Patent Application
20150057442
This invention relates to a method for producing cellulose, wherein cellulose based fibre raw material is processed mechanically to separate microfibrils. The invention also relates to an apparatus for preparing nanocellulose.
In the refining of lignocellulose-containing fibres by, for example, a disc refiner or a conical refiner at a low consistency of about 3 to 4%, the structure of the fibre wall is loosened and fibrils or so-called fines are detached from the surface of the fibre. The formed fines and flexible fibres have an advantageous effect on the properties of most paper grades. In the refining of pulp fibres, however, the aim is to retain the length and strength of the fibres. In post-refining of mechanical pulp, the aim is partial fibrillation of the fibres by making the thick fibre wall thinner by refining, for detaching fibrils from the surface of the fibre.
Lignocellulose-containing fibres can also be totally disintegrated into smaller parts by detaching fibrils which act as components in the fibre walls, wherein the particles obtained become significantly smaller in size. The properties of so-called fibril cellulose thus obtained differ significantly from the properties of normal pulp. It is also possible to use fibril cellulose as an additive in papermaking and to increase the internal bond strength and tensile strength of the paper product, as well as to increase the compactness of the paper. Fibril cellulose also differs from pulp in its appearance, because it is gel-like material in which the fibrils are present in a water dispersion. Because of the properties of fibril cellulose, it has become a desired raw material, and products containing it would have several uses in industry, for example as an additive in various compositions.
Fibril cellulose can be isolated as such directly from the fermentation process of some bacteria (including Acetobacter xylinus). However, in view of large-scale production of fibril cellulose, the most promising potential raw material is raw material derived from plants and containing cellulose fibres, particularly wood and fibrous pulp made from it. The production of fibril cellulose from pulp requires the decomposition of the fibres further to the size class of fibrils. In processing, a cellulose fibre suspension is run several times through a homogenization step that generates high shear forces in the material. This can be achieved by guiding the suspension under high pressure repeatedly through a narrow opening where it achieves a high speed. It is also possible to use refiner discs, between which the fibre suspension is introduced several times.
In practice, the production of fibril cellulose from cellulose fibres of the conventional size class can, at present, only be implemented by disc refiners of laboratory scale, which have been developed for the needs of food industry. This technique requires several refining runs in succession, for example 2 to 5 runs, to obtain the size class of nanocellulose. The method is also poorly scalable up to industrial scale.
With the refining surfaces of a disc refiner, the aim is to convey the refining work to the material to be treated by means of hard particles, “grits”, attached to the surface. When trying to produce fibres of a small size class, such as the size class of fibril cellulose, the problem is particularly that the efficiency of the refining work carried out by the surfaces is poor, if the refining gap is wide. On the other hand, if the refining gap is made narrower, the hard grits used in the refining, placed at varying heights in the adhesive of the surface, due to the manufacturing method, are easily worn out or detached. Thus, in practice, the grits are abraded to the basic level of the refining surface or, when detached, leave large pits on the refining surface, wherein points of larger clearance are formed in the refining gap, in which the fibres avoid the refining work.
Due to the large size of the grits used, the surface roughness also becomes so great that free spaces are left on the refining surface which do not process the fibres.
It is an aim of the invention to present a method for fibrillation of cellulose-containing materials, particularly for producing fibril cellulose, by which a better refining efficiency is achieved per unit area of refining surface, and abrasion of the surface can be avoided. For achieving this aim, the method according to the invention is primarily characterized in that the fibres to be refined are subjected to the action of a surface roughness Ra produced by grits elevated from the refining surface, said surface roughness being smaller than 3 μm, advantageously smaller than 2 μm. The refining is preferably performed in a refining gap, whose opposite surfaces have the above defined structure.
Advantageously, the surface roughness Ra on the refining surface is between 0.5 and 1 μm.
With such a “micro roughness” and sharp-cornered grits distributed sufficiently evenly and densely, providing sufficiently hard leading edges of particles in the direction of movement of the refining surface, it is possible to avoid too large a free volume, through which the fibres could pass and avoid the refining work.
In practice, the above described method can be implemented in different refiners, in which the refining gap can be provided between two relatively movable refining surfaces equipped with the above described “micro roughness”. Normally, one of the surfaces is static and the other surface is movable for implementing the above-mentioned relative movement.
In an embodiment, the fibre suspension to be processed is introduced into the refining gap by applying a constant volume supply, and for setting the refining gap optimally in view of the refining work, closing force is applied to the gap. When a constant volume supply is used, which can be implemented e.g. with a positive displacement pump, a given volumetric flow (output) is always obtained, and as a second variable it is possible to use a gap-closing force produced by an external actuator.
The apparatus according to the invention is, in turn, characterized in that the refining surface delimiting the refining gap has a surface roughness Ra provided by elevated grits on the surface, said surface roughness being smaller than 3 μm, advantageously smaller than 2 μm.
Advantageously, the surface roughness Ra of the refining surface is between 0.5 and 1 μm.
Most advantageously, both of the refining surfaces delimiting the refining gap have the above defined structure. They can have, for example, exactly the same surface roughness Ra.
The grits consist of densely placed hard sharp-cornered particles which provide high leading edges advancing with the movement of the refining surface. The particle size of the particles is smaller than 10 μm. The particles can consist of carbide, for example tungsten carbide.
According to an advantageous embodiment of the apparatus, it comprises a fixed displacement pump for feeding the fibre suspension to be processed into the refining gap, as well as an actuator for setting the refining gap by means of the force generated by the actuator.
The shape of the refiner itself can vary. It can be a disc refiner type apparatus, in which the gap between the refining surfaces extends in the radial direction from the axis of rotation of the refiner disc, or a conical refiner type apparatus, in which the gap is formed between conical refining surfaces.
The apparatus can also comprise several refining zones functioning simultaneously and formed by refining gaps of the above described type, in which zones the same pulp to be processed is supplied in parallel, to increase the capacity. The refining surfaces of these refining zones can be coupled mechanically to the same rotation shaft (driving shaft), by which the energy required for refining is introduced into the refining gaps. Pulp to be processed can be supplied to each refining zone with a fixed displacement pump of its own. The refining surfaces of all the refining zones are advantageously similar in surface roughness.
The invention will be described in the following with reference to the appended drawings, in which:
In this application, fibril cellulose refers to cellulose microfibrils or microfibril bundles separated from cellulose based fibre raw material. These fibrils are characterized by a high aspect ratio (length/diameter): their length may exceed 1 μm, whereas the diameter typically remains smaller than 200 nm. The smallest fibrils are in the size class of so-called elementary fibrils, where the diameter is typically 2 to 12 nm. The dimensions and size distribution of the fibrils depend on the refining method and efficiency. Fibril cellulose can be characterized as a cellulose based material, in which the median length of particles (fibrils or fibril bundles) is not greater than 10 μm, for example between 0.2 and 10 μm, advantageously not greater than 1 μm, and the particle diameter is smaller than 1 μm, suitably ranging from 2 nm to 200 nm. Fibril cellulose is characterized by a large specific surface area and a strong ability to form hydrogen bonds. In water dispersion, fibril cellulose typically appears as either light or almost colourless gel-like material. Depending on the fibre raw material, fibril cellulose may also contain small amounts of other wood components, such as hemicellulose or lignin. Often used parallel names for fibril cellulose include nanofibrillated cellulose (NFC), which is often simply called nanocellulose, and microfibrillated cellulose (MFC).
In this application, the term “refining” generally refers to comminuting material mechanically by work applied to the particles, which work may be grinding, crushing or shearing, or a combination of these, or another corresponding action that reduces the particle size. The energy taken by the refining work is normally expressed in terms of energy per processed raw material quantity, in units of e.g. kWh/kg, MWh/ton, or units proportional to these.
The refining is performed at a low consistency for the mixture of fibre raw material and water, the fibre suspension. Hereinbelow, the term pulp will also be used for the mixture of fibre raw material and water subjected to refining. The fibre raw material subjected to refining may refer to whole fibres, parts separated from them, fibril bundles, or fibrils, and typically the pulp is a mixture of such elements, in which the ratios between the components are dependent on the stage of refining.
One possible coating which does not, however, limit the invention, is a tungsten carbide (WC)/cobalt chrome (10Co-4Cr) coating, in which the latter component acts as a binder. By the ratio of the hard particles (grit) and the binder, it is possible to affect the grit density on the refining surface.
The refining surface 1 can be advantageously formed by spraying a mixture containing grits and binder in a suitable ratio onto a substrate.
The particles are as uniform in size as possible, wherein spraying by a suitable spraying method provides a micro coarseness of the surface, which is determined primarily according to the size of the grit particles which rise substantially onto the same level on the surface. As a rule of thumb, it can be said that the surface roughness Ra is about half the particle size of the grits.
Thanks to the small particle size, detachment of single particles does not leave large pits, impairing the refining efficiency, on the refining surface, and the surface roughness remains on a constant level.
If necessary, the surface roughness can be reduced to a desired level by wearing new surfaces against each other, for example by running only water through the refining gap.
The Ra value is measured by a known method with a stylus profilometer, and it is the arithmetic mean value of absolute values of profile deviations from the mean value line of the profile.
The load generated by the actuator A can be based on the pressure of pressurized air or liquid, wherein the load can be measured directly by measuring the pressure of such a medium. The aim is to keep this pressure constant during the refining. The loading actuator A can be coupled, for example, to the rotating shaft of the rotor of the refiner by known mechanical solutions for transmitting a linear movement to the shaft.
Pulp supply by the principle of constant volumetric flow, combined with the force closing the gap and sufficiently smooth (low roughness) surfaces provides a very narrow and uniform gap without direct contact of the refining surfaces. Transient fluctuations in the properties of the pulp supplied are reflected primarily as pressure fluctuations in the supply, which does not substantially disturb the refining. The gap is formed by opposite disc-shaped or conical refining surfaces to have a substantially uniform width so that the refining zone does not comprise a blade grooving or the like which would reduce the surface carrying out the refining work by which the grit affects the fibres.
One variable by which it is also possible to influence the refining result is the speed of motion of the refining surface, which can be changed to change the amount of refining energy introduced in the pulp.
The arrangement shown in
The pulp to be supplied for refining is a mixture of water and fibre material where the fibres have been separated from each other in the preceding manufacturing processes of mechanical pulp or chemical pulp, where the starting material is preferably wood raw material. In the manufacture of fibril cellulose, it is also possible to use cellulosic fibres from other plants, where cellulose fibrils are separable from the fibre structure. A suitable consistency of the low-consistency pulp to be refined is 1.5 to 4.5%, preferably 2 to 4% (weight/weight). The pulp is thus sufficiently dilute so that the starting material fibres can be supplied evenly and in sufficiently swollen form to open them up and to separate the fibrils.
The cellulose fibres of the pulp to be supplied may also be pre-processed enzymatically or chemically, for example to reduce the quantity of hemicellulose. Furthermore, the cellulose fibres may be chemically modified, wherein the cellulose molecules contain functional groups other than in the original cellulose. Such groups include, for example, carboxyl groups or quaternary ammonium (cationic pulp).
Description of a product made by the method.
Fibril cellulose suspension is a gel with strongly shear thinning properties. Typically, its viscosity is measured by a Brookfield viscometer. Complete fibrillation of the fibres takes place as a function of energy consumption, and the proportion of non-degraded pieces of fibre wall contained in fibril cellulose is measured by, for example, a Fiberlab device.
As an example of fibril cellulose made by applying constant volumetric supply,
As shown in the figures, fibril cellulose can be obtained by refining, whereby the viscosity of the aqueous dispersion of the pulp increases as a function of the specific energy (energy consumption), that is, as the specific energy used for refining increases. Consequently, the viscosity of the product and the specific energy used in the method have a positive correlation. It has also been found that fibril cellulose can be obtained by refining, whereby the turbidity and the content of fibre particles reduces as a function of specific energy (energy consumption).
Typically in the method, the aim of low-consistency refining of pulp is to obtain fibril cellulose whose Brookfield viscosity, measured at a consistency of 0.8%, is at least 5,000 mPa.s. In the case of pulp which has been oxidized catalytically before the refining (pulp containing carboxyl groups), for example oxidized by means of TEMPO catalyst, the aim is to obtain fibril cellulose whose Brookfield viscosity, measured at a consistency of 0.8%, is at least 15,000 mPa.s. In addition to the high viscosity, the aqueous fibril cellulose dispersions obtained are also characterized by so-called shear thinning; that is, the viscosity decreases as the shear rate increases.
All refining surfaces were first used for gentle mixing refining at a low specific energy, by which the fibre pulp was first homogenized (fibre aggregates were disintegrated), wherein the specific energy was lower than 0.6 kWh/kg and the viscosity remained lower than 5,000 mPa.s (points in the lower left corner of the graph). Two successive actual fibrillating refining runs (uppermost circles) were performed by the refining surface “W2 opened” (measured Ra=2.83 μm). On the first run through, the viscosity exceeded 25,000 mPa.s, whereas the second run had no longer any effect and the viscosity obtained even decreased slightly. One run was performed on each of the two refining surfaces with different surface roughness, “FS opened” (gentle mixing refining and actual fibrillating refining), the two uppermost squares indicating their end points in the graph. With the surface roughness value Ra of 3.07 μm, a viscosity of about 18,000 to 20,000 mPa.s was obtained, whereas with the surface roughness value of 3.51 μm, the final viscosity was 13,000 to 15,000 mPa.s. With the smooth refining surface “W2 worn down” (Ra=0.49 μm), two successive actual refining runs were performed, in which the viscosity remained at a level of about 10,000 mPa.s. With the smooth refining surfaces “FS smooth” (Ra=0.15 μm), two runs were performed, in which the final viscosity remained lower than 4,000 mPa.s.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20115667 | Jun 2011 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI2012/050642 | 6/20/2012 | WO | 00 | 3/31/2014 |
