Case Study of a Multiple Sand Waterflood, Hewitt Unit, OK

Abstract

Summary Synthetic two-component mixtures (uraninite and iron sulfide) as well as native uranium ores obtained from Texas and Wyoming have been examined. Physical/chemical ore properties are correlated with observed laboratory leach response. Data show a large inherent selectivity of oxidant for uranium in the early stages of a leach period. Uranium head grade was found to increase in a nearly linear fashion with hydrogen peroxide concentration in the leach solution. As uranium in the ore is depleted, uranium response decreases and the oxidant serves mainly to leach iron sulfide gangue material. Introduction Uranium historically has been mined by two methods, open pit (strip) or underground shaft mining. These two methods require high-grade uranium deposits to justify the large investment of personnel and material required to process the uranium into saleable form. New reserves of high-grade uranium ore are not being discovered in large quantities, indicating the need to locate new types of uranium deposits to fill future demands of the nuclear power industry. New sources of uranium exist in the form of low-grade ore deposits approximately 60 to 600 m below ground level. Recovery of some of these low-grade deposits became economically feasible when the price of uranium increased in the early 1970's. It is too costly, however, to mine these uranium deposits by conventional techniques (i.e., open pit or underground shaft), so a new mining method was developed. This new method is known as in-situ leaching or solution mining and involves selective leaching of the uranium from the ore deposit by use of a special solution injected into the ore body. Ore body composition and leach solution chemistry have a large effect on uranium recovery and are investigated in this study. Chemistry Uranium present in an ore body is generally in the reduced tetravalent (U+4) state and first must be oxidized to the more soluble hexavalent (U+6) state before it can be leached. Various oxidants are available, with hydrogen peroxide and molecular oxygen being the most widely used in commercial practice. Aside from uranium, an ore body contains other oxidant-consuming species such as pyritic iron sulfides and carbonaceous material (Table 1). Uranium leach chemistry can be described best by the following equations, with hydrogen peroxide (H2O2) as oxidant. (1) (2) (3) (4) (5) Eqs. 1 and 2 describe uranium oxidation and solubilization, respectively. Oxidant is added to a leach solution composed of either sodium or ammonium carbonate, and is injected into the ore body. Dissolved uranium is transported through the ore body to the recovery wells in the form of a uranyl carbonate complex. Eqs. 1 through 5 show that more oxidant can be consumed by oxidation of the iron sulfide and carbonaceous gangue materials than by uranium itself. JPT P. 937^