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      Pre-grinding systems        
      3   Roller Press (suite 6)        
         3.15 Modification of the ball mill:        
        - The modification of the internals of the ball mill is often part of the package of modifications required in order
          to meet the guarantees.        
        - Depending on the case, this modification at the level of the mill will be important or not.  
         3.15.1 Case of a roller press in open circuit        
        - The modification at the level of the mill is not important.      
        - One recommends often a finer 1st Chamber ball charge, possibly without balls 90mm, but not too much if
          components such as limestone are not treated in the press.      
        - One can also reduce the length of the first compartment, but it is quite rare.    
         3.15.2 Case of a roller press with recirculation of cakes      
        - The specific power of the first chamber of a ball mill is always between 9 and 12 kWh/t.  
        - With a pre-grinding system, the required specific power is no more than 6-7,5 kWh/t.  
        - The first chamber is therefore shortened according the new requirement.    
        - Here below is an example of chamber 1 length modification:      
        - The second modification to do is to replace the ball charge of chamber 1 with a finer one.  
        - In fact, the fresh material entering the mill is much finer with a significant proportion of particles lower than 90μ.
        - Also, many of the grains of material have micro cracks, then are easy to crush.    
        - Consequence, the 90 mm diameter of balls is not used.      
        - The charge will be composed with balls of 80, 70, 60 and 50 mm.      
        - 60 and 50 mm balls represent almost 50% of the total.      
        - The average weight is less than 1000 g / ball.      
        - We must always keep in mind that fresh feed going to the roller press can be diverted to the ball mill because
          due to the presence of metal particles.        
        - If the stand-by period of the roller press is long enough, the ball mill may be completely polluted with unground
          grains in the medium term, with negative consequences in terms of performance and production.
         3.15.3 Case of a roller press in closed circuit with a high-efficiency separator    
        - The ball mill receives the fines of the press separator as feed.      
        - In this case, the first chamber is useless.        
        - The mill becomes a monochamber mill.        
        - Even the classifying lining is useless because the ball charge consists only of small balls.  
        - This charge will consist of balls of diameter 30, 25, 20 and 17 (15) mm.    
        - The average weight of the charge will vary between 30 and 35 g/ball.    
        - The problem of unplanned downtime of the press should always be considered.
         3.16 Advantages of the roller press:        
        - The possibility of production increases can be very large, up to more than 100% of the existing capacity.
        - Efficiency gains are also very consistent and allow to reduce the size of the ball mill which is a big consumer
          of energy.        
        - The return on investment is very interesting.      
        - The inter-particle grinding that creates micro-cracks in the particles increases the grindability of the material
          before its introduction into the ball mill.        
        - The finer material entering in the ball mill reduces the wear rate of the internal equipments (grinding media).
        - The space required for the installation of a pre-grinding system is reasonable.    
        - The press can accept up to 2% moisture (even improving its efficiency).    
         3.17 Disadvantages of the roller press:        
        - The initial investment is enough important especially for the semi-finished grinding system.  
        - The operation at the level of the feed is more complicated than a ball mill.    
        - Maintenance costs are high (repair and replacement of the rollers).    
        - Maintenance workforce must be highly technical.      
         3.18 Conclusion:        
        - The roller press is today, with the vertical mill which may be concurrent, the milling system that knows more
          success in the cement industry, both as pre-grinding system than as full integral grinding system.
        - The mining industry also adopts more and more this system.      
        - Manufacturers offer a wide range of solutions because the possibilities are endless.  
        - Finally, studies and tests are ongoing to improve the resistance of the rollers surface which is the weak part
          of the machine.        
        - The roller press probably has a bright future on the horizon.      
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