Method and apparatus for producing a multilayer paper web

(54) Title: Procedure and apparatus for producing a multilayer paper web (57) Sanman features:

In the manufacture of multilayer paper in a double-layer in transformers of the kind in which the wires (13, 15) define a curved forming zone (23), preferably a roll type mold with smooth surface forming roll (11), a significantly improved layer unit is obtained by the draft of a first and at least one subsequent portion of the curved forming zone (23) is collected separately as separate fractions. The first fraction, closest to the multilayer inlet box (21), is collected in a first separate circuit for use in diluting a first thick mass to a first stock of inlet box concentration, from which stock a first layer is to be formed directly on the relative to the curved forming zone ( 23) outer front (15). At least one they! of a second fraction collected at a greater distance from the multilayer inlet box is returned in a second separate circuit for use in diluting a second thick mass to a second stock of inlet box concentration, from which a second layer is to be superimposed on the first layer.

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(56) Publications cited:

8107215-9

The present invention relates to a process for producing a multi-layered paper web in a double wire former of the kind in which the wires define a curved forming zone, to which at least two different layers are emitted from a multi-layer inlet box, from which the forming zone rear water is ejected, collected and substantially is returned to the papermaking process.

The invention also relates to a device for producing multi-layered paper web for carrying out the method, comprising a double wire former of the kind in which the wires define a curved forming zone, a multi-layer inlet box for delivering at least two different layers to the curved forming zone. and means for intercepting and returning to the papermaking process at least a major portion of the backwater discharged from the curved forming zone upon forming the multilayer paper web.

The term forming zone is used here in its generally accepted sense among those skilled in the art, that is, a zone which starts from the point where the dewatering of the stock through at least one wire extends to the point where the dewatering has progressed so far that the paper web forming fibers can no longer float about in the suspension fluid without abutting essentially unobtrusive to each other.

Water that is separated from a fiber suspension or fiber web in a paper machine is called backwater. Backwater usually contains fiber debris, sometimes also fillers, dyes, resin adhesives and the like, and is generally returned to the papermaking process. The flow circuit arranged for the return of the backwater to the process, which includes pipelines, storage tanks, purification devices and control equipment for return, is usually referred to as backwater systems. A backwater system is said to be open if a large portion of the total backwater flow leaves the system and the end if only a small portion of the total backwater flow leaves the system. A flow circuit for so-called short backwater around a former is often referred to as short circulation, whereby short bath water refers to backwater which is returned to the process step from which it originates. Analogously, the term long circulation denotes a flow circuit for so-called long backwater, ie backwater that is returned to a different process step than the one from which it is separated.

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The backwater returned in short circulation is used to dilute the maiden from a higher concentration to the inlet box concentration, e.g. in newsprint style Operation from over 2.5 weight percent to less than 1 weight percent, and this reversal occurs without purification of the backwater. In newsprint use 5 with flat wire machines, it is known that backwater separated at the end of the flat wire portion has a substantially lower content of suspended material than the backwater separated at the beginning of the flat wire section, also compare SE 366 567 and SE 392 491 (both D 21 F 1 / 66). This cleaner backwater is usually returned in long circulation to the mill's grinder, but some of it can be led to final cleaning before discharge into the appropriate recipient. It is also known to divide the backwater into three fractions with towards the end of the planvira portion increasing purity, whereby the purest fraction is led to final purification. The state of the art regarding system shaping and system gradient for planvira type newsprint machines is shown in the report Forestry Industry's 1 1 care project from SSVL (Foundation for Forest Industries Water and Air Conservation Research), pages 151-155 and 178-190.

Further, US 3,846,232 (= SE 397,695) discloses a double roll type machine at which the forming zone curves along the mantle surface of a suction roll type forming roll, the forming zone is followed by a slightly curved dewatering zone having a length of no more than three times the forming width . In order to achieve the weak curvature, the inner wire is supported along the dewatering zone by a few guide members, such as rotatable rollers or one or more solid and narrow foils or deflectors. It is stated that the backwater separated by the molding is trapped in a first trough and a second trough, which are located on each side of the wires, and the backwater is led to the inlet of a mixing pump for the inlet box for use as dilution water. It is also stated that backwater, which is separated downstream from the forming zone from the formed web and generally has a lower content of fibers than the backwater from the forming web, is collected in separate trays to be directed to a point in the water system where a lower fiber content is sought.

By forced dewatering of the stock at a forming roller compared to a flat wire part, the excess backwater content of suspended material is higher at the forming roller, and compared to the backwater from the end of the flat wire part, the backwater from the forming roller can have more than 50% higher content of suspended material. Therefore, in the manufacture of multilayer paper in a double roll type wrap former, the backwater may be relatively fiber rich and contain both long and short fibers. In cases where it is desired to manufacture a multilayer paper with at least one layer of long fibers and at least one layer of short fibers and dilute partly non-fibrous thick pulp and partly short fibrous thick pulp with backwater to form a stock of predetermined inlet box concentrations, and this is the backwater with which it is diluted. rich in both short and long fibers, no significant layer purity can be achieved. The layer of short fibers will contain a large proportion of long fibers and the layer of long fibers a large proportion of short fibers. This unfavorable result can be aggravated if the inlet box is not such that it keeps the bays apart not only through the inlet bins but also a distance downstream of its outlet openings for the bins. In the worst case, the layer purity has been reduced to such an extent that when determining the proportion of long and short fibers in the layers through fiber counting, no significant difference between the different layers could be detected.

The object of the present invention is to produce in a double-wire former with a curved forming zone a multilayer paper web with considerably improved layer purity.

In accordance with one aspect of the invention, this object is achieved in the process stated by introducing the drafts of a first and at least a subsequent portion of the curved forming zone separately as separate fractions, that the first, the fraction collected closest to the inlet box is returned to a first separate circuit for use in diluting a first thick mass to form a first stock of inlet box concentration from which a first layer is to be formed directly on the outer membrane with respect to the curved forming zone, and part of another, at a greater distance from the inlet box, the fraction collected is returned in a second separate circuit for use in diluting a second thick mass to form a second stock of inlet box concentration, from which a second layer is to be superimposed on the first layer.

According to another aspect of the invention, the object of the device initially stated is achieved by said means including means for collecting separately as separate fractions ejected from a first and at least a subsequent portion of the curved forming zone, a first separate circuit for returning the first nearest the inlet box collected the fraction for use thereof in diluting a first thick mass to form a first

8107215-9 measured by the inlet box concentration, from which a first layer is to be formed directly on the outer wall relative to the curved forming zone, and a second separate circuit for returning at least a portion of a second fraction collected at a greater distance from the inlet box for using the same when diluting a second thick mass to form a second stock of inlet box concentration, from which a second layer is to be formed superimposed on the first layer.

An improvement of the layer purity makes it possible to produce new paper grades that were previously unthinkable. First, the mechanical properties of the paper can be improved. For example, a soft and flexible tissue can be made with a strong mid-layer of long-fiber pulp and soft surface layers of short-fiber pulp, or a printing paper which, despite having a low surface weight, is relatively rigid by surrounding two strong outer layers of long-fiber material with a weaker mid-layer of short-fiber material. Second, cheaper raw materials can be used for certain paper grades. For example. For example, the middle layer may sometimes consist of recycled fibers while the surface layer consists of fine fibers. Third, in the production of certain paper grades, process benefits such as improved machine drivability can be achieved. For example, in the manufacture of two-layer tissue, the tissue web adjacent to the Yankee cylinder may be made up of number 1 sulfate pulp to achieve a good adhesion of the web to the Yankee cylinder, and let the other side of the tissue web consist of abrasive pulp, which usually provides increased abrasion, in the present case does not come into contact with the cabinet edge.

Preferably, in the backwater trough which collects the draft from the forming zone, a machine-wide screen is placed which is positioned such that the first backwater fraction becomes at most large to fully dilute the first thick mass to form the first the stock of a predetermined inlet box concentration. Thereby, the entire fiber content of the first fraction can be returned in short circulation to the first thick mass to dilute it to the stock from which the first, directly applied wire layer is to be formed. For example, if this layer consists of long-fiber pulp, the first fraction comprises long fibers, all of which are returned to the correct inlet box channel.

If the first fraction is not large enough to permit the desired dilution of the thick mass, it is convenient that a diluent supplement is supplied from the backwater of the second fraction or from the backwater in an even later one.

8107215-9 fraction if it were richer with respect to the right fiber type and poorer with respect to incorrect fiber type.

In order to provide the greatest opportunity for fractional collection of the backwater, it is advisable for the double-wire former to be of a roll type and have a smooth surface forming roll along which the mantle surface of the forming zone curves. As a result, only a negligible amount of backwater passes through the inner wire, so that all the backwater removed from the forming zone is ejected from the outer wire during unilateral dewatering of the wedges squeezed between the wires.

Preferably, the inlet box comprises at least one machine-wide partition, which keeps two mounts separate when dispensed from the inlet box. Such an inlet box is described in e.g. CA 1 107 111 and makes a particularly effective contribution to the desired high layer purity.

The invention will be described in more detail below with reference to the accompanying drawing, which is a principle sketch of the wet portion of a dual-wire machine with short-circuit water flow circuits designed in accordance with a preferred embodiment of the invention.

The double wire former illustrated in the drawing is of a roll type and thus comprises a rotatable forming roller 11, which in the illustrated embodiment consists of a roller with a smooth casing surface, an inner wire 13 which runs in an endless loop around the forming roller 11 and is supported by a portion of the this, and an outer wire 15 which runs in an endless loop and via the inner wire 13 is supported by said portion of the forming roller 11. The wires 13 and 15, which are held stretched by stretch rolls 17 and 19, join together on the forming roller 11 to form a space converging in the direction of rotation to receive a multilayer stock jet from a multilayer inlet box 21. The stock is dewatered by squeezing between the strands 13 as they follow the circumference of the rotary forming roll 11, forming a web of paper. The area between the point where the dewatering begins and the point where the fibers no longer flow around the stock but are essentially immobile in relation to each other is called the forming zone. The forming zone 23 curves along the circumferential surface of the molding 11. The suspension fluid which is squeezed out of the wires 13 and 15 by squeezing the stock between the wires 15 is removed by being ejected as a result of the rotation of the roller 11 and is trapped in a backwater trough 25 arranged inside the outer winding loop 15. A squeeze of suspension fluid or backwater through the inner wire 13 is blocked by the smooth casing surface of the forming roller 11. The draft

8107215-9 is intercepted and deflected by means of machine wide, curved screen plates 27 with perforated trailing edge arranged in the rear water trough 25, as described in US 4,028,174. The drawing further shows that the trough 25 is included in what can be termed means for capture and return to the paper-making process at least a major portion of the backwater ejected from the curved forming zone 23 upon forming the multilayer paper web.

According to the invention, the drafts from a first and at least a subsequent portion of the curved forming zone 23 are collected separately as separate fractions. The first fraction closest to the inlet box 21 is collected in a first separate circuit 29 for use in diluting a first thick mass to form a first stock of inlet box concentration, from which a first layer is to be formed directly on the relative to the curved forming zone. 23 exterior we ran 15- At least part of a second, at a greater distance from the inlet box 21, the fraction collected is returned to a second separate circuit 31 for use in diluting a second thick mass to form a second stock of inlet box concentration from which a second layer is to be superimposed on the first layer.

For carrying out this method, said backwater capture and return means comprises means for collecting separately as separate fractions the ejections from said portions of said curved forming zone 23 and said first separate circuit 29 and said second separate circuit 31. In the preferred embodiment shown in the drawing, the aforementioned means includes a machine-wide screen 33 arranged in the backwater trough 25 which extends down to the bottom of the trough and also divides the outlet from the trough 25 so that the fractions are not inadvertently mixed with each other. Also shown in the drawing is a second fraction splitting screen 35 located at a greater distance from the inlet box than screen 33, but otherwise similar in construction and function.

The screens 33 and 35 thus divide the draft from the forming zone 23 into three separate fractions. The first fraction is discharged through the first circuit 29 which includes a backwater tank 37 and a mixing pump 39 in which said first thick mass from a thick massager 41 is mixed with the first backwater fraction to form the first stock of a predetermined inlet box concentration. Correspondingly, the second and third fractions are discharged through the second circuit 31 and a similar third circuit 43, each of which comprises its rear water tank 45 and 51 respectively, and each of its mixing pumps.

8107215-9 and 53, respectively. In pump 47, said second thick mass from a thick massager 49 is mixed with the second backwater fraction to form the second stock of a predetermined inlet box concentration, and in pump 53 a third thick mass from a thick massager 55 is mixed with the third backwater fraction. formation of a third stock of a predetermined inlet box concentration. From the mixing pumps 39, 47 and 53, the feeders go to their respective cross distributors 57, 59 and 61, respectively, which extend across the machine direction and have a cross section decreasing in the flow direction to deliver a uniformly distributed stock flow to the inlet box 21.

In the preferred embodiment shown, the inlet box 21 is a three-layer inlet box which is preferably of the type described in CA 1 107 111. The three bearings run apart through the inlet box, which has a nozzle chamber converging toward a lip opening. The nozzle chamber is divided by two machine-wide partitions 63 and 65 into three nozzle channels, one for each stock. The partitions 63 and 65 are rigid and thick, and each is configured to allow an air wedge not shown at its downstream end which projects out of the lip opening. The air wedges keep the bores apart from each other after ejection through the lip opening a further distance in the direction of the forming zone. By delaying the onset of a mixture of at least the boundary layers between the stock jets, a notable increase in the layer purity is achieved. A further improvement can usually be achieved if an unseen, preferably relatively rigid, ski wall extension in the form of a film of slightly greater length than the air wedge is anchored at the downstream end of the partition to prevent the jets of the jet beams from advancing towards each other, , causes an undesirable mixing. If desired, such a film may extend all the way into the forming zone 23. A limit to its length is set by the risk of the film being clamped between the wires 13 and 15 and pulled.

Preferably, the first screen 33 in the backwater trough 25 is positioned in such a position that the first backwater fraction becomes at most large enough to dilute the first thick mass to form the first stock of the predetermined inlet box concentration. In case the position of the screen 33 is such that the first fraction is not large enough to effect the desired dilution of the thick mass, it is appropriate that means 67 be arranged to allow a controllable flow of backwater from generally the second but in some cases the the third fraction to the first fraction.

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Since with the thick masses more water is added to the wet portion than the seam is removed as water in the newly formed wet paper web when it leaves the wet portion, a backwater excess occurs. This is removed in the illustrated embodiment by a vented conduit $ 9 connected to the backwater tank 51 of the third circuit, since the third fraction usually contains the lowest proportion of valuable fibers.

The preferred embodiment of the invention described and shown in the drawing is merely an illustrative example of the practice of the invention in practice. Thus, the invention is not limited to said preferred embodiment but a plurality of variations and modifications to those of ordinary skill in the art are possible within the scope of the appended claims. For example, the backwater tanks 37, 45 and 47 may be co-assembled into a single large container in which partitions keep the different backwater fractions apart. In this case, it may be convenient that the means 67 shown as a vented conduit for supplementing the first fraction with a dilute water shot from the second fraction as needed instead consist of a hatch, overflow or the like arranged in one of the container's longitudinal walls.

Furthermore, in the manufacture of three-layer paper, it is common for the two outer layers to be formed from one and the same stock. In this case, the thick mass separator 41 and 55 can be combined into a single vessel, from which the mass goes to a single mixing pump common to the two outer channels of the inlet box 21. The first backwater fraction was used in conjunction with a required portion of the third to provide the dilution of the thick mass to the desired stock concentration for the outer channels. The remaining portion of the third backwater fraction is conveniently returned as long backwater to the thick mass preparation.

If a two-ply paper is to be made in the illustrated former, the pellets 41 and 49 or 49 and 55 can be provided with thick pulp from a common source. Alternatively, either the screen 33, the first circuit 29 and the thick massager 41 or also the screen 35, the third circuit 43 and the thick massager 55 can be fired at the same time as the three-layer inlet box 21 with two partitions 63 and 65 is exchanged for a two-layer inlet box with a single such partition.

Although the absolute greatest effect of the invention is obtained when the double-wire former is of a roll type and has a smooth mantle forming web, the invention provides an increased layer purity also in such applications where the forming roll is circumferentially traced or made as a suction roll. One of ordinary skill in the art will readily recognize

8107215-9 that the invention can be applied to all double-wire molders with curved forming zone, e.g. of the kind shown in CA 960 496, to provide an improvement of the layer purity of the paper produced in such a former.