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      Grinding technology of limestone with ball mills
             
      7. Ventilation:    
             
      7.1. Introduction:    
      The ventilation has 3 objectives:  
       - to insure The cooling of The mill and The material
       - to dedust The mill    
       - to remove The fine particles From The mill
      7.2. Cooling of the mill:    
      A good cooling of the mill and the material inside the mill is necessary to maintain a proper working
      temperature.    
      The temperature of reference is the one of the material at the mill outlet.
      For the cement mills, one can not exceed a temperature of 105-110°C according the type of circuit.
      For the limestone, a temperature lower of 100°C should be better.
      If one goes higher, we will probably observe a coating on the lining and the balls.
      7.3. Problem of coating:    
      The coating is an agglomeration of material on the lining and/or grinding media.
      It is due to the superficial cohesion forces and the forces generated by the static electricity.
      As limestone has the particular propriety to produce a big amount of very fine particles
      (less than 2 microns) when grounded, the problem of coating can become crucial.
      Coating is an agent of efficiency loss which can be very important (up to 30%) because:
       - coating is disturbing The segregation of The Ball charge In case of classifying lining.
       - coating is also reducing the attrition's efficiency of the small balls which are not more in contact
       with the material.    
      7.4. Dedusting of the mill:  
      This function seems to be evident if we want to avoid the accumulation of dust in the whole system.
      7.5. Ventilation usual values:  
      When we want to define the necessary ventilation in a grinding installation, it is calculated in
      terms of air velocity in the free section of the mill, i.e:
         
     
     
             
             
      The reference values which are generally admitted are divided in two cases and are the following:
             
      - Open circuit:     
         
     
     
             
             
      - Closed circuit:    
         
     
     
             
             
      Remark: we can also define the ventilation in term of Nm3/kg of finished product.
      Normally: 0.3 to 0.45 Nm3/kg.   
      But this method is less usual.  
      7.6. Air speeds into the mill:  
      Besides the basis figures given hereabove, it is very important to respect some velocities in other
      parts of the mill in order to:  
       - allow The passage of The required air
       - decrease as far as possible The pressure drop
      These velocities are:    
      - mill inlet: 20-25 m/sec    
      - intermediate and outlet diaphragm: 15-20 m/sec
      - mill outlet: 20-25 m/sec    
      7.7. Mill's pressure drops:  
      The less pressure drops are important and the more:
       - It will be easy to let The required air through The mill
       - The energetic consumption will decrease (kW of The fan)
      The standard pressure drops if the mill is well equiped are:
         
     
     
             
             
             
             
             
             
      7.8. False air:    
      The false air is an air entry at mill outlet (at the outlet hood or at the peripheral discharge for
      the birotator mills) which is added to the air really passing through the mill.
      This supplementary air does not bring anything, overloads the fan or reduces the ventilation of
      the mill if the existing fan is too short. This false air is coming from some badly adjusted airtightness
      or from deficient discharge devices.  
      We evaluate the false air percentage in function of the type of mill and its transmission.
      These values are given in percentage of the mill's required air:
         
     
     
             
             
             
             
             
             
      In bad condition, a birotator mill can have up to 60% false air!
             
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