METHOD AND APPARATUS FOR SHARPENING THE SURFACE OF A GRINDSTONE IN A PULP GRINDER

Method and apparatus for sharpening the surface of a grindstone in a pulp grinder The object of the invention is a method of sharpening the surface of a grindstone in a pulp grinder, wherein material is removed from the surface of the grindstone, from part of the surface of the grindstone at a time, and essentially from the whole surface area.

Another object of the invention is an apparatus for carrying out the method, comprising sharpening means for removing material from the surface of the grindstone, from part of the surface area at a time.

In pulp grinders fibre is detached from wood by pressing wood material, such as blocks of wood, chips, etc., against the surface of the grindstone rotating in its longitudinal direction, simultaneously spraying water thereonto, whereby the combined effect of grinding by a grindstone and softening by water is that wood fibres become detached from the blocks, formed ing a pulp suspension with water. Although the grindstone is much harder than wood fibres, the surface of the grindstone wears slightly all the time during grinding. This changes the surface pattern and roughness of the grindstone, whereby the grinding capacity and properties of the grindstone change.

Consequently, the properties of the fibres formed and accordingly the properties of the pulp suspension change at longer intervals, whereby the usefulness of the pulp e.g. in the manufacture of paper and the properties of the paper produced vary. In order that these drawbacks might be avoided, the grindstone is conditioned by sharpening, which in this case means that material is removed from the surface of the grindstone to give it the desired quality.

Conventionally, this is carried out by passing a burr along the surface of the grindstone, the burr being pressed against the surface of the grindstone while said grindstone is being rotated. Consequently, material becomes detached from part of the surface of the grindstoner i.e. from the contact area of the burr and the grindstone, and thus by moving the burr in the axial direction of the grindstone as said grindstone rotates, material can be removed from the whole surface area of the grindstone. A blunted grindstone can be resharpened with a burr of a suitable shape. A solution of this kind is known e.g. from Finnish Patent 26,584.

The drawback of the known solutions is that during the sharpening stage the burrs not only remove mineral aggregate as they rotate but also break abrasive grains, whereby the edges of the surfaces of the broken abrasive grains are very sharp and almost function like knifes. Consequently, after the sharpening the pulp produced by the grindstone has shives and contains a large number of short broken fibres, which impairs the usefulness of the pulp produced immediately after the sharpening. Therefore, burring is avoided and carried out at moderately long intervals. From this follows that the so called freeness value, which typically describes variation in the properties of pulp, varies significantly between two sharpenings.

The purpose of this invention is to provide a method and an apparatus for sharpening a grindstone, by which method and apparatus sharpening can be carried out easily and simply and breaking of abrasive grains can be prevented, whereby variation in the properties of pulp caused by sharpening can be essentially limited from what it is now. It is characteristic of the method of the invention that at least one water jet is directed to the surface of the grindstone, the feed pressure of the water jet being so high that the water jet detaches from the surface of the grindstone bonding material found between abrasive grains, and that the whole surface of the grindstone is treated at the entire width thereof by a water jet of this kind while the grindstone is being rotated.

It is characteristic of the apparatus according to the invention that the sharpening means comprise at least one nozzle and a high pressure pump arranged to pump a high pressure water jet through the nozzle against the surface of the grindstone while the grindstone is rotated during the sharpening thereof.

The essential idea of the invention is that the surface of the grindstone is sharpened by subjecting it to water jets affecting a small surface area and having such high pressure that bonding material found between abrasive grains becomes detached from the surface of the grindstone, whereby the abrasive grains come better into sight and, when sufficient bonding material has been removed, the utmost and worn abrasive grains become detached.

According to a preferred embodiment of the invention, the water jet or jets are moved during the sharpening while the grindstone rotates slowly in the direction of the axle thereof, whereby the whole grindstone can be sharpened in the same manner and the sharpening can be carried out by a suitable embodiment of the apparatus even during the grinding, which simplifies the sharpening and eliminates the break in production normally needed for sharpening.

It is previously known to spray water onto the grindstone and the grinding zone at a pressure higher than the pressure prevailing in the grinder, as disclosed e.g. in Finnish Patent 68,433. However, this water is intended to enhance grinding and has no effect on the sharpness of the grindstone since according to the method of this reference the grindstones must be sharpened regularly by a known burring method.

By the method and apparatus of the invention, sharpening of the grindstone can be carried out often, removing each time a small amount of the surface layer yet without breaking abrasive grains during the sharpening. The freeness value of pulp brought about by sharpening changes less than in burring and the sharpening can be carried out more often, whereby the overall variation in the freeness value can be maintained significantly smaller than in burring. Consequently, both the quantity and quality of production can be maintained more even than is conventional.

The invention is described in greater detail in the attached drawings wherein Figure 1 shows a schematic side view of sharpening by a water jet, Figure 2 shows a top view of the sharpening depicted in Figure 1, Figure 3 shows a schematic view of controlling of water jet sharpening, Figure 4 shows schematically the effect of water jet sharpening and, on the other hand, of burring on the freeness value of pulp, Figure 5 shows schematically the effect of water jet sharpening on the production rate of a grinder, Figure 6 shows the freeness value of pulp as a function of time in both burring and water jet sharpening, measured in connection with a practical test, and Figure 7 shows the sharpness index of a grindstone as a function of time in both burring and water jet sharpening, measured in connection with a practical test.

Figure 1 shows a grindstone 1 rotating about an axle la. Today on the surface of the grindstone 1 is typically an actual grinding surface ib consisting of grinding blocks made of ceramics or a ceramic mixture or the like, the grinding surface grinding fibre off the wood. In the manner disclosed in the invention, a high pressure water jet 2 is directed onto the surface of the surface layer ib of the grindstone from a nozzle 3. The nozzle 3 is moved by a separate transfer apparatus or transfer means 4 in the direction of the axle of the grindstone 1 in order that the water jet might be directed onto the whole surface of the grindstone 1 in a desired manner and at a desired angle.

From the transfer apparatus 4 a water pipe 5 leads to a high pressure pump 6, which raises the pressure of the water coming from the water feed point, such as a container 7 or the like, sufficiently high. On testing the method of the invention, a water jet was used in which the distance between the nozzle and the surface of the grindstone was about 2 cm and the width of t-he water jet at the point where it hits the surface was from 2 to 2.5 cm, the thickness of the water jet being about 0.5 cm. The values of water feed pressure used in the tests varied between 200 and 2,500 bar, the typical quantity of water supplied being about 5 1/mien. Although in the tests the water jet had a kind of chisel-shaped cross section, this is not in any way necessary but water jets of any other shape can also be used.

Figure 2 shows a top view of the apparatus according to the figure, showing how the surface layer 1b of the grindstone 1 consists of separate blocks. The transfer apparatus 4 is arranged such that the nozzle 3 and thereby the water jet 2 extend over the entire width of the grindstone 1. For grinding, the nozzle 3 is positioned within a certain distance, whereby the shape of the water jet and the kinetic energy generated by the feed pressure are sufficient and suitable to detach bonding and filling material found between abrasive grains such that the abrasive grains come better into sight and worn abrasive grains are capable of becoming detached from the surface lb of the grindstone.

The transfer means or transfer apparatus 4 presented in Figures 1 and 2 can be any kind of apparatus known per se, having a cross beam 4a in the axial direction of the grindstone extending over the width thereof, and a carrier or other such member 4b moving therealong, the actual nozzle or nozzles 3 being attached thereto to move along the transfer beam 4a therewith. The carrier 4b can be moved along the transfer beam 4a either by means of a feed screw, a chain mechanism, a cylinder, some other motor or transmission solution, or in any other manner generally known per se. Further, the nozzle or nozzles 3 can be attached to the carrier 4b to move transverse to the axle of the grindstone 1, whereby the distance of the nozzle 3 from the surface of the grindstone 1 can be adjusted in a desired manner.

The nozzles 3 can be attached movably with respect to the carrier 4b in any manner known per se, by means of glide, screw, etc. structures, and the nozzles can be arranged to be moved either manually or automatically by means of a power unit. Since structures like this are generally known per se, and so are the structures and operation of the apparatuses suitable for use as transfer apparatuses, it has not been considered necessary to describe either of these any closer herein.

Figure 3 shows schematically automatic or almost automatic sharpening of a grindstone by a grinder such that the sharpening can be carried out at suitable intervals. In a grinder 8 wood 9 is pressed against the grindstone 1 by means of plungers 10 in a manner known per se while water is sprayed conventionally onto the grinding zone, whereby pulp 11 is formed. Instead of having a feed apparatus based on the plunger structure disclosed above, the grinder can naturally also be provided with any other known wood feed mechanism, such as different chain feed structures or screw feed solutions. From the pulp 11 and the grinder 8 are measured different factors, such as a production rate 12, a pressure 13 of hydraulic fluids of the cylinders pressing the press plungers 10, power of a motor 14 (not shown) rotating the grindstone, and a freeness value 15 of the pulp 11.

In addition, the values of measurement 16 obtained in the measuring of pulp 11 carried out in a laboratory and the set and threshold values-17 defined for the operation of the apparatus are combined e.g. by means of a control means 18, obtaining a control signal 19 for sharpening the grindstone 1 by means of a water jet 2. On the basis of this, the pressure pump 6 is started and sharpening by means of a water jet 2 by moving the nozzle 3 is carried out until the values measured indicate that the grinder is in the desired area of operation and the sharpening can be stopped.

Further, sharpening can be carried out by treating the surface of the grindstone for a predetermined time, whereafter grinding is continued and the freeness value and the other values to be measured are monitored, whereafter this kind of periodic sharpening can be repeated at suitable intervals, e.g. in about every two hours, until the desired values of operation are reached as a result of sharpening.

Figure 4 shows schematically the variation in the freeness value of the pulp produced by a grinder, in both burring and water jet sharpening. Figure 4 shows the line A, which forms a sawlike pattern, illustrating as a function of time the change in the freeness value of pulp brought about by burring. The line B illustrates the desired value or set value where the freeness, in theory, should be, and the curve C illustrates as a function of time the variation in the freeness value in water jet sharpening. As illustrated by the curve A, the freeness value increases heavily in burring, whereafter it decreases fairly steadily as the grinding progresses until the grindstone again needs sharpening.

In practice the interval between sharpenings is several days, typically about two weeks, during which time the freeness value may vary e.g. i 30 - 40% of the mean value. This affects significantly the properties of the pulp produced and the range of variation is quite significant. In water jet sharpening illustrated by the curve C sharpening can be carried out at shorter intervals, which means that water jet sharpening can be carried out e.g. at an interval of two or three days. Since in burring extra material must be in practice removed from the surface of the grindstone in order that one might be sure of the result of the sharpening, water jet sharpening, which removes essentially less material, makes it possible to repeat sharpening more often without decreasing the operating life of the grindstone.

As illustrated by the curve C, in water jet sharpening the roughness and sharpness of the grindstone remains such that the variation in the freeness value of pulp is e.g. about '-5t. Accordingly, the properties of pulp can be quite well maintained as desired and consequently the properties of the paper manufactured from the pulp, more uniform.

Figure 5 shows schematically the effect of burring and water jet sharpening on output obtained by a grinder, the curve D illustrating the output obtained by burring as compared with the theoretic maximum output, the line E illustrating schematically the theoretic maximum output, and the curve F illustrating the output obtained by water jet sharpening. The curves in Figure 5 illustrate a situation where the maximum output obtained by sharpening is set to correspond to the the oretic maximum output E. The output brought about by the sharpness reached by burring, illustrated by the curve D, is very wavy, and between the line E and the curve D remains a biggish area nP, which represents the loss in production brought about by incapability of maintain- ing the production high owing to the properties of the surface of the grindstone.

This is due to the fact that on attempting to maintain the freeness value of pulp within the limits desired, one has to adjust the quantity produced to the properties of the surface of the grindstone. The curve F in turn illustrates the output obtained by water jet sharpening as compared with the theoretic maximum output E. As illustrated by the curve F, variation in output is very little in water jet sharpening and only slightly wavy. Consequently, when water jet sharpening is used for a grinder, the areas LP between the curves D and F marked with lines can also be utilized, the areas representing an addition in output obtained by using water jet sharpening and thus the advantages of water jet sharpening over burring. The measure T in the figure, like in Figure 4, illustrates a normal period of time between sharpenings.

Figure 6 shows a measuring result obtained, in practice, of the variation in the freeness of pulp when sharpening is performed with a burr on the one hand and by a water jet on the other hand. Figure 6 shows that when burring is used (curve G), the freeness varies between the values 225 and 110 (ml) between two sharpenings. When water jet sharpening is used (curve H), the freeness varies, according to the curve, from about 135 to 155 (ml), the variation being less than 1/5 of the variation in the freeness brought about by burring.

Figure 7 shows in the corresponding manner the calculatory sharpness index of the grindstone, the value being known per se to one skilled in the art, in burring on the one hand and in water jet sharpening on the other hand. As shown in Figure 7, the sharpness of the surface of the grindstone varies between the values 0.7 and 0.45 when burring is used (curve I), being at it is greatest after the sharpening and decreasing considerably toward the following sharpening. Consequently, immediately after the sharpening the edges of the abrasive grains of the grindstone are sharp and tend to cut the fibres instead of detaching them longer and thus more suitable for manufacture of paper.

When water jet sharpening is used (curve J), the sharpness index varies between 0.48 and 0.55, whereby cutting sharpness of the type occurring in burring is essentially absent and accordingly the grindstone can be used more effectively on the basis of the information given above.

The specification and the drawings describe the invention only by way of example. The invention can be applied in different ways, e.g. by using one or more jet nozzles to achieve the desired sharpening rate and effect. It is characteristic that essentially the whole surface of the grindstone is sharpened in the same way in order that the sharpening result might be even. Further, since it is typical that grooves are provided on the surface of the grindstone, essentially evenly on the surface area, to improve the grinding properties of the grindstone and to facilitate removal of pulp and water, a corresponding grooving can be provided, if necessary, on the surface of the grindstone by the sharpening apparatus of the invention. Likewise, use of different pump and nozzle structures in the manner described in the invention is possible within the scope of the claims.

Instead of mere water jet feed, it is naturally possible to feed different abrasive or otherwise suitable solid particles in the water jet to intensify the sharpening, but this is-not in any way necessary.