METHOD AND APPARATUS FOR MAKING PAPER

FIELD OF THE INVENTION

The invention relates to a method defined in the preamble of claim 1 for removing water during pa- permaking, wherein paper is formed from a fibrous source material by feeding pulp to a wire section through a headbox, a web is formed at the wire section, and the web is led from the wire section to a drying section through a press section, and to an apparatus defined in the preamble of claim 6 for making paper.

BACKGROUND OF THE INVENTION Known from prior art are various methods and apparatuses for making different kinds of papers.

It is known that the web formation section of a paper machine comprises a headbox section with a feed piping of the headbox and the headbox, and a wire section. The purpose of the wire section is to form a web from fibre pulp arriving from the headbox, and to remove water from the fibre pulp by filtrating it through a wire cloth, bringing the web to a sufficiently high dry content. A filtrate flow is produced by the pressure difference acting over a filtrated layer of fibres and the wire. When the ^ web has a sufficiently high dry content, transferring the web from the wire section to a press in easy and good runnabil- ity is achieved at the press section. From the headbox to the wire, the fibre pulp has a thickness of about 0.2 - 1.2%, and the dry content of the web as it leaves the wire is about 15 - 20%. After the press section, the dry content of the paper web rises to a level of 40 - 55%. However, only a small portion of the initial water content is removed at the press sec-

tion compared to the wire section. It is important to achieve as high dry content as possible in the press section, so that runnability at a drying section would be good and not too much energy would be consumed. At the drying section, moisture of the paper web is reduced by means of cylinders heated with steam. Depending on the further processing of the paper, the purpose of the drying section is to achieve a predetermined moisture level for the paper. It is known to use e.g. drying cylinders and/or suction cylinders at the drying section.

After the headbox, papermaking is principally dewatering and providing the paper with a desired moisture content e.g. prior to printing or other fur- ther processing.

The increase of speed in paper machines has contributed to a situation where the desired dry content of the paper web has not been achieved e.g. in the press section, due to a shorter dwell time between the pressing nips. Conseguently, the reduced dry content level easily produces problems, e.g. relating to runnability and quality, at the drying section, and later during coating or printing. Due to higher speeds in paper machines, the runnability of the drying sec- tion is a limiting factor in increasing the production. This is why the objective is to achieve as high dry content level as possible already before the drying section.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new, cost-effective method and apparatus for making paper and enhancing the removal of water from a paper web. One specific objective of the invention is to provide a method and apparatus for making paper and

removing water from a web substantially without interfering with the surface of the web.

SUMMARY OF THE INVENTION

The method and apparatus according to the invention are characterized by what has been presented in the claims. The invention is based on a method for removing water during papermaking, wherein paper is formed by feeding pulp to a wire section through a headbox, a web is formed at the wire section and the web is led from the wire section to a drying section through a press section. According to the invention, dewatering the web is enhanced and moisture content of the paper is reduced by removing water from the web that is being formed by at least one additional step selected from the following group: in a gap formed by the wires, an enhanced filtration over the wires is directed to the wires by raising the tension level of the wires clearly above the normal tension level; at least one suction cylinder is used to remove water from the web after the web formation, preferably for pre-pressing purposes; at least one static shoe press unit is used in conjunction with the wire section; at least one static shoe press unit is used between the wire section and the press section, e.g. after the gap or after the suction cylinder; at least one static shoe press unit is used in conjunction with the press section; at least one static shoe press unit is used between the press section and the drying section; at least one static shoe press unit is used in conjunction with the drying section; and/or at least one dewatering unit is arranged in conjunction with the drying section in order to remove water.

Furthermore, the invention is based on an apparatus for making paper, the apparatus comprising a headbox section and a wire section, a press section and a drying section. According to the invention, the apparatus comprises at least one additional dewatering means for enhancing the removal of water from the web that is being formed and for reducing the moisture content of the paper, the dewatering means being selected from the following group: in the wire section of the apparatus, the wires form a gap, and the apparatus comprises means for adjusting the tension level of the wires, preferably by raising the tension level clearly above the normal level, and for directing an enhanced filtration over the wires; at least one suction cylin- der for removing water from the web after the web formation; at least one static shoe press unit arranged in conjunction with the wire section; at least one static shoe press unit arranged between the wire section and the press section; at least one static shoe press unit arranged in conjunction with the press section; at least one static shoe press unit arranged between the press section and the drying section; at least one static shoe press unit arranged in conjunction with the drying section; and/or at least one dewatering unit in conjunction with the drying section.

The invention is specifically based on a method and apparatus for enhancing the drying of the web and for optimizing the moisture content of the paper that is being formed, as well as for facilitating the production of the paper and for reducing the production costs, yet providing good quality to the paper. The objective of the invention is to raise the dry content of the paper web preferably already prior to the drying section. The method and the apparatus can be arranged to be extremely flexible relative to different paper grades. Preferably, dewatering is per-

formed substantially with as little interference as possible with the surface of the web, substantially e.g. without breaking, modifying or heavy calendering of the surface. In this context, paper refers to any fibre- based paper, board or fibre product or the like. The paper may be made from chemical pulp, mechanical pulp, chemimechanical pulp, recycled pulp and/or the like.

The paper may be in the form of a wet web, dried web or sheet, or in other form which is suitable for the purpose. The paper may comprise suitable fillers and additives .

In conjunction with the web formation, water is preferably removed in the wire section by the vari- ous dewatering elements of the wire section known per se .

In one embodiment, the wire section comprises a gap former. Rising speeds in paper machines and tightened quality criteria have increased the use of the gap formers as wire parts. Different kinds of gap formers are known, e.g. MB-, blade and cylinder gap formers and combinations thereof.

In the apparatus according to this invention, any devices and means known per se in which the web passes between nips, cylinders, rolls and/or the like which perform the pressing can be used as the pressing devices and means.

The press section may comprise one or more presses known per se, e.g. a suction cylinder press, a smooth press, a shoe press and/or a similar press. The structure of the press section may be completely known per se.

In one embodiment, pre-pressing is applied for reducing water in the paper web arriving from the wire section. The pre-pressing may be performed in

conjunction with the wire section and/or between the wire and the press section.

The drying section may have a structure that is completely known per se. The drying section may comprise e.g. a condebelt dryer, a Yankee dryer, a Yankee cylinder, an impingement dryer and/or the like. The drying section may include a number of drying devices or drying units.

The headbox used in the apparatus according to the invention may be any headbox known per se, e.g. a hydraulic headbox, a high consistency pulp headbox, a multiply headbox or the like.

In one embodiment of the invention, a modified wire section is used, i.e. an enhanced filtration is directed over the wires in the gap formed by the wires, preferably in the dewatering gap, substantially during the web formation, by raising the tension level of the wires. Preferably, the modification of the gap, the narrowing/broadening of the gap, allows one to control the wire tension. By using this dewatering step, the web is very little disturbed during the web formation.

In one embodiment, adjusting the tension level of the wires and providing an enhanced filtration are applied at a consistency of the headbox that is higher than normally, e.g. at a headbox consistency of 3 - 4%. In an alternative embodiment, adjusting the tension level and providing an enhanced filtration are applied at a normal headbox consistency. In one embodiment of the invention, the web is arranged to pass over at least one large-radius suction cylinder designed for removing water after the web formation in order to control the dewatering. This is done by utilizing the centrifugal force by forming the web over at least one large-radius suction cylinder, e.g. in conjunction with the wire section, between the wire

section and the press section, between the press section and the drying section or in conjunction with the press section or the drying section. With the large- radius suction cylinder, the dewatering is performed by the centrifugal force with as little interference with the web as possible. The use a large-radius suction cylinder does not limit the speed of the paper machine. In one embodiment, a disk cylinder is used as the suction cylinder. '• In one embodiment, there may be several serial suction cylinders, e.g. large-radius suction cylinders, in order to remove water in both directions on both sides of the web.

In one embodiment of the invention, a static shoe-type dewatering means is used in the dewatering in the wire, press and/or drying section and/or therebetween. Any shoe press device known per se may be used as the static shoe press unit. In one preferred embodiment, a long-nip press device, e.g. the long-nip press device according to patent publication FI 94368 (inventor Teuvo Rajamaki) is used as the static shoe press. The shoe press unit may comprise a static shoe press means on both sides or only on one side of the press nip, in which case an opposing cylinder, a roll, a smooth surface or the like is used on the other side of the nip that is being formed. The static shoe press unit may comprise one, two or more belt circulations per side.

In one embodiment of the invention, the dewa- tering unit comprises a belt circulating member with a belt, e.g. a metal, steel or textile fabric belt, and at least two rolls or cylinders over which the belt is arranged to pass and run, thus forming a belt- circulated dewatering member. The belt circulating mem- ber may be structurally of any shape, depending on the number of rolls or cylinders and/or their positioning.

In one embodiment, the paper web may be sucked against the belt e.g. by impingement.

In one embodiment of the invention, the dewa- tering unit comprises an opposing cylinder for forming a nip together with the belt circulating member. In one embodiment, the opposing cylinder is selected from the group of: a roll, a cylinder, a slide shoe, a static shoe, another belt circulating member according to the invention or the like. In one embodiment, the belt cir- culating member may operate as a calender nip with the opposing cylinder.

In one embodiment, the belt circulating member comprises at least three or more rolls or cylinders over which the belt is arranged to pass. In one embodiment, the belt-circulated dewa- tering unit according to the invention may operate as a drying cylinder with optionally a paper web and a drying fabric on top of it.

In one embodiment, heating means may be ar- ranged in conjunction with the belt circulating cylinder above and/or below the belt to heat one or both sides of the belt and the web. Warming or heating of the web/belt, and thereby the removal of moisture, are provided e.g. by steam, condensate, recovered heat of the drying section, hot air, high pressure air, by radiation or in a similar manner. In heating the web/belt, the steam and/or condensate system of the drying section, steaming boxes, impingement units, heat pump technology or the like may be utilized. In one embodiment, the static shoe press unit arranged in the drying section may be arranged to be part of the dewatering unit, e.g. as an opposing cylinder, to form a nip with the belt circulating cylinder of the dewatering unit. In one embodiment, the drying section may comprise one or more dewatering units. Multiple dewatering

units may be arranged structurally in different manners, e.g. successively, two laid one above the other, and/or in a combination of these.

In one embodiment of the invention, the web is pre-filtered by thickening the mid part, i.e. the core part, of the web and by thinning the surface layers. In one preferred embodiment, the pre-filtration is performed in a slice chamber. The slice chamber is part of the headbox through which the stock flows to the wire. In one embodiment of the invention, the pa- permaking comprises a web calendering step in a calendering section. The calendering section may comprise devices that are known per se, e.g. a long nip calender, a belt calender or the like. According to the in- vention, the calendering section may comprise at least one static shoe press unit for removing water.

The apparatus according to the invention may comprise one or more static shoe press units placed separately or serially in different sections of the apparatus and/or therebetween.

In addition to the dewatering members according to the invention, the web may be dried by the de- watering and drying methods and means known per se in conjunction with the wire section, press section, dry- ing section and/or calendering section and therebetween. Accordingly, the pres section, drying section, and/or calendering section may only comprise solutions according to the invention.

In one embodiment, the formed paper web may be surface-treated by surface-sizing and/or coating in conjunction with the apparatus according to the invention, e.g. between the sections and/or in conjunction with the sections of the apparatus and/or after the apparatus . In one preferred embodiment, the web is led to the press section in a closed system in such manner

that there is a closed transfer from the wire section to the press section. In one embodiment, the web is led to the drying section from the press section by using a closed system. In one embodiment, additives, such as pigments, bonding agents and chemicals, are added to the fibrous source material. Any agents and chemicals known in the art may be used as pigments, bonding agents and chemicals. The method and apparatus according to the invention provides considerable advantages compared to the prior art .

Thanks to the invention, it is possible to remove water and moisture more efficiently from the web and to achieve the desired dry content of the paper more easily. Furthermore, the bulk of the paper may be saved according to the invention.

The invention provides a simple and cost- effective manner of making quality paper. The method and apparatus according to the invention are applicable for making different kinds of papers .

DETAILED DESCRIPTION OF THE INVENTION In the following section, the invention will de described with the aid of a detailed example of an embodiment with reference to the accompanying Fig. 1. Fig. 1 represents an apparatus according to the invention for making paper. The apparatus for making paper presented in

Fig. 1 comprises a hydraulic headbox section and a modified wire section 2 as well as modified press 3, drying 4 and calendering sections 15. Furthermore, the apparatus comprises a number of dewatering means for en- hancing the removal of water from the web that is being

formed and for reducing the moisture content of the paper.

In the wire section 2 of the apparatus, formed from a gap former known per se, the wires form a gap and the apparatus comprises means (not shown in the figure) for adjusting the tension level of the wires to be clearly higher than what is known, and for directing an enhanced filtration over the wires. Furthermore, the apparatus comprises large-radius suction cylinders 6 for removing water from the web after the web formation and prior to the press section for pre-pressing purposes. At the wire section 2, the large-radius suction cylinders are arranged serially in such manner that they can be used for removing water in both directions both above and below the web.

The press section 3 comprises two static shoe press units 7. The static shoe press units comprise two belt circulations per side; an outer felt circulation against which the paper web is arranged, and an inner belt circulation comprising a grooved belt around the cylinders. The belt may be grooved or perforated or alternatively smooth at the outer surface, i.e. at the surface against the felt. The purpose of the grooves or perforations is to transfer the water removed by the felt from the paper further away from the felt.

The drying section 4 comprises four dewatering units 8. The dewatering units comprise a belt circulating member 10 with a metal belt 11 and five rolls 12 over which the belt is arranged to pass and run, thus forming the belt-circulated dewatering member 10. In the dewatering unit 8, another belt circulating member 10 operates as an opposing cylinder 13 in order to form a nip. Heating of the metal belt and the web is performed using impingement units 16 and steaming boxes 17 in such manner that in the first and third belt circulating member, the impingement units 16 heat the upper

side and the steaming boxes 17 heat the lower side of the web, and in the second and fourth belt circulating member, the impingement units 16 heat the lower side and the steaming boxes 17 heat the upper side of the web.

Furthermore, the calendering section 15 comprises one static shoe press unit 9 for removing moisture from the paper.

In this example, paper having a grammage of about 80gsm is formed from fibre pulp known per se by the method and apparatus according to the invention. Pigment and typically used bonding agents and chemicals are added to the fibrous source material.

The papermaking is performed in a manner known per se and is not described in more detail in this context. The dewatering is substantially performed with as little interference as possible with the surface of the web that is being formed.

It was observed that with the method and ap- paratus according to the invention, it is possible efficiently to remove water from the paper to be formed and to optimize the moisture content of the paper. Further, it was observed that it is possible cost- effectively to make paper and a paper product of the desired type with good properties.

The method and apparatus according to the invention are applicable in different embodiments for making the most different kinds of papers. The invention is not limited merely to the example referred to above; instead many variations are possible within the scope of the inventive idea defined by the claims.