A method for manufacturing micro fibrillated cellulose from a cellulosic raw material

Abstract

A method for manufacturing micro fibrillated cellulose from a cellulosic raw material comprises treating the cellulosic raw material by a steam explosion at a temperature of 170°C to 215°C and thereafter refining the treated cellulosic raw material.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a method for manufacturing micro fibrillated cellulose from a cellulosic raw material.BACKGROUND OF THE INVENTION

[0002] One of the disadvantages associated with the prior art methods is that chemicals, such as enzymes, must be used to pretreat cellulose. Without any chemical pretreatments energy consumption may be challenging.BRIEF DESCRIPTION OF THE INVENTION

[0003] An object of the present invention to provide a method so as to overcome the above disadvantages. The objects of the invention are achieved by a method which is characterized by what is stated in the independent claim. The preferred embodiments of the invention are disclosed in the dependent claims.

[0004] The invention is based on the idea of manufacturing micro fibrillated cellulose (MFC) with a low energy consumption. Micro fibrillated cellulose is a material made up of cellulose fibrils obtained from a cellulosic raw material.

[0005] The method possesses a few advantages: No chemicals are required. Energy consumption is decreased compared to conventional methods in such a manner that the same amount of fine fibrils can be achieved with less energy, or more fine fibrils may be achieved by using the same amount of energy. Further, as the method utilizes steam that may be available in large amounts in an industrial scale production there is a reasonable use to the steam. However, although the steam is produced specially for the method the overall energy consumption (steam explosion and refining) is less than in a conventional refining alone. The method is not just based on laboratory tests but the method is scalable, i.e. manufacturing in different industrial scales is possible. Cellulose does not degrade during implementing the method.

[0006] The method is for manufacturing micro fibrillated cellulose from the cellulosic raw material. The cellulosic raw material may be any suitable cellulosic raw material but raw materials that are originated from wood are preferred. The cellulosic raw material may be a wood pulp, such as a kraft pulp. The cellulosic raw material may originate from hardwood and / or softwood species, such as eucalyptus, birch, spruce, or pine species. Non-wood species, such as hemp or cotton, may come into question.

[0007] The method comprises treating the cellulosic raw material by a steam explosion and thereafter refining the treated cellulosic raw material. The method may comprise treating the cellulosic raw material by a steam explosion and after treating the cellulosic raw material refining the treated cellulosic raw material in a refiner without any other step between the steam explosion and the refining.

[0008] The steam explosion takes place at a consistency of 30 to 60 %, preferably at 40 to 45 %. The method may comprise more than one steam explosion cycle in a row. However, one steam explosion cycle has been noticed to be adequate. The aim of the steam explosion is to open the structure of the cellulosic raw material. The steam explosion makes the cellulosic raw material more responsive to the refining.

[0009] The steam explosion takes place in a reactor. The steam explosion is performed below 220°C since hemicellulose decomposes at temperatures over 220°C. The temperature range is from 170°C to 215°C, preferably 185°C to 205°C.

[0010] The steam explosion requires a certain residence time, i.e. a time which the raw material is in the reactor. The residence time may often be from 2 to 6 minutes, preferably from 3 to 5 minutes.

[0011] However, there is a correlation between the temperature and the residence time of the steam explosion. The steam explosion has an intensity. The intensity of the steam explosion may be calculated by using a severity factor R 0 . R 0 = ∫ a b e T t − 100 14.75 ⋅ dt wherein T is the temperature (°C) and t is the residence time (min).

[0012] When there is a constant temperature the severity factor may be calculated by a simplified equation: log 10 R 0 = log 10 t ⋅ e T − 100 14.75

[0013] The severity factor of the steam explosion may be from 2.50 to 3.90, preferably from 2.60 to 3.20.

[0014] The method comprises low consistency refining at a consistency of 3 to 6 % in a refiner after the steam explosion. The consistency may preferably be from 3.5 to 4.5 %. The method may comprise more than one refining cycle in a row. There may be more than one refiner in a row, or the cellulosic raw material is treated in the same refiner more than once. The refiner may be a disc refiner or a cone refiner. Rotation speed of the refiner may be 500 to 2000 rpm, preferably 1200 to 1500 rpm.

[0015] Pilot tests have shown that there is neither essential decomposition of cellulose nor hemicellulose. Thus, cellulosic material is not lost during implementing the method. The microfibrillated cellulose fibers also keep their original properties, such as the fiber length.

[0016] Instead of the low consistency refining the method may comprise high consistency refining at a consistency of at least 20 %. However, the low consistency refining is preferred.

[0017] Due to the steam explosion step before the refining step it is possible to exert more energy in a unit of time on the cellulosic raw material in the refining step. In a consequence one may get more fine fibrils by the same energy consumption, or one may get the same fine fibril content with less energy.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the following the invention will be described in greater detail with reference to the accompanying drawing, in which Figure 1 shows Fines content as a function of the net energy consumption in the LC-refining.DETAILED DESCRIPTION OF THE INVENTION

[0019] Figure 1 shows results of tests explained below. Thus, Fig. 1 and the related description show a few examples how to carry out the invention.

[0020] Six tests were carried out. One of the tests was Reference that was not treated in a steam explosion process.

[0021] Baled bleached eucalyptus kraft pulp (BEKP) was used as a raw material. However, any cellulosic pulp is applicable as the raw material. The bales were disintegrated in a pulper to a mass and the disintegrated BEKP mass was pumped to a tank. The BEKP mass was conveyed from the tank to a roll press. The BEKP mass was dewatered in the roll press to a consistency which was at least 40 %. 500 - 2500 kg of dry pulp were used in each steam explosion test.

[0022] Thereafter the BEKP mass was conveyed to a steam explosion reactor by suitable means, such as a force feed screw and a plug screw feeder that is used for transporting the mass from atmospheric conditions to pressurized conditions. The steam explosion reactor in these tests was a horizontal reactor.

[0023] The mass was disintegrated by a plug breaker in the reactor. The mass entered in a conveyor screw inside the reactor. The conveyor screw transported the mass at a certain speed that was adjusted so that a desired residence time was achieved, i.e. the equation presented above was used for determining the residence time.

[0024] The temperature of the reactor was set by injecting steam in the reactor. The mass was discharged from the reactor through an orifice plate. When the mass was discharged the steam explosion took place due to a pressure drop and thus, fibers in the mass were fibrillated. After the reactor there were a cyclone through which the treated mass was conveyed. Thus, the steam explosion step was completed.

[0025] Some of the tests include more than one steam explosion cycle, see Table 1.

[0026] After the steam explosion a low consistency refining was carried out in a cone refiner at a consistency of 3 to 6 %. Fillings of the cone refiner had the following characteristics: RotorStatorBar width (mm)1.3 - 1.41.3 - 1.4Groove width (mm)2.0 - 2.22.0 - 2.2Groove depth (mm)4.5 - 6.04.5 - 6.0Bar angle32°- 25°33°- 15° However, another type of the refiner, or another type of the fillings may be used. Table 1. Steam explosion conditions for tests 1-5. Tests1 (SE3)2 (SE6)3 (SEO1)4 (SEO2)5 (SEO3)Temperature (°C)170190170190190Pressure (bar(g))* )< 6.911.56.911.511.5Residence time (min)53.53.53.53.5Severity, Log R 0 2.763.192.613.193.19Cycles33112 *) gauge pressure, i.e. pressure above the atmospheric pressure

[0027] After the steam explosion the BEKP mass was tested in regard to furfural, HMF, acetic acid, formic acid, levulinic acid and lactic acid that are decomposition products of cellulose. Such chemicals were not detected so one can conclude that the decomposition did not take place. When there is no decomposition there is not a loss in a yield and the fibers keep their original properties, such as the fiber length.

[0028] Length weighted fines content F1(l) was measured by Valmet Fiber Furnish Analyzer (Valmet Map Q) and expressed in percentages. The length weighted fines include a percentage of smaller fiber-like particles that are small enough to pass through Bauer-McNett 200 mesh screen.

[0029] Fines A % includes a percentage of flakelike fines of the projection area, particles that are shorter than 0.2 mm are included in fines A %.

[0030] Fines B % includes a percentage of lamella-shaped fines. Particles with a width of less than 10 µm and a length of over 0.2 mm are included. The sum length of these particles is divided by the sum length of all measured particles longer that 0.2 mm and then multiplied with 100. Table 2. Reference: Fines content and net energy consumption.Number of LC-refining loops03510121516Fines A (%)14.918.317.920.021.122.324.1Fines B (%)0.440.911.232.583.214.524.91F1(l)22.037.939.546.749.652.757.4SEC net (kWh / t B.D. )** )< 088142264309375437** )< B.D. = bone dry Table 3. Test 1: Fines content and net energy consumption. (SE3) Number of LC-refining loops035101215Fines A (%)24.533.142.566.669.470.9Fines B (%)0.482.374.419.0910.612.1F1(l)42.669.677.588.890.290.5SEC net (kWh / t B.D. )02384188569651005 Table 4. Test 2: Fines content and net energy consumption. (SE6) Number of LC-refining loops035101215Fines A (%)27.045.159.372.876.679.4Fines B (%)0.342.854.055.776.317.68F1(l)48.678.585.891.192.693.6SEC net (kWh / t B.D. )03325599059671045 Table 5: Test 3: Fines content and net energy consumption. (SEO1) Number of LC-refining loops03581012Fines A (%)20.919.621.726.029.635.2Fines B (%)0.461.463.367.0210.214.1F1(l)34.747.455.462.966.771.5SEC net (kWh / t B.D. )0195345533657804 Table 6. Test 4: Fines content and net energy consumption. (SEO2) Number of LC-refining loops03581012Fines A (%)20.121.725.636.550.962.0Fines B (%)0.382.014.49.8414.717.6F1(l)33.454.062.172.881.986.5SEC net (kWh / t B.D. )02173736448801123 Table 7. Test 5: Fines content and net energy consumption. (SEO3) Number of LC-refining loops03581012Fines A (%)24.931.037.154.663.867.8Fines B (%)0.482.154.398.079.2510.9F1(l)42.166.673.183.086.888.4SEC net (kWh / t B.D. )02524227148931046

[0031] Fig. 1 is a graphical view of the length weighted fines F1(l) and net energy consumption SEC net that are shown in tables 2 to 7. Fig. 1 illustrates length weighted fines F1(l) content as a function of the net energy SEC net applied in the LC-refining stage. The net energy SEC net represents a cumulative energy of the LC-refining loops.

[0032] As one can see from Fig. 1, Reference had the lowest content of the length weighted fines when the same amount of the net energy was consumed. Test 3, which included one steam explosion cycle at 170°C, was not remarkably better than Reference. Tests 1 and 2, which included three steam explosion cycles at 170°C and 190°C, respectively, reached the fines content > 90 %. Test 4, which included one steam explosion cycle at 190°C, reached almost the same fines content as Tests 1 and 2. Test 5, which included two steam explosion cycles at 190°C, had almost the same fines content than Test 1, Test 2 and Test 4. Therefore, it was concluded that one steam explosion is sufficient to reach the goal of the invention, i.e. manufacturing MFC in such a manner that the net energy consumption is low.

[0033] It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A method for manufacturing micro fibrillated cellulose from a cellulosic raw material, characterized in that the method comprises treating the cellulosic raw material by a steam explosion at a temperature of 170°C to 215°C and thereafter refining the treated cellulosic raw material.

2. The method according to claim 1, characterized in that the steam explosion takes place at a consistency of 30 to 60 %.

3. The method according to claim 1 or 2, characterized in that the method comprises low consistency refining at a consistency of 3 to 6 %.

4. The method according to any preceding claim, characterized in that the method comprises more than one steam explosion cycles in a row.

5. The method according to any preceding claim, characterized in that the method comprises more than one refining cycles in a row.

6. The method according to any preceding claim, characterized in that the temperature is linked with a residence time by an equation log 10 R 0 = log 10 t ⋅ e T − 100 14.75 7. The method according to claim 1 or 2, characterized in that the method comprises high consistency refining at a consistency of 30 to 45 %.

8. The method according to any preceding claim, characterized in that the cellulosic raw material is eucalyptus.

Images