WO2024172669A1

WO2024172669A1 - Method of recycling wood fibres, especially from wood-based boards

Info

Publication number: WO2024172669A1
Application number: PCT/PL2023/050064
Authority: WO
Inventors: Bartlomiej MAZELA; Mehrnoosh TAVAKKOLI; Izabela SIEMIŃSKA
Original assignee: Przedsiebiorstwo Wielobranzowe Sp. Z O.O.
Priority date: 2023-02-15
Filing date: 2023-08-07
Publication date: 2024-08-22
Also published as: PL443796A1

Abstract

A method of recycling wood fibres, especially from wood-based boards, is conducted in such a way thatin the first step, the waste containing wood fibres cured with a binding agent is comminuted to a chip size of 0.5 to 5 cm and placed in a sealed pressure vessel, and then the pressure vessel is closed and the reaction chamber is depressurised to a value of 0.1 to 0.95 kPa while heating its contents to a temperature of 20 to 95ºC. In the second step, either an alkaline solution or an acidic solution heated to a temperature of 40 to 90ºC is introduced into the reaction chamber and the pressure in the reaction chamber is brought down to atmospheric pressure, and then, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 100 to 180ºC and a pressure of 1.0 do 1.5 MPa is maintained in the reaction chamber for 1 to 5 hours. In the third step, the pressure in the reaction chamber is reduced and the impregnation liquid, together with the extraction lye, is evacuated. In the fourth step, the reaction chamber is emptied, and in the fifth step, the obtained pulp is washed and dried.

Description

Method of recycling wood fibres, especially from wood-based boards

The object of the invention is a method of recycling wood fibres from the waste of mechanical processing of furniture boards, in particular from medium density fibreboard, MDF, high-density fibreboard, HDF, or chipboard.

The use of moulded products based on fibres, more specifically on wood fibres, is generally known. As the availability of native wood fibres may be limited in the future, the production of moulded products from aftermarket wood fibres is becoming increasingly important. From the description of the application for invention EP1498241A2, a moulded product is known, which is in particular MDF or HDF and which consists of wood fibres bonded with a binding agent or polymer. In such moulded products, the wood fibres are bonded together using a binding agent such as uncured melamine resin, which is cured between the wood fibres. Such moulded products contain, for example, 85% wood fibres, 13% cured melamine resin and 2% additives that affect the bonding or binding of the melamine resin and wood fibres. Due to the presence of the already cured binding agent, the waste in the form of particles is no longer suitable for re-bonding to form a moulded product.

Given that the average service life of MDF or HDF products is around 10 years, it is believed that a large amount of waste will be generated in the future. Today, most MDF waste is incinerated or buried in the ground, which is a major environmental problem. Numerous attempts are being made to retrieve wood fibres from MDF by mechanical methods, but in doing so, the wood fibre becomes damaged and is not suitable for re-use in existing applications, particularly for the production of board materials. From the description of the application for invention US2003056873A1, a method of making dry-processed fibreboard is known, which uses waste composite wood products such as chipboard, fibreboard, plywood, fibreboard and oriented strand board, OSB, as raw material.

The description of the application for invention WO2018169230A1 discloses a device for retrieving MDF and a method of retrieving MDF which comprises a step of supplying pulverised MDF pieces to an agitator, a step of pulping consisting in supplying water and heat to the agitator and simultaneous or successive mixing of MDF pieces, and a step of discharging consisting in discharging the MDF wood fibre comminuted in the pulping step. In this known method, damage to the wood fibres has been minimised and the fibrillation time, thanks to the pre-heating process carried out prior to the fibrillation process, has been reduced, but it requires the raw material to be comminuted into a powder, which results in a shortening of the fibres and thus a reduction in the strength properties of the new product. Furthermore, the fibrillation step is characterised in that heat is supplied with water at a temperature below 100°C and, in addition, heat is supplied with steam at a temperature of 150°C or lower. The described fibrillation step also requires the use of very high-frequency microwaves, ensuring homogeneous distribution of heat.

From the description of the application for invention GB2465202A, a method of converting waste chipboard and fibreboard into useful component materials is known, which includes: introduction of planed sections of waste wood into a high-pressure water jet and mechanical pulping of the raw material under the pressure of the water jets. The method further includes separating the wood particles or wood fibres from the effluent water generated during the reduction of the raw material and pumping the suspended wood particles or wood fibres into a separation tank in order to extract most of the effluent: feeding the wet particles or fibres into a press where further water is extracted; transferring a batch of wet wood particles or fibres from the wet material hopper and feeding the wet material into a dryer. Patent description EA021142B1 discloses a method of recycling wooden structural panels, particularly fibreboard, in order to retrieve component wood fibres for reuse as a substitute for original wood fibres. In this known method, the wooden panels are comminuted and mixed with water in order to produce a suspension, which is then heated by passing an electric current in order to weaken the binding between the wood fibres. Heating can take place at a pressure higher than atmospheric. Once heated, the suspension can be rapidly expanded, especially by spraying through a nozzle in order to dry and separate the fibres. The advantage of using electric heating is that it is a homogeneous and penetrating thermal effect generated by this known method without burning or carbonising the wood fibres. However, the use of this known method requires mechanical comminution of the waste by grinding before applying heat or by adding water in order to improve the efficiency of the process.

From patent description GB2410746B, a method of retrieving a component of a board material containing lignocellulosic elements is known, which includes subjecting the material to electromagnetic radiation at frequencies from 10 MHz to 300 GHz and soaking in water before retrieving the component. The board material is exposed to electromagnetic radiation, in the form of microwaves, before or during immersion in water which has an elevated temperature, particularly between 60 and 90°C.

The description of the application for invention CN107476103 A discloses a method of extracting cellulose from wood fibre board using an ionic liquid.

The description of the application for invention CN104641033A discloses a method of refining a cellulose-based fibres by using an enzymatic composition for treating an unprocessed cellulose-based fibres or a finished product made from these fibres. The enzymatic composition comprises one or more types of enzymes - pectinases capable of releasing the pectic substance from the untreated cellulose-based fibres. An enzymatic composition and a saponified fatty acid, of which oleic acid is the main component, are used to carry out the refining.

The aim of the invention is to develop a new method of recycling wood fibres from wood-based board materials, in particular from MDF, HDF or chipboard, which allows the wood fibres to be purified from the binding agent and does not require the raw material to be finely comminuted beforehand, while the end product can be used, in particular, for the production of moulded 3D paper products.

The method of recycling wood fibres, in particular from wood-based boards, according to the invention is characterised in thatln the first step, the waste containing wood fibres cured with a binding agent, in particular phenolformaldehyde resin or urea-formaldehyde resin or melamine-urea-formaldehyde resin, is comminuted into chips between 0.5 and 5 cm in size and these chips are placed in a sealed pressure vessel, and then the pressure vessel is closed and the pressure in the reaction chamber is reduced to between 0.1 and 0.95 kPa while heating the contents to a temperature between 30 and 95°C, after which, in the second step, either an alkaline solution or an acidic solution heated to a temperature of 40 to 90°C is introduced into the reaction chamber and the pressure in the reaction chamber is brought down to atmospheric pressure, and then the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, the reaction chamber is maintained at a temperature of 100 to 180°C and a pressure in the range of 1.0 to 1.5 MPa for 1 to 5 hours, in the third step, the pressure in the reaction chamber is reduced, the impregnation liquid together with the extraction lye is evacuated, after which the reaction chamber is emptied in the fourth step and the pulp obtained in the fifth step is washed and dried.

Preferably, in the first step, the waste containing wood fibres cured with a binding agent is comminuted into chips of 2 to 4 cm in size, the pressure in the reaction chamber is reduced to a value of 0.45 to 0.65 kPa and the contents of the reaction chamber are heated to a temperature of 20 to 70°C.

Further advantages are obtained if, in the second step, an aqueous solution containing 2.5% by weight of sodium sulphate is used as the alkaline solution, or anaqueous solution containing 1.75% by weight of sodium sulphate (IV) and 0.75% by weight of sodium hydroxide is used as the alkaline solution, or an aqueous solution containing 2.5% by weight of sodium sulphate (IV) and 0.14% by weight of anthraquinone is used as the alkaline solution, or an aqueous solution containing 1.12% by weight of sodium hydroxide and 0.7% by weight of hydrogen peroxide is used as the alkaline solution, or an aqueous solution containing 13.33% by weight of formic acid is used as the acidic solution, wherein the alkaline solution or acidic solution introduced into the reaction chamber is used at a temperature of 60 to 80°C and, in addition, the reaction system is heated to a temperature of 110 to 135°C and the reaction system is maintained at a pressure of 1.1 to 1.3 MPa for 2 to 3 hours.

Further advantages are obtained if, in the third step, hot steam at a temperature of 100°C is introduced into the reaction chamber for 15 minutes and the pressure in the chamber is increased to a value between 0.8 and 1 MPa, or compressed air is introduced into the reaction chamber.

Further advantages are obtained if, in the fifth step, the pulp is washed in a drum washer for 5 to 55 minutes, preferably 10 to 20 minutes, the pulp being washed in the drum washer at a drum speed of 0.5 to 3 rev/minute and the drum washer being used with a drum diameter of 2000 to 3000 mm, and, in addition, in the fifth step, the pulp is dried in a drum dryer for a period of 15 to 90 minutes at a temperature of 45 to 120°C, preferably at a temperature of 70 to 90°C, the pulp being dried in the drum dryer at a rotational speed of 500 to 1500 rev/minute.

A new method of recycling wood fibres, especially from wood-based boards, according to the invention makes it possible to purify wood fibres from a pre-cured binding agent, especially phenol-formaldehyde resin, ureaformaldehyde resin or melamine-urea-formaldehyde resin in a process of a hot water bath with a non-neutral pH, especially in an alkaline environment, under high pressure, in a sealed chamber. The starting raw material for recycling does not need to be homogenised and ground to a fine fraction. It only undergoes an initial comminution on a chipper to a particle size of 2 to 4 cm. As a result of refining - boiling and then washing - the resulting wood fibres show a desirable tendency to bond together through cross-linking, especially during the production of a 3D moulded wood fibre product, which in turn translates into a high papermaking capacity of the dried and pressed fibre pulp. The non-neutral reaction environment in the new method of wood fibre recycling, under conditions of high temperature and pressure, causes softening and extraction of the lignin fraction from the wood fibres and contributes to the softening and dissolution of the cured resin, especially the phenol-formaldehyde resin that binds the wood fibres together. The lignin fraction is a natural polymer whose monomers are phenolic alcohol derivatives - coniferyl, synapin, coumaryl. Under elevated temperature conditions, the vapour pressure of the extracted alcohols increases, allowing the cured resin to soften and detach from the fibre surface. Then, the wood fibres are released from both the synthetic and natural binder, exposing the dominant cellulose fraction, promoting the interactions of the purified fibres. Unexpectedly, it was also found that heating to 100°C of a sealed system consisting of MDF chips immersed in an aqueous solution, especially an alkaline solution, results in the release of certain amounts of resin decomposition products, especially those of phenol-formaldehyde resin, which catalyse the further refining of the raw material. As a result, temperature and pressure increase during the process, making the reaction auto-thermal, thereby increasing the reaction's efficiency in terms of energy and economy. Unexpectedly, it was also found that raw material in the form of wood-based board waste, especially from MDF or HDF, subjected only to initial comminution in a chipper and then refined according to the method of recycling wood fibres, especially from wood-based boards, showed better mouldability on the paper machine screen than raw material subjected to homogeneous grinding and sorting. Indeed, it was found that the size of the particles subjected to dissolution does not affect the effect of the refining process carried out according to the method, while larger particles allowed the natural length of the wood fibres to be retained. The new method of recycling wood fibres, according to the invention, does not require the use of any additional and energy-consuming treatment processes, in particular heavy comminution to a very fine fraction by grinding, exposure of the raw material to electromagnetic radiation, microwaves, ultrasound or initial moistening of the raw material. It also does not require the use of any expensive chemical components such as ionic liquids, DES-type eutectic solvents, nor does it require the use of additional timeconsuming biological treatment of the raw material, consisting in particular in enzymatic dissolution using bacteria or fungi, for example.

The object of the invention is shown in embodiments.

The method of recycling wood fibres, especially from wood-based boards, according to the invention in the first embodiment is carried out in such a way that, in the first step, the MDF waste is comminuted into chips of 0.5 to 5 cm in size and these chips in an amount of 1,000 g are introduced into a sealed pressure vessel, which is then closed and the pressure in the reaction chamber is reduced to 0.1 kPa in order to evacuate the air from the free spaces, while the reaction system is simultaneously heated to a temperature of 30°C. Then, in the second step, an alkaline solution with a pH of 8.4 is introduced into the reaction chamber, which is an aqueous solution containing 2.5% by weight of sodium sulphate (IV) Na2SOs pre-heated to a temperature of 40°C, with the pressure in the reaction chamber being brought to atmospheric pressure. The weight ratio of the individual components of the reaction mixture is 6.67% by weight of MDF woodchips, 91.26% by weight of water, 2.07% by weight of sodium sulphate (IV) ISfeSCh. The ratio of working fluid to woodchips is 14: 1. In turn, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 100°C and a pressure of 1.0 MPa is maintained in the reaction chamber for 5 hours. Subsequently, in the third step, the pressure in the reaction chamber is reduced and the impregnation liquid is evacuated together with the extraction lye, followed by the introduction of hot steam at a temperature of 100°C into the reaction chamber over a period of 15 minutes and at the same time the pressure in the reaction chamber is increased to 1 MPa. The violent decompression of the system causes steam to be ejected from the chamber, thereby releasing the fibres from the refining products. Then, in the fourth step, the reaction chamber is emptied, and in the fifth step, the obtained pulp is washed in a drum washer at a drum speed of 0.5 to 3 rev/minute, with the drum size ranging from 2000 to 3000 mm in diameter, and then dried in a drum dryer for 15 to 60 minutes at a temperature of 40 to 75°C at a drum speed of 500 to 1500 rev/minute. The lubricity of the obtained pulp in Schopper-Riegler degrees is 9 SR, and 71 SR after grinding.

The method of recycling wood fibres, especially from wood-based boards, according to the invention in the second embodiment is carried out in such a way that, in the first step, the MDF waste is comminuted into chips of 2 to 4 cm in size and these chips in an amount of 1,000 g are introduced into a sealed pressure vessel, which is then closed and the pressure in the reaction chamber is reduced to 0.95 kPa in order to evacuate the air from the free spaces, while the reaction system is simultaneously heated to a temperature of 95°C. Then, in the second step, an alkaline solution with a pH of 9.6 is introduced into the reaction chamber, which is an aqueous solution containing 1.75% by weight of sodium sulphate (IV) Na2SOs and 0.75% by weight of sodium hydroxide NaOH, pre-heated to a temperature of 90°C, with the pressure in the reaction chamber being brought to atmospheric pressure. The weight ratio of the individual components of the reaction mixture is 6.25% by weight of MDF woodchips, 91.325% by weight of water, 1.698% by weight of sodium sulphate (IV) ISfeSCh and 0.727% by weight of sodium hydroxide NaOH. The ratio of working fluid to woodchips is 15: 1. In turn, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 180°C and a pressure of 1.5 MPa is maintained in the reaction chamber for 1 hour. Then, in the third step, the pressure in the reaction chamber is reduced and the impregnation liquid, together with the extraction lye, is evacuated, followed by the introduction of compressed air into the reaction chamber. Then, in the fourth step, the reaction chamber is emptied, and in the fifth step, the obtained pulp is washed in a drum washer at a drum speed of 0.5 rev/minute, with the drum size of 3000 mm in diameter, and then dried in a drum dryer for 90 minutes at a temperature of 60°C at a drum speed of 500 rev/minute or more. The lubricity of the obtained pulp in Schopper- Riegler degrees is 5 SR, and 66 SR after grinding.

The method of recycling wood fibres, especially from wood-based boards, according to the invention in the third embodiment is carried out in such a way that, in the first step, the MDF waste is comminuted into chips of 2 to 4 cm in size and these chips in an amount of 1,000 g are introduced into a sealed pressure vessel, which is then closed and the pressure in the reaction chamber is reduced to 0.45 kPa in order to evacuate the air from the free spaces, while the reaction system is simultaneously heated to a temperature of 50°C. Then, in the second step, an alkaline solution with a pH of 8.8 is introduced into the reaction chamber, which is an aqueous solution containing 2.5% by weight of sodium sulphate (IV) Na2SOs and 0.14% by weight of anthraquinone, pre-heated to a temperature of 60°C, with the pressure in the reaction chamber being brought to atmospheric pressure. The weight ratio of the individual components of the reaction mixture is 6.25% by weight of MDF woodchips, 91.207% by weight of water, 2.41% by weight of sodium sulphate (IV) ISfeSCh and 0.133% by weight of anthraquinone. The ratio of working fluid to woodchips is 15: 1. In turn, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 110°C and a pressure of 1.3 MPa is maintained in the reaction chamber for 3 hours. Then, in the third step, the pressure in the reaction chamber is reduced and the impregnation liquid, together with the extraction lye, is evacuated, followed by the introduction of compressed air into the reaction chamber. Then, in the fourth step, the reaction chamber is emptied, and in the fifth step, the obtained pulp is washed in a drum washer for 10 minutes at a drum speed of 2 rev/minute or more, with the drum size of 2500 mm in diameter, and then dried in a drum dryer for 60 minutes at a temperature of 70°C at a drum speed of 1000 rev/minute. The lubricity of the obtained pulp in Schopper-Riegler degrees is 5.5 SR, and 62 SR after grinding. The method of recycling wood fibres, especially from wood-based boards, according to the invention in the fourth embodiment is carried out in such a way that, in the first step, the MDF waste is comminuted into chips of 2 to 4 cm in size and these chips in an amount of 1,000 g are introduced into a sealed pressure vessel, which is then closed and the pressure in the reaction chamber is reduced to 0.65 kPa in order to evacuate the air from the free spaces, while the reaction system is simultaneously heated to a temperature of 70°C. Then, in the second step, an alkaline solution with a pH of 9.5 is introduced into the reaction chamber, which is an aqueous solution containing 1,12% by weight of sodium hydroxide NaOH and 0.7% by weight of hydrogen peroxide H2O2, pre-heated to a temperature of 80°C, with the pressure in the reaction chamber being brought to atmospheric pressure. The weight ratio of the individual components of the reaction mixture is 6.25% by weight of MDF woodchips, 92.01% by weight of water, 1.071% by weight of sodium hydroxide NaOH and 0.669% by weight of hydrogen peroxide H2O2. The ratio of working fluid to woodchips is 15: 1. In turn, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 135°C and a pressure of 1.1 MPa is maintained in the reaction chamber for 2 hours. Subsequently, in the third step, the pressure in the reaction chamber is reduced and the impregnation liquid is evacuated together with the extraction lye, followed by the introduction of hot steam at a temperature of 100°C into the reaction chamber over a period of 15 minutes and at the same time the pressure in the reaction chamber is increased to 1 MPa. The violent decompression of the system causes steam to be ejected from the chamber, thereby releasing the fibres from the refining products. Then, in the fourth step, the reaction chamber is emptied, and in the fifth step, the obtained pulp is washed in a drum washer at a drum speed of 3 rev/minute or more, with the drum size of 2000 mm in diameter, and then dried in a drum dryer for 50 minutes at a temperature of 90°C at a drum speed of 1500 rev/minute or more. The lubricity of the obtained pulp in Schopper-Riegler degrees is 5.5 SR, and 70 SR after grinding. The method of recycling wood fibres, especially from wood-based boards, according to the invention in the fifth embodiment is carried out in such a way that, in the first step, the MDF waste is comminuted into chips of 2 to 4 cm in size and these chips in an amount of 1,000 g are introduced into a sealed pressure vessel, which is then closed and the pressure in the reaction chamber is reduced to 0.55 kPa in order to evacuate the air from the free spaces, while the reaction system is simultaneously heated to a temperature of 60°C. Then, in the second step, an acidic solution with a pH of 2.5 is introduced into the reaction chamber, which is an aqueous solution containing 13.33% by weight of formic acid HCOOH, pre-heated to a temperature of 70°C, with the pressure in the reaction chamber being brought to atmospheric pressure. The weight ratio of the individual components of the reaction mixture is 6.25% by weight of MDF woodchips, 81.25% by weight of water, 12.5% by weight of formic acid HCOOH. The ratio of working fluid to woodchips is 15: 1. In turn, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 120°C and a pressure of 1.2 MPa is maintained in the reaction chamber for 2 hours. Then, in the third step, the pressure in the reaction chamber is reduced and the impregnation liquid, together with the extraction lye, is evacuated, followed by the introduction of compressed air into the reaction chamber. Then, in the fourth step, the reaction chamber is emptied, and in the fifth step, the obtained pulp is washed in a drum washer at a drum speed of 0.5 to 3 rev/minute, with the drum size ranging from 2000 to 3000 mm in diameter, and then dried in a drum dryer for 40 minutes at a temperature of 60°C at a drum speed of 500 to 1500 rev/minute. The lubricity of the obtained pulp in Schopper- Riegler degrees is 5.5 SR, and 58 SR after grinding.

Claims

1. A method of recycling wood fibres, especially from wood-based boards, characterised in thatin the first step, the waste containing wood fibres cured with a binding agent, especially phenol-formaldehyde resin or urea-formaldehyde resin or melamine-urea-formaldehyde resin, is comminuted into chips between 0.5 and 5 cm in size and these chips are placed in a sealed pressure vessel, and then the pressure vessel is closed and the reaction chamber is depressurised to a value of 0.1 to 0.95 kPa while heating its contents to a temperature of 20 to 95°C, after which, in the second step, either an alkaline solution or an acidic solution heated to a temperature of 40 to 90°C is introduced into the reaction chamber and the pressure in the reaction chamber is brought down to atmospheric pressure, and then, the reaction system is heated gradually until the exothermic reaction starts and, once the reaction is complete, a temperature of 100 to 180°C and a pressure of 1.0 do 1.5 MPa is maintained in the reaction chamber for 1 to 5 hours, in the third step, the pressure in the reaction chamber is reduced, the impregnation liquid together with the extraction lye is evacuated, after which, in the fourth step, the reaction chamber is emptied, and the obtained pulp is washed and dried in the fifth step.

2. The method according to claim 1, characterised in that in a first step, the waste containing wood fibres cured with a binding agent is comminuted into chips of 2 to 4 cm in size.

3. The method according to claim 1 or 2, characterised in that in the first step, the pressure in the reaction chamber is reduced to a value of 0.45 to 0.65 kPa.

4. The method according to any one of claims 1 to 3, characterised in that in the first step, the contents of the reaction chamber are heated to a temperature of 30 to 70°C.

5. The method according to any one of claims 1 to 4, characterised in that in the second step an aqueous solution containing 2.5% by weight of sodium sulphate (IV) is used as the alkaline solution.

6. The method according to any one of claims 1 to 4, characterised in that in the second step an aqueous solution containing 1.75% by weight of sodium sulphate (IV) and 0.75% by weight of sodium hydroxide is used as the alkaline solution.

7. The method according to any one of claims 1 to 4, characterised in that in the second step an aqueous solution containing 2.5% by weight of sodium sulphate (IV) and 0.14% by weight of anthraquinone is used as the alkaline solution.

8. The method according to any one of claims 1 to 4, characterised in that in the second step an aqueous solution containing 1.12% by weight of sodium hydroxide and 0.7% by weight of hydrogen peroxide is used as the alkaline solution.

9. The method according to any one of claims 1 to 4, characterised in that in the second step an aqueous solution containing 13.33% by weight of formic acid is used as the acidic solution.

10. The method according to any one of claims 1 to 9, characterised in that in the second step, the alkaline solution or acid solution introduced into the reaction chamber is used at a temperature of 60 to 80°C.

11. The method according to any one of claims 1 to 10, characterised in that in the second step, the reaction system is heated to a temperature of 110 to

12. The method according to any one of claims 1 to 11, characterised in that in the second step, the reaction system is maintained at a pressure of 1.1 to 1.3 MPa for 2 to 3 hours.

13. The method according to one of the claims 1 to 12, characterised in that in the third step, hot steam at 100°C is introduced into the reaction chamber for 15 minutes and the pressure in the chamber is increased to a value of 0.8 to 1 MPa.

14. The method according to any one of claims 1 to 12, characterised in that in the third step, compressed air is introduced into the reaction chamber.

15. The method according to any one of claims 1 to 14, characterised in that in the fifth step, the pulp is washed in a drum washer for 5 to 55 minutes.

16. The method according to claim 15, characterised in that in the fifth step, the pulp is washed in a drum washer for 10 to 20 minutes.

17. The method according to any one of claims 1 to 16, characterised in that in the fifth step, the pulp is washed in a drum washer at a drum speed of 0.5 to 3 rev/minute.

18. The method according to any one of claims 1 to 17, characterised in that in the fifth step, the drum washer is used with a drum diameter of 2000 to 3000 mm.

19. The method according to any one of claims 1 to 18, characterised in that in the fifth step, the pulp is dried in a drum dryer for 15 to 90 minutes at a temperature of 45 to 120°C.

20. The method according to claim 19, characterised in that in the fifth step, the pulp is dried in a drum dryer at a temperature of 70 to 90°C.

21. The method according to any one of claims 1 to 20, characterised in that in the fifth step, the pulp is dried in a drum dryer at a drum speed of 500 to 1500 rev/minute.