Optimisation of the calcrete-uranium process flowsheet: the implications of metallurgical testwork for the Marenica project

Marenica Energy is carrying out a study of the recovery of uranium from the Marenica deposit in Namibia. The Marenica deposit is located in the Damara Province in the Erongo region of the Karibib district, some 70 km east of Henties Bay and 60 km west of Usakos. The regional geology of the area is dominated by the northeast trending Damara Mobile Belt, which typically comprises high-grade metamorphics with granitic intrusions. Tertiary aged calcrete deposits, as well as Karoo sediments, also host uranium mineralisation. Deep weathering and erosion of the uranium-rich granites and felsic volcanics, under semi arid to arid conditions, has created valley fill sedimentary deposits within incised Tertiary drainage channels. Uranium was leached from source rocks and transported by slightly alkaline oxidising groundwaters, with subsequent precipitation leading to the formation of surficial calcrete deposits. Calcrete-hosted uranium deposits form within fluvial sediments under arid conditions, where near-surface groundwater and high evaporation promotes the deposition of calcite (± barite, dolomite and celestite) as an intergranular cement. Carnotite is the main uranium mineral and occurs as coatings on grains, along fractures, and as disseminations throughout the calcrete and gypcrete. Carnotite is readily leached in alkaline solution. Alternative processing techniques were examined in laboratory tests and included ore sorting, heap leach amenability tests and scrubbing/size separation. Although the options are still open, the ore has been demonstrated to be amenable to upgrading by scrubbing and to alkaline leaching. Agitated slurry leach optimisation and uranium recovery testwork is underway and up-to-date results on the progress of this work at ANSTO, and the flowsheet implications, are presented and discussed.