WO2021209586A1

WO2021209586A1 - Delignification process

Info

Publication number: WO2021209586A1
Application number: PCT/EP2021/059874
Authority: WO
Inventors: Lasse Hernesniemi
Original assignee: Bim Kemi AB
Current assignee: Bim Kemi AB
Priority date: 2020-04-16
Filing date: 2021-04-16
Publication date: 2021-10-21
Anticipated expiration: 2022-10-16

Abstract

A process for delignification of wood fibers is provided, comprising a) providing a lignin-containing cellulosic pulp selected from chemical pulp and semi-chemical pulp, said pulp having a weight content of magnesium of at least 0.001 % by weight; b) reacting said pulp with an oxidizing delignification agent to form a first reacted pulp having a pH of from 3 to less than 9.5; c) adding an alkaline agent to said first reacted pulp to adjust the pH to at least 9.5; and d) reacting said first reacted pulp with the alkaline agent added in step c) and an oxidizing delignification agent to form a delignified pulp, wherein the delignified pulp has a pH of at least 9.5. The above process can provide substantial benefits in terms of selectivity, and/or allow an increase of the pH during the second reaction to provide a higher degree of delignification, without loss of selectivity.

Description

DELIGNIFICATION PROCESS

Field of the invention

The invention relates to processing of wood pulp e.g. for papermaking, and in particular to delignification of wood fibers.

Background

Wood fibers for use in papermaking are manufactured either by a chemical pulping process or a mechanical process or a combination of the two. In the processing of wood for preparing pulp suitable for paper and board production, the intermediate material is often contacted with an oxidzing substance. One example of such a process is oxygen delignification, and another example is bleaching of pulp. Lignin is a heterogeneous class of biopolymers abundantly present in the cell walls of plants, especially in wood, providing structure and rigidity. However, for papermaking, lignin is often undesirable, and is usually removed in chemical or semichemical pulping processes. Oxygen delignification is the process of digestion of the lignin present in the wood fibers by the action of oxygen. The process is typically carried out after cooking and washing of the pulp, in two reaction steps performed at elevated temperature and pH. However, the reaction conditions also favor undesirable degradation of carbohydrates in the pulp. Oxygen delignification is a free radical process involving a complex interplay between superoxide, 0₂ ^‘ , and the hydroxyl radical, H0^‘. The process is carried out at pH>10 to ascertain that phenolic groups are ionized. This facilitates one-electron transfer reactions between phenolate and oxygen by formation of phenoxyl radicals and superoxide. This is the starting point for further reactions eventually leading to formation of water-soluble lignin fragments, e.g. muconic acid derivatives. However, superoxide is also a precursor of hydrogen peroxide (H2O2) formation, and this may lead to hydroxyl radical formation by superoxide driven Fenton-type reactions due to transition metal ions present in the pulp. Hence, while superoxide is responsible for delignification by formation of water-soluble lignin fragments, it also generates unselective hydroxyl radicals in the pulp matrix, causing carbohydrate degradation and loss of fibre strength properties as measured by reduced viscosity. 'Viscosity” refers to average chain length of carbohydrates of pulp, and reduced viscosity in this context means a reduced, unwanted reduction in carbohydrate yield.

It has previously been shown that a small amount of added MgSC may reduce damage to cellulose and hemicellulose. Two mechnisms have been proposed to account for this protective effect: 1 ) Reaction of MgSC with alkali to produce insoluble Mg(OH)2, which upon precipitation may capture transition metal ions; and 2) formation of magnesium-carbohydrate complexes and reduction of cellulose chain cleavage. Later, it was found that although MgSC may provide a certain degree of protection, the gain in viscosity may be counteracted by an adverse effect on delignification (kappa number). Addition of Mg(OH)2 did not provide any net positive effect. Addition of MgSC as well as Mg(OH)2 negatively affected the degree of delignification in laboratory tests. (Bouchard et al. , Holzforschung 2011, 65, 295-301)

Thus, there remains a need in the art for improvements with regard to delignification of pulp.

Summary of the invention

It is an object of the present invention to at least partly overcome the problems in the prior art, and to provide a process offering improvements in the context of oxygen delignification of pulp.

According to a first aspect of the present invention, this and other objects are achieved by a process for delignification of wood fibers, comprising a) providing a lignin-containing cellulosic pulp selected from chemical pulp and semi-chemical pulp, said lignin-containing cellulosic pulp having a weight content of magnesium of at least 0.001 %, calculated as weight percent of Mg²⁺ based on dry weight of the lignin-containing cellulosic pulp; b) reacting said lignin-containing cellulosic pulp with an oxidizing delignification agent to form a first reacted pulp having a pH in the range of from 3 to less than 9.5, such as from 3 to 8.5; c) adding an alkaline agent to said first reacted pulp; and d) reacting said first reacted pulp with the alkaline agent added in step c) and an oxidizing delignification agent, to form a delignified pulp, wherein the delignified pulp has a pH of at least 9.5.

The present inventor has found that the above oxygen delignification process involving a first reaction performed in the presence of some magnesium at a pH of less than 10, in particular less than 9.5, can increase selectivity of a subsequent delignification step carried out at a more alkaline pH. The pH referred to is the end pH of the reaction. Herein, “end pH” may be defined as the pH of the pulp coming out of the reaction, which in the case of steb b is the first reacted pulp. The process can provide substantial benefits in terms of selectivity in delignification, and/or allow an increase of the pH during the second reaction to provide a higher degree of delignification, without loss of selectivity. A higher pH of the delignified pulp coming out of the delignification process also facilitates further processing such as bleaching with hydrogen peroxide.

During step b, organic moieties present in the pulp will react with oxygen to form organic acids, which decreases the pH over the course of the reaction. At the unconventionally low pH thus obtained at the end of the first reaction with oxygen, magnesium ions are available to form magnesium-carbohydrate complexes that may protect cellulose fibers from degradation. Compared to the first step of a conventional two-step oxygen delignification process, step b of the present process uses less alkali and has a lower end pH. In the inventive process, the pH of the reactions of steps b and d are controlled independently.

Not wishing to be bound by any particular theory, the effect of the first delignification reaction (step b) resulting in an end pH<9.5 may be explained, at least partly, by one or more of the following mechanisms:

1. Magnesium ion impregnation of the pulp at intermediary pH may cause substitution of transition metal ions trapped in the pulp. This would eliminate the fundamental reason for hydroxyl radical formation in the pulp.

2. Under the same conditions, magnesium ions are free to form carbohydrate complexes that may protect the carbohydrates from hydroxyl radical attacks during oxidizing treatment.

3. At pH<10 phenol moieties present in the pulp are not ionized and are thus almost unreactive towards molecular oxygen. Hence, during the first reaction, oxygen reacts mainly with other components or lignin moieties.

Furthermore, it is hypothesized that magnesium could improve lignin solubility in the presence of calcium ions, which are believed to be able to form bridges between lignin molecules and thus prevent their solubility.

Preferably, the second reaction with an oxidizing delignification agent (step d) may be performed at a pH of 10 or higher, such as 10.5. Thereby, unreacted magnesium present in the pulp precipitates to form magnesium hydroxide, Mg(OH)2, and may thus capture transition metal ions.

Unless otherwise specified, all references herein to weight content or percent are based on dry weight of the pulp. Dry weight contents and proportions indicated herein are based on 100 % dry mass, and not on air dry pulp weight. The degree of delignification of chemical and semi-chemical pulps can be assessed by determining the residual lignin content referred to as the Kappa number according to ISO 302:2004. In the context of delignification, the term selectivity is a measure of the degree of delignification in relation to the loss of cellulose fibers (measured as change in vicosity) and is defined according to Equation 1 : kappa

Selectivity - —

Aviscosity

Equation 1

In embodiments, the lignin-containing cellulosic pulp reacted in step b may have a content of magnesium in the range of from 0.005 to 0.5 %, preferably from 0.05 to 0.1 %, calculated as Mg²⁺, based on the dry weight of the lignin- containing cellulosic pulp. The magnesium content may represent the natural content of magnesium, or may include added magnesium. In embodiments, step a may comprise adding a magnesium compound to said lignin-containing cellulosic pulp. The magnesium compound added may be selected from MgSC and Mg(OH)_¾ and may preferably be Mg(OH)2. Adding Mg(OH)2 rather than MgSC may be helpful in controlling the sodium/sulfur content balance of the pulp mill.

In embodiments, prior to reaction with the oxidizing delignification agent (that is, prior to step b), the lignin-containing cellulosic pulp may have a pH in the range of from 6 to 10.5, such as from 6.5 to 10.5, such as from 7 to 10.5. Step b may comprise adding an alkaline agent to provide a pH in said range. As an example, the alkaline agent may be NaOH and may be added at a content of from 0.5 to 1 % by weight of the lignin-containing cellulosic pulp.

After the first reaction with oxygen, the pH of the first reacted pulp obtained by step b is increased by addition of an alkaline agent. The pH of the first reacted pulp may be increased from a value of less than 9.5 to for instance at least 9. In embodiments, step c may comprise increasing the pH to more than 9.5, such as at least 10, such as at least 10.5, such as at least 11, such as at least 11.5. The purpose of alkali addition in step c may be to ensure an end pH of step d of at least 9.5, such as at least 10, such as at least 10.5, such as at least 11 , such as at least 11.5.

Thus, in embodiments, step d may be carried out at a pH of at least 10, such as at least 10.5, such as at least 11 , such as at least 11.5. Correspondingly, the pH of the delignified pulp obtained in step d) is at least 10, such as at least 10.5, such as at least 11 , such as at least 11.5.

In contrast to conventional two-step oxygen delignification processes, according to the present process the pH is thus increased for the second reaction with oxygen relative to the first reaction. Also, step d of the present process may be carried out at a higher pH relative to the second step of a conventional two-step oxygen delignification process. During this step, phenolic moieties of the lignin are deprotonated and thus susceptible to react with oxygen. Advantageously, a higher pH during the reaction may improve the delignification as such (kappa number). Furthermore, a higher pH of the pulp at the end of the second reaction may enhance further processing of the pulp, such as subsequent bleaching.

In embodiments, the alkaline agent of step c may be NaOH or oxidized white liqor. For instance, NaOH may be added at a content of at least 1 % by weight of the first reacted pulp.

Optionally, a chelating agent may be added to the pulp prior to step d, but after step b.

In embodiments, the temperature at which step b is performed and the temperature at which step d is performed are controlled independently. The oxidizing delignification agent used in step b and step d may be an oxygen-containing delignification agent. The oxidizing delignification agent used in step b and d may be independently selected from oxygen gas (O2), hydrogen peroxide, and a mixture thereof. In embodiments, the delignification agent of both steps b and d is oxygen gas (O2).

Typically, step b may be carried out for time period in the range of from 1 to 60 minutes, such as from 10 to 30 minutes. Further, step d may be carried out for longer time period than step b. For example, step d may be carried out for a time period of from 10 minutes to 240 minutes.

In embodiments, step b may provide a reduction of kappa number representing from 1 % to 45 % of the total reduction of kappa number achieved by steps a-d. In embodiments, the delignified pulp obtained by step d may have a kappa value in the range of from 6 to 18, such as from 6 to 12 or from 8 to 18, such as from 8 to 12. It is conceived that the first and second reaction with oxygen together may reduce the kappa number of the pulp by up to 85 %, such as up to 75 %, up to 60 %. In embodiments the first and second reaction steps with oxygen may together reduce the kappa number by at least 20 %, such as at least 30 %, at least 40 %, at least 50 %, at least 55 %, or at least 60 %.

In another aspect, there is provided a reacted, lignin-containing cellulosic pulp having a pH in the range of from 3 to less than 9.5 after being subjected to a first reaction with an oxidizing delignification agent as described herein.

It is noted that the invention relates to all possible combinations of features recited in the claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Brief description of the drawings

The present invention will hereafter be described with reference to the appended drawings, in which: Fig. 1 is a flow chart illustrating a process according to the present invention.

Fig. 2 is a schematic illustration of a reactor system that can be used for carrying out the process of the invention. Detailed description

Preferred embodiments of the invention will now be described in more detail. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

The delignification process of the invention may be carried out after cooking and washing of the cellulosic pulp. The delignification process is typically followed by one or more washing steps, and optionally by one or more bleaching steps. In washing steps immediately following the delignification process, the process water may be at least partially circulated. Process water, optionally in the form of condensate obtained after evaporation of washing filtrate, from one or more washing steps following the delignification process may also be recirculated back to an earlier washing step preceding the delignification process. Optionally, the delignification process may be followed by one or more washing steps and subsequently by one or more bleaching steps.

The process of the invention is represented as a flow chart depicted in Fig. 1. In a first step S1 , lignin-containing cellulosic pulp is provided, typically as an aqueous slurry. The lignin-containing cellulosic pulp may be a chemical or semi-chemical pulp. Typically, the lignin-containing cellulosic pulp has a weight content of magnesium of at least 0.001 %, calculated as weight percent Mg²⁺ based on the dry weight of the pulp.

In a second step S2, the lignin-containing cellulosic pulp is reacted under alkaline or moderately alkaline conditions with an oxidizing delignification agent, to form a first reacted pulp. Typically, the first reacted pulp obtained in this step has a pH of between 3 and 9.5. An alkaline agent may be added to ensure a desirable pH at the end of reaction step S2. For example, an alkaline agent may be added to the lignin-containing cellulosic pulp prior to addition of the delignification agent.

In a third step S3, an alkaline agent is added to the first reacted pulp, to increase the pH. Typically, the pH may be increased to a value of more than 9.5.

In a fourth step S4 the first reacted pulp is once more reacted, under alkaline conditions as provided by addition of the alkali in step S3, with an oxidizing delignification agent. The oxidizing delignification agent may be added separately to the pulp, and the pulp is then maintained under conditions allowing delignification, such as to form a delignified pulp. Typically, the delignified pulp has a pH of at least 9.5. The fourth step S4 may be referred to as a delignification step.

Following step S4, the delignified pulp may be further processed in accordance with conventional pulp treatment processes, such as by bleaching.

The pulp to be subjected to the delignification process decribed herein is a lignin-containing cellulosic pulp. The pulp may originate from softwood or hardwood. The pulp may be selected from chemical pulp and semi-chemical pulp. The chemical pulp may be a kraft pulp. The pulp is typically has not been subjected to any prior delignification step using oxygen (O2) or hydrogen peroxide. The pulp may have a natural content of metal ions, including transition metals as well as alkaline earth metals such as magnesium and calcium. The content of metal ions naturally present in the pulp, prior to any process or step designated to remove such ions, such as a addition of a chelating agent, or acid wash, may be referred to as the in situ metal ion content.

In embodiments, the pulp provided in step S1 has not been subjected to a prior chelating treatment, e.g. by addition of a chelating agent. In other embodiments, the pulp provided in S1 may have been subjected to a chelating step, and/or may be optionally subjected to a chelating step prior to step S2.

In some embodiments, the pulp provded in step S1 may initially have a magnesium content (calculated as Mg²⁺) of at least 0.001 % by weight without addition of magnesium from an external source.

In case the pulp as such contains less than 0.001 % by weight of magnesium, addition of a magnesium-containing material may be needed or desirable. However, also pulp having an in situ weight content of magnesium of 0.001 % or more, such as 0.005 % or more, may benefit from addition of more magnesium. Hence, in embodiments, step S1 may comprise a first step S1a of providing a lignin-containing cellulosic pulp optionally having an in situ weight content of magnesium of less than 0.001 %, and a step of adding a magnesium-containing material to the pulp to provide a magnesium content of at least 0.001 %, such as at least 0.005 %, such as at least 0.05 % by weight. The magnesium content may be up to 1 %, such as up to 0.5 % such as up to 0.1 %. To increase the magnesium content, a magnesium compound such as

Mg(OH)2or MgSC may be added to the pulp. In embodiments, a sulfur-free magnesium compound, such as Mg(OH)2, may be preferred over a sulfur- containing magnesium compound such as MgSC . When adding a sulfur- containing magnesium compound to the pulp of a closed loop of a pulp mill, sulfur may have to be removed from the process at a later stage, downstream of the oxygen delignification process. A sulfur-free magnesium compound may allow for the process water of the process to be circulated repeatedly in the process.

For example, Mg(OH)2 may be added at a content of from 0.002 % to 0.24 % by weight of the lignin-containing cellulosic pulp to achieve a desirable level of magnesium in the pulp.

Optionally prior to, but at least during step S2, at least some of the magnesium contained in the pulp is present as free magnesium ions, Mg²⁺.

Mg(OH)2 has low solubility in water (12 mg/I at 25 °C at equilibrium pH) and is typically provided as an aqueous dispersion. When added to an aqueous system, such as an aqueous pulp slurry having a pH of less than about 10, magnesium hydroxide hydrolyses to form magnesium ions, Mg²⁺, and hydroxide ions, OH-, of which the latter may contribute to the activation of an oxidising treatment agent (e.g., oxygen or hydrogen peroxide), if present.

The pulp provided in step S1 may have a pH in a wide range, e.g. in the range of from 6 to 10.5. In embodiments, the pulp may have a pH of about 10 or above 10, even above 10.5, prior to the reaction of step S2. Addition of an alkaline compound, such as Mg(OH)2, sodium hydroxide (NaOH) or oxidized white liquor may be used to provide a pH within this range.

During the reaction with the delignification agent the pH of the pulp will typically decrease, as a result of the formation of one or more organic acid(s) by reaction of oxygen with organic compounds originating from the pulp. Examples of organic acids that may be formed include, for instance, gluconic acid and citric acid. The pH of the pulp towards the end of the reaction of step S2 (the end pH, that is, the pH of the first reacted pulp) may be in the range of from 3 to 9.5, or from 3 to less than 9.5, such as from 7 to 9, such as from 7 to less than 9, or from 3 to 8.5, such as from 7 to 8.5. Optionally, addition of alkali as described above may be used to adjust the end pH of step S2 to within the desired range. The end pH of the reaction may be monitored and controlled by appropriate addition of alkali to the pulp stream upstream of the reactor in which step S2 is carried out, as appreciated by persons of skill in the art. Appropriate dosing of alkali may be achieved by varying alkali dosing and monitoring of changes in end pH.

Furthermore, during step S2, the magnesium present in the pulp may form magnesium-carbohydrate complexes than can protect the cellulose fibers from degradation by reactive radical species in the subsequent delignification reaction in step S4, which is carried out at a higher pH.

In embodiments, an organic acid present in the pulp may enhance the cellulose protection by chelating metals other than magnesium. In embodiments, an organic acid, in particular citric acid, present in the pulp may enhance the cellulose protective effect by improving the formation of magnesium ions from Mg(OH)2 and/or chelating metals other than magnesium.

The reaction of step S2 may serve to degrade fast reacting lignin compounds present in the pulp. By “fast reacting lignin compounds” is meant lignin compounds which do not contain phenolic moieties, and/or lignin compounds which do not require of pH of 10 or higher.

The reaction of step S2 may be carried out at a temperature in the range of 100°C or less, referring to the temperature at the end of the reaction. Hence, the first reacted pulp obtained in this step may have a temperature of 100°C or less, such as in the range of from 80°C to 100°C, such as from 80°C to 90°C.

In embodiments, the reaction of step S2 may be carried out for a time period (referred to as retention time) of from 1 minute to 60 minutes, such as from 20 to 40 minutes, or from 10 to 30 minutes, such as about 20 minutes, or about 30 minutes. Typically, the retention time of the step S2 is shorter than the subsequent delignification reaction of step S4.

The kappa number of the pulp may be in the range of from 12 to 45 and may differ depending on the type of wood used. For softwood pulp, the kappa number prior to delignification may be in the range of from 18 to 45. For hardwood pulp, the kappa number prior to delignification may be in the range of from 12 to 15.

The delignified pulp obtained in step S4 may have a kappa number in the range of from 6 to 18, such as from 6 to 12 (in particular for hardwood pulp) or from 8 to 18 (in particular for softwood pulp).

The reaction of step S2 provides a first reacted pulp, which may be an at least partially delignified pulp. In embodiments, step S2 may result in a Akappa number corresponding to from 1 % to 45 % of the total reduction in kappa number achieved by the inventive process as a whole.

Following the reaction of step S2, an alkaline agent may be added as a step S3 to the first reacted (and partially delignified) pulp in order to increase the pH of the pulp. The alkaline agent may be any agent conventionally employed for raising the pH prior to a delignification process, such as sodium hydroxide, NaOFI. The pH of the pulp may be adjusted to be in the range of from 9.5 to 13, such as from 10 to 12, such as at least 10.5, or in the range of from 10.5 to 11.5. A high pH may favor the delignification reaction. Furthermore, at a pH above 10, magnesium present in the pulp may precipitate in the form of Mg(OH)_2.

Optionally, a chelating agent may be added to the pulp after step S2, typically in step S3 or between step S3 and S4. The second delignification reaction, step S4 of the present process, is thus carried out at a pH of at least 9.5, that is, starting with a first reacted pulp having a pH of at least 9.5. Typically, a delignification agent such as described above is added to and allowed to mix with the pulp. The delignification agent may be the same as in step S2 or different. In embodiments, the delignification agent may be oxygen (O2) for both steps S2, S4. During the delignification reaction of step S4, which may be carrier out for a longer time period than the reaction of step S2, slowly reacting lignin components, such as components containing phenolic groups may be digested. For example, the reaction of step S4 may be carried out for at least 30 minutes, and up to 240 minutes, such as up to 180 minutes, for example from 60 to 90 minutes, such as about 60 minutes. The reaction may be carried out at a temperature slightly higher than the step S2, such as at 120°C or less, referring to the temperature at the end of the reaction step. Hence, for example, the delignified pulp obtained in this step may have a temperature in the range of from 85°C to 120°C, such as from 90°C to 100°C. Typically, higher temperatures may provide a higher degree of delignification, as represented by a greater decrease in kappa number, which however requires a higher amount of alkali to be added. Also, higher reaction temperatures may be used to achieve a desired reduction of kappa number using a shorter reaction time (retention time).

After the delignification reaction of step S4, a delignified pulp is obtained. The delignified pulp may have a kappa number in the range of from 6 to 18, such as from 6 to 12 (in particular for hardwood pulp) or from 8 to 18 (in particular for softwood pulp).

Overall, the selectivity of the delignification process described herein, including both steps S2 and S4, may constitute an improvement of from 5 to 50 % compared to the selectivity of a conventional two-step oxygen delignification process utilizing a high pH first step. The pH of the delignified pulp may be at least 10.5, and advantageously more than 10.5, such as at least 11 , for example at least 11.5 and up to 12. In an example, the pH may be about 11.5, which is about 1 unit higher than a conventional oxygen delignification process. The end pH of the reaction of step S4 may be monitored and controlled by appropriate addition of alkali to the pulp stream upstream of the reactor in which step S4 is carried out, but downstream of the reactor in which step S2 is carried out.

An exemplary process according to embodiments of the invention is illustrated with reference to Fig. 2, which schematically shows a delignification process equipment 200 comprising an inlet feed 201 , a first reactor 202, an intermediate feed 203, a second reactor 204 and an outlet feed 205. The process equipment 200 is typically connected at the upstream end to a pulp processing equipment from where the pulp is fed into the delignification process. Correspondingly, at the downstream end, the outlet feed 205 is connected to further pulp processing equipment, such as bleaching equipment for further treatment of the delignified pulp.

In an exemplary process, lignin-containing cellulosic pulp is fed from a digerster and pulp washing equipment (not shown) via the pulp inlet 201. If desired a magnesium-containing compound is added to the pulp via an inlet 211 a, to increase the magnesium content of the pulp. The magnesium- containing compound may be Mg(OH)2. However, the addition of a magnesium-containing compound may optionally be omitted if the pulp has an adequate in situ content of magnesium, such as at least 0.001% by weight.

If desired, an alkaline agent such as NaOH or oxidized white liqor can be added via inlet 211 b to increase the pH, with the purpose of controlling the end pH of the first reactor 202.

Next, oxygen gas is added to the pulp via inlet 211c, and the pulp is fed into the first reactor 202. Going in to the first reactor the pulp may have a temperature of up to 100°C in order to reach a desired reduction in Kappa number adfter the first reactor 202. Hot steam may be added by conventional means (not shown) to provide a desired high temperature.

The pulp may be retained in the first reactor 202 for a time period of from 1 to 60 minutes, for example 30 minutes, to form a first reacted pulp.

Leaving the first react reactor 202, the first reacted pulp is fed via an intermediate feed 203 towards a second reactor 204. Leaving the first reactor 202, the first reacted pulp may have a temperature of 80-90°C and a pH in the range of 7-9. Next, the pulp is then fed via the intermediate feed 203 towards the second reactor 204. An alkaline agent such as NaOH or oxidized white liqor is added via inlet 212a to the pulp to increase the pH, to control the end pH of the second reactor 204 at above 9.5. Oxygen is added via an inlet 212b. Hot steam may be added by conventional means, optionally via inlet 212b , to provide a desired high temperature. Going into the second reactor 204 the pulp may have a temperature of 80-120°C. The pulp may be retained in the second reactor 204 for a time period of 10-200 minutes, for example about 90 minutes, undergoing delignification to yield a delignified pulp. The delignified pulp may have a pH in the range of 9.5-11.5, for example about 10.5 or about 11. The pulp is subsequently fed through the outlet feed 205 to further pulp processing equipment (not shown).

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A process for delignification of wood fibers, comprising a) providing (S1) a lignin-containing cellulosic pulp selected from chemical pulp and semi-chemical pulp, said lignin- containing cellulosic pulp having a weight content of magnesium of at least 0.001 %, calculated as weight percent of Mg²⁺ based on dry weight of the lignin-containing cellulosic pulp; b) reacting (S2) said lignin-containing cellulosic pulp with an oxidizing delignification agent to form a first reacted pulp having a pH in the range of from 3 to less than 9.5; c) adding (S3) an alkaline agent to said first reacted pulp; d) reacting (S4) said first reacted pulp with the alkaline agent added in step c) and an oxidizing delignification agent, to form a delignified pulp, wherein the delignified pulp has a pH of at least 9.5.

2. The process of claim 1 , wherein step a) comprises adding a magnesium compound to said lignin-containing cellulosic pulp.

3. The process of claim 2, wherein the magnesium compound is selected from magnesium MgSC and Mg(OH)_¾ and preferably is Mg(OH)2.

4. The process of any one of the preceding claims, wherein the lignin- containing cellulosic pulp reacted in step b) has a content of magnesium in the range of from 0.005 to 0.5 %, preferably from 0.05 to 0.1 %, calculated as Mg²⁺, based on the dry weight of the lignin-containing cellulosic pulp.

5. The process of claim any one of the preceding claims, wherein the lignin-containing cellulosic pulp prior to reaction with the oxidizing delignification agent has a pH in the range of from 6 to 10.5, such as from 6.5 to 10.5, such as from 7 to 10.5, prior to step b).

6. The process of any one of the preceding claims, wherein step b comprises adding an alkaline agent to provide a pH in the range of from 6 to 10.5, such as from 7 to 10.5.

7. The process of any one of the preceding claims, wherein the first reacted pulp obtained in step b has a pH in the range of from 3 to less than 8.5.

8. The process of any one of the preceding claims, wherein in step c) the pH is increased from a value of less than 9.5 to a value of 9.5 or more, such as to a value of at least 10, such as at least 10.5, such as at least 11 , such as at least 11.5.

9. The process of any one of the preceding claims, wherein the alkaline agent of step c) is NaOH and is added at a content of at least 1 % by weight of the first reacted pulp.

10. The process of any one of the preceding claims, wherein step d) is carried out at a pH of at least 10, such as at least 10.5, such as at least 11 , such as at least 11.5, and/or the pH of the delignified pulp obtained in step d) is at least 10, such as at least 10.5, such as at least 11 , such as at least 11.5.

11. The process of any one of the preceding claims, wherein the temperature at which step b is performed, and the temperature at which step d is performed are controlled independently.

12. The process of any one of the preceding claims, wherein the oxidizing delignification agent of step b) and d) is independently selected from oxygen gas (O2), hydrogen peroxide, and a mixture thereof.

13. The process of claim 12, wherein the oxidizing delignification agent step of b) and d) is oxygen gas (O2).

14. The process of any one of the preceding claims, wherein step b) is carried out for time period in the range of from 1 to 60 minutes.

15. The process of any one of the preceding claims, wherein step d) is carried out for longer time period than step b).