PROCESS FOR MAKING WOOD CHIPS

Abstract: Process for the manufacturing of wood chips as raw material in the production of mechanical pulp in order to reduce the energy consumption in the production of pulp, characterized in that the wood fibres, including the summerwood fibres, are irreversibly deformed across the grain, i.e. in a direction perpendicular to the longitudinal axis of the fibres. Preferably this is done through mechanical treatment with one or several tools, arranged so that the forces created in the treatment act in a direction substantially perpendicular to the longitudinal axis of the fibres.

Description: PROCESS FOR MAKING WOOD CHIPS 1 Field of the invention The present invenuon concerns a process for making wood chips as a raw material in the production of mechanical pulp, characterized in that irreversible deformation of the wood fibres, including the summerwood fibres, is accomplished in a direction across the grain, i.e. perpendicular to the longitudinal axis of the fibres, to thereby create favourable deformation of the fibres and reduce the energy consumption in the production of mechanical pulp. Background of the invention Traditionally, the fibre raw material has been subjected to mechanical treatment in two steps in the production of mechanic pulp. Firstly, a disintegration of the wood, so-called chipping has been preformed, followed by fibre separation and flexibilization of the fibres in one or two refiners, so-called refining. Currently used processes for chipping were often developed for the production of chemical pulp, before the introduction of mechanical pulp in the pulp and paper industry. Fibre deformation is unwanted in the production of chemical pulp and therefore minimized, as the fibre otherwise is attacked during the chemical cooking (defibration) and liable to weakening and shortening at the sites that have been mechanically deformed. Apparatuses and processes for chipping have focused on this and try instead to achieve chips of uniform size with minimal deformation of the fibres. For example US-P-3 304 970 describes a new way to produce wafers to avoid the deformation encountered in conventional chipping. The damage on the fibres, e.g. bruising which occurs in chipping are thought to reduce the quality of the wood chips. Other patent publications, concerning chips both for production of paper and board, underline the benefit of splitting the wood raw material in parallel to the longitudinal axis of the fibres as being less severe to the fibres. For example TJS-P-3 407 854 describes it as preferred to produce chips that are not compressed or damaged. SE 463 295 concerns the adjustment of the cutting tools in chipping, expressly to avoid "the risk of forming longitudinal defects, caused by the pressure, in the fibre walls." When the industry increasingly started to produce mechanical pulp, the traditional methods of chipping have nevertheless remained largely unchanged. With some variations for different methods of chipping, the thickness of the chip has been in the order of 3 to 7 mm. Chips produced accordingly have been fed into refiners of different construction for exposing and flexibilizing the fibres. The refiner has been developed and adjusted to the type of chip conventionally used in production of chemical pulp. The energy consumption in refining is considerable and, further, the refining step is a very non-selective way of treating the fibres. During more prolonged refining an unwanted fibre shortening takes place. In spite of this, refining has to be preformed up to a certain degree to guarantee that the main part of the fibres are exposed and flexibilized to a sufficient extent. One special quality of the fibre raw material has gained larger importance in mechanical pulping in the recent years. This is that the wood fibres, dividable as they are into fibres with thin walls and thick walls, so-called springwood fibres and summerwood fibres respectively, are very different in relation to their mechanical properties. The thick- walled fibres in the summerwood are resistant to strong loads in a direction perpendicular to their length direction, whereas the springwood fibres are more easily deformed in the same situation. The summerwood fiber can resist more than 10 times higher loads than the springwood fiber. All non-selective treatment of fibres leads to more or less unwanted results. Refining in the extent needed to flexibilize the summerwood fibres, reduces the springwood fibres in a large extent to fines, which leads to inferior dewatering properties of the pulp and weaker paper. With less extensive treatment, the summerwood fibres retain unreduced stiffness, which leads to unevenness in the paper surface and weaker paper. The length and axial strength of the wood fibres (both tensile strength and compression strength) should be conserved to as large an extent as possible. The transversal strength of the fibre should nevertheless be reduced, making the fibre more easily collapsible. As an alternative, the fibres should be deformed irreversibly, making them flatter, i.e. collapsed. A collapsed fibre has a substantially lower bending stiffness. The lower bending stiffness makes the fibre more easily adaptable to surrounding fibres. The number of contact points between the fibres in the paper thus increases and so does the density of the paper. A collapsed fibre is also more flat and this increases the area of the contact surface with other fibres. Attempts with rolling the chips, i.e. subjecting them to forces perpendicular to the fibre 's length direction, in order to reduce the energy consumption in the production of mechanical pulp has shown little effect. This is due to the fact, that the weaker fibres in the springwood are compressed first and function as a deformation buffer protecting the stronger fibres of the summerwood. Summary of the invention The present invention concerns a new process for production of disintegrated raw material for the production of mechanical pulp. Already during the disintegration step involving the wood raw material to chips according to the invention, the wood is subjected to such stress across the grain, i.e. in a direction perpendicular to the longitudinal axis of the fibre, that also the summerwood fibres are irreversibly deformed. Thereby a more uniform pulp is achieved in refining. Further, the previously common too far reaching treatment of the springwood fibres is avoided. Simultaneously, the energy consumption of the refining is decreased. A less extensive refining leads to a lower degree of fibre cutting, which gives a pulp with higher average fibre length and thereby better paper qualities. Characteristics of the process according to the present invention is that irreversible deformation of the wood fibres across the grain, i.e. in a direction perpendicular to the fibre length is achieved, according to the attached claims. Preferably, said irreversible deforming of the fibres across the fibre length is substantially achieved without an accompanying fibre shortening. Short description of the drawings Fig. 1 is a schematic view of a cutting tool, arranged substantially parallel with the fibre length, and fig. 2 shows an electron microscope photography of a fibre section in undeformed wood (A) and in chips, produced according to the invention (B). Description of preferred modes The present invention will be described in the following in further detail with reference to preferred modes and fig. 1. The irreversible deformation of the fibres across the grain, i.e. in a direction perpendicular to the longitudinal axis of the fibres according to the present invention, comprises a substantial deformation also of the summerwood fibres across the grain. This is achieved e.g. through processing of the wood through a cutting procedure, where the cutting tool or tools are positioned so, that the edge or edges of the cutting tool or tools are substantially in parallel to the length of the fibres and that the combination of shearing strain and compressing strain, created in the treatment, acts substantially perpendicular to the longitudinal axis of the fibres. The cutting tool or tools are positioned so, and the cutting operation performed in such a manner, that the irreversible deformation defined above is maximized while the fibre shortening is minimized. By optimization of the design of the cutting tool and the position and operation of the same, a splitting zone can be created, which zone runs through the wood slightly ahead of the cutting tool, in the direction of the tool and in parallel with the fibre length. Simultaneously, the forces acting across the grain must be kept sufficiently strong to acheive the irreversible deformation. Optimization of this kind can be performed without deviating from the scope of the invention as set forth in the appended claims. Preferably, the cutting tool or tools is/are positioned so that the chip angle 3 is less than about 60°, preferably between about 2° and about 30° and most preferably about 5°. The cutting tool or tools are further preferably positioned so, that the chip thickness in the interval of 0.2 to 2.0 mm, preferably about 0.5 mm is achieved. When the cutting depth is larger, the chip comprises both summerwood and springwood in alternating layers, whereby primarily the weaker springwood is deformed. Naturally the cutting depth is adjusted to the wood raw material and possible species or location dependable variations, e.g. variations in the width of the annual rings. The irreversible deformation of fibres in a direction perpendicular to the longitudinal axis of the fibres according to the present invention can also be achieved through other mechanical treatments, such as pressing or rolling of sufficiently thin chips produced by any optional method or such treatment in combination with the previously described process. The present invention comprises an apparatus for irreversible deformation of wood fibres across the grain, i.e. in a direction perpendicular to the longitudinal axis of the fibres according to the previous description. Such an apparatus for large scale use can be designed as a drum with internal cutting tools e.g. knives or projecting blades. The design and confiuration of said cutting tools are then chosen so, that the objectives and parameters specified in the attached claims are met. Such apparatus can have some apparent similarity to a barking drum, but the mode of operation together with the specific parameters are completely different and novel for this application. Such apparatus can comprise a revolving drum with inward oriented cutting tools and means for receiving logs, wherein the edge or edges of the cutting tools is/are arranged substantially parallel to the longitudinal axis of the logs and the cutting angle and chip thickness are adjusted to values in the interval of 0 to 30° and 0.2 to 2.0 mm, respectively. The cutting tool or tools can also be positioned on the outer perimeter of one or several revolving drums. An apparatus according to the present invention can also comprise means for pressing or rolling wood chips, either in combination with the above described apparatus or in combination with a conventional apparatus for the production of wood chips in the prouction of mechanical pulp. According to one embodiment of the invention, different fibre fractions are collected. According to this embodiment the logs are treated sequentially whereby the fibres are removed layer by layer using e.g. an apparatus as described above. This embodiment makes it possible to use different fibre fractions depending on the sought qualities of the end product. According to a preferred mode of the invention, the wood raw material 1, which preferably is supplied as debarked logs, is treated with one or several cutting tools 2, arranged so that the chip angle 3 is less than about 60°. Preferably the chip angle is in the interval of 2 to 30°, most preferably 5°. Even a minor negative chip angle is possible, i.e. an arrangement of the tool so that the angle between the cutting tool and the wood raw material is less than 90° in the direction of the path of the tool 4. In treating the wood raw material according to the process of the present invention the cutting tool or tools is/are arranged so, that the chips 5 formed have a thickness in the interval of 0.2 to 2.0 mm, preferably about 0.5 mm. In this way it is guaranteed, that the chip does not comprise such a number of layers of alternating spring- and summerwood, that the springwood functions as a deformation buffer and absorbs all deformation while the summerwood escapes deformation. The properties of the wood raw material, such as temperature and humidity, influence the result of the treatment. A person skilled in the field can, within the scope of the invention, optimize the properties of the wood, such as temperature and humidity, or through optimization of the treatment process, take these properties into account. Apart from cutting depth and chip angle, also cutting speed and the exact design of the cutting tool and the friction coefficient between the knife and the wood be varied can within the scope of the present invention. Examples Example 1. The influence of chip thickness on fibre length and deformation Chips of Swedish spruce were produced in small scale experiments by fixing a piece of wood with a length of 2 cm in a lathe. The wood had a humidity of about 40 %, corresponding to industrially used wood. A cutting tool was applied to the revolving piece of wood in a manner making it possible to vary the chip angle and cutting depth / chip thickness. The peripheral velocity was between 2 and 6 m/s. The samples were examined with an electron microscope and compared with electron microscope images of undeformed control samples. The degree of deformation was estimated visually. The fibre length was measured conventionally, using pulp prepared in the laboratory from the experimental chips. Treatment with cutting depths in the interval 0.1 to 0.7 mm was examined. Satisfying results were achieved within this entire interval. However, with smaller cutting depths the fibre length was reduced. The results are summarized in table 1. Examples of an undeformed and a highly deformed cross section are given in fig. 2(A) and (B). Table 1. Chip thickness Fibre length Degree of deformation (mm) (mm) 0.2 very low high 1.0 1.74 1.6 1.86 low Example 2. The influence of chip angle on deformation The influence of the chip angle on the degree of deformation was studied using the same experimental set up as previously described. The chip angle was varied incrementally in an interval from 0 to 30°. A cross section of each sample was examined with an electron microscope and the degree of deformation visually estimated. It was shown that strong and irreversible deformation of the summerwood fibres was achieved in the entire interval from about 0 to 30°. The fibre condition corresponded to a condition, usually observed only after the refining step. Within the previously mentioned interval, a chip angle of about 5° was shown to be most preferable under the prevailing experimental conditions. Although the invention has been described with regard to its preferred embodi¬ ment, which constitute best mode presently known to the inventors, it should be under- stood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth in the claims appended hereto.

Claims: Claims 1. Process for production of wood chips as raw material for the production of mechanical pulp, characterized in that the wood fibres are irreversibly deformed across the grain, i.e. in a direction peφendicular to the longitudinal axis of the fibres. 2. Process according to claim 1, characterized in that the irreversible deformation of the fibres across the grain includes substantial deformation of the summer¬ wood fibres. 3. Process according to one of claims lor2, characterized in that the fibres are irreversible deformed across the grain through a cutting treatment of the wood, whereby the cutting tool (2) or tools is/are arranged so, that the edge or edges of the cutting tool or tools is/are substantially parallel to the longitudinal axis of the fibres and that the shearing strain achieved in the treatment acts in a direction substantially perpendicular to the longitudinal axis of the fibres. 4. Process according to one of claims lor2, characterized in that the fibres are irreversible deformed across the grain through a cutting treatment of the wood, whereby the cutting tool (2) or tools is/are brought to advance in relation to the bulk of the wood, in a manner, including orientation and speed, forming a splitting zone ahead of the advancing cutting tool. 5. Process according to any one of claims 3 and 4, characterized in that the cutting tool or tools is/are arranged so that the chip angle (3) is less than about 60°, preferably from about 2° to about 30° and most preferably about 5°. 6. Process according to any one of claims 3 and 4, characterized in that cutting tool or tools is/are arranged so that chips with a thickness preferably in the interval of 0.2 to 2.0 mm are formed. 7. Process according to one of claims lor2, characterized in that the fibres are irreversibly deformed across the grain through other mechanical treatment, such as pressing or rolling or such treatment in combination with the process according to claim 3. 8. Apparatus for irreversible deforming of wood fibres across the grain according to any one of the previous claims. 9. Apparatus for irreversible deforming of wood fibres across the grain, characterized in that said apparatus comprises a revolving drum with inward oriented cutting tools and means for receiving logs, wherein the edge or edges of the cutting tools is/are arranged substantially parallel to the longitudinal axis of the logs and the cutting angle and chip thickness are adjusted to values in the interval of 0 to 30° and 0.2 to 2.0 mm, respectively. 10. Apparatus for irreversible deforming of wood fibres across the grain, c h a r a c t e r i z e d in that said apparatus comprises one or several revolving drums with outward oriented cutting tools and means for receiving logs, wherein the edge or edges of the cutting tools is/are arranged substantially parallel to the longitudinal axis of the logs and the cutting angle and chip thickness are adjusted to values in the interval of 0 to 30° and 0.2 to 2.0 mm, respectively.

Inventors: UHMEIER ANDREAS, UHMEIER, ANDREAS
Applicants: SVENSKA TRAEFORSKNINGSINST, UHMEIER ANDREAS, STFI, UHMEIER, ANDREAS
Application year: 1996
Main classification: B27L11
IPC codes: B27L11