This phenomenon is mainly due to the small feeding particle size, short residence time of ore in the ultrafine vertical mill and fast speed. 

It should be noted that when the grinding products are required to be coarse, the variation of mill productivity with feed particle size is obvious, while when the grinding products are required to be fine, the variation of mill productivity with feed particle size is not very prominent. 

When the feeding particle size, i.e. the grinding particle size, is small, the grinding fineness is easy to achieve. However, the fineness and productivity are contradictory. 

Under the condition of the same feed particle size, the production capacity of the mill decreases with the increase of the fineness of grinding products.