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Results of the experimental investigation of the influence of surface forces on high gradient magnetic separation of iron ores will be presented. Surface characteristics of the slurries of pure hematite and hematite ores ground below 100 μm and 40 μm were varied from pH = 1.5 to 11 and these test samples were treated in high gradient magnetic separator at magnetic fields from 0.1 T to 0.7 T. The experimental data indicate that the maximum of grade and recovery of magnetic fraction of the ore can be obtained at pH corresponding to the point of zero charge, while at other pH the grade and recovery decrease. Since pH of point of zero charge for iron oxide is different from pH corresponding to water used in iron dressing plant, adjustment of pH of the slurry could lead to a substantial increase in grade and recovery. These considerable variations of grade and recovery can be attributed to a higher probability of retention of the particles on the matrix, higher resistance of the deposited particles against the shear stress and to selective magnetic and ionic flocculation of magnetic or gangue particlec.