EP0169140A2 - Wet-grinding apparatus - Google Patents

Abstract

The apparatus comprises, on the one hand, a receptacle (1) containing grinding elements and traversed by a stream of a pulp of the product to be ground and, on the other hand, an agitator (5) arranged inside the said receptacle and intended to impart movement to the said grinding elements. According to the invention, this grinding apparatus is characterised in that the said agitator (5) has the shape of a body of revolution, the outer wall (6) of which defines, with the inner wall (2) of the said receptacle (1), a peripheral annular grinding chamber (17), the volume of which increases in the direction in which the said pulp flows. <IMAGE>

Description

The present invention relates to a wet phase grinding apparatus comprising on the one hand a container containing grinding elements, such as balls, and traversed by a stream of a slurry of the product to be ground and, on the other hand, a agitator arranged inside said container and intended to put said grinding elements in motion.

Already known, for example from patent DE-A-2 125 888, such a wet phase grinding apparatus.

however, this known grinding device is complex, expensive and its performance is not optimal. Also, in patent FR-A-2 347 981, it is proposed to remedy the drawbacks of the prior apparatus by compartmentalizing the space between the container and the agitator into a plurality of grinding zones, the volume of each increasing in the direction of the flow of the slurry and adapting the diameter of the agitator blades to the diameter of each of the grinding zones.

• - l'effet de broyage se produit lorsqu'une particule à broyer est écrasée entre deux billes qui se heurtent ;
• - au fur et à mesure que se développe le processus de broyage, le nombre de particules à broyer, pour un volume donné, augmente alors que les dimensions desdites particules diminuent;le nombre de surfaces de contact entre billes et donc le nombre de billes, doit être proportionnel au nombre de particules à broyer et, par conséquent, proportionnel à l'augmentation du nombre de particules au fur et à mesure du déroulement du processus de broyage ;
• - la durée de broyage dans la phase initiale du processus est plus courte que dans les phases suivantes et à plus forte raison, que dans les phases terminales ;
• - la zone de broyage doit contenir le maximum possible de billes, pour augmenter la probabilité de chocs ;
• - le nombre de billes devant être proportionnel à tout moment au nombre de particules à broyer, ceci implique que le volume de la zone de broyage augmente au fur et à mesure que la quantité de particules à broyer devient plus grande ;
• - l'effet de broyage étant lié à l'énergie de choc de deux billes, et cette énergie liée elle-même à la somme de leurs énergies cinétiques, il importe qu'à une diminution du diamètre de ces billes corresponde une augmentation de leur vitesse, ce qui implique une augmentation du diamètre des pales de l'agitateur au fur et à mesure de l'avancement du processus de broyage.

The design of the device of patent FR-A-2,347,981 is based on the following observations:

• - the grinding effect occurs when a particle to be ground is crushed between two colliding balls;
• - as the grinding process develops, the number of particles to be ground, for a given volume, increases while the dimensions of said particles decrease; the number of contact surfaces between balls and therefore the number of balls, must be proportional to the number of particles to be ground and therefore proportional to the increase in the number of particles as the grinding process proceeds;
• - the duration of grinding in the initial phase of the process is shorter than in the following phases and a fortiori, than in the terminal phases;
• - the grinding zone must contain the maximum possible of balls, to increase the probability of impacts;
• - The number of beads to be proportional at all times to the number of particles to be ground, this implies that the volume of the grinding zone increases as the quantity of particles to be ground becomes greater;
• - the grinding effect being linked to the impact energy of two balls, and this energy itself linked to the sum of their kinetic energies, it is important that a reduction in the diameter of these balls corresponds to an increase in their speed, which implies an increase in the diameter of the agitator blades as the grinding process progresses.

At equivalent useful volume, and for identical products, an apparatus respecting the preceding observations has a yield approximately twice that of the preceding apparatuses.

• - complexité des pièces d'usure en mouvement (notamment de l'agitateur) ;
• - prix de revient élevé ;
• - et, surtout, difficultés de refroidissement, faute de surface d'échanges thermiques suffisante par rapport à la quantité de travail que l'on peut déployer. Ceci limite les dimensions, comme les performances, dudit appareil qui ne peut donner toutes ses possibilités, l'échauffement inéluctable du produit en cours de broyage interdisant l'exploitation à pleine puissance de l'appareil broyant. Il en résulte de plus que les produits plastiques qui ne tolèrent pas des températures excédant 35 à 40°C, ne sont pas susceptibles d'être efficacement traités par de tels appareils.

However, the device conforming to document FR-A-2 347 981 still has serious drawbacks, namely:

• - complexity of moving wear parts (especially of the agitator);
• - high cost price;
• - And, above all, cooling difficulties, lack of sufficient heat exchange surface compared to the amount of work that can be deployed. This limits the dimensions, as well as the performance, of said device which cannot give all its possibilities, the inevitable heating of the product during grinding prohibiting the full power operation of the grinding device. It further results that plastic products which do not tolerate temperatures exceeding 35 to 40 ° C, are not likely to be effectively treated by such devices.

The object of the present invention is to define an apparatus which overcomes these various drawbacks, while making it possible to comply with the laws resulting from the observations mentioned above.

In addition, the shapes of its working chamber, entirely original, translate even more rigorously in their architecture the laws mentioned above and the considerable increase for a given volume of the heat exchange surface allows in particular a strong increase in a same time of the work provided, thereby resulting in a higher yield and the possibility of processing any product, even of high sensitivity to heat.

As for the manufacturing costs of the device according to the invention, they are significantly lowered, all of the wearing parts, by the simplicity of their shapes, allowing a very large use of foundry.

For these purposes, according to the invention, the wet phase grinding apparatus comprising, on the one hand, a container containing grinding elements and traversed by a stream of a slurry of the product to be grinded, and, on the other hand, an agitator arranged inside said container and intended to set said grinding elements in motion, is remarkable in that said agitator has the shape of a body of revolution whose outer wall defines with the inner wall of said container a peripheral annular grinding chamber whose volume increases in the direction of flow of said slurry.

Thus, according to the invention, the agitator consists of a body of revolution which can have a large surface facing the container, instead of being a simple shaft provided with fins or blades as in the prior art. Consequently, in the apparatus according to the invention, the agitator has a large heat exchange surface allowing the elimination of the heat generated during grinding. The performance of the device is therefore improved.

Furthermore, due to the design of the agitator in body of revolution, it is also possible to provide a cooling system, associated with said agitator, which further improves performance. Indeed, the agitator can be produced in the form of a hollow body, which is crossed by a stream of cooling fluid passing through the shafts used for mounting said agitator relative to said container.

In order to be as efficient as possible, it is advantageous for said flow of coolant to be in the opposite direction to that of said slurry; thus, the hottest part of the device is traversed by a cooling fluid which has not yet been heated.

Preferably, a cooling system is also provided for said container.

In order to improve as much as possible the setting in motion of the grinding elements, the periphery of the agitator carries stirring elements projecting into said grinding chamber. It is advantageous that the periphery of the container then carries elements for braking the movement of the grinding elements, also projecting into said grinding chamber and arranged between said stirring elements linked to the agitator.

If we start from the minimum assumption that each particle to be ground passing through a point of contact between two grinding elements gives rise to two particles, it is advantageous that the increase in the number of these points of contact, in the direction of the flow of said slurry, increases at least as about a geometric progression of reason 2. It is therefore necessary that the increase in the volume of the grinding chamber, in the direction of flow of said slurry, allows such an increase the number of contact points. Of course, the actual increase in the volume of said grinding chamber will depend on the fact that said balls can all be identical or even be smaller and smaller as the volume of the grinding chamber increases, taking into account that the entire volume of this grinding chamber is filled with grinding balls.

In a particularly advantageous embodiment, the interior wall of the container and the exterior wall of the agitator are coaxial conical surfaces, converging in the same direction.

The single figure of the appended drawing will make it clear how the invention can be implemented. In this figure, there is shown in schematic axial section an embodiment of the apparatus according to the invention.

The apparatus according to the invention, shown schematically in this single figure comprises a stator 1 constituted by a frustoconical envelope 2 of axis X-X, closed at its ends by flanges 3 and 4, respectively.

Inside the stator 1 is arranged a rotor 5, constituted by a frustoconical envelope 6, closed at its ends by flanges 7 and 8, respectively. The rotor 5 is coaxial with the stator 1 and is mounted rotatable relative to the latter by means of shafts 9 and 10, for example integral with flanges 7 and 8 respectively. Shafts 9 and 10 pass through bearing liners 11 and 12, sealed under pressure.

The stator 1 has an inlet 13 and an outlet 14 for the slurry of the product to be ground, as well as an inlet 15 and an outlet 16 for the grinding balls (not shown in the drawing for clarity).

The frustoconical envelopes 2 and 6 converge in the same direction and they delimit between them a volume 17 in the form of a frustoconical crown constituting the grinding chamber containing the balls.

The shafts 9 and 10 are hollow, so that they can be used to circulate inside the casing 6 of the rotor 5 a cooling fluid, for example water from tree 9 to tree 10.

Similarly, the casing 2 of the stator 1 is provided with a double wall provided with a network of channels 18 for the circulation of a cooling fluid.

It can thus be seen that, for a given volume, the heat exchange surface is optimal and that the walls of each of the frustoconical walls 2 and 6, cooled by a circulation of water, play their role of heat exchanger as much as possible.

The angles at the top of each of the thermal walls 2 and 6 are chosen so that the variation in the volume of the frustoconical crown 17 constituting the working chamber increases in the direction of the flow of the slurry between the inlet 13 and the outlet 14 It is of course possible to adjust said angles at the top so that this increase in the volume of the frustoconical crown 17 follows any desired law.

The outer surface of the casing 6 of the rotor 5 carries an archimedean screw whose turns 19 are arranged in such a way that each of them is each time in the slot defined by two consecutive turns 20 of the inner wall of the stator 1 which also carries an archimedean screw of identical pitch.

Between the turns 19 of the rotor 5 and those 20 of the stator 1 there is, in the grinding chamber 17, a space which allows under the reverse action of the two archimedes screws an adequate play of the balls within the product filling said chamber. grinding.

Of course, all of the turns 19 and 20 are subject to numerous variants, making it possible to obtain the grinding action sought for the said balls. For example, the active elements for the agitation provided by the rotor 5 may be in the form of discontinuous turns, of equal pitch but in opposite directions, alternated on the external surface of the rotor. In this case and vice versa, the internal surface of the stator carries turns of the same pitch, located in the spaces delimited by the turns of the stator.

To avoid the creation of dead zones between the flanges 3 and 7 on the one hand and 4 and 8 on the other hand, the flanges 7 and 8 of the rotor 5 carry stirring fins 21 and 22, respectively.

The apparatus shown in the figure is associated with a motor for driving the rotor 5, as well as pumps for circulating the slurry of product to be ground and the coolants. The motor and these pumps are not shown in the figure.

Claims (7)

1.- Wet phase grinding apparatus comprising, on the one hand, a container (1) containing grinding elements and passes through a stream of a slurry of the product to be ground and, on the other hand, an agitator (5 ) disposed inside said container and intended to set said grinding elements in motion, said agitator (5) having the shape of a body of revolution whose external wall (6) delimits with the internal wall (2) of said container (1) a peripheral annular grinding chamber (17),
characterized in that the volume of said peripheral annular grinding chamber (7) increases in the direction of flow of said slurry and in that said agitator (5) is hollow and is traversed by a stream of cooling fluid passing through through the shafts (11,12) used for mounting said agitator (5) relative to said container (1). 2. A grinding apparatus according to claim 1, wherein the entire grinding chamber (17) is filled with grinding elements in the form of balls, characterized in that the increase in the volume of the chamber (17) is such that the number of points of contact between balls increases, in the direction of flow of said slurry, at least as about a geometric progression of reason 2. 3. A grinding apparatus according to any one of claims 1 or 2, in which the inner wall (2) of the container (1) and the outer wall (6) of the agitator (5) are coaxial conical surfaces, converging in the same direction,
characterized in that said conical surfaces have different apex angles. 4.- grinding apparatus according to one of claims 1 to 3,
characterized in that said coolant flow is in the opposite direction to that of said slurry. 5.- grinding apparatus according to any one of claims 1 to 4,
characterized in that a cooling system is associated with said container (1). 6.- grinding apparatus according to any one of claims 1 to 5,
characterized in that the periphery of the container (1) carries, in addition to stirring elements (19) projecting in said grinding chamber (17), braking elements (20) projecting in said grinding chamber (17) and arranged between the stirring elements (19) linked to the agitator. 7.- grinding apparatus according to any one of claims 1 to 6,
characterized in that stirring elements (21,22) are carried by the ends of the agitator (5) opposite the ends of said container (1).

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