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      Pre-grinding systems        
      2   Vertical shaft impactor (suite 1)        
         2.5 Case Study for a raw mill in Pakistan:        
         2.5.1 Description of the circuit:        
        - This is a classical airswept raw mill.        
        - The customer wanted a capacity increase of 50%.      
        - The proposed solution has been pre-grinding system of the limestone with a vertical shaft impactor and other
          significant modifications of the mill circuit (new 3rd generation separator, bucket elevator) and the mill itself
          (new lining, outlet diaphragm and new ball charge).      
        - The circuit is similar to the VSI flowsheet of Chapter 2.4.      
        - The fresh material is stored in three small silos.      
        - Three weighfeeders feed a belt conveyor up to a vibrating screen.      
        - A metal detector and a metal extractor are installed on this conveyor to avoid unplanned stoppages.
        - The vibrating screen has the following capacities:      
          * Capacity: 300 t/h        
          * Inclination angle of 15°        
          * A first deck of 25mm and a second one of 10mm (due to moisture).    
        - Le matériau est tamisé dans cette section afin d'obtenir une granulométrie de 0 à 10mm.  
        - The rejects (or material greater than 10 mm) fall into a conveyor belt which moves towards the crusher.
        - The vertical shaft impactor has the following dimensions and capacities:    
          * Diameter of the central body: 2,200 m        
          * Distributor diameter: 960 mm        
          * Number of impellers: 5        
          * Maximum speed: 1500 rpm        
          * Installed power (1 motor): 200 kW        
          * Feed size: 0-130 mm        
          * Maximum output: 350 t/h        
        - The raw material is crushed and the product output is collected by another belt conveyor up to the fresh material
        - All components of the pre-grinding system are thus interconnected by belt conveyors.  
        - This system is more extensive in relation with a vertical configuration because the people of the cement plant
          did not want to use a bucket elevator in this circuit.      
        - Finally, the passing of the vibrating screen feeds the ball mill.      
        - Both pregrinding and mill circuits are directly online without intermediate silo.    
         2.5.2 Granulometries in the pre-grinding circuit:        
        - The sampling was performed after the circuit was fully optimized and in steady operation.  
        - Sample names:        
          1. Fresh Feed        
          2. Feed of vibrating screen        
          3. Feed of crusher        
          4. crusher product        
          5. Screen product        
        - Analysis results:        
        - RRSB Diagram of the samples:        
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