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.
Several ways to comminute a soil have been devised: using pestles, mortars, and sieves, by hand; using milling machines, with rods, balls, hammer, and air jets applied to soils in a jar; using mechanical crushers, powered by electricity to apply jaws, gyrating rods, rollers, and disc and edge runners. Comminution mechanisms have not been precisely explained for each type of mill machine. Advantages and disadvantages of previous comminution methods were discussed by Powel and Morrison [
Gay [
Liu [
Grady [
The performance of our rod-mill device is sufficiently satisfactory compared with conventional machines; however, the fragmentation mechanism in the jar is not clear. Thus, to help improve the performance of the device, we observed what happens in the jar and measured the magnitude of forces generated by the device. Also considering improvement of crushing events, Saeidi et al. [
We developed a new type of rod mill (Japanese patent No. 5055524) (
1) Put the sampled clods and two rods in a jar (
2) Put the jar on a turntable (
3) Soil particles that pass through the sieve fall cleanly into a collector cup, without the escape of soil dust.
4) Collected soil particles enter the analysis phase. A turning bed, tilted at a 20˚ angle, causes the clods and rods in the jar to move randomly. Their movement is more complex than if motion were just in a horizontal plane because of the addition of rotational motion around the horizontal axis.
We tried to separate the powdering process by the compression and the wear processes. To be focusing on the compression, we made a vibrating container (
Our original wear-testing machine, the abrasion-resistance tester BIK6556, is shown in
reciprocated sinusoidally, with an amplitude of 25 mm. The frequency of the cantilever motion is changeable within the range 10 - 60 rpm. The details of specification are shown in
In our experiment, we used a soil sample from the Kanto district in Japan. Derived from volcanic ash, the sample contained ferric oxide (12.1%) and clay (25.0% - 37.5%). Because results are influenced by a soil’s water content, usually crushing tests are done on dry clods. Therefore, first, we dried the testing soil samples at 105˚C for 5 days. To measure its water content after the initial drying, we used a Karl Fischer Moisture Titrator, which combines vaporization apparatus EV-6 and measuring apparatus AQV-7 (Hiranuma Co.). We found that the treated soil included 10.7 wt.% water.
The chemical composition of the soil (
We used the laser-diffraction particle-size analyzer SALD-3100 (Shimadzu Co.) to measure the size distribution of the soil particles after crushing.
We recorded the clods-crushing process by using a high-speed CCD camera and an LED light, which were attached to the jar cover. Clods weighing 0.150 kg were stirred 2 min. in the jar as it rotated by a planetary gear. When only clods were in the jar, most of the clods were still as intact as before stirring, with only a small amount of powder falling through the sieve (
Characteristic | Specification |
---|---|
Stroke | 2 - 120 mm (1-mm precision) |
Frequency | 10 - 60 per minute |
Repeat count | 1 - 100,000,000 times |
User options | 15 cases |
Power supply | AC 100 - 240 V, 50/60Hz |
Dimensions | W380 mm × D590 mm × H310 mm |
Weight | 22 kg |
Component | Amount (mass percent) |
---|---|
MgO | 1.7277 |
Al2O3 | 33.6425 |
SiO2 | 47.5489 |
P2O5 | 0.2339 |
SO3 | 0.4039 |
Cl | 0.8168 |
K2O | 1.3569 |
CaO | 0.4612 |
TiO2 | 1.3894 |
MnO | 0.2673 |
Fe2O3 | 12.0768 |
CuO | 0.014 |
ZnO | 0.0191 |
Br | 0.0077 |
SrO | 0.0083 |
ZrO2 | 0.0257 |
When clods and two rods are combined in the jar (
crushing process, as shown in the blurry photo in
Because a rod mill normally needs many rods, we questioned why our preliminary experiments showed that the combination of two rods was best for crushing quickly in our rod mill. We have tried the change the number of rods in a jar for the crushing. The results by using two rods show the advantage to reduce the working time and to break into smaller particles (<2-mm diam). However, the mechanism of crushing using only two rods in the rotating jar remains unclear. This has been our motivation to carry out fundamental clods-crushing experiments as the impacting and the wearing processes. We sought to find out what kinds of strains cause crushing of clods in the rotating jar, whether or not the maximum strength exceeds the yield stress of the soil, and by what mechanism rods achieve quick crushing. The sections following show the results of our fundamental experiments to investigate these questions.
To understand what is happening during repeated collisions of a clod with a wall, we observed the change of states of a clod in a vertical oscillating container (
Starting with an initial clod mass of 2.6 × 10−3 kg, we noted that the clod mass decreases logarithmically with the passage of time but that its mass did not change within 2 minutes (
the clod. In contrast, the maximum impact force in the horizontal oscillating container was 0.068 N, as described previously. However, the yielding force of the tested soil is about 52 N, based on the results of our tests using the universal material test equipment Instron 5566. Therefore, the impact force given by the oscillating container is ~8/1000 in the vertical case and ~1/1000 in the horizontal case smaller than the yielding force. From these results, it is clear that the clods never crush under this experimental condition. Given that the oscillating motion of the container simulated that of the rotating jar of our rod mill, this result shows that clods in the rotating jar do not get crushed.
Nakata et al. [
We also considered whether clod strength might decrease as a result of the effect of fatigue [
As discussed in Section 3.1, crushing was not observed when only clods were in the rotating jar but occurred successfully when clods together with two rods were in the jar. From observation of the combined motions of clods and rods in the jar, the differences of their motions apparently caused abrasion between them. We simply simulated that interaction by using the abrasion-resistance tester BIK 6556.
first, the fine particles are ground from the surface of a clod. Small pieces of the clod begin to separate after 37.7 seconds. A typical large inclined crack due to shearing stress is seen in pictures of the clod at 37.8 and 38.5 seconds. Small spaces and/or small cracks between particles trigger large cracks under shear stress [
The height of the cantilever measured from a surface at the base of the device through time did not change much until 37.7 s (
clod to break into small fragments ranges from the initial 7.3 s to as long as 37.7 s, when the crush appears to occur catastrophically (
To make clear the crushing mechanism in a rotating jar of a newly developed rod-mill machine, we investigated crushing by means of fundamental experiments. Our questions concerned 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. We carried out compressive 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. The experimental crushing tests were recorded visually. Centrifugal forces produced by rotation of our machines’ jar were found to be much smaller than the critical forces of the clods’ amorphous yield points. Crushing did not happen if there were no rods with the clod in the jar but occurred in a short time if two rods were in the jar with the clod. Initially in the crushing process, soil dust due to abrasion was observed in the jar. This early-stage soil dust apparently arose because the clod’s surface initially is worn by shearing forces caused by the rods. After the surface of a clod is scraped, it fragments into small particles catastrophically. The shearing forces produced by the rods are more effective than the compressive forces in comminution by our rod-mill machine. These results suggest that the crushing of clods needs to consider the sequence of adding the shearing and the compressive forces to develop the milling device.
The authors declare no conflicts of interest regarding the publication of this paper.
Oishi, M., Kubota, Y. and Mochizuki, O. (2019) Investigation of Fundamental Mechanism of Crushing of Clods in a Rod Mill. Engineering, 11, 703-716. https://doi.org/10.4236/eng.2019.1110045