METHOD AND APPARATUS FOR WASHING FIBRE PULP MIXTURE

Abstract: The invention relates to a method of washing a fibre pulp mixture, in which method the fibre pulp mixture is fed between two filter wires (1, 2) running in the same direction. There is a convergent dewatering space (3) in the direction of travel of the wires, where water is removed from the pulp. According to the idea of the invention, the thickness of the pulp layer being fed between the wires is adjusted during operation, since the ash and fines content of the washed fibre pulp is essentially dependent on the basis weight of the fibre web being washed. In a preferred embodiment of the invention, the height of the slice (23) at the output end of the dewatering space is adjusted. The essential idea of another preferred embodiment of the invention is that the speed of the wires is adjusted.The invention also relates to an arrangement comprising means for adjusting the thickness of the pulp layer being fed between the wires, i.e. the basis weight of the fibre web being washed.

Description: METHOD AND APPARATUS FOR WASHING FIBRE PULP MIXTURE The invention relates to a method of washing a fibre pulp mixture, in which method a fibre pulp mixture which is substantially constant in consistency is fed between two filter wires running in the same direction, at a substantially constant input flow, the wires being run into a closed dewatering space which converges in their direction of travel, whereby the filter wires support themselves on to surfaces equipped with dewatering holes, and water is removed from the fibre pulp mixture through the wires and the surfaces of the dewatering space, after which the washed fibre pulp mixture exits, supported by the wires, the dewatering space through a slice at its output end. The invention also relates to an arrangement for washing a fibre pulp mixture, which arrangement comprises two water permeable filter wires arranged to run in the same direction, the wires being run into a closed dewatering space which converges in their direction of travel, in which dewatering space the filter wires are arranged to support themselves against surfaces equipped with dewatering holes, and means for feeding the fibre pulp mixture between the wires, whereby the water in the fibre pulp mixture is arranged to drain through the wires and the surfaces of the dewatering space and whereby the washed fibre pulp is arranged to exit, supported by the wires, the dewatering space through a slice at its output end. Fibre pulp, such as waste fibre, contains not only fibre-like raw materials, but also filling agents, such as ash and also fines originating from the fibre-like material. To be able to reuse such a fibre pulp in paper-making, it needs to be washed in a de-inking plant. There, a certain part of the fine fraction of the fibre pulp is removed by means of water, and the aim is to leave only useful fibres in the pulp. Thus, ash and other particle-like materials, for instance, are washed away by means of filtrate waters. Further, when washing fibre pulp, water is removed from it with the aim of affecting the consistency of the washed pulp. The washed accept is lead on in the process to be used as raw material for newsprint and soft tissue, for instance. There are several different apparatuses on the market for the above-mentioned pulp washing. The known apparatuses remove fines relatively well. The problem is, however, that fines loss cannot be adjusted during operation in the present apparatuses, but this always requires a shutdown. When process conditions change, the apparatus is stopped and washing is changed by changing the wires, for instance. A further problem may be that when attempting maximum ash removal, fines loss becomes too high, which is uneconomical with respect to efficient use of the pulp. Changes in the process impair the running of the washers, producing a varying washing result which then causes problems in the phases after the washers. The object of this invention is to provide a method and an arrangement which prevent the problems occurring in prior art solutions. The method of the invention is characterized in that the thickness, i.e. pulp weight per surface area, of the fibre pulp layer being fed between the wires is adjusted during operation, whereby the ash and fines content of the washed fibre pulp can be adjusted to desired values. Further, the arrangement of the invention is characterized in that the arrangement comprises means for adjusting during operation the layer thickness of the fibre pulp mixture being fed between the wires, to adjust the ash and fines content of the washed fibre pulp to desired values. The essential idea of the invention is that the fibre pulp is fed between two wires which are arranged to run towards a slice at the output end of a convergent dewatering space. In the dewatering space, liquid in the pulp drains through the wires and further through the holed surfaces of the dewatering space. Fibre pulp is fed into the apparatus substantially at a constant flow. According to the idea of the invention, the grammage of the fibre pulp being fed between the wires, i.e. the thickness of the pulp layer running between the wires during washing, is adjusted. Ash removal ability is essentially dependent on the thickness of the fibre web being washed. Further, the essential idea of a preferred embodiment of the invention is that the grammage of the fibre pulp being fed between the wires is adjusted by adjusting the height of the slice, i.e. the quantity of the fibre pulp being fed between the wires. The idea of a second preferred embodiment of the invention is that the speed of the wires, i.e. the running speed of the washer, is adjusted. Another essential idea is that the layer thickness of the pulp being fed in is adjusted on the basis of the ash and fines content measured from the washed fibre pulp, the fibre mixture fed between the wires and/or the removed water. The invention provides the advantage that this way it is possible to adjust the ash and fines content of the washed fibre material to be suitable for each use. Large quantities of waste fibre are used in newsprint and soft tissue, for instance, and the requirements for their ash and fines content are different. Soft tissue requires maximum ash removal, whereas newsprint does not require complete ash removal, but a certain level with which the desired strength and other technical properties can be achieved. With the solution of the invention, it is possible to exactly adjust the ash content. It is then possible to manufacture pulp having different fines contents simply and quickly in the same apparatus without complex changing of settings and wires. In pulp of this kind, the relative proportion of ash and fines in the washed fibre pulp, and correspondingly the proportion of other fibres, is as desired. Thus, the same kind of apparatus can be used in various factories, as the paper and process engineering properties of the washed pulp can always be tuned as required by each factory. Further, other processes of the factory can be kept as they are or the processes can be adjusted quite freely independent of the washing. In addition, the solution of the invention is more environment-friendly than before, because ash and other fines are not unnecessarily washed from the pulp. This means that smaller quantities of fines find their way outside the process with filtrate waters. Because the washing is, due to the adjustment, done at exactly the right efficiency, the power consumption of washing is smaller than before. This matter is naturally significant for both the environment and the price of the final product. Further, the process becomes more stable and easier to run due to measurements and the adjustments made on the basis thereof, whereby a fibre pulp having a more even quality exits the washer for the following phases of production. This way, process variations occurring during the phases prior to washing and sudden peaks caused by process disturbances can also be evened out. Owing to the adjustment according to the invention, the washing can be adjusted without needing to change the flow or the consistency of the pulp being fed into the washer. Thus, the washing does not require changes in the prior process phases and expensive additional equipment. The invention is described in greater detail in the attached drawing which shows a schematic sectional side view of the principle of a washing apparatus of the invention. The apparatus comprises a first wire 1 and a second wire 2 having a dewatering space 3 between them. The first wire 1 rotates in a closed loop around guide rolls 4a to 4d and the second wire 2 correspondingly around a turning roll 5 and guide rolls 6a and 6b. The number and location of guide rolls can be as deemed suitable, which matter is known per se and obvious to a person skilled in the art. The fibre pulp mixture to be washed is fed into a feeding chamber 7 from which it is preferably fed through for instance a turbulence generator 8 known per se to the dewatering space 3 between the first wire 1 and the second wire 2. On both sides of the dewatering space 3 against the outer surface of the wires 1 and 2, there are dewatering boxes 9, 10 or the like, through which water in the fibre pulp mixture in the dewatering space 3 is drained as shown by arrows 9a and 10a. The dewatering space 3 is most preferably a closed chamber formed by sides and support structures between the sides along which the wires 1 and 2 run through the dewatering chamber. The dewatering boxes 9, 10 or the like are installed on the support structures in such a manner that the water drained through the wires flows through the support structures to the dewatering boxes or the like. Such wire support structures, which may be made of a perforated plate, various foil structures or the like, are known to a person skilled in the art and they or their operation need, therefore, not be explained in more detail. In this application and its claims, a closed dewatering chamber refers specifically to a structure where fibre pulp and the water contained in it can only exit the dewatering space along desired dewatering routes. Since both sides of the dewatering space are closed and the wires 1 and 2 are made tight beside the guide rolls 4a and 6b in such a manner that the fibre pulp mixture cannot escape elsewhere, the fibre pulp mixture has to flow from the dewatering space 3 onward in the same direction as the wires 1 and 2. At the same time, water drains continuously from it. Dewatering in a dewatering space occurs, for instance, in such a manner that water is drained in the forward end of the dewatering space 3, i.e. in the input end of the fibre pulp, preferably more in proportion to the travel length of the fibre pulp than at the output end of the dewatering space 3. A turbulence formed in advance in the fibre pulp mixture can also be maintained by adjusting the dewatering on the basis of the speed ratio difference, and the fibres remain suitably mixed without a significant filtering onto the surfaces of the wires 1 and 2 taking place. When the fibre pulp mixture arrives at the turning roll 5, water still drains from it, due to the centrifugal force and the tightness of the wires, upwards through the wire 1 to a dewatering tray 11 , whereby a fibre pulp having a suitable dry stuff content may be achieved as a result. Because the consistency of the fibre pulp mixture coming out of the dewatering space 3 is quite high, the fibre pulp mixture remains between the wires 1 and 2 without trying to gush out from the side. Thus, this apparatus does not require separate sealings after the dewatering space. To make the fibre pulp follow the lower wire 2 requires a force affecting into said direction. This is preferably achieved by using a smooth turning roll 5. Thus a negative pressure is formed in the space, marked 12 in the figure, between the wire 2 and the turning roll 5 on the left side of the turning roll 5 when the wire 2 separates from the roll. This, for its part, causes the negative pressure to suck the fibre pulp against the wire 2 and consequently, the fibre pulp is separated from the top wire 1. For clarity's sake, a slice 23 at the output end of the dewatering space 3 is drawn in a highly simplified manner in the figure. The slice comprises a top lip and a lower lip, whose operation is known per se to a person skilled in the art. According to the idea of the invention, the size, i.e. height, of the slice 23 can be adjusted by means of suitable actuators 24. Adjustable mechanical limiters, against which the top and lower lip can be set during operation, can also be installed to the slice. This adjustment of the slice makes it possible to control the dry stuff content of the fibre pulp exiting the apparatus as desired. The slice of the present washing apparatuses cannot be adjusted during operation. The figure also shows the input opening of the apparatus, marked in the figure schematically with a round bracket 25, and a second actuator 26 for adjusting the height of the input opening of the apparatus by means of an operating coupling 27. In the figure, the actuator 26 is marked to control the height of the input opening on the side of the lower lip only, but a similar actuator can correspondingly be connected to the side of the top lip so that both can preferably symmetrically be adjusted simultaneously in relation to the input channel of the fibre pulp flow. Similarly, it is possible to use only one input opening actuator 26 which is connected to adjust both lips simultaneously in the same way as the actuator 24. By adjusting the height of the input opening, it is possible to correspondingly adjust the ash wash and the basis weight of the washed pulp. Adjusting the height of the input opening makes it possible to change the discharge rate of the fibre pulp being washed in relation to the wires. Thus, when reducing the height of the input opening, the flow rate of the fibre pulp increases in relation to the wires, and correspondingly, when widening the input opening, the flow rate of the fibre pulp decreases in relation to the wires and speed. In the currently used apparatuses, the discharge of the fibre pulp being fed between the wires is adjusted according to the speed of the wires, and their relation with respect to each other, and consequently, the basis weight of the fibre pulp, cannot be adjusted. The figure also shows drive units 11 and 13 arranged with the rolls 4a and 6b, which units run said rolls by means of suitable shafts, gears and other necessary power transmission equipment to move the wires 1 and 2. The drive units are preferably electric motors whose speed can exactly and preferably steplessly be adjusted by means of frequency converters or corresponding regulating units 12, 14. Moving the wires can naturally also be arranged by using another roll. The apparatus also comprises a control unit 15 which is arranged to control the above-mentioned regulating units 12 and 14 of the drive units 11 and 13 by means of operating couplings 16 and 17 and/or the actuator 24 of the slice 23 by means of an operating coupling 18. The operating couplings are marked by a thicker dotted line in the figure. Measuring connections 19 to 22 are marked by a lighter dotted line in the figure. The ash and fines content of the filtrate waters exiting the dewatering space 3 are measured and the measurement data is forwarded by means of the measuring connections 19 and 20 to the control unit 15. It is also possible to measure the ash and fines content of the fibre pulp being fed from the feeding chamber 7 and to forward the obtained measurement data to the control unit by means of the measurement connection 22. It is further possible to adjust the washing on the basis of the properties of the washed fibre pulp. The measurement result obtained on the ash and fines content of the washed pulp is then forwarded by means of the measurement connection 21 to the control unit. Making measurements on waters is usually easier than on accept. The measuring devices suitable for the purpose are known to a person skilled in the art and need thus not be presented herein. The control unit can be a computer, for instance, or another suitable control device, such as a programmable logic. It should be noted that, for adjustment, it is possible to measure either only one of the above parameters or alternatively all of them at the same time. Similarly, it is possible to have the control unit control the speed of the wires and the slice simultaneously, in which case the washing result can be affected by adjusting the relation between the running speed and the slice size. Tests made on the apparatus show that increasing the feed consistency reduces ash removal and fines loss in the washed pulp. In addition, it reduces power consumption and, naturally, increases the consistency of the washed pulp. Increasing wire speed reduces ash and fines removal as well as power consumption. A change in wire speed does not seem to affect the consistency of the washed pulp. In principle, it should be possible to adjust the washing result by changing the consistency of the fibre mixture fed into the apparatus, but in most cases, the process is run in factories at a standard consistency, and the factories do not have equipment for adjusting the consistency, which means that a dilution water cycle or a corresponding apparatus would have to be build separately for this purpose. If such an adjustment option exists, however, it can be used so that when a careful fines-saving wash is required, the feed consistency is increased, and correspondingly, when a strong was is required, the feed consistency is reduced. Wire speed can in normal use be adjusted in the speed range below 500 m/min. When a gentle wash is required, i.e. a wash that washes away less fines particles of the pulp, wire speed is set to correspond to the rate of the slice discharge. The rate of the slice discharge can even be set higher than wire speed. When an efficient fines wash is required, i.e. when a strong wash / high fines loss is required, wire speed is set considerably higher than the rate of the slice discharge (250 m/min, for instance). The thickness of the pulp layer is then smaller and fines are efficiently washed away from the thin layer. The drawing and the related description is only intended to illustrate the idea of the invention. The invention may vary in detail within the scope of the claims. Actuators used as the means for controlling the slice and input opening include pressure intermediate agent cylinder or a combination of a motor and gears.

Claims: CLAIMS 1. A method of washing a fibre pulp mixture, in which method a fibre pulp mixture which is substantially constant in consistency is fed between two filter wires (1 , 2) running in the same direction, at a substantially constant input flow, the wires being run into a closed dewatering space (3) which converges in their direction of travel, whereby the filter wires support themselves on to surfaces equipped with dewatering holes and water is removed from the fibre pulp mixture through the wires and the surfaces of the dewatering space, after which the washed fibre pulp mixture exits, supported by the wires, the dewatering space through a slice (23) at its output end, characterized in that the thickness of the fibre pulp layer being fed between the wires, i.e. the pulp weight per surface area, is adjusted during operation, whereby the ash and fines content of the washed fibre pulp can be adjusted to desired values. 2. A method as claimed in claim 1, characterized in that the height of the input opening (25), and thus also the quantity of fibre pulp mixture flowing between the wires (1, 2), is adjusted. 3. A method as claimed in claim 1 or 2, characterized in that the speed of the wires (1 , 2) is adjusted. 4. A method as claimed in any one of the preceding claims, characterized in that the ash and fines content of the washed fibre pulp is measured and that the measurement result is used in adjusting the fibre pulp weight per surface area ratio. 5. A method as claimed in any one of the preceding claims, characterized in that the ash and fines content of the fibre pulp mixture being fed is measured and that the measurement result is used in adjusting the fibre pulp weight per surface area ratio. 6. A method as claimed in any one of the preceding claims, characterized in that the ash and fines content of the water drained from the fibre pulp mixture is measured and the measurement result is used in adjusting the fibre pulp weight per surface area ratio. 7. An arrangement for washing a fibre pulp mixture, which arrangement comprises two water permeable filter wires (1, 2) arranged to run in the same direction, the wires being run into a closed dewatering space (3) which converges in their direction of travel, in which dewatering space the filter wires are arranged to support themselves against surfaces equipped with dewatering holes, and means (7, 8) for feeding the fibre pulp mixture between the wires, whereby the water in the fibre pulp mixture is arranged to drain through the wires and the surfaces of the dewatering space and whereby the washed fibre pulp is arranged to exit, supported by the wires, the dewatering space through a slice (23) at its output end, characterized in that the arrangement comprises means for adjusting during operation the layer thickness of the fibre pulp mixture being fed between the wires, to adjust the ash and fines content of the washed fibre pulp to desired values. 8. An arrangement as claimed in claim 7, c h a ra cte rized in that the arrangement comprises means for adjusting the input opening (25), whereby it is also possible to adjust the quantity of fibre pulp mixture being fed between the wires (1 , 2). 9. An arrangement as claimed in claim 7 or 8, characterized in that the arrangement comprises means (11 to 14) for adjusting the speed of the wires (1, 2). 10. An arrangement as claimed in claim 9, characterized in that the arrangement comprises frequency converters (12, 14) for controlling electric motors (11, 13) running the wires and thus also controlling the speed of the wires (1,2). 11. An arrangement as claimed in claim 7, characterized in that the arrangement comprises measuring means for measuring the ash and fines content of the washed fibre pulp to adjust the fibre pulp weight per surface area ratio. 12. An arrangement as claimed in claim 7 or 11, characterized in that the arrangement comprises measuring means for measuring the ash and fines content of the fibre pulp mixture being fed to adjust the fibre pulp weight per surface area ratio. 13. An arrangement as claimed in claim 7, 11 or 12, characterized in that the arrangement comprises measuring means for measuring the ash and fines content of the water drained from the fibre pulp mixture to adjust the fibre pulp weight per surface area ratio.

Inventors: JUUTINEN VESA, JUUTINEN, VESA
Applicants: JUUTINEN VESA, VALMET CORP, JUUTINEN, VESA, VALMET CORPORATION
Application year: 2000
Main classification: D21C9
IPC codes: B08B3, D21C9, D21D1, D21F1