Environmental Geochemistry Of Uranium In Coal Of Eastern Yunnan Province

This dissertation aims to study the geochemistry of uranium in coals from eastern Yunnan Province using state-of-art analytic techniques including sequential extraction experiment and laboratory-simulated combustion experiment. Basing on the theories of coal geochemistry, environmental geochemistry and environmental science, and related mathematical modeling, this study will illustrate the distribution characteristics, modes of occurrence, enrichment patterns and environmental hazards of uranium in eastern Yunnan Province. In addition to improve our understandings on the environmental geochemistry of uranium in coals, the results of this study will enhance our knowledge on exploration of coal-bearing uranium and inform the decision-makers to enforce relevant environmental protections. The following aspects are obtained in this dissertation:1) The abundance and distribution of uranium in coals from eastern Yunnan Province are obtained, and the variations of uranium in coals of different coal-forming periods and different ranks are illustrated. The uranium varies from 0.33 to 8.25 μg/g, with an average value of 3.76 μg/g. On average, uranium in studied coals are higher than those in Chinese and world coals, belong to uranium-slightly enriched coals. Uranium in coals of different coal-forming periods has the following order: Wanshoushan Formation>Permian Xuanwei Formation>Neogene>Permian Longtan Formation. Uranium decreases with the increase of coal ranks: lignite>coking coal>anthracite.2) The modes of occurrence and the chemical structure of uranium has been revealed by sequential extraction experiment and mathematically statistical analysis. Uranium is dominated by organic forms in medium-high sulfur coals. In low sulfur coals, uranium is dominated by mineral forms followed by organic forms. The sources of uranium probably came from source sediment input.3) The uranium/thorium ratio shows that uranium can be oxidized into to UO22+ in oxidizing environment. UO22+ can easily migrate, adsorb onto coal matrix, and precipitate under reducing environment. Other factors such as coal-forming plants, source rocks, and volcanic eruptions are all the underlying factors contributing uranium enrichment in coals.4)Based on indoor static combustion experimetion, the distribution and release characteristics of uranium in combustion products is revealed. The volatilization ratio of U in coal is correlated with temperature of combustion, ash yield and the type of coal. The maximum volatilization ratio occurs at 815℃, the enrichment ratio of U during combustion is correlated with ash yield, decreasing with increasing ash yield. When under the same temperature, the volatilization ratio of different coal differs, i.e. under lower temperature, coking coal < lignite < anthracite; under medium temperature, coking coal < anthracite < lignite; while under high temperature, lignite < coking coal < anthracite. Compare the migration rule of the uranium to its associated elements in coals, plumbum, stibium, stannum, zinc, cadmium are volatile elements, but vanadium, nickel, cobalt, chromium, copper and molybdenum belong to semivolatile elements with the same to uranium, thorium is less volatile element.5)The radioactivity level of radioactive elements(238U, 232 Th, and 40K) is assessed, the radioactivity level exceeds previous the values in Yunnan coals reported, among which the radioactivity level of 238 U is higher than its average value in Chinese coal. It is demonstrated from the correlation analysis between 238 U and coal ash that the radioactivity of 238 U is mainly derived from U that is associated with minerals, while 232 Th and 226 Ra originated from inorganic minerals and organic matter, respectively.