Cement Milling Circuit

Cement milling circuit

A schematic representation of the cement milling circuit is depicted in Fig.1. Cement milling circuit is an industrial process, which takes raw material as input and which produces cement having the desired fineness. The raw material enters to the classifier after grinding process in the mill. The classifier separates the incoming material into two parts. The refused material i.e. the material that is not in the desired fineness is sent back to the mill for regrinding. Accepted material goes to the other stages of the production as the output of the cement milling circuit.

Cement mill grinding process

In steady-state operation, the product flow rate yf is necessarily equal to the feed flow rate u while the tailings flow rate yr and the load in the mill z may take any arbitrary constant values. The load in the mill depends on the input feed (fresh feed plus tailings flow rate) and on the output flow rate that depends in a nonlinear way, on the load in the mill and on a very important and time-varying quantity: the hardness of the material. Sometimes this nonlinearity may destabilize the system and the obstruction of the mill (a phenomenon called “plugging”), which then requires an interruption of the cement mill grinding process.

The load in the mill must be controlled at a well chosen level because too high a level of the load in the mill leads to the obstruction of the mill, while too low a circulating load contributes to fast wear of the internal equipment of the mill.

The efficiency of a cement grinding circuit

Moreover, the energy consumption of the mill (i.e., the ratio energy per unit product) depends on the output of the mill that is related to the load in the mill. A usual approach is to control the tailings flow rate by using the feed flow rate as control input. This control strategy is, however, not fully satisfactory since it indirectly induces a loss of control of the product flow rate. A correct fineness of the product is also very important. The fineness depends on the composition of the mill feed, but also on the rotational speed and on the air flow rate of the classifier. A natural control objective would therefore be to keep the fineness as close as possible to a desired value by controlling the rotational speed of the classifier.

The efficiency of a grinding circuit is dependent on three key conditions:

• An optimum and constant level of material in the mill
• Constant air to material ratios for the separator material
• A constant and optimum ratio between fresh feed

The designer could choose two of the three state variables independently, as the behavior of the third state variable would be determined upon the selection of other two.