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      Grinding technology of limestone with ball mills
             
      8. Heat balance:    
             
      The biggest part of the energy introduced in a ball mill is converted into heat.
      As a consequence, this heat can induce very high temperatures. 
      These temperatures will affect the grinding process.
      Then, it is important to make the heat balance of the installation.
      For all heat balances, there must be an equilibrium between what goes in and what goes out
      of the system.    
         
     
     
             
             
             
             
      Regardless to the characteristics and production data of the installation that we have to introduce,
      the heat balance turns on 3 values:  
       - temperature of the product at mill outlet
       - amount of air    
       - water to inject    
             
      9. Problems of drying:    
             
      In all the cement plants, the most important problem of the raw mill section is the drying of the
      limestone and the other components.  
      In dry process, when the material has more than 1% moisture, the efficiency of the mill drops
      drastically.      
      Some theories say that the lost of efficiency is 10% every 1% moisture higher than 1%.
      In consequence, it is very important to get the drying capacities to avoid that.
      In the cement plants, it is always possible to recuperate the heat of the kiln and all the ball mills
      receive hot gas at 350°C.    
      An additional burner can be indispensable in some cases.
      It is also possible to dry in a separate dryer but the cost is higher.
      Very often, the drying is realized in the ball mill where there is or not a drying chamber.
      9.1. The drying chamber:    
      This chamber helps the drying of the material with the help of lifters.
      The material is lifted and after falls in the hot gas stream.
      Which length must have a drying chamber?
      According to industrial experiences, it is  possible to dry a maximum of 220-230 l/m3.h of water
      to be efficient. This figure is generally accepted to size the drying chamber.
      Example:      
      - Mill diameter: 4.6 m    
      - Mill output: 230 t/h    
      - Limestone moisture: 4%  
      => quantity of water: 230 * 1000 * 4% = 9200 l/h
      => volume requested: 9200 / 225 = 40.89 m3
      => length of the drying chamber: 40.35 / (p * 4.62 / 4) = 2.46 m (with the transfer diaphragm)
      We will take a length of 2.50 m as it is DIN.
      9.2. Temperature and quantity of gas:  
      The drying effect is more efficient with a higher temperature than with a higher quantity of gas.
      But due to mechanical problems (bearings), we can be limited in temperature.
      On the other side, the quantity of gas must respect some limitations in term of gas velocity
      inside the mill.    
      These limitations are:    
      - Compound mill: 1.5-2 m/sec  
      - Birotator mill: 3-4 m/sec  
      - Airswept mill: 6-9 m/sec  
      Then, the drying capacities must be a compromise between temperature and quantity of gas
      in order to run in good conditions of efficiency.
             
      End of the presentation    
             
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