Issue34

Q. Like et alii, Frattura ed Integrità Strutturale, 34 (2015) 543-553; DOI: 10.3221/IGF-ESIS.34.60 548 Effect of irradiation time on mineral boundary failure Fig. 4 shows the distribution of the mechanical state of rock at different irradiation times under the following conditions: mineral content of 5%, mineral crystal size of 0.6 mm, and microwave power density P d = 10 9 W/m 3 . The figure demonstrates that as irradiation time increases, the rock changes from a completely elastic to a yield failure state. The boundary element around galena crystals near the rock border is destroyed first due to the free boundary of the mineral. Other boundary elements of galena crystals are gradually destroyed as irradiation time increases. Meanwhile, the failure elements among different crystals begin to link with each other. The development process of failure elements is similar to that obtained by Mamdouh Omran [14]. Fig. 4 also shows that the failure regions in the rock are concentrated at the boundary elements of galena crystals, which clearly signifies that microwave irradiation benefits mineral liberation. As indicated by the failure type, both shear and tensile failures occur. Tensile failure primarily occurs at the boundary regions of the sample and at the connecting regions of different mineral crystals. The rest of the failures are tensile. Fig. 5 depicts the graph of mineral boundary failure rate over the irradiation time. The model consists of 728 mineral boundary elements. When irradiation time is 7.9 ms, some mineral boundary elements begin to step into a failure state. By contrast, when irradiation time is 12.5 ms to 20 ms, the mineral boundary failure rate rapidly increases from 11.67% to 91.07%, and mineral boundary element failure rate grows fastest during this irradiation period. When irradiation time is 28 ms, the boundary failure rate reaches 99.31%, after which time the rate does not increase further. This condition shows an optimal irradiation time for microwave-assisted mineral liberation. An irradiation time that is shorter than the optimal irradiation time averts the attainment of the mineral liberation effect. By contrast, an irradiation time that exceeds the optimal irradiation time wastes energy. Therefore, microwave-assisted mineral liberation and energy saving should critically determine the beginning and completion times of mineral boundary failure. Figure 5 : Relationship between failure rate around mineral boundary and irradiation time (Pd=10 9 W / m 3 ). Effect of microwave power density on failure rate Fig. 6 illustrates the relationship between mineral failure rate and irradiation time at different microwave power densities under the condition in which mineral content is 5% and mineral crystal size is 0.6 mm. The figure demonstrates that as the power density increases during the optimal irradiation time, the growth of the slope is larger and the boundary element requires a shorter time to complete the failure. When the power density reaches 12×10 9 W/m 3 , mineral boundary element failure requires 3.5 ms from beginning to completion. Moreover, when the power density reaches 3×10 9 W/m 3 , the time required is 0.72 ms. Meanwhile, as the power density increases, the beginning and completion times of mineral boundary element failure occur earlier. Fig. 7 illustrates the relationship between irradiation time and power density when the mineral boundary failure reaches 95%. As shown in the figure, as power density increases, irradiation time decreases. When the power density increases from 0.5×10 9 W/m 3 to 3×10 9 W/m 3 , irradiation time decreases most obviously from 83 ms to 4.95 ms. Furthermore, when the power density is greater than 3×10 9 W/m 3 , irradiation time decreases within a very small range. Fig. 8 shows the relationship between energy consumption and power density when the boundary failure reaches 95%. When the power density increases from 0.5×10 9 W/m 3 to 12×10 9 W/m 3 , energy consumption decreases rapidly. When the power density is greater than 3×10 9 W/m 3 , energy consumption decreases slightly, and when it is more than 12×10 9 W/m 3 , energy consumption increases slowly. In general, when the microwave power density is greater than 3×10 9 W/m 3 , irradiation time and energy consumption slightly change. This condition signifies that irradiation time and energy consumption cannot be