Modeling and optimization of fine grinding of minerals in high-pressure roll mill - ball mill hybrid comminution circuits

Fine grinding of solids is an energy-intensive process, primarily because of the poor energy efficiency prevalent in comminution technology. The energy efficiency of particle bed comminution as carried out in the recently invented high-pressure roll mill, on the other hand, is second only to that of single-particle comminution under slow compression, which is the most energy-efficient mode of comminution. In high-pressure roll mill grinding, size reduction results from confined particle-bed comminution under compressive stresses such that the daughter particles produced are highly fractured, stressed or otherwise weakened. The product usually comes out briquetted, especially for the grinding of soft and easily deformable materials. Subsequent grinding of the roll mill product in a ball mill to deaggregate the briquettes and to achieve further size reduction should require significantly lower energy expenditure. Such a two-stage grinding configuration is referred to as a hybrid grinding system in this dissertation.; Nearly all industrially installed high-pressure roll mills are operated in conjunction with ball mills in various circuit configurations. However, very little quantitative data are available in the literature on such hybrid grinding systems. The first task in this research work was therefore to study the characteristics and kinetics of grinding the high-pressure roll mill product in a ball mill. Open-loop hybrid grinding studies were undertaken, where coal samples were first ground in the high-pressure roll mill with various levels of energy expenditure and then subsequently ground in a ball mill in the batch mode. Because high-pressure roll mill grinding results in daughter particles with varying degrees of damage, a distributed grinding rate constant model was developed next to describe the batch grinding kinetics and the energy-size reduction relationships. This model was then used to simulate successfully, in the predictive mode, the steady-state performance of various closed-loop continuous hybrid grinding circuits for a wide range of operating conditions. Model-based simulations were carried out to determine the optimal energy expenditure and its partitioning between the high-pressure roll mill and the ball mill for maximum energy savings. Based on these studies we concluded that closed-circuit hybrid grinding, in general, is more energy efficient than closed-circuit ball grinding. Moreover, there is an optimum energy expenditure and partitioning of the energy between the mills that result in maximum energy saving at a given recycle ratio.; Finally, we attempted to relate the comminution/compaction behavior for confined particle-bed under compression in a piston-die press with the macroscopic variables in high-pressure roll mill comminution. A model describing the pressure-densification relationship was developed and used to accurately simulate the macroscopic variables in high-pressure roll mill grinding, utilizing experimental data for particle-bed comminution with the piston-die press.; The accuracy of the predictive simulations strongly suggests that, starting with a few batch-mode laboratory-scale experiments, it should be possible not only to identify the optimal operating conditions for existing operations but also to successfully design new industrial hybrid grinding systems.