In the defibering of wood chips, which generally have been pretreated by heat or chemicals, so-called refiners are utilized. Similarly, such refiners are utilized for refining cellulose and other mechanical pulps of various kinds when it is desired to develop the paper-forming properties of these materials by treating them mechanically. In all such defibering or refining operations the desired results are achieved by treating the fiber material mechanically while it is passing through the refiner. Such treatment is generally effected by passing the fiber material, after they have been fed into the refiner by various types of devices, through a clearance between two refining surfaces, which for this purpose are generally provided with grooves and ridges. At least one of these surfaces and at times both such surfaces rotate so that the material is refined in the desired manner and conveyed out of the refiner by the forces of rotation. While generally the refiners are of the disc refiner type, the so-called cone refiners can also be employed.
The intensity and mode of refining these fibrous materials is determined both by the number of ridges and grooves on the refining surfaces and by the width of the clearance therebetween. Thus a narrower clearance as well as a greater number of ridges renders the refining more intensive. Also, in order to enable the performance of the refining work in the refiner and to convey the material therethrough, a certain amount of water must be added to the fiber material, so that a fiber concentration suitable for the particular circumstances in question is obtained. The amount of water supplied and the amount of energy consumed at those particular circumstances in question is decisive as to the temperature which will prevail during the various refining phases, which will thereby effect the refining result obtained.
Consequently, a substantially homogeneous mixture of fiber material, water, and water vapor is conveyed through that narrow clearance between the refining surfaces. The forces of rotation advance the material through the clearance between the refining surfaces at a considerable speed, and the rotating refining surfaces therefore are subjected to heavy wear which may be of both a chemical and mechanical nature.
The particular chemical atmosphere encountered by refining surfaces is relatively well defined for each set of conditions encountered, by the kind of fibers employed, the temperature conditions, the amounts of water and the type of water employed, and thereby also the chemical attack on the material which forms the refining surfaces is also well defined. This material employed for preparing the refining surfaces must therefore be selected accordingly for each such set of conditions. The mechanical breakdown of the profiles and structure of the refining surfaces is also determined by the speed with which each fiber/water mixture is conveyed through the clearance between the refining surfaces, i.e. by the size of the clearance therebetween and by the production which is mirrored by the load on the motor driving the refiners or the amount of energy consumed by the process. This amount of energy is thus transferred from the electric drive motors to the fiber/water mixture via the refining disc surfaces, and converted into heat.
Therefore, an increase in production increases the energy consumption and thereby the wear on the refining surfaces in exactly the same manner as would a reduction in the clearance between the refining surfaces. In other words, for a given machine size (or given refining surface area) and a given atmosphere, the wear on the refining surface is a function of the specific energy transfer expressed, for example, by kWh per cm.sup.2 of available active refining surface. For the production under each set of conditions the material used for the refining surfaces must therefore be selected with the greatest care so as to have the highest possible resistance to the combination of mechanical and chemical acts which take place upon the refining segments. This attack may also be described in terms of corrosion-erosion and a certain gradual wear of the refining surfaces cannot be avoided. A disc refiner therefore is normally divided into refining segments, for obvious practical reasons. These so-called refining segments may thus be exchanged after a certain time when the wear has proceeded so far that it causes process disturbances, or when the refining result is unsatisfactory. These refining segments are manufactured with a pattern and profile in accordance with the kind of work to be carried out in the refiner. The energy transferred to the fibers, and other materials to be refined, via the refining segments provided with ridges, is effected partly by the edges of the ridges and partly be the ridge surfaces. An edge which is therefore sharp and geometrically well defined can transfer more energy than a rounded or irregular edge. This implies, that optimally, the ridge edges must be sharp and intact for as long a period of time as is possible. As wear is unavoidable, wear should optimally take place as far as possible while maintaining the ridge profile. It has thus been attempted to avoid the greatest possible extent of wear by a suitable material selection for the refining segments, since such wear produces rounded ridge edges or heavy break-down of both ridge edges and entire ridges, which rapidly renders proper refining of the fiber material impossible and necessitates frequent exchange of refining segments.
The refining segment ridges should therefore wear uniformly over the edges as well as over the surface of the ridges. It has been attempted to achieve this by choosing for the segments and material which has wear properties on the ridge edge and the ridge surfaces so that they are in balance with the specific energy transfer via the ridge edge and the ridge surface and thus result in a uniform wear. The greater the production through the refiner or the higher the specific energy transfer, the greater will be the wear, as already noted above, on the segment ridges and the easier it generally becomes to find suitable alloys to provide the desired strength balance on the edges and surfaces of the ridges.
When, however, the specific load on the refining surfaces is lighter, a material must be chosen which is less resistant to errosion-corrosion. Otherwise, the wear will be concentrated on the ridges edges which will become rounded after a short time, because the automatic sharpening of the edges as a result of the wear also being distributed over the ridge surfaces would not take place. Since a very large part of the energy is transferred first over the sharp ridges, it will be difficult to transfer sufficient energy for refining the fiber material via the motors of the refiner, and consequently, an unsatisfactory result would be achieved. When it is attempted to compensate for this insufficient refining by reducing the refining clearance which itself renders it possible to transfer more energy to the fibers, difficulties in conveying the fiber material in water through this reduced clearance rapidly arise. In this case, the only remedy is to exchange the segments, because the segment material employed have proven to be too strong. In such case, the effect produced on the ridge surfaces predominantly is that of polishing. In such cases, it is only possible to start up the refining work with these segments because the very sharp ridges of a re-ground segment permits sufficient energy transfer to the fiber materials. Furthermore, a re-ground segment has a certain grinding pattern on the ridge surfaces, which also provides the surface with a certain "roughness" with friction properties resulting therefrom. This grinding pattern, however, is generally rapidly worn off.
It is therefore necessary in each particular application to select a material for the refining segments which is adapted for the purposes in question and from which the segments are to be cast. After casting, the segments are generally ground with a high degree of precision to an accurate profile and correct dimensions before use. The grinding process, however, such as generally carried out, requires that for most materials the surface of the segment ridges is subjected to polishing and therefore covered with a thin layer of material having a structure other than that of the material employed to prepare the segments, this layer being generally of a thickness of some hundreds of millimeters. Thus, while the selected material is generally suitable in all respects for the work for which it is intended to be used, and for the specific energy transfer involved, the ridge surface in ready state nevertheless has been covered by the grinding operation with a layer of polished material, which renders it unsuitable for the energy transfer aspect.
In certain cases, when the production through the refiner is sufficiently great, or the specific energy transfer via the ridges is sufficiently high, this thin layer of polished surface material can be worn off. The higher the specific energy transfer, the more rapidly the working of the polished layer proceeds. During this "adaptation period", substantial process disturbance may arise because the polished layer does not have the properties suitable for the fiber refining as does the material beneath this surface layer. The layer of polished material thus renders the employment of such segments difficult if not impossible.