Liquid purification or separation – Processes – Separating
Patent
1997-05-12
1999-11-16
Popovics, Robert J.
Liquid purification or separation
Processes
Separating
210784, 210400, 210402, 210406, 162 56, 1623581, 162368, 100 37, 100116, 100121, 100153, B01D 3304, B01D 33052, B01D 33073, D21F 166
Patent
active
059851597
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method and a device for concentrating a suspension, i.e., increasing the dry contents of the suspension, the suspension during increase of its dry contents moving between a first, relatively tight surface, and a second, relatively pervious surface, through which suction takes place.
The invention also concerns a device for carrying out this method, the device including a first, relatively tight surface and a second, relatively pervious surface, which is movable relative to the first surface, means for forming on the second surface a continuous web of suspension, and means for providing suction through the relatively pervious surface.
The increasing use of recycled fibre pulp involves a need of washing the pulp, i.e., to remove fine-material from the waste paper stock, particularly filler material, such as clay, printing ink and fines. This is achieved by utilizing a diluted suspension which is screened through a wire gauze such that a thin fibre mat having such low grammage that its filtering capability is moderate. In order to achieve sufficient volume capacity the apparatuses have a high operational speed and produce a fibre mat having a relatively high concentration.
Presently, mainly two types of thin film washers are used, viz., "Vario-Split" from Escher-Wyss and "DNT Belt Washer" from Black Clawson.
Vario-Split is based on experiences from the kind of wire part for paper that is called roll former, and the low initial retention these give for fine material. A fibre suspension having a low concentration (0.4-1.5%) is sprayed into the nip between a massive roll and a surrounding wire. Water and accompanying fine material pass through the wire, while larger particles are retained thereon, so that a fibre mat is built up under the wire. The separation of fine particles from the larger ones rapidly becomes less effective during the continued filtration. After an enclosing angle of about 180.degree. the wire is brought back via a system of pulley rolls etc. The fibre web adheres to the roll and is removed by a doctor blade, the pulp then having a dry content of 7-8%. This apparatus works according to the "constant pressure" principle, i.e., that the pressure against the suspension/fibre mat is constant during the entire process.
"DNT washer" exhibits two rolls having equal diameters, one being grooved and the other smooth. The rolls are placed side by side at a centre distance larger than twice the diameters, and are surrounded by a wire having a direction of movement from the top of the grooved roll, down around it, to the smooth roll, up around it and back to the top of the grooved roll. The nozzle of a head box sprays a diluted suspension into the nip between the wire and the grooved roll. Water and fine material pass through the wire in a similar manner as in the "Vario-Split", but the grooves involve a lower pressure against the fibre mat, so that a smaller part of the water is removed during passage around the roll. The grooves cause the pulp web to be thrown off the roll at the nip exit and to follow the wire up to the nip towards the smooth roll. More water and fine material is here pressed out of the pulp. At the outlet from the latter nip, at the top of the smooth roll, the pulp web adheres to the roll and follows it, is removed by a doctor blade, and falls down into a channel parallel to the rolls for discharge by a conveyor screw.
Also this apparatus works according to the "constant pressure" principle, with the exception, however, that there is a first relatively low pressure during the passage of the first roll, and a second, relatively high pressure during the passage of the second roll.
The "constant pressure" principle has as an inherent peculiarity that practically all dewatering occurs at the beginning of the pressing between the wire and the roll, the retained fibres arranging themselves in a static manner which is then not changed during the continued passage and which very rapidly makes the fibre mat so tight that any fine material no longer passes thr
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Horlyk Lars
Oswaldsson Rolf
Strid Kent
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