Abstract: In In order to improve the pre-concentration accuracy of 20~2 mm grain size before grind⁃ ing in Meishan Iron Mine，in view of the problems existing in the existing pre-concentration process，such as gangue inclusion in concentrate and large loss of tailings metal，the separation test was carried out on the raw ore of this grain size. The test results show that the concentrate with iron grade of 48.96% and iron recovery of 83.97% can be obtained by pre-concentration of 20~2 mm magnetic pulley concentrate before grinding，and the iron grade is increased by 5.86 percentage points. The average iron grade of concentrate reached 36.79% and tailings grade is 11.01%，respectively，using a new type of high magnetic field force 2RTGX0612 cylindrical permanent magnetic separator. Compared with the existing roller high intensity magnetic separator concentrate iron grade increased by 2.59 percentage points，tailings iron grade de⁃ creased by 2.19 percentage points，separation efficiency improved significantly; under the premise that the metal recovery rate is 0.93 percentage points higher than that of the roller high intensity magnetic separa⁃ tor，the concentrate quantity (grinding quantity) of the roller high intensity magnetic separator is reduced by 4.09 percentage points.

Key words: preselection, magnetic separation, magnetic field intensity, metal recovery