Biohydrometallurgy Of Uranium Dump, Heap and Insitu

Transcription

1 Lecture 17 Biohydrometallurgy Of Uranium Dump, Heap and Insitu Leaching Keywords: Dump, Heaps, In-situ Leaching Dump, Heap, in situ leaching Dump millions of tons of over burden / waste rock Dumps often built in a valley to take advantage of natural slope not inoculated- flooding / sprinkling the top surface. Heap-rocks are often crushed to avoid solution contact problems - built up on impermeable pads to prevent loss of solution to underground - aeration systems installed to increase flow of air. Inplace leaching for low grade ores present in inaccessible sites abandoned workings depleted mine working leach solution directly applied to walls and roofs of an intact stope (an underground excavation from which ore has been removed ) or to the rubble of fractured workings. Percolation bacterial leaching methods for the treatment of uranium ores In-situ leaching: Submarginal ores obtained by blasting; ore treated for 5 25 years with an operational capacity of 4 million tons of ore. Fig illustrates design aspects of heap bioleaching for uranium. 1

2 A B C Fig 17.1: Design aspects (A-B) of heap and (C) shallow stope leaching for uranium. 2

3 Surface ore heaps and underground mined stopes can be efficiently bioleached as shown in fig Fig. 17.2: Bioleaching of surface ore heaps and underground mined stopes. In situ leaching concepts are illustrated in fig

4 A B Fig. 17.3: In situ leaching concepts for uranium from underground deposits. 4

5 Dump leaching: Submarginal ores obtained by bulldozing; ore treated for 3 20 years with an operational capacity of 5 million tons of ore. Heap leaching: Low grade ores obtained by crushing - ore treated for 1-2 years with an operational capacity of 3 x 10 5 t of ore. Vat leaching: Similar to leaching in heaps but for days and with an operational capacity of 5 x 10 3 t of ore. Mine waters: Water collected after rain in open pit mines. In situ ore leaching from injection wells and producing wells There may be difficulties connected with ecology since it is necessary to collect the leach liquor after it has passed through the ore beds. Unsuitable silting may lead to large amounts of the leaching fluid escaping underground. There are some bacterial leaching processes which in a broader sense can be termed in situ leaching. For example, percolating of a worked-out mine with residual ore. In Canadian uranium mines after they were worked-out, the walls, roofs and floors were hosed down at intervals of several months. The water was collected and the uranium extracted. Bioleaching of uranium ores depends wholly on indirect oxidation in presence of ferric/ferrous iron system. Leaching of uranium ores containing pyrite as an iron source is most economical. Otherwise one has to add pyrite or another source of iron to facilitate bioleaching. Heap or basin leaching can be applied. The ore is set up in basins. The mode of operation is preferably a two stage leaching: the out-flowing liquor, in which the iron is largely in the ferrous form, is treated in an oxidation pond. The leach systems are aerated to enable Acidithiobacillus ferrooxidans to oxidize ferrous iron and to obtain the ferric iron required for oxidation of uranium. The oxidized liquor is then pumped back to the dump or basin. 5

6 Microbially mediated uranium recovery from low-grade ores dates back to the 1950 s. Initial work in early 1950 s directed towards preventing solubilization Urgeiric Uranium ore (Portugal) Problem due to U-solubilisation by Fe and S bacteria and washout by rain water. During , U-commercial bioleaching at Urgeirica (Portugal). Subsequently, bioleaching processes for commercial extraction of U by heap, dump and stope leaching of mine waste rocks and worked out stopes in the Elliot lake Area, Ontario, Canada in the early 1960 s. Leach solutions containing acidic ferric sulfate circulated through surface heaps and underground stopes. Laboratory, pilot and commercial scale bioleaching have been evaluated in many uranium producing countries. Potential of this biotechnology to low-grade U-ores containing < 0.50 % U 3 O 8 recognized. 6