Method for an Arrangement at a Paper Machine

Abstract

The present invention relates to a method for reducing the spreading of additional agent(s) which is/are introduced into the process at a papermaking machine. A flowable paper pulp including fibres and a process liquid and having a consistency of not more than 10% is conducted towards the paper machine's headbox (5). Prior to the introduction (14) of said additional agent the consistency of the pulp is increased to a value higher than 10% by removing (8) process liquid and conducting (11) it in the upstream direction, while compensating process liquid is introduced (13) downstream. The present invention also relates to an arrangement at a paper machine, where separating means (8) are arranged in the process direction before means (14) for introducing additional agent (s). The arrangement further includes means (11) for conveying process liquid separated from the process in an upstream direction in the process, as well as means (13) for a downstream introduction of process liquid from a short circulation in order to compensate for the separated process liquid.

Description

The present invention relates to a method at a paper machine for reducing the spreading of agents added to the process in a continuous process for preparation of pulp for a paper machine, whereby a flowable paper pulp including fibres and process liquid and having a consistency of not more than 10% is conducted to the headbox of the paper machine.

The present invention also relates to an arrangement at the pulp preparation for a paper machine in order to restrict a spreading of additional agents in the further process, whereby the process comprises an inlet for a flowable pulp having a consistency of not more than 10% and including a fibre material and a process liquid, a closed cycle system including a headbox for the paper machine, as well as means for adding said additional agent.

In the present patent application a paper machine is defined as a machine for manufacturing paper as a continuous web. The expression pulp preparation as used in the present patent application indicates a process where starting from a pulp mass comprising fibres, usually a cellulose material, and having a more or less firm consistency, a paper pulp mass is prepared under mixing to be transferred further to the head-box, whereby the pulp mass includes not more than 10% fibre material, normally at least 1%, suitably 2 to 7% and usually 3 to 5 %, as well as added process liquid, usually water. Typically, such a pulp preparation includes a so called pulper where a fibre material from a cellulose process is resolved during decomposition and agitation, and mixed with process water.

Traditional papermaking includes a process where a pulp mass which essentially comprises a fibre material and process liquid is conducted via a headbox for de-watering on a wire. A multitude of further steps and arrangements are associated to the process, including vessels where the consistency typically is 3 to 5%, whereby much surplus liquid is conducted in an upstream direction back into the process.

In production of special quality paper such as decor paper, colored paper or other paper grades where valuable and perhaps process sensitive additional agent(s) such as titanium dioxide (TiO₂), pigment or the like often very expensive components are added to the process, the problem occurs that the retention of these agents in the process is low, so that the so called white water typically can contain up to 1% or more of such agents. In order to regain such agents a traditional process includes a recovering by means of, e.g., flotation in recovering devices, after which the clear water from the flotation is recycled in the pulp preparation.

At the manufacture of such special quality paper it is usual that said additional agents are added to the process already during the pulp preparation, in order to obtain a sufficient mixing and contact time with the actual papermaking pulp. In practice this means that the whole process, in practice, will be dyed throughout. At a change of grade the process thus must initially achieve a balance, which results in a long building-up time, while even a change of color requires an extensive cleaning of the system and results in high losses of material. Even as such a colored paper, including pigment colored white paper, is difficult to produce, and especially for decor paper the setting of the right color is difficult. Additionally there is the fact that a web break as such under these conditions lead to instability.

Since a decor paper, for example, will be impregnated at the further refinement its optical properties will also change. The refractive index for the fibres and the impregnating agent are about the same, and thus such factors as the concentration, distribution and optical properties of the pigment are critical for the quality of the product. For this reason there is usually a need for making a test laminate prior to the start of the actual production, and this takes about half an hour, during which time the pulp is run in a circulation on the machine.

As such the process can partially be made more effective by means of such arrangements disclosed, i.a., in the applicant's international patent applications No PCT/FI93/00214, PCT/FI94/00578, PCT/FI96/00052 and PCT/FI01/00365, but some problems still remain, although said arrangements make it possible to chose a suitable white water for each specific use. However, in a typical traditional process about 20 to 30 kilograms of the additive titanium dioxide (TiO₂) for each 100 kilograms of paper will pass the fibre recovery (see also Table I below). In a traditional fibre recovery plant using a flotation technique the chemicals will cause an agglomeration which reduces the optical effect of the pigments. Thus the titanium dioxide will lose in optical effect, and yet a part will be lost in the form of, for example, deposition in the system. Additionally, there are, of course, the drawbacks caused by separate arrangements for fibre recovery, costs of investments and operating and costs depending on a contamination and cleaning of the systems, as well as the more complicated process chemistry which a recovery in line with prior art requires.

Thus, the object of the present invention is to avoid the drawbacks which until now have been connected to the use of pigments and especially in connection with a change of the paper grade.

The present invention is based on the astonishing realization that by raising the consistency of the pulp at an early stage of the process it is, in practice, possible to cut the arrangement which in traditional papermaking is called the long circulation, i.e. a circulation system where white water in accordance with traditional arrangements can run from the wire pit of the paper machine upstream in the system up to the point where pulp is added, for example to a pulper. By interrupting this circulation a flow of valuable additional agents directed upstream can be prevented, for example a flow of titanium dioxide and other pigments which otherwise partially would be lost and partially contaminate the whole chain of process.

The characteristics of the present invention are disclosed in the respective characterizing portion of the appended independent claims, while the dependent claims disclose favorable embodiments of the present invention. Thus, the method according to the present invention is characterized in that the consistency of the pulp is increased, prior to the introduction of said additional agent and suitably at a separate process stage, to a value of more than 10% by removing process liquid, in that this process liquid is conveyed upstream in the process, while compensating process liquid, which suitably can contain additional agents, is added downstream in the direction of the process.

Correspondingly, a process according to the present invention is characterized in that separating means are arranged prior to the means for introducing the additional agent, seen in the direction of the process, but after said inlet, whereby these separating means are arranged to separate process liquid from the flowable pulp so that the consistency of the pulp will increase to a value of more than 10%, in some cases favorably even up to 50%. The arrangement further includes means for conveying the liquid which has been separated from the process liquid in an upstream direction in the process, as well as means for a downstream introduction of process liquid from said circulation in order to compensate the separated process liquid. This latter process liquid can, as such, contain more or less additional agent(s)

Below the present invention is presented in more detail by reference to some favorable embodiments and the appended drawings, where

FIG. 1 as a principle drawing discloses an arrangement according to one embodiment of the present invention, and

FIG. 2 generally discloses a paper machine arrangement in accordance with an especially favorable embodiment.

According to FIG. 1 a paper machine typically includes an inlet for a fiber containing material, i.e. cellulose which usually is provided in the form of dry sheets or, at so called integrated plants, directly from a cellulose plant in the form of a fibre containing process liquid. If the incoming material is in the form of sheets the process starts with a pulp preparation including a so called pulper 2 where the sheets are disintegrated and mixed with process water, for example from a tank 3, to constitute a so called stock or papermaking pulp. The pulp passes through different process stages 4a, 4b, 4c, 4d, 4e . . . which in this context are essentially irrelevant, and finally it is spread via the paper machine's headbox 5 onto a wire 6. The wire runs through a machinery 7 where the process liquid to an extensive portion is exhausted by suction in the form of so called white water, while the moist remaining pulp gradually by means of pressing 9 and drying is formed to paper.

Besides water the white water also to some degree contains chemicals that will do be retained in the paper. These chemicals follow with the process water which to some parts is used in the so called short circulation for dilution purposes at such process stages where said chemicals are needed as such, or where they in any case cause only minor harm. White water used for other purposes is cleaned in different processes and cleaning stages, on one hand to recover the chemicals and, on the other hand, to obtain a clean water which can be used in other parts of the process. Each such cleaning process, is, however, to some extent incomplete and additionally requires both energy, time and apparatus.

Especially for special paper such as decor paper and other colored paper the manufacturing volumes are relatively small. This makes the production of such grades fairly non-profitable in integrated paper mills, and for this reason special grade paper is usually manufactured in paper mills which receive the fiber material essentially in dry form, which leads to a need for process liquid already in the beginning of the process. In known processes this process liquid is, as a rule, taken in the form of said long circulation from the cleaned white water. However, in practice also this cleaned process liquid contains a higher or lower portion of impurities, i.e. mainly material which belongs to some other part of the process. For colored paper a part of these impurities are constituted by, i.a., pigment such as titanium dioxide, which in order to obtain a good mixing is usually added already in the pulper 1, whereby the system in practice will be fully dyed throughout. At a change to, e.g., a different color, this fact in combination with the tendency of these components, which are expensive as such, to deposit as sediments in the different parts of the system leads, for example for colored paper, in practice to a situation, where the whole system must be cleaned before the following quality can be manufactured.

The system according to the present invention differs essentially from the known technology described above. By momentarily raising the consistency of the paper pulp by introducing separator means at least at one point of the process so that an additional separation 8, 8a of the process liquids is achieved, which interrupts the long flows back up in the system of such process liquid which is contaminated with valuable additional agents. Earlier a separation has only been adopted in order to prevent chemicals that disturb lower processes from flowing downwards, while the idea is quite new that a prevention of valuable additive agents from contaminating upstream also may be realized in a corresponding manner.

The principle behind the present invention is presented in Table I, where the material balance within different parts of the system is illustrated by means of the pulp consistency (in .), the volume (“X”=liters of liquid pro kilogram of pulp mass) and dry substance (in kilograms) for, on one hand, a typical process according to prior art (T) and, on the other hand, a process according to one embodiment of the present invention (U), where A indicates the values for the pulp mass during the initial stages 1 . . . 4a of the process in accordance with prior art, A₁ indicates the values for the pulp mass after the separation 8 in accordance with the present invention, B indicates the situation at the headbox 5, C indicates the situation when the paper has left presses 9, D indicates “surplus water”, i.e., water that has to be cleaned before it can be re-used, and E indicates the situation at the short circulation:

	TABLE I
					dry
	Consistency		Volume		substance
	%		X		kg
Point	T	U	T	U	T	U
A	4.0		25.0		100
A1		30		3.3		100
B	2.0	2.0	100.0	100	100	100
C	45	45	2.2	2.2	100	100
D	1.0	0.5	22.8	1.1	22.8	0.6
E		2		100		200

From the above Table I it is evident that about 20 to 30 kilograms of TiO₂ pro 100 kilograms of paper passes, i.a., a fibre recovery 10, while the volume of TiO₂-containing surplus water and its share of dry substance will be radically reduced when the consistency of the pulp mass is momentarily increased in accordance with the present invention.

In practice, the removing separation of process liquid is favorably effected by means of such a thickening technique which is known per se from, for example, production of cellulose, bleaching and de-inking. Such arrangements are usually capable of increasing the consistency of the fibre containing pulp mass from an initial value of 2 to 10%, in the present case typically in the order of about 4 to 6%, to a final value of even up to 50% and possibly even higher.

According to a favorable embodiment the separation is performed by means of a pressing which favorably takes place prior to a beating 4c. The pressing at the thickening favorably then takes place in a screw or band press 8a of a type known per se, so that the slushed pulp after pressing obtains a dry matter content of more than 10%, especially within an interval of 20 to 50%, suitably 25 to 40%, favorably at least 30%.

The process liquid which is separated during the pressing is conveyed 11 back in the upstream direction into the process, suitably back to the pulper 2. The liquid which has been separated from the process is suitably replaced 13 in a dispersing device 12 with an essentially corresponding amount of liquid taken from the circulation of the paper machine. Favorably this takes place in an arrangement according to the applicants international patent application No PCT/FI99/00143, or alternatively by means of a screw arrangement or a mixer of some other type. In order to obtain an optimal intermixing the dispersing device 12 is favorably arranged in the process direction essentially directly behind the separation 8, 8a.

According to a favorable embodiment of the present invention the main introduction 14 of critical added agents such as very expensive pigments and coloring agents takes place in connection with the addition of liquid to the dispersing device 12 (see FIG. 2), which provides an effective intermixing of the additional agents in as early a stage as possible after the concentration. FIG. 2 shows that additional agents also can be introduced 14a, 14b at later stages in the process. Since the introduction of at least a part of the additional agent(s) favorably takes place at the same time as the introduction 13 of replacing liquid it is of little consequence if the replacing liquid is taken from such a part of the process where the residual content of additional agent(s) could be even high.

In practice, the above discussed arrangement where the consistency of the pulp mass is increased will lead to a situation where a removal and returning of process liquid that functionally belongs to an earlier process stage in a way “makes room” for an introduction of such additional agent material one wants to prevent from spreading in the upstream direction. Thereby the latter additional agent material, and thus, e.g., the colored portion of the process, can be restricted to the short circulation of the paper machine, while the long circulation, to the extent that such a circuit possibly might be needed, is kept free from valuable and/or contaminating material.

Since at the especially favorable embodiment according to FIG. 2 the system is essentially self-cleaning, there is, in practice, a good supply of white water of different fractions, as well as a supply of relatively clean surplus liquid to the extent such liquid is needed in other portions of the system. Thus the correct clear fraction can be chosen for such needs, which further cuts down the losses of valuable additional agent material. The minor amount of surplus water still remaining can be treated in a fine filter to provide spray water etc, whereby the filter cake is returned to the dispersing device.

Thus, FIG. 2 discloses an especially favorable arrangement where the amount of water taken out from the system is diverted at a point where the concentration of material is as low as possible. According to this embodiment this concentration can be reduced by arranging a suction box 15 under the wire at such a point where a fibre web already has been formed, so that the fibre web functions as a filter. This suction box 15 is worked by a light vacuum whereby a careful filtration and a very clean white water is achieved. This filtration reduces the amount of material so that this white water normally can be conducted 16 to an overflow 17 without any need for any additional recovering.

By means of the arrangement according to the present invention a nearly 100% retention of the valuable additional agents can be achieved, while at the same time, e.g., the requirement for separate recovering devices is eliminated, the no-load operation during the test laminating procedure is reduced and the building-up time after a change of color is shortened.

Above some favorable embodiments of the present invention have been disclosed, but for the professional it is clear that the present invention also can be effected in many other ways within the scope of the appended claims.

Claims

1. A method for reducing the spreading of additive agent(s) introduced into a a continuous process in pulp preparation for a paper machine, wherein a flowable paper pulp including fibres and process liquid and having a consistency of not more than 10% is conveyed towards a headbox of a paper machine, additive agent the consistency of the pulp is increased to a value of more than 10% by removing process liquid and conveying this liquid upstream, while compensating process liquid is added downstream.

2. A method as defined in claim 1, wherein the consistency of the pulp is increased so that the pulp corresponds to a dry matter content of 20 to 50%, suitably about 25 to 40%, favorably at least 30%, by separating process liquid from the pulp, suitably by means of pressing, which process liquid is conveyed back upstream into the process.

3. A method as defined in claim 1 wherein, prior to conveying the pulp forwards towards the headbox, a compensating liquid is added in the form of additive agent(s) containing process liquid from the short circulation of the paper machine.

4. A method as defined in claim 1, characterized in that the additive agent(s) is introduced in connection with a dispersing (12) of the pulp which has a increased consistency.

5 A method as defined in claim 1, characterized in that said additive agent(s) include(s) pigment or other color agents.

6. An arrangement at a pulp preparation for a paper machine, for restricting a spreading of additive agent(s) upstream in the further process, where the process includes an inlet for a flowable mass having a consistency of not more than 10% and including a fibre material as well as process liquid, a closed circulation system including a headbox for a wire for the paper machine as well as means for introducing said additive agent(s), wherein separating means are arranged in the direction of the process ahead of the means for introducing the additive agent(s) but behind said inlet, which separating means are arranged to separate process liquid from the flowable mass so that the consistency of the mass increases to more than 10%, means for conveying the process liquid which has been separated from the mass upstream into the process, as well as means for introducing process liquid from said closed system downstream in order to compensate the separated process liquid.

7. An arrangement as defined in claim 6, wherein said separating means are capable of increasing, suitably by means of a pressing, the consistency of the fibre containing mass from an initial value of 2 to 10%, typically in the order of about 4 to 6%, to a final value of 20 to 50%, suitably 25 to 40%, favorably at least 30%.

8. An arrangement as defined in claim 6, characterized in that means are arranged to add, in connection with or after said inlet for additive agent(s), compensating liquid from the short circulation system of the paper machine.

9. An arrangement as defined in claim 6, wherein dispersing means are arranged to re-disperse, suitably under addition of said compensating process liquid, the mass from which excess liquidhas been separated, whereby the means for introducing said additive agent(s) favorably are arranged in context with said dispersing means, suitably so that said dispersing means are arranged, in the direction of the process, essentially immediately behind said separating means.

10. An arrangement as defined in claim 6, wherein a separate suction box with a slight vacuum is arranged at such a portion of the wire where a fiber web already has been formed, and that means are arranged for conveying water from such a separate suction box a high up in the system towards a separate overflow.

11. A method as defined in claim 3, wherein said compensating process liquid is introduced in connection with a dispersing of the pulp which has a increased consistency.