Abstract:
The invention relates to a method and to apparatus for crushing brittle material for grinding using a roller mill and a material bed roll mill for crushing the material for grinding falling downwards over the edge of the grinding track before it is delivered to the sifter. By the use of a material bed roll mill a considerable increase in the throughput capacity of the apparatus and a significant saving of energy can be achieved.

Description:
The invention relates to a method and apparatus for crushing brittle material for grinding. 
     It is frequently desired to increase significantly the throughput capacity of apparatus of the aforementioned type (containing a roller mill, sifter and bucket elevator). 
     The object of the invention, therefore, is to provide a method and apparatus improved in such a way that the throughput capacity of the apparatus is significantly increased with comparatively low expenditure on plant. 
     This object is achieved according to the invention by the provision of a roll mill between the roller mill and bucket elevator. Advantageous embodiments also form parts of the invention as set forth hereinafter. 
    
    
     THE DRAWINGS 
     One embodiment of the invention is shown in the drawings, in which: 
     FIG. 1 shows an overall diagram of apparatus according to the invention. 
     FIG. 2 shows a section through the roller mill and sifter of the apparatus according to FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     The apparatus shown in FIG. 1 for crushing brittle material for grinding, for example cement clinker, contains a roller mill 1, a sifter 2 arranged above it, a material bed roll mill 3, a bucket elevator 4, a fines separator 5 and a plurality of further parts which will be described in connection with the functioning of the apparatus. 
     The roller mill 1 which can be seen in detail in FIG. 2 has a horizontal grinding track 7 which runs around the vertical axis 6 and on which grinding rollers 8, 9 press. 
     The grinding track 7 is driven by a drive motor 10 and a gear unit which is not shown in detail in the support of the roller mill 1. 
     An adjustable nozzle ring 11 through which an air stream (arrow 12) is delivered to the grinding track 7 is arranged on the periphery of the grinding track 7. 
     The material for grinding which is not yet sufficiently crushed falls downwards over the edge of the grinding track 7 (i.e. against the air stream) and passes via a chute 13 to the roll mill 3 (arrow 14). 
     The dynamic sifter 2 arranged above the roller mill 1 contains a fixed (advantageously adjustable) guide vane ring 15 and sifting cage 18 which is driven by a motor 17 via a shaft 16. The shaft is mounted at its lower end by means of radial struts 19. 
     The shaft 16 also has above the sifting cage 18 a distributor plate 20 to which the material to be sifted is delivered (arrow 23) from the bucket elevator 4 via short pipes 21, 22. 
     Below the guide vane ring 15 the sifter 2 has a hopper 24 through which the tailings can be returned to the grinding track 7 of the roller mill 1. The raw material is delivered to the grinding track 7 through an opening 25 (arrow 26). 
     A tailings screw 27 which can be regulated in speed is also connected to the lower end of the hopper 24 and delivers an adjustable proportion of the tailings direct to the roll mill 3 (arrow 28 in FIG. 1). 
     The air stream conducting the fines is drawn off from the sifter 2 via a pipe 29 (arrow 30). 
     The apparatus functions using the method according to the invention as follows: 
     The material for grinding which is first to be crushed is delivered to the roller mill 1 from a silo 31 via a proportioning belt scale 32 and an air excluding device 33. The material for grinding which has been sufficiently crushed on the grinding track is conveyed to the sifter 2 by the air stream. The fines leaving the sifter together with the air stream are precipitated in the separator 5. 
     A proportion of the tailings precipitated in the sifter 2 is returned to the grinding track 7 of the roller mill 1. A further, adjustable proportion of the tailings passes via the tailings screw 27 direct to the roll mill 3. The material for grinding falling downwards over the edge of the grinding track 7 is also delivered to this material bed roll mill. In the roll mill 3, the rolls of which are pressed against one another with a high pressure, the material undergoes material bed crushing with noticeable formation of agglomerates. 
     The material for grinding discharged from the roll mill 3 is conveyed upwards via the bucket elevator 4 and by means of a valve 34 is either delivered to the distributor plate 20 of the sifter 2 or to some other application (vehicle 35) (if required, this stream of material delivered via the bucket elevator 4 can also be divided). 
     The agglomerates of material for grinding which are formed in the roll mill 3 undergo disagglomeration on the rotating distributor plate 20 of the sifter 2. The agglomerates which are broken up in this way are thus delivered evenly to the sifting air. 
     The finished material which is already present after passing through the roll mill 3 is thus directly sifted and no longer returned to the roller mill 1. In this way the roller mill 1 is substantially relieved of strain. 
     As a result there is a considerable reduction in the pressure loss for the roller mill, which leads to a marked saving in energy in the pneumatic conveying of the material for grinding. 
     It is also a particular advantage that the material recirculated by the bucket elevator is already largely dried, since in the roll mill 3 the material for grinding falling downwards over the edge of the grinding track 7 is ground together with tailings (delivered via the tailings screw 27) which are already largely dried. 
     The apparatus is controlled essentially by the use of the following control circuits: 
     A first control circuit (regulator 36) regulates the 
     quantity delivered to the roller mill 1 via the proportioning 
     belt scale 32 as a function of the drive 
     power of the roller mill 1. 
     A second control circuit (regulator 37) keeps the 
     electric current consumption of the material bed roll 
     mill constant by appropriate regulation of the crosssection 
     of the adjustable nozzle ring 11 and thus the 
     gas speed in the nozzle ring (the proportion of 
     material for grinding falling downwards against the 
     air stream, and with it the quantity of material 
     recirculated by the bucket elevator, alters with the 
     gas speed in the nozzle ring). 
     A third control circuit (regulator 38) keeps the 
     quantity of gas drawn through the apparatus constant 
     by influencing the speed of a fan 39 arranged after 
     the fines separator 5. 
     A fourth control circuit (regulator 40) controls a 
     circulating air valve 41 and thus keeps the negative 
     pressure prevailing in the air stream before the 
     roller mill 1 constant. 
     Only a proportion of the tailings precipitated in the sifter 2 is advantageously removed via the tailings screw 27 and delivered direct to the material bed roll mill 3. This part quantity is regulated by adjusting the speed of the tailings screw 27 which operates as an overflow screw. The quantity removed can be weighed and thus the speed of the screw can be controlled. 
     The use of pneumatic means for conveying the material for grinding in a conventional grinding apparatus using a roller mill takes a comparatively large proportion of the total energy used for grinding. By means of the invention the material for grinding falling downwards through the nozzle ring 11 is subjected to further crushing by means of the material bed roll mill 3 and delivered via the bucket elevator 4 to the sifter 2. Up to 50% finished material is contained in this material for grinding which is delivered directly to the sifter (depending upon the type of material to be ground). This proportion of material is thus withdrawn from the pneumatic conveying. This results in a pressure loss for the grinding apparatus which is up to 40% lower, which results in a correspondingly high energy saving on the mill fan.