Addition of pebbles to a ball-mill to improve grinding efficiency

Abstract Nkwanyana and Loveday (2017) used batch grinding experiments in a 0.6 m diameter mill to test partial replacement of steel balls (37.5 mm) for secondary grinding, by partly rounded pebbles (19–75 mm) from a SAG mill. At the optimum pebble content of 25% by volume, a 25% saving in steel ball consumption and a 15% saving in energy consumption was achieved, with no change in productivity. These very encouraging results were discussed with operators and designers of SABC circuits. This paper compares the performance of a 25/75 pebble/ball (volume ratio) composite charge to ball-milling, under conditions typical for a ball-mill in SABC circuits, i.e. a large ball top size (75 mm), a high mill speed (75% of critical speed) and a coarse feed top size (19 mm). The volume of pulp in some experiments was increased to mimic pulp filling in an overflow discharge ball-mill. All the experiments in this phase were conducted using a silicate ore. The rate of production of material finer than 75 µm, when using a 25/75 mixture, was virtually identical to that of the corresponding ball-milling experiment. The average saving in energy consumption was about 13% and a saving in ball consumption of 25% applies. It was noted that the replacement of balls by pebbles reduced the rate of grinding of coarse particles. This problem was solved by retaining the original quota of large balls, and replacing only smaller ball sizes. This method had a minimal effect on the rate production of fines (−75 µm) and makes composite secondary milling even more attractive, for reducing the cost of balls. Concerns about accumulation of ‘scats’ (coarse particles) have also been analysed. The effect of scats on ball-mill productivity was tested and it was concluded that scats may not have an adverse effect on productivity in secondary ball-mills and composite mills, if large balls are present. The use of a trommel on the ball-mill is suggested to solve related problems in the classification circuit.