NO178348B - Method and apparatus for producing cellulose pulp with improved quality properties - Google Patents

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

The invention relates to a method and device for treating cellulose fiber suspensions, which are to be purified and concentrated, the purification also comprising the elimination of light contaminants.

Purification of cellulose suspensions from heavy contaminants, such as sand, bark and short chips, now normally takes place with the help of hydrocyclones, where contaminants are removed in the tip fraction and purified suspension in the base fraction. Such cleaning requires a low fiber content, 0.1-3%, preferably 0.2-2%.

Purification of light pollution using hydrocyclones is also previously known, particularly when it comes to the treatment of recycled paper which often contains significant amounts of plastic waste as well as printing ink residues etc. The hydrocyclones that are normally used for this are sized and arranged so that the base fraction contains light contaminants and the tip fraction the essential part of added cellulose fibre. Hydrocyclones for this purpose are also known, where both fractions are removed at one and the same end in relation to the outlet.

The first-mentioned type of hydrocyclones is referred to below as A cyclones, while the latter is referred to as B cyclones.

As hydrocyclones are not perfect separators, recovery units are always required for economic reasons that recover cellulose fibers from the reject quantity. Such recycling is previously known. Regarding recovery by B cyclones, reference is made to Swedish patent application 8802580-4 of 8 July 1988, which describes hydrocyclones which are particularly suitable for such recovery.

In current facilities suitable for the production of paper and market pulp, it is common that the one way or another

(boiling, defibrating, bleaching, grinding down recycled paper)

prepared suspension is first sped to be able to be sieved and/or cyclone cleaned, after which the purified suspension is dewatered to be able to be stored temporarily. This dewatering normally takes place in open dewatering facilities. In order to be able to feed the mass to a recording machine, it must be diluted once more, admittedly as a rule to a concentration that is higher than that used in the purification step. This thickening, intermediate storage and screening results in an extra

costs that can be eliminated by thickening the mass directly from the cyclone cleaning stage and leading it to the recording machine, in particular by using one and the same pump for the primary stage of the cyclone cleaning and a pressure-operated, closed thickener/dewaterer, as stated in Swedish design document 8305036-9.

However, as mentioned, there is an increasing need for the removal of light pollution, primarily plastic. It goes without saying that this need exists for recycled paper, but also wood raw material, especially from populated areas, contains more and more of such plastic pollution. Plastic particles also enter internally in production. In papermaking, these contaminants cause difficulties even in small amounts, e.g. when coating, as streaks may occur in the paper web.

It is an aim of the invention to provide an improved system for the production of good quality pulp, particularly with regard to the elimination of contaminants, both in the form of heavy and light particles (by light particles is meant not only particles of low density, but also such as, due to their shape, in hydrocyclones behave as lighter than cellulose fibres).

It is another object to provide a system in which one avoids thickening already cleaned pulp suspension with a view to intermediate storage, and instead in an essentially continuous process in order carries out cleaning, thickening and final processing in a recording machine or paper machine.

These and other purposes and advantages are achieved according to the invention by a method according to claim 1 and by a device according to claim 9.

A particular advantage of the invention is that a synergistic effect is achieved by combining the functions of thickening and cleaning from light contamination, preferably during the simultaneous elimination of an intermediate storage step and a screening step.

What makes the invention possible is that a suspension fraction that comes out of the spisis in a hydrocyclone arranged and constructed for the elimination of light pollutants has a higher consistency compared to the supplied suspension, as a lot of water is included with the light pollutants in the base fraction.

A particularly high consistency can be achieved, with very good screening of light contaminants, if one accepts that some cellulose fibers end up in the base fraction, as recycling is facilitated by the fact that the number of recycling steps can be reduced by using hydrocyclones of the type described in Swedish patent application 8802580-4, which is not yet generally available.

According to the invention, after combined cleaning from light impurities and thickening, it is possible to achieve a consistency between e.g. 1 and 3%, i.e. for many types of recording machines and paper machines suitable input consistency. Hydrocyclones, in accordance with their nature, must be operated with an essentially constant liquid flow, while the output consistency is determined by the consistency of the supplied mass flow. The current to the recorder/paper machine can later be further varied through a return line that leads back to an adjustable or adjustable part of the output current to the cleaning system's input side.

It is appropriate to let the recorder or paper machine's backwater silo be equipped with an overflow to keep the level constant, and from there to collect the necessary leaching water for the process.

Hydrocyclone systems usually work best under the condition that the liquid flows and pressure conditions are kept fairly constant at their rated values. However, there may be a need to, in different operating cases, vary the flow to a connected recording machine or paper machine. It is therefore appropriate to arrange a diversion of the treated suspension, which returns such unused but purified and concentrated suspension. This return can take place to a point immediately in front of the main pump, without the fiber flow to the recording machine or the paper machine being changed, but with the result that the consistency of the suspension to the inlet box is increased. Alternatively, the return can take place to a point before the concentration regulation that occurs in connection with a first discharge, as the consistency of the suspension in the inlet box becomes constant, and the flow to the inlet box is reduced.

The invention will now be described on the basis of a non-limiting exemplary embodiment which is schematically shown in diagram form in the drawing figure.

The schematic figure shows an example of a plant that works in accordance with the invention. The main part of the fiber material that passes through goes from a conventional silo 1 to a recording machine or paper machine 9 in a thick-lined main road. Mass with a consistency of 10-12% is poured into the lower part of the silo 1 to approx. 3.5%, is sped at the entrance to a pump 103 to a regulated consistency of 3% and is led to a silo 2 with a level that is kept constant. Waste water is collected via a pump 12 from the waste water of the recording or paper machine. How the regulation of level and consistency for the silo 2 is arranged is realized by the person skilled in the art, why it is sufficient to state that the level is regulated through the influence of the valves 100 and 101 of the sensor 102 and the level through the level sensor 2 00 which affects the speed regulator 104 of the pump 103. 105 are regulating means which open the valves 100 and 101 in parallel and control the flow through them in a quota ratio. In the silo 2, a constant level and a constant consistency for the suspension is thereby maintained.

The suction side of the pump 3 is connected to the silo 2, and for further suction from the pump 12 via the valve 300 which is controlled by the sensor 301 which senses the consistency of the suspension that comes out on the pressure side of the pump 3. The consistency is thereby regulated to 2.5%. Through a valve 400 under the control of a flow sensor 401, the flow of the supplied suspension is controlled to a desired value. Thereby, the flow of fibers through the valve 400 is kept constant.

A regulated flow of suspension with a regulated consistency (fibre concentration) is thus directed towards the suction side of the pump 4.

One can for the moment disregard the line 13 which concerns 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 of the backwater container 10, which level is kept constant in a known manner. The pump 4 emits as long as its back pressure is stable a specific flow, with a consistency that is determined by the amount of fiber that per time unit passes through the valve 400. With the values according to the example, a consistency of 0.5% is achieved on the pressure side of the pump 400, which is an appropriate value for cyclone cleaning, although both lower and higher consistencies are possible to work with. The thus diluted suspension is led via a strainer 5, which is primarily intended to eliminate scrap particles and other coarser particles that could damage the cyclones, to a multihydrocyclone unit 6 with A cyclones whose base fraction is led on. The consistency of the base fraction is 0.45%, i.e. somewhat lower than the supplied suspension. This base fraction is led on without intermediate pumping to a multihydrocyclone unit 7 with B cyclones, where the fiber is taken out as tip fraction, while leader particles go in the base fraction.

In this tip fraction, the cellulose fibers are enriched to a higher consistency, in the example case to 1.5%, i.e. by a factor of 3.33. This consistency is suitable for the subsequent recorder or paper machine 9 to whose inlet box 8 the suspension is directed via a valve 800, controlled by a sensor 801, in this case a pressure sensor. The inlet box thereby receives the suspension with regulated pressure. If it is more appropriate, the sensor 801 can be a level sensor or a flow sensor.

The two multihydrocyclone aggregates 6 and 7 include recovery units, shown here with damage-coupled aggregates each with their own pumps on the inlet, and their suction sides connected to a common level in the silo 10. Although only two stages in each unit are shown, the number of stages is determined taking into account desired degree of recovery and purification. The recovery units for aggregate 6 consist of an aggregate of A cyclones, while those for aggregate 7 are an aggregate of B cyclones. The number of steps in the recovery from the aggregate 7 is beneficially affected, as are the individual cyclones in the aggregates, if the latter is constructed in accordance with Swedish patent application 8802580-4.

In the example, through the unit 7, both a purification from light contamination and a thickening of the thus purified suspension takes place, which combination is the great merit of the invention. This thickening can be arranged to varying degrees depending on cyclone dimensions, pressure drop and distribution of the tip and base flow of incoming suspension. The degree of thickening, which always exceeds 1, and in the example case is 3.33, can be driven significantly higher, e.g. to 5, if necessary, and this without the separation effect for light impurities diminishing: on the contrary, it tends to further increase. Thereby, the base fraction's fiber content can increase, so that the processing work in the recycling steps increases, which can nevertheless be mastered by using hydrocyclones in these according to Swedish patent application 8802580-4.

The aforementioned thickening is the result of separation between water and fibres. The degree of thickening in a hydrocyclone is determined by this separation efficiency. To achieve a high degree of thickening, the hydrocyclone is dimensioned with a smaller base opening than the tip opening. When it comes to this separation work, the conical separation space should also 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 one fifth of the largest diameter of the separation space.

As the main flow of fiber has now been explained, it can be noted that the recovery flows from the recovery steps via the line 11' go back to the suction side of the pump 4, and this implies, if the line 13 did not exist, that under stability conditions the entire fiber flow entering via the valve 400 be discharged to the inlet box 8. The flow is then determined by the capacity of the pump 4 as well as the outflows determined by the normally constant pressure conditions in the tip fraction of the aggregate 6 and in the base fraction of the aggregate 7.

Hydrocyclones should, in order to work optimally, work with predetermined liquid flows. However, it is not obvious that this is the case with the inlet box of a recorder or paper machine, where it may be necessary to vary both volume flow, pressure and consistency. With a constant current, the consistency can be regulated by regulating the flow and consistency of the suspension supplied via the valve 400. The volume flow to the inlet box 8, on the other hand, is regulated through a shunt line 13

with valve 13 0, controlled by a pressure sensor 131. In accordance with the first embodiment, this shunt line 13 can open out at the suction side of the pump 4, which means that the fiber flow to

the inlet box 8 remains unchanged, but the consistency is changed.

In accordance with a second embodiment, shown with a broken line in the figure, this return flow via the line 13' returns to a point in front of the pump 3, in this case in front of the flow control valve 300, but in each case to a point in front of the consistency - and flow regulation that determines the fiber flow that is supplied to the pump's 4 suction side. A regulation of the liquid flow to the inlet box 8 by means of the valve 130 does not then affect the consistency of the supplied suspension.

The invention is now described on the basis of a non-limiting exemplary embodiment. It is clear from the example that such a facility can be regulated within wide limits. Although the capacity in the stream for the cyclone equipment is given by the construction, it is possible within wide limits to vary the capacity in the mass flow and the consistency of the discharged suspension.

As mentioned, it is also possible through construction and regulatory measures within the framework of the invention to vary, e.g. the degree of thickening in the plant.

In the plant, as shown, one can particularly notice that a single main pump 4 leads the diluted suspension to the series-connected aggregates 6 and 7 and on to the inlet box 8. In a representative example, the pressure drop between inlet and base in an A- cyclone 1 bar, and the pressure drop in a B cyclone between inlet and tip 1.5 bar. These pressure drops are essentially a function of the flow. The pump 4 is dimensioned so that it can both compensate for these pressure drops and provide the necessary pressure to the inlet box 8.

The regulation of the valves 800 and 130 must be done in such a way that the working conditions for the multihydrocycling aggregates are not adversely affected. The main pump 4 is a centrifugal pump, driven at an essentially constant speed, with a decreasing head with increasing flow. The construction is arranged in such a way that at rated flow the pressure head is sufficient for the pressure drop in the multihydrocyclone aggregates 6 and 7 as well as the valve 800 at a normal working point, and a regulation within reasonable limits of the flow through the valve 800 to maintain pressure or level in the inlet box 8 does not cause any significant disturbance. Larger changes in the pressure drop across the valve 800 cannot, however, be tolerated, as the working conditions for the aggregates 6 and 7 are then adversely affected. In order to make significant changes in the supply flow to the box 8, a diversion via the shunt valve 130 is therefore required, so that the pressure on the suspension can be kept reasonably constant. The shunt valve 130 can then either be regulated by hand against a manometer or pressure regulated in the manner shown in the figure. The various regulations in the figure are shown as mutually independent, local regulations. However, it will be realized that such regulation can take place under joint data management.

Claims (15)

1. Process for the production of cellulose pulp with improved quality properties, particularly with regard to the content of light, solid contaminants such as plastic particles and the like, and where a fiber suspension of low consistency under the influence of a main pump (4) is brought to pass a first multihydrocyclone unit (6) for the separation of heavy particles and by means of raising the pulp consistency, at least part of the thus treated pulp suspension being directly, without intermediate pumping, then led to the inlet box (8) in a recording machine or paper machine, characterized in that a second multihydrocyclone unit (7) is included in the same pump circuit for the separation of light particles, whereby at the same time the mentioned medium function for raising the mass consistency is fulfilled. 2. Method according to claim 1, characterized in that the suspension led to the recording machine or the paper machine passes through a controllable valve (800) which is controlled by a sensor (801) for pressure, flow or level in the inlet box (8) of the recording or the paper machine. 3. Method according to claim 1, characterized in that part of the purified stream coming out of the second multihydrocyclone unit (7) is returned to be re-intake upstream of the pump (4). 4. Method according to claim 3, characterized in that the returned flow is controlled by means of a valve (130), in order to achieve the desired pressure for the purified suspension coming from the second multihydrocyclone unit. 5. Method according to claim 3, characterized in that the re-intake is arranged at the suction side of the main pump (4). 6. Method according to claim 3, characterized in that the re-intake is arranged to a discharge device (300) arranged in front of the suction side of the main pump (4). 7. Method according to claim 1, characterized in that a suspension controlled with respect to its flow rate and consistency is led to the suction side of the main pump (4) by means of a second pump (3), the suction side of the main pump (4) being connected to a level-regulated silo (10) which is part of the recording machine's or paper machine's tailwater system, whereby discharge takes place with tailwater from there. 8. Method according to claim 1, characterized in that the second multihydrocyclone unit is made up of hydrocyclones with a conical separation chamber with a largest diameter less than 125 mm and preferably less than 100 mm, and with a base opening diameter less than one fifth of the separation chamber's largest diameter and a tip opening which is larger than the base opening. 9. Device for the production of cellulose pulp with improved quality properties, particularly with regard to the content of light, solid contaminants, such as plastic particles and the like. of a fiber suspension of low consistency, comprising in combination a pump (4), a multihydrocyclone unit (6) for separating heavy particles, means for thickening the purified suspension and the inlet box (8) of a recorder or paper machine (9) connected in order one after the other, characterized in that the device comprises a second multihydrocyclone unit (7) for separating light particles, the second multihydrocyclone unit simultaneously constituting the means for thickening the purified suspension. 10. Device according to claim 9, characterized by a controllable valve (800) which is connected between the second multihydrocyclone unit (7) and the inlet box, as well as means (801) for sensing and through the influence of the valve (800) regulates pressure, flow or level in the inlet box. 11. Device according to claim 9 or 10, characterized in that a shunt line (13, 13') for diversion of suspension is connected in a diversion point between the second multihydrocyclone unit (7) and the inlet box (8) and leads to a supply point which is located upstream for the aforementioned pump (4). 12. Device according to claim 11, characterized in that there is a valve (130) in the shunt line (13, 13'). 13. Device according to claim 12, characterized by a control device (131) which is arranged to control the valve (130) in dependence on the pressure at the said diversion point. 14. Device according to claim 11, characterized in that the aforementioned shunt line (13, 13') leads to a discharge device arranged upstream of the pump (4). 15. Device according to claim 9, characterized in that the second multi-hydrocyclone unit is made up of hydrocyclones with a conical separation chamber with the largest diameter of no more than 125 mm, preferably no more than 100 mm, and with a base opening and a tip opening where the base opening diameter is less than a fifth of the separation space's largest diameter, and the tip opening is larger than the base opening.