Analisis Conductrimetrico

Minerals Engineering 20 (2007) 84–91 This article is also available online at: www.elsevier.com/locate/mineng

Experimental analysis of charge dynamics in tumbling mills by vibration signature technique
B. Behera, B.K. Mishra
*,1,

C.V.R. Murty

Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, India Received 2 February 2006; accepted 15 May2006 Available online 9 August 2006

Abstract 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 themill 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 byfollowing 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. Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Ball mill; FFT technique; Vibration signature; Charge motion

1. Introduction Characterization of the motion of the charge inside a ball mill is one of the mostimportant requirements for analyzing the performance of the mill. The important characteristics of the charge motion are conventionally termed as (a) cascading, (b) cataracting, and (c) surging. Surging of the charge may take place at all speeds and it is particularly damaging at high speeds. If the motion of the grinding media were not controlled then a greater fraction of energy would be wasted inimpacts, which do not break particles, or consumed in the generation of unwanted product sizes such as ultra-fines. Therefore, in order to improve

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1

Corresponding author. E-mail address: bk@iitk.ac.in (B.K. Mishra). Currently Director, Regional Research Laboratory, Bhubaneswar.

the utilization of the energy, it is essential to have a good understanding of the mechanics of the motion ofthe tumbling charge. A vast amount of research work has been done on motion analysis of ball charge by visual means in smaller mills. Among these, the work of Vermeulen et al. (1984) is noteworthy. These authors claim that the slip of the charge in a rotary mill with smooth liners causes it to surge, while the grinding media adjacent to the mill shell do not ever move counter to the mill shell.However, under lowball fillings, surging may resemble the oscillating behavior of the charge as first proposed by Rose and Sullivan (1958). In a very similar kind of approach, Agrawala et al. (1997) have shown the surging behavior of the charge in a 90-cm diameter mill where they clearly identified the periods of peak surge and subsequent charge collapse. In larger mills, obviously, motion analysis canonly be carried out by indirect means.

0892-6875/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.05.007

B. Behera et al. / Minerals Engineering 20 (2007) 84–91

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Indirect assessments of the profile of the charge have been done in two ways: imbedding strain gauges and integrated circuitry inside the ball, or alternatively, imbedding sensors on...