A GRINDSTONE

Abstract: The invention relates to a grindstone for production of ground-wood pulp. The grinding surface is formed by grinding grits and a concrete matrix binding together the grits. The matrix comprises a cement mixture, and the hardness of the cement mixture in relation to the grits is so controlled that the grinding surface is self-sharpening during the grinding process. The invention is also concerned with a method for the manufacture of a casting mixture for a grind-stone. The method comprises the steps of mixing together water and a slowhardening, sulphate-resisting low-heat Portland cement mixture, the water/cement ratio being about 0.30 to 0.35; effecting the rheology of the casting mixture by a plasticizing agent instead of excess water; and compacting the mixture by stirring, vibrating, compressing or in some other way.

Description: A grindstone The present invention relates to a grindstone in¬ tended for the production of groundwood pulp, the grind- ing surface of said grindstone being substantially formed by grinding grits and a concrete matrix binding the grits together. In the manufacture of paper pulp it has long been known to use a rotating grindstone cast of concrete for mechanically separating fibres from each other. In prin¬ ciple, such a grindstone is formed as a body of reinfor¬ ced concrete and hard grinding grits embedded evenly therein. Conventional Portland cement has been used as a binder and arenaceous quartz as grinding grits. Normal- ly, a concrete cylinder is attached to a rotating shaft by clamping it at the end faces thereof between two flanges. A grindstone structure of this type has been used throughout the 20th century and it is still partly in use. Pulpwood logs are introduced through a verrical pit against the shell of the grindstone so that they make contact with the shell over the whole length there¬ of due to the weight of the wood contained in the piτ. This process, still in use, is known as a GV_ process. With increasing production requirements, a grind- stone structure of the above type has no longer proved advantageous. For obtaining the necessary strength, the manufacture of the grindstone has to be carried out very carefully; for instance, it is necessary to store a grindstone for two to three years before it is taken in- to use in order that strains caused by the shrinking of concrete would die out. For achieving a sufficient strength, it is also necessary to use relatively great amounts of cement, which, in turns, requires greater amounts of water than usual for achieving a good casta- bility. The excess water deteriorates the streng.n cf the structure and increases the shrinking and the forma¬ tion of strains. The concrete surface of the grindstone wears out relatively rapidly in the grinding process be¬ cause it has not been possible to obtain a cement compo- sition strong enough to hold the protecting grinding grits. As a result of this the grinding grits come off prematurely, i.e. when still capable of grinding, leav¬ ing the softer concrete surface free for wear. Conse¬ quently, the sharp local edges formed on the grinding surface grind the logs rather into "sawdust" than into desired fibres. In addition, a conventional concrete grinding s.iell is not particularly resistant to the ef¬ fect of the water used in the process. Especially sul¬ phate ions contained in the water affect adversely the concrete. For these reasons, it is necessary to sharpen the grindstone, i.e. provide it with grooves of a deter¬ mined shape, even three to five times in a .day. The above drawbacks have become even more criti¬ cal in new methods increasing the production rate. In these methods, the pulpwood logs are introduced and pressed against the shell of the grindstone over the whole length thereof in closed chambers by means of hyd- raulically operated pistons. Fibres so obtained are sprayed off with process water directed to the surface of the grindstone and having a temperature of up to 140°C on account of the increased pressure. The high temperature of the water contributes to the formation of fibres. A grinding process of this type is known as a PGW process, in which the grindstone is subjected to strains considerably greater than in the earlier GW pro¬ cess. The concrete stone described above is practically unusable in a PGW process. For the elimination of these drawbacks, the com¬ position and structure of the grindstone have been under study. As to the first-mentioned, the properties of the material binding together the grinding grits is of cru¬ cial imporrance. From now on, this structural parr of the grindstone will be called a matrix. More specifical¬ ly, a matrix is a physico-chemical structural part of a grindstone that binds together the grinding grits and the possible reinforcements into a strong grindstone functioning as an integral body. In the grindstone de¬ scribed above, the matrix is formed by concrete based on Portland cement. In other known grinding methods, the matrix is formed by ceramic materials. The shell of a concrete grinding drum is thereby covered with grindstone seg¬ ments about 200 x 200 x 300 mm in size. These consist of a matrix bound by sintering, which matrix, in turn, binds together the grinding grits. Since the wearing strength of a ceramic material is considerably greater than that of ordinary concrete, it is economical that the grinding grits embedded in a ceramic grindstone are considerably harder than quartz, such as the known AI2O3, SiC, etc. Ceramic segments are usually fastened to the drum by means of a hook or loop provided therein by embedding the hook or loop in the load-bearing mid portion of the concrete drum. Each segment should be able to receive forces exerted thereon in the peripheral direction, wherefore interspaces between the segments are filled with a flexible and force absorbing material. An advantage of a ceramic grindstone is the high wear resistance thereof. Instead of coming off easily and prematurely, the grinding grits in general stay in place until they wear out. After the grinding grits have worn to the level of the matrix, the grindstone -is no longer effective, and in order to restore the formation of fibres the grindstone has to be sharpened, i.e. pro¬ vided with grooves having a determined shape, similarly as in the case of a concrete grindstone. The sharpening has to be repeated about every other wee r which" is a major improvement as compared with a. concrete stone. Moreover, the ceramic shell is not liable to the" disad- ; vantageous effects of the process water, but withstands well the corrosive effect of the concrets corroding sub¬ stances contained therein. It has also been found out that a ceramic grinding shell retains its circular cross-section better than a concrete grindstone. Unne¬ cessary much material has to be removed from a concrete grindstone for retaining the circular shape. For this reason, among others, a concrete grindstone may wear out in six months, whereas a ceramic shell may endure fcr about two to four years. Despite the high wearing strength of a ceramic grindstone, it does not fulfil all the requirements of the present-day large-scale production. Being of a cera¬ mic material, it is fragile. It cracks easily, and since it is very difficult to replace individual segments, the entire drum has to be replaced after the cracking of a determined number of the segments, For a major part, the' grinding is carried out by means of inferior grindstones decreasing the production rate. For the missing tough¬ ness, a ceramic grindstone does not withstand any greater concentrated or linear loads, such as loads transmitted through the fastening means of the shell during the deformation of the shell. No unnecessary strains are allowed, wherefore the temperature of the grindstone, for instance, has to be elevated to the op¬ erating temperature very cautiously and kept at this temperature rather accurately. Due to the great mass thereof, a grindstone is liable to temperature varia¬ tions, and the unequal coefficients of heat expansion of the concrete body, the ceramic segments and the fasten¬ ing irons thereof cause disadvantageous relative move- ments and strains to occur between the different parts. At worst, the ceramic shell may be wholly stripped cff in case of instantaneous excessive load. In spite of these drawbacks, ceramic grindstones are today used almost without exception in the grinding of wood. The object of this invention is to provide a new and improved grindstone, which combines the advantages of the concrete and ceramic grinding elements without taking over the disadvantages. The grindstone according to the invention is mainly characterized in that the matrix is formed by a cement mixture containing slow-hardening, sulphate-re¬ sisting, low-heat Portland cement, slag cement or the like, strength-increasing silica additive and plasti- cizing agent, and that the grinding grits are cf a mate¬ rial having a hardness - 2,000 Vickers, whereby the hardness of the cement matrix in relation to the hard¬ ness of the grinding grits is so controlled that the grinding surface is at least substantially self-sharpen- ing during the grinding process. The grinding grits preferably consist cf sole aluminium oxide or silicon carbide grits cr conglomera¬ tes thereof. Contrary to ceramic grindstones, the matrix cf the grindstone according to the invention may further contain e.g. metallic reinforcing fibres, which is a ve¬ ry important advantage, giving the matrix an excellent toughness compared to the brittle constitution of cera¬ mic materials. That portion of the grindstone which forms the grinding surface preferably contains slow-har¬ dening, sulphate-resisting low-heat Portland cement, slag cement or the like in an amount of about 25 to 35% by weight, silica fume or the like about 5 to 8% by weight, a plasticizing agent about 1 to 2% by weight, grinding grits about 50 to 65% by weight and reinforcing fibres about 0 to 10% by weight. In the grindstone according to the invention, the grinding surface is preferably formed as segments, simi¬ larly as in ceramic grindstones. Distinct from a ceramic grindstone having a concrete body, the grindstone seg¬ ments according to the invention can be secured to the body portion, e.g. a hollow cylinder of steel, from the outside by means of threaded bars or the like inserted through the segments and embedded therein and pinching nuts attached thereto, whereby it is not necessary to fill the interspaces between the segments. The new type of grindstone concrete matrix ac¬ cording to the invention combines the advantages cf prior concrete and ceramic matrices without the dra - backs of prior matrices. The advantages of the matrix are specified as ollows: - an extremely dense structure, i.e. little if any strength-reducing pores; - a great strength as well as a tough structure; - in the form of a segment, high resistance tc concentrated and linear loads caused by the fastening means thereof; - optimized wear resistance with respect tc the properties of the used grinding grits so that the matrix and the grinding grits wear out in an advantageous pro¬ portional manner with respect to each other for the for¬ mation of a fibre as good as possible without any sharp¬ ening and blunting routines; - resistant to the corrosive effect of the pro- cess water; - separately replaceable segments and the inter¬ spaces at the joints of the shell formed by the segments need not necessarily be filled with a material balancing the forces. These advantaσes have been accuratelv croved in tests carried out. In practice, a property of great im¬ portance to the production is the self-sharpening pro¬ perty of the grinding surface which enables the grinding process to be carried out substantially continuously un- til the grinding segments are thoroughly worn out. There are some highly surprising results probably also to be attributed to the self-sharpening property, i.e. that the specific consumption of energy as compared with- a ceramic grindstone was reduced as much as 20 to 30%, and, in increase of 25 to 40% was obtained in the production; and the so called chest shive content, i.e. the amount of groundwood containing inferior fibres, was re¬ duced about 40%. The self-sharpening property can be explicitly explained by the fact that when the initially sharp grinding grits wear out and get round, the matrix simul¬ taneously wears in an advantageous and structurally con¬ trolled manner so that it releases continuously new grinding grits. In this way the sharp grinding surface advantageous for the fibre formation remains unchanged. This is not possible with a ceramic matrix, because it is throughout too hard. The increase in the production rate, i.e. the increase in the amount of acceptable fibre per time unit, is probably to be explained on the same grounds. The solution according to the invention is based on the main points of cement chemistry well-known per se. When conventional ordinary cement reacts with water, plenty of calcium hydroxide (Ca(0H) 2 ) i formed at the initial stage. This is a reaction product cf tri- calsium silicate (C S) and dicalcium silicate (C ? S) con¬ tained in cement. The content cf the first-mentioned in ordinary cement is about 60% by weight and that cf the last-mentioned about 20% by weight. Another reaction product obtained in these processes is hydrated calcium silicate (3CaO-2Siθ2"3H 2 0) , also known by the name To- bermorit. This is the most important final product of the hardening process, and it determines the properties of the finished concrete. Calcium hydroxide, in turn, has a low strength due to its crystal structure (clea¬ vage planes), and is soluble and very reactive in acidic and sulphatic conditions. The pH of the spray water used in the grinding process is about 4.5 to 5; a relatively high temperature, about 100°C, intensifies the reaction. Consequently, water dissolves calcium hydroxide from the concrete, i.e. pores are formed. Sulphates contained in the spray water are also liable to react with Ca(0H) ? so that an expansive component called ettringite is formed. Other components of cement also participate in the hard¬ ening process, but these are not of primary importance in view of the present invention. In the manufacture of ordinary concrete, the amount of water in relation to the amount of cement is tried to be kept as small as possible for the achievement of good strength proper¬ ties, whereby the rheology of the casting mixture is bad. The amount of water has to be increased fcr improv¬ ing the rheology, which is necessary e.g. in the manu- facture of concrete grindstones. However, excess water that is not needed in the hardening process is formed and gradually evaporated after the hardening, which leads to the formation of strength-reducing pores. In the solution according to the invention the object has been to affect all parametres determining the properties of the matrix. The selected hydraulic binder is inexpensive and advantageous in strength. A well- known binder called LHSR cement with slow strength deve¬ lopment and low evolution of heat is preferably used as a binder. These properties ensure that the state of ini- tial strain formed in connection with the casting nearly nonexistent. The percentages by weight of C S and C 2 S contained therein are about 20% and 60%, respective¬ ly, i.e. the reverse as compared with ordinary cement. For this reason, disadvantageous calcium hydroxide is formed considerably less than in the case of ordinary cement: C 2 S produces this disadvantageous substance in an amount only half of the amount produced by CgS. Due to the low tricalcium aluminate 3Ca0 2 'Al Og content thereof (<C2%), this cement also gives the concrete an excellent sulphate resistance. According to the invention, disadvantageous Ca(0H) 2 is removed in such a way that it is replaced with components reinforcing the matrix, i.e. strong si- licate bridges combining the particles of the matrix. Calcium hydroxide does not only form a cover around the cement particles but obviously also around the grinding grits. The cavities formed therebetween are also partly filled with calcium hydroxide and partly with the excess water which is gradually evaporated. According tc the invention the calcium hydroxide is replaced and the wa¬ ter amount used is considerably reduced and replaced volumetrically with silica fume. Trades names include MICROPOZ" with the composition Si0 2 _86 to 96%; C: £ 2.0%; Fe 2 0 3 : - 2.2%; Na 2 0: - 1.8%; A1 2 0 3 : - 2.2%; K 2 0: - 2.5%; MgO: - 2.0%; and S0 3 : - 1.5% (annealing loss - 3.0%). Silica fume is known per se as a concrete additive, and its grain size is smaller than 0.5 ,um and, accordingly, it has a very large specific area. By virtue thereof, it has a high water binding ability. When metered suitably in relation to the amount of water, it thus binds and removes the water which stoichiometrically is not needed in the hardening process. It reacts with water ana cal¬ cium hydroxide, thus forming strong Tobermorit bridges between the grinding grits. As to the grinding surfaces. similar to crystal surfaces, this mechanism has not yet been proved; on the other hand, it is known that this kind of binding takes place e.g. on the crystal surfaces of metals, so there is reason to assume that the binding mechanism applies to grinding grits as well. In any case it has been found, that by proportioning the amounts of silica fume and water according to the invention to the desired properties of the matrix in question, the weak calcium hydroxide crystals in the matrix so obtained have been converted into Tobermorit bridges. No pores formed by water occur in the matrix; instead, these are replaced with a component which considerably improves the strength properties. The matrix is very compact. Reduction in the amount of water normally dete- riorates the rheology and castability of the mixture to be cast. This is a drawback in that the mixture does thereby not spread efficiently around irregular grinding grains or into the cavities therebetween. Voids filled with water or air may be formed. To avoid this, the con- sistence of the casting mixture is improved by adding thereto a plasticizing agent known per se, sold e.g. under the names PARMIX, MIGHTY, MELMENT, etc. If re¬ quired, it is also possible to carry out the casting cf the grindstone in vacuum by stirring, compressing or vibrating the casting mixture so that air is removed and the mixture is compacted. If needed, it is also possible to add to the mixture suitable, e.g. metallic reinforc¬ ing fibres in order to improve the toughness of the grindstone. This is not possible with ceramic grind- stones, because the metallic fibres would melt. Another improvement according to the invention relates to the structure of the grinding grits. Natural¬ ly, a small grit does not stick to the matrix as well as a big one. Grinding grits of utmost size may, on the other hand, be teared off premataurely, i.e. before they have lost their grinding ability. According to a preferred embodiment of the pre¬ sent invention, this drawback is overcome by using al¬ ternative conglomerates of grinding grits. As said be- fore, ceramics have excellent wear resistance and bind¬ ing features. By sintering together grinding grits with ceramics, the weak binding due to small contact surfaces is compensated by an excellent binder and the weak bind¬ ing to the concrete matrix is compensated by a larger contact surface. The conglomerates are optimally distri¬ buted in the matrix to obtain natural grooves, for εelf- sharpening and for receiving the fibres ground off, as explained before. The self-sharpening of the grindstone is obtained by controlling the proportional wear of the matrix and the conglomerates. Practically, this is achieved by optimally distributing individual grinding grits between the conglomerates to form the desired groove paths. According to a preferred embodiment of the inven- tion the conglomerates are given a rodlike shape. Each rod may then extend through the whole radial depth of the grinding segment or at least along a part of it. In the following the invention will be described with reference to the attached drawing. Figure 1 is a partial view of one embodiment of the invention in a cross-section. Figure 2 is a top view of a part of a grinding surface formed by grinding segments. Figures 3 and 4 illustrate differences between a grindstone according to the invention and a ceramic grindstone in view of the wear of the grinding surface and the energy consumption, respectively. Figures 5 and 6 are strongly enlarged views cf the surface of a ceramic grindstone and the grindstone according to the invention, respectively. Figure 7 illustrates a conglomerate embodiment, including the rodlike alternative, and small grits, dis¬ tributed between these. In principle, the grindstone according to the in- vention could be entirely formed by a one-piece matrix and grinding grits, but this is uneconomical in . most cases. Alternatively, the matrix and the grinding grits. thereof could form a hollow cylinder within which a body portion of ordinary concrete is arranged. It would, also be possible to attach grinding segments to such a body portion similarly as in prior ceramic grindstones. Figures 1 and 2 show schematically an embodiment advantageous over these. The body portion of the grindstone is formed as a hollow steel cylinder 1 which may be closed at the ends and attached to an operating shaft. The grinding surface consists of grinding segments 2 each one of which is fastened to the metal cylinder 1 e.g. by means of two bolts 3 to be inserted through the segment 2 from- the. outside. The bolts 3 are anchored in the cylinder 1 and the segments 2 are tightened in place by means of nuts 4. The nuts 4 are sunk in recesses 5 formed in the sur¬ face of the segments 2, so that the nuts can be reached from the outside, whereby the segments are easily repla- ceable separately, if required. There is no need to po¬ sition a flexible supporting agent or a medium in inter¬ spaces 6 between the segments 2 as in the case of cera¬ mic grindstones, instead, these can be open. The inter¬ spaces 6, similarly as the recesses 5, are thereby able to receive fibres ground off so as to avoid any unneces¬ sary grinding of the fibres on the grinding surface. This is a major advantage over grindstones provided with a concrete body and ceramic segments. When the grinding surface is provided with grooves, fibres ground off will get into these grooves so that they will not be exposed to constant breaking pressure between the grinding sur¬ face and the logs. The spray water flushes them out of the grooves. In this respect a ceramic grinding surface is relatively smooth, and it takes a lot of time and re- quires special equipment to make the grooves sufficient¬ ly deep in connection with the sharpening. The optimized matrix according to the invention enables the grinding surface and the grinding segments to be provided with advantageous recesses for this purpose as early as at the casting stage. The recesses of the fastening bolts, for instance, are advantageous in this respect. During the grinding new grooves are shaped due to the self- sharpening process. Another major advantage of the invention over ce- ramie segments is the direct contact between the fasten¬ ing bolts 3 and the spray water. By virtue of the greater thermal conductivity of the bolts, it is easier to remove any undesired heat from the grindstone struc¬ ture than from a ceramic structure in which the material between the bolts and the spray water has a lower ther¬ mal conductivity. The grinding segments 2 can, natural¬ ly, be positioned arbitrarily with respect to each oth¬ er. One example would be the embodiment similar to a ma¬ sonry wall shown in Figure 2. In the example of Figure 2, the recesses 5 are connected to the interspaces 6 and to each other by means of grooves 7. Of course a lot of other configurations can be obtained. Example Square grinding segments 150 x 150 x 70 mm in si- ze and having a matrix according to the invention were manufactured. Each segment was fastened to a cylindrical steel body by means of two bolts going through the seg¬ ment in such a way that a cylindrical grinding shell having a thickness of 70 mm, and outer diameter of 980 mm and a length of 660 mm was formed around the drum. No kind of medium for balancing the peripheral forces was provided in the interspaces formed at the joints. This grindstone was rotated in a PGW grinding process at a peripheral speed of 35 m/s, while the temperature of the spray water was 105°C and the pressure exerted on the logs 250 kPa. The composition of the stone was as follows: Cement 33% by weight Grinding grits, Al 2 0g 58% " Silica fume (MICROPOZ) 8% Plasticizing agent 1% " Figure 3 illustrates the self-sharpening property of the grindstone according to the invention as compared with a ceramic grindstone in a set of time-production rate coordinates. The unbroken curve 8 represents a stone according to the invention and the broken-line curve 9 a ceramic stone. Initially, i.e. after the sharpening, a ceramic grindstone requires a "running-in" of a couple of days, during which time the production rate is low. After a fully efficient grinding for about two weeks, the groundwood begins to get excessively fine, on account cf which the grindstone has to be sharpened again. The initial time required for the running-in of the grindstone according to the invention is practically nonexistent, the production rate is clearly higher than that of a ceramic grindstone and tests carried out by now confirm that no resharpening is needed. When considering the features of the object of invention it should also be kept in mind, that the steel/concrete combination exhibits a good interaction as to relative movements between the integral parts. It is well known from the structural engineering that the concrete and the steel reinforcement have almost the same coefficient of thermal expansivity. This means, that the grinding stone according to the invention is not sensitive to temperature shocks or temperature vari¬ ations in general, as consisting of concrete and steel only. A ceramic stone, on the contrary, is hypersensiti- ve to temperature variations and thermal stress, since it consists of materials not interacting in an advan¬ tageous way. This explains the careful treatment of ce¬ ramic grinding stones, mentioned on page 4. Figure 4 illustrates, in more detail as compared with Figure 3, the obtained energy savings, i.e. the in¬ crease in the production rate in a set of coordinates representing the specific energy consumption and the de¬ gree of fineness. The unbroken curve 10 represents the new grindstone, the broken-line curve 11 a ceramic grindstone. The energy consumption per one ton of groundwood is about 20 to 25% smaller when using the grindstone according to the invention. Figure 5 illustrates the surface of the ceramic grindstone and Figure 6 the surface of the grindstone according to the invention in a strongly enlarged view. The matrices are designated with the reference numerals 12 and 13, respectively; the grinding grits with the re¬ ference numerals 14 and 15; and the fibres with the re¬ ference numeral 16. In Figure 5, the dark areas designa- tes with the reference numeral 17 represent empty space, which does not occur in Figure 6, illustrative of the invention. In Figure 7, a part of a curved segment sur¬ face, the conglomerates are marked with a reference nu¬ meral 18, the rodlike alternatives with 19 and the wear controlling individual grits with 20. 21 designates in¬ dividual grains in the conglomerate and 22 the ceramic binder. 23 designate grooves.

Claims: Claims 1. A grindstone intended for the production σf groundwood pulp, the grinding surface of said grindstone being substantially formed by grinding grits and a con¬ crete matrix binding together the grits, c h a r a c ¬ t e r i z e d in that the matrix is formed by a cement mixture containing slow-hardening, sulphate-resisting, low-heat Portland cement, slag cement or the like, strength-increasing silica additive and plasticizing agent, and that the grinding grits are of a material having a hardness - 2,000 Vickers, whereby the hardness of the cement matrix in relation to the hardness of the grinding grits is so controllled that the grinding sur- face is at least substantially self-sharpening during the grinding process. 2. A grindstone according to claim 1, ch r ¬ a c t e r i z e d in that the cement mixture is of a. low-heat, sulphate-resisting kind, the tricalcium sili- cate content thereof being no more than about 30% by- weight and the tricalcium aluminate content no more than about 2% by weight. 3. A grindstone according to claim 1, c h a r ¬ a c t e r i z e d in that the matrix further comprises reinforcing fibres, preferably metallic fibres. 4. A grindstone according to claim 1, c h a r ¬ a c t e r i z e d in that the grinding grits comprice grits of aluminium oxide or silicon carbide. 5. A grindstone according to claim 1, c h a r - a c t e r i z e d in that the grinding grits comprice conglomerates of distinct grinding grits bond together. by ceramics. 6. A grindstone according to claim 5, c h a r ¬ a c t e r i z e d in that the conglomerates are of a rodlike shape throughout the whole depth of the grinding segment, or at least a part thereof. 7. A grindstone according to any of the claims 1 to 6, c h a r a c t e r i z e d in that the portion thereof forming the grinding surface comprises slow-hardening, sulphate-resist¬ ing low-heat Portland cement, slag cement or the like about 25 to 35% by weight silica fume or the like about 5 to 8% " plasticizing agent about 1 to 2% " grinding grits about 50 to 65% " reinforcing fibres about 0 to 10% " 8. A grindstone according to the claims 5 to , c h a r a c t e r i z e d in that wear controlling grinding grits are distributed between the conglome- rates. 9. A grindstone according to claim 1, c h a r ¬ a c t e r i z e d in that the grinding surface thereof comprices segments (2) fastened to a body portion (1) of the grindstone from the outside by means of bolts ( 3 ) or the like inserted through the segments (2) and sunk therein. 10. A grindstone according to claim 9, c h a r ¬ a c t e r i z e d in that the body portion of the grindstone is a hollow metal cylinder ( 1 ) . 11. A grindstone according to claim 9, c h a r ¬ a c t e r i z e d in that there is no medium provided in interspaces (6) defined between the segments (2) . 12. A grindstone according to claim 11, c h a r ¬ a c t e r i z e d in that grooves (7) are formed on the surface of the segments (2) for joining the sinking re¬ cesses (5) of the bolts (3) or the like and the inter¬ spaces ( 6) defined between the segments. 13. A grindstone according to claim 1, c h a r ¬ a c t e r i z e d in that the grinding surface thereof is integral. 14. A method for the manufacture of a casting mixture for a grindstone intended for the production of groundwood pulp, c h a r a c t e r i z e d by the steps of mixing together water and a slow-hardening, sulphate-resisting low-heat Portland cement mixture hav¬ ing a tricalcium silicate content of no more than about 30% by weight and a tricalcium aluminate content of no more than about 2% by weight so that the water/cement ratio is no more than about 0.30 to 0.35; . effecting the known effect of excess water neces¬ sary for the consistence of the casting mixture by a plasticizing agent instead of said excess water; and compacting the mixture in connection with the casting by stiring, vibrating, compressing or in some other way. 15. A method according to claim 14, c h a r a c ¬ t e r i z e d in that the mixing and casting of the mixture is carried out in a vacuum.

Inventors: KEPPO MARTTI, NURMI TOIVO, PALM CARL-OLOF, SUNDEN FRED, KEPPO, MARTTI, NURMI, TOIVO, PALM, CARL-OLOF, SUNDEN, FRED
Applicants: PARTEK AB, OY PARTEK AB
Application year: 1987
Main classification: B24D3
IPC codes: B24D3, B24D18, C04B14