On the leaching behavior of uranium-bearing resources in carbonate-bicarbonate solution by gaseous oxidants

Abstract

Uranium is recognized to be a critical commodity in the context of satisfying the global energy-demands for the twenty-first century and beyond. In 2013, a significant supply gap of 17% between worldwide production and consumption of uranium was identified. Consequently, low-grade uranium-bearing limestone rock, including mining waste, may represent a significant future uranium-resource, provided an economically-viable method of extraction is developed. Alkaline leaching of uranium from limestone media has been practiced extensively in the past; however, cutting-edge research during the last 20 years has been rather meager. The research reported in this thesis is an investigation into the use of an alternative gaseous oxidant (oxygen/ozone), which may serve to improve the economics of alkaline leaching; specifically, heap leaching. Components of the research include the employment of computational software (PHREEQ), which allowed the equilibrium distribution of the aquo-species in solution to be determined, as well as solubility behavior of candidate condensed-species (precipitates). Also, two suites of leaching experiments were performed, one with synthetic UO2 particles, which served as a model system, and the other with comminuted uranium-bearing Todilto limestone. The aqueous-phase lixiviant for each was comprised of 40 gpl Na2CO3 and 15 gpl NaHCO3. The invariant leaching-configuration was: continuously-stirred batch-reactor with the ratio of uranium-bearing particulate-source to aqueous-phase such that a hypothetical-maximum uranium concentration in solution of 1.0 gpl (~0.0040 molar) could be achieved. The duration of the two suites of experiments was 48-hours and 72-hours, respectively. A robust two-parameter non-linear function, Recovery (%) = 100 [1- exp(-a *(t^b)], was formulated to regress (smooth) the data acquired from the leaching experiments. The regressed-equations (containing optimal values for the parameters "a" and "b") served as a quantitative tool for relative assessment of the leaching-rate characteristics of each member-experiment within a suite of experiments performed. The work reported in this thesis is considered to be a significant contribution to the knowledge-base required for planning cost-effective strategies for uranium recovery from low-grade alkaline uranium-bearing resources. Additionally, it provides process-engineers with information for developing appropriate recovery-methods (flowsheets), while at the same time being able to assess potential environmental impacts.