Patent Publication Number: US-2010116720-A1

Title: Apparatus and method for sifting feedstock

Description:
The invention relates to an apparatus for sifting feedstock, having a static sifter comprising an aeration base which is oriented at an angle to the vertical and through which sifting gas flows, and a dynamic sifter which is arranged downstream and which comprises at least one rotor having a horizontal rotor axis. 
     DE 10 2005 045 591 A1 discloses a grinding installation in which a static sifter is operated directly in front of a dynamic sifter, and a material bed roller mill and/or a tube mill is(are) used as the mill. This type of static-dynamic sifter has proved its worth for specific tasks. The feedstock passes via feeding devices (conveyor belts/chutes) onto the aeration base of the static sifter and then slides downwards via the aeration base. 
     The sifting air flowing through the feedstock in transverse flow carries the fine material to the dynamic sifter, while the coarse material of the static sifter is discharged by gravity at the lower outlet. The fineness of the fine material of the static sifter can be influenced by altering the sifting air volume flow. In the dynamic sifter, the desired product fineness is adjusted by means of the sifting volume flow and the speed of the rotor. 
     Owing to structural measures, the sifting air should flow against the rotor substantially tangentially in order to support the centrifugal field built up by the rotor. The tangential incident flow is achieved by a volute configuration of the housing surrounding the dynamic sifter in combination with a dynamic rotor arranged eccentrically with respect thereto. The resultant flow conditions can also be gathered, for example, from DE 103 50 518 A1. The centrifugal force acting on the particles and the sweeping force of the sifting air acting in the direction towards the rotor separate the feedstock of the dynamic sifter into product and coarse material. 
     Furthermore, DD 263 468 A1 discloses a pneumatic sifter in the sifting chamber of which at least two rod baskets operated in opposite directions of rotation are arranged one above the other in two planes which are perpendicular to the axis of the sifting chamber. 
     The object of the invention is to improve the sifting efficiency of a static-dynamic sifter. 
     According to the invention, that object is achieved by the features of claim  1 . 
     The apparatus according to the invention for sifting feedstock basically comprises 
     a. a static sifter having an aeration base which is oriented at an angle to the vertical and through which sifting gas flows,
 
b. an inlet opening for feeding the feedstock onto the aeration base,
 
c. an outlet opening for the coarse material,
 
d. a dynamic sifter which is arranged downstream and which comprises at least one rotor having rotor blades and a horizontal rotor axis,
 
e. at least one outlet opening for the sifting gas charged with fine material,
 
f. and also a housing in which the static and the dynamic sifter are arranged, the region of the housing surrounding the dynamic sifter being in the form of a housing volute so that a substantially tangential flow of sifting gas against the rotor results.
 
     The direction of rotation of the rotor is counter to the direction of flow of the sifting gas in the housing volute. 
     Further forms of the invention are the subject-matter of the subordinate claims. 
     The rotor blades of static-dynamic sifters are normally oriented radially. 
     A further increase in the sifter efficiency of the dynamic sifting stage can be achieved by also inclining the rotor blades by from 10 to 50° to the radial direction. The sifter efficiency of the dynamic sifting stage can thereby be improved by 10% or more. 
     According to a preferred form, guide plates for optimising the tangential flow against the rotor are provided in the region between the static and the dynamic sifter, it being possible for at least one of the guide plates to be arranged in such a manner as to be adjustable. 
     During the operation of the apparatus for sifting, it has also been found to be especially advantageous if the circumferential speed of the rotor is markedly increased compared with conventional operation, a circumferential speed in the range of from 15 to 35 m/s, preferably in the range of from 20 to 30 m/s, being regarded as especially advantageous. 
     The above-described apparatus for sifting is especially suitable in a grinding installation having a mill. If, in addition, the mill is formed by a material bed roller mill, the static sifter can be used at least in part to break up, or deagglomerate, the scabs coming from the material bed roller mill. 
     Further advantages and forms of the invention will be explained in more detail hereinafter by means of the description and the drawings. 
    
    
     
       In the Drawings 
         FIG. 1  is a diagrammatic sectioned view of the apparatus according to the invention for sifting feedstock, 
         FIG. 2  is a detailed view in the region of the rotor, and 
         FIG. 3  is a flow diagram of a grinding installation having an apparatus according to the invention for sifting feedstock. 
     
    
    
     The apparatus  100  shown in  FIG. 1  for sifting feedstock  1  basically comprises a static sifter  2  having an aeration base  4  which is oriented at an angle to the vertical and through which sifting gas  3  flows, and a dynamic sifter  5  which is arranged downstream and which comprises at least one rotor  6  having a horizontal rotor axis  7 . 
     The static sifter  2  and the dynamic sifter  5  are arranged in a housing  8  which has an inlet opening  9  for feeding the feedstock  1  onto the aeration base  4 , and an outlet opening  10  for the coarse material. Furthermore, an outlet opening  11  is provided for the sifting gas charged with fine material. 
     The region of the housing  1  surrounding the dynamic sifter  5  is in the form of a housing volute, so that substantially tangential flow against the rotor results (see arrows  12 ,  13 ). Therefore, in the embodiment shown, the sifting gas charged with fine material flows substantially clockwise into the housing volute. 
     The direction of rotation  14  of the rotor  6  is counter to the direction of flow (arrows  12 ,  13 ) of the sifting gas into the housing volute, that is to say, in the view according to  FIG. 1 , the rotor rotates anticlockwise. 
     It can be seen from the detailed view according to  FIG. 2  that the rotor  6  has rotor blades which are so set that they are at an angle α of from 10 to 50°, preferably from 25 to 35°, relative to the radial direction  16 , with the rotor blades  15  being offset at their outer circumference relative to the radial orientation in the direction of rotation  14  of the rotor. 
     During the sifting operation, large portions of the rotor  6  are subjected to tangential incident flow and, as a result of the direction of rotation of the rotor, a centrifugal field rotating in the opposite direction builds up. It therefore becomes necessary for the sifting air (arrow  13 ) and the particles  1   a  contained therein to perform a sharp turn-around from the clockwise direction into the opposite direction. As a result, a significantly improved sifting outcome becomes apparent. The coarse material of the dynamic stage consequently contains markedly fewer fines, as a result of which the throughput can be substantially improved. The coarse material entrained with the sifting air passes around the rotor and is drawn off via a duct  17  to the outlet opening  10 . Optionally, a medium-grain fraction could instead be drawn off separately. 
     In order to optimise the tangential flow against the rotor  6 , guide plates  18  can be provided in the region between the static and the dynamic sifter  2 ,  5  and are preferably arranged to be adjustable. The guide plates are so oriented that the majority of the sifting air volume flow streams into the housing volute in the clockwise direction. Only a minor portion is drawn in anticlockwise. 
     The sifting efficiency can be further substantially increased if the rotor  6  rotates substantially faster than in the case of the conventional clockwise direction of rotation, which produces turbulence. The power consumption of the rotor consequently increases accordingly. The higher product fineness which normally results from the higher speed of rotation is avoided by the set of the rotor blades. In the tests on which the invention is based, operation of the rotor  6  at a circumferential speed in the range of from 15 to 35 m/s, preferably in the range of from 20 to 30 m/s, has proved to be especially advantageous. 
     The above-described apparatus  100  for sifting is suitable for use in a grinding installation together with a mill, especially a material bed roller mill  200 . As can be seen from  FIG. 3 , the coarse material passes from the apparatus  100  via the outlet opening  10 , optionally together with fresh material  19 , into the material bed roller mill  200 . The comminuted material is guided by suitable conveying means, for example a bucket conveyor, to the inlet opening  9  of the apparatus  100  for sifting the feedstock. The fine material is drawn off by way of the outlet opening  11  and conveyed to a separator  100  for separating the sifting air from the fine material. 
     With the above-described apparatus  100  for sifting feedstock, the sifter efficiency of the dynamic sifting stage can be increased by 10% or more compared with conventional sifters, as described, for example, in DE 10 2005 045 591. The throughput and the electrical energy requirement of a grinding installation having a material bed roller mill can consequently also be substantially improved.