Dynamic Analysis of a Small Scale BallMill Structure used For Gold Mining

Abstract

Ball mills are used as grinding machines in mining, cement, chemical and agricultural industries. They are the most commonly used grinding machines due to their ability to producefineparticlesofthematerial. Thecommunitionprocess,whichisthereductionof material to smaller particle size, takes place by abrasion, crushing and impacting through therotationoftheballmilldrum. However,themotionofgrindingmediaupontherotation oftheballmillcausesundesirablevibrationinthemillstructurewhichmayleadtoenergy losses, reduction in the performance of the ball mill and fatigue failure of the ball mill structure. Although a lot of research has been conducted to analyze the vibration problem of a ball mill, the tumbling of grinding media charge have not been considered as the excitation load because of the complexity of the dynamics of charges inside the ball mill drum. Therefore the aim of this research work is to analyze the dynamic characteristics of structure of the ball mill developed in JKUAT, by combining Discrete element and Finite element analysis. This is done in order to establish the likelihood of the structure to undergo resonance, and the vibration amplitudes that would result while examining if theyarewithinallowablevaluesbasedonmaterialandstructuralpropertiesoftheballmill structure. The ball mill was modeled on Autodesk Inventor 17. The dynamic analysis of the charge inside the ball drum was carried out using Discrete element EDEM software, which resulted in the time varying load applied on the drum. This load was then applied to the FE model using ANSYS software for dynamic analysis of the ball mill structure. Modal analysis was conducted to obtain insight on the possibilities of resonance to occur, by extracting the first ten modes of vibration. Harmonic analysis was conducted to obtain resonance frequencies of the structure. Transient analysis was also conducted and the response of the ball mill structure under time varying load was obtained and compared with the standard limit. Experimental work was carried out and the results were used to validate the FE model which showed a good agreement with a mean error of 5.9 %. It was observed that the ball mill was safe in its range of operation as far as resonance is concerned,asthefirstnaturalfrequency(7.02Hz)wasfarabovetheoperatingfrequencies (1.67 Hz from the rotation speed and 2.4 Hz from the impact load). The results also showedthattheballmillwassubjectedtosmallvibrationamplitudesandthattheresulting maximumstresswaswithinallowablestrengthhencewouldnotaffectthedurabilityofthe ball mill structure.