In situ leaching

Aquifers with double porosity (e.g. sandstones with fractures and pore volume) require special considerations with regard to transport modeling even if no reactive mass transport in its proper sense is taken into account. This problem is demonstrated with the following example of an aquifer regeneration in an uranium mine. The ore was leached in this mine by in-situ leaching (ISL) using sulfuric acid. The hydrochemical composition of the water that is in the aquifer after this in-situ leaching process is shown as ISL in Table 40  

The simulation will be done in a zone of 200 m between an infiltration well and a pumping well. This zone shows a kf value of 5TO 5 m/s along the fracture and 10 8 m/s within the pores (those kf values are only for orientation and are not needed directly for the modeling). The flow velocity is 10 m/day due to the potential head. 

The dispersivity is 2 m. Natural groundwater ( GW in Table 40) will be infiltrated in the infiltration well and extracted at the pumping well. 

Assume that the exchange between pores and fractures only takes place by diffusion (2TO 10 m2/s). The fracture volume is 0.05, and the pore volume is 0.15. Presuppose that the fractures are planar and that the distance between them is 20 cm. Thus, on average each fracture has a pore matrix of 10 cm thickness to each side. Homogeneous and heterogeneous reactions shall be ignored. 

The simulation time is supposed to be 200 days. Thus, the water of the fractures will be exchanged 10 times in the 200 m long aquifer section. 

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In Situ Leaching. Copper and uranium ores are sometimes leached ia place by circulatiag acidified mine water through the underground deposit. This process is known as solution mining. 

Domestic. Estimates of U.S. uranium resources for reasonably assured resources, estimated additional resources, and speculative resources at costs of 80, 130, and 260/kg of uranium are given in Table 1 (18). These estimates include only conventional uranium resources, which principally include sandstone deposits of the Colorado Plateaus, the Wyoming basins, and the Gulf Coastal Plain of Texas. Marine phosphorite deposits in central Elorida, the western United States, and other areas contain low grade uranium having 30—150 ppm U that can be recovered as a by-product from wet-process phosphoric acid. Because of relatively low uranium prices, on the order of 20.67/kg U (19), in situ leach and by-product plants accounted for 76% of total uranium production in 1992 (20). 

Figure 5.5 Various cases of in-situ leaching of ore deposits below the water table (I = injection of leach solutions P = recovery of pregnant solutions). 

In situ encapsulation processes, 16 445 In situ leaching, 16 153 In situ metal—matrix composite processing, 16 173-175... 

Nuclear power plants are based on uranium mined in surface mines, or by in situ leaching. 

In this paper, the selectivity of the ECH method for the reduction of nitro compounds to the corresponding amines on RCu electrodes will be compared with that of reduction by RCu alloy powder in alkaline aqueous ethanol. In the latter method (termed chemical catalytic hydrogenation (CCH)), chemisorbed hydrogen is generated in situ but by reduction of water by aluminium (by leaching of the alloy) (equation [12]). The reductions by in situ leaching must be carried out in a basic medium in order to ensure the conversion of insoluble Al(OH)3 into soluble aluminate (equation [12]). The selectivity and efficiency of the electrochemical reduction of 5-nitro-indoles, -benzofurane, and -benzothiophene at RCu electrodes in neutral and alkaline aqueous ethanol will also be compared with that of the classical reduction with zinc in acidic medium. 

In the early 1990s a new copper extraction technology, known as in situ leach mining, was under investigation. This methodology was proposed, and demonstration plants were lo be constructed to test the proposal. These were jointly sponsored projects by the U.S. Bureau of Mines and two mining companies. If successful, leaching would cut ihe costs and hazards of the traditional open-pit methods currently used. 

They also indicate the feasibility of in situ leaching and eventual recovery in conjunction with an oxidation method. This may be economically compatible with conventional thermal degradation and retorting methods. 

Data from in-situ leach mining and restoration of roll-front uranium deposits also provide information on the potential mobility of the waste if oxidizing ground water should enter the repository. Uranium solids probably will be initially very soluble in carbonate ground water however, as reducing conditions are re-established through water/rock interactions, the uranium will reprecipitate and the amount of uranium in solution will again equilibrate with the reduced uranium minerals ... 

Roll-front uranium deposits in confined aquifer systems are amenable to extraction by in situ leach techniques. This method of mining was first tested in Wyoming approximately twenty years... 

We obtained samples of pregnant lixiviant containing 52 ppm uranium from an operating in situ leach facility in south Texas. Also, the reduced sediments downgradient from the ore zone were sampled. The dominant mineral in these sediments that contains components in a reduced valence state is pyrite, which makes up a small percentage of the sediment (7). Marcasite is also present, but at a relativey low concentration. 

This work was sponsored by the Nuclear Regulatory Commission, Office of Nuclear Research. It was conducted at the Pacific Northwest Laboratory, which is operated by Battelle Memorial Institute for the Department of Energy under contract number DE-AC06-76RL0 1830. Additional data and information are given in the report "Aquifer Restoration at In-Situ Leach Uranium Mines -Laboratory Evidence for Natural Restoration Processes," NUREG/CR-3136, PNL-4604, 1983. We appreciate the efforts of Wayne Martin who did much of the laboratory work. 

Deutsch, W. J. Serne, R. J. Bell, N. E. and Martin, W. J. "Aquifer Restoration at In-Situ Leach Uranium Mines ... 

Larson, W. C. "Uranium In Situ Leach Mining in the United States" Information Circular 8777 U. S. Dept, of the Interior, Bureau of Mines Washington, D. C., 1978. 

Goddard, J. B. and Brosnahan, D. R. "Rate of Consumption of Dissolved Oxygen During Ammonium Carbonate In Situ Leaching of Uranium" Mining Engineering 1982 34 (11), 1589-1596. 

Table 40 Water analysis of a natural groundwater (GW) and groundwater influenced by in-situ leaching (ISL) (concentrations in mg/L)...

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