WO1991009707A1

WO1991009707A1 - Grindstone

Info

Publication number: WO1991009707A1
Application number: PCT/FI1990/000313
Authority: WO
Inventors: Carl-Olof Palm
Original assignee: Oy Partek Ab
Priority date: 1989-12-29
Filing date: 1990-12-27
Publication date: 1991-07-11
Also published as: FI896334A0; FI82491B; AU6908991A; FI82491C

Abstract

The object is to provide a novel grindstone intended for the production of woodpulp, wherewith groundwood pulp of better quality than hitherto is obtained. The grinding surface of the grindstone includes grinding grains bound to a concrete matrix, sintered together from so called grits. The individual grits in the grinding grains have rounded edges.

Description

Grindstone

The invention relates to a grindstone intended for the production of woodpulp, the grinding surface of said grindstone essentially comprising grinding grains bound to a matrix and sintered together from so called grits.

The structural elements called grits herein¬ below are onograins, i.e. fine crushed aggregate of a mineral, usually aluminum oxide or silicon carbide. Thus they are not composed of smaller structural ele¬ ments. On account of the crushing, they are very sharp-edge .

As grinding grains are called hereinbelow clusters produced synthetically from grits. In the professional literature, such terms as conglomerate, agglomerate, hybrid etc. are used.

Thus, grit hereinbelow is the smallest indiv¬ isible element of the actual grinding material. Today grindstones usually have a matrix of a ceramic material, but the matrix may also be of con¬ crete; Finnish Patent 75 611 discloses an advantage¬ ous grindstone with a concrete matrix, the grinding surface of which can be regarded as self-sharpening. Ceramic grindstones must be smoothed and sharp¬ ened prior to being taken into use and later at pre¬ determined intervals, in which connection grooves of a desired depth are produced in the grinding surface with a special tool. When these grooves are produced, the portions of the grinding surface which are loc¬ ated between the grooves and which actually grind the wood become so coarse that they must be separately ground fine prior to the use of the grindstone for groundwood pulp production, or one has first to ac- cept poor-quality groundwood pulp for a period of time. At any rate, when the portions of the grinding surface located between the grooves gradually wear away to the level of the bottom of the grooves during the grinding process, new very sharp-edged grits will emerge in the grinding surface, which for some time, before appropriate wearing out, will disconnect wood fibers in an undesired manner by tearing.

The grindstone according to Finnish Patent 75 611 has the significant advantage over ceramic grindstones that the expensive sharpening of the stone is eliminated, but despite self-sharpening the grits coming up in the grinding surface are likewise initially too sharp.

The shives torn off by the sharp-edged grits namely consist of several fibers and are therefore too large, whereby difficulties will arise in sus¬ taining homogeneous woodpulp. Only when the grits have worn off to such an extent that they will in a way knead the fibers off one another, they will take part in the production of useful fiber. Since, how¬ ever, new sharp-edged grits constantly come up in the grinding surface as the surface wears off, this res¬ ults in that a certain amount of shives will also be produced continually. One has sought to alleviate this problem by blunting the grits for instance by means of a calcium sand tile which has been drawn reciprocally over the grinding surface after the sharpening. However, this is a rather futile proce¬ dure in view of the time spent and the result. The grits are so hard that the effective blunting thereof requires an unreasonably long time.

The object is presently to provide a novel grindstone eliminating the above-stated drawback.

The grindstone of the invention is mainly char- acterized in that the grinding grains include grits the edges of which have been rounded prior to the binding thereof to the matrix. Preferably the dis¬ tribution of the grits in the grinding surface cor¬ responds to a grinding frequency of about 50 kHz. With the grindstone of the invention, the op¬ timum condition is achieved during the entire grind¬ ing process according to the prevalent grinding the¬ ories, in which condition the fibers of the wood are subjected to moderate impacts of compression and ex- pansion at a frequency of about 50 kHz, so that the lignin in the wood is softened at an elevated temper¬ ature and the wood fibers are disconnected without being damaged. The grits having rounded edges in a way knead or shiver the fibers so that the temper- ature of the wood material increases and the lignin softens. According to experiments, this is most ad¬ vantageously achieved when the distribution of the grits and the rotating speed of the grinding drum are so selected that the impact frequency for the grits is approximately the above-stated 50 kHz. With the velocities employed today, there are about 2 grits for one centimeter. The preferred radius of curvature of the grit is of the order 0.1 mm, and the preferred grain size thereof is 60-80 mesh, i.e. 0.4-0.27 mm. The grinding grain employed in the grindstone according to the invention may be produced from sharp-edged grits. Sharp-edged grits of a suitable size and quality are selected. By directing grits of this kind at a suitable speed through a plasma flame or other hot flame, the sharp edges of the grits can be melted so that they are rounded off. Such a method can be applied e.g. to the sharp-edged grits employed in accordance with WO publication 89/05712.

The grits obtained which have rounded edges are sintered into grinding grains according to the inven- tion, which grinding grains are preferably bound to a matrix of compacted concrete. The combination has the appearance of the combination described in WO pub¬ lication 89/05712. However, the important and de- cisive difference is now that the grits have rounded edges from the start and that the grinding grains produced therefrom by sintering are embedded in a concrete matrix in order to attain the above-stated impact frequency of 50 kHz. The grinding grains ac- cording to the invention are about 5-10 mm in length and have a diameter of about 2-4 mm at the thickest part thereof. The grits are sintered to one another very strongly on account of ceramic binding, and the binding of the entire grinding grain to the concrete matrix is also very strong owing to the greater sur¬ face and the irregular shape of the grinding grain. Individual grits having rounded edges cannot be dir¬ ectly embedded in the concrete matrix, since they would not stick to the matrix on account of their small surface. Yet this fact is utilized in that by means of such individual grits having rounded edges, disposed between grinding grains and fixεd to a matrix, the grooves produced between the grinding grains can be optimized so that they advantageously serve as flush-away passages for the disconnected fibers.

Claims

Claims:

1. A grindstone intended for the production of woodpulp, the grinding surface of said grindstone essentially comprising grinding grains bound to a matrix, said grinding grains comprising so called grits sintered together, c h a r a c t e r i z e d in that the grinding grains include grits the edges of which have been rounded prior to the binding thereof to the matrix.

2. A grindstone as claimed in claim 1, c h a r a c t e r i z e d in that the distribution of the grits in the grinding surface corresponds to a grinding frequency of about 50 kHz.

3. A grindstone as claimed in claim 1, c h a r a c t e r i z e d in that the grinding grains are about 5-10 mm in length and about 2-4 mm in diameter at the thickest part thereof, the effect¬ ive diameter of the grits having rounded edges being 0.4-0.27 mm and the radius of curvature of the edges being about 0.1 mm.

4. A method of producing grinding grains for use in a grindstone as claimed in claim 1, c h a r a c t e r i z e d in that initially sharp- edged grits are directed through a plasma flame or other high-temperature field to melt the sharp-edged parts and thus to produce rounded grinding surfaces, prior to the sintering of the grits together into grinding grains.