Procedure for the debarking of logs

Abstract: The invention concerns a procedure for the debarking of logs, in which procedure the logs are treated with enzymes to weaken the bonds between bark and wood; an apparatus for the debarking of logs, comprising a conveyor (1) for conveying the logs to a debarker (2), in conjunction with the conveyor a piping system (3) with nozzles (4) for spraying the logs with an enzyme solution, a device (5) for separating the enzyme solution from the logs and conducting it back into the piping system (3), and a feed device (9) for supplying enzyme solution into the circulation system.

Description: The present invention relates to a procedure for the debarking of logs. In the production of chemical and/or groundwood pulp from logs, the fibre yield from the bark of the logs is small and of low quality. In addition, the presence of bark in the pulp causes problems relating to the use of extractives and an increased consumption of chemicals. Therefore, the logs are usually debarked prior to defibration. The need for debarking depends in the first place on the intended product, but also on the equipment and process used for defibration. The process used for the production of bleached softwood sulphate tolerates the presence of small quantities of bark, whereas the production of groundwood pulp generally requires complete debarking of the logs. At present, drum debarkers are used for the debarking of pulpwood. Other methods include debarking using cambio debarkers, rotor debarkers or high-pressure water jets. Trees have a cambium layer between the bark and the wood. This is the living and continuously growing part of the tree. The cells in this part divide continuously, which is why they have a lower mechanical strength than the cells elsewehere in the tree. In debarking, the aim is to remove the bark including the cambium. Characteristically, the cambium has a high pectin content. Pectin polymers consist of galacturonic acid, ramnose, arabinose and galactose. Besides pectin, the cambium contains hemicellulose, cellulose and protein. In current mechanical debarking methods and equipment, problems result from the fact that, in order to achieve a desired degree of debarking, although the logs are almost barkfree at the later stage of the debarking process, the debarking has to be continued to remove the bits of bark that stick fastest to the trunks. This results in wood losses in the completely debarked parts of the trunks. Besides, it leads to longer debarking times and an increased energy consumption. The object of the present invention is to eliminate the drawbacks referred to. A specific object of the invention is to introduce a procedure that enables the bark to be removed from logs faster, with less energy and more accurately in such a way that the amount of wood fibres removed along with the bark is minimized. Another object of the invention is to introduce an apparatus for implementing the procedure. As for the features characteristic of the invention, reference is made to the claims. The invention is based on the principle that, in order to weaken the bonds between the wood and the bark, the logs are treated with enzymes during debarking. The weakening of the bonds is implemented using enzymes which break down polymers present in the cells of the cambium and/or which weaken the bonds between the cells. In prior art, enzymes have been used in the treatment of woodpulp or chemical pulp e.g. for detaching the fibers in the pulping of bark fiber (Improved enzymatic pulping of bark fiber, JP 63042988) or for the drainage of pulp (Treatment of paper pulp with hemicellulase, EP 262040). Enzymes have not been used for weakening the bonds between wood and bark. In the procedure of the invention, it is possible to use e.g. pectin breaking enzymes, hemicellulases, cellulases and/or proteases, and other enzymes weakening the bonds between wood and bark and/or breaking down polymers present in the cambium. The enzymes are used in concentrations varying with the enzymatic activities of the preparation used. The enzyme concentration is not a critical factor because the effect of the enzyme treatment depends, in addition to the enzyme concentration, also on the treating time and other treatment conditions. Thus, the desired effect can be achieved e.g by using a lower enzyme concentration and a longer treating time or a higher enzyme concentration and a shorter treating time. The solution used for the treatment may contain e.g. a polygalacturonase activity, which may vary between 5 000 - 5 000 000, suitably between 24 000 - 1 200 000, preferably between 180 000 - 600 000 nkat/l of solution. The solution used for the treatment may contain a pectin lyase activity, which may vary between 20 - 20 000, suitably between 80 - 4 000, preferably between 600 - 2 000 nkat/l of solution. The solution used for the treatment may contain a xylanase activity of 50 - 60 000, suitably 260 - 13 000, preferably 2 000 - 7 000 nkat/l of solution. The solution used for the treatment may contain an endoglucanase activity of 150 - 200 000, suitably 700 - 36 000, preferably 5 400 - 18 000 nkat/l of solution. For the weakening of the bonds between wood and bark, especially polygalacturonase and pectin lyase activities are important. Especially advantageous is a solution that, in addition to the above-mentioned activties, has a xylanase activity or an endoglucanase activity or both. The pH of the treatment solution is such that the enzymes in question are able to weaken the bonds between wood and bark, to break down polymers present in the cambium and/or to weaken the bonds between the cells of the cambium. The pH is e.g. 2 - 8, suitably 3 - 7, preferably approx. 5. If necessary, the treatment solution can be buffered to a desired pH level, using e.g. sodium citrate or any other buffering substance known in enzyme technology. The treating time is 1 - 24 h, even longer, preferably 2 - 6 h. The treatment temperature, i.e. the temperature of the treatment solution, is e.g. 5 - 80 °C, suitably 10 - 65 °C, preferably approx. 20 - 40 °C. According to the invention, enzymes are used to assist mechanical debarking. The logs may be subjected to enzyme treatment prior to debarking by known metods. If desirable, the enzyme treatment can also be effected after the debarking, i.e. part of the bark is first removed, possibly after enzyme treatment, whereupon the logs are subjected to an enzyme treatment designed to weaken the bonds between the wood and the remaining portions of the bark so as to enable the remaining bark portions to be removed during a second treatment, which may consist of mechanical or some other kind of treatment. The enzyme treatment can also be implemented in other ways in conjunction with the debarking. The enzyme treatment can be implemented by immersing the logs in the treatment solution, or by flushing and/or spraying them with the solution. The enzyme treatment of the invention has the effect of reducing the detaching resistance of the bark, i.e. it makes the bark more loose. This facilitates mechanical debarking and makes it faster. The fact that the bark is more easily removed reduces the amount of energy needed for the debarking. A higher and more constant degree of debarking is achieved. Furthermore, enzyme treatment helps reduce the wood losses occurring in mechanical debarking as a result of differences in barking resistance between different trunks or logs. The apparatus of the invention for the debarking of logs comprises a conventional conveyor for conveying the logs to a debarking machine, e.g. a barking drum, a high-pressure water debarker etc., and, arranged e.g. in conjunction with the conveyor, a piping system with nozzles for spraying the logs with an enzyme solution. Furthermore, the apparatus preferably comprises a device for separating the enzyme solution from the logs and conducting it back into the piping carrying the enzyme solution, allowing the solution to be reused. The apparatus also comprises feed connections for supplying fresh water and enzyme solution e.g. into the piping. The piping system is naturally provided with a circulation pump for creating a sufficient pressure. In the following, the invention is described in detail by the aid of examples of its embodiments, reference being made to the appended drawings, in which Fig. 1 presents a diagram representing an embodiment of the procedure and apparatus of the invention, Fig. 2 presents another embodiment of the procedure and apparatus of the invention, Figs. 3 - 6 present graphs representing experimental results obtained by certain embodiments of the procedure of the invention. Fig. 1 illustrates the procedure of the invention as applied in conjunction with the conveyor 1 of an ordinary barking drum 2. The logs are placed in bundles 20 on a chain conveyor 1 which conveys them slowly into the drum. Arranged in conjunction with the conveyor is a system of pipes 3 carrying an enzyme solution. The pipes are provided with nozzles 4 for spraying the logs with the enzyme solution. The enzyme treatment takes place while the logs are slowly moving on the conveyor 1 towards the barking drum. The equipment comprises a special means 5 for separating and recovering the used enzyme solution. Further, the solution is conducted to a de-watering conveyor 6 for separating the bark and other impurities from it. The solution is collected in a settling tank 7, from where it is passed back into the circulation system 3. The circulation system is provided with a pump 8 and a feed connection 9 for fresh enzyme solution and a feed connection 10 for fresh water. In short, the logs 20 to be debarked are sprayed with an enzyme solution, the solution is recovered, bark and other impurities are removed from it, the solution is allowed to settle and then sprayed again onto the logs. Fresh water and fresh enzyme solution are added into the circulation system. In the embodiment illustrated in Fig. 1, the nozzles 4 for enzyme solution are placed in a covering 11 provided for the conveyor 1. The covering forms a log treatment chamber. In the cold season, this chamber can be used for the thawing and warming of icy logs by spraying them with warm water through the nozzles. As a result of the enzyme treatment, the bonds between bark and wood are weakened and the bark is more easily removed from the logs in the barking drum. This means that the time needed for drum debarking is shortened. The capacity of the plant increases and, due to the shorter debarking time, the wood losses occurring in the drum are reduced. Fig. 2 illustrates another embodiment of the procedure and apparatus of the invention, in which pre-barked logs 20 are conveyed by a conveyor 21 into a special intermediate storage, i.e. an enzyme treatment chamber 22. The chamber is connected to a piping system 3 circulating an enzyme solution and provided with nozzles 4. Placed on the bottom of the chamber is a special scraper conveyor 1 which slowly conveys the pre-barked logs from the chamber to a high-pressure water debarker 2. The enzyme treatment takes place in the treatment chamber 22, where the logs are sprayed with an enzyme solution from the circulation pipe system 3. The circulation pipe system is continuously supplied with fresh enzyme solution via feed connection 9 and with fresh water via feed connection 10. The system is provided with a pump 8 which pumps the solution into the nozzles 4. The conveyor 1, e.g. a scraper conveyor, is provided with a drainage means 5 for separating the water from the logs and passing it e.g. into a settling tank 23 for reuse. The water in the settling tank 23 is also used to feed a high-pressure water debarker 2 via a high-pressure pump 24. The high-pressure water debarker is provided with a drainage means 25 for separating the water and returning it into the settling tank 23. When the apparatus presented in Fig. 2 is used, the pre-barked logs obtained from a debarker, e.g. a barking drum, are conveyed into a treatment chamber 22, treated with an enzyme solution and conveyed further into a high-pressure water debarker 2, which performs the final debarking. From here, the logs are taken e.g. to a groundwood plant. Example 1 Unbarked birch logs were treated with a pectinase solution. After two days of treatment, the enzyme treated samples were compared with samples similarly soaked in water to see if the bark had been loosened. The bark of the enzyme treated samples had partly come off during the treatment and the rest of the bark could be easily detached by tearing it off manually, whereas the bark on the water soaked samples sticked tightly to the wood. Example 2 Unbarked fir logs were treated with a pectinase preparation having a polygalacturonase activity of 180 000 nkat/ml. The amount of enzyme preparation used was 1 ml/l of treatment solution. The treatment solution had a pH value of 5 and its temperature was 20 °C. After 24 h of treatment, the energy required for removing the bark was measured using a device in which, by applying a shear force, a fixed blade detaches a piece of bark from the surface of a slowly rotating disc of wood. The shear stress applied to the bark was measured using a force sensor and registered by means of a recorder. From the time integral of the shear force, the energy required for detaching the piece of bark was calculated as from the beginning of the application of force to the moment of detachment of the bark. As a reference experiment, a corresponding relative E value was determined for untreated log samples. The results are shown in Fig. 3, in which column diagram 1 represents the relative energy E required for detaching the bark from an untreated log sample and column diagram 2 represents the relative energy E in the case of an enzyme treated log sample. In this experiment, the enzyme treatment reduced the required energy by approx. 23 %. Example 3 Unbarked fir logs were treated with a pectinase preparation having a polygalacturonase activity of 120 000 nkat/ml, a pectin lyase activity of 400 nkat/ml, a xylanase activity of 1 300 nkat/ml and an endoglucanase activity of 3 600 nkat/ml. The treatment was implemented by immersing the logs in 10 mM sodium citrate buffered treatment solutions having a pH of 5, into which had been added 0, 0.3, 1.5 and 7.5 ml of said pectinase preparation /l. The samples were treated for 24 h at a temperature of 20 °C. The energy required for removing the bark was measured as in example 2. The measurement results obtained in the experiment are presented in Fig. 4, showing the relative energy (%) as a function of enzyme content (ml/l). When the amount of enzyme preparation used was 0.3 ml/l of treatment solution, the energy required for detaching the bark was 38 % lower than the corresponding energy without enzyme treatment. When the amount of enzyme preparation used was 7.5 ml/l of treatment solution, the energy required was 80 % lower than the energy without enzyme treatment. Example 4 Logs were treated with a pectinase preparation as specified in the previous example. The logs were immersed in a mM sodium citrate buffer with pH 5 and a pectinase preparation content of 1.5 ml/l. The treating time was 24 h and the treatment was performed at temperatures of 20 °C and 40 °C. The reference experiments were performed in the same circumstances without using enzymes. After the treatment, the relative energy (E) required for removing the bark was measured as in example 2. The measurement results are shown in Fig. 5, in which diagram 3 represents the relative debarking energy without enzyme treatment, at 20 °C; diagram 4, enzyme treatment at 20 °C; diagram 5, treatment without enzyme at 40 °C; and diagram 6, enzyme treatment at 40 °C. - It is obvious that raising the temperature enhances the effect of the treatment both with and without enzymes, but in the case of treatment with enzymes, the effect of increasing the temperature is clearly stronger. Example 5 Unbarked fir logs were treated with an enzyme preparation as described in example 3. The treatment was performed by immersing the logs in a water solution having 1.5 ml of said pectinase preparation /l. The treatment temperature was 20 °C and the energy required for removing the bark was measured after 2, 12 and 24 h as described in example 2. The reference sample was a fir log treated in the same conditions for 24 h without enzyme. The measurement results are presented in Fig. 6. Two hours of enzyme treatment reduces the required debarking energy by about 5 % as compared to soaking without enzymes. After 12 h of enzyme treatment, the debarking energy is reduced by approx. 35 %, and after 24 h by approx. 50 %. Increasing the treating time enhances the effect of the enzyme treatment. In the foregoing, the invention has been described in detail by referring to the experiments carried out, but it is obvious that it can be implemented in various ways within the scope of the idea of the invention as defined in the following claims.

Claims: Procedure for the debarking of logs, characterized in that the logs are treated with enzymes to weaken the bonds between bark and wood, and that the logs are debarked after the enzyme treatement. Procedure according to claim 1, characterized in that it uses enzymes that break down polymers present in the cambium of trees, preferably e.g. pectinase, hemicellulase, cellulase, protease or a mixture of these. Procedure according to claim 1 or 2, characterized in that the solution used for the treatment has a polygalacturonase activity of 5 000 - 5 000 000, suitably 24 000 - 1 200 000, preferably 180 000 - 600 000 nkat/l of solution, a pectin lyase activity of 20 - 20 000, suitably 80 - 4 000, preferably 600 - 2 000 nkat/l of solution, a xylanase activity of 50 - 60 000, suitably 260 - 13 000, preferably 2 000 - 7 000 nkat/l of solution, and/or an endoglucanase activity of 150 - 200 000, suitably 700 - 36 000, preferably 5 400 - 18 000 nkat/l of solution. Procedure according to any one of claims 1 - 3, characterized in that the treatment solution has a pH of 2 - 8, suitably 3 - 7, preferably approx. 5. Procedure according to any one of claims 1 - 4, characterized in that the treatment solution is buffered to pH 3 - 7, preferably approx. 5. Procedure according to any one of claims 1 - 5, characterized in that the duration of the enzyme treatment is 1 h - 3 days, suitably 1 - 24 h, preferably 2 - 6 h. Procedure according to any one of claims 1 - 6, characterized in that the temperature of the treatment solution is 5 - 80 °C, suitably 10 - 65 °C, preferably approx. 20 - 40 °C. Procedure according to any one of claims 1 - 7, characterized in that the enzyme treatment is effected by immersing the logs in the treatment solution containing enzymes, by flushing and/or spraying the logs with the solution. Procedure according to any one of claims 1 - 8, characterized in that the logs are debarked mechanically in conjunction with the enzyme treatment, after it and/or possibly partly before it. Procedure according to claim 9, characterized in that the logs to be treated are brought to a debarker, e.g. onto a conveyor feeding a barking drum, that the logs are sprayed with an enzyme solution, the solution is recovered, the logs are conveyed to a debarker for mechanical debarking, and enzymes are added to the enzyme solution for reuse. Procedure according to claim 9, characterized in that the logs are barked mechanically e.g. in a barking drum, the logs thus barked are treated with an enzyme solution by spraying and/or immersing, the enzyme solution is recovered, the logs are debarked using a high-pressure water jet and enzymes are added to the enzyme solution for reuse. Apparatus for the debarking of logs, comprising a conveyor (1) for conveying the logs to a debarker (2), characterized in that, arranged in conjunction with the conveyor, the apparatus has a piping system (3) carrying an enzyme solution and nozzles (4) for spraying the logs with the solution, a device (5) for separating the enzyme solution from the logs and conducting it back into the piping system (3), and a feed device (9) for supplying fresh enzyme solution into circulation system. The use of enzymes in the debarking of logs to weaken the bonds between bark and wood, the enzymes being selected from the groups of pectinases, hemicellulases and cellulases and proteases.

Inventors: KANTELINEN ANNE, RAETTOE MARJAANA, VIIKARI LIISA, KANTELINEN, ANNE, RAETTOE, MARJAANA, VIIKARI, LIISA, Rättö, Marjaana
Applicants: KONE OY
Application year: 1990
Main classification: B27L3
IPC codes: B27L3