The efficiency of the early stages of grinding in a ball mill is much greater than that for formation of ultra-fine particles, so ball mills operate most efficiently by making a coarse product, the fine fractions of this then are being separated, and the coarse part being returned to the mill inlet.
The proportion of the mill-exit material returned to the inlet may vary from 10-30% when ordinary cement is being ground, to 85-95% for extremely fine cement products. It is important for system efficiency that the minimum amount of material of finished-product fineness is returned to the inlet. Modern separators are capable of making a very precise size "cut" and contribute significantly to the reduction of energy consumption, and have the additional advantage that they cool both the product and the returned material, thus minimizing overheating.

The efficiency of the early stages of grinding in a ball mill is much greater than that for formation of ultra-fine particles, so ball mills operate most efficiently by making a coarse product, the fine fractions of this then are being separated, and the coarse part being returned to the mill inlet.
The proportion of the mill-exit material returned to the inlet may vary from 10-30% when ordinary cement is being ground, to 85-95% for extremely fine cement products. It is important for system efficiency that the minimum amount of material of finished-product fineness is returned to the inlet. Modern separators are capable of making a very precise size "cut" and contribute significantly to the reduction of energy consumption, and have the additional advantage that they cool both the product and the returned material, thus minimizing overheating.