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      Types of Tube Mills                
                             
      1 Introduction                  
        - This presentation intends to describe the different types of tube mills encountered in the industry.    
        - We will not talk about old mills mot more used like conical mills.         
        - Different types of classification are possible.            
        - For example, we can perform a first classification of different types of mills by type of grinding media used:  
          * Rod mills                  
          * Ball mills                  
          * SAG mills                  
          * AG mills                  
          * Pebbles mills                
        - One can classify also according the mills working in discontinuous mode (laboratory) or continuous mode  
          (cement industry, mining, pigments, food...etc).            
        - In this presentation, we have first to split in two processes:          
          * Wet Process                
          * Dry Process                
                             
      2 Tube mills in wet process                
        - Wet process is the process where grinding is performed with material forming a pulp with addition of water (minimum 30%).
        - Mills presented in this chapter:              
          * Rod mills                  
          * AG mills                  
          * SAG mills                  
          * Pebbles mills                
          * Ball mills                  
      2.1 Rod mills:                  
        - Rod mills are usually used as the first grinding stage after crushing in the mining industry. Since the growth of the AG, SAG mill
          circuits (with much higher capacities),  rod mill technology is almost fallen into disuse.      
          Their use is then generally limited to specialized cases such as:          
          * a very coarse product size is required              
          * overgrinding is to be minimized              
          * the ore is very hard                
          NB: Note that rod mills can be utilized in dry process to grind coke or iron ore.        
        - Different discharge arrangements are possible:            
          * Trunnion discharge                
          * End peripheral discharge              
          * Center peripheral discharge              
        - Sketch of rod mill:                
         
     
         
               
               
               
               
               
               
               
               
               
               
               
               
        - Rod mills are always monochamber and are working in overflow mode (without grate at the outlet).    
        - Speed of rotation is lower than ball mills in order to avoid tangling of the rods.       
          Speed of rotation varies from 62 to 70% of the critical speed.          
        - The ratio L/D must be well defined in order to avoid tangling of the rods (not to be lower than 1,3-1,4).     
          For bigger diameters (> 2m), L/D = 1,4-1,8            
          For smaller diameters (< 1,5m), L/D = 2-2,5            
        - The length of the rods is limited to 20ft (6,08m).            
          A 150 mm clearance being required to the ends of the mill.          
        - Rods density is varying from 5400 kg/m3 to 6400 kg/m3.          
        - Available diameters of the rods are generally: 105,95,80,50 and 25mm.        
        - Due to the restriction of rod length, the maximum shell diameter allowed will be 4,4-4,6m (to keep the right L/D ratio!).
        - Unlike ball mills, the volume load can be drastically higher.           
          Values between 35 and 40% (even more) are normal.          
        - Linings of steel or rubber are accepted.            
        - Concerning the fresh feed, F80 (size in µ of the 80% passing in the fresh feed) must be comprised between 5 and 20mm.
          It is generally accepted that the optimum fresh feed (80%) is 15-17mm.        
          Allis Chalmers developed a formula to define the right fresh feed:        
         
     
                   
          Where:                  
          F80 is the size in µ of the 80% passing in the fresh feed and          
          Wi is the Bond Index                
        - Concerning the final product, we consider the following:          
          * P80 (size in µ of the 80% passing in the product) is normally between 1 to 2mm      
          * Minimum P80 can reached 0,5mm (and even 0,3mm in closed circuit)        
          * NB: Rod mills are not producing very fine particles.          
        - Reduction Ratio:                
          According to Bond, it is around 16:1 (between 12:1 and 20:1 as maximum).        
        - Calculation of the absorbed power:              
          Bond developed the following formula:            
         
     
               
          Where:                  
          P is the mill absorbed power at the shaft in kW,            
          Tr is the mass of grinding charge in tons            
          D is the internal diameter of the mill,              
          J is the volume load in % and              
          Vcrit is the percentage of the critical speed            
        - In the majority of cases, rod mills are operating in open circuit.          
        - Sizing a rod mill, see the link below:              
          http://www.themininggrindingoffice.com/rodballmillsizing.html        
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      2.2 AG and SAG mills:                
        - AG and SAG are the diminutives of (FULLY) AUTOGENOUS and SEMI AUTOGENOUS mills.      
        - AG and SAG mills circuits are commonly and advantageously applied in the high capacity circuits for processing ores  
          with lower metal grades.                
      2.2.1 AG mills:                  
        - AG mills are used as first grinding step.              
        - Fresh feed can be:                
          * Run of mine ore: F80 is between 200 and 400mm (Mined ore of a size that can be processed without further crushing)
          * Crushed ore: F80 is between 150 and 250mm            
        - AG mills operate with rocks of the ore itself as grinding media.          
        - For that reason, the feed has to contain a sufficient amount of coarse grinding stones.      
      2.2.2 SAG mills:                  
        - Like AG, SAG mills are used as first grinding step.            
        - Fresh feed is usually:                
          * Run of mine ore: F80 is between 200 and 400mm (Mined ore of a size that can be processed without further crushing)
          * Crushed ore: F80 is between 150 and 250mm            
        - SAG mills operate with big rocks of the ore itself plus steel balls to improve efficiency.      
        - The grinding media volume load is between 4 and 15%.          
        - Normally, balls of 125 and 100mm are used.            
        - The mill diameter is usually bigger than the length (L/D from 0,3 to 0,6). But SAG mills can also have a L/D > 1.  
        - Sketch of a SAG mill:                
         
     
             
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
        - Biggest SAG mill circuit in the World:              
          * Copper/Gold mine in Perou - Conga              
          * Metso + ABB                
          * One 42 × 25 feet, (12,8 × 7,6 m) 28 MW gearless SAG mill          
          * Two 26 × 42 feet, (7,9 × 12,8 m) 15.6 MW ball mills            
      2.3 Pebbles mills:                  
        - Pebbles mills are used as secondary grinding step.            
        - Fresh feed is the product of rod/SAG mills.            
        - Grinding media (pebbles) consists of screened out fractions of the ore itself, synthetic pebbles, or manufactured alumina
          oxide.                  
        - Dimension of pebbles from 50 to 90mm.            
        - Pebbles mills are operating with a grate discharge.            
      2.4 Slurry mills (ball mills):                
        - Slurry mills are used as secondary grinding step of a mining plant and in the raw section of cement plants.  
        - These mills are operating in open or closed circuit with hydrocyclones.        
        - Linings of steel or rubber are accepted.            
        - Mill discharge can be in overflow process or with a grate discharge.        
          Some raw mills are also equipped with an outlet flow control diaphragm.        
        - Sketch of a slurry mill:                
         
     
         
               
               
               
               
               
               
               
               
               
               
               
               
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