Modeling and control of cement grinding processes

In this study, a nonlinear dynamic model of a cement grinding process, including a ball mill and an air separator in closed loop, is developed. This gray-box model consists of a set of algebraic and partial differential equations containing a set of unknown parameters. The selection of a model parametrization, the design of experiments, the estimation of unknown parameters from experimental data, and the model validation are discussed. Based on the resulting model, a dynamic simulator can be developed, which appears as a useful tool to analyze the process behavior and to understand the origin of instabilities observed in real-life operations. As a result, a cascaded control structure for regulating the mill flow rate, and a proportional integral controller for regulating the cement fineness are designed. Experimental data demonstrate the effectiveness of this control scheme. Alternatively, if on line measurements of the recirculated flow rate are available, a feedforward control of the feed flow rate is described, which ensures a better decoupling of mass flow rate and fineness regulation.