WO1998011297A1

WO1998011297A1 - A method and a plant for treating of a contaminated pulp suspension

Info

Publication number: WO1998011297A1
Application number: PCT/SE1997/001503
Authority: WO
Inventors: Roine Andersson; Jan Backman
Original assignee: Alfa Laval Ab
Priority date: 1996-09-16
Filing date: 1997-09-08
Publication date: 1998-03-19
Also published as: SE507387C2; NO982225D0; KR19990067424A; CA2235384A1; SE9603345D0; ID21089A; NO982225L; BR9706751A; JP2000500832A; EP0870084A1; SE9603345L

Abstract

Relatively heavy contaminants are separated from a contaminated pulp suspension by pumping the latter through a multihydrocyclone unit (6) having a multiplicity of hydrocyclones. Each hydrocyclone is provided with turbulence creating means adapted to counteract the formation of fibre network in radially outer liquid layers in the hydrocyclone. The pulp suspension separated from heavy contaminants is dewatered in a pulp wet machine (2). According to the invention, the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit (6) is at least 0,9 %.

Description

A method and a plant for treating of a contaminated pip r. suspension

The present invention relates to a method of treating a contaminated pulp suspension, in which relatively heavy contaminants are separated from the pulp suspension by pumping the latter through a multihydrocyclone unit containing a multiplicity of hydrocyclones, each of which is provided with turbulence creating means adapted to counteract formation of fibre network in radially outer liquid layers in the hydrocyclone. The pulp suspension separated from heavy contaminants is dewatered in a pulp wet machine. The invention also relates to a plant for treating a contaminated pulp suspension, comprising a multihydrocyclone unit of the kind stated above, a pump for pumping the pulp suspension through the multihydrocyclone unit, and a pulp wet machine for receiving cleaned pulp suspension.

In a factory for producing paper pulp the pulp wet machine, also called pulp drying machine, constitutes a last treatment step, in which the pulp is dewatered to a fibre concentration which makes the pulp suitable for drying and storing or for further transportation to a factory for paper production. The pulp which is dewatered in the pulp wet machine usually contains no filling compounds (so called filler) or insignificantly small amounts of filling compounds if the pulp is produced from recycled paper. Such a pulp free from filling compounds has the property that fibre network in the form of flocks of fibres easily are formed, since the pulp is particularly easy to dewater, which can disturb the hydrocyclone cleaning of the pulp by the flocks clogging the outlets of the hydrocyclones for contami- nants. However, the tendency to the formation of such fibre flocks reduces with reducing fibre concentration. In addition the cleaning efficiency reduces with increasing fibre concentration. For these reasons the pulp is conventionally diluted to a fibre concentration which is about 0,4 - 0,75 %. The fibre concentrations mentioned throughout the text all relate to weight percentage. The formation of fibre flocks is also counteracted actively by the turbulence creating means of the multihydrocyclone unit. Such turbulence creating means may comprise vibration creating means, for instance means for generating ultrasound or means arranged to mechanically knock against the walls of the hydrocyclones. Hydrocyclones with parti- cularly efficient turbulence creating means in the form of radially outwardly directed steps in the separation chambers of the hydrocyclones, as disclosed in WO 93/10908, are marketed by Alfa Laval Celleco AB under the designation StepRelease™ .

Most types of pulp wet machines have a limited de- watering capacity such that the fibre concentration of the pulp suspension which is supplied to the pulp wet machine usually has to be higher than the fibre concentration which is achieved at the cleaned pulp suspension leaving the multihydrocyclone unit. For this reason the cleaned pulp suspension first has to be thickened by means of a thickening device of any kind before the pulp suspension can be supplied to the pulp wet machine.

The investments and operation costs for plants with said multihydrocyclone unit and said necessary thickening device are considerable, because the multihydrocyclone unit and the thickening device have to be dimensioned for very large flows, between 40 000 and 180 000 litres/minute is usual. A plant comprising a multihydrocyclone unit for separating heavy contaminants, a thickening device and a pulp wet machine are known from EP-B-0 422 314.

The object of the present invention is to provide a method and a plant, respectively, for treating of contaminated pulp suspension which in comparison with the conventional technique described above results in substantially reduced flows of the pulp suspension which is treated, whereby the investments and operation costs are substantially reduced.

This object is obtained by the method mentioned initially, which is characterized in that the fibre concentration of the pulp suspension which is supplied to the multihydrocyclone unit is at least 0,9 %. It has surprisingly been proved that the hydrocyclones with the turbulence creating means are capable of cleaning a pulp suspension with the relatively high fibre concentration of 0,9 % without the cleaning efficiency becoming un- acceptably low. As a matter of fact, the cleaning efficiency can be satisfactorily maintained with increasing fibre concentration, up to about 1,5 %.

The fibre concentration of a pulp suspension which can be increased from for example 0,45 % to 0,9 % thanks to the invention thus results in a reduction of the flow through the multihydrocyclone unit by 50 % . The halved flow in turn results in that the number of hydrocyclones in the multihydrocyclone unit can be halved. Normally, fibres are recovered from the reject flow from the multihydrocyclone unit by means of a number of stages of hydrocyclones connected in cascade. Since the reject flow also is reduced thanks to the invention, the number of hydrocyclones connected in cascade in this example can be reduced by about 30 % .

According to the invention, the increase in the fibre concentration of uncleaned pulp suspension to at least 0,9 % has the consequence that it is not necessary to thicken the pulp suspension before the latter is supplied to certain types of pulp wet machines. Preferably, said fibre concentration is at least 1,0 % and at most 1,5 %, whereby the cleaned pulp suspension does not need to be thickened when using most types of pulp wet machines. In certain cases said fibre concentration may exceed 1,5 % , however not more than 3,0 % .

If relatively light contaminants are present in the pulp suspension, for instance if the pulp is produced from recycled paper, the light contaminants are advantageously separated from the pulp suspension by means of an additional multihydrocyclone unit before the pulp sus- pension is fed into the first mentioned multihydrocyclone unit for separating relatively heavy contaminants. Suitably, the pulp suspension separated from light contaminants is pumped from said additional multihydrocyclone unit to the first mentioned hydrocyclone unit. However, it is possible to conduct the separated pulp suspension directly to the first mentioned hydrocyclone unit. In this connection light contaminants also comprise particles which in themselves are heavier than fibres but because of their shape behave as lighter than fibres in the hydrocyclones.

Alternatively, light contaminants may be separated from the pulp suspension by means of said additional multihydrocyclone unit after that the heavy contaminants have been separated from the pulp suspension by means of the first mentioned multihydrocyclone unit and before the pulp suspension is dewatered in a pulp wet machine. In this case the pulp suspension separated from heavy contaminants is conducted from the first mentioned multihydrocyclone unit via a pump to said additional multihydrocyclone unit.

The object of the present invention is also obtained by the plant mentioned initially, which is characterized in that means are arranged to keep the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit at at least 0,9 %. Preferably, the multihydrocyclone unit is directly connected to the pulp wet machine via a transport conduit for transporting separa- ted pulp suspension from the multihydrocyclone unit to the pulp wet machine.

Suitably, said means are arranged to affect the fibre concentration of the pulp suspension such that pulp suspension which is fed to the multihydrocyclone unit has a fibre concentration which is at least 1,0 % and 3 % at maximum, preferably 1,5 % at maximum.

When needed an additional multihydrocyclone unit for separating relatively light contaminants from the pulp suspension is arranged upstream of the first mentioned multihydrocyclone unit and downstream of said pump. Said additional multihydrocyclone unit is suitably connected to the first mentioned multihydrocyclone unit via a connection conduit with an additional pump for transferring pulp suspension separated from light contaminants .

Suitably, a control device is adapted to control the capacity of said additional pump in response to the pressure in the connection conduit upstream of said additional pump. Alternatively, a control device may be adapted to control the capacity of said additional pump in response to the pressure in the inlet box of the pulp wet machine.

With advantage, a return conduit for cleaned pulp suspension extends from said transport conduit to the suction side of said pump and a control device is adapted to control a control valve in the return conduit in response to the pressure in the inlet box of the pulp wet machine.

Upstream of said additional pump and downstream of said additional multihydrocyclone unit said connection conduit is preferably provided with a counterpressure device adapted to maintain a constant counterpressure in an outlet for an accept fraction of said additional multihydrocyclone unit. The counterpressure device enables a return flow with recovered fibres from the reject fraction of the first mentioned hydrocyclone to be transferred back to the first mentioned multihydrocyclone unit without loading said additional multihydrocyclone unit with said return flow.

Further features of the plant according to the invention are defined in the attached claims.

The invention is described in more detail in the follo- wing with reference to the accompanying drawings, in which

figure 1 is a diagram showing how the cleaning efficiency depends on the fibre concentration of a pulp suspension in hydrocyclones with and without turbulence creating means, respectively,

figure 2 shows a flow chart of a plant according to a first embodiment of the invention,

figure 3 shows a modification of the embodiment according to figure 2,

figures 4, 5 and 6 show a flow chart of a plant according to a second, a third and a fourth embodiment of the invention, respectively,

figure 7 shows a modification of the embodiment accor- ding to figure 6, and

figure 8 shows a flow chart of a plant according to a fifth embodiment of the invention.

In the figures, identical components have been provided with the same reference numerals.

In figure 1 there is shown a diagram, in which the dependence of the cleaning efficiency η upon the fibre concentration C of a pulp suspension for an older conventional regular hydrocyclone is illustrated by a continuous curve line and for a newer regular hydrocyclone, which is provided with turbulence creating means of the kind shown in WO 93/10908, is illustrated by a dotted curve line. The pressure difference dP between the inlet and the accept outlet of each hydrocyclone in this case is 120 kPa. As is evident from the diagram the cleaning efficiency of a conventional hydrocyclone reduces from about 92 % at a fibre concentration of 0,5 % to about 87 % at a fibre concentration of 0,9 %, whereas the cleaning efficiency of the newer hydrocyclone still is as high as about 92 % at a fibre concentration of 0,9 %. This means that only 8 % contaminants are left in a pulp suspension, which has a fibre concentration of 0,9 % and which has been separated by the newer hydrocyclone, whereas 13 % contaminants are left in the same pulp suspension which has been separated by the older hydrocyclone. Thus the older hydrocyclone allows about 60 % more contaminants to pass through than the newer hydrocyclone when separating pulp suspensions having 0,9 % fibre concentration.

As is evident from the diagram, at a higher fibre concentration of 1,0 % the cleaning efficiency of the newer hydrocylone still is about 92 %, whereas the cleaning efficiency of the older hydrocyclone has reduced further to about 85 %. At fibre concentrations higher than 0,9 % the risk of clogging of the apex outlet of the older hydrocyclone with fibre network is dramatically increased, and for this reason in practice the older hydrocyclone is only utilized for pulp suspensions having a fibre concentration less than 0,9 %. However, without risking clogging the newer hydrocyclone can be utilized for the separation of pulp suspensions having a fibre concentration of up to 1,5 %. The limiting factor here is the poorer cleaning efficiency, not the risk of clogging.

In figure 2 there is shown a plant according to a first embodiment of the invention comprising a container 1 for a pulp suspension, which contains relatively heavy contaminants, a pulp wet machine 2 with an inlet box 3 for receiving cleaned pulp suspension, and a white water container 4 for receiving white water from the pulp wet machine 2. From the white water container 4 a conduit 5 extends to a multihydrocyclone unit 6, which comprises hydrocyclones with turbulence creating means of the kind shown in WO 93/10908 coupled in parallel. Each hydrocyclone in the multihydrocyclone unit 6 is conical with an inlet 6a for pulp suspension, an apex outlet 7 for a reject fraction containing relatively heavy contaminants and a base outlet 8 for an accept fraction containing cleaned pulp suspension. From the base outlet 8 a transport conduit 9 extends directly to the inlet box 3 of the pulp wet machine 2.

The conduit 5 is provided with a pump 10 and a protective screen 11 situated downstream of the pump 10. From the container 1 a conduit 12, which is connected to the conduit 5, extends upstream of the pump 10. Also the conduit 12 is provided with a pump 13, the capacity of which is controlled by a control device 14 in response to the basis weight of dewatered pulp leaving the pulp wet machine 2. The basis weight is sensed by a means 14a somewhere after the pulp wet machine. The capacity of the pump 10 is controlled by a control device 15 via a signal conduit 16. The control device 15 comprises pressure sensors, which sense the pressure in the inlet 6a via a signal conduit 17 and the pressure in the outlet 8 via a signal conduit 18.

A return conduit 19 for separated pulp suspension extends from the transport conduit 9 to the conduit 5. The return conduit 19 is provided with a control valve 20, which is controlled by a control device 21 adapted to sense the pressure in the inlet box via a signal conduit 22.

The reject fraction flowing through the apex outlet 7 of the multihydrocyclone unit 6 contains some fibres which are recovered by a multihydrocyclone unit 23 comprising hydrocyclones of the same kind as in the multihydrocyclone unit 6 coupled in parallel. (The fibres in the reject fraction are usually recovered by several stages of multihydrocyclone units coupled in cascade, but for reasons of simplicity only one such stage is shown here ) .

A conduit 24, which is connected to the conduit 5 upstream of the connections between the latter and the conduits 12 and 19, extends to the multihydrocyclone unit 23. In the conduit 24 there is a pump 25. The apex outlet 7 of the multihydrocyclone unit 6 is connected via a conduit 26 to the conduit 24 upstream of the pump 25. In the conduit 26 there is a control valve 27 which is controlled by a control device 28. The control device 28 comprises pressure sensors, which sense the pressure in the apex outlet 7 via signal conduits 29 and 30 and the pressure in the base outlet 8 (via the conduit 9). A base outlet 31 in the multihydrocyclone unit 23 is via a conduit 32 connected to the conduit 5 downstream of the connection between the conduit 5 and the conduit 24, for supplying recovered fibres to the pump 10. An apex outlet 33 of the multi- hydrocyclone unit 23 is connected to a container, not shown, for separated heavy contaminants.

During operation of the plant according to Fig . 2 pulp suspension is pumped from the container 1 by the pump 13 to the suction side of the pump 10 where the pulp suspension is diluted with white water from the white water container, so that the fibre concentration of the pulp suspension becomes at least 0,9 % and at most 3 %. The capacity of the pump 13 is controlled by the control device 14 so that the desired basic weight of the dewatered pulp in the pulp wet machine is achieved. The diluted pulp suspension is further pumped by the pump 10 via the protective screen 11 to the multihydrocyclone unit 6. The protective screen 11 separates possible coarse contaminants from the pulp suspension. The capacity of the pump 10 is controlled by the control device 15, so that the desired pressure difference between the inlet 6a and the base outlet 8 for operating the multihydrocyclone 6 at an optimum is maintained.

From the base outlet 8 the pulp suspension separated from relatively heavy contaminants is conducted to the inlet box 3 of the pulp wet machine 2. The pressure in the inlet box 6 is controlled by the control device 21, so that a part flow of the flow in the transport conduit 9 is brought back to the pump 10. Thus, the control device 21 increases said part flow by the control valve 20 if the pressure in the inlet box exceeds a set value and reduces said part flow by the control valve 20 if the pressure in the inlet box is less than said set value.

The pressure difference between the base outlet 8 and the apex outlet 7 of the multihydrocyclone unit is kept at a desired value by the control device 28 which controls the opening degree of the control valve 27. The reject fraction from the apex outlet 7 is diluted with the white water in the conduit 24 and is pumped by the pump 25 to the multihydrocyclone unit 23, which delivers an accept fraction with recovered fibres through the base outlet 31 and a reject fraction with heavy contaminants through the apex outlet 33. The accept fraction is conducted through the conduit 32 back to the pump 10. The fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit 6 is controlled by the control devices 14, 15 and 21. According to the invention these control devices 14, 15 and 21 cooperate so that the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit 6 is kept at at least 0,9 %.

In figure 3 there is shown the same plant as in figure 2 but with modified control devices. Thus, the capacity of the pump 10 is controlled by a control device 34 in response to the pressure in the inlet box 3. By means of a control device 35 the opening degree of the control valve 20 is controlled in response to the pressure difference between the inlet 6a and the base outlet 8 of the multihydrocyclone unit 6. In this case the control devices 14, 34 and 35 cooperate so that the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit 6 is kept at at least 0,9 %.

In figure 4 there is shown a plant according to a second embodiment of the invention which basically differs from the plant according to figure 2 by the fact that a multihydrocyclone unit 36 for separating relatively light contaminants from the pulp suspension is arranged upstream of the multihydrocyclone unit 6 and downstream of the pump 10. The multihydrocyclone unit 36 comprises a multiplicity of hydrocyclones of reverse type coupled in parallel, for instance Tripac 90 Reverse™ marketed by Alfa Laval Celleco AB and having an apex outlet 37, which via a connection conduit 38 is directly connected to the inlet 6a of the multihydrocyclone unit 6, and a base outlet 39, which is connected to a drain conduit 40. As opposed to the plant according to figure 2 the capacity of the pump 10 is not controlled to achieve a desired pressure difference in the multihydrocyclone unit 6. Instead said pressure difference is achieved by means of a control device 41, which controls the opening degree of a control valve 42 in the drain conduit 40, in response to the pressure in the inlet 6a and the pressure in the base outlet 8 of the multihydrocyclone unit 6.

The light reject fraction flowing through the base outlet 39 of the multihydrocyclone unit 36 contains some fibres which are recovered by a multihydrocyclone unit 43 comprising parallelly coupled conical hydrocyclones of the same kind as in the multihydrocyclone unit 36. (The fibres in the reject fraction are usually recovered by means of several stages of multihydrocyclone units coupled in cascade, but for reasons of simplicity only one such stage is shown here). A conduit 44 with a pump 45 extends from the conduit 24 to the multihydrocyclone unit 43. The drain conduit 40 is connected to the conduit 44 upstream of the pump 45. An apex outlet 46 of the multihydrocylone unit 43 is via a conduit 47 connected to the conduit 5 downstream of the connection between the conduit 5 and the conduit 24, for supplying recovered fibres to the pump 10. A base outlet 48 of the multihydrocyclone unit 43 is connected to a container, not shown, for separated light contaminants .

During operation of the plant according to figure 4, the pulp suspension is diluted with white water so that the fibre concentration of the pulp suspension which is supplied to the multihydrocyclone unit 36 becomes about 0,7 %, which gives a fibre concentration of about 1,5 % at the pulp suspension flowing through the apex outlet 37. The control device 41 controls the control valve 42 so that the desired pressure difference for operating the multihydrocyclone unit 6 at an optimum is maintained. In this case the fibre concentration becomes about 1,49 % at the pulp suspension flowing through the base outlet 8. The light reject fraction from the base outlet 39 is diluted with white water in the conduit 44 and is pumped by means of the pump 45 to the multihydrocyclone unit 43, which gives an accept fraction with recovered fibres through the apex outlet 46 and a light reject fraction with light contaminants through the base outlet 48. The accept fraction is conducted through the conduit 47 back to the pump 10. In addition to this, the plant according to figure 4 is operated in the same manner as the plant according to figure 2.

In figure 5 there is shown a plant according to a third embodiment of the invention which basically differs from the plant according to figure 4 by the fact that a pump 49 is arranged in the conduit 38. The capacity of the pump 49 is controlled by a control device 50 in response to the pressure in the apex outlet 37 of the multihydrocyclone unit 36. The capacity of the pump 10 is controlled by a control device 51 in response to the pressure in the inlet to the multihydrocyclone unit 36. The advantage of the plant according to figure 5 is that the energy consumption of the pumps 10 and 49 will be substantially less than for the single pump 10 in the plant according to figure 4.

During operation of the plant according to figure 5, the capacity of the pump 10 is controlled by the control device 51, so that the pressure in the inlet to the multihydrocyclone unit 36 is maintained at a desired value, and the capacity of the pump 49 is controlled by the control device 50, so that the pressure in the apex outlet 37 of the multihydrocyclone unit 36 is maintained at a desired value. In other words, the control devices 50 and 51 control the pressure difference between the inlet and the apex outlet 37 of the multihydrocyclone unit 36. In addition to this, the plant according to figure 5 is operated in the same manner as the plant according to figure 4.

In figure 6 there is shown a plant according to a fourth embodiment of the invention which basically differs from the embodiment according to figure 5 by the fact that the pump 49 and the control valves 18 and 42 are controlled differently. Thus, the connection conduit 38 comprises a first part 38a, which extends upwardly from the apex outlet 37 of the multihydrocyclone unit 36 to a first open container 52 with an overflow 53, and a second part 38b, which leads from a second open container 54 to the pump 49, the container 54 being arranged to receive pulp suspension from the container 52 via the overflow 53. The containers 52 and 54 and the overflow 53 constitute a counterpressure device adapted to maintain a constant counterpressure in the apex outlet 37 of the multihydrocyclone unit 36.

A level control means 55 is adapted to control the control valve 42 in response to the liquid surface level in the second container 54, so that said level is below the overflow 53. The capacity of the pump 49 is controlled in this case by a control device 56 in response to the pressure in the inlet box 3. The control valve 20 is controlled by a control device 57 in response to the pressure difference between the inlet 6a and the base outlet 8 of the multihydrocyclone unit 6. Thanks to the open container 54 the accept fraction with recovered fibres can be conducted via a return conduit 58 from the multihydrocyclone unit 23 to the container 54, whereby the pump 10 and the ultihydro- cyclone unit 36 do not need to be loaded by said accept fraction. The return conduit 58 is connected to the conduit 32 via a valve 59. There is also a valve 60 in the conduit 32. By means of the valves 59, 60 desired flows in the return conduit 58 and the conduit 32 can be adjusted. In addition to this, the plant according to figure 6 corresponds to the plant according to figure 5.

In figure 7 there is shown a modification of the plant according to figure 6. Thus, the return conduit 19 is replaced by a return conduit 61 extending from the transport conduit 9 via a control valve 62 to the open container 54. A control device 63 is adapted to control the control valve 62 in response to the pressure difference between the inlet 6a and the base outlet 8 of the multihydrocyclone unit 6. Hereby, there is no need for the cleaned pulp suspension, which is returned from the transport conduit 9, to load the pump 10 and the multihydrocyclone unit 36.

In figure 8 there is shown a plant according to a fifth embodiment of the invention which basically differs from the plant according to figure 2 by the fact that a multihydrocyclone unit 64 of the same kind as the above described multihydrocyclone unit 36, for separating relatively light contaminants from the pulp suspension, is arranged downstream of the multihydrocyclone unit 6 and upstream of the pulp wet machine 2. A conduit 65 extends from a pump 66 to an inlet 67 of the multihydrocyclone unit 64. From the base outlet 8 of the multihydrocyclone unit 6 a conduit 68 extends via a valve 69 to the suction side of the pump 66. A transport conduit 70 extends from an apex outlet 71 of the multihydrocyclone unit 64 to the inlet box 3. A control device 72 is adapted to control the capacity of the pump 66 in response to the pressure difference between the inlet 67 and the apex outlet 71 of the multihydrocyclone unit 64.

The fibres in the light reject fraction from the multi- hydrocyclone unit 64 is recovered by a recover stage comprising a multihydrocyclone unit 73 of the same kind (but having fewer hydrocyclones) as the multihydrocyclone unit 64. (More recover stages comprising multihydrocyclone units coupled in cascade may of course be arranged). A conduit 74 with a pump 75 extends from the conduit 5 to the multihydrocyclone unit 73. A drain conduit 76 with a control valve 77 is connected to the conduit 74 upstream of the pump 75. The control valve 77 is controlled by a control device 78 in response to the pressure in the transport conduit 70. An apex outlet 79 of the multihydrocyclone unit 73 is via a conduit 80 connected to the conduit 5 and to the suction side of the pump 66.

A return conduit 81 for separated pulp suspension extends from the transport conduit 70 to the suction side of the pump 66. The return conduit 81 is provided with a control valve 82, which is controlled by a control device 83 in response to the pressure in the inlet box 3.

During operation of the plant according to figure 8 the multihydrocyclone unit 6 is supplied with a pulp suspension having a fibre concentration in the range of 0,9 - 1,3 %. The pulp suspension separated from heavy contaminants will have a fibre concentration in the range of 0,85 - 1,25 % and is diluted with the accept fraction from the multihydrocyclone unit 73 to a fibre concentration in the range of 0,6 - 0,7 %. The pulp suspension diluted in this manner is pumped by the pump 66 to the multihydrocyclone unit 64, which via the apex outlet 71 delivers a fraction of the pulp suspension separated from light contaminants with a fibre concentration in the range of 1,3 - 1,7 %. Via the transport conduit 70 the pulp suspension separated from heavy and light contaminants is transported directly to the inlet box 3 of the pulp wet machine 2.

Claims

1. A method of treating a contaminated pulp suspension, in which relatively heavy contaminants are separated from the pulp suspension by pumping the latter through a multihydrocyclone unit (6) which comprises a multiplicity of hydrocyclones, each of which is provided with turbulence creating means adapted to counteract formation of fibre network in radially outer liquid layers in the hydrocyclone, and the pulp suspension separated from heavy contaminants is dewatered in a pulp wet machine (2), c h a r a c t e r i z e d i n that the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit ( 6 ) is at least 0,9 %.

2. A method according to claim 1, c h a r a c t e r i z e d i n that the pulp suspension which is fed into the multihydrocyclone unit ( 6 ) has a fibre concent- ration which is at least 1,0 %.

3. A method according to claim 1 or 2, c h a r a c t e r i z e d i n that the fibre concentration of the pulp suspension which is fed into the multihydro- cyclone unit (6) is 3,0 % at maximum.

4. A method according to claim 1, c h a r a c t e r i z e d i n that the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit (6) is at least 1,0 % and 1,5 % at maximum.

5. A method according to any one of claims 1-4, c h a r a c t e r i z e d i n that relatively light contaminants are separated from the pulp suspension by an additional multihydrocyclone unit (36) before the pulp suspension is fed into the first mentioned multihydrocyclone unit (6) for separating heavy contaminants

6. A method according to claim 5, c h a r a c t e - r i z e d i n that the pulp suspension separated from relatively light contaminants is conducted from said additional multihydrocyclone unit (36) directly to the first mentioned multihydrocyclone unit ( 6 ) .

7. A method according to claim 5, c h a r a c t e r i z e d i n that the pulp suspension separated from relatively light contaminants is conducted from said additional multihydrocyclone unit (36) via a pump (49) to the first mentioned multihydrocyclone unit ( 6 ) .

8. A method according to any one of claims 1-7, c h a r a c t e r i z e d i n that the pulp suspension separated from relatively heavy contaminants is conducted from the multihydrocyclone unit ( 6 ) for separating heavy contaminants directly to the pulp wet machine ( 2 ) without undergoing any intermediate thickening.

9. A method according to any one of claims 1-4, c h a r a c t e r i z e d i n that relatively light contaminants are separated from the pulp suspension by means of an additional multihydrocyclone unit ( 64 ) after that the heavy contaminants have been separated from the pulp suspension by the first mentioned multihydrocyclone unit ( 6 ) for separating heavy contaminants and before the pulp suspension is dewatered in the pulp wet machine ( 2 ) .

10. A method according to claim 9, c h a r a c t e r i z e d i n that the pulp suspension separated from relatively heavy contaminants is conducted from the first mentioned multihydrocyclone unit ( 6 ) via a pump ( 66 ) to said additional multihydrocyclone unit ( 64 ) .

11. A plant for treating a contaminated pulp suspension, comprising a multihydrocyclone unit (6), which has a multiplicity of hydrocyclones, for separating relatively heavy contaminants from the pulp suspension, each hydrocyclone being provided with turbulence creating means adapted to counteract the formation of fibre network in radially outer liquid layers in the hydro- cyclone, a pump (10) for pumping the pulp suspension through the multihydrocyclone unit, and a pulp wet machine ( 2 ) with an inlet box ( 3 ) for receiving pulp suspension separated from heavy contaminants, c h a r a c t e r i z e d i n that means (14, 15, 21; 14, 34, 35; 14, 21, 41; 14, 56, 57; 14, 56, 63; 14, 15) are arranged to keep the fibre concentration of the pulp suspension which is fed into the multihydrocyclone unit (6) at at least 0,9 %.

12. A plant according to claim 11, c h a r a c t e r i z e d i n that said means are arranged to keep said fibre concentration at at least 1,0 %.

13. A plant according to claim 11 or 12, c h a - r a c t e r i z e d i n that said means are arranged to keep said fibre concentration at 3,0 % at maximum.

14. A plant according to claim 11, c h a r a c t e r i z e d i n that said means are arranged to keep said fibre concentration at at least 1,0 % and at 1,5 % at maximum.

15. A plant according to any one of claims 11-14, c h a r a c t e r i z e d i n that the multihydrocyclone unit ( 6 ) for separating heavy contaminants is directly connected to the pulp wet machine ( 2 ) via a transport conduit ( 9 ) for transporting pulp suspension to the pulp wet machine.

16. A plant according to claim 15, c h a r a c t e r i z e d i n that a return conduit (19) for cleaned pulp suspension extends from said transport conduit (9) to the suction side of said pump (10), a control valve (20) is arranged in the return conduit, and a control device (21) is adapted to control the control valve in response to the pressure in the inlet box ( 3 ) of the pulp wet machine ( 2 ) .

17. A plant according to claim 15 or 16, c h a r a c t e r i z e d i n that the multihydrocyclone unit ( 6 ) comprises an inlet ( 6a ) for pulp suspension to be separated and an outlet ( 8 ) for separated pulp suspension, and that a control device (15) is adapted to control the capacity of said pump (10) in response to the pressure difference between the inlet and the outlet of the multihydrocyclone unit.

18. A plant according to claim 15, c h a r a c t e r i z e d i n that a control device ( 34 ) is adapted to control the capacity of said pump ( 10 ) in response to the pressure in the inlet box (3) of the pulp wet machine ( 2 ) .

19. A plant according to claim 15 or 18, c h a r a c t e r i z e d i n that a return conduit ( 19 ) for cleaned pulp suspension extends from said transport conduit (9) to the suction side of said pump (10), a control valve (20) is arranged in the return conduit, the multihydrocyclone unit ( 6 ) comprises an inlet ( 6a ) for pulp suspension to be separated and an outlet ( 8 ) for separated pulp suspension, and a control device (35) is adapted to control the control valve in response to the pressure difference between the inlet and the outlet of the multihydrocyclone unit.

20. A plant according to any one of claims 11-14, c h a r a c t e r i z e d i n that an additional multihydrocyclone unit (36) for separating relatively light contaminants from the pulp suspension is arranged upstream of the first mentioned multihydrocyclone unit ( 6 ) for separating heavy contaminants and downstream of said pump ( 10 ) .

21. A plant according to claim 20, c h a r a c t e r i z e d i n that said additional multihydrocyclone unit (36) is directly connected to the first mentioned multihydrocyclone unit (6) via a connection conduit (38) for transferring pulp suspension separated from light contaminants .

22. A plant according to claim 20, c h a r a c t e r i z e d i n that said additional multihydrocyclone unit (36) is connected to the first mentioned multihydrocyclone unit (6) via a connection conduit (38) for transferring pulp suspension separated from light contaminants, and that an additional pump (49) is arranged in the connection conduit.

23. A plant according to claim 22, c h a r a c t e r i z e d i n that a control device ( 50 ) is adapted to control the capacity of said additional pump (49) in response to the pressure in the connection conduit (38) upstream of said additional pump.

24. A plant according to claim 22, c h a r a c t e r i z e d i n that a control device (56) is adapted to control the capacity of said additional pump (49) in response to the pressure in the inlet box ( 3 ) of the pulp wet machine ( 2 ) .

25. A plant according to claim 22 or 23, c h a r a c t e r i z e d i n that upstream of said additional pump (49) and downstream of said additional multihydrocyclone unit (36) said connection conduit (38) is provided with a counterpressure device (52-54) adapted to maintain a constant counterpressure in an outlet (37) for an accept fraction of said additional multihydrocyclone unit.

26. A plant according to claim 25, c h a r a c t e r i z e d i n that the counterpressure device comprises a first open container ( 52 ) with an over- flow (53), and a second open container (54) arranged to receive pulp suspension from the first container via the overflow.

27. A plant according to claim 26, c h a r a c t e - r i z e d i n that the connection conduit (38) has a first part ( 38a ) , which extends upwardly from said additional multihydrocyclone unit (36) to the first open container (52), and a second part (38b), which extends from the second open container ( 54 ) to said additional pump ( 49 ) .

28. A plant according to claim 27, c h a r a c t e r i z e d i n that said additional multihydrocyclone unit (36) comprises a base outlet (39) for a light reject fraction, that a drain conduit (40) with a control valve (42) extends from the base outlet (39), and that a level control means (55) is adapted to control the control valve ( 42 ) in response to the level of the liquid surface in the second container ( 54 ) , such that said level is below the overflow ( 53 ) .

29. A plant according to claim 27 or 28, c h a r a c t e r i z e d i n that the firstmentioned multihydrocyclone unit ( 6 ) comprises an outlet ( 7 ) for a reject fraction, which outlet is connected to at least one multihydrocyclone unit (23) for recovering fibres from the reject fraction, and that a return conduit (58) is arranged to transfer an accept fraction with recovered fibres from said multihydrocyclone unit ( 23 ) for recovering fibres to said second open container ( 54 ) .

30. A plant according to any one of claims 27-29, c h a r a c t e r i z e d i n that the first mentioned multihydrocyclone unit ( 6 ) comprises an inlet (6a) for pulp suspension and an outlet (8) for separated pulp suspension, that a return conduit (61) with a control valve (62) extends from said transport conduit ( 9 ) to said second open container ( 54 ) , and that a control device ( 63 ) is adapted to control the control valve in response to the pressure difference between the inlet and the outlet of the first mentioned multihydrocyclone unit.

31. A plant according to any one of claims 20-30, c h a r a c t e r i z e d i n that the first mentioned multihydrocyclone unit ( 6 ) is directly connected to the pulp wet machine ( 2 ) via a transport conduit ( 9 ) for transporting pulp suspension separated from heavy contaminants from the multihydrocyclone unit to the pulp wet machine.

32. A plant according to any one of claims 11-14, c h a r a c t e r i z e d i n that an additional multihydrocyclone unit (64) for separating relatively light contaminants from the pulp suspension is arranged downstream of the first mentioned multihydrocyclone unit ( 6 ) for separating heavy contaminants and upstream of the pulp wet machine ( 2 ) .

33. A plant according to claim 32, c h a r a c t e - r i z e d i n that an additional pump (66) is adapted to pump pulp suspension from the first mentioned multihydrocyclone unit (6) to said additional multihydrocyclone unit ( 64 ) .

34. A plant according to claim 33, c h a r a c t e r i z e d b means (64, 76, 75, 73, 80) for diluting the pulp suspension that said additional pump (66) is adapted to pump.