Objectives and Requirements of the magnetic separation.
If the abrasive surface have spots, when grinding the work-piece it is not only affects the appearance of quality, but also easily to cause burns. Therefore, in order to control the spots the national standard made strict restrictions to abrasive grains.

Iron which exists in the abrasives is the internal factor to produce abrasive spot. The iron largely in the presence of magnetic material, but it could be measured in various ways, sometimes there is no obvious correlation between the abrasive spot and the abrasive magnetic content. The reasons for this situation is manifold: 
1, the external conditions and other conditions, such as firing conditions, binders quality, generated largely affected in spots.
2, in the presence of iron in the abrasive, such as a portion of the oxide, under certain conditions does not exhibit significant magnetic properties, and this abrasive spots will appear also. This spot produced by a non-magnetic material with the current (1987, editor's note) detection methods can not be controlled.
3, the distribution of iron in abrasive conditions, such as iron dispersed or concentrated, free or fused, is wrapped in an internal or adhesion of the abrasive spots generated in the abrasive surface, but also is a factor to influence the spot take shape. These different forms of iron present in the abrasive, often can not be properly identified in the check.
Thus, iron is present in the abrasive in different forms, in different external conditions, affecting the possibility to form an abrasive spot, but on the macro level, the presence of iron is one of the root causes of abrasive spots. In this level, pickling iron and magnetic separation should be an important step in the process of an abrasive machining.

Although pickling can be take away iron, but that does not mean magnetic separation is not necessary. Because the magnetic abrasive material is usually iron and its alloys with silicon, titanium, aluminum, and it often presents mixed crushed particulate abrasive, both free form and adhered to the abrasive surface, but also blend in abrasive interior. Pickling can only clean abrasive surface and free of other iron impurities, for iron impurities that blending in grain interior, pickling are powerless. It can not completely remove the coarse particles for the free iron alloy particles. Therefore, magnetic separation for de-iron step is also necessary.

Abrasive iron impurities mainly come from two aspects: First, the crystalline block, including alloy ingot which mixed in classification, the second is iron pieces and iron that mixed in the mechanical crushing process. ■