EP0102645B1 - Grinding process, and roller grinding mill for carrying out this process - Google Patents

Abstract

An improved milling process, wherein the material to be crushed is processed within a circular milling path under the action of at least one roller rolling along said path. The material is fed evenly distributed around the circumference of the milling path and enters it with a predetermined initial speed transversely to the extension of the path such that each piece to be crushed is subjected at least once to the action of one of the rollers. Under the action of an air current which is guided across the milling path pulverized material below a given grain size is immediately removed from the milling path, thereby avoiding the build-up of a thick and energy consuming layer on the milling path such that the milling forces are limited to the breaking forces for crushing the pieces fed into the milling path.

Description

The invention relates to a grinding process, the material to be ground being distributed evenly over the circumference of a circular grinding path and entering into it and being crushed under the action of at least one rolling body rolling along this path, as well as a pan mill and a roller mill for carrying out this method.

In known grinding processes or mills of this type, as can be seen from CH-PS 558 678 or CH-PS 406 795, regrind is fed through a peripheral feed opening at a point above the grinding track. It gets into the grinding chamber from the side, is partly gripped by the grinding rollers and partly falls uncrushed between them into a collecting container that also holds the shredded material. From there, the regrind is thrown upwards into the area of the rotating grinding rollers according to CH-PS 406 795 by means of rotating blades. An air stream that passes through the entire grinding chamber from bottom to top leads the sufficiently shredded material upwards. In mills of this type, as are also shown in US Pat. No. 1,499,516 and US Pat. No. 3,955,766, the ground material is thrown around the grinding chamber in a relatively uncontrolled manner. In addition to the newly introduced regrind, material that has already been shredded and is still in the grinding chamber is repeatedly captured by the grinding rollers. The grinding process is therefore carried out with high friction, which is caused by the displacement of the ground material and the internal friction that occurs under the influence of the grinding rollers. The one-sided regrind and the uncontrolled movements of the regrind in the grinding chamber also lead to less than optimal efficiency of the grinding process and increased wear.

The publication US Pat. No. 1,499,624 shows a mill of the type mentioned at the outset, in which the above-mentioned disadvantage is remedied by the fact that both the material crushed by the grinding rollers and the material not captured after passing through the grinding path by means of a strong air stream is returned outside the grinding area and then exposed again to the action of the grinding rollers, provided that it has not yet reached the required particle size. This mill has the disadvantage that the newly fed and the recycled material get into the grinding path in an uncontrolled manner, which results in poor efficiency. In particular, the material must therefore be fed several times to the grinding area by means of an energy-intensive, strong air flow until it is even gripped by a grinding roller.

Finally, a mill is known from US Pat. No. 2,189,441, in which a controlled supply of material into the grinding path takes place. The material reaches the grinding path distributed over the circumference and is gripped there by the grinding rollers. In this case, no measures are provided for, in particular, to lead the comminuted particles out of the grinding path, to which particles the adhesive forces that occur have a stronger effect than the gravitation. This mill results in a relatively thick grinding bed due to inadequate removal of the particles from the grinding track and uncontrolled displacement and swirling effects which counteract the controlled feed and make it partially illusory.

The object of the present invention is to provide a grinding process and a roll mill of the type mentioned at the outset, which do not have the disadvantages explained and result in a grinding process with a controlled material flow.

This is achieved according to the invention in that, prior to entry into the grinding path, acceleration forces act on the grinding stock, which impart an entry speed into the grinding path which is directed transversely to the grinding path and which is matched to the rolling body speed in such a way that essentially every grinding material piece is gripped by the rolling body at least once is and that in the area of the grinding track a transverse air stream is generated, by means of which crushed ground material is led away from the grinding track.

The mill is preferably designed as a roller mill with a plurality of grinding rollers which are suspended from a central axis and can be rolled under pressure on a cylindrical lateral surface which is concentric with the axis and forms a grinding path, and with a distribution device arranged above the grinding path for evenly distributing the ground material over the circumference the grinding path and for the continuous introduction of the same in a direction approximately parallel to the roller axes into the grinding path and a device for generating an air flow, air guide walls being provided which are arranged parallel to the grinding path and form an air gap with it, through which the air flow transversely is guided over the grinding path, the air guide walls having openings for the grinding rollers, through which they protrude into the air gap, and that the grinding path is arranged in such a way that the ground material exits the grinding path after detection by the grinding rollers.

The procedure according to the invention has the effect that the conventional grinding process with displacement and compression of the ground material is essentially replaced by a breaking process which takes place uniformly and in a controlled manner along the grinding path. This is attributed to the controlled feeding of the material into the grinding track and the removal of the shredded material by means of a directed air flow from the area of the grinding track. This enables a very thin grinding bed to be achieved with a relatively high throughput. The rolling resistance of the grinding rollers and the energy consumption of the mill can be reduced and their efficiency improved.

• Fig. 1 eine schematische Darstellung einer als Rollenmühle ausgebildeten Walzmühle zur Ausführung des Mahlverfahrens, in perspektivischer Darstellung;
• Fig. 2 eine Schnittansicht eines Teils einer anderen Ausführungsform einer Rollenmühle zur Ausführung des Mahlverfahrens,
• Fig. 3 eine schematische Schnittansicht einer Kollermühle für das Mahlverfahren,
• Fig. 4 eine mehrstufige Rollenmühle gemäss der Erfindung in Schnittansicht,
• Fig. 5 eine schematische Aufsicht auf einen in der Rollenmühle von Fig. 4 vorgesehenen Vorbrecher und
• Fig. 6 eine perspektivische Ansicht der Mahlrollenanordnung von Fig. 4

Embodiments of the invention are explained in more detail below with reference to the drawings. In it show:

• Figure 1 is a schematic representation of a roller mill designed as a roller mill for performing the grinding process, in a perspective view.
• Fig. 2 is a sectional view of part of another embodiment of a roller mill for performing the Grinding process,
• 3 shows a schematic sectional view of a pan mill for the grinding process,
• 4 shows a multi-stage roller mill according to the invention in a sectional view,
• Fig. 5 is a schematic plan view of a primary crusher provided in the roller mill of Fig. 4 and
• FIG. 6 shows a perspective view of the grinding roller arrangement from FIG. 4

1 and 2, the structure and mode of operation of roller mills, by means of which the grinding process can be carried out, are first described.

Fig. 1 shows schematically a first construction variant to explain the principle. On a central axis 1, which is designed as a drive star 2 at its upper end, a plurality of grinding rollers 3 with vertically aligned axes are pivotably mounted on pivot arms 4. The grinding rollers 3, which are driven by the drive star 2, run along a horizontal grinding path on a cylindrical outer surface 5 and are pressed against them by centrifugal force or, if necessary, additionally by hydraulics or pneumatics. A housing 6 is arranged above the drive star 2 and the mowing rollers 3, which rotates with it and leaves an annular channel open laterally against the lateral surface 5, the function of which will be explained below. Through openings 7 in the housing 6, the grinding rollers emerge laterally therefrom. Below the grinding area there is a collecting device for regrind that emerges downwards, to which a bucket elevator (not shown) for returning this regrind is connected via a feed opening through which the regrind enters the mill from above. If several mills are connected in series, the emerging regrind is fed to the next mill instead (see FIG. 4).

Through the feed opening, the possibly pre-comminuted ground material entering the mill first passes centrally to a spreading plate 11 which acts as a distribution device and is preferably formed by the top of the housing 6. The ground material is placed in the center of the spreading plate 11 and is distributed over the spreading plate 11 in such a way that it is conveyed outwards under the action of the centrifugal force and is thrown radially away from it. The spreading plate 11 can be provided with essentially tangential ribs or a corresponding border 12 (FIGS. 4 and 6), so that a regrind layer forms over the plate 11, via which the further regrind rolls outwards. The wear of the spreading plate 11 can thus be reduced. The ground material, which is fed continuously to the spreading plate 11, is distributed evenly along the periphery of the spreading plate in this way. It thereby reaches an annular surface 14, which is adjustable along the periphery above the grinding track (FIG. 2). The purpose of this is to catch the material thrown away by the spreading disc and to guide it into the grinding path at a defined falling speed. According to the exemplary embodiments according to FIGS. 1 and 2, the annular surface is formed by a horizontal annular shoulder 14 above the grinding track. The regrind deposited on it forms a cone (FIG. 2), from which the further material is then taken up in operation to roll down and finally reach the grinding area in free fall. The inclination of the flank of the pouring cone adjusts itself automatically to the angle of repose of the corresponding material, so that a steady state occurs with a defined falling speed of the individual pieces in a certain area. Other configurations of the distribution device will be discussed later with reference to FIGS. 3 and 4.

und dem Umstand, dass an einem bestimmten Umfangsort eine Zeit T = ih zwischen zwei Mahlrollendurchgängen verstreicht, ergibt sich als Bedingung für mindestens einmaliges Erfassen für die Anfangsgeschwindigkeit V_µ:

• V_o < h.n.f. - 2,9-f

(unter Vernachlässigung des Luftstroms im Spalt), wobei diese Bedingung aus zu erläuternden Gründen nur näherungsweise gelten kann. In Fig. 1 ist eine Ausführung mit vier Mahlrollen dargestellt. Es sind jedoch ebenso Ausführungen mit nur zwei Rollen und entsprechend höherer Umdrehungszahl vorgesehen (Fig. 4).The arrangement according to FIGS. 1 and 2 is such that each piece is gripped by a grinding roller 3 at least once when the grinding path falls through. This requires matching the number of grinding rollers n and their height h and the speed f of the central axis 1 to the initial speed V of the pieces of regrind as they enter the grinding path. From the case law:

and the fact that a time T = ih elapses between two grinding roller passes at a certain circumferential location, results as a condition for at least one detection for the initial speed V _µ :

• V _o <hnf - 2.9-f

(neglecting the air flow in the gap), whereby this condition can only apply approximately for reasons to be explained. In Fig. 1, an embodiment with four grinding rollers is shown. However, versions with only two rollers and a correspondingly higher number of revolutions are also provided (FIG. 4).

The grinding process runs evenly over the entire circumference. The pieces falling into the grinding path from above are gripped by one of the grinding rollers 3 and subjected to a compressive stress between the latter and the outer surface 5, which leads to their bursting. The fragments are then removed from the grinding path, and at the same time a separation of fragments from a certain grain size takes place by means of an air stream. The air flow can be directed both from bottom to top (FIG. 1) and in the opposite direction (FIGS. 2 and 4), the last-mentioned embodiment having the advantage that the ground material enters the grinding path with the air flow, which is why Fall height can be reduced and the removal effect of the fine material is better, since it does not take place against the gravitation.

The heavy pieces fall from the grinding path into a collecting device 8 (FIG. 2), while the lighter pieces are carried along by the air flow and are removed and collected through an exhaust air duct.

• Z = M f2 R 4 712

The contact pressure of the grinding rollers 3 against the lateral surface 5 is basically exerted by the centrifugal force of the grinding rollers 3, which are arranged to be horizontally pivotable for this purpose. Run the centers of the grinding rollers on a radius R at the speed f and each of them has the mass M, the contact pressure Z results as:

• Z = M f2 R 4 712

• Z = 4 _n3 p h _f2_r2 _R

wobei zusätzlich die oben erwähnte Bedingung bezüglich des Erfassens des Mahlgutes zu beachten ist.If the grinding rollers 3 with the radius r and the height h are formed from a material of the specific weight p, this further results

• Z = 4 _n 3 ph _f 2 _r 2 _R

the condition mentioned above with regard to the detection of the ground material must also be observed.

In the arrangement according to FIG. 1, shock absorbers (not shown) can be arranged between the swivel arms 4 and the corresponding legs of the driving star in order to dampen any jumping of the grinding rollers. Depending on the regrind, this is superfluous, since this itself takes over this function. If very high contact forces are required for special applications, additional hydraulic-pneumatic elements can be provided for this.

As already mentioned, the material to be ground is partially screened in the area of the grinding path using an air stream. The air flow is generated by a fan (not shown). It enters the grinding chamber through suitable openings and flows through the relatively narrow space between the rotating housing 6 and the lateral surface 5 over the grinding path, since the remaining part of the cross section is closed by the housing 6. This allows a relatively high air speed to be achieved in the area of the grinding track with a relatively low fan output. This effect can be further improved by cover plates 29 between the grinding rollers 3 (FIG. 6), with which a further reduction in the grinding bed thickness can be achieved, with the possibility that relatively large pieces of the grinding path can enter the grinding path. The housing 6, of which the surrounding parts are largely surrounded, counteracts air turbulence, so that in the grinding area there is an air flow which is relatively rectified on average. By controlling the air speed, the separation effect, in particular the fineness of the shredded ground material, can be adjusted. The air flow has little influence on the uncrushed ground material when it enters the grinding path, even if its free fall is slowed down when the air flow is directed upwards (FIG. 1).

2 shows a further exemplary embodiment of a roller mill according to the invention. Each grinding roller 3 is suspended from a column 17 pivotably mounted on the housing 6. The housing 6 is closed, but has openings 18 on the underside, in particular in the region of its periphery, for releasing pieces of regrind which have penetrated through the roller openings 7. The spreading plate 11 with tangential strips 12 is arranged above the housing, as already explained above. The height of the annular shoulder 14 is adjustable in order to adjust the drop height of the ground material and thus the initial speed V of the ground material as it enters the grinding path. This enables the number of revolutions of the grinding rollers and the grinding material speed V _{o to be} optimally adjusted.

The rotating inner area is driven by a disk 22 which is connected to the housing 6 and the rotating discharge tube 15 by elements 23 transmitting torque. Opposing, stationary drive wheels 24, which are connected to a motor (not shown), drive the disk 22.

The air for generating the air flow in the area of the grinding path is sucked in through air inlets 27 arranged centrally in the area of the discharge pipe 15. The air flow through the grinding track can be controlled by an adjusting device 25 arranged on the underside. The air loaded with ground material comes out of the mill through outlet 15, to which a fan (not shown) is connected, for further processing. The arrangement is such that there is a slight negative pressure in the inside of the mill compared to the surroundings, so that no grinding dust escapes to the outside. In the space below the housing 6, the rotation of the air flow receives a tangential component, so that this space also acts as a cyclone.

In contrast to the embodiment according to FIG. 1, the air flow flows through the grinding area from top to bottom and has an accelerating effect on the material falling from the annular shoulder 14, so that the drop height can be reduced or the material throughput can be increased (with a correspondingly increased speed of the grinding rollers according to FIG relationships given above). The space below the housing 6 with the central discharge pipe 15 also serves as an air classifier for the material carried by the air flow.

The grinding rollers 3 can have a circumferentially grooved working surface, in particular for high rotational speeds, in order to reduce the air displacement in the rollers. Furthermore, a section 26 of smaller diameter can be provided in the upper region of each grinding roller 3, which section serves as a primary crusher for pieces of larger size that may be present in the material to be ground.

The described devices are operated continuously, i.e. a flow of regrind is continuously fed through the feed opening, which is distributed over the circumference and falls into the grinding path there. The speed of the grinding rollers is adjusted in relation to the falling speed of the grinding material so that each piece of the grinding material is gripped by a grinding roller at least once. The fineness of the ground material is adjusted with the air speed. The material that has not yet been sufficiently shredded during the first grinding cycle can be returned to the feed opening. If larger throughputs are to be achieved, a plurality of mills which are coordinated with one another can be arranged one behind the other, the regrinding material emerging downwards being fed to the next stage, which can be dimensioned correspondingly smaller (FIG. 4).

FIG. 3 shows a further type of mill, which realizes the method according to the invention, in the form of a Schollergangs shown schematically. The rollers 35 are arranged in a stationary manner in a housing 30 and roll on a rotating turntable 31, which both acts as a distribution device and forms the grinding track. An air gap is formed between the housing 30 and the grinding path thus formed, through which an air stream flows radially inwards in order to discharge the ground material, which has been sufficiently finely comminuted, from the grinding path. The regrind laden air passes through air ducts to a discharge pipe 32 to which a fan is connected. Air passages 33 are provided on the side of the housing 30 and can be adjusted such that the desired proportion of air flows through the air gap in the grinding path.

A metering device 34 for introducing the ground material to prevent the escape of air is arranged centrally above the rotating plate 31, e.g. in the form of a wing cross. By means of this metering device 34, the ground material arrives centrally on the turntable 31 and is distributed there over the turntable by the centrifugal forces that occur. The regrind enters the grinding path at a radial speed influenced by the rotational speed, such that each regrind piece is gripped by one of the rollers 35 at least once. Pieces which have not been comminuted sufficiently reach the outside and fall over the edge of the plate into a receiving device, from which they are in turn fed to the metering device.

The space above the housing 30, together with the central discharge pipe 32, serves as an air classifier, whereby the air flow in the area is caused to rotate by vanes 36.

The mode of operation otherwise corresponds to that which was already explained in detail above.

4, a multi-stage cement roller mill is described, which works according to the principle explained above with reference to FIGS. 1 and 2. The multi-stage mill has an impact crusher 40 and two grinding stages 41 and 42, all of which are arranged on a common axis 45. The axis 45 is driven at its upper end via a gear 44 or directly by an electric motor 43 and is supported at its ends. The material entering the mill through an upper filler neck 46 with a piece diameter of max. Approx. 15 cm from a corresponding primary crusher first reaches a rotating spreading plate 47, as indicated by a double arrow. From there, the material is conveyed horizontally outwards by the centrifugal force and reaches an annular shoulder 48, on which a pouring cone is formed, over which the individual pieces fall evenly distributed over the circumference into the path of the vertically rotating impact crusher 40, which is shown in FIG 5 is shown schematically partly in section. It has two massive baffles 49 pivotally attached to the axis and an outer ring provided with baffle surfaces 50, against which the fragments of the pieces gripped by one of the baffles are thrown in order to be further crushed there. The height of the shoulder 48 above the path of the baffle vanes is coordinated with their number of revolutions and height in such a way that each piece is grasped, as already explained above in connection with the grinding process. The material released from the impact crusher has a max. Piece size of about 1 cm and passes over a further shoulder 52, on which a cone of material is formed, onto the spreading plate 11 of the first mill 41 and from there onto the ring surface 14. A height-adjustable wall 53, behind which there is a contact on the ring surface Forming cone, allows the drop height of the material to be adjusted into the grinding path, as explained, where it is gripped by one of the two grinding rollers 3 arranged in the housing 6 (FIG. 6). Between the grinding rollers 6 cover plates 29 (FIG. 6) are arranged on the housing, which concentrate the air flow crossing the mill onto the grinding bed. The grinding process takes place in the manner described.

Below the first grinding stage 41, an air inlet or outlet arrangement 55 is provided which serves to control the air throughput and can be connected to the fan. The air flow through the mill, which is indicated by simple arrows and runs from top to bottom, is generated by a fan (not shown) arranged on the outlet side. The air is sucked in through the filler neck and first crosses the first grinding stage 41. Between the first and the second grinding stage, part of the air flow can be drawn off or air can be supplied through the inlet and outlet arrangement 55. In the first case the air flow in the second grinding stage is weakened, in the second case the air flow in the first grinding stage. Based on the fact that there is a constant pressure drop across the entire mill, its proportions can be adjusted in this way over the individual grinding stages.

Before the second grinding stage, a second filler neck 56 for regrind is also provided, which comes from the air classifier to which the regrind is fed. This material, which has not yet reached the desired fineness, is fed together with the starting material of the first grinding stage to the spreading plate 11 'of the second grinding stage 42, which essentially corresponds to the first and therefore does not need to be described in detail here. The air flow through this stage can be adjusted to the increased fineness, as explained.

The mill is operated continuously by adapting the continuously supplied amount of material to the speed of rotation and the finer fraction returned to the lower grinding stage.

The method according to the invention and explained on the basis of several exemplary embodiments results in lower energy consumption than known grinding methods in that the ground material is continuously removed from the grinding path. The controlled material flows inside the mill mean that the optimal amount of material for effective grinding is available in the grinding area and the grinding rollers only have to do a little displacement work. The guidance of the air flow allows the use of less energy-consuming fans.

Claims (10)

1. Milling process, wherein the material to be crushed enters a circular milling path evenly distributed around the circumference of the same and is crushed by the action of at least one roller rolling along said path, characterized in that the material to be crushed is subjected to acceleration forces before entering said milling path, which impart to the same a speed of entering directed transversely to the milling path which speed is adjusted to the speed of the rollers such that substantially each piece of the material is subjected to the action of the roller at least once and in that in the zone of the milling path a transverse air current is generated by means of which the crushed material is carried off the milling path.

2. Edge-runner mill for carrying out the process of claim 1, having at least one edge-runner (35), a distributing device (31, 34) for evenly distributing the supplied material around the circumference of a circular, horizontal milling path and a device for generating an air current through the mill, characterized in that the milling path is disposed on a rotating disk (31), onto which the material to be crushed is supplied centrally and is accelerated radially towards the milling path and in that the edge-runner (35) substantially is surrounded by a stationary casing (30) which together with the milling path forms an air gap for the air current.

3. Roller mill for carrying out the process of claim 1, comprising several rollers, which are suspended at a central axis (1; 45) to roll on and being pressed against a cylindrical surface (5) concentric to said axis which forms the milling path, further comprising a distributor device (11, 14) arranged above the milling path for evenly distributing the material to be crushed around the circumference of the milling path and for continuously introducing the material into the milling path in a direction substantially parallel to the axes of the rollers and comprising a device for generating an air current, characterized in that air guiding walls (6; 30) are provided, which are disposed parallel to the milling path to form an air gap therewith, by which the air current is guided across the milling path, the air guiding walls having openings for the rollers extending therethrough into the air gap, and in that the milling path is arranged so that the material to be crushed leaves the milling path after being subjected to the action of the rollers.

4. Roller mill as defined in claim 3 comprising a central, rotatively driven shaft (45), the milling path being formed by a cylindrical surface (5) arranged concentrically to said shaft and the rollers (3) being hingedly suspended at and driven by this shaft, so that they roll on the cylindrical surface being pressed against it by centrifugal forces characterized in that the distributing device (11, 12) is arranged above the cylindrical surface and cooperates with a concentric shoulder (14), from which the material to be crushed falls into the milling path, the air gap arranged at the milling path being open at the bottom so that the crushed material leaves the milling path downwards.

5. Roller mill as defined in claim 4, characterized in that the distributing device comprises a horizontal distributor plate (11) with elements (12) for retaining a portion of the material to be crushed on the plate, so that a material layer is formed, on which the supplied material is distributed and moved radially outwards by means of centrifugal force, so that the material is collected on said shoulder (14).

6. Roller mill as defined in claim 5, characterized by a housing (6) fixed to and rotating with the shaft (45) which substantially encloses the rollers and comprises a cylindrical outer wall concentric to the milling path and forming the air gap, said wall having openings for the rollers.

7. Roller mill as defined in claim 6 characterized in that the housing (6) has a cover wall (11) forming said distributor plate.

8. Roller mill as defined in claim 3 with several milling stages arranged one above the other, each of which comprising a distributing device and a milling path, on which the corresponding rollers roll thereby being pressed against it, characterized in that for all milling stages there is a common device for generating an air current, which by means of air guiding walls provided at each stage and forming an air gap at the respective milling path, is guided through all stages and in that the milling path of each milling stage is arranged so that the material to be crushed after its being subjected to the action of the rollers leaves the milling path towards the distributing device of the following stage or towards an outlet.

9. Roller mill as defined in claim 8, characterized in that there is a common driving shaft (45) for all stages, wherein every milling path forms a cylindrical surface arranged concentrically to this shaft and wherein the rollers (3) of all milling stages are hingedly suspended at and driven by this shaft, so that they roll on the respective milling path being pressed against the same by centrifugal forces.

10. Roller mill as defined in claim 9, characterized in that there is a precrusher (40) arranged above the first milling stage and comprising two impact crusher beaters (49) hingedly connected to said shaft, wherein a distributing device (47, 48) is provided for distributing the supplied material to be crushed over the path of the beaters and wherein an outer baffle ring is arranged concentrically to the shaft in the plane of the beater path for receiving the material thrown radially outwards by the action of the beaters.

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