The present invention concerns a method for facilitating the debarking of logs of wood which are difficult to debark. In the pulp and paper industry, the logs are barked before chipping. Some of the wood used as raw material have a very tough bark, so that the normal drum debarking is not sufficiently quick and effective. Especially many tropical sorts of wood like Eucalyptus are difficult to debark and long barks also cause problems on the conveyors. In addition, the bark of Eucalyptus tends to loosen as long strips, so that also fresh bark requires pretreatment in order to avoid problems after the drum. Long barks are difficult to remove from the bark-discharge openings and they come out together with the logs to the feeding line of the chipper. Drum debarking has in some cases proved to be ineffective also in debarking of birch logs.

Also other types of methods than drum debarking are used for barking of logs. A disadvantage of those methods is, however, the small capacity and the adjustment of the efficiency of the pretreatment. One known method of this kind has been described in the barking machine patent publication US 4,685,498 of Fuji Kogyo. There the logs are transported through a chute-like part in a longitudinal direction. On the bottom of the chute-like part there are longitudinal rollers. When the rollers are rotating, the teeth mounted thereto break the bark off from the surface of the log. One method is to break the bark surface by means of a high pressure water jet. This kind of a method requires, however, some other method for finally removing the bark. A known log pretreatment method of this kind has been described in patent publication US 4,180,109. In order to achieve sufficient water pressure, a strong pump is required in this method. In addition, a high purity of the water is required.

In a method in accordance with the present invention, the bark of the logs to be finally debarked in a debarking drum will be pretreated or pre-shredded before the drumming. When the bark skin is partly broken, the actual debarking of the log starts immediately after the logs have entered the drum. Without the pretreatment, the logs that are difficult to debark rotate in the drum for some time before the bark skin is broken, so that the logs will not be totally debarked during the drumming. In case of Eucalyptus it is preferable to break the bark skin of the log at certain intervals in the longitudinal direction

of the log, so that the barks do not loosen out as long strips in the longitudinal direction of the log. When only a pretreatment of the bark is done, and the debarking itself is effected in the drum by a method in accordance with the invention, the same capacities are achieved as with logs that are easier to debark by a drum debarking method alone. Figure 1 illustrates a drum feeding line provided with a pretreatment equipment in accordance with the invention,

Figure 2 shows a cross section of the pretreatment apparatus in accordance with the invention, showing also the logs to be treated,

Figure 3 shows a cross section of a shredding wheelwork, Figure 4 shows a cross section of a pretreatment apparatus comprising a shredding chain-conveyor,

Figure 4b shows an enlarged view of a chain of a shredding chain-conveyor, a shredding blade mounted thereto,

Figure 5 illustrates a pretreatment apparatus mounted between the drum feeding conveyor and the drum,

Figure 6 shows section A-A of figure 5, and

Figure 7 shows an enlarged section B-B of figure 5.

Bundles of logs 1 are loaded onto a log receiving conveyor 2 crosswise. The receiving conveyor is preferably a scraper conveyor (figures 1 and 2). The receiving conveyor feeds the logs further crosswise to a pretreatment equipment 3. The pretreatment equipment comprises shredding wheelworks 10, 1 1 and 12, covered with a bottom 4 carrying the logs. Driving units 5 of the shredding wheelworks are on the sides of the equipment. One shredding wheelwork comprises a shaft 6 on which shredding wheels 7 are mounted at certain intervals. In the bottom plate 4, there are holes 8 at the positions of the wheels, so that the outer peripheries of the wheels are above the upper surface of the bottom plate 4 (figure 3). Shredding blades 9 are mounted to the wheels 7, for shredding off the bark from the surface of the log. As the wheels equipped with blades are located at certain intervals from each other, the bark on the surface of the log is shredded at corresponding places in the longitudinal direction of the log. The purpose is to decrease the production of long bark strips. The shredding blades 9 are changeable, so that worn blades can be replaced by new ones. The equipment may comprise more sequential shredding wheels 10, 11, 12 than those three shown in figures 1 and 2.

The rotating direction of the wheelworks can be chosen so that it is most advantageous in each case, so that also the logs can be made rotate. In figures 1 and 2 the first wheelwork 10 rotates clockwise and the following two 1 1, 12 counter-clockwise. From the last wheelwork 12 the logs fall to the feeding conveyor 13 of the drum which transports the logs to the drum 14. By slowing down the speed of the feeding conveyor 13, the quantity of logs in the pretreatment equipment 3 can be increased and the operation of the equipment made more effective.

A barrier 15 can be mounted between the last wheelwork 12 and the feeding conveyor 13 of the drum, said barrier preventing the logs to be pretreated from rolling too early from the shredding wheelwork and thus securing the efficiency of the pretreatment. As the wheelworks rotate in accordance with figure 2 during the batch processing, the bundle of logs 16 rotate as shown by arrow 17. The logs 1 rotate above the shredding wheels 7 also about their longitudinal axis. After a certain period of time the barrier 15 is lowered and the logs drop from the pretreatment equipment to the feeding conveyor 13 of the drum. After this, the receiving conveyor 2 feeds new logs to the pretreatment equipment 3. The batch processing of logs in question can also be made automatic.

If the receiving table 2 is wide enough, shorter logs can be loaded onto it two bundles side by side. For increasing the capacity need, the pretreatment method can also be mounted on the opposite side of the feeding conveyor 13 of the drum. In case that dirty logs come to the mill, water jets 18 above the pretreatment equipment 3 wash the logs (figure 2). The water and loosened pieces of bark fall through the openings of the shredding blades to a collecting conveyor 19 under the pretreatment equipment.

A pretreatment equipment 3' can also comprise a chain conveyor 20, to the chains of which shredding blades 21 are mounted (figure 4). In figure 4b, arrow N shows the direction of motion of the chain provided with a shredding blade. The shredding chain conveyor 20 follows immediately after a receiving conveyor 2'. The amount of logs arriving to the pretreatment equipment 3' is controlled by means of the receiving conveyor 2'. When the barrier 15 or the feeding conveyor 13 of a full drum prevents the logs from leaving the pretreatment equipment 3', a slowly moving pile of logs is formed on the shredder chain conveyor 20 rotating clockwise, so that the faster rotating blades 21 of the chain conveyor shred off the bark from the logs.

A pretreatment equipment 3" can also be placed between the feeding conveyor 13 of the drum and the drum 14 (figures 5, 6 and 7). In this case the shafts 6 of the shredding wheelworks are parallel with or slightly oblique in relation to the direction of motion of the logs. The logs are fed side-obliquely from the feeding conveyor 13 to the pretreatment equipment 3" by means of gravitation. The pretreatment equipment 3 " is inclined towards the direction of motion of the logs (figure 6) and thus the logs move in the longitudinal direction towards the drum 14 at the same time as the blades of the wheelworks 7 rotate the logs around their longitudinal axis. Otherwise the pretreatment equipment is of the same type as those described above. An advantage of the equipment as described in figures 5, 6 and 7 is that through the slow feeding motion of the logs, many cuts are made into the barking surface of the logs in the longitudinal direction thereof, and the debarking will be very effective after the treatment. Additionally, the pretreatment time can be controlled by means of the filling degree of the drum 14, so that a long pretreatment time can be achieved when the filling degree is high. A method in accordance with figures 5, 6 and 7 is especially suitable in cases when the debarking properties of logs change quickly and still the same debarking line is desired to be used.