Improved cement quality and grinding efficiency by means of closed mill circuit modeling

摘要

Grinding of clinker is the last and most energy-consuming stage of the cementmanufacturing process, drawing on average 40% of the total energy required to produceone ton of cement. During this stage, the clinker particles are substantially reduced insize to generate a certain level of fineness as it has a direct influence on suchperformance characteristics of the final product as rate of hydration, water demand,strength development, and other. The grinding objectives tying together the energy andfineness requirements were formulated based on a review of the state of the art of clinkergrinding and numerical simulation employing the Markov chain theory.The literature survey revealed that not only the specific surface of the finalproduct, but also the shape of its particle size distribution (PSD) is responsible for thecement performance characteristics. While it is feasible to engineer the desired PSD inthe laboratory, the process-specific recommendations on how to generate the desiredPSD in the industrial mill are not available.Based on a population balance principle and stochastic representation of theparticle movement within the grinding system, the Markov chain model for the circuitconsisting of a tube ball mill and a high efficiency separator was introduced through thematrices of grinding and classification. The grinding matrix was calculated using theselection and breakage functions, whereas the classification matrix was defined from theTromp curve of the separator. The results of field experiments carried out at a pilotcement plant were used to identify the model's parameters. The retrospective process data pertaining to the operation of the pilot grinding circuit was employed to validate themodel and define the process constraints.Through numerical simulation, the relationships between the controlled (freshfeed rate; separator cut size) and observed (fineness characteristics of cement;production rate; specific energy consumption) parameters of the circuit were defined.The analysis of the simulation results allowed formulation of the process controlprocedures with the objectives of decreasing the specific energy consumption of the mill,maintaining the targeted specific surface area of the final product, and governing theshape of its PSD.