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      Different ways to get an efficient grinding plant       
      1 Introduction              
        - Everybody knows that the cement industry is a huge consumer of energy.    
        - The global cement production was about 3,6 billion tonnes in 2012.     
        - If we take an average of 110 kWh to produce 1 ton of cement, it means a consumption of electricity of about
          400 billions of kWh.            
        - Considering that grinding departments are responsible for 65% of the total consumption (23% Raw meal
          and 42% Cement), it is then necessary to optimize these installations!    
        - The world's fleet is divided into four types of grinding machines:    
          * Tube Mills              
          * Roller Presses            
          * Vertical Roller Mills            
          * Horizontal Roller Mills            
        - You can see on the following table the estimated grinding efficiency for each type:  
        - The grinding efficiency of these machines is very poor and the greater part of the energy supplied by the
          absorbed power of the devices is lost into heat, vibration, friction wear or sound noise.  
        - The tube mill is the less efficient but is still the most common equipment in the world despite the emergence
          of more efficient devices like the vertical roller mill.       
        - And it is also commonly accepted that there is still a huge potential for possible improvements regarding ball mills.
        - We will now review all the solutions to improve a ball mill circuit.    
        - Here is the list:            
          * Increase the filling degree          
          * Right lifting lining in chamber 1          
          * Flow control intermediate diaphragm        
          * Classifying lining in second chamber          
          * Right ball charge gradation          
          * From open to closed circuit          
          * High Efficiency separator          
          * Pre-crushing system            
          * Pre-grinding system            
          * Automated control            
          * Predictive maintenance          
          * Grinding aids            
      2 Increase the filling degree          
        - It seems to be evident but not for everybody.         
        - This is an example:            
          * Ball mill diameter: 4m            
          * Total length: 13m            
          * Speed: 15,8 rpm or 74,7%Vcr          
          * 183t of balls or 28%VL => 2440 kW          
        - Now, if the filling degree is 32%.          
          * 210t of balls or 32%VL => 2660 kW          
        - Representing a gain of 9% which will be a maximum in this case. Why a maximum?  
        - This can be explained by the fact that the grinding efficiency also depends of the filling level.
    - See the figure below where it is possible to see that the efficiency is optimum at 27-28% volume load:
      3 Right lifting lining in chamber 1          
        - The lining must have a good lifting effect in order to crush properly the fresh material.  
        - On the figure below, one sees the balls falling in cataract and a dead zone that must be the smallest possible:
        - The choice of the lining depends of the kind of material, fresh material granulometry and hardness.
        - While the choice of the alloy is important to avoid breakage, decrease the wear rate and increase the lifetime.
        - The different linings are: STEP, Single wave lining, Double wave lining, Lorain lining, lifting-classifying lining...etc
      4 Flow control intermediate diaphragm        
        - A flow control diaphragm is often necessary because the first chamber is empty of material, especially near the
        - Laboratory trials and industrial tests showed 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%).
        - Different types:            
          * Adjustable ring FCD (Ch.Pfeiffer)          
          * Adjustable scoops FCD (Slegten)          
          * Adjustable lifters FCD (FL Smidth)          
      5 Classifying lining in second chamber          
        - A classifying lining is necessary because at the inlet of the second chamber, the material is coarser.
        - Then, we need impact forces, then bigger balls to crush the last big particles.     
        - At the contrary, at the chamber outlet, the material is finer, we need attrition forces and smallest balls.
        - The classifying lining is the most widely used in the world.      
        - Here below a sketch of this lining.          
      6 Right ball charge gradation          
        - It is very important but often neglected!        
        - The first compartment has the following objective: CRUSHING, then impact forces. Balls from ø90 to 60mm are
        - Factors influencing the composition are:        
          * granulometry            
          * hardness              
          * type of lining            
          * specific power consumption of the chamber        
          * slag              
          * additives              
          * moisture              
          * clinker              
        - The second compartment has the following objective:   FINISHING, then attrition forces. Balls from ø(60-50-40)30 to
          15mm are recommended.          
        - Factors influencing the composition are:        
          * fineness target of the cement          
          * ratio length/diameter (L/D)          
          * Type of circuit            
          * Type of separator            
        - Used balls are welcome, but balls of diameter 90mm must be NEW!    
      7 From open to closed circuit          
        - In the past, the majority of tube mills were operating in open circuit.    
        - After that, separators of the first, second and third generation have been developed.  
        - Depending of the separator and the fineness of the cement, the gain in kWh/t can be very important:
      8 High Efficiency separator            
        - A big jump in quality was achieved with the emergence of high-efficiency separators.  
        - We will not insist today on the principle of the three forces and the rotor.    
        - But we can see the differences on the following sheets:      
      9 Pre-crushing system            
        - These equipments have been developed in the 80-90’ties, like the VSI (Vertical Shaft Impactor).
        - On the following figure, you can see the operating mode with the rotor and its impellers which send the material
          against the anvils and cause the breakage.        
        - This kind of system is operating in open circuit or closed circuit with a vibrating screen.  
        - This is a very simple system, with small investment but big problem of wear!    
        - Then the success was very limited.          
      10 Pre-grinding system            
        - Everybody knows Roller Presses used for clinker and other materials, with very high pressures between 2 rollers
          (one fix, one moving).            
        - Different circuits are possible with RP, from open circuit up to complete grinding system.  
        - Here two circuits:            
      11 Automated control            
        - The target of the automated control is to optimize the installation with a steady system.  
        - Solutions are RP feed control, Elevator control, Ear control, Reject flow control, Total feed control...etc
        - Tools are (this list is not exhaustive):          
          * controller ON-OFF            
          * controller PID            
          * fuzzy logic              
          * Expert system            
        - Suppliers are (this list is not exhaustive):        
          * SCAP              
          * Milltronics            
          * ECS              
          * K-Tron/Hasler            
          * Slegten Go Control            
      12 Predictive maintenance            
        - A rigorous method of predictive maintenance will decrease downtime volume and will allow a production at a stable
          level over time.            
      13 Grinding aids              
        - As you know, the action of a GA is to decrease the surface energy forces created on the cement particles during
          the comminution work.            
        - In other words, it helps to counteract the charges present on the surface of cement particles.
        - Grinding aids action consequences are the following:      
          * Eliminate coating on balls and linings        
          * Improve the cement flowability          
          * Improve the separator efficiency          
          * Increase the production          
          * Improve the cement quality          
          * Prevent cement pack set upon storage        
      14 Possible improvements Summary table        
          (*) On the total production     
    All rights reserved © 2012-2015 The Cement Grinding Office