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GRINDING TECHNOLOGY JULY 2017 • E&MJ; 33 www.e-mj.com 8.2 m in diameter and an effective grinding length of 14.6 m), driven by a ring motor with a peak output (for ball mills) of 22 MW. In the course of more than 100 years of ball mill evolution, two mill design types have become firmly established: the trunnion-supported mill, and shell-sup- ported design that has been used with in- creasing frequency in mining [Figure 2]. thyssenkrupp considers itself a pioneer in the field of the shell-supported design, having employed this design approach for about 30 years in all tube mill types. The generally applied combination of welded and cast elements used for the conven- tional trunnion-supported design has been greatly simplified. The completely welded design of the shell-supported mills consists of large cylinders with welded-in end walls, both ends of which rotate on a sliding shoe bearing assembly [Figure 3]. However, when compared with trun- nion-supported mills, the shell-supported mills have design differences, which have positive effects on operating and capital expenditures (OPEX/CAPEX) as well as on delivery and operating reliability. For example, the inlets and outlets of trunnion-supported mills also serve as bearing surfaces for the mill cylinder, leading to design restrictions. Because no static restrictions need to be applied to the inlet and outlet of the shell-sup- ported mill, it can be designed entirely in compliance with the customer's process requirements. For this reason, the length of the inlets can be significantly shorter as shown in Figure 4. Apart from the shorter overall length, the customer benefits from an improved material flow and easier access with a lin- er handler, which has a positive effect on the overall availability of the plant. Shell-supported mills allow a high- er filling ratio (38%) over trunnion-sup- ported units, which increases the grind- ing energy applied to the ore [Figure 5]. Shell-supported ball mills also offer savings up to 20% in installation and footprint requirements. The mills can be designed more compactly with the same power input. Alternatively, while maintaining the design size, more pow- er can be introduced into the material to be ground and the throughput thus increased. The compact design allows savings of approximately 15% to 20% on installation and footprint [Figure 6]. In addition, the forces inside the cyl- inder due to weight are directly absorbed where they are produced and do not need to be directed via flanged, cast- end walls and bearing trunnions. More than 35 years ago, thyssenkrupp decided, as one of the first mill manufacturers, to use the finite element calcu- lation method [Figure 7]. This method was relatively unknown at that time but has since be- come an industrial standard practice. Based on its decades of experience thyssenkrupp is now in a position to use significantly optimized plate thicknesses for dimensioning large mills of nonetheless rugged but cost-ef- fective design. Additional design and cost benefits can be achieved if the mill cylinder is not bolted but welded together on site. Because mills of the size installed at Cerro Verde and elsewhere, such as Si- erra Gorda, cannot now be manufactured and transported in a single assembly, they usually must be delivered in flanged seg- ments made to be bolted together on site. The mill segments delivered in the case of Cerro Verde and Sierra Gorda, however, were not bolted but welded together on site. For this, the mobile POLWELD meth- od developed by thyssenkrupp more than 20 years ago was applied. This design does not require flanges, thus increasing rigidity and reducing weight. The information contained in this article was provided by thyssenkrupp Industrial Solutions. Figure 4—Comparison of inlet lengths for trunnion- supported and shell-supported mills. Figure 5—Illustration of the superior filling ratio flexibility of shell-supported ball mills. Figure 6—Comparison of installation footprint. Figure 7—FEM display of a shell-supported ball mill cylinder.