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      Pre-grinding systems        
                   
      3   Roller Press (suite 2)        
         3.5 Methods of use and types of circuits (suite):      
         3.5.6 Pre-grinding - Roller press with recirculation of cakes      
        - Conventional flowsheet here below:        
         
     
           
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
        - The fresh material supplies the roller press.      
          There are cases where only the clinker passes through the press, the other components go directly to the ball mill.
        - Before going into the press, a magnetic metal extractor removes foreign objects which could be in the feed.
        - Then, a metal detector can switch on the bypass of the material if necessary.    
        - The material is then treated in the roller press.      
        - At the outlet, a part of the material of cakes returns to the press while the other part goes to the mill.
        - In this circuit, the roller press can supply several mills if only clinker is pre-ground.  
        - It is this circuit which was predominant during the introduction of this technology in the 80s.  
         3.5.7 Semi-finished grinding - Press and mill in closed circuit      
        - Conventional flowsheet here below:        
         
     
           
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
        - The fresh material supplies the roller press.      
        - Normally in this type of circuit, all the components of the feed pass through the roller press.  
        - Before going into the press, a magnetic metal extractor removes foreign objects which could be found in the feed.
        - Then, a metal detector can switch on the bypass of the material if necessary.    
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        - The material is then treated in the roller press.      
        - The product of the press is sent to a separator via a bucket elevator.    
        - Before going into the separator, the cakes of material are deagglomerated in a suitable device, which can be
          independent or forms an integral part of the separator.      
        - The separator is of the 3rd generation.        
        - The rejects of the separator go back to the press.      
        - The fines of the separator move towards the mill circuit (usually towards the mill itself).  
        - In some cases, a two-stage separator may be used for the press and the mill.    
         3.6 Problem of the feed:        
        - The common feed of the roller press is called "choke feed the press" in the sense of feeding continuously.
        - In other words, this is a column of material which feeds the press.      
        - And to ensure its smooth operation, the material must be aerated before entering the compression zone.
        - The feed must be constant.        
        - This is why power systems offered by suppliers are adjustable.      
        - The mosaic picture here below shows the adjustable power systems offered by the 4 large roller press suppliers:
         
     
     
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
           
        - Description:        
          1. Feed hopper (vertical)        
          2. Control gate        
          3. Feed gate        
        - The control gate (2) is composed of two plates (tongues) which allow the stopping of the press feed and the material
          retention in the feed hopper.        
        - They are activated hydraulically or pneumatically.      
        - The feed gate (3) is normally composed of two plates that regulate the flow of material supplied and also the
          eccentricity of distribution of fresh material  between the rollers.      
        - The experience has in fact shown that the efficiency of the press was better if the material flow was offset towards
          the fixed roller.        
        - Of course, the optimum setting is found by performing tests.      
        - The feed gate is either motorized or with manual adjustment.      
        - There is also an alternative feed called "starve" mode.      
        - That consists in optimizing the feed rate so that there is a maximum average spacing between the rollers without
          a column of material above them.        
                   
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