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      Grinding technology of limestone with ball mills
             
      5. Mill's internals:    
             
      5.1. Compound mill:    
      This kind of mill has normally 2 compartments.
      The coumpound mill is generally used to grind cement and pure limestone in some cement plants.
      5.1.1. First compartment:    
      The first compartment is to prepare the material before to go in the second chamber.
      Generally, the lenght is the third of the total usefull lenght. But it can be different due to the
      granulometry of the fresh feed.  
      The target is to have at the end of the chamber: maximum 5% residue on 2,5mm.
      Lining:      
      The lining is a lifting effect lining.  
      The shape can be different according:  
      * the granulometry of the limestone  
      * the hardness of the limestone  
      * the ball charge    
      * the length      
      * the speed of rotation    
      Ball charge:      
      The ball charge is depending of:  
      * the granulometry of the limestone  
      * the hardness of the limestone  
      * the lining      
      Generally, the ball charge is a gradation of balls diameter: 90mm, 80mm, 70mm, 60mm.
      Intermediate diaphragm:    
      Very often, the first chamber is empty of material, especially near the diaphragm.
      By laboratory trials and industrial tests, we got the following diagram:
     
     
         
             
             
             
             
             
             
             
             
             
             
             
             
      This diagram shows that the optimum of efficiency is reached when the ratio
      weight of material/weight of balls is around 18%.
      Practically, it means that the material must be at the level of the balls (to fill the voids which
      represent 42%).    
      The flow control diaphragm (FCD) has the function to regulate the material level of the 1st chamber.
      The diaphragm must also be designed in order to allow the ventilation of the mill (big center).
      The slots of the grate plates must have 6mm width if the moisture content of the limestone is
      lower than 1%, if this content is higher, slots than 8mm must be forecast.
      5.1.2. Second compartment:  
      The second chamber is the finishing chamber. It means an attrition work in order to produce fineness.
      Lining and ball charge:    
      For that reason, the lining is a classifying lining. The biggest balls are at the beginning of the
      compartment where the material coming from the first chamber is coarser, and the finest balls
      at the end of the chamber where the material is the finest.
      Laboratory trials were realized in the past and the conclusion is that it is better to classify the balls
      by dimension.    
      Also, finer is the ball, better is the efficiency.
      According to these trials, balls of 10mm diameter are the best. But it is inacceptable to put these
       balls in an industrial ball mill because the retention time of the material would be excessive.
      Then balls of 15mm minimum are used in the cement mills in open circuit, balls of 17mm or 20mm
      in closed circuit.    
      Concerning the limestone, it is better to limit the smallest dimension at 20mm because problems
      of coating (see hereafter).  
      The ball charge will consist in a gradation of balls: 60mm, 50mm, 40mm, 30mm, 25mm and 20mm
      diameter.      
      15% of balls 60, 50 and 40mm are used to break the last big particles coming from the 1st chamber.
      The rest of the ball charge depends of:
      * the fineness of the finished product  
      * the kind of separator    
      * other factors like: ventilation, temperature, compartment length
      Outlet diaphragm:    
      The slots much be bigger than the ones of the intermediate diaphragm.
      5.2. Double rotator mill:    
      This kind of mill is used in the raw mill section of a cement plant.
      Limestone is prepared in order to produce the clinker in the kiln.
      The fineness requested is generally between 10 and 20%R90m and 1-2%R210m.
      The double rotator mill is composed of:
       - one Drying chamber with lifters  
       - one transfer diaphragm which allows important ventilation
       - one coarse grinding chamber with a lifting lining and a ball charge with 90-80-70-60-50mm.
       - one Peripheral discharge diaphragm.
       - one fine grinding chamber on the other side with a classifying lining and a ball charge of 50-40-30-25mm.
      The proportion of the lengths is generally 40% for the coarse grinding chamber and 60% for the 
      fine grinding chamber.    
      The volume of the drying chamber depends of the output of the mill and the moisture of the
      limestone.      
      5.3. Air-swept mill:    
      This mill has only one grinding chamber with or without drying chamber.
      The configuration is the following:  
       - 40% of the length with a lifting lining
       - 60% of the rest with a classifying lining
       - From time to time, it is interesting to add a retaining ring between both linings.
       - The ball charge is composed of: 90-80-70-60-50-40-30mm diameter balls.
       - The filling degree is often low In order to insure a large passage For The hot gas.
             
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