Experimental analysis of charge dynamics in tumbling mills by vibration signature technique

Up until now, real time identification of the dynamics of the charge in a tumbling mill has not been accomplished. This paper examines the possibility of correlating the vibration signature of tumbling mills to characterize the motion of the charge and the state of grinding. Vibration signals were picked up using accelerometers mounted directly on the mill shaft of a 90-cm diameter mill. The time domain signals were transformed to frequency domain by using fast Fourier transform (FFT). The Fourier spectra in the frequency domain were methodically interpreted and correlated to establish the prevailing mode of the charge motion under any operating condition. The grinding behavior under dry as well as wet grinding conditions were analyzed by following the variations in the vibration signature as a function of speed of the mill, volumetric filling, powder loading, and time of grinding.Experimental results clearly show that the dominant peak in the FFT spectra is quite sensitive to the variations in any mill operating parameter. This feature has been employed to detect undesirable operating conditions such as surging, mill over-load, etc. Finally, it is demonstrated that by proper interpretation of the vibration signature of the mill, it is possible to predict the charge dynamics and establish the state of grinding.