The invention relates to a disc refiner comprising a refiner disc rotating around an axis and forming together with an opposite refiner disc a refining gap which is transverse to the axis and has an adjustable width; as well as an inlet for feeding material to be refined into the refiner disc, and an outlet for discharging the refined material passed through the refining gap. The invention also relates to a method for using the refiner. For example, a disc refiner is known which is intended for refining dry material and comprising hard carbide material on the surfaces of the refiner discs forming the refining gap. In such a disc refiner, the material is supplied through a funnel by means of gravity through the centre of one of the refiner discs into the refining gap where it is transferred by the centrifugal force from the centre towards the outer rim. An example of such a refiner type, primarily intended for use in the food industry, is a refiner manufactured by Masuko Sangyo under the product name Supermass colloider®. For example US patent 5,620, 1 5 discloses a refiner of this type, intended for pulverizing particulate foods (for example, spices) by a high-speed pulverizing method. By said refiner, it is possible to pulverize materials at room temperature.

Nanofibril cellulose can be made by refining from lignocellulose-containing fibres. By sufficiently efficient refining, lignocellulose-containing fibres can be totally disintegrated into smaller parts by disintegrating the fibrils acting as components in the fibre walls, wherein the particles obtained become significantly smaller in size scale. The properties of nanofibril cellulose thus obtained differ significantly from the properties of normal pulp. It is also possible to use nanofibril cellulose as an additive in papermaking and to increase the internal bond strength (interlaminar strength) and the tensile strength of the paper product, as well as to increase the tightness of the paper. Nanofibril cellulose also differs from pulp in its appearance, because it is gellike material in which the fibrils are dispersed in water. Because of the properties of nanofibril cellulose, it has become a desired raw material; products containing it would have several uses in industry, for example as an additive in various compositions. Nanofibril 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 nanofibril 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 manufacture of the product from pulp requires the decomposition of the fibres further to the size scale of fibrils. In processing, a cellulose fibre suspension is run several times through a homogenization step that generates high shear forces on the material. This can be achieved by guiding the suspension under high pres- sure repeatedly through a narrow gap where it achieves a high speed. It is also possible to use a disc refiner by introducing the fibre suspension several times between its refiner discs.

Because the aim is to refine the material to very fine powder, the refining gap, i.e. the blade gap, should be almost zero, wherein the material between the refiner discs prevents the refining surfaces from touching each other. The material to be refined should be supplied as evenly as possible, and the distance between the refiner discs should be adjusted very accurately. Furthermore, the work applied in the grinding also produces a lot of heat which has to be transferred away from the refiner, because otherwise the refiner discs are overheated and may be damaged. This is particularly important if the refiner is used for refining for long times.

When nanofibril cellulose is refined mechanically from raw material that con- tains cellulose fibres, it is done in a water suspension where the content of fibre material is relatively low, in the order of a few per cent. When most of the material passing through the process is water, and large amounts of specific energy (in the order of 2 MWh per ton of fibre raw material, or more) are used for achieving a good refining result, a lot of heat is produced. When too much heat is developed, steam bubbles may be produced in the water of the suspension, which bubbles reduce the refining efficiency when entering the refining gap, and break the water film which is essential for keeping the refining surfaces apart. The aim of the invention is to present a refiner in which the refining conditions can be controlled and adjusted accurately to achieve a specific refining grade, for example in the fibrillation of cellulose fibres. The aim of the inven- tion is particularly to present a refiner which can be used at high specific energy values without problems caused by the heating. The aim of the invention is also to present a refiner which can be used continuously. The aim is also to present a refiner which is efficiently cooled.

The aim of the invention is also to present a method for using a refiner, wherein the refining conditions are accurately controlled and adjusted. Furthermore, the aim is to present a continuous refining method. The refiner according to the invention is characterized in what will be presented in the characterizing part of the appended claim 1. The method according to the invention for using the refiner, in turn, is characterized in what will be presented in the characterizing part of the appended claim 1 . The cooling of the material to be processed can be implemented in two ways:

- by placing a cooling element inside the supply chamber, upstream of the refining gap, and providing it with a cooling agent circulation, or

- by providing cooling in the wall of the chamber surrounding the refining gap, downstream of the refining gap.

Consequently, the cooling of the cellulose fibre containing water suspension that enters the refining gap can be implemented in the supply chamber placed upstream of the refining gap and situated between the refiner discs, and/or the cooling is arranged in the wall of a chamber surrounding the refining gap, onto which the material passed through the refining gap is thrown and along which it flows down. For implementing the cooling, this wall can be equipped with a cooling agent circulation.

Both methods can be used simultaneously during the refining process; in other words, the material is cooled in contact with the cooling surfaces, both shortly upstream of and immediately downstream of the refining gap.

In the first alternative, the refiner comprises a cooling element placed in the supply chamber which is situated centrally with respect to the rotation axis and which is limited in the radial direction by an annular refining zone with a given width, where the surfaces of the refiner discs are placed against each other, forming the refining gap. The heat transfer surface of the cooling element is placed approximately concentrically with the rotation axis. The cooling element can be placed, for example, in a lid closing the stator of the refiner, on the inside of the lid. Through the same lid, it is possible to introduce an inlet for the material to be refined, a possible return duct for the same, as well as the ducts required for the cooling agent circulation of the cooling element. Heat transferred from the refiner discs to the suspension in the supply chamber can be removed through a heat transfer surface in contact with the suspension. In another advantageous embodiment, the refiner comprises a cooling agent circulation in the wall of the chamber surrounding the refiner discs, said wall forming the upper part of the outlet.

In yet another embodiment, the refiner can be equipped with cooling in such a way that a return duct is provided from said supply chamber that is upstream of the refining gap in the supply direction, through which duct part of the material supplied through the inlet can be returned. Thus, the refiner, particularly the refiner discs, can be cooled by that flow of the material supplied into the refiner which is not passed via the refining gap. The return line of the material is advantageously provided with a controllable valve, by which the pressure in the supply chamber can be controlled by causing a flow resistance in the flow in the return linr. The valve can also be totally closed, wherein there is no return flow and the cooling is implemented by means of a cooling element placed in the supply chamber.

The return circulation of material is also provided with cooling, for cooling the material which was heated up in the supply chamber. The cooling can be arranged in the return line upstream of the supply container, in the supply container, or in the supply line between the supply container and the inlet.

Advantageously, the refiner comprises a feeding pump which is in contact with the inlet for supplying the material to be refined to the refining gap, and for generating a pressure in the inlet and in the supply chamber downstream of the inlet. Thanks to its rotating speed, the refiner is capable of sucking material to be refined through the refining gap; as a result, it would be possible to use a conventional supply funnel for feeding the material. By means of a pump, it is possible to generate pressure between the refiner discs. If the pump used is a fixed displacement pump, however, it is possible to keep the amount of material to be refined in the refining process more constant and to achieve a product of uniform quality. By controlling the fixed displacement pump, it is possible to set the material flow passing through the refining process as desired with respect to the power input in the refining.

According to an embodiment, the refiner also comprises a control unit arranged to define the refining power and to change one or more of the control variables of the refining on the basis of the refining power.

For example, it is possible to set a desired set value for the refining power, or a desired range for the refining power, in the control unit. When the refining power is used in this way as the primary control parameter, it can be determined from, for example, an electric variable relating to the drive of the motor, which variable can be measured quickly and to which a response can be given in a short response time. The refining power can be determined, for example, from the power input in the motor that rotates the rotor of the refiner. In particular, as a variable to be measured it is possible to use the power input in the motor by an inverter (frequency converter) that controls the motor.

As the control variables for influencing the power input, it is possible to use, for example, the rotation speed of the rotor, or the refining gap (blade gap). The refiner is provided with an accurate regulating motor for adjusting the relative position of the rotor and the stator, to achieve a stepless adjustment of the refining gap at the accuracy of micrometers.

The refining power used as a basis for the adjustment can be determined by calculation, advantageously so that from the measured power input in the motor, a so-called zero power is subtracted, which has been obtained by driving the refiner without a material supply and with a refining gap (blade gap) larger than the refining gaps used in the refining process, at the same rotation speed (rpm). Determinations can be made with different rotational speeds.

When the pump connected to the inlet of the refiner is a fixed displacement pump, a fixed volumetric flow through the refiner and a constant output are obtained. When the refining power is used as a control parameter in this context, it is possible to make the specific energy (work per processed pulp) constant in the refining process and to obtain a product of uniform quality, when the dry matter content of the material to be processed is known.

For carrying out the refining process with one refiner in a continuous manner, the refiner also comprises two supply containers connected to the same inlet of the refiner. Furthermore, the supply containers are connected to the same outlet of the refiner. Moreover, the ducts between the containers and the inlet, as well as the ducts between the containers and the outlet, are provided with valves, by which the containers can be selectively connected to the inlet of the refiner, and the outlet of the refiner can be selectively connected to the containers, respectively. In this way, one of the containers can be used at a time as the container that supplies material to be refined to the refining gap, whereby the other container thus acts as a container that receives refined material passed through the refining gap. When the material to be supplied from the first container has been diminished to a given level, the valves can be used to change the supply to occur from the other container containing material previously passed through the refiner, and to change the emptied container to receive material passed through the refining gap. In this way, the number of refining runs can be increased to a given number by using the same refiner in a continuous manner. Furthermore, only one feeding pump is needed in the supply line between the containers and the inlet downstream of the valve, which pump can be the above-mentioned fixed displacement pump.

Depending on the starting material and/or the refining power (specific energy) used, the refining process can also be performed by running the material only once through the refining gap. In this case, no alternation of the supply containers is needed, but a return circulation can be used for cooling. The invention also comprises an alternative, in which refiners are connected in series for implementing two refining runs. In this case, both refiners can be equipped with the above-mentioned cooling arrangement. Also, the above- mentioned control system can be used in both refiners, and both refiners can be supplied by a fixed displacement pump. It is also possible to use three refiners in series with analogous arrangements. In the following, the invention will be described in more detail with reference to the appended drawings, in which:

Fig. 1 shows an arrangement for controlling the temperature in the supply chamber of a refiner,

Fig. 2 shows another solution in the supply chamber of the refiner,

Fig. 3 shows the principle of controlling the refiner according to the invention in a schematic view,

Fig. 4 shows a refiner for implementing several refining runs in a continuous manner, and

Fig. 5 shows a refiner equipped with a servo motor.

In this application, the term "refining" generally refers to comminuting material mechanically by applying work on 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, the specific energy, 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. Figure 1 shows the structure of the refiner in a cross-section in the direction parallel to the rotation axis. The component performing the actual refining is a disc refiner 1 comprising two refiner discs with a relative mutual rotating movement. One of the discs is a stator 1 a placed in a fixed position, and the other is a rotor 1 b placed opposite to it and rotating in relation to a rotation axis A. A refining gap 1 c is formed between the rotor and the stator, in a direction perpendicular to the axis A. This refining gap is also called a blade gap. The material to be refined, which is preferably lignocellulosic fibres in an aqueous suspension, is supplied concentrically with the rotation axis A through the stator 1 a via an inlet 2 to a supply chamber 8, from which it is radially distributed into the refining gap 1c and discharged via an outlet 3 on the outer periphery of the discs. In the refining gap 1 c, the material is subjected to refining energy which disintegrates it and which can be expressed, for example, in terms of specific energy consumption (energy applied / processed pulp).

The supply chamber 18 tapers in a wedge-like manner towards the refining gap. This part of the rotor is provided with grooves for intensifying the guiding of the material towards the refining gap by the effect of the centrifugal force (not shown). The refining zone itself, i.e. the refining gap 1c, is a relatively short annular zone in the radial direction, at the marginal zone of the refiner discs, formed between the refining surfaces facing each other.

The refining surfaces can, in a way known as such, comprise hard wear- resistant particles or "grits" fastened to a binder. However, the method is not limited to refiner surfaces with a given structure. For absorbing heat produced in the refining, the inside of the supply chamber is provided with a cooling element 19 which is fastened to the lower side of a lid 20 covering the opening of the stator 1 a. The cooling element 19 is an annular structure placed concentrically with the rotation axis A, and its outer surfaces act as heat transfer surfaces in contact with the material in the supply chamber 18. Inside the annular structure, a channel is provided for the cooling agent which is supplied through an inlet 19a and discharged through an outlet 19b. The inlet and the outlet are provided in the same lid 20 as the inlet 2 for the material to be refined. The annular channel is closed by an intermediate wall between the inlet and the outlet, so that cooling agent can circulate a whole circle. Reference numeral 19c denotes an insert, by means of which the flow of cooling agent can be limited to the vicinity of the outer surfaces of the element 19, and the linear flow velocity can be increased to intensify the heat transfer. The heat transfer medium can be water or another cooling agent which is cooled in a circulation outside the element 19.

The material passed through the refining gap is thrown into a chamber 3a that encircles the outer periphery of the discs 1a, 1b and constitutes the upper part of the outlet, from which it flows to an annular space located partly below the rotor 1 b and forming the lower part 3b of the outlet; the bottom of the annular space being connected to an outlet line 5. For immediate cooling of the material coming from the refining gap c, the outer wall 3c of the outlet, onto which the material is thrown and along which it flows down, can be provided with cooling; for example, the outer wall of the outlet can be equipped with a jacket 3d (shown with a broken line) provided with a cooling agent circulation. Figure 2 shows another arrangement for cooling the material in the supply chamber. A return duct 9 is connected to the supply chamber, through which duct part of the material supplied to the supply chamber 18 returns, forming a return circulation for such material that has not passed through the refining gap 1 c. This material is capable of transferring heat produced by the refiner discs away from the refining chamber. As seen in the figure, the orifice of the inlet 2 opening into the supply chamber 18 is oriented towards the space that tapers in a wedge-like manner at the edges of the supply chamber, to guide the material to be supplied to the vicinity of the refining gap 1 c. The inlet 2 is introduced in the supply chamber through the cooling element 9. In a corre- sponding manner, the return duct 9 opens more centrally into the supply chamber 18, inside the ring formed by the cooling element 19. Depending on the rotation speed and the supplied volume flow, it is thus possible that the supply chamber 18 is not filled completely with the material, but all the material is guided to the edges of the feeding chamber 18 because of the centrifu- gal force, and an empty space is left around the rotation axis A. Thus, the cooling takes place through the heating element 19, because there is no return flow via the return duct 9. Both the inlet 1 and the return duct 9 are placed in the lid covering the opening of the stator 1 a. Figure 3 shows schematically a system for controlling the refiner, and here the same parts are denoted with the same reference numerals. The rotor 1 b is rotated by an electric motor M1 , which is supplied with energy by an inverter, i.e. a frequency converter INV. The position of the rotor 1 b with respect to the stator 1 a and simultaneously the width of the refining gap 1 c are adjusted with a separate motor M2 which is capable of adjusting the gap in the precision of micrometers in a stepless manner.

The material to be refined is supplied along a supply line 4 to the inlet 2 from a supply container (not shown). Refined material that has passed through the refining gap 1 c is, in turn, removed via an outlet line 5 to an outlet container (not shown). The supply line 4 is provided with a feeding pump P1 which is a fixed displacement pump, whereby it produces a given pressure in the inlet 2, and the material is carried through the refining gap 1c by the pressure difference between the inlet 2 and the outlet 3. The volumetric flow generated by the pump P1 can be adjusted and set to a constant value; in other words, if desired, material can be run with a predetermined output through the refiner. In this way, a product with a uniform quality is achieved.

Figure 3 shows a control unit C for controlling the refining process. The control unit is arranged to determine the power taken by the refining, to compare it with a pre-set value of the power or power range, and to control one or more actuators of the refiner which influence such control variables of refining, by which the power can be changed, for achieving a set value or a set value range of the power in this way. The system in question is a closed control loop, i.e. feedback control. The control unit C makes the adjustments automatically on the basis of the data entered by the user and the measurement data received.

The control unit C receives the signal needed for determining the power from the inverter INV along a signal transmission line 6. The control unit C communicates with the actuators via signal transmission lines 7, 8. Via the signal transmission line 7, the control unit C is configured to control the motor M2 that adjusts the width of the refining gap 1c. In a corresponding manner, via the signal transmission line 8, the control unit C controls the inverter INV in such a way that this control affects the rotation speed of the motor M1 and, correspondingly, the rotation speed of the rotor 1 b. The pressing of the rotor and the stator towards each other as the rotor rotates affects the power input in the inverter INV and thereby the refining power. In practice, a desired refining power can be achieved by adjusting the refining gap 1c by means of the motor M2. Also, the shear rate in the refining gap, that is, the rotation speed of the rotor 1 b, affects the power intake, so that by changing the rota- tion speed it is also possible to adjust the refining power.

The motor M2 is configured to transfer the rotor 1 b accurately in the direction of the rotation axis A. The power transmission without a clearance between the motor and the rotor consists of a screw parallel to the axis and a planar toothed gear, by which the height of the rotor disc can be adjusted, and the mechanism will be described in more detail hereinbelow. When determining the refining power, the zero power is taken into account, which is subtracted from the power taken by the inverter. The zero power is determined for different rotation speeds (rpm) of the rotor with a refining gap larger than normal (about 1 mm), the refining gap being empty. In the control unit C, the zero power obtained for a given rotation speed is subtracted from the power value obtained from the inverter INV at the same rotation speed, resulting in the value of the refining power that is used as a basis for the control. The control unit C is based on a microprocessor, and it also comprises data input means for entering and changing values and data relating to the control. The control can be implemented with a P controller, a PI controller, or a PID controller.

Figure 4 shows a system, by which material flows entering and having passed the refining can be controlled as a whole in such a way that, on one hand, good cooling of the refiner is achieved, as already described above, and on the other hand, the same refiner can be used for refining the same material batch several times in succession in a continuous manner. In addition to the control system shown in Fig. 3, the refiner comprises two supply containers 10a and 10b, which are both connected to the supply line 4 and the return line 5. During the refining, one of the containers is always used as a supply container, from which material is supplied for refining, while the other container is used as an outlet container, into which material originating in the supply container and having passed through the refiner (the refining gap 1c) is discharged.

The containers 10a, 10b are connected to the supply line 4 via a valve 12, by means of which either one of the containers can be selectively set in flow connection with the supply line 4. In a corresponding manner, both of the containers 10a, 10b are connected to the outlet line 5 via a valve 13, by means of which either one of the containers can be selectively set in flow connection with the outlet line 5. The feeding pump P1 is downstream of the valve 12 in the supply line 4. Furthermore, each of the containers 10a, 10b is provided with a sensor S, for example a level sensor, for detecting the quantity of material therein. The sensors S are connected via signal transmission lines 14 to the control unit C which is, in turn, configured to control said valves 12, 13 via signal transmission lines 15 in such a way that when the quantity of material in the container acting as the supply container decreases below a given limit, as detected by the sensor S, the position of the valves 12, 13 is changed in such a way that the container that received the refined material starts to act as the supply container, and the container that acted as the supply container and is becoming empty starts to act as the outlet container. When the new supply container becomes empty, a corresponding sequence is followed. Acting in this way, the number of refining runs with a single refiner and a single pump can be increased as desired without discontinuing the process.

The valves 2, 13 can be normal electrically controlled directional valves. Furthermore, the outlet line 5 is provided with an intermediate container 0c, to which the material discharged via the outlet 5 is first introduced and from which it can be pumped by a discharge pump P2 into the containers 10a, 10b. Figure 4 also shows a cooling system for the refiner. Part of the material flow supplied via the inlet 2 is returned via the return duct 9 and the return line 1 1 to the supply container 10a, 10b from which it was supplied, without entering the refining gap 1 c. This material is used in the refiner as an internal cooling agent for the supply chamber 18, as shown above in Fig. 2. The control unit C controls a control valve 16 in the return line 1 1 , wherein the return flow from the supply chamber 18 can be controlled, and simultaneously a desired pressure level can be adjusted for the supply chamber. Furthermore, the return line 1 1 is provided with a valve 17 downstream of the control valve 16, for guiding the return flow of the material that was used as the cooling agent to the container 10a, 10b used as the supply container at the time. The control unit C is in control connection via the signal transmission line 15 with the valve 17 which may also be an electrically controlled directional valve.

By means of the control valve 16, a given resistance is provided in the return flow, and in this way it is also possible to maintain such an overpressure in the supply chamber 18, if necessary, that it is possible to determine the temperature at which steam bubbles begin to form in the water suspension, and the magnitude of this overpressure can be simultaneously adjusted by the control valve 16.

The containers 10a, 10b and the intermediate container 10c are provided with agitators for keeping the material contained in the containers homogeneous over the whole volume. The containers 10a, 10b are also provided with cooling, for cooling particularly the material returning to the supply chamber 18 via the return circulation (return duct 9 - return line 1 1 supply chamber 10a, 10b - supply line 4). The cooling can be arranged, for example, in the jacket of the containers 10a, 10b. The cooling can also be arranged downstream of the return duct 9 in the return line 1 1 , or in the supply line 4. For cooling the material coming from the refining, the intermediate container 10c which is simultaneously used as a buffer container in the system, is also provided with cooling.

Moreover, a pressure sensor is introduced in the supply chamber 18, for measuring the pressure prevailing inside the supply chamber and thereby upstream of the refining gap. This pressure can be adjusted by the control unit C by controlling, for example, the valve 16 in the return line 1 1 on the basis of the pressure data.

Figure 5 shows the assembly of a regulating motor. The regulating motor 21 , which is an actuator for varying the width of the refining gap 1 c (the distance between the opposite refining surfaces) and corresponds to the motor M2 shown in Fig. 3, is a servomotor which is mounted outside the frame of the refiner, and it is connected by a chain 22 to a toothed gear 23 provided around the driving shaft of the rotor, with the plane of the gear ring perpendicularly to the rotation axis A. The toothed gear 23 is rotated, controlled by the regulating motor 21 , concentrically around the rotation axis A, and it rotates a cylindrical piece 24 which is concentric with the rotation axis A and whose height level is changed during the rotary movement because it has an outer thread engaging the inner thread having a fixed position with respect to the frame. The pair of the outer and inner threads is denoted with the reference numeral 25. The upper end of the cylindrical piece is fastened to the supporting plate 26 of the rotor, which can thus be lifted and lowered by the cylindrical piece 24 and simultaneously the rotor 1 b itself can be lifted and lowered. Even if the refiner presented above comprises horizontal refiner discs, and the upper disc is the stator and the lower disc is the rotor, the situation can also be vice versa. Similarly, the refiner discs can be located upright, that is, the rotation axis A being horizontal. Because material is supplied to the refining gap by the high rotation speeds of the rotor as such, material can be supplied into the supply chamber from above, from below or from the side, depending on the position of the refiner, and the material to be refined will be evenly distributed in the refining zone encircling the supply chamber.

Furthermore, the refining gap does not need to be straight and perpendicular to the rotation axis A, as in the figures, but it may also be conical, in which case the facing surfaces of the stator and rotor are conical, the rotation axis A being the central axis.

After the last refiner in a series of refiners, cooling of the product is not necessarily needed, because the elevated temperature is advantageous in view of some processes of further treatment. The same applies to the measures after a refiner refining the product in a single run through.

The refiner is used for making nanofibril cellulose from a cellulose based raw material. In this application, nanofibril 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 pm, whereas the diameter typically remains smaller than 200 nm. The smallest fibrils are in the scale of so-called elementary fibrils, the diameter being typically 2 to 12 nm. The dimensions and size distribution of the fibrils depend on the refining method and efficiency. Nanofibril 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 pm, for example between 0.2 and 10 pm, advantageously not greater than 1 pm, and the particle diameter is smaller than 1 pm, suitably ranging from 2 nm to 200 nm. Nanofibril cellulose is characterized by a large specific surface area and a strong ability to form hydrogen bonds. In water dispersion, nanofibril cellulose typically appears as either light or almost colourless gel-like material. Depending on the fibre raw material, nanofibril cellulose may also contain small amounts of other wood components, such as hemicellulose or lig- nin. Often used parallel names for nanofibril cellulose include nanofibrillated cellulose (NFC), which is often simply called nanocellulose, and microfibril- lated cellulose (MFC). In the manufacture of nanofibril cellulose, the material to be supplied into a refiner is a mixture of fibre raw material and water, a fibre suspension, particularly low-consistency pulp. In the material to be supplied to the refiner for the first time, the fibres have already 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 nanofibril cellulose, it is also possible to use cellulose 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) in an aqueous medium. The pulp is thus suffi- ciently 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. In the case of several refining runs, the material to be fed to refining may refer to whole fibres, parts separated from them, fibril bundles, or fibrils, typically a mixture of such elements, in which the ratios between the components are dependent on the refining run.

The cellulose fibres of the material to be supplied may also be pre-processed enzymatically or chemically, for example to reduce the quantity of hemicellu- lose. 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 (for example, cellulose oxidized by N-oxyl-mediated oxidation) or quaternary ammonium (cationic pulp). As the final result, the nanofibril cellulose suspension obtained after several refining runs 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-disintegrated pieces of fibre wall contained in nanofibril cellulose is measured by, for example, Fiberlab equipment. Among other things, it is typical of the obtained product that when the material to be refined is an aqueous suspension that contains cellulose fibres, there is a clear positive correlation between the viscosity of the nanofibril cellulose product and the specific refining energy. As a guideline value, which is not to be considered limiting, it can be said that the aim of the refining is a product whose Brookfield viscosity, measured at a consistency of 1.5%, is at least 2000 mPa.s, advantageously at least 5000 mPa.s (10 rpm). By the method, values higher than 7000 mPa.s can be easily obtained, measured under the same conditions. The viscosity was measured by a Brookfield rotation viscometer at a rotational speed of 10 rpm by using a so-called "vane spindle" sensor which is suitable for testing heterogeneous viscose materials.

The fibre analysis can be performed by a method based on a high-precision high-resolution microscopy and an image analysis which is suitable for the quantitative analysis of micro and nano scale fibres in the NFC, and in which the non-fibrillated fibrous material is determined from the NFC. The quantity of detectable fibres or fibrous particles is measured in a known pulp sample quantity, and the rest of the sample is considered to belong to the non- detectable category, that is, particles in the micro and nano scale. For characterizing non-fibrillated fibrous material in the nanofibril cellulose, it is possible to apply commercial fibre analyzers. For example, Kajaani Fibrelab equipment and FS-300 equipment is suitable, but it is also possible to use other similar fibre analyzers having a similar resolution.

The fibre analysis comprises steps in which the dry mass of the sample is determined for analysis, after which the volume is scaled by diluting and sampling. If necessary, it is possible to use a larger sample size than with normal pulp mass samples, for increasing the number of fibres detected in the analysis.

For simplicity, the quantitative measure of particles / milligram is used.

By way of example, it can be mentioned that the Fiberlab values for the obtained nanofibril cellulose, determined in the above presented way, are advantageously lower than 20,000 particles per mg, more advantageously lower than 10,000 particles per mg. Examples

The devices and conditions presented in the examples should not be considered to limit the invention, but they are only presented as one way of applying the invention.

The refiner used was a "Supermasscolloider" refiner manufactured by Masuko Sangyo KK under the type number MKZB20-100J, having refiner discs with a diameter of 500 mm. The refiner was modified as described above.

The raw material was bleached and lightly pre-refined (70 kWh/t) birch pulp.

As shown in the table above, cooling can be used for supplying energy to refining efficiently in even one step. With a specific energy consumption higher than 2.0 MWh/t, it has been possible to achieve a Brookfield viscosity clearly higher than 7000 mPa.s, measured at a consistency of 1.5% ( 0 rpm). Thanks to its Theological properties, fibril strength properties, as well as the translucency of the products made from it, the nanofibril cellulose obtained by the method can be applied in many uses, for example as a Theological modifier and a viscosity regulator, and as elements in different structures, for example as a reinforcement. Nanofibril cellulose can be used, among other things, in oil fields as a rheological modifier and a sealing agent. Similarly, nanofibril cellulose can be used as an additive in various medical and cos- metic products, as a reinforcement in composite materials, and as an ingredient in paper products. This list is not intended to be exhaustive, but nano- fibril cellulose can also be applied in other uses, if it is found to have properties suitable for them.