The raw material used in the manufacture of paper pulp is generally wood chips obtained by chipping pulp wood.

In brief, the chips are digested in a cellulose digester that contains caustic soda, i. e. sodium hydroxide. The caustic soda is regenerated after use.

The cook is carried out at an overpressing and at a temperature of, e. g., 175°C over a period of, e. g., two to three hours.

The lignin that binds the cellulose fibres together is dissolved by the cooking liquor, so as to separate the fibres. Subsequent to separation of the cellulose fibres, the cooking liquor is washed away from the fibres so as to leave a finished paper pulp.

One problem with the use of chips is that the chips have different size fractions. The largest chip fraction is the deciding factor as to when digestion of the pulp is complete. Furthermore, overcooking of the smaller size fractions has a deleterious effect on the quality of the finished pulp, due to the fact that the lignin in the smaller fractions is dissolved long before the cook is terminated. Excessive cooking is harmful to the separated cellulose fibres.

Another problem with chipping is that many cellulose fibres are mechanically damaged in the process, i. e. shortened, because chipping does not take into account the orientation of the cellulose fibres in relation to the wood chipper.

It is desirable to be able to monitor and control the pulping process in a cellulose digester so that all fibres are cooked for essentially the same length of time and so that the cook can be stopped at a specific stage of the cook.

One serious problem encountered with chips produced in a chipper is that the chips contain severed knots. Knots are essentially chemically resistant to the cooking process. In known processes, these knots must be screened-off after the cooking process. The presence of knots also increases the amount of caustic soda required. Knots also discolour the pulp.

Consequently, it is an advantage when the material charged to the digester is knot-free.

The ability to remove knots from the wood prior to the cook is thus highly desirable.

The present invention solves the problem of removing knots and also produces a relatively homogeneous raw material.

The present invention thus relates to a method of producing paper-pulp raw material wherein disintegrated pulp wood is cooked in a cellulose digester with caustic soda, or some corresponding cooking process, to produce paper pulp, or alternatively wherein the pulp wood is ground to produce

groundwood pulp, and is characterised in that the raw material consists of pulp wood splits that are split from the wood in its axial direction by means of a wood splitting tool such that the fibres will lie essentially in the plane of the split; in that the splits are given a thickness of about 3-10 millimetres; and in that the splits are passed between several mutually sequential rolls so as to bend or flex said splits backwards and forwards and therewith cause knots to fall from said splits.

The present invention will now be described in more detail with reference to exemplifying embodiments thereof and also with reference to the accompanying drawings, in which -Figure 1 is a block schematic that illustrates an inventive process; -Figure 2 is a schematic illustration of a cellulose digester; -Figure 3 is a schematic illustration of a first embodiment of an arrangement used in accordance with the inventive method; -Figures 4 and 5 are schematic illustrations of a second embodiment of an inventive arrangement; and -Figure 6 is a schematic illustration of a rotary chipper.

Figure 1 illustrates schematically an inventive method of producing raw material and thereafter paper pulp. According to the method, disintegrated pulp wood is cooked in a cellulose digester with caustic soda, or in accordance with a corresponding cooking process, to produce paper pulp. The present invention is not restricted to the so-called sulphite process or to the sulphate process, but can be applied with equal benefit in both processes.

In the Figure 1 illustration, block 1 denotes de-barking of the wood, block 7 denotes cutting of the wood into lengths suitable for subsequent disintegration, block 2 denotes disintegration of the wood, block 3 denotes charging of the cellulose digester, and block 4 denotes cooking of the pulp wood in caustic soda in a cellulose digester. The arrow 5 indicates tapping-off the paper pulp.

According to the present invention, the raw material is comprised of wood splits that have been split from the wood in its axial direction by means of a wood cleaving or splitting tool, wherewith the fibres lie essentially in the plane of the split. This is the disintegration of the wood included in step 2 in Figure 1.

The splitting tool shall not be sharp, for instance as sharp as a planing tool or wood chisel, but will be sufficiently blunt, or dull, to split the wood along the fibres, roughly in the same way as when splitting a log into firewood. This process can be achieved with a device corresponding essentially to a typical wood splitter. Alternatively, there can be used a frame structure in which mutually parallel wodd-splitting tools are fixedly mounted and through which the wood is pressed.

The pieces split from the wood in accordance with the invention, said pieces being referred to here as splits, have a thickness of about 3-10 millimetres. Naturally, both thinner and slightly thicker splits are conceivable.

According to one very essential feature of the invention, the wood splits are passed between a plurality of mutually

sequential rolls and therewith bent or flexed forwards and backwards so that any knots present in the wood will fall therefrom. The knots present in the wood are in fact conical in shape and much more difficult to deform than the remainder of the wood. Thus, as the splits are bent or flexed forwards and backwards, the knots are loosened from the surrounding wood. As a result of the conicity of the knots and also of the wood sockets in which the knots are seated, the knots will fall from the wood under the influence of gravity, in the direction in which the cone opens.

This means that when the cone opens upwards, it will be necessary to turn the splits upside down after having bent said slits forwards and backwards, so that the knots are able to fall out. However, the splits are preferably placed between the rolls in a position in which the cones open downwards.

According to one preferred embodiment of the invention, the splits are given a length of about 1-1.5 metres. The splits, however, may be both longer and shorter than this.

According to one preferred embodiment, the splits are given a maximum width corresponding to the diameter of the wood, wherein excessively wide splits are cut to a suitable width in the longitudinal direction of the slits with the aid of a cutter. By suitable width is meant a width that corresponds to narrower slits, with the intention of giving the majority of splits roughly the same width. A suitable width is about 0.1-0.25 m.

A typical split may then have a thickness of 5 mm, a width ot 25 cm and a length of 1 m.

Figure 3 illustrates an arrangement for bending or flexing the splits forwards and backwards. This corresponds to the process step referenced 6 in Figure 1. The arrangement shown in Figure 3 includes a roller conveyor 13 and a roller conveyor 14. The split infeed direction is shown by arrow 16.

The roller conveyor 14 is an outfeed conveyor. Located between the conveyors are a number of cylindrical rolls 17- 20, where each alternate roll 17,19 is offset relative to adjacent rolls 18,20, so that an object fed through the roll paths 17-20 will lie against the undersurface of each alternate roll and the upper surface of each other alternate roll, thereby bending or flexing the object between said rolls. A roll path of this nature can be likened to a roller- equipped straightening mill for straightening metal sheet.

The rolls 17-20 are, of course, motor driven. Anvil rollers 21,22 may be required to increase the friction between the splits and the rolls 17-20.

The splits are thus flexed backwards and forwards so as to free the knots and cause them to fall. The knots are conveniently collected in a collecting vessel 24.

According to one preferred embodiment of the invention, the splits are whipped with a whipping device or brushed with a rotating brush that acts against one or both flat surfaces of the splits while or after introducing the splits in between the rolls 17-20. This facilitates removal of the knots, subsequent to having loosened the knots from the surrounding wood, ie their sockets.

This feature is illustrated in Figure 3 with a brush 25,26 positioned adjacent each of the two centre rolls 18,19. The brushes 25,26 will preferably rotate opposite to the conveying direction of the splits, as indicated by the arrows.

Instead of being offset in relation to one another, as in the Figure 3 embodiment, the rolls may be placed in pairs 27-29 along a straight drive-line, as illustrated in Figure 4. This arrangement also includes an infeed and an outfeed conveyor path 13,14. In this latter arrangement, a roll pair is comprised of a convex roll 30 and a concave roll 31. When the roll pair 28 includes a convex upper roll 30, a surrounding roll pair will include a concave upper roll. Thus, a split that is fed in between the rolls will be flexed forwards and backwards in accordance with the curvature of the nip or gap between the rolls of respective roll pairs, with a successive transition from one roll pair to following roll pairs. The embodiment shown in Figure 4 may also include brushes 25,26.

Roll pairs according to Figure 5 may replace the rolls 17-20 in Figure 3, wherewith the splits will take a double-curved surface between two mutually adjacent roll pairs.

According to one preferred embodiment of the invention, the splits are cut transversely to their fibre direction, i. e. transversely to their longitudinal direction, subsequent to the knots having been removed therefrom, so as to obtain slits that have a width of about 25-50 millimetres, where the length of the slits corresponds to their width prior to cutting said slits. This is illustrated with process step 32 in Figure 1. The slits may be cut with an appropriate rotary

chipper, for instance of the kind shown in Figure 6. The illustrated chipper includes a rotary cylinder 34 provided with a radially projecting knife 35 that severs a split 15 in co-action with an anvil surface 36.

Provided that the splits have roughly the same width and thickness prior to chipping, all splits will have roughly the same dimensions and be clear of knots.

Because the splits have been produced by a wood splitting process, they will have an uneven surface in relation to shavings, produced by a planing method. The slits will thus have a relatively large specific surface area in relation to their volume. Because of this relatively large specific surface area in relation to volume, as compared with the specific surface area of chips, the lignin present in the slits will be dissolved more rapidly. This enables one or more of the parameters cooking time, temperature and digester pressure to be changed in a beneficial sense. The cooking process will therefore be more effective when applying the present invention.

Raw material in the form of slits thus constitutes a highly homogenous raw material as opposed to chips of varying size.

This homogenous raw material means that essentially the same process will continue simultaneously throughout the whole of the pulp in the cellulose digester during a cook. This enables the process to be monitored and controlled much more easily than when chips are charged to the digester.

The pulp in the digester is observed during the cooking process with respect to the occurrence of fibre bundles.

These occur shortly before the pulp is fully cooked. As before mentioned, the fibres may be damaged when subjected to excessively long cooking times. It is therefore essential to stop the cook precisely at the time when the lignin has been dissolved, but before the fibres are damaged.

This observation can be made by taking a small sample of the pulp and studying this sample with respect to the presence of fibre bunches. Figure 2 is a schematic illustration of a cellulose digester 8 in the form of a simple vessel. The bath surface is referenced 9.

A sample is thus taken from the pulp 10 and passed to an analyzing stage 11, which may be a manual or an automatic stage. An electric signal is sent to a control circuit 12, when fibre bunches are observed.

When a fibre bunch is observed, the pulp 10 is vibrated mechanically at a frequency and amplitude at which the fibres in the bunch will separate. This is achieved by virtue of cellulose and lignin having different specific weights, causing the fibre bunch to fall apart in response to the vibrations in the pulp.

Subsequent to observing the fibre bunch and said control circuit having received said signal, the control circuit functions to activate vibrators 13,14 in the bath 9. These vibrators may be of any suitable known kind that are powered electrically. For instance, vibrators used to vibrate concrete may be used. The person skilled in this art is fully capable of dimensioning the vibrators with regard to number, position and the magnitude of their mechanical energy to

generate vibrations in the bath with which the fibre ouncnes will fall apart.

Because the raw material is highly homogenous, fibre bunches will occur generally simultaneously in the bath. This enables the cook to be terminated after vibrating the bath, therewith drastically reducing the danger of fibres becoming damaged as a result of excessively long cook times.

In addition to avoiding the drawbacks associated with the presence of knots in the resultant pulp, the present invention also enables the pulping process in a cellulose digester to be monitored and controlled to ensure that substantially all fibres will be cooked over essentially the same length of time, such that the cook can be stopped at a determined stage of the cooking process.

This results in shorter cook times, which, in turn, results in higher capacity. Energy consumption is also lower.

Furthermore, fibre yield is increased and fibres are, on average, stronger.

Removal of the knots means that smaller quantities of chemicals are required, therewith reducing pressure on the environment.

The removed knot material may be used for those purposes disclosed in Swedish Patent Specification No. 466 897. This patent specification teaches a method of treating wood with powder consisting of ground knot material. This treatment enables attack by wood fungi to be avoided, among other things.

Although the invention has been described with reterence La pulping wood in a digester, it will be understood that the knot-free slits may alternatively be used in a known process in which pulp wood is ground to produce groundwood pulp.

The present invention shall not therefore be considered restricted to the aforedescribed exemplifying embodiments thereof, since variations and modifications can be made within the scope of the following Claims.