Some observations on modeling and control of cement grinding circuits

Based on system analysis and experimental data, a dynamic model of a closed-loop cement grinding circuit, which consists of a mixed set of algebraic and partial differential equations, is developed and validated. The model equations are solved numerically using the method of lines and the resulting simulation program is used to gain some insight into the process dynamics and to design and compare control loops to achieve product specifications. The influence of the model nonlinearities, which are related to the dependency of the rates of breakage on the mill hold-up, is highlighted. In particular, this nonlinearity introduces a strong coupling between PI control loops using the fresh feed flow rate and the louver position of the classifier as manipulated variables. Several variations of this basic control scheme are thoroughly analyzed, and the necessity of an efficient mill flow rate control for the stability of the fineness control loop is demonstrated.