METHOD FOR DEFIBRING RECYCLED FIBRE MATERIAL IN A DRUM PULPER, AND A DRUM PULPER FIELD OF THE INVENTION [0001] The invention relates to a method for defibring recycled fibre material in a drum pulper provided with a substantially horizontal drum, the method comprising feeding recycled fibre material and water to the drum pulper and simultaneously rotating the drum of the drum pulper, whereby the recycled fibre material rises up in the rotating drum and finally, due to gravita- tional force, falls back down, whereupon, after falling and hitting to the bottom, it is defibred, thus forming a fibre pulp mixture with water, and measuring a value of at least one parameter describing the defibration process or fibre pulp properties on the basis of the drum pulper or the resultant fibre pulp mixture, and by controlling the rotation speed of the drum of the drum pulper on the ba- sis of said parameter value.

[0002] The invention also relates to a drum pulper, which comprises a substantially horizontal drum rotatably mounted around a substantially hori- zontal axis, to which drum recycled fibre material is fed to defibre it, a pit below the drum for a fibre pulp mixture which is formed of the recycled fibre material supplied to the drum pulper in the defibration and water, means for removing the fibre pulp mixture from the pit of the drum pulper, and a motor for rotating the drum, and it comprises at least one measuring means for measuring a value of at least one parameter describing the defibration process or fibre pulp properties on the basis of the drum pulper or the resultant fibre pulp mixture and that it comprises control means for controlling the rotation speed of the drum of the drum pulper on the basis of said parameter value.

BACKGROUND OF THE INVENTION [0003] In drum pulpers, recycled fibre material is defibred to fibre pulp. The process is carried out so that a pulper drum rotates substantially horizontally or in a position slightly inclined from the material feed end to the exit end and material to be defibred rises up in the rotating drum and at some point, falls down to the bottom and is defibred. The size and rotation speed of the pulper drum are designed according to the pulper capacity so that material fed to one end of the drum would be defibred as well as possible when it reaches the exit end. Current drum pulpers do not have actual controls, and the only typical variables are chemicals used and the amount thereof as well

as consistency of pulp to be pulped, which is a constant value regarded as suitable for the pulper and the pulp type.

[0004] The aim of drum pulping is to separate fibres from each other as well as possible, thereby keeping the size of different impurities, such as <BR> <BR> plastic, glues etc. , as large as possible to facilitate their separation from fibre pulp. By using current drum pulper solutions, however, it is not possible to op- timise the process in practice.

BRIEF DESCRIPTION OF THE INVENTION [0005] It is an object of the present invention to provide such a method for defibring recycled fibre material in a drum pulper that the defibra- tion process can be controlled and, if desired, optimised according to a desired parameter. Furthermore, it is an object of the present invention to provide a drum pulper, by which a defibration process can be controlled.

[0006] The method of the invention is characterized by measuring at least the power taken by a motor of the drum pulper and by controlling the ro- tation speed of the drum of the drum pulper on the basis of the measured power value.

[0007] The drum pulper of the invention is characterized in that at least one measuring means is arranged to measure the power taken by the motor of the drum pulper and that the control means are arranged to control the rotation speed of the drum of the drum pulper on the basis of the measured power value.

[0008] The essential idea of the invention is that the rotation speed of the drum pulper is controlled on the basis of a predetermined, measurable parameter. According to the essential idea of a preferred embodiment of the invention, the rotation speed of the drum pulper is increased, until the power taken by the motor of the drum pulper starts to decrease, whereby the found maximum point of power consumption is regained by reducing the rotation speed. According to a second preferred embodiment of the invention, the rota- tion speed of the drum pulper is controlled on the basis of its specific energy consumption SEC. According to a third preferred embodiment of the invention, the rotation speed of the drum pulper is controlled so that the specific energy consumption can be kept as high as possible during the defibration.

[0009] The invention provides the advantage that defibration can be controlled and adjusted so that a desired process result and desired defibration

of recycled material are achieved. By controlling the rotation speed on the ba- sis of specific energy consumption, defibration can be controlled by taking into account how much energy is consumed in the defibration for a certain amount of defibred material. Especially when specific energy consumption is used as a parameter value in the defibration such that as large an amount of specific en- ergy as possible is used per one defibred material unit, the defibration is car- ried out in the most efficient way.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] The invention will be described in greater detail in the at- tached drawings, in which Figure 1 schematically shows a partially sectional side view of a drum pulper of the invention, and its control devices, Figure 2 schematically shows a cross section of the drum pulper of Figure 1, taken along the line A-A of Figure 1, and Figure 3 schematically illustrates specific energy consumption of a drum pulper as a function of pulp flow defibred therein and of rotation speed, when two different raw materials are used.

DETAILED DESCRIPTION OF THE INVENTION [0011] Figure 1 shows a drum pulper 1 of the invention, comprising a rotating drum 2. The drum 2 is rotated by a motor 3 using transmission equipment 4. Below the drum 2 there is a pit 5. Recycled material to be proc- essed is fed into the drum 2 of the drum pulper 1 via a feed channel 6 and the finished pulp is removed from the drum pulper via a discharge channel 7 at the other end. Recycled material, such as papers, cardboard or other recyclable material and water, is fed to the drum pulper 1 in order to be pulped. Recycled material can be fed to the pulper either as continuous feeding or in batches.

The drum 2 is rotated by the motor 3, and protrusions 8 on the inner surface of the drum 2 convey the material to be pulped upwards in the drum, until the ma- terial falls back down and, after falling and hitting to the bottom, is defibred more and more each time it falls down, until it forms a fibre pulp suspension with water while exiting from the drum at the other end of the drum 2. Part of the defibration is caused by the rubbing of fibres against each other when the recycled material is rotated in the drum pulper. To convey the material to be pulped along the drum in its axial direction from the feed end to the exit end, the drum 2 can be provided with slanting rail-like or shorter blade-like protru-

sions 8, which are known per se and aim at transferring the material to the exit end when the drum 2 is rotating. Correspondingly, the drum 2 can be pivoted in a manner known per se to be slightly inclined so that after the material has risen up on the surface of the drum, it falls a distance forwards axially to the drum. Inside the drum, in its cross-direction, there are annular spacer plates 9 around the inner surface of the drum, which prevent the fed pulp and water from flowing too fast through the drum 2. The pass-through time and process- ing time of pulp can be adjusted by the structure and inclination angle of the inner parts of the drum.

[0012] The drum pulper of Figure 1 also comprises a control unit 10, which controls the operation of the drum pulper. The electricity required for the motor is fed along a cable line 11 to a frequency converter 12, which is con- nected to control the rotation of the motor 3, i. e. its rotation speed. The control unit 10, for its part, is connected to control the frequency converter 12. The control unit 10 is further connected to measure electric power, i. e. used en- ergy, fed to the motor 3. The drum pulper also comprises measuring means 13 for continuously measuring the amount, or mass, of recycled fibre material to be fed to the drum pulper 1. At the same time, the amount of water to be fed to the pulper is also measured with one or more flow meters. Water is fed to the forward end of the drum pulper preferably via the feed channel 6. At the drum pulper's forward end, where material to be defibred is still solid and possibly sticked together as compact bales, the material does not have a clear material surface. At a later stage of defibration, the material is formed to pulp, which is more uniform and the surface of which can be somehow defined.

[0013] At the forward end of the drum 2, the drum cover is made of a solid material, and pulp to be defibred and water remain inside the drum. In the embodiment of Figure 1, the rear end of the drum 2 comprises a screening stage 14 known per se, under which there is a pit 5. The screening stage 14 is provided with a screen cylinder 15 in a manner known per se, comprising gaps or openings, through which the accepted fibres and water can pass to the pit 5 below, thus forming a fibre pulp suspension. The part of the recycled fibre ma- terial and other material conveyed with it that is not passed through the screen cylinder is finally transferred after the screening stage out of the end of the drum 2 to a separate conveyor 16, for instance, to be removed from the proc- ess. Instead of the conveyor 16, there can also be a separate reject pit or a transport carriage or other corresponding solution.

[0014] In the case of Figure 1, water is fed to the drum pulper by means of a nozzle 17 firstly to the feed channel 6. The flow rate of water is measured with a measuring sensor 18, which is connected to the control unit 10 via a separate measuring and control unit 19. The measuring and control unit 19, for its part, is connected to control a water feed valve 20, by which the amount of water to be fed to the drum 2 is controlled. Water is also fed by wa- ter jets 21 to the screening stage 14, in which the water jets clean the screen cylinder by removing fibres or other materials possibly stuck to the gaps or openings of the screen cylinder and flushes the accepted fibres to the pit 5.

The flow rate of water is measured correspondingly with a measuring device 22, which is connected to the control unit 10 via a second measuring and con- trol unit 23. The second measuring and control unit 23, for its part, is con- nected to control a second water feed valve 24, by which the amount of water to be fed to the screening stage 14 is controlled. Via a nozzle 25, the required amount of chemicals possibly needed for defibration can also be fed to the feed channel 6. The control unit 10 receives the measured values from differ- ent sensors and/or the measuring and control units and controls the process correspondingly on the basis of the measured values.

[0015] According to the invention, the rotation speed of the drum 2 of the drum pulper is controlled by measuring the mass of the recycled fibre material to be fed in a time unit and another parameter relating to drum pulping or resulting from drum pulping and by controlling the rotation speed of the mo- tor 3 on the basis of said parameter so that a desired defibration result is achieved. For instance, a point at which the intensity, i. e. the processing power is at its highest is searched for the rotation speed of the drum pulper. Simula- neously fuzzy logic seeks the dependence between the intensity and the rota- tion speed. After the dependence is detected, the processing power is man- aged and a desired intensity is selected, the intensity preferably being 70 to 100% of the maximum power taken by the motor 3 during the operation. By using this programming principle, a rotation speed corresponding to the high- est power can be searched again when recycled fibre materials and process circumstances change. The processing power can be measured with power of the motor 3, torque of the drum or by using the measured energy consumption, which are variables directly proportional to each other. Process circumstances can be considered to change when the pulp flow through the drum pulper weakens or increases or when the amount of material rotating in the drum

pulper changes substantially or when the frictional properties of the material change owing to a change in consistency, temperature, chemicals or raw ma- terial, for instance. In the case of Figure 1, the parameter used is specific en- ergy consumption per one defibred unit of recycled fibre material (SEC), which is obtained by dividing the power fed to the motor 3 by the mass of the recy- cled fibre material fed to the pulper and passed through it. When this parame- ter is used, it is easy to implement the control, because, in practice, it can be carried out entirely as an online measurement.

[0016] On the basis of experiments conducted, when SEC control is used, an optimal implementation is such that the specific energy consumption per a mass unit is as high as possible. With these controls, the rotation speed of the drum 2 and, correspondingly, the rotation speed of the motor 3 in a cer- tain drum pulper have specific, substantially constant maximum values, when the same materials and the same pulp flow are employed.

[0017] Other applicable parameters may include a screening ratio, i. e. a ratio between an accept and a reject, which is formed in the next screen- ing stage of the fibre pulp mixture exiting from the drum pulper 1 and obtained by measuring, in the embodiment of Figure 1, the flow rate and consistency of the accept flowing out of the discharge channel 7 with measuring devices 26 and 27, which are connected via a third measuring and control unit 28 to the control unit 10. A reject ratio is obtained by subtracting the obtained values from the pulp flow fed to the drum pulper, on the basis of which either a reject ratio or an accept ratio can be calculated. By using this information, the control unit 10 can calculate the screening ratio and use it for controlling the rotation speed of the drum 2. If desired, SEC control and screening ratio control can be used at the same time.

[0018] Figure 2 shows a cross section of an embodiment of the drum pulper of the invention schematically, seen from the end of the drum. It shows schematically a drum 2, which is supported on typically several succes- sive wheels of transmission elements 4. The transmission elements 4 are ro- tated by one or more motors, which are not shown. Inside the drum 2 there are blade-like protrusions 8, by which material is transferred when the drum is ro- tating, such that after the material has risen up, it falls down and, as a result of an energy pulse caused by the hit to the bottom, it is defibred. Typically, the filling degree of a drum pulper is about 30% of the cross-sectional area, in which case the material to be defibred can be conveyed sufficiently freely with

the blades and fall down. As a result of rotation, the material to be defibred is settled at the end section of the drum in practice so that its surface is at an an- gle of approximately 45°C but it can naturally vary greatly, depending on the rotation speed of the drum. By way of example, the material to be defibred falls downward at a point of approximately 11 to 11.30 o'clock when the drum ro- tates clockwise, i. e. approximately 5 to 15° before the highest point. This falling point naturally depends on raw material, consistency, rotation speed and ele- ments controlling the fibre flow and is thus not strictly limited to a specific point during the rotation of the drum. The blades 8 of Figure 2 show one embodi- ment, but they may vary in many ways, depending on applications. As a physi- cally effective quantity, the controlling of rotation speed of a drum pulper means that the circumferential speed of the drum must be changed. The circumferential speed which is produced in the pulp suspension and is in pro- portion to the circumferential speed of the drum determines the centrifugal force affecting the suspension and forcing the suspension outwards in the di- rection of the drum radius. The pulp suspension is simultaneously affected by gravitational force, which aims at dropping the suspension down to the bottom of the drum. The falling point of the pulp in the drum is also affected by these forces.

[0019] Figure 3 schematically illustrates the ratio of specific energy consumption to the rotation speed of a drum in a drum pulper with a diameter of 2.0 metres, when certain pulp flow of recycled fibre material and two differ- ent raw materials are used. As Figure 3 shows, the specific energy consump- tion increases until a certain rotation speed is achieved, after which it starts to decrease. By using SEC control, the purpose is to achieve a state where the specific energy consumption is as high as possible, i. e. where the rotation speeds denoted by arrows SECmax/a and SECmax/b, respectively, of Figure 3 are achieved. Since the structure and other properties, such as mass and practically also feed rate of the recycled fibre material vary, the rotation speed cannot be fixed to this value but is controlled continuously or periodically on the basis of measurement results such that the specific energy consumption is as high as possible.

[0020] In the above specification and drawings, the invention is only described by way of example and is not restricted thereto in any way. Various technologies known per se can be employed for controlling a drum pulper and measuring and adjusting parameters and other values. Therefore, instead of a

separate control unit, various decentralized technologies, a computer control- ling a larger process etc. can be used. Accordingly, measuring devices and sensors can be connected directly to the control unit or the like without sepa- rate measuring and control units. What is essential is that the rotation speed of the drum of the drum pulper is controlled by means of a parameter relating to pulping or to the formed fibre pulp mixture so that a desired pulping result, i. e. a defibration result is achieved.