EP1702100A2

EP1702100A2 - Process for removing interfering substances in the production of mechanical pulp and process for producing mechanical pulp

Info

Publication number: EP1702100A2
Application number: EP04805208A
Authority: EP
Inventors: Petteri Kotonen; Toni VÄKIPARTA; Jukka JÄKÄRÄ; Mikael Svedman
Original assignee: Kemira Oyj
Current assignee: Kemira Oyj
Priority date: 2003-12-31
Filing date: 2004-12-31
Publication date: 2006-09-20
Anticipated expiration: 2024-12-31
Also published as: FI121381B; WO2005064074A3; CA2549834A1; CA2549834C; EP1702100B1; FI20031937A0; WO2005064074A2; FI20031937L

Abstract

The invention relates to a method for removing an extractive during the production of mechanical pulp, the method comprising a step for impregnating chopped ligno-cellulosic material intended for use in pulp production before mechanical defibration with a liquid containing a peracid at a maximum pH value of 7, the amount of peracid being 0.5-20 kg/ton of dry lignocellulosic material, indicated as 100 % peracid. The invention also relates to a method for producing mechanical pulp.

Description

Process for removing interfering substances in the production of mechanical pulp and process for producing mechanical pulp

The invention relates to a method for removing extractives and optionally electrolytes during mechanical pulp production and a method for producing mechanical pulp.

Methods for producing and bleaching mechanical pulps differ from methods for de- fibrating and bleaching chemical pulps with respect to efforts to avoid delignifica- tion. The pulp yield is maintained on a high level. Bleaching is intended to decolourise the chromophore groups of lignin.

Mechanical pulps can be divided into two main classes, purely mechanical pulps and chemi-mechanical pulps. Pulps can be further divided into subgroups, with mechanical pulps including groundwood (SGW), pressure groundwood (PGW), re- fined mechanical pulp (RMP), thermomechanical pulp (TMP) and others, such as TRMP and PRMP. Accordingly, chernimechanical pulps include low sulphonated (chemithermomechanical pulp CTMP and BCTMP), chemically modified (OPCO) and high sulphonated pulps (CMP, TJHYS). Chernimechanical pulps typically differ from purely mechanical pulps in that the wood chips are pre-treated with alkaline solutions containing sulphite or alkaline solutions containing peroxide before mechanical defibration. The pre-treatment is intended to enhance the defϊbrating process e.g. by decreasing energy consumption or to improve the technical quality of the paper.

Mechanical pulps are used in the production of e.g. newsprint and magazine paper and porous paper grades (tissue papers). A number of highly bleached chemi- mechanical pulps (BCTMP qualities) are also used as a replacement of chemical pulp in bleached printing papers.

Wood entrains a number of substances that interfere with the papermaking process, degrade the pulp and paper quality and hamper closed water circulation. The main interfering substances derived from wood comprise lipophilic extractives, i.e. wood pitch and polyvalent metals. The most frequent problems caused by lipophilic extractives include various contamination and clogging problems, impaired runnabil- ity of the papermaking machine and impaired quality of the end product. Polyvalent metals, again, are problematic, because they generate deposits e.g. together with lipophilic extractives, oxalates and inorganic anions, reduce the washing effect of extractives, hamper the activity of chemicals used in paper production and hamper the peroxide bleaching process.

In conventional chip wash, the amount of extractives and polyvalent metals entering the process decreases by only 1-2%. Chip compression and impregnation methods used in the most recent process solutions increase the removal efficiency of extractives and metals to about 15%. Despite the improved chip washing effect, extractives and polyvalent metals are nevertheless in the most recent processes chiefly controlled by pulp wash and removal of a filtrate from the pulp production process. The filtrate from the process results in removal of a maximum of 30% of the wood extractives, with the filtrate removed at a rate of 5-20 m³/t.

Under increasingly stringent laws and green values, the paper industry has been forced to decrease its water consumption, and will be increasingly forced to do so in the future. As a consequence of reduced water consumption and increasingly strin- gent discharge limits, the paper industry will have to adopt new methods in order to prevent increased concentrations of interfering substances. In the practice, this will mean that various purification methods within the process will be introduced. Such purification methods may comprise e.g. ultra and nano-filtering, evaporation and chemical impregnation.

US patent specification 4,756,799 discloses a method for producing chemi- mechanical pulp from chips, comprising steaming, single-step impregnation with alkali and peroxide, liquid removal, preheating, one or two-step refining and bleaching of the chips. The alkaline treatment softens the material, and this favours defi- bration, given the better preservation of the geometrical shape of the fibres. It is also known that alkaline treatment gives the pulp a darker shade.

US patent specification 4,356,213 discloses a method for treating wood chips, which comprises impregnation of the chips with an impregnating solution containing sodium sulphite, sodium hydroxide and/or sodium bisulphite. The impregnated chips are dewatered and the chips are washed with a washing liquid. This patent sets fort that the impregnating solution fills up the cavities in the chips, thus avoiding absorption of washing liquor into the chips.

US patent specification 4,486,267 discloses a method for preparing chemi- thermomechanical pulp (CTMP) from hardwood, the method comprising a two-step chemical treatment preceding refining. In the first treatment, the chips are impreg- nated with an alkaline solution having a pH of at least 7.5 and containing sodium hydroxide, whereby the chips soften and swell by chemical means. After this, the alkaline solution is removed and a second chemical treatment is carried out, in which the chips are impregnated with a sulphite solution containing sodium sulphite and/or sodium bisulphite. Then the chips are digested with a sulphite solution in order to sulphonise the lignin contained in the chips, followed by defibration of the chips thus obtained.

US patent specification 3,428,520 discloses a method for producing chemical pulp, the method comprising impregnation of chips with a solution of an organic peroxygen compound, such as peroxygen acid, and then the chips are digested in the vapour phase, the organic peroxygen compound oxidising the lignin so that it can be extracted with an alkali solution. The organic peroxygen compound has the explicit purpose of converting the lignin into a form that can be extracted, in other words, the method aims at delignification.

FI patent specification 107545 B discloses a method for treating mechanical pulp by adding aliphatic percarboxylic acid at a rate of 0.5-5 kg/ton of pulp as an agent acting on the opacity of the pulp. The percarboxylic acid is added into the peroxide bleaching of defibrated mechanical pulp or into the after-treatment after peroxide bleaching.

EP patent specification 0 464 110 Bl discloses a method for bleaching mechanical or chemi-mechanical pulp, the method comprising a first step for treating the pulp with a reducing agent such as sodium bisulphite or sodium borohydride, and a second step for treating the pulp with a peroxygen compound, and a third step for treat- ing the pulp with a peroxygen compound. This peroxygen compound may be e.g. hydrogen peroxide or peractic acid. This method relates to the bleaching of mechanically defibrated pulp.

US patent specification 6,364,999 Bl discloses a method for removing an extractive from wood chips by extracting the wood chips with organic solvents, among which acetone is said to be particularly advantageous. Acetone extraction removes up to 65% of the extractive. The weight ratio of solvent to wood chips is in the range 6:1-1:1, in other words, very large amounts of solvent are required for extraction. The extractive can be removed from the solvent and purified solvent, can be recirculated to extraction. Example 1 of this patent specification gives a comparative ex- ample using peracetic acid, Caron acid and hypochlorous acid as the solvent. The patent specification notes with respect to these solvents that the effect of oxidised acids is based on chemical reactions, which modify the wood resins so that they can be extracted, which is not efficient in terms of thermodynamics.

It is known from many publications and standards that peracids can be utilised in chip soaking and further in digestion, such as Milox digestion and in the treatment of various wood components, when it is desirable to analyse extractives, cellulose components etc.

The article S. Rothenberg et al., Effect of chemical modification on the properties of lignin-containing fibres, Paperija Puu, No 3, 1981, pages 111-119, describes the effect of peracetic acid, acidic sodium chlorite and ozone on the strength properties of mechanical pulp .

The invention has the purpose of providing a method allowing the removal of extractives and optionally electrolytes during the production of mechanical pulp, while allowing pulp production with a small amount of water.

In accordance with the invention, a method has thus been provided for removing extractives during the production of mechanical pulp, the method comprising a step for impregnating chopped lignocellulosic material used in the pulp production before mechanical defibration with a solution containing a peracid at a maximum pH value of 7, the amount of the peracid being 0.5-20 kg/ton of dry lignocellulosic ma- terial, indicated as 100% peracid. This method can also comprise a liquid removal step, wherein a liquid containing unreacted feed liquor, soluble reaction products, e.g. reaction products of extractives, and optionally electrolytes, is removed from the impregnated material.

In addition, in accordance with the invention, a method has been provided for pro- ducing mechanical pulp, the method comprising the following steps:

i) chopped lignocellulosic material is impregnated with a solution containing a peracid at a maximum pH value of 7, the amount of the peracid being 0.5-20 kg/ton of dry lignocellulosic material, indicated as 100% peracid,

ii) a liquid containing unreacted feed liquor, soluble reaction products, e.g. reaction products of extractives, and optionally electrolytes, is removed from the impregnated material,

iii) the material having reduced extractive and electrolyte concentration is defibrated mechanically in one or more steps, and iv) the mechanical pulp thus obtained is bleached.

Said chopped lignocellulosic material mainly implies chip softwood or hardwood and or a material having typically an average length of 5-60 mm and an average length of typically 1-10 mm.

The typical softwood species comprise the following, for instance:

- spruces, such as e.g. Picea abies, Picea mariana, Picea glauca,

- firs, such as e.g. Abies balsamea, Abies alba,

- pines, such as e.g. Pinus radiata, Pinus sylvestris, Pinus strobes, Pinus resinosa, Pinus banksiana, Pinus contorta, Pinus ponderosa, Pinus caribaea, Pinus patula, Pinus virginiana,

- southern pines, such as e.g. Pinus taeda, Pinus palustris, Pinus rigida,

- hemlocks, such as e.g. Tsuga heterophylla, Tsuga canadensis.

Other softwood species:

- Douglas-fir Pseudotsuga menziesii,

- cedars, such as e.g. Thuja occidentalis, Thuja plicata,

- larches, such as e.g. Larix laricina, Larix decidua,

Typical hardwood species comprise the following, for instance:

- birches, such as e.g. Betula papyrifera, - aspens, such as e.g. Populus tremula, Populus tremuloides, Populus deltoides,

- hybride aspens, such as e.g. P. tremula x tremula, P. tremula x tremuloides,

- eucalyptus, such as e.g. E. grandis, E. globulus, E. viminalis,

- acacia.

The extractive mainly consists of wood pitch, containing among other things fatty acids, resin acids, triglycerides, steryl esters, sterols and lignans.

The electrolytes chiefly consist of polyvalent metals derived from wood, such as e.g. calcium, copper, magnesium, manganese and iron.

The peracid impregnation of the invention does not substantially affect lignin.

The peracid may be an organic or an inorganic peracid. The peracid is preferably an organic peracid, such as peracetic acid, performic acid or perpropionic acid, or a mixture of these peracids. As organic peracid, one may use peracid solutions puri- fied either by distillation or in some other manner, or equilibrium solutions of per- acids. Caron acid is a preferred inorganic peracid. It is also possible to use different mixtures of peracids.

The amount of peracid is preferably about 0.5-15 kg/ton of dry fibre material, more advantageously about 0.5-10 kg/ton of dry fibre material and particularly advantageously about 0.5-7 kg/ton of dry fibre material. The amount of peracid is indicated as 100% peracid.

Impregnation can be performed using methods and devices known per se, with impregnation carried out e.g. after the material has been treated with a device of screw press type. The pulp is compressed in a screw press, and as the pulp is subsequently released from pressure, i.e. underpressure is generated, the pulp absorbs impregnating liquid.

The impregnation period may be approx. in the range 1-60 minutes, preferably approx. 5-60 minutes, this effective period including the duration of an optional post-steaming and the liquid removal period. In other words, the impregnation period signifies the period from the moment impregnation starts until liquid removal has been completed.

Impregnation and liquid removal can be performed in one or several steps.

In addition, the method of the invention may comprise one or more steaming steps, which are performed before and/or after impregnation. Thus, the material can be steam-treated before or after impregnation, or the material can be steam-treated both before impregnation and after impregnation. Steaming preceding impregnation aims at preheating of the material, reducing the air content and homogenising the material. Steaming after impregnation and liquid removal aims at heating of the material and enhancing peracid reactions with respect to interfering substances.

Impregnation is preferably performed in the presence of a chelating agent. The che- lating agent is preferably added into the impregnation. A particularly suitable chelating agent is a chemical whose chelating capacity is preserved in the presence of peracid.

The amount of chelating agent is preferably about 0.1-2 kg/ton of dry lignocellulosic material, more preferably about 0.5-1 kg/ton of dry lignocellulosic material. The following chelating agents are suitable:

(a) (poly)aminophosphonic acids, such as aminotri(methylene phosphonic acid), ethylene diaminetetra(methylene phosphonic acid) and diethylene triamine pen- ta(methylene phosphonic acid),

(b) (poly)aminopolycarboxylic acids, such as ethylene diamine tetraacetic acid, i.e. EDTA, diethylene triamine pentaacetic acid, i.e. DTP A, 2,2'-iminodisuccinic acid, i.e. ISA, and ethylene diamine-N,N'-disuccinic acid, i.e. EDDS, and

(c) N-bis- or N-tris-[(l,2-dicarboxy ethoxy)ethyl]amine derivatives, such as N-bis- [(1,2-dicarboxy ethoxy) ethyl] amine and N-bis[(l,2-dicarboxy ethoxy) ethyl] as- partic acid, which have been described in WO 97/45396 and FI 106258.

The chelating agents in group (c) are particularly advantageous.

The chelating agents my appear in acid form or as alkali metal and alkaline earth metal salts.

During impregnation, the pH value is maximally 7, preferably about 2-7, more ad- vantageous approx. in the range 3-5 and particularly advantageously approx. in the range 3.5-5.

The impregnation temperature is preferably maximally about 140 °C and at least about 20 °C and more advantageously maximally about 120 °C. The preferred impregnation temperature is about 20-100 °C and a particularly advantageous impreg- nation temperature is about 20-80 °C.

In accordance with the invention, at least a portion of the liquid obtained in the liquid removal step, containing unreacted feed liquor, soluble reaction products, e.g. reaction products of extractives, and optionally electrolytes, can be returned to any of the preceding steps, such as washing of chopped lignocellulosic material, during which sand and other solids are removed from the material. The liquid obtained in the liquid removal step or a portion of this can also be conducted to waste water purification or evaporation and the dry matter to combustion or any other purpose of use.

In the method for producing mechanical pulp in accordance with the invention, the liquid removal step ii) can be performed by means of a screw press or by means of other known methods and/or devices. After this step, the pH value of the material can be regulated to the range of approx. 4.5-8. The pH value is preferably raised by means of a base, such as sodium hydroxide, e.g. to the pH value 7 or a value close to this. A pH increase is advantageous, because defibration at a low pH value results in inadequate strength properties.

The liquid removal step ii) comprises liquid removal preferably so as to obtain liquid in the impregnated material at a rate of 0.2-2 ton/ton of dry lignocellulosic material, preferably 0.3-1.6 ton/ton of dry lignocellulosic material.

In step iii) the material is mechanically defibrated using one or more refiners, e.g. a disc refiner. One may use e.g. two serially connected refiners. The pulp obtained in step iii) can be screened/classified and thickened.

The pulp obtained in step iv) is bleached. The bleaching is preferably carried out using a process that comprises alkaline peroxide bleaching. The bleaching may comprise one or more alkaline peroxide steps and/or a combination of an alkaline peroxide bleaching and a reducing bleaching step, such as e.g. dithionite bleaching. If necessary, the stock can be treated with a chelating agent before bleaching. The chelating treatment is followed by precipitation before bleaching, if necessary.

Chemi-mechanical pulps are prepared by the method of the invention.

The invention also relates to the use of a peracid for removing extractives and optionally electrolytes from chopped lignocellulosic material, which is intended for use in mechanical pulp production, the peracid batch being 0.5-20 kg/ton of dry lignocellulosic material, indicated as 100% peracid. The peracid batch is preferably about 0.5-15 kg/ton, more advantageously about 0.5-10 kg/ton and particularly ad- vantageously about 0.5-7 kg/ton.

The invention allows removal of extractives and polyvalent metals from the chips before the actual pulp production process owing to peracid impregnation. This achieves interfering substance removal with higher efficiency than present effi- ciency, yet with considerably smaller amounts of water than those currently employed. In laboratory tests, the chip extractive concentration has been decreased to almost half in the optimal case, and the chip metal concentration to about 40% from the original concentration. At the same time, the water amount to be removed was as low as 0.7-0.8 m³/t, which constitutes only about 3—16% of the water amount to be removed at the present time. At the same rate of water consumption in a normal case, a large and expensive water-treatment plant would be required, and especially metal removal would require e.g. nano-filtering or evaporation in order to allow the equivalent ratio metal removal/water consumption. In addition peracid impregnation has the advantage of modifying extractives e.g. by oxidation/polymerisation, resulting in a lower (COD) (TOC) concentration of water fraction to be removed than in current process solutions, such as sulphite impregnation. This results in lower loads on the waste water plant, and a decrease in the amount of poorly biodegradable extractives facilitates waste water purification.

The percentages are indicated as weight percentage in this description, unless oth- erwise indicated.

The invention is described in further detail below by means of the accompanying drawing, in which the figure is a bloc diagram of a method using peracid impregnation in accordance with the invention.

In figure 1 the chips 1 are conducted to washing 2 and from there to impregnation 3, to which a peracid solution 4 and a chelating agent 5 are fed. Liquid 6 is removed from chip wash 2. From impregnation 3, the chips are conducted to liquid removal 7, during which liquid 8 containing extractives and electrolytes is removed. The liquid 8 can be removed as a flow 9 from the system, or a portion of the liquid 8 can be recirculated to chip washing 2 and/or impregnation 3. Liquid 10 can also be removed from the impregnation step 3. Line 11 illustrates an optional recycling, stirring or liquid replacement provisions. From the liquid removal step 7, the chips are conducted to defibration 12, from where the pulp is then taken to further processing, such as bleaching.

The following examples are given with the purpose to further illustrate the invention.

Example 1

Chips were impregnated at a temperature of 70 °C with peracetic acid (peracid A), perpropionic acid (peracid B) or performic acid (peracid C), without pH regulation and using an impregnation period of 45 minutes. The feed liquor had a concentration of peracetic acid of about 10 g/1. The ratio water: chips was 6:1. With peracids, a chelating agent was used at a rate of 0.2%> by weight of dry chips. The chelating agent was of a mixture of N-bis[(l,2-dicarboxyethoxy)ethyl]amine and N-bis[(l,2- dicarboxy ethoxy)ethyl]aspartic acid. Comparative tests were conducted, in which chips were impregnated at a temperature of 70 °C with pure water alone, with sulphite (CTMP process) or with formic acid, without pH regulation, using an impregnation period of 45 minutes.

The amounts of extractives, calcium and manganese, respectively, were determined in the original chips and the impregnated chips.

The test results are given in the following table 1.

Table 1

The results show that water impregnation does not remove extractives or polyvalent metals. Sulphite yields higher removal efficiency, which does not, however, reach the level yielded by peracids. Formic acid removes electrolytes, but not extractives. Peracid impregnation removes about 30% or more of the extractives, removing also electrolytes more efficiently than sulphite does.

Example 2 Chips were impregnated with peracids in the same manner as in example 1, but the pH was regulated to 3, 5 and 7, respectively. The test results are shown in the following table 2.

The results show that peracids decompose extractives more efficiently when the pH is low. The washing effect of electrolytes also improves when the pH drops. Example 3

Chips were impregnated at a temperature of 70 °C with peracetic acid (peracid A) at pH 2-3. The effect of the peracid amounts, the impregnation periods and the chelating agent on the extractive concentration was examined. The fed liquor had a peracetic acid concentration of about 10-20 g/1. The ratio water:chips was 6:1. The test results are given in the following table 3. The chelating agent was the same substance as in example 1.

Table 3

The results show that increased impregnation duration and a chelating agent addition enhance the peracid activity, which appears as decreased extractive and peracid concentrations. An increased amount of peracid also reduces the extractive concentration.

Example 4

Chips were impregnated with sulphite (comparison), peracetic acid (peracid A) and performic acid (peracid C) and were subjected to post-steaming. A short impregnation period was used. The impregnation temperature was 70 °C in the last array and 90 °C in the remaining ones. Steaming was carried out under overpressure of 2 bar and at a temperature of 120 °C. The fed liquid had a peracetic acid concentration of about 10-20 g/1. The ratio waterxhips was 6:1. Steaming was carried out after impregnation and liquid removal. The test results are given in table 4. Table 4

The results show that short impregnation combined with post-steaming yields good results at a low pH value.

Example 5

Spruce chips were impregnated with a equilibrium peracetic acid solution containing 15% of peracetic acid under acidic conditions, followed by refining under neutral conditions. Impregnation with sulphite (CTMP process; test A) and water (TMP process; test F) were used as a reference. Finally the pulps were bleached with alkaline peroxide. The total extractive concentration of unbleached pulps and peroxide- bleached pulps, respectively, were determined. The test results are indicated in table 5. Table 5

The peracid batches in table 5 and also table 6 below are indicated as 100% peracetic acid/chips, calculated as oven-dry chips, and the peroxide batch of peroxide bleaching is indicated as 100% hydrogen peroxide/ton of pulp, calculated as dry pulp. As shown in table 5, in this case, alkaline sulphonation (30 kg of 100% Na2S03/t of chips, calculated as dry) gives the pulp roughly as low or lower an extractive concentration compared to peracetic acid impregnation under acidic conditions and subsequent refining under neutral conditions. When the pulps from acidic peracid impregnation and from subsequent refining were bleached with alkaline pe- roxide, the bleached pulp received a lower extractive concentration compared to the reference pulps, i.e. alkaline sulphonation-impregnated refined pulp further bleached with alkaline peroxide, or water-impregnated, refined pulp further bleached with alkaline peroxide. In all the cases, the refined pulps were washed before peroxide bleaching, so that residual chemicals from impregnation did not reach the peroxide-bleaching step. Peroxide bleaching was performed by using NaOH for pH regulation and water glas as a stabiliser at a rate of 30 kg of product/ton of pulp, calculated as dry pulp.

Example 6

The brightness and strength properties of the impregnated and refined pulps A-F in example 5 were determined before and after the peroxide bleaching, and so was the refining energy consumption of the different impregnated pulps. The results are given in the following table 6.

Table 6

Table 6 indicates that the examined peracid batches did not show as high a brightness of peracid-impregnated pulps before alkaline peroxide bleaching compared to the brightness of the sulphonation-impregnated CTMP reference or the water- impregnated TMP reference. This is due to the formation of chromophore groups during peracid impregnation and the chromophore groups formed with refiners under oxidising conditions, resulting in decreased brightness of the pulp. During reducing sulphonation or subsequent refining, oxidising conditions are avoided and chromophore groups do not form as readily as under oxidising conditions at high temperature during refining after equilibrium peracid impregnation. In the TMP reference, only air oxygen was present during refining, in other words, the conditions were less oxidising than after equilibrium peracid impregnation. It is not obvious to those skilled in the art that peracid impregnation enhances the brightness of the pulp obtained - in the literature, pulp brightness has been improved by treating mechanical and chemical pulps with peracids.

Alkaline peroxide bleaching after peracid impregnation enhances pulp brightness appreciably. The brightening potential for peroxide bleaching of coloured chromophore groups formed in the pulp during peracid impregnation and subsequent refining is not obvious to those skilled in the art. Table 6 also shows that neutralisation of peracid-impregnated pulps for refining is advantageous. A low neutralising pH of 3.5 after peracid impregnation yields a clearly poorer strength of the refined pulp than does neutralisation to a pH range of 6-7. Assumedly peracid impregnation at a very low pH value (pH <3.5) impairs the quality of refined fibres unless neutralisation is performed after the impregnation. On the average, alkaline peroxide bleaching improves the strength of impregnated and refined pulps. The brightness and the strength of peracid-impregnated pulps further refined at a pH level of 6-7 improved during peroxide bleaching to values above those of the reference sulphonation pulp. Raising the peracid batch of peracid impregnation above 7 kg PAA/t did not yield better results. Peracid impregnation dissolves the organic substances. In addition, equilibrium peracid contains acetic acid and acetic acid is released from peracid during the reactions, and hence it is advantageous to avoid excessive peracid batches.

Claims

1. A method for removing an extractive during the production of mechanical pulp, characterised in comprising a step in which chopped lignocellulosic material used for pulp production is impregnated before chemical defibration with a liquid containing a peracid at a maximum pH value of 7, the amount of the peracid being 0.5-20 kg/ton of dry lignocellulosic material, indicated as 100% peracid.

2. A method as defined in claim 1, characterised in comprising additionally a liquid removal step, wherein a liquid containing unreacted feed liquor, soluble reaction products, e.g. reaction products of extractives, and optionally electrolytes, is removed from the impregnated material.

3. A method as defined in claim 1 or 2, characterised in that the peracid is an organic peracid, such as peracetic acid, performic acid or perpropionic acid, or a mixture of these peracids.

4. A method as defined in claim 1 or 2, characterised in that the peracid is an inorganic peracid, such as Caron acid.

5. A method as defined in any of the preceding claims, characterised in that the amount of peracid is 0.5-15 kg/ton of dry lignocellulosic material, indicated as 100% peracid.

6. A method as defined in any of the preceding claims, characterised in that the impregnation is performed after the material has been treated with a device of screw press type.

7. A method as defined in any of the preceding claims, characterised in comprising additionally one or more steaming steps performed before and/or after the impregnation.

8. A method as defined in any of the preceding claims, characterised in that the impregnation is performed in the presence of a chelating agent.

9. A method as defined in claim 8, characterised in that the chelating agent is N-bis[(l,2-dicarboxyethoxy)ethyl]amine or N-bis[(l,2-dicarboxyethoxy)- ethyl]aspartic acid.

10. A method as defined in claim 8 or 9, characterised in that the amount of chelating agent is 0.1-2 kg/ton of dry lignocellulosic material.

11. A method as defined in any of the preceding claims, characterised in that the pH value during impregnation is in the range 2-7, preferably 3-5 and more advantageously 3.5-5.

12. A method as defined in any of the preceding claims, characterised in that the temperature during impregnation is in the range 20-140 °C, preferably 20-100°C, and particularly advantageously 20-80 °C.

13. A method for producing mechanical pulp, characterised in comprising the following steps:

i) impregnating chopped lignocellulosic material with a liquid containing a peracid at a maximum pH value of 7, the amount of the peracid being 0.5-20 kg/ton of dry lignocellulosic material, indicated as 100% peracid,

ii) removing a liquid containing unreacted feed liquor, soluble reaction products, e.g. reaction products of extractives, and optionally electrolytes, from the impregnated material,

iii) defibrating the material having a decreased concentration of extractives and electrolytes mechanically in one or more steps, and

iv) bleaching the mechanical pulp obtained.

14. A method as defined in claim 13, characterised in that step i) occurs at a temperature in the range 20-140 °C, preferably 20-100 °C, particularly advantageously 20-80 °C.

15. A method as defined in claim 13 or 14, characterised in that step i) is completed in a period of 1-60 min, preferably 5-60 min.

16. A method as defined in any of claims 13-15, characterised in that the pH of the material is regulated to a value in the range 4.5-8 between step ii) and iii).

17. A method as defined in any of claims 13-16, characterised in that step ii) is performed by removing liquid so as to obtain liquid in the impregnated mate- rial at a rate of 0.2-2 ton/ton of dry lignocellulosic material, preferably 0.3-1.6 ton/ton of dry lignocellulosic material.

18. A method as defined in any of claims 13-17, characterised in that the bleaching is performed with a process comprising an alkaline peroxide bleaching.

19. Use of a peracid for removing an extractive from chopped lignocellulosic material intended for use in mechanical pulp production, the amount of the peracid being 0.5-20 kg/ton of dry lignocellulosic material, indicated as 100% peracid.