It is known that in paper production the paper or board product is generated by removing water from the solids slush. The quantity of water is clearly the largest of the raw materials and the aim is to remove it as rapidly as possible from the finished product (uncoated or coated paper or board) by using a wire, a press and a drying section. Typically, in paper production “high consistency pulp” is first generated mainly from fibres, water and inorganic fillers or pigments. The high consistency pulp is diluted (typically to a consistency of 0.2-1.5%) in order to achieve better quality properties, before the pulp is spread from the headbox and before the dewatering is started in the wire section.
The process of dewatering and the attachment of detrimental substances to the fibres are among the most important factors affecting the economy of paper production, and it is attempted to affect these chemically, among others, using various flocculants and coagulants. Mechanically, it is attempted to affect the dewatering at the wire, press and drying sections (in the wire section for instance by means of suction boxes and drainage foils, which are designed to accelerate the dewatering by means of pulsation). More effective dewatering also reduces energy consumption needed for drying in the drying section.
Over decades, the wire sections of paper and board machines have changed considerably. Earlier, in Fourdrinier machines, water was removed only through one wire. In modern gap formers, water is removed simultaneously through two wires. After the wire section, the dry matter percentage of paper or board is generally 15-25%. At this stage, the water lies mainly between the fibres. The remainder of the water is mainly in the lumens of the fibres, the pores and the walls of the fibres.
In the press section, it is possible to raise the dry matter percentage to as high as approximately 50%. The most important task of the press section is to increase the tensile strength of paper or board in order to improve the runnability of the machine. In the drying section, the remaining water, which is mainly in the lumens of the fibres, the pores and the walls of the fibres, is evaporated. The percentage of dry matter is generally increased from 35-45% to approximately 95%.
Paper is generated from the pulp, which can be either mechanical pulp or chemical pulp, or recycled fibre pulp.
Here, mechanical pulps mean groundwood pulp, refiner groundwood pulp, thermomechanical pulp (TMP), pressure groundwood (PGW) and chemi-mechanical pulp (CTMP). Chemical pulp is pulp which is prepared from cooked wood chips. Recycled fibre may be deinked (DIP) or undeinked (for instance OCC). The most typical deinking methods are wash deinking, enzymatic deinking, flotation, and combinations of these three. The essential difference between these various pulps is that the mechanical and chemical pulps are made from “virgin” fibres, i.e. fibre from which paper or board has not yet been manufactured. Recycled fibre, in turn, is made from finished paper or board by recycling it for production of a new paper or board product. The pulps can be bleached or unbleached. The most typical bleaching methods are peroxide bleaching and dithionite bleaching.
In the production of chemical pulp, mechanical pulp and recycled fibre pulp, various wood-based and other dissolved and colloidal substances are released into the process waters. In mechanical pulps, dissolved and colloidal substance means mainly wood-based soluble and colloidal compounds (hemicelluloses, lipophilic extractives and compounds such as lignin), particularly resin. Resin is sourced from wood and comprises various fatty acids, esters, resin acids and sterols. The soluble and colloidal materials which accompany the recycled fibre and which are detrimental to the production of paper and board, are generally called gunges. A dissolved and colloidal substance is called a detrimental substance because it increases the consumption of chemicals, is generally very small-sized, anionic and easily generates precipitates. Typically, the gunges are thermoplastic impurities such as glue, latex, waxes, printing inks, anti-foaming agents and plastic. The gunges may include for instance compounds such as vinyl acetate, polyamides, polyethylene, polybutadiene, caoutchouc and styrene acrylate. The gunges may also comprise residues of beater-sizing (AKD, ASA and resin gluing), wood-based dissolved and colloidal substance and resin. Both resin and the gunges are hydrophobic. They have a tendency to agglomerate in water into large precipitates. This agglomeration is encouraged by variations in pH and temperature, and strong shear forces. In paper and board machines, the gunges stick to metal surfaces, wires and felts. Over time, they may also accumulate in the piping of the white water system and then unpredictably break free, thereby causing numerous breaks in the wet section, press section and drying section. On the wires and felts, they can reduce the water drain and thus the productivity of the paper or board machine. Dark hydrophobic precipitates also reduce the level of brightness, because in water they attract components of wood, such as tannins, which readily attach to them. In a final paper or board, these may be visible as dark patches. Typically, for paper and board machines in which it is not possible to efficiently keep low in the circulating water the amounts of dissolved and colloidal substance which accompany especially mechanical pulp or recycled fibre, shutdowns for cleaning must be arranged frequently because to avoid quality and runnability problems. In some production processes of mechanical pulp or recycled fibre, the fibres are additionally bleached with hydrogen peroxide or dithionite. Peroxide bleaching in particular substantially increases the amount of dissolved and colloidal detrimental substance in the waters of paper and board machines.
Typical chemical methods of removing the detrimental effects of hydrophobic substance are stabilisation, i.e. dispersing of the hydrophobic substance, attachment to the fibre and adsorbing to an active surface. To reduce the amounts of hydrophobic detrimental substance, they are dispersed, in which case their agglomeration is prevented. The problem with this is that over time the percentages of the hydrophobic substances may grow to the extent that the paper or board machine suffers from runnability problems. Preferably, the hydrophobic substance is attached, preferably small-sized, to the fibre, and removed from the process along with the finished paper or board. Adsorbing the hydrophobic substance onto an active surface prevents agglomeration and adherence to the surfaces. Minerals such as talc and bentonite are used for this. Here, it is important to remove the minerals from the process by means of good wire retention, otherwise the runnability problems will recur, for instance when the dispersion method is used. The most reliable method is, and this is achieved by attaching the hydrophobic substance to the fibres, to remove the hydrophobic substance as close as possible to the point where the hydrophobic substance enters the white water system of the paper or board machine. This is the purpose of the invention of the application.
By using different screens and cleaners which employ centrifugal force, the largest agglomerates of hydrophobic substance are removed mechanically—often before a chemical treatment. It is also possible to use combinations of all of the above-mentioned means. The surfaces of paper or board machines, on which surfaces most of the precipitates accumulate, are generally treated with different chemicals, in which case attachment of precipitates onto the surfaces are prevented. Examples of such chemicals are organic solvents, acids and alkalis.
By storing the raw wood and also by applying certain enzyme treatments it is also possible to reduce the detrimental effects of hydrophobic substance. It is also important to separate the circulating waters of the pulp production from the white water system of the paper or board machine, in which case it is possible that part of the hydrophobic substance left inside the pulp production. In fact, nowadays this is the usual way in most paper and board mills. Also, a carefully designed and executed wash program of the white water system of a paper or board machine, used in conjunction with effective use of biocides prevents problems which are caused by dissolved and colloidal substance. A lot of air and foam in the pulp also increases problems caused by the hydrophobic substance.
The process water is the dilution water of the consistent pulp obtained from the production of mechanical pulp (for instance at a groundwood mill and refinery) or the production of recycled fibre (for instance at a deinking plant), and which water is taken from the white water system of the paper or board machine. The process water used is often circulating water having a low consistency. Consistent pulp in the production of the different pulps mentioned above is often concentrated by mechanical means, to avoid the waters of the pulp production being carried into the white water system of the paper or board machine. In this stage, the consistent pulp is called a high-consistency pulp, because its consistency generally exceeds 8%. Often, the high-consistency pulp is moved to the storage tower of the paper or board mill, from which it is diluted with fetch waters for further use in the production process of paper or board. In the present application, the water-based composition which is formed of colloidal carbonate particles and bicarbonates and other forms of carbonate (the pH value remaining essentially between 6.0 and 8.3), and which is prepared into the fetch water, is called acidic water.
In order to attach the hydrophobic soluble colloidal substance to the fibre it is advantageous that the so called acidic water is brought to react with a pulp which has as high a consistency as possible, at the earliest possible stage, in the white water system of the paper or board machine. The first point at which the chemical pulp or the mechanical pulp or the pulp coming from the production of recycled fibre enters the white water system of the paper or board machine is the containers for storing the consistent pulp, from which containers the pulp is moved forward, having been diluted with fetch water, to the paper or board production process.
The aim is to affect the economy and quality of the production of the paper and board by using different mineral fillers. These improve the quality properties, particularly opacity, brightness and printability. They often improve the economy because they are cheaper than fibre and they bind water to themselves less than fibre does. A lower water adsorption capacity is expressed in the wire, press and drying sections as faster dewatering, which in turn lowers energy costs in the drying stage.
Paper qualities such as copying papers and certain magazine papers, the filler percentages of which are large, generally require greater rigidity. The demand for lower grammages in the production of paper and board also places a premium on rigidity. Generally, the rigidity of paper declines as amount of filler in the paper rises or when the grammage is lowered. In fact, this reduction of rigidity and the lower strength together present the most important quality challenges when using fillers.
For instance, the following mineral fillers (or coating pigments) can be included are examples of the fillers used: kaolin, titanium dioxide, gypsum, talc, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC) and satin white. The most used fillers are GCC, PCC and kaolin.
The reduction in strength and rigidity of paper and board products that occurs when fibre is replaced with a filler is mainly caused by fillers decreasing the generation of hydrogen bonds between fibres, because the surface of the fillers do not form hydrogen bonds.
Nowadays, the filler is directly added into the fibre slush. In the wire section, only part of the filler added is attached to the finished paper or board web. The rest of the filler is carried through the white water system to ultimately form part of the finished paper or board structure, but in that case the risks of different runnability problems increase, mainly because of attachment of different hydrophobic substances to the fillers in the white water system. Generally, the resulting runnability problems appear in the paper or board machine for instance as fouling of the wires and felts, i.e. breaks. Part of the filler in the white water system also eventually overloads the sewage treatment plant, because the filler never travels out from the process along with the finished paper or board.
Because of the several disadvantages mentioned above, patents have been applied for during the last two decades, which patents are particularly related to precipitation of calcium carbonate directly into the fibre structure in the production process of paper or board. The aim of these known solutions is mostly to precipitate calcium carbonate either into the fibre structure or into its lumen.
Numerous such patents exist which relate to the precipitation of calcium carbonate directly into the fibre structure during the production process of a paper or board mill, and it is not appropriate to go through all of them here one by one. However, in the following, we refer briefly to a few interesting publications.
U.S. Pat. No. 4,510,020 is a patent related to the process of precipitating into the fibre lumens. According to the publication, powerful mixing is used to force precipitated calcium carbonate particles inside the lumens of fibre. The calcium carbonate particles which adhere to the outer surfaces of the fibres are detached from the surface of the fibres during the washing stages which follow the mixing. The calcium carbonate particles are detached more rapidly from the surface of the fibres than from inside the lumens, in which case the result is an outer surface of fibre which generates hydrogen bonds, and a fibrous structure, the brightness, the opacity and the rigidity of which are better.
U.S. Pat. No. 5,223,090 describes how calcium oxide or calcium hydroxide is mixed among fibres using high shear speed mixing, while carbon dioxide is simultaneously fed into the mixer.
WO published patent application 03033815 A2 describes how precipitated calcium carbonate is precipitated into a diluted fibre pulp, and onto the surface of the fibres, by using in this precipitation calcium carbonate slurry which is partly dissolved to calcium bicarbonate, and calcium hydroxide, or calcium hydroxide and carbon dioxide.
EP publication 0791685 A2 describes the precipitation of calcium carbonate onto the surfaces of fibre and fines by means of adding carbon dioxide into a mixture of calcium hydroxide and fibre material. As a final result, on average, 500 nanometer calcium carbonate crystals are precipitated onto the surfaces of the fibre.
In general, for reasons of cost or technical reasons these solutions have not been put into practice.
Water-based compositions and how they are used in the production of paper and board are described in FI publication 20085969, FI application 20096098, FI application 20105437 and FI application 20105627. These publications demonstrate that by using a composition which comprises forms of carbonate and calcium and/or magnesium ions it is possible to achieve good adhesion of a filler to the fibrous web, rapid dewatering, the attachment of hydrophobic particles to the fibrous web, and also improved opacity, rigidity and printability of the finished paper or board.
In particular, FI publication 20085969 demonstrates that by means of colloidal calcium carbonate and bicarbonate, and aqueous solutions of other forms of carbonate, an improved dewatering, retention and formation are achieved in the production of paper, within the pH range of 6-9, when a charged polymer is used. According to this published method, burnt lime or calcium hydroxide is first added into the process waters, after which the pH value is lowered, by applying carbon dioxide, to the range of 6-9. This sequence of addition, which is described both in the examples and the claims of the publication, and in particular the fact that the pH value is measured only after the addition of the other components, leads to pH variations in the solution during the production. It is known that variation in pH is a factor which causes agglomeration of hydrophobic detrimental substance. However, the publication makes no mention of any addition of a charged polymer and/or inorganic chemical either into the process water of a paper or board mill prior to the preparation of the acidic water, or into the water-based composition (acidic water) before diluting the pulp.
FI application 20096098 is similar to the previous publication except in that the lowest percentage of the colloidal calcium carbonate and bicarbonate and other forms of carbonate is lowered more than in FI publication 20085969. However, also in this application, charged polymer and/or inorganic chemical is not added into the process water of a paper or board mill prior to the preparation of the acidic water, nor into the water-based composition (acidic water) before diluting the pulp.
FI application 20105437 differs from the preceding publications in that the pH variations in the colloidal calcium carbonate and bicarbonate, and other forms of carbonate are removed during the production. However, in the application, it is still a fact that the waters of the paper or board machine, which waters are changed into water-based compositions, according to the application, are directly used for diluting the paper or board pulps—charged polymers and/or inorganic chemicals are not added into the process water of the paper or board mill prior to the preparation of the acidic water, nor into the water-based composition (acidic water) before diluting the pulp.