1. FIELD OF I VEΝTIOΝ.

The invention concerns mainly removal of fibre fragments, fibre fines and other particles from fibre suspensions, for example in connection with the production of pulp and paper.

Fibre fragments, fines and other, minor particles are often unwanted in fibre suspensions due to the fact that they retard paper drainage in the paper machine and thereby also reduce the machine's production capacity. Likewise, fibre fragments and fines in the finished paper reduce the strength of the paper and cause unwanted dusting from the paper web in printing shops.

Fragments and fines also often cause de-lamination of the paper sheet, which again leads to paper rupture.

All these facts reduce the paper quality, and may, under circumstances, make the fibre suspension unsuitable for paper production. These problems are increased by increased consumption of secondary- fibres because new fibre fragments and new fines are peeling off from secondary fibres every time they are re-used for the production of paper.

Therefore, there is a great and steadily increasing demand for a suit ^¬ able apparatus capable of fractionation of fibre suspensions for sepa ^¬ ration of fibre fragments, fines and other, unwanted small particles out of fibre suspensions.

Due to the more stringent environmental regulations there will also be a general, increased demand for separation of suspended solids from liquids, also in many other fields. This invention aims at meeting these demands.

In addition, the invention is also applicable for general recovery of fibres and solids from suspensions.

2. STATE OF TECHNOLOGY.

The machinery applied today for the separation of fibres, fibre fragments and fines from fibre suspensions are constructed as ow screens or as rotating drum- or disc filters. These filters are very varied in design and mode of operation, and are

less suited for fibre fractionation.

Whereas the bow screen's filtering area consists of many, parallel metal rods packed together at certain, internal intervals, thus forming a re¬ latively? smooth filtering surface, whereby the intervals between the rods constitute the perforation or open area of the filtering surface, the filtering areas of the drum- and disc filters are woven clothes of high mesh, usually made out of monofilament threads of synthetic material (plastics), the filtering area rotating slowly within a vat filled with the suspension to be filtrated. These screens' mode of operation are also different:

The static bow screen is functioning with the fibre suspension flushing over the filtering area, across the metal rods and their intervals, either by having the suspension flushing over an inclined, bowed sur¬ face by its own momentum or being sprayed onto the filtering surface, approximately tangential to the bow's form. Drum- and disc filters are functioning differently: during rotation in the fibre suspension a filtering layer consisting of fibres is being formed on the one side of the high-mesh screen cloth through which the filtrate is penetrating. This filtering layer, formed onto the the filtering cloth, consists of fibres from the suspension to be filtered, or is also added to the suspension on beforehand, as socalled filtering pulp. By each revolution of the drum- or disc filters, this filtering layer, which contains those fragments and those fines which has been filtered out of the suspension, is being washed off the filtering cloth by means of water or air. This filtering layer retains not only the fibres but also fibre fragments and fines. Both drum- and disc filters are therefore fitted for socalled barrier screening of fibre suspensions, but less suitable for fractionation of same.

The bow screen is due to its relatively coarse, mechanical construction and coarse perforation also not suited for fractionation, as the screen permits too many usable fibres to slip through together with the fragments and the fines which is wanted separated from the fibre suspen¬ sion.

3. DESCRIPTION OF THE INVENTION.

The invention takes into use a new, perforated screening material, which has not previously been applied in the pulp and paper industry, and which, by virtue of its special characteristics, opens up

new opportunities in connection with screening and fractionation of fibrecontaining suspensions.

The screening material is electrolyttically produced, as a continuous, thin and smooth metal foil with extremely small perforation holes, preferably with diameters in the order of 10 - 80/un and a thickness of the foil in relation to the hole diameter in relationship 1:1 or 1,0- 1,5:1, and an open area in the order of 5% to 40%.

Thus, the metal foil has, as a screening material, technical dimensions and specifications which are unachievable by today'ε modes of production for conventional filters, these being bow screens, where the width of the intervals hardly can be less than 150 _/ km, or the screen clothes in drum- or disc filters, where the surface is comparatively rough and un¬ even due to the weaving texture of the cloth, as a consequence of an obvious "fishing net effect".

In contrast hereto, the screening material of the invention, being a thin metal foil, has a very smooth an even surface. The perforation holes have a selfcleaning capability by virtue of their short lengths and round edges, which counteract a clogging of the holes. The construction of the invention can be seen in Fig. 1-8. The screening material (ll,Fig.l) with its special perforation is attached to an underlying material (12), which has substantially coarser perforation or openings, preferably in the form of a woven texture of comparatively coarse, monofila net wires of synthetic material (plastics) or metal, as shown in Fig.l.

The texture is further attached to a stiff, underlying material (13) containing still coarser perforation in the form of a plate or pipe provided with holes or ribs in a framework (31,Fig.3) provided with large openings.

The suspension to be filtered or fractionated is conducted onto the screening foil through suitable pipes (21,Fig.2) which are equipped with spray nozzles or slits (22) thus formed that the outgoing suspension jet (23) gets a certain spreading before the suspension liquid hits the screening foil (11).

The magnitude of the perforation openings in the screen foil (11) is a limitation for which solid particles in the suspensions are being let through the openings and for which solid particles (24) are being retained at the creening foil and which are continuously pushed away from the area (25) of the screening foil which is being hit by the jet and where the filtrate is being hurled through the perforation openings

due to the momentum of the jet (23) .

The screening foil is preferably placed vertically in the screening apparatus, either in the form of standing plates (32,Fig.3) or standing cylinders (41,Fig.4) from where the separated particles (24,Fig.2) on the screening foil can fall down between the row og spraying nozzles (22) and the jets (24) into a outlet device (33,Fig.3), (51,Fig.5) for separated particles, apart from the outlets of the filtrate (34)(52). The apparatus may consist of more plate sections as shown in Fig.3 which contain respectively the screening foil (32) or the parallel, vertically placed jetpipes (21) which are packed together, and which at top and bottom are connected to the inlet pipes (35) (36) for the suspension, and thus placed together into packs, and where separated outlet devices (34) (33) respectively for filtrate and separated particles, are placed under these packages.

A plate (37) stops the filtrate after it has been hurdled through the screening foil and leads the filtrate down to the outlet (34) . Fig. 4, 5 and 6 show the apparatus formed as a vertical standing cylinder (41), placed within a tube jacket (42), on which inside is attached a pipe (21) with spray nozzles or slits (22) which comπunicate with a distribution chamber (53,Fig.5) for suspension at top and bottom (54), and which is connected to the inlet pipes (55) (56) for suspension within the chambers.

Another form of performance is shown in Fig.6, where the pipes with spray nozzles or slits are replaced with an inner tube jacket (61) which is provided with spray nozzles or slits (62) which spray suspension as jets (23) onto the inner cylinder (41) which is clad with the sceening foil (11) over the underlying material (13), which further is attached to a framework (63) . By this performance are eliminated the pipes for the spray nozzles and slits and the separated distribution chambers on top and bottom, as the volume between the inner tube jacket (61) and outer tube jacket (42) becomes one, integrated chamber of distribution for the suspension.

Also by this performance the filtrate and concentrate (separated, solid particles) leave the apparatus as described for Fig.5. A device for oscillation (57), not shown in detail in the drawing, cause the direction of the ingoing jet (23,Fig.7) for the screen openings (72) to be changed by the oscillation from position A to position B, which further cause the particles which tend to clog the holes (72) in a position (A), to be hurdled out of the opening again when the direction

of the ingoing jet is changed by oscillation in the other, outer po¬ sition (B) .

4. MODE OF OPERATION.

The suspension to be filtered or fractionated is put under pressure by a pump, nor shown on the drawings, and is conducted through spray nozzles or slits, which give the thus formed jets a certain spraying, and where¬ by the pressure gives the jet a certain speed and momentum. The suspension jet is moved at high speed, preferably exceeding 10 meters per second onto the screening foil, which preferably is arranged in a vertical position.

Dependant upon the suspension's consistencey, the size of the particles and perforation holes and the open area of the screening foil, a certain amount of the suspension is hurdled through the openings of the screening foil (11) -and thereafter continue through the coarse openings of the layers behind (12) (13).

Retained on the screening foil are left as a concentrate those particles of the suspension which because of their magnitude were not let through the perforation openings of the screening foil. These separated particles are continuously hurdled out of the zone (25) on the screening foil which the suspension is sprayed onto, and form accumulations (24) of separated particles between the jets.

By an open and non-pressurized construction of the screening apparatus, these accumulations will grow and then fall down due to own weight, when the weight exceeds the adhesion forces between particles and screening material by those constructions where the screening foil is placed vertically.

By a closed and pressurized construction the apparatus may also have an inclined or horizontally positioned screening foil, whereby separated particles are moved out of the apparatus towards the outlet by the pressure drop created within the closed construction, through an outlet channel (80,Fig.8), which walls are formed by the screening foil (11), the suspension jets (23) and pipe walls (21) of the spray nozzles or slits (22) .

The oscillation device causes continuous purification of the screening foil (11) by moving the frames (31) with the screening foil (32) by a separate oscillation device (38), not shown in detail on Fig.3, to and fro in front of the nozzles or slits, suspended by wheel rails (45) and rolls (46), or by moving the nozzles or slits in a similar way

in front of stationary screen plates, or by slightly moving the round cylinder (41, Fig. 4, 5 -and 6) in both directions in the bearing (59) around its own axis by a separate device (57), not shown in detail on the drawings.

For both performances (Fig.3, resp. Fig. 4, 5 and 6) the oscillation measures 2 to 3 times the distance between the row of nozzles or slits.