Only part of the ore discharge product of the mill is of acceptable grade, and the unqualified part is usually returned to the mill for regrind, which constitutes closed-circuit grinding. In closed-circuit grinding, the ratio of the steady amount of sand returned from the classifier to the mill and the original ore is called the cyclic load or the return ratio. It should be pointed out that excessive circulation load causes overloading of the grinding machine and the classifier, so that the grinding and grading process cannot be carried out normally, the ball mill will produce a swollen belly, and the classification efficiency of the classifier will decrease, so the circulating load should have a quantity. limit. Production practice proves that when the sand return ratio is increased from 100% to 400-500%, the production capacity of the grinding machine generally increases by 20-30%; if the sand return ratio increases to more than 500%, the increase of production capacity is not significant. The situation is as shown in the figure below. [next] Generally speaking, the coarse grinding back sand ratio is smaller, and the fine grinding back sand ratio is larger. The table below lists the appropriate sand ratio ranges for different grinding conditions based on production experience.  The most suitable cyclic load under different grinding conditions Configuration condition Grinding section Feed grain size upper limit Grinding fineness upper limit C value (%) (mm) (mm) Grinder and grading I 　 0.5 to 0.3 150～350 0.3 to 0.1 250 to 600 Machine self-flow configuration II 0.3 0.1 200~400 Grinder and swirl I 　 0.4 to 0.2 200～350 0.2 to 0.1 300~600 Self-flow configuration II 0.2 0.1 150～350 PP Synthetic Paper Label Materials
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The figure above shows a closed-circuit grinding grading process. According to the material flow balance principle of the raw ore entering the grinder and the graded overflow, the sampling is performed at the three levels of the grinder discharge and the graded overflow grading back to the sand for analysis. The contents are α%, β%, θ%, and the following equilibrium equations can be listed:
(θ+S)α=Qβ+Sθ