Uranium Minerals

Kadapa district of Andhra Pradesh

Oxidative pressure leaching of uranium from a dolomitic limestone ore:

India has a medium-tonnage, low-grade uranium ore deposit of siliceous dolomitic phosphatic limestone type, in Kadapa district of Andhra Pradesh. Detailed exploration carried out in a stretch of about 9 km in this area, established a resource of 29000 t of U3 O8 with a cut-off grade of 0.025% U3 O8. Mineralogical studies on an exploratory mine ore sample from this area, indicated the occurrence of uranium values predominantly as ultra-fine dissemination, in lighter gangue minerals (specific gravity less than 3.2). It also occurs, albeit to a minor extent, in the form of ultra-fine pitchblende in association with pyrite, as disseminations in collophane-rich parts, coffinite and as U-Ti complex. Carbonate minerals constitute the major gangue present in the form of dolostone (~80%). Siliceous minerals in the ore are quartz, feldspar and chlorite (13%). Collophane (4%) is the only phosphate bearing phase. Pyrite is the predominant sulphide ore mineral, along with few grains of chalcopyrite and galena. The iron bearing oxides are magnetite, ilmenite and goethite. Heavy media separation of various closely-sized feed fractions, using bromoform (BR) and methylene iodide (MI) liquids, have indicated that about 91% of the uranium values are present in lighter minerals (specific gravity <3.2), as ultra-fine disseminations. The remaining 9% of uranium values reported in methylene iodide heavy fraction, are accounted by discrete pitchblende, which is mostly associated with pyrite and collophane. Pitchblende occurring with pyrite is present as fine orbicular cluster, separated by thin disconnected rims of pyrite or as garlands around pyrite.

Leaching Chemistry of Uranium Minerals

The common oxidation states of uranium, in its minerals like uraninite, pitchblende, coffinite and numerous others, are +4 and +6. Amongst the two oxidation states, U+6 is soluble in aqueous media under suitable EH – pH conditions, while U+4 is practically insoluble. The uranium minerals occurring in various ore deposits consist predominantly of uranous ion (U+4), necessitating the use of an oxidant and other lixiviants, for quantitative dissolution during leaching. The type of leaching - acid or alkaline mode depends upon the host rock. Sulfuric acid is the common leachant in acid leaching process, while Na2 CO3 - NaHCO3 , (NH4 ) 2CO3 and NH4 HCO3 are the widely used lixiviants in alkaline leaching of uranium ores. The oxidant reagents could be either chemical or gaseous in nature. A typical chemical reaction in alkaline leaching of UO2 with carbonate ions and oxidant (X) is given in Equation 1, a similar equation can be written for the sulfuric acid leaching process. UO2 + 3CO3 -2 + X  [UO2(CO3) 3 ] -4 + X-2 .

Atmospheric alkaline leaching studies, carried out on this ore sample, by varying important process parameters like mesh-of-grind, temperature, contact time, dosages of leachants - sodium carbonate and sodium bicarbonate, solids concentration and type of oxidant, gave a maximum U3O8 leachability of 65%. Studies with other oxidants like NaOCl, Cu-NH3, oxygen and air gave poor leachability in comparison to KMnO4, emphasizing the need for strong oxidizing conditions during the dissolution process. However, as KMnO4 cannot be used as an oxidant on commercial scale due to its expensive nature, the only alternative is to carryout the leaching reaction in a pressure reactor, using a gaseous oxidant. Since the solubility of oxygen diminishes with increasing temperature, adoption of higher partial pressure aids in increased dissolved oxygen concentration. Detailed analysis of the leach residue obtained in the atmospheric leaching experiments indicated, that uranium values associated with pyrite are not completely leached at temperatures <1000 C. Further, some of the locked-up uranium values in various gangue phases, require more aggressive diffusion conditions for penetration of the leachant to the desired mineral interface. Both these requirements can be realized only at elevated temperature and under sustained oxidizing conditions, possible in an autoclave reactor. Leaching at elevated temperature and pressure was initially carried out in a laboratory, 5 liter S.S. autoclave reactor equipped with necessary instrumentation and control to maintain preset temperature, overpressure and agitation speed of the impeller. All the autoclave leaching experiments were carried out, at optimum dosage combination of sodium carbonate and sodium bicarbonate evolved in atmospheric leaching, that is - 50 kg/ton and - 70 kg/ton respectively. The autoclave leaching studies mainly addressed the dissolution of uranium associated with pyrite and the scope of replacing KMnO4 with industrial oxygen. Figs. 3 and 4, illustrate the effect of temperature and contact time on the leachability of uranium values, observed under aggressive conditions. About 75% of uranium values were leached at a reaction temperature of 125 - 130°C in 3 h of contact time, using a feed ground to 65% weight finer than 200#. Increasing the fineness of grind in -200# to 85% showed, an enhancement in leachability to about 80%. Based on these results, large-scale leaching studies were carried out, both on batch and continuous leach reactor, to generate necessary scale-up and engineering data for industrial scale reactor, besides verifying the reproducibility of results at higher-scale of operation. Both the batch and cigar type continuous reactor were of 850 liter capacity with inconel 600 as material of construction. Largescale studies confirmed the results generated in batch scale experiments. At present, DAE is setting-up a 3000 tpd capacity uranium mill at site, wherein two 720 m3 capacity autoclave reactors with inconel 600 cladding for the wetted parts will be used. This will be the first uranium plant, using autoclave leaching technology in India.