DEVICE FOR DEWATERING AND WASHING OF A FIBROUS SUSPENSION

Abstract: A device for dewatering and washing a fibrous suspension, comprising two rolls (1), which together form a nip, in which a seal (8) is located. A head chamber (2) for the suspension is placed symmetrically directly beneath the nip, and a common washing chamber (10) for the supply of washing liquid to at least on first washing zone (25) is placed symmetrically directly above the nip. The shell surface of the rolls (1) is divided into compartments (3) with screen bottoms for receiving the suspension from the head chamber (2) and for draining filtrate into each roll in the washing zone (25). The washing zone (25) is defined by each roll (1) and a washing flap (12), which extends along the shell surface of each rool (1).

Description: Device for dewatering and washing of a fibrous suspension. This invention relates to the dewatering and washing of a fibrous suspension, such as pulp of cellulosic fibrous material. The invention, more precisely, relates to a device, which comprises two counter-rotating rolls, which on their shell surface are provided with compartments for the fibrous suspension. At the manufacture of papermaking pulp from cellulosic fibrous material, in one or several places in the process there is a demand for dewatering and washing the pulp. According to previously known devices for washing and dewatering papermaking pulp, for example of the kind shown in the patent specification SE 442 127, the pulp is led as a continuous pulp web between a rotating drum and a surrounding shell. The washing liquid is passed repeatedly through this pulp web. The washing liquid is collected in compartments located beneath the shell surface of the drum, and the bottoms of the compartments are connected to a restricted plane valve, which is located at one end of the drum and co-operates with a stationary contact portion. Such a valve solution, however, which has been used also previously at this kind of devices, is not satisfactory because of the great diameter of the drum and a restricted valve, which contribute to long filtrate lines, which phase displaces and mixes together filtrate in different washing steps. Long lines also cause increased pressure drop in the system, which adds to constriction losses through the pulp web in respective washing steps. With only one filtrate pump to the last washing step and further through other steps, the pressure becomes much too high for an effective sealing of the valve and above all for sealing in the final steps. In the patent specifications SE 467 161 an alternative to the aforesaid solution is shown. According to this device the bottoms of the compartments are inclined to the end of the drum where the valve system is located. The valve is ring-shaped and has substantially the same diameter as the shell surface of the drum. The end portions of the filtrate compartments are connected to the valve system. This kind of valve system has been known for decades, usually for devices with several washing zones. The weaknesses of this system are wear of the valve seats and high effect consumption. There further remain phase displacements and mixing together of filtrate in different steps, because filtrate lines from the valve to the shell about the drum are long. The present invention offers a solution of the aforesaid problems in that the device is formed with two rolls provided with compartments and counter-rotating, which between themselves form a nip with a symmetric inlet chamber for the fibrous suspension and a washing chamber located above the nip. The characterizing features of the invention are defined in the attached claims. According to the present invention two counter-rotating rolls are used, which can have a substantially smaller diameter with maintained capacity compared with the devices described above and, thus, also can have limited dimensions of the valve system. Another object is to shorten the filtrate lines from the valve sector of the second washing step to the axial connection of the first step, whereby the risk of phase displacement and mixing together is reduced. A further object is to achieve an effective device for dewatering and washing, for better forming of the fibrous suspension, which results in capacity increase alternatively possibilities to construct a smaller device with the same capacity as a greater device according to known state of art. According to one aspect, the inlet for the fibrous suspension consists of a head box, which is equipped with seals defining a head chamber to a transfer zone. The device is easily adjustable in vertical direction in order to abut the seals against the rolls. The head box can be dismantled for seal exchange. According to another aspect, the head chamber is sealed against the washing zone by an adjustable wedge-shaped seal, which distributes the fibrous suspension and washing liquid into two flows. The seal is placed between the rolls and constitutes a portion of a common washing chamber where washing liquid is supplied axially at the end wall valves of the rolls. The washing flaps in a first washing step terminate with a longitudinal seal, which is enclosed by a contracted washing chamber for a second washing step, the overpressure of which pressurizes the seal. According to still another aspect, a conveying screw is placed between and below the rolls, the trough sweep of which with end walls is utilized as hold for vertically adjustable arms, and where seals are connected as sealing between a second washing step and the transfer zone. The invention is described in greater detail in the following, with reference to the' accompanying Figure, which is a cross-section of a device according to the invention. The device shown comprises two opposed rolls 1, the shell surface of which is divided into compartments 3, which are defined by radial wall elements extending in the axial direction of the rolls. The compartments 3 are formed with screen bottoms for draining liquid into each roll. The rolls 1 together form a nip. Beneath the nip a head chamber 2 is symmetrically located. The head chamber is defined by a head box 5 with inlet 4 for the fibrous suspension and compartments 3 on the shell surfaces of the rolls. The head chamber 2 is pressurized and sealed against the rolls in that the head box 5 is vertically movable and has two inclined seals 6 abutting the rolls. The inlets 4 of the head box are placed axially between the opposed rolls. For cleaning the compartments 3 before the head chamber 2, spray pipes 7 are placed on the vertical end walls of the head box 5. In the nip between the rolls 1 a wedge-shaped seal 8 is located, which is controlled vertically by adjusting means 9. Above the nip between the rolls a common washing chamber 10 is located with axial connection 11 for the supply of washing liquid. The common washing chamber 10 communicates with a first washing zone 25 on each roll 1. The washing zones 25 are defined by the two rolls 1 and washing flaps 12, which extend along the shell surface of each roll so that the height of the washing zones successively decreases. The washing flaps 12 terminate with seals 13 against a second washing chamber 14 on each roll. These second washing chambers 14 are connected to second inlets 15 and communicate with a second washing zone 26 on each roll 1. These second washing zones 26 are defined by each roll 1 and second washing flaps 16, which extend along the shell surfaces of the rolls so that the height of the washing zones successively decreases. The second washing zones 26 terminate with seals 19. After these seals 19, counted in the peripheral direction, a transfer zone 28 is located where the dewatered and washed fibrous suspension is removed from the compartments 3 of the rolls to a trough 17 located below, in which a conveying screw 21 is provided. The seals 19 preferably are placed on arms 18, which are vertically adjustable in guides 20 on the trough for setting and adjusting the seals. The washing flaps 12, 16 can be provided with perforated cover plates 27, which abut the compartments of the rolls to distribute the washing liquid into the compartments for constricting the liquid in the fibrous suspension through the screen bottoms of the compartments into the rolls. The rolls should have a diameter of 1-3 metre, preferably 1,5 - 2,5 metre. The head chamber 2 is a thickening and forming zone for the ingoing fibrous suspension, which at the inlet normally has a concentration of 3-8%. The head chamber is sealed against the common washing chamber 10 by the wedge-shaped seal 8, which distributes the fibrous suspension into two equally great portions in the compartments 3 on the roll in question. The pulp concentration in the compartments 3 is there suitably 12-16%ι. The seal 8 constitutes also a portion of the common washing chamber 10, from which the washing liquid is distributed into two equally great flows in the first washing zones 25. The washing liquid is supplied axially to the washing chamber 10, the connection 11 of which is placed at the end wall valves of the rolls 1. The washing flaps 12 in the first washing zones 25 should extend along about a third or more of the circumference of the rolls. The washing flaps 16 in the second washing zones 26 should also extend along a third or more of the circumference of the rolls. The washing flaps 12 and 16 together can enclose 250 - 300° of the roll in question. The second washing chambers 14 are arranged both to distribute the washing liquid and to pressurize the longitudinal seals 13, which are located at the end of the washing flaps 12. In the transfer zone 28 after the seals 19 the treated fibrous suspension is removed from the compartments 3 and moved away by the conveying screw 21 to a subsequent treatment step. The compartments 3 are thereafter rinsed by means of the spray pipes 7. According to an alternative embodiment, the device can be formed with a single washing zone on each roll. This washing zone extends from the common washing chamber 10 to the transfer zone 28. The washing principle at the device according to the invention is to lead washing liquid to the washing zones 25, 26 where the washing liquid is distributed over the fibrous suspension in the compartments 3 of the rolls. The filtrate constricted through the screen bottoms of the compartments is advanced axially in the rolls to valves mounted on the end walls of the rolls. When the device is designed for two washing steps, the filtrate is pumped by a pump 30 from the second washing zone 26 directly to the axial connection 11 to the common washing chamber 10 and to the first washing zone 25. The filtrate from the first washing zone is taken out through the end wall valves of the rolls and is moved to some other place in the process. It is clear to an expert in the field, that the different application forms of the invention are not restricted to the embodiments described, but that they can be varied within the scope of the claims with reference to description and Figure.

Claims: Claims 1. A device for dewatering and washing of a fibrous suspension, comprising a head chamber (2) for the suspension, rotary rolls, the shell surface of which is divided into compartments (3) for receiving the suspension from the head chamber (2), a washing zone with a washing chamber for the supply of washing liquid, the compartments being formed with screen bottoms for draining filtrate into each roll, and a transfer zone (28) for the treated fibrous suspension, characterized in that two rolls (1) together form a nip, in which a seal (8) is located, that the head chamber (2) is symmetrically placed directly beneath the nip, that a common washing chamber (10) is symmetrically placed directly above the nip, and this washing chamber (10) communicates with at least one first washing zone (25) on each roll (1), which washing zones are defined by the two rolls and washing flaps (12), which extend along the shell surface of each roll (1). 2. A device as defined in claim 1, characterized in that the seal (8) in the nip between the rolls (1) is symmetrically wedge-shaped and controllable by adjusting means (9). 3. A device as defined in any one of the preceding claims, characterized in that a second washing chamber (14) and a second washing zone (26) with a washing flap (16) are located at each roll (1) directly after the first washing zone (25), and a pressurized seal (13) defines the two washing zones (25, 26). 4. A device as defined in claim 3, characterized in that the second washing chamber (14) is formed both for distributing washing liquid to the second washing zone (26) and for pressurizing the seal (13) between the washing zones (25,26). 5. A device as defined in any one of the claims 3-4, characterized i n that a pump (30) is provided to pump filtrate from the second washing zones (26) directly to an axial connection (11) for the supply of washing liquid to the common washing chamber (10). 6. A device as defined in any one of the preceding claims, characterized in that the transfer zone (28) for the treated fibrous suspension is located after the washing zones (25, 26) and defined therefrom by seals (19). 7. A device as defined in claim 6, characterized in that the seals (19) are located adjustable on a trough (17) for receiving the transferred fibrous suspension beneath the rolls (1). 8. A device as defined in any one of the preceding claims, characterized in that the washing flaps (12, 16) are provided with perforated cover plates (27), which abut the compartments (3) of the rolls (1) to distribute the washing liquid into the compartments.

Inventors: FORSLUND KJELL, FORSLUND, KJELL
Applicants: FORSLUND KJELL, METSO PAPER INC, FORSLUND, KJELL, METSO PAPER, INC
Application year: 2001
Main classification: D21C9
IPC codes: D21C9