Modelling studies of a rotary offset crusher

Nghipulile, Titus
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The quest for efficiency in comminution is an on-going concern as it usually constitutes a major cost component in the metal production industry. Such improvements can be made by circuit optimization or development of more efficient equipment. The rotary offset crusher (ROC), a novel crushing equipment, invented in 2002 by Michael Hunt, Henry Simonsen and Ian Sinclair is simple by design with only two moving parts (the cylindrical discs) that are parallel to each other, but not vertically aligned. This crusher employs the same crushing mechanism as the HPGR which is said to be energy efficient as compared to tumbling mills, with energy savings of 10 – 30 % reported. In addition, unlike the HPGR, in the ROC centrifugal motion guides the transportation of particles in the crushing zone and therefore, showing the potential to be a high throughput crusher. This study aimed at building the laboratory prototype to demonstrate the concept and study the principles guiding the operation of this equipment. The original design concept was re-ignited, and a redesigned laboratory crusher has been built. The crusher was instrumented with sensing devices to pick up signals that allow measurement of the speeds for the two discs, motor drive torque and mass on the conveyor belt and thereby allowing computation of the crusher power draw and feed rates. Laboratory breakage tests with the drop weight and piston-die apparatus on coal and quartz were conducted and the results were correlated to those of the ROC. Following the fabrication and commissioning of the crusher, experiments were conducted with coal to investigate the effect of feed size distribution, horizontal offset of the discs and vertical exit gap between the top and bottom discs. The rotational speed was fixed at 330 rpm in all tests. The horizontal offsets were 5 and 10 mm while the vertical exit gaps were 1.5 and 3 mm. The two feed size fractions used were -13.2+9.5 mm and 19+13.2 mm. Results from the 8 experiments conducted showed no well-defined relationships between different operating variables (horizontal offset, vertical exit gap and feed size distribution) and d80 and throughput. More experiments still need to be conducted to establish sustained trends. The size reduction ratios were in the range of 2 with acicular particle discharged from the crusher. Those results were attributed to the absence of corrugated profiles on the crushing surfaces as well as the mineralogical characteristics of coal. To investigate the effects of rotational speed and feed rate, 8 experiments were conducted with quartz. The horizontal offset between the discs and vertical exit gap were fixed at 10 and 3 mm respectively. What came to light was that the performance of the ROC is highly dependent on the speed. The size reduction ratios as high as 7 were recorded at the speed of 550 rpm and feed rate of 1 tph. It is recommended that many experiments be conducted with quartz at this speed for various offsets, particle sizes, feed rates and crusher exit gaps to help with optimization of the crusher. Thereafter, higher speeds can be tried to establish the relationship between speed and size reduction. The discrete element method (DEM) was used to study the transportation of particles in the ROC. Simulation results showed that the throughput is highly dependent on the rotational speed of the discs. This agrees with the experimental data generated using the laboratory prototype. Simulation using the DEM for various design configurations is worth considering in future as this would improve the understanding of the flow behaviours of the particles in the crusher. The design of disc profiles of various configurations needs to be undertaken using the DEM to study both the breakage and transportation of particles in the crusher. This would guide the future modifications of the crushing faces of the crusher. Importantly, it is recommended that comparative studies with competing comminution machines such as HPGR, short head cone crusher and Loesche mill be undertaken to establish benefits in terms of energy efficiency, throughput, size reduction, if any, for the ROC over the existing machines.
A dissertation submitted to the School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa, in fulfillment of the requirements for the Master of Science Degree in Engineering October 2019
Nghipulile,Titus (2019) Modelling studies of a Rotary Offset Crusher, University of the Witwatersrand,