Crystalline silica in compressed rice hull coke ash
AbstractCompressed rice hull coke was produced by compressing rice hulls ground to less than 200 Âµm diameter at 1.22x106 N/m2 and 310ËšC with an oxygen deficient atmosphere. The compressed rice hull coke had a 5 cm outer diameter, a 1.5 cm inner diameter and 35 cm length. The density of the coke is 1.2, and, the heat value is 16570 kJÂ·kg-1. In this study, 2 kg coke was combusted at the average temperature of 850ËšC for 104 min with a maximum temperature of 860ËšC. The ash was colored black-grey, and the form remained coke. The ash contained 95.4% silica with 9.2% cristobalite and 86.2% amorphous silica. The occupational exposure limits of respirable fraction of crystalline silica such as cristobalite and quartz in floating dust less than the diameter of 7 Âµm is 0.025 mg/m3; however, no respirable cristobalite was detected during combustion of the compressed rice hull coke. Cristobalite was not present in floating dust because it melted during the crystallization of amorphous silica to cristobalite.
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