Selective uranium leaching and separation from Ochre-Umm Greifat, Red Sea coast, Central Eastern Desert, Egypt

The Ochre-Umm Greifat area is one of the Red Sea areas with high concentrations of iron and zinc, which is formed from hydrothermal solutions as a result of the structural activity that occurred in the Red Sea zone during the Pleistocene period. These deposits are also accompanied by deposits of low to high uranium grade in addition to zinc, lead and copper anomalies, particularly in fault zones and their branches affecting the studied area. Although there are numerous zinc minerals in the Ochre-Greifat area, uranium minerals are scarce, with only one mineral, compreignacite, being recorded, and the majority of the uranium is present as an adsorbed element on iron and/or clay stones. In addition, uranothorite is extremely rare and occurs as fine grains embedded in rocks. In this study, a technological sample was taken from an iron-rich clay area in a fault zone and was found to assay 700 ppm uranium. The leachability of uranium from the sample was investigated using an alkaline solution based on the chemical and mineralogical composition of the sample. The selected ore was treated with sodium carbonate and sodium bicarbonate in the presence of hydrogen peroxide as oxidant. Several digestion factors were studied and optimized. Under the optimum leaching conditions, the uranium dissolution efficiency was around 84 percent. For uranium separation, the pH of the leach liquor was adjusted at 10, then subjected to a solvent extraction step using 4 percent Aliquat 336/kerosene in the presence of isodecanol as third-phase prevention. The loaded organic solvent was then treated with a sodium hydroxide/hydrogen peroxide solution as a stripping solution. Finally, the resultant solution was subjected to a precipitation step using ammonia solution. Background The Ochre-Umm Greifat area on the Red Sea coast in Central Eastern Desert, Egypt, has high concentrations of iron and zinc (Fig. 1). In general, radioactive anomalies have been concentrated along the northwest-southeast trending normal fault that cuts the Miocene clastic-carbonate sediments at Ochre-Umm Greifat and other northwest locations such as Wadi Abu Gherban, Wadi Wizr and Wadi El-Sharm El-Bahari [1-2]. These radioactive anomalies are structurally controlled, and their association with hydrothermal mineral deposits gives support to their hydrothermal origin. A study by Abu-Elatta and Mansour [3] concluded that the iron, manganese, zinc and uranium deposits at Ochre-Greifat and Wadi Wizr are hydrothermal in nature. Fig. 1 Ochre-Umm Greifat sample location. Previous researchers discovered many zinc-bearing minerals in the studied area, as well as lead and copper-bearing minerals. It lacks uranium minerals, despite the high uranium concentrations found in some zones, with the exception of compreignacite (a secondary uranium mineral) and uranothorite (which is extremely rare and occurs as fine grains embedded in rocks). As a result, clay minerals and iron oxides absorb uranium species. These deposits are also accompanied by deposits of low to high uranium grade in addition to zinc, lead and copper anomalies, particularly in fault zones and their branches affecting the studied area. In our study, we used a technological sample taken from an iron-rich clay area from a fault zone, and it was found to assay 700 ppm uranium (Table 1). The leachability of uranium from the chosen sample was investigated using an alkaline solution based on the chemical and mineralogical composition of the studied sample. The selected ore was treated with sodium carbonate (Na2CO3) and bicarbonate (NaHCO3) in the presence of hydrogen peroxide as an effective oxidant. Methods In general, in the absence of bicarbonate, uranium reprecipitation occurs at a pH of about 11. As a result, the bicarbonate ion is added during the uranium dissolution from the sample from the Ochre-Umm Greifat area: 2Na4UO2(CO3)3] + 6NaOH+ →Na2O2O7↓ + 3NaCO3 + 3H2O (1)HCO3− + OH− → CO32− + H2O (2) In this work, hydrogen peroxide was used for oxidation of the uranium content in the studied sample because the uranium in carbonate solutions differ from acid liquors in that they lack oxidative activity when no oxidants are present. In general, only the hexavalent uranium species is dissolving during the alkaline treatments, while the tetravalent remains insoluble. Hence, the oxidation of tetravalent uranium to a hexavalent state is required for economic recovery. The peroxide’s primary function in the carbonate medium is to convert unreachable U(IV) to leachable U(VI): UO3(oxidized form) + 3CO32− + H2O →[UO2(CO3)3]4− + 2OH− (3)UO2(nonoxidized form) + 3CO32− + ½O2 →[UO2(CO3)3]4− + 3H2O (4) Using carbonate/bicarbonate as a leaching solution to avoid the limitations of the previous works for uranium dissolution using mineral acids or sodium hydroxide solutions, the use of carbonate solution is a good choice for uranium dissolutions from carbonate-bearing rocks such as calcite, dolomite and others [4]. In addition to these benefits, metals such as iron, titanium and silicon in the original sample do not dissolve in carbonate solution, allowing for the recovery of high-purity dissolved uranium [5]. Furthermore, the carbonate as leachate has low environmental impact (no gas emissions) and high uranium dissolution. Results and discussion For high uranium leaching using the alkaline solution, several factors including carbonate and peroxide concentrations, solid/liquid phase ratio, digestion time, grain size and reaction temperature were investigated and optimized. Under the obtained optimum leaching conditions, the uranium dissolution efficiency was around 84 percent. Fig. 2 Flow sheet for uranium separation from the Ochre-Umm Greifat sample. For uranium separation, an experiment was performed under the optimized leaching conditions. The resultant leach liquor is then subjected to pH adjustment at 10 using sulfuric acid and sodium carbonate solutions. The resultant leach liquor is subjected to a solvent extraction step using an Aliquat 336/kerosene solvent in the presence of isodecanol as a third-phase prevention. The extraction of U(VI) with Aliquat 336 (sulfate) takes the form indicated by [6]: 2(R4N)+.SO4−org + UO2.(CO3)34− →2[R4N UO2(CO3)3]org + 2SO42− (5) The loaded organic solvent was then treated with sodium hydroxide/hydrogen peroxide (NaOH/H2O2) solution as stripping solution. Finally, the resultant solution was subjected to a precipitation step using ammonia solution. Conclusion The final leaching, extraction and precipitation process of the uranium content from the Ochre-Umm Greifat area is presented in a flow sheet (Fig. 2). Selected references Hassaan M (1990) Studies on lead zinc sulphide mineralization in the Red Sea coastal zone, Egypt. In: Proc. 8th Symp. IGADO, Otowa, Canada, 835–847 Ramadan T, El-Lithy B, Nada A, Hassaan M (1999) Application of remote sensing and GIS in prospecting for radioactive minerals in the Central Eastern Desert of Egypt. J. Remote Sens. Space Sci 2:141–151 Abu-Elatta S, Mansour G (2018) The hydrothermal uranium and some other metal deposits of the extensional faulting during the advanced opening of the Red Sea, Central Eastern Desert, Egypt, Beni-Suef. University Journal of Basic and Applied Sciences 7(4):752–766 Elizângela A., Santos E., Ladeira A. (2011), Recovery of uranium from mine waste by leaching with carbonate-based reagents. Environ Sci., Technol 45(8):3591–3597, https://doi.org/10.1021/ es2002056 El-Ansary A., Abd El Wahab G., Bayomi E., Nouhb E. (2017), Purification of Abu-Zenima wet crude yellow cake using alkaline leaching of U(VI). Egypt J Pet (in press), https://www.scien cedirect.com/science/article/pii/S1110062117301927. Manaa E. and Lasheen T., (2016,), Liquid–Liquid Extraction of Sulfuric Acid from Aqueous Sulfate Waste Solution using Alamine 336/Kerosene/TBP Solvent, Journal of Dispersion Science and Technology, 37:137–143, https://doi.org/10.1080/01932691.2015.1034362

Selective uranium leaching and separation from Ochre-Umm Greifat, Red Sea coast, Central Eastern Desert, Egypt

Authored by: El‑Sayed A. Manaa, Soliman Abu Elatta Mahmoud and Elham Awny

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