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4. Conclusions
In conclusion, we have developed a performance model for impact crushers that is able to predict the product size distribution at steady-state operating conditions and contains a reasonable number of parameters. The specific behaviour of impact crushers is modelled through classification and breakage functions that both depend on the rotor radius and angular velocity as well as on the feed rate. The introduction of variable minimum size of the breakable particles and proportion of fine fraction in the product seems to be very important for successful modelling of the impact crushing.
The simulation results are in a good agreement with the experiment and show that at higher rotor velocity the product size distribution becomes finer, provided that the feed rate and size are kept unchanged. Alternatively, higher feed rate at constant rotor velocity results in a coarser product.
The model can be easily implemented in the existing commercial codes for mineral processing simulations. It can be used for prediction of the steady-state performance of hammer and/or vertical-axis impact crushers integrated in complex flowsheets. Further work is required to adapt the model for unsteady and transient operating regimes.
Acknowledgements
The Belgian Walloon Region government and the European Community have jointly funded this research as an Objective 1 European project. The author thanks Dr. A. Attou for his contributions at the early stage of the project, Mr. Chr. Lucion for the useful discussions as well as Mr. R. Lemaire for providing the experimental results.
Abstract
The PFC3D (particle flow code) that models the movement and interaction of particles by the DEM techniques was employed to simulate the particle movement and to calculate the velocity and energy distribution of collision in two types of impact crusher: the Canica vertical shaft crusher and the BJD horizontal shaft swing hammer mill. The distribution of collision energies was then converted into a product size distribution for a particular ore type using JKMRC impact breakage test data. Experimental data of the Canica VSI crusher treating quarry and the BJD hammer mill treating coal were used to verify the DEM simulation results.
Upon the DEM procedures being validated, a detailed simulation study was conducted to investigate the effects of the machine design and operational conditions on velocity and energy distributions of collision inside the milling chamber and on the particle breakage behaviour.
Keywords: Crushing, Comminution, Modelling, Simulation
1. Introduction
Impact-induced rock fragmentation is relevant for many fields of science and technology. The length scale involved in this process span from domain of astrophysics to domain of geophysics and finally to the scale of fragments and agglomerates in the chemical and pharmaceutical industries. Impact crushers have been applied in mineral, food, coal and cement industries for a long time. The literature shows that substantial effort has been expended in understanding the impact crusher performance in relation to machine configuration and operational conditions through experimental work and mathematical modelling (Callcott, 1960; Austin et al., 1979; Gotsis et al., 1985; Shi, 2002; Shi et al., 2003). However, due to lack of detailed knowledge on velocity and energy distributions of collision inside a milling chamber, the mechanisms are still not clear.
The discrete element method (DEM) was employed in the present work to study the kinematics of the particle motion within the impact crushers. The DEM was first proposed by Cundall and Strack (1979) to model the behavior of soil particles subject to dynamic loading conditions. Since its inception this technique has been adapted to model a variety of physical systems. Mishra (1991) and Mishra and Rajamani, 1992 and Mishra and Rajamani, 1994 pioneered the application of DEM to grinding mills and demonstrated that despite the DEM simulations were two-dimensional (2D), the technique was able to predict the power draw of mills with reasonable accuracy over a wide range of mill diameters. Over 10 years since then, the DEM technique has been widely applied to ball mills (e.g. Cleary, 1998 and Cleary, 2001; van Nierop et al., 2001), SAG mills (e.g. Rajamani et al., 2000; Morrison et al., 2001), centrifugal mill (Inoue and Okaya, 1996; Cleary and Hoyer, 2000). Meanwhile the DEM code has been extended from 2D to 3D, and the contact parameters involved in the DEM model have been studied and corrected to improve the simulation accuracy (e.g. Zhang and Whiten, 1996 and Zhang and Whiten, 1998; Mishra and Murty, 2001).