EP0309998B1

EP0309998B1 - A method in the activation of lignocellulosic material with a gas containing nitrogen dioxide

Info

Publication number: EP0309998B1
Application number: EP88115896A
Authority: EP
Inventors: Hans Olof Prof. Samuelson
Original assignee: Mo och Domsjo AB
Current assignee: Mo och Domsjo AB
Priority date: 1987-09-28
Filing date: 1988-09-27
Publication date: 1994-02-23
Anticipated expiration: 2008-09-27
Also published as: NO884279D0; ES2049234T3; ATE101886T1; DE3887948D1; NZ226104A; EP0309998A2; FI884432A; NO884279L; FI93231C; EP0309998A3; CA1301412C; ZA887229B; NO172698B; SE460543B; JP2510424B2; SE8703718D0; DE3887948T2; FI884432A0; NO172698C; AU2283288A

Abstract

Method of activating aqueous lignocellulosic material with a gas containing nitrogen oxide, followed by a delignification stage for the purpose of obtaining a high quality end product with no or only a slight effect on the environment. In accordance with the inventive method, the lignocellulosic material is brought into contact (18) with an oxygen-containing gas (19), and activating gas is separated from the lignocellulosic material during and/or subsequent to the activating process (9). The method is characterized by a) controlling the supply of oxygen-containing gas (12) such that the separated gas (27) contains at least 2 kg nitric oxide (NO) calculated on 1000 kg of dry lignocellulosic material, and by b) reacting the separated gas (27) in one or more stages (26) with absorption solution (25) whose original pH lies within the range 3-13.5, and by passing (28) the gas purified with the aid of absorption solution to atmosphere, or to a destruction plant optionally subsequent ot further purification of the gas.

Description

Technical field

The present invention relates to a method in which lignocellulosic material is subjected to a delignifying treatment. By lignocellulosic material is meant primarily different lignocellulosic pulps, preferably such pulps as those in which, e.g., wood has been converted to cellulose pulp either completely or partially with the aid of chemicals. The invention is particularly suited for application with chemical cellulose pulps produced in accordance with both alkaline methods and the sulphite method. Among alkaline digestion methods can be mentioned the sulphate method, the polysulphide method and the soda (= sodium hydroxide) method, with or without chemical additives of, e.g, the quinone compound type. Although the invention is preferably applied with unbleached cellulose pulp, the invention may also be applied successfully to cellulose pulp which has previously been bleached and/or treated in some other way, e.g. alkali treated.

Background prior art

It has been found that by treating aqueous lignocellulosic material with a gas containing nitrogen dioxide (NO₂) in one or more so-called activation stages prior to one or more delignifying stages it is possible to carry out delignification of the cellulosic material in a highly selective manner to a much greater extent than was previously considered possible without the use of chlorine or chlorine compounds as a delignifying agent.
The activation process is influenced by a number of factors. These factors include pulp consistency, the amount of nitrogen dioxide charged, time and temperature. Different temperature profiles, i.e. different temperatures at different stages of the activation process also influence the end result. In addition, the nitrate content and the hydrogen-ion content during activation also have a decisive significance on the activation process. The need to supply expensive nitrogen dioxide can be drastically reduced by supplying nitrate ions or hydrogen ions to the activation stage. The selectivity in the delignification of the cellulose pulp can also be optimized, by optimizing, inter alia, the aforementioned parameters. This can be utilized to carry out an extremely extensive delignification.
The activation process can also be influenced by introducing an oxygen-containing gas, for instance gaseous oxygen. From an environmental aspect it is always advantageous to add gaseous oxygen, since it has been found that when activation is completed the gas will always contain a low proportion of both nitrogen dioxide (NO₂) and nitric oxide (NO). (see e.g. US-A-4 439 271) Furthermore, the case is that the addition of gaseous oxygen during activation of the cellulose pulp will under certain conditions contribute to improved selectivity in the delignification of cellulose pulp.

Summary of the invention

Technical problem

It has been found, however, that under other conditions the addition of excessive quantities of oxygen can result in impaired selectivity in the delignification of the cellulose pulp. A low oxygen addition results in turn in environmental problems, since the gas in such cases contains a high proportion of nitric oxide (NO) in particular upon completion of the activation process.
The present invention solves these problems and relates to a method in the manufacture of cellulose pulp, in which aqueous lignocellulosic material is activated with a gas containing nitrogen dioxide (NO₂) in at least one stage, subsequent to and/or while adding an oxygen-containing gas to the material, followed by delignification of the lignocellulosic material in at least one step, and in which gas is separated from the lignocellulosic material during and/or subsequent to the activation process. The method is characterized by controlling the supply of oxygen-containing gas such that the separated gas will contain at least 2 kg nitric oxide (NO) calculated on 1000 kg absolutely dry lignocellulosic material, and by reacting the separated gas in one or more steps with an absorption solution whose original pH-value lies within the range 3-13.5, and recovering the absorption solution subsequent to its reaction with the separated gas and introducing said solution into the lignocellulosic material prior to and/or during the activating process; and by passing the gas purified with the aid of said absorption solution to atmosphere, or to a destruction plant, optionally after further purification of the gas.
The mixture of lignocellulosic material (hereinafter called cellulose pulp) with water shall be such that during the activation process the pulp consistency will lie within the range of 2-80%, suitably 3-40%, preferably 5-30%.
Nitrogen dioxide (NO₂) is introduced to the activating stage either as substantially pure nitrogen dioxide, or is allowed to form immediately before or in the activating reactor, by supplying nitric oxide (NO) and oxygen. Both nitrogen dioxide and nitric oxide can be introduced into one and the same cellulose pulp. The term nitrogen dioxide is also meant to include dinitrogen tetroxide (N₂O₄) and other polymeric forms of nitrogen oxides. One mole of dinitrogen tetroxide is calculated as two moles of nitrogen dioxide. Addition products which include nitric oxide are calculated in the same way as nitric oxide. Thus, dinitrogen trioxide (N₂O₃) is calculated as one mole of nitric oxide and one mole of nitrogen dioxide. Addition products which include oxygen are probably present as intermediates. Similarly, nitrous acid (HNO₂) is calculated as active nitric oxide. Similar to dinitrogen trioxide, nitrous acid is volatile and difficult to separate analytically from nitrogen dioxide and nitric oxide. Dinitrogen oxide (N₂O) on the other hand is not calculated as an active nitrogen oxide.
The amount of nitrogen oxides charged to the system is adapted, inter alia, according to the lignin content of the cellulose pulp, tolerable attack on the carbohydrates of the pulp, and the desired degreee of delignification. Calculated as monomers, the amount of nitrogen oxides charged is normally from 0.1-2 kilomoles for each 100 kg of lignin in the cellulose pulp.
The temperature during the activating process can be chosen relatively freely, e.g. within the range of 20-110°C. If the activating process is carried out in a single stage, the optimum temperature will lie within the range of 50-95°C. When the activating process is divided into two stages, the preferred temperature will lie within the range of 25-40°C in the first stage, whereas the temperature in the second stage will lie within the range of 80-100°C.
The time is partly contingent on the temperature. If the pH is very low and the temperature high, it is necessary to choose a short activating period. In other cases, the activating result is normally improved when the activating process is carried out over a long time period.
The amount of oxygen charged prior to and/or during the activation of the cellulose pulp shall be kept low. According to one preferred embodiment of the invention no oxygen-containing gas is intentionally charged during the activation of the cellulose pulp. Unless special preventative measures are taken, a certain amount of air will always accompany the cellulose pulp into the activating reactor, and the oxygen contained in the air is often sufficient. In some cases it may even be necessary to reduce the amount of air accompanying the cellulose pulp. Air can be removed by compressing the cellulose pulp prior to introducing the pulp into the activating stage, or by heating and/or evacuating the cellulose pulp. When nitric oxide (NO) is charged as an active nitrogen oxide, oxygen is preferably charged solely in a quantity below the stoichiometric quantity required to oxidize nitric oxide (NO) to nitrogen dioxide (NO₂).
The gas separated from the activating stage - this gas having a certain lowest content of nitric oxide (NO) - is recovered for treatment. This treatment process comprises at least two phases, namely the introduction of an oxygen-containing gas and the reaction of the gas with an absorption solution. The oxygen addition is normally made first, although it is fully conceivable to carry out both phases in one and the same treatment stage.
In order to achieve an optimum result, it has been found that the amount of oxygen charged shall be from 0.10 to 0.35, preferably 0.20-0.28 mole O₂ calculated per mole nitric oxide (NO) in the separated gas.
The absorption solution may be any suitable solution having a pH within the range of 3-13.5 and the ability to remove the nitrogen oxides to a very high degree from the separated gas.
According to preferred embodiments of the invention each of two solutions is used as the absorption solution. It is particularly expedient to use one of the solutions in one absorption stage followed by a second absorption stage in which the other of said solutions is used.
One of the solutions may be weakly acid, up to neutral, and comprises waste liquor derived from the activation of the cellulose pulp. The solution may either comprise solely waste liquor of this kind or also a mixture of said waste liquor with some other liquid, e.g. a liquid which increases the pH of the resultant solution. The other solution will have a pH within the range of 7-13.5, and suitably comprises waste liquor derived from the delignification of the cellulose pulp with alkali. A particularly preferred waste liquor is one obtained from an alkaline oxygen-gas bleaching stage, and particularly a waste liquor obtained from an alkaline oxygen bleaching process in which the cellulose pulp has been activated with nitrogen dioxide (NO₂) in accordance with the invention. This results in a low consumption of active nitrogen oxides and avoids the precipitation of lignin. These absorption solutions are recovered and charged to the cellulose pulp prior to and/or during the activating process. The solutions may be used advantageously for impregnating and/or diluting the cellulose pulp immediately prior to and particularly during the activation process, i.e. the treatment with gas containing nitrogen dioxide (NO₂).
The pH-values recited in this document refer to measurements made with glass electrodes on samples which were cooled to room temperature (approx. 20°C) in the absence of vaporization. In the case of samples taken during the activation process, the cellulose pulp was separated out prior to determining the pH. When samples were taken at a pulp consistency above 8%, the consistency was brought down to 8% by diluting with pure water, whereafter the cellulose pulp was separated. The pH-values recited with regard to absorption solutions, e.g. various waste liquors, relate to cooled undiluted samples.
With regard to the purification of the separated gas, good results were obtained when the relative quantities of the separated gas and the absorption solution, and optionally the amount of oxygen (alternatively oxygen-containing gas) charged to the system were adapted so that the pH of the absorption solution used was within the rage 5-12. The recovery of active nitrogen oxide was particularly favoured when using an absorption solution within this pH-range. When recovering this absorption solution it is preferred to use the solution solely for impregnation and/or dilution of the cellulose pulp prior to commencing the activating process.

Advantages

When practising the method according to the invention it has been found possible, as distinct from prior art technique, to maintain the consumption of both oxygen and newly charged nitrogen dioxide (alternatively nitric oxide) at a low level while maintaining high selectivity during delignification of the cellulose pulp.
When using certain conditions it has even been found possible to achieve a slightly improved quality, e.g. improved strength properties, despite a low consumption of nitrogen oxides, as distinct from the prior art techniques. These advantages are obtained with no effect or only a slight effect on the environment.

Brief description of the drawings

Fig. 1 is a flow sheet describing a first embodiment of the method according to the invention, and Fig. 2 is a flow sheet describing a preferred second embodiment of the method according to the invention.

Preferred embodiment

A plurality of other parameters significant to the inventive method will be recited in the following description of the aforesaid flow sheets.
In the case of the inventive method illustrated in Fig. 1 chemical cellulose pulp, e.g. unbleached chemical cellulose pulp, is passed to a liquor removing apparatus 2, e.g. a press, through a conduit 1. The cellulose pulp may be screened or unscreened, and is normally freed from the major part of the spent digestion liquor accompanying the pulp from the digester. The spent digestion liquor is displaced normally with waste bleaching liquors, e.g. waste liquor deriving from oxygen-gas bleaching stages. It is also possible to use for this purpose a given quantity of waste liquor deriving from the activating process. The pulp consistency is normally low (one or a few %) when the pulp is introduced into the press 2. The pulp consistency is increased in the press 2, e.g. to 30% and above. Liquor pressed from the cellulose pulp is carried away through a conduit 3 and used, for instance, to wash and/or dilute unbleached pulp. The cellulose pulp is passed to a diluting plant 5, through a conduit 4. At this location of the process, the conduit 4 may be replaced with a chute, or a feed screw. The consistency of the pulp is lowered in the plant 5 to, e.g. 5%. The cellulose pulp is then passed through a conduit 6 (e.g. with the aid of a pump) to a mixer 7. A nitrogen oxide, e.g. nitrogen dioxide (NO₂) is supplied through a conduit 8. Desired temperature during the activating process is normally obtained by injecting steam into the cellulose pulp flow. The cellulose pulp is then caused to pass upstream through an activating reactor 9. The temperature at which activation is permitted to take place is dependent on a number of other parameters, although in the case of this embodiment of the invention the temperature used will normally lie within the range of 50-95°C. Temperatures of approx. 90°C are particularly applicable in the case of the illustrated single-stage method, with regard to activation. The height of the reactor 9 and the rate at which pulp passes through the reactor are determined by the residence time (the treatment time). The residence time is normally from 60-360 minutes. Residence times of 180 minutes have been found to give good results at temperatures of approx. 90°C.
A gas containing nitric oxide (NO) is separated from the cellulose pulp suspension at the top of the reactor 9. The gas is passed to an oxidation reactor 11, through a conduit 10. The gas, which contains inter alia nitric oxide (NO) is reacted in the reactor with an oxygen-containing gas, preferably oxygen gas, which is supplied through a conduit 12. The amount of oxygen charged is suitably from 0.10-0.35 mole O₂ calculated per mole nitric oxide (NO) in the gas transferred to the reactor 11.
No oxygen-containing gas is supplied intentionally to the cellulose pulp when practising this embodiment of the inventive method. On the other hand, larger or smaller quantities of air will always accompany the cellulose pulp into the activating reactor. With the aid of the apparatus arrangement illustrated in Fig. 1, it is possible to maintain the amount of air accompanying the pulp on a suitable level (and therewith also the amount of oxygen charged).
In order to achieve an optimum activating effect, it is important to monitor and control the pH of the cellulose pulp prior to and during the activating process. At position 7, i.e. at the location immediately prior to introducing nitrogen oxide in some form or another through the conduit 8, the cellulose pulp will normally have a pH within the range of 5-12. A pH within the range of 5.5-8 is preferred. Subsequent to introducing nitrogen oxide into the pulp the pH falls, and it has been found that the pH of the cellulose pulp during the final stage of the activating process and thereafter should lie within the range of 1.5-4.5. Particularly good results have been achieved with a final pH of 1.8-2.8. Suitable low pH-values can be obtained in several ways. For example, a suitably low pH can be obtained by adding nitric acid or some other acid, preferably a strong mineral acid. The addition of a small quantity of oxygen gas, e.g. 0.5-2 kg for each 1000 kg of lignin accompanying the cellulose pulp can also be used to lower the pH.
The cellulose pulp leaves the reactor 9 through the top of the reactor and is passed to a liquor-separating apparatus 14, through a conduit 13. This apparatus may have the form of a press by means of which a considerable part of the activating liquor can be removed from the cellulose pulp. Instead of pressing the waste liquor from the cellulose pulp, the liquor can be displaced from the pulp with the aid of a liquid, preferably re-cycled activating waste liquor and/or fresh water. The displacing liquid may also contain waste bleaching liquor, e.g. waste liquor derived from an oxygen-gas bleaching stage. The waste liquor is carried away through a conduit 15 and passed to the diluting apparatus 5 and/or to the cellulose pulp at a position upstream of said plant. The cellulose pulp is then passed to the impregnating plant 16, in which alkali in some form or another, e.g. sodium hydroxide, is charged to the pulp. If oxygen-gas bleaching waste liquor has not already been charged to the cellulose pulp, it is suitable to introduce such liquor into the pulp, preferably in conjunction with adding a magnesium salt. The cellulose pulp is passed through a conduit 17 to an intensive mixer 18, to which oxygen gas is supplied through a conduit 19. The oxygen gas disperses in finely divided form throughout the cellulose pulp suspension, the consistency of which suitably lies within the range of 5-10%. The suspension passes upstream through an oxygen-gas bleaching reactor 20. The oxygen-gas pressure at the bottom of the reactor 20 is, to some extent, determined by the height of the reactor. The pressure of the oxygen gas supplied can be selected freely, meaning that the oxygen-gas pressure at the top of the reactor 20 is equal to atmospheric pressure or higher than atmospheric pressure. It is also conceivable to use a pulp consistency greater than 10% during the oxygen-gas bleaching stage, implying so-called high-consistency oxygen-gas bleaching.
The cellulose pulp is then passed through a conduit 21 to a liquor separating plant 22, in which the cellulose pulp is freed from oxygen-gas bleaching waste-liquor in a known manner, e.g. by pressing and/or washing. The cellulose pulp leaves the plant through a conduit 23, for further treatment.
A certain amount of the waste oxygen-bleaching liquor is transported through a conduit 25, with the aid of a pump 24, to the top of a gas absorption plant (scrubber) 26. The waste oxygen-gas bleaching liquor normally has a pH within the range of 9-12. Gas taken from the oxidation reactor 11 is passed to the bottom of the scrubber 26 through a conduit 27. Contact between the gas and the absorption liquid frees the gas from the major part of its nitrogen-oxide content. If a very high degree of absorption is achieved in the scrubber 26, the treated gas may be discharged to atmosphere through a conduit 28. This is not to be preferred. Instead, and alternatively, the gas is transported to a soda recovery unit in which, inter alia, cooking waste liquor is combusted. In those cases when the requirements placed on the environment are very high, it may be necessary to further purify the gas in the conduit 28 prior to discharging the gas to atmosphere or introducing the gas into the soda recovery unit.
The absorption liquid, which contains several nitrogen compounds, is removed from the scrubber 26 through the bottom thereof, and is transported to the diluting plant 5, through a conduit 29. A larger or smaller amount of the absorption liquid can also be used for other purposes. It is preferred, however, to introduce the absorption liquid into the cellulose pulp at a position upstream of the activating reactor 9, so as to use the nitrogen compounds present during the activating process.
It may be advantageous to recycle both the gases and liquids through the various stages, even though this is not shown in the figure. For example, part of the gas flow in conduit 10 and/or conduit 27 can be returned to the activating reactor 9. Furthermore, it is preferred to recycle to the activating reactor 9 part of the gas carried through the conduit 28. Oxygen can be supplied to said gas flow, during passage of the flow to the reactor 9. Furthermore, part of the absorption solution at the bottom of the scrubber 26 can be cycled back to the scrubber, and preferably to one or more locations along the outer shell of the scrubber in the vertical extension thereof.
The preferred embodiment of the inventive method illustrated in Fig. 2 coincides initially with the embodiment of the inventive method described with reference to Fig. 1.
Chemical cellulose pulp is transported through a conduit 30 to a liquor removal apparatus 31. Liquor extracted from the cellulose pulp is carried away through a conduit 32. The cellulose pulp is transported through a conduit 33 to a diluting plant 34. The cellulose pulp is then transported through a conduit 35 to a mixer 36, in which the cellulose pulp is brought into contact with nitric oxide (NO) and oxygen (O₂) via a conduit 37. The molar proportion between these gases may, e.g. 2.5:1.
The introduction of these gases initiates the activating process. In this case the activating process is divided into two stages with intermediate dilution of the pulp suspension. Cellulose pulp having a consistency of e.g. 10-15% is passed upstream through a first activating reactor 38. The temperature in this stage is advantageously comparatively low and the time comparatively short. For example, a temperature of 35°C and time of 20 minutes can be used. The cellulose pulp is then transported through a conduit 39 to a diluting arrangement 40, in which the cellulose pulp is thinned, e.g. to a consistency of 4-9%. The cellulose pulp is then transported through a conduit 41 to a second activating reactor 42. This second activating stage shall be carried out at a high temperature (e.g. 90°C) and over a long period of time (e.g. 180 minutes). This second treatment stage can be referred to as the ripening stage. The cellulose pulp is then conducted through a conduit 43 to a gas separator 44, e.g. a cyclone separator. The nitrogen-oxide containing gas separated from the cellulose pulp suspension is passed through a conduit 45 to an oxidation reactor 46, to which there is connected a conduit 47 for supplying oxygen gas to the reactor.
Subsequent to this gas extraction, the cellulose pulp is transported to a liquor separating plant 49, through a conduit 48. The cellulose pulp is then impregnated in a plant 50 with alkali, e.g. in the form of sodium hydroxide, necessary for oxygen-gas bleaching, and optionally a protector, e.g. in the form of a magnesium salt. Waste oxygen-gas bleaching liquor may also be supplied to the cellulose pulp at positions 49 and 50. The cellulose pulp is then transported through a conduit 52 to an intensive mixer 51, to which oxygen gas under overpressure is supplied through a conduit 53.
The cellulose pulp is then caused to pass upstream through an oxygen-gas bleaching reactor 54. The temperature and time are not critical, and these parameters, together with oxygen-gas pressure and alkali charge can be selected in accordance with conventional techniques.
The cellulose pulp is transported from the oxygen-gas bleaching reactor 54 through a conduit 55 to a liquor separating plant 56. Subsequent to extracting waste oxygen-gas bleaching liquor from the cellulose pulp, the pulp is transported through a conduit 57 to some further treatment location, e.g. one or more final bleaching stages.
Part of the waste oxygen-gas bleaching liquor, which has a pH of 9-12, is transported, with the aid of a pump 58, through a conduit 59 to the top of a first gas absorption plant 60 (scrubber). The liquid is finely divided in a known manner, e.g. with the aid of spray nozzles, or is caused to pass through the scrubber in the form of a thin liquid film on solid packing bodies arranged in the scrubber, e.g. saddle packing bodies or so-called Raschig rings. The gas is transported from the oxidation plant 46 through a conduit 61 to the bottom of a scrubber 60. Purified gas is removed from the scrubber 60 and passed through a conduit 62 to the bottom of a second gas absorption plant 63 (scrubber). Further oxygen is supplied through a conduit 64, which is connected to the conduit 62. An advantage is gained when part of the gas flowing through the conduit 62 is removed at a location immediately upstream of the connecting conduit 64 or downstream thereof and returned to one (or both) of the activating reactors 38 and 42, through a conduit herefor. Absorption liquid is passed through a conduit 65 from the liquor separating plant 49 to the top of the scrubber 63. This liquid has a pH which lies within the range 3.5-6.5. The cellulose pulp suspension introduced into the plant 49 normally has a pH well below 3. When the displacement liquid used is totally or partially waste liquor from the oxygen-gas bleaching stage, the waste liquor will have a pH which lies within the aforesaid range. When the activating waste liquor is extracted from the cellulose pulp with the aid of a press, the extracted waste liquor can normally be mixed with an alkaline liquid (e.g. waste oxygen-gas bleaching liquor), so that the mixture will function well as an absorption liquid in the scrubber 63.
The gas cleansed in two stages can be removed from the system, through a conduit 66, and discharged, e.g. to atmosphere or to a soda recovery unit, or to some other form of combustion plant. It is also possible to purify the gas in a third purification stage of any form, prior to finally discharging the gas from the system. Absorption solution from the first scrubber 60 is passed through the conduit 67 to the diluting plant 34, while absorption solution from the second scrubber 63 is passed to the diluting arrangement 40 through the conduit 68.
There is obtained wlth the aforedescribed method a residual gas which is extremely pure, i.e. has a practically negligible content of nitrogen oxides, while at the same time obtaining two absorption solutions which can both be used effectively in the activating stage. By introducing these solutions into the cellulose pulp prior to and during activation of the pulp together with nitrogen dioxide (NO₂) containing gas, there is obtained an activated cellulose pulp which can be delignified in a highly selective manner in a subsequent stage (e.g. an oxygen-gas bleaching stage). It has also been found possible to lower the lignin content of the cellulose pulp from a kappa number of 30-35 to 3-4 while maintaining a viscosity of about 950 dm³/kg, when applying the aforedescribed method.

Claims

A method in the manufacture of cellulose pulp, in which aqueous lignocellulosic material is activated in at least one stage with a gas containing nitrogen dioxide (NO₂), subsequent to and/or while supplying an oxygen-containing gas to said pulp, followed by delignification of the lignocellulosic material in at least one stage, and in which gas is separated from the lignocellulosic material during and/or subsequent to the activating process, characterized by controlling the supply of oxygen-containing gas such that the separated gas contains at least 2 kg nitric oxide (NO) calculated on 1000 kg absolutely dry lignocellulosic material; and by reacting the separated gas in one or more stages with absorption solution whose original pH lies within the range of 3-13.5 and recovering the absorption solution subsequent to its reaction with the separated gas and introducing said solution into the lignocellulosic material prior to and/or during the activating process; and by passing the gas purified with the aid of absorption solution to atmosphere or to a destruction plant, optionally after further purification of the gas.
A method according to claim 1, characterized by introducing oxygen-containing gas to the separated gas prior to and/or during treatment of the gas with said absorption solution.
A method according to claims 1 to 2, characterized by using an absorption solution whose major component comprises waste liquor derived from the activation of lignocellulosic material.
A method according to claims 1 to 2, characterized in that the original pH of the absorption solution lies within the range of 7-13.5.
A method according to claim 4, characterized in that the major component of the absorption solution contains waste liquor derived from the delignification in alkaline medium of lignocellulosic material activated with a gas containing nitrogen dioxide (NO₂).
A method according to claim 5, characterized in that the absorption solution comprises waste liquor deriving from an alkaline oxygen-gas delignification process.
A method according to claims 1 to 6, characterized by adapting relative quantities of the separated gas and absorption solution, and optionally also the amount of oxygen-containing gas, so that the absorption solution used has a pH within the range of 5-12.
A method according to claims 1 to 7, characterized by recycling part of the separated gas, after being caused to react with the absorption solution in at least one stage, to the lignocellulosic material activating stage, optionally after adding oxygen.