The present invention concerns an apparatus and equipment for grinding of paper; cardboard or similar continuous webs made of fibrous raw material.
The invention also concerns a grinding method.
The present invention relates to treatment of paper and cardboard and similar fibre-based webs, in particular by grinding one or both sides thereof. In our earlier patent applications, PCT/FI98/00341 and FI 980044 we have dealt with the advantages obtainable by grinding of paper surfaces and by means of which it is possible partially or totally to substitute the calendering used for smoothening of the surface without impairing the strength properties of paper and cardboard and without deteriorating opacity. We have also shown that it is easier to attach various coating layers to a ground surface and that similarly a coating polymer film is more easily attached to a ground surface than to a conventional surface. This is, e.g., because fibrils are released from the surface fibres of the web, whereby there is formed a very fine fluff on the surface which increases the surface available for attachment of the coating pigments. Typically, the surface coarseness of paper and cardboard is about 6 microns and by grinding it can be reduced to even one micron. Since the internal structure of the paper is not compressed, the stiffness and strength properties are preserved and, in some cases, even improved by 10%. Coarsed or risen fibres are nearly completely removed. This is important for the manufacture of, e.g. liquid container cardboard and corresponding coated packaging material.
In principle, grinding can be carried out in many different ways. Thus, it is possible to use a grinding belt or a grinding roll, which is coated with an abrasive or grinding agent, or a grinding stone. These solutions represent purely mechanical grinding processes. It is also possible to grind by using different pigments which simultaneously or later on are used for coating for example by wedging pigments at great speed against the paper web.
If, however, the aim is to achieve grinding by simultaneously subjecting the paper surface to calibration, a belt or roller grinder or a grinding stone are the only alternatives worth considering. The essentially most advantageous of these are the grinding rolls or sets of grinding rolls.
A problem associated with the known wood grinders is that they become clogged or blocked by the wood resins together with hemicelluloses and possibly lignin and in combination with the fine dust formed during grinding. If grinding is too effective, the surface temperature of the web becomes too high which gives rise to the above mentioned clogging. It is important to regulate the grinding pressure also so that it becomes possible to remove only a part of the surface layer of the web without otherwise damaging the web.
The present invention aims at eliminating the problems of the prior art and to provide a completely novel kind of technical solution for grinding of paper and cardboard webs and similar continuous, fibrous webs.
The present invention is based on the idea that the grinding material is selected in such a way that the grinding particles, i.e. the particles on the surface of the grinding roll or belt, become charged and they provide triboelectrically the removed paper dust or fibres with a charge that is the same or which is close to the charge adopted by the fibres. Paper is always slightly negatively charged during brusing, rubbing and grinding. By contrast, it may be mentioned as an example that asbestos fibres always become strongly positively charged.
A second and very important finding of the invention is that the grinding pressure can be more easily arranged by subjecting the web to tension by using stretch rolls or by arranging a different stretching angle between each grinding roll and the paper and by further regulating the inherent tension in the paper. A cascade of rolls or a belt grinder are more advantageous, because it is rather difficult exactly to calibrate the dimension and shape of one grinding means, and the errors of the preceding grinding means can be levelled by the following grinding means.
Aluminium oxide, beryl and many other aluminium oxide compounds also become negatively charged. In connection with our tests we have surprisingly found that, when this is the situation, grinding dust and fibres and fibrils released from the surface fly away from the ground surface and the ground surface does not become clogged when wood-free webs are being treated. This is also true for surface sized papers when the surface size used comprises known alkyl ketene-dimers, anhydrides of alkyl succinic acid and/or starch or cationized starch and combinations thereof. All of these will become electrically charge and fly away from the surface, whereby it is simple to remove them by suction produced by, a vacuum extractor close to the grinding roll. Due to electrical charging, the dust is so strongly released and removed that there are no dustings problems in the environment.
For wood-free papers complete triboelectric cleaning can be obtained with aluminium oxide (Al2O3) or a corresponding grinding surface.
Identical results have also been reached with papers made partially from wood-containing fibres or from chemical pulp which still contained residual lignin. The grinding rolls were not contaminated and the interstices between the grinding particles were not blocked during testing which involved running over 40 km of different papers through a grinder. It should, however, be emphasised that for wood-containing papers or for papers containing resins and polymers the other methods described in the present invention may become necessary.
The matter was studied also by using coloured papers, whereby it was found that the papers having coloured fibres did not at all change the colours of the grinding rolls.
Since the hardness of alumina on the Mohs scale is 9 and, e.g. that of beryl (Al2Be3SiO18) is 8, beryl powder is a very suitable additive for alumina because then the wear of the grinding pigments becomes uneven and the grinding surface becomes self-sharpening when the wear of the grinding surface progresses. Any additive or binding agent softer than alumina is suitable provided that it does not make the grinding layer electrically conducting. The following examples of additives of said kind can be mentioned: titanium dioxide and iron oxide. It is also possible to use mixtures of said compounds but only to the extent that the electrical resistivity of the grinding layer is not essentially lowered.
The grinding surface of all grinders is continuously slightly worn, and consequently the worn surface takes up less and less material from the material which is subjected to grinding. This makes it possible, and this is also an essential feature of the present invention, to make each worn grinding roll the last roll of the grinding roller cascade when there are 2 to 8 successive rolls in use. The first two rolls will do most of the grinding of, e.g., the paper surface and the next ones will grind less and less. The rollers following the two first rollers will subject the paper surface to removal of fibres and fibrils, and after said rollers the surface of the ground paper contains less released fibres than a completely untreated raw paper. A paper surface ground with a number of rolls exceeding two is free from risen fibres and fibrils to a larger extent than even a conventional sized surface. This feature is readily comprehended by the paper makers.
According to the present invention an apparatus is therefore used which comprises a plurality for grinding means which are in a deviated position from the straight direction of motion of the web and which can be contacted with the web. The grinding means are fitted in such a way that they can be moved transversally towards the web and away from the web in order to regulate the web tension and, thus, the effectiveness of the grinding. Alternatively, the grinding means can be in fixed position, and the web tension is adjusted by mobile rolls or other support means, such as press rolls. Advantageously, there is on an average a maximum of 1.5 microns of material removed with each grinding means.
The surface which has been subjected to grinding can be polished and easily provided with desired patterns, such as micropatterns which can be provided with an identification medium, such as a fluorescent powder. The apparatus according to the present invention also incorporates, in addition to the above-described grinding unit, a post-treatment unit for modification of the surface of the ground surface.
The invention will provide considerable advantages. By means of the invention it becomes possible exactly to separate the desired amount of substance from the surface of a paper or cardboard web. By regulating the grinding pressure the temperature increase and the clogging of the grinding means can be reduced. By controlling the temperature increase it becomes possible to avoid clogging of the grinding surface. In connection with the present invention there are also provided solutions for cleaning of the grinding means, which will provide for rapid and efficientxe2x80x94and even continuousxe2x80x94removal of substance which has been adhered to the grinding surface. The surface of the web becomes extremely even, because the grinding means will remove material from the highest (thickest) portions of the web. The levelling effect is of a different kind than in calendering in which the thickest parts of the web are compressed together and the density of the web increases more at these portions than at the thinner portions. The microcoarseness of the surface fibres of the web improves the result of coating carried out subsequent to grinding and bonds the coating particles to the web by forming a larger bonding surface while still removing the loose fibres from the surface.
By combining a post-treatment with the grinding it is also possible to achieve considerable advantages. Thus, the post-treatment of a ground surface is much more easily effected than the post-treatment of the surface which has not been subjected to grinding. The post-treatment can be carried out by methods known per se, such as soft calendering and belt calendering. The post-treatment of the surface can also be effected by other means, such as by pressing into the surface 2 to 10 micron notches or imprints, which can later on be filled with a coating or with another indicator chemical or identification colour. Thus, a pattern is formed on the surface which cannot be seen with the bare eye but which can be identified with fluorescence or another analysis. A normal calendered, smooth surface cannot be subjected to a surface forming method of this kind because the surface will swell when it takes up water and then the even embossing pattern will disappear.
Further processing of the surface can be performed by grinding in such a way that a shaping roll rotates with exactly the same speed as the web and grinds the surface by vibrating in the directions of the x, y or z axes.
Next the invention will be more closely studied with the aid of a detailed description and with reference to the attached drawings.