EP0198825B1

EP0198825B1 - Pulp washer

Info

Publication number: EP0198825B1
Application number: EP19840904213
Authority: EP
Inventors: Lars Eyolf NÄSMAN; Alf Inge LINDSTRÖM
Original assignee: Sunds Defibrator Industries AB
Current assignee: Valmet AB
Priority date: 1983-12-02
Filing date: 1984-11-12
Publication date: 1990-09-05
Also published as: SE8306658D0; WO1985002424A1; SE8306658L; CA1265691A; FI76848C; FI76848B; FI854787A0; EP0198825A1; SE452344B; JPS61500558A; DE3483161D1; NZ210131A; AU575883B2; FI854787A; AU3611684A

Abstract

The apparatus comprises an endless perforated steel web (1), which runs about two end rolls (2, 3). A head box (6) for the pulp is located at the first roll (2) for distributing a pulp layer on the web. Along the web (1) means (11) are located for supplying treatment liquid to the pulp layer, and vessels (8) for receiving liquid from the lower surface of the pulp layer. In order to additionally increasing the dry content of the pulp, means (16) are provided for pressing-out liquid, which means co-operate with the steel web at its travel about the second roll (3). In connection to said roll (3) also a device (14) for transferring the pulp layer is located.

Description

This invention relates to an apparatus for the liquid treatment of cellulose pulp, for example washing and dewatering, The invention, more precisely, refers to an apparatus where the pulp from a head box is allowed to flow out and form a layer on a plane, endless liquid permeable web. The web runs over two end rolls, and treatment liquid is supplied on the upper surface of the pulp layer at the same time as liquid is sucked away from the lower surface of the pulp layer to vacuum vessels beneath the web.
In such apparatuses the web usually is a wire manufactured of plastic wires. The wire is driven by one end roll and slides over the perforated covers on the underlying vacuum vessels. Owing to the friction between the wire and vessel covers, great forces are required for driving the wire, This gives rise, at the same time, to high tensile stresses in the wire, which thereby is restricted as to its possible length, because the tensile stress increases with the length of the contact surface between wire and vessel covers. When it is desired in an existing mill to increase the capacity, it is therefore necessary in most cases to increase the wire width instead of the wire length. This would imply substantial re-construction of the mill, If a lengthening of the wire would have been possible, this could have been effected by relatively simple measures and at reasonable costs.
Due to the fact, that the wire is manufactured of interplaited plastic wires, a damage in the form of broken wires renders a very troublesome repair work necessary, which in most cases is so expensive that it is more economic to exchange the entire wire.
In FI-C-25 895 a pulp washer is disclosed in which washing of the pulp is accomplished with the use of a pair of wire belts between which the pulp layer is provided. A number of tensioning rolls and guide rolls are provided for the wire belts in order to provide the necessary flexibility for the belt to pass about the various rolls.
Further, in US-A-3 121 660, CH-A-392 239 and FR-C-14 477 603 are disclosed different kinds of papermaking machines where very thin and flexible metal sheets can be used for a fourdrinier- type wire. This type of thin sheets is necessary since in paper-making applications the fourdrinier wire or band is driven at a very high speed and is bent around a number of separate rolls. However, such a band could not satisfactorily be employed in a pulp washing apparatus.
The present invention has the object to eliminate the aforesaid problems and at the same time to render further advantages possible. According to the invention, the plastic wire referred to above is replaced by a perforated steel web. A steel web having a thickness of between 0.5 and 1.5mm and an open area of between 8 and 25% has a substantially higher tensile strength than a wire. It is, further, relatively simple and inexpensive to repair damages in the web. Since a steel web also has a higher resistance to pressure, the dewatering of the pulp layer according to the invention can be carried out much more effectively. A further advantage is that the open area in the covers of the vacuum vessels can be increased due to the inherent carrying capacity of the steel web.
The characterizing features of the invention are defined in greater detail in the attached claims.
The invention is described in the following, with reference to an embodiment thereof and to the accompanying drawings, in which Fig. 1 shows a liquid treatment apparatus according to the invention, and Fig, 2 is a cross-section along II-II in Fig, 1.
The embodiment shown of the apparatus comprises an endless perforated steel web 1, which runs about a first and a second end roll 2 and, respectively, 3. The rolls are mounted in a stand 4 so that the first roll 2 is movable by means of a stretching device 5, preferably a hydraulic one, for adjusting the web tension. The driving is effected via the second roll 3, and both rolls preferably are coated with a friction-increasing surface layer.
At the first roll 2 a head box 6 is located for distributing the cellulose pulp in the form of a uniform layer on the web 1, In the head box a rotor 7 is located for deflocculating the pulp. Beneath the web a plurality of vacuum vessels 8 are arranged one after the other in the longitudinal direction of the web. The vessels are provided with perforated covers acting as a support for the web 1.
Outlets 9 and, respectively, 10 are connected to the vessels 8 for the discharge of liquid and, respectively, gas. Above the web means 11 for the supply of treatment liquid are located. Over the pulp layer a pressure difference is maintained for effecting liquid transport crosswise through the pulp layer. The width of the pulp layer on the web is limited by two endless deckle straps 12, which follow along with the web 1. Also other means for limiting the width, of course, can be imagined. A hood 13 is placed over the web 1.
At the second roll 3 a transfer device 14 for the pulp layer after its completed treatment is located, which for example has the form of a doctor blade. Beneath the same a conveying screw 15 is provided for conveying the pulp to subsequent processing steps. Said second roll 3, also co-operates with means for pressing liquid from the pulp layer. According to the embodiment shown, these means are a liquid-permeable web 16, which is held pressed against the steel web 1. The web 16 runs over two small rolls 17,18 and a stretch roll 19. The second end roll 3 is provided with grooves extending all about the roll to permit liquid to be pressed through the steel web 1. The liquid pressed off is collected in a trough 20 beneath the end roll 3. It is, of course, also possible to use other types of pressing means. The pressing-off implies that a high dry content of the pulp can be obtained. The pulp concentration, for example, can be increased from about 8% to about 30% by this pressing-off. This implies that a greater part than normal of the chemical content of the pulp can be removed already in connection with the liquid treatment. It is possible from a chemical content of 6 - 10% at a conventional fourdrinier wire washing to come down to 1 - 3%. It is thereby possible, for example, to pass the pulp from the treatment plant directly to the bleach plant without further dewatering in special thickeners.
The steel web used preferably shall have a thickness of 0,5 - 1,5 mm. The diameter of the end rolls must be adapted to the thickness of the web, because the bending stress in the web is directly proportional to the web thickness and reciprocally proportional to the roll diameter. A suitable dimension of the rolls can be about 2 m diameter.
The hole size must be adapted to the pulp to be treated, but should be in the interval 0,5 - 1,5 mm diameter. It is also imaginable, of course, to form the holes as slits. The open area must under all conditions be between 8 and 25%.
The length and width of the web are not critical, but can be adapted at option. The desired width can be brought about by welding a number of narrow webs longitudinally to a wider web, because it is not possible to directly manufacture steel webs having widths here in question, for example about 6 m. Steel webs normally are not manufactured in widths exceeding much more than one meter.

Claims

1. An apparatus for liquid treatment of cellulose pulp, comprising a pair of spaced-apart end rolls (2, 3) one of which is a drive roll (3), an endless perforated web (1) arranged about said pair of end rolls (2, 3) so as to provide a substantially horizontally disposed upper portion, said perforated web (1) being adapted to be driven by said end rolls, head box means (6) for supplying cellulose pulp to be treated directly onto said upper portion of said web (1) at the first roll (2), liquid collection means (8) arranged between said pair of end rolls (2, 3) for collecting liquid displaced from said cellulose pulp through said web (1) as said web is driven by said drive roll (3) relative to said liquid collection means, said liquid collection means (8) having a substantially horizontally disposed surface arranged beneath and in direct contact with said upper portion of said web (1), liquid inlet means (11) arranged above said upper portion of said web (1) for supplying a treatment liquid to said cellulose pulp supplied onto said upper portion of said web (1), means for applying a pressure differential across said upper portion of said web to force liquid from said cellulose pulp through said web into said liquid collection means (8) as said web is moved relative to said liquid collection means and removal means (14) for removing treated cellulose pulp from said web (1) after said cellulose pulp has been subjected to said liquid treatment,
characterized in that said web (1) is made of steel having a thickness of between 0.5 and 1.5 mm and being perforated so as to have an open area of between 8% and 25% and that means (16) for pressing-out liquid are arranged so as to cooperate with the steel web (1) at its travel about the second roll (3).

2. The liquid treatment apparatus of Claim 1 wherein said endless steel web (1) is supported by said pair of spaced-apart end rolls (2, 3) and said liquid collection means (8) arranged therebetween; wherein said endless steel web (1) includes a lower portion extending between said end rolls; and wherein said liquid treatment apparatus further includes means (5) for adjusting the spacing between said end rolls to adjust the tension on said endless steel web (1) and maintain said lower portion of said endless steel web substantially horizontally disposed.

3. The liquid treatment apparatus of Claim 2 wherein the length of said horizontally disposed surface of said liquid collection means (8) substantially corresponds to the spacing between said end rolls (2, 3).

4. The apparatus of Claim 2 wherein said liquid collection means (8) comprises a plurality of vessels (8) arranged between said pair of end rolls (2, 3), said vessels (8) each being arranged transversely of the longitudinal direction of movement of said endless web (1) and said vessels (8) collectively extending one after the other in said longitudinal direction of movement of said web, each of said vessels being provided with perforated covers in contact said upper portion of said web (1).