FR2507101A1 - Granular abrasive particles mfr. - by forcing mixt. of abrasive particles, filler and liq. binder through sieve, hardening and passing through sieve - Google Patents

Abstract

Granular abrasive particles are made by mixing abrasive particles, inert and/or active fillers and liq. binders and forcing the mixt. through a sieve of defined mesh size. The resulting moist cylindrical granulate of uniform dia. is dried and cured, then the dried and cured granulates are forced through a sieve of at least equal mesh size. Suitable abrasive materials are corundum, zirconium, corundum or silicon carbide. Typical active filler with abrasive action is cryolite. Suitable inert filler is chalk. The binder is e.g. a heat-curable phenolic, urea or melamine formaldehyde resin, epoxy resin, alkyd resin, unsatd. polyester resin or urethane resin. Process allows the prodn. of abrasive granules of uniform size without problems of dust formation and atmospheric pollution with solvent vapours associated with prior art processes. The prods. may be bonded to a suitable substrate e.g. in the prodn. of flexible prods.

Description

Process for producing granules of abrasive grains.

The subject of the present invention is a process for the production of granules of abrasive grains constituted by abrasive grains of corundum and / or of zirconium-corundum or of silicon carbide linked by a binder in synthetic resin, preferably a phenol-formaldehyde resin, with the addition of fillers with an abrasive action, such as for example cryolite, and / or with inert behavior, such as, for example, chalk.

It is known from U.S. Patent No. 2,194,472 to manufacture a flexible abrasive, the support of which is packed with an agglomerate of abrasive grains formed from a large number of individual grains. The actual manufacturing process for the aggregate particles is that the individual constituents such as the abrasive grains, the binder and the fillers are mixed, hardened into a compact mass which is finally ground and the pieces are removed by sieving.

Independently of a significant release of dust during grinding and sieving, the yield of agglomerate particles of suitable particle size usable for the formation of the coating layer is economically too low. According to the applicant's checks, the yield of usable agglomerate particles is, with this manufacturing process, below 25% of the quantity used. A process capable of technical application is not indicated. The teachings of US Pat. No. 2,194,472 have therefore found no application in practice. On the other hand, it has been described in the German published application No. 2,608 .273 a flexible abrasive element in the form of a sheet or strip on the support of which are anchored abrasive bodies in the form of balls which are constituted by a large number of abrasive grains which are linked by a binder mixed with fillers. This roughly corresponds to what was known by US Patent No. 2,194,472. The production of a ball-shaped abrasive body from a large number of abrasive grains, a binder and fillers according to demand published German NO 2,608,273, is characterized in that the abrasive grains and the binder forming the matrix in the liquid state are dispersed in an organic solvent phase - preferably perchlorethylene - and are kept in suspension therein until spherical abrasive grains are formed which are stabilized in their spherical form by hardening of the binder forming the matrix.

Regardless of the fact that this method corresponds to the method described in German published application No. 2,447,520 for the formation of spherical bodies bonded by a synthetic resin, the production of abrasive bodies with a uniform diameter usable to form the coating layer and under the quantity necessary for the technical application, presents significant difficulties so that this process also has not hitherto been implemented on a technical scale. In addition, significant pollution of the process and of the environment resulting from the vapors of the organic solvent must be taken into account, in particular during the discharge of highly porous abrasive bodies from which the organic solvent must be evaporated.

The object of the present invention is to provide a process which makes possible the advantageous production on an industrial scale of agglomerated particles containing abrasive grains (granules of abrasive grains) of an appropriate size without having the drawbacks of a low yield and pollution of the environment by dust or solvent vapor.

It is in fact moreover already known that, in avoiding the above-mentioned drawbacks, agglomerates of abrasive grains are obtained in a sittpie manner when they are pressed or compressed through a sieve cloth or a perforated sheet with a defined mesh or hole size, with the aid of a steel bar, for example a squeegee, a watery aqueous pasty mixture of abrasive grains and a binder, preferably an aqueous liquid phenol-formaldehyde resin, and then dries and hardens, in a heating flue, the cylindrical agglomerated particles extruded through the sieve.

For the production of a cylindrical aggregate, the principle of the production of the aggregate with steel bars, respectively squeegees, on a flexible screen fabric is implemented from the point of view of the apparatus with the bars of steel scraping the screen arranged at the periphery of a cylindrical rotor parallel to the axis of rotation, the screen fabric being stretched to surround the lower part of the rotor in a semicircle so that the mixture of abrasive grains to be granulated can be The driven bar rotor brushes through its bars, during rotation, the screen cloth and thereby presses through the screen cloth the mixture of abrasive grains, moistened with the binder. which is fed therein, whereby cylindrical granules fall continuously which can be dried and hardened in a closed flue heating pipe. This mechanical installation has the drawback, however, that both the rigid steel bars of the rotor and the mesh of wire or plastic wire are worn relatively quickly due to the abrasive action of the mixture of abrasive grains. . During dry granulation carried out using the same device for grinding and sieving the dried cylindrical granular particles, wear is also observed to be high.

In addition, it would be particularly desirable to obtain a flow rate of aggregates of approximately 150 kg per hour in order to reach the quantities necessary on an industrial scale and this with a particle size as uniform as possible. It has now been surprisingly found that with another construction of the rotor bars, not only the wear of the bars and the screen cloth was greatly reduced but also the yield per unit of time could be increased. 3 to 4 times, that is to say carried between 500 to 600 kg of granules per hour.

This considerable technological improvement is obtained by the fact that the rigid bars scraping the screen cloth are replaced by corresponding dimensioning tubes mounted with free rotation on a shaft (round bar). Therefore we succeed in transforming the scraping movement of the bars on the sieve fabric which generates wear in a rolling movement on the sieve fabric corresponding to the speed of rotation of the entire rotor. This rolling rotor allows a significant reduction in wear and an increase in the aggregate yield from 3 to 4 times. In this embodiment the process allows the production on an industrial scale of granules of abrasive grains of all dimensions so that with this technical progress for obtaining granules of abrasive grains one can find for the first time a 'access applicable on an industrial scale for abrasive agents based on these aggregates.

The uniformity of particle size can be further increased by another improvement in the process whereby the dried and hardened aggregate particles are pressed through a screen cloth having at least the same mesh width as that used for wet granulation , whereby the cylindrical aggregate is broken up into shorter particles and at the same time sieving takes place based on the size desired for the aggregate. The yield in dimensions of particles usable directly for coating corresponds to 85 to 95%, that is to say is situated at a level several times higher than with the other methods known hitherto. Furthermore, it is easily possible, by modifying the mesh width of the sieve fabric chosen for use, to vary the granulometry of the aggregate as necessary or at will.

The reduction in wear during this dry granulation process with a roller rotor is even more sensitive than during the granulation of the wet mixture containing abrasive grains.

Claims (3)

Claims 1. A process for the production of granules of abrasive grains with a granulation device constituted by a rotor which circulates passing over a screen cloth, characterized in that the rigid bars known hitherto of the rotor are put in the form of tubes mounted with free rotation around an axis so that during the rotation of the rotor, the scraping movement of the rigid bars of a bar rotor on the screen cloth is transformed into a rolling movement of a roller rotor, whereby the mixture of abrasive grains, inert and / or active fillers and liquid binder is continuously pressed through the sieve with defined mesh width and the abrasive aggregate of uniform diameter but of different lengths is pressed, after drying and hardening in a closed circulation heating flue, with a roller rotor through a screen fabric having at least the same mesh width. 2. A method according to claim 1, characterized in that the binder consists of a thermosetting resin, for example a phenolic resin, a urea resin and a melamine-formaldehyde resin or an epoxy resin, an alkyd resin, a resin unsaturated polyester and urethane resin. 3. A granule of abrasive grains obtained by the method according to any one of claims 1 and 2, characterized in that it is applied to a fabric support, vulcanized fibers, nonwoven or paper.