A DIFFUSER OF AN INJECTOR IN A FLOTATION CELL AND A METHOD FOR FEEDING FIBRE SUSPENSION INTO THE FLOTATION CELL

BACKGROUND OF THE INVENTION

[0001] The invention relates to a diffuser of an injector in a flotation cell for feeding a fibre suspension flow into the flotation cell, the diffuser including a diffuser element, which comprises at least one first surface, at least one second surface and at least one gap between the first surface and the second surface in the transverse direction in relation to the flow direction of the fibre suspension flow to be fed into the diffuser, from which gap the fibre suspension flow is arranged to be discharged from the diffuser.

[0002] The invention also relates to an injector in a flotation cell, the injector comprising an inlet tube for feeding the fibre suspension flow into the injector, at least one air supply point for supplying air to be mixed with the fibre suspension flow into the injector, a mixing tube for mixing together the fibre suspension flow and air fed into the injector and a diffuser for feeding the flow of the mixture of fibre suspension and air out of the injector.

[0003] The invention further relates to a method for feeding fibre suspension into a flotation cell, which comprises an injector including an inlet tube for feeding a fibre suspension flow into the injector, at least one air supply point for supplying air to be mixed with the fibre suspension flow into the injector, a mixing tube for mixing together the fibre suspension flow and air, and a diffuser for feeding the mixture of fibre suspension and air into a basin space of the flotation cell, and the method comprising feeding the fibre suspension flow into the injector, supplying air into the injector, mixing air into the fibre suspension flow and feeding the flow of the mixture of fibre suspension and air into the basin space of the flotation cell.

[0004] Flotation cells, or flotation containers, are used for removing printing inks and possible impurities from a fibre suspension made of decomposed and assorted waste paper; the procedure also being referred to as deinking. The aim of deinking is to provide a recycled fibre pulp that is as white and pure as possible. Flotation cells are used in flotation deinking, whereby flotation is carried out in cells such that soap or another chemical improving flotation and reducing surface tension is added as a flotation chemical to a diluted about 1% fibre suspension. Air is also mixed into the fibre suspension. The printing ink and impurities attach to air bubbles, which are carried to the

surface of the fibre suspension in the basin space, and the printing ink and impurities can be removed as overflow or by scraping.

[0005] US publication 5465 848 discloses a flotation cell comprising an injector for feeding a fibre suspension flow into the cell and for mixing air into the fibre suspension flow to be fed into the cell. What is typically used with the injectors in the lower part of the injector in the flotation cell according to the publication is a slit nozzle, or a radial d iff user, which is circular in cross-section, for feeding the pulp aired in the injector into the fibre suspension volume already provided in the cell.

[0006] Figure 1 schematically shows in cross-section the slit nozzle, or diffuser, of the type illustrated in US publication 5 465 848 provided with a gap restricted by two surfaces in the transverse direction in relation to the flow direction of the fibre suspension flow to be fed into the diffuser, the gap being typically dimensioned such that the aired fibre suspension flow C discharged from the gap is provided with a sufficiently significant flow rate and turbulence level. A bottom plate forming the lower part of the slit nozzle causes a hydraulic blow to the fibre suspension flow bumping thereto, which further increases the turbulence level of the flow and prevents the air bubbles in the flow from attaching to one another. When correctly dimensioned the slit nozzle causes a fairly insignificant pressure loss and provides a pulp jet of homogeneous quality, the discharge rate of which is typically 2 to 3 m/s. Owing to the significant flow rate thereof the jet discharged from the slit nozzle penetrates thoroughly into the surrounding basin space of the flotation cell. In an adequately large basin space this does not cause any problems to the control of the flow field, but it may be difficult to provide a controlled flow field in a small basin space or even impossible, which results in an inferior quality of purified pulp fraction obtained from the flotation cell.

[0007] Small basin volumes require a slow flow rate of the jet in order to achieve a controlled flow field. Increasing the height of the gap of the slit nozzle as schematically shown in Figure 2, the flow rate can easily be reduced to an adequately low level. However, enlarging the gap easily causes the flow to be disengaged from the upper plate of the nozzle, whereby whirls designated by reference B are created as shown schematically in Figure 2. Such whirls disrupt the homogeneous flow and collect air to form large air bubbles. Large air bubbles rapidly rise to the surface of the fibre suspension in the basin of the flotation cell and break the froth layer formed thereto.

BRIEF DESCRIPTION OF THE INVENTION

[0008] It is an object of the present invention to provide a new type of diffuser.

[0009] A diffuser according to the invention is characterized in that the diffuser comprises at least two diffuser elements, which are arranged successively in relation to one another in the flow direction of the fibre suspension flow to be fed into the diffuser and which are arranged to divide the fibre suspension flow to be fed into the diffuser into at least two sub-flows separate from one another for discharging the sub-flows separate from one another from the gaps in the diffuser elements.

[0010] An injector according to the invention is characterized in that the diffuser is the diffuser according to claim 1.

[0011] A method according to the invention is characterized by dividing the flow of the mixture of fibre suspension and air in the diffuser into at least two sub-flows separate from one another and discharging the sub-flows separate from one another from the diffuser to the basin space of the flotation cell.

[0012] In accordance with the essential idea of the invention the diffuser of the injector in the flotation cell comprises at least two diffuser elements, which are arranged successively in relation to one another in the flow direction of the fibre suspension flow to be fed into the diffuser, and each diffuser element comprising at least one first surface, at least one second surface and at least one gap between the at least first surface and the second surface in the transverse direction in relation to the flow direction of the fibre suspension flow to be fed into the diffuser, from which gap the fibre suspension flow is arranged to be discharged from the diffuser. Furthermore, according to an essential idea, the diffuser elements are arranged to divide the fibre suspension flow to be fed into the diffuser into at least two sub-flows separate from one another for discharging the sub-flows separate from one another from the gaps in the diffuser elements. According to an embodiment of the invention, the diffuser element comprises a tube or a tube portion, which is arranged to extend in respect of the flow direction of the fibre suspension flow substantially in the opposite direction and which is arranged to divide the fibre suspension flow to be fed into the diffuser into at least two sub-flows separate from one another. According to a second embodiment of the invention, the tube portion of the diffuser element is arranged inside the tube portion of a preceding diffuser

element in the flow direction of the fibre suspension flow. According to a third embodiment of the invention the diameters of the tube portions placed within each other are dimensioned such that the flow cross-sectional areas of the flow channels restricted by the walls of the tube portions placed within each other are substantially equal in size.

[0013] An advantage of the diffuser of the type shown above is that it allows the fibre suspension to be fed into the basin space at a slow rate such that the pulp jet still remains homogeneous. The diffuser shown above is particularly well suited to flotation cells, in which the basin volume is small and in which high output flow rates of the fibre suspension flow cannot be employed. When the tube portion of the diffuser element is placed within a tube portion of a preceding diffuser element in the flow direction of the fibre suspension flow, the fibre suspension flow to be fed into the diffuser is divided efficiently into sub-flows. When the diameters of the tube portions placed within each other in the diffuser are dimensioned such that the flow cross-sectional areas in the flow channels restricted by the walls of the tube portions placed within each other are substantially equal in size, then equal amounts of fibre suspension can easily be directed into the gap of each diffuser element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the following, embodiments of the invention will be explained in greater detail in the accompanying drawings, in which

Figure 1 schematically shows a side view in cross-section of a prior art diffuser,

Figure 2 schematically shows a side view in cross-section of another prior art diffuser,

Figure 3 schematically shows a side view in cross-section of a flotation cell

Figure 4 schematically shows a side view of a diffuser,

Figures 5 and 6 schematically show a side view in cross-section of the diffuser shown in Figure 4,

Figure 7 schematically shows a second diffuser, and

Figure 8 schematically shows a side view in cross-section of a third diffuser.

[0015] For clarity, the figures show embodiments of the invention in simplified form. Similar parts in the figures are designated with the same reference numerals.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0016] Figure 3 schematically shows a side view in cross-section of a flotation cell 1 for removing printing ink and possible impurities from a fibre suspension. The flotation cell 1 comprises an injector 2, through an inlet tube 3 of which a fibre suspension flow and a flotation chemical added thereto, such as soap, are fed into the flotation cell 1. The injector 2 according to Figure 3 also comprises at least one air supply point 4 and at least one air supply channel 5 for supplying air to be mixed with the fibre suspension flow within the injector 2. The injector 2 also comprises a mixing tube 6 and the mixing tube 6 may be provided with a mixing apparatus 7, typically arranged within the tube for mixing together the fibre suspension flow and air. The injector 2 is placed in relation to a surface 8 of the fibre suspension in the flotation cell 1 such that a diffuser 9 arranged at least in connection with the lower part of the injector 2, or in practice as an extension to the mixing tube 6, remains beneath the surface 8 of the fibre suspension. The diffuser 9 is a nozzle that allows turning the flow direction of the fibre suspension flow to be fed into the flotation cell and that allows feeding the fibre suspension flow to a basin space 10 of the flotation cell 1 at a desired speed.

[0017] In the flotation cell 1 the printing ink and other impurities attach to the bubbles formed of air and flotation agent that rise to the surface 8 of the fibre suspension, from where the printing ink and other impurities can be removed using a scraper that is not shown in Figure 3 or by means of overflow. The overflow mentioned in Figure 3 is formed using a dam plate 11 , whereby the froth created on the surface of the fibre suspension and the printing ink as well as other impurities therein are transferred over the dam plate 11 into a reject state 12, from where they are forwarded through a reject discharge channel 13 for further processing. The flotation cell according to Figure 3 also shows schematically a discharge channel 14 for feeding the purified fibre suspension forward in the pulp making process. The basic structure and the operation mode of the flotation cell 1 are known per se for those skilled in the art and are therefore not discussed in more detail in this context.

[0018] Figure 4 schematically shows a side view of a d iff user 9 and as arranged at a mixing tube 6 of the injector 2, and Figures 5 and 6 schematically show the diffuser 9 according to Figure 4 in cross-section. The diffuser 9 according to Figures 4 to 6 comprises four diffuser elements 15. Each diffuser element 15 includes a first surface 16 and a second surface 17, between which a gap 18 is provided, which is in the transverse direction in relation to flow direction A of the fibre suspension flow to be fed into the diffuser 9. What is meant with a transverse direction in relation to flow direction A of the fibre suspension flow to be fed into the diffuser 9 is a direction, which is not parallel with flow direction A of the fibre suspension flow to be fed into the diffuser 9. The diffuser elements 15 are arranged successively in relation to one another in flow direction A of the fibre suspension flow to be fed into the diffuser 9. The diffuser element 15 according to Figures 4, 5 and 6 is formed of two plate elements 19 and 19' placed at a distance from one another provided with an opening 22 and a gap 18 between them. The plate elements 19 and 19' are supported to one another using support elements 20 arranged between the plate elements 19. Each diffuser element 15 further includes at least one tube or a tube portion 21 that is directed substantially in the opposite direction in respect of flow direction A of the fibre suspension flow to be fed into the diffuser 9, i.e. upwards in Figures 4 to 6. The tube portions 21 are arranged in relation to one another within each other such that the tube portion 21 of the last diffuser element 15, or the lowest diffuser element in Figure 5, in flow direction A of the fibre suspension flow to be fed into the diffuser 9 is the innermost tube portion and the tube portion in the preceding diffuser element 15 is the second innermost tube portion, as clearly shown in Figure 5. The tube portions 21 , i.e. the walls of the tube portions 21 , divide the fibre suspension flow fed into the diffuser into sub-flows separate from one another and direct such sub-flows to the gaps 18 of the diffuser elements 15, from which gaps 18 the sub-flows are discharged from the diffuser 9 as separate flows as shown by arrows D.

[0019] The tube portions 21 thus divide the flow cross-sectional area of the diffuser 9 into several, in the case shown in Figures 4 to 6 into four, separate rotationally symmetrical flow channels 23 placed in respect of one another within each other and restricted by the wall surfaces of the tube portions 21. The outermost wall of the outermost flow channel is formed of the mixing tube 6 of the injector or an extension thereof. After the vertical portion

formed of the tube portions 21 the flow channels 23 turn outwards in the direction of the diffuser radius, into the gaps 18 restricted by the first surfaces 16 and the second surfaces 17 of the diffuser elements 15, the cross-sectional area of the gaps increasing constantly while moving towards the outer periphery of the diffuser. Such a sharp turn provides a hydraulic blow to the fibre suspension flow and maintains a high turbulence level and the flow cross- sectional area that evenly increases in the flow direction prevents the creation of harmful whirls. After this the fibre suspension flow is discharged from the diffuser 9 from the overlapping flow channels 23 formed of gaps 18 as shown by arrows D.

[0020] The diffuser shown above allows feeding the fibre suspension into a basin space at a slow rate, but still maintaining a homogeneous pulp jet. The diffuser is particularly well suited to such flotation cells, in which the basin space used is small and where high discharge rates of the fibre suspension flow cannot be used.

[0021] The fibre suspension flow to be fed into the diffuser 9 may be a mixture of fibre suspension and air, if the supply of fibre suspension to the flotation cell 1 takes place as shown in Figure 3. The fibre suspension flow fed into the diffuser 9 may also be merely fibre suspension, if another solution is used for aerating the fibre suspension.

[0022] The diffuser 9 shown in Figures 4 to 6 is therefore implemented so as to comprise four diffuser elements 15, whereby the discharge of fibre suspension from the diffuser 9 starts at four levels, but depending on the application the diffuser may be provided with two or more diffuser elements 15.

[0023] In the solution shown in Figures 4 to 6 the tube portions 21 are dimensioned in the longitudinal direction such that the upper ends 24 thereof, i.e. the ends in the opposite direction in respect of flow direction A of the fibre suspension flow to be fed into the diffuser 9, are substantially at the same level and partly extend to the vertical portion of the injector 2, where the flow rate of the fibre suspension downwards ranges between 1 and 5 m/s. In Figures 4 to 6 the fibre suspension flow to flow channels 23 directed to different diffuser elements 15 in the diffuser 9 is thus formed of a single level. However, the fibre suspension flow to the flow channels 23 directed to the different diffuser elements 15 in the diffuser 9 can also be formed of different levels, for instance so that the flow to the gap 18 of the uppermost diffuser element 15 can be formed in the flow direction of the fibre suspension to be fed into the

diffuser from a first or uppermost dividing point and to the second uppermost layer from a second or second highest dividing point, or so that the flow to the gap 18 of the highest diffuser element 15 can be formed in the flow direction of the fibre suspension to be fed into the diffuser from the last or lowest dividing point and to the second highest layer from the second last or second lowest dividing point.

[0024] In the diffuser shown in Figures 4 to 6 the flow channels 23 restricted by the walls of the tube portions 21 in the diffuser are all provided with the same flow cross-sectional area. In some embodiments the diameters of the tube portions 21 can, however, be implemented in such a manner that the different flow channels 23 are provided with different flow cross-sectional areas, whereby the amount of flow in the sub-flows D discharged from the gaps 18 of the diffuser elements 15 at different levels deviates from one another. This can be implemented for instance such that the sub-flow, having either the greatest volume flow or the smallest volume flow, to be directed to the middlemost flow channel 23.

[0025] In the diffuser 9 shown in Figures 4 to 6 the gaps 18 of the diffuser elements 15 are arranged to widen towards the outer periphery of the diffuser 9 such that some of the gaps 18 are directed upwards and some downwards to feed the fibre suspension flow. The gaps 18 can also be formed such that the height of the gaps 18, i.e. the first surface 16 and the second surface 17 of the diffuser element 15, constantly remain at the same distance from one another. Furthermore, the gaps 18 can also be formed such that sub- flows D from the gaps 18 of the diffuser 9 can be directed downwards from all the gaps. The gaps 18 may also be formed such that all or some of sub-flows D arrive from the diffuser in the horizontal direction or the jets can be directed also in the horizontal direction or such that sub-flows D are discharged from the diffuser 9 like a fan at different height angles.

[0026] The cross-section of the diffuser 9 according to Figures 4 to 6 is circular. Diffusers that are circular in cross-section are generally also referred to as radial diffusers. Instead of a circular cross-section the diffuser may have for instance an elliptic, triangular, square or rectangular cross-section or a cross-section that preferably has another equilateral and polygon form. For example, Figure 7 schematically shows a diffuser 9 having a square cross- section, in which the gaps 18 of the diffuser elements 15 are formed at the two sides of the diffuser 9 and the two other sides of the diffuser 9 are enclosed.

The fibre suspension flow out of the diffuser can therefore be arranged in one direction of the periphery angle, in two directions of the periphery angle as shown in Figure 7, in several directions of the periphery angle or to be directed on the entire periphery.

[0027] In the diffusers shown in Figures 3 to 7 the diffuser elements 15 are formed such that the gaps 18 of the diffuser elements 15 extend equally far from an imaginary centre axis of the diffuser, but the diffuser elements 15 can also be formed such that the gaps 18 of the diffuser elements 15 extend at different distances from the imaginary centre axis of the diffuser. Then, the form of the diffuser 9 may resemble a ball. Furthermore, selecting the cross- sectional areas of the flow channels of the diffuser elements 15 appropriately, a larger or a smaller volume flow or then again a flow at a higher or lower discharge rate can be directed through the flow channels into different periphery angle directions and at different heights according to need,. Thus, sub-flows D discharged from the diffuser 9 can be arranged such that the volume of the surrounding basin space 10 is used as much as possible as regards the success of the flotation.

[0028] In the diffuser according to Figures 4 to 6 the plate elements 19 are formed of a single annular plate element, the plate elements 19 being supported to one another with support elements 20 arranged between them, the support elements 20 extending only partly in the direction of the radius of the plate elements 20. Irrespective of whether the plate elements 19 are annular or of another shape, they can be made of several separate parts, which are joined together to form a single plate element, in which case the plate element is provided with one or more first and/or second surfaces. In addition, the support elements 20 can be extended over the entire width of the plate element 19 in the direction of the plate element, which is in the transverse direction in respect of flow direction A of the fibre suspension flow to be fed into the diffuser, whereby several gaps 18 are formed between the support elements 20 in the single diffuser element 15.

[0029] Figure 8 schematically shows a side view in cross-section of a third diffuser 9. In the diffuser 9 shown in Figure 8 the first surfaces 16 and the second surfaces 17 of the diffuser elements 15 are arched surfaces, meaning that the plate element 19 comprising the first surface 16 and the plate element 19' comprising the second surface 17 are arched in such a manner that the plate elements 19 and 19' arch towards the feed direction of the fibre sus-

pension flow. The diffuser elements 15 of the diffuser 9 shown in Figure 8 do not comprise particular tubes or tube portions for dividing the fibre suspension flow to be fed into the diffuser into sub-flows, instead the arched ends facing the centre axis of the diffuser 9 in the plate elements 19 and 19', in other words the plate elements 19 and 19' themselves, divide the fibre suspension flow into sub-flows without any auxiliary means. For clarity, Figure 8 does not show the support elements used to support the plate elements.

[0030] In the diffuser 9 according to Figure 8 the sub-flows rotate smoothly, in which case the hydraulic blow remains small. Such a solution is applicable to be used particularly when the turbulence level of the flow in the inlet tube 6 preceding the diffuser 9 is so high that in order to avoid the froth bubbles from joining, no need arises to significantly increase the turbulence level in the diffuser 9. The parts associated with the diffuser according to Figure 8 can each be made of a single part by moulding, whereby the number of parts in this solution as a whole remains very small.

[0031] The diffusers 9 shown in Figures 3 to 8 and the diffuser elements 15 therein all have a symmetric structure in relation to the centre axis of the diffuser 9. However, the diffuser 9 may also be implemented in such a manner that the diffuser elements 15 remain asymmetric in relation to the centre axis of the diffuser 9 so that the sub-flows of the fibre suspension flow are fed asymmetrically into the basin space 10 of the flotation cell 1 in respect of the centre axis of the diffuser 9.

[0032] In some cases the properties shown in the present application can be used as such, irrespective of other properties. Then again, the properties shown in the present application can if necessary be combined in order to form different combinations.

[0033] The drawings and the description associated therewith are merely intended to illustrate the idea of the invention. As to the details, the invention may vary within the scope of the claims. In the embodiments shown in Figures 3 to 8 the diffuser is a fixed part of the injector such that the diffuser is formed to be a fixed part of the injector 2. However, it is apparent that the diffuser may be formed as a structure that is completely separate from the rest of the injector structure, which is then connected to the vertical portion of the injector, i.e. for example to the mixing tube 6, either in a fixed manner for instance by welding or detachably for instance by means of a flange joint. It is also obvious that even though all the figures of the examples show that the

injector and the diffuser in connection therewith are placed in a vertical position, the injector and the diffuser in connection therewith or a diffuser separate from the injector may also be placed in the flotation cell horizontally or in different oblique angle positions.