Modelling the comminution process in the rotary offset crusher

Abstract

There is always a search for size reduction solutions due to the inherent energy inefficiency associated with comminution devices. The rotary offset crusher (ROC) is a new comminution device with a promising performance potential in terms of throughput due to enhanced speed of transportation induced by the centrifugal force of the discs and high frequencies of closure and opening of crushing chamber. The lack of fundamental understanding of the micro- processes that are facilitating both comminution and material transport in this new crusher necessitated a thorough investigation of key factors that drive comminution in this equipment. A combination of experimental and numerical modelling techniques was used to study the effect of key operating variables such as speed of the discs, offset between the vertical axes of the discs, feed size distribution and feed rate. The offset provides some flexibility in the system, but it does not significantly improve the crushing efficiency or the throughput. The speeds of the discs proved to be the key determinants of the degree of breakage achieved in the crusher. Both compressive and shear energies are active, with compressive energy dominating at lower speeds and decreasing with gradual increase in speed while the shear force increases with the increasing speed. Those trends render the device operating at higher speeds to be analogous to a laboratory pulveriser. Feed rates of at least 10 tph and feeds with wide size distributions (including near-vertical exit gap particles) are recommended for future studies in the quest to optimize the throughput of the crusher. Further work is recommended to investigate the effect of crusher profiles on the product quality (size and shape), power draw and throughput. It is anticipated that modifying the crusher profile can further intensify the crushing forces imposed on the particles. Consideration should also be given to increase the opening of the comminution cavity in order to be able to choke feed the crusher and thereby promoting rock-on-rock crushing. It is also recommended that the vibratory motion for the top disc be considered in order to intensify the compressive loads experienced by the particles. Overall, the redesign of this engineering device is necessary to reduce comminution by attrition due to disc-particle contacts and intensify the compressive forces.