DEVICE FOR DEWATERING A PULP SUSPENSION

Abstract: A device for dewatering a pulp suspension, comprising an open-work movable surface (1) for forming and dewatering a pulp web. In an inlet (2) for the pulp suspension to the movable surface (1), a cylindric chamber (3) is located which encloses a rotatable cylindric rotor (4), the shaft of which extends transverse to the flow direction of the pulp. The rotor (4) is provided on its surface with a plurality of crosswise arranged channels (7). The pulp is forced to flow from an inlet (2) in the chamber (3) through the channels (7) to an outlet (6) located on the opposite side of the chamber (3).

Description: Device for dewatering a pulp suspension This invention relates to a device for dewatering a pulp suspension. It comprises an open-work movable surface for forming and dewatering the pulp suspension, an inlet for the pulp suspension and an outlet for the dewatered pulp. The pulp suspension is supplied with a relatively low concentration, usually 3-5%, and is formed on the open- -work movable surface, which consists, for example, of a perforated roll, through which liquid is sucked and pressed off. Such presses usually comprise two perforated rolls, which form between themselves a nip for pressing off liquid. The pulp normally is dewatered to a concen¬ tration of 35-50%. A large problem at dewatering is that, due to low flow rate, floeculation (lump formation) of the pμlp easily occurs when the pulp suspension is being supplied to the concentrator. The pulp suspension, thus, is supplied through a pipe line, in which the flow must be adapted to the capacity of the concentrator. At the subsequent pressing between the rolls, the fibres in these fibre lumps are subjected to a pressing force higher than on the remaining fibres, which can cause the fibres to be crushed. This has a negative effect on the pulp quality and- cannot be compensated for in subsequent processing steps. The floeculation problem can be reduced by lowering the pulp concentration. Then, instead, the fibre losses in¬ crease and a greater liquid amount nust be removed, at the same time as the dewatering zone increases in length. According to the present invention, the aforesaid problem is eliminated in that the pulp inlet to the concentrator is formed so that the supply takes place at a higher rate by means of cross-wise arranged channels. Turbulence is thereby brought about in the suspension, so that floecul¬ ation is prevented or lumps formed are broken up. In certain cases it is even possible to increase the pulp concentration to about 8%, without inconvenience. The characterizing features of the invention become app¬ arent from the attached claims. The invention is described in greater detail in the foll¬ owing, with reference to the accompanying drawings, in which Fig. 1 shows a pulp inlet to a concentrator according to the invention, Fig. 2 shows the surface of a rotor provided in the pulp inlet, Fig. 3 shows a detail of the rotor surface according to a section III-III in Fig. 2, Fig. shows an alternative configuration of the pulp inlet. Fig. 5 shows an alternative configuration of the rotor surface. The device comprises an open-work movable surface 1 , on which the pulp suspension is intended to be formed. The movable surface can be a perforated roll, wire or the like. A pulp inlet 2 is provided for■ • the supply of the suspension. The pulp inlet 2 is formed with a cylindric chamber 3, which encloses a cylindric rotor rotatable about its shaft. The chamber 3 and rotor are located transversely to the flow direction of the pulp and have a length substantially corresponding to the width of the movable surface 1. The chamber 3 is provided with an inlet 5 and an outlet 6 located substantially directly in front of each other. The cylindric rotor 4 is provided on its surface with a plurality of cros s-wise arranged channels 7. The channels suitably are arranged helicoidally with opposedpitch angles . The channels can be arranged so that the cros sings form a right angle or nearly a right angle . Due to the fact , that the rotor 4 is enclosed with a small clearing by the chamber 4 ι the pulp suspension is forced to flow through the channels 7. The rotatable rotor 4 suitably rotates at a low rotation speed , for example 1 -20 rp . The driving of the rotator should be reversible . The rotor diameter can be 50-1 00 mm . The channels are to be dimensioned so that the pulp suspension flows at a sufficient rate . The cross section of the channels 7 can be of different design, for instance with rectangular ( fig 2 and 3) or triangular cross section ( fig 5) . A suitable size of rectangular channels can be a width of 2-10 mm, preferably 3-7 mm, and a depth of 2-8 -mm, preferably 3-6 mm. the device operates as follows. T h e pulp suspension is supplied through the pulp inlet 2, whereby the pul flows via the inlet 5 of the chamber 3 into the channels 7 , passes throug the same and flows out through the outlet 6 of the chamber 3. The outlet 6 communicates directly with the movable surface 1 . The pulp flows through the channels 7 at a high rate , which at every crossing between the channels 7 is increased instantaneously and thereby gives rise to intensive turb¬ ulence . Floeculation of the pulp is thereby prevented , and lumps already formed are broken up . Due to the rotation of the rotor 4 about its shaft at a low speed of rotation , continously or reversed , the flow direction through each channel 7 changes periodically . Clogging of the channels is thereby prevented . The rotor speed shall be so low in relation to the flow rate of the pulp through the channels 7 , that the flow in one direction or the other about the rotor 4 is of substantially equal size. The rotor thereby is not subjected to uneven load, and at the same time the formation of fibre lumps is prevented in an optimum manner. According to the embodiment shown in Fig. 4, an outer return line 8,9 is coupled between the outlet 6 and inlet 5 of the chamber 3. In this line 8,9 a pump 10 is located. This arrangement has the objetttto maintain in the channels 7 a flow rate higher than corresponding to the capacity of the concentrator. A portion of the pulp flow is then circulated through the line 8,9 from the outlet 6 to the inlet 5. In this way, a homogenous flow of pulp free from fibre lumps to the formation zone on the movable surface 1 is ensured, irrespective of the capacity of the concentrator. The invention, of course, is not restricted to the embod¬ iments described and shown, but can be varied within the scope of the invention idea.

Claims: Claims 1. A device for dewatering a pulp suspension, compris¬ ing an open-work movable surface (1) for forming and dewatering a pulp web, and an inlet (2) for the pulp susp¬ ension to the movable surface (1), c h a r a c t e r ¬ i z e d i n that in said.pulp inlet (2) a rotatable cylindric rotor (4) is located which with its shaft extends transverse to the flow direction of the pulp and the length of which substantially corresponds to the width of the movable surface (1), that the rotor (4) on its surface is provided with a plurality of cross-wise arr¬ anged channels (7) , and that the rotor (4) is enclosed by a cylindric chamber (3), which has an inlet (5) and an outlet (6) located substantially directly in front of each other, so that the pulp is forced to flow through the channels (7) . 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 rotor (4) is arranged for slow rotation, continuous or reversible. 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 channels (7) are arranged helicoidally. 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 channels (7) at their crossings form a substantially right angle with one another. 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 rotor (4) is rotatable at a speed of 1-20 rpm. 6. 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 diameter of the rotor (4) is 50-100 mm. 7. 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 outlet (6) of the chamber (3) communicates via an outer return line (8, 9) with the inlet (5) of the chamber (3) for circulation of a portion of the pulp flow.

Inventors: LINDSTROEM ALF, LINDSTROEM, ALF
Applicants: SUNDS DEFIBRATOR IND AB, SUNDS DEFIBRATOR INDUSTRIES AKTIEBOLAG
Application year: 1992
Main classification: D21F1
IPC codes: D21F1