EP3808896A1 - Method for producing paper containing waste paper and grass fibres - Google Patents

Abstract

A method is disclosed for producing paper containing waste paper fibers and grass fibers. In a pulper, waste paper material and grass material and process water are introduced. A mass with waste paper fibers provided by the waste paper material introduced into the pulper and with grass fibers provided by the grass material introduced into the pulper is applied to at least one carrier screen with separation of at least part of the process water in order to form a paper web. In any case, part of the process water is fed back into the pulper for reintroduction. The process water is originally obtained from fresh water, which has a pH value in the range from 7.0 to 8.5. Probiotic bacteria, but not biocides, are added to the process water to reduce the formation of process-damaging microorganisms. The pH value of the process water is monitored, and if the pH value is detected to fall below a threshold value, an increased dose of a basic agent is added in order to adjust the pH value of the process water to a target value and to increase the pH value to keep the target value. The threshold value of the pH value is at least 0.2, in particular at least 0.3, greater than the target value.

Description

The invention relates to a method for producing paper containing waste paper fibers and grass fibers. In particular, it relates to a process for the production of paper which is obtained exclusively from waste paper fibers and grass fibers.

It is known to produce paper from waste paper fibers with the addition of grass fibers. For example, a method for producing a multilayer paper formed from waste paper fibers and grass fibers is described in the previously unpublished European patent application 19151777.0 by the present applicant.

The advantage of paper made using grass fibers can be seen in the sustainable use of raw materials. Because grass is an easily available, rapidly renewable raw material that is produced locally and can be used accordingly in papermaking. The use of recovered paper fibers in paper production should also be seen as conserving resources. This opens a cycle in which paper material that has already been used once or even several times can be returned to the economic cycle and used again for the production of paper. The use of fresh cellulose fibers, which are usually obtained from wood from trees felled for this purpose, can thus be dispensed with, and the consumption of such fresh fibers can in any case be significantly reduced. This helps to conserve the forests that are important for air pollution control and CO _{2 regulation.}

In paper production, the starting materials for the production of paper from waste paper and grass fibers, if necessary with the addition of fresh fibers, shredded waste paper and grass particles, are first put into a pulper, into which process water is also introduced. In this way, a suspension of waste paper and grass particles is formed in the process water in the pulper, which is circulated and made pumpable by means of a whip, while the coarse cargo is crushed. The suspension is then extracted from the pulper, possibly via a so-called refiner, in which the particulate fractions of the suspension are once again comminuted and defibrated, and also optionally via sorting to separate out too coarse material, e.g. via so-called sorting baskets, brought to an application unit, which applies the pulpy suspension to a sieve support. Process water flows or drips off through the screen support, which can also be referred to as a carrier screen, which is collected and returned to the pulper. Thus, a paper web forms on the screen support, which typically runs around in an endless belt, which is separated from the screen support and dried in further steps and, if necessary, further treated, e.g. by means of an application of glue or starch to set a Cobb ₆₀ value, which is a measure of the liquid absorption of the paper and its size, for example with regard to the printability of the finished paper, plays an essential role. The paper web is further dried and wound onto a roll at the end of the manufacturing process.

In the production of paper, there is an increased temperature in the area of the paper machine, to which the aforementioned technical units are summarized, in particular due to the heat used for drying the paper web.

• Sie bilden Biofilme aus, die insbesondere in Bereichen der Papiermaschine, die schwer zugänglich sind, zu Problemen wie etwa Leitungsverstopfungen oder dergleichen führen können.
• Sie verursachen bei der Verstoffwechselung der Nährstoffe entstehende Faulgase, die zu einer überaus unangenehmen Geruchsentwicklung im Bereich der Papiermaschine führen können.
• Sie bedingen aufgrund der bei ihrer Umsetzung der Nährstoffe durch Verstoffwechslung entstehenden Stoffwechselprodukte eine Versauerung des Prozesswassers. Dies kann zu einer erheblichen Absenkung des pH-Werts führen, was nicht nur einen negativen Einfluss auf die Qualität des hergestellten Papiers hat, sondern auch zu nicht unerheblichen Problemen in der Behandlung des aus der Papiermaschine austretenden Abwassers führen kann. Ist dieses zu sauer, d.h. weist es einen zu geringen pH-Wert auf, so kann es z.B. von einer kommunalen Kläranlage zurückgewiesen werden. Untere Grenzwerte für pH-Werte des für die Einleitung in mit biologischer Klärung arbeitenden Kläranlagen vorgesehenen Abwassers liegen typischerweise bei pH 4, da ansonsten die Gefahr besteht, dass die für die Abwasseraufbereitung, insbesondere eine Verstoffwechslung der biologischen Fracht, eingesetzten Mikroorganismen absterben oder jedenfalls nicht mehr effektiv die biologische Fracht abbauen können.

Temperatures of 25 ° C and more typically prevail in the area of the paper machine. These temperatures together with those present in the process water due to the addition of waste paper, but in particular also of grass Nutrients, such as proteins released from the grass material, form an excellent environment for the growth of microorganisms, such as the bacteria that metabolize nutrients. Such bacteria or other microorganisms are undesirable in the field of paper machines. There are several reasons for this:

• They form biofilms which can lead to problems such as line blockages or the like, particularly in areas of the paper machine that are difficult to access.
• When the nutrients are metabolized, they cause fermentation gases which can lead to an extremely unpleasant odor in the area of the paper machine.
• Due to the metabolic products that arise when the nutrients are converted, they cause the process water to become acidic. This can lead to a considerable lowering of the pH value, which not only has a negative influence on the quality of the paper produced, but can also lead to not inconsiderable problems in the treatment of the wastewater emerging from the paper machine. If this is too acidic, ie if the pH value is too low, it can be rejected by a municipal sewage treatment plant, for example. Lower limit values for the pH values of the wastewater intended for discharge into sewage treatment plants that work with biological treatment are typically pH 4, as otherwise there is a risk that the microorganisms used for wastewater treatment, in particular the metabolism of the biological load, will die off or at least no longer can effectively break down the biological load.

Accordingly, biocides are added to the process water in paper manufacture in order to prevent the formation and / or reproduction of process-damaging microorganisms in the process water or in the paper machine and thus to avoid the problems described above.

The addition of biocide is viewed critically from an environmental point of view. They can be discharged from the paper machine with the wastewater and thus get into the environment.

Biocides and their use in the production of paper containing grass fibers, which is also known as "grass paper" and which also enjoys a special reputation from an ecological point of view, are to be assessed as sustainable and from an ecological point of view better than paper made from fresh fibers, for example viewed critically, as these biocides in turn result in a poorer assessment of the life cycle assessment of the paper produced in this way.

The aim of the invention is therefore to specify a production method for paper containing waste paper fibers and grass fibers that can dispense with the use of biocides in the process water.

To solve this problem, a method with the features of claim 1 is proposed. Advantageous developments of such a method are given in claims 2 to 11.

According to the invention, in a method for producing waste paper fibers and paper containing grass fibers, waste paper material and grass material as well as process water are first introduced into a pulper in a conventional manner and treated there in a known manner and shredded and treated to break up fibers. Furthermore, a mass with waste paper fibers provided by the waste paper material introduced into the pulper and grass fibers provided by the grass material introduced into the pulper is deposited onto at least one carrier screen, e.g. in the form of a screen support, which can also be a continuous endless belt, with separation of at least part of the process water, applied to form a paper web. Here, between the pulper and the application of the mass to the carrier screen, the further steps known per se can be carried out, such as further processing of the fibers in a refiner, separation of excessively coarse material in screens or sorting baskets or the like. Also in a manner known per se, a part of the process water separated when the mass is applied to the carrier screen is in any case removed returned for a new introduction into the pulper. As is also customary, this process water is originally obtained from fresh water, which fresh water has a pH value in the range from 7.0 to 8.5. According to the invention, in order to reduce the formation of process-damaging microorganisms, biocides are not added to the process water, as is otherwise usual in the prior art, but rather probiotic bacteria. These probiotic bacteria prevent or at least block the formation and / or the multiplication and / or the growth of process-damaging microorganisms, for example by depriving them of nutrients or colonizing the available habitat and thus depriving the process-damaging microorganisms of the possibility of settlement. For example, a product from the PIP Aquatec series from the Belgian supplier Chrisal NV, which is an agent based on probiotic bacteria, can be used for the purposes described here.

According to the method according to the invention, the pH of the process water is also monitored. Because without the use of biocide and using probiotic bacteria in its place, a change, in particular a decrease in the pH value, can still occur, which, as mentioned above, is disadvantageous for the manufacturing process and possibly also for the paper produced. Thus, according to the method according to the invention, if the pH value has been found to fall below a threshold value, an increased dose of a basic agent is added in order to adjust the pH value of the process water to a target value and to keep the pH value at the target value . In this way, a pH value is set for the process water which, on the one hand, allows the probiotic bacteria to be active, but on the other hand does not drop into such a low value range that the problems described above are caused. The threshold value of the pH value is selected to be at least 0.2, in particular at least 0.3, greater than the target value. This is done because it has been shown that an early introduction of the basic agent and therefore timely countermeasures against over-acidification of the process water is necessary to ensure the pH value To be able to set the target value. Contrary to what was initially expected, it has been shown that counteracting by adding the basic agent does not allow this target value to be achieved until the target value for the pH value is in the immediate vicinity. Then, despite the addition of the basic agent, the acid content of the process water surprisingly gets out of control, resulting in an uncontrolled drop in the pH value well below the target value.

The addition of a basic agent in increased dosage means that if a basic agent is added in a "basic amount" anyway to control the pH value, the added amount of the basic agent is increased accordingly. If, however, no basic agent is added in the basic setting, the increased dosage can also indicate the beginning of the addition of the basic agent at all.

By means of the measure according to the invention of introducing the basic agent in good time, however, the target value can be set without the pH value "shooting" below this value.

The combination of the means specified with the invention, that is, once the use of probiotic microorganisms and also the special control of the pH value of the process water described above, now makes it possible, especially in the production of grass paper, in which by the introduction of the Due to the high input of biological nutrients into the process water associated with grass particles, the formation and growth of process-damaging microorganisms poses a particular problem in not using biocides in the manufacture of paper for the process water without losing production continuity. If the pH value drops too much, production on the paper machine does not have to be stopped and the process water diluted or even replaced. Instead, production can continue continuously with the control of the pH value described above. From time to time (or in continuous admixture) probiotic bacteria may have to be replenished and added to the process water. Also a regular one, anyway repeated or even continuous addition of the basic agent may be necessary in order to be able to carry out the continuous operation and the continuous production of paper by the process according to the invention.

It has proven to be an advantageous variant for carrying out the method according to the invention if the target value is selected at a pH of 6.5 to 7.0. For example, it can be pH 6.7. With such a target value, the problems otherwise observed when the pH value is lowered, as described at the beginning, are avoided, and the probiotic bacteria are particularly effective against the process-damaging microorganisms.

In the method according to the invention, the threshold value from which the basic agent is added must be at least 0.2 above the target value for the pH. However, it should also advantageously not be too high above the target value, in order not to receive premature addition of the basic agent. In particular, the threshold value can be at most 0.5 higher than the target value for the pH value of the process water.

As a basic agent to be added, basically any agent that is harmless for paper production can be considered. However, the invention advantageously proposes using milk of lime for this purpose.

In the method according to the invention, fresh water can be fed to the process water to replenish its volume. In the continuous production of grass paper, part of the process water is always drained from the paper machine and ends up in the wastewater. This "loss" of process water can be compensated for by adding fresh water accordingly. The supply of fresh water can also have an influence on the management of the pH value of the process water, since fresh water will typically have a higher pH value than the process water that is already in use. Fresh water that is added to the process water can typically have a pH of 7.0 to 8.5.

The monitoring of the pH value of the process water can in particular take place continuously, in particular with a pH probe. This allows seamless, close-knit monitoring and thus timely and targeted intervention by adding the basic agent when the threshold value for the pH value is reached. Alternatively, the pH value can also be monitored by means of individual measurements made at predetermined intervals.

In the method according to the invention, a solution containing glue and / or starch dissolved in water can be applied to the paper web formed in order to set a Cobb _{60 value for the paper.} This can be done, for example, in a so-called application of glue or starch. Such an application of glue or starch also requires water for the formation of the solution, this water usually being separate from the process water of the other paper machine. The solution is typically applied at an elevated temperature, for example 135.degree. Since glue and starch in particular bring with them a considerable input of nutrients for harmful microorganisms and, in the area of the glue or starch application, correspondingly increased temperatures also favor the growth of microorganisms, there is also a considerable problem in the glue or starch application and the associated machine parts with the formation and growth of such microorganisms and the consequences outlined above. Correspondingly, in conventional paper machines, biocide is also used at this point and added to the solution or to the water used for this. In order to be able to work in grass paper production without biocide and therefore in an ecologically justifiable manner, probiotic bacteria, but not biocides, can also advantageously be added to the solution for at least a reduction in the formation of process-damaging microorganisms, and the pH value of the solution can be monitored become. If the pH value of the solution falls, in particular to a value below 4.5, a warning can advantageously be output so that measures can be taken to set a higher pH value.

While in the method according to the invention in principle also fresh fibers (cellulose fibers that are not obtained from waste paper, but from fresh material are) can be used, in a preferred variant of the process, paper is produced using exclusively recovered paper fibers and grass fibers, and without using fresh cellulose fibers.

In the method according to the invention, the grass fibers used can in particular be fibers which are obtained by appropriate processing from dried, semi-dried or fresh sweet and / or sour grass and / or seaweed and / or algae.

Fig. 1

eine schematische Darstellung des zeitlichen Verlaufs gemessenen pH-Wertes des Prozesswassers (obere Kurve) und der Zugabemenge eines basischen Mittels (untere Kurve) aufgetragen über in der Darstellung synchronisierte Zeitachsen.

The invention described and claimed here is explained in more detail below with reference to the attached figure. It shows:

Fig. 1

a schematic representation of the pH value of the process water measured over time (upper curve) and the amount of a basic agent added (lower curve) plotted over time axes that are synchronized in the representation.

The figure, which is a purely schematic representation, illustrates how the addition of the basic agent (the amount added per unit of time is shown as a curve over time in the lower representation with a dotted line) is carried out in an increased dose when the pH Value (the time course of which is shown in the upper illustration in a dashed line) is still above the target value drawn in as a continuous line in the upper illustration (also referred to as the target value in the figure). On the left in the upper curve, the pH value reaches the threshold value, which is a value of at least 0.2, preferably at least 0.3 above the target value (target value), and the addition of the basic agent with increased dosage is initiated.

This ensures that the pH value initially drops further, but does not overshoot into the range of lower pH values (into the acidic range), but is captured very well and then fluctuates around the target value with a comparatively constant addition or Dosage of the basic agent.

The target value for the pH value of the process water can be, for example, pH 6.5 to pH 7.

Claims (11)

A process for the production of paper containing recovered paper fibers and grass fibers, with recovered paper material and grass material as well as process water being introduced into a pulper and a mass of recovered paper fibers provided by the recovered paper material introduced into the pulper and grass fibers made available by the grass material introduced into the pulper with separation of at least part of the process water is applied to at least one carrier wire in order to form a paper web, in any case part of the process water is returned for renewed introduction into the pulper, the process water originally being obtained from fresh water which has a pH value in the range from 7.0 to 8.5, with probiotic bacteria, but not biocides, being added to the process water for any reduction in the formation of process-damaging microorganisms, and with the pH value of the process water being monitored and if the pH-W If a basic agent is added in an increased dosage below a threshold value in order to adjust the pH value of the process water to a target value and to keep the pH value at the target value, the threshold value of the pH value at least 0.2, in particular, at least 0.3 is greater than the target value. Method according to Claim 1, characterized in that the target value is selected at a pH value of 6.5 to 7.0. Method according to one of the preceding claims, characterized in that the threshold value of the pH value is at most 0.5 greater than the target value. Method according to one of the preceding claims, characterized in that milk of lime is added as the basic agent. Method according to one of the preceding claims, characterized in that fresh water is fed to the process water to fill its volume. Process according to Claim 5, characterized in that the fresh water supplied has a pH of 7.0 to 8.5. Method according to one of the preceding claims, characterized in that the monitoring of the pH value of the process water takes place continuously, in particular with a pH probe. Method according to one of Claims 1 to 6, characterized in that the pH value is monitored by means of individual measurements carried out at predetermined intervals. Method according to one of the preceding claims, characterized in that a solution containing glue and / or starch dissolved in water is applied to the formed paper web to set a Cobb ₆₀ value of the paper, the solution in any case reducing the formation of process-damaging microorganisms probiotic bacteria, but not biocides, are added and the pH of the solution is monitored. Method according to Claim 9, characterized in that a warning is issued if the pH value of the solution falls to a value below 4.5. Method according to one of the preceding claims, characterized in that the paper is produced using exclusively recovered paper fibers and grass fibers, in particular without using fresh cellulose fibers.

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