GRINDING BLOCK FOR A REFINER

Abstract: Grinding block (1) for a refiner, consisting of a sector or part of a sector of a grinding disc or cone made of stainless steel. In known refiners, the worst drawback with the kinds of steel (with a carbon content of 1.2 - 1.5 %) used in the grinding blocks is the brittleness of the material, entailing a low resistance against shocks and tensile stress. According to the invention, this problem is avoided in that at least part of the grinding surface of the block is constituted by one or more detachable segments (2) made of a substantially harder and more durable material than the rest of the grinding block (1), and that the block is provided with means (3, 4) for securing this detachable segment.

Description: GRINDING BLOCK FOR A REFINER The present invention relates to a grinding block for a re¬ finer, consisting of a sector or a part of a sector of a grinding disc or cone made of stainless steel. The most important phase in the process of pulp production is the grinding of the wood material, which is done by refi¬ ners specially designed for the purpose. Among these, the disc refiner, which grinds the wood chips between two discs placed face to face, is the most commonly used type of refi¬ ner, although the conic refiner is also important. Since the grinding takes place under high pressure, with water used for fluidization and lubrication, and since plenty of orga¬ nic acids are produced from the disintegrating wood pulp, the material from which the grinding discs and cones are manufactured deserves .special attention. The grinding ele¬ ments must be hard and durable, but they must also have good resistance to corrosion. Moreover, they must be made of a material that is tough enough to withstand the various strains caused by pressure shocks and other disturbances. The steel material most commonly used in the grinding ele¬ ments is stainless martensite steel containing 14-18% of chrome, depending on the carbon content. Such steel is rela¬ tively inexpensive and hard and allows further heat treat¬ ment and hardening. In addition, it has good resistance to the chemical conditions present in the refiner. However, the worst drawback, especially with the hardest kinds of steel (with a carbon content of 1.2 - 1.5 %), is the brittleness of the material, entailing a low resistance against impacts and tensile stress. Anyway, these steels have provided the best compromise to meet the conflicting requirements, but when e.g. a disturbance in a disc refiner causes the grin¬ ding discs to collide so that the grinding profiles on the disc surfaces suddenly hit against each other, the result is a severe impact that invariably destroys the discs, worth several tens of thousands of FIK. Also, the mere centri¬ fugal force caused by the fast rotation of the heavy disc may produce an excessive tensile stress on the disc if " it is entirely made of very brittle steel. Traditionally, the entire grinding disc or cone used to be made of the same corrosion resistant, hard and durable ma¬ terial required by the pulp refining process. Nowadays, the discs and cones are composed of several so called grinding blocks, which in fact are sectors or parts of a sector of a disc or cone. Further, e.g. US patent 4,423,845 proposes a grinding sector with a locally hardened grinding surface, thus allowing a better control of the refining process. Also, the practice of incorporating various substances, e.g. ceramics, and parts made of such substances, in the desired areas of the grinding element to achieve a better refining result is known from the above-mentioned patent and also from the British patent publication 1,228,046 as well as from the Soviet inventor's certificate 502,993. Although, as implied by the above remarks, the use of diffe¬ rent materials with differing degrees of hardness in the sa¬ me grinding element must be regarded as a known practice, nowhere has a solution been proposed that makes use of this feature so as to decisively lengthen the useful life of the grinding elements or to reduce the down-time in production to a minimum in cases when the refiner is damaged. The object of the present invention is to eliminate the drawbacks mentioned and to provide a new type of grinding block for a disc or conic refiner in which the questions referred to above have been taken into consideration. To achieve this, the grinding block of the invention is cha¬ racterized in that at least part of the grinding surface of the block is constituted by one or more detachable segments made of a substantially harder and more durable material than the rest of the grinding block, and that the block is provided with means for securing this detachable segment. The advantages of the grinding block of the invention inclu¬ de an almost complete immunity to brittle fracture, the pos¬ sibility of partial replacement and renewal of the grinding elements and a high degree of hardness and durability of the detachable segments. Naturally, the detachable parts will best serve their purpose if mounted near the periphery of the grindng block, where the radial velocity is highest and, therefore, the strain greatest. The other advantageous embodiments of the invention are characterized by what is presented in the claims to follow. In the following, the invention is described in greater detail by the aid of examples, reference being made to the drawings attached, wherein: Fig. 1 is a front view of the grinding block of the invention as used in a disc refiner, with two detachable- grinding segments. Fig. 2 shows an embodiment of the grinding block of the invention in lateral view. Fig. 3 shows another embodiment of the grinding block of the invention in perspective. The grinding block 1 shown in fig. 1 is provided with two detachable parts or grinding segments 2, which are made of a substantially harder and more durable material than the rest of the block 1. For instance, the grinding block 1 may be made of stainless artensite steel with a medium carbon con¬ tent (CrSteel 25 or 50, hardness H β = a pprox. 500 ) and the detachable segments 2 of a high-carbon martensite steel (CrSteel 150), which has a high carbide content even if it is hardened, so that the steel is of a very hard quality, H-, = approx. 600. The hardening may be effected on a forged bloom either before or after it is fitted in the grinding block. Other materials and combinations of materials that may be used to produce the detachable segments are non-metallic materials (ceramics) and metals coated with a durable ma¬ terial (ceramic) . Ceramics suiting the purpose are various oxides, carbides and nitrides, several kinds of which are commercially available, nitrides being especially suitable for use as a coating material. The embodiment in figure 2 is provided with means 3 for securing the detachable grinding segments. The grinding block 1 is provided with shoulders 3 which constitute a dovetail joint to secure the segments 2 in a form-locked manner. When several blocks 1 are placed side by side to form a grinding disc, the detachable segments 2 are locked in place in the sideways direction as well, so that no other securing means are needed. Figure 3 shows in perspective an embodiment employing bolts 4 to ensure that the cutter segments are firmly secured. Be¬ cause of the great centrifugal force generated by the fast rotation of the grinding block in practical use, it is al¬ ways preferable to provide the outer edge of the block 1 with a shoulder 3 to support the detachable segment 2, thus enabling the compressive strength of the segment 2 to be effectively utilized while at the same time firmly securing the segment in the grinding block 1. Further, the detachable segments may be secured in the grin¬ ding block by means of a solid joint, e.g. a soldered or brazed one, either as a sole or an auxiliary means of secu- rement. When the block is damaged, this kind of oints are generally also disjoined, so that they will not present a notable obstruction to the replacement of the segments. ' The segments may also be fixed in place by means of an adhesive medium, such as glue. If the detachable segments 2 are to be secured in the manner shown in fig. 2, it is also possible to make the segments from a durable cast iron, such as white martensite cast iron, by casting the segments in the space between the shoulders 3 in the grinding block. In this case it is natu¬ rally necessarry to use a casting mould, which is placed over the grinding block 1. The mould is not shown in the drawing. To facilitate the removal of the cast segments 2, a suitable loosening agent may be applied to the grinding block before the casting is performed. It is obvious to a person skilled in the art that the invention is not restricted to the above examples, but that its embodiments may vary in the scope of the following claims.

Claims: CLAIMS 1. Grinding block (1) for a refiner, consisting of a sector or part of a sector of a grinding disc or cone made of - stainless steel, characterized by that at least part of the grinding surface of the block is constituted by one or more detachable segments (2) made of a substantially harder and more durable material than the rest of the grinding block (1), and that the block is provided with means (3,4) for securing this detachable segment. 2. Grinding block accoding to claim 1, characterized by that the said detachable segment (2) is made of a durable kind of steel, such as stainless martensite steel. 3. Grinding block according to claim 1 , characterized by that the said detachable segment (2) is made of a non- metallic durable material, e.g. a ceramic. 4. Grinding block according to claim 1 t characterized by that the said detachable segment (2) is made of a metal coated with a durable material, e.g. a ceramic. 5. Grinding block according to one of the claims 1-4, cha¬ racterized by that the said detachable segment (2) is essen¬ tially secured in the block (1 ) by means of a form-locked joint. 6. Grinding block according to one of the claims 1-4, cha¬ racterized by that the said detachable segment (2) is essen¬ tially secured in the block (1) by means of separate secu¬ ring means, e.g. bolts (4). 7. Grinding block according to one of the claims 1-3, cha¬ racterized by that the said detachable segment (2) is essen- tially secured in the block (1) by means of a solid joint, e.g. a soldered or brazed one. 8. Grinding block according to one of the claims 5-7, cha¬ racterized in that an adhesive medium, e.g. glue, is used to ensure that the detachable segment (2) is firmly secured in the grinding block. 9. Grinding block according to claim 1 , characterized in that the said detachable segment (2) is made of a durable kind of cast iron, e.g. white martensite cast iron, and cast in place in the grinding block (1). 10. Grinding block according to one of the claims 1, 2 or 5- 8, characterized in that the said detachable segment (2) is forged into the proper shape and then hardened. 11. Grinding block according to one of the above claims, characterized in that it constitutes part of the grinding surface of a disc refiner. 12. Grinding block according to one of the above claims, characterized in that it constitutes part of the grinding surface of a conic refiner.

Inventors: BLOMSTER KARI, VILJAKAINEN ESA, BLOMSTER, KARI, VILJAKAINEN, ESA
Applicants: SUNDS DEFIBRATOR JYLHA OY, SUNDS DEFIBRATOR JYLHAE OY
Application year: 1987
Main classification: D21D1
IPC codes: D21B1, D21D1