ROLLER MILL FIELD OF THE INVENTION The invention relates to a roller mill with a virtually horizontal grinding path on a rotating grinding drum, which has stationary grinding rolls that roll on a grinding bed formed by the grinding material on the ground. milling path of the drum, and with retaining devices that are located between the grinding rolls to influence the movement of the material in milling. DESCRIPTION OF THE PREVIOUS TECHNIQUE In roller mills, which are also known as air-swept roller mills or roller drum mills, a flow of material for milling, supplied to the center of a milling drum and containing fresh material and coarse particles from a classifier, generally integrated in the roller mill, moves by centrifugal force towards the side of the grinding drum and towards a ring of vanes that surrounds the latter. The centrifugal force acting on the material being milled depends, in particular, on the diameter and speed of the grinding drum. The production capacity of a roller mill is essentially determined by the available grinding surface, e.g., the number and dimensions of the grinding rolls used and the grinding speed. This circumstance presupposes an adequate supply to each roller of material to be ground. In principle, by increasing the speed, the production capacity of a roller mill can be increased with respect to the same predetermined geometries and the same number of grinding rollers. As a result, a smaller roller mill can be used, which entails lower costs. However, an increase in speed is associated simultaneously with an increase in centrifugal force, which results in the grinding drum being partially or completely empty in the area between two successive crushing rollers, so that these are no longer they feed with material to be ground and can not achieve the intended production increase. DE 36 42 814 A1 discloses a roller mill which is provided with a retainer device between two crushing rollers for controlling the movement of the material in rooling in the vicinity of the milling path and on it. The retainer device is constructed as an arched deflector, which is fixed by means of fastening devices to the roller mill housing and is adjustable in regard to height and inclination.
A roller mill, described in the German patent 1,507,579, has a deflector facing the feeding of the material and between two crushing rollers, by means of which it is intended to establish a classification of the material that moves in the direction of the side of the drum for be ground The form and the height of the deflector produce a division of the material to be ground and a separate transport of the flow of fine material and a retention of the coarse material on the grinding path. The disadvantages of these known devices are a friction of relatively high magnitude between the material in grinding and the deflector used as a retainer device, which results in a greater need for motive power. As a result of the friction, greater wear occurs on the deflectors and also on the retention devices, which requires corresponding measures of maintenance and hardening of the surface of the deflectors and of the retention devices. SUMMARY OF THE INVENTION The object of the invention is to provide a roller mill with an effective, low-wear retaining device, which ensures an adequate supply of material to the grinding rolls to be ground, both at normal speed of material grinding and at speed increased, which ensures a greater production capacity with a reduced effort and lower expenditure. According to the invention, this object is achieved thanks to a roller mill having a virtually horizontal milling path on a rotating milling drum, with stationary crushing rollers, which roll on a grinding bed formed by a material in grinding on the path milling of the drum, and having retaining devices, which are placed between the grinding rollers to influence the movement of the material being grinded, in which, as retaining devices, damming rollers are formed, which roll over the material that it is being milled and has at least one face of damming or containment, which retains the material in grinding between the grinding rolls in the grinding path for complementary grinding purposes. The invention is based on the idea of placing an extraordinarily low friction retaining device, which is obviously reduced compared with that of known deflectors between the grinding rollers of a roller mill. According to the invention, the roller mill has a retainer device between two grinding rollers and one or more damming rollers which exert a damming or containing action on the material being milled and moving out in the milling path of the milling drum. This action of damming or containment is produced by means of the containment faces on the damming rollers, which are generally mounted diametrically to the movement of the material in milling, so that the material that is milled between the grinding rollers, due to the centrifugal force, can not reach the edge of the drum and, on the contrary, is retained on the grinding bed and is fed to follow the grinding processes. In a particularly suitable and simple construction, a dampening roller has, on an inner face facing the longitudinal axis of the roller mill, a dam or containment face. Suitably, one or more recessing rollers are positioned and dimensioned in such a way that their faces form a rotating damper front wall or a rotary lock against the outflow of the material being milled. Conveniently, thanks to the damming rollers rotating on the grinding bed and being placed between two grinding rollers, a rotary dam or rotary lock front wall is formed, because the individual rollers are virtually adjacent to each other on their faces and have a minimum separation with respect to the adjacent crushing roller. In principle, the damming rollers do not produce grinding action and, consequently, are generally exposed to very little or no force, so they do not "jump". Therefore, for a very good damming or containment action, the friction is maintained at an extraordinarily low level which produces a convenient influence on the energy balance. For the intended damming action or for the action of a lock against an outflow of the grinding material, with a particularly low friction, it is appropriate to build the damming rolls with a very low weight. The damper roller does not require a defined additional force other than its own weight, because the damper roller does not perform any grinding. Suitably, a spring damping system is constituted with which the damper roller is kept on the grinding bed. If a downward retention force is produced, e.g., by means of a spring damping system, it can be prevented that the damming rollers skip. It is convenient to limit the free movement of the damming rollers in the downward direction towards the milling path of the grinding drum with the aid of a mechanism. A limiting mechanism, particularly simple, has a lever with a stop, an adjusting screw or a shock absorber. In order to form a very tight lock or a tight damped front wall, the damming rollers are positioned so that their axes of rotation have an angle of attack or an exit angle with respect to a radial arrangement. Depending on the construction of the damper rollers, eg, like a conical, cylindrical or also curved grapple roller, the damming rollers are positioned in such a way that their projected rotational axes form an intersection with the axis of rotation of the drum. of milling or with the longitudinal axis of the roller mill. Depending on the geometry of the mill, the material to be ground and the desired production capacity, the retraction rollers can be placed in such a way that the containment surfaces of the faces of the inner ends have a different radial separation and / or a different inclination with respect to the axis of rotation of the grinding drum or the longitudinal axis of the roller mill and mutually superimposed and / or with the grinding rollers. According to another embodiment of the invention, the damming rollers are formed with more than one containment face. The complementary containment faces are constructed inside dam rings, which project radially from the circumferential surface of a damming roller. Suitably, the containment faces of the damming rings form almost a right angle with respect to the axis of rotation or the circumferential surface of the damming roller. As a result of said complementary dam faces, the retention action of a damming roller increases even more. The material that is being milled, which as a result of the centrifugal action is displaced from the grinding path in the direction of the edge of the grinding drum, the said movement taking place following a spiral path, is first blocked by the containment surface of the face of the inner end of the damming roller and subsequently retained on one or more containment faces of the damming rings. Therefore, the damming rings act as lock or locking device and, thanks to the rotary construction of the retaining wall, an extremely low friction is produced. In a particularly convenient construction, in addition to an internal containment face, a damming roller has a dam ring in the vicinity of the face of the inner end, at least on the face of the outer end facing the wall of the housing of the roller mill, whose ring forms an outer airlock or an external containment face. The dam rings can subsequently be adopted in an appropriate manner to the damming rollers. A removable device allows the adaptation of the containment faces of the dam rings according to the particular needs. Suitably, the dam rings are made of a wear resistant material and roll on the grinding bed with a circumferential surface of a relatively small radial width. As a variant to a purely rolling movement or with a rolling movement superimposed on a sliding movement, as a function of the arrangement of the retraction rollers and of the intersections of the axes of rotation with the axes of rotation of the milling drum above or below the plane of the grinding path, the damming roller can be operated in a rotating manner in addition to the drum. Additionally, the circumferential surface areas of the damper roller, formed between the dam rings, can perform a leveling or also a smoothing and a precompression of the bed of roolturación, if the repressor roller has a corresponding weight or is subjected to an additional force action.
The advantages of the roller mill according to the invention, with the damper rollers acting as retaining devices, constitute an extremely efficient grinding material retention between the grinding rollers. As the material in grinding is prevented from passing over the edge of the drum and arrives in the vicinity of the paddle ring, an inconvenient load of the sorting air flow with improperly ground grinding material is avoided. In this way, the load of the mill circuit is relieved and only a reduced discharge work is necessary, which improves the energy balance. Since the grinding rollers retain the material in grinding very efficiently, the rotation speed of the grinding drum can be increased and, consequently, production can be increased. This circumstance entails the convenient possibility of using smaller and less expensive mills. Additional hardening of the surfaces is unnecessary for wear protection purposes, particularly if the damper rollers only rest and roll with their own weight on the grinding bed. BRIEF DESCRIPTION OF THE DRAWINGS The invention is described below in more detail with reference to the accompanying drawings in which:
Figure 1 is a plan view of a grinding drum of a roller mill with grinding rollers and with damming rollers as retaining devices. Figure 2 is a partial sectional view of a roller mill in the vicinity of a damming roller that rolls on a milling drum. Figure 3 is a view of an alternative construction of a damper roller according to Figure 2. Figure 4 is a view of a cylindrical damper roll. DETAILED DESCRIPTION OF THE INVENTION The schematic representation of Figure 1 shows a grinding drum 2, which rotates by means of a motor mechanism. The material to be ground, fed from above or from the side to the grinding path 8 of the grinding drum, is crushed between grinding rollers 6, resiliently tightened, and the grinding path 8 of the drum 2. The resilient pressure generally occurs thanks to a tilting lever and a cooperating hydraulic cylinder device, not shown. The grinding rollers 6 generally operate exclusively by frictional contact with the grinding bed 7 located in the grinding path and do not have separate transmission. Thanks to a paddle ring 4 (see also FIG. 2), a fluid, generally air, is supplied via a supply line. The fluid, flowing through the paddle ring 4, conveys the mixture of material in grinding, constituted by fine and coarse material, violently thrown as a result of the centrifugal force of the grinding path 8 and after passing through the grinding rolls 6. to get to the vicinity of a classifier not illustrated. The fine material is removed from the roller mill, while the coarse material is fed, by means of a thick material outlet not illustrated to the center of the milling drum. Due to the centrifugal movement, the coarse material, together with fresh material, passes to the grinding path 8 and forms the grinding bed 7. The coarse material can be extracted, at least partially, from the classifier or mill and fed, by means of mechanisms of feed, to an external circuit in front of the grinding rollers 6. To avoid that, at a correspondingly high rotational speed of the grinding drum 2, grinding material is improperly transported between the grinding rolls 6, on the rim of the drum 11 towards the paddle ring 4, damming rollers 10 are constituted in the form of retaining devices. In contrast to the grinding rolls 6, the damming rollers 10 rest on the grinding bed 7 exclusively by their own weight and optionally by means of a spring damping system. The damming rollers 10 do not participate in the grinding of the material being milled. The main task of the damming rollers 10 is to "seal" the free space between the grinding rollers 6, to prevent the improperly shredded material from being violently thrown from the grinding path 8 and loaded into the mill circuit. Figure 1 presents a first variant of the damming rollers 10, which have only one containment surface 12 on the face of the inner end 14. The total of the containment surfaces 12 in the vicinity of the faces of the inner ends 14 form a damming wall 13, almost hermetic, or lock, thanks to which the grinding material is retained between the grinding rollers 6 and the grinding path 8 of the following grinding roller 6 is supplied. Only in the vicinity of the grinding rollers 6 and consequently following the grinding process the grinding material can be moved in the direction of the paddle ring 4 and be a conveyor to the sorting chamber. Figure 1 shows that the damming rollers 10 are arranged with an angle of attack or output to perform an effective containment action of the grinding material moving along a spiral path. Figure 2 shows a second damper roller 10, particularly effective, in longitudinal section and its axis of rotation 20 intersects, in projection, the axis of rotation 3 of the drum 2 at the same level. The mounting and support of the damming roller 10, which is not illustrated, can take place in the same way as the supporting assembly of the grinding rollers 6. If there are several damming rollers 10 and if they have a particularly low weight compared to the grinding rollers, it is also possible to fix the rollers 10 to the housing. The damper roller 10, according to Figure 2, has a containment surface 12 on one side of the inner end 14, as well as a dam ring 17. This dam ring 17 runs on one side of the outer end 18. facing the wall of the casing of the roller mill 5 and directed, with another containment face 15, towards the axis of rotation 3 of the drum. Characteristics that are the same are indicated by the same reference numbers. The damper roller 10, according to Figure 2, has a truncated cone-shaped construction and, as a result of the two damming faces, that is the inner face 12 and the outer face 15 formed in the dam ring 17, produces a particularly good retention action.
Figure 3 shows another alternative damper roller 10. This conical damper roller 10 is constructed with three damper rings 17. They can also be installed in a removable manner on the circumferential surface 19 of the damper roller 10. The faces The corresponding containment elements 15 are directed radially outwardly from the circumferential surface 19 of the repressor roller 10 and, in each case, are arranged in parallel. The containment faces 15, formed in the dam rings 17, run approximately at right angles to the axis of rotation 20 of the damper roller 10, whereby the material in milling that moves towards the edge 11 of the milling tarobor 2 is blocked and retained. The separation space between the grinding path 8 and the circumferential surfaces 19 of the repelling rollers 10 is suitably adjustable with the aid of a device not illustrated and is dimensioned so that the circumferential surfaces 19 contribute to a leveling or also to a compression of the grinding bed. The dam rings 17 are made of a wear resistant material. The containment faces 15 of the dam rings 17 can be flat, concave or convex. A cylindrical damming roller 10 according to FIG. 4 has an axis of rotation 20 parallel to the grinding path 8. The damming roller 10 is provided with two dam rings 17, so that, together with a containment surface 12 in one In the end face 14, a total of three damming areas 12, 15 are formed. Even though the grinding drum 2 rotates at a speed, in the case of a virtually adjacent arrangement of the damming rolls 10 and the grinding rolls 16, effective retention and grinding are ensured, so that a roller mill equipped in this way has a large production capacity with reduced energy consumption, wear and tear of the retainer device and reduced maintenance costs.