AU575883B2

AU575883B2 - Pulp washer

Info

Publication number: AU575883B2
Application number: AU36116/84A
Authority: AU
Inventors: Alf Inge Lindstrom; Lars Eyolf Nasman
Original assignee: SUNDS DEFIBRATOR; Sunds Defibrator AB
Current assignee: Valmet AB
Priority date: 1983-12-02
Filing date: 1984-11-12
Publication date: 1988-08-11
Anticipated expiration: 2004-11-12
Also published as: WO1985002424A1; SE8306658L; SE8306658D0; JPS61500558A; FI854787A; NZ210131A; CA1265691A; AU3611684A; EP0198825B1; FI854787A0; DE3483161D1; FI76848B; SE452344B; FI76848C; EP0198825A1

Description

Pulp washer

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 vac¬ uum vessels beneath the web.

In such apparatuses the web usually is a wire manufact¬ ured 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 betw¬ een the wire and vessel covers, great forces are requir¬ ed for driving the wire. This gives rise, at the same time, to high tensile stresses in the wire, which there¬ by 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 capa¬ city, 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 inter- plaited 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. 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 perfor¬ ated steel web. A steel web 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 press¬ ure, 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 inher¬ ent 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 refer¬ ence to an embodiment thereof and to the accompanying drawings, in which Fig. 1 shows a liquid treatment appar¬ atus according to the invention, and Pig. 2 is a cross- -section along II-II in Pig. 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 3₃ and both rolls preferably are coated with a friction-increasing surface layer.

At^" the first roll 2 a head box 6 is located for distrib¬ uting 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.

OMPI 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 diff¬ erence 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?ό to about 30?ό 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 longit¬ udinally 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.

The invention is not restricted to the embodiment shown, but can be varied within the scope of the invention idea.

, _j, _ OMPI røSy, WTPO Λ»J ?i

Claims

1. An apparatus for liquid treatment of cellulose pulp, comprising an endless liquid-permeable web running about a first and a second end roll (2 and, respectively, 3)» a device (6) for supplying the pulp to the web at the first roll (2), means (11) above the web for supplying treatment liquid to the pulp, vessels (8) for collecting liquid under the web, and means (14) for transferring treated pulp at the second roll (3)_s c h a r a c t e r i z e d i n that the web is a perforated steel web (1), and that means (16) for press¬ ing-out liquid are arranged so as to co-operate with the steel web (1) at its travel about the second roll (3).

2. An apparatus as defined in claim 1, c h a r a c t ¬ e r i z e d i n that the means (16) for pressing-out liquid comprise a liquid-permeable movable web, which is pressed against the steel web (1) on the second roll (3)

3. An apparatus as defined in claim 1 or 2, c h a r ¬ a c t e r i z e d i n that the second roll (3) is provided en the surface with grooves extending all about the roll for draining pressed-out liquid.

4. An apparatus as defined in any one of the claims 1-3, c h a r a c t e r i z e d i n that both rolls (2,3) are coated with a friction-increasing surface layer.

5. An apparatus 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 steel web (1) has a thickness of 0,5 - 1,5 mm.

6. An apparatus 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 perforated steel web (1) has an open area of 8 - 25$.

7. An apparatus 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 steel web (1) consists of a plurality of narrow webs assembled by longitudinal welds.