EP1887130B1 - Process for removing tensides during the manufacture of paper in a papermaking machine and papermaking machine for carrying out the process - Google Patents

Abstract

The process for fabricating a paper (2) on a machine, comprises separating surface-active agents through dispersed air (9) flotation in a wet part, and eliminating flotation foam containing the surface-active agents. The separation of the agents is carried out on all or part of filtration water from the paper formation and on all or part of clear water from a fiber recuperator. Recycled or mechanical slurry is used in the process. The flotation foam is treated with ozone and then evacuated in a purification station and a drainage table. An independent claim is included for a machine with paper.

Description

TECHNICAL AREA

The present invention relates to an improved method of making paper on machine, as well as the paper machine adapted to the implementation of such a method.

More precisely, the invention uses the technique of dispersed air flotation, making it possible to rid the water used in the manufacture of paper of surfactants.

This results in a better operation of the paper machine and a higher quality product paper.

PRIOR STATE OF THE TECHNIQUE

It is known that the presence of surfactants has many disadvantages in the manufacture of paper.

First, these substances tend to stabilize any air bubble introduced into the head circuits, especially when the dough is highly sheared as in a binder, a pump, .... This therefore requires the availability of deaerators usually placed just before the headbox.

Moreover, since surfactants are also responsible for the foam observed in the head circuits, they require the use of antifoam agents, added in almost all circuits of existing machines.

In addition to these two visible effects (air in the dough and foam), the surfactants remain within the suspension or process water and accumulate in the top circuits.

As a result, surfactants can considerably reduce or even destroy the effectiveness of several chemicals introduced at the machine head, such as bonding agents, retention and dewatering agents ... Thus, the publication " Influence of active surface substances on AKD sizing ", Zeno et al., Nordic Pulp and Paper Research Journal, Vol 20 no 2/2005, pp 252-57 reports the adverse effects of surfactants on bonding. Because of this, their presence can also have very important consequences, not only on paper properties but also on productivity.

There is therefore an obvious need to selectively and effectively remove surfactants present in the paper-making waters.

The document DE-A-199 14 779 discloses a method of making paper on a machine comprising, in the wet part, a white water flotation step, as well as a paper machine comprising, in its wet part, a white water flotation cell.

OBJECT OF THE INVENTION

In the first place, the invention therefore relates to a method for producing paper on a machine comprising in the wet part at least one step of separating the surfactant substances present in the white water by dispersed air flotation.

The term "surfactant substances" means chemical compounds which, introduced into a liquid, lower the surface tension, which has the effect of increasing the wetting properties.

It can for example be fatty acids or nonionic surfactants. These substances are found in large quantities in the process water when the pulp used for the manufacture of the paper is recycled pulp. Indeed, in addition to the substances added voluntarily during deinking, other surfactants are "salted out" by the recovered papers (surfactants put into the formulation of printing inks, in the formulation of the layers applied to the surface of the paper for manufacturing magazine papers) and also released by mechanical pulp fibers when they are obtained (defibration, refining) or bleaching.

In the present application, it appears that the surface-active substances have no potential beneficial effect, even if present in a small amount. The object of the invention is therefore the total elimination of the surface-active substances present in the waters used for papermaking, after separation.

In the context of the invention, the step of separating the surfactant substances is carried out using the technique of dispersed air flotation.

This technique consists in separating the surfactants from the water containing them, dragging them to the surface by the insufflation, within the liquid mass, of air bubbles having a diameter that may range from a few hundred microns a few millimeters. This results in a flotation foam rich in isolated surfactant substances. This foam is then separated from the treated liquid, for example by suction or overflow.

It has been shown in the context of the invention that this technique was the most suitable, in particular the most selective and the most effective, for the separation of surfactants from water intended for the preparation of pulp.

Another contribution of the invention has been to determine the most judicious places of the papermaking process, where the dispersed air flotation treatment has to be carried out.

Indeed, the separation of this flow must be done by minimizing as much as possible the loss of suspended solids and water losses, factories having to reduce their water consumption more and more.

Thus, in order to minimize solid losses, the treatment is carried out on a stream with a low content of suspended solids. In addition, to reduce water losses, the surfactant removal process must concentrate the rich surfactant-rich rejection as much as possible.

In general, the dispersed air flotation treatment is carried out on all or part of the white water.

In a first embodiment, the step of separating surfactants by dispersed air flotation is carried out directly on the filtration water, from the web or table forming paper, before their recirculation and recycling.

A second possibility, even more efficient, is to implement this air flotation dispersed on the water of the fiber recuperator, at the machine head, at the level of the white water circuit of the paper machine. The recovery of the fibers can be carried out either with the aid of a disk filter or with a dissolved air flotation cell. The water from this fiber recuperator is therefore depleted of suspended solids, which makes it possible to limit the losses in solids.

In practice, the dispersed air flotation generates a so-called flotation foam in which the surface-active substances are trapped in concentrated form. As already stated, these foams are intended to be eliminated after recovery, typically by overflow or by suction.

A method according to the invention provides for the evacuation of this flotation foam directly in a purification plant. Advantageously, the flotation foam is pretreated with ozone, which facilitates the degradation of the surfactants.

Another possibility is to introduce this flow in the middle of the sludge dewatering table, to drain the surfactants into the sludge.

According to a second aspect, the invention relates to a paper machine adapted to the implementation of the method described above.

A paper machine according to the invention is therefore characterized by the presence in its wet part of at least one cell adapted to dispersed air flotation allowing the separation of surfactants present in white water.

In practice, such a cell is designed to have contact time between air bubbles and water to be treated, sufficiently long. Thus, all surfactants, even with slow adsorption kinetics, have time to migrate to the air / water air bubble interfaces.

The rate of air introduced into the cell must be large enough for the adsorption surface of the surfactants to be sufficient. In order to minimize water losses, several stages are necessary. One of the ways of minimizing water losses is also to have a cell with a sufficient foam thickness to dry the concentrated foam in surfactants.

The dispersed air flotation cell, as described above, is advantageously positioned at the level of the white water circuit, in particular at the level of the filtration water coming from the paper forming fabric, before it is put back into circulation and recycled.

Even more advantageously, the flotation cell is located on the clarified water circuit from the fiber recuperator, so as to minimize material losses.

In addition, the paper machine according to the invention is advantageously provided with a system for recovering foams generated by the dispersed air flotation cell. According to a first system, it is a connection between the flotation cell and a purification plant, preferably via an ozone treatment unit. According to a second alternative, the flotation cell is connected to a sludge dewatering table.

The present invention relates to any type of paper machines. More particularly targeted by the present invention, paper machines or processes consuming small amounts of fresh water, or even operating in a closed circuit. It is in these conditions that the waters are the most loaded with surfactants.

As already stated, the invention is also very useful and efficient when the paper is formed from recycled (especially de-inked) or mechanical pasta (wood pulp, ...).

A third application also seems particularly suitable: during the manufacture of coated papers from virgin pastes, the process can be implemented on the thickening waters, generated during the recycling of the manufacturing breakages to be thickened.

• l'amélioration du fonctionnement de la machine à papier ;
• la réduction des débits de produits chimiques en tête de machine ;
• la réduction des défauts inhérents à l'air dans la pâte (tête d'épingle,...).

The present invention therefore has advantages on several levels:

• improving the operation of the paper machine;
• reducing the flow of chemicals at the machine head;
• the reduction of defects inherent in the air in the dough (pinhead, ...).

EXEMPLARY EMBODIMENT

The embodiment which follows, in support of the appended figure, is intended to illustrate the invention but is in no way limiting.

The figure 1 illustrates a paper machine according to the invention for the implementation of the advantageous paper manufacturing method as described.

As illustrated in figure 1 , the paper machine according to the invention is conventionally equipped with a headbox (1) providing a homogeneous distribution of the paste over the entire width of the paper forming table (2), a press section ( 3) to remove water by pressing and a dryer (4) to remove the residual water by evaporation before winding the roll paper (5).

The filtration water beneath the forming fabric is recovered for recycling and to dilute the pulp in the head circuits, before the hydrocyclones (6) and the machine head files (7) for removing contaminants with a higher density. to 1 on the one hand and the contaminants of size greater than 150 - 200 microns on the other hand.

The filtration water under the forming fabric is either recycled directly without any treatment, or clarified by the clarifier, in this case a dissolved air flotation cell (8).

The flotation treatment of water, implemented using a dispersed air flotation cell (9), can then be carried out on all or part of the stream of white water recovered under the formation web (9A ), or all or part of the flow of water free of suspended matter, at the outlet of the clarifier (9B).

Claims (14)

1. Method for manufacturing paper in a papermaking machine including in the wet part at least one step of separating the surface active substances present in the white waters by dispersed air flotation.
2. Method for manufacturing paper in a papermaking machine as claimed in claim 1 characterized in that the surface active substance separation step is implemented on all or part of the white waters.
3. Method for manufacturing paper in a papermaking machine as claimed in one of claims 1 and 2 characterized in that the surface active substance separation step is implemented on the filtration waters, produced at the paper forming stage.
4. Method for manufacturing paper in a papermaking machine as claimed in one of claims 1 and 2 characterized in that the surface active substance separation step is implemented on all or part of the clarified waters produced by a fibre reclaiming device.
5. Method for manufacturing paper in a papermaking machine as claimed in one of the previous claims characterized in that it uses recycled or mechanical pulp.
6. Method for manufacturing paper in a papermaking machine as claimed in one of the previous claims characterized in that it includes a step of removing the flotation froth containing the separated surface active substances.
7. Method for manufacturing paper in a papermaking machine as claimed in claim 6 characterized in that the flotation froth is treated with ozone before being discharged to a treatment plant.
8. Method for manufacturing paper in a papermaking machine as claimed in claim 6 characterized in that the flotation froth is discharged on a sludge dewatering table.
9. Papermaking machine including in its wet part at least one dispersed air flotation cell (9A, 9B) allowing the surface active substances present in the white waters to be separated.
10. Papermaking machine as claimed in claim 9 characterized in that the flotation cell (9A) is located at the output of the machine wire (2), and upstream of the white water recirculation circuit.
11. Papermaking machine as claimed in claim 9 characterized in that the flotation cell (9B) is located on the clarified water circuit, at the output of a fibre reclaiming device (8).
12. Papermaking machine as claimed in one of claims 9 to 11 characterized in that it consumes small quantities of fresh water, or even that it operates in a closed circuit.
13. Papermaking machine as claimed in one of claims 9 to 12 characterized in that the flotation cell (9A, 9B) is connected to a treatment plant for the removal of the flotation froths.
14. Papermaking machine as claimed in one of claims 9 to 12 characterized in that the flotation cell (9A, 9B) is connected to a sludge dewatering table.

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