A method and device for the production of cellulose pulp of improved quality

Abstract

 In a process and a device, a cellulose suspension of thin consistency, the consistency and flow rate of which is known, is conducted to the suction side of a main pump (4), to the pressure side of which there are coupled in sequence a first multi-hydrocyclone unit (6) for cleaning out heavy impurities, a second multi-hydrocyclone unit (7) for cleaning out light impurities, and a headbox (8) for a wet machine or paper machine (9). This provides, on one hand, cleaning and pulp or paper production uninterrupted by intermediate storage, and on the other hand a thicken­ing of the pulp in the second multi-hydrocyclone unit from a suitable consistency for cleaning to a thicker consist­ency suitable for spreading on a wire or the like.

Description

[0001] The present invention relates to a method and a device for the treatment of suspensions of cellulose fibers, which are to be cleaned and concentrated, wherein the cleaning also encompasses the elimination of light impurities.

[0002] The cleaning of cellulose suspensions from heavy im­purities, such as sand, bark and short shives, is now normally done in hydrocyclones, in which the impurities are taken out in the apex fraction and the purified suspension in the base fraction. Such cleaning presupposes a low fiber content, 0.1-3%, preferably 0.2-2%.

[0003] Cleaning out light impurities by means of hydroclones is also previously known, particularly in the treatment of recycled paper, which often contains significant amounts of waste plastic and ink etc. The hydrocyclones normally used in this case are dimensioned and disposed in such a way that the base fraction contains the light impurities and the apex fraction the major portion of the input cellolose fibers. Hydrocyclones for this purpose are also known in which both fractions are taken out at the same end through separate outlets.

[0004] The first mentioned type of hydrocyclone is referred to below as an A-cyclone, while the second is referred to as a B-cyclone.

[0005] Since hydrocyclones are not perfect separators, recycling units are always required for economic reasons, which recover cellulose fibers from the reject material. Such recovering is previously known. As regards recycling with B-cyclones, reference is made to Swedish patent applica­ tion 8802580-4 from 8 July 1988, which describes hydro­cyclones particularly suited for such recycling.

[0006] In plants presently used for manufacturing paper and market pulp, it is usual that the suspension produced by whatever method (cooking, defibration, bleaching, beating of recycled paper) is first diluted to be able to be screened and/or cyclontreated, whereafter the cleaned suspension is thickened to be able to be intermediately stored. This thickening is normally done in open thick­eners. In order to thereafter be able to conduct the pulp to a wet machine it must be diluted once again, as a rule to a concentration which is higher that that used for the cleaning step. This thickening, intermediate storage and dilution involves extra costs, which can be eliminated by concentrating the pulp coming from the cycloning step and directly leading it to the wet machine, particularly by using the same pump for the primary cycloning step and a closed thickener working under pressure, as disclosed in Swedish Lay-Open Print 8305036-9.

[0007] As mentioned aboved, however, there is an increasing need for removal of light impurities, especially plastic, and this applies not only to recycled paper but also to wood, particularly from populated areas, which contains in­creasing amounts of plastic impurities. Plastic particles can also come in during production in the plant. Even small amounts of these impurities can present difficulties in paper production. Stripes in the paper web can appear for example when coating.

[0008] One purpose of the present invention is to provide an improved system for manufacturing paper pulp of good quality, especially as regards elimination of impurities, both in the form of heavy and light particles. (Light particles are meant to include not only particles of low density, but also particles which due to their shape perform as lighter than cellulose fibers in hydrocyclones).

[0009] Another purpose is to provide a system which eliminates the need for thickening for intermediate storage of al­ready cleaned pulp suspension. Instead, the invention provides an essentially continuous process which performs, in one sequence, cleaning, concentration and final treatment in a wet machine or paper machine.

[0010] These and other purposes and advantages are achieved according to the invention by the process according to claim 1 and by a device according to claim 8.

[0011] One particular advantage of the invention is the syner­getic effect achieved by the combination of the functions of thickening and cleaning from light impurities, prefer­ably whilst simultaneously eliminating an intermediate storage step and a diluting step.

[0012] What makes the invention possible is the fact that the suspension fraction coming from the apex in a hydrocyclone designed for eliminating lighter impurities has a consist­ency which is increased in relation to that of the sus­pension supplied, since much water accompanies the light impurities in the base fraction.

[0013] A particularly high consistency can be achieved, with very good separation of light impurities, if one can accept that some cellulose fibres will accompany the base fraction, whereby the recovery is simplified by reducing the number of recycling steps by using hydrocyclones of the type described in Swedish patent application 8802580-4 (not yet publicly available).

[0014] According to the invention, it is possible to achieve a consistency of between 1 and 3% after combined cleaning of the light impurities and thickening, which is a suit­able starting consistency for many types of wet machines and paper machines. Hydrocyclones must of course be operated with an essentially constant flow of liquid, and the output consistency is determined by the consistency of the pulp flow supply. The flow to the wet machine or paper machine can then be additionally varied by a feedback which feeds a controllable portion of the output flow to the input side of the cleaning system.

[0015] It is suitable to provide the white water tank of the wet or paper machine with a spillway to keep the water level constant, and to take the necessary diluting water from there for the process.

[0016] As a rule, hydrocyclones function best if the liquid flows and pressure ratios are kept rather constant at the rated values. However, in different operating situations it can be necessary to vary the flow to a wet machine or paper machine. It is therefore suitable to arrange a diversion of the treated suspension, which returns the unused but cleaned and concentrated suspension. This recycling can be done to a point immediately in front of the main pump, without changing the fiber flow to the wet machine or paper machine, but with the result that the consistency of the suspension to the headbox will be increased. Alterna­tively, the feedback can be to a point prior to the con­sistency control in connection with a first dilution, thus keeping the consistency of the suspension to the headbox constant and reducing the flow to the headbox.

[0017] The invention will now be described with reference to a non-limiting example, shown schematically in diagram form in the drawing.

[0018] The schematic figure shows an example of a plant function­ing in accordance with the invention. The major portion of the fiber material passing through it flows from a conven­tional silo 1 to a wet machine or paper machine 9 along a main path drawn as a thick line. The pulp with a consist­ency of 10-12% is diluted in the lower portion of the silo 1 to about 3.5%, and is diluted at the input side of a pump 103 to a controlled consistency of 3.0% and is then led to a silo 2, where the level is kept constant. The diluting water is taken via a pump 12 from the white water of the wet or paper machine. The person skilled in the art will understand how the level and consistency in the silo 2 will be controlled and therefore it is sufficient to state that the dilution is controlled by actuating the valves 100 and 101 by the sensor 102 and the level by the level sensor 200, which controls the speed regulator 104 to the pump 103. 105 is a control unit which opens the valves 100 and 101 in parallel and controls the flow therethrough in a predetermined ratio. In the silo 2, there is thereby achieved a constant level and a constant consistency for the suspension.

[0019] The suction side of the pump 3 is connected to the silo 2, and it receives diluting water from the pump 12 via the valve 300, which is controlled by the sensor 301, which senses the consistency of the suspension exiting from the pressure side of the pump 3. The consistency is controlled thereby to 2.5%. The flow of the suspension supplied is led through a valve 400 under the control of a flow sensor 401 at a desired level. The flow of fibers through the valve 400 is thus kept constant.

[0020] A controlled flow of suspension with a controlled con­sistency (fiber concentration) is thus led to the suction side of the pump 4.

[0021] For the present, one can disregard the line 13, which refers to a particular aspect of the invention which will be described in more detail below. The pump 4, a centri­fugal pump, provides an essentially constant flow, and the pressure on the suction side is determined by the level in the white water tank 10, the level of which is kept con­stant in a known manner. The pump 4, as long as its coun­terpressure is stable, gives a predetermined flow with a consistency determined by the amount of fibers per unit of time which passes through the valve 400. With the values according to the example, a consistency of 0.5% is attain­ed on the pressure side of the pump 4, which is a suitable value for cycloning, even though thinner and thicker con­sistencies are possible to work with.

[0022] The suspension thus diluted is led through a screen 5, which is primarily intended to eliminate scrap particles and other larger particles, which could damage the cyclones, to a multi-hydrocyclone unit 6 with A-cyclones, the base fraction of which is conducted further. The consistency of the base fraction will be 0.45%, i.e. somewhat thinner than the suspension supplied. This base fraction is moved on without intermediate pumping to a multi-hydrocyclone unit 7 with B-cyclones, where the fiber is removed as an apex fraction, which the lighter par­ticles go into the base fraction. In this apex fraction, the cellulose fibers are enriched to a higher consistency, in the example to 1.5%, thus thickened by a factor of 3.33. This consistency is suitable for the following wet or paper machine 9, to the headbox 8 of which the sus­pension is conducted via a valve 800, controlled by a sensor 801, in this case a pressure sensor. The headbox thus receives the suspension at a controlled pressure. If so desired, the sensor 801 can be a level sensor or a flow sensor.

[0023] Recovery units are connected to the two multi-hydro­ cyclone units 6 and 7. The recovery units are shown here as cascade coupled units with individual pumps at the inlets and with the outlets connected to a common level in the tank 10. Although only two steps in each unit have been shown, the number of steps is determined by the desired degree of recovery and purification. The recovery units to the unit 6 consist of A-cyclones, while the recycling units to unit 7 are B-cyclones. The number of steps in the recycling from unit 7 is positively affected, as is the number of individual cyclones in the units, if the latter are constructed in accordance with Swedish patent application 8802580-4.

[0024] In the example, the unit 7 provides both a cleaning from light impurities and a thickening of the suspension thus cleaned, which combination is the great advantage of the invention. This thickening can be done in different degrees depending on the cyclone dimensions, the pressure drop and the distribution of apex and base flows of the incoming suspension. The thickening factor, which always exceeds 1, and in this example is 3.33, can be increased to 5 for example, if so required, and this can be done without negatively effecting the separation of light impurities; on the contrary, the separation effect will tend to increase. The fiber content in the base fraction may thereby increase, so that the processing work in the recovery units increases, but this can be dealt with by using hydrocyclones according to the Swedish patent application 8802580-4.

[0025] The above-mentioned thickening is a result of separation of water and fibers. The degree of thickening in a hydro­cyclone is determined by the effectiveness of this separa­tion. In order to achieve a high degree of thickening, the hydrocyclone is made with a smaller base opening than the apex opening. For the separation work, the conical separa­ tion chamber should have a largest diameter of less than 125 mm and preferably less than 100 mm. Furthermore, the diameter of the base opening should be less than a fifth of the largest diameter of the separation chamber.

[0026] The main flow of fibers has now been explained. The return flows from the recycling steps go through the line 11′ back to the suction side of the pump 4. This means, that if the line 13 were not there, the entire flow of fibers, under stability conditions, coming in via the valve 400, would be fed out to the headbox 8. The flow is then determined by the capacity of the pump 4 and by the flow-offs, determined by the normally constant pressure ratios, in the apex fraction of unit 6 and in the base fraction of unit 7.

[0027] For optimal operation, hydrocyclones should be operated with predetermined liquid flows. However, it is not certain that this is the case with the headbox of a wet machine or paper machine, where the flow, pressure and consistency may need to be varied. The consistency, at constant flow, can be controlled by controlling the flow and consistency of the suspension supplied via the valve 400. The flow to the headbox 8 can be controlled by a return line 13 with the valve 130 controlled by a pressure sensor 131. This return line 13 can, in accordance with the first embodiment, open into the suction side of the pump 4, which will cause the fiber flow to the headbox 8 to be unchanged while the consistency is changed.

[0028] In accordance with a second embodiment, drawn with dashed lines in the Figure, the return flow is effected via a line 13′ back to a point prior to the pump 3, in this case upstream of the dilution control valve 300, but in any case to a point upstream of the consistency and flow control which determined the fiber flow supplied to the suction side of the pump 4. Controlling the liquid flow to the headbox 8 by means of the valve 130 does not effect the consistency of the suspension supply.

[0029] The invention has now been described on the basis of a non-limiting example, from which it is clear that such a system can be controlled within side limits. Even if the flow capacity for the cyclone equipment is given by the design, it is possible to vary within wide limits the capacity of the pulp flow and the consistency of the sus­pension outflow.

[0030] As mentioned, it is also possible by designing control measures within the scope of the invention to vary for example the degree of thickening in the system.

[0031] One should note in particular that a single main pump 4 pumps the diluted suspension to the units 6 and 7 coupled in series and then to the headbox 8. In a representative example, the pressure drop between the inlet and the base in a A-cyclone is 1 bar and the pressure drop in a B-cyclone between the inlet and the apex is 1.5 bar. These pressure drops are essentially a function of the flow. The pump 4 is dimensioned so that it can both compensate these pressure drops and produce sufficient pressure to the headbox 8.

[0032] Control of the valves 800 and 130 must be done in such a manner that the operating conditions for the multi-hydro­cyclone units are not affected negatively. The main pump 4 is a centrifugal pump, driven at essentially constant rotational speed, with falling pressure head as flow increases. The system is designed so that at the rated flow the pressure head is sufficient for the pressure drops in the multi-hydrocyclone units 6 and 7 and the valve 800 at a normal work point, and controlling within reasonable limits of the flow through the valve 800 to keep the pressure or the level in the headbox 8 does not involve any appreciable disruption. Major changes in the pressure drop over the valve 800 cannot be tolerated, however, since the working conditions for the units 6 and 7 would be adversely affected. In order to make signi­ficant changes in the feed flow to the box 8, there must therefore be a diversion via the shunt valve 130 so that the pressure on the suspension can be kept reasonably constant. The shunt valve 130 can then either be control­led manually with reference to a manometer or can be pressure controlled in the manner shown in the drawing.

[0033] The various controls in the drawing have been shown as mutually independent local controls. It will be under­stood, however, that such controls can be effected by a common computer.

Claims

1. Process for manufacturing cellulose pulp with improved quality properties, especially as regards the content of light, solid impurities such as particles of plastic and the like, characterized in that a fiber suspension of low consistency is caused by a main pump (4) to pass through a first multi-hydrocyclone unit (6) to separate out heavy particles and a second multi-hydrocyclone unit (7) in­cluded in the same pump circuit for separating out light particles at the same time as the pulp consistency is made higher during treatment in the second multi-hydrocyclone unit (7), and that at least a portion of the pulp suspen­sion thus treated is thereafter conducted directly without intermediate pumping to the headbox (8) of a wet machine or paper machine.

2. Process according to claim 1, characterized in that the suspension conducted to the wet machine or paper machine passes through a controllable valve (800), which is con­trolled by a sensor (801) for pressure, flow or level in the head box (8) of the wet machine or paper machine.

3. Process according to claim 1, characterized in that a portion of the flow which emerges cleaned from the second multi-hydrocyclone unit (7) is recycled for intake up­stream of the main pump (4).

4. Process according to claim 3, characterized in that the recycled flow is controlled by means of a valve (130) to achieve the desired pressure for the cleaned suspension coming from this second multi-hydrocyclone unit.

5. Process according to claim 3, characterized in that the reintroduction is arranged at the suction side of the main pump (4).

6. Process according to claim 3, characterized in that the reintroduction is arranged to a diluting device (300) upstreams of the suction side of the main pump (4).

7. Process according to claim 1, characterized in that to the suction side of the main pump (4) by means of a second pump (3) there is pumped a suspension which is controlled as to its flow rate and consistency, the suction side of said main pump (4) being coupled to a level regulated tank (10) included in the white water system of the wet machine or paper machine, dilution being effected with white water therefrom.

8. Process according to claim 1, characterized in that the second multi-hydrocyclone unit is provided with hydro­cyclones with conical separation chambers with a greatest diameter of less than 125 mm and preferably less than 100 mm, and with a base opening diameter which is less than a fifth of the largest diameter of the separation chamber and an apex opening which is greater than the base opening.

9. Device for manufacturing cellulose pulp with improved quality properties, particularly as regards the content of light, solid impurities such as particles of plastic and the like, characterized by the combination of a pump (4), a first multi-hydrocyclone unit (6) for separating out heavy particles, a second multi-hydrocyclone unit (7) for separating out light particles, and the headbox (8) of a wet machine or paper machine (9) coupled in series with each other, said second multi-hydrocyclone unit serving at the same time as a means for thickening the cleaned suspension.

10. Device according to claim 9, characterized by a con­trollable valve (800) which is coupled between the second multi-hydrocyclone unit (7) and the headbox, and means (801) for sensing and, by actuating the valve (800), con­trolling pressure, flow or the level in the headbox.

11. Device according to claim 9 or 10, characterized by a shunt line (13, 13′) which, for diverting suspension, is connected at a diversion point between the second multi-­hydrocyclone unit (7) and the headbox (8) and leads to an introduction point which is located upstream of said pump (4).

12. Device according to claim 11, characterized in that in the shunt line (13, 13′) there is a valve (130).

13. Device according to claim 12, characterized by a control device (131) which is disposed to control the valve (130) in response to the pressure at said diversion point.

14. Device according to claim 11, characterized in that said shunt line (13, 13′) leads to a diluting device disposed upstream of the pump (4).

15. Device according to claim 9, characterized in that the second multi-hydrocyclone unit is made of hydrocyclones with conical separation chambers with a largest diameter of at most 125 mm, preferably at most 100 mm, and with the base opening and an apex opening, the base opening dia­meter being less than a fifth of the largest diameter of the separation chamber and the apex opening being larger than the base opening.

Drawing