METHOD AND PLANT FOR TREATING A CONTAMINATED PULP SUSPENSION

Abstract: With the aid of hydrocyclones, relatively light and heavy contaminants are separated from a pulp suspension. According to the invention, at first light contaminants are separated from the pulp suspension and the fibre concentration of the pulp suspension is substantially increased by means of a first multihydrocyclone unit (6) comprising a multiplicity of parallelly coupled hydrocyclones designed for separating light contaminants, and thereafter heavy contaminants are separated from the pulp suspension with the increased fibre concentration by a second multihydrocyclone unit (13) comprising a multiplicity of parallelly coupled hydrocyclones designed for separating heavy contaminants.

Description: Method and plant for treating a contaminated pulp suspension The present invention relates to a method of treating a pulp suspension containing relatively light and heavy contaminants, the contaminants being separated from the pulp suspension with the aid of hydrocyclones. The invention also relates to a plant for treating such a pulp suspension comprising a first niuitihydrocyclone unit having a multiplicity of parallelly coupled hydrocyclones designed for separating light contaminants from the pulp suspension under substantial thickening of the pulp suspension, and a second niuitihydrocyclone unit having a multiplicity of parallelly coupled hydrocyclo- nes designed for separating heavy contaminants from the pulp suspension. The plant further comprises at least one pump for pumping the pulp suspension to be separated to the niuitihydrocyclone units. With the expression "relatively light and heavy contaminants" is meant such contaminants which are light and heavy relative to the fibres of the pulp suspension. In this connection, relatively light contaminants also comprise particles which in themselves are heavier than fibres but because of their shape behave as lighter fibres in the hydro- cyclones. Conventionally, light and heavy contaminants are separated from pulp suspensions by first removing the heavy contaminants by means of a "regular" type of hydrocyclone, whereafter the light contaminants are separated by means of a "reverse" type of hydrocyclone. A hydrocyclone plant comprising such a regular hydrocyclone and such a reverse hydrocyclone arranged downstream of the regular hydrocyclone is disclosed in for instance CA 1 203 778. Since the density of light contaminants, such as plastic and glue fragments, is close to the density of wood fibres, the concentration of fibres of a pulp suspension which is separated from such light contaminants by a reverse hydrocyclone has to be relatively low, about 0.4 - 0.7 %, (the fibre concentrations mentioned throughout the text all relate to weight percentage) in order to make the separation efficiency good. For this reason, the regular hydrocyclone is conventionally fed with a pulp suspension having a fibre concentration in the range of 0.4 - 0.75 %, which gives an outgoing pulp suspension which is suitable for feeding the reverse hydrocyclone, since the regular hydrocyclone somewhat dilutes the pulp suspension. Thus, the flows of pulp suspension which are supplied to the regular hydrocyclone and the reverse hydrocyclone are of about the same sizes, which makes it advantageous to utilize one single pump to pump the pulp suspension both through the regular hydrocyclone and the subsequent reverse hydrocyclone, see for instance EP-B-0 422 314. The investment and operation costs for plants of the kind described above are significant, since the ulti- hydrocyclone units have to be dimensioned for very large flows, between 40 000 and 180 000 litres/minute is usual . The object of the present invention is to provide a method and a plant of the kinds here presented, which in comparison with the above described conventional technique results in substantially reduced flows of the pulp suspension which is treated, whereby investments and operation costs are substantially reduced. This object is obtained by the method stated initially, which is characterized in that at first light contaminants are separated from the pulp suspension and the fibre concentration of the pulp suspension is substan- tially increased by means of a first niuitihydrocyclone unit comprising a multiplicity of parallelly coupled hydrocyclones designed for separating light contaminants, and thereafter heavy contaminants are separated from the pulp suspension with the increased fibre con- centration by means of a second ultihydrocyclone unit comprising a multiplicity of parallelly coupled hydrocyclones designed for separating heavy contaminants. As a result, the number of hydrocyclones in the second niuitihydrocyclone unit can be substantially reduced, since the flow of pulp suspension, which is separated from light contaminants, from the first niuitihydrocyclone unit is substantially smaller than the flow of pulp suspension which is fed into the first niuitihydrocyclone unit. The reduced flow through the second niuitihydrocyclone unit also results in a reduced need for energy to pump the flow. For instance, it is quite possible to increase the fibre concentration of a pulp suspension from 0.4 % to 0.8 % by means of the first niuitihydrocyclone unit, which reduces the necessary number of hydrocyclones in the second multihydrocyclone unit by 50 %, since the flow through the second niuitihydrocyclone unit is halved. The pulp suspension with the increased fibre concentra- tion is advantageously conducted from the first multi- hydrocyclone unit via a pump to the second multihydro- cyclone unit. Each hydrocyclone of the second niuitihydrocyclone unit is preferably provided with turbulence increasing means adapted to counteract the formation of fibre net-work in radially outer liquid layers in the hydrocyclone, and the fibre concentration of the pulp suspension is increased to at least 0.9 % by the first ultihydro- cyclone unit before the pulp suspension is separated by the second niuitihydrocyclone unit. It has surprisingly been proved that hydrocyclones with such turbulence creating means are capable of separating a pulp suspension from relatively heavy contaminants without the separation efficiency becoming unacceptably low. As a matter of fact the separation efficiency can be satisfactorily maintained with increasing fibre concentration, up to about 1.5 %. Hydrocyclones with particularly efficient turbulence creating means in the form of radially outwardly directed steps in the separation chambers of the hydrocyclones, as shown in WO 93/10908, are marketed by Alfa Laval Celleco AB under the designation StepRelease™. The object of the present invention is also achieved by means of the plant described initially, which is characterized in that the pump is adapted to pump the pulp suspension to be separated through the first multihydro- cyclone unit, and that the first niuitihydrocyclone unit is connected to the second niuitihydrocyclone unit for supplying thickened pulp suspension, which is separated from light contaminants, to the second niuitihydrocyclone unit. Preferably, each hydrocyclone of the second niuitihydrocyclone unit is provided with turbulence creating means adapted to counteract the formation of fibre net-work in radially outer liquid layers in the hydrocyclone, and the first niuitihydrocyclone unit is adapted to thicken the pulp suspension to a fibre concentration of at least 0.9 %, preferably at least 1.0 % and maximally 1,5 %. The first niuitihydrocyclone unit is advantageously con- nected to the second niuitihydrocyclone unit via a connection conduit with an additional pump, for transferring said thickened pulp suspension from the first to the second niuitihydrocyclone unit. The energy consumption of said two pumps will be substantially less than the energy comsumption of a single pump which is utilized to operate the first and the second niuitihydrocyclone unit. Upstream of said additional pump and downstream of the first niuitihydrocyclone unit, said connection conduit can advantageously be provided with a counter pressure device adapted to maintain a constant counter pressure in an outlet for accept fraction of the first niuitihydrocyclone unit. The counter pressure device enables a return flow with recovered fibres to be transferred from the reject fraction of the second niuitihydrocyclone unit back to the second niuitihydrocyclone unit without loading the first niuitihydrocyclone unit with said return flow. Further advantageous features of the plant according to the invention are defined in the attached claims. The invention is described more closely in the following with reference to the accompanying drawings, in which figure 1 is a diagram showing how the cleaning efficiency depends on the fibre concentration of the pulp suspension at hydrocyclones with and without turbulence creating means. figure 2 shows a flow chart of a plant according to a first embodiment of the invention, and figure 3 shows a flow chart of a plant according to a second embodiment of the invention. In the figures identical components have been provided with the same reference numerals . In figure 1 there is shown as an example 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 is in this case 120 kPa. As is evident from the diagram the cleaning efficiency of a conventional hydrocyclone decreases 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 supension which has been sepa- rated by the older hydrocyclone. Consequently, the older hydrocyclone allows about 60 % more contaminants to pass through than the newer hydrocyclone when separating pulp suspensions with 0.9 % fibre concentration. As is evident from the diagram, the cleaning efficiency of the newer hydrocyclone at a higher fibre concentration of 1.0 % is still about 92 %, whereas the cleaning efficiency of the older hydrocyclone has decreased further to about 85 %. At higher fibre concentrations than 0.9 % the risk of clogging of the apex outlet of the older hydrocyclone with fibre net-work is dramatically increased, and for this reason in practice the older hydrocyclone is only utilized for pulp suspensions with a fibre concentration less than 0.9 %. The newer hydrocyclone, on the other hand, can be utilized for separating pulp suspensions with a fibre concentration of up to 1.5 %, without risk for clogging. The limiting factor here is the poorer cleaning efficiency, not the risk of clogging. It should be noted that the diagram according to figure 1 relates to a certain kind of pulp suspension. Other kinds of pulp suspensions of course give other curve lines in the diagram, but the principal differences which are evident from the example above are also valid for such other kinds of pulp suspensions. In figure 2 there is shown a plant according to a first embodiment of the invention comprising a feed conduit 1 for a pulp suspension, which contains relative light and heavy contaminants, a machine 2 for receiving and dewa- tering separated pulp suspension, e.g rotary filter, bow sieve or paper machine, and a container 3 for receiving water from the machine 2. From the container 3 a conduit 4 with a pump 5 extends to a niuitihydrocyclone unit 6, which comprises a multiplicity of parallelly coupled conical hydrocyclones of reverse type, e.g. Tripac 90 Reverse™ which are marketed by Alfa Laval Celleco AB. The niuitihydrocyclone unit 6 has an inlet 7 for pulp suspension, an apex outlet 8 for an accept fraction containing pulp suspension separated from relatively light contaminants and a base outlet 9 for a light reject fraction containing relatively light conta- inants. From the apex outlet 8 a connection conduit 10 with a pump 11 extends to an inlet 12 of a multihydrocyclone unit 13, which comprises parallelly coupled conical hydrocyclones with turbulence creating means of the kind shown in WO 93/10908. The multihydrocyclone unit 13 has an apex outlet 14 for a heavy reject fraction containing relatively heavy contaminants and a base outlet 15 for an accept fraction containing pulp suspension separated from relatively heavy contaminants . From the base outlet 15 a transport conduit 16 extends directly to the machine 2. A return conduit 17 for separated pulp suspension extends from the transport conduit 16 to the conduit 4. The return conduit 17 is provided with a valve 18 for adjusting the return ' flow in the return conduit 17. The heavy reject fraction flowing through the apex outlet 14 of the multihydrocyclone unit 13 contains some fibres which are recovered by means of a multihydro- cyclone unit 19 comprising parallelly coupled hydrocyclones of the same kind as in the multihydrocyclone unit 13. (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 a stage is shown here). A conduit 20, which is connected to the conduit 4 upstream of the connection between the latter and the conduit 17, extends to the multihydrocyclone unit 19. In the conduit 20 there is a pump 21. The apex outlet 14 of the multihydrocyclone unit 13 is connected via a conduit 22 to the conduit 20 upstream of the pump 21. A base outlet 23 in the multihydrocyclone unit 19 is connected via a conduit 24 to the conduit 4 downstream of the connection between the conduit 4 and the conduit 20, for supplying recovered fibres to the pump 5. An apex outlet 25 of the multihydrocyclone unit 19 is connected to a container, not shown, for separated heavy contaminants. The light reject fraction flowing through the base outlet of the multihydrocyclone unit 6 contains some fibres which are recovered by a multihydrocyclone unit 26 comprising parallelly coupled conical hydrocyclones of the same kind as in the multihydrocyclone unit 6. (The fibres in the light reject fraction are usually recovered by several stages of multihydrocyclone units coupled in cascade, but for resons of simplicity only one such a stage is shown here). A conduit 27 with a pump 28 extends from the conduit 20 upstream of the connection between the conduits 20 and 22 to the multihydrocyclone unit 26. A drain conduit 29 from the base outlet 9 is connected to the conduit 27 upstream of the pump 28. An apex outlet 30 of the multihydrocyclone unit 26 is connected via a conduit 31 to the conduit 4 down- stream of the connection between the conduit 4 and the conduit 20, for supplying recovered fibres to the pump 5. A base outlet 32 of the multihydrocyclone unit 26 is connected to a container, not shown, for separated light contaminants . During operation of the plant according to figure 2 the pulp suspension which comes in via the feed conduit 1 is deluted with water from the container 3 so that the fibre concentration of the pulp suspension supplied to the multihydrocyclone unit 6 becomes about 0.6 %, which gives a fibre concentration of about 1.2 % at the accept fraction of the pulp suspension flowing through the apex outlet 8. The light reject fraction from the base outlet 9 is deluted with the water in the conduit 27 and is pumped by the pump 28 to the multihydrocyclone unit 26, which gives an accept fraction with recovered fibres through the apex outlet 30 and a light reject fraction with light contaminants through the base outlet 32. The accept fraction is conducted via the conduit 31 back to the pump 5. From the base outlet 15 of the multihydrocyclone unit 13 an accept fraction, which contains pulp suspension separated from relatively heavy contaminants and which in this case has a fibre concentration of about 1.15 %, is conducted to the machine 2. Depending on the capacity of the machine 2 the valve 18 is adjusted so that a part flow of the flow in the transport conduit 16 is returned to the pump 5. The heavy reject fraction from the apex outlet 14 of the multihydrocyclone unit 13 is deluted with water in the conduit 20 and is pumped by the pump 21 to the multihydrocyclone unit 19, which gives an accept fraction with recovered fibres through the base outlet 23 and a reject fraction with heavy contaminants through the apex outlet 25. The accept fraction is conducted via the conduit 24 back to the pump 5. In figure 3 there is shown a plant according to a second embodiment of the invention, which is identical to the plant according to figure 2, except that means are arranged to bring back the accept fraction with recovered fibres from the multihydrocyclone unit 19 without loading the pump 5 and the multihydrocyclone unit 6, and that suitable control devices are implemented. Consequently, the connection conduit 10 comprises a first part 10a extending upwardly from the apex outlet 8 of the multihydrocyclone unit 6 to a first open container 33 with an overflow 34 and a second part 10b extending from a second open container 35 to the pump 11, the container 35 being arranged to receive pulp suspension from the container 33 via the overflow 34. The containers 33 and 35 and the overflow 34 constitute a counter pressure device adapted to maintain a constant counter pressure in the apex outlet 8. A level control means 36 is adapted to control a control valve 37, which is arranged in the drain conduit 29 from the base outlet 9 of the multihydrocyclone unit 6, in response to the level of the liquid surface in the second container 35, so that said level is below the overflow 34. A return conduit 38 extends from the con- duit 24 via a valve 39 to the container 35. A valve 40 is arranged in the conduit 24. By the valves 39 and 40 desired part flows of the accept fraction from the multihydrocyclone unit 19 can be adjusted in the conduits 38 and 24. For instance, a major part flow or the entire accept fraction may be conducted via the return conduit 38 to the container 35, whereby energy can be saved for the operation of the pump 5 and the number of hydrocyclones in the multihydrocyclone unit 6 can be reduced. A control device 41 is adapted to control the capacity of the pump 5 in response to the pressure in the inlet 7 of the multihydrocyclone unit 6. Since it exists a constant counter pressure in the apex outlet 8 of the multihydrocyclone unit 6, because of the arrangement with the open container 33, which is located at a certain height above the apex outlet 8, the control device 41 only needs to control the pressure in the inlet 7 in order to maintain a desired difference pressure between the inlet 7 and the apex outlet 8. A control device 42 is adapted to control the capacity of the pump 11 in response to the difference pressure between the inlet 12 and the base outlet 15 of the multihydrocyclone unit 13. A control valve 43 is arranged in the conduit 22 and is controlled by a control device 44 in response to the pressure difference between the accept outlet 15 and the apex outlet 14 of the multihydrocyclone unit 13. Since the accept fraction with recovered fibres from the multihydrocyclone unit 19, during operation of the plant according to figure 3, is supplied to the pulp suspension from the multihydrocyclone unit 6 via the container 35 the fibre concentration of the pulp suspension is increased to about 1.3 % before the pulp suspension is pumped into the multihydrocyclone unit 13. As a result, the fibre concentration of the pulp suspension separated from relatively heavy contaminants and flowing through the base outlet 15 of the multihydrocyclone unit 13 becomes about 1.15 %. In addition to this, the plant according to figure 3 is operated in the same manner as the plant according to figure 2. As an alternative, one of or both of the control devices 42 and 44 of the plant according to figure 3 may also be installed in the plant according to figure 2.

Claims: Claims 1. A method of treating a pulp suspension containing relatively light and heavy contaminants, the contami- nants being separated from the pulp suspension with the aid of hydrocyclones, c h a r a c t e r i z e d i n that at first light contaminants are separated from the pulp suspension and the fibre concentration of the pulp suspension is substantially increased by a first multi- hydrocyclone unit (6) comprising a multiplicity of parallelly coupled hydrocyclones designed for separating light contaminants, and thereafter heavy contaminants are separated from the pulp suspension with the increased fibre concentration by a second multihydro- cyclone unit (13) comprising a multiplicity of parallelly coupled hydrocyclones designed for separating heavy contaminants . 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 with the increased fibre concentration is conducted from the first multihydrocyclone unit (6) via a pump (11) to the second multihydrocyclone unit (13). 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 each hydrocyclone of the second multihydrocyclone unit (13) is provided with turbulence creating means adapted to counteract the formation of fibre net-work in radially outer liquid layers in the hydrocyclone, and that the fibre concentration of the pulp suspension is increased to at least 0.9 % by the first multihydrocyclone unit (6) before the pulp suspension is separated by the second multihydrocyclone unit. 4. A method according to claim 3, c h a r a c t e r i z e d i that the fibre concentration of the pulp suspension is increased to at least 1.0 % and maximally 1.3 % by the first multihydrocyclone unit (6) before the pulp suspension is separated by the second multihydrocyclone unit (13). 5. A method according to anyone of claims 1-4, c h a r a c t e r i z e d i n that the pulp suspension separated from light and heavy contaminants is conducted from the second multihydrocyclone unit ( 13 ) directly to a machine (2) for dewatering the pulp suspension. 6. A plant for treating a pulp suspension containing relatively light and heavy contaminants, comprising a first multihydrocyclone unit (6) having a multiplicity of parallelly coupled hydrocyclones designed for separating light contaminants from the pulp suspension under substantial thickening of the pulp suspension, a second multihydrocyclone unit ( 13 ) having a multiplicity of parallelly coupled hydrocyclones designed for separating heavy contaminants from the pulp suspension, and at least one pump ( 5 ) for pumping the pulp suspension to be separated through the multihydrocyclone units, c h a - r a c t e r i z e d i n that the pump ( 5 ) is adapted to pump the pulp suspension to be separated through the first multihydrocyclone unit (6), and that the first multihydrocyclone unit is connected to the second multihydrocyclone unit ( 13 ) for supplying thickened pulp suspension, which is separated from the light contaminants, to the second multihydrocyclone unit. 7. A plant according to claim 6, c h a r a c t e r i z e d i n that each hydrocyclone of the second multi- hydrocyclone unit (13) is provided with turbulence creating means adapted to counteract the formation of fibre network in radially outer liquid layers in the hydrocyclone, and that the first multihydrocyclone unit ( 6 ) is adapted to thicken the pulp suspension to a fibre concentration of at least 0.9 %. 8. A plant according to claim 7, c h a r a c t e r i z e d i n that the first multihydrocyclone unit ( 6 ) is adapted to thicken the pulp suspension to a fibre concentration of at least 1.0 % and maximally 1.5 %. 9. A plant according to anyone of claims 6-8, c h a r a c t e r i z e d i n that the second multihydrocyclone unit (13) is directly connected to a machine (2) for dewatering the pulp suspension via a transport conduit (16) for transporting pulp suspension separated from heavy contaminants from the second multihydrocyclone unit to said machine. 10. A plant according to claim 9, c h a r a c t e r i z e d i n that a return conduit (17) for separated pulp suspension extends from said transport conduit (16) to the suction side of said pump (5). 11. A plant according to anyone of claims 6-10, c h a r a c t e r i z e d i n that the first multihydrocyclone unit ( 6 ) is connected to the second multihydrocyclone unit (13) via a connection conduit (10) with an additional pump (11), for transferring said thickened pulp suspension from the first to the second multihydrocyclone unit. 12. A plant according to claim 11, c h a r a c t e r i z e d i n that the second multihydrocyclone unit (13) comprises an inlet (12) for pulp suspension to be separated and an outlet (15) for an accept fraction, and that a control device (42) is adapted to control the capacity of said additional pump (11) in response to the pressure difference between said inlet and said outlet for accept fraction of the second multihydrocyclone unit. 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 upstream of said additional pump (11) and downstream of said first multihydrocyclone unit ( 6 ) said connection conduit ( 10 ) is provided with a counter pressure device (33-35) adapted to maintain a constant counter pressure in an outlet ( 8 ) for an accept fraction of the first multihydrocyclone unit ( 6 ) . 14. A plant according to claim 13, c h a r a c t e r i z e d i n that the counter pressure device comprises a first open container (33) with an overflow (34), and a second open container (35), which is arranged to receive pulp suspension flowing across said overflow ( 34 ) . 15. A plant according to claim 14, c h a r a c t e r i z e d i that said connection conduit (10) comprises a first part ( 10a ) , which extends upwardly from the first multihydrocyclone unit ( 6 ) to the first container ( 33 ) , and a second part ( 10b ) , which extends from the second container (35) via said additional pump (11) to the second multihydrocyclone unit (13). 16. A plant according to claim 15, c h a r a c t e r i z e d i n that the first multihydrocyclone unit (6) comprises an outlet (9) for a light reject fraction, that a drain conduit (29) with a control valve (37) extends from the outlet for light reject fraction, and that a level control means ( 36 ) is adapted to control the control valve ( 37 ) in response to the level of the suspension surface in the second container (35), such that said level is below the overflow (34). 17. A plant according to anyone of claims 14-16, c h a r a c t e r i z e d i n that the second multihydrocyclone unit (13) comprises an outlet (14) for a heavy reject fraction connected to at least one multihydrocyclone unit (19) for recovering fibres from the heavy reject fraction, and that a return conduit (38) is adapted to transfer an accept fraction with recovered fibres from said multihydrocyclone unit (19) for recovering fibres to said second open container (35).

Inventors: ANDERSSON ROINE, BACKMAN JAN, ANDERSSON, ROINE, BACKMAN, JAN
Applicants: ALFA LAVAL AB, ANDERSSON ROINE, BACKMAN JAN, ANDERSSON, ROINE, BACKMAN, JAN
Application year: 1997
Main classification: D21D5
IPC codes: B04C5, D21D5