WO2008037840A1

WO2008037840A1 - A method for adding filler to a fibre suspension

Info

Publication number: WO2008037840A1
Application number: PCT/FI2007/050465
Authority: WO
Inventors: Jukka Selin
Original assignee: Upm-Kymmene Corporation
Priority date: 2006-09-25
Filing date: 2007-08-30
Publication date: 2008-04-03
Also published as: DE112007000876T5; GB0816304D0; GB2455168A; CN101432483A; FI20065589L; FI20065589A0

Abstract

A method for adding filler to a fibre suspension, in which method at least one aluminium salt is dissolved into the fibre suspension, and aluminium hydroxide acting as filler is precipitated onto the surface of the fibre by means of the pH conditions of the fibre suspension.

Description

A METHOD FOR ADDING FILLER TO A FIBRE SUSPENSION

The invention relates to a method according to the preamble of the appended claim 1 for adding filler to a fibre suspension.

In addition to fibres which contain cellulose, an important part of the composition of particularly certain paper grades consists of fillers used in it. The fillers have an effect on the appearance and the technical properties of the paper. The fillers are normally inorganic and are based on minerals, and they consist of smaller particles that fit in the spaces of the fibre network of the paper. Fillers can be selected from a variety of inorganic minerals, and their content in the paper may be quite high.

According to common practice, a filler that is available from a filler supplier meets certain quality requirements and has a suitable particle size distribution, and it is admixed with other pulp components, and following this, the pulp having a suitable consistency is led from the headbox to the forming section, in which dewatering of the pulp sus- pension and the formation of paper to a cohesive fibre web are started. One object in papermaking is to achieve good retention; in other words, as large an amount as possible of the paper pulp entering the wire should remain in the paper web forming on the wire.

However, fillers admixed with the paper pulp are left circulating in the circulation water of the paper machine, where problems are caused by them. When the fillers circulate in the water circulations of paper machines, organic material is accumulated on their surface, causing discolouration of the fillers and thereby affecting the optical properties of the final product. Furthermore, fillers left in the water circulations hinder the treatment of effluents from the paper mill, because the fillers are not burnt or degraded in biological effluent treatment.

It is an object of the present invention to disclose a method for adding filler to a fibre suspension, resulting in an improvement in the optical properties of the final product and in filler retention, and thereby result- ing in a reduction of solid matter discharges from the process. To attain this object, the method according to the invention is primarily characterized in that at least one aluminium salt is dissolved into the fibre suspension, and aluminium hydroxide used as filler is precipitated onto the surface of the fibre by means of the pH conditions of the fibre suspension.

The method according to the invention is based on the idea that the fibre suspension is treated with an aluminium salt to form filler for the papermaking process. In the method, aluminium hydroxide acting as filler can be precipitated onto the surface of the fibre. In this way, the filler can be attached to the fibre and it does not remain circulating in the water circulations of the paper machine. Compared with the conventional method, in which ready-made filler is admixed with paper pulp, the present invention is different in that a starting material for the filler is admixed with the fibre suspension, after which the filler is formed and precipitated directly onto the surface of the fibre in the fibre suspension.

In this context, the term fibre suspension refers to an aqueous suspension which contains fibre material and whose consistency may vary. Thus, the fibre suspension also refers to a clear filtrate that contains mostly water but also a small amount of short fibre. The term aluminium salt is used for a compound that can be dissolved into an aqueous phase, wherein aluminium ions are released into the solution. In this context, aluminium salt refers, for example, to aluminium sulphate, which is the most commonly used aluminium salt.

The fibre suspension prepared by the method according to the inven- tion can be used as a raw material for paper or paperboard.

The most significant advantage of the method according to the invention is the improvement in the optical properties of the final product. The precipitation of aluminium hydroxide onto the surface of the fibre improves the optical properties by way of the brightness of the aluminium hydroxide and as a result of the fact that the fillers do not circulate in the water circulations of the paper machine, which causes their discolouration. Furthermore, the method according to the invention has an opacity-improving effect.

The precipitating aluminium hydroxide is also used for binding other fillers to the surface of the fibre, for example kaoline, GCC (Ground Calcium Carbonate), titanium dioxide, and other possible fillers which have an anionic surface charge or which have been turned anionic. As a result, the content of other fillers can also be reduced in the water circulations of the paper machine, where they have previously caused problems. Furthermore, the better binding of the fillers to the fibres facilitates the treatment of effluents from the paper mill, because the fillers are not degraded in biological sewage treatment. Thus, one advantage of the invention to be mentioned is the better filler retention compared with conventionally used methods, and thereby the reduction of solid matter discharges from the process. Yet another advantage of the method according to the invention is a reduction in transportation costs, because the method makes it possible to prepare the filler from the starting materials in situ at the mill.

In the following, the invention will be described in more detail with reference to the appended images which have been captured by a scanning electron microscope (SEM) and in which

Fig. 1 shows aluminium hydroxide precipitated onto the surface of a fibre in a clear filtrate,

Fig. 2 shows a fibre in a clear filtrate, totally covered with aluminium hydroxide,

Fig. 3 shows aluminium hydroxide particles precipitated onto the surface of an unbleached refined fibre,

Fig. 4 shows an aluminium hydroxide layer precipitated onto the surface of an unbleached refined fibre, Fig. 5 shows single aluminium hydroxide particles precipitated onto the surface of an unbleached refined fibre, and Fig. 6 shows kaoline particles precipitated together with aluminium hydroxide onto the surface of a fibre. An aluminium ion and the components formed by it are strongly dependent on the pH of the liquid phase. Consequently, changes in the pH of the liquid phase are utilized when precipitating the aluminium salt to aluminium hydroxide. Aluminium is present as aluminium hydroxide in water primarily in the pH range from 6.5 to 8. Partly in this range, aluminium can also be present in other forms.

In the following, we shall discuss the precipitation of aluminium hydroxide in aqueous phase in more detail, using aluminium sulphate as a reactant. At first, aluminium sulphate (AI₂(SO₄)₃) is dissolved into the aqueous phase, in this case a fibre suspension, wherein a trivalent aluminium ion and a sulphate ion are produced in the solution. Thus, the pH of the solution is on the acidic side. The precipitation of alumin- ium hydroxide AI(OH)₃ is possible by raising the pH of the solution, because aluminium is present in the form of aluminium hydroxide in aqueous phase in the pH range from 6.5 to 8.0. Therefore, the pH of the solution is raised with an alkaline solution, for example sodium hydroxide (NaOH), wherein aluminium is precipitated to aluminium hydroxide. The most optimal pH for the precipitation of aluminium hydroxide is 7.5.

Figures 1 to 6 show the precipitation of aluminium hydroxide onto the surface of fibres captured by a scanning electron microscope (SEM). 100 ml of aluminium sulphate, whose relative density was 11.8 ⁰Be (degrees Baume), i.e. about 1088 kg/m³, was added to two different samples, 200 ml of clear filtrate and unbleached TMP (Thermo Mechanical Pulp). The fibre content of the samples was 100 mg/l. After the addition of aluminium sulphate, the pH of the clear filtrate dropped from about 5 to 3.1. The pH of the solution was then raised to 7.5 by adding sodium hydroxide, wherein the aluminium was precipitated to aluminium hydroxide. According to the appended figures, aluminium hydroxide seems to precipitate as particles having primarily a rhombo- hedral shape. Figures 1 and 2 show a fibre in a clear solution, and Fig- ures 3 to 6 show a single unbleached refined fibre captured from a TMP sample. Figures 1 , 3 and 5 show aluminium hydroxide precipitated on the surface of a fibre, partly in the form of single particles (Fig. 5). When the local concentrations of aluminium sulphate increase, aluminium hydroxide is precipitated forming thick layers on the surface of the fibre, as shown in Fig. 2. Figure 4 also shows precipitated aluminium hydroxide forming a layer on the surface of an unbleached refined fibre. Furthermore, when precipitated, aluminium hydroxide binds other fillers to the surface of the fibre, as shown in Fig. 6, where kaoline par- tides (flat large particles) are attached to the surface of the fibre.

In practice, the precipitation is performed by dissolving aluminium salt into the fibre suspension, after which the precipitation may take place by two different principles:

- already at the time of addition, the fibre suspension has such pH conditions that result in the precipitation of aluminium hydroxide, or

- after the addition, the pH conditions of the fibre suspension are changed so that aluminium hydroxide will precipitate.

In the first alternative, no separate pH adjustment will be needed after the addition of the aluminium salt. In the latter alternative, the pH conditions of the fibre suspension can be changed by purpose for precipi- tating aluminium hydroxide, or they become suitable in connection with the normal treatment process of paper pulp. The pH conditions favourable for the precipitation are obtained, for example, by admixing an alkaline solution with the fibre suspension. The content of the addition depends on the pH conditions of the fibre suspension at the time. The precipitation to form aluminium hydroxide takes place in the pH range from 6.5 to 8.0.

The dissolution of the aluminium salt into the fibre suspension and the precipitation to form aluminium hydroxide are carried out in the short circulation of a paper or paperboard machine, as close to the headbox as possible. Thus the delay between the precipitation of the aluminium hydroxide and the spreading of the pulp onto the wire section is as short as possible, at the most about 15 s. In this way, good adhesion of fillers to the forming paper or paperboard web is achieved in the wire section, and thereby the filler contents in the water circulation of the paper mill can be reduced.

The aluminium salt can be dosed and dissolved into the aqueous phase also in a side flow or in a separate water circulation, after which the aqueous solution that contains aluminium salt is admixed with the fibre suspension in which aluminium hydroxide is precipitated either by changing the pH conditions of the fibre suspension, or the fibre suspension already has pH conditions that are favourable for the precipitation. The dosage into the fibre suspension and the precipitation to form aluminium hydroxide take place in the short circulation as close to the headbox as possible, as presented above.

An alternative method is to admix aluminium salt directly with mechanical pulp before the aqueous fibre suspension is formed. After this, the precipitation to form aluminium hydroxide can be carried out as close to the headbox as possible, by changing the conditions to be favourable for the precipitation by the above-described methods.

The method does not set any constraints on the consistency of the pulp, at which the precipitation of aluminium hydroxide can take place. The precipitation of aluminium hydroxide can take place, for example, in a clear filtrate which consists mostly of water but also contains short fibres. In this case, some of the aluminium ions will form aluminium hydroxide that is not attached to the surface of the fibre. However, the risk of alkaline discolouration is smaller than when the precipitation takes place in a high consistency pulp, because the brightness of the pulps will decrease when the pH rises above six.

The content of the aluminium salt to be dissolved into the pulp can be used to adjust the content of aluminium hydroxide precipitating onto the surface of the fibres. Normally, excess quantities of aluminium salt are admixed with the fibre suspension to provide sufficient precipitation. The precipitating aluminium hydroxide may also bind other fillers, which thus improves the retention of the fillers. Such fillers include, for example, kaoline, GCC, titanium dioxide, and other possible fillers which have an anionic surface charge or which have been turned anionic. The binding between aluminium hydroxide and other fillers is assumed to take place as chemical binding.

By the method according to the invention, the filler retention can be improved by 20 to 60%, depending on the fillers used, compared with the conventionally used method, in which finished filler is admixed with the pulp. The filler retention will depend on the pulp consistency, at which the precipitation of aluminium hydroxide has been performed. A better filler retention is obtained in high consistency pulps than, for example, in a clear filtrate, because in high consistency pulps most of the aluminium hydroxide is attached to the fibre.

The optical properties of the paper pulp are improved by way of the brightness of the aluminium hydroxide, because the ISO brightness of aluminium hydroxide is 98. The improvement in the optical properties is also a result from the fact that when aluminium hydroxide is formed, it is attached to the surface of the fibre and does not remain circulating in the circulation waters, which causes discolouration of fillers. Furthermore, the method according to the invention has an opacity- improving effect on the final product.

The method according to the invention is suitable for a variety of pulps, including chemical, mechanical, chemimechanical, or thermomechani- cal pulps, as well as recycled pulp, or mixtures of these pulps.

The paper pulp formed by the fibre suspension according to the invention, treated with aluminium salt, is led to the forming section of a paper or paperboard machine, and paper or cardboard is made of it in the form of a continuous web by letting it infiltrate through a forming medium that is permeable to water. The fibre suspension according to the invention can thus be used as a raw material for paper, paper- board, or another corresponding web or pulp product made of a fibrous material.

As an alternative to the above-described methods, we can also men- tion the direct precipitation of aluminium hydroxide in water, wherein the filler can be made in situ at a paper mill. Aluminium hydroxide is precipitated by adding e.g. aluminium sulphate to water, for example circulation water, and by adjusting the pH of the water to be suitable for precipitation, for example by using sodium hydroxide. The formed salt can be ground to a desired particle size and shape. After this, the aluminium hydroxide can be admixed with the pulp components according to common practice. By this method, economic benefits are achieved, because the preparation of aluminium hydroxide in situ is economically more viable than when it is purchased from a chemical supplier. With this alternative, however, the filler retention cannot be improved, but the optical properties of the final product can be improved.

The invention is not limited solely to certain embodiments or sections in the papermaking process presented above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims.

Claims

Claims:

1. A method for adding filler to a fibre suspension, characterized in that at least one aluminium salt is dissolved into the fibre suspension, and aluminium hydroxide acting as filler is precipitated onto the surface of the fibre by means of the pH conditions of the fibre suspension.

2. The method according to claim 1 , characterized in that after the aluminium salt addition, the pH conditions of the fibre suspension are brought such that the aluminium hydroxide is precipitated onto the surface of the fibre.

3. The method according to claim 2, characterized in that the pH conditions of the fibre suspension are changed by admixing an alkaline substance with the fibre suspension.

4. The method according to claim 3, characterized in that the alkaline substance that causes the precipitation is sodium hydroxide.

5. The method according to claim 1 , characterized in that already at the time of addition, the fibre suspension has such pH conditions that aluminium hydroxide will precipitate.

6. The method according to any of the preceding claims, character- ized in that the precipitation takes place in the pH range from 6.5 to 8.

7. The method according to any of the preceding claims, characterized in that the aluminium salt is aluminium sulphate.

8. The method according to any of the preceding claims, characterized in that the aluminium hydroxide is also used for binding other fillers onto the surface of the fibre.

9. The method according to claim 8, characterized in that the filler binding with aluminium hydroxide is, for example, one of the following: kaoline, GCC, titanium dioxide, or another filler which has an anionic surface charge or which has been turned anionic.

10. The method according to any of the preceding claims, character- ized in that the precipitation of aluminium hydroxide onto the fibre is performed in the short circulation of a paper or paperboard machine.

11. The method according to any of the preceding claims, characterized in that the paper pulp formed by the fibre suspension is led to the forming section of a paper or paperboard machine, and paper or cardboard is made of it in the form of a continuous web by letting the paper pulp infiltrate through a forming medium that is permeable to water.

12. The use of a fibre suspension made by the method according to any of the preceding claims 1 to 10, as a raw material for paper or paperboard.