Method and apparatus for screening granular materials

Abstract: A method for separating a collection of particles according to size, shape and/or density, whereby the collection of particles being fed through a vibrating space with classifying elements (1,7-11) in the form of screen cloths, wires or bars. The invention is characterized by that the classifying elements have a more vigorous movement (3) mostly perpendicular to the classifying elements near the inlet end of the space and a more gentle movement (4) which is more horizontal at the outlet end of the space.

Description: The present invention relates to an apparatus for separating a collection of particles according to the pre-characterizing portion of claim 1. Such vibrating screening apparatuses handle the material to be screened in a mainly vertical loose flow - in the following called Sizers. An example of such a Sizer is described, e.g. in the US Patent 3 710 940. In that case the loose flow of particles is created by the apertures of the classifying elements, below called screen cloths, being larger than the particles arriving on them. A loose flow of particles can also be achieved when a minor portion of them are larger than these apertures. DE-C-29 23 662 discloses an apparatus for separating according to the pre-characterizing portion of claim 1 and therefore comprises only one vibrator motor, which by means of a heavy duty element places the vibratory centre in the vicinity of the gravity centre of the apparatus. Thereby, a movement pattern is achieved which is quite uniform through the whole apparatus. From DE-A-19 26 143 an apparatus for separating particles is known. This apparatus as well has a vibratory centre at the proximity of the gravity centre of the apparatus. This apparatus is also equipped with two vibrator motors rendering a movement pattern, which also is quite uniform along the vibrating space of the apparatus. Conventional screening apparatuses have screen cloths where the apertures are usually smaller than a greater portion of the particles. In such a case it is much more difficult for the particles to pass the screen cloth: the particles form a bed on top of the screen cloth, which obstructs the movement of the particles. Particles, which have nearly the same size as the apertures may get stuck in the screen cloth. Sizers depend upon the formation of such beds of particles being avoided as far as possible; by using several layers of often steeply sloping screen cloths, satisfactory separations are obtained, despite the apertures being large in relation to the separation size. In practice, however, Sizers do suffer from beds being formed, normally when the flow of particles meets the first part of the screen cloths, where the flow is relatively concentrated. If such a bed is allowed to spread over a substantial segment of the screen cloth, the danger of reduced screening efficiency becomes imminent in that the congestion of the bed prevents the finer particles from working their way down through the screen cloths; they contaminate the coarser products. The object of the present invention is to prevent the formation of such beds, or, to segregate the particles in the beds. This is achieved in that said unique vibratory centre is placed rear of said vertical plane. The motion of Sizer has so far been linear and of approximately the same attitude (stroke, direction and frequency) over the entire Sizer. Such a vibration is normally created by two vibrator motors or eccentric elements, where the eccenters rotate in opposite directions, or by a linear magnetic vibrator. The movement of the Sizer, according to this invention, must not be uniform. At the initial part of the screen cloths, it should be vigorous and approximately perpendicular to the surface of them so that the segregating action is enhanced. An elliptic motion could be advantageous, in that its direction of rotation can be varied, according to the screening duty. This motion has not the same impact as a linear motion and could, therefore, be larger and more effective. Towards the outlet end of the Sizer, where the particles leave it, a more transporting and less agitating movement is to be preferred; it should be more horizontal, having a direction with an acute angle, relative to the surface of the screen cloths. The invention will be described by way of examples with reference to the drawings Figure 1 illustrates the principle of the invention. Figure 2 schematically illustrates a Sizer according to the invention in longitudinal section. Figure 3 illustrates the Sizer seen from the rear end. Figure 4 illustrates an external side view of the invention. Figure 5 and 6 are diagrams showing the development of the cleanliness and recovery respectively. Figure 1 illustrates the principle. 1 is a screen cloth, seen from the side. 2 indicates the general direction of the flow of particles arriving on the screen cloth. The primary part of the screen cloth, near its inlet end, moves, more or less, perpendicularly to its surface, in the figure illustrated by the ellipse 3, while the secondary part of the screen cloth, near its outlet end, moves almost linearly, at an angle, as indicated by the ellipse 4. Such movements can be for example created by means of mounting the Sizer and/or the screen cloth in a suspension system, which restricts the motion by means of links, leaf springs or other types of guides, the position of the vibrator motor not being critical. Another way to achieve the desired movement is to use one vibrator motor located underneath the inlet of the Sizer and well to the rear of its centre of gravity, relative to the direction of the flow of particles. By arranging the vibrator in this fashion and suspending the Sizer by means of springs, its perpendicular stroke at the inlet end becomes very much larger than, say, that at the centre of gravity, which further enhances the vigorous action. The position of the vibrator motor in the Sizer is critical. When operating a Sizer with one vibrator motor, the position of the vibrator in relation to the Sizer's centre of gravity, as well as the Sizer's rotational inertia, are important factors influencing its movement. Sometimes it is necessary to equip a Sizer with accessories of such considerable weight or location that the centre of gravity becomes very much displaced. This may have to be compensated by adding motion restricting guides, as mentioned above, or by fixing the vibrator in a different position; alternative vibrator locations could be a standard feature. The same criteria would apply, e.g., with wider Sizers which need more than one vibrator motor, if their shafts are in line and the vibrators have the same setting and direction of rotation. A series of comparative tests has been made, where dry sand 0-8 mm was separated at 0.25, 0.5. 1, 2 and 4 mm, by means of a Sizer with 5 screen cloths. The Sizer was driven by either two vibrators or by one vibrator, according to the invention. The stroke of the 2-vibrator unit was uniform, with the same magnitude as the stroke at the 1-vibrator unit's centre of gravity. The inlet end of the 1-vibrator unit had an elliptical motion, which was more than twice as large as the motion of the 2-vibrator unit. No other parametres were changed, but several feed rates were tried and the obtained products analyzed. The developments of two features were plotted in diagrams. One was the cleanliness, i.e., the amount of the total feed that was brought to the respective products and being within the desired size limits. The other was the recovery or the mean value of the amount of each particle size range that was recovered in the respective product, e.g. the amount of <0,25 mm that ended up in the fines through the bottom screen cloth. Figure 5 shows the development of the cleanliness and Figure 6 that of the recovery. Initially the 2-vibrator and the 1-vibrator cases are nearly similar, but when the feed rate exceeds some 5 t/h the 1-vibrator unit manages with less deterioration. For instance, at about 8 t/h the 1- vibrator unit has the same cleanliness (85%) as the 2-vibrator unit at about 6 t/h. Other advantages of using one vibrator instead of two are that the production and energy costs are reduced and the risk of running the Sizer with two vibrators incorrectly - either with the vibrators having different settings, or rotation, or one vibrator standing still - is largely eliminated. Referring to Figures 2-4 the Sizer consists of a frame 5, which has a feed plate 6 at the upper rear, or inlet end, where the flow of material enters the apparatus, as illustrated by the arrow 2. A number of screen cloths 7, 8, 9, 10 and 11, are slopingly arranged inside the frame 5, one below the other. An eccentric vibrator motor 13 is mounted on a transversal structure 14, which is located underneath the feed plate 6, to the rear of the centre of gravity 12 of the Sizer. This location gives the motion as shown in Figure 1, described above. The Sizer is suspended on helical rear springs 15 and helical front springs 16. The side walls of the frame 5 are reinforced by beams, an example of which is identified by 17. The finest particles leave the Sizer after having passed the bottom screen cloth 11, while larger particles only to varying degrees manage to pass the screen cloths and are moved out of the Sizer, to the right in the drawing. If the vibrator motor were located in the centre of gravity, the entire machine would have an almost circular motion, and the desired effect would not be achieved. If the vibrator motor were located on top or forward of the centre of gravity, the movement would be more uniform or even counter-productive in that the inlet section may only have a transporting movement. Another example of the invention is shown schematically in Figure 7, which is a longitudinal section of the machine, and in Figure 8, which is an external side view of the machine. The Sizer consists of a frame 5, which is agitated by a vibration generating device, here consisting of a flywheel 19 and a piston 20. It should be mentioned that this device could be almost any kind of vibrator, or vibrators. The Sizer is reinforced by a transversal beam 21. The movement of the Sizer is here mechanically guided by swinging brackets 18. The free ends of the brackets are to be mounted in fixed points, which could be placed in different positions. It should be emphasized, that these brackets illustrate the principle of forced control of the movement only; their location, shape and number could be very different. Similar to the unit in Figure 2, the Sizer here has a feed plate 6 and screen cloths 7, 8, 9, 10 and 11. Suspension springs, as in Figure 4, are not shown. It should be underlined that the features of the invention could be obtained in different ways and with different devices and linear magnetic motors could also be used. Comparative studies, as described above, have so far only been made with Sizers, but it is believed that similar, beneficial effects would be achieved with conventional screens, using the same motion.

Claims: Apparatus for separating a collection of particles according to size, shape and/or density, said apparatus including a single vibrating means (13, 19) placed below an inlet and mounted on a transversal structure (14, 21) of a frame (5) forming a vibrating space having classifying elements (7 - 11) in the form of cloths, wires or bars, slopingly arranged in said space one below the other, the collection of particles being fed through said inlet into said space, the gravity centre (12) of said apparatus being situated relative to the direction of particle flow, in front of a vertical plane passing through the point of transfer of particles from the inlet to the classifying elements, characterized in that said single vibrating means (13, 19) is placed rear of said vertical plane. Apparatus according to claim 1, characterized in that said transversal structure is a transversal tube (14, 21). Apparatus according to claim 1, characterized in that said transversal structure is a transversal beam (14, 21). Apparatus according to any of the preceeding claims, characterized by a suspension system in the form of brackets (18) mechanically guiding the movements of the classifying elements (7 - 11). Apparatus according to any of the preceeding claims, characterized in that said vibrating means (13, 19) has alternative locations to compensate for the centre of gravity (12) being displaced by accessories fitted to the apparatus. Apparatus according to claim 4 or 5, characterized in that said suspension system is adjustable to allow variation of the motion.

Inventors: MOGENSEN PEDER, MOGENSEN, PEDER
Applicants: MOGENSEN FREDRIK AB, FREDRIK MOGENSEN AB
Application year: 1991
Main classification: B07B1
IPC codes: B07B1