Engineering & Mining Journal

MAY 2019

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

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Page 39 of 83

GRINDING CIRCUITS 38 E&MJ • MAY 2019 crushing and, strictly speaking, a second grinding stage. As the damp and quite sticky mate- rial tends to accumulate on the grinding media, drying would need to be done in a separate dryer ahead of the tumbling mill(s). Dry screening at 500 µm is not simple and requires a large screening area, making air classification a suitable alternative. The increased complexity of a conventional solution entails not only more equipment, but also has other con- sequences like a larger footprint or a more complex operation system, among others. The Initial Test Work Before tendering to supply the equip- ment, Loesche ran tests with the ore at its test center in Neuss, Germany. A stan- dard test provided the data for the layout of the mill and the process. Modifications to the process, mill and classifier in re- sponse to the coarse product size were tested and reviewed. In addition, several settings were run to identify the impact of the online parameter's working pressure, table rotation speed and classifier rota- tion speed on the operation performance of the mill and the product quality, and to assess the transferability of the observed results to an industrial scale operation. The tests clearly showed that the VRM does grind the phosphate rock at a high throughput, with low grinding energy con- sumption, low wear and a very nar- row particle size distribution, with only small modifications being needed to the equipment and the process. This allowed Loesche to design the process very effi- ciently around an LM 24.2 M. Thanks to the efficient drying inside the mill and the flexibility of the hot gas gen- erator, the system can handle the fluctua- tions in the feed moisture without restric- tions. The impact of the online parameter's working pressure, table rotation speed and classifier rotation speed on to the mill per- formance was evident. This meant they are suitable tools to compensate for fluctua- tions in the feed material characteristics also at industrial scale and can ensure very stable values for the throughput, product size and product particle-size distribution. The Production Experience After blasting, the ROM ore is transported by truck to the mineral processing plant. The material is crushed to a size (P90) of 80 mm, the feed size for the VRM, and is taken to the grinding plant (See Figure 3). From the feed hopper (1), the material is fed into the grinding circuit. After tramp metal has been removed (2), the material is trans- ported to a small mill feed hopper (3) that is discharged using a dosing belt to guarantee a precise, constant feed rate to the mill. Key Loesche mill 24.2 M with classifier LSKS 37 M.

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