5:15 PM - 6:30 PM
[SVC27-P02] Selective pulverization of pumiceous groundmass in pumice pulverization processes in a laboratory
Keywords:pumice pulverization and sieving experiments, volcanic ash hazard, phenocryst, volcanic glass
The pulverization characteristics of volcanic rocks have recently been the subject of much experimental research from the viewpoint of volcanic ash generation during the eruption processes. The pulverization characteristics are also important for preparing artificial volcanic ash by pulverizing volcanic rocks and sieving the pulverized materials for experimental studies on the adverse effects of volcanic ash on equipment and facilities. The author has been experimentally investigating the pulverization characteristics of porous volcanic rocks for manufacturing artificial volcanic ash (Takeuchi, 2020). In this presentation, I investigate the variation in the ratio of crystal to glass at each φ-scale sieve class by pulverizing phenocryst-bearing pumice samples.
The Osumi pumice (Os), which is available in large quantities and is expected to produce pumiceous particles, was used as a raw material for the pulverization experiments. Lapilli with -1φ (2000 μm) class were prepared as starting samples by a jaw crusher and manual sieving. The intensity of a disk mill used to pulverize the starting samples was varied by adjusting the slit width between the rotating metal discs. A Vibratory Sieve Shaker (FRITSCH A-3PRO) was used to sieve the pulverized material. The Vibratory Sieve Shaker is electro-magnetically controlled and can be loaded with vertical vibrations of 3000 times/min in the amplitude range of 0.1 to 3.0 mm, and the mass variation with the sieving time can also be confirmed to maintain objectivity in judging the end of sieving.
The difference in pulverization intensity produced a difference in the particle size distribution of the pulverized material as follows. For the wide and narrow slit widths, the median particle sizes (f50) are -1.0φ (2000 μm) and 1.5φ (354 μm), and the mass fractions with a sieve class of less than 4φare 9% and 34%, respectively.
Based on the bulk rock composition obtained by XRF analysis and the average composition of each crystal and glass obtained by EPMA analysis, the mass fraction of crystal and glass for each class was quantified by mass balance calculation. As a result, the following characteristics of the crystal content, which changed during the pulverization and sieving processes, were clarified. Compared to crystal content (about 10%) of the raw material, the crystal content increases (up to ca. 30%) in the 1φ (500 μm) to 3φ (125 μm) particles. Crystal content decreases down to ca. 5% in the pulverized material of less than 4φ(63 μm). Weak pulverization tended to result in a greater crystal content in sieve classes larger than 1φ. The -1φ(2000 μm) sieve class in the pulverized material has a lower crystal fraction than the starting sample (-1φ, 2000 μm). The size distribution of the crystals (phenocryst) in the Os pumice obtained from image analysis of the elemental and electronic images of EPMA for the polished section of pumice lapilli was consistent with the crystal-rich sieve class of the pulverized material. I interpret that when the material is subjected to disk milling, the phenocryst in pumice is likely to be separated as free crystals, and the separated phenocryst is relatively difficult to be pulverized, and the pumiceous glassy matrix was selectively pulverized to a sieve class of less than 4φ(63 μm). This variation in crystal content in pulverization and sieving processes of phenocryst-bearing pumices is an important note on the properties of artificial volcanic ash when using for experiments.
The Osumi pumice (Os), which is available in large quantities and is expected to produce pumiceous particles, was used as a raw material for the pulverization experiments. Lapilli with -1φ (2000 μm) class were prepared as starting samples by a jaw crusher and manual sieving. The intensity of a disk mill used to pulverize the starting samples was varied by adjusting the slit width between the rotating metal discs. A Vibratory Sieve Shaker (FRITSCH A-3PRO) was used to sieve the pulverized material. The Vibratory Sieve Shaker is electro-magnetically controlled and can be loaded with vertical vibrations of 3000 times/min in the amplitude range of 0.1 to 3.0 mm, and the mass variation with the sieving time can also be confirmed to maintain objectivity in judging the end of sieving.
The difference in pulverization intensity produced a difference in the particle size distribution of the pulverized material as follows. For the wide and narrow slit widths, the median particle sizes (f50) are -1.0φ (2000 μm) and 1.5φ (354 μm), and the mass fractions with a sieve class of less than 4φare 9% and 34%, respectively.
Based on the bulk rock composition obtained by XRF analysis and the average composition of each crystal and glass obtained by EPMA analysis, the mass fraction of crystal and glass for each class was quantified by mass balance calculation. As a result, the following characteristics of the crystal content, which changed during the pulverization and sieving processes, were clarified. Compared to crystal content (about 10%) of the raw material, the crystal content increases (up to ca. 30%) in the 1φ (500 μm) to 3φ (125 μm) particles. Crystal content decreases down to ca. 5% in the pulverized material of less than 4φ(63 μm). Weak pulverization tended to result in a greater crystal content in sieve classes larger than 1φ. The -1φ(2000 μm) sieve class in the pulverized material has a lower crystal fraction than the starting sample (-1φ, 2000 μm). The size distribution of the crystals (phenocryst) in the Os pumice obtained from image analysis of the elemental and electronic images of EPMA for the polished section of pumice lapilli was consistent with the crystal-rich sieve class of the pulverized material. I interpret that when the material is subjected to disk milling, the phenocryst in pumice is likely to be separated as free crystals, and the separated phenocryst is relatively difficult to be pulverized, and the pumiceous glassy matrix was selectively pulverized to a sieve class of less than 4φ(63 μm). This variation in crystal content in pulverization and sieving processes of phenocryst-bearing pumices is an important note on the properties of artificial volcanic ash when using for experiments.