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      Pre-grinding systems        
      3   Roller Press        
         3.1 Introduction:        
        - As already mentioned in the introduction, roller presses have been very successful as soon they appeared and
          despite reliability problems with grinding rollers, especially in the case of clinker.  
        - These problems resolved, the field of application has been expanded as shown on the list below (non-exhaustive):
          * clinker        
          * slag        
          * limestone        
          * coal        
          * iron ore        
          * copper ore        
          * quartzite        
          * limestone        
          * gold ore        
          * diamonds        
          * phosphate        
        - The roller press is used for crushing, for coarse grinding and fine grinding.    
        - Even today, with the emergence of other grinding systems like the vertical mill, the roller press seems to be
          the alternative that brings the most advantages.      
         3.2 Operating principle:        
        - Operating diagram here below:        
        - The roll press is mainly composed of two grinding rollers arranged horizontally.    
        - As seen in the drawing above, the 2 rollers rotate in opposite directions.    
        - One is movable and the other is fixed.        
        - The material arrives from above and falls by gravity between the two rollers.    
        - The installation has an adjustable feed hopper which provides a steady feed to the rollers.  
        - The movable roller exerts the pressure required to achieve the action of grinding of particles.  
        - The pushing of the movable cylinder is performed by a hydraulic system and amortized with nitrogen gas.
        - This crushing action is often called "grinding in a material bed".      
        - The pressure is maximum at the location where the rollers are closest to each other.  
        - Then the pressure decreases rapidly and the material comes out in the form of compressed cakes.
        - The material then passes through a deagglomerator.      
        - We will see after the progress of the material after the roller press because there are many possible circuits.
        - In the next chapter, we will analyse a little more the grinding action of the roller press.  
         3.3 Explanation of the grinding action:        
        - Diagram of working zones:        
        - The material comes out the hopper from the line AA'.      
        - The material feed between the rollers is carried out by means of the force of gravity and the friction between
          the roller and the material.        
        - This is the beginning of the introduction area which goes up to the line BB'.    
        - The material is not yet subject to pressure in this area AA '-BB' and its bulk density is always the same.
        - From AA ', the velocity of the material increases up to be closer that the one of the rollers in BB'.
        - The angle αi of the figure corresponding to this introduction zone is dependent of the regulation of the feed.
        - Then, the material enters in the compression zone which is located between BB 'and DD'.  
        - In this area, the pressure on this material bed rises very rapidly to reach its maximum just before the line DD'.
        - The material is crushed and ground in this area.      
        - The bed of material between the rollers is ground by an interparticle failure mechanism.  
        - In other words, the particles are reduced in size by compression forces therebetween.  
        - Illustration here below:        
        - Note that the line DD' corresponds to the center line of the rollers.      
        - The angle αc corresponds to this area.        
        - The compression zone obviously corresponds to an increase of the bulk density of the material.
        - Material densities are related by the following relationship:      
          ρo is the initial density at BB'        
          Ho is the distance between B and B'        
          ρ is density at the point of maximum compaction, either at DD'      
          H is the distance between D and D'.        
        - In the compression zone (we can also say compaction) is the area CC'-DD'.    
        - In this area CC'-DD' which is the last part of the compaction zone, there is a phenomenon of plastic deformation,
          and the emergence of microcracks for fragile particles (brittle).      
        - A interparticle grinding takes place and each particle transmits the force imposed on another particle.
        - The angle αf corresponds to this area and is also called angle of friction, wherein the lamination process of the
          material ends.        
        - From the line CC', the sign of the friction force changes and the material is pushed out of the compaction zone.
        - From the line DD', the pressure decreases rapidly until it becomes zero.    
        - Below the line DD', the product that comes out is a compact slab, consisting of a large proportion of fines particles
          and micro-cracked particles.        
        - Microcracks illustration here below:        
        - It is also important to note that the material that has only been elastically deformed in the area CC'-DD' returned
          to its original state as soon as the pressure disappears.      
        - In this last zone DD'-EE', corresponding to the angle αd, on can see that the material in expansion thus goes from
          a thickness H to a thickness Hd.        
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