WO2011059376A1

WO2011059376A1 - Method of producing pulp from lignocellulosic material containing silica

Info

Publication number: WO2011059376A1
Application number: PCT/SE2010/051157
Authority: WO
Inventors: Christina Jansson; Per-Åke SÖDERSTEN; Rolf Boman; Rune Larsson
Original assignee: Metso Paper, Inc.
Priority date: 2009-11-11
Filing date: 2010-10-26
Publication date: 2011-05-19
Also published as: CN102597366A; BR112012010746A2; SE534885C2; SE0901436A1; CN102597366B

Abstract

The present disclosure relates to a process of producing a pulp from lignocellulosic material, especially non-wood raw material, wherein the dissolution of silica is minimized. The process comprises the steps of cooking lignocellulosic material to a pulp and subjecting said pulp to an oxygen delignification stage in which an alkali addition is made such that the pulp has a final pH of equal to or less than 10.0 after said stage.

Description

Method of producing pulp from lignocellulosic material containing silica .

The present disclosure relates in general to a process of producing a pulp, suitable for end bleaching, from lignocellulosic material having a high silica content, such as non-wood raw material. The process comprises cooking lignocellulosic material followed by oxygen delignification of the pulp.

BACKGROUND

In the wood based pulping industry significant efforts have been made over the course of several decades to reduce environmental impact and notable improvements have been achieved, especially regarding effluent emissions.

However, due to some ecological damage still being caused, there is a growing interest in the use of non-wood raw materials as alternative or supplementary fiber sources for pulp making.

While the wood based chemical pulp industry has been able to

significantly decrease emissions the situation is quite different when looking at the non-wood based pulp industry. For non-wood pulp, chlorine and hypochlorite are still commonly used in pulp bleaching and recovery of spent cooking liquor does not take place or is incomplete.

The most frequently used non-wood raw materials are straw, reed and bagasse. Also bamboo is often used as raw material. These raw materials are easily delignified in alkaline cooking processes. The soda cooking process predominates but Kraft cooking is also used to some degree for non-wood pulping.

Non-wood raw materials generally comprise high silica contents. Table 1 specifies typical silica contents for some non-wood raw materials.

In literature, silica is generally used as a short convenient designation for silicon dioxide in all its crystalline, amorphous and hydrated forms. In itself, solid silica does not dissolve but is hydrolyzed to form monomeric silicic acid. Silica content, S1O₂, in raw material and pulp is usually analyzed by a gravimetric method, while filtrates are analyzed by atomic absorbance spectroscopy for elementary silicon, Si.

The combination of an alkaline cooking process and a raw material having high silica content results in a high concentration of silicon in the spent cooking liquor. The consequences are extreme difficulties in evaporation, combustion and recaustisizing, which make recovery incomplete if it is attempted at all.

The increasing awareness of environmental hazards related to effluent emission from non-wood pulp mills combined with the lack of adequate technology for recovery of spent cooking liquors has meant the non-wood pulp industry recognizes it has not yet reached its full potential.

Table 1

Various potential options have been considered to abate the silica problem. Some of them are removal of silica prior to pulping, precipitation and removal of silica prior to evaporation, possible new techniques for recovery of spent cooking liquors and new cooking technologies.

WO 2006/103317 discloses one example of such a new cooking technology. The document discloses a process for the production of pulp from comminuted lignocellulosic material, such as any type of wood, straw or bamboo. The comminuted lignocellulosic material is impregnated with reactant chemicals and thereafter heated to a suitable reaction temperature for delignification reactions using the heat released by condensation of a gaseous organic agent. The process results inter alia in very rapid reactions, high yield and lowered energy demands.

When utilizing the cooking method disclosed in WO 2006/103317 on non- wood raw materials, it is possible to obtain a pulp comprising a low amount of dissolved silica. However, when subjecting the pulp to a subsequent oxygen delignification by conventional methods it has been observed that high amounts of silica is dissolved in the oxygen delignification stage. For example, 33% dissolved silica has been observed in the spent liquor at pH 10.9 for a pulp based on straw and cooked by the method. Such a high content of silica would cause great problems in a recovery process.

Figure 1 schematically illustrates a process for producing a pulp in accordance with prior art. Lignocellulosic material is cooked 1 to a chemical pulp which is washed in a first washer 2. Fresh water or condensate is fed to the washer, which is illustrated by arrow 3. The filtrate is transferred to a recovery process, which is illustrated by arrow 4. The washed pulp is thereafter subjected to an oxygen delignification stage 5 in which sodium hydroxide (NaOH) is used as alkali, which typically results in a final pH of about 10.5-1 1 . The oxygen delignified pulp is washed in a second washer 6 and thereafter subjected to a bleaching process starting with for example a D₀ stage 9. Naturally, the end bleaching process may also start with another kind of bleaching stage. Fresh water is fed to the second washer, which is illustrated by arrow 7 and the filtrate from the washer is transferred to a sewer, which is illustrated by arrow 8. The remaining bleaching stages of the final bleaching sequence are not shown in the figure.

As can be seen from the figure, the process requires fresh water and the filtrate from the washers after oxygen delignification is not reused. This is a consequence of the high silica content in the filtrate.

Thus, in order for the non-wood pulping industry to reach its full potential the problems associated with silica have to be overcome.

SUMMARY

The primary object of the present invention is to develop a process for producing a pulp, suitable for end bleaching, which process at least reduces the problems associated with silica in case of a raw material having a high content of silica, such as a non-wood raw material. It is also an object of the present invention to reduce the environmental impact of such a process.

These objects are achieved by means of the process for producing a pulp from lignocellulosic material comprising at least 0.5 % of S1O2 in accordance with independent claim 1 . Embodiments of the process are defined by the dependent claims.

The process for producing a pulp from lignocellulosic material comprising at least 0.5 % S1O2 comprises cooking the lignocellulosic material to form a chemical pulp, wherein said cooking is performed such that the pulping liquor has a pH of equal to or less than 1 1 after said cooking, washing the chemical pulp and thereafter subjecting the pulp to a first oxygen delignification stage in which an alkali addition is made such that the pulp has a final pH of equal to or less than 10.0 after said stage.

The present invention is based on the principle of keeping the pH of the pulp as low as possible during the oxygen delignification of the pulp and thereby minimizing the amount of silica dissolved. This is achieved by cooking under such conditions that the resulting pulping liquor will have a pH of equal to or less than 1 1 after said cooking, and conducting the first oxygen delignification stage under such conditions that the resulting pulp has a pH of equal to or less than 10.0. The pH is controlled by addition of a suitable alkali. The first oxygen delignification stage is suitably performed such that a delignification of at least 35 %, preferably at least 40 %, is achieved in this stage.

The alkali used during the first oxygen delignification stage is preferably carbonate. The carbonate may be any suitable alkaline carbonate, but is preferably chosen among Li, Na and K carbonates or mixtures thereof. More preferably, the carbonate is sodium carbonate. Carbonate is a relatively weak alkali and it is thus possible to achieve the desired low pH. Moreover, the first oxygen delignification stage may preferably be conducted at a pressure of 0.6-1 .2 MPa, more preferably 0.8-1 .1 MPa. A high pressure increases the degree of delignification obtainable in said stage.

When relatively low production amount of pulp is conducted, such as up to about 300 t/d, one single oxygen delignification stage may be sufficient in order to obtain the desired degree of delignification. However, in case of high production amount of pulp, it is preferred that two oxygen delignification stages are

performed. Therefore, according to an embodiment of the invention, the pulp is after the first oxygen delignification stage subjected to a second oxygen delignification stage.

The pulp may optionally be washed between the first and second oxygen delignification stages. The filtrate obtained in such a washing stage can be recovered, and thus the environmental impact of the process may be further reduced. However, including a washing stage increases the investment cost of the plant and the washing stage may therefore be left out if desired. However, it is believed that the degree of delignification is improved in case a washing stage is included between the first and second oxygen delignification stages.

Moreover, the degree of delignification obtainable during the first oxygen delignification stage may in some cases be insufficient for the pulp to be suitable for cost-efficient end bleaching sequences, for example if the first oxygen delignification stage is conducted at normal pressures, such as about 0.5 MPa, and/or conventional periods of time, such as about 80-100 minutes. Therefore, a second oxygen delignification stage may also be included in such cases. When the process comprises two oxygen delignification stages, the second oxygen delignification stage is adapted to achieve the desired degree of delignification to make the pulp suitable for end bleaching by conventional methods. For example, the second oxygen delignification stage may preferably be performed such that the kappa number of the pulp will be in the range of about 8-14.

In accordance with one embodiment of the invention, the second oxygen delignification stage may be performed such that the resulting pulp has a pH of equal to or less than 10.0 after said stage. Alternatively, the second oxygen delignification stage may be performed such that the resulting pulp has a final pH of above 10.0. The pH is controlled by the alkali used in the stage. Preferably, a new alkali addition is made to the second oxygen delignification stage. It is however also possible to make the entire alkali addition to the first oxygen delignification stage and performing the second oxygen delignification stage in the presence of the alkali added to the first oxygen delignification stage. In this case, no washing stage is present between the delignification stages. Carbonate, hydroxide or both carbonate and hydroxide may be used in the second oxygen delignification stage depending on the desired pH in the resulting pulp.

The fact that the process according to one embodiment of the present invention comprises two separate oxygen delignification stages facilitates the use of a low pH in the first oxygen delignification stage. The relatively low pH minimizes the dissolution of silica during the first stage which in turn enables recovery of the filtrate as mentioned above. The filtrate may for example be used as wash liquid in a preceding washing stage.

Moreover, the total discharge of COD to an external recipient may be reduced, compared to a conventional process only utilizing one oxygen delignification stage wherein sodium hydroxide is used as alkali addition, due to the possibility of recovering filtrates.

The use of a low pH during oxygen delignification of the pulp may require a longer period of time for the oxygen delignification than conventional processes in order to obtain the desired degree of delignification. Thus, in accordance with a preferred embodiment of the process according to the invention, the first oxygen delignification stage is performed during at least 150 minutes when the process comprises only one single oxygen delignification stage. When the process comprises two oxygen delignification stages, the total time of the first and second oxygen delignification stages should preferably be at least 150 minutes.

As previously mentioned the cooking is performed such that the pulping liquor has a pH of equal to or less than 1 1 after the cooking. Thereby, the amount of silica dissolved during this part of the process is minimized.

According to a preferred embodiment of the present invention, cooking is performed by impregnating comminuted lignocellulosic material with reactant chemicals and heating the impregnated lignocellulosic material to a temperature sufficient for delignification reaction using heat released from condensation of a gaseous organic agent.

The pulp obtained by the process according to the present invention can easily be bleached in accordance with previously known bleaching methods in order to obtain a final desired brightness. For example, the pulp may be subjected to bleaching sequences comprising chlorine dioxide stages, ozone stages, extraction stages, and/or peroxide stages etc.

Even though the present process has been developed mainly for the treatment of non-wood materials which often have high silica contents, such as the annual plants straw, bagasse and bamboo, it may also be used for other types of lignocellulosic materials comprising high amounts of silica, i.e. above 0.5 % S1O2.

BREIF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates schematically a process for producing a pulp in

accordance with prior art.

Figure 2 illustrates schematically a process for producing a pulp in

accordance with one embodiment of the present invention. Figure 3 shows the solubility of silica at different pH and different

temperatures.

Figure 4 shows the kappa number, as a function of the total amount of sodium carbonate added, obtained after oxygen delignification in a laboratory test on a straw-based pulp according to three different embodiments of the invention.

DETAILED DESCRIPTION

The process will be further described with reference to the accompanying drawings. It should however be noted that the invention is not limited to the embodiments described below and shown in the drawings, but may be modified within the scope of the appended claims.

The process for producing a pulp in accordance with the present invention comprises the steps of cooking a lignocellulosic material comprising at least 0.5 % S1O2 under a relatively low pH to a chemical pulp followed by washing, and subjecting the cooked pulp to a first oxygen delignification stage wherein alkali is added such that the pulp has pH of 10.0 or less after said stage, preferably the pulp has a pH of less than 10.0 after said stage. The relatively low pH used during the oxygen delignification stage minimizes the amount of silica dissolved during the oxygen delignification.

In accordance with a preferred embodiment, carbonate is used as alkali in the first oxygen delignification stage. The carbonate may be any suitable alkaline carbonate, but is preferably chosen among Li, Na and K carbonates or mixtures thereof. More preferably, the carbonate is sodium carbonate. Alternatively, hydroxide, or a mixture of carbonate and hydroxide, may be used as alkali in said first oxygen delignification stage if used in such a small amount that the pH of the pulp after said stage is equal to or less than 10.0.

Performing oxygen delignification at a low pH in accordance with the present invention reduces the degree of delignification obtainable in such a stage compared to conventional processes if the processes are performed under the same conditions, such as time, temperature and pressure. However, if the time and/or the pressure are increased it is possible to achieve a sufficiently low kappa number of the pulp such that it is suitable for end bleaching processes, i.e. a kappa number of about 8-14. For example, in a conventional process the oxygen delignification stage is typically performed during about 80-100 minutes and such that the pulp has a pH of about 10.5-1 1 . However, when the oxygen delignification is performed in accordance with the present invention, i.e. such that the pulp has a final pH of 10.0 or less after the first oxygen delignification stage, oxygen delignification should preferably be performed for at least 150 minutes, suitably 150-200 minutes, in order to achieve sufficient delignification of the pulp.

According to one embodiment of the process according to the invention, the pulp is subjected to a second oxygen delignification stage after the first oxygen delignification stage. The pulp may optionally be washed between the first and second oxygen delignification stages. The filtrate obtained in such a washing stage can be recovered, and thus the environmental impact of the process may be further reduced. It is believed that the degree of delignification could be improved if a washing stage is included between the first and second oxygen delignification stages. However, including a washing stage increases the investment cost of the plant and the washing stage may therefore be left out if desired.

When the process is used for relatively low production amounts of a pulp, such as up to about 300 t/d, it might be sufficient with only one single oxygen delignification stage. Using a single oxygen delignification stage keeps the investment cost of the plant low since only one tower is needed. However, in case of relatively high production amounts of pulp, the tower in which the oxygen delignification is to be conducted would have to be very high and thus a process comprising two oxygen delignification stages would be preferable. The difference between a process utilizing only one oxygen delignification stage and a process utilizing two oxygen delignification stages, in addition to the number of towers, is that a new alkali addition may be made in the second oxygen delignification stage. A process comprising two oxygen delignification stages would have substantially the same chemical cost, if no washing stage is conducted between the two oxygen delignification stages, as a process having only one single oxygen delignification stage. When a washing stage is included between the two oxygen delignification stages, the chemical cost for alkali would be substantially the same, but the chemical cost for oxygen would be higher compared to a process with only one single oxygen delignification stage.

Moreover, the degree of delignification may under certain conditions be insufficient for the pulp to be subjected to cost-efficient end bleaching processes if only one single oxygen delignification stage is conducted. This may for example be the case if the first oxygen delignification stage is performed at normal pressures, such as about 0.5 MPa, or for conventional periods of time, such as 80- 100 minutes. The incorporation of a second oxygen delignification stage is advantageous also in such cases. The second oxygen delignification stage is then adapted to achieve the desired kappa number of the pulp to make the pulp suitable for end bleaching processes, i.e. a kappa number of 8-14.

When the process according to the present invention comprises two oxygen delignification stages, the entire amount of alkali may not necessarily be added to the first oxygen delignification stage. Instead, a second alkali addition may be made to the second oxygen delignification stage. However, when the process according to the present invention comprises two oxygen delignification stages without any intermediate washing stage and the oxygen delignification stages are performed under a high pressure, it may in some cases be difficult to make an addition of alkali to the second oxygen delignification stage. In such a case, the entire amount of alkali necessary for the process may naturally be added to the first oxygen delignification stage.

When the process comprises two oxygen delignification stages, the stages should preferably be conducted for a period of time in total of at least 150 minutes, suitably 150-200 minutes, since the relatively low pH used at least in the first oxygen delignification stage may require a longer period of time in order to obtain the desired degree of delignification.

Figure 2 illustrates one embodiment of the process for producing a pulp according to the invention. Lignocellulosic material is cooked 10 to form a pulp which is washed in a first washer 1 1 . The filtrate from the first washer 1 1 is transferred to a recovery process, which is illustrated by the arrow 17.

After the pulp has been washed, it is subjected to a first oxygen

delignification stage 12 wherein an alkali addition is made such that the pH is equal to or less than 10.0. The pulp is thereafter washed in a second washer 13. The filtrate from the second washer 13 is reused in the process as wash liquid in the first washer and thus is transferred from the second washer 13 to the first washer 1 1 , as illustrated by the arrow 18. The pulp is thereafter subjected to a second oxygen delignification stage 14 and washed in a third washer 15. Fresh water is used in the third washer 15 and is therefore fed to said washer as illustrated by arrow 19.

In case sodium hydroxide has been used as alkali in the second oxygen delignification stage 14, the pH of the pulp will be so high that a significant amount of silica may have been dissolved. In such a case, the alkaline filtrate from the third washer is suitably transferred to a sewer, as illustrated by arrow 20. However, in case a weaker alkali addition has been made, the dissolution of silica is minimized and the filtrate can therefore suitably be transferred to the second washer 13, as illustrated by arrow 21 , to be reused as wash liquid in said washer 13. If the filtrate from the third washer 15 can not be reused in the second washer 13, the second washer is provided with fresh water or a condensate from the end bleaching process, as illustrated by arrow 22.

When the pulp has been washed in the third washer 15, the pulp can be subjected to end bleaching in accordance with previous known methods in order to achieve the final desired brightness. Only the first stage of such an end bleaching process is illustrated in Figure 2, wherein the end bleaching sequence starts with for example a DO stage 16. The DO stage is only one example of an initial stage of an end bleaching sequence and it will be readily apparent to the skilled person that the end bleaching sequence may instead start with another kind of bleaching stage.

Cooking the lignocellulosic material can for example be made by a conventional soda cooking method, preferably followed by a process stage for removal of silica from the spent liquor. However, cooking can also advantageously be performed by means of any previously known alkaline cooking method resulting in a pulping liquor having a pH of equal to or less than 1 1 .

According to one preferred embodiment of the process according to the present invention, the raw material is cooked using a cooking method wherein comminuted lignocellulosic material is impregnated with reactant chemicals and thereafter heated to a suitable reaction temperature using heat released by condensation of a gaseous agent.

The impregnation can for example be performed by submersing the material in a solution containing the chemicals, followed by removal of excess liquid. Examples of solutions for impregnating the lignocellulosic material are aqueous solutions of hydroxide, sulfide, sulfite, bisulfite, carbonate, sulphur dioxide, anthraquinone, amines or acids. Preferably, the solution is an aqueous solution comprising carbonate, such as sodium carbonate. The impregnation can also be performed by contacting the material with gaseous delignifying agents, for example sulphur dioxide gas.

The heat required for the delignification reactions is provided by heating with a gaseous organic agent, condensating and releasing energy to the impregnated lignocellulosic material. The gaseous agent is not necessarily in a completely gaseous state but may comprise various amounts of vapor or droplets. Examples of suitable gaseous agents are lower alkyl alcohols, ketones and aldehydes or mixtures thereof. The gaseous agent may also contain water in addition to the organic agent. According to a preferred embodiment, the gaseous organic agent is selected from the group consisting of methanol, ethanol, propanol, butanol, acetone and any mixture thereof, and possibly comprising water. The organic agent content in the cooking liquor contributes further to the reduced dissolution of silica.

According to one embodiment, the impregnated lignocellulosic material is heated up to maximally 200°C. Preferably, the lignocellulosic material is heated to 120-200°C.

The cooking method described above is as such previously known and described in detail in WO 2006/103317, which is hereby incorporated by reference.

It has been found that by using the cooking method described above on non-wood material comprising high contents of silica, the amount of dissolved silica is substantially lower compared to if a conventional soda cooking method is utilized. This effect is particularly pronounced where sodium carbonate is used as a reactant chemical when impregnating the raw material. For a non-wood raw material, such as straw, the cooking method described above will typically result in a pulp having a kappa number in the order of about 25-30.

After the lignocellulosic material has been cooked, the obtained chemical pulp is washed and oxygen delignified to a kappa number which is suitable for subsequent end bleaching of the pulp to a final desired brightness, i.e. a kappa number of about 8-14. As mentioned above, the present invention is based on the principle of keeping the pH of the pulp as low as possible during the oxygen delignification of the pulp. Figure 3 (Her, R.K, The chemistry of silica, 1979, p. 48) shows the interrelationship between solubility of amorphous silica and pH at different temperatures. It is clear from the figure that the dissolution of silica is strongly dependent on the pH, and increases with increasing pH. As shown in the figure, the dissolution increases drastically at a pH of about 10. Even though Figure 3 relates to the dissolution of silica in general and not specifically to the conditions in a pulp, the relationship between the dissolution of silica and pH is also valid for the conditions of a pulp. Thus, it is of great importance to control the pH of the pulp in order to control the dissolution of silica.

It is evident from Figure 3 that the dissolution of silica also depends on the temperature. However, in order to achieve the required delignification of the pulp in an oxygen delignification stage the temperature must be kept at a required minimum temperature in order to achieve the desired delignification reactions. Moreover, below a certain temperature, oxygen delignification is not cost-efficient since it takes too long. Therefore, the temperature of the oxygen delignification stage(s) should not be lowered below the temperature generally considered necessary within the technical field of oxygen delignification of a pulp.

In order to minimize the dissolution of silica during the oxygen

delignification of the pulp while still achieving a sufficient degree of delignification, the oxygen delignification could suitably be performed in two separate oxygen delignification stages, optionally with an intermediate washing stage, as discussed above.

In the first oxygen delignification stage, an addition of a relatively weak alkali is made such that the pulp has a pH of 10.0 or less after said first oxygen delignification stage for a delignification typically in the order of about 40-50 %. The oxygen delignification may preferably be performed at a high pressure, such as 0.6-1 .2 MPa, preferably 0.8-1 .1 MPa. Thanks to the high pressure, it is possible to obtain a desired degree of delignification despite the weak alkali used.

Carbonate is preferably used as the alkali addition during the first oxygen delignification stage. The carbonate may be any suitable alkaline carbonate, but is preferably chosen among Li, Na, and K carbonates or mixtures thereof. Most preferably, sodium carbonate (Na2CO3) is used. Carbonate is a weaker base than sodium hydroxide and the pulp will therefore have a lower pH compared to if sodium hydroxide is used. Thereby, the dissolution of silica in the pulp during this stage is minimized and the filtrate produced can be recycled to a preceding washing stage, e.g. the washing stage prior to the first oxygen delignification stage.

The use of carbonate in the first oxygen delignification stage may typically enable a delignification to a kappa value of about 17-21 when conducted for conventional periods of time, such as 80-100 minutes. In order to achieve a kappa value suitable for final bleaching of the pulp, such as a kappa value of 8-14, a second oxygen delignification stage may thus in some cases be conducted, optionally after an intermediate washing stage. Sodium hydroxide may be used as alkali during the second oxygen delignification stage to achieve a sufficient delignification. The use of sodium hydroxide will give a final pH of the pulp which is higher than if sodium carbonate is used, typically a pH in the range of 10.5-1 1 .

Silica will be dissolved during the second oxygen delignification stage, when sodium hydroxide is used as alkali, as a result of the high pH of the pulp. The filtrate produced from this stage is therefore unsuitable for recovery and is consequently suitably transferred to external purification.

It is also possible to utilize a mixture of sodium hydroxide and carbonate, or only carbonate, as alkali during the second oxygen delignification stage. The carbonate may be any suitable alkaline carbonate, but preferably sodium carbonate is used. The use of a mixture of sodium hydroxide and carbonate, or only carbonate, will cause a somewhat lower pH of the filtrate, typically a pH of equal to or less than 10.0, compared to if only sodium hydroxide is used and thus a lower dissolution of silica. Moreover, this will in some cases result in a degree of delignification which is less than if only sodium hydroxide is used if the second oxygen delignification stage is performed under conventional process parameters, such as a time of 80-100 minutes and a pressure of about 0.5 MPa. However, if the time and/or pressure are increased, a kappa number suitable for end bleaching may easily be achieved. Moreover, the filtrate can be recovered when a lower pH is used.

The oxygen delignification stage or stages are performed at a temperature which is sufficient for the delignification reactions to occur at an acceptable rate. However, the temperature is preferably kept at a level which minimizes the dissolution of silica. Therefore, the first and/or second oxygen delignification stages may suitably be performed at a temperature of 80-130 °C, preferably 90- 105 °C.

According to an alternative embodiment, the first oxygen delignification stage is performed at a slightly lower temperature than the second oxygen delignification in order to minimize the dissolution of silica in the first oxygen delignification stage. This is especially suitable in case sodium hydroxide is used as alkali in the second oxygen delignification stage since silica will dissolve during the second stage and the temperature therefore can be higher in order to reduce the time required for said stage. For example, the first oxygen delignification stage can be performed at a temperature of about 85-100 °C and the second oxygen delignification stage can be performed at a temperature of about 100-1 15 °C.

After oxygen delignification, the pulp may be subjected to end bleaching processes in accordance with conventional techniques, such as bleaching sequences comprising stages using chlorine dioxide, ozone, peroxide or the like, in order to achieve a final desired brightness.

The process according to the present invention overcomes the obstacles presented by the presence of silica since it ensures that the majority of the silica present in the raw material is dissolved, if at all, only at a late stage of the process. When the process comprises two oxygen delignification stages, the process enables recycling of the filtrate from at least the washing stage after the first oxygen delignification stage. Moreover, the load on the recovery process is significantly reduced compared to previously known processes for producing a pulp from non-wood raw material.

For example, it has been observed that using the cooking method comprising impregnating the raw material and heating using a gaseous agent, followed by a single oxygen delignification stage wherein sodium hydroxide is used as alkali results in about 33 % dissolved silica at pH 10.9. Such a high content of silica will impair the recovery process significantly. However, by using the same cooking method followed by two separate oxygen delignification stages in accordance with one embodiment of the present invention, the filtrate from the first oxygen delignification stage can be reused in the preceding washing stage. Moreover, the generation of COD in the filtrate of the second oxygen delignification stage is significantly reduced compared to the process comprising only one oxygen delignification stage using sodium hydroxide as alkali.

The process of the present invention is mainly developed for

lignocellulosic raw materials comprising high contents of silica, such as non-wood raw materials (also known as annual plants). Examples of such raw materials are straw, bagasse, reed and bamboo. However, the process can also be used for other types of lignocellulosic raw material comprising high amounts of silica.

From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as departures from the spirit and scope of the invention, and all such modification as would be obvious to one skilled in the art are intended for inclusion within the scope of the appended claims.

EXPERIMENTAL DATA 1

Laboratory tests were performed on a pulp which had been cooked to a kappa value of 32.6. The pulp comprised 16.4 g/kg of silica.

The pulp was subjected to four different oxygen delignification stages, Cases 1 to 4, with different additions of alkali followed by subsequent washing. In all of the cases, oxygen delignification was performed during 90 minutes and at a pressure of 1 .0 MPa. The alkali used and the amount thereof as well as the obtained final pH of the pulp is specified in Table 2. The kappa number achieved by the different oxygen delignification stages, i.e. Cases 1 to 4, as well as the amount of silica in the pulp and in the filtrate resulting from the washing, are also specified in Table 2.

The results of Case 1 , which constitutes a reference process, showed that the pulp was delignified to a kappa value of 12.5 with the addition of 30 kg

NaOH/odt, which resulted in a pH of 10.6. This caused 5.3 g/kg silica in the filtrate.

However, in Case 2 where 30 kg Na2CO3 was used, the pH of the pulp was 8.8, and the silica in the filtrate was substantially lower. The kappa value was reduced to 17.9 in this case.

Thus, it is clear that a substantial delignification is achieved even in the case of using sodium carbonate as alkali during the oxygen delignification.

However, the kappa number achieved is still too high for making the pulp suitable to be subjected to a cost-efficient and environmental friendly end bleaching process. Therefore, the pulp may suitably also be subjected to a second oxygen delignification stage or the first oxygen delignification stage could be performed during a longer period of time.

Cases 3 and 4, wherein both sodium hydroxide and sodium carbonate were used, showed similar results as when only sodium carbonate is used, i.e. Case 2. Thus, it is clear that it is also possible to perform the oxygen delignification in the presence of both sodium hydroxide and sodium carbonate while achieving a significantly lower content of silica in the filtrate compared to if only sodium hydroxide is used.

To summarize, the test results show that it is possible to more than halve the amount of silica in the filtrate by reducing the pH of the pulp from 10.6 to 8.8.

Table 2

Moreover, the amount of COD in the filtrate was investigated. In case the oxygen delignification was conducted in accordance with Case 1 specified in Table 2, the filtrate comprised about 91 kg/odt COD for a reduction of the kappa number of 20.

However, in case the oxygen delignification was conducted in accordance with Case 2, the filtrate comprised about 68 kg/odt COD for a reduction of the kappa number of about 15. In accordance with the present invention, said filtrate can suitably be recovered and thus the overall COD to recipient of the process according to the invention will be significantly lower than in the case of the prior art.

Based on the results above it is possible to estimate the COD per deltakappa to about 4.5. As previously discussed, the process may according to one embodiment comprise a second oxygen delignification stage after the first oxygen delignification stage. In case of conducting the second oxygen

delignification stage to achieve a deltakappa of 6, the COD in the filtrate can therefore be estimated to about 27 kg/odt using the estimated COD/deltakappa of 4.5. The desired reduction of kappa number in the second oxygen delignification stage can easily be controlled by the addition of a proper amount of alkali during this stage.

Thus, in case the second oxygen delignification stage is performed using only sodium hydroxide, the pH of the pulp after said stage will be above 10. In such a case, the COD in the filtrate which is transferred to recipient can be estimated to about 27 kg/odt. The filtrate would not be suitable for recovery since it will most likely have high silica content.

However, if the second oxygen delignification stage is performed at a pH of 10 or less, the silica content of the filtrate will be sufficiently low to enable recovery of the filtrate. Thus, the filtrate comprising the estimated 27 kg/odt of COD is suitably recovered and the overall COD of the process is minimized.

From the above it is clear that it is possible to achieve an efficient and environmental friendly process by the process according to the present invention. EXPERIMENTAL DATA 2

Laboratory tests were performed on a straw-based pulp which had been cooked to a kappa value of 32.2. The pulp comprised 36.5 g/kg of silica. The pulp was oxygen delignified in accordance with three different processes according to the invention, Cases 5-7.

Case 5 comprised oxygen delignification in one single stage for 180 minutes. Case 6 comprised two oxygen delignification stages of 90 minutes each without any intermediate washing stage. Case 7 comprised two oxygen

delignification stages of 90 minutes each with an intermediate washing stage. In each of the cases, the oxygen delignification was performed at a pressure of 1 .0 MPa and at a temperature of 105 °C, and sodium carbonate was used as alkali.

Figure 4 shows the result of the delignification in the form of the kappa number versus the total amount of sodium carbonate used. In Case 6 and 7, i.e. where two oxygen delignification stages were performed, the amount of sodium carbonate added to the first oxygen delignification stage was 30 kg/odt and the amount of sodium carbonate added in the second oxygen delignification stage was varied.

As shown in Figure 4, all three processes Cases 5-7 resulted in

substantially the same degree of delignification for a given total amount of sodium carbonate. The results show that a kappa number of approximately 13, which in this case corresponds to a delignification of about 60 %, is obtainable for a total charge of sodium carbonate of about 60 kg/odt. Previous test on the same pulp had shown that a single oxygen delignification for 90 minutes at 1 .0 MPa using sodium hydroxide in an amount of 30 kg/odt resulted in a delignification of about 62.5 %. This previous test is hereafter denoted Case 8.

Case 7, which comprised an intermediate washing stage, showed however that the intrinsic viscosity of the pulp was lowered by approximately 50 ml/g after the oxygen delignification compared to Cases 5 and 6. The intrinsic viscosity of the pulp before oxygen delignification was 840 ml/g. After oxygen delignification, the intrinsic viscosity for Cases 5 and 6 was about 985 ml/g at a kappa number of about 13.

Even though no significant difference in delignification was observed in the laboratory test between the two processes comprising two oxygen delignification stages, i.e. Cases 6 and 7, it is believed that a washing stage between the two oxygen delignification stages would give a higher degree of delignification when the process is performed in a plant since the carry-over is significantly higher in a plant than in a laboratory test.

Furthermore, the silica content in the oxygen filtrate for Case 5 was compared to Case 8. The results are summarized in Table 3. It is clear from the results that the amount of silica is considerably less in Case 5 compared to Case 8.

Table 3

Case 5 Case 8

Alkali Na₂CO₃ NaOH

Time, oxygen delignification [min] 180 90

Final pH 9.0 10.2

Dry content (DS) in oxygen filtrate [%]^* 1 .6 1 .5 Silica in oxygen filtrate [kg/odt]^* 4.7 8.8

Silica of DS [%] 4.0 7.5

^*in filtrate at 12% pulp consistency, before laboratory washing

Claims

A process for producing a pulp suitable for end bleaching from

lignocellulosic material comprising at least 0,5 % S1O2, the process comprising the following steps:

a. cooking the lignocellulosic material to form a chemical pulp,

wherein said cooking is performed such that the pulping liquor has a pH of equal to or less than 1 1 after said cooking,

b. washing the pulp,

c. subjecting the pulp to a first oxygen delignification stage in which an alkali addition is made such that the pulp has a final pH of equal to or less than 10.0 after said stage.

Process according to claim 1 wherein carbonate, preferably sodium carbonate, is used as alkali in said first oxygen delignification stage.

Process according to claim 2 wherein said first oxygen delignification stage is performed at a pressure of 0.6-1 .2 MPa, preferably 0.8-1 .1 MPa.

Process according to any of claims 1 to 3 wherein the process comprises one single oxygen delignification stage and wherein said oxygen delignification stage is conducted for at least 150 minutes.

Process according to any of the claims 1 to 3 wherein the process further comprises the following steps:

d. optionally washing the pulp after said first oxygen delignification stage, and

e. subjecting the pulp to a second oxygen delignification stage in the presence of alkali.

Process according to claim 5 wherein a second alkali addition is made to the second oxygen delignification stage. Process according to claim 6 wherein an alkali selected from the group consisting of carbonate, preferably sodium carbonate, hydroxide, preferably sodium hydroxide, or a mixture thereof is used in the second oxygen delignification stage.

Process according to claim 7 wherein carbonate, preferably sodium carbonate, or both hydroxide and carbonate, is used as alkali during the second oxygen delignification stage such that the final pH of the pulp after the second oxygen delignification stage is equal to or less than 10.0.

Process according to any of claims 5 to 8 wherein said second oxygen delignification stage is performed at a pressure of 0.6-1 .2 MPa, preferably 0.8-1 .1 MPa.

Process according to any of claims 5 to 9 wherein the total time for oxygen delignification in said first oxygen delignification stage and said second oxygen delignification stage is at least 150 minutes.

Process according to any of the preceding claims wherein the cooking step comprises the steps of:

a1 . impregnating a comminuted lignocellulosic material with reactant chemicals; and

a2. heating the impregnated lignocellulosic material to a temperature sufficient for delignification reaction using heat released from condensation of a gaseous organic agent.

12. Process according to claim 1 1 wherein the organic agent is selected from the group consisting of aliphatic alcohols, ketones and aldehydes. 13. Process according to any of the claims 5 to 12 wherein a filtrate obtained during the washing stage d is recycled and used as wash liquid in the washing stage b. Process according to claim 8 wherein the pulp is subjected to a washing stage after said second oxygen delignification stage and a filtrate obtained during said washing stage is recovered.