Operation & Maintenance of Tailings Earth En Dam Specially For Disposal of Fly Ash from Thermal Power Stations
Several tailings earthen dams have failed during monsoon. Due to failure of tailings dam, stored tailings waste gets release in downstream area causing environmental hazard. Ash produced by combustion of coal in thermal power stations are stored in ash pond. No well-defined provision of operation and maintenance exists for ash pond. Therefore, this paper describes various issues related to operation and maintenance of ash pond.
DENSIFICATION OF DEPOSITED ASH SLURRY
In India, ash content in coal is usually high and percentage utilization for construction is limited. As a result, about 90 to 95 percentage of the ash generated has to be deposited in ash pond. The present ash generation is about 75 million tonnes and it is likely to exceed I 00 million tonnes by the end of this century. During the last decade, several laboratory studies have been made on densification of deposited ash slurry by vibration, application of vacuum, electro osmosis, etc. In last few years, with assistance from Department of Science and Technology, field experiments have been carried out to densify the deposit by vtbro flotation technique and also by explosives. This paper describes brief literature review on the subject andsummary of compaction achieved in field by vibroflotation as well as explosives. Based on the study canied out so far, it appears that densification in ash pond is very effective for rehabilitation of the area for housing or other use. It also eliminates chan~ Qf liquefaction during earthquake.
O2 constraints from Paleoproterozoic detrital pyrite and uraninite
Redox-sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the Archean and early Paleoproterozoic atmosphere. Today, these minerals are rapidly chemically weathered within short transport distances. Prior to the rise of oxygen, low O2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes. After the rise of oxygen, redox-sensitive detrital grains effectively vanish from the sedimentary record. To get a better understanding of the timing of this transition, we examined sandstones recorded in a scientifi c drill core from the South African 2.415 Ga Koegas Subgroup, a mixed siliciclastic and iron formation–bearing unit deposited on the western deltaic margin of the Kaapvaal craton in early Paleoproterozoic time. We observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section, indicating the rise of oxygen is younger than 2.415 Ga. To better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain Earth surface redox conditions, we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental O2 concentrations. Even conservative model calculations for deltaic depositional systems with suffi cient transport distances (approximately hundreds of kilometers) show that redox-sensitive detrital grains are remarkably sensitive to environmental O2 concentrations, and they constrain the Archean and early Paleoproterozoic atmosphere to have <3.2 × 10 –5 atm of molecular O2. These levels are lower than previously hypothesized for redox-sensitive detrital grains, but they are consistent with estimates made from other redox proxy data, including the anomalous fractionation of sulfur isotopes. The binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric O2 concentrations rose substantially at this time and were never again suffi ciently low (<0.01 atm) to enable survival and preservation of these grains in short transport systems.