Investigation of load behaviour of an industrial grinding mill

Abstract

The dynamic behaviour of the ball load (specifically toe and shoulder positions) within a dry Φ4.74m by 7.4m long, 2.15MW powered, ball mill was investigated as a function of worn and new liners. The mill operating conditions and charge composition where kept constant. Three types of experimental probes were designed, manufactured and mounted strategically on the mill to determine the orientation of the load inside the mill. These included a combination of electrical conductivity, movement and photo-detector probes. Data recorded from each probe was processed, analysed and compared with each other and to Millsoft’s 2D theoretical numerical simulation model. Achieving good electrical contact proved to be a very difficult task, hence conductivity did not compare well to Millsoft simulations. It was deduced that the conductivity probe design and electronics data capturing system limited accurate recording of the ball trajectory positions and tended to represent more of the load ‘locked-in-state’ region (or the period where maximum pressure is applied to the mill walls) instead of the extreme load toe and shoulder positions. The movement probe data compared best with Millsoft predicted load toe and shoulder results at 133 and 282 degrees, respectively. The probe design was however susceptible to forces from different directions thus decreasing its accuracy. Only two tests from only one photo detector probe (measuring ball reflection) were successfully recorded. The one test showed high variances while the other was affected by an inaccurate datum signal; the low number of effective detector data resulted in defining the detector data as unreliable. Measured experimental power was compared to three published power prediction models and Millsoft. The relative error percentages concluded that only Millsoft best predicted mill input power for both the worn and new liners, at 1871kW and 1887kW, respectively. These results have provided valuable information on probe design for dry industrial type mills and mill internal load dynamics.