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
According to
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:
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
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
Thus,
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.
Number | Date | Country | Kind |
---|---|---|---|
20040884 | Jun 2004 | FI | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/FI05/50246 | 6/28/2005 | WO | 00 | 8/27/2007 |