GB2051600A - Paper pulp screen - Google Patents

Abstract

A screening device for paper pulp has a closed tank (1) containing a fixed sieve (2) separating an inner chamber (4) from an outer chamber (3). The inner chamber (4) has a rotor (5) provided with blades or vanes which move in the vicinity of the sieve (2). The outer chamber (3), between the sieve (2) and the tank (1), comprises a helical wall (7) providing the outer chamber (3) with the shape of a helical channel which extends round the sieve (2) from a pulp inlet (8) to a rejects outlet (9) which is controlled by a valve. An inlet (10) opens into the inner chamber and supplies water at below 20% of the pulp flow rate. <IMAGE>

Description

SPECIFICATION A device for cleaning and recovering paper pulp This invention relates to a device for cleaning and recovering paper pulp.

In the paper industry, the recovery of waste paper is currently carried out as follows. The waste paper is first dispersed in water and reduced into pulp in an apparatus known as a pulper which effects a rough cleaning by elimination of the heavy parts of density higher than that of the pulp (a density of about 1.027). The pulpers operate in general at a low concentration of less than 5%.

At the outlet of the pulper, the pulp is fed through at least one screen or strainer which acts as a sieve. The object of this operation is to clear the pulp of wood, plastics and metal contaminants which it may contain. Generally, a series of two apparatus has to be used, one of them comprising at least one fine slit sieve which eliminates all elements having a particle size exceeding the width of the slits, while letting through fibres and the flat and very thin elements and the other apparatus comprises at least one hollow sieve for eliminating the flat elements.

It is usual to operate with an apparatus comprising a sieve with holes having a diameter of the order of 4 to 5 mm, followed by an apparatus comprising a sieve having slits of a width of the order of 0.3 to 0.4 mm.

The screens or strainers, which may, for example, be of the type disclosed in French Patent No. 77.35151, produce at one outlet a cleaned pulp containing water and fibres but no contaminants, and at another outlet waste material with high content of contaminants.

By way of example and with an apparatus such as that disclosed in French Patent No.

77.35151 into which is fed pulp having a concentration of about 3 to 5% of dry materials and containing about 5% of pollutants, there is obtained at one outlet a pulp containing 0.05% of pollutants with a concentration which is always about 3 to 5% of dry materials, and at the other outlet waste material containing about 30% to 50% of pollutants, the remainder being pulp with a concentration of the order 3 to 5% of dry materials.

It will be seen that waste material contains appreciable quantities of pulp and that it is highly diluted. Thus, the waste material cannot be discarded untreated and is very difficult to deal with.

The presently most prevalent process for treating the waste material consists in feeding it into an auxiliary apparatus known as a sorters which is generally made of vibrating sieves in which the waste material is washed in a high diluted state and in an open circuit thereby producing on the one hand washed waste material and on the other hand a very diluted pulp which can be recycled.

Such a sorter has serious disadvantages, first of all because, by operating in an open circuit, it requires a pumping system and a supply of clean water and is therefore a consumer of energy and water and produces very diluted products which have to be thereafter reconcentrated. Another disadvantage is that, as all vibrating apparatus, they require large masses of support concrete for absorbing the vibrations, and their maintenance costs are therefore high. Finally, if the cleaning has been carried out by means of fine slit sieves, the washing of the waste material is practically ineffective since a vibrating sieve is necessarily a hollow sieve, the slit sieves not offering a sufficient mechanical strength for use in a vibrating apparatus. Due to this fact, a large part of the waste material eliminated by the strainer gets through the vibrating sieve and is recycled.

An object of the present invention is to provide a sorter device capable of continuously cleaning a paper pulp by supplying washed waste material containing no more than 10% of paper pulp.

According to the invention there is provided a cleaning and recovery device for paper pulp, comprising a closed tank containing a fixed sieve separating an inner chamber from an outer chamber, the inner chamber comprising a rotor provided with blades or vanes movable in the vicinity of the sieve, and the outer chamber, between the sieve and the tank, comprising a helical wall providing the outer chamber with the shape of a helical channel which is wrapped around the sieve and extends from a pulp inlet to a waste outlet controlled by a gate.

Embodiments of the invention may include the following features: (a) An additional water inlet is provided in the inner chamber, on the side of the pulp arrival.

(b) The helical outer chamber is placed such that it is wrapped around the fixed sieve from the pulp inlet to the waste outlet, with a rotation direction consistent with that of the liquid in said outer chamber.

(c) This rotation direction is opposite to that of the rotation of the rotor.

In the accompanying drawings: Figure 1 is an axiai sectional schematic view of a device according to the invention; Figure 2 is an end view of the same device with an end piate removed; and Figure 3 is a detailed sectional view, perpendicular to the rotor axis, showing a preferred embodiment of blades and perforations.

The illustrated device comprises a closed tank 1 in which a revolution sieve 2 separates two chambers 3 and 4, a rotor or rotating drum 5 being provided inside the inner chamber 4. The drum 5 is provided with longitudinal blades 6. Such a device is discussed in French Patent No. 77.35151.

The outer chamber 3 defined between the sieve 2 and the wall of the closed tank 1, is partitioned by a helical wall 7. Thus, the chamber 3 is helical in shape and extends from the pulp inlet 8 to the waste outlet 9.

The inlet 8 and the outlet 9 are located at opposite ends of the helical chamber 3. A water inlet 10 is provided in the inner chamber 4, on a wall 11 which is opposite an outlet 12.

The operation of the device is as follows.

The pulp to be cleaned reaches the apparatus through the inlet 8. As it proceeds inside the chamber 3, it is cleaned. The fibres pass through the sieve 2 and the pulp which is deprived of its impurities emerges at the outlet 1 2. The waste material progresses slowly along chamber 3 and is concentrated gradually until it reaches an end space 1 3 which is immediately upstream of a gate 1 4 where it is retained as long as the gate 14 is closed. By periodically opening the gate 14, waste material completely washed and concentrated to a mass or a kind of pulp may be removed, while the pulp is entirely recovered at the outlet 12.

It will be seen that the washing of the waste material is thus carried out in a closed apparatus which is under pressure, thereby avoiding all the disadvantages of an open apparatus, particularly as regards the pulp and waste circulation means. On the other hand, the apparatus operates continuously as regards the inlet and outlet of the pulp, only the outlet of the waste material being carried out discontinuously.

These results had never been achieved hitherto, since with an apparatus closed and operating continuously and under pressure, one could only separate a cleaned pulp from a pulp containing a concentration of pollutants, but without being able to wash away the pollutants.

In the preferred embodiment which is illustrated, the helical wall 7 forms a spiral extending from the inlet 8 to the outlet 9 by following the direction of rotation of the pulp in the chamber 3. The pulp which reaches the chamber 3 moves within that chamber by effecting a rotating movement around the sieve. This rotating movement is caused by the rotation of the drum 5 in combination with the shape of the sieve perforations and the disposition of the inlet 8 which is, for example, tangential.

Particularly, if the holes or slits which form the perforations 1 5 of the sieve 2, have in cross-section the shape shown in Fig. 3, as well as the blades 6, the rotation of the drum in the direction of arrow F1 causes a rotation of the pulp inside chamber 3 along arrow F2 due to the reflection of the streams of liquid on the radial walls 1 6 of the perforations 1 5.

The wall 7 is wrapped around the sieve 2 so as to extend from the inlet 8 to the outlet 9 by turning around the sieve 2 in a direction contrary to the direction of rotation of the drum 6. As is shown in Fig. 2, the inlet 8 and the outlet 9 are placed tangentially to tank 1 to correspond to a rotation of the liquid in a reverse direction to the rotation direction F1 of drum 6.

The pitch of the helix formed by wall 7 is preferably larger than the diameter of the inlet 8.

The water inlet 10 provides an auxiliary water supply which makes it possible to obtain a better washing of the waste material.

This water inlet is controlled by a gate 1 7 and the additional water flow rate is preferably less than 20% of the pulp flow rate arriving at the inlet 8. Thus, the pulp is not diluted and may be recycled as it stands from the outlet 12.

The apparatus according to the invention may be used in various ways. For example, it may be associated with a strainer the waste material of which has to be washed with a view to recovering the pulp which the waste material contains. Such a strainer that disclosed in the aforementioned French Patent No. 77.35151.

The apparatus according to the invention may also be used directly as an improved strainer to which is fed pulp prepared in a pulper. However, experience shows that the latter solution provides results which are not as good since the optimum dimensional characteristics of a given pulp are not the same at the various cleaning stages. Therefore, it is often preferable to use in succession a hollow strainer followed by a slit cleaner and then by a sorter according to the present invention.

With such a succession of equipment, one may use a sorter having a drum of a diameter slightly less than that of the strainer, thereby allowing carrying out the washing of the waste material with an economically acceptable consumption of energy.

It is thus possible to exclude the waste material which contains little or no pulp from the waste material of the preceding strainer which contained from 30 to 50% of pollutants in a pulp having a dry material content of the order of 3 to 5%

Claims (6)

1. A cleaning and recovery device for paper pulp, comprising a closed tank containing a fixed sieve separating an inner chamber from an outer chamber, the inner chamber comprising a rotor provided with blades or vanes movable in the vicinity of the sieve, and the outer chamber, between the sieve and the tank, comprising a helical wall providing the outer chamber with the shape of a helical channel which is wrapped around the sieve and extends from a pulp inlet to a waste outlet controlled by a gate.

2. A device according to claim 1, further comprising an additional water inlet emerging into the inner chamber on a wall which is opposite an outlet for cleaned pulp.

3. A device according to claim 2, wherein the additional water inlet is arranged to supply water at a flow rate which is less than 20% of the flow rate of the pulp to be cleaned.

4. A device according to any one of the preceding claims, wherein the helical wall forms a spiral which is wrapped from the inlet to the outlet by following the rotation direction of the liquid inside the outer chamber.

5. A device according to claim 4, wherein the helical wall is wrapped around the sieve from the inlet to the outlet with a rotation direction which is contrary to that of the rotor.

6. A cleaning the recovery device for paper pulp, substantially as herein described with reference to the accompanying drawings.