DEVICE AT FLOTATION OF FIBRE SUSPENSIONS

Abstract: The device is intended for admixing gas bubbles into a fibre suspension in order to render possible flotation for removing impurities from the suspension. It comprises an inlet portion (1), a mixing portion (2) and a diverging outlet portion (3). The mixing portion (2) is divided into two parallel slit-shaped passages (4) by means of a wing member (5). Each passage (4) communicates with an air supply means (6) with an adjustable slit aperture (7) substantially directly in front of the narrowest portion of the passage (4).

Description: Device at flotation of fibre suspensions This invention relates to a device for removing impurities in the form of solid particles from a fibre suspension by means of flotation. For effecting flotation, gas bubbles are admixed to and distributed in the suspension, which gas bubbles adhere to the impurities and rise to the surface and form a foam mixed with the impurities. The foam can thereafter be removed from the suspension. A flotation process of this kind is known for removing printer's ink from paper pulp of newspaper waste, at which process air is injected into a * .suspension of paper pulp, which thereafter is passed into the lower part of a flot¬ ation container. The air can be admixed in a separate mixing chamber, for example of the kind disclosed in SE-PS 77θ4203----_ Prior to its feed into the flotation container, the suspension is passed through the mixing chamber in the form of a thin layer at the same time as air is injected into this layer in transverse direction. It was found difficult, however, to bring about a uniform distribution of the bubbles, especially at a high flow rate of the suspension. This applies, for example, when the suspension is to be fed tangentially into a cylindric flotation container.-.at a rate so high that the suspension in the container is caused to rotate. The object of maintaining the rotation of the suspension is to facilitate the removal of the foam rich in impurities from the suspension surface in the container. The present invention has the object of proposing a new design of the mixing chamber, at which the aforesaid disadvantages are eliminated. The characterizing features of the invention become apparent from the attached claims. The invention is described in greater detail in the follow¬ ing, with reference to the accompanying drawings illustrating an embodiment of the invention, in which Pig. 1 is a partially sectional view from above of a mixing chamber, and Fig. 2 is a vertical longitudinal section through the mixing chamber along II-II in Fig. 1. The illustrated mixing chamber is intended for the admixture of air into a pulp suspension with a concentration of 1-2$. The mixing chamber comprises an inlet portion 1 in the form of a passageway, the cross-section of which in the flow direction transforms successively from circular to oblong rectangular shape by decreasing height and increasing width. The inlet portion 1 transforms to a mixing portion 2, which in its turn transforms to an outlet portion 3. The width of the passageway is maintained through the mixing portion 2 and outlet portion 3 9 while the height, and therewith the cross-section, successively increase in the outlet portion 3. The upper and lower defining walls of the outlet portion 3 diverge, at a constant angle, suitably between 5° and 10°, preferably about 7°. The mixing portion 2 is divided into two parallel slit-shaped passages 4 by means of a wing member 5, which is located centrally in the mixing portion 2 and extends across the entire width of the passageway. The wing member 5 has a thickness, which first increases successively in the flow direction and thereafter decreases successively, in such a manner, that the thickest portion of the wing member 5 defines the narrowest portion of the passageway and therewith also of the passages 4. The two passages 4 diverge from this narrowest portion substantially symmetrically in relation to the longitudinal direction of the passageway. The angle is substantially the same as in the subsequent outlet portion, i.e. 5-10° and preferably about 7°. "ST_R__4 _O PI Each passage 4 communicates with an air supply means 6, the slit aperture 7 of which extends in transverse direction of the passage across its entire width. The width of the slit aperture 7 is adjustable, and the aperture is located substantially directly in front of the thickest portion of the wing member which also is the narrowest portion of the passage. Each passage has a height of 4-16 mm, preferably 6-10 mm, in order to bring about an effective air admixture. The air slit should be 0,1-1,0 mm, preferably 0,2*-O,5 mm. The air slit preferably is directed obliquely in the flow direction of the suspension. Each air supply means 6 comprises two nozzle portions 8,9, which define the air slit. The distance between the nozzle portions 8,9 is adjustable, for example by means of insert plates. The nozzle portions 8,9 are located in a transverse . recess 10 in the upper and, respectively, lower defining wall of the mixing portion 2, and the nozzle portions 8,9 are retained in place by a bar 11 and screws 12. In the bar 11 inlet apertures 13 for air are located. Sealings 14 are located between the walls of the mixing portion 2 and the nozzle portions 8,9 and, respectively, between the nozzle portions 8,9 and the bar 11. The length of the outlet portion 3 is to be adjusted to the rate the suspension is desired to have when it leaves the outlet portion and flows into the subsequent flotation container (not shown). The pulp concentration in question also is to be taken into consideration. A suitable length of the outlet portion 3 should be so that the area ratio between the narrowest portion of the passageway and the end of the outlet portion is between 1:4 and 1:6. The object of the configuration of the outlet portion is to smooth the flow of the pulp suspension, because the air admixture in the mixing portion 2 gives rise to substantial turbulence. By designing the mixing device according to the invention, a fibre suspension with a very uniform distribution of air bubbles of a suitable size for a subsequent flotation process is obtained. The flow leaving the mixing device also is substantially free of turbulence,.and a highflow rate can be obtained which promotes the subsequent flotation process. The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the invention idea.

Claims: Claims 1. A device for admixing gas bubbles into a fibre suspension in order to render possible flotation for removing impurities from the fibre suspension, which device comprises an inlet portion (1), a mixing portion (2) and an outlet portion (3), where the mixing portion (2) has oblong rectangular cross-section and the outlet portion diverges from the mixing portion (2), c h a r a c t e r i z e d i n that the mixing portion (2) is divided into two parallel slit- shaped passages (4) by means of a wing member (5), which is located centrally in the mixing portion (2), that the wing member (5) has a thickness at first increasing succesively in the flow direction of the suspension and thereafter decreasing successively, that each passage (4) diverges substantially symmetrically in the flow direction from the thickest portion of the wing member (5), that each passage (4) communicates with an air supply means (6) with an adjustable slit aperture (7), which extends in transverse direction of the passage (4) substantially directly in front of the thickest portion of the wing member (5) and therewith of the narrowest portion of the passage (4), and that the outlet portion (3) diverges at the same degree as each passage (4). 2. A device as defined in claim 1, c h a r a c t e r i z e d i n that the outlet portion (3) like each passage (4) diverges at an angle of 5-10°: 3. A device as defined in claim 1 or 2, c h a r a c t e r ¬ i z e d i n that the narrowest portion in each passage (4) has a height of 4-l6 mm, preferably 6-10 mm. 4. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the area ratio between the narrowest portion in the mixing portion (2) and the end of the outlet portion (3) is between 1:4 and 1:6. 5. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the slit aperture (7) of the air supply means (6) is directed obliquely in the flow direction " of the pulp suspension.

Inventors: BYLEHN BO VALTER SVANTE, GRANQVIST TAGE HUBERT, BYLEHN, BO, VALTER, SVANTE, GRANQVIST, TAGE, HUBERT
Applicants: SUNDS DEFIBRATOR, SUNDS DEFIBRATOR AKTIEBOLAG
Application year: 1984
Main classification: B01F5
IPC codes: B01F5, B03D1, D21B1