NO300929B1 - Process for bleaching lignocellulosic materials - Google Patents

Abstract

(i) Wood chips are chemically digested to provide lignocellulosic pulp followed by e.g. washing and screening (ii) pulp is then O2 delignified and (iii) comminuted and O3 treated for further delignification and bleaching. Pulp prod. obtd. in (iii) can be further brightened by addn. of CO2 or a peroxide. Typically at end of step (ii) pulps have a K number of 10 or less and viscosity above 13cps.. Corresp. values at completion of (iii) are 5 or less and above 10cps.. Digestion in (i) is e.g. by Kraft pulping. Pulp pH in (iii) is pref. 1-4 and temp. is ambient.

Description

The present invention relates to a new method for bleaching lignocellulosic materials which does not require the use of elemental chlorine and which forms a mass of acceptable strength. Application of this method also reduces the amount of environmental pollution.

Wood consists of two main components, a fibrous carbohydrate, i.e. a cellulosic part and a non-fibrous component. The polymeric chains that form the fibrous cellulosic part of the wood are aligned in relation to each other and form strong associated bonds with adjacent chains. The non-fibrous part of the wood comprises a three-dimensional polymeric material formed mainly of phenylpropane units, known as lignin. Part of the lignin is between the cellulose-containing fibres, binding these into a solid mass, despite the fact that a significant part of the lignin is also distributed between the fibers themselves.

For use in papermaking processes, the wood must first be reduced to pulp. Pulp can be defined as wood fibers that can be slurried or suspended and then deposited on a screen to form a sheet, e.g. of paper. The procedures used to achieve the pulping step usually involve either physical or chemical treatment of the wood, or a combination of these two treatments, to change the chemical form of the wood and to give the resulting product the desired properties. There are therefore two main types of pulping techniques, i.e. mechanical pulping and chemical pulping. In mechanical pulping, the wood is physically separated into individual fibers. In chemical pulping, the wood chips are split with chemical solutions to dissolve part of the lignin and thus enable its removal. The commonly used chemical pulping processes are roughly classified as: (1) the soda process, (2) the sulphite process and (3) the Kraft process, the latter process being the one that is usually used and which can be subjected to well-known modifications as described below.

The soda process is well known in this field. It uses sodium hydroxide (NaOH) as an active reagent to break down the lignin and assist in its removal. The sulphite process is also well known in the field (see e.g. Handbook For Pulp & Paper Technologists - Chap. 6: Sulphite Pulping (TAPPI, USA).

The Kraft process, together with its many variations, is the most important chemical process used for the production of paper. The Kraft process as described in the Handbook For Pulp and Paper Technologists - Chap. 7: Kraft Pulping (TAPPI, USA), involves splitting the wood chips in an aqueous solution of sodium hydroxide (NaOH) and sodium sulphide (Na2S). This process is very effective when it comes to pulping even difficult woods such as southern softwood, as well as the other more easily pulped species of wood such as northern hardwood and softwood. The Kraft process generally produces a relatively high strength pulp due to the fact that its application results in reduced attack on the cellulose component of the wood.

The modified Kraft techniques can result in even less degradation of the polymeric structure of the cellulosic fibers during pulping and therefore the strength loss in the resulting paper product is reduced compared to that which occurs with the standard Kraft process. A modified Kraft pulping process is known as "extended delignification", which is a broad term used in the art to encompass a variety of the modified Kraft techniques, such as the addition of pulping chemicals in a specifically defined order, or at different locations within the splitting apparatus, or at different time periods, or with a removal and re-injection of cooling liquor in a prescribed sequence, to more effectively remove a greater amount of lignin, reducing the degree of pulping liquor's chemical attack on the cellulosic fibers. Another modification of the Kraft process is the Kraft-AQ process, in which a small amount of anthracdone is added to the Kraft pulping liquor to accelerate delignification while limiting the attack on the cellulosic fibers of which the wood is composed.

Various further extended delignification techniques are known in the art and include Kamyr Modified Continuous Cooking (MCC) as described by V.A. Kortelainen and E.A. Backlund in TAPPI, volumes 68 (11), 70 (1985); Beloit Rapid Displacement Heating (RDH) as reported by R.S. Grant in TAPPI, Vol 66 (3), 120 (1983); and Sund's Cold Blow Cooking as reported by B. Pettersson and B. Ernerfeldt in Pulp and Paper, Vol. 59 (11), 90 (1985).

Cleavage of the wood by a Kraft or modified Kraft process results in the formation of a dark colored slurry of cellulosic fibers known as "brown pulp". The dark color of the brown pulp is due to the fact that not all lignin has been removed during the cleavage and has been chemically modified in the pulp production for the formation of promophore groups. In order to lighten the color of the brown pulp, i.e. to make it suitable for use as printing and writing and other uses of white paper, it is necessary to continue the removal of residual lignin by adding the lignifying material and by chemically converting any remaining lignin into colorless compounds by a process known as "bleaching" or "brightening".

Prior to bleaching the pulp, the split material is conveniently transferred to a separate bladder tank after the chemical treatments involved in the pulping process have been completed. In the blowing tank, the pressure developed during the first chemical treatment of the lignocellulosic material is relieved and the pulp material is separated into a fibrous mass. The resulting fibrous pulp is then subjected to a series of washing steps to remove the combination of any residual chemicals and soluble materials (such as lignin) that were separated from the fibrous materials in the pulping process. The pulp also often undergoes one or more separation steps designed to separate the larger parts of undefined wood for special processing (reboiling, mechanical grinding, etc.).

The residue obtained from the washing process, usually referred to as black liquor, is collected, concentrated and then incinerated in an environmentally safe way in an insulating boiler. The technique for collecting, concentrating and burning black liquor is conventional and well known within the field.

The delignification and bleaching processes are carried out on the washed fibrous pulp in a series of steps using selected combinations of chemical reactants. According to the prior art, various combinations of chemical treatments have been proposed. Individual treatment steps have further been rearranged in an almost unlimited number of combinations and permutations. Therefore, to simplify the explanation of the various bleaching processes and systems, letter codes are appropriately used in combination to describe the particular chemical reactants that are used and the order of the steps in the process.

The letter codes that will be used below where appropriate are as follows: C = Chlorination - Reaction with elemental chlorine in acid

medium.

E = Alkaline - Dissolution of reaction products

extraction with NaOH.

E0 = Oxidative - Dissolution of the reaction products alkaline with NaOH and oxygen.

extraction

D = Chlorine - Reaction with CIO2 in acidic medium

dioxide

P = peroxide - Reaction with peroxides i

alkaline medium.

0 = oxygen - Reaction with elemental oxygen i

alkaline medium.

0m = modified - Even alkali treatment of pulp oxygen with low to medium consistency followed by reaction of pulp with high consistency with oxygen.

Z = ozone - Reaction with ozone.

Zm = modified - Smooth reaction with ozone.

ozone

C/D = - Mixtures of chlorine and

chlorine dioxide.

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

It has been common for many years to delignify and bleach wood using elemental chlorine. Examples of bleaching of lignocellulose-containing wood pulps are the methods described in e.g. US Patent Nos. 1,957,937 to Campbell et al., 2,975,169 to Cranford et al., and 3,462,344 to Kindron et al.; and Handbook for Pulp and Paper Technologists - Chap. 11: Bleaching (Jll.3) (TAPPI, USA).

Despite the fact that elemental chlorine has proven to be an effective bleaching agent, it is difficult to handle and potentially harmful to both mill personnel and equipment. Effluents from chlorine bleaching processes contain large amounts of chlorides produced as by-products of these processes. These chlorides easily corrode the processing equipment and therefore require the use of expensive materials for the construction of such mills. Formation of chlorides inside the mill precludes recycling the wash filtrate after a chlorination step in a closed operating system without the use of isolation systems that require extensive, and therefore expensive, modifications. In addition, concerns about the potential environmental effects of chlorinated organics in the effluents that the U.S. The Environmental Protection Agency assumes to be toxic to humans and animals, has caused significant changes in government requirements and allows bleach mills to meet standards that may be impossible to meet with conventional bleaching or pollution control technology.

To avoid these disadvantages, the paper industry has attempted to reduce or eliminate the use of elemental chlorine and chlorine-containing compounds from multi-stage bleaching processes for ligno-cellulosic pulps. What makes these experiments difficult is the requirement for a high degree of pulp with lightness, which is necessary for many of the applications for which such pulps are to be used.

In this context, attempts have been made to develop a bleaching process where chlorine-containing agents are e.g. replaced by oxygen to bleach the pulp. The use of oxygen enables recycling of the effluent from this stage for isolation and enables a significant reduction in the amount of elemental chlorine used. A number of methods for bleaching and delignifying pulp with oxygen have been proposed, such as Richter U.S. Pat. Pat. 1,860,432, Grangaard et al. U.S. Pats. 2,926,114 and 3,024,158, Gaschke et al. U.S. Pat. 3,251,730, Rerolle et al. U.S. Pat. 3,423,282, Farley U.S. Pat. 3,661,699, Kooi U.S. Pat. 4,619,733, and P. Christensen in "Bleaching of Sulphate Pulps with Hydrogen Peroxide", Norsk Skogindustri, 268-271 (1973). Pretreatments of pulp prior to oxygen delignification have been proposed by U.S. Pat. Pat. No. 4,806,203 to Elton.

The use of oxygen, on the other hand, is not a completely satisfactory solution to the problems that arise with elemental chlorine. Oxygen is not as selective a delignifier as elemental chlorine, and the K No. of the pulp, using conventional oxygen delignification methods, can only be reduced by a limited amount before there is a disproportion, i.e., unacceptable attack on the cellulosic fibers . After the oxygen delignification, residual lignin has previously usually been removed by the chlorine bleaching method to obtain a fully bleached pulp, but using very reduced amounts of chlorine. But even with such reduced chlorine concentrations, corrosive chlorides will quickly reach unacceptable concentration levels in a closed-loop operation.

In order to avoid the use of chlorine bleaches, the removal of such remaining lignin by the use of ozone in the bleaching of chemical pulp has previously been attempted. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost have until now limited the development of satisfactory ozone bleaching processes for lignocellulosic materials, especially southern conifer. Ozone will readily react with lignin to effectively reduce the K no., but it will also, under most conditions, aggressively attack the carbohydrate comprising the cellulosic fibers and substantially reduce the strength of the resulting pulp. Ozone is likewise extremely sensitive to process conditions such as pH with respect to its oxidative and chemical stability, and such changes can considerably change the reactivity of ozone with respect to the lignocellulosic materials.

Since around the turn of the century, when the delignifying abilities of ozone were first discovered, substantial and continuous work has been done by many people in this field to develop a commercially viable process using ozone in the bleaching of lignocellulosic materials. materials. Numerous articles and patents have been issued in this area and there have been reports of attempts to carry out ozone bleaching on a non-commercial pilot scale basis. E.g. describes the U.S. pat. 2,466,633 to Brabender et al. a bleaching process where ozone is sent through a mass that has a moisture content (adjusted to an oven-dry consistency) of between 25 and 55$ and a pH adjusted to the range 4 to 7.

Other non-chlorine bleaching sequences are described by S. Rothenberg, D. Robinson & D. Johnsonbaugh, "Bleaching of Oxygen Pulps with Ozone", Tappi, 182-185 (1975) - Z, ZEZ, ZP and ZPa(Pa-peroxyacetic acid ); and N. Soteland, "Bleaching of Chemical Pulps With Oxygen and Ozone", Pulp and Paper Magazine of Canada; T153-58 (1974) - OZEP, OP and ZP.

U.S. pat. No. 4,196,043 to Singh also describes a multi-step bleaching process that also attempts to avoid the use of chlorine compounds, and includes examples specifically directed at hardwoods. It is well known to professionals that hardwood is easier to bleach than most conifers. This method is characterized by from one to three ozone bleaching steps and a final treatment with alkaline hydrogen peroxide, where each step is separated by an alkaline extraction. Such a sequence can be described in the abbreviated nomenclature of the paper industry as ZEZEP. According to this method, the effluent from each treatment stage is collected and recycled for use in bleaching operations, preferably at an earlier stage than that from which it is obtained. This patent also provides a so-called countercurrent outlet flow.

Despite all the research carried out in this area, no commercially feasible method for the production of ozone-bleached lignocellulosic pulps, especially southern softwood, has yet been described, but many unsuccessful attempts have been reported.

The present invention provides new combinations of pulping and bleaching steps which solve the problems encountered in the prior art discussed herein and which substantially eliminate the release of chlorinated organic compounds and minimize color and BOD releases to form "bleached pulp at a commercially feasible manner.

It is an aim according to the present invention to provide a multi-stage method for delignification and bleaching of lignocellulosic pulp without the use of bleaching agents with elemental chlorine to significantly reduce or eliminate pollution of the environment, with optimization of the physical properties of the pulp in an energy-efficient, cheap procedure. The present invention can be applied to almost any type of wood, including southern U.S. softwoods which are difficult to bleach.

The present invention therefore relates to a method for producing a substantially uniformly bleached pulp from a lignocellulosic material comprising the formation of an intermediate pulp with a reduced lignin content by chemical boiling (4) of the lignocellulosic material, followed by oxygen delignification (26) of the pulp which is thereby achieved, characterized by that

a) the pH of the intermediate mass is adjusted (42) to the range of 1 to 4, b) the consistency of the intermediate mass is adjusted (48) to a high consistency of 20% to 50%, c) the intermediate mass is pulverized into separate particles, where most have a size of less than

5mm,

d) the intermediate mass obtained after steps a), b) and c) is subjected to a delignification treatment by

turbulent mixing of this with an ozone-containing

gaseous mixture in a dynamic reaction zone (58) for a sufficient time and at a sufficient temperature to enable access of ozone to substantially the entire intermediate mass for reaction thereby, while said mass particles are carried (62) through substantially the entire reaction zone in a manner which exposes substantially all the mass particles to the ozone in a uniform manner during

the advance of the pulp to thereby achieve a further substantially uniform delignification of most of the pulp particles without significant degradation of the cellulose components of the pulp, so that for intermediate pulp of southern softwood with a GE lightened of 25% to 45$ after oxygen delignification the pulp leaving the dynamic reaction zone a GE lightened of at least 50% and for intermediate hardwood pulp the pulp leaving the dynamic reaction zone shows a GE lightness greater than 55%.

Wood chips can be delignified into a lignocellulosic pulp using any of several chemical pulping processes, followed by removal by washing of most of the dissolved organics and cooking chemicals for recycling and isolation. It usually involves a selection of the pulp to remove fiber knots that have not been separated in pulp production. This delignification step is performed so that, for a southern U.S. conifers, becomes e.g. mass with a K No. in the range of 20-24 (target of 21), a cupriethylenediamine ("CED") viscosity in the range of 21-28, and a GE brightness in the range of about 15-25 are usually obtained. For southern U.S. hardwood becomes pulp with a K No. in the range of about 10-14 (target 12.5) and a CED viscosity of about 21-28 usually obtained.

Effective embodiments are therefore:

a. Kraft-pulp production using either a continuous or batch cleavage step; b. Continuous cleavage Kraft-pulp production with extended delignification using stepwise alkali addition and countercurrent final boiling; c. Batch digestion Kraft-pulp production with extended delignification using rapid liquor transfer and cold blast techniques, or d. Kraft-AQ pulp production to achieve extended delignification using either a continuous or batch digestion step.

The extended delignification techniques described in (b) and (c) above can e.g. include Kamyr MCC, Beloit RDH and Sund's cold blast cooking techniques described in the background section of this description. Depending on the type of lignocellulosic material being used, the soda and sulfite processes described above may be used.

This is followed by an oxygen delignification treatment to further remove lignin without an associated significant loss in cellulosic fiber strength. This may include the removal by washing of dissolved organic substances and alkali for recycling and isolation. Mass selection is also performed after the oxygen delignification.

During the oxygen delignification process, K Nr. to the increased consistency of the pulp reduced by at least about 45% (for 0) to at least about 60% (for 0m) without significant damage to the cellulosic component of the pulp. The relationship between K No. and the viscosity of the pulp is usually reduced by at least 25%. For softwood pulp described above when using 0m, a K Nr. of about 7 to 10 and a viscosity above about 13 readily obtained. For hardwood pulp, a K No. of about 5 to 8 and a viscosity above about 13 obtained after the oxygen delignification step.

Possible embodiments of this step are:

a. Conventional oxygen delignification, consisting of an alkaline oxygen treatment of the pulp at either low, medium or high pulp consistency (0); or

b. The preferred embodiment of an alkaline treatment at low to medium pulp consistency, i.e., less than about 10 percent by weight, followed by

oxygen treatment at high pulp consistency, i.e. higher than about 20$ (0m).

For pulp end uses that do not require lightness above about 35$ GEB (often referred to as semi-bleached pulp), it is possible to use pulp that has only been processed through step 2 directly in the papermaking process.

This is followed by an acidic, gaseous ozone bleaching treatment (Z i" or Zm) under defined parameters for the method in order to

providing a highly selective removal and bleaching of lignin with minimal degradation of cellulose. Among the process parameters are chelating agents for metal ion control, pH

control, mass particle size control, mass consistency, !5 ozone concentration and gas/mass contact control. Before treatment with ozone, a chelating agent, e.g. oxalic acid, diethylenetriamine pentaacetic acid ("DTPA") or ethylene diaminetetraacetic acid ("EDTA") be added to the pulp for

substantially to be bound to the metal ions contained therein. The pH f of the pulp is preferably adjusted over a range of between about 1-4 before the third step. This can be achieved by adding to the mass a sufficient amount of an acidic material. The consistency of the mass is preferably

increased to between about 35 - 45 percent by weight and the particle*5 size of the fiber flocks is reduced to a size of about 5 mm or less before the ozone delignification step. Included is a dissolved organic wash step for recycling and isolation.

!0 During the ozone step, the mass is preferably maintained at room temperature or at least at a mass temperature of less than approximately 49°C. The ozone can be provided from an ozone-containing gas such as e.g. may include oxygen

or air. When an ozone/oxygen mixture is used, the <!>5 ozone concentration is preferably between about 1 and 8

volume percent, while for the ozone/air mixture, an ozone concentration of between approximately 1 and 4 volume percent is acceptable.

Within the ozone reactor vessel, the substantially delignified mass is advanced in a manner that leads to substantially all of the mass particles being led to the ozone in a uniform manner.

It has been discovered that masses with K No. which are greater than about 10 after the second step are not suitable for this third step, because of the significant amounts of ozone required to reduce the K No. to the desired level, which usually results in the properties of the pulp being adversely and detrimentally affected by extensive ozone degradation of the cellulosic fibers of the pulp. The mass that has a K No. of less than 10 is ozone treated, a smaller ozone concentration is used, and which only leads to a minimal amount of cellulose degradation. The product from this ozonation step for either southern U.S. softwood and hardwood described above which is the starting point is a mass with a K No. of less than about 5 and generally in the range of about 3 to 4 (measure 3.5), a viscosity greater than about 10 and a GE lightness of at least 50$ (typically about 5456 or higher for softwood and 63$ or higher for hardwood ).

Effective embodiments of this step are:

a. Treatment of acidified pulp by countercurrent contact of

ozone in an oxygen or carrier gas; or

b. Treatment of the acidified mass by countercurrent contact of the ozone in an oxygen or carrier gas.

A further bleaching step may then be employed to bring the pulp to a desired fully bleached state, i.e. having levels of GE brightness of approximately 70 to 95$ using any number of possible well-known bleaching and extraction processes. Effective embodiments are: a. A conventional extraction step with washing followed by a peroxide step with washing; (ie

EP);

b. Conventional alkali extraction and washing step followed by a conventional chlorine dioxide step with washing, (ie ED);

c. A conventional alkali extraction and washing step followed by a conventional chlorine dioxide step with washing, followed by a repetition of the extraction and chlorine dioxide step (ie EDED) or

o d. An extraction step, supplemented with either oxygen or oxygen and peroxide, followed by a conventional chlorine dioxide step; i.e. , (EC)D or (Eop)D.

The extraction step may in a further embodiment 5 comprise combining the substantially delignified mass with an effective amount of an alkali material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature correlated with the amount of alkaline material to dissolve a substantial portion of any o lignin remaining in the pulp. Then a portion of the aqueous alkaline solution may be extracted to remove substantially all dissolved lignin therefrom.

After the extraction step, the substantially delignified pulp may be processed in a further bleaching step to increase the GE brightness of the resulting pulp to at least about 70%. Preferred brightening agents include chlorine dioxide or a peroxide.

o (E0)D, (Eop)D or EDED forms will provide the highest levels of brightness. For the ED forms, the chlorine dioxide stage filtrate cannot be recycled for chemical isolation without treatment due to the presence of inorganic chlorides.

Because this is the only required filtrate from the process, dramatic reductions in effluent volume, color, COD, BOD and chlorinated organics are achieved. Color of less than 0.907 kg per ton, BOD5 of less than 0.907 kg per tonnes and total organic chloride (TOC1) of less than 0.907 kg and preferably less than 0.3629 can be achieved. It is thus possible to treat the chlorine dioxide stage filtrate with a membrane filtration process which will enable substantially complete recycling. In the EP form, no chlorinated materials are used in the bleaching process and essentially the entire amount of liquid filtrate can be recycled and isolated to create an almost outlet-free process.

Brief description of the drawings

Fig. 1 is a block flow diagram of the preferred methods of this invention where a solid line represents mass flow and a dashed line represents outlet flow; Fig. 2 is a schematic representation of a preferred method according to the invention; Fig. 3 is a cross-sectional drawing of a part of an ozone apparatus shown in Fig. 2, seen along the line 3--3; Fig. 3A is a cross-sectional drawing of part of a preferred ozone apparatus shown in Fig. 2, viewed along the line 3-3; and Fig. 4 is a comparison of recycle and waste streams from various pulping processes.

Below are definitions of several parameters involved in the various steps of any delignification/bleaching process.

A. General definitions.

In the description, the following definitions apply:

"Consistency" is defined as the amount of pulp fiber in a slurry, expressed as a percentage of the total weight of oven-dried fibers and water. The term also sometimes denotes mass concentration. The consistency of a mass will depend on the operation of and type of dewatering equipment that is used. The following definitions are based on those found in Rydholm, Pulping Processes, Interscience Pulbishers, 1965, pp. 862-863 and TAPPI Monograph Nr. 27, The Bleaching of Pulp, Rapson, Ed. , The Technical Association of Pulp and Paper Industry, 1963, pp. 186-187.

o "Low consistency" includes ranges up to 656 , usually between 3 and 556. It is a suspension that is pumpable using a

ordinary centrifugal pump and can be achieved by using covers and filters without pressure rollers.

5 "Medium consistency" is between approximately 6 and 2056. 1556 is a dividing point between the medium consistency range. Below 1556, consistency can be achieved by filters. This is the consistency of the pulp mat which provides a vacuum drum filter in the washing system for the brown log and the bleaching system. The consistency of a 0 slurry from a washer, either a brownwood washer or a bleach stage washer, is 9 - 1556. Above about 1556, press rollers are required for dewatering. Rydholm claims that the usual range for medium consistency is 10 - 1856, while Rapson claims

that it is 9 - 1556. The slurry is pumpable by means of 5 special machinery despite the fact that it is still a coherent liquid phase at higher temperatures and under some compression.

"High consistency" is above about 2056 up to about 5056. 0 Rydholm claims the usual range is 25 - 3556 and Rapson claims the range is from 20 - 3556. These consistencies are only achievable using presses. The liquid phase is completely absorbed by the fibers and the pulp can be pumped in

only very short distances.

5

In this specification, "pulping" is used in the conventional sense to denote a splitting of ligno-cellulosic material to form brownwood. Mass production includes e.g. Kraft, the Kraft-AQ process and forms of extended delignification.

The term "modified Kraft process" is used herein to include extended delignification and all other modified Kraft processes with the exception of the Kraft-AQ process, since this process has achieved a special status and acceptability within the field and is separately known by this name. The oxygen delignification step after completion of pulping would not be considered an extended delignification, but instead would be termed a first step of a delignification process for leaching or lightening the pulp.

There are two main types of measurements to determine the completeness of the pulping or bleaching process, i.e. "degree of delignification" and "lightness" of the pulp. Degree of delignification is usually applied in conjunction with the pulping process and the previous bleaching steps. It tends to be less accurate when only small amounts of lignin are present in the pulp, i.e. in the later bleaching steps. The lightness factor is normally used in connection with the bleaching process because it tends to be more accurate when the mass is lighter in color and the reflectivity is high.

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

There are also a number of methods for measuring mass brightness. This parameter is usually a measure of degree of reflectivity and its value is expressed as a percentage of a degree. A standard method is GE brightness which is expressed as a percentage of a maximum GE brightness determined by TAPPI Standard Method TPD-103.

Where appropriate, the letter codes described in the background section will be used to denote the various stages of pulp processing throughout this detailed description of the invention.

B. The method steps according to the invention

The values (ie, K No., viscosity and GE lightness) obtained using the present pulping, delignification and bleaching process, as set forth below, demonstrate the ability of this process to enhance the degree of removal of lignin from the pulp while minimizing the resulting degradation of the cellulose. After the oxygen delignification step and before lighting, the pulp has been partially delignified to a K No. of about 5 to 10, preferably between about 7 to 10 for the U.S. conifers and about 5 to 7 for the U.S. leaf-wood. This partially delignified pulp has a viscosity of above about 10, usually more than 13 and preferably at least 14 (for softwood pulp) or 15 (for hardwood pulp). This partially delignified material therefore has good strength and suitable viscosity so that it can withstand the effects of ozone. The partially delignified pulp is exposed to ozone to further delignify the pulp, thereby reducing the K Nr. to the pulp to about 3 to 4 for both softwood and hardwood, while the GE lightness of the pulp is increased to at least about 50 - 7056. For softwood pulp, a GE lightness of about 5456 or higher is usually achieved, while for hardwood pulp values of approximately 6356 or more achieved. The lightness of the pulp is then further increased by an alkali extraction and a further bleaching step using chlorine dioxide or peroxide.

In order to understand the present invention, Fig. 1 describes, in schematic form, the various steps used in pulp production, delignification and brightening of a pulp. The process includes the following steps: (a) pulping the lignocellulosic material whereby the pulping chemicals can be isolated and reused in a well-known manner; (b) washing the pulp to remove chemical residues from the pulping end together with residual lignin and usually including a screening of the pulp to remove fiber bundles that have not been separated during pulping; (c) alkaline oxygen delignification (ie 0 or 0m) of the pulp; (d) washing the partially delignified pulp obtained in step (c) above to remove dissolved organics from the oxygen treatment; optionally, the screening may be carried out at this time, with recycling of at least part of the effluent from this stage to a previous stage; (e) chelating and acidifying the pulp to bind metal ions and to adjust the pH to a preferred level; (f) contacting pulp with ozone (ie Z or Zm) to further delignify and partially bleach this material; (g) washing the ozonated pulp, recycling at least a portion of the effluent from this step to a previous step; (h) caustic extraction to remove residual lignin; (i) washing the extracted mass by recycling at least part of the effluent to a previous step; (j) adding another bleaching agent (ie D or P to lighten and bleach the pulp; (k) washing the bleached pulp to obtain a bleached product having a GE brightness of about 70 - 90%; and (1 ) recycling at least part of the effluent from the P bleaching stage to an earlier stage; or diverting the effluent from the D bleaching stage or, after appropriate treatment, recycling this effluent to an earlier stage.

1. Mass production.

The first step in the process where procedures can be used that enhance the amount of lignin removed from the lignocellulosic material, while minimizing the amount of degradation of the cellulose, is in the pulping step. The particular pulping process used is, to a large extent, dependent on the type of lignocellulosic materials and, in particular, the type of wood used as a starting material. As can be seen from Fig. 1, the lye for pulping which is used in the techniques for chemical pulping can be isolated and used again in a well-known way. This step is usually followed by washing to remove most of the dissolved organics and chemical boiling for recycling and isolation, as well as a screening step where the pulp is passed through a screening apparatus to remove fiber bundles that have not been separated in pulping.

The Kraft process is generally acceptable for use with all types of wood compared to the other specified processes, as the final pulps obtained from the Kraft process have acceptable physical properties, despite the fact that the brown stock pulp is also darker in colour.

Depending on the lignocellulosic starting material, the results obtained with conventional Kraft processes can be enhanced by using extended delignification techniques or the Kraft-AQ process. These techniques are preferred for achieving the greatest degree of reduction in K No. of the mass without adversely affecting the strength and viscosity properties of the mass.

When using the Kraft-AQ technique, the amount of anthraquinone 1 cooking liquor should be in an amount of at least about 0.01 percent by weight, based on the oven-dried weight of the wood to be pulped, amounts of from about 0.02 to about 0.156 being generally preferred. The inclusion of anthraquinone in the Kraft pulp process contributes considerably to the removal of lignin without negatively affecting the desired strength characteristics of the remaining cellulose. The additional cost of anthraquinone can be ignored due to the savings in costs for chemicals in the subsequent Zm, E and D or P steps.

Alternatively, or perhaps in addition to Kraft-AQ, is the application of extended delignification techniques such as Kamyr MCC, Beloit RDH and Sunds Cold Blow Methods for batch splitters. These techniques also enable the ability to remove more of the lignin during pulping without adversely affecting the desired strength characteristics of the remaining cellulose.

2. Oxygen delignif icas. ion.

The next step comprises the removal of remaining lignin from the brownwood pulp which is being processed, i.e. oxygen delignification step. The solids removed in this step are oxygenated materials which, like black liquor, can be collected, concentrated and then incinerated in an environmentally safe manner in a conventional digester for insulation. At least part of the liquid phase is recycled as illustrated in Fig. 1.

It has been discovered that the oxygen delignification step can be carried out in a manner which enables the removal of increased percentages of residual lignin in the brownwood pulp without causing an unacceptable corresponding reduction in the viscosity of the pulp. The process that has been identified is practiced by treating the brownwood pulp from the pulping process at low to medium consistency as described below, with the necessary amount of alkali necessary for the oxygen delignification step to ensure uniform application of alkali, and then increasing the consistency and delignification at high consistencies. Although high consistency delignification is preferred, low or medium consistency oxygen delignification techniques may be used instead of high consistency delignification.

The high consistency oxygen delignification step is preferably carried out in the presence of an aqueous alkaline solution at a pulp consistency of from about 25% to about 35%, and more preferably is at about 27%. This improved process (0m) enables the removal of at least 60% of the remaining lignins from the brownwood pulp, compared to 45 - 50% removal with conventional oxygen delignification steps, without the previously expected undesirable reduction in relative viscosity. Because of the unique process capabilities of this modified process, it constitutes the clearly preferred oxygen process for use in the method of this invention.

The treatment step of the modified oxygen process (0m) comprises substantially uniformly combining wood pulp, preferably Kraft brown log pulp, with an aqueous alkaline solution maintaining the consistency of the pulp at less than about 1056 and preferably less than about 5 percent by weight. The aqueous alkaline solution is preferably present in an amount sufficient to provide from about 0.5 weight percent to about 4 weight percent active alkali after thickening based on the oven-dried bulk weight of the base stock, and even more preferably about 2.5 weight percent active alkali after thickening based on the oven dry weight to the brownwood pulp.

This step distributes the aqueous alkaline solution evenly throughout the low consistency brown pulp and ensures that substantially all of the brown log fibers are exposed for a uniform application of the alkaline solution. It is surprising that the brownwood pulp treated in this way is not significantly delignified in the treatment step, but is more effectively delignified in the subsequent high consistency oxygen delignification step than brownwood which is treated with alkaline solutions at high consistency according to the methods that are usually used. Localized inhomogeneities in the distribution of alkali in conventional high consistency pulp are avoided and therefore eliminate accompanying uneven oxygen delignification.

This homogeneous distribution step therefore preferably comprises uniform combination of pulp with an aqueous alkaline solution for at least about 1 minute and preferably not more than about 15 minutes. It is believed that treatment times of less than about 1 minute will generally not provide sufficient time to retain substantially even distribution, while treatment times in excess of about 15 minutes are not expected to produce significant additional benefit.

The preferred alkaline treatment of the pulp can be carried out over a wide range of temperature conditions. According to a preferred embodiment, the treatment step is carried out at a temperature of from about room temperature to about 66°C, with temperatures ranging from about 32°C to about 66°C being more preferred. Atmospheric pressure or elevated pressure may be used. The treatment step is completed when the aqueous alkaline solution is substantially uniformly distributed in the low consistency mass. The amount of aqueous alkaline solution present in the treatment step can vary widely according to the particular process parameters of the delignification reaction. The amount of alkaline solution effective in accordance with the present invention will depend primarily on the degree of delignification desired in the oxygen bleaching step and the strength of the particular solution used. The aqueous alkaline solutions which are preferably used comprise a sodium hydroxide solution having a concentration of from about 20 to about 120 g/l. This solution is mixed with low consistency mass, so that the whole mixture has a concentration of alkaline material of between approximately 6.5 and 3.5 g/l, and preferably around 9 g/l. For a 5 to 15 minute treatment of a 3 to 5% consistency mass with temperatures between 49° C. to 66° C. at these concentrations of alkaline material, an even distribution of such alkaline material is achieved in the mass.

An aqueous sodium hydroxide solution may be added to low consistency pulp in an amount sufficient to provide from about 15 weight percent to about 30 weight percent sodium hydroxide based on dry pulp weight. Other alkaline sources that have a similar sodium hydroxide content, such as the oxidized white liquor from conventional Kraft isolation and regeneration cycle, can also be used.

After the low consistency caustic treatment step described above, the consistency of the treated pulp is increased to greater than about 20%, preferably from about 25% to about 35%. Several methods exist and are well known in the art for increasing the consistency of the pulp, such as pressing wood pulp to remove liquid from it.

Then the oxygen delignification is carried out on the high consistency pulp. The methods are available and well known in the field for dissolving gaseous oxygen into the liquid phase into a high consistency mass to achieve delignification thereof. It is contemplated that any of these well-known methods can be adapted for use according to the present invention. However, it is preferred that oxygen delignification according to the present invention comprises the introduction of gaseous oxygen at approximately 648 KPa to approximately 9653 KPa into the liquid phase of the high consistency mass by maintaining the temperature of the mass between approximately 90°C and 130°C. The average contact time between the high consistency pulp and aqueous oxygen is preferably from about 20 minutes to about 60 minutes.

By following the preferred method according to the present invention, it is possible to achieve a reduction in K No. for the pulp after the oxygen delignification step of at least about 6056 with substantially no damage to the cellulosic portion of the pulp. Compared to this, conventional oxygen delignification can only achieve reductions in K No. of about 5056 before degradation of cellulose occurs. The present preferred method therefore unexpectedly provides an increase of at least 20% in delignification compared to previous delignification processes: i.e. from 5056 to at least about 6056 reduction of K No. for incoming mass. Reductions of 7056 or more can even be achieved with minimal cellulose degradation. Avoidance of destruction of the cellulose component in the pulp results from the minimal change in the viscosity of the pulp which is treated according to the present invention.

When entering the oxygen delignification step, mass K Nr varies. for the particular pulp range from about 10 - 26 depending on the type of wood (e.g. for Kraft-pulp production, about 10 - 14, measure 12.5 for hardwood and about 20 - 24, measure 21, for softwood), while after oxygen delignification is K No. generally in the range of about 5-10.

A processing scheme for carrying out the method according to the present invention is shown in schematic form in Fig. 2. The steps indicated therein represent a preferred operating system which maximizes certain advantages of the present invention. Wood chips 2 are fed into boiler 4 where they are boiled in a lye such as a lye of sodium hydroxide and sodium sulphide. The boiling unit 4 produces a Kraft brown stock 8 and a black liquor 6 containing the reaction products of the lignin solubilization. The brown log is processed in washing units which preferably comprise a blow tank 10 and washers 12 where the remaining lye contained in the pulp is removed. Many methods are available and well known in the art for washing brownwood, such as diffusion washing, rotary pressure washing, horizontal belt filtration and dilution/extraction. Sieving of brown matter is often carried out either before or after the washing steps to remove large parts of undefined wood for special processing.

The washed brown log is fed into a treatment unit 14 where it is treated with an alkaline solution and maintained at a consistency of less than about 10% and preferably less than about 5%. The method according to the present invention preferably includes devices for introducing supplementary caustic 16 into the treatment step in order to maintain the desired caustic application level. The treated mass 18 is taken to a thickening unit 20 where the consistency of the mass is increased, by pressing e.g. to at least about 20% by weight and preferably to about 25% to about 35%. The liquid 22 removed from the thickening unit 20 is preferably returned to the washing unit 12 for further use. High consistency "pressed" lignite 24 produced in the thickener 20 is fed to the oxygen delignification reactor vessel 26 where it is contacted with gaseous oxygen 28. The delignified lignite 30 is preferably passed through blow tank 32 and then to a second washing unit 34 in which the pulp is washed with water for to remove any dissolved organic matter to produce pulp 36 with high quality and little color. At least part of the outlet 38 from this washing step is preferably returned to the washing unit 12 for use therein. The outlet 13 from the washing unit 12 can be recycled alone or possibly with all or part of the outlet 38 to either blow tank 10 or black liquor line 6. In addition, the partially delignified pulp obtained after oxygen delignification can be sieved to remove fiber bundles from the pulp that are not been separated for further processing such as mechanical grinding. From here, mass 36 can be sent to subsequent bleaching steps to produce a fully bleached product.

In a particular preferred method according to the present invention as shown in Fig. 2 to successfully apply ozone bleaching, Kraft pulping of the wood can be carried out, followed by the modified low-consistency alkali treatment/high-consistency oxygen delignification process (0m ) described above. For conifers, as indicated above, this combination results in a pulp with a K No. of about 8 to 10, preferably 9, and a viscosity greater than about 13 to 14. Alternatively, it is possible to subject the wood to Kraft-AQ pulping followed by a conventional oxygen delignification step (ie, 0, high consistency alkaline treatment followed by high consistency oxygen delignification) to obtain a pulp that has similar character traits. Instead of Kraft-AQ pulp production, it is also possible to use extended delignification processes, followed by a standard oxygen delignification step to obtain pulp with the desired properties. It is also useful, but less preferred due to increasing costs or process steps, to combine Kraft pulping with extended delignification techniques such as Kamyr MCC, Beloit RDH or Sund's Cold Blow Cooking process, as described in the background section of this description, followed by conventional oxygen delignification .

Any of the many different pulping and oxygen delignification steps may be used in combination if they provide the above K No. and the viscosity values before the ozone step.

Conventional Kraft pulp production followed by conventional delignification is generally not acceptable in the present invention, with the exception of certain hardwoods such as aspen which are relatively easy to delignify and bleach, due to the fact that for a given wood species the combination of these conventional techniques normally requires the application of the greatest amount of ozone in the ozone stage with simultaneously higher cellulose breakdown.

By applying the present invention, ozone consumption can be reduced by using a number of alternative routes, such as standard Kraft cooking followed by a modified oxygen delignification step (0m) or modified Kraft pulp production by extended delignification (such as Kamyr MCC, Beloit RDH or Sunds Cold Blow) followed of a conventional oxygen delignification step (0), or by Kraft-AQ boiling followed by a conventional oxygen delignification step (0) as described above. Even a higher reduction in ozone consumption will be achieved both when using modified Kraft pulp production with extended delignification (Kamyr MCC, Beloit RDH or Sunds Cold Blow) followed by a modified oxygen delignification step (0m), or alternatively when a Kraft-AQ cooking process with extended delignification (Kamyr MCC, Beloit RDH or Sunds Cold Blow) is followed by a conventional oxygen delignification step (0) using all these techniques together in one process, i.e. Kraft-AQ cooking modified by extended delignification (Kamyr MCC, Beloit RDH or Sund's Cold Blow), followed by a modified oxygen delignification step (0m), further reduces the amount of ozone consumed. Reduction in the amount of ozone consumed generally enables the viscosity of the pulp to be maintained at acceptable levels.

The advantages of using the modified high consistency oxygen delignification bleaching step (0m) described above are clearly illustrated by comparison of K No. and the viscosities obtained using southern conifers with related processes under otherwise essentially identical process conditions. Using a conventional Kraft pulping procedure and conventional high consistency oxygen delignification bleaching, the resulting pulps 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 enable later delignification when applying the ozone step. Application of conventional Kraft pulp production with the modified consistency oxygen bleaching surprisingly results in a pulp having a K No. of less than about 9, while the viscosity of the pulp is above about 12 to 14. This preferred pulp K No. enables the use of the ozone delignification bleaching step according to the invention.

3. The ozone layer

The next step according to the method in the invention is ozone delignification and bleaching of the oxygen-delignified brown cane mass. This ozone treatment takes place in an ozone reactor which is described in detail below and illustrated in Figs. 2, 3 and 3A. For treating the pulp with ozone, the pulp is treated to achieve the most effective selective delignification of the pulp and to minimize the chemical attack of ozone on the cellulose. The incoming mass 36 is fed into a mixing container 40 where it is diluted to a low consistency. An acid 42 such as sulfuric acid, formic acid, acetic acid or the like is added to the low consistency pulp to reduce the pH of the pulp in the mixing vessel 40 to a degree of about 1 to 4, preferably between 2 and 3. The pH is adjusted as described above at because it is known that the relative effectiveness of ozone bleaching of the pulps is dependent on the pH of the pulp mixture. Lower pH values do not appear to have any beneficial effect on the further processing of the pulp, while increasing the pH above about 4 to 5 causes a reduction in viscosity and an increase in ozone consumption.

The acidified pulp is treated with chelating agent 44 to complex any metals or metal salts that may be present in the pulp. This chelation step is used to render such metals unreactive or harmless in the ozone reactor so that they will not cause decomposition of the ozone and thereby reduce the efficiency of the lignin removal and also reduce the viscosity of the cellulose.

The chelating agents are known per se and include e.g. polycarboxylate and polycarboxylate derivatives such as di-, tri- and tetra-carboxylates, amides and the like. Preferred chelating agents for this ozone treatment for economic and efficiency reasons include DTPA, EDTA and oxalic acid. Amounts of these chelating agents ranging from about 0.1 weight percent to about 0.2 weight percent of oven dry pulp are generally effective although additional amounts may be required when high metal ion concentrations are present.

The effectiveness of the ozone bleaching process is controlled by a number of interrelated process parameters, including the pH level and the amount of metal salts in the pulp as indicated above. Another very important parameter is the consistency of the pulp during the ozone bleaching process. The pulp to be bleached must contain sufficient water so that the water exists as a continuous phase throughout the individual fibers, i.e. the fiber should be sufficiently saturated with water. The water in the fiber allows the transfer of the ozone from the gaseous ozone atmosphere to both treat the outer surface of the fibers and, possibly more importantly, for the ozone to be transferred via the water phase to the less accessible interior of the individual fibers thereby providing more complete removal of lignin from the fibers. However, the consistency should not be so low that the ozone is diluted and chemically broken down instead of bleaching the mass.

The preferred consistency range, especially for the southern U.S. conifers, have been found to be between about 2856 and about 50%, with optimum results being obtained at between about 3856 and 4556. Within the above ranges, preferred results are obtained as indicated under relative amounts of delignification, the relatively low amount of degradation of cellulose and the clear increase in lightness of the treated masses.

The reaction temperature at which the ozone bleaching is carried out is likewise an important controlling factor in the method according to the present invention. The ozone step can be effectively carried out at temperatures up to a certain critical temperature, where the reaction begins to cause extensive degradation of the cellulose. This critical temperature will vary considerably depending on the particular type of wood used to form the pulp and how the pulp has been previously treated. The maximum temperature of the pulp fibers at which the reaction should be carried out should not exceed the temperature at which extensive degradation of cellulose occurs, as with southern U.S. conifers are at a maximum of about 49°C - 66°C.

The ozone gas used in the bleaching process can be used as a mixture of ozone with oxygen and/or an inert gas, or can be used as a mixture of ozone with air. The amount of ozone that satisfies this can be incorporated into the treatment gases is limited by the stability of the ozone in the gas mixture. Ozone gas mixtures typically containing about 1-8 weight percent ozone in an ozone/oxygen mixture, or about 1-456 ozone in an ozone/air mixture, are suitable for use in this invention. The high concentration of ozone in the ozone gas mixture enables the use of relatively smaller reactors and shorter reaction times to treat equivalent amounts of mass, thereby reducing the capital costs required for the equipment. The ozone gas mixtures containing lower amounts of ozone are usually cheaper to manufacture and can reduce operating costs.

A further controlling factor is the relative weight of ozone used to bleach a given weight of pulp. This amount is determined, at least in part, by the amount of lignin to be removed during the ozone bleaching process, balanced against the relative amount of cellulose degradation that can be tolerated during the ozone bleaching process. According to the preferred method of this invention, an amount of ozone will be used which reacts with about 50% to 70% of the lignin present in the pulp. The entire amount of lignin in the pulp is not removed in the ozone bleaching step as specified by K Nr. of about 3 to 4 obtained after this step, because the absence of all lignin in the reaction zone will result in the ozone reacting extensively with the cellulose to substantially reduce the degree of polymerization of the cellulose. In the preferred method of this invention, the amount of ozone added, based on the oven-dried weight of the pulp, will generally be from about 0.2% to about 1% to reach the lignin levels of a 3-4 K No. Higher amounts may be required if significant amounts of dissolved solids are present in the system.

The reaction time used for the ozone bleaching step is determined by the desired completion rate of the ozone bleaching reaction as indicated by complete or substantially complete consumption of the ozone being used. This time will vary depending on the concentration of the ozone in the ozone gas mixture, since relatively more concentrated ozone mixtures react more quickly, and the relative amount of lignin that is desired to be removed. The time required is preferably less than 2 minutes, but the procedure may take significantly longer depending on other reaction parameters.

An important feature is that the pulp is bleached evenly. This feature is achieved in part by comminuting the mass into discrete flocculent particles of a size which has a sufficiently small diameter and which has a sufficiently low bulk density so that the ozone gas mixture completely penetrates most of the fiber flocculates, i.e. which comprise agglomerations of fibres. During the fining, it was not possible to completely separate the pulp fibers into specific fibers. The flocculent particles that resulted from the fining have a relatively compact central core surrounded by a multitude of outgoing fibers. According to this invention, the flocculant particle size is determined by measuring what was stated to be the smallest diameter of this relatively uninflated central core.

The evenness of the bleaching is also largely dependent on certain other process parameters, but it has been discovered that if the flocculant particle size is limited to a maximum of 5 mm, and preferably even smaller, e.g. 3 mm, even treatment of a substantial majority of these particles can easily be achieved, as is evident from the observation of a considerable number of darker unbleached floc centers. When the floc particle size was greater than about 5 mm, the bleaching was uneven and this appeared with a lot of darker unbleached floc centers. It is therefore important to achieve sufficient fine division so that a major part of the flocs have a size below an average of approximately 5 mm for uniform ozone treatment.

A further important method parameter is that during the ozone bleaching process the particles to be bleached should be exposed to the mixture for ozone bleaching by mixing in order to enable the ozone gas mixture to have access to all floccule surfaces and for the ozone gas mixture to have equal access to all floccules. Mixing the mass in the ozone gas mixture gives good results with regard to uniformity compared to the results obtained with a static layer of flocculates where some of the flocculates are isolated from the ozone gas relative to other flocculates and thereby bleached less than other flocculates.

Moving the flocs to expose them to the ozone gas mixture causes even treatment of the flocs with respect to each other. This treatment results in the desired amount of lignin being removed evenly from the pulp without extensive degradation of the cellulose in the fibers comprising the flocculants. Control of the ozone treatment according to this invention by using a controlled particle size and by turbulent movement through the ozone treatment has been shown to result in a final mass which usually has less than about a 5% variation in GE brightness, K No. and viscosity. If the treatment is not uniform, as in static bed reactors, (i.e. reactors in which the particles are not agitated during the ozone treatment), some parts of the bed are significantly bleached while other parts remain relatively unaffected due to the fact that the flow of the ozone gas mixture through the static the bed reactor is not uniform.

Treatment of pulp at high consistency with ozone without paying particular attention to the comminution of the pulp fibers or proper contact between the individual fibers and the reactant gas flow results in an uneven ozone bleaching of the fibers. The present application indicates such uneven ozone treatment with the letters "Zm". Application of a modified ozone technique according to the present invention, as indicated above, in which the fibers are comminuted to a size of about 5 mm or less and which are properly and uniformly contacted with the ozone gas stream, has been designated "Zm".

Pulp coming out of the ozone reactor has a GE lightness of about at least 50$ and generally around 50 to 70$, with hardwoods usually above about 55$. The pulp (for hardwoods or conifers) also has a K No. of between about 3 and 4 (measurement 3.5), which is completely satisfactory for a mass at this stage of the process.

An apparatus which is particularly suitable for ozone bleaching according to the present invention is illustrated in Figs. 2, 3 and 3A. As described above, washed pulp 36 is led to mixing container 40 where it is treated with an acid 42 and a chelating agent 44. The acidified, chelated low-consistency pulp 46 is fed into thickening unit 48 for removing excess liquid 50 from the pulp, such as a double roller where the consistency of the mass is increased to the desired level. At least a portion of this excess liquid 50 can be recycled to the mixing vessel 40, with a remaining portion being directed towards the blow tank 32. The resulting high consistency mass 52 is then sent through screw feeder 54 which acts as a gas seal for the ozone gas and then through a comminution unit 56, such as a fluffer, where the pulp is comminuted into pulp fiber flocculates 60 of a predetermined size which, as indicated above, should measure about 5 mm or less in size. The finely divided particles are then fed into a dynamic ozone reaction vessel 58 which, as illustrated, is a conveyor 62 driven by motor 64. Conveyor 62 is specially designed for mixing and transporting the pulp particles 60 to enable the entire surface of the particles to be exposed for ozone gas mixture 66 through displacement of the mass. As shown in Fig. 2, the pulp fiber flocculates 60 after treatment are allowed to fall into dilution tank 68. Fig. 3 is a cross-section through the ozone reactor 58 illustrating the arrangement of the pulp particles 60 as they are carried through the reactor by conveyor 62. Fig. 3A is a cross-section of a preferred conveyor which uses a paddle-like arrangement to move finely divided particles through reaction chamber 58. The process in Fig. 2 shows the mass being treated with ozone simultaneously with the ozone gas mixture. Alternatively, however, the part of the pulp which has been bleached to the greatest extent can first be contacted with the newly introduced ozone mixture containing the maximum amount of ozone by passing the ozone-containing gas in a direction opposite to the flow of pulp 60. The pulp entering in the reactor has the highest lignin content and first contacts the outgoing, mostly utilized ozone mixture, thereby providing an optimal chance of consuming almost all of the ozone. This is an effective method for stripping ozone from the ozone/oxygen or ozone/air mixture.

When ozone 66 is contacted with the mass together as shown in Fig. 2, the remaining spent ozone gas 70 can be isolated from dilution tank 68. In tank 68, dilution water 72, which also acts as an ozone gas seal, is added to reduce the consistency of the mass to a low level to facilitate movement of the bleached mass 74 through subsequent process steps.

Spent ozone gas 70 from dilution tank 68 is directed to a carrier gas pretreatment step 76 where a carrier gas 78 of oxygen or air is added. This mixture 80 is directed towards the ozone generator 82 where the appropriate amount of ozone is formed to achieve the desired concentration. The correct ozone/air mixture 66 is then directed towards ozone reactor 58 for delignification and bleaching of the pulp.

After completion of the ozone bleaching step, the substantially delignified mass 74 is again thoroughly washed in washers 84 as shown in

Fig. 2 and at least part of the water 86 that was isolated is recycled to the washing unit 34 according to the method, thereby creating significant environmental benefits from the elimination of used liquid.

The bleached low consistency pulp 74 will, after ozone treatment, have a reduced amount of lignin, and therefore a lower K Nr. and an acceptable viscosity. The exact values for K Nr. and the viscosity which is obtained depends on the particular processing to which the pulp has been subjected. A southern U.S. Softwood pulp that is treated with a conventional Kraft method, initially delignified by modified high consistency oxygen delignification (0m), and then further delignified with ozone, preferably by a modified uniform ozone treatment (Zm), will usually have a K Nr. of about 3 - 4 and a viscosity of about 10. Southern U.S. Softwood pulp subjected to Kraft-AQ pulping and then to modified high consistency oxygen bleaching (0m) and modified uniform ozone treatment (Zm) will usually have a K No. of about 2 and a viscosity greater than about 12.

The resulting mass 74 will be clearly lighter than the starting mass. Southern conifers have e.g. after the pulping process a GE brightness of about 1556 to 2556; after the oxygen bleaching process, a GE brightness of about 2556 to 4556; and after the ozone bleaching process, a GE brightness of approximately 5056 to 7056.

4. Alkaline extraction.

The washed pulp 88 from the ozone step was then combined with a sufficient amount of alkaline material 90 in extraction vessel 92 to achieve extraction. Pulp 88 is therefore exposed to an aqueous alkaline solution for a predetermined time and at a predetermined temperature correlated to the amount of alkaline material, to dissolve a substantial part of any lignin remaining in the pulp, in container 92. This extraction process increases also the brightness of the mass, usually by about 2 GE brightness points. Next, the alkali-treated pulp 94 is directed to washing unit 96 and the aqueous alkaline solution is washed from the pulp to remove substantially all dissolved lignin from the pulp, thereby forming a substantially lignin-free pulp. This step is well known to those skilled in the art and no further comment is necessary. The examples illustrate preferred extraction parameters for this step of the process. At least a portion of the alkaline solution 98 which is isolated is recycled to the washing unit 84. Substantial environmental benefits are achieved by avoiding sewer passage of this solution.

In some cases, especially where higher final brightness is the goal, the extraction step can be supplemented by incorporating an oxygen treatment in the caustic extraction (EQ) step. This option, which is also well known to those skilled in the art, does not need to be commented on further here.

5. Additional bleaching step.

For most papermaking purposes, a final lightness in the range of 50 to 65 is not satisfactory. To further increase the GE lightness to the more desired range of approximately 70 to 95%, the pulp is subjected to lightening bleaching, which will mainly convert the chromophoric groups on the lignin remaining in the pulp into a colorless state. After extraction and washing of the pulp, the brightening bleaching of the ozone-bleached and extracted pulp can be carried out using different materials. As illustrated in Fig. 2, the washed pulp 100 is combined with the selected bleaching agent 102 in bleaching container 104. The preferred bleaching agent is chlorine dioxide or peroxide. After bleaching, pulp 106 is washed with water 114 in washing unit 108 and the effluent is either recycled 110 or directed to sewer 112. When recycled, at least a portion of wash water stream 110 is directed to washing unit 96. The resulting bleached pulp 116 can then be collected and applied as needed.

One of the main materials that has been used until now and which is generally very effective is chlorine dioxide (D) (see Fig. 1). According to the invention, an appropriate amount of chlorine dioxide enables pulps with high strength and having a value of GE lightness greater than about 8056. Due to the fact that the pulps entering the chlorine dioxide stage have a relatively low content of lignin, the brightening bleaching with chlorine dioxide be carried out in the presence of only from about 0.2556 to about 1% chlorine dioxide based on the oven dry weight of the pulp.

The chlorine dioxide that is used in the lightening process should preferably be produced by a method that is free of elemental chlorine. Alternatively, but less preferably, chlorine dioxide containing a smaller amount of elemental chlorine can be used without any significant increase in the relative amount of the desired contaminants due to the relatively low amount of lignin present in the ozone-bleached pulp. The discharge to the last bleaching step according to the invention using chlorine dioxide is exceptionally low and can be removed in a safe way as shown in Fig. 2.

If sewage disposal of the effluent from the final chlorine dioxide bleaching step is unacceptable, the stream can, on the other hand, be further purified by being treated with a membrane filtration process such as reverse osmosis. This technique produces a clean filtrate that can be recycled back to the previous bleaching step for further use. This has the advantage that the use of fresh water is reduced. The concentrated chlorine streams resulting from the membrane filtration have a relatively low volume.

There may be some cases where extremely high lightness of the mass is desirable, e.g. 92 - 9556 GEB, where additional bleaching steps may be necessary. A further extraction and chlorine dioxide treatment may be a common choice to thereby form an OmZmEDED bleaching sequence.

Instead of using chlorine dioxide for final bleaching, the brightening bleaching can be performed with hydrogen peroxide, as also shown in Fig. 1. This technique provides a completely chlorine-free bleaching cycle (such as an OmZmEP sequence), in which no chlorinated materials are formed in the bleaching process and the liquid extraction product can be easily recycled without the necessity of laborious filtration techniques. When peroxides are used as bleaching agents, K No. until the pulp from either conifers or hardwoods is reduced to a level of about 6 before the ozonation step to obtain, as a final product after the peroxide bleaching step, a pulp of acceptable lightness, i.e. a GE lightness higher than about 80%, due to the fact that peroxide is not as effective at bleaching as chlorine dioxide is. When a completely chlorine/chlorine dioxide-free process is desired, peroxide gives acceptable results.

Typical peroxide brighteners and their use in this step are conventional, and those skilled in the art are familiar with the appropriate concentration, types and use of such peroxide agents. Hydrogen peroxide is preferred.

The washed, further brightened pulp has a GE brightness of between about 70 and 95%, and preferably between about 80 and 95%. The physical properties of this pulp are consistent with those obtained from pulp produced by conventional CEDED or OC/DED processes.

6. Washing of outlet recycling material.

In any pulp process, filtrate treatment is an important factor in the overall economics or cost of operating the process. The water used in the process requires both access to a suitable source and treatment of the discharge before release.

In an attempt to reduce the water requirement of the process, it is desirable to recycle as much of the effluent as possible. This practice cannot be used with processes that use chlorine or multiple stages of chlorine dioxide, due to the fact that effluent produced by these processes contains large amounts of chlorides produced as by-products from such chemicals. Recycling of these effluents therefore causes an accumulation of chlorides which in turn causes either corrosion of the processing equipment or the use of expensive construction materials. In addition, such recycled effluents require substantial treatment before these effluents can be discharged from the mill and thus require further consumption of equipment and treatment chemicals.

As illustrated in Fig. 4, application of either the conventional CEDED process or the OC/DED technique results in a significant waste problem with regard to the effluents produced from the washing steps due to the high levels of chloride-containing compounds contained therein. As indicated above, these streams can be recycled, and are preferably treated prior to release into the environment. Recirculation of the effluent can be used to reduce the amount of water used, but then the process equipment must be exposed to increased corrosion rates due to the increased chloride level in the recycled effluent.

Application of the OmZmED process according to the invention, on the other hand, results in the formation of only a minimal amount of chlorinated material in the washing water and the water can be safely discharged, i.e. into the sewer, within most standards for environmental protection. This effluent can alternatively be treated with reverse osmosis to provide itself with an even cleaner filtrate which can be recycled to previous bleaching steps as shown for further applications without the formation of chlorides. When a D bleaching step is desired, steps must be taken that reduce the demand for chlorine dioxide. An EG step can allow the pulp to achieve higher lightness levels despite additional costs being incurred by the use of additional sodium hydroxide and oxygen in this step. There are also known industrial procedures for the production of chlorine dioxide whereby residual chlorine levels are minimized (eg the R8 process vs. the R3 process). These reduced chlorine level chemicals are preferred for use in the D stage to reduce the chloride levels of the wash water effluent.

Instead of OmZmED, the OmZmEP process according to the invention can be used to achieve further significant advantages in relation to the state of the art due to the fact that no chlorinated compounds are produced. This enables the entire amount of wastewater to be recycled without problems with chloride build-up in the process wash water streams becoming a problem.

The method according to the present invention leads to significant advantages with regard to reductions in effluent volume, colour, COD, BOD and chlorinated organic compounds. Because the effluent used in the washing steps contains significantly reduced chloride levels compared to previous processes using chlorine, the washing unit effluent will not contain chlorinated organic compounds or gases that require treatment before discharge.

EXAMPLES

Unless otherwise stated, all chemical percentages are calculated on the basis of the weight of oven-dried (OD) fibres. Those skilled in the art are also aware that the brightness values set as targets do not necessarily need to be achieved exactly, as GEB values of plus or minus 2% from the target are acceptable. In all the examples with a D step, with the exception of Example 11, an R-3 type chlorine dioxide solution, known to contain a 6:1 ratio of dioxide to elemental chlorine, was used.

EXAMPLE 1 (Comparative)

Loblolly pine shavings were lab batch cooked according to the conditions in Table I to produce a conventional Kraft pulp. The resulting pulp had a K No. of 22.6 and a viscosity of 27.1 eps. The kraft pulp was then subjected to conventional oxygen treatment (Tables II and V) followed by bleaching to a final target brightness of 83 GEB using both a conventional OC/DED sequence (Table III) and an OZmED bleaching sequence (Tables IV and V). The ozone bleach step was run at 3556 consistency with an ozone application of 0.6156. As shown in Tables VI and VII below, the OZmED bleach under these conditions produces a mass with acceptable strength ratios compared to the 8356 GE target lightness OC/DED baseline mass. Under these conditions, the OZmED mass had a marginal viscosity of 9.7 eps. The strength properties were measured on an OZmED mass where the last D-stage application was 2.5SÉ. The target brightness was only reached with extensive chlorine dioxide addition. The response of the OZmED pulps to the chlorine dioxide treatment shows that higher lightness can only be achieved by increasing the use of ozone considerably, which then causes the pulp to experience a considerable loss of viscosity and strength.

EXAMPLE 2

A Kraft/AQ brown log was prepared in a laboratory batch cooker from loblolly pine shavings as described in Table VIII. K No. to the resulting base stock was 18.3 and the viscosity was 20.6 eps. The Kraft/AQ pulp conditions produced in a pulp which has a considerably lower lignin content than in Example 1 as it appears from K No., without unacceptable destruction of the pulp strength as shown by the viscosity. Kraft/AQ brownwood was then subjected to further bleaching using the conventional OC/DED sequence and OZmED sequence as shown in Tables II, III, IV and V to a target lightness of 8356 GEB. Application of the Kraft AQ pulp processing technology achieved the goal of producing a starting pulp with a low K No., which has acceptable viscosity properties, for the ozone bleaching sequence. The ozone bleach stage was operated at 35% consistency with an ozone usage of 0.3556 and 1.656 CIO2 was used in the final D stage to achieve the lightness target.

As shown in Tables IX and X below, the optical properties as measured by brightness response in the final chlorine dioxide step were improved and the strength properties were acceptable compared to the OC/DED baseline.

EXAMPLE 3 (Comparative)

A pine Kraft brown log with a K No. of about 24 was pressed to a consistency of about 30-36 weight percent to produce a high consistency mat. The brown cane mat was sprayed with a 10% sodium hydroxide solution in an amount sufficient to produce approximately 2.5 weight percent sodium hydroxide based on pulp dry weight. Dilution water was added in an amount sufficient to adjust the brownwood mat to approximately a consistency of 27%. The high consistency log mat was then subjected to oxygen delignification using the following conditions: 110°C, 30 minutes, 648 KPa 02.

EXAMPLE 4

Furu Kraft brown log according to Example 3 was introduced into a treatment vessel together with a sufficient volume of 1056 NaOH solution to obtain a 3056 NaOH addition based on oven-dried pulp. Sufficient dilution water was added to achieve a brown log consistency of approximately 3% by weight in the treatment vessel. The brown stock and the aqueous sodium hydroxide solution were uniformly mixed at room temperature by a ribbon mixer for about 15 minutes. The treated brown log was then pressed to a consistency of approximately 27% by weight. After pressing, the sodium hydroxide on the fibers was approximately 2.556 as in Example 3. The treated brown log was then delignified according to the oxygen delignification procedure described in Example 3. A comparison is shown in Table XI.

As can be seen from a comparison of Examples 3 and 4, a preferred method according to the present invention using a low consistency alkali addition followed by a high consistency oxygen treatment (0m) led to the production of a bleached brown log with greater delignification (lower K Nr .) than previous methods without any significant change in the strength properties.

As a result of lower K No. pulp produced by this process, subsequent bleaching steps can be adjusted to accommodate higher lightness, lower lignin containing pulp. The bleaching steps for such pulp thus require less bleaching agents or shorter bleaching times than for pulps that are not treated according to the present invention.

EXAMPLE 5

Pulp produced from pine according to the 0m method in Example 4 of the present invention is compared with that produced conventionally (0) (ie without a low consistency alkaline treatment step). The average caustic dose for high consistency oxygen delignification of the brownwood pulp was found to be 20.4 kg per kiln-dried ton (kg/t) or 2.3%. At this level, the average reduction in K Nr. in the oxygen delignification reactor ten units. For the same level of caustic applied to the pulp according to a preferred treatment step, an average K No. reduction during delignification found to be 13 units: a 30% improvement compared to the conventional method.

This advantage in delignification selectivity can also be shown by a comparison of the pulp viscosity. Average K No. and viscosity for conventional pulp was 2.1 and 14.4 eps respectively. For the preferred treatment process according to the invention, the average K No. at essentially the same viscosity (14.0 eps) 8.3.

The delignification selectivity can also be expressed as change in viscosity versus change in K No. between brown logs and similarly treated pulps. Oxygen delignification selectivity is reduced rapidly when change in K Nr. begins to exceed 10 K No. units. Reduction in selectivity is observed as a rapid increase in change in viscosity for a given change in K No. As an example, for a change in K No. of 12 units, the corresponding change in viscosity exceeds 12 to 13 eps. For the same change in K No. (12) obtained with delignified pulps which have been treated using the preferred method, the change in viscosity was found to be 6 eps. Change in viscosity per change in K No. appears to be constant up to a 16 to 17 K No. unit change for masses obtained using the preferred treatment process of the invention. The results are shown in Table XII.

EXAMPLE 6

A southern pine pulp was prepared in a 600 TPD fine paper mill in operation using the modified oxygen delignification process (0m) with the conditions of Table II in combination with the steady alkali treatment described in Examples 4 and 5 and the conditions described in Table XIII below. The 0-stage pulp produced by this new method had the properties necessary for a successful way to complete the bleaching process using ozone, as described in the embodiment according to this invention. The oxygen level mass had a K No. of 7.9 (compared to a typical conventional O-stage K No. of about 12). The viscosity of the delignified pulp was 15 eps and was not significantly reduced by the high degree of delignification achieved using the modified oxygen process. This pulp could then be further bleached with ozone, using any of the many methods described herein to produce a pulp of acceptable final strength and optical properties.

The C/DED bleaching of this pulp was completed in the laboratory, as described in Table XIV, to provide a baseline for comparison of properties.

The ozone bleaching step was carried out in a pilot plant reactor as shown in Fig. 2. The conditions for operating the pilot plant reactor are shown in Table XV.

The ozone-bleached pulp formed in the pilot plant reactor was then processed in the extraction and chlorine dioxide steps in the laboratory, as described in Table V above, to produce a final bleached pulp product of target brightness. A final D stage addition of only 1.0$ CIO2 was applied to the fiber.

The strength and optical properties of the ozone bleached pulp were acceptable compared to the conventional OC/DED baseline and the results of the comparison are shown in

Tables XVI and XVII below.

EXAMPLE 7

To further exemplify the application and application area of the method according to this invention, a southern hardwood fiber, from mixed hardwood comprising mainly eucalyptus and oak, was bleached with ozone in the pilot plant described in Example 6 above. A conventional oxygen stage mass produced in the 600 TPD mill was treated with ozone in the pilot plant reactor. The oxygen step compound had a K No. of 5.7 and a viscosity of 14.1.

A portion of the 0 stage pulp was finally bleached by the conventional C/DED sequence in the laboratory to provide a baseline for comparison. The C/DED conditions are shown in Table XVIII.

The ozone reactor treatment conditions are shown in Table XIX. Pilot plant Zm step the pulp was then finally bleached at conventional E and D steps as shown in Table XX to a target lightness. A D-stage ClOg addition of only 0.35$ was applied to OD fiber. Wear and lightness properties were acceptable compared to the baseline as shown in Tables XXI and XXII.

EXAMPLE 8:

Comparison tests similar to those in Example 5 were carried out for laboratory-produced Kraft hardwood pulp, from mixed hardwood comprising mainly eucalyptus and oak. It was again found that a considerably larger K Nr. reduction over the oxygen delignification reactor using the modified oxygen process (0m) is achieved compared to conventional oxygen processing (0). The average caustic dosage for hardwood was 12.2 kg/h or 1,456. This produced a K No. drop of approximately 5 units during the oxygen stage. For the same caustic level used according to the modified oxygen process of the present invention, an average K No. fall of about 7.3 units achieved and amounts to an increase of almost 5056.

This advantage in delignification selectivity can also be shown by comparing the pulp viscosity. The average hardwood K No. and the viscosity was found to be 7.6 and 16 eps. For the invention, a K No. of 6 and a viscosity of 17.7 obtained. K No. at the same viscosity as untreated pulp (16 eps) was found to be 5.8.

The selectivity of the delignification can also be expressed in terms of change in viscosity versus change in K No. between brownwood and the correspondingly modified oxygen-treated pulps. When comparing pulps that are conventionally oxygen treated with those according to the invention, there is a greater reduction in delignification selectivity for increased delignification degrees. For a change in K No. of four units the average change in viscosity was 4 eps for pulps prepared by the conventional process. In contrast to this, the change in K No. for the same change in viscosity for pulps prepared by the modified oxygen method of 7 units. Expressed in terms of a delignification-selectivity ratio, the selectivity of the modified method was 1.8 K Nr./eps and that of the conventional method was 1 K Nr./eps, an increase of 80%. The results are shown in Table XXIII.

EXAMPLE 9

A series of experiments were conducted in the pilot plant reactor using pulp from a 600 TPD fine paper mill with a conventional oxygen delignification stage (0). These experiments were conducted to illustrate the effect of pH on the ozone bleaching process using southern hardwoods. Reactor operating conditions were held constant at the conditions shown in Table XXIV with the pE of the ozone stage being the only variable.

As can be seen from Table XXV below, the effect of pH on the ozone bleaching process is considerable and lower pH improves the selectivity of the bleaching process.

EXAMPLE 10

A number of comparative characteristics are of interest to illustrate the beneficial effects of producing fully bleached pulps using the OZjjjED process. Common operating data and measurements of the effluent were obtained from operation of the mills using the CEDED and OC/DED bleaching sequences on southern pine. These properties were compared to those of the effluents produced by the 0ZmED sequence using the 0ZmED pulp and effluent prepared in Example 1. For the conventional CEDED sequence seen in Table XXVI, for the conventional OC/DED sequence see Tables II and III above and for the 0ZmED the sequence see Tables IV and V above. It is noted that the CEDED sequence drains are combined C, E^, D^, Eg and D2 drains. The OC/DED drain is the C/D, E and D combined drain and the OZjjjED drain is the D stage drain, each representing the other drain characteristics. As shown in Table XXVII below, the ozone bleaching sequence significantly reduces the environmental impact of the effluent from the bleaching process. To determine color, EPA method 110.2 was used. From this data it can be seen that the present invention provides an effluent with a color no higher than about 0.907 kg per ton, a BOD5 value that is not higher than approximately 0.907 kg per tons and an amount of total organic chloride not greater than about 0.907 kg and preferably less than about 0.36 kg.

EXAMPLE 11

Southern pine Kraft pulp is bleached using three modifications of the main OZED sequence. In the first sequence (OZmED) the pulp was bleached as in Tables IV and V with conventional oxygen, modified ozone, caustic extraction and chloride dioxide as prepared in the R-3 sequence with a ClOg/Clg ratio of 6:1. In the second sequence, the modified oxygen process (0m) was used and again used in the final stage R-3 type chlorine dioxide. In the third sequence, the modified oxygen process (0m) was used again, and an R-8 chlorine dioxide solution was used with a 95:1 ratio in the final step. Table XXVIII demonstrates the positive environmental impact provided by the use of the modified oxygen process (0^). R-8 bleach also had a positive effect.

EXAMPLE 12

Southern loblolly pine pulps were prepared by the Kraft and Kraft/AQ pulping processes as described in Tables I and VIII above. These pulps were further subjected to conventional and modified oxygen delignification as described in Examples 4 and 5 to show the effect of combining these methods (for extended delignification with minimal impact on pulp strength) on the ozone bleaching sequence. As can be seen from Table XXIX, these processes form an additive effect. Extremely low OmZmE K No. can be achieved with little effect on the final viscosity. In contrast, the amount of ozone needed to achieve a target 0mZmE K Nr. of approximately 3.5 for the previously described ozone bleaching process be substantially reduced. In addition, the additive effect produces a southern pine pulp that can be completely bleached by an OmZmEP process where a very low 0mZmE K Nr. is required for a functional peroxide step.

EXAMPLE 13

Southern softwood, i.e. loblolly pine, was bleached to a lightness target value of 83 GEB using the conventional CEDED sequence as shown in Table XXVI, using the conventional OC/DED sequence as shown in Tables II and III above and by using the 0ZmED sequence as shown in Tables IV and V above. Wood-based waste was refined and added to the 0ZmED starting brownstock at a level of 0.75 weight percent to investigate the ability of this sequence to remove waste compared to CEDED and OC/DED bleaching. The waste properties of the three sequences, measured as effective black area, bark and chips, were equivalent.

EXAMPLE 14

This example illustrates the application area of the ozone bleaching process according to the invention. Bleached pulps can be formed over a wide range of product lightness, using appropriate combinations of ozone and chlorine dioxide additions to minimize environmental impacts and operating costs. As shown in Table XXX below, products with brightness above 65$ GEB can be produced by various combinations of ozone and chlorine dioxide while retaining considerable strength properties.

Claims (121)

Process for the production of a substantially uniformly bleached pulp from a lignocellulosic material comprising the formation of an intermediate pulp with a reduced lignin content by chemical boiling (4) of the lignocellulosic material, followed by oxygen delignification (26) of the resulting pulp, characterized in that a) the pH of the intermediate mass is adjusted (42) to the range of 1 to 4, b) the consistency of the intermediate mass is adjusted (48) to a high consistency of 2056 to 5056, c) the intermediate mass is pulverized into separate particles, where most have a size of less than 5 mm, d) the intermediate mass obtained after steps a), b) and c) is subjected to a delignification treatment by turbulent mixing of this with an ozone-containing gaseous mixture in a dynamic reaction zone (58) for a sufficient time and at a sufficient temperature to enable access of ozone to substantially the entire intermediate mass for reaction thus, while n possible pulp particles are passed (62) through substantially the entire reaction zone in a manner that exposes substantially all of the pulp particles to the ozone in a uniform manner during the advance of the pulp to thereby achieve further substantially uniform delignification of most of the pulp particles without significant degradation of the cellulose components of the pulp, so that for southern softwood intermediate pulp with a GE lightness of 2556 to 4556 after oxygen delignification, the pulp leaving the dynamic reaction zone shows a GE lightness of at least 5056 and for hardwood intermediate pulp, the pulp leaving the dynamic reaction zone shows a GE brightness greater than 5556. 2. Method according to claim 1, characterized by chemical cooking of the lignocellulosic material by Kraft pulp treatment, Kraft AQ pulp treatment or extended delignification. 3. Method according to claim 1, characterized in that the oxygen delignification step comprises the formation of a mass of low to medium consistency; treatment of the pulp with low to medium consistency with an aqueous solution of an alkaline material for a predetermined time and at a predetermined temperature relative to the amount of the alkaline material in order to achieve substantially uniform distribution of the alkaline material in the pulp with low to medium consistency; increasing the consistency of the pulp to a high consistency of 20 to 50$ and subjecting the resulting high consistency pulp to high consistency oxygen delignification to obtain the intermediate pulp. 4. The method of claim 1, characterized in that the final determined GE lightness of the bleached pulp is at least about 5056, the viscosity thereof is greater than about 10 eps, and wherein the specified amount of lignin is indicated by an intermediate mass K No. of 10 or less and the viscosity of the intermediate mass is higher than 13 eps. 5. Method according to claim 1, characterized in that the lignocellulosic material is a softwood, the final GE lightness of the bleached pulp is at least 5056, the viscosity of this is higher than 10 eps, and in which the specified amount of lignin is indicated by an intermediate mass K No. of 7 - 10 and the viscosity of the intermediate mass is higher than 13 eps. 6. Method according to claim 1, characterized in that the lignocellulosic material is a hardwood, the final GE lightness of the bleached pulp is at least 5556, the viscosity of this is higher than 10 eps and in which the amount of lignin is indicated by an intermediate pulp K Nr . of 5 - 8 and the viscosity of the intermediate mass is higher than 13 eps. 7. Method according to claim 4, 5 or 6, character i-s ert in that the amount of lignin included in the mass after ozone delignification is indicated with a bleaching mass K No. at 3 to 4. 8. Method according to claim 1, characterized in that it further comprises bleaching the pulp after ozone delignification with a lightening agent to increase the GE brightness of the bleached pulp. 9. Method according to claim 8, characterized in that it further comprises combining the bleached mass with an effective amount of alkaline material in an alkaline aqueous solution at a predetermined temperature, corresponding to the amount of alkaline material that is combined, to dissolve a substantial part of any lignin remaining in the bleached pulp; and then extracting a portion of the aqueous alkaline solution to remove substantially all dissolved lignin therefrom and form an extracted pulp prior to bleaching with the brightening agent. 10. Method according to claim 8, characterized in that the brightening agent is chlorine dioxide or a peroxide. 11. Method according to claim 1, characterized in that the ignocellulose-containing material is chemically boiled to form a mass having first K no. and first viscosity values; said pulp is delignified with oxygen to form a partially delignified pulp having a second K No. which is lower than the aforementioned first K No. and which is sufficient to enable the partially delignified pulp to be further delignified with ozone while maintaining the viscosity at a level such that the cellulosic components of said partially lignified pulp are not significantly chemically degraded by oxygen delignification; and the partially delignified pulp is further delignified by the ozone to obtain a substantially delignified pulp which has a third K No. which is significantly reduced under the aforementioned second K No. to said partially delignified pulp and which has a GE lightness value that is significantly above that of the partially delignified pulp while maintaining the viscosity and without aggressively chemically attacking cellulose components to the pulp in order to avoid a significant reduction in the strength of the pulp. 12. Process according to claim 11, characterized in that the partially delignified pulp contains an amount of lignin which, after ozone delignification, enables the pulp to achieve the specified GE lightness, and also has a viscosity that is sufficiently high to compensate for reductions in viscosity in the course of ozone delignification. . thereby enabling the substantially delignified mass to achieve a certain strength. 13. Method according to claim 11, characterized in that it further comprises combining the substantially delignified mass with an effective amount of alkaline material in an alkaline aqueous solution at a predetermined temperature, corresponding to the amount of alkaline material that is combined, in order to dissolve a substantial part of any lignin remaining in the substantially delignified mass; and then extracting a portion of the aqueous alkaline solution to remove substantially all dissolved lignin therefrom and form a substantially lignin-free pulp. 14. Method according to claim 13, characterized in that it further comprises bleaching the substantially lignin-free pulp to a lightness value that is substantially higher than that of the substantially delignified pulp. 15. Method according to claim 11, characterized in that the viscosity of said partially delignified mass is maintained at higher than 13 eps. 16. Method according to claim 15, characterized in that said partially delignified mass is maintained by a reduction in viscosity of 30% or less of said first value. 17. Method according to claim 11, characterized in that said lignocellulose-containing material is a hardwood. 18. Method according to claim 17, characterized in that said first K No. is in the range 10 - 14. 19. Method according to claim 17, characterized in that said first value for viscosity is in the range 21 - 28 eps. 20. Method according to claim 17, characterized in that said second K No. is in the range 5-8. 21. Method according to claim 20, characterized in that said third K No. is less than 5. 22. Method according to claim 11, characterized in that said lignocellulose-containing material is a conifer. 23. Method according to claim 22, characterized in that said first K No. is in the range 20 - 24. 24. Method according to claim 22, characterized in that said second K No. is in the range 7-10. 25. Method according to claim 11, characterized in that the oxygen delignification treatment is carried out on pulp of medium consistency. 26. Method according to claim 11, characterized in that the partial delignification step further comprises: treatment of said pulp with an amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material to substantially complete a substantially uniform distribution of the alkaline material in the pulp; increasing the consistency of the pulp after completion of the processing step; and subjecting the pulp of increased consistency containing alkaline materials to high consistency oxygen delignification to obtain a partially delignified pulp. 27. Method according to claim 11, characterized in that the viscosity of said substantially delignified mass is maintained at higher than 10 eps. 28. Method according to claim 27, characterized in that said substantially delignified mass is maintained by a reduction in viscosity of approximately 30% or less of the viscosity of said partially delignified mass. 29. Method according to claim 11, characterized in that the further delignification step additionally comprises: increasing the consistency of said partially delignified pulp; and finely grind said increased consistency mass to a predetermined particle size and to uniformly contact said finely divided mass with said effective amount of ozone when the mass goes through the process. 30. Method according to claim 29, characterized in that the particle size of said pulp with increased consistency is finely divided to one that facilitates uniform contact with ozone without causing considerable degradation of the cellulose components of the pulp. 31. Method according to claim 30, characterized in that the mass is finely divided to approximately 5 mm and carried through the process in such a way that an uneven application of ozone to the mass is avoided. 32. Method according to claim 31, characterized in that the finely divided mass is advanced simultaneously with ozone. 33. Method according to claim 31, characterized in that the finely divided mass is carried countercurrently in relation to ozone. 34. Method according to claim 14, characterized in that the essentially lignin-free mass is bleached with chlorine dioxide. 35. Method according to claim 14, characterized in that the substantially lignin-free mass is bleached with a peroxide. 36. Method according to claim 34 or 35, characterized in that the bleaching step increases the GE lightness of said substantially lignin-free pulp to at least 7056. 37. Method according to claim 34 or 35, characterized in that the bleaching step increases the GE lightness of said substantially lignin-free pulp to at least 80%. 38. Method according to claim 34 or 35, characterized in that the bleaching step increases the GE lightness of said essentially lignin-free pulp to at least 9056. 39. Method according to claim 1, characterized in that the oxygen delignified mass has a K.No. of about 10 or less and a viscosity greater than about 13 eps after oxygen delignification; and the ozone delignified pulp has a K No. of 5 or less, a viscosity greater than 10 and a GE brightness of at least about 5056 after ozone delignification. 40. Method according to claim 39, characterized in that the lignocellulose-containing material is a conifer and is partially delignified into a pulp with a K No. of 7 to 10 and a viscosity above 13 before further delignification with ozone. 41. Method according to claim 40, characterized in that the softwood pulp has a K No. of 3 to 4, a viscosity tet of above about 10 and a GE brightness of at least 50$ after said further delignification with ozone. 42. Method according to claim 41, characterized in that the lignocellulosic material is a hardwood and is partially delignified into a pulp which has a K No. of 5 to 8 and a viscosity above 13 before further delignification with ozone. 43. Method according to claim 42, characterized in that the hardwood pulp has a K No. of 3 to 4, a viscosity above about 10 and a GE brightness of at least 555é after further delignification with ozone. 44. Method according to claim 39, characterized in that it further comprises combining the substantially delignified mass with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material to dissolve a substantial portion of any lignin remaining in the pulp, and then extracting a portion of the aqueous alkaline solution to remove substantially all soluble lignin therefrom and form a substantially lignin-free pulp. 45. Method according to claim 44, characterized in that said extraction step increases the lightness of the mass by approximately 2%. 46. Method according to claim 44, characterized in that it further comprises bleaching the substantially lignin-free mass with a chlorine dioxide or peroxide to increase the GE brightness to at least about 70%. 47. Method according to claim 46, characterized in that the GE brightness is increased to at least 80%. 48. Method according to claim 46, characterized in that the GE brightness is increased to at least 90%. 49. Method according to claim 44, characterized in that the lignocellulosic material is partially delignified by oxygen delignification treatment. 50. Method according to claim 49, characterized in that the oxygen delignification treatment is carried out on pulp of medium consistency. 51. Method according to claim 44, characterized in that the lignocellulosic material is partially delignified by forming a mass which has a relatively low consistency of less than 10 percent by weight; treating the pulp having a low consistency with an amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material to substantially complete a substantially uniform distribution of the alkaline material in the pulp; to increase the consistency of the pulp to at least about 20 percent by weight after completion of the processing step and subjecting the pulp with consistency containing alkaline material to high-consistency oxygen delignification to obtain a partially delignified pulp having a K No. of 9 or less and a viscosity of 13 or greater. 52. Method according to claim 1, characterized in that the oxygen delignifying step comprises: reducing the consistency of the pulp to a low consistency of less than 10 percent by weight; treating the pulp having a low consistency with an amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material to substantially complete a substantially uniform distribution of alkaline material in the pulp; increasing the consistency of the pulp to at least about 20 percent by weight after completion of the processing step; and subjecting the pulp of increased consistency containing alkaline material to high consistency oxygen delignification to obtain a partially delignified pulp with a K No. of 9 or less and a viscosity greater than 13 eps; whereby the oxygen delignifying step means that a substantially delignified mass is obtained which has a K No. of about 5 or less, a viscosity greater than about 10 and a GE lightness of at least $50; and that the method further comprises: combining the ozone delignified mass with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material in order to dissolve a substantial part of any lignin which is left in the mass; extracting a portion of the aqueous alkaline solution to remove substantially all soluble lignin therefrom and form substantially lignin-free pulp; and bleaches the substantially equation-free mass to increase the GE lightness to at least 70%. 53. Method according to claim 52, characterized in that the GE brightness is increased to at least 80%. 54. Method according to claim 52, characterized in that the GE brightness is increased to at least 90%. 55. Method according to claim 52, characterized in that the lignocellulose-containing material is a conifer and is partially delignified to a K No. of 8 to 9 and a viscosity higher than 14 before further delignification with ozone. 56. Method according to claim 55, characterized in that the softwood pulp has a K No. of between 3 to 4, a viscosity of over 10 and a GE lightness of at least 54$ after said further delignification with ozone. 57. Method according to claim 52, characterized in that the lignocellulose-containing material is a hardwood and is partially delignified to a K No. of 6 to 7 and a viscosity of over 15 before said further delignification with ozone. 58. Method according to claim 57, characterized in that the hardwood pulp has a K No. of about 34, a viscosity above 10 and a GE brightness of at least 63% after said further delignification with ozone. 59. Method according to claim 52, characterized in that the partially lignified lignocellulosic material is obtained by Kraft pulp treatment, Kraft-AQ pulp treatment or extended delignification of a lignocellulosic material. 60. Method according to claim 52, characterized in that it comprises reduction of K No. to the pulp with increased consistency by at least 60% during the oxygen delignification step without considerable damage to the cellulose components of the pulp. 61. Method according to claim 52, characterized in that the pulp is exposed to high-consistency oxygen delignification without significant change in the viscosity of the pulp. 62. Method according to claim 52, characterized in that it comprises reduction of the ratio between K No. and the viscosity of the pulp during the oxygen delignification step by at least 25%. 63. Method according to claim 52, characterized in that the consistency of the mass which is treated with the aqueous alkaline solution before oxygen delignification varies between 1 and 4.5 percent by weight. 64. Method according to claim 52, characterized in that the consistency of the mass is increased to between 25 and 35 percent by weight before the oxygen delignification step. 65. Method according to claim 52, characterized in that the amount of alkaline material distributed in the pulp with a low consistency before the oxygen delignification varies from 15 to 30 percent by weight based on the dry weight of the pulp. 66. Method according to claim 65, characterized in that the aqueous alkaline solution has a concentration of alkaline material of between 20 and 120 g/l, so that the concentration of the alkaline material in the mass with low consistency varies from 6.5 to 13 g/l. 67. Method according to claim 52, characterized in that the alkaline treatment step is carried out for a time of between 1 and 15 minutes at a temperature of between 32°C and 66°C. 68. Method according to claim 52, characterized in that the originally formed pulp is brown pulp and further that at least part of the liquid obtained from the alkaline solution during the pulp consistency increasing step is recycled to the alkaline treatment step. 69. Method according to claim 1, characterized in that the mass is initially formed as a brown mass with a K No. on from 10 - 24 with Kraft pulp treatment, Kraft-AQ pulp treatment or extended delignification of a lignocellulosic material; and wherein the oxygen delignification step comprises reducing the consistency of said pulp to 1 to 4.5 percent by weight; treating the reduced consistency mass with an amount of alkaline material in an aqueous alkaline solution having a concentration of alkaline material of between 20 and 120 g/l, for a time of between 1 and 15 minutes and at a temperature of between 32°C and 66 °C so that the concentration of the alkaline material in the mass with reduced consistency during this treatment step varies from 6.5 to 13 g/l, in order to substantially achieve a substantially uniform distribution of the alkaline material in the mass; increasing the consistency of the alkaline treated pulp to between 25 to 35 percent by weight; subjecting the pulp having an increased consistency to high-consistency oxygen delignification without substantially changing the viscosity of the pulp to form a partially delignified pulp having a K No. of about 10 or less and a viscosity greater than about 13, wherein the ratio of K No. and the viscosity of said pulp is reduced by at least 25% during oxygen delignification; and where the ozone delignifying step results in a substantially delignified pulp which has a K No. of 5 or less, a viscosity greater than 10 and a GE lightness of at least about 50$; and the method further comprises: combining the substantially delignified mass with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material in order to dissolve a significant part of any lignin which is left in the mass; extracting a portion of the aqueous alkaline solution to remove substantially all dissolved lignin therefrom and to form a substantially lignin-free pulp; and bleaches the substantially lignin-free pulp to increase the GE brightness to at least 70%. 70. Method according to claim 69, characterized in that the GE brightness is increased to at least 80%. 71. Method according to claim 69, characterized in that the GE brightness is increased to at least 90%. 72. Method according to claim 69, characterized in that it comprises reducing K Nr. to the pulp with increased consistency by at least b0% during the oxygen delignification step without significantly damaging the cellulose components of the pulp. 73. Method according to claim 69, characterized in that the essentially lignin-free mass is bleached with chlorine dioxide or a peroxide. 74. Method according to claim 73, characterized in that the peroxide is hydrogen peroxide. 75 . Method according to claim 69, characterized in that it further comprises the addition of a chelating agent to said mass before the ozone delignification in order to make the metal ions substantially non-reactive towards ozone. 76. Method according to claim 75, characterized in that the chelating agent is DTPA, EDTA or oxalic acid. 77. Method according to claim 69, characterized in that it further comprises adjusting the pH of the mass to a range of 1 to 4 by adding to the mass a sufficient amount of an acidic material before the ozone delignification. 78. Method according to claim 69, characterized in that it further comprises increasing the consistency of the mass to the range of 25 - 50 percent by weight before ozone delignification. 79. Method according to claim 78, characterized in that the consistency of the mass is increased to the range of 35 - 45 percent by weight before the ozone delignification. 80. Method according to claim 69, characterized in that it comprises comminution of said mass to a diameter of less than 5 mm after oxygen delignification and before ozone delignification. 81. Method according to claim 69, characterized in that it further comprises maintaining the mass at a temperature of less than 49° C during the ozone delignification. 82. Method according to claim 69, characterized in that ozone is provided by a mixture consisting of ozone and oxygen. 83. Method according to claim 82, characterized in that the ozone concentration in the mixture is between 1 and 8 percent by volume. 84. Method according to claim 69, characterized in that the ozone is provided from a mixture of ozone and air. 85. Method according to claim 84, characterized in that the ozone concentration is between 1 and 4 percent by volume. 86. Method according to claim 69, characterized in that the partially delignified mass is advanced during the ozone delignification step in a way that substantially exposes the entire mass to ozone. 87. Method according to claim 86, characterized by introducing ozone countercurrently in relation to the carrying mass. 88. Method according to claim 86, characterized by introducing ozone co-currently with the carrying mass. 89. Method according to claim 1, characterized in that the oxygen delignified mass has a K No. of 10 or less and a viscosity greater than 13 eps; the ozone delignifying step comprises: adding a chelating agent to said mass to make the metal ions therein substantially non-reactive towards ozone; adjusting the pH of said pulp to a range of 1 and 4 by adding a sufficient amount of an acidic material thereto; increasing the consistency of said pulp to between 25 and 50$; comminutes said mass of increased consistency to a diameter of less than 5 mm; delignifies said pulp with increased consistency with an effective amount of ozone for a sufficient time by advancing the finely divided pulp in a way that exposes substantially the entire pulp to ozone to obtain a substantially delignified pulp having a K No. of 5 or less, a viscosity greater than 10 and a GE lightness less than about 50$; and the method further includes that one: combining the substantially delignified pulp with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material to dissolve a substantial portion of any lignin remaining in the pulp; extracting a portion of the aqueous alkaline solution to remove substantially all dissolved lignin therefrom and form a substantially lignin-free pulp; and bleaching the substantially lignin-free pulp with chlorine dioxide to increase the GE brightness to at least about 70$. 90. Method according to claim 89, characterized in that the GE brightness is increased to at least 80$. 91. Method according to claim 89, characterized in that the GE brightness is increased to at least 90$. 92. Method according to claim 89, characterized in that the pulp treatment step includes Kraft pulp treatment and the oxygen delignification step reduces K Nr. to the pulp by at least b0% without damaging the cellulose components of the pulp or without significantly changing the viscosity of the pulp. 93. Method according to claim 92, characterized in that the pulp treatment step comprises Kraft-ÅQ pulp treatment. 94. Method according to claim 93, characterized in that the oxygen delignification step reduces K Nr. to the pulp by at least 60$ without significantly damaging the cellulose components of the pulp or without significantly changing the viscosity of the pulp. 95. Method according to claim 89, characterized in that the pulp treatment step comprises the combination of Kraft-AQ pulp treatment and extended delignification and the oxygen delignification step reduces K Nr. to the pulp by at least 60$ without significantly damaging the cellulose components of the pulp or without significantly changing the viscosity of the pulp. 96. Method according to claim 89, characterized in that during the ozone delignification the mass is advanced in such a way that the mass is maintained at a temperature of less than 49°C. 97. Method according to claim 89, characterized in that the chelating agent and the acid are added to said mass in a mixing container. 98. Method according to claim 97, characterized in that at least part of the liquid that is separated from the mass during the consistency-increasing step is recycled to the mixing container. 99. Method according to claim 97, characterized in that the mass is carried co-currently with the ozone. 100. Method according to claim 97, characterized in that the mass is carried countercurrently with the ozone. 101. Method according to claim 1, characterized in that the oxygen partially ignited mass has a K No. of 10 or less and a viscosity greater than 13 eps; the ozone delignifying step comprises: adding a chelating agent to said mass to make the metal ions therein substantially non-reactive towards ozone; adjusting the pH of said pulp to a range of 1 and 4 by adding a sufficient amount of an acidic material thereto; increases the consistency of said pulp to between about 25 and 50%; comminutes said mass of increased consistency to a diameter of less than 5 mm; delignifies said pulp with increased consistency with an effective amount of ozone for a sufficient time to obtain a substantially delignified pulp which has a K No. of 5 or less, a viscosity greater than 10 and a GE brightness of at least 50%; and the method further comprises: combining the substantially delignified mass with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material in order to dissolve a significant part of any lignin which is left in the mass; extracting a portion of the aqueous alkaline solution to remove substantially all dissolved lignin therefrom and form a substantially lignin-free pulp; and bleaches the substantially lignin-free pulp with peroxide to increase the GE brightness to at least 70%. 102. Method according to claim 101, characterized in that the GE brightness is increased to at least $80. 103. Method according to claim 101, characterized in that the GE brightness is increased to at least 90$. 104. Method according to claim 101, characterized in that the pulp treatment step comprises Kraft pulp treatment and the oxygen delignification step reduces K No. to the pulp by at least 60$ without damaging the cellulose components of the pulp or without significantly changing the viscosity of the pulp. 105. Method according to claim 101, characterized in that the pulp treatment step comprises Kraft-AQ pulp treatment. 106. Method according to claim 105, characterized in that the oxygen delignification step reduces K Nr. to the pulp by at least 60% without significantly damaging the cellulose components of the pulp or without significantly changing the viscosity of the pulp. 107. Method according to claim 101, characterized in that the pulp treatment step comprises the combination of Kraft-AQ pulp treatment and extended delignification, and the oxygen delignification step reduces K Nr. to the pulp by at least 60% without significantly damaging the cellulose components of the pulp or without significantly changing the viscosity of the pulp. 108. Method according to claim 89 or 101, characterized in that the ozone delignification reduces K Nr. to the mass by at least 50%. 109. Method according to claim 89 or 101, characterized in that the bleaching step increases the lightness of the pulp by at least 50%. 110. Method according to claim 109, characterized in that the GE lightness of the mass is increased to at least 83%. 111. Method according to claim 1, characterized in that the mass is washed between the chemical boiling and the oxygen delignification steps, in the said mass after the oxygen delignification step has a K No. of 10 or less and a viscosity greater than 13 eps; and the method further comprises: washing the oxygen-delignified mass; performs the ozone delignification step to obtain a substantially delignified pulp having a K No. of 5 or less, a viscosity greater than 10 and a GE brightness of at least 50%; washes the substantially delignified pulp; combining the substantially delignified pulp with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature corresponding to the amount of alkaline material to dissolve a substantial portion of any lignin remaining in the pulp; extracting a portion of the aqueous alkaline solution to remove substantially all dissolved lignin therefrom and form a substantially lignin-free pulp; washing the substantially lignin-free pulp; and bleaching the substantially lignin-free pulp with either chlorine dioxide or a peroxide to increase the GE brightness to at least 70% and washing the bleached pulp. 112. Method according to claim 111, characterized in that the GE brightness is increased to at least 80%. 113. Method according to claim 111, characterized in that the GE brightness is increased to at least 90%. 114. Method according to claim 111, characterized in that the washing step with bleached pulp comprises washing said pulp with fresh water, and separating the pulp from the resulting washing water drain. 115. Method according to claim 111, characterized in that the bleaching step uses chlorine dioxide and the bleached mass washing water effluent is removed. 116. Method according to claim 111, characterized in that the bleaching step uses chlorine dioxide and the bleached pulp washing water effluent is treated by reverse osmosis to form a treated filtrate and at least part of said treated filtrate is then led to the substantially lignin-free pulp washing step. 117. Method according to claim 111, characterized in that the bleaching step uses a peroxide and at least part of the bleaching mass is recycled to the step in which the substantially lignin-free mass is washed. 118. Method according to claim 116 or 117, characterized in that the substantially lignin-free pulp washing step comprises washing said pulp with bleached pulp washing water, separating pulp from the resulting washing water and directing at least part of said washing water to the substantially delignified pulp washing step . 119. Method according to claim 118, characterized in that the substantially delignified pulp washing step comprises washing said pulp with the substantially lignin-free pulp washing water, separating the pulp from the resulting washing water and directing at least part of said washing water to the substantially delignified pulp washing step. 120. Method according to claim 119, characterized in that the substantially delignified pulp washing step comprises washing said pulp with substantially delignified pulp washing water, separating the pulp from the resulting washing water and directing at least part of said washing water to the pulp washing step. 121. Method according to claim 120, characterized in that the pulp washing step comprises washing said pulp with partially delignified pulp washing water, separating the pulp from the resulting washing water and collecting and concentrating said washing water before combustion in an insulating boiler. 122. Method according to claim 111 or 121, characterized in that the bleaching step uses chlorine dioxide with a minimal chlorine content. 123. Method according to claim 111 or 121, characterized in that the water requirement for the washing steps is significantly reduced compared to conventional CEDED or OC/DED processes. 124. Method according to claim 115, characterized in that the waste effluent has a color that is not greater than 0.907 kg per ton, a BOD5 value that is not greater than 0.907 kg per tonnes and a quantity of total organic chlorides not greater than 2.125. Method according to claim 1, characterized in that the oxygen delignified mass has a K No. of about 14 or less and a viscosity greater than 10 eps after the oxygen delignification; and the ozone delignified pulp has a K No. of 6 or less, a viscosity greater than 7 eps and an initial GE brightness of at least 35% after ozone delignification.