In addition to the fibres containing cellulose, a very important part of the composition, especially of certain paper grades, is comprised of the fillers used in the paper, whose concentration in paper can be very high. Fillers influence the appearance and the technical properties of the paper, because they can improve its optical properties, for example by adding opacity or improving printability.

The fillers used are generally inorganic and mineral based, and they are composed of small particles that fit into the spaces in the fibrous web of the paper. Fillers can be selected from among several inorganic minerals.

Increasing the filler content of paper also reduces raw material costs, since the price of fillers is usually lower than that of fibre material. In terms of process technology, the fillers can facilitate dewatering at the initial phase of web formation, but in this case the retention of the fillers in the forming paper web is important.

According to general practice, the filler is received from the filler supplier as meeting certain quality requirements and having the appropriate particle size distribution. It is then mixed into the other pulp components, and after this the pulp is conveyed at an appropriate consistency from the head box to the forming section, where the dewatering of the pulp suspension and the forming of the paper into a cohesive fibrous web begins. There are also inorganic fillers, the chemical structure of which enables their production from base solutions in situ, for example at the paper mill. Calcium carbonate in particular is a filler of this kind, and several examples have been presented in the patent literature on the use of precipitated calcium carbonate (PCC) as a filler in different papers.

The PCC production integrated into papermaking is disclosed in U. S. Pat. No.

5,558,782, according to which, by precipitating calcium carbonate in circulation water containing fines, the fines residing in the circulation water can be separated, and the PCC containing fines can thus be used as a filler. In addition, a special object of interest has been the precipitation of calcium carbonate in an aqueous medium, in which a selected fibre material of paper is already suspended, and thus the so-called lumen-loading technique in particular has been the object of study. EP0791685, for example, discloses the precipitation of calcium carbonate in situ from calcium hydroxide with carbon dioxide into the pores and lumens in the walls of cellulose fibres when the pulp is in a fluidised state. U. S. Pat. No. 5,223,090 discloses the precipitation of calcium carbonate with carbon dioxide into the pores and lumens of the walls of the cellulose fibres in a pressurised refiner. U. S. patents No. 5,096,539 and 5,275,699 for their part disclose the precipitation of calcium carbonate into the walls of fibres by treating the fibres at the first stage with a solution of a soluble calcium salt, for example a solution of calcium chloride, and at the second stage with a solution containing a carbonate anion.

U. S. Pat. No. 5,679,220 discloses yet another in situ. method of precipitating calcium carbonate into the fibre material. In this method, the paper raw material fibre slurry is mixed with calcium hydroxide slurry in a tubular reactor, and carbon dioxide gas is introduced into the mixture flowing in the tube.

International patent WO 90/09483, patents corresponding to which are, for example, Finnish patent 96336 and U. S. Pat. No. 5,262,006, describes a method for removing dissolved gypsum from recycled pulp by adding a salt containing carbonate or hydrogen carbonate anions, for example sodium carbonate, into the slush containing recycled pulp, as a consequence of which calcium carbonate is precipitated to form a filler, and the soluble sodium sulphate is discharged with the water.

As an example of the use of recrystallised calcium sulphate as a filler, U. S. Pat.

No. 4,801,355 can be mentioned.

The in situ precipitation of fillers into the fibre raw material for paper naturally requires considerable changes in the pulp preparation system of the paper mill.

In addition, providing the paper with fillers follows, in a sense, the conventional technique: the filler and the fibres have already been mixed together when the paper raw material is conveyed as a dilute suspension to paper forming.

In the coating of paper, on the other hand, the purpose is expressly to adjust the surface properties of the paper in order to make them suitable for printing, for example, and therefore pigment is added with a binding agent to the surface of the paper web by using a suitable spreading method. The whole surface of the paper is intended to be covered by using these methods.

International patent WO 00/03092 (p. 6, p. 12) discloses the transfer of calcium oxide onto the surface of the paper web by using, for example, the press section, after which calcium hydroxide is formed from it with the help of humidity and the web is transferred into a carbon dioxide atmosphere, as a consequence of which calcium carbonate particles are formed on the surface of the web.

In the traditional spreading methods, binding agents are used to cause the pigment to take on to the surface of the paper web. In the technique disclosed in the above-mentioned patent, WO 00/03092, which aims at the use of minimum amounts of water in coating and at applying the idea, for instance, to dry web forming, carbon dioxide gas must be used and a carbon dioxide atmosphere must be arranged at some point on the track of the web.

The use of carbon dioxide to prevent the calcium carbonate that has been added to paper raw material pulp from dissolving at low pH values is known from Finnish patent 103520, to which the international patent WO 99/35333 corresponds.

The object of the present invention is to present a new method for adding pigment to paper, a method which offers new possibilities in particular for

providing the surface of the paper with small amounts of pigment (usually less than 2 g/m2). The object is to present a method which does not require great changes in the paper making process and which is suited especially for smoothing the paper surface and coating it with pigment. To realise this object, it is mainly characteristic of the method of the present invention that pigment raw material is fed in a soluble state into or towards the moving paper web, which is at least partly formed, and after the feeding, the raw material is brought into conditions that make it possible to cause the pigment to precipitate.

The raw material, which is in the form of a solution, can be sprayed or spread onto the surface of the moving paper web by utilising a suitable method. The conditions which enable the raw material to be caused to precipitate to form pigment may be the conditions of the paper web, which may be appropriate already at the moment of adding or may subsequently be rendered such. The conditions that cause precipitation may be the temperature, the pH, or the ions added to the paper web. In addition, when spraying the solution, it is possible to cause conditions promoting the precipitation of the pigment already in the spray, for example by spraying another solution into it.

The pigment is precipitated at a phase where the grammage of the paper has already been determined and some water has been removed from the forming or formed web. The precipitation can be carried out at an appropriate phase on the paper machine, or on the finishing machine, where the paper which is already complete as to its body structure is finished. The time delay, from the moment when the solution containing raw material is fed into or towards the web to the moment when the precipitation is caused, is as short as possible, 0- 1.0 s, whereby the pigment takes better onto the surface of the web.

With this invention, a novel in situ precipitation of pigment can be realised in such a way that the pigment takes onto one surface of the web on the side from which the raw material has been added in a soluble state. In this way the surface pores of the web can be filled and the surface of the web smoothed, and especially the quality of the web surface can be affected, which for its part

has the greatest effect on the printing properties among other things. When compared to coating, the method differs from it in that the pigment particles are not ready in the composition that is to be spread on the surface of the web, and in that very small amounts of pigment (less than 2 g/m2) can be spread by using the method. No binding agent is needed to attach the particles, because the pigment particles precipitated onto the moving paper web by the in situ principle at the same time adhere tightly to the fibres.

What is meant here by the pigment raw material which is in a soluble state, is an agent which in soluble state contains one component of the pigment (in the case of salts at least one of its ions) or all of its components. In practice the precipitation is carried out in such a way that the soluble salt, usually an inorganic salt, which contains at least one of the components of the pigment, is added to the moving paper web, after which the precipitation can be carried out according to either of the following principles: a) already at the moment of adding the conditions of the paper web are such as to cause the precipitation of the pigment, or, b) after adding the conditions of the paper web are changed so that they are such as to cause the pigment to precipitate In the latter alternative the circumstances can be changed intentionally, expressly with a view to the precipitation of the pigment, or the conditions become suitable as a consequence of the normal paper manufacturing and finishing process.

Alternatively, the precipitation of pigment can be started already when its raw material is fed in the form of a solution towards the paper web, i. e. the conditions causing precipitation are already caused in the solution sprayed towards the paper web in the air, for example by feeding another agent into the spray.

The conditions with which the precipitation of pigment can be affected are the pH, the temperature and/or the concentration of the ions originating from another source, or their quality. Several factors can be adjusted simultaneously.

The choice of the precipitation method depends on the pigment itself and its precipitation chemistry. By adjusting the conditions to be appropriate at or after the adding point, several different inorganic pigments can be precipitated.

These pigments are described in the following.

The inorganic pigment to be precipitated is a salt. The precipitation of a salt can be carried out in two different ways: 1) The aqueous solution of the salt is added to the paper web at a certain concentration, and the precipitation takes place with the help of the web conditions, which cause the salt to change from a dissolved state to a solid state.

2) The aqueous solution, which contains one of the ions of the salt at a certain concentration, is added to the paper web, and the precipitation takes place in the web as a consequence of the effect of a certain concentration of an ion of different quality. In the case of salts, these ions are an anion and a cation, which separately form soluble salts but together produce a salt that is insoluble in water.

In what follows the invention is described with reference to certain pigments and their precipitation methods. The invention is not, however, limited to these fillers.

According to the invention, appropriate salts are dissolved to a soluble state at the aqueous phase, and a neutral or an acid paper is treated with this water either on the paper machine or in the finishing process, for example at the supercalender. After this, the salts are precipitated on the surface of the paper in certain conditions (temperature, pH and/or solubility product, i. e. ion

concentration). On the paper machine, the soluble salt can be added to the moving, continuous paper web with damping rolls, spreading rolls, or in the water of the spraying damping device. In a finishing machine, such as the supercalender, damping or steaming or even spreading can be used.

In what follows the precipitation of three salts, i. e. gypsum, calcium carbonate and aluminium hydroxide, is examined.

Gypsum The behaviour of gypsum (calcium sulphate) is well known to be very divergent from that of other pigments. In normal circumstances approximately 2 g/l of gypsum always dissolves in the aqueous phase. The solubility of gypsum is highly dependent on the temperature of the system and to some extent also on the pH (when the pH is less than 2, the gypsum begins to precipitate). In the present invention the solubility vs. the precipitation of gypsum at a high temperature is exploited at the pH values normally used in papermaking. Less than 2 g/l of gypsum is added, for example, to the damping water of the paper machine, and thus the gypsum is caused to transfer into the paper in a dissolved state with the water. After this, in the thermal treatment of the paper, which as such can be a part of the papermaking process (for example hot calendering), the surface temperature of the paper rises so high that it causes the calcium sulphate to precipitate. The temperature is advantageously not lower than 150°C. The gypsum can be dissolved in the damping water which is added after the drying section before the hot calendering. The time delay between damping and the hot calender roll nip is kept as short as possible, 0- 1.0 s.

As in the supercalender, so also in the finishing machine, dissolved gypsum can be added with the steam or the damping water to the paper, which is unwound from the paper reel and run to the calender, where the precipitation takes place by means of temperature. This application requires high temperatures in the supercalender : approximately 150°C or advantageously higher. At the above- mentioned temperature the solubility of gypsum is only about 500 ppm, so the

yield of gypsum is around 60-75% (from dissolved state to solid state). To cause the gypsum to precipitate onto the surface of the paper, the aqueous solution is added before the calender nip, and the time delay from damping to the hot calender roll nip is kept as short as possible, 0-1.0 s.

It is also possible to promote the precipitation of gypsum in paper by adding a surplus amount of calcium or sulphate ions to the aqueous phase containing dissolved calcium sulphate, which has already been sprayed onto the surface of the paper. That is, the precipitation of gypsum can be adjusted by the ratio of ion concentration to solubility product. A good agent for this is sodium sulphate (a source of sulphate ions), but calcium carbonate or calcium bicarbonate (a source of calcium ions) can also be used.

In this method, many different types of raw gypsum which are in a solid state can be used as base materials, because the particle shape of the raw gypsum to be dissolved does not affect the form of the gypsum precipitating onto the surface of the paper web.

Aluminium hydroxide Alum, i. e. aluminium sulphate, is dissolved in the aqueous phase, whereby a trivalent aluminium ion and a sulphate ion are obtained in the solution.

Consequently, the solution is acid. This can be done with the damping waters of the paper machine and through the damping carried out on the finishing machine, for example in the supercalender. After this, alkaline water is added to the track, whereby the pH of the track is caused to rise and free aluminium forms aluminium hydroxide in the neutral zone. AI (OH) 3 is a very bright sheet- like pigment which has a large specific surface area. On the other hand, if the pH value of the track is already neutral or suitable for precipitation, it is not necessary to alter the pH with water; i. e. the conditions of the track are already favourable for precipitation. This depends on the buffer capacity of the track as regards pH.

For example, after the drying section the damping waters fed into the web may contain alum, and before the hot calender, alkaline water is fed into the web.

In addition, it is possible to add alkaline water to the alum water spray already in the air, i. e. in practice these two solutions are sprayed together.

Calcium carbonate A method for precipitating CaCO3 into paper is to add a solution containing calcium ion and immediately after that damping water which contains soda or sodium bicarbonate. In this way, the precipitation is caused due to the effect of the calcium and carbonate ions.

The solution containing calcium ions can be derived expressly from gypsum. In this case, it is possible to improve the yield by precipitating at least a part of the dissolved calcium that has been added to the paper web in the form of calcium carbonate, when calcium sulphate is precipitated from a dissolved state by means of temperature.

Precipitated salts, i. e. pigments, have the following effects on the quality of paper: -the pigment can be brought more easily onto the surface of the paper -brightness increases, opacity (absorption and scattering) increases, surface roughness diminishes, the surface of the paper becomes denser (printability improves) -the grain size or the grain shape of the utilised pigments are of no importance, because they dissolve in the aqueous phase and they do not necessarily take their previous crystalline form when precipitating. If gypsum is used, for example raw gypsum is excellent for the purpose, as long as the purity of the gypsum is sufficient.

-a special advantage of gypsum is the fact that its solubility is independent of pH in the pH ranges used in papermaking, whereby it works in both neutral and acid systems.

The present invention is especially suitable for use in such papermaking and finishing processes where the paper surface is treated by bringing it under a certain pressure, for example in the calender roll nip, whereby the precipitated/precipitating pigment particles also sink into the surface pores of the paper web and fill them. In calendering, a suitably high temperature can also be arranged on the surface of the web, if needed to cause the precipitation.

Any single or multiple nip calender utilising one or more hot rolls can be used for this purpose. The heat can be applied to the surface of the web by other means as well, for example by infrared heating.

The scope of the present invention is not limited only to certain phases in the papermaking line and the finishing process. The precipitation can also be carried out in the press section or the drying section by spraying or spreading a solution containing pigment raw material on the surface of the web.