QUESTION 2 [20 Marks] A hard rock (specific gravity = 3.8) is fed at 12000 kg per hour (F2) to a ball mill operating with a classifying screen. The overhead from the classifying screen is recycled back (8000 kg/h) to the feed. Oversize Recycle, R 1 Classifying 1 Screen Raw Feed, F1 F2 Ball Mill Crusher discharge. P: 2 Undersize Product, P2 The ball mill produces the following particle size distribution and a work index of 15.3 kWh/ton. Mass fraction Mesh (um) F1 F2 P2 Oo 0.1 0.2 0.4 0.2 0.1 0 -6680 +4699 -4699 +3327 -3327 +2362 -2362 +1651 -1651 +1168 -1168 +833 -833 +589 -589 +417 -417 +295 -295 +208 -208 +147 -147 +104 -104 +74 0.033 0.067 0.133 0.067 0.047 0.027 0.040 0.167 0.200 0.133 0.040 0.027 0.020 0 0 0 0 0.03 0.06 0.09 0.35 0.30 0.08 0.05 0.04 0 0 0 0 0 0 a) Determine the average particle size (mm) of the feed and product from the ball mill. (16) b) Estimate the energy required (kW) to archive the particle size reduction determined in part a). (2) c) If the total power supplied to drive the crone crusher is 52 kW, determine the energy efficiency of the mill. (2)