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      Pre-grinding systems        
      3   Roller Press (suite 3)        
         3.7 Problem of the roller pressure:        
        - The force generated on the rollers to complete the work of comminution comes from 3 factors:
          * The number of hydraulic cylinders        
          * The diameter of the cylinder        
          * Operating pressure        
        - The formula calculating the force is as follows:      
          F is the force generated in kN        
          d is the roll diameter in m        
          n is the number of cylinder        
          p is the pressure in bar        
        - The specific force is given by the formula:        
          S is the specific force in kN/m2        
          D is the diameter of the rollers in m        
          l is the width of the rollers (or the bed of material) in m      
        - Example:        
          * Rollers diameter: 1,6 m        
          * Rollers Width: 0,9 m        
          * Diameter of pistons (hydraulic cylinder): 0,53 m      
          * Number of cylinders: 4        
          * Operating Pressure: 110 bar        
          Calculate the force generated and the specific strength:      
        - Generally, the hydraulic pressure is between 50 and 150 bar.      
        - The specific force generated is between 6500 and 8000 kN/m2 depending on the manufacturer.
        - Obviously, the permitted specific force must consider the problem of wear roller which is still current.
        - The constant shock of the rollers is absorbed by a nitrogen accumulator system installed behind the moving roller,
          allowing smooth operation of the press.        
         3.8 Operating parameters:        
         3.8.1 Feed granulometry        
        - The maximum size of the feed recommended varies from one manufacturer to another.  
        - But it is in any way dependent on the diameter of the rollers.      
        - A commonly accepted value is 50 mm.        
        - Another means for determining the maximum size is given by the formula:    
         3.8.2 Rollers speed        
        - The circumferential speed of the rollers is a key parameter of the operation of the press.  
        - Its metric unit is m/s (meters per second).        
        - This speed determines the throughput of the press and consequently the power consumption of the press.
        - But the speed of the rollers must also adapt to the particle size of the fresh feed.    
        - Indeed, if the feed includes a large proportion of fine particles (90μ), the press tends to vibrate if the roller speed
          is high (1,7-1,9 m/s).        
        - Result, a regular operation is not possible.        
        - This can happen in the case of the circuit with recirculation of cakes, because there are usually a large proportion of 
          fines in it.   
        - The diagram here below shows the influence of the recirculation of cakes on the speed of the rollers:
        - We see that the greater the circulation factor and the more it is necessary to reduce the speed, so the output.
        - The circumferential speed of the rollers is between 1 et 1,6 m/s depending on the conditions of use.
         3.8.2 Gap between rollers        
        - The gap between the two rollers equals the thickness of the material bed.    
        - This is a critical parameter since it is directly proportional to the total flow rate of the press.  
        - The distance between the rollers is primarily dependent on the press dimensions.  
        - Is estimated to be between 0,013.D and 0,02.D (D being the diameter of the rollers in mm)  
        - If this method is used, we have:        
         3.8.3 Operating Pressure        
        - It is known that the grinding efficiency depends on the specific power transmitted to the material bed and the same
          specific energy depends on the absorbed power.      
        - The absorbed power is also correlated to the transmitted force itself correlated to the operating pressure.
        - We must therefore find the best setting for this pressure which actually depends on three parameters:
          * Initial settings of the nitrogen accumulators      
          * Pressure setting of the hydraulic system        
          * Initial gap between rollers        
        - A compromise must be found between these 3 parameters to achieve the proper operation pressure.
         3.8.4 Problem of moisture        
        - Some materials can be very wet such as limestone, slag, coal, shale, pozzolan or clay.  
        - Obviously, a drying plant must be installed in order to dry these materials prior to their introduction in the press.
        - However, a certain percentage of residual moisture (up to 2%) can be beneficial in order to stabilize the feed.
        - This is particularly the case for grinding slag material.      
         3.9 Wear problems of the rollers:        
        - As was mentioned in the introduction, the beginnings of the roller presses have been quite difficult following
          the big problems of premature wear of material at the surface of the rollers.    
        - In addition to the costs associated for the repair or replacement of the rollers and the decrease of production,
          it was not recommended to optimize the ball mill.      
        - Great efforts have therefore been made to find lasting solutions.      
        - Although manufacturers continue their research, we can say today that the roller press is a reliable machine.
        - Without going into complicated metallurgical considerations, the proposed solutions are:  
          * Forged rollers reloaded        
          * Rollers with bainitic bandages        
          * Rollers with composite bandages        
          * Rollers with hard alloy pins (abrasive)        
          * Rollers with hardened surfaces (full-hard-facing CHF)      
          * Rollers with Hexadur profile        
          (informations collected in the suppliers technical brochures and available online)  
        - The roller surfaces thus can have the following profiles:      
        - According to the same manufacturers, the lifetime of some of these profiles are tens of thousands of hours.
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