Abstract: In the largescale transformation of medium and fine crushing equipment in Nanfen Concentrator,it is planned to improve the grinding grade and reducing cost by dry magnetic preseparation.According to the medium crushing and oversize products test results of dry magnetic preseparation and the economic indexes of the concentrator,the results of analyzing economic benefit showed the reasonable grade of magnetic separation.The results show that the critical magnetic iron grade defined in this study is about 3%,which means the increament of dry discarding magnetic iron grade should lower than 3% in theory.The magnetic iron grade 0.94% can be used as the reference index of dry magnetic preseparation waste.The waste disposal grade is not fixed,it should be controlled flexibly according to the mining situation.When the stope has sufficient supply and crushing capacity can pass smoothly,the profit margin and total profit of the concentrator can be improved by dry magnetic preseparation discarding more poor conjunctions.In order to determine the waste grade accurately,the test of dry magnetic preseparation should control the waste yield span and improve the test precision,as "small step slow walk".With the condition that mining enterprises generally attach importance to economic and environmental benefits,concentrator should advocate the concept of economic benefits.The waste throwing behavior should be standardized,and the irrational waste throwing behavior should be eliminated.