

The present process reduces the amount of lignin as much as is practical in the front end of the process (as evidenced by a corresponding decrease in the K No. of the pulp) without a concomitant substantial (and therefore unacceptable) decrease in pulp strength. This, in turn, ensures that the viscosity of the pulp exiting the ozone delignification bleaching stage remains sufficiently high to permit the pulp to withstand the effects of the subsequent peroxide bleaching treatment, thus enabling the formation of a final bleached pulp product having sufficient strength and GE brightness ("GEB") for its intended application.

Following the ozone delignification stage, the substantially delignified pulp preferably has a GEB of from about 59-65. The subsequent hydrogen peroxide bleaching treatment raises the GE brightness value to about 75 or, in an alternate embodiment, to at least about 83-86. The peroxide treatment phase of the invention may be carried out along a number of paths as described below, with the option chosen depending, at least in part, upon the final GE brightness which is desired for the finished pulp.

Brief Description of the Drawings

Fig. 1 schematically illustrates the pulping, oxygen delignification and ozone delignification/bleaching steps which comprise the front end of the process of the invention;

Fig. 2 schematically illustrates the peroxide bleaching stage at the back end of applicants' process; and

Fig. 3 is a plot of ozone GEB vs. peroxide ceiling GEB obtained with a Kraft-AQ/O_m/Z_m/P pulp. petailed Description of the Preferred Embodiments

A. The "Front End" of the Process 1. Pulping

The first stage in the method of the present invention is the pulping step. Here, procedures may be utilized which improve the amount of lignin removed from the lignocellulosic material, while minimizing the amount of degradation of the cellulose.

A processing scheme for carrying out the "front end" of the method of the present invention is illustrated in schematic form in Fig. 1. Wood chips 2 are introduced into a digester 4 together with a white liquor 6 comprising sodium hydroxide, sodium sulfide and, in the preferred embodiment, an anthraquinone additive. Sufficient white liquor should be introduced into digester 4 to substantially cover the wood chips. The contents of digester 4 are then heated at a temperature and for a time sufficient to allow the liquor to substantially impregnate the wood chips.

The use of the Kraft/AQ pulping technique is preferred since the inclusion of the anthraquinone additive contributes significantly to the degree of lignin removal without causing significant adverse affects upon the desired strength characteristics of the remaining cellulose. The amount of anthraquinone in the cooking liquor should be at least about 0.01% by weight, based upon the oven dried ("OD") weight of the wood to be pulped, with amounts of from about 0.02 to 0.1% generally being preferred. Although the Kraft/AQ technique costs more to perform than, for example, an unmodified Kraft treatment, this additional cost is at least partially offset by the savings in the cost of chemicals needed for the subsequent oxygen, ozone and peroxide stages.

Alternately, or perhaps even in addition to the use of the Kraft/AQ process, the pulping stage can be carried out with the use of techniques for extended delignification such as the Ka yr MCC, Beloit RDH and Sunds Cold Blow methods. These techniques also offer the ability to remove more of the lignin during cooking without adversely affecting the desired strength characteristics of the remaining cellulose to a significant degree. Still further, the pulping stage may be carried out, if desired, with the use of an unmodified Kraft process. The brownstock pulp which results from the use of this technique is darker in color than that obtained with the Kraft/AQ and extended delignification processes described above due to the presence of a greater amount of unbleached lignin remaining in the pulp. The Kraft process is therefore not particularly practical for use in the present invention unless it is coupled with an oxygen treatment which is sufficient to remove correspondingly more lignin in the oxygen delignification step without adversely affecting the strength of the oxygen delignified pulp. This combination is capable of producing a pulp of sufficient brightness and viscosity to permit effective ozone and peroxide bleaching to the high GEB values stated above. In contrast, the combination of Kraft pulping plus "standard" oxygen delignification (described below) produces a pulp which does not retain sufficient viscosity, i.e., strength, to form a useful product upon completion of the remainder of applicants' process as described herein.

The pulping step is conducted so that, for a southern U.S. softwood, for example, a K No. in the range of about 16-20, a cupriethylenediamine ("CED") viscosity in the range of about 18-28, and a GE brightness in the range 15-25 is typically obtained. For southern U.S. hardwood, one may obtain pulp with a K No. in the range of about 10-14 and a CED viscosity of about 21-28.

Turning again to Fig. 1, the digester 4 produces a black liquor containing the reaction products of lignin solubilization together with the brownstock pulp 8. The cooking step is typically followed by washing to remove most of the dissolved organics and cooking chemicals for recycle and recovery, as well as a screening stage (not shown) in which the pulp is passed through a screening apparatus to remove bundles of fibers that have not been separated in pulping. The brownstock 8 is treated in 5 washing units comprising, in sequence, a blow tank 10 and washing unit 12 where residual liquor 14 contained in the pulp is removed.

2. Oxygen Delignification 10 The next stage in the process of the present invention, i.e., the oxygen delignification step, primarily involves removal of residual lignin from the brownstock pulp. In one oxygen delignification technique (i.e., "0") well-known in the prior art, the washed pulp

15 is pressed to a high consistency of at least about 25% and an aqueous alkaline solution is then sprayed onto the resultant fiber mat to deposit from about 0.8 - 7% by weight of the alkaline material onto the pulp. The high- consistency alkaline fiber mat is then subjected to oxygen

20 delignification to remove a substantial portion of the lignin from the pulp. Since, however, the procedure, when used to obtain substantial decreases in K No., i.e., greater than 50%, is known to result in a substantial decrease in pulp viscosity, i.e., strength, it is

25 important to couple this technique with one of the more efficient pulping processes, such as Kraft/AQ and/or extended delignification or Kraft plus extended delignification, in order to obtain pulp with sufficiently low K Nos. for use in the remainder of applicants'

30 process.

It has been found that the oxygen deligni¬ fication treatment may be modified and conducted in a manner which allows for the removal of increased __. percentages of the lignin remaining in the brownstock pulp without causing an unacceptable corresponding decrease in the viscosity of the pulp. This allows conventional Kraft pulping to be used with such modified oxygen delignification techniques while still obtaining the desired K Nos. and viscosities.

In one process, designated herein as 0_m (m=modified) , the brownstock pulp is treated at low to medium consistency with an amount of alkali necessary to ensure uniform application thereof upon the pulp. The brownstock is maintained at a pulp consistency of less than about 10% and preferably less than about 5% by weight. The consistency of the pulp is generally greater than about 0.5%, however, since lesser consistencies are not economical to process in this manner. A most preferred consistency range is 0.5 to 4.5%. Thereafter the consistency of the pulp is raised to at least 18 percent. Preferably the pulp consistency is raised from about 20-25% to about 35%, and even more preferably, to about 27%. Thereafter the high consistency pulp is directed to an oxygen reactor for delignification using conventional conditions.

The advantage of using the O_m process is illustrated by comparison of the K Nos. and viscosities obtained using southern softwoods to those obtained with the O process under otherwise substantially identical process conditions. Using a conventional Kraft pulping procedure and conventional high consistency oxygen ("0") delignification bleaching, the pulp thus obtained will typically have a K No. of about 12 to 14 and a viscosity of about 15. This K No. is too large to permit later delignification using the ozone stage of the present invention. However, the use of conventional Kraft pulping with the modified high consistency oxygen bleaching surprisingly results in a pulp having a K No. of less than about 9, while the viscosity of the pulp is maintained above about 12 to 14.

These two values, i.e., K No. and viscosity, are related in that the ratio of the change in viscosity to the change in K No. , referred to as the "delignification selectivity" of the process, is a measure of the efficiency of the O_m technique for removing lignin while maintaining adequate levels of viscosity therein. The use of the 0_m process, as described above, thus results in an enhanced degree in the selectivity of the delignification, signified by a reduction in K No. of at least about 20% greater than that obtained with the use of an "O" stage. Thus, the combination of Kraft pulping and 0_m oxygen delignification will result in an enhanced delignification selectivity, i.e., a sufficiently low K No. and a sufficiently high viscosity, to permit further delignification and bleaching by ozone and peroxide.

Further details of the 0_m oxygen delignification process are disclosed in application serial no. 07/489,845 of Bruce F. Griggs entitled METHODS FOR HIGH CONSISTENCY OXYGEN DELIGNIFICATION, the disclosure of which is expressly incorporated herein by reference.

Alternately the oxygen delignification treatment may be carried out using a two-stage "O." (s = split) alkali addition. In this stage a first amount of alkaline material is applied to pulp at low consistency by combining the pulp with a quantity of alkaline material in an aqueous alkaline solution. The consistency of the pulp is then increased to a high consistency of at least about 18%. Next, a second amount of alkaline material is applied to the high consistency pulp to obtain a total amount of alkaline material applied to the pulp. After this treatment, the pulp is then subjected to oxygen delignification whereby the enhanced delignification selectivities of the 0_m process are achieved.

While the 0_m process is preferred over the standard "0" method, the alternate 0_S technique is most preferred because a lower proportion of the alkaline material (i.e., than with the 0_m process) is applied to the low consistency pulp. This, in turn, reduces the amount of alkaline material utilized in mixing chest 18 and also reduces the amount of this material removed via pressate discharge 32 (see below) . Thus, splitting the application of the alkaline material between the high and low consistency pulp reduces the amount of pressate discharge 32 which, in turn, reduces the amount of alkaline material which must be reintroduced, thus saving chemical. Further the high consistency alkaline treatment portion of the 0. method permits rapid modification of the amount of the alkaline material present in the pulp entering the oxygen delignification reactor to compensate for changes in the properties (i.e., wood type. Kappa or K. No. and viscosity) of the incoming brownstock, or to vary the degree or extent of oxygen delignification for a particular pulp.

Referring again to Fig. 1, washed brownstock 16 is introduced into a mixing chest 18 where it is substantially uniformly treated with sufficient alkaline material 20 for a time sufficient to distribute a first amount of alkaline material throughout the pulp. The low consistency treatment portion of this 0, process is carried out in the same manner as the 0_m process, but less alkaline material (i.e., about half as much) is applied to the pulp. In the 0_m process, an aqueous sodium hydroxide solution is combined with the low consistency pulp in an amount sufficient to provide essentially the same amounts on the OD pulp as was achieved by the O process. In the 0. process, at least about 0.4% to about 3.5% by weight of sodium hydroxide is deposited on the pulp, based on oven dry pulp after thickening with the balance applied to the high consistency pulp. Other alkali sources having equivalent sodium hydroxide content can also be employed instead of sodium hydroxide if desired. Oxidized white liquor is a convenient plant stream which may be utilized.

The alkaline treated pulp 22 is forwarded to a thickening unit 24 such as a twin roll press where the consistency of the pulp is increased to the desired value. The pulp consistency increasing step also removes residual liquid or pressate 26. A portion 28 of this pressate 26, ay be directly recycled back to brownstock washer 12.

Alternately, a portion 30 may instead be directed to mixing chest 18 for use in the low consistency pulp alkaline treatment step. Since the consistency of the pulp is increased in the thickening unit 24, a certain amount 32 of pressate may continually be discharged to the plant liquid recovery system to maintain water balance in the mixing chest 18.

Additional alkaline material 36 is applied to the high consistency brownstock 34 produced by the thickening unit 24 to obtain the desired total amount of alkaline material on the pulp prior to oxygen delignification. This total amount of alkaline material is selected to achieve the desired extent of delignification in the subsequent oxygen delignification step which is carried out on the alkaline material treated high consistency pulp. The total amount of alkaline material actually applied onto the pulp will generally be between 0.8 and 7% by weight based on oven dry ("OD") pulp, and preferably between about 1.5 and 4% for southern softwood and between about 1 and 3.8% for hardwood. About half these amounts are preferably applied in each of the low consistency and high consistency treatments. Thus, about 0.4 to 3.5% by weight, preferably about 0.5 to 1.9% for hardwood and 0.75 to 2% for softwood, is applied onto the pulp during each of the low and high consistency alkaline treatments.

Further details concerning the split addition, i.e., "0_", process are set forth in application serial no. of S. Terrett et al. filed on even date herewith and entitled SPLIT ALKALI ADDITION FOR HIGH CONSISTENCY OXYGEN DELIGNIFICATION, the disclosure of which is expressly incorporated herein by reference.

The alkaline treated pulp 38 is then forwarded to the oxygen delignification reactor 40 where it is contacted with gaseous oxygen 42. Suitable conditions for oxygen delignification according to either the O, O_m or 0_S processes comprise introducing gaseous oxygen at about 80 to about 100 psig to the high consistency pulp while maintaining the temperature of the pulp between about 90 and 130°C. The average contact time between the high consistency pulp and the gaseous oxygen ranges from about 15 minutes to about 60 minutes.

After oxygen delignification in reactor 40, the partially delignified pulp 44 is forwarded to washing unit 46 wherein the pulp is washed with water 48 to remove any dissolved organics and to produce high quality, low color pulp 50. A first portion 54 of the oxygen stage washer 46 filtrate 52 can be used to advantage in a first shower on the brownstock washer 12. This improves washing and reduces the pressate portion 55 which is used in a second shower on washing unit 12 and later returns into the residual liquor 14 which is sent to the plant recovery without further reuse. A second portion 56 of filtrate 52 is discharged directly to the plant recovery system. Upon completing the oxygen delignification stage, the delignification selectivity of the pulp is enhanced in that the K No. of the pulp is decreased by at least about 50%, compared to the decrease of no more than about 50% with conventional oxygen delignification systems, without significantly damaging the cellulose component of the pulp. For the softwood pulp described above, a K No. of about 7-10 and a viscosity of above about 13 is readily achieved. For hardwood pulp, a K No. of about 5-8 and a viscosity above about 13 is obtained after the oxygen delignification step.

3. Ozone Delignification/Bleaching

The next step in the process of the invention is ozone delignification and bleaching of the oxygen- delignified brownstock pulp. Treating pulp at high consistencies with ozone without paying particular attention to the comminution of the pulp fibers or to the contact between the individual fibers and the reactant gas stream invariably results in a non-uniform ozone bleaching of the fibers. Such a non-uniform ozone treatment is designated in the prior art with the letter "Z". While the use of a Z stage is not desirable due to the non- uniformities produced, there are situations where the resulting pulp is useful. However, it is preferred to use a modified ozone technique in which the fibers in a desired size range are uniformly contacted with the ozone gas stream. This ozone treatment has been designated herein as "Z_m".

Prior to treatment with ozone, the pulp is conditioned so as to ensure the most effective selective delignification and to minimize the chemical attack of the ozone on the cellulose. As illustrated in Fig. 1, the incoming pulp 50 is directed into a mixing chest 58, where it is diluted to a low consistency. An organic or inorganic acid 60 such as sulfuric acid, formic acid, acetic acid or the like, is added to the low consistency pulp to decrease the pH of the pulp in mixing chest 58 to the range of about 1 to 4 and preferably between 2 and 3.

The acidified pulp is treated with chelating agent 62 to complex any metals or metal salts which may be present therein. This chelating step is used to render such metals non-reactive or harmless in the ozone reactor so that they will not cause breakdown of the ozone, thus decreasing the efficiency of the lignin removal and also reducing the viscosity of the cellulose. Preferred chelating agents for this ozone treatment, for reasons of cost and efficiency, include diethylenetriamine pentacetic acid ("DTPA") , ethylenediamine tetraacetic acid ("EDTA") and oxalic acid. Amounts of these chelating agents ranging from about 0.1% to about 0.2% by weight of oven dry pulp are generally effective, although additional amounts may be needed when high metal ion concentrations are present.

The acidified, chelated, low-consistency pulp 64 is introduced into a thickening unit 66, such as a twin roll press, for removing excess liquid 68 from the pulp, wherein the consistency of the pulp is raised to a level above about 20%. At least a portion of this excess liquid 68 may be recycled to mixing chest 58 with a remaining portion 68a being directed to the plant recovery. The resultant high consistency pulp 70 is then passed through compaction device 72 such as a screw feeder which acts as a gas seal for the ozone gas and thereafter through a comminuting unit 74, such as a fluffer, for use in reducing the pulp particle size as described below. A preferred range of consistency, especially for southern U.S. softwood, has been found to be between about 28% and 50%, with the optimum results being obtained at between about 38% and 45% prior to contact with ozone. Within the above ranges, preferred results are obtained as indicated by the relative amount of delignification, the relatively low amount of degradation of the cellulose, and the noticeable increase in the brightness of the treated pulps.

The reaction temperature at which the ozone bleaching is conducted is likewise an important factor in the process of the present invention. The maximum temperature of the pulp at which the reaction should be conducted should not exceed the temperature at which excessive degradation of the cellulose occurs, which with southern U.S. softwood is a maximum of about 120°F - 150°F.

An important feature of the ozone stage of the invention is that the pulp be uniformly bleached by the ozone. This uniform bleaching is obtained, in part, by comminution of the pulp into discrete floe particles of a size which is of a sufficiently small diameter and of a sufficiently low bulk density so that the ozone gas mixture will completely penetrate a majority of the fiber floes. Generally, a comminuted pulp particle size of 10mm or less has been found to be acceptable. During the ozone bleaching process, the particles to be bleached should be exposed to the gaseous ozone bleaching agent by mixing so as to allow access of the ozone gas mixture to all surfaces of the floes and equal access by the ozone gas mixture to all floes. The mixing of the pulp in the ozone gas mixture gives superior results with regard to uniformity as compared to the results obtained with a static bed of floes which results in channeling wherein some of the floes are isolated from the ozone gas relative to other floes and are thereby bleached less than other floes.

Upon exiting fluffer 74, the oxygen delignified pulp particles 76 enter a reactor apparatus 78 adapted for bleaching these particles from a first GE brightness to a second, higher GE brightness. The pulp fiber particles 76 are bleached by the ozone in reactor 78 typically to remove a substantial portion, but not all, of the lignin therefrom. A preferred apparatus comprises a paddle reactor as described in application serial no. 07/604,849 of David White et al. entitled PULP BLEACHING REACTOR, the disclosure of which is expressly incorporated herein by ref rence.

As the pulp particles are advanced through this reactor, an internal conveyor 80, preferably in the form of a rotating shaft 82 to which is attached a plurality of paddle members 84, powered by motor 86, is used to provide intimate contact and mixing between the pulp particles and the ozone gas. These conveying means displace and toss the pulp particles in a radial and forward direction while also inducing the ozone to flow and surround the displaced and tossed pulp particles, to expose substantially all surfaces of a majority of these particles to the ozone. This facilitates substantially complete penetration of all surfaces of these particles by the ozone.

At low RPMs, the paddles move the pulp in a manner such that it appears to be "rolling" or "lifted and dropped" through the reactor. At higher RPMs, the pulp is dispersed into the gas phase in the reactor, with the pulp particles uniformly separated and distributed throughout the gas, causing uniform bleaching of the pulp. The overall bleaching rate of the pulp particles is thus significantly improved compared to prior art bleaching methods utilizing fast-reacting gaseous bleaching agents such as ozone.

The forward movement of the dispersed pulp approximates plug flow and facilitates a high degree of bleaching uniformity. The reactor is designed to simultaneously control pulp contacting, pulp residence time and gas residence time while effectively consuming up to 99 percent of the ozone. In this way the pulp is bleached to the desired degree while a preferably high conversion of ozone is achieved.

The ozone gas which is used in the bleaching process may be employed as a mixture of ozone with oxygen and/or an inert gas, or it can be employed as a mixture of ozone with air. The amount of ozone which can satisfactorily be incorporated into the treatment gases is limited by the stability of the ozone in the gas mixture. Ozone gas mixtures which typically contain about 1-8% by weight of ozone in an ozone/oxygen mixture, or about 1-4% ozone in an ozone/air mixture, are suitable for use in this invention. The ozone gas can be introduced at any position through the outer wall of the shell of the reactor.

As shown in FIG. 1, ozone gas 88 is introduced into the reactor 78 in a manner such that it flows, in one embodiment of the invention, countercurrent to the flow of the pulp.

Any residual ozone gas 90, as it exits reactor 78, is directed to a carrier gas pretreatment stage 92 where a carrier gas 94 of oxygen or air is added. This mixture 96 is directed to ozone generator 98 where the appropriate amount of ozone is generated to obtain the desired concentration. The proper ozone/air or mixture 100 is then directed to reactor vessel 78 for delignification and bleaching of pulp particles 76.

Pulp fiber floes 102, after treatment, are permitted to fall into tank 104 from which they can be collected and transported to the peroxide bleaching treatment stage described below. Upon completion of the front end of the process, the ozonated pulp, to be acceptable for further bleaching treatment in the process of the application, has a brightness of at least about 59, a K No. of less than about 5 and a viscosity of at least about 9 to 10.

A further description and discussion of the reaction conditions utilized in the ozone delignification/ bleaching stage of the invention can be found in application serial no. 07/525,804 of Griggs et al. entitled ENVIRONMENTALLY IMPROVED PROCESS FOR BLEACHING LIGNOCELLULOSIC MATERIALS, the disclosure of which is incorporated herein by reference.

B. The "Back End" of the Process For most papermaking processes, a final pulp brightness in the range of 50-63 GEB, such as that achievable upon completion of the Z_m stage described above, is unsatisfactory. Accordingly, in order to further raise the pulp brightness to a more desirable range of about 75-86 GEB, the substantially delignified pulp from the Z_m stage is subsequently subjected to further bleaching, primarily intended to convert the chromophoric groups on the lignin remaining in the pulp into colorless derivatives. Since pulp 92 exiting the ozone reactor is already of sufficient minimum initial GE brightness (i.e., at least about 59) for further bleaching, in the present invention no intermediate extraction (i.e., E) stage prior to peroxide bleaching is needed. 1. The Peroxide Stage

The consistency and pH of the pulp 102 exiting the ozone stage must be adjusted prior to carrying out the peroxide bleaching treatment. The consistency is thus raised to a preferred range of between about 10-15% while the pH of the pulp is adjusted upwardly to ensure a final pH of about 9.5-10.5. A peroxide stabilizing agent, selected from sodium silicate, magnesium sulfate, a chelate (such as EDTA or DTPA) or mixtures thereof, is added in an amount sufficient to prevent the undesirable decomposition of the hydrogen peroxide bleaching agent. The preferred stabilizing agent is a mixture of magnesium sulfate and sodium silicate. The stabilizing agents are added on a weight percent basis based upon the weight of the pulp, with preferred ranges of use being up to 3% of sodium silicate, up to 0.2% magnesium sulfate, i.e., as magnesium (Mg⁺⁺) and up to 0.2% of the chelate.

At this stage of the process, several different peroxide bleaching treatments may be selected with the particular one chosen depending on the GEB desired for the final product. In the first instance, a semi-bleached pulp having a final GEB of about 75 can be produced while, along an alternative path a final pulp product having a GEB of at least about 83-86 can be produced.

Where a bleached pulp having a final GEB of 83- 86 is desired, an ozonated pulp with a GEB of 59-65 is contacted with at least about 0.9%, preferably from about 1-1.5% and most preferably about 1.1% by weight of a peroxide solution, preferably hydrogen peroxide, based upon the weight of the pulp. The reaction is permitted to continue in a bleaching tower for approximately three hours, with no further mixing of the pulp and peroxide once they are initially combined. The pulp must be at a GEB of at least about 59 prior to the peroxide bleaching stage in order to achieve a final GEB of at least 83.

If, however, the ultimate end use of the pulp produced by the process of the invention requires only a se i-bleached pulp having a final GEB of about 75, the process for forming GEB pulp of 83+ as described above may be modified by reducing the concentration of the peroxide bleaching agent by about 2/3, i.e., to between about 0.20%-0.65% and preferably about 0.4% by weight of the pulp. The pulp is placed in contact with this material (without additional mixing) for about three hours. In order to obtain optimum results with this procedure, the GEB of the ozonated pulp which serves as the starting material must be at least about 59.

In an alternate, but less preferred, process for forming 75 GEB pulp, pulp having an ozone GEB of about 65 is contacted with at least about 0.7% by weight of hydrogen peroxide for a truncated period of about 2-15 minutes, preferably about 5-7 minutes, in contrast to the three hour interval in the procedure described above, with continuous mixing. At levels of about 0.7 wt % and above of hydrogen peroxide, an increase of about ten points in GEB results from the ozone stage to the peroxide stage. Thus, the ozone stage GEB must be at least about 65 to permit the formation of a semi-bleached pulp of about 75 GEB.

In another method for forming 83+ GEB pulp, the preferred technique combines the truncated process described above (utilizing at least about 0.3% by weight of peroxide) to initially raise the GEB of the pulp by at least about 7 points, and preferably by about 10 points, i.e., to a a GEB of about 70-75, followed by tower bleaching this bleached pulp for about three hours (with from about 0.6% by weight peroxide) so as to obtain a final product having a GEB of at least about 83. Thus, the truncated step may be utilized as an initial bleaching stage in the formation of 83+ GEB pulp. An important feature of this preferred process is that the effluent from the final peroxide bleaching stage (i.e., to 83 GEB) is recycled into the ozone delignified pulp prior to the initial bleaching stage. Thus, in the initial stage, not only is the pulp mixed with fresh peroxide, it is also continuously blended with the effluent from the final P- stage. Moreover, before the caustic (i.e., for pH adjustment) and peroxide are added in the final peroxide bleaching stage, the effluent of the initial stage is discharged.

Recycling the effluent from the final bleaching stage to the initial peroxide stage serves two purposes. First, residual peroxide in the bleaching effluent which is recycled may be consumed in the initial stage (see, e.g.. Example 6). Additionally, recycling the effluent to the initial stage helps to boost the ozone pulp brightness level prior to the final peroxide bleaching stage. As a result, the total amount of fresh hydrogen peroxide required in the bleaching operation is significantly reduced, thus providing an economic advantage tied to the use of the process. Peroxide levels in the effluent discharge are also significantly reduced.

Bleached pulp with a GE brightness of 83+ can also be produced from ozone delignified pulp with a GEB of 55+, i.e., in contrast to the 59+ GEB pulp used in the systems described above. To achieve this result, a Kraft- AQ/O_m or 0_S/Z_m pulp is subjected to two consecutive three hour peroxide bleaching treatments, each utilizing at least about 0.9% and perferably from about 1-1.5% of peroxide by weight, which increases the ceiling brightness by about 4 points and thus provide a sufficient bleaching action to raise the GEB of the pulp from 55+ to 83+. As in the systems described above, no extraction is carried out between the ozone and the peroxide stages. One requirement of using this technique, however, is that the pulp, upon exiting the oxygen delignification stage, must have a K No. of 9 or less and a viscosity of at least about 17 cps. This ensures that the pulp retains sufficient strength after oxygen delignification to permit it to withstand the effects of the modified ozone treatment carried out prior to the peroxide bleaching stage.

Turning now to Fig. 2, there is illustrated the "back end" of applicants' process, i.e, the peroxide bleaching stage of the present invention. In an embodiment for forming 70-75 GEB semi-bleached pulp (or alternately, for use in conjunction with an additional subsequent bleaching step to form a pulp having a GEB of at least 83) , ozonated pulp 102 is passed through washer 106 to remove the by-products 108 of the ozone delignification reaction. The washed, ozonated pulp 110 then enters reaction vessel 112 equipped with agitation means such as impeller 114, wherein it is combined with the hydrogen peroxide bleaching solution 116 for about 2- 15, and preferably 5-7 minutes. Pulp 118 having, for example, a GEB of about 70-75 and an acceptable viscosity of at least about 9 cps, may be removed from reaction vessel 112.

In forming a final pulp product of GEB 83+ one may select, as a starting material, either: 1) ozonized pulp 102a or 2) partially bleached (i.e. GEB 70-75) pulp 120 from reaction vessel 112. The pulp (102a or 120) is treated with water 122 in (optional) washer 124. Effluent 126 from washer 124 may be recycled to washer 106 in order to conserve the peroxide bleaching agent. Washed pulp 128, having a GEB of about 70-75, is transported at a low consistency, i.e., about 10%, to reactor vessel 130.

In vessel 130, pulp 128 is contacted with the peroxide bleaching agent 132 for about three hours and bleached to a GEB of at least about 83. The consistency of bleached pulp 134 is thereafter raised to about 45% by, for example, pressing in a thickening unit such as twin roll press 136. A portion 138 of this pressate is recycled to reaction vessel 112 for use in the initial bleaching treatment as described above, thus significantly reducing the amount of fresh peroxide 116 which must be added. Finally, the high consistency bleached pulp 140 (GEB=83+) is washed with water 142 in washer 144 and the effluent 146 of that wash is discarded by sewering. Alternately, effluent 146 may be recycled for use in washer 124. The final pulp product 148, having an acceptable viscosity of at least about 9 cps and a GE brightness of at least about 83 may thereafter be collected for use.

EXAMPLES

The scope of the invention is further described in connection with the following examples which are set forth for purposes of illustration only and which are not to be construed as limiting the scope of the invention in any manner. Unless otherwise indicated, all chemical percentages are calculated on the basis of the weight of OD fiber. Also, one skilled in the art would understand that the target brightness values do not need to be precisely achieved, as GEB values of plus or minus 2% from the target are acceptable.

The pulp subjected to the peroxide bleaching sequences described below was initially prepared in the laboratory by a Kraft-AQ/O_m/Z_m process using laboratory equipment to obtain pulp having appropriate K nos., viscosity and GE brightness values to permit it to be used in subsequent peroxide bleaching stages performed according to the present invention.

General Procedure for

Pulp Bag-Bleaching with Peroxide.

1. Eight oven-dried (O.D.) grams of pulp of about 35% consistency were placed in one end of a plastic bag. A clamp was affixed to the plastic bag, thereby isolating the pulp from the from the opposite end of the plastic bag. 2. A volume of water, which when added to the pulp reduces the consistency of the pulp to about 10%, was calculated and prepared. To the calculated volume of water was added the desired amount of stabilizer. The required H₂0₂ was then added to the volume of water, followed by a sufficient amount of sodium hydroxide to adjust the final pH to about 9.5-10.5. 3. The above solution was then poured into the unsealed end of the plastic bag, and that end of the bag was also sealed in such a way that the pulp and the solution were isolated from each other by the position of the clamp between the two ends of the plastic bag.

4. The bag and its contents was then placed in a water bath at 80°C and allowed to equilibrate at 80°C.

5. The bag was then removed from the water bath, the clamp was removed, thereby allowing contact between the pulp and the solution, and the pulp and solution were thoroughly mixed by hand. The bag was then unsealed and excess air was removed to minimize the total volume. The bag was then resealed and returned to the water bath at 80°C.

The mixing procedure of step 5 was repeated at 30 minute intervals throughout the reaction. At the end of the reaction, the bag and contents were removed from the water bath and the temperature was lowered by placing the bag in a room temperature water bath.

8. The pulp was then pressed, thereby removing the effluent from the pulp, which effluent was then tested for pH and residual H₂0₂. Example 1: The Effect of Reaction Time on GEB in the Peroxide Stage

A total of 10 bags containing eight O.D. grams of pulp were prepared for bleaching, according to the general bag-bleaching procedure set forth above. The amounts of stabilizer added were .85% sodium silicate and .05% magnesium sulfate, while the amount of hydrogen peroxide added was 1.3%. All bags were placed in the water bath at 80°C, allowed to equilibrate, and then were initially mixed according to the general procedure. The bags were then removed at intervals over a 3-hour time period. Tappi handsheets were then made in order to measure the GEB of the bleached pulp.

The results (Table 1) show a rapid increase in brightness after 5 minutes, followed by a gradual increase in brightness over the next three hours. This suggests that an intermediate peroxide stage could be used between the ozone and peroxide stages to consume residual peroxide from the peroxide stage.

TABLE 1

EFFECT OF H0₂ REACTION TIME ON GEB Ozone Bleached Pulp: Initial GEB = 63.4

Final GEB

75.4 76.1 77.9 79.0 79.0 80.2 79.6 81.6 81.9

82.2 Example 2: Effect of Hydrogen Peroxide Concentration on GEB

A series of five plastic bags, each containing eight O.D. grams of pulp, were prepared according to the general bag-bleaching procedure described above. Sodium silicate was added at a level of .67% by weight of the level of hydrogen peroxide used in the sample. Magnesium sulfate (as Mg⁺⁺) was added at a level of .05% by weight, based on the weight of the O.D. pulp. The hydrogen peroxide concentration was varied from .5% to 2.5% (Table 2) , based on the weight of the pulp. The plastic bags were placed in a hot water bath at 80°C, initially mixed per the general bag-bleaching procedure, and then allowed to react for a total time of three hours. Tappi handsheets were then made to determine the GEB. The results (Table 2) show that it is possible to produce a fully-bleached pulp with a GEB of at least 83 using neither chlorine nor an alkaline extraction stage between the ozone and peroxide bleaching stages. The results further show that there comes a point at which increased levels of hydrogen peroxide no longer result in corresponding increased GEB. This point is typically referred to as the "ceiling brightness." In this case, where the ozone GEB is about 63.5, the ceiling brightness is about 85. The optimum level of hydrogen peroxide, then, to achieve a GEB of about 83 in this instance, is about 1% based on the weight of the pulp.

TABLE 2

EFFECTS OF THE HYDROGEN PEROXIDE CONCENTRATION ON GEB Ozone Bleached Pulp: Initial GEB = 63.5 %_H₂0₂ GEB (3 hrs.

0.5 1.0 1.5 2.0 2.5

Example 3: Effects of Ozone Stage GEB on

Peroxide Stage GEB

An (0..) oxygen delignified Kraft/AQ pulp having a GEB of approximately 41 was further bleached with ozone to varying ozone GEB levels (Table 3) . For each different ozone bleached pulp sample the peroxide ceiling GEB was ascertained using the procedure of Example 2 (3-hour reaction time) . The results in Table 3 demonstrate that the peroxide ceiling GEB is dependent upon the ozone stage pulp GEB. Greater additions of peroxide will not increase the peroxide ceiling GEB level; therefore, in order to achieve a final peroxide stage GEB of at least 83, an ozone stage pulp GEB of about 59 or greater is required.

TABLE 3 EFFECTS OF OZONE GEB ON FINAL PULP GEB

Initial Oxygen Delignified GEB = 41.1 Ozone GEB Peroxide Ceilin GEB

By plotting the peroxide ceiling GEB against the ozone GEB, as shown in Fig. 3, one can determine what minimum ozone GEB is required in order to get the final desired peroxide GEB. For instance, in order to achieve a final peroxide GEB of at least 83, an ozone GEB of about 59 or greater is required. In order to get a final perox¬ ide GEB of 86, an ozone GEB of about 65 is required. To go from an ozone GEB of about 63 to a peroxide GEB of about 83 requires only about 1% of hydrogen peroxide. Example 4 :

Both Kraft and Kraft/AQ pulps were prepared uti¬ lizing the same (soft) wood chip supply. Approximately 500g of the Kraft/AQ pulp were bleached using the 0_mZ_m bleaching sequence. Approximately 500g of the Kraft pulp was bleached using the O_mC/DED bleaching sequence.

Approximately 500g of the Kraft pulp was bleached using the O_mZ_mED bleaching sequence. The P stage, C/DED stage and ED stage were all performed using a ribbon mixer. After bleaching, four pulp samples were prepared to show varying levels of freeness in the range of 150 ml to 700 ml Canadian Standard Freeness (CSF) . Tappi handsheets were then tested for various strength properties of the pulp, i.e., tear factor, tensile breaking length, etc. (Table 4A) . Each strength property was then plotted versus the freeness level, and the value of that property at 7 km breaking length was taken as the representative strength property value (Table 4A) . The GEB and viscosity were also determined for the Kraft/AQ 0_mZ_m pulp (Table 4B) . The above procedure was duplicated for both the Kraft O_mZ_mD and Kraft O_mC/DED bleaching sequences. The results of these tests indicate that the chlorine-free 0_mZ_m bleaching sequence, when resulting from a Kraft/AQ pulp, will pro¬ duce pulps which have GEB values comparable to or higher than the O_mC/DED or the O_mZ_mD bleaching sequences, while at the same time maintaining adequate fully-bleached pulp strength.

PHYSICAL PROPERTIES VERSUS BLEACHING SEQUENCE

Scattering coeff. 240 250 255 (cm²/g)

Tappi opacity 71 70.3 71

Zero span breaking 11 11.5 11.6 length (km)

TABLE 4B

* = not determined — = not applicable

Example 5: Truncated Peroxide Bleaching Stage

A series of ozone bleached pulp samples were prepared for peroxide bleaching according to the general bag-bleaching procedure set forth above. Varying levels of hydrogen peroxide (Table 5) were added to each plastic bag. The bags were then placed in a water bath at 80°C, allowed to equilibrate to that temperature, removed from the water bath after equilibration, and mixed according to the general bag-bleaching procedure. The samples were allowed to react for about 5-7 minutes and then were removed from the water bath. Tappi handsheets were then made in order to measure the GEB of the peroxide bleached pulp.

The results in Table 5 indicate that an increase of approximately eight points in the GEB from ozone to peroxide stage can be achieved in 5-7 minutes when at least about .4% of peroxide is used in the bleaching step. When at least about .7% of peroxide is used in the bleaching step, the corresponding increase in GEB is about ten points. Therefore, in a truncated peroxide bleaching stage, the minimum required ozone stage GEB can be about 8-10 points lower than the desired peroxide stage GEB.

Table 5

Δ GEB IN TRUNCATED PEROXIDE STAGE

Z stage GEB % H₂O_z

P stage GEB Δ GEB

Example 6: Recycle of Peroxide Stage Filtrate to an Intermediate Peroxide Stage

The filtrate from a three hour peroxide bleaching of a 63.5 GE brightness pulp was recycled to an intermediate peroxide stage wherein it was fortified with additional fresh peroxide to a concentration of 0.3% by weight of the pulp. This solution was applied to the pulp with mixing and was permitted to react therewith for seven minutes. The pulp was then washed and contacted with 0.6% by weight of peroxide in a three hour final stage to 83.3 GE brightness. The dissolved solids from the final peroxide bleaching stage were thus also recycled to the intermediate mixing step.

As shown in Table 6 below, the total peroxide applied to the pulp in the process outlined above is 0.9 (0.3 + 0.6) percent by weight of the pulp. About 0.1% is, however, recovered from the recycle. The amount of fresh peroxide which must therefore be added is about 0.8% by weight. Thus, about 0.2% fresh peroxide is added to the intermediate stage, thus reducing the total amount of fresh chemical required when the recycle process is used. TABLE 6

RECYCLE OF P-STAGE FILTRATE TO INTERMEDIATE STAGE

Ozone Bleached Pulp: Initial GEB = 63.7

% Residual Reaction Time Stage % H₂02₂ Peroxide (minutes) GEB Intermediate 0.3 0.16 7 71.5

Final 0.6 0.26 180 83.3

While it is apparent that the invention herein disclosed is well calculated to fulfill the objectives stated above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art. It is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.

Claims

CLAIMS We claim:

1. A process for the manufacture of a bleached pulp having a final GE brightness of at least about 75 which comprises the steps of: chemically digesting a lignocellulosic material to provide a brownstock pulp; oxygen delignifying the brownstock pulp to remove a substantial portion of the lignin therefrom to provide an intermediate pulp containing a specified amount of lignin as evidenced by a first K No. and having a first specified viscosity sufficiently high to permit formation of a bleached pulp having acceptable strength; further delignifying the intermediate pulp by advancing the intermediate pulp while intimately contacting and mixing said intermediate pulp with an amount of ozone sufficient to remove a substantial portion, but not all, of the lignin remaining in said intermediate pulp, to obtain a substantially delignified pulp having a specified amount of lignin as evidenced by a second K No. sufficiently reduced to impart a predetermined intermediate GE brightness to the substantially delignified pulp and a second specified viscosity sufficiently high to permit formation of a bleached pulp having acceptable strength; and bleaching the substantially delignified pulp having said specified amount of lignin with an amount of a peroxide compound sufficient to form a bleached pulp having a final GE brightness of at least about 75.

2. The process of claim 1 wherein the first K No. is about 10 or less and the first specified viscosity is greater than about 12 cps, and wherein the ozone delignification step produces a substantially uniformly delignified pulp, having a K No. of less than about 5, a second specified viscosity greater than about 9-10 cps and an intermediate GE brightness of at least about 59.

3. The process of claim 1 wherein said oxygen delignification step comprises: forming a low consistency pulp and combining the low consistency pulp with a sufficient amount of an alkaline material to substantially uniformly distribute the alkaline material throughout the low consistency pulp; forming a high consistency pulp having a total amount of alkaline of between about 0.8 and 7% by weight based on the weight of oven dry pulp; and subjecting the resulting high consistency alkaline material containing pulp to high consistency oxygen delignification to obtain the intermediate pulp.

4. The process of claim 3 wherein the low consistency pulp has a consistency of less than about 10% by weight and wherein said high consistency pulp has a consistency of at least about 18% by weight.

5. The process of claim 3 wherein the low consistency pulp has a consistency between about 1 and 4.5% by weight.

6. The process of claim 3 wherein the high consistency pulp has a consistency between about 25-35% by weight.

7. The process of claim 3 wherein a first amount of said alkaline material is combined with said low consistency pulp and wherein a second amount thereof is applied to said high consistency pulp to obtain said total amount thereof prior to oxygen delignification.

8. The process of claim 1 wherein said oxygen delignification step comprises: forming a low consistency pulp and combining the low consistency pulp with a first amount of alkaline material to obtain a substantially uniform distribution of alkaline aterial throughout the pulp and increasing the consistency of the pulp to at least about 18% by weight to obtain high consistency pulp including said first amount of alkaline material substantially uniformly dispersed throughout; applying a second amount of alkaline material onto the high consistency pulp to thus obtain a total amount of alkaline material of at least about 0.8 to 7 percent by weight based upon the oven dry weight of the pulp; and substantially delignifying the alkaline material- containing high consistency pulp with oxygen.

9. The process of claim 1 wherein the further delignifying step comprises advancing the intermediate pulp in an axial direction in a plug flow-like manner while intimately contacting and mixing said intermediate pulp with said ozone by lifting, displacing and tossing the intermediate pulp in a radial direction.

10. The process of claim 1 which further comprises comminuting the intermediate pulp to a predetermined particle size prior to further delignifying said pulp with ozone.

11. The process of claim 1 wherein chemical digesting is by Kraft/AQ pulping.

12. The process of claim 1 wherein said substantially delignified pulp is bleached by contacting said pulp with from about 0.20-0.65% by weight of said peroxide compound for a sufficient time to raise the GE brightness thereof to at least about 75.

13. The process of claim 12 wherein said substantially delignified pulp has an intermediate GE brightness of at least about 59 and said peroxide solution is contacted with said substantially delignified pulp for up to about ^'3 hours.

14. The process of claim l wherein said substantially delignified pulp is bleached by contacting said pulp with at least about 0.9% by weight of a peroxide compound for a sufficient time to raise the GE brightness thereof to at least about 83.

is. The process of claim 14 wherein said substantially delignified pulp has an intermediate GE brightness of at least about 59 and said peroxide solution is contacted with said substantially delignified pulp for up to about 3 hours.

16. The process of claim 1 wherein said substantially delignified pulp is bleached by contacting said pulp with a sufficient amount of said peroxide compound for a sufficient time with continuous mixing to raise the intermediate brightness of said substantially delignified pulp to at least about 75.

17. The process of claim 16 wherein said substantially delignified pulp has an intermediate GE brightness of at least about 65 and which further comprises contacting at least about 0.7% by weight of said peroxide compound with said substantially delignified pulp with substantially continuous mixing for between about 2- 15 minutes.

18. The process of claim 1 wherein said bleaching step comprises: contacting, in an initial bleaching stage, said substantially delignified pulp with a sufficient amount of said peroxide compound for a sufficient time with substantially constant mixing to raise the intermediate brightness of said substantially delignified pulp by at least about 7 GEB points; and contacting, in a final bleaching stage, said bleached pulp with a sufficient additional amount of said peroxide compound for up to about three hours to further raise the GE brightness of said substantially delignified pulp to at least about 83.

19. The process of claim 18, wherein said substantially delignified pulp having said intermediate brightness is contacted with at least about 0.3% by weight of said peroxide compound and mixed for between about 2-15 minutes.

20. The process of claim 19 wherein said pulp having a GE brightness raised by at least 7 points is contacted with an additional amount of at least about 0.4% by weight of said peroxide compound for up to about three additional hours.

21. The process of claim 18 which further comprises recycling effluent from said final bleaching stage to said initial bleaching stage to reduce the total fresh peroxide requirement.

22. The process of claim 1 wherein said peroxide compound is hydrogen peroxide.

23. The process of claim 1, which further comprises adjusting the pH and consistency of the substantially delignified pulp, prior to bleaching said pulp, to ensure a final pH of between about 9.5-10.5 and a consistency of between about 10-15%, respectively.

24. The process of claim 23 which further comprises adding a sufficient amount of a peroxide stabilizing agent to said substantially delignified pulp prior to bleaching said pulp to prevent decomposition of said peroxide compound.

25. The process of claim 24 wherein said stabilizing agent is selected from among sodium silicate, magnesium sulfate, a chelate and mixtures thereof.

26. The process of claim 25 wherein said chelate is selected from among EDTA, DTPA and oxalic acid.

27. The process of claim 25 wherein said stabilizing agent is added in an amount of up to about 3% of said sodium silicate, up to about 0.2% of said magnesium sulfate, as magnesium and up to about 0.2% of said chelate, based upon the weight of the substantially delignified pulp.

28. The process of claim 27 wherein said stabilizing agent comprises from up to about 3% of said sodium silicate and up to about 0.2% of said magnesium sulfate, as magnesium.

29. The process of claim 1 wherein the pulp is softwood pulp having a K No. of about 16 to 20 prior to oxygen delignification and further wherein the K No. is reduced to about 8 to 10 after oxygen delignification.

30. The process of claim 1 wherein the pulp is hardwood pulp having a K No. of about 10 to 14 prior to oxygen delignification and further wherein the K No. is reduced to about 5 to 7 after oxygen delignification.

31. A process for delignifying and bleaching a lignocellulosic pulp which comprises: forming a substantially delignified pulp having a first GE brightness; and contacting said substantially delignified pulp in an initial bleaching stage with a sufficient amount of a peroxide compound for a sufficient time with substantially continuous mixing to form a bleached pulp having a second GE brightness higher than said first GE brightness.

32. The process of claim 31 wherein said second GE brightness is at least about 10 units higher than said first GE brightness.

33. The process of claim 32 wherein said bleached pulp has a second GE brightness of at least about 75.

34. The process of claim 31 wherein said substantially delignified pulp is contacted with at least about 0.3% by weight of said peroxide compound and mixed for between about 2-15 minutes to form said bleached pulp.

35. The process of claim 31 which further comprises contacting, in a final bleaching stage, said bleached pulp from said initial bleaching stage with a sufficient additional amount of said peroxide compound for a sufficient time to further raise the second GE brightness of said bleached pulp to at least about 83.

36. The process of claim 35 wherein the pulp formed in said initial bleaching stage is contacted with an additional amount of between about 0.4 - 0.8% by weight of said peroxide compound for up to about three additional hours in said final bleaching stage.

37. The process of claim 35 wherein effluent from said final bleaching stage is recycled to said initial bleaching stage to reduce the total fresh peroxide requirement.

38. A process for the manufacture of a bleached pulp having a GE brightness of at least about 75, said process comprising: chemically digesting a lignocellulosic material to provide a brownstock pulp; sequentially treating said brownstock pulp with oxygen and then with ozone to form a substantially delignified pulp having a K No. less than about 5, a viscosity of at least about 9-10 cps and a GE brightness of at least about 50; and contacting said substantially delignified pulp, without subjecting it to an intervening extraction treatment, with a sufficient amount of a peroxide compound for a sufficient time with substantially continuous mixing to bleach said substantially delignified pulp to a GE brightness of at least about 75.