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A new planetary-type rod-mill machine that we developed for use at a research institute improves the working efficiency of comminution. It also prevents the generation of soil dust, and the jar is easy to keep clean. This device breaks up clods into soil of a small particle size (<2-mm diam.) within two minutes. The performance of our device is sufficiently satisfactory compared with other conventional machines. However, the exact crushing mechanism remains unclear. We sought to answer questions such as what kinds of strains cause the crushing of clods in the rotating jar, whether or not the maximum strength exceeds the yield stress of the soil, and what is the function of rods during quick crushing. An objective of this study was to understand fundamental mechanisms of crushing clods to further improve the milling device. We carried out compression and shear stress tests using a vibrating container and shearing device to observe the crushing mechanism due to a single stress acting on a clod. We used a charged-coupled device digital camera to visually capture the crushing event in these tests. We found the centrifugal forces produced by rotation of our machine's jar to be much smaller than the critical forces of the amorphous yield point of the clods. The crushing occurs actually in a short time if two rods are in the jar. Soil dust observed in the early stage of the crushing process is produced because the surface of a soil clod is worn initially by shearing forces caused by the rods. After the surface of a clod is scraped, it fragments into small particles catastrophically. The shearing forces exerted by the rods are more effective than the compressive forces in comminution by our rod-mill machine. These results suggest that the cause of crushing clod is the sharing forces acting on the clod for the initial stage.

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We developed a planetary-type rod mill that can improve the comminution working efficiency, prevent the generation of soil dust, and make it easy to clean the used jars. Clods are broken by this device into fine-grained soil particles with sizes of less than 2 mm in diameter within 2 min. One objective of this study is to evaluate the performance of the mill by observing the fragmentation process of the clods. The process is visualized experimentally via a high-speed camera installed on a rotating jar of the device. The breakage of the clods in the rod mill is considered morphologically according to the crushing patterns obtained by previous studies. The rod moves together with the clods in the jar. Therefore, violent collisions are not thought to be a major reason for the breakage of the clods. The surfaces of the clods are first scraped and then appear to become fragile. However, if there are no rods in the jar, the clods never break into small pieces. Therefore, it is likely that collisions with the rod trigger the catastrophic fragmentation of the clods into small pieces. This suggests that the design of the device could be modified to improve its performance in the near future.