Technical Field

The invention relates to production of lignocellulosic material from raw wood by mechanical grinding. In particular, the invention relates to an apparatus comprising a grinding wheel and means for feeding raw wood material against the grinding wheel to produce fine material. In addition, the invention relates to a corresponding grinding method.

Background Art

Cellulosic nano- or microfibrillated materials are today among the most interesting renewable raw materials. In addition to the obvious environmental reasons, its nontoxicity and availability of cellulose, the interest for fibrillar material is due to its extraordinarily high specific strength, thermal stability, hydrophilicity, and broad capacity for chemical modification. Particularly in polymer composite technology, cellulosic small fibrillar materials have potential in novel products with a range of enhanced properties. One substantial production expense of nano- or microcellulose comes from the energy required for defibrillation of the starting materials.

Mechanical pulping is a common method for producing fibrillar material. Mechanical pulps are produced by specified attrition processes. Groundwood (GW), pressure groundwood (PGW), and thermomechanical pulp (TMP) all apply similar shear and compression forces to fresh wood and fibres. In grinding, a high strain rate of cyclic compressive loads heat, loosen and fatigues the wood, whereas shear loads free the hot fatigued fibres from the solid wood. As a result, a competent pulp is obtained. As a result of the process, mechanical pulps comprise a continuum of different kinds of particles, conventionally from long fibres of up to 3 mm to shorter fibres, and debris or fine materials of very small dimensions. In papermaking, these very small objects down to a characteristic size of only few micrometres play an important role in the development of tensile strength and stiffness as well as of the light scattering and the formation and structure of the paper web. Depending on the production process, the fines can be flaky or fibrillar. Fibrillar fines are like yarns in that they have a larger specific surface area and are, therefore, favoured over flake and chunky fines as regards strength development.

Grinding, invented in 1843 by F.G. Keller in Germany, is the oldest mechanical pulping process and it is still a viable process that produces fibres and fines with high yield and low cost compared to other mechanical pulping processes and to chemical pulping. In grinding, the surface of wood is typically treated using a ceramic grinding wheel, also called grinding stone, having an abrasive grinding surface, in the presence of water. As a result, fibres and fines are separated from the wood. Conventional grinding surfaces are flat or contain small grooves in the axial direction of the grinding wheel to enhance the process. Recently, a grinding surface with wave-like profile in the circumferential direction has also been disclosed for example in EP 1896651 to improve energy efficiency of the surface. In more detail, this concept features a grinding surface superposed with sinusoidal radius modulation that is hypothesized to generate additional fatigue in the wood compared to a traditional flat grinding stone.

To introduce another kind of a grinding arrangement, US 4,560,439 discloses a grinding system where a feeder pushes wood chips into a chamber wherein a piston compresses the chips into a grinder zone between grinding members. The compression direction of the piston is parallel to an axis of a rotatable grinding member which has grinding protrusions in saw teeth configuration or otherwise overlapping each other in the direction of movement. DE 3210321 discloses a system which utilizes relative oscillation of two working means, between which there is a narrow column and through which column the feedstock is moved, which causes defibering of the material.

In so called transversal wood grinding, logs are pressed against a rotating stone, and the log axis as well as the longitudinal axis of the fibres is aimed parallel to the rotation axis of the grinding stone. Thus, the stone surface velocity is perpendicular to the fibres axis. There can be deviation from transversal grinding in the stone tangential plane or out from this plane, or both simultaneously. If the fibres are aligned so that the surface velocity is parallel to the fiber axis, potentially with an additional angle between the fibre length and the stone tangent, the grinding is called longitudinal. The aspects of wood alignment have been studied earlier in for example Brauns, O., and Gavelin, G. (1959). "Groundwood quality at different angels between stone surface and wood." Svensk Papperstidn. 62(3), 67-70. The article discloses that freeness decreased very sharply as the angle between the wood and the stone tangent and the energy consumption increased. Accordingly, the wood angle has neither an effect on pulp strength nor on energy consumption if the pulp is ground to the same freeness level. On the other hand, the article Alfthan, v. G. (1970). "Influence de la position du bois", Revue A. T.I.P. 24(6), 241-259, discusses the effect of the wood angle in the tangential plane of the grinding stone and out of this plane. It was found that longitudinal grinding increases the long fibre fraction and fines fraction, while the amount of middle fractions decreases. An article by Beath, "The Varying Angle Between Stone Surface and Wood - A Cause of Wide, Uncontrolled Freeness Variation in Groundwood Production", 1958, has also studied grinding under various feed angles of the wood material, using a conventional cylindrical grinding stone.

Longitudinal grinding has been studied by Stationwala, M, "Disk Grinding", 2003 International Mechanical Pulping conference, Quebec, Que, Canada, 2-5 June 2003, pp 153-162. The aim of this study was to increase long fiber content of pulp produced in longitudinal feed orientations.

Properties of fibrous masses has also been altered by different methods, for example using a supermass colloider, such as in Dooley, N. and Weinberg, G., 63rd Appita Annual Conference and Exhibition, 19-22 Apr. 2009, Melbourne, Australia, pp 115-120. The coarse grinding method disclosed therein is, however, relatively slow and required a lot of energy.

Grinding is still the most energy- efficient method of mechanical pulp production. One problem of prior art, including the publications cited above, is that there is no industry- scale method for producing a large proportion of fine fibrils compared with other particle sizes using grinding. Existing methods suffer from production capacity, energy efficiency or the presence of relatively large amounts of unwanted particles. Pure fibrillar fines as large specific surface area lignocellulosic particles are, however, becoming more and more appealing in many paper and board applications and other forest industry applications too. Firstly, they can be used to produce surface layer material for board and paper grades. In foam forming, a thin and pure layer of fines could be attached on the surface of bulky base material. Secondly, composites will benefit from new kinds of lignin-rich high-aspect ratio materials. Also pure flaky fines improve bulk and light scattering properties acting as organic recyclable filler.

There is thus an industrial need for novel methods for producing large amounts of fine lignocellulosic fibrillar material.

Summary of the Invention

Technical Problem It is an aim of the invention to provide a novel apparatus and method for production of fine fibrillar or fine flaky lignocellulosic particles. A particular aim is to provide an apparatus and method which suit for industry-scale use.

An addition aim of the invention is to provide an apparatus and method whose

implementation costs are low in existing pulp-production environments.

The invention is based on the idea of providing a grinding apparatus with a grinding wheel with a surface profile serrated in the axial direction of the wheel. The serration comprises surface features which are at an oblique angle with respect to the rotation axis of the wheel. This is in contrast with prior art surface profiling schemes, in which there may be variation only in the circumferential direction of the wheel (although such profiling additionally is not excluded). When raw wood material comprising fibers at least mainly oriented along the longitudinal direction of the raw wood material are fed to the presently profiled wheel in transverse or tangentially rotated oblique angles, the fibers of the raw wood material meet the grinding surface at an oblique angle at the regions of the oblique surface features. This prevents fibrous material from separating from the wood as entire long fibers or fiber sections, but as short sections, i.e., fine particles. Fiber liberation is prevented because fibers are stuck by the neighboring fibers. This leads to emphasized production of fine lignocellular material.

Preferably, the serrated surface profile covers the entire cylindrical surface of the wheel (or at least the portion where the raw material is fed), whereby a vast majority of the fibers meet the surface at oblique angle. This is true even if the fibre axis of the raw wood material is parallel to the rotation axis (transverse grinding setup), as is the case in a preferred embodiment of the invention. The apparatus according to the invention comprises a rotatable grinding wheel having a rotation axis and around the rotation axis a grinding surface capable of separating fine material from wood by grinding effect and a feeder for feeding raw wood material against the grinding surface during rotation of the grinding wheel. The grinding surface comprises a serrated surface profile in the axial direction of the wheel. In other words, when viewed in the radial cross-sectional plane of the wheel, the edge of the wheel has a serrated shape. In particular, the serration comprises a pattern of surface features having oblique angle with respect to the rotation axis. In this way, even fibers fed to the grinder in purely transversal grinding orientation, meet the grinding surface in oblique angle and are ground to finer particles.

The present method for producing fine lignocellulosic material comprises rotating a generally cylindrical grinding wheel having a grinding surface, and feeding raw wood material against the grinding surface for producing said fine material by grinding. In the method, the grinding wheel comprises a serrated grinding surface profile having a pattern of surface features at oblique angle with respect to the rotation axis. Feeding is preferably carried out such that so that average fiber orientation of the raw wood material is in a tangential plane of the grinding surface.

More specifically, the invention is characterized by what is stated in the independent claims.

The invention provides considerable advantages. First, the grinding wheel according to the invention results in considerably higher amount of fine material in the produced pulp as compared with other fiber fractions. The invention thus allows for separation of the fine material production from long fiber production. As the fine fraction is larger or even close to 100%, less or no post-processing at all is needed in order to obtain an essentially pure fines-containing mass. This provides significant cost- savings.

Second, the invention has very low investment and operating costs, as it can exploit existing grinding apparatuses, with only a straight-forward modification to or replacement of a conventional grinding wheel as a minimum. Optional modifications include replacement or provision of a serrated finger bar and a wheel profile-maintaining

("sharpening") arrangement. In a typically-sized pulp production facility, the estimated small particle production rate is 50 t/d, which exceeds previous rates significantly. Estimated cost saving for producing light-weight coat (LWC) are 30€/t, to mention one example only.

In addition, flaky fines are interest as far as properties of light scattering and bulk are concerned. Flaky fines are also organic filler and therefore environmentally beneficial. By adjusting grinding parameters the process can produce fibrillar or flaky fines.

Brief Description of the Drawings Fig. 1 shows a grinding apparatus set-up according to one embodiment of the invention. Figs. 2A-C shows a detailed illustration of grinding wheel surface profiles according to alternative embodiments of the invention.

Fig. 3 illustrates a profiled finger bar arrangement according to one embodiment of the invention.

Figs. 4A and 4B depict grinding wheel sharpening arrangements according to alternative embodiments. Figs. 5A-C shows schematically how material, in this specific case fibrillar material, is separated from wood fibers with different surface angles of the grinding wheel.

Figs. 6A and 6B illustrate alignment of wood with respect to a grinding wheel used in grinding trials.

Figs. 7A and 7B illustrate as a graph the percentage of Bauer McNett Fractions shown as a function of radial and tangential alignment between fibre and stone, respectively.

Figs. 7C and 7D show graphs of length- weighted average fibre length of pulp and specific sedimentation volume of fines, respectively, versus the radial and tangential grinding angle.

Fig. 7E shows a graph of specific energy consumption, fines amount, and fines quality in grinding versus radial alignment between wood and a grinding stone.

Figs. 8A and 8B and

Figs. 9A to 9C illustrate the effect of environmental parameters on the energy consumption of the grinding process and properties of the resulting fibrillar product.

Embodiments

Definitions In the present context:

"Grinding wheel" refers to a rotatable member having a cylindrical outer surface having an abrasive surface suitable for grinding. The term "cylindrical" refers to the general form of the wheel (excluding the serrated profile).

"Axial" directions refer to the direction of the rotational axis, which is typically also the longitudinal axis of symmetry of the grinding wheel. This direction is occasionally also called "transverse", as it is transverse to the direction of movement of the wheel surface. "Radial" refers to the direction along a radius of the grinding wheel. "Circumferential" direction means along the circumference of the grinding wheel. "Tangential" direction refers to a direction parallel to a tangent of the cylindrical grinding wheel. "Tangential plane" is a plane defined by a tangent of the cylinder and axial direction intersecting the tangent.

"Wood angle is the angle between reference and the longitudinal axis of wood fiber.

"Radial (grinding) angle" refers to deviation of wood fiber orientation from the axial direction in the radial direction. "Tangential (grinding) angle" refers deviation of wood fiber orientation from the axial direction in the tangential direction. "Raw wood material" refers to wood material in which the original natural fiber orientation and structure are preserved. In particular, the term refers to logs and other forms of wood material also conventionally used for industrial scale grinding processes.

"Serrated surface profile" of the grinding wheel refers to a surface shape having radial height variations on the surface along the axial direction (in the radial plane) of the grinding wheel. In particular, the serrated surface profile comprises repeating pattern of oblique-angle features such that the whole grinding surface is covered and causing transversely fed fiber to meet the stone surface at a radial angle. According to a primary aspect of the present disclosure, the wheel radius is essentially constant along the circumference of the wheel at each particular axial location. In other words, the grinding wheel is rotationally symmetric along its full length.

In some embodiments the grinding wheel is only partially rotationally symmetric. In such embodiments, the grinding wheel is partially and preferably mainly (at least 50 % of the axial length) rotationally symmetric and the wheel radius is essentially constant along the circumference of the wheel at most axial locations. Based on the latter embodiments, according to this secondary aspect of the present disclosure, which may be the subject of divisional applications, there may be for example "circumferential profiling" in addition to serrated "axial profiling". "Oblique angle" between fiber orientation and grinding surface means any angle between but not including zero angle and normal angle. Particularly preferred angle ranges are disclosed elsewhere in this document. The term "majority of fibers" meeting the grinding surface at oblique angle is used to take into account the fact that in practice, all serrated surfaces comprise also non-oblique surface portions for example at the tops of ridges or bottoms of grooves of the serration. In addition, in the raw wood material, there is always a distribution fiber angles around the length direction of the wood material, which causes some minor portion of fibers to necessarily be parallel to the surface.

"Flaky" stands for plate-like particles having generally a smaller height that width and length. Typically, the flakes have an average height of 0.01 to 3 mm, in particular 0.05 to 1.5 mm. The average width of the flakes is on the order of 0.1 to 30 mm, in particular 0.15 to 10 mm. The average length of the flakes is 0.1 to 75 mm, typically 0.5 to 50 mm.

"Fibres" generally have an aspect ratio (length to average diameter) of more than 6, in particular more than 10. Typically, the fibres have an average length of 0.1 to 150 mm, for example about 0.2 to 100 mm. As shown in Figure 7C, the length- weighted fibre length can be roughly in the range of 100 to 900 μιη.

As was discussed above, generally the present apparatus for producing lignocellulosic fibrillar material from wood comprises a rotatable grinding wheel which has a rotation axis and exhibiting about the rotation axis a grinding surface which is capable of separating fibrillar material from wood by grinding effect. The apparatus has a feeder for feeding raw wood material and for forcing it against the grinding surface during rotation of the grinding wheel. The grinding surface of the grinding wheel has a surface profile which is serrated in the direction of the rotation axis. The serration preferably comprises a pattern of surface features having oblique angle with respect to the rotation axis.

The feeder is typically arranged to apply a force to the raw material, the force being directed towards the rotation axis of the grinding wheel, i.e. at a normal angle with respect to the generally cylindrical shape of grinding wheel. If the raw material includes logs, the longitudinal axes of the logs are typically parallel to the axis of the grinding wheel according to the transversal grinding principle, whereby the serration alone produces the desired oblique grinding angle.

According to one embodiment, the serrated surface profile comprises one or more zones of similar surface features repeating on the grinding surface in the axial direction of the grinding wheel. Similarity of the features and periodicity of the profile is beneficial as concerns the initial forming and maintenance of the profile ("sharpening" of the wheel), and ensures production of even-quality fibrillar material.

According to one embodiment, the surface features have a cross-sectional form of isosceles triangles in a plane defined by the rotation axis and a radial direction of the grinding wheel. That is, the features rise as symmetric triangles with sides in the above-mentioned oblique angle with respect to the axial direction of the wheel. Put periodically, the whole surface therefore has "zigzag" profile.

According to one embodiment, the surface features comprise an equal amount of oblique surfaces towards both ends of the grinding wheel in order to cancel out reaction forces parallel to the rotation axis due to feeding of the raw material. This embodiment minimized potentially harmful axial forces experienced by the grinding wheel, its shaft, bearings and rotation motor. The abovementioned embodiment comprising periodic isosceles triangles can be made to satisfy this requirement but there are also other possibilities, as will be described later.

According to one embodiment, the feeder is adapted to feed the raw wood material against the grinding surface such that fiber orientation of the raw wood material is in the plane tangential to the grinding wheel, i.e., an imaginary cylinder placed around the grinding surface. The oblique-angled surface features ensure that there is a radial angle between the wood fibers and the grinding surface. In addition, there may be tangential angle, which is defined by the relative orientation of the feeder and the wheel. The tangential angle is preferably 0-45°. In one embodiment, the grinding is purely transversal, that is, the fiber orientation of the raw wood material is parallel to the rotation axis of the grinding wheel (tangential angle = 0°).

According to one embodiment, the apparatus comprises a finger bar placed at a distance from the grinding wheel and having an edge towards the grinding wheel shaped to match the serrated surface profile of the grinding wheel.

According to one embodiment the apparatus comprises a burr comprising a surface profile matching at least a portion of the serrated surface profile of the grinding wheel and means for pressing the burr against the grinding wheel for maintaining the serrated surface profile of the grinding wheel.

The raw wood material fed to the apparatus preferably comprises logs, in particular debarked logs.

According to one embodiment, the fine lignocellulosic fibrillar material produced using the principle of the invention has a percentage of Bauer McNett fraction F<200 of at least 50 %. This kind of end material has been shown to be completely achievable using the present invention, contrary to any previous mechanical grinding method.

According to one embodiment, the serrated surface profile covers essentially the whole grinding surface, that is, at least the portion of the grinding wheel that the feeder is capable of feeding wood material to. In one embodiment, the profile covers the entire axial width of the grinding wheel. In one embodiment, the serrated surface profile covers at least a part of the grinding surface, preferably at least 50 % of the grinding surface, in particular essentially all the grinding surface, such as at least 90 % of the grinding surface.

According to one embodiment, the oblique angle is in the range of 5-75°, in particular 10- 45°. The angle is preferably constant (but may incline in either axial direction, depending on the axial location), but may also vary so that the angle of at least portion, preferably all, of the profile is within one of the abovementioned ranges. In one embodiment, the grinding surface is made of ceramic material, in which case the oblique angle is preferably 5-60°, in particular 5-45°. In another embodiment, the grinding surface is made of metal, in which case the oblique angle is preferably steeper, that is, 60- 75°. These material and angle combinations have shown to produce best results, as concerns the quality of material and energy consumption.

In some embodiments there are provided means for adjusting environmental parameters further affecting the quality of material and energy consumption. For example, in one embodiment the feeder comprises means for adjusting the speed of feeding the raw wood material against the grinding surface and or means for adjusting the rotation speed of the grinding wheel. Instead or in addition to these, there may be provided means for adjusting the temperature prevailing at the grinding zone where the raw wood material meets the grinding surface, means for adjusting grinding consistency and/or means for dosing chemical to the grinding zone and/or to the separated fibrillar material.

Turning now to the drawings, it can be noted that Figure 1 shows main parts of the present apparatus according to one embodiment. The apparatus comprises a grinding wheel 10 and a feeder piston 18 for pressing logs 16 or other raw wood material against the grinding wheel 10 so that the fiber orientation of each log 16 is the same. The feeder typically comprises walls (not shown) keeping the logs 16 in the desired position between the feeder piston 18 and the grinding wheel 10.

The grinding wheel is supported by a shaft 11 so that it is rotatable about its longitudinal axis 11. The shaft 11 is connected to drive means (not shown) for rotating the grinding wheel 11. The cylinder surface of the grinding wheel 10 is provided with serrated surface profile 12, in this case covering the whole cylinder surface. The wheel 10 in the illustrated case has also rotational symmetry, i.e. the surface profile is independent of the radial angle of the wheel. Fig. 2A shows the surface profile 22A of Fig. 1 in more detail on a grinding wheel 20A. The profile 22A comprises a repeating pattern of surface features having a cross-sectional shape of isosceles triangles with constant ascent and descent angles a. The features repeat one another so as to form an alternating pattern of ridges and grooves of similar geometry. The period of the profile is denoted with L and height with H. This way, the whole surface is serrated and oblique in one or the other direction (excluding the optimally infinitesimally short ridge tops an groove bottoms). In addition, transverse reaction forces during grinding cancel each other, as there is obliqueness equally much in both axial directions.

The angle a is preferably 5-75°, in particular 10-75°, typically 10-45°.

To mention some examples, the period L may be for example 2-200 mm, in particular 5- 20 mm, and the height H for example 0.5-100 mm, in particular 1-20 mm.

Fig. 2B shows an alternative embodiment comprising on a grinding wheel 20B profile 22B with features having a cross-sectional shape of essentially right-angled triangles one after another so as to form slanted serration taking reminding sawtooth profile. The profile preferably comprises triangles in both orientations so that axial reaction forces cancel out during grinding. Like in the embodiment of Fig. 2A, preferably essentially the whole surface is profiled so that the whole surface is at an oblique angle with respect to the axial direction. Fig. 2C shows still another variation of the surface profile on a grinding wheel 20C. The profile 22C is otherwise similar to that of Fig. 2A, but comprises a variable angle in the oblique portions. It is, however, preferable that the angle does not fall below a certain minimum angle a is 5° so that there will not be surface portions which are nearly parallel with the wood fibers.

To mention some examples, the half-period or period L of the profile may be for example 2-100 mm, in particular 5-30 mm, and the height H for example 1-100 mm, in particular 2-10 mm. The diameter of the grinding wheel can be for example 20-300 cm and length 10-300 cm. It should be noted that the dimensions of the surface features are exaggerated in Figures 1 and 2A-C. The surface profile may take also other forms than those discussed above and illustrated in the drawings within the scope of the invention. The profile may also be a combination of two or more of the above examples. In one embodiment, the surface profile is independent on the radial angle. That is, there are no surface height variations in the circumferential direction of the grinding wheel.

In an alternative embodiment applicable to secondary aspects of the invention, the surface height is different depending on the radial angle of the grinding wheel. That is, there is a surface profile in the circumferential direction of the grinding wheel in addition to the axial profile discussed above. A circumferential profile may bring additional advantages in terms of grinding speed or energy consumption, for example.

The serrated grinding wheels of the present kind suit for normal atmospheric or pressurized grinders, normal feeding method and normal raw material. In addition, the wheels can be made from normal grinding stone materials (typically ceramics) and using normal stone conditioning. Change is needed only in a stone preparation ("sharpening") phase and optionally in the finger bar of the grinder. Fig. 3 shows a potential finger bar arrangement with a serrated finger bar 39 having an edge profile matching with the serrated surface profile of the grinding wheel 32.

It is preferred to maintain sharpness, i.e. the desired surface properties of the serrated wheel at all times during grinding, because a worn profile having axially oriented surface portions, may produce long fibers and therefore deteriorate the quality of the fibrous mass obtained.

Figs. 4A and 4B illustrate alternative configurations for maintaining the surface profile of in particular ceramic grinding wheels 40, using one or more conical burrs 47A, 47B. In conventional sharpening, a flat burr travels continuously by the stone as in normal lathe. Opposite to that, the present concept according to Fig. 4A trues and sharpens the stone in stepwise and burring tool wide steps. There is thus provided a burring tool 47 A having a surface profile matching with the desired surface feature geometry of the grinding wheel 40. The burring tool 47 A is moved one period length at a time and engaged with the wheel 40 as it rotates about is axis 41 in order to true and sharpen one axial section of the wheel at a time. As a result, the wheel gets the desired serrated profile. This sharpening can be performed to a normal grinding stone and it can be conditioned with water jet as usual. Serrated sharpening has low costs and enable a wide range of angles.

In the alternative sharpening configuration of Fig. 4B, there is provided a wide burring tool 47B which is capable of treating a larger surface or in this case the whole surface of the wheel 40 at one time. A combination of the embodiments of The burring tool 47A, 47B can be for example a shaped (conical) roll or a blade. In case of a blade, the burring is carried out according to the lathe principle.

As an alternative to a serrated ceramic grinding wheel, a serrated metallic wheel, in particular steel wheel, can be used. In particular, grit impregnated steel can be used. The serration is such wheel may be axially symmetric with respect to axial centre point of the wheel to cancel out transverse reaction forces due to the helical, screw-like, shape. A correspondingly modified finger bar can also be provided.

A ceramic grinding wheel is preferred if the grinding angle a is 1-60°, in particular 5° - 45°, whereas a metallic, optionally serrated, grinding wheel is most suitable for high grinding angles a of 60° and more.

In principle, the larger the grinding angle a, the more small particles are produced. A quality optimum for strengthening purposes is around 15°. A sharp stone is expected to provide good balance between energy consumption and amount of small particles. The production of small particles is flexible regarding the log moisture and quality.

The feeding means may be arranged to produce pressure pulses on the raw material so as to improve bonding ability of the fibrillar fines produced.

In one embodiment, the grinding process is carried out in the presence of water. The water is provided to the grinding zone where the grinding wheel and the wood to be ground meet. The water can be brought in the form of a shower directed to the wheel, to the raw wood material, or both (directly to the grinding zone).

As briefly mentioned above, the apparatus may additionally comprise means for adjusting environmental parameters of the grinding process. This way the quality of the fibrillar material and/or the energy consumption of the process can be varied over a wide range with a given surface profile of the grinding wheel. For example the mechanical properties (e.g. strength, bulk), chemical properties (e.g. binding capability) and optical properties (e.g. scattering properties) of the resulting fibers can be affected. To mention one example, trade-off between binding capacity and light scattering properties can be controlled. The ability to produce different grades of fibrillar material without having to change the massive grinding wheel has huge economic advantages.

The environmental parameters may comprise one or more of the following: feeding speed of the wood, tangential speed of the surface of the grinding wheel (i.e. peripheral speed), grinding temperature, grinding consistency, and chemical dosing to water shower directed to the grinding area (during treatment) and/or to the ground fibril mass (post treatment). These mechanisms are discussed below in more detail. In one embodiment there are provided means for adjusting the feeding speed of the wood material against the grinding wheel. This can be achieved by controlling the movement speed of the feeder piston 18 shown in Fig. 1. This can be achieved for example by a directly controllable drive coupled to the piston or by arranging an adjustable gearing between the piston and its drive. The drive can be in particular electric, hydraulic or pneumatic motor. When other parameters are kept constant, higher feeding speeds results in lower energy consumption values, whereas lower speeds may produce material with improved chemical, mechanical and/or optical properties . The feeding speed may be variable e.g. between 0.1 and 10 mm/s, in particular 0.2 - 2 mm/s, or any subrange thereof. In one embodiment, there are provided means for adjusting the rotation speed of the grinding wheel for setting its peripheral speed to a desired value. Again, this adjustment can be achieved by directly controllable drive for rotating the wheel or by adjustable gearing between wheel and its drive. Higher rotational speed typically consumes slightly more energy but may again have positive impact on the chemical, mechanical and/or optical properties of the resulting fibrous mass. According to one embodiment, there are provided means for adjusting, typically elevating, the grinding temperature, i.e. the temperature of the wood material arriving at the grinding zone. This adjustment can be carried out by various techniques. For example, the raw wood material may be heated before it enters the grinding zone, a heated water shower may be directed to grinding zone, the grinding wheel can be heated, or pressure between the wood and the wheel can be adjusted to increase of decrease heat-producing friction. Adjustment of the temperature of a water shower has proven to be a particularly effective and easily controllable way of reaching desired grinding temperatures in the range of 20 - 170 degrees Celsius. The casing can be pressurized and this makes it possible to use higher shower water temperatures than in atmospheric grinding, as a means to prevent

evaporation of water.

In one embodiment, there are provided means for adjusting the grinding consistency. Such means can comprise e.g. a changeable surface mesh of the grinding wheel. Finally, in one embodiment there are provided means for adding chemical to the grinding zone or to the ground mass immediately after grinding. For example, there may be provided an aqueous jet or shower directed at the grinding zone and adjustable dosing equipment for adding one or more chemicals to the aqueous jet or shower. In one embodiment, the one or more chemicals comprise alkaline peroxide, which is suitable both for application during grinding (to the grinding zone) and as a post treatment chemical (e.g. to a collector of the fibrillated material). In particular in the case of post treatment also other oxidizing chemicals and e.g. ozone treatment are effective.

Adjusting one or more of the abovementioned environmental parameters provides considerable advantages as concerns the variety of products the fibrillated mass, obtained from a single grinding apparatus, can be used in. For example, one can produce strong and well light scattering material for papers based on mechanical mass, decrease the amount of refining needed for TMP fibers, achieve chemimechanical grinding of hardwood for fine paper products and the like or an additive for improving the retention and strength of papers and cardboards. Additionally, the chemical compatibility of lignin-containing fibrous mass can be improved for composite materials.

Examples

Example 1 (effect of grinding angle) The following experimental data illustrate the effect on oblique-angled grinding on the resulting mass. The data are obtained using a laboratory-size ceramic-wheel grinder, but similar results can be expected in industrial size grinders, if the angles grinder material and roughness are used. Measurement setup

Three spruce logs were cut to 34x34 mm blocks with various angles in relation to the axis of the logs. Each batch, 400 to 500 g of wood with a moisture content of 45 %, included the same number of blocks from each log, and the aberrant material such as branches was excluded. Diameter of the grinding stone of the custom-built laboratory grinder used was 300 mm, with a grinding area of 35 mm both in length and in width. The peripheral speed of the stone was 15 m/s, and the stone type was Norton A601-N5VG. A water shower with water temperature between 60 °C and 68 °C was used, and the wood feeding rate was kept constant at 0.5 mm/s.

Two types of wood alignment were studied. In principle, all possible wood alignments can be represented by the radial angle a (fibre orientation deviation from stone surface, see e.g. Fig. 2A (industrial serrated setup) and Fig. 6A (laboratory setup)) and the tangential angle β (fibre orientation deviation from axial direction in the tangential plane, see Fig. 6B). Both a and β are always between 0° and 90°. If both angles a and β are simultaneously non-zero, the rotational symmetry is lower and different situations are found for both angles in the range from -90° to 90°. Conventional transversal grinding was used as reference, having both the radial and tangential angles zero (α = β = 0°).

Grinding trials were performed for six radial angles a; 0°, 5°, 10°, 15°, 30°, and 45°. In the radial alignment series, the tangential angle β was set to 0°. In the series of the varied tangential angle β (0°, 15°, 45°, and 90°), the radial alignment was kept constant at 0°. The resulting pulps were characterized by specific energy consumption, fractional composition and fibre length. So as to obtain optical and strength properties, laboratory handsheets were made according to EN ISO 5269-1 (with recirculated fines). The tensile strength was measured according to ISO 5270 and EN ISO 1924-2, and the light-scattering coefficient was determined according to ISO 9416.

The fractional composition was determined with a Bauer-McNett apparatus (SCAN-CM 6) using the Tyler series: 28-mesh, 48-mesh, 100-mesh, and 200-mesh wires. The pulp fibre lengths were measured by the Valmet FS300 fibre analyser and given as length- weighted averages. The fines character was determined by measuring the specific sedimentation volume (Marton, R., and Robie, J.D. (1969). "Characterization of mechanical pulps by a settling technique". Tappi J. 52(12), 2400-2406; Heikkurinen, A., and Hattula, T. (1993). "Mechanical pulp fines - Characterization and implications for defibration mechanisms". 1993 Mechanical pulping Conference, Technical Association of the Norwegian Pulp and Paper Industry, Oslo, Norway, 294-308).

Macroscopic wood surfaces after grinding were captured by a photo camera using the macro tool with the closest possible distance to the samples.

In order to analyse changes in terms of specific surface area in the grinding zone, layers from never-dried wood blocks were separated using a Leica microtome equipped with a metal knife. Two different layers were cut, one from the surface up to 300 μιη, and the other from a depth 600 to 900 μιη. The microtome cuttings were solvent-exchanged to acetone in a continuous extracting vessel. The wood cuttings in acetone were then critical point-dried with C0 ₂ in a Baltec CPD 030 apparatus. The specific surface area of the wood cuttings was determined by the BET -method (Brunauer, S., Emmett, P.H., and Teller, E. (1938). "The adsorption of gases in multimolecular layers". J. Am. Chem. Soc, 60(2), 309-319) based on the physical adsorption of nitrogen molecules (ISO 9277). In the present trials, the critical point-dried samples were absorbed by nitrogen gas at T = -196°C with a partial pressure of between 5 and 35 %. The specific surface area was calculated according to the amount of adsorbed nitrogen assuming that the sample is covered by a monolayer of nitrogen molecules. One nitrogen molecule covers an area of 16.2 A2. The BET-analysis was performed with the Micrometrics Tri Star surface area analyzer. Results

The wood alignment in grinding had a substantial effect on energy consumption and pulp quality. The results in Table 1 show that even a small radial angle of five to ten degrees between log and stone cause a considerable change in pulp properties.

Table 1. Specific energy consumptions, pulp and handsheet, properties, and fines characteristics for the groundwood pulps investigated

The changes in the fractional composition of the pulps are shown in Figs. 7A and 7B. With small radial grinding angles, the proportion of the long fibres (McNett fraction F>28) decreased, while other fibre fractions remained constant (Fig. 7A). A further increase of the radial grinding angle caused the proportion in the fraction F28-48 to first decrease, then this was followed by a decrease in F48-100,and finally by a decrease in fraction F100-200. The amount of fines increased constantly until the fines content of the pulp was close to 90%.

The effect of grinding angles in the tangential plane on the pulp composition was not as drastic as in radial grinding (Fig. 7B). With increasing tangential angle, the long fibre fraction F>28 and the fines fraction F<200 increased, while middle fractions diminished. In fibre lengths (Fig. 7C), a small change in radial grinding angle caused a drastic decrease in the average fibre length, while in tangential grinding the average fibre length remained steady even when the pulp composition changed. In radial grinding, small alignment angles a up to 15° increased the specific sedimentation volume of the fines markedly (Fig. 7D). This development of the fines character stands for an improved bonding potential, which could be confirmed by the tensile index and light- scattering coefficient (Table 1). With radial angles a exceeding 15°, the pulp consisted mainly of fines (86-87%) obviously of non-fibrillar shape, and the specific sedimentation volume dropped to 130 cm ³ /g at a=45°. In tangential direction, the specific sedimentation volume was the highest at a grinding angle of β=45° and remained at a high level up to a grinding angle of β=90°.

As shown in Fig. 7E, the fines content and the specific energy consumption in grinding followed each other, but the surface area of the particles (measured as specific

sedimentation volume) was not so straightforward.

In summary, grinding was found to be surprisingly sensitive to the wood and stone radial angle. If this angle differs from zero, the process starts to require more energy, produces shorter fibres and more fines. With radial angles higher than 30°, almost pure fines are produced. The radial angle largely determines the fibre length and the quality of fines. Thus, pulp composition and fines quality in grinding can be controlled by the wood alignment angle against the stone surface. Radial grinding with small angles (5-15°) leads to fatigue-based refining, in which the fibre structure is loosened by fatigue before the fibres are bent onto the surface. Pressure pulses produce fibrillar fines and fibres of good bonding ability. When the angle becomes bigger, the fibres are worn and crushed immediately on the surface into small particles with low bonding ability. The change of the tangential grinding angle does not have as drastic an effect as the change in the radial angle. An angle of 90° in a tangential direction produces higher amounts of fines and of long fibre fraction compared to transversal grinding. Example 2 (effect of environmental parameters)

Figs. 8A-B and 9A-C and Table 2 illustrate results of experiments carried out to test the influence of adjustment of environmental parameters, in this case particularly the wood feed speed, temperature, peripheral speed of the stone and stone surface mesh on the energy consumption of grinding and resulting fines quality.

The experiments comprised grinding different woods (spruce, aspen and birch) using two different serrated wheel ("V-stone" with angle of 15 degrees) surface meshes ("Stone 1": 60 and "Stone 2": 70-open (70-open was sharper than 60 in this case), two different peripheral speeds (10 and 20 m/s) and two different temperatures (17 and 65 C) and different wood feed speeds (0.3-0.9 mm/s) in selected combinations and measuring the energy consumption (see Figs. 8A-B) and properties of products manufactured from the resulting fibrillar material (see Table 2 and Figs. 9A-C).

Fig. 8A illustrates the influence of stone sharpness and the feed speed on the energy consumption. The feed speed is kept at a constant level of 0.5 mm/s (apart from two reference measurements with a V-stone and a flat stone). One can note from Fig. 8B a strong dependence on the stone surface sharpness and wood feed speed of two different stones on energy consumption, also peripheral speed and temperature have noticeable effects on the energy consumption.

Table 2 shows that the properties, in particular tensile strength, light scattering properties, binding strength and air permeance can be varied over a wide range by changing the environmental parameters. Table 2. Properties of Spruce products obtained

In the above table, the "V-fines 1" column corresponds to "triangles" product (combined sample of all "triangles") of Fig. 8A, where dynamic drainage jar (DDJ)-fines content was 95%, and the "V-fines 2" column to "Spruce 70-20 m/s-65C" product of Fig. 8B, where Bauer McNett (BMcN)-fmes content was 87%.

Figs. 9A to 9C illustrate the effect of 15-angle serration and surface mesh on the fines content, light scattering coefficient and tensile index of the ground products, respectively, compared with that achievable with a flat stone (60 mesh). Fig. 9A shows that with the serrated stones the fines content was considerably higher than that obtained with a flat stone with the same wood feed angle. Fig. 9B shows that the light scattering coefficient was significantly lower, depending on the mesh. Finally, Fig. 9C shows that the tensile index was higher with lower mesh value and vice versa. Thus, in a sense, the quality of the product can be changed with surface sharpness and production. Higher sharpness and higher production rate reduced energy consumption and strength properties but improved light scattering. Industrial Applicability

The novel apparatus and method are suitable for use on an industrial scale for defibered pulp, in particular mechanical or semimechanical pulp. In particular the present technology is suitable for production of fine lignocellulosic particles. This material may be utilized as large range in strength, bulk, and optical improvement agent. This will improve the economics.

Reference Signs List

10, 20A, 20B, 20C grinding wheel

32, 40

18 feeder piston

16 logs

11 shaft

12, 22A, 22B, 22C surface profile

39 finger bar

41 axis

47A, 47B conical burrs

Citation List

Patent Literature

US 4,560,439

DE 3210321 Non-Patent Literature

Brauns, O., and Gavelin, G. (1959). "Groundwood quality at different angels between stone surface and wood." Svensk Papperstidn. 62(3), 67-70.

Alfthan, v. G. (1970). "Influence de la position du bois", Revue A. T.I.P. 24(6), 241-259. Beath, "The Varying Angle Between Stone Surface and Wood - A Cause of Wide, Uncontrolled Freeness Variation in Groundwood Production", 1958. Stationwala, M, "Disk Grinding", 2003 International Mechanical Pulping conference, Quebec, Que, Canada, 2-5 June 2003, pp 153-162.

Dooley, N. and Weinberg, G., 63 ^rd Appita Annual Conference and Exhibition, 19-22 Apr. 2009, Melbourne, Australia, pp 115-120.

Marton, R., and Robie, J.D. (1969). "Characterization of mechanical pulps by a settling technique". Tappi J. 52(12), 2400-2406

Heikkurinen, A., and Hattula, T. (1993). "Mechanical pulp fines - Characterization and implications for defibration mechanisms". 1993 Mechanical pulping Conference,

Technical Association of the Norwegian Pulp and Paper Industry, Oslo, Norway, 294- 308).

Brunauer, S., Emmett, P.H., and Teller, E. (1938). "The adsorption of gases in

multimolecular layers". J. Am. Chem. Soc, 60(2), 309-319).