Optimization of Uranium Leach Mining

The results of the first restoration effort for a pilot-scale acidic leaching of a uranium ore body in the U.S. are given. The project was performed under a cooperative agreement between the USBM and a joint venture consisting of Rocky Mountain Energy, Mono Power Co., and Halliburton Co. The leaching phase is described only briefly; it was reported earlier by Tweeton et al. Leaching was done with H2SO4 (pH range of 1.6 to 3.9) and H2O2 (80 to 1,000 ppm) over an 11-month period. The restoration phase begun in September 1978 and was completed in 11 months. Water samples were taken from observation wells located between injection and production wells while the leaching solution was decreased in strength. Groundwater quality in the leached ore zone was restored by diluting and neutralizing effects of injecting native groun dwater and treated water produced from recovered solution. Recovered water was purified by a combination of chemical precipitation and reverse osmosis. In-situ measurements of pH, Eh, dissolved oxygen, conductivity, and temperature were made by a downhole probe in one of the wells. Water samples were taken and analyzed for U. V, Na, K, Ca, Mg, SO4, P, Cl, F, Fe, Mn, Si, Zn, As, and Se. Introduction The USBM and the companies listed previously entered into a cooperative agreement aimed at modeling in situ leaching (ISL) of uranium with sulfuric acid at the venture's Nine-Mile Lake test site near Casper. WY (Fig. 1). Acid leaching of uraniferous ores is relatively unproved because most earlier ISL systems involved use of alkaline leachants. This report describes the first restoration of a pilot-scale acidic-leached ore body inthe U.S. The results of an 11-month leaching phase conducted on afive-spot pilot test area (Pattern 2, Fig. 2) were previously reported bythe Bureau of Mines. The study presented here consisted of weekly in-situ determination of water quality parameters within the ore zone being restored plus additional water analysis in the laboratory. Geology The uranium mineralization occurs in Teapot sandstone members of the Upper Creataceous Mesaverde formation. The project site is located approximately 15 km (9 miles) north of Casper in Natrona County. The leached uranium ore (and aquifer) restored to baseline in this study was a bedded deposit ranging from 155 to 165 m (508 to 541 ft) deep. The sandstone contains quartz, feldspar, and minor amounts of mica, black (organic-containing) minerals, glauconite, carbon aceous fragments, blue to earthy clayinclusions, and disseminated argillaceous materials. The deposit isamenable to acidic leaching because of the relatively low carbonate content. The mineralization is typical of a roll-front type deposit. The principal uranium mineral is uraninite (UO2), with minor amounts of coffinite U(SiO4) 1-X (OH) 4X. Further de tails are found in the operators environmental report to the U.S. Nuclear Regulatory Commission (NRC). Hydrology The Teapot sand stone is an artesian aquifer confined by the overlying Lewisshale and the underlying Pumpkin Buttes shale. The formation dips to theeast-northeast at about 7 with ground water movement in that direction at about 4 m/a (14 ft/yr). The aquifer is recharged at the out crop approximately 3 km (2 miles) west of the test area. Groun dwater in the project area is marginally acceptable for live stock watering or industrial use. SPEJ P. 382^