ONE-STAGE VS TWO-STAGE MILLING PROCESS

The main factors affecting the determination of the number of grinding sections are: the grindability of ore and the embedding characteristics ofminerals, the feed size of the grinding mill, the required particle size of the grinding products, the production scale of the concentrator, the necessity of separately treating the ore sand and slime, and the necessity of stage separation. It has been proved by practice that the ore can beground to any size required by beneficiation economically by adopting one or two stages of grinding process, without using more grinding sections. The increase of the number of grinding sections to more than two is usually determined by the requirements of the stage selection.Compared with two-stage milling process, the main advantages of one-stage milling process are: fewer classifiers and lower investment; easy production operation and simple adjustment; without intermediate product transportation between sections, multiple series of mills can be placedon the same level, so the equipment configuration is relatively simple; it will not be affected by the shutdown of one stage mill or classifier It will affect the work of another milling section, so the shutdown loss is small; larger equipment can be installed in each series. The disadvantagesof the first stage grinding process are: the feeding particle size range of the pulverizer is very wide, reasonable ball loading is difficult, and the grinding efficiency is low; the grading overflow fineness in the first stage grinding process is generally – 200 mesh, accounting for about 60%, so it is not easy to get finer products.According to the above characteristics, when the particle size of final grinding product is required to be greater than 0.2-0.15 mm (that is – 200 mesh accounts for 60-72%), a grinding process should be adopted. In order to simplify the grinding process and equipment configuration in a small concentrator, a grinding process can also be adopted when the grinding fineness requirement is – 200 mesh, accounting for 80%.The outstanding advantage of the two-stage grinding process is that the ore can be rough and fine grinded in different grinding stages. Different grinding conditions can be used in the two grinding stages: in coarse grinding, larger steel balls and higher rotating speed are conducive to improving the grinding efficiency; while in fine grinding, smaller steel balls and lower rotating speed can also improve the grinding efficiency.Another great advantage of the two-stage milling process is that it is suitable for stage separation. In the treatment of non-uniform disseminated ores and ores containing large specific gravity minerals, the separation operation in the grinding cycle can timely separate the separated minerals, prevent the occurrence of over crushing, and improve the quality index of mineral processing; at the same time, it can reduce the accumulation of heavy metal minerals in the grading return sand, and improve the classification efficiency of the classifier.Therefore, when the grinding fineness is less than 0.15 mm in medium and large concentrators, it is more economical to use two-stage grinding. At this time, the power consumption of grinding per ton of ore is less; the particle size composition of grinding products is relatively uniform, and the phenomenon of over crushing is less, which can improve the separation index.

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