Abstract: The actual selected ore in mixed iron ore beneficiation system of Nanfen concentrator is greatly different in property aspect from the ore which is used for design， resulting in the difficult to reach the design value and a final iron concentrate with iron grade of only about 63% and iron recovery of only about 58%. In order to understand the causes for low throughput of the semi-automatic grinding mill and find out the links in the site process which is obviously not suitable for the properties of the selected ore， the ore grinding characteristics and beneficiation process were studied to provide ideas for process transformation. The results indicated that:①The ore is difficult to be ground， which is the main cause for the low throughput of the semi-automatic grinding mill； ②The insufficiently liberated high-magnetic separation concentrate was forced to discard tailing using spiral sluice gravity-separation， which is the major cause for metal loss； ③Large amounts of gangue and slime liberated from the second stage ball grinding and directly entered the reverse flotation， thus seriously affected the quality improvement and impurity reduction of the reverse flotation， which was the main cause for the low iron grade of the reverse flotation concentrate. On the basis of the research results and combining the reality， the gravity separation by spiral sluice in the original process was canceled on site. Meanwhile， a tailing discarding operation by low intensity magnetic separation and pulsating high gradient high intensity magnetic separation was added between the second stage closed-circuit ball grinding and the reverse flotation. After the process transformation， the site production runs smoothly and the production indexes improves significantly with the concentrate iron grade increasing by 1.98 percentage points and the recovery rate increasing by 6.11 percentage points.

Key words: Mixed iron ore, Low intensity magnetic separation, fine screen screening, High intensity magnetic separation, Reverse flotation